Comparative geochemistry of jasperoids from Carlin-type gold deposits of the western United States

Journal o[ Geochemical Exploration, 36 (1990) 171-195

171

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Comparative geochemistry of jasperoids from Carlin-type gold deposits of the western United States CARL E. NELSON*

7500 East Quincy C-203, Denver, CO 80237, U.S.A. (Received January 17, 1989; revised and accepted June 20, 1989)

ABSTRACT Nelson, C.E., 1990. Comparative geochemistry of jasperoids from Carlin-type gold deposits of the western United States. In: J.W. Hedenquist, N.C. White and G. Siddeley {Editors), Epithermal Gold Mineralization of the Circum-Pacific: Geology, Geochemistry, Origin and Exploration, II. J. Geochem. Explor., 36: 171-195. Exploration for Carlin-type gold orebodies in the western United States typically involves sampling and analysis of jasperoid, a distinctive alteration type formed by intense silicification of marine sediments. In this study, rock suites were collected from six orebodies and four similar but barren systems. Jasperoids at all ten systems contain episodically silicified breccias, quartz vein stockworks, elevated As, Sb, Hg, Ba and Tl, and, locally, anomalous Au and Ag. Jasperoids from the four barren systems are as anomalous as jasperoids from the six orebodies in all members of an epithermal geochemical suite, including gold and silver. A database suite containing 272 samples from six of the ten systems was analyzed for 45 elements. Q-mode factor analysis shows that geochemical variance in the epithermal geochemical suite is related to geochemical variance in Li, P, Mn, Ba, Mo, Cr, Co, V, Cd, Ni, U, Zn and Pb. Metalliferous marine black shales are enriched in these elements and are spatially related to Carlin-type deposits. A test suite containing 109 samples from the four remaining systems was analyzed to determine whether a discriminant function derived from the database suite could be applied to exploration. An 8-element function correctly assigned 40 of 42 jasperoid samples collected from the surface at the Carlin and Horse Canyon gold deposits. These results indicate that elements characteristic of metalliferous marine black shales can be used to identify Carlin-type systems with associated gold ore. Gold in Carlin-type orebodies may have been leached from a source-rock sequence which contains metalliferous marine black shales.

INTRODUCTION

Precious-metal exploration in the Great Basin of the western United States focuses on Carlin-type gold deposits. These are variably oxidized, bulk mine*Present address: Apdo. 2155-1000, San Jose, Costa Rica.

0375-6742/90/$03.50

© 1990 Elsevier Science Publishers B.V.

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able orebodies hosted by argillized and silicified marine sediments. Micronsize gold is disseminated with fine-grained sulfides in carbonaceous limestones, dolomites and shales. The median deposit size cited by Bagby and Berger (1986) is 5.1 million tonnes averaging 2.5 ppm Au. Newmont's Carlin deposit has produced over 133,000 kg Au (4.3 million ounces) at an average grade of 10 ppm (Wilkins, 1984). Past production plus published reserves through 1989 at 33 Carlin-type deposits total over 1800 tonnes (60 million ounces). Carlin-type gold reserves have been increasing since 1976 at a rate of close to 200 tonnes (6.5 million ounces) per year. Exploration for Carlin-type gold orebodies typically involves sampling of jasperoid, a distinctive alteration type formed by intense silicification of marine sediments. Jasperoids provide evidence of hydrothermal activity, are resistant to erosion, and occur in all Carlin-type systems. Samples are analyzed for a suite of elements including Au, Ag, As, Sb, Hg, Ba and T1. These elements are enriched in Carlin-type gold orebodies and in epithermal precious-metal deposits in general. In this study, jasperoids from barren Carlin-type systems, where drilling has

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Fig. 1. Location map showing the distribution of rock units in the Basin and Range Province which are reported to contain metalliferous marine black shale. Locations of Carlin-type orebodies sampled for this study are numbered as follows: Cortez (I), Mercur (2), Jerritt Canyon (3), Alligator Ridge (4), Carlin (5), and Horse Canyon (6). Locations of barren Carlin-type systems sampled for this study are numbered as follows: Black Cliff (7), Hot Creek (8), Iron Point (9), and MeCluskey Peak (10). Outcrop pattern for the Valmy, Comus, Vinini and Woodruff Formations and the Chainman Shale is taken from the 1:500,000 scale geologic map of Nevada compiled by Stewart and Carlson (1978). A rectangular box outlines the area shown in Fig. 4.

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1000

100

A 10 E

> 1.0

0.1

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Mercur (14)

Gold orebodles ~Au ~Ag

~c~201k =

Barrensystems

Fig. 2. Geochemical data from surface exposures of jasperoid sampled at five orebodies and four barren systems. The number of jasperoid samples taken at each system is given in parentheses. The range in values is shown as patterned bars. Arrows pointing up indicate that values continue above the range plotted. Arrows pointing down indicate that values extend below the range plotted or below the lower detection limit. Mean values are shown as triangles.

eliminated potential for an associated gold orebody, were collected and compared to jasperoids from orebodies. Samples were collected at six Carlin-type orebodies and four barren systems, located on Figure 1. The gold orebodies include Alligator Ridge, Carlin, Cortez, Horse Canyon, Jerritt Canyon, and Mercur. The barren systems are Black Cliff, Hot Creek, Iron Point and McCluskey Peak. As many as 60 drill holes in each of the four barren systems failed to reveal more than trace amounts of gold mineralization. Data for the epithermal geochemical suite in surface exposures ofjasperoid from nine of the ten systems sampled are shown on Figure 2. The Cortez deposit is not represented since surface exposures ofjasperoid have all been mined away. These data are similar to what would likely be obtained by an exploration team sampling each system prior to development. Figure 2 shows that gold rarely exceeds one ppm in jasperoid samples collected from the surface at Carlin-type gold orebodies. Figure 2 also shows that the epithermal geochemical suite is as enriched in jasperoids from the four barren systems as it is in jasperoids from the six Carlin-type orebodies. Apparently, elevated As, Sb, Hg, Ba and T1 and, locally, anomalous Au and Ag in jasperoid indicate only that alteration occurred in an epithermal environment.

