江南造山带黄金洞金矿蚀变岩型金矿化形成机制研究

蚀变岩型矿石是热液型金矿床中重要的矿石类型，其形成与水岩反应密切相关。江南造山带黄金洞金矿中的蚀变岩型矿石多发育在褪色化蚀变带中，目前其形成机制尚不清楚。野外勘查、岩相学观察和TIMA分析表明，蚀变带中最主要的特征是广泛发育菱铁矿、绢云母和隐晶质石英。蚀变岩型矿石局部可见较多硫化物，与含金石英脉中的硫化物具有相似的化学成分，说明为同一期流体作用的产物。部分硫化物切穿碳酸盐矿物，说明褪色化蚀变形成于成矿前。TIMA和μ-XRF分析表明，蚀变带发生硅化和绢云母化，且Fe聚集成点状。因此，推测成矿前的水岩反应形成大量菱铁矿斑点，为成矿提供良好的化学圈闭。成矿期含金流体与菱铁矿发生化学反应，通过硫化作用促进金的沉淀，为黄金洞金矿蚀变岩型矿石的成矿机制。

Study on the Formation Mechanism of Altered Rock Type Gold Mineralization of Huangjindong Gold Deposit in Jiangnan Orogenic Belt

Quartz （carbonate） vein type and altered rock type ores are the most important ore types in hy-drothermal gold deposits，and the quartz vein type ores are widely studied. However，altered rock type ores are much less well studied due to the complexity of mineral compositions and unavailability for fluid inclusion and geochemical analysis. Compared with quartz veins，altered rock ores generally has a lower grade but larger reserve，which is of great significance for gold exploration. 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 mostly developed in the bleaching alteration zone.Previous research demonstrates that the alteration associated with mineralization mainly include sericitization，silicification，carbonatization and sulfidation，but the gold precipitation mechanism of altered slate ores remains indistinct. Based on field work， and petrographic observations，altered rock type ores in the Huangjindong gold deposit commonly occur in the bleaching alteration zone with a remarkable color transformation from greenish grey to yellow-pale grey.The bleaching alteration zone generally distribute symmetrically along carbonate-quartz veins and mainly characterized by the occurrence of siderite spots，as well as sericites and cryptocrystalline quartzes. Abundant gold-bearing sulfide in altered slates aggregate near carbonate spots，sharing similar geochemical compositions with those in quartz veins viaElectron Probe Microanalysis （EPMA）.These sulfides locally crosscut siderite grains，as well as the cementing of partially dissolved siderite grains by quartz，demonstrating that the fading alteration took place before gold mineralization and likely generated by the reaction of CO₂-rich fluids with host rocks. Tescan Integrated Mineral Analyzer （TIMA） analysis shows that two alteration zones have been observed in the altered slate，from the proximal to the distal side of the carbonate-quartz vein are silicification and sericitization，respectively. Micro area X-ray Fluorescence Surface Scan （μ-XRF） elemental mapping on carbonate-quartz vein and adjacent alteration zone suggest that most elements have obvious zoning. Mg，Fe and Mn are aggregated into spots in the alteration zones because of the appearance of siderite. Si is abundant in silicification zone，K and Al contents are high in sericitization zone. 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. The chemical reaction between gold-bearing fluid and siderite spot occurs during the ore-forming period，and contributes to gold mineralization by triggering sulfidation，which is the major genesis for the altered slate ores in the Huangjindong gold deposit.