A copper mine started late 1989. Owned by Arimetco of Tucson, AZ.
Commodity: Ore Materials: chalcopyrite, pyrite, bornite, covellite, chrysocolla, cuprite, tenorite, malachite, azurite, chalcocite (minor), native copper, melaconite, magnetite. Gangue Materials: the ore body has low pyrite content, essentially no Au or Mo. Molybdenite has been seen in the mine only 12 to 15 times by Anaconda geologists. There is no Au, Ag, Mo trace element zoning. K-spar, biotite, quartz veining. Bornite is associated with high K-spar alteration. There is no trace of pyrrhotite, sphalerite, or galena in the ore body. In the root zone (present east end) albite flooding is recognized. The west end (top) was higher in pyrite, especially along certain structural zones. Along these zones a chalcocite enrichment blanket was formed. Bornite is present with chalcopyrite in the "middle" zone. The top (now west) is characterized by sericite, chalcopyrite, and pyrite. 50 % Of The Ore Is Copper Oxide Minerals. Production Was About 25,000 - 30,000 Tpd Ore, And An Equal Amount Of Waste Was Mined. Sulfide Finely Disseminated And In Narrow Seams

Deposit: General Location Is About 53 Km SE Of Carson. Mining Method Is Open Pit. Mill Is Dismantled. Past Prod. 177,000 Mt Cu From 144 Million Mt Ore (179). Pit Extends Omtp M/2 Sec. 21 And The Se/4 Sec. 17. The Yerington ore body has been tilted on its side, until the original top of the body is now at the west end. The tilting dips west 70 degrees, as measured by the Tertiary-pre-Tertiary contact which lies at this angle to the west of the mine. 6,000 to 7,000 feet of vertical section of the deposit is exposed because of this tilting. MacArthur forms part of the famous Yerington porphyry copper district which includes three deposits with historical resources totaling approximately 1.2 billion tons of ore averaging approximately 0.45% Cu. The root zone and core of the deposit produced oxide minerals high in copper-silicates with the silica derived from the decomposition of plagioclase. No chalcocite blanket developed because of low original pyrite content of the deposit. Chrysocolla is the dominant oxide mineral here. About half of the ore is copper oxide minerals. Production was about 25,000 - 30,000 tpd ore, with an equal amount of waste mined. Sulfide minerals are finely disseminated and in narrow seams. Major Alteration Types: Pre-main stage endoskarn (garnet, albite ? clinopyroxene); main stage potassic (biotite ? K-feldspar), sodic-calcic (actinolite, sodic plagioclase), propylitic (actinolite, chlorite); late stage sericitic, sodic, chloritic. Alteration Zoning: Likely contemporaneous central potassic with deeper and lateral sodic-calcic with peripheral propylitic; albitic (albite-chlorite) and sericitic alteration overprint the earlier alteration with sericitic alteration increasing at shallower levels. Major Vein types and Relative Ages: Common veins oldest to youngest in ore zone: quartz?K-feldspar; chalcopyrite ? pyrite or bornite ?quartz ? chlorite; epitdote ? quartz ? chlorite + chalcopyrite ? pyrite or bornite; epidote + quartz ? chlorite + chalcopyrite ? pyrite or bornite; pyrite ? quartz ? chalcopyrite ? tourmaline. Metal Zoning: central magnetite-bornite ?chalcopyrite, or ?chalcocite/digenite. Outer chalcopyrite- ?magnetite ?pyrite and outermost pyrite. Associated Deposits: Casting Copper (skarn), Douglas Hill (skarn), Bluestone (skarn), Mason Valley (skarn), McConnel Mine (skarn), MacArthur (porphyry), Bear-Lagomarsino (porphyry), Buckskin Mine (Au-Cu veins), Ludwig Mine (carbonate replacement/skarn)

