Date of Award

Spring 2017

Degree Type

Thesis

Degree Name

Master of Science (MS)

Committee Chair

Chris Gammons

First Advisor

Larry Smith

Second Advisor

Katherine Zodrow

Abstract

The Butte porphyry Cu-Mo-Ag deposit has been considerably altered by supergene processes that enriched Cu and Ag, and mobilized other metals. Supergene enrichment provided early miners with excellent ore grades, and contributes significantly to current mining operations in Butte. The Continental Pit exhibits the classic leached and enriched vertical zonation that is common in near-surface porphyry systems around the world. Although studied by the Anaconda Copper Mining Company during the mid-20th century, significant and economically relevant questions remain regarding this deposit’s supergene heavy metal geochemistry and its relationship to the local hydrology and tectonic history.

This thesis investigates the paragenesis of supergene and leach-cap mineralogy, its distribution throughout the Continental Pit and provides recommendations for future mining. Mineral compositions and textures were determined using modern analytical techniques including portable X-ray fluorescence, shortwave infrared spectroscopy, Raman micro-spectroscopy, X-ray diffraction, and scanning electron microscopy/energy dispersive spectroscopy. Sulfur isotopic analysis of chalcocite and pyrite mixtures was performed at the University of Nevada-Reno, and copper isotopic analysis of various secondary minerals was performed at the University of Arizona.

Structural control of supergene geochemical zonation is apparent throughout the deposit; the distribution of hypogene veins and alteration assemblages, as well as faults and joints, leads to varying degrees of enrichment both vertically and horizontally in the Butte system. Supergene argillic alteration has resulted in areas of intense replacement of feldspars by clay minerals, commonly kaolinite. Major supergene ore minerals include chalcocite, acanthite, and native copper, with minor amounts of Cu-carbonates, phosphates, oxides, and sulfates. δ34S varies between +2.8 to +3.5‰ for mixtures of supergene chalcocite and hypogene pyrite; slightly depleted when compared to pure hypogene pyrite at +3.9‰. δ65Cu varied widely; with values of -5.5‰ for chalcocite, -1.2‰ for turquoise, +0.48‰ for malachite, and +7.3‰ for native copper. A ferricrete, located above oxidized Butte granite, and overlain by alluvial gravel, has been described and is interpreted as having formed due to acidic springs and seeps along the Continental Fault.

Significant exposures of secondary fracture-filling copper phosphates were identified, including the minerals turquoise and chalcosiderite. Arsenic has been released from hypogene arsenic-bearing sulfosalts and incorporated into clays and secondary phosphates with measured concentrations up to 0.15 wt% arsenic in turquoise from the supergene enrichment blanket of the Continental Pit. Geochemical modeling suggests, and field observations corroborate, that turquoise forms in the reducing zone below the water table, and chalcosiderite is an alteration product of turquoise that forms when an enrichment blanket is uplifted and exposed to oxygenated conditions. This paragenetic relationship provides an important tool for understanding the tectonic and hydrologic history of the Butte orebody, and could be applied as a prospecting guide.

Comments

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geoscience: Geology Option

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