Economic Geology and the Bulletin of the Society of Economic Geologists 1916-06: Vol 11 Iss 4

1916 · Historical

Economic Geology and the Bulletin of the Society of Economic Geologists 1916-06: Volume 11 , Issue 4. Digitized from IA1518511-02 . Previous issue:…

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Economic Geology

With Which Is Incorporated

The American Geologist

VoL XI JUNE, 1916 No. 4

Conservation Of The Oil And Gas Resources Of The Americas.

RatpH ARNOLD. Part II.

CoNnTENTS.

Central America

West India Islands

South America .

Summary Table of Oil Resources of the Americas Methods of Conservation

(Continued from Vol. XI., No. 3. Concluded in this number.)

Central America

Little is known of the oil possibilities of the Central American countries, but from the information available it does not seem probable that any commercially important fields will ever be de- veloped in them. Certain regions showing surface evidences of petroleum have been examined by private interests, but little or no effort has been put forth by the governments themselves to explore or exploit their oil resources.

Guatemala.

The oil-bearing formations of Chiapas and Campeche extend southeastward into eastern Guatemala, where they are said to yield indications of petroleum, especially near the Mexican bound-

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300 Ralph Arnold.

ary. The only exploratory work so far carried on has been done by the International Railroad of Central America, which: has developed a small production for its own use.

Honduras.

Good seepages of oil are reported as occurring in limestone (presumably of Cretaceous age) near Comayagua, in the Guare Mountains, Honduras. Indications are also said to exist at points along the Caribbean coastal plain. No development, so far as known, has been attempted.

Costa Rica.

Seepages of oil are known along the east coast of Costa Rica, and efforts to procure concessions have been made at one time or another in the past. No detailed examinations and no develop- ment work have been attempted, so that the possibilities of the country are at present unknown.

Panamia.

Seepages of oil occur at San Miguel Bay, on the Pacific side of the Isthmus of Panama, 50 to 60 miles northwest of the Colombian boundary. They are probably in northward exten- sions of the oil-bearing formations of Colombia. Asphalt de- posits also occur on the Mosquito Gulf, abut 100 miles south- west of Colon, and also in the Gulf of Montijo, on the opposite or Pacific side of the isthmus. Superficial examinations have been made at these localities, but no detailed surveys or develop- ment work have been attempted. From the evidence in hand it sems probable that commercial deposits of oil exist in this Republic.

West India Islands.

General Statement.

In the West India Islands, which lie to the north and east of the Caribbean Sea, between North and South America, oil indi- cations have been found in Cuba, Haiti or Santo Domingo, Porto

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Oil And Gas Resources Of The Americas. 301

Rico, Barbados, and Trinidad. Drilling for oil has been done in all these islands except Porto Rico, but important commercial results have been obtained only in Trinidad.

Cuba.

Indications of petroleum exist in all the provinces of Cuba, but the most productive localities are confined to Pinar del Rio, Havana, Matanzas, and Santa Clara. The surface evidences con- sist of oil seepages in serpentine or at the contact of serpentine and limestone. The limestone is of Cretaceous age; the serpen- tine is probably pre-Cretaceous. The deposits occur in crevices or joint cracks. Such of the oil as has been recovered is of a re- markably high grade, ranging from 55° to 70° Baumé (0.7568 to 0.7000 specific gravity). Only one well is at present produc- ing in the island. This well, which belongs to the Cuban-Ameri- can Sugar Co., is at Motembo, in the Province of Santa Clara, is about 1,900 feet deep, and yields 10 gallons of 70° Baumé oil daily. Other wells from 300 to 700 feet deep have been drilled in the same region. The most that has so far been taken from a single well (Cardefias) does not exceed 100,000 gallons. An- other well is said to have produced 18% gallons daily.

Drilling is now being done at Esperanza, in the Province of Pinar del Rio, where a well has reached a depth of 830 feet in limestone, with no results. At Minas, 15 miles east of Havana, a well put down near some seepages reached a depth of 350 feet but encountered no oil. At Puentes Grandes a well sunk to a depth of 600 or 700 feet in 1913 encountered considerable quanti- ties of gas. Another well is now being sunk at the same locality. Several wells have been sunk both east and west of Cardeijias, attaining depths of 1,000 to 2,385 feet, the latter the maximum depth for the island. They have all started in limestone and finished in serpentine. Commercial production was not obtained in any of the wells.

To judge from personal observations made by C. B. Osborne,

® Owing to the writer’s professional connections in Trinidad he deems it inexpedient to discuss the oil resources or possibilities of that island.

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the writer's assistant, and other geologists, from the results obtained in the tests already made, and from the theoretically unfavorable conditions for extensive accumulation that surround the oil deposits of Cuba, it seems improbable that any consider- able yields of oil will ever be obtained in this island.

Haiti and Santo Domingo.

Two localities in the island of Haiti yield indications of petro- leum—one 3 miles north of Agua and the other on the cost near San Cristobal, 10 to 15 miles west of the town of Santo Domingo. The rocks from which the oil comes are said to be of Cretaceous age. The record of but one well, that at Agua, 7 miles north- west of Ocoa Bay, is available. This well attained a depth of 940 feet and is said to have flowed commercial quantities of an oil of about 20° Baumé gravity (0.917 specific gravity), carry- ing 2.5 per cent. of sulphur. No remunerative results have yet been obtained on the island, and whether this is due to unfavor- able natural or commercial conditions is unknown to the writer. At best this island can be said to offer only insignificant potenti- alities so far as oil is concerned.

Porto Rico.

According to Redwood, exudations of petroleum are reported as occurring at several points on the island of Porto Rico, being possibly derived from the carbonaceous Tertiary beds. As no indications worthy of a serious test have yet been discovered, it seems unlikely that the island will ever yield oil in commercial quantities.

Barbados.

The deposits of petroleum and manjak (a type of asphalt) in Barbados are confined to the Scotland district, on the east side of the island. The containing beds are much-disturbed sand- stones and shales of Oligocene age, which are known locally as the Scotland beds and are probably related to the oil-bearing Tertiary formations of Trinidad. Shallow wells, some dug and

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Oil And Gas Resources Of The Americas. 303

others drilled, have been put down at three or four localities, yielding small quantities (1 or 2 barrels daily) of a moderately heavy black to greenish oil ranging from about 14° to 30%° Baumé (0.971 to 0.872 specific gravity). As the most favorable localities have been tested with only mediocre results, it can be assumed that Barbados will never become an important factor in the production of petroleum.

SOUTH AMERICA. General Conditions.

Indications of petroleum were noted in South America as far back as 1788, when Humboldt described the oil seepages and mud volcanoes of northern Colombia, but little was done toward oil development until 1896, when active operations began in Peru. Even at the present time commercial production is confined to Peru and Argentina, and the combined yield of the two coun- tries is 1914 was only about 2% million barrels, or 0.6 per cent. of the world’s production. Practically all the countries of South America afford oil indications of more or less importance, but the principal attention is now being given to Colombia, Vene- zuela,‘ Peru, and Argentina.

Colombia.

General Features.—Although development of oil resources has been carried on in Colombia for many years no marked commer- cially successful results have yet been obtained. The oil ranges from the heavy oil of asphaltic type to a paraffin base oil of 41° Baumé (0.8187 specific gravity) and is believed to be derived largely from Cretaceous and lower Tertiary formations. The prospective territory may be divided into four general districts as follows:

7 Owing to the writer’s professional connections in Venezuela he deems it inexpedient to discuss the oil resources or possibilities of that country at the present time.

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TABLE VIII. AreAS INCLUDED IN THE Prospective Ort Districts oF CoLoMBIA, IN SQUARE Mites. District. Area. Possible Oil Ter- Proved (il Ter- ritory. ritory . Magdalena-Santander 10,000 200 I 34,300 618 2

Caribbean District.

The Caribbean district extends from Rio Hacha, on the west edge of the Guajiro Peninsula, southwestward along the Carib- bean coast to the Gulf of Darien and Gulf of Uraba, and inland to include the Tubara and Turbaco fields and the region as far south as 30 miles south to Chima, on Sinu River, and Monpos, on San Jorge River, and up the Arato River valley for 90 miles from Punta Arena. It occupies portions of the departments of Magda- lena, Bolivar, and Cauca. It is approximately 300 miles long and 50 miles wide and contains 15,000 square miles. Of this area probably 300 square miles contain oil possibilities, while less than I square mile is as yet questionably proved as commercially pro- ductive. The rocks carrying the oil are mostly coal-bearing also and are of early Tertiary, probably Oligocene age. The great bulk of the sediments are dark-colored shales, with sandstone members; the oil occurs in the sandstone and also, to judge by the type of certain seepages, in joints in the shale. The structure is that of broad to sharply folded and faulted anticlines, and the surface evidences of oil are usually, though not invariably, con- fined to the anticlinal areas. The gravity of the oil ranges from 16° to 41° Baumé (0.9587 to 0.8187 specific gravity), the bulk of it, as now known, falling between 20° and 30° Baumé (0.9333 to 0.8750 specific gravity). In the only fields in which wells have been drilled the production of the individual wells has usually been under Io barrels daily. However, these wells, of which only ten are known to the writer, should not be taken as criteria for judging the possibilities of the country, as pioneer wells are

Oil And Gas Resources Of The Americas. 305

usually shallow, are often not advantageously located structurally, and are always drilled under adverse mechanical conditions. It is the belief of those who have seen the various fields in this dis- trict that at least some of them give promise of yielding com- mercial quantities of oil of medium to high grade. The following are among the more promising localities in the district:

The Turbaco field lies 12 to 15 miles south of Cartagena, and its surface evidence of oil consists largely of mud volcanoes. Here Diego Martinez & Co. and their associates, the Standard Oil Co., have drilled five wells, ranging in depth from 500 to 2,200 feet, some of which are said to yield a medium-grade oil. This same group owns a small refinery in Cartagena in which oil imported from America is refined. The Standard Oil Co. is also drilling in the Sinu River valley near Lorica.

The Tubara field is 20 miles east of Cartagena. A Canadian company is operating here and has drilled three wells, ranging in depth from 700 to 3,018 feet, at least one of which yielded 7 or 8 barrels daily of oil having an asphalt base and a gravity of 22° to 26° Baumé (0.9211 to 0.8974 specific gravity). As many as 100 mud volcanoes are said to occur in an area of 3 acres in the vicinity of the wells. Sulphur springs and oil seepages occur at Usiacari, 2 miles south of Tubara, and oil seepages also are found 8 miles west of Tubara.

A seepage yielding 7 barrels daily of oil having a paraffin base and a gravity of 41° Baumé (0.8187 specific gravity) is reported by Mr. Vassar as occurring 10 miles south of Punta Arena, or 4 miles east from the Gulf of Uraba. This high-grade crude oil is used in lamps by the natives.

An area of 800 square miles along the west side of the Gulf of Darien yields coal and indications of oil having a gravity of 16° Baumé (0.9589 specific gravity). Indications of oil are also found at Rio Hacha, on the north coast, near Calamar, on Magda- lena River, and near Baranca, back of Salgar on the Puerto Colombia Railway, in the region of Galera Zamba and Savana Larga.

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Pacific District.

The Pacific district includes a belt 60 or 70 miles long extena- ing up the Pacific coast north of Buenaventura to Baudo River, and reaching inland to Atrato River at Quibdo and as far south as Cali, on Cauca River. The district is included in the Depart- ment of Cauca and occupies an area of 1,800 square miles, of which a very small area only—perhaps I per cent., or 18 square miles—has possibilities of oil. The rocks in this region are pre- dominantly shale probably of middle Tertiary age, and are quite sharply folded. Many hot springs and salt springs exist and a few emit gas of some sort. No oil occurs along the coast. The oil on Baudo River is of 31° to 37° Baumé gravity (0.8696 to 0.8383 specific gravity), is associated with the “coal series” and probably occurs in a southwestward extension of the Caribbean coastal belt. Oil has also been floating on Andagueda River. a tributary of the Atrato.. Heavy oil is reported east of Quibdo, where the Quibdo-Medellin trail crosses Tutendnendo River; and also 20 miles from Santa Rosa and 4 miles north of Porce River, which is south of Nechi River.

Magdalena-Santander District.

A district that is probably as important but more inaccessible and less well known than that along the Caribbean coast includes the southern part of the Department of Magdalena, the Depart- ment of Santander, and the western edge of the Department of Boyaca, extending from Magdalena River to the eastern Cordil- leras. It also occupies an area in the southeastern part of the Department of Bolivar. It covers a belt approximately 200 miles and 50 miles wide, or 10,000 square miles, of which possibly 200 square miles contains oil possibilities. The only proved ter- ritory is less than a square mile in extent and is found at Pam- plona, near the Venezuelan frontier, where a small refinery handles the high-grade product of a few wells for the local trade. The oil in this district occurs in the Cretaceous limestones and sand- stones and the coal-bearing lower Tertiary (probably Oligocene) beds, where sandstones are the reservoirs. Long, well-defined,

Oil And Gas Resources Of The Americas. 307

and in places overturned anticlines and possibly fault zones in the Cretaceous rucks are the advantageous structures for accumula- tion. The oil is of both high and low grade. The yield of the producing wells, which are all very shallow, is small, but greater yields will probably be encountered at greater depths. This dis- trict gives promise of some good fields of both high and low grade oil. The country is largely inaccessible at present, and its development will require immense capital and a long time. Among the localities that have received special mention for their oil indications are Pamplona near the Venezuelan boundary, a belt between Cesar River and the Venezuelan frontier, touching the Magdalena above £1 Banco; an area near La Gloria, on the Magdalena, where heavy oil and asphalt are found; and the seep- ages of Simitt, in the Department of Bolivar. A line of seepages, some of which are reported to yield a barrel of oil a day, extend along the Magdalena flank of the eastern Cordilleras from a point south of Girardot northward to Bucaramanga. The beds along this line are sharply folded and in many places fractured.

Tolima District.

Under the Tolima district may be grouped the occurrences in the upper Magdalena basin, in the Departments of Cundinamarca and Tolima, and on the edge of the San Martin and Casanare plains. The oil-bearing rocks are probably of Cretaceous or Tertiary age and form a continuation of those in the Santander belt, to the north. In the Province of Medina, at the foot of the extinct volcano of Guaycarame, in that part of the Cordil- leras which terminates in the plain of Medina, 7%4 miles from Upia River, tributary to the Meta, petroleum of about 22° Baumé gravity (0.926 specific gravity) comes from shale fissures. At a point 140 miles south of Nevea, on the east side of the Andes, Mr. Vassar collected samples of paraffin-base oil testing 31° to 38° Baumé (0.8696 to 0.8333 specific gravity). He believes that this deposit may extend into Brazil. Oil is also reported from Santa Rosa, on the right bank of Magdalena River.

These deposits are all far from transportation facilities, and their

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308 Ralph Arnold.

development will entail great expense; furthermore the markets will be entirely local until adequate communication is opened to the outside world. It is therefore obvious that these remote regions probably will remain undeveloped for a very long time.

Conclusion.—The surface evidence and what drilling has been done leads to the conclusion that valuable deposits of petroleum of various grades exist in the Caribbean and Santander districts of Colombia. The testing and development of certain parts of the Caribbean district can be accomplished rather easily, but through the major portion of Santander and the other districts many natural obstacles will have to be overcome before com- mercial production can be attained.

Dutch Guiana.

Oil seepages or indications have been known in Dutch Guiana since the eighteenth century. There are three ‘localities about which information is available at present. One is on the south side of Surinam River, 6 miles below Kabele station and 97 miles by rail south of Paramaribo. Here there are exposures of shale and some sandstone yielding small quantities of a high-grade amber-colored oil. A concession has been issued covering 1,700 acres in the region of the seepage.

Another area is situated on Marowijne River 100 miles above Albina. A shale formation cut by serpentine dikes occupies this region. The oil seepages, which occur along a stream course, are small and are found in the shale about 5 miles from the nearest serpentine. The quality of the exuded oil is excellent.

A third belt lies between Surinam River and the railroad, about 48 miles above the head of deep-water navigation (16 feet). The oil here has a gravity of 45° Baumé (0.800 specific gravity) and occurs in small seepages.

To judge by the small size of the seepages and the little that is known of the geology of the country, it will be surprising if the oil deposits of Dutch Guiana are ever found to be more than un- important local pools.

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Oil And Gas Resources Of The Americas. 309

French Guiana.

Oil seepages occur in French Guiana southeast of Marowijne River, in formations which are the continuations of those that yield the seepages in Dutch Guiana. It is believed that the possi- bilities here arc insignificant.

Ecuador.

The best-known oil field of Ecuador is that of Santa Elena about 64 miles west of Guayaquil. The principal surface indica- tions, consisting of “gum” deposits and oil seepages, occur at San Raimondo, where shallow dug wells have been put down near the coast; and at Santa Paula and Achagian, 2 or 3 miles inland. The field is believed to be a northward continuation of the Peru- vian petroliferous area, and the oil comes from sandstones and shales of Eocene age. Forty dug wells at Santa Paula yield small quantities of heavy oil which is taken to the coast on donkeys. A number of dug wells were formerly operated at Achagian. The oil ranges in gravity between 12° and 22° Baumé (0.985 to 0.928 specific gravity). Oil springs are reported on the east side of the Andes, 130 miles north of east of Guayaquil, and at points in the coastal plain north of the same port, particularly at Atacamas.

As the known deposits of Ecuador have not been thoroughly tested, any predictions as to their ultimate possibilities would obvi- ously not be justified. However, as the surface evidences are rather meager it does not seem likely that this country will ever play a very prominent part in the oil industry of South America.

General Conditions—Peru was the first country in South America in which oil was produced on a commercial scale. The yield in that country for 1914 was 1,917,802 barrels; the total production from 1896, when records of commercial quantities were first obtained, to 1914 was 14,306,972 barrels.

8 The data regarding Peru are largely furnished by Mr. Campbell M. Hunter, and the quotations are taken from a recent letter from him to the writer.

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310 Ralph Arnold.

The petroliferous areas of Peru can be separated into two general provinces—that of the Andes, comprising at present the Titicaca field; and that of the coastal belt, in which are the more productive Zorritos, Lobitos, and Negritos fields. The coastal belt extends southward along the Pacific Ocean from the Ecuador frontier for 180 miles, to and beyond Payta, and is bounded on the east by spurs of the Andes. It is about 30 miles wide and occupies the Province of Tumbis and the northern part of the Province of Piura. Oil is found at several other points in the Andes, as in the Huallanca region, in Cerro de Pasco, and in the Provinces of Jauja and Pariiiacochas, but little or no development work has been done in these isolated regions. The total area in- cluded in the oil belts is over 5,000 square miles, of which about 100 square miles has oil possibilities, and 200 square miles, ac- cording to Mr. Hunter, can be said to be proved. The oil-bearing rocks are largely sandstones that are sufficiently porous to render shooting unnecessary. The formation is of Eocene age and its fossils closely resemble those of the California Eocene.

“While the structure of the oil fields is very broken, the formation is essentially monoclinal, with the beds dipping to the eastward. Anti- clines have been located in some places, but, generally speaking, the pro- duction is not obtained from them’ so much as from monoclinal struc- ture with local saturation increased due to both dip and strike faults.

“No very big gushers have as yet been encountered, but wells yield- ing up to 800 barrels a day have been struck. The initial production is not very long-lived, and after a few weeks or months it settles down to from 4 to 7 barrels a day, which appears to be maintained for several years.

“The specific gravity of the oil is from 32° to 43° Baumé (0.8642 to 0.8092 specific gravity), its average contents being gasoline, 15 per cent.; kerosene, 35 per cent.; residuum, 50 per cent. The residuum is capable of treatment into high-grade lubricants. The oil has an asphaltic base, though its asphalt content is remarkably small, being in certain places only about 1.5 per cent.

“The depth of the wells ranges from 700 to 3,000 feet, the average being probably about 1,500 feet. The standard cable rig with calf-wheel attachment has been generally adopted and found to give very satis- factory results. The rotary has been tried, but owing to the amount of sandstone that is encountered, it has not been proved very satisfactory.

Oil And Gas Resources Of The Americas. 311

“Most of the production is exported, 80 per cent. going to the United States, the remainder being sent to Chile, where it is consumed prin- cipally on the nitrate fields as liquid fuel.”

The following figures of production of crude petroleum in Peru were compiled by Anne B. Coons, of the United States Geo- logical Survey.

TABLE IX. PropucTion oF CrupE IN Peru, 1896 To I914.

Zorritos Field.

The Zorritos is the northernmost of the Peruvian fields and lies a few miles south of Tumbez. The producing territory ex- tends along the coast for about 4 miles, most of the producing wells being drilled at the water’s edge. The Zorritos field is con- trolled by Sefior Piaggio and produces from 70,000 to 140,000 barrels a year. The oil is of asphalt base and ranges from about 37° to 43° Baumé (0.840 to 0.810 specific gravity). The wells are from 600 to 850 feet deep and yield an average of 6 barrels a day. Initial yields of several hundred barrels are reported.

Lobitos Field.

The Lobitos field lies south of the Zorritos field, about 60 miles north of Payta, and comprises a proved area of 25 square miles. In character of the oil and. productivity of the wells this field is similar to Zorritos. It is controlled by the Lobitos Oil Co. and yields about 560,000 barrels annually.

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Negritos Field.

The southernmost of the developed coastal areas is the Negritos field, which lies 40 miles north of Payta and includes about 150 square miles of proved territory comprised in the hacienda La Mina Brea and Parifias. The oil is brown and ranges in gravity from 35° to 38° Baumé (0.848 to 0.834 specific gravity). The wells range in depth from 500 to 3,000 feet and in individual production from over 800 barrels daily to an average settled pro- duction of 4 to 7 barrels. The Negritos field is controlled by the London & Pacific Petroleum Co., Ltd., and yields about 1,260,- ooo barrels annually.

Titicaca Feld.

