Economic Geology and the Bulletin of the Society of Economic Geologists September-October 1925: Vol 20 Iss 6

1925 · Historical

Economic Geology and the Bulletin of the Society of Economic Geologists September-October 1925: Volume 20 , Issue 6. Digitized from IA1518511-02 . Previous…

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Economic Geology

VoL. XX. SEPTEMBER-OCTOBER, 1925. No. 6

Silicification Of Erosion Surfaces. C. K. Leith.

By silicification is here meant broadly the replacement of rocks by quartz, chert, chalcedony and jasperoid, the filling of joints and other openings by these minerals, and also the mechanical concentration of fragmentary masses of them by leaching of associated soluble substances, followed in some cases by siliceous recementation. No sharp distinction will be made between quartz, chert, chalcedony and jasperoid. Most of the so-called cherts, when examined under the microscope with a high power objective, are found to consist of finely crystalline quartz, even though originally they may have been truly amorphous cherts. The adjective cherty, when applied to quartz, usually signifies an extremely fine and irregular texture suggestive of chert and the absence of uniform crystalline texture characteristic of much vein quartz and quartzite. Seldom, however, do such “ cherty ” rocks contain much real chert in a mineralogical sense. The term jasperoid implies more or less iron with the quartz, giving it reddish, brownish or black colors. In nearly all the cases of silicification described in this paper, there is more or less concen- tration of iron oxide with the quartz, indicating some parallelism of behavior of the two minerals in weathering, although their relative concentrations vary widely under different conditions. In certain cases, noted later, the coloring matter in the jasperoid is hydro-carbonaceous.

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514 C. K. Leith.

Pre-Cambrian Unconformities——Pre-Cambrian unconformi- ties in the Lake Superior region exhibit many cases of silicifica- tion of old erosion surfaces in granites, green schists, slates, quartzites and dolomites.

About half a mile northwest of Biwabik and a mile northeast of Eveleth on the Mesabi range of Minnesota, Animikie (Upper Huronian) quartzite rests with gentle dips on the eroded edges of a vertically-dipping series of slates and graywackes of the Knife Lake series (Middle-Lower Huronian). The top surface of the Knife Lake formation carries numerous irregular patches and veins of cherty and jaspery quartz, filling and evening up the irregularities in the old erosion surface and dying out rapidly below, usually within two or three feet, although some of the veins extend down much farther. -Locally these quartz patches are brecciated. The immediately overlying conglomerate, con- stituting the base of the Animikie series, spreads thinly and ir- regularly over this silicified surface and most of its pebbles are clearly derived from the quartz patches and veins in the under- lying surface; every phase of texture and color in the quartz pebbles can be matched below.

The Animikie series rests unconformably on the Giants Range granite on the east Mesabi range near the plant of the Mesabi Iron Company. The basal member of the Animikie is here iron formation. The surface of the granite is irregularly silicified by veins and patches of quartz. When the writer first saw this contact in 1900, he interpreted it as an intrusive contact, because of the extensive silicification, the local absence of coarse con- glomerate at the base of the Animikie, and the extensive recrystal- lization which the sediments had undergone, but now that exten- sive surfaces have been cleared of vegetation, conglomerates have been found and it is apparent that the relations are those of erosion unconformity, as reported by Grout and Broderick.’

Another good illustration of silicification along an unconform- ity may be seen at the contact of the Animikie or Upper Hu- ronian quartzite with Archean (Laurentian) granites on the

1Grout, Frank F., and Broderick, T. M., “ The Magnetite Deposits of the East- ern Mesabi Range, Minnesota,” Bull. 17, Minn. Geol. Survey, 1919, pp. 5-6.

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Silicification Of Erosion Surfaces. 515

Gogebic range of Michigan, and particularly along the contact east of the Newport Mine (Ironwood). Here erosion has stripped off the Animikie series and has exposed the pre-Animikie erosion surface of the granite. This exhibits rounded and hum- mocky forms and semi-detached and completely detached boul- ders, which are obviously the result of weathering of the granite in place. On this surface are patches and veins of cherty and jaspery quartz, filling the openings and depressions. They tend to spread out along the erosion surface, but projecting down- ward are root-like veins, which usually end less than ten feet be- low the surface. In spots the granite is completely permeated by vein quartz which has entered between the grains, leaving the feldspars and a few biotites as fragments in a matrix of quartz. This is either the result of cementation of disintegrated granite of the kind so often seen on granite erosion surfaces or replace- ment mainly of non-feldspathic minerals. The microscopic evi- dence favors the latter interpretation. The hummocks and semi- detached boulders of the old granite surface often project through the quartz masses, giving the appearance of a coarse conglomer- ate with a vein quartz cement. The Animikie conglomerate rests on this surface and contains many rounded pebbles of quartz which can be matched in the vein material below. It ranges in thickness from nothing to several feet.

On the Menominee range of Michigan, at the contact of the iron-bearing series (probably Negaunee) with the underlying Randville dolomite (Lower Huronian), there is silicification of the pre-Negaunee surface, which is much more extensive than in the other cases mentioned. Over an irregular surface of dolo- mite, next to its contact with the overlying series, there is an al- most continuous mass of cherty quartz, drusy in places, light red or dark purple, more or less brecciated and recemented, and in- cluding a few grains of hematite, magnetite and other iron min- erals, as well as a few rounded grains of quartz. The mass not only mantles the old surface but extends down into old crevices; its thickness is therefore highly irregular, from a few feet to perhaps a hundred feet. It is not an upper stratigraphic horizon of the dolomite itself, because here and there it lies diagonally

516 C. K. Leith.

across the layers of dolomite. The best exposures may be seen in the vicinity of Norway, and at Iron Hill about two miles to the north. The siliceous capping on the dolomite is interpreted as the product of weathering, which has leached the dolomite and thereby concentrated syngenetic bands of chert as fragments on the surface, later to be recemented by secondary chert, and which has at the same time caused silicification of immediately underlying portions of the dolomite. In the folding which fol- lowed the deposition of the Upper Huronian series, part of the siliceous capping close to the contact was sheared, the result being a siliceous talc schist. In Monograph 52, of the U. S. Geological Survey, this cherty upper surface of the dolomite was described as representing a separate series called Middle Huronian. The irregularity of its under surface was taken to indicate an erosion unconformity. Re-study of these exposures has brought us to the present interpretation.

The upper surface of the Kona dolomite in the Marquette dis- trict (correlated with the Randville dolomite) is also highly siliceous, consisting of brecciated chert cemented by later chert, features which are probably to be explained by silicification of the upper surface of the dolomite, combined with slump and brecciation of original chert layers due to leaching out of dolo- mite from between them.

Illustrations of the silicification of the erosion surface along pre-Cambrian unconformities of the Lake Superior region could be multiplied, but the ones given will serve to indicate the nature of the phenomenon. It is clear that the silicification took place mainly prior to the deposition of overlying sediments, because in all cases the chert is conspicuously represented in all its phases in pebbles in the basal conglomerate. The manner in which the chert follows the details of the old erosion surface, filling joints and other irregularities, crossing the secondary structures and bedding of the underlying rocks, shows clearly that it was intro- duced after the underlying rocks were deformed, and during or after their erosion. The fact that it appears in so many different kinds of rocks would seem to indicate that it was related to a widespread condition like weathering rather than to local and

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Silicification Of Erosion Surfaces. 517

special conditions dependent upon the type of rock, but, as would be expected, it is much more extensive and thicker in the rocks most susceptible to siliceous alteration (the limestones and dolomites) and least conspicuous, though present, in quartzitic rocks. It varies in amount, also in the same rock, or in different regions, or in different unconformities, and is completely absent over some areas.

It is an interesting fact that the bases of the pre-Cambrian series of the Lake Superior region (and, in fact, the bases of sedimentary series of many ages and districts) are very com- monly marked by a few feet of fine-grained shaly sediments, even though the overlying rocks be coarsely fragmental. Where these rocks have undergone folding, shearing has naturally been con- centrated along the contact and particularly in the zone of soft sediments immediately over the conglomerate, often producing talcose and micaceous quartz schists. Both the soft shaly layers and their sheared equivalents yield to erosion faster than the adjacent rocks and are represented by depressions on the erosion surface, making it difficult to find contacts in natural exposures. In such cases it is rare to find the contact at the base of the escarpment of overlying beds. If exposed at all, it is likely to be on the stripped surface of the underlying rocks, where patches of conglomerate, more or less sheared, adhere to the old surface. In the search for these patches, if any marked silicification of the surface is noted, it may be regarded as indicating an approach to the contact. Where the shearing has been intense, the con- glomerate may not be easily distinguished from the brecciated and cherty surface of the rocks below, but so common is this association that field workers familiar with its occurrence usually recognize the conditions. While detailed study usually discloses the actual contact, there are places where the weathered debris on the old surface has become cemented in its original place by fragmental sediments of the new series, and thus is to be inter- preted as constituting both the top of the old rocks and the basal conglomerate of the new.

If we are right in believing that the silicification along the above described pre-Cambrian unconformities is the result of

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518 C. K. Leith.

weathering on old erosion surfaces, the same results should be noted on later unconformities and on the present erosion sur- face. A few cases may be cited to show that silicification of erosion surfaces is by no means confined to the pre-Cambrian.

Erosion Surfaces in Paleozoic Rocks of the Mississippi Val- ley.—The Paleozoic limestones of the Mississippi Valley exhibit widespread, though not universal, silicification of their erosion surfaces. Ulrich states that within his experience “ most cherts in Paleozoic rocks are subaerial weathering products and mainly surficial, extending but short distances into the beds of limestone however abundantly their exposed surfaces and edges may be studded with chert... .I have observed many hundreds of cases indicating surface silicification.” ®? He says also that some of the occurrences of chert in deep wells have been proved to lie in zones of known unconformities, and that he has observed many cases of silicification of either the under or upper sides or both sides of unconformable contacts. Thwaites,* who has studied in much detail the Paleozoic rocks of the Upper Mississippi Valley, particularly in Wisconsin, confirms this statement, and add that well drillings show distinct diminution in the amount of chert deep below the erosion surface. Of course, surface silicification locally extends down along fissures, caverns and water courses deep within the limestone formations.

A phase of surface silicification of the Paleozoic limestones is the concentration of chert fragments on the erosion surfaces by solution of the limestone, in places where erosion does not sweep them off as fast as they are formed. This is to be observed in the driftless areas of southwestern Wisconsin. The chert frag- ments in such a case may be derived from both syngenetic chert in the limestone, and secondary chert formed by replacement near the erosion surface, brought into a zone of solution (usually above water table) by the downward migration of the erosion surface. Twenhofel* reports the accumulation of masses of

2 Ulrich, E. O., Personal communication. See also: Bain, H. Foster, and Ulrich, E. O., “ Copper Deposits of Missouri,” Bull. 267, U. S. Geol. Survey, 1905, p. 29.

8 Thwaites, F. T., Personal communication, 4 Personal communication.

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Silicification Of Erosion Surfaces. 519

chert fragments of this kind over the Pennsylvanian limestones in Kansas, in a belt extending northeast and southwest across the state in the general region of the Flint Hills. Locally it is in such abundance that it forms an important source for road ma- terial, thicknesses of fifteen feet or more being known. Such present accumulations of chert at the surface suggest a source for the abundant chert fragments seen in basal conglomerates marking unconformities. They may later serve as the frag- mental base or “ basal breccia” of transgressing sediments, and therefore show confusing gradations to true basal conglomerates, as in cases cited below.

One of the most interesting cases of concentration of chert on erosion surfaces is in the tri-state lead and zinc district of Mis- souri, Oklahoma and Kansas, where the cherts have a close genetic association with the lead and zinc ores. The Boone lime- stone (Mississippian), the ore-carrying formation, carries ex- tensive chert breccias on the present erosion surface, at the un- conformable contact between the limestone and the overlying Chester or Cartersville formation, at the unconformable contact between the Boone and the Cherokee shales of Pennsylvanian age (where Chester has been removed), and in the Grand Falls chert bed, a more or less continuous horizon within the Boone lime- stone, which is interpreted by Ulrich® as marking an uncon- formity. Perhaps there should be mentioned in addition the chert gravels (Lafayette) developed on the pre-Tertiary erosion surface, though these have undergone more local transportation and rounding. On the surface of unconformity original chert bands in limestone may be seen in all stages of breaking up, through solution of the limestone and slump of the chert bands, resulting in the accumulation of considerable masses of brec- ciated chert on the irregular surfaces of limestone, particularly in depressions. Uusally these fragments are angular, but occa- sional well-rounded boulders are to be seen. Weathering of the chert fragments has locally softened and whitened them, pro- ducing “cotton cherts,” “chalky cherts,” “dead cherts,” or

‘5 Ulrich, E. O., “ Revision of the Paleozoic Systems,” Bull. Geol. Soc. Am., vol. 22, 1911, pp. 281-680. See especially Plate 29.

520 C. K. Leith.

“ tripoli.” The chert breccias have been more or less recemented by a variety of minerals, the most common being a dark colored chert, locally called jasperoid, owing its color to its content of carbonaceous material and containing the ore minerals. Other cements are shale, sandstone and limestone in the “ basal brec- cias ” of the Cherokee and Chester formations, and clay and sand on the present erosion surface and in the Lafayette gravels. Geologists who have studied the origin of the lead and zinc deposits have been pretty well agreed that the chert breccias on the present erosion surface, and along the unconformable con- tact at the base of the Cherokee shale and Chester sandstone are residual deposits on old erosion surface, caused by solution and slump. Curiously enough, however, they have usually inter- preted the continuous Grand Falls chert layer as a friction brec- cia, due to slight movements parallel to the bedding. That the Grand Falls chert is not a friction breccia seems to be shown by the fact that the Grand Falls chert bed is continuous over practi- cally the entire area of the Joplin and Miami districts, that it- is irregular in thickness (15 to 60 feet), being thickest in depres- sions on the underlying surface, that it rests on a “ karst” topo- graphy of the underlying limestone, that the underlying lime- stone itself is irregular in thickness, that in the Miami field it is overlain by more or less solid limestone and not generally con- nected with the surface, that complete gradation can be observed to original beds of chert broken by solution of the limestone, that some of its fragments show whitening characteristic of weathering, that limestone is almost completely absent, and finally, that it locally contains well-rounded pebbles and boulders. In short, the Grand Falls chert seems to show all the evidences of surficial weathering origin that are shown by the “ basal” cherts of the Cherokee shale, and like them, to mark a “ solution unconformity.” Brecciation caused by movement along faults and joints is locally to be seen, both as a later disturbance of the main chert bed and as a phase of its development, but this does not seem to be at all adequate to account for the main mass of the cherts,—just as it fails to account for all of the chert breccias

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at the base of the Cherokee formation or on the present erosion surface.

The successive erosion surfaces noted for the tri-state district are close together, and later ones truncate earlier ones, resulting in an actual physical connection between the chert breccias of different ages. For instance, the cherts at the base of the Chero- kee shale, at the base of the Chester sandstone, and the Grand Falls cherts are continuous, one with the other, in places in the Miami camp, as at the Barr Mine. In the Joplin district the present erosion surface is locally superposed directly upon that at the base of the Cherokee shale and on the Grand Falls cherts, causing local difficulty in assigning some of the chert breccias to one or another erosion period.

The above facts have a bearing on the origin of the lead and zinc ores. This problem is outside of the scope of the present discussion, but attention may be called to one of its aspects. The chert breccias, which have furnished the favorable locus for the concentration of the ore, mark several successive unconformities, but the jasperoid cement of these breccias, which came in with the ore-bearing solutions, may have been and usually has been assumed to be, of one period. The fact that the accumulation of chert breccias on erosion surfaces is so often accompanied by silicification of the adjacent limestone, and by the cementation of the chert fragments by secondary silica, naturally raises the question whether there may have been more than one period of introduction of the jasperoid cement carrying the ore in this dis- trict, and particularly whether the jasperoid cement in the Grand Falls chert, so widespread in its occurrence and so generally covered as it is by Boone limestone, was introduced at the same time as the jasperoid of the breccias on the later erosion surfaces. In the Miami district, where the Grand Falls chert and its con- tained ore are covered by Boone limestone, and this in turn by the unconformable and impervious Cherokee shale, it would seem that the pre-Cherokee erosion working down on the bituminous Chester sandstones and the top of the Boone limestone, may have accomplished ore concentration quite as well as any post-Chero- kee erosion.

522 C. K. Leith.

Weathering of Glacial Tills—The weathering of glacial tills is marked by the concentration of siliceous and clayey minerals on the surface, caused by the leaching out of the more soluble minerals, particularly the carbonates. Where the deposits of successive ice invasions have been superposed, these weathered, silicified zones become valuable criteria for the separation of suc- cessive till deposits.®

Siliceous Capping of Sulphide Deposits—Silicification of erosion surfaces is exhibited in the oxide zones of siliceous sul- phide deposits, where, by solution of sulphides and other soluble materials, quartz and iron oxides remain as protective cappings at or near the erosion surface. ;

Case-hardening.—Case-hardening by quartz and iron oxide is a well-known phenomenon. In some places this is due to direct deposition of these substances at the surface; in others what has been called case-hardening is probably due to the arrest of the erosion surface on some part of the underlying materials which has been previously silicified and hardened.

Siliceous Deposits in Iron Ores.—The weathering of the later- itic iron ores of Cuba shows leaching of silica from the soft ores above, and a tendency for concentration near the contact of the soft ores and the apparent serpentine rock. This has been checked by analysis of a series of samples covering the alteration of the serpentine to ore.‘

A more or less parallel case is observed in the Iron River dis- trict of Michigan and elsewhere in, the Lake Superior region, where the iron ores have been largely concentrated by the leach- ing of silica. Locally this is deposited near the contact of the residual soft ore with the wall rock. If erosion had swept off the soft ores as they formed, a distinct concentration of quartz would appear on the remaining surface. Abundant vein quartz is regarded as a good indication in drilling for ore.

General Discussion—Many more cases of silicification of ero- sion surfaces could be cited, but it is enough for our purpose to

6 Kay, George F., and Pearce, J. Newton, “ The Origin of Gumbotil,” Jour. Geol., vol. 28, 1920, pp. 89-125.

7 Leith, C. K., and Mead, W. J., “ Origin of the Iron Ores of Central and Northeastern Cuba,” Trans. Am. Inst. Min. Engrs., vol. 42, 1912, pp. 90-102.

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Silicification Of Erosion Surfaces. 523

show that the process is a common one. It does not follow that it is universal, for there are many surfaces in which no evidence of silicification can be found. The exact reasons for this dif- ference are not clear; though climate and erosional conditions doubtless are factors. Limestones show silicification much more commonly than other formations, due to their frequent high con- tent of original chert, which by solution of the limestone becomes concentrated mechanically and chemically at the surface, but silicification is not confined to limestones, as shown by the silici- fied upper surfaces of granite, schist, and quartzite in the Lake Superior region and by silicified cappings of many ore veins.

The chemistry involved in the silicification of erosion surfaces is a long story in itself which requires consideration of practically all the processes and conditions of weathering. It is enough for our present purpose merely to call attention to the fact that quantitative study of rock alterations, ground and surface water analyses, determinations of colloids in soils, and laboratory ex- periments have now firmly established the mobility of silica in large amounts, notwithstanding the geologic tradition of the insolubility of silica in rocks.

Looked at broadly, the silicification of erosion surfaces may be regarded as a defensive reaction against the agents of weather- ing. While silica may be removed both mechanically and in solution, it nevertheless in many places holds back as compared with other rock constituents. There are erosion surfaces, both new and old, which do not show silicification, raising an interest- ing question as to just what environmental conditions determine its presence or absence. The answer to this question requires a much wider range of pertinent facts than have been brought to- gether in this paper.

The silicified erosion surfaces have proved helpful as diag- nostic criteria for unconformities and in the interpretation of certain phases of ore concentration, but there seem to be much wider possibilities for their use, and particularly for the light they may ultimately throw on the interpretation of environ- mental conditions during the erosion represented.

Univ. oF WISCONSIN, Mapison, WIs.

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Relation Of Earth Temperatures To Buried Hills And Anticlinal Folds."

W. T. Thom, Jr2

It seems reasonable to believe that structural irregularities of the earth’s crust are reflected by corresponding variations in the distribution of heat in the lithosphere, and that under certain con- ditions the discovery of oil and gas pools may be facilitated by measurements of temperatures in deep wells. Darton’s* work suggests that thermal gradients above hills buried beneath an un- conformity are lower than above adjacent erosional or structural depressions, and Van Orstrand* has found abnormally steep thermal gradient in wells drilled near the crests of the Salt Creek and other structural uplifts in Wyoming.

