Apatite geochemistry and development of REE minerals related to coupled dissolution-reprecipitation process in the Gazestan deposit, Central Iran

The Gazestan iron-apatite deposit in the Bafq mining district, is hosted in the upper Proterozoic-lower Cambrian volcanic-sedimentary sequence, known as Rizu series, and shallow intrusions. In backscattered electron (BSE) images, the apatite displays irregular light and dark zones with variable compositions. The dark zones contain inclusions of monazite and xenotime, and are distinguished by depletions in LREE+Y, Na and Cl, and enrichments in Ca and P. Development of reaction rims around apatite crystals, the occurrence of monazite and xenotime as scattered fine grains as well as filling microfractures in the dark zones, and the occurrence of the REE minerals at apatite-magnetite boundary provide evidence for alteration of the original apatite crystals in Gazestan. There is evidence that the apatite crystals formed during original magnetite-apatite ore formation, interacted and reequilibrated with fluids. During a coupled dissolution-reprecipitation process, the original apatite partially reacted with fluids of external origin. This led to leaching/reprecipitation of certain components, in particular the REEs, and development of dark apatite zones with fine, scattered monazite and xenotime grains, as well as the occurrence of fine REE mineral grains across microscopic fractures in apatite, calcite and quartz.