Date of Award

Fall 12-12-2025

Degree Type

Thesis

Degree Name

PhD Earth Science and Engineering

Department

Earth Science

Committee Chair

Christopher H. Gammons

First Advisor

Colleen G. Elliott

Second Advisor

Karen Lund

Third Advisor

Courtney A. Young

Fourth Advisor

Stella Capoccia

Abstract

The Philipsburg district, renowned for its rich mining history, has contributed significantly to Montana's mineral production, ranking as the state's second-most productive district. Active from 1865 to the 1980s, it yielded an impressive array of Ag (50-60 million oz), with notable outputs of Au, Zn, Pb, Cu, and battery-grade MnO2. The district features Cordilleran polymetallic veins, some of which are along bedding in Mississippian and Devonian limestone, and others are steeply dipping quartz veins in silicate-rich sedimentary rocks and in the Philipsburg batholith. The veins exhibit a district-wide mineral zonation, marked by Cu- and Ag-rich ores near granite porphyries, with increasing concentrations of Zn, Pb, and Au as the distance from the porphyries increases. The zonation is akin to Butte, Montana, and other well-studied Cordilleran polymetallic vein deposits. Veins in the Tower area contain sphalerite that fluoresces bright colors under longwave UV (365 nm) light. LA-ICP-MS analysis reveals that the fluorescence is linked to high concentrations of Cu (up to 8500 ppm), Ge (up to 580 ppm), Ga (up to 5000 ppm), In (up to 2100 ppm), W (up to 2800 ppm), and Ag (up to 100 ppm). The fluorescence coincides with a high sulfidation state as indicated by the mineral assemblage low-Fe sphalerite, enargite, and tennantite. Temperature and pressure assessments from fluid inclusions suggest vein formation occurred at 200-500 °C and at about 5-6 kilometers deep. Stable sulfur isotope values near 0‰ δ34SVCDT in sulfide minerals suggest a magmatic origin for the mineralizing fluids. Two quartz-feldspar porphyry intrusions at the north of the district, and theorized to be part of the porphyry system that is the source of the polymetallic veins, were dated at ~66 Ma (U-Pb zircon). Zircon grains have trace element compositions that are consistent with the existence of a fertile Mo-Cu deposit. Although the age of the polymetallic veins could not be determined, the vein deposits are inferred to be associated with the emplacement of multiple bodies of granite porphyry concealed by the sedimentary rocks, especially below the high-sulfidation veins located in the center of the district. The potential for undiscovered porphyry Mo-Cu deposits underscores the need for further exploration in the Philipsburg district.

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