Gold-bearing skarns
In recent years, a significant proportion of the mining industry's interest has been centered on discovery of gold deposits; this includes discovery of…
Public-domain full text preserved in the Mountain Man Mining Library. Original source: pubs.usgs.gov.
Gold-Bearing Skarns U.S. GEOLOGICAL SURVEY BULLETIN 1930
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Gold-Bearing Skarns By TED G. THEODORE, GRETA J. ORRIS, JANE M. HAMMARSTROM, and JAMES D. BLISS U.S. GEOLOGICAL SURVEY BULLETIN 1930
U.S. 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, WASHINGTON : 1991 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center, Box 25425 Denver, CO 80225 Library of Congress Cataloging-in-Publication Data Gold-bearing skarns I by Ted G. Theodore ... [et al.} p. em.- (U.S. Geological Survey bulletin : 1930) Includes bibliographical references. Supt. of Docs. no.: I 19.3: 1930 1. Skarn. 2. Gold ores. I. Theodore, Ted. G. II. Series. QE75.89 no. 1930 [QE475.A2} 557.3 s-dc20 [553.4'1} CIP
CONTENTS Abstract 1 Introduction Acknowledgments 1 Data 2 Geology 6 General deposit definition 6 Associated deposits 7 Tectonostratigraphic setting and paleodepths Age range 11 Host and associated rocks Ore minerals 13 Gangue mineralogy W allrock alteration Structural setting 18 Dimensions of ore in typical deposits 18 Dimensions of alteration or distinctive haloes 18 Effect of weathering 18 Effect of metamorphism Geochemical signatures Isotopic signatures 20 Fluid inclusions 20 Geophysical signatures 21 Ore controls/exploration guides 21 Grades and tonnages of gold-bearing skarns References cited 23 Bibliography of additional gold-bearing skarn references 32 FIGURES Graphs showing distributions of tonnage for Au-bearing skarn deposits 2 Graphs showing distributions of gold grade for Au-bearing skarn deposits 3 Graphs showing gold skarn classification schemes based on metal ratios 4 Schematic cross sections of Au-skarn deposits in north-central Nevada 8 Map showing distribution of Au-skarn districts and geological provinces in the cordillera of western North America 10 Map showing worldwide distribution of major Au-bearing skarn deposits and fold belts Graphs showing chemical compositions of igneous rocks associated with major types of mineralized skarn Photomicrographs showing textural relations of gold and electrum in selected Au-skarn deposits Photomicrographs showing complexly zoned garnets from oxidized skarn, Surprise Mine, Nevada 16 Ternary diagrams showing ranges of garnet compositions for representative samples from five Au-bearing skarn systems in north-central Nevada 16 Ternary diagrams showing ranges of pyroxene compositions for representative samples from three Au-bearing skarn systems in north-central Nevada 17 Photomicrograph showing massive garnet partly replaced by pyrrhotite and chalcopyrite 18 Graphs showing distributions of silver grade for Au-bearing skarns 22 Graphs showing gold grade compared with copper grade and silver grade 23 Contents
Contents TABLES Abbreviations used in tables 38 Gold-bearing skarns in which gold and silver are major commodities exploited 40 Gold-bearing skarns in which gold and silver are byproduct commodities 46 Gold-bearing skarn deposits and deposits purported to be gold-bearing skarns for which grade and tonnage data are unavailable 54 Mineral abundances for gold-bearing skarns 59 Analytical data for some igneous rocks associated with gold-bearing skarn deposits in north-central Nevada 59 · Representative data for minerals in gold skarns from north-central Nevada 60 Chemical signatures of nontronite clay layers associated with gold-bearing skarns 61
Gold-Bearing Skarns By Ted G. Theodore, Greta j. Orris, jane M. Hammarstrom, and james D. Bliss Abstract In recent years, a significant proportion of the mining industry's interest has been centered on discovery of gold deposits; this includes discovery of additional deposits where gold occurs in skarn, such as at Fortitude, Nevada, and at Red Dome, Australia. Under the classification of Au-bearing skarns, we have modeled these and similar gold-rich deposits that have a gold grade of at least 1 g/t and exhibit distinctive skarn mineralogy. Two subtypes, Au-skarns and byproduct Au-skarns, can be recognized on the basis of gold, silver, and base-metal grades, although many other geologic factors apparently are still undistinguishable largely because of a lack of detailed studies of the Au-skarns. Median grades and tonnage for 40 Au-skarn deposits are 8.6 g/t Au, 5.0 g/t Ag, and 213,000 t. Median grades and tonnage for 50 byproduct Au-skarn deposits are 3.7 g/t Au, 37 g/t Ag, and 330,000 t. Gold-bearing skarns are generally calcic exoskarns associated with intense retrograde hydrosilicate alteration. These skarns may contain economic amounts of numerous other commodities (Cu, Fe, Pb, Zn, As, Bi, W, Sb, Co, Cd, and S) as well as gold and silver. Most Au-bearing skarns are found in Paleozoic and Cenozoic orogenic-belt and island-arc settings and are associated with felsic to intermediate intrusive rocks of Paleozoic to Tertiary age. Native gold, electrum, pyrite, pyrrhotite, chalcopyrite, arsenopyrite, sphalerite, galena, bismuth minerals, and magnetite or hematite are the most common opaque minerals. Gangue minerals typically include garnet (andradite-grossular), pyroxene (diopside-hedenbergite), wollastonite, chlorite, epidote, quartz, actinolite-tremolite, and (or) calcite. INTRODUCTION Gold exploration efforts of the mining industry in the last few years have centered on discovery of skarn deposits, such as Battle Mountain Gold Company's Fortitude deposit in Nevada, and Elders Resources' Red Dome deposit in Queensland, Australia, as well as on discovery of disseminated, carbonate-hosted, or Carlin-type deposits. Carbonate-hosted gold deposits are generally much larger deposits than skarns but are of much lower grade. Bagby and others (1987) report median tonnage and grade values Manuscript approved for publication, January 11, 1990. of 5.1 million tonnes and 2.5 g/t Au, respectively, for 35 Carlin-type deposits. Median tonnage and grade values for the 90 skarn deposits we report in this study are 0.279 million tonnes and 5.7 g/t Au, respectively. Some major gold skarns, such as the Lower Fortitude deposi4 Nevada (5.1 million tonne, 10.45 g/t Au), and the deposit at Bau, Malaysia (2.4 million tonnes), however, contain more gold than many of the large, disseminated-type deposits and are thus extremely attractive as exploration targets. The geologic characteristics of gold-bearing skarn deposits have only recently been addressed (Meinert, 1988a,b, 1989). This paper presents descriptive and grade-tonnage information obtained from more than 90 deposits that have been referred to in the literature as "Au-bearing skarns," "Au-rich skarn," or "Auskarn," in a format somewhat similar to models in Cox and Singer (1986) but as modified by P.B. Barton (written commun., 1986). These and many other deposits, generally referred to as gold skarns in the literature, are occasionally further differentiated into contact, or proximal, skarns and distal skarns (Sillitoe, 1983, 1987; Bonham, 1985). Special attention has been given to the mineral chemistry of gangue skarn minerals as they have previously proved useful in distinguishing skarn types. This paper consists of a geologic description of Aubearing skarns, presented in a form modified from that established previously for Cu-, Zn-Pb-, and Fe-skarn descriptive models (Cox and Singer, 1986) to allow rapid comparison and contrast; grade-tonnage distributions of Aubearing skarns; and a combination references-bibliography section. Acknowledgments We thank the following people who contributed to this paper: William C. Bagby, Donald A. Singer, Greg E. McKelvey, W. David Menzie, Dan L. Mosier, Robert C. Pearson, James E. Elliott, and James J. Rytuba of the U.S. Geological Survey; David W. Blake, R.G. Benson, Kirk W. Schmidt, and Patrick R. Wotruba of Battle Mountain Gold Company; Edward I. Bloomstein of Santa Fe Pacific Mining, Inc.; Gail M. Jones of Western Geologic Resources, San Rafael, Calif.; Bruce A. Kuyper of Echo Bay Mining Co.; JeffS. Loen and John Childs of Lupine Minerals; N. Eric Pier of Pegasus Gold Corporation; Larry Hillesland of West Introduction
Gold; Fess Foster, Golden Sunlight Mines, Inc.; Sharon E. Lewis, Montana Bureau of Mines and Geology; Nolan Smith, Philipsburg, Montana, and Robert G. Russel, Coeur d'Alene, Idaho. DATA In our examination of the geologic literature for over 300 skarns, we determined that about 65 percent of those reported detectable gold in amounts ranging from a trace to approximately 157 grams per tonne (g/t). Data are compiled for approximately 125 skarn deposits, and the deposit tonnage and gold grade distributions of 90 of those with a (J) 1A a.. w WE u. 0:: w CD
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,, MILLIONS OF TONNES, IN LOG UNITS Figure 1. Distributions of tonnage for Au-bearing skarn deposits. A, Tonnage histogram for 90 Au- and byproduct Au-skarn deposits. 8, Tonnage model, same data set. C, Tonnage model for 39 Au-skarns. 0, Tonnage model for 59 byproduct Au-skarns. Gold-Bearing Skarns
1837 and 1857 (see discussion in Shawe, 1988). All gold-bearing skarns can, as a first approximation, be treated as deposits in one of two subtypes that have different gold, silver, and base-metal grade distributions: (1) skarns in which gold is the primary commodity and (2) skarns in which gold had been or is being recovered as a byproduct However, in some already mined out deposits wherein gold was recovered as a byproduct, changes in metal prices to those prevailing during the late 1980's would result in gold assuming the role of primary commodity because of sufficient gold grade. A set of criteria has been established to determine whether a deposit should be classified as an Au-bearing skarn: (/) w a: w al :J z (/) ll.. w z
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~q, 1. The deposit must have an average gold grade of at least 1 g/t. 2. The mineral assemblage(s) of the deposit must include mineralogy that indicates that a skarn environment was genetically associated with introduction of gold Meinert (1988a) emphasized that a critical mineralogic feature of Au-bearing skarn is the presence of pyroxene and garnet. However, as we discuss below, introduction of most gold in such deposits does not necessarily occur during prograde pyroxene- and garnet-stages of skarn development Among skarns that meet these two criteria, some were mined primarily for their precious-metal content, whereas others were either mined primarily for their base0 (/) ll.. w z .4 a: 0 a: (/) ll.. w z .4 a: 0 a: Median 8.6 grams per tonne D Median 3.7 grams per tonne GOLD GRADE, IN LOG UNITS OF GRAMS PER TONNE Figure 2. Distributions of gold grade for Au-bearing skarn deposits. A, Gold grade histogram for 90 Au- and byproduct Au-skarn deposits. B, Gold grade model, same data set. C, Gold grade model for 39 Au-skarns. D, Gold grade model for 59 byproduct Au-skarns. Data
and ferrous-metal content or were mined for precious metals but contained very large amounts of base and ferrous metals. The presence of some gold in Cu-, Fe-, and W-skarn was characterized appropriately by Lindgren (1933): "Gold is present in traces in almost all sulphide deposits of the pyrometasomatic type, and a few ounces of silver to the ton is likewise not unusual " Skarn deposits with byproduct gold that average at least 1 g/t and with base-metal grades less than the lowest tenth percentile of a grade model of copper (0.7 percent Cu) in Cu skarns (Jones and Menzie, 1986), of zinc (2.7 percent Zn) or lead (0.87 percent Pb) in a Zn-Pb skarn model (Mosier, 1986), or of iron (36 percent Fe) in Fe skarns (Mosier and Menzie, 1986) are included in an Au-skarn data subset (table 2). Skarn deposits with greater than 1 g/t gold and higher base- and ferrous-metal grades that fit existing models of base- and ferrous-metal skarndeposit types are assigned to a byproduct Au-skarn data subset (table 3). Orris and others (1987) presented these criteria for classification of Au-bearing skarns along with a preliminary compilation of deposits. Much of the information in that report has been updated and revised because of subsequent availability of newly released data, and a number of deposits have been added. The geologic characteristics of many deposits in our byproduct Au-skarn subset are as important from the viewpoint of a gold explorationist in the late 1980's as are the characteristics of the Au-skarn subset. Many of the byproduct Au-skarn deposits exploited at their respective grades of gold greater than 1 g/t before 1950 (table 3) undoubtedly would have been evaluated only for their precious-metal content if first discovered in the late 1980's. Admittedly, the classification scheme above is for drilled out deposits currently in production or for deposits that have been mined out, and the classification scheme strictly cannot be used to classify precious-metalmineralized, unexploited skarns (Ettlinger and Ray, 1989). Nonetheless, the classification schem~ provides a data base of precious-metal-mineralized skarns to which data from unexploited skarns may be compared. Studies by Myers and Meinert (1988), G.L. Myers (written commun., 1988), and Ettlinger and Ray (1989) have shown that metal ratios in metallized skarns may be used to discriminate effectively among types of Au-bearing skarn. Myers and Meinert (1988) suggested that true copper skarns have Au/Cu ratios (where gold grade is in grams per tonne and copper grade is in weight percent) less than about 3. On the one hand, all 20 skarns classed as Au-skarns, as we defined above, for which gold and copper grades are available, show Au/Cu ratios greater than 3 (table 2). On the other hand, 22 of 50 byproduct Au-skarns have Au/Cu ratios greater than 3 (table 3). Therefore, adoption of the classification scheme of Myers and Meinert (1988) based on that ratio would result in the addition of the 22 deposits to our Au~skarn data set (table 2). These 22 deposits, however, fit existing models for basemetal skarns. Within gold-bearing skarn, deposits of our Au ... skarn subset cluster in a domain showing elevated overall Gold-Bearing Skarns abundances of gold and an increased Au/Cu ratio relative to most deposits in the byproduct Au-skarn subset for which data are available (tables 2, 3; fig. 3A). Ettlinger and Ray (1989, fig. 51) defined fields for gold (silver), copper, iron, and silver-poor gold-rich skarns based on Cu/Ag and Cu/ Au ratios. Of the 20 deposits in our Au-skarn subset (table 2) that report grade values for Au, Ag, and Cu, 16 plot within the gold (silver) field (fig. 3B). The Surprise deposit,
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EXPLANATION o Byproduct Au-skarn Au-skarn '1o 1oo 10oo 1o:ooo 1oo.ooo Cu (WEIGHT PERCENT)/ Au (WEIGHT PERCENT) Figure 3 .. Gold skarn classification schemes based on metal ratios. A, Gold-copper ratios compared with contained gold (average grade multiplied by tonnage) for 20 Au- and for 47 byproduct Au-skarn deposits for which copper grades are available. B, Copper/silver ratios compared with copper/gold ratios for the same data set. Fields for gold (silver), copper, iron, and silver-rich skarns are from Ettlinger and Ray (1989).
which we included as an Au-skarn rather than a byproduct skarn, plots on the boundary of the copper skarn field. Although the Surprise deposit is copper-rich and has an average grade of 0.85 percent Cu (above the 0. 7 percent Cu grade for the lowest tenth percentile grade for copper in the copper skarn model of Jones and Menzie, 1986), we included it in table 2 because it is presently being mined for gold alone. The 42 deposits in our byproduct subset (table 3) for which grade data are available for all three elements are scattered about the central region of figure 3B; nearly half of the deposits cluster in and near the copper skarn field. Interestingly, the one iron skarn in our byproduct subset, Larap, plots in the gold (silver) region of figure 3B, at a much lower Cu/ Au ratio than that deemed for iron skarn. Four of the seven Zn-Pb byproduct gold skarns plot outside of any of the fields deemed by Ettlinger and Ray due to their low Cu/Ag ratio; one deposit plots within the gold (silver) field and two plot within the copper skarn field. All three methods for classifying gold-bearing skarn deposits (this study, Myers and Meinert's Au/Cu ratio, and Ettlinger and Ray's Cu/Ag vs. Cu/Au ratio) converge on the conclusion that most deposits that can be mined primarily for their gold content have log Cu/Au ratios less than about 3 to 3.5 and have a slightly lower Cu/Ag ratio than copperrich skarns. By restricting our byproduct Au-skarn data (table 3) to those known skarn systems wherein gold concentrations are greater than or equal to 1 g/t, we provide composited cumulative-distribution relations only for the Au-enriched part of Cu, Pb-Zn, and Fe skarns as defined by Jones and Menzie (1986), Mosier (1986), Mosier and Menzie (1986), and Meinert (1988a, b). According to our definitions, Aubearing skarn may include both Au-rich and Ag-rich variants of skarn as employed in the terminology of Ray and others (1986a). An example of geologic linkage between Au-skarn deposits and byproduct Au-skarn deposits is present at Copper Canyon, Nevada (Wotruba and others, 1986; 1987a, b; Myers and Meinert, 1988). At Copper Canyon, the West ore body is an Au-bearing Co-skarn that formed adjacent to a 38 Ma granodiorite (Theodore and Blake, 1978). However, its gold grade (approximately 0.7 g/t) is less than the 1 g/t cutoff that we use in this report for deposits to be included in our byproduct Au-skarn subset (table 3). Nonetheless, at Copper Canyon the Fortitude Au-skarn (table 2) formed in the same stratigraphic sequence of rocks as the West ore body, but at a much greater distance from the granodiorite (Wotruba and others, 1986; 1987a, b). Deposits for which some geologic or grade-tonnage data are available are listed in tables 2 through 4. Grades, tonnages, and some geologic data available for 40 Au-skarns are listed in table 2; most of these skarns were (or are being) exploited primarily for their precious-metal content. Byproduct Au-skarns that can be classified under other skarn types, including Cu, Zn-Pb, and Fe skarns, and that have grade-tonnage and some geologic data are included in table 3. Deposits that have been described as "gold skarns" in the literature and for which complete grade-tonnage data are not available but are suspected to have average gold grades greater than or equal to 1 g/t are described in table 4. Although Schrader (1947) from his studies in the 1920's, cited production in the 1880's of extremely high grade gold (622 g/t for 272 t) from gossaniferous skarn at the Mottini Mine, IXL Mining District, Nevada (table 4), subsequent geochemical studies have failed to confmn the occurrence there of such concentrations of gold (Vanderburg, 1940; David A. John, written common., 1989). The Cable Mine in the Southern Flint Creek Range, Montana, has long been recognized as a gold skarn. Knopf (1933) classified the Cable Mine, along with deposits in the Hedley district of British Columbia, as a pyrometasomatic gold deposit. The production history of the Cable Mine (table 4) provides a good example of the difficulties of assigning reliable tonnage or grade values to long-lived deposits. The mine was discovered in 1866, and Emmons (1907) reported that 9,000 tons of ore produced $172,000, mostly in gold, in 1867. Emmons and Calkins (1913) report $400,000 from production up to 1872, including $30,000 from one ton of ore, a single gold nugget valued at $375, and more than $2,000,000 in gold produced from 1877 to 1891. They also described average tailings from upper levels and partly oxidized ore of $2.97 per ton in gold, 0.15 ounces per ton in silver, and 3.06 percent copper. Earll (1972) noted that 90 percent of the production from the district took place prior to 1900, and reported district production, including placer, of $3,535,820 from 165,127 oz of gold and 134,904 oz of silver. The Cable area and nearby veincontrolled and oxidized ores at the Southern Cross, Gold Coin, and Pyrenees deposits are currently under exploration (Nolan Smith, oral common., 1989) as a joint venture by Magellan Resources and Chevron Resources Company. Paired grade-tonnage values are available for 90 ore bodies in 89 skarn systems mineralized to gold concentrations equal to or greater than 1 g/t. At Tillicum, British Columbia, two entries (table 2) are included in the statistical calculations: estimated reserves of 2 million tonnes at 6.9 g/t Au in the East Ridge zone of the deposit and proven reserves of 0.05 million tonnes at 35 g/t Au in the Heino-Maney zone of the deposit (Ettlinger and Ray, 1989). The deposit tonnage estimate consists of any known production plus reserves (proven, estimated, or drillingindicated) at a given point in time; the grade is an estimated average grade for the total tonnage. For some deposits, tonnage and grade are based on known production only. These values probably are representative of the entire ore bodies for many of those small deposits mined during the late 1800's and early 1900's. It should be noted that most of the production for over one-half of the skarns was concluded prior to 1950, and we cannot be sure that many of those deposits of base-metal skarn did not have a gold content that would be significant under today's (1989) economics. Values of tonnage and ore grades qualified as Data
"greater than" in tables 2 and 3 were used in statistical calculations and graphs of data described below. This results in two tonnage values, one gold grade value, and two silver grade values being substituted by an unqualified numerical value. Values qualified as "less than" were not considered further in either statistical calculations or graphs of data. Iron skarn dominates the mineralized skarns worldwide, comprising approximately one-third of the deposits; however, gold contents for most of these skarns were reported in the literature as "trace," "minor," or "detectable." Only two deposits of iron skarn with grade and tonnage figures reported average gold grades exceeding 1 g/t (table 3), and 11 deposits reported grades lower than 1 g/t as deposit averages or in selected parts of a compositionally zoned skarn body. In a comprehensive data compilation for Alaskan skarns, Newberry (1986) classified 109 deposits as Fe-Auskarns. He reported typical grades for these deposits of 40 percent Fe, 1 percent Cu, 0.1 oz Au per ton (3.4 g/t), 10 oz Ag per ton (343 g/t), and 50 ppm Co. Several additional deposits have been described as gold skarns in one or more publications listed in the bibliography but were not included in the above tables for the following reasons: inadequate description of the deposit; inaccessibility to the publication; description(s) of the deposit showed the deposit to be inappropriately classified as a gold-bearing skarn according to the classification scheme we have adopted; or the gold grade was less than 1 g/t. These deposits include: Tennant Creek, Australia; LanduskyZortman, Montana; Ertsberg, Indonesia; Andacollo, Chile; Equity (Sam Goosly), British Columbia; Salsigne, France; Pamlico, Nevada; Red Cloud, Nevada; Island Copper, British Columbia; and others. Ertsberg has an average gold grade below 1 g/t. Wedekind (1988) and Wedekind and others (1988) did not include garnet or pyroxene as part of the composite mineral assemblages of the deposits at Tennant Creek. Andacollo has been cited under other deposit types, and a detailed geologic description of the area is not available. Inappropriate or alternate classification of deposits and (or) lack of detailed geologic data have excluded the other deposits. Some deposits with grade and tonnage data reported were placed in table 4 because the tonnage and grade information conflicted with other known data and we were unable to resolve the conflict; an example of this situation is Mt. Biggenden, Australia. Although the Mt. Biggenden magnetite-bismuth-gold skarn is classified as an Au-skarn by Meinert (1988a) and assigned a size of 500,000 tons and a gold grade of 15 g/t, we have not included it with either our Au-skarn or byproduct Au-skarn subtypes primarily because of our uncertainty about the gold grade and tonnage of mined ore. For example, total gold production to 1969 from Mt. Biggenden is more than 7,000 oz, of which 5,751 oz was produced before 1901 (Clarke, 1969). The corresponding tonnage of ore mined is not reported. As of 1917, Dunstan (1917) calculated magnetite ore reserves as 500,000 tons, which apparently includes only "a few Gold-Bearing Skarns grains of gold per ton" (Clarke, 1969), because all of the "actinolite rock" that contained most of the gold and bismuth had been already mined out by that time. If 500,000 tons is a correct tonnage for the gold ore, then 14,500 oz of total gold production is required for an average grade of 1 g/t, and over 200,000 oz of production would be needed for a grade of 15 g/t. If the grade of 15 g/t is correct, 7,000 oz of gold could have been produced from about 16,000 tons of ore. Pegasus Gold Corporation's Beal Gold deposit in the Siberia Mining District near Butte, Montana, described as a 9.2-million-tonne, low-grade (1.509 g/t Au), bulk minable precious-metal reserve (Hastings and Harrold, 1989), has some characteristics of skarn (N. Eric Pier, oral common., 1989), but shows no extensive calcsilicate exoskarn gangue mineral assemblage at the present levels of exposure. Precious metals and sulfides (pyrrhotite, pyrite, chalcopyrite, trace arsenopyrite and molybdenite) are disseminated in metaconglomerate, quartzite, diopside hornfels, and potassium feldspar hornfels and also are present in veins with chlorite, quartz, adularia, and carbonate minerals. Gold is present as free gold and in association with Pb- and Bi-tellurides. GEOLOGY General Deposit Definition Smirnov (1976) suggested that classification of skarns be based upon the composition of the original protolith of the skarn: calcareous, magnesian, or silicate. However, we follow the nongenetic definition of skarn proposed by Einaudi and others (1981): "replacement of carbonate [or other sedimentary or igneous rocks] by CaFe-Mg-Mn silicates [resulting from] (1) metamorphic recrystallization of silica-carbonate rocks, (2) local exchange of components between unlike lithologies during high-grade regional or contact metamorphism, (3) local exchange at high temperatures of components between magmas and carbonate rocks, and ( 4) large-scale transfer of components over a broad temperature range between hydrothermal fluids predominantly carbonate rocks." Most Au-bearing skarns owe their genesis to processes largely involving the fourth process. Thus we follow an overall classification of skarns based upon their sought-for metal content (see also Shimazaki, 1981, and Zharikov, 1970). As recognized by Meinert (1988a), many deposits referred to as Au-skarns in the literature have been classified, or could be classified, under skarn deposit models such as Cu- and Fe-skams by their dominant base- or ferrous-metal contents. For these deposits, gold production may be considered a byproduct of base- or ferrous-metal mining. Furthermore, Au-bearing skarn deposits commonly may be gradational into skarn that contains no gold but does contain
