Understanding controls on gold mobility in large hydrothermal systems: an example from the Pebble porphyry Cu-Au-Mo deposit, Alaska

The gold distribution in porphyry deposits can be highly variable due to the changing physiochemical conditions associated with multiple hydrothermal alteration events that commonly overprint each other. Geochemical modelling of mineralogical data from the Pebble porphyry Cu-Au-Mo deposit, Alaska, provides insights into the controls on variations in gold distribution. During potassic alteration at Pebble, gold and silver are transported as bisulfide complexes in the vapor phase and have low solubility, resulting in both metals being deposited together as electrum inclusions in chalcopyrite. During illite alteration, gold and silver are also transported as bisulfide complexes but gold solubility is higher compared with silver solubility. Gold solubility is highest during weak illite alteration of K-feldspar and lower during intense illite alteration. As a result, gold is locally remobilized during illite alteration, being redeposited as pure gold inclusions in pyrite. During advanced argillic alteration, the acidic conditions enhance the separation of gold and silver which partition to the vapor and brine, respectively, and both metals have relatively low solubilities. As a result pure gold inclusions are deposited with the dominant sulfide minerals in the alteration assemblage. Overall, these results show that the pH of the alteration mineral assemblage controls the degree to which gold is transported or deposited by low salinity fluid phases in porphyry deposits.