| 其他摘要 | Tin distribution and its mineralization often have close relation with granitic magmatism. Previous studies have indicated that the majority of tin deposits are intimately associated with highly differentiated S-type granite or continental crust transformation series granitoids. Only in recent years have people begun paying attention to the study of A-type granites and their relations with tin metallogenesis since the discovery of some tin deposits occurring in A-type granites. In contrast to S-type granites, a few studies have focused on mechanisms of A-type granite related tin mineralization and the data concerned in this respect are less available.
South Hunan region, located in middle Nanling polymetallic mineralization belt, is an important non-ferrous metal mineralization belt in South China, in which the newly discovered Furong tin polymetallic deposit is world famous. Recently published papers indicate that the deposit has intimate spacial and temporal relationship to Qitianling granite, which has been evidenced to be A-type granites. In this paper, with the Furong tin deposit and the related Qitianling granites as the object of study, based on the research published data and considerations, detailed investigations have been conducted into the geochemical characteristics and evolution mechanism of the ore-forming fluids by means of the theories and approaches of petrology, mineralogy, fluid inclusions, trace elements and stable isotope geochemistry. On the basis of the data and considerations obtained above, the paper attempts to probe into the relationship between Qitianling granites and Furong tin deposit and the mineralization mechanism of the deposit. From the above studies, the following achievements have been obtained:
1. By means of mineralogy, petrology, trace elements geochemistry and isotope geochemistry, it has been evidenced that the Qitianling granite is possessed of characteristics of A-type granites and characterized by high Si, alkali, and K enrichment, metaluminous. The two stages of granites of Qitianling instusion have the same magma origin and assigned to A2-type granite. Isotopic data show that the source of magma is affected by EMII-type enriched mantle, which implies that the Qitianling granite is emplaced in the tectonic setting of crustal extension, lithosphere thinning of South China and there existed mantle components involved during lithogenesis.
2. According to microscope observation of the structure and compounent of the ore minerals and SEM, EPMA analysis, it is concluded that the primary skarns in Furong deposit is formed in oxidized environment and its diagenesis is mainly related to the hornblende-biotite monzogranite intruded earlier. Tin dominantly occurs as Sn4+ and is diffused in the crystal lattices of skarn minerals. Regressive metamorphosed skarn, greisen, and altered granite mineralization constitute the main mineralization stage, and the hydrothermal fluids that responsible for these three types of ores are characterized by similar geochemical properties and are enriched in Cl, Ti, Sn. The formation of cassiterite sulfide ores is distinctly later than the main mineralization stage.
3. By means of the theories and approaches of fluid inclusion geochemistry inconjunction with the Laser Raman technique, the nature and physical chemistry of the ore-forming fluids of the Furong tin deposit have been defined. These fluids have the composition of an unmixing solution system of CO2-CH4-CaCl2- NaCl-KCl and have salinity of 0~50.63 wt%NaCl eq., density of 0.31~1.12g/cm3. The main-stage ore-forming fluids have temperatures mainly distributing between 300℃ and 450℃, pressures of 179-1800bars. The later-stage ore-forming fluids have the temperature mainly concentrating in 150~300 ℃, pressure of 400-600bars. From the main stage, later stage to post stage of the mineralization , the hydrothermal fluids show a decreasing tendency of salinity, and the ingredients also transform from CaCl2-NaCl-KCl-H2O solution with CO2, CH4 to simple NaCl KCl-H2O solution without carbon species.
4. The origin of ore-forming fluid has been revealed according to analyze the REE and stable isotopic geochemistry of gangue minerals of the main types of mineralizeation. It is deduced that the hydrothermal fluids have inherited the REE and stable isotope geochemical characteristics of the Qitianling granite, and that the magmatic-hydrothermal fluid concentrated by crystallization of the Qitianling granite intrusion is assumed to be a significant source of the ore-forming hydrothermal fluid in the Furong tin deposit. Much of deep circulated meteoric water is involved during mineralization process.
5. Combined with published considerations and the new findings of this study, the author has delt with the genetic relations between Qitianling granites and Furong tin deposit, as well as the mineralization mechanism of the deposit. The ore-forming fluids of the Furong tin deposit has close genetic association with the magmatic-hydrothermal fluid enriched in Cl and Sn concentrated by crystallization of the biotite granite. Tin predominately presents as complex Sn(II)-Cl species in hydrothermal fluid. Mixing of granitic magma-derived tin ore fluids with meteoric waters would cause temperature-, salinity-, pressure-reducing, CO2 degassing, and oxygen fugacity ascending, and subsequently the Sn(II)-Cl complex is decomplexed and Sn is oxidized to Sn4+. Due to the raise of pH resulted from CO2 degassing, and the decreasing of the temperature and the salinity of the ore-forming fluids, the solubility of SnO2 would reduce rapidly and cassiterite would precipitate as a result. Such a fluid mixture is efficient mechanism of the precipitation of cassiterite and tin mineralization. |
修改评论