Thermodynamic Analysis of Ore-forming Conditions and Sulfur Isotopic Systematics of Dachang Ore Field

On the basis of ore-forming periods and stages of the Daehang ore field,the pH and f;conditions andthe S isotopic systematics during ore formation have been thermodynamically treated in thispaper.Calculations show a progressively decreased pH,an increased oxidation regime and an intensifiedactivity of sulfur from the early to the late stage.Owing to the unreliability of inferring the S source fromδ;S;,δ;S;has been calculated using the Ohmoto’s model.Results indicate that the δ;S;frequencydistribution is more concentrated than that of δ;S;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 Ssources.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 fromcalculations are supported by many lines of geological evidence.