Thermodynamic analysis of ore-forming conditions and sulfur isotopic systematics of Dachang ore field
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Abstract:
On the basis of ore-forming periods and stages of the Dachang ore field, the pH and $$f_{0_2 } $$ conditions and the S isotopic systematics during ore formation have been thcrmodynamically treated in this paper. Calculations show a progressively decreased pH, an increased oxidation regime and an intensified activity of sulfur from the early to the late stage. Owing to the unreliability of inferring the S source from δ34Smin, $$\delta ^{34} S_{\Sigma _S } $$ has been calculated using the Ohmoto’s model. Results indicate that the δ34 min frequency distribution is more concentrated than that of δ34Smin and the peak value shifts to negative region by 2.5%. The sulfur in the whole ore field seems to be of multiple source, i.e., different deposits have their own S sources. But the S isotopic composition pertaining to each stage is nearly constant, suggesting that the ore-forming system be open to sulfur and the supply of sulfur be sufficient. The conclusions deduced from calculations are supported by many lines of geological evidence.