Hydrothermal alteration of chevkinite-(Ce) in the Shuiquangou syenitic intrusion, northern China

A rare-earth iron titanosilicate mineral closely associated with amphibole was found in two altered syenite samples from the Shuiquangou syenitic complex. Based on electron microprobe analyses, the mineral is chevkinite-(Ce). The mineral is altered and mantled by a corona of allanite–ilmenite–titanite–epidote–quartz. Mass balance calculations reveal that the former chevkinite was replaced directly by an allanite and ilmenite association (allanite–ilmenite zone) in the inner part of the corona texture. It appears that during the replacement, Si and Ti were immobile, whereas Al, Fe, Mn, Mg and Ca were externally supplied and a proportion of REE and Th were transferred out of the former chevkinite. The released REE and Th become incorporated in allanite and titanite in the allanite–ilmenite–titanite mantle where allanite contains higher Nd, Sm and Y but lower La and Ce than that in the allanite–ilmenite zone, suggesting that redistribution of these elements took place and the mobility of Nd, Sm and Y is enhanced during hydrothermal alteration.