Late-trassic Cu Mineralization In Porphyry Environment,Northwest Yunnan,China

Northwest Yunnan located at middle of Sanjiang metallogenic belt, where is a important basement of porphyry-skarn copper and multi-metal in China. The study selected Pulang and Yangla deposite as representative, through data analysis, field investigation, indoor appraisal, geochemical test methods, to discusse magmatic origin, evolution and magma-fluid dissolved in late magmatism, migration and enrichment of copper element in fissure vein system. The main results are as follows:1. According to the hand specimens, the microscopic residual structure of gray altered rock in Yangla copper deposit, the original rock of which is quartz monzonitic porphyry, and by means of LA-ICP-MS zircons U-Pb isotopic dating, gaining its diagenetic age as228.1±0.3Ma, which consistent with predecessor’s mineralization age who use molybdenite Re-Os method, which provides the basis for the ore-forming parent rock.2. The research of oxidability-reducibility of porphyry indicate that the value of Ce(IV)/Ce(III) is great than250,8Eu is less than0.38which is the geochemical indicators of porphyritic type mineralization in arc environment; the value of Ce(IV)/Ce(III) is greater than150, SEu is less than0.13which is the geochemical indicators of skarn type mineralization in continental arc environment.3. The occurrence of MME and its geochemical characteristics show that the MASH process of Yangla complex is felsic magma which come up with assimilation and contamination with surrounding rock. The in Pulang complex is mainly through massing basic magma into acid magma, and it’s the result of mafic magma degassing and convection together with felsic magma.4. Pulang copper deposit have large amount of pyrrhotite and owning lots of reducing compositions, such as CO2, CO and CH4in the ore-forming fluid, and with fO2values below the FMQ buffer, characterized by reduced porphyry copper deposits. The copper solubility in reduced fluid is lower than that in the oxidized fluid, but the gold solubility is not affected, that makes Cu grade low but with massive Au, Mo mineralization.5. Yangla copper deposit is typical skarn deposit, and ore forming rock is QMP. Although, the deposit is not a product of carbonate rocks direct contact with magmatic rocks, but a replacement of mainly magmatic fluid carrying ore-forming elements and lens marble in layer.6. Under the background of oblique subduction, the interbending fault and shear of Yubo anticlinorium made the shape of the Yangla deposit ore like as "bended comb". Arc parallel structures, tension viens and fractures made the shape of Pulang despite like as "plurality of gourd".