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摘要: 河南省东沟钼矿床是东秦岭钼矿带新发现的燕山期超大型斑岩钼矿床,是大陆碰撞成矿理论预测在先,勘查突破在后的成功范例。该矿床的形成与东沟A型花岗斑岩有关,矿体产于斑岩体外接触带的熊耳群火山岩中。以岩体为中心发育典型的斑岩蚀变分带,由内到外依次是钾化、绢英岩化和青磐岩化。流体成矿过程包括早、中、晚3个阶段,分别以石英-钾长石组合、石英-(钾长石)-多金属硫化物组合和石英-碳酸盐-萤石组合为标志,矿石矿物主要沉淀于中阶段。早、中阶段热液石英中发育CO2-H2O型包裹体(C型)、水溶液包裹体(W型)和含子晶多相包裹体(S型),但晚阶段只发育水溶液包裹体(W型)。早阶段C型和W型包裹体均一温度集中于380~550℃,盐度为7.70%~18.28% NaCleqv;S型包裹体中常见石盐、黄铜矿、方解石和未知透明子矿物,其均一温度范围为318~516℃,加热过程中除石盐外其他子矿物不熔;不包括不熔子矿物的贡献,该类包裹体盐度变化于12.85%~17.87 和35.55%~47.67% NaCleqv。中阶段C型和W型流体包裹体均一温度集中于260~410℃,盐度为4.62%~18.28% NaCleqv;除不熔子矿物外,S型包裹体均一温度为197~436℃,盐度变化于7.45%~19.30% NaCleqv和31.71%~49.22% NaCleqv。晚阶段流体包裹体均一温度集中于125~225℃,盐度介于0.5%~7.25% NaCleqv之间。估算的早、中阶段流体捕获压力分别为63~117MPa和12~67MPa,推测最大成矿深度为4.7km。上述流体包裹体研究表明成矿流体由早阶段高温、富CO2的岩浆热液演化为晚阶段低温、贫CO2的大气降水热液。这种高温、富CO2的岩浆热液可视为大陆内部体制斑岩成矿系统的标志性特征,以区别于岩浆弧区同类矿床高温、贫CO2的岩浆热液。通过对比东秦岭-大别钼矿带典型斑岩成矿系统,认为成矿流体中CO2等挥发组分的含量和围岩性质(化学成分、结晶程度、抗剪抗压程度等)是控制矿体空间定位的重要因素。Abstract: The recently discovered Donggou Mo deposit, Henan Province, is a giant Yanshanian porphyry Mo system in the East Qinling molybdenum belt. The deposit was discovered by prospecting after a successfully theoretical prediction according to the tectonic model for collisional orogeny, metallogeny and fluid flow. Mo mineralization is associated with the Donggou granite porphyry which has been classified as A-type granite. Both the porphyry stock and wallrocks underwent intense hydrothermal alteration. The alteration ranges outwardly from potassic, phyllic to propylitic alteration zones with increasing distance from the intrusion. Molybdenum mineralization presents as numerous veinlets in the altered wallrocks instead of the causative porphyry. The hydrothermal ore-forming process includes the early, middle and late stages, characterized by mineral assemblages of quartz-potassic feldspar, quartz-(potassic feldspar)-polymetal sulfides and quartz-carbonate-fluorite, respectively. Ore minerals were mainly precipitated in the middle stage. The hydrothermal minerals in the early and middle stages contain three types of fluid inclusions, i.e. NaCl-H2O (W-type), CO2-H2O (C-type) and daughter mineral-bearing (S-type) fluid inclusions; while the late-stage hydrothermal quartz contains only NaCl-H2O fluid inclusions (W-type). The homogeneous temperatures of the C-type and W-type fluid inclusions in the early stage are mainly 380~550℃, with salinities ranging from 7.70% to 18.28% NaCleqv. S-type fluid inclusions contain variable daughter minerals including halite, chalcopyrite, calcite and unidentified transparent crystal. Except halite, other daughter minerals do not dissolve in the heating process. These fluid inclusions generally homogenize at temperatures of 318~516℃, and yield salinities of 12.85%~17.87% NaCleqv and 35.55%~47.67% NaCleqv. In the middle stage, the C-type and W-type fluid inclusions mainly yield homogeneous temperatures of 260~410℃ and salinities of 4.62%~18.28% NaCleqv. S-type fluid inclusions are generally homogenized at 197~436℃, with salinities of 7.45%~18.28% NaCleqv and 31.71%~49.22% NaCleqv. W-type fluid inclusions in the late stage are homogenized at temperatures of 125~225℃, yielding salinities of 0.50%~7.25% NaCleqv. The estimated pressures range from 63~117MPa in the early stage to 12~67MPa in the middle stage, with the greatest formation depth of 4.7km. In a word, the ore-forming fluids evolved from high temperature, CO2-bearing magmatic fluid to low temperature, CO2-poor meteoric fluid. It seems that high temperature, CO2-rich fluids can be regarded as typical features of porphyry deposits developed in intra-continental setting, contrasting to the CO2-poor NaCl-H2O fluids commonly observed in volcanic arcs. Based on comparison of typical porphyry systems in the East Qinling-Dabie area, we suggest that the volatile concentration in the initial ore-forming fluids and the features of wallrocks (including chemical composition, crystallization degree, mechanical strength, etc.) control the spatial location of orebodies in porphyry systems.
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