Age and Petrogenesis of Volcanic and Intrusive Rocks in the Sulphur

Home , Elko Formation, Latite, Roberts Mountains, Sulphur Spring Range

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. Ryskamp Department of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA Jeffrey T. Abbott Golden Gryphon Explorations, 1400 Tanager Place, RR 21, Roberts Creek, British Columbia V0N 2W1, Canada Eric H. Christiansen* Jeffrey D. Keith Department of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA Jeffrey D. Vervoort School of Earth and Environmental Sciences, Washington State University, Pullman, Washington 99164, USA David G. Tingey Department of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA

ABSTRACT The nature of this suite and its potential for Oligocene of the western United States that mineralization is elucidated via comparisons promoted the production of oxidized mafi c Widespread base- and precious-metal to other Eocene age volcanic rocks associated magma in an arclike setting, but far inland as anomalies, oxidized sulfi de veins, silicifi ed with much larger gold and copper deposits a result of the rollback of the Farallon slab; calcareous shales and carbonates, and altered in the Great Basin. The East Sulphur Spring (2) the mafi c magmas intruded or erupted porphyry intrusions occur in the northeast- suite is more similar to Eocene igneous rocks separately, or mixed with more silicic magma ern Sulphur Spring Range, Nevada, 80 km found along and near the Carlin trend than generated by fractional crystallization and south of important gold deposits in the it is to those erupted while the Bingham por- assimilation of crustal materials; and (3) Carlin trend. The small historic mines and phyry copper deposit developed 300 km far- these mafi c magmas may have delivered sig- prospects in the area are spatially and per- ther to the east. For example, the East Sul- nifi cant amounts of sulfur and chalcophile haps genetically related to a suite of vari- phur Spring suite and the Eocene magmatic metals to upper crustal magma chambers ably altered dikes, small lava fl ows, silicic rocks along the Carlin trend are less alkaline and eventually to Paleogene ore deposits in domes, and related pyroclastic rocks. New than the Bingham suite and lack its unusual the eastern Great Basin. major- and trace-element data and U-Pb enrichment of Cr, Ni, and Ba in intermedi- zircon ages show that the East Sulphur ate composition rocks (58–68 wt% silica). Keywords: Eocene, economic geology, igneous Spring volcanic suite is Eo-Oligocene in age Nonetheless, the Bingham and East Sulphur rock, Carlin-type, porphyry copper. (36–31 Ma) and ranges in composition from Spring volcanic suites both preserve evidence high MgO-basaltic andesite to peraluminous of mixing that created intermediate composi- INTRODUCTION rhyolite. The major- and trace-element com- tions. For example, an andesite has obvious positions of the volcanic rocks are character- mineral disequilibrium with plagioclase, bio- The Great Basin of the western United States istic of continental margin subduction zone tite, clinopyroxene, orthopyroxene, olivine, contains a multitude of ore deposits and asso- magmas and form a high-K, calc-alkaline and amphibole coexisting with extensively ciated igneous rocks. Studies of the ages and suite with low Fe/Mg ratios. In addition, the resorbed megacrysts of quartz, K-feldspar, compositions of the volcanic and intrusive rocks have negative Nb and Ti anomalies and garnet—indicative of mixing basaltic rocks have shown that many of the deposits are and elevated Ba, K, and Pb on normalized andesite or andesite and largely crystallized not only spatially associated with magmatism, trace-element diagrams. Crustal melting is garnet-bearing rhyolite. On the other hand, but temporally and genetically linked to igne- indicated by the eruption of a peraluminous we found no evidence for mixing with a mafi c ous processes as well. In many cases, magmas garnet-bearing ignimbrite and as a compo- alkaline magma like that in the Bingham and their solidifi ed equivalents were important nent in hybridized andesite. Canyon magma-ore system. sources of heat to drive hydrothermal convec- We conclude that: (1) an unusual tectonic tion, of sulfur used as a complexing agent in the setting prevailed during the Eocene and fl uids and then deposited in sulfi des and sulfates, *Corresponding author.

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

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and of the ore metals themselves. In this paper, ward sweep of magmatism that passed through Because of its broad similarities to much we consider this paradigm in light of the rela- this area in the Eocene (Seedorf, 1991; Hofstra larger Eocene porphyry copper and Carlin- tionships between Paleogene magmatism and et al., 1999; Cline et al., 2005; Ressel and Henry, type gold deposits, the deposits in the Sulphur ore deposition in north-central Nevada. 2006). However, the exact nature of the relation- Spring Range have recently been the site of The Sulphur Spring Range is ~80 km south of ship between Carlin-type deposits and Eocene grass-roots exploration for base metals and Au. large gold deposits in the Carlin trend (Fig. 1). magmatism is the subject of debate. To help assess the potential for and further our 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, 116°30' 115°30' silver, and copper (Lincoln, 1923). Mineral Hill is best described as a small polymetallic replace- 41°00' 41°00' ment deposit (19A of Cox and Singer, 1986) and Dee is probably related to the distal effects of middle Post Cenozoic magmatism according to McKee and Carlin trend Moring (1996). Small base metal–silver replacement bodies, which would now be identifi ed as carbonate replacement deposits (Megaw, 1999), Carlin are found in the Union district. Prospects contain- Elko ing Au and As were explored in the 1980s. The Sulphur Spring Range is underlain by a CarlinCarlin thick sequence of east-dipping Paleozoic sedi- I-80 mentary rocks. Prior to recent mapping, a 2 km2 area of undifferentiated Tertiary volcanic rock RainRain 40°30' 40°30' 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 BBullionullion intrude or overlie the Paleozoic deposits. Some of the igneous rocks are spatially associated with mineralization and exhibit key charac- Ruby Mountains teristics of porphyry deposits (cf. Beane and Tit- ley, 1981; Richards, 2003; Seedorf et al., 2005), UUnionnion PassPass including substantial amounts of phyllic and argillic hydrothermal alteration, pebble dikes, breccia MineralMineral Study AreaAre pipes, and disseminated and vein-related mineral- HillHill ization. Altered mafi c and intermediate composition dikes have geochemical anomalies of As, Ba, 40°00' H-278 40°00' 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 Sulphur Spring Range Canyon deposit 300 km to the east (Maughan et al., 2002) and is reexamined here. Mt Hope In addition, the Sulphur Spring Range has Diamond Valley several features in common with Carlin-type gold deposits (Fig. 1), which contain the most productive gold mines in North America (Jensen H-50 et al., 1995). The geology and origin of Carlin- Eureka type gold deposits are described in detail by 39°30' 39°30' Hofstra and Cline (2000) and Cline et al. (2005). 116°30' 115°30' Mineralized rocks in the Sulphur Spring Range, 0 10 20 30 miles like most Carlin-type gold deposits, occur below Au deposits the Roberts Mountains thrust at intersections of a Nevada N complex array of structures with permeable and 0 10 20 30 40 50 kilometers Sampled dikes reactive strata, usually Devonian carbonate rocks or calcareous clastic sediments. Small bodies of Figure 1. Shaded relief map of central Nevada, showing the location of the Sulphur Spring jasperoid have anomalous concentrations of Au, Range relative to the Carlin trend of gold deposits. Circles identify known gold deposits. As, Hg, Sb, and Tl. The deposits are spatially The locations of samples taken from dikes (unit Tba on Fig. 4) in the central part of the associated with volcanism that is part of a south- range are shown as triangles. Shaded relief base map from Chalk Butte, Inc.

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understanding of these important ore deposits, 2004; Cline et al., 2005). In the late Devonian In the Great Basin, Jurassic magmatism included we compare the igneous rocks of the Sulphur through late Mississippian, the Antler orogeny metaluminous to peraluminous granitoids and Spring Range to those from other Eocene mag- affected the western margin of the North Ameri- sparse lamprophyre dikes (e.g., Ressel and matic centers related to mineralization in the can plate (Carpenter et al., 1994; Dickinson, Henry, 2006; Cline et al., 2005). By the end of the Great Basin—those near the Carlin trend in 2006). This contractional orogeny produced Cretaceous, the Farallon plate was subducting at Nevada and the Bingham porphyry system in the Roberts Mountains thrust (Fig. 2) that jux- a very low angle under North America and arc Utah. We conclude that there are several impor- taposes the Ordovician Vinini Formation and magmatism essentially shut off in Nevada, but tant similarities in these ore-related magma sys- some Mississippian clastic rocks over Devonian small volumes of strongly peraluminous granite tems, including their ages, tectonic settings, and carbonate rocks. In the Sulphur Spring Range, were intruded along the Utah-Nevada border the potential for mafi c magmas to have deliv- these are typically capped by the Devils Gate (e.g., Kistler et al., 1981). During the early Ceno- ered metals and sulfur to upper crustal hydro- Limestone or the Telegraph Canyon Formation zoic, the Farallon plate apparently detached from thermal systems. (Carlisle and Nelson, 1990). The structural con- the lithosphere in a southward-sweeping fash- tact between carbonate rocks and clastic rocks in ion allowing hot asthenospheric mantle to fl ow GEOTECTONIC SETTING the Roberts Mountains allochthon is one of the between the subducting slab and the lithospheric typical settings for gold mineralization along the mantle (Best and Christiansen, 1991). This cre- During the late Proterozoic, central Nevada lay Carlin trend, including the Rain mine, the near- ated a continental magmatic arc that extended on the western edge of the rifted North American est of the Carlin-type mineral deposits (Longo far inland (Lipman et al., 1972; Severinghaus continent (Wooden et al., 1998; Grauch et al., et al., 2002). The Roberts Mountains thrust fault and Atwater, 1990). Middle Tertiary magmatism 2003). The former continental margin, defi ned has also been delineated in various locations related to this event may have resulted from (1) by rifting in the Neoproterozoic and identifi ed throughout the Sulphur Spring Range (Carlisle dehydration of the Farallon plate, which induced by the 87Sr/86Sr = 0.706 line, lies just west of the and Nelson, 1990; Johnson and Visconti, 1992). hydrous melting of the mantle wedge, (2) heat- Carlin and Battle Mountain–Eureka mineraliza- The Sonoma orogeny in the Triassic created the ing lithospheric mantle by hot asthenospheric tion trends (Fig. 2; Tosdal et al., 2000; Grauch Golconda thrust (Dickinson, 2006), which lies mantle or by wedge-derived magma, and (3) et al. 2003). Neoproterozic and Cambrian sili- to the west (Fig. 2). decompression melting of hot mantle associ- ciclastic sediment were deposited during the rift During the Mesozoic, subduction beneath ated with the pattern of fl ow in the wedge. These phase. From the Ordovician through Devonian, western North America created a protracted mafi c, mantle-derived magmas rose and pow- thick shelf-type sediments accumulated on the series of contractional events (Dickinson, 2006). ered crustal magma systems in which more fel- continental margin and silty carbonate rocks The Sevier orogeny thickened the crust beneath sic magmas evolved by fractional crystallization accumulated on the slope to the west (Madrid, Nevada and western Utah, and a series of thrust and by partial melting and assimilation of conti- 1987; Finney et al., 2000; Cook and Corboy, sheets formed to the east (e.g., DeCelles, 2004). nental crust (Ressel and Henry, 2006).

