Rare earth element mineralization in the linear carbonatites of the Arshandeposit, western Transbaikal region (Russia)

The Arshan deposit is a typical example of carbonatite complexes of the lin ear type. It has a distinct rare-earth element geochemical specialization. REE mineralization is expressed in a high content of bastnasite, a Ce-rich REE fluorcarbonate. The ores exhibit an extensive replacement of bastnasite with orthite, a principally new relationship found for minerals. The analy sis of mineral assemblages and thermobarogeochemical data show that the ore mineralization originated during the carbonatite formation, a medium-tempe rature hydrothermal-metasomatic process.