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Carlin-type systems that exhibit anomalous As, Sb, Hg, Ba and T1 and lowlevel Au and Ag are drilled by most exploration companies. This procedure has resulted in numerous discoveries in the past and will undoubtably lead to more discoveries in the future. However, the level of risk associated with such a program is high since there are many more barren systems with anomalous epithermal geochemistry than there are orebodies. Anderson (1981) estimates that 500 prospects need to be evaluated in order to be 75% certain of a single 45 tonne (1.5 million ounce) discovery. This estimate is based on the actual experience of companies exploring for gold in the western United States. The odds against finding an orebody are high because most Carlin-type systems are barren or contain a subeconomic gold resource. It is also true that Carlin-type systems with associated ore are sometimes drilled and abandoned by several exploration companies prior to discovery. Many explorationists have given up for lack of encouragement on at least one prospect that later developed into a producing gold deposit. An exploration tool is needed which is capable of distinguishing the Carlintype systems which have an associated gold orebody from the numerous barren Carlin-type systems with anomalous epithermal geochemistry. Such a tool would serve to both reduce the number and improve the quality of targets that are tested by drilling. An exploration company would also be encouraged to persevere at those properties where, although initial drill results are discouraging, an eventual discovery is likely. SAMPLE SELECTIONAND PETROGRAPHY The ten systems sampled were divided into a database suite and a test suite. The database suite includes 272 samples from four orebodies (Alligator Ridge, Cortez, Jerritt Canyon and Mercur) and two barren systems (Hot Creek and Black Cliff). The test suite includes 109 samples from two orebodies (Carlin and Horse Canyon) and two barren systems {Iron Point and McCluskey Peak). Host rock at Carlin, Cortez and at Hot Creek is the Silurian-Devonian Roberts Mountains Formation, a thin- to medium-bedded calcareous siltstone with minor carbonaceous material and early diagenetic pyrite (Wells et al., 1969; Radtke et al., 1980). Jerritt Canyon is hosted by the Roberts Mountains Formation and by the underlying Ordovician-Silurian Hanson Creek Formation (Birak and Hawkins, 1985). Horse Canyon is hosted by the Devonian Wenban Limestone and the Ordovician Vinini Formation (Coppinger, 1986). McCluskey Peak is hosted by the Devonian Denay Limestone and the Vinini Formation. Iron Point is hosted by the Ordovician Comus and Vinini Formations and the Cambrian Preble Formation (Erickson and Marsh, 1974). Alligator Ridge and Black Cliff are hosted by the lower member of the Devonian-Mississippian Pilot Shale, a fissile carbonaceous shale to siltstone with minor carbonate cement (Tapper, 1986). Mercur is hosted by a shale member of the

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Great Blue Limestone (Lenzi, 1973 ). Host-rock types at the six orebodies sampled include fossiliferous limestone, silty limestone, silty dolostone and shale. The two barren systems sampled for the database suite are located in southern and eastern Nevada (Fig. 1). Although many additional barren systems are known in southeastern Nevada and in western Utah, these were passed over as candidates for the test suite. Instead, test systems were selected in established gold belts close to known Carlin-type gold deposits. McCluskey Peak is in the Battle Mountain-Eureka mineral belt, roughly 30 km from Horse Canyon and 10 km from Tonkin Springs. Iron Point is on the Getchell trend, roughly 10 km from the Preble mine. The database suite and the test suite plus standards and blanks together total 413 samples. The sample distribution by deposit and by rock type is shown in Table 1. Representative rock textures are shown in Figure 3. An average of 38 samples were collected from each of the systems in order to provide a daTABLE 1

Sample suites for six orebodies and four barren systems Deposit

Unaltered Altered host host

Ore Replacement Fragmental jasperoid jasperoid

Episodically Totals silicified jasperoid

Database suite: Gold Acres standard Devils Gate blank Barren systems: Hot Creek Black Cliff Ore deposits: Alligator Ridge Jerritt Canyon Mercur Cortez

Total

11 11 7 7

8 8

0 0

9 5

7 2

8 14

39 36

7 6 5 1 33

19 7 8 16 66

4 12 11 4 31

4 5 6 7 36

9 11 6 7 42

10 14 11 7 64

53 55 47 42 294

Test suite: Gold Acres standard Barren systems: Iron Point McCluskey Peak

10 2 4

0 0

0 0

7 5

6 5

9 10

24 24

Ore deposits: Horse Canyon Carlin Total

2 4 12

1 1 2

5 6 11

7 5 24

6 8 25

10 6 35

31 30 119

Grand Total

45

68

42

60

67

99

413

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C.E. NELSON

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Fig. 3. Representative samples for six sample categories: A. Unaltered Hanson Creek siltstone from the Jerritt Canyon deposit. B. Altered pilot shale from Black Cliff. C. Replacement jasperoid from the Cortez deposit.

GEOCHEMISTRYOF JASPEROIDSFROMCARLIN-TYPEGOLDDEPOSITS,WESTERNU.S.

D. Fragmental jasperoid from the Alligator Ridge deposit. E. Episodically silicified jasperoid breccia from Hot Creek. F. Ore from the Mercur deposit.

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TABLE 2 Statistical summary for the database suite Element

SiO2 (%) A1203 (%) Na20 (%) MgO (%) CaO (%) K20 (%) TiO2 (%) P~O5 (%) Total C (%) Carbonate (%) LOI (%) Total S ( % ) Sulfate (%) Sulfide (%) Total Fe (%) FeO (%) Fe203 (%) Au (ppb) Ba (ppm) Hg (ppb) Sr (ppm) As (ppm) Zn (ppm) Sb (ppm) V (ppm) Rb (ppm) T] (ppm) Ag (ppm) Zr (ppm) Cr (ppm) Ni (ppm) Mo (ppm) Cu {ppm) Pb (ppm) Cd (ppm) Co (ppm) U (ppm) Se (ppm) Te (ppm) W (ppm) Nb (ppm) Li (ppm) Bi (ppm) Be (ppm) Mn (ppm)

Standard replicates below limit of detection

Coeff. of Range variation in (272 sample database) standard replicates Low Value High Value

0 0 0 0 0 0 0 0 0 0 0 0 11 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 11 7 8 10 0 0 9 9 0

5 4.1 17.2 9.1 3.1 8.1 13.5 24.6 7.0 6.1 1.9 14.7 16.4 6.3 40.5 10.1 6.9 12.2 16.4 10.7 8.9 5.9 10.4 9.8 9.1 12.1 16.8 3.6 8.8 14.4 30.2 7.2 15.7 8.7 16.1 54.1 47.3 37.4 18.4 8.7 50.8 34.2 5.5

1.00 0.02 0.01 0.02 0.06 0.01 0.03 0.01 0.01 0.03 0.20 0.01 0.03 0.00 0.04 0.05 0.03 2.50 3.00 2.50 2.50 2.50 0.50 2.50 0.50 2.50 0.25 0.25 2.50 4.00 0.50 0.50 0.50 2.50 0.50 1.00 5.00 2.50 5.00 5.00 2.50 0.50 1.00 0.50 12.00

97.50 15.66 0.23 11.59 54.50 4.20 0.90 1.22 10.94 54.74 42.14 12.67 35.92 2.49 22.59 0.70 21.25 93740.00 570000.00 490000.00 4420.00 9700.00 1098.00 20000.00 771.00 200.00 280.00 108.34 470.00 603.00 166.00 210.00 110.00 109.00 16.00 31.00 27.00 97.00 67.00 98.00 33.00 77.00 8.00 4.00 35400.00

Range (142jasperoid samples) Low Value

High Value

1.00 0.02 0.01 0.02 0.06 0.01 0.03 0.01 0.01 0.03 0.20 0.01 0.24 0.00 0.14 0.10 0.03 2.50 23.00 25.00 7.00 2.50 0.50 2.50 0.50 2.50 0.25 0.25 2.50 6.00 2.00 0.50 O.50 2.50 O.5O 1.00 5.00 2.50 5.00 5.00 2.50 0.50 1.00 0.50 25.00

97.50 8.47 0.16 5.08 24.64 2.23 0.28 1.00 5.18 25.92 19.57 12.67 35.92 1.85 10.87 0.50 10.40 13060.00 570000.00 330000.00 4420.00 1145.00 827.00 20000.00 407.00 83.00 150.00 108.34 210.00 603.00 69.00 210.00 51.00 109.00 8.10 31.00 14.00 14.00 67.00 98.00 33.00 77.00 4.00 4.00 35400.00