118 o 116 o 114 o o 42 Carlin-type Au deposits Other Au deposits NNR GA Sulphur Spring Range

RMA GT = Getchell trend Carlin Trend W G T NNR = Northern Nevada Rift Battle Mt - Eureka Trend GA = Golconda allochthon BM RMA = Roberts Mtns allochthon W= Winnemucca Accreted Sulphur Spring Range BM = Battle Mtn o Terranes 40 E = Eureka

E N

Sr i=0.706 0 km 50

Map Area Miogeocline Nevada GA 38 o

Figure 2. Structural elements and gold deposits of northern Nevada. The area underlain by Precambrian continental 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).

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Cenozoic arc magmatism in the northern Although still controversial, some geologists the Miocene (Zoback et al., 1994; Ressel and Great Basin began in the Eocene, between 40 have concluded that Eocene stress relaxation and Henry, 2006). Bimodal basalt-rhyolite volca- and 36 Ma and then the front swept farther extension accompanied this southward sweep of nism is typical of this time (John, 2001). south (Ressel and Henry, 2006; du Bray, 2007). arc magmatism across western North America Magma compositions ranged from basaltic (Gans et al., 1989; Feeley and Grunder, 1991; Relationship of Paleogene Mineralization to andesite to rhyolite (or their intrusive equiva- Seedorf, 1991; Hofstra et al., 1999). For exam- Magmatism lents), and volcanism was widespread through- ple, Henry et al. (2001), Haynes (2003), Sata- out northern Nevada (Brooks et al., 1995; Henry rugsa and Johnson (2000), Cline et al. (2005), Although the close spatial and tempo- and Boden, 1998; Henry and Faulds, 1999; du and Henrici and Haynes (2006) conclude that ral relationships of Eocene igneous rocks to Bray, 2007). Apparently several magma sys- the Eocene-age Elko Formation accumulated Carlin-trend gold mineralization has suggested tems were episodically active beneath the Carlin in an extension-related basin (Fig. 3). The for- a probable link (Ressel et al., 2000; Ressel and trend during the Eocene and Oligocene (Grauch mation consists of a lower conglomerate unit, Henry, 2006), the presence or absence of mag- et al., 2003; Ressel and Henry, 2006). Concur- lacustrine limestone, and organic-rich shale, matic ﬂ uids in these meteoric water-dominated rent magmatism occurred at similar latitudes in but also has interlayered volcanic rocks. The hydrothermal systems is controversial. In some Utah, including that responsible for the Bing- extension that created the Northern Nevada rift Carlin-type districts (e.g., Getchell and Gold ham, Clayton, and Alta stocks (Vogel et al., (Fig. 2) and continued to form the present-day Bar), Eocene intrusions are absent (Hofstra and 2001; Deino and Keith, 1997). Basin and Range topography probably began in Cline, 2000). Cline et al. (2005) conclude that

Nevada >38 - <34 Utah 42-35

38-35 36 - <34 41 38-32 > 38 34 38-32 <34 39? 39? 40 - >34 <34 Bingham

38 36 - <34 36-34

36-34 35-34 34-37

36-34 Eocene volcanic rocks

---- Regions of possible Eocene extension

Sulphur Spring Range, Nevada N Bingham volcanic field, Utah

Emigrant Pass volcanic field, Nevada

North Carlin trend volcanic field, Nevada

Tuscarora volcanic field, Nevada

Figure 3. Eocene volcanic ﬁ elds of Nevada and Utah. Ages of igneous rocks (numbers) are expressed in Ma. Dashed red lines and numbers represent the age of southward progressing volcanic front. Black lines and numbers represent average ages of extension according to Seedorf (1991). Shaded relief base map from Chalk Butte.

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Eocene magmatism, in conjunction with deep method for most trace elements, but were fused and Tera-Wasserburg concordia were calculated crustal melting and metamorphism, and a pulse with a fl ux before digestion for rare-earth ele- using IsoPlot 3.0 (Ludwig, 2003). of extension-released ore fl uids were channeled ment (REE) analyses. Zr concentrations reported Total uncertainty for each spot analysis of an upward along basement-penetrating faults and from the four-acid dissolution technique were unknown was combined quadratically with the mixed to varying degree with exchanged mete- much lower than the XRF analyses and are not uncertainty in the measured isotope ratios and oric water. In contrast, Emsbo et al. (2006) sug- used here. Other element concentrations (includ- the uncertainty in the fractionation factors cal- gest that the ore fl uid was meteoric water that ing REE, Nb, and Th) agreed favorably by the culated from the measurement of standards. For extracted gold and sulfur from sedimentary various techniques. Representative analyses are individual analyses we estimate that the accuracy rocks already enriched in gold by Devonian presented in Table 1, and the complete data set is and precision are better than 4% at the 2 sigma exhalative processes. available in Supplemental Table S11. level, with the largest contribution in uncertainty The Bingham porphyry Cu-Mo-Au system U-Pb zircon ages were determined by laser from the measurement of the standards. Based has a much clearer connection to igneous rocks ablation ICP-MS (LA-ICP-MS) in the GeoAna- on a comparison of LA-ICP-MS ages with ages and processes with much of the sulfur, met- lytical Lab at Washington State University. All determined by thermal ionization mass spec- als, and fl uids being of magmatic origin. The zircon samples were processed and separated trometry, Chang et al. (2006) estimated the accu- magmatism related to the deposit is 38–36 Ma using standard gravimetric and magnetic tech- racy of age determinations using this technique (Deino and Keith, 1997) and is comparable in niques at Brigham Young University. Zircon to be on the order of 1% or better. However, age to the dikes and gold mineralization in the grains, both standards and unknowns, were there are unresolved contributions to uncertainty Carlin trend (Ressel et al., 2000; Ressel and mounted in a 1-inch-diameter epoxy puck that from the lack of a common Pb correction and Henry, 2006). Moreover, the porphyry system is was ground and polished to expose the interi- from potential matrix effects between standards associated with distal disseminated gold depos- ors of the grains. Cathodoluminescence images and unknowns. Consequently, we analyzed the its that have much in common with Carlin-type acquired at the University of Idaho were used as Temora zircon as an independent check on the gold deposits (Cunningham et al., 2004). The base maps for recording laser spot locations and accuracy and precision. The Temora zircon has role of deep, basement-penetrating faults may to reveal growth and compositional zonation, been proposed as a zircon standard by Black not be as important as along the Carlin trend, inclusions, and to look for inherited cores. Chang et al. (2003), who reported a weighted aver- but Bingham is near an old east-trending con- et al. (2006) present a comprehensive overview age 206Pb/238U age of 416.8 ± 1.1 Ma based on tinental margin to which Proterozoic terranes of the laser ablation techniques, and a brief over- 21 isotope dilution-thermal ionization mass accreted (Whitmeyer and Karlstrom, 2007). view is given below. The analytical results are spectrometric (ID-TIMS) analyses and 416.8 summarized in Table 2, and the complete data ± 1.8 Ma based on 50 sensitive high-resolution SAMPLING AND ANALYTICAL set is available in Supplemental Table S22. ion microprobe (SHRIMP) analyses. Chang et TECHNIQUES al. (2006), using the same instrument and ana- GEOCHRONOLOGY lytical conditions as used here, report an age of More than 200 samples of igneous rocks were 416 ± 9 Ma for Temora. During the course of collected and a geologic map (Fig. 4) was con- New U-Pb zircon ages were acquired for six our analyses, 12 LA-ICP-MS analyses on seven structed for this study. Hand sample descriptions samples from the eastern Sulphur Spring Range. grains of Temora were collected in two sepa- and locations are presented by Ryskamp (2006). To avoid problems associated with alteration rate analytical sessions. All analyses, corrected Thin sections cut from 25 samples of the main and zircon inheritance from older rock units, for fractionation and incorporating fraction- lithologic units were studied. Garnet and olivine we obtained U-Pb ages on zircons using LA- ation factor uncertainty, give a weighted mean phenocryst compositions were determined with ICP-MS. Zircon ages were determined using 206Pb/238U age of 416.9 ± 5.6 Ma (mean square an upgraded Cameca SX50 electron microprobe. a New Wave UP-213 laser ablation system in of weighted deviates [MSWD] = 0.49), which is A 20 micron beam and 15 kV acceleration volt- conjunction with a Thermo Scientifi c Element2 within error of the ID-TIMS age (Table 2). age were used for analyses, which are reported single collector, double-focusing magnetic sec- Four samples of the principal volcanic units in Ryskamp (2006). tor ICP-MS. Zircons were analyzed using a from the Sulphur Spring Range gave middle Major- and trace-element analyses for 87 of 30-μm-diameter beam operating at 10 Hz; the Tertiary 206Pb/238U ages ranging from 35.9 ± 0.5 the freshest samples were obtained by X-ray ablated material was delivered to the torch by to 31.4 ± 0.5 Ma (Table 2; Fig. 5) that correlate fl uorescence (XRF) analysis at Brigham Young a mixed He and Ar gas. Laser-induced time- with the stratigraphic sequence where it can be University using a Siemens SRS-303 spectrome- dependent fractionation was corrected by nor- interpreted (Fig. 6). Two samples show no evi- ter. Analyses of representative international stan- malizing measured ratios in standards and sam- dence of older zircon grains, while the other two dards together with our estimates of precision and ples to the beginning of the analysis using the have inherited zircons with Precambrian ages. A accuracy are available from the authors. Trace- intercept method. Static fractionation, including dacitic sample (04EB86) from the biotite por- element analyses (including rare-earth elements) that caused by laser ablation and due to instru- phyry unit had one zircon that yielded 207Pb/206Pb of 50 of these samples were also performed by mental discrimination, was corrected using ages of ca. 1.8 Ga. An andesite lava (04EB123) ALS Chemex using inductively coupled plasma external zircon standards. In our case, we used with considerable evidence for contamination, mass spectrometry (ICP-MS). Rocks were dis- FC1 and Peixe (Paces and Miller, 1993; Dickin- as described below, had multiple grains with solved using a four-acid “near-total” digestion son and Gehrels, 2003). Weighted average ages “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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A. Geologic map 116°

