Coeur Rochester Mine

Site Name

Primary: Coeur Rochester Mine
Secondary: Rochester Mine
Secondary: Nenzel Hill
Secondary: East pit
Secondary: West pit

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Commodity

Primary: Silver
Primary: Gold
Secondary: Copper
Secondary: Antimony
Secondary: Lead
Secondary: Zinc

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Location

State: Nevada
County: Pershing
District: Rochester District

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Land Status

Land ownership: BLM Administrative Area
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.
Administrative Organization: WInnnemucca BLM Administrative District

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Holdings

Not available

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Workings

Not available

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Ownership

Owner Name: Coeur-Rochester Mines, Inc, wholly owned subsidiary of Coeur d'Alene Mines Corp.
Info Year: 2004

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Producer
Deposit Type: hydrothermal stringer zone
Operation Type: Surface-Underground
Year First Production: 1912
Discovery Year: 1905
Years of Production:
Organization:
Significant: Y
Deposit Size: L

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Physiography

Not available

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Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein
Model Name: Epithermal vein, Comstock

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Orebody

Form: tabular, pipelike, blanket, irregular, disseminated

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Structure

Type: R
Description: Rocks are folded into a broad, asymmetrical anticlinal arch.

Type: L
Description: There are 2 major normal fault systems and post- mineral faults.

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Alterations

Alteration Type: L
Alteration Text: Silicification and oxidation are prevalent in the deposit.

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Rocks

Name: Mixed Clastic/Volcanic Rock
Role: Host
Description: volcanosedimentary rocks
Age Type: Host Rock
Age Young: Early Triassic

Name: Rhyolite
Role: Host
Description: tuff
Age Type: Host Rock
Age Young: Early Triassic

Name: Trachyte
Role: Host
Age Type: Host Rock
Age Young: Late Permian
Age Old: Early Triassic

Name: Volcanic Breccia (Agglomerate)
Role: Host
Age Type: Host Rock
Age Young: Late Permian
Age Old: Early Triassic

Name: Rhyolite
Role: Host
Description: flows
Age Type: Host Rock
Age Young: Late Permian
Age Old: Early Triassic

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Analytical Data

Not available

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Materials

Ore: Pyrite
Ore: Chalcophanite
Ore: Melanterite
Ore: Anglesite
Ore: Chalcanthite
Ore: Pyrrhotite
Ore: Teallite
Ore: Owyheeite
Ore: Electrum
Ore: Feldspar
Ore: Sericite
Ore: Limonite
Ore: Hematite
Ore: Goethite
Ore: Silver
Ore: Embolite
Ore: Chlorargyrite
Ore: Sphalerite
Ore: Tetrahedrite
Ore: Arsenopyrite
Ore: Chalcopyrite
Ore: Galena
Ore: Covellite
Ore: Chalcocite
Ore: Stromeyerite
Ore: Polybasite
Ore: Pyrargyrite
Ore: Acanthite
Ore: Argentite
Ore: Argentojarosite
Ore: Clay
Gangue: Quartz

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Comments

Comment (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).

Comment (Identification): This record includes material from MRDS record M060371, an earlier record for the deposit. The Nevada Packard mine reserves are included with Coeur Rochester Mine, although the Nevada Packard mine is described in a separate record.

Comment (Location): The Coeur Rochester Mine is located on Nenzel Hill near the crest of the Humboldt Range.

Comment (Workings): Earlier mining was underground, especially along 3 long tunnels, but mining has been by open pit methods since 1986.

Comment (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

Comment (Commodity): Gangue Materials: quartz, k-feldspar, sericite, limonite, hematite, goethite, clays

Comment (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.

Comment (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 dore 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.

