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Corresponding author: e-mail, eholley@mines.edu

Present address: Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401.

Abstract

The Marigold Au deposits are located in the Battle Mountain mining district at the northern end of Nevada’s Battle Mountain-Eureka trend. The Marigold deposits currently make up the second largest Au accumulation in the district with over 320 tonnes (10.35 Moz) of Au in oxidized rock in a N-trending series of mineralized zones approximately 7.5 km long. Ore is hosted primarily in oxidized Paleozoic siliciclastic rocks between the Roberts Mountain and Golconda thrusts. Most of the ore occurs in quartzite of the Ordovician Valmy Formation. Higher grades but lower tonnages of ore are present in the overlying Pennsylvanian-Permian Antler sequence, including the Battle Formation conglomerate, the Antler Peak Limestone, and debris flows and siltstone of the Edna Mountain Formation. Sedimentary rocks at Marigold are crosscut by a series of WNW- to N-striking quartz monzonite dikes (zircon U-Pb chemical abrasion-thermal ionization mass spectrometry ages 97.63 ± 0.05–92.22 ± 0.05 Ma) and a lamprophyre (biotite 40Ar/39Ar age 160.7 ± 0.1 Ma).

Marigold displays many classic Carlin-type characteristics although the deposits are predominantly hosted in relatively unreactive, carbonate-poor siliciclastic rocks. Sulfidation, minor silicification, and possibly pyritization occurred in association with Au mineralization in quartzite and argillite. Chemically reactive but volumetrically minor carbonate rocks also display these alteration styles as well as significant decarbonatization. Argillic alteration occurred proximal to faults in mudstone and siltstone and at the margins of intrusions. Gold, As, Sb, and Tl are enriched along high-angle structures and structural intersections in the sedimentary host rocks and in faulted dike margins. Gold is present in Au-, As-, and Sb-rich pyrite overgrowths on pre-gold stage trace element-poor pyrite grains. Oxidation extends to depths of 150 to 500 m below surface, and above the redox boundary Au is present natively with iron oxides in voids and fractures. In the cores and margins of the Cretaceous dikes and fault zones, a distinct geochemical association of base metal and Ag minerals is identifiable, characterized by Ag-bearing tetrahedrite-tennantite, chalcopyrite, gersdorffite, pyrite, sphalerite, stannite, and galena.

Sericite 40Ar/39Ar ages of 88.0 ± 0.46 and 79.59 ± 0.16 Ma indicate that hydrothermal alteration occurred along the dike margins at least 4 m.y. after emplacement. On the basis of similarities to other deposits in the district, the base metal and Ag mineralization may have occurred at this time. The Au mineralization occurred sometime after the base metal and Ag event, possibly in conjunction with the Eocene magmatism that occurred elsewhere in the district, although this study found no definitive evidence for a magmatic-hydrothermal origin of the Au.

Introduction

Marigold is located in the Battle Mountain mining district at the northern end of Nevada’s Battle Mountain-Eureka trend. This trend (Fig. 1) is a conspicuous alignment of ore deposits containing in excess of 2,800 tonnes (t) (90 Moz) Au primarily in disseminated deposits hosted in sedimentary rock, including Carlin-type and porphyry-related systems (Berger et al., 2014; Davis and Muntean, 2014). In the Battle Mountain district, many Au deposits (Fig. 2) are spatially associated with Jurassic, Cretaceous, or Eocene intrusions, and several are clearly porphyry related, including skarns at Copper Canyon (Theodore et al., 1973) and the Converse project (Cleveland, 2000), distal disseminated and skarn mineralization at Buffalo Valley (Kizis et al., 1997; Reid et al., 2010; Fig. 2), and polymetallic porphyry deposits at Buckingham, Copper Basin, and Elder Creek (King, 2011). Several of the deposits in the Battle Mountain district have been described as distal disseminated, including Buffalo Valley, Lone Tree, Trenton Canyon, North Peak, and Marigold (Fig. 2; Doebrich and Theodore, 1996;Theodore, 1998, 2000; Reid et al., 2010).

Fig. 1.

Location of the Marigold mine in north-central Nevada relative to major mineral trends; modified after Wallace et al. (2004).

Fig. 1.

Location of the Marigold mine in north-central Nevada relative to major mineral trends; modified after Wallace et al. (2004).

Fig. 2.

(A) General stratigraphic sequence in the northern Battle Mountain district. (B) Geologic map of the northern Battle Mountain mining district, showing the locations of major deposits; modified after Theodore (1998) with additional data from mapping in this study and unpublished data from Marigold Mining Company. Rocks historically described as Scott Canyon Formation belong to the Valmy and Harmony Formations (Ketner, 2008, 2013) and are shown as Valmy-Harmony Formation. (C) From north to south, deposits at Marigold (filled black circles) include 5 North, 8 North, Terry North, 8 South, Old Marigold, Terry, Mackay, Target 1, Target 2, Antler, and Basalt. (D) Location of core drill holes with samples discussed in text. Drill holes from the 2014–2015 deep drilling program are shown with black markers. (E) Location of intrusions (pink squares) identified in this study that are too small to be mapped on this scale. A-A′ shows the location of Figure 3. Grid coordinates are UTM (NAD83). Abbreviations: BP = Basalt Pit intrusion, LS = limestone, MP = Mackay Pit intrusion, MS= Moonshine intrusion, TII = Target 2 intrusion.

Fig. 2.

(A) General stratigraphic sequence in the northern Battle Mountain district. (B) Geologic map of the northern Battle Mountain mining district, showing the locations of major deposits; modified after Theodore (1998) with additional data from mapping in this study and unpublished data from Marigold Mining Company. Rocks historically described as Scott Canyon Formation belong to the Valmy and Harmony Formations (Ketner, 2008, 2013) and are shown as Valmy-Harmony Formation. (C) From north to south, deposits at Marigold (filled black circles) include 5 North, 8 North, Terry North, 8 South, Old Marigold, Terry, Mackay, Target 1, Target 2, Antler, and Basalt. (D) Location of core drill holes with samples discussed in text. Drill holes from the 2014–2015 deep drilling program are shown with black markers. (E) Location of intrusions (pink squares) identified in this study that are too small to be mapped on this scale. A-A′ shows the location of Figure 3. Grid coordinates are UTM (NAD83). Abbreviations: BP = Basalt Pit intrusion, LS = limestone, MP = Mackay Pit intrusion, MS= Moonshine intrusion, TII = Target 2 intrusion.

Distal disseminated Au ± Ag deposits are the disseminated equivalents of porphyry-related polymetallic vein deposits (Cox and Singer, 1990; Cox, 1992; Peters et al., 2004). In Nevada they occur in magmatic arcs of Jurassic, Cretaceous, and mid-Tertiary age, as Au and base metal anomalies that form concentrically around causative intrusions (Hofstra and Cline, 2000). These deposits have many similarities with Carlin-type Au deposits, including host rocks, orebody morphology, structural setting, and alteration styles (Hofstra and Cline, 2000). Carlin-type Au deposits are hydrothermal replacement bodies of disseminated ore, predominately hosted in decarbonatized argillaceous limestone of Paleozoic age below major thrusts (Hofstra and Cline, 2000; Cline et al., 2005). Carlin-type Au deposits in Nevada are thought to have formed exclusively during the Eocene, tracking the southwest sweep of magmatism through the region (Arehart et al., 2003; Ressel and Henry 2006; Muntean et al., 2011).There is considerable debate about whether Au in Carlin-type deposits had a magmatic-hydrothermal origin (e.g., Sillitoe and Bonham, 1990; Johnston and Ressel, 2004; Ressel and Henry, 2006; Muntean et al., 2011) or was sourced from the sedimentary host-rock package (e.g., Ilchik and Barton, 1997; Emsbo et al., 2003; Large et al., 2011).

Johnston and Ressel (2004) proposed that Carlin-type Au deposits represent even more distal expressions of magmatichydrothermal systems than distal disseminated deposits and that a continuum exists between the two deposit types. Both deposit types can occur in carbonate or clastic sedimentary host rocks, with argillization, silicification, and decarbonatization during deposit formation. However, relative to Carlin-type deposits, distal disseminated deposits show a more definitive magmatic signature that includes zoning of alteration relative to felsic hypabyssal intrusions, significantly higher Ag/Au ratios, and distinctive hypogene ore mineralogy (e.g., base metal sulfides, native Au and Ag, electrum, Ag sulfides, and Ag sulfosalts; Cox and Singer, 1990; Cox, 1992; Hofstra and Cline, 2000).

Marigold is a group of several Au deposits that together compose the second-largest Au accumulation in the Battle Mountain district. The mine has produced over 100 t (3 Moz) of Au at an average grade of 0.68 g/t; the deposits host over 10.35 Moz of contained Au (mined ounces + resource) in oxidized rock (Silver Standard Resources, 2014, 2015, 2016, 2017). Despite the economic significance of the Marigold deposits, many of their characteristics were unknown or undocumented prior to this study. The Au mineralization was assumed to have a distal disseminated origin based on porphyry-related deposits elsewhere in the district (Doebrich and Theodore, 1996; Theodore, 1998, 2000), porphyritic felsic dikes within the mine area, and sulfur isotope values of barite interpreted to have a magmatic signature. However, the link between Au mineralization and causative intrusions at Marigold had not been conclusively demonstrated. The purpose of study presented here is to characterize the geology of the Marigold deposits in comparison to Carlin-type Au and distal disseminated Au ± Ag deposits. Particular attention was given to observable and measurable features that would indicate whether Au mineralization occurred distal to a causative porphyry system.

Materials and Methods

The present study comprised field mapping and geologic modeling of all current open pits and exposures in the mine area as well as outcrop and float near the mine, logging of 58,000 m of reverse circulation chips and drill core, sampling, whole-rock geochemical analyses, petrography, electron probe microanalyses, and geochronology. These data were supplemented by mine data, including a database of 7,482 multielement analyses of predominantly oxidized core and drill cuttings and over 1,000,000 Au assays of chip samples, as well as historic maps, lithologic logs, and petrographic reports.

Whole-rock geochemistry

All samples in the Marigold Mine multielement database were analyzed for Au by inductively coupled plasma-mass spectrometry (ICP-MS) by third-party laboratories with various detection limits over the course of the 28-yr mine history.

Core holes DDH6008, DDH6018, DDH6029, DDH6066, and DDH6110 were analyzed for 36 elements by ICP-MS by American Assay Laboratories in Sparks, Nevada. Fifty-four samples were collected specifically for whole-rock geochemistry in this study, including open-pit transects of host rock and intrusions, intrusions exposed outside of mining areas, and high-grade core samples from oxidized and unoxidized zones. All of these samples were analyzed for 49 elements by inductively coupled plasma-optical emission spectrometry (ICP-OES), ICP-MS, and instrumental neutron activation analysis (INAA) by Activation Laboratories, Inc. in Ancaster, Ontario, Canada.

Petrography

Fifty-seven samples were selected for petrographic analysis based on field relationships, hand sample observations (alteration, paragenetic relationships, and sulfide mineralogy), and results of whole-rock geochemical analyses. All thin sections were prepared at the Colorado School of Mines, Golden, Colorado. Initial petrographic observations were made in transmitted and reflected light. Samples were subsequently analyzed using a scanning electron microscope (SEM), and a subset of two samples was examined using QEMSCAN automated mineralogy. The SEM analyses were conducted at the Colorado School of Mines using a FEI QUANTA 600i environmental SEM equipped with a PGT energy dispersive X-ray spectrometer attachment. Backscatter electron images were collected at an accelerating voltage of 20 kV, a 5-μm beam width, and a working distance of 9.7 to 10.3 mm. The QEMSCAN analyses were conducted on a Zeiss EVO 50 SEM equipped with four Bruker X275HR silicon drift X-ray detectors using an accelerating voltage of 25kV, a 5-nA specimen current, and working distance of 7 to 50 mm.

Electron probe microanalysis

Electron probe microanalysis (EPMA) were conducted at the U.S. Geological Survey Microbeam Laboratory in Denver, Colorado, on a JEOL 8900 microprobe. The accelerating voltage was 20 kV with a beam current of 50 nA and a focused beam. The following elements were analyzed: Ag, As, Au, Co, Cu, Fe, Hg, Ni, S, Sb, Sn, Tl, and Zn. The associated X-ray lines, detection limits, and standards are presented in Table 1. Counting time was increased for analyses of trace elements.

Table 1.

Representative Electron Probe Microanalysis (EPMA) of Gold-Stage Minerals (values given in wt %)

 EMP settingsMain-stage pyrite
   Drill hole 4919Drill hole 5031
   523.6 m571.5 m472.4 m
Detection limitStandardX-raySample MC-13MC-10  MC-12  
Ag0.0095Silverla0000000000
As0.013GaAsla4.164.316.216.73.943.80.784.223.732.93
Au0.012Goldma0.030.010.020.090.050.070.020.090.10.07
Co0.33Cobaltla0000000000
Cu0.27Copperka0.080.1600.330.190.250.030.30.190.28
Fe0.016Pyriteka44.9543.5141.4743.2844.3643.8244.7343.7843.3243.85
Hg0.017Cinnabarma000000.020000
Ni0.12Milleritela00.1200.2300.290.40.3700.16
S0.0068Pyriteka49.8648.8148.9447.6549.650.5752.150.8949.3850.58
Sb0.0093Sbla00.140.160.040.010.210.010.120.080.21
Sn0.0096Tinla0000000000
Tl0.036TlBrma0.060.140.160.080.190.120.220.110.080.07
Zn0.036Zincka0000000000
Total   99.2297.2196.9998.498.3699.1698.2999.9196.9898.17
 EMP settingsMain-stage pyrite
   Drill hole 4919Drill hole 5031
   523.6 m571.5 m472.4 m
Detection limitStandardX-raySample MC-13MC-10  MC-12  
Ag0.0095Silverla0000000000
As0.013GaAsla4.164.316.216.73.943.80.784.223.732.93
Au0.012Goldma0.030.010.020.090.050.070.020.090.10.07
Co0.33Cobaltla0000000000
Cu0.27Copperka0.080.1600.330.190.250.030.30.190.28
Fe0.016Pyriteka44.9543.5141.4743.2844.3643.8244.7343.7843.3243.85
Hg0.017Cinnabarma000000.020000
Ni0.12Milleritela00.1200.2300.290.40.3700.16
S0.0068Pyriteka49.8648.8148.9447.6549.650.5752.150.8949.3850.58
Sb0.0093Sbla00.140.160.040.010.210.010.120.080.21
Sn0.0096Tinla0000000000
Tl0.036TlBrma0.060.140.160.080.190.120.220.110.080.07
Zn0.036Zincka0000000000
Total   99.2297.2196.9998.498.3699.1698.2999.9196.9898.17
ElementGold-poor, arsenic-rich pyriteGold-poor, arsenic-poor pyrite 
Drill hole5031491950315031 
m Sample471.7 MC-5472.4 MC-12523.6 MC-13 523.6 MC-13 571.5 MC-10471.7 MC-5472.4 MC-12Detection limit
Ag0000000000000000.0095
As27.838.4519.1838.1636.80000000000.150.013
Au0000000.0100000.010000.012
Co0000000000000000.33
Cu0.15000000000000.0400.050.27
Fe37.1235.4741.0235.7836.946.9846.1346.2446.4646.0946.8546.7345.945.4145.970.016
Hg0000000000000000.017
Ni0.270.130.310.16000.640.680.1300.1300000.12
S36.0523.5640.9424.1425.1453.8152.7352.7753.7952.5853.7953.4753.5952.1652.940.0068
Sb0.030.140.140.01000000.02000000.0093
Sn0000000000000000.0096
Tl0.110.080.160.070.120.150.150.260.080.070.120.160.130.120.180.036
Zn0000000000000000.036
Total101.5397.85101.7898.3499.0799.6399.82100.78100.9599.399.6799.96100.67100.85101.16 
ElementGold-poor, arsenic-rich pyriteGold-poor, arsenic-poor pyrite 
Drill hole5031491950315031 
m Sample471.7 MC-5472.4 MC-12523.6 MC-13 523.6 MC-13 571.5 MC-10471.7 MC-5472.4 MC-12Detection limit
Ag0000000000000000.0095
As27.838.4519.1838.1636.80000000000.150.013
Au0000000.0100000.010000.012
Co0000000000000000.33
Cu0.15000000000000.0400.050.27
Fe37.1235.4741.0235.7836.946.9846.1346.2446.4646.0946.8546.7345.945.4145.970.016
Hg0000000000000000.017
Ni0.270.130.310.16000.640.680.1300.1300000.12
S36.0523.5640.9424.1425.1453.8152.7352.7753.7952.5853.7953.4753.5952.1652.940.0068
Sb0.030.140.140.01000000.02000000.0093
Sn0000000000000000.0096
Tl0.110.080.160.070.120.150.150.260.080.070.120.160.130.120.180.036
Zn0000000000000000.036
Total101.5397.85101.7898.3499.0799.6399.82100.78100.9599.399.6799.96100.67100.85101.16 

Geochronology

Biotite grains from a single lamprophyre sample (6029-L) were separated and dated using 40Ar/39Ar geochronology at the New Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico. Methods followed those described in detail in McIntosh et al. (2003). Zircon grains from four altered quartz monzonite intrusions were separated for U-Pb analysis using standard crushing, grinding, and concentrating techniques. Single grains were analyzed by laser ablationinductively coupled plasma-mass spectrometry (LA-ICP-MS) to screen zircon age populations prior to high-precision analysis of select grains by chemical abrasion-isotope dilutionthermal ionization mass spectrometry (CA-TIMS). All analyses were conducted at the Isotope Geology Laboratory at Boise State University, Idaho, with CA-TIMS analysis using a method modified from Mattinson (2005). Weighted mean 206Pb/238U ages were calculated using Isoplot 3.0 (Ludwig, 2003), and uncertainties are presented at the 2σ level. The internal error of the weighted mean average is based on analytical uncertainty only and includes the error associated with blank and common lead, tracer solution, and counting statistics. Dates of weighted mean averages are given in the format age ± internal error.

Regional Geology

Three primary tectonostratigraphic assemblages are present in the Battle Mountain district (Fig. 2):

  1. Ordovician eugeoclinal siliciclastic rocks of the Roberts Mountain allocthon, including the Valmy Formation, which were thrust over the miogeoclinal carbonate sequence during the Devonian to Mississippian Antler orogeny (Roberts, 1964);

  2. autochthonous Pennsylvanian to Permian shallow water facies of the Antler sequence, consisting of chemical sediments and clastic rocks shed into a foreland basin that developed in response to lithostatic loading during the Antler orogeny (Cline et al., 2005; McGibbon, 2005);

  3. Mississippian to Permian deep-water siliciclastic rocks and basalts of the Golconda allocthon, composing the Havallah sequence, which were thrust on top of the overlap sequence during the Permo-Triassic Sonoma orogeny (Theodore, 2000).

A recent deep drilling program has identified the presence of Cambro-Ordovician miogeoclinal carbonate shelf-slope rocks, such as the Preble Formation (H. Cook, unpub. data, 2015), described in subsequent sections.

Much of the Au in Nevada’s Carlin-type deposits is localized in carbonate rocks below major thrusts, leading to the conclusion that thrusting of siliclastic units over carbonates during the Antler and Sonoma orogenies may have concentrated Au-bearing hydrothermal fluids in the reactive rocks beneath the thrusts during subsequent mineralizing events (Cline et al., 2005; Emsbo et al., 2006). In contrast, the ore mined at Marigold is primarily hosted in siliciclastic rocks between the Roberts Mountain and Golconda thrusts. The stratigraphic position of the deposits and the siliciclastic-dominated host rocks may partly explain why Marigold has not previously been closely compared with Carlin-type deposits.

In northern Nevada, most of the plutonic rocks including stocks, dikes, and sills intruded during three periods (du Bray, 2007): the Late Jurassic (ca. 160–155 Ma), the Late Cretaceous (ca. 115–80 Ma), and the Eocene (ca. 40–35 Ma). Prior to this study, Jurassic magmatism had not been documented in the Battle Mountain district, whereas magmatic rocks of Late Cretaceous and Eocene ages are known to be present in association with Au and polymetallic deposits at a number of locations in the Battle Mountain district (Fig. 2; Theodore et al., 1973; Doebrich and Theodore, 1996; Theodore, 1998, 2000). The district hosts a wide range of deposit styles that are clearly related to Eocene porphyry-type magmatic-hydro-thermal activity, including porphyries and skarns at Copper Canyon and Elder Creek (Theodore et al., 1973; Keeler, 2010; King, 2011), skarn and base and precious metal veins at Copper Basin (Loucks and Johnson, 1992), skarn at Converse (Cleveland, 2000), skarn and distal disseminated-type mineralization at Buffalo Valley (Kizis et al., 1997; Reid et al., 2010), as well as skarn and polymetallic vein mineralization farther south at Cove and McCoy (Johnston et al., 2008). Gold mineralization at Lone Tree crosscuts Eocene dikes (Holley et al., 2015). Magmatic-hydrothermal mineralization also occurred in association with Cretaceous intrusive activity in the district, including molybdenum porphyry mineralization at the Trenton Canyon prospect (Theodore et al., 1973) and the Buckingham porphyry molybdenum deposit (Roberts and Arnold, 1965; Loucks and Johnson, 1992; McKee, 1992) and hypogene porphyry Cu at Copper Basin (Blake, 1992). At Marigold, Au-mineralized zones are present in Paleozoic sedimentary rocks that have been intruded by felsic dikes of previously unknown age; the genetic relationship between the dikes and the mineralization was also unknown.

Extensional faulting commenced in the Eocene and continues through the present in the Battle Mountain district (Roberts, 1964; Christiansen and McKee, 1978; Dohrenwend and Moring, 1991). The SW-striking, NW-dipping Oyarbide fault, which is the most prominent normal fault in the northern part of the district, truncates the northern flank of the Battle Mountain Range (Roberts, 1964). Roberts (1964) estimated dip-slip displacement of the hanging wall of the Oyarbide fault to be approximately 750 to 1,050 m. This fault physically separates Au deposits at Marigold from polymetallic deposits to the south (Fig. 2). Doebrich and Theodore (1996) and Theodore (1998, 2000) suggested that the Au deposits at Marigold represent upper levels of Cretaceous or Eocene porphyry systems such as those south of the Oyarbide fault (Fig. 2).

Deposit Geology

The rocks at Marigold are mineralized over a N-trending 7.5- × 1-km area. From north to south, current and historically mined orebodies and occurrences at Marigold are 5 North, 8 North, 8 Deep, Terry Zone North, 8 South, 8 South extension, HideOut, Old Marigold (Terry zone), Terry Complex, Red Dot, Mackay, Mud, Target 1, Target 2, Valmy, Antler, and Basalt (Fig. 2). Mineralized rock is nearly continuous from the Basalt pit to the undeveloped 8 North deposit. The 5 North orebody is located approximately 2.5 km northeast of 8 North and has been offset approximately 750 m to the east from the deposits to the south by an inferred NE-striking, dextral strike-slip fault.