Tbp Tpd Qc Ta Tpd

Tbd Qc Tpd A A' Tba Tpd Tpd Tpd Qa Tba Ql Tbd Qc Ta Qc Qa Tbr Tl Tpd Td

Te Tbd Tpd Tbd Qc Tav Qa Tpd Ql Ta

Tpd Tbd Ta Ov Te 40° 07' 30" 40° 07' 30"

116°

Map Units Qa Alluvium 0 km 1 Quaternary Qc Colluvium Ql Lake sediments Study ^ Ta Andesite dike and lava flows area Tav Altered volcanic rocks, undifferentiated Nevada Tba Basaltic andesite dikes Eocene - Tpd Dacite domes N Tbd Biotite dacite tuff Oligocene Tbp Biotite porphyry intrusion Tl Latite lava flow Tbr Banded rhyolite lava flow Te Elko Formation, sedimentary units Faults

Paleozoic Ov Vinini Formaion

A A' B. Cross section 9000'

Dm 8000' Du Dlu Dm 7000' Du Tba Dt Td Dlm Dm Mc-d Ov Dlu Ov Dlu Dw Tec Tbd 6000' Dll Dlu Du Tbd Dt Mc-d Tpd TvaTa Dt Tpd 5000' Dw West 4000' Graben Fault East 3000' Graben 2000' Fault 1000'

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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Andesite Miocene

dike Basaltic andesite

. 3 O 2 dacite Plagioclase

—total Fe as 3 tuff

O ROCKS FROM THE NORTHERN SULPHUR SPRING RANGE, NEVADA 2 Biotite dacite

14 8 27 –22 27 18 – porphyry

)—below detection limit; Fe )—below – Union tuff Biotite 5 3 2 4 41 39 8 6 28 6 8 39 2 41 3 4 5 – 10 – 10 22 27 22 13 16 quartz porphyry

Latite Square

TABLE 1. CHEMICAL ANALYSES OF REPRESENTATIVE VOLCANIC rhyolite rhyolite banded

: LOI—Loss on ignition at 1000 °C for 4 hours; dashes ( 1.33 5.15 1.42 3.85 3.41 2.82 4.64 11.90 8.68 8.50 6.11 7.22 13.16 17.09 14.26 16.91 16.92 16.03 16.20 16.19 11.81 13.83 15.27 15.09 3 0.07 0.36 0.08 0.23 0.30 0.21 0.30 0.89 0.21 0.46 0.41 74.58 61.04 74.59 64.73 63.92 68.82 62.71 50.26 51.83 53.49 62.48 60.14 3 0.13 0.79 0.07 0.76 0.72 0.59 0.77 2.12 1.09 1.04 0.88 0.99

5 2 O 3.29 3.11 3.59 2.94 3.12 3.28 2.81 3.38 1.14 2.25 2.50 2.34 2 O 2 O 2 O O 5.04 4.46 4.69 4.69 2.86 3.63 3.21 1.55 0.96 1.61 4.23 3.86 2 2 2 Note Ba 1482 1744 231 1945 1755 1540 1545 1135 767 1214 1267 1010 1267 1214 Total 767 100.06 100.47 100.64 1135 99.99 99.18 100.51 1545 99.69 1540 100.38 99.68 100.28 99.59 99.53 1755 1945 Rb 231 224 170 272 121 127 111 1744 Nb 99 Ba 1482 39 La 19 17 45 150 49 132 19 15 17 16 15 31 44 13 20 13 14 13 13 45 43 44 39 35 28 29 Sample no. Unit symbol 04EB 097 Rock type 04EB 105 Tbr 04EB 162a Flow- 04EB 102 SiO 04EB 143 TiO Tl Al 04EB 041a Fe 04EB 137 MnO MgO 03EB 25 CaO Tu 0.03 0.04 0.05 Na 04EB 050 0.02 0.06 0.03 0.16 1.74 0.23 K 04EB 049 0.37 0.60 0.70 0.07 1.35 4.90 0.48 P 0.17 0.13 0.14 0.10 0.10 3.55 4.03 3.58 04EB 154 1.54 5.07 12.868.45 2.39 3.19 LOI 04EB 073 Tu 4.61 8.29 9.48 8.34 4.66 5.89 5 18 0.93 1.79 1.19 1.95 3.24 0.82 Tbp – 12 V 2.83 -0.27 2.19 1.82 0.50 1.05 Cr 71 5 Co 10 22 130 36 10 2 Ni 14 7 6 440 5 6 220 5 Tbd 95 65 54 136 Cu 5 490 8 142 181 Zn 90 194 2 3 38 Ga 3 2 144 6 7 166 34 84 23 2 Tpd 47 60 36 Sr 16 21 19 20 19 74 135 67 84 88 74 20 231 725 22 17 18 19 19 61 675 689 696 Tba 735 671 592 691 649 671 Nd Tba Sm Pb 22 34 – 3 8 2 Th 30 34 5 3 4 Ta U 34 22 18 35 17 20 21 7 11 36 26 33 22 17 10 25 9 6 7 9 14 18 16 18 7 4 16 4 5 8 9 18 Ta 14 9 9 10 5 7 3 2 3 4 8 4 2 wt% 13 ppm Sc – Y Zr 19 24 31 53 95 38 17 21 14 116 218 79 88 87 60 212 234 204 21 93 100 Ce 41 27 28 23 25 90 74 66 65 214 258 184 202 186

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to yield three 207Pb/206Pb ages of ca. 2.8 Ga; two and 8 show the elemental compositions of the other zircon grains gave four separate 207Pb/206Pb volcanic rocks. ages of ca. 1.8 Ga; two other zircon grains gave three separate 207Pb/206Pb ages of ca. 1.4–1.2 Ga; Structure and one grain yielded late Cretaceous 206Pb/238U ages. We interpret the eruption age of the andes- A deformed succession of Paleozoic sedi- ite to be best represented by ten 206Pb/238U ages mentary rocks is in high-angle fault contact with from four separate zircon grains that range from a complex suite of east-dipping volcanic units 0.08 Ga

alyses. 33.3 to 30.7 Ma. Because the age range is larger in the studied area (Fig. 4). The Paleozoic strata 2.8, 1.8, 1.4, than expected for analytical error alone (as indi- dip eastward and are cut by thrust faults related cated by an elevated MSWD value), we used the to the Roberts Mountains thrust (Fig. 4; Carlisle TuffZirc routine in IsoPlot to estimate an age of and Nelson, 1990). The eastward tilt is probably 31.4 +1.3/−0.5 Ma. Thus, this lava is most likely the result of displacement on range-bounding Oligocene in age and signifi cantly younger than normal faults that developed during the Mio- the other Paleogene volcanic rocks in the volca- cene. Within the map area, the most prominent nic fi eld. For the other samples, our preferred faults are the north-trending West Graben and age was the weighted mean of the 206Pb/238U East Graben faults, both of which have appar- ages (Table 2). A weighted mean age for 16 ent normal displacements of hundreds of meters analyses yielded an age of 35.1 ± 0.5 Ma for one (Fig. 4; Carlisle and Nelson, 1990). Post-Oligo- of the plagioclase dacite domes (Fig. 5). A bio- cene movement on the East Graben Fault cut tite porphyry intrusion and a biotite dacite tuff the Paleozoic thrusts and dropped the Paleogene have indistinguishable ages of 35.9 ± 0.5 and volcanic section against lower Paleozoic sedi- 35.5 ± 0.4 Ma, respectively (Fig. 5). mentary rocks. However, kinematic indicators in Zircon separated from a distinctive rhyolitic the fault zones demonstrate that the faults expe- clast (sample 04JA156) of what we have called rienced oblique and strike-slip displacement as the “square quartz porphyry” was analyzed in well as normal displacement. Two other observa- an attempt to constrain the eruptive age of a tions suggest that these faults have a protracted tuffaceous interval low in the volcanic section history. Facies changes in some of the Devonian (the Union tuff described below). However, the carbonate units across the East Graben Fault 206Pb/238U age of the zircons in the clast is 157.4 suggest that it may have controlled sedimenta- ± 2.2 Ma or Late Jurassic (Table 2). Jurassic tion patterns in the Paleozoic, as described in the plutonic rocks are exposed farther south in the northern Carlin trend (Volk et al., 1995; Emsbo – 0.5 97.9 10 24 14 Sulphur Spring Range and have similar textures et al., 1999). In addition, the East Graben Fault to these porphyritic clasts. This sample also may have localized the emplacement of Eocene contained inherited zircons with Paleozoic and dikes and plugs (Fig. 4). An extended history Proterozoic ages. of activity along these faults suggests that they Zircon from a nonwelded pyroclastic deposit are major structures, possibly of crustal-scale. of rhyolitic composition (sample 03JA122) These faults may have guided the emplacement yielded a Miocene age of 14.0 ± 0.3 Ma, the and eruption of the middle Tertiary volcanic THE NORTHERN SULPHUR SPRING NEVADA SULPHUR SPRING RANGE, THE NORTHERN same age as rhyolitic volcanism associated with rocks and provided conduits for hydrothermal the Basin and Range province. Although the fl uids. A similar interpretation has been made zircons are compositionally zoned, we found no for north- to northwest-trending structures in the evidence of age inheritance in the zircons of this southern Carlin trend (Longo et al., 2002). sample (Table 2). The northern Sulphur Spring Range lies on a strong gradient in the Bouguer gravity anomaly. PALEOGENE GEOLOGY OF THE The gradient trends northeast and separates a NORTHERN SULPHUR SPRING RANGE broad gravity low on the south from higher values to the north (Grauch et al., 2003). The Below we use mapping, structural, strati- gradient is interpreted to separate Proterozoic graphic, and petrological information, together continental basement on the east from young with the new U-Pb ages, to reconstruct the his- accreted terranes on the west. An irregular clus- tory of magmatism in the northeastern part of ter of aeromagnetic highs marks the northern the Sulphur Spring Range. We have informally Sulphur Spring Range; the largest is ~10 km Age uncertainty at 2 sigma. MSWD—Mean square of weighted deviates. Grains—Number of zircon grains analyzed. Spots—Number of an grouped the Paleogene volcanic rocks into the across and is centered in the valley just east of TABLE 2. SUMMARY TABLE 2. SUMMARY OF U-PB LASER ABLATION INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY AGES OF IGNEOUS ROCKS FROM East Sulphur Spring suite to distinguish them the volcanic outcrops (Grauch et al., 2003). The

