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Age and Petrogenesis of Volcanic 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. 496 For permission to copy, contact [email protected] © 2008 Geological Society of America Paleogene magmatism in the Sulphur Spring Range, Nevada 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 replace- ment 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 CCarlinarlin 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 BullionBullion intrude or overlie the Paleozoic deposits. Some of the igneous rocks are spatially associ- ated 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 PasPasss including substantial amounts of phyllic and argil- lic hydrothermal alteration, pebble dikes, breccia MineralMineral Study AreaAre pipes, and disseminated and vein-related mineral- HillHill ization. Altered mafi c and intermediate composi- tion 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. Geosphere, June 2008 497 Ryskamp et al. 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.
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