Ore‐forming Fluid Characteristics of the Saishitang Cu‐polymetallic Deposit in Qinghai Province, China
Authors: Jianqing LAI , Peijiao JU , Yin MAO , Jianghua AN , Xiongjun WANG 
Published: April 2015

Saishitang Cu‐polymetallic deposit is located in the southeast section of Late Paleozoic arcfold in the southeastern margin of Qaidam platform. Accoring to the geological process of the deposit, four mineralization episodes were identified: melt/fluid coexisting period (O), skarn period (A), first sulfide period (B) and second sulfide period (C), and 10 stages were finally subdivided. Three types of inclusions were classified in seven stages, namely crystal bearing inclusions (type I), aqueous inclusions (type II) and pure liquid inclusions (type III). Type I and II inclusions were observed in stage Ol, having homogenization temperature from 252 to 431°C, and salinities ranging from 24.3% to 48.0%. Type I inclusion was present in stage A1, having homogenization temperature from 506 to 548° C., and salinities ranging from 39.4% to 44.6%. In stage B1, type II and III inclusions were observed, with homogenization temperature concentrating between 300–400°C, and salinities from 0.4% to 4.3%. Type II inclusions were present in stage B2, with homogenization temperature varying from 403 to 550° C. In stage C1, type I and II inclusion commonly coexisted, and constituted a boiling inclusion group, having homogenization temperatures at 187–463°C, and salinities in a range of 29.4%–46.8% and 2.2%‐11.0%. Type II and III inclusions were developed in stage C2, having homogenization temperature at 124–350°C, and salinities ranging between 1.6% and 15.4%. In stage C3, type II and III inclusions were presented, with a homogenization temperature range of 164–360°C, and salinities varying from 4.0% to 11.0%. The results of micro‐thermal analysis show that fluids are characterized by high temperature and high salinity in stage O1 and A1, and experienced slight decrease in temperature and dramatic decrease in salinity in stage B1 and B2. In stage C1, the salinity of fluid increased greatly and a further decrease of temperature and salinity occurred in stage C2 and C3. Fluids boiled in stage C1. With calculated pressure of 22 MPa from the trapping temperature of 284–289°C, a mineralization depth of 2.2 km was inferred. Results of Laser Raman Spectroscopy show high density of H2O, CH4 and CO2 were found as gas composition. H‐O isotope study indicates the ore‐forming fluids were the mixture of magmatic water and meteoric water. Physicochemical parameters of fluids show oxygen and sulfur fugacity experienced a decrease, and redox state is weakly reducing. Along with fluid evolution, oxidation has increased slightly. Comprehensive analysis shows that melt exsolution occurred during the formation of quartz diorite and that metal elements existed and migrated in the form of chlorine complex. Immiscible fluid separation and boiling widely occurred after addition of new fluids, bringing about dissociation of chlorine‐complex, resulting in a great deal of copper precipitation. In conclusion, Saishitang deposit, controlled by regional tectonics, is formed by metasomatism between highly fractionated mineralization rock body and wall rock, and belongs to banded skarn Cu‐polymetallic deposit.