IDENTIFICATION OF BISMUTH AND TELLURIUM-BEARING MINERALS WITHIN A CARLIN-TYPE GOLD DEPOSIT, NEVADA

The goals of this study were to identify minerals within gold ore from the world-class Cortez (Carlin-type) Au mine, Lander County, Nevada that contains bismuth and/or tellurium and determine the association, if any, between the bismuth, tellurium, and gold. At Cortez, anomalous concentrations of Bi and Te generally occur in addition to the usual pathfinder suite of elements (Sb, Ag, As, Ba, Hg, Tl) that typify Carlin-type Au deposits.

Forty-four samples of rock core from six different drill holes in/around the Cortez mine were collected for this study; forty-two samples represent variably-altered and mineralized Devonian Wenban Formation and Silurian Roberts Mountain Formation rocks and two “type” samples represent unaltered and unmineralized rocks from those formations. Only a few altered and mineralized samples contain abundant, visible (macroscopic) sulfides; the rest contain sulfides unobservable to the unaided eye.

Multi-element geochemical analyses on all 44 core samples confirmed a generally positive correlation between anomalous concentrations of Bi and Te and Au, Cu, Ag, Pb, Zn, Sb, and Se, and concentrations of Bi, Te, and Au are generally highest in samples that contain the highest concentration of sulfides.

Transmitted and reflected light microscopy and electron microprobe analyses identified several Bi and/or Te-bearing phases within the altered and mineralized rocks, including tetradymite, native Bi, bismuthinite, alisonite, fahlore, and at least four other undetermined Bi+S±Te±Pb±Cu-bearing phases commonly intergrown with one another. In sulfide-poor samples Bi and Te do not appear to be associated with one another, as some crystals within the rock (e.g., actinolite or tremolite) appear to have been subsequently replaced by or altered to Bi±Fe±As±Pb±Cu-bearing phases while the surrounding host rock contains a higher concentration of Te compared to that of the remnant crystals. Conversely, Bi and Te clearly occur together in rock samples rich in macroscopic sulfides. Gold was not directly observed in any of the sample material and its association with Bi and Te is still undetermined; quantitative and qualitative data collection via electron microprobe are ongoing, as is the construction of detailed element maps of samples with highly-anomalous Au, Bi, and Te.