The Miller Creek, Homestake, and Daisy Pass resources contain 7,406,260 metric tons (t) of mineralized rock averaging 7.44 to 11.97 g/t Au and 0.44 to 0.90 percent Cu. They are spatially associated with the 41 Ma Homestake intrusive complex within the New World district near Cooke City, Park County, in south-central Montana. The Homestake intrusive complex consists of numerous dacitic, porphyry intrusive rocks, and phreatomagmatic breccia bodies that have intruded Cambrian sedimentary rocks and Precambrian crystalline rocks. Diatreme breccia cuts earlier intrusion breccia and contains small zones rich in pulverized and altered rock-flour matrix and volumetrically more abundant clast-rich zones. Clasts within diatreme breccia include Cambrian sedimentary rocks, Precambrian crystalline rocks, and porphyritic intrusive rocks. Although moderate mixing and milling of clasts is present within diatreme breccia, relict Cambrian stratigraphy is commonly preserved.
Alteration may be separated into three stages: initial contact metamorphism of sedimentary rocks related to the emplacement of the intrusive rocks and intrusion breccias, an early hydrothermal stage that occurred after emplacement of the intrusive rocks, and a late hydrothermal stage that is postdiatreme. Early hydrothermal alteration of sedimentary rocks and breccia is laterally zoned relative to the Homestake intrusive complex from narrow zones of proximal prograde skarn to more widespread distal calc-silicate hornfels. Calc-silicate hornfels consists of banded epidote, actinolite, chlorite, and orthoclase, lies stratigraphically above and below the mineralized replacement zones, occurs up to 3.3 km away from the intrusive complex, and is barren of Au-Cu mineralization. Intrusive rocks, intrusion breccia, and diatreme breccia are dominated by late, vertically zoned alteration assemblages that destroyed early alteration and consist of deep advanced argillic alteration (including hydrothermal breccia), intermediate-level intermediate argillic alteration, and distal upper-level propylitic alteration. Trace fine-grained epidote associated with early alteration is present locally in the mineralized Homestake replacement zones but is absent at Miller Creek.
Replacement zones within the Homestake resource are hosted by diatreme and intrusion breccia containing abundant clasts of Cambrian limestone and are spatially associated with intermediate argillic alteration. Tabular subhorizontal replacement zones at Miller Creek and Daisy Pass are hosted by dolomitic limestone of the Cambrian Meagher Formation at the contact with the intrusive complex and are spatially associated with sills and dikes with argillic and propylitic alteration.
Replacement zones associated with late alteration are dominated by magnetite + rutilated (i.e., rutile-bearing) quartz and were partially replaced by later pyrite, clear quartz, carbonate minerals, chlorite, clay, and chalcopyrite. Almost all magnetite in the mineralized replacement zones partially to completely replaced earlier acicular hematite and retains the distinct acicular habit. Native gold and electrum typically occur as micron-sized inclusions and fracture fillings in pyrite and, to a lesser extent, in chalcopyrite. The abundance of chlorite, carbonate, magnetite, and gold increase in the upper part of the hydrothermal system, whereas kaolinite, Cu/(Au × 1,000) and Ag/Au increase in the lower part. Homogenization temperatures of fluid inclusions in clear quartz associated with Au-Cu-Ag mineralization suggest a thermal zoning from the proximal Homestake resource (~369°C) to the peripheral Miller Creek resource (~344°C). A phreatomagmatic event and related structures created permeable zones and conduits for the hydrothermal fluids that selectively replaced nearby limestone-rich protoliths. The alteration mineralogy, geochemistry, fluid inclusion homogenization temperatures, and paragenesis of the replacement zones and breccia-hosted resources are similar, suggesting that they were related to the same hydrothermal system centered on the Homestake intrusive complex.