The three tectonostratigraphic assemblages described above are exposed at Marigold: Ordovician eugeoclinal siliciclastic rocks of the Roberts Mountain thrust plate, locally present autochthonous Pennsylvanian-Permian shallow water rocks of the Antler overlap sequence, and Mississippian-Permian deep-water siliciclastic rock and basalt of the Golconda thrust plate (Figs. 2, 3). Although rocks belonging to the lower plate of the Roberts Mountain thrust have not been formally recognized at Marigold, beds of limestone, limy mudstone, siliciclastic, volcaniclastic, and carbonate debris flows, and turbidites have been observed in deep drill core. These rocks bear many similarities to units of the Preble Formation that have been intersected in the lower plate of the Roberts Mountain thrust during mining in the Getchell trend 30 km north of Marigold. Rocks of the Valmy Formation, Antler sequence, and Havallah sequence are exposed on the surface and in open pits at Marigold and are intruded by plagioclase-biotite-hornblende phyric quartz monzonite dikes and sills.

Fig. 3.

Cross section looking to the north; section line shown in Figure 2E. Gold is hosted in quartzite, argillite, and chert of the Valmy Formation and mixed carbonate and clastic rocks of the Antler sequence. The Havallah sequence is typically unmineralized or weakly mineralized.

Fig. 3.

Cross section looking to the north; section line shown in Figure 2E. Gold is hosted in quartzite, argillite, and chert of the Valmy Formation and mixed carbonate and clastic rocks of the Antler sequence. The Havallah sequence is typically unmineralized or weakly mineralized.

Preble-Comus Formation

In 2014, a deep drilling program was initiated consisting of five core holes drilled to depths of 1,050 to 1,200 m with the intent to confirm the presence of prospective carbonate host rocks inferred to exist below the Roberts Mountain thrust based on widely accepted regional tectonostratigraphic relationships. At depth, all five core holes intersected limy mudstone, limestone, siliciclastic, volcaniclastic, and carbonate debris flows, siliciclastic and carbonate turbidites, and volcanic rocks. Clasts within carbonate units contain fossils interpreted to be Nuia and Codiacean algae (H. Cook, unpub. data, 2015). Nuia packstones are present at the Emigrant Canyon-type section for the Preble (formerly Comus) Formation proposed by Cook (2015), suggesting a Cambro-Ordovician age for these units at Marigold. The presence of alternating sequences of siliciclastic and carbonate debris flows and turbidites at Marigold indicates the potential for more than one sedimentary source, as described by Cook (2015) for similar rocks on the Getchell trend. Carbonate debris flows and turbidites at Marigold are likely derived from the Comus carbonate seamount (Cook, 2015) to the north, whereas siliciclastic debris flows and turbidites are interpreted to have originated from a terrigenous source.

Valmy Formation

The Valmy Formation is the major ore host at Marigold. The Valmy Formation consists of quartzite, argillite, chert, and lesser metabasalt. It is interpreted to be the shallower, lateral facies equivalent to the carbonaceous, shale-rich Ordovician Vinini Formation found farther to the east in northern Nevada (Gilluly and Gates, 1965). At Marigold, most Valmy Formation encountered during mining consists of massive beds of dark-gray to buff-colored quartzite with interbedded (centimeter to tens of meters thick) lenses of maroon to light green-gray argillite. At the outcrop scale, these rocks appear complexly deformed and include discontinuous, folded blocks from meters to tens of meters wide, juxtaposed by both highand low-angle faults. Geologic modeling of lithologic drill hole data has resulted in recognition of deposit-scale, E-verging, overturned anticlinal folds in the Valmy Formation. This deformation is largely attributed to thrusting during the Mississippian Antler orogeny with secondary influences from Mesozoic to Cenozoic tectonic and magmatic events and an extensional tectonic regime operating from the mid-Tertiary through the present (Roberts, 1964; Theodore, 1998). The Valmy Formation at Marigold was transported at least 90 km eastward to its present position as part of the Roberts Mountain allochthon during the early Mississippian Antler orogeny (Evans and Theodore, 1978).

Antler sequence

The Pennsylvanian-Permian Antler overlap sequence is a package of conglomerate, limestone, sandstone, siltstone, and debris flows unconformably deposited on the Valmy Formation at Marigold. The Antler sequence hosts high-grade Au at Marigold and is also an important host at the Lone Tree deposit. Rocks of the Antler sequence are relatively undeformed, despite their stratigraphic position between two major thrust plates (McGibbon, 2005). This has been cited as evidence that emplacement of the Golconda allochthon did not deform the rocks of the underlying Valmy Formation (McGibbon, 2005).

The Antler sequence consists of three units: Battle Formation, Antler Peak Limestone, and Edna Mountain Formation (Fig. 2). The Battle Formation includes a gray to brick-red conglomerate, composed of subangular to rounded clasts of quartzite, chert, and argillite in a fine-grained sand-clay matrix. Clast size generally decreases in the younging direction, although discrete successions of fining sequences are present that suggest multiple pulses of deposition. The conglomerate is locally interbedded with and overlain by siltstone but is directly overlain by Antler Peak Limestone in many places. Conglomerate of the Battle Formation is present locally throughout the Marigold property and is up to 130 m thick (McGibbon and Wallace, 2000).

The Antler Peak Limestone is a micritic to sandy limestone deposited disconformably on the Battle Formation. The Antler Peak Limestone is as much as 65 m thick at Marigold (McGibbon and Wallace, 2000), and like the Battle Formation it is not laterally continuous in the Marigold mine area. Fossils from the basal section of the Antler Peak Limestone bracket its age between the Late Pennsylvanian and Early Permian (Roberts, 1964).

Debris flows and intercalated calcareous siltstone of the Permian Edna Mountain Formation overlie the Antler Peak Limestone. The debris flows grade upward into phosphatic siltstone that contains as much as 1% P2O5 (Bloomstein et al., 2000; McGibbon, 2005). At Marigold, the Edna Mountain Formation is truncated by the Golconda thrust, precluding estimation of the original thickness of the formation at this location. The Edna Mountain Formation was at least 200 m thick prior to emplacement of the Golconda thrust plate during the Sonoma orogeny (McGibbon, 2005).

Havallah sequence

The Mississippian-Permian Havallah sequence is interpreted to have formed in a deep-water basin that gradually shallowed during deposition and has been temporally correlated with the Antler sequence (Roberts, 1964). The Havallah sequence was transported eastward along the Golconda thrust during the Permo-Triassic Sonoma orogeny. The basal contact of this thrust is clearly exposed at Marigold in the Old Marigold pit, where a distinct dark-brown chert is in contact with rocks of both the Antler sequence and Valmy Formation. Geologic modeling of lithologic data from drill holes suggests this unit is only present north of the Terry Complex, in agreement with previous mapping (Theodore, 1991).

The overlying units are predominantly gray-green to lightbrown siltstone, chert, and rare metabasalt. The basal brown chert is at least 40 m thick, whereas the siltstone is of considerably greater thickness. These rocks are exposed west of the deposits at Marigold, where large-scale open folds are evident in outcrop and road cuts throughout the Havallah hills. The distinctive Jory Member of the Havallah sequence, a pebbly lime conglomerate, is well exposed to the southwest of the deposits at Marigold where it is a significant ridge-forming unit. The clasts within the Jory Member are typically rounded to subrounded and composed of quartzite and chert. The matrix is sand with a calcareous cement. Doebrich and Theodore (1996) interpret the Golconda thrust to be a major tectonostratigraphic control on Au mineralization, as rocks of the Havallah sequence are essentially devoid of Au on the Marigold property. Recent drilling has identified a small volume of ore-grade Havallah sequence rock overlying the highestgrade zones of the Antler sequence-hosted HideOut orebody. The Havallah sequence is a significant Au host at the nearby Lone Tree and Buffalo Valley mines (Fig. 2; Kizis et al., 1997;Bloomstein et al., 2000; McGibbon, 2005; Reid et al., 2010).Minor amounts of malachite and chalcopyrite are present in chert of the Havallah sequence approximately 300 m west of Marigold and 60 m north of outcrops of N-striking quartz monzonite intrusions.

Intrusive rocks

Plagioclase-biotite-hornblende phyric quartz monzonite dikes and sills intrude rocks of the Valmy Formation, Antler sequence, and Havallah sequence at Marigold and can be traced along strike for tens to hundreds of meters. The intrusions are typically no wider than 10 m and strike west-north-west to north. These intrusions have been identified in five open pits (Basalt, Valmy, Target 2, Mackay, and Old Marigold) and one unmined deposit (Red Dot; Fig. 2) and in some cases have spatial associations with ore zones. The intrusions are typically localized along minor fault zones. Collectively, the intrusions define a north-northwest trend. The ages of these intrusions, their alteration styles, and relation to mineralization are described in separate sections below.

In addition to monzonitic rocks, a 2-m interval of biotitedominant lamprophyre (minette-kersantite) was intersected approximately 1,100 m below the premining surface in the area of the Target 2 pit. The lamprophyre contains abundant phenocrysts of minimally altered, wavy biotite laths with interstitial apatite and heavily altered phenocrysts of an unknown mafic phase set in a very fine grained groundmass composed of biotite, plagioclase, and orthoclase. The lamprophyre is calcareous, containing disseminated calcite in the matrix and in veins. Minor amounts of chlorite are present.

Extrusive rocks

Basaltic andesite is present to the southwest of the Marigold mine area. Two whole-rock K-Ar dates from olivine-augite phyric basaltic andesite yielded ages of 31.8 ± 0.8 and 31.4 ± 1.0 Ma (McKee, 2000). Outcrops of basaltic andesite are relatively small, and ~0.1 km2 total area is exposed at the Marigold locality. A ~1-km2 outcrop of Miocene (12 ± 0.4 Ma) tholeiitic basalt is exposed farther to the north (S32 T35N R43E), approximately 5 km northeast of Lone Tree (McKee, 2000; Theodore, 2000). Unmineralized, biotite-bearing rhyolite tuff is exposed in several road cuts and open pits on the Marigold property. The tuff is typically white in color and is extremely friable where interbedded with gravels; however, there is an outcrop of tuff west of Trout Creek and southwest of the Basalt-Antler pit that is a substantially more competent, ridge-forming unit. In the 8 South pit, rhyolite tuff unconformably overlies debris flows of the Edna Mountain Formation (McGibbon and Wallace, 2000; Theodore, 2000). Elsewhere on the property, tuff layers are intercalated with Tertiary alluvium, but the individual tuff layers have not been dated. Potassium-argon dating of biotite from unmineralized tuff in the 8 South pit yielded an age of 22.9 ± 0.7 Ma (McKee, 2000), constraining the minimum age of Au mineralization.

Structure

Host rocks at Marigold record an extensive history of folding, thrusting, and extensional faulting during superimposed tectonic events from the Devonian through to the present. Major Au-bearing NW- to NE-striking high-angle structures (Fig. 2) are thought to have formed initially during thrusting of the clastic assemblage over the carbonate assemblage during the Devonian-Mississippian Antler orogeny (McGibbon, 2005). Large-scale graben-bounding normal faults appear to have vertically offset the rocks of the Valmy Formation, Antler, and Havallah sequences by 150 m or more. The orebodies also are cut by these structures, suggesting most displacement occurred during the Cenozoic. Marker beds of rhyolite tuff (22.9 ± 0.7 Ma; McKee, 2000) and contemporaneously formed gravel have been offset by as much as 60 m in the 8 South area (McGibbon and Wallace, 2000) and by as much as approximately 20 m on the south end of the property, indicating that a set of N-trending structures were active into the Miocene. A series of NE-striking, NW-dipping normal faults subparallel to the normal Oyarbide fault offset beds of tuff and alluvium on the northern edge of the Target 2 area and the southern edge of the Mackay area. A notable NE-striking strike-slip fault has offset the 5 North orebody by approximately 750 m to the northeast. In addition, a series of small, W-striking, high-angle faults with little to no displacement is present throughout the property. Faults with this orientation are generally unmineralized, although a WNW-striking dike in the Target 2 pit is strongly mineralized at its margins. The intrusions appear to have intruded along small-scale highangle faults, as evidenced by gouge along intrusion margins.

The surface of the Valmy Formation forms a linear, N-oriented horst-like structural high (Fig. 3) that is buried by progressively thicker alluvium to the north of Marigold. The geometry of this feature is inferred by modeling of the upper contact of the Valmy Formation from drill hole data. This geometry is significant because much of the Au-mineralized rock within the Valmy Formation coincides with N-, NE-, and NW-striking faults within this structural high.

Alteration Styles

At Marigold, distinctive alteration styles are identifiable in faults and intrusions, as well as in sedimentary host rocks above and below the redox boundary. Oxidation extends to depths of 150 to 500 m below surface (Fig. 3), commonly increasing in depth near high-angle faults and shallowing away from fault surfaces. The following sections describe the major alteration styles in the sulfide and oxide zones (Figs. 4, 5) as well as in faults and intrusions.

Fig. 4.

Representative alteration styles at Marigold. (A) High-density quartz veining in silicified argillite of the Valmy Formation (DDH6008, 722.7–723.9 m) resulting in local brecciation; this alteration style is not indicative of Au mineralization. (B) Volcaniclastic debris flow of the Preble Formation (DDH6029, 774.5–775 m) with thin calcite veinlets. Note volcaniclast replaced by pyrite on lower left. (C) Oxidized pyrite veinlets in Valmy quartzite (DDH6008, 297 m). The iron oxide filled fractures and veinlets crosscut quartz flooding (white) in quartzite. (D, E) Bleached argillite of the Valmy Formation (DDH5331, 216.7 m) cut by native Au-bearing quartz and indigenous limonite microveinlets. Note maroon potassic alteration halo around veinlets. (F) Oxidized gouge from a high-angle mineralized structure in the Mackay Phase 1 pit. Illite is developed in mineralized structures. (G) Jasperoid with quartz veinlets in Havallah sequence carbonate from west of the 5 North deposit. (H) Pervasively argillically altered margin of quartz monzonite intrusion with iron oxide pseudomorphs after pyrite. Abbreviations: bt = biotite, cc = calcite, FeOx = iron oxide, qtz = quartz.

Fig. 4.

Representative alteration styles at Marigold. (A) High-density quartz veining in silicified argillite of the Valmy Formation (DDH6008, 722.7–723.9 m) resulting in local brecciation; this alteration style is not indicative of Au mineralization. (B) Volcaniclastic debris flow of the Preble Formation (DDH6029, 774.5–775 m) with thin calcite veinlets. Note volcaniclast replaced by pyrite on lower left. (C) Oxidized pyrite veinlets in Valmy quartzite (DDH6008, 297 m). The iron oxide filled fractures and veinlets crosscut quartz flooding (white) in quartzite. (D, E) Bleached argillite of the Valmy Formation (DDH5331, 216.7 m) cut by native Au-bearing quartz and indigenous limonite microveinlets. Note maroon potassic alteration halo around veinlets. (F) Oxidized gouge from a high-angle mineralized structure in the Mackay Phase 1 pit. Illite is developed in mineralized structures. (G) Jasperoid with quartz veinlets in Havallah sequence carbonate from west of the 5 North deposit. (H) Pervasively argillically altered margin of quartz monzonite intrusion with iron oxide pseudomorphs after pyrite. Abbreviations: bt = biotite, cc = calcite, FeOx = iron oxide, qtz = quartz.

Fig. 5.

Representative vein styles at Marigold. (A) Sphalerite-pyrite-quartz veinlets in Valmy quartzite (DDH6029, 857 m). Note vuggy euhedral quartz. (B) Sphalerite-bearing quartz veinlet in Valmy quartzite (DDH6029, 871 m). In rare instances, small (<0.5 cm) sphalerite grains are observable in hand sample at Marigold, whereas other base metal sulfides such as chalcopyrite can only be detected under magnification. (C) Calcite veinlet cutting carbonate clasts and pyrite in the matrix of a carbonate debris flow (DDH6008, 1139 m). (D) En echelon Au-bearing quartz veinlets with a biotite selvage cut bleached argillite of the Valmy Formation (DDH5331, 216.7 m). Relationship between these veinlets and the overall Au mineralizing system is unknown. (E) Ankerite vein and ankerite-flooded matrix in conglomerate of the Battle Formation. Ankerite flooding occurs proximal to the Target 2 quartz monzonite dike. (F) Ankerite veins with barite core and barite-ankerite veinlets in Havallah siltstone above the unmined Red Dot deposit approximately 50 m southwest of the INT-5 quartz monzonite dike. (G) Quartz-chalcopyrite vein in chert of the Havallah sequence. Most of the copper occurs as malachite; however, minor amounts of quartz-encapsulated chalcopyrite remain. Sample location is 50 m north-northeast of a quartz monzonite dike, approximately 285 m west of Old Marigold. (H) Coarse stibnite and barite on a fracture surface of Valmy quartzite (DDH6029, 823 m). Abbreviations: ank = ankerite, bar = barite, bt = biotite, cc = calcite, FeOx = iron oxide, mal = malachite, py = pyrite, qtz = quartz, sph = sphalerite, stbn = stibnite.

Fig. 5.

Representative vein styles at Marigold. (A) Sphalerite-pyrite-quartz veinlets in Valmy quartzite (DDH6029, 857 m). Note vuggy euhedral quartz. (B) Sphalerite-bearing quartz veinlet in Valmy quartzite (DDH6029, 871 m). In rare instances, small (<0.5 cm) sphalerite grains are observable in hand sample at Marigold, whereas other base metal sulfides such as chalcopyrite can only be detected under magnification. (C) Calcite veinlet cutting carbonate clasts and pyrite in the matrix of a carbonate debris flow (DDH6008, 1139 m). (D) En echelon Au-bearing quartz veinlets with a biotite selvage cut bleached argillite of the Valmy Formation (DDH5331, 216.7 m). Relationship between these veinlets and the overall Au mineralizing system is unknown. (E) Ankerite vein and ankerite-flooded matrix in conglomerate of the Battle Formation. Ankerite flooding occurs proximal to the Target 2 quartz monzonite dike. (F) Ankerite veins with barite core and barite-ankerite veinlets in Havallah siltstone above the unmined Red Dot deposit approximately 50 m southwest of the INT-5 quartz monzonite dike. (G) Quartz-chalcopyrite vein in chert of the Havallah sequence. Most of the copper occurs as malachite; however, minor amounts of quartz-encapsulated chalcopyrite remain. Sample location is 50 m north-northeast of a quartz monzonite dike, approximately 285 m west of Old Marigold. (H) Coarse stibnite and barite on a fracture surface of Valmy quartzite (DDH6029, 823 m). Abbreviations: ank = ankerite, bar = barite, bt = biotite, cc = calcite, FeOx = iron oxide, mal = malachite, py = pyrite, qtz = quartz, sph = sphalerite, stbn = stibnite.

Sulfide zone

Unoxidized quartzite and argillite of the Valmy Formation show little evidence of alteration other than silicification due to their chemically unreactive nature. Local silicification resulted in a slightly darker color and smoother texture due to quartz flooding of interstitial space. Quartz veinlets of 1- to 3-mm width are more common than in other rock types, although they are volumetrically minor and not ubiquitous in mineralized zones. In some cases these quartz veinlets create local zones of hydrothermal breccia due to high vein density (Fig. 4A). The veinlets typically cut the rocks at high angles with respect to bedding planes and lithologic contacts. The quartz veinlets contain chalcopyrite and sphalerite in some cases (Fig. 5A, B) but are difficult to relate paragenetically to Au-bearing sulfide minerals, as they do not commonly occur in close textural association.

Unoxidized rocks of the Antler sequence typically are gray to black, are carbonaceous, and contain pyrite. In the Antler Peak Limestone, very fine grained pyrite is disseminated throughout, giving a dull-gray appearance in hand sample. In reflected light, the fine-grained pyrite gives the rock a faint metallic luster. Circular clots of dark-gray to black-rimmed pyrite, up to 5 mm in diameter, are present in some samples and appear to be replacements of fossilized crinoid stems. Limestone, limy mudstone, and carbonate debris flows have been locally leached of carbonate minerals. Beds of massive, micritic limestone of the Antler Peak Limestone are variably decarbonatized from dissolution of carbonate minerals and silicified where intersected by mineralizing structures. In those cases, minor fine-grained quartz has precipitated in the decarbonatized zones. This quartz is most notable in small fractures and vugs, where fine-grained crustiform quartz is overgrown by euhedral, prismatic quartz crystals typically no larger than 1 mm. The coarse quartz is coated with minor amounts of calcite and white clay. Small clots of bituminous material are present in unoxidized rocks, occurring as discrete masses in interstitial space. Quartz veins are extremely rare to absent in Antler sequence rock, despite local silicification and jasperoid formation. However, calcite veins, calcite matrix breccia, and euhedral calcite crystals are common in Antler Peak Limestone and debris flow of the Edna Mountain Formation in the HideOut and 8 South extension deposits as well as in carbonate debris flow of the Preble Formation (Fig. 5C).

Pyrite is the most common sulfide mineral below the redox boundary. It is present as disseminations, as stringer veinlets <0.5 mm wide, and less commonly as massive replacements. Unoxidized and mineralized quartzite most commonly has pyrite on fracture surfaces without quartz veinlets. Disseminated pyrite also occurs in interstitial space between quartz grains. In conglomerate and debris flows, more pyrite occurs in iron-rich pelitic and volcanic clasts than in quartzite and chert clasts (Fig. 4B). Gold is associated with pyrite below the base of oxidation.

Oxide zone

Pervasive oxidation has occurred to as much as 500-m depths at Marigold, overprinting all other alteration styles in these rocks. Only oxide ore is currently (2017) mined. Oxidation has resulted in destruction of sulfide and mafic phases, as well as the development of goethite and hematite. Iron oxides are best developed above zones of unoxidized rock that contain high abundances of pyrite. Iron oxides are ubiquitous on fracture surfaces in all rock types—in the Valmy Formation quartzite in particular. In all rock types, iron oxides are variably colored from deep crimson to light yellow but are most commonly orange. Above the redox boundary throughout the Marigold mine area, intensity of iron oxide staining is generally positively correlated with gold grade.

In oxidized Valmy Formation quartzite of the Terry Zone North deposit (Fig. 2), rare vuggy, euhedral quartz crystals <1 mm in length are coated by red to orange iron oxides in Au-mineralized intervals. At least one generation of thin, 1- to 2-mm-wide quartz veinlets are cut by vuggy quartzbearing fractures. Some quartzite contains linear, penetrating, iron oxide stains cutting millimeter-scale quartz veinlets. These iron oxides may represent oxidized sulfide veinlets that formed after the quartz veinlets (Fig. 4C), although it is difficult to be certain given the oxidation, and these textures were not observed in the available unoxidized samples. In argillite beds of the Valmy Formation, iron oxides are present along microfractures and as disseminations. Potassic alteration and silicification are present in the form of minor biotite and quartz veinlets in the argillite (Figs. 4D, E, 5D). The fine-grained biotite appears as fuzzy maroon-colored selvages ranging in width from <1 to approximately 5 mm on submillimeter-scale quartz veinlets. In one drill hole in oxidized argillite, several generations of native Au-bearing quartz veinlets are present in thin section. The extent of this alteration and its relationship to the main Au mineralizing system are unknown. Carbonate veins are exceedingly rare in the Valmy Formation.