The Titicaca field lies high in the Andes, 8 miles from Lake Titicaca, near the Bolivian frontier. It is controlled by the Titicaca Petroleum Co. and in 1910 yielded 50,000 barrels of oil. An anticline is said to extend through the field in a N. 40° W. direction, and the formations on its flanks are much disturbed. Ten wells had been drilled up to 1908, with varying results. Although the initial production in some wells has been as high as goo barrels daily, the output falls rapidly. The oil is unlike that of the northern fields of Peru, as it contains about 5 per cent. of paraffin wax and 40 to 50 per cent. of kerosene.

Conclusion.—The evidence in hand indicates good possibili- ties for the Negritos field and probably a less important future for Zorritos and Lobitos. The Andean petroliferous areas are little known, but no very promising fields are believed to exist in this province.

Bolivia.

The oil fields of Bolivia are believed to form in general a con- tinuation of those found along the eastern base of the Andes in northwestern Argentina. The fields extend from the vicinity of Santa Cruz, on the north, through Sauces to Piquirenda, Plata, and Guarazuti, in the Province of Tarija, near the Argentine boundary. Springs and seepages of petroleum are found at sev- eral points throughout this general region, coming chiefly from

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Oil And Gas Resources Of The Americas. 313

Lower Cretaceous dolomites. Some exploratory work and shal- low test drilling has been done in the region, resulting in the find- ing of some high-grade oil, 35° to 47° Baumé (0.850 to 0.790 specific gravity). Mr. Hunter, under whose direction the test drilling was done, says:

“The formation is very broken, being sharply folded and much faulted. Owing to the inaccessibility of the oil fields there is little likelihood of serious development work being initiated until better trans- port facilities exist.”

Still more recently geologic investigations in the area between the Incahuasi and Aguaraygua ranges, south of Sucre, have shown the presence of a considerable area of prospective oil land. It is the writer’s belief that these fields, owing to their com- parative inaccessibility, will not be an important factor outside of the local Bolivian market, at least for a very long time.

Chile.

Chile offers little inducement to the oil prospector, on account of its general unfavorable geologic conditions and paucity of surface evidences of hydrocarbons. Indications of petroleum are found south of Patillos, in the Province of Tarapaca, northern Chile, and oil has been reported as occurring at Puerto Porvenir and Agua Fresca, in Magallanes Territory. There is also an ex- tensive area south of Maullin River which gives indications of gas in Tertiary beds. These facts, derived from Redwood, lead to the conclusion that Chile is destined to play little or no part in the oil industry of South America.

Argentina.

General Conditions——Argentina first attracted attention as a possible producer of petroleum in 1907, when oil was discovered, entirely by accident, in drilling for water at Comodoro Riva- davia, on the coast of Patagonia. Previous to that time test drilling had been done in the Andean portion of the republic, but no significant results were obtained. The area included in the

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petroliferous districts exceeds 8,000 square miles, and of this territory probably 400 square miles gives superficial evidence of petroleum. The proved area does not exceed 2 square miles. The petroleum occurs in rocks of Jurassic, Cretaceous, and Eo- cene age, limestones, dolomites and sandstones predominating. Both anticlines and practically structureless areas yield commer- cial quantities. The oil is asphaltic and ranges in gravity from I1° to 24° Baumé (0.996 to 0.9091 specific gravity). The wells attain depths from 200 to more than 4,000 feet and are rather small producers, so far averaging less than 100 barrels daily. The production, which was approximately 275,000 barrels in 1914, comes entirely from the Comodoro Rivadavia district and is consumed in the country, principally for fuel.

The petroliferous area of Argentina is divisible into three prin- cipal districts—the Comodoro Rivadavia district, on the Atlantic coast, and the Salta-Jujuy and Mendoza-Neuquen districts, in the Andean region.

Comodoro Rivadavia District.

The Comodoro Rivadavia district, the best known and only commercially productive field in Argentina, is situated on the Gulf of St. George, 850 miles south-southwest of Buenos Aires. According to the government geologists who have studied the dis- trict, the possible petroliferous area is extensive, possibly includ- ing 20 square miles; some other geologists are still more opti- mistic, but a few think that the productive area will be confined pretty closely to the present proved territory of I or 2 square miles. The oil-yielding formation is of Cretaceous age, the oil coming from coarse pebbly sandstone, which lies on schist and granite and is unconformably overlain by Eocene and later Terti- ary tuffaceous and fossiliferous beds. The origin of the oil is obscure. The beds dip at a low angle, not exceeding 12 feet to the mile, and are said to occupy a broad syncline or shallow down- ward warp. The oil is of asphaltic base and ranges in gravity from 2114° to 24° Baumé (0.925 to 0.9091 specific gravity). The average daily production of the wells is between 50 and 100

Oil And Gas Resources Of The Americas. 315

barrels, although some of the wells have had an initial yield of over 1,000 barrels daily. There are now about 20 wells in the district, of which about Io can be rated as successful producers. They range in depth from 1,500 to 1,800 feet, being fairly uni- form in this respect. The oil is used for fuel on the national railways.

The following figures relating to the Comodoro Rivadavia field are copied from La Nacién of Buenos Aires, issue of April 8, 1915, with eauivalents in American barrels:

TABLE X. PropucTIoN oF CRruDE PETROLEUM IN THE Comoporo RivaApAviA FIED, ARGENTINA. Year Liters. American Barrels of 42 Gallons.

There is a great divergence of opinion as to the possibilities of this district, owing no doubt to the fact that the mode of occur- rence of the oil is unique, so that further development will be followed with interest. It seems probable that the production of individual wells will never be great, and therefore, the field can not become a large producer unless the producing land is found to have a wide extent.

Salta-Jujuy District.

The Salta-Jujuy district occupies a roughly triangular area with sides approximating 100 miles in length in the Andean region of northwestern Argentina, adjoining the Bolivian frontier between 800 and 900 miles northwest of Buenos Aires. Its fields are practically continuous with those of Bolivia. The district in- cludes approximately 5,000 square miles, of which about 250 square miles contain evidences of petroleum; less than a square

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316 Ralph Arnold.

mile has actually been tested and the results have been indifferent. The oil reservoirs are sandstones and dolomites of Jurassic and possibly Cretaceous age; the accumulations of oil are in long, fairly well defined anticlines. The anticline dominating the Sierra Aguaraygua extends into Bolivia. The oil is black and asphaltic and ranges in gravity from about 11° to 14° Baumé (0.996 to 0.975 specific gravity). The production of the wells so far drilled has been very small.

In the Tartagal region the anticlines extend north and south. Two wells, both of which were poorly located with regard to the structure, reached depths of 150 and 450 feet, without results. At Quebrada Galarce four wells, showing some oil and gas, have been drilled to depths ranging between 127 and 574 feet. Seep- ages are also known at Yavi Chico, Tejada, Abre de la Cruz, Garrapatal, I-aguna de la Brea, and Cerro de Calilegua.

The presence of well-defined anticlines and good oil seepages indicates the probable presence of commercial deposits of oil; the tests already made were only partial and tell little or nothing of the possibilities of the district.

Mendoza-Neuquen District.

Evidences of petroleum have been found in a series of shales for nearly 300 miles along the eastern Andean slope, beginning 40 miles north of Mendoza and extending southward to latitude 42° in the Neuquen region. The district is from 600 to 800 miles southwest of Buenos Aires, and only its north end is accessible by rail. At least 6,000 square miles are included in the district; the area showing oil possibilities probably does not exceed 300 square miles, and the proved area is less than I square mile. The beds yielding the oil in the north end of the district are said by Dr. Windhausen to be of upper Jurassic age; those in the south end, according to Dr. Bailey Willis, are of Eocene age. The oil comes principally from sandstones and marls and has a gravity of about 20° Baumé (0.935 specific gravity). The wells yield only a small production and are short lived. East of the main Andean belt the lower Jurassic limestones and Cretaceous beds are also

s

Oil And Gas Resources Of The Americas. 317

said to be petroliferous, especially along the upper part of Salado River in latitude 35°, longitude 70°.

The principal developments are at Cachenta, near Mendoza, where 12 wells had been sunk up to 1893, the total yield of the wells up to that year being 10,000 barrels. A pipe line was built to Mendoza, but owing to the exhaustion of the wells the venture was not a success. Two wells also were drilled by the Neuquen Oil Syndicate at Cerro Tolena between 1908 and 1910. These are said to have yielded a little oil. Four or more test holes put down at Covunco, 105 miles west of Neuquen, attained depths between 262 and 1,180 feet and encountered some gas and oil. In 1913 a well was put down at San Cristobal, in the Province of Santa Fe, and attained a depth of more than 4,000 feet. It is reported that some oil was found in this well.

Seepages and other evidences of petroleum, including rafaelite, a kind of asphalt, have been reported from San Rafael, south of Mendoza; Cerro Auca Mahuida; Rio Barrancas; Curileuva; Gar- rapatal; La Brea; Vachenta; La Carena; and Plaza Huincul or Challaco, “north of kilometer 81, on the railroad.”

The development in the Mendoza-Neuquen district, like those in the area farther north, lead to pessimistic conclusions regard- ing its future; but, like pioneer work in most other oil fields, this development kas probably been carried through under more or less adverse conditions, and it is to be assumed that the results do not stand as a final test of the whole region.

Conclusions —With the exception of the region about the Comodoro Rivadavia field the country between the Atlantic Ocean and the front of the Andes offers little chance for the accumula- tion of oil in commercial quantities. Furthermore, is is question- able whether or not the productive area at Comodoro Rivadavia will be extended ; if it can be, the field is certainly ideally located for development and marketing its product. There are favorable indications for the development of commercial production in the Andean districts, probably the most encouraging areas being in the northern fields. In general the sharp and in places overturned folds of the mountain districts do not suggest extensive deposits or possibilities for large wells, but small wells yielding oil of

nt. in ng, if the ind mé ells ith. the Its. ive uz, the of les ° les ble CUS 4 Tele) ‘he by ith oil of nly an lso

318 Ralph Arnold.

various grades are to be expected when test drilling is systemati- cally and thoroughly attempted. In general it is doubtful whether any great fields of oil exist in this country, although detailed in- vestigations may change this conclusion.

Brasil.

The present known oil resources of Brazil are apparently con- fined to the oil shales of the Bahia region and a rather indetermi- nate area of favorable oil indications somewhere in the interior. The oil shales are of Eocene age and extend intermittently from Porto Alegre along the coast for over 1,200 miles, nearly to the mouth of the Amazon. Portions of the shales suitable for distil- lation have been reported at the following, among other localities: North of Ilheos, on Itahipe River; on the island of Joao Thania, in Marahu River, 80 miles south of Bahia; on Tinhare Island, 30 miles south of Bahia; at Riachadoce and Camarajibe, 25 and 45 miles respectively north of Maceio; in the Province of Alegoas; and in the Sierra de Araripe, in Ceara. It seems probable that the formations that are oil-bearing in southeastern Colombia, especially south of Nevea, may extend over the boundary into Brazil. Even if such is the case the deposits will be inaccessible for some time to come. Dr. J. C. Branner, the authority on Bra- zilian geology, believes that the chances for the recovery of com- mercial quantities of petroleum are rather problematic except in the regions mentioned, which means that the enormous expanse of territory in Brazil will probably remain a negligible factor in the oil industry of South America, at least during the present generation.

Summary Table Of The Oil Resources Of The Americas.

The following table is intended to summarize the present know- ledge of the oil resources of the Americas by countries. Attention is called to the fact that of an estimated total of 8,429 square miles of oil land on both continents, 4,365 square miles, or over one half, has already been proved. The estimated future supply of the United States is 5,763,100,000 barrels; outside of this

or

Oil And Gas Resources Of The Americas. 319

country the data are so meager that even rough quantitative esti- mates are out of the question.

Table Xi.

Summary oF Resources oF THE AMERICAS.

Proved Prospective — production to 1914. p Present Yearly

Country: i Area, 4; Area, ate of Production. Sq. Miles. Sq. Miles. Barrels. Barrels. Newfoundland Small (2) 25 1,000+ 23,493,610 214,805 United States: 4,109 946 3:335:457:130 265,762,535 25 1,000+ 90,359,869 21,188,427 Guatemala r+ @) (?) (?) 9 2,069,430 643,533 Perce 200 100 14,306,972 1,917,802 2 400 600,000+ 275,500 4,365 4,064 3,466,287,021 290,002,592

Methods Of Conservation.

The principal facts concerning the oil deposits of North and South America, as we now know them, having been presented, the next question to be considered is how best to utilize the present out- put and how most economically to discover and develop the gas and petroleum resources which are yet hidden in the earth. The fol- lowing suggestions are offered in the hope that they may guide those who are interested in the future development of these re- sources away from some of the troubles that have beset the oil industry in the past. As others have discussed some of these methods of conservation in detail, the writer will confine himself to the broader generalities.

Titles.

Conservation in developing the oil resources of any region should begin with an investigation of the validity of the title to the ® Owing to the writer’s professional connections in Trinidad and Venezuela

he deems it inexpedient to discuss the oil resources or possibilities of these countries at the present time.

n- yr m he 1a, US iat la, e ra- m- in of in ent ion are ver ply j

320 Ralph Arnold.

land containing the oil, for drilling oil wells on land with unsound titles might be likened to building a house on the sand. If the property is situated in a region of fee or freehold titles (“ patented lands,” as it is often called in the United States) the quality of the title can be determined by procuring an abstract. Care should be taken to see that the mineral rights are included in the title to the land in regions where mineral and surface rights may or may not go together. In most countries outside of the United States the mineral rights are alienated from the surface rights, so that separate titles must be obtained for the minerals. Such titles should always include the right to utilize the surface so far as is necessary for all purposes of development by paying reasonable damages to the owner of the surface rights, and if possible a fixed amount of damages should be agreed on before development starts, as the term “ reasonable” is capable of many interpretations.

If the oil is to be developed under concessions or leases the operator should be absolutely certain that the contract comes from parties authorized to grant it. Furthermore, assurances should be had that the wording of the concession or lease will be interpreted along broad lines; quibbling or taking advantage of technicalities usually results disastrously for both parties. The terms of the contract should be as explicit as possible, and the more liberal the conditions the easier will it be for those who intend to carry on the projected development to operate. The conditions of a conces- sion or lease should be framed to facilitate rather than retard the development. As a general statement, it is not fair to ask intend- ing operators to pay for the privilege of risking their money in proving up prospective land, and therefore it is much more equi- table for the owners of the mineral rights to obtain their remunera- tion through royalties than through bonuses or excessive rentals. A sliding scale of royalty is often more desirable than a fixed per- centage. For self-protection the owners necessarily should place a rental clause in the concession or lease, to guarantee development within a reasonable time. An observance of the spirit as well as the letter of such contracts on the part of both the grantor and the grantee usually works to the best advantage of all concerned and furthers the ends of conservation.

(

Oil And Gas Resources Of The Americas. 321

Exploration.

Much of the money that is expended uselessly in the oil industry is wasted in the drilling of dry holes. However, dry holes are as much a part of oil development as waste lumber is an ac- companiment of house building. It is impossible for anyone to say definitely from surface evidence whether oil does or does not exist below the surface of the earth at any particular spot—the drill is the final arbiter of this question, and even the drill does not always tell the truth—but there is a way in which explora- tion may be conducted to bring about the most satisfactory re- sults with the least expenditure of energy and money. The cost of exploratory work is charged against the capital account in any field, so that what is saved in the preliminary stages of develop- ment is a cut in the ultimate cost of the field’s product.

It ought to be as obvious that exploration with the drill should be preceded by careful geologic studies as it is that railroad con- struction should be based on surveys. These studies should in- clude such subjects as topography, stratigraphy, structure, and surface evidence of petroleum in the regions to be tested. The work divides itself into two stages—preliminary reconnaissances and detailed surveys.

The preliminary reconnaissance should consist in procuring all the available published and hearsay evidence regarding the occur- rence of oil or gas seepages or hydrocarbon deposits in the region; in making preliminary geologic surveys to determine from which formations the oil is to come and the areal distribution of these formations; in determining those general regions in which the surface evidence is supposed to be most favorable for the accu- mulation of hydrocarbons; and in determining the best routes and methods of transportation.

The second stage includes detailed geologic surveys of those regions where the surface evidence indicates that petroleum is most likely to be found and the location of test holes at favorable points. By working out the surface distribution and structure of the formations it is usually possible to select the areas offering the best chances of success. Geology should always be the dominant

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7

322 Ralph Arnold.

factor in determining the location of test holes, although modifica- tions to meet natural conditions must sometimes be made. Noth- ing is so encouraging as a successful first hole; nothing so dis- heartening as the failure of an initial test; so every other con- sideration should be sacrificed to make the first effort at the place where success is most likely. Just what character or degree of evidence should determine whether or not drilling is advisable must be decided separately for each area. So many factors enter into decisions of this kind that no set rules of action can be laid down. As a prerequisite to carrying on such work the investi- gator should have a general knowledge of the occurrence of oil in the other fields of the world, and, if possible, definite information concerning the interpretation of the surface evidence in the par- ticular type of deposit under examination. As has been tersely stated by one of the most successful oil men in the world, “ Where- ever the Lord put a deposit of oil He left a little on top to show its presence in the ground below.” It is hazardous enough to drill with favorable evidence in hand; it is many times more so to locate a well without reference to the surface evidence and geology of the region. Drilling blindly without a definite basis for action is working against conservation, and for every well so drilled that has been successful, a hundred can be cited that have resulted in failure.

Development.

After locating the best point for the sinking of a test well in any new field the next questions which confront the operator are those of development. These include the choice of proper equip- ment, its transportation and installation, the building of camps, and preparation for the proper care of the oil in case the well is successful. Prior to the proving of an area by a successful test well, the less that is expended in the way of equipment, road building, etc., the better. It is therefore often more desirable to put up with crude methods of transportation, temporary camps, and improvised tools than to spend great sums in installing a plant that may have to be almost immediately abandoned. Ordinarily it is much better to drill a test hole with a standard churn drill

tk la

Oil And Gas Resources Of The Americas. 323

than with a rotary, for the reason that the well record secured by the former is as a rule much more reliable than that yielded by the latter—and the log or record of the test well is its greatest asset. Furthermore proper gates or safety casing-head equipment should be installed when the oil zone is approached, for it is often pos- sible to control the strong initial flows or “gushes” of a par- ticular sand or zone by such means where lax methods of drilling would cause a great loss. Even under the most favorable circum- stances, however, and with the assistance of specially designed gates or casing heads, oil is liable to be lost when a well is brought in, particularly if the gas pressure is strong, for the action of fluids under great pressure is very sudden and is sometimes most disastrous to the apparatus installed for its control. After the drilling of test holes, which it is assumed will give a general idea of the location and character of occurrence of the oil deposits, it is comparatively easy to decide upon proper equipment for the drilling of future wells. In general, the standard or churn drill is the most satisfactory in a region of hard rocks, while for soft formations and for the control of strong gas pressures above the oil sands the rotary is often the most efficient. In the matter of camps it always pays to make the lives of the men in the field as attractive as possible. Oil men are of the highest type and most discriminating of artisans and as long-continued service is the most efficient and economical, it is the best policy to make them centented by providing good food, proper shelter, and a reason- able amount of diversion.

Production.

When the wells have been satisfactorily drilled and finished the matter of production presents itself. It is easy to handle the product of wells that flow from a moderate natural-gas pressure, but in general the great bulk of the wells of the world do not flow but have to be pumped, and some tap deposits under pres- sure so great as to render their operation difficult. Where the gzs pressure is high great care must be exercised in finishing the wells and in their constant control, for a “wild well” is usually the most wasteful thing about an oil field. Furthermore, high

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324 Ralph Arnold.

pressure implies the presence of great quantities of valuable gas, which should be utilized, if possible. The installation of proper gas traps and the economical use of the gas for fuel, illuminants, or the manufacture of gasoline are therefore other problems fac- ing the operator who wishes to conserve his resources. In pump- ing wells the method adopted and the skill with which it is con- ducted often determine whether or not the well is a paying one or is operated at a loss. In general, shallow wells can be operated by the ordinary lift pump, several wells being handled from a common “power.” If the wells frequently “sand up” or give trouble from caving, it is usually economical to maintain a drill- ing engine at each well in order to remove the tubing and casing when necessary for the purpose of cleaning the hole. Under cer- tain conditions, where water is present with the oil, the air-lift method of recovering the oil is satisfactory and economical. The utilization of waste gas for the development of power or the utilization of electricity generated by water power is often more economical than the burning of the oil. It has been estimated that 10 to 15 per cent. of the oil produced in California is either used as fuel in the fields or is wasted in production and through careless methods of handling. The greater part of this waste could be saved by the use of hydrogenerated electricity which is as cheap as the oil if not cheaper, and the practice of greater care in the fields. The waste of oil that is questionably excusable in the strenuous early period of any field is seldom excusable and should be made criminal in the later stages of development.

Transportation and Storage.

After the oil has reached the surface it is necessary to store it and transport it to markets. The storage of oil in open reser- voirs, especially those made of dirt, is most wasteful, but is often unavoidable in new fields and in places where flush production mounts higher than the steel storage facilities. If possible, how- ever, oil should be kept in cement-lined covered reservoirs or covered steel tanks. Tanks of the ordinary type, holding 37,500 and 55,000 barrels, have been found the most economical for general purposes. Cement-lined or reinforced-concrete reservoirs

t t

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Oil And Gas Resources Of The Americas. 325

holding as much as 7,000,000 barrels are in successful use. The tanks should be set in large earthen reservoirs to prevent the loss of oil in case of accident, and should be widely enough separated to keep fire from spreading if the tanks are struck by lightning, or fired from other causes. Painting the tanks white is also a method of conserving oil, as it keeps down the internal tempera- ture of the tank in hot countries, thus lowering the percentage of less through volatilization.

