The Chemical Composition of Uraninite in Variscan Granites of the Erzgebirge, Germany1

H.-J. Förster
GeoForschungsZentrum Potsdam, Telegrafenberg, D - 14473 Potsdam, Germany
1This paper is Part III of a series on REE-Y-Th-U-rich accessory minerals in peraluminous granites.

Abstract: 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 ThO2 (0.8–6.5 wt.%), Y2O3 (0–0.8 wt.%) and REE2O3 (0.1–0.6 wt.%). In contrast, biotite granites from the Kirchberg pluton contain uraninite which is enriched in these components (in wt.%) (ThO2 = 5.6–11.0, Y2O3 = 0.6–5.5, Ce2O3 = 0.1–0.6, Dy2O3 = 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%.

Keywords: uraninite • composition • accessory minerals • granites • electron microprobe

Mineralogical Magazine; April 1999 v. 63; no. 2; p. 239-252; DOI: 10.1180/002646199548466
© 1999, The Mineralogical Society
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