Clasts and matrix of Battle Formation conglomerate are visibly oxidized and crosscut by calcite and ankerite veins. Ankerite veinlets grade into zones of pervasive ankerite flooding of the matrix, giving the conglomerate a unique orange-brown color (Fig. 5E). West of the Antler pit (Fig. 2), a mineralized intercept of Battle Formation is oxidized at a depth of more than 450 m, below hundreds of meters of unmineralized, refractory rock. Silicified zones of limestone of the Antler Peak Limestone are typically stained red by iron oxide. Gold-mineralized and pervasively iron oxide stained jasperoidal limestone is typically present at the base of the limestone package. Gold-mineralized limestone above the silicified beds is commonly pervasively iron oxide stained and decarbonitized, and the remaining material is a red to orange, friable silty clay with coarse, euhedral calcite crystals.

Milky white quartz veins are common in chert of the Havallah sequence in the oxide zone; these veins have not been observed in the sulfide zone due to the stratigraphic position of the redox boundary. The chert is commonly bleached to a light-brown color proximal to these veins. This generation of quartz vein is not iron oxide stained and contains no textures suggestive of relict sulfides. Neither the presence of quartz veins or high vein densities in the Havallah chert correlate to presence or intensity of Au mineralization. Ankerite and calcite veins and veinlets are present in greenish-gray to brown siltstone of the Havallah sequence (Fig. 5F). Where present in outcrop, ankerite veinlets with a barite core as wide as 1 cm cut perpendicular to bedding. Ankerite veins are subparallel, with small ankerite vein splays that branch from larger veins. Barite is present in the core of only the largest ankerite veins. Ankerite-barite veins in the Havallah siltstone do not spatially correlate with Au mineralization.

Sooty black carbon of unknown provenance coats fracture surfaces of a volumetrically small proportion of oxidized rocks. This carbon is evident in rocks of the Antler sequence in the Target 2 pit (Fig. 2) and is primarily restricted to the Edna Mountain Formation, where a plume of sooty carbon crosscuts bedding in the sedimentary host rocks and is itself crosscut by the Target 2 intrusion.

Faults

Argillic alteration of host rocks is pronounced in mudstone, siltstone, and intrusive rock proximal to faults both above and below the redox boundary. Steeply dipping Au-bearing faults approximately 0.5 m wide contain iron oxide-stained illite gouge with coarse euhedral calcite crystals up to several centimeters wide, calcite veins, and locally present barite (Fig. 4F). Samples of this fault material routinely assay over 34 g/t; the grade quickly diminishes with increasing distance from the fault. Muscovite and kaolinite are also present in the cores of fault zones and matrices of fault breccias, and these zones are marked by increased rock friability. Argillized fault zones commonly grade into silicified wall rock, and mudstones and muddy debris flows adjacent to fault zones are silicified for several meters outboard of fault zones. Within fault zones, silicified rock fragments may preserve clay coatings. At surface, outcrops of red-brown jasperoid and quartz matrix jasperoid breccia are extensively developed along fault zones to the west of the Marigold deposits in the Havallah hills, as well as several kilometers north of the mine proximal to the 5 North deposit (Fig. 4G). The jasperoids are interpreted to represent areas of high-fluid flow proximal to faults. Limestone and siltstone of the Antler sequence are locally altered to jasperoid proximal to mineralized structures, including a small body of ore-grade jasperoid that was mined in the area of the Antler pit (McGibbon, 2005).

Intrusions

All quartz-monzonite intrusions identified to date have been affected by intense argillic alteration. This alteration is most pervasive at intrusion margins. The intrusions commonly are present along or near minor fault zones, and argillic alteration is also pervasive where faults intersect or are parallel to margins of the intrusions. Argillic alteration has bleached quartzmonzonite intrusions to a white to buff color and rendered the rock extremely friable (Fig. 4H). In zones of argillic alteration, nearly all phenocrysts except quartz have been replaced by a mineral association of sericite, calcite, epidote, chlorite, and clays. Euhedral biotite phenocrysts are replaced by sericite and lesser chlorite. Plagioclase phenocrysts are replaced by calcite, epidote, sericite, and clays. Sulfidation is manifested by relict pyrite pseudomorphs after amphibole. Relics of the characteristic 56°/124° cleavage of amphiboles may be preserved by pyrite pseudomorphs.

Alteration geochemistry

A number of most and least altered samples were selected from oxidized exposures of the Battle Formation conglomerate and the Valmy Formation quartzite, unoxidized drill core from the Valmy quartzite and argillite, and oxidized exposures of four felsic intrusions. The degree of alteration was determined by field and petrographic observations.

The least altered felsic intrusive sample was obtained from the Moonshine intrusion, a large dike northwest of the Old Marigold pit (MS; Fig. 2), and plots in the quartz monzonite field of the Le Maitre (2002) total alkalis vs. silica diagram (Fig. 6A). Increasing alteration intensity correlates with decreasing total alkalis, and the more altered intrusions plot in the granodiorite field. In Figure 6A, the Marigold intrusions are also compared to other intrusions in the Battle Mountain mining district.

Fig. 6.

Alteration geochemistry in Marigold samples. (A) Whole-rock geochemical classification of felsic dikes (see Fig. 2 for dike names and locations) using the total alkalis vs. silica classification diagram of Le Maitre (2002); some fields intentionally left blank for visual clarity. Compositions of select intrusions from nearby deposits are from Roberts (1964), Loucks and Johnson (1992), and McKee (2000). (B-H) Isocon diagrams plotting whole-rock geochemistry of a single most altered sample against a single least altered sample for individual rock types collected as part of this study, following the method of Grant (2005). The immobility isocon is the best-fit line through the typically immobile elements Al (Al2O3), Ti (TiO2), and Zr for B-E, Ti (TiO2), and Zr for F, and Al (Al2O3) and Ti (TiO2) for G-H. Elements that plot above the immobility isocon are enriched in the altered rock. Altered samples are consistently enriched in Au, As, and Sb (B-H). Additional analyses were conducted for Tl and Hg in the unoxidized samples (G, H), and these elements are also enriched by alteration.

Fig. 6.

Alteration geochemistry in Marigold samples. (A) Whole-rock geochemical classification of felsic dikes (see Fig. 2 for dike names and locations) using the total alkalis vs. silica classification diagram of Le Maitre (2002); some fields intentionally left blank for visual clarity. Compositions of select intrusions from nearby deposits are from Roberts (1964), Loucks and Johnson (1992), and McKee (2000). (B-H) Isocon diagrams plotting whole-rock geochemistry of a single most altered sample against a single least altered sample for individual rock types collected as part of this study, following the method of Grant (2005). The immobility isocon is the best-fit line through the typically immobile elements Al (Al2O3), Ti (TiO2), and Zr for B-E, Ti (TiO2), and Zr for F, and Al (Al2O3) and Ti (TiO2) for G-H. Elements that plot above the immobility isocon are enriched in the altered rock. Altered samples are consistently enriched in Au, As, and Sb (B-H). Additional analyses were conducted for Tl and Hg in the unoxidized samples (G, H), and these elements are also enriched by alteration.

Whole-rock geochemical analyses of these samples were plotted on isocon diagrams following a method described by Grant (2005) wherein a single most altered sample is compared to a single least altered sample to determine geochemical modification associated with alteration (Fig. 6B-H). Both samples of the Basalt pit intrusion were pervasively altered, but the most altered sample (from a faulted margin of the dike) is marked by increased amounts of Au, As, and Sb. Major elements appear relatively unchanged despite pervasive destruction of primary igneous phases in both samples. Barium is the only minor element considerably depleted in the most altered Basalt pit intrusion sample relative to the least altered sample. In the Target 2 intrusion, elevated Au in the most altered sample (from the faulted margin of the intrusion) is accompanied by increased concentrations of As, Sb, W, and Co, whereas MnO, Na2O, CaO, MgO, Sr, and Ba are depleted. Alteration of the Mackay pit intrusion is characterized by slight As enrichment and minor depletion of MgO and Na2O. Alteration of the Battle Formation conglomerate coincides with the addition of Au, As, Sb, and Ba, as well as W and Co. In the oxide zone, altered quartzite of the Valmy Formation is strongly enriched in Au and As relative to unaltered quartzite. Below the redox boundary, altered Valmy quartzite and argillite are enriched in Au, As, Sb, and Tl.

Samples were collected along an 88-m transect in the Target 2 pit to examine spatial distribution of alteration relative to a quartz monzonite dike and its wall rocks (Figs. 7, 8). The transect included quartzite and argillite of the Valmy Formation, a subvertical quartz monzonite dike, conglomerate of the Battle Formation, a low-angle fault, and a high-angle fault. The lowangle fault intersects the bench face in the Battle Formation, causing bleaching and argillic alteration of the conglomerate. The high-angle fault intersects the bench face in the Valmy Formation, and the fault zone is characterized by dark-gray to black angular clasts of Valmy quartzite in a matrix of ground quartzite fragments and clay. Samples were collected on the 5,170-ft bench (Fig. 7B) at the center of the intrusion, at the intrusion margins, and 5, 10, 15, 20, and 40 m outboard of the intrusion on both sides (Fig. 8). Relative to sedimentary host rocks, the dike is enriched in all major elements analyzed with the exception of SiO2 (Fig. 8A). Alteration at the dike margins has caused depletion in Na2O, CaO, and MnO relative to the dike’s core. One of the dike margins is weakly enriched in K2O relative to the core. Metal enrichment is discussed in the mineralization section below.

Fig. 7.

Plan view maps of ore shells and lithology in the Target 2 pit. (A) 5,320-ft level. Preferential mineralization of the Antler Peak Formation relative to the rest of the Antler sequence and significant Au enrichment along the quartz monzonite dike. (B) 5,170-ft level. Quartz monzonite dike with no Au enrichment relative to other host rocks. The location of the geochemical transect is shown in the southeast wall on the 5,170-ft level of the Target 2 pit. LS = limestone.

Fig. 7.

Plan view maps of ore shells and lithology in the Target 2 pit. (A) 5,320-ft level. Preferential mineralization of the Antler Peak Formation relative to the rest of the Antler sequence and significant Au enrichment along the quartz monzonite dike. (B) 5,170-ft level. Quartz monzonite dike with no Au enrichment relative to other host rocks. The location of the geochemical transect is shown in the southeast wall on the 5,170-ft level of the Target 2 pit. LS = limestone.

Fig. 8.

Major element (A) and trace element concentrations (B, C) along the Target 2 pit transect across the Valmy Formation, quartz monzonite intrusion, and Battle Formation. Samples from fault intersections are marked with an asterisk.

Fig. 8.

Major element (A) and trace element concentrations (B, C) along the Target 2 pit transect across the Valmy Formation, quartz monzonite intrusion, and Battle Formation. Samples from fault intersections are marked with an asterisk.

Mineralization

Silver and base metals

Whole-rock geochemical analyses indicate that Ag and base metal sulfides are a trace constituent of the rocks at Marigold. Base metal sulfides are present as disseminated clots of grains that are typically less than 50 μm in diameter and constitute a very small percentage of the rock, identifiable only in thin section and by whole-rock geochemistry. Sphalerite is the only base metal sulfide observable in hand sample.

Silver and base metal mineralization is characterized by a distinct association of chalcopyrite, members of the tetrahedrite-tennantite series including argentiferous tetrahedrite-tennantite, pyrite, weakly auriferous pyrite, the sulfosalt gersdorffite (NiAsS), a stannite group mineral (Cu2[Fe,Zn,Hg] SnS4; Fig. 9A), arsenopyrite, sphalerite, and galena. These minerals are present most commonly in some fault zones crosscutting sedimentary rocks, calcite veinlets in dikes, as well as metabasaltic intervals of the Valmy Formation.

Fig. 9.

Base metal sulfide minerals at Marigold. (A) Base metal sulfide clot with stannite overgrown by chalcopyrite, tetrahedrite-tennantite, and weakly auriferous pyrite, hosted in a vuggy breccia with clasts of basalt and mudstone in a carbonate matrix (reflected light). (B) Chalcopyrite and digenite in a vuggy breccia with clasts of basalt and mudstone in a carbonate matrix (reflected light). Digenite is typically present as thin rims on chalcopyrite grains in oxidized rock. (C) Sphalerite with chalcopyrite disease and minor arsenopyrite in silicified mudstone and siltstone from a fault zone (reflected light). (D) Sphalerite partially replaced by pyrite with minor tennantite (backscattered electron [BSE] image). (E) Pyrite overgrown by chalcopyrite and an Ag-bearing member of the tetrahedrite-tennantite series (BSE). Data are from electron probe microanalysis (EPMA) point analyses of pyrite and tennantite. <d.l.= less than detection limit of ~134 ppm Au. (F) Pyrite, chalcopyrite, Ag-bearing tetrahedrite-tennantite, and stannite (BSE). Data are from EPMA point analyses of pyrite and tennantite. <d.l.= less than dection limit of ~135 ppm Au, 105 ppm Ag. (G) Gersdorffite intergrown with chalcopyrite, tetrahedrite-tennantite, and pyrite (BSE). Data are from EPMA point analyses of gersdorffite, pyrite, and tennantite. <d.l.= less than detection limit of ~142 ppm Au, 93 ppm Ag. (H) Sphalerite and Ag-bearing tetrahedrite-tennantite, overgrown by pyrite with fuzzy As and Au-enriched rims. Abbreviations: asp = arsenopyrite, cpy = chalcopyrite, dg = digenite, gdf = gersdorffite, py = pyrite, sph = sphalerite, st = stannite, tn = tetrahedrite-tennantite.

Fig. 9.

Base metal sulfide minerals at Marigold. (A) Base metal sulfide clot with stannite overgrown by chalcopyrite, tetrahedrite-tennantite, and weakly auriferous pyrite, hosted in a vuggy breccia with clasts of basalt and mudstone in a carbonate matrix (reflected light). (B) Chalcopyrite and digenite in a vuggy breccia with clasts of basalt and mudstone in a carbonate matrix (reflected light). Digenite is typically present as thin rims on chalcopyrite grains in oxidized rock. (C) Sphalerite with chalcopyrite disease and minor arsenopyrite in silicified mudstone and siltstone from a fault zone (reflected light). (D) Sphalerite partially replaced by pyrite with minor tennantite (backscattered electron [BSE] image). (E) Pyrite overgrown by chalcopyrite and an Ag-bearing member of the tetrahedrite-tennantite series (BSE). Data are from electron probe microanalysis (EPMA) point analyses of pyrite and tennantite. <d.l.= less than detection limit of ~134 ppm Au. (F) Pyrite, chalcopyrite, Ag-bearing tetrahedrite-tennantite, and stannite (BSE). Data are from EPMA point analyses of pyrite and tennantite. <d.l.= less than dection limit of ~135 ppm Au, 105 ppm Ag. (G) Gersdorffite intergrown with chalcopyrite, tetrahedrite-tennantite, and pyrite (BSE). Data are from EPMA point analyses of gersdorffite, pyrite, and tennantite. <d.l.= less than detection limit of ~142 ppm Au, 93 ppm Ag. (H) Sphalerite and Ag-bearing tetrahedrite-tennantite, overgrown by pyrite with fuzzy As and Au-enriched rims. Abbreviations: asp = arsenopyrite, cpy = chalcopyrite, dg = digenite, gdf = gersdorffite, py = pyrite, sph = sphalerite, st = stannite, tn = tetrahedrite-tennantite.

Chalcopyrite-bearing quartz veins (Fig. 5G) crosscut chert of the Havallah sequence in outcrop approximately 300 m northwest of the Old Marigold pit. This site has a small (<20 m2) prospect of unknown age. The fracture surfaces of rock in this location are coated with malachite. The quartz veins are up to 0.75 cm in width with sharp boundaries and no apparent alteration selvage and cut near perpendicular to bedding. The quartz veins are crudely parallel; however, smaller stringer veinlets are crosscut by larger veins. Malachite, digenite (Fig. 9B), and goethite are present in the quartz veins, and quartz-encapsulated chalcopyrite is present. Chalcopyrite is not known to be present within quartz veins or otherwise anywhere else on the property; however, chrysocolla and malachite are very rarely present on fracture surfaces in conglomerate of the Battle Formation in the 8 South extension area.

Quartz-pyrite-sphalerite veinlets (Fig. 5B, C) are present in Valmy quartzite in DDH6029 approximately 975 to 1,035 m below the original land surface. The veinlets are approximately 2 mm wide and are cut by at least one generation of smaller, barren quartz veinlets. Euhedral quartz crystals are common in small vugs and are typically coated with a white clay, stibnite, and barite. Stibnite and barite also occur on fracture surfaces that cut the quartz veinlets (Fig. 5H).

A number of consistent textural relationships were observed for base metal and Ag sulfides, allowing the development of a paragenetic sequence (Fig. 10). In the sedimentary rocks, arsenopyrite is present as an intergrowth, with sphalerite affected by chalcopyrite disease (Fig. 9C), and in some cases the sphalerite is partially replaced by pyrite and minor tetrahedrite-tennantite (Fig. 9D). Framboidal, cubic, or anhedral fuzzy Au-poor pyrite grains are commonly overgrown by chalcopyrite and argentiferous tetrahedrite-tennantite (Fig. 9E). The stannite group mineral is also commonly overgrown by chalcopyrite and argentiferous tetrahedrite-tennantite (Fig. 9A, F). Rare anhedral gersdorffite is present in argillically altered fault zones proximal to metabasalt of the Valmy Formation, intergrown with chalcopyrite, argentiferous tetrahedrite-tennantite, and pyrite (Fig. 9G). Gold-poor, Cu-bearing pyrite has overgrown the chalcopyrite and argentiferous tetrahedrite-tennantite (Fig. 9A, F). In the quartz monzonite intrusions, Au-poor pyrite is most commonly intergrown with galena and very fine grained chalcopyrite.

Fig. 10.

Paragenetic sequence inferred from petrography of Marigold ores. (A) Alteration. (B) Mineralization.

Fig. 10.

Paragenetic sequence inferred from petrography of Marigold ores. (A) Alteration. (B) Mineralization.

Gangue minerals associated with the base metals include quartz, calcite, potassium feldspar, biotite, muscovite, sericite, kaolinite, and barite. In fault breccias in the Valmy Formation, muscovite, potassium feldspar, and biotite are present in the matrix, with small quantities of biotite on the margins of quartz grains. Muscovite and potassium feldspar are major constituents of the groundmass, which contains trace chromite. In quartzite, white clays, likely kaolinite, are present in void space in sphalerite-bearing quartz veins. In quartz monzonite intrusions, muscovite is present on the margin of calcite veins, some of which host small quantities of galena and pyrite with minor barite overgrowths. Sericite is present throughout the groundmass of these intrusions and also within altered biotite and feldspar phenocrysts, although the relationship to carbonate veins is uncertain. The textural observations suggest the following paragenetic sequence (Fig. 10): quartz formed prior to the development of kaolinite, sericite, muscovite, and potassium feldspar, which may be related to carbonate veining. Barite overgrowths on pyrite within calcite veins suggest paragenetically late precipitation of barite.

Pyrite associated with base metal sulfides typically does not contain detectable Au based on electron probe microanalyses (average analysis less than the detection limit of 120 ppm; Table 2). Gold was detected in three spot analyses of pyrite associated with base metals. Gold is elevated in the cores of base metal-associated pyrite grains rather than the rims (maximum concentration of 283 ppm). The primary Ag-bearing phase at Marigold is a mineral in the tetrahedrite-tennantite series (as much as 2,322 ppm Ag; Table 2), which also contains minor amounts of Sb, Fe, Zn, and Hg.

Table 2.

Representative Electron Probe Microanalyses of Base Metal-Stage Minerals (values given in wt %)

ElementBase metal-stage pyriteTennantite-tetrahedrite
Drill hole5031491950314919
m Sample471.7 MC-5523.6 MC-13471.7 MC-5523.6 MC-13571.5 MC-10
Ag0000000000.020.030.020.010.03000.040.23
As2.5502.332.863.142.662.142.792.3116.5316.0117.5617.4217.6417.0410.7418.5616.47
Au000.020.030000.020.01000000000
Co0000000000000000.2500
Cu2.510.320.711.310.190.170.230.150.738.7739.8539.840.4839.5739.538.0639.6840.27
Fe41.7946.4945.2344.6645.8145.6645.6845.4445.12.83.191.712.057.955.82.995.862.3
Hg0.03000000001.030.740.760.741.043.194.573.830.04
Ni0.2300000000.03000000000
S51.4453.4852.1550.9651.1951.1851.7851.1451.6627.5428.1428.9928.2429.628.3527.3328.5128.04
Sb0.29000.0100000.043.744.582.352.441.923.2111.730.454.6
Sn0000000000.030.030000.020.0700
Tl0.090.180.070.150.070.180.110.150.130.0800.05000.160.1900.12
Zn0.2800000000.036.094.2265.760.931.554.390.225.83
Total99.21100.55 100.52100.01100.4399.92100.0399.82100.0696.6296.8397.2597.1798.6898.82100.3297.1697.93
ElementBase metal-stage pyriteTennantite-tetrahedrite
Drill hole5031491950314919
m Sample471.7 MC-5523.6 MC-13471.7 MC-5523.6 MC-13571.5 MC-10
Ag0000000000.020.030.020.010.03000.040.23
As2.5502.332.863.142.662.142.792.3116.5316.0117.5617.4217.6417.0410.7418.5616.47
Au000.020.030000.020.01000000000
Co0000000000000000.2500
Cu2.510.320.711.310.190.170.230.150.738.7739.8539.840.4839.5739.538.0639.6840.27
Fe41.7946.4945.2344.6645.8145.6645.6845.4445.12.83.191.712.057.955.82.995.862.3
Hg0.03000000001.030.740.760.741.043.194.573.830.04
Ni0.2300000000.03000000000
S51.4453.4852.1550.9651.1951.1851.7851.1451.6627.5428.1428.9928.2429.628.3527.3328.5128.04
Sb0.29000.0100000.043.744.582.352.441.923.2111.730.454.6
Sn0000000000.030.030000.020.0700
Tl0.090.180.070.150.070.180.110.150.130.0800.05000.160.1900.12
Zn0.2800000000.036.094.2265.760.931.554.390.225.83
Total99.21100.55 100.52100.01100.4399.92100.0399.82100.0696.6296.8397.2597.1798.6898.82100.3297.1697.93
ElementStanniteChalcopyriteGersdorffite
Drill Hole491950314919
m523.6471.7523.6Detection limit
SampleMC-13MC-5MC-5MC-6
Ag0000000.0095
As00045.2846.1646.540.013
Au0000000.012
Co0003.21.592.410.33
Cu26.3826.133.012.4800.040.27
Fe4.674.729.420.480.510.580.016
Hg12.3613.3800.03000.017
Ni00041.244.5944.530.12
S29.1728.9534.7618.6819.5619.120.0068
Sb0000.290.10.10.0093
Sn23.9123.7400000.0096
Tl00.0500000.036
Zn4.434.0500.2000.036
Total100.99100.9797.23111.84112.54113.35 
ElementStanniteChalcopyriteGersdorffite
Drill Hole491950314919
m523.6471.7523.6Detection limit
SampleMC-13MC-5MC-5MC-6
Ag0000000.0095
As00045.2846.1646.540.013
Au0000000.012
Co0003.21.592.410.33
Cu26.3826.133.012.4800.040.27
Fe4.674.729.420.480.510.580.016
Hg12.3613.3800.03000.017
Ni00041.244.5944.530.12
S29.1728.9534.7618.6819.5619.120.0068
Sb0000.290.10.10.0093
Sn23.9123.7400000.0096
Tl00.0500000.036
Zn4.434.0500.2000.036
Total100.99100.9797.23111.84112.54113.35 

Petrographic observations indicate that the base metals and Ag predominately are present in intervals different from the Au-associated minerals. This is supported by deposit-scale geochemical data (Table 3) demonstrating a lack of correlation between Au and base metals. In this data set, Ag shows strong positive correlations with Cu, Mo, Pb and Zn, whereas Au is most closely correlated with As, Ba, Hg, Sb, and Tl. However, both geochemical associations (Au-As-Hg-Sb-Tl and Ag-Cu-Pb-Sn-W-Zn) were observed to occur in some fault zones. This is evident in the geochemical transect across a dike and its wall rocks (Fig. 8B, C). In general the sedimentary rocks contain the highest concentrations of Au, As, Sb, and Tl (Fig. 8B), whereas Ag, Sn, Cu, Pb, Zn, and W occur preferentially in the dike (Fig. 8C). Both suites of metals occur where the Battle conglomerate is cut by a low-angle fault. Petrographic evidence for the temporal relationship between the Au and Ag base metal suites is limited, because they so rarely occur in the same rocks. However, in a drill core interval of argillite from the Valmy Formation below the redox boundary, pyrite with fuzzy Au- and As-rich rims (as detected by EPMA) was observed to overgrow sphalerite and an Ag-bearing member of the tetrahedrite-tennantite series (Fig. 9H), suggesting that Au mineralization followed base metal mineralization.

