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Age and petrogenesis of volcanic and intrusive rocks in the Sulphur Spring Range, central Nevada: Comparisons with

ore-associated Eocene magma systems in the Great Basin

Elizabeth B. RyskampDepartment of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA

Jeffrey T. AbbottGolden Gryphon Explorations, 1400 Tanager Place, RR 21, Roberts Creek, British Columbia V0N 2W1, Canada

Eric H. Christiansen*Jeffrey D. KeithDepartment of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA

Jeffrey D. VervoortSchool of Earth and Environmental Sciences, Washington State University, Pullman, Washington 99164, USA

David G. TingeyDepartment of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA

496

Geosphere; June 2008; v. 4; no. 3; p. 496–519; doi: 10.1130/GES00113.1; 17 fi gures; 2 tables; 2 supplemental tables.

*Corresponding author.

ABSTRACT

Widespread base- and precious-metal anomalies, oxidized sulfi de veins, silicifi ed calcareous shales and carbonates, and altered porphyry intrusions occur in the northeast-ern Sulphur Spring Range, Nevada, 80 km south of important gold deposits in the Carlin trend. The small historic mines and prospects in the area are spatially and per-haps genetically related to a suite of vari-ably altered dikes, small lava fl ows, silicic domes, and related pyroclastic rocks. New major- and trace-element data and U-Pb zircon ages show that the East Sulphur Spring volcanic suite is Eo-Oligocene in age (36–31 Ma) and ranges in composition from high MgO- basaltic andesite to peraluminous rhyolite. The major- and trace-element com-positions of the volcanic rocks are character-istic of continental margin subduction zone magmas and form a high-K, calc-alkaline suite with low Fe/Mg ratios. In addition, the rocks have negative Nb and Ti anomalies and elevated Ba, K, and Pb on normalized trace-element diagrams. Crustal melting is indicated by the eruption of a peraluminous garnet-bearing ignimbrite and as a compo-nent in hybridized andesite.

The nature of this suite and its potential for mineralization is elucidated via comparisons to other Eocene age volcanic rocks associated with much larger gold and copper deposits in the Great Basin. The East Sulphur Spring suite is more similar to Eocene igneous rocks found along and near the Carlin trend than it is to those erupted while the Bingham por-phyry copper deposit developed 300 km far-ther to the east. For example, the East Sul-phur Spring suite and the Eocene magmatic rocks along the Carlin trend are less alkaline than the Bingham suite and lack its unusual enrichment of Cr, Ni, and Ba in intermedi-ate composition rocks (58–68 wt% silica). Nonetheless, the Bingham and East Sulphur Spring volcanic suites both preserve evidence of mixing that created intermediate composi-tions. For example, an andesite has obvious mineral disequilibrium with plagioclase, bio-tite, clinopyroxene, orthopyroxene, olivine, and amphibole coexisting with extensively resorbed megacrysts of quartz, K-feldspar, and garnet—indicative of mixing basaltic andesite or andesite and largely crystallized garnet-bearing rhyolite. On the other hand, we found no evidence for mixing with a mafi c alkaline magma like that in the Bingham Canyon magma-ore system.

We conclude that: (1) an unusual tectonic setting prevailed during the Eocene and

Oligocene of the western United States that promoted the production of oxidized mafi c magma in an arclike setting, but far inland as a result of the rollback of the Farallon slab; (2) the mafi c magmas intruded or erupted separately, or mixed with more silicic magma generated by fractional crystallization and assimilation of crustal materials; and (3) these mafi c magmas may have delivered sig-nifi cant amounts of sulfur and chalcophile metals to upper crustal magma chambers and eventually to Paleogene ore deposits in the eastern Great Basin.

Keywords: Eocene, economic geology, igneous rock, Carlin-type, porphyry copper.

INTRODUCTION

The Great Basin of the western United States contains a multitude of ore deposits and asso-ciated igneous rocks. Studies of the ages and compositions of the volcanic and intrusive rocks have shown that many of the deposits are not only spatially associated with magmatism, but temporally and genetically linked to igne-ous processes as well. In many cases, magmas and their solidifi ed equivalents were important sources of heat to drive hydrothermal convec-tion, of sulfur used as a complexing agent in the fl uids and then deposited in sulfi des and sulfates,

Paleogene magmatism in the Sulphur Spring Range, Nevada

Geosphere, June 2008 497

and of the ore metals themselves. In this paper, we consider this paradigm in light of the rela-tionships between Paleogene magmatism and ore deposition in north-central Nevada.

The Sulphur Spring Range is ~80 km south of large gold deposits in the Carlin trend (Fig. 1). The Mineral Hill district, on the west side of the range, and the Union district, on the east side, were mined in the late 1800s to early 1900s for gold, silver, and copper (Lincoln, 1923). Mineral Hill is best described as a small polymetallic replace-ment deposit (19A of Cox and Singer, 1986) and is probably related to the distal effects of middle Cenozoic magmatism according to McKee and Moring (1996). Small base metal–silver replace-ment bodies, which would now be identifi ed as carbonate replacement deposits (Megaw, 1999), are found in the Union district. Prospects contain-ing Au and As were explored in the 1980s.

The Sulphur Spring Range is underlain by a thick sequence of east-dipping Paleozoic sedi-mentary rocks. Prior to recent mapping, a 2 km2 area of undifferentiated Tertiary volcanic rock on the east side of the range (Carlisle and Nel-son, 1990) contained the only known outcrops of volcanic rock. Our mapping identifi ed numer-ous small exposures of igneous rocks that either intrude or overlie the Paleozoic deposits.

Some of the igneous rocks are spatially associ-ated with mineralization and exhibit key charac-teristics of porphyry deposits (cf. Beane and Tit-ley, 1981; Richards, 2003; Seedorf et al., 2005), including substantial amounts of phyllic and argil-lic hydrothermal alteration, pebble dikes, breccia pipes, and disseminated and vein-related mineral-ization. Altered mafi c and intermediate composi-tion dikes have geochemical anomalies of As, Ba, Bi, Cr, Cu, Mo, Ni, Pb, Sb, Tl, and Zn. We have also identifi ed evidence of magma mixing in the intermediate composition volcanic rocks. This may be an important feature of porphyry copper deposits such as the enormous Eocene Bingham Canyon deposit 300 km to the east (Maughan et al., 2002) and is reexamined here.

In addition, the Sulphur Spring Range has several features in common with Carlin-type gold deposits (Fig. 1), which contain the most productive gold mines in North America (Jensen et al., 1995). The geology and origin of Carlin-type gold deposits are described in detail by Hofstra and Cline (2000) and Cline et al. (2005). Mineralized rocks in the Sulphur Spring Range, like most Carlin-type gold deposits, occur below the Roberts Mountains thrust at intersections of a complex array of structures with permeable and reactive strata, usually Devonian carbonate rocks or calcareous clastic sediments. Small bodies of jasperoid have anomalous concentrations of Au, As, Hg, Sb, and Tl. The deposits are spatially associated with volcanism that is part of a south-

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Figure 1. Shaded relief map of central Nevada, showing the location of the Sulphur Spring Range relative to the Carlin trend of gold deposits. Circles identify known gold deposits. The locations of samples taken from dikes (unit Tba on Fig. 4) in the central part of the range are shown as triangles. Shaded relief base map from Chalk Butte, Inc.

ward sweep of magmatism that passed through this area in the Eocene (Seedorf, 1991; Hofstra et al., 1999; Cline et al., 2005; Ressel and Henry, 2006). However, the exact nature of the relation-ship between Carlin-type deposits and Eocene magmatism is the subject of debate.

Because of its broad similarities to much larger Eocene porphyry copper and Carlin-type gold deposits, the deposits in the Sulphur Spring Range have recently been the site of grass-roots exploration for base metals and Au. To help assess the potential for and further our

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498 Geosphere, June 2008

understanding of these important ore deposits, we compare the igneous rocks of the Sulphur Spring Range to those from other Eocene mag-matic centers related to mineralization in the Great Basin—those near the Carlin trend in Nevada and the Bingham porphyry system in Utah. We conclude that there are several impor-tant similarities in these ore-related magma sys-tems, including their ages, tectonic settings, and the potential for mafi c magmas to have deliv-ered metals and sulfur to upper crustal hydro-thermal systems.

GEOTECTONIC SETTING

During the late Proterozoic, central Nevada lay on the western edge of the rifted North American continent (Wooden et al., 1998; Grauch et al., 2003). The former continental margin, defi ned by rifting in the Neoproterozoic and identifi ed by the 87Sr/86Sr = 0.706 line, lies just west of the Carlin and Battle Mountain–Eureka mineraliza-tion trends (Fig. 2; Tosdal et al., 2000; Grauch et al. 2003). Neoproterozic and Cambrian sili-ciclastic sediment were deposited during the rift phase. From the Ordovician through Devonian, thick shelf-type sediments accumulated on the continental margin and silty carbonate rocks accumulated on the slope to the west (Madrid, 1987; Finney et al., 2000; Cook and Corboy,

2004; Cline et al., 2005). In the late Devonian through late Mississippian, the Antler orogeny affected the western margin of the North Ameri-can plate (Carpenter et al., 1994; Dickinson, 2006). This contractional orogeny produced the Roberts Mountains thrust (Fig. 2) that jux-taposes the Ordovician Vinini Formation and some Mississippian clastic rocks over Devonian carbonate rocks. In the Sulphur Spring Range, these are typically capped by the Devils Gate Limestone or the Telegraph Canyon Formation (Carlisle and Nelson, 1990). The structural con-tact between carbonate rocks and clastic rocks in the Roberts Mountains allochthon is one of the typical settings for gold mineralization along the Carlin trend, including the Rain mine, the near-est of the Carlin-type mineral deposits (Longo et al., 2002). The Roberts Mountains thrust fault has also been delineated in various locations throughout the Sulphur Spring Range (Carlisle and Nelson, 1990; Johnson and Visconti, 1992). The Sonoma orogeny in the Triassic created the Golconda thrust (Dickinson, 2006), which lies to the west (Fig. 2).

During the Mesozoic, subduction beneath western North America created a protracted series of contractional events (Dickinson, 2006). The Sevier orogeny thickened the crust beneath Nevada and western Utah, and a series of thrust sheets formed to the east (e.g., DeCelles, 2004).

In the Great Basin, Jurassic magmatism included metaluminous to peraluminous granitoids and sparse lamprophyre dikes (e.g., Ressel and Henry, 2006; Cline et al., 2005). By the end of the Cretaceous, the Farallon plate was subducting at a very low angle under North America and arc magmatism essentially shut off in Nevada, but small volumes of strongly peraluminous granite were intruded along the Utah-Nevada border (e.g., Kistler et al., 1981). During the early Ceno-zoic, the Farallon plate apparently detached from the lithosphere in a southward-sweeping fash-ion allowing hot asthenospheric mantle to fl ow between the subducting slab and the lithospheric mantle (Best and Christiansen, 1991). This cre-ated a continental magmatic arc that extended far inland (Lipman et al., 1972; Severinghaus and Atwater, 1990). Middle Tertiary magmatism related to this event may have resulted from (1) dehydration of the Farallon plate, which induced hydrous melting of the mantle wedge, (2) heat-ing lithospheric mantle by hot asthenospheric mantle or by wedge-derived magma, and (3) decompression melting of hot mantle associ-ated with the pattern of fl ow in the wedge. These mafi c, mantle-derived magmas rose and pow-ered crustal magma systems in which more fel-sic magmas evolved by fractional crystallization and by partial melting and assimilation of conti-nental crust (Ressel and Henry, 2006).