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TABLE 3 Statistical summary for the test suite Element

Standard replicates below limit of detection

SiO2 (%) 0 A1203 (%) 0 Na20 (%) 0 MgO (%) 0 CaO (%) 0 K20 (%) 0 TiO2 (%) 0 P205 (%) 0 Total C (%) 0 Carbonate (To) 0 LOI (%) 0 Total S (TO) 0 Sulfate (TO) 7 Sulfide (TO) 0 Total Fe (TO) 0 FeO (%) 0 Fe203 (TO) 0 Au (ppb) 0 Ba (ppm) 0 Hg (ppb) 0 Sr (ppm) 0 As (ppm) 0 Zn (ppm) 0 Sb (ppm) 0 V (ppm) 0 Rb (ppm) 0 Tl (ppm) 0 Ag (ppm) 0 Zr (ppm) 0 Cr (ppm) 0 Ni (ppm) 0 Mo (ppm) 0 Cu (ppm) 0 Pb (ppm) 0 Cd (ppm) 0 Co (ppm) 0 U (ppm) 4 Se (ppm) 7 Te (ppm) 9 W (ppm) 0 Nb (ppm) 0 Li (ppm) 0 Bi (ppm) 1 Be (ppm) 0 Mn (ppm) 0

Coeff. of Range variation in (109 sample test suite) standard replicates Low Value High Value

Range (84 jasperoid samples) Low Value

High Value

1.7 2.5 4.9 3.1 3.1 6.7 4.5 15.0 5.2 9.1 2.5 67.2 54.4 80.2 8.2 29.3 4.8 13.2 4.4 19.0 5.8 7.6 9.0 26.1 8.4 5.7 9.1 24.5 1.9 13.0 5.9 19.9 8.3 13.4 15.0 15.1 51.3 93.7 43.6 23.3 15.3 12.7 59.8 29.3 7.3

60.50 0.21 0.01 0.01 0.05 0.03 0.01 0.01 0.01 0.05 0.60 0.01 0.03 0.00 0.04 0.05 0.03 2.50 10.00 40.00 5.00 8.00 4.00 2.50 0.50 2.50 0.25 0.25 42.00 0.50 0.50 0.50 0.50 2.50 0.50 0.50 5.00 2.50 5.00 5.00 2.50 1.00

97.99 16.64 0.42 4.60 7.96 5.31 0.63 5.48 3.07 13.40 12.60 3.88 6.90 1.62 21.11 3.80 21.05 1150.00 190000.00 300000.00 660.00 4600.00 6160.00 1944.00 2148.00 177.00 16.00 370.00 210.00 246.00 96.00 44.00 397.00 2415.00 138.00 36.00 15.00 51.00 15.00 171.00 24.00 98.00

5.28 0.21 0.01 0.01 0.05 0.03 0.01 0.01 0.01 0.05 0.60 0.01 0.03 0.00 0.04 0.05 0.03 2.50 10.00 30.00 5.00 2.50 4.00 2.50 0.50 2.50 0.25 0.25 32.00 0.50 0.50 0.50 0.50 2.50 0.50 0.50 5.00 2.50 5.00 5.00 2.50 1.00 1.00 0.50 0.50

97.99 16.64 0.42 9.23 49.99 5.31 0.63 5.48 10.02 51.03 41.10 3.88 6.90 2.02 21.11 3.80 21.05 30218.00 190000.00 300000.00 660.00 5500.00 6160.00 1944.00 2148.00 177.00 27.00 370.00 210.00 246.00 228.00 44.00 397.00 2415.00 138.00 36.00 25.00 55.00 15.00 171.00 24.00 98.00 21.00 4.00 1977.00

1.00

21.00

0.50 0.50

4.00 796.00

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C.E. NELSON

tabase that adequately represents the spectrum of geochemical variabilityat each deposit (Tables 2 and 3). However, too few samples were taken to address jasperoid paragenesis or geochemical zoning relativeto ore. Samples of unaltered host rock are included in both the database suite and the test suite. These samples provide a basis from which to assess chemical and mineralogical changes accompanying alterationand mineralization.Samples of unaltered host can also be used to assess what differences (ifany) exist between the host rocks in Carlin-type orebodies and the host rocks in barren Carlin-type systems. Various explorationistshave suggested that a host rock needs to be thin-bedded (physicallypermeable), carbonate-cemented (chemically reactive) or rich in organic matter in order to serve as an appropriate host. Samples of altered host reveal more intense (than in the unaltered host group) recrystallizationof carbonate (dolomitization), more abundant carbonate veining and replacement, and hydrothermal quartz veining and replacement. Organic carbon is redistributedand concentrated along fractures. None of these alterationeffectsis unique to mineralized systems. Dolomitization is better developed in samples of hydrothermally altered host than in samples of unaltered host rock. Dolomite is coarser than primary carbonate, is typicallyeuhedral, and shows rhombohedral outlines that cut fossil boundaries. These observations suggest that dolomitization is,at leastin part, epigenetic in origin. Dolomitization, whether diagenetic or epigenetic,would contribute to the permeability of the host rock environment. However, dolomitizationis also observed in thebarren systems. Ore occurs in all of the rock types listedunder unaltered host. Not all of the ore-grade carbonates are dolomitized nor are all ore samples originallycalcareous. M a n y ore samples exhibit only weak alterationconsistingof minor quartz replacement of carbonate with cross-cuttingveinletsof pyrite and carbon. Native gold was observed in polished sections of high-grade samples (greaterthan 30 p p m Au), both in veinlets and as disseminations in the altered carbonate host rock. Three varieties of jasperoid were distinguished in the field.All are so intensely silicifiedthat insufficientcarbonate remains to produce a noticeable reaction with dilute hydrochloric acid. The three jasperoid categories are: replacement silicifiedhost rocks, silicifiedrocks which are fragmental but which contain evidence for only one episode of alteration,and episodicallysilicified veins and breccias. The episodicallysilicifiedveins and breccias category includes cross-cutting veins and vein stockworks as well as variably oxidized, episodicallysilicifiedjasperoid breccias.These categoriesare based on textural differences that can be easilyrecognized in hand specimen. The intensity of silicificationfor all of the jasperoid categoriesvaries in response to primary permeability. Siltstones are more intensely silicifiedthan

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shales and dolomitized units are better silicified than unrecrystallized carbonates. Replacement jasperoids exhibit partial to complete quartz replacement of an originally calcareous host rock. Microveinlets of quartz are visible in thin section and are well developed in both barren and mineralized systems. The main coloring agent giving jasperoids their dark color is microscopic disseminated carbon, present as inclusions in fnely crystalline quartz. The second jasperoid category includes fragmental rocks which show evidence for only one episode of silicification.These are clast-supported breccias with angular to subrounded fragments. Fragmental jasperoids may be tabular relative to bedding or may be cross-cutting.The fragmental character of some tabular jasperoids increases with increasing intensity of silicifcation.These breccias probably formed by dissolution and replacement of the carbonatebearing host rock. Cross-cutting fragmental jasperoids may have developed by a similar solution brecciation mechanism or may simply have formed by silicificationof pre-existing faults and fractures. Episodically silicifiedveins and breccias are the third and the most diverse of the jasperoid categories. Samples exhibit evidence for multiple generations of quartz veining and replacement. Rock types include episodically silicified and variably oxidizedjasperoid breccias,veins and vein stockworks. Veins contain variable proportions of quartz, barite and calcite. Barite occurs as both vein and cavity fillings.Calcite and dolomite occur as cavity fillingswith quartz. Other minerals observed in episodically silicifiedveins and breccias include apatite, fluorite,gypsum, alunite and jarosite. HYDROCARBONS

