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Mineralogical Magazine; April 1999; v. 63; no. 2; p. 239-252
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The chemical composition of uraninite in Variscan granites of the Erzgebirge, Germany

Hans-Juergen Foerster

GeoForschungsZentrum Potsdam, Potsdam, Federal Republic of Germany

Uraninite is widespread as an accessory mineral in the Erzgebirge granites. It occurs throughout the entire comagmatic series of strongly peraluminous S-type Li-mica granites and has been discovered in more evolved transitional I-S type biotite and two-mica granites, but is rare in those of A-type affinity. Textural relationships and chemical ages imply that uraninite is of magmatic origin. Its composition is variable with a proportion of U plus radiogenic Pb between 71 and 99 mol.%. Uraninite has incorporated Th, Y, and the REE in total amounts between 1 and 29 mol.%. Elements such as P, Si, Al, Ca, and Fe are subordinate. Uraninite from two-mica and Li-mica granites is low in ThO 2 (0.8-6.5 wt.%), Y 2 O 3 (0-0.8 wt.%) and REE 2 O 3 (0.1-0.6 wt.%). In contrast, biotite granites from the Kirchberg pluton contain uraninite which is enriched in these components (in wt.%) (ThO 2 = 5.6-11.0, Y 2 O 3 = 0.6-5.5, Ce 2 O 3 = 0.1-0.6, Dy 2 O 3 = 0.2-1.1). Commonly, the lanthanide and actinide contents in uraninite correlate poorly with those in the host granite. In S-type Li-mica granites as well as fractionated two-mica and biotite granites, uraninite is the dominant contributor to the bulk-rock U content. Here the proportion of U approaches 80-90%.

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