| 其他摘要 | The research on the relationship between geodynamics and mineralization is a frontier of the earth sciences, however the research on the relationship between intracontinent lithospheric extension and mineralization is still in the beginning in general. It took place strong lithospheric extension and thining in the Cretaceous-Paleogene in South China, which had close genetic relationship to many metal and nonmetal deposits formed in the same time. Especially, it is believed that the granite-type uranium deposits in South China may have relationships to lithospheric extension in various aecpects. The Xiazhuang uranium orefield, located in the south of Nanling polymetallic mineralization zone, is a typical granite-type uranium orefield in South China. On the base of detailed field investigation we have made a systematic study on the character of mineralizing fluid, the activity rule of element during mineralizing, the sources and evolution of ore-forming fluids and so on with the method of fluid inclusion geochemistry, element geochemistry and isotopic geochemistry. We discussed primarily the restraining mechanism of lithospheric extension to uranium minelization and proposed a possible genetic modle of the uranium deposits in Xiazhuang orefield. The main conclusions are listed as follows:
(1) The studies on the microthermometry, Raman microspectrometry and composition of fluid inclusions in some gangue minerals indicated that the premetallogenic fluids are characterized by medium-high temperature (200℃~350℃), low salinity (the salinity ranges from 0.72% to 5.95% NaCl), medium density (the density ranges from 0.703g/cm3 to 0.830g/cm3), larger depth (the depth ranges from 2.29km to 5.74km), enriched in ∑CO2, relative reductive condition. In contrast, the metallogenic fluids are characterized by low-medium temperature (154℃~250℃), low salinity (the salinity ranges from 0 to 1.83% NaCl), medium density (the density ranges from 0.628g/cm3 to 0.867g/cm3), smaller depth (the depth ranges from 0.19km to 1.62km), enriched in F-, relative oxidizing condition.
(2) The systematic studies of the electron-probe analysis of ore minerals and the trace elements geochemistry of rock and ores indicated that major uranium minerals are pitchblende, coffinite and brannerite in Xiazhuang orefield, whose formation was related to Si, Ca and W, however, other metal elements were not enriched. The rare earth elements composition of ore and gangue minerals was partly similar to the host rocks, and evolved highly on the base of the host rocks.
(3) The analyses of C, O isotopes method of the carbonate and He, Ar isotopes method of pyrite were employed in this paper. The results show that the mineralizer ∑CO2 mainly comes from mantle same as He, Ar. The deep faults may control the involving of the mantle volatile.
(4) The studies on the Sr, Nd isotopes of carbonate and fluorite indicated that the ore-forming fluids derived mainly from the crust, and the gangue minerals in south of Xiazhuang uranium orefield are the products of water-rock interaction between hydrothermal fluids and the mafic dikes. Moreover, according to the studies on Pb isotopes of carbonate, uranium might come from the metasomatized granite by later fluids.
(5) The relation between the lithospheric extension and uranium mineralization was close. Uranium deposits in Xiazhuang orefield are obviously controlled by extensional structures that are not only the transmitting structures but also the depositing structures. The increasing of geothermal gradient induced by lithosphere extension drove the heat fluids to flow and provided major power. Huge faults induced by lithosphere extension connected the crust with the mantle and promoted the mantle derived volatiles (mostly of ∑CO2) to ascend along faults. The volatiles entered the crust hydrothermal circulating system, and took part in uranium mineralization.
(6) “A possible genetic model of hydrothermal uranium deposits in continental ho circulating system in lithospheric extension zone” is proposed. Lithospheric extension during the Cretaceous-Paleogene induced a large scale fluids circulating in Xiazhuang uranium orefield. These volatiles, deriving from mantle degassing, joined into the hydrothermal fluids of meteoric origin with absence of the mineralizer and uranium. The mineralizer-rich hydrothermal fluids had been formed and leached uranium from the U-rich granite. So the fluids were transformed into the mineralizer-rich, U-rich hydrothermal fluids. Then the fluids ascended along the structures forced by the thermodynamic driving of lithospheric extension. Since the influence of the geochemical barrier and pressure releasing of the hydrothermal fluids when the fluids reached shallow environment, U was precipitated and enriched in the favorable position to form the deposits. |
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