Quantitative evaluation of the distribution of a variable within a solid by a method using constitution and distribution heterogeneities of P. Gy, application to textural study of ores.

The textural description of a solid can be seen as a two stages job. The first one consists in determining the mineralogical composition and the size of the components. Information collected during this step will be named Granularity Parameter (GP). The second stage is a study of the spatial distribution of the components, called below Distribution Parameter (DP). The proposed method quantifies GP and DP by respectively constitution heterogeneity (HC) and distribution heterogeneity (HD). HC and HD are functions defined by P. Gy, 1982 and 1988 in his “Theory of particule material sampling”

Crystallisation in flow Part II: Modelling crystal growth kinetics controlled by boundary layer thickness.

International audienceAs in several other AMS studies, the main direction of the magnetic lineation analysed in Part I of this work, as well as crystal elongation, have been found to be roughly aligned with the direction of the surrounding flow. In order to explain the mechanisms responsible for such crystal shape anisotropy in a hydrodynamic context, we derive a mathematical model based on Falkner-Skan self-similar boundary layers at high Reynolds numbers. The model allows calculating local growth rates out of diffusion processes in the concentration boundary layer for crystal faces orientated arbitrarily in the range 90° to -18° with respect to the flow direction, and for any flow velocity. Hence, our work generalizes rationally previous attempts already done in the case of a flow parallel to the crystal face. This crystal growth model is applied to a natural case of calcite growth rate in 2D section perpendicular to the axis. The reconstructed calcite growth reproduces the texture of a natural case observed in Part I, although the local Reynolds numbers are quite low. This approach may be applied for various geological settings, from deep metasomatism to flowing on the earth surface

Textural characterization using P. Gy heterogeneity functions.

International audienceA solid can be regarded as a set of contiguous elementary units. The distribution within the solid of any properties, measurable within each elementary unit, can be characterized using two parameters. These parameters are built using the constitution and distribution heterogeneities of P. Gy (1982, 1988). The former account for the granularity of the elementary units, whereas the latter assess the spatial distribution of the property. A texture which definition involves several properties can be described using a diagram where both parameters work as variables. Potential applications encompass: (i) the textural classification of soils, ore, breccia and concrete and (ii) the monitoring of textural transformation during process like dolomitization, metamorphism, weathering, deformation or annealing

Anisotropie de susceptibilité magnétique (ASM) et chimie des traces : une nouvelle méthodologie pour démêler processus hydrothermaux et supergènes. Application au gisement à Ba-Fe-F de Chaillac (Indre, France)

International audienceAnisotropy of magnetic susceptibility (AMS) determinations and trace-element analyses have been firstly carried out, at Chaillac, on two ferruginous sandstone sections of hydrothermal and lateritic origins, respectively. The goethite has been identified as the main AMS carrier. Distinct AMS patterns and characteristic Ti-As anomalies have been observed, which allows to quantify the importance of hydrothermal and lateritic processes when both are superimposed. These preliminary results show that the 'Fe-Ba complex' of hydrothermal origin at Chaillac has been lateritized for a few meters from the present erosion surface

Anisotropie de la Susceptibilité Magnétique : Théorie et exemples d'application pour la mis en évidence de paléocirculations de fluides.

21-23 mai 2008L'Anisotropie de la Susceptibilité Magnétique (ASM) est utilisée depuis une trentaine d'années pour déterminer la fabrique des roches (Hrouda, 1982 ; Tarling et Hrouda, 1993). Elle est aussi largement utilisée lors de travaux de géologie structurale en particulier ceux relatifs à la détermination des structures des plutons granitiques (Talbot et al., 2005). L'objectif de cette communication est de montrer, à partir de deux études de cas, comment le couplage entre l'ASM et l'analyse texturale permet de retracer le(s) trajet(s) de circulation des paléo-fluides

Magnetic fabrics and fluid flow directions in hydrothermal systems. A case study in the Chaillac Ba-F-Fe deposit (France).

This study presents a possible use of anisotropy of magnetic susceptibility (AMS) to describe the mineralizing process in hydrothermal systems. Ba–F–Fe-rich deposits within the Chaillac Basin are on the southern border of the Paris Basin. In these deposits hydrothermal textures and tectonic structures have been described in veins, sinters, and sandstone cemented by hydrothermal goethite. 278 oriented cores from 24 sites have been collected in these formations. In addition, a lateritic duricrust superimposed on the hydrothermal formation has been sampled. Rock magnetic investigations show that the principal magnetic carrier is goethite for the hydrothermal mineralization and for the laterite level. The AMS measurements show distinguishable behaviors in the different mineralogical and geological contexts. The K1 magnetic lineation (maximum axis) is strongly inclined for the vertical veins. For the horizontally mineralized sinters, the magnetic lineation is almost horizontal with an azimuth similar to the sedimentary flow direction. The AMS of goethite-rich sandstone close to the veins shows strongly inclined K1 as they are probably influenced by the vertical veins; however, when the distance from the vein is larger than 1 m, the AMS presents rather horizontal K1 directions, parallel to the sedimentary flow. The laterite has a foliation dominance of AMS with vertically well-grouped K3 axes and scattered K1 and K2 axes. Field structural observations suggest that the ore deposit is mainly controlled by EW extension tectonics associated with NS trending normal faults. Combining the AMS results on the deposit with vein textures and field data a model is proposed in which AMS results are interpreted in terms of hydrothermal fluid flow. This work opens a new investigation field to constrain hydrodynamic models using the AMS method. Textural study combined with efficient AMS fabric measurements should be used for systematic investigation to trace flow direction in fissures and in sand porosity