A thermal gradient contour map of the Salt Creek field, pre- pared by the writer from Van Orstrand’s data, was exhibited at the International Petroleum Exposition at Tulsa, Okla., in Sep- tember, 1924, and has been combined with an ordinary structure contour map of the Salt Creek dome to form Fig. 1. This figure shows that thermal gradients in the several wells studied by Van Orstrand are steepest in those wells near the crest of the fold, and are progressively less steep as the flanks of the fold are ap- proached, the difference in gradients between two wells being roughly proportional to the difference in structural elevation be- tween them. Therefore, in this field it is possible to draw a fairly accurate structure map of the fold on the basis of well- temperature measurements alone, without reference to other lines of evidence, and scattering temperature measurements made by

1 Published by permission of the Director, U. S. Geological Survey. Presented before the Society of Economic Geologists, New York meeting, May, 1925.

2 Geologist in Charge, Geology of Fuels.

3 Darton, N. H., “ Geothermal Data of the United States,” U. S. Geol. Survey Bull. 701, 1920.

4Van Orstrand, C. E. Manuscript in Preparation.

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EARTH TEMPERATURES AND BURIED HILLS. 525 Van Orstrand on the Lost Soldier, Ferris, and Warm Spring domes suggests that in those fields also there is a direct relation-

ship between the shape of the uplift and the local distribution of heat in the earth’s outer crust.

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Fic. 1. Salt Creek Dome, Wyoming. Dashed contours show eleva- tion of top of Second Wall Creek sand above sea level; solid lines are contours of equal temperature gradient; dots and heavy figures show

locations of wells, and temperature gradients (showing rate of increase in feet per 1° F.).

The abnormally high temperatures found beneath the Salt Creek, Warm Springs, Ferris, and Lost Soldier domes might conceivably be due to local heating by laccolithic intrusions; to heating by fluids migrating through porous beds underlying the

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folds ; to heat developed by friction during the folding which pro- duced the domes; or to the fact that the truncation of the folds by erosion has exposed relatively hot strata near their tops. The last explanation seems to be the correct one, in view of the rela- tionships shown by Fig. 2, which presents in cross-section the relative position of surfaces of equal temperature in the Salt Creek field, both with reference to the surface of the earth and to the “‘ Second Wall Creek sand.” This cross-section and other similar ones seem to indicate that the present distribution of heat in the rocks at Salt Creek is most easily explainable on the basis of in- complete cooling of the uplifted strata.

The structural character and relationships of folds such as the Salt Creek dome are regarded by the writer ° as indicating that the tangential compression which produced them was not applied directly to their surface rocks, but so far as they were concerned resulted in vertical elevation and tension, under thrust transmitted in very deep-seated rocks, rather than in folding with attendant friction between folded beds, under direct lateral thrust trans- mitted by strata near the surface. With such a mode of origin, frictional heating would be slight, except, perhaps, along fault planes, where indeed Van Orstrand’s measurements suggest that minor heating effects exist. The general insignificance of fric- tional heating is believed to be illustrated by the relation of the isogeothermal surfaces and axial plane of the Salt Creek fold, as shown by Fig. 2 for it seems improbable that the 120° tempera- ture surface, for example, would lie so far below the steep limb of the “ Second Wall Creek sand,” were friction the chief cause of the local high temperatures.

The abnormal temperatures of the Salt Creek dome are also not due in an important degree to artesian water movement, or to the migration of other fluids through porous beds beneath the oil-bearing sands of the field, for the syncline separating the Salt Creek dome from the zone of artesian intake along the Big Horn Mountains is so shallow that any important and long-con-

5 Thom, W. T., Jr., “ The Relation of Deep-seated Faults to the Surface Struc-

tural Features of Central Montana,” Am. Assoc. Petroleum Geologists Bull., vol. 7, No. I, pp. 1-13, 1923.

528 W. T. Thom, Jr.

tinued passage of artesian water beneath the dome would soon tend to chill rather than heat the overlying strata.

The crescentic line of steep dips bounding the Salt Creek dome on the southwest, west, and northwest suggests to the writer the presence beneath this belt of a rift or tear in the deep-seated rocks corresponding to one margin of a deep-seated intrusion beneath the dome. However, the observed earth temperatures are be- lieved to be quite independent of heat radiating from such an in- trusion, for, even with the steepest thermal gradient observed in the Salt Creek field, the temperature of molten rock would only be attained at a depth of several miles, or considerably below the

center of curvature of the dome, and were the observed heat ef-_

fects produced by radiation from so deep a source there would be a much greater uniformity of temperature gradients within the field than exists, and a much less evident relation between steep- ness of gradient, and structural position of the point of observa- tion.

The writer therefore concludes that in the Salt Creek, and prob- ably in the Lost Soldier, Ferris, and Warm Springs domes (all formed during the late Cretaceous and Eocene period of orogenic activity, and soon after truncated by erosion), the temperatures of the rocks near the surface are roughly proportional to the amount of their original overburden and therefore are hottest now where the greatest cover was removed. Subsequent cooling has caused a gradual lowering and flattening of the isogeothermal surfaces, but has not yet lowered them to a depth beneath the surface such as they occupied before the folding took place.

In the light of temperature relationships found at Salt Creek and in the eastern Dakota artesian basin, and quite apart from any particular explanation of the causes of these relationships, it seems possible that measurements of earth temperatures may fa- cilitate the delimitation of concealed uplifts or of buried hills, and hence in places greatly reduce the amount of exploratory drilling required to discover oil and gas pools associated with such features.

In an area containing relatively large and high anticlines or

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Earth Temperatures And Buried Hills. 529

domes, a single well showing an abnormally steep temperature gradient would suggest the existence of an uplift nearby; two wells showing such gradients would give a possible clue as to their relative position upon such an uplift; and temperature measure- ments in three wells would give a suggestive guide as to the gen- eral direction in which the apex or crest of such a concealed uplift would lie. Areas covered by shale or by blankets of young de- posits, such as exist in the Lost Soldier district; and areas of thick, poorly bedded and poorly consolidated Tertiary rocks, would presumably be places where the measurement of tempera- tures in wells would aid most effectively in the search for new oil fields beneath hidden folds. Much of the folding in the Cal- ifornia oil fields occurred in Pliocene, or even in Pleistocene time, and because of its newness should be more clearly shown by earth temperatures than the older folding which produced the Salt Creek dome. Hence it seems reasonable to suppose that uplifts of relatively slight structural relief in late Tertiary rocks may be detected and outlined, with a minimum of effort, by the systematic study of earth temperatures in exploratory wells. Theoretically, it might also be possible to make some estimate of the position of a well with reference to the flank of an intrusive salt core, in salt dome exploration, as soon as enough temperature observa- tions have been made to afford a basis for comparison.

In contrast with hidden anticlines, buried hills such as control oil production in south-central Oklahoma, may be marked by abnormally low rather than by abnormally high thermal gradients, on the basis of the relations pointed out by Darton,’ although the thermal contrasts in Oklahoma would presumably be much less sharp, owing to the more prolonged burial of the concealed hills.

If the temperature relations observed by Darton in eastern North and South Dakota hold good over the remainder of the Great Plains States, the systematic measurement of temperatures in existing deep borings may yield enough evidence to justify wildcat testing of the oil possibilities of pre-Dakota rocks in Plains areas where such rocks are not now seriously considered as possible sources of commercial oil or gas production because

6 Prev. cit.

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530 W.T. Thom, Jr.

under present methods exploration could proceed only by random drilling without geologic guidance.

Summary.

Theoretical considerations, and such observations as are avail- able, suggest that there is a systematic relationship between earth temperatures and structural uplifts, and that further observations of temperatures encountered in deep wells may afford a relatively cheap and effective method of finding oil fields otherwise discov- erable only by the unsatisfactory and costly process of random drilling. In so far as other deposits of economic value are di- rectly related to hidden magmatic intrusions, to structural uplifts, or to buried topographic features, it would also seem possible that temperatures measured in drill holes might give valuable clues to the location of such deposits.

U. S. GEoLocicaL SURVEY, D. C.

( fet

Quantitative Standards For Hardness Of The Ore Minerals?

Sterling B. Talmage.

PREFATORY NOTE. L. C. Graton.

Since, and even before, the publication in 1916 of Murdoch’s “ Micro- scopical Determination of the Opaque Minerals ” it has been evident that this important branch of geological inquiry suffers through lack of quan- titative standards analogous to those available in thin-section petrography. Numerous efforts have been made to improve the situation; in the Har- vard laboratory this has been a constant objective.

Among the properties of minerals which to that end have been receiv- ing attention, hardness seemed to be eminently constant and suitable for diagnostic use, provided only it could be reduced to reliable measurement. Murdoch endeavored to standardize hardness for his determinative tables, using a scratch-sclerometer generously built to our design by the Geo- physical Laboratory of Washington. It was hoped thereby to secure such precise hardness values as greatly to simplify mineral identification; but to our surprise the results proved so variable and unreliable as to dis- courage further effort at that time.

Last fall, Mr. Talmage resurrected the old instrument and discovered that results varied not because of hardness variability in the minerals but because of easy dulling of the scratching point. By substitution of a diamond blade for the steel needle formerly used, coupled with accurate calibration of intensity of the scratch, gratifying’ constancy has been secured and the initial hopes for the method evidently realized. In addi- tion to the extensive tests and checks applied by Mr. Talmage, this means of measurement has been used during several months by other workers in this laboratory and has been found to be essentially practical in appli- cation and independent of the personal equation.

It is hoped, therefore, that this method and the quantitative constants it provides may take their place as a fundamental step in. opaque-mineral identification not unlike the réle of refractive index for transparent min- erals. The simplicity of the scheme, however, must not obscure the necessity of application under rigidly standardized conditions.

Introduction.

OF the physical characteristics used as an aid in the identification

of minerals, one of the most evident is hardness. Early in the 1 Progress Report, Emmons Memorial Fellow, 1924-25

532 Sterling B. Talmage.

modern science of mineralogy, there was proposed by Mohs the scale that has continued in use for over a century. The Mohs scale gives a convenient and satisfactory criterion for rough de- terminations, especially in field tests, but it has long been recog- nized that this scale is at best but semi-quantitative, and various workers have endeavored to established a more definite numeri- cal relationship than is furnished by the arbitrary numbers of the Mohs scale. VARIATION IN RESULTS.

The results of tests on the minerals adopted as standard for the Mohs scale, as tabulated by Jaggar, Iddings,* and Dana,‘ are far from being concordant; much of the apparent disagreement, however, is probably due to the different methods of testing used. It would appear that workers in this field are not agreed as to just what hardness really is. It is variously defined as “that quality of a mineral that resists scratching;”° “ the re- sistance which a smooth surface offers to abrasion; ”’ ® “the re- sistance offered by the cohesion of the molecules to their being torn apart by abrasion;”’* and as the resistance which a sub- stance opposes to permanent deformation by abrasion, penetra- tion, friction or fracture.* Methods of testing hardness are even more varied than the wording of definitions. Scratching, boring, grinding, pressure, impact and other means have been used in the endeavor to measure the hardness of minerals and metals. Jaggar, who made his tests by boring with a diamond point, very properly objected to some methods of testing hard- ness “on the ground of the interference of tenacity and plastic- ity.” With such diversity of testing methods, concordance of results is scarcely to be expected; or, otherwise stated, scratch-

2 See list tabulated by Jaggar, “ A Microsclerometer for Determining the Hard- ness of Minerals,” Am. Jour. Sci., 4th Ser., Dec. 1897, p. 399.

8 Iddings, “‘ Rock Minerals,” 1911, p. 84.

4 Dana, “ Textbook of Mineralogy,” 1922, p. 193.

5 Standard Dictionary.

6 Dana, op. cit., p. 191.

7 Iddings, op. cit., p. 84.

8 Jaggar, op. cit., p. 400.

As

Standards For Hardness Of Ore Minerals. 533

hardness, bore-hardness, wear-hardness, pressure-hardness and impact-hardness may have distinctly different values on the same material.

The hardness of the ore minerals is important chiefly to assist in the identification of minerals. Tests of the hardness of min- erals in polished sections must be made quickly, and the methods used must be applicable to small areas. Consequently grinding and boring methods, involving precise and time-consuming meas- urements, are not satisfactory; neither are pressure or impact methods, which, up to the present time, have been applied only to areas larger than many mineral grains seen under the micro- scope. Wear-hardness observations are useful to a limited ex- tent; when a section is polished, hardness differences sometimes develop relief between the softer minerals and the upstanding harder constituents. As polishing methods are improved, this relief decreases. Even though it shows at a glance which of two adjacent minerals is the harder, it gives no measurable result. Scratch-hardness remains as the only promising means by which quantitative improvements in hardness measurements can be achieved and utilized in this branch of study; and, so far as known, scratch-hardness is as distinctive and significant as is the hardness measured in any other way.

In the more extensive tables for the determination of minerals in polished sections published to date ° there are recognized three degrees of hardness, as determined by relief and by scratching with a steel needle. Murdoch recommended a needle-holder five inches long, weighing a quarter of an ounce. The three recog- nized degrees of hardness are defined as: (1) Soft, or low hard- ness, including minerals that can be scratched with the weight of the handle alone, or very easily with slight pressure; (2) medium hardness, including minerals that can be scratched only faintly with slight pressure, but easily with moderate or heavy pressure; and (3) hard, or high hardness, including minerals that can be scratched slightly or not at all with heavy pressure on the needle.

. 8 Murdoch, Joseph, “ Microscopical Determination of the Opaque Minerals,” New York (Wiley) 1916; Davy, W. M. and Farnkam, C. M., “ Microscopic Ex- amination of the Ore Minerals,” New York (McGraw-Hill), 1920.

534 Sterling B. Talmage.

Since the opaque minerals included in the tables range in hard- ness according to Mohs scale from 1-1.5 for molybdenite up to 6-7 for cassiterite, it is evident that the divisions proposed by Murdoch are only half as many as are provided over the same range by the admittedly inadequate Mohs scale. Murdoch says:*° “It has not been considered advisable in this work to carry this determination too far, lest confusion arise from the overlapping of hardness ranges of various minerals.” Many de- terminations of hardness of minerals in polished sections do not check with previously published Mohs scale figures; in the fol- lowing examples of such disparity, the designations by Murdoch are compared with the Mohs scale values in parentheses: covel- lite (1.5-2) is described by Murdoch as equal in hardness to bornite (3) and harder than chalcocite (2.5-3); molybdenite (1-1.5) harder than chalcocite (2.5—3) ; pyrrhotite (3.5-4.5) is placed by Murdoch in the high-hardness class, while breithauptite (5.5) goes in the medium-hardness group; native copper (2.5- 3) is called medium, while stannite (4) and horsfordite (4-5) are placed in the low-hardness group. There are other discrep- ancies of this same type.** Generally, but not invariably, the recent quantitative tests check better with Murdoch’s ratings than with the Mohs scale figures. Perhaps too close a check should not be expected, since tests according to the two scales are not strictly comparable. Mohs scale determinations are stand- ardized on the natural faces of crystals, while hardness tests in polished sections are made on planes of random orientation that have been given artificially a high degree of polish. Tests of scratch hardness under the microscope cannot be made satisfac- torily on the standard minerals of the Mohs scale as a basis for better correlation on account of the great difference in luster.

The three grades of hardness proposed by Murdoch have not proved entirely satisfying in use, largely on account of unstand- ardized measuring methods. Different workers use needles of

10 Murdoch, op. cit., p. 29.

11 For purposes of comparison, Murdoch’s ratings and Mohs scale ratings are listed in Table III. at the end of this article, along with the new ratings based on the writer’s recent measurements.

é (

Standards For Hardness Of Ore Minerals. 535

different types; some prefer a stiff straight needle; in the Har- vard laboratory we use a fine needle bent at an angle near the point.’ Some push the needle against the mineral, plowing up a furrow, while others draw the needle away with its point in- clined to the rear, impressing a smooth groove. Some will use only a sharp needle, discarding it as soon as it shows signs of wear, while others continue to use a needle long after its point has become considerably blunted. In view of these facts, it is scarcely surprising that hardness determinations by such variety of methods are neither concordant nor quantitatively satisfying.

Instrument For Quantitative Measurements.

While the three grades of hardness proposed by Murdoch have seemed more applicable to polished section work than the Mohs

scale figures, they are subject to two disadvantages. They are.

based on judgment rather than on measurement, and they are too broad, each group including too many minerals for really deter- minative work. In the attempt to overcome these difficulties, there has been constructed an instrument for scratching minerals under the microscope, based on the principle of a measured weigh

several years ago at the Geophysical Laboratory, according to a design proposed by Prof. L. C. Graton and workers in the Har- vard Laboratory of Economic Geology. The foundation of the instrument is a broad brass base-plate, provided with a clamp by which the microscope may be firmly fixed in place. From the end of this base-plate rises the supporting pillar, consisting of telescoped tubes of brass. The inner tube can be clamped firmly at such a height that the graduated beam is horizontal when the point rests on the polished surface of the mineral. Into the inner tube fits a rod, carrying at its top the supports for the graduated beam. This rod, which rotates within the inner

12 This bending can be done quite easily by grasping the extreme point—z2 or 3 mm. only—in a pair of pliers, heating the needle to redness in a blowpipe flame just beyond the end of the pliers, and bending to the desired angle. If heavy nosed pliers are used, the heat is conducted away so rapidly that the temper of the needle point is not affected.

sep 15 1905:

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Standards For Hardness Of Ore Minerals. 537

tube, is provided with a lug and appropriate stops so that the graduated beam may be swung horizontally through a quadrant; at one extreme, the beam is stopped when the scratching point is directly under the microscope objective, and at the other end of its swing the beam points away from the operator and is com-

Fic. 2. Detailed view of scratching mechanism.

pletely clear of the microscope stage. The graduated beam, like the beam of a balance, is pivoted on a cross-piece that fits into the supporting bearings. It is provided with a sliding weight, and with a counterweight on the opposite end (See Fig. 2) ad- justable so that the beam is just balanced when the sliding weight is set at zero on the scale. A thumbscrew, set in the supporting frame, bears on the graduated beam just to the counterweight side of the pivot, and permits the point to be let down on the min- eral, or held stationary just above it. The point used to do the scratching is a small diamond, ground at one end to a semicircular edge or blade with an included angle of 45°, and set in a small metal shaft. The microscope is provided with a mechanical stage so that the mineral may be drawn away under the point,

538 Sterling B. Talmage.

and with a revolving stage so that the same area on the mineral may be tested in different directions.

Use of Instrument—In using this instrument, the polished section is first examined under the microscope with the graduated beam clear of the stage. The area to be tested for hardness is brought near the right-hand edge of the field of view. The beam is swung around so that the point is under the microscope, but held above the mineral by the thumbscrew. The height adjust- ment is made, if necessary,"* by the pillar clamp, so that the beam is horizontal when the point rests on the mineral surface. When the beam is in position for scratching, the point is just visible in the right hand edge of the microscopic field. The particular place on the surface to be tested is brought exactly opposite the point, and then moved back under the raised point until it is just out- side the visible field. The sliding weight is set at any desired position, and the diamond point lowered on to the mineral sur- face. By means of the mechanical stage, the mineral under test is drawn back into the microscopic field, and as it comes into view from under the diamond point, the scratch is clearly seen. The scratch is compared with the standard-limit scratches in the eyepiece disk (described below, and see Fig. 4) and if the scratch on the mineral is either too heavy or too light to be standard, the point is lifted, the weight is shifted appropriately, and another scratch made. Generally not more than three trials are necessary to produce a scratch within the standard limits. The point is then lifted, the stage is turned through a quadrant, and another scratch is made at right angles to the first one.

In addition to permitting classification by measurement, this in- strument enables the worker to make comparisons of relative hard- ness of adjacent minerals very easily and satisfactorily. When a single scratch is made across a boundary between two minerals, it will be of different intensity on the two sides of the boundary if the minerals differ materially in hardness. Even with min-

13 With the Harvard system of mounting, by which the polished sections are held in cement in rectangular brass cases, the distance of the polished surfaces above the stage is so nearly uniform that this adjustment for height can be made once for all.

