ALTERATION MINERALOGY, AIRBORNE GEOPHYSICAL SIGNALS, AND CRITICAL MINERALS POTENTIAL OF THE RADERSBURG AND GIANT HILL 7.5’ QUADRANGLES, SOUTHWESTERN MONTANA

New USGS-funded airborne geophysical datasets of the Boulder Batholith region in southwestern Montana provide an opportunity to investigate the magnetic and radiometric response of hydrothermal alteration in the well-mineralized Radersburg and Giant Hill 7.5’ quadrangles. This USGS-funded Earth Mapping Resources Initiative (Earth MRI) mapping and geochemistry project targets magmatic-hydrothermal mineral systems including porphyry, epithermal, skarn, and carbonate-replacement ore types, and allows for a critical minerals assessment of the eastern margin of the Boulder Batholith. Data generated from this project includes whole-rock geochemistry of fresh and altered igneous rocks and mineralization, geochronology, and ore and alteration mineralogy. Handheld field radiometric and magnetic susceptibility measurements have been made to ground-truth airborne surveys.

Laramide magmatic-hydrothermal activity altered and mineralized host rocks in the project area, including the Late Cretaceous Elkhorn Mountain Volcanics (EMVs), which are a well-exposed sequence of andesitic to rhyolitic lavas, ignimbrites, and volcaniclastic rocks. Hydrothermal alteration includes widespread epidote-chlorite alteration in the volcanic sequence, localized muscovite-chlorite, K-feldspar-biotite, and calc-sodic (albite-calcite) alteration, with calc-silicate (epidote-garnet-pyroxene) alteration at skarn occurrences. Broad halos of pyrite, pyrrhotite, and lesser chalcopyrite are associated with many of these alteration zones. The airborne radiometric signatures of fresh and altered EMVs and mineralizing intrusions provide useful local exploration tools, with the K/Th ratio highlighting sericitic and potassic alteration types in intermediate composition host rocks; this is corroborated by whole-rock geochemistry and handheld radiometric measurements. The critical minerals potential for the Radersburg and Giant Hill quadrangles includes W, Bi, and Te at porphyry-skarn occurrences, Zn and sphalerite-hosted Ga, Ge, and In at skarns and carbonate-replacement deposits, and magnetite-rich paleo-placers with minor REE content in the pre-EMV volcaniclastic sequence. Geologic and alteration mapping, geochemical sampling, and geophysical ground-truthing is ongoing in this project area.