All of the orebodies sampled are enriched in carbon relative to the background carbon content of unaltered host rock. Carbonaceous ores typically run 1-5% C, with some samples containing as much as 20% C. Although carbon was observed in pyrite veinlets cutting ore, carbon was not present in veinlets in any of the jasperoid samples. This observation suggests that hydrocarbon enrichment occurred before hydrothermal alteration and mineralization. Detailed paragenetic and fluid inclusion studies, where available, provide additional evidence for pre-hydrothermal carbon enrichment. Kuehn and Gize (1985) and Hausen and Park (1986) report that organically derived hydrocarbon enrichment occurred prior to hydrothermal gold mineralization at the Carlin gold deposit. Kuehn and Rose (1987) report immiscible hydrocarbon globules (petroleum) in paragenetically early quartz veins at Carlin. Similar results, indicating that organically derived hydrocarbon enrichment occurred prior to hydrothermal gold mineralization, are reported by Levanthal et al. (1987) for the JerrittCanyon deposit. Paragenetic studies document a pre-hydrothermal accumulation of organi-

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cally derived hydrocarbon. Salinity measurements as high as 10 eq. wt.% NaC1 in fluid inclusions from jasperoids at the Jerritt Canyon deposit (Hofstra et al., 1987) provide evidence for an associated brine. Saline fluids, dolomitized host rock, and pre-hydrothermal organically derived hydrocarbon enrichment suggest that Carlin-type gold deposits are also fossil petroleum reservoirs. No high-salinity fluid-inclusion measurements have yet been reported for the Carlin deposit (Kuehn and Rose, 1987; Rose and Kuehn, 1987). Studies focusing on the character of the organic material indicate that hydrocarbon had matured (to pyrobitumin at Carlin and to graphite at Jerritt Canyon) prior to the introduction of gold-bearing hydrothermal fluids (Rose and Kuehn, 1987; Leventhal et al., 1987). The timing of hydrocarbon introduction and evidence that hydrocarbons had matured prior to mineralization indicate that organic complexes are not a likely transporting agent for gold in Carlin-type deposits. However, mixing of gold-bearing hydrothermal fluids with a pre-existing petroleum reservoir would result in thermal maturation of hydrocarbon (Simoneit, 1983) and would provide a mechanism for gold precipitation. Petroleum generation took place during the Mesozoic (Poole et al., 1983) at which time present-day northeastern Nevada was buried by several kilometers of overlying sediment (Kuehn and Bodnar, 1984 ). Gold was introduced by Tertiary hydrothermal fluids which, after rising through a thick section of marine carbonates, encountered a fossil petroleum reservoir. GEOCHEMICALANALYSES All samples were checked for consistent assignment of rock categories prior to crushing and analysis. This review provided an opportunity to even the distribution of samples between deposits and among rock types. Replicate splits of a standard collected at the Gold Acres mine and a Devils Gate Limestone blank were added to the database suite. Additional splits of the same standard were added to the test suite. All of the samples were renumbered with a randomized number series. A standard sample preparation procedure was followed. Samples were first jaw crushed and then cone crushed to minus 10 mesh. A riffle splitter was used to separate a 250-gr split which was pulverized to minus 100 mesh using ceramic plates. A list of elements analyzed and technique employed is provided in Table 4. All samples were run at a single laboratory. Analytical techniques were chosen to provide a reproducible total value for each element over its expected geochemical range. For instance, a low detection limit was achieved for gold by using a fire-assay preparation procedure followed by atomic adsorption analysis. A total value for barium required analysis using X-ray fluorescence. The

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4

Analytical procedures

Element (lower detection limit )

Analytical procedure

Ba ( 1 ppm) Rb, St, Nb, Zr (5 ppm)

X-ray fluorescence

Loss on Ignition - LOI (0.01% )

Gravimetric analysis after heating to 900 ° C

Au (5 ppb)

Fire assay preparation, atomic absorption finish

Ag (0.5 ppm) Cd, Be, Li, Co, Cr, Cu (1 ppm) Mn, Mo, Ni, Zn, V ( 1 ppm ) Bi (2 ppm) As, Se, Pb, Sb (5 ppm) Ta (8 ppm) Te, W, U, Sn (10 ppm) Total Iron (0.1%), CaO (0.01% ) A1203, K20, MgO, Na20 (0.01%) Si02, Ti02, P205 (0.01%)

Four acid digestion (nitric, perchloric, hydrofluoric, hydrochloric), analysis by direct coupled plasma - atomic emission spectroscopy

Hg (5 ppb)

Hot acid digestion, cold vapor extraction and atomic absorption analysis

Tl (0.05 ppm)

Four acid digestion, organic dissolution and atomic absorption analysis

Total carbon (0.02%)

Induction furnace, gravimetric analysis

Carbonate (0.05 %)

H3P04 extraction, gravimetric analysis

Organic carbon (0.02 % )

By subtraction

Ferrous iron (0.10 % )

Three acid digestion, titration

Ferric iron (0.05 % )

By subtraction

Total sulfur (0.02 %)

Induction furnace, gravimetric analysis

Sulfate (0.05 %)

Na2CO3 extraction, gravimetric analysis

Sulfide (0.02 % )

By subtraction

a l t e r n a t i v e D i r e c t C o u p l e d P l a s m a - E m i s s i o n S p e c t r o g r a p h a n a l y t i c a l techn i q u e relies o n a n acid digestion w h i c h is i n c o m p l e t e for sRmples c o n t a i n i n g barite. T a b l e s 2 a n d 3 s u m m a r i z e a n a l y t i c a l results for t h e s t a n d a r d a n d t h e overall raw g e o c h e m i c a l v a r i a b i l i t y in b o t h t h e d a t a b a s e suite a n d t h e t e s t suite. Results for t h e s t a n d a r d p r o v i d e a c h e c k o n p r e c i s i o n for e a c h e l e m e n t analyzed. B l a n k s a n d a r a n d o m i z e d n u m b e r series p r o v i d e a m e a n s o f d e t e c t i n g c o n t a m i n a t i o n . N o e v i d e n c e o f c o n t a m i n a t i o n was d e t e c t e d in t h e s a m p l e p r e p a r a t i o n or a n a l y t i c a l p r o c e d u r e . A coefficient o f v a r i a t i o n ( s t a n d a r d d e v i a t i o n as a p e r c e n t a g e o f t h e m e a n ) was c a l c u l a t e d for e a c h e l e m e n t using d a t a for repli-

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C.E. NELSON

cate splits of the standard. This value provides a measure of the error introduced during sample preparation and analysis. A low coefficient of variation indicates a narrow distribution of values around the mean. Less reproducible results are indicated by larger coefficients. Elements which report below the detection limit were arbitrarily assigned a value of one-half the detection limit. In the 413-sample database, only tin and tantalum report values less than the detection limit for a large number of samples. Tin and tantalum were removed from the database prior to data reduction. Organic carbon was calculated from the difference between total carbon and carbonate carbon, but was eliminated from the database when standard replicates revealed an unacceptably high coefficient of variation {106). Sulfide sulfur was calculated from the difference between total sulfur and sulfate sulfur. Ferric iron was calculated from the difference between total iron and ferrous iron. FACTOR ANALYSIS