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Figure 2. Structural elements and gold deposits of northern Nevada. The area underlain by Precambrian conti-nental crust is east of the initial strontium 0.706 line (red dashed line). The eastern edges of allochthonous terranes and the middle Miocene northern Nevada rift are also shown (modifi ed from Grauch et al., 2003).



Cenozoic arc magmatism in the northern Great Basin began in the Eocene, between 40 and 36 Ma and then the front swept farther south (Ressel and Henry, 2006; du Bray, 2007). Magma compositions ranged from basaltic andesite to rhyolite (or their intrusive equiva-lents), and volcanism was widespread through-out northern Nevada (Brooks et al., 1995; Henry and Boden, 1998; Henry and Faulds, 1999; du Bray, 2007). Apparently several magma sys-tems were episodically active beneath the Carlin trend during the Eocene and Oligocene (Grauch et al., 2003; Ressel and Henry, 2006). Concur-rent magmatism occurred at similar latitudes in Utah, including that responsible for the Bing-ham, Clayton, and Alta stocks (Vogel et al., 2001; Deino and Keith, 1997).

Although still controversial, some geologists have concluded that Eocene stress relaxation and extension accompanied this southward sweep of arc magmatism across western North America (Gans et al., 1989; Feeley and Grunder, 1991; Seedorf, 1991; Hofstra et al., 1999). For exam-ple, Henry et al. (2001), Haynes (2003), Sata-rugsa and Johnson (2000), Cline et al. (2005), and Henrici and Haynes (2006) conclude that the Eocene-age Elko Formation accumulated in an extension-related basin (Fig. 3). The for-mation consists of a lower conglomerate unit, lacustrine limestone, and organic-rich shale, but also has interlayered volcanic rocks. The extension that created the Northern Nevada rift (Fig. 2) and continued to form the present-day Basin and Range topography probably began in

the Miocene (Zoback et al., 1994; Ressel and Henry, 2006). Bimodal basalt-rhyolite volca-nism is typical of this time (John, 2001).

Relationship of Paleogene Mineralization to Magmatism

Although the close spatial and tempo-ral relationships of Eocene igneous rocks to Carlin-trend gold mineralization has suggested a probable link (Ressel et al., 2000; Ressel and Henry, 2006), the presence or absence of mag-matic fl uids in these meteoric water-dominated hydrothermal systems is controversial. In some Carlin-type districts (e.g., Getchell and Gold Bar), Eocene intrusions are absent (Hofstra and Cline, 2000). Cline et al. (2005) conclude that

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Ryskamp et al.


Eocene magmatism, in conjunction with deep crustal melting and metamorphism, and a pulse of extension-released ore fl uids were channeled upward along basement-penetrating faults and mixed to varying degree with exchanged mete-oric water. In contrast, Emsbo et al. (2006) sug-gest that the ore fl uid was meteoric water that extracted gold and sulfur from sedimentary rocks already enriched in gold by Devonian exhalative processes.

The Bingham porphyry Cu-Mo-Au system has a much clearer connection to igneous rocks and processes with much of the sulfur, met-als, and fl uids being of magmatic origin. The magmatism related to the deposit is 38–36 Ma (Deino and Keith, 1997) and is comparable in age to the dikes and gold mineralization in the Carlin trend (Ressel et al., 2000; Ressel and Henry, 2006). Moreover, the porphyry system is associated with distal disseminated gold depos-its that have much in common with Carlin-type gold deposits (Cunningham et al., 2004). The role of deep, basement-penetrating faults may not be as important as along the Carlin trend, but Bingham is near an old east-trending con-tinental margin to which Proterozoic terranes accreted (Whitmeyer and Karlstrom, 2007).

SAMPLING AND ANALYTICAL TECHNIQUES

More than 200 samples of igneous rocks were collected and a geologic map (Fig. 4) was con-structed for this study. Hand sample descriptions and locations are presented by Ryskamp (2006). Thin sections cut from 25 samples of the main lithologic units were studied. Garnet and olivine phenocryst compositions were determined with an upgraded Cameca SX50 electron microprobe. A 20 micron beam and 15 kV acceleration volt-age were used for analyses, which are reported in Ryskamp (2006).

Major- and trace-element analyses for 87 of the freshest samples were obtained by X-ray fl uorescence (XRF) analysis at Brigham Young University using a Siemens SRS-303 spectrome-ter. Analyses of representative international stan-dards together with our estimates of precision and accuracy are available from the authors. Trace-element analyses (including rare-earth elements) of 50 of these samples were also performed by ALS Chemex using inductively coupled plasma mass spectrometry (ICP-MS). Rocks were dis-solved using a four-acid “near-total” digestion

method for most trace elements, but were fused with a fl ux before digestion for rare-earth ele-ment (REE) analyses. Zr concentrations reported from the four-acid dissolution technique were much lower than the XRF analyses and are not used here. Other element concentrations (includ-ing REE, Nb, and Th) agreed favorably by the various techniques. Representative analyses are presented in Table 1, and the complete data set is available in Supplemental Table S11.

U-Pb zircon ages were determined by laser ablation ICP-MS (LA-ICP-MS) in the GeoAna-lytical Lab at Washington State University. All zircon samples were processed and separated using standard gravimetric and magnetic tech-niques at Brigham Young University. Zircon grains, both standards and unknowns, were mounted in a 1-inch-diameter epoxy puck that was ground and polished to expose the interi-ors of the grains. Cathodoluminescence images acquired at the University of Idaho were used as base maps for recording laser spot locations and to reveal growth and compositional zonation, inclusions, and to look for inherited cores. Chang et al. (2006) present a comprehensive overview of the laser ablation techniques, and a brief over-view is given below. The analytical results are summarized in Table 2, and the complete data set is available in Supplemental Table S22.

GEOCHRONOLOGY

New U-Pb zircon ages were acquired for six samples from the eastern Sulphur Spring Range. To avoid problems associated with alteration and zircon inheritance from older rock units, we obtained U-Pb ages on zircons using LA-ICP-MS. Zircon ages were determined using a New Wave UP-213 laser ablation system in conjunction with a Thermo Scientifi c Element2 single collector, double-focusing magnetic sec-tor ICP-MS. Zircons were analyzed using a 30-μm-diameter beam operating at 10 Hz; the ablated material was delivered to the torch by a mixed He and Ar gas. Laser-induced time-dependent fractionation was corrected by nor-malizing measured ratios in standards and sam-ples to the beginning of the analysis using the intercept method. Static fractionation, including that caused by laser ablation and due to instru-mental discrimination, was corrected using external zircon standards. In our case, we used FC1 and Peixe (Paces and Miller, 1993; Dickin-son and Gehrels, 2003). Weighted average ages

and Tera-Wasserburg concordia were calculated using IsoPlot 3.0 (Ludwig, 2003).

Total uncertainty for each spot analysis of an unknown was combined quadratically with the uncertainty in the measured isotope ratios and the uncertainty in the fractionation factors cal-culated from the measurement of standards. For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards. Based on a comparison of LA-ICP-MS ages with ages determined by thermal ionization mass spec-trometry, Chang et al. (2006) estimated the accu-racy of age determinations using this technique to be on the order of 1% or better. However, there are unresolved contributions to uncertainty from the lack of a common Pb correction and from potential matrix effects between standards and unknowns. Consequently, we analyzed the Temora zircon as an independent check on the accuracy and precision. The Temora zircon has been proposed as a zircon standard by Black et al. (2003), who reported a weighted aver-age 206Pb/238U age of 416.8 ± 1.1 Ma based on 21 isotope dilution-thermal ionization mass spectrometric (ID-TIMS) analyses and 416.8 ± 1.8 Ma based on 50 sensitive high-resolution ion microprobe (SHRIMP) analyses. Chang et al. (2006), using the same instrument and ana-lytical conditions as used here, report an age of 416 ± 9 Ma for Temora. During the course of our analyses, 12 LA-ICP-MS analyses on seven grains of Temora were collected in two sepa-rate analytical sessions. All analyses, corrected for fractionation and incorporating fraction-ation factor uncertainty, give a weighted mean 206Pb/238U age of 416.9 ± 5.6 Ma (mean square of weighted deviates [MSWD] = 0.49), which is within error of the ID-TIMS age (Table 2).

Four samples of the principal volcanic units from the Sulphur Spring Range gave middle Tertiary 206Pb/238U ages ranging from 35.9 ± 0.5 to 31.4 ± 0.5 Ma (Table 2; Fig. 5) that correlate with the stratigraphic sequence where it can be interpreted (Fig. 6). Two samples show no evi-dence of older zircon grains, while the other two have inherited zircons with Precambrian ages. A dacitic sample (04EB86) from the biotite por-phyry unit had one zircon that yielded 207Pb/206Pb ages of ca. 1.8 Ga. An andesite lava (04EB123) with considerable evidence for contamination, as described below, had multiple grains with “anomalous” ages. One grain was large enough

1If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00113.S1 (Table S1) or the full-text article on www.gsajournals.org to view Supplemental Table S1.

2If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00113.S2 (Table S2) or the full-text article on www.gsajournals.org to view Supplemental Table S2.



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Figure 4. Geologic map (A) and cross section (B) of the northeastern Sulphur Spring Range, Nevada. West side of the cross section is modifi ed from Carlisle and Nelson (1990). EGF—East Graben Fault; WGF—West Graben Fault. Unit labels on west side of cross section: Ov—Ordovi-cian Vinini Formation; Dll—Devonian Lone Mountain Dolomite Lower; Dlm—Devonian Lone Mountain Dolomite Middle; Dlu—Devonian Lone Mountain Dolomite Upper; Dm—Devonian McColley Canyon Formation; Du—Devonian Union Mountain Formation; Dt—Devonian Telegraph Formation; Dw—Devonian Woodruff Formation; Mc-d—Mississippian Chainman–Dale Canyon Formation.

