| Sandstone-type uranium deposits are products of water-rock interaction in sedimentary basins.The geological process from sedimentation,diagenesis to uranium mineralization is genetically related.Whether during the sedimentation or mineralization period,the uranium-rich provenance from the source area is transported by physical and chemical processes to the basin where it is deposited,diagenesized and mineralized,which is a typical process from "source" to "sink".The "source-sink" system research has become a hot topic and frontier in the field of sedimentology.However,its concept and idea on how to apply it to the study of sandstone-type uranium deposits to solve the coupling ore-control mechanism between key elements of uranium mineralization,such as the evolution of the provenance and uranium source of the uranium-containing rock system,the sand body of the uranium reservoir and the spatial positioning of the oregenesis flow field.This system research is still in the exploration.This thesis selects the Cretaceous uranium-bearing rock series in the Tiefa area on the southeast margin of the Songliao Basin as the research object,draws on the concept of the "source-sink" system analysis in sedimentary basins,and focuses on the three key scientific questions of "source(provenance and uranium source)","path(physical channel and mineralization flow field)",and "sink(spatial configuration of controlling mineralization factors and enrichment mechanisms)" for uranium mineralization.Systematic studies of sedimentology,petrology,mineralogy,and geochemistry of elements and isotopes were conducted,and the spatiotemporal coupling mineralization relationships of key mineralization factors were summarized.The "source-sink" system of sandstone-type uranium deposits in the study area was elucidated,providing a systematic sedimentary geological basis for regional uranium exploration and prediction.The main findings of this thesis are as follows:The construction of the isochronous stratigraphic framework of the uranium-bearing rock series shows that the third segment of the Quantou Formation of the Upper Cretaceous in the study area is the main uranium-producing layer,followed by the gravelly rock segment at the bottom of the Fuxin Formation of the Lower Cretaceous.The lithology of the ore-bearing section is mainly gray conglomerate,conglomeratic sandstone and sandstone,and the environment-sensitive parameters indicate that the orebearing sand bodies have reducing properties.The study found that the uranium reservoir sand bodies near the basin margin underwent post-oxidative alteration marked by hematization and goethitization,and the uranium minerals are strictly controlled by the redox environment,occurring as colloidal,granular or adsorbed states in the pores of gray sandstone gravel around the periphery of the oxidation zone.These phenomena have the characteristic of "hydrothermal uranium deposits" and "uranium cementation" diagenesis.Based on this,the type of uranium mineralization in the study area can be determined as interlayer oxidation zone type.The geochemical analysis of the uranium-bearing sandstone reservoirs and zircon U-Pb geochronology shows that the sedimentary parent rock of the uranium-bearing rock series is mainly medium-acidic igneous rock,and the tectonic background of the source rock area belongs to an active continental margin and island arc.Further petrological and zircon geochemical analysis,combined with regional geological surveys,revealed that the early Triassic felsic rock(Baijiagou)in the western part of the basin has the highest zircon U content,with an average value of 1074ppm(maximum value of 2617ppm).The rock Th/U ratio is as high as 17.69(maximum value of 41.63),indicating that the rock has a higher U leaching rate and is the best uranium source rock.Comparison shows that there is a good correlation between the clastic zircon U content of uranium reservoirs and the zircon U content of rock masses in the source rock,which can effectively record the original uranium enrichment degree of the source rock.It also shows that the high-quality uranium source in the source rock can well match the original accumulation(pre enrichment)of trace uranium in the uranium bearing rock series,indicating that the uranium in the source rock during the sedimentary period can effectively accumulate in the potential uranium reservoir sand body.The transportation of uranium and its deposition in sandstone-type uranium deposits also relies on the porous media of the uranium reservoir sand bodies.The study of sand dispersal systems can not only demonstrate the spatial distribution of uranium mineralization in terms of "radius",but also indicate the direction of sediment transport.The study found that the ore-bearing layer in the gravelly rock section of the Fuxin Formation is a fluvial fan sedimentary system,which is locally distributed on the western edge of the basin,and its source is from the Diaobingshan uplift in the western periphery.The uranium reservoir sand bodies in the third member of the Quantou Formation are a river sedimentary system,with sources from both the