Q-mode factor analysis was used to reduce the 45-element database to a smaller number of objectively derived factors. Each factor is a linearly independent combination of the original geochemical elements. Q-mode factors were derived from a similarity matrix calculated using a cosine theta measure of similarity. In the alternative R-mode procedure, factors are derived from a matrix of correlation coefficients. Both Q-mode and R-mode procedures reduce the raw database to a smaller number of basis vectors or factor axes. Factor axes may then be rotated towards the original element axes (varimax rotation). These techniques are described in standard texts on statistical methods, e.g. Davis (1986). The software used is described in Grundy and Miesch (1989). Q-mode factors represent end-member sample compositions whereas R-mode factors focus on interrelationships between elements. A Q-mode routine was chosen for this study since end-member sample compositions are easier to relate to geologic processes than are groups of interrelated elements. Each geochemical element contributes to one or more factors in proportion to its factor score. Various options are available for data transformation (log base 10, percent of range, standardization, etc.) prior to factor analysis. Standardization, for example, allows percent-level variability in silica to be compared to ppm- or ppb-level variability in other elements such as arsenic or gold. A choice from among these options is governed in part by the resulting factor model. A preferred transformation is one which results in factors which exhibit high scores for a small number of elements. An equal mean and standard deviation transformation, described by Miesch (1980), provided a well defined factor model and was used to standardize all of the geochemical data prior to factor analysis. A ten-factor Q-mode model derived from the 272-sample, 45-element data-

185

G E O C H E M I S T R Y OF JASPEROIDS F R O M CARLIN-TYPE G O L D DEPOSITS, W E S T E R N U.S. TABLE

5

Factor model for the database suite Factors

1

2

3

4

5

6

7

8

9

10

Cr Si02 Li P20s Bi FeO Ag Sb Mo Hg Au

CaO C03 LOI Ctot MgO (SiO2)

Zr K20 A1203 Ti02 Rb Nb V Ni MgO Co Cu (Na20)

S04 Stot Ba Sr Nb

T1 Fe208 Fetot As Pb P2Os (Na20) (FeO) (MgO)

W Mn Be Co (FeO)

Zn Cd Ni U Mo V (Nb) (Li)

Ag Sb T1 Te (FeO) (Mo) (Cr) (P2Os)

Pb Zr V Ba MgO (Mo) (Co) (Na20) (FeO) ( Sulfide ) (Li)

Bi Au Na20 Hg As (Pb) (Be) (Mo) (Li)

1.3

1.7

1.0

0.6

0.4

0.5

91.7

93.4

94.4

95.0

95.4

96.0

Variance accounted for by each factor:

35.5

25.8

22.7

6.4

Cumulative variance accounted for:

35.5

61.3

84.0

90.4

Major contributing elements to each factor are listed in order of decreasing factor score. Elements with negative factor scores are shown in parentheses.

base sample suite is presented in Table 5. The major contributing elements to each factor are listed in order of decreasing factor score. Only the highest scoring elements are listed. Negative factor scores, indicated by parentheses, indicate that geochemical variance for that element increases in a direction opposed to the direction of increase for positive scoring elements. Variance accounted for by each factor is also shown along with cumulative variance accounted for as additional factors are added. A ten factor model accounts for 96% of the overall geochemical variance (total sum of squares) in the 45-element database. Factor I is heavily influenced by SiO2 and accounts for more variance (35.5%) than any other factor. Factor 1 is interpreted as hydrothermal silicification, since most of the samples in the database are jasperoid (Table 1). Elements which accompany hydrothermal silica include Cr, Li and P2Os plus Au, Ag, Hg and Sb. These elements also score high on factors 7, 8, 9 and 10. Factor 2 is composed of CaO, MgO and carbonate carbon and accounts for almost 26% of the variance in the database. It likely represents the variably dolomitized carbonate fraction of the host rock. No minor elements occur on this factor. However, dolomitization may have played an indirect role in the formation of ore by creating a permeable trap for hydrocarbon enrichment.

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TABLE 6

Metal content of marine black shales Element

Average black shale

Highest values in metalliferous black s h a l e s

Metal-rich black shale

Vinini F m . Co,g ( % )

Ag ( p p m ) As Cd Co Cr Cu

(ppm) (ppm) (ppm) (ppm) (ppm) Pb ( p p m ) Mo (ppm) Ni (ppm) U (ppm) V (ppm) Zn ( p p m ) Mn (ppm) Ba (ppm)

3.2 < 1

10 100 70 20 10 50 150 < 300 150 300

5 . 20 500 150 70 150 200 20 700 1000 700 700

Woodruff Fm.

22 5 .

11.5 3 105 .

170 150 200 4400 1800 -

Chainman Shale

15 -

. 500 < 200 700 1330 55 3140 18000 -

200 700 20 150 200 3000 3000 150 -

Data compiled from Desborough and Poole ( 1 9 8 3 ) , Poole and Desborough ( 1 9 8 5 ) , Vine and Tourtelot ( 1 9 7 0 ) and Davidson and Lakin ( 1 9 6 2 ) . Values not reported are shown as dashes ( - ) .

Factor 3 has high scores for K 2 0 , MgO, Rb, V, and A1203, a group of elements enriched in sheet silicates plus minor elements such as TiO2, Nb and Zr, suggestive of refractory minerals such as rutile and zircon. Factor 3 accounts for 23% of the overall geochemical variance in the database and probably represents the fine-grained detrital component of argillaceous marine sedimentary host rocks. Factors 2 and 3 together account for close to 50% of the overall geochemical variability in the database. However, these factors account for a very small proportion of the geochemical variability in members of the epithermal suite (Au, Ag, As, Sb, Hg, Ba and T1). Geochemical variability of minor elements and metals is largely accounted for by factors 4 through 10. Factor 4 is essentially Ba plus sulfate sulfur; it loads heavily on hydrothermal barite-cemented veins and breccias. Factor 5, composed of As, Pb, T1 and ferric iron, is particularly well developed in oxidized jasperoid breccias. Factors 6 and 7 contain Mn, Co, Zn, Cd, Ni, U, Mo and V, members of a suite of metals that Poole and Claypool (1984), Poole and Desborough (1985), Desborough and Poole (1983), and Desborough et al. (1987) report are concentrated in kerogen-rich shales of the Great Basin. Data on the metal content (other than gold) of marine black shales are provided in Table 6. A reasonable average figure for gold is 14.7 ppb (range 6.7-65 ppb) taken from 529 analysis of carbonaceous shales reported by Korobeynikov (1986).

GEOCHEMISTRY OF JASPEROIDS FROM CARLIN-TYPE GOLD DEPOSITS, WESTERN U.S.

