Miocene and Early Pliocene Epithermal Gold-Silver Deposits in the Northern Great Basin, Western United States: Characteristics, Distribution, and Relationship to Magmatism

Numerous important Miocene and early Pliocene epithermal Au-Ag deposits are present in the northern Great Basin. Most deposits are spatially and temporally related to two magmatic assemblages: bimodal basaltrhyolite and western andesite. These magmatic assemblages are petrogenetic suites that reflect variations in tectonic environment of magma generation. The bimodal assemblage is a K-rich tholeiitic series formed during continental rifting. Rocks in the bimodal assemblage consist mostly of basalt to andesite and rhyolite compositions that generally contain anhydrous and reduced mineral assemblages (e.g., quartz + fayalite rhyolites). Eruptive forms include mafic lava flows, dikes, cinder and/or spatter cones, shield volcanoes, silicic flows, domes, and ash-flow calderas. Fe-Ti oxide barometry indicates oxygen fugacities between the magnetitewustite and fayalite-magnetite-quartz oxygen buffers for this magmatic assemblage. The western andesite assemblage is a high K calc-alkaline series that formed a continental-margin arc related to subduction of oceanic crust beneath the western coast of North America. In the northern Great Basin, most of the western andesite assemblage was erupted in the Walker Lane belt, a zone of transtension and strike-slip faulting. The western andesite assemblage consists of stratovolcanoes, dome fields, and subvolcanic plutons, mostly of andesite and dacite composition. Biotite and hornblende phenocrysts are abundant in these rocks. Oxygen fugacities of the western andesite assemblage magmas were between the nickel-nickel oxide and hematite-magnetite buffers, about two to four orders of magnitude greater than magmas of the bimodal assemblage. Numerous low-sulfidation Au-Ag deposits in the bimodal assemblage include deposits in the Midas (Ken Snyder), Sleeper, DeLamar, Mule Canyon, Buckhorn, National, Hog Ranch, Ivanhoe, and Jarbidge districts; high-sulfidation gold and porphyry copper-gold deposits are absent. Both highand low-sulfidation gold-silver and porphyry copper-gold deposits are affiliated with the western andesite assemblage and include the Comstock Lode, Tonopah, Goldfield, Aurora, Bodie, Paradise Peak, and Rawhide deposits. Low-sulfidation Au-Ag deposits in the bimodal assemblage formed under relatively low oxygen and sulfur fugacities and have generally low total base metal (Cu + Pb + Zn) contents, low Ag/Au ratios, and notably high selenide mineral contents compared to temporally equivalent low-sulfidation deposits in the western andesite assemblage. Petrologic studies suggest that these differences may reflect variations in the magmatic-tectonic settings of the associated magmatic assemblages—deposits in the western andesite assemblage formed from oxidized, water-rich, subduction-related calc-alkaline magmas, whereas deposits in the bimodal assemblage were associated with reduced, water-poor tholeiitic magmas derived from the lithospheric mantle during continental extension. The contrasting types and characteristics of epithermal deposits and their affinities with associated igneous rocks suggest that a genetic relationship is present between these Au-Ag deposits and their temporally associated magmatism, although available data do not prove this relationship for most low-sulfidation deposits. Economic Geology Vol. 96, 2001, pp. 1827–1853 † E-mail: [email protected] mineral assemblages transitional between highand lowsulfidation deposits. Based on deposits in the Great Basin and elsewhere, Hedenquist et al. (2000) suggested that these two subtypes of low-sulfidation deposits should be renamed intermediate-sulfidation and end-member low-sulfidation types to emphasize their mineralogical, magmatic, and tectonic differences. Most epithermal gold-silver deposits in the northern Great Basin are hosted by middle to late Tertiary igneous rocks (Fig. 2). With the exception of the giant late Oligocene Round Mountain deposit and much smaller deposits in the Atlanta, Fairview, Tuscarora, and Wonder mining districts that are hosted by late Eocene to early Miocene caldera complexes (Fig. 2a; McKee and Moring, 1996), these deposits are associated with two distinct Miocene to Quaternary magmatic assemblages, the western andesite and bimodal basalt-rhyolite assemblages (Fig. 2b and c, Table 1; Christiansen and Yeats, 1992; Ludington et al., 1996b; John et al., 1999). The western andesite assemblage is a subduction-related, continentalmargin volcanic arc that formed along the western coast of North America between about 22 to 4 Ma. By contrast, the bimodal basalt-rhyolite assemblage is related to continental rifting (Basin and Range extension) that began about 17 Ma and formed the present physiography of the Great Basin (McKee, 1971; Noble, 1972). Numerous low-sulfidation deposits are present in both magmatic assemblages, whereas high-sulfidation gold-silver deposits are restricted to the western andesite assemblage (John et al., 1999). This paper is a summary of late Cenozoic magmatism and epithermal gold-silver deposits in the northern Great Basin (north of latitude 37°30'N). The focus is on variations in the composition, tectonic setting, and eruptive style of the Miocene to early Pliocene magmatic assemblages and how these variations may have influenced the types and characteristics of epithermal gold-silver deposits formed in the northern Great Basin. The characteristics of low-sulfidation deposits of the western andesite and bimodal assemblages are compared, and the distinction of intermediate-sulfidation deposits from other low-sulfidation deposits, as proposed by Hedenquist et al. (2000), is discussed. The magmatic relationships and deposit distinctions have important implications for exploration for epithermal deposits in the Great Basin and elsewhere. Pre-Middle Cenozoic Geologic History of the Northern Great Basin The northern Great Basin has had a complex and varied geologic history that is part of the evolution of the North American Cordillera (Fig. 3; Burchfiel et al., 1992; Ludington et al., 1996a). In the Late Proterozoic, breakup of the supercontinent Rodinia led to development of a west-facing passive margin at the rifted edge of Precambrian continental crust and a westward-thickening wedge of miogeoclinal sediments on the continental slope and shelf (Stewart, 1980; Karlstrom et al., 1999). The location of the rifted continental margin has been inferred to correspond to the 0.706 isopleth of initial 87Sr/86Sr in Mesozoic granitic plutons (Figs. 2 and 3; Kistler, 1991). The rifted continental margin was the focus of several episodes of contractional and extensional deformation during Paleozoic to early Cenozoic time (Fig. 3). The Late Devonian to Early Mississippian Antler and the Late Permian to Early Triassic Sonoma orogenies thrust eugeoclinal sedimentary rocks of the Roberts Mountains and Golconda allochthons eastward over coeval miogeoclinal rocks of the continental shelf. Mesozoic and early Cenozoic deformation of the Nevadan, Elko, Sevier, and Laramide orogenies was associated with an east-dipping subduction zone beneath western North America, accretion of island-arc terranes, and progressive contraction of the miogeocline from west to east, resulting in younger deformation affecting rocks farther to the east. Extensional deformation has affected the region since the late Eocene (see below). Subduction-related calc-alkaline magmatism was widespread in the northern Great Basin and most intense in the Middle and Late Jurassic, the Cretaceous, and in the middle Cenozoic (Miller and Barton, 1990; Christiansen and Yeats, 1828 DAVID A. JOHN 0361-0128/98/000/000-00 $6.0