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Standards For Hardness Of Ore Minerals. 539

erals so near together in the scale of hardness as to give no visible relief in polished section, this change in intensity of scratch is in some cases clearly evident. Minerals classified in the same group in the tables following often show by this means which of the two is the harder. With very small grains, not differing greatly in hardness from the mineral by which they are surrounded, the appearance of relief may be difficult to judge, and in some cases even appears reversed; scratching clear across the small grain and into the surrounding mineral on both sides often gives an unmistakable difference in scratch intensity on the two minerals, and a consequent positive indication as to their relative hardness.**

Standardization of Scratching Point—When this instrument was first constructed, it was provided with a socket to take a Sharp’s No. 10 sewing needle, the same as was used in the holder for scratching by hand; the needle was held at an angle, and the mineral drawn under and away from it. Dr. Murdoch’s first at- tempts at quantitative tests with such needle points showed wide discrepancies; it appeared that the hardness variation in single minerals was surprisingly great. Such results made it appear in- advisable to establish a scale of hardness with finer divisions than the three originally suggested; and with only three sub- divisions, hand-scratching seemed sufficiently accurate.

Recent work has shown that much of the apparent variation encountered by Murdoch was due to blunting of the needle. A new needle scratched a given mineral with much less weight on the point than was necessary with a needle that had been used; a few scratches as was shown by close examination, curled up the end, like the toe of a ski.

The need of a harder and tougher point was indicated, and ex- periments were made with a sapphire-pointed phonograph needle, but, if it was made sharp enough, it was not sufficiently strong. Finally a diamond was ground to a semi-circular blade-like edge

14 With less refinement, this method may be extended to the hand-held needle. A sapphire-pointed phonograph needle, in a suitable holder, works better than a sewing needle. On account of the unsteadiness of the human hand, such tests cannot be made as delicately as can tests with the instrument described.

15 This shape was suggested by Mr. F. L. Huston, and made under his direc- tion by the Arthur A. Crafts Company, Diamond Cutters, of Boston.

“

540 Sterling B. Talmage.

(See Fig. 3) small enough to make the scratch directly under the microscope, and the improved consistency of the results indicated that this was the most desirable type of point yet tried. This point is sufficiently sharp to scratch the hardest of the ore min- erals with an actual weight on the point of less than three grams.

ba

Metal

FDiamond”

included Angle

Side View Edge View DIAGRAM OF DIAMOND POINT

Fic. 3. Diagram of diamond point.

Even with such an edge, the possibility of blunting by wear had to be considered. After some three thousand scratches had been made with this point, on minerals of varying hardness, tests were repeated on the same mineral specimens used for the first measurements of this series. There was no observable differ- ence in the results. Inasmuch as each scratch is only about three millimeters long, the total distance traveled by the point over the mineral surfaces is something less than ten meters for the three thousand scratches. The wear on such a point appears to be negligible. The point looks delicate and fragile, and could prob- ably be broken easily by rough or careless handling; but its

ar - a

Standards For Hardness Of Ore Minerals. 541

strength seems ample for the purpose intended, if reasonable care be taken to avoid subjecting it to unnecessary strains or shocks.

Standard Intensity of Scratch—To judge the intensity of scratches, a standard for direct comparison was found necessary. This was provided by ruling a thin glass disk and inserting it in the eyepiece in the position ordinarily occupied by cross-hairs. Two scratches were ruled in line, one light and the other heavier, with a gap between, as indicated diagrammatically in Fig. 4.

Standard Limit Scratches on Glass Disk

Standard on Mineral

Fic. 4. Diagram showing use of standard-limit scratches in eyepiece disk.

These two lines represent the limits of the standard scratch; in- stead of trying to match one line exactly, it was found more satis- factory to make the standard scratch on the mineral more dis- tinct than the light line on the eyepiece disk, and less distinct than the heavy one. This plan supplies a little latitude for the stand- ard scratch, and takes care of variations within the limits of such latitude. The standard so established is entirely arbitrary, it is true, but it is a standard, and it works satisfactorily. The two standard-limit scratches appear as though scored directly on the mineral, and the scratch made with the diamond edge can be brought alongside or into alignment with the standard-limit lines, so that the comparison is easily made.

' The principle of shifting the weight to obtain scratches of standard intensity was considered preferable to the principle of

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542 Sterling B. Talmage.

using a fixed weight and measuring the width of the scratch pro- duced which has been used in testing; the latter method en- tails the use of a high-power micrometer eyepiece, and a change from low to high power objectives in order to measure the width of the scratch with great accuracy. However, the desideratum sought for our purpose was not a means for determining hard- ness with the greatest possible refinement, but a quick and easy method of determining, under working conditions, the relative hardness of the ore minerals within limits only sufficiently narrow to be of assistance in identification.

Another objection to the fixed weight method is that, because of the great hardness range of the ore minerals, a weight suffi- cient to make a visible impression on the harder minerals would gouge the softer ones so deeply as to damage their surfaces seri- ously. In the effort to preserve the polished surface so as to permit further work on it, the first attempts with the diamond edge were aimed at determining the least weight that would make a faintly visible scratch, but this proved to be an unsatisfactory standard; conditions of illumination, direction of scratch, and more particularly perfection of polish made a great difference in the visibility of these very faint scratches. The standard-limits scheme avoids both these difficulties.

Further Standardization—The point and the scratch having been standardized, it was noticed that variation in illumination caused irregular results. The first work with this instrument was done by daylight, but the difference in visibility of scratches on bright and dull days was found to be surprisingly great. Therefore a Spencer no. 372B microscope lamp was used, with a consequent increase in consistency of results. In fact, all the difficulties of this problem seem to have been overcome by estab- lishing standard conditions, many of them quite arbitrary, but governed by considerations of speed and accuracy, and by similar- ity to usual working conditions with polished sections. The standards finally established are: (1) Standard point, a diamond

16 Bierbaum, C. H., “ A Study of Bearing Metals,” Trans. A. I. M. E., vol. 69 (1923), p. 972. Pé, Viktor, “ Die Harte der festen Korper,” Dresden, 1909.

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Standards For Hardness Of Ore Minerals. 543

edge with an included angle of 45°; (2) standard light, from a no. 372B Spencer microscope lamp; (3) standard direction of scratch, parallel to plane of illuminator; (4) standard optical equipment, 16 mm. short-mounted objective, 5X eyepiece, and prism illuminator; (5) standard tube-length, 160 mm.; (6) standard width of scratch, between limits ruled on eyepiece disk.

With these standards established, highly consistent results were obtained. It was found that all the ore minerals, from the soft- est to the hardest, could be scratched to the degree determined as standard by shifting the weight within the limits on the graduated beam. Instead of trying to measure the hardness of every min- eral to the nearest division on the beam, it was found preferable to classify the minerals by groups. The hardness difference be- tween type members of adjacent groups depended on a clearly visible difference in scratch intensity at the same weight setting. The difference between groups was taken as approximately equal to the difference between the standard-limit scratches in the eye- piece disk. Since this difference was represented by a shorter shift of the weight with the softer minerals than with the harder ones, the weight-settings for adjacent groups are more closely spaced near the lower end of the scale. Of course, these weight- settings are arbitrary, and, as must be the case when any arbitrary classification is applied to a continuously varying series, some min- erals will fall on the border lines, or show an overlap into two or more groups.

New Scale Of Hardness.

The results of this work appear to warrant the proposal of a new scale of hardness for use with polished sections, consisting of seven well-defined degrees into which the ore minerals may be graded, to replace the three poorly-defined degrees in use to date. These seven degrees are believed to be as finely drawn as is fea- sible without introducing such great refinement of manipulation as to defeat the main purpose of establishing this scale—to aid in the practical identification of the ore minerals.

. To avoid confusion with the Mohs scale, these groups are

designated by letter instead of by number. To provide a basis

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544 Sterling B. Talmage.

for comparison and for calibration of instruments, each group has also been designated by a type mineral. The group letter designations and the type minerals for the various groups are listed in Table I., along with the scale and weight figures: for the particular instrument used in this work.

TABLE I. New Scate oF HarpNess Proposep FoR ORE MINERALS IN POLISHED SECTIONS. Weighting at Standard Scratch. Position of Weight on Actual Weight on Point. Beam Scale. A Argentite 0.2 0.105 gms. B Galena 0.4 Cc Chalcopyrite 0.7 0.365 D Tetrahedrite 1.0 E Niccolite 0.775 F Magnetite 2.0 1.035 “ G Ilmenite 3.0

Most of the ore minerals can be placed with reasonable cer- tainty in one of the seven gtoups proposed under this new rating. Some border-line minerals overlap into two groups, as do a few minerals that show a difference of hardness in different speci- mens, and minerals in which the hardness differs markedly with change of crystal orientation. A very few minerals are so vari- able as to extend into three or even four classes; this extreme variability may at times be used as an added factor for the identi- fication of these few species.

Another source of apparent variability is difference in perfec- tion of polish. These tests were made, wherever possible, on well-polished surfaces. It was noted that a very smooth surface in a polished section could be scratched more easily than a sur- face of the same mineral not so smooth. This was first noted with pyrite; imperfectly polished portions seemed harder than very smooth areas on the same grain. Schneiderhohn has recently revived the suggestion that there is produced in the proc-

17 Schneiderhéhn, Hans, “ Anleitung zur mikroskopischen Bestimmung und Untersuchung von Erzen,” etc., Berlin, 1922, pp. 59, 65, 111, 138.

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Standards For Hardness Of Ore Minerals. 545

ess of polishing an amorphous slip-film or smeared coating which may conceal the underlying structure; the question naturally arose as to whether these scratch-measurements on polished surfaces represented hardness of the mineral itself, or only hardness of the slip-film, which might be quite a different thing. Therefore a scratch-measurement was made on an exceptionally perfect crys- tal face of a pyrite cube which had never been polished artifici- ally; it was scratched at the same weight-setting required for a perfectly polished section of pyrite, while a less perfect face of the same crystal required the heavier weighting such as was neces- sary for an imperfectly polished area. The conclusion is, there- fore, that the ease of scratching is to some extent a function of smoothness of surface, and that in applying scratch tests to areas that are microscopically rough, a somewhat higher result must be expected than that indicated by the rating given in the tables following. Schneiderhohn states that the slip-film appears to be without effect on phenomena visible with polarized light ; its effect also seems to be negligible in connection with hardness tests.

The Quantitative Data.

The accompanying tables give the results of tests with the in- strument described and illustrated on the minerals of the original suite assembled and studied by Dr. Murdoch. The figures given in Table I. relate directly to this particular instrument, and may not be strictly duplicable under slightly changed conditions; they are given here for purposes of comparison, but the influence of small changes in certain factors is of such a nature that it is be- lieved that any other instrument would have to be calibrated against the type minerals before comparable results could be ob- tained.

In Table II. the ore minerals are listed in groups, and arranged, so far as present knowledge makes possible, in the order of in- creasing hardness. This arrangement is made on the basis of tests on all the available material, but in a number of cases only a single specimen of a particular mineral could be obtained. Each specimen was scratched on three areas, and in two direc-

4 :

546 Sterling B. Talmage.

tions on each area; except in the rare cases where the specimen was a single crystal unit, the test would represent six scratches of random orientation.

Some minerals, as noted, showed distinct differences of hard- ness in different directions; in several cases this results in over- lap into two or more classes, while in others the variation, though perceptible, is confined within the limits of a single class. With most minerals, however, the variation with changing orientation is negligible under measurements of the degree of accuracy used in this work. Quantitative study of these variations, on an abundance of material and with more accurate measurements than were needed for this classification, would probably develop some information of value.

Hereafter as measurements accumulate on more material, it is possible that certain minerals will be found to have a somewhat wider range of hardness than is here indicated, and also that slight shifts in the relative position of some of the minerals may be found necessary. It is believed, however, that this arrange- ment in classes is substantially correct. After tests had been made on all the specimens in the original Murdoch suite, repre- senting only a single specimen of each mineral species, the min- erals were classified tentatively in these seven groups according to hardness; the later testing of all Dr. Murdoch’s duplicate specimens, most of them from other localities than the corre- sponding minerals in the original set, resulted in changing the group rating of only three minerals. The group classifications given are based on results of tests on all the Mudoch minerals and some others.

Minerals close together in Table II. are of nearly the same hardness; minerals very close together may have enough varia- tion in individual specimens to reverse the tabulated order in any particular case. But it is believed that minerals fairly well separated in this table can be relied on in most cases to show the hardness relations indicated.

In Table III., which is really an index to Table II., the ore

n nN s u t t

Standards For Hardness Of Ore Minerals. 547

minerals are listed alphabetically, followed by new and old hard- ness ratings for comparison.

Throughout this work the writer has been greatly assisted by friendly criticisms and constructive suggestions from Professor L. C. Graton, under whose supervision these tests were made.

TABLE II. Ore MINERALS ARRANGED IN GROUPS IN ORDER OF INCREASING HARDNESS.

The bases of this tabulation are:

Minerals that were scratched to standard intensity with the same weight set- ting at every trial are placed under the group letter assigned to that weight setting.

Minerals that in most cases were scratched to standard intensity at a particular weight setting, but in some instances required a heavier weighting are placed under the appropriate group letter, with the addition of a plus sign.

Minerals that in most cases were scratched to standard intensity at a particular weight setting, but in some instances gave a standard scratch under lighter weighting are placed under the appropriate group letter with the addition of a minus sign.

Minerals that in an equal number of instances were scratched to standard in- tensity at two adjacent weight settings are placed under the harder rating, with the addition of a minus sign.

Minerals thus carrying either a plus or a minus sign after the group letter represent the border-line cases between the arbitrary groups, or else minerals in which the hardness differs markedly according to orientation, or is variable in the different specimens studied.

While these minerals are arranged approximately in the order of increasing hardness, so far as known, all may be subject to some slight variability, and the arbitrary group-separations between the members of this continuously varying series must not be interpreted too rigidly; for instance, the hardness difference between tetradymiie, the hardest mineral listed under B+, and pyrargyrite, the softest mineral listed under C—, may be no greater than the difference between two adjacent minerals in any other part of the table.

In the “Notes” column, following the listing of figures, the designations “harder than —” or “softer than —” are in each case the result of measure- ments on adjacent minerals in the same polished section.

The term “ variable”’ refers to differences in hardness between different speci- mens, or in different directions on the same specimen.

The term “irregular” refers to visible differences in intensity of a single scratch over an apparently continuous and otherwise uniform surface.

The figures under the group-letter headings following the “ Group-Rating ” column represent the actual number of scratches made at the weight-settings indi- cated; in nearly all cases, six scratches were made on each specimen.

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548 STERLING B. TALMAGE. TABLE II.—Continued. Rating. A+ |22] Slightly irregular. Kalgoorlite A+ 5] Clausthalite A+ 7) 5|/—|!]—]—|—|—] Varies with orientation. wie A+ |19] 5] 1] Very irregular. B+ 2] 7] Very irregular. Molybdenite B+ 2] 2] 1] 1]—|—]—| Very irregular. Lehrbachite B- 2) ‘ Tiemannite B— Slightly irregular. Berzelianite B Lorandite B Prankeite B |—j—]—|— ; Steinmannite B B B Varies slightly with orienta- tion. Kermesite B Metacinnabarite B Naumannite B j tion. Stephanite B — Varies slightly with orienta- ’ tion. ‘ Stromeyerite B Whitneyite B Miargyrite B ( B+ |—]15| 3}—]—]—]|— Slightly variable. Guanajuatite B+ 5] Variable. ‘ B+ 5] Slightly irregular. ] Jamesonite B+ 9] Varies slightly with orienta- ] tion. ] Lengenbachite ]

Standards For Hardness Of Ore Minerals.

TABLE II.—Continued.

Mineral.

Notes.

Bismuthinite

Dufrenoysite

Chalcopyrite

Chalcostibite

Epiboulangerite. ...

Tetradymite Pyrargyrite Coloradoite (Cylindrite. Guitermanite Horsfordite Seligmannite Semseyite Meneghinite

Polybasite

Areyrodite. Beegerite Baumhauerite Livingstonite Plagionite. +. Rezbanyite Schirmerite

Emplectite

Andorite.

Alabandite. Calavetste

Chalmersite Krennerite Algodonite Famatinite Melonite Sphalerite

Ne

Berthierite Hnargite.

Ownu

w

Pyrrhotite 5...

°

Varies with orientation. Varies with orientation.

Varies with orientation.

Variable and irregular.

Harder than proustite.

Slightly irregular.

Harder than Galena; no relief.

Varies with orientation.

Same as chalcopyrite. Softer than sphalerite.

Varies with orientation.

Varies with orientation; ir- regular.

Varies greatly with orienta- tion.

a Rating.

Bournonite ) C—

Wittichenite ] C —j|—|— :

550 STERLING B. TALMAGE. TABLE II.—Continued. Pentlandite 5|—]—]—] Slightly harder than pyr- rhotite. D+ 6] 4] 2}]—|—J]Two specimens very dif- ferent. Stylotypite D 6/—|—|— Tennantite D Variable; harder than sphal- erite. Polytelite D Willyamite E 12) — — Delafossite E 6|—|—] Somewhat variable. Maucherite E Rammelsbergite E Breithauptite E Hauchecornite E 6|—]—| Harder than millerite. Hematite (micro- crystalline) E+ 5] Chloanthite E+ |—|/]—|—|—]19] Softer than cobaltite. Erythrozincite E+ 4] Gersdorffite F— ©] Two specimens; different. Polydymite 5] 7/— Marcasite F-— 8]10}]—| Somewhat variable. F-— 5]13|—| Varies with orientation, and polish. DGllingite. 3]15|—| Somewhat variable. Arsenopyrite Fa 2] 7] 3] Variable. Magnetite F F Franklinite F+ 9 Psilomelane G— 12] 24] Variable with banding. Cobaltite. Harder than ldllingite. Hematite G 12 Well-crystallized specularite. Gs G+ Variable; some scratches weak. Cassiterite. G+ —|—|—]—]|12 Sub-standard scratches. G+ Sub-standard; harder than cassiterite.

ee

: dee

Standards For Hardness

OF ORE MINERALS. 551 TABLE III. ALPHABETICAL List OF THE ORE MINERALS wiITH NEw anD Harpness RATINGS. Mineral. New Rating. Murdoch Rating. Mohs Scale Rating.

B Low Brongniargite. . . B+ Low 3.0+ Chaleostipite Cc Medium 3.0-4.0 Cuprodescloizite D Medium 3-5 Cc Low 2.0 Epiboulangerite B+ Low Erythrozincite E+ Low Medium B- Low 2.5

id te. les an

552 Sterling B. Talmage.

TABLE III.—Continued.

Mineral. New Rating. Murdoch Rating. Mohs Scale Rating. B+ Low 2.5-3-5 sdanchecormite E High Medium 5.0 Hematite (microcrystalline) . E+ High 5.5-0.5 Hematite (specularite) G Lengenbachite B+ Low Lorandite B Low 2.0-2.5 Metacinnabarite B Low 3.0 B+ Low 1.5-2.0 Polyargyrite A Very Low 2.5 Medium

Standards For Hardness Of Ore Minerals.

TABLE III.—Continued. Mineral. New Rating. Murdoch Rating.

G- High 5.0-6.0 Rammelsbergite E High 5.5-6.0 Realgar B Low I.5-2.0 E+ High 5.5-0.0 C+ Medium 3-5-4.0 Steinmannite B Low Stromeyerite. B Very Low 2.5-3.0 Bao Low 2.0-2.5 B- Low 2.5 E High 5.5-6.0 B Medium 3-5 D- 4.0-4.5

Foxcroft House, CAMBRIDGE, Mass.

NOTES ON SILVER-LEAD DEPOSITS OF SLOCAN DISTRICT, BRITISH COLUMBIA, CANADA.*

Alan M. Bateman.

In the Slocan district are silver-lead deposits, both fissure veins and replacement bodies, that have contributed to the wealth of the province of British Columbia for some thirty years. They are situated well up on the hillsides of a region with strong relief and steep slopes—a topography youthful in the physiographic sense. The relation between these deposits and the topography involves some points not only of geologic interest but also of pos- sible commercial importance. In addition, the character of the ore, the form of the deposits, and their origin, attract the inter- est of the geologist.

This region has claimed the attention of geologists in the past, and descriptions of the deposits and their geologic setting are numerous,’ though unfortunately meagre. A few seasons ago, I spent a brief period in the field and during this work and sub- sequently, a few thoughts occurred to me that have not been mentioned before, and they form the excuse for these notes.

The setting of the ore deposits may be dealt with briefly.

General Features

History.—Mining has been carried on in the Slocan district for over thirty years although no one property has been steadily operated. The earlier ones have been exhausted, others have come into existence, and older mines have been reopened so that continuous production has been maintained. At present there are perhaps a dozen shipping mines.

