西藏尼木岗讲铜矿床不同成矿阶段流体包裹体特征及其对成矿的指示意义

Characteristics of Fluid Inclusions in Different Metallogenic Stages and Their Implications for Ore Genesis in Nimu Gangjiang Cu Deposit, Tibet

摘要: 岗讲斑岩型铜矿床是西藏冈斯成矿带一个大型铜矿床。成矿作用主要与黑云二长花岗岩、石英二长斑岩和闪长斑岩密切相关。根据包裹体在室温下的相态充填度以及是否含有子矿物等，将岗讲矿床的流体包裹体分为3大类：富液相包裹体(I)、富气相包裹体(II)和含子矿物多相包裹体(III)。其中，黄铜矿–黄铁矿化阶段流体包裹体主要为I、II和III类包裹体，均一温度在173.2℃~493.2℃之间，盐度为1.74%~36.2% NaCleqv；辉钼矿–黄铜矿化阶段的流体包裹体主要为I、II和III类包裹体，均一温度介于179.8℃~390℃之间，盐度为7.13%~35.2% NaCleqv，早晚成矿阶段流体包裹体均一温度略有降低，盐度变化范围基本保持一致，表示流体发生过多次不混溶作用，预示压力的逐渐降低和硫化物的沉淀。成矿流体总体表现为中温、低盐度的NaCl-H₂O体系，可能与青藏高原快速隆升过程中大量大气降水参与成矿活动有关。激光拉曼探针分析结果表明，流体包裹体液相成分主要为H₂O，气相成分含有CO₂，子矿物含少量黄铜矿。但是该矿床品位总体偏低，规模有限，可能是原始岩浆流体“先天不足”造成。

Abstract: The Gangjiang deposit is a large copper deposit in Tibet. Mineralization mainly occurs in bio- adamellite, quartz-monzonite-porphyry and diorite porphyry. A detailed fluid inclusion study was conducted for veins in the different host rocks to investigate the relationship between fluid evolution and ore-forming processes. According to the phase filling characteristics of inclusions at room temperature and whether they contain sub minerals, three types of fluid inclusions were recognized: aqueous fluid inclusions (I type), H2O-CO2 inclusions (II type), and daughter mineral- bearing fluid inclusions (III type). Measurement of inclusions trapped in quartz revealed that the total homogenization temperatures were 173.2˚C~493.2˚C in chalcopyrite-pyrite mineralization stage, 179.8˚C~390˚C in molybdenite-chalcopyrite mineralization stage, with corresponding salinities of 1.74%~36.2% NaCleqv and 7.13%~35.2% NaCleqv. The homogenization temperature of fluid inclusions decreased slightly in the early and late metallogenic stages, and the variation range of salinity remained basically the same, indicating that the fluid has been immiscible many times, with the decrease of pressure, the sulfide gradually precipitates. The ore-forming fluids is NaCl-H2O system fluid with medium temperature, which may be related to the participation of a large amount of atmospheric precipitation in ore-forming activities during the rapid uplift of the Qinghai Tibet Plateau. The laser Raman probe analysis shows that the main component of fluid wrapped body fluid phase is H₂O, with the gas phaes containing CO₂, and containing a small amount of chalcopyrite. However, the grade of the Gangjiang deposit is generally low, and with limited scale, it may be caused by the “congenital deficiency” of primitive magmatic fluid.