Table 3.

Spearman Correlation Matrix Constructed Using Whole-Rock Analyses of 9,153 Drill Core and Drill Cutting Samples

Gold

Gold-mineralized rock is largely confined to steeply dipping, NW- to NE-striking structures and their intersections at Marigold (Fig. 11). Approximately 30% of the Au is present in the Antler sequence or in dikes and structures crosscutting these rocks (Fig. 11A). The remaining 70% of the Au is present in the Valmy Formation or in dikes and structures crosscutting these rocks (Fig. 11B). Where faults intersect limestone and debris flows of the Antler sequence, the area of Au-mineralized rock extends laterally along bedding (Fig. 3). Above the Golconda thrust, the Havallah sequence is generally unmineralized; however, a small proportion of ore-grade Havallah sequence rock is present in the Old Marigold and HideOut areas. Gold-mineralized rock is present below the redox boundary, but given the refractory nature of material it is not currently economic to mine. The difference in Au grade between oxidized ore and unoxidized rock is negligible on the basis of a data set of more than 15,000 samples.

Fig. 11.

Structures in the 1-ppm-grade shell overlain on premining topographic surface, showing distribution of mineralized rock in the Antler sequence (A) and Valmy Formation (B). Gold is the only element routinely analyzed. Grid coordinates are UTM (NAD83).

Fig. 11.

Structures in the 1-ppm-grade shell overlain on premining topographic surface, showing distribution of mineralized rock in the Antler sequence (A) and Valmy Formation (B). Gold is the only element routinely analyzed. Grid coordinates are UTM (NAD83).

Gold grades are locally elevated in fault zones as well as in some faulted dike margins. At pit scale, ore blocks follow dike margins in some exposures (Fig. 7A) and not in others (Fig. 7B). In the geochemical transect across a dike, faults, and host rock (Fig. 8), highest concentrations of Au are present with As, Sb, and Tl where a fault intersects the reactive Battle conglomerate. Within the dike, these elements are enriched at the margins relative to the core, suggesting the intrusion is not the source of the Au, although its margins may represent post-dike-emplacement fluid pathways. In representative intervals of drill core, Au, As, and Sb are most enriched in faulted sedimentary rock (Fig. 12A) and faulted quartz monzonite intrusions (Fig. 12B). Silver and base metals are elevated in some Au-bearing structures but commonly are present in intervals separate from Au mineralization (Figs. 8B, C, 12A, B).

Fig. 12.

(A,B) Representative samples of drill core geochemistry from two drill holes, showing the spatial correlation of Au, As, and Sb in fault zones and faulted and argillically altered quartz monzonite dikes. Silver and base metals are most abundant outside of these zones.

Fig. 12.

(A,B) Representative samples of drill core geochemistry from two drill holes, showing the spatial correlation of Au, As, and Sb in fault zones and faulted and argillically altered quartz monzonite dikes. Silver and base metals are most abundant outside of these zones.

The Au-As-Sb association reflects the hypogene mineralogy of the Au. In unoxidized rocks, Au is present in As- and Sb-rich overgrowths on pyrite (Fig. 13A-D); native Au was not observed. Cores of the pyrite grains are typically Au and As poor. The cores display euhedral or framboidal morphology and commonly contain voids. The framboidal pyrite occurs as disseminations of single grains or clusters of multiple grains, and the euhedral pyrite occurs within veinlets and as disseminations of single grains. The early pyrite cores have been overgrown by two geochemically distinct pyrite types: Au-poor arsenopyrite and Au-rich arsenian pyrite. The Au-poor arsenopyrite overgrowths contain 191,820 to 384,474 ppm As, 0 to 1,368 ppm Sb, and Au below the detection limit (~136 ppm). This type of overgrowth ranges in width from <1 to ~500 μm (e.g., Fig. 13A). The Au-As pyrite overgrowths are typically less than 5 μm in width (e.g., Fig. 13B, C) and are characterized by an average of 559 ppm Au, with a maximum value of 958 ppm on the basis of 10-point analyses. Arsenic contents are less than those in arsenopyrite overgrowths (≤67,018 ppm), whereas Sb contents are higher (≤2,106 ppm). In each grain analyzed, As, Au, and Sb are elevated relative to cores of the pyrites (e.g., Fig. 13D). Unlike pyrite in Carlin-style ores at the Cove mine, which contain 12 ± 5 to 681 ± 173 ppm Ag (Johnston, 2000), Au-bearing pyrite analyzed from Marigold does not contain detectable Ag (Table 1). Arsenopyrite overgrowths occur along grain margins surrounding the Au-As–poor early pyrite, as well as in voids and fractures.

Fig. 13.

(A-C) Backscatter electron images of gold-stage minerals with locations of selected electron probe microanalyses. (A) Small grain on left is composed of Au- and As-enriched pyrite (Au-As py) overgrown on arsenopyrite (asp). Large grain is composed of arsenopyrite overgrown on Au-poor, arsenic-poor (GPAP) and Au-poor, arsenic-rich (GPAR) pyrite from DDH4919 522.4–524 m; 0.27 ppm Au. <d.l.= less than detection limit of ~132 ppm Au, 131 ppm As. (B) Au-As pyrite over-growth on GPAP pyrite from DDH5031 471.8–473.0 m; 0.86 ppm Au. <d.l.= less than detection limit of ~116 ppm Au, 132 ppm As. (C) Arsenopyrite and Au-As pyrite overgrowths on GPAP pyrite; the paragenetic relationship between arsenopyrite and Au-As pyrite is ambiguous. DDH5031 471.8–473.0 m; 0.86 ppm Au. <d.l.= less than detection limit of ~127 ppm Au, 134 ppm As. (D) Electron probe microanalyses from A to A′ in image C. (E) Reflected light image of iron oxide (FeOx) pseudomorph after pyrite and fuzzy pyrite overgrowth in Valmy quartzite (qtz). Hand sample, Au below detection. (F-H) DDH5331 216.1–217.4 m; 5.38 ppm. (F) Reflected light image of iron oxides and native Au in argillite of the Valmy Formation. (G, H) Backscatter electron images of iron oxides and native Au within argillite of the Valmy Formation. Native Au grains are high (974–993) fineness.

Fig. 13.

(A-C) Backscatter electron images of gold-stage minerals with locations of selected electron probe microanalyses. (A) Small grain on left is composed of Au- and As-enriched pyrite (Au-As py) overgrown on arsenopyrite (asp). Large grain is composed of arsenopyrite overgrown on Au-poor, arsenic-poor (GPAP) and Au-poor, arsenic-rich (GPAR) pyrite from DDH4919 522.4–524 m; 0.27 ppm Au. <d.l.= less than detection limit of ~132 ppm Au, 131 ppm As. (B) Au-As pyrite over-growth on GPAP pyrite from DDH5031 471.8–473.0 m; 0.86 ppm Au. <d.l.= less than detection limit of ~116 ppm Au, 132 ppm As. (C) Arsenopyrite and Au-As pyrite overgrowths on GPAP pyrite; the paragenetic relationship between arsenopyrite and Au-As pyrite is ambiguous. DDH5031 471.8–473.0 m; 0.86 ppm Au. <d.l.= less than detection limit of ~127 ppm Au, 134 ppm As. (D) Electron probe microanalyses from A to A′ in image C. (E) Reflected light image of iron oxide (FeOx) pseudomorph after pyrite and fuzzy pyrite overgrowth in Valmy quartzite (qtz). Hand sample, Au below detection. (F-H) DDH5331 216.1–217.4 m; 5.38 ppm. (F) Reflected light image of iron oxides and native Au in argillite of the Valmy Formation. (G, H) Backscatter electron images of iron oxides and native Au within argillite of the Valmy Formation. Native Au grains are high (974–993) fineness.

In thin section, stibnite was observed proximal to As-rich pyrite. Macroscopic stibnite is intergrown with barite on fracture surfaces of barren quartzite in DDH6029. Stibnite- and barite-coated fracture surfaces cut sphalerite-pyrite–bearing quartz veins. Realgar and orpiment were not identified in Marigold samples, but the presence of arsenopyrite indicates arsenic was a significant component of the hydrothermal fluid. A simple paragenetic sequence can be drawn from these textural observations (Fig. 10): Au-As pyrite and arsenopyrite formed after preexisting pyrite, and these rims were followed by stibnite.

The Au-rich pyrite rims are geochemically distinct from the base metal-associated generation of pyrite. In comparison, the base metal-associated pyrite is typically massive and chemically distinct from other pyrite generations, containing a greater concentration of copper and zinc and an unusually limited range of arsenic values compared with pyrite with Au-As rims. Figure 14 shows the Au, As, and Cu concentrations in the base metal vs. Au-stage pyrites.

Fig. 14.

Pyrite geochemistry at Marigiold. (A) Au (ppm) vs. As (wt %) contents in pyrite. For graphic purposes, electron probe microanalyses point analyses below detection limit (~133 ppm As, ~117 ppm Au, ~265 ppm Cu) are given a value of 0. Early pyrite (black) contains very minor amounts of arsenic and generally low Au values. Texturally, the pyrite is coarse and typically euhedral, but may be overgrown by Cu- or Au-bearing pyrite. The chemical affinity of this early pyrite is enigmatic due to Au concentrations between below detection limit (<d.l.) and 200 ppm and relatively low concentrations (<d.l.–1 wt %) of arsenic. Base metal-related pyrite (dark gray) has a restricted range of arsenic values and generally low Au relative to Au-stage pyrite. (B) Copper vs. As (mol %) for Au-stage pyrite (light gray), base metal associated pyrite (dark gray), and Au-poor, arsenic-poor pyrite (black).

Fig. 14.

Pyrite geochemistry at Marigiold. (A) Au (ppm) vs. As (wt %) contents in pyrite. For graphic purposes, electron probe microanalyses point analyses below detection limit (~133 ppm As, ~117 ppm Au, ~265 ppm Cu) are given a value of 0. Early pyrite (black) contains very minor amounts of arsenic and generally low Au values. Texturally, the pyrite is coarse and typically euhedral, but may be overgrown by Cu- or Au-bearing pyrite. The chemical affinity of this early pyrite is enigmatic due to Au concentrations between below detection limit (<d.l.) and 200 ppm and relatively low concentrations (<d.l.–1 wt %) of arsenic. Base metal-related pyrite (dark gray) has a restricted range of arsenic values and generally low Au relative to Au-stage pyrite. (B) Copper vs. As (mol %) for Au-stage pyrite (light gray), base metal associated pyrite (dark gray), and Au-poor, arsenic-poor pyrite (black).

Characterization of the relative importance of sulfidation and pyritization would require more extensive multielement data than are available for mineralized rocks below the redox boundary. The limited dataset of geochemical analyses from unoxidized rock collected in this study suggests that both Fe and S were added to some mineralized samples. Other common alteration minerals associated with Au include quartz, illite, and calcite. Textural observations suggest that quartz and illite formed with the Au-As overgrowths, followed by calcite and potentially barite (Fig. 10). A consistent temporal relationship could not be determined for barite, although in some Au-mineralized fault zones, illite gouge is cut by calcite veins ± barite.

In oxidized rocks at Marigold, native Au is present in close spatial association with iron oxide minerals such as hematite and goethite. Red iron oxide staining typically correlates with the highest grades, although high-grade quartzite in the Terry Zone North and Valmy deposits typically has a more brown color. In many cases, the iron oxides preserve the textures of pyrite, and iron oxide pseudomorphs of pyrite grains and fuzzy overgrowths are visible in thin section (Fig. 13E). The native Au grains are rounded and irregularly shaped (Fig. 13F-H). In one sample only, Au was observed to occur as 10-μm or smaller native Au blebs in quartz veinlets and within void spaces (Fig. 13F). It is not clear whether this is the result of oxidation of quartz veinlet-hosted Au-bearing pyrite, or whether quartz veinlets contain rare hypogene native Au grains. Given the rarity of this mineralization style, its relationship to the main Au-mineralizing system is unknown. Electron probe microanalyses indicated that Au in the oxide zone is high fineness. No other elements were detectable in native Au grains, with the exception of 1 to 2.5 wt % Fe, likely from texturally associated iron oxides (Table 1).

Ag/Au ratio

One of the key measurable differences between Carlin-type Au deposits and distal disseminated Au ± Ag deposits is the Ag/Au ratio. Carlin-type deposits typically contain less Ag than Au (Cline et al., 2005), whereas the Ag/Au ratio in distal disseminated deposits can be up to 400:1 (Cox, 1992). At Marigold, the low-Ag content of the ore was traditionally assumed to be a product of oxidation, and significantly higher Ag/Au ratios were inferred below the redox boundary (Doebrich and Theodore, 1996; Theodore, 1998, 2000). In the present study, a geochemical database of 2,494 drill hole samples was used to evaluate the relationships between Au and Ag in Au-bearing rocks above and below the redox boundary (Fig. 15A), as well as in a single representative drill hole (Fig. 15B). In the large dataset there is a trend toward higher Ag/Au ratios with depth, but no abrupt change is observed at the redox boundary in either dataset (Fig. 15A, B). These data suggest that the low Ag concentrations in Marigold ores reflect the geochemistry of hypogene Au mineralization rather than oxidation but that there may be some metal zoning with depth.

Fig. 15.

Comparison of Ag and Au concentrations with depth. (A) Depth vs. Ag/Au ratios for 2,494 drill hole samples. (B) Gold and Ag concentration vs. depth in DDH6008; Au and Ag are commonly enriched in separate intervals. Note slight enrichment of Ag in the transition zone, and low Ag values in the reduced zone.

Fig. 15.

Comparison of Ag and Au concentrations with depth. (A) Depth vs. Ag/Au ratios for 2,494 drill hole samples. (B) Gold and Ag concentration vs. depth in DDH6008; Au and Ag are commonly enriched in separate intervals. Note slight enrichment of Ag in the transition zone, and low Ag values in the reduced zone.

Geochronology

Ar-Ar geochronology

Biotite from the lamprophyre (6029-L) gave a plateau age of 160.7 ± 0.1 Ma based on 10 steps and 86% of the cumulative 39Ar. The argon release pattern and plateau data are given in Figure 16A. The age of the biotite is interpreted to represent the cooling of the sample below the approximate closure temperature of diffusion of Ar in biotite (~300°C; Hodges, 1991) and is considered close to the age of formation.

Fig. 16.

40Ar/39Ar step heating plots. (A) Biotite from lamprophyre 6029-L. (B) Sericite from intrusion T2-QM1. (C) Sericite from intrusion T2-QM2. MSWD = mean square of weighted deviates.

Fig. 16.

40Ar/39Ar step heating plots. (A) Biotite from lamprophyre 6029-L. (B) Sericite from intrusion T2-QM1. (C) Sericite from intrusion T2-QM2. MSWD = mean square of weighted deviates.

In a previously unpublished study by Marigold Mining Company, grains of sericite that replaced biotite were dated from two samples (T2-QM1, T2-QM2) of the Target 2 quartz monzonite dike using 40Ar/39Ar geochronology. The sericite grains ranged from tens to hundreds of micrometers along the a-b axial plane. The samples gave integrated 40Ar/39Ar ages of 88.0 ± 0.46 and 79.59 ± 0.16 Ma; no plateau could be defined for these samples according to the criteria of Fleck et al. (1977). The argon release patterns are given in Figure 16B and C. The closure temperature of white mica is largely dependent on the size of the crystals, duration of heating event, and rate of cooling (Hofstra et al., 1999; Arehart et al., 2003). White micas with a diameter of 5 μm along the a-b axial plane will begin to reset at 350°C during a heating event of ≥100,000 years or at approximately 425°C during a heating event of ≥1,000 years. Only sericite grains of ≤0.5 μm along the a-b axial plane, which is much smaller than those dated here, can be expected to record resetting from the temperatures of hydrothermal systems that formed Carlin-type Au deposits (Arehart et al., 2003). The Cretaceous ages from these two samples are thus interpreted to record alteration from heating to temperatures significantly higher than those associated with Carlin-type deposit formation (~180°–240°C; Hofstra et al., 1991; Hofstra and Cline, 2000; Cline et al., 2005).

U-Pb zircon geochronology

The results of the CA-TIMS analyses of four quartz monzonite intrusions are presented in Table 4, and the LA-ICP-MS results are presented in the Electronic Appendix. The CA-TIMS ages are shown in Figure 17.

Table 4.