1 Presented before the Society of Economic Geologists, New York meeting, May, 1925.

2 To be found mostly as brief papers in the Summary Reports of the Geological Survey of Canada. They will be referred to specifically throughout this paper.

Silver-Lead Deposits Of Slocan District. 555

Since 1891 the district has yielded approximately $45,000,000. Most of the mines are small and individual ones have produced from a few thousand dollars to over $8,000,000. For the cap- ital involved, however, the return has been large and most of the mines have “ paid their way.”

The ore is of high value so that the mines have been “ ship- pers” from the start. Reserves have rarely been accumulated and little development has been carried in advance of mining, consequently the working out of the exposed ore shoots often resulted in closing the mines.

Location.—A glance at a map of British Columbia will show in the center of the southern part a series of long, narrow, north- ward-trending lakes. Between the two easternmost of these, Kootenay Lake and Slocan Lake, is a narrow, steep, cross valley through which is a winding railroad, and here lies the Slocan min- ing section. The town of Sandon, one of the centers of the dis- trict, lies in a small tributary valley. The place can be reached by either lake, from the south via Nelson, or from the north, via Revelstoke, both points being served by the Canadian Pacific Railway.

Topography.—The town of Sandon is centrally located in the district and the towns of Silverton and New Denver lie on Slocan Lake on the western side of the mining area. Carpenter Creek and Silverton Creek, with their numerous tributaries, form the drainage channels of most of the area. The majority of the mines lie well up on the valley slopes of Carpenter Creek and this valley, in the vicinity ot Sandon, is steep-sided and narrow—so narrow, in fact, that there was not sufficient room for both the town and the creek, and the latter is covered over to form the main street. The tributaries likewise flow in steep-sided, nar- row valleys. Both master streams and tributaries have steep gradients; waterfalls are numerous and the streams are still ac- tively cutting. The relief is quite pronounced; the elevation of Slocan Lake is 1,730 feet and that of the highest peaks is over 8,000 feet.. The mines are, therefore, opened by adit tunnels,

3 See map No. 1641, Can. Geol. Surv.

556 Alan M. Bateman.

and aerial tramways are necessary for the transportation of the ore. The problem of transportation’ is consequently serious; roads are few and steep; the railroad has to contend with steep grades; and washouts and snow slides are frequent.

The present youthful topography was carved by streams out of an elevated erosion surface that had considerable relief. Plu- tonic igneous rocks outcrop on the tops of some of the higher hills, consequently the erosion that caused the present topography was preceded by one sufficiently prolonged to reveal intrusive rocks, which by their texture show that they crystallized at con- siderable depth.

Rock Formations.~—Most of the mines lie within a series of interbedded slates or argillites, quartzites (argillaceous and ar- kosic), and finely crystalline limestone, known as the Slocan series, and considered by Bancroft ° to be of upper Carbonifer- ous age. There are all gradations between the individual rock types so that the slates may have a considerable lime or sand content, and vice versa. The slates, in many places, are highly carbonaceous and where mashed become graphitic slates, locally called graphite.

The Nelson batholith, of supposed Upper Jurassic age,° has invaded the Slocan series, and its main mass forms a half-moon- shaped area around the southern border of the metallized sec- tion. The mines are thus partly surrounded by the intrusive and no one is at any great distance from the surface exposure of the batholith. Numerous offshoots from the batholith, in the form of acidic and basic masses, dikes, and sheets, cut the Slocan series in the vicinity of the mines. It is not unlikely that the batholith underlies the whole metallized area, and the ore deposits are lo- callized in a roof rock that has been punched here and there by

4 Bancroft, M. E., Geol. Surv. Can. Sum. Rep. 1919, Pt. B, pp. 41-46; idem, 1917, pp. 28-41.

Leroy, O. E., Geol. Surv. Can. Sum. Reports 1908, 1909, 1910. Can. Rec. Sc., Vol. 6, p. 494, 1896; Can. Geol. Surv. Guide Book No. 9, 1913, p. 98.

Brock, R. W., Can. Geol. Surv. Reports 1899, 1900, 1901.

Drysdale, C. W., Geol. Surv. Can. Sum. Rep. 1916, pp. 56-57, and map No. 1667.

5 Loc. cit., p. 42. 6 Idem.

al ti ft it K n

] (

Silver-Lead Deposits Of Slocan District. 557

cupolas of the batholith and into whose cracked mass, magma was injected to form dikes and sills.

The batholithic rock is a granodiorite with facies of granite and quartz diorite. In the metallized section it is prominently porphyritic and large phenocrysts of feldspar attract one’s atten- tion. The dikes are also porphyritic and vary in composition from quartz porphyry to basic lamprophyres. All of them are greatly altered.

Structural Features—The Slocan series has been intricately folded and squeezed and the beds have a general northwesterly strike, but this folding is complicated by a northerly cross-fold- ing. These larger structural features have been depicted graph- ically by Drysdale.’

The stresses that produced the intricate folding of the Slocan

series also found relief in rupture, forming innumerable faults of

northeasterly and northwesterly trends. Some of the faults have great length and are uniform in strike and dip, particularly the northeasterly ones. Others are discontinuous, horizontally and vertically. Faults of one trend show a tendency to terminate against or swing along those of another trend.

Most of the faults are earlier than the mineralization and have localized the ore deposits. Post-mineral faults that cut or dis- place the veins are also to be seen, and post-mineral movement has occurred along earlier fault veins.

The character and displacement of pre-mineral faults is diffi- cult to determine, but both normal and reverse faults have been recognized. Even with the post-mineral faults, the monotonous repetition of the beds of the Slocan series prohibits, as a rule, a determination of their displacement.

Ore DEposits.

General_—Three types of deposits have long been recognized in the district. They are:

7 Drysdale, C. W., Can. Geol. Surv. Map, No. 1667. Probably owing to the untimely death of Dr. Drysdale this map was not accompanied by text matter relating to the structure. ;

8See also Uglow, W. L., “ Gmeissic Galena Ores of Slocan, B. C.,” Econ. Geot., vol. XII., 1917, p. 644.

ig ¢ 4H 42° SEP 151 Be

558 Alan M. Bateman.

1. Narrow quartz veins chiefly in the granodiorite, frequently referred to locally as “dry ores.” They have high silver, and low lead and zinc content, and are now relatively unimportant. They are quite distinct from the more valuable silver-lead depos- its and need not be referred to further.

2. Massive zinc deposits, of which the Lucky Jim mine is an example. These have been formed by the process of replace- ment along fractures that traverse the sparse beds of limestone. Consequently they display those irregular outlines that are gen- erally considered as characteristic of replacement deposits in lime- stone. They are elongated in response to the control exerted by the fissures and so have the appearance of veins. Their width may vary from one to forty feet. Several wide bodies, con- nected with each other by narrow bands of ore, may occur along a single fissure. These large swellings seem to have been local- ized by cross fractures that afforded more ready opportunity for the replacing solutions to work outward from the master control fissures. The ore consists of zinc blende and pyrite with minor amounts of galena, in a gangue of altered limestone, quartz, and siderite. Such mineral composition, combined with the small silver and lead content, causes the ores to have little value at present even though their zinc content is large.“*

3. Silver-lead fissure veins. These constitute the chief source of wealth of the district and the remainder of the discussion will be confined largely to them.

Character of Ore—The ore is valuable chiefly for silver and lead with lesser zinc. Gold is almost negligible. The metals are obtained largely from galena, tetrahedrite, and blende, all of which are argentiferous. The galena contains 1.5 to 2 oz. sil- ver to each per cent. of lead, and the blende about 1 oz. or less. In rare cases these ratios may be much higher. Pyrite and chal- copyrite are rare and pyrrhotite is occasionally found. The pyrite increases as the lead content decreases, but the chalcopyrite is not present in sufficient amount for the copper to be paid for. Ruby silver is not uncommon and a little argentite and native silver were seen. There are some oxidized ores and these contain little zinc

8a Since the above was written, the Lucky Jim mine has been reopened.

Ce a AY

Silver-Lead Deposits Of Slocan District. 559

and much lead, a considerable part of which is in the form of the carbonate, cerrusite. The gangue matter is largely the altered country rock, or fault gouge, along with quartz, siderite, or rarely calcite.

Both direct shipping ore and mill ore are mined. The shipping ore and concentrates yield from 40 per cent. to 70 per cent. lead, and 60 to 100 ounces silver. With shipping ore, as much zinc as possible is eliminated, and mill ores yield a separate zinc concen- trate with considerable silver.

Nature of Deposits—tThe silver-lead veins occupy fault-fis- sures, though a few are localized by master joints. The fault- fissures cut across the bedding of the slates of the Slocan series and die out in a gradual distortion of the slates or by abutment against other faults. More rarely the displacement has been ab- sorbed by movement along bedding planes. In a few places, as at the Sovereign mine, the fault-fissures follow narrow dikes, thus being localized along lines of previous rupture. In such cases the fissure usually follows one wall of the dike, though rarely they may angle across the dike. The metallization has taken place by both filling and replacement in the fissures.

The resulting veins commonly are sharply marked against gouge walls, crushed slate, or rarely porphyry dikes. They vary in length up to several thousand feet and in width up to 40 feet. Some of them are remarkably continuous and straight; others show a tendency to swing from one fault-fissure to another, or from a fault to a joint or bedding plane. The veins strike pre- dominantly northeasterly, though a few trend east and west. The dip is almost invariably southerly.

Most of the veins show continuous mineralization in the form of quartz, altered wall rock, or traces of sulphides, but the valuable minerals have been concentrated in only a few places to form ore shoots.

Ore Shoots—The workable parts of the veins, or the ore shoots, form but a small part of the total volume of a vein. Though the veins are continuous, the ore shoots, on the other

hand, are quite restricted. They range in length from a few

560 Alan M. Bateman.

feet to over 450 feet and may occupy only a part of the width of the vein. Most of the shoots have a pronounced pitch or rake within a vein. They are sporadically distributed along the veins, both laterally and vertically, and usually the unworkable parts of the vein between shoots is of greater length than the shoots themselves. Presumably the same is true in depth.

The distribution of the ore in the shoots is also somewhat ir- regular; the width varies considerably and horses of waste and lean places are numerous. A stope may have a back of almost solid ore one day, only to disappear with a few more days of stoping. Occasionally good ore may occupy the full width of the vein up to twenty feet or more, as in the Silversmith and Standard Mines. Usually, however, where the veins are very wide, only a part of the width is mineable and bands of crushed country rock or bunches of calcite and quartz lie between the bands of ore. Thus, in places the walls are only apparent and other ore bodies lie beyond them. It is advisable, therefore, in mining operations that the walls be tested frequently in order that such parallel bands of ore be not missed. The ore shows a tend- ency to be concentrated somewhat on the hanging-wall sides of the veins, but this tendency is not so marked that the footwall can be overlooked.

Usually within the shoots there are massive bunches or bands of nearly pure galena up to five or, in places, ten, feet in width and these may be flanked by sorting ore, mill ore, or waste. All of the shoots contain more or less clean shipping ore in addition to mixed ores of galena and blende from which the galena can be hand sorted. The mill ore contains finely scattered grains or blebs of sulphides within the gangue matter.

The ore shoots end by a gradual fading out of the ore and the extremities are marked by diminishing galena and increasing quartz, siderite, and pyrite. Beyond the margins of an ore shoot the veins are almost barren of ore minerals and are difficult to follow ; some crushed rock, a little quartz or calcite, or rare traces of sulphide, alone mark them. Occasional sporadic bunches of mill ore may occur in the inter-shoot areas, but usually they are too small to be stoped.

a IRE

+

Silver-Lead Deposits Of Slocan District. 561

A search for the cause of the localization of the ore shoots proved elusive. No companionship between ore and any particu- lar kind of rock could be detected; wall rocks similar to those bounding the ore are also in the barren intershoot areas, and ore shoots cross more than one variety of slate without visible change. The variety or abundance of gouge seemed to be no different in the ore shoot areas and barren stretches. Neither does a change in the strike or dip of a fissure appear to affect its metal content. Nor do cross fractures or shear zones appear to localize the ore; the ore shoots are much longer than the width of any cross-frac- turing and also cross-fracturing is frequently absent in the vicin- ity of shoots, and where it does occur it is no different from that to be observed beyond the ore limits. Pinches and swells of the original fissures offer no better hope as an explanation. I greatly doubt, anyway, that open swells could be maintained be- tween such yielding walls.

The cause of ore shoots is usually a baffling question in most mining camps and is particularly so in the Slocan district.

Mineralogy of Deposits—The ore minerals that have been ob- served both by the naked eye and by means of the microscope are: galena, sphalerite, chalcopyrite, pyrite, tetrahedrite and freiber- gite, ruby silver (both proustite and pyrargyrite), argentite, pyr- rhotite (one instance), native silver, and covellite. Considerable cerrusite occurs in places and also a little calamine and smithson- ite. A few small fragments of boulangerite were identified, and numerous specks of a soft creamy to grayish white mineral, too small to be identified, were seen under the microscope. The in- troduced gangue minerals consist of quartz, siderite, and calcite.

The most striking feature of the silver-lead ore is its banded or gneissic texture—a feature that has been described in an ex- cellent paper by Dr. Uglow.® The term, gneissic, aptly describes the texture since, except for the composition, it looks like a coarse granite gneiss.*° The galena is arranged in bands, discernible by

9 Uglow, W. L., “ Gneissic Galena Ore from the Slocan District, B. C.,” Econ. GeEoL., vol. 12, 1917, pp. 643-662. 10 See plates accompanying Dr. Uglow’s paper.

ihe

562 Alan M. Bateman.

the difference in the size and orientation of the galena grains. It gives the impression of flowing and slicing under compression, and the microscope brings out forcibly, especially upon etched pol- ished surfaces, the curving and distortion that the galena crystals have undergone. The galena plates curve around eye-like knots 1 of tetrahedrite, sphalerite, quartz, or siderite, indicating that the 1 softer galena flowed around the other harder and more resistant minerals. The origin of this structure has been taken up by Uglow.** The crushing of the ore must be considered in connec- . tion with the age relationship of the different minerals.

The impression conveyed in a microscope study of the ores is that the galena is later in age than the quartz, siderite, sphalerite, tetrahedrite, and pyrite. Most of the latter minerals are shat- tered and traversed by galena, but in the case of the crushed ores this cannot be used as a criterion of later age of the galena, for the harder minerals have undergone fracturing while the galena has undergone flowage and recrystallization and, therefore, gives the appearance of being of later age, whereas it may not have been so. Age relations of different minerals, then, in shattered ores of this type may be no criterion whatever of the original sequence of mineral deposition—a point which should not be overlooked in building deductions upon mineral sequences. Similarly, the numerous inclusions of wall rock inclosed by galena may not be replacement residuals as they appear to be, but fragments incor- porated during the rearrangement of the galena.

In the unchanged, or only slightly crushed ores, the quartz, pyrite, and tetrahedrite appear to have formed earlier than the other minerals. Sphalerite and chalcopyrite were formed simul- taneously and the galena was formed contemporaneously with, and also slightly later than, the sphalerite; there appears to have been a slight overlapping in deposition. The siderite was contem- poraneous with the blende and galena. Siderite, blende, and galena were all observed to replace quartz, tetrahedrite, and country rock.

Much of the sphalerite contains haphazardly arranged pin-

nr

11 Idem,

‘

Silver-Lead Deposits Of Slocan District. 563

points of chalcopyrite visible only under higher magnifications— a mode of occurrence familiar to all who have examined micro- scopically ores containing zinc and copper.

The high silver content of these ores may be accounted for by the abundant presence of minute specks of silver-bearing minerals that can be seen only under the microscope. The ruby silvers, and the unknown mineral, presumably a silver ore, are confined almost entirely to the galena. They occur in shapeless forms with smooth outlines against the galena. Some of them contain inclusions of galena. They, and the galena, apparently were formed at the same time.

All of the minerals, with the exception of covellite and native silver, are believed to be primary or hypogene. The covellite is present only in occasional minute microscopic amount in ores that are partially oxidized and in which the galena is altering to cerrusite. It occurs as small secondary veinlets that follow the cleavage planes of galena and its occurrence is similar to the structures to be found in secondary or supergene enriched ores. It is unquestionably secondary. The native silver was observed as thin flakes and scales along cracks in the sulphides and it also probably owes its presence to surface alteration. With these ex- ceptions, there is, in the uncrushed ores, an entire absence of later crossing veinlets and other features that are characteristic of su- pergene enrichment. The texture is that which is typical of ores of undoubted hypogene origin and my microscopic examinations leave no doubt whatever in my mind that they are of primary or hypogene origin. In so far as secondary enrichment is con- cerned, therefore, no mineralogic change need be expected with depth.

The zone of oxidation, in most of the mines, is negligible, and primary sulphides outcrop at or near the surface. Ina few cases, of which the Queen Bess mine is an example, superficial alteration is more pronounced and extends to a depth of a couple of hundred feet. Along fractures the pyrite and chalcopyrite are partially converted to limonite; galena is partly altered to cerussite. Blende has been removed at the surface, causing a local lead en-

When

564 Alan M. Bateman.

richment and there are small amounts of calamine, smithsonite, and malachite. The gossan is negligible. Such oxidation occurs only in places where the most recent erosion has not been active, as on the tops of inter-tributary ridges.

Relation of Ore to Rocks.—The areal relation of the silver-lead deposits to the Nelson batholith must be more than casual. Here is an area of rocks of the Slocan series, about ten miles in diam- eter, that embays the batholith. If one were to draw a line con- necting the tips of the crescentic outcrop of the granodiorite, the area of sediments so enclosed would contain most of the silver- lead mines of the district. Moreover, the sedimentary embay- ment is punched by a dozen projections of irregular masses of granitic rocks, and the number of porphyry dikes and sills in this locality is legion. If the dikes and sills have a genetic connection with the batholith, then so have the veins, and the former is proved by the petrographic similarity of the two classes of rocks. There thus seems little doubt that the veins are a manifestation of ihe same igneous activity that resulted in the batholith, the ir- regular intrusions, and the dikes.

The field relations sketched above suggest strongly that the batholith underlies the sediments inclosing the veins and the dikes, and at no great depth. This conclusion is strengthened further by Bancroft’s observation that the dip of the granodiorite be- neath the sediments is usually quite flat.

The dikes may be seen, from a study of Drysdale’s map,** to have a pronounced northwesterly strike, whereas the adjacent veins strike predominantly northeasterly and cut across the dikes. Therefore the fractures occupied by the veins were formed later than those occupied by the dikes, else the dikes would also strike northeasterly.

The area of greatest abundance of both dikes and veins lies at a distance of about two miles from the surface contact of the main mass of granodiorite; nearer to the contact there are only a few veins, and the ores of these veins are much more siliceous

12 Loc. cit., p. 46B. 13 No. 166.

t a n t t n a n a ti a

Silver-Lead Deposits Of Slocan District. 565

than the distant ones. The inference is that the conditions for the deposition of the silver-lead ores were more favorable at some distance from the contact than close to it.

The formation of the silver-lead ores may then be considered a part of the igneous activity of the district of which the se- quence of major events was as follows: First, intrusion of the batholith, the top of which was irregular in outline and from which numerous cupolas extended up into the folded roof sedi- ments ; second, fracturing of the overlying rocks and contempora- neous injections of magma, giving rise to porphyritic dikes ; third, further fracturing of the roof rocks and dikes and expulsion from the unconsolidated interior of the batholith of the metallizing solu- tions that gave rise to the ore deposits. These solutions travelled through fractures in the outer consolidated parts of the batholith and extended upward into the roof rocks. They lost a part of their metallic content in the granodiorite, and in the sediments im- mediately overlying it, but the major part of their metallic load was retained until the solutions had passed through a few thou- sand feet of the overlying rocks.

