| 其他摘要 | 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. |
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