Geochemistry, Petrologic Evolution, and Ore Deposits of the Miocene
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1 REVISION 1 2 Geochemistry, petrologic evolution, and ore 3 deposits of the Miocene Bodie Hills Volcanic 4 Field, California and Nevada 5 By Edward A. du Bray1, David A. John2, Brian L. Cousens3, Leslie A. Hayden4, and Peter G. 6 Vikre5 7 1 U.S. Geological Survey, MS 973; Box 25046, DFC; Lakewood, CO 80225 8 2 U.S. Geological Survey, MS 901; 345 Middlefield Rd., Menlo Park, CA 94025 9 3 Department of Earth Sciences; Carleton University; 1125 Colonel By Drive; Ottawa, ON. 10 K1S5B6 Canada 11 4 U.S. Geological Survey, MS 910; 345 Middlefield Rd., Menlo Park, CA 94025 12 5 U.S. Geological Survey, Reno Office; Mackay School of Earth Sciences and Engineering; 13 University of Nevada, Reno; Reno, NV 89557-0047 14 15 16 17 Abstract 18 The southern segment of the ancestral Cascades magmatic arc includes numerous 19 volcanic fields; among these, the Bodie Hills volcanic field (BHVF), astride the California- 20 Nevada border north of Mono Lake, is one of the largest (>700 km2) and most well studied. 21 Episodic magmatism in the BHVF spanned about 9 million years between about 15 and 6 Ma; 22 magmatic output was greatest between ca. 15.0 to 12.6 Ma and ca. 9.9 to 8.0 Ma. 23 About two dozen contiguous and coalescing eruptive centers above middle- to shallow- 24 crustal-level reservoirs generated several trachyandesite stratovolcanoes and numerous silicic 25 trachyandesite to rhyolite flow dome complexes whose compositional variations are consistent 26 with fractionation of observed phenocryst phases. BHVF rocks have high-potassium calc- 27 alkaline compositions consistent with generation of subduction-related continental margin arc 28 magmas beneath thick continental crust. Radiogenic isotope ratios in BHVF rocks vary 29 considerably but suggest somewhat enriched, crustal sources; isotopic ratios for some of the 30 more primitive units are consistent with more depleted, mantle sources. Neither age nor whole- 1 31 rock compositions of BHVF rocks are well correlated with isotopic variations. Textures and 32 compositions of phenocrysts in BHVF rocks are in accord with the associated magma reservoirs 33 evolving via open-system behavior. Reservoir recharge and subsequent incomplete 34 homogenization are evidenced by the broad compositional diversity characteristic of many 35 BHVF eruptive units. Significant compositional diversity among the products of coeval eruptive 36 centers further suggests that centers responsible for BHVF magmatism were underlain by small, 37 discrete, compositionally distinct, and closely spaced reservoirs. 38 Volcanic rocks of the BHVF host quartz-adularia and quartz-alunite epithermal gold- 39 silver deposits, from which about 3.4 Moz. of gold and 28 Moz. of silver have been produced. 40 The volcanic rocks and contained deposits are broadly coeval, which suggests that the associated 41 magmas are the sources of heat, fluids, and metals involved in deposit genesis. Characteristics of 42 the quartz-adularia deposits are consistent with derivation from near-neutral pH fluids at ≤250°C, 43 whereas those of the quartz-alunite systems require more acidic, oxidized, and sulfur-rich fluids 44 at temperatures <250°C. Economically viable precious metal accumulations are in fault-hosted 45 vein deposits in the Bodie and Aurora districts. Circulation of hydrothermal fluids through 46 permeable pyroclastic deposits but lacking prominent structural conduits resulted in large areas 47 of altered but unmineralized rock. 48 49 Keywords: arc magmatism, geochemistry, petrogenesis, mineral deposits, tectonic setting 50 51 Introduction 52 The middle to late Miocene Bodie Hills volcanic field (BHVF) in the Bodie Hills north of 53 Mono Lake, in western Nevada and eastern California, represents a large (>700 km2), long-lived 54 (≈9 m.y.), but episodic field in the southern segment of the ancestral Cascades arc (John et al., 55 2012; du Bray et al., 2014) (Fig. 1). In the BHVF, erupted volumes, duration of magmatism, 56 compositional diversity, and degree of eruptive center preservation are unusual relative to other 57 southern segment, ancestral Cascades arc volcanic fields (John et al., 2012; du Bray et al., 2014). 58 The geochemical evolution of the BHVF is documented by compositional data for 21 major and 59 10 minor (eruptive units with map areas of less than 2 km2; John et al., 2015) centers. The large 60 dataset synthesized as part of this study provides a unique opportunity to evaluate the processes 61 that contributed to the evolution of subduction-related magmatism and related precious metal 2 62 mineral deposit formation in the southern segment of the ancestral Cascades arc. The goal of this 63 paper is to assess overall compositional-temporal variation within the BHVF, compositional 64 evolution within its individual eruptive centers, and relations between magmatism and 65 mineralizing processes. 