| 湘西沃溪Au-Sb-W矿床矿石矿物的矿物学和地球化学研究 | |
| 祝亚男 | |
| Thesis Advisor | 彭建堂 |
| 2014 | |
| Degree Grantor | 中国科学院研究生院 |
| Place of Conferral | 北京 |
| Degree Name | 博士 |
| Degree Discipline | 地球化学 |
| Keyword | 矿物学 微量元素 稳定同位素及稀有气体同位素 流体包裹体 沃溪au-sb-w矿床 |
| Abstract | 湘西雪峰山一带，脉型金矿床广泛分布，与国外常见的单金石英脉型金矿床不同，该区金矿常与钨、锑共生。其中，沃溪矿床作为该区最大、也是唯一发育Au-Sb-W组合的金矿床，是研究此类独特元素组合脉型金矿的天然实验室。本论文主要以矿石矿物为研究对象，运用矿物学、矿物化学（EPMA）、微量元素地球化学、同位素地球化学（S、Pb及稀有气体同位素）以及流体包裹体地球化学等方法，对沃溪矿床的成矿物质来源、成矿流体性质与演化以及矿床成因等问题进行了系统研究。本论文主要获得以下几方面的重要认识： （1）沃溪矿区黑钨矿矿脉以顺层的含矿石英脉为主，同时发育各种节理脉；节理脉常相互错断，表现出多阶段成矿特征。矿脉中黑钨矿常与石英、碳酸盐及硫化物共生，矿石组构显示出热液充填特点。该矿黑钨矿的REE含量很低（1.62 ~4.58μg/g），明显低于南岭与花岗岩有关的黑钨矿；其HREE相对富集，并具有Eu、Sm、Gd、Tb异常及MW复合型四分组效应等特征，这可能与成矿流体的氧逸度及络合物的稳定性有关。同时，该矿黑钨矿的Y/Ho比值均小于28，指示其成矿流体中以CO32-（HCO3-）络合物为主；其高含量Sc可能与成矿流体对深部岩石的淋滤有关。这些特征明显有别于南岭石英脉型黑钨矿，暗示其成矿与岩浆作用无直接关系。 （2）沃溪矿区的黄铁矿与湘西区域马底驿组板岩、赋矿围岩以及深部岩石的REE分布特征均十分相似，可能反映围岩或/和深部岩石为其形成提供了物质。该矿金属硫化物的S同位素组成为-7.40~+0.92‰，与区域板岩中黄铁矿的d34S值（-5.50~+19.12‰）部分吻合但整体偏低，这可能与硫化物的形成机制（流体沸腾）有关；成矿流体总S同位素组成为-9.19~+3.09‰，其分布具有两个明显的峰，可能指示黄铁矿的形成受流体沸腾与水-岩反应的双重影响，而导致由其d34SPy计算的d34SH2S较由辉锑矿计算的偏低，或指示成矿流体中的S可能来自深部流体与围岩。硫化物的Pb同位素组成显示其主要来自于造山带，可能是围岩与含较高放射成因Pb的深部岩石的混合。 （3）利用红外显微技术，首次研究了该矿中矿石矿物（辉锑矿）的流体包裹体特征，并结合对白钨矿和石英中包裹体的研究，确定了沃溪矿床的成矿流体性质以及矿石的沉淀机制。白钨矿及共生石英（早阶段）与辉锑矿及共生石英（晚阶段）中的流体包裹主要有4种类型：I型，富液相的气液两相水溶液包裹体；II型，富CO2气液两相或三相包裹体；III型，富气相的气液两相水溶液包裹体；IV型，纯液相水溶液包裹体。矿石矿物与共生石英中的流体包裹体类型相似，且均一温度和盐度也大致相同。沃溪矿床的成矿流体为中低温（140~240°C）、低盐度（<7.0 wt.% NaCl equiv.）、富CO2并含N2的水溶液，与雪峰山地区其他脉型金矿的成矿流体特征相吻合。钨成矿阶段矿石形成机制以流体混合作用为主，而金锑成矿阶段则以流体沸腾作用为主；此外，水-岩反应也可能对该区金沉淀有影响。 （4）首次对沃溪矿床进行了稀有气体同位素研究。该矿的黄铁矿和辉锑矿中流体包裹体的3He/4He值为 0.002~0.281 Ra，40Ar/36Ar值为229.9~2585.9，幔源物质没有参与该矿的成矿作用，成矿流体为深部非岩浆成因的地壳流体，向浅部大气饱和水的贡献增加；成矿流体的He、Ar同位素组成受基底及赋矿围岩影响。 （5）沃溪矿床的地质地球化学特征与造山型金矿十分吻合，但其还具有W、Sb成矿，故其很可能是一个非典型的造山型金矿床。 （6）探讨了沃溪矿床的成矿机理。晚加里东时期，雪峰山地区处于陆内造山的构造背景下，发生了一系列的变形变质作用，使深部含矿质及CO2的流体沿断裂上涌，期间与围岩相互作用，并最终于层间断裂中与浅部流体混合而导致W矿的富集沉淀。W矿脉形成以后，晚阶段的深源含矿质流体再次上涌，由于其流体压力高于静岩压力，将迫使早阶段形成的W矿脉重新张开，或在围岩中产生新的破裂；进而该流体及一些围岩中的大气饱和水迁移至破裂带中，导致流体压力骤减，发生沸腾作用，从而沉淀黄铁矿、自然金及辉锑矿等矿物。此外，成矿过程中伴随的强烈水-岩反应也可能对Au的析出有一定贡献。 |
| Other Abstract | The lode gold deposits occurred in the Xuefengshan Range, western Hunan, are distinguished from typical gold-only lode deposits. Their ore-forming element associations are predominated by Au-Sb-W, Au-W and Au-Sb. The Woxi Au-Sb-W deposit, as the largest gold deposit in the Xuefengshan Range, is representative of these gold deposits with unique metal associations in this region, and can provide a natural laboratory for investigating the nature and source of ore-forming fluid of lode gold deposits in the Xuefengshan Range. In this study, the mineralogy, trace elements, S-Pb-He-Ar isotopes, and fluid inclusions of the ore minerals from the Woxi deposit were systematically studied in order to constrain the origin of the ore-forming materials, the nature and evolution of the ore-forming fluids, and the ore genesis of this deposit. The main