藏南折木朗造山型金矿成矿流体地球化学和成矿机制

Geochemistry of ore-forming fluid and metallogenic mechanism for Zhemulang gold deposit in southern Tibet,China

折木朗金矿位于青藏高原雅鲁藏布江缝合带东段的南侧,矿体受大型脆-韧性剪切带的次级断裂控制。系统的显微测温和激光拉曼测定显示折木朗金矿矿石中存在3类流体包裹体:NaCl-H2O溶液包裹体(类型Ⅰ);含CO2盐水溶液包裹体(类型Ⅱ),此类包裹体又分为两相(Ⅱa)和三相(Ⅱb)2个小类;Ⅲ纯气相包裹体。折木朗金矿床中流体包裹体显微测温显示该矿成矿流体的盐度范围为2.31%～7.39%NaCleqv,平均值为5.33%%NaCleqv,峰值为4.0%～7.0%NaCleqv;均一温度的范围为164.5～273.1℃,峰值为220～240℃,平均值为221.0℃。相对应的密度范围为0.82～0.93g.cm-3,峰值为0.84～0.90g.cm-3,平均值为0.88g.cm-3。折木朗金矿床成矿流体具有富含CO2、低盐度、低密度、中低温度的特征,与造山型金矿成矿流体相似。此外,同位素测定显示成矿流体的氢氧碳同位素组成分别为δDH2O=-36.7‰～-107.5‰,δ18OH2O=4.1‰～5.5‰,δ13C=-9.6‰～-11.5‰,说明成矿流体主要为变质水,但有地幔流体的加入。综合成矿地质特征和成矿流体的证据,提出折木朗金矿为陆陆碰撞造山型金矿。

The Zhemulang gold deposit is located to the south of the east section of the Yarlung Zangbo tectonic suture zone in the southern Tibet.The gold ore bodies are controlled by the secondary brittle fractures of the large-scale brittle-ductile shear zone.Microthermometric measurements and Laser Raman analysis show that auriferous quartz veins of the Zhemulang gold deposit contain three types of fluid inclusions：NaCl-H2O inclusions（type I）;CO2 brine inclusions（type II）,which can be subdivided into two-phase（type IIa） and three-phase（type IIb） inclusions;Pure gaseous inclusions（type III）.The NaCl-H2O inclusions have salinity values of 2.31%~7.39% NaCleqv,with a peak of 4.0%~7.0% NaCleqv and an average of 5.33% NaCleqv,homogenization temperature values of 164.5~273.1℃,with a peak of 220~240℃ and an average of 221.0℃,and density values of 0.82~0.93g·cm-3,with a peak of 0.84~0.90g·cm-3 and an average of 0.88g·cm-3,suggesting that the ore-forming fluids of the Zhemulang gold deposit is characteristics by high content of CO2,low salinity,low to moderate homogenization temperature and low density,which are similar to those of typical orogenic gold deposits.H-O-C isotopic analyses show that δDH2O=-36.7‰^-107.5‰,δ18OH2O=4.1‰~5.5‰,δ13C=-9.6‰^-11.5‰ in Zhemulang gold deposit,indicating that the ore-forming fluids is composed mainly of metamorphic fluid,with addition of mantle-derived fluid.Geologic and geochemical features show that the Zhemulang gold deposit may be an orogenic gold deposit formed under continental collisional background.