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.

Rare-earth minerals in ankeritic ferrocarbonatite and associated magnetite ore, as well as in hematite-calcite (dolomite) carbonatite (rodberg) derived from the ferrocarbonatite by post-magmatic alteration, include monazite, parisite and bastnasite, as well as an aluminous silicate with allanite composition and possibly synchysite. The REE distribution patterns change from being LREE-enriched in magnetite ore and ferrocarbonatite to less LREE-enriched in the most strongly re-equilibrated carbonatites. Two distinct mineral varieties, monazite-(Ce,Nd) and a Nd-fluorocarbonate, have been found in rodberg. 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.-R.A.H.