achievements obtained in this study are as follows: (1) Most wolframite ore veins in the Woxi deposit occur as bedding quartz veins, whereas the others appear as various crosscutting joint veins, there probably exist multi-stage tungsten mineralization events in this area. Wolframite, frequently intergrown with carbonate and sulfide, always fills in open-space and fissures. The REE concentrations of wolframites from the Woxi deposit (1.62~4.58μg/g) are obviously lower than those of other wolframites related with granite in the Nanling Range. The REE distribution patterns of these wolframites are characterized by HREE enrichment, Eu-, Sm-, Gd- and Tb-anomalies, as well as an unusual composite M- and W-type Tetrad Effect. All of these features are probably controlled by the oxygen fugacity and the stabilities of complexing agent in the ore-forming fluid. Moreover, the Y/Ho ratios (<28) of wolframites from the Woxi deposit reveal REE and Y speciation is dominated by (bi)carbonate-complexes in wolframite-precipitating fluid. In addition, the high content of Sc of wolframite from this deposit is probably resulted from the leaching of the underlying older continental rocks by the hydrothermal solution. The geological and geochemical characteristics of wolframite in the Woxi deposit are obviously different from those of quartz vein-type tungsten deposits associated with granite intrusions in the Nanling Range, which provides support for that no direct relationship between the tungsten mineralization and magmatism in the Woxi deposit. (2) Trace element geochemistry features suggested that the Woxi pyrite is possibly originated from the wall rock and/or the underlying older rocks, since all pyrite samples from the Woxi deposit and those slate samples collected from the Banxi and Lengjiaxi Groups share similar LREE-enriched pattern. The δ34SSulfide values of sulfide from Woxi (-7.40~+0.92‰) are lower than the δ34Spyrite values of diagenetic pyrite from the Banxi Group (-5.50~+19.12‰). The relative negative span of the isotopic values of the ore-forming fluids can be explained by fluid oxidation as a consequence of boiling. The δ34SH2S values (-9.19~+3.09‰) calculated for the ore-forming fluids of the Woxi deposit, are characterized by bimodal distribution, which are indicative of boiling and extensive fluid-rock interaction, or imply that sulfur from host rock may have been involved during fluid-rock interaction. The Pb isotope composition of the Woxi sulfides show similarities to the orogenic Pb, and most likely indicate mixing between a less lead source (host rock) and a more radiogenic lead source (deep rock). (3) An infrared microscope was first introduced to investigate the petrographic and microthermometric features of the fluid inclusions in stibnite collected from the Woxi deposit. Besides, fluid inclusions in scheelite