Zonal Distribution.—The character of the resulting ores shows a response to the distance of the travel of the solutions; in other words there is a zonal distribution with respect to the batholith. The ores in the granodiorite (type 1) are highly siliceous, zinc is scarce, lead is subordinate, and the chief metal is silver. The minerals are quartz, tetrahedrite, argentiferous galena, with small amounts of sphalerite, ruby silver, chalcopyrite, pyrite, pyrrhotite, calcite, and siderite. The total amount of metallic minerals is small. The ores in the roof sediments near the contact are less siliceous, contain more galena, sphalerite, siderite, and calcite, with less tetrahedrite and ruby silver and about the same propor- tions of pyrite and chalcopyrite. In the more distant deposits the relative proportion of galena and sphalerite to quartz is greater, and siderite and calcite are more abundant while chalcopyrite and pyrite remain about the same. Tetrahedrite and ruby silver are still present, but the proportion of these minerals with respect to the amounts of galena and sphalerite is much less than in the ores

: :

566 Alan M. Bateman.

in the granodiorite. In brief, the zonal arrangement here is char- acterized by increasing amounts of galena, sphalerite, and siderite, and decreasing relative amounts of quartz, tetrahedrite and ruby silver, outward from the mother rock of the metals. The rela- tive amounts of chalcopyrite and pyrite do not change appreciably, though the latter has been observed by Argall to increase with de- creasing galena.** Argall also notes that zincblende increases in proportion to galena as depth is gained, and these pass into side- rite and quartz and finally into quartz.

Depth Of Ore And Relations To Topography.

The idea is prevalent that the ore deposits of the Slocan District do not extend deep beneath the surface; it is a part of the heritage of the camp and has found expression in the literature.*° Most of the mines have been limited to the first few hundred feet be- neath the surface, though a few exceed 1,000 feet, and LeRoy re- ports one that reaches 1,270 feet depth. This conclusion has

without question influenced the development of the camp; it has:

acted as a brake upon deeper and more expensive exploration: It so happens that in most of the mines the ore shoots pitch, or the veins dip, into the hills, consequently the adits (by which the mines are developed) must be much longer at lower elevations; and therefore more costly. Some long adits gave disappointing re- sults, others were abandoned because of lack of funds before their objectives were reached. Thus the accumulated experience to date has shown that the ore bodies have only shallow depth; and the deeper exploration has not dispelled this conclusion—a conclu- sion, which, if correct, spells gloom for the future of the camp. But let us examine critically the data upon which this conclu- sion has been based. It is true that the mines play out at shallow depth beneath the surface, but it is also true that the exploration at depth has been too desultory to prove the non-existence of more ore beneath the present bottoms of the mines. If the fissure veins 14 Report of Zinc Commission, Mines Branch Canada, 1912, p. 168.

15 LeRoy, O. E., Inter. Geol. Cong. 1913, Guide Book No. 9, p. 100. 16 Idem.

t ] (

: t : ‘

Silver-Lead Deposits Of Slocan District. 567

play out at such shallow depth, it suggests that with primary ores of this type, the mineralization was a shallow-seated type and therefore related to the surface, or that only the roots of former veins are now left.

With respect to the former, there is nothing in the mineralogy to suggest shallow vein formation related to the present surface. If the age of the ore deposits and the physiographic development of the region be taken into consideration it will be seen that there can be no connection between the metallization and the present surface. The metallization was a phase of the Jurassic igneous activity. If one were to restore the surface that existed at that time he would have to fill up the great valleys whose flanks con- tain the ore deposits. This would add 5,000 feet of cover to the tops of some of the veins. And on this filled levelled surface there would outcrop coarse-grained plutonic igneous rocks, formed only at great depths beneath the surface. Additional cover would have to be added to bring the surface to the top of the batholith, and then another few thousand feet to allow for the depths at which granular rocks crystallize. Thus many thousand feet of cover. lay on top of the present ore bodies at the time they were formed; and their relation to the now existing surface is due solely to accidental erosion.

Another line of evidence throws light on whether the mineral- ization was restricted to a narrow vertical range and the work- able bodies are but the roots of veins of which the greater part has been eroded—that is, the elevations of different deposits. One finds pronounced differences in elevation within relatively short horizontal distances. For example, in the ridge between Carpenter Creek and Silverton Creek, the Bosun mine is at an elevation of 2,000 feet; Emily Edith mine, 2,500; Hartney and Standard mines, 4,000; Queen Bess mine, 5,000; Idaho, Alamo, and Wakefield mines, 6,000 feet; and the Ivanhoe, 6,500 feet. Within this area, then, there is a maximum difference in eleva- tion of 4,500 feet. On the slope north of Sandon within a radius of a mile are the Sovereign at 5,000 feet, the Payne at 6,000, the Noble Five at 6,500, and the Last Chance and Rico mines at 7,000

568 Alan M. Bateman.

feet, a maximum difference of 2,000 feet. Probably other de- posits would have been found at lower depths were it not that in this district the lower slopes are heavily mantled by debris and forests.

These differences in elevation, greatly exceed the depths at- tained in any of the mines, and the ore deposits, obviously, are not restricted to a narrow vertical zone. Neither can they be con- sidered the erosion remnants or roots of veins, in the light of the differences in elevation, for if so the roots have a vertical range that exceeds that of most known veins; the fortuity of the eleva- tion of the outcrops has already been remarked.

It therefore appears that the physical’ conditions at an eleva- tion of 4,000 feet were just as favorable for ore formation as those at 7,000 feet and that within the district there was a ver- tical range of 5,000 feet congenial for ore formation. Conse- quently, increase of depth alone appears insufficient to explain why an orebody outcropping at say 7,000 feet in elevation should dis- appear at 6,500 or 6,200 feet above sea level. Some other ex- planation than the playing out of the veins must be sought.

The answer, I think, is that the fissures do not play out but that the ore shoots do, and that a clear distinction has not been made between the ore shoots and the fissures that contain them. Un- fortunately, it was impossible to examine personally as many of the lowest levels as could be desired, but those I did see and the accounts I gathered of others indicate that although the bottom of workable metallization was reached, the fissures themselves, and with some mineralization, still continued below. The ques- tion, then, might properly be raised whether other ore shoots may be expected at greater depth.

It has already been pointed out that, in the Slocan district more than one shoot has been found along a fissure in a horizontal di- rection and, in one or two instances, in depth. The intershoot intervals, from the little that is known of them, are only slightly mineralized or barren, and are longer than the shoots themselves. In this district, where the fissures are known to extend horizon- tally a few thousand feet, and where there is some evidence of

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Silver-Lead Deposits Of Slocan District. 569

repetition of ore shoots, added to the fact that both fissuring and metallization are shown to have a vertical range of several thou- sand feet, it is not unlikely that other ore shoots may occur in strong fissures below the present bottoms of the mines, but at widely spaced intervals. There is little actually known about the possibility of deeper ore shoots and the above suggestion can be considered only from theoretical grounds. The evidence bearing on this may, then, be examined.

The ore shoots that already have been mined - bottomed are those which chance erosion revealed at the surface, under condi- tions for ready discovery. Most discoveries occurred at the higher elevations where the bedrock is fairly well exposed. Prob- ably other ore shoots outcrop on the lower slopes but have re- mained hidden because of the heavy mantle of debris and forests that clothe them. Had erosion in Carpenter Creek been less deep many ore shoots would be buried beneath the surface and only the barren fissures above the shoots would outcrop. Had erosion been deeper, it is not unlikely that deeper shoots would be revealed on the same fissures that are now abandoned, provided the fissures are strong and continuous. The expectation of deeper shoots would be slight in the case of mineralized joints, or fissures that are short or turn along beddings. But with long fault fissures that occur at the higher elevations and that continue strongly be- yond or below ore shoots, there seems a reasonable expectancy of other ore shoots at depth. The deeper development to date does not disprove the possibility, for practically no reliable deep tests have been made. That fissures extend to lower elevations has already been shown, and that there is a vertical range of metalliza- tion of several thousand feet has also been pointed out. Should a fissure with an ore shoot that outcrops at 7,000 feet be strong enough to continue to an elevation of 4,000 feet, then that same fissure is just as likely to have another shoot at 4,000 feet as is a nearby fissure that contains an ore shoot outcropping at the same elevation. To test this feature requires boldness and cap- ital, both of which have not been over-abundant in the past. The tendency, naturally enough, has been to explore only those

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570 Alan M. Bateman.

fissures in which ore shoots cropped at the surface, and never to let exploratory work get far from the ore.

If the above possibility of deeper ore shoots exists, then one may reasonably ask why should not outcropping barren fissures also contain ore shoots at depth, on the assumption that the for- tuity of erosion has revealed only an unmineralized interval? This is a possibility, in so far as strong continuous fissures are concerned, although the expectation of shoots in such fissures would not be so great as in the case of fissures that are already known to have been the channelways for the metallizing solu- tions. They offer possible prospecting ground, however, and where they occur within the reach of a diamond drill, might jus- tify the gamble of some exploration work from underground workings where depth already has been attained.

Origin Of Ore Deposits.

The remarkable dependence of the ores upon the proximity of the Nelson batholith has already been pointed out, as has also the time interval separating the batholithic intrusion from the vein formation. It follows, therefore, that the metals were not ex- haled directly from the granodiorite that outcrops adjacent to the veins and that they came either from the unconsolidated interior of the batholith or from the same source that supplied the igneous rocks. Consequently, the vehicle by means of which they were transported to their present resting place must have been either magma, gases, or aqueous solutions.

There is nothing to suggest that the ores may have come in as amagma. Also, the minerals of the deposits are not diagnostic of high temperature gaseous conditions, although it is probable that in the first part of their upward journey the solutions were in a gaseous state, but in the colder rocks above they became aque- ous. The kinds of minerals, the structure of the vein fillings, and the evidences of replacement that will be mentioned later, are features usually considered to be characteristic of deposition from aqueous solutions. The substances that the solutions carried, and the fact that they were able to dissolve quartz, limestone,

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Silver-Lead Deposits Of Slocan District. 571

slate, and porphyry, indicate that they were hot, though their tem- perature was not high because the minerals are all characteristic of deposits formed at intermediate temperatures and pressures. The solutions must also have been attenuated—at least thin enough to penetrate dense and relatively impervious slates and to travel through microscopic cracks.

Two processes entered into the formation of the ores, 1, re- placement, and 2, direct deposition in open cavities. Evidence of the latter is to be seen where thin lens-like veins are banded by successive crusts of quartz, sphalerite, siderite, and galena, each with inward projecting crystals, and the centers of which may contain vugs, lined by crystals of galena. There is no evidence in the larger veins to suggest cavity filling, and I doubt if wide fissures could remain open in such yielding rocks. The cavity filling mode of deposition is relatively insignificant and most of the ore has found its resting place by replacement of the country rock. Evidence of this may be seen both by the naked eye and by the microscope. All steps may be traced from incipient re- placement along veinlets to more massive replacement in which rounded and angular residuals of unreplaced rock are completely enclosed by ore. The preserved structure of the slates may occa- sionally be seen in the uncrushed fine-grained, galena-blende ores. Large cubes of pyrite were observed with tightly fitting contacts in massive and uncontorted slate near the Lucky Jim mine—a condition unsusceptible of explanation except by simultaneous solution and deposition during which the growing pyrite cube re- placed the slate particle by particle. Also in the Alamo and Sil- versmith mines replaced porphyry was observed in which unre- placed grains of the porphyry remained in the ore. The Alamo occurrence is also of interest because it shows sharply angular residual nuclei of fine-grained porphyry inclused by blende and galena. In advance of the more massive ore, cubes of galena, similar to the pyrite cubes mentioned above, made a place for themselves in the porphyry. The above features leave no doubt

17See “Angular Inclusions and Replacement Deposits,’ Alan M. Bateman, Econ. GEOL., vol. 19, 1924, pp. 504-521.

572 Alan M. Bateman.

that the process of replacement has operated in the deposition of the ores.

The sequence of the deposition of the minerals in which the later ones replace the earlier formed minerals and in which there is a certain amount of overlapping and reversal in deposition, does not necessarily imply successive pulsations of solutions of different chemical content. Rather, it may be considered to have been brought about by changing concentrations of the different components in the same solution. The relative concentrations of say lead and zinc in a solution will determine whether zinc or lead will be deposited. The deposition of one would change the relative concentration and yield conditions suitable for the dep- osition of the other. Accession of new material, dissolving of wall rocks, or other causes, might change again the respective con- centrations, perhaps reversing once more the order of deposition. In this way may be explained the corrosion, overlapping, reversal of order, or simultaneous deposition of several ore minerals from one solution.*®

18 For further discussion of this see “ Primary Chalcocite, Bristol Copper Mine,” Econ. GEoL., vol. 18, 1923, pp. 158-166.

YALE UNIVERSITY, New Haven, Conn.

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Deformation In Ores, Coeur D’Alene District, Idaho.

W. A. Waldschmidt.

Introduction.

Tuis article deals with the secondary deformation of some galena ores from the East Hecla, Star and Morning mines, of the Coeur d’Alene district, Idaho. The specimens were collected by Dr. Waldemar Lindgren, to whom acknowledgment is due for notes on the occurrence of the specimens. The deformation by pres- sure causes a flow structure in which the galena acts as a plastic mass containing fragments, frequently rolled and transported, of the other harder minerals.

As well known, similar “schistose” or “ gneissic” or “pressed ”’ ores in which the original texture has been more or less lost have been described by W. H. Emmons,’ W. Lindgren and J. D. Irving,’ and by W. L. Uglow,* but such described in- stances are few and the following description may be of some interest.

The material examined came from the East Hecla, the Star, the Morning, and the Interstate Callahan mines. The general association of minerals is that common in the district, that is, siderite, quartz, occasionally barite, galena, sphalerite, tetrahedrite (with freibergite), and less commonly pyrite, pyrrhotite, and chalcopyrite. Ransome in 1908 called attention to the fact that “the fine-grained, so-called ‘ steel galena,’ which in many places is clearly the result of crushing and recrystallization of coarser

1 Economic GEoLoecy, vol. 4, 1909, pp. 755-781.

2 Idem, vol. 6, 1911, pp. 303-313.

3 Idem, vol. 12, 1917, pp. 643-662.

4F, L, Ransome and F. C. Calkins, “The Geology and Ore Deposits of the Coeur d’Alene District, Idaho,” Prof. Paper 62, U. S. Geol. Survey, 1908, pp. 108, 91, 112, and 168.

: ‘4

574 W. A. Waldschmidt.

galena along recent fissures, is richer in silver than the rest of the ore.” He also says that “much of the massive granular galena of the Last Chance Mine exhibits a faint banding and rough schistosity which is probably also due to pressure. . . . The galena has in part an irregular banded structure due to the produc- tion of lamellar twinning by pressure.” None of this banded ore is, however, described or figured. He further says that much of the ore of the Morning Mine carries galena of a fine-grained texture.

It may be concluded that the fine-grained, crushed galena occurs quite generally in most of the veins of the district, though by no means universally. The typical ore is quite coarse-grained and exhibits the usual depositional succession of siderite, pyrrhotite, sphalerite, and galena. The fine-grained galena is not continu- ous nor is there often any persistently visible schistose or banded structure.

The Hecla East Vein.

The Hecla East vein, or the Russell vein as its surface part was called, is now the property of the Hecla Mining Company. The vein outcrops at an elevation of 5,261 feet or about 1,400 feet above the river level at Burke. The lower part has been opened by a branch of the Hecla No. 3 tunnel from the level of the main Hecla workings (elev. 3, 816), and by incline to a depth of goo feet below this tunnel. It is, therefore, known over a vertical interval of about 2,300 feet. The strike of the vein is N. 75° W., and its dip 70° S. S. E. At tunnel No. 3 the vein splits in two or three branches, which continue to the goo—foot level. The vein is from 1 to 20 feet wide and is entirely con- tained in quartzite of the Burke formation. The Hecla East vein is cut by a fault supposed to be the O’Neil (strike N.—-S., dip moderate towards the west) which seems to be an overthrust with a horizontal displacement of 80 feet and a vertical throw of not less than 200 feet.

An important ore shoot was mined from the lower part of the Hecla East vein from an elevation of 4,250 to 3,600 or 200 feet

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Deformation In Ores. 575

below the No. 3 tunnel. On the west it was terminated by the O’Neil fault; to the east the vein tends to pinch. The shoot intersects the lower side of the fault for 1,000 feet in length; west of this the vein again tends to pinch.

The stope length varied from 100 feet near the top to 500 feet on No. 3 tunnel. At the fault the ore was partly oxidized at least to a depth of 1,200 feet below the surface. Otherwise only sulphide ore was seen. Undoubtedly oxidation penetrated deeper along the fault than elsewhere. In the upper section of the vein complete oxidation reached at least to 350 feet below the surface This ore shoot was unusually rich in silver as well as in lead. Some stopes averaged 35 ounces of silver per ton and much higher assays were often obtained. The width averaged about 6 feet and was in places up to 20 feet. The quartzite is crushed, con- tains many gouge seams, and the lead stringers widen and con- tract irregularly. Below No. 3 tunnel the ore became poorer and stringy and the silver assays approached the average for the dis- trict, 7.e., a few ounces per ton. On the hanging side of the fault the ore shoot was not found.

Throughout the shoot, except close to the fault, the ore is un- oxidized. It is massive and irregular, never drusy, and evi- dently of the kind formed by replacement. There is much fine- grained (steel) galena, but in many places also coarse aggregates. The gangue is siderite with occasional horizontal veinlets of quartz. There is much dark brown sphalerite, and more or less pyrite, pyrrhotite, tetrahedrite, and chalcopyrite. The unusual richness in silver is in part due to the presence of tetrahedrite (or freibergite), and boulangerite.

Examination In Polished Sections.

In order to avoid repetition the principal characteristics of the minerals will be first described.

Galena is the most common sulphide. Much of it is coarse with cleavage faces 2 to 4 millimeters square, but more generally it is finer-grained and of the kind known as steel galena. The steel galena is an intimate mixture of predominant galena with

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576 W. A. Waldschmidt.

more or less sphalerite, tetrahedrite, pyrite, gangue, and other minerals present in the ore.

Boulangerite occurs in small lens-shaped masses several milli-

ga meters in length in the planes of flowage or schistosity of the ‘af galena. In polished sections it is distinguished from galena by isi its slight creamy or yellowish color, and is readily perceived when si the galena is etched wth HCl, FeCl; or H:O.. The reactions were thought to be more like those of boulangerite than jameson- ™ ite but without an analysis the positive identification of the vari- ae ous lead sulphantimonides is always a matter of some difficulty.

Microchemical tests showed that the mineral contained some it

silver, copper, and arsenic, besides the main constituents of lead, ‘e antimony, and sulphur. For further remarks on the occurrence

of boulangerite see below. 7 Tetrahedrite is abundant in some parts of the vein and occurs hs

always in small particles which have resulted from breaking and di

replacement. It was impossible to make microchemical tests on

the tetrahedrite for silver, because the particles are so small that sas

they can not be cleanly separated from the other minerals. Tetra-

hedrite is difficult to distinguish in the hand specimens, but its ia

presence is sometimes shown by the blue color of some surfaces, we due to the alteration of the tetrahedrite to covellite. Freibergite occurs in small particles throughout the ore, and is .

closely associated with the tetrahedrite. It can not be readily de- ° tected in polished sections unless it is first etched with nitric acid 4 th or potassium cyanide. These reactions distinguish it from tetra- 4 hedrite. d th Sphalerite occurs in relatively small amounts. It is massive i th and usually in small particles which have resulted from the crush- : O ing in the ore body. The breaking of the sphalerite is quite clearly shown by the numerous small particles which have been - broken away from and remain near the larger particles of sphaler- ‘ ite. Pyrrhotite occurs massive and usually is quite abundant. It ee is often extremely broken into small particles and occurs scattered ge

throughout the ore, and is also strung out in somewhat parallel fe rows in the galena flowage zones.

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Deformation In Ores.

. Pyrite occurs massive and quite abundantly, especially where the ore comes in contact with the gangue. Where it occurs in the galena ore it is usually in small individual cubic crystals. Many of the larger crystals have been broken and subsequently filled with galena and gangue. The smallest individual pyrite crystals were found included in the boulangerite lenses.

Chalcopyrite is quite common and occurs in stringers and large masses, replacing and cutting the other minerals of the ore. It is abundant near the contact of the ore and gangue.

Covellite was observed only in the partly oxidized ore where it occurred in very small amounts, replacing the galena or tetra- hedrite.

Cerussite occurs abundantly in the oxidized portions of the vein, both massive in the limonite-stained crushed ore and as well- defined crystals in vugs. The massive cerussite shows that it is a direct alteration product of galena.

Limonite occurs finely divided in the oxidized portions of the vein and is the result of the oxidation of the pyrite and pyrrhotite.

The non-metallic gangue minerals are quartz, siderite, calcite, and chlorite. The latter occurs only where crushing and slipping’

has been greatest.

In describing the various ore samples, it has been found con-

venient to discuss them in descending order of their occurrence in the vein, describing first, those of the upper levels, and lastly, those of the lowest level.