significant other metal(s), including the Ag-rich skarns as defined by Ray and others (1986a), sediment-hosted disseminated Au-Ag deposits (also known as carbonatehosted and Carlin-type), porphyry Cu or Cu-Mo deposits, or polymetallic replacement deposits (exemplified by the McCoy megasystem in Nevada), as well as other deposit types related to felsic to intermediate plutonic emplacement or volcanic activity. The Cove deposit, McCoy Mining District, Nevada, has been classified recently as a distal disseminated Ag-Au deposit according to a scheme proposed by Dennis P. Cox (written commun., 1989). Polymetallic veins are one of the other deposit types that may be present on the fringes of Au-bearing skarn deposits. Therefore, we have chosen to use the term "Au-bearing" skarn as most aptly describing such skarn deposits and related mineralization commonly distal to the immediate contact zone. Other commodities produced by Au-bearing skarns include silver, copper, zinc, iron, lead, arsenic, bismuth, tungsten, and tin as principal or byproduct commodities and cobalt, cadmium, and sulfur as byproducts. In addition, we have provisionally restricted our working model of this deposit type to those Au-bearing skarns that have more than 1 g/t gold. This figure is based largely on cutoff grades that were reported as low as 1 g/t for many Au-bearing skarn operations in production in 1988 that required milling of their ore to a very fine grain size for efficient gold recovery. Some Au-skarn operations, such as McCoy, Nevada, that utilize heap-leach extraction procedures for their ores, have cutoff grades as low as 0.3 g/t for oxidized ore (Bruce A. Kuyper, oral commun., 1987), but the average deposit grade is greater than 1 g/t. Deposits with average gold grades below 1 g/t and without other economic mineralization are rarely reported in a quantitative manner in the literature and thus result in an artificially truncated data set. In an attempt to limit the influence of this reporting problem when comparing Au-bearing subsets, we have limited all our data to those with gold concentrations greater than 1 g/t or reasonably inferred by cited reporters to be greater than 1 g/t. Gold-bearing skarns are generally calcic exoskarns with gold associated with intense retrograde hydrosilicate alteration, although Au-bearing magnesian skarns are known and in some areas are dominant. Some economically significant Au-bearing skarns (Hedley, British Columbia, and Suian, South Korea), however, are partly in endoskarn (Barr, 1980; see also Lee, 1951; Lee, 1981). Reported pyrrhotite, chalcopyrite, and "augite" enclosed in quartz monzonite at the Golden Curry deposit, Montana, may be endoskarn (Knopf, 1913; Pardee and Schrader, 1933). Significant concentrations of gold-bearing endoskarn also are present at the Nambija, Ecuador, Au-skarn· deposit (table 4). In some districts, our data set includes deposits that are significantly distant from igneous contacts at current levels of erosion but still exhibit high-temperature, prograde mineral assemblages composed of garnet and (or) pyroxene. Gold-bearing skarns show diverse geometric relations to genetically associated intrusive rocks and nearby premetallization structures (fig. 4). As presently constituted (tables 2, 3), our compilation includes some deposits that were previously considered as Cu, Fe, or Zn-Pb skarns in the classification schemes of Einaudi and others (1981) and Meinert (1988a). In some cases when establishing deposit size or grade, we have included other styles of genetically related, generally latestage mineralization adjacent or continuous to known skarn mineralization under the size estimate and description of the Au-bearing skarn deposit when demarcation between the mineralization styles would be arbitrary. Associated Deposits Deposit types most commonly associated with Aubearing skarn include Cu, Fe, Zn-Pb, and porphyry Cu skarn-related deposits. Other deposit types include porphyry Cu-Mo or Cu-Au deposits, porphyry Cu deposits, carbonatehosted Au-Ag (see Sillitoe, 1983), polymetallic replacement and polymetallic veins, distal disseminated Ag-Au deposits (Dennis P. Cox, written commun., 1989), W skarns, Sn skarns and greisens, Au placers, and other deposits related to felsic and intermediate intrusions (Cox and Singer, 1986), including stockwork molybdenum systems such as at Red Dome, Australia, and Buckingham, Nevada. The Carissa and the Surprise Cu-Au-Ag skarn deposits are on the northern fringes of the Late Cretaceous (86 Ma) Buckingham, Nevada, stockwork molybdenum system, and they appear to be related genetically to emplacement of potassic-altered monzogranite porphyry (Schmidt and others, 1988; Theodore and others, 1989). Other examples of deposits associated with Au-skarn include skarn mineralization at Katanga, Peru, which becomes porphyry Cu-Mo mineralization at depth, and the deposit at Bau, Indonesia, that includes a large component of sedimenthosted gold mineralization as well as that hosted by skarn. Other areas that probably document transition from a skarn environment into mostly sediment-hosted systems are silver and gold mineralization at the McCoy-Cove mineralized system in north-central Nevada, gold mineralization in the general area of the Broadway, Montana, Au-skarn deposit (Sahinen, 1939), and mostly gold at the Kavak-tau area in Kirghiziya, U.S.S.R. (Dolzhenko, 1974). Near the Broadway deposit and other nearby Au-skarn-related occurrences, Aubearing jasperoid mantles epidote-rich endoskam that formed at the contact of Cretaceous quartz monzonite and Cambrian limestone (Sahinen, 1939). At Kavaktau, most of the gold mineralization is apparently associated with "secondary silicates," probably jasperoids in North American terminology, that are present in marble and silicate-carbonate rock beyond the outer limit of well-developed skarn assemblages. Placer gold deposits are found associated with Associated Deposits
copper and gold deposits of the Battle Mountain Mining District, Nevada, of the Helena, Bannack, and Cable Mining Districts, Montana, and in the Zeballos area, Vancouver Island, British Columbia. Gradational changes from Aubearing skarn mineralization to another deposit type (Myers and Meinert, 1988), relatively small areas of gold enrichment within or peripheral to base- or ferrous-metal skarn mineralization, the presence of minerals that can be attributed to weak or distal development of skarn in deposit types not in a contact-metamorphic aureole, and continuous gold mineralization through multiple deposit types related to a single intrusion or series of events are common to Aubearing skarn environments. In many gold-enriched skarn deposits of British Columbia, Ettlinger and Ray (1988) noted multiple types of gold mineralization within single deposits. For example, at the Discovery deposit at Banks Island, gold is present in skarn with massive pyrrhotite that replaces marble, as well as in brecciated quartz-pyrite veins that cross-cut skarn and marble. Ettlinger and Ray suggested that skarn and quartz-pyrite mineralization may be genetically linked. Similarly, high-grade gold mineralization (Parnell gold shoot) overprints earlier formed copper-gold skarn at Carr Fork in the Bingham district, Utah (Cameron and Garmoe, 1987). WEST METERS A Triassic Agusta Formation (clastic facies) Tertiary monzogranite and tonalite Further studies are needed to address the problem of whether all of the gold, or some of the gold in a few deposits, represented a much later epithermal overprint on an earlier skarn system or was deposited as a continuum near the fmal stages of the skarn process along structures that permitted extensive development of retrograde assemblages. In a number of mining districts that contain gold skarn deposits, ore deposits are zoned from a core area (sometimes, but not always, a porphyry copper or other stock) of Cu±Au and Ag mineralization, to an intermediate zone of Au-skarn or other types of gold mineralization, to an outermost area of dominantly Zn+Pb+Ag±Au mineralization. Blake and others (1984) demonstrated such a zonation about the middle Tertiary altered granodiorite stock of Copper Canyon in the Battle Mountain Mining District, Nevada, where the Tomboy-Minnie and Fortitude gold skarn deposits lie between an area of Cu+Au+Ag and Pb+Zn+Ag mineralization. El-Shatoury and Whelan (1970) described a zonal arrangement of ore deposits in the Gold Hill Mining District, Utah, from a central zone of W+MO+Cu, through Cu, Cu+Au, Cu+Pb+As and Pb+Zn+Au mineralization. The Alvarado, Cane Spring, and Bonnemort skarn deposits all lie within the Cu+Au wne in the Gold Hill Mining District. In the Elkhorn Mining District, 100 METERS EXPlANATION Skarn Contact Triassic Agusta Formation (carbonate facies) EAST METERS Figure 4. Schematic cross sections of Au-skarn deposits in north-central Nevada. A, McCoy Mine, modified from Lane (1987). B, fortitude Mine, modified from Myers (1988). C, Surprise Mine, modified from Schmidt and others (1988). Gold-Bearing Skarns
Montana (Klepper and others, 1957), the distribution of deposits around the eastern edge of the stock of the Black Butte area suggests that the Klondyke and Dolcoath gold skarn deposits, and possibly the Golden Curry deposit to the west, represent a gold-rich zone interior to a zone of Pb+ Ag mineralization. SOUTH METERS Permian and Pennsylvanian Havallah Sequence Permian and Pennsylvanian Antler Peak Limestone NORTHWEST METERS Pennsylvanian Battle Formation (middle unit) Cambrian Harmony Formation Cambrian Harmony Formation EXPlANATION 11111111111111·111 Skarn Contact 300 METERS Fault-Dashed where approximate. Arrows show relative movement EXPlANATION Skarn Contact Pennsylvanian Battle Formation (lower unit) NORTH METERS SOUTHEAST METERS o 0 50 METERS Figure 4. Continued. Associated Deposits
Tectonostratigraphic Setting and Paleodepths In North America, Au-bearing skarn is present most commonly in Mesozoic and Cenozoic orogenic-belt and island-arc settings (fig. 5); a few Au-bearing skarns have been found in rifted craton. The regional distribution of Aubearing skarns may have been confined partly by emplacement of Au-enriched magmato-hydrothermal systems possibly controlled by long-active rifts intersecting the craton's edge in the continental-margin environment of western North America (Roberts, 1966). Such magmatism may be related to onset of regional-scale extensional tectonism in the northern Great Basin. Ettlinger and Ray (1989) examined the distribution of 126 precious-metal-enriched skarns in British Columbia in terms of tectonic belt and tectonic terrane. They found that gold- and silver-bearing skarns are present throughout the four westernmost, mobile tectonic belts in British Columbia, but are absent from the easternmost, stable Foreland belt. Of the 14 terranes in which precious-metalenriched skarns are present, Ettlinger and Ray (1989) showed that most occurrences and most producing deposits are in the Wrangellia and Quesnellia terranes. Most of the gold produced from skarns in British Columbia comes from deposits in the Quesnellia tectonic terrane, which includes the world-class gold skarn deposit at Hedley and the Greenwood Mining District. A recently announced gold skarn occurrence in northern Washington, the Buckhorn Mountain deposit (table 4 ), lies within the southern extension of the Quesnellia terrane into the United States (Silberling and others, 1987). A similar analysis of the terrane distribution for the 106 Fe-Au-skarn occurrences (34 producers) reported by Newberry (1986) reveals 54 occurrences (24 producers) in the Alexander terrane, followed by 26 occurrences ( 6 producers, including the large Nabesna deposit) in Wrangellia, and 15 occurrences (2 producers) in the Peninsula terrane. Less than 5 occurrences in each are reported for the Tracy Arm, Chulitna, Dillinger, Mystic, and Nixon Fork terranes. Island-arc volcanic sequences, clastic sediments, and comagmatic calc-alkaline intrusions are common features of the terranes that host the largest proportions of known Au-skarn deposits in British Columbia and in Alaska (Ettlinger and Ray, 1989; Monger and Berg, 1987; Jones and others, 1987). In the conterminous United States, the important gold skarn districts of north-central Nevada lie in the Roberts terrane (Silberling and others, 1987), in a geographic position analogous to Quesnellia to the north, just west of ancestral North America proper. However, the Figure 5. Distribution of Au-skarn districts and geological provinces in the cordi II era of western North America. Modified from Monger and others (1972). Gold-Bearing Skarns 1\t
3T 114° 250 KILOMETERS EXPlANATION Au-skarn districts: Banks Island/Discovery Zeballos Texada Island Vancouver Hedley Dividend-Lakeview Tillicum Mountain Greenwood , Thrust fault-Sawteeth on upper plate Ophir/Elliston Elkhorn Silver Star Bannack Battle Mountain McCoy White Pine Gold Hill
gold skarn districts of southwestern Montana, and Utah occur to the east of the accreted terrane boundary. Some of the most productive Au-skarn systems in western North America apparently formed in relatively shallow seated geologic environments, probably at 1.5-3.0 km below their respective paleosurfaces. Other Au-bearing skarn systems formed as much as 5 km below their paleosurfaces. At the Mottini Mine in the IXL Mining District, Nevada (table 4; also see Schrader (1947) and Vanderburg (1940)), gossaniferous Pb-Zn-Cu skarn with some gold is associated with emplacement of a 28-Ma, zoned granodiorite that is cogenetic with a tilted caldera (David A. John, oral commun., 1989). The Au-bearing PbZn-Cu skarn apparently developed approximately 5 km below the 28-Ma paleo surface on the basis of removal of the present-day tilts in the rocks of the caldera. The 38- to 39-Ma Au-skams at McCoy, Fortitude, Tomboy-Minnie, and Labrador, all in Nevada, regionally are clustered not far from the 34-Ma erosion surface upon which the 34-Ma Oligocene Caetano Tuff was deposited. This relation suggests that those four Au-skarn systems must have formed in a relatively shallow geologic environment-a conclusion confirmed by study of fluid-inclusion relations in the Auskarn deposits (see below). Much less abundant are Tertiary Au-bearing skarns in cratonic environments (Bright Diamond and Iron Clad, Colorado, see Irving, 1905; Irving and Cross, 1907). In the Soviet Union, most reported data on Aubearing skarns seem to indicate development in geologic environments deeper than those in western North America. As such, they have been classified as medium-depth deposits according to the scheme of Bodaevskaya and Rozhkov (1977). Furthermore, according to them, Au-bearing skarns are associated with deformed Paleozoic early-eugeoclinalstage batholiths of granite-granodiorite composition or with minor Paleozoic late-eugeoclinal stage gabbro-plagiogranite or gabbro-syenite intrusive complexes. In Australia, most known Au-bearing skarns are in the Paleozoic Tasman geoclinal belt, and some of the most significant deposits (Red Dome) are associated with late Paleozoic stocks. Worldwide distribution of some important Au-bearing skams relative to major fold belts is shown in figure 6. Age Range Gold-bearing skarns are generally Mesozoic or Tertiary in the cordillera of western North America, probably middle Tertiary in the rifted cratonic regions (Bright Diamond, Iron Clad, Colorado), and probably middle Tertiary in West Sarawak, Malaysia (Bau), according to Wolfenden (1965). Several significant systems of early Paleozoic age are also known in the Soviet Union, and a significant Au-bearing skarn in Australia (Red Dome) is late Paleowic in age. The base-metal-dominated deposits at Falun and Garpenberg Oda, in Sweden, are present in Proterowic rocks (table 3). Host and Associated Rocks Gold-bearing skarn may be hosted by a wide variety of sedimentary and igneous rocks, including limestone, dolomite, shale, conglomerate, rhyolitic to andesitic tuff, and granitoids; however, a premetamorphic calcareous component is commonly present. Meinert (1988b) further noted that the overwhelming bulk of the Au-skarns are present in clastic or volcaniclastic-rich sequences. Pearson and others (1989) showed that gold-bearing skams in the Dillon, Montana, 1 o x 2° quadrangle have the same gangue minerals and same kinds of associated plutons as tungsten skams in the area but that the tungsten skams are mostly hosted by the Mississippian and Pennsylvanian Amsden Formation whereas gold-bearing skarns in the Bannack and Silver Star Mining Districts are in Mississippian Mission Canyon Limestone. In general, compositionally expanded I-type (Chappell and White, 1974) felsic and intermediate plutons, dikes, sills, or stocks that may or may not be porphyritic are associated with Au-bearing skarn. Some deposits (for example, Tumco, California) may be associated with weakly to strongly peraluminous calcic granite (Smith and Graubard, 1987). In north-central Nevada, Au-skarns (Fortitude, McCoy, Northeast Extension, Surprise, Carissa, Labrador) are associated with monzogranite stocks (table 5), whereas in British Columbia many Au-bearing skams (Tillicum Mountain, Oka) are associated with diorite to gabbro stocks (see Ray and others, 1987a, b). In addition, Keith and Swan (1987) have shown that an area in north-central Nevada with plutons that have reduced ferric:ferrous ratios (less than 0.85) correlates in part with the regional distribution of Aubearing sediment-hosted and porphyry deposits. According to them, such reduced ratios may reflect minor assimilation of reduced crust during magma genesis. Leveille and others (1988) showed that most Au-associated plutons have low oxidation state and (or) high alkalinity when plotted in terms of an alkalinity index (~O+Na20-0.57 Si02) and ratio of Fe20 3 to FeO. Meinert (1983) presented mean compositions for igneous rocks associated with different types of mineralized skarn and noted that the most distinctive chemical trends are for parameters that reflect magmatic oxidation state and degree of differentiation, notably ferric:ferrous ratios and alkali contents. The mean igneous rock composition associated with Au-bearing skams (J.M. Hammarstrom, unpub. data, 1989) and with other types of mineralized skams (Meinert, 1983) is shown in figure 7. Gold-bearing skams appear to be associated with slightly less siliceous rocks than other skarn types, and in terms of alumina, total alkalis, and calcium they are most similar to granitoids associated with iron and copper skams (fig. 7). Host and Associated Rocks
C') 0 c:: c., ttl
:;· CIQ & :J 80° 40" 40° 160° 120° PACIFIC OCEAN EXPlANATION D Shield areas Paleozoic fold belts Mesozoic/Cenozoic fold belts 80° 40° ATLANTIC OCEAN t) Figure 6. Worldwide distribution of major Au-bearing skarn deposits (solid dots) and fold belts. 40° 80° INDIAN OCEAN 120°
160° Pacific Ocean
Jj
1-z w () a: w
w
M N
1-z w () a: w
w
u.i g N " N
EXPLANATION A Hedley
"' Nevada
Si02, IN WEIGHT PERCENT I I ' I ' I f B I '
(/) (/) (/) (/) (/)
(/) (/) (/) (/) (/) Q) ..Q 3':
c: + N EXPLANATION
(/)
t (/) K20+Na20 (/) c .a. CaO (f) I t Si02, IN WEIGHT PERCENT
Sn sk<un Cu skarn
W skarn' Zn-Pb skarn
Au skarn
Fe skarn Fe 20 3/Fe0 Figure 7. Chemical compositions of igneous rocks associated with major types of mineralized skarn. A, Al 20 3 versusSi02 , in weight percent, for unaltered igneous rocks associated with Aubearing skarn deposits in the Hedley district, British Columbia (Ray and others, 1987a), in the Battle Mountain and McCoy districts, Nevada (this study), and in other districts. B, Mean compositions for igneous rocks associated with major skarn classes, in terms of weight percents. Squares, Al20 3; filled circles, K20+Na20; triangles, CaO. Data for Au-skarn, this study; data for other skarns, from Meinert (1983). C, Mean compositions for igneous rocks associated with major skarn classes in terms of alkali and oxidation ratios. Same data sources as in B. Our preliminary compilation also suggests that the intrusions associated with Au-bearing skams appear to be more reduced than intrusions associated with copper and (or) iron skarns, also noted by Keith and Swan (1987), and are less evolved than those associated with tin skarn mineralization. These associations do not necessarily imply that all gold in skarn originates in the nearby genetically associated pluton . In southwestern Montana, a number of gold-bearing skarn districts lie at the periphery of the Cretaceous Boulder batholith and appear to be associated with satellite bodies and with sodic series rocks of the batholith rather than with main series rocks, as deemed by Tilling (1973) on the basis of rock chemistry. Ore Minerals Ore minerals typically found in Au-bearing skarn include native gold, electrum, pyrite, chalcopyrite, pyrrhotite, arsenopyrite, sphalerite, galena, bismuth minerals (especially bismuthinite and native bismuth), magnetite or hematite, tellurides (commonly those of Au, Ag, Ni, and Pb), tetrahedrite, tetradymite, bornite, marcasite, loellingite, stibnite, and W- and Mo-bearing minerals. Mineral abundances for ore and gangue assemblages (table 5) were compiled for our Au-skarn data (table 2) and for our byproduct Au-skarn data (table 3), along with the minerals reported by Newberry (1986) for 106 Alaskan Fe-Au-skarn deposits. This compilation is based on the assemblages reported in tables 2 and 3 from the references cited therein. We emphasize that these data are not modal and are probably incomplete, so the actual percentages of various minerals reported are not significant. However, the relative abundance of a given mineral, the frequency of occurrence of some unusual minerals, and apparent differences in mineralogy between deposits mined primarily for gold and those where gold is recovered as a byproduct may be significant in characterizing gold skarn deposits. Meinert ( 1988a, b) stated that the most abundant sulfide minerals in gold skams are arsenopyrite, pyrrhotite, and marcasite and also noted the common occurrence of bismuth and telluride minerals. R.G. Russell (written commun., 1989) reported pyrrhotite as the principal sulfide mineral in gold exoskam, with lesser amounts of arsenopyrite and traces of chalcopyrite, but noted that the major gold skarn deposits in the Hedley district, on which much of his model is based, are unusually arsenic-rich. Our compilation (table 5) suggests a different conclusion. Chalcopyrite is the most common sulfide mineral reported; it is reported from 85 percent or more of the deposits in all three data sets. For the Au-skarn data set, the next most common ore minerals reported (in decreasing order of occurrence) are pyrite, pyrrhotite, gold (or electrum), arsenopyrite, sphalerite, magnetite, galena, tellurides, bismuth (or bismuthinite), hematite (or specularite), molybdenite, hedleyite, and Ore Minerals
scheelite. For the byproduct Au-skarn data set, following chalcopyrite, the sequence is pyrite, magnetite, pyrrhotite, sphalerite, gold (or electrum), galena, hematite (specularite), molybdenite, arsenopyrite, scheelite, tellurides and bismuthinite. None of the byproduct Au-skarn deposits report hedleyite. As might be expected, magnetite is the most commonly reported ore mineral in the Alaskan Fe-Au-skarn data set, galena is uncommon, and no bismuth minerals, tellurides, free gold or electrum, or scheelite are reported. In many of the deposits that report no free gold or electrum, gold is present as auriferous pyrite, gold tellurides, and auriferous jasperoid, and in some cases the mineralogic residence of gold in the system is not identified. In some deposits, silver occurs in Bi-bearing galena. Some free gold and native bismuth occur in galena, all as probable latestage reaction products from breakdown of cosalite (ideally Pb2Bi2S5) or galenobismutite (ideally PbBi2S4) near the northern, distal edge of the Fortitude Au-skarn deposit (T.G. Theodore, unpub. data, 1989). These samples show prominent myrmekitic or eutectoid-type intergrowths between native bismuth and galena. Some domains of mostly intergrown native bismuth and galena at the Fortitude deposit include small anhedral blebs of gold. Other phases present in very minor amounts include bismuthinite, tellurobismutite, and possibly schirmerite (ideally 3(Ag2,Pb)S2 Bi2S3). In addition, many other minerals have been reported for skarns studied in detail, including scorodite, wittichenite, sperrylite, and malayaite. Textural relations of electrum in massive pyrrhotite and in association with native bismuth and galena in clinopyroxene at the Fortitude, Nevada, deposit; gold in late-stage quartz-potassium feldspar-garnet assemblages that cut Jurassic granodiorite at the Nambija, Ecuador, Au-skarn deposit; gold in iron oxide(s) that replace pyrite and (or) pyrrhotite at the Surprise, Nevada, deposit; and gold in pyrite at the McCoy, Nevada, deposit are shown in figure 8. Gangue Mineralogy Typical composite assemblages in Au-bearing skarn include garnet (andradite-grossular), pyroxenes (diopsidehedenbergite), wollastonite, chlorite, epidote-clinozoisitezoisite, scapolite, quartz, actinolite-tremolite, prehnite, potassium feldspar, plagioclase, calcite and serpentine as gangue. Additionally, various micas, ilvaite, vesuvianite, talc, sphene, fluorite, apatite, and abundant clays have been reported from several deposits (tables 2, 3). Garnet and epidote, its typical retrograde alteration product, are the most commonly reported minerals in goldbearing skarns (table 5), followed by pyroxene, amphibole, and chlorite. Of the 39 deposits in our gold skarn subset, 5 (13 percent) report boron minerals in the gangue assemblage, including axinite and ludwigite. No boron minerals are Gold-Bearing Skarns reported in the byproduct Au-skarn subset or in Newberry's (1986) Alaskan Fe-Au-skarn compilation Many deposits include both garnet and pyroxene, but others report only one mineral or the other or are zoned from proximal garnetrich to distal pyroxene-rich assemblages. Pyroxene tends to be dominant in unoxidized, pyrrhotite-rich, more distal skarns, such as the Fortitude deposit, Nevada (Myers and Meinert, 1988). Massive hedenbergite skarn formed in the Black pit of the Broadway Mine in the Silver Star Mining District, Montana, distal to mineralized jasperoid at the granodiorite contact (Larry Hillesland, oral commun., 1989). Gamet is the characteristic prograde silicate mineral of many calcic Au-bearing skams (rocks are commonly massive garnetite); garnet is later than and replaces pyroxene. Mineral chemistry studies show that garnets are andraditegrossular solid solutions (mostly A~0 to Ad1oJ with less than 5 mole percent pyralspite components. Both isotropic and anisotropic varieties are common (fig. 9). Multiple generations of garnet are present in some deposits (for example, Fortitude, Surprise, and McCoy, Nevada). In some deposits from north-central Nevada, early garnet is colorless, anisotropic, zoned toward more Fe-rich rim compositions, A 20 MICROMETERS Figure 8. Textural relations of gold and electru min selected Auskarn deposits. Au, gold; E, electrum; Q, quartz. A, Electrum in massive pyrrhotite (po} from the Lower Fortitude, Nevada, Auskarn deposit. Plane-polarized light. 8, Electrum associated with native bismuth (B) and galena (G) hosted by clinopyroxene (cpx} from the Lower Fortitude, Nevada, Au-skarn deposit. Backscattered electron micrograph. C, Gold in quartz associated with a quartz-garnet (ga}-potassium feldspar assemblage that alters granodiorite at the Nambija, Ecuador, Au-skarn deposit. Plane-polarized, reflected light. 0, Gold in pyrite (py} associated with quartz from the McCoy, Nevada, Au-skarn deposit. Backscattered electron micrograph. f, Electrum (Au6Ag4} in limonite (L} from the 5,595-ft bench, Surprise, Nevada, Auskarn deposit. Plane polarized, reflected light; reflectivity differences due to variations in content of silica.