Diaobingshan uplift in the southwestern periphery of the basin and the Kaiyuan uplift in the eastern part,but mainly from the former.The source of the Diaobingshan uplift mainly spreads towards the center of the basin along the sand body high-value area of the Jintaitun-Dasisijia-Diaobingshan line,thus forming a high-quality and potential uranium reservoir porous media,which lays a good physical space for the development of the ore-bearing flow field and the formation of uranium deposits.The spatial location of the epigenetic interlayer oxidation zone in the uraniumbearing sandstone reservoir is an important indicator for recovery and reconstruction,as well as tracing the ore-controlling fluid flow field.Mineralogical and petrogeochemistry studies of the uranium-bearing sandstone reservoirs indicate that the post-genetic oxidation alteration in the study area is characterized by typical indicator minerals such as hematite and limonite.It has lower TOC,Fe O,△Eh,and higher Fe2O3,Fe2O3/Fe O environmental index parameters compared to the primary reduced sandstone.Based on these criteria,the interlayer oxidation zones of the third member of the Quantou Formation were identified,which are mainly distributed along the Jintiantun-DasijiaziDiaobingshan line in the west and the Xiaertai-Quantou area in the east.Ore-forming fluid tracing shows that the high-value areas of the oxidized sandstone in the western of the study area are consistent with the high-value areas of the uranium-bearing sandstone,revealing that the Ore-bearing fluid during the mineralization period was consistent with the ancient water flow system during the sedimentation period.Moreover,the Oreforming fluid selectively diffused to the periphery of the basin in areas with larger physical space of the uranium reservoir.From the perspective of spatial allocation of ore-controlling factors and uranium enrichment mechanism-"sink",the spatial allocation relationships of uranium reservoirs,oxidized sand bodies,and reduced geological bodies have been quantitatively studied.It is found that the transition zone where the uranium reservoir sand body in the study area changes from thick to thin,the sand content ratio changes from high to low,the ratio of oxidized sand body thickness to oxidized sand body changes from large to small,and the external reducing geological body(gray mudstone and coal seam)changes from thin to thick is the most favorable area for uranium mineralization.Specifically,the physical space "radius" of uranium mineralization constrains the trajectory of interlayer oxidation zone development.The internal reducing media such as carbonaceous debris and pyrite,combined with the external reducing geological bodies such as thin coal seams and gray mudstones,enhance the reducing capacity of the entire uranium reservoirs,thereby restricting the range and front line position of the interlayer oxidation zone advancement.The organic coupling of these three factors promotes uranium mineral enrichment and mineralization.By systematically analyzing the elements of "source","path",and "sink",the“source-sink" system for uranium mineralization in the southeast margin of the Songliao Basin has been constructed.During the Late Early Cretaceous to Early Late Cretaceous,sediment from the source areas on the east and west sides of the basin were transported by surface runoff into the basin for deposition and diagenesis.With favorable uranium source conditions,uranium was pre-enriched in the target layers through physical(clastic uranium)and chemical(dissolved uranium)processes.In the Late Late Cretaceous,the reverse tectonic event of the Songliao Basin triggered uranium mineralization in the area.The exposure of the uranium reservoir in the Fuxin Formation and Quantou Formation promoted the development of the regional mineralized flow system of "replenishmentpathway-discharge".The oxidation and release of dissolved uranium in the source areas and the trace uranium in the uranium reservoirs provided sufficient material guarantee for the enrichment and mineralization of uranium under the oxidation-reduction geochemical barrier.It can be seen that the formation and development of sandstone-type uranium deposits has a "source-trap" process consisting of two stages.Although the migration and precipitation of uranium during the sedimentary and mineralization periods have different characteristics,they together form an indispensable part of the uranium mineralization"source-trap" system.The characteristics and innovation of this article are as follows: Drawing on the concept of "source-sink" system analysis in sedimentary basins,a "source-sink" system for uranium mineralization has been constructed,providing a keyto reveal the interaction between the evolution and migration of the uranium reservoir provenance and uranium sources,the tracing of the physical space of mineralization and the mineralizing flow field,the spatial coupling of key controlling factors,and the uranium enrichment mechanism.This also offers new ideas for exploration and prediction of sandstone-type uranium deposits. |
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