Comparison of the major- and trace-element compositions of the Kang-dian and Rössing-Husaib uraniferous leucocratic rocks

Abstract

The Rössing-Husaib area is well-known for its low-grade and large-reserve U deposits hosted by leucogranites. The leucocratic dykes in the Kang-dian area of China resemble the Rössing-Husaib uraniferous leucogranites in occurrences and mineral assemblages, however, it remains disputable if the Rössing-Husaib and Kang-dian leucocratic rocks have consistent U-mineralizing processes. A comparison of the major and trace element data of the leucocratic rocks from both areas may help to better understand the ore-forming mechanisms and provide implications for exploration in the Kang-dian area. The positive correlation between Rb and P₂O₅ contents for the Rössing leucogranite samples is typical of S-type granitoids in spite of the low A/CNK ratios (∼1.0). The Rössing-Husaib uraniferous leucogranites are generally rich in K₂O and Rb, and have high Rb/Sr and Rb/Ba ratios. In contrast, the Kang-dian leucocratic dykes have lower K₂O and Rb contents, and lower Rb/Sr and Rb/Ba ratios, suggesting a clay-poor source. The U contents of the Rössing-Husaib and Kang-dian dyke samples are positively correlated with TiO₂ contents, but show no correlation with Rb/Sr ratios, which is inconsistent with fractional crystallization. Low-degree partial melting of a U-rich protolith may account for primary U enrichment in the leucocratic rocks. The variable major- and trace-element compositions of the Kang-dian dyke samples may reflect wall-rock assimilation and/or alteration to varying degrees, which further enhanced U enrichment. Relatively low Th/U ratios (∼0.1) favored the final crystallization of uraninite. The uraniferous leucogranites in the Kang-dian and Rössing-Husaib areas are commonly undeformed and emplaced along regional large faults and/or domes at the late stage of orogeny. We propose that decompression and uplift of basement rocks facilitated the generation of uraniferous melts, and regional large faults provided a pathway for the ascent of leucocratic magma from depth.