Geochemical Characteristics Of Emeishan Basalt And Mineralization Of Native Copper Deposit In Junction Area Of Yunnan-Guizhou Province, China

Emeishan Basalt spreads widely in Yunnan , Guizhou and Sichuan Province. There are variations between the different parts of the Emeishan Basalt. In the junction area of Yunnan and Guizhou Province , the basalt is special for far-ranging nature copper mineralization. This study focus on Emeishan basalt in the junctional area of Yunnan and Guizhou Province. Based on systematic studies on geological, stratigraphic and geochemical characteristics of the volcanic rocks and copper deposits hosted in the rocks, the composition of primary magma and their changes and the time of volcanic activity have been discussed respectively in this paper. Isotope also has been proposed to explain the evolution of Emeishan basalt copper deposits. From this study, following achievements have been obtained:1. Based on geochemical characteristics of major elements, trace elements and rare earth elements of the basalts, it is indicated that the primary magma of the basalts is derived from enrich mantle II (EM II) with little contamination, and the basalts end mineral composition is garnet lherzolite, and picrite and olivine fractional crystallization have been occured during magma ascent or emplacement. Intensive activity of hydrothermal fluid has also been suggested to explain Rb's intensive negative anomaly.2. The age of the the shale, which overlays Emeishan Basalt Group, have been accessed by iterative reconstruction and regression of Rb-Sr binary isotope paleomixing line. This is different from conventional Rb-Sr dating. From the research, the shale's model age of 255±12 Ma have been dated. This age indicates Emeishan basalts end-eruption happened before 255Ma. It not only supports the study of paleomagnetism of stratum and stratum chronology but also Emeishan basalt main eruption time is nearly 258Ma. This study indicates Rb-Sr isotope chronology is applicable in Mesozoic stratum and 255Ma effectively constraints the upper limit time of Emeishan basalt.3 ²⁰⁶pb/ ²⁰⁴pb , ²⁰⁷pb/ ²⁰⁴ pb and ²⁰⁸ pb/ ²⁰⁴ pb of minerals from copper depositsare generally from 18.078 to 18.923, from 15.463 to 15.690 and from 38.301 to 39.036, respectively. Pb isotopic compositions of the minerals resemble their host rocks-Emeishan basalt, which indicates the matter of basalt copper deposit originated from Emeishan basalt.4. δ¹³C_v-pdb of bitumen and carbonaceous matter from copper ore in basalts range from -32.3%o to -21%o, conforming to the range of Sedimentary organic carbon and Biomass in Nature δ¹³C reservoirs, which suggested that carbon of the bitumen and the carbonaceous matter did not come from mantle source but bio-source. δ¹⁸Osmow and δ¹³C_pdb of calcite from copper ore are generally from 13.1‰ to 22.9‰ and from -32.3‰ to -13.5‰, respectively. It is special in that δ¹⁸O is rich while δ¹³C depleted. Different ore deposit has same carbon isotope composition respectively, which suggests the carbon comes from same source -organic carbon.5. δ¹⁸O_{矿物-sMOW}, ¹⁸O_{h2o- smow} and δD_v-smow of fluid inclusions in quartz and calcite from copper ore are from 14.3‰ to 18.9‰, from 2.8‰ to 7.2‰ and from -63.6‰ to-80.6‰ respectively ,with a medium-low temperature(151℃to201℃). All...