甘肃寨上金矿床成矿流体性质与成矿作用探讨

Zhaishang gold deposit in Gansu Province:Characteristics,evolution of ore-forming fluids and their metallogenic implications.

位于西秦岭成矿省礼(县)-岷(县)成矿带西段的寨上金矿床,是近年发现的一个大型微细浸染型金矿。对矿床流体包裹体的研究表明,热液石英脉及方解石脉中两类流体包裹体主要是水溶液包裹体和含 CO_2包裹体。流体包裹体均一温度一般介于110～300℃,240℃为主成矿阶段温度峰值。从热液作用的早阶段到晚阶段,具有成矿温度递减、盐度升高的趋势,两者呈负相关性变化,指示流体发生过不混溶作用。流体密度集中变化于0.8～1.0g/cm^3之间,压力及成矿深度略大于邻区同类矿床。成矿流体是以'盐水-CO_2-CH_4'为主要组分的热液体系,流体包裹体气相成分以 H_2O 和 CO_2为主,次为 N_2、CH_4等有机还原性气体,液相成分中 Na^+、Ca^(2+)远高于其它金属阳离子,阴离子以 SO_4^(2-)、Cl^-为主,NO_3^-和 F^-次之。寨上金矿床成矿流体总体表现为中低温、低盐度、低密度且富含碳氢化合物组分的热液。从成矿流体性质及其演化来看,深源热液参与可能对金的活化迁移起到重要作用,构造体制转换使流体稳定体系发生改变而造成金的富集成矿。

Located in the west of Li-Min metallogenic belt, western Qinling Mountains, the Zhaishang gold deposit is newly found as a large micro-disseminated gold deposit in this early century. Two main types of fluid inclusions, namely aqueous inclusions and CO2 beating inclusions are recorded in quartz and calcite hypothermal veins. Final homogenization in two-phase fluid inclusions occurs between 110 and 300℃ with most of the ore-forming temperatures around 240℃. Salinities （7.18% on average） represent reverse correlation with fluid temperatures as it increase in hypothermal fluids system from early to late stage, which indicates a boiling event. Calculated densities of hypothermal fluids concentrate in the range of 0. 8 - 1. 0g/cm^3. The Zhaishang gold deposit is partly not similar to other adjacent gold deposits since it has a higher pressure and deep depth. Fluid inclusions analyses show that H2O, CO2 are main compositions of gas phase, with minor of N2, CH4 ; Na^ ＋ , Ca^2 ＋ , anion SO4^2- and Cl^- are dominating in liquid phase, with minor of other cation and NO3^- , F^-. All these data demonstrate a ＂brine-CO2-CH4＂ ore-forming fluids combination with moderate to low temperature, low salinity, low density and rich organic compositions. Based on characteristics, evolution associated with the ore- forming fluids of the Zhaishang gold deposit, we believe that deep origin （magmatic and/or metamorphic） hypothermal fluids may take a more important role in gold activation, though structural transition leads to the gold and sulfides precipitation by changing the conditions of the primary well-evolved fluids.