THE REE MINERALS OF THE BASTNAS-TYPE DEPOSITS, SOUTH-CENTRAL SWEDEN

Abstract

On the basis of characteristic assemblages, the Bastnas-type Fe–(Cu–)REE skarn deposits in the Bergslagen mining region of south-central Sweden can be divided into two subtypes: one almost exclusively with LREE enrichment (subtype 1, mainly in the Riddarhyttan–Bastnas area), and another showing enrichment of both LREE and Y + HREE (subtype 2, Norberg District). New data have been collected on twenty REE species including three unnamed ones, using electron-microprobe analysis, X-ray diffraction and Mossbauer spectroscopy. Cerite-(Ce) is, together with ferriallanite-(Ce), the most important REE mineral at Bastnas, but less common at the subtype-2 deposits. Compared to other occurrences worldwide, the present samples of cerite-(Ce) are poor in Ca and enriched in Mg and F. Calcium and (REE + Y) in the samples are negatively correlated, and the upper limit for Ca is close to 1 apfu. Iron ranges from 0.01 to 0.30 apfu, and is mainly in the trivalent state. Cerite-(Ce) is a major carrier of Y (up to 3.5 wt.% Y2O3). Fluorbritholite is found only in the subtype-2 deposits, and is low in P, Na and the actinides compared to other major occurrences in the world. Significant inter- and intrasample variations in REE and Y occur, ranging from fluorbritholite-(Ce) to the unaccredited member fluorbritholite-(Y). Fluorbritholite is non-stoichiometric, with Ca 3 apfu. Vastmanlandite-(Ce) is another important host for the REE in subtype 2. By means of the substitutions Mg2+ + F– Fe3+ + O2– and Mg2+ Fe2+, it forms solid solutions with an unnamed Fe-dominant member that is found in the subtype-1 deposits. Dollaseite-(Ce), which forms a partial solid-solution series with dissakisite-(Ce), is restricted to subtype 2. Gadolinite is very rare at the subtype-1 deposits, and more widespread in the Norberg District. All samples have a significant hingganite component in solid solution (0.10–0.35 molar fraction). Single crystals are commonly zoned with respect to REE and Y, with compositions corresponding to gadolinite-(Ce), gadolinite-(Y) and an Nd-dominant member. The substantial fractionation of REE and Y on a local scale within the Bastnas-type deposits is dependent both on crystal-chemical factors and on fluctuations in fluid composition during crystallization. The initial precipitation of REE silicates in type-1 and type-2 deposits (mainly cerite and fluorbritholite, respectively) was a reaction between relatively acidic solutions carrying major amounts of REE complexed mainly by ligands of F and Si, and dolomitic host-rocks.