Uranium-rich pyrochlore, thorite and associated minerals in the Muluozhai rare-earth deposit (Sichuan, SW China): implications for the geochemistry of high-field-strength elements in carbonatites and mineral exploration

The Muluozhai rare-earth deposit in the Mianning-Dechan metallotect (China) is hosted by carbonatites genetically linked to subduction and plate collision in the eastern Tibet. The Muluozhai carbonatites comprise variable proportions of bastnäsite-(Ce), fluorite, barite and calcite, along with abundant xenoliths of nordmarkite and metabasalt wall-rocks. Muluozhai is unusual among similar deposits in plate-collision zones owing to its locally high levels of Nb, Th and U arising from the presence of U-rich pyrochlore (+ minor betafite, 19.4–29.4 wt% UO2) and thorite (5.5–15.4 wt% UO2) associated with zircon (up to 3.0 wt% ThO2 and 2.0 wt% UO2), rutile (up to 16.3 wt% Nb2O5 and 1.7 wt% V2O3) and molybdenite. The chemical variation of pyrochlore at Muluozhai records contemporaneous crystallization of zircon, thorite, fluorite and other F-rich minerals, followed by rutile, and culminates with anion-deficient Sr-rich compositions indicative of hydrothermal alteration. Pyrochlore-group phases are rare in carbonatites from plate-collision zones, and the few known examples can be distinguished from those in anorogenic occurrences, including previously and currently active Nb mines, using a combination of discrimination criteria (e.g., their U, Ti and Ta contents, Th/U, Ta/Ti and Ti/Nb ratios). Using a global database of carbonatite compositions, we suggest that these differences reflect, to some extent, the unusual geochemistry of plate-collision carbonatites and their mantle sources with respect to high-field-strength elements. However, some geochemical characteristics of these rocks (e.g., variations in Th/U and Nb/Ta ratios) may stem from “nugget effects” caused by early pyrochlore or thorite fractionation. Implications of these data for niobium exploration are discussed.