Gold deposits in metamorphic rocks: Part I

DeWitt, Ed, Waegli, Jerome, Light, Thomas D., Brew, David A. dbrew@usgs.gov · 1989 · Report

Contents<br/> Gold in the United Verde massive sulfide deposit, Jerome, Arizona<br/> The Alaska-Juneau and Treadwell lode gold systems, southeastern Alaska

This 1989 document, Gold deposits in metamorphic rocks: Part I, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

Gold Deposits in Metamorphic Rocks-Part I U.S. GEOLOGICAL SURVEY BULLETIN 1857-D

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Chapter D Gold Deposits in Metamorphic Rocks-Part I Gold in the United Verde Massive Sulfide Deposit, Jerome, Arizona By ED DEWITI and JEROME WAEGLI The Alaska-Juneau and Treadwell Lode Gold Systems, Southeastern Alaska By THOMAS D. LIGHT, DAVID A. BREW, and ROGER P. ASH LEY U.S. GEOLOGICAL SURVEY BULLETIN 1857 GEOLOGY AND RESOURCES OF GOLD IN THE UNITED STATES DANIEL R. SHAWE and ROGER P. ASHLEY, Scientific Editors L.M.H. CARTER, Technical Editor

DEPARTMENT OF THE INTERIOR MANUEL LUJAN, JR., Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. UNITED STATES GOVERNMENT PRINTING OFFICE: 1989 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center Box 25425 Denver, CO 80225 Ubrary of Congress Cataloging-In-Publication Data Gold deposi~ in metamorphic rocks. (Geology and resources of gold in the United States) (U.S. Geological Survey bulletin ; 1857) Bibliography: p. Supt. of Docs. no.: I 19.3: 1857- D Contents: pt. 1. Gold in the United Verde massive sulfide deposit, Jerome, Arizona I by Ed DeWitt and Jerome Waegli. The Alaska-Juneau and Treadwell Lode gold systems, southeastern Alaska 1 by Thomas D. Ught, David A. Brew, and Roger P. Ashley. 1. Gold ores- Arizona-Jerome Region. 2. Gold ores- Alaska-Juneau Region. 3. Rocks, Metamorphic- Arizona-Jerome Region. 4. Rocks, Metamorphic- Alaska-Juneau Region. 5. Geology- Arizona-Jerome Region. 6. Geology- Alaska-Juneau Region. I. DeWitt, Ed. II. Geology Survey (U.S.) Ill. Series. IV. Series: U.S. Geological Survey bulletin ; 1857. QE75.B9 no. 1857 557.3 s (553.4'1'0973] 89-600195 (TN423.A75]

CONTENTS Gold in the United Verde massive sulfide deposit, Jerome, Arizona Abstract Dl Introduction Dl History D2 Production D3 Similar deposits D3 Acknowledgments DS Geology DS Stratified metavolcanic and metasedimentary rocks DS Intrusive rocks D8 Regional structure and age of metamorphism and deformation D9 Ore deposits DlO Structure DlO Wallrock alteration Dll Mineralogy D14 Lead and sulfur isotope data DlS Gold-silver zonation DlS Conclusions D20 Distinguishing characteristics D20 Ore controls D21 Origin D22 References cited D23 The Alaska-Juneau and Treadwell lode gold systems, southeastern Alaska Abstract D27 Introduction D27 History, production, and reserves D27 Comparisons with similar deposits D30 Alaska-Juneau system D30 Treadwell system D33 Conclusions D34 References cited D35 FIGURES Dl. Location map of the Jerome district, central Arizona D2 D2. Plots of production data for the lifetime of the United Verde mine, Arizona D6 Contents

03. Plot of gold grade versus copper grade for different time periods, United Verde mine D9 04. Plot of Ag:Au ratio versus tonnage for different time periods, United Verde rome DlO 05. Plot of gold grade and Ag:Au ratio versus Cu:Zn ratio for ore, 1944-1953, United Verde mine Dll 06. Map showing generalized geology of the Jerome region D12 07. Chemical classification diagram for volcanic units in the Jerome and Prescott areas Dl3 08. Map showing geology of the Unjted Verde mine area D14 09. Element-element plots for the 2400 level of the United Verde massive sulfide deposit D 17 010. Element-element plots in ore of the United Verde massive sulfide deposit D20 Dll. Index map of southeastern Alaska showing location of Juneau D28 012. Generalized geologic map of Juneau area showing locations of mines in the A-J and Treadwell systems D29 013. Cross section through the North ore body of the Alaska-Juneau mine D31 014. Diagram showing paragenesis of minerals in the Alaska-Juneau South ore body D32 TABLES Dl. United Verde mine production records D4 02. Average grade of ore types, United Verde massive sulfide deposit D16 03. Correlation coefficients data for metals in ore types from the Unjted Verde mine D24 Contents

GEOLOGY AND RESOURCES OF GOLD IN THE UNITED STATES Gold Deposits in Metamorphic Rocks-Part I Gold in the United Verde Massive Sulfide Deposit, Jerome, Arizona By Ed DeWitt and Jerome Waegli1 Abstract The Early Proterozoic volcanogenic massive sulfide deposit at the United Verde mine, Jerome, Arizona, is one of the largest massive sulfide deposits in North America and has produc~ more than 1.3 million troy ounces of gold. The unmined, zinc-rich part of the deposit is estimated to contain about 400,000 troy ounces of gold; the entire unmined part of the deposit may contain more than 1.0 million ounces of gold. The deposit is localized at the top of a quartz porphyry, a rhyolitic flow complex in the Cleopatra Member near the top of the Deception Rhyolite, a sequence of 1.76-1.78 billionyear-old rhyolitic flows, tuffs, and intrusive rocks. Alteration and mineralization styles in the deposit are typical of Archean and Proterozoic volcanogenic massive sulfide deposits in North America. From stratigraphic bottom to top, the ore body consists of a sericitic and chloritic alteration pipe in the Cleopatra, a zone of massive chlorite and stringer-zone ore rich in copper, pyritic massive sulfide ore that is zoned from a copper-rich bottom to a zinc-rich top, and chert-rich ore that caps the deposit. Alteration studies indicate that volume loss during chloritization and mass transfer of constituents during hydrothermal alteration in the chloritic alteration pipe have drastically changed concentrations of CaO, Na20, K20, Al20 3 , Si02 , volatile constituents, total iron, rubidium, strontium, cesium, uranium, thorium, manganese, scandium, zirconium, niobium, yttrium, tantalum, hafnium, and rareearth elements (especially light rare earths}. Sulfur isotope ratios of pyrite, sphalerite, and galena cluster between - 2.8 and + 0.2 per mil and indicate either an overwhelming contribution of igneous-related sulfur or derivation of sulfur Manuscript approved for publication February 18, 1989. 1Phelps Dodge Corporation, Morenci, AZ 85540. from restricted or stagnant bottom water that was not in contact with a sulfate-rich ocean reservoir. Lead isotope data are most easily explained by postulating a mixing of lead from sedimentary and igneous sources during evolution of the hydrothermal system. Gold within the United Verde deposit is concentrated in the pyritic massive sulfide ore and chert-rich ore; stringerzone (chlorite schist) ore contains very low grades of gold. Concentrations of gold for various ore types are: pyritic massive sulfide and zinc-rich massive sulfide ore, 0.02 to 0.06 oz Au/ton; chert-rich ore, as high as 0.15 oz Au/ton; chlorite schist, 0.002 to 0.006 oz Au/ton. Statistical analysis of different ore types indicates the best correlation of gold to be with combined copper and zinc. A major observation is that zinc-rich massive sulfide ore contains grades of gold as high as or higher than those of zinc-poor ore. INTRODUCTION The United Verde mine and the surrounding Verde mineralized district (Keith and others, 1983; Keith and others, 1984) are on the eastern slope of the Black Hills, near Jerome, Yavapai County, Ariz., 64 km southwest of Flagstaff (fig. D1) and about 140 km north of Phoenix. Small massive sulfide deposits extend 9 km south-southeast from Jerome, but more than 98 percent of the production from the Verde district has come from the United Verde and United Verde Extension mines at Jerome. Although other deposits will be referred to, only the massive sulfide deposit of the United Verde mine will be discussed in this report. Gold Deposits In Metamorphic Rocks- Part I

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Phoenix Wickenburg ~DeSoto mine crown King 112° Figure 01. Location map of the Jerome area, central Arizona. Mine localities shown are mentioned in the text. History Prehistoric American Indians and, later, Spanish explorers in the late 1590's knew of the deposits at Jerome and recovered some copper by surface mining, Geology and Resources of Gold In the United States but the modern history did not begin until 1876--1877 with the location of claims that would later become the holdings of the United Verde Copper Company (Hamilton, 1884; Rickard, 1918). Senator W.A. Clark of Montana purchased the United Verde holdings in 1888

and operated the United Verde mine until 1935, when Phelps Dodge Corporation purchased the property. The deposit at the United Verde mine, which consists of the mined ore and lower grade ore remaining in the ground, is the largest known massive sulfide deposit in the United States and one of the largest in North America. The mine closed in 1953, but has since been leased to several parties conducting small-scale surface mining and leaching. Production Production records of the Phelps Dodge Corporation indicate that from 1889 until 1974 the United Verde mine produced nearly 33,000,000 short tons of ore that yielded more than 2,921,000,000 lb of copper, 52,891,000 lb of zinc, 49,279,000 troy ounces of silver, and 1,350,000 troy ounces of gold (table D1). Unpublished data from the Arizona Bureau of Geology and Mineral Technology indicate that from 1884 through 1888 and in 1975 an additional 27,000 tons of ore was produced that yielded about 6,900,000 lb of copper, 324,000 oz of silver, and 4,200 oz of gold. Unpublished data from the same source indicate an additional 45,000,000 lb of zinc and 459,000 lb of lead were produced during the lifetime of the United Verde mine. From 1920 through 1940 the open pit operations produced 9,708,923 tons of ore that contained 674,734,000 lb of copper, 20,529,100 oz of silver, and 689,260 oz of gold (Alenius, 1930, 1968; fig. D2A). From 1900 through 1975 the silver grade of ore decreased from more than 4.0 oz Ag/ton to 1.0 oz Ag!ton (fig. 02 B). During the same time gold grade decreased from approximately 0.085 oz Au/ton to 0.010 oz Au/ton, resulting in an increase of the Ag:Au ratio from 25 to 75 (fig. D2 C). From 1884 through 1975 the ore averaged 1.49 oz Ag/ton and 0.041 oz Au/ton, and had a Ag:Au ratio of 36.3. From 1903 through 1953 the gold grade of the ore had a roughly positive correlation with copper content (fig. D3); the highest grade copper ores of 1903-1911 contained the highest gold grades, and the lowest grade copper ores of 1940-1953 contained the lowest gold grades. This correlation is partly the result of the mining of oxidized ores with high gold content early in the history of the deposit. Gold grades from 1960 through 1975 did not follow the rough positive correlation with copper content, as much of the material mined during that time was from mineralized quartz porphyry and black schist (hydrothermally altered quartz porphyry), and not from massive sulfide ore. The Ag:Au ratio of mined ore has varied over time and with the tonnage of ore produced (fig. D4). From 1901 through 1920, Ag:Au ratios increased with increasing underground production; as tonnage increased the gold grade decreased more than the silver grade (figs. D2A, D2 C). When open pit production began in 1920 and overtook underground production in 1922-1923, the Ag:Au ratio stabilized at 30-40 and was independent of tonnage produced through 1939 (fig. 04). From 1940 through 1944 the Ag:Au ratio increased to 50-70 and was inversely related to production; gold grades decreased more than silver grades (figs. D2 B, D2 C). When zinc production began in 1944-1945, the Ag:Au ratio stabilized at about 50 and was independent of tonnage produced. Notably, the gold grade increased slightly from 1945 through 1953 (fig. D2 C), and zincrich ore contained as much gold as did copper-rich ore (fig. D5). Because only the copper-rich part of the massive sulfide lens and surrounding rock was mined at the United Verde, a very large tonnage of low grade, probably subeconomic, ore remains in the ground. Reber (1922, 1938) estimated that only 14-20 percent of the original 90,000,000-ton body was of commercial grade (greater than 2 percent copper). Both Paul Handverger (written commun., 1974) and Paul Lindberg (written commun., 1977) estimated that the deposit contained 80,000,000 to 100,000,000 tons of mineralized material, 75 percent of which remains in the ground. If the areas indicated by Anderson and Creasey (1958, fig. 14) are used to calculate tonnages, approximately 115,000,000 tons of low-grade massive sulfide ore and 38,000,000 tons of mineralized black schist remain in the United Verde mine from the surface to the 4500 level. The grade of the massive sulfide material is unknown, but a conservative estimate, based on past production data, is 0.5-1.0 percent Cu, 2-4 percent Zn, traces of lead, 0.01-0.015 oz Au/ton, and 0.5-1.0 oz Ag/ton. Mcilroy and others (1974) indicated a resource of about 25,000,000 tons of ore that averages 0.52 percent Cu, 0.90 percent Pb, 4.72 percent Zn, and 2.05 oz Ag/ton, but they gave no values for gold grade. Using a cutoff grade of 4 percent zinc, Waegli and Jonathon Duhamel (unpub. data, 1981) estimated that the 500 through 3000 levels contain nearly 21,000,000 tons of ore that averages 0.52 percent Cu, 6.6 percent Zn, 0.61 oz Ag/ton, and 0.02 oz Au/ton. Similar Deposits The ore in the United Verde mine is interpreted to be a stratiform syngenetic deposit associated with submarine rhyolitic volcanism and hydrothermal alteration (Anderson and Nash, 1972). The ore body and associated volcanic rocks are very similar to Archean massive sulfide deposits of the Canadian Shield (Hutchinson, 1973; Sangster, 1980; Franklin and others, 1981) and the Miocene Kuroko deposits of Japan (Ishihara, 1974). In the southwestern United States similar deposits of Proterozoic age are noted near Pecos, N. Mex. (Krieger, 1932; Riesmeyer, 1978; Robertson and Gold Deposits In Metamorphic Rocks-Part I

