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Location: 2 km E from Butte, Montana, United States
600 Shields Ave.ButteMontana, United States59701
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Two orebodies within the Butte district have been evaluated and are considered feasible for open pit mining based on current economics and conservative economic projections.The polymetallic ore complex in Butte consists of different types of mineralization hosted in the late Cretaceous Boulder Batholith, composed mostly of Butte Quartz. The Butte copper porphyry deposit is best known as a high-grade vein system that was mined underground from the late 1800’s until the mid-1970’s (Gammons et al. 2006). With development of the Berkeley Pit commencing in the 1950s, the focus of mining shifted to a zone of Quartz-Sericite-Pyrite Alteration with pronounced supergene enrichment. In the 1980’s, mining was initiated in the Continental Pit in order to blend the high-arsenic Berkeley Pit ore with low-arsenic Continental ores, making the combined concentrate more marketable to smelters. The majority of the Continental Pit that lies east of the Continental Fault is within older mineralization that predates the Main Stage polymetallic vein system. Each of these deposits are geochemically different. The Continental Pit consists of lower copper and higher molybdenum grades, lower pyrite content, and higher calcite content than is found in either the vein system or the Berkeley Pit deposit.Continental Pit In the Continental Pit area, 160 to 200 feet of leached cap and overburden are draped over a massive copper-molybdenum orebody bounded on the west by the Continental Fault and on the east by the Klepper Fault. The deposit is a typical porphyry copper system except that the enriched zone is less pronounced than is commonly observed. Continental Primary Zone copper occurs as chalcopyrite in interlacing veinlet swarms and as disseminations in the Butte Quartz Monzonite, a granitic intrusive some 78 million years old. Continental Primary Zone molybdenum occurs in younger subparallel veinlet swarms and slicks that offset the early copper veinlets. More recent mesothermal veins overprint the early mineralization, but these contain mostly iron, lead, and zinc. Continental Primary Zone copper immediately below the leached cap is weakly enriched by surface weathering where secondary chalcocite occurs as coatings on pyrite and chalcopyrite.Central Zone The Central Zone orebody is situated between the Berkeley Pit and the Continental Pit and is bounded on the east by the Continental Fault. This major, north-south trending basin and range fault dips steeply to the west and offsets the Continental orebody by some 3,500 feet. Porphyry-style mineralization has been intercepted in the hanging wall of the Continental Fault at that depth and mapped in underground workings at elevations of 2,000 feet above mean sea level (amsl), beneath the vertical extent of the Berkeley Pit. As in the Berkeley Pit and in the Continental area, mesothermal veins overprint the early mineralization. In localized areas, shear couples have developed “horsetail zones” of minor importance compared to those along the Leonard-Belmont axis that were exploited by underground mining and ultimately mined out in the Berkeley Pit.Weathering of mesothermal veins and veinlets, horsetail structures, and porphyry-style mineralization, along with the supergene enrichment of the pyritic alteration halo associated with the deep copper-molybdenum mineralization, has resulted in a tabular orebody in the Central Zone conforming to the paleotopography. The Central Zone orebody is between 200 and 400 feet thick and is overlain by a leached cap ranging in thickness between 150 and 300 feet. The overburden includes a sequence of alluvial sand and gravel varying in thickness from 200 feet on the north end of the valley near the McQueen Booster Pump House to over 1,000 feet along Continental Drive.