and poikilitically encloses relict diopsidic pyroxene. Late garnet pods and veins are inclusion-free, are less altered than early garnets, and have distinctly yellow (in thin section), isotropic, andradite cores and colorless, anisotropic rims that have oscillatory zoning with respect to AI and Fe. Contents of 0.4 to 3 weight percent Ti02 are common for early garnet, whereas late garnet is nearly Ti-free. Gamet compositions for representative samples of some Au-bearing skams from north-central Nevada (fig. 10) fall within the compositional fields outlined for garnets from copper and magnetite skarns and are distinct from garnets associated with tungsten, tin, zinc, and molybdenum skarns, primarily due to more oxidized, less manganiferous compositions. Ettlinger and Ray (1989) reached similar conclusions for garnet compositions in precious-metal-enriched skarns from British Columbia. B Figure 8. Continued. 10 MICROMETERS 0.24 MILLIMETER Meinert (1988a) suggested that garnets associated with gold skarns may be more aluminous than those associated with many other skarn types. Bin and Barton (1988) inferred from a study of mineralized skarns in China that andradite components in andradite-grossular garnets of calcic skarn will decrease gradually in the following order of associated metals: W-Zn-Cu, Fe-Cu, W-Bi-Cu-Mo, Fe, Sn-Mo-Bi-W, Cu-Zn, Sn, Pb-Zn, W. Compositions as aluminous as Gr60 to Gr70 are observed for some zones in garnets from the McCoy and Surprise deposits; however, nearly pure andradites are present within the same domains at a thin-section scale. Myers and Meinert (1988) have shown that garnet in the distal Fortitude Au-skarn has compositions (Ad90-too cores; rims) that contrast with garnets in the West orebody Cu-Au-Ag skarn (Ad7a-1~, which is proximal to altered granodiorite of Copper Canyon D E 20 MICROMETERS 20 MICROMETERS Gangue Mineralogy
(Theodore and Blake, 1978). Brooks and others (1989) reported garnet compositions of Ad1S-too for the McCoy deposit, Nevada, which includes the range of compositions encountered in our study of selected samples (fig. 10). Reported hand-specimen colors for garnets from Auskarns vary from buff to yellow to yellow-green to red to brown. Different colors can be used to distinguish among different generations and different compositions within some deposits, however, correlations of a particular color with a particular range of composition are highly variable. Einaudi A 2 MILLIMETERS B 2 MILLIMETERS Figure 9. Photomicrographs showing complexly zoned garnets (ga) from oxidized skarn, Surprise Mine, Nevada. A, Planepolarized light; note growth zone in garnet rim. 8, Crossed nicols; garnet has isotropic, andradite core (C) and sectortwinned, anisotropic, oscillatorily zoned rim (R). Gold-Bearing Skarns (1982) noted that garnets in skarns associated with porphyry copper deposits are commonly reddish brown proximal to the stock and greenish distal to the stock. Meinert (1988a) found yellowish-tan to brown garnet in skarn formed in limestone and reddish-brown garnet in skarn formed in dolostone in moderately gold-bearing porphyry-Cu skarn in the Whitehorse Mining District, Canada. Torrey and others (1986) reported brown and green garnet (A~ to Ad1oo> with pyroxene (Hd10 to Hd15) in early-metasomatic stage skarn at Red Dome, Australia, red-brown garnet (Ad40 to Ad~ with pyroxene (Hd5 to H~5) in late-metasomatic wollastonitegarnet endo-exoskarn associated with rhyolite porphyry, and pale-green garnet (Ad50 to Ad100) with minor pink garnet (Ad60 to Ad8J associated with early retrograde alteration and the minerals vesuvianite, epidote, quartz, fluorite, calcite, chlorite, sphene, orthoclase, magnetite, and hematite. At Red Dome, most of the primary copper-gold-silver ore is in wollastonite-garnet skarn. Red, brown, yellow, and green garnets, all more iron-rich than Ad90, formed in limestone at Carr Fork, Utah (Atkinson and Einaudi, 1978). Callow Grossular Pyrope +almandine +Spessartine Buffalo Valley Mine MCCoy Mine Fortitude Mine Surprise Mine Carissa Mine
n 91 n 27 Andradite Figure 10. Ternary diagrams showing ranges of garnet compositions for representative samples from five Au-bearing skarn systems in north-central Nevada. n, number of samples.
(1967) presented an andradite analysis for brown garnet in garnet-clinozoisite skarn at the Thanksgiving Mine in the Philippines. Late, coarse, zoned andradite (Ad8s to Ad1oc) is reported as the latest skarn mineral in some Au-bearing skarn from the Altai-Sayan region (Vakhrushev, 1972). The wider range of compositions reported for more recent (1980's) studies reflects data acquired by electron microprobe, wherein compositional data for different grains and different zones within a grain can be obtained, whereas much of the earlier data represents wet chemical analysis of a garnet separate. Many recent studies have examined garnet zoning patterns and protolith effects on garnet composition for goldmineralized skarn systems (for example, Beddoe-Stephens and others, 1987; Hammarstrom in Theodore and others, 1989; Ettlinger and Ray, 1989; Brooks and others, 1989). These studies show that (1) garnets commonly remain stable throughout extensive retrograde alteration processes, (2) Ti02 contents of a few weight percent are typical of many garnets, especially those formed from impure carbonates or noncarbonates, and (3) although the normal zoning trends (core to rim increases in andradite content) typical of copper and base-metal skarn garnets are observed, aluminous zones and aluminous rims may be a feature peculiar to goldmineralized systems. Ettlinger and Ray (1989) suggest that the deposition of Al-rich zones in both garnets and pyroxenes (see below) in precious-metal-enriched skarns reflects changes in availability or solubility of aluminum in the system. Alternatively, fluctuations in ferric ironaluminum availability in the system could reflect changes in sulfidation state (J. Remley, oral commun., 1989); that is, ore minerals (predominantly iron sulfides) could effectively deplete the iron available at silicate-hydrothermal Johannsenite Diopside Hedenbergite Figure 11. Ranges of pyroxene compositions for representative samples from three Au-bearing skarn systems in north-central Nevada. fluid interfaces, resulting in growth of relatively aluminous zones. Pyroxene in Au-bearing skarns is typically a diopside-hedenbergite solid solution having low manganese contents. V akhrushev (1972) described diopside (pure to Hd20) as the characteristic pyroxene of the Altai-Sayan gold skarns. Pyroxene in garnet skarn at the middle Tertiary McCoy deposit is diopside-rich (Hd10 to HdSO' percent johannsenite ). Pyroxene coexisting with massive pyrrhotite, other sulfides, and late garnet at the Fortitude deposit is more iron-rich (Hd40 to Hd60) whereas pyroxene in pale-green gametite skarn from the 5,770-ft bench of the Buffalo Valley Mine is nearly pure hedenbergite (Hd80 to H~) (table 7; fig. 11). Skarn mineral assemblages in the gold-enriched part of the Mam property, Yukon, contain iron-rich pyroxene (Hd40 to Hd8J (Brown and Nesbitt, 1987). Brooks and others (1989) noted the presence of narrow aluminous zones in pyroxenes from the McCoy deposit. Ettlinger and Ray (1989) recognized similar zones in pyroxenes in precious-metalenriched skarns from British Columbia and suggested that (1) the presence of high Al20 3+ Ti02 (>1.25 weight percent) in skarn pyroxene may be an indication of precious-metal potential, and (2) the presence of very iron rich (>26.0 weight percent FeO) or very iron poor (<3.5 weight percent FeO) pyroxenes may indicate a low precious-metal potential for a given skarn. Amphibole typically replaces pyroxene as pseudomorphs in Au-skarns and is present with sulfides; reported compositions include actinolite, tremolite, ferrotremolite, and hornblende. Representative amphibole compositions for some Nevada gold skarns are given in table 6, along with data for other minerals. In sulfidized skarn at the Fortitude deposit, ferro-actinolite (low fluorine, as much as 1 percent MnO, 2 percent Al20 3) is intergrown with or replaces pyroxene; pyroxene is present adjacent to massive garnet that is replaced partly by pyrrhotite and chalcopyrite (fig. 12). Actinolite is present with epidote and chlorite in sulfidized retrograde skarn at the Northeast Extension Mine, and in pyrite in garnet skarn at the Carissa Mine. Wallrock Alteration Metasomatic, anhydrous calcic (or magnesian) skarn assemblages in Au-bearing skarn are typically superposed on preceding contact-metamorphic assemblages and followed paragenetically in most deposits by hydrous assemblages with abundant sulfide(s) and (or) magnetite. Some deposits (Bau, Malaysia) show lateral gradation and subsequent replacement by jasperoid (Wolfenden, 1965; W.C. Bagby, oral commun., 1987). Calcic Au-bearing skarns typically are zoned from marble, wollastonite, diopside-hedenbergite, and finally grossular-andradite with or without retrograde tremolite-actinolite-epidote-chlorite assemblages. Watanabe (1943) reported in his study of the Suian Mining District, Wallrock Alteration
North Korea, that magnesian Au-bearing skarn may show dolomite followed by marble bearing kotoite [M~(B03)2] and ludwigite [(Mg,Fe2+)2Fe3+BOs]; a narrow fluoborite [Mg3(B03)(F,OH)3]-bearing reaction zone marking the contact between skarn and marble; a marked concentration of native gold, bismuth, chalcopyrite, pyrrhotite, and cubanite just inside the reaction zone; diopside; clinohumite; and, finally, diopside partly replaced by phlogopite-all zones developed across 25-35 em. At the Surprise, Nevada, gold skarn, limonite, fine-grained quartz, copper oxide(s), and calcite occur interstitial to massive garnet; garnet is crosscut and replaced by veins of limonite and chlorite (Schmidt and others, 1988). In this deposit, gold is present as electrum in limonite (fig. 8) associated with quartz, calcite, and secondary copper minerals. The only sulfides remaining in extensively oxidized high-grade ore currently (1989) exposed at the Surprise Mine are pyrite remnants in limonite and tiny blebs of various sulfides encapsulated in late, euhedral quartz crystals. The Buffalo Valley, Nevada, gold skarn shows widespread development of nontronite throughout much of the exposed ore. Structural Setting Gold-bearing skarn may occur in the immediate vicinity of, or relatively distal from, weakly mineralized intrusive rocks, commonly where wallrocks are extensively brecciated or faulted (fig. 4). On a local scale, gold-enriched dikes and small plutons astride hinge regions of broad anticlinal arches seem to have been an important structural control (Madrid, 1987). The Bau Mining District, Malaysia, lies along the axis of a major anticline flanked by synclinal basins (Wolfenden, 1965). 0.2 MILLIMETER Figure 12. Photomicrograph showing massive garnet (ga), partly replaced by pyrrhotite (po) and chalcopyrite (cp) and separated from a pod of actinolite (act) grains by pyrrhotite; Fortitude Mine, Nevada. Gold-Bearing Skarns Dimensions of Ore in Typical Deposits Overall dimensions of ore in Au-bearing skarn are highly variable; dimensions possibly increase with distance from the genetically associated intrusive rock and as grade decreases. Geologic configuration of such deposits is largely a function of respective geometries of mineralizing magma and premineralization structures, favorable replacement sequences, and impermeable barriers to fluid flow, if present. However, eventual configuration of economic dimensions of deposits results from cut-off grades that are influenced highly by factors such as pre-mining topography (R.G. Russell, written commun., 1989). Dimensions of Alteration or Distinctive Haloes Alteration haloes that surround Au-bearing skarn are highly variable in size, from very restricted to as much as several kilometers from inferred loci of mineralizing systems. In some systems, the overall size of the alteration zone has been enhanced by the presence of premineralization structures that channeled fluid flow. Nonetheless, in a largely carbonate terrane, the Au-bearing skarns are almost always found within the outer limit of conversion of carbonate sequences to marble. Effect of Weathering The economic limits of some deposits are entirely within the oxide zone. In fact, gold grade is commonly higher in the oxide zone than in the equivalent sulfide zone. The oxide zone in some deposits includes coarsely crystalline vivianite along fractures in areas showing limited overall amounts of iron oxide development and limited amounts of subjacent iron sulfide(s) (R.G. Benson, written commun., 1988). At the McCoy, Nevada, Au-skarn, samples from the 5,080-ft bench show some extremely small, micrometersized crystals of greenockite (CdS) concentrated at interfaces between chalcopyrite and chalcocite. In this deposit, some chalcocite also appears to be associated paragenetically with a silver-selenide mineral, possibly Ag2(S, Se). Nontronite layers are commonly interbedded with some garnet skarn and locally concentrated along fractures in some deposits. At Browns Creek, Australia, gold-bearing nontronite was the major target of mining activity inasmuch as it typically contained greater than 10 g/t gold (Creelman and others, 1988). The term "nontronite" is used as a field term for iron-rich, yellow-green montmorillonite that swells upon treatment with ethylene glycol; a Mossbauer spectrometic study of one such clay from skarn in the Harmony Formation near the Surprise Mine shows that nearly all of the iron present in the sample is ferric iron. Thus, nontronite is the main component of the clay layer there. Clay layers include
quartz and calcite and may include relict skarn silicates (pyroxene, garnet, and epidote). Oxidized karst-collapse breccia developed in marble as a result of marble reacting with acidic ground water at Red Dome (Torrey and others, 1986). At this deposit, acidic ground water probably resulted from breakdown of sulfides in the surrounding pyritic halo of the Au-skarn. Effect of Metamorphism Gold-bearing skarn systems could undergo regional metamorphism to yield gneiss-hosted Au deposits with a resultant loss of most contact-metasomatic features. The Tumco deposit, California, which has been metamorphosed to amphibolite grade and is provisionally included by us with Au-bearing skarn (Smith and Graubard, 1987; Tosdal and Smith, 1987), may be an example of such a process. However, some relatively extensive tin-tungsten-base-metal skarns in Alaska show readily recognizable prograde and retrograde contact-metasomatic assemblages through a superposed greenschist dynamothermal event (Newberry and others, 1986). In these Sn skarns, strain is confined largely to 1-m-wide wnes at the margins of skarn where calc-silicate porphyroclastic mylonite is present. Skarn away from the contact shows some kinked chalcopyrite-bornite exsolution lamellae, but no cleavage or foliation. The Falun deposit in Sweden is hosted by Proterowic granite, amphibolite, and quartz porphyry (Grip, 1978). Greenstone-hosted Au-AgW-As deposits in the Southern Cross greenstone belt of western Australia may represent Archean analogues of Phanerozoic gold skarn deposits (Mueller, 1988). Geochemical Signatures Geochemical signatures for Au-bearing skarn include anomalous gold primarily in an environment of retrogradealtered, sulfidized skarn. The associated pyrite in some Auskarn deposits is reported to contain 0.1 to 250 ppm Au (Vakhrushev, 1972). At Bau, Malaysia, anomalous antimony (in stibnite) and arsenic (in scorodite) are present with gold in wollastonite-bearing skarn assemblages and in colloformbanded quartz and jasperoid, all distal to quartz- and calciteflooded, calc-silicate gold ore (Wolfenden, 1965; W.C. Bagby, oral commun., 1987). In other Au-bearing skarn systems, quartz-calcite veins contain anomalous gold. In addition, gold mineralization and highly anomalous concentrations of gold in some skarn systems (Akshiryak Range, U.S.S.R.) are found mostly in fme-grained, gray to light-gray, highly silicified sequences of rock in carbonate beyond the outer limit of established skarn (Dolzhenko, 1974). Many Au-bearing skarns in British Columbia contain elevated abundances of arsenic, bismuth, and tellurium (Ray and others, 1987b; Ettlinger and Ray, 1989). The bismuth minerals reported from some Au-skarns include native bismuth,.bismuthinite, wittichenite, hedleyite, maldonite, and Bi-bearing galena (Meinert, 1988b). Theodore and others (1989) report major-element and trace-element data for garnet skams associated with gold mineralization at Copper Basin, Nevada, including low-grade, oxidized ore from the Surprise Mine (29 ppb Au, 6 ppm Ag, <10 ppm Bi, 57 ppm As, 4 ppm Sb, 3 ppm Co, 25 ppm Cu). Finally, surface expression of some Au-skarn systems (Red Dome, Australia; Surprise, Nevada) includes relatively abundant, fracturecontrolled secondary copper minerals (Torrey and others, 1986; Schimdt and others, 1988). The Au/Ag ratio in rock apparently increases laterally outward (away from the center of the associated intrusion) in some productive copper-bearing calcic skarn systems toward ore (Fortitude, Nevada) that is approximately 0.6 km from the exposed, genetically associated intrusion. The Fortitude Au-skarn is close to a relatively sharp boundary between marble and sulfidized calc-silicates (Blake and others, 1984; Theodore and others, 1986; Wotruba and others, 1987a, b; Myers and Meinert, 1988). In other Auskarn systems that are predominantly wned vertically close to the related intrusive rocks (Red Dome, Australia}, much of the gold ore is near the original intrusion-wallrock contact and interior to massive magnetite developed at the calcsilicate-marble interface (Torrey and others, 1986). Surrounding rocks in many systems typically show high local thresholds for many associated base and ferrous metals and, for some deposits, arsenic, bismuth, selenium, and tellurium values in particular may be relatively high both within and peripheral to the Au-bearing skarn (Ray and others, 1987b). Zonation of gold in Au- and Pb-Zn-bearing skarn (Ban Ban, Australia; Thanksgiving, Philippines; TomboyMinnie, Nevada) seems to show inconsistent patterns. At Ban Ban, gold in unreported trace abundances may coincide with known distribution of silver, which varies directly with lead and zinc concentrations that are, in tum, constrained tightly to the central part of associated garnet skarn (Ashley, 1980). At Thanksgiving, irregularly distributed sphaleritepyrite pods that replace andradite skarn show higher gold contents than pyrite-magnetite replacement pods (Callow, 1967). At Tomboy-Minnie, local metal zoning of the gold orebodies shows high concentrations of gold (more than 0.05 troy oz/ton or more than 1. 7 g/t}; these high concentrations of gold show increased abundances of zinc and silver (more than 500 ppm and more than 0.1 troy oz/ ton, or more than 3.4 g/t respectively) on the granodiorite side of the gold orebody. Such metal-zoning relations constitute a local reversal of the district-wide wning from Cu+Au+Ag, through Au+Ag, to finally Pb+Zn+Ag (Theodore and others, 1986). Zonation of gold in some Fe-skam systems that contain byproduct gold (Benson Lake, British Columbia) seems to be related directly to the abundance of sulfide Geochemical Signatures