04EB041 Biotite dacite tuff Weighted mean 35.5 ± 0.4 1.30 11 18 1 04EB044 Dacite domes Note: Rejected—Number of analyses not used in age calculation. of Inheritance—Ages of old grains in billions years. Weighted mean 35.1 ± 0.5 1.30 11 16 0 04EB123 Andesite TuffZirc +1.3 31.4 04EB86 04JA156 Biotite porphyry Square quartz porphyry Weighted mean Weighted mean 157.1 ± 1.8 35.9 ± 0.5 1.30 1.16 10 22 8 18 6 0.4, 1.1 Ga 3 1.8 Ga 03JA122 tephra Rhyolite Weighted mean 14.0 ± 0.3 1.18 9 14 0 Sample Unit Type Age (Ma) MSWD Confidence Grains Spots Rejected Inheritance Rejected Spots Grains Confidence MSWD (Ma) Sample Unit Age Type

from potentially distinctive igneous rocks to magnetic highs are probably due to the higher the west. Figures 4 and 6 show the geologic magnetic susceptibility of the volcanic and and stratigraphic relationships between the subjacent intrusive rocks and may reveal the units as determined by superposition, cross- extent of the shallow intrusive system beneath cutting relations, and isotopic ages. Figures 7 the volcanic ﬁ eld. The size and amplitude of the

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. A. 03JA122 Rhyolitic ash B. 04EB123 Andesite 0.08 Mean = 14.0 ± 0.3 Ma Mean = 31.4 +1.3 -0.5 Ma 0.07

0.06 Pb Pb

206 0.06 206 28 24 20 16

Pb / / Pb 12 0.04 Pb / / Pb 207 207 0.05 0.02 35 34 33 32 31 30 29

0.00 0.04 200 300 400 500 600 700 800 180190200210220230 238 U / 206 Pb 238 U / 206 Pb 0.060 0.064 . C. 04EB044 Plagioclase dacite D. 04EB041 Biotite dacite tuff Mean = 35.10 ± 0.5 Ma Mean = 35.5 ± 0.4 Ma 0.056 0.060

0.056 0.052 Pb Pb 206 206 0.052 Pb / / Pb

Pb / / Pb 0.048 207

207 0.048 38 36 34 32 30 40 38 36 34 0.044 32 0.044

0.040 0.040 160 170 180 190 200 210 155165175185195205 238 U / 206 Pb 238 U / 206 Pb

0.068 E. 04EB86 Biotite porphyryy F. 04JA156 Clast in Union tuff (square quartz porphyry) Mean = 35.9 ± 0.5 Ma 0.07 0.064 Mean = 157.1 ± 1.8 Ma

0.060 Pb 0.06 206

Pb 0.056 206 Pb / / Pb 0.052 207 Pb / / Pb

207 0.048 0.05 180 170 160 40 38 36 34 32 150 140 0.044

0.040 0.04 155 165 175 185 195 205 34 36 38 40 42 44 46 48 238 U / 206 Pb 238 U / 206 Pb

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.

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Possible roded K-feldspar, indicative of disequilibrium. correlatives Tr In some locations, lobes of latite appear to have fl owed down paleo-streambeds and banked Tpd Ta against the fl ow-banded rhyolite. The fl ow-banded rhyolite fl ow is older than Tiw Tbp the nearby plagioclase dacite lava domes; Indian Well Fm Tbd contacts are not exposed, but it crops out on the western edge of the east-dipping volcanic Tu Te sequence butted against the East Graben Fault. Ta Elko Fm Tba It is correlative with and older than part of the Elko Formation, as there are outcrops of the Tl conglomeratic facies of the Elko Formation Tbr stratigraphically above the banded rhyolite Tr Rhyolite tephra 14.0 +/- 0.3 Ma (Fig. 4). The latites are somewhat younger than Ta Andesite dike and flows 31.4 +/- 0.5 Ma the rhyolite, but are also within the lower part of Tba Basaltic andesite dikes the Elko Formation in this area. Tpd Dacite domes 35.1 +/- 0.5 Ma Union tuff (Tu). Exposures immediately Tbd Biotite dacite tuff 35.5 +/- 0.4 Ma south and southeast of the Union district contain Tbp Biotite porphyry intrusion 35.9 +/- 0.5 Ma a poorly welded, orange to maroon, dacite ash- Tu Union tuff fl ow tuff, herein informally named the Union Tl Latite lava flow Tbr Banded rhyolite lava flow tuff. Plagioclase and quartz are the most prominent phenocrysts in this dacitic tuff. The tuffa- Tiw Indian Well Formation ~37 - 30 Ma ceous interval also includes quartzite cobbles Te Elko Formation ~46 - 37 Ma and well-rounded pebbles and cobbles of rhyolite with quartz phenocrysts that are probably Figure 6. Stratigraphy of Eo-Oligocene rocks in the East Sulphur Spring volcanic suite, the same as the porphyritic rhyolite identifi ed Nevada. by Henrici and Haynes (2006). The porphyritic clasts contain bipyramidal quartz phenocrysts along with plagioclase, potassium feldspar, and anomaly are similar to others along the Carlin sedimentary strata and give ages that range sparse biotite. The U-Pb zircon ages of these trend (Ressel and Henry, 2006). from 46 to 38 Ma. clasts are Late Jurassic (Table 2). A porphyritic In the eastern Sulphur Spring Range, the beds granitic intrusion very similar to the clasts crops Stratigraphy we correlate with the Elko Formation crop out out in the southeastern Sulphur Spring Range east of the East Graben Fault (Fig. 4) and con- ~40 km away. Clast sizes and the apparent Elko Formation (Te) sist of red-brown conglomerate and arkose with source of the clasts are consistent with paleo- We correlate the oldest Paleogene strata in pebbles and cobbles of quartzite and green chert. current indicators found by Henrici and Haynes the eastern Sulphur Spring Range with the Elko Lava fl ows and tuff are interlayered with the (2006), indicating the source area of the sedi- Formation. These clastic sedimentary rocks clastic sedimentary rocks, as described below. ments was southeast of the Elko Hills. were mapped by Carlisle and Nelson (1990) as The combination of nonvolcanic conglomerate, The Elko Formation is Eocene to Oligocene the Permian Garden Valley Formation, but the arkose, and tuff is very similar to the lower mem- according to Smith and Ketner (1978) and presence of Jurassic quartz porphyry cobbles ber of the Elko Formation as described by Ket- middle to late Eocene according to Henrici (Table 2) in the Union tuff member shows that ner and Alpha (1992) and Henrici and Haynes and Haynes (2006). Radiometric ages on tuffs this age assignment is not correct. Instead, the (2006). The overlying Indian Well Formation is in the unit range from 46.1 to 38.6 Ma (Solo- overall lithologic character, including the pres- dominated by volcaniclastic sedimentary rocks mon et al., 1979; Haynes, 2003; Henrici and ence of volcanic rocks, suggests correlation (Ketner and Alpha, 1992). Haynes, 2006) and suggest that the Union tuff with the Elko Formation, a prominent clastic Flow-banded rhyolite and latite lava is Eocene in age. unit of Eocene age in central Nevada. Else- fl ows (Tbr). The basal part of the Elko Formation where, the Elko Formation is a fi ning-upward in the Sulphur Spring Range includes a crystal- Biotite Porphyry (Tbp) and Biotite Dacite Tuff series of fl uviolacustrine beds. The base is pri- poor rhyolite lava fl ow with distinctive alternat- (Tbd) marily red-brown pebble to cobble conglom- ing dark-gray and light-gray to pink fl ow-layers A shallow dike or vent-fi lling intrusion of erate and locally arkosic sandstone, but the ~10 cm thick. Although it is only found in one porphyritic dacite is found near the East Graben middle and upper parts are dominated by fi ne- location in the mapped area (Fig. 4), a very similar Fault (Fig. 4). It contains abundant phenocrysts grained clastic sediments, lacustrine limestone, rhyolite lava fl ow crops out ~11 km to the south. of coarse biotite, plagioclase, sanidine, quartz, and oil shale (Henrici and Haynes, 2006). The The rhyolite has extremely small phenocrysts of and altered magnetite. The large (~2 mm) book- conglomerate clasts in the lower member are quartz, plagioclase, and magnetite in a glassy lets of biotite are particularly distinctive. Some predominantly reworked Paleozoic chert and matrix containing fl ow-aligned microlites. quartz phenocrysts are resorbed. Phyllic altera- quartzite, but also include rare (<1%) porphy- A crystal-poor, black, scoriaceous latitic lava tion has converted most sanidine to sericite. ritic rhyolite (Smith and Ketner, 1978; Henrici fl ow crops out near the banded rhyolite. The The medium- to fi ne-grained matrix consists and Haynes, 2006). Locally, pyroclastic fallout lavas have phenocrysts of pyroxene, sieved of intergranular quartz, plagioclase, biotite, and tuffs and ignimbrites are interlayered with the plagioclase, quartz with reaction rims, and cor- small amounts of glass.

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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).