Comment (Economic Factors): Production The Coeur Rochester mine had record silver and gold production in 1997; a total of 6,690,704 ounces of silver and 90,019 ounces of gold from a record 8.7 million tons of ore. Average gold grade increased, also to 0.0119 ounces per ton, up from 0.010 the previous year. In 2002, the Coeur Rochester mine produced 6.4 million ounces of silver and 71,905 ounces of gold in 2002. Coeur Rochester Mine reached a major milestone in January 2002 by pouring more than one million ounces of gold and 88 million ounces of silver since commencing production in 1986. In 2003 the Coeur-Rochester mine in Pershing County remained the state's largest silver producer at just under five and one half million ounces produced in 2003. The Coeur-Rochester mine in Pershing County remains the state?s largest silver producer at 5.7 million ounces in 2005. Reserves At the end of 1997, mine reserves stood at 74,216,000 ounces of silver and 603,000 ounces of gold. Year-end 2000 reserves at Rochester and Nevada Packard combined totaled 54 million tons grading 0.93 ounces of silver per ton and 0.008 ounces of gold per ton, for almost 50 million ounces of silver and 410,000 ounces of gold 2003 reserves at Rochester totaled: 32.7 million tons of ore grading 0.01 ounces of gold per ton, 0.91 ounces of silver per ton in proven and probable reserves as well as 40.3 million tons of mineralized material grading 0.01 opt Au and 0.77 ounces of silver per ton

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References

Reference (Deposit): Strusacker, E., (ed.), 1993, Gold and Silver Deposits of Western Nevada: Geological Society of Nevada Special Publication No.18, 1993 Fall Field Trip Guide

Reference (Deposit): Schrader, F.C., 1914, The Rochester Mining District, Nevada: USGS Bulletin 580-M

Reference (Deposit): Knopf, A., 1924, Geology and Ore Deposits of the Rochester District, Nevada: USGS Bulletin 762, pp.78-81

Reference (Deposit): Bonham, H.F. and Hess, R.H., 1993, in, The Nevada Mineral Industry-1992: NBMG Special Publication MI- 1992-2002 p26

Reference (Deposit): Vikre, P.G., 1981, Silver Mineralization of the Rochester District, Pershing County, Nevada: Economic Geology, v.76, pp 580-609

Reference (Deposit): Johnson, M.G., 1977, Geology and Mineral Deposits of Pershing County, Nevada: NBMG Bulletin 89, Pp.78-80

Reference (Deposit): Tingley, J.V., 1985, Nenzel Hill, NBMG Sample 2645, Pershing County, Rochester District; NBMG Unpublished Mining District (Tingley) Files

Reference (Deposit): Nevada Bureau Of Mines And Geology, Mining District Files, Various Clippings, 1983-1985

Reference (Deposit): Minsearch Annual, 1984-1985, v. vii, pp 380-384; Metals Economics Group, Boulder, CO

Reference (Deposit): Bonham, H.F., Garside, L.J., Jones, R.B., Papke, K.G., Quade, J., Tingley, J.V., 1985, A Mineral Inventory of the Paradise-Denio and Sonoma-Gerlach, Winnemucca BLM District, Nevada: NBMG Open File Report 85-5

Reference (Deposit): NBMG, 1994, MI-1993

Reference (Deposit): Nevada Division of Minerals, 1994

Reference (Deposit): Coeur d'Alene Mines Corp.,1997 Annual Report.

Reference (Deposit): Long, K.R., DeYoung, J.H., Jr., and Ludington, S.D., 1998, Database of significant deposits of gold, silver, copper, lead, and zinc in the United States; Part A, Database description and analysis; part B, Digital database: U.S. Geological Survey Open-File Report 98-206, 33 p., one 3.5 inch diskette.

Reference (Deposit): Crafford, A.E.J. (ed.), 2003, GSN Road Log 44, Coal Canyon Road East from I-80 Exit 112.

Reference (Deposit): Coeur d'Alene Mines Corp, press release, 5/14/02

Reference (Deposit): The Mining Record, 8/13/02.

Reference (Deposit): Harvey, David, Chadwick, Thomas, and Krewedl, Dieter, 2002, Geology and Mineralization in the Rochester and Nevada Packard Deposits, in Geological Society of Nevada Fall 2002 Field Trip Guidebook, Precious Metals Deposits of the Humboldt Range, New Discoveries in an Old District; GSN Special Publication No. 36.

Reference (Deposit): Vikre, P.G., 1977, Geology and Silver Mineralization of the Rochester District, Pershing County, Nevada: Stanford University, Unpublished Phd. Dissertation

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Nevada Gold

Nevada has a total of 368 distinct gold districts. Of the of those, just 36 are major producers with production and/or reserves of over 1,000,000 ounces, 49 have production and/or reserves of over 100,000 ounces, with the rest having less than 100,000 ounces. Read more: Gold Districts of Nevada.