西藏布东拉金矿床浅成低温热液成矿作用:来自流体包裹体和H-O同位素的证据

Epithermal mineralization at Budongla gold deposit in Zhongba County of Tibet:Evidence from fluid inclusions and H-O isotopes

西藏自治区仲巴县布东拉金矿床位于中拉萨地块西段,矿体受北西向的断裂构造控制,呈脉状、透镜状产出。矿石产出类型主要为石英脉型与蚀变岩型,主要矿石矿物为自然金。矿化蚀变过程经历了3个阶段,包括石英-自然金-黄铁矿阶段(S1)、石英-自然金-多金属硫化物阶段(S2)和石英-碳酸盐矿物阶段(S3)。文章在野外地质调查的基础上,对不同成矿阶段的石英脉进行了流体包裹体岩相学观测、包裹体显微测温、包裹体显微激光拉曼分析、群包裹体成分分析和群包裹体H-O同位素测试。结果表明,S1阶段流体均一温度为330~350℃,盐度w(NaCl_(eq))为9.0%~11.0%,密度0.82~0.86 g/cm^(3),压力为(100~140)×10^(5)Pa,深度为1.0~1.4 km;S2阶段流体均一温度为300~309℃,盐度w(NaCl_(eq))为7.0%~9.0%,密度为0.80~0.85 g/cm^(3),压力为(70~120)×10^(5)Pa,深度为0.7~1.2 km;S3阶段流体均一温度为210~230℃,盐度w(NaCl_(eq))集中为2.0%~3.0%,密度为0.84~0.90 g/cm^(3),压力为(50~80)×10^(5)Pa,深度为0.5~0.8 km。包裹体成分分析表明,布东拉金矿床的流体包裹体气相成分以H_(2)O为主,含有少量CO_(2)、SO_(2)、N_(2)和CH_(4);液相中阴阳离子主要为Ca^(2+)、Na^(+)、C1^(-)和SO_(4)^(^(2-))。各热液脉体石英中流体包裹体的δDH_(2)O值为-101.3‰~-90.7‰,δ18OHO值为-0.75‰~5.06‰,表明成矿流体主要来源于地下水及少量岩浆水。研究表明,含2金成矿流体沿着断裂从深部封闭体系运移到浅部的开放体系时,迅速突破临界状态,减压沸腾导致金属物质的沉淀,形成各种类型的矿脉及矿化。布东拉金矿床的成矿流体为中低温、低盐度、中低密度、含少量CO_(2)、SO_(2)、N_(2)、CH_(4)的NaCl-CaSO_(4)-H_(2)O体系的浅成地下热水,表明其可能属于浅成低温热液型金矿床。

The Budongla gold deposit in Zhongba County of Tibet is located in the western segment of the Central Lhasa Block.The gold orebodies are veined or lenticular in form and are controlled by NW-trending faults.The gold ores are dominated by quartz-vein and altered-rock types.The ore mineral is mainly native gold.Three main mineralization stages have been identified:The quartz-native-gold-pyrite stage(S1),the quartz-native-gold-polymetallic-sulfides stage(S2),and the quartz-native-gold-carbonate stage(S3).Based on field investigation,the quartz veins from different mineralization stages were selected for fluid inclusion and H-O isotopic studies.The homogenization temperatures,salinities w(NaCl_(eq)),and densities of fluid inclusions in S1 stage range from 330 to 350℃,from 9.0%to 11.0%,and from 0.82 to 0.86 g/cm^(3),respectively.The calculated pressure and depth vary from 100×10^(5) to 140×10^(5) Pa and from 1.0 to 1.4 km,respectively.These corresponding values in S2 stage are from 300 to309℃,from 7.0%to 9.0%,from 0.80 to 0.85 g/cm^(3),from 70×10^(5) to 120×10^(5) Pa,and from 0.7 to 1.2 km,respectively.In S3 stage,these values are from 210 to 230℃,from 2.0%to 3.0%,from 0.84 to 0.90 g/cm^(3),from 50×10^(5) to 80×10^(5) Pa,and from 0.5 to 0.8 km,respectively.The component analysis shows that the gas phase primarily contains H_(2) O with a small amount of CO_(2),SO_(2),N_(2),and CH_(4),and the anions and cations in liquid part mainly contain Ca^(2+),Na^(+),Cl-,and SO_(4)^(2-).TheδDH_(2) Oandδ18OH_(2) Ovalues of fluid inclusions in quartz veins from various stages range from-106.1‰to-97.5‰,and from-7.33‰to-7.13‰,respectively,which suggests that the ore-forming fluids were mainly groundwater,with a subordinate contribution from magmatic fluid.The fluid boiling caused by decompression led to the precipitation of metal sulfides and the various types of ore-bearing veins.The current study shows that the ore-forming fluid of the Budongla gold deposit contains a little CO_(2),SO_(2),N_(2),and CH_(4),and the fluid has low-moderate homogenization temperature,low-salinity,and low-moderate density,so that this deposit could be most likely classified as an epithermal gold deposit.