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A number of sandstone-type uranium deposits, hosted in interlayer oxidation zones, have been discovered in the southern margin of the Yili Basin, which makes the region one of the largest uranium mineral provinces in China. The recently-discovered large Mengqiguer uranium deposit contains many high-grade roll-front orebodies in the Jurassic sandstone bodies, with some special features distinct from typical interlayer oxidation zone sandstone-type uranium deposits. According to the lithological characteristics, the Jurassic Sangonghe Formation can be divided into two sedimentary successions, separated by relatively impermeable layer. However, previous exploration revealed that the impermeable layer is locally missing, coinciding with the depths of high-grade uranium orebodies. In these places, the interlayer oxidation zone pinchout lines in the upper succession sandstone bodies of the Sangonghe Formation showed a mushroom-shaped pattern in profile and an ellipsoidal distribution in plane, with the orebodies behaving as an enclosed to semi-closed elliptical shape in plane and some of roll-front head orebodies overturned in profile. Field hydrogeological measurements showed that the groundwater pressure level of the lower succession of the Sangonghe Formation (at drill hole No. ZK416) is 1349.61 m in elevation, and that in the upper succession is 1339.53 m. As a result, there is over 10-m difference in the groundwater pressure level between the upper and lower succession, suggesting that the locally missing of impermeable layer may have led to the formation of a hydrodynamic window, which allowed the groundwater in the lower succession layer to rise up to the upper succession. Redox reactions may have occurred between the up-welling, oxygen- and uranium-bearing groundwater with the reducing and permeable sandstone bodies, leading to the formation of roll-front together with overturned uranium orebodies in the upper succession sandstone bodies. Therefore, a new sandstone-type uranium genetic model, named as up-welling leakage-recharge model was proposed to explain the formation of the sandstone-type uranium deposits in the southern margin of the Yili Basin, which can also provide guidance to future uranium exploration in the region and similar sedimentary basins around the world. |
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