Cove, McCoy

Nevada, USA

Main commodities: Ag Au
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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 comprises two distinct ore types: i). a central core of polymetallic vein-type ore, which consist of pyrite-sphalerite-galenaĞdominated 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 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).

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 Cove McCoy mines closed in early 2002.

The most recent source geological information used to prepare this summary 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.

  References & Additional Information
   Selected References:
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
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

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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