Ore-forming fluids and metallogenesis of the Qibaoshan Cu-Au deposit in the Wulian County, Shandong Province, China

Qibaoshan mine, located on the east side of the Yishu fault, is an epithermal Cu-Au deposit in southwestern Shandong Province. Fluid inclusion studies indicate that the inclusion types are comparatively simple, with abundance of very small inclusions which have low gas-liquid ratios at the pre-mineralization stage, a great number of low gas-liquid ratios gas-liquid inclusions dominating at the mineralization stage and large amount of low gas-liquid ratios and pure liquid inclusions after the mineralization. Homogenization temperatures decreased with time from 290 degrees C before the mineralization to 160 degrees C during the main mineralization stage and then reached 130 degrees C at the end. A gradually downward salinities from 14.7% NaCl at the early stage to 5.6% NaCl at the main ore-forming stage and 2.9% NaCl at the end, show to us the evolution process of the fluids. Observations made on this deposit suggest it is an epithermal Cu-Au deposit, in which gold was mostly transported in the form of gold sulfide complex within the fluids which concentrated near the earth surface and crypto-explosion, resulting in gold and copper precipitation. Considering the tectonic history of this area, it is believed that Qibaoshan Cu-Au deposit was formed under a great influence of tectonic system transformation and lithospheric thinning events in the eastern China in Cenozoic, and during that time extensive magma activities combined with tectonically weak belts (faults) near the Yishu fault leaded to the metallogenesis.