and their coexisting quartz were examined in ordered to constrain the ore precipitation mechanisms of different stages. Four types of fluid inclusions were identified based on petrography, including type I (two-phase, liquid-rich aqueous inclusions), type II (two- or three-phase, CO2-rich inclusions), type III (two-phase, vapour-rich aqueous inclusions), and type IV (single-phase aqueous inclusions). The fluid inclusions in ore minerals (scheelite and stibnite) and their coexisting quartz largely share similar characteristics in terms of their types, homogenization temperatures and salinities. The ore-forming fluid of the Woxi deposit is characterized by a low-to-moderate temperature (140~240°C), low salinity (<7.0 wt.% NaCl equiv.), CO2-rich, N2-bearing aqueous ore-forming fluid, which is consistent with other lode gold deposits in the Xuefengshan Range. W ore precipitation was probably associated with fluid mixing; however, Au and Sb ore deposition probably resulted from boiling. Moreover, fluid-rock interaction probably played a role during gold precipitation in Woxi. (4) Noble gas isotopes (He and Ar) were first applied to study the fluid inclusions hosted in the pyrite and stibnite from the Woxi deposit. The low 3He/4He ratios (0.002~0.281 Ra) and 40Ar/36Ar ratios (229.9~2585.9) of the ore-forming fluids indicate that they are dominated by deeply amagmatic crustal origin fluids, with an increasing contribution of meteoric water from deep to shallow. The He isotope compositions in sulfide samples in this study are similar to the basement, and the Ar isotope compositions are influenced by the host rock. (5) The Woxi Au-Sb-W deposit is probably an atypical orogenic gold deposit with significant tungsten and antimony mineralization. (6) The Xuefengshan Range has experienced a series of deformation and metamorphism in an intracontinental environment during the Late Caledonian. Several shear zones and metamorphic fluids are formed during deformation- metamorphism. These deep metamorphic fluids upwelling along the fault, mixed with shallow fluids at the interlayer faults, leading to W ore deposition. After the W ore veins were formed, these metamorphic fluids upwelled along the fault again. As the fluid pressure exceeded the lithostatic pressure, reopening of the W ore veins or a new phase of fracturing occurred. With the fluids migrating award the fractures, boiling caused by the marked pressure drop resulted in gold and stibnite deposition. Besides, fluid-rock interaction probably played a role during gold precipitation in Woxi. |
| Subject Area | 矿床地球化学 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.gyig.ac.cn/handle/352002/5923 |
| Collection | 研究生_研究生_学位论文 |
| Recommended Citation GB/T 7714 | 祝亚男. 湘西沃溪Au-Sb-W矿床矿石矿物的矿物学和地球化学研究[D]. 北京. 中国科学院研究生院,2014. |
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