滇东南洒西钨铍矿床成矿流体来源和演化:来自流体包裹体和H-O同位素证据

Sources and evolution of ore-forming fluids from the Saxi tungsten-beryllium deposit in southeastern Yunnan Province: Evidence from fluid inclusions and H-O isotopes

洒西钨铍矿床位于滇东南老君山钨锡多金属成矿区。确定早期似层状矿体和晚期脉状矿体的流体性质、来源和演化过程,并对其流体地球化学和同位素进行约束,可以有效探讨洒西钨铍矿床成矿机制,为老君山矿集区下一步找矿工作提供理论思考。洒西钨铍矿床脉状矿体的形成经历了硅酸盐阶段、氧化物-硫化物阶段和碳酸盐-萤石阶段,白钨矿和绿柱石主要形成于前两阶段。对洒西钨铍矿床脉状矿体不同阶段石英中的流体包裹体进行了岩相学特征、显微测温、激光拉曼光谱分析等研究,并对早期似层状矿体和晚期脉状矿体矿石中石英的氢-氧同位素组成进行了测试。结果表明:该矿床内脉状矿体的流体包裹体主要有富液相包裹体、富气相包裹体和含子矿物多相包裹体3种类型。流体包裹体的气相成分以H_(2)O主,含少量还原性气体如C_(2)H_(2)等,液相成分也以H_(2)O为主。从硅酸盐阶段到碳酸盐-萤石阶段,包裹体的均一温度和盐度(NaCl_(eq))峰值范围分别为240~360℃、2.35%~13.81%;220~310℃、4.03%~9.86%和190~270℃、2.41%~6.88%。从硅酸盐阶段到碳酸盐-萤石阶段,成矿流体的温度呈现降低趋势,盐度也呈降低趋势。成矿流体总体上属中-高温度、低盐度、贫CO_(2)、含部分还原性气体的NaCl-H_(2)O流体体系。早期似层状矿体石英样品δD_(V-SMOW)值变化范围小,为-102.8‰~-99.0‰,δ^(18)O_(V-SMOW)值为11.7‰~13.0‰,δ^(18)OH_(2)O值为3.16‰~6.46‰;晚期脉状矿体氧化物-硫化物阶段石英样品δD_(V-SMOW)值变化范围较大,为-99.6‰~-69.5‰,δ^(18)O_(V-SMOW)值为11.2‰~14.1‰,δ^(18)OH_(2)O值为3.08‰~6.73‰。综合表明成矿流体主要是岩浆水,混合有少量大气降水或有机水,流体可能发生了沸腾作用,加之温度的降低,导致晚期脉状矿体氧化物-硫化物阶段主要成矿物质的沉淀。洒西钨铍矿床属于中高温热液矿床。

The Saxi tungsten-beryllium deposit is located in the Laojunshan tungsten-tin polymetallic metallogenic area in southeastern Yunnan. Determining the fluid nature, source, and evolution process of early layered orebodies and late vein-like orebodies and constraining their fluid geochemistry and isotopes can effectively explore the metallogenic mechanism of the Saxi tungsten-beryllium deposit, which is a collection of Laojunshan deposits. This study provides theoretical thinking for future prospecting work in the district. The formation of vein-like ore bodies in this deposit experienced a silicate stage, an oxide-sulfide stage, and a carbonate-fluorite stage. Scheelite and beryl were mainly formed in the first two stages. This article studies the fluid inclusions in quartz at different stages of the orebodies of the Saxi tungsten-beryllium deposit, including petrographic characteristics, microscopic temperature measurements, and laser Raman spectroscopy. The quartz sample′s hydrogen-oxygen isotope composition in the ore-like ore body was tested. Three primary types of fluid inclusions(FIs) in vein-like ore bodies are detected, i.e. : liquid-rich FIs, gas-rich FIs, and aughter minerals bearing FIs. The gas phase composition of FIs is mainly H_(2)O, with a small amount of reducing gas such as C_(2)H_(2), and the liquid phase composition is also mainly H_(2)O. From the silicate stage to the carbonate-fluorite stage, the peak homogenization temperature and salinity(NaCl_(eq)) of FIs are 240-360℃, 2.35%-13.81%;220-310℃, 4.03%-9.86%, and 190-270℃, 2.41%-6.88%. From early to late, both homogenization temperature and salinity of the ore-forming fluid decreased obviously. The ore-forming fluid is generally a medium temperature, low salinity, CO_(2)-poor NaCl-H_(2)O fluid system. The δD and δ^(18)O values of the quartz samples from early quasi-layered ore body are-102.8‰ to-99.0‰ and 11.7‰-13.0‰ respectively, and the corresponding δ^(18)O value of the H_(2)O value is 3.16‰-6.46‰. The δD and δ^(18)O values of the quartz samplesfrom bulk oxide-sulfide section are-99.6‰ to-69.5‰ and 11.2‰-14.1‰ resoectively, and the corresponding δ^(18)O value of H_(2)O value is 3.08‰-6.73‰. This indicates that the ore-forming fluid mainly sourced from magmatic water mixed with a small amount of atmospheric precipitation or organic water.The fluid may boil, forming the precipitation of the main ore-forming minerals in the late vein-like ore body oxide-sulfide stage. The Saxi tungsten-beryllium deposit belong to a medium-to high-temperature hydrothermal system.