Pipe-line transportation is usually the most economical and satisfactory, although under certain conditions tank cars and barges are more satisfactory if the oil is exceedingly heavy or the quantity small. The latter type of transportation is often economical if waterways are adjacent to the oil fields and market terminals, as in the Panuco field of Mexico. A careful study of transportation questions, especially those relating to pumping, will often lead to a great saving over the method of hit-or-miss adop- tion of a transportation system without investigation. Each field and each type of oil has its own problems, and before putting in pipe lines it is always an economy to procure the latest informa- tion regarding the experiences of others in similar matters.

Utilization.

After the oil has been produced and is ready for refining or for the market the forms in which it is to be used should be a matter of the most careful investigation. Naturally, oil of the heavier grades is best suited for the manufacture of asphalt, for use as fuel, and for such other minor uses as road dressing and the manufacture of roofing materials. Oil of the lighter grades should always be refined, as the lighter constituents, which can be taken off even by a crude process of topping, often yield a greatly increased price over that of the crude oil. The utilization of the tops in the manufacture of staple and by-products usually results in the maximum profit that is made in the oil business. Probably the greatest waste of oil at the present time comes through its use in connection with the steam engine, for were this same oil used in the internal combustion engine of the Diesel type. the saving would be from 75 to 80 per cent. This saving will

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ift he re ed ler gh ste ire re er- en or ‘or Irs a

326 Ralph Arnold.

come about only with the perfecting of the internal combustion engine and a marked rise in the price of oil, and then only under conditions where the engine of the Diesel type can be satisfac- torily substituted for the steam engine.

In conclusion, it should be borne in mind that the best way to meet the demands of conservation is to hunt for and produce t’ oil with as little loss and as cheaply as possible, to utilize it for those purposes for which it has no substitute, and to avoid using it for those purposes, as for fuel to produce steam, for which such substitutes as coal or water power are available. This sentiment, so happily expressed by Dr. C. W. Hayes as “utilization with a maximum efficiency and a minimum of loss,” should actuate all who have the best interests of the oil industry of the Americas at heart.

Observations On The Geology Of The Broken Hill Lode, New South Wales.

E. S. Moore.

Introduction.

The general characters of the Broken Hill field are more or less familiar to geologists, the world over. It is situated near the northwest corner of the state of New South Wales and forms a low ridge something over 1,000 feet above sea-level—no definite figures seem to appear in the Australian reports—in the Barrier Ranges. The lode strikes approximately northeast and southwest, and mining operations are conducted along the strike for a dis- tance of over two and one-half miles. The deposits were dis- covered in 1883, and their enormous wealth is indicated by the figures of a little over 70,000,000 pounds sterling for the produc- tion since that date. The first values were in silver then the mines were operated for silver and lead, while in recent years silver, lead, and zinc are all produced in large quantities. Since the introduction of the flotation process enormous piles of tail- ings have been repassed through the mills for the recovery of the zinc.

Much literature has appeared concerning this field, and those desiring a more extensive discussion on the subject than is af- forded by this paper are referred to the works of Jacquet,) Maw- son,” and the officials of various mines, including Messrs. Donald- son, Matters, Slee, Coldham, Wallman, Smith, and Davies. The results of their observations are published under the title “Interim Report” by the geological subcommittee of the late Scientific So-

1“ Broken Hill Lode and Barrier Ranges Mineral Field, N. S. W.,” by J. B. Jacquet. Memoirs of the Geol. Surv. of N. S. W., No. 5, Geology, 1804.

2“ Geological Investigations in the Broken Hill Area,” by D. Mawson. Memoirs of the Royal Society of S. Australia, 1912.

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328 E. S. Moore.

ciety of Broken Hill* and where reference is made to the report in this article it is referred to simply as the “ Report of the Sub- committee” with page locations specified.

The writer visited the famous field at an important moment in its history. During the week spent there the European war broke out, and many of the mines closed, or partially closed, for a time owing to lack of a market in Europe for the zinc concentrates which had hitherto gone to Germany for treatment. Such a cessation of work was a serious blow in a city of about 40,000 people dependent almost entirely upon mining operations, since Broken Hill is situated within the southern border of the great arid region and receives less than nine inches average rainfall per year. An attempt was made to keep most of the mines working at least part time so as to keep the men, especially those with families, from being entirely thrown out of work. At the same time many men left the city, a number to join the army and some to enter other mining fields in which the demand for labor increased under war conditions. Since that time some efforts have been made to have the German contracts cancelled with a view to erecting permanent smelting plants in Australia and at- tempts have also been made to take over and enlarge one of the smelters already in existence and to have this handle the lead ores for all the mines not previously provided for by their own plants. At the time of writing no definite information has been received concerning the final stages in these negotiations.

The writer would take this opportunity of expressing his sin- cere thanks to the president and members of the Mine Managers Association for their generous hospitality during his visit to Broken Hill and to all the officials who so freely furnished infor- mation concerning the geology around the mines. His personal thanks are also due to Professor A. Lacroix, Secretaire perpetuel de l’Academie des Sciences, in whose laboratory at Paris some of the rocks were studied.

3 Proceedings of the Australasian Institute of Mining Engineers, Vol. VI, No. 2, 1910, pages 17-61. Discussion on this paper, Vol. VII., pages 238-270.

Geology Of The Broken Hill Lode. 329

The Pre-Cambrian Rocks At Broken Hill.

There has been much speculation in the past concerning the age and character of the schists and gneisses with which the ores are associated. Jacquet* considered them of sedimentary origin and of Paleozoic age, while some other writers have regarded them as igneous. Mawson? points out clearly the sedimentary character of the bulk of the rocks and shows that they are pre- Cambrian because of their relations to the Torrowangee tillites of Cambrian age. He applies the name Willyama Complex to this series, which he describes as consisting of a great variety of amphibolites, gneisses, granites, and schists. He mentions quartz- feldspar gneiss, cordierite gneiss, garnet-sillimanite gneiss, scapo- lite gneiss, phyllites, and marble among other types occurring in the Barrier Ranges. All of these types are not represented im- mediately at Broken Hill, where there is, however, a consider- able variety of them which will later be discussed in detail.

The similarity of the rocks in this field to the pre-Cambrian types of northern Canada immediately strikes the geologist who has worked on the North American pre-Cambrian. The green and gray schists and gneisses interbedded with amphibolites and intruded by granites are typical of the pre-Cambrian. The pres- ence of arkoses, apparently the quartz-feldspar gneisses men- tioned by Mawson, is also of frequent occurrence in the rocks of that age, and they may be considered as representing the disinte- grated but only partially decomposed rocks such as granites and others of similar composition.

The bulk of the series at Broken Hill is undoubtedly of sedi- mentary origin, but the writer is not in accord with Dr. Mawson® regarding the origin of the sillimanite gneiss if he understands that it represents a metamorphosed igneous rather than a sedi- mentary rock. The characters of this rock under the microscope point to its sedimentary origin and, further, many geologists con- sider that sillimanite seldom, if ever, occurs in a meta-igneous rock.

4 Loc. cit.

5 “ Geological Investigations in the Broken Hill Area,” p. 236. 6 Ibid., p. 238.

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330 E. S. Moore.

There might also be added to the above list a spinel gneiss or schist which I believe has not been mentioned previously and which occurs near the lode.

No conglomerates were seen by the writer, although at the top of the Wilson shaft, in the Proprietary property, there is a nar- row band of what was at first mistaken for water-worn pebbles. These, however, on closer observation seem to be crystals of bluish, rather opalescent quartz, similar to much quartz in the lode, and they look as if they had suffered a slight rounding by shearing in the enclosing rocks. The specimens collected do not show any definite indication of water action and their miner- alogical characters seem to relate them to the vein material. Limestone occurs in the Barrier Ranges north of Silverton and about fifteen miles northwest of Broken Hill. Another band of this Ettlewood limestone also occurs about thirty-five miles north of Broken Hill, and from the strike of the strata in the two areas it seems probable that one represents the continuation of the other band. While this formation might readily be duplicated in the Broken Hill folds, so far no sign of it has been found. The replacement of such a band would more readily explain the origin of the ore deposits and the presence of such large quanti- ties of calcite in parts of the lode, but the characters of the garnets so far as studied and thé nature of other minerals asso- ciated with the ore bodies do not give any suggestion that lime- stone has been replaced.

The amphibolites so commonly found intruding the sediments are metamorphosed diorites and diabases.

PETROGRAPHY OF THE LODE AND COUNTRY ROCKS. (a) Igneous Rocks in Genetic Order.

Amphibolites——Just northwest of the British Mine there is a large band of so-called amphibolite. This is either a dike in- truded into the sediments along the strike, or a sill intruded be- tween the beds before the sediments were folded. It has the appearance in the hand specimen of a quartz-diorite with much amphibole, and in thin section it is seen to consist chiefly of am-

Geology Of The Broken Hill Lode. 331

phibole of blue and blue-green colors grading over into ordinary hornblende. Some of it approaches riebeckite in color, but the properties are not those of riebeckite but rather those of a mix- ture of the hornblende and glaucophane molecules with an extinc- tion angle lying between the two. Magnetite, pyrite, and con- siderable garnet are present, and there is also a yellowish and brown, isotropic mineral in small masses which is probably zinc blende. It seems to have developed late in the history of the rock. Most of the feldspar is highly striated, and it has the index of refraction of albite and oligoclase. There is considerable orthoclase and much quartz, but it is believed that quite a propor- tion of the latter has been introduced secondarily. The rock is regarded as a highly altered granodiorite.

From a depth of 1,200 feet in a bore hole in the country rock another section was obtained which shows the same blue-green amphibole. Professor Lacroix stated that this type of amphibole is a common associate of glaucophane schists in other regions. The rock as a whole is more basic than the one described above,,. and the percentage of feldspar is small. A little garnet and mag- netite and considerable clinozoisite, with small amounts of epi- dote, also occur.

On the 600-foot level of the Proprietary Mine a greenish, speckled, fine grained dike is impregnated with garnets. Under the microscope the minerals are chiefly amphiboles, with smaller amounts of epidote and titanite. A little actinolite and common hornblende occur, the latter passing to a type with low birefrin- gence and considerably less color than the normal type, but show- ing the typical cleavages of amphibole, viz., 56 and 124 degrees.

A section of highly altered diorite was also obtained from the country rock on Block 14 at a depth of 1,500 feet.

Granite-gneiss and Pegmatite—A specimen taken from the small hill on Argent Street near the southwest end is a typical biotite-granite gneiss with a rather high percentage of sodic feld- spar. It has suffered considerable dynamo-regional metamor- phism and strongly resembles the type of Laurentian gneiss so common in this country. There are a few grains of garnet which have that same fragmenta] appearance seen in some of the other

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332 E. S. Moore.

rocks in the vicinity of the lode. In a few cases they give the impression of being a part of a larger crushed fragment, but in general they seem to be individuals which developed in that form. Since they are not more fragmental in appearance than the biotite in the same rock this appearance may be due to the particular con- ditions under which they grew in the rock.

Associated with these granitic masses are feldspar- and quartz- pegmatites. The granites are considered later. than the diorites described above, but the question may well be considered as to whether the two types of rocks may not be comagmatic.

Diabase and Basalt Dikes.—In thin section the rock from a dike in the Proprietary Mine proves to be a fine-grained diabase almost completely altered to an amphibole schist. The feldspars illustrate the ophitic texture, and there is a little augite present, but the ferromagnesian minerals are almost entirely altered to uralite and other secondary amphiboles.

In another section taken from a core at the 650-foot level of the same mine the rock is a basalt-porphyry in which the calcic feldspars have at least partly recrystallized and inclose small grains of biotite and hornblende. Some zoisite occurs as a sec- ondary mineral.

In the open cut on the Proprietary Mine a dike of fine-grained diabase may be seen. This appéars to cut the ore, but on closer examination is found to be earlier than the ore. It cuts across the projected strike of some of the other rocks in the area and is considered later than the granites, although it was not seen actu- ally cutting them. In thin section the ophitic texture may be seen, and almost all the pyroxene is altered to bluish-green hornblende. The dikes are considered later than the granites because they cut across the strike of the sediments with which the granite-gneisses and amphibolites are associated more or less conformably, and they do not seem to have suffered so much dynamo-regional meta- morphism as the latter; but the writer feels that it would be difficult for anyone to establish the fact that all these dikes are not closely related in origin.

Pegmatites——There are in the vicinity of Broken Hill some pegmatites distinctly connected with the granite and some later

Geology Of The Broken Hill Lode. 333

ones closely associated with the ore bodies and particularly with the zone of disturbance along which it is believed that the ore solu- tions must have moved. The latter type of pegmatite is quite distinct from the granite pegmatites and seems in all cases where observed to be characterized by a greenish or bluish orthoclase intergrown with quartz. Sometimes this feldspar is of a very striking light green color. This color is largely lost by heating the specimen to a red heat in a burner, as already observed by Dr. Beck.?

Two specimens taken from the 600-foot level of the British Mine were examined in detail. The first one consists largely of quartz and gray feldspar with sericite along the cracks. Galena and sphalerite occur in the specimen in such a way as to suggest that the pegmatite is distinctly older than and has been impreg- nated by the ore. Under the microscope the feldspar is found to be chiefly orthoclase with a little albite. These feldspars are associated with quartz grains large and small, the former in such association as to suggest their primary character, and the latter, their later introduction into the rock. Some biotite is undergoing weathering and passing to chlorite. Along the cracks in the feld- spar are biotite, muscovite, sphalerite, and galena. These min- erals are so distributed as to indicate their introduction along cracks rather than an occurrence as inclusions in the feldspar.

‘The quartz is full of small inclusions of gas and minerals, some of the latter being square, some round, and others oval, but they are too small to be identified.

The other specimen shows a bluish to gray feldspar intergrown with quartz and muscovite. Under the microscope the orthoclase shows little flakes of sericite developed along the cleavage planes, and in many patches the mineral is cloudy with submicroscopic, irregular specks. These may be of organic character. Dr. Beck speaks of a light-brown liquid with movable gas bubbles in the specimens which he examined. A chemical examination failed

7“ Contributions to Our Knowledge of Broken Hill,” by R. Beck. Records of the Geol. Surv. of N. S. Wales, Vol. VIL., Pt. I., 1900, p. 21. (Trans. from

Zeit. fiir prak. Geol., March, 1899, pp. 65-71.) 8 Loc. cit.

ane it d e

334 E. S. Moore.

to show metallic content indicating a source for the pigment. Some specimens show the presence of garnet in small crystals. All observations seem to indicate that the pegmatites preceded the deposition of the ore.

(b) Sedimentary Rocks.

Two types of original sediments were observed in the im- mediate vicinity of the lode, these being arkoses and quartzites, the one type grading into the other with decrease of feldspar con- tent. From these two types, however, have been developed, by metamorphism, garnet-spinel-sillimanite schist or gneiss, garnet rock, and other related types. The presence of the sillimanite gneiss has already been mentioned by Mawson, and the garnet rock or “ garnet-sandstone,” as it has frequently been called, has been of interest to everyone who has visited the field because of its close association with the ore bodies. The occurrence of zinc spinel is here mentioned for the first time.

The Quartzites—A specimen taken from the foot of the knoll along the east side of the lode on Block 10 has the appearance of a brownish sandstone stained along fractures with manganese oxide and limonite. Under the microscope the grains of quartz have intergrown to some extent, but their former outlines may be seen and along these are small crystals of a pink to colorless garnet and some green spinel with a better cleavage than that in most of the other specimens of this mineral which were seen. A little sillimanite also occurs, and small zircon crystals form inclu- sions in the quartz. Fluorite is fairly common, and it is often associated with the spinel, garnet and ore minerals along lines of weakness in the rock or in cracks in the quartz grains.

In specimens from the South Mine containing bluish opalescent quartz the grains of the quartzite are all recrystallized and inter- locked so that only some fragments show the original grains. This rock also contains a little garnet, muscovite, sillimanite, and zoisite, and it is impregnated with galena and chalcopyrite. In the quartz, flakes of biotite are included, probably during the recrys- tallization process.

Geology Of The Broken Hill Lode. 335

These specimens with bluish opalescent quartz grade over into distinct quartz veins.

In a specimen from the Central Mine, galena, chalcopyrite, sphalerite, and opal all occur along lines indicating that they had been introduced later than the quartz and probably all at the same time. Some of the yellowish, isotropic mineral with low index taken for opal, may be fluorite, but no sign of cleavage can be recognized in any of the specimens, and there is no violet shade.

The garnets in these quartzites can generally be seen to develop along lines indicating the former outlines of the quartz fragments, but in some cases they seem to develop in the midst of a grain as if they had grown by virtue of their crystallizing power or that the quartz had inclosed them in recrystallizing.

Sillimanite-garnet-spinel-biotite-rhodonite Gneiss—A number of thin sections were made of the drill cores from the country rock near the ore in Block 14 Mine. These show evidences of being metamorphosed arkoses, containing a varying proportion of orthoclase, albite and quartz as original minerals with a consider- able development of such secondary minerals as garnet, green spinel, sillimanite and biotite and in one case rhodonite.

The garnet is pinkish or colorless, and it is intergrown with spinel and sillimanite. The sillimanite, in long fibrous crystals, is intergrown with the spinel and occasionally the garnet, and the spinel is always surrounded by what is considered a halo of minute needles of sillimanite which stands out when the nicols are crossed. The garnet sometimes contains a few quartz inclusions, while the spinel is often crowded with them, even up to one fifth of its area. The larger fragments of orthoclase contain many inclusions of quartz, biotite, garnet, and rhodonite. There are also crystals of zircon, and what is either ilmenite or magnetite occurs in one specimen.

Rhodonite is uncommon in the specimens examined, but is well developed in one section. It has a well-marked cleavage, high extinction angle, low birefringence, and the plane of the optic axis is inclined to the prominent cleavage. Some frag- ments show a peculiar alteration by a brownish shade beginning along the cleavage planes and extending over other portions of the

y t t S f f n

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336 E. S. Moore.

crystal so that in some cases the fragment becomes completely changed to a brown, isotropic body which is believed to be a man- ganese-bearing garnet. The grains of rhodonite often appear as if the ends had been broken off the fragment, but as no cases of crushed crystals were actually seen, it is believed that this is due to the way in which the fragments developed in an already solid rock by replacement, and it is observed that some of the garnets exhibit the same irregularities in outline.

Other examples of arkose altered to sillimanite-garnet gneiss were found in the country rock of the North Mine where much sillimanite is developed and may be found cutting completely through the garnet and biotite. As in other cases the larger pieces of feldspar often contain numerous inclusions of quartz and biotite giving the appearance of the feldspar so often seen in contact metamorphic rocks. A little magnetite is present, and where surrounded by garnet there is a brownish-red zone between them.

Another specimen taken from the country rock closely adjoin- ing the ore on the 300-foot level in the Proprietary Mine is an arkose containing much orthoclase and quartz. A dirty, grayish- brown biotite with alteration products of iron oxide is present and many small crystals of garnet. From this specimen there seems to be a slight tendency for the quartz to be more readily replaced than the feldspar.

Garnet Rock.—This rock is often known by the name of “ gar- net-sandstone”’ because of its granular sandstone character and the fact that it is full of little garnet crystals so loosely cemented that it crumbles very readily with handling. It is reddish-brown in color, and it is characteristically associated with the ore in all the mines visited so a number of specimens were collected and studied.

A specimen from the so-called saddle structure on the 500-foot level of the Central Mine shows the transition from the arkoses to a quartzite largely replaced by garnet. In it the garnet and the biotite show a tendency to be bunched rather than to be uniformly distributed. There is a little pyrite and some magnetite or ilmen- ite, a determination being impossible from the specimens found.

Geology Of The Broken Hill Lode. 337

From the 300-foot level of the Proprietary Mine the garnet rock near the ore showed the following characters. It consists almost entirely of garnets, the crystals of which are closely inter- grown. In the garnet there is considerable fluorite in little violet and colorless inclusions and filling the interstices between the garnets as irregular masses. There are as a rule, although not always, ore minerals associated with the fluorite, and there are a few small inclusions of the ore minerals in the garnets.

Fic. 19. Photomicrograph showing the intergrowth of rutile (dark) and titanite, in a small vein in the garnet rock of the Central Mine. (30 diameters.)

In a section from the North Mine there seems to be more of the ore minerals where the rock is mostly garnet. An interesting specimen was collected from the 500-foot level near the ore body in the Central Mine. In thin section it is a garnet rock with the crystals closely interlocked. Running through the rock is a vein- let in which occur rutile, titanite, epidote, clinozoisite, zoisite, fluorite, quartz, muscovite, chalcopyrite, and zinc blende.

The rutile occurs in tabular and elongated, irregular masses.

;

ly

of 1€ id SS id ad ull ot to he ly ‘adi

338 E. S. Moore.

It is striated and twinned; one geniculate type was observed and the lamelle of other twins. It is of a yellowish-brown color with a greenish tinge. The rutile is intergrown with the titanite in pegmatitic texture and the other minerals fill the interstices around these minerals.

Another section from the same level showed a good deal of epidote intergrown with the garnets, and it had the appearance of having been developed at the same time.

THE OCCURRENCE OF GAHNITE, THE ZINC SPINEL (Zn Al, O,).

It seems appropriate that special attention should be paid to this occurrence of the zinc spinel in the country rock because of the possible bearing it might have upon the origin of the minerals associated with it.

As already mentioned it occurs with garnet in the sillimanite- gneiss. I believe it does not occur in any specimens studied, in which feldspar is not present. This mineral might very readily be mistaken for garnet since it is isotropic, has similar index of refraction and fractures in much the same way. It is, however, green in color, an apple green but not intense in thin section and in grains a dark green. It shows a slightly better cleavage than garnet in some cases. The index of refraction is above 1.74, while that of samples of gahnite is 1.76. The index of ordinary spinel is considerably less than this.

