Evolution of sulfur isotopes in porphyry Cu-Mo ore-magmatic systems of Siberia and Mongolia

Paleozoic-Mesozoic (D-J(3)) porphyry Cu-Mo deposits in Siberia and Mongolia are characterized by a narrow range of delta(34)S values of sulfides (pyrite, chalcopyrite, molybdenite), -2.0...+10.2parts per thousand. Sulfides of essentially copper deposits, whose ore-bearing porphyry complexes have (Sr-87/Sr-86)(0) = 0.7039-0.7046, show the least delta(34)S values, -2.0...+3.8parts per thousand, close to those in the meteoritic standard. Deep-seated horizons of essentially copper deposits (Erdenetuin-Obo, Aksug) abound in anhydrite with delta(34)S = 10-16parts per thousand. Sulfides of essentially molybdenum deposits with (Sr-87/Sr-86)(0) = 0.7046-0.7078 are enriched in S-34. Elevated delta(34)S values might be due to a partial borrowing of sulfur from sulfur-carbon compounds and syngenetic pyrite of sedimentary strata that host intrusions in porphyry Cu-Mo ore regions. At some deposits, the ores show a rather uniform isotopic composition of sulfur, which is probably due to its perfect homogenization in high-temperature magma chamber. Such favorable conditions might have appeared first of all in large ore-magmatic systems producing large deposits. The revealed minor variations in delta(34)S in sulfides, as well as the isotopic fractionation of sulfur in the system sulfide-sulfate, might have been due to the interaction of fluids with the host rocks, which resulted in fluctuation in pH and/or f(O2).