西藏甲玛铜多金属矿床成因研究——来自流体包裹体的证据

甲玛铜多金属矿床位于西藏冈底斯斑岩铜矿带东段,是近年来勘探发现的超大型斑岩-矽卡岩型铜多金属矿床。通过冷热台显微观察与测温、扫描电镜、激光拉曼探针测试,对甲玛矿床各成矿阶段典型矿物的流体包裹体研究表明,成矿流体富含挥发分,临界相均一的流体来自岩浆超临界流体出溶,主成矿阶段具有沸腾包裹体组合特征,有机质包裹体荧光效应显著。显微测温结果显示,岩浆-热液阶段斑岩中石英斑晶的流体包裹体均一温度范围为250～540℃,含石盐子晶高盐度包裹体盐度范围为35～61(wt%)NaCl.eq,中等盐度的临界均一的气液包裹体盐度范围为3～29(wt%)NaCl.eq,岩浆期后热液阶段斑岩、角岩中石英脉的流体包裹体均一温度范围为210～410℃,盐度范围为33～41(wt%)NaCl.eq,与其不混溶共生的中低盐度气液两相流体包裹体盐度范围为5～25(wt%)NaCl.eq。矽卡岩阶段矿物均一温度范围为130～360℃,盐度范围为3～41(wt%)NaCl.eq,从岩浆热液过渡阶段到石英-硫化物阶段均一温度与盐度呈阶梯式降低趋势。斑岩体石英的流体包裹体中含有较多黄铜矿子矿物,岩浆结晶分异过程中已经具成矿元素的富集。激光拉曼探针测试结果显示,成矿早期至主成矿期矿物流体包裹体气相成分主要为CO2、CH4和N2,各阶段矿物流体包裹体气相成分具有继承性。成矿流体为高温度高盐度,富含CO2、CH4的流体。成矿流体主要源于岩浆,后期混有大气降水。当岩浆热液上升时因压力的突然释放造成高温含矿热流体发生减压沸腾,CO2和CH4等气体大量逃逸,导致成矿物质快速沉淀。矿床在成因上与岩浆-热液成矿作用密切相关。

A Study of Fluid Inclusions and Their Constraints on the Genesis of the Jiama(Gyama) Copper Polymetallic Deposit in Tibet

The Jiama(Gyama) copper polymetallic deposit in Tibet,which is located in the eastern part of the Gangdise porphyry copper belt,is a superlarge porphyry-skarn deposit explored in recent years.A study of fluid inclusions in typical minerals of the mineralization stage from the Jiama(Gyama) deposit through heating/freezing system TEM and Laser Raman analysis shows that the ore-forming fluid was full of volatiles.Fluid homogenization of the critical phase originated from the exsolution of fluid supercritical fluid.The fluid inclusions of the main stage were assemblages of boiling inclusions.Fluorescent effect of organic inclusions are notable.Microthermometry shows that the homogenization temperature of fluid inclusions in quartz phenocrysts from the magmatic-hydrothermal stage was 250-540℃,the salinity concentration of fluid inclusions with halite minerals was 35-61(wt%)NaCl.eq,the salinity concentration of fluid inclusions of homogeneous phase was 3-29(wt%)NaCl.eq,the homogenization temperature in quartz vein from porphyry and hornfels after the magmatic stage was 210-410 ℃,the salinityconcentration was 33-41(wt%)NaCl.eq,the salinity concentration of fluid inclusions in unmixing association with them was 5-25(wt%)NaCl.eq.Homogenization temperature of skarn minerals was 130-360℃,the salinity concentration was 3-41(wt%)NaCl.eq.The homogenization temperatures and salinity decreased step by step from the magmatic-hydrothermal transition stage to the quartz-sulfide stage.There were abundant chalcopyrite daughter minerals in fluid inclusions from quartz phenocrysts,suggesting the enrichment of metallogenic elements at the magma crystallization and differentiation stage.The results of Laser Raman show that the gas compositions were mainly CO 2,CH 4 and N 2 in fluid inclusions from the early mineralization stage to the main mineralization stage.The gas compositions of fluid inclusions in minerals of various stages had close succession.Ore-forming fluids had high temperature and high salinity and were enriched in CO 2 and CH 4.The ore-forming fluids came from magma and were subsequently mixed with atmospheric water.When the magma-hydrothermal solution rose,high temperature ore-forming fluid experienced decompression and boiling due to the sudden release of pressure.Lots of CO 2 and CH 4 escaped,magmatic fluid was mixed with atmospheric water,causing the deposition of metallogenic elements.The deposit was genetically related to magmatic-hydrothermal mineralization.