Table 01. United Verde mine production records [Data from Phelps Dodge Corporation except as footnoted; published with permission. Unpublished production records for 1884-1888 and 1975 available only from Arizona Bureau of Geology and Mineral Technology. U.V., United Verde production; P.O., Phelps Dodge production; Big Hole, composite of various small-scale leased operations; blank, no record) Metals recovered Year Tons mined Lbs Cu Lbs Zn Oz Ag Oz Au Lbs Pb 1884-18881 26,900 6,869,075 323,726 4,242 10,000 1,910,890 154,260 9,697.5 26,000 5,257,421 423,714 37,000 7,360,000 50,000 9,845,666 109,878 2,442.7 50,000 9,222,,141 58,990 11,043,542 276,258 7,778.8 60,201 16,522,181 263,080 4,627.9 116,994 22,366,425 151,266 31,355,027 526,114 9,773.6 219,831 42,453,316 254,138 43,995,733 486,470 13,956.7 245,352 39,888,472 523,723 15,943 248,646 34,438,441 504,277 15,695 133,219 19,407,080 306,784 9,551 156,970 23,771,597 450,603 15,038 268,412 29,274,610 668,612 23,762 273,523 32,683,951 486,041 15,915 274,181 38,836,441 428,317 12,913 253,566 33,015,457 356,939 11,734 36,183,089 494,574 20,334 280,534 36,695,455 495,477 17,021 323,569 38,663,880 563,132 19,267 304,949 33,164,520 461,168 15,239 351 817 31,570,152 480,518 15,069 395,674 35,344,694 641,074 20,666 397,227 32,448,170 646,573 21,400 491,992 45,127,832 903,051 28,221 694,053 58,299,573 1,030,851 26,416 813,176 71,726,634 1,223,310 29,230 861,250 77,501,595 1,292,109 29,281 469,353 42,927,666 665,327 16,838 718,851 64,952,270 910,345 18,759 132,353 13,581,488 202,716 4,090 423,543 40,845,407 554,587 12,603 1,101,456 97,560,882 2,113,162 67,541 1,257,714 98,246,081 1,846,621 48,235 1,289,350 108,210,901 2,108,348 50,196 1,285,461 107,388,418 2,242,788 55,906 1,352,387 99,969,654 2,096,681 55,897 1,580,312 118,151,126 2,113,174 55,395 1,800,607 142,290,460 2,092,418 62,096 941,196 70,720,014 1,413,333 42,938 229,789 19,891,777 309,650 10,350 1935 u.v. through 2/18 58,546 4,679,333 66,954 2,154 1935 P.o. from 2/19 725,020 59,817,830 867,390 27,337 1,302,974 91,514,608 1,826,875 64,531 1,433,330 85,290,903 1,907,898 65,743 772,527 55,579,814 1,040,611 41,877 740,022 55,417,033 920,091 25,652 795,784 68,111,01l 917,950 17,984 1,078,462 80,858,441 1,314,725 26,649 1,212,933 86,554,613 1,279. 650 24,011 882,214 68,192,842 934,927 16,362 502,247 52,774,095 547,873 8,100 377,579 39,228,810 463,540 8,475 336,197 31,485,167 414,387 8,055 D4 Geology and Resources of Gold In the United States

Table 01. United Verde mine production records-Continued Metals recovered Year Tons mined Lbs Cu Lbs Zn Q.z Ag Oz Au Lbs Pb 346,311 31 ,063,468 391,993 7,573 348,048 29,833,400 436,721 11 , 166 409,313 33,957,890 8,005,488 485,301 9,497 351,992 15,426,569 15,157,169 272,342 6,117 299,436 19,830,228 17,915,943 399,689 7,000 155,642 10,475,722 9,613,749 249,015 4,300 1953 End P.D. 29,164 1,938,803 2,199,620 49,058 1,258 Start 1954 Big Hole 5,004 605,319 3,604 13,447 1 ,371 ,056 9,453 14,929 1,545,434 10,843 12,872 1,644,400 14,773 16,010 2,067,960 27,724 9,824 969,108 9,627 17,010 2,299,801 10,088 18,533 2,436,505 16,187 10,477 1,770,601 8,794 15,076 2,326,437 13,414 8,203 876,828 7,974 9,577 1,216,466 9,603 5,552 543,943 4,174 5,006 528,227 3,751 5,218 565,219 5,649 9,299 1,118,820 9,950 11,062 1,392,423 12,946 7,238 758,239 8,428 6,214 615,609 6,732 1,705 173,120 1 ,829 3,923 376,019 4,190 91 ,292 Total U.V. 20,685,383 1,978,768,862 34,360,185 971,801 Total P.D. 12,099,195 917,312,277 52,891,969 14,720,036 381 ,687 206,1 so 25,201,534 199,793 2,877.7 Total Big H~le Grand Total 32,990,728 2,921,291,673 52,891,969 49,279,954 1,356,366 ABGMT3 33,018,255 2,928,252,040 49,604,334 1,360,614 459,100 ABGMT Total4 32,588,782 2,844,403,043 97,352, 100 49,732,425 1,376,108 459 ,100 1includes data from Arizona Bureau of Geology and Mineral Technology (ABGMT). 2Excludes data from ABGMT. 3includes 1884-1888, 1975 data from ABGMT. 4Total from records of ABGMT. Moench, 1979), and in central and western Arizona (Anderson and Guilbert, 1979; Gilmour and Still, 1968; DeWitt, 1979; Baker and Clayton, 1968; Anderson and others, 1955; Stensrud and More, 1980; and Donnelly and Hahn, 1981). In central and western Arizona similar but smaller volcanogenic massive sulfide deposits include the Iron Zonia, United Verde Extension, Old Dick- Bruce, Bluebell, Copper Chief, Desoto, and numerous much smaller deposits. Acknowledgments Phelps Dodge Corporation contributed unpublished production data and assay results from the 2400 level of the United Verde mine. Anna Wilson aided in the computer plotting of the data from Storms (1955). The manuscript was reviewed by Phillip Anderson, Jonathon Duhamel, Gary Landis, and D.M. Sheridan, and benefited from their comments. GEOLOGY Stratified Metavolcanic and Metasedimentary Rocks The United Verde massive sulfide deposit lies at the top of a sequence of interbedded rhyolitic to basaltic Gold Deposits In Metamorphic Rocks-Part I

CJ) ::J 'I Shut down during the Depression , "" /Total mine production BOO z H IJ I Total mine z producti on H u ::J

/ a: Open pit production I y 0 / A Total mine I production I (1931) YEAR Figure 02 (above and following pages) . Data for the lifetime of the United Verde mine. Data are from table D1, and unpublished data from the Arizona Bureau of Geology and Mineral Technology. A, plot of production versus time. 8, plot of copper and silver grades versus time. C, plot of Ag:Au and gold grade versus time. Dashed lines between X's in A connect data for intermittent years; dashed curves in 8 and Care average trends. volcanic rocks which have been metamorphosed to lower greenschist facies. Mixed sedimentary and volcaniclastic rocks overlie the massive sulfide deposit. Because original volcanic textures and structures are well preserved and deformation has not obliterated stratigraphic relations, the metamorphosed lithologies will be described by their protoliths. Detailed stratigraphy was described in Anderson and Creasey (1958) and Anderson and Nash (1972). According to data in Anderson and Creasey (1958), volcanic rocks older than the massive sulfide deposit are a bimodal suite of basalt and basaltic andesite (40 percent) and rhyolite (55 percent), with only minor amounts (5 percent) of andesite and dacite (fig. D6). The Geology and Resources of Gold In the United States mafic rocks include the Gaddes Basalt and Shea Basalt, both of which are pillow lava flows with minor tuff and rhyolite breccia beds. The intermediate rocks are represented only by the Brindle Pup Andesite, a flow unit containing intercalated basalt and rhyolite flows. The felsic rocks include the Buzzard Rhyolite, dacite of Burnt Canyon, and Deception Rhyolite and allied minor intrusive rocks. These felsic extrusive units consist of flows, flow breccias, jasper-rich flows, and crystal and lithic tuffs. Porphyritic rhyolite (Cleopatra Quartz Porphyry) once thought to intrude the Deception Rhyolite (Reber, 1922; Anderson and Creasey, 1958) is now included as the Cleopatra Member, an extrusive unit within the Deception (Anderson and Nash, 1972).

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the Depression YEAR Figure 02-Continued. Lifetime of the United Verde mine. The analytical data for 11 unaltered metavolcanic rocks in the Jerome area (Anderson and Creasey, 1958), however, do not indicate such a bimodal suite. (See discussion in DeWitt and Waegli, 1986.) A chemical classification of these rocks (fig. D7), based on the major element method of De Ia Roche and others (1980), indicates a gradational range of compositions from rhyolite to basalt instead of the bimodal suite of rhyolite and basalt-basaltic andesite. The major-element chemistry of unaltered volcanic units in the Jerome area is similar to that in the Prescott area to the west (fig. D7) where calc-alkalic, iron-rich, low-to medium-potassium rocks predominate (DeWitt and Waegli, 1986). Volcanic rocks that predate the ore deposit are predominantly calc-alkalic (Peacock, 1931), as shown on figure D7. The Shea and Gaddes Basalts are lowpotassium rocks and the rest of the units are either lowor medium-potassium rocks according to the classification ofPeccerillo and Taylor (1976). The suite is predominantly metaluminous (Shand, 1927), but felsic rocks are slightly peraluminous due to alkali depletion related to hydrothermal alteration near ore bodies. All the volcanic units that predate the ore deposit are ironrich (Miyashiro, 1974). Magnesium-rich rocks are noted only in hydrothermally altered areas near ore bodies. Metasedimentary and metavolcanic units that were deposited with or postdate the massive sulfide deposit include coarseand fine-grained tuffaceous rocks, volcanic breccia, chert and jasper beds, and dacite and andesite of the Grapevine Gulch Formation (Anderson and Creasey, 1958; Anderson and Nash, 1972). Chemically the dacite in the Grapevine Gulch is an Gold Deposits In Metamorphic Rocks-Part I

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(!) YEAR Figure 02-Continued. Lifetime of the United Verde mine. alkali-calcic rhyolite, unlike older calc-alkalic units in the Jerome area (fig. D7). Crystal tuffs are locally abundant in the upper part of the Grapevine Gulch Formation, indicating that rhyolitic volcanism continued after deposition of tuffaceous rocks, minor limestone, and some agglomerate (Reber, 1922). Breccia beds in the basal Grapevine Gulch contain mineralized fragments of Deception Rhyolite (Anderson and Nash, 1972). However, a change in style of alteration within rocks of the upper part of the Cleopatra led Gustin (1986, 1987) to suggest that the time horizon represented by the massive sulfide deposit is within the Cleopatra, not at the overlying contact with the rocks of the Grapevine Gulch. Volcanic rocks associated with the United Verde deposit have been described as having formed in a greenstone belt (Anderson and Silver, 1976), and in volcanic arcs with distinct polarities and chemical evolution (Anderson, 1978). Vance and Condie (1986) suggested that the volcanic rocks formed in a combination of arc and back-arc settings. Geology and Resources of Gold In the United States Intrusive Rocks All pre-ore metavolcanic and metasedimentary units have been intruded by minor quartz porphyry dikes, a large gabbro body (United Verde Diorite of Reber, 1922), and later andesite(?) porphyry dikes. In the southern part of the Jerome area, a quartz diorite pluton (Anderson and Creasey, 1967) ir.trudes those units. The age of the pluton is 1,740± 15 Ma (Anderson and others, 1971; date recalculated using decay constants in Steiger and Jaeger, 1977). The gabbro in the United Verde mine area was intruded as a sill near the contact of the Grapevine Gulch Formation and Cleopatra Member of the Deception Rhyolite, and locally it cut out and isolated pieces of the massive sulfide deposit (Haynes ore body; pl. 7 and fig. 24 of Anderson and Creasey, 1958; pl. X of Reber, 1938; Anderson and Nash, 1972). The gabbro is not foliated as extensively as the rocks it intrudes, but it has been metamorphosed to the same degree as the stratified rocks. Chemically, the gabbro has been

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s9 62;.-- ... COPPER GRADE, IN PERCENT Figure 03. Plot of gold grade versus copper grade for different time periods of production in the United Verde mine. Number indicates year (37 1937). extensively altered (high H 20, C02, A120 3 ) and cannot be chemically classified as are other rock units on figure 07. A swarm of east-trending, low-grade metamorphosed andesite(?) dikes intrudes the gabbro a.nd massive sulfide lens in the United Verde mine (Provot, 1916; Reber, 1922). The dikes are undeformed, but they do contain sparse pyrite and chalcopyrite (Anderson and Creasey, 1958). Apparently the dikes were emplaced after regional deformation ceased, and the heat from the dikes caused very local remobilization of sulfide minerals. Regional Structure and Age of Metamorphism and Deformation The stratified volcanic sequence that underlies the massive sulfide deposit has a minimum age of 1,770 to 1,780 Ma, which is the U-Pb zircon age of the Cleopatra Member of the Deception Rhyolite (L.T. Silver, written commun., 1982). Regional deformation and low-grade metamorphism of this 1,770- 1,780 Ma sequence, and the younger Grapevine Gulch Formation and gabbro, occurred between 1,770 Ma and 1,690 Ma, which is the Gold Deposits In Metamorphic Rocks-Part I D9

