RARE-EARTH MINERALS YTTRIAN ZIRCONOLITE AND ALLANITE-(CE) AND ASSOCIATED MINERALS FROM KOBERG MINE, BERGSLAGEN, SWEDEN

A study of mine dump material from Koberg, Bergslagen, Sweden has revealed a varied mineral assemblage, including Zr-, REE-, Th- and U-bearing minerals associated with the sulphide ore deposit. Detailed descriptions and microprobe analyses are presented of the rare earth minerals yttrian zirconolite and allanite-(Ce), and the zirconium oxide baddeleyite. Yttrian zirconolite is patchily zoned in the actinide elements Th and U, and the low analytical totals suggest the presence of several weight per cent of H2O. The implications of these features with respect to synthetic zirconolite in SYNROC and to radioactive waste management are briefly discussed. Chemical zoning of individual REE in yttrian zirconolite is described and the REE distribution compared with those of zirconolites from other localities. Allanite-(Ce) is generally unzoned, but contains REE-rich rims which are considered to be sub-micron intergrowths of allanite plus a bastnasite-type mineral phase. The MgO content of allanite-(Ce) is unusually high and appears to be a characteristic feature of allanite from other cerium and mineral occurrences of Central Sweden. The rare earth minerals at Koberg are interpretated as having been formed as a result of localized remobilization of elements including Ti and Zr with H2O- and CO2-rich fluids.