"THE RICHEST HILL ON EARTH"
The vast copper deposits in Butte, Montana are responsible for the production of one of the major mining districts of the worldboth underground and open pit. These operations began in 1864 and finally ended in June 1983 for a total of 119 years! Berkeley pit, located in NE Butte, was once the largest open pit mine in the world.--see a map of Butte, Mt.
Butte Copper Deposit Description:
There are two main vein types:Pre-main-stage veins Main-stage veinslow grade, dome-shaped constituted 80% of the mining disseminated copper molybdenum cut pre-main-stage veinsupper limit 850 m below surface mined to depths of 5000 ft.small veins, few 10's of feet 3 types of veins: Anaconda, few inches thick Blue fault, and Steward veins.alkali feldspar, quartz, sericte, biotite, Anaconda veins are most chalcopyrite, andalusite. productive, EW trendingquartz molybdenite, chalcopyrite, pyrite. chalcocite, enargite, bornite covellite, digenite, djurleite minerals in a --mineral reference glossary quartz-pyrite gangue. Blue fault veins are NW in trend and displace older Anaconda veins. Steward veins are dinky. There is an interesting zonal distribution of metals within the Butte deposits:Central Copper zone: chalcocite, bornite, covellite, enargite with quartz-pyrite gangue and some silver.Intermediate zone: chalcopyrite, sphalerite, galena, as well as copper minerals and rhodochrosite, rhodonite (Mn minerals).Peripheral zone: rhodochrosite, sphalerite, galena, silver, acanthite, stephanite, prousite, pyrargyrite, pearceite, and polybasite (no Cu minerals). So, looking from the inside out: Cu+Ag to Cu+Pb+Zn+Mn to Mn+An+Pb+Ag.
HOST ROCK DESCRIPTION AND HISTORY:
The host rock for the veins of the Butte district is the Butte Quartz Monzonite. This is part of the largest pluton of the Boulder Batholith, emplaced between 68 and 78 Ma.Quartz and orthoclase compose 45% of the rock, plagioclase makes up 40% and Biotite/hornblende compose 15-20% of the total rock. Emerging from this plutonare aplite and pegmatite dikes and irregular masses with hydrothermally altered rocknext to the veins. These are where the copper rich deposits are. Tectonic Background:The location of the Butte copper deposits is in the middle of the Northern RockyMountains. This is one of seven distinct provinces of the Cordilleran Orogenic System.
The Rockies really began with the Laramide Orogeny. This was the result of the Farallonplate buoyantly subducting beneath North America during the late Mesozoic to theearly Cenozoic. This formed major eastward thrust faults. There were few terrancesadded to the continent following the late Mesozoic accretionary period, though subduction continued along the west coast. This became more restricted in the Mid-Tertiary when the East Pacific Rise itself subducted causing the San Andreas faultto form in response to transcurrent and extensional tectonics.The dip angle of the subducting plate decreased from 45° to 25° around 75 Ma causing subduction related volcanics to move eastward into Colorado, Wyoming, and Montana. Volcanism in these places ceased when the subduction angle steepened again in the mid Tertiary shifting subduction related volcanics westward again. Pluton emplacement:There were several different sites/times and types of volcanism/plutonismduring the late Mesozoic to early Cenozoic. This was controlled in part by changesin tectonic style, particularly major accelerations of plate convergence (80-55 Ma) and changes in the nature of plate boundaries (30 Ma). These intrusive events represent major pulses of magmatic activity.The batholith intrusion which led to the copper deposits of Butte are part of the composite batholiths (tonalitic to granitic) that represent exhumed roots of ancient volcanic arcs. The border faults of these ancient calderas provided channels for hydrothermal fluids. The resulting quartz pegmatites appear to have been injected during the late stages of magmatic crystallization. Following the Laramide Orogeny, the Rocky Mountain Province was subjected to intenseerosion. This helped to bring the volcanic roots closer the the surface. A late Pliocene upliftbrought the Rocky Mountains to their current elevation and stature. Faulting during this time also helped to expose many of the valuable mineral deposits throughout the Rocky Mountain Province. sources used: Condie, Kent C. and Sloan, Robert C. Origin and Evolution of the Earth. Principles of Historical Geology. copyright 1998, by Prentice-Hall, Inc. French, Don E. and Grabb, Robert F. eds. 1989 Field Conference Guidebook: Montana Centennial Edition, Geologic Resources of Montana vol 1. copyright 1989, by the Montana Geological Society. Oldow, John S., Bally, Albert W., and Ave' Lallemant, Hans G. Phanerozoic Evolution of the North American Cordillera; United States and Canada. in The Geology of North America - An Overview. ed. by Bally, A. W. and Palmer, A. R. copyright 1989, by The Geological Society of America, Inc. Zeinen, Lester G., Berg, Richard B. and McClernan, Henry G. Geology of the Butte Mining District, Montana. in Rocky Mountain Section of the Geological Society of America Centennial Field Guide, vol. 2. ed. by Beus, Stanley S. copyright 1987, by The Geological Society of America, Inc.