Deposit type: Skarn Cu

Development: Copper mineralization was discovered in the Singatse Range west of Yerington in 1865 resulting in the development of many small hand-dug mines. In 1941, International Smelting and Refining Company, a subsidiary of Anaconda Copper Company, acquired the property. During the ensuing four years, exploration confirmed the presence of approximately 60,000,000 tons of ore with copper content ranging from 0.9% to 0.95%. In 1951, Anaconda took direct control of the Yerington site and began construction and installation of equipment for mining operations. Anaconda actively mined the site from 1953 into 1978. In 1977, the Atlantic Richfield Company (ARCo) purchased a number of holdings from Anaconda, including the Yerington site, just as it was planning to close down. ARCo closed the site on June 30, 1978. In 1978, ARCo sold all of its holdings at the site, including the townsite of Weed Heights, to Don W. Tibbals. Tibbals subsequently leased portions of the property site to other companies who conducted various mining and copper processing activities. From 1978 through the present, the U.S. Bureau of Land Management ( BLM), the U.S. Geological Survey and the Nevada Division of Environmental Protection (NDEP) and the U.S. Environmental Protection Agency (EPA) have identified a variety of environmental concerns at the Yerington mine site including contamination of groundwater with heavy metals and, some cases, radionuclides (uranium and thorium). Approximately half of the land at the Yerington mine site was and is currently managed by the BLM. In 1988, Tibbals sold a large portion of his land and unpatented mining claims to the Arizona Metals Company (dba Arimetco) who operated the site from 1989 to 1996, producing copper from a solvent extraction electrowinning (SX-EW) process. In January 2000, faced with government enforcement actions, fines, falling copper prices, and other financial problems, Arimetco abandoned the Yerington operations and declared bankruptcy. In March 2002, NDEP, BLM and EPA signed a Memorandum of Understanding (MOU) headed by NDEP but also defining how the agencies would work together in the oversight of the old mine site. In January 2005, the EPA agreed to become the regulatory lead agency for investigation and cleanup at the Yerington Mine site. EPA will continue to coordinate with NDEP and BLM as it proceeds forward ensuring the cleanup progresses. Primarily as a result of Arimetco's failure at Yerington, substantially stronger requirements for corporate guarantees as an acceptable instrument for financial assurance were adopted in 2001. Annual reviews of corporate guarantees were required by regulation in 2002. ?The BLM regulations governing mining were first adopted in 1981 and amended in 1998 and 2000. Among other things, these require quarterly inspections of mines on public lands to ensure compliance with all permit conditions. ?Every mining operation on public land is now subject to the National Environmental Policy Act, which provides opportunity for public comment during the planning process for a mine.

Geology: Silver Is Negligible: Gold And Silver Content Is Inconsequential The Yerington Ore Body Is A Porphyry Copper Deposit. It Is Roughly Conformable To Its Host, A Quartz Monzonite Stock That Pitches About 5 Degrees Nw. Originally, In Mesozoic Time, The Cylindroid Shaped Stock Was Emplaced As A Steeply Pitching Body With Its Long Axis Dipping To The East. Since Miocene Time, The Body Has Been Rotated 60 Degrees To The West Resulting In Its Present, Gently Pitching Position. The Northern Edge Of The Ore Body Dips Steeply To The North, While The Southern Edge Dips North At A More Modderate Angle. A North-South Cross-Sectional View Varies From A Flat-Lying Lens On The East To A Distorted Inverted V Shape Toward The West. General Size Of Ore Body Is Large. The "A" porphyry is altered quartz monzonite porphyry. The "B" porphyry is high in mafic minerals and plagioclase; it is pre-mineralization and accompanies the porphyritic quartz monzonite. Age dating (K-Ar) at Yerington indicates 150 Ma for all events related to intrusion and mineralization. -can't separate main intrusion from later mineralization age, etc. The host rock granodiorite and biotite and hornblende porphyries are indistinguishable by K-Ar age dating methods. The whole system evolved over 4-5 m.y. Copper is associated separately with each porphyry - that is, although the sulfides do not come in with the magma, they follow it very closely, and each porphyry has its own related copper mineralization. Contacts between different porphyry phases are knife-edge sharp, indicating multiple intrusion. Definite E -W-trending swarm of pre- and post-mineralization dikes. No deep rocks were brought to the surface by venting, etc....with the exception of the Yerington mine, essentially all of the districts production has been from rocks of Triassic age.

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