Gold solubility in oxidized and reduced, water-saturated mafic melt

Abstract

We have performed experiments to evaluate Au solubility in natural, water-saturated basaltic melts as a function of oxygen fugacity. Experiments were carried out at 1000 °C and 200 MPa, and oxygen fugacity was controlled at the fayalite-magnetite-quartz (FMQ) oxygen fugacity buffer and FMQ + 4. All experiments were saturated with a metal-chloride aqueous solution loaded initially as a 10 wt% NaCl eq. fluid. The stable phase assemblage at FMQ consists of basalt melt, olivine, clinopyroxene, a single-phase aqueous fluid, and metallic Au. The stable phase assemblage at FMQ + 4 consists of basalt melt, clinopyroxene, magnetite-spinel solid solution, a single-phase aqueous fluid, and metallic Au. Silicate glasses (i.e., quenched melt) and their contained crystalline material were analyzed by using both electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Measured Au concentrations in the quenched melt range from 4.8 μg g −1 to 0.64 μg g −1 at FMQ + 4, and 0.54 μg g −1 to 0.1 μg g −1 at FMQ. The measured solubility of Au in olivine and clinopyroxene was consistently below the LA-ICP-MS limit of detection (i.e., 0.1 μg g −1). These melt solubility data place important limitations on the dissolved Au content of water-saturated, Cl- and S-bearing basaltic liquids at geologically relevant fO 2 values. The new data are compared to published, experimentally-determined values for Au solubility in dry and hydrous silicate liquids spanning the compositional range from basalt to rhyolite, and the effects of melt composition, oxygen fugacity, pressure and temperature are discussed.