高度富Cl矿化斑岩体系高盐度水盐流体的直接出溶

High-salinity hydrosaline liquids are highly efficient at dissolving and transporting large quantities of ore metals. So, they play important roles in the formation of hydrothermal mineralization. For porphyry systems, hydrosaline liquids were commonly thought to be formed through unmixing of the first exsolved low-salinity aqueous fluids. However, experimental and theoretical research show that hydrosaline liquids can also exsolve directly from extremely Cl-enriched silicate melts, which may partly explain why some porphyry deposits are giant ones. In this proposal, silicate melt inclusions trapped in the quartz phenocrysts of Dexing ore-forming porphyritic granodiorites will be the research object. Following on detailed geological surveys of the Dexing deposit, petrography and the homogenization of the melt inclusions, we will use electron microprobe to analyze the concentrations of the major elements and Cl; utilize the Fourier Transform Infrared Spectroscopy to analyze the contents of H2O of the homogenized melt inclusions. In addition, we will conduct hydrothermal experiments to determine and develop a model on the distribution of Cl and OH/H2O between felsic melts, fluid phases, apatites, micas, amphiboles at shallow crustal pressures and magmatic temperatures. The compositions of the volatile-bearing minerals apatites, biotites and amphiboles of the Dexing deposit will be interpreted by this new partitioning model to calculate the concentrations of Cl and H2O of Dexing ore-forming magmas. The calculated results will be compared with those of the melt inclusions. Geochemical modeling that predicts Cl concentrations of hydrosaline liquid-saturated silicate melt will be compared with the above obtained Cl and H2O concentrations of the Dexing ore-forming magmas in order to study the likelihood of direct exsolution of hydrosaline liquids from the Dexing system.

高盐度水盐流体能够高效的溶解和运移大量的成矿金属。因此，其在热液矿化的形成过程中扮演着重要的角色。对于斑岩体系，水盐流体通常被认为是由初始出溶的低盐度液相流体不混溶形成。然而，实验和理论研究表明水盐流体也可以自高度富Cl硅酸盐熔体中直接出溶。本申请项目以德兴斑岩矿床成矿花岗闪长斑岩石英斑晶中的熔体包裹体为研究对象，在详细野外调查、室内岩相学和熔体包裹体均一的基础上，使用电子探针测定其主量元素和Cl的含量；使用傅里叶变化红外光谱测定其H2O含量。此外，开展高温高压热液实验探究Cl、OH/H2O在硅酸盐熔体-热液流体-磷灰石-黑云母-角闪石多相体系中的分配行为。运用构建的分配模型解译德兴体系磷灰石、角闪石、黑云母的组成可以计算德兴成矿岩浆Cl、H2O含量。将计算和测试获得的Cl、H2O含量与预测高盐度流体出溶时熔体的Cl、H2O含量的模型进行对比，探究水盐流体自德兴体系直接出溶的可能性。

斑岩铜系统供给世界将近1/3的Cu、1/2的Mo、1/5的Au和大部分的Re等金属。因此，研究超大型斑岩矿床的形成对于资源的供给具有重要的意义。本次研究主要聚焦于影响德兴超大型斑岩矿床形成的两个重要因素：(1) 反演成矿岩浆Cl、H2O含量，判断高盐度水盐流体自德兴成矿体系直接出溶的可能性；(2)查明石英闪长玢岩岩石成因以及幔源岩浆与斑岩矿床成矿的关系。.花岗闪长斑岩磷灰石氯含量为0.21-2.53%，根据已有挥发分分配模型反算得出与磷灰石共存岩浆的氯含量为0.03-0.41%，这与熔体包裹体氯含量0.11-0.44%高度一致。大部分熔体包裹体氯含量 (0.3-0.43 %) 达到或者高度接近选定850℃和50MPa条件下的氯溶解度（0.36-0.46 %），同时也接近850℃和200MPa条件下的溶解度（0.42-0.54 %），指示德兴成矿岩浆可以在较深的位置 (50-200Mpa) 直接出溶高盐度水盐流体，系超大型斑岩矿床形成的重要因素之一。.石英闪长玢岩LA-ICP-MS锆石U-Pb定年结果分别为169.28±0.62Ma和168.45±0.59Ma，略晚于德兴成矿花岗闪长斑岩，系中侏罗世岩浆活动的产物。全岩SiO2含量为58.41-63.12%，显示高Al2O3 (14.75-15.40%) 特征，属于准铝质I型安山质岩石。富集大离子亲石元素、亏损高场强元素，显示高Yb特征，构造判别图解显示石英闪长玢岩形成于火山弧环境。岩体具有高的Mg#值 (50-57)及Cr (96.44-178.09 ppm)、Ni (34.65-54.56 ppm)含量，εHf(t)值介于2.20-7.93之间，指示幔源岩浆端元的重要贡献。中侏罗世岩石圈伸展阶段，软流圈上涌导致新元古代俯冲改造的岩石圈熔融并经分离结晶作用形成德兴石英闪长玢岩。石英闪长玢岩氧逸度参数（Fe2O3/FeO）指示岩浆最高氧逸度已达到磁铁矿-赤铁矿等级，幔源岩浆高的氧逸度特征有利于堆晶作用形成的硫化物的二次熔融，对于成矿金属的再富集起到重要的作用。石英闪长玢岩磷灰石羟基摩尔分数介于0.55-0.65之间，明显高于成矿花岗闪长斑岩的0.30，指示幔源铁镁质熔体的注入为长英质岩浆房注入了大量的水。综上所述，幔源岩浆的注入对于德兴超大型斑岩矿床的形成起到了重要的作用。