Geochemical Study On The Minerogenetic Background And Mineralization Of Gold Deposits In The Yiyang-Yuanling Region Of Hunan Province

A series of gold deposits occuring in Proterozoic strata in Yiyang-Yuanling region of Hunan Province, can be divided into four categories as follow :Au,Au-Sb,Au-W and Au-Sb-W according to their valuable element association. These deposits as well as the Mobin-type gold deposits occuring in Proterozoic strata in southwest of Hunan province and northeast of Guizhou province form the Hunan-Guizhou gold metallogenic belt.Petrological, petrochemical and rock associative characters of the Proterozoic and lower palaeozoic sedimentary rocks and igneous rocks indicate that Yiyang-Yuanling region is the convergent zone of ocean plate and Yangzi plate in Proterozoic Era to early Palaeozoic Era The convergence which made the Proterozoic strata deformation to form the close linear folds and thrust faults provided the favorable structure conditions for gold deposit, and also provided energy for the formation of the Xuefeng-Caledonian period gold deposits.Trace element geochemistry studies show that Au, Sb and W contents of Proterozoic strata are more than the average vaules of them in the upper continental crust, and most of them exist in rocks with easy activiting forms, so it can be concluded that the Proterozoic strata is a Au-Sb-W associative ore-bearing formation. The regional geochemical studies indicate that thy regional gold-depleted zones occuring in outer area of gold dewposits and the orebodies with their surrounding gold primitive anomalies formed the depletion-enrichment geochemistry conjugate system. This suggests that the gold came from the wallrocks, It can be further demonstrated by the S, C and Pb isotopic studies. Evidence from mineral assemblage, wallrock alteration and mierothermometry of fluid inclusion suggest that the gold deposits in the area belong to middle- to low- temperature deposits. These deposits formed in the weak reducing and alkalic fluids originated from mixture of metamorphic solution and formation water. Au[HS]₂^- was the main migrating form of gold in the fluids, and Au[Cl]₂^- also was the migrating form of gold in minority gold deposits.