Inner Mongolia, China

Main commodities: U
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The Dongsheng group of uranium deposits are located in south-central Inner Mongolia (Nei Mongol) in mid northern China, approximately 100 km south of Baotou and 150 km south-west of Hohhot.

Mineralisation is hosted within fluvial sandstones on the northern margin of the Ordos Basin which covers over 250 000 sq. km of Shaanxui, Shanxi, Gansu and Inner Mongolia. This Basin, which is located at the transition between the Tethys Tectonic System to the west and the Circum-Pacific Tectonic System to the east, contains major coal units as well as commercial gas reservoirs.

Basement to Ordos Basin includes Archaean to Paleoproterozoic crystalline rocks of the North China Craton and Mesoproterozoic to Triassic sediments of the North China Platform which laps onto the northern margin of that craton. Throughout much of this time the platform was stable and accreted a thick section of marine dominated sediments. Since the late Triassic the basin has progressed from a platformal regime to an intra-cratonic basin, resulting in a transition from the marine sediments of the Palaeozoic to continental sequences in the Mesozoic. The northern uplift margin of the Ordos Basin is occupied by widespread metamorphic rocks and Jurassic to Cretaceous U-rich granites which may have been the provenance of the host sediments.

The fluvio-lacustrine Mesozoic sedimentary rocks in the Ordos Basin comprise the Triassic Yanchang Formation; the Lower Jurassic Fuxian Formation; the Mid-Jurassic Yan'an, Zhiluo and Anding Formations, and the Lower Cretaceous Zhidan or Liupanshan Group. The sequence is predominantly composed of sandstones, with occasional intercalated mudstone or siltstone, coal seams, and intervals of poorly cemented sandstones with abundant carbonaceous plant remains, coal debris and sulphides. Interlayered greenish and bluish oxidation zones are well developed in Yan'an and Zhiluo Formation.

Uranium ore bodies are mostly located over an area of 200 sq. km hosted by fluvial sandstones at or near the contact between grey sandstones and grey-greenish sandstones in the lower member of the Middle Jurassic Zhiluo Formation, and to a lesser degree within the Yan'an Formation.

The Yan'an Formation is composed of 40 to 76 m of fluvial-lacustrine sediments deposited in a humid climate, comprising grey to dark grey coarse to medium-grained sandstone or gravelly coarse-grained sandstone intercalated with siltstone and mudstone, with an upper section of mainly greyish-white medium to fine-grained sandstone with kaolin indicating a weathering crust.

The overlying Zhiluo Formation has been subdivided into 3 members, namely:
i). A lower unit of braided fluvial sediments, which is the main host to uranium mineralisation, interpreted to have formed as NNW-striking, proximal, braided fluvial zones, possibly within a large-scale humid alluvial fan developed when the northern part of the basin underwent regional uplift. The braided channels eroded into underlying coal-bearing rocks that contributed to the sediments deposited in the channels. The fluvial sediments are mainly coarse to medium-grained feldspar-quartz sandstone, with minor fine-grained sandstone;
ii). The intermediate suite of a low sinuosity meandering fluvial system and,
iii). The upper high sinuosity meandering fluvial system as the uplift to the north weakened and a regime of subsidence and sedimentation predominated.

During the deposition of the intermediate and upper units, and uplift to the north, the NNW trending valleys became north-east directed sags as a result of south-east compression. During this time, the lower braided stream sandstones remained in hydrological connection with surface waters due to the discontinuous nature of the overlying impermeable lenses. As a result, both oxidised grey-green and reduced grey zones are evident in the upper parts of the lower braided stream sandstones. The grey zones of the lower braided stream unit are characterised by abundant carbonaceous plant remains, coal debris and sulphides, while the overprinted oxidised grey-green zones and upper units only have minor plant remains and pyrite, although clay alteration, especially chlorite, was intensely developed, with a high contents of Fe2+.

The initial tabular orebodies formed at this stage at redox fronts.  At the end of the Early Cretaceous, the area was even more intensely uplifted, the overlying sediments were denudated and the lower braided steam unit was partially exposed on the surface. In this time, a 40 km long east-west striking redox front was formed and pre-existing tabular ore bodies were locally reworked and turned into roll front deposits of the main mineralisation. The ore forming process ceased in the late Early Cretaceous when further capping sediments were deposited.

Petrological, mineralogical and geochemical studies are said to have indicated that the grey-green sandstones are the product of epigenetic reducing alteration by introduced fluids of previously oxidised sandstones. The reduction process protected uranium accumulations from the destruction by younger infiltrating oxidising water. Red and/or yellow coloured sediments are not evident in the sub-surface.

Commercial uranium mineralisation is principally hosted by greenish and bluish green sand bodies of the Zhiluo Formation, stretching more than 20 km long and hundreds of metres in width as a regional redox front. The uranium in the mineralised sandstones is mostly in form of adsorption coffinite, minor brannerite and pitchblende while secondary uranium minerals such as autunite and torbernite are visible on surface. The U-Pb isotopic dating gave a primary mineralisation age of 107±14 Ma corresponding to a known period of Mesozoic tectonic activity, although U-Pb isotopic ages of coffinite in high grade ores of the uranium deposit are centred at Miocene time between 20 and10 Ma.

A multistage mineralisation model has been proposed divided into four main stages, namely: i). pre-concentraion during diagenesis, ii). interlayer infiltration mineralisation during the Late Jurassic to Paleocene; iii). reconcentration in the Miocene, and iv). post Pliocene late stage of ore preservation by reducing fluids.

At least three U-mineralised sections have been discovered discontinuously along the 20 km long EW-striking redox front. Individual ore bodies are several tens to one hundred metres long, 1 to 20 m thick and have average ore grades of 0.033 %U.

The 20 km ore zone, which contains the individual deposits over a cumulative strike length of 15 km, increases in depth from east to west. The shallowest section is at 77 m below the surface in the east, plunging west to where it is at a depth of 185m, closely following the dip of the Zhiluo Formation. The tabular ore lies towards the centre of the ore zone, while the roll front mineralisation is developed to the east and west. The ore zone is around 11.5 m thick in the east, 4.75 m in the centre and 3 to 7.5 m in the west. Grades average 0.02% to 0.05%U, with a maximum value of 0.235%U.

The most recent source geological information used to prepare this summary was dated: 2005.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Chunfang Cai, Hongtao Li, Mingkuan Qin, Xiaorong Luo, Feiyu Wang and Guangxi Ou,  2007 - Biogenic and petroleum-related ore-forming processes in Dongsheng uranium deposit, NW China: in    Ore Geology Reviews   v32 pp 262-274

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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