Rio Tinto mine (Mountain City Copper mine), Mountain City Mining District, Elko County, Nevada, USAi

Regional Level Types
Rio Tinto mine (Mountain City Copper mine)	Mine
Mountain City Mining District	Mining District
Elko County	County
Nevada	State
USA	Country

Structure: The Roberts Mountains Thrust fault is exposed at the surface less than 3 miles from the mine. faults

Alteration: Alteration types affecting the host rocks are chloritic, argillic, and silicic.

Commodity: Ore Materials: chalcopyrite, bornite, chalcocite, covellite, sphalerite, galena, malachite, cuprite Gangue Materials: quartz, pyrite

Deposit: The orebody is a hydrothermal exhalative massive sulfide replacement in the upper plate of the Roberts Mountains thrust fault. The original hypogene ore body consisted of quartz-chalcopyrite-pyrite and minor amounts of sphalerite and galena. The ore body is stratabound, it is limited to a particular sequence of black and gray shales with minor quartzite lenses. Supergene processes produced a gossan which was 400 feet by 100 feet at the surface and extends to a depth of 200 feet. Starting at approximately the 200 foot level and extending locally to below the 400 foot level secondary copper sulfides have formed. Locally these form a blanket from a few inches thick to 50 feet thick and in spots running as much as 50 percent copper. Below the zone of secondary sulfides is the primary quartz-chalcopyrite, pyrite ore. The ore body occurs in the upper plate of the Roberts Mountains thrust in eugeosynclinal rocks transported from the west. The primary orebodies are lenticular in shape and are composed largely of quartz, pyrite, and chalcopyrite. The ore lenses, in general, strike northwestward and dip northward; they are in shales with associated minor quartzite lenses. The ore is epigenetic, and the principal orebody was leached to the 200-ft level; supergene copper sulfide ore was immediately below the barren gossan. THE RIO GRANDE COPPER "WEST" CLAIM GROUP CONSISTS OF 4 CLAIMS: GOLDEN COPPER AND GOLDEN COPPER NOS. 1-3. THERE ARE CONTIGUOUS WITH THE WEST END OF THE RIO TINTO CLAIMS. THE RIO GRANDE COPPER "EAST" CLAIM GROUP CONSISTS 11 CLAIMS: RIO GRANDE NORTH, RIO GRANDE EXTENSION, RIO GRANDE EXTENSION NO. 1, RIO GRANDE, RIO GRANDE NOS. 1-7. THESE ARE CONTIGUOUS WITH THE EAST END OF THE RIO TINTO CLAIMS AND PARTLY WRAP AROUND THE EAST END. THE 1989 ELKO COUNTY MASTER TITLE PLATS SHOW THAT THE "WEST" CLAIMS ARE INCLUDED IN A MUCH LARGER CLAIM GROUP KNOWN AS THE PAT CLAIMS. THE PAT CLAIMS INCLUDE THE PAT NOS. 1-25, SPIRO CLAIMS, AND BJH CLAIMS. THE "WEST" CLAIMS ARE PARTLY INCLUDED IN THE LABERTA CLAIMS (ALBERTA-UTOPIA CLAIM GROUP) AND LUCKY NOS. 1-11 CLAIMS. PART OF THE "WEST^quot; CLAIMS ARE NOW ON OPEN OR UNCLAIMED GROUND (1989). ACCORDING TO EMMONS, P. 84, SEVERAL HUNDRED THOUSAND DOLLARS WORTH OF SILVER ORE, SOME LEAD, WAS TAKEN FROM THE MOUNTAIN CITY MINE IN THE 1870'S, THROUGH A SHAFT, NOW INACCESSIBLE.

Deposit type: Massive sulfide, Besshi (Japanese deposits)