0 20 40 kilometers

r

187

~ ~t " l q ) i

''~

'

Fig. 4. Outcrop distribution of rock units which contain metalliferous marine black shales in northeastern Nevada. Western assemblage rock units including the Valmy, Comus, Vinini, and Woodruff Formations are shown in a solid pattern. The Chainman Shale is shown in a stippled pattern. Carlin-type ore deposits are numbered as follows:Getchell (1), Pinson (2), Jerritt Canyon (3), Dee (4), Bootstrap (5), Goldstrike (6), Bluestar (7), Carlin (8), Gold Quarry and Maggie Creek (9), Rain (10), Gold Acres (11), Cortez (12), Horse Canyon (13), Tonkin Springs (14), Windfall (15), Alligator Ridge (16), Victorine (17), and Northumberland (18). Five factors (6, 7, 8, 9 and 10) account for less than 5% of the total sum of squares, b u t account for 75% of the variance in gold. Factor 8 looks initially like a strong epithermal factor (Ag, Sb, T1), but is also heavily influenced by Mo, Cr and P205. Factor 9 contains Li, Na, FeO, sulfide sulfur, Co, Pb, Mo, V, Ba and Zr. Factor 10, which accounts for most of the gold variability in the database, has high scores for Bi, Na20, Pb, Mo and Li. Factors 6 through 10 together document an association of epithermal elements (including gold) with a geochemical suite characteristic of metalliferous marine black shales {Li, P, Mn, Ba, Mo, Cr, Co, V, Bi, Cd, Ni, U, Zn and P b ) .

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C.E. NELSON

This association suggests that a stratigraphic section containing metalliferous marine black shales provides a source of gold for Carlin-type orebodies. The hypothesis is supported by a close correspondence between Carlin-type orebodies and the outcrop distribution of rock units containing metalliferous marine black shales (Figs. 1 and 4). Hydrothermal solutions which circulate through a favorable source rock sequence leach gold plus the distinctive suite of minor elements. Hydrothermal fluids which do not circulate through a favorable source rock, or which are chemically incapable of transporting significant quantities of gold, form barren systems. These conclusions are supported by: (1) the association of gold, silver and related epithermal elements with a suite of elements characteristic of metalliferous marine black shales (Tables 5 and 6); (2) the observation that Carlintype orebodies are spatially related to formations which contain metalliferous marine black shales (Figs. 1 and 4); and (3) the observation that a suite of elements which are enriched in metalliferous marine black shales can be used to distinguish jasperoids sampled at barren Carlin-type systems from jasperoids sampled at Carlin-type orebodies (Figs. 5 through 8 and following section on Discriminant Analysis). There is low potential, according to this model, for Carlin-type gold discoveries in southeastern Nevada and western Utah. DISCUSSION REGARDING METALLIFEROUS MARINE BLACK SHALES

The Ordovician Vinini Formation and the correlative Valmy Formation contain metalliferous marine black shales (Poole and Desborough, 1985) and are an established source of petroleum in the Great Basin (Poole et al., 1983). These formations and lithologically equivalent Devonian units, including the Comus Formation, Woodruff Formation, and Slaven Chert, consist of siliceous (western assemblage) marine sediments. All were deposited on the continental rise or in a marginal ocean basin along the west side of early and middle Paleozoic North America. A1 were thrust eastward by approximately 150 km during the Late Devonian and Early Mississippian Antler orogeny. In detail, the Valmy-Vinini sequence includes argillite, quartzite and chert with lesser greenstone, tuff and feldspathic quartzite. Minor stratabound leadzinc mineralization is reported (Ketner, 1983) as well as bedded barite and minor limestone. Greenstone and stratabound syngenetic base-metal sulfide mineralization represent a possible source of gold and indicate an environment where associated sediments, including black shales, are likely to be metalliferous. The vast majority of Carlin-type reserves are contained in deposits which are spatially associated with western assemblage rock units (Figs. 1 and 4). The Mississippian Chainman Shale is locally enriched in metals (Table 6) and is also an established source of petroleum in the Great Basin (Poole et al., 1983). However, in contrast to western assemblage rock units, the Chainman Shale was deposited in a flysch trough and pro-delta basin that developed east

GEOCHEMISTRY OF JASPEROIDS FROM CARLIN-TYPE GOLD DEPOSITS, WESTERN U.S.

189

of the Antler orogenic highland. Lithologies vary from mudstone and calcareous shale to siltstone with up to 5% organic carbon. The Chainman Shale contains no greenstones or stratabound base-metal sulfides such as those found in the Vinini, Valmy and Woodruff Formations. A few of the smaller Carlintype gold deposits, such as Alligator Ridge, are spatially related to the Chainman Shale. Marine shales characterized by relatively fast sedimentation rates (e.g., Chainman Shale) seem to be poor candidates for metal concentration and consequently make less attractive source rocks for gold. In contrast, a slow sedimentation rate (e.g., Vinini Formation) favors concentration of metals from seawater. The Chainman Shale and correlative units in the eastern Great Basin contain a small fraction of the metal concentrated in the eugeosynclinal setting represented by the Vinini, Valmy, and Woodruff Formations. Mercur, located far to the east of most Carlin-type orebodies, may have derived its gold from the Lower Mississippian Deseret Limestone, a locally metalliferous, phosphatic shale deposited in a starved basin along the western margin of the Mississippian carbonate platform. Hydrothermal leaching of gold and associated elements from a rock sequence containing metalliferous marine black shales results in a fluid capable of forming a gold orebody. A permeable reservoir enriched in organically derived hydrocarbon provides a favorable environment for gold precipitation. Evidence presented here suggests that Carlin-type gold orebodies form where gold-bearing hydrothermal fluids encounter a pre-existing petroleum reservoir. Alternative interpretations are certainly possible. For example, the metalliferous marine black shale geochemical suite represents a group of elements which are weakly adsorbed by hydrocarbons. Factors 6 through 10 (Table 5) could be the result of metal adsorbtion on hydrocarbons at the site of gold deposition rather than a result of metal leaching. This interpretation avoids the question of gold source, but helps to explain the existence of barren systems which occur close to Carlin-type orebodies. Barren systems may simply lack a pre-existing petroleum reservoir. Rock packages containing metalliferous marine black shales do provide a source of petroleum, could provide a source of metal, and probably provide both. Both models account for the spatial association between Carlin-type orebodies and rock units which contain metalliferous marine black shales (Figs. 1 and 4). DISCRIMINANT ANALYSIS

Quantitative application of a metalliferous marine black shale geochemical suite to exploration requires testing of its reliability. The following section describes a discriminant function derived from the database suite and applied

190

C.E.NELSON

to a test suite ofjasperoids from two additional orebodies and two additional barren systems. Discriminant analysis establishes whether separation exists between two known sample populations and provides a function which describes that separation. The two known sample populations in this application are jasperoids from barren Carlin-type systems and jasperoids from Carlin-type orebodies. The software used is provided in Davis (1986). Figure 5 shows that discriminant analysis can be used to distinguish jasperoids sampled at barren systems from jasperoids sampled at orebodies. All of the jasperoid samples in the database suite are correctly assigned by a 45-element function. Geochemical data used to generate Figure 5 were log base-10 transformed prior to discriminant analysis. Chi-square tests show that log base-10 transformed data more closely approximate a normal distribution for 36 of the 45 elements in the database. A log base-10 transformation does not eliminate the compromises inherent in applying discriminant analysis to a database that includes normal, log normal and mixed populations. However, selective transformation of individual elements was not adopted since chi-square results are sensitive to the addition or subtraction of small numbers of samples to the database. A log base-10 transform for all elements was judged preferable to selected transformations. Samples from each of the three jasperoid categories (replacement, fragmental and episodically silicified jasperoids) were plotted separately. Replacement jasperoids are distinguished as effectively as fragmental jasperoids, which are distinguished as effectively as episodically silicified veins and breccias. Jasperoids are the only sample category that show reliable separation. There is no significant separation in the host rock or the altered rock group for barren systems versus orebodies. Apparently, the composition of the unaltered host rock at orebodies is not different from the unaltered host rock at barren systems. The 45-element discriminant function used to plot Figure 5 was reduced to 15

l

I

I

I

60

70

No Q.

