大兴安岭南段白音查干Sn-Ag-Zn-Pb矿床电气石矿物学特征及对岩浆-热液演化过程的启示

内蒙古西乌旗白音查干矿床是大兴安岭南段锡矿带内最典型、规模最大的Sn-Ag-Zn-Pb矿床。电气石在成矿岩体花岗斑岩和围岩地层中均广泛发育，依据其产状可分为四类：Ⅰ团斑状电气石；Ⅱ热液角砾岩胶结物中电气石；Ⅲ热液脉状电气石；Ⅳ弥散状电气石。在详细的岩相学基础上，利用电子探针点分析和面扫描分析对不同产状和结构的电气石进行了详细的成分分析。结果显示，花岗斑岩体深部的团斑状电气石（Ⅰa类）以自形、环带发育为特征，至少可见三期生长环带：核部电气石（Ⅰa-1）极高的Fe/（Fe+Mg）和Al值暗示其岩浆成因；边部电气石（Ⅰa-3）较富Mg，且与热液矿物共生，是从早期热液流体中沉淀形成的；幔部电气石（Ⅰa-2）的结构和成分显示其形成可能与不混溶的富B-Fe-Na的熔体或流体有关。因此，电气石从核部到边部的生长记录了从晚期岩浆到早期热液阶段的演变过程。花岗斑岩体中上部的团斑状电气石（Ⅰb类）环带不发育，其与热液矿物共生的组合及成分暗示其形成更倾向于与热液过程相关，可能是岩浆顶部聚集的早期流体释放之后被固结岩浆"圈闭"的残余流体结晶的产物。随后，大规模释放的富B流体形成了大量以电气石为主要胶结物的热液角砾岩（Ⅱ类）、成矿前电气石-石英阶段脉系（Ⅲa类）及伴随围岩蚀变而形成的弥散状电气石（Ⅳ类）。对Ⅱ类和Ⅲa类电气石内存在的生长环带分析显示，成矿前可能存在多个脉冲期次且成分有差异的流体的叠加作用。同时，电气石从早期到晚期向富Mg方向的演化，及成分明显受围岩地层影响的现象，暗示岩浆热液流体与围岩地层发生的水岩反应可能在金属成矿过程中起了重要作用。此外，本研究显示，不同产状电气石的结构和成分信息能够有效记录矿床内岩浆-热液转变及热液演化过程的众多细节信息，为深入理解成矿过程提供了依据。

Mineralogical features of tourmaline in Baiyinchagan Sn-Ag-Pb-Zn deposit, southern Great Xing'an Range, and its implications for magmatic-hydrothermal evolution

Baiyinchagan deposit, which is located at West Ujimqin Banner, Inner Mongolia, is the largest and most typical Sn-Ag-Zn-Pb deposit in the tin ore belt of southern Geart Xing'an Range. Abundant tourmalines occur in ore-related granite porphyries and wallrocks in this deposit. They can be classified into four different occurrences, including: Type Ⅰ tourmaline nodules, type Ⅱ cement tourmalines in hydrothermal breccias, type Ⅲ tourmalines in hydrothermal veins and type Ⅳ disseminated tourmalines. In this paper, chemistry of tourmalines from various occurrences and textures are studied by using electron microprobe analysis and map scanning based on their petrographic features. The type Ⅰ tourmalines in nodules from deep granite porphyries are featured by euhedral shape and complex growth zonings. At least three zonings are identified from core to rim. The core tourmalines have very high values of Fe/(Fe+Mg) and Al, implying their magmatic origin, whereas the rim tourmalines are rich in Mg and intergrowth with hydrothermal minerals, indicating they precipitated from hydrothermal fluids. The textures and chemistry of mantle tourmalines should relate to immiscible B-Fe-Na melt or fluid. Thus, the tourmaline nodules may record the whole evolution process from late magmatic to very early hydrothermal stage. However, the type Ⅰ tourmalines in nodules from the upper granite porphyry are lack of zoning, and the mineral assemblages and chemistry indicate these tourmalines should be associated with hydrothermal process. It is possible they represent the residual early hydrothermal fluids that accumulated at the top of magma and locked by crystallizing magma. Subsequently, massive released B-rich fluid from granite porphyry generates hydrothermal breccias cement by tourmalines, pre-ore mineralization tourmaline-quartz veins and disseminated tourmalines via wallrock alteration. Based on the analysis of tourmaline growth zonings existing in hydrothermal breccias and veins, it is possible that multiple pluses of fluids with varying composition may overprint at pre-ore mineralization stage. Meanwhile, the Mg-rich evolution trend of tourmaline from early to late stage and the wallrock-controlled composition feature, implying that the water-rock interaction between hydrothermal fluid and wallrock plays an important role on the metal mineralization in this deposit. Furthermore, according to the various textures and chemistry among different occurrence of tourmalines, this study reveals that tourmaline can record some detailed information related to magmatic-hydrothermal transition and hydrothermal evolution, and it is helpful to promote our understanding of ore-forming process.