Chemical Abrasion-Thermal Ionization Mass Spectrometry Analyses of Zircons

Radiogenic isotopic ratiosIsotopic ages
Sample1Th/U2206Pb ×10−13 mol3mol % 206Pb☼3Pb/Pbc3Pbc (pg)3206Pb/204Pb4208Pb/206Pb5207Pb/206Pb5% err6207Pb/235U5% err6206Pb/238U5% err6Correlation coeffient207Pb/206Pb7±6207Pb/235U7±6206Pb/238U7±6
MAR-INT-BP
z10.1501.221799.78%1250.228,3070.0480.0480060.1180.1000380.1720.0151140.0700.86299.552.7896.810.1696.700.07
z20.1420.721099.65%770.215,1050.0460.0479140.1590.0997830.2110.0151040.0720.80695.003.7596.580.1996.640.07
z30.2500.416199.53%590.163,8360.0800.0479100.2290.0997360.2790.0150980.0790.71694.855.4296.530.2696.600.08
z40.1723.773899.92%3550.2522,8780.0550.0716750.0890.2444860.1750.0247390.1220.880976.731.81222.090.35157.540.19
z50.2032.925199.87%2150.3114,0490.0650.0479970.0810.1001500.1440.0151330.0710.93499.141.9396.910.1396.820.07
z60.2261.179299.80%1380.208,9920.0720.0480800.1070.1004370.1640.0151500.0710.878103.232.5497.180.1596.930.07
z70.5850.410199.16%360.292,1450.1870.0480400.3320.0999240.3800.0150860.0800.677101.227.8496.710.3596.520.08
MAR-INT-MP
z1a0.1050.581899.61%680.194,5990.0330.0479250.1710.0984930.2220.0149050.0710.79595.584.0695.380.2095.380.07
z1b0.0751.640599.86%1920.1913,0540.0240.0479600.0880.0997670.1490.0150870.0690.92897.302.0896.560.1496.530.07
z2a0.1210.980999.80%1360.169,1420.0390.0479060.1270.0964380.1810.0146000.0710.84794.613.0093.480.1693.440.07
z2b0.1221.147799.66%800.325,3670.0390.0479210.1360.0965740.1890.0146160.0710.83795.383.2193.610.1793.540.07
z30.4431.212899.78%1370.228,3840.1420.0478880.1180.0955920.1730.0144780.0700.86193.722.7892.700.1592.660.06
z40.3213.043699.91%3350.2221,1580.1030.0479110.0760.0956120.1380.0144740.0700.94294.881.8092.720.1292.630.06
z50.6010.475099.55%680.184,0220.1920.0479220.2090.0955620.2650.0144630.0930.71395.444.9492.670.2392.560.09
z60.3452.087199.84%1820.2711,4430.1100.0478950.0850.0955880.1460.0144750.0700.93394.072.0092.700.1392.640.06
MAR-INT-5
z1a0.2100.309799.46%510.143,3390.0670.0478880.2560.0951230.3070.0144060.0740.74793.766.0692.260.2792.210.07
z1b0.3910.881799.79%1400.158,6800.1250.0478420.1190.0950530.1730.0144100.0710.84691.452.8292.200.1592.230.06
z20.2730.324199.42%480.163,1110.0870.0479330.2610.0975350.3100.0147580.0750.71995.956.1994.500.2894.440.07
z30.2770.124897.96%140.228830.0890.0471131.0050.0938771.0720.0144510.1080.65054.9823.9791.110.9392.490.10
z40.4410.909999.81%1570.149,6180.1410.0480170.1100.0959160.1670.0144880.0700.880100.092.6193.000.1592.720.06
z50.3522.229799.76%1210.447,6040.1130.0479910.0990.0957170.1550.0144650.0690.89298.832.3492.820.1492.580.06
z60.4761.190499.75%1210.247,3130.1520.0479370.1330.0957780.1860.0144910.0720.82996.173.1492.870.1792.740.07
z70.3910.823999.78%1320.158,2090.1250.0479330.1270.0956570.1810.0144740.0700.84995.953.0092.760.1692.640.06
MAR-INT-T2
z1a0.0762.946999.92%3400.1923,0310.0240.0480080.0770.1010540.1390.0152660.0690.94799.681.8297.750.1397.670.07
z1b0.0872.074499.88%2190.2114,8390.0280.0479920.1140.1009430.1640.0152550.0720.80798.882.7097.650.1597.600.07
z20.0880.863299.74%1010.196,8160.0280.0480920.1580.1015090.2060.0153080.0730.756103.823.7498.170.1997.940.07
z30.0781.069499.78%1210.208,1970.0250.0480900.1330.1029190.1850.0155220.0700.828103.683.1499.470.1899.290.07
z4a0.1540.814499.63%730.254,8330.0490.0480940.1600.1018770.2130.0153630.0740.805103.913.7798.510.2098.280.07
z4b0.1160.786499.72%960.186,4140.0370.0537030.1200.1487940.1760.0200950.0690.880358.622.70140.850.23128.260.09
z4c0.1300.631499.50%540.263,6180.0420.0479800.1930.1017590.2440.0153820.0720.77598.304.5798.400.2398.400.07
Radiogenic isotopic ratiosIsotopic ages
Sample1Th/U2206Pb ×10−13 mol3mol % 206Pb☼3Pb/Pbc3Pbc (pg)3206Pb/204Pb4208Pb/206Pb5207Pb/206Pb5% err6207Pb/235U5% err6206Pb/238U5% err6Correlation coeffient207Pb/206Pb7±6207Pb/235U7±6206Pb/238U7±6
MAR-INT-BP
z10.1501.221799.78%1250.228,3070.0480.0480060.1180.1000380.1720.0151140.0700.86299.552.7896.810.1696.700.07
z20.1420.721099.65%770.215,1050.0460.0479140.1590.0997830.2110.0151040.0720.80695.003.7596.580.1996.640.07
z30.2500.416199.53%590.163,8360.0800.0479100.2290.0997360.2790.0150980.0790.71694.855.4296.530.2696.600.08
z40.1723.773899.92%3550.2522,8780.0550.0716750.0890.2444860.1750.0247390.1220.880976.731.81222.090.35157.540.19
z50.2032.925199.87%2150.3114,0490.0650.0479970.0810.1001500.1440.0151330.0710.93499.141.9396.910.1396.820.07
z60.2261.179299.80%1380.208,9920.0720.0480800.1070.1004370.1640.0151500.0710.878103.232.5497.180.1596.930.07
z70.5850.410199.16%360.292,1450.1870.0480400.3320.0999240.3800.0150860.0800.677101.227.8496.710.3596.520.08
MAR-INT-MP
z1a0.1050.581899.61%680.194,5990.0330.0479250.1710.0984930.2220.0149050.0710.79595.584.0695.380.2095.380.07
z1b0.0751.640599.86%1920.1913,0540.0240.0479600.0880.0997670.1490.0150870.0690.92897.302.0896.560.1496.530.07
z2a0.1210.980999.80%1360.169,1420.0390.0479060.1270.0964380.1810.0146000.0710.84794.613.0093.480.1693.440.07
z2b0.1221.147799.66%800.325,3670.0390.0479210.1360.0965740.1890.0146160.0710.83795.383.2193.610.1793.540.07
z30.4431.212899.78%1370.228,3840.1420.0478880.1180.0955920.1730.0144780.0700.86193.722.7892.700.1592.660.06
z40.3213.043699.91%3350.2221,1580.1030.0479110.0760.0956120.1380.0144740.0700.94294.881.8092.720.1292.630.06
z50.6010.475099.55%680.184,0220.1920.0479220.2090.0955620.2650.0144630.0930.71395.444.9492.670.2392.560.09
z60.3452.087199.84%1820.2711,4430.1100.0478950.0850.0955880.1460.0144750.0700.93394.072.0092.700.1392.640.06
MAR-INT-5
z1a0.2100.309799.46%510.143,3390.0670.0478880.2560.0951230.3070.0144060.0740.74793.766.0692.260.2792.210.07
z1b0.3910.881799.79%1400.158,6800.1250.0478420.1190.0950530.1730.0144100.0710.84691.452.8292.200.1592.230.06
z20.2730.324199.42%480.163,1110.0870.0479330.2610.0975350.3100.0147580.0750.71995.956.1994.500.2894.440.07
z30.2770.124897.96%140.228830.0890.0471131.0050.0938771.0720.0144510.1080.65054.9823.9791.110.9392.490.10
z40.4410.909999.81%1570.149,6180.1410.0480170.1100.0959160.1670.0144880.0700.880100.092.6193.000.1592.720.06
z50.3522.229799.76%1210.447,6040.1130.0479910.0990.0957170.1550.0144650.0690.89298.832.3492.820.1492.580.06
z60.4761.190499.75%1210.247,3130.1520.0479370.1330.0957780.1860.0144910.0720.82996.173.1492.870.1792.740.07
z70.3910.823999.78%1320.158,2090.1250.0479330.1270.0956570.1810.0144740.0700.84995.953.0092.760.1692.640.06
MAR-INT-T2
z1a0.0762.946999.92%3400.1923,0310.0240.0480080.0770.1010540.1390.0152660.0690.94799.681.8297.750.1397.670.07
z1b0.0872.074499.88%2190.2114,8390.0280.0479920.1140.1009430.1640.0152550.0720.80798.882.7097.650.1597.600.07
z20.0880.863299.74%1010.196,8160.0280.0480920.1580.1015090.2060.0153080.0730.756103.823.7498.170.1997.940.07
z30.0781.069499.78%1210.208,1970.0250.0480900.1330.1029190.1850.0155220.0700.828103.683.1499.470.1899.290.07
z4a0.1540.814499.63%730.254,8330.0490.0480940.1600.1018770.2130.0153630.0740.805103.913.7798.510.2098.280.07
z4b0.1160.786499.72%960.186,4140.0370.0537030.1200.1487940.1760.0200950.0690.880358.622.70140.850.23128.260.09
z4c0.1300.631499.50%540.263,6180.0420.0479800.1930.1017590.2440.0153820.0720.77598.304.5798.400.2398.400.07
1

z1, z2, etc. are labels for analyses composed of single zircon grains that were annealed and chemically abraded (Mattinson, 2005); labels with same numbers (e.g., z1a, z1b) denote fragments from the same grain; labels in bold denote analyses used in weighted mean calculations

2

Model Th/U ratio calculated from radiogenic 208Pb/206Pb ratio and 207Pb/235U date

3

Pb and Pbc are radiogenic and common Pb, respectively; mol % 206Pb is with respect to radiogenic and blank Pb

4

Measured ratio corrected for spike and fractionation only; fractionation correction is 0.16 ± 0.03 (1 sigma) %/amu (atomic mass unit) for single-collector Daly analyses, based on analysis of EARTH-TIME 202Pb/205Pb tracer solution

5

Corrected for fractionation, spike, common Pb, and initial disequilibrium in 230Th/238U; common Pb is assigned to procedural blank with composition of 206Pb/204Pb = 18.04 ± 0.61%; 207Pb/204Pb =15.54 ± 0.52%; 208Pb/204Pb = 37.69 ± 0.63% (1 sigma); 206Pb/238U and 207Pb/206Pb ratios corrected for initial disequilibrium in 230Th/238U using Th/U (magma) = 3.0 ± 0.3 (1 sigma)

6

Errors are 2 sigma, propagated using algorithms of Schmitz and Schoene (2007) and Crowley et al. (2007)

7

Calculations based on decay constants of Jaffey et al. (1971); 206Pb/238U and 207Pb/206Pb dates corrected for initial disequilibrium in 230Th/238U by Th/U [magma] = 3.0 ± 0.3 (1 sigma)

Fig. 17.

Chemical abrasion-thermal ionization mass spectrometry U-Pb zircon data for four felsic dikes at Marigold. (A) Target 2 intrusion (MI-T2). (B) Basalt Pit intrusion (MI-BP). (C) Intrusion 5 (MI-5). (D) Mackay intrusion (MI-MP). MSWD = mean square of weighted deviates.

Fig. 17.

Chemical abrasion-thermal ionization mass spectrometry U-Pb zircon data for four felsic dikes at Marigold. (A) Target 2 intrusion (MI-T2). (B) Basalt Pit intrusion (MI-BP). (C) Intrusion 5 (MI-5). (D) Mackay intrusion (MI-MP). MSWD = mean square of weighted deviates.

Zircon grains separated from sample MI-T2 (intrusion in Target 2 pit; Fig. 2) have the greatest variation in grain size and morphology of the four intrusions examined in this study. Internal patterns within the zircon grains show growth, resorption, and subsequent overgrowth of the original crystal and complex compositional banding. The oldest grains in the separate, determined by LA-ICP-MS, are Paleoproterozoic to Mesoproterozoic in age (1724 ± 54–1004 ± 35 Ma). These grains are subrounded to rounded with bright cores when imaged by cathodoluminescence (CL). The largest grains in the separate are fragmented and considerably more equant than prismatic. These grains were analyzed by LA-ICP-MS and gave Jurassic 206Pb/238U ages (165 ± 4–148 ± 7 Ma; Electronic App. Table A1-A3). The smaller zircon populations are more euhedral and prismatic. There is no consistent core to rim relationship between the zircon grains when imaged by CL. Thirteen of these grains have high Th/U ratios (0.36–0.90) and when analyzed by LA-ICP-MS gave early Cretaceous ages (114 ± 5–105 ± 6 Ma). Seven grains with Late Cretaceous ages determined by LA-ICP-MS, including two whole zircon grains and five fragments from two additional grains, were analyzed by CA-TIMS (Fig. 17A; Table 4). The igneous crystallization age was determined from two fragments of a single zircon grain (Z1a, b), which yielded a 206Pb/238U weighted mean average age of 97.63 ± 0.05 Ma. Four of the other analyses yield slightly older ages from 99.29 ± 0.07 to 97.94 ± 0.10 Ma.

Zircon grains from sample MI-BP (intrusion in Basalt pit; Fig. 2; Electronic App. Table A4) show significantly less variation in morphology than those in sample MI-T2. The majority of the zircon grains are euhedral and prismatic, and overall they tend to have dark CL signatures. Smaller, more equant grains show evidence of resorption and have distinct cores that are CL bright. These grains have LA-ICP-MS 206Pb/238U ages older than 1 Ga and so are considered inherited. Seven zircon grains from MI-BP were analyzed by CA-TIMS. An igneous crystallization age of 96.59 ± 0.04 Ma was determined from a weighted mean of the three youngest dates (Fig. 17B; Table 4). Dates yielded from three other grains ranged from 96.93 ± 0.07 to 96.70 ± 0.07 Ma.

Zircon grains separated from sample MI-MP (intrusion in Mackay pit; Fig. 2; Electronic App. Table A5) are generally euhedral and prismatic but also contain anhedral grain fragments. A small population of subhedral grains has resorbed cores and was not analyzed by CA-TIMS. One of these grains has a CL-bright core with a CL-dark overgrowth that was analyzed by LA-ICP-MS and gave a Jurassic age (159 ± 4 Ma). Eight CA-TIMS dates were obtained from four whole zircon grains and two grains broken into two pieces and analyzed separately (Fig. 17D; Table 4). Four dates are interpreted to represent the age of magmatic crystallization, with a weighted mean average of 92.63 ± 0.03 Ma. One of the grains, split into two pieces, yielded slightly older but age-equivalent dates of 93.44 ± 0.07 and 93.54 ± 0.07 Ma. The remaining grains yielded older dates of 95.38 ± 0.07 and 96.53 ± 0.07 Ma.

Zircon grains separated from MI-5 (intrusion immediately northwest of Mackay pit; Fig. 2; Electronic App. Table A6) appear very similar to zircons from MI-MP in terms of size, shape, and CL signature. The zircon grains are generally euhedral and prismatic. One grain with a CL-bright core analyzed by LA-ICP-MS gave a Mesoproterozoic age (1113 ± 46 Ma). Grains with darker CL signatures show more variation in zoning. Two grains with CL-dark signatures analyzed by LA-ICP-MS gave Jurassic ages (157 ± 6–154 ± 6 Ma). Seven zircon grains were analyzed by CA-TIMS (Fig. 17C; Table 4), one of which was broken into two fragments and analyzed separately. The two youngest dates were from the two grain fragments; the whole zircon grains yielded slightly older ages. The weighted mean average of the two grain fragments is 92.22 ± 0.05 Ma and is interpreted to represent the magmatic crystallization age. The whole zircon grains yielded slightly older dates ranging from 94.44 ± 0.07 to 92.49 ± 0.10 Ma.

The four intrusions included in this study crystallized in the Late Cretaceous (97.63 ± 0.05–92.22 ± 0.05 Ma), a period of magmatism spanning approximately 5.4 m.y. However, inherited zircon grains in some samples indicate Jurassic magmatic rocks at depth. The two oldest intrusions, T2 and BP, were emplaced approximately 1 m.y. apart, whereas the two youngest intrusions, MI-MP and MI-5, were emplaced approximately 400 k.y. apart. The temporal gaps between dike emplacements are well outside the range of error for the four weighted mean average ages (Fig. 17).

Discussion

Magmatism in the Battle Mountain mining district

Previous descriptions classified the Marigold deposits as distal disseminated, genetically related to shallow magmatism (Doebrich and Theodore, 1996; Theodore, 1998, 2000). Several authors (e.g., Sillitoe and Bonham, 1990; Johnston and Ressel, 2004; Muntean et al., 2011) have suggested that Carlin-type Au deposits also form in magmatic-hydrothermal environments. The question of whether Au mineralization at Marigold could be linked to magmatism motivated study of the intrusive rocks exposed in the deposit area. The following section places the ages of the igneous units identified at Marigold in regional context (Fig. 18). The relationship between these units and mineralization is discussed in subsequent sections.

Fig. 18.

Composite geochronologic history of the Marigold mine area recorded by magmatic zircon grains (this study), 40Ar/39Ar analyses of biotite (this study), and sericite (Marigold, unpub. data, 2004). The frequency of intrusions in northern Nevada (NV) is plotted on the left based on data from du Bray (2007); regional ages for Buffalo Mountain from Neff (1973); Mill Canyon stock from Stewart and McKee (1977); Goldstrike from Arehart et al. (1993b); Richmond stock from Ressel and Henry (2006); Buckingham stock from Theodore et al. (1992); Trenton Canyon from Theodore et al. (1973); Osgood stock from Silberman et al. (1974); Carlin-type Au from Cline et al. (2005); Battle Mountain (BM) from King (2015); Marigold rhyolite and Treaty Hill basalt from McKee (2000). CA-TIMS = chemical abrasion-thermal ionization mass spectrometry, LA-ICP-MS = laser ablation-inductively coupled plasma mass spectrometry.

Fig. 18.

Composite geochronologic history of the Marigold mine area recorded by magmatic zircon grains (this study), 40Ar/39Ar analyses of biotite (this study), and sericite (Marigold, unpub. data, 2004). The frequency of intrusions in northern Nevada (NV) is plotted on the left based on data from du Bray (2007); regional ages for Buffalo Mountain from Neff (1973); Mill Canyon stock from Stewart and McKee (1977); Goldstrike from Arehart et al. (1993b); Richmond stock from Ressel and Henry (2006); Buckingham stock from Theodore et al. (1992); Trenton Canyon from Theodore et al. (1973); Osgood stock from Silberman et al. (1974); Carlin-type Au from Cline et al. (2005); Battle Mountain (BM) from King (2015); Marigold rhyolite and Treaty Hill basalt from McKee (2000). CA-TIMS = chemical abrasion-thermal ionization mass spectrometry, LA-ICP-MS = laser ablation-inductively coupled plasma mass spectrometry.

Three main pulses of intrusive magmatism, which occurred in the Jurassic, Cretaceous, and Eocene, have been recognized across northern Nevada (Cline et al., 2005). Prior to this study, Jurassic intrusions were unknown in the Battle Mountain district. The lamprophyre, dated here using 40Ar/39Ar biotite geochronology, and the population of inherited Jurassic zircon grains in the quartz monzonite dikes represent the first reported Jurassic magmatic ages from this district. The nearest known occurrences of Jurassic igneous rocks are reported at Buffalo Mountain in the range approximately 18 km west of Marigold (Neff, 1973), the McCoy stock 25 km south of the Battle Mountain district (Emmons and Eng, 1995), and in the Cortez

Mountains approximately 80 km southeast of Marigold (Stewart and McKee, 1977). The Jurassic magmatic ages from this study (Fig. 18) are coincident with the ages of the Buffalo Mountain pluton (160–146 Ma; Neff, 1973), the Mill Canyon stock in the Cortez range (158 Ma; Stewart and McKee, 1977), and the Goldstrike pluton on the Carlin trend (158 Ma; Arehart et al., 1993b). The inherited zircon grains in the dikes provide evidence for Jurassic felsic magmatic rocks present at depth in the Marigold area, whereas the lamprophyre dike indicates that deep ultrapotassic melts were being generated at or near the same time. While limited conclusions can be drawn from the presence of a single lamprophyre in the northern Battle Mountain district, Jurassic lamprophyres are common in Au deposits on the Carlin trend (Ressel and Henry, 2006) and have been cited as evidence of deeply penetrating crustal structures critical to localizing magmatism and subsequent episodes of hydrothermal fluid circulation (Emsbo et al., 2006).

The ~98 to 92 Ma Cretaceous quartz monzonite dikes at Marigold are coeval with the monzogranite Buckingham stock on the east side of the district (99–92 Ma; Keeler, 2010) and other known Cretaceous intrusions in the district (Fig. 18), including the Trenton Canyon monzogranite 5 km south of Marigold (87 Ma K-Ar age of biotite; Theodore et al., 1973). Quartz monzonite at the Buffalo Valley molybdenum deposit is assumed to be Cretaceous based on compositional similarities to the intrusions at Trenton Canyon and Buckingham (Doebrich and Theodore, 1996). King (2011) demonstrated that granodiorite porphyries and monzogranite at Elder Creek are Eocene in age (38–35 Ma), as is the major pulse of magmatism at Copper Canyon (Theodore et al., 1973), so magmatism of this composition was not limited to the Cretaceous in the district. The location of the source pluton for the Cretaceous-age Marigold dikes is not known, but it is possible that the dikes are related to one of the larger known intrusive systems of equivalent age elsewhere in the district. The Marigold dikes are also of similar age to the Osgood stock 30 km north of Marigold near the Getchell, Turquoise Ridge, and Twin Creeks Carlin-type deposits (~92 Ma; Silberman et al., 1974; Groff et al., 1997).

Mineralization styles

The spatial, textural, and geochemical relationships indicate that there were two styles of mineralization at Marigold: base metals + Ag, and Au-As-Sb-Tl. Each style occurs with a particular suite of alteration minerals (Fig. 10) and each with distinctive pyrite geochemistry (Fig. 14). The two styles may represent two separate events or a single system that evolved in composition over time. An estimated 8 km3 of rock are Au mineralized at Marigold. In contrast, the base metal and Ag distribution is not spatially extensive or continuous based on available data. Based on the significant difference in spatial extent of the two styles, as well as their apparent preferential occurrence in different rocks, two separate events may be more likely. However, both styles of mineralization can occur in fault zones; this could be evidence of either a single system with evolving fluid composition or the continued exploitation of the same fluid pathways by fluids of different ages and origins.

The spatial association of base metals and Ag with the dikes suggests a possible genetic relationship between the intrusions and this style of mineralization at Marigold. No evidence was found in this study to link the Au mineralization to uppercrustal magmatism. Although a magmatic origin for the Au cannot be ruled out, the salient characteristics of mineralization caused by shallow felsic intrusions in distal disseminated settings (Table 5) were not observed, such as zoned alteration with respect to intrusions, characteristic ore mineralogy of distal disseminated deposits, or stable isotope signatures indicative of magmatic fluids.

Table 5.

Comparison of Key Characteristics of the Marigold Deposit with Published Characteristics for Distal Disseminated and Carlin-Type Deposits

FeatureCarlin-typeDistal disseminatedMarigold
AlterationDecarbonatization, argillization, silicification, sulfidationArgillization, silicification, decalcification, potassic, phyllicDecarbonatization, argillization, silicification, pyritization, sulfidation
Geochemical signatureAu, As, S, Sb, Hg, TlAg, Au, Sb, As, Hg, Te, Pb, Zn, Cu, Na, K, Mn ± BaBase metal stage: Cu, Ag, Pb, Zn, Ni, Sn, Au, As, Na, K, Mn Gold stage: Au, As, S, Sb, Hg, Tl
Nature of goldSolid solution or submicron inclusions within arsenian pyrite, oxide zone native AuNative, electrum, solid solution, or inclusions in chalcopyrite, tetrahedrite, pyrite, and arsenopyriteSubmicron particles within arsenian pyrite, oxide zone native Au
Ag/Au<1:1Up to 400:1<1:1 above and below redox
Associated rocksNo genetic relationship to felsic hypabyssal intrusionsGenetically related to felsic hypabyssal intrusionsNo shallow magmatic signature
FeatureCarlin-typeDistal disseminatedMarigold
AlterationDecarbonatization, argillization, silicification, sulfidationArgillization, silicification, decalcification, potassic, phyllicDecarbonatization, argillization, silicification, pyritization, sulfidation
Geochemical signatureAu, As, S, Sb, Hg, TlAg, Au, Sb, As, Hg, Te, Pb, Zn, Cu, Na, K, Mn ± BaBase metal stage: Cu, Ag, Pb, Zn, Ni, Sn, Au, As, Na, K, Mn Gold stage: Au, As, S, Sb, Hg, Tl
Nature of goldSolid solution or submicron inclusions within arsenian pyrite, oxide zone native AuNative, electrum, solid solution, or inclusions in chalcopyrite, tetrahedrite, pyrite, and arsenopyriteSubmicron particles within arsenian pyrite, oxide zone native Au
Ag/Au<1:1Up to 400:1<1:1 above and below redox
Associated rocksNo genetic relationship to felsic hypabyssal intrusionsGenetically related to felsic hypabyssal intrusionsNo shallow magmatic signature

Data for distal disseminated deposits from Cox and Singer (1990), Cox (1992), and Hofstra and Cline (2000); data for Carlin-type deposits from Hofstra and Cline (2000) and Cline et al. (2005)

The shallow magmatic-hydrothermal activity (0.5–8 km; Hofstra and Cline, 2000) that produces distal disseminated deposits would be expected to generate alteration zonation with respect to a causative magma body, but no such pattern has been observed at Marigold. The geochemical signature of distal disseminated deposits contains many elements in common with Carlin-type deposits, but K and Mn are useful indicators. These elements are typically enriched in ore zones in distal disseminated deposits, whereas they are unchanged or depleted in Carlin-type Au deposits (Albino, 1993; Bloomstein et al., 2000; Peters et al., 2004). On the deposit scale at Marigold, Au does not have a consistent relationship with Mn and K, whereas base metals such as Cu do have a positive relationship with Mn and K. The relationship of microscopic potassic alteration to Au mineralization remains unknown, as this alteration style was only identified in one sample. It is possible that subtle potassium metasomatism of host rocks may be more pervasive than currently recognized.

In distal disseminated Au ± Ag deposits, ore-stage veins host native Au, native Ag, electrum, argentite, Ag sulfosalts, tetrahedrite, stibnite, galena, sphalerite, chalcopyrite, pyrite, marcasite, arsenopyrite, ± stannite, ± canfieldite (Cox and Singer, 1990). The Ag to Au ratio of ores in distal disseminated Au ± Ag deposits is as high as 143:1, such as at Taylor, Nevada (Cox, 1992). Gold can occur natively, as well as in chalcopyrite, tetrahedrite, arsenopyrite, and pyrite (Hofstra and Cline, 2000). The hypogene Au deportment in disseminated arsenic-rich pyrite at Marigold does not provide definitive evidence for a particular origin. However, veins are not a significant host to ore at Marigold. In addition, native Ag, electrum, argentite, and Ag sulfosalts are not present, and the base metal sulfides appear to be unrelated to and likely predating the introduction of the majority of Au. Previous workers (Doebrich and Theodore, 1996; Theodore, 1998, 2000) attributed the low Ag/Au ratio at Marigold to leaching during protracted oxidation beginning in the Tertiary, but comparison of data from drill core above and below the redox boundary (Fig. 15) indicates that the ratio has not been significantly changed by oxidation. However, a gradual increase in Ag/Au ratios with depth (Fig. 15A) suggests the possibility that there may be vertical zoning in mineralization styles at Marigold.

