Non-Metamict Uranoan Pyrochlore and Uranpyrochlore from Tuff Near Ndale, Fort Portal Area, Uganda

D. D. Hogarth and J. E. T. Horne*
Department of Geology, University of Ottawa and Ottawa-Carleton Geoscience Centre, Ottawa, Canada K1N 6N5
Formerly of the Atomic Energy Division, Geological Survey of Great Britain
*Present address: 58 Westend, Kemsing, nr Sevenoaks, Kent TN15 6QB

Abstract: A thin (100 m) cover of flat-lying, Recent, calcite-rich tuff at Ndale near Fort Portal, Uganda, unconformably overlies steeply dipping Precambrian metamorphic rocks. It is locally radioactive owing to uranium-rich pyrochlore minerals and lesser amounts of zircon, monazite, titanite, and an unidentified thorium phosphate. In one concentrate, four grains of uranpyrochlore and one grain of uranoan pyrochlore showed a positive linear correlation of Ti with U, and negative linear correlations of Ti with Na, F and Sr. Ta remained high and relatively constant [11 anal., ave. 14.5 (0.6)% Ta2O5]. In the same concentrate the composition of a separate grain of uranoan pyrochlore did not plot on these lines and Ta was comparatively low [2 anal., ave. 4.5 (0.3)% Ta2O5]. The data suggest two separate paths of differentiation. However, zoned grains were not observed. Unit cells were cubic with a = 10.351 ± 0.002 Å for a grain with 12.9% UO2tot. and 10.333 ± 0.002 Å for a grain with 26.6% UO2tot.. On heating in air the cell size decreased, possibly due to oxidation of U4+. The crystalline nature of these minerals can be attributed to a very young (4000–5000 yr) geological age.

Keywords: Fort Portal • Ndale • Uganda • tuff • pyrochlore • uranpyrochlore • uranium • tantalum • titanium • non-metamict • radioactivity

Mineralogical Magazine; April 1989 v. 53; no. 370; p. 257-262; DOI: 10.1180/minmag.1989.053.370.14
© 1989, The Mineralogical Society
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