66 FIGURE 1 NEAR HERE 67 Previous studies 68 Geologic studies of the Bodie Hills have focused on mineral deposits in the Bodie, 69 Aurora, and Masonic mining districts (e.g., Hill, 1915; Chesterman et al., 1986; Herrera et al., 70 1991, 1993; Osborne, 1991; Silberman and Chesterman, 1991; Breit, 2000; Vikre and Henry, 71 2011; Vikre et al., 2015), volcanic stratigraphy in the Bodie 15′ quadrangle (Chesterman, 1968; 72 Chesterman and Gray, 1975), late Cenozoic development of the Mono Basin (Gilbert et al., 73 1968; Al-Rawi, 1969), geology of the Bodie Hills volcanic field (John et al., 2012), and 74 evolution of the post-subduction Pliocene-Pleistocene Aurora volcanic field (Lange et al., 1993; 75 Lange and Carmichael, 1996). John et al. (2015) compiled a new geologic map of the Bodie 76 Hills from existing data and refined geologic relations based on extensive new fieldwork and 77 geochronology. Geophysical studies of the BHVF, summarized by John et al. (2012) pertain to 78 region-scale Basin and Range geology and tectonism in the area surrounding the Bodie Hills 79 (e.g., Eaton et al., 1978; Blakely and Jachens, 1991; Saltus and Jachens, 1995) and expressions 80 of gold-silver deposits and large areas of altered rock (e.g., Kleinhampl et al., 1975; 81 Smailbegovic, 2002; Rockwell, 2010). 82 Geology of the Bodie Hills volcanic field 83 The wealth of new geochemical, geochronologic, and petrographic data synthesized 84 herein, in concert with geologic map relations summarized by John et al. (2015), distinguish the 85 BHVF as one of the few, well documented large volcanic fields within the ancestral Cascades 86 arc. The Bodie Hills straddle a roughly 40 by 30 km area along the California-Nevada border and 87 rise to an elevation of 3112 m at Potato Peak, about 1100 to 1400 m above Bridgeport and 88 Fletcher Valleys, Mono Valley, and the East Walker River (Fig. 2). The central Sierra Nevada 89 rises nearly 2000 m above Bridgeport Valley and Mono Valley about 8 km west of the Bodie 90 Hills. The volcanic field includes the products of overlapping and coalesced eruptive centers in 91 which stratigraphic relations are poorly developed, requiring geochronologic data to establish 92 relative ages. The BHVF includes at least 31 volcanic map units associated with 21 significant 3 93 volcanic eruptive centers and several smaller centers (Fig. 2). The eruptive centers include 94 several trachyandesite stratovolcanoes that were emplaced along the margins of the volcanic 95 field and numerous silicic trachyandesite to rhyolite flow dome complexes that were generally 96 localized more centrally (Supplemental Fig. 1). Volcanism in the BHVF was episodic, with two 97 peak periods of eruptive activity, including an early period between ca. 15.0 to 12.6 Ma that 98 formed several large stratovolcanoes and a later period from ca. 9.9 to 8.0 Ma dominated by 99 emplacement of large silicic trachyandesite-dacite lava domes but also including formation of the 100 trachyandesite stratovolcano centered on Mount Biedeman (Fig. 2). Magmatism in the BHVF 101 concluded with formation of dispersed, structurally controlled, small-volume silicic lava domes 102 at about 6 Ma. 103 FIGURE 2 NEAR HERE 104 Miocene igneous rocks of the BHVF were erupted onto pre-Tertiary basement, 105 discontinuously exposed throughout the Bodie Hills (John et al., 2015). Pre-Tertiary rocks 106 consist of (1) lower Paleozoic hornfelsed argillite, sandstone, chert, and pebble conglomerate; (2) 107 Triassic(?) metamorphosed sandstone, siltstone, chert, tuff, and pillow basalt; (3) Mesozoic 108 (Jurassic?) meta-andesite, metatuff, and metavolcaniclastic rocks; and (4) Late Cretaceous (98– 109 83 Ma; John et al., 2015) granitic rocks that are part of the Sierra Nevada batholith (Chesterman 110 and Gray, 1975; Stewart et al., 1982; Robinson and Kistler, 1986). The Paleozoic and Mesozoic 111 metasedimentary rocks crop out in the southwestern and northern parts of the Bodie Hills, 112 Mesozoic metavolcanic rocks are exposed in the northern and eastern parts, and the Late 113 Cretaceous granitic rocks crop out around the margins of the area. 114 The Bodie Hills are in a complex tectonic setting near the west edge of the Walker Lane 115 and the Basin and Range physiographic province and at the northwest limit of the Mina 116 deflection (Fig. 1; Stewart, 1988; Faulds and Henry, 2008; Busby, 2013); associated structures 117 related to these tectonic features might have localized BHVF magmatism. The Walker Lane fault 118 system is a broad, northwest-striking, dextral high-strain zone that accommodates about 20 119 percent of the right-lateral motion between the Pacific and North American plates (Oldow, 2003; 120 Faulds and Henry, 2008; John et al., 2012; Busby, 2013). The Mina deflection (Fig. 1) is a 60- 121 km right step in the Walker Lane (Stewart, 1988; Oldow, 1992; Faulds and Henry, 2008), in 122 which slip is partitioned across an approximately 80-km-wide, complex array of northwest- 123 striking right-lateral faults, northeast-striking normal faults, and east- to east-northeast-striking 4 124 left-lateral faults (Oldow, 1992; Wesnousky, 2005).