Geology And Ore-Forming Fluids Geochemistry Of The Dongguashan Porphyry Cu Deposit,Tongling, Anhui Province

The Dongguashancopper deposit is a porphyry-skarn multiple copperdeposit in Tongling, Anhui province. This deposit is typical in Tongling due to its larger scale and deeper burial depth.A detailed study has been carried out on the ore deposit geology and fluid geochemistry of the deposit, leading to discrimination of ore types andalteration zones, division of metallogenic periods with stages, and revealing of ore-forming fluid evolution and mineralization process.Geological investigation and drillingrecord compiling indicate existence of the Dongguashan porphyry copper deposit ina superimposedand composited position of the Qingshan NE-trending anticlinebythe NNE-trending fracture system. The orebodiesarehosted in the Qingshanjiao and Baocun plutons, with lithology of quartz diorite porphyritebelonging to the Yanshanian high potassium calc-alkaline rock series. According to mineral assemblage and texture and structure, oresare divided into two types of copper-bearingaltered rocks and copper-bearing quartz sulphide veins. The former is characterized by disseminated,spotted,and taxitic structures, while the latter by vein structure.Field investigation and microscopic observationshowdistribution of wall rockalterationsas a concentric-ring shape.From center to edge of each pluton occur in ordersilication, potassic, phyllic, and propyliticalteration zones. According to veins crosscutting relationships and mineral assemblage characteristics, the alteration and mineralization process is divided into pre-minerization, mineralization, and post-mineralizationperiods.The pre-minerization period contains only a potassic alteration stage. The mineralization period consists ofquartz-pyrite, quartz-chalcopyrite, and quartz-galena-blende stages, in the quartz-chalcopyrite stage of which took place major copper mineralization. The post-mineralizationperiod includes only a carbonatization stage.Average homogenization temperature and salinity of fluid inclusions in the potassic alteration, quartz-pyrite, quartz-chalcopyrite, quartz-galena-blende, and carbonatization stages are defined in the ranges of 403℃ and 47.91 wt.% Na Cl eq.,393℃ and 44.44 wt.% Na Cl eq., 339℃ and 37.57 wt.% Na Cl eq., 226℃ and 7.72 wt.% Na Cl eq., and 158℃ and 1.27 wt.% Na Cl eq, respectively. These data indicate gradual evolution of ore-forming fluids from a high temperature and salinity fluid in the pre-mineralization period to a low temperature and salinity fluid in the post-mineralization.As mentioned above, the Dongguashan porphyry copper deposit is a typical moderate-high temperature depositof gas-liquid metasomatismand filling. The ores of copper-bearingalterated rock type weregenerated by high temperature gas liquid metasomatism,whilethose of copper-bearing quartz sulphide veins type by high temperature liquid filling. In the early mineralization period, ore-forming fluids were mainly magmatic water; while in the late mineralization period,meteoric water increased and gradually became the main ingredient. Although ore-forming material was mainly derived from magma, surrounding rocks may have some contribution.