老挝南巴坦矽卡岩型锡多金属矿床成矿流体特征与演化

Ore-forming fluid characteristics and evolution of Nam Pathene skarn tin polymetallic deposit in Laos

中文关键词:成矿流体  矽卡岩型锡矿  老挝南巴坦

keywords:Ore-forming fluid  skarn tin deposit  the Nam Pathene in Laos

中文摘要:老挝南巴坦锡矿床作为长山成矿带典型矽卡岩型矿床，矿体赋存于石炭系—二叠系灰岩中，与黑云母二长花岗岩和闪长玢岩表现出较为密切的时空关系，显示明显的多阶段演化特征。文章通过系统的矿物学及流体包裹体分析，揭示该矿床成矿过程可划分为：（1）矽卡岩阶段：发育钙铝-钙铁榴石系列及钙铁辉石-普通辉石系列的矽卡岩矿物组合，流体包裹体均一温度390～452℃，盐度w(NaCl_eq)6.71%～11.10%；（2）退化蚀变阶段：发育绿泥石、绿帘石、锡石和石英等矿物组合，均一温度降至283～392℃，盐度w(NaCl_eq)4.34%～10.11%；（3）石英-锡石-硫化物阶段：具有典型的沸腾包裹体群，均一温度202～332℃，盐度w(NaCl_eq)显著分异（1.41%～40.70%）；（4）石英-方解石阶段：均一温度147～252℃，盐度w(NaCl_eq)0.35%～4.96%。激光拉曼分析显示流体气相以H₂O和CH₄为主，液相均为H₂O。成矿流体经历明显降温过程（452℃→147℃），成矿流体盐度最初为中等盐度，因流体沸腾导致盐度显著分异，最后演化为低盐度流体。锡在成矿流体中以Sn（Ⅱ）Cl络合物主导迁移，可能伴随少量Sn（Ⅳ）Cl络合物进行迁移。Ⅰ→Ⅱ阶段流体冷却和氧逸度升高（H₂O-NaCl-CH₄系统变为H₂O-NaCl系统）导致少量锡石沉淀；Ⅱ→Ⅲ阶段流体沸腾与冷却引发的氧化还原反应促使锡石与金属硫化物大规模富集；Ⅲ→Ⅳ阶段流体混合引发后期硫化物沉淀。这种温度-盐度梯度变化与多阶段流体作用共同控制了金属元素的迁移-沉淀过程，为理解矽卡岩型锡矿床成矿机制提供了重要实例。

Abstract:The Nam Pathene tin deposit in Laos, as a typical skarn deposit in the Truong Son metallogenic belt, is hosted within Carboniferous-Permian limestone. The mineralization exhibits a relatively close spatial and temporal relationship with biotite monzogranite and diorite porphyrite, and shows distinct multi-stage evolutionary characteristics. Through systematic mineralogical and fluid inclusion analyses, this study reveals that the mineralization process of this deposit can be divided into four stages:（1） Prograde or skarn stage: characterized by the development of garnet series ranging from andradite to grossular and pyroxene series ranging from hedenbergite to diopside, with fluid inclusion homogenization temperatures of 390℃ to 452° C and salinities w(NaCl_eq) of 6.71% to 11.10%;（2） Retrograde stage: marked by the formation of chlorite, epidote, cassiterite, and quartz assemblages,with homogenization temperatures decreasing to 283℃ to 392℃ and salinities w(NaCl_eq) of 4.34% to 10.11%;（3） Quartz-cassiterite-sulfide stage: featuring typical boiling inclusion assemblages, with homogenization temperatures of 202℃ to 332℃ and highly variable salinities w(NaCl_eq)（1.41% to 40.70%）;（4） Quartz-calcite stage: with homogenization temperatures of 147℃ to 252℃ and salinities w(NaCl_eq) of 0.35% to 4.96%. Laser Raman analysis shows that the gas phase of the fluids was mainly composed of H₂O and CH₄, while the liquid phase was entirely H₂O. The ore-forming fluids underwent a significant cooling process（from 452℃ to 147℃）, initially possessing moderate salinity levels which significantly differentiated due to fluid boiling, eventually evolving into low salinity fluids. Tin was primarily transported in the ore-forming fluids as Sn（Ⅱ） Cl complexes, possibly accompanied by minor amounts of Sn（Ⅳ）Cl complexes. Fluid cooling and an increase in oxygen fugacity from Stage Ⅰ to Ⅱ（transition from the H₂O-NaCl-CH₄ system to the H₂O-NaCl system） resulted in minor cassiterite precipitation;boiling and cooling-induced redox reactions during Stage Ⅱ to Ⅲ led to substantial cassiterite and base metal sulfides deposition; mixing of fluids in Stage Ⅲ to Ⅳ triggered later sulfide precipitation. This temperature-salinity gradient variation, coupled with multi-stage fluid activity, jointly controlled the migration and precipitation processes of metallic elements, providing a crucial case study for understanding the genesis of skarn tin deposits.