Determination of formation age (0.14 Ma) of the Pengyang sandstone-type uranium deposit in the Ordos Basin, China: Using pitchblende in situ femtosecond LA-MC-ICPMS method

Hu-lei Zho, Jin-guo Li, Zhi-bin Xio, Pei-sen Mio, Qing-hong Si,b, Lu-lu Chen,b, Reng-n Yu,b, Yin Chen,b

a Tianjin Center, China Geological Survey, Ministry of Natural Resources, Tianjin 300170, China

b Key Laboratory of Uranium Geology, China Geological Survey, Tianjin 300170, China

c Institute of Exploration Techniques, China Academy of Geological Sciences, Langfang 065000, China

1. Objective

Because of the characteristics of multistage mineralization, long duration, relatively new age, and complicated occurrence of ore minerals, dating of sandstonetype uranium deposits has been a challenging problem. For a long time, the whole-rock U-Pb isochronal method has been used in the dating of sandstone-type uranium deposits, but its accuracy has been questioned by experts. With the development of large-scale equipment and improved analytical techniques, it is now possible to identify mineralsin situfor U-Pb isotope dating. Fayek M’s team at the University of Manitoba, Canada, has developed a method forin situUPb isotope analysis using secondary ion mass spectrometry,achieving good results in the dating of uranium minerals from unconformable and pegmatite-type uranium deposits (Fayek M et al., 2002). However, secondary ion mass spectrometry has many disadvantages, such as expensive equipment, slow analysis speed, a high matrix effect, a lack of standard materials, and demanding sample requirements. Because of this, the author’s team introduced anin situdating method by femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry (femtosecond LA-MC-ICP-MS),which has advantages such as high accuracy, reduced matrix effects, fast analysis speed, and wide adaptability. Research group used this method to test the uraninite in Chenjiazhuang granite in the Qinling area and obtained results consistent with isotope dilution-thermoionization mass spectrometry. With this method, the Pengyang uranium deposit was studied to provide a basis for the study of uranium metallogenic theory and prospecting.

2. Methods

The samples in this study were collected from four industrial uranium ore holes in the Pengyang uranium deposit.First, a scanning electron microscope and electron probe were used to identify the large particles of uranium minerals and to determine their composition and types. Second, appropriate locations were selected to calibrate the suitable minerals.Finally, the femtosecond LA-MC-ICP MS system was used to analyze the circled uranium minerals. The experiment was conducted at the Key Laboratory of Uranium Geology, China Geological Survey. A spot size of 5 μm in width and 20 μm in length was used throughout this study. At a scan speed of 1 μm/s, the laser (257 nm) was in line mode at a fluence of 2 J/cm2and a frequency of 3 Hz. The national standard material (GBW04420) for the U-Pb isotope age of pitchblende was used as the standard sample to calibrate the age.

3. Results

Thein situfemtosecond LA-MC-ICPMS dating results for pitchblende are given in the attached table. A total of 13 age data were obtained, all of which were valid dating data. The detailed analysis points are shown in Fig. 1. The results show that the Pb isotopic compositions of the analysis points are very similar to that of common lead, indicating that the content of radiogenic Pb was very low, whereas the uranium minerals have a very high U content (average = 73.96%),implying that the formation age is very young. The 13 data fall on the same concordant curve (Fig. 2), and the lower intersection point age with the concordant curve is 0.14 ± 0.20 Ma. Although the dating result is very young and with a larger error, the qualitative analysis of U-Pb system dating in this work is acceptable. It can be inferred that the Pengyang uranium deposit had a mineralization stage during the Pleistocene.

Fig. 1. In situ femtosecond LA-MC-ICP MS pitchblende dating points noted on the backscattering photograph. Pit-pitchblende;Ab-albite; Ant-anatase; Qtz-quartz; Cal-calcite.

Fig. 2. Pitchblende U-Pb isotopic concordant diagram for the Pengyang uranium deposit.

The metallogenic period peaks of sandstone-type uranium deposits are mainly Cretaceous and Cenozoic, and some deposits are multiperiod or continuous. The dating results for the Pengyang uranium deposit also exhibit this feature. Cheng YH et al. (2019) summarized the metallogenic ages of numerous sandstone-type uranium deposits around the world and concluded that the global tectonic uplift events since the Miocene formed the modern landform and controlled the formation and distribution of many sandstone-type uranium deposits. Thein situfemtosecond LA-MC-ICPMS dating results from the Pengyang uranium deposit also seem to confirm this view to a certain extent. It is inferred that after the formation of the landform in the southern Ordos Basin,under the control of landform, the Pengyang uranium deposit began to mineralize gradually, and its mineralization may be sustained.

4. Conclusion

Thein situfemtosecond LA-MC-ICPMS dating results for pitchblende indicate that the Pengyang uranium deposit has undergone a mineralization stage during the Pleistocene(0.14±0.2Ma). It is inferred that after the formation of the landform in the southern Ordos Basin, under the control of landform, the Pengyang uranium deposit began to mineralize gradually, and its mineralization may be sustained. On this basis, close attention should be paid to metallogenesis occurring in the aeolian sandstone formed under similar geological conditions since the Early Cretaceous. Thein situfemtosecond LA-MC-ICPMS dating method of uranium minerals is effective and will have a good application prospect in the dating of sandstone-type uranium deposits.

CRediT authorship contribution statement

Hua-lei Zhao conceived of the presented idea. Hua-lei Zhao and Zhi-bin Xiao wrote the manuscript in consultation.Qing-hong Si, Lu-lu Chen, Reng-an Yu and Yin Chen participated in the preliminary work of sample collection and scanning electron microscopy. Jian-guo Li supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgment

The authors are very grateful to Professor Guo-xiang Chi for his comments and suggestions that have greatly improved the scientific level of this paper. This study is supported by the National Key Research and Development Project(2018YFC0604200), the International Geoscience Programme(IGCP 675), and the project of Geological Survey China(DD20190119).