The mineral was further tested by microchemical methods and the presence of Al and Mg determined. A test for zinc was then applied. To carry out this experiment it was necessary to sepa- rate the mineral from the remainder of the rock, a difficult task owing to the similar characters of the garnet and spinel. The rock was crushed and everything but the garnet, sillimanite and spinel disposed of by hydrofluoric acid. The separation of the sillimanite and most of the garnet from the spinel was accom- plished by means of the method suggested by M. René Bréon® in which he employs a mixture of lead chloride and zinc chloride.

9“ Separation des mineraux Microscopiques lourds,” Bull. de la Societé Mineralogique, Tome 3-4, 1880-81.

Geology Of The Broken Hill Lode. 339

Since it was impossible to obtain a complete separation of the garnet and spinel by this means on account of the similar specific gravities of these minerals, the microscopic was finally employed.

To determine the presence of zinc H. C. Bradley’s nitro-prus- side of sodium test’® was used, and similar results were obtained from the specimen from Broken Hill and Brazilian gahnite.

The occurrence of this spinel is interesting, since it is another associate of the garnet, and it indicates the presence of slight traces of zinc at the time it was developed. It is also interesting to observe that this mineral is often found associated with pegma- tites in the other areas where it occurs.

Form And Genesis Of The Ore Bodies.

In the papers already mentioned in the introduction to this article the question of genesis of the ore bodies and their form has been extensively discussed. Messrs. Pittman and Jacquet, as well as a few others, strongly contend that the deposit is a typical saddle structure. It has not been the writer’s privilege to see the articles by Gregory and Andrews, but their views are quoted in the report of the subcommittee already mentioned. The members of this committee are of the opinion that the main lode is of meta- somatic origin, but the possibility of folding developing a certain amount of saddle structure is recognized.

From field and laboratory observations, strengthened by and correlated with the opinions of the various other writers on the subject, the author advances the following statements simply as his conclusions regarding the history of the field and not with the belief that they settle all the vexed questions which have worried workers for many years.

The oldest rocks exposed in the Broken Hill ridge are the quartzites and arkoses which on metamorphism have given rise to the various crystalline schists and gneisses by dynamo-regional and igneous processes. Before the folding this sedimentary series was intruded by sills or dikes of diorite and gronodiorite which

10 “ Color Reaction for Copper and a Micro-chemical Test for Zinc,” Amer. Jour. of Sci., 4th Series, Vol. XXII., 1906, p. 326.

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340 E. S. Moore.

have been altered to amphibolites. These were followed by the granites and granitic pegmatites, but as to how close a magmatic relationship there is between the granites and the diorites there is no certainty. Great dynamo-regional metamorphism followed, developing from these rocks, gneisses, and schists, with the be- ginning of a shear zone along the general line of the present lode. The diabase dikes, such as those cutting the deposits on the Pro- prietary Mine, do not seem to have suffered the same amount of dynamic action as the other rocks and seem to represent a later injection cutting across, to some extent, the earlier gneisses and schists.

Succeeding this eruption or contemporaneous with it orogenic movements developed further shearing and faulting along a pre- viously developed zone of weakness, since these dikes are offset in some cases by the movements, and probably soon afterwards the green and gray orthoclase pegmatites were injected along this zone and in the immediate vicinity. The development of the so- called country rock garnets, the gahnite and the sillimanite prob- ably occurred about this time and the rhodonite soon after, or in some cases at the same time, being closely followed by the highly manganiferous garnets, the ore minerals, and fluorite. Analyses of the garnets’! show that though the country rock garnets and the garnet sandstone garnets are largely Fe and Al types, while those in the gangue are high in manganese, they also show that there are gradations between them, and no sharp division can be made. The large development of the garnet and sillimanite seems to be due to the action of igneous emanations rather than simply dynamo-regional metamorphism, because the cases mentioned by Mawson where these minerals occur in large quantities in the Barrier Ranges demonstrate their association with similar areas of intense loca] disturbance and the development of ore deposits.

Any differences which may exist between the country rock garnets and those in the gangue proper might be due to the fact that the early solutions came in contact, in the country rocks, with more iron than the later solutions, which may be regarded as

11 Report of the Subcommittee, pp. 43-44.

Geology Of The Broken Hill Lode. 341

being unable to develop certain chemical combinations with the country rock because certain elements were already exhausted in developing stable compounds. Also the reason that in some cases the great masses of rhodonite and silica occur in some of the mines instead of the garnet rock may be due to the lack of alumina and the very high proportion of silica in the country rock in those areas.

There are all gradations from country rock with little garnet, rhodonite, and ore minerals through that consisting almost en- tirely of garnet in some cases and rhodonite in others, or the two combined, to the ore in which the ore minerals reach a maximum proportion, the garnet and rhodonite a small proportion and the country rock has entirely disappeared.

In thin sections examined, Dr. Skeats,12 Professor Beck,!® and the writer all found garnets distinctly inclosing inclusions of sul- phide ore minerals as well as quartz and fluorite, and these same minerals are also found filling the interstices between the garnets and intergrown with them. This fact shows that apparently now one and now the other developed first, and it seems natural to think of the invasion of the country rock by the solutions and its metasomatic replacement as partaking more or less of a pulsating action. This invading mass of liquid would in some cases be sufficiently charged with gases and at sufficiently high temperature and pressure to absorb all the country rock met, while at other times it would be halted until fresh acquisitions of gas and an increase in pressure and temperature would permit it to advance again. In such cases precipitation might occur on the outskirts of the invaded area and solution follow when fresh supplies of the solutions arrived.

The ore bodies are therefore regarded as being due to meta- somatic replacement. It does not seem probable in view of the great amount of mashing, shearing, and recrystallization which these rocks have suffered that any extensive cavities would remain

12 Report of the Subcommittee, p. 42. 13 Loc, cit.

le ’ id d le at 1S ry 1e AS th

342 E. S. Moore.

open in the strata. The apparent saddle structure in some of the mines may possibly be explained by selective replacement of cer- tain beds of the sediments which would thus give an apparent conformable arrangement of the enclosing rock and the ore. From some of the thin sections evidences may be found of a greater amount of replacement of quartz than feldspar for ex- ample, and it seems as if bands or lenses of quartzite may in some cases have been completely replaced, while the arkoses have been turned into garnet sandstone.

The numerous diagrams published in the report of the sub- committee and the various models which have been prepared to illustrate the form of the lode seem to leave no doubt that there is a tabular zone of ore along one side of the mineralized area to which all the other and more irregular masses are somewhat closely related. The many evidences of faulting also indicate that this tabular zone is a zone of faulting and shearing along which the ore solutions have risen to replace and extend into the beds in the hanging wall, replacing them in such a way as to give in some cases a saddle form. In other cases the solutions have developed semi-detached bodies by working along bedding planes or in fissures to points where replacement might again be carried on. In the discussion on the Report of the Subcommittee,’* Pro- fessor David mentions that he observed segregations of garnet and ore in the country rock some distance from the main body. The writer thinks the larger detached masses may simply be such bodies on a much larger scale.

The absence of sulphides along the main fau!t plane is puzzling. There may be certain explanations offered, however. One sug- gestion is that along certain of the faces examined the movement on that particular plane has occurred since the ore was deposited. In other areas the ore has been oxidized and removed in solution by ground water. Finally it seems probable that along part of this faulted zone no sulphides were ever deposited because it was too directly over the source of supply, kept at too high a tempera-

14 Page 250.

Geology Of The Broken Hill Lode. 343

ture, and the solutions were too highly charged with gases to permit deposition until] the solutions were depleted of their metal- lic contents by the diffusion of the solution into the surrounding rocks and being precipitated partly through the mass action of the metals concentrated in the cooler portions of the solutions. Sulphides do occur in some places, and some one of the above hypotheses might help to explain the other cases.

As to the presence of the calcite gangue principally near the extremities of the lode and associated with great quantities of rhodonite a satisfactory explanation is difficult. As previously stated there does not seem to be any evidence of the replacement and partial recrystallization of a mass of limestone, since no sign of this rock appears in the immediate vicinity, and the associated minerals do not suggest in their composition such a replacement. It looks as if the calcite had been developed later than the garnets, unless it can be shown that the associated garnets are of the cal- careous type rather than the iron or manganese types found else- where. The writer is not aware that any careful study of this subject has been made. There is the possibility that the calcite has been carried in solution farther from the source of supply owing to its greater solubility, as similar segregations seem to have taken place among minerals in other ore regions.

The blebs, blotches, and veinlets of bluish, opalescent quartz are interesting because of the fact that bluish, opalescent quartz is of such common occurrence in rocks affected by contact meta- morphic processes in some other regions, and everything in the Broken Hill deposits seems to point to deposition by high tempera- ture solutions except the report made by Dr. F. E. Wright on the quartz submitted to him by Mawson. In this report he states that he does not consider that either the quartz in the ore or in the vein had been heated above 575 degrees.!®> This being the case much of the replacement work must have been due to the solutions charged with such active gases such as fluorine.

15 “ Geological Investigations in the Broken Hill Area,” p. 310.

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344 E. S. Moore.

Secondary Enrichment Of The Ores.

The effect of secondary enrichment in this field has been of great importance, enormous bodies of sulphides having been altered to carbonates, bromides, chlorides, oxides, etc. Owing to the peculiar form of the ore bodies, which may reach over 300 feet in width in some places and whose vertical dimensions are much less than the horizontal, a great opportunity has been given for alteration by meteoric waters. While complete oxidation of the sulphides extends to between 500 and 600 feet in some parts of the field small amounts of oxidized ores are found to a depth of over 1,000 feet where a deep opening occurs owing to faulting. The deepest mine shaft at the present time is probably less than 1,800 feet.

The metallic contents of the sulphide ores vary somewhat with the type of gangue. For example, in one mine the rhodonite, fluorite gangue carries II oz. of silver, the quartz rhodonite gangue about 8 oz., and the garnet sandstone about 3 oz. In another mine the rhodonite gangue may carry as much as 20 oz. per ton. In still another the figures run 14-15 per cent. lead, 11 per cent. zinc, and 11 oz. of silver. It may be considered that in general the silver in the primary ores will average from 3 to about 14 0z.; the lead, which is usually fairly uniform, 14 to 16 per cent.; and the zinc, 8 to 18 per cent. The zinc is as a rule lower in the calcite gangue, and the silver is considerably lower.

In discussing the depth of secondary enrichment the data here given were obtained from the mine managers, and in many cases verified by personal observations in the various mines. The writer is therefore deeply indebted to the various officials for the information which they so kindly supplied. In such a discussion it seems best to give the depth of the various oxidized minerals in each mine as a group for purposes of comparison under similar conditions and it seems best in most cases to refer to the various mines by letter in order that there may be no possibility of giving out information regarding the ores which might be considered confidential.

One of the most striking oxidized ore bodies is the famous

1S

Geology Of The Broken Hill Lode. 345

pipe of lead carbonate which has been previously mentioned in literature on the Broken Hill region and which extended to the 600-foot level in the Central Mine. It was approximately 50 feet in diameter, and it is reported that on the 600-foot level a cavern 25 feet by 40 feet existed. In the same mine tons of pyromor- phite have been found on the 700-foot and extending almost to the 800-foot level. White zinc sulphide is said to occur with lead carbonate on the goo-foot level, and along the fault plane bleaching of the rhodonite occurs as low as 1,300 feet. Since some sulphides occur as high up as the 200-foot level in the same mine the great irregularity of the oxidation in this particular mine is evident.

In Mine A:

Bromyrite (AgBr) was found on the 200-foot level.

Other haloids not below 300 feet.

Anglesite and pyromorphite to 350 feet.

Cerussite in large crystals to 300 feet and more massive to 400 feet.

Native copper and cuprite to 400 feet.

Zinc in oxidized zone from I to 5 per cent.

High lead and high silver do not go together.

Silver usually high in the oxides except where lead is high and has gone up to 200 oz. per ton. It often reaches 40 oz.

Smithsonite in gossan with manganese and iron oxides but not below about 100 feet.

White zinc sulphide at 500 to 700 feet.

Mine B: Carbonates to 240 feet. Native silver to 200 feet. Haloids not important but a little cerargyrite as low as 100 feet. At zone of transition between the galena and lead carbonate zinc usually low and lead high.

Mine C:

Depth of ore in mine not over 600 feet. Lead carbonate to 230 feet.

h n t e e n q 1S

346 E. S. Moore.

Explanation To Plate Xv.

Fic. 1. Arrowhead-shaped crystals of cerussite covered with a thin coat- ing of secondary galena. Proprietary Mine, Broken Hill. (% natural size.)

Fic. 2. Secondary galena on mass of ore found in caverns and in stopes or 1,250-foot level of the North Mine, Broken Hill. (% natural size.)

PLATE XV. Economic GeoLocy. Vot. XI.

“3 Bie be : Fic. 1.

a ) bit FIG. 2.

Geology Of The Broken Hill Lode. 347

Chlorides and bromides to 200 feet. In the upper 100 feet the silver chloride occurs only as smearings in cracks but from 100 to 200 feet it becomes evident in the ore.

Native copper to 230 feet and scarce above 100 feet.

In the highly oxidized zone the zinc has not for years run more than I per cent.

Pyromorphite occurs down to the contact of oxides and sulphides.

White zinc sulphide is more common in the calcite gangue than in the rhodonite. This mineral often occurs near the con- tact of the sulphides and oxides but it has been found quite isolated in the sulphides.

Near the contact of the oxidized and sulphide zones, lead and zinc are almost invariably high, although they have occa- sionally been found as high in other parts of the deposit.

There are some evidences of the formation of secondary galena in these deposits as illustrated in Plate XV. One example is found in the arrowheaded crystals of cerussite, a form charac- teristic of many of the crystals of this mineral in the Broken Hill field. These crystals have a distinct coating of galena on the exterior where the cerussite has altered, giving definite evidence of the secondary origin of the galena.

The other example was found in the North Mine at a depth of 1,250 feet where specimens of crystallized, secondary galena are abundant in some of the stopes. Some of this mineral has developed since the opening of the stope and is found along cracks in the roof and in other situations indicating that it is undoubtedly of secondary origin.

Although the exact nature of the solution depositing the sul- phide is not known it is supposed to be from the carbonate.

Regarding the white zinc sulphide mentioned, the writer did not have an opportunity to see much of the mineral. In one case a specimen which was presented as amorphous zinc sulphide proved. on testing, to be zinc carbonate, and while the occurrence of white zinc sulphide has been well established it seems probable that the more common mineral, smithsonite, is confused with it in some cases at least.

Bea

348 E. S. Moore.

From the above figures it is seen that in certain mines there is a striking regularity in the depth of most of the oxidized ores, but there may be some great exceptions, as for example, in the Central Mine. There is also one striking feature, viz., the leach- ing of zinc from the oxidized zone, and it is difficult to explain what has become of the metal in some cases. In the Report of the Subcommittee’® much stress is laid upon the fact that so much zinc has been leached from the upper zone and there is no appar- ent enrichment of the sulphide zone. While this zinc has appar- ently been lost in some cases the statements above show that such is not the case in all mines and further the figures published by the committee can in some mines be construed as showing an en- richment just below the oxidized zone. In certain cases the figures given are for too great depths to show any appreciable effects of the enrichment even if it had occurred just below the oxidized zone.

In those mines where the zinc has actually disappeared, as it no doubt has, owing to its greater solubility than that of the lead, the explanations offered by Mawson would probably satisfy the condi- tions. He suggests that much of the zinc salts had been carried away by mine waters and considerable had risen in solution by capillarity and disappeared by action of surface waters.'7

16 Pages 51-56.

17 “ Geological Investigations in the Broken Hill Area,” p. 311.

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( ‘ :

Laboratory Studies On Secondary Sulphide Ore Enrichment.

1. THE CopPpER SULPHIDES AND HyDROGEN SULPHIDE.

S. W. Younc and Neit Preston Moore. INTRODUCTION.

The commercially workable portions of many deposits of copper sulphide ores, as well as sulphide deposits of other metals usu- ally show evidences of comparatively recent formation, and it is now commonly considered that many of them have resulted from the concentration, in some way or other, of disseminated deposits, such as pyrite beds of low copper content. The phenomena of secondary enrichment have been quite extensively, investigated by geologic and petrographic methods, and at the present time offer a most fruitful field of study.

Apart from their geologic interest these phenomena also pre- sent an interesting field to the chemist. The commoner sul- phides of copper and of copper and iron, namely chalcocite (Cu,S), covellite (CuS), chalcopyrite (CuFeS.) and bornite (Cu;FeS,), form a series of salts, which the geological data indi- cate are transformable under certain conditions one into another. There is no reason to believe that these physical-chemical trans- formations would not lend themselves to solution from the stand- point of the Gibbs phase rule. While there are many experi- mental difficulties to be encountered in such work, there are prob- ably none that cannot in the end be overcome. In fact one has only to refer to the work of the Geophysical Laboratory at Wash- ington, of Doelter, and of a number of others, both in this country and abroad, to realize that considerable progress has already been made in this direction. A summary of the results of research

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1e h- r r- n- 1e le he e od

ig

350 S. W. Young And Neil Preston Moore,

along this line is contained in Clark’s “ Data of Geochemistry ’? and they need not be enumerated here.

This paper contains the results of the first of a series of in- vestigations on the synthesis of the copper sulphide minerals and their decompositions and mutual transformations. It may be stated that these researches have resulted in the synthesis by very simple methods of covellite, chalcocite and chalcopyrite, and pos- sibly, although not quite certainly, of bornite. The results of these investigations will soon be ready for publication.

The great majority of the mineral syntheses in this field have heretofore involved the use of high temperatures, either fusion or heating in the autoclave from 150 to 400 degrees Centigrade. The geological evidence seems to show conclusively, however, that most secondary enrichment has taken place below 100 degrees largely probably between 20 and 50, and sometimes as low as zero in frozen water-saturated beds.? It was deemed advisable therefore to carry out our experiments at temperatures more in conformity with natural conditions. We however permitted our- selves to improve on nature (probably) in the matter of the con- centrations of some of the mineralizing reagents used. As most of the reactions with which we are concerned are very slow, this use of very high concentrations of certain “ mineralizers” is necessary in order to obtain results within a reasonable time. Full allowance must be made for the fact that in some cases such practice may lead to results slightly different from those which take place in nature.

The present paper contains the results of a rather extended in- vestigation of the conduct of natural chalcocite, covellite, chalco- pyrite, and bornite in alkaline, neutral and acid solutions, in the presence of a maximum concentration of hydrogen sulphide at 30 degrees Centigrade. The maximum concentration of hydrogen sulphide was maintained by liquefying and sealing up some of the

1F. W. Clarke, “ The Data of Geochemistry,” Bull. U. S. Geol. Survey, No. 491, IQII.

2In frozen beds at Bonanza Mine, Alaska, azurite (melaconite), malachite,

covellite, and white chalcocite are seemingly being continuously formed. (Personally communicated by C. F. Tolman, Jr.)

Secondary Sulphide Ore Enrichment. 351

gas in a tube with the mineral and other reagent. The temper- ature was maintained by means of a thermostat, and 30 degrees was necessary since at lower temperatures the crystalline hydrate (H,S-6H,O) appears as an ice-like mass, solidifying the whole contents of the tube, and making observations impossible.

Experimental.

The general method of procedure in these experiments was as follows:

A large number of glass tubes, fairly thickwalled, of an inside diameter of about eight millimeters, and a length of about twelve centimeters were sealed off at one end. Fragments of the min- erals to be investigated were placed in the tubes, which were constricted near the open end to facilitate sealing off later. A sufficient amount of the aqueous solution of the reagent to be employed was introduced through the constriction by means of a thin capillary tube, and the tubes and contents were placed in an ether bath which was cooled to below the liquefying temper- ature of hydrogen sulphide by means of liquid air. While thus cooled, a stream of hydrogen sulphide from a generator was passed into the tube. When a sufficient amount of the liquefied gas (enough to insure a permanent layer after the tubes had warmed up) had accumulated, the open end of the tube was sealed off. When the tubes warmed up to room temperature they not infrequently exploded, and such tubes were replaced until full sets were obtained. If a tube was due to explode it usually did so quite promptly on warming up, explosions being quite rare among tubes that had resisted for a few hours.

The reagents used were in all cases four different concentra- tions of potassium sulphide (10N, N, N/1o, and N/100), pure water, and N/1o sulphuric acid. Thus there were six tubes pre- pared for each mineral.

Observations were carried out on these tubes, beginning imme- diately after they had warmed up, and thereafter at intervals, the intervals at first being short (a day or two), and later being extended to a week or more. The observations were made by

be ry of : ve on de. hat : ees as

ble in : ur- on- lost this me. uch lich Ico- the

yen the , No. chite, ‘med.

352 S. W. Young And Neil Preston Moore.

means of a hand lens, goggles also being used as a protection to the eyes. There follows a résumé of the recorded observations, which are too voluminous to present in full.

Chalcocite.

The chalcocite used was a fairly pure specimen, containing about two per cent. iron as impurity. Microscopic examination showed practically no chalcopyrite, bornite or covellite. The mineral did, however, show the presence of both the white and blue chalcocite when examined by the polished section method. The white chalcocite appeared in irregular stringers throughout the ground mass which was blue chalcocite.

In all the chalcocite tubes, the first change observed was the for- mationofa deep brown colloidal solution of copper sulphide® ( prob- ably also containing iron sulphide). At the same time the surface of the mineral was found to be undergoing attack, and considerable amounts of black, amorphous, flocculent material were produced, some of which settled in the tube on standing, while some re- mained adhering to the mineral, forming a close velvety deposit. After one or two days the brown colloidal solution had floccu- lated and settled to the bottom. This conduct was characteristic of all the tubes, irrespective of the reagent they might contain. From this time onward there was, however, considerable varia- tion in conduct, and the different tubes must be treated individ- ually.

