杭带锡田合江口锡钨矿床流体包裹体和稳定同位素特征

合江口锡钨多金属矿为钦杭缝合带上锡田矿田中的岩浆热液型矿床。为探讨矿床的成矿流体性质、演化及成因机制,文章分别研究了3个成矿期中流体包裹体的特征,并重点对第Ⅱ期的流体特征进行了分阶段剖析。发现第Ⅰ期为L+V型包裹体,成矿流体为中高温、中盐度H_2O-CO_2-Na Cl体系,流体不混溶作用不明显;第Ⅱ期有L+V、Cl、Cl+Cv、Aq+Cl+Cv和S型几类包裹体,初始成矿流体为高温、中盐度的H_2S-N_2-CO_2-H_2O-Na Cl体系,本期不仅新发现了大量的N_2包裹体,还见到了萤石子矿物,同时检出了大量的CO_2、H_2S、CH_4、N_2、CO、F、Cl等挥发分,挥发分对矿质的析出、溶解和搬运起了重要作用,温度-盐度的降低、挥发分逃逸、流体不混溶作用、还原至氧化环境的改变、大气水的参与是导致矿质沉淀的重要机制,本期的成矿深度约为4. 5 km,N_2的检出为幔源流体参与成矿提供了证据。第Ⅲ期主要为L+V型包裹体,流体为高温、中—高盐度的H_2O-NaCl体系。稳定同位素研究表明第Ⅰ期流体以壳源为主,第Ⅱ期为壳幔混合源,初始成矿流体主要源自岩浆水,后期有大气水混入。

Fluid inclusions and stable isotopes features of Hejiangkou tin-tungsten deposit of Qinzhou-Hangzhou structure, China

The Hejiangkou Sn-W-polymetallic deposit is a magmatic-hydrothermal mineral deposit located in the Xitian orefield within the Qinzhou-Hangzhou structure. In order to understand the nature of ore-forming fluids and the genesis and evolution of the deposit, in this study, we investigated fluid inclusions formed in three different ore-forming periods. The characteristics of the fluid of Stage II are the focus of this study and were analyzed in stages. The fluid inclusions of Period I are predominantly L+V inclusions and the ore-forming fluid in this period is characterized by a moderate-temperature, moderate-salinity H₂O-CO₂-NaCl system. There was no obvious fluid immiscibility during Period I. The fluid inclusions of Period II can be divided into L+V, Cl, Cl+Cv, Aq+Cl+Cv, and S inclusions. The initial ore-forming fluid was a high-temperature, moderate-salinity H₂S-N₂-CO₂-H₂O-NaCl system. Not only abundant N₂-bearing inclusions but also fluid inclusions containing fluorite daughter minerals occurred during this period. The presence of abundant volatiles such as CO₂, H₂S, CH₄, N₂, CO, F, and Cl may imply that volatiles played an important role in precipitation, dissolution and transport of ore-forming materials. Moreover, decrease in temperature and salinity, escape of volatiles, fluid immiscibility, change in redox condition, and participation of meteoric water are important mechanisms for ore precipitation. The depth of mineralization of Period II is roughly 4.5 km. The presence of N₂ indicates that the mantle-derived fluids participated in the ore-forming process. The fluid inclusions of Period III are predominantly L+V inclusions and the ore-forming fluid was represented by a high-temperature, moderate to high salinity H₂O-NaCl system. Stable isotopic analysis reveals that the fluid of Period I was dominated by crust-derived fluid and that the fluid of Period II was formed through mixing between crust and mantle-derived fluids. In addition, the initial fluid of Period II was dominated by magmatic water and latterly meteoric water was added into the ore-forming fluid.