Genetic Simulation of Roll-Front Uranium Deposits - a Reactive Transport Approach
Résumé
Roll-front type uranium deposits are a category of sedimentary uranium deposits of great economic interest. A correct representation of these geometrically complex deposits is fundamental for a rationalized exploitation. We propose here an alternative to the geostatistical approaches: the genetic simulation of the deposit, using reactive transport tools. The simulations should represent the chemical processes that lead to the formation of the mineral deposit, coupled with complex paleoflow in heterogeneous sandstone aquifers. To mimic complex flows in an aquifer hydrodynamically equivalent to the target aquifer, a geostatistical permeability model was used. A geochemical model was also elaborated: it represents a reduced aquifer, rich in pyrite and organic matter, and its reaction with oxidizing water. Several simplifying assumptions were made, and the competition between kinetics and hydrodynamics has been carefully monitored. Reactive transport 2-D simulations were then performed with the code HYTEC, using several random realizations for the permeability distribution. The results show the formation of a redox front, progressing through the aquifer, where uranium accumulates. Due to the local variations in the permeability field, the flow distribution is heterogeneous, which leads to locally variable front velocities. As a result, the front displays complex structures. Some loops are observed, with deviated directions from the mean flow direction; also, gaps are observed over the mineralized front. These characteristics are well known in natural rollfronts. The study demonstrates the possibility to develop genetic simulations based on reactive transport tools for roll-front type deposits. They could ultimately lead to wider applications with predictive potential for characteristic values for the shape of the front or the mass of uranium in the deposit. The data required to calibrate the simulations are easily available (geochemistry and permeability of the aquifer, not necessarily in the roll itself).