Crystallisation in flow Part I: paleo-circulation track by texture analysis and magnetic fabrics.

International audienceIn order to better define the dependence of the mineral texture on flow, Anisotropy of Magnetic Susceptibility (AMS) and microscopic observations have been performed on calcite rich precipitations occurring in a horizontal pipe where the flow direction is clearly defined. Sixty-five cubes were cut from 5 slices, magnetic studies identified-pseudo-single domain magnetite as the major AMS carrier. Horizontal foliation characterizes the magnetic fabrics and the lineation is parallel to the pipe axis, i.e. the flow direction. The origin of this lineation is discussed and is interpreted to be mainly the consequence of elongated pseudo single domain magnetite. To complete the magnetic fabric studies, shape preferred orientation statistics were performed on 563 calcite sections in the (0001) calcite plane. It shows elongated shapes with a general orientation parallel to the pipe axis. The mean shape orientation is the average of two distinct sub-populations that deviated slightly from the pipe axis. Observation on calcite shapes and the direction of the magnetic lineation are coherent, suggesting that it is possible to track hydrothermal paleo-circulation using magnetic lineation and petrographic fabrics

Development of a Darcy- Brinkman model to simulate water flow and tracer transport in a heterogeneous karstic aquifer (Val d'Orléans, France)

International audienceModelling karstic aquifers is problematic because the equation of references (i.e. Darcy) is adapted to describe hydrodynamics of flow in a rock where porosity is low. The modelling of the karstic drains requires to use a macroscopic equation representative of the physics of flows occurring in rock where pores are important in size. To answer this question, our study presents an example of the hydrodynamic model adapted to the karstic aquifer of the Val d'Orléans starting from two equations characterizing the dynamics of the fluids: i) the Darcy law used to describe the hydraulic behaviour of massive limestone, and ii) the equation of Brinkman models the flows in vacuums of big sizes within the karstic drain. In the second objective, the flow equations are coupled with the transport equation to predict the karst properties. The results are tested by using six tracer tests carried out in the Val d'Orléans. The simulations show that the draining permeability ranges from 5 10-6 to 5.5 10-5 m2, the limestone permeability ranges from 8 10-11 to 6 10-10 m2. The dispersivity coefficients in the drains range from 23 to 43 m and in the fractured zone from 1 to 5 m

Internal structures and dating of non-sulphide Zn deposits 5 using rock magnetism: insights from the Moroccan High Atlas

International audienceThe renewal of interest in Zn-Pb non-sulphide ores has been induced by mineral processing improvement and leads to new exploration and mining projects in the world. Although the mineralogy is often precisely known, and despite several studies linking ore deposition to regional tectonics, absolute dating of non-sulphide stages is rare and structure of ore bodies was largely disregarded. Geochronological data from non-sulphide ores are essential to timely constrain alteration episodes and to insert supergene ore genesis in the climate and tectonic evolution of the metallogenic province. The access to internal organization of ore could reveal post-mineralization episodes related to supergene evolution. Thus, a rock magnetism study combining anisotropy of magnetic susceptibility (AMS) and palaeomagnetism was performed on four non-sulphide deposits from the Moroccan High Atlas. AMS generally shows similar horizontal magnetic fabrics for ores and the clayey and carbonaceous internal sediments filling karstic cavities. The palaeomagnetic directions of ores and internal sediments are compatible, and the calculated poles are consistent with the last 30 Ma of the Africa apparent polar wander path, with an upper age at 0.78 Ma. The proposed three-step scenario is placed within the evolution of the Moroccan High Atlas belt. Deposition of primary sulphides is contemporaneous with opening of the Tethyan and Atlantic oceans. During the Tertiary, intracontinental deformation gave rise to the High Atlas fold-and-thrust belt and to regional uplift. Finally, Zn-Pb sulphides hosted in carbonates experienced oxidation under an arid climate to form karst-related Zn-Pb non-sulphide ores. These promising results pave the way for an efficient method to constrain the internal fabrics and age of Zn supergene deposits

Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS): An example from Aptian carbonates of La Florida (Northern Spain)

International audienceThe present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn-Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn-Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K2. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterised by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ‹c› axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ‹c› axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE-SW elongation interpreted as the global circulation direction and a NW-SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterised by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric