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Rochester

Nevada, USA

Main commodities: Ag Au
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The Rochester gold deposit is located within the Humboldt Range of central-western Nevada, like the Relief Canyon and Florida Canyon deposits which are 12 km to the south and 32 km to the north respectively. It is approximately 85 km to the SSW of the town of Winnemucca and is in Pershing County, Nevada, USA.

The Rochester district encompasses a number of precious metal vein deposits which were mainly exploited between 1912 and 1928. The largest was at Nenzel Hill where a larger, bulk-low grade Ag-Au deposit has more recently been defined. Both the veins and the bulk low grade mineralisation are hosted by rhyolites of the Permo-Triassic Koipato Group (Douglas, 1987).

The Koipato Group includes mafic to siliceous volcanic and intrusive rocks which are scattered throughout north-central Nevada. Overlying Mesozoic sediments, which had a pre-erosional thickness of 3500 m in the Rochester district, have been severely eroded during the Basin and Range deformation. The host Koipato rhyolites have been hydrothermally altered on a regional scale to quartz-sericite-pyrite. This alteration is interpreted to be related to the emplacement of the co-magmatic Koipato intrusives and late Cretaceous stocks. The introduction of precious metals has been assumed to have been related to the late Cretaceous event (Douglas, 1987).

The major veins at Nenzel Hill comprise two mineral assemblages which fill and replace faults and strongly fractured rhyolite. The stage 1 veins consist of quartz, K-feldspar and base and precious metal sulphides. These are confined to the deeper portions of the Nenzel Hill mineralised zone. Stage 2 veins have a similar minerals assemblage, but cut stage 1 veins and display a strong vertical zonation. With increasing elevation Ag becomes more abundant and sericite becomes prevalent. The bulk-low-grade mineralisation consists of 80% oxidised, and 15% un-oxidised assemblages of quartz, K-feldspar and base and precious metal sulphides which fill thin, randomly oriented, closely spaced fractures between major veins. It has been suggested that buffering of the ore bearing media by an assemblage of K-feldspar-sericite-quartz in the wall rocks allowed dispersion of metals over a greater distance forming a large volume low grade deposit rather than confined veins. Studies indicate that the mineralisation formed at a temperature of around 300°C and a pressure of 1 kb (Douglas, 1987).

Published reserve and resource figures are:
    65 Mt @ 0.34 g/t Au, 47 g/t Ag (Reserve, 1986, Douglas, 1987).
    9.3 Mt @ 0.38 g/t Au, 29.5 g/t Ag (Proven + probable reserve, 2006, Coeur d'Alene Mines. 2006).
    14.25 Mt @ 0.38 g/t Au, 35.3 g/t Ag (Indicated + inferred resource, 2006, Coeur d'Alene Mines. 2006).

Remaining reserves and resources at the end of 2015 (Coeur Mining Inc, 2016) were:
    Proved + probable reserves - 136.7 Mt @ 18.2 g/t Ag, 0.1 g/t Au;
    Measured + indicated resources - 127.87 Mt @ 16.5 g/t Ag, 0.1 g/t Au;
    Inferred resources - 56.07 Mt @ 17.8 g/t Ag, 0.1 g/t Au.
  Resources are exclusive of reserves.

The most recent source geological information used to prepare this summary was dated: 2006.    
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:
Douglas I,  1987 - The Rochester district, Pershing County, Nevada: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada    pp 175-196


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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