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 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

Modèle de formation du gisement d'argent d'Imiter (Anti-Atlas oriental, Maroc). Nouveaux apports de l'analyse structurale et minéralogique. Model of formation of the Imiter silver deposit (eastern Anti-Atlas, Morocco). New structural and mineralogical constraints .

Based on a combined geometrical and mineralogical analysis, a three-stage model of formation of the mineralized veins of the giant Imiter silver deposit (Anti-Atlas, Morocco) is herein proposed. A first episode is characterized by the development of quartz, pink dolomite and Ag-rich minerals veins formed during a dextral transpressive event. The second episode is associated with a normal left-lateral motion that re-opens previous structures, filled by pink dolomite gangue. Alteration stages contribute to a local Ag enrichment. Basé sur l'étude de nouvelles données géométriques et minéralogiques, un modèle de formation en trois stades des veines du gisement d'argent d'Imiter (Anti-Atlas oriental, Maroc) est proposé. Le premier stade est caractérisé par le développement des veines les plus riches, à remplissage de quartz puis dolomite rose, dans un contexte transpressif dextre associé à une direction de raccourcissement NW–SE à WNW–ESE. Le second stade, à remplissage de dolomite rose, est associé à un cisaillement senestre normal, contrôlé par une direction de raccourcissement nord–sud, réutilisant certaines des structures du stade 1. Des phénomènes d'altération contribuent à un enrichissement local en Ag

Significance of a combined approach for replacement stones in the heritage buildings' conservation frame

Stone substitution is a conventional operation during heritage buildings' restoration, but becomes problematic for architects and restorers when the quarry is mined out. The compatibility of the substitution stones with the original ones has been for long mainly based on the aesthetical aspect, this resulting too often in a patchwork of original and substitution stones with different patina after several years because of differences of properties. In this study, the objective is to show how substitution stones can be selected by combining aesthetic criteria and stones properties that are relevant for analyzing their compatibility. A couple of French limestones with their potential substitution stones were selected for the study. Our results showed that potential substitution stones selected on their aesthetic criteria require to be rejected because of their differences of physical properties. On the other hand, our results showed also the possibility to select substitution stones with satisfactory aesthetic aspect and properties that enable to expect a satisfactory compatibility with the original stone

Zinc-rich clays in supergene non-sulfide zinc deposits

International audienceThe nature and the origin of zinc clays are poorly understood. With the example of the Bou Arhous Zn-Pb ore deposit in the Moroccan High Atlas, this study presents new data for the mineralogical and chemical characterization of barren and zinc clays associated with non-sulfide zinc ores. In the field, white to ocher granular clays are associated with willemite (Zn2SiO4), while red clays fill karst-related cavities cutting across the non-sulfide ore bodies. Red clays (kaolinite, chlorite, illite, and smectite) present evidence of stratification that reflects internal sedimentation processes during the karst evolution. White clays contain 7-Å clay mineral/smectite irregular interstratified minerals with less than 20 % of smectite layers. Willemite is partially dissolved and is surrounded by authigenic zinc clay minerals. Together with XRD results, WDS analyses on newly formed clay aggregates suggest that this interstratified mineral is composed of fraipontite and sauconite. CEC measurements support that zinc is only located within the octahedral sheets. These new results support the following process: (i) dissolution of willemite, leading to release of Si and Zn, (ii) interaction between Zn-Si-rich solutions and residual-detrital clays, and (iii) dissolution of kaolinite and formation of interstratified zinc clay minerals that grew over detrital micas

"Memory loss" during mineral processing: Application to base metals traceability

International audienceTraceability of concentrates is required to introduce transparency in the trade of raw minerals. In this context traceability may be considered as a kind of inversion process: studying the product sold (i.e. the concentrate) in order to identify the original ore, in terms of ore deposit-type and if possible, location. The difficulty of making this inversion from concentrate toward bulk ore corresponds to the "memory loss" of the crude ore which occurs during mineral processing. Based on textural characterization and the chemical composition of the material at different steps of processing, as well as the minimum residence corresponding to each step, an estimation of this "memory loss" is proposed and the relations between memory loss and global kinetic rate of flotation are established. "Memory loss" calculations are applied to the Neves Corvo plant. Throughout the process, the parameter of memory loss increases respectively from 0 to 195.06 for Cu; 0 to 46.15 for Zn and 0 to 0.43 for Fe. The "global memory loss", namely as the "experimental memory loss". For the Neves Corvo plant at the moment of the study this "experimental memory loss" was 14,146 min for Cu, 3408 min for Zn and 36 min for Fe. The results show that "memory loss" is greater for Cu than for Zn, thus emphasizing the importance of secondary elements for traceability purposes

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

Rare earth elements as proxies of supergene alteration processes from the giant Imiter silver deposit (Morocco)

International audienceThe giant Imiter silver mine located at the northern edge of the West African craton in Morocco is assumed to be a late Neoproterozoic epithermal deposit mainly characterized by a hypogene paragenesis of Agrich sulphides and sulfosalts, and Ag-Hg alloys occuring preferentially in quartz-rich veins. The secondary enrichment zone at Imiter reaches a thickness of 50 to 150 m below ground surface. The upper levels, famous because of giant native silver crystals, grade up to 300 kg/t. Metallographic observations, SEM-EDS and XRD analyses reveal the presence of a quite complex secondary paragenesis made of acanthite, cinnabar, imiterite, perroudite, cerussite, mimetite, iron oxyhydroxides, synchisite and coronadite. Supergene alteration processes of the giant Imiter silver mine deposit consist of the remobilisation of the primary hypogene paragenesis by (i) deep and old basinal brines and (ii) downward infiltrations of surficial waters becoming progressively more reduced and F-enriched in response to fluid-rock interactions. Development of such a supergene mineralization strongly suggests prevalence of arid to semiarid conditions