内蒙古东不拉格钼铅锌矿床的成矿年代及成矿流体特征

通过野外调研、辉钼矿Re-Os定年和流体包裹体研究,对内蒙古西乌旗东不拉格钼铅锌矿床的成矿时代、成矿背景和成矿流体特征及流体演化进行了探讨。研究结果表明,矿区发育斑岩型钼矿化和热液脉状铅锌矿化,二者为同一构造岩浆事件的产物。钼、铅锌成矿均与区内石英二长斑岩有关,辉钼矿Re-Os定年结果表明,成矿时代为中侏罗世(165.2±2.8 Ma),是中国东部燕山期大规模成矿事件的产物。流体包裹体测试结果表明,成矿流体主要来自岩浆水,早期成矿流体具有高温、高氧逸度、富Fe和含CO_2的特征。从成矿早阶段至晚阶段成矿流体从高温(312~534℃)、高盐度(高达63.9%NaCleq)向低温(158~166℃)低盐度(低至0.35%NaCleq)演化。隐爆造成的岩浆减压沸腾是岩浆流体出溶的主要机制,早期岩浆流体由于减压和降温过程造成热液发生CO_2与水的不混溶及流体沸腾,这是早期钼沉淀的主要机制,而岩浆流体与还原性地层的水岩反应引起的氧逸度降低及大气水的加入则是铅锌沉淀的主要机制。

Molybdenite Re-Os Age and Characteristics of Ore-forming Fluid of the Dongbulage Mo-Pb-Zn Deposit,Inner Mongolia

The Dongbulage Mo-Pb-Zn deposit is located in Xi Ujimqin county,Inner Mongolia.Through field geological survey,molybdenite Re-Os dating and fluid inclusion study,the age of ore formation and the characteristics of the ore-forming fluid are discussed.The results showed that the porphyry molybdenum mineralization and the vein-type hydrothermal lead-zinc mineralization are related to the same tectonic-magmatic event.The Mo-Pb-Zn mineralization are closely related to the quartz monzonite porphyry.Molybdenite Re-Os analytical results showed that the mineralization took place at 165.2±2.8 Ma,as part of the large-scale mineralization event during the Yanshanian period of the eastern China.The results of fluid inclusions indicate that the ore-forming fluid is mainly derived from magmatic water,and the early ore-forming fluid has the characteristics of high temperature,high oxygen fugacity,rich Fe and CO2.From the early to the late mineralization stage,the ore-forming fluids evolved from high temperature (312-534 ℃) and high salinity (up to 63.9％NaC1eq) to low temperature (158-166 ℃) and low salinity (low to 0.35％NaCleq).Boiling associated with crypto-explosion is likely the main cause of magmatic fluid exsolution.The pressure reduction and cooling of the early stage magmatic fluid may lead to the immiscibility of water and CO2 and fluid boiling,which is the most important process controlling the early molybdenum mineralization.The interaction between the magmatic fluid and country rock led to the decrease of oxygen fugacity,and coupled with the input of meteoric water,caused the precipitation of lead and zinc.