An Approachment In Genesis Of Polymetal Ore Deposits In Machangqin, Yunnan Province, China

Cu-Mo-Au deposits in Machangqin are typical ores relevant to alkali-rich porphyries of Himalayan movement in"Sanjiang"metallogenic belt, locating at collision environment of Yangtz and Indian plates. Preexisting studies suggest that this area has suffered oceanic crust subduction of Early Permain, collision orogenesis of Middle Triassic, apotectonic extension of Late Triassic, as well as intracontinental convergence orogenesis of epoch-Tertiary. Influenced by the Himalayan movement, series of strike-slip basins, Ailaoshan-Jinshajiang deep fault and porphyry belt formed. Active magma and crust-mantle interaction provide approved metallogenic conditions.Based on the researches of alteration wall-rocks,①two stages of Mo-mineralization are confirmed, microlite silication and micrite carbonatation in early phase, and pyritization and molybdenition with aphanitic silication in late phase.②Cu-mineralization is closely related with silication, pyritization and carbonatation.③Au-mineralization is related with silication and carbonatation.What also observed is that black opaque material is common in alteration and mineralization, which is mainly silicate by the EPMA. Preliminary studies suggest that it may be the vestiges of mantle fluid which initiates mineralization and alterations. It is advocated that this black opaque material results of rapid cooling of mantle fluid of a melt nature to sub-microcrystalline.Isotopic trace studies exhibit that mineralizing fluid and metallogenetic elements of Machangqin (Mo-Cu) ore block come from mantle, while Luandongshan (Cu-Mo) o ore block f mantle-crust contamination, and Jinchanqing (Au) with a much higher degree of crustal fluid. Such a conclusion is got that from Mo→Cu→Au, crustal fluid plays a more and more role.Through the investigation into minor elements and REE of ores, veins and ?mineralized wall-rocks show that normalized pattern of REE with distinct enrichment of LREE inclining to left and slight minus abnormality of Eu, which are prominent feature of mantle fluid metasomatism. Normalized pattern of minor elements exhibit some similarity.Comprehensive researches suggest that mineralizing process of Machangqing Mo-Cu-Au deposits may under the control of mantle fluid rich in LREE:The alkali-rich porphyries were synchronically self-replaced and alterated by the mantle fluid going with the process of alakali-rich magma crystallizing while the fluid and magma was trapped well, and the Mo deposit was formed in magma body or its deep-seat; if perturbation of tectonization was happened in the process of diagenesis and metallogeneis, the mantle fulid would enter into the contact zone between magma body and wall rock or strata next to the contact zone replacing and alterating to form contact-metasomatic deposit, Mo-Cu-deposit, existing with Au-deposit, which exists mainly in atrsta rocks; if the Au-deposit exists in porphyry body, it is commonly controlled by the fractures after diagenesis; if the deep fractures ,as magma and fluid channels ,are well developed , and environment is relatively open, the ore-bearing mantle fluid will flow far from alkali-rich magma along branch fractures and enter into different strata and rocks ,replacing and alterating to go with the process of diagenesis, and Au deposit formed.