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TABLE 1. CHEMICAL ANALYSES OF REPRESENTATIVE VOLCANIC ROCKS FROM THE NORTHERN SULPHUR SPRING RANGE, NEVADA

Sample no. 04EB 097 04EB 105 04EB 162a 04EB 102 04EB 143 04EB 041a 04EB 137 03EB 25 04EB 050 04EB 049 04EB 154 04EB 073 Unit symbol Tbr Tl Tu Tu Tbp Tbd Tpd Tba Tba Ta Ta Rock type Flow-

banded rhyolite

Latite Square quartz

porphyry

Union tuff Biotite porphyry

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Plagioclase dacite

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dike

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Andesite Andesite Miocene basalt

wt% SiO2 74.58 61.04 74.59 64.73 63.92 68.82 62.71 51.83 53.49 60.14 62.48 50.26 TiO2 0.13 0.79 0.07 0.76 0.72 0.59 0.77 1.09 1.04 0.99 0.88 2.12 Al2O3 13.16 17.09 14.26 16.91 16.92 16.03 16.20 11.81 13.83 15.09 15.27 16.19 Fe2O3 1.33 5.15 1.42 3.85 3.41 2.82 4.64 8.68 8.50 7.22 6.11 11.90 MnO 0.03 0.04 0.05 0.02 0.06 0.03 0.07 0.13 0.14 0.10 0.10 0.17 MgO 0.16 1.74 0.23 0.37 0.60 0.70 1.54 12.86 8.45 3.19 2.39 5.07 CaO 1.35 4.90 0.48 3.55 4.03 3.58 4.61 9.48 8.34 5.89 4.66 8.29 Na2O 3.29 3.11 3.59 2.94 3.12 3.28 2.81 1.14 2.25 2.34 2.50 3.38 K2O 5.04 4.46 4.69 4.69 2.86 3.63 3.21 0.96 1.61 3.86 4.23 1.55 P2O5 0.07 0.36 0.08 0.23 0.30 0.21 0.30 0.21 0.21 0.41 0.46 0.89 LOI 0.93 1.79 1.19 1.95 3.24 0.82 2.83 2.19 1.82 1.05 0.50 -0.27 Total 100.06 100.47 100.64 99.99 99.18 100.51 99.69 100.38 99.68 100.28 99.59 99.53 ppm Sc – 13 – 10 10 – 10 27 22 16 13 22 V 6 130 5 95 65 54 90 194 181 142 136 220 Cr 5 10 2 14 7 6 8 490 440 36 22 71 Co 10 12 18 5 – 14 8 – 27 18 22 27 Ni 2 3 3 3 2 2 2 166 144 7 6 38 Cu 5 5 – 5 3 2 4 41 39 8 6 28 Zn 34 84 23 47 60 36 74 84 88 74 67 135 Ga 16 21 19 20 20 19 20 17 18 19 19 22 Rb 224 170 272 121 127 111 99 39 49 132 150 19 Sr 231 725 61 675 689 696 735 691 649 671 592 671 Y 19 24 31 17 21 14 21 27 28 25 23 41 Zr 116 218 60 212 234 204 214 202 202 186 184 258 Nb 19 17 45 15 17 16 15 13 14 13 13 20 Ba 1482 1744 231 1945 1755 1540 1545 1135 767 1214 1267 1010 La 31 44 13 45 43 45 44 35 35 29 28 39 Ce 53 95 38 79 88 87 93 90 74 65 66 100 Nd 22 34 – 30 34 34 35 33 33 25 26 36 Sm 3 8 2 5 3 4 7 9 6 9 7 11 Pb 34 22 18 17 20 21 18 8 9 14 18 9 Th 22 17 10 14 18 16 16 9 10 7 8 2 U 7 7 4 4 5 4 5 2 3 4 4 3 Note: LOI—Loss on ignition at 1000 °C for 4 hours; dashes (–)—below detection limit; Fe2O3—total Fe as Fe2O3.

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eosphere, June 2008 503

to yield three 207Pb/ 206Pb ages of ca. 2.8 Ga; tw

o other zircon grains gave four separate 207Pb/ 206Pb ages of ca. 1.8 G

a; two other zircon grains gave

three separate 207Pb/ 206Pb ages of ca. 1.4–1.2 Ga;

and one grain yielded late Cretaceous 206Pb/ 238U

ages. W

e interpret the eruption age of the andes-ite to be best represented by ten 206Pb/ 238U

ages from

four separate zircon grains that range from

33.3 to 30.7 Ma. B

ecause the age range is larger than expected for analytical error alone (as indi-cated by an elevated M

SWD

value), we used the

TuffZirc routine in IsoPlot to estim

ate an age of 31.4 +1.3/−0.5 M

a. Thus, this lava is m

ost likely O

ligocene in age and signifi cantly younger than the other Paleogene volcanic rocks in the volca-nic fi eld. For the other sam

ples, our preferred age w

as the weighted m

ean of the 206Pb/ 238U

ages (Table 2). A

weighted m

ean age for 16 analyses yielded an age of 35.1 ± 0.5 M

a for one of the plagioclase dacite dom

es (Fig. 5). A bio-

tite porphyry intrusion and a biotite dacite tuff have indistinguishable ages of 35.9 ± 0.5 and 35.5 ± 0.4 M

a, respectively (Fig. 5).Z

ircon separated from a distinctive rhyolitic

clast (sample 04JA

156) of what w

e have called the “square quartz porphyry” w

as analyzed in an attem

pt to constrain the eruptive age of a tuffaceous interval low

in the volcanic section (the U

nion tuff described below). H

owever, the

206Pb/ 238U age of the zircons in the clast is 157.4

± 2.2 Ma or L

ate Jurassic (Table 2). Jurassic plutonic rocks are exposed farther south in the Sulphur Spring R

ange and have similar textures

to these porphyritic clasts. This sam

ple also contained inherited zircons w

ith Paleozoic and Proterozoic ages.

Zircon from

a nonwelded pyroclastic deposit

of rhyolitic

composition

(sample

03JA122)

yielded a Miocene age of 14.0 ± 0.3 M

a, the sam

e age as rhyolitic volcanism associated w

ith the B

asin and Range province. A

lthough the zircons are com

positionally zoned, we found no

evidence of age inheritance in the zircons of this sam

ple (Table 2).

PAL

EO

GE

NE

GE

OL

OG

Y O

F TH

E

NO

RT

HE

RN

SUL

PH

UR

SPR

ING

RA

NG

E

Below

we use m

apping, structural, strati-graphic, and petrological inform

ation, together w

ith the new U

-Pb ages, to reconstruct the his-tory of m

agmatism

in the northeastern part of the Sulphur Spring R

ange. We have inform

ally grouped the Paleogene volcanic rocks into the E

ast Sulphur Spring suite to distinguish them

from potentially distinctive igneous rocks to

the west. Figures 4 and 6 show

the geologic and

stratigraphic relationships

between

the units as determ

ined by superposition, cross-cutting relations, and isotopic ages. Figures 7

and 8 show the elem

ental compositions of the

volcanic rocks.

Structure

A deform

ed succession of Paleozoic sedi-m

entary rocks is in high-angle fault contact with

a complex suite of east-dipping volcanic units

in the studied area (Fig. 4). The Paleozoic strata

dip eastward and are cut by thrust faults related

to the Roberts M

ountains thrust (Fig. 4; Carlisle

and Nelson, 1990). T

he eastward tilt is probably

the result of displacement on range-bounding

normal faults that developed during the M

io-cene. W

ithin the map area, the m

ost prominent

faults are the north-trending West G

raben and E

ast Graben faults, both of w

hich have appar-ent norm

al displacements of hundreds of m

eters (Fig. 4; C

arlisle and Nelson, 1990). Post-O

ligo-cene m

ovement on the E

ast Graben Fault cut

the Paleozoic thrusts and dropped the Paleogene volcanic section against low

er Paleozoic sedi-m

entary rocks. How

ever, kinematic indicators in

the fault zones demonstrate that the faults expe-

rienced oblique and strike-slip displacement as

well as norm

al displacement. Tw

o other observa-tions suggest that these faults have a protracted history. Facies changes in som

e of the Devonian

carbonate units across the East G

raben Fault suggest that it m

ay have controlled sedimenta-

tion patterns in the Paleozoic, as described in the northern C

arlin trend (Volk et al., 1995; E

msbo

et al., 1999). In addition, the East G

raben Fault m

ay have localized the emplacem

ent of Eocene

dikes and plugs (Fig. 4). An extended history

of activity along these faults suggests that they are m

ajor structures, possibly of crustal-scale. T

hese faults may have guided the em

placement

and eruption of the middle Tertiary volcanic

rocks and provided conduits for hydrothermal

fl uids. A sim

ilar interpretation has been made

for north- to northwest-trending structures in the

southern Carlin trend (L

ongo et al., 2002).T

he northern Sulphur Spring Range lies on a

strong gradient in the Bouguer gravity anom

aly. T

he gradient trends northeast and separates a broad gravity low

on the south from higher

values to the north (Grauch et al., 2003). T

he gradient is interpreted to separate Proterozoic continental basem

ent on the east from young

accreted terranes on the west. A

n irregular clus-ter of aerom

agnetic highs marks the northern

Sulphur Spring Range; the largest is ~10 km

across and is centered in the valley just east of the volcanic outcrops (G

rauch et al., 2003). The

magnetic highs are probably due to the higher

magnetic

susceptibility of

the volcanic

and subjacent intrusive rocks and m

ay reveal the extent of the shallow

intrusive system beneath

the volcanic fi eld. The size and am

plitude of the

TABLE 2. SUMMARY OF U-PB LASER ABLATION INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY AGES OF IGNEOUS ROCKS FROM THE NORTHERN SULPHUR SPRING RANGE, NEVADA

Sample Unit Type Age (Ma) MSWD Confidence Grains Spots Rejected Inheritance 03JA122 Rhyolite tephra Weighted mean 14.0 ± 0.3 1.18 9 14 0

04EB123 Andesite TuffZirc 31.4 +1.3 – 0.5 97.9 10 24 14 2.8, 1.8, 1.4,

0.08 Ga 04EB044 Dacite domes Weighted mean 35.1 ± 0.5 1.30 11 16 0 04EB041 Biotite dacite tuff Weighted mean 35.5 ± 0.4 1.30 11 18 1 04EB86 Biotite porphyry Weighted mean 35.9 ± 0.5 1.16 8 18 3 1.8 Ga 04JA156 Square quartz porphyry Weighted mean 157.1 ± 1.8 1.30 10 22 6 0.4, 1.1 Ga Note: Age uncertainty at 2 sigma. MSWD—Mean square of weighted deviates. Grains—Number of zircon grains analyzed. Spots—Number of analyses. Rejected—Number of analyses not used in age calculation. Inheritance—Ages of old grains in billions of years.

Ryskamp et al.


180 170 160 150 140

0.04

0.05

0.06

0.07

34 36 38 40 42 44 46 48

35 34 33 32 31 30 29

0.04

0.05

0.06

0.07

180 190 200 210 220 230

.

40 38 36 34 32

0.040

0.044

0.048

0.052

0.056

0.060

0.064

0.068

165 175 185 195 205

28 24 20 16 12

0.00

0 .02

0 .04

0 .06

0 .08

200 300 400 500 600 700 800

38 36 34 32 30

0 .040

0 .044

0 .048

0 .052

0 .056

0 .060

160 170 180 190 200 210

40 38 36 34 32

0 .040

0 .044

0 .048

0 .052

0 .056

0 .060

0 .064

155 165 175 185 195 205

.

A. 03JA122 Rhyolitic ash B. 04EB123 Andesite

D. 04EB041 Biotite dacite tuff

E. 04EB86 Biotite porphyry F. 04JA156 Clast in Union tuff (square quartz porphyry)

C. 04EB044 Plagioclase dacite

207 P

b / 20

6 Pb

207 P

b / 20

6 Pb

207 P

b / 20

6 Pb

207 P

b / 20

6 Pb

207 P

b / 20

6 Pb

207 P

b / 20

6 Pb

238 U / 206 Pb

238 U / 206 Pb 238 U / 206 Pb

238 U / 206 Pb238 U / 206 Pb

238 U / 206 Pb

155

yMean = 35.9 ± 0.5 Ma

Mean = 157.1 ± 1.8 Ma

Mean = 35.5 ± 0.4 Ma Mean = 35.10 ± 0.5 Ma

Mean = 31.4 +1.3 -0.5 Ma Mean = 14.0 ± 0.3 Ma

Figure 5. Tera-Wasserburg U-Pb concordia plots for zircons separated from igneous rocks of the Sulphur Spring Range. (A) Rhyolitic ash; (B) andesite (Ta); (C) plagioclase dacite dome (Tpd); (D) biotite dacite tuff (Tbd); (E) biotite porphyry intrusion (Tbp); and (F) clast in Union tuff (Tu). Ovals represent 1 sigma error ellipses. Ages given are means of the 238Pb/206Pb ages.



anomaly are similar to others along the Carlin trend (Ressel and Henry, 2006).

