REE mineralisation and metasomatic alteration in the Olserum metasediments

Abstract: The Olserum mineralisation is situated in the metasediments of the Västervik formation, which repre-sents the southernmost continental margin of the Svecokarelian Domain. After deposition between 1.88 – 1.85 Ga these sediments were subsided to a depth of about 10 km and intruded by a series of granitoid melts. The resulting HT/LP metamorphism, with a peak at upper amphibolite facies, transformed the sediments to quartzites, quartzitic gneiss and gneiss. REE mineral-rich magnetite bands and veins occur in the mica-rich quartzite and have been in-terpreted as former heavy mineral-rich alluvial beds that now form placer deposits. Infiltration of enriched hydrothermal fluids into the Västervik metasediments resulted in assimilation of primary sedimentary minerals, extensive metasomatic alteration and the subsequent reprecipitation of enriched material. At Olserum this process resulted in the formation of REE-bearing, magnetite-rich biotite veins, with monazite and xenotime constituting the main REE-bearing minerals. Subsequent magmatism-associated fluid events resulted in extensive metasomatic alteration, including the enrichment/depletion of REE in monazite/xenotime and the forma-tion of secondary uranium minerals. Electron microprobe analysis suggests that efficient leaching and transport processes, as well as coupled/uncoupled substitution mechanisms, were integral in the alteration of monazite, xeno-time and uraninite. Textural evidence and chemical analysis suggests a close relationship between REE-bearing minerals and U-Th-minerals at Olserum. The close relationship was reflected in the electron microprobe analysis results in this study, with some monazite and xenotime showing significant U and/or Th concentrations and some U-minerals having substantial REE content. Based on electron microprobe dating of monazite and xenotime, metasomatic alteration at Olserum is interpreted to have been largely associated with intrusion of granites belonging to the 1.85-1.65 Ga Transscandinavian Igneous Belt (TIB), and in particular the 1.76-1.81 Ga Småland granites. However observations also indicate a later thermal event, interpreted as being possibly associated with the 1.46-1.42 Ga Hallandian orogeny.