associated with magnetite (Eastwood, 1965). At the Merry Widow pit of the Benson Lake cluster of magnetite skarns, concentrates of chalcopyrite were reported to contain as much as 1 oz gold per ton of chalcopyrite. Significant concentrations of gold have been reported, although specifics are unavailable, in many of the Paleozoic W skarns in the Soviet Union (table 4). Gold is an associated minor metal in approximately one-half of the W-skarn deposits in the Ural Mountains, U.S.S.R. (Rabchevsky, 1988). These skarns are reported to be associated with Devonian- to Permian-age granitoid bodies (Rabchevksy, 1988). In addition, selected samples of Mesozoic W skarns from Alaska are reported to contain as much as 30 ppm gold (R.J. Newberry, oral commun., 1987; Newberry and others, 1987). Tin skarns in China and Australia have reported significant Au or Au-enriched areas (see Stormont and Ge Jiou, table 4). At a more detailed level, nontronite layers from some Au-bearing, calcic skarn deposits show significant concentrations of silver and copper and variable, but enhanced levels of other trace elements, such as tin (table 8). Spectral analyses of garnets from four Au-bearing skarn deposits in the Altai-Sayan study (Vakhrushev, 1972) show trace-element signatures distinct from those of garnets from Fe skarns: copper and zinc (tens to hundreds of parts per million), molybdenum, scandium, gallium, and tin (10 to 50 ppm each) are present in all the garnets from Au-skarn; some garnets carry several hundred parts per million arsenic, as much as 30 ppm lead, and similar concentrations of silver as well. In contrast, garnets from Fe skarns have titanium, chromium, vanadium, nickel, cobalt, and germanium as a characteristic trace-element suite and lack the elements associated with Au-skarn garnets or show inconsistent distributions of them. The single report of platinum associated with gold skarn that we found is in northern Sumatra, where Bowles and others (1985) described a reference to 8 ppm Pt and 4 ppm Au in wollastonite-garnet skarn; however, they point out that some confusion exists over the precise locality of the occurrence. Isotopic Signatures Isotopic data are not available for a great number of Au-bearing skarns. However, the range in o34S values for sulfides is clustered tightly in one examined system: +2.7 to +4.7 permil for the Tomboy-Minnie deposit (Theodore and others, 1986). Such values suggest a magmatic source, and minimal contribution from heavy, crustal sulfur that was highly homogenized. An associated Cu skarn adjacent to the intrusion, the West orebody, shows more scattered values of o34S, +1.1 to +5.1, in sulfides there, possibly reflecting disequilibria resulting from passage of retrograde fluids. Derivation of the associated altered granodiorite Gold-Bearing Skarns apparently was primarily from crustal components, to judge from initial neodymium isotopic compositions (Farmer and DePaolo, 1984). Fluid Inclusions Boiling, high-salinity fluids are associated with the early, prograde paragenetic stages of many Au-bearing skarn systems. The fluid-inclusion signature of skarn probably is most easily inferred from fluid inclusions trapped in quartz in the associated intrusive rocks if optical limitations preclude study of fluid inclusions in garnet or pyroxene. For example, possible involvement of high-salinity fluids some time during the generation of Au-bearing skarn may be implied by occurrence of halite-bearing fluid inclusions in quartz phenocrysts of a genetically associated granitoid. In some deposits (Tomboy-Minnie, Nevada), early fluids associated with diopside-quartz assemblages were dominantly CaC12 -brines and were boiling at temperatures higher than 500 OC. Fluids then were progressively enriched in sodium and potassium over time, and during hydrosilicate stages, temperatures ranged from 320 to 500 °C at the time actinolite formed, and from 220 to 320 OC at the time chlorite became dominant in the assemblages (Theodore and others, 1986). Much of the gold is paragenetically late, deposited from NaCl-rich brines at temperatures less than 300 °C. However, genetic association of highly saline brines with skarn does not guarantee presence of a metal-bearing deposit somewhere in the environment of the skarn. Some Tertiary garnet-pyroxene skarn in the northern Battle Mountain Mining District shows fluid-inclusion signatures highly suggestive of many porphyry copper systems, yet the skarn is barren of any associated metal deposits (Theodore and Hammarstrom, 1989). At the Fortitude, Nevada, deposit, initial fluid-inclusion studies indicate that the Au-skarn was formed by fluids ranging from 300 to 450 oc and with salinities much less than 26 weight percent NaCl equivalent (Myers and Meinert, 1988). At Red Dome, Australia, coppergold-silver ores apparently were deposited during a retrograde stage attendant with the circulation of relatively low-salinity (less than 10 weight percent NaCl equivalent), possibly meteoric-dominant fluids at temperatures in excess of 350-380 °C (Torrey and others, 1986; Ewers and Sun, 1988). In other skarn systems, gold also was deposited mostly during low-temperature stages: Alae-Sayan, U.S.S.R. (250-150 °C), Central Tadzhikistan, U.S.S.R. (350-250 OC), Sayakskig, U.S.S.R. (greater than 250-225 °C), and Kochulak, U.S.S.R. (270-240 °C; 190-170 °C) (table 4). Deposition of most gold close to the calc-silicate-marble interface, as reported in many Au-bearing skarns (Myers and Meinert, 1988), may reflect a combination of protracted solubility of gold in bisulfide complexes and build-up of HC03- in the fringe environment of evolving skarn (Gumenyuk and Glyuk, 1983), thereby decreasing the
solubility of gold owing to a change in pH (Henley, 1984). Gold solubility relations at 250 °C, a temperature considered by many to approximate thermal conditions in most Aubearing skarns during paragenetic stage(s) of gold deposition, culminate at oxygen activity-pH conditions compatible with pyrite stability (Romberger, 1988). As Romberger (1988) further noted, if most gold is transported as a bisulfide complex, gold deposition may be accomplished by any chemical reaction or physicochemical process that decreases chemical activity of sulfur components dissolved in aqueous fluids circulating through skarn, including deposition of sulfide minerals and loss of sulfur components because of boiling. Geophysical Signatures Well-developed, local magnetic highs result from increased abundance of pyrrhotite and (or) magnetite in some Au-skarn systems (see Wotruba and others, 1987a, b). However, other Au-skarn systems are associated mostly with pyrite in their unoxidized parts (McCoy) and show no distinctive magnetic signatures (Bruce A. Kuyper, oral commun., 1987). Ore Controls/Exploration Guides In established mining districts zoned from mostly proximal copper-dominant deposits to distal precious-metaldominant and base-metal-dominant veins, all stratigraphic sequences favorable for development of skarn in the zone of precious-metal deposits should be considered as permissive hosts for development of Au-bearing skarn. Polymetallic veins and polymetallic replacement deposits showing geochemical signatures and sulfide mineral assemblages similar to those at many Au-bearing skarns (for example, the Fe-As-Zn-Cu-Bi-Au- and Sb-bearing ores at the Matsuo Mine, Japan; Matsukuma, 1962) may be highlevel or lateral reflections of Au-bearing skarn. Other guides include: reported gold in base- and ferrous-metal skarn systems; gold placers in regions permissive for the formation of skarn (R.G. Russell, written commun., 1989), especially if the placer gold is intergrown with bismuth minerals, including bismuth oxides or bismuth tellurides (Theodore and others, 1987; Theodore and others, 1989). Anomalous values of bismuth, tellurium, arsenic, selenium, and cobalt are useful geochemical signatures for some gold-bearing skarns (tables 2, 3; Brooks and Meinert, 1989). Metal ratios in jasperoids, which commonly occur in or on the fringes of gold skarn systems, may also provide useful geochemical signatures for exploration. Faults cutting skarns and intersecting structures are important pathways along which retrograde assemblages and associated ores are concentrated. R.G. Russell (written commun., 1989) distinguishes between barren, early, high-temperature contact skarn formed adjacent to intrusive rocks and mineralized, fracture-enhanced exoskarn developed in Au-skarn systems. Although pyroxene (hedenbergite )- and pyrrhotiterich distal skarns host gold mineralization in some deposits, such as the Fortitude, garnet-pyroxene (diopsidic) and chalcopyrite or pyrite-rich proximal skarns are the locus of gold mineralization at other deposits, such as McCoy. Further studies on Au-bearing skarn deposits may reveal relatively reduced (Fortitude) and oxidized (McCoy) types of goldbearing skarn, such as have been recognized for tungsten skarns (Einaudi and others, 1981). GRADES AND TONNAGES OF GOLD-BEARING SKARNS Graphs of grades and tonnages of 40 Au-skarns from table 2 and 50 byproduct Au-skarns from table 3 are shown in figures 1, 2, and 13. Gold grade must be 1 g/t or higher to be included, as described above. Median tonnage for the Au-skarn subtype is about 213,000 tonnes (fig. lC), and median tonnage for the byproduct Au-skarn subtype is about 330,000 tonnes (fig. 1D). For the Au-skarn subtype there is a strong negative correlation between gold grade and tonnage (linear correlation coefficient -0.69); this relation is slightly weaker for the byproduct Au-skarn subtype (linear correlation coefficient= -0.54). The Au-skarn subtype has a median gold grade of about 8.6 g/t and a median silver grade of about 5.0 g/t (figs. 2C and 13A). The determination of median silver grade for the Au-skarn subtype is based upon values of silver grade available for 29 of 40 deposits (table 2). Meinert (1988a) tabulated Au, Ag and Cu grades for various types of skams. The fourteen deposits he classified as gold skarns all have gold grades greater than 1 g/t Au and largely overlap our data set. Median gold grade for Meinert's gold skarn set is 6.5 g/t; median silver grade for the nine deposits that report silver is 9 g/t. For the byproduct Au-skarn subtype, the medians are 3. 7 g/t gold and approximately 34 g/t silver. Nearly 90 percent of the byproduct Au-skarns report silver (table 3). Silver content appears to have a strong correlation with base-metal content. As a comparison, the median gold grade for 14 porphyry copper-related Cu skarns, as reported by Meinert (1988a), is approximately 0.3 g/t and the median silver grade is approximately 8 g/t (note that these values are higher than those reported by Singer, 1986) for gold in porphyry copperrelated skarns. We found wide variations in gold grade distributions. In fact, values of gold grade reported during various stages of exploration and development of many deposits typically show significant adjustments, usually in a descendent manner. Furthermore, tests of the gold grade distribution for Au-skarns indicate that the addition of approximately 40 deposits with grades less than 3.7 g/t would be required Grades and Tonnages of Gold-Bearing Skarns
to change the median to a value approximately the same as that of the byproduct Au-skarn subtype. As already described, skarns that contain byproduct gold show no statisticaily significant differences in tonnage distributions from Au-skarns exploited almost exclusively for their precious-metal content (figs. 1, 2). This relation is primarily a reflection of the highly variable exploitability of many polymetallic skarn systems under a wide range of economic circumstances. Tonnages of deposits that comprise the Au-skarn subtype vary widely, from approximately 9 tonnes to 15 million tonnes (table 2), primarily because of a combination of both differing economic circumstances and A en (j) a.. oc UJ u. -1.5 z
B a: .9 a.. a: a.. ?:, ?:,
?:, SILVER GRADE, IN LOG UNITS OF GRAMS PER TONNE Figure 13. Distributions of silver grade for Au-bearing skarns. A, Silver grade model for 29 Au-skarn deposits. 8, Silver grade model for 44 byproduct Au-skarn deposits. Gold-Bearing Skarns advances in metaiiurgical techniques over the many years these types of deposits have been mined. However, there is a marked difference in cumulative distributions for gold and silver grades of Au-skarn and byproduct Au-skarn as deemed previously: Au-skarns have a median gold grade of 8.6 g/t and byproduct Au-skarns have a median grade of 3.7 g/t Median silver in Au-skarns is 5 g/t as compared to 37 g/t in byproduct Au-skarns (fig. 13). Both gold (greater than 1 g/t) and silver grade populations as currently reported are significantly different between the Au-skarn and byproduct Au-skarn subtypes. Median silver grades were determined from silver data available for 29 of 40 deposits included in the Au-skarn subset, and for 44 of 50 deposits included in the byproduct Au-skarn subset. It should also be noted that in element-versus-element plots for byproduct Au-skarn and Au-skarn subtypes, byproduct Au-skarns (or Au-rich Cu-skarn deposits in the terminology of many others), for instance, plot in a cluster spatiaiiy separate from most Au-skarns and tend to represent the Cu-rich part of the domain with gradational and overlapping relations with other skarns; these types of relations hold for many other elements (fig. 14A). However, as shown by the graph of these data (fig. 14A), strengths of association of gold grade for copper grade in the two subsets of Au-bearing skarn seem to show extremely weak correlations between gold grade and copper grade; correlation coefficients for the two subsets of types of Au-bearing skarn are less than 0.2. Gold and copper grades are available only for 20 of 40 Au-skarn deposits (table 2), which may in part be an indication of an underreporting of copper contents for some deposits because of its economic insignificance during the time many of those deposits were being mined. Nonetheless, very significant gold grades can occur in some very Cu-rich skarns. Many Au-skarn deposits show a strong spatial association between gold and copper within the deposits themselves. A plot of gold grades versus silver grades for both subsets of Aubearing skarn shows that most Au-skarns have silver grades lower than byproduct Au-skarns (fig. 14B). Gold grades compared with silver grades for the Au-skarns have a correlation coefficient of approximately +0.4, and for the byproduct Au-skarns a correlation coefficient of approximately +0.2. One important exploration implication is that economicaiiy viable Au-bearing skarn deposits may be associated with Cu, Pb-Zn, Fe, or W skarn, and although the median gold grade of these byproduct Au-skarns is lower, the highest grades are similar to those of the Au-skarn subtype. Perhaps the only types of metal-bearing skarn that might be excluded from consideration as permissive for the occurrence of significant concentrations of gold and silver are tin-skarn and lithophile-element (beryllium, fluorine, tungsten, molybdenum, tin, and zinc) skarn associated with two-mica granite. Significant gold or silver mineralization is not known in classic Sn-skarn regions in Cornwall (Hosking, 1964) or Malaysia (Hosking, 1977, 1979). However, some Au- and Bi-bearing skarns (Stormant) are
known in the Moina Mining District, Tasmania, Australia, which is largely known for its Sn-W skarn and greisen deposits (Collins and Williams, 1986). Lithophile-element skarn is associated with numerous Late Cretaceous, peraluminous, two-mica granitoids across a broad region in the eastern Great Basin of the United States (Barton, 1987; Barton and others, 1988). Significant concentrations of gold have not been reported from this lithophile-elementskarn environment. However, silver is present in many of these lithophile-element skams in apparently genetically 1. 5 , r r T
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1.25 1.5 1.75 GOLD GRADE, IN LOG UNITS OF GRAMS PER TONNE Figure 14. Gold grade compared with copper grade and silver grade. A, Gold grade compared with copper grade for Au-skarns and byproduct Au-skarns; 8, Gold grade compared with silver grade for Au-skarns and byproduct Au-skarns. associated silver-base-metal, quartz-carbonate veins (Barton, 1987). REFERENCES CITED Argall, G.O., Jr., 1986, The golden glow at Battle Mountain; Pennzoil spin-off starts life nearly debt free as third largest in US gold: Engineering and Mining Journal, v. 187, no. 2, p. Ashley, P.M., 1980, Geology of the Ban Ban zinc deposit, a sulfide-bearing skarn, southeast Queensland, Australia: Economic Geology, v. 75, no. 1, p. 15-29. Atkinson, W.W., Jr., and Einaudi, M.T., 1978, Skarn formation and mineralization in the contact aureole at Carr Fork, Bingham, Utah: Economic Geology, v. 73, p. 1326-1365. Bagby, W.C., Menzie, W.D., Mosier, D.L., and Singer, D.A., 1987, Grade and tonnage model of carbonate-hosted Au-Ag, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 175-177. Baker, A.A., Calkins, F.C., Crittenden, M.D., Jr., and Bromfield, C.S., 1966, Geologic map of the Brighton quadrangle, Utah: U.S. Geological Survey Geologic Quadrangle Map GQ-534, 1 sheet Barr, D.A., 1980, Gold in the Canadian cordillera: Canadian Institute of Mining and Metallurgy Bulletin, v. 73, no. 818, p. 5976. Barton, M.D., 1987, Lithophile-element mineralization associated with Late Cretaceous two-mica granites in the Great Basin: Geology, v. 15, no. 4, p. 337-340. Barton, M.D., Battles, D.A., Debout, G.E., Capo, R.C., Christensen, J.N., Davis, S.R., Hanson, R.B., Michelsen, C.J., and Trim, H.E., 1988, Mesozoic contact metamorphism in the Western United States, in Ernst, W.G., ed., Metamorphism and crustal evolution of the Western United States: Englewood Cliffs, New Jersey, Prentice-Hall, p. 110-178 . Bazhenov, V J., 1968, Zones of increased fracture and their role in localization of gold mineralization in Maryinskaya Taiga: International Geological Review, v. 10, no. 2, p. 208-214. Beddoe-Stephens, B., Shepherd, T.J., Bowles, J.F.W., and Brook, M., 1987, Gold mineralization and skarn development near Muara Sipongi, West Sumatra, Indonesia: Economic Geology,v.82,p. 1732-1749. Bergeat, Alfred, 1910, La granadiorita de Concepcion del Oro en el Estada de Zacatecas y sus formaciones de contacts: Instituto Geologico Mexico, Bulletin 27. Bevan, P.A., 1973, Rosita Mine-A brief history and geological description: Canadian Institute of Mining and Metallurgy Bulletin,v.66,no. 736,p.80-84. Billingsley, P., and Hume, C.B., 1941, The ore deposits of Nickel Plate Mountain, Hedley, British Columbia: Canadian Institute of Mining and Metallurgy, Bulletin, v. XLN, p. 524590. Bin, Zhao, and Barton, M.D., 1988, Compositional characteristics of garnets and pyroxenes in contact-metasomatic skarn deposits and their relationships with metallization: Chinese Journal of Geochemistry, v. 7, no. 4,p. 329-335. Blake, D.W., Wotruba, P.R., and Theodore, T.G., 1984, Zonation in the skarn environment at the Minnie-Tomboy gold deposReferences Cited
