Synmetamorphic uranium mineralization from Tiraun, Graubuenden, Switzerland

Petrographic, microscopic, and X-ray investigations of a uranium occurrence near Tiraun, Graubuenden, Switzerland, are described. The uranium mineralization occurs in greenschist facies metamorphic rocks of the Tavetsch massif and consists essentially of uraninite of various habits with minor chalcopyrite, linnaeite, millerite, galena, marcasite, pyrite, hematite, magnetite, and skutterudite. The ore structure, characterized by concentrations along foliation planes, boudinage, and rotated (helicitic) textures of uraninite porphyroblasts, is indicative of a synmetamorphic (Alpine) remobilization, recrystallization and enrichment of the ore, which was probably derived from an original 'protore' in psephites and psammites (sandstone-type mineralization) of at least Variscan age. The synmetamorphic emplacement of the ore took place under medium- to high-temperature conditions (approx. 350 to 400 deg C), as demonstrated by the lattice constants of the uraninite phases and by the ore paragenesis. Late to postmetamorphic tectonism resulted in a partial remobilization (over cm distances) and redistribution of the ore along veinlets that cut the foliation planes. The characteristics of the ore from Tiraun are similar to those from the synmetamorphic ore from Preit, northern Italy, as described by Cevales (1961). (author)