Ore-controlling structures of sandstone-hosted uranium deposit in the southwestern Ordos Basin: Revealed from seismic and gravity data

Abstract

We present the seismic and 3-D gravity study to define the ore-controlling structures of sandstone-hosted uranium deposits in the southwestern Ordos Basin, the second largest sedimentary basin in China. The instantaneous amplitude and frequency are extracted from seismic reflection data, and the seismic impedance is obtained by joint inversion of well-log and post-stack seismic data. Results of the seismic attributes and inversion show that the uranium-bearing deposits exhibit low instantaneous frequencies, low instantaneous amplitudes and medium–low seismic impedances. The scratch analysis method is employed on the residual gravity anomaly generated from the gravity bouguer anomaly by using the potential field separation. The edge coefficient image of the residual gravity anomaly shows that the fault structures are composed of NE-trending faults in the west and EW-trending faults in the east of the study area. The 3-D gravity inversion constrained by the seismic data was performed to reveal that uranium-bearing deposits are characterized by low gravity anomaly, which is related to the larger porosity of the uranium-bearing sandstones. Geophysical results show that the uranium-bearing deposits are controlled by flower structures, EW-trending normal faults and NW-trending thrust faults. Flower structures are produced by the tectonic inversion as a result of the uplift and NE-directed growth of the Tibetan Plateau. Ore-controlling faults cut crossing the floor of the ore-bearing strata as channels, along which the ore-forming fluids move upward and are enriched in the sandstone strata with high permeability and porosity in Luohe Formation. In addition, the roof and floor of the ore-bearing strata in Luohe Formation are characterized by high impedance and gravity anomaly, which has a sealing effect on uranium deposits. Then we predicted the thickness of the sandstones in Luohe Formation, as the sandstone is a prerequisite for mineralization.