Gold solubility in arc magmas: Experimental determination of the effect of sulfur at 1000 掳C and 0.4 GPa

详细信息	查看全文 | 推荐本文 |

• 作者：Sé ; bastien Jé ; go^a ; ^b ; ^c ; ^{Sebastien.Jego@rice.edu} ; Michel Pichavant^c
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2012
• 期刊代码：27_00167037
• 类别：ear
• 出版时间：1 May, 2012
• 卷：84
• 期：Complete
• 页码：560-592
• 文件大小：1518 K

摘要

To investigate the behaviour of gold in sulfur-bearing hydrous intermediate calc-alkaline melts under different redox states typical of subduction-zone settings, we have determined the solubility of Au at 0.4 GPa and 1000 掳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 (鈭糔NO鈭?), moderately oxidizing (鈭糔NO+1.5) and strongly oxidizing (猢綨NO+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 鈭? 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鈭?) 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 鈭糔NO+1.25 to NNO+1.6) due to the destabilization of sulfides and the increase of melt S^{2鈭?/sup> 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 X_FeS - 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+/S_total 鈭?#xA0;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 螖NNO<鈭? 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.}