%0 Journal Article
%T Ilmenites and their alteration products, sinkholes for uranium and radium in roll-front deposits after the example of South Tortkuduk (Kazakhstan)
%A Boulesteix, T.
%A Cathelineau, M.
%A Deloule, E.
%A Brouand, M.
%A Toubon, H.
%A Lach, P.
%A Fiet, N.
%J Journal of Geochemical Exploration
%D 2019
%V 206
%@ 0375-6742
%F Boulesteix_etal2019
%O exported from refbase (http://www.uhydro.de/base/show.php?record=181), last updated on Fri, 26 Jan 2024 13:19:04 +0100
%X The approximate determination of average Ra/U disequilibria in orebodies is one of the most common causes of errors in U reserve estimations. In roll-front deposits, the disequilibria are however frequently distributed following complex geometries, which must be fully understood to prevent major U reserve overestimates and costly unproductive extractive operations. The processes responsible for disruption of the radioactive equilibria and the U and Ra carriers in such complex natural systems remain poorly constrained. In this contribution, we propose an innovative approach, mixing orebody to sub-grain scale studies to unravel the distribution of U and Ra and the processes responsible for their concentration and uncoupling. Using mineral separations, gamma spectrometry and mineral-chemical analyses, we identified the Fe-Ti clusters (altered ilmenite + pyrite/marcasite) as the microsites for coffinite precipitation and Ra concentration. To understand the influence of such clusters on the distribution of U and Ra at the deposit scale, whole-rock Ra/U disequilibria were measured and mapped at a series of ten drill holes along a profile crosscutting the studied roll-front. The main Ra/U disequilibria are encountered around the mineralization in low U content zones. They are controlled by two main processes. (1) In the oxidized zones, the immobility of 230Th with respect to the U produces patches of Ra disequilibria (carried by the altered U minerals). (2) In the immediate vicinity of the roll-front, the dissolution of the mineralization produces an Ra flux trapped by the alteration products of ilmenites, as definitely confirmed by direct SIMS measurements. Such a process is responsible for the Ra disequilibria envelope located downstream of the richest ores, also known as Ra halo. The highest Ra/U ratios correspond to oxidized upstream samples, but most other high Ra/U ratios are from reduced downstream samples close to the mineralization. Such a low to medium U content envelope with high Ra/U ratios constitutes the main cause of U reserve overestimations.
%U https://www.sciencedirect.com/science/article/pii/S0375674219300950
%P 106343