Formation of Pt, Pd and Ni tellurides: experiments in sulfide-telluride systems.

We evaluate experimentally the role of temperature and metal/Te ratio on the composition and crystallization temperature of sulfides and tellurides in the Fe-Cu-Ni-Pd-Pt-Te-S system. The monosulfide-sulfide melt partition coefficients decrease with increasing Te concentration and decreasing S/Te ratio of the bulk composition because Pt and Pd are strongly complexed by Te and thus stabilized in the melt phase. The solubility of Te in Fe-rich monosulfide solid solution and Cu-rich intermediate solid solution is around 0.2 wt% and largely insensitive to temperature down to 320 degrees C, indicating that Te substitutes in sulfides as an anion replacing S. Solid solution between moncheite PtTe2 and merenskyite PdTe2 is more limited than implied by natural telluride phase compositions. Solid solution between tellurides with variable metal/Te stoichiometry also appears to be limited, again in contrast to natural phase compositions. Natural tellurides are compositionally more flexible than the experimental compositions synthesized here. It is argued, therefore, that many natural tellurides coexisting with sulfides may be metastable, i.e. modified by exsolution of a Ni-Te component from the coexisting high-temperature sulfides. From Te concentrations in monosulfide solid solution it is deduced that natural sulfide-telluride assemblages record equilibration temperatures as low as 200-250 degrees C. With respect to Te and precious metal mineralization, no systematic temperature difference exists between sulfide-telluride ores referred to in the descriptive ore petrography literature as magmatic and ores termed hydrothermal in origin.