Stratigraphy

Elko Formation (Te)We correlate the oldest Paleogene strata in

the eastern Sulphur Spring Range with the Elko Formation. These clastic sedimentary rocks were mapped by Carlisle and Nelson (1990) as the Permian Garden Valley Formation, but the presence of Jurassic quartz porphyry cobbles (Table 2) in the Union tuff member shows that this age assignment is not correct. Instead, the overall lithologic character, including the pres-ence of volcanic rocks, suggests correlation with the Elko Formation, a prominent clastic unit of Eocene age in central Nevada. Else-where, the Elko Formation is a fi ning-upward series of fl uviolacustrine beds. The base is pri-marily red-brown pebble to cobble conglom-erate and locally arkosic sandstone, but the middle and upper parts are dominated by fi ne-grained clastic sediments, lacustrine limestone, and oil shale (Henrici and Haynes, 2006). The conglomerate clasts in the lower member are predominantly reworked Paleozoic chert and quartzite, but also include rare (<1%) porphy-ritic rhyolite (Smith and Ketner, 1978; Henrici and Haynes, 2006). Locally, pyroclastic fallout tuffs and ignimbrites are interlayered with the

sedimentary strata and give ages that range from 46 to 38 Ma.

In the eastern Sulphur Spring Range, the beds we correlate with the Elko Formation crop out east of the East Graben Fault (Fig. 4) and con-sist of red-brown conglomerate and arkose with pebbles and cobbles of quartzite and green chert. Lava fl ows and tuff are interlayered with the clastic sedimentary rocks, as described below. The combination of nonvolcanic conglomerate, arkose, and tuff is very similar to the lower mem-ber of the Elko Formation as described by Ket-ner and Alpha (1992) and Henrici and Haynes (2006). The overlying Indian Well Formation is dominated by volcaniclastic sedimentary rocks (Ketner and Alpha, 1992).

Flow-banded rhyolite and latite lava fl ows (Tbr). The basal part of the Elko Formation in the Sulphur Spring Range includes a crystal-poor rhyolite lava fl ow with distinctive alternat-ing dark-gray and light-gray to pink fl ow-layers ~10 cm thick. Although it is only found in one location in the mapped area (Fig. 4), a very similar rhyolite lava fl ow crops out ~11 km to the south. The rhyolite has extremely small phenocrysts of quartz, plagioclase, and magnetite in a glassy matrix containing fl ow-aligned microlites.

A crystal-poor, black, scoriaceous latitic lava fl ow crops out near the banded rhyolite. The lavas have phenocrysts of pyroxene, sieved plagioclase, quartz with reaction rims, and cor-

Tpd

TbrTl

Tu

Tbd

TaTba

Ta

Tbp

Te Elko Fm

TiwIndian Well Fm

Possible correlatives Tr

Tr Rhyolite tephra 14.0 +/- 0.3 MaTa Andesite dike and flows 31.4 +/- 0.5 MaTba Basaltic andesite dikesTpd Dacite domes 35.1 +/- 0.5 MaTbd Biotite dacite tuff 35.5 +/- 0.4 MaTbp Biotite porphyry intrusion 35.9 +/- 0.5 MaTu Union tuffTl Latite lava flowTbr Banded rhyolite lava flow

Tiw Indian Well Formation ~37 - 30 MaTe Elko Formation ~46 - 37 Ma

Figure 6. Stratigraphy of Eo-Oligocene rocks in the East Sulphur Spring volcanic suite, Nevada.

roded K-feldspar, indicative of disequilibrium. In some locations, lobes of latite appear to have fl owed down paleo-streambeds and banked against the fl ow-banded rhyolite.

The fl ow-banded rhyolite fl ow is older than the nearby plagioclase dacite lava domes; contacts are not exposed, but it crops out on the western edge of the east-dipping volcanic sequence butted against the East Graben Fault. It is correlative with and older than part of the Elko Formation, as there are outcrops of the conglomeratic facies of the Elko Formation stratigraphically above the banded rhyolite (Fig. 4). The latites are somewhat younger than the rhyolite, but are also within the lower part of the Elko Formation in this area.

Union tuff (Tu). Exposures immediately south and southeast of the Union district contain a poorly welded, orange to maroon, dacite ash-fl ow tuff, herein informally named the Union tuff. Plagioclase and quartz are the most promi-nent phenocrysts in this dacitic tuff. The tuffa-ceous interval also includes quartzite cobbles and well-rounded pebbles and cobbles of rhyo-lite with quartz phenocrysts that are probably the same as the porphyritic rhyolite identifi ed by Henrici and Haynes (2006). The porphyritic clasts contain bipyramidal quartz phenocrysts along with plagioclase, potassium feldspar, and sparse biotite. The U-Pb zircon ages of these clasts are Late Jurassic (Table 2). A porphyritic granitic intrusion very similar to the clasts crops out in the southeastern Sulphur Spring Range ~40 km away. Clast sizes and the apparent source of the clasts are consistent with paleo-current indicators found by Henrici and Haynes (2006), indicating the source area of the sedi-ments was southeast of the Elko Hills.

The Elko Formation is Eocene to Oligocene according to Smith and Ketner (1978) and middle to late Eocene according to Henrici and Haynes (2006). Radiometric ages on tuffs in the unit range from 46.1 to 38.6 Ma (Solo-mon et al., 1979; Haynes, 2003; Henrici and Haynes, 2006) and suggest that the Union tuff is Eocene in age.

Biotite Porphyry (Tbp) and Biotite Dacite Tuff (Tbd)

A shallow dike or vent-fi lling intrusion of porphyritic dacite is found near the East Graben Fault (Fig. 4). It contains abundant phenocrysts of coarse biotite, plagioclase, sanidine, quartz, and altered magnetite. The large (~2 mm) book-lets of biotite are particularly distinctive. Some quartz phenocrysts are resorbed. Phyllic altera-tion has converted most sanidine to sericite. The medium- to fi ne-grained matrix consists of intergranular quartz, plagioclase, biotite, and small amounts of glass.

Ryskamp et al.


Figure 7. Chemical compositions of igneous rocks from the Sulphur Spring Range, Nevada. Rock compositions normalized to 100% on a volatile-free basis. (A) International Union of Geological Sciences (IUGS) chemical classifi cation for volcanic rocks (Le Maitre, 1989). (B) Modifi ed alkali lime index of Frost et al. (2001). (C) FeO/(MgO + FeO) discriminant from Miyashiro (1974) using terminology of Frost et al. (2001). (D) Volcanic rocks from the Sulphur Spring Range are dominantly metaluminous, but some of the more silicic rocks are peraluminous. (E) SiO2 versus K2O variation diagram with fi elds from Ewart (1979).



A biotite-bearing ash-fl ow tuff is the most widespread unit in the mapped area. However, it crops out poorly as porphyritic fragments that litter the ground. Roadcuts provide the best exposures, where the unit is seen to be a non-welded ignimbrite with abundant lithic clasts. In most places, it is argillically altered, whitish-yellow, and laced with small oxidized veins containing pyrite. The tuff contains phenocrysts of plagioclase, coarse biotite (~2 mm across), quartz, Fe-rich garnet, and megacrysts of quartz. Its major-element composition ranges from dac-ite to rhyolite (Fig. 7).

The biotite porphyry intrusions and the bio-tite dacite tuff have indistinguishable ages and elemental compositions and similar textures featuring coarse biotite. U-Pb zircon ages of these units are 35.9 ± 0.5 and 35.4 ± 0.4 Ma, respectively (Table 2 and Fig. 5), and their compositions suggest the two units may be comagmatic (Figs. 7, 8, and 9). As such, the biotite dacite tuff may be an explosive product, whereas the biotite porphyry may form a vent-fi lling dome or intrusion.

The biotite dacite tuff overlies the Union tuff in the Elko Formation (Fig. 6). Thus, the tuff and units overlying it may be correlative with the Indian Well Formation, an Eocene- Oligocene series of ash-fl ow tuffs interbedded with vol-caniclastic sediments mapped in the valley east of the Pinion Range to the north of the Sulphur Spring Range (Smith and Ketner, 1978). The U-Pb ages are similar to the ages of tuffs within the Indian Well Formation.

Dacite Lava Domes (Tpd)A group of dacite lava domes with distinctive

large plagioclase phenocrysts overlies the biotite dacite tuff. These “plagioclase dacite” domes comprise the majority of the knolls in the east-ern portion of the volcanic fi eld (Fig. 4). This is the major Tertiary volcanic unit described and mapped by Carlisle and Nelson (1990).

The dacite domes consist of pink or orange phenocryst-rich, fl ow-foliated or brecciated lava. A vitrophyre is present in some locations along dome margins. Matrix-supported fl ow breccias developed along some shear planes. Plagioclase is the primary phenocryst with lesser quartz and clinopyroxene, along with sparse amphibole, bio-tite, and tiny euhedra of magnetite as inclusions and in the groundmass. Chlorite, clay minerals, and iron-stains are characteristic of altered rocks.

The U-Pb zircon age of 35.1 ± 0.5 Ma obtained from one dome (Fig. 5) is consistent with fi eld relations showing that the dacite lavas erupted onto or through older biotite dacite tuff.

Basaltic Andesite Dikes (Tba)The most mafi c of the Paleogene igneous

rocks found in the East Sulphur Spring suite are dikes that generally trend north-northeast (Fig. 4). The dikes cut the biotite dacite tuff (Tbd) as well as the dacite domes (Tpd). Most of the dikes are dense, black porphyritic basaltic andesite containing phenocrysts of clinopyrox-ene, orthopyroxene, olivine (Fo

84), and sparse

plagioclase. The matrix is composed of plagio-clase, pyroxene, Fe-Ti oxides, and minor glass.

A distinct trachytic fl ow foliation is defi ned by elongate plagioclase microphenocrysts. Dikes with shoshonite and biotite-bearing latite com-positions are rare (Fig. 7). Whole-rock composi-tions range from basaltic andesite with MgO as high as 13 wt% to as low as 3 wt% in the latite; Cr concentrations follow suit ranging from nearly 500 to 15 ppm (Table 1).

Andesite Dikes and Lava Flows (Ta)The youngest volcanic unit in the East Sulphur

Spring suite is a series of crystal-rich andesite dikes and near-vent fl ows with unique textural aspects. The exposed dikes trend NNE, like most other dikes in the northern Sulphur Spring Range. Rounded, sieve-textured plagioclase is the domi-nant phenocryst. Other prominent phenocrysts include biotite, clinopyroxene, orthopyroxene, olivine (Fo

78), and magnetite. These mafi c phe-

nocrysts coexist with megacrysts of sanidine and unstrained quartz as much as 3.5 cm long. Smaller quartz phenocrysts are extensively resorbed, and many of these crystals also have reaction rims of clinopyroxene (Fig. 10). The andesite also con-tains grains of resorbed and oxidized garnet. The garnet is Mn-rich compared to that found in the biotite dacite (Fig. 11). Dike and fl ow margins have perlitic glass in their matrices.