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Thompson, K.C., 1973, Mineral deposits of the Deep Creek Mountains, Utah: Utah Geological and Mineralogical Survey Bulletin 99. Tilling, R.I., 1973, Boulder batholith, Montana: A product of two contemporaneous but chemically distinct magma series: Geological Society of America Bulletin, v. 84, p. 3879-3900. Tingley, J.V., and Smith, P., 1982, Mineral inventory of EurekaShoshone Resource Area: Nevada Bureau of Mines and Geology Open-File Report 82-10. Tooker, E.W.,1989, Gold in the Bingham district, Utah, in Shawe, D.R., Ashley, R.P., and Carter, L.M.H., eds., Geology and resources of gold in the United States: U.S. Geological Survey Bulletin 1857-E, p. E1-E27. Torrey, C.E., Karjalainen, H., Joyce, P.J., Erceg, M., and Stevens, M., 1986, Geology and mineralization of the Red Dome (Mungana) gold skarn deposit, north Queensland, Australia, in Macdonald, A.J., ed., Proceedings of Gold '86, an International Symposium on the Geology of Gold: Toronto, Geological Association of Canada, 1986, p. 3-22. Tosdal, R.M., and Smith, D.B., 1987, Descriptive models for gneisshosted kyanite gold and gneiss-hosted epithermal gold: A supplement to U.S.G.S. Bulletin 1693: U.S. Geological Survey Open-File Report 87-272B, 6 p. Traumerman, C.J., and Reyner, M.L., 1950, Directory of mining properties-1949: Montana Bureau of Mines and Geology Memoir 31, 125 p. TRM Engineering Ltd., 1986, Resource assessment for coastal and western British Columbia and the development of a portable modular mill design: Trader Resources CorporationFlat Development Ltd., and the British Columbia Ministry of Energy, Mines and Petroleum Resources, 223 p. Tveritinov, Y.l., 1966, Relation of skarn to mineralization in gold deposits of northeastern Altay Region: International Geological Review, v. 8, no. 10, p. 1215-1217. Umbgrove, J.H.F., 1947, The pulse of the Earth: The Hague, Netherlands, Martinus Nijhoff, 358 p. U.S. Bureau of Mines, 1950, Strategic minerals examination: Washington, D.C., Minerals Yearbook, 1, 690 p. Vakhrushev, V.A., 1972, Mineralogiya, geokhimiya i obrazovaniye mestorozhdeniy skarnovo-zolotorudnoy formatsii [Mineralogy, geochemistry, and genesis of gold-bearing skarn formations]: Akademiya Nauk SSSR, Sibirskoye Otdeleniye, Institut Geologii i GeofiZiki, 238 p. (in Russian). Vakhrushev, V.A., and Tsimbalist, V.G., 1967, Raspredeleniye zolota v sul'fidakh skamovykh mestorozhdeniy Altaye-Sayanskoy oblasti [Gold distribution in the sulfides of skarn deposits in the Altai-Sayan region]: Geokhimiya, no. 10, p. 1076-1081 (in Russian with English swnmary). Vanderburg, W.O., 1940, Reconnaissance of mining districts in Churchill County, Nevada: U.S. Bureau of Mines Information Circular 7093, 57 p. Watanabe, Takeo, 1943, Geology and mineralization of the Suian district, Yuosen (Korea): Journal of the Faculty of Science, Hokkaido Imperial University, Series N, Geology and Mineralogy,v.6,no.3-4,p.205-303. Wayland, R.G., 1943, Gold deposit near Nabesna: U.S. Geological Swvey Bulletin 933-B, p. 175-195. Wedekind, Richard, 1988, Petrology, sulfur isotope, and geochemistry of the Warrego gold-bismuth-copper mine, Tennant Creek, Northern Territory, in Goode, A.D.T., Smyth, E.L., Birch, W.O., and Bosma, L.l., compilers, Bicentennial Gold 88, Extended Abstracts, Poster Programme, v. 2: Geological Society of Australia, Abstract Series, no. 23, p. 489-491. Wedekind, Richard, Large, Ross, Zaw, Khin, Horvceth, Harry, and Gulson, Brian, 1988, The composition and source of ore depositing fluids in the Tennant Creek Goldfield, in Goode, A.D.T., Smyth, E.L., Birch, W.O., and Bosma, L.l., compilers, Bicentennial Gold 88, Extended Abstracts, Poster Programme, v. 2: Geological Society of Australia, Abstract Series,no.23,p.492-494. Wilkins, J.D., 1971, The Benson Lake mine--Operating practice: Canadian Institute of Mining and Metallurgy Bulletin, v. 64, no. 708, p. 71-77. Wilson, S.R., 1959, Mining history and mineralogy of ores of the Clifton district, Gold Hill, Tooele County, Utah: Mineralogical Society of Utah Bulletin, v. 9, no.1, p. 5-11. Winchell, A.N., 1914, Mining districts of the Dillon quadrangle, Montana, and adjacent areas: U.S. Geological Swvey Bulletin 574, 191 p. Wolfenden, E.B., 1965, Bau mining district, west Sarawak, Malaysia, part I, B au: Geological Survey of Malaysia (Borneo Region) Bulletin 7, pt. 1, 147 p. Wotruba, P.R., Benson, R.G., and Schmidt, K.W., 1986, Battle Mountain describes the geology of its Fortitude gold-silver deposit at Copper Canyon: Mining Engineering, v. 38, no. 7, p.495-499. ---1987a, Geology of the Fortitude gold-silver skarn deposit, Copper Canyon, Lander County, Nevada [abs.]: Geological Society of Nevada, Bulk Mineable Precious Metal Deposits References Cited
of the Western United States, Symposium, Reno, Nev., April 6-8, 1987, Program with Abstracts, p. 39-40. ---1987b, The Fortitude gold-silver deposit, Copper Canyon, Lander CoWlty, Nevada, in Johnson, J.L., ed., Bulk Mineable Guidebook for Field Trips: Geological Society of Nevada Symposium, Reno, Nev., April6-8, 1987, Guidebook, p. 343347. YoWlg, G.A., and Uglow, W.L., 1926, The iron ores of Canada; Volume 1, British Columbia and Yukon: Geological Survey of Canada, Economic Geology Series 3, v. 1. Zharikov, V.A., 1970, Skams: International Geology Review, v. 12, p. 541-559,619-647,760-775. BIBLIOGRAPHY OF ADDITIONAL GOLD-BEARING SKARN REFERENCES Abdullaev, K.M., AdelWlg, A.S., Kalabina, M.G., Malakoy, A.A., Matsokina, T.M., Mirkhodzhaev, I.M., Radzhabov, F .S.L., and V oronich, V.A., 1958, Osnovnye cherty magmatizrna i metallogenii Chatkalo-Kuraminskikh gor [Main features of magmatism and metallogeny of the Chatkalo-Kuraminsky mountain range]: Tashkent, U.S.S.R., Akademiya Nauk Uzbekskoy SSR, Institut Geologicheskych Nauk, 289 p. (in Russian). Abulgazina, S.D., Kuznetsova, Y.I., and Slyusarev, A.P., 1975, Sostav i svoystva dvukh vismutovykh sul'fosoley medi iz skamovykh mestorozhdeniy Sayakskoy gruppy [Composition and properties of the bismuth sulfosalts of copper from skarn deposits of the Sayak Group]: Moscow, U.S.S.R., Akademiya Nauk SSSR Doklady, v. 222, no. 1, p. 183-185 (in Russian). Addie, G.G., 1985, Self-potential tests at the Silver Queen Prospect near Tillicum Mountain and the Hailstorm MoWltain gold prospect, in Geological fieldwork 1985: British Columbia Ministry Energy, Mines and Petroleum Resources Paper 1985-1, p. 48-52. Agostini, A., 1984, Nyngan 1:250,000 sheet; A preliminary geological interpretation from regional aeromagnetic and gravity data: Geological Survey of New South Wales Quarterly Notes, v. 54, p. 13-23. AkhWldzhanov, R., and Turesebekov, A.K., 1985, Svyaz' skarnovo-polimetallicheskikh i medno-molibdenovykh mestorozhdeniy Karamazara s intruziyami (Kuraminskiye gory) [The relationship of the skarn-polymetallic and coppermolybdenum deposits of Karamazar to intrusions; Kurama Range]: Uzbekskiy Geologicheskiy Zhurnal, v. 3, p. 6-9 (in Russian). Andrusenko, N.I., Kosovets, T.N., Ushakova, L.K., Shugurova, N.A., and Bochek, L.I., 1978, Conditions of formation of gold mineralization in a complex field: International Geology Review, v. 20, no. 8, p. 916-926. Aristov, V.V., and Lyakhov, L.L., 1982 (1983), Surface and subsurface prospecting for concealed solid-mineral deposits, part 2: International Geology Review, v. 25, no. 9, p. 1060-1074. Arutyunyan, M.A., and Kukulyan, M.A., 1985, Vremya vydeleniya zolota v protsesse skamo i rudoobrazovaniya na Kefahenskom skarnovom medno-molibdenovom proyavlenii Zangezurskogo rudnogo rayona (Armyanskaya SSR) [Deposition of gold in Gold-Bearing Skarns processes of skarn and ore formation in the Kefashen coppermolybdenum skarn of the Zangezur ore region, Armenia]: Izvestiya Akademii Nauk Armyanskoy SSR, Nauki o Zemle, v. 38, no. 3, p. 62-66 (in Russian). Baker, J.H., and Hellingwerf, R.H., 1988, Rare-earth element geochemistry of W-Mo-(Au) skams and granites fromWestem Bergolagen, Central Sweden: Mineralogy and Petrology, v. 39, p. 231-244. Baksht, F.B., 1972, Geofizicheskiye metody kak sredstvo izucheniya zolotorudnykh stolbov na skamovykh mestorozhdeniyakh Gonogo Altaya [Geophysical methods as a means of studying gold-ore shoots in skarn deposits of Gorny Altai], in Problemy obrazovaniya rudnykh stolbov: Novosibirsk, U.S.S.R., Akademiya Nauk SSSR, Sibirskoye Otdeleniye, Institut Geologii i Geofiziki, p. 165-168 (in Russian). Barton, M.D., Ruiz, J., and Ito, E., 1982, Preliminary tracer studies of the fluorine-rich skarn at McCullough Butte, Eureka Co., Nevada [abs.]: Geological Society of America Abstracts with Programs, v. 14, no. 7, p. 440. Beane, R.E., Bloom, M.S., and Jaramillo, L., 1974, Skarn and disseminated mineralization in the Jarilla MoWltains, Otero CoWlty [abs.], in Silver anniversary guidebook: Ghost Ranch, central-northern New Mexico; base-metal and fluorspar districts of New Mexico; a symposium: New Mexico Geological Society Annual Field Conference Guidebook, no. 25, p. Bekmukhametov, A.Y., Dzhaminov, K.D., Zhunusov, A.A., and Tulenova, Z.S., 1984, 0 zolotosoderzhashchickh piritakh Kacharskogo magnetitovogo mestorozhdeniya [Gold-bearing pyrite in the Kacharsk magnetite deposit]: Akademii Nauk Kazakhskoy SSR lzvestiya, Seriya Geologicheskaya 1984, v. 3, 43 p. (in Russian). Blake, D.W., and Kretschmer, E.L., 1983, Gold deposits at Copper Canyon, Lander County, Nevada, in Kral, V.E., Hall, J.A., Blakestad, R.B., Bonham, H.F., Jr., Hartley, G.B., Jr., McClelland, G.E., McGlasson, J.A., and Mousette-Jones, Pierre, eds., Papers given at the Precious-Metals Symposium, Sparks, Nevada, November 17-19, 1980: Nevada Bureau of Mines and Geology Report 36, p. 3-10. Blokhina, N.A., 1974, Bornaya mineralizatsiy v skamakh zolotosul'fidnykh mestorozhdeniy Taborskoy gruppy, Tsentral'nyy Tadzhikistan [Boron mineralization in skams of gold-sulfide deposits, Tabor Group, central Tadzhikistan]: Akademiya Nauk Tadzhikskoy SSR, Doklady, v. 17, no. 8, p. 47-50 (in Russian). ---1984, Mineralogiya, geokhimiya i usloviya obrazovaniya zoloto-sul'fidnykh mestorozhdeniy v formatsii magnezial'nykh skarnov (Tsentral'nyy Tadzhikistan) [Mineralogy, geochemistry and genesis of gold sulfide deposits during the formation of magnesian skams; central Tadzhikistan]: Izdatel'stvo "Donish," 256 p. (in Russian). Boyle, R.W., 1968, The geochemistry of silver and its deposits, with notes on geochemical prospecting for the element Geological Survey of Canada Bulletin 160, 264 p. Brown, I.J., 1985, Gold-bismuth-copper skarn mineralization in the Mam Skarn, Yukon: Edmonton, Canada, University of Alberta, M.S. thesis, 158 p. Burdokov, G.P., Popov, Y.V., and Tarnovskiy, Y.V., 1975, Geologiya skamovo-mednykh mestorozhdeniy Sayakskogo graben-sinklinoriya [The geology of skarn copper deposits of
the Sayak graben-synclinorium]: Soviet Geology, v. 4, p. 4858 (in Russian). Buryak, V.A., 1970, Zolotonosnost' zapadnogo i severo-zapadnogo Pribaykal 'ya [Gold of western and northwestern Baikal region], in Geologiya zolotorudnykh mestorozhdeniy Sibiri: Novosibirsk, U.S.S.R., Akademiya Nauk: SSSR, Sibirskoye Otdeleniye, Institut Geologii i Geofiziki, p. 31-41 (in Russian). Cameron, D.E., and Garmoe, W.J., 1983, Distribution of gold in skarn ores of the Carr Fork Mine, Tooele, Utah [abs.]: Geological Society of America Abstracts with Programs, v. 15, no. 5, 299p. Chemyshev, V.G., and Korin, IZ., 1973, Osobennosti stroyeniya i zakonomernosti razmeshcheniya endogennykh mestorozhdeniy v Zeravshano-Gissarskoy gomoy oblasti [Structural characteristics and distribution patterns of endogene deposits in the Zervshan-Hissar mining district], in Lukin, L.l., ed., Strukturnyye usloviya formirovaniya endogennykh rudnykh mestorozhdeniy: lzdatel'stvo Nauk:a, p. 58-94 (in Russian). Chmyrev, V.M., Stazhilo-Alekseev, K.F., Mirzad, S.Kh., Dronov, V.I., Kazakhani, A.R., Salah, A.S., and Teleshev, G.l., 1973, Mineral resources of Afghanistan, in Geology and mineral resources of Afghanistan: Kabul, Afghanistan Department Geological Surveys, p. 44-86. Church, B.N., 1976, Geology in the vicinity of the Oro Denoro Mine (82E/2E): British Columbia Ministry of Energy, Mines and Resources Geology in British Columbia, 1976, p. 1-13. ---1984, Geology and self-potential survey of the Sylvester K gold-sulphide prospect (82E/2E), in Geological fieldwork, 1983; A summary of field activities: British Columbia Ministty Energy, Mines and Resources Paper 1984-1, p. 7-14. ---1985, Geology of the Mount Attwood-Phoenix area, Greenwood, in Geological fieldwork 1985: British Columbia Ministry of Energy, Mines and Petroleum Resources Paper 1985-1, p. 17-21. Dawson, K.M., Godwin, C.l., and Gabites, J., 1985, Lead isotope analyses from silver-rich deposits in the Cassiar, Midway, and Ketza River areas of the Northern Cordillera, in Silver '85: Vancouver, British Columbia, Geological Association of Canada, Cordilleran Section, p. 5-6. Diggles, M.F., 1984, Tungsten skarn delineated by USGS geochemical sampling program, White Mountains, California [abs.]: Geological Society of America Abstracts with Programs, v. 16, no. 6, 489 p. Elliot, I.E., 1982, Model for contact metasomatic tungsten/copper/ gold deposits, in Erickson, R.L., ed., Characteristics of mineral deposit occurrences: U.S. Geological Survey Open-File Report 82-795, p. 49-54. Entin, A.R., 1975,0 zolotonosnosti arkheyskikh zhelezorudnykh mestorozhdeniy tsentral 'noy chasti Aldanskogo shcita [Gold content of Archean iron-ore deposits in the central part of the Aldan Shield]: Akademiya Nauk: SSSR Doklady, v. 223, no. 3, p. 722-725 (in Russian). Filimonova, A.A., and Vakhrushev, V.A., 1969, Melonit iz zolotonosnykh skamov Sinyuk:hinskogo mestorozhdeniya v Gornom Altaye [Melonite from gold-bearing skarns of the Sinyuk:ha deposit in the Gorny Altai]: Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, v. 98, no. 2, p. 175-182 (in Russian). Fleming, J., Walker, R., and Wilton, P., 1983, Mineral deposits of Vancouver Island; Westmin Resources (Au-Ag-Cu-Pb-Zn), Island Copper (Cu-Au-Mo), Argonaut (Fe), in Geological Association of Canada, Mineralogical Association of Canada, Canadian Geophysical Union, joint annual meeting, Field trip guidebook, v. 2, trips 9-16: Geological Association of Canada, Victoria Section, 41 p. Foster, R.P., 1984, A bibliography of gold; Geology, geochemistry, and metallurgy: University of Zimbabwe Institute of Mining Research Report 53, 56 p. Gibbons, G.S., 1974, Mineralogical studies at Mount Morgan, Queensland: Australasian Institute of Mining Metallurgy, Conference Series 3, p. 445-463. Golovanov, I.M., 1978, Mednorudnyye formatsii zapadnogo Tyan'- Shanya [Copper ore formations of western Tien Shan]: lzdatel'stvo Fan, 239 p. Grant, F.S., 1985, Aeromagnetics, geology and ore environments; II, Magnetite and ore environments: Geoexploration, v. 23, no. 3, p. 335-362. Griffith, J.R., and Walker, J.S., 1983, Metallogeny of hydrothermal gold deposits in British Columbia [abs.]: Geological Association of Canada Program with Abstracts, v. 8, 29 p. Harnish, D., and Brown, P.E., 1984, Porphyry copper related mineralization in the Terre Neuve District, Haiti [abs.]: Geological Society of America Abstracts with Programs, v. 16, no. 6, 530p. Hickman, R.G., and Craddock, C., 1976, Mineral occurrences near Cantwell, south-central Alaska: Alaska Division of Geological and Geophysical Surveys Special Report 13, 7 p. Hosking, K.F .G., 1973, Primary mineral deposits, in Geology of the Malay Peninsula (West Malaysia and Singapore): New York, Wiley-Interscience, p. 335-390. Il'yenok, S.S., 1970, Geneticheskiye svyazi orudeneniya s intruziyami [The genetic relationship between mineralization and intrusions], in Geologiya zolotorudnykh mestorozhdeniy Sibiri: Akademiya N auk SSSR, Sibirskoye Otdeleniye, Institut Geologii i Geofiziki (Novosibirsk), p. 3-30 (in Russian). Ishaq, S., 1985, Gold in Queensland: Queensland Government Mining Journal, v. 86, no. 1000, p. 72-77. Ivanov, Yu.G., 1974, Geokhimisheskiye i mineralogicheskiye kriterii poiskov vol'framovogo orudeneniya [Geochemical and mineralogical criteria of prospecting for tungsten ores]: lzdatel'stvo Nedra, 213 p. (in Russian). Jackson, D., 1982, How Duval transformed its Battle Mountain properties from copper to gold production: Engineering and Mining Journal, v. 183, no. 10, p. 95, 97, 99. Johnson, L.C., 1983, The Ellison District: Alteration-mineralization associated with a mid-Tertiary intrusive complex at Sawmill Canyon, White Pine County, Nevada: Tucson, University of Arizona, M.S. thesis, 123 p. Kalbskopf, S., and Treloar, P., 1983, The geology and calc-silicate assemblages of Stemblick Quarry, Harare: Annuals of the Zimbabwe Geological Survey, v. 9, p. 87-107. Kim, S.E., and Kim, S.Y., 1981, Geology and ores of Concession No. 17 of Jecheon Sheet: Korea Research Institute of Geoscience and Mineral Resources Report on Geoscience and Mineral Resources Report 12, p. 61-75 (in Korean with English summary). Korobeynikov, A.F., 1979, Sostav i svoystva mineraloobrazuyushchikh rastvorov zoloto-rudnykh mestorozhdeniy Sayano-Altayskoy skladchatoy oblasti po vklyucheniyam v Bibliography of Additional Gold-Bearing Skarn References
mineralakh [The composition and properties of mineralforming solutions of gold deposits of the Sayan Altai folded region according to inclusions in minerals], in Kuznetsov, V.A., Berzina,A.P., Distanov, E.G., Dymkin, A.M., Zolotukhin, V.V., Kolonin, G.R., Obolenskiy, A.A., Pavlov, A.L., Smimov, V.E., Sotnikov,. V J., and Shcherbak.ov, Yu.G., eds., Osnovnyye parametry prirodnykh protsessov endogennogo rudoobrazovaniya; Olovyano-vol'framovyye, kolchedanno-polimet allicheskiye, zolotorudnyye, sur'myanortutnyye mestorozhdeniy a [Principal parameters of natural processes of endogenic ore deposition; Tin-tungsten, basemetal, gold, mercury-antimony deposits]: lzdatel'stvo Nauka, v. 2, p. 161-174 (in Russian). ---1982, Gold in pyroxenes in intrusive and contact-metasomatic rocks: Geochemistry International, v. 19, no. 2, p. 13-24. ---1983, Zakonomernosti formirovaniya mestorozhdeniy zolotoskarnovoy formatsii [Conditions of formation of gold ores in skams], in Kumetsov, V.A., ed., Skamy i rudy [Skams and ores]: Trudy Instituta Geologii i Geofiziki (Novosibirsk), v. 546, p. 50-55 (in Russian). Korobeynikov, A.F., and Matsyushevskiy, A.V., 1976, Zoloto v intruzivnykh i kontaktovo-metasomaticheskikh porodakh Tardanskogo skarnovogo polya Tuvy [Gold in intrusive and contact-metasomatic rocks of the Tardan skarn field, Tuva]: Geokhimiya 1976, v. 9, p. 1409-1416 (in Russian). Kosals, Y.A., Dmitriyeva, A.N., Dorosh, V.M., and Simonova, V.I., 1976, Geokhimiya redkikh elementov v protsesse obrazovaniya izvestkovykh skamov (Zapadnoye Zabaykal'ye) [The geochemistry of rare elements in genetic processes of calcareous skams, Western Transbaikalia], in Shcherbak.ov, Y.G., ed., Zoloto i redkiye elementy v geokhimicheskikh protsessakh [Gold and rare elements in geochemical processes]: Trudy Instituta Geology i Geofiziki (Novosibirsk), v. 255, p. 196234 (in Russian). Kozlovskaya, Z.A., Kozlovskiy, G.M., and Kosyak, Ye.A., 1974, Mineralogicheskiye osobennosti rud zoloto-skarnovogo mestorozhdeniya Sary-Adyr v Tsentral'nom Kazakhstane [Mineralogy of ores of the Sary-Adyr gold-skarn deposit in central Kazakhstan]: Akademiya Nauk Kazak.hskoy SSR, Izvestiya, Seriya Geologicheskaya, v. 4, p. 67-73 (in Russian). Ksenofontov, O.K., and Davydov, Ye.V., 1971, Petrologiya, geokhimiya i metallogeniya Barambayevskogo plutona (Zapadnyy Turgay) [Petrology, geochemistry, and metallogeny of the Barambay Pluton, western Turgai], in Geologiya i poleznyye iskopayemyye Turgayskogo progiba: Trudy V sesoyuznyy N auchno-Issledovatel 'skogo Geologichesko Instituta, no. 169, p. 70-89 (in Russian). Kulichikhina, R.D., and Gubanov, A.M., 1977, K issledovaniyu prirodnogo soyedineniya medi i zolota iz skamovorudnogo redkometal'nogo mestorozhdeniya [Study of the natural copper and gold compounds from skarn rare metal deposits], in Semonov, Ye.I., and Chvileva, T.N., eds., Metodicheskiye mineralogicheskiye issledovaniya: Izdatel' stvo N auka, p. 6264 (in Russian). Kurgan'kov, S.P., Chesnokov, B.P., and Sergutkin, A.M., 1981,0 nekotorykh aspektakh zolotoorudeneniya kontak.tovo-metasoma ticheskikh zhelezorudnykh mestorozhdeniy yuga Krasnoyarskogo kraya [Aspects of gold mineralization of iron metasomatic and contact ores in Krasnoyarsk], in Kuznetsov, Gold-Bearing Skarns V.A., ed., Skarny i rudy [Skams and ores]: Trudy Instituta Geologii i Geofiziki (Novosibirsk), v. 546, p. 50-55 (in Russian). Kwong, Y.T., and Addie, G.G., 1982, Tillicum Mountain gold prospect, in Geological fieldwork 1981, a summary of field activities: British Columbia Ministry of Energy, Mines and Petroleum Resources Paper 1982-1, p. 39-45. Large, R.R., 1975, Zonation of hydrothermal minerals at the Juno Mine, Tennant Creek goldfield, central Australia: Economic Geology, v. 70, p; 1387-1413. Larichkin, V.A., 1978, Osobennosti otsenki rudnykh mestorozhdeniy na ranney stadii ikh izucheniya [Analysis of ore deposits in their early stages]: Razvedka i Okhrana Nedr, v. 5, p. 1418 (m Russian). Maksudov, M., 1969, Osobennosti raspredeleniya zolota i serebra v sul'fidakh rudoproyavleniy basseyna reki Koksu (Chatkal'skiy khrebet, Zapadnyy Tyan'-Shan') [Characteristics of the distribution of gold and silver in sulfide ore occurrences of the Koksu River basin]: Uzbekskiy Geologicheskiy Zhumal no. 2, p. 10-17 (in Russian). Mazurov, M.P., Kalinin, Y.A., Roslyakov, N.A., Titov, A.T., and Yakovleva, N.A., 1985, Mineralogical and geochemical characteristics of the Tomurtai iron-ore deposit (Mongolia): Soviet Geology and Geophysics, v. 26, no. 3, p. 58-65. McLemore, V.T., and North, R.M., 1984, Occurrences of precious metals and uranium along the Rio Grande Rift in northern New Mexico, in Baldridge, W.S., Dickerson, P.W., Riecker, R.E., and Zidek, J., eds., Rio Grande Rift, northern New Mexico: New Mexico Geological Society Guidebook 35, p. Meinert, L.D., 1983, Mineralogy and petrology of iron skams in western British Columbia: Geological Association of Canada Program with Abstracts, v. 8, 46 p. ---1984, Mineralogy and petrology of iron skams in western British Columbia, Canada: Economic Geology, v. 79, p .. 869882. Metz, P.A., and Halls, C., 1982, Ore petrology of the Au-Ag-SbW -Hg mineralization of the Fairbanks Mining District, Alaska: Journal of the Geological Society of London, v. 139, pt. 5, 662p. 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Tveritinov, Y.I., 1972, Strukturnyye usloviya lokalizatsii rud skarnovogo tipa na primere mestorozhdeniy Gornogo Altay [Structural conditions of ore localized in a skarn, exemplified by the Gorny Altai deposits], in Problemy obrazovaniya rudnykh stolbov: Akademiya N auk SSSR, Sibirskoye Otdeleniye, Institut Geologii i Geofiziki (Novosibirsk), p. 15636 Gold-Bearing Skarns 160 (in Russian). Usenko, I.S., Kravchenko, G.L., and Sakhats'kiy, I.I., 1973, Osoblivosti rozpodilu zolota v zalizisto-kremenistikh ta deyakikh inshikh kristalichnikh porodakh Priazov'ya [Characteristics of gold distribution in ferruginous-siliceous and other crystalline rocks of the Azov region]: Geologichniy Zhurnal, v. 33, no. 5, p. 58-66 (in Ukranian). Utter, T., 1982, Geological setting of primary gold deposits in the Andes of Colombia (South America), in Foster, RP., ed., Gold '82: The geology, geochemistry and genesis of gold deposits: Geological Society of Zimbabwe Special Publication 1, p. 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Tables 1-8
