广西大厂花岗斑岩黑云母成分特征及其成岩成矿意义

广西大厂锡多金属矿床是世界上最大的锡多金属矿床之一，区内花岗斑岩主要分布于铜坑-长坡-巴里-龙头山矿区的东侧，受近SN向张扭性断裂构造控制，并对早期形成的矿体产生了叠加改造作用.为了探讨花岗斑岩的成岩成矿条件，对花岗斑岩中以斑晶、包体以及基质形式产出的黑云母进行了详细的电子探针分析.结果显示:花岗斑岩中的黑云母主要为岩浆成因的铁质黑云母，且指示岩体具有高碱度的特征.与斑晶和包体黑云母相比，基质黑云母相对富铁、锰、锂，贫镁、钛.斑晶和包体黑云母结晶温度为688~715℃，结晶压力为1.7~2.3 kbar，相当于6.3~8.4 km的侵位深度；基质黑云母的结晶温度为630~673℃，结晶压力为1.6~2.0 kbar，相当于6.0~7.4 km的侵位深度.斑晶、包体和基质黑云母的氧逸度分别为-17.0~-16.1、-17.0~-16.6和-19.0~-17.4，指示在侵位的过程中，伴随着岩浆的上涌，氧逸度随之降低，成岩条件由相对氧化向相对还原的方向变化.与花岗斑岩有关的共存流体性质研究显示，斑晶黑云母中的lg（f_H2O/f_HF）Fluid、lg（f_H2O/f_HCl）Fluid和lg（f_HF/f_HCl）Fluid值分别为3.41~3.82、3.78~4.07和-0.54~-0.17；包体黑云母中的lg（f_H2O/f_HF）Fluid、lg（f_H2O/f_HCl）Fluid和lg（f_HF/f_HCl）Fluid值分别为3.90~4.11、3.86~4.11和-0.74~-0.43；基质黑云母中的lg（f_H2O/f_HF）Fluid、lg（f_H2O/f_HCl）Fluid和lg（f_HF/f_HCl）Fluid值分别为3.90~4.39、3.85~4.96和-0.61~0.25.结果显示，在花岗斑岩上侵的过程中，与不同阶段结晶生成的斑晶、包体和基质黑云母共存的流体在成分上有一定的差异性，不是单一的流体.大厂花岗斑岩中黑云母成分的变化指示岩浆结晶过程早期阶段具有高温、高碱度和低氧逸度的特征，有利Sn元素的活化、迁移、富集，进而对大厂锡多金属矿床起到叠加改造作用. 

Characteristics of Biotite in the Granite Porphyry and Its Significance for Petrogenesis and Mineralization of Dachang Sn-Polymetallic Ore Deposit,Guangxi

The Dachang Sn-polymetallic ore deposit,Guangxi,is one of the largest tin deposits in the world. The granite porphyry occurs in the eastern side of the Tongkeng-Changpo-Bali-Longtoushan area,which is mainly controlled by nearly SN extensional tectonics and has superimposed and reformed the ore bodies formed in the early stage. To further evaluate the petrogenesis and associated hydrothermal mineralization of the granite porphyry,detailed electron microprobe analyses were performed on three modes of biotite occurrence:inclusions enclosed in quartz,phenocrysts and matrix as one of rock-forming minerals. The biotites of different occurrences are all classified as magmatic ferrobiotites,reflecting that their host rock has high basicity. Biotite matrixes contain higher FeO_t,MnO and Li₂O but lower MgO and TiO₂ than those occur as phenocryst and inclusion. The estimated crystallization temperatures for phenocryst and inclusion biotites are 688-715℃ and pressure conditions range from 1.7 to 2.3 kbar,equivalent to emplacement depth of 6.3-8.4 km. In contrast,the calculated crystallization temperatures for matrix biotites are 630-673℃ and pressure conditions range from 1.6 to 2.0 kbar,equivalent to emplacement depth of 6.0-7.4 km. Systematic differences in oxygen fugacity between biotites related to inclusion (-17.0 to -16.6),phenocryst (-17.0 to -16.1) and matrix (-19.0 to -17.4) are found,which are characterized by a trend of decreasing oxygen fugacities in the direction of magma upwelling,suggesting that the diagenetic conditions change from relative oxidizing condition to reducing condition. Magmatic biotite composition is useful in the calculation of the halogen fugacity in magma during its crystallization,which are represented by lg(f_H2O/f_HF)Fluid,lg(f_H2O/f_HCl)Fluid and lg(f_HF/f_HCl)Fluid values. These values for phenocrystys are 3.41-3.82,3.78-4.07 and -0.54 to -0.17 respectively; for inclusions are 3.90-4.11,3.86-4.11 and -0.74 to -0.43 respectively; and for matrix are 3.90-4.39,3.85-4.96 and -0.61-0.25 respectively,which indicates that the coexisting hydrothermal fluids associated with phenocrysty,inclusion and matrix biotite crystallization are not continuous episode but differences in composition. The changes in the composition of biotites regarding to inclusion,phenocrysts and matrix,indicate that the early stage of the magma crystallization is characterized by high temperature,high basicity and low oxygen fugacity. These characteristics could have been beneficial to the activation,migration and concentration of the Sn element,as well as to further transformation on the earlier formed deposit.