GEOCHEMISTRY AND HYPERSPECTRAL MINEROLOGY OF THE BOULDER PORPHYRY CU-MO PROSPECT, SOUTHWESTERN MONTANA

The Boulder Porphyry Prospect is located in the heart of the Late Cretaceous Boulder Batholith in southwestern Montana, and is within a NNE-trending structural lineament that extends between the world class Butte porphyry-epithermal system and the Montana Tunnels epithermal Au-Ag deposit. Porphyry-style mineralization and alteration is hosted at depth in the Butte Granite, and is concealed in a possible fault-bounded caldera filled with at least 700 remaining meters of the Eocene Lowland Creek Volcanics (LCVs), dominantly rhyolitic ignimbrites. This property has had 14 surface holes drilled, of which 12 holes intersected porphyry-style veining and alteration at depth. Early potassic alteration with biotite, K-feldspar, and magnetite is overprinted by quartz-pyrite±chalcopyrite veinlets with quartz-white mica halos; disseminated sulfides are common and are accompanied by locally intense white mica-kaolinite alteration.

This project prioritized 39 core samples from one 3000-foot deep drillhole for hyperspectral shortwave infrared (SWIR) scanning with a HySpex imaging system at the Center for Advanced Subsurface Earth Resource Models (CASERM). Preliminary results from this work identify two dominant alteration assemblages: (1) carbonate, montmorillonite, illite, epidote, chlorite, and biotite, with relatively less white mica, and (2) a white mica and kaolinite-dominated assemblage. These zones alternate multiple times with depth in the drillhole, suggesting complicated overprinting relationships and structurally controlled zones of hydrothermal fluid flow.

Planned future work includes S-isotopes of sulfide and sulfate minerals,, fluid inclusions in quartz veins, and a comparison with existing data for nearby deposits, including Butte. MicroXRF scanning is planned for a subset of core slabs, and a handheld pXRF will be used to guide selection of samples for future petrography work. This porphyry property is located within a recently-flown USGS-funded magnetic and radiometric survey, with published results anticipated in early 2024.