We interpret the mineral assemblage (e.g., forsteritic olivine coexisting with quartz), the resorption of the felsic phases, and the reaction rims to be the result of mixing silicic magma (containing quartz and sanidine megacrysts) and mafi c magma (containing olivine and pyroxene). Clinopyroxene halos around resorbed quartz are also diagnostic of magma mixing (Fig. 10; Coombs and Gardner, 2004).

We interpret the U-Pb zircon data to show that the andesite has an Oligocene eruptive age of ca. 32 Ma, although the error and the MSWD are rather large (Table 2). The 2 sigma uncertainty does not overlap with the ages of the other igneous rocks in the East Sulphur Spring suite (Fig. 12). Stratigraphic relationships also suggest that this is the youngest exposed unit; it appears to over-lie the dacite domes (Tpd) because it outcrops farthest to the east in this east-dipping sequence of strata (Fig. 4). The large spread in U-Pb ages of zircon xenocrysts may show that some of the crystal cargo was derived from already solid rock by assimilation of the Paleoproterozoic (2.5 and 1.7 Ga) basement (Table 2).

Mafi c Dikes (Tmd)In addition to the dikes in the East Sulphur

Spring suite, a series of weakly to extremely altered dikes crops out west and northwest of the mapped area (Fig. 1). The overwhelming major-ity of the dikes are basaltic andesite, but sho-shonite and latite have also been found (Fig. 7).

0.1

1.0

10.0

100.0

1000.0

Ba Rb Th U Nb K La Ce Pb Sr Nd P Sm Zr Ti Y

Roc

k/P

rim

itive

Man

tle

Plagioclase dacite

Biotite porphyryBiotite dacite tuff

AndesiteBasaltic andesite & mafic dikes

Rhyolite lava flowLatite lava flow Union tuff

Figure 8. Normalized trace-element patterns for igneous rocks from the Sulphur Spring Range, Nevada. Normalization values from prim-itive Earth from McDonough and Sun (1995).

Ryskamp et al.


La, and Ce and higher SiO2, Al

2O

3, K

2O, Rb, Nb,

and Ba than the Jurassic lamprophyres described by Ressel and Henry (2006). These differences suggest that the latites are more fractionated rocks and that they have different parentage than the Jurassic lamprophyres. The Sulphur Spring dikes are also unlike Neogene basalt fl ows that crop out to the south—the dikes are magnesian and have lower TiO

2 and Nb concentrations than

the young basalts (Table 1). These mafi c dikes are similar to the Paleogene basaltic andesite dikes on the east side of the range in their orien-tations, major- and trace- element compositions, mineral assemblages, and alteration styles. Con-sequently, we presume that they are Paleogene in age.

The altered mafi c dikes have anomalous con-centrations of As, Ba, Cr, Cu, Ni, Pb, Sb, and Zn. These dikes are believed to lie within the hydrothermally altered outer carapace of the buried Eocene intrusive system. The high con-centrations of Cr are especially noteworthy. Cr concentrations range from 60 to 630 ppm with an average of 355 ppm. Since Cr is relatively immobile during hydrothermal alteration, these high concentrations show that even the highly altered dikes were mafi c.

Mineralization: Gossan Veins and JasperoidA distinctive set of veins is found within and

surrounding the core of the northern Sulphur Spring Range and includes those in the Union Pass and Mineral Hill districts (Fig. 1). Their compositions show that large quantities of S, Au, Ag, As, Cu, Mo, Pb, Sb, and Zn were deposited by hydrothermal fl uids. Most veins are oxidized, sulfi de-rich quartz veins or gossans. Veins range in width from 1 to 100 cm, are often banded, and have been brecciated and then healed by younger deposits of silica; some veins contain unoxidized fragments of sulfi de minerals, including pyrite or galena. These are highly sulfi dic polymetallic veins (now completely oxidized in most cases), and might be equivalent to “D” veins in the “alpha” system originally established for the El Salvador deposit by Gustafson and Hunt (1975). The lack of sericite-rich margins is due to their formation in carbonate rocks.

The dominant trend of the gossan veins is northeast, which is the same as the dated Paleo-gene, and presumed Paleogene, mafi c dikes described above. Their relationship to the Paleo-gene magmatism is not certain because they are not directly adjacent to any dikes, but because the gossan veins are found in the same part of the range as the dikes and share a common orienta-tion, we speculate that they are distal to but con-temporaneous with the major intrusive cluster.

The east side of the Sulphur Spring Range also has a number of small bodies of jasperoid,

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Roc

k/C

hond

rite

Basaltic andesite and latite dikesSulphur Spring Range

Jurassic lamprophyre dikes, Carlin Trend

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Roc

k/C

hond

rite

Plagioclase dacite

Biotite porphyryBiotite dacite tuff

Andesite

Rhyolite lava flowLatite lava flow Union tuff

B

A

Figure 9. Rare-earth element (REE) patterns of igneous rocks from the Sulphur Spring Range normalized to the composition of chon-dritic meteorites (McDonough and Sun, 1995). (A) Intermediate and silicic volcanic rocks. (B) Basaltic andesite and latite dikes (unit Tba) from the East Sulphur Spring suite compared to Jurassic lampro-phyre dikes from the Carlin trend (Ressel and Henry, 2006).

The dikes occur largely in two east-northeast- trending swarms that transect the northern Sul-phur Spring Range where they cut pre-Cenozoic rocks. Most of the dikes are altered to argillic or phyllic mineral assemblages and crop out as light-orange to tan fragments. Several propyliti-cally altered dikes preserve phenocrysts of oliv-ine and clinopyroxene.

The ages of these dikes are uncertain because they do not cut any of the Paleogene units. How-ever, the elemental compositions and mineralogy of the freshest rocks show that these dikes are not

like the Jurassic lamprophyres in the region. The basaltic andesites from the Sulphur Spring Range have lower alkalis, P, Rb, Sr, and Ba; their light REE (LREE) patterns are also different than those of the Jurassic lamprophyres (Fig. 9). The basal-tic andesites also lack mica and amphibole phe-nocrysts, but they have plagioclase phenocrysts. Moreover, most of the Jurassic lamprophyres along the Carlin trend strike NW, not NE to ENE like most of the Paleogene dikes. Even the latite samples are unlike the Carlin lamprophyres; they have signifi cantly lower MgO, CaO, Cr, Ni, Sr,



particularly along the contact between carbon-ate and clastic rocks with silty carbonates and calcareous shales. These silicifi ed bodies have anomalously high concentrations of Au, As, Hg, Sb, and Tl and are very similar to those found in Carlin-type systems (e.g., Wilson et al., 1994). The Paleozoic host rocks are complexly faulted, and the mineralization is spatially associated with the Eocene igneous rocks. These structural, geochemical, and stratigraphic characteristics are similar to those of Carlin-type gold depos-its elsewhere in Nevada. (Indeed, Carlin-style alteration hosted in calcareous shales and car-bonates has been intercepted in a drill hole just north of the mapped area, but the results of these investigations are still proprietary.)

PETROCHEMISTRY OF THE EAST SULPHUR SPRING SUITE

During its ~4 million year lifetime, the Eocene-Oligocene magma system beneath the northern Sulphur Spring Range erupted basal-tic andesite to rhyolite as lava fl ows, domes, and small pyroclastic deposits, apparently fed by a series of dikes. Dacite is the dominant erupted volume. Much of the volcanic section is interlayered with contemporary sedimentary deposits. Modal and whole-rock chemical com-positions of the igneous rocks provide some general insights into the nature and evolution of the magma system.

Subduction Zone Origin

The Eocene-Oligocene volcanic rocks of the Sulphur Spring Range have much in common with other subduction-related continental mar-gin suites (Fig. 7). They are largely calc-alkalic using the classifi cation of Frost et al. (2001). Only some of the basaltic andesite dikes and the biotite dacite tuff are calcic. The suite is overwhelmingly magnesian (using the dividing line of Miyashiro [1974] and the terminology of Frost et al. [2001]); this lack of Fe-enrichment is characteristic of crystallization at relatively high f O

2.

Likewise, even the most primitive basal-tic andesite dikes have relatively low TiO

2,

less than ~1.4 wt%, similar to volcanic rocks found in arcs. Most of the volcanic units form a high-K series, but the hybridized andesite and some of the intermediate composition dikes range widely from medium K

2O to shoshonitic

(Fig. 7). REE patterns of the rocks in the East Sulphur Spring suite are similar to those of igne-ous rocks from continental margin subduction zones (Ewart, 1979) with relatively steep slopes and small negative Eu anomalies (Fig. 9). The latites have the highest REE concentrations,

Garnet

Garnet Garnet

Quartz

A

B C

D

1 mm 1 mm

Olivine

Quartz

Clinopyroxene

Plagioclase

1 mm

1 mm

Figure 10. Photomicrographs of andesite (unit Ta) showing dis-equilibrium mineral assemblage, textures, and reactions indica-tive of magma mixing. Transmitted light. (A) Sieved plagioclase, oxidized olivine, clinopyroxene glomerocrysts, and quartz with extensive resorption and a reaction rim of clinopyroxene (sample 04 EB 064). (B) Garnet replaced by magnetite along rim (sample 04 EB 064). (C) Euhedral garnet has no rim (sample 04 EB 168, lava fl ow sample). (D) Garnet rim and fractures replaced by mag-netite; quartz crystal is rounded and rimmed with clinopyroxene (sample 04 EB 123).

Ryskamp et al.


and the most silicic rhyolite has lower LREE concentrations and a larger Eu anomaly. On the tectonic discrimination diagrams of Pearce et al. (1984), which are based on Rb, Nb, and Y abun-dances, the rhyolite lavas and more voluminous dacite lavas and tuffs are similar to subduction-related volcanic arc granites (Fig. 13). Based on Zr-Ti-Ce-P systematics, mafi c rocks in the Sulphur Spring suite are also of a continental arc-type (Fig. 13). Finally, like volcanic rocks erupted in other subduction settings, all of the rocks have negative Nb and Ti anomalies and positive anomalies for Pb on primitive-mantle normalized diagrams (Fig. 8). We conclude that the East Sulphur Spring suite is related to subduction and that the parental magmas were hydrous and oxidized. In addition, arc magmas like these are typically rich in S and chalcophile metals (e.g., Richards, 2003).

Biotiteporphyryintrusion

Biotitedacitetuff

Plagioclasedacitedome

Andesite Bingham Cu-Mo-Au

Deposit,Utah

CarlinTrend,Nevada

Tuscarora,Nevada

EmigrantPass,

Nevada

Rain-Railroad, Nevada

30

32

34

36

38

40

42

Age

(M

a)

Sulphur Spring suite, Nevada Comparison Suites

Figure 11. Compositions of garnet in the biotite dacite tuff (Tbd) and in the andesite (Ta) of the East Sulphur Spring suite. Older, biotite dacite tuff con-tains garnets with lower Mn contents than garnet xenocrysts in the andesite.