1. Sample from the 30th Floor above No. 3 Tunnel Close to the Fault.—On this level, the ore has been largely removed. On the east side, the vein pinches to a two-inch stringer of quartz. On the west, the vein is cut by the O’Neil fault.

The ore is a high grade silver ore, assaying 55.2 per cent. lead and 32.8 ounces of silver. Only poor sections could be obtained, because the ore is oxidized, but even these show interesting fea- tures. The most characteristic feature is the alteration of galena to cerussite, which could be seen in all stages, from nearly pure galena with a small amount of cerussite or all cerussite with a few tiny residual specks of galena. Where alteration has been

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578 W. A. Waldschmidt.

active, the ore has a sooty appearance due to the presence of covellite. Other parts of the ore contain well-developed cerussite crystals in vugs, while most of it is stained yellow by limonite. Sphalerite occurs sparingly as residual rounded particles. Pyrite usually occurs as unaltered individual crystals in the quartz, but some smaller crystals occur in the galena. As no tetrahedrite or chalcopyrite were observed, the small amount of covellite replac- ing the galena may represent their alteration product.

The high silver values can only be accounted for by assuming that it is included in the remaining cerussite, and partly altered galena, as a result of the oxidation of freibergite, and silver- bearing boulangerite, of which only a few minute particles re- main. Just in what state it is can not be determined in this sample, but very likely it is metallic or as cerargyrite and so finely divided that it can not be observed in polished sections.

2. Sample from the 31st Floor of Raise 68.—This is a rela- tively high grade ore, assaying 20 ounces of silver and 4o per cent. lead. It is a typical steel galena consisting of galena, gangue, pyrite, tetrahedrite, pyrrhotite, sphalerite, chalcopyrite, and freibergite. All of these minerals, except some of the galena and the chalcopyrite, appear to be primary because they all show the effects of breaking. The rounded particles of sphalerite, the borders of many tetrahedrite particles and the gangue appear to be replaced by galena, which indicates galena of a later age than that present before the breaking and crushing of the vein. Pyrite occurs in small individual crystals throughout the ore. Pyr- rhotite is also quite abundant and is extremely broken. Chal- copyrite is the latest mineral to be deposited in this ore and is seen cutting all of the other minerals present.

The high silver content of this ore can be assigned to the frei- bergite which is present in small particles having the same ap- pearance as the tetrahedrite. Tetrahedrite is quite abundant and possibly some of the silver may be included in it. No boulanger- ite was observed in the sample.

3. Sample from the 18th Floor above No. 3 Tunnel.—This specimen is a fine-grained steel galena, which shows distinctly the

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Deformation In Ores. 579

effects of crushing. Minor fractures planes are present and are somewhat slickensided from slight movement. The ore also shows the effect of percolating solutions which in places have re- moved the galena, sphalerite, and softer gangue minerals, leaving the pyrite with a honeycomb structure. Besides the gangue min- erals, pyrite, pyrrhotite, and galena are the most abundant, with sphalerite, tetrahedrite, chalcopyrite, and freibergite in smaller amounts. Hand specimens do not show any marked structure, ; but the polished sections reveal a definite flow structure, in which the broken particles of the primary minerals are suspended in the flowage zones of the galena.

This ore is also quite high grade, assaying 17 ounces of silver

and 19.1 per cent. lead. The silver can again be assigned to the freibergite and possibly partly to the tetrahedrite. The ore is so extremely crushed that little can be said definitely about the order of deposition of the minerals. There are, how- ever, indications of two ages of galena, and the chalcopyrite again shows clearly that it is secondary and is the last mineral to be de- posited.

4. Sample from the 18th Floor above No. 3 Tunnel, close to foot of the fault. This ore is a steel galena with an unusual amount of pyrrhotite. Between the quartz and chloritic gangue, and the galena ore is a considerable amount of siderite. The ore itself shows a roughly banded structure of alternating bands of galena, pyrrhotite, and chalcopyrite. In polished sections, this banded flow structure is much more pronounced. (See Plate II. A.) Tetrahedrite, pyrrhotite, pyrite, and the gangue min- erals all show an arrangement of their broken particles in some- what parallel rows in the flowage zones of the galena. Often the tetrahedrite particles are completely surrounded by broken parti- cles of pyrrhotite in the flowage zones and this is conclusive proof that the tetrahedrite is a primary mineral. Galena also shows a second stage of deposition in that it replaces the siderite, and sphalerite. Tetrahedrite is very abundant, especially where the pyrrhotite occurs. In these portions the broken particles of tetrahedrite and pyrrhotite are cemented together by a ground-

j

580 W. A. Waldschmidt.

mass of galena. Chalcopyrite again appears to be the latest min- eral deposited and can be seen cutting the other minerals present. In this sample the replacement, of galena and siderite by chalco- pyrite is very distinct, and also the alteration of tetrahedrite to chalcopyrite as described under the minerals.

Freibergite is present and occurs associated with the tetra- hedrite. Other than this, no more silver minerals were found in the sample. No assay values were given for this sample, but it is probably of the same grade as the sample from the 18th Floor, just described.

5. Sample from Stope, 16th Floor, above No. 3 Tunnel.—This specimen proved to bé of considerable interest because of the abundance of silver-bearing boulangerite. The boulangerite is confined entirely to the flowage or schistosity planes of the rather coarse galena and is, therefore, of secondary origin. If cut perpendicular to the flowage, the boulangerite appears as elongated oval areas arranged roughly end to end, but if cut parallel to the flowage, the areas of boulangerite exposed are much larger: This indicates that the boulangerite occurs in roughly lens-shaped areas in the flowage planes. Slight subsequent movement may have aided in the formation of these lenses. The galena of this sample is much coarser than that of the other ore specimens but it shows a very definite flow or schistose structure. Pyrite, sphalerite, chalcopyrite, and tetrahedrite are nearly absent in the coarse galena but near the contact of the galena and gangue, they appear more abundantly. Near the contact the galena also be- comes much finer grained.

The silver in this specimen can be assigned entirely to the boulangerite because it is silver-bearing, and occurs in consider- able quantities.

At the 16th Floor above the No. 3 tunnel, the vein is strong, averaging about three feet in width. Toward the east it fingers out into poor seams while toward the west it reaches a width of 15 to 20 feet, and is especially large below the O’Neil fault which cuts it off.

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Deformation In Ores. 581

6. Sample from the 600—Foot Level.—On the 600-—foot level, 600 feet below the tunnel level, the vein is divided and the ore is not of good grade. The sample examined is another typical steel galena, formed by intense crushing. It breaks freely along sur- faces which have been formed by minor slipping and along these, descending solutions have deposited a thin layer of clayey ma- terial, and has caused some of the tetrahedrite to be altered to covellite which gives the surface a bluish-tint. Flow structure shows up well in the polished sections, but it is obscured in the hand specimens because of the intense crushing which took place. Galena is the chief mineral and in it are suspended the broken particles of the other minerals. Tetrahedrite is abundant and has been extremely broken and partly replaced by galena and chalcopyrite. Chalcopyrite again appears as a secondary mineral of later deposition than the others. Sphalerite occurs more abundantly and in larger masses than in the ore farther up in the vein. It shows characteristic breaking due to movement and also rounding of particles due to replacement by galena. Pyrite and pyrrhotite are also abundant, and are broken into small particles. The pyrite also occurs in small cubic crystals.

Neither boulangerite or freibergite were noticed in this ore.

Paragenesis Of Hecla East Ores.

The general succession of minerals appears quite normal. In specimens not affected by pressure it may be determined as (1) (Oldest) pyrite; (2) pyrrhotite; (3) chalcopyrite, tetrahedrite, and sphalerite; and (4) galena.

When movement took place along the O’Neil fault and pos- sibly along other minor slips, the ore body was greatly crushed, during which time the galena acted as a putty-like mass for the suspension of broken fragments of the harder minerals. This breaking resulted in the formation of the typical steel galena. At the same time, numerous cracks were formed in the quartz and siderite, and a chloritic rock and gouge were developed along the larger planes of movement.

The arrangement of the mineral fragments shows clearly that

: : ae

582 W. A. Waldschmidt.

the galena acted as a plastic mass and also shows conclusively the primary character of some of the minerals. The broken pyrite crystals, the particles of tetrahedrite, freibergite, sphalerite, and pyrrhotite all show a stringing out and a definite arrangement parallel to the flowage of the galena. The primary character of the tetrahedrite is more clearly brought out by the fact that some of the particles are completely surrounded by broken pyrrhotite particles. (See Plate. II. B.)

After the breaking and crushing of the ore body, mineral bear- ing solutions could circulate through it freely. This resulted in a deposition of the silver-bearing boulangerite along with very small pyrite crystals, in the flowage or schistosity planes of the coarser galena.

Two ages of galena are also evident. First, the galena of the original ore body which was deformed and pressed by the dynamic movements ; second, a later stage, probably soon after the break- ing occurred, which can be seen replacing the siderite and sphaler- ite. This second generation of galena has also filled some of the cracks in the gangue which were caused during the faulting. It is assumed that these stringers or veinlets of galena are later than the original galena because they do not show transverse cracks or displacements which would surely show if they had been pres- ent before movement took place in the vein.

Some chalcopyrite was deposited later than the faulting of the vein and was the last of the secondary sulphide minerals to be precipitated. It is seen cutting and replacing the galena, tetra- hedrite, sphalerite, and siderite, and filling many small cracks, especially in the gangue.

Slight movement took place subsequent to the deposition of the chalcopyrite and developed small cracks which have since been filled with calcium or lead carbonates. These small stringers of carbonate can be seen cutting across chalcopyrite stringers, sphalerite particles and across the general direction of flowage of the galena.

Later, oxidization took place, chiefly along the fault plane, re- sulting in the alteration of (1) galena to cerussite; (2) pyrite

é : “3

Economic GEOLoGy. VOL.

A. Steel galena, East Hecla mine. Boulangerite lenses in galena. Galena, BI—Boulangerite. 4o.

B. Steel galena, East Hecla mine. Boulangerite lenses in galena. Gn— Galena, Bl—Boulangerite. 4o.

C. Steel galena, East Hecla mine. Note arrangement of mineral particles parallel to flow structure. T—Tetrahedrite, Gn—Galena, Sph—Sphalerite, Pyrr— Pyrrhotite, Py—Pyrite. x 4o.

D. Steel galena, East Hecla mine. Note pyrrhotite particles surrounding a tetrahedrite particle. T—Tetrahedrite. Large dark gray masses, quartz. Small light gray particles, pyrrhotite. Matrix, galena. X 100.

PLATE I. x x.

A Sph

PLATE Il. Economic GEOLOGY. VOL. XX.

A. Steel galena, Morning mine. Note broken hard minerals in galena. Black particles, quartz; light gray particles, pyrrhotite; matrix, galena. X 40.

B. Steel galena, Morning mine. Illustrating flowage around a quartz grain. Black particles, quartz; matrix, galena; light gray particles, pyrrhotite. Xx 40.

C. Steel galena, Star mine. Illustrating replacement of sphalerite by galena. Sph—Sphalerite, Gn—Galena. 4o.

SEP 15 1925F g

Deformation In Ores. 583

and pyrrhotite to limonite; (3) tetrahedrite and chalcopyrite to covellite; and (4) the oxidization of freibergite and boulangerite leaving the silver finely divided in the oxidized ore.

The Morning Vein.

The Morning vein is quite sufficiently described by Ransome.*® The principal minerals are generally deposited by replacement of quartzite and gangue. They comprise siderite, barite, quartz, pyrite, sphalerite, and galena. Commonly the galena is pressed to curly aggregates and to steel galena and the above described phenomena of pressure are observed. The sphalerite is crushed and strung out in aggregates of fragments. Tetrahedrite, pyr- rhotite, and chalcopyrite are rare. The paragenesis of the normal ore is (1) gangue; (2) pyrite; (3) sphalerite; and (4) galena. The Morning ores are poor in silver. The galena contains micro- scopic inclusions of argentite (?), and some small veinlets of native silver were observed. Also a late mineral was noted in one place which was identified, with some doubt, as polybasite. It was thought that some small particles of boulangerite were present but about this there is also some doubt. Figs. 4 and B, Plate II., illustrate the appearance of the pressed ore from the Morning Mine.

The Star Vein.

The Star vein lies a short distance north of the Morning vein, It is a small, poorly defined, stringy and metasomatic vein con- tained in Revett quartzite. It is likewise poor in silver. Sphaler- ite, pyrite, and galena are the principal minerals. The latter is mostly present as steel-galena showing the usual effects of press- ing but including few other crushed minerals, except sphalerite which is embedded in the galena in coarser aggregates drawn out in streaks parallel to the structure.

Effects of Differential Pressure—It is evident that long after the primary vein structure was completed, mainly by replacement, the vein matter was subjected to differential pressure. As Adams

- and Bancroft have pointed out this resulted in deformation of

5U. S. Geol. Surv., Prof. Paper 62. 6 Journal Geology, vol. 25, 1917 p. 637-

ee :

584 W. A. Waldschmidt.

the softer minerals by movement due to slipping within the con- stituent crystals or their gliding planes. It is, therefore, essen- tially a plastic flow. In harder minerals, up to 5 in the usual hardness scale the deformation is accompanied by granulation, the texture developed being similar to that found in mylonite. The first effect on these harder minerals such as pyrite, pyr- rhotite, tetrahedrite, and sphalerite was a crushing and the result- ing grains were caught up in the movement of the softer galena and rolled along as in a plastic flow.

In the present case no extensive recrystallization of these harder minerals seems to have taken place, and the pressure seems to have been less than, for instance, in the case of Rammelsberg where the sphalerite also suffered extensive slipping on its gliding planes and was drawn out to aggregates similar to those of the galena.

There was, however, also some recrystallization as shown by the second generation of galena and chalcopyrite, and the evidence seems clear to the writer that the boulangerite was also formed during the deformation in flat aggregates parallel to the gliding planes of the galena. Therefore, as the boulangerite contains a considerable amount of silver, there was also some secondary concentration of the latter metal during the pressing. But no new tetrahedrite or freibergite appears to have been formed.

It is reasonable to attribute the differential pressure to move- ments in connection with the extensive faulting which occurred in the district long after the epoch of mineralization, and is is probable that this movement took place under no excessive load.

Long afterwards oxidation penetrated to a depth of several hundred feet, and even more along the important fault planes. This oxidation was accompanied by the development of cerussite and other oxysalts and also by the formation of a moderate amount of covellite. The silver was converted to native metal and chloride and, especially in the deepest zone of oxidation there took place a notable enrichment.

The shoot in the Hecla East Mine is dependent upon the local, hypogene development of tetrahedrite and freibergite, but the shoot was in part enriched by processes accompanying the dyna-

ren

Deformation In Ores. 585

mometamorphic movement, and still more by supergene processes accompanying the oxidation and taking place directly under the oxidized fault gouge.

Remarks On The Lead Sulphantimonides.

It has frequently been noted that minerals like jamesonite, boulangerite, and geocronite, are among the very latest formed in any given paragenesis. This is, for instance, very marked in the Bolivian silver-tin veins. Lately, Earl V. Shannon’ has done much work on the, lead-sulphantimonides and has shown that boulangerite is the most common of these minerals. He gives the formula of boulangerite as 5 PbS. 2 Sb.S;._ Recently he has shown that a mineral provisionally named stibnite by Ransome from the Gold Hunter Mine in the Coeur d’Alene district was really boulangerite. He has also found that boulangerite occurs in the Wood River district, Idaho, in the Star and Independence mines. He believes that it is a primary, though the latest, min- eral. He has also shown its presence in ore from the Bunker Hill and Sullivan mines in the Coeur d’Alene district, and points out a similar occurrence in the Przibram mines in Bohemia.

In this connection mention should be made of a lead-sulphanti- monide determined as “‘ geocronite” in galena from Tintic, Utah, by Whitehead and Means.* It is well possible that this “ geo- cronite”’ may really be boulangerite as no quantitative analysis could be made of it. It occurs in galena of the Colorado Mine occurring close to the oxidized ore. It replaces galena along the cleavage planes and forms feathery aggregates. '

In the Eagle and Blue Bell Mine of the same district a veinlet of “ geocronite”” was found cutting across a vein of anglesite. From these occurrences it was held that this lead sulphantimonide was formed, with covellite, during the processes of oxidation.

It would, therefore, seem as if lead sulphantimonides can be

7 Proc. U. S. Nat. Mus., vol. 58, 1920, pp. 589-607. 8 W. Lindgren and G. F. Loughlin, “ Geology and Ore Deposits of the Tintic

District, Utah,” Prof. Paper 107, U. S. Geol. Survey, 1919, p. 163, and Pl. XXIX,, E.

:

586 W. A. Waldschmidt.

formed (1) during the very latest stages of hypogene vein for-

mation; (2) during differential pressure and deformation in

primary veins containing lead and antimony [if the writer’s inter-

pretation is correct]; and (3) during some stages of oxidation

where oxygen was in scant supply; perhaps also in the zone of

the supergene sulphides below the zone of complete oxidation. LABORATORY OF Economic GEOLoGcy,

Mass. INSTITUTE OF TECHNOLOGY, CAMBRIDGE, Mass.

Be :

rie

Some Magnetite-Hematite Relations.

Geoffrey Gilbert.

DurinG the past few years there have appeared several papers dealing with various phases of the question of the relations of the iron oxide minerals magnetite and hematite in ores in which they occur together. Broderick,’ for instance, has discussed the micro- scopical relations of the two minerals as exhibited in specimens from several different localities, and Gruner? has described vari- ous types of replacement of one by the other in the Mesabi ores. In the course of studies made recently in the Harvard laboratories during the preparation of a doctor’s thesis, I did some work along similar lines; and this paper is a record of observations and con- clusions resulting from a microscopical study of the two minerals in ores of contact metamorphic and hydrothermal origin. No attempt is made to cover the subject completely, and some of the facts set forth are merely confirmations of those already pub- lished. It is hoped, nevertheless, that the notes and discussion which it contains will be of sufficient interest to justify their publication. IDENTIFICATION OF THE MINERALS.

Magnetite is described as grayish white under the reflecting microscope, but when perfectly homogeneous it usually has a brownish tinge, sometimes with a suggestion of purple. Its hardness is high but somewhat variable, and it can generally be scratched with a sharp needle. It is practically negative to the ordinary reagents. It is somewhat soluble in HCl but rarely shows etch structures; a solution of SnCl, in hot HCl, however, affects it more strongly. The powder adheres to a magnetized

1 Broderick, T. M., “ Some of the Relations of Magnetite and Hematite,” Econ.

Geot., XIV., 1919, pp. 353-366.

2Gruner, J. W., “ Paragenesis of the Martite Ore Bodies and Magnetites of the Mesabi Range, Econ. Geou., XVII., 1922, pp. 1-14.

588 Geoffrey Gilbert.

needle. It is generally found in subhedral grains, but sharp octahedra are not rare, and various modes of association with hematite, to be described later, also occur.

Hematite is pure white or bluish white under the microscope, with hardness high, and is negative to all the common reagents. Where it can develop its own crystal outlines the form is charac- teristically platy, and the majority of the plates, being inclined to the plane of the section, appear as laths. It is distinctly harder than magnetite and ‘shows the higher relief where the two occur together.

The two minerals are thus readily distinguishable one from the other, and in most specimens of the type with which I worked no other minerals likely to cause confusion are found. Ilmenite is rarely present in contact metamorphic or hydrothermal deposits. Limonite is gray, usually dark gray, and easily recognizable. Goethite is said by Davy and Farnham to be gray and chemically like limonite, and “turgite” to be much lighter in color than limonite.* The manganese oxides are all listed by Thiel* as grayish white, except pyrolusite which is brownish white. In a few very badly oxidized specimens which I examined a grayish white supergene mineral was observed which appeared to be an iron hydrate with less water than ordinary limonite, but it is not common. No manganese minerals were definitely identified in my specimens.

Age Relations.

The identification of magnetite and hematite is a compara- tively simple matter. The determination of their relative ages, where they occur together, usually involves but little difficulty, but in some cases there is an element of doubt. They do occur together very frequently. It is rare to find hematite completely free from magnetite, and while magnetite more often occurs with-

3 While the work of Posnjak and Merwin has discredited limonite as a distinct mineral species, it remains a useful term to cover the amorphous hydrated ferric oxides.

