Les images satellitaires Radar. Utilisation en cartographie géologique et exploration minière (Guyane, Côte-d'Ivoire).

International audienceThe aim of the AURROS project conslsted in demonstrating the use of satellite radar data on sites representative of the problems encountered in geological and mineral exploralion. The objective is to improve characterisation methods for target zones, parlicularly in the early stages of the process leading to mining permit application. The dalabase integrales ERS (l and 2) data, RADARSAT (broad-swath Seansar mode and fine bi-angular viewing made) data, DEMs acquired by various means, and geophysical dala (airborne magnelics, spectral radiomelry). Various me/hods (fil/ering, geocoding and stereoscopy) have heen integrated or developed by BRGM to create differenl map types. These dala are analysed at various scales and validated using hvo approaches, namely checking in Ihe field and fusion wilh airhome geophysical data (spectral radiometry and aeromagnetics). The first results showed the radar to be a powerful mapping and deposit-prediction tool, and notably revealed that 1) the raw data con be used directly by geologists either for standard geological studies or as an exploration aid, and 2) fusion with geophysical data enables the identification of deformation structures where geophysics alone only reveals large masses, and rapidly and efficiently increases the possibilities of lithological discrimination. Correct use of the data nevertheless involves taking into consideration the scales used and currenl restrictions concerning the most suitable study environments. The development possibilities associated with radar stereoscopy constitute a major progress factor for the near future

Pre-mineralization thermal evolution of the Palaeoproterozoic gold-rich Ashanti belt, Ghana

International audienceThe region of the gold-rich Ashanti belt in southern Ghana was chosen as the subject for a detailed regional thermal modelling study. Geological studies, in addition to laboratory measurements of thermal properties and heat-production rates, allow us to constrain a finite-element thermal modelling. Scenarios integrating variations of the structure of the crust and various chronological settings were examined. We calculated the thermal regime before and after the thrust tectonism that affected the region during the Eburnean orogeny (2130 to 2095 Ma), just before ore deposit formation. This gives a new insight into the regional thermal state of the crust before the mineralizing events. To satisfy the thermobarometric observations, the most probable mantle heat flow must be 60 mW.m-2, which is at least three times greater than the present-day value. At shallow depths, our results also indicate anomalies of lateral heat flow reaching 25 mW.m-2, focused on the margins of each lithological unit, including the Ashanti belt. These anomalies are related to the distortion of the isotherms in the first few kilometres that can be mostly explained by lateral contrasts in thermal conductivity. Such anomalies could be of importance for the mineralizing events, as they would favour fluid circulation locally

La mine médiévale d'argent du Pontet, une fente alpine contemporaine de la surrection des massifs cristallins ?

International audienceThis study of the Pontet mine aimed at updating the understanding of ore-structural geology setting and, in terms of archaeology, at pointing out how the geometry of structures influenced the medieval workings and the shape of the galleries. The mineralisation and the WNW–ESE dextral normal faults controlling its emplacement are assigned to the Eocene by analogy with the neighbouring La Gardette structure. This confirms the Late Eocene to Oligocene age classically proposed for Phase 1 and places it in the Eocene. The Prégentil-type P1 folds described by Lameyre (1958) are here attributed to Phase 1, which raises the question as to whether the contemporaneous tectonic uplift of the crystalline basement should also be assigned to Phase 1 rather than to Phase 2Présenté par Zdenek Johan Résumé La mine du Pontet a été étudiée dans le but d'actualiser la connaissance gîtologique et, en termes d'archéologie, de préciser l'influence de la géométrie des structures sur la conduite des travaux médiévaux et la forme des chantiers. L'étude structurale permet de replacer la minéralisation dans l'évolution tectonique. La minéralisation et les failles WNW-ESE dextre-normales contrôlant sa mise en place sont rapportées à l'Éocène par analogie avec le faisceau voisin de La Gardette. Ceci confirme l'âge Éocène supérieur à Oligocène classiquement proposé pour la phase 1 et amