The cobalt deposits of Modum, Norway, which were worked from 1772 to 1898, are described. The geology of the area is discussed with particular reference to the Modum Formation which forms the host rock for the greater part of the cobalt mineralisation. The petrography and field relations of this formation suggest that it represents a series of ancient sediments which have been metamorphosed to the upper amphibolite facies of regional metamorphism. The origin of the word 'fahlband' is discussed and a suggested definition of the term is given. The fahlbands of the original locality of Kongsberg are compared with those present in the Modum area. It is postulated that the fahlband zones represent metamorphosed sulphidic black muds. Olivine gabbro bodies have intruded the Modum Formation; the mineralogical changes occurring on amphibolitisation of the gabbros are discussed. From a review of the field relations, petrography and chemistry of the amphibolites in the region, it is concluded that some, art least, of these rocks are derived by metamorphosism of basic intrusives. The cobalt mineralisation occurs as a low grade impregnation in the amphibolites and metasediments. The structural controls of the mineralisation are discussed and a possible zonal arrangement of tie cobalt minerals is recognized. Optical properties and textural relations of the cobalt minerals have been determined. Possible genetic hypotheses are reviewed and it is concluded that the deposits probably represent an occurrence originally of the Cobalt, Ontario type which has undergone high grade metamorphism
