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Gold Solubility in Arc Magmas: Experimental Determination of the Effect of Sulfur at 1000°C and 0.4 GPa

Abstract : To investigate the behaviour of gold in sulfur-bearing hydrous intermediate calcalkaline melts under different redox states typical of subduction-zone settings, we have determined the solubility of Au at 0.4 GPa and 1000°C for three dacitic magmas (two adakites and one calc-alkaline composition) from the North-Luzon Arc (Philippines). The experiments were performed over an oxygen fugacity (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, serving also as the source of gold, in presence of variable amounts of H2O and ~1 wt% of elemental sulfur (S). Concentrations of Au in glasses were determined by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). Gold solubility in S-bearing melts is drastically enhanced compared to S-free melts, by up to two orders of magnitude. In addition, very high gold solubilities are reached under reducing conditions (< NNO-1) in Fe-poor, S-rich, sulfide-saturated melts probably as a result of an increase of fH2S, and a strong increase of gold solubility is observed at the sulfide/sulfate transition (from ~NNO+1.25 to NNO+1.6) due to the destabilization of sulfides and the increase of melt S2- concentration. Thermodynamic modelling of the experimental results suggest that the dissolution of gold in silicate melt is the result of a combination of various gold species (Au0, Au2O, Au2O3, Au2S3, Au2(SO4)3, and Au2FeS2) present in the melt in variable proportions, depending on the three parameters considered in the model - log fO2, log fS2 and log XFeS - which are the main variables controlling the dissolution of gold in melt under our experimental PTX conditions. Our modelling shows that Au2FeS2 is the main gold species dissolved under reducing conditions (i.e., S6+/Stotal ~ 0), whereas at sulfate saturation gold is mainly dissolved as Au metal and Au2O. The present study shows that sulfide undersaturation of primary mantle magmas or/and highly oxidizing conditions are not required for metal mobilization from the source, since gold enrichment in evolving arc magmas and exsolving fluid phases is likely to occur over a wide range of fO2 at sulfide saturation, from NNO < -1 to the sulfide-sulfate transition (i.e., NNO+0.5-2.0); nevertheless it is critically controlled by variations of fS2 and fH2S. The role of Au-enriched slab partial melts and slab-derived aqueous fluids, and the importance of the abundance of sulfur in the source for an early gold enrichment in the melt, are emphasized.
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Sébastien Jégo, Michel Pichavant. Gold Solubility in Arc Magmas: Experimental Determination of the Effect of Sulfur at 1000°C and 0.4 GPa. Geochimica et Cosmochimica Acta, Elsevier, 2012, 84, pp.560-592. ⟨10.1016/j.gca.2012.01.027⟩. ⟨insu-00671964⟩

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