1. Chalcocite, 1oN and H,S.

Two days after filling, the amorphous material began to show very minute crystalline faces throughout the mass. After two weeks crystallization had progressed considerably, while at the end of one month the whole mass of previously amorphous ma- terial had become seemingly well crystallized, while on the sur- face of the mineral were a considerable number of well-defined crystalline individuals. As will be shown later, these were chal- cocite. The material at the bottom of the tube was mainly chalcocite crystals. Changes occurring after this time were not particularly noteworthy.

3 Cf. J. D. Clark, Bull. Univ. New Mexico, Chem. Series, 1, No. 2.

ns,

Secondary Sulphide Ore Enrichment. 353

2. Chalcocite, N, K,S and H,S.

This tube differed from the preceding in that there was less sloughing of the amorphous material, and a greater tendency for this to adhere to the mineral, forming a very compact velvety coating. This tube was also much slower in developing crystals. The preceding tube showed distinct signs of recrystallization on the second day while this phenomenon did not appear in the pres- ent one until seven weeks had elapsed. At the end of two months much of the floating material was still amorphous, and remains so to the present time (four months after setting up the ex- periment).

3. Chalcocite, N/10, K,S and H.S.

This tube differed from the two preceding in showing much less sloughing from the mineral. After ten days the amount of floating material seemed to have decreased and the mineral had a velvety coating. After sixteen days the floating material was considerably crystallized and a few crystal faces of chalcocite had appeared on the mineral. Since that time recrystallization has proceeded regularly but slowly.

4. Chalcocite, N/100, K,S and H,S.

In this case the sloughing from the mineral was less marked than in the preceding one. Also the recrystallization began much more promptly. Three days after starting there was but little floating material, and the mineral surface showed distinct signs of recrystallization. After five days all floating material had disap- peared, and the whole surface of the mineral was distinctly though minutely crystalline. From this time on the crystallization pro- gressed regularly and fairly rapidly, until, at the end of sixty days when the experiment was interrupted, a very beautiful coating of brilliant crystals (afterward shown to be chalcocite) had devel- oped, the amorphous material having been nearly all reabsorbed on the mineral.

ion ‘he ind out or- ob- ace ible ed, sit. cu- ria- two the ned ; 1al- nly not

354 S. W. Young And Neil Preston Moore,

5. Chalcocite, H.O, and H,S.

In this tube the initial sloughing was very slight, although quite noticeable. At the same time the recrystallization of chalcocite began much more promptly than in any of the preceding tubes, so that after three days the mineral was distinctly coated with a film of minute but very brilliant crystals of chalcocite, and as time went by the crystals grew at a more rapid rate than in any other tube under observation.

In connection with this tube another very interesting phenom- enon was observed. Upon very careful scrutiny it seemed at first that the minute crystals of chalcocite were imbedded in an olive brown drusy and probably amorphous material which was ad- herent to the surface of the mineral. After a few days this changed gradually to a bright golden appearance, much resem- bling chalcopyrite, which it was afterward proved to be.

6. Chalcocite, N/10 Sulphuric Acid, and H,S.

Immediately upon warming up, very marked colloidal dispersion was evident. Ina day or two the flocculation of this material was complete. In case of this tube there is up to the present time (four months after setting up) .no evidence of any tendency to the recrystallization of chalcocite, while the sloughing process seems to have been steadily progressing, until a very considerable portion of the mineral has become disintegrated.

After the tubes had been under treatment for some two months or more, a number of them were opened, and the mineral frag- ments submitted to a microscopic examination. The crystals which we called chalcocite showed very clear-cut pseudohexagonal forms which were not to be distinguished from the rather uncom- monly occurring natural crystals. The surfaces were extremely bright and the edges very sharp. Some attempts to photograph them did not succeed very well on account of their intense glitter. Fig. 20 is a photograph of a specimen first dulled by treating with chromic acid. The crystals were seriously injured by this treat- ment and the photograph is not very satisfactory.

Secondary Sulphide Ore Enrichment. 355

It was stated that in the case of tube No. 5 there was very con- siderable evidence of the development of chalcopyrite along with chalcocite. Upon microscopic examination all specimens which showed chalcocite recrystallization also showed the chalcopyrite phenomenon. This chalcopyrite formation was most marked in cavities and fissures, and always had the appearance of a smooth coating, almost as though it were plated on. The chalcocite crystals, on the other hand, were almost always in protuberant

Fic. 20. Chalcocite crystals grown on natural massive chalcocite. Magnifi- cation about twenty diameters.

clusters and individuals, so that the appearance was that of a chal- cocite gem in a chalcopyrite setting. There seems to be no doubt as to justification for considering this material to be really chal- copyrite. It is of course impossible to isolate any of the material for analysis, but reasonably certain evidence has been obtained in another manner. Some of the mineral fragments were ground off after imbedding in wax, then carefully polished and examined by the vertical illumination microscope. In cross-section where the chalcopyrite had deposited there showed a beautifully distinct thin

a

s y h t-

356 S. W. Young And Neil Preston Moore.

border of material which fulfilled perfectly the petrographic properties of chalcopyrite. Furthermore, the chalcopyrite had in- filtrated into the fragment for very considerable distances, fol- lowing the courses of small veinlets. These veinlets which pre- viously contained mainly chalcocite had become completely filled with chalcopyrite, and could not be distinguished from forma- tions frequently found in nature.

The petrographic work was carried out by Mr. Joseph Beeson, of the department of geology and mining, and our thanks are hereby expressed to him. Much more work has been done along similar lines and will be reported in a later paper.

Discussion Of The Results On Chalcocite.

We shall here discuss some of the main results indicated by the above study of the conduct of chalcocite under maximal concen- tration of hydrogen sulphide.

1. The Initial Sloughing.

This phenomenon occurred in every case, and is referable to the dispersion of some of the chalcocite into the amorphous colloidal form. In neutral and alkalirie solutions the sloughing ceased after a few days and the dispersed material redeposited on the mineral fragment, usually in crystalline form, although it seemed in some cases to first form a tightly adherent velvety coating, which later became crystalline.

The first problem is to explain the origin of this amorphous material. It is inconceivable that it should have come from the breakdown of the ordinary crystalline, pseudohexagonal chalco- cite since the breakdown of this into amorphous, followed by re- crystallization into the same form would conflict with the second law of thermodynamics. Thus we must look to some other source than pseudo-hexagonal chalcocite.

As the chalcocite used showed the phenomenon of “blue,” and “white” chalcocite, it was thought that in this fact an explanation

n-

ion

Secondary Sulphide Ore Enrichment. 357

might be found. Our thanks are due to Mr. Manji Yoshimura for carrying out the following experiments with the object of throwing some light on the matter.

Polished sections were prepared and the polished surface care- fully examined with a vertical illuminator. An area was chosen which showed the blue and white phenomenon very distinctly. The orientation of this area was carefully determined by means of characteristic veinlets, and the area photographed. The spec- imen was then treated by immersion in hydrogen sulphide water for five minutes, and carefully washed and dried. The “blue” areas were distinctly dulled, while the “white” stringers remained bright, indicating that the sloughing had taken place from the blue and not from the white chalcocite. The specimen was then again placed in hydrogen sulphide water, and allowed to remain for ten days, when it was again examined. Minute pseudo- hexagonal crystals had formed on the surfaces, but were located like a network of ridges, which under the binocular were dis- tinctly elevated above the much etched surface. The small area which had been originally photographed was carefully sought out, and again photographed. On comparing the two photographs it was found that the ridges of chalcocite crystals which had de- veloped during treatment coincided entirely with the original stringers of white chalcocite, which shows clearly that during treatment the blue chalcocite had sloughed off, in part at least as amorphous material, and had later deposited in crystalline form on the white. The obvious conclusion is that white chalcocite is identical with the crystalline pseudo-hexagonal material, while the blue is something else.

As to the nature of the blue chalcocite, several hypotheses may be made. The essential thing is that it be something less stable than pseudo-hexagonal chalcocite under the conditions of the ex- periments. The most obvious hypotheses are: (1) That it is a crystalline form of chalcocite less stable than the pseudo-hexag- onal. (2) That it is amorphous cuprous sulphide.

The first hypothesis does not seem very probable. While chemists recognize two crystalline forms of cuprous sulphide

ba ig 1e he lal ed h e ed 1g, he re- nd e re ind %

:

358 S. W. Young And Neil Preston Moore.

with a transition point at 103 degrees Centigrade,* the form stable above the transition point being regular, there are, so far as we know, no suspicions of the occurrence of the second form in nature. But even if found, its identity with the blue chalcocite would be very doubtful, since the blue chalcocite breaks down into colloidal material under the conditions of our experiments. That a crystalline form in an unstable field should do this is perhaps not impossible, but it is not very probable.

The second hypothesis, namely that the blue chalcocite is amor- phous cuprous sulphide, seems to be supported in a quite positive manner by all the phenomena which we have observed. If this is the case, the initial sloughing and formation of colloidal solu- tions becomes merely the dispersion of already colloidal material by means of hydrogen sulphide. This is a phenomenon for which there is ample precedent from chemical researches and some in- vestigations carried out in this laboratory along this line will shortly be published elsewhere.

The conception of the origin of the blue and the white chalco- cite combination which seems to us most plausible is as follows: At one time the whole material was merely a mass of amor- phous cuprous sulphide more or less contaminated with iron. There are many ways in which such masses may have originated. They may have been merely accumulations in favorable locations from sulphides floating in underground waters. We have seen irregular, kidney-like nodules of chalcocite which seem to indi- cate such origin. On the other hand they may have originated from the breakdown of bornite in situ. This is a more common and more rapid reaction than might seem probable, and we have found one specimen of bornite which after treatment for a few week at 100 degrees Centigrade in water showed, after being sectioned, not the least appearance of bornite, but the uniform blue gray chalcocite color. Unfortunately we have not found time to investigate this point further.

However, the origin of the mass of impure amorphous chalco-

Bellati and Lussana, Atti. Inst. Venet. (6), 7, 9, 1880.

Secondary Sulphide Ore Enrichment. 359

cite need not especially concern us at present. Being present and exposed to neutral or slightly alkaline waters carrying hydro- gen sulphide, recrystallization of the chalcocite would set in, as our experiments have shown. Since the whole mass, being amor- phous, would be permeable to: mineralizing solutions, crystalliza- tion would start at many points throughout the mass. On ac- count of the universal tendency for crystals to be purer than the material from which they were formed, there would be continu- ally occurring a concentration of the impurities into the spaces between the crystals. The rate of crystallization would thus be retarded, and after a time the concentration of the impurities would probably become so great that crystallization would be as good as stopped, just as sugar syrup ceases to crystallize when the impurities become sufficiently high. We might expect in time a material consisting of a spongelike mesh of crystallized chalco- cite, the interstices being filled with residual, impure, amorphous cuprous sulphide, the main impurity in our case being iron. This is a condition which approximates very closely to what we find in the material with which we have worked, as is interestingly shown by Mr. Yoshimura’s experiments mentioned above. An- other experiment performed by Mr. Yoshimura is of consider- able interest in this connection. As has been shown above, the chalcocite when treated in acid solution with hydrogen sulphide shows no tendency to recrystallize even after months. Mr. Yoshimura therefore treated a specimen of our chalcocite with such reagents for several days. The amount of sloughing pro- duced was great. After washing and drying the specimen, it was found that the patches of blue chalcocite were deeply eaten out into pits and fissures, the residual spongelike skeleton of white chalcocite remaining intact. In this case the white chal- cocite was clearly residual, showing no distinct sharp-edged indi- viduals, as was the case in the treatments in alkaline and neutral solutions.

One important point must still be considered before this hy- pothesis as to the origin of the blue and white chalcocite combi- nation may be considered as wholly plausible. Why does not the

) Ss

n

360 S. W. Young And Neil Preston Moore.

iron present unite with the cuprous sulphide to produce chalco- pyrite? This happened in every case in our experiments, except possibly in acid solution, in which case, unfortunately, the tube exploded, and the mineral was lost before it could be removed for observation. There was considerable evidence, however, be- fore the tube exploded, that chalcopyrite was actually forming, and we have observed its formation in numerous other experi- ments in acid solution. Besides, these conditions could scarcely fill the needs of the situation, since chalcocite itself does not crystallize at any measurable rate in acid solution. The answer to the above question must therefore be that the blue and white chalcocite could scarcely have formed under hydrogen sulphide conditions. Considerable evidence has been gathered, however, which shows that it might have formed in sulphur dioxide con- ditions. It has been found that chalcocite slowly recrystallizes in sulphur dioxide solutions, and that chalcopyrite breaks down, yielding chalcocite (and sometimes covellite) in the presence of this reagent. Data on this point will be given in a future paper.

Our experiments further show a connection between the amount of initial sloughing and the acidity or alkalinity of the solution. The sloughing is at a maximum in the acid solution, and is seemingly permanent. It is least in neutral solution and again increases with the alkalinity.

2. The Rate of Recrystallization of Chalcocite.

Our experiments bring out quite clearly the influence of the acidity and alkalinity of the solution on the rate of crystalliza- tion of chalcocite from the amorphous form in the presence of hydrogen sulphide. The rate is zero so far as we have been able to observe in tenth normal acid solution. It is greatest in pure water, and thereafter falls off as the alkalinity increases, except that in very high concentrations of alkaline sulphide it again in- creases very noticeably. Such strongly alkaline solutions, how- ever, are unlikely to occur in nature.

#2

Secondary Sulphide Ore Enrichment. 361

Chalcopyrite, Bornite and Covellite.

Tubes containing fragments of chalcopyrite, bornite, and covel- lite, but otherwise identical with those described under chalcocite were set up and subjected to observation. Since the experimental method and the system of observation has been fully indicated under chalcocite it is not considered necessary to go into similar detail concerning these minerals. We shall therefore in the re- mainder of this paper, confine ourselves to a discussion of the results we have obtained with the three remaining minerals, not even going to the extent of recording categorically the conduct of individual tubes.

Results on Chalcopyrite.

None of the tubes showed any very marked sloughing of col- loidal material, although this phenomenon was present to some extent. After a few days, in all cases black patches appeared here and there on the surface of the mineral fragment, and it became evident that the sloughing had occurred from these points, which were undoubtedly areas of inclusions of chalcocite or bor- nite. Previous microscopic examination of the chalcopyrite showed small amounts of these minerals to be present. After three months of observation it became evident that in no case was the chalcopyrite itself attacked. Its surface gradually brightened somewhat, as though it were perhaps sufficiently soluble to allow very slow superficial purification and recrystallization. Chalco- pyrite is thus seemingly, under the conditions of our experiments, a perfectly stable substance. Its formation under the same con- ditions from iron-bearing chalcocite indicates the same thing.

Results on Bornite.

Initial sloughing was much more marked with bornite than with chalcopyrite, although not so marked as with chalcocite. After from five to fifteen days, all tubes containing neutral or alkaline solutions showed distinct blue velvety films on the surface of the mineral. These films gradually developed, and in time became more and more certainly crystalline. After two or three months they had become interspersed with very brilliant patches

xe e f he e

362 S. W. Young And Neil Preston Moore.

of chalcopyrite. Some specimens were removed and examined

microscopically. The observations showed very distinctly three

different things:

(1) Chalcopyrite patches.

(2) Some chalcocite crystals.

(3) Large quantities of very thin scale-like crystals showing blue in some lights and purple in others.

These plates showed a marked tendency to expose the thin edges to view, and this, together with their glitter, made them unsatisfactory for photographing. The specimens when directly viewed were of great beauty. These blue scales were formed not only on the surface of the mineral, but throughout the solu- tion, on the walls of the tubes, and at the junction between the liquid hydrogen sulphide and solution. There seems to be not the slightest doubt that these plates were covellite. The fact that covellite itself under similar experimental conditions, récrystal- lizes in just such forms (see later under covellite) confirms this view.

As in the case of the chalcocite, the rate at which these changes occurred was greatest in neutral and slightly alkaline conditions, the stronger alkaline solutions retarding the action. In acid solu- tion bornite showed only insignificant change. We may there- fore conclude that in the presence of hydrogen sulphide bornite is, in neutral and alkaline solutions, unstable while in acid solu- tions, if not stable, it may at least persist for relatively very con- siderable periods of time.

The decomposition products of bornite under our experimental conditions raise some interesting questions. They seem to be chalcopyrite, covellite and chalcocite. The bornite used contained some little chalcocite, which accounts at least in part, and perhaps wholly, for the chalcocite crystals which were found; but the development of covellite in such large quantities seems rather sur- prising. If we take for granted the usual formula for bornite (Cu;FeS,), its breakdown yielding chalcopyrite would take place according to the equation,

Bornite Chalcopyrite Chalcocite Cu,FeS, CuFeS, + 2Cu.S

[]

ee

Secondary Sulphide Ore Enrichment. 363

Why then should we get covellite? Either the bornite must have contained a considerable quantity of excess sulphur (as solid solutions or otherwise) or the formula for bornite is incorrect. If the bornite formula is correct pyrrhotite must have formed, which is highly improbable. We have at present no data upon which to decide this point, but investigations are being continued.

Results on Covellite.

Covellite showed considerable tendency to recrystallize in neu- tral and alkaline solutions, giving thin scales, absolutely identical in appearance with those obtained from the breakdown of bornite, but this was restricted to localized portions of the mineral, pre- sumably where impurities were present, being probably merely the result of the tendency of impure materials to purify them- selves by recrystallization. We should thus conclude that covel- lite is a perfectly stable substance under the conditions of our experiments. In acid solutions little change was noticeable. One thing must however be mentioned here, to which we have not before referred, and that is the rather frequent appearance of crystalline sulphur, not only in tubes containing covellite, but in all others except chalcocite.

This appearance of sulphur seems to us at present rather fortu- itous, and strangely it sometimes appears only to disappear again. In the case of covellite and of bornite (which yields covellite) the appearance of the sulphur might be explained as due to a break- down of covellite to chalcocite and sulphur.2 However, the whole matter of the role of free sulphur is at present quite ob- scure to us, although we hope to be able to throw some light on the matter soon.

It is only fair to state that in the original sealing off of the tubes a minute amount of sulphur was formed by the decompo- sition of a little hydrogen sulphide by heat. This amount how- ever was so small that it seems to bear no relation to the quantities which afterward appeared in many tubes.

5 Cf. J. D. Clark, Bulletin, University of New Mexico, No. 75.

d ue in m ly ed u- he : ot t a 11S es 1S, We ite lu- n be ed Ps he ir- ite ice

364 S. W. Young And Neil Preston Moore.

Summary.

The results of the investigations discussed in this paper may be summarized as follows:

1. Hydrogen sulphide is a very powerful mineralizing agent for the four commoner sulphides of copper, namely chalcocite, chal- copyrite, bornite, and covellite. Under the influence of high concentrations of this reagent very fundamental changes occur in these minerals at 30 degrees Centigrade, in periods of time so short as to be easily within the demands of laboratory investiga- tion. The conduct of the minerals in acid solution is quite dif- ferent from that in alkaline or neutral ones. The main reactions observed may be compiled as follows:

A, In Acid Solutions.

White—Unattacked.

Blue—Disperses into colloidal and flocculent amor- phous material showing no signs of recrystalli- zation.

(2) Chalcopyrite—Sloughs out certain localized impurities.

(3) Bornite—Seems to be a persistent substance, possibly stable.

(4) Covellite—Is stable, showing practically no signs even of recrys-

tallization.

(1) Chalcocite

B. In Alkaline Solutions.

White form‘is stable.

Blue form sloughs off into amorphous material, some going into colloidal solution. Much of this material is reattracted to the mineral fragment, forming first a sooty layer which afterwards re- crystallizes to the white pseudo-hexagonal form. The rate of recrystallization of the white chal-

(1) Chalcocite + cocite is at a maximum in absence of alkaline

sulphide, decreases as the alkaline sulphide in-

creases up to a certain point (about normal so far as we have determined) and thereafter in- creases. The amount of sloughing increases as the concentration of alkaline sulphide increases.

The iron present as impurity in all cases com-

bines with some chalcocite to form chalcopyrite.

(2) Chalcopyrite. Is stable and shows only a tendency to sloughing

off of localized impurity.

(3) Bornite. Breaks down rapidly into covellite, chalcocite, and

chalcopyrite, all of which are, in a couple of months, obtained

Secondary Sulphide Ore Enrichment. 365

in well recrystallized forms. Both the rate of breakdown (as shown by rapidity of sloughing) and the rate of recrystalliza- tion seem to be at a maximum in water or faintly alkaline solution.

(4) Covellite. Is stable showing only a tendency to recrystallize in thin scales, and this only locally, probably at points of impurity. There is some evidence that seems to indicate under certain conditions, the possibility of covellite breaking down into chal- cocite and free sulphur, but we prefer to postpone the discus- sion of this point to a later time.

2. It has been shown that replacements having all the appearance of natural ones may be brought about in the laboratory in com- paratively short time. Thus Mr. Beeson found perfect infiltra- tions of chalcopyrite into veinlets of melaconite in chalcocite, in specimens which we had treated. This would seem to open up a new and interesting method of investigation. Minerals may be sectioned and carefully studied and mapped, then submitted to treatment with various mineralizers and the change noted after a repolishing. We have already done considerable work in this manner, with interesting results, which will be reported at a later time. For want of a better term we may call this the “ method of artificial replacement.” Our thanks are due to Professor C. F. Tolman, Jr., and A. F. Rogers for many kindly and helpful suggestions.

r

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d

Origin Of Copper Ores Of The “Red Beds” ‘Type.

Austin F. Rocers.

During the microscopic examination of polished sections of copper sulphide minerals in reflected light, the writer found that specimens of so-called chalcocitized lignite from the Sierra Oscura in New Mexico contained hematite, pyrite, bornite, and only a minor amount of chalcocite. The presence of these min- erals and their relations throw some light upon the origin of these interesting ores.’ Such light should be welcome, for the field relations give no decided evidence as to their origin.