Sulfur isotopes of vein barite are an additional line of evidence previously used to assign a shallow magmatic-hydrothermal signature to Marigold (Doebrich and Theodore, 1996). Barite is elevated in Au-bearing fault zones at Marigold; however, the paragenetic relationships between Au and vein barite are unclear. At least six different morphological types of barite are documented by McGibbon and Wallace (2000), three of which are of the vein variety. Graney and McGibbon (1991) observed that barite veinlets crosscut mineralized rock, and McGibbon and Wallace (2000) suggest that late silica, barite, and calcite flooded mineralized structures, diluting Au grade. McGibbon (2005) notes that barite is very rarely documented in drill hole logging, and barite was not observed in association with Au-stage pyrite in this study. Howe and Theodore (1993) and Howe et al. (1995) report δ34S values from 8.2 to 13.7‰ for vein barite from the Antler sequence-hosted 8 South, 8 North, and 5 North deposits. Barite from the primarily Valmy-hosted Top and East Hill zones (Terry Complex) has markedly lighter δ34S values from –0.7 to 1.3‰. Those authors suggest that the isotopically heavier barite formed due to progressive oxidation of the hydrothermal fluid. According to this line of reasoning, barite of the Terry Complex must have formed from a relatively unevolved magmatic-hydrothermal fluid, presumably adjacent to a porphyry system, no other evidence of which has been encountered in exploration and mining. Doebrich and Theodore (1996) compare δ34S of vein barite from Marigold and the Carlin trend, concluding that the isotopically lighter barite at Marigold evidences a greater magmatic component to the hydrothermal fluid. Doebrich and Theodore (1996) also recognized that the isotopic signature of vein barite on the Carlin trend is nearly identical to that of bedded barite in the region, an observation that supports the hypothesis of Arehart et al. (1993a) that S in ore fluids was, in part, derived from lower Paleozoic seawaterderived sulfate. Bedded barite is undocumented in the strata proximal to the Marigold mine. However, scavenging of S from the host rock may explain the measured values of δ34S of vein barite from the 8 South, 8 North, and 5 North deposits, as the reported δ34S values of 8.2 to 13.7‰ are consistent with the δ34 values of seawater sulfate in the Pennsylvanian to Permian (Claypool et al., 1980; Chang et al., 2008), during which time the sedimentary rocks of the Antler sequence were deposited. Considering that a wide range of δ34S is exhibited by barite in Carlin-type Au deposits and that late-stage barite exhibits lower δ34S than early barite (Arehart et al., 1993a; Cline et al., 2005), a similar interpretation to Arehart et al. (1993a) can be made for the barite at Marigold. In summary, the δ34S values from vein barite at Marigold do not provide conclusive evidence for a magmatic-hydrothermal origin of the Au.

The classic distal disseminated Au ± Ag deposits occur within relatively short distances of major porphyry systems. For example, the Purísima Concepción distal disseminated Au deposit is within hundreds of meters of base metal replacement bodies that are adjacent to a porphyry stock in the Yauricocha district, Peru (Alvarez and Noble, 1988). The Jeronimo distal disseminated Au deposit is 4.3 km from the El Hueso porphyry system in the Potrerillos district, Chile (Gale, 1999). At Bau in Malaysia, disseminated Au mineralization occurs within 2 km of the nearest known porphyritic stock (Sillitoe and Bonham, 1990). The disseminated Au deposits at Melco and Barney’s Canyon are 6 and 7.5 km, respectively, from the giant porphyry deposit at Bingham Canyon, Utah (Cunningham et al., 2004). At the Cove mine in the Fish Creek Mountains south of the Battle Mountain district, polymetallic veins and distal disseminated ores are located 1.6 km from the contemporaneous McCoy skarn (Johnston et al., 2008). The most cogent evidence for distal disseminated Au in the Battle Mountain mining district is from Buffalo Valley, where alteration, base metal, and precious metal mineralogy are zoned within hundreds of meters of Tertiary (36.9 ± 1.1–33.7 ± 1.1 Ma) granodioritic intrusions (Kizis et al., 1997; Reid et al., 2010). On the southern end of the Marigold property the Basalt-Antler pit is over 6 km from the nearest known porphyry at the Cretaceous Trenton Canyon molybdenum prospect. The northernmost Marigold deposit, 5 North, is approximately 10 km from the nearest known porphyry deposit at Converse. However, there may be unexposed plutons in the Marigold area that have not been encountered during exploration and mining.

At Lone Tree 7 km northwest of Marigold’s 5 North deposit, Au occurs in the altered margins of felsic dikes; a distal disseminated origin is generally assumed for the deposit (Doebrich and Theodore, 1996; Panhorst, 1996; Hofstra and Cline, 2000), but a genetic link between Au mineralization and magmatism has not been conclusively demonstrated. The base metals occur as galena, sphalerite, chalcopyrite, Pb-Ag selenides, polybasite, and Ag-bearing tetrahedrite in calc-silicate altered Havallah sequence siltstone (Bloomstein et al., 2000). Similar to Marigold, there is strong structural control of ore in Au-mineralized zones, and Au is associated with As. However, there are key differences from Marigold: at Lone Tree the Au occurs in veins and silicified breccias with abundant coarse ore-stage pyrite and arsenopyrite; adularia, sericite, and calcite are paragenetically associated with the Au (Bloomstein et al., 2000). In addition, significant ore occurs above the Golconda thrust, whereas rocks above this thrust are essentially barren at Marigold. It is possible that the Au deposits at Marigold represent distal expressions of the hydrothermal system responsible for mineralization at Lone Tree, but further data are needed to evaluate this hypothesis.

In the absence of a clear link to causative shallow magmatism as would be expected for distal disseminated deposits, the geochemistry, mineralogy, and alteration at Marigold are most consistent with Carlin-type deposits (cf. Fleet and Mumin, 1997; Hofstra and Cline, 2000; Cline, 2001; Heitt et al., 2003; Cline et al., 2005, 2008; Large et al., 2009; Almeida et al., 2010). Similar to published characteristics of known Carlintype deposits, at Marigold the Au mineralization occurred in fault zones, faulted dike margins, and in the sedimentary host-rock package. Below the redox boundary at Marigold the Au mineralization occurs with arsenic in fuzzy pyrite rims in association with decarbonatization, minor silicification, and argillization; these features are characteristic of Carlin-type Au deposits but not diagnostic.

The most apparent differences between Marigold and typical Carlin-type Au deposits are the low Au grade, the uncommon position in stratigraphy (upper plate of Roberts Mountain thrust), the dominantly siliciclastic host lithology, presence of minor quartz veinlets, and setting in a porphyrydominated district. The Au grade from past production records at Marigold is 0.68 g/t, which is significantly lower than the 10th percentile grade of 0.83 g/t for Carlin-type deposits (n = 88; Carlin-type deposit data from Berger et al., 2014). The ore at Marigold is economic in part because Au is present on fracture surfaces in oxidized Valmy quartzite and can be heap leached without crushing. Despite its low grade, the Marigold deposits contain more Au than the largest distal disseminated Au ± Ag deposits (Cox, 1992; Hofstra and Cline, 2000). Marigold is large enough to be considered a world-class (>100 t Au) Carlin-type deposit, of which there are only 15 in Nevada (Cline et al., 2005; Berger et al., 2014). Figure 19 shows Marigold grade and tonnage in comparison to Carlin-type deposits classified based on host-rock type.

Fig. 19.

Ore grade (g/t Au) vs. metric tonnes of ore for Carlin-type deposits in Nevada. Blue icons represent deposits hosted by slope rocks on rifted western margin of North American craton. Purple icons represent deposits hosted in “complex tectonic settings of imbricate thrust structures at craton margins” (Berger et al., 2014, p. 20). Orange icons represent deposits hosted in rocks related to the foreland basin of the Antler orogeny. Marigold is shown in red; grade is the total grade of the Marigold deposits, and tonnage is the total of past production and resource. Higher grades but lower tonnages of Marigold ore are hosted in Antler orogeny foreland basin rocks, whereas lower grades and higher tonnages are hosted in eugeoclinal siliciclastic rocks of the Roberts Mountain allocthon. Marigold is most similar to the lower plate-hosted Mike deposit and has grades similar to Carlin-type deposits hosted by rocks of foreland basin of the Antler orogeny. Grade, tonnage, and classification of Carlin-type deposits from Berger et al. (2014).

Fig. 19.

Ore grade (g/t Au) vs. metric tonnes of ore for Carlin-type deposits in Nevada. Blue icons represent deposits hosted by slope rocks on rifted western margin of North American craton. Purple icons represent deposits hosted in “complex tectonic settings of imbricate thrust structures at craton margins” (Berger et al., 2014, p. 20). Orange icons represent deposits hosted in rocks related to the foreland basin of the Antler orogeny. Marigold is shown in red; grade is the total grade of the Marigold deposits, and tonnage is the total of past production and resource. Higher grades but lower tonnages of Marigold ore are hosted in Antler orogeny foreland basin rocks, whereas lower grades and higher tonnages are hosted in eugeoclinal siliciclastic rocks of the Roberts Mountain allocthon. Marigold is most similar to the lower plate-hosted Mike deposit and has grades similar to Carlin-type deposits hosted by rocks of foreland basin of the Antler orogeny. Grade, tonnage, and classification of Carlin-type deposits from Berger et al. (2014).

The majority of Carlin-type deposits above and east of the Roberts Mountain thrust are hosted in Devonian-Mississippian limestone and shale, and the dominance of chemically inert Valmy quartzite at Marigold may explain the low average grade. The argillaceous “dirty” carbonates that host most large Carlin-type deposits contain significant concentrations of ferrous iron that was sulfidized by Au-bearing hydrothermal fluids (Hofstra et al., 1991). Desulfidation of hydrothermal fluids results in decreased Au solubility as the amount of sulfur available to form the ligand of Au bisulfide complexes is decreased (Hofstra et al., 1991; Fortuna et al., 2003; Kesler et al., 2003). At Marigold, the relatively unreactive, siliciclastic rocks of the Ordovician Valmy Formation likely contained insufficient concentrations of ferrous iron to precipitate high concentrations of Au (e.g., Hofstra and Cline, 2000). For comparison, the now-mined-out 8 South deposit at Marigold, which was hosted primarily by Antler sequence rocks including the Antler Peak Limestone, was the highest-grade ore on the property (averaging 2.8 g/t; Graney and McGibbon, 1991). Classic Carlin-style alteration characteristics were present in those rocks, including evidence of decarbonatization and preferential Au mineralization of silty limestone beds.

Timing of mineralization

Given the available data, the absolute ages of the two mineralizing styles at Marigold cannot be conclusively determined. The base metals at Marigold must be younger than the Cretaceous dikes (~92 Ma) and could be either Late Cretaceous or Eocene by correlation with other events in the district. From limited textural evidence at Marigold, the major episode of Au mineralization appears to have postdated base metal and Ag mineralization. Given the observations collected in this study, it is not possible to determine whether Au mineralization occurred immediately afterward or in a separate, later event.

Base metal events occurred elsewhere in the district in both the Cretaceous and the Eocene. At the Trenton Canyon molybdenum prospect 6 km southwest of the Basalt-Antler pit, Cu-Mo mineralization is associated with Cretaceous monzogranite in the footwall of the Oyarbide fault (Theodore et al., 1973). The Late Cretaceous Buckingham stockwork molybdenum deposit contains an estimated 1 Mt of molybdenum (Loucks and Johnson, 1992; McKee, 1992) and includes base metal-bearing phases such as freibergite (in the argentotennantite-tetrahedrite solid solution series), chalcopyrite, sphalerite, arsenopyrite, and cassiterite (Loucks and Johnson, 1992; Theodore, 2000).

Thirty kilometers north of the Battle Mountain district in the Getchell trend, base metal mineralization occurred on the flanks of the Osgood Mountains in association with Cretaceous intrusions. There the base metal veins occur within zones of 92 to 75 Ma sericitic alteration (Groff et al., 1997). Similar to the Getchell trend, at Marigold sericite occurs with base metals in the Cretaceous felsic dikes. In the absence of any other data, it is hypothesized that the Marigold base metal mineralization occurred in association with the Late Cretaceous sericitic alteration dated at 88.0 ± 0.46 and 79.59 ± 0.16 Ma.

Eocene intrusive rocks were not identified at Marigold but are present and associated with mineralization elsewhere in the district, including at Buckingham (Keeler, 2010), Elder Creek (King, 2011), Copper Basin (Keeler, 2010), Buffalo Valley (Doebrich, 1995; Kizis et al., 1997; Reid et al., 2010), and Copper Canyon (Theodore et al., 1973). At Lone Tree, Bloomstein et al. (2000) noted an undated base metal and Ag event occurred prior to Au mineralization, and the Au event is Eocene or younger based on crosscutting relationships with Eocene dikes (Holley et al., 2015).

The characteristics of mineralization at Marigold are most strikingly similar to other Carlin-type Au overprints on Mesozoic hydrothermal activity, as at Getchell, Turquoise Ridge, and Twin Creeks on the Getchell trend (Cline, 2001) and Goldstrike on the Carlin trend (Emsbo et al., 2003). If the Au mineralization is Carlin-type, it may be Eocene by correlation with the ages of other Carlin-type Au deposits in Nevada (i.e., 40–33 Ma; Arehart et al., 2003; Cline et al., 2005; Ressel and Henry 2006; Muntean et al., 2011). Figure 20 summarizes a possible chronology given the available data at Marigold: Cretaceous base metal mineralization followed by Eocene Au mineralization; however, absolute ages for the mineralizing events at Marigold remain unclear.

Fig. 20.

Summary of the depositional, tectonic, magmatic, and mineralization history of the Battle Mountain mining district; red arrows highlight the ages of intrusive magmatism and the hypothesized ages of mineralization at Marigold.

Fig. 20.

Summary of the depositional, tectonic, magmatic, and mineralization history of the Battle Mountain mining district; red arrows highlight the ages of intrusive magmatism and the hypothesized ages of mineralization at Marigold.

Conclusions

The Marigold deposits constitute minor Ag-base metal mineralization overprinted by major Au-As-Sb-Tl mineralization. Preparation of the host rock was associated with emplacement of Late Cretaceous quartz monzonite intrusions, when acidic hydrothermal fluids generated fluid pathways and sericitic alteration. The base metal mineralization may have occurred at this time.

During Au mineralization, hydrothermal fluids appear to have ascended some of the same structures that accommodated the dike emplacement. The Au fluids were not hot enough to reset Late Cretaceous 40Ar/39Ar ages recorded by secondary white micas in the dikes, suggesting temperatures <350°C. This study did not identify a clear affiliation between Au mineralization and magmatic-hydrothermal activity at Marigold. The Au mineralization could have been a later product of the same event that produced the base metal mineralization or the result of a subsequent event of a different origin. It is likely that the Marigold Au event occurred in the Eocene, given the similarities between Au mineralization at Marigold and Carlin-type deposits elsewhere in northern Nevada.

The Au deposits at Marigold are best classified as Carlintype Au deposits, although the host rocks are atypical. Most of the Au at Marigold is hosted in siliciclastic rocks of the Roberts Mountain allochthon. Higher grades but lower tonnages are hosted in carbonates and debris flows of the autochthonous Antler sequence. Alteration of the siliciclastic host rocks is characterized by cryptic silicification, argillization, sulfidation, and potentially pyritization, whereas chemically reactive carbonates and debris flows display these alteration styles more strongly, as well as significant decarbonatization. In the oxide zone, Au occurs natively with iron oxides on fracture surfaces and along high-angle structures and structural intersections. Below the redox boundary, the majority of the Au occurs within As-enriched pyrite overgrowths on arsenopyrite and pre-gold stage pyrite.

Acknowledgments

Acknowledgments

This project was part of an M.S. thesis by the lead author, who gratefully acknowledges support by Goldcorp, Inc. During field work, Marigold Mining Company, Silver Standard Resources, and James Carver provided critical support. At Colorado School of Mines (Golden, Colorado), Dante Huff, Katharina Pfaff, John Skok, and Gary Zito provided assistance with sample analyses and Yvette Kuiper and Ric Wendlandt gave valuable insight on data interpretation. The authors thank Heather Lowers at the U.S. Geological Survey in Denver, Colorado, for assistance with electron probe microanalyses, Jim Crowley at the Boise State Isotope Geology Laboratory (Idaho) for U-Pb geochronology, and Matt Heizler and the New Mexico Geochronology Research Laboratory (Socorro, New Mexico) for 40Ar/39Ar analyses. Reviews by John Muntean, Doug McGibbon, and Ted Theodore greatly improved the manuscript.

This project was part of an M.S. thesis by the lead author, who gratefully acknowledges support by Goldcorp, Inc. During field work, Marigold Mining Company, Silver Standard Resources, and James Carver provided critical support. At Colorado School of Mines (Golden, Colorado), Dante Huff, Katharina Pfaff, John Skok, and Gary Zito provided assistance with sample analyses and Yvette Kuiper and Ric Wendlandt gave valuable insight on data interpretation. The authors thank Heather Lowers at the U.S. Geological Survey in Denver, Colorado, for assistance with electron probe microanalyses, Jim Crowley at the Boise State Isotope Geology Laboratory (Idaho) for U-Pb geochronology, and Matt Heizler and the New Mexico Geochronology Research Laboratory (Socorro, New Mexico) for 40Ar/39Ar analyses. Reviews by John Muntean, Doug McGibbon, and Ted Theodore greatly improved the manuscript.

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Figures & Tables

Fig. 1.

Location of the Marigold mine in north-central Nevada relative to major mineral trends; modified after Wallace et al. (2004).

Fig. 1.

Location of the Marigold mine in north-central Nevada relative to major mineral trends; modified after Wallace et al. (2004).

Fig. 2.

(A) General stratigraphic sequence in the northern Battle Mountain district. (B) Geologic map of the northern Battle Mountain mining district, showing the locations of major deposits; modified after Theodore (1998) with additional data from mapping in this study and unpublished data from Marigold Mining Company. Rocks historically described as Scott Canyon Formation belong to the Valmy and Harmony Formations (Ketner, 2008, 2013) and are shown as Valmy-Harmony Formation. (C) From north to south, deposits at Marigold (filled black circles) include 5 North, 8 North, Terry North, 8 South, Old Marigold, Terry, Mackay, Target 1, Target 2, Antler, and Basalt. (D) Location of core drill holes with samples discussed in text. Drill holes from the 2014–2015 deep drilling program are shown with black markers. (E) Location of intrusions (pink squares) identified in this study that are too small to be mapped on this scale. A-A′ shows the location of Figure 3. Grid coordinates are UTM (NAD83). Abbreviations: BP = Basalt Pit intrusion, LS = limestone, MP = Mackay Pit intrusion, MS= Moonshine intrusion, TII = Target 2 intrusion.

Fig. 2.

(A) General stratigraphic sequence in the northern Battle Mountain district. (B) Geologic map of the northern Battle Mountain mining district, showing the locations of major deposits; modified after Theodore (1998) with additional data from mapping in this study and unpublished data from Marigold Mining Company. Rocks historically described as Scott Canyon Formation belong to the Valmy and Harmony Formations (Ketner, 2008, 2013) and are shown as Valmy-Harmony Formation. (C) From north to south, deposits at Marigold (filled black circles) include 5 North, 8 North, Terry North, 8 South, Old Marigold, Terry, Mackay, Target 1, Target 2, Antler, and Basalt. (D) Location of core drill holes with samples discussed in text. Drill holes from the 2014–2015 deep drilling program are shown with black markers. (E) Location of intrusions (pink squares) identified in this study that are too small to be mapped on this scale. A-A′ shows the location of Figure 3. Grid coordinates are UTM (NAD83). Abbreviations: BP = Basalt Pit intrusion, LS = limestone, MP = Mackay Pit intrusion, MS= Moonshine intrusion, TII = Target 2 intrusion.

Fig. 3.

Cross section looking to the north; section line shown in Figure 2E. Gold is hosted in quartzite, argillite, and chert of the Valmy Formation and mixed carbonate and clastic rocks of the Antler sequence. The Havallah sequence is typically unmineralized or weakly mineralized.

Fig. 3.

Cross section looking to the north; section line shown in Figure 2E. Gold is hosted in quartzite, argillite, and chert of the Valmy Formation and mixed carbonate and clastic rocks of the Antler sequence. The Havallah sequence is typically unmineralized or weakly mineralized.

Fig. 4.

Representative alteration styles at Marigold. (A) High-density quartz veining in silicified argillite of the Valmy Formation (DDH6008, 722.7–723.9 m) resulting in local brecciation; this alteration style is not indicative of Au mineralization. (B) Volcaniclastic debris flow of the Preble Formation (DDH6029, 774.5–775 m) with thin calcite veinlets. Note volcaniclast replaced by pyrite on lower left. (C) Oxidized pyrite veinlets in Valmy quartzite (DDH6008, 297 m). The iron oxide filled fractures and veinlets crosscut quartz flooding (white) in quartzite. (D, E) Bleached argillite of the Valmy Formation (DDH5331, 216.7 m) cut by native Au-bearing quartz and indigenous limonite microveinlets. Note maroon potassic alteration halo around veinlets. (F) Oxidized gouge from a high-angle mineralized structure in the Mackay Phase 1 pit. Illite is developed in mineralized structures. (G) Jasperoid with quartz veinlets in Havallah sequence carbonate from west of the 5 North deposit. (H) Pervasively argillically altered margin of quartz monzonite intrusion with iron oxide pseudomorphs after pyrite. Abbreviations: bt = biotite, cc = calcite, FeOx = iron oxide, qtz = quartz.

Fig. 4.

Representative alteration styles at Marigold. (A) High-density quartz veining in silicified argillite of the Valmy Formation (DDH6008, 722.7–723.9 m) resulting in local brecciation; this alteration style is not indicative of Au mineralization. (B) Volcaniclastic debris flow of the Preble Formation (DDH6029, 774.5–775 m) with thin calcite veinlets. Note volcaniclast replaced by pyrite on lower left. (C) Oxidized pyrite veinlets in Valmy quartzite (DDH6008, 297 m). The iron oxide filled fractures and veinlets crosscut quartz flooding (white) in quartzite. (D, E) Bleached argillite of the Valmy Formation (DDH5331, 216.7 m) cut by native Au-bearing quartz and indigenous limonite microveinlets. Note maroon potassic alteration halo around veinlets. (F) Oxidized gouge from a high-angle mineralized structure in the Mackay Phase 1 pit. Illite is developed in mineralized structures. (G) Jasperoid with quartz veinlets in Havallah sequence carbonate from west of the 5 North deposit. (H) Pervasively argillically altered margin of quartz monzonite intrusion with iron oxide pseudomorphs after pyrite. Abbreviations: bt = biotite, cc = calcite, FeOx = iron oxide, qtz = quartz.

