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Tuscarora |
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Nevada, USA |
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Au Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Tuscarora volcanic field and low-sulphidation gold deposits are located ~50 km NW of Elko in north-central Nevada, ~25 km west of Jerritt Canyon and ~25 km NE of the Carlin Trend.
Placer gold was discovered in the Tuscarora district in 1867 and production of silver-gold lode deposits began in 1875 (LaPointe, et al., 1991). Total precious metal production between 1867 and 1990 was 6.345 t of gold and 236 t of silver from quartz veins and quartz stockwork mineralization (Castor, et al., 2003). This includes up to 1.06 t of placer gold estimated by Nolan in 1936 (Johnson, 1973).
The Tuscarora volcanic field is part of a broad area of Eocene (~43 to 34 Ma) magmatism that occurred throughout northeastern Nevada and northwestern Utah. It is the largest (~2000 km2) Eocene volcanic field in Nevada and one of the more diverse and complex (Berger et al., 1991; Boden et al., 1993; Henry and Boden, 1998, 1999; Henry et al., 1998, 1999) and underwent six major pulses of activity between about 39.9 and 39.3 Ma. Rhyolite lavas and dykes were emplaced at ~35 Ma in the western part of the field. From oldest to youngest, the six pulses produced i). early rhyolitic to dacitic ash-flow tuffs in the western part of the field at 39.9 Ma; ii). andesitic to dacitic lava flows and tuffaceous sedimentary rocks of the Pleasant Valley volcanic complex between 39.9 and 39.7 Ma; iii). dacitic domes, with a small volume of pyroclastic- flow deposits, and epiclastic deposits of the Mount Blitzen volcanic center from 39.8 to 39.7 Ma; iv). rhyolitic ash-flow tuff of the Big Cottonwood Canyon caldera at 39.7 Ma; v). a granodiorite pluton and andesitic to rhyolitic dykes and small intrusions of the Mount Neva intrusive episode between 39.5 and 39.3 Ma; and vi). andesitic to dacitic lava flows of Sixmile Canyon at 39.3 Ma. Gold-silver mineralisation in the Tuscarora district was emplaced at ~39.3 Ma, contemporaneous with the Mount Neva intrusions during the last phases of magmatism (Castor, et al., 2003).
The Mount Neva intrusive episode represents the most significant magmatism within the Tuscarora volcanic field. It developed in three distinct pulses (Castor, et al., 2003):
i). Early porphyritic dacite - which occur as numerous irregularly shaped intrusions along the western, southern and eastern margins of the Mount Blitzen volcanic centre and a few dykes within the centre, suggesting most intrusions rose along the faults that bound that centre. Early dacite is abundant in the district, intruding volcaniclastic sedimentary rocks of the Pleasant Valley complex, and is the other major host rock in the district.
ii). The Mount Neva granodiorite, which is the largest intrusion in the area, cutting the southwestern margin of the Mount Blitzen centre. It covers an ENE elongated area of ~5 × 1 to 2 km, and has steep contacts with surrounding rocks.
iii). Late dykes - numerous late dykes, ranging from andesite to low-silica rhyolite, were the last event of the Mount Neva episode.
The basement to the Tuscarora volcanic field comprises a complexly deformed sequence of Lower Palaeozoic rocks that comprise the upper
plate of the Roberts Mountains allochthon (Roberts, 1964). Two Palaeozoic packages of rocks are exposed in the Tuscarora volcanic field, i). northern suite that includes a wedge of Ordovician and Silurian rocks immediately north of the Tuscarora district, composed of
quartzite, siltstone, chert, and minor limestone (Coats, 1987; Henry and Boden, 1998); and ii). southern Silurian and Devonian
package which is composed of siltstone and chert, and minor sandstone and limestone. Carbonate-bearing, lower-plate rocks that are the major hosts for gold deposits in the Carlin trend and Independence Mountains, do not crop out in the Tuscarora volcanic field but are interpreted to underlie the upper-plate rocks.
This sedimentary basement is overlain by up to 1500 m of Eocene Mt. Blitzen and Pleasant Valley volcanic rocks. Overlying these rocks are up to 500 feet of Tertiary to Quaternary-age alluvium gravels and lacustrine deposits that thicken southward. The base of the volcanic sequence is a thick moderately-welded, latitic, lithic and pumice lapilli tuff. The tuff fines gradationally upward without apparent depositional breaks. The tuff is overlain by volcaniclastic and sedimentary rocks that vary greatly in thickness, continuity, and distribution. Intercalated sedimentary rocks range from siltstone to conglomerate, and mostly comprise reworked volcanic rocks and some clasts of Palaeozoic quartzite, chert and shale. The volcaniclastic rocks include clast-rich breccia and fine pumiceous ash-flow tuffs. Dacitic lava flows unconformably overlay the volcaniclastic sequence. The volcanic sequence has a consistent dip of 10 to 45°SE, except where disrupted by faulting. In the vicinity of the dacite intrusions, sedimentary rocks are deformed and layering dips in a variety of directions. Porphyritic biotite-hornblende dacite dykes, sills and small stocks intrude the volcanic rocks. These intrusions are in contact with the lithic-pumice lapilli tuff along faults. Contacts are marked by clay-rich rubble zones (Hunsaker, 2018).
