Abstract:The Lianshanguan area is located in the northeast part of Northern China Platform. It lies in the Lianshanguan brachy anticline composed of strata of the Lower Proterozoic sequence called Liaohe Group. The core of the anticline is made up of an approximately EW striking migmatitic granite. In recent years a number of radioactive anomalies and uranium occurrences have been discovered along the contacts between Liaohe Group and the migmatitic complex massif on both flanks of the anticline. In this paper the migmatitic metasomatism as an important metallogenetic controlling factor is discussed in some detail, which is to be summarized as follows: 1. According to the isotopic ages and the field occurrences, the red migmatitic granite and the white migmatite developed in this region can be considered as products of different periods. The former (2320-2336 m. y.) was formed during Late Anshan orogeny, comprising the basement of Liaohe Group, while the latter (1890-1933m. y.) was formed during Luliang orogeny, migmatizing and replacing the strata of Liaohe Geoup. Uranium mineralization formed at 1900 m. y. was closely related to the white migmatites. 2. The characteristics of different kinds of migmatites are described with respect to their mineral constituents, petrotexture and petrochemical composition. The homogeneous red migmatitic granite is rich in potash feldspar, whereas the white migmatite is rather complex in lithology, having well-developed taxitic texture and relatively abundant plagioclase. 3. The evolution of the migmatitic metasomatism in time and space are studied through the observation of compositional variation of different migmatites. Drmng Luliang orogeny, the Liaohe Group underwent regional metamorphism of epidote-hornblende facies. In this process the tectonic emplacement of the red migmatitic granite underlying Liaohe Group caused tectonic faulting and thermodynamic metamorphism along their contact zone. Meanwhile, a Na-bearing alkalic solution was produced due to the metamorphic differentiatiou and the local remobilization of the red migmatitic granite. Migrating through the fracture zones developed along the periphery of the red migmatitic granite and the interstratified brecciated zone in Langzishan Formation of Liaohe Group, the Na-bearing solution replaced the wall-rocks-quartzites, mica-quartz-schists of basal part of Langzishan Formation and a part of the red migmatitic granite-on both sides and produced an elongated belt of white migmatites 0-100 m in width. This migmatization,therefore, is actually an alkalic metasomatic process, xvhich can be divided into three stages: the main migmatitic stage, the residual-migmatitic metasomatic stage and the stage oi migmatitic-hydrothermal solution. It started from Na-metasomatism which subsequently gave way to K-metasomatism. Potassium had been obtained from wall rock itself. Na-metasomatism and K-metasomatism occurred alternatively with their intensity weakened gradually. The difference in composition of the original rocks and in intensity of the metasomatism resulted in the spatial zonation of white migmatites. A transitional belt of Light red or white migmatitic granite was formed along the periphery of the red migmatitic granite. From this belt to the metamorpho-sedimentary rocks of Liaohe Group there can be roughly recognized three subzones: homogeneous migmatite, taxitic migmatil:e and migmatitic metasediments. But such a zonation often extend discontinuously. Usually the interstratified white migmatitic belts represent mainly Na-metasomatism and are characterized by lower intensity of metasomatism. 4. The relationship of uranium mineralization to migmatitic metasomatism is investigated. The white migmatite, xvhose U abunclance reaches 6-16 ppm on average and whose uranium-extracting rate is higher than that of other rocks in the region, provided part of uranium source. Uranium mineralization mainly appears in the fracture zones along the borders of the interstratified-white migmatitic metasediments. Jnhomogeneity of the metasomatism in these zones can be regarded as a favorable tectonic condition for uranium mineralization. The late Na-bearing alkalic hydrothermal solution, be/ng an important transport agent for uranium migration, caused the mobilization and concentration of uranium previously scattered in the rocks. 5. The genetic type and mineralization model of uranium deposits are put forward tentatively. The mineralization of economic significance seems localized in the subzone of white migmatitic metasediment and in the basal part of Langzishan formation. The isotopic ages suggest that the uranium mineralization formed at 1898 m. y., i, e., slightly later than the stage of migmatitic metasomatism. The processes of the formation of uranium deposits may be broadly stated as follows: After Anshan orogeny the Lianshanguan red migmatitic granite massif was exposed at the surface and underwent weathering erosion. Part of uranium was separated out from the rocks and carried by runoff to the base trough near the seacoast. In the transitional environment of redox uranium could be reduced and precipitated from water or be absorbed, giving rise to a horizon of low-grade uranium at the basal part of Langzishan formation. During Luliang orogeny, Liaohe group underwent a lower middle grade regional metamorphism and part of the syngenetic uranium could be reconcentrated to form the poorly graded minera lization of sedimentary-metamorphic type. At the late stage of metamorphism the migmatitic metasomatism and alkalic hydrothermal alteration took place which stimulated the mobilization of uranium from surrounding rocks and, under the favorable physico-chemical conditions, deposited it in. the suitable fracture zone to form the rich orebodies of migmatitic hydrothermal type or to reconstruct the low grade orebodies of sedimentary-metamorphic type into the superimposed mineralization. All this indicates that the deposits are likely to be of the multistage, polygenetic vein-like type, which underwent a complex processes of sedimentation, metamorphism and migmatitic metasomatism during their formation. They are, to a certain extent, stratabound, but the commercial uranium mineralization mainly occurred at the migmatitic hydrothermal stage.
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钟家蓉.1983.连山关地区下元古界中混合交代作用与铀成矿的关系[J].矿床地质,2(2):77~86.1983.Relationship of uranium mineralization to migamtitic metasomatism in lower proterozoic strata in Lianshanguan area, Liaoning Province[J].Mineral Deposits2(2):77~86
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