Geological and Geochemical Characteristics of the Hetai Gold Deposit, South China: Gold Mineralization in an Auriferous Shear Zone

Abstract

The Hetai gold deposit is contained within a ductile shear zone in low-rank metamorphosed rocks of the late Proterozoic Yunkai Group. The ores occur as veins or as lenticular bodies within altered mylonite and ultramylonite host rocks. Native gold is disseminated in silicified, pyritized, and sericitized phyllonites. The auriferous shear zone has undergone two stages of evolution. The early stage was characterized by ductile deformation during which auriferous mylonite containing n x 106 Au was formed. The later stage was developed in brittle-ductile and brittle environments in which magmatic hydrothermal mineralization was superimposed, resulting in local gold enrichments. Studies of fluid inclusions show that the ore-forming temperature varied from 200° C to 280° C, the pressure was 57.4 MPa, and f(O2) varied from 10-37 to 10-39. The ore-forming fluids are rich in HCO3− and CO2, and have a relative proportion of Na+ > K+ > Ca2+ and HCO3− > Cl− > F−> ΣS. The δDH2O of inclusion water ranges from −57 %o to −82 %o and the from 5.71 ‰ to 4.28 ‰, implying multiple sources for the ore-forming fluids. The Pb-isotope composition of ores suggests that the Pb came from the upper crust-i.e., mainly from metamorphosed rocks and granites. The δ34S has a range of −3.06‰ to −1.73 ‰ and is in agreement with the δ34S value of the wall-rock mylonite, indicating a wall-rock source. On the basis of the above characteristics, the authors propose the following metallogenic model for the Hetai gold deposit: (1) early ductile shear deformation and metamorphism in the Hercynian-Indosinian epoch formed altered mylonite-type gold deposits; (2) the superimposition of a magmatic hydrothermal solution occurred in the late Indosinian epoch and resulted in local gold enrichment.