河南省嵩县鱼池岭斑岩钼矿床成矿流体特征及其地质意义

Characteristics of ore-forming fluids of the Yuchiling porphyry Mo deposit, Songxian county, Henan province, and its geological significance.

河南省嵩县鱼池岭钼矿床产于合峪复式花岗岩基内部,含矿岩体为黑云母二长花岗斑岩。矿体呈透镜状或似层状产出于侵入体中,含矿斑岩全岩矿化。矿石构造主要为网脉浸染状,网脉矿物组合有石英±钾长石±绿帘石、石英-萤石±方解石、萤石以及大量的石英-黄铁矿、石英-辉钼矿、石英-多金属硫化物等。根据网脉矿物组合和穿切关系,将矿化过程分为早、中、晚3个阶段,其特征性围岩蚀变分别是高温硅化、钾长石化、绿帘石化,硅化、绢云母化、绿泥石化、硫化物化,以及低温碳酸盐化、萤石化。脉石英中广泛发育流体包裹体,包括水溶液包裹体(W型)、纯CO_2包裹体(PC型)、H_2O-CO_2包裹体(C型)及含子矿物多相包裹体(S型)。早期无矿石英脉中捕获了H_2O-CO_2-NaCl体系的初始成矿流体,其温度集中于280～420℃、盐度为30.1 wt%～54.1 wt%NaCl.eqv,压力最大值为194MPa(约6.8km深)。成矿流体上升至5.6km左右、温度降低至400℃即发生沸腾,CO_2大量逸失,黄铁矿及少量辉钼矿沉淀,形成石英-黄铁矿-辉钼矿脉;在280～380℃、～137MPa、约4.8km深处流体强烈沸腾,辉钼矿大量沉淀。在200～340℃、约0.8～4.3km深时,黄铁矿、辉钼矿、黄铜矿、闪锌矿等多种金属硫化物沉淀,形成石英-多金属硫化物脉。晚期流体为H_2O-NaCl体系,脉石英中仅见水溶液包裹体,均一温度为130～200℃。上述结果表明,由酸性岩浆分异出高温、高盐度、富CO_2流体,CO_2可大量存在于浆控高温热液体系。结合区域地质演化,认为包括鱼池岭含矿斑岩在内的合峪花岗岩基形成于加厚造山带地壳的快速隆升过程;考虑到成矿年龄为144Ma,确定144Ma以来的最大剥蚀厚度不超过7km,平均剥蚀速率不超过0.05mm/a。

The Yuchiling Mo deposit, Songxian County, Henan Province, is a large porphyry deposit recently discovered in the famous East Qinling Mo belt. It is spatially associated with a biotite monzogranite porphyry intruding into the Heyu multistage granite batholith. Ore-bodies are lentiform or stratiform occurring in the porphyry which was universally mineralized. The ores are characterized by disseminated multistage stockworks of quartz +/- potassic feldspar +/- epidote, quartz-fluorite +/- calcite, fluorite and quartz-sulfide including quartz-pyrite, quartz-molybdenite, quartz-polymetallic sulfides. Paragenesis and crosscutting relationship of the stockworks suggest the ore-forming process includes the early, middle and late stages, accompanied by characteristic wall-rock alterations of silicification-potassic feldspathization-epidotization, silicification-sericitization-chloritization-sulfidization, and low-temperature carbonation-fluoritization, respectively. In the vein quartz, four types of fluid inclusions are distinguished from aqueous water (W-type), pure CO(2) (PC-type), H(2)O-CO(2) (C-type) and daughter mineral-bearing (S-type). The early-stage barren quartz veins trapped the initial high-salinity (30.1 wt% similar to 54.1 wt% NaCl eqv.) H(2)O-CO(2)-NaCl ore-fluids at temperatures of 280 similar to 420 degrees C with the pressure up to 194MPa, corresponding to depth of < 6.8 km. Due to the high CO(2) content and low sulfur fugacity, little sulfide precipitated in the early-stage fluid process. The quartz-pyrite-molybdenite veins formed where high temperature, CO(2)-rich fluids ascended to depth of ca 5.6km and cooled to ca 400 degrees C, due to fluid boiling and CO(2)-escape. Further fluid ascending resulted in intense boiling and formation of quartz-molybdenite or molybdenite stockworks whose fluid inclusions yield homogenization temperature of 280 similar to 380 degrees C and pressure of up to 137MPa, corresponding to depth of up to 4.8km. Quartz-polymetallic sulfide veins progressively formed at lower temperature and shallower depth, i.e. 200 similar to 340 degrees C and 0 8 similar to 4.3km, The late-stage barren quartz veins contain only aqueous fluid inclusions with homogenization temperatures between 130 and 200 degrees C. Fluid inclusions are rare in the latest quartz-fluorite calcite and fluorite veins. The results above show that fluid-system forming intracontinental intrusion-related hypothermal deposit; are likely characterized by high salinity and high CO(2) content. Considering regional tectonic evolution, we conclude that the Heyu granite batholith where Yuchiling deposit located formed during a rapid uplift of a thickened orogenic crust. The erosion thickness is estimated to be less than 7km and erosion rate is less than 0.05mm/a since 144Ma which is interpreted as the formation age of the Yuchiling deposit.