Geological characteristics and geochemistry of ore-forming fluid in the Tongchang copper-gold polymetallic deposit, Shaanxi

Abstract：The Tongchang copper-gold polymetallic deposit is a typical deposit of the Tongchang ore field in the Mian-Lue-Yang metallogenic province in China. Based on the study of the geological characteristics of the deposit, the ore-forming process may be divided into the pneumatolyto-hydrothermal period and reworked hydrothermal period. The latter may be subdivided into the pyrite-arsenopyrite-calcite-quartz stage, polymetallic sulfide stage and pyrite-carbonate-quartz stage. It is especially the case with the Cu-Au-(Co) polymetallic quartz-carbonate orebody occurring in NE-trending schistose zones of the diorite, where metallic mineralization is featured by horizontal zoning: from east to west there appear Cu→Co-Cu (Au)→Cu→Au-Au mineralizations and their tectono-geochemical anomalies. Four types of fluid inclusion are developed: vapor-liquid inclusions, liquid inclusions, vapor inclusions and liquid CO2-bearing polyphase inclusions, of which the first two types are dominant. Medium- and low-temperature, medium-pressure, moderate-oxygen fugacity and nearly neutral ore fluid conditions are favorable to the formation of the reworked hydrothermal deposits. The ore fluids were derived from reworked water (heated seawater and precipitation), magmatic water and metamorphic water. From the early to late stages of the reworked hydrothermal period, the reworked water was more and more dominant. So the deposit is regarded as a reworked meso-and epithermal deposit.