Alkali metals control the release of gold from volatile-rich magmas

Magmatic-hydrothermal ore deposits are a major resource of gold. Volatiles exsolving from magmas rising through the Earth's crust are the key agent for enrichment of gold in these deposits. We report the first experimental determination of gold solubilities in the magmatic volatile phases at conditions typical of most magmas associated with gold deposits (T = 1000 °C, P = 150 MPa). We show that gold hydrosulfide complexes supersede gold chloride complexes, and more importantly, that the stability of gold hydrosulfide complexes is greatly increased by the presence of minute concentrations of KCl or NaCl (0.1–0.5 mol/kg H2O). The amplifying effect of alkali chlorides on the solubility of gold in H2S bearing volatiles may explain the preferential association of many giant hydrothermal gold deposits with high-potassium alkaline mafic to intermediate igneous rocks, which exsolve volatiles that simultaneously contain both H2S and alkali chlorides in significant concentrations.