Geological Feature And Ore-controlling Factors Of The Chang’an Cu-Mo-Au Ore Concentration Region, Southwestern Ailaoshan Orogenic Belt

The Chang’an Cu-Mo-Au ore concentration region, consisting of Chang’an gold deposit, Tongchang Cu-Mo deposit and Changanchong Cu-Mo deposit, is located in the south of the Jinping nappe, southwest of the Ailaoshan orogenic belt, which is an important repository of precious metal and nonferrous metal resources in China.Magmatism in the ore concentration region mainly occurred in the Himalayan period. Igneous rocks vary from basic to acidic rocks, including gabbro, diabase, syenite, syenite porphyry, granite porphyry and a small amount of granodiorite. Most of them belong to the metaluminous to weak peraluminous alkaline high potassium rock, occur as batholith or small stock, comprising of composite intrusions as the result of multiple-episode magmatism. The geochemical characteristics of these rocks are consistent with those of alkaline magmatic rocks in the southern Ailaoshan-Jinshajiang alkalic rock mass belt, which are interpreted to be formed by the location of fractionated melts of potassic alkaline magma, derived from partial melting of spinel phase mantle with phlogopite. Zircon LA-ICP-MS U-Pb dating indicate the magmatic intrusion in the ore concentration region occured between 37~21Ma. Granite porphyry and syenite porphyry forming in 35~21Ma directly or indirectly control the formation of the porphyry Cu-Mo deposit and the fracture altered rock type gold deposit.The structures in the ore concentration area are dominated by thrust faults striking northwest and dipping northeast and interstratified strike-slip faults striking northeast and dipping northwest. Both of them are ore-transporting and ore-contained structures, and control the spatial distribution of intrusions and orebodies.The copper and molybdenum ore bodies in the Chang’an Ore concentration area are mainly hosted in the contact zone of porphyry and carbonate, most of them are plate, imbricate, lenticular, banded, taxitic and block output. The zoning of wall-rock alteration is evident, greisenization zone, skarnization zone and carbonatization can be identified from intrusion to sedimentary rock.Molybdenum orebodies mainly distribute in greisenization zone, while copper orebodies mainly distribute in skarnized zone. Gold deposits mainly distribute in the fault near the intrusions. The wall rock are mainly aleurolite, with obvious silicification, pyritization and carbonatization, while mineralization is mainly related with silicification and pyritization. The stable isotopes show that the ore-forming metals and fluids of the copper and molybdenum deposits are mainly originated from magma. For the gold deposit, the ore-forming metals originated from the postmagmatic hydrotherm, ore-forming fluids is the mixture of postmagmatic hydrotherm and groundwater. Mineralization was taken placed from 35 to 21Ma。The data of structural geology, petrology and isotope geology show that Cenozoic magma intruded along the intersection zone of the northwest and northeast faults. The metasomatism happened when postmagmatic hydrotherm and wall rocks met, forming greisenization, skarnization and carbonation alteration. The ore-forming metals such as Cu and Mo precipitated in the form of sulfides, forming the copper and molybdenum deposits. Meanwhile, low to medium temperature fluids moved along fractures, and groundwater added into the fluids in the fractures far away from intrusion, which resulted in obvious changes of physical and chemical conditions of the ore-forming fluids. As a result, the clathrate carrying gold element decomposed. Gold element precipitated with crystallization of the quartz, pyrite and calcite, and alteration of the silicification, pyritization and carbonatization, and formed the gold deposit.