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104 / " 1 ./ BOO PRODUCTION. IN THOUSANDS OF TONS OF ORE Figure 04. Plot of Ag:Au ratio versus tonnage for different time periods of production in the United Verde mine. Number indicates year {52 1952). K-Ar cooling age of the 1,740±15 Ma late-tectonic quartz diorite batholith south of Jerome (fig. D6). The syngenetic stratiform massive sulfide deposit in the United Verde mine is therefore dated at about 1,770 to 1,780 Ma. Varied structural interpretations of the Jerome area proposed by Anderson and Creasey (1958), Anderson and Nash (1972), Lindberg and Jacobsen (1974), and Norman (1977) involve from one to three major deformational events. The interpretation favored in this report incorporates much of Anderson and Nash's (1972) stratigraphy and follows Lindberg and Jacobsen's (1974) model for two generations of folding (figs. D6 and D8). In the area south of Jerome, the northwest-trending Jerome anticline (F1, fig. D6) has been crossfolded about west-northwest-trending axes. The regional foliation in the metamorphosed lithologies parallels the second generation of folds (F2 , fig. D6). Locally, as near the United Verde mine, F1 and F2 structural features are paraJJel and the effects of refolding are not apparent. Geology and Resources of Gold in the United States ORE DEPOSITS Structure The United Verde massive sulfide deposit is localized near the top of the Cleopatra Member of the Deception Rhyolite, and in the overlying Grapevine Gulch Formation (fig. D8). The deposit and country rocks have been folded about north-northwest-trending axes, resulting in a pipe-shaped stratiform deposit that trends N. 20° W. and plunges 65° N., parallel to minor folds and axial plane lineations (Anderson and Creasey, 1958). The plunge of the deposit increases with depth. Above the 1100 level the plunge averages 45°; from the 1100 l~vel to the 3300 level it averages 70°; and from the 3300 level to the 4500 level it is vertical or reverses plunge to the southeast. A sill-like body of gabbro which isolates and cuts off parts of the top of the ore body (see pis. 7 and 10 of Anderson and Creasey, 1958) was emplaced along the contact between the Cleopatra Member and the Grapevine Gulch Formation.

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L J H <l: a: ::J O'l <l: L 10 Zinc-rich Copper-rich Cu: Zn RATIO Figure 05. Plot of gold grade and Ag:Au ratio versus Cu:Zn ratio for ore mined from 1944 through 1953 from the United Verde mine. X, Ag:Au ratio; circled dot, gold grade; number indicates year (44 1944); tie line joins data points for a specific year. The deposit is zoned from stratigraphic bottom (Cleopatra Member of the Deception Rhyolite) to top (Grapevine Gulch Formation) and consists of: (1) chloritized rhyolite or quartz porphyry; (2) copper-rich black schist that is hydrothermally altered rhyolite containing massive chlorite; (3) copper-rich massive sulfide ore containing varied amounts of zinc; (4) copper-poor, zinc-rich massive sulfide ore that forms most of the deposit; (5) copper-and zinc-poor, pyrite-rich massive sulfide ore; and (6) jaspery chert lenses (Reber, 1922; Anderson and Creasey, 1958). The black schist ranges from 0 to 60 m thick, the massive sulfide ore from 0 to 120 m thick, and the chert from 0 to 40 m thick. The massive sulfide deposit grades downward into black schist and ultimately to rhyolite, but its upper contacts with chert, with rocks of the Grapevine Gulch, or with rhyolite are sharp. Laterally, the massive sulfide grades into and is interbedded with rhyolite and rocks of the Grapevine Gulch. The sulfide mass is exposed from the surface to the 4500 level of the mine, and undoubtedly once extended upward to the overlying Cambrian Tapeats Sandstone, an additional vertical distance of about 120 m. The sulfide body therefore had a minimum length of 1,600 m. The massive sulfide part of the deposit varies in thickness from 12 to 18m on the 4500 level to more than 120m on the 1650 level. It ranges in width from discontinuous zones 75-160 m wide on the lower levels of the mine to a continuously mineralized zone more than 420 m wide on the 3000 level. The massive sulfide ore deposit is approximately pipe-or molar-shaped from the surface to the 1650 level, crescent-shaped from there to the 3300 level, and lens-shaped in the lowest parts of the mine (Anderson and Creasey, 1958, pl. 7). This variation in shape reflects both the original configuration of the deposit and the effects of folding. Wallrock Alteration Both the Cleopatra and tuffaceous units of the Grapevine Gulch Formation below and adjacent to the massive sulfide lens have been variably chloritized and sericitized (Anderson and Creasey, 1958). Chloritization of rhyolite by the action of hot seawater brine converted rhyolite to black schist in the mine area (Anderson and Nash, 1972). Regionally, sericitization of the rhyolite was Gold Deposits In Metamorphic Rocks- Part I

EXPLANATION r::7B lJii:J

Cenozoic and Paleozoic sedimentary rocks Quartz diorite (1740±15Ma) Volcaniclastic rocks-- Grapevine Gulch Formati on l Rhyolitic rocks- -Buzzard Rhyo 1 i te, Deception Rhyolite, dacite of Burnt Canyon Dacitic rocks-- Brindle Pup Andesite Basaltic and andesitic rocks --Gaddes and Shea Basalts F1mTrace of earl i er fold axis --Dotted where pro j ected F2Trace of later fold axis --Dotted where projected Fault Contact--Dashed where inferred, dotted where concealed 3 KILOMETERS 2 MILES u H N 0 a: w f-- 0 a: n. Figure 06. Generalized geology of the Jerome region. Modified from Anderson and Creasey (1958). Mapped units listed by rock type rather than by age. more widespread than chloritization, but it was not as pervasive and it may not have been related entirely to the ore-forming process. Instead, some of the sericite may represent premetamorphic devitrification and alteration of glass and pumice within the rhyolitic units (Anderson and Creasey, 1958). Microprobe analyses indicate that chlorite in the black schist from the United Verde mine is ripidolite (classification of Hey, 1954) with an Fe: Fe+ Mg ratio that ranges from 0.37 to 0.49 (Nash, 1973). Chlorite of this composition is classified as prochlorite (Saggerson and Turner, 1982). The magnesium content of the ripidolite in the Cleopatra increases south to north toward the United Verde mine. Similar magnesium-rich chlorite (Fe:Fe + Mg ratio of 0.31-0.56) has been reported from the alteration pipe of the Bruce massive sulfide deposit near Bagdad, Ariz. (Larson, 1984, 1987). Geology and Resources of Gold in the United States Many major, minor, and trace elements have been mobilized within the chloritic and sericitic alteration zones. In massive chlorite or black schist zones, Si02, Na20, CaO, K20, rubidium, strontium, and barium show depletion relative to least altered rhyolitic rocks (Vance and Condie, 1987). Si02 and K20 show the greatest relative depletion. Chloritized zones show relative enrichment of MgO, A120 3 , volatile constituents, total iron, cesium, uranium, thorium, manganese, scandium, cobalt, rare-earth elements (especially the light rareearths), zirconium, niobium, yttrium, tantalum, and hafnium. Some of this apparent enrichment is due to volume Joss during alteration of the rhyolite, but much enrichment is due to mass transfer of constituents in the hydrothermal system. In zones characterized by sericitic alteration, N~O and CaO are depleted relative to least altered rhyolite, and MgO, Si02, total iron, volatile

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lC) lC) R1 4Si-11 Figure 07. Chemical classification diagram (De Ia Roche and others, 1980) for volcanic units in the Jerome and Prescott areas. Data from Anderson and Creasey (1958), Anderson and Blacet (1972), Krieger (1965), and Vrba (1980). Only those analyses (in weight percent) are plotted that contain less than 3.0 percent H2 0 and less than 1.0 percent C02 , and that have total oxide weight percents between 98.5 and 100.5 percent. Numbers designate rock units in the Jerome area: 1, gabbro at United Verde mine; 2, Shea Basalt; 3, Brindle Pup Andesite; 4, dacite from Grapevine Gulch Formation; 5, Buzzard Rhyolite; 7, dacite of Burnt Canyon; 8, Cleopatra Member; 9, other quartz porphyries constituents, and locally are enriched. Light-ion lithophile elements, rare-earth elements, and high-fieldstrength elements are not measurably enriched or depleted to the levels of those elements in least altered rhyolite in sericitic zones (Vance and Condie, 1987). On the basis of alteration studies, Vance and Condie (1987), Gustin (1986, 1987), and Lindberg and Gustin (1987) suggested that the contact at the top of the probably correlative with Cleopatra. Letters designate rock units in the Prescott area: B, rhyolite near the Binghampton mine; C, basalt east of the DeSoto mine; E, rhyolite southwest of Iron King mine; F, andesite northwest of DeSoto mine; H, basalt south of Iron King mine; J, rhyolite south of Binghampton mine; L, basalt near Bluebell mine; M, rhyolite north of Bluebell mine; N, rhyolite northeast of Iron King mine; P, andesite southwest of Iron King mine; R, basalt near Binghampton mine; T, basalt east of DeSoto mine. Approximate boundaries between calcic, calc-alkalic, alkalicalcic, and alkalic suites shown by heavy, dashed lines. Cleopatra Member, not the contact between the upper unit of the Deception Rhyolite and the Grapevine Gulch Formation, marks the time horizon of massive sulfide deposition. At the inferred time horizon, chloritized and sericitized rocks are overlain by hematite-rich and carbonate mineral-and epidote-bearing assemblages that contain lesser amounts of sericite and chlorite (Gustin, 1987). Also, chlorite from the hematite-rich rocks is Gold Deposits In Metamorphic Rocks-Part I

500 METERS 2000 FEET N EXPLANATION l: ti : : / Cenozoic and Paleozoic rocks Proterozoic Massive sulfide ore/ black schist ore

Chloritized rock Gabbro Volcaniclastic rocks. tuff . chert. Grapevine Gulch Formation Rhyolitic rocks-- Cleopatra Member of Deception Rhyolite and upper unit of Deception Rhyolite Rhyolitic rocks-- Bedded breccia/lower unit of Deception Rhyolite Andesite agglomerate (7) member of Deception Rhyolite Basaltic and andesitic rocks-- Shea basalt

Fault--Dashed where inferred; dotted where concealed

Contact--Dashed where inferred Figure DB. Geology of the United Verde mine area. Modified from Anderson and Creasey (1958) and Lindberg and Jacobsen (1974). more iron rich than the prochlorite reported from beneath the deposit. In addition, hematite-rich rocks contain recognizable feldspar phenocrysts, whereas the chlorite-rich rocks have no preserved feldspar phenocrysts (Gustin, 1986, 1987), suggesting that a significant change in volcanic activity took place at the horizon of the United Verde massive sulfide ·deposit. Mineralogy The primary mineralogy of the United Verde massive sulfide deposit is simple, and it has been modified by weathering or supergene processes. Naturally oxidized (weathered) ores above the 160 level and ores oxidized by mine fires down to the 600 level contained cuprite, chalcocite, azurite, malachite, native copper, wire silver, minor copper hydroxide minerals, limonite, and hydrous copper sulfate minerals (Reber, 1922; Lindgren, 1926; Anderson and Creasey, 1958). Ore minerals in the massive sulfide lens are, in decreasing order of abundance, pyrite, sphalerite, chalcopyrite, bornite, arsenopyrite, galena, tennantite, and Geology and Resources of Gold in the United States gold (probably electrum). Gangue minerals include quartz, carbonate minerals, chlorite, sericite, and minor hematite. Pyritic massive sulfide ore typically averages about 50-60 percent sulfide mineral , is nonbanded, fine grained (0.04-0.2 rom diameter range; 0.1 mm diameter average), and consists of pyrite, minor sphalerite and chalcopyrite, very minor arsenopyrite, and interstitial quartz and carbonate minerals, chiefly dolomite and ankerite (Lindgren, 1926; Ralston, 1930; Slavin, 1930; Anderson and Creasey, 1958). Sulfide minerals other than pyrite typically make up less than 5 volume percent of the pyritic massive sulfide ore. Basal parts of massive sulfide ore contain local concentrations of chlorite-rich ore. Zinc-rich massive sulfide ore typically is more banded than pyritic massive sulfide ore and consists of alternating zinc-and pyrite-rich layers about one-quarter inch thick (Anderson and Creasey, 1958). Sphalerite in the ore is light brown and contains about 6.5 percent iron. The amount of sulfide minerals relative to gangue minerals is about the same as in pyritic massive sulfide ore, but carbonate minerals form less of the

