Geological and geochemical characteristics of hydrothermal alteration in the Wangu deposit in the central Jiangnan Orogenic Belt and implications for gold mineralization

Abstract Hydrothermal alteration commonly occurs in lode gold deposits, and contributes to metal precipitation through water–rock interactions. In the Wangu gold deposit in northeastern Hunan, South China, bleaching of slates enclosing auriferous quartz veins widespreadly occur, with the colour transformed from dark-grey into pale yellow-grey. The bleaching has been used to guide gold exploration, but its nature and genesis, as well as the relationship with gold mineralization, remains unclear. Field and petrographic work, combined with Tescan Integrated Mineral Analyzer (TIMA), demonstrates that the bleaching is mainly caused by the combination of sericitization and carbonatization. Combined with fluid inclusion studies, it is inferred that bleaching is caused by the pre-ore hydrothermal event characterized with CO2-bearing fluids. Large amounts of siderite are distributed within the bleaching zones, and they are locally crosscut by sulfides, suggesting that the bleaching happened before gold mineralization. Mass balance calculation and μ-XRF analysis indicate that despite the abundance of siderite, Fe is not gained during alteration. Geochemical path modeling further suggests that siderite is efficient in precipitating Au by decreasing S contents in fluids and destroying Au-bearing complexes. In addition, the siderite contents in altered slates and Au contents in sulfides suggest that the chemical interaction between siderite and ore fluids can produce ores with economic significance. The siderite developed during the pre-mineralization alteration, provided ideal chemical traps and prepared the ground for gold mineralization. When the gold-bearing fluids overprinted the altered rocks, sulfides formed through reaction with siderite. It is concluded that the sulfidation, which resulted in the destruction of Au-S complexes, is the major mechanism for gold mineralization within the bleached slates.