Textural relations and mineral chemistry of REE in the Grangesberg apatite-iron oxide deposit, Sweden: the role of fluids

The metamorphosed and deformed Kirunatype deposits of the Bergslagen province in south central Sweden not only represent the biggest iron ore resource in central and southern Sweden, but also a significant occurrence of REE's as well as phosphorous. Here we present data from comprehensive mineralogical, textural and mineral chemistry studies of the major REE-bearing assemblages in the Grangesberg deposit, primarily comprising REE-substituted fluorapatite, allanite-(Ce), monazite-(Ce) and xenotime-(Y). It is evident that relatively intense fluid-mediated overprinting has led to alteration of the more reactive phases, such as apatite. This likely occurred in two stages, where the first introduced acidic fluids along apatite grain boundaries during the later stages of crystallization of the apatite-iron oxide deposit. This resulted in the formation of monazite and xenotime inclusions in the apatite. The second stage may have occurred in association with fluid mobility related to the amphibolite-facies metamorphism of the ores and host rocks. This resulted in mass transfer of (Y+REE+P) (due to leaching from the apatite grain rims) and elements from the surrounding groundmass, which in turn led to the formation of allanite along with minor monazite and xenotime in the metavolcanic host rocks.