HYDROUS SULFIDE MELTING IN THE SYSTEM FES-PBS-ZNS-H2O AT 1.5 GPA

The solubility of H₂O in sulfide melts and hence the effect of H₂O on the melting point of sulfides has been of interest to petrologists and economic geologists for decades. Inherent difficulties associated with performing sulfide-bearing experiments have prevented conclusive determination of the effect of H₂O on sulfide melting. However, development of a new experimental method has permitted (re)investigation of the effect of H₂O on sulfide melting in the FeS-PbS-ZnS system.

Cold-pressed polycrystalline galena capsules are a departure from traditional capsule methods, wherein the capsule is a sealed, inert, and (relatively) impermeable container of the experiment. In contrast, galena capsules actively participate in the experiment, providing a PbS component to the melt. FTIR spectroscopy of hydrous silicate glasses have demonstrated galena capsules are capable of sealing free H₂O to produce H₂O-bearing silicate melts. Cold-pressed polycrystalline capsules show promise for experiments involving reactive components (sulfides, arsenides etc.), and polyphase polycrystalline capsules may serve as both a capsule and an fS₂/ fO₂ buffer assemblage.

New experimental data for the FeS-PbS-ZnS system reveals that the addition of H₂O depresses the solidus by 35 ±5 °C relative to the dry eutectic of 900 °C at 1.5 GPa. The presence of dissolved H₂O is confirmed by the occurrence of vesicles in the sulfide melt quench intergrowth.

It has been suggested that the quartz-sulfide veinlets, associated hydrothermal alteration and high-salinity fluid inclusions from the Strathcona Deep Copper Zone, Sudbury, represent an aqueous fluid exsolved from a cooling sulfide melt. Confirmation of the solubility of H₂O in sulfide melts permits reconciliation between experimental results and field observations. Recent recognition of sulfide melting in response to high-grade metamorphism presents a situation in which hydrous sulfide melts could be expected.