Stable isotopic compositions of the Hongshan skarn copper deposit in the Zhongdian area and its implication for the copper mineralization process.

The Hongshan Cu deposit is located in the Zhongdian area which lies on the southern part of the Yidun island arc.It is a medium sized skarn copper deposit formed during the westward subduction of the Ganzi-Litang oceanic basin in Late Triassic. Generally,the skarn bodies are copper orebodies or copper mineralized bodies occurred in stratoid,bedded,veining and lentoid forms mainly in the contact between marble and hornfels with minor in hornfels.No direct contact between the skaru and intrusive bodies has been observed.The stable isotopic compositions of various kinds of minerals including garnet,magnetite,pyrrhotite,pyrite, chalcopyrite,calcite,which were formed in different stages,and marble have been analyzed respectively.The results indicate that the δ~(18) O_(SMOW) values of the garnet,major mineral component of the skaru,are varied from 6.2‰ to 8.3‰,indicating the oxygen isotopic compositions of skarn could inherited those of the concealed porphyry body.The oxygen isotopic compositions of magnetite have been used to calculate the δ~(18) O_(SMOW) values of the ore-forming fluid at magnetitization stage.They are 13.1～14.7‰ at assumed temperature of 400℃ or 12.5～14.1‰ at assumed temperature of 500℃ respectively.This indicates that the δ~(18) O-enriched CO_2 was dissolved into the ore-forming fluid.The δ~(34) S_(CDT) values of sulfides,varying from 4.45‰ to 6.20‰,indicate that the sulfur of the sulfides in the deposit is from a highly homogeneous source,with very weak sulfur isotopic fractionation in the process of sulfide crystallization and precipitation in the ore-forming fluid.Thus,it is suggested that the δ~(34) S_(∑S) value of the ore-forming fluid of the main ore-stage is 5.6± 0.6%0.The δ~(13) C_(PDB) value of 2.0～2.2‰ and the δ~(18) O_(SMOW) value of 24.0～24.8‰ for the marble in the deposit indicate that the marble was re-crystallized from the oceanic carbonate.The δ~(13) C_(PDB) value of-2.4～1.7‰ and the δ~(18) O_(SMOW) value of 16.3～22.4‰ for the calcite vein formed in the late ore-stage indicate that the carbon and oxygen of calcite vein are mainly sourced from the marble. In general,it is suggested that the Hongshan skarn could be formed in the lithological transition zone between the elastic rocks and carbonate with the crystallization of a SiO_2,Al_2O_3,Fe_2O_3,CaO-rich secondary magma which was formed through the assimilation of carbonate by the underneath porphyry magma.It is also believed that the ore-forming materials and the ore-forming fluid of the skarn mineralization in the Hongshan deposit are congenetic to those of the porphyry copper mienralization underneath.