The metasedimentary rocks of the Modum Complex contain concordant, sulphide-rich zones, commonly referred to as 'fahlbands'. Two of these fahlbands contain cobalt minerals. Petrographic and microprobe analyses of the cobalt minerals have been performed on selected samples, to gain insight into the genesis and post-mineralization history of the cobalt minerals. It is suggested that these minerals precipitated from an oxidizing fluid, and that certain minerals in the uraninite-thorianite series precipitated simultaneously. The fahlband silicates, and later the cobalt minerals, were recrystallized during Sveconorwegian penetrative deformation and thermal maximum (~ 1 .2- ~ 0.9 Ga). Prolonged heating during this event eradicated any compositional zoning which may originally have been present. Neither textural nor mineral chemical analyses indicate whether the observed cobalt minerals are the result of a primary precipitation, or of remobilization.

H. F. Grorud, Institute for Geology, University of Oslo, P. O. Box 1047, N-0316, Oslo, Norway.