zinc-rich ore than the pyritic ore. Sphalerite commonly makes up less than 15 volume percent of zinc-rich ore. Although much of the sphalerite is the same age as pyrite, some sphalerite veins cut pyritic ore. Chalcopyrite (normally 1-2 volume percent) in the ore is generally surrounded by sphalerite, as are minute grains of galena. The fine-grained nature and intergrown texture of the zinc-rich ore made beneficiation of zinc-rich ore difficult, as the ore had to be ground finer than 300 mesh (less than 0.02 mm) for efficient liberation of zinc (Anderson and Creasey, 1958). The same sulfides present in the massive sulfide lens also are present in the black schist ore and quartz porphyry ore, but pyrite is much less abundant (about 20 volume percent) and chalcopyrite, especially in veinlets associated with quartz and chlorite, is much more abundant (7-15 volume percent). Very fine grained chlorite (0.002-0.01 mm diameter) forms 30-65 volume percent of the ore. Quartz is less abundant in both ore types than it is in massive sulfide ore. Zircon (relict from Cleopatra Member) and rutile (hydrothermal?) are trace constituents. Bornite is present locally in black schist ore. Black schist ore, in places, appears to grade into massive sulfide ore; chalcopyrite-rich veinlets typically cut the basal parts of massive sulfide ore. Chert ore that overlies the massive sulfide lens contains less pyrite and more chalcopyrite, sphalerite, hematite, and magnetite than do the other ores. The chert ore is very slightly coarser than massive sulfide ore. Gold is apparently present as electrum in microscopic inclusions within most sulfide minerals, and the electrum may contain significant silver. Unfortunately no studies have been made of the mineralogy or occurrence of gold and silver. Much silver is present in late-stage tennantite in quartz-carbonate veins and carbonate-rich massive sulfide ore (Anderson and Creasey, 1958, table 16). This tennantite contains significant gold also, most probably as microscopic inclusions. Lead and Sulfur Isotope Data Galena from the United Verde deposit was first analyzed for lead isotope ratios by Mauger and others (1965), who reported a 1,767 Ma single-stage model lead date (date recalculated using data in Appendix C, Doe and Stacey, 1974) for the apparently conformable base metal deposit. Subsequently, Stacey and others (1976) reported galena from the deposit to have the following lead isotopic ratios: 206Pbf 04Pb 15.725; 207Pbf04 Pb 15.270; 208Pbf04Pb =35.344. A singlestage model lead date calculated for this galena, again using data in Doe and Stacey (1974), is 1,729 Ma, but the point falls slightly off the single stage growth line on a 207Pbf04Pb versus 206Pbf04Pb diagram. Stacey and others (1976) indicated that the galena analysis from the United Verde deposit falls between their curves for lead from the mantle and lead from a well-mixed oro gene, and they suggested that their two-stage model isochron date (Stacey and Kramers, 1975) of 1,645 Ma is a result of lead from the mantle being . mixed with lead from an orogenic or marine source that was of a slightly more radiogenic nature than the mantle lead. The age discrepancy of about 130 m.y. between the preferred age of the host rocks (about 1,775 Ma) and the two-stage galena date likely is a function of isotopic mixing in an orogenic setting. Pyrite, sphalerite, and galena from massive sulfide and black schist ore in the United Verde depo it have o34S values that range from - 2.8 to +0.2 per mil (Gustin, 1986; Eastoe and others, 1987). The restricted range of o34S from sulfide minerals in both the United Verde deposit and other Proterozoic volcanogenic massive sulfide deposits in Arizona (127 sulfide minerals from all major and most minor deposits are in the range - 2.0 to + 3.2 per mil; Eastoe and others, 1987) contrasts with a much wider range and heavier sulfur in Phanerozoic deposits. The restricted range for Proterozoic volcanogenic massive sulfide deposits in Arizona could indicate that igneous sulfur was in much greater supply than seawater sulfur, or that recharge of the hydrothermal system was from a widespread, but stagnant and stratified layer of seawater that was isolated from more normal, sulfate-rich seawater (Eastoe and others, 1987). Gold-Silver Zonation The most obvious contrast in content of precious metals in the deposit, as previously noted, is from unoxidized to oxidized ore: in oxidized ore, gold and silver grades are almost an order of magnitude higher (0.03 oz Au/ton in unoxidized ore to 0.2 oz Au/ton or greater in oxidized ore, and 1.2 oz Ag/ton in unoxidized ore to 15.0 oz Ag/ton or greater in oxidized ore). However, gold and silver contents also vary considerably within the unoxidized ore where their grades are related to ore type. Smith and Sirdevan (1921, table 3) were perhaps the first to show that silica-rich ("converter flux" ore in metallurgical terminology) mas ive sulfide ore contained more gold and silver than normal massive sulfide (iron) or black schist (silica) ore. Subsequently, Hansen (1930), Barker (1930), and Ralston and Hunter (1930), enlarging upon these findings, indicated that precious metal contents ranged from low in quartz porphyry and black schist ore to high in massive sulfide and siliceous massive sulfide ore. Their data suggested that the deposit was zoned and that precious metal contents were greatest at the stratigraphic top. This variation of gold and silver within ore types is further quantified by using the data of Storms (1955) for Gold Deposits In Metamorphic Rocks-Part I

Table 02. Average grades of ore types, United Verde massive sulfide deposit [Number of samples averaged, in parentheses] Ore type Mine Copper level (percent) United Verde ore body:

Massive sulfide--- Chert, siliceous massive sul f ide-- 3.450 6) Black schist 2) Massive sulfide---

Black schist 1.02( 17) Massive sulfide--- 1.800°9) Black schist

Chert 3. 70( 4) Siliceous massive sulfide-- 3. Massive sulfide---

Black schist

Haynes ore body: Massive sulfide---

various levels within the United Verde deposit. Massive sulfide ore on the 700 level averages 10 times as much gold and 4-5 times as much silver as black schist ore on the same level (table 02). Massive sulfide ore on the 3000 level averages three-four times as much gold and two times as much silver as black schist ore on the same level. Grades range from 0.002 oz Au/ton and 0.71 oz Ag/ton in black schist ore, to 0.116 oz Au/ton and 4.0 oz Ag/ton in chert ore on the 4500 level. Clearly the concentration of precious metals is highest at the stratigraphic top of the deposit and gold is more enriched than silver near the top. Correlations between precious and base metals in the deposit are not obvious if distinctions are not made among ore types (fig. 09 A- C). Copper, zinc, gold, and silver analyses for a suite of samples on the 2400 level show no high positive or negative correlations among the four variables (fig. 09 A - C). An important feature to note from figure 09 is that the gold and silver grades, although not positively correlated with either copper or zinc content, are just as high in zinc-rich ore (fig. D9A, C) as in copper-rich ore (fig. 09 B). If the data of Storms (1955) are grouped into the various ore types and plotted, correlations are noted (fig. 010, table 03). The Ag:Au ratio and precious metal grades vary with location in the deposit (fig. D10A). In Geology and Resources of Gold In the United States Zinc Gold Silver (percent) (oz/ton) (oz/ton) 10. 5) 4) 4) o.oo504) 0.75< 34 ) 3.41 <88)

) 0.71( 9 ) 4.29< 28 ) o. o5o06 o.5o06> the black schist ore the Ag:Au ratio is highest (average 170-400) and gold content lowest (about 0.005 oz Au/ton). In massive sulfide ore the Ag:Au ratio decreases to 30--100 (average for unoxidized ore about 55) and the grade increases to about 0.04 oz Au/ton. Chert ore has the lowest Ag:Au ratio (30) and the highest gold grade (0.10 oz Au/ton), and is similar in grade, although somewhat lower, to ore from the "gold stope" of the United Verde Extension mine (Anderson and Creasey, 1958; Arizona Bureau of Geology and Mineral Technology, unpub. production data, 1984; White, 1986a, 1986b ). Positive correlations of gold with zinc and gold with combined zinc and copper (fig. 010 B, C; table 03) are noted for massive sulfide ore. The best correlation between precious and base metals in massive and siliceous massive sulfide ore is between gold and Figure 09 (facing and following pages). Element-element plots for the 2400 level of the United Verde massive sulfide deposit. Data from unpublished assay results made available by Phelps Dodge Corporation. Individual ore types are not plotted separately. +, one analysis; numbers 2-9, two through nine analyses plot at one point; #, more than nine analyses plot at one point. A, copper versus zinc (top); silver versus zinc (bottom).

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EXPLANATION o 700 level. black schist ore X 3000 leve l, black schist ore A 4500 level. black schist ore o 4500 level, chert ore / / / / / /

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/ / / f/ / / / / / / / Average for 700 level. black schist ore Average for 3000 level. black schist ore Average for 4500 level. black schist ore Average for 4500 level. chert ore A GOLD, IN OZ/TON Figure 010 (above and following pages). Element-element plots in ore of the United Verde massive sulfide deposit. Data from Storms (1955). A, gold versus silver in black schist and chert ore. combined copper plus zinc (0.570 for 106 samples). Except for data from the 3000 level, the correlation between zinc and gold is as good as or better than that between gold and combined zinc plus copper (table D3). Gold distribution is not well correlated with copper except on the 3000 level. The addition of silver to gold decreases the correlation with combined copper plus zinc, indicating that silver must be present in another mineral, probably galena, in addition to the assumed electrum. CONCLUSIONS Distinguishing Characteristics The deposit at the United Verde mine in many respects is typical of Archean, Proterozoic, and Phan020 Geology and Resources of Gold in the United States erozoic volcanogenic massive sulfide deposits (Ohmoto and Skinner, 1983; Franklin and others, 1981; Sangster, 1980, 1972; Ishihara, 1974; Hutchinson, 1973; and Gilmour, 1965). The ore deposit occurs at the top of a submarine rhyolite dome and flow breccia unit (Cleopatra Member of the Deception Rhyolite; Anderson and Nash, 1972) that is at the top of a predominantly calcalkalic, low-to medium-potassium, iron-rich suite, which ranges in composition from rhyolite to basalt. The deposit is zoned and consists, from top to bottom, of capping chert ore and siliceous massive sulfide ore, pyrite-rich massive sulfide, zinc-and copper-zinc-rich massive sulfide, chloritic stringer ore and chloritized quartz porphyry ore in the Cleopatra. Gold and silver are present in all ore types but are concentrated at the stratigraphic top, in massive sulfide, siliceous massive sulfide (both zinc-and copper-rich), and chert ore. The grade of precious metals in the deposit (0.041 oz Au/ton;

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/ / / / / Numbered contours Ag: Au ratios (t:P / / / / / are () Average f or 700 level. massive sulfide ore / / / / / / Average for 3000 leve l , X massive sulfide ore Average for 4500 level, ll massive sulfide ore / / GOLD, IN OZ/TON Figure 010-Continued. Element-element plots in ore. 8, gold versus silver, massive sulfide ore. 1.49 oz Ag/ton) is comparable to that of other volcanogenic massive sulfide deposits in Arizona (DeWitt, 1983) and was exceeded only by that of the Iron King mine near Humboldt, Ariz. (0.073 oz Au/ton; 2.67 oz Ag/ton; Arizona Bureau of Geology and Mineral Technology, unpub. production data). The deposit is the largest known volcanogenic massive sulfide in the United States (33 million tons mined; minimum of 50-70 million tons of low-grade material remaining) and one of the largest in North America. Only the Kidd Creek (Walker and others, 1975) and Brunswick 12 deposits (Sangster, 1984; McAllister and others, 1980) in Canada have greater reserves plus production tonnages. Ore Controls The localization of the stratiform massive sulfide body was controlled by a hydrothermal conduit now represented by the chlorite schist and chloritic alteration pipe in the Cleopatra quartz porphyry. Paleotopographic controls, such as caldera moats, flanks of domes, and so on, have been postulated (Lindberg, 1986a, 1986b) as the structural controls for hydrothermal fluids at the United Verde deposit. Many such structural features have probably been masked by deformation of the metavolcanic rocks. Why such a large sulfide body was localized at the top of the Cleopatra quartz porphyry, as opposed to lower in the volcanic pile, is unknown, but the genesis of the deposit must have been intimately associated with the evolution of the submarine volcanic suite, and may reflect the end-products of differentiation in the source region of the magmas. The deposit is overlain by a sequence of volcaniclastic sedimentary rocks, tuffs, cherty sedimentary rocks, and jasper-rich beds (Grapevine Gulch Formation) that is compositionally unlike the underlying flows, breccias, domes, and intrusives. Therefore, formation of the massive sulfide deposit appears to have marked the end of the period of submarine calc-alkalic Gold Deposits In Metamorphic Rocks-Part I

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H N

GOLD, IN OZ/TON Figure 010-Continued. Element-element plots in ore. C, gold versus zinc, massive sulfide and chert ore. volcanic activity and signaled the beginning of volcaniclastic activity that was chemically more evolved (alkali-calcic) and was certainly in a different geologic environment (partly subaerial; tuffs and immature sedimentary rock dominant instead of flows). Base and precious metals are zoned within the deposit; the alteration pipe contains average to high copper but very little zinc, gold, or silver. The massive sulfide body has average to high copper, zinc, gold, and silver. The capping siliceous massive sulfide horizon and chert layers contain base and precious metals in concentrations similar to those of the highest parts of the massive sulfide ores. If all the metals were transported through the alteration pipe to the paleosurface by hydrothermal solutions, zinc and gold were retained in solution Geology and Resources of Gold In the United States longer than copper and silver, and were precipitated only upon reaching the seawater-sediment interface. This zonation is consistent with decreased pH, Eh, and temperature of the hydrothermal solution at the inferred top of the deposit. Origin The United Verde deposit was formed in a Proterozoic submarine environment during deposition of rhyolite flows, tuffs, and pyroclastic material. The massive sulfide body and underlying chloritic alteration pipe are the end products of hydrothermal solutions enriched in copper, zinc, lead, gold, and silver that