The specimens in question are representatives of a widely dis- tributed type of copper ore which may for convenience be called the “Red Beds” type. These copper ores are found principally in the southwestern part of the United States. They have been described from Arizona, Colorado, New Mexico, Oklahoma, and Texas, but seem to be especially common in New Mexico, in which state they have been reported from nine counties. They were first described from Abiquiu, near Santa Fe, New Mexico, by Newberry in a report on the Macomb Expedition of 1859, published in 1876.

Similar occurrences of copper ores are of world-wide distribu- tion, for they have been described from Central Asia (Kirghiz Steppes), Turkestan, Bohemia, England, Nova Scotia, and Bo- livia. The Mansfeld (Germany) deposits, though perhaps syn- genetic rather than epigenetic, are closely related.

A general account of the American deposits has been given by S. F. Emmons* and by Lindgren.* The occurrences in

1 Since this paper was written A. E. Fath (Econ. Grot., Vol. 5, pp. 140-150,

1915) has briefly described polished sections of copper ore from the “Red Beds ” of Oklahoma. 2 Bulletin No. 260, U. S. Geol. Surv., pp. 221-232 (1904). 3 Econ. Geox., Vol. 6, pp. 356-381 (1911) ; “ Mineral Deposits,” pp. 360-376 (1913).

] (

‘

Origin Of Copper Ores Of The Red Beds Type. 367

New Mexico are described by Lindgren, Graton, Gordon, and Schrader.* A bibliography of publications relating to the western deposits is given by Hill.®

The copper ores of the “ Red Beds” type occur in sandstones and shales as disseminated specks or as nodules or replacements of plants, the latter often large tree-trunks. While these ores occur in the “Red Beds” series, which range from the Carbon- iferous through the Permian into the Triassic, they are usually directly associated with light-colored sandstones from which the red color has probably been bleached.

The minerals recorded in the literature are principally chalco- cite and its oxidation products, malachite and azurite, but pyrite, chalcopyrite, and bornite are occasionally mentioned and also barite and gypsum.

Ores From The Sierra Oscura, New Mexico.

The Sierra Oscura specimens, with which this paper is mainly concerned, were collected by Mr. H. W. Turner in 1901 and through his kindness have been presented to Stanford University. Mr. Turner* wrote a short account of the geology of the Sierra Oscura deposits. The copper ores occur in sandstones and shales interbedded with red sandstones and with Carboniferous lime- stone. The writer has examined a thin section and polished sec- tion of one of the copper-bearing sandstones. It is a fine-grained arkose with quartz, orthoclase, plagioclase, biotite, chlorite, apa- tite, and white opaque spots which are probably kaolinite. Chal- cocite occurs in irregular specks between, and also replacing, the quartz and silicates. Hematite with brown limonitic stains also occurs in much the same way as the chalcocite. A coarse red arkose with calcite cement contains magnetite and hematite, but no copper minerals.

The copper ore specimens from the Sierra Oscura are of two kinds. Some, which range up to three or four inches in size, are

4 Prof. Paper No. 68, U. S. Geol. Surv. (1910). 5 Bulletin No. 580-D, U. S. Geol. Surv., p. 57 (1913). 6 Trans. Am. Inst. Min. Eng., Vol. 33, pp. 678-681 (1903).

hat rra and 2 1n- lese eld dis- lled ally een and hey ICO,

59, bu- hiz Bo- : yn- ven I 50, Red

368 Austin F. Rogers.

Explanation To Plate Xvi.

Fic. 1. Wood replacement (1a) and nodule (1b) from Sierra Oscura, New Mexico. .

Fic. 2. Polished section of nodule from Sierra Oscura, New Mexico. (p) pyrite; (b) bornite; (cc) chalcocite; (m) melaconite; (h?) hematite of second generation.

Fic. 3. Polished section from Sierra Oscura, New Mexico (h) hematite; (p) pyrite; (b) bornite; (cc) chalcocite; (m) melaconite; (q) quartz.

Fic. 4. Polished section of wood replacement from Sierra Oscura, New Mexico. pyrite; (b) bornite with chalcocite border; (m) melaconite.

Pla

PLaTe XVI. Economic GeoLocy. VoL XI.

Fic. 1 b. Fic. I a. FIG. 2.

:

(p)

FIG. 3. FIG. 4. ae

:

Origin Of Copper Ores Of The Red Beds Type. 369

undoubtedly plant replacements for the wood structure is plainly visible. Fig. 1a represents a typical specimen. The other speci- mens are more or less spherical nodules or concretions about an inch or so in diameter. They are represented by Fig. 1b. The exterior of both kinds of specimens is dull brownish black with a little malachite or azurite. In the interior bornite and pyrite are the only minerals distinctly recognizable with the hand lens. Polished sections are necessary to identify the other minerals.

The study of polished sections in reflected light reveals a num- ber of minerals with interesting relations. The minerals identi- fied are hematite, pyrite, bornite, chalcocite, chalcopyrite, covel- lite, melaconite, limonite, malachite, azurite and quartz. The accompanying micro-photographs were made by means of the large Leitz metallographic microscope.

Fig. 2 (of one of the nodules) and Fig. 3 (of one of the wood replacements ) show the general relations of the minerals with low magnification. The smooth light-colored areas are bornite (b), which is bordered by chalcocite (cc), and it in turn by melaconite, which is the dull gray mineral making up the main mass of both photographs. The dark veinlets of Fig. 2 are quartz and hema- tite. The dark spots within the bornite areas which are a little out of focus are residual specks of pyrite and hematite. Though the nodules (Fig. 2) and plant replacements (Fig. 4) have the same minerals there is some difference in the structural relations. In Fig. 4 the bornite occurs along anastomosing channels. The spaces between the channels show a fine detailed structure which on higher magnification gives Fig. 5. This is evidently cell- structure of wood. My colleague, Professor L. D. Burlingame, of the botany department of Stanford University, who kindly examined these sections, says “this suggests coniferous wood.” The cell structure occurs only over part of the area of Fig. 4. The areas not occupied by the cell structure probably represent hydrocarbons formed before the replacement or possibly represent decayed areas. With the access of ore-bearing solutions both hydrocarbons and undestroyed wood with cell structure were re- placed. As will be seen later, Fig. 4 is a late stage of replacement

ae

370 Austin F. Rogers.

Explanation To Plate Xvii.

Fic. 5. Polished section of wood replacement, Sierra Oscura, New Mex- ico, showing cell structure. (h) hematite; (m) melaconite.

Fic. 6. Polished section of wood replacement from Sierra Oscura, New Mexico. (h) hematite; (p) pyrite; (b) bornite; (cc) chalcocite; (m) melaconite.

Fic. 7. Polished section of nodule from Sierra Oscura, New Mexico. (h) hematite; (p) pyrite; (b) bornite; (cc) chalcocite; (m) melaconite.

Fic. 8. Polished section of wood replacement from Sierra Oscura, New Mexico, showing a speck of bornite bordered by covellite (cv) and late chalcopyrite (cp).

:

[ex-

Yew

(h)

late

PiaTe XVII.

Economic GeoLocy. VOL. XI.

a FIG. 5. Fic. 6. : FIG. 7. Fic. 8. s

Origin Of Copper Ores Of The Red Beds Type. 371

and the explanation is that the cell walls afford capillary channels for the easy access of ore-forming solutions.

Figs. 6 and 7 are higher magnifications showing residual spots of hematite (/1) and pyrite in bornite (b). Both these min- erals have high relief, but hematite has a smooth surface and pyrite a rather rough surface and dark borders. Of the minerals present hematite was the first to be formed, for it occurs as residual specks in all the other minerals. It will be noticed that the pyrite has the same general shape as the hematite but is usually smaller. This suggests that the pyrite was formed by the replacement of hematite. In proof of this note that areas which were evidently all hematite are now in part pyrite. The hematite often has vermicular markings and along these pyrite is some- times developed. Ina few cases distinct veinlets of pyrite cutting the hematite were recognized.

The next stage was the formation of bornite at the expense of pyrite.‘ During this stage hematite was apparently not affected and simply remained as a residual mineral. The replacement of pyrite by bornite seems to have been more regular than the re- placement of hematite by pyrite.

The bornite areas are everywhere fringed by a narrow rim of blue chalcocite and occasional veinlets of blue chalcocite pene- trate the bornite. This is true of specimens represented by Figs. 2, 4, 6, and 7, but they do not show in the photographs. It is necessary to photograph the sections with colored screens to bring out the chalcocite. Fig. 3, which was taken with a deep blue screen (Wrotten), gives an idea of the relative amount of chal- cocite in this so-called chalcocitized lignite. The chalcocite was undoubtedly formed from bornite, and there is no evidence that any of it came directly from the pyrite. While chalcocite is the usual alteration product of bornite, covellite (cv) and chalco- pyrite (cp) are sometimes formed. Fig. 8 is a minute area of bornite bordered by covellite and chalcopyrite. This was taken with a 7s oil immersion lens. Immersed in the oil the covellite turns purple, and this is an aid in photography. The chalcopy-

7 The replacement of pyrite by bornite at Collahuasi, Chile, has been de- scribed by the writer (Min. and Sci. Press, Vol. 109, p. 684 (October 31, 1914).

372 Austin F. Rogers.

Explanation To Plate Xviii.

Fic. 9. Polished section of nodule from Payne County, Oklahoma, show- ing pyrite (p) replaced by chalcocite (cc).

Fic. 10. Polished section of wood replacement from Payne County, Okla- homa, showing cell structure. (cc) chalcocite; (m) melaconite.

Fic. 11. Polished section of wood replacement from Nacimiento district, New Mexico, showing cell structure. (p) pyrite; (cc) chalcocite; (cv) covellite.

Fic. 12. Polished section of wood replacement from Red Gulch, Colorado, showing cell structure. (cc) chalcocite.

Plate

Economic GEoLocy. VOL. XI.

Plate Xviii.

Fig. 9.

Fig. 11.

FIG. 10. Fic. 12.

Origin Of Copper Ores Of The Red Beds Type. 373

rite resembles chalcopyrite of the second generation commonly observed in copper ores, and it sometimes occurs in typical gashes in bornite.

The dull gray substance exterior to the chalcocite and formed at its expense is probably melaconite, as it is rapidly attacked by hydrochloric acid, and the solution gives a good copper test, while the bornite and chalcocite are scarcely affected. The mela- conite is especially well shown in Fig. 4. The minute bright spots are principally remnants of chalcocite, but some are hema- tite. In its earlier stages the melaconite occurs in fairly regular anastomosing channels. As Fig. 4 shows, the presence of cell structure favors the development of melaconite.

The veinlets along the centers of the melaconite channels are quartz (q) and hematite of the second generation (h?). These are shown in Figs. 2 and 3. The quartz veinlets do not show well in the photograph because they reflect very little light. The hematite veinlets are very narrow and have high relief. Limo- nite also occurs in veinlets and as an alteration product of residual specks of hematite. The carbonates on the exterior of the speci- mens of quartz and hematite of the second generation represent the last stages of alteration.

The order of succession of the various minerals, as determined by the metallographic microscope, is as follows:

ORDER OF SUCCESSION OF MINERALS, . Hematite. Pyrite. . Bornite. Chalcocite, covellite, and late chalcopyrite.® . Melaconite. . Hematite of the second generation, limonite, and puartz.®

7. Azurite and malachite.

The history of this ore-deposit may be interpreted somewhat as follows: Before any replacement occurred the partial destruc- tion of cell structure of the wood was effected either by decay or by the formation of hydrocarbons, while the wood was perhaps

8 The exact order of the minerals under 4 and 6 could not be determined.

An

:

374 Austin F. Rogers.

partly changed to lignite. The replacement of organic matter by iron-bearing solutions probably gave rise to hematite on account of the dehydrating character of salt solutions under arid condi- tions. The nodules, which show no evidence of organic structure, were probably originally limonite or siderite, although concre- tionary hematite is a possibility. Siderite, especially, is common in concretions in shales and forms in the presence of organic mat- ter. Furthermore, siderite has been reported from deposits near Archer City, Texas,® which are similar to those under discussion. The hematite was next changed to pyrite by alkaline sulphur- bearing solutions. Copper-bearing solutions then converted the pyrite into bornite and later the bornite into chalcocite with occa- sionally a little covellite and chalcopyrite. At a still later stage the chalcocite was oxidized to melaconite and finally a little hema- tite of the second generation was formed along with quartz.

Before the origin of these ores is further discussed three other similar occurrences are briefly described.

Ores From Nacimiento District, New Mexico.

Through the courtesy of Dr. J. D. Clark, of the University of New Mexico, the writer has been able to examine a specimen of chalcocite ore from the Nacimiento Mountains (the locality is seven miles southeast cf Senorita) New Mexico. This specimen is evidently a replacement of wood or lignite. The polished specimens reveal three minerals: pyrite, chalcocite, and covellite. Fig. 11 is a photograph of one of the sections. The chalcocite is formed at the expense of the pyrite. This probably means long-continued action of the solutions and in itself is evidence of the action of meteoric waters. There are occasional blotches of covellite within the chalcocite, and covellite has also formed along a transverse fracture. The cell structure of the wood is well preserved in the chalcocite. Cell structure is rarely preserved in pyrite, but the cell walls furnish capillary channels for later solu- tions which alter the pyrite to chalcocite. There are also areas of hydrocarbons which do not show the cell structure, and chalcocite

9E. J. Schmitz, Trans. Am. Inst. Min. Eng., Vol. 26, p. 97 (1806).

Origin Of Copper Ores Of The Red Beds Type. 375

has also replaced these to some extent. The Nacimiento deposits have been described by Schrader.?°

Ores From Payne County, Oklahoma.

A few specimens of chalcocitized wood and concretions from near Stillwater, Payne County, Oklahoma were kindly furnished by Mr. W. A. Tarr of the University of Missouri through my colleague, Professor C. F. Tolman, Jr. A paper on these copper ores by Mr. Tarr appeared in this journal several years ago,'? but he made no microscopic examination of polished sections.

The specimens consist of nodules and wood replacements. The concretions are made up largely of chalcocite, pyrite, and quartz with a little covellite. On the exterior pyrite predominates and in the center chalcocite. Fig. 9 is a micro-photograph taken along the transition zone between the two minerals. The replacement of pyrite by chalcocite is evident. In the interior of the nodule, pyrite is practically absent and in its place there is chalcocite with included quartz grains. This indicates that the nodules were formed by the replacement of a sandstone, principally of the cement but also of the sand grains to some extent. The replace- ment has reached an advanced stage because of the capillary channels around the sand grains. On the exterior of the nodule the solid mass of pyrite was replaced with difficulty.

A transverse polished section of the wood replacement is repre- sented by Fig. 10, which shows cell structure of the wood. The principal mineral here is chalcocite with gray non-metallic melaco- nite on the exterior of the cells. Covellite is also present in occasional patches. No pyrite is visible in the specimens formed by the replacement, but it was probably abundant at one time and has since been completely replaced because of the cellular texture which furnishes capillary channels for the ingress of solutions.

ORES FROM RED GULCH, COLORADO. Professor W. Lindgren, of the Massachusetts Institute of Technology, has kindly furnished the writer specimens of chal-

11 Econ. Geor., Vol. 5, pp. 221-226 (1910). 10 Prof. Paper No. 68, U. S. Geol. Survey, pp. 141-149.

fe is n e. te ns of of ne ell in u- of ite

376 Austin F. Rogers.

cocite from carbonaceous shale of Red Gulch, Colorado, which have been described’? by him. The polished sections were made

from the specimens figured by Lindgren.1* The only mineral .

visible in the sections :s chalcocite, but it is not unreasonable to suppose that the chalcocite is due to the complete replacement of pyrite by long-continued action. It is very doubtful whether chal- cocite is ever formed in any amount except by the replacement of other sulphides. This specimen is not a replacement of shale, but a replacement of wood for the micro-photograph (Fig. 11) shows clearly the cell structure of wood. The cell structure is absent in spots, where possibly the wood had decayed or resins were formed. The apparent lamination of the shale in the chalco- cite of Lindgren’s photograph is due to linear areas of cell structure.

Origin Of Copper Ores Of The “‘ Red Beds” Type.

It seems clear from the published descriptions that most, if not all, of the copper ores of the Red Beds type have a similar origin. What applies to one deposit will, with some modification perhaps, probably apply to all.

The origin of ores of this type has been discussed by S. F. Emmons, Lindgren, Turner, Graton, Schmitz and Hill. Both Turner'* and Schmitz'® incline to the syngenetic origin, but Lindgren considers that the epigenetic origin is practically proved. While the geological evidence perhaps favors the epigenetic origin of the copper ores, the microscopic evidence conclusively proves it, for the copper minerals were introduced at a comparatively later stage.

Whether these ores were deposited by cold descending meteoric waters or by hot ascending solutions is the next question to be settled. Lindgren sees in these copper ores the work of meteoric waters. As to the Sierra Oscura occurrence, Graton believes that copper-bearing solutions have come up from below along faults,.

12 Bull. 340, U. S. Geol. Survey, pp. 170-174 (1907). 13 “ Mineral Deposits,” Fig. 115, p. 371.

14 Loc. cit.

15 Loc. cit.

( ‘

Origin Of Copper Ores Of The Red Beds Type. 377

Turner, on the other hand, thinks that the ores are later than the main faults. In the Tularosa (New Mexico) district copper ores occur in sandstone of the “Red Beds” and also in an under lying igneous rock (diorite-porphyry). Graton,?® who describes these deposits, believes that these two types of ores are closely related in origin. He dissents from the views of Lindgren and decides in favor of the formation of both the Tularosa and Sierra Oscura copper ores by hot, ascending solutions.

The geological evidence, while perhaps favoring the meteoric origin, is not conclusive. It is, moreover, probable that Lind- gren’s views are partly based upon the supposed reducing action of the organic matter on copper solutions to form chalcocite. But we know that in the case of the Sierra Oscura and Nacimiento deposits chalcocite was not precipitated by organic matter, and it is not at all certain that such was the case in the Red Gulch deposit. As the copper ores occur in the nodules as well as in the plant replacements at Sierra Oscura, the role of carbon as a reducing agent is very improbable. As Lindgren himself has said, “ The importance of precipitation by carbonaceous material has been overestimated. . . .”78 In this connection it is interest- ing to note that Clark’® failed to precipitate copper sulphide on coal from a solution of copper and ferrous sulphates even at the end of 122 days. Lindgren’s remark “It is necessary to explain why chalcocite is characteristic of these deposits and chalcopy- rite of the fissure veins’’”° loses its force after the results of the microscopic examination are obtained.

All students of ore-deposits must admit that at least some types of ore-deposits, though perhaps greatly in the minority, are formed from meteoric waters, though, as Lindgren” says,

“The prevailing influence of igneous intrusions on ore deposition is, however, so strong that it is difficult to establish the proofs of the less conspicuous deposition by purely meteoric water.”

16 Prof. Paper No. 68, pp. 188-189 (1910).

17“ Mineral Deposits,” p. 174.

18 Bull. Univ. of New Mexico, Chem. Series, Vol. I., p. 117 (1914). 19 “ Mineral Deposits,” p. 375.

20“ Mineral Deposits,” p. 387.

21 Min, and Sci. Press, Vol. 100, p. 685.

ch de ral . to of al- nt le, I) is Ins cO- ell not rin. DS ps, oth but ed. gin ves hat Its,.

378 Austin F. Rogers.

Now let us consider the evidence furnished by the microscope for it is in cases of this kind, when the geological evidence is not convincing, or when the geology admits of several interpretations (e. g., Turner’s and Graton’s interpretation of the Sierra Oscura deposit), that we may turn to the microscope with some degree of confidence. For let it be emphasized that the history of an ore-deposit is written in the mineralogical changes revealed by the microscope, as well as in the broad geological features, and when we have made a little more progress interpretations based upon microscopic work will probably be more exact than those based upon field geology though of course microscopic work and field work should go hand in hand.

The later stages of the Sierra Oscura ores, and I speak more particularly of the Sierra Oscura ores for the microscopic record is more complete for them, are clearly the result of meteoric waters. The replacement of bornite by chalcocite does not in any way suggest the irregular “upward enrichment” structur: recently described by the writer.2!_ The fact that bornite is bordered by chalcocite and penetrated by veinlets of chalcocite is clear evidence of descending solutions. As the chalcocite im- mediately preceded the melaconite stage it is certainly formed by solutions directly in advance of the oxidizing solutions and these must have been meteoric waters. The chalcocite, then, is certainly the product of downward enrichment.

Next let us consider the earlier stages. Of the earlier formed minerals hematite furnishes the most conclusive evidence as to the origin of the Sierra Oscura ores. There are two fairly well defined kinds of hematite. The well-crystallized hematite (specu- larite) is a high temperature mineral occurring in igneous rocks as a magmatic mineral,?* in contacts, in deep veins and in vol- canic rocks as a sublimate. Crypto-crystalline and amorphous hematite occurs in sedimentary iron-ores such as the Clinton ore, in concretions, in pseudomorphs after other iron minerals, and in the gossans of ore-deposits of arid regions. The hematite of the

22 One of the clearest examples of magmatic hematite is found at the Engels

mine in Plumas County, Cal. See Turner and Rogers, Econ. GEot., Vol. 9, Pp. 359-391 (July, 1914).

Origin Of Copper Ores Of The Red Beds Type. 379

Sierra Oscura specimens belongs to the second kind of hematite. Though probably crypto-crystalline by slow, long-continued crys- tallization, there is no indication of the crystalline condition, and it does not have the appearance of specular hematite. Under arid conditions, and we know that at the time of the deposition of the Red Beds arid conditions prevailed in the southwest, hema- tite was probably formed instead of limonite on account of the presence of salts in solutions or limonite or siderite might have been converted into hematite under these conditions.?*

Pyrite, while usually formed by ascending thermal waters, is at times undoubtedly formed from meteoric waters. It is fairly common in sedimentary rocks and sedimentary pyrite de- posits are known.** A large pyrite deposit at Leona Heights, in Alameda county, California, has been assigned a meteoric origin by Whitman,”* and while his arguments are not convincing for the meteoric origin of this particular deposit, his experiment to produce pyrite under vadose conditions was successful. Sie- benthal has observed pyrite in springs at Sulphur Springs, Ark., which has undoubtedly formed from cold alkaline solutions.

