Cataclastic deformation bands, redox fronts and vadose zone
Present day mining exploration readdresses the old problem about the structural control on 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 « roll front » model, can we map the subsurface in order to find the redox fronts where the Uranium ore deposits should be located? Into a structurally-controlled vadose zone (biphasic), the truth is often much more complicated than the basic « roll front » model.
What is a Catasclastic Deformation Band (CDB)?
Cataclastic deformation bands (CDB) are common structures in porous sandstones (Aydin 1978; Antonellini et al. 1994). These structures are tabular zones of finite width that have experienced grain rotation, crushing, cataclasis or cementation and they correspond to the localization of strain in porous rocks. Field and laboratory measurements exhibit evidence of porosity reduction in the CDB (ratio of ½ to ¼ of the host rock porosity) and that the CDB permeability is significantly reduced by one to six orders of magnitude relative to the host rock (Fossen & Bale 2007). In fluid saturated rocks, CDBs seem to behave like transient barriers, whereas in non saturated rocks, their effect on fluid flow is not clear. In the vadose zone, a barrier effect was observed for paleofluids (Eichhubl et al. 2004) whereas laboratory tests suggest that CDB could be capillarity conduits (Sidga & Wilson 2003).
In this article, we examine the spatial distribution of oxidation fronts and CDB in porous sandstones to discuss interaction between fluid flow and CDB in the vadose zone. These observations/interpretations can be interesting and inspiring for hydrogeologists as well as exploration geologists.
A basic outcrop description
With respect to CDB, oxides are always localized along the bands and surface oxidations are always more important in the hanging wall of the structures than in their footwall. The intersection between two CBD leads to « bucket » oxide concentrations in the upper part, and unaltered shadow zones underneath the intersections of the bands. In terms of microstructures, the non-polarized light image of the thin-section shows a clear loss of porosity.
How could we use this outcrop in exploration?
This work is crucial for the understanding of metalbearing deposits. We observe that the roll-front geometry (Deffeyes and MacGregor, 1980) does not exist in such structurally-controlled setting. The study shows that the sand reddening is attributed to iron oxidation and remobilization by oxidant fluid transfers in the vadose zone after the extensional tectonic deformation. 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. We provide field evidences that CDB behave as permeability barrier, which is demonstrated by the oxide asymmetries relative to the band. In most cases, fluids seem to accumulate along CDB to flow downward in the sandstone as a result of gravity forces.
To summarize, localizing the oxidation with respect to fault helps to (i) understand the hydraulic behavior of the structural heterogeneity network at the time of the water migration in the vadose zone, (ii) to efficiently predict the oxidation location at a subseismic scale where core data cannot provide such 10 m – 100 m scale understanding of the mineralization locations.
The constructive comments are always very welcome.
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Antonellini, M. & Aydin, A. 1994. Effect of faulting on fluid flow in porous sandstones: petrophysical properties.
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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.
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