Precipitation of Uraninite in Chlorite-Bearing Veins of the Hydrothermal Alteration Zone ( Argile De Pile ) of the Natur
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PRECIPITATION OF URANINITE IN CHLORITE-BEARING VEINS OF THE HYDROTHERMAL ALTERATION ZONE (ARGILE DE PILE) OF THE NATURAL NUCLEAR REACTOR AT BANGOMBE, REPUBLIC OF GABON P. EBERLY*, J. JANECZEK**, and R.EWING* *Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New
Mexico 87131, U.S.A. (e-mail: [email protected]) "**Department of Earth Sciences, Silesian University, Bedzinska 60, 41-200 Sosnowiec, Poland ABSTRACT
This paper describes the mineralogy of a phyllosilicate/uraninite/galena-bearing vein located within the hydrothermal alteration halo associated with the Bangomb6 reactor. Phyllosilicates within the vein include a trioctahedral Al-Mg-Fe chlorite (ripidolite), Al-rich clay (kaolinite and/or donbassite) and illite. Textural relations obtained by backscattered-electron imaging suggest that ripidolite crystallized first among the sheet silicates. Uraninite is spatially associated with ripidolite and probably precipitated at a later time. While energy-dispersive x-ray analyses suggest that the uranium phase is predominantly uraninite, coffinite or other phases may also be present. INTRODUCTION
The natural nuclear reactor at Bangombd is similar to nuclear reactors at Oklo and Okelobondo, approximately 30 kilometers to the north, and consists of a pocket (meters lateral, centimeters vertical) of concentrated uranium ore containing fission products, a low content of 235 U, and a hydrothermal alteration halo (argilede pile) enriched in clays. The reactor sustained critical fission reactions approximately 2 Ga ago and is located within the clastic FA formation of the Francevillian Series of the Franceville Basin of eastern Gabon" 2. The natural nuclear reactors of Gabon are unique. Study of the reactor zone and associated rocks permits the evaluation of the stability of uraninite (a natural analogue for the U0 2 in spent nuclear fuel) and
of uranium migration in fluids on a time scale considerably greater than that of laboratory experiments (years) or the planned lifetime of engineered repositories (104 to 106 years). This study was undertaken to investigate the mineralogy of phyllosilicate/uraninite/galenacontaining veins first noted within the Bangomb6 hydrothermal alteration halo by Bros and others3 and Janeczek and Ewing 4. This mineralogical association is significant because it provides empirical evidence of the retardationof uranium in ground water in the presence of phyllosilicates.
The alteration halo at Bangomb6 is less than a meter thick; the sample described below was drawn from borehole BAX3 (BAX03-5D2) and is located immediately above the reactor core (Figure 1). ANALYTICAL METHODS
Electron microprobe analyses were performed using a JEOL Superprobe equipped with Oxford Link automation and data-reduction equipment and an Oxford detector for energy-dispersive analysis; the instrument is also equipped with five spectrometers for wavelength-dispersive analysis. The electron microprobe was operated at an accelerating voltage of 15 kV and a sample current of 20 nA. Th
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