Study On Stable Isotope, Fluid Inclusions And Mineralization Of Saishitang Cu-polymetallic Deposit,Qinghai Province

Saishitang Cu-polymetallic deposit is located in the Late Paleozoic faulted depression zone of southeastern margin of the Qaidam Block. The producing formations include sericitized phyllite/zoned biotite phyllite, metasandstone/meta-siltstone, marble and quartzite of the Middle-Lower Triassic, with the main mineralized intrusion of quartz diorite. The orebodies occur in the contact zone of the intrusion and the Lower Triassic as form of stratoid and lenticular shapes, with main ore minerals of chalcopyrite, pyrrhotite and pyrite. According to the geologic characteristics, four mineralization periods have been identified: melt-fluid coexisting period(O), skarn period(A), first sulphide period(B) and second sulphide period(C), which are subdivided into10mineralization stages.The S, Pb, H and O isotopic studies indicate that the mineralization materials are mainly sourced from magamatic system in depth, Pb and other related metallic elements are derived from crust-mantle mixtures, and the ore-forming fluids originated from the mixing process of magmatic and meteoric waters.The fluid inclusions in the district can be classified into crystal bearing inclusions(type Ⅰ), vapor-liquid two-phase inclusions(type Ⅱ) and liquid inclusions(type Ⅲ). According to the research, the ore-forming fluids of O1and A1are characterized by hyperthermal and high-salinity, then the fluids temperature decrease slightly and the salinity decrease greatly in B1and B2, while the salinity increase greatly during C1, and in the last stages(C2, C3) the temperature and salinity of the fluids decrease further.The hydrothermal fluid boils in C1stage, with a pressure of220bar calculated from the trapping temperature of284℃-289℃, inferring a2.2km mineralization depth, which is an epithermal environment. Laser Raman spectroscopic analysis reveals that H2O, CH4and CO2are contained in vapour phase of the inclusions, and bulk composition analysis indicates that Ca2+、Na+、K+are the main cations in ore-fluid, with main anion of Cl-. The calculated physical-chemical parameters of ore fluids indicate that the oxygen and sulfur fugacity of ore fluid decrease with the time, the redox state is weakly reducing, and increases a little bit with time. By the comprehensive analysis, the melt exsolved from quartz diorite, in which the metals were complexed by Cl and transported as form of Cl complex. Boiling process happened widely during the unmixing of different fluids, where the Cu-Cl complex significantly discomposed, which is the main factor of Cu precipitation. A metallogenic model is established for this ore deposit based on above analysis results.