湘西董家河铅锌矿床成矿流体和成矿物质来源:来自流体包裹体和C-O-Sr同位素的制约

Sources of ore-formig fluids and materials of the Dongjiahe lead-zinc deposit in the Western Hunan, China: Constraints from data of fluid inclusions and C-O-Sr isotopes

董家河铅锌矿是湘西地区赋存于震旦系地层中唯一的大型铅锌矿床。为探讨成矿流体及成矿物质来源,系统开展了野外调研、流体包裹体测温、激光拉曼探针及C-O-Sr同位素研究。研究结果表明,矿床成矿阶段可划分为石英-黄铁矿阶段(Ⅰ)、多金属硫化物阶段(Ⅱ)及碳酸盐阶段(Ⅲ)。原生流体包裹体类型单一,仅见气液两相、纯液相流体包裹体及少量纯气相包裹体。3个成矿阶段的流体包裹体均一温度分别为121.5~255.2℃、120.6~232.7℃和102.5~167.9℃,盐度分别为6.20%~14.77%Na Cl_(eqv)、3.39%~11.93%NaCl_(eqv)和2.07%~4.18%NaCl_(eqv)。激光拉曼测试显示流体中含有CH_(4)、C_(2)H_(6)及沥青等有机质,成矿流体属于中低温、中低盐度的H_(2)O-NaCl±CH_(4)±C_(2)H_(6)体系。方解石C-O同位素组成表明,成矿流体中的碳部分来自碳酸盐岩的溶解,部分来自于基底地层中的有机碳。方解石的^(87)Sr/^(86)Sr值(0.708237~0.710326)高于碳酸盐岩围岩,表明热液中存在来自于基底的Sr。综合分析显示以陆源碎屑为主的陡山沱组及其下伏地层为董家河铅锌矿成矿提供了物质支持。富含有机质的热卤水携带大量金属物质,在震旦系白云岩层合适构造空间内,与地层中先存的含硫酸盐的建造水混合,发生硫酸盐热化学还原作用(TSR)产生还原硫,还原硫与铅锌等金属离子结合从而导致卸载成矿。

The Dongjiahe Pb-Zn deposit is the only large-scale Pb-Zn deposit hosted in the Sinian strata in the Western Hunan.Aiming to discuss the sources of ore-forming fluids and materials,field investigation,fluid inclusionmicrothermometry and Laser Raman spectrometry,and C-O-Sr isotopes of the deposit have been systematically performed in this paper.Three stages of hydrothermal mineralization of the deposit,including the quartz-pyrite stage (I),polymetallic sulfides stage (II),and carbonate stage (III),have been identified.Fluid inclusion petrographic study shows that there are simple types of primary fluid inclusions including liquid-vapor two-phase inclusions (L+V),pure liquid inclusions,and minor pure vapor inclusions.The fluid inclusions in minerals of three stages have homogenization temperatures ranging from 121.5℃ to 255.2℃,from 120.6℃ to 232.7℃,and 102.5℃to 167.9℃,respectively,and have corresponding salinities varying from 6.20% to 14.77% NaCl_(eqv),from 3.39% to 11.93% NaCl_(eqv),and from 2.07% to 4.18% NaCl_(eqv),respectively.The Laser Raman spectrometry study reveals that the inclusion fluids contain organic matters,such as CH,CH,and asphalt.It is believed that the ore-forming fluid belongs to the medium-low temperature and medium-low salinity H_(2)O-NaCl±CH_(4)±C_(2)H_(6)system.The C-O isotopic compositions of calcites demonstrate that carbon in ore-forming fluids was derived partly from the dissolution of carbonate rocks and partly organic carbon in the basement strata.TheSr/Sr ratios of calcites ranging from 0.708237 to 0.710326 are higher than the those of carbonate wallrocks,suggesting that the ore-forming hydrothermal fluid probably contains basement-derived Sr.The comprehensive analysis indicates that the Sinian Doushantuo Formation dominated by terrigenous clastic rocks and its underlying strata could have provided material support for mineralization of the Dongjiahe Pb-Zn deposit.The mixing of organic-rich hot brine,which carried a large amount of metal elements,and sulfate-bearing formation water,which was pre-existed in the ore-hosted strata in the appropriate structural space of the Sinian dolomite layer,had resulted in the formation of reduced sulfur by thermochemical sulfate reduction (TSR),and then the combination of reduced sulfur with lead and zinc cations from the hot brine had resulted the unloading of those cations and the mineralization of the Dongjiahe Pb-Zn deposit.