The Economic Petrology research lab has the overarching theme of using petrological techniques to investigate the interactions of fluids with rocks during ore formation.

Characterization of the geology, alteration and mineralization of the high-sulfidation epithermal deposits in Antelope Valley, CA:

Antelope Valley in Sierra County, CA has long been known to host a series of high-sulfidation epithermal features, known as the Golden Dome deposit. First explored in the 1970s, this deposit was determined to contain precious metal and native sulfur resources, though not in sufficient concentrations to be mineable in the current economic climate. Subsequent mapping and increased geotectonic understanding of the region have revealed a line of high sulfidation epithermal deposits hosted within the southern ancestral Cascades arc, from Goldfield, NV, up to the active hydrothermal system in Lassen Volcanic National Park, CA. The Economic Petrology lab group is working to characterize the Golden Dome deposit in detail through a combination of 1:24,000 scale field mapping (geology, alteration and mineralization), petrography (transmitted and reflected light), fluid inclusion studies and geochemical techniques.

This research is funded by a Student Award for Research and Creativity (to SC in Fall 2016) and by an EdMap award (to HMA in Spring 2017).

Distribution of precious metals in a porphyry deposit:

Determining the exact ore minerals that host an economic commodity is vital for understanding the processes by which the ore was deposited and for choosing an appropriate method to process the ore. The Ann Mason Cu-porphyry deposit in Nevada is in the economic assessment phase of mine development. The extractable resources of the deposit comprise mostly copper with minor molybdenum, gold and silver. Copper is primarily hosted by the Cu-bearing sulfide minerals chalcopyrite (CuFeS2) and bornite (Cu5FeS4), in veins or disseminated throughout the host rock. Molybdenum is hosted by the sulfide mineral molybdenite (MoS2), which formed later than the Cu-mineralization (Dilles et al., 2000). The precise locations of the precious metals in terms of host minerals and textural distribution is still unknown. The Economic Petrology lab group is working to determine the ore mineral assemblages that host the precious metals, and their textures to the surrounding rocks through a combination of petrography, EDS analysis by SEM, and WDS analysis by EPMA.

This research is funded by an Award for Research and Scholarly Creativity (to HMA in Spring 2016).

S10-142 ref light

Peridotite zone sulfides viewed under reflected light, surrounded by carbonate and silicate minerals. Abbreviations: Ccp chalcopyrite; Po pyrrhotite; Cpx clinopyroxene; Opx orthopyroxene; Cc calcite; Do dolomite.

Sulphide assemblages as a tracer for PGE-enrichment processes:

The Stillwater Complex in Montana hosts a world-class platinum-group element (PGE) deposit: the only significant source of platinum currently being mined in the USA. Although the J-M Reef (the sulfide-associated, Pt-Pd-rich ore zone of the Stillwater Complex) has been well characterized over a number of decades, little work other than bulk rock geochemistry has been carried out on the footwall and hangingwall rocks. During HMA’s doctoral research, she sought to investigate the “sulfur-poor” areas above and below the reef, with particular reference to the texture of the sulfides and their associated accessory minerals in an attempt to clarify the mechanism by which the platinum and palladium were concentrated into the J-M Reef.

This research was funded by Sigma Xi’s Grants in Aid of Research G20100315152413 and G20111015158486 (to HMA); the Geological Society of America’s Graduate Student Research Grant 9402-10 (to HMA); a 2012 student research grant from the Hugh E.
McKinstry Fund, from the Society of Economic Geologists Foundation (to HMA); a Duke University Undergraduate Research Independent Study Grant (to MLL); and a Duke University Dean’s Summer Research Fellowship Award (to KMF).

This work was published in Mineralium Deposita in July 2016.

Please find the article here.

Fig 3 - 32143-49 #11A

Electron microprobe composition map of sulfides in association with high temperature carbonates, from gabbronorite zone I in the Stillwater Complex. Abbreviations: Po pyrrhotite; Do dolomite; Cc calcite; Opz orthopyroxene; Cpx clinopyroxene; Qz quartz; Pl plagioclase.

High-temperature hydromagmatic carbonates in a layered igneous intrusion:

During analysis of the Stillwater samples by electron microprobe, HMA found these sulphide-associated carbonates. That they were formed at high temperature was inferred from the following lines of evidence: they demonstrate exsolution of calcite from dolomite; they are surrounded by a rim of Ca-rich pyroxene formed by reaction of the carbonates with their host orthopyroxene; they give temperatures of >950°C on the calcite-dolomite geothermometer; when plotted on a MnO-FeO graph their compositions closely overlap with those of mantle calcite, extending past the compositions of typical carbonatite carbonates.

This research was funded by Sigma Xi’s Grants in Aid of Research G20100315152413 and the Geological Society of America’s Graduate Student Research Grant 9402-10 (both to HMA).

This paper was published in Contributions to Mineralogy and Petrology in July 2013.

Please find the article here.