云南南秧田钨矿床流体包裹体特征及其意义

Characteristics and significance of the fluid inclusions from the Nanyangtian tungsten deposit in Yunnan Province

对南秧田矽卡岩型钨矿床的石英和石榴石流体包裹体的岩相学特征研究表明,与成矿有关的包裹体主要有3类:富液相、富气相和含子晶的多相包裹体。石英包裹体均一温度范围为232～337℃,盐度w(NaCl)=0.53%～9.98%;石榴石包裹体的均一温度范围为228～306℃,盐度w(NaCl)=6.45%～14.04%。激光拉曼探针分析表明,南秧田白钨矿的成矿流体中气相成分以H2O为主,含少量CO2、CH4、H2S和N2等气体,液相成分以H2O为主,属NaCl-H2O流体体系。成矿溶液的密度为0.72～0.87g/cm3,表明形成这种矽卡岩型矿床的成矿流体均属于中温、低盐度、低密度的流体。成矿压力为18～32MPa,成矿深度约为0.6～1.2km。石英包裹体水的δD为-72.16‰～-65.10‰,δ18O为7.98‰～8.45‰,钨矿床中硫化物δ34S为6.6‰。成矿流体主要来自燕山晚期的岩浆热液作用。

South China is the largest and the most specific tungsten metallogenic belt in the world, Jurassic is the main ore-forming period of tungsten ore deposits in South China, and Cretaceous is the main mineralization period of tin ore deposits in China. Nevertheless, the Nanyangtian skarn-type scheelite deposit in Yunnan Province is closely linked with the Cretaceous granites in space and time and is the only skarn-type tungsten deposit discovered in Cretaceous strata. The ore-bearing rock series of the Nanyangtian skarn-type tungsten deposit comprise alternating layers composed of skarns, gneiss, granulite, schist and some other rocks, in which skarn layers are the main ore-hosting rocks and skarn scheelite is the main ore type. This paper deals with micro temperature measurement and Laser Raman spectroscopy of fluid inclusions in the Nanyangtian tungsten deposit, aimed at discriminating ore-forming fluid properties of this deposit and clarifying its ore-forming material source and metallogenic mechanism based on petrographic characteristics for the further study of the Nanyangtian skarn-type tungsten deposit. There are three dominant types of fluid inclusions related to metallogensis, as evidenced by petrographic characteristics of fluid inclusions of quartz and garnet in the Nanyangtian skarn-type tungsten deposit; they are liquid-rich inclusions, gas-rich two-phase inclusions and daughter minerals-bearing polyphase inclusions. The homogenization temperatures and salinity of fluid inclusions in quartz range from 232℃ to 337℃ and from 0.53% to 9.98 wt% NaCleqv respectively, and those of the inclusions in garnet vary from 228℃ to 306℃ and from 6.45% to 14.04 wt% NaCleqv respectively. Laser Raman spectroscopy shows that the gas composition of ore-forming fluids in Nanyangtian scheelite is mainly H2O, with a small amount of CO2, CH4, H2S, N2 and some other gases, whereas liquid composition is mainly H2O, thus suggesting a NaCl-H2O fluid system. The density of the fluids is 0.72～0.87 g/cm3. These features demonstrate that the fluids in this tungsten deposit are of medium temperature, low salinity and low density. The pressure of the fluid is 18～32 MPa, the depth of the fluid is 0.6～1.2 km, the δD values range from -72.16‰ to -65.10‰, the δ18O water values vary from 7.98‰ to 8.45‰, and the δ34S value of sulfides in the tungsten deposit is 6.6‰. These data imply that ore-forming fluids were derived from the magmatic hydrothermal process in late Yanshanian period, and magmatism caused the remobilization and enrichment of ore-forming elements to form the Nanyangtian skarn-type tungsten deposit.