Mineralogy and geochemistry of pyrochlore from the Shaxiongdong Nb-REE deposit, northwestern Hubei Province: Implications for the niobium enrichment mechanism in carbonatites

Abstract

Objective The Shaxiongdong carbonatite-alkaline related Nb-REE deposit is located in the Wudang area of the south Qinling Belt, but the studies on Nb mineralization in this deposit are scarce. Methods Detailed petrographic observations were made by using polarizing microscope, optical cathodoluminescence and backscattered electron testing. The structure, composition and symbiotic relationship of the minerals were identified. The composition of two types of pyrochlore was analyzed by electron probe and the mechanism of niobium enrichment was discussed. Results The Shaxiongdong complex consists of three types of rocks that are spatially closely related: Meta-pyroxenite, syenite and carbonatite, of which the carbonatites are the major hosts of Nb mineralization characterized by main pyrochlore. This implies that Nb was concentrated to economic values in the carbonatite stage after extensive fractionation of the parental carbonated silicate magma. The pyrochlore in the deposit can be classified into two types on the basis of micro-textures: ① The primary ones (Pcl1) are euhedral to subhedral and display oscillatory zoning, implying their direct crystallization from carbonatite melts; ② The altered grains (Pcl2) exhibit patchy zonation under BSE images due to hydrothermal alteration. EPMA analytical results indicate that both types are rich in Nb and F at the B and Y sites, respectively, and at the A site, they are similarly mostly rich in Ca, thus named F-Ca-pyrochlore, with minor Na, thus named F-Na-pyrochlore. Conclusion The good positive correlations between F and Nb in both types of pyrochlore indicate that F plays a key role in the enrichment of niobium during magma differentiation. On the other hand, compared to the primary pyrochlore, the altered ones have relatively low Nb2O5, CaO, Na2O and F but high UO2, Ta2O5, SrO and SiO2, indicating that the fluids responsible for pyrochlore alteration are rich in Sr, Si and U, and the alteration tends to mobilize Nb in early pyrochlore, a process that is able to lower the Nb budgets of the deposit.

Keywords

• pyrochlore
• nb-ree mineralization
• carbonatite
• shaxiongdong