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1000.0 A distinct trachytic ﬂ ow foliation is deﬁ ned by elongate plagioclase microphenocrysts. Dikes with shoshonite and biotite-bearing latite compositions are rare (Fig. 7). Whole-rock composi- 100.0 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). 10.0 Andesite Dikes and Lava Flows (Ta) Andesite The youngest volcanic unit in the East Sulphur

Rock/Primitive Mantle Rock/Primitive Basaltic andesite & mafic dikes Spring suite is a series of crystal-rich andesite Plagioclase dacite dikes and near-vent fl ows with unique textural 1.0 Biotite dacite tuff Biotite porphyry aspects. The exposed dikes trend NNE, like most Rhyolite lava flow other dikes in the northern Sulphur Spring Range. Latite lava flow Union tuff Rounded, sieve-textured plagioclase is the dominant phenocryst. Other prominent phenocrysts 0.1 include biotite, clinopyroxene, orthopyroxene, Ba Rb Th U Nb K La Ce Pb Sr Nd P Sm Zr Ti Y olivine (Fo78), and magnetite. These mafi c phenocrysts coexist with megacrysts of sanidine and Figure 8. Normalized trace-element patterns for igneous rocks from unstrained quartz as much as 3.5 cm long. Smaller the Sulphur Spring Range, Nevada. Normalization values from prim- quartz phenocrysts are extensively resorbed, and itive Earth from McDonough and Sun (1995). many of these crystals also have reaction rims of clinopyroxene (Fig. 10). The andesite also contains grains of resorbed and oxidized garnet. The garnet is Mn-rich compared to that found in the A biotite-bearing ash-fl ow tuff is the most Dacite Lava Domes (Tpd) biotite dacite (Fig. 11). Dike and fl ow margins widespread unit in the mapped area. However, A group of dacite lava domes with distinctive have perlitic glass in their matrices. it crops out poorly as porphyritic fragments large plagioclase phenocrysts overlies the biotite We interpret the mineral assemblage (e.g., that litter the ground. Roadcuts provide the best dacite tuff. These “plagioclase dacite” domes forsteritic olivine coexisting with quartz), the exposures, where the unit is seen to be a non- comprise the majority of the knolls in the east- resorption of the felsic phases, and the reaction welded ignimbrite with abundant lithic clasts. ern portion of the volcanic fi eld (Fig. 4). This is rims to be the result of mixing silicic magma In most places, it is argillically altered, whitish- the major Tertiary volcanic unit described and (containing quartz and sanidine megacrysts) and yellow, and laced with small oxidized veins mapped by Carlisle and Nelson (1990). mafi c magma (containing olivine and pyroxene). containing pyrite. The tuff contains phenocrysts The dacite domes consist of pink or orange Clinopyroxene halos around resorbed quartz of plagioclase, coarse biotite (~2 mm across), phenocryst-rich, fl ow-foliated or brecciated lava. are also diagnostic of magma mixing (Fig. 10; quartz, Fe-rich garnet, and megacrysts of quartz. A vitrophyre is present in some locations along Coombs and Gardner, 2004). Its major-element composition ranges from dac- dome margins. Matrix-supported fl ow breccias We interpret the U-Pb zircon data to show that ite to rhyolite (Fig. 7). developed along some shear planes. Plagioclase the andesite has an Oligocene eruptive age of The biotite porphyry intrusions and the bio- is the primary phenocryst with lesser quartz and ca. 32 Ma, although the error and the MSWD are tite dacite tuff have indistinguishable ages and clinopyroxene, along with sparse amphibole, bio- rather large (Table 2). The 2 sigma uncertainty elemental compositions and similar textures tite, and tiny euhedra of magnetite as inclusions does not overlap with the ages of the other igneous featuring coarse biotite. U-Pb zircon ages of and in the groundmass. Chlorite, clay minerals, rocks in the East Sulphur Spring suite (Fig. 12). these units are 35.9 ± 0.5 and 35.4 ± 0.4 Ma, and iron-stains are characteristic of altered rocks. Stratigraphic relationships also suggest that this respectively (Table 2 and Fig. 5), and their The U-Pb zircon age of 35.1 ± 0.5 Ma is the youngest exposed unit; it appears to over- compositions suggest the two units may be obtained from one dome (Fig. 5) is consistent lie the dacite domes (Tpd) because it outcrops comagmatic (Figs. 7, 8, and 9). As such, the with fi eld relations showing that the dacite lavas farthest to the east in this east-dipping sequence biotite dacite tuff may be an explosive product, erupted onto or through older biotite dacite tuff. of strata (Fig. 4). The large spread in U-Pb ages whereas the biotite porphyry may form a vent- of zircon xenocrysts may show that some of the fi lling dome or intrusion. Basaltic Andesite Dikes (Tba) crystal cargo was derived from already solid rock The biotite dacite tuff overlies the Union tuff The most mafi c of the Paleogene igneous by assimilation of the Paleoproterozoic (2.5 and in the Elko Formation (Fig. 6). Thus, the tuff and rocks found in the East Sulphur Spring suite 1.7 Ga) basement (Table 2). units overlying it may be correlative with the are dikes that generally trend north-northeast Indian Well Formation, an Eocene- Oligocene (Fig. 4). The dikes cut the biotite dacite tuff Mafi c Dikes (Tmd) series of ash-fl ow tuffs interbedded with vol- (Tbd) as well as the dacite domes (Tpd). Most In addition to the dikes in the East Sulphur caniclastic sediments mapped in the valley east of the dikes are dense, black porphyritic basaltic Spring suite, a series of weakly to extremely of the Pinion Range to the north of the Sulphur andesite containing phenocrysts of clinopyrox- altered dikes crops out west and northwest of the

Spring Range (Smith and Ketner, 1978). The U- ene, orthopyroxene, olivine (Fo84), and sparse mapped area (Fig. 1). The overwhelming major- Pb ages are similar to the ages of tuffs within the plagioclase. The matrix is composed of plagio- ity of the dikes are basaltic andesite, but sho- Indian Well Formation. clase, pyroxene, Fe-Ti oxides, and minor glass. shonite and latite have also been found (Fig. 7).

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1000 A La, and Ce and higher SiO2, Al2O3, K2O, Rb, Nb, Andesite and Ba than the Jurassic lamprophyres described Plagioclase dacite by Ressel and Henry (2006). These differences Biotite dacite tuff suggest that the latites are more fractionated Biotite porphyry Rhyolite lava flow rocks and that they have different parentage than 100 Latite lava flow the Jurassic lamprophyres. The Sulphur Spring Union tuff dikes are also unlike Neogene basalt ﬂ ows that crop out to the south—the dikes are magnesian

and have lower TiO2 and Nb concentrations than the young basalts (Table 1). These maﬁ c dikes

Rock/Chondrite are similar to the Paleogene basaltic andesite 10 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. 1 The altered maﬁ c dikes have anomalous con- La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 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- 1000 B centrations of Cr are especially noteworthy. Cr concentrations range from 60 to 630 ppm with Jurassic lamprophyre dikes, Carlin Trend an average of 355 ppm. Since Cr is relatively immobile during hydrothermal alteration, these high concentrations show that even the highly 100 altered dikes were maﬁ c.

Mineralization: Gossan Veins and Jasperoid A distinctive set of veins is found within and surrounding the core of the northern Sulphur Basaltic andesite and latite dikes Rock/Chondrite Sulphur Spring Range Spring Range and includes those in the Union 10 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 1 in width from 1 to 100 cm, are often banded, and La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu have been brecciated and then healed by younger deposits of silica; some veins contain unoxidized Figure 9. Rare-earth element (REE) patterns of igneous rocks from fragments of sulfi de minerals, including pyrite the Sulphur Spring Range normalized to the composition of chon- or galena. These are highly sulfi dic polymetallic dritic meteorites (McDonough and Sun, 1995). (A) Intermediate and veins (now completely oxidized in most cases), silicic volcanic rocks. (B) Basaltic andesite and latite dikes (unit Tba) and might be equivalent to “D” veins in the from the East Sulphur Spring suite compared to Jurassic lampro- “alpha” system originally established for the El phyre dikes from the Carlin trend (Ressel and Henry, 2006). Salvador deposit by Gustafson and Hunt (1975). The lack of sericite-rich margins is due to their formation in carbonate rocks. The dikes occur largely in two east-northeast- like the Jurassic lamprophyres in the region. The The dominant trend of the gossan veins is trending swarms that transect the northern Sul- basaltic andesites from the Sulphur Spring Range northeast, which is the same as the dated Paleo- phur Spring Range where they cut pre-Cenozoic have lower alkalis, P, Rb, Sr, and Ba; their light gene, and presumed Paleogene, mafi c dikes rocks. Most of the dikes are altered to argillic REE (LREE) patterns are also different than those described above. Their relationship to the Paleo- or phyllic mineral assemblages and crop out as of the Jurassic lamprophyres (Fig. 9). The basal- gene magmatism is not certain because they are light-orange to tan fragments. Several propyliti- tic andesites also lack mica and amphibole phe- not directly adjacent to any dikes, but because cally altered dikes preserve phenocrysts of oliv- nocrysts, but they have plagioclase phenocrysts. the gossan veins are found in the same part of the ine and clinopyroxene. Moreover, most of the Jurassic lamprophyres range as the dikes and share a common orienta- The ages of these dikes are uncertain because along the Carlin trend strike NW, not NE to ENE tion, we speculate that they are distal to but con- they do not cut any of the Paleogene units. How- like most of the Paleogene dikes. Even the latite temporaneous with the major intrusive cluster. ever, the elemental compositions and mineralogy samples are unlike the Carlin lamprophyres; they The east side of the Sulphur Spring Range of the freshest rocks show that these dikes are not have signifi cantly lower MgO, CaO, Cr, Ni, Sr, also has a number of small bodies of jasperoid,

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particularly along the contact between carbon- A Olivine 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 Plagioclase 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- Quartz its elsewhere in Nevada. (Indeed, Carlin-style alteration hosted in calcareous shales and car- Clinopyroxene bonates has been intercepted in a drill hole just north of the mapped area, but the results of these investigations are still proprietary.) 1 mm PETROCHEMISTRY OF THE EAST SULPHUR SPRING SUITE B C During its ~4 million year lifetime, the Eocene-Oligocene magma system beneath the northern Sulphur Spring Range erupted basal- Garnet Garnet 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 1 mm 1 mm is interlayered with contemporary sedimentary deposits. Modal and whole-rock chemical com- D positions of the igneous rocks provide some general insights into the nature and evolution of the magma system. Garnet Subduction Zone Origin