4 Thiel, G. A., “Manganese Minerals: their Identification and Paragenesis,” Econ. Geor., XIX., 1924, pp. 107-145.

J y ;

Magnetite-Hematite Relations. 589

out hematite it very commonly shows at least the beginnings of hematitic alteration. The associations of the two minerals may thus be divided into two main classes: (1) those in which the magnetite is the older and is altering to hematite, and (2) those in which the hematite is the older and is altering to magnetite. The first class has been well described by Broderick,® and the paragraphs immediately following, though based primarily on my own observations, are largely a reiteration of Broderick’s re- sults.

Magnetite the Older—Magnetite, whether found in large masses or disseminated, usually occurs in rounded grains, roughly equidimensional, each grain surrounded by other similar grains, by sulphides,.or by gangue. The individual grains are generally broken by irregular cracks. When hematite replaces magnetite it ordinarily forms along the grain borders and the cracks, work- ing inward with a smooth front which shows no dependence on the crystal structure of the magnetite. When such relations are observed in polished section, there can be no reasonable doubt that the magnetite is the older and that the hematite replaces it. When there is only a very small amount of hematite it may be scattered in minute dots along the edges of the grains.

In many specimens, however, quite a different relationship is seen. The hematite appears in small, clean-cut, parallel plates, running in two, three, or four directions through the magnetite and undoubtedly oriented by the octahedral planes of the magnet- ite. The relations are very much like those often seen in titanic ores, in which ilmenite or some other titanic mineral is similarly oriented in a magnetite host. Warren,°® after studying these titanic ores, decided that they were intergrowths formed by the unmixing in the solid of an originally homogeneous mixture, an interpretation with which my very limited study of such ores leads me to agree. In the case of the magnetite-hematite struc- tures, however, a replacement origin seems more probable. The

5 Op. cit. 6 Warren, C. H., “On the Microstructure of Certain Titanic Iron Ores,” Econ. XIII., 1918, pp. 419-446.

ae ny: kg

:

590 Geoffrey Gilbert.

two cases are not entirely similar. In the titanic ores the ilmenite plates are scattered through the magnetite with remarkable even- ness ; in magnetite-hematite ores, on the other hand, the hematite plates are erratic in their distribution and are usually most abun- dant toward the edges of the magnetite grains.

Some cases have been seen in which a magnetite grain, whose center is completely unaltered, is replaced around the edges by solid hematite, while there is an intermediate zone consisting of magnetite filled with minute hematite plates. This does not seem to be a case of the separation of a homogeneous solid into two components. It bears some analogy to the zoning seen in some plagioclase crystals, which is considered to be the result of changes in the solutions during the growth of the crystal.. Conditions, however, are quite different, for the magnetite has grown by the replacement of some pre-existing solid and not by solidification from a melt. Further, the zoning is seen only occasionally, is erratic in distribution, and the hematite forms not only around the borders but in cracks in the magnetite, so that it appears that it is a matter of ordinary hydrothermal replacement. That is, it is reasonably certain that the hematite has formed by replacing magnetite and not by growing around it. If the hematite in these peripheral bands is replacing magnetite, it is to be supposed that the hematite plates scattered through the magnetite near the borders are also replacing it.

In a few cases the hematite neither follows the octahedral planes of the magnetite nor works from definite cracks, but pene- trates the magnetite in a delicate and complicated network which can only be described as an intergrowth. Even this, however, seems to be a replacement effect.

Hematite the Older.—In practically all the ores studied by Broderick the original mineral is magnetite. Dr. Alfred Wandke, formerly of Harvard, was the first, so far as I know, to investi- gate the replacement of hematite by magnetite. He discovered several cases—in ores from Nacozari, Virgilina, and elsewhere— in which hematite is reduced to magnetite, and when I commenced to work on the question I was given the benefit of his results.

é — Ky

PLATE Ill.

Economic GEoLoGy. VOL. XX.

A. Marginal replacement of magnetite (gray) by hematite (white). 14th level, Lowell mine, Bisbee, Ariz. X 80.

B. Hematite plates largely replaced by magnetite (dark gray). Black is gangue. Bright Diamond mine, Ouray, Colo. X 235.

C. Linear replacement of hematite plate by magnetite. 12th level, Pileares mine, Nacozari. X 8o.

“D. Magnetite (mg, light gray) replaced by limonite (Im, dark gray) and con- : is taining veinlets and irregular grains of hematite (white). Ragged hematite prob- & aoe ably supergene. Open cut, Shannon mine, Morenci. X 80. ee

—

Magnetite-Hematite Relations. 591

Hematite, except when the crystals are so crowded together that they interfere, nearly always exhibits its characteristic platy form, and the plates appear in polished section as needles or laths, with two parallel edges. Magnetite sometimes replaces these laths by working inward from the borders, but frequently it forms well within the interior. A lath whose outer portion con- sists of unaltered hematite may have its center almost entirely re- placed by magnetite, which occurs in rather irregular streaks parallel to the long edges of the laths. When this replacement is far advanced there may be a noticeable loss of volume, revealed by irregular cracks filled with gangue—quartz, calcite, or some- times siderite.. When it is not far advanced all that can be seen is a few small patches of magnetite scattered here and there through the interior of the plates.

In the latter case there may be no definite evidence that the magnetite is the younger. It is quite possible that it may have formed simultaneously with or even earlier than the hematite; in fact that is the explanation sometimes advanced. Gruner,’ study- ing such relations in some of the Mesabi ores, even concluded that he was dealing with a crystal which had formed as hematite, had altered completely to magnetite, and then altered back almost completely to hematite. However, it seems certain that for some unknown reason magnetite may and often does form by replace- ment of the interior of plates of hematite, leaving the marginal portions intact. The same sort of thing takes place in the re- placement of sphalerite by chalcocite, and of garnet by chlorite or sulphides ; in fact it is a well-known phenomenon.

In some specimens the edges of plates of hematite are broken into minute saw-teeth by a row of very small octahedra of mag- netite. It is noteworthy that in this case the magnetite manages to develop its own crystal form, partly at the expense of the hematite and partly at that of the surrounding gangue.

7 Op. cit.

: :

592 Geoffrey Gilbert.

Discussion.

A few general statements may appropriately be made here. The idea that magnetite and hematite can form solid solutions, advanced by Sosman and Hostetter as a result of chemical anal- yses of crystals of Elba hematite, has been destructively criticized by Broderick,® who showed that their conclusions, whether cor- rect or not, were certainly not borne out convincingly by the evi- dence they advanced. The existence of such solid solutions would seem to be still unproven and not very probable. Magnetite varies slightly in color, as might naturally be expected, since it is isomorphous with other members of the spinel group and the color differences presumably reflect slight differences of composi- tion. There seems to be no reason to suppose, however, that these color differences are caused by the presence of excess ferric oxide in the magnetite. As for hematite, when it replaces mag- netite it is usually quite white (in reflected light). The platy hematite is likewise usually white, but it frequently has a bluish- gray tinge. It may be that this is due to the presence of ferrous oxide, but there is at least one other possible explanation which should be taken into account. Several specimens which I have examined, which seemed at first to be nearly solid hematite, very much off-color, gave appreciable amounts of water in the closed tube. In many cases incipient hydration is at least as plausible an explanation of the bluish tinge as the presence of ferrous oxide.

The main difficulty in deciding these questions lies in the fact that the relations are very local and not at all uniform even in a single specimen. A single field of the microscope may contain a grain of unaltered magnetite, a plate of unaltered hematite, and a plate of hematite: almost entirely replaced by magnetite. Or, one side of the field may be mostly hematite and the other side mostly magnetite. There is, therefore, no guarantee that the

8 Sosman, R. B. and Hostetter, J. C., “ Zonal Growth in Hematite and its Bear- ing on the Origin of Certain Iron Ores,” Trans. Am. Inst. Min. Eng., LVIIL., 1918, PP. 434-444.

9 Op. cit.

Magnetite-Hematite Relations. 593

material on which chemical tests are made is identical with that studied under the microscope. The change from one mineral to the other involves, of course, merely the addition or removal of a small amount of oxygen, which may be a purely local affair. Another point on which I can corroborate Broderick is the question of magnetic hematite. Much hematite is magnetic, but so far as my experience goes all such hematite has more or less magnetite scattered through it. The quantity of included mag- netite seen under the microscope may sometimes appear to be too small to account for the magnetic properties ; but, considering the local variations just referred to and the difficulties of estimating the amount of magnetism, the accordance seems satisfactory.

Supergene Hematite.

One of the most perplexing problems in the study of the two oxide minerals is the determination of the time at which the re- placement of one by the other took place. Suppose, for example, the polished section shows a grain of magnetite partly altered to hematite. Even if it be possible to state definitely the age of the original magnetite, it is frequently impossible to say when the hematite formed, and unfortunately it is generally arguable that the change may be due to surface oxidation. In fact, there is a tendency in much of the literature to assume that such is the case. Again and again hematite is listed as a product of the supergene alteration of magnetite.

Moore cites the experimental work of Spring and Wittstein as showing that iron hydroxides can dehydrate and become crys- talline at ordinary temperatures and pressures. Van Hise was of the same opinion, though he regarded the more important hematite deposits as formed in the zone of anamorphism. Gruner ” advocates a supergene origin for some of the Mesabi

10 Moore, E. S., “ Occurrence and Origin of Some Bog Iron Ore Deposits in the District of Thunder Bay, Ont.,” Econ. Grot., V., 1910, pp. 528-537.

11 Van Hise, C. R., “ Treatise on Metamorphism,” U. S. G. S. Mon. 47, 1904, p.

12 Op. cit.

504 Geoffrey Gilbert.

hematite, and remarks that “of course it has long been known that limonite changes to hematite by simple cold dehydration.”

On the other hand, it is the view of many geologists that specular hematite, which in its broadest meaning can include all pure crystalline hematite, is not a supergene product. The iron ores of Daiquiri, Cuba, have been much discussed in this connec- tion. The hematite which makes up a large part of the Daiquiri ore is in part pseudomorphous after magnetite. Lindgren and Ross were of the opinion that it has formed chiefly by surface oxidation of the magnetite, principally because it is more abun- dant in the upper workings than in the lower. Singewald,™* on the other hand, believed it to be hypogene, and Roesler,*® who made the most detailed study, was also of that opinion. In the discussion following these papers, emphasis was laid on the fact that the oxidation, if it is supergene, has taken place to a depth of several hundred feet, a depth at which the sulphides associated with the magnetite are perfectly fresh. Considering the ease with which most sulphides are known to oxidize, and the inert- ness of magnetite exposed to oxidation in placers and beach sands, this seemed unreasonable, and the balance of evidence appeared to favor the belief that the hematite is hypogene.

I have examined a few specimens from Daiquiri, thanks to the kindness of Dr. Lindgren, and a good many specimens from other hematite-bearing deposits of high temperature origin. In this work the question of the hypogene or supergene origin of the hematite arose again and again, and it was only after much painful weighing of the evidence that any conclusions were reached. These conclusions, while they make no claim to final- ity, have become fairly definite, in my own mind at least, and they are given for what they may be worth.

The iron oxide which forms as a result of the surface oxida- tion of sulphides is limonite; that is, a more or less hydrous

13 Lindgren, W. and Ross, C., “ The Iron Deposits of Daiquiri, Cuba,” Trans. A. I. M. E., 53, 1915, pp. 40-66.

14 Singewald, J. T., Jr., Discussion of Roesler’s paper.

15 Roesler, M., “ Geology of the Iron-ore Deposits of the Firmeza District, Oriente Province, Cuba,” Trans. A. I. M. E., 56, 1916, pp. 77-127.

te q

Magnetite-Hematite Relations. 595

oxide. It may be any color from yellow through all shades of brown to red. Under the reflecting microscope it may be vari- ous shades of gray or even grayish white, but it never has the pure white color or the platy form of specularite. I have never seen any platy specularite or any pure white hematite which gave the faintest indication of having been formed by the surface oxi- dation of sulphides or of any mineral except magnetite.

Of the hematite which replaces magnetite, I think that over- whelmingly the greater part is hypogene. Much of it is found well below the water level, in the presence of perfectly fresh sul- phides, and while it usually forms around the borders of mag- netite grains and along cracks in them it does not show the de- pendence on major fractures which one would expect from sur- face alteration at low pressures.

Magnetite is fairly resistant to limonite alteration, much more so than pyrite. Hematite is much more resistant even than mag- netite. It is common in oxidized specimens to see magnetite crystals with their borders replaced by hematite and their interiors by limonite. It is easy in such cases to assume that both hematite and limonite are supergene, but as a rule a more reasonable ex- planation is that the hematite is hypogene and the limonite is re- placing the magnetite selectively.

A further point may be touched on now, though I hope to de- velop it more fully in a later paper. There is a broad general connection between the amount of hematite in an ore and the amount of sulphides. The replacement of magnetite by hematite is most vigorous in ores which are sulphur-poor; in pyrrhotite ores, so far as my experience goes, it is not found at all. Now there is no obvious reason why the presence or absence of sul- phides (and especially of pyrrhotite) should have any effect on the mode of alteration of magnetite under surface conditions, but there is a very obvious connection if the hematite is formed at high temperatures by the ore solutions themselves.

These arguments, of course, do not prove that all hematite is hypogene. Some of my specimens from Cananea contain white hematite which can hardly be interpreted otherwise than as a

596 Geoffrey Gilbert.

product of surface alteration, and one or two from Morenci are in the same category, but in each case it is amorphous, structure- less stuff which bears little resemblance to that which I interpret as hypogene.

To sum up, I do not believe that platy (micaceous) hematite is ever supergene. I do not believe that true hematite forms as a product of surface oxidation after any mineral except magnet- ite, and only very rarely after magnetite. The ordinary decom- position product of magnetite is a hydrous oxide, but it does ap- pear that under some conditions—such possibly as a dry and very hot climate, which would produce warm concentrated vadose solu- tions—hematite may form.

LeuicH UNIVERSITY, BETHLEHEM, Pa,

iz

‘ £

Editorial

New Methods For The Study Of Granitic Intrusives.

In recent years Professor Hans Cloos, of the University of Breslau, has been developing, with the aid of his assistants and students, new methods for the study and interpretation of a number of the great intrusive masses of granite in central Eu- rope. The strongly gneissoid granites of the oldest formations have not been objects of investigation because the foliation is quite certainly the result of pressure and shearing which operated long after the time of consolidation. These later rearrangements disguise or destroy the phenomena which may be detected in in- trusives of a less complicated history, and which are the ones rec- orded and interpreted. Professor Cloos has developed a tech- nique which involves, both in methods and results, much that is of interest to American students of the later intrusives, the more because these bodies are so often associated with ore deposits. For example, in several European instances, which had been re- garded as batholiths,—that is, as great masses which had risen from the depths, forcing and stoping an essentially vertical way upward, and which were believed to maintain a series of succes- sive cross-sections in depth, similar to the outcrops exposed at the surface,—Professor Cloos has concluded that they are actually great sills which had come up along faults and had then turned sidewise so as to cover many square miles, although derived from comparatively limited vents. Instead of everywhere extending to vast depths, the granite throughout the larger part of the area may be only two or three thousand feet or less in thickness, and may rest with an intrusive contact on older, supporting strata. Fundamental to a grasp of the methods and their significance is a conception of the uprising granite magma, as a fluid mass,

598 Editorial.

propelled by a tectonic pressure from behind, a vis a tergo, which, as the magma cools and passes through a viscous stage into a finally crystallized solid, manifests itself in structures, sometimes rather obscure and elusive, sometimes outstanding and easily recognizable. The direction from which has come the predomi- nating pressure is the influential feature, and when it has once been determined, we may interpret the consequent line of move- ment and the relations of the intrusive to its wall, alike on the sides, below and above. An observer cannot credit to an up- rising batholith a widely spread body of igneous rock, whose structural lines of movement are linear, are in part horizontal, and are predominantly parallel in direction. A sill or a laccolith alone meets the conditions.

Professor Cloos seeks, records, and plots on his areal map the following phenomena, noting, where possible, the strike and the dip of the particular structures, the larger number of which are genetically involved with the flowing movement of the crystalliz- ing magma. A “ Streckung” or stretching of certain minerals may be observed, although at times obscurely. Thus, in a mica granite, the mica scales have parallel’ orientation. The longest dimensions of the feldspar crystals may be parallel among them- selves and with the mica, so that the so-called “ stretching” re- sults. When plotted it has been found to be impressively uni- form in direction, and is believed to indicate the direction of movement of the magma. There are, however, at times local phases of the granite in which the massive texture may make the stretching phenomena obscure. Platy minerals, such as mica, which have their flat sides parallel in arrangement, possess not only a linear strike but, unless horizontal, an inclination or dip. If we have a strike to the north because of a propelling force operating from the south in a north direction, we may have a dip to the east or to the west. In a particularly favorable case, the granite at Strehlen, Silesia, Professor Cloos discovered that with a strike of the stretching, running north, the dip on the east side was to the east, and on the west side, to the west, while in the middle of the mass the stretching was horizontal. The dips,

f

Editorial. 599

therefore, were anticlinal in their large structural relations, and the intrusive must have had, at its upper surface, some such shape in its original, uneroded, and deeply buried state as an arch, or elongated dome. This is described as “ Aufwoelbung ” or dom- ing. A most interesting point in all these relations will appear when jointing is taken up below.

The granites may also manifest a platy structure which is co- incident with the stretching and which is called “ Schieferung ” by Professor Cloos. Gneissoid structure, or schistosity, is not meant by his use of the term, but flow-bands which are sometimes emphasized by the “ Schlieren ” or streaks of a mineralogy which is contrasted with that of the general rock. More basic streaks are darker; more siliceous ones, lighter, in color. Flowage lines (Fliess-gefuege) are also indicated in porphyritic varieties by the alinement of the feldspar phenocrysts. The long axes of the phenocrysts run parallel with one another and with the linear di- rections already described. Besides the phenocrysts, xenoliths may be strung out in the same directions and with the same sig- nificance.

Another group of phenomena is specially revealed in the quar- ries and is the cleavage or “ Theilbarkeit” of the granite. In the geological term often used in America, it is called “ rifting.” (See, R. S. Tarr, ‘The Phenomena of Rifting in Granite,” American Journal Science, April, 1891, 267.) The American quarry-men speak of the “rift” and the “lift” for the two easy cleavages, parallel in direction, but the first approximately vertical and the other horizontal. The third cleavage, at right angles to the rift and the lift, is much less perfect. At Cape Ann, Professor Tarr records that it was called the “ cut-off,” but at Barre, Vt., Dr. Robert Balk has found the workmen naming it the “hard way,” as it was the hardest of the three to develop. The two easy cleavages lie parallel with the flow lines, the “ hard way ” across them.

Under jointing, or Klueftung, Professor Cloos groups first a ‘series of joints which strike with the stretching and therefore with the “ schlieren” (a word now quite well naturalized in the English language) and with the flow-lines, but which are found

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600 Editorial.

to always dip at right angles to the dip of the “ stretching” or Streckung. The joints are evidently of later origin than the other three phenomena already mentioned, and must date from the stage of consolidation. Since the cracks strike in the direc- tion of the tectonic pressure, there was nothing to prevent them being open fissures and such they prove to be on study. If we imagine a cross-section of the igneous mass at right angles to the line of flow, we would see upon it the lines of intersections made with it by the joints. On account of the anticlinal relations of the dips shown by the stretching, the intersections of the joints would have a radiating or fan-shaped arrangement. As a matter of great structural interest, Professor Cloos has observed that in these longitudinal joints are found the aplite dikes, the pegmatite dikes with their extremes, the quartz dike-veins, sometimes with metallic ores of characteristic pegmatitic affinities, and even the later basic dikes which so often cut bodies of granitic intrusives. All these are grouped as Gaenge. We can justifiably picture to ourselves, as we would say using the descriptive terms current in North America, the juices of the magma in the last stages of consolidation, exuding into these fissures and supplying the aplites, pegmatites and quartz: veins. Latest of all, the final basic dif- ferentiates or dregs of the magma discover the same paths of weakness for their travels to the upper world. We may even perhaps detect an explanation of composite dikes, if we admit the migration along the same paths, of successive, deep-seated dif- ferentiates with contrasted compositions.

From what has been said of the radiating dips of these longi- tudinal joints, and the complementary relations of their dips to the dips of the stretching, it follows that where the stretching is obscure, its dips may be inferred to be at right angles to those of the better displayed joints.

In addition to the longitudinal and gaping joints, it is possible at times to observe cross-joints, which strike at right angles to them and, therefore, to the line of application of the impelling force, or pressure. As might be anticipated, the cross-joints are tight.

