Basement Feature Of Junggar Basin Sandstone Type Uranium Mineralization

Junggar large superimposition basin is the second major sedimentary basin in Xinjiang Autonomous Region with the area of about13xl04km2. Its tectonic location, basin types, main sedimentary facies of covering strata and regional uranium mineralization conditions are similar to those of Yili typical uranium-producing basin. In terms of this reason, the basin generally possesses a number of basic favorite conditions for the occurrence of sandstone type uranium mineralization. However, successful breakthrough in U exploration has not been achieved up-to-date. Therefore, there are some crucial problems that the uranium exploration decision-making departments and production units are facing to and urgent to understand or answer:a) how is the sandstone type uranium mineralization prospect? b) is it potential to form large or superlarge sandstone type uranium deposit? c) if it has, where should we go to explore? and d) how to achieve a fast breakthrough?Based on the previous research work, the basement of the Junggar basin has been selected as one of the research objectives. In this paper,4aspects will be studied including the basement structure properties, tectonic properties, folded-basement uranium aboundance and its crustal maturity and tectonic metallogenic favourability degree. Under the guide of the plate tectonics and regional sandstone type uranium mineralization theory, the relationship between the basement characteristics and sandstone-type uranium mineralization in Junggar basin are to be discussed compared to those of Yili uranium-producing basin, and the sandstone type uranium metallogenic potential is to be analysed. What’s more, the work of metallogenic prediction will also be carried out.The field geological survey work was carried out combined with indoor chemical analysis and test, moblile uranium leaching experiment and zircon LA-ICP MS analysis and test. The results are shown as follows:(1) The basement of Junggar basin is of continental crust property composed of Precambrian crystalline basement and the Hercynian fold basement. The evidences are as follows:a) Bouguer gravity anomaly shows negative value (average value=65x10-5m/s2) in basin range reflecting the crustal thickness of35km at least. The average Moho depth of the basin is more than39km which is close to or more than that of the world stable platform (39.614km); b)3large anomalies were indentified during airborne magnetic survey in the basin corresponding to the Precambrian crystalline basement. Moreover, the source information indicating the presence of Precambrian crystal basement and fold basement under the covering strata was obtained by the method of magnetotelluric sounding and artificial earthquake, c) At the same time, isotopic geochronology gave the ages from Late Ordovician to Early Precambrian providing evidence for the above inference. Such "double" layer basement structure is comparable to that in Yili, Turpan-Hami and Erdos typical uranium-producing basin, and indicates that the Junggar basin posseses basic conditions for the metallogenesis of sandstone type uranium deposits to some extent.(2) Isotopic chronological data analysis show that tectonic magmatic activities around the Junggar basin margin began in Late Devonian, and peaked Late Carboniferous. Also, the activities occurred in Permian at the northern margin of Altai and Dalabute belt in western Junggar, which led to wide development of igneous rock belts around the basin margin forming the main body of the folded-basement.At periphery of the basin, the U-rich formations and/or geochemical blocks enriched in uranium are developed widely because of multistage tectonic-magmatic activities.12zones (ares) with high or moderately elevated uranium content have been identified by Airborne Radioactive Survey in the basin. Those are intermediate-acidic intrusions, volcanics and strata enriched in uranium have also been discovered by the ground gamma spectrometric survey in Kelameili area, eastern Junggar, Aletai area in the northern part of the basin and south-western Caladaban region. The further analytic results of geochemical samples and mobile uranium leaching experiments show that, the uranium abundance of those intrusive and volcanics at the periphyry of the basin margin is close to or slightly higher than the average content of granite of crust but a little bit less than those in Yili uranium-producing basin. The remove of uranium did happen in above rocks or strata, but it does not reach the level as much as in Turpan-Hami basin. In general, the folded-basement of Junggar basin could provide some components for sandstone type uranium ore-formation, but its uranium metallogenic potential might be a little worse as compared to that for the southern margin of Yili and Turpan-Hami basin.(3). Geochemistry analysis results of typical granite in Kelameili area, Eastern Junggar, Dalabute belts, Western Junggar and Aletai-Fuyun-Buergen region, Northern Junggar show that rocks studied are generally rich in silicon, alkali, large-ion lithophile elements (LIL) and high field strength elements (HFS) with high ratio value of K20/Na20(>1) and high FeO/MgO ratio (>1). They belong to high K calc alkaline series with the ancient crust thickness mostly being more than35km, when rocks are formed. That is to say, the magma evolution at the periphery of the basin reached a matured stage and the uranium is continuously accumulated in crust during the evolution process and finally U-rich granites originate from matured crust appeared thus providing material base for the sandstone type uranium mineralization.The characteristics of major and trace elements in most granitic rocks are in accordance with those in A2type alkaline granite, and they belong to the post collisional granitoids or with in plate ones which indicate that the basin at that time entered into the post collision tectonic environment at the end of magmatic activity. In other words, a stable weakly extensional tectonic setting being favorable for the formation of sandstone type uranium deposits.Sr, Nd, Pb and O isotopic studies indicate that the material source of studied granite rocks are originated from varies including mantle materials, young crust partial melting products as well as crustal materials. Then, the materials from mantle are accounted for more than65%. That is to say, in a certain extent, as compared with the continental crust-originated (especially matured ancient continental crust) transformed-type granite, the matured crust stated above will not enrich much abundant uranium for "rock material being congenitally deficient". Zircon from studied rocks contains mostly uranium less than1000x10"6which indicates the paleo uranium background value is low. This fact is in accordance with low U-content in granites. This might be the reason why the exploration for superlarge sandstone type uranium deposit in Junggar basin still is quite difficult.(4). During the basin evolution process, Junggar basin generally experienced5stages, it was uplifted and eroded from Permian to Middle Triassic, subsides and received deposition during the period from Late Triassic to early stage of Middle Jurassic, then again uplifted and eroded from late stage of Middle Jurassic to Late Jurassic and subsided and received deposition again in Cretaceous and finally uplifted and eroded in Cenozoic era. During the geological period from Late Triassic to the end of Xishanyao period, the primary reduction sedimentary formations, ore-hosting formations and mult-types of uranium mineralization were developed; The tectonic environment was changed from compressional to extensional during the period from Middle Jurassic to Cretaceous which led to the uranium ore formation again. The strong uplifting of Northern Tianshan from Neogene period destructed basin preexisting metallogenic conditions (especially the southern margin of the basin) even "shoveled" inherent uranium mineralization, which might be one of the important reasons for unsuccessful prospectings for large-sized sandstone type uranium deposit in Junggar basin.(5). Based on the above research results, the suggestions can be obtained as the following. Junggar basin has possessed certain metallogenic conditions for sandstone type uranium mineralization, such as:"double-layerd" structure of the basement, relatively stable tectonic environment from Late Devonian to Eogene, and matured Hercynian igneous rocks which can provide certain source material for uranium mineralization. In different direction slopes of Beisantai covex and Luliang convex, there are four slope zones where the formation of interlayer oxidation sandstone type uranium deposit could be developed. These are perspective areas for uranium exploration. However, the uranium abundance in folded basement is limited, the uranium migration ability is weak, in addition the deficiency of ore mineral sources and negative effect of Cenozoic tectonic movement, the metallogenic scale of sandstone uranium deposit is limited in Juggar basin as compared with U-highbearing Yili and Tuke basins. Nevertheless, on the basis of systematic detailed research and exploration work, it mighted be possible to find median-large scaled sandstone-hosted U-deposits.