Fig. 5.

Representative vein styles at Marigold. (A) Sphalerite-pyrite-quartz veinlets in Valmy quartzite (DDH6029, 857 m). Note vuggy euhedral quartz. (B) Sphalerite-bearing quartz veinlet in Valmy quartzite (DDH6029, 871 m). In rare instances, small (<0.5 cm) sphalerite grains are observable in hand sample at Marigold, whereas other base metal sulfides such as chalcopyrite can only be detected under magnification. (C) Calcite veinlet cutting carbonate clasts and pyrite in the matrix of a carbonate debris flow (DDH6008, 1139 m). (D) En echelon Au-bearing quartz veinlets with a biotite selvage cut bleached argillite of the Valmy Formation (DDH5331, 216.7 m). Relationship between these veinlets and the overall Au mineralizing system is unknown. (E) Ankerite vein and ankerite-flooded matrix in conglomerate of the Battle Formation. Ankerite flooding occurs proximal to the Target 2 quartz monzonite dike. (F) Ankerite veins with barite core and barite-ankerite veinlets in Havallah siltstone above the unmined Red Dot deposit approximately 50 m southwest of the INT-5 quartz monzonite dike. (G) Quartz-chalcopyrite vein in chert of the Havallah sequence. Most of the copper occurs as malachite; however, minor amounts of quartz-encapsulated chalcopyrite remain. Sample location is 50 m north-northeast of a quartz monzonite dike, approximately 285 m west of Old Marigold. (H) Coarse stibnite and barite on a fracture surface of Valmy quartzite (DDH6029, 823 m). Abbreviations: ank = ankerite, bar = barite, bt = biotite, cc = calcite, FeOx = iron oxide, mal = malachite, py = pyrite, qtz = quartz, sph = sphalerite, stbn = stibnite.

Fig. 5.

Representative vein styles at Marigold. (A) Sphalerite-pyrite-quartz veinlets in Valmy quartzite (DDH6029, 857 m). Note vuggy euhedral quartz. (B) Sphalerite-bearing quartz veinlet in Valmy quartzite (DDH6029, 871 m). In rare instances, small (<0.5 cm) sphalerite grains are observable in hand sample at Marigold, whereas other base metal sulfides such as chalcopyrite can only be detected under magnification. (C) Calcite veinlet cutting carbonate clasts and pyrite in the matrix of a carbonate debris flow (DDH6008, 1139 m). (D) En echelon Au-bearing quartz veinlets with a biotite selvage cut bleached argillite of the Valmy Formation (DDH5331, 216.7 m). Relationship between these veinlets and the overall Au mineralizing system is unknown. (E) Ankerite vein and ankerite-flooded matrix in conglomerate of the Battle Formation. Ankerite flooding occurs proximal to the Target 2 quartz monzonite dike. (F) Ankerite veins with barite core and barite-ankerite veinlets in Havallah siltstone above the unmined Red Dot deposit approximately 50 m southwest of the INT-5 quartz monzonite dike. (G) Quartz-chalcopyrite vein in chert of the Havallah sequence. Most of the copper occurs as malachite; however, minor amounts of quartz-encapsulated chalcopyrite remain. Sample location is 50 m north-northeast of a quartz monzonite dike, approximately 285 m west of Old Marigold. (H) Coarse stibnite and barite on a fracture surface of Valmy quartzite (DDH6029, 823 m). Abbreviations: ank = ankerite, bar = barite, bt = biotite, cc = calcite, FeOx = iron oxide, mal = malachite, py = pyrite, qtz = quartz, sph = sphalerite, stbn = stibnite.

Fig. 6.

Alteration geochemistry in Marigold samples. (A) Whole-rock geochemical classification of felsic dikes (see Fig. 2 for dike names and locations) using the total alkalis vs. silica classification diagram of Le Maitre (2002); some fields intentionally left blank for visual clarity. Compositions of select intrusions from nearby deposits are from Roberts (1964), Loucks and Johnson (1992), and McKee (2000). (B-H) Isocon diagrams plotting whole-rock geochemistry of a single most altered sample against a single least altered sample for individual rock types collected as part of this study, following the method of Grant (2005). The immobility isocon is the best-fit line through the typically immobile elements Al (Al2O3), Ti (TiO2), and Zr for B-E, Ti (TiO2), and Zr for F, and Al (Al2O3) and Ti (TiO2) for G-H. Elements that plot above the immobility isocon are enriched in the altered rock. Altered samples are consistently enriched in Au, As, and Sb (B-H). Additional analyses were conducted for Tl and Hg in the unoxidized samples (G, H), and these elements are also enriched by alteration.

Fig. 6.

Alteration geochemistry in Marigold samples. (A) Whole-rock geochemical classification of felsic dikes (see Fig. 2 for dike names and locations) using the total alkalis vs. silica classification diagram of Le Maitre (2002); some fields intentionally left blank for visual clarity. Compositions of select intrusions from nearby deposits are from Roberts (1964), Loucks and Johnson (1992), and McKee (2000). (B-H) Isocon diagrams plotting whole-rock geochemistry of a single most altered sample against a single least altered sample for individual rock types collected as part of this study, following the method of Grant (2005). The immobility isocon is the best-fit line through the typically immobile elements Al (Al2O3), Ti (TiO2), and Zr for B-E, Ti (TiO2), and Zr for F, and Al (Al2O3) and Ti (TiO2) for G-H. Elements that plot above the immobility isocon are enriched in the altered rock. Altered samples are consistently enriched in Au, As, and Sb (B-H). Additional analyses were conducted for Tl and Hg in the unoxidized samples (G, H), and these elements are also enriched by alteration.

Fig. 7.

Plan view maps of ore shells and lithology in the Target 2 pit. (A) 5,320-ft level. Preferential mineralization of the Antler Peak Formation relative to the rest of the Antler sequence and significant Au enrichment along the quartz monzonite dike. (B) 5,170-ft level. Quartz monzonite dike with no Au enrichment relative to other host rocks. The location of the geochemical transect is shown in the southeast wall on the 5,170-ft level of the Target 2 pit. LS = limestone.

Fig. 7.

Plan view maps of ore shells and lithology in the Target 2 pit. (A) 5,320-ft level. Preferential mineralization of the Antler Peak Formation relative to the rest of the Antler sequence and significant Au enrichment along the quartz monzonite dike. (B) 5,170-ft level. Quartz monzonite dike with no Au enrichment relative to other host rocks. The location of the geochemical transect is shown in the southeast wall on the 5,170-ft level of the Target 2 pit. LS = limestone.

Fig. 8.

Major element (A) and trace element concentrations (B, C) along the Target 2 pit transect across the Valmy Formation, quartz monzonite intrusion, and Battle Formation. Samples from fault intersections are marked with an asterisk.

Fig. 8.

Major element (A) and trace element concentrations (B, C) along the Target 2 pit transect across the Valmy Formation, quartz monzonite intrusion, and Battle Formation. Samples from fault intersections are marked with an asterisk.

Fig. 9.

Base metal sulfide minerals at Marigold. (A) Base metal sulfide clot with stannite overgrown by chalcopyrite, tetrahedrite-tennantite, and weakly auriferous pyrite, hosted in a vuggy breccia with clasts of basalt and mudstone in a carbonate matrix (reflected light). (B) Chalcopyrite and digenite in a vuggy breccia with clasts of basalt and mudstone in a carbonate matrix (reflected light). Digenite is typically present as thin rims on chalcopyrite grains in oxidized rock. (C) Sphalerite with chalcopyrite disease and minor arsenopyrite in silicified mudstone and siltstone from a fault zone (reflected light). (D) Sphalerite partially replaced by pyrite with minor tennantite (backscattered electron [BSE] image). (E) Pyrite overgrown by chalcopyrite and an Ag-bearing member of the tetrahedrite-tennantite series (BSE). Data are from electron probe microanalysis (EPMA) point analyses of pyrite and tennantite. <d.l.= less than detection limit of ~134 ppm Au. (F) Pyrite, chalcopyrite, Ag-bearing tetrahedrite-tennantite, and stannite (BSE). Data are from EPMA point analyses of pyrite and tennantite. <d.l.= less than dection limit of ~135 ppm Au, 105 ppm Ag. (G) Gersdorffite intergrown with chalcopyrite, tetrahedrite-tennantite, and pyrite (BSE). Data are from EPMA point analyses of gersdorffite, pyrite, and tennantite. <d.l.= less than detection limit of ~142 ppm Au, 93 ppm Ag. (H) Sphalerite and Ag-bearing tetrahedrite-tennantite, overgrown by pyrite with fuzzy As and Au-enriched rims. Abbreviations: asp = arsenopyrite, cpy = chalcopyrite, dg = digenite, gdf = gersdorffite, py = pyrite, sph = sphalerite, st = stannite, tn = tetrahedrite-tennantite.

Fig. 9.

Base metal sulfide minerals at Marigold. (A) Base metal sulfide clot with stannite overgrown by chalcopyrite, tetrahedrite-tennantite, and weakly auriferous pyrite, hosted in a vuggy breccia with clasts of basalt and mudstone in a carbonate matrix (reflected light). (B) Chalcopyrite and digenite in a vuggy breccia with clasts of basalt and mudstone in a carbonate matrix (reflected light). Digenite is typically present as thin rims on chalcopyrite grains in oxidized rock. (C) Sphalerite with chalcopyrite disease and minor arsenopyrite in silicified mudstone and siltstone from a fault zone (reflected light). (D) Sphalerite partially replaced by pyrite with minor tennantite (backscattered electron [BSE] image). (E) Pyrite overgrown by chalcopyrite and an Ag-bearing member of the tetrahedrite-tennantite series (BSE). Data are from electron probe microanalysis (EPMA) point analyses of pyrite and tennantite. <d.l.= less than detection limit of ~134 ppm Au. (F) Pyrite, chalcopyrite, Ag-bearing tetrahedrite-tennantite, and stannite (BSE). Data are from EPMA point analyses of pyrite and tennantite. <d.l.= less than dection limit of ~135 ppm Au, 105 ppm Ag. (G) Gersdorffite intergrown with chalcopyrite, tetrahedrite-tennantite, and pyrite (BSE). Data are from EPMA point analyses of gersdorffite, pyrite, and tennantite. <d.l.= less than detection limit of ~142 ppm Au, 93 ppm Ag. (H) Sphalerite and Ag-bearing tetrahedrite-tennantite, overgrown by pyrite with fuzzy As and Au-enriched rims. Abbreviations: asp = arsenopyrite, cpy = chalcopyrite, dg = digenite, gdf = gersdorffite, py = pyrite, sph = sphalerite, st = stannite, tn = tetrahedrite-tennantite.

Fig. 10.

Paragenetic sequence inferred from petrography of Marigold ores. (A) Alteration. (B) Mineralization.

Fig. 10.

Paragenetic sequence inferred from petrography of Marigold ores. (A) Alteration. (B) Mineralization.

Fig. 11.

Structures in the 1-ppm-grade shell overlain on premining topographic surface, showing distribution of mineralized rock in the Antler sequence (A) and Valmy Formation (B). Gold is the only element routinely analyzed. Grid coordinates are UTM (NAD83).

Fig. 11.

Structures in the 1-ppm-grade shell overlain on premining topographic surface, showing distribution of mineralized rock in the Antler sequence (A) and Valmy Formation (B). Gold is the only element routinely analyzed. Grid coordinates are UTM (NAD83).

Fig. 12.

(A,B) Representative samples of drill core geochemistry from two drill holes, showing the spatial correlation of Au, As, and Sb in fault zones and faulted and argillically altered quartz monzonite dikes. Silver and base metals are most abundant outside of these zones.

Fig. 12.

(A,B) Representative samples of drill core geochemistry from two drill holes, showing the spatial correlation of Au, As, and Sb in fault zones and faulted and argillically altered quartz monzonite dikes. Silver and base metals are most abundant outside of these zones.

Fig. 13.

(A-C) Backscatter electron images of gold-stage minerals with locations of selected electron probe microanalyses. (A) Small grain on left is composed of Au- and As-enriched pyrite (Au-As py) overgrown on arsenopyrite (asp). Large grain is composed of arsenopyrite overgrown on Au-poor, arsenic-poor (GPAP) and Au-poor, arsenic-rich (GPAR) pyrite from DDH4919 522.4–524 m; 0.27 ppm Au. <d.l.= less than detection limit of ~132 ppm Au, 131 ppm As. (B) Au-As pyrite over-growth on GPAP pyrite from DDH5031 471.8–473.0 m; 0.86 ppm Au. <d.l.= less than detection limit of ~116 ppm Au, 132 ppm As. (C) Arsenopyrite and Au-As pyrite overgrowths on GPAP pyrite; the paragenetic relationship between arsenopyrite and Au-As pyrite is ambiguous. DDH5031 471.8–473.0 m; 0.86 ppm Au. <d.l.= less than detection limit of ~127 ppm Au, 134 ppm As. (D) Electron probe microanalyses from A to A′ in image C. (E) Reflected light image of iron oxide (FeOx) pseudomorph after pyrite and fuzzy pyrite overgrowth in Valmy quartzite (qtz). Hand sample, Au below detection. (F-H) DDH5331 216.1–217.4 m; 5.38 ppm. (F) Reflected light image of iron oxides and native Au in argillite of the Valmy Formation. (G, H) Backscatter electron images of iron oxides and native Au within argillite of the Valmy Formation. Native Au grains are high (974–993) fineness.

Fig. 13.

(A-C) Backscatter electron images of gold-stage minerals with locations of selected electron probe microanalyses. (A) Small grain on left is composed of Au- and As-enriched pyrite (Au-As py) overgrown on arsenopyrite (asp). Large grain is composed of arsenopyrite overgrown on Au-poor, arsenic-poor (GPAP) and Au-poor, arsenic-rich (GPAR) pyrite from DDH4919 522.4–524 m; 0.27 ppm Au. <d.l.= less than detection limit of ~132 ppm Au, 131 ppm As. (B) Au-As pyrite over-growth on GPAP pyrite from DDH5031 471.8–473.0 m; 0.86 ppm Au. <d.l.= less than detection limit of ~116 ppm Au, 132 ppm As. (C) Arsenopyrite and Au-As pyrite overgrowths on GPAP pyrite; the paragenetic relationship between arsenopyrite and Au-As pyrite is ambiguous. DDH5031 471.8–473.0 m; 0.86 ppm Au. <d.l.= less than detection limit of ~127 ppm Au, 134 ppm As. (D) Electron probe microanalyses from A to A′ in image C. (E) Reflected light image of iron oxide (FeOx) pseudomorph after pyrite and fuzzy pyrite overgrowth in Valmy quartzite (qtz). Hand sample, Au below detection. (F-H) DDH5331 216.1–217.4 m; 5.38 ppm. (F) Reflected light image of iron oxides and native Au in argillite of the Valmy Formation. (G, H) Backscatter electron images of iron oxides and native Au within argillite of the Valmy Formation. Native Au grains are high (974–993) fineness.

Fig. 14.

Pyrite geochemistry at Marigiold. (A) Au (ppm) vs. As (wt %) contents in pyrite. For graphic purposes, electron probe microanalyses point analyses below detection limit (~133 ppm As, ~117 ppm Au, ~265 ppm Cu) are given a value of 0. Early pyrite (black) contains very minor amounts of arsenic and generally low Au values. Texturally, the pyrite is coarse and typically euhedral, but may be overgrown by Cu- or Au-bearing pyrite. The chemical affinity of this early pyrite is enigmatic due to Au concentrations between below detection limit (<d.l.) and 200 ppm and relatively low concentrations (<d.l.–1 wt %) of arsenic. Base metal-related pyrite (dark gray) has a restricted range of arsenic values and generally low Au relative to Au-stage pyrite. (B) Copper vs. As (mol %) for Au-stage pyrite (light gray), base metal associated pyrite (dark gray), and Au-poor, arsenic-poor pyrite (black).

Fig. 14.

Pyrite geochemistry at Marigiold. (A) Au (ppm) vs. As (wt %) contents in pyrite. For graphic purposes, electron probe microanalyses point analyses below detection limit (~133 ppm As, ~117 ppm Au, ~265 ppm Cu) are given a value of 0. Early pyrite (black) contains very minor amounts of arsenic and generally low Au values. Texturally, the pyrite is coarse and typically euhedral, but may be overgrown by Cu- or Au-bearing pyrite. The chemical affinity of this early pyrite is enigmatic due to Au concentrations between below detection limit (<d.l.) and 200 ppm and relatively low concentrations (<d.l.–1 wt %) of arsenic. Base metal-related pyrite (dark gray) has a restricted range of arsenic values and generally low Au relative to Au-stage pyrite. (B) Copper vs. As (mol %) for Au-stage pyrite (light gray), base metal associated pyrite (dark gray), and Au-poor, arsenic-poor pyrite (black).

Fig. 15.

Comparison of Ag and Au concentrations with depth. (A) Depth vs. Ag/Au ratios for 2,494 drill hole samples. (B) Gold and Ag concentration vs. depth in DDH6008; Au and Ag are commonly enriched in separate intervals. Note slight enrichment of Ag in the transition zone, and low Ag values in the reduced zone.

Fig. 15.

Comparison of Ag and Au concentrations with depth. (A) Depth vs. Ag/Au ratios for 2,494 drill hole samples. (B) Gold and Ag concentration vs. depth in DDH6008; Au and Ag are commonly enriched in separate intervals. Note slight enrichment of Ag in the transition zone, and low Ag values in the reduced zone.

Fig. 16.

40Ar/39Ar step heating plots. (A) Biotite from lamprophyre 6029-L. (B) Sericite from intrusion T2-QM1. (C) Sericite from intrusion T2-QM2. MSWD = mean square of weighted deviates.

Fig. 16.

40Ar/39Ar step heating plots. (A) Biotite from lamprophyre 6029-L. (B) Sericite from intrusion T2-QM1. (C) Sericite from intrusion T2-QM2. MSWD = mean square of weighted deviates.

Fig. 17.

Chemical abrasion-thermal ionization mass spectrometry U-Pb zircon data for four felsic dikes at Marigold. (A) Target 2 intrusion (MI-T2). (B) Basalt Pit intrusion (MI-BP). (C) Intrusion 5 (MI-5). (D) Mackay intrusion (MI-MP). MSWD = mean square of weighted deviates.

Fig. 17.

Chemical abrasion-thermal ionization mass spectrometry U-Pb zircon data for four felsic dikes at Marigold. (A) Target 2 intrusion (MI-T2). (B) Basalt Pit intrusion (MI-BP). (C) Intrusion 5 (MI-5). (D) Mackay intrusion (MI-MP). MSWD = mean square of weighted deviates.

Fig. 18.

Composite geochronologic history of the Marigold mine area recorded by magmatic zircon grains (this study), 40Ar/39Ar analyses of biotite (this study), and sericite (Marigold, unpub. data, 2004). The frequency of intrusions in northern Nevada (NV) is plotted on the left based on data from du Bray (2007); regional ages for Buffalo Mountain from Neff (1973); Mill Canyon stock from Stewart and McKee (1977); Goldstrike from Arehart et al. (1993b); Richmond stock from Ressel and Henry (2006); Buckingham stock from Theodore et al. (1992); Trenton Canyon from Theodore et al. (1973); Osgood stock from Silberman et al. (1974); Carlin-type Au from Cline et al. (2005); Battle Mountain (BM) from King (2015); Marigold rhyolite and Treaty Hill basalt from McKee (2000). CA-TIMS = chemical abrasion-thermal ionization mass spectrometry, LA-ICP-MS = laser ablation-inductively coupled plasma mass spectrometry.

Fig. 18.

Composite geochronologic history of the Marigold mine area recorded by magmatic zircon grains (this study), 40Ar/39Ar analyses of biotite (this study), and sericite (Marigold, unpub. data, 2004). The frequency of intrusions in northern Nevada (NV) is plotted on the left based on data from du Bray (2007); regional ages for Buffalo Mountain from Neff (1973); Mill Canyon stock from Stewart and McKee (1977); Goldstrike from Arehart et al. (1993b); Richmond stock from Ressel and Henry (2006); Buckingham stock from Theodore et al. (1992); Trenton Canyon from Theodore et al. (1973); Osgood stock from Silberman et al. (1974); Carlin-type Au from Cline et al. (2005); Battle Mountain (BM) from King (2015); Marigold rhyolite and Treaty Hill basalt from McKee (2000). CA-TIMS = chemical abrasion-thermal ionization mass spectrometry, LA-ICP-MS = laser ablation-inductively coupled plasma mass spectrometry.

Fig. 19.

Ore grade (g/t Au) vs. metric tonnes of ore for Carlin-type deposits in Nevada. Blue icons represent deposits hosted by slope rocks on rifted western margin of North American craton. Purple icons represent deposits hosted in “complex tectonic settings of imbricate thrust structures at craton margins” (Berger et al., 2014, p. 20). Orange icons represent deposits hosted in rocks related to the foreland basin of the Antler orogeny. Marigold is shown in red; grade is the total grade of the Marigold deposits, and tonnage is the total of past production and resource. Higher grades but lower tonnages of Marigold ore are hosted in Antler orogeny foreland basin rocks, whereas lower grades and higher tonnages are hosted in eugeoclinal siliciclastic rocks of the Roberts Mountain allocthon. Marigold is most similar to the lower plate-hosted Mike deposit and has grades similar to Carlin-type deposits hosted by rocks of foreland basin of the Antler orogeny. Grade, tonnage, and classification of Carlin-type deposits from Berger et al. (2014).

Fig. 19.

Ore grade (g/t Au) vs. metric tonnes of ore for Carlin-type deposits in Nevada. Blue icons represent deposits hosted by slope rocks on rifted western margin of North American craton. Purple icons represent deposits hosted in “complex tectonic settings of imbricate thrust structures at craton margins” (Berger et al., 2014, p. 20). Orange icons represent deposits hosted in rocks related to the foreland basin of the Antler orogeny. Marigold is shown in red; grade is the total grade of the Marigold deposits, and tonnage is the total of past production and resource. Higher grades but lower tonnages of Marigold ore are hosted in Antler orogeny foreland basin rocks, whereas lower grades and higher tonnages are hosted in eugeoclinal siliciclastic rocks of the Roberts Mountain allocthon. Marigold is most similar to the lower plate-hosted Mike deposit and has grades similar to Carlin-type deposits hosted by rocks of foreland basin of the Antler orogeny. Grade, tonnage, and classification of Carlin-type deposits from Berger et al. (2014).

Fig. 20.

Summary of the depositional, tectonic, magmatic, and mineralization history of the Battle Mountain mining district; red arrows highlight the ages of intrusive magmatism and the hypothesized ages of mineralization at Marigold.

Fig. 20.

Summary of the depositional, tectonic, magmatic, and mineralization history of the Battle Mountain mining district; red arrows highlight the ages of intrusive magmatism and the hypothesized ages of mineralization at Marigold.

