Compositional variation of some rare earth minerals from the Fen complex (Telemark, SE Norway): implications for the mobility of rare earths in a carbonatite system

Abstract

Rare earth minerals in ankeritic ferrocar-bonatite and associated magnetite ore, as well as in hematite-calcite-(dolomite) carbonatite (rødberg) derived from the ferrocarbonatite by post-magmatic alteration, have been studied by electron microprobe. The minerals found are monazite, parisite, and bastnäsite (identifications confirmed by X-ray diffraction) as well as an aluminous silicate with allanite composition, and possibly, synchysite. The rare earth element (REE) distribution patterns of the minerals change from light rare earth (LREE)-enriched in magnetite ore and ferrocarbonatite to less LREE-enriched, or middle REE-enriched in the most strongly re-equilibrated carbonatites. Two distinct mineral varieties, monazite-(Ce,Nd) and a Nd-fluorocarbonate, have been found in rødberg. The LREE were leached from their host minerals during re-equilibration of the carbonatite, most likely because of the influence of fluoride-bearing hydrothermal solutions.