Contrasting effects of S and Cl on the gold enrichment of natural magmas

Volatile-rich magmas are very important sources for highly siderophile elements (i.e., Pt group elements and Au), which they transport from depths of magma generation to subsurface Cu-Au-porphyry or Ni-Cu-PGE sulphide ore deposits. The behaviour of highly siderophile elements (HSE) is strongly controlled by the redox conditions in magmas and by the activity of volatiles, acting as mobilization and transport agents for HSE. It has been already experimentally established that Au solubility in silicate melts is significantly increased with increasing fO2 in S- and Cl-bearing systems in comparison with S- and Cl-free magmas (e.g., Botcharnikov et al., 2010; Jégo et al., 2010). Here we present new experimental data illustrating that S and Cl have contrasting effects on the enrichment of Au in silicate melts as fO2 increases from reducing to highly oxidizing conditions. The experiments at 200 MPa and at 1050 and 1200°C conducted in basaltic, andesitic and rhyodacitic systems show that in S-bearing magmas the highest gold solubility (by upto an order of magnitude) is observed within a narrow “window” of redox conditions characterized by sulfide-sulfate transition (i.e., Au concentration exposes a pronounced maximum). Within this redox range, Au is found to be particularly mobile and is dissolved in silicate melts as a sulphide-bearing component. With further increase in fO2, the solubility of Au drops dramatically to the level observed in S-free melts. On the contrary, the solubility of gold in S-free but Cl-bearing magmas increases linearly with the concentration of Cl. The Cl-enriched magmas can dissolve exponentially increasing amounts of gold with increasing fO2. The highest solubility values (even higher than that in S-bearing systems) have been determined at logfO2 corresponding to FMQ+3 and it is expected that this dependence holds also at more oxidizing conditions. The observed contrasting solubility of Au in S- and Cl-bearing magmas at different redox conditions implies that the presence of S would favour gold transport in a narrow range of fO2 from 0.5<FMQ<1.5, whereas Cl is expected to take control over the mobility of gold with progressive magma oxidation. At FMQ<-0.5, the change in volatile composition is not expected to affect Au mobility significantly. Thus, small variations in the activities of volatiles and redox conditions in natural magmas can lead to mobilization or fixation of gold during magmatic evolution. Hence, the relative changes in these parameters should be considered for the interpretation of natural magmatic processes leading to the formation of ore deposits. References: Botcharnikov et al. (2010) Solubility of Au in Cl- and S-bearing hydrous silicate melts. Geochim. Cosmochim. Acta 74, 2396-2411. Jégo et al. (2010) Controls on gold solubility in arc magmas: An experimental study at 1000°C and 4 kbar. Geochim. Cosmochim. Acta 74, 2165-2189 (2010).