湘东北杨山庄金矿床流体成矿机制

Mineralization mechanism of Yangshanzhuang gold deposit,northeastern Hunan Province

杨山庄金矿床位于江南造山带中部,矿体严格受NW-NWW向断裂控制,赋存于新元古代浅变质的绢云母板岩和粉砂质板岩中,成矿期分为三个阶段:石英-毒砂-黄铁矿(少量)-自然金-白钨矿-白云石-白云母(阶段Ⅰ)、石英-毒砂-黄铁矿-自然金-黄铜矿-方铅矿-闪锌矿-绢云母(阶段Ⅱ)和石英-方解石(阶段Ⅲ)。成矿Ⅰ、Ⅱ阶段流体包裹体以两相水溶液包裹体为主,含有少量的气相包裹体和含CO_2的三相包裹体,气相包裹体成分为CH_4-N_2-CO_2、N_2-CO_2和N_2。成矿温度集中在240~320℃,盐度集中在7%~9%Na Cleqv,成矿流体为中-低温、中低盐度H_2O-Na Cl-CO_2体系。在成矿Ⅰ、Ⅱ阶段,成矿流体与围岩发生交代反应,流体发生相分离,使液相CO_2含量降低,引起Au(HS)^(2-)溶解度降低,导致金发生沉淀。

The Yangshanzhuang gold deposit,located in the middle part of the Jiangnan orogenic belt,is hosted by the Neoproterozoic low-grade metamorphic sericite slate and silty slate and is strictly controlled by NW-NWW-trending faults. According to intergrowth and crosscutting relationships among different minerals,three paragenetic stages were identified, which are quartzarsnopyrite-pyrite-native gold-scheelite-dolomite-muscovite（ stage Ⅰ）; quartz-arsnopyrite-pyrite-native gold-chalcopyrite-galenasphalerite-sericite（ stage Ⅱ） and quartz-sericite（ stage Ⅲ）. Fluid inclusions in stage Ⅰ and Ⅱ are mainly aqueous inclusions with minor gaseous inclusions and H2O-CO2 inclusions. Raman spectroscopy analysis indicates that gaseous inclusions are consist of N2,CH4 and CO2. Mineralization temperature and salinity of the ore-forming fluid are restricted in ranges of 240 - 320℃ and 7% - 9%Na Cleqv,respectively. The combined microthermometry and Raman spectroscopy studies suggest a H2O-Na Cl-CO2 fluid system characterized by medium-low temperature and medium-low salinity. Fluid phase separation occurred during the process of wall rock metasomatism in Stage Ⅰ and Stage Ⅱ. The concentration of liquidus CO2 decreased because of phase separation might further facilitate the deposition of gold.