Origin and metamorphic evolution of carbonate-hosted Mn-Fe oxide deposits

Dan Holtstam

The so-called Långban-type deposits in south-central Sweden comprise carbonate-hosted Mn-Fe mineralizations that were of economic importance in the past. According to contemporary knowledge, they are metamorphosed, exhalative-sedimentary deposits that exhibit chemically well separated, but spatially closely associated Mn and Fe oxide ore bodies; In association with these, mineralogically and chemically complex skarn masses and veins with few counterparts in the world, occur. An almost unique feature is the development of extensive assemblages of Pb, As and Sb oxyminerals during regional metamorphism. The aim of this project is to refine the model for deposition of protoliths and enrichment of key elements, and to elucidate the formation of skarn mineral assemblages (As- and Sb-rich parageneses in particular) during the different stages of diagenesis and metamorphism. For this purpose the Sjögruvan locality (Hällefors, Örebro) has been selected for a comprehensive case study. The work includes: field collecting and mapping; examination of petrographic sections using optical microscopy and the scanning-electron microscope; mineral-chemical analyses with electron microprobe and back scattered electron imaging; powder X-ray diffraction for phase identification; major- and trace-element analyses of bulk ores, skarns and country rocks; and stable (C-O) and radiogenic (Pb) isotope analyses. A genetic model is formulated based on compositional and textural evidence, geochemical signatures etc.

bild1_kopia.jpg (116125 bytes)

Hausmannite ore with manganberzeliite (yellow) from Sjögruvan.
Specimen is 80 x 50 mm.

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