Metallagenic Regularities Of Sandstone-type Uranium Deposits In Sedimentary Basins

Uranium is an important resource of strategy and energy which influences the peace and development in present world. Nuclear power has already been a strategic choice of national readjustment of the energy structure in china, so enhancing study of uranium metallogenesis and investigating large and rich in-situ teachable sandstone-type uranium deposits have become the strategic mission of nulcear geologists. Chagandelesu region which belong to inner Mongolia lieing in north side of Langshan mountain are one of most important prospective base of sandstone-type uranium deposits, several deposits have been discovered in the past years and indicated tremendous potential resource of the region. So, strengthening uranium metallogenesis research of this region have important realistic significance and practical value.Analyzing the metallogenesis of sandstone-type uranium deposits, we know that uranium deposits are the outcome of dynamics process of groundwater flow. It is clear that water plays important role in processes of transportation and accumulation of uranium material, water not only are the carrier of uranium transportation but also transmitter of energy, even it can say the nature of the uranium deposits is the result of water-rock interaction. In this case, the research of the uranium metallogenesis should attach importance to effection of water. However, current domestic and international research status about sandstone-type uranium deposit show it far from been attentioned. So, this paper try to study the metallogenic regularities of uranium existed in sedimentary basin with the method of analysis of paleo-hydrogeology.Firstly, the detailed description of history and current development status of sandstone-type uranium deposit worldwide were summarized and reviewed. Three main shortage which exist in the research on epigenetic uranium deposits at the present time were pointed out:â‘ attaching importance to the single factor of geology,but shortage of guidance of system theory;â‘¡much research only focused on deposit, no regional scale which demand by the theory of metallogenic system;â‘¢"static"analysis, shortage of idea of dynamic analysis and evolution of space-time. Based on these viewpoints, a new idea which was enlightened by the theory of metallogenic system was brought forward: under the guidance of the theory of system, with the analysis of dynamics of transportation and accumulation of uranium material acting as main clue , starting the study from evolution of regional geology especially evolution of tectono-dynamic and sedimentary facies, and then reconstruct the regional paleo-topography of different geology period which was relative to evolution of metallogenic system, further can applying for the theory of paleo-hydrogeological analysis to reconstruct the flow system of surface water and ground water according to the regional paleo-topography. the evolution of flow system on regional scacle can help us to research the dynamics processes of metallogenic of uranium which generally consist by resource, transport, collection and change. Detailed contents as follow:(1) The uranium source of sandstone-type uranium deposit in the region has been discussed in-depth. First of all, two different scale source has been put forward based on system-level theory, one is the source for single deposit, the other is the source for regional scale. the two can not be discussed together. Generally speaking, the latter has more significance to the our research. At the same time, Pointed out that the research of past often confused the two different levels and led to the problem of source can not solved convincingly. According to previous understanding, three principles has been brought forward to search for the source of sandstone-type uranium deposit, and then using several methods to investigate the evolution of regional geology and the dynamic process of transportation and accumulation of uranium material of the region, we determined the granite of Yanshannian cycle and Variscan cycle in the region are the main source of sandstone-type uranium deposits.(2) Based on groundwater flow system theory and mineralization theory of sandstone-type uranium, we put forward that the front of the first local groundwater flow system which was near to the source of uranium in ancient mineralization times is the site of uranium collection. According to the viewpoint, we reconstructed the regional paleo-topography of different geology period, and then further analyzed the groundwater flow system of these different geology period which was relative to evolution of metallogenic system, at last, we determined the pluvial fan facies, delta facies of Celaomiao basin and Maimawusu basin are the potential prospective area. Actually, it have been fully confirmed by past prospecting practice.(3) Analyzed the dynamic process of uranium mineralization which generally consist by resource, transport, collection and change according to the analysis of the flow system of surface water and ground water, we discovered the history of the uranium mineralization can be divided three phase : collected in early Cretaceous, protected in late Cretaceous and denuded and destroyed since the Tertiary(4) the shape and spatial distribution of uranium deposit in sedimentary Basin were discussed. Based on the groundwater flow system theory and the geochemical characteristics of uranium, the forming dynamics mechanism of roll-type ore-body of sandstone-type uranium deposit is analyzed. And the new model is put forward. The author thinks the shape of ore-body is controlled by sedimentary environment and hydrodynamic process in the mineralization period. Some characteristic of sedimentary environment, such as the permeability, thickness and shape of sediment, as well as distribution of deoxidize material, are important factors affected ore body shape. And the permeability and spatial distribution of organic matter are the most important factors. Ore body outcroped in roll, tubular or other shape, which depends on hydrodynamic process in mineralization period and later transformation process.(5) The the relation and differences between syngenetic deposit and epigenetic deposit were discussed. the author pointed out that the theory that the sandstone-type uranium deposits related to oxidation zone are all belong to epigenetic deposit need to be reflected again. Actually, the viewpoint is the result of the fixed pattern of thinking which rooted from the "law of cross-cutting relationships". Furthermore, on the ground the theory of metallogenic system, we think mineralization often is successive and the forming process of one single deposit had the characteristics of multi-stage, multi-phase and multi-causes, syngenetic deposit and epigenetic deposit have in the given region have important relation and often are belong to same metallogenic system, so the syngenetic period may be the main stage of mineralization. According to these understanding, the author put forward an idea of deep exploration for prospecting sandstone-type uranium deposit.(6) The relationship between climate and uranium mineralization were discussed. With regional environmental evolution of Chagandelesu region since the Early Cretaceous and the temporal and spatial distribution characteristics of uranium material, the relationship between climate and uranium mineralization were discussed according to the dynamics of the uranium migration with the groundwater flow. The author found that the Early Cretaceous humid climate is the most favorable period to mineralization in this region. Then the theory that the arid climate is the prerequisite for mineralization of sandstone-type uranium deposits is worthed to be reflected. Analyzed from the perspective of the metallogenic system, the occurrence of mineralization is the result of multi-factor synergies including climate. The relation of climate and uranium mineralization needs to be researched further.(7)At last, the conclusions of the study were summarized and some suggestions for further prospecting of sandstone-type uranium deposits in Chagandelesu region were put forward.