10o.or EXPLANATION o 700 level, massive sulfide ore X 3000 level, massive sulfide ore 4500 level, massive sulfide ore D 4500 level, chert ore I-z w u a: w a. z H u z H N + a: w a. a. 0 u 1.0~: L j D GOLD. IN OZ/TON Figure 010-Continued. Element-element plots in ore. D, gold versus copper +zinc, massive sulfide and chert ore. traveled up and through the quartz porphyry of the Cleopatra Member and ultimately precipitated their sulfide minerals near or at the seawater interface. Chemical studies have not been completed that would indicate the ultimate source of metals in the deposit, whether that source be the underlying volcanic pile (leaching and redistribution of metals) or the magma reservoir of the Cleopatra quartz porphyry (primary enrichment of metals in the magma). Manuscript received by scientific editors May 1985 REFERENCES CITED Alenius, E.M.J., 1930, Methods and costs of stripping and mining at the United Verde open-pit mine, Jerome, Arizona: U.S. Bureau of Mines Information Circular 6248,33 p. 1968, A brief history of the United Verde open pit, Jerome, Arizona: Arizona Bureau of Mines Bulletin 178, 34 p. Anderson, C.A., and Blacet, P.M., 1972, Precambrian geology of the northern Bradshaw Mountains, Yavapai County, Arizona: U.S. Geological Survey Bulletin 1336, 82 p. Anderson, C.A., Blacet, P.M., Silver, L.T., and Stem, T.W., 1971, Revision of the Precambrian stratigraphy in the Prescott-Jerome area, Yavapai County, Arizona: U.S. Geological Survey Bulletin 1324-C, p. C1-C16. Anderson, C.A., and Creasey, S.C., 1958, Geology and ore deposits of the Jerome area, Yavapai County, Arizona: U.S. Geological Survey Professional Paper 308, 185 p. 1967, Geologic map of the Mingus Mountain quadrangle, Yavapai County, Arizona: U.S. Geological Survey Geologic Quadrangle Map GQ- 715, scale 1:62,500. Anderson, C.A., and Nash, J.T., 1972, Geology of the massive sulfide deposits at Jerome, Arizona-A reinterpretation: Economic Geology, v. 67, p. 845- 863. Anderson, C.A., Scholz, E.A., and Strobell, J.D., Jr., 1955, Geology and ore deposits of the Bagdad area, Yavapai County, Arizona: U.S. Geological Survey Professional Paper 278, 103 p. Anderson, C.A., and Silver, L.T., 1976, Yavapai Series-A greenstone belt: Arizona Geological Society Digest, v. 10, p. 13-26. Anderson, Phillip, 1978, The island arc nature of Precambrian volcanic belts in Arizona: Geological Society of America Abstracts with Programs, v. 10, no. 3, p. 156. Gold Deposits In Metamorphic Rocks-Part I

Table 03. Correlation coefficients for ore types in the United Verde massive sulfide deposit [A, no zinc analyses; B, no gold and silver analyses; number of samples in parentheses) Mine level, ore type Cu vs Zn Cu vs Au All levels, massive sulfide (92)- All levels, siliceous massive sulfide (14) All levels, massive plus siliceous massive sulfide (106) All levels plus Haynes ore body, massive sulfide (120) 700, massive sulfide (15) 3000, massive sulfide (29) 4500 massive sulfide (48) Haynes 3000, massive sulfide (28) All levels, black schist ( 115)--- A All levels, quartz porphyry (15)- A All levels, massive sulfide (137) All levels, black schist (22) Anderson, Phillip, and Guilbert, J.M., 1979, The Precambrian massive sulfide deposits of Arizona-A distinct metallogenic epoch and province: Nevada Bureau of Mines and Geology Report 33, p. 39-48. Baker, Arthur III, and Clayton, R.L., 1968, Massive sulfide deposits of the Bagdad district, Yavapai County, Arizona, in Ridge, J.D., ed., Ore deposits of the United States, 1933-1967 [Graton-Sales Volume]: American Institute of Mining and Metallurgical Engineers, New York, p. 1311-1327. Barker, L.M., 1930, Concentrating plant of the United Verde Copper Company: Mining Congress Journal, v. 16, p. De Ia Roche, E., Leterrier, J., Grandclaude, P., and Marchal, M., 1980, A classification of volcanic and plutonic rocks using R 1 R:.~-diagram and major-element analyses-Its relationships with current nomenclature: Chemical Geology, v. 29, p. 183-210. DeWitt, Ed, 1979, New data concerning Proterozoic volcanic stratigraphy and structure in central Arionza and its importance in massive sulfide exploration: Economic Geology, v. 74, p. 1371-1382. 1983, Precious metal content of Proterozoic massive sulfide deposits in Arizona: Geological Society of America Abstracts with Programs, v. 15, p. 298. DeWitt, Ed, and Waegli, Jerome, 1986, Gold in the United Verde massive sulfide deposit, Jerome, Arizona: U.S. Geological Survey Open-File Report 86--{)585, 41 p. Geology and Resources of Gold In the United States B B Correlation coefficients Zn vs Au Cu+Zn vs Au Ast. vs Au Cu+Zn vs Ag:+Au A A A A A A B B B B B B B B Doe, B.R., and Stacey, J.S., 1974, The application of lead isotopes to the problems of ore genesis and ore prospect evaluation-A review: Economic Geology, v. 69, p. Donnelly, M.E., and Hahn, G.A., 1981, A review of the Precambrian volcanogenic massive sulfide deposits in central Arizona and the relationship to their depositional environment, in Dickinson, W.R., and Payne, W.D., eds., Relations of tectonics to ore deposits in the southern Cordillera: Arizona Geological Society Digest, v. XIV, p. Eastoe, C.J., Gustin, M.S., Hurlbut, D.F., and Orr, Richard, 1987, Sulfur isotopes in Proterozoic syngenetic sulfide deposits, Arizona; implications for geochemistry of seawater-rock interaction: Geological Society of America Abstracts with Programs, v. 19, p. 650. Franklin, J.M., Lydon, J.W., and Sangster, D.F., 1981, Volcanic-associated massive sulfide deposits: Economic Geology, 75th Anniversary Volume, p. 485--627. Gilmour, Paul, 1965, The origin of the massive sulphide mineralization in the Noranda district, northwestern Quebec: Geological Association of Canada Proceedings, v. 16, p. 63-81. Gilmour, Paul, and Still, A.R., 1968, The geology of the Iron King mine, in Ridge, J.D., ed., Ore deposits of the United States, 1933-1967 [Graton-Sales Volume]: American Institute of Mining and Metallurgical Engineers, New York, p. 1238-1257.

Gustin, M.S., 1986, A preliminary report on a petrological, geochemical, and stable-isotope study of the orebody and associated alteration, United Verde volcanogenic massive sulfide deposit, Jerome, Arizona, in Beatty, Barbara, and Wilkinson, P.A.K., eds., Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest, v. XVI, p. 339-342. 1987, Geochemistry and mineralogy of alteration associated with the United Verde syngenetic massive sulfide, Jerome, Arizona: Geological Society of America Abstracts with Programs, v. 19, p. 688. Hamilton, Patrick, 1884, Resources of Arizona, 3rd edition: San Francisco, A.L. Bancroft and Company, 414 p. Hansen, M.G., 1930, Geology and ore deposits of the United Verde mine: Mining Congress Journal, v. 16, p. 306--311. Hey, M.H., 1954, A new review of the chlorites: Mineralogical Magazine, v. 30, p. 277-292. Hutchinson, R.W., 1973, Volcanogenic sulfide deposits and their metallogenic significance: Economic Geology, v. 68, p. 1223-1246. Ishihara, S., ed., 1974, Geology of the Kuroko deposits: Society of Mining Geologists of Japan Special Issue 6, 437 p. Keith, S.B., Gest, Donald, DeWitt, Ed, Woode-Toll, Netta, and Everson, B.A., 1984, Metallic mineral districts and production in Arizona: Arizona Bureau of Geology and Mineral Technology Bulletin 194, 58 p., 1 sheet, scale 1:1,000,000. Keith, S.B., Schnabel, Lorraine, DeWitt, Ed, Gest, D.E., and Wilt, Jan, 1983, Map, description, and bibliography of the mineralized areas of the Basin and Range province in Arizona: U.S. Geological Survey Open-File Report 84-0086, 129 p., 1 sheet, scale 1:500,000. Krieger, M.H., 1965, Geology of the Prescott and Paulden quadrangles, Arizona: U.S. Geological Survey Professional Paper 467, 127 p. Krieger, Philip, 1932, Geology of the zinc-lead deposit at Pecos, New Mexico: Economic Geology, v. 27, Pt. I, p. 344-364; Pt. II, p. 450-470. Larson, P.B., 1984, Geochemistry of the alteration pipe at the Bruce Cu-Zn volcanogenic massive sulfide deposit, Arizona: Economic Geology, v. 79, p. 1880--1896. 1987, Geology and oxygen isotope geochemistry of the Bruce volcanogenic massive sulfide deposit, Bagdad, Arizona: Geological Society of America Abstracts with Programs, v. 19, p. 740. Lindberg, P.A., 1986a, Genesis of Proterozoic massive sulfide deposits and structural geology of the Jerome district, Arizona: Geological Society of America Abstracts with Programs, v. 18, p. 390. 1986b, A brief geologic history and field guide to the Jerome district, Arizona, in Nations, J.D., Conway, C.M., and Swann, G.A., eds., Geology of central and northern Arizona: Geological Society of America Rocky Mountain Section Guidebook, p. 127-139. Lindberg, P.A., and Gustin, M.S., 1987, Field-trip guide to the geology, structure, and alteration of the Jerome, Arizona ore deposits, i11 Davis, G.H., and VandenDolder, E.M., eds., Geologic diversity of Arizona and its margins; excursions to choice areas: Arizona Bureau of Geology and Mineral Technology Special Paper 5, p. 176--181. Lindberg, P.A., and Jacobson, H.S., 1974, Economic geology and field guide of the Jerome district, Arizona, in Karlstrom, T.N.V., Swann, G.A., and Eastwood, R.L., eds., Geology of northern Arizona with notes on archeology and paleoclimate; Part II, area studies and field guides: Geological Society of America, Cordilleran section annual meeting, Trip 8, p. 794-804. Lindgren, Waldemar, 1926, Ore deposits of the Jerome and Bradshaw Mountains quadrangles, Arizona: U.S. Geological Survey Bulletin 782, 192 p. Mauger, R.L., Damon, P.E., and Giletti, B.J., 1965, Isotopic dating of Arizona ore deposits: Transactions, Society of Mining Engineers, v. 232, p. 81-87. McAllister, A.L., Strong, D.F., Davies, J.L., Lamarche, R.Y., Sangster, D.F., and Zentilli, M., 1980, Summary data on stratabound sulphide deposits in the Canadian Caledonides {Appalachians): Canadian Geological Survey Open File Report 677, 40 p. Mcilroy, P., Coffman, J.S., and Rice, W.L., 1974, Availability of U.S. primary lead resources: U.S. Bureau of Mines Information Circular 8646, 46 p. Miyashiro, Akiho, 1974, Volcanic rock series in island arcs and active continental margins: American Journal of Science, v. 274, p. 321-355. Nash, J.T., 1973, Microprobe analyses of sericite, chlorite, and epidote from Jerome, Arizona: U.S. Geological Survey Journal of Research, v. 1, p. 673-678. Norman, G.W.H., 1977, Proterozoic massive sulfide replacements in volcanic rocks at Jerome, Arizona: Economic Geology, v. 72, p. 642-656. Ohmoto, Hiroshi, and Skinner, B.J., eds., 1983, The Kuroko and related volcanogenic massive sulfide deposits: Economic Geology Monograph 5, 604 p. Peacock, M.A., 1931, Classification of igneous rock series: Journal of Geology, v. 39, p. 54-67. Peccerillo, A., and Taylor, S.R., 1976, Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, northern Turkey: Contributions to Mineralogy and Petrology, v. 58, p. 63- 81. Provot, F.A., 1916, Jerome

district geology: Engineering and Mining Journal, v. 102, p. 1028-1031. Ralston, O.C., 1930, Possibilities of zinc production in Arizo na: Mining Journal, v. 14, p. 11. Ralston, O.C., and Hunter, W.C., 1930, Activation of sphalerite for flotation: American Institute of Mining and Metallurgical Engineers Transactions, v. 87, p. 411-416. Reber, L.E., Jr., 1922, Geology and ore deposits of Jerome district: American Institute of Mining and Metallurgical Engineers Transactions, v. 66, p. 3-26. 1938, Jerome district, i11 Some Arizona ore deposits: Arizona Bureau of Mines Bulletin 145, p. 41-65. Rickard, T.A., 1918, The story of the U.V.X. bonanza: Mining and Scientific Press, v. 116, p. 9-17,47-52. Riesmeyer, W.D., 1978, Precambrian geology and ore deposits of the Pecos mining district, San Miguel and Santa Fe Counties, New Mexico: Albuquerque, N. Mex., University of New Mexico M.S. thesis, 215 p. Gold Deposits in Metamorphic Rocks-Part I