Figure 12. Ages of Eo-Oligocene igneous rocks from the East Sulphur Spring, Bingham, Carlin, and Tuscarora suites. One sigma error bars are shown for the U-Pb zircon ages from the East Sulphur Spring suite. Bingham, Utah, ages from Deino and Keith (1997), Maughan et al. (2002), Carlin trend, Emigrant Pass, and Rain-Railroad Pass, Nevada, ages from Ressel and Henry (2006), Tuscarora ages from Henry and Boden (1998).



Crustal Melting and Felsic Peraluminous Magmas

An important characteristic of the felsic members of the East Sulphur Spring suite is the inclusion of peraluminous silicic rocks. Two of the volcanic units contain Fe-rich magmatic garnet—a mineralogical indicator of the excess of Al over Ca + Na + K. The andesite lavas (Ta) have rimmed crystals of Mn-rich garnet, and the biotite dacite tuff (Tbd) has sparse Mn-poor grains of euhedral garnet. Most of the individual samples of the tuff are peraluminous with the alumina saturation index ranging from 0.95 to over 1.1. Although peraluminous silicic rocks in the Great Basin are typically Cretaceous in age (Miller and Bradfi sh, 1980; Lee and Chris-tiansen, 1983), Eocene granites (ca. 39–34 Ma) in the nearby Ruby Mountains are strongly per-aluminous and locally contain garnet (Kistler et al., 1981; Barnes et al., 2001), as do Eocene rhyolites at Mount Hope (Westra and Riedell, 1995) to the south (Fig. 1). Strongly peralumi-nous silicic magmas like these are most likely generated as a result of partial melting of pelitic or semipelitic metasedimentary rocks in thick-ened continental crust. They show that many of the Paleogene magma systems of north-central Nevada include large proportions of crustal magma. This implies that suffi cient mantle-derived magma was inserted into the crust to induce melting by breakdown of hydrous min-erals. Some of these crustal melts were probably assimilated in the more mafi c magmas as well.

Fractional Crystallization

Fractional crystallization appears to have been an important differentiation process for both felsic and mafi c magmas in the East Sul-phur Spring suite. Even though the alkalis have been perturbed by slight alteration, it is appar-ent that the biotite dacite tuff (unit Tbd) ranges from dacite to rhyolite in composition (Fig. 7). Its chemical variation is consistent with frac-tional crystallization of the observed phases—

Figure 13. Tectonic discrimination diagrams for rocks from the East Sulphur Spring volcanic suite compared to other Eocene volcanic suites from Bingham, Utah, and along the Carlin trend, Nevada. (A) Mafi c rocks (Müller and Groves, 2000). (B) Silicic rocks (Pearce et al., 1984). Compositions of Bingham samples from Pulsipher (2000) and Maughan et al. (2002). Carlin analyses from Ressel and Henry (2006) and Henry et al. (1999).

feldspars and mafi c silicates and oxides. As SiO2

increases, Al2O

3, CaO, TiO

2, Fe

2O

3, and MgO all

decline in concentration. Increases in the incom-patible elements (Rb, Nb, Pb, and Th) suggest that 25% to 35% fractional crystallization could have created these changes. REE concentrations change little or decrease, perhaps as a result of the removal of garnet.

The intermediate composition dikes (Tba) range in composition from olivine-bearing basaltic andesite (with as much as 13% MgO, 175 ppm Ni, and 500 ppm Cr) to latite (with 2% MgO, 20 ppm Ni, and 15 ppm Cr). These steep declines in compatible element concentra-tions (Fig. 14) suggest fractional crystallization of mafi c mineral phases that have high partition coeffi cients for these elements. Other compat-ible elements, such as Ti, Fe, Mg, Ca, Sc, and V, decrease in concentration as SiO

2 increases. On

the other hand, incompatible elements like P, Zr, LREE, Pb, Rb, and Ba increase from the basal-tic andesites to the latites. The curved trends and sharp decreases of compatible element concen-trations are not typical of magma mixing, which

produces linear trends on two-element variation diagrams. Consequently, we conclude that the basaltic andesite, shoshonite, and latite dikes are probably related to one another by fractional crystallization of pyroxene, olivine, plagioclase, and oxides. The extent of fractionation from parental basaltic andesite to derivative latite was ~50% based on the enrichments of incompatible elements (Nb, Zr, Pb, Th, and Rb).

Magma Mixing

Even though fractional crystallization seems to have been the dominant process leading to latite, a few of the basaltic andesite dikes have trace-element compositions that are consis-tent with mixing of mafi c and silicic magmas. Anomalously high concentrations of Cr and Ni are found in at least two dikes (Fig. 14). How-ever, this remains a tentative conclusion because it is diffi cult to distinguish fractionation from mixing using other elements because compo-sitional trends produced by both processes are linear and overlapping. In fact, Ba variations do

Ryskamp et al.


not clearly show the effects of mixing basaltic andesite and rhyolite, which would pull hybrid-ized magma off the fractionation trend to lower concentrations of Ba (Fig. 14).

A less ambiguous example of magma mixing is found in the andesite fl ows and dikes (Ta). They form a tight compositional array on most variation diagrams. Their high K

2O contents

render them the most consistently shoshonitic unit in the area (Fig. 7). On the other hand, the andesites have lower concentrations of Al

2O

3,

Zr, and Ba (Figs. 14 and 15). Their anomalous positions and linear trends on these variation diagrams suggest that they formed by mixing of mafi c and silicic magmas. The disequilibrium mineral assemblage, which includes forsteritic olivine and quartz, is compelling evidence for magma mixing. This is substantiated by reac-tion rims, and extensive resorption of phases that may have come from the silicic magma—quartz, sanidine, and Mn-rich garnet. The sili-cic end member must have been a high-silica rhyolite to explain the presence of quartz, sani-dine, and Mn-rich garnet. (Garnet in the older dacite tuff is not as rich in Mn [Fig. 11].) In addition, mixing of mafi c magma with a highly evolved rhyolite could explain the low Zr, Ba, and Sr in the andesite compared to other inter-mediate composition rocks from the volcanic fi eld (Fig. 15). Although peraluminous, garnet- bearing silicic magmas are not common in the Great Basin, it is clear that such magmas were generated during the Eocene as noted above. Identifying the mafi c end member is more prob-lematic, but mineral assemblages, olivine com-positions, and trends on most silica variation diagrams (Ti, Al, Fe, Mg, Ca, Na, and K) sug-gest that it was a member of the basaltic andesite to latite suite described above. Elemental trends require that the mafi c end member was moder-ately evolved, with high P

2O

5 (>0.5%) and low

Cr (<50 ppm) and Ni (<25 ppm). Based on these

Figure 14. Variation diagrams comparing compositions of East Sulphur Spring, Car-lin, and Bingham volcanic suites. Blue lines show the results of mixing basaltic andes-ite (Tba) with rhyolite, and red arrows are schematic paths for fractional crystalliza-tion of basaltic andesite. Yellow line shows a mixing trend for the andesite (unit Ta); this line is omitted from (A) and (B) for clarity, but would connect rhyolite with low-Cr and low-Ni magma that plot near the bottom of the graphs. Analyses from Pulsipher (2000), Maughan et al. (2002), Henry et al. (1999), and Ressel and Henry (2006). (A) Cr versus SiO2. (B) Ni versus SiO2. (C) Ba versus SiO2.



end members, major-element mass balance cal-culations show the proportion of rhyolite in the hybridized andesite ranged from ~15% to 35% by weight.

PETROCHEMICAL COMPARISONS

The potential for mineralization, styles of alteration, ages, and overall compositional char-acteristics of the Paleogene volcanic rocks in the Sulphur Spring Range invite comparison with other volcanic suites associated with mineraliza-tion in the Great Basin. Below, we compare the East Sulphur Spring suite with Eo-Oligocene volcanic rocks at the Bingham Canyon Cu-Mo-Au porphyry deposit farther east in Utah and then to magmatic rocks associated with gold deposits in the Carlin trend of northern Nevada.

Comparison with Bingham Canyon Volcanic Rocks

The igneous rocks, structures, and alteration styles of the Sulphur Spring Range are some-what similar to those of porphyry copper sys-tems, such as that at the giant Bingham Canyon deposit in the eastern Great Basin (Fig. 3). Dike swarms, vent-facies volcanic rocks, pebble dikes, local bleaching and marbleization of the carbon-ate rocks, and oxidized sulfi dic veins (gossan veins) straddle and surround an aeromagnetic anomaly. Collectively these features cover an area of at least 5 km × 7 km, comparable in size to that associated with the Bingham deposit, and suggest that the northern Sulphur Spring Range overlies a shallow intrusive center. Similar tec-tonic regimes and structural histories also shaped the features of the hydrothermal and magmatic systems. Both are near paleocontinental mar-gins. The Sulphur Spring Range (and the Car-lin trend) are near an accretionary boundary on the western edge of the Proterozoic basement in central Nevada (e.g., Emsbo et al., 2006) marked by the 87Sr/86Sr = 0.706 line and the Paleozoic-age Roberts Mountains and Golconda thrusts (Fig. 2). Bingham lies near a hypothetical suture between Archean and Proterozoic terranes (e.g., Whitmeyer and Karlstrom, 2007), and Mesozoic thrust faults related to the Sevier Orogeny are cut by the Bingham intrusions. Thus, deep, crust-penetrating faults may have formed anciently in both areas. On the other hand, igneous rocks in the Sulphur Spring Range are ca. 2–3 Ma younger than (Fig. 12) those associated with the Bingham porphyry copper deposit.

Both volcanic suites have compositions that are generally consistent with a subduction zone origin—low Fe/Mg ratios, high oxygen fugaci-ties, high K

2O, and similar “spiky” trace-element

patterns, for example. Apparently, both regions

Figure 15. The compositions of the andesite unit (Ta) are different than the rest of the East Sulphur Spring suite and appear to be consistent with mixing of silicic magma with more mafi c magma. (A) Al2O3 versus SiO2. (B) Zr versus SiO2.

were affected by the same Paleogene detachment or roll back of the Farallon plate, which contrib-uted to continental arc magmatism over a wide area of western North America. These magmas appear to have played a central role in the min-eralization in both areas. Fractionation from basaltic andesite to shoshonite and latite also occurred in both suites, although latite is rare and found only in a small lava fl ow and a few dikes in the East Sulphur Spring suite (Fig. 16). A similar fractionation trend is also important in the Eocene Absaroka volcanic fi eld of Wyoming and Montana (Bray, 1999; Feeley and Cosca, 2003) and appears to be a common process in these continental interior magma systems.

In spite of these similarities, the major- and trace-element compositions of the East Sulphur

Spring suite defi ne arrays that are distinct from those of the Bingham volcanic suite (Figs. 14 and 16). The Bingham suite has high total alkali contents and includes melanephelinite, minette, shoshonite, latite, trachyte, and rhyolite (Maughan et al., 2002), whereas the Paleogene East Sulphur Spring suite is dominated by basal-tic andesite, andesite, dacite, and rhyolite; latite is rare as noted above (Fig. 16). Silica-under-saturated magmas are unknown in the East Sul-phur Spring suite, but melanephelinite intruded as dikes and stocks and erupted as lava fl ows at Bingham and appears to have been an important source of S, Cu, and Au in the deposits.

