Christian Le Carlier de Veslud

3D modeling of uranium-bearing solution-collapse breccias in Proterozoic sandstones (Athabasca Basin, Canada)-Metallogenic interpretations

Unconformity-related uranium deposits are the highest grade, large tonnage uranium resources in the world. In the Athabasca Basin (northern Saskatchewan, Canada), which is the premier host for unconformity-type deposits, the ore deposits are frequently hosted and surrounded by breccias in sandstone. The significance of these breccias and their relation to mineralization are of major importance for the genesis of these high-grade deposits. Therefore, a modeling study, integrating results from structural geology and petrology, was performed with the gOcad 3D modeling software, in order to decipher geometrical and geological relationships between breccias, faults and mineralization zones. Mineralized bodies and the sudoite-dravite breccia bodies display strong spatial correlations. They appear to be controlled by reverse shear zones cross-cutting the unconformity and containing graphite in the basement. Geochemical computations evidenced that volumetric water-rock ratios up to 10,000 could be obtained in these breccia bodies for volume losses of up to 90%. Assuming reasonable values for quartz saturation, hydraulic conductivity and connected porosity, the minimal fluid volume and the time duration necessary to generate the sudoite-dravite breccia bodies were estimated at ca. 2km^3 and ca. 1Myr, respectively. The comparison of these results with literature data suggests that the formation of sudoite-dravite breccia and mineralization could have been coeval. It may be proposed that within the space created by the quartz dissolution in the breccia body, a mixing between basement and basinal fluids could have induced U deposition and allowed the development of high-grade mineralization. The first-order uranium solubility that this coeval formation would imply is consistent with literature data, which suggests that this conceptual model is reasonable.

3D stratigraphic and structural synthesis of the Dannemarie basin (Upper Rhine Graben)

The Dannemarie basin is the southwesternmost depocentre of the Upper Rhine Graben, which belongs to the West-European Tertiary rift system. It is bounded to the west by the Vosges mountains, to the south by the folded Jura belt and to the east by the Mulhouse block. The rifting reached its maximum activity during the Priabonian and early Rupelian (35-31 Ma). In the framework of the GeoFrance 3D project “Fosse rhenan”, a 3D geometrical model of the Dannemarie basin was built in the gOcad 3D modeler. It incorporates the BRGM well database and geological maps, and 40 seismic cross-sections. These data are used to study the structure and geological history of the area. Seismic data have been converted from time to depth using a 1D time-to-depth polynomial law deduced from the analysis of the Bellemagny borehole. The Dannemarie basin is bounded to the west by the Vosges fault zone and to the east by the Illfurth fault zone. On both borders, basin subsidence was controlled by normal faults and associated syn-rift flexures. The minimum throw on the Vosges fault zone is about 1400 m to the north of the model, decreasing to the south, where it is replaced by a syn-rift flexure. On the Illfurth fault zone, subsidence is accommodated by faults (with about 1000 m throw) and by a flexure (about 300 m). Stratigraphic data indicate that these flexures were active during Priabonian and early Rupelian extension. These monoclinal flexures are interpreted as fault-propagation folds that developed above upward propagating normal faults in the basement. As displacement accumulates, the fault propagates upwards and cuts the overlying fold. Similar fault-fold geometries have been described on the western border of the Rhine graben, close to Colmar and in other extensional tectonic contexts. In the Colmar area, the Vosges fault zone cuts through the basin margin fold, while further south along the western border of the Dannemarie basin, displacement on the fault decreases and subsidence is accommodated on a major flexure. Flexure locations correspond to gravimetric discontinuities attributed to Variscan structures, suggesting reactivation of deep structures during rifting. The Illfurth fault zone displays upwardly divergent fault geometries that resemble “flower structures”. The data can be interpreted as follows, either that (a) the Illfurth fault zone accommodated a minor sinistral strike-slip component due to a post-Miocene NW-SE compressive regional stress field or (b) these faults developed in association with the fault propagation folds.

L'altération naturelle des scories de la métallurgie ancienne : un analogue de déchets vitrifiés

The study of the natural alteration of ancient vitreous slags (100 to 4 000 years) suggests a single global mechanism. In a first stage, weathering consists of a selective extraction of the modifier cations of glass (including Pb and Ba). This phenomenon is associated with an increase of the solution pH, inducing the glass dissolution. The elements with a weak limit of solubility remain in place and can form hydroxides (Al, Fe). Pb and Ba are extracted from weathered glass. Pb is located in Fe-hydroxides, Ba may form sulfides. The rate of alteration varies from 20 to 180 μm per 1 000 years.

3D Modeling of Complex Natural Objects

The aim of 3D modeling is to define the geometry and properties (such as permeability, porosity, .. .) of natural complex structures which are investigated from irregular sampling (boreholes, sections, pOints). Once a geometrical model is built, the evolution of various parameters, such as coupled heat and mass transfers, can be studied using numerical modeling. The geometry and the physical parameters of the objects are modeled using the GOCAD software developed at the Ecole de Geologie de Nancy. Several studies are presented to illustrate this approach, including the 3D modeling of an underground chromite mine, the thermal field and the fluid flow patterns around a fractured geothermal site at Soultz in the Rhine Graben, and the 3D seismic data collected during an injection experiment.