The Tuscarora district contains two deposit types (Emmons, 1910; Nolan, 1936). A silver zone occurs to the north and is characterised by relatively high Ag/Au ratios, narrow alteration zones, and quartz-carbonate veins, commonly hosted by early porphyritic dacite. A gold zone is found to the south and has relatively low Ag/Au ratios, more widespread alteration, quartz-fissure veins, and areas of stockwork veining, commonly in tuffaceous sedimentary rocks. Although other differences are recognised, Ag/Au ratio is the most distinctive difference between the zones.
The Tuscarora silver zone covers an area of ~1200 x 1500 m centred ~500 m NW of Tuscarora. Mineralisation in this area is typified by multiple vein types including quartz + sulphide veins, carbonate + sulphide ±quartz veins, and late-stage comb-quartz veins. Carbonate is all calcite, although minor iron, magnesium and manganese are indicated in the calcite. Sulfides are predominantly pyrite, although base metal- and Ag-bearing sulphides are common. Iron and manganese oxide and hydroxide minerals are locally abundant, while locally abundant Ag halide minerals have been reportes (Castor, et al., 2003).
The Tuscarora gold zone occupy a NE_SW elongated area of ~1500 x 3000 m to the SW of the Silver Zone. Host rocks are tuffs, lavas and intrusions of the Pleasant Valley volcanic complex and early porphyritic dacite. In addition to relatively low Ag/Au ratios, the ore is characterised by stockwork to sheeted or en echelon fissure veins of finely granular to comb quartz. Carbonate is absent, although quartz after bladed carbonate crystals are locally reported. Sulphides are common, mainly pyrite, in unoxidised rock, whilst arsenopyrite is alos locally observed. Base metal sulfides are rare. The most productive historical working was the dexter Mine, where Ag and Au mineralisation is associated with widespread silica and adularia alteration in lapilli-fall or pumice-flow tuffs and fine-grained tuffaceous sedimentary rocks, which dip moderately to steeply SE. Crosscutting early porphyritic dacite occurs beneath and along the north wall of the open pit. In the latter area a large mass of this dacite overlies the tuffaceous rocks along an east-striking contact that dips shallowly to moderately northward. The overlying dacite is cut locally by shear zones with adularia-silica alteration, although the dacite is mostly propylitic altered and typically not a host to ore. The deposit contained numerous randomly oriented veinlets, in some cases containing adularia, and locally rich in Au within a strongly fractured, north-dipping, 400 × 60 m zone with pockets of rich ore. Veins and veinlets include randomly oriented limonitic comb quartz ±adularia ±pyrite, quartz-cemented breccia with pyrite, and quartz-free veinlets of limonite, hematite and Mn oxide. Ore minerals are electrum, acanthite and secondary minerals. Pyrite is replaced or rimmed by acanthite ±argentojarosite with interlayered textures that indicate alternating sulphide and oxide deposition during supergene Ag enrichment (Castor, et al., 2003).
Both silver and gold deposit types at Tuscarora formed at 39.3 Ma, contemporaneous with the only major intrusive activity in the volcanic field. This date falls within the age range at which most Carlin-trend gold deposits formed in the Eocene, generally between 42 and 36 Ma (Arehart et al., 1993; Thorman et al., 1995; Emsbo et al., 1996; Leonardson and Rahn, 1996; Phinisey et al., 1996; Groff et al., 1997; Hall et al., 1997, 2000; Teal and Jackson,1997; Hofstra et al., 1999; Hofstra and Cline, 2000; Johnston, 2000; Ressel et al., 2000; Tretbar et al., 2000; Cline, 2001).
The most recent source geological information used to prepare this decription was dated: 2018.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Castor, S.B., Boden, D.R., Henry, C.D., Cline, J.S., Hofstra, A.H., McIntosh, W.C., Tosdal, R.M. and Wooden, J.P., 2003 - The Tuscarora Au-Ag District: Eocene Volcanic-Hosted Epithermal Deposits in the Carlin Gold Region, Nevada: in Econ. Geol. v.98, pp. 339-366.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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