Rare Earth Minerals in the Rare-Metal Greisens of the Verkhnee Deposit in the Khingan–Olonoisky District of the Amur Region, Russia

Rare minerals represented by oxides, fluorides, F-carbonates and REE arsenates in association with monazite-Ce, monazite-Th, xenotime-Y and zonal REE fluorite were identified in the rare-metal ores of the Sn–W deposits of Lesser Khingan, Amur region. Rare lanthanide minerals are described, including fluocerite, bastnaesite, and gasparite-Ce, which was found for the first time in Russia in the Sn–W greisens, and chernovite-Y, the second find in the greisens of the Russian Far East. The influence of arsenic fluids at the late hydrothermal stage caused the replacement of xenotime-Y and monazite-Ce by diverse REE minerals with different lanthanide proportions. It is found that these minerals differ in chemical composition from the same minerals described in other deposits elsewhere. Due to the variable valence state of arsenic and the complete isomorphism between rare earths, arsenates can be used as indicators of redox conditions of their deposition.