Bornite, like pyrite, has usually been formed by ascending thermal solutions, but it, like pyrite and chalcopyrite, is probably a persistent mineral ranging in occurrence from igneous rocks to deposits formed near the surface from cold meteoric waters. Bornite has recently?® been described as nodules in shales from Mashonaland, southern Rhodesia. It is interesting to note that Mennell believes that the bornite is a replacement of pyrite. While this bornite occurs in Silurian or Lower Devonian sand- stone instead of in Red Beds, it is suggestive that the shales show pseudomorphs after cubic crystals of salt.

The relation of the bornite to the pyrite is practically that of the “exploding bomb” structure described by Graton and Mur-

23 Gagel, Zeit. f. prakt. Geol., Vol. 18, pp. 18-23. Abstr. in Econ. Grot., Vol. 5, P. 499 (1910).

24 Lindgren, “ Mineral Deposits,” p. 232.

25 Econ. GEo., Vol. 8, pp. 455-468 (1913).

26 Mennell, Mineralogical Magazine, Vol. 17, p. 111 (1914).

ot ra in id se in is 1S n- ed nd 1S d to the gels 9,

380 Austin F. Rogers.

dock,?* but this structure is probably produced by bot hypogene and supergene solutions.

While pyrite and bornite may have been formed either by meteoric or thermal waters, the occurrence of hematite of the low-temperature type in concretions constitutes the most con- clusive single argument in favor of the meteoric origin of the Sierra Oscura ores. The presence of hematite in two genera- tions between which periods the sulphides were formed, suggests arid climate followed by humid climate and finally, by another period of arid climate. The dependence upon the climate is addi- tional evidence in favor of the meteoric origin.

In conclusion, then, it may be said that the microscopic evi- dence, and the geological evidence as far as it goes, suggest that the copper ores of the “ Red Beds” type were formed by meteoric waters without the agency of igneous rocks. The ores were formed by circulating solutions, which may have been locally as- cending, during a long-continued period of time. The solutions were alkaline, at least during the earlier stages. As with most ores, there were definite stages of mineralization.

In view of the fact that the prevailing trend in the study of ores is towards the increasing importance of magmatic waters (and the writer is fully in accord with this view) it is interesting to find evidence of the influence of meteoric waters in the deposi- tion of ores and to find an application of the theoretical principles of ore-deposition so ably set forth by Van Hise.?® The Red Beds type of copper ores, is probably one of the few cases in which Van Hise’s principles of ore deposition will apply.

27 Trans. Am. Inst. Min. Eng., Vol. 45, pp. 26-81 (1913).

28 Trans. Am. Inst. Min. Eng., Vol. 30, pp. 27-176 (1901); also Mono- graph 47, U. S. Geol. Surv., pp. 123-191 (1904).

( ( t

‘ i ‘ ‘ ?

THE ORIGIN OF CLAY SLIPS. W. B. WIzson.

Clay slips may be defined as clay-filled fissures cutting a coal seam and extending ‘nto the roof. They are referred to inci- dentally in many coal reports, but few articles have been pub- lished which attempt to describe them accurately or to explain their origin. Keyes! was perhaps one of the first to point out some of the more significant features. He considers the basin character of coal deposits and the effect which the greater settling in the center of the coal-forming basins has had on the beds overlying the coal. His conclusion is’ “that in the case of many of the small faults the stresses may have been superinduced by the diminution in bulk of lenticular coal beds at horizons some- what lower than those in which they occur.”

In a paper on certain features of coal basins in Iowa, Bain states that the formation of slips is to be referred to volume changes in the coal seam with which they are immediately asso- ciated. He says:?

“Tn the coal bed itself the settling effects may be seen in the small slips or faults that are encountered. As the coal became older and less plastic it was able to resist the pressure for some time. Finally the accumulated force would cause it to yield and this yielding would cause it to take the form of a series of slips or cracks.”

Gresley found no existing relation either general or local between the clay slips and dip of the underlying strata. He writes® “that it may he possible that the same causes that pro- duced the fissuring of the Coal Measures may also have been the origin of some of the well known fissure veins of metalliferous mining regions.”

1 Keyes, Bull. G. S. A., Vol. 5 (1803), p. 231.

2 Bain, Jour. of Geol., Vol. 3 (1805), pp. 651-652. 3 Gresley, Bull. G. S. A., Vol. 9 (1807), p. 57.

4, a

382 W. B. Wilson.

According to Crane‘ the evidence in the Kansas coal fields does not clearly indicate the source of the stresses that caused the slips.

“Long after the coal was consolidated almost to its present state, vibratory movements of one kind or another fissured the strata includ- ing the coal beds. . . . The general results were of a nature to stretch and elongate the strata horizontally rather than to compress them.”

While discussing clay slips in the Illinois coal field Savage® assumes

“that there would be unequal contraction in different parts of the seam owing to the lack of homogeneity of the vegetal materials making up the coal bed. . . . The combined strength of the roof shale and the cap rock was not sufficient to stand the unequal strain to which they were subjected, and fissuring of the beds resulted.”

It is evident from this belief review of the literature that there is lack of agreement as to the causes of clay slips. While a student at the University of Missouri, the writer had access to mines in which slips were well developed and so numerous as seriously to retard mining operations. Certain relations that are clearly shown in this‘locality have not been noted by writers in other areas. It cannot be said, however, that all clay slips were due to a single cause, and the theory advanced in this paper may hold only for the area investigated. The writer wishes here to express his thanks to Prof. W. A. Tarr, of the University of Missouri, for suggestions and assistance in the field work and to Prof. R. T. Chamberlin, of the University of Chicago, for further suggestions during the preparation of the manuscript.

The data for this paper were collected from mines operating on the Bevier seam of the Cherokee formation in Boone County, Missouri. This is a bituminous coal which averages three feet in thickness but varies from twenty-eight to forty-two inches in the mine examined in greatest detail. Above the coal are twenty or more feet of shale and beneath it is a fire clay that rests upon a three-foot limestone bed.

A description of the clay slips falls into four parts: the way

4Crane, Kansas Univ. Geol. Report, Vol. 3 (1808), p. 211. 5 Savage, Econ. Grox., Vol. 5 (1910), pp. 185-186.

oes ps. ate,

The Origin Of Clay Slips. 383

in which they affect (1) the roof, (2) the coal, (3) the floor, and (4) the constitution and structure of the clay filling. In the roof the slip planes are well defined and in most cases the dips are not far from the vertical, but may range as low as 45 degrees. The thickness of the filling varies from a foot or more to a mere crack containing a film of clay. As a rule the projections of the roof shales into the filling on one side have corresponding reéntrants on the opposite side. Such breaks in the roof require extensive timbering during the mining operations. Where the timber has been removed caving has exposed certain slips to a height of fifteen or twenty feet and in such cases they give evidences of dying out vertically.

In general slips cut to the botom of the coal but some of small size pinch out within the coal. In most cases the average thick- ness of the filling is considerably greater in the coal than in the roof. Near them, especially in the case of the larger slips, the coal tends to be shattered and broken so that much of it is lost in the slack as a result, but the composition of the coal does not appear to have been changed, since approximate analyses failed to show any appreciable difference in the quality of the coal near the slips and that farther away.

The relation of the clay to the floor is of small concern to the miner but of great interest to the geologist, for it seems that they do not cut below the base of the coal. It was impracticable to make certain of this in every case but a typical slip was selected and the fire clay beneath the coal removed with a charge of dynamite. The limestone member beneath the fire clay showed no sign of a break.

The material in the slips consists largely of clay somewhat lighter in color than the roof shales of this locality. Incorpor- ated in it are bits of sandstone, pieces of roof shales, and below the level of the top of the coal, fragments of coal. Slickensided surfaces are a characteristic feature of this clay filling. They are so numerous that scarcely a hand specimen can be found that does not show them. Slickensided surfaces were found also at the contact of the coal and the fire clay at a distance of many

ge® the cing the they hat hile to as are in yere nay 2 to of and for ting nty, tin the ona way

384 W. B. Wilson.

yards from any slip. They are developed to a lesser degree at the contact of the coal and the roof shale.

The slips are associated with thin coal. In the mine with coal ranging from twenty-eight to forty-two inches the greater num- ber intersected coal less than thirty-two inches thick and not one was found in coal of more than thirty-eight inches.

Origin Of Clay Slips.

The distribution of clay slips is so widespread that if they all take their origin from a single cause that cause must be closely related to the coal-making processes. Let us now consider a cause that must have had very general applicability. Although some coal seams can be traced over hundreds of square miles, basin structure is a characteristic feature, and there are present local variations that suggest the following interpretation. At the beginning of the period in which the Bevier seam, for example, was formed, the general region was low and poorly drained. The surface was not without some relief and many small lake or swamp basins were separated by divides whose crests were five, ten, or even twenty or more feet above the adjacent depressions. The coal-forming materials may have been deposited first in the depressions, spreading with a gradually rising water level, or the advance of the vegetable materials may have been due to plant growth about the margins of lakes supplemented by floating

Peat

Fic. 20.

forms. Just what amount of unconsolidated materials was re- quired to form one foot of coal has not been thoroughly worked out. It is improbable that the ratio was as low as Io to 1, but with that assumption the thirty feet required for the Bevier seam would have filled the basins and extended over the divides in a region with this low relief. Fig. 20 illustrates the conditions that

Ve

The Origin Of Clay Slips. 385

existed just before the beginning of the deposition of the roof. It is assumed that the upper surface of the peat bog was essentially level at that time.

It has been stated that the slips are associated with thin coal and it is now to be noted that the thickness of the coal is related to the rise and fall of the floor. The thickest coal is in the bottom of the depressions and the thinnest at the top of the divides with a rather uniform gradation between. The slips are found, there- fore, along the crests or on the adjacent slopes, but not in the thick coal near the bottom of the basins. Fig. 21 represents the change from conditions shown in Fig. 20 brought about by the deposition of the overlying beds and the consolidation of the peat into coal.

For diagrammatic purposes the vertical scale in Figs. 1 and 2 was made twice the horizontal since the true scale would not show relief enough to be effective. As drawn the central roll

in the floor in Fig. 20 has above it about twenty-five feet of ma- terial to be consolidated while there are nearly forty-five feet over the center of the basin to the right. If the peat shrank to one fifteenth of its original volume the roof descended almost twenty-three feet over the crest and forty-two feet over the center of the basin. As a result of this differential vertical movement that which was essentially a plane surface at the top of the peat became a warped surface at the top of the coal. This caused lengthening and in response to the tension stresses set up, the coal, and particularly the roof, had to stretch or fracture. The stresses were most effective at or near the crest of the rolls and there breaks resulted that amounted to a sort of open faulting. Where a divide is sharp a slip has developed at its crest in many places, but if it is somewhat flat on top there has been a tendency for a slip to develop on each side at the place where the floor

‘ Coal Floor Fic. 21. mn at

385 W. B. Wilson.

begins to drop rather rapidly. The slickensided surfaces at the contact of the coal with the floor point to the movement of the coal down the slopes when the slips were forming. They are limited to the slopes and were not found in the bottoms of the basins. Horizontal shrinking due to the escape of volatile con- stituents may have increased somewhat the tension stresses, but the irregularities of the floor determined the location of the slip planes.

The material that pressed in to fill the fissures was chiefly clay from the overlying beds. The slickensided surfaces of this filling indicate that it came in under pressure in a somewhat indurated form and did not enter as a plastic flow. It is conceivable that later movements may have formed them but there seems to be no reason to postulate any other theory than that they were formed as the clay entered the fissures. In some coal fields part of the filling at least seems to have been derived from the beds below the coal, but in this region the rigid limestone in the floor prob- ably checked any tendency toward a movement similar to the “squeezes” that occur in the floors of the workings of deep mines.

In isolated basins where the peat was not thick enough to pass over the divides slips appear to have been formed on the slopes by tensional stresses set up by the greater settling in the deeper portions. They seem, however, to be more typically developed and more numerous in this area in which there is marked varia- tion in the thickness of the coal and the irregularities in the floor are equally pronounced.

The Original Peat Bog,

Although somewhat apart from our subject it is of interest to enquire with what certainty we can calculate the dimensions of the original peat bogs from the ratios found to exist between the thickness of the coal and the elevation of the floor. Bain® has cited a coal seam in which a rise in elevation of sixty feet was accompanied by a decrease of four feet in the thickness of the

6 Bain, op. cit., p. 65.

onn

The Origin Of Clay Slips. 387

coal. From this he infers that each foot of coal now present there occupies one fifteenth of the vertical space which it origin- ally filled. A profile along a road in one of the mines examined by the writer gave the following data: in a distance of about 300 feet there was an increase of ten feet in the elevation of the floor and the coal thinned from thirty-six to twenty-six inches. This portion of the profile is given in Fig. 22.

It appears from these data that where the peat was ten feet thicker it resulted in the formation of ten inches more of coal. This gives a ratio of 12 to I, and reconstructing the bog on this basis thirty-six feet of peat were required to make three feet of coal and the upper surface of the bog was originally at the height

Upper surface of bog on assumption that volume changes varied with depth

‘Upper surface of bog as ordinarily calculated

Fic. 22.

of the broken line in Fig. 22. It appears, however, that applying the ratios directly in this manner may lead to serious error. To do so implies three assumptions that are in part at least unwar- ranted, (1) that the attitude of the floor has not been changed by warping since Pennsylvanian times, (2) that there was no creep of the coal-forming materials down the slopes as the de- scending beds came to bear more heavily on the crests of the rolls, and (3) that the volume changes did not vary with the depth of the bog.

In scores of cases the writer found no exception to the rule that the elevations of the floor are associated with thin coal and this is a general indication that there have been no marked warp-

36” r

388 W. B. Wilson.

ing movements since the formation of the coal. There is par- ticular reason to believe that there have been none along the line of the profile given in Fig. 22. This profile was runalonga slope parallel to the plunging crest of a divide and about 100 feet from it. A second profile was made normal to the first and intersect- ing it and the crest of the roll. The ratios between the elevation and thickness of the coal were essentially the same in the two pro- files. To influence both profiles equally a special type of warp- ing or doming must be postulated and warping along any other axis than 45 degrees to the direction of the profiles would not have affected both to the same degree.

It is not believed that creep of the peat down the slopes has been an important factor. The average distance between the crests of rolls is measured in hundreds of feet and the rigidity of the overlying shale beds was not such as to cause them to with- stand stresses across spans of that width and thus rest more heavily on the crests as the beds descended. To whatever extent movement of this kind took place, applying the simple ratios will lead to underestimation of the original thickness of the bogs.

The fact that shrinkage will vary with the depth of the basins cannot be neglected. This point has been referred to by Ashley‘ but he does not point out explicitly its bearing on the problem of reconstructing the bogs. On the basis of certain assumptions let us calculate the importance of this factor. Ashley® has been fol- lowed by White® in estimating that at a depth of fifteen or twenty feet, one foot of surface peat will have been reduced to 1% inches of compressed peat. As the bog which formed the coal shown in Fig. 22 certainly was deeper than twenty feet, we may assume that the material in the deepest parts was compressed more than that amount, possibly to about one inch, before any of the roof shales were deposited. We will assume also that there was a uniform gradation of materials from top to bottom and that the coal-forming power of the peat was inversely proportional to its bulkiness which we will consider equal to 12 at the top and

7 Ashley, Econ. Grot., Vol. 2 (1907), p. 43.

8 Ashley, op. cit., p. 39.

® David White, Bull. 38, U. S. Bureau of Mines, p. 87.

The Origin Of Clay Slips. 389

I at the bottom. Now the profile in Fig. 22 shows a decrease in coal at a rate of ten inches for ten feet of elevation and, since the coal is three feet thick, ten thirty-sixths of the total coal-form- ing power of the bog lay within this lowest ten feet of depth. With these data and with these assumptions a close approxima- tion of the height of the bog can be found by the aid of the formula,

in which a is the original height of the bog, and N is the number of feet that forms a fraction, 1, of the total thickness of the coal. In this problem N is ten feet and 2 is ten thirty-sixths. Substi- tuting these values in the formula the value of a is found to be a little more than 66 feet, as indicated by the top line in Fig. 22. This result adds more than 83 per cent. to the estimate as it would ordinarily be computed.

Summary.

In the area examined clay slips were formed by stretching the coal-forming materials and overlying beds over the divides be- tween the basins when the coal was consolidating. Fissures formed by the relief of tension stresses were filled by clay from the overlying beds and this material now constitutes the slicken- sided filling of the clay slips. This theory does not appeal to un- usual forces of unknown origin but presupposes and is dependent on the accepted theory of the origin of coal.

In reconstructing the original peat bogs from ratios found to exist between the elevation of the floor and the thickness of the coal, a marked underestimation will result if the fact is ignored that the volume changes have varied with the depth of the bog.

N d f e t 1] a Z

y d a d

A PECULIAR TYPE OF ORE FROM THE TYEE COPPER DEPOSIT OF VANCOUVER ISLAND.

Victor DoLMAGE.

The following is a description of a copper silver ore of an unusual type from the Tyee-Lenora Copper Deposit of Van- couver Island, B. C. A suite of specimens of this deposit was studied by the writer at the Massachusetts Institute of Tech- nology. This collection of about thirty specimens of ore and associated rock was sent to the Institute some years ago by C. S. Hurter, who was employed in the metallurgical department of the Tyee Copper Company.

The deposit, at one time an important copper producer, has been exhausted and is now abandoned. It has been described by Weed in the Engineering and Mining Journal of January 25, 1908, and by Dr. C. H. Clapp in Memoir 13 of the Canadian Geo- logical Survey. The deposit consists of a lense-shaped ore body occupying a synclinal trough in a much altered quartz-talc-seri- cite schist. The ore contains chalcopyrite, zincblende, and galena in a gangue of chiefly barite with some calcite and less quartz. The ore body and surrounding rocks are cut by many large quartz veins.

Among the specimens were two, which consist chiefly of bor- nite and galena, and are quite different from the others which con- sist largely of chalcopyrite. They were labelled “ Bonanza Ore from a 12-inch Vein,” and the analysis given as:

A recent assay of the ore by Professor Bugbee of the Massa- chusetts Institute of Technology gave:

]

39°

R

A Peculiar Type Of Ore. 391

No mention is made of this ore in the literature on that de- posit, and nothing is known of the occurrence beyond what is contained on the label. Mr. Hurter thinks it came from the Lenora mine, and probably near the surface, but he did not visit that mine when this ore was being extracted. The writer visited the locality when assisting Dr. Clapp in his work on the Duncan Quadrangle in 1913, but was unable to see any ore re- sembling it. There is no reason to doubt that the ore occurred as a vein in or near the main ore body.

This ore is dark, massive and dense. It appears to consist -of bornite, galena, and a [ittle chalcopyrite in a gangue of barite, calcite, and quartz. The sulphides are more plentiful than the gangue, and impart to the ore a general dark color. The ore gives evidence of having been crushed and shows an indistinct banding of light and dark gangue. In places the gangue is cut by numerous fine cracks filled with later sulphides.

Under the microscope this ore was seen to contain pyrite, zinc- blende, glena, bornite, chalcopyrite, argentite, gold, chalcocite, covellite, and an unknown yellowish-brown mineral in the bornite. These sulphides replace or occupy fine cracks in the barite, cal- cite, and quartz. The gangue is largely barite and calcite; quartz occurring only as small rounded grains scattered throughout the ore. The replacement of the calcite and quartz by the sulphides is clearly indicated. The sulphide veins form smooth rounded contacts with the gangue, are extremely irregular and vary greatly in width. Small hair-like veinlets of sulphides cut through grains of quartz and calcite or run along contacts and send off many smaller ramifying veinlets which fade out beyond the limit of visibility or expand into thickened rounded nodules which again pinch down to the original size. They may contain any single sulphide or any combination of them. This is shown in figure 23.

The pyrite is rare, only a few rounded grains were seen. These are corroded and replaced by bornite and zincblende.

The zincblende is the most plentiful sulphide. It is of dark color and forms large rounded and irregular grains in the gangue. It is replaced by bornite and galena and contains small particles of chalcopyrite.

‘ n d 1S ) y y

392 Victor Dolmage.

The bornite and galena are abundant and form an important part of the ore. They replace the gangue, forming irregular rounded grains, and small veins, and also occupy a system of thin sharply angular cracks, which they have apparently filled. The two sulphides are always closely associated; their contacts are smooth, rounded, and very irregular, forming in places an inter- fingering pattern. Each mineral also occurs as small irregular grains enclosed by the other and both are found together occu- pying the most minute crack. There seems little doubt but that they were deposited simultaneously.

Fic. 23. Drawing showing replacement of calcite by the sulphides. (G) galena; (A) argentite (black) bornite; (C) calcite. Magnification fifty diameters.

Argentite is an important mineral, being the source of the high content of silver. It is present as small rounded grains in the galena, bornite, and also in the gangue. These grains are .fre- quently cut by small veins of galena or bornite, which proves the argentite to be of earlier origin. It has a grayish color, takes smooth polish, and is blackened by hydrochloric acid. The ore was carefully tested for arsenic and antimony but none was found.

; 6) : %

A Peculiar Type Of Ore. 393

Chalcocite was seen in one of the sections in conspicuous amounts. It occupies the small veins along with the bornite and galena. It is readily distinguished from the galena by a slightly grayer color and absence of triangular pits. It is of blue color, etches with nitric acid and does not appear to be of secondary origin.

Chalcopyrite occurs sparingly as small particles in the bornite and zincblende and covellite were seen as minute veinlets cutting the veins of bornite and galena.

