Controls on Gold Solubility in Arc Magmas: An Experimental Study at 1000°C and 4 kbar
Résumé
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 (not, vert, similarNNO-1), moderately oxidizing (not, vert, similarNNO+1.5) and strongly oxidizing (not, vert, similarNNO+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 not, vert, similarNNO-1, gold solubility reaches much higher values, from not, vert, similar1200 to 4300 ppb, and seems to correlate with melt S content. No systematic difference in gold solubility is observed between the adakitic and 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.
Origine : Fichiers éditeurs autorisés sur une archive ouverte
Loading...