Table 1.

Representative Electron Probe Microanalysis (EPMA) of Gold-Stage Minerals (values given in wt %)

 EMP settingsMain-stage pyrite
   Drill hole 4919Drill hole 5031
   523.6 m571.5 m472.4 m
Detection limitStandardX-raySample MC-13MC-10  MC-12  
Ag0.0095Silverla0000000000
As0.013GaAsla4.164.316.216.73.943.80.784.223.732.93
Au0.012Goldma0.030.010.020.090.050.070.020.090.10.07
Co0.33Cobaltla0000000000
Cu0.27Copperka0.080.1600.330.190.250.030.30.190.28
Fe0.016Pyriteka44.9543.5141.4743.2844.3643.8244.7343.7843.3243.85
Hg0.017Cinnabarma000000.020000
Ni0.12Milleritela00.1200.2300.290.40.3700.16
S0.0068Pyriteka49.8648.8148.9447.6549.650.5752.150.8949.3850.58
Sb0.0093Sbla00.140.160.040.010.210.010.120.080.21
Sn0.0096Tinla0000000000
Tl0.036TlBrma0.060.140.160.080.190.120.220.110.080.07
Zn0.036Zincka0000000000
Total   99.2297.2196.9998.498.3699.1698.2999.9196.9898.17
 EMP settingsMain-stage pyrite
   Drill hole 4919Drill hole 5031
   523.6 m571.5 m472.4 m
Detection limitStandardX-raySample MC-13MC-10  MC-12  
Ag0.0095Silverla0000000000
As0.013GaAsla4.164.316.216.73.943.80.784.223.732.93
Au0.012Goldma0.030.010.020.090.050.070.020.090.10.07
Co0.33Cobaltla0000000000
Cu0.27Copperka0.080.1600.330.190.250.030.30.190.28
Fe0.016Pyriteka44.9543.5141.4743.2844.3643.8244.7343.7843.3243.85
Hg0.017Cinnabarma000000.020000
Ni0.12Milleritela00.1200.2300.290.40.3700.16
S0.0068Pyriteka49.8648.8148.9447.6549.650.5752.150.8949.3850.58
Sb0.0093Sbla00.140.160.040.010.210.010.120.080.21
Sn0.0096Tinla0000000000
Tl0.036TlBrma0.060.140.160.080.190.120.220.110.080.07
Zn0.036Zincka0000000000
Total   99.2297.2196.9998.498.3699.1698.2999.9196.9898.17
ElementGold-poor, arsenic-rich pyriteGold-poor, arsenic-poor pyrite 
Drill hole5031491950315031 
m Sample471.7 MC-5472.4 MC-12523.6 MC-13 523.6 MC-13 571.5 MC-10471.7 MC-5472.4 MC-12Detection limit
Ag0000000000000000.0095
As27.838.4519.1838.1636.80000000000.150.013
Au0000000.0100000.010000.012
Co0000000000000000.33
Cu0.15000000000000.0400.050.27
Fe37.1235.4741.0235.7836.946.9846.1346.2446.4646.0946.8546.7345.945.4145.970.016
Hg0000000000000000.017
Ni0.270.130.310.16000.640.680.1300.1300000.12
S36.0523.5640.9424.1425.1453.8152.7352.7753.7952.5853.7953.4753.5952.1652.940.0068
Sb0.030.140.140.01000000.02000000.0093
Sn0000000000000000.0096
Tl0.110.080.160.070.120.150.150.260.080.070.120.160.130.120.180.036
Zn0000000000000000.036
Total101.5397.85101.7898.3499.0799.6399.82100.78100.9599.399.6799.96100.67100.85101.16 
ElementGold-poor, arsenic-rich pyriteGold-poor, arsenic-poor pyrite 
Drill hole5031491950315031 
m Sample471.7 MC-5472.4 MC-12523.6 MC-13 523.6 MC-13 571.5 MC-10471.7 MC-5472.4 MC-12Detection limit
Ag0000000000000000.0095
As27.838.4519.1838.1636.80000000000.150.013
Au0000000.0100000.010000.012
Co0000000000000000.33
Cu0.15000000000000.0400.050.27
Fe37.1235.4741.0235.7836.946.9846.1346.2446.4646.0946.8546.7345.945.4145.970.016
Hg0000000000000000.017
Ni0.270.130.310.16000.640.680.1300.1300000.12
S36.0523.5640.9424.1425.1453.8152.7352.7753.7952.5853.7953.4753.5952.1652.940.0068
Sb0.030.140.140.01000000.02000000.0093
Sn0000000000000000.0096
Tl0.110.080.160.070.120.150.150.260.080.070.120.160.130.120.180.036
Zn0000000000000000.036
Total101.5397.85101.7898.3499.0799.6399.82100.78100.9599.399.6799.96100.67100.85101.16 
Table 2.

Representative Electron Probe Microanalyses of Base Metal-Stage Minerals (values given in wt %)

ElementBase metal-stage pyriteTennantite-tetrahedrite
Drill hole5031491950314919
m Sample471.7 MC-5523.6 MC-13471.7 MC-5523.6 MC-13571.5 MC-10
Ag0000000000.020.030.020.010.03000.040.23
As2.5502.332.863.142.662.142.792.3116.5316.0117.5617.4217.6417.0410.7418.5616.47
Au000.020.030000.020.01000000000
Co0000000000000000.2500
Cu2.510.320.711.310.190.170.230.150.738.7739.8539.840.4839.5739.538.0639.6840.27
Fe41.7946.4945.2344.6645.8145.6645.6845.4445.12.83.191.712.057.955.82.995.862.3
Hg0.03000000001.030.740.760.741.043.194.573.830.04
Ni0.2300000000.03000000000
S51.4453.4852.1550.9651.1951.1851.7851.1451.6627.5428.1428.9928.2429.628.3527.3328.5128.04
Sb0.29000.0100000.043.744.582.352.441.923.2111.730.454.6
Sn0000000000.030.030000.020.0700
Tl0.090.180.070.150.070.180.110.150.130.0800.05000.160.1900.12
Zn0.2800000000.036.094.2265.760.931.554.390.225.83
Total99.21100.55 100.52100.01100.4399.92100.0399.82100.0696.6296.8397.2597.1798.6898.82100.3297.1697.93
ElementBase metal-stage pyriteTennantite-tetrahedrite
Drill hole5031491950314919
m Sample471.7 MC-5523.6 MC-13471.7 MC-5523.6 MC-13571.5 MC-10
Ag0000000000.020.030.020.010.03000.040.23
As2.5502.332.863.142.662.142.792.3116.5316.0117.5617.4217.6417.0410.7418.5616.47
Au000.020.030000.020.01000000000
Co0000000000000000.2500
Cu2.510.320.711.310.190.170.230.150.738.7739.8539.840.4839.5739.538.0639.6840.27
Fe41.7946.4945.2344.6645.8145.6645.6845.4445.12.83.191.712.057.955.82.995.862.3
Hg0.03000000001.030.740.760.741.043.194.573.830.04
Ni0.2300000000.03000000000
S51.4453.4852.1550.9651.1951.1851.7851.1451.6627.5428.1428.9928.2429.628.3527.3328.5128.04
Sb0.29000.0100000.043.744.582.352.441.923.2111.730.454.6
Sn0000000000.030.030000.020.0700
Tl0.090.180.070.150.070.180.110.150.130.0800.05000.160.1900.12
Zn0.2800000000.036.094.2265.760.931.554.390.225.83
Total99.21100.55 100.52100.01100.4399.92100.0399.82100.0696.6296.8397.2597.1798.6898.82100.3297.1697.93
ElementStanniteChalcopyriteGersdorffite
Drill Hole491950314919
m523.6471.7523.6Detection limit
SampleMC-13MC-5MC-5MC-6
Ag0000000.0095
As00045.2846.1646.540.013
Au0000000.012
Co0003.21.592.410.33
Cu26.3826.133.012.4800.040.27
Fe4.674.729.420.480.510.580.016
Hg12.3613.3800.03000.017
Ni00041.244.5944.530.12
S29.1728.9534.7618.6819.5619.120.0068
Sb0000.290.10.10.0093
Sn23.9123.7400000.0096
Tl00.0500000.036
Zn4.434.0500.2000.036
Total100.99100.9797.23111.84112.54113.35 
ElementStanniteChalcopyriteGersdorffite
Drill Hole491950314919
m523.6471.7523.6Detection limit
SampleMC-13MC-5MC-5MC-6
Ag0000000.0095
As00045.2846.1646.540.013
Au0000000.012
Co0003.21.592.410.33
Cu26.3826.133.012.4800.040.27
Fe4.674.729.420.480.510.580.016
Hg12.3613.3800.03000.017
Ni00041.244.5944.530.12
S29.1728.9534.7618.6819.5619.120.0068
Sb0000.290.10.10.0093
Sn23.9123.7400000.0096
Tl00.0500000.036
Zn4.434.0500.2000.036
Total100.99100.9797.23111.84112.54113.35 
Table 3.

Spearman Correlation Matrix Constructed Using Whole-Rock Analyses of 9,153 Drill Core and Drill Cutting Samples

Table 4.

Chemical Abrasion-Thermal Ionization Mass Spectrometry Analyses of Zircons

Radiogenic isotopic ratiosIsotopic ages
Sample1Th/U2206Pb ×10−13 mol3mol % 206Pb☼3Pb/Pbc3Pbc (pg)3206Pb/204Pb4208Pb/206Pb5207Pb/206Pb5% err6207Pb/235U5% err6206Pb/238U5% err6Correlation coeffient207Pb/206Pb7±6207Pb/235U7±6206Pb/238U7±6
MAR-INT-BP
z10.1501.221799.78%1250.228,3070.0480.0480060.1180.1000380.1720.0151140.0700.86299.552.7896.810.1696.700.07
z20.1420.721099.65%770.215,1050.0460.0479140.1590.0997830.2110.0151040.0720.80695.003.7596.580.1996.640.07
z30.2500.416199.53%590.163,8360.0800.0479100.2290.0997360.2790.0150980.0790.71694.855.4296.530.2696.600.08
z40.1723.773899.92%3550.2522,8780.0550.0716750.0890.2444860.1750.0247390.1220.880976.731.81222.090.35157.540.19
z50.2032.925199.87%2150.3114,0490.0650.0479970.0810.1001500.1440.0151330.0710.93499.141.9396.910.1396.820.07
z60.2261.179299.80%1380.208,9920.0720.0480800.1070.1004370.1640.0151500.0710.878103.232.5497.180.1596.930.07
z70.5850.410199.16%360.292,1450.1870.0480400.3320.0999240.3800.0150860.0800.677101.227.8496.710.3596.520.08
MAR-INT-MP
z1a0.1050.581899.61%680.194,5990.0330.0479250.1710.0984930.2220.0149050.0710.79595.584.0695.380.2095.380.07
z1b0.0751.640599.86%1920.1913,0540.0240.0479600.0880.0997670.1490.0150870.0690.92897.302.0896.560.1496.530.07
z2a0.1210.980999.80%1360.169,1420.0390.0479060.1270.0964380.1810.0146000.0710.84794.613.0093.480.1693.440.07
z2b0.1221.147799.66%800.325,3670.0390.0479210.1360.0965740.1890.0146160.0710.83795.383.2193.610.1793.540.07
z30.4431.212899.78%1370.228,3840.1420.0478880.1180.0955920.1730.0144780.0700.86193.722.7892.700.1592.660.06
z40.3213.043699.91%3350.2221,1580.1030.0479110.0760.0956120.1380.0144740.0700.94294.881.8092.720.1292.630.06
z50.6010.475099.55%680.184,0220.1920.0479220.2090.0955620.2650.0144630.0930.71395.444.9492.670.2392.560.09
z60.3452.087199.84%1820.2711,4430.1100.0478950.0850.0955880.1460.0144750.0700.93394.072.0092.700.1392.640.06
MAR-INT-5
z1a0.2100.309799.46%510.143,3390.0670.0478880.2560.0951230.3070.0144060.0740.74793.766.0692.260.2792.210.07
z1b0.3910.881799.79%1400.158,6800.1250.0478420.1190.0950530.1730.0144100.0710.84691.452.8292.200.1592.230.06
z20.2730.324199.42%480.163,1110.0870.0479330.2610.0975350.3100.0147580.0750.71995.956.1994.500.2894.440.07
z30.2770.124897.96%140.228830.0890.0471131.0050.0938771.0720.0144510.1080.65054.9823.9791.110.9392.490.10
z40.4410.909999.81%1570.149,6180.1410.0480170.1100.0959160.1670.0144880.0700.880100.092.6193.000.1592.720.06
z50.3522.229799.76%1210.447,6040.1130.0479910.0990.0957170.1550.0144650.0690.89298.832.3492.820.1492.580.06
z60.4761.190499.75%1210.247,3130.1520.0479370.1330.0957780.1860.0144910.0720.82996.173.1492.870.1792.740.07
z70.3910.823999.78%1320.158,2090.1250.0479330.1270.0956570.1810.0144740.0700.84995.953.0092.760.1692.640.06
MAR-INT-T2
z1a0.0762.946999.92%3400.1923,0310.0240.0480080.0770.1010540.1390.0152660.0690.94799.681.8297.750.1397.670.07
z1b0.0872.074499.88%2190.2114,8390.0280.0479920.1140.1009430.1640.0152550.0720.80798.882.7097.650.1597.600.07
z20.0880.863299.74%1010.196,8160.0280.0480920.1580.1015090.2060.0153080.0730.756103.823.7498.170.1997.940.07
z30.0781.069499.78%1210.208,1970.0250.0480900.1330.1029190.1850.0155220.0700.828103.683.1499.470.1899.290.07
z4a0.1540.814499.63%730.254,8330.0490.0480940.1600.1018770.2130.0153630.0740.805103.913.7798.510.2098.280.07
z4b0.1160.786499.72%960.186,4140.0370.0537030.1200.1487940.1760.0200950.0690.880358.622.70140.850.23128.260.09
z4c0.1300.631499.50%540.263,6180.0420.0479800.1930.1017590.2440.0153820.0720.77598.304.5798.400.2398.400.07
Radiogenic isotopic ratiosIsotopic ages
Sample1Th/U2206Pb ×10−13 mol3mol % 206Pb☼3Pb/Pbc3Pbc (pg)3206Pb/204Pb4208Pb/206Pb5207Pb/206Pb5% err6207Pb/235U5% err6206Pb/238U5% err6Correlation coeffient207Pb/206Pb7±6207Pb/235U7±6206Pb/238U7±6
MAR-INT-BP
z10.1501.221799.78%1250.228,3070.0480.0480060.1180.1000380.1720.0151140.0700.86299.552.7896.810.1696.700.07
z20.1420.721099.65%770.215,1050.0460.0479140.1590.0997830.2110.0151040.0720.80695.003.7596.580.1996.640.07
z30.2500.416199.53%590.163,8360.0800.0479100.2290.0997360.2790.0150980.0790.71694.855.4296.530.2696.600.08
z40.1723.773899.92%3550.2522,8780.0550.0716750.0890.2444860.1750.0247390.1220.880976.731.81222.090.35157.540.19
z50.2032.925199.87%2150.3114,0490.0650.0479970.0810.1001500.1440.0151330.0710.93499.141.9396.910.1396.820.07
z60.2261.179299.80%1380.208,9920.0720.0480800.1070.1004370.1640.0151500.0710.878103.232.5497.180.1596.930.07
z70.5850.410199.16%360.292,1450.1870.0480400.3320.0999240.3800.0150860.0800.677101.227.8496.710.3596.520.08
MAR-INT-MP
z1a0.1050.581899.61%680.194,5990.0330.0479250.1710.0984930.2220.0149050.0710.79595.584.0695.380.2095.380.07
z1b0.0751.640599.86%1920.1913,0540.0240.0479600.0880.0997670.1490.0150870.0690.92897.302.0896.560.1496.530.07
z2a0.1210.980999.80%1360.169,1420.0390.0479060.1270.0964380.1810.0146000.0710.84794.613.0093.480.1693.440.07
z2b0.1221.147799.66%800.325,3670.0390.0479210.1360.0965740.1890.0146160.0710.83795.383.2193.610.1793.540.07
z30.4431.212899.78%1370.228,3840.1420.0478880.1180.0955920.1730.0144780.0700.86193.722.7892.700.1592.660.06
z40.3213.043699.91%3350.2221,1580.1030.0479110.0760.0956120.1380.0144740.0700.94294.881.8092.720.1292.630.06
z50.6010.475099.55%680.184,0220.1920.0479220.2090.0955620.2650.0144630.0930.71395.444.9492.670.2392.560.09
z60.3452.087199.84%1820.2711,4430.1100.0478950.0850.0955880.1460.0144750.0700.93394.072.0092.700.1392.640.06
MAR-INT-5
z1a0.2100.309799.46%510.143,3390.0670.0478880.2560.0951230.3070.0144060.0740.74793.766.0692.260.2792.210.07
z1b0.3910.881799.79%1400.158,6800.1250.0478420.1190.0950530.1730.0144100.0710.84691.452.8292.200.1592.230.06
z20.2730.324199.42%480.163,1110.0870.0479330.2610.0975350.3100.0147580.0750.71995.956.1994.500.2894.440.07
z30.2770.124897.96%140.228830.0890.0471131.0050.0938771.0720.0144510.1080.65054.9823.9791.110.9392.490.10
z40.4410.909999.81%1570.149,6180.1410.0480170.1100.0959160.1670.0144880.0700.880100.092.6193.000.1592.720.06
z50.3522.229799.76%1210.447,6040.1130.0479910.0990.0957170.1550.0144650.0690.89298.832.3492.820.1492.580.06
z60.4761.190499.75%1210.247,3130.1520.0479370.1330.0957780.1860.0144910.0720.82996.173.1492.870.1792.740.07
z70.3910.823999.78%1320.158,2090.1250.0479330.1270.0956570.1810.0144740.0700.84995.953.0092.760.1692.640.06
MAR-INT-T2
z1a0.0762.946999.92%3400.1923,0310.0240.0480080.0770.1010540.1390.0152660.0690.94799.681.8297.750.1397.670.07
z1b0.0872.074499.88%2190.2114,8390.0280.0479920.1140.1009430.1640.0152550.0720.80798.882.7097.650.1597.600.07
z20.0880.863299.74%1010.196,8160.0280.0480920.1580.1015090.2060.0153080.0730.756103.823.7498.170.1997.940.07
z30.0781.069499.78%1210.208,1970.0250.0480900.1330.1029190.1850.0155220.0700.828103.683.1499.470.1899.290.07
z4a0.1540.814499.63%730.254,8330.0490.0480940.1600.1018770.2130.0153630.0740.805103.913.7798.510.2098.280.07
z4b0.1160.786499.72%960.186,4140.0370.0537030.1200.1487940.1760.0200950.0690.880358.622.70140.850.23128.260.09
z4c0.1300.631499.50%540.263,6180.0420.0479800.1930.1017590.2440.0153820.0720.77598.304.5798.400.2398.400.07
1

z1, z2, etc. are labels for analyses composed of single zircon grains that were annealed and chemically abraded (Mattinson, 2005); labels with same numbers (e.g., z1a, z1b) denote fragments from the same grain; labels in bold denote analyses used in weighted mean calculations

2

Model Th/U ratio calculated from radiogenic 208Pb/206Pb ratio and 207Pb/235U date

3

Pb and Pbc are radiogenic and common Pb, respectively; mol % 206Pb is with respect to radiogenic and blank Pb

4

Measured ratio corrected for spike and fractionation only; fractionation correction is 0.16 ± 0.03 (1 sigma) %/amu (atomic mass unit) for single-collector Daly analyses, based on analysis of EARTH-TIME 202Pb/205Pb tracer solution

5

Corrected for fractionation, spike, common Pb, and initial disequilibrium in 230Th/238U; common Pb is assigned to procedural blank with composition of 206Pb/204Pb = 18.04 ± 0.61%; 207Pb/204Pb =15.54 ± 0.52%; 208Pb/204Pb = 37.69 ± 0.63% (1 sigma); 206Pb/238U and 207Pb/206Pb ratios corrected for initial disequilibrium in 230Th/238U using Th/U (magma) = 3.0 ± 0.3 (1 sigma)

6

Errors are 2 sigma, propagated using algorithms of Schmitz and Schoene (2007) and Crowley et al. (2007)

7

Calculations based on decay constants of Jaffey et al. (1971); 206Pb/238U and 207Pb/206Pb dates corrected for initial disequilibrium in 230Th/238U by Th/U [magma] = 3.0 ± 0.3 (1 sigma)

Table 5.

Comparison of Key Characteristics of the Marigold Deposit with Published Characteristics for Distal Disseminated and Carlin-Type Deposits

FeatureCarlin-typeDistal disseminatedMarigold
AlterationDecarbonatization, argillization, silicification, sulfidationArgillization, silicification, decalcification, potassic, phyllicDecarbonatization, argillization, silicification, pyritization, sulfidation
Geochemical signatureAu, As, S, Sb, Hg, TlAg, Au, Sb, As, Hg, Te, Pb, Zn, Cu, Na, K, Mn ± BaBase metal stage: Cu, Ag, Pb, Zn, Ni, Sn, Au, As, Na, K, Mn Gold stage: Au, As, S, Sb, Hg, Tl
Nature of goldSolid solution or submicron inclusions within arsenian pyrite, oxide zone native AuNative, electrum, solid solution, or inclusions in chalcopyrite, tetrahedrite, pyrite, and arsenopyriteSubmicron particles within arsenian pyrite, oxide zone native Au
Ag/Au<1:1Up to 400:1<1:1 above and below redox
Associated rocksNo genetic relationship to felsic hypabyssal intrusionsGenetically related to felsic hypabyssal intrusionsNo shallow magmatic signature
FeatureCarlin-typeDistal disseminatedMarigold
AlterationDecarbonatization, argillization, silicification, sulfidationArgillization, silicification, decalcification, potassic, phyllicDecarbonatization, argillization, silicification, pyritization, sulfidation
Geochemical signatureAu, As, S, Sb, Hg, TlAg, Au, Sb, As, Hg, Te, Pb, Zn, Cu, Na, K, Mn ± BaBase metal stage: Cu, Ag, Pb, Zn, Ni, Sn, Au, As, Na, K, Mn Gold stage: Au, As, S, Sb, Hg, Tl
Nature of goldSolid solution or submicron inclusions within arsenian pyrite, oxide zone native AuNative, electrum, solid solution, or inclusions in chalcopyrite, tetrahedrite, pyrite, and arsenopyriteSubmicron particles within arsenian pyrite, oxide zone native Au
Ag/Au<1:1Up to 400:1<1:1 above and below redox
Associated rocksNo genetic relationship to felsic hypabyssal intrusionsGenetically related to felsic hypabyssal intrusionsNo shallow magmatic signature

Data for distal disseminated deposits from Cox and Singer (1990), Cox (1992), and Hofstra and Cline (2000); data for Carlin-type deposits from Hofstra and Cline (2000) and Cline et al. (2005)

Contents

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