E 10--

J~

5"-

s

Z

0

30

40

50

80

90

Discriminant function value

Fig. 5. Discriminantplot for the database suite using log base- 10 transformed data for 45 elements. The stipled pattern represents 97 jasperoid samples from four orebodies. The cross-hatched pattern represents 45 jasperoid samples from two barren systems. The midpoint between mean values for the two samples populations is Ro.

GEOCHEMISTRYOFJASPEROIDSFROMCARLIN-TYPEGOLDDEPOSITS,WESTERNU.S.

191

8 elements for application to exploration. An 8-element function correctly assigns 86% of jasperoid samples from the database suite (Fig. 6). Elements which contribute to the 8-element function include Au, Ag, Sb, Mo, Ba, Ni, P and Mn. A test of the 8-element discriminant function using samples collected at additional Carlin-type systems is shown in Figure 7. All 21 jasperoid samples from the Carlin deposit are correctly assigned. At the smaller and lower grade Horse Canyon deposit, 19 out of 21 jasperoid samples are correctly assigned. The smaller size and lower grade of the Horse Canyon orebody and the distance of some jasperoid samples from ore (up to 850 m) may have contributed to a weaker signature for the two incorrectly assigned samples. Maximum gold values in the jasperoid suites were 0.75 ppm at Carlin and 1.15 ppm at Horse Canyon. Jasperoids collected from the two barren test systems (Fig. 8) plot over the entire range of the discriminant function. Although the plot is distinctly different from that generated for two Carlin-type orebodies (Fig. 7), jasperoid samples from the two barren systems are not clearly discriminated. An explanation may be that all four test systems are partially hosted by western assemI

10

I

I I

I

I

0

3

6

E

5 5 Z

o

m

-9

-6

-3

Discrirninant function value

Fig. 6. D iscriminant plot for jasperoid samples of the database suite using log base-10 transformed data for 8 elements. See Fig. 5 for an explanation of symbols.

ii I -9

' -6

o7 -3

0

3

6

9

Dlacrlmlnant function value

Fig. 7. Discriminant plot for a test suite of 42 jasperoid samples from the Carlin and Horse Canyon orebodies.

o3. •Ea o E

~- o

_~ Oiacrlminant function value

Fig. 8. Discriminant plot for a test suite of 42 jasperoid samples from two barren systems.

192

C.E.NELSON

blage rock units (the proposed source rock for gold) whereas the six systems sampled for the database suite are hosted by eastern assemblage rock units. The barren test systems apparently failed for reasons other than absence of an appropriate source rock; both are located in established mineral belts close to producing Carlin-type deposits. The hydrothermal solution may have been chemically incapable of transporting significant quantities of gold or the host environment may have lacked a pre-existing concentration of organically derived hydrocarbon. In summary, an 8-element function reliably discriminates betweens jasperoids collected from four orebodies and two barren, lower plate hosted systems (Fig. 6). The function also reliably assigns jasperoids collected as a test from two additional orebodies, but is less reliable in its assignment of jasperoids from two barren, western assemblage hosted, test systems. Results of the test indicate that jasperoid samples collected from Carlin-type prospects with an associated gold orebody will be reliably discriminated as ore-related. Jasperoid samples collected from barren prospects will plot as barren or, in the case of barren systems hosted by western assemblage rock units, will fail to cluster at all. A minimum of twenty jasperoid samples should be taken at each prospect in order to produce an interpretable plot. Quantitative application of a discriminating geochemical signature requires careful monitoring of data quality. A single, carefully homogenized standard should be used throughout the process of discriminant function development, testing and application. This procedure permits evaluation of data quality both within and between sample submittals. The eight elements used to construct Figure 6 are reproducible within and vary by less than 15% between sample submittals (Tables 2 and 3 ). The discriminant function used to plot Figures 6, 7 and 8 is not the only function capable of distinguishing jasperoids collected at barren systems from jasperoids collected at orebodies. Coefficients will change if a different database is used, if analytical procedures are changed, or if new elements are substituted. A1 of the elements which appear on factors 6 through 10 (Table 5) are potential contributors. Useful discriminant functions will include elements from the metalliferous marine black shale geochemical suite (Li, P, Mn, Ba, Mo, Cr, Co, V, Bi, Cd, Ni, U, Pb, Zn) supplemented by elements from an epithermal geochemical suite (Au, Ag, As, Sb, Hg, Ba, T1). CONCLUSIONS Exploration for Carlin-type gold deposits in the western United States relies heavily on analysis for a suite of epithermal elements which turn out to be as anomalous in jasperoids collected from many barren systems as they are jasperoids collected from orebodies. Multielement geochemical analysis of 272 rock samples collected at four orebodies and two barren systems provides a

GEOCHEMISTRY OF JASPEROIDS FROM CARLIN-TYPE GOLD DEPOSITS, WESTERN U.S.

193

database which can be used to improve upon existing tools for evaluating ore potential. Q-mode factor analysis was used to reduce a 45-element database suite. Five of ten factors generated document an association between an epithermal suite of elements (Au, Ag, As, Sb, Hg, Ba, T1 ) and a geochemical suite including Li, P, Mn, Ba, Mo, Cr, Co, V, Bi, Cd, Ni, U, Zn and Pb. This association suggests a genetic link between Carlin-type orebodies and metalliferous marine black shales. One interpretation of the data presented here is that mineralizing hydrothermal systems leach gold from a rock sequence which contains metalliferous marine black shales. Alternatively, the metalliferous marine black shale geochemical signature may be a response to adsorption of gold on pre-enriched, organically derived hydrocarbon at the site of gold deposition. Either or both processes may be responsible for a geochemical signature that distinguishes jasperoids sampled at Carlin-type gold deposits from jasperoids sampled at barren Carlin-type systems. An 8-element discriminant function was derived and tested for its applicability as an exploration tool. Forty of 42 jasperoid samples collected at two Carlin-type orebodies were correctly identified as ore related. Only one of the samples contained over one ppm gold. The 8-element function is composed of Au, Ag, Sb, Mo, Ba, Ni, P and Mn. Evaluation of ore potential using a discriminant function for jasperoids could be particularly helpful when information from other sources is incomplete. A positive jasperoid signature might provide the encouragement needed to continue assessment of Carlin-type systems that have been abandoned after a few discouraging drill holes. The discriminant function can also be used to evaluate untested targets and targets which are only partially exposed. Overall, application of a discriminant function to exploration should help to focus exploration efforts on those systems which have high potential for an associated gold orebody. ACKNOWLEDGEMENTS This study was funded during 1984 and 1985 by The Superior Oil Company and during 1986 and 1987 by Arctic Precious Metals, Inc. I am grateful to both companies, and especially to Robert Perkins, for financial support and permission to publish these results. I would also like to thank the the mine staffs and corporate owners of Alligator Ridge, Carlin, Cortez, Horse Canyon, Jerritt Canyon, and Mercur for permission to visit and sample their deposits. Russell Honea did the petrographic work. Bondar-Clegg and Company, Ltd. performed the geochemical analyses. Substantial improvements to the text were made in response to reviews by William Bagby, Colin Farrelly, Richard

