吉林夹皮沟金矿集区三道岔金矿床成矿流体来源与演化——流体包裹体和H-O同位素的制约

Origin and Evolution of Ore-Forming Fluids of the Sandaocha Gold Deposit in the Jiapigou Glod Ore Concentration Area, Jilin Province: Constraints from Fluid Inclusions and H-O Isotopes

夹皮沟金矿集区位于华北克拉通北缘东段与兴蒙造山带的拼贴部位,是我国最主要的产金地之一。然而该区金矿床的成矿流体来源和矿床成因类型目前仍存有广泛争议;此外,成矿过程中流体的演化以及成矿机制方面也有待深入研究。为解决上述问题,本文选取该区规模最大且最具代表性的三道岔金矿床为研究对象,在详细野外工作的基础上,对不同成矿阶段的石英开展了系统的流体包裹体显微测温、激光拉曼光谱分析以及H-O同位素测试。矿床地质和岩相学特征表明,三道岔金矿床的成矿过程可以划分为(Ⅰ)乳白色石英阶段、(Ⅱ)石英-黄铁矿阶段、(Ⅲ)石英-多金属硫化物阶段和(Ⅳ)石英-碳酸盐阶段,其中阶段Ⅱ和Ⅲ代表成矿主阶段。在各阶段石英中识别出3种类型的原生流体包裹体,依次为水溶液包裹体(NaCl-H_(2)O,W型)、含CO_(2)水溶液包裹体(NaCl-H_(2)O-CO_(2),C型)和纯CO_(2)包裹体(PC型)。早阶段与主阶段石英均捕获C型和W型包裹体,主阶段石英还发育少量PC型包裹体,而晚阶段石英中仅见W型包裹体。早、主、晚阶段流体包裹体的均一温度依次为283~411、210~288和131~210℃,盐度(w(NaCl))依次为4.26%~17.48%、2.07%~15.76%和2.57%~14.04%。H-O同位素结果表明,初始成矿流体来自深源岩浆提供的岩浆水与少量大气降水的混合,且成矿过程中不断有大气降水的加入。自成矿早阶段至晚阶段,成矿流体逐渐由富含CO_(2)的中温、中低盐度NaCl-H_(2)O-CO_(2)体系演化为以H_(2)O为主的低温、低盐度NaCl-H_(2)O体系。流体上升演化过程中与围岩发生了广泛的水岩反应,压力突降与大气降水的持续加入所引发的流体不混溶导致CO_(2)和其他气相成分(如H_(2)S、CH_(4)等)大量逸出与成矿流体物理化学条件变化,进而造成Au-S络合物分解、金溶解度降低,从而有利于金和其他成矿元素的卸载。综合矿床地质、流体包裹体和H-O同位素研究结果,三道岔金矿床属于中温热液脉型金矿床。

The Jiapigou gold ore concentration area,located in the collage section between the northeastern margin of the North China craton and the Xing’an-Mongolian orogenic belt,is one of the most important gold-producing areas in China.However,the origin of the ore-forming fluids and genetic types of the gold deposits in the Jiapigou gold ore concentration area are still widely disputed.Moreover,there is still a chronic lack of systematic research on evolution of the ore-forming fluids and ore-forming mechanism.To address the problems above,this paper selects the most representative Sandaocha gold deposit in the Jiapigou gold ore concentration area as an example to carry out a detailed analysis of fluid inclusions,laser Raman spectroscopy and H-O isotopes.Four stages of mineralization have been identified in the Sandaocha gold deposit:(Ⅰ)milky quartz,(Ⅱ)quartz-pyrite,(Ⅲ)quartz-polymetallic sulfide,and(Ⅳ)quartz-carbonate.The stagesⅡandⅢrepresent the main gold mineralization.Three types of primary fluid inclusions(FIs)are identified in quartz formed at different stages:NaCl-H_(2)O-CO_(2)(C-type),NaCl-H_(2)O(W-type)and pure CO_(2)(PC-type).The early-stage quartz contains C-and W-type FIs,which have homogenization temperatures of 283-411℃and salinities of 4.26%-17.48%NaCl equiv.The main-stage quartz contains all three types of FIs,with homogenization temperatures of 210-288℃and salinities of 2.07%-15.76%NaCl equiv.The late-stage quartz contains only W-type FIs with homogenization temperatures of 131-210℃and salinities of 2.57%-14.04%NaCl equiv.The H-O isotope results indicate that the initial ore-forming fluid is the mixture of magmatic water and a small amount of meteoric water,and the meteoric water was continuously added to the ore-forming fluids during mineralization.The ore-forming fluid system evolved from a moderate-temperature and moderate-to low-salinity NaCl-H_(2)O-CO_(2) system in the early stage to a NaCl-H_(2)O system with a low temperature and salinity in the late stage.Water-rock interactions generally occurred between ore-forming fluids and metamorphic wall-rocks during fluids migration.The sudden decompression and the addition of meteoric water led to fluid immiscibility and the exsolution of CO_(2),H_(2)S,and other volatiles.This process destroyed the stability of Au-S complexes and facilitated the precipitation of gold and other ore-forming elements.Comprehensive studies of geology,fluid inclusions and H-O isotopes confirm that the Sandaocha gold deposit is a mesothermal hydrothermal vein gold deposit.