The longitudinal joints sometimes pass into faults (Verschie-

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Editorial. 601

bungen) as is shown by the occasional slickensides (Rutschstrei- fen) on their walls. The displacement is, however, slight.

The methods of Professor Cloos require much patient work because the plotted records may reach up into the thousands. In one great area of Silesian granite, he and his assistants recorded upwards of 50,000 observations, but they obtained some very striking and instructive structural results. In-general, it may be said that for a number of the later granites which were of a char- acter to lend themselves to the studies, sills and laccoliths were indicated rather than batholiths. In two ways the conclusions affect inferences regarding the size and persistence of veins. Ex- perience in America has already shown that after-effect veins in sills and laccoliths are not often of persistent and valuable char- acter. Certainly deep-seated batholiths are more favorable. On the other hand, as has been remarked by Professor Cloos, veins older than sills and laccoliths, and already developed in forma- tions known to lie beneath them, might continue and be of value despite the presence of the overlying igneous rock. By it the veins would only be cut off above, not necessarily on the strike or in depth.

Dr. Robert Balk, one of Professor Cloos’ assistants, has now been a year or more in the United States, and has made a study of the granites at Barre and Bethel, Vt., Quincy, Mass., and Westerly, R. I., all of which have periods of intrusion, pre- sumably in the Paleozoic. His maps appear highly significant. Dr. Balk set forth the methods at the Ithaca meeting of the Geo- logical Society, last December. It is to be hoped that the New England studies will find an avenue of publication. The writer is greatly indebted to Dr. Balk from whom in many discussions of the subject much that is suggestive and illuminating has been gained. A few citations are given below.’

James F. Kemp.

1 Hans Cloos, “ Tektonik und Magma,” Untersuchungen sur Geologie der Tiefen. Band I. Abhandl. der Preussichen Geologischen Landesanstalt. Neue Folge. Heft 89, Berlin, 1922. A general statement and introduction by Professor Cloos is followed by the descriptions of eight special cases by himself and his students.

Hans Cloos, “ Das Batholithenproblem.” Berlin, 1923. Hans Cloos, “ Granitgeologie und Lagestaeten. Stahl und Eisen,” 1924. No. 24.

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Discussion And Informal Communications

Angular Inclusions In Ore Deposits.

Sir: The article on the above subject by Alan M. Bateman, that recently appeared in this journal,’ invokes the following dis- cussion: Inclusions of rock and mineral fragments are a well- known feature of igneous rocks and of ore bodies. These in- clusions are often angular and may be so separated from one an- other as to indicate that they were completely enclosed by igneous rocks or by ore minerals.

The presence of such inclusions in igneous rocks offers no dif- ficulties since magma solutions are in the main mutually dissolved silicates, and as such have a comparatively high viscosity and are capable of buoying up rock fragments. Most ore depositing solutions however have been commonly regarded as highly aque- ous, dilute, and of low viscosity as compared with rock magmas. The presence in ores of angular, wholly enclosed fragments is therefore a more difficult problem. Spurr has interpreted such fragments as evidence that the ore solutions were concentrated and possessed of a viscosity comparable with that of magmas. Bateman has marshalled the evidence to show that wholly en- closed fragments, even when angular, very commonly are rem- nants, isolated through replacement by solutions which were probably highly aqueous and of low viscosity.

It is perhaps worthy of mention that in some ore deposits a third alternative exists, namely the supporting of fragments by

1“ Angular Inclusions and Replacement Deposits,” by Alan M. Bateman, Econ. GEOL., vol. 19, pp. 504-520, 1924.

2Spurr, J. E., “ Ore Magmas,” McGraw-Hill Co., New York, 1923, chap. 2. 3 Loc. cit.

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Discussion And Informal Communications. 603

solutions that are highly viscous and perhaps highly aqueous and yet are of lower specific gravity than the fragments they support. Solutions that are in the gelatinous state are of this type. There is ample evidence which need not be detailed here of the occasional existence of silica gels in mineral veins. They may be of pri- mary as well as secondary formation. There is evidence also that certain ore textures, notably rhythmic banding, are the re- sult of diffusion, presumably through some once gelatinous medium, that has since crystallized. Where such gels exist they may conceivably support fragments of greater specific gravity than themselves. How aqueous such natural colloidal solutions are we do not know, but the experiments described below in which silica jellies containing 95 per cent. water supported galena fragments may be taken to represent the extreme of this method of support.

In the first experiment the gel was allowed to set and then two pieces of galena were placed on its surface. The gel supported these fragments and there was no indication of settling even after several days. In the second experiment two pieces of galena were suspended by cords in the still liquid silicic acid. The gel on setting completely surrounded the galena fragments, after which the cords were cut. The gel held the inclusions sus- pended without any sign of their sinking during the time of the

experiment which extended over two weeks.

It is evident from these experiments that silicic acid of high water content can support mineral inclusions of considerable spe- cific gravity. Insofar as highly aqueous gels are formed in the early stages of primary mineralization their components must be supposed later to crystallize slowly with loss of water, but the crystalline framework thus formed may still serve effectively to separate the enclosed fragments.

Also in accordance with the principles of dialysis, if colloids were present in the mineralizing solutions, there would be a tend- ency for them to be left behind in the diffusion of the solutions through the wall rocks. Thus a concentration of colloidal ma- terial in the fissure vein might go hand in hand with extensive

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604 Discussion And Informal Communications.

penetration and alteration of the walls by the non-colloidal por- tions of the solutions. C. A. MERRITT.

University Of Chicago,

The Geological Age Of The Homestake Ore Bodies.

Sir: Judging by recent geological reports on the Homestake deposits there exist two rather definite but widely divergent opin- ions regarding their geological age. Paige,* of the U. S. Geo- logical Survey, considers them Pre-Cambrian while Hosted and Wright,’ geologists of the Homestake Mining Company, believe that they were formed in the Tertiary period at the time the Tertiary rhyolites and porphyry were intruded. Having had an opportunity to see something of these deposits, at the surface and underground, during a visit to this camp in the summer of 1923, I would like to add to the discussion certain conclusions reached as a result of my observations of them. My first conclusion supports that of Hosted and Wright regarding the Tertiary age of the ores in the Pre-Cambrian rocks as well as those in the over- lying Paleozoic strata. The close relationship of the intrusions to the ore bodies, illustrated by their proximity to one another and the influence of impervious slates and quartzites in damming back solutions from the intrusions, supports the view that the ore minerals came from the Tertiary intrusions. A specimen secured from the 1,000-foot level of the Pierce ore body illustrates very nicely the relations which can be observed at a number of points in the mine between the rhyolite and the ore. It shows that the ore in that section at least is later than the rhyolite as it has been injected into the rhyolite and it surrounds fragments of both

1Sidney Paige, “ The Geology of the Homestake Mine,” Econ. Geot., vol. 18, Pp. 205-237, 1923. Also, “ Geology of the Region Around Lead, S. D., and its Bearing on the Homestake Ore Body,” U. S. Geol. Surv. Bull. 765, 1924.

2J. O. Hosted and L. B. Wright, “ Geology of the Homestake Orebodies and

the Lead Area, S. D.,” Eng. and Min. Journal-Press, vol. 115, pp. 793-799 and 836-843, 1923.

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Discussion And Informal Communications. 605

brecciated rhyolite and schist. The presence of free gold and of tellurides of gold in the ore bodies of both Pre-Cambrian and Paleozoic formations favors a common origin and although the relative proportions of these minerals vary in the different for- mations this variation may be accounted for by differences in temperature of the depositing solutions and differences in compo- sition of the wall rocks. The pyrite in the Cambrian conglomer- ate is undoubtedly of igneous origin and most of the gold in this formation is probably of the same type.

A study of the paragenesis of the ore minerals gave results similar to those which Paige obtained. The order of deposition found was: arsenopyrite, pyrrhotite, pyrite, gold. Hosted and Wright consider the pyrrhotite and pyrite as contemporaneous. A number of assays made by K. C. Gray, one of my students, showed that the pyrrhotite carried more gold than the arseno- pyrite. This is contrary to the results secured by Hosted and Wright. One should probably conclude from these varying re- sults that conditions vary in different parts of the ore bodies.

E. S. Moore.

UNIVERSITY OF TORONTO, Toronto, ONTARIO.

Shattering By Replacement."

Sir: In the course of his studies by means of polished sur- faces of ore deposits, the writer, on seeing crystals of pyrite shattered and veined by the younger sulphides, has ascribed this shattering to a period of shearing which took place after the de- position of the pyrite and before the deposition of the younger sulphides. Recent studies have shown that shearing need not be necessary to produce a shattered crystal of pyrite but that this shattering can result from replacement.

In looking over some vuggy specimens of Butte ore a number of tarnished crystals of pyrite were seen projecting into the open spaces. The tarnishing suggested a coating of bornite, and to

1 Presented before Society of Economic Geologists, New York, May, 1924.

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606 Discussion And Informal Communications.

satisfy the question as to the depth of this coating, crystals were broken out of several different vugs, mounted in sealing wax and polished. On examining the polished surface of the pyrite it was surprising to find that the crystal instead of being merely coated by bornite was also intricately veined by bornite and a little chalcopyrite. (See Fig. 1.) The pattern of the polished

Fic. 1. Free-growing crystal of pyrite veined by bornite and chal- copyrite. Veinlets are due to replacement along crystallographic planes of weakness and do not follow cracks produced by shearing. 300.

surface was identical in every respect with those patterns hereto- fore thought of as due to shearing. In the case of these free growing pyrite crystals from Butte it is obviously impossible for the crystal to have been sheared, but the “shattering” appears to have been produced by the replacing solutions.

From the Caridad mine, Sonora, Mexico, a similar occurrence has been observed, only here enargite rather than pyite is the “shattered” mineral. The enargite, in part, occurs in unmistak- able crystals. A polished surface of some of these crystals shows the crystals to be coated by a thin film of tennantite and also to

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Discussion And Informal Communications. 607

be crisscrossed by countless tiny veinlets of tennantite and bornite which in part follow crystallographic directions of the enargite. Here again it seems out of question to think of these free grow- ing enargite crystals as having been “shattered” in any way other than by replacement.

Conclusions.—Possibly others than the writer have in the past explained the occurrence of one mineral veining another by sup- posing a period of shearing to have opened up a pathway for the later mineral. As a result of the above described observations, the writer is inclined to modify his views. Instead of crowding a number of periods of shattering into the history of an ore de- posit in order to get the veining relationships, it now seems prob- able that in many cases the minerals follow each other in orderly sequence, but that by replacement an earlier mineral may be “shattered” as though it had been sheared. Why an earlier min- eral is thus “shattered” in this way is hard to say. The explana- tion that comes to mind is that a given mineral or a group of minerals crystallizes under a certain set of temperature and pres- sure condtions. During the period of mineralization a change in temperature may occur and thus set up strains in the minerals already crystallized. As the chemical nature of the ore solutions changes, the minerals already crystallized may become susceptible to replacements, the replacing solutions finding their easiest path- ways along crystallographic directions, which are the natural loci of strains produced by temperature changes.

ALFRED WANDKE. GUANAJUATO, MEXICo.

Metal Content In Magmas.

Sir: In his interesting discussion of “‘ The Content of Metals in Intrusive Magmas ” in Economic GEo.oey, vol. XIX., 1924, page 89, Mr. Spurr refers to the foothill belt of the copper de- posits of the Sierra Nevada (Jackson folio) as being associated with quartz-porphyrite intrusions, “ which quartz-porphyrite he (Turner) lists as of the same general age as the granodiorite.”

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608 Discussion And Informal Communications.

I judge that Mr. Spurr is referring to the statement in the Jackson folio, as follows:

Some of the quartz porphyrite is, however, plainly later than the augitic porphyrite series, for at two points it was noted as unquestionable dikes in the series. One of these dikes is in the bed-rock (augite-porphyrite) of the gravel mine 2 miles east of Jenny Lind, and the other 3 miles southeast of Jenny Lind. The coarsely crystalline, granitoid quartz-por- phyrite at the western base of Gopher Ridge may likewise be later.

Whatever inference may be drawn from this statement, there was no evidence obtained that any of the porphyrites are of the same age as the granodiorite. The rocks called porphyrites at that time are metamorphic andesites and for that reason I sug- gested later that such rocks should be called meta-andesites and this suggestion was adopted by the Geological Survey. Most of these old andesites represent original surface flows but like the Tertiary andesites were accompanied by dikes from the same magma which in both cases intruded the surface flows of the same general age. The granodiorite of the Sierra Nevada, in so far as I have observed, is everywhere later than the old por- phyrites (meta-andesites) and this is likewise true of the gab- bros and peridotites and in fact of all the old granular igneous rocks.

H. W. Turner.

‘ Fey

Reviews

La Détermination des Plagioclases dans les Coupes Minces. By L. Duparc and M. Reinuarp. Mémoires de la Société de physique et (histoire naturelle de Genéve. 1924. Pp. 1-140.

Since the French have been pioneers in the development of optical methods in mineralogy, it seems appropriate that there should appear in the French language an attempt at a revision of all the known methods for the optical determination of the plagioclase feldspars. This is true even though the data for the work have been taken from Austrian publi- cations. The new curves and stereograms with which the book abounds have been based upon eight type feldspars taken from the contributions of F. Becke and his coworkers. These are as follows:

(1) Albite, Greenland. (O. Grosspietsch.) 0.5% An (2) Acid Oligoclase, Bamle, Sobot. (F. Becke.) 13.0%-An (3) Oligoclase, Bakersfield. (F. Becke.) 20.0% An (4) Basic Oligoclase, Twedestrand. (F. Becke.) 25.0% An. (5) Andesine, Hohenstein. (O. Grosspietsch.) 35.0% An (6) Labradorite, Various localities. (F. Becke.) 52.0% An. (7) Naradal, (F. 73.0% An.

(8) Anorthite, Vesuvius. (J. Kratzert.)

These eight points corresponding to the above percentages of anorthite are noted repeatedly on the various curves and stereograms throughout the work. The authors express regret that there is not yet available a greater amount of material for the more basic series of the plagioclases than is shown in the list.

With the optical and chemical data thus obtained, all the standard curves and stereograms are revised and many curves showing the re- lations of twinning laws less often used, have been introduced. A special feature of the work is the introduction of new curves and stereograms for the interpretation of the results obtained from the method of Fedorow.

The economic geologist as well as the petrographer has frequent oc- casion to make accurate feldspar determinations and he should not de- pend, as is often done, upon one or two methods. The section may not be favorable for these methods. With the large number of procedures

at of, 97.0% An. :

610 Reviews.

described in this work, the investigator should never be at a loss to orient his section properly and to fit it into the appropriate curve.

F. N.

Non-Metallic Minerals: Occurrence—Preparation—Utilization. By Ray- MOND B. Lapoo. McGraw-Hill Book Co., 1925, 675 pages.

This volume is essentially a short, concise statement of the composi- tion, properties, uses, specifications, values, and occurrence of the Non- Metallic minerals. In treating some of the subject matter, the author expands on the methods of mining and preparation, and the extent and nature of the markets. The material presented is not new, but is a valu- able compilation of the available data on the subject. Naturally in covering such a vast field in a single volume, considerable condensation was necessary. Each chapter is appended with a bibliography, and there is also a general one at the end of the volume. This in itself adds greatly to the usefulness of the book. The work is not a complete treatise on the subject, but as a reference book it will be found to have great value to any one interested in Non-Metallic minerals. No previous work has attempted the broad scope followed by the author, and he covers a field in which the need of such a general work has long been felt. The volume is well written in a clear concise manner affording easy reading. There is also an extensive index making the material easily available.

E. Grim.

Lead Deposits of Pend Oreille and Stevens Counties, Washington. By OraF P. Jenkins. Bulletin No. 31, Geological Series, pp. 146, 15 text figures, 4 plates. Division of Geology, Department of Conservation and Development, State of Washington.

This bulletin gives the results of two months of field study of the de- posits of this region, especially about Metaline Falls and Leadpoint (the most important deposits), supplemented by records of earlier studies in the same field. After brief preliminary chapters giving general facts regarding the mineralogy, distribution and uses of lead, and the geologi- cal history of the region, the geographical and geological features of the different districts are discussed, with a final chapter of conclusions and recommendations. A bibliography is included.

B. W.

ane Bes

Scientific Notes And News

Sir Edgeworth David has resigned as professor of geology and phys- ical geography in the University of Sydney, Australia, in order to de- vote his time to the completion of a work on the geology of Australia. He has completed thirty-three years of service with the University, which has conferred on him the title of emeritus professor of geology.

L. A. Cotton, acting professor of geology in the University of Sydney, Australia, has been appointed successor of Sir Edgeworth David in that office.

Clarence A. Fredell, of the geological department of the Verdi Copper Company, has resigned to become chief engineer for the American Smelt- ing and Refining Company at Angangueo, Mexico.

W. E. Cookfield, of the Canadian Geological Survey, with a party of students from the University of British Columbia, has been making a reconnaissance survey of the extreme northwestern part of British Columbia.

Olaf P. Jenkins has resigned his position as associate professor of economic geology in the State College of Washington, to take up work for the Nederlandsche Koloniale Petroleum Maatschappy, Sluisburg, Weltevreden, Batavia, Dutch East Indies.

E. F. Burchard, of the U. S-: Geological Survey, is spending four months in examining iron ore deposits in Argentina for the Argentine .Government.

H. C. Boydell delivered a series of lectures during the March-May term of the Massachusetts Institute of Technology on “ Colloid Chemistry Applied to Geology.”

E. T. Dumble, consulting geologist for the Southern Pacific Company and its subsidiaries since 1897, was retired June Ist, having passed the prescribed age limit for active service. :

Clyde E. Williams, of the Bureau of Mines station at Seattle, has re- turned from a trip to South America, where with H. Foster Bain he has been working for the Argentine Government.

Bancroft Gore, of Rapid City, South Dakota, is in South America making an examination of the lead mines in the Famatina district of the Province of Rioja, Argentina, which are now under development.

J. P. Dunlap, the head of the Metals Section of the Geological Survey’s Mineral Resource Division, will make Joplin his permanent headquarters in future.

E. H. T. Plant, of Queensland, Australia, a well-known mining man and one of the pioneers in gold mining in Australia, has recently retired from business at the age of more than seventy years.

612 Scientific Notes And News.

Frederick G. Clapp, who went to Australasia late in 1923, is still in New Zealand, mapping prospective oil fields for Australian companies.

S. P. Kinney has been put in charge of work on ferrous metallurgy in the Bureau of Mines, and will supervise the work at Pittsburgh, Minn- eapolis and Tuscaloosa.

C. E. Sims, of the Bureau of Mines, has been transferred from Seattle to Pittsburgh, where he will be in charge of the metallurgical section of the station.

H. I. Jensen, who has served as geologist for the Australian Common- wealth Government and on the Queensland Geological Survey, is spend- ing nine months in New Guinea as geologist for the Ormildah Oil Devel- opment Co., Ltd.

Alexander Anderson, of Fullerton, Calif., has invented and is operating some new devices for obtaining data on deep oil wells, also a new method of handling impression blocks and fishing tools.

Felix E. Wormser, assistant editor of Mining-Journal-Press, has re- signed in order to take up mining work as engineer for Col. H. H. Arm- stead of New York. He is now engaged in field examinations in Alaska and elsewhere with Colonel Armstead.

Daniel H. Braymer, editorial director of Industrial Engineer, a Mc- Graw-Hill publication, has resigned this position in order to give his time to the direction of the D. H. Braymer Equipment Company, re- cently organized, with headquaters in Omaha, Nebraska. He will con- tinue as consulting editor of Industrial Engineer.

R. I. Rutherford of the University of Alberta, with a surveying party, has been making a survey of the McLeod River, starting from Bliss, and working eastward.

William A. Haswell died recently in Sydney, New South Wales, at the age of seventy-one.

Donaldson B. Dowling, of the Canadian Geological Survey, died at his home in Ottawa, on May 26th, at the age of 67 years.

The Carnegie Institute of Technology and the Pittsburgh Experiment Station of the United States Bureau of Mines offer for the coming year four research fellowships in metallurgy. The research program for the year will be a continuation of the plan effected a year ago between Car- negie Tech, the Bureau of Mines, and an advisory board of men prominent in metallurgical industries in the Pittsburgh district.

John T. Lonsdale has been appointed associate geologist in the Texas Bureau of Economic Geology and Technology.

Warren D. Smith, professor of geology of the University of Oregon, has been giving a special course on the Geology of the Pacific in the University of Michigan Summer School.