Groundwater processes and sedimentary uranium deposits

Hydrologic processes are fundamental in the emplacement of all three major categories of sedimentary uranium deposits: syngenetic, syndiagenetic, and epigenetic. In each case, the basic sedimentary uranium-enrichment cycle involves: (1) leaching or erosion of uranium from a low-grade provenance; (2) transport of uranium by surface or groundwater flow; and (3) concentration of uranium by mechanical, geochemical, or physiochemical processes. Although surface flow was responsible for lower Precambrian uranium deposits, groundwater was the primary agent in upper Precambrian and Phanerozoic sedimentary uranium emplacement. Meteoric or more deeply derived groundwater flow transported uranium in solution through transmissive facies, generally sands and gravels, until it was precipitated under reducing conditions. Syndiagenetic uranium deposits are typically concentrated in reducing lacustrine and swamp environments, whereas epigenetic deposits accumulated along mineralization fronts or tabular boundaries. The role of groundwater is particularly well illustrated in the bedload fluvial systems of the South Texas uranium province. Upward migration of deep, reducing brines conditioned the host rock before oxidizing meteoric flow concentrated uranium and other secondary minerals. Interactions between uranium-transporting groundwater and the transmissive aquifer facies are also reflected in the uranium mineralization fronts in the lower Tertiary basins of Wyoming. Similar relationships are observed in the tabular uranium deposits of the Colorado Plateau.Original Abstract: Les processus hydrologiques sont determinants dans la localisation des trois principaux types de depots sedimentaires d'uranium: syngenetique, syndiagenetique et epigenetique. Dans chaque cas, le cycle fondamental d'enrichissement en uranium sedimentaire met en jeu: 1) le lessivage ou l'erosion de l'uranium a partir d'un materiau a faible teneur, 2) le transport de l'uranium par un ecoulement d'eau de surface ou souterraine, et 3) la concentration de l'uranium par des processus mecaniques, geochimiques ou physico-chimiques. Bien que les ecoulements de surface aient etea l'origine des depots d'uranium au Precambrien inferieur, les eaux souterraines ont ete l'agent principal de la mise en place des formations uraniferes sedimentaires du Precambrien superieur et du Phanerozoieque. Des ecoulements souterrains proches de la surface ou meme plus profonds ont transporte l'uranium en solution au travers de fa