Thermodynamic Study For The Association And Fractionation Of Copper And Gold In Shizishan Orefield, Tongling, Anhui Province

Shizishan orefield is the most representative copper-gold orefield in Tongling ore cluster region, Anhui Province, and contains several types of copper deposits and gold deposits. Copper and gold exit separately or in paragenetic in the deposits. Copper mineralization and gold mineralization existed in association or fractionation either temporally or spatially.Based on systematic collecting and comprehensivel summarizing of geological characteristics, and geochemical features of fluid inclusion of copper deposits and gold deposit, the solubility of Cu, Au transportation complexes species has been calculated according to obtained thermodynamic parameters. The relationship between precipitation of ore-forming elements and change of physicochemical conditions has been discussed, and the geological processes resulting in the change of physicochemical conditions of ore-forming fluid have also been discussed in the ore-forming fluids of Shishan orefield. The main conclusions are drawn as follows:Complexes species of copper and gold in ore-forming hydrothermal fluids were both chlorine and sulfide in Shizishan orefield. The change of Cu and Au migration forms in the hydrothermal fluids was closely related to physicochemical conditions of ore-forming fluid. The dominant Cu complexes species were CuCl0 and CuCl2. The procipitation of Cu in ore-forming fluid was mainly controlled by temperature, pH,fO2, the solubility of Cl-, and was not obvious related toΣS. The solubility of Cu decreased with decreasing temperature, fO2, Cl- and increasing pH. The major Au complexes species were Au(HS)2-,and chlorine complexes species was not important. The solubility of Au was closely related to temperature, fO2, pH,ΣS, and not Cl-. Au decreased with decreasing temperature,ΣS and increasing pH. When the fluid was alkalescent (pH=7), the solubility of Au was lagest.Changes of physicochemical conditions hold the key to the association and fractionation between copper and gold in ore-forming hydrothermal solutions. The great difference between copper and gold in soluble species and mineral solubility responded distinctly to physicochemical conditions and led to their precipitations at different period of mineralization and localities during hydrothermal evolution.