Date of Award

Spring 2022

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geosciences

Committee Chair

Chris Gammons

First Advisor

Kaleb Scarberry

Second Advisor

Gary Wyss

Abstract

Butte, Montana, is a famous example of a zoned, polymetallic, porphyry-lode deposit. Whereas over a century of previous work has focused on the porphyry-style mineralization and centrally located, copper-rich lode deposits, relatively little has been written on the outer, silver-rich veins. The Marget Ann mine, in the northern portion of the district, was chosen for this study because of ease of access to the mine dumps as well as archived samples of high-grade vein mineralization. In addition, the Marget Ann mine is somewhat unique in Butte because gold has been found there and previously recorded in the veins. The main methods used in this study include optical mineralogy, scanning electron microscopy, and examination of fluid inclusions in quartz. The 76 Ma Butte Granite is the host to mineralization at Marget Ann. Primary ore minerals in the veins include pyrite, sphalerite, galena, chalcopyrite, Ag-bearing tetrahedrite, pearceite-polybasite, acanthite/argentite, jalpaite, electrum, and uytenbogaardtite. Gangue minerals include quartz, rhodonite, rhodochrosite, calcite, dolomite, adularia, and sericite. Supergene minerals include secondary acanthite, native silver, covellite, and unidentified Mn- and Fe-oxides. High-grade ore shoots containing electrum and fine-grained Ag-Au-Cu sulfides occur with rhodonite + rhodochrosite + adularia, and may have formed by boiling. In contrast, most of the base-metal sulfides and coarse, comb-textured quartz likely formed by simple cooling or fluid mixing. Overall, the mineral assemblage is indicative of an “intermediate sulfidation” style of vein deposit. Most of the fluid inclusions in quartz are simple two-phase, water-rich inclusions, and many are quite large (> 50 μm diameter). Homogenization temperatures range from 160 to 300°C (average of 251°C) with a salinity range of 0.5 to 7 wt% NaCleq (average of 2.2 wt%). No direct evidence for CO2 was shown in any of the inclusions, other than the occasional presence of anisotropic daughter minerals believed to be dawsonite. The overall spread in the data suggests mixing of a high temperature, higher salinity fluid (possibly a magmatic fluid) with a lower salinity fluid (heated meteoric water) during growth of quartz. The Marget Ann veins have a lower average homogenization temperature and salinity compared to the famous copper-rich Main Stage veins of Butte, which might indicate a greater involvement of meteoric waters on the outer edge of the district. This could also help to explain the higher gold content of the veins.

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