| Southwest Sanjiang(Nu river, Lancang river, Jinsha river)region is located in theeastern Tibetan Plateau, and is part of Alpinei-Himalayan orogen,too. The depositoccurs eastern of Tethyan tectonic which is the connection position of Indian plate andEurasian plate, spreading from NNW to SN trending. The area has experienced aseries of tectonic events, such as oceanic subduction, continental-arc collision andintracontinental with the characteristics of long-term acticities. In the tectonic region,magmatic activity is frequent and intense, meatllogenic condition is superior with richmineral resources, and is one of Chinese major nonferrous metals and precious metalsdeposits with the formation of large, super-large deposits, having giant prospectingpotential.Shaxi Ag-Pb-Zn deposit is located in the middle zone of Yidun island arc foldbelt which is in the north of southwest Sanjiang area, with frequent magmatism,developed folds and faults. In addition, the deposit is the second oversized silverpolymetallic deposit after Gacun deposit (volcanogenic massive sulphide deposits),Xiasia silver polymetallic deposit in Yidun island arc. Since deposit has been found,As a result of the limitation of field environment,a few scholars to systematicallystudy its ore-forming geological background, deposit geological characteristics,source area, geochemical characters and so on. Based on studying and analyzing theprevious results material, through a great deal of field work and laboratory study,guided by the theory of regional metallogeny and geochemical, explore theore-forming sources and fluid of Shaxi deposit, and understand the deposit genesis.The ore body occurs in the form of vien, lenticular and cystic of Shaxi silverpolymetallic deposit, which is controlled by two faults cutting, Thum strata group andits staggered position, one fault is NNW trending, and the other is NW trending. Themain minerals are composed of galena, sphalerite, chalcopyrite, pyrrhotite, containinga small amount of chalcopyrite, arsenopyrite, pyrite. These minerals are represented ina very high temperature conditions as pyrrhotite, but also under low temperatureconditions as galena, sphalerite, indicating that the formation of the depositexperienced different mineralization periods. According to the study of the petrography, rock geochemistry and chronology inorefield, the age of Rong Yicuo granite ranges from97.5to98.18Ma. Because thetime is concentrated, it is the product of acidic magma activity. Gabbro vein’s zirconU-Pb age is108Ma. The10Ma years gap between Rong Yicuo granite and Gabbrovein may reflect the region’s basic magma evolution experienced by acidic magmacrystallization differentiation of duration.The characteristics of sulfur and lead isotope indicates that the source of sulfuris magma and partly could be mixed with the formation of organic sulfur. Theresearch of lead isotope concludes that the lead material should be the mixed lead,mainly comes from supracrustal meterial. In the process of ore deposition, it maybemixed with a certain amount of radioactive origin of lead in surrounding rock. Thecharacteristics of carbon isotope indicates that the carbon source of ore-formingsolution is diversitily. Most of them are derived from magma carbon and inorganiccarbon, and almost do not contain organic carbon. Oxygen isotope research concludesthat during the process of deposit formation,it may be exchanged with deephydrothermal fluid isotopic, and went through fluid mixing effect. It plays animportant role in the concentration of ore-forming metal elements.The study of fluid inclusion indicates that the ore-forming fluid of Shaxi deposithave the characteristics of middle-low salinity, middle-low salinity temperature andmiddle-low density, its ore-forming fluid may mainly comes from magmaichydrothermal fluid. The diagenetic mineralization pressure is51.86~55.65MPa,Buried depth ranges form1.9to2.1Km. This outcomes prove that Shaxi deposit has ashallow metallogenic depth.By further research, we conclude that Shaxi Ag-Pb-Zn deposit is a lowtemperature magmatic hydrothermal deposit. In the early age of mineralization, theore magma hydrothermal is in a high temperature of supercritical state, During theprocess of migration, along with the high temperature hydrothermal boiling water,leads to a large amount of metal elements precipitated. Further with the reduction oftemperature and pressure, and join the atmospheric precipitation, carbonate mineralsin the hydrothermal and metal sulfide separate out and mineralize. |
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