Tectonic Evolution At The Western Margin Of Ordos Basin With Respect To Metallogenesis Of Sandstone Type Uranium Deposits

The thrust-fold belt at the western margin of Ordos Basin is the tectonic transformation belt between eastern and west structures of Northern China. It is of important significance for understanding the structural evolution of Ordos Basin and the tectonic transformation of the North of China, for summarizing uranium metallogenic regularity and evaluating the potentials of uranium metallogenesis. Thus, guided by geodynamics theory and the theory of hydrogenic uranium metallogenesis, this paper undertakes a comprehensive and systematic research on the tectonic evolution of the western margin of Ordos Basin and its control over uranium mineralization applying the approaches of tectonic analysis, isotope geochronology and geochemistry.According to the U-Pb dating and Lu-Hf isotope analytic results of detrital zircons from the provenance at the western margin of Ordos Basin, it is concluded that the paleo-continental nucleus was formed upon the crust-mantle differentiation in the provenance at 3.5Ga. After two-time considerable crustal accretion during 2.7~2.6Ga and 1.1~0.8Ga, as well as the reformation by the Lvliangian and Hercynian magmatic activities, the crust became higher in maturity and was abundant in uranium-rich intermediate-acidic volcanics and alkaline granites, which would supply the Basin with sufficient uranium sources.During Mesozoic and Cenozoic, divergence-convergence processes of the surrounding plates and the conversion of tectonic stress field resulted in 7-phase tectonic evolution of the western margin of the Basin. The oblique crossing superimposition of Yanshanian Meridional Tectonic Belt by Himalayan Liupanshan Arc-shaped Tectonic Belt led to the remarkable differences between the southern and northern sections of the western margin of Ordos Basin. Late Triassic fan delta sand bodies and Middle Jurassic braided stream sand bodies in lower accommodation space are stably distributed and enriched in organic substances and pyrites, which are favorable for the sandstone uranium mineralization. The intense structural deformation, however could reduce the uranium metallogenic potential of sand bodies. Early Cretaceous braided stream sand bodies formed under the compressive regime are of certain size and benificial to the infiltration of groundwater and uranium ore formation.There are three types of uranium mineralization at the western margin of the Basin: (1) Interlayer Oxidation Zone Uranium Mineralization: higher in grade and larger in size, occurring in the lower member of Zhiluo Formation (J2zh) and Madongshan Formation (K1m), of which the epigenetic alteration zoning is expressed obviously. Precipitation of uranium is associated with the alkalic-reducing geochemical barrier at the redox front. (2) Phreatic Oxidation Zone Uranium Mineralization: lower in grade and smaller in size, controlled by local reducing environment. (3) Sedimentary Diagenetic Superimposed Reformation Uranium Mineralization: also lower in grade with variable thickness, controlled by lithologies and lithofacies and may be further concentrated by epigenetic infiltration and reworking.Structures are the most predominant ore control factors. Not only the development and the outcroping of uranium source rocks in the provenance, but also the upgrowth and deformation of prospective target layers are controlled by tectogenetic movements. Infiltration of ground water and uranium metallogenesis are constrained by the characteristics of sand bodies, the structural slopes and the discharge structures. According to the mineralization features and ore-controlling factors, three type of uranium metallogenic models, i.e. CYB, GJW and BTJ, are established by the author. The uranium potentials are also evaluated and prognosis. Four prospective areas are have been outlined for further mineral exploration.