Table 1. Abbreviations used in tables Term Abbreviation Term Abbreviation Term Abbreviation Mineral name actinolite act forsterite fo prehnite preh amphibole am ph galena gal pyrite py andradite an garnet gar pyrolusite pyr ankerite ank goethite goe pyroxene px apatite ap grossular gros pyrrhotite po argentite arg hedenbergite bed quartz qtz arsenopyrite apy hedleyite hedl realgar real azurite azur hematite hem sea polite scap biotite biot hessite hes scheelite sch bismu thinite bism hornblende hom scorodite scor bornite bor jasper jas sericite ser calcite cal K-feldspar k-spar serpentine serp carbonate minerals carbs leucopyrite leucopy siderite sid cerargyrite erg limonite lim spadaite spa cerussite cer loellingite loel specularite spec chalcocite ludwigite lud sphalerite sph chalcopyrite cpy magnesite mags spinel spin chlorite chl magnetite mag stibnite stib chrysocolla chr malachite mal stilpnomelane stilp clinopyroxene clinopx maldonite mald telluride(s) tell clinozoisite clinoz marcasite marc tenorite ten covellite cov molybdenite moly tetradymite tetd cubanite cub muscovite muse tetrahedrite tet cummingtonite cum native bismuth Bi tourmaline tour cuprite cup native copper Cu tremolite trem diopside diop native gold hi vermiculite ver dolomite dol native silver Ag vesuvianite ves electrum elec nontronite non white mica wm enargite enar orpiment orp wolframite wolf epidote ep oxide(s) ox wollastonite wol feldspar feld phlogopite
zoisite zoi fluorite fl plagioclase plag Rock type agglomerate agglom hornfels hfs sandstone sst andesite and limestone Is sedimentary sed argillite argl manganiferous mang sediments seds calcareous calc marble mar shale sh carbonaceous carb monzonite monz siltstone
conglomerate congl mudstone
skarn skn dolostone do los porphyry porph slate sl greenstone ernst quartzite qtzite volcanic rocks voles Gold-Bearing Skarns
Table 1. Abbreviations used in tables-Continued Term Abbreviation Term Abbreviation Term Abbreviation Age Tertiary Tert. Carboniferous Car b. Proterozoic Prot. Miocene Mio. Pennsylvanian Penn. Early E. Eocene Eo c. Mississippian Miss. Middle M. Mesozoic Mes. Devonian Dev. Late Cretaceous Cret. Silurian Sil. early e. Jurassic Jur. Ordovician Or d. middle m. Triassic Tri. Cambrian Cam b. late Paleozoic Pal. Precambrian Prec. Upper u. Permian Perm. Other average avg million M sequence seq Formation Fm. million years Ma short ton (2000 lb) st gram g tonne(s) (metric tons) trace tr Group Gp. Country Country code Country Country code Afghanistan AFGH Philippines PLPN Australia, New South Wales AUNS Papua New Guinea PPNG Australia, Queensland AUQL South-West Africa (Namibia) SAFR Australia, Tasmania AUfS Spain SPAN China CHNA Sweden SWDN Colombia CLBA Thailand 1lll.D Canada, British Columbia CNBC United States, Alaska USAK Canada, Quebec CNQU United States, California USCA Canada, Yukon Territory CNYf United States, Colorado usco Ecuador ECDR United States, Idaho USID Indonesia INDS United States, Montana USMf Japan JPAN United States, Nevada USNV Mexico MXCO Soviet Union USSR Malaysia MYLA United States, Utah usur Nicaragua NCRG United States, Washington USWA North Korea NKOR Federal Republic of Germany WGER Peru PERU Table 1
Table 2. Gold-bearing skarns in which gold and silver are major commodities exploited [p, metal present in unquantified amount; other abbreviations listed in table 1] Name Location Host lithology Formation age/ Igneous rocks Age Ore minerals Gangue (mining district) name minerals Bau MYLA mar, sh L. Jur./Bau Ls. acid porph stocks & Mio. Au, apy, py, sph, chi, diop, ep, gar, dikes stib, real, orp, wol, ves, qtz, cal, scor rare plag, ap, preh Beano CNBC Is, and tuff Tri.-Jur./ diorite-rhyolite Jur. po, cpy, apy, act, qtz, cal, chi, (Zeballos) Quatsino Fm. porph sill mag, hedl clinopx, Cl-amph Bonanza voles. Broadway USMT Is Miss./Mission Boulder batholith, Cret. auriferous jas, jas, lim, ep, gar, cal, (Victoria) (Silver Star) Canyon Ls. Rader Creek pluton, cup, mal, py, serp hed, chi, qtz monz argentiferous non, diop lead ore, py, po, cpy Brown's AUNS Is & in tuff Ord./ Carcoar Granite Dev. Au, apy, cpy, py, act, diop, ep, gar, Creek Angullong Tuff diorite po, ten, tet, bor wol, ves, trem, clinoz, , sid, clay, chl Buffalo USNV chert, argl, Is Penn.-Perm./ granodiorite Tert. py (Au?), hem, qtz, lim, px, cal, ep, Valley (Battle Havallah seq. porph Au, cpy, Ag, gar, non Mountain) mal, chr Canty CNBC Is, , limy L. Tri./ Hedley intrusions, E. Jur. Au, apy, py, cpy, clinopx, cal, gar, ep, (Hedley) argl, tuff Nicola Gp. qtz diorite sills po qtz, scap, k-spar, albite Discovery CNBC mar Pal.(?) Coast plutonic Cret.(?) py, po, cpy, mag, gar, px, chi, qtz, cal, (Banks Island) complex apy, sph am ph DividendCNBC Is, ernst, qtzite, Perm.-Tri./ Kobau Nelson/Osoyoos Cret. mag, cpy, py, po, qtz, cal, ep, gar, chi, Lakeview and flows & tuffs Gp. or batholith, qtz apy, Au, Bi, Ch-amph, act, Perm/Anarchist Gp. diorite-granodiorite marc, hedl sphene, clay, minor diop Esmeralda USMT Is, and, granoPal., Cret. granodiorite, and Cret. Au, cpy, mal, jas gar, ep (Ophir) diorite, mar Excelsior USMT Is Miss./ Bannack stock auriferous py, gar, cal, ep, qtz (Bannack) Madison Ls. granodiorite Au in goe, Ag, bor, tetd, spec, hem Fortitude USNV calc seds Antler Sequence granodiorite stock Tert. po, cpy, apy, clinopx(hed), (Lower (Battle elec, bism, Bi, gar( an), act, chi, Fortitude) Mountain) tell, marc, gal, preh, ep, qt mag, sph French CNBC Is boulder congl, L. Tri./ Hedley intrusions, E. Jur. po, cpy, py, bor, clinopx, gar, ep, qtz, (Oregon) (Hedley) limy argl, tuff Nicola Gp. porph qtz diorite, apy, Cu, sch cal, axinite, wol, sills clinoz, biot, trem-act, k-spar Golden USMT Is, qtz monz Cam b./ Boulder batholith, Cret. po, bism, tetd, gar, diop, cal, ep Curry (Elkhorn) Wolsey Fm. qtz monz cpy, mag Gold-Bearing Skarns
Ore control Tonnage Au (g/t) Ag (g/t) Base metals Comments References (millions of tonnes) contact zone, fractures, Area includes skn, vein, and replacement 1, 2, 3, 4 permeable lithology mineralization in irregular pods and lenses along joints and fractures; contains 0.002% Sb 0.16% Cu Contains Au, Ag, Cu,Bi, Te, Co; 5, 6, 7 Au-poor iron skams nearby; similar setting for nearby Hiller property (potential Au skn based on assays 1 g Au/t) contact zone between p CupPbp Ore concentrated in jasper zone along 8, 9 qtz granodiorite-Is contact; px-gar skn locally 1st-tuff contact, 0.44% Cu Contains As, Sb fractures intense qtz-py Skn assemblages are rare in pit but more 11, 12, 13 silicification in (0.0027 Cup) common at depth; produced approx 9,200 fractured skn near oz Au during fiscal1988; geologic gold porph reserves, 110,900 oz in 1988 (1924-51 production: 1,380 oz Ag from 0.0024 Mt, 0.80% Cu from 542 t of ore, 1937-39); Horizon Gold Inc./Chevron Resources Cahill Creek fault zone, p Size based on 1939 and 1941 production; 5, 7, 14, 15 16 lithology mineralization probably hosted by upthrown, fault-bounded sediments within the fault zone; contains Au, W, As, Co, Ag; Mascot Gold Mines Ltd. fracture zone at p p Probable reserves indicated by drilling; Au granodiorite-mar in skn and in qtz-py veins; additional contact reserves (0.0955 Mt of ore at 16.21 g!t) for a massive sulfide vein (Tel zone) that cuts mar and metapelite; Trader Resources Corp. contact zone, structure 0.06% Cu Gar-ep skn replaces voles and mar; contains 5, 7, 17, 18, 19 Pb p As, Co, Bi, Te Znp contact zone between 0.00000907 Skn explored by 120-m-long adit; surface granodiorite and Is cuts contain Is, and, jas-all of which probably contain Au contact zone between Cup Production reported for 1902, 1917-19; 21, 22 granodiorite and Is 385 t contained 3.2% Cu; estimate 0.0058 Mt produced 36.5 g Cult before 1914 favorable lithology 0.2% Cu Porph Cu skn, polymetallic veins; Battle 23, 24, 25, 26 Mountain Gold Co. hinge zone of anticline 0.03% Cu 8700 t of unmined ore reported to contain 5, 7, 14, 27 as much as 85 g!t Ag and 2% Cu; contains W, As, Mo, Bi, Co, Sb contact zone 0.33% Cu; Production data for 1904-51. Four types 28,29 Fe p of ore present: mag veins, jasperoid lodes, massive mag-po, and massive po-cpy in px gangue Table 2
Table 2. Gold-bearing skarns in which gold and silver are major commodities exploited-Continued Name Location Host lithology Formation age/ Igneous rocks Age Ore minerals Gangue (mining district) name minerals Golden USMT ls Miss./ Bannack stock Tert. Au, Ag, tet, erg, qtz, gar, py, cal, ep, Leaf (Bannack) Madison Ls. granodiorite cpy, bor, mal, chi, sid, ves gal, cer, sph, mag Good CNBC tuff, argl, Is L. Tri./ Hedley intrusions, E. Jur. apy, py, cpy, po, clinopx, gar, cal, Hope 1 (Hedley) Nicola Gp. qtz diorite Bi, moly, hedl wol, ep, biot, qtz Hardcash USMT Is Camb./Park Sh. Boulder Cret. bism, tetd, cpy gar, ep, diop, cal (Dolcoath) (Elkhorn) batholith Labrador USNV calc sh, calc sst, U. Camb./ granodiorite Tert. Au, py, po, lim gar, ep, chi, cal, qtz (Battle arkose, calc Harmony Fm. porph Mountain) congl M. Penn./ Battle Fm. La Luz NCRG Is, limy sh, Mine Series granodiorite Tert. Au, cpy, py, hem ep (Siunna) agglom, tuff Lebedskoe USSR Is, calc sh, dolos E. Pal. diorite Pal. Au, apy, sph, tet, gar (an, gros ), diop, (Kaurchak) Pb tell, mag, cpy, bed, trem, ep, hem, py, cc, po, clinoz, wol, act gal, sph, ten, bor Marshall CNBC Is, siliceous , L. Tri./ microdiorite L. Tri. cpy, py, po, sph, chl, gar, diop, (Greenwood) argl, congl Brooklyn Gp. granodiorite Au, minor mag, amph, ep hem, gal, marc Mascot CNBC , ls, congl, L. Tri./ Hedley intrusions, E. Jur. apy, po, cpy, px, gar, wol, biot, Fraction (Hedley) tuff Nicola Gp. porph qtz diorite, mag, bor, mald, k-spar, cap, cal, qtz, (Hedley Fm.) gabbro, sills & dikes hedl preh, ap, axinite McCoy USNV ls, dolos, qtzite Tri./ Augusta Brown stock Tert. Au, py, cpy, cc, gar, px, ep, cal, qtz, (McCoy) Mountain Fm. granodiorite (39.7 Ma) cov, sph, gal, po, chl, amph, ves, feld Tri./Cane Spring hem, mag Fm. Midas USUT ls Manning qtz monz Tert. py, apy, Cu wol, diop, gar, ves (Gold Hill) Canyon Sh. & sulfides Oquirrh Fm. Molly B CNBC tuff, argl, ls Jur./ Coast Range Cret.(?) py, po, cpy, moly, gar, ep, px, qtz Haselton Gp. batholith sch Mt. USNV sh, ls, calc sh Camb./ Secret Seligman and Cret. Au, sch, moly, gar, qtz, ep, cal Hamilton (White Pine) Canyon Sh. Monte Cristo cpy, py, apy, sph, stocks, granodiorite gal, bor, tetd, py, hem Navachab SAFR turbidites, L. Prec. 2 mica granite Cam b. clastics Nickel CNBC Is, limy argl, L. Tri./ Hedley intrusions, E. Jur. Au, elec, apy, gar, clinopx, cal, Plate (Hedley) qtzite, tuffs, , Nicola Gp. qtz diorite, gabbro, (180 Ma) cpy, py, po, tetd, axinite, scap, ap, congl (Hedley Fm.) sills & dikes sph, marc, gal, clinoz, ep, biot, moly, mag, trem-act, qtz, preh, titanite, hedl, wol tell, cobaltite, ecythrite, platinum, Bi, maldonite, gersdorffite, Cu, pyrargyrite Northeast USNV calc congl M. Penn./ granodiorite stock M. Tert. po, cpy, py, Au act, ep, sphene, Extension (Battle Battle Fm. k-spar, chi Mountain) Pagaran INDS Is, and voles Perm./ Muara Sipongi L. Jur. Ag, Au, cpy, bor, gar, wol, diop, qtz, Siayu (Muara Silungkang Fm. intrusions, tetd, tell, apy, preh, chi, cal Sipongi area) granodiorite, diorite sph, gersdorffite Gold-Bearing Skarns
Ore control Tonnage Au (g/t) Ag (g/t) Base metals Comments References (millions of tonnes) contact zone between Production figures for 1909-41; Cu, Pb, 21,22 granodiorite and ls Cu; Zn production for lower tonnages of ore; Trace Zn oxidized to 100-m depth 0.06% Pb fault cutting skn Same stratigraphic horizon as French mine 5, 7, 14, 15, 16 replaced bed 0.20% Cu 28,29 0.006% Pb northeast- & Minable reserves. Ore associated with northwest-striking oxidized py along faults cutting gar skn; faults Battle Mountain Gold Co. fault/hanging wall 0.44% Cu 31, 32, 33 andesite Estimated tonnage and grade (ref. 37). Au 1, 34, 35, 36, in py, 0.8 to 30 ppm; Au in cpy, 13.6 ppb; 37,38 low quantity of sulfide in deposit (few percent) crest isoclinal fold at 0.24% Cu; Described as Au-enriched Cu skn 5, 7, 17, 39 contact of ls with 0.29% Zn; underlying siliceous 1.19% Pb
skn-mar contact 0.14% Cu Forms part of same deposit as Nickel Plate; 7, 14, 15, 16, As-Au skn; Mascot Gold Mines Ltd./Corona 40,41 Corp. contact between ls and 0.08% Cu Distal disseminated Cove Ag-Au deposit 42, 43, 44, 45, stock, endoskarn, nearby with 16.4 Mt of 2.6 g Au/t and 111 g 46, 47 shears, bedding planes, Aglt; Echo Bay Minerals Co. faults bedding p Production estimated for 1902 data; estimate 86 t higher grade ore pre-1897; lower grade production reported for 1904 0.13% Cu 7,49 contact zone between Cup Au associated mostly with intense granodiorite and calc Wp retrograde alteration of gar-py skn; sh; retrograde Mop sulfide-bearing qtz veins overprint skn; alteration Westmont Mining Inc. Purported to be a skn deposit; Erongo 51, 52 Mining and Exploration Co. Pty. Ltd. contact zone 0.03% Cu Forms part of same deposit as Mascot 1, 7, 14, 53, Fraction; As-Au skn; Mascot Gold Mines 54, 55, 56, 57, Ltd./ Corona Corp. favorable lithology 0.11% Cu Porph Cu , polymetallic veins; Battle Mountain Gold Co. contact zone regional 0.2% Cu Production for 1936-39; numerous 3, 60, 61 faults Au-Ag-Cu skarns in this area of West Sumatra; N. V. Mijnbouw Maatschappij Moeara Sipongi Table 2
Table 2. Gold-bearing skarns in which gold and silver are major commodities exploited-Continued Name Location Host lithology Formation age/ Igneous rocks Age Ore minerals Gangue (mining district) name minerals Red Dome AUQL sst, chert, Sil.-Dev./ qtz feldspar Carb.- Au, bor, mag, gar, wol, clinopx (Mungana) (Chillagoe) andesite, lithic Chillagoe Fm. porph dikes & sills Perm. sph, Pb & Ag congl, Is tells, cc, wittichenite, cpy, moly Rokuromi JPAN biot schist, Is Carb./ qtz diorite, Mes. po,apy gar, ep, others Utakai Fm. granodiorite Second CNBC basalts, tuffs L. Jur./ Nelson batholith py, po, cpy, mag, gar, ep, qtz, amph, Relief Rossland Gp. diorite porph dike moly clinopx, biot, carb (Elise Fm.) SheahenAUNS , sh, sl, Ord./ granodiorite Dev.(?) Au, po, maid, py, px, hom, bio, ep, Grants mostly calc Malongulli Fm. apy, cpy, sph, trem, preh, cal, qtz, Au-Bi-sulfide chi Silverado CNBC metavolc, Is granodiorite Jur.-Tert. sph, cpy, po, mag px, gar, ep, qtz, cal Suian NKOR schist, qtzite, sl, Suian granite stock Mes.(?) Au, apy, cpy, gal, gar, diop, , act, dolos, Is py, po, sph, lud, chi, talc, trem, bism, tetd, loel, wol moly, borate minerals Surprise USNV calc sh, calc sst U. Camb./ Cret.(?) elec, py, cpy, gar, diop, chi, cal, (Battle Harmony Fm. mal, lim, hem, qtz, amph, ap, Mountain) sph k-spar, non Tillicum CNBC tuffaceous seds, E. Jur./ qtz monz E. Jur.(?) Ag, Au, gal, po, act, gar, feld, trem, (Heino- (Till cum calc , argl Rossland Gp. sph, clinoz, py, apy, cpy, Money, Mountain) marc, biot, qtz, cal, East k-spar tetd, elec Ridge) TomboyUSNV calc congl M. Penn./ granodiorite Tert. Au, cpy, gal, py, act, chi, ep, trem, Minnie (Battle Battle Fm. porph po,sph,apy clays, muse Mountain) TulMi NKOR schist, qtzite, sl, Suian granite stock Au, apy, cpy, gal, gar, diop, , act, Chung dolos, Is py, po, sph, lud, talc, trem, wol, bism, tetd, loel chi 1. Boyle, 1979 16. Ray and others, 1988 32. Plecash and others, 1963 2 Wolfenden, 1965 17. Ettlinger and Ray, 1988 33. R.H. Sillitoe, oral common., 1987 3. Bowles, 1984 18. Cockfield, 1935 34. Vakhrushev, 1972 4. W.C. Bagby, written common., 1987 19. McKechnie, 1964 35. Ivankin and Rabinovich, 1972 5. British Columbia Ministcy of Energy, 20. McCleman, 1976 36. Vakhrushev and Tsimbalist, 1967 Mines and Petroleum Resources, 1981 21. Geach, 1972 37. E.L. Bloomstein, written common., 1987 6. Stevenson, 1950 22. Winchell, 1914 38.1Veritonov, 1966 7. Ettlinger and Ray, 1989 23. Argall, 1986 39. Canada Department of Energy, Mines 8. Gilbert, 1935 24. Wotruba and others, 1987a and Resources, 1986 9. Sahioen, 1939 25. Wotruba and others, 1987b 40. Dolmage and Brown, 1945 10. Taylor, 1983 26. Myers and Meinert, 1988 41. Billingsley and Home, 1941 11. Coffey and others, 1988 27. Canada Department of Energy, Mines 42. Kuyper, 1987 12 Horizon Gold Shares, Inc., 1988 annual report and Resources, 1980 43. Tingley and Smith, 1982 13. Roberts and Arnold, 1965 28. Roby and others, 1960 44. Schrader, 1934 14. Ray and others, 1987a 29. Klepper and others, 1957 45. Kral, 1947 15. Ray and others, 1986b 30. Schmidt and others, 1988 46.L'lne, 1987 31. Sillitoe, 1983 47. Emmons and Coyle, 1988 Gold-Bearing Skarns
Tonnage Ore control (millions of tonnes) intrusive contact dike contact, fault contact multistage contact metasomatism of reactive bed faults contact zone favorable beds, faults contact zone, permeable lithology contact zone 48. Nolan, 1935 49. Hanson, 1935 50. Dennis and others, 1989 51. Mining Magazine, 1987 52. Mining Activity Digest, 1987 53. Lee, 1951 52. Barr, 1980 55. Northern Miner, 1986 56. Northern Miner, 1987 57. Northern Miner, 1989a 58. Northern Miner, 1989b 59. Wotruba and others, 1986 60. Bowles and others, 1985 61. Beddoe-Stephens and others, 1987 62. Torrey and others, 1986 63. Murray, 1986 Au (g/t) Ag (g/t) Base metals Comments References 0.46% Cu; Deposit includes mineralization in breccia 62,63 1% Zn and qtz vein stockwork; associated with Cu and Zn-Pb ; Murray reports a Zn grade of 1.8%; Elders Resources Ltd. Fe (mag) skams; adjacent to Kamaishi 0.0098% Production data for 1900-48; anomalous Cu· ' As, Bi; skn overprints voles and porph 0.0005% Pb; diorite; skn cut by later qtz veins, sulfides 0.0002% Zn Tonnage and grade composited from 65,66 recorded production and drill-proven geologic reserve 0.07% Cu Skn cut by faults 7, 67, 68, 69 Znp Cu ; worked from ancient times; grade 11, 70,71 and (or) tonnage may be underestimated 0.85% Cu Mineable reserves; associated with Late Cret. porph Cu and polymetallic veins; Battle Mountain Gold Co. p Zn p; Pb p Proven reserves (ref. 7) for the 7, 72, 73, 74, p Zn p; Pb p Heino-Maney zone, and estimated reserves 75, 76, 77 for the East Ridge deposit (0.34-Mt core of deposit grades 10.3 g!t Au); two phases of metal deposition-Au, apy, ±sph, qtz, calc silicates followed by Ag-gal, apy, sph; Esperanza Exploration Ltd. 0.03% Cu; Associated with Mid.-Tert. porph Cu 31, 78 Znp; and polymetallic veins; Battle Mountain Pb p Gold Co. 70, 71 64. Grant, 1950 65. Stevens, 1975 66. R. Overton, written commun., 1987 67. Muller and others, 1981 68. Carson, 1969 69. Carson, 1973 70. GalL'lgher, 1963 71. Watanabe, 1943 72. Ray and others 1985 73. Ray and others, 1986a 74. McClintock and Roberts, 1984 75. Roberts and McClintock, 1984a 76. Roberts and McClintock, 1984b 77. Mining Journa~ 1989 78. Blake and others, 1984 Table 2
