西藏恰功矽卡岩铁矿床成矿流体特征及演化:对Fe-Pb矿化的约束

Ore-Forming Fluids Signature and Evolution in the Qiagong Fe Skarn Deposit of the Gangdese Belt, Tibet: Implications for Fe-Pb Mineralization

目前关于恰功矽卡岩型铁矿床的流体演化过程及成矿机制,尤其是铁-铅矿体的成矿作用尚缺研究.对不同阶段的主要矿物进行包裹体均一温度-盐度、激光拉曼光谱分析以及H-O同位素测试.进矽卡岩阶段包裹体均一温度为400~550℃;盐度为15.5%~20.9%NaCleqv,其中S型盐度高达56.5%NaCleqv;气液相成分均为H2O.退化蚀变阶段包裹体均一温度为350~420℃;盐度集中于14.1%~16.6%NaCleqv,少量为2%~8%NaCleqv,而S型包裹体盐度亦高达55.8%NaCleqv;气液相成分均为H2O,液相富含HCO3-和CO32-.石英-方铅矿阶段包裹体均一温度范围为238~343℃,对应盐度为3.1~13.9%NaCleqv,其中含CO2三相包裹体完全均一温度集中在290~310℃,盐度为1.6%~11.2%NaCleqv.石英-方解石阶段包裹体均一温度与盐度分别为242~360℃和1.7%~11.8%NaCleqv,气液相成分均为H2O.H-O同位素显示:进矽卡岩阶段δDH2O为-106.4‰^-113.2‰,δ18OH2O为6.2‰~8.0‰;退化蚀变阶段δDH2O为-84.8‰^-130.1‰,δ18OH2O为2.7‰~5.5‰,退化蚀变阶段δ18OH2O值相对进矽卡岩阶段低;石英-方铅矿阶段δDH2O为-95.3‰^-103.8‰,δ18OH2O为-1.6‰^-0.7‰;石英-方解石阶段δDH2O为-67.4‰^-101.0‰,δ18OH2O为-0.8‰~0.6‰.结果表明流体整体具有从高温、中-高盐度逐渐向低温、低盐度演化的特征,矽卡岩期成矿流体来源于岩浆出溶;矽卡岩期流体的不混溶作用并与围岩发生反应是磁铁矿沉淀的重要机制,石英-方铅矿阶段流体温压下降是方铅矿沉淀的根本原因.

There is still lack of research on the evolution of ore-forming fluids and ore-forming mechanism of the Qiagong Fe skarn deposit in the Gangdese Belt,especially the mineralization of the iron-lead ore body.We studied the homogenization temperature,salinity,laser Raman spectroscopy and H-O isotopic compositions of major minerals in different stages.The homogeneous temperatures of the fluid inclusions that in the prograde stage range from 400 to 550℃,and the salinities vary from 15.5%to20.95%NaCleqv.Besides,the salinity of S-type fluid inclusion in this stage is up to 56.5%NaCleqv.Both the vapor and the liquid phases in these fluid inclusions are H2O.The homogeneous temperatures of fluid inclusions during the retrograde stage range from350℃to 420℃,and the salinities are primarily concentrated in 14.1%-16.68%NaCleqv,partly in 2%-8%NaCleqv.However,the salinity of type-S inclusion is up to 55.8%NaCleqv.Both the vapor and the liquid phases in these fluid inclusions are H2O.In addition,the liquid phase is also rich in HCO3-and CO32-.The homogeneous temperatures of the fluid inclusions in quartzgalena stage are 238-343℃,corresponding to the salinities of 3.1%-13.9%NaCleqv.Additionally,the total homogenization temperatures of CO2-bearing three-phase fluid inclusions are 290-310℃,corresponding to the salinities of 1.6%-11.2%NaCleqv.Lastly,during the quartz-calcite stage,the homogeneous temperatures and salinities of the fluid inclusions vary in 242-360℃and 1.7%-11.8%NaCleqvrespectively.The component of liquid phase in fluid inclusion is dominated by H2O,as well as the vapor phase.The H-O isotopes show that theδDH2O andδ18OH2O are-106.4‰--113.2‰and 6.2‰-8.0‰during the prograde stage,and-84.8‰--130.1‰and 2.7‰-5.5‰during the retrograde stage,respectively.Theδ18OH2O compositions in the retrograde stage are lower than those in the prograde stage.TheδDH2O andδ18OH2O are-95.3‰--103.8‰and-1.6‰--0.7‰in the quartz-galena stage and-67.4‰--101.0‰and-0.8‰-0.6‰in the quartz-calcite stage,respectively.These data indicate that the fluids evolved from high temperature,medium-high salinity to low temperature and low salinity.The ore-forming fluids are mainly derived from magma exsolution in skarn stage.The fluid immiscibility and wall-rock interaction during skarn stage are the main mechanisms of magnetite precipitation.Decreased pressure and temperature during quartz-galena stage might be the prime reasons for precipitation of galena.