Research On Metallogenic Conditions Of Sandstone Type Uranium Deposit Of Cretaceous In Southern Songliao Basin

The south of Songliao basin is not only a very important oil and gas accumulating area, but also a metallogenic area of sandstone type uranium.Based on comprehensive research on basic geology,well drilling,logging,seismic and aeromagnetic materials ,takeing the metallogenic theory of modern sandstone type uranium as the guide, using the methods of sequence stratigraphy and sedimentary system analysis with a combination of samples analyzing and testing in the laboratory, a systematic studies on Mesozoic tectonic activities, sequence stratigraphic frame work, the distribution of depositional system and facies, environment of sandstone type uranium deposit,sandbody conditions,features of epigenic ransformation,metallogenic conditions of uranium deposit,mineralization features and mineralization controlling factors have been done in this paper.According to the three sequence interfaces ( T3 ,T11 and T03) which were formed during the three episodes(episodeⅠ,Ⅱ,Ⅲ)of late Yanshan movement,combined with the identification of sequence boundary,the sequence stratigraphic framework of southern Songliao Basin has been established. Upper Cretaceous can be divided into three super sequences(namely, thermal subsidence depression ,depression expansion ,and inverse atrophy) , ten third-order sequences and a series of system tracts . According to the integrity,layering ,connectivity and permeability of preserved sandbodies ,it is suggested that LST in Yaojia Formation and the 1st member of Quantou Formation are favourable metallogenic strata in the south of the basin . The braided river sand bodies are favourable for uranium metallization.According to the analysis of sedimentary system ,sedimentary facies and favourable facies for uranium mineralization, it is demonstrated that the provenance of late Cretaceous is mainly from the long axis of basin , minorly from short axis of basin, and with the features of intermittence.It is mentioned that Baokang water system developed along the long axis provenance of basin consists of Tongliao and Shuangliao water system,the two water systems merged into lake and formed large-scale delta system at Baokang—Changling area. Meanwhile, during the run off process , braided river channels deposit developed along Yuliangbao—Kulun—Tongliao—Baxing area,and deposited a large sacle of braided river sand bodies in which uranium mineralization has been found, which indicates that braided river facies is the major sedimentary facies for sandstone type uranium exploration in southern Songliao basin.The research on uranium mineralization features of BX deposit suggest that: 1)uranium pre-enriching of primary gray layer is the basis of sandstone type uranium mineralization .2)gray sandstone layers which were formed by epigenetic reducing alteration of red sandbodies whose reducing fluid came from deep fluid intrusing upward and diffusing along through-faults are the precondition of sandstone type uranium mineralization. 3) the formation of local oxidation zone is the key to sandstone type uranium mineralization .According to the analysis of tectonic evolution in Mesozoic, it is demonstrated that uranium mineralization in southern Songliao basin is connected with the tectonic inversion of the end stage of Formation. A new understanding that two tectonic positions control two kinds of oxidation zones, and form two kinds of uranium mineralization has been put forward . The slope zone formed by tilting at southeastern margin of Songliao basin controlled the regonial interlayered oxidation zone and formed roll or roll-like uranium ore bodies. But the uranium mineralization scales are small because of the fierce uplifting in southeastern margin of the basin and incompletely preserved oxidation zones. Structure fenster formed by inverse uplifting and denudation controlled the local oxidation zones and formed tabular uranium ore bodies.Based on comprehensive research, a new understanding that"four-layer structures"prospecting marks of Yaojia Formation in southern Songliao basin is established, i.e., from the bottom to the up is the top of lower member of Yaojia Formation is red mudstone,upper oxidized sand bodies ,gray uranium–bearing layers ,lower oxidized sand bodies from top to bottom vertically.Research on metallogenic regularity of sandbodies uranium deposit in southern Songliao basin indicate that: 1)the shapes of uranium body is different from that of traditional typical interlayered oxidation sandstone type uranium deposit. The shapes of ore body are tabular, but not roll. 2)uranium metallogenesis is controlled by the scale and degree of local interlayered oxidation zones.Based above ,a renovating idea that"local oxidation zone controls tabular uranium body""local interlayered oxidation type uranium deposit"has been put forward, which enriched the theory of interlayered oxidation type uranium metallogenic. A new understanding that"structure denudation fenster controls local oxidation zone, fault restricts alteration zone, and uranium mineralization enriches in braid river sand-bodies in two wings of tectonic fenster"has been first put forward .The quaternity metallogenic model of"oil-gas bearing fine clastic rock depositional construction in deep, depositional construction of braid river coarse clastic rock in low,fault, and structure denudation fenster"has been established in this paper by comprehensive studying on metallogenic environment in southern Songliao basin, and characteristics of uranium mineralization ,mineralization–controlling factors and metallogenic rules of important areas.Three uranium prospecting target areas ,Qian Jiadian-baokang area,Shuangliao-Xie Ersu area ,Yang Dachengzi Area are forecasted according to the selection of the target layers , forecasting criteria of target area and metallogenic model of uranium deposite. thirdmore, key breakthrough has been made in uranium deposit exploitation.