Date of this Version
Henderson Mountain, near Cooke City, Mont., about 4.5 mi northeast of the northeast entrance to Yellowstone National Park (the Park), hosts identified resources of at least 2.3 million ounces of gold, 8.9 million ounces of silver, and 130 million pounds of copper. The mineral deposits formed by selective replacement of calcareous blocks and clasts in a complex breccia pipe (the Homestake deposit), and concurrent skarn development replaced limestone adjacent to the breccia pipe (the Miller Creek deposit). The breccia pipe and mineral deposits formed in the middle Eocene during intrusion of the dacitic Homestake stock.
The geologic history of the Homestake and Miller Creek deposits involves Precambrian tectonics, Phanerozoic sedimentation, Eocene magmatism and hydrothermal processes, and Quaternary erosion and sediment deposition. The stocks of the Cooke City area (New World mining district) represent one of several mineralized centers of Eocene intrusive activity associated with the extensive Eocene Absaroka volcanic field in northwestern Wyoming and southwestern Montana. The Cooke City structural zone is a northwest-trending synformal zone developed in the Precambrian basement rocks of the Beartooth uplift. It was a zone of weakness that permitted calc-alkaline magmas to ascend through the crust during the Eocene to form mineralized centers. At the northern end of Henderson Mountain, intrusion of the Homestake stock during the middle Eocene was accompanied by metamorphism. The argillic Cambrian rocks locally were altered, whereas the interlayered Cambrian limestones were only weakly recrystallized and bleached. Intrusion breccias formed during magmatic stoping and injection of dacitic magma, and roof pendants composed of fractured Cambrian sedimentary rocks were lifted above the rising stock. Multiple events of hydrothermal shattering accompanied a flooding of relatively oxidized sulfur-rich fluids upward through intrusion breccias and shatter breccias (collectively and informally called the “Homestake breccia”). Intense argillic alteration formed in deep shatter zones, and limestone clasts and blocks in upper portions of the Homestake breccia were selectively replaced by silver- and copper-bearing sulfide minerals, coeval iron-oxide minerals, and trace native gold with silver. Lateral flow of the fluids into Cambrian limestones along the perimeter of the breccia pipe formed the stratabound and tabular Miller Creek deposit.
Mineral deposits of the Homestake breccia were the target of two significant mining ventures in the 1900s. An eastern part of the Homestake breccia was explored by a haulage adit in the early 1920s, and a power plant, a smelter, and an aerial tramway to haul ore from the mine to the smelter were nearly completed and then abandoned in 1925. The adit stopped a few hundred feet from high-grade gold deposits discovered later by drilling in the 1990s. An extensive drilling program in the northern New World district by Crown Butte Mines, Inc., led to the “blind” discoveries of the Miller Creek and Homestake deposits in 1989 and 1990. In 1990, Crown Butte Mines submitted a plan to State and Federal agencies to mine these deposits by underground methods. The proposed mining venture caused a strong, organized, anti-mine opposition that drew national attention. Because of the close proximity of the proposed mine to Yellowstone National Park, the environmental legacy of historic mines, and concerns about potential threats to the environment, the Federal Government negotiated a settlement with the mining company and the majority claim owner to halt mining in the area. On August 7, 1998, all properties and mineral rights were formally transferred to the U.S. Forest Service. Later, in August 1998, the U.S. Departments of the Interior and Agriculture announced a 20-year withdrawal of Federal locatable minerals in an area of 19,100 acres (7,735 hectares) surrounding Cooke City (the New World district and a buffer zone).