Development: In 1919, Samuel Franklyn Hunt, filed claims on some traces of copper a few miles south of Mountain City. He named his claims Rio Tinto after the famed copper mines in Spain and spent the next 12 years attempting to get financial backing to develop the claims. He was grubstaked during this time by Walt and Jack Davidson who ran a store in Mountain City. In 1931, Ogden Chase became interested in backing the venture, and Hunt and Chase organized the Rio Tinto Copper Company with Samuel Hunt as president and Ogden Chase as manager. In 1932 an inclined shaft was sunk and ore was hit at 227 ft. Production began in 1932. The ore assayed at 40% and gradually rose to 47%.On June 30, 1932, the International Smelting and Refining Company, a subsidiary of the Anaconda Copper Company, purchased the Rio Tinto Copper Company for $300,000. The company was renamed the Mountain City Copper Company. A flotation mill was built in 1937 to process the ore. It closed in 1949. In 1965, G.M. Wallace Co. of Denver leased the property from Bieroth, and leached copper from old tailings. Cliff's Copper Corp.of Rifle, Colo. purchased the mine in the early 1970s and continued leaching operations and did some underground work, investigating the feasibility of in-place leaching. The 1960s and early 1970s leaching operations yielded another 7.7 million pounds of copper. The mine closed again in 1975 with the fall in copper prices. Cominco bought the mine in 1976 but did not initiate any mining. The mill was dismantled for scrap by the Loveland Construction Company in 1979. Ore processing during historic mine operations included generated tailings that were emplaced in the Mill Creek drainage. In 1993, due to concerns about environmental damage to surrounding surface waters, four of the previous owners and operators of the site jointly formed the Rio Tinto Working Group (RTWG) to remediate the site in cooperation with the State of Nevada, Department of Conservation and Natural Resource and the Division of Environmental Protection (NDEP). The Rio Tinto Working Group consists of the Atlantic Richfield Company, Cleveland Cliffs Iron Company, E. I. DuPont de Nemours and Company, and Cominco American Inc. The RTWG was able to fast track site investigation and remedial designs, leading to a timely start up of remedial construction activities. AFTER 2 YRS RIO TINTO PREPROD MINE WILL HAVE A TOTAL LIFE OF 20 YRS, C ARE FOR AN IN-SITU-LEACH, ALL COSTS FROM L. MOORES NOTES. REC-120 + 151 BASE YR OF COSTING 1970. PHASE 1 MINING OPERATION - IN SITU LEACHING OF MINE TO 500 ANNUAL PRODUCTION 4,800,000 LBS, LIFE- 10.4 YRS NOTE- PROD. STARTED 1972 , SINK WELL BETWEEN 300 TO 500 L ELARGE PRESENT CEMENTATION PLANT ACQ 0 EXP $ 0 DEV 0 EQPT 244,600 LEACH PLANT 180,000 WORK CAP 200,100 OP COST (ANNUAL DIRECT, IND, TAX, INS) $605,100 PHASE 2 MINING OP - IN SITU LEACHING OF LOW GRADE ORE BRO UNDERCUTTING ANNUAL PRODUCTION 4,000,000 LBS COPPER, 10 YR LIFE NOTE- NEW WORK CONSISTS OF UNDERCUTTING ORE BLOCK AND DRIL SOLUTION DISTRIBUTION HOLES ABOVE CAVED ORE. MINE EQPT AND DEVEL $6,077,400 245,900 OP COSTS (ANNUAL DIRECT, IND, TAX, INS) $821,30 RESERVE ESTIMATE GIVEN FOR INDICATED ONLY-75% PROB INTERVAL 50% P I = 25% P I = 10% P I = 1.33 X 75% P I Mountain City mine: discovery year - 1820's, year first production - 1869, year last production - 1947, production years - 1869-1947.

Geology: The Rio Tinto mine is developed in the Ordovician Valmy Formation. Several plutons of Cretaceous age intrude this and other Paleozoic formations; this sequence was eroded and covered with Miocene, Pliocene, and possibly older volcanic rocks. The primary mineralization of the area is believed to postdate the Mississippian Nelson Formation and pre-date the Cretaceous intrusives. The primary orebodies are lenticular in shape and are composed largely of quartz, pyrite, and chalcopyrite. The ore lenses, in general, strike northwestward and dip northward; they are in shales with associated minor quartzite lenses. The ore is epigenetic, and the principal orebody was leached to the 200-ft level; supergene copper sulfide ore was immediately below the barren gossan. The supergene enrichment of the ore may have required a large part of Tertiary time. THE MOUNTAIN CITY MINE ORE VEIN IS A FISSURE FILLING, CEMENTING ANGULAR FRAGMENTS OF LIMESTONE. AT SOME SITES THE VEIN IS SHEETED, CONTAING LIMESTONE PARTITIONS BETWEEN.

Ore(s): bedding

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Link to USGS MRDS:	10310316

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