The Eocene-Oligocene volcanic rocks of the Sulphur Spring Range have much in common with other subduction-related continental margin 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 Quartz Frost et al. [2001]); this lack of Fe-enrichment is characteristic of crystallization at relatively high f O . 1 mm 2 Likewise, even the most primitive basal-

tic andesite dikes have relatively low TiO2, less than ~1.4 wt%, similar to volcanic rocks Figure 10. Photomicrographs of andesite (unit Ta) showing dis- found in arcs. Most of the volcanic units form equilibrium mineral assemblage, textures, and reactions indica- a high-K series, but the hybridized andesite and tive of magma mixing. Transmitted light. (A) Sieved plagioclase, some of the intermediate composition dikes

oxidized olivine, clinopyroxene glomerocrysts, and quartz with range widely from medium K2O to shoshonitic extensive resorption and a reaction rim of clinopyroxene (sample (Fig. 7). REE patterns of the rocks in the East 04 EB 064). (B) Garnet replaced by magnetite along rim (sample Sulphur Spring suite are similar to those of igne- 04 EB 064). (C) Euhedral garnet has no rim (sample 04 EB 168, ous rocks from continental margin subduction lava ﬂ ow sample). (D) Garnet rim and fractures replaced by mag- zones (Ewart, 1979) with relatively steep slopes netite; quartz crystal is rounded and rimmed with clinopyroxene and small negative Eu anomalies (Fig. 9). The (sample 04 EB 123). latites have the highest REE concentrations,

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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 Figure 11. Compositions of garnet in the biotite dacite tuff (Tbd) and in the metals (e.g., Richards, 2003). andesite (Ta) of the East Sulphur Spring suite. Older, biotite dacite tuff contains garnets with lower Mn contents than garnet xenocrysts in the andesite.

30 Sulphur Spring suite, Nevada Comparison Suites

36 Age (Ma) Age

42 Biotite Biotite Plagioclase Andesite Bingham Carlin Emigrant Rain- Tuscarora, porphyry dacite dacite Cu-Mo-Au Trend, Pass, Railroad, Nevada intrusion tuff dome Deposit, Nevada Nevada Nevada Utah 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).

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Crustal Melting and Felsic Peraluminous feldspars and maﬁ c silicates and oxides. As SiO2 produces linear trends on two-element variation

Magmas increases, Al2O3, CaO, TiO2, Fe2O3, and MgO all diagrams. Consequently, we conclude that the decline in concentration. Increases in the incom- basaltic andesite, shoshonite, and latite dikes An important characteristic of the felsic patible elements (Rb, Nb, Pb, and Th) suggest are probably related to one another by fractional members of the East Sulphur Spring suite is the that 25% to 35% fractional crystallization could crystallization of pyroxene, olivine, plagioclase, inclusion of peraluminous silicic rocks. Two have created these changes. REE concentrations and oxides. The extent of fractionation from of the volcanic units contain Fe-rich magmatic change little or decrease, perhaps as a result of parental basaltic andesite to derivative latite was garnet—a mineralogical indicator of the excess the removal of garnet. ~50% based on the enrichments of incompatible of Al over Ca + Na + K. The andesite lavas (Ta) The intermediate composition dikes (Tba) elements (Nb, Zr, Pb, Th, and Rb). have rimmed crystals of Mn-rich garnet, and range in composition from olivine-bearing the biotite dacite tuff (Tbd) has sparse Mn-poor basaltic andesite (with as much as 13% MgO, Magma Mixing grains of euhedral garnet. Most of the individual 175 ppm Ni, and 500 ppm Cr) to latite (with samples of the tuff are peraluminous with the 2% MgO, 20 ppm Ni, and 15 ppm Cr). These Even though fractional crystallization seems alumina saturation index ranging from 0.95 to steep declines in compatible element concentra- to have been the dominant process leading to over 1.1. Although peraluminous silicic rocks tions (Fig. 14) suggest fractional crystallization latite, a few of the basaltic andesite dikes have in the Great Basin are typically Cretaceous in of mafi c mineral phases that have high partition trace-element compositions that are consis- age (Miller and Bradfi sh, 1980; Lee and Chris- coeffi cients for these elements. Other compat- tent with mixing of mafi c and silicic magmas. tiansen, 1983), Eocene granites (ca. 39–34 Ma) ible elements, such as Ti, Fe, Mg, Ca, Sc, and V, Anomalously high concentrations of Cr and Ni

in the nearby Ruby Mountains are strongly per- decrease in concentration as SiO2 increases. On are found in at least two dikes (Fig. 14). How- aluminous and locally contain garnet (Kistler the other hand, incompatible elements like P, Zr, ever, this remains a tentative conclusion because et al., 1981; Barnes et al., 2001), as do Eocene LREE, Pb, Rb, and Ba increase from the basal- it is diffi cult to distinguish fractionation from rhyolites at Mount Hope (Westra and Riedell, tic andesites to the latites. The curved trends and mixing using other elements because compo- 1995) to the south (Fig. 1). Strongly peralumi- sharp decreases of compatible element concen- sitional trends produced by both processes are nous silicic magmas like these are most likely trations are not typical of magma mixing, which linear and overlapping. In fact, Ba variations do generated as a result of partial melting of pelitic or semipelitic metasedimentary rocks in thickened 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 minerals. 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 apparent that the biotite dacite tuff (unit Tbd) ranges from dacite to rhyolite in composition (Fig. 7). Its chemical variation is consistent with fractional 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).

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not clearly show the effects of mixing basaltic andesite and rhyolite, which would pull hybridized 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 K2O contents render them the most consistently shoshonitic unit in the area (Fig. 7). On the other hand, the

andesites have lower concentrations of Al2O3, 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 reaction rims, and extensive resorption of phases that may have come from the silicic magma— quartz, sanidine, and Mn-rich garnet. The silicic end member must have been a high-silica rhyolite to explain the presence of quartz, sanidine, 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 intermediate 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 compositions, and trends on most silica variation diagrams (Ti, Al, Fe, Mg, Ca, Na, and K) suggest 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 P2O5 (>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 andesite (Tba) with rhyolite, and red arrows are schematic paths for fractional crystallization 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.

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end members, major-element mass balance cal- were affected by the same Paleogene detachment Spring suite defi ne arrays that are distinct from culations show the proportion of rhyolite in the or roll back of the Farallon plate, which contrib- those of the Bingham volcanic suite (Figs. 14 hybridized andesite ranged from ~15% to 35% uted to continental arc magmatism over a wide and 16). The Bingham suite has high total by weight. area of western North America. These magmas alkali contents and includes melanephelinite, appear to have played a central role in the min- minette, shoshonite, latite, trachyte, and rhyolite PETROCHEMICAL COMPARISONS eralization in both areas. Fractionation from (Maughan et al., 2002), whereas the Paleogene basaltic andesite to shoshonite and latite also East Sulphur Spring suite is dominated by basal- The potential for mineralization, styles of occurred in both suites, although latite is rare tic andesite, andesite, dacite, and rhyolite; latite alteration, ages, and overall compositional char- and found only in a small lava fl ow and a few is rare as noted above (Fig. 16). Silica-under- acteristics of the Paleogene volcanic rocks in the dikes in the East Sulphur Spring suite (Fig. 16). saturated magmas are unknown in the East Sul- Sulphur Spring Range invite comparison with A similar fractionation trend is also important in phur Spring suite, but melanephelinite intruded other volcanic suites associated with mineraliza- the Eocene Absaroka volcanic fi eld of Wyoming as dikes and stocks and erupted as lava fl ows at tion in the Great Basin. Below, we compare the and Montana (Bray, 1999; Feeley and Cosca, Bingham and appears to have been an important East Sulphur Spring suite with Eo-Oligocene 2003) and appears to be a common process in source of S, Cu, and Au in the deposits. volcanic rocks at the Bingham Canyon Cu-Mo- these continental interior magma systems. Characteristically high Cr and Ni contents of

Au porphyry deposit farther east in Utah and In spite of these similarities, the major- and the intermediate composition (58%–65% SiO2) then to magmatic rocks associated with gold trace-element compositions of the East Sulphur volcanic rocks from Bingham (Maughan et al., 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 somewhat similar to those of porphyry copper systems, 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 carbonate 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 tectonic regimes and structural histories also shaped the features of the hydrothermal and magmatic systems. Both are near paleocontinental margins. 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 Figure 15. The compositions of the andesite unit (Ta) are different than origin—low Fe/Mg ratios, high oxygen fugaci- the rest of the East Sulphur Spring suite and appear to be consistent with

ties, high K2O, and similar “spiky” trace-element mixing of silicic magma with more maﬁ c magma. (A) Al2O3 versus SiO2.

patterns, for example. Apparently, both regions (B) Zr versus SiO2.

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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.