Robertson, J.M., and Moench, R.H., 1979, The Pecos greenstone belt-A Proterozoic volcano-sedimentary sequence in the southern Sangre de Cristo Mountains, New Mexico: New Mexico Geological Society Guidebook 30th field conference, Santa Fe, p. 165-173. Saggerson, E.P., and Turner, L.M., 1982, General comments on the identification of chlorites in thin sections: Mineralogical Magazine, v. 46, p. 469-473. Sangster, D.F., 1972, Precambrian massive sulphide deposits in Canada- A review: Canadian Geological Survey Paper 72-22, 44 p. 1980, Distribution and origin of Precambrian massive sulphide deposits of North America: Geological Association of Canada Special Paper 20, p. 723-740. 1984, Grade-tonnage summaries of massive sulfide deposits relative to paleotectonic settings in the Appalachian-Caledonian orogen: Economic Geology, v. 79, p. 1479-1482. Shand, S.J., 1927, The eruptive rocks: New York, John Wiley, 488 p. Slavin, Morris, 1930, Ore-dressing microscopy at United Verde: Mining Congress Journal, v. 16, p. 386-387. Smith, H.D., and Sirdevan, W.H., 1921, Mining methods and costs at the United Verde mine: American Institute of Mining and Metallurgical Engineers Transactions, v. 65, p. 127- 181. Stacey, J.S., Doe, B.R., Silver, L.T., and Zartman, R.E., 1976, Plumbotectonics IIA, Precambrian massive sulfide deposits: U.S. Geological Survey Open-File Report 76-476, 26 p. Stacey, J.S., and Kramers, John, 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Sciences Letters, v. 26, p. 207-221. Geology and Resources of Gold In the United States Steiger, R.H., and Jaeger, Emile, 1977, Subcommission on geochronology; convention and use of decay constants in geo-and cosmochronology: Earth and Planetary Sciences Letters, v. 36, p. 359-362. Stensrud, H.L., and More, Suver, 1980, Precambrian geology and massive sulfide environments of the west-central Hualapai Mountains, Mohave County, Arizona-A preliminary report: Arizona Geological Society Digest, v. XII, p. 155-166. Storms, W.R., 1955, Selenium samples from the United Verde Branch, Phelps Dodge Corporation, Jerome, Arizona: U.S. Bureau of Mines Open-File report, 55 p. Vance, R.K., and Condie, K.C., 1986, Geochemistry and tectonic setting of the early Proterozoic Ash Creek Group, Jerome, Arizona: Geological Society of America Abstracts with Programs, v. 18, p. 419. 1987, Geochemistry of footwall alteration associated with the Early Proterozoic United Verde massive sulfide deposit, Jerome, Arizona: Economic Geology, v. 82, p. Vrba, S.L., 1980, Precambrian geology of the Cleator area, Yavapai County, Arizona: Flagstaff, Ariz., Northern Arizona University M.S. thesis, 96 p. Walker, R.R., Matulich, A., Amos, A.C., Watkins, J.J., and Mannard, G.W., 1975, The geology of the Kidd Creek mine: Economic Geology, v. 70, p. 80-89. White, Don, 1986a, Gold distribution at the United Verde Extension, a massive base-metal sulfide deposit, Jerome, Arizona, in Beatty, Barbara, and Wilkinson, P.A.K., eds., Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest, v. XVI, p. 330--338. 1986b, Gold exploration at the United Verde Extension mine, Jerome, Arizona: Geological Society of America Abstracts with Programs, v. 18, p. 422.

The Alaska-Juneau and Treadwell Lode Gold Systems, Southeastern Alaska By Thomas D. Light, David A. Brew, and Roger P. Ashley Abstract The Alaska-Juneau and Treadwell lode-gold systems (or deposits) in southeastern Alaska produced more than 6.7 million ounces of gold, about 75 percent of the total lode gold production from Alaska. Each system consists of four separate contiguous ore bodies aligned in a northwesterly direction along the regional structural trend. The composition and competence of the stratigraphic units as well as the structural setting were major factors in localizing the individual ore bodies. At the Alaska-Juneau deposit, gold, silver, and lead occur in quartz veins in hydrothermally altered amphibolite, phyllite, and schist. Albite diorite sills at the Treadwell deposit host gold-bearing calcite and quartz veins and stringers as well as disseminated gold. Both lode-gold systems were emplaced, probably between 50 and 60 million years ago, at or near the end of a major regional compressive stress event and before or during the uplift of the Coast Mountains. Fluid inclusion and isotopic compositions suggest that the ore-forming fluids were derived from the metamorphism of deeper rocks rather than from circulating meteoric waters. These physical, chemical, and temporal conditions coincide with the transition from strongly compressive subduction to transpressive collision along the western margin of North America, and indicate an environment of formation similar to that of the California Mother Lode deposits. INTRODUCTION The Alaska-Juneau (A-J) and Treadwell systems represent the two largest lode gold deposits in Alaska, and together produced more than 6.7 million oz of gold. These deposits are located about 6 km apart on opposite sides of Gastineau Channel near Juneau, along the western flank of the Coast Mountains in southeastern Alaska (fig. 011). The deposits lie in the western metamorphic zone of the northern Coast plutonicmetamorphic complex (Brew and Ford, 1984a). The A-J system comprises four separate ore bodies in Silverbow Basin in upper Gold Creek: the Ebner mine on the northwest end of the system, the North and South ore bodies of the Alaska-Juneau mine, and the Perseverance mine to the southeast (fig. 012). The Treadwell system on Douglas Island includes, from northwest to southeast, the Treadwell, the 700-Foot, the Mexican, and the Ready Bullion mines, located contiguously along 1,200 m of a mineralized albite diorite sill. HISTORY, PRODUCTION, AND RESERVES Gold was first discovered in Gold Creek, at the present site of Juneau, Alaska, during the ummer of 1880 by Richard T. Harris and Joseph Juneau. Subsequently, placer deposits in Gold Creek and other nearby streams were worked for several years. The Alaska-Juneau Gold Mining Company was organized in 1897, and it mined open pits on both the North and South ore bodies in Silverbow Basin east of Juneau. Underground operations began in 1903 and continued until 1944 (Twenhofel, 1952). The Ebner mine was worked intermittently until 1912, when U.S. Smelting and Refining Co. acquired the property. The property was idle from 1917 to 1925, when arrangements were made with the Alaska-Juneau Gold Mining Co. to mine and mill ore from the Ebner (Twenhofel, 1952). Spencer (1906) listed total production from the Ebner mine at about 29,000 oz gold. No figures are available for production after 1905. The Perseverance mine began production in 1895, and was worked intermittently until it was acquired by the Alaska-Gastineau Mining Co. in 1912. From 1895 to 1921, approximately 500,000 oz gold was recovered from the Perseverance mine. The property was idle from 1921 until1933, when it was purchased by the Alaska-Juneau Gold Mining Co. and was integrated into the AlaskaJuneau mine (Twenhofel, 1952). Total lode production from the A-J system was approximately 3.5 million oz gold, 1.9 million oz silver, and 40.2 million lb lead (Berg, 1984). In addition, more than 120,000 oz gold was recovered from placer deposits · in Gold Creek and Silverbow Basin (Spencer, 1906). The A-J deposit was mined predominantly by modified block-caving, and hand-sorting was used to eliminate much of the waste. This hand-cobbing of the sulfide-bearing quartz veins eliminated approximately 45 percent of the rock mined, and raised the tenor of the ore from 0.045 to 0.086 oz Au/ton (Wayland, 1939; Gold Deposits In Metamorphic Rocks-Part I

EXPLANATION Co11st plutonicmetamorphic complex coast plutonic complex sill belt Juneau gold belt

Coast

Range megaline!lent 50 KILOMETERS Figure 011. Index map of southeastern Alaska showing the location of Juneau. Twenhofel, 1952). Mining at the Alaska-Juneau mine ceased in April 1944. At that time a total of 99,000,000 tons of ore had been mined from the A-J system, and the known ore reserves were about 1.4 million oz gold (Stone and Stone, 1980). The Alaska Mill and Mining company was founded in 1882 and worked the Treadwell mine, of the Treadwell Geology and Resources of Gold In the United States system. This property was acquired by the Alaska Treadwell Gold Mining Company in 1889. The Alaska Mexican Gold Mining Company owned the Mexican mine, and the Alaska United Gold Mining Company owned the 700Foot and Ready Bullion mines. The companies were financially separate, but the Alaska Treadwell Gold Mining Company managed all the properties. The

3 KILOMETERS EXPL ANATION Tonalite sill (Tertiary end Cretaceous) Phyllite (Tertiary end Cretaceous)

Chlorite schist

Hornblende schist

(Tertiary end Cretaceous) ~Marble and calc-silicate grenofels

(Tertiary end Cretaceous) Hornblende diorite (Cretaceous) Stephens Passage Group

Seymour Canal Formation

Mine. open pit Mine, adit or shaft Fault Figure 012. Generalized geologic map of Juneau area showing locations of mines in the A-J and Treadwell systems (from Brew and Ford, 1985). Treadwell system, comprising the Treadwell, 700-Foot, Mexican, and Ready Bullion mines, was worked to a depth of about 950 m, below which the gold grade became uneconomical. In addition to the underground workings, the Treadwell mine operated an open pit until 1906. At that time the glory hole was 550 m long, 140m wide, and 150 m deep (Stone and Stone, 1980). More than half the ore produced from the Treadwell system came from the Alaska-Treadwell mine (Stone and Stone, 1980). The 700-Foot mine was named for the width of the strip of land which separated the Alaska-Treadwell and the Mexican mines. Approximately 247,000 oz gold was produced from the 700-Foot mine (Wells and others, 1985). The Ready Gold Deposits In Metamorphic Rocks-Part I

Bullion mine was separated from the Mexican mine by 1,000 m of nearly barren ground on the surface, but productive underground .workings from the two mines extend nearly this entire distance (Spencer, 1905). The Ready Bullion and the Mexican mine each produced more than 500,000 oz gold (Wells and others, 1985). The Alaska-Treadwell, Mexican, and 700-Foot mines partially caved and were flooded by seawater in 1917. The Ready Bullion mine continued to operate until 1922, when decreasing ore tenor forced closure. In 1928 the Treadwell complex was sold to the Alaska-Juneau Gold Mining Co. Total production for the mines in the Treadwell system was approximately 29 million tons of ore containing 3.2 million oz gold, for an average concentration of 0.11 oz Au/ton (Stone and Stone, 1980; Berg, 1984). No figures are available for total silver or lead production. In 1972 the Alaska Electric Light and Power Company and the City and Borough of Juneau acquired all the properties that constitute the A-J and Treadwell systems (Stone and Stone, 1980). In 1988 the properties were under lease to Echo Bay Mining Company to evaluate the potential for additional development. COMPARISONS WITH SIMILAR DEPOSITS The A-J and Treadwell systems comprise a group of mesotherma1 to hypothermal quartz veins that postdate deformation and regional metamorphism in an accretionary continental margin setting. In the A-J, hydrothermal alteration, vein mineralogy, and chemistry of mineralizing fluids have been considered similar to the California Mother Lode system (Leach, Goldfarb, and Light, 1987). Both the A-J and Treadwell deposits have many features in common with low-sulfide, gold-quartz veins (Cox and Singer, 1986); also described as orogenic gold-quartz veins (Bohlke, 1982); but both deposits also have important features that are not typical of goldquartz veins. In the case of the A-J deposit, all major features of geologic setting, ore and gangue mineralogy, alteration effects, and ore controls are typical of lowsulfide gold-quartz veins, except for the effects of unusual biotitic alteration, and the effects of carbonate alteration that are less prominent than in most other deposits. In addition, grade and tonnage characteristics of the A-J are strikingly different from those of more than 300 low-sulfide gold-quartz deposits used by Cox and Singer (1986) to define cumulative grade and tonnage curves for this deposit type. The gold grade of ores from the A-J is lower and the tonnage higher than in any deposit in the Cox and Singer data base, and data for the A-J were not included in their grade and tonnage curves. Geology and Resources of Gold in the United States The Treadwell system has a number of features that are unusual for low-sulfide gold-quartz veins, including character of alteration effects, gangue, and ore minerals. Carbonate is a prominent alteration and gangue phase in the Treadwell system and is typical of low-sulfide gold-quartz deposits. However, at the Treadwell the carbonate is calcite rather than the more common ferroan carbonate. The ore mineral suite includes arsenic-bearing minerals such as realgar, orpiment, and native arsenic, which are more commonly associated with shallower and lower temperature deposits. The most unusual feature, perhaps, is the paucity of quartz in the gangue assemblage. Tonnage and grade values for the Treadwell, however, fall on the curves of Cox and Singer (1986), and the Treadwell was included in their low-sulfide gold-quartz-deposit data base. Smaller deposits are scattered throughout the Juneau gold belt, which extends for more than 160 km along the western flank of the Coast Mountains, from Comet on the north to Holkham Bay on the south. The gold deposits that define the Juneau gold belt occur along both sides of the Coast Range megalineament and along the western edge of a tonalite sill in the western Coast plutonic-metamorphic complex (Brew and Ford, 1984b ). Although little detailed information is available on ore and gangue mineralogy, wallrock alteration effects, grades, production, or reserves for these deposits, they appear to be mostly typical low-sulfide gold-quartz veins, with quartz as the major gangue mineral. Sulfide mineralogy commonly is relatively simple, and is dominated by arsenopyrite, pyrite, pyrrhotite, galena, chalcopyrite, and sphalerite (Spencer, 1906). ALASKA-JUNEAU SYSTEM The A-J system occurs in the structurally lowest part of the Perseverance Slate, a 1.5-km-wide belt of carbonaceous and graphitic quartz-sericite phyllite, schist, and black slate with minor carbonaceous limestone and numerous sill-like lenses of amphibolite (fig. D13). The slates are bounded by greenstones to the west and schists to the east, all striking N. 50°-70° W., parallel to the regional structural trend. The slates dip from vertical in Sheep Creek on the south, to about 60° NE. in Silverbow Basin, and to 40° NE. farther to the northwest (Wayland, 1939). The age of the protoliths of the Perseverance Slate and adjacent metavolcanic rocks is interpreted to be Permian(?) and Late Triassic (Brew and Ford, 1977). The amphibolite lenses within the Perseverance Slate range in size from a few centimeters wide and several meters long to more than 300 m wide and 3 km long. The amphibolites were derived from a gabbroic protolith (composed of hornblende, augite, and

