Experimental Study On Sulfide Stability At Lower Crust Conditions:Constraints On The Migration Behaviors Of Copper,Gold And Sulfur

In order to investigate the behavior of Cu,Au and S during deep magmatic-hydrothermal processes of porphyry deposits,this study performed high temperature and high pressure experiments with piston-cylinder apparatus to determine the solubility of S in hydrous silicate melts and S partitioning coefficients between fluids and silicate melts under various oxygen fugacity?fO₂?conditions at lower crust conditions.Two synthetic silicate compositions,a K-feldspar glass and a Dacite glass were used as starting materials.These experiments were conducted at 1.0 GPa?corresponding to³0km of the lower crust?and 950 ^oC with run duration of 48–72 h.The oxygen fugacity was controlled by solid oxygen fugacity buffer such as Co-CoO?CCO?,Ni-NiO?NNO?and Re-ReO₂?RRO?,covering the reducing to oxidizing range.The run products include quenched melts,sulfides?pyrrhotites?and fluid bubbles.The compositions of quenched melts and sulfides and the concentration of S were measured by EMP.Our experimental results show that the solubility of S is not affected by the amount of sulfides,but inceases with the increase of FeO in the silicate melt.At the condition of sulfide saturation,sulfur contents in felsic melt increase from²00 ppm to⁸000 ppm with oxygen fugacity increasing from FMQ-1.5 to FMQ+2.5.In addition,the S solubility also increases with increasing fS₂ and the water content in the silicate melt.The experimental results also show that the fluid/melt partition coefficient of S decreases from⁴00 to⁵0 with increase of oxygen fugacity.We calculated the S content in the exolved fluid of silicate melt when the sulfide is saturated:¹0 wt%at reducing conditions?CCO and NNO?and²0 wt%at the oxidizing condition?RRO?.That means the capability of magmas containing fluid bubbles in transfer of sulfur depends crucially on fO₂,sulfur solubility in melt and the amount of exsolved fluid when magmatic sulfide is saturated in the source region.At oxidizing conditions,it mainly depends on the amount of exsolved fluid and at the reducing condition,it depends on both melt sulfur solubility and the amount of exsolved fluid.The intermediate-felsic magma related to porphyry deposits has a high Sr/Y,which caused by partial melting of mafic rocks at lower crust or crystallization differentiation of the mafic magmas.The magma of island arc basalt contains 2–6 wt%H₂O,and the H₂O content in the evolved intermediate-felsic magma can reach 20–30 wt%by crystallization differentiation,thus leading to the saturation of H₂O.The exsolved fluid can reach 10 wt%in the residual melt,enhancing the sulfides destruction.The S-rich fluid bubbles play a key role in the migration and enrichment of Cu,Au and S,making the ore-forming magma.