Genesis of altered slate type ores in the Huangjindong gold deposit, Jiangnan Orogenic Belt, South China

Altered rock ores, closely associated with fluid-rock interactions, represent a typical mineralization style in hydrothermal gold deposits. In the Huangjindong gold deposit of the Jiangnan Orogenic Belt, South China, altered slate ores are generally characterized with bleaching, but the genesis remains not understood. Based on field work, petrographic observations and Tescan Integrated Mineral Analyzer (TIMA) analysis, the alteration of the slate can be defined as carbonatization-sericitization, characterized with the abundant siderite spots and sericite, as well as locally occurred cryptocrystalline quartz. Abundant gold-bearing sulfides are locally present in altered rocks, which constitute the altered slate ores. In altered slates, pyrite and arsenopyrite aggregate near carbonate spots, sharing similar geochemical compositions with those in quartz veins. These sulfides locally crosscut siderite grains, demonstrating that the carbonatization-sericitization alteration took place before gold mineralization. μ-XRF analysis and mass balance calculation show that Fe is not prominently gained or lost but aggregated to form siderite spots in the alteration zone. In addition, K contents increased during alteration, consistent with the presence of sericitization. Thermodynamic modeling of the dissolution of siderite manifests that the chemical reactions between siderite and S-Au-bearing fluids leads to the co-precipitation of gold, pyrite, arsenopyrite and quartz. Moreover, taking account of the siderite contents in altered slates and Au contents in sulfides, the water-rock interaction could generate altered slate ores with economic significance. These results show that large amounts of siderite spots are generated during the pre-mineralization fluid-rock reaction, and thus provide favorable chemical traps for gold mineralization. Abundant siderites contribute to gold mineralization by triggering sulfidation, which is the major genesis for the altered slate ores in the Huangjindong gold deposit.