sw NE 750 METERS 0 SEA LEVEL EXPLANATION Metagabbro, amphibolite schist, and greenstone Slate and quartzite Ore body Fault Contact Figure 013. Cross section through the North ore body of the Alaska-Juneau mine (from Wernecke, 1932). labradorite) during regional metamorphism (Twenhofel, 1952). Two types of amphibolite have been recognized. One is a green, relatively unaltered rock composed of about 50 percent hornblende in a groundmass of plagioclase, quartz, and zoisite. The other type is a brown amphibolite derived from the green amphibolite by hydrothermal alteration associated with the emplacement of the mineralized quartz veins. This hydrothermal alteration formed biotite, ankerite, and sericite with some chlorite and albite at the expense of hornblende and feldspar, and destroyed the schistosity of the amphibolite (Twenhofel, 1952). The brown amphibolite is composed of about 35 percent biotite, 35 percent ankerite, 20 percent feldspar (andesine to oligoclase with minor albite), and varied amounts of pyrrhotite, chlorite, and sericite. Most of the ankerite is later than and follows the quartz veins (Wayland, 1939). Effects of hydrothermal alteration have been traced as much as 1 km away from the mineralized zone of the A-J system, and they increase progres ively toward the center of the sy tern. Initially magnetite, and closer to the deposits both magnetite and ilmenite, di appear at the expense of pyrrhotite±pyrite (Newberry and Brew, 1988). Numerous faults cut the slates and greenstones in the vicinity of the ore bodies. Most of the faults are parallel or subparallel with the foliation and compositional layering, but splitting and converging of the faults are common (Wayland, 1939). Three set of joints that predate the mineralized veins have been recognized: N. 30° E., 76° NW.; N. 34° W., 40° SW.; N.- S., 4SO W. (Twenhofel, 1952). The Alaska-Juneau deposit has been cut by the Silverbow fault, trending nearly east-west and dipping 75° N., and hundreds of subsidiary faults (Twenhofel, 1952). The Silvcrbow is an oblique normal fault with the north side (hanging wall) moved about 550 m to the west and downward about the same distance (Wayland, 1939). The A-J vein system extends laterally 6 km from Mt. Juneau southeast to Sheep Mountain, and i restricted to the lower 100 m of the Perseverencc Slate. The system extends vertically from the surface to a depth of more than 700 m and contains numerous gold-bearing quartz veins and stringers from a few centimeters to 1 rn in width. Individual veins extend horizontally for a few tens to several hundred meters, and have height-to-width ratios ranging from 1:1 to more than 50:1 (Barton and Light, 1987). Early barren veins reportedly tend to follow the trend of the schistosity (Wayland, 1939). The goldbearing quartz veins have well-defined walls within both the slate and amphibolite. The veins commonly make an angle of about 20° to the schistosity, strike from north to N. 70° W., and dip nearly vertical (Wayland, 1939). The veins are boudinaged in both the vertical and horizontal planes, indicating extension in more than one direction, and in places have been sheared and rotated (Barton and Light, 1987). Because the veins have a fairly consistent gold content, the most favorable ground is that with the greatest density of veining. The gold grade may be higher in veins near interfingering slate and amphibolite. High gold concentrations do not occur in veins restricted either to the massive green amphibolite or to slate without amphibolite lenses (Twenhofel, 1952). The veins of the A-J system are about 95 percent quartz with subsidiary ankerite, pyrrhotite, galena, sphalerite, arsenopyrite, pyrite, and gold (Twenhofel, 1952). Wayland (1939, 1960) described the paragenetic sequence of base and precious metals at the AlaskaJuneau mine (fig. D14). The gold in the A-J deposit is contained almost entirely within the quartz stringers and veins (Wayland, 1939). Gold is the last of the metals to have been deposited, and it tends to be concentrated with Gold Deposits In Metamorphic Rocks-Part I

Biotite Quartz Ankerite

- - - Pyrite Arsenopyrite Specu Ia rite Pyrrhotite

Sphalerite

Chalcopyrite

Galena

Gold

Early Late Figure 014. Paragenesis of minerals in the Alaska-Juneau South ore body (from Wayland, 1960). sulfide minerals along the edges of the larger quartz veins, along the boundaries of sulfide minerals, and in late fractures in ankerite and quartz. Although much of the gold occurs along sulfide boundaries, approximately 90 percent is free gold and is easily released during crushing (Twenhofel, 1952). Silver occurs in recoverable quantities in the A-J d~posit. Native silver is not known to occur, and it has been estimated that 23 percent of the silver occurs with gold in electrum, and the remainder is combined with galena (Twenhofel, 1952). Pyrrhotite is by far the most common sulfide mineral, and occurs in the altered wallrocks as well as in the quartz veins (Wayland, 1939). Pyrrhotite constitutes almost 1 percent of the vein material in the AlaskaJuneau mine, but does not appear to be correlated with gold distribution (Twenhofel, 1952). The abundance of pyrrhotite in the A-J system was interpreted by Newberry and Brew (1987) as one line of evidence supporting an epigenetic rather than syngenetic mineralization. Sphalerite is abundant in the Alaska-Juneau mine; it is essentially contemporaneous with pyrrhotite and predates most of the galena. Galena also is abundant in the Alaska-Juneau mine, and is one of the last sulfides to be deposited. It normally occurs near the center of quartz stringers (Wayland, 1939). Pyrite and arsenopyrite are Geology and Resources of Gold In the United States common sulfide minerals in the A-J deposit and were among the earliest sulfides to be deposited (Wayland, 1939). Chalcopyrite occurs in minor amounts in the A-J deposit, commonly associated with sphalerite (Twenhofel, 1952). Ankerite, aside from quartz, is the most abundant gangue mineral, and it normally occupies the edges of veins, separating the quartz from the country rock. Ankerite formed later than all the other minerals except gold (Twenhofel, 1952). Sericite occurs both in the host rocks as a metamorphic mineral and as a late-stage hydrothermal mineral along vein edges (Twenhofel, 1952). In general, the abundance of sulfide rrunerals in the veins increases from the Ebner deposit, where sulfides are minor and pyrrhotite is the dominant species, to the South ore body, where sulfides are abundant and varied. Southeast of the South ore body, at the Perseverance mine, pyrrhotite is still the dominant sulfide, but sphalerite and galena are more abundant and arsenopyrite reaches its greatest abundance. The sulfide content decreases in Sheep Creek (immediately south of the Perseverance mine), where sphalerite and galena dominate. The gold:silver ratio gradually decreases to the southeast. Gold:silver ratios from the Ebner mine were 7:1 (Spencer, 1906) and 6.5:1 (Scott, 1932). Smith (1941)

reported a gold:silver ratio of 5.6:1 for the South ore body. At the Perseverance mine, gold:silver ratios vary from 1:1 (Wayland, 1939) to 1:3 and 1:10 (Spencer, 1906). Analyses of fluid inclusions in sulfide-bearing quartz veins from the South ore body of the AlaskaJuneau mine indicate that the ore fluids can be characterized by the H 20-C02-N2-NaCl system (Goldfarb and others, 1986). C02 is the dominant gas and is estimated to compose 30-60 bulk mole percent of the ore-forming fluids at the Alaska-Juneau mine (Leach, Goldfarb, and Hofstra, 1987). H 20 -rich and C02-rich inclusions, interpreted as being coevally trapped, represent immiscible fluids trapped in a boiling system. A minimum trapping pressure of 1.5 Kb, a minimum depth of emplacement of 5.5 km, and a minimum trapping temperature of 230 °C were determined from the intersection of isochores for the two types of fluid inclusions (Goldfarb and others, 1986). The intense hydrofracturing of the metasedimentary host rocks, which contributed to the development of abundant quartz-carbonate veins at the Alaska-Juneau south pit, was probably caused by fluid immiscibility (boiling) of the mineralizing fluids (Leach, Goldfarb, and Light, 1987). A preliminary oxygen-isotope study yielded o180 values ranging from + 17.1 per mil to + 18.1 per mil for quartz vein material from the Alaska-Juneau mine (four samples). Assuming isotopic equilibrium between quartz and fluid at a minimum temperature of 230 oc for deposition of the vein quartz, the hydrothermal fluids that produced the deposit would have had o180 values in the range of + 6 to + 8 per mil, which is within the range of values inferred for primary magmatic waters (Taylor, 1979). However, if temperatures of deposition of vein quartz significantly exceeded 230 °C, the o180 of hydrothermal fluids would be greater than the maximum value of + 9 per mil for primary magmatic waters. Fluids with o180 in the range of + 9 to + 11 per mil, which would result from temperatures of deposition in the vicinity of 300 oc, match the range of values for waters isotopically equilibrated with volcanic rocks, volcanogenic sediments, or graywackes under greenschistto amphibolite-facies conditions ( + 5 to + 13 per mil; Taylor, 1979; Fyfe and Kerrich, 1984). Analyses of hydrothermal sericite from the Alaska-Juneau mine give oD values of -61 per mil and calculated values for fluids in equilibrium of -20 to -30 per mil. These values fall well within the metamorphic water field (R.J. Goldfarb, written commun., 1988). The age of mineralization at the Alaska-Juneau South pit is believed to be about 55.3±2.1 Ma, based on a K-Ar age of sericite from a gold-bearing vein (Goldfarb and others, 1987). This age is supported by a Rb-Sr isochron at 57 Ma on vein materials (Rainer Newberry, written commun., 1987). Because the gold-bearing quartz veins cut the metamorphic foliation in the slates and phyllites, the maximum age of mineralization in the A-J system is also constrained by the age of the metamorphism, which has been dated at 57-60 Ma by U-Pb and K-Ar techniques (Newberry and Brew, 1988). The tonalite sill of the .western Coast plutonicmetamorphic complex (Brew and Ford, 1984b) has been dated at about 67 Ma (Gehrels and others, 1984); it was emplaced shortly after the peak of metamorphism in the area of most intense deformation (Brew and others, 1976; Brew and Ford, 1984b ). Compression related to the emplacement of the tonalite sill continued until approximately 60 Ma. Emplacement of the mineralized quartz veins in fractures was postkinematic and followed relaxation of the compressional stress field, and therefore the maximum age of the A-J system must be less than 60 Ma. Igneous rocks dated at 50 Ma (Gehrels and others, 1984) in the Juneau vicinity were emplaced at relatively shallow levels, compared with mm1mum vein emplacement depths of 5.5 km determined from fluid inclusion trapping conditions (Leach and others, 1986; Leach, Goldfarb, and Light, 1987), suggesting that these intrusions are younger than the veins and thus constrain the minimum age of the mineralizing system. TREADWELL SYSTEM The mines in the Treadwell system are in a series of altered and mineralized albite diorite sills that intrude the Seymour Canal Formation along the contact between the black slates that form the footwall (Treadwell Slate) and a greenstone unit that forms the hanging wall (Spencer, 1905). The black slates of the Seymour Canal Formation contain abundant carbonaceous material and are Jurassic to Cretaceous in age (Brew and Ford, 1977). In general, the foliation in the Seymour Canal Formation trends northwest, parallel to the regional structure, and dips about 50°-70° NE. Locally the slate beds have been distorted into small folds a meter or so across, probably formed during the intrusion of the diorite sills ('I'wenhofel, 1952). The greenstone, which forms the hanging wall of the ore deposits, is about 100 m thick, and is a highly altered diabase or andesite. It is locally schistose, and follows the strike and dip of the foliation in the encompassing slates (Spencer, 1905). The mineralized diorite sills are several kilometers long and more than a hundred meters thick, and they parallel the regional structure. They are composed mainly of albite, calcite, and quartz, with lesser amounts of biotite, chlorite, epidote, hornblende, magnetite, pyrite, pyrrhotite, rutile, sericite, stibnite, and zoisite (Spencer, 1905). Intrusion of the sills into the Treadwell slate caused the development of some hornfels texture, but contact aureoles are limited in areal extent (Twenhofel, 1952). Gold Deposits In Metamorphic Rocks-Part I