Table 3. Gold-bearing skarns in which gold and silver are byproduct commodities [p, metal present in unquantified amount; other abbreviations listed in table 1] Location Host Formation age/ Associated Ore Gangue Name igneous Age (mining district) lithology name rocks minerals minerals Fe skarns with byproduct gold Larap PLPN calc seds Eoc.(?)/ diorite, granoMio. mag, hem, py, gar, px, ep, including Is, calc Universal Fm. diorite, syenite po, moly, cpy, amph, cal, chi, sh, arkose stocks, dikes bor, gal, sph, ap, k-spar, scap cobaltite albite Nabesna USAK ls, dolos, marl, Tri./ monzodiorite py, mag, cpy, Au, gar, wol, ves, ep, mafic voles Chitistone Ls. stock Ma po, gal, sph, apy, act, hom, chl, stib scap, ap, serp, qtz Cu skarns with byproduct gold Apex CNBC Is, voles Tri./Kunga Fm. San Christoval E. Jur. py, cpy, mag qtz, cal, gar, ep (Queen or Kamutsen Fm. batholith -qtz Charlotte) diorite Bailey Day USNV calc sst U. Camb./ Tert. (?) py(Au), cpy, mal, ep, k-spar, (Battle Harmony Fm. chr, azur, ten, sphene, gar, chi, Mountain) hem, lim, Au, biot Ag Benson Lake CNBC ls, voles U. Tri. & L. Jur./ · Benson Lake Jur. cpy, mag, bor, gar, ep, cal, chi (Vancouver) Vancouver Series stock py, po, apy Quatsino Ls. & qta diorite Karmutsen voles. Blue Bell USMT qtz monz Cret. qtz monz Cret. mal, azur, Au, gar, ep, qtz, scap (Elliston) moly, py Bluestone USNV Is Tri. granodiorite Jur. cpy, py ep, gar, minor (Yerington) diop Carissa USNV sh, calc sh, Is U. Camb./ granodioriteTert. or Cu carbs, Fe ox, ep, diop, gar, (Battle Harmony Fm. monzogranite Cret. py, cpy, po, sch qtz, cal, chl, clays Mountain) Coast Copper CNBC Is, and voles U. Tri. & L. Jur./ diorite-gabbro cpy, bor, mag (Vancouver) Vancouver Series Quatsino Ls. & Karmutsen voles. Concepcion MXCO Is, granodiorite stock Mes. Eoc. cpy, py, mag, an, chi, diop, ep, del Oro hem, enar, tet, px, plag, ves, zoi, ga~ sph, po, ten, scap, act, ilvaite bism, cosalite, wittichenite Copper Queen CNBC Is, mar Tri./ diorite porph Mes. (?) bor, cpy, Ag, tet, gar, px, ep, cal (Texada Marble Bay Ls .. moly, sch Island) Cornell CNBC ls, mar Tri./ gabbroic Mes. (?) cpy, ~or, po, cal, gar, diop, ep, (Texada Marble Bay Ls. suite-diorite mag, marc, Ag, serp Island) porph py, moly, tet, sch Crevice Creek USAK ls, argl, chert, U.Tri./ granodiorite Jur. (?) py, cpy, mag ep, gar (McNeil) metavolcs Kamishak Fm. stock of Pilot Jur.{falkeetna Fm. Knob Cyclone USMT Is Pal. porphyritic Cret. mal, Au gar, qtz, ver (Ophir) granite Empire USID dolomitic ls Miss. granite, porph cpy, py, po, gar, px (Alder Creek) dikes secondary Cu minerals Gold-Bearing Skarns
Tonnage Base metals Ore control (millions of Au (g/t) Ag (g/t) and Fe Comments References tonnes) Fe skarns with byproduct gold fractures, sediment >18 0.12% Cu Deposit grades into Cu-Mo porph at lower 1, 2, 3, 4, contact with 43% Fe levels ; 0.08% Mo, 0.02% Ni, 0.03% Co; unconformably Pim Bessemaer overlying voles contact zone p Fep Main Au ores are py veins along fractures; 6, 7 minor Au in mag and po bodies; no Fe production; Nabesna Mining Corp. Cu skarns with byproduct gold 0.70% Cu Based on 1945 production; note that 8, 9, 10 Ettlinger and Ray (1989) report reserves of 163,000 t for Ag-Cu Alpine (Apex Star) skn of 34% Fe, 0.90% Cu, 24.6 g!t Ag, no Au reported favorable bed, structure 1.2% Cu Additional Au-ore tonnages discovered in 1980's at this deposit; associated with Cu and Au , polymetallic veins Is-and contact 1.6% Cu Coast Copper Co., Ltd. 8, 9, 12, 13 0.0000045 1.62% Cu Gar apparently veins qtz monz 3.34 2.08% Cu Associated with a porph Cu deposit 15, 16, 17 favorable bed 2.96% Cu Associated porph Cu deposit; Battle 11, 18 Mountain Gold Co. lst-and contact 1.56% Cu Adjacent to Benson Lake mine; Quatisino 33.3% Fe Copper-Gold Mines Limited and Empire Development contact zone 2% Cu Fe p In Zactecas; endoskam present 1, 2, 20, 21 PbpZnp contact zone 4.4% Cu Based on 1903-17 production; reported 9, 22,23,24 Mo, W contact zone 3.4% Cu Based on 1897-19 production; Vanada 9, 22, 23, 24 Mining Co., Ltd. (1943) 17.5% Cu Ep-gar skn bodies in Is adjacent to stock; 5, 25, 26 mag zones; magnetic anomalies around stock contact zone Based on 1942-61 production; Ag grade from 23 t; Cu grade from 22 t contact zone 53.89 3.64% Cu Table 3
Table 3. Gold-bearing skarns in which gold and silver are byproduct commodities--Continued Location Host Formation age/ Associated Ore Gangue Name igneous Age (mining district) lithology name rocks minerals minerals Cu skarns with byproduct gold-Continued Esashi JPAN Is Carb./Shiba & qtz porph, Cret. cpy, py, bism, Bi, diop, ep, gar, (Akagane) Yonezato Fms. granodiorite sch, cub tour, amph, bed Gold Bug USMT Is Miss. granodiorite Tert. auriferous py, qtz, cal, gar, goe, (Bannack) Au, auriferous ep, mal tetd ll'mensk USSR diorite(?) gar, px (ul'ma) Jumbo USAK mar L. Pal./ granodiorite E. Cret. Au, cpy, sph, diop, gar, wol, (Jumbo) Wales Gp. stock moly, hem, py, ep, act, hom, po, spec chl, scap, plag, qtz Katanga PERU Is, sh Cret./equivalent of qtz dioritie, Tert. cpy, py, bor, gar, trem Ferrobamba & qtz monz chal, chr, mal, Tintaya Ls. brochantite, mag, Au, Ag Klondyke USMT dol Dev./Jefferson Fm. Boulder Cret. tetd, py, cpy, lim, diop(?), gar, (Elkhorn) batholith mal trem, cal, chl near Black Butte stock-gabbro diorite Lily CNBC , Is L. Tri./ Jedway stock L. Cret. mag, po, cpy, py, gar, act, chl, cal Karmutsen Fm. sph E. Jur./Kunga Fm. Little Billie CNBC Is, mar Tri./ felsic Mes. (?) cpy, bor, Ag, Au, cal, diop, gar, ep, (Vananda) (Texada Marble Bay Ls. granodiorite, py, po, sph, mag, act, wol, amph, Island) qtz monz moly, sch, marc, ves, qtz gal, tell Lucky Mike CNBC , Is, L. Tri./ acidic dikes, Jur. (?) py, po, cpy, sch gar, px, ep, cal (Last Chance) breccia, agglom Nicola Gp. granite-diorite Marble Bay CNBC Is, mar Tri./Marble Bay Ls. diorite Mes. (?) cpy, bor, po, ep, gar, cal, diop, (Texada mag, marc, Ag, wol?, trem, qtz Island) Au, py, sph, moly, tet Morning CNBC Is, basalt L. Tri./ Collision Bay L. Cret.- mag, py, cpy, po, gar, px, qtz (Ikeda) Karmutsen Fm. diorite stock, E. Tert. apy, bor Kunga Fm. Carpenter qtz monz stock Mother Lode CNBC sharps tone L. Tri./ Wallace Creek L. Jur. cpy, py, hem, ep, gar, cal, qtz, (Greenwood) congl, Is Brooklyn Gp. granodiorite mag, Au act, trem, chl Natalevskoe USSR Is, calc sh, dolos Pal. diorite, Pal. cpy, bor, apy, py, an-gros, diop, syenite, aplite po, bism, sph, trem, wol, mag, moly, cc, ep-clinoz, fo, Au, gal, elec, Bi, phg, serp, ves, cub, Pb tell, Ni scap, spin, fl, selenide, tet, ten, chondrodite, Ag clinohumite, preh, ap, chl, sphene Gold-Bearing Skarns
Ore control granodiorite-Is contact; ore generally exterior to gar skn contact zone contact, fissures fractures, bedding planes granodiorite-mar contact faults, intrusive contacts faults steep fracture-skn intersection Tonnage (millions of tonnes) Base metals Au (g/t) Ag (g/t) and Fe Comments Cu skarns with byproduct gold-Continued 0.8% Cu 23.7% Fe 0.76% Cu 0.31% Pb 5% Cu 4.1% Cu Fep 3.5% Cu Fep 1.04% Cu 1.58% Pb 4.28% Cu 1.3% Cu Fep 3.65% Cu 3.31% Pb 3.4% Cu Fep 2.49% Cu Zn p Fe p 1.6% Cu Deposit has 5 ore bodies, largely skn but some disseminated veins; 5.9%pyrite, reported Bi Tonnage and grade figures represent 1922-41 production; originally staked as Dakota claim Estimated tonnage and grade (ref. 31); in northeastern Altai Mountains; Au-Cu mineralization in skn may predate associated diorite Production data for 1907-44; estimated 0.28 Mt ore reserves of 45% Fe, 0.73% Cu In Chillioroya region; Mitsui Mining and Smelting Co.(?) Based on 1915-57 production. Cu grade for 375 tore; Pb grade for 16.3 tore; similar to nearby Hardcash deposit Veinlike masses in altered and sheared Deposit size based on production; reported Mo; Texada Mines Ltd. Deposit size based on production; Ideal Basic Industries, Inc. Production from Cu claims; ore in dikelike sulfide bodies along faults and contacts Includes Sunset property; Gold Mines Resources, Ltd. Estimated tonnage and grade (ref. 31); 3 stage magnesian skn; numerous small podlike bodies of ore form at intersection of steeply dipping fractures; gold is relatively fine ( avg about 930); only a few percent sulfide in deposit References 29, 30 31, 32, 33 5, 6, 34, 35 36,37 38,39 9, 10, 40 22, 23 9, 44 22,23 9, 10,42 8, 9, 22, 43, 44,45 31, 46, 47, 48, 49, 50 Table 3
Table 3. Gold-bearing skarns in which gold and silver are byproduct commodities-Continued Location Host Formation age/ Associated Ore Gangue Name igneous Age (mining district) lithology name rocks minerals minerals Co skarns with byproduct gold-Continued Old Sport CNBC Is, and voles L. Tri./ Coast Copper Jur. cpy, po, bor, cpy, cal, ep, gar, Quatsino Ls. Stock diorite/ py, mag, apy amph, diop, chi, gabbro qtz Pauline USAZ Is Cret. qtz latite Tert.- gal, cer, cpy, py, gar, qtz, ep (Helvetiaporph Cret. sph, moly, Au, Rosemont) Ag, spec Phoenix CNBC sharpstone L. Tri./ granodiorite Cret. po, cpy, py, hem, amph, ep, gar, (Greenwood) congl, Is, argl, Brooklyn Gp. of Nelson spec, minor mag, qtz, cal, chi tuff batholith Ag,Au PioneerUSCA hfs, mar hom granoMes. py, cpy, bor, gar, ep, qtz, feld, Lilyama (ElDorado, diorite mag, hem, ves, cal CO) minor sch Rosita NCRC Is, mar Cret. diorite, monz Tert. cpy, py, po, mag, gar, ep bor, cc, mal, cup Seven Devils USID Is Tri./Martin Bridge Deep Creek Cret. cpy, bor, cc, mal, gar, ep, diop, district Ls. (?) stock-qtz azurite, chr, cov, bed, trem, act diorite sch-powellite Sinyuzhinskoe USSR Is, calc sh, dolos Pal. diorite Pal. Au, apy, Ni & gar(an-gros), Pb tells, ten, tet, diop-hed, wol moly, mag, bor, cc, cpy, py, gal, sph, po Vieja CLBA lbaque cpy, gal, py, spec cal, ep, batholith Ma marmetite, qtz Yaguki JPAN sh, Is Perm. granodiorite Cret. cpy, po, cub, ep, gar(an), qtz, mag, W, Bi, px, preh, hem, bism, babingtonite, chi, cobaltite, sph, act, plag gal, moly Yreka CNBC limy tuffs, and L. Jur./ qtz-feldspar po, cpy, py, sph, Is Bonanza Gp. porph dikes & mag, spec sills Zackly USAK mar Tri. qtz Cret. cpy, born, py, gar (an), wol, px, monzodiorite Au, Cu, mal, clinopx lim, chalcedony Porphyry Co skarn related byproduct gold Carr Fork USUT Is L. Penn./ Bingham Tert. py, cpy, po, apy, gar, diop, qtz, Parnell ore (Bingham) Bingham Mine Fm., stock-qtz hem, mag, sid, clay body Parnell Is. monz porph tet-tennantite Gold-Bearing Skarns
Ore control Is-vole contact Is-dike contacts congl-lst contact, faults faults Is, xenoliths in qtz diorite, fractures limy tuffs contact zone, faults faults, distance to stock, elevation Tonnage (millions of tonnes) Base metals Au (g/t) Ag (g/t) and Fe Comments References Cu skarns with byproduct gold-Continued 4.41 1.55% Cu Ls beds in voles; may be Cu-rich part of 8, 9, 22, 51 20.6% Fe zoned system; nearby Merry Widow, Kingfisher, and Ravel deposits produced 3.4 Mt iron ore; grab samples from Merry Widow assay as high as 19 g Au/t; Coast Copper Co. Ltd. (1968) 2% Cu Although Is and qtz latite porph both 52, 53 2%Pb silicated, bulk of sulfides is in skn; erratic py in porph 7.12 0.85% Cu Includes Knob Hill and several other claims; 8, 9, 22, 44, Pb p mine was closed in 1978 and reclaimed; 45,54, 55 Granby Co. 2.3% Cu Bulk of sulfides associated spatially with 56,57 mag; some syenite porph as dike 3% Cu Gar-ep skn is deeply weathered; Rosita 1, 46, 58, 59 Mines, Ltd. 16.1% Cu Composited production from 1943-51 for 60,61 10.0% Pb Arkansas-Decorah, South Peacock, and Helena mines; Pb grade for 131 t of ore; Pb-rich zones in some mines; W present. 2.5% Cu Au in stage II py, 0.1 to 1.6 ppm; Au in cpy; 46, 47, 48, 0.93 ppb; deposit has only small amount of sulfide (a few percent) 1.7% Cu 0.8% Cu Deposit is 1 km by 310 m and as much as 70 m thick 2.6% Cu Uke Resources, Ltd.
p 2.6% Cu Estimated reserves. Assays up to 6.6% Cu, 4.4 g!t Au. Zoned Porphyry Cu skarn related byproduct gold 1.02% Drill-indicated resource for Parnell gold 63, 64, 65 shoot. Multistage mineralization: high-grade Au in py-qtz and py-clay overprint on Cu-Au-Ag skn. Avg 1.9 g Au/t in gametized ls. Byproduct Au was produced 1979-81 at Carr Fork mine from estimated reserves of 61 Mt of ore of avg grade 1.89% Cu, 0.38% g Au/t, 10.6 g Ag/t, and 0.027% Mo Table 3
Table 3. Gold-bearing skarns in which gold and silver are byproduct commodities-Continued Name Location (mining district) Host lithology Formation age/ name Associated igneous rocks Age Ore minerals Gangue minerals Porphyry Cu skarn related byproduct gold-Continued Ok Tedi Chichibu El Sapo Falun Garpenberg Oda Maxfield SE Afghanistan PPNG JPAN CLBA SWDN SWDN USUT (Big Cottonwood) AFGH Thanksgiving PLPN (Baguio) Tsumo JPAN WoodlawnUSUT Kentuck)'-Utah (Big Cottonwood) 1. Einaudi and others, 1981 2. Philippine Bureau of Mines and Geosciences, 1986 3. Bryner, 1969 4. Frost, 1965 5. Nokleberg and others, 1987 6. Newberry, 1986 7. Wayland, 1943 Zn-Pb skarns with byproduct gold sl, sst, chert, Is Pal. qtz diorite lbaque batholith Is, qtzite Prot./Leptite Series granite, qtz porph dikes, amphibolite dolos, qtzite, Prot./Leptite Series granite mica, schist Is Miss./ diorite Gardison Ls. ls, minor congl, Mio./Zig-Zag Series diorite porph sst, sh, lithic tuff dolomitic mar ls Pal./Koseiso Fm. Miss./ Deseret Ls. diorite, granodiorite, granite Alta stock, granodiorite 17. Harris and Einaud~ 1982 18. Roberts and others, 1971 19. Canada Department of Energy, Mines and Resources, 1984 20. Buseck, 1966 21. Bergeat, 1910 22. Ettlinger and Ray, 1988 23. Little and others, 1970 8. British Columbia Ministry of Energy, Mines and Petroleum Resources, 1981 24. McConnell, 1914 25. Martin and Katz, 1912 9. Ettlinger and Ray, 1989 26. Richter and Herreid, 1965 10. Sutherland Brown, 1968 27. Koschman and Bergendahl, 1968 11. Roberts and Arnold, 1965 28. Shoj~ 1978 12. L1znicka, 1985 29. Geach, 1972 13. Wilkins, 1971 30. Winchell, 1914 14. McCleman, 1976 31. E.I. Bloomstein, written commun., 1987 15. Knopf, 1918 32. Bulynnikov, 1948 16. Einaud~ 1982 33. 'I'veritinov, 1966 Gold-Bearing Skarns cpy, gal, py, po, sph, mag, marc act, cal, gar, px, trem, talc Mio. 7.9, cpy, sph, py, mag gar, clinopx, ep, 8.2Ma act, cal, qtz cpy, gal, py, mag, gar, wol, Ma Prot. Prot. Tert. Cret. Tert. bor py, cpy, po, sph, gal, mag, Au, gahnite, weibullite sph, gal, py, po, cpy, Au py, gal, tet, sph, Cu-stained oxide minerals py, cpy, cc, rare Mo marmetite, qtz trem, talc, act, diop, qtz, biot, anthophyllite, chi, almandine, cummingtonite, ophicalcite, cordierite, andalusite trem, qtz, mica, talc, fl, tour, diop, gahnite cal, qtz, sepiolite, diop, ep, mica, wol, gar py, sph, apy, cpy, chl, gar, cal, qtz, gal, hem, mag, clinoz, ep, Au tell, Au act-trem, ves cpy, mag, malayaite qtz, clay, , gar, trem, chondrodite, wol, hed mag, cpy, py, gal, trem, cer, arg, Au, sph calcsilicates 34. Kennedy, 1953 35. Herreid and others, 1978 36. Sociedad Mineria y Petroles, 1969 37. Frank Simon, written commun., 1987 38. Roby and others, 1960 39. Klepper and others, 1957 40. British Columbia Minister of Mines Annual Report, 1981 41. Cockfield, 1948 42. Young and Uglow, 1926 43. C.·mada Department of Energy, Mines and Resources, 1980 45. Peatfield, 1978 44. Church, 1986 46. Boyle, 1979 47. Vakhrushev, 1972 48. Ivankin and Rabinovich, 1972 49. Vakhrushev and Tsimbalist, 1967
Tonnage Ore control (millions of tonnes) lithology, contact zone structure, contact zone fissures contact, favorable beds & structures contact 50.Bazheno~ 1968 51. TRM Engineering Ltd., 1986 52. Keith, 1974 , 1981 53. Schrader, 1915 54. Hedley, 1981 55. Little, 1983 56. Clark and Carlson, 1956 57. Cox and others, 1948 58. Bevan, 1973 59. Plecash and others, 1963 60. Cook, 1954 61. C. Field, oral commun., 1989 62. Jurada, 1982 63. Cameron and Garmoe, 1987 Base metals Au (g/t) Ag (g/t) and Fe Comments Porphyry Cu skarn related byproduct gold-Continued 1.5% Cu Zn-Pb skarns with byproduct gold 0.45% Cu Cu is restricted to gar-bearing skn; 12% 6%Zn Pb p pyrite; Nitchitsu Mining Co., Ltd. Fe P 5.1% Cu 16.21% Pb 1.06% Cu Associated with Fe skn; deposit is zoned 4.1% Zn 1.4% Pb 0.3% Cu Deposit has well-developed zoning 5.2% Zn 3.6% Pb 19.7% Pb Average grades reported for 1902--40 1.4% Cu production
3%Zn Average composition reported from Pb-Zn 9% Pb exploration of Cu-skam mineralization 40.55 0.36% Cu Mined largely for Au-Ag; reported Cd; 4.47% Zn Banquet Explorations, Ltd. 0.68% Cu Two ore bodies: Tsumo, Maruyama 2.43% Zn 11.9% Pb 2.64% Zn Pb-Ag-Zn production from bedded replacement fissure 64. Tooker, 1989 66. R.H. Sillitoe, oral commun., 1987 67. Imai, 1978 68. Grant, 1950 69. DA. Singer, oral commun., 1988 70. Grip, 1978 71. James, 1979 72. Calkins and Butler, 1943 73. Bybochkin and Kats, 1972 74. Callow, 1967 75. Geological Survey of Japan, 1980 References 1, 66 67,68 62, 69 71,72 2, 74 67, 75 71,72 Table 3
Table 4. Gold-bearing skarn deposits and deposits purported to be gold-bearing skarns for which grade and tonnage data are unavailable [Abbreviations listed in table 1] Mine name Akshiyryak Range Alae-Sayan Alvarado Ban Na Lorn Blue Grass Bright Diamond Buckhorn Mountain Bumblebee Cable Cadia Cane Springs Location (mining district) USSR (Kirghiziya) USSR USUT (Gold Hill) THLD USMT (Bannack) us co USWA USMT (Ophir) USMT (Cable) AUNS USUT (Gold Hill) Gold-Bearing Skarns Description 280-Ma granite (K/Ar, biot) intrudes carbonate-siliceous sequence. Au mineralization is associated with skarnoid & secondary silicified rocks in mar & silicate-carbonate rocks gradationally farther out than skarn. Au is mainly in highly silicified rock that contains wol, locally ves, px. Dark-gray highly silicified rock contains po, py, Au (as 0.1-mm-wide flakes). Skn includes px, gar, amph, serp, and late qtz with Cu, Pb, Zn, As, Sb, Cd. Fluids: high Cl; Na/K=l.1 to 1.5:1. Early skarns formed at 480-890 oc. Au deposited at 250-150 oc. Cu-Au skn formed in Miss. Ochre Mountain Ls. near Tertiary Gold Hill qtz monz stock. Ore includes Au, py, cpy, gal, bor, cc, mag, mal, lim, chalcanthite, jarosite. Gangue consists of wol, diop, ap, gar, spa, zoi, ves, trem, serp, qtz. Nolan (1935) reports $120,000 in Au produced 1892-1895.Channel sample assays range from tr to 5.8 g Au/t, tr to 387 g Aglt, tr to 1.9% Cu. Woodman Mining Co. Au, py, cpy associated with qtz deposition during or after retrograde skn formation. Although 3 skn types are recognized, Au mineralization is associated with relatively coarse grained gar-ep skn. Calcic skn is thought to be a metasomatically transformed tuff sequence. Gar-ep-cal skn veined by qtz as much as 5 m wide; veins not continuous. Ore along contact between granodiorite & Is mined from 40- by 7-m open cut. Ore in flat shoots about 3 m thick; Au localized in mag-py; porph dike as much as 10 m wide nearby. Described as a gold-bearing skn in Okanogan Co., Washington, that displays geological similarities to some major gold skn deposit in Nevada. Exploration in progress. Crown Resources Corp. and Gold Texas Resources Ltd. Gar skn showing superimposed faults. Explored by 30-mdeep shaft. Mal in qtz stringers in skn. Mineralized Is pendant of Camb. Hasmark dol, calc sh of Silver Hill Fm. in Eoc. Cable granodiorite stock. Primary ore: py, cpy, mag, po, Au, gal, sph, apy, tetd, marc. Oxidized ore: lim, hem, cc, bor, mal, azurite, Cu, Mn ox. Gangue minerals: gar, px, amph, wol, qtz, cal, dol, sid, mica, scap. Mag skn (Pomeroy mine) nearby. Most production pre-1900; total production for district, which includes Cable placer, estimated at 165,127 oz Au, 134,583 oz Ag. Nearby structurally controlled vein mineralization and oxidized ores occur at Southern Cross, Gold Coin, and Pyrenees deposits; Magellan Resources-Chevron Resources Co. Au-bearing skn in area of Fe skn. Cu-Au skn formed in Miss. Ochre Mountain Ls. near Tert. Gold Hill qtz monz porph stock. Ore: Au, py, cpy, bor, cc, cov, moly, mal. Gangue: gar, wol, diop, ves, zoi, qtz, cal, spa. Produced $50,000 to $70,000 gold 1892-95; 42 t ore assayed at 36.6 g Au/t, 103 g Aglt, and 5.5% Cu in 1914; 1,479 t ore produced 1931-35. Recent assays reported by El-Shatoury and Whelan (1970) range from tr to 21.2 g Au/t, tr to 6.8 g Aglt, and 0.12 to 1.03% Cu. Reference(s) Dolzhenko, 1974 Indukaev, 1977 Nolan, 1935; Wilson, 1959; El-Shatoury and Whelan, 1970 Pisutha-Armond and others, 1984 Geach, 1972 Irving, 1905; Irving and Cross, 1907 Mining Journal, 1989 McCleman, 1976; Mineral Resource Data System, 1989, record DC09691 Earll, 1972; Emmons and Calkins, 1913; Emmons, 1907; Holmes, 1982; Meinert, 1988a; Holser, 1950 McLeod, 1965 Nolan, 1935; Wilson, 1959; El-Shatoury and Whelan, 1970
