Coeur Rochester Mine, Rochester Mining District, Pershing County, Nevada, USAi

Regional Level Types
Coeur Rochester Mine	Mine
Rochester Mining District	Mining District
Pershing County	County
Nevada	State
USA	Country

A large Ag-Au occurrence/open pit mine centered on the Nenzel Hill area. See entry for Nenzel Hill. A subsidiary pit is at the site of the Nevada Packard Mines.

Structure: Rocks are folded into a broad, asymmetrical anticlinal arch. There are 2 major normal fault systems and post- mineral faults.

Alteration: Silicification and oxidation are prevalent in the deposit.

Commodity: Ore Materials: Pyrite, sphalerite, argentian tetrahedrite, arsenopyrite, chalcopyrite, galena, covellite, chalcocite, stromeyerite, polybasite, pyrargyrite, acanthite, argentite, argentojarosite, chlorargyrite, embolite, native silver, chalcophanite, melanterite, anglesite, manganese oxides, chalcanthite, pyrrhotite, teallite, owyheeite, electrum Gangue Materials: quartz, k-feldspar, sericite, limonite, hematite, goethite, clays

Deposit: Mineralization is distributed within an area of regional quartz-sericite-pyrite alteration of the rhyolitic volcanics of the Permian-Triassic Koipato Group. Two Late Cretaceous intrusive events took place in the Humboldt Range, and K-Ar dates suggest that the silver mineralization was related to one or perhaps both events. Low-grade silver-gold mineralization occurs in two stages in thin, randomly oriented, closely spaced fractures associated with major veins along high-angle faults. The Rochester deposit has some of the characteristics of the adularia-sericite USGS model, but studies show that deposit was probably formed at greater depth (>12,000 feet), temperature (300-400c), and pressure (1kb) than adularia-sericite model. It has a low volume of clay and no evidence of boiling. It is possibly a mesothermal deposit; richer veins (like east and west veins) are genetically linked with lower grade, higher tonnage ore, but structural differences are reported. Gold grade is expected to rise near the end of the mine life as a high-grade vein is mined near the present tertiary crusher. Most Rochester ore is disseminated in stockworks and siliceous breccia. High-grade, fault-controlled veins provide smaller quantity but richer ore. Veins are replacement, tight (few vugs), closely spaced and often laminar. N-S veins cut E-W veins. Two broad west vein zones and at least 6 separate east vein sets have been identified. Intersections of these zones are excellent hosts, especially when the vein set intersections cut the Weaver/Rochester contact. During early mining (1912-1915), the East and West veins averaged 6 feet in width and carried 12 ounces of silver and 0.100 ounces of gold per ton. Veins narrowed (and some dips flattened) and became lower grade with depth. Model name could also be Comstock epithermal veins. Gold and silver are strongly zoned. The deposit has a silver-rich core, following northeast-trending structures, with a peripheral gold halo. Gold shows a stronger affinity for the Rochester/Weaver contact. Quartz is the most common host mineral. Unoxidized blocks of ore are found throughout the deposit and drilling has shown oxidation continuing to at least 1,000 feet below Nenzel Hill.

Deposit type: Epithermal vein, Comstock

Development: In 1905 a group of prospects was located on Nenzel Hill which later became Rochester Mines in 1912, one of the largest producers in the district. Early (1912 ) underground mining of the deposit occurred on the East and West veins on the northwest side of Nenzel Hill. Between 1912 and 1913, ore had to average more than 35 ounces of silver per ton to justify shipping costs, but by 1915, ore assaying as low as 8 ounces of silver per ton could be extracted profitably, due to the construction of a cyanide mill and the Nevada Short Line railway. A 12,016 foot tramway was built in 1917 to haul ore from the foot of Nenzel Hill to the mill in Lower Rochester Canyon. Major production ended by 1923, caused by grade and price fluctuations and by litigation. Limited mining continued until 1928. In 1929 it closed down. Approximately $7 million in silver and gold was produced from veins in the Rochester District between 1912 and 1928. The veins are opened in depth by three long adit tunnels. In 1960-69, Silver State Mining Co held the property, and from 1969-1984 ASARCO owned it and drilled 485 holes totalling 160,000 feet. In 1983 Coeur d'Alene mines purchased property and began to operate it as an open pit, heap leach mine in 1986. Relatively uniform grades and simple metallurgy, minimal stripping, and downhill haulage help keep production costs low. 1993 recovery rates averaged 55% of the silver and 80-85% of the gold in heap leach pads. In 1993, the mine employed about 290 people. Weekly ore production averaged 100,000 troy ounces of 98% silver. Concurrent reclamation is being done on some former leach pads and mine dumps. Remaining mine life was 10 years in 1993 but Coeur has been successful in finding additional mineable silver mineralization at both the Rochester mine, and the nearby Nevada Packard property. A total of 61,500 feet were drilled in 161 holes around the margins of the Rochester mine. Calculations are in progress to bring this new mineralization into reserve status. At the end of 1999 the reserve at Rochester was 48.3 million tons averaging 1.09 opt Ag and 0.01 opt Au. Coeur's Rochester Mine reached a major milestone in the third week of January 2002 by pouring more than one million ounces of gold and 88 million ounces of silver since commencing production in 1986. Mining at the Nevada Packard satellite deposit, just 1.5 miles to the south was to begin early in 2003 with road construction and development of access to the pit underway. In 2003 Coeur Rochester filed a proposal to include expansion and deepening of the existing Rochester Open Pit, increasing the height of the heap leach pad, new haul roads, and closure/reclamation plans. The Coeur-Rochester mine in Pershing County remains the state?s largest silver producer at 5.7 million ounces in 2005.

Geology: Quartz-sericite-pyrite alteration is believed to be associated with emplacement of stocks, particularly those in western Pershing County. The regional anticline trends NNW with the axial plane dipping west. Bedding dips shallowly east, as indicated by epiclastic Weaver units. Both the Weaver and Rochester formations are parts of the Permian-Triassic Koipato group. Quartz-sericite-pyrite alteration is associated with mineralized quartz veins in the deposit and is difficult to distinguish from the regional-type hydrothermal alteration. Au, Ag, Sb, As, Hg, and S concentrations in mineralized Weaver rhyolite increase sharply with degree of alteration. K/Ar dates of the deposit range from 85 to 114 Ma, averaging 99 Ma. At least 3 trends of post-mineral movement have been noted: (1) E-W, dipping steeply N or S; (2) N30W-N50W, Dipping 30-70W, (3) N20W-N20E, 70-80E (Late Miocene Basin and Range).

Ore(s): High angle faults control ore formation.

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Link to USGS MRDS:	10310354

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References