Uranium migration and retention during weathering of a granitic waste rock pile F. Boekhout a,⇑ , M. Gérard a , A. Kanzari a , A. Michel b , A. Déjeant a , L. Galoisy a , G. Calas a , M. Descostes c a Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Universités, UPMC Univ Paris 06, CNRS, IRD UMR 206, 4 place Jussieu, F-75005 Paris, France b Institut du Physique du Globe, 75005 Paris, France c AREVA, BG Mines, R&D, Paris la Défense, France article info Article history: Available online 25 February 2015 Editorial handling by M. Kersten abstract This study investigates the post-mining evolution of S-type granitic waste rocks around a former uranium mine, Vieilles Sagnes (Haute Vienne, NW Massif Central, France). This mine was operated between 1957 and 1965 in the La Crouzille former world-class uranium mining district and is representative of intra- granitic vein-type deposits. 50 years after mine closure and the construction and subsequent re-vegeta- tion of the granitic waste rock pile, we evaluate the environmental evolution of the rock pile, including rock alteration, neo-formation of U-bearing phases during weathering, and U migration. Vertical trenches have been excavated through the rock pile down to an underlying paleo-soil, allowing the investigation of the vertical differentiation of the rock pile and its influence on water pathways, weathering processes and U migration and retention. Arenization dominantly drives liberation of U, by dissolution of uraninite inclusions in the most alterable granitic minerals (i.e. K-feldspar and biotite). Retention of U in the matrix at the base of the waste rock pile, and in the underlying paleo-soil most likely occurs by precipitation of (nano-) uranyl phosphates or a combination of co-precipitation and adsorption reactions of U onto Fe (oxy)hydroxides and/or clay minerals. Even though U-migration was observed, U is retained in stable sec- ondary mineral phases, provided the current conditions will not be modified. Ó 2015 Published by Elsevier Ltd. 1. Introduction In the near future, the combination of an intensifying demand for uranium resources, declining ore grades, and the exploitation of lower commodity grade ores will lead to an exponential increase of the annual volume of waste rock (Kahouli, 2011). The largest volume of waste products are produced in the nuclear fuel-cycle during mining and milling (Abdelouas, 2006), triggering a growing interest in waste rock management (e.g., Kipp et al., 2009; Miao et al., 2013; Schindler et al., 2013; Neiva et al., 2014). The term ‘waste rock’ is defined as untreated rocks that do not contain enough U to be economically processed. Waste rock piles are highly heterogeneous in their nature, comprising barren rock remobilized from the mine surroundings (access roads, mining works), overburden from overlying soils and rock covering the ore deposit and un-reclaimed, sub-economic ore extracted from the mine. A major characteristic of waste rock piles is their increased erosional surface compared to natural granitic outcrops, inducing an accelerated weathering rate. Apart from being poten- tially harmful for the environment this enhanced weathering pro- vides an end-member example of accelerated continental alteration processes. Rock pile weathering overprints the late hydrothermal alteration of the rock and the natural supergene pre-mining alteration of the site. For a long period, U vein-type deposits yielded the bulk of glo- bal U production, whereas less than 10 percent of the U was pro- duced from deposits of this type at the end of the 1980s. This mining activity left behind a significant amount of granitic waste rock that form the oldest U-bearing rock piles in many inhabited areas, hence their importance for the evaluation of the weathering evolution of mine wastes for remediation strategies. One of the most representative regions with granitic vein-type deposits is the ‘La Crouzille’ district in the French Massif Central. S-type gran- ites of the Massif Central region have an elevated U concentration, often larger than 20 ppm (Barbier, 1970; Cuney, 2014). Such a high background concentration dominantly arises from resistate U- bearing phases, located in accessory phases (zircon, sphene, allan- ite, monazite, apatite, or magmatic uraninite, ect.) (Bajo et al., 1983; Berzina et al., 1975; Cuney, 2009), as opposed to secondary U phases within primary minerals (Speer et al., 1981; Tieh et al., http://dx.doi.org/10.1016/j.apgeochem.2015.02.012 0883-2927/Ó 2015 Published by Elsevier Ltd. ⇑ Corresponding author at: Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität, 48149 Münster, Germany. Tel.: +33 1 44 27 50 84. E-mail address: floraboekhout@hotmail.com (F. Boekhout). Applied Geochemistry 58 (2015) 123–135 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem