U-Pb Geochronology of Fersmite: Potential Time Constraints on Magnesite Formation, Sparry Dolomitisation, and MVT Pb-Zn Mineralisation in SE British Columbia, Canada

Fersmite ([Ca,Ce,Na][Nb,Ta,Ti]2[O,OH,F]6) from the Mount Brussilof magnesite deposit, British Columbia, Canada occurs as accessory brittle, black, submetallic to vitreous lustre, acicular to platy crystals up to 2 cm long, developed in sparry dolomite, which lines cavities in sparry magnesite. Fersmite also occurs as smaller crystals (<3 mm) enclosed by dolomite, where it is commonly fractured or broken, formed during the final stage of dolomite crystallisation. Electron microprobe (WDS) major element data indicate that the grains confirmed to be fersmite by X-ray diffraction contain >50% Nb and are atypically Ta-poor. Fersmite contains significant U and Th (up to 4700 ppm and 6 wt.%, respectively) and therefore is a viable mineral for U-Pb geochronology. A series of laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) spot analyses and maps were collected on fersmite grains. Although the fersmite grains have considerable common Pb and have experienced Pb loss, the U-Pb spot data suggest growth or pervasive resetting at ca. 190 Ma. Some 40Ar/39Ar ages (two of four samples) are consistent with the ~190 Ma U-Pb date. Electron microprobe and LA-ICP-MS mapping indicate that the fersmite is middle to heavy rare earth element-rich. The ~190 Ma fersmite age estimate provides an approximate upper time constraint on the age of sparry magnesite mineralisation, sparry dolomitisation, and, indirectly, on the formation of MVT deposits in the Kicking Horse Rim area and possibly elsewhere in southeastern British Columbia.