THE PARTITIONING OF FE, NI, CU, PT, AND AU BETWEEN SULFIDE, METAL, AND FLUID PHASES - A PILOT-STUDY

This paper describes new experimental and analytical techniques to stu dy element partitioning behavior between crystalline material and a la te- to post-magmatic fluid phase. Samples of the fluid phase are isola ted at experimental run conditions as synthetic fluid in quartz. Indiv idual fluid inclusions are later analyzed for dissolved metals using P roton Induced X-ray Emission (PIXE). Back reactions between fluid and solid phases during quenching are prevented because the fluid is isola ted at the experimental pressure, temperature (P, T) conditions before quenching occurs. The technique is applied to study the partitioning of chalcophile elements (Fe, Ni, Cu, Pt, and Au) between sulfide phase s, metal alloys, and supercritical SiO2-NaCl-saturated H2O +/- CH4-CO2 -H2S fluids. Synthetic Ni-Cu-rich monosulfide solid solution (mss) dop ed with PtS or Au is packed in a quartz capsule and, together with a h ydrogen buffer capsule and compounds to generate a fluid phase, welded shut in an outer Pt or Au metal capsule. The fluid phase is generated by combustion and reaction of various C-H-O fluid components during h eating. Depending on capsule material and sample composition, the run products consist of platiniferous or auriferous mss, Pt-Fe, or (Au, Cu ) alloy phases, PtS, Fe3O4, sometimes a Cu-rich sulfide melt, and a fl uid phase. Samples of the fluid are trapped in the walls of the quartz sample capsule as polyphase fluid inclusions. All phases are now avai lable for analysis: fluid speciation is analyzed by piercing the outer metal capsule under vacuum and feeding the released fluid into a mass spectrometer. Phases and components within fluid inclusions are ident ified with Raman spectroscopy. Platinum and gold in solid solution in mss are determined with a CAMECA SX50 electron microanalyser. Metal co ntents trapped in selected fluid inclusions are determined quantitativ ely by in situ analysis with a proton microprobe using PIXE and a corr ection procedure specifically developed for quantitative fluid inclusi on analysis. Initial results of metal solubilities in the fluid are as follows. Iron decreases from above 6,000 ppm under reduced conditions in the presence of H2S in the fluid, to less than 1,000 ppm if hemati te is stable in the crystalline run product. Copper and gold concentra tions in the fluid range from about 600 to over 1200 and from 150 to a bout 270 ppm, respectively. The solubilities of these two metals in Na Cl-saturated fluids are apparently independent of fluid speciations co vered here. Nickel is mostly below detection limit (<10 ppm) and appar ently poorly soluble in high-temperature fluid phases. Platinum concen trations in fluid inclusions are highly variable even among fluid incl usions of single runs, possibly because Pt tends to form multi-atom co mplexes in fluid phases.