T-P-X evolution path of ore forming fluid in Qiagong polymetallic deposit, Tibet, China

Qiagong polymetallic deposit is genetically related to monzogranite porphyry. The mineralization styles include skarn type Fe(-Cu), distal vein type Pb-Zn-Ag(-Cu) and carbonate replacement type Pb-Zn-Ag. Based on field observation and petrographic results the ore-forming process in this deposit can be divided into at least six stages including garnet-magnetite stage (I), epidote-magnetite stage (II), quartz-hematite stage(III), fluorite-chalcopyrite stage (IV), calcite-galena-sphalerite stage (V) and calcite-quartz stage(VI). Petrographic, microthermometric and laser Raman microprobe results of fluid inclusions in quartz, garnet, epidote, calcite and fluorite from these six stages demonstrate that the pressure, temperature, density and salinity of the ore forming fluid decrease from early to later stage, and the fluid system change from early H2O-NaCl(I-II) via H2O-NaCl-FeCl2-3 +/- MgCl2(III), H2O-CO2-NaCl(IV) to late H2ONaCl-CaCl2(V&VI). The deposition of Cu in Qiagong was triggered by the boiling of the ore-forming fluid of H2OCO2-NaCl system from stage IV. Proton induced X-ray emission analyses show that the concentration of the metallic elements (such as Pb, Zn etc) in the fluid inclusions also decreased with the fluid evolution.