Epigenetic Alteration Features Of The Daying Uranium Deposit And Its Significance To The Follow-up Prospecting

The Daying uranium deposit is a sandstone-type uranium deposit with the largest resource reserves found in China at present.The post-genetic alteration of the lower part of the Middle Jurassic Zhiluo Formation has its unique characteristics.This paper mainly studies the characteristics of post-genetic alteration of the target layer of Daying uranium deposit and its relationship with uranium mineralization,summarizes the regularity of uranium mineralization,and puts forward the next step of prospecting directions and suggestions.The uranium mineralization of Daying uranium deposit mainly occurred in the lower and upper part of Zhiluo Formation.The lower part of Zhiluo Formation was composed by braided river and braided river delta sedimentation with direction of NE-SW-trending with average thickness of 63.46m.The upper part was mainly meandering river and floodplain environment deposit,and strip spread with direction of SW-to NE-trending.Moreover,it was the superimposing river.The average thickness of the sand body was up to 52.35m with significantly lateral migration of sandstone bodies and strong heterogeneity.The distribution of orebodies are closely related to the epigenetic alteration features of the lower and upper part of Zhiluo Formation.Ore-bearing sandstone bodies were dominated by dark gray,light gray sandstone with low cementation degree,loose structure,and development of the parallel bedding and small bedding.Sandstone was mainly composed of quartz,feldspar,and contained more debris,mica,organic debris and the heavy minerals.The metagenetic alteration of the deposit can be divided into oxidation zone,transition zone and reduction zone.The mineral assemblages are basically the same in the lower and upper part of Zhiluo Formation.Uranium mainly precipitates in the transition zone.Intersertal material content in Daying sandstone type uranium deposits was about 8%to 33%,which was mainly composed of matrix and cement.Matrix heteroaryl was mainly illite,kaolinite,water mica.And there was higher content of intersertal material up to about 18%in calcareous sandstone.Cement was mainly composed of calcite,pyrite,and a very small amount of goethite and limonite with the manner of contact,pore cementation up to 80.8%.,and showing the base cementation and higher debris content of 79%to 98%,angular-subangular,medium sorting,grain size dominated by medium-grained,followed by the fine-and coarse-grained.Geochemical type from target layer of lower part of Zhiluo Formation showed reducing features of dark gray,gray,and green sandstone;Upper part of Zhiluo Formation was dominated by green,gray sandstone and purple,brown mudstone,siltstone,which deposited in transition environment and mostly reduced.Sandstone body from lower part of Zhiluo Formation experienced several times of epigenetic alteration.Therefore,there was red and yellow oxidation.It was a massive transformation of epigenetic reduction in late stage.So that red or yellow oxidized rocks formed in the early stage of oxidation were transformed into a green or light green as discovered nowadays by later stage of reducing.There were still some red and yellow residual traces of ancient oxidation in those.Through the field research,altered rocks under mineralogy microscope observation,and with some analysis and testing tools,etc.,we think that the role of epigenetic oxidation was the important condition for uranium migration and enrichment.While reducing geochemical barrier form by epigenetic and original reducing was not only the key factor in the precipitation of uranium mineralization,but also the important condition for preservation of uranium deposits.The mineral composition,element contents and characteristics of environmental parameters from ancient layer oxidation zone in studied area showed significant zonation.The constant elements from the position between the complete oxidation zone and reduction zone were no significant changes.There was slightly lower SiO₂ content in the ore in the transition zone.And FeO content decreased gradually from greenish sandstone from the oxidation reduction zone to the transition zone.The??Fe₂O₃?/??FeO?ratio in gray transitional sandstone was higher than that in green sandstone from oxidation zone and gray sandstone from reduction zone.Low sulfur content was the highest content in the ore than in others.Organic carbon content and?Eh value gradually increased from the oxidation zone to transition zone to reduction zone.According to the study on epigenetic alteration features in Daying uranium deposit,it was indicated that this region has a good prospect of uranium mineralization.There was a feature of multilayer nature of ore-bearing position.It was still relatively low degree of research,and there was a possibility of each band of ore-body connecting as one.Ore-bodies in this uranium deposit extend well to both sides.