ADVANCES IN GEOPHYSICAL METHODS USED FOR URANIUM EXPLORATION AND THEIR APPLICATIONS IN CHINA

J. DENG, H. CHEN, Y. WANG, H. LI, H. YANG, Z. ZHANG East China University of Technology, Nanchang, China

1. INTRODUCTION

Geophysics is one of the most useful techniques for uranium exploration. It supports the development of geological models through the definition of lithological, structural and alteration characteristics of metallogenic environments under evaluation. With exploration at increasing depths, the traditional radiometric method is no longer effective for uranium exploration. Uranium mineralization is not closely related with observable gravity, magnetic and impedance anomalies. Gravity, magnetic and electromagnetic techniques can be used to survey the subsurface geological background of an area and can be efficient in detecting deeper uranium deposits [1]. The progression and development of geophysical methods, measurement techniques, data processing, computer modelling and inversion have permitted improvements in the field of uranium exploration [1, 2].

It is well known that progress in geophysical methods has contributed to successful field investigations in the exploration for deeper deposits (including uranium). Xu et al. [2] have reviewed the latest advances and developing trends in geophysical and geochemical methods and techniques applied to uranium resources exploration in China [2]. Following this work, this paper summarizes the research carried out by the East China University of Technology during the past decade. This includes 3-D inversion of magnetic data and 3-D electromagnetic methods tested in the Xiazhuang area over granite type uranium deposits [3, 4] and in the Xiangshan area over volcanic type uranium deposits [5]. Results indicate that some of the objectives, including mapping of basement structures, rock interface and lithology recognition, can be achieved.

2. APPLICATION TO DEEP URANIUM DEPOSITS IN THE XIAZHUANG AREA

The Xiazhuang uranium orefield where granite-related uranium deposits are present was selected as a case study for the use of magnetic 3-D inversion and electromagnetic imaging. In this area, uranium deposits related to diabase are mainly located in the eastern part of the Guidong granite body [6–8]. This type of uranium deposit was named ‘intersection type’ and is located at the intersection of diabase dyke swarms with a near E–W strike and a silicified fault system with a NNE extension. After the discovery of new mineralization at depth, attention focused on the Xiazhuang uranium orefield.

Integrated geophysical methods, including gravity, magnetic and electromagnetic, were used to delineate granitic rocks and faults at depth. The goal of this study was to test the applicability of the abovementioned methods to exploration for uranium deposits and to predict the presence of mineralization during prospection. For this purpose, eight profiles were assigned for an AMT and gravity survey and a 20 km2 magnetic survey was performed along an area on which several uranium orebodies were known from deep drill hole data.

After surveying the profiles, the data were processed using 2-D or 3-D inversed and interpreted. The results effectively identified the granitic rocks at significant depths and the imaged distribution of these units was consistent with information obtained from the local geology. Finally, by integrating the results from the gravity, magnetic and electromagnetic data, three locations were suggested for drilling. Two of these intersected uranium mineralization at a depth of around 1000 m while one was unmineralized. After drilling of the three locations, cores were studied and compared with the results obtained from the geophysical

124 methods which resulted in confirmation of the geophysical findings. Furthermore, this work showed the efficiency of the geophysical methods in exploring for deeper granite hosted uranium deposits.

3. APPLICATION TO THE GEOLOGICAL STRUCTURE OF THE XIANSHAN VOLCANIC BASIN

As the third largest volcanic type uranium orefield in the world, the Xiangshan volcanic basin is attracting great research interest and important investment from the mining industry. A 3-D geological structure survey and modelling project was undertaken in 2011. The purpose of the project was to delineate the volcanic calderas (limits still not confirmed over the past 60 years) and to investigate the deeper geological structure of the basin. It was based on the physical property measurements of around 1400 samples from drill cores and of rock samples taken along the geological profiles. Also, 3-D inversion of regional gravity and magnetic data were conducted and 19 magnetotelluric profiles covered the Xiangshan volcanic basin. These magnetotelluri data were inverted using 2-D and 3-D inversion algorithms developed in-house [9, 10]. With the integration of the geophysical survey results and with information from drill holes and local geology, a 3-D geological and geophysical model was developed. It was then concluded that the Xiangshan volcanic basin comprises two basement units, a metamorphic one and a granitic one of Caledonian age. A low resistivity layer exists between the metamorphic basement and the overlying volcanic–sedimentary formations, corresponding to an unconformity interface. A mushroom shaped low resistivity geological body with a radius of around 2 km located in the Xiangshan Mountains is inferred as corresponding to the Ehuling Formation. Seven NE striking, four NW striking and one N–S striking faults were delineated on the basis of the geophysical results.

4. CONCLUSIONS

Integrated geophysical surveys for deeper uranium deposit exploration and 3-D geological structure surveys were initiated in south China. Combined with geological and borehole data, these results support the need for deeper uranium exploration in these areas. It is concluded that geophysical methods can give results useful for uranium exploration. The selection of the geophysical methods to be used depends on the physical properties of the target and on the nature of the rock formations, the geological setting and the environment. Integration of several geophysical methods and other disciplines is necessary in most cases to achieve better results.

AKNOWLEDGEMENTS

These research works are financially supported by the National Natural Science Foundation of China (Nos 41404057, 41674077 and 411640034), and the Nuclear Energy Development Project of China.

REFERENCES

[1] INTERNATIONAL ATOMIC ENERGY AGENCY, Advances in Airborne and Ground Geophysical Methods for Uranium Exploration, IAEA Nuclear Energy Series No. NF-T-1.5, IAEA, Vienna (2013). [2] XU, G.L, et al., Latest advance and developing trend of geophysical and geochemical methods and techniques applied in uranium resources sxploration, Uran. Geol. 29 6 (2013) 344–351(in Chinese). [3] LI, M., et al., The application of ground high-precision magnetic survey to the exploration of intersection-type uranium deposits, Geophys. Geochem. Expl. 36 2 (2012) 355–359 (in Chinese). [4] GE, K.P., et al., Rock magnetic investigation and its geological significance for vein-type uranium deposits in southern China, Geochem. Geophys. Geosys. 18 (2017) 1333–1349. [5] GUO, F.S., et al., Study on the geological structure of Xiangshan uranium-bearing volcanic basin: Evidence from magnetotelluric sounding and GOCAD modeling, Chin. J. Geophys. 60 4 (2017) 1491–1510.

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