The Mineralogy and Genesis of Uranium in Rhyolitic Ignimbrites of Precambrian Age from Duobblon, Sweden

John A. T. Smellie
Geological Survey of Sweden, Box 801, Luleå, Sweden

Abstract: The Duobblon rhyolitic ignimbrites, of middle Precambrian age are 60 m thick. They consist of at least three flows with varying degrees of welding and have undergone devitrification, producing lithophysal and spherulitic textures. They are overlain by thick fluviatile red-bed-type conglomerates and sandstones, which in turn are capped by terrestrial acid volcanics.

Uranium enrichments of up to 3000 ppm U occur within two or three peneconcordant tabular horizons which are mostly lithophysae-bearing. Fission-track investigations of the ignimbrites and overlying conglomerates and sandstones, which supplement earlier mineralogical studies, show that U occurs as fine pitchblende disseminations; as complex uranotitanates associated with Fe-Ti-Mn oxides; and as coatings associated with matrix sericite. Small amounts of O are present in such primary accessory minerals as sphene, apatite, and zircon.

It is suggested that oxidizing U-bearing solutions, generated partly during devitrification of the ignimbrites and partly from the overlying volcano-sedimentary pile, produced the U enrichments with later sulphide deposition, along the more permeable lithophysal horizons in the ignimbrites.

Mineralogical Magazine; June 1982 v. 46; no. 339; p. 187-199; DOI: 10.1180/minmag.1982.046.339.05
© 1982, The Mineralogical Society
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