Study on the physical and chemical conditions of ore formation of Hetai ductile shear zone-hosted gold deposit and discovery of melt inclusions

Abstract

The Hetai ductile shear zone-hosted gold deposit occurs in the deep-seated fault mylonite zone of the Sinian-Silurian metamorphic rock series. In this study there have been discovered melt inclusions, fluid-melt inclusions and organic inclusions in ore-bearing quartz veins of the ore deposit and mylonite for the first time. The homogenization temperatures of the various types of inclusions are 160°C, 180 – 350°C, 530°C and 870°C for organic inclusions, liquid inclusions, two-phase immiscible liquid inclusions and melt inclusions, respectively. Ore fluid is categorized as the neutral to basic K+-Ca2+-Mg2+ -Na+ − SO 4 2− -HCO 3 − -Cl− system. The contents of trace gases follow a descending order of H2O> CO2> CH4> (or CO> C2H2 > C2H6> O2>N2. The concentrations of K+, Ca2+, SO 4 2− , HCO 3 − ,. Cl−, H2O and C2H2 in fluid inclusions are related to the contents of gold and the Au/Ag ratios in ores from different levels of the gold deposit. This is significant for deep ore prospecting in the region. Daughter minerals in melt inclusions were analyzed using SEM. Quartz, orthoclase, wollastonite and other silicate minerals were identified. They were formed in different mineral assemblages. This analysis further proves the existence of melt inclusions in ore veins. Sedimentary metamorphic rocks could form silicate melts during metamorphic anatexis and dynamic metamorphism, which possess melt-solution characteristics. Ore formation is related to the multi-stage forming process of silicate melt and fluid.