PARTITIONING OF ORE METALS AMONG FE-S-O MELT, RHYOLITE MELT, AND PYRRHOTITE

Sulfide minerals and melts can sequester chalcophile elements in arc magmatic systems. At magmatic temperatures, small amounts of an immiscible iron sulfide-oxide (Fe-S-O) liquid may sequester disproportionately large amounts of metals from silicate melts, and may affect the ultimate metal budget in the evolving magma. In this study, silver partitioning among pyrrhotite, Fe-S-O and a silicate glass of rhyolitic composition was investigated, along with the partitioning of other significant trace elements present in the starting materials.

Experimental starting materials consisted of powdered rhyolite glass (glass) (Bishop Tuff) and magnetite (mt) (Essex, NY); pyrrhotite (po) (Sudbury, Ontario); and metallic silver. All charges were sealed in evacuated, fused quartz tubes. Experiments were performed at temperatures from 1000 to 1070°C in a tube furnace. The final assemblage was mt+po+glass+quenched Fe-S-O melt. Vapor pressure was established by the composition of the starting materials. Run times varied from 15 minutes to 3 days, with runs quenched in a water bath. Phase compositions were determined by using a JEOL JXA-8900 superprobe for EDS/WDS and an Element 2 single collector LA-ICP-MS at the University of Maryland.

Experiments at and above 1039°C showed rounding and reduction in grain size of the po and mt, accompanied by the appearance of complex, fine-textured patches of sulfide-oxide intergrowth. The boundary of these patches and the silicate melt are consistent with a liquid-liquid contact, suggesting the intergrowth was a homogenous immiscible Fe-S-O liquid at temperature which subsequently formed two phases (mt + po) on quench. Following established S₂ and O₂ fugacity relationships in systems containing mt + po, final compositions suggest ƒO₂ ~FMQ. Silver concentration in the Fe-S-O melt was found to be at higher levels than in pyrrhotite and was measured at 5700 ppm ±700. Preliminary data show: ^Fe-S-O/poD_Ag= 2.5±0.8; _Cu = 0.9±0.2; _Zn = 2.4 ±0.3; _Pb = 70±30. ^Fe-S-O/glassD_Ag = 400±200; _Cu = 500±200; _Zn = 130±50; _Pb = 7±3.

The Fe-S-O melt is seemingly more efficient than po at concentrating Ag, Zn, and Pb; and as efficient at concentrating Cu. Fe-S-O melts will control substantially more of the metal budget than silicic melts.