Movement of the redox front at the Osamu Utsumi uranium mine, Poços de Caldas, Brazil

Abstract

In the open pit uranium mine at Poços de Caldas the uranium oxide ore has accumulated at a redox front. The redox front forms and moves by the oxidation of pyrite in the rock by oxygen dissolved in the infiltrating rainwater. The phonolitic host-rock was formed about 90 million years ago. It is possible that between 3 and 12 km of rock has been eroded since then. The redox front lies between tens and several hundred meters below the ground surface. The front is very irregular and locally there are oxidation “fingers” which extend well ahead of the main front. The redox fingers are often associated with fractures or fracture zones. An attempt has been made to model the formation and evolution of the redox front at Poços de Caldas by applying chemical transport models of different complexity. One class of models used is based on advective flow in a porous medium; the continuum models. Another class of models used is based on fracture flow with transport of oxygen into the rock matrix by molecular diffusion. In the latter class of models, channels with different flowrates are modelled and the flowrate is allowed to vary over time stochastically to account for the changing properties of the hydraulic network. The erosion of the surface is as fast as the redox front movement which stabilizes at a distance below the ground surface because the water flux decreases with depth. Another possible cause for stabilization of the tips of the redox fingers is that the flow channels form a network with variable flow properties. The main chemical reactions, i.e. pyrite oxidation, clay formation and the accumulation of uranium oxide at the redox front, are well in accordance with expectations and with what a chemical transport model predicts. The formation of redox fingers may partly be caused by variable hydraulic conductivity of the rock and partly by the presence of highly conductive fractures and channels in fractures. Several mechanisms contribute to the formation of the complex shape of the redox front. Some of the processes have been conceptualized and some have been quantified in the form of mathematical models which were used to make quantitative calculations. The calculations have given considerable support to the determination of which mechanisms may be important. The overall oxidized zone can be explained by advective transport in a porous medium. The fingered zone around fractures would be explained by flow through fractures (channelling) and indicates that matrix diffusion is clearly present as a mechanism to access the interior of the rock.