湘中杏枫山金矿床流体包裹体特征及其对矿床成因的指示

Characteristics of fluid inclusions of the Xingfengshan Gold Deposit,central Hunan,and its genetic implications

杏枫山金矿是湘中盆地典型的石英脉型金矿床,矿床位于白马山复式岩体的外接触带,主要赋存于新元古界板岩—千枚岩中。为了查明杏枫山金矿床的成矿流体特征,并揭示其矿床成因,本文在对该金矿的矿床地质特征、矿物共生关系进行了野外调查和室内镜下研究的基础上,利用岩相学、显微测温以及激光拉曼显微探针分析等技术手段,对该金矿的不同期次石英中的包裹体开展了系统研究。研究结果表明:成矿期石英脉呈席状产出,其流体包裹体以富液相为主,含少数富气相包裹体和CO2包裹体,流体包裹体的均一温度在220~420℃范围内,盐度为0.35%~11.94%NaCleqv;成矿后石英中流体包裹体的均一温度和盐度均明显小于成矿期。该金矿床的成矿流体属中高温、贫CO2的还原性H2O—NaCl(±KCl)—CO2—CH4—N2体系,减压沸腾作用造成成矿流体的氧逸度、pH值改变,是导致该区金矿石沉淀的主要原因。湘中杏枫山金矿的成矿温度高,成矿压力较小,成矿流体及地质特征均明显有别于国内外典型的造山型金矿床。结合其围岩蚀变类型和矿物共生组合等特征,可推断杏枫山金矿床属于与侵入岩有关的金矿体系(IRGS)。

Objectives:The Xingfengshan lode gold deposit is a representative of gold deposits occurred in the Xiangzhong Basin,central Hunan.The deposit,located in the external contact zone of Baimashan granitic pluton,is hosted by Neoproterozoic slate and phyllite rocks.This study devotes to ascertain the characteristics of ore-forming fluids and further understand the ore genesis of the Xingfengshan gold deposit.Methods:Based on field investigation and indoor microscopic study,the petrographic observation,microthermometric measurements and laser Raman spectroscopy on the fluid inclusions hosted in different stage quartz from the Xingfengshan deposit,were systematically carried out in this study.Results:The fluid inclusions hosted in sheeted quartz vein of the ore-forming stage mainly include liquid-rich aqueous inclusions,and a few of vapour-rich aqueous inclusions and less CO2-rich inclusions,its homogenization temperatures vary between 220℃and 420℃,and its salinities change from 0.35%to 11.94%Na Cleq;while the homogenization temperatures and salinities for the fluid inclusions in the quartz veins of post-ore-forming stage are obviously less than those of fluid inclusions in auriferous quartz.The ore-forming fluid in this study belongs to the meso-and hyper-thermal,CO2-poor and reduced H2 O—Na Cl(±KCl)-CO2—CH4—N2 system,the change of fO2 and p H caused by decompression boiling is the key factor to gold precipitation.Conclusions:The geological characteristics of the Xingfengshan deposit and its ore-forming fluid signatures with high-temperatures and the low-pressure are obviously different from the orogenic gold deposits.Combined with the type of wall rock alterations and its mineral assemblages,it can be concluded that the Xingfengshan gold deposit belongs to a typical one of the intrusion-related gold systems(IRGS).