Two Occurrences of Uranium in Sweden — the Los Cobalt Deposit and the Iron Ores of the Västervik Area

Abstract

Several small sulphide disseminations and vein fillings, characterized by the presence of Ni and Co minerals, occur in a greenstone area around the community of Los, Hälsingland. The largest vein filling, i.e. the Los cobalt deposit, was mined during the 18th century and contains a gangue of calcite, quartz, small amounts of baryte, fluorite and tourmaline in which the ore minerals cobaltite, gersdorffite, pyrrhotite, chalcopyrite, pitchblende, pyrite, arsenopyrite, bismuthinite and marcasite, are found. The wall rock is altered and minerals of the ore paragenesis are deposited in it. These disseminations contain euhedral uraninite and granular pitchblende which apparently represent two generations, the uraninite being older than the main deposition of the sulphides. The genetic relationship of the Co-Ni-Cu-Bi-U-paragenesis is discussed. One age determination by the U/Pb method on pitchblende gives an almost concordant age of 1,690 m.y. which coincides with the age of the rapakivi granites in Finland regarded as contemporaneous with the sub-Jotnian granites (Dala-granites) in the Los area.

The iron ores of the Västervik area belong to the quartzite formation and consist of thin horizons of biotite or hornblende quartzite containing abundant magnetite or hematite. The iron ores have been explained as original limonite depositions in sandy sediments. Much new evidence, however, point to a detrital origin of the ores, which, through the high-grade of metamorphism associated with the emplacement of the Småland granites, finally assumed their present shape and composition. The uranium-bearing minerals are uraninite and monazite. Uraninite is always found in the iron ore horizons and preferentially in the magnetite-rich thin beds. Monazite prefers the quartz-rich beds of the ore horizons but occurs also in aplite veins conformable to or cutting the quarzitic sediments. It is concluded that uraninite and monazite probably are of syngenetic origin and possibly mechanical accumulations in black sand layers.