Table 4. Gold-bearing skarn deposits and deposits purported to be gold-bearing skarns for which grade and tonnage data are unavailable--Continued Mine name Carles (Salas, Asturias) Carr Fork Central Tadzhikistan Charmitan Chihuahua district Chihuahua Chillioroya area Chumbivilcas CulveiWell Dutro East Sayan Mtns. (Medrezhye and Konstan tinovskoe deposits) El Fenomeno First Chance Ge Jiou Yun Nan Geunteut area Sumatra Gissaro-Alay Glassford Creek Goldstrike Location (mining district) SPAN USUT (Bingham) USSR USSR MXCO PERU USNV (Pennsylvania) USMT (Blue Cloud) USSR (Siberia, middle Asia) MXCO USMT CHNA INDS USSR (central Tadzhikistan) AUQL USNV (Carlin) Description Apy-cpy-py-Au in qtz-veined skn; 5- to 100-14m-size Au associated with apy. Gar-diop-act-ep-wol skn associated with porph Cu mineralization. Au-Cu-As in px & gar-px skn associated with L. Miss.- E. Perm. granodiorite & qtz diorite rocks; overall trapping temperatures of fluid inclusions range from 450° to 7500C; Au ores deposited paragenetically late in two stages: early py-apy, late tet-cpy at 2500-350°C. Four ore-forming stages: Au-Bi-tell, py-apy, Au-sulfide polymetallic, qtz-cc. Au-Ag-Pb-Cu skn occurs at Is-diorite contact. Over 2,100 kg Au was produced 1928-49 from approx 60,000 tore, with grade ranging from 0.1 to 100 g Au/t. Cu-Ag-Au ore occurs in small bodies of gar-mag skn at ls-qtz monz porph contact. Ore minerals include cpy, py, bor, cc, Au. Grades are reported as "few" g Au/t, about 5% Cu, 100 g Aglt for 2-10 Mt ore. Katanga (table 2) is found in this area. Skn associated with mag replacement pods in Camb. Is. Ep replaces monz-diorite; some skn includes copper ox, py, cpy. Grab samples assay as much as 85 g Au/t. Free Au, cassiterite, Bi reported from hydrosilicate altered skn including qtz, jas, trem, opal. Au ores preferentially formed in calcic skn from Cl-S04-Ca Na-bearing fluids (Na/K=2:1 to 6:1; Cl/F=31:1) at 220°-420°C; Cl F in leachates from productive Au skn. Au, Cu are reported to occur in this W skn mined during WWI and 1937-44; possible additional ore exists. Sch, secondary Cu minerals, py, po, cpy, apy occur in gangue of gar, ves, axinite, dio, q tz, cal. Main ore body was fan-shaped tactite zone at contact of L. Pal. mar & Cret.-Tert. diorite. In northern Baja California. Au reportedly produced, together with W, from xenoliths of Dev. Jefferson Ls. in granodiorite. Sn-Au skn formed during Mes. contact metasomatism. Geunteut granodiorite (14.3 Ma) intrudes L. Jur. & E. Cret. Is of Woyla Gp. Mineralization includes cpy, py, bor, azur, mal. W-apy-Au-Cu skn containing p:x, gar, qtz, feld, amph, dol, wm. Gar-mag-ep-wol-hem-act skn in Is at granite contact; ore minerals include cpy, bor, po, hes, Bi, Au, secondary Cu minerals; 735 t Cu, 80 kg Au 725 kg Ag were produced Au-bearing skn occurs at the No. 9 Pit & at Skarn Hill at the Goldstrike Mine (includes several types of ore bodies); skn formed in Dev. Is, informally named the Popovich Ls., beneath the Roberts Mountain thrust. At West No. 9 Pit, Au mineralization occurs in 160-Ma(?) granodiorite. Skn assemblages include gar, diop, act, chi, Au. American Barrick Resources, Inc. Reference(s) Rau-Figueroa and others, 1985 Atkinson and Einaudi, 1978; Reid, 1978 Morozov, 1976; Morozov and others, 1974; Morozov and others, Proskuryakov and others, D.L. Mosier (oral commun., 1987) Mineral Resource Data System, 1981, record W002200; Frank Simon (written commun., 1960) Mineral Resource Data System, 1984, records M241646 and M032085 Knopf, 1933 Korobeynikov, 1976a, b, c; Korobeynikov and Chernyaev, 1976; Korobeynikov and Matsyushevskiy, 1973 Salas, 1975; Fries and Schmitter, 1945; Leonard, 1989 Pardee, 1918; Kaufmann, Sang and Ho, 1987 Bowles, 1984 Khasanov, 1982 Murray, 1986 R.J. Roberts (oral commun., 1989); Schafer and Buffa, Table 4
Table 4. Gold-bearing skarn deposits and deposits purported to be gold-bearing skarns for which grade and tonnage data are unavailable-Continued Mine name Gould-Corry Hua Tong An Hui Huarca Hudson Group Iron Clad Kaliostrovskoe Kochulak Dalnagorsk region Kommunar district Kaznetskiy Alatau and Gornyi Altai La Gloria La Sonora Lucky Strike Many Peaks Marn (Mini Grid) Midas Midas (Berg Creek) Mottini Mount Biggenden Location (mining district) USMT (Red LionHidden Lake) CHNA PERU (Cuzco) USMT (Silver Star) us co USSR USSR USSR (Altai-Sayan) USSR MXCO MXCO AUNS AUQL CNYT USUT (Gold Hill) USAK USNV (IXL) AUQL Gold-Bearing Skarns Description Au reportedly produced from Au-Ag-Cu-W skn in Camb. ls intruded by Cret. granodiorite; gar, ep, goe, qtz present. Cenozoic Cu-Mo-Au skn. Irregular patches of Cu-Ag-Au ore in gametite at contact of qtz monz porph with ls. Minerals include cpy, mag, minor bor & cc. Average grades of 1 to 2 g Au/tare reported for 1-10 Mt of ore. Reference(s) Earll, 1972 Sang and Ho, 1987 Frank Simon (written commun., 1960) Gar-ep skn developed in Camb. ls near contact with Cret. qtz Sociedad Nacional de monz of Boulder batholith; average grade reportedly 32-42 Mineria y Petroleo g Au/t, 42-52 g Ag/t; serp, sid, cal, asbestos present. (Peru), 1969 Ore in flat shoot; Au localized in mag-pyas replacement with Winchell, 1914; Sahinen, silicates of blue-gray ls. Large blocks of ls engulfed totally by granitoid rock. Au-tell-tet stage formed at homogenization temperatures of 240°-270°C and 170°-190°C, together with Ag, Pb, Cu tell at lower temperatures (130°-150°C). In 8 deposits, Au associated with py, po, cpy, mag primarily in qtz-act veined skn developed in Camb. sed-voles sequence as result of emplacement of L. Camb. px diorite & monz. Au-bearing skn formed at 280°-700°C from homogenization temperatures in qtz & gar. Irving, 1905; Irving and Cross, 1907 Ivankin and Rabinovich, Genkin and others, 1983 Lobanov, 1972 Small Au-bearing W skn with Cu, Mo. Sch cpy, auriferous py, Pavlova, 1983 apy, moly occur in E. Cret. ls adjacent to granite. Gangue minerals include gar, ep, tour. Reserves of W-Mo ore are estimated at 25,000 t. In State of Sonora. A small Au-Cu skn in Pal. ls associated with L. Cret.-E. Tert. Perez Segura, 1985; granite. Minerals include Cu-ox. In State of Sonora. Radelli, 1985; Leonard, 1989 South of Bathurst, near Beuraga. Gar-mag-cal skn in shear zone with py, cpy; 8,650 t Cu, 130 kg Au were produced from 1910-18. Assemblage elec (AuS0-40)-Bi-bism-hes associated with cub exsolution in cpy or as blebs in apy, all hosted by px (diop20-40)-act (trem25_85)-po skn; avg grade 1.4 g Au/t, 2.8 g Ag/t. Ls beds in Manning Canyon Fm. altered to wol-gar-diop-ves skn near qtz monz; associated with oxidized and sulfidized Cu & Pb-Ag ore. Minor production (86 t, avg $56 Au per st produced before 1897). Cu-Au skn in Tri. Nizina Ls adjacent to Jur. granodiorite-qtz monzodiorite pluton. Ore: mag, py, cpy, Au. Gangue: ep, gar,qtz. Grab samples assay as high as 8 g Au/t, 10 g Ag/t, 20%Cu. Au skn in U. Tri. sh, sst, associated with 28-Ma granodiorite. Produced 272 t ore, avg $375 Au per st (est. 564 g Au/t, assuming a price of $20.67/troy oz); Ag, Cu, Pb, Ni present. Gangue of gar, cal, qtz, mag, spec, Fe and Mn ox. Ore consists of free Au, Ag & sulfides (py, gal, cpy). Gar-cal skn at granite contact; ore minerals include mag, bism, Bi, cpy, py, apy; 185 kg Au, about 235 t Bi produced 1890-1901; 378,725 t magnetite produced from adjacent skn from 1967 to present. Perez Segura, 1985; Leonard, 1989 Murray, 1986 Brown and Nesbitt, 1984, 1987 Nolan, 1935; Thompson, Nokleberg and others, Schrader, 1947; D.A. John (oral commun., 1989) Murray, 1986
Table 4. Gold-bearing skarn deposits and deposits purported to be gold-bearing skarns for which grade and tonnage data are unavailable-Continued Mine name Nambija Natal Sumatra New Calumet New World district Nixon Fork-Medfra Novo Brdo Oka Olkhovskii West Siberia Primor'ye area Sara Alicia Location (mining district) ECDR INDS CNQU USMT USAK YUGO CNBC USSR USSR (Far East) MXCO Description Gar skn, including k-spar altered to chi, ep, cal; includes Au, sch, auriferous py, apy; related to emplacement of Jur. batholithic rocks; grade reportedly may be as high as 30 g Au/t; notable concentrations of Au at qtz-qtz boundaries and qtz-gar-k-spar flooded portions of endoskam. Skn has formed where L. Cret. Manunggal batholith (87 Ma) intrudes E. Cret. Soma Fm. & U. Jur., Lower Cret. Woyla Gp. sed rocks; both include meta-vols, Is, meta-ls members. Skn has formed at margins of batholith & in xenoliths of Is. Mineralization includes py, mag, Au, Ag, Cu-Pb-Zn minerals. Pb-Zn-Ag ores; Grenville province. Ore shoots, masses occur in Grenville biot gneiss near its contact with an overlying amphibolite. Minerals include sph, py, marc, po, gal, cpy, apy, Ca & Mg silicates. Production (1943-68): 3.74 Mst at 6% Zn, 1.7% Pb. Reserves (1968): 0.282 Mst at 4.51% Zn, 1.08% Pb, 2.34 oz Agist, 0.014 oz Au/st. Consolidated Professor Co. Cu-Pb-Au skn in Camb. Is & shaly Is associated with Tert. rhyodacite porph & other intrusions. Ore: Au, gal, sph, py, cpy, spec. Gangue: gar, ep, trem, ves, qtz, ank. Daisy mine produced 13 carloads of gold ore in 1888 that averaged $50 per ton (est. 83 g Au/t). Skn gangue reported at other mines, prospects in district. Crown Butte Mines, Inc. developing New World Project for Au, Cu in 1987. Group of Cu-Au skn deposits at contact (of Is) of Ord. Telsitna Fm. with L. Cret. monz pluton; in fractures, roof pendants. Ore: cpy, py, bor, Bi, lim, mal, Au. Gangue: diop, gar, ep, plag,qtz, ap, act. Produced 1.24 to 1.87 Mg Au, with Cu, Ag. Some deposits have grades as high as 113 g Au/t, 1.5 to 2.0% Cu. Skn & replacement mineralization in Is along schist-Is & dacite-Is contacts; ore minerals include sph, gal, po, cpy, marc; main skn mineral is gar. Ore contains 1-5% Pb, 1-8% Zn, about 100 g Aglt, 3-4 g Au/t. Au in sulfide pods along skn-mar contacts & in faults; associated with diorite sills & reported to be similar to Mascot deposit 50 km to southwest. Fairfield Minerals Ltd. has identified several areas of mineralization along 5-km soil geochemical anomaly through mapping, chip sampling, trenching. Future drilling is planned on basis of chip sample analyses with 0.24-1.12 oz Au/st. Contains Au, Ag, Cu, As, Zn. Ord. diabase, diorite porph, qtz porph, aplite dikes intrude M. Camb. carbonate & tuffaceous rocks. Au-bearing mineralization occurs in gar-px skn & in qtz-sulfide veins. Ore minerals include cpy, sph, gal, bism; Au, Ag tells. Alteration includes chloritization, sericitization, slight serpentinization. Deposit is considered to have formed at medium depth, moderate temperature. Sch-Au-py skn formed under relatively reducing conditions; some associated Au-wolf deposits; mafic granite intruded into Sikhole Alin folded belt; some apy, mica, po; bed, gros, ves, cum, ep, act, tour, stilp, bustamite; mag, cc assemblages in skn. Small Au-Cu skn in Jur.-Cret. Is(?) associated with L. Cret.-E. Tert. granitic intrusives. In State of Sonora. Reference(s) E. Salazar (written commun., 1987); Minera Nambija (written commun., 1988) Bowles, 1984 Boyle, 1982; Canada Department of Energy, Mines and Resources, 1980 Lovering, 1929; Reed, 1950; Elliott, 1979; Elliott (oral commun., 1989); Lawson, 1988 Nokleberg and others, 1987; Newberry, 1986 Jankovic, 1982 Skillings' Mining Review, 1987; Ettlinger and Ray, 1988, 1989 Smimov and others, Stepanov, 1977, 1981; Stepanov and others, 1976a, 1976b; Stepanov and Kucyakova, 1973; Makiyevskiy, 1978, 1979; Efimova and others, 1982; Piskunov and Makiyevskiy, 1978 Perez Segura, 1985; Leonard, 1989 Table 4
Table 4. Gold-bearing skarn deposits and deposits purported to be gold-bearing skarns for which grade and tonnage data are unavailable-Continued Mine name Sayakskiy region Shul Kou Shan Spring Hill Stormont Sylvester K Terrazas TP (claims) Union Amalgamated West Park USSR Location (mining district) CHNA USMT (Helena) AUTS CNBC MXCO CNBC USNV (Manhattan) USUT (Snake Creek) Gold-Bearing Skarns Description High-temperature zones of skn contain three assemblages: (1) Au1: gersdorffite (NiS2 XN~)-apy-cobaltite (>250° C); (2) Au2: Bi-cpy-po (250°C) with ep, act; (3) Au3: wittichenite (Cu3BiS3)-moly-bor-cpy (225°C). Pal. contact metasomatic deposit. Skn mined for Pb, Zn, Au. Gar-px skn developed in Miss. Madison Ls; skn altered to qtz-ank-cal-chl assemblage with Au-apy-po-py-cpy-gal inore; much of ore reported to be px-rich. A u-Bi skn in Moina district, area known primarily for Sn-W skn, veins, greisens. Deposit is in Ord. Gordon Ls., associated with Dev. granite. Had minor production 1928-34. Reported Au values at Moina skn, 4.5 ppm. Lenticular bodies of Au-bearing skn concordant with enclosing rocks of Tri. Brooklyn Fm., associated with micro-diorite stocks, dikes of L. Jur. to E. Cret. age. Mineralization consists of massive py, mar, po, minor cpy in calcic exoskn. Deposit has lim-goe cap several meters thick. Au-bearing Cu skn along Is-diorite contact. Skn runs 2-3% Cu as cup, azur, mal, cpy, Cu. Associated Pb-Ag veins. In State of Chihuahua. As much as 15 g Au/t, 3.9% Co in chip samples from mag-calgar-amph skn in pre-Tri. gneiss, schist, mar of Yukon Gp. Skn zone is 15 by 200 m, controlled by two NW.-trending fracture zones. Four types of skn present. Sulfide-bearing skn veined by Au-bearing qtz; developed in Pal. marine sed & metamorphic rocks; possibly related to 16-Ma caldera or Cret. intrusion. Skn formed along contact of Miss. Is with Tert. granodiorite Peak Stock); ore includes cpy, bor, mag; average grade production 1946-50: 3% Cu, 17.2 g Ag!t, 1.57 g Au/t. Reference(s) Fomichev and Kuznetsova, 1972 Sang and Ho, 1987 Pardee and Schrader, Green, 1975; Collins and Williams, 1986; Kwak and Askins, 1981 Church, 1984; Canada Department of Energy, Mines and Resources, 1986 Salas, 1975; Gonzalez, 1956; Clark and Goodell, 1983; Leonard, 1989 Ettlinger and Ray, 1988 Shawe and others, 1986 U.S. Bureau of Mines, Strategic Minerals Examination, 1950; Mineral Resource Data System, 1984, record D011978
Table 5. Mineral abundances for gold-bearing skarns [Data are reported as a percentage of the number of deposits in the data set that report a given mineral present] Data set Au-skarn Byproduct Alaskan Au -skarn Fe-Au skarn Reference This study, This study, Newberry, table 2 table 3 Number of deposits Ore minerals (in percent) Au/electnun 6 2 pyrite 7 2 pyrrhotite 6 7 chalcopyrite 8 5 arsenopyrite magnetite 3 3 hematite/specularite sphalerite galena 28 Bi or bismuthinite 23 hedleyite 13 telluride minerals molybdenite 18 scheelite Gangue minerals (in percent) garnet 82 pyroxene 72 epidote 72 amphibole 46 chlorite 46 prehnite 13 vesuvianite wollastonite 31 scapolite boron minerals Table 6. Analytical data for some igneous rocks associated with gold-bearing skarn deposits in north-central Nevada [n.d., not detected;-, no data] Analysis Si A1 fJ3 FeO MgO CaO N~O K20 Ti P205 MnO Other2 Au (ppb ) n.d.
Cu (ppm) 990 1,500 K20/Na20 Altered granodiorite sill at north edge of West ore body, a Cu-Au skarn deposit related to 38-Ma altered granodiorite of Copper Canyon. Fortitude Au-bearing skarn deposit lies just north of West ore body and is also probably genetically related to altered granodiorite of Copper Canyon. Analysis from Theodore and others (1973) (loc. 12, sample MB-40). Altered granodiorite of Copper Canyon. Analysis from Theodore and others (1973)(1oc. 9, sample MB-18). McCoy granodiorite stock, 5,300-ft bench McCoy Mine sample 87JH013. XRF analysis by J. Taggart, A. Bartel, and D. Siems. Gold determined by INAA, G. Wandless, analysL Granodiorite dike at South shaft, Buffalo Valley Mine, sample 87TT228. Same methods as 3. 1Total iron as Fe203. 20ther =H2o+, H20-, for analyses 1, 2; =loss on ignition for analyses 3, 4. Tables S and 6
Table 7. Representative data for minerals in gold skarns from north-central Nevada [Total iron as FeO for pyroxene, idocrase, amphibole; total iron as Fe10 3 for gameL Values are in weight percent. n.d., not detected] Analysis Mineral Pyroxene Garnet ldocrase Amphibole Si02 50.4 Al20J FeO (F~OJ)- 18.2 MgO CaO 23.2
K20 n.d. n.d. n.d. n.d. Ti~ MnO F n.d. n.d. n.d. n.d. n.d. n.d. a n.d. n.d. n.d. n.d n.d. n.d. Total Fortitude deposit sample 85TT243; averafT of 7 grains in massive pyrrhotite ore. McCoy dea,osit, samples 86TT134 and 86 137; average of 5 grains in massive oxidized garnet skarn. Fortitude eposit, diill core sample of garnet-bearing sulfidized skarn; colorless, anisotropic zone. yell ow isotropic andradite zone in same armet grain as 3. Surtlcrise deposit, drill core sample of oxi 'zed garnet skarn; colorless, anisotropic rim of large, euhedral zoned grain. Ye ow isotropic andradite core in same grain as 5. McCoy de~sit, sample 87TT137; average of 3 grains in idocrase-rich pod in garnet skarn. Fortitude eposit, sample 85TTI43; ferro-actinolite in phcoxene-bearing sulfiruzed skarn. Northeast Extension deposit, sample 87TT2; actinolitic ornblende in epidote-amphibole-quartz-chlorite-sulfide skarn; no garnets or pr;;oxenes present. Actina ite in sample 87TT2. Gold-Bearing Skarns
Table 8. Chemical signatures of nontronite clay layers associated with gold-bearing skarns [-,not detected; N.D., not detennined] Sample1 85JH115 85jH142 86TT135 88TT63 87jH004 Method2 a b a b b b Weight percent Al Ca Fe Mg Na K p Ti Parts per million Mn Ag
As <10 B Ba Be
Bi <10 <10 <10 Cd
Ce
Co Cr Cu Ga
La
Li Ni Pb Sc
Sn <10 <10 Sr v y Zn Zr Hg3 N.D. N.D. N.D. Au3 N.D. Pd3 N.D. N.D . N.D. Pt3 N.D. N.D. N.D. Rh3 N.D. N.D. N.D. w3 N.D. N.D. N.D. 1 Analyses were done on bulk samples of earthy, yellow-green clay layers in skams. X -ray diffraction studies show that all samples are mixtures of clay and significant amounts of quartz and calcite or pyroxene. All samples have characteristic smectite peaks at 14 angstrom that expand to about 17 angstrom with glycolation. Microprobe work on 85JH 115 confirms the Fe-rich nature of clay. Samples are from skams in the Harmony Formation (85lli115) and in the Battle Formation (85JH142) in Battle Mountain Mining District, the McCoy Mine (86TT135) in McCoy Mining District, the Buffalo Valley Mine (88TT63), and the Sutprise Mine (87JH004). 2 Elements sought for but not detected at limit of methods a and b include Au, Mo, and W. (a) Six-step direct current arc semi-quantitative spectrographic analyses; analyses performed in U.S. Geological Survey exploration research laboratories by Betty Adrian and Olga Ehrlich; X-ray studies by Steve Autley and Ted Botinelly. (b) Quantitative inductively coupled plasma direct reader emission spectroscopy by M. Malcolm in U.S. Geological Survey analytical laboratories; X-ray work by Karen Gray. 3 Trace analysis and chemical separation by C. Gent, R. O'Leary, B. Libby, N. Rait, and S. Wilson in U.S. Geological Survey analytical laboratories. Table 8
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Pre-1971 maps show bedrock geology in relation to specific mining or mineral-deposit problems; post-1971 maps are primarily black-and-white maps on various subjects such as environmental studies or wilderness mineral investigations. Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planirnetrit.. bases presenting a wide range of geohydrologic data of both regular and irregular areas; principal scale is 1:24,000 and regional studies are at 1:250,000 scale or smaller. Catalogs Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under the conditions indicated below from the U.S. Geological Survey, Books and Open-File Reports Section, Federal Center, Box 25425, Denver, CO 80225. (See latest Price and Availability List) "Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a set of microfiche. 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Selected copies of a monthly catalog "New Publications of the U.S. Geological Survey" available free of charge by mail or may be obtained over the counter in paperback booklet form only. Those wishing a free subscription to the monthly catalog "New Publications of the U.S. Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092. Note.--Prices of Government publications listed in older catalogs, announcements, and publications may be incorrect Therefore, the prices charged may differ from the prices in catalogs, announcements, and publications.