194

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Glanzman, Jeffrey Hedenquist, George Koch, Sherman Marsh, Forrest Poole and Art Rose. REFERENCES Anderson, .A., 1981. Gold - Its history and role in the U.S. economy and Homestake Mining Company's U.S. exploration program. Am. Mineral. Cong. presentation, Denver, CO. Bagby, W.C. and Berger, B.R., 1986. Geologic characteristics of sediment-hosted, disseminated precious metal deposits in the western United States. In: B.R. Berger and P.M. Bethke (Editors), Geology and Geochemistry of Epithermal Systems. Soc. Econ. Geol., Rev. Econ. Geol., 2: 169-202. Birak, D.J. and Hawkins, R.B., 1985. The geology of the Enfield Bell mine and the Jerritt Canyon district, Elko County, Nevada. U.S. Geol. Surv., Bull., 1646: 95-106. Coppinger, W.W., 1986. Geology of the Horse Canyon disseminated gold deposit, Eureka County, Nevada. Geol. Soc. Am. Abstr. Programs, 18: 547. Davidson, D.F. and Lakin, H.W., 1962. Metal content of some black shales of the western conterminous United States. U.S. Geol. Surv., Prof. Pap. 450-C, 74 pp. Davis, J.C., 1986. Statistics and Data Analysis in Geology. Wiley, New York, NY, 646 pp. Desborough, G.A. and Poole, F.G., 1983. Metal concentrations in some marine black shales of the United States. In: W.C. Shanks (Editor), Unconventional Mineral Deposits. Soc. Min. Eng., London, pp. 99-110. Desborough, G.A., Poole, F.G., Hose, R.K. and Green, G.N., 1987. Metalliferous oil shale in the Upper Devonian Gibellini facies of the Woodruff Formation, southern Fish Creek Range, Nevada. In: A.L. Bush (Editor), Contributions to Mineral Resource Research. U.S. Geol. Surv., Bull., 1694-H: 91-104. Erickson, R.L. and Marsh, S.P., 1974. Geologic map of the Iron Point Quadrangle. U.S. Geol. Surv., Geol. Map Set., GQ-1175. Grundy, W.D. and Miesch, A.T., 1988. Brief descriptions of STATPAC and related statistical programs for the IBM Personal Computer. U.S. Geol. Surv., Open File Rep. 87-411-A, 24 pp. Hausen, D.M. and Park, W.C., 1986. Observations on the association of gold mineralization with organic matter in Carlin-type ores. In: W.E. Dean (Editor), Proceedings of the Denver Region Exploration Geologists Symposium - Organics and Ore Deposits. Denver Region Explor. Geol. Soc., Denver, CO, pp. 119-136. Hofstra, A.H., Landis, G.P. and Rowe, W.A., 1987. Sediment-hosted disseminated gold mineralization at Jerritt Canyon, Nevada. IV - Fluid geochemistry. Geol. Soc. Am. Abstr. Programs, 19: 704. Ketner, K.B., 1983. Strata-bound, silver-bearing iron, lead, and zinc sulfide deposits in Silurian and Ordovician rocks of allochthonous terranes, Nevada and northern Mexico. U.S. Geol. Surv., Open File Rep. 83-792. Korobeyniko, A.F., 1986. Gold distributions in black shale associations. Geochem. Int., 23 {5): 114-124. Kuehn, C.A. and Bodner, R.J., 1984. P-T-X characteristics of fluids associated with the Carlin sediment-hosted gold deposit. Geol. Soc. Am., Abstr. Programs, 16: 566. Kuehn, C.A. and Gize, A.P., 1985. Textural and P-T-X characteristics of the hydrocarbon-bearing stages of the paragenesis at Carlin, Nevada. Geol. Soc. Am., Abstr. Programs, 17: 635. Kuehn, C.A. and Rose, A.W., 1987. Temporal relationships between hydrocarbon introduction, maturation and gold mineralization/alteration at Carlin, Nevada: A tale of two fluids. Am. Inst. Mining, Metal. Pet. Eng., Abstr. Programs, p. 38. Lenzi, G.W., 1973. Geochemical reconnaissance at Mercur, Utah. Utah Geol. Mineral. Surv., Spec. Stud., 43, 16 pp.

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Leventhal, J.S., Hofstra, A.H., Vuletich, A.K. and Mancuso, T.B., 1987. Sediment-hosted disseminated gold mineralization at Jerritt Canyon, Nevada. I I I - Role of organic carbon. Geol. Soc. Am., Abstr. Programs, 19: 745. Miesch, A.T., 1980. Scaling variables and interpretation of eigenvalues in principal components analysis of geologic data. Math. Geol., 12: 523-538. Poole, F.G. and Claypool, G.E., 1984. Petroleum source-rock potential and crude-oil correlation in the Great Basin. In: J. Woodward, F.F. Meissner and J.L. Clayton (Editors), Hydrocarbon Source Rocks of the greater Rocky Mountain region. Rocky Mtn. Assoc. Geol., pp. 179-229. Poole, F.G. and Desborough, G.A., 1985. Metal concentrations in marine black shales. In: K. Krafft (Editor), USGS Research on Mineral Resources - 1985. U.S. Geol. Surv., Circ., 949: 43-44. Poole, F.G., Claypool, G.E. and Fouch, T.D., 1983. Major episodes of petroleum generation in part of the northern Great Basin. Geothermal Res. Council, Spec. Rep., 13: 207-213. Radtke, A.S., Rye, R.O. and Dickson, F.W., 1980. Geology and stable isotope studies of the Carlin gold deposit, Nevada. Econ. Geol., 75: 641-672. Rose, A.W. and Kuehn, C.A., 1987. Ore deposition from acidic CO2-rich solutions at the Carlin gold deposit, Eureka County, Nevada. Geol. Soc. Am., Abstr. Programs, 19: 824. Simoneit, B.R.T., 1983. Organic matter maturation and petroleum genesis - Geothermal versus hydrothermal. Geothermal Resour. Council, Spec. Rep., 13: 215-241. Stewart, J.H. and Carlson, J.E., 1978. Geologic map of Nevada. U.S. Geol. Surv. and Nev. Bur. Mines Geol. Tapper, C.J., 1986. Geology and genesis of the Alligator Ridge Mine, White Pine County, Nevada. Nevada Bur. Mines Geol., Rep., 40: 85-103. Vine, J.D. and Tourtelot, E.B., 1970. Geochemistry of black shale deposits - a summary report. Econ. Geol., 65: 253-272. Wells, J.D., Stoiser, L.R. and Elliott, J.E., 1969. Geology and geochemistry of the Cortez gold deposit, Nevada. Econ. Geol., 64: 526-537. Wilkins, J., Jr., 1984. The distribution of gold- and silver-bearing deposits in the Basin and Range Province, western United States. Ariz. Geol. Soc. Dig., 15: 1-27.