Ore zones in the Treadwell system consist of base and precious metals disseminated (1) in the albite diorite sills and (2) in reticulated veins and stringers of calcite and quartz from a few millimeters to a few centimeters in width and constituting about one-fifth of the volume of the sills (Spencer, 1905). Calcite, by far the most abundant mineral, occurs in the veins and stringers and is disseminated throughout the sills. Disseminated quartz rarely occurs in the sills and is restricted essentially to the veins and stringers. Calcite commonly occurs without quartz, but where both minerals occur in the same vein let, calcite normally occupies the border of the quartz veinlet and is disseminated into the sill rock (Spencer, 1906). Locally, gold-bearing veins and stringers extend out into the slates that form the footwall of the mineralized albite diorite sills (Spencer, 1905). Three different types of ore have been recognized in the Treadwell system: (1) "white ore" is the most abundant and consists of mineralized and altered albite diorite sill rock; (2) "mixed ore" is formed where small fine-grained sills and stringers of albite diorite have pervaded the slate and where sulfides are disseminated throughout both the slate and sills; (3) "brown ore" is altered and mineralized slate adjacent to the sills and represents a minor portion of the total production (Spencer, 1905, 1906). Metallic minerals constitute about 2 percent of the Treadwell ores. Pyrite, the most abundant sulfide, both is disseminated throughout the sill and occurs in the veinlets. Gold, chalcopyrite, galena, sphalerite, molybdenite, native arsenic, realgar, and orpiment have all been observed in small quantities (Spencer, 1905). Because the gold is irregularly distributed throughout the albite diorite sills, the major control of ore localization was probably the fracture-induced permeability of the rock. In general, higher gold <:concentrations tend to be associated with a greater density of quartz and calcite veinlets (Spencer, 1905). Although 60-75 percent of the gold is free milling, visible gold is rare in both the sill rock and the veinlets. Some of the gold is associated with the pyrite, but the distribution of pyrite and the distribution of gold seem to be largely independent (Spencer, 1905). A preliminary oxygen-isotope study yielded o 180 values ranging from + 16.3 to + 20.8 per mil for quartzcalcite-albite vein material from the Treadwell system. Values of oD of - 62 per mil for hydrothermal chlorite yield calculated values of -20 to -30 per mil for fluids in equilibrium, and fall well within the metamorphic water field (R.J. Goldfarb, written commun., 1988). These values suggest that temperatures of vein deposition and isotopic composition of fluids were similar to those at the A-J system, and that the mineralizing fluids were released during metamorphism of rocks subducted beneath the Coast Mountains. Geology and Resources of Gold in the United States Neither the age of the albite diorite sills nor the age of mineralization in the Treadwell system is known. The sills may be as old as or younger than a 90 Ma intrusive event known from northeastern Admiralty Island, some 25 km to the southwest. The sills are not foliated and therefore postdate the major regional metamorphism of the adjacent rocks. Regional studies show that the Seymour Canal Formation, which is Late Jurassic and Early Cretaceous in age (Loney, 1964), underwent low-grade greenschist facies dynamothermal metamorphism sometime before 100 Ma. Metamorphism associated with the deformation of the Perseverance slate (62- 69 Ma) overprints the effects of the earlier metamorphic event, and the two may not be distinguishable in the vicinity of the Treadwell system. The maximum age of mineralization in the Treadwell system is therefore interpreted to be no older than 90 Ma and may be younger than 60- 69 Ma. The similarity in isotopic composition and paragenesis between the Treadwell and A-J systems suggests that the Treadwell system is equivalent in age to the A-J system. CONCLUSIONS The A-J (Alaska-Juneau) system is restricted to a 6-km-long zone in the lowest 100 m of the carbonaceous Perseverance Slate, just above the greenstones of the Gastineau Volcanic Group. The highest gold concentrations are found in quartz veins and stringers near where altered amphibolite lenses finger out into the slate. The slate has both well-developed joint sets and a locally well developed axial plane cleavage. These relationships suggest that lithology and permeability were important factors in localizing ore deposition. The Treadwell system occurs as reticulated veins and stringers in a highly fractured albite diorite sill 70 m wide and 3 km long. Gold is disseminated throughout the sill and also is hosted in calcite and quartz veins and stringers cutting the dike and footwall slates. The placement of the albite diorite sill between ductile greenstone and slate walls probably intensified the fracturing of the sill and greatly increased the permeability, which contributed to the localization and concentration of the ores. The postmetamorphic intrusive rocks of the Coast plutonic-metamorphic complex have been thought to be related to the genesis of gold deposits in the Juneau gold belt (Dawson, 1889; Spencer, 1906). Twenhofel and Sainsbury (1958) postulated that mineralization in the Juneau gold belt was spatially related to prominent northwest-trending features, one of which is known as the Coast Range megalineament (Brew and Ford, 1978); the megalineament is interpreted to represent the surface expression of the buried contact between the slates, phyllites, and greenstones to the southwest and the

more coarsely crystalline plutonic and metamorphic rocks to the northeast. The location of the A-J and Treadwell systems on either side of this major lineament suggests that it had some regional influence on the migration of ore fluids. Ore distributions in both the A-J and Treadwell systems suggest that the mineralization was postkinematic and structurally controlled, and because the age of regional metamorphism may be the same at both deposits, both systems may have formed at about the same time. The tonalite sill body closest to the the A-J deposit is well foliated and is enclosed in highly deformed rocks. It is currently interpreted to be prekinematic in relation to the major compressive deformation, while slightly more distant tonalite sill bodies are younger and are late synkinematic with respect to the regional metamorphism (Ford and Brew, 1981). The more distant sill bodies have been dated at 62-67 Ma (Gehrels and others, 1984). The fractures along which the mineralizing fluids were channeled were developed in a noncompressive regime, and postdate the regional metamorphism, both types of tonalite sill bodies, and also the slightly foliated plutons northeast of the tonalite sill that are dated at about 60 Ma (Gehrels and others, 1984). Nearby, 50 Ma intrusions are relatively shallow, based on regional considerations. The estimated emplacement depths of these intrusions are less than those inferred from the study of fluid inclusions in vein material. Still younger intrusions known to occur elsewhere in the Coast Mountains have been dated at about 20 Ma; they are very shallow and certainly postdate the mineralization. These relations are interpreted to limit the age of mineralization at the A-J deposit between 60 Ma and 50 Ma. During the interval from 56 to 43 Ma, the relative motion along the western margin of southeastern Alaska changed from strongly convergent, with the northeasterly subduction of the Kula Plate at about 120 km/m.y., to transpressive, with the northward migration of the Pacific plate at about 40 km/m.y. (Engebretson and others, 1985). If the mineralization of the A-J and Treadwell systems occurred at about the same time, then both systems probably developed early in this period and were accompanied by uplift of the Coast Mountains. Relaxation of the regional stress field allowed the influx of large volumes of hydrothermal fluids from depth. The rapid migration of fluids caused intense fracturing in the more competent amphibolite lenses in the A-J system and in albite diorite sills in the Treadwell sy tem. Significant decrease in the confining pressure and temperature also promoted boiling of the fluids and precipitation of metals to form the A-J and Treadwell deposits. Manuscript received by scientific editors February 1986 REFERENCES CITED Barton, C.C., and Light, T.D., 1987, Structural fabric analysis of the Perseverance slate and gold-bearing quartz veins in the south ore body of the Alaska-Juneau lode system, southeastern Alaska, in Hamilton, T.D., and Galloway, J.P., eds., Geologic studie in Alaska by the U. S. Geological Survey during 1986: U.S. Geological Survey Circular 998, p. 110-112. Berg, H .C., 1984, Regional geologic summary, metallogenesis, and mineral resources of southea tern Alaska: U.S. Geological Survey Open-File Report 84-572, 298 p. Bohlke, J.F.P., 1982, Orogenic (metamorphic-ho ted) goldquartz veins, in Erickson, R.L., 1982, Characteri tics of mineral deposit occurrences: U.S. Geological Survey Open-File Report 82-795, p. 62-68. Brew, D.A., and Ford, A.B., 1977, Preliminary geologic and metamorphic-isograd map of the Juneau B- 1 quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-846, calc 1:31,680. 1978, Megalineament in southeastern Alaska marks southwest edge of Coast Range batholithic complex: Canadian Journal of Earth Science, v. 15, p. 1763-1772. The northern Coast plutonic complex, southeastern Alaska and northwestern British Columbia, in Coonrad, W.L., and Elliott, R.L., eds., The United States Geological Survey in Alaska-Accomplishments during 1981: U.S. Geological Survey Circular 868, p. 120-124. 1984b, Timing of metamorphism and deformation in the Coast plutonic-metamorphic complex, near Juneau, Alaska: Geological Society of America Annual Meeting, Cordilleran Section, 80th, Anchorage, Alaska, May 30-31, June 1, 1984, Abstracts with Programs, v. 16, no. 5, p. 272. 1985, Preliminary reconnaissance geologic map of the Juneau, Tak."U River, Atlin, and part of the Skagway 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 85-395, 23 p. Brew, D.A., Ford, A.B., Grybeck, D., Johnson, B.R., and Nutt, C.J ., 1976, Key foliated quartz diorite sill along southwest side of Coast Range complex, northern southeastern Alaska, in Cobb, E. H., ed., The United States Geological Survey in Alaska-Accomplishments during 1975: U.S. Geological Survey Circular 733, p. 60. Cox, D.P., and Singer, D. A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p. Dawson, G.M., 1889, Notes on the ore deposit of the Treadwell mine, Alaska: American Gemologist, v. 4, p. 84-88. Engebretson, D.C., Cox, Allan, and Gordon, R.G., 1985, Relative motions between oceanic and continental plates in the Pacific Basin: Geological Society of America Special Paper 206, 59 p. Ford, A.B., and Brew, D.A., 1981, Orthogneiss of Mount Juneau-An early phase of Coast Mountain plutonism involved in Barrovian regional metamorphism near Gold Deposits in Metamorphic Rocks-Part I

Juneau, in Albert, N.R.D., and Hudson, Travis, eds., The United States Geological Survey in Alaska-Accomplishments during 1979: U.S. Geological Survey Circular 823-B, p. B99-B102. Fyfe, W.S., and Kerrich, R., 1984, Gold-Natural concentration processes, in Foster, R.P., ed., Gold '82, The geology, geochemistry, and genesis of gold deposits: Rotterdam, A.A. Balkema, p. 99-127. Gehrels, G.E., Brew, D.A., and Saleeby, J. B., 1984, Progress report on U- Pb (zircon) geochronologic studies in the Coast plutonic-metamorphic complex east of Juneau, southeastern Alaska, in Reed, K.M., and BartschWinkler, Susan, eds., The United States Geological Survey in Alaska-Acomplishments during 1982: U.S. Geological Survey Circular 939, p. 100-102. Goldfarb, RJ ., Leach, D.L., Light, T.D., Patterson, C.J., Pickthorn, L.B., and Pickthorn, W.J., 1987, The Juneau gold belt-A mother lode-type deposit in southeastern Alaska: Geological Society of America Abstracts with Programs, v. 19, no . 6, p. 382. Goldfarb, R.J., Light, T.D., and Leach, D.L., 1986, Nature of the ore fluids at the Alaska-Juneau gold deposit, in Bartsch-Winkler, Susan, and Reed, K.M., eds., Geologic studies in Alaska by the U.S. Geological Survey during 1985: U.S. Geological Survey Circular 978, p. 92-95. Leach, D.L., Goldfarb, R.J., and Hofstra, A.H., 1987, Fluid inclusion characteristics· of the Juneau gold belt, southeastern Alaska: Geological Society of America Abstracts with Programs, v. 19, no. 6, p.398. Leach, D.L., Goldfarb, R.J., and Light, T.D., 1987, Fluid inclusion constraints on the genesis of the Alaska-Juneau gold deposit, in Elliott, I.L., and Smee, B.W., eds., Geoexpo/86: Exploration in the North American Cordillera, Association of Exploration Geochemists, Ontario, Canada, p. 150-159. Leach, D.L., Light, T.D., and Goldfarb, R.J., 1986, Fluidinclusion constraints on the genesis of the Alaska-Juneau gold deposit [ abs.]: Association of Exploration Geochemists Annual Meeting, Vancouver, Canada, May 12-14, 1986, p. 53-54. Loney, R.A., 1964, Stratigraphy and petrography of the PybusGambier area, Admiralty Island, Alaska: U.S. Geological Survey Bulletin 1178, 103 p. Geology and Resources of Gold in the United States Newberry, R.J., and Brew, DA., 1987, The Alaska-Juneau gold deposit; remobilized syngenetic versus exotic epigenetic origin, in Hamilton, T.D., and Galloway, J.P., eds., Geologic studies in Alaska by the U.S. Geological Survey during 1986: U.S. Geological Survey Circular 998, p. 1988, Alteration zoning and origin of the AlaskaJuneau gold deposit, in Galloway, J.P., and Hamilton, T .D., eds., Geologic studies in Alaska by the U.S. Geological Survey during 1987: U.S. Geological Survey Circular 1016. Scott, W.P., 1932, Milling methods and ore-treatment equipment: Engineering and Mining Journal, v. 133, p. Smith, P.S., 1941, Fineness of gold from Alaska placers: U.S. Geological Survey Bulletin 910-C, p. 147-272. Spencer, A.C., 1905, The Treadwell ore deposits, Douglas Island: U.S. Geological Survey Bulletin 259, p. 69- 87. 1906, The Juneau gold belt, Alaska: U.S. Geological Survey Bulletin 287, 137 p. Stone, D., and Stone, B., 1980, Hard rock gold-The story of the great mines that were the heartbeat of Juneau: City and Dorough of Juneau Centennial Committee, Juneau, Alaska, 108 p. Taylor, H.P., 1979, Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits, in Barnes, H.L., ed., Geochemistry of hydrothermal ore deposits, 2nd edition: New York, John Wiley, p. 236-277. Twenhofel, W.S., 1952, Geology of the Alaska-Juneau lode system, Alaska: U.S. Geological Survey Open File Report 52-160, 180 p. Twenhofel, W.S., and Sainsbury, C.L., 1958, Fault patterns in southeastern Alaska: Geological Society of America Bulletin, v. 69, p. 1431-1442. Wayland, R.G., 1939, Geology of the Juneau region, Alaska, with special reference to the Alaska Juneau ore body: Minneapolis, Minn., University of Minnesota Ph. D. thesis, 93 p. 1960, The Alaska Juneau gold ore body: Neues Jahrbuch fur Mineralogic Abhandlungen, v. 94, p. 267- 279. Wells, D.E., Pittman, T.L., Brew, D.A., and Douglass, S.L., 1985, Map and description of the mineral deposits in the Juneau, Tak-u River, Atlin, and part of the Skagway quadrangles, Alaska: U.S. Geological Survey Open-File Report 85-717, 332 p. Wernecke, L., 1932, Geology of the ore zones: Engineering and Mining Journal, v. 133, p. 494-499. -(:( U.S. GOVERNMENT PRINTING OFFICE: 199(). n~716027

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DeWitt, Light, and others-GOLD IN METAMOR ~HIC ROCKS, I-U.S. Geological Survey Bulletin 1857- D

Plates & figures from the original