Study On Characteristics Of Lower Cambrian Strata And Genesis Of Uranium Enrichment/Mineralization In The Strata In Xiushui Area

The carbonaceous-siliceous-pelitic rock type uranium deposits in the Xiushui area are a significant proportion of this type of uranium deposits in south China.Previous studies considered that besides uranium enrichment during formation of the sedimentary rock,further enrichment of uranium by hydrothermal fluids or illuviation through meteoric waters play an important role in the formation of this type of uranium deposits.In this area,this type of uranium deposits/mineralization are hosted within the uranium-rich strata without displacement,indicating that depositional settings constrains the location of the deposits.However,previous studies only focused on studying the large-scale sedimentary facies of the strata;but ignored the specific sedimentary environment that might exert mn influence on the uranium content in the sediments to some degree.Based on microscopic investigation on drilling bore hole samples of the Lower Cambrian Series which consists of the Wangyinpu Formation and lower part of the Guanyintang Formation(uranium content ranging from 9.37?202ppm)in Xiushui area,we found that the borehole samples have features that reflect sedimentation and diagenesis rather than hydrothermal process or supergene leaching/illuviation process.We further analyzed the relationship between uranium content of the stratum and various aspects of environment factors such as redox condition of bottom water,primary productivity and degree of water communication between the sedimentary basin and the open ocean.In most of the samples,uranium content is positively correlated with total organic carbon content(TOC).The uranium content(202ppm)in one sample that is located in the upper part of the Wangyinpu Formation is the highest,as well is the productivity reflected by organic matter.In the sample,pitchblende has been found within the carbonized organic matter by using SEM and EPMA in this study.The uranium may have deposited with the organic matter into the sediments and formed the pitchblende and carbonized organic matter during diagenesis of the sediments.The composition feature of diagenetic chlorites in the sample shows that:the chlorites have a few vacant octahedral sites;the tetrahedral A1 amount is significantly smaller than the octahedral A1 amount.Also,there is no sign of typical minerals forming in the process of hydrothermal superimposition or supergene leaching in the sample.The strong rock weathering indicates that current climate is mild and humid in source region.The Wangyinpu Formation has higher uranium than the lower part of the Guanyintang Formation.It is also revealed that the Wangyinpu Formation was deposited mainly under sulfidic bottom waters and the lower part of the Guanyintang Formation was deposited mainly under anoxic bottom waters without sulfide.Under sulfidic condition,sediments deposited in an environment with less restriction would have higher uranium content than sediments deposited in an environment with more restriction.In summary,during the period of deposition of the upper part of the Wangyinpu Formation,sedimentation and degradation of organic matter produced by high bioproductivity in the photic waters of the basin consumes much of the oxygen in the bottom seawaters of the basin,strongly reducing sulfidic environment would prevail.At the same time,appropriately well connection of the sedimentary basin with the open ocean would provide enough uranium into the sulfidic environment to deposit,resulting in highly enrichment or even mineralization of uranium in the sediments.