Abstract

The Duobblon occurrence is a stratabound uranium mineralization within rhyolitic ignimbrites of middle Precambrian age. The ignimbrites and an underlying basal breccia lie unconformably on a deeply weathered granite basement of about 1,790 m.y., and are overlain by thick fluviatile deposits of red-bed-type conglomerates and sandstones which are in turn capped by acid to intermediate terrestrial volcanics. The ignimbrite unit is about 60 m thick and 4 to 5 km of strike length is known. The richest uranium concentrations, on an average 200 to 300 ppm U (over 50 cms of drill core), occur as several 1- to 25-m-thick and about 1,000-m-long horizons within lithophysae-bearing parts of the ignimbrite unit. The uranium, mostly present in the groundmass, has undergone intrastratal leaching and redistribution and there is a tendency for enrichment downdip as well as along lithological boundaries. The location of enrichment areas may have been guided by the proximity of interbedded and overlying conglomerates acting as physicochemical traps. The descending transporting solutions have been oxidizing in character and the uranium has been precipitated under favorable conditions as fine granular pitchblende and complex uranotitanates, within the chlorite-sericite components of the groundmass.The highest U enrichments are generally associated with high Pb, V, and Mo, but the correlation is poor in detail. The exploration methods employed are briefly described.

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