Fate of trace elements during alteration of uraninite in a hydrothermal vein-type U-deposit from Marshall Pass, Colorado, USA Artur P. Deditius, Satoshi Utsunomiya, Rodney C. Ewing * Department of Geological Sciences, University of Michigan, 1100 N. University, Ann Arbor, MI 48109-1005, USA Received 22 January 2007; accepted in revised form 15 August 2007; available online 11 September 2007 Abstract Alteration of uraninite from a hydrothermal vein-type U-deposit in Marshall Pass, Colorado, has been examined by elec- tron microprobe analysis in order to investigate the release and migration of trace elements W, As, Mo, Zr, Pb, Ba, Ce, Y, Ca, Ti, P, Th, Fe, Si, Al, during alteration, under both reducing and oxidizing conditions. The release of trace elements from ura- ninite is used to establish constraints on the release of fission product elements from the UO 2 in spent nuclear fuels. Uraninite occurs with two different textures: (1) colloform uraninite and (2) fine-grained uraninite. The colloform uraninite contains 1.04–1.75 wt% of WO 3 , 0.16–1.70 wt% of As 2 O 3 , 0.06–0.88 wt% of MoO 3 ; whereas, the fine-grained uraninite retains 2.25–4.93 wt% of WO 3 , up to 5.76 wt% of MoO 3 , and 0.26–0.60 wt% of As 2 O 3 . The near constant concentration of incom- patible W in the colloform uraninite suggests W-incorporation into the uraninite structure or homogeneous distribution of W-rich nano-domains. Incorporation of W and Mo into the uraninite and subsequent precipitation of uranyl phases bearing these elements are critically important to understanding the release and migration of Cs during the corrosion of spent nuclear fuel, as there is a strong affinity of Cs with W and Mo. Zoning in the colloform texture is attributed to variation in the amount of impurities in uraninite. For unaltered zones, the calculated amount of oxygen ranges from 2.08 to 2.32 [apfu, (atom per formula unit)] and defines the stoichiometry as UO 2+x and U 4 O 9 ; whereas, for the altered zones of the colloform texture, the oxygen content is 2.37–2.48 [apfu], which is probably due to the inclusion of secondary uranyl phases, mainly schoepite. The supergene alteration resulted in precipitation of secondary uranyl minerals at the expense of uraninite. Four stages of colloform uraninite alteration are proposed: (i) formation of an oxidized layer at the rim, (ii) corrosion of the oxidized layer, (iii) precipitation of U 6+ -phases with well-defined cleavage, and (iv) fracture of the uraninite surface along the cleavage planes of the U 6+ -phases. Ó 2007 Elsevier Ltd. All rights reserved. 1. INTRODUCTION During the last two decades, a number of studies of uraninite (UO 2+x ; space group Fm3m; Z = 4; number of units per one unit cell) in natural U-deposits have been used to establish constraints on the corrosion of used nuclear fuel and the subsequent release of fission product and trans- uranium elements (e.g., Grandstaff, 1976; Langmuir, 1978; Parks and Pohl, 1988; Finch and Ewing, 1992; Janeczek and Ewing, 1992a,b; Pearcy et al., 1994; Fayek et al., 1997a; Jensen et al., 1997; Finch and Murakami, 1999; Jensen and Ewing, 2001; Bruno and Ewing, 2006). The migration of U and other trace metals during corrosion (i.e., dissolution) of natural uraninite is of particular inter- est, because it provides information that can be used to understand the alteration of used nuclear fuel, mainly UO 2 , over long periods in geologic repositories. Previous studies have shown the importance of the trace element geochemistry of uraninite in determining the alteration phases that form (Finch and Ewing, 1992). Impurities, such 0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.08.008 * Corresponding author. Fax: +1 734 647 5706. E-mail address: rodewing@umich.edu (R.C. Ewing). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 71 (2007) 4954–4973