Controls on gold solubility in arc magmas: An experimental study at 1000°C and 4kbar

Abstract

In order to (1) explain the worldwide association between epithermal gold–copper–molybdenum deposits and arc magmas and (2) test the hypothesis that adakitic magmas would be Au-specialized, we have determined the solubility of Au at 4 kbar and 1000 °C for three intermediate magmas (two adakites and one calc-alkaline composition) from the Philippines. The experiments were performed over a fO2 range corresponding to reducing (∼NNO−1), moderately oxidizing (∼NNO+1.5) and strongly oxidizing (∼NNO+3) conditions as measured by solid Ni–Pd–O sensors. They were carried out in gold containers, the latter serving also as the source of gold, in presence of variable amounts of H2O and, in a few additional experiments, of S. Concentrations of Au in glasses were determined by LA-ICPMS. Gold solubility in melt is very low (30–240 ppb) but increases with fO2 in a way consistent with the dissolution of gold as both Au1+ and Au3+ species. In the S-bearing experiments performed at ∼NNO−1, gold solubility reaches much higher values, from ∼1200 to 4300 ppb, and seems to correlate with melt S content. No systematic difference in gold solubility is observed between the adakitic and the non-adakitic compositions investigated. Oxygen fugacity and the sulfur concentration in melt are the main parameters controlling the incorporation and concentration of gold in magmas. Certain adakitic and non-adakitic magmas have high fO2 and magmatic S concentrations favorable to the incorporation and transport of gold. Therefore, the cause of a particular association between some arc magmas and Au–Cu–Mo deposits needs to be searched in the origin of those specialized magmas by involvement of Au- and S-rich protoliths. The subducted slab, which contains metal-rich massive sulfides, may constitute a potentially favorable protolith for the genesis of magmas specialized with respect to gold.