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Cove, McCoy |
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Nevada, USA |
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Ag Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The McCoy skarn gold deposit is located in the northern Fish Creek Mountains, Lander County, Nevada, USA, approximately 45 km to the south-west of the town of Battle Mountain, and 28 km SSW of the Fortitude skarn gold deposit of the Battle Mountain mine. The Cove deposit is located approximately 1.6 km from the McCoy orebody.
Mineralisation at McCoy is predominantly hosted by limestones and siltstones of the Triassic Augusta Mountain Formation, with minor amounts in the basal conglomerate of the overlying Cane Spring Formation. The skarn is related to a complex of dykes and sills on the periphery of the Eocene Brown stock which is of granodioritic composition. This igneous activity took place in two pulses. The first occurred at ~41.5 Ma and consisted of relatively oxidised magnetite-series magma, forming the magnetite bearing biotite>hornblende granodiorite central stock at McCoy and related dykes that extend outwards to the Cove deposit, producing subeconomic skarn at McCoy. The second pulse consisted of relatively reduced ilmenite-series magma, producing a subordinate, ilmenite bearing hornblende>biotite granodiorite and resulted in the formation of economic skarn ore at McCoy (Johnston, et al., 2008). Both have a bi-modal, porphyritic texture. Hornfels and skarn development proceeded through the following stages, in chronological order: i). formation of biotite hornfels; ii). development of an Fe-poor skarnoid in sediments and quartz-pyroxene alteration in the igneous rocks; iii). alteration to a prograde Fe-rich skarn which was generally composed of garnet>pyroxene ±epidote ±K-feldspar; in detail its composition was controlled by lithology, with limestone producing garnet>>pyroxene, and siltstone forming garnet+pyroxene+epidote+K feldspar; iv). a retrograde assemblage of chlorite > amphibole ±biotite ±epidote; v). quartz+pyrite ±adularia alteration; and vi). argillisation & oxidation (Brooks, et al., 1990).
Gold mineralisation accompanied prograde and retrograde skarn development and the quartz-pyrite-adularia stages, although most accompanies the retrograde and quartz-pyrite-adularia stages. The highest grades occur where the quartz-pyrite-adularia stage overprints the skarns. The ore is found in the proximal garnet rich skarn in contrast to the Fortitude orebody at Battle Mountain. The sulphide content of the ore is generally >2%, with pyrite<<chalcopyrite, pyrrhotite, sphalerite and galena. Gold occurs as electrum with associated anomalous As, Bi and Te, moderately anomalous Se, Zn and Pb and correlates with the higher sulphide developments (Brooks, et al., 1990). The ore zone follows the intrusive contacts and persists to a depth of 200 m and up to 10 m outwards from the contact (Bonham, 1988). The ore appears to persist up to 50 m from the intrusive contact.
The Cove deposit is hosted by limestone with lesser dolostone and clastic units of the middle to early Late Triassic Augusta Mountain Formation, and locally by Eocene porphyritic granodiorite dykes and sills. It is structurally connected to McCoy by a series of NE striking faults and comprises two distinct ore types: i). a central core of polymetallic vein-type ore, which consist of pyrite-sphalerite-galena dominated Au- and Ag-bearing veins, veinlets, stockworks, crustifications and disseminations within clastic and carbonate beds and locally in the intrusions and ii). an outer aureole of what some authors interpret as Carlin-style ore which is relatively rich in Ag, which comprises disseminated Fe ±As sulphides with arsenian, argentiferous and auriferous components ±native Au-electrum in silty to sandy carbonate units (Johnston, et al., 2008). The ore averages 5% sulphides, but may locally contain up to 90% (Bonham, 1988).
Alternatively it has been interpreted as a structurally controlled intermediate sulphidation epithermal system with abundant silver-rich base metals. The upper high grade zone comprised a swarm of thin base metal veins that followed bedding in the upward coarsening Panther Canyon fluvial sequence. This zone was controlled both by drag folding in the footwall of the Lighthouse Fault, as well as a bedding-controlled intrusive sill which capped the mineralisation. High grade orebodies occurred in the hanging wall of the major NE trending faults. Alteration comprised illite-sericite grading to distal propylitic, that was most obvious in intrusive dykes. The principal visible sulphides in veins were sphalerite-galena-pyrite, although petrography has revealed ~30 sulphides and sulphosalts and native elements, some carbonates and oxides, including acanthite, canfieldite, cassiterite, chalcopyrite, cinnibar, electrum, friebergite, gersdorffidite, kesterite, marcasite, pearcite, pyrargyrite, pyrrhotite, rhodochrosite, native silver, several varieties of stannite and tetrahedrite. The sulphide veins are generally free of gangue such as quartz, and primarily comprise friable sulphides. Paragenesis consisted of early metal-poor micro-veins that were cut by native gold-quartz veins, which were, in turn, cut and offset by the main stage sulphide veins. (This paragraph was based on a Linkedin post dated 2 Feb 2023 by Consulting geologist Rick Streiff who had extensive experience working on the mine from 1990 to 1994, and later throughout the Great Basin in Nevada.)
Massive sulphide mantos have been drilled below the main high grade mineralisation in the open pit at Cove, which are, in turn, apparently underlain by quartz-chalcocite veining with electrum and base metals (Hill, 2023).
Published resource figures include.
63 Mt @ 1.2 g/t Au (Proven + Probable Reserve, McCoy + Cove, 1994, AME, 1995).
17.2 Mt @ 1.27 g/t Au = 22 t Au (Reserve, McCoy, 1986, Brooks, et al., 1990).
12.3 t Au, 389 t Ag (Production, Cove + McCoy, 1993, Nevada Bureau of Mines, 1994).
5.1 t Au, 383 t Ag (Production, Cove + McCoy, 2001, Nevada Bureau of Mines, 2001).
120 t Au, 5135 t Ag (Pre-mining, in situ reserves at Cove - Johnston, et al., 2008).
The combined production from both mines amounted to 109.636 tonnes of gold and 3578 tonnes of silver (USGS).
The Cove McCoy mines closed in early 2002.
The most recent source geological information used to prepare this decription was dated: 1996.
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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Johnston M K, Thompson T B, Emmons D L and Jones K, 2008 - Geology of the Cove Mine, Lander County, Nevada, and a Genetic Model for the McCoy-Cove Hydrothermal System: in Econ. Geol. v103 pp 759-782
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Kuyper B A, 1988 - Geology of the McCoy gold deposit, Lander County, Nevada: in Schafer R W, Cooper J J, Vikre P G (Eds), 1988 Bulk Mineable Precious Metal Deposits of the Western United States Geol Soc of Nevada, Reno, pp 173-185
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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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