Roll-fronts and oxidation fronts: Can we predict their subsurface distribution?
Present day mining exploration readdresses the old problem about the geological control on subsurface fluid flow, which is still debated in hydrogeology and petroleum research, in particular in porous media. Can we predict where are located the oxidation fronts? Assuming a classical « Wyoming roll-front » model, can we map the subsurface in order to find the redox fronts where the Uranium ore deposits could be located? Back to the outcrop, back to the concept ..
What is a roll-front?
Bates and Jackson (1980) define a roll-front orebody as a structure which « is bounded on the concave side by oxidized altered rock typically containing hematite or limonite, and on the convex side by relatively reduced altered rock typically containing pyrite and organic matter« . Roll-fronts are commonly related to gravity-driven water circulation into the Vadose zone. They can also host selenium (Se), molybdenum (Mo), and tellurium (Te) as a function of the available elements in the geological system (e.g. Harshman, 1974; Bullock and Parnell, 2017 and herein references). Uranium ore deposits have been extensively described in the « nose » of the roll-fronts (e.g. Rubin, 1970). Redox zone units can therefore be localized using the rock gamma signature (Gamma Ray).
What is the Vadose zone?
The zone of aeration that is commonly located above the water table and out of the zone of saturation. Both water and air fill the porosity in such rock/soil volume, mostly depending on the moisture content and the local temperature. Relevant synonyms are « non-saturated zone » and « unsaturated zone ».
How could we use this outcrop in exploration?
Layering, cross-bedding stratification
These outcrop picture and related-interpretation are crucial for understanding the layering control on roll-fronts geometries. Indeed, differences in permeability between granulometries (grain-size) and lithologies (sands vs clays) lead to complex subsurface roll-fronts geometries (without any structural heterogeneities such as faults or fractures).
Oxidation fronts locations
On the outcrop, the sand reddening is attributed to iron oxidation and remobilization by oxidant fluid transfers in the vadose (Cavailhes et al., 2009). Sandstones are oxidized by gravity driven ground water saturated in atmospheric oxygen infiltrated from the surface. In a non water saturated zone, the main mechanism of oxidant solute transfer in high porosity sandstone seems to be advection (Taylor & Pollard 2000). We therefore make the hypothesis that the oxidation fronts underline preferential pathways for the ground water.
Implications for water flow
The main water flow seems to come from the right corner of this picture. The clay layer (k) in the middle of the picture probably localizes a vertical loss of porosity/permeability/connectivity. The roll-fronts below this layer seem « in late » i.e. showing an offset with the upper ones. This observation demonstrating a reservoir-compartimentalization has to be taken in account during exploration. The vertical changes in permeability values commonly lead to roll-front « offsets ». For a same water flow rate, if the host rock permeability increases, the distance between the water entrance point and the roll-front increases. If the sandstone matrix permeability decreases, the distance between the water entrance point and the roll-front decreases.
To summarize, localizing the oxidation helps to (i) understand the hydraulic behavior of the stractigraphic package at the time of the water migration into the vadose zone, (ii) to efficiently predict the oxidation location at a subseismic scale in order to predict mineralization locations.
The constructive comments are always very welcome.
Bates, R. L., & Jackson, J. A. (1980). Glossary of Geology: Falls Church. Virginia, American Geological Institute, 167.
Cavailhes, T., Soliva, R., Benedicto, A., Loggia, D., Schultz, R. A., & Wibberley, C. A. J. (2009, September). Are cataclastic shear bands fluid barriers or capillarity conduits? Insight from the analysis of redox fronts in porous. In 2nd EAGE International Conference on Fault and Top Seals-From Pore to Basin Scale 2009 (pp. cp-136). European Association of Geoscientists & Engineers.
Deffeyes, K. S., & MacGregor, I. D. (1980). World uranium resources. Scientific American, 242(1), 66-77.
Harshman, E. N. (1974). Distribution of elements in some roll-type uranium deposits. In Formation of uranium ore deposits.
Liam A. Bullock, John Parnell, Selenium and molybdenum enrichment in uranium roll-front deposits of Wyoming and Colorado, USA, Journal of Geochemical Exploration, Volume 180, 2017, Pages 101-112, ISSN 0375-6742, https://doi.org/10.1016/j.gexplo.2017.06.013.
Rubin, B. (1970). Uranium roll front zonation in the southern Powder River Basin, Wyoming.