Uranium deposits in sedimentary basins can be formed at various depths,from near surface to the basement.While many factors may have played a role in controlling the location of mineralization,examination of various examples in the world,coupled with numerical modeling of fluid flow,indicates that the hydrodynamic regime of a basin may have exerted a major control on the localization of uranium deposits.If a basin is strongly overpressured,due to rapid sedimentation,abundance of low-permeability sediments or generation of hydrocarbons,fluid flow is dominantly upward and uranium mineralization is likely limited at shallow depths.If a basin is moderately overpressured,upward moving fluids carrying reducing agents may meet downward moving,oxidizing,uranium-bearing fluids in the middle of the basin,forming uranium deposits at moderate depths.If a basin is weakly or not overpressured,either due to slow sedimentation or dominance of high-permeability lithologies,minor topographic disturbance or density variation may drive oxidizing fluids to the bottom of the basin,leaching uranium either from the basin or the basement,forming unconformity-type uranium deposits.It is therefore important to analyze the hydrodynamic regime of a basin in order to predict the most likely type and location of uranium deposits in the basin.
1.Introduction Most metallic mineral deposits formed from hydrothermal fluids,and the mineralization processes include both chemical (e.g.,fluid-rock and fluid-fluid interactions)and physical(e.g., fluid flow)aspects.More attention has been paid to the chemical processes than the physical ones,even though the latter is equally important in controlling the formation of mineral deposits and their localization.Nevertheless,a number of studies have been carried out regarding the hydrodynamics of fluid flow associated with ore mineralization(e.g.,Cathles,1981,1997; Garven and Raffensperger,1997;Cathles and Adams,2005; Cox,2005;Ingebritsen et al.,2006),and it is timely to eval-
Guoxiang Chi Department of Geology,University of Regina, Regina,Saskatchewan,Canada Chunji Xue State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences, Xueyuan Road 29,Beijing 100083,China