In the bornite of one of the sections were seen small rounded areas of a yellowish-brown mineral. It tarnishes slightly by nitric acid, and answers the description of the unknown mineral found in bornite and described by Murdoch in his “Tables for Microscopic Determination of Opaque Minerals.”

In the five specimens, each of which was polished twice, gold could be detected in only one, and in that with difficulty. Several small particles were found occupying irregular replacement cavi- ties in the calcite, and one small mass associated with zincblende. It appears to be later than the zincblende but this could not be proven. The gold was distinguished from the chalcopyrite by a difference in color and a much brighter luster. It was further identified by yielding a black film on treatment with potassium cyanide.

Because of the difficulty in finding the gold in the sections it was thought that there might be tellurides present and for this reason a quantitative analysis was made. Mr. N. F. Hamilton, of the Massachusetts Institute of Technology, analyzed three samples of ten grams each, and found no tellurium or selenium to be present. The bulk of the gold must, therefore, be present in a very finely divided state.

The relations of the minerals, as seen in the polished section, indicates the paragenesis to be:

1. Quartz and calcite 2. Zincblende 3. Argentite

By Gan at

ty h e es re

394 Victor Dolmage.

Gold Bornite

4.2 Galena Chalcopyrite Chalcocite

5. Covellite

If the scant data available are to be relied upon, it is clear that this peculiar ore came from a vein in the main deposit. That the ore is due to local enrichment from the main ore body seems probable from the composition of both. The average of all the analyses made by the Tyee Copper Company and the analysis of the bonanza ore are given in the following table.

4.56 per cent. 10 per cent. 6.66 per cent. (15 per cent.)1

The silver and gold have evidently been concentrated in far greater Gegree than the base metals.

I. Average of analyses made by Tyee Copper Company. II. Analysis of bonanza ore.

There is little, if any, evidence in the deposit of secondary enrichment due to descending solutions, and no secondary min- erals are found in any amount. The water level is so high at present that enrichment from above would hardly be expected. It, therefore, seems most probable that this peculiar ore is due to local enrichment by solutions from below, which took place soon after the formation of the deposit.

Few, if any, ores of this unusual composition are recorded in the literature of ore deposits.

1The zinc was not given in the analysis but from the large amount of sphalerite seen in the slide, 15 per cent. seems a close estimate.

ar

Discussion

This department has been established by the editors in order to afford to those interested in questions relating to economic geology an opportunity for informal discussion. Contributions are cordially invited either in the form of discussion of more formal papers appearing in earlier numbers or bearing upon matters not previously treated. Letters should be directed to the Editor, Sheffield Scientific School of Yale University, New Haven, Conn. The full name of the author should he attached to all communications.

THE LOCALIZATION OF VALUES OR OCCURRENCE OF SHOOTS IN METALLIFEROUS DEPOSITS.

Sir: One of the simplest theoretical probabilities in the for- mation of shoots in veins seems to have received no clear state- ment in the discussion which has been conducted on ore shoots some years ago. The matter seems of sufficient importance to warrant a separate presentation. This involves the circulation of the solutions, assumed to be uprising from their magmatic source, at depth.

1. The main fissure, once formed, is filled only as material is supplied to it. The feeding channels, often inconspicuous and minute, may be more abundant in one part of the base of the vein than in another.

2. Even if the supply were well distributed along the roots of the vein, the escape of solutions above—assume hot springs or some similar result—would be considerably localized. It is rare to find a long fissure with water issuing all along. In fact, it becomes noteworthy if more than three hot springs are in align- ment.

3. Such being the nature of intake and outlet, the most direct line between them will be, even with a perfectly uniform vein fissure, the direction of the maximum current of solutions. This general idea is, of course, radically modified by irregularities of the width of the vein, and any obstructions. But the effect of ir-

rit at 1e of ry n- at d. ue ce in of

396 Discussion.

regularities on the size of the ore shoot may be even less important, sometimes, than their effects on circulation. Such localized cur- rents were mentioned by Sales! in cases where fault gouge inter- feres with free movement of solutions, but gouge must certainly be a local and rare factor as compared with localized intake and outlet of solutions. One would expect the uniformly filled vein to be an exception, possibly due to complete filling during a single stage of mineralization.

4. With the differences in rates of circulation will come dif- ferences in speed of deposition, in different parts of the vein. If the maximum original circulation represents the best condition for deposition the part of the vein first filled will be the original channel of rapid circulation. If rapid circulation is unfavorable to deposition, the other portions will be first filled.

5. It is generally admitten that the quality of ore-bearing solutions varies during vein filling, and if the earliest solutions carried the best ore, the channel first filled would be the ore shoot, and vice versa.

6. As the channel or region of deposition was filled, circula- tion would be gradually retarded in that region, and confined to open zones. In the last open zone there would be no chance for selective action, except that it would remain open until the con- ditions were right for deposition.

7. The periods and zones must not be thought of as wholly distinct, as above outlined. The channels fill gradually; the shift in maximum circulation may be gradual or even oscillatory; there may be several progressive shifts in the channel.

8. The reactions mineralizing the wall rock, replacements, and related processes, would be affected in a way similar to vein fillings.

g. In practise it is seldom that the intake or outlet involved in a particular vein filling can be located. They were probably variable, the outlet is now eroded, and the roots seldom explored. So the idea may not commonly be of service in exploring for shoots. However, it may well be taken as a confirmatory fact

1 Econ. Vol. 3, p. 329.

Discussion. 397

that many shoots show a general parallelism down the pitch, as if controlled by the direction of circulation.

10. Reversing the logic, it might in some cases be of scien- tific interest to study the position of the shoots to see if it would indicate the locus of intake or outlet of solutions. The trend of shoots reaching down into the deep vein zone, might indicate the probable locus of the magmatic source, or of contact deposits.

11. The idea of variable circulation seems sufficient to account for Irving’s class of shoots of “unknown origin,” but it is not wholly conclusive. Many factors affecting shoots are still unknown.

FRANK F. Grout. SHEFFIELD SCIENTIFIC SCHOOL,

Yate University, New Haven, Conn.

The Origin And Occurrence Of Certain Crystallographic Intergrowths.

Sir: In an investigation of the copper nickel ores of the Insizwa range, East Griqualand, S. Africa, that I have been conducting recently, I have found interesting examples of the different kinds of intergrowths discussed by Mr. M. J. Segall. They are of special interest in that the collateral geological evidence estab- lishes conclusively the genetic conditions under which the veins were formed from which the samples were obtained.

Briefly, the Insizwa range is built of a large basin-shaped mass of gabbro averaging between two and three thousand feet in thickness and injected into argillaceous sediments that have been extensively metamorphosed into hornfels in the neighborhood of the intrusion. The gabbro has undergone magmatic differentia- tion, resulting in the development of basal ultrabasic phases, olivine picrite, and a mineralized zone situated just above the contact. Cutting across the lower hornfels-gabbro contact are numerous small fissures filled almost entirely with sulphides of copper, nickel and iron. The general geological evidence ob- tained by the S. African geological survey points to the gabbro having been a dry melt and special note is made in the survey report that there is no evidence anywhere along the range point- ing to the formation of ore deposits by the action of solutions.

;

r y e n d St

398 Discussion.

The petrographical evidence from the mineralized zone shows that the sulphides and biotite micas were the last minerals in the rock to solidify. The corresponding petrographical evidence of the silicates in the veins shows that these were filled at high temperatures immediately after the freezing of the felspars in the mineralized zone but before the solidification of the micas and sulphides. It appears that the sulphides which exhibit these struc- tures were deposited in the veins by the gradual exclusion of molten sulphides from the overlying mineralized zone.

The Insizwa sulphides are somewhat remarkable in that their order of crystallization in the mineralized picrite is just the re- verse of that found to obtain in the somewhat similar copper nickel occurrences in Sudbury, Norway, and elsewhere.

The order of separation at Insizwa is (1) chalcopyrite, (2) pentlandite, (3) pyrrhotite, whereas the order at the other locali- ties is (1) pyrrhotite, (2) pentlandite, (3) chalcopyrite.

It appeared strange to the writer that there should be this departure at Insizwa from what appears to be the normal order of crystallization of these three minerals. Chemical analysis of the supposed chalcopyrite from these veins showed that it was not a normal chalcopyrite but approached closely to a composition represented by the formula 4Cu,S-3Fe,S,, and that it contained about 1% per cent. of nickel that is not present as pentlandite. The closer resemblance of the iron sulphide constituent of the system to pyrrhotite than is the case for a normal chalcopyrite, is perhaps of more importance than the excess of copper sulphide. The powdered mineral is appreciably attracted to an ordinary hand magnet, a fact that tends to confirm the closer approxima- tion to pyrrhotite of the iron sulphide of the system. The mineral as a whole is of lighter color than chalcopyrite and inclined to have a greenish tint, moreover it is softer. Hitherto it has been regarded as chalcopyrite, but there seems little room to doubt that this abnormal constitution is a contributory cause of the reversed order of crystallization. The sample analyzed was specially chosen as one showing the triangular banded intergrowth similar to that figured in Mr. Segall’s paper (Fig. 1). The banded structure of the Insizwa sulphides is commonly much coarser

Discussion. 399

than the cases cited in the paper and is quite noticeable even in the rough unpolished hand specimens of the mineral complex where the bands may be seen as thin continuous plates giving rise to a sort of sandwich structure throughout the mass. It has not been found possible to determine the precise composition of these interstitial bands, but there are grounds for thinking that the nickel is concentrated in them and that they may be richer in copper than the matrix.

Polishing the banded copper complex shows that the interstitial bands are harder than the general matrix and somewhat lighter in color. The bands polish well with a finely pitted surface but the matrix polishes badly on the whole, for although it tends to take a very high polish it was found practically impossible to obtain a surface free from numbers of deep irregular pits. No difficulty was experienced in obtaining good surfaces from samples of the mineral which did not show the banded structure. The impression conveyed by polishing operations is that the matrix of the banded variety has been strained and its cohesion impaired. Another interesting point noticed in the banded speci- mens was the not infrequent presence of isolated rounded and subangular pellets, about the size of peas, of another copper mineral which has a faint pinkish tinge on polishing and which appears to be a sulphide similar to bornite.

A goodly proportion of the pentlandite and copper mineral occurring in these veins appears as fairly well defined masses embedded in pyrrhotite and sometimes showing crystal bound- aries towards it. Besides occurring in this manner both the pent- landite and copper mineral are apt to occur as thin sinuous plate- like partings in the pyrrhotite while the copper mineral often shows both the triangular banded type of intergrowth and a mutual intergrowth with pyrrhotite of the micropegmatitic type. On etching polished surfaces of mixed sulphides from the veins the etched pyrrhotite areas are apt to grade imperceptibly into totally unaffected areas of undoubted pentlandite and copper mineral, while these poorly individualized minerals frequently surround large pyrrhotite areas as a marginal fringe. This in- definiteness of the boundaries is not suprising in view of the

ws he ae of gh he nd eIr EG= eT 2) his ler of 10t ed the de. iry 1a- ral to en hat sed Ily lar led ser

400 Discussion.

constitution of the copper mineral and the close association of nickel indicated in the analysis of the banded variety.

Some oxidation has occurred in the outcrops of these veins but there is no evidence to even suggest that the vein material at the points whence these samples were obtained had undergone partial re-arrangement or alteration by means of solutions acting subse- quently to the primary deposition. While there can be no doubt that the intergrowths in this instance result from cooling of mixed sulphides from a state of fusion, it is possible that differential liquation in virtue of the different melting points of the various minerals may have complicated matters and that the structures may in consequence not be the result of strictly contemporaneous intergrowth. The order of crystallization of the sulphides in the mineralized zone may thus have exerted an important influence on the arrangement of the vein minerals in respect to one another. It seems not entirely improbable that towards the close of the free- ing period, when the temperatures were not far-removed from the melting points of the mixed sulphides, the most easily fusible mineral, pyrrhotite in this instance, would tend to be squeezed out ahead of the copper and nickel sulphides just as an excess of mercury can be squeezed from amalgam. The last portions excluded from the mineralized zone might thus be enriched in copper and nickel and then be squeezed along any lines of weak- ness in the partially solidified mass in the veins.

Cleavage planes and the boundaries between masses of solidi- fied sulphides would probably afford such lines of weakness for the deposition of such a residuum. The micropegmatitic type of intergrowth is suggestive too of what might result from the squeezing of one non-miscible viscous fluid into another. The marginal fringes can also be readily conceived as the result of a final squeezing of more highly nickeliferous and cupriferous sul- phides into the partially crystallized mass. The interstitial char- acter of these sulphides between masses of pyrrhotite is other- wise not easily explained if the order of crystallization is constant in the veins, but even this is uncertain for a reversal of the order may be brought about by variations in the local composition of the melt. It should be mentioned in this connection that while the

a fe fe t a t

Discussion. 401

sulphides in the overlying mineralized zone are fairly evenly dis- tributed, the veins beneath vary greatly in composition from point to point although the average ratio of copper to nickel appears to be about the same in both types of deposit. Some kind of liquation or diffusion must therefore have taken place to bring about these variations of vein composition. Physical considerations of the development of strains in the contact zone during the cooling of the mass indicate that the opening and filling of these fissures was a slow continuous operation accompanying the cooling process.

Differential liquation may have occurred both in the mineral- ized zone and in the veins themselves and such a process appears to afford a fairly complete explanation of the whole of the struc- tural arrangement of the minerals in respect to one another.

Owing to the absence of adequate experimental data on the liquation of sulphides under high pressure it would be premature to definitely conclude that the intergrowths observed in this in- stance necessarily result in their entirety either from pressure in- jection of one kind of sulphide into another or from strictly con- temporaneous intergrowth.

The fact that Messrs. Graton and Murdock found indications of the triangular banded structure in their simple fusion experi- ments seems to lend color to Ray’s “ purification” theory, par- ticularly in the case of these banded Insizwa copper-iron sul- phides, which contain notable quantities of nickel in addition.

The outstanding feature of the Insizwa intergrowths is that they unquestionably result from cooling of mixed sulphides from a state of fusion. In view of this fact and the analogies with fused silicate intergrowths, the question arises whether the mani- festation of these structures does not afford a strong presumption that similar intergrowths observed elsewhere indicate deposition of ore from a state of fusion rather than as a result of “ mineral- izing solutions.” The term “ mineralizing solution” is so indefi- nite that it is apt to be misleading if no distinction is drawn between molten sulphides or melts in which sulphides predomi- nate over the other constituents, and solutions of sulphides in a non-sulphide solvent. The vague and general ascription of all sorts of phenomena observed in ore occurrences to deposition

lig

402 Discussion.

from mineralizing solutions, or replacements effected thereby, irrespective of the other intrinsic physico-chemical properties of the substances that may be intimately connected with their trans- fer from one point and deposition at another, appears to me to be a dangerous feature of many modern applications of the theory of deposition of ore from solutions.

The subject appears well worthy of closer critical investi- gation for it is by such researches that it would seem possible to obtain a clearer insight into those factors that have influenced the distribution of minerals within ore deposits. This is certainly as important from the economic standpoint, if not more so in most cases, than the formulation of general theories as to how any particular deposit may have originated.

W. H. Tinsspury PavEMENT House, London, E. C.

Sir: My attention has only just been drawn to the paper on the “Origin and Occurrence of Certain Crystallographic Inter- growths,” by Julius Segall, in Vol. X., number 5 of the journal, in the first paragraph of which it is stated that F. B. Laney was the first to recognize crystallographic intergrowths in sulphide minerals, the reference being to the microscopic intergrowths of bornite and chalcocite described in his paper published in 1911 in Vol. VI., No. 4, pages 407 and 408 of the journal. The matter of priority is of little our no importance, but it may be of interest to mention that a similar micropegmatitic intergrowth of bornite and galena apparently resulting from the solidification of a eutec- tic solution was described by me from Mexico in a paper read be- fore the Mineralogical Society of Great Britain and Ireland in June, 1903, and published in the Mineralogical Magazine, Vol. XIII., page 356. The particular interest of the occurrence in question lies in the fact that the intergrowth is not upon a micro- scopic scale but the two minerals can be most distinctly separated by the unaided eye, as in the case of graphic granite itself, without even the use of a low power hand lens. The outlines of both surround large pyrrhotite areas as a marginal fringe. This in- definiteness of the boundaries is not surprising in view of the

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Discussion. 403

alteration of either, more especially taking into account that the two minerals are so dissimilar in chemical composition, the only element common to them both being sulphur.

Henry F. CUEVA DE LA Mora, VALDELAMUSA, HUELvA, SPAIN.

The Scientific Value Of Economic Geology And Double Specialization.

Sir: The death of my friend, Dr. C. A. Davis, inspires me to call attention to the way in which his career illustrates two prin- ciples which are not always sufficiently valued: First, the scien- tific value of economic investigation when thoroughly done, and second, the peculiar value which often comes of being a specialist in two departments; these may be, and in the case of Davis were allied, botany and geology, but might not always be so. I was told one time by Rosenbusch that he and three other professors in German universities were once gathered around a small table in a meeting of a union of German scientists, when they discovered that not one of them took his degree in the subject of which he was then a professor. Rosenbusch had taken his as a classicist, Helmholtz had passed from medicine to physic, Klein was the third and I have forgotten the fourth.

Davis’ investigation into the origin of what was known in the cement factories as “marl” came in this way. The substance which the factories call marl is a nearly pure carbonate of lime. It might be called “limestone dough.” It has also been called “Shell marl’; but a casual observation showed that, while in many cases there were shells, there was a sharp line between them and the mass of the deposit; and there was no such seriate grada- tion of broken fragments as would be likely if it was really com- posed of comminuted shells. This was recognized by I. C. Rus- sell. The next suggestion as to its origin was that it was a direct chemical precipitate. It seemed to me however that the deposit was forming in places where the water was not saturated with calcium carbonate. The marl was at that time supposed to be a valuable source of Portland cement, and the question came up as to how it was formed, as this would give the key to its distri-

404 Discussion.

bution, quality and possible reproduction or growth. I saw the problem but could not answer it. I put it up to Davis, who was not only a sound geologist, but had also a good botanical train- ing. He suggested a solution which, after being subjected by him to a combination of geological, botanical and chemical investiga- tion was absolutely proven, 7. ¢., that plant life, in the shape of blue alge and also especially Charas, were the principal agents. By a combination of geology and botany with chemistry, the identifica- tion of calcium succinate in marl beds and plant juices not only settled the question, but also indicated a mode of forming lime- stones which seems to me responsible for the Pre-Cambrian lime- stones and dominates. If this hypothesis is true, and it is I be- lieve accepted by Walcott, the small economic investigation has lead to results of large geologic import.

In the same way numerous factories were starting the develop- ment of peat and were making more money selling stock than by manufacturing peat. It was important that a review of the sub- ject should be given showing what had been and could be done, in order that there might not be an endless repetition of failures along the same line. This investigation, started in Michigan, broadened until it became Davis’ life-work. Not only was it of economic value but it has radically modified the customary dia- gram showing the mode of formation of peat deposits, has shown the many different types of peat formed, of which sphagnum is not nearly as important as was supposed, and has thrown much light on the origin of coal and the problem of subsidence.

It is not, however, my object to go into the discussion of what Davis had done in the discussion of the problem of subsidence nor in organizing the forestry department at Ann Arbor, but simply to point out that when a man has two specialties it does not matter how far apart they are, the two circles are sure to intersect, and make a field in which he may be the acknowledged master; and second, that any economic investigation thoroughly done may lead to scientific results of wide interest, if given into the hands of a man of keen scientific mind, of broad vision, and of competent training like my friend Charles Albert Davis.

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Reviews

Building Stones of Ohio. By J. A. Bownocker. 160 pp., 11 pl., 11 figs.

Geological Survey of Ohio, Bulletin 18, 1915.

The report contains three chapters, of which the first deals with “ The General Character and Properties of Ohio Building Stones,” the second, with limestones suitable for building stone, treated in the order of their age, and the third, with the sandstones suitable for the same purposes— a rather happy division, inasmuch as a north-and-south line through the center of the state divides it, roughly, into a western limestone half and an eastern sandstone half.

In a period of frequent voluminous reports, appears this one of modest length, sufficiently long to adequately treat the subject under consideration, but not so long as to discourage reading. It is well written and clearly illustrated—in fact is [really] inviting. Its value has been greatly enhanced by the introduction of short historical sketches of public and private buildings in which the stone has withstood the test of time. The report will fill a widespread need for the informa- tion thus brought together for the first time.

CLIFFORD Morse.

Washington University,

Saint Louts.

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Scientific Notes And News'

N. H. Darton, of the U. S. Geological Survey, has returned from Cuba, where he has been investigating prospects for artesian water for irrigation on sugar plantations.

Burton Hart_ey has resigned from the geological staff of the Roxana Petroleum Co. to join the Cosden Oil & Gas Co.

Epwarb L, EstaBrook has returned from geological explora- tion in north China, and has accepted a position as geologist for the Wisconsin Zinc Co. at Platteville, Wis.

THE nineteenth annual session of the American Mining Con- gress will be held at the LaSalle Hotel, Chicago, during the week of November 13.

GILBERT Rice, recently of the New Jersey Zinc Co., has gone to Australia to become associated with the Broken Hill Associated Smelters Proprietary, Ltd. He has been engaged to establish a zinc smelting industry on a large scale and is to furnish an outlet for Australia’s zinc concentrates within the British Empire to take the place of the German market.

O. B. Perry, consulting engineer for the Yukon Gold Com- pany, has returned from Alaska.

W. E. Hopper, of the geological department of the Michigan College of Mines, will spend his vacation in professional work in the Oklahoma oil district.

Geologists, mining engineers and others interested in applied geology are invited to keep the editor informed of new investigations of mining districts or scientific studies undertaken by them, together with such other scientific and personal items as may come to their notice.