Cordilleran Section - 113th Annual Meeting - 2017

Paper No. 33-5
Presentation Time: 8:30 AM-5:00 PM


PURCELL, Ceara K.Q.1, AIRD, Hannah2 and DE WITT, Nancy1, (1)Geologic and Environmental Sciences Department, CSU Chico, 400 W 1st St, Chico, CA 95929, (2)Geological and Environmental Sciences, California State University, Chico, Chico, CA 95929-0205,

Copper porphyry deposits are a major economic resource hosting Cu and other precious metals in ore minerals, typically in the form of sulfides. Understanding the specific mineral assemblages present in these deposits and how they formed is key in selecting the most efficient mining and refining processes. This study focuses on the Ann Mason Deposit in Nevada which is yet to be developed as a mine. The Ann Mason ore is hosted in quartz monzodiorite and quartz monzonite intrusions which have been cross cut by granite porphyry dikes associated with hydrothermal mineralizing fluids. Related mines with similar structure from the same area have already been exploited. The deposit contains all the common alteration zones and is a classic example of a copper porphyry, making the study of its ore mineral assemblages ideal.

There are four different mineralization zones in the deposit: chalcopyrite-bornite, chalcopyrite-pyrite, chalcopyrite>pyrite, and chalcopyrite<pyrite. Assay analyses by Entree Gold, who own the rights to the deposit, show an association of bornite-rich zones with higher concentrations of precious metals, including Au and Ag. The goal of this project is to determine the assemblages which host precious metals within the deposit, particularly in the bornite-rich region, by using petrographic descriptions, SEM EDAX analysis, and WDS microprobe analysis of thirty thin sections of drill core samples from the deposit. Our study finds that Au and Ag occur in telluride minerals along the borders of bornite grains in the chalcopyrite-bornite zone. Specifically, Ag and Au are found within a Ag-Te-Pb-Se-Au mineral, with Au concentrated along the outer rim of the grain. Other samples show Ag hosted in rutile alongside U in apatite. All Au and Ag have been found in the chalcopyrite-bornite zone.

In summary, accessory metals are hosted within different mineral assemblages in distinct mineralization zones within the deposit. These findings indicate that Au and Ag are transported and deposited separately from the major ore elements in the Ann Mason Deposit, or have been remobilized by later fluids. The conclusions of this research will be used in the extraction and processing of ore at Ann Mason when the deposit is developed, and contribute new data to the scientific literature on trace assemblages in porphyry deposits.