PRECIOUS METAL DEPORTMENT IN THE ANN MASON PORPHYRY COPPER DEPOSIT, YERINGTON, NEVADA

Porphyry copper deposits provide a significant portion of the global supply of gold, yet the mechanisms of precious metal enrichment are still debated. Minor precious metals may upgrade the value of a copper deposit if they are present in a form that is economic to extract. This study investigates the deportment of gold and silver in the Ann Mason deposit, including distribution, mineral species, textural relationships, grain size and host mineralogy. Ann Mason is one of several copper porphyry deposits hosted within the Yerington batholith. Copper mineralization at Ann Mason is primarily associated with potassic alteration, and has been classified into the chalcopyrite-bornite, chalcopyrite-pyrite and pyrite>chalcopyrite zones. Assay data shows that precious metals are most closely associated with bornite, although they are found in every mineralization zone. Analysis by petrographic microscope, SEM-EDS and EPMA-WDS shows that silver-bearing minerals (78% of grains analyzed) are much more common than gold-bearing minerals (16%). Gold is predominantly hosted as electrum, while silver is most common in silver telluride minerals, especially hessite. Over 85% of the 415 grains measured are hosted by a sulfide mineral, with 65% hosted by bornite. Electrum and telluride minerals are mostly observed in the chalcopyrite-bornite zone, and hosted within or on the edge of bornite grains. Silver selenide minerals are equally common in the chalcopyrite-bornite and the chalcopyrite-pyrite zones and may contain subordinate Pb. Only three grains of native gold are observed, and all are as rounded blebs within pyrite, associated with propylitic alteration in the chalcopyrite-bornite and the pyrite>chalcopyrite zones. Results suggest that the previous metals may have originally been collected by crystallizing sulfide minerals, and exsolved at lower temperatures. The association of native gold with pyrite and propylitic alteration suggests low temperature remobilization of gold from other higher temperature phases. These findings have implications for ore processing at Ann Mason and similar deposits with minor precious metal contents around the world, with the potential to inform degree of milling required for precious metal liberation and the most effective extraction processes.