Characteristics And Evolution Of Ore-Forming Fluid Of Copper Deposits And Gold Deposits In Shizishan Ore Field, Tongling, Anhui Province

Shizishan ore field is the most representative copper-gold ore-field in the Tongling Mineralization Cluster Area, Anhui Province. This ore-field contains many types of copper deposits and gold deposits. Copper and gold exist separately, or in paragenetic and associated. The matallogenetic processes are very complex. Geologisits always focus on the geological characteristics and the evolution process of ore-forming fluids.Basing on the detail field geological studies and systematical laboratory analysis, it is carried out for the comparativation of the ore geological characteristics and geochemical characteristics of fluid inclusions of copper deposits and gold deposits in Shizishan ore-field. The main conclusions are drawn as follows:In the geology, the copper deposits and gold deposits in Shizishan ore-field have both similarities and differences. The metallogenic geological setting, ore-field structure and host strata of the copper deposits and gold deposits are similar. But in the spacial distribution, the copper deposits are seat in deep relatively. In the forming time, the copper deposits are earlier than the gold deposits. The genetic types of copper deposits are mainly interformational skarn type, strata-bound skarn type, porphyry type and crypto-explosive breccia pipe-type, those of the gold deposits are contact-metasomatic skarn type, moderate- low temperature hydrothermal type. The host strata of copper deposits are mainly Huanglong and Chuanshan Formation of upper Carboniferous, Dalong Formation of upper Permian, Helongshan, Yinkeng and Nanlinghu Formationin of lower Triassic; those of the gold deposits are mainly the Helongshan and Nanlinghu Formationin of lower Triassic, East-Ma'anshan Formation of middle Triassic. The main ore-controlling structures of the copper deposits are bedding decollement structure, interstratified crack and crypto- explosive breccia pipe, and those of the gold deposits are contact zone between magmatic body and wall-rock, fault fracture zone and fissure structure. The copper deposits are associated with granodiorite, quartz monzodiorite and monzodiorite intrusions, and the gold deposits with the pyroxene-monzodiorite intrusions.The formation of the copper deposits and gold deposits progresses through the skarn stage, quartz-sulfide stage and carbonate stage. The analysis and comparative study of fluid inclusions of the major mineralization stages shows that the characteristics and evolution of ore-forming fluids also exist similarities and differences between the copper deposits and gold deposit. The homogenized temperatures of copper deposits have a wide range and are higher than those of the gold deposits, which indicated the copper deposits had a higher ore-forming temperature and longer duration of fluid evolution. The salinity peak values of copper deposist occurre in a lower value range, and the gold deposits have a lower and a higher salinity range. The pH and Eh indicated that the fluids of copper deposits are weak acid and weak reduction environment, and the gold deposits formed in neutrality and weak oxidation environment. Analytical data of gas and liquid phases of fluid inclusions demonstrated that the content of CO₂, H₂S, Cl^- and SO₂^4- of quartz-sulfide stage of copper deposits are lower than those of the gold deposit. Otherwise the copper and gold deposits have the similar chorndrite- normalized REE distribution patterns and Chinese continental crust-normalized trace element spider diagram, which indicated that the hydrothermal fluid of copper deposits and gold deposits were all derived from the magmatic differentiation. Hydrogen, oxygen and carbon isotopic compositions imply that the ore-forming fluid is likely to be mixed with meteoric waters.Based on the above geological and geochemical studies, it is concluded that the copper and gold seprated deposition may be mainly caused by the changes of temprature, salinity and the gas-liquid contents of the ore-forming fluid. At the earlier ore-forming stage and the deep part, the hydrothermal solutions had a high temprature, low salinity, and low contents of H₂S, Cl^- and SO₂^4- , copper began deposition in weak acid and weak reduction environment. With the fluid ascend, the temprature decreases, the gold-bearing fluid eluviated more strata components, and the contents of H₂S, Cl^- and SO₂^4- increases, which brought about favorable conditions for migration and enrichment of gold, when the salinity of ore-forming fluid increace to be saturated, the gold began deposition in weak oxidation and neutrality environment and favourable places, and further forms gold ore deposits.