内蒙古西山湾羊场火山岩银矿床流体包裹体研究

Study on fluid inclusions of the Xishanwanyangchang silver ore deposit in volcanic rocks,Inner Mongolia

西山湾羊场火山岩银矿床位于内蒙古中西部,大地构造位置处于华北地台西北部。银矿体上盘围岩为二叠纪黑云母二长花岗岩,下盘围岩为寒武纪英云闪长岩,矿体呈透镜状或似层状产出于下白垩统白女羊盘组及其与上下盘的接触带,赋矿岩石主要为具硅化、褐铁矿化的碎裂流纹岩、流纹斑岩。矿石结构主要有自形-半自形粒状结构、他形粒状结构、交代残余结构、碎裂结构等;矿石构造主要有角砾状构造、浸染状构造、脉状构造、块状构造等。主要矿化现象为褐铁矿化、黄铁矿化、萤石矿化,局部见铅锌矿化。矿石矿物主要为黄铁矿、辉银矿、螺状硫银矿、闪锌矿、方铅矿等,矿物生成顺序:黄铁矿-闪锌矿(黄铜矿)-方铅矿-赤铁矿-辉银矿。根据矿物间的相互交代关系,将矿化过程分为早、中、晚三个阶段,分别为石英-黄铁矿、石英-多金属硫化物和石英-碳酸盐组合为标志,特征性围岩蚀变主要为褐铁矿化、硅化、绿泥石化、碳酸盐化等。脉石矿物主要有石英、蛋白石、玉髓、萤石、钾长石及少量绢云母等。矿石的石英中主要可见气液两相水溶液包裹体,还可见少量含石盐子晶三相包裹体,偶尔可见含硫化物子晶多相包裹体。早阶段流体包裹体均一温度大于330℃,盐度0.35%~5.86%NaCleqv,主要发育气液两相的水溶液包裹体,为高温、低盐度流体特征;中阶段流体包裹体均一温度集中在250~330℃,盐度为0.35%~31.90%NaCleqv,除水溶液包裹体外,还可见含石盐子晶三相包裹体和含硫化物子晶多相包裹体,以低盐度的水溶液包裹体和高盐度的含子矿物多相包裹体并存为特征;晚阶段流体包裹体均一温度集中在169~250℃,盐度0.71%~32.66%NaCleqv,除水溶液包裹体外,还可见含石盐子晶三相包裹体,以低盐度的水溶液包裹体和高盐度的含石盐子矿物包裹体并存为特征。中阶段的含硫化物子矿物多相高盐度流体包裹体是"岩浆二次沸腾"形成的,晚阶段的含石盐子矿物高盐度流体包裹体是直接从结晶的熔体中出溶的,流体混合是该矿床的可能成矿机制,金属沉淀主要是因含金属高盐度流体与冷的地下水在矿石沉淀部位混合而最终沉淀富集成矿。

The Xishanwanyangchang silver ore deposit in volcanic rocks, lied in mid-west area of Inner Mongolia, is located in the northwest of the North China Plate. Hanging wall rock of the silver deposit is Permian biotite adamellite, and footwall rock is Cambrian tonalite. Silver polymetallic ore-bodies are lentiform or stratiform occurring in the rhyolite, rhyolite porphyry, and rhyolite breccia of lower series of Cretaceous Bainvyangpan Formation （K1bn） as well as contact zone of the volcanic rock and granite. The ores mainly have euhedral to subhedral granular, anhedral granular, metasomatie-relict and cataclastic textures and brecciated, disseminated, vein and massive structures. The mineralization mainly is ferritization, pyritization, fluoritization and mineralization associated with lead-zinc. The sulfide mineral mainly consists of pyrite, and a small amount of argentite, acanthite, sphalerite and galena. The hydrothermal ore-forming process includes three stages, i. e. the early, middle and late stages, characterized by quartz-pyrite stage, quartz-polymetallic sulfides stage and quartz-carbonate stage, respectively. The main types of wall rock alteration include quartzitification, sericitization, chloritization, pyritization, limonitization, kaolinization, carbonatization. The gangue minerals are mainly composed of quartz, opal, chalcedony, fluorite, potash feldspar and a small amount of sericite. In quartz phenocrysts of rhyolite, three types of fluid inclusions can be observed, i.e. NaCl-H2O solution, the fluid inclusion containing halite daughter mineral and the fluid inclusion containing chalcopyrite daughter mineral. In early stage, homogeneous temperatures of fluid inclusions are mainly above 330℃, with fluid salinities ranging from 0.35% to 5.86% NaCleqv, and the liquid-rich fluid inclusions are mainly observed, with high temperature and low salinity. In middle stage, homogeneous temperatures of fluid inclusions are mainly ranging from 250℃ to 330℃, with fluid salinities ranging from 0.35% to 31.90% NaCleqv, and the fluid inclusions containing halite daughter mineral and the fluid inclusions containing chalcopyrite daughter mineral can be observed except liquid-rich fluid inclusions, characterized by the daughter mineral-bearing inclusions of high salinity coexisting with the liquid-rich fluid inclusions of low salinity. In late stage, homogeneous temperatures of fluid inclusions are mainly ranging from 169℃ to 250℃, with fluid salinities ranging from 0.71% to 32.66% NaCleqv, and the fluid inclusions containing halite daughter mineral can be observed except liquid-rich fluid inclusions, characterized by the fluid inclusions containing halite daughter mineral of high salinity coexisting with the liquid-rich fluid inclusions of low salinity. The fluid inclusions containing chalcopyrite daughter mineral of high salinity in middle stage are resulted from boiled fluid, and the fluid inclusions containing halite daughter mineral of high salinity in late stage are preferably interpreted as being generated directly from a water-saturated crystallizing magmatic melt. The mineralization should be triggered by the temperature dropping and fluid mixing. Metal precipitation is mainly due to mixing between metal-bearing, high-salinity fluid and cold groundwater in the place where ores were deposited, resulting in enrichment and mineralization of the metal elements.