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2002) are not found in the unaltered rocks of the Instead, the “maﬁ c” component must have been Like those from the Sulphur Spring area, the

East Sulphur Spring suite (Fig. 14). The volcanic a moderately evolved (>55% SiO2) basaltic Carlin-trend rocks are dominantly calc-alkalic, suite at Bingham has elevated concentrations of andesite or shoshonite, similar to those found magnesian, and high-K. REE patterns of the Cr and Ni across the full range of silica content. in a few dikes. As a consequence, the igneous Sulphur Spring suite are also similar to those of Only the least silicic rocks from the Sulphur rocks of the East Sulphur Spring Range have igneous rocks along the Carlin trend (Ressel and Spring Range contain high Cr and Ni contents lower concentrations of Ni, Cr, Cu, and prob- Henry, 2006), which also have relatively steep and their concentrations decrease to very low val- ably S and Au (which follow Cu concentrations) slopes and small negative Eu anomalies (Fig. 9). ues (>50 ppm) in the more evolved andesites and than the intermediate composition stocks and In other words, both regions erupted magmas dacites. For example, dacites from the Bingham lavas associated with the Bingham intrusion. with subduction zone characteristics—low Fe/

volcanic center with 65% SiO2 have as much as Mg ratios, oxidized, hydrous, and high large-ion 200 ppm Cr and 75 ppm Ni. Dacites from the Comparison with Igneous Rocks along the lithophile to high-ﬁ eld-strength element (LIL/ Sulphur Spring Range (and from the Carlin Carlin Trend HFSE) ratios—produced during Paleogene trend) have less than 40 ppm Cr and 20 ppm Ni. rollback of the Farallon slab. Reduced, ilmenite- The low Cr and Ni concentrations are typical of The setting of the Sulphur Spring Range is dominated silicic magmas are probably rare in many continental calc-alkaline suites; the high Cr similar to that of nearby, intensely mineralized the region, but the strongly peraluminous, mus- and Ni in the Bingham volcanic suite is a fairly localities to the north along the Carlin trend. The covite- and garnet-bearing Harrison Pass intru- unique feature. Another distinguishing charac- Roberts Mountains thrust is an important ele- sion in the Ruby Range is one example (Barnes teristic of the Bingham magmas is the very high ment of mineralization along the Carlin trend; et al., 2001). Ba concentration (Fig. 14). In the East Sulphur it is also exposed in the northern Sulphur Spring The igneous rocks of the Carlin trend also Spring suite, only the fractionated latite dikes Range where it juxtaposes Ordovician strata have concentrations of key trace elements—Cr, have Ba contents (>2500 ppm) approaching, but on Devonian carbonate rocks and is in turn Ni, and Ba—much more similar to those of the not equaling, the maximum Ba contents of the cut by high-angle, N-S–, NW-SSE–, NE-, and Sulphur Spring suite than to those at Bingham volcanic rocks at Bingham. Moreover, the Ba E-W–trending normal faults (Carlisle and Nel- (Fig. 14; Ressel and Henry, 2006; C.D. Henry, content of the Bingham suite is high across the son, 1990). These structural and stratigraphic 2005, written commun.). Low concentrations of silica range, but is the highest in the maﬁ c and elements appear to be conducive to genesis of Cr and Ni in the intermediate magmas and the

intermediate composition rocks (<65 wt% SiO2). Carlin-type gold deposits (Hofstra and Cline, association with peraluminous magmas imply In the East Sulphur Spring suite, Ba increases as 2000; Grauch et al., 2003; Cline et al., 2005; that fractional crystallization and assimilation

SiO2 increases, whereas in the Bingham suite Ba Ressel and Henry, 2006). In addition, we have of pelitic crustal materials were the predomi-

generally decreases as SiO2 increases. noted the presence of jasperoid and altered nant magmatic processes in both areas (Fig. 14). Both the Bingham and the East Sulphur rocks with anomalous concentrations of As, Hg, Magma mixing played a lesser role and did not Spring suites preserve petrographic and chemi- Sb, and Tl, which are also features of mineral- involve mafi c alkaline magma as an end mem- cal evidence of mafi c magma having mixed with ization along the Carlin trend. ber. Rather, it involved mixing of basaltic andes- silicic magma to create intermediate composi- Ages, as revealed by new U-Pb data and ite or andesite with rhyolite. Mixing of compositions. At Bingham, the high concentrations of stratigraphic correlations, show that the mag- tionally similar magmas along the fractionation Cr, Ni, and Ba in the intermediate composition matic history of the northern Sulphur Spring trends is not ruled out and is in fact quite likely. volcanic and intrusive rocks have been traced to Range is also similar to that along the main The only differences between the East Sul- mixing with a mafi c alkaline magma—silica- Carlin trend as outlined by Ressel and Henry phur Spring volcanic suite and the magmatic undersaturated melanephelinite (or minette) with (2006). They identifi ed Jurassic plutonic rocks rocks along the Carlin trend that we have iden- high concentrations of Ni, Cr, Ba, Cu, Au, and S. (158 Ma), Cretaceous granite (112 Ma), abun- tifi ed are that the volcanism in the Sulphur Altered olivine and pyroxene rimmed by amphi- dant Eocene dikes, lavas, tuffs, and stocks Spring Range is a few million years younger bole in intermediate composition rocks at Bing- (40–36 Ma) thought to be directly related to Au (Fig. 12). Some of the intermediate magmas ham also demonstrate mixing of mafi c alkaline mineralization, and Miocene rhyolite (15 Ma). in the East Sulphur Spring suite have higher

magma with more silicic magma. Other evidence Inherited and primary zircons from the igneous K2O contents and are more shoshonitic, if includes dacite clasts in block and ash fl ows that rocks in the Sulphur Spring Range record each the handful of potassic samples from Sulphur contain cuspate mafi c clots, large resorbed potas- of these episodes. Clasts in the Elko Formation Spring is compared with the small set of “rep- sium feldspar phenocrysts in silicic rocks with are derived from a Late Jurassic granitic rock; resentative” analyses of Carlin-trend igneous elevated Cr and Ni concentrations, and adjacent, inherited zircon in the andesite lava has a Creta- rocks published by Ressel and Henry (2006). same-aged minette and quartz latite dikes (Pul- ceous age; most of the volcanic rocks and dikes However, a larger compilation of igneous sipher, 2000; Maughan et al., 2002). The Sulphur are Paleogene in age but slightly younger (36– rock compositions from north-central Nevada Spring unit that most vividly expresses magma 32 Ma) than those to the north along the Carlin (C.D. Henry, 2005, written commun.) has a mixing is the andesite (unit Ta), with its dis- trend (40–36 Ma; Fig. 12); and fi nally, a rhyolite few Eocene latites, but no shoshonites. There equilibrium mineral assemblage and anomalous pyroclastic deposit has a Miocene age. are also a few potassic Eocene plutonic rocks elemental composition. More subtle chemical Igneous rocks in the Sulphur Spring Range from northeastern Nevada in du Bray’s (2007) evidence of mixing is found in a few dikes in the are compositionally and genetically similar to database (eight of 643 samples after removing a basaltic andesite suite (Tba) as well. those associated with the Carlin trend, which we few obviously altered rocks) in the 50% to 65% In spite of the evidence for magma mixing, take to include the Emigrant Pass and Tuscarora range, and three of those are lamprophyre dikes in the East Sulphur Spring suite, the mafi c com- volcanic fi elds (Henry et al., 1999; Ressel and from the Fish Canyon Range whose age is not ponent was not silica-undersaturated, nor was Henry, 2006). For example, both suites consist clearly established as Eocene. Thus, it appears it especially rich in Ni, Cr, or Ba compared to of subalkaline basaltic andesite to rhyolite, and that potassic igneous rocks of Eocene age are the melanephelinite and minette at Bingham. include only minor latite and trachyte (Fig. 16). sparsely found across northern Nevada.

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Given the similarities in structure, stratigra- Waite et al. (1997), Hattori and Keith (2001), or as underplated and mixed magma may have phy, alteration styles, geochemical anomalies, and Maughan et al. (2002) concluded that por- contributed large quantities of sulfur, fl uids, and magmatic history, and petrology of the Eo- phyry and Carlin-like mineralization in the metals to the ore-forming systems, in addition Oligocene igneous rocks, it is reasonable to Bingham district could not have formed without to any heat they would have released to drive infer that economic Carlin-type mineralization involvement of mafi c alkaline magmas rich in S, convecting fl uids. This concept has been pro- might exist in the Sulphur Spring Range. Cu, and Au. Intermediate “calc-alkaline” mag- posed for multiple magma-ore systems, includ- mas at Bingham were simply too poor in these ing porphyry Cu deposits at Santa Rita in New Potential Importance of Mafi c Magma and elements for any reasonable volume of magma to Mexico (Audetat and Pettke, 2006), Questa Mo Magma Mixing for Mineralization have served as a source of metal in the deposits. deposit in New Mexico, Nukay Au-Cu deposit Although only small amounts of mafi c rock are in Mexico, Las Bambas porphyry Cu deposit Intermediate and felsic rocks, such as those known near Bingham, evidence of magma mix- in Peru (Jones, 2002), the Farallón Negro that typically host porphyry Cu-Au systems, have ing involving alkaline olivine-bearing magmas Cu-Au deposit in Argentina (Halter et al., low concentrations of sulfur and of chalcophile is widespread. Apparently during the Eocene, 2005), as well as the Bingham porphyry cop- ore metals compared to more mafi c magmas. mafi c alkaline magmas intercepted and mixed per system emphasized here. The presence Sulfur concentrations are limited by the solubil- with evolved calc-alkaline magmas in shallow of mafi c volcanic and/or dike rocks (Cr- and ity of sulfur in felsic magmas, which decrease subvolcanic settings. This mafi c alkaline magma MgO-rich dikes) in the Sulphur Spring Range as silica concentrations increase causing sulfi de was richer in compatible ore elements (e.g., Cu and along the Carlin trend during the Paleogene and sulfate minerals to precipitate. A sulfur-satu- and Au) and was probably the main source of is permissive evidence for the operation of this

