CRITICAL MINERALS PROSPECTIVITY, GEOCHEMISTRY, AND ALTERATION MINERALOGY OF MAGMATIC-HYDROTHERMAL MINERAL SYSTEMS IN THE RADERSBURG AND GIANT HILL 7.5’ QUADRANGLES, SOUTHWESTERN MONTANA

The Radersburg and Giant Hill 7.5’ quadrangles are the focus of a new USGS-funded Earth Mapping Resources Initiative (Earth MRI) geologic mapping and geochemistry project in southwestern Montana. This focus area provides an opportunity to assess the critical minerals prospectivity of magmatic-hydrothermal mineral systems along the eastern margin of the Late Cretaceous Boulder Batholith, where mineralization styles include porphyry, epithermal, skarn, and carbonate-replacement ores. Preliminary geochemical sampling targeted inactive mines, intrusions, and hydrothermally altered rocks; a subset of samples has been scanned with the HySpex shortwave infrared (SWIR) hyperspectral imaging camera at the CASERM Core Research Laboratory. Recent Earth MRI airborne magnetic and radiometric surveys cover this area and provide important context for interpreting these mineral systems.

Hydrothermal alteration includes significant areas of epidote-chlorite alteration within the Late Cretaceous Elkhorn Mountain Volcanics (EMVs) and associated intrusions. Epidote replaces plagioclase and occurs in breccia clasts and matrix, vesicle infill in lavas, as veins, and more rarely as orbicules. Skarns in the area contain significant well-crystallized epidote, along with garnet, diopside, specular hematite, and Cu-Fe sulfides. At Diamond Hill, porphyry-style quartz±pyrite±chalopyrite±magnetite stockwork veining is associated with chlorite-muscovite alteration, and chlorite alteration near the intrusive contact with EMVs. The EMVs have locally experienced intense muscovite-pyrite-quartz alteration at the contact with the Diamond Hill Stock, and the Diamond Hill Au(W-Bi-Te) skarn occurrence is developed in the lower EMVs. Magnetite is locally abundant in stockwork veins that outcrop along the southern margin of the Diamond Hill Stock and appear to produce a distinct magnetic anomaly. A broad footprint of muscovite-pyrite±pyrrhotite alteration occurs at the Ohio-Keating mine area where a distinct potassium anomaly is observed. Carbonate replacement deposits in the area typically contain Pb-Zn±Cu mineralogy, with supergene additions of V and Mo. Geochemical and spectral mineralogy studies are ongoing and will continue to inform the metallogenic interpretation of this area.