内蒙古白乃庙铜-金-钼矿床成矿机制——来自流体包裹体和He-Ar同位素的证据

白乃庙铜-金-钼矿是华北板块北缘中段一个重要矿床,其矿化特征既显示斑岩型蚀变特征,同时又表现出明显的后期造山作用改造的特点。本次研究通过系统的流体包裹体显微测温、激光拉曼和气液相色谱分析揭示其成矿流体为中—低温、中低盐度的CO_2(CO)-H_2O-CH_4(C_2H_2+C_2H_4)-NaCl-CaCl_2体系,流体混合作用和CO_2逃逸为成矿主要因素。富还原性流体的存在可能促使气相流体携带大量的Cu、Au等成矿元素迁移至较远点的白乃庙群地层,沿着片理或裂隙沉淀成矿。对不同成矿阶段矿石中黄铁矿的He-Ar同位素组成测试结果显示白乃庙矿床黄铁矿流体包裹体的~3He/~4He比值在0.06~0.71Ra之间,~(40)Ar/~(36)Ar比值为375.5~1436.3,指示成矿体系由不同性质和组成的两个端元流体混合而成,即混入了类似于MORB型地幔端元的高温高盐度流体和富含地壳放射成因氦但具有空气氩同位素组成特征的低温大气降水。白乃庙矿床属受后期造山作用改造的斑岩型铜-金-钼矿床,形成于古亚洲洋板块持续向南俯冲背景。

Mineralization Mechanism of the Bainaimiao Cu-Au-Mo Deposit in Inner Mongolia: Evidence from Fluid Inclusions and He-Ar Isotopes

The Bainaimiao Cu-Au-Mo deposit is an important ore deposit in the middle section of the north margin of North China Block, and the mineralization characteristics of the Bainaimiao deposit show both porphyry alteration features, as well as the characteristics of late orogenesis transformation. Systematic micro-thermometry, laser Raman and chromatography analysis are carried out on fluid inclusions, which reflect the ore-forming fluid is middle-low temperature CO2 (CO)-H2O-CH4 (C2H2 C2H4)-NaCl-CaCl2 system, and fluid mixing and CO2 escaping are the main factors of mineralization. Reducing fluid may urge the vapor fluid carrying ore-forming elements such as Cu, Au migrating to distant Bainaimiao group, then, deposits along the foliation or fissures. The 3He/4He and 40Ar/36Ar values (3He/4He=0.06~0.71 Ra, and 40Ar/36Ar=375.5~1436.3, respectively) of the fluid inclusions in pyrites from the different mineralization phases in the Bainaiiao deposit suggest that the ore-forming fluid system is mixed by two end members, one is the high temperature deep-derived fluid congenetic with the porphyries generated by crust-mantle mixing; another is the low-temperature meteoric water which is rich in crustal radiogenic He with the component of atmospheric Ar. We consider that the Bainaimiao deposit belongs to the porphyry deposit that suffered the late orogenic transformation, and the deposit formed under the background of the paleo-Asian Ocean plate continued southward subduction.