rated andesite with 60% SiO2 may have less than the ore metals and sulfur. process here as well. These mafi c rocks, with 200 ppm S, whereas mantle-derived basalt may In light of these conclusions, the obvious their higher concentrations of compatible ele- have over 1000 ppm S (Liu et al., 2007). Gold question is: Did mafi c magmas play such a role ments (including chalcophile elements), could and copper are also enriched in mafi c magmas in the Sulphur Spring Range or for that matter have been an additional source of ore metals compared to silicic magmas; copper concentra- along the Carlin trend? and fl uids in the deposits. At most Carlin-type tions in alkaline mafi c magmas can be as much While there is no direct evidence for the deposits, the isotopic heritage of the magmatic as 120 ppm and in rhyolite concentrations are involvement of mafi c magma in mineralization components may have been overwhelmed by only a few ppm (e.g., Maughan et al., 2002). In along the Carlin trend (e.g., Cline et al., 2005; later meteoric water interaction (Cline et al., fresh igneous rocks of the East Sulphur Spring Emsbo et al., 2006), the presence of olivine- 2005). In any case, such mafi c magmas would suite, Cu concentrations are as much as 50 ppm bearing basaltic andesites along the Carlin trend be much better sources of Au and S than the in the mafi c dikes and basaltic andesite dikes, (Ressel and Henry, 2006) and in the Sulphur andesites and dacites that dominate the erup- but typically less than 20 ppm in the andesites Spring Range shows that mafi c magma was tive record. Thus, from the evidence found in and more silicic rocks. Gold concentrations are present. MgO contents as high as 13% have been the northern Sulphur Spring Range and that likely to be on the order of 1.7–0.5 ppb based found in dikes from the Sulphur Spring Range. previously published for the Carlin trend, we on a copper:gold ratio of ~30,000 (Rudnick and The mafi c component of the andesite of the Sul- concur with the hypothesis that Paleogene Gao, 2004). Copper and gold behave as com- phur Spring Range is also clear evidence that magmas may have served as important sources patible elements—in differentiated magma sys- mafi c magmas were involved in this magma sys- of S and Au (Cline et al., 2005; Ressel and tems their concentrations usually co-vary with tem. These mafi c magmas form coherent com- Henry, 2006) in addition to the isotopically strongly compatible elements like Ni and Cr. positional trends relating them to the rest of the identifi ed crustal sources of these elements Immiscible sulfi de melts and minerals and mag- volcanic rocks, demonstrating that mafi c mantle- (e.g., Arehart et al., 1993; Hofstra and Cline, netite have high partition coeffi cients for chal- derived magmas were at the “roots” of these 2000; Emsbo et al., 2006). cophile elements like gold and copper and, once Eocene magma systems, as in all subduction evolving magmas become sulfi de-saturated, the related magmatic arcs. Finally, the gold deposits CONCLUSIONS residual melt becomes strongly depleted in these and the igneous rocks have similar ages. elements (Jugo et al., 1999: Simon et al., 2003). Other geologists emphasize the role of Paleo- A suite of Eo-Oligocene lava fl ows, domes, Thus, the potential for the development of a por- gene magmas as simple heat sources to drive and pyroclastic rocks is interlayered with clas- phyry Cu-Au system hosted by felsic intrusions fl uid fl ow (e.g., Tosdal, 1998). However, the tic sediments and cut by dikes in the northern may be strongly linked to the extent of mixing close spatial association of the basaltic andesite Sulphur Spring Range of central Nevada. These with mafi c magma to yield higher than “normal” dikes in the Sulphur Spring Range with miner- rocks form a dominantly high-K, calc-alkalic concentrations of ore metals and sulfur. On the alized veins suggests an even closer genetic link suite, with low Fe/Mg ratios similar to those other hand, disseminated Carlin-type gold depos- between the magmas and ore deposits. These found in subduction settings worldwide. The its are typically hosted by sedimentary rocks and dikes are not volumetrically signifi cant at the volcanic and subvolcanic rocks range from not by intermediate composition igneous stocks. current level of exposure, but they may have olivine-bearing, high-MgO basaltic andesite to In most deposits, the S, O, H, and C isotopic dominated at deeper levels of the Eo-Oligocene garnet-bearing dacite, and high-silica rhyolite. compositions are closely linked to sediments magma system. Many of the dikes and lava fl ows The intermediate to silicic rocks have spiky and meteoric fl uids (e.g., Cline et al., 2005). are Cu-, Cr-, Ni-, and MgO-rich, indicating little trace-element patterns with Nb-Ti depletions Therefore, if elements from mafi c magmas are fractionation occurred since they left their man- and enrichment of Pb, also similar to those important to their generation, mixing may not be tle sources (Table 1; Fig. 14). Consequently, formed at convergent margins. The oldest Paleo- as critical as the mere presence of mafi c magma if they are like other oxidized mafi c magmas, gene volcanic rocks are rhyolite and latite lava that in the process of crystallizing and “degas- they would have been enriched in Au and S fl ows interlayered with fl uvial conglomerates sing” can give off signifi cant sulfur and gold to compared to more silicic magmas. “Degassing” and dacitic tuff that probably correlate with the upper crustal hydrothermal systems. of mafi c magma in dikes and volcanic conduits Eocene Elko Formation. This interpretation is

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based on lithologic similarities and on new U-Pb include shoshonite and latite as well. The dikes fraction of the mafi c magma was able to erupt. zircon ages of overlying units, which show the are parallel to a distinctive set of oxidized quartz- Structural boundaries (including Proterozoic- basal unit is probably middle Eocene in age. A sulfi de veins in the central part of the range. The age basement-penetrating faults, Paleozoic and porphyritic dacite intrusion (35.9 ± 0.5 Ma) and veins have anomalous concentrations of Au, Ag, Mesozoic thrust faults, and magma-generated probably cogenetic rhyolite to dacite tuff (35.5 As, Cu, Mo, Pb, Sb, and Zn and surround two fractures) guided magma emplacement routes, ± 0.4 Ma) overlie this succession. The tuff is per- historically mined polymetallic vein deposits. controlled levels of stagnation, promoted hydro- aluminous and has sparse phenocrysts of garnet. Jasperoid bodies on the margins of the range also thermal fl uid fl ow, and placed reactive wall A series of dacite lava domes (35.1 ± 0.5 Ma) have Carlin-like trace-element signatures. rocks in the fl ow paths. “Degassing” of mafi c caps the Eocene sequence. The youngest volca- These characteristics suggest the magma magma may have contributed sulfur and chalco- nic unit is an extensively hybridized Oligocene system was rooted in a subduction zone that phile metals to the mineralized veins and small andesite (31.4 ± 0.5 Ma). It has a disequilibrium formed as the Farallon plate steepened during ore deposits. phenocryst assemblage of plagioclase, bio- the Paleogene (Fig. 17). As hot asthenosphere The Eo-Oligocene igneous rocks of the East tite, clinopyroxene, orthopyroxene, amphibole, came in contact with the slab, dehydration of Sulphur Spring suite are compositionally akin to and olivine, along with resorbed megacrysts the plate produced an oxidized aqueous fl uid Eocene igneous rocks associated with large gold of quartz, potassium feldspar, and garnet. Lin- enriched in S, Cu, Au, Pb, and other soluble deposits in the Carlin trend. When other simi- ear trends on variation diagrams are consistent elements. The fl uid lowered the melting tem- larities in structure, stratigraphy, and alteration with mixing between intermediate composition perature of the overlying mantle wedge, gen- are considered, we conclude that the Sulphur magma and peraluminous rhyolitic magma to erating distinctly arclike magmas as a result Spring Range is prospective for Carlin-type gold form the andesite. The dated units probably cor- of hydrous partial melting. This hot, hydrous, deposits. Johnston and Ressel (2004) suggested relate in time with the Indian Well Formation oxidized magma rose buoyantly into the crust, that porphyry Cu-Au deposits, Carlin-type of Eo-Oligocene age that is mapped in adjacent stagnated because of density differences, and deposits, and distal disseminated deposits are areas. Dikes (dominantly of basaltic andesite) promoted partial melting of the continental crust all part of a continuum with differences depend- cut this unit and trend NNE. We correlate these to form peraluminous silicic magma. These ing mostly on spatial relations to the magmatic mafi c dikes, which cut Paleogene volcanic and disparate mantle- and crust-derived magmas hydrothermal system. We agree and suggest sedimentary rocks, with dikes of similar orien- mixed, rose into shallow chambers, differenti- that, like Sulphur Spring and Bingham, Carlin- tation and composition that cut only Paleozoic ated by fractional crystallization, and eventually related Eocene magmatic systems include rela- rocks in the central part of the range. These mafi c erupted or fi lled fractures to form dikes. As a tively mafi c magmas that are vital to generating dikes are also dominated by basaltic andesite but consequence of crustal trapping, only a small the large quantities of gold in the ore deposits.

A. 120 to 45 Ma Flat Slab Subduction

W California Nevada Utah E Figure 17. Plate tectonic reconstruction of the western United States during the Paleo- GT RMT gene. Gt—Golconda thrust; RMT—Roberts Mountains thrust. (A) A period of low-angle Oceanic Young Accreted Terranes Precambrian Continental Lithosphere Crust subduction during the Cretaceous and early Paleogene thickens the crust, but magma Lithospheric Mantle Ancient Veined Mantle generation is hindered. (B) The subducting oceanic lithosphere rolled back inducing asthenosphere counterfl ow. As it heated, Asthenosphere the slab dehydrated and initiated the production 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 B. 42 to 30 Ma Slab Rollback concentrations of S and chalcophile metals, W California Nevada Utah E and may be key to generation of ore deposits Sulphur Spring in the shallow crust. East of the Proterozoic Range craton margin, mantle-derived magmas (orange) interact with ancient lithospheric Oceanic Differentiation Lithoshere Young Accreted Terranes and mixing mantle and overlying crust. Partial melting Partial melting of the crust produces felsic, peraluminous Lithospheric Mantle Ancient Veined Mantle 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 Slab Dehydration & Arc Magmatism Hot Asthenospheric of the Proterozoic craton, magmas (blue) Wet-Oxidized Magma Counterflow interact with younger lithosphere and less felsic crust.

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chemistry Geophysics Geosystems, v. 7, p. 1–14, doi: Grauch, V.J.S., Klein, D.P., Rodriguez, B.D., and Wooden, We readily acknowledge that the mere presence 10.1029/2005GC001100. J.L., 2003, Geophysical and isotopic constraints on of mafi c magmatic rocks does not demonstrate Cline, J.S., Hofstra, A.H., Muntean, J.L., Tosdal, R.M., and crustal structure related to mineral trends in north- conclusively that they contributed to the miner- Hickey, K.A., 2005, Carlin-type gold deposits in Nevada: central Nevada and implications for tectonic history: Critical geologic characteristics and viable models: Eco- Economic Geology and the Bulletin of the Society of alization, but combined with the close spatial nomic Geology 100th Anniversary Volume, p. 451–484. 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Halter, W.E., Heinrich, C.A., and Pettke, T., 2005, Magma Coombs, M.L., and Gardner, J.E., 2004, Reaction rim growth evolution and the formation of porphyry Cu-Au ore fl u- esis that merits further investigation. on olivine in silicic melt: Implication for magma mix- ids: Evidence from silicate and sulfi de melt inclusions: ing: American Mineralogist, v. 89, p. 748–759. Mineralium Deposita, v. 39, p. 845–863, doi: 10.1007/ ACKNOWLEDGMENTS Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit s00126-004-0457-5. models: U.S. Geological Survey Bulletin 1693, 379 p. 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