| 花岗岩型和火山岩型铀矿化的成因 |
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| 引用本文：.1983.A model of the genesis of granite-type and volcanite-type uranium mineralization[J].Mineral Deposits,2(2):59~67 |
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| A model of the genesis of granite-type and volcanite-type uranium mineralization |
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| Abstract:Through an investigation on the geological setting and geochemical environments of granite-type and volcanite-type uranium mineralization in south China, a genetic model of ore-forming processes has been formulated and summarized as follows: During the Yenshan movement, due to the subduction of the Pacific Plate beneath the Chinese continent, the old strata intercalated with uranium-rich beds in the middle and lower parts of the side of south China were repeatedly compressed and infiltrated with such constituents as fluids and hot flows of the ascending mantle, giving rise to extensive anatexes and generating subsequently substantial amounts of uranium-rich acid magma. Having undergone several times of compressive actions, the crust of south China turned into a state of tension, and at the same time, the anatectic zone lost its ability of further remelting and cooled down gradually. The superfluous volatiles of supercritical content such as CO2, F, Cl, H2S and H2O could not be solved uniformly in the acid magma. Uranium, volatiles and other materials were fractionated out again and again as a result of the repeated occurrence of the deep tensile fracture. The fractional order of various volatiles seems to have depended .not only on the' sorts of volatiles, but on their contents as well. During the earlier stage, the superfluous CO2 of supercritical content was first fractionated out and, together with Na, U and H2O, formed uranium-bearing fluids, during the later stage, the volatiles F and Cl, which were also in a state of supercritical content, became fractionated out along with SiO2, K, U, H2O and minor amounts of CO2 and H2S, composing another uranium-bearing fluid or even forming the third uranium-bearing fluid rich in F, Cl and U while depleted in SiO2 and K. These uranium-bearing fluids, when ascending to a certain level of the crust, would surely meet with circulating ground water, forming in turn uranium-rich or uranium-poor, Na-, CO2- containing alkaline hydrothermal solutions, strong acid uranium-bearing hydrothermal solutions abundant in F, Cl, SiO2 and subordinate amounts of K and CO2, as well as acid uranium-bearing hydrothermal solutions containing abundant F, Cl and lesser amounts of CO2 and H2S. These uranium-bearing fluids would conduct neutralization reaction, silicification, hydromicazation and argillization. When the temperature of hydrothermal solutions fell to or even below 200℃, uranium precipitated in the form of pitchblende, making up uranium ore deposits fo different mineralization types. It is worth while to note that for microcrystal quartz filling-metasomatic type, uranium was poor in the early-stage silicification zone and was not concentrated until the late stage. A possible explanation for this is that SiO2 migrated in the form of soluble K-silicate ( xK2O, ySiO2) in the strong acid K-bearing solutions and when the temperature fell to about 300℃, it became unstable and its precipitates formed a gigantic silicification zone while uranium was concentrated in the residual hydrothermal solutions; a further decrease in temperature and acidity of these solutions would cause the concentration and precipitation of uranium in late stage minor fissure zones. My basic idea about the genesis of the granite-type and volcanite-type uranium ore deposits agrees fairly well with Chen Zhaobo' s "double mixing" genetic model of volcanite-type uranium ore deposits. However, instead of over-emphasizing the role of transitional magma chambers, my model only lays stress on the important part that the anatectic zone played in the formation and migration of various uranium-bearing fluids, the dependence of the fractional order of a variety of volatiles on the limited solubility and concentration of these volatiles in silicate melts and some other factors, |
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