Characteristically high Cr and Ni contents of the intermediate composition (58%–65% SiO

2)

volcanic rocks from Bingham (Maughan et al.,

Ryskamp et al.


Figure 16. Comparison of the compositions of volcanic rocks from the Sulphur Spring Range with other Eocene volcanic rocks related to mineralization in the Great Basin. International Union of Geo-logical Sciences (IUGS) classifi cation of Le Maitre (1989). Rock compositions normalized to 100% on a volatile-free basis. (A) Carlin and Tuscarora volcanic fi elds. Analyses from Henry et al. (1999), and Ressel and Henry (2006). The compositions of the volcanic rocks overlap on this diagram. (B) Bingham volcanic fi eld. Analyses from Maughan et al. (2002) and Pulsipher (2000). The East Sulphur Spring suite plots mainly in the lower part of the diagram, whereas the Bingham suite is more alkaline.



2002) are not found in the unaltered rocks of the East Sulphur Spring suite (Fig. 14). The volcanic suite at Bingham has elevated concentrations of Cr and Ni across the full range of silica content. Only the least silicic rocks from the Sulphur Spring Range contain high Cr and Ni contents and their concentrations decrease to very low val-ues (>50 ppm) in the more evolved andesites and dacites. For example, dacites from the Bingham volcanic center with 65% SiO

2 have as much as

200 ppm Cr and 75 ppm Ni. Dacites from the Sulphur Spring Range (and from the Carlin trend) have less than 40 ppm Cr and 20 ppm Ni. The low Cr and Ni concentrations are typical of many continental calc-alkaline suites; the high Cr and Ni in the Bingham volcanic suite is a fairly unique feature. Another distinguishing charac-teristic of the Bingham magmas is the very high Ba concentration (Fig. 14). In the East Sulphur Spring suite, only the fractionated latite dikes have Ba contents (>2500 ppm) approaching, but not equaling, the maximum Ba contents of the volcanic rocks at Bingham. Moreover, the Ba content of the Bingham suite is high across the silica range, but is the highest in the mafi c and intermediate composition rocks (<65 wt% SiO

2).

In the East Sulphur Spring suite, Ba increases as SiO

2 increases, whereas in the Bingham suite Ba

generally decreases as SiO2 increases.

Both the Bingham and the East Sulphur Spring suites preserve petrographic and chemi-cal evidence of mafi c magma having mixed with silicic magma to create intermediate composi-tions. At Bingham, the high concentrations of Cr, Ni, and Ba in the intermediate composition volcanic and intrusive rocks have been traced to mixing with a mafi c alkaline magma—silica-undersaturated melanephelinite (or minette) with high concentrations of Ni, Cr, Ba, Cu, Au, and S. Altered olivine and pyroxene rimmed by amphi-bole in intermediate composition rocks at Bing-ham also demonstrate mixing of mafi c alkaline magma with more silicic magma. Other evidence includes dacite clasts in block and ash fl ows that contain cuspate mafi c clots, large resorbed potas-sium feldspar phenocrysts in silicic rocks with elevated Cr and Ni concentrations, and adjacent, same-aged minette and quartz latite dikes (Pul-sipher, 2000; Maughan et al., 2002). The Sulphur Spring unit that most vividly expresses magma mixing is the andesite (unit Ta), with its dis-equilibrium mineral assemblage and anomalous elemental composition. More subtle chemical evidence of mixing is found in a few dikes in the basaltic andesite suite (Tba) as well.

In spite of the evidence for magma mixing, in the East Sulphur Spring suite, the mafi c com-ponent was not silica-undersaturated, nor was it especially rich in Ni, Cr, or Ba compared to the melanephelinite and minette at Bingham.

Instead, the “mafi c” component must have been a moderately evolved (>55% SiO

2) basaltic

andesite or shoshonite, similar to those found in a few dikes. As a consequence, the igneous rocks of the East Sulphur Spring Range have lower concentrations of Ni, Cr, Cu, and prob-ably S and Au (which follow Cu concentrations) than the intermediate composition stocks and lavas associated with the Bingham intrusion.

Comparison with Igneous Rocks along the Carlin Trend

The setting of the Sulphur Spring Range is similar to that of nearby, intensely mineralized localities to the north along the Carlin trend. The Roberts Mountains thrust is an important ele-ment of mineralization along the Carlin trend; it is also exposed in the northern Sulphur Spring Range where it juxtaposes Ordovician strata on Devonian carbonate rocks and is in turn cut by high-angle, N-S–, NW-SSE–, NE-, and E-W–trending normal faults (Carlisle and Nel-son, 1990). These structural and stratigraphic elements appear to be conducive to genesis of Carlin-type gold deposits (Hofstra and Cline, 2000; Grauch et al., 2003; Cline et al., 2005; Ressel and Henry, 2006). In addition, we have noted the presence of jasperoid and altered rocks with anomalous concentrations of As, Hg, Sb, and Tl, which are also features of mineral-ization along the Carlin trend.

Ages, as revealed by new U-Pb data and stratigraphic correlations, show that the mag-matic history of the northern Sulphur Spring Range is also similar to that along the main Carlin trend as outlined by Ressel and Henry (2006). They identifi ed Jurassic plutonic rocks (158 Ma), Cretaceous granite (112 Ma), abun-dant Eocene dikes, lavas, tuffs, and stocks (40–36 Ma) thought to be directly related to Au mineralization, and Miocene rhyolite (15 Ma). Inherited and primary zircons from the igneous rocks in the Sulphur Spring Range record each of these episodes. Clasts in the Elko Formation are derived from a Late Jurassic granitic rock; inherited zircon in the andesite lava has a Creta-ceous age; most of the volcanic rocks and dikes are Paleogene in age but slightly younger (36–32 Ma) than those to the north along the Carlin trend (40–36 Ma; Fig. 12); and fi nally, a rhyolite pyroclastic deposit has a Miocene age.

Igneous rocks in the Sulphur Spring Range are compositionally and genetically similar to those associated with the Carlin trend, which we take to include the Emigrant Pass and Tuscarora volcanic fi elds (Henry et al., 1999; Ressel and Henry, 2006). For example, both suites consist of subalkaline basaltic andesite to rhyolite, and include only minor latite and trachyte (Fig. 16).

Like those from the Sulphur Spring area, the Carlin-trend rocks are dominantly calc-alkalic, magnesian, and high-K. REE patterns of the Sulphur Spring suite are also similar to those of igneous rocks along the Carlin trend (Ressel and Henry, 2006), which also have relatively steep slopes and small negative Eu anomalies (Fig. 9). In other words, both regions erupted magmas with subduction zone characteristics—low Fe/Mg ratios, oxidized, hydrous, and high large-ion lithophile to high-fi eld-strength element (LIL/HFSE) ratios—produced during Paleogene rollback of the Farallon slab. Reduced, ilmenite-dominated silicic magmas are probably rare in the region, but the strongly peraluminous, mus-covite- and garnet-bearing Harrison Pass intru-sion in the Ruby Range is one example (Barnes et al., 2001).

The igneous rocks of the Carlin trend also have concentrations of key trace elements—Cr, Ni, and Ba—much more similar to those of the Sulphur Spring suite than to those at Bingham (Fig. 14; Ressel and Henry, 2006; C.D. Henry, 2005, written commun.). Low concentrations of Cr and Ni in the intermediate magmas and the association with peraluminous magmas imply that fractional crystallization and assimilation of pelitic crustal materials were the predomi-nant magmatic processes in both areas (Fig. 14). Magma mixing played a lesser role and did not involve mafi c alkaline magma as an end mem-ber. Rather, it involved mixing of basaltic andes-ite or andesite with rhyolite. Mixing of composi-tionally similar magmas along the fractionation trends is not ruled out and is in fact quite likely.

The only differences between the East Sul-phur Spring volcanic suite and the magmatic rocks along the Carlin trend that we have iden-tifi ed are that the volcanism in the Sulphur Spring Range is a few million years younger (Fig. 12). Some of the intermediate magmas in the East Sulphur Spring suite have higher K

2O contents and are more shoshonitic, if

the handful of potassic samples from Sulphur Spring is compared with the small set of “rep-resentative” analyses of Carlin-trend igneous rocks published by Ressel and Henry (2006). However, a larger compilation of igneous rock compositions from north-central Nevada (C.D. Henry, 2005, written commun.) has a few Eocene latites, but no shoshonites. There are also a few potassic Eocene plutonic rocks from northeastern Nevada in du Bray’s (2007) database (eight of 643 samples after removing a few obviously altered rocks) in the 50% to 65% range, and three of those are lamprophyre dikes from the Fish Canyon Range whose age is not clearly established as Eocene. Thus, it appears that potassic igneous rocks of Eocene age are sparsely found across northern Nevada.

Ryskamp et al.


Given the similarities in structure, stratigra-phy, alteration styles, geochemical anomalies, magmatic history, and petrology of the Eo- Oligocene igneous rocks, it is reasonable to infer that economic Carlin-type mineralization might exist in the Sulphur Spring Range.

Potential Importance of Mafi c Magma and Magma Mixing for Mineralization

Intermediate and felsic rocks, such as those that typically host porphyry Cu-Au systems, have low concentrations of sulfur and of chalcophile ore metals compared to more mafi c magmas. Sulfur concentrations are limited by the solubil-ity of sulfur in felsic magmas, which decrease as silica concentrations increase causing sulfi de and sulfate minerals to precipitate. A sulfur-satu-rated andesite with 60% SiO

2 may have less than

200 ppm S, whereas mantle-derived basalt may have over 1000 ppm S (Liu et al., 2007). Gold and copper are also enriched in mafi c magmas compared to silicic magmas; copper concentra-tions in alkaline mafi c magmas can be as much as 120 ppm and in rhyolite concentrations are only a few ppm (e.g., Maughan et al., 2002). In fresh igneous rocks of the East Sulphur Spring suite, Cu concentrations are as much as 50 ppm in the mafi c dikes and basaltic andesite dikes, but typically less than 20 ppm in the andesites and more silicic rocks. Gold concentrations are likely to be on the order of 1.7–0.5 ppb based on a copper:gold ratio of ~30,000 (Rudnick and Gao, 2004). Copper and gold behave as com-patible elements—in differentiated magma sys-tems their concentrations usually co-vary with strongly compatible elements like Ni and Cr. Immiscible sulfi de melts and minerals and mag-netite have high partition coeffi cients for chal-cophile elements like gold and copper and, once evolving magmas become sulfi de-saturated, the residual melt becomes strongly depleted in these elements (Jugo et al., 1999: Simon et al., 2003). Thus, the potential for the development of a por-phyry Cu-Au system hosted by felsic intrusions may be strongly linked to the extent of mixing with mafi c magma to yield higher than “normal” concentrations of ore metals and sulfur. On the other hand, disseminated Carlin-type gold depos-its are typically hosted by sedimentary rocks and not by intermediate composition igneous stocks. In most deposits, the S, O, H, and C isotopic compositions are closely linked to sediments and meteoric fl uids (e.g., Cline et al., 2005). Therefore, if elements from mafi c magmas are important to their generation, mixing may not be as critical as the mere presence of mafi c magma that in the process of crystallizing and “degas-sing” can give off signifi cant sulfur and gold to upper crustal hydrothermal systems.

Waite et al. (1997), Hattori and Keith (2001), and Maughan et al. (2002) concluded that por-phyry and Carlin-like mineralization in the Bingham district could not have formed without involvement of mafi c alkaline magmas rich in S, Cu, and Au. Intermediate “calc-alkaline” mag-mas at Bingham were simply too poor in these elements for any reasonable volume of magma to have served as a source of metal in the deposits. Although only small amounts of mafi c rock are known near Bingham, evidence of magma mix-ing involving alkaline olivine-bearing magmas is widespread. Apparently during the Eocene, mafi c alkaline magmas intercepted and mixed with evolved calc-alkaline magmas in shallow subvolcanic settings. This mafi c alkaline magma was richer in compatible ore elements (e.g., Cu and Au) and was probably the main source of the ore metals and sulfur.

In light of these conclusions, the obvious question is: Did mafi c magmas play such a role in the Sulphur Spring Range or for that matter along the Carlin trend?

While there is no direct evidence for the involvement of mafi c magma in mineralization along the Carlin trend (e.g., Cline et al., 2005; Emsbo et al., 2006), the presence of olivine-bearing basaltic andesites along the Carlin trend (Ressel and Henry, 2006) and in the Sulphur Spring Range shows that mafi c magma was present. MgO contents as high as 13% have been found in dikes from the Sulphur Spring Range. The mafi c component of the andesite of the Sul-phur Spring Range is also clear evidence that mafi c magmas were involved in this magma sys-tem. These mafi c magmas form coherent com-positional trends relating them to the rest of the volcanic rocks, demonstrating that mafi c mantle-derived magmas were at the “roots” of these Eocene magma systems, as in all subduction related magmatic arcs. Finally, the gold deposits and the igneous rocks have similar ages.

Other geologists emphasize the role of Paleo-gene magmas as simple heat sources to drive fl uid fl ow (e.g., Tosdal, 1998). However, the close spatial association of the basaltic andesite dikes in the Sulphur Spring Range with miner-alized veins suggests an even closer genetic link between the magmas and ore deposits. These dikes are not volumetrically signifi cant at the current level of exposure, but they may have dominated at deeper levels of the Eo- Oligocene magma system. Many of the dikes and lava fl ows are Cu-, Cr-, Ni-, and MgO-rich, indicating little fractionation occurred since they left their man-tle sources (Table 1; Fig. 14). Consequently, if they are like other oxidized mafi c magmas, they would have been enriched in Au and S compared to more silicic magmas. “Degassing” of mafi c magma in dikes and volcanic conduits

or as underplated and mixed magma may have contributed large quantities of sulfur, fl uids, and metals to the ore-forming systems, in addition to any heat they would have released to drive convecting fl uids. This concept has been pro-posed for multiple magma-ore systems, includ-ing porphyry Cu deposits at Santa Rita in New Mexico (Audetat and Pettke, 2006), Questa Mo deposit in New Mexico, Nukay Au-Cu deposit in Mexico, Las Bambas porphyry Cu deposit in Peru (Jones, 2002), the Farallón Negro Cu-Au deposit in Argentina (Halter et al., 2005), as well as the Bingham porphyry cop-per system emphasized here. The presence of mafi c volcanic and/or dike rocks (Cr- and MgO-rich dikes) in the Sulphur Spring Range and along the Carlin trend during the Paleogene is permissive evidence for the operation of this process here as well. These mafi c rocks, with their higher concentrations of compatible ele-ments (including chalcophile elements), could have been an additional source of ore metals and fl uids in the deposits. At most Carlin-type deposits, the isotopic heritage of the magmatic components may have been overwhelmed by later meteoric water interaction (Cline et al., 2005). In any case, such mafi c magmas would be much better sources of Au and S than the andesites and dacites that dominate the erup-tive record. Thus, from the evidence found in the northern Sulphur Spring Range and that previously published for the Carlin trend, we concur with the hypothesis that Paleogene magmas may have served as important sources of S and Au (Cline et al., 2005; Ressel and Henry, 2006) in addition to the isotopically identifi ed crustal sources of these elements (e.g., Arehart et al., 1993; Hofstra and Cline, 2000; Emsbo et al., 2006).

CONCLUSIONS

A suite of Eo-Oligocene lava fl ows, domes, and pyroclastic rocks is interlayered with clas-tic sediments and cut by dikes in the northern Sulphur Spring Range of central Nevada. These rocks form a dominantly high-K, calc-alkalic suite, with low Fe/Mg ratios similar to those found in subduction settings worldwide. The volcanic and subvolcanic rocks range from olivine-bearing, high-MgO basaltic andesite to garnet-bearing dacite, and high-silica rhyolite. The intermediate to silicic rocks have spiky trace-element patterns with Nb-Ti depletions and enrichment of Pb, also similar to those formed at convergent margins. The oldest Paleo-gene volcanic rocks are rhyolite and latite lava fl ows interlayered with fl uvial conglomerates and dacitic tuff that probably correlate with the Eocene Elko Formation. This interpretation is



based on lithologic similarities and on new U-Pb zircon ages of overlying units, which show the basal unit is probably middle Eocene in age. A porphyritic dacite intrusion (35.9 ± 0.5 Ma) and probably cogenetic rhyolite to dacite tuff (35.5 ± 0.4 Ma) overlie this succession. The tuff is per-aluminous and has sparse phenocrysts of garnet. A series of dacite lava domes (35.1 ± 0.5 Ma) caps the Eocene sequence. The youngest volca-nic unit is an extensively hybridized Oligocene andesite (31.4 ± 0.5 Ma). It has a disequilibrium phenocryst assemblage of plagioclase, bio-tite, clinopyroxene, orthopyroxene, amphibole, and olivine, along with resorbed megacrysts of quartz, potassium feldspar, and garnet. Lin-ear trends on variation diagrams are consistent with mixing between intermediate composition magma and peraluminous rhyolitic magma to form the andesite. The dated units probably cor-relate in time with the Indian Well Formation of Eo-Oligocene age that is mapped in adjacent areas. Dikes (dominantly of basaltic andesite) cut this unit and trend NNE. We correlate these mafi c dikes, which cut Paleogene volcanic and sedimentary rocks, with dikes of similar orien-tation and composition that cut only Paleozoic rocks in the central part of the range. These mafi c dikes are also dominated by basaltic andesite but

include shoshonite and latite as well. The dikes are parallel to a distinctive set of oxidized quartz-sulfi de veins in the central part of the range. The veins have anomalous concentrations of Au, Ag, As, Cu, Mo, Pb, Sb, and Zn and surround two historically mined polymetallic vein deposits. Jasperoid bodies on the margins of the range also have Carlin-like trace-element signatures.

These characteristics suggest the magma system was rooted in a subduction zone that formed as the Farallon plate steepened during the Paleogene (Fig. 17). As hot asthenosphere came in contact with the slab, dehydration of the plate produced an oxidized aqueous fl uid enriched in S, Cu, Au, Pb, and other soluble elements. The fl uid lowered the melting tem-perature of the overlying mantle wedge, gen-erating distinctly arclike magmas as a result of hydrous partial melting. This hot, hydrous, oxidized magma rose buoyantly into the crust, stagnated because of density differences, and promoted partial melting of the continental crust to form peraluminous silicic magma. These disparate mantle- and crust-derived magmas mixed, rose into shallow chambers, differenti-ated by fractional crystallization, and eventually erupted or fi lled fractures to form dikes. As a consequence of crustal trapping, only a small

fraction of the mafi c magma was able to erupt. Structural boundaries (including Proterozoic-age basement-penetrating faults, Paleozoic and Mesozoic thrust faults, and magma-generated fractures) guided magma emplacement routes, controlled levels of stagnation, promoted hydro-thermal fl uid fl ow, and placed reactive wall rocks in the fl ow paths. “Degassing” of mafi c magma may have contributed sulfur and chalco-phile metals to the mineralized veins and small ore deposits.

The Eo-Oligocene igneous rocks of the East Sulphur Spring suite are compositionally akin to Eocene igneous rocks associated with large gold deposits in the Carlin trend. When other simi-larities in structure, stratigraphy, and alteration are considered, we conclude that the Sulphur Spring Range is prospective for Carlin-type gold deposits. Johnston and Ressel (2004) suggested that porphyry Cu-Au deposits, Carlin-type deposits, and distal disseminated deposits are all part of a continuum with differences depend-ing mostly on spatial relations to the magmatic hydrothermal system. We agree and suggest that, like Sulphur Spring and Bingham, Carlin-related Eocene magmatic systems include rela-tively mafi c magmas that are vital to generating the large quantities of gold in the ore deposits.

B. 42 to 30 Ma Slab Rollback

Slab Dehydration& Arc MagmatismWet-Oxidized Magma

Nevada UtahCalifornia

OceanicLithoshere

W

Hot Asthenospheric

Counterflow

A. 120 to 45 Ma Flat Slab Subduction

E

E

Nevada

RMTGT

Sulphur SpringRange

UtahCalifornia

Lithospheric Mantle

Young Accreted Terranes

Young Accreted Terranes

Ancient Veined Mantle

Lithospheric Mantle Ancient Veined Mantle

OceanicLithosphere

W

Asthenosphere

Precambrian ContinentalCrust

Differentiation and mixing

Partial melting

Figure 17. Plate tectonic reconstruction of the western United States during the Paleo-gene. Gt—Golconda thrust; RMT—Roberts Mountains thrust. (A) A period of low-angle subduction during the Cretaceous and early Paleogene thickens the crust, but magma generation is hindered. (B) The subducting oceanic lithosphere rolled back inducing asthenosphere counterfl ow. As it heated, the slab dehydrated and initiated the pro-duction of hydrous, oxidized magmas in the overlying mantle wedge over a broad area (green). Mafi c magmas generated in the subduction-modifi ed mantle have high concentrations of S and chalcophile metals, and may be key to generation of ore deposits in the shallow crust. East of the Proterozoic craton margin, mantle-derived magmas (orange) interact with ancient lithospheric mantle and overlying crust. Partial melting of the crust produces felsic, peraluminous magma that is assimilated into the magma system in the lower crustal zone of hybrid-ization or rises and mixes with more mafi c magma in shallow crustal chambers. West of the Proterozoic craton, magmas (blue) interact with younger lithosphere and less felsic crust.

Ryskamp et al.


We readily acknowledge that the mere presence of mafi c magmatic rocks does not demonstrate conclusively that they contributed to the miner-alization, but combined with the close spatial and temporal association of mafi c lava fl ows, mafi c dikes, and mineralized veins in the Sul-phur Spring Range (and in the broader Carlin trend), we suggest that this is a viable hypoth-esis that merits further investigation.

ACKNOWLEDGMENTS

We are grateful for the assistance of Greg Melton in the fi eld and Michael Dorais in the electron micro-probe laboratory. The comments of W. Bagby and the assistance of C.D. Henry are also appreciated. The thorough reviews of Edward du Bray and John Dilles and the editorial assistance of Albert Hofstra were vital. Their careful examination of our manuscript focused our ideas on the essentials and tremendously improved the presentation. The research was sup-ported by funds from Brigham Young University and Golden Gryphon Explorations.

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Age and petrogenesis of volcanic and intrusive rocks in ...geology.byu.edu/.../files/ryskamp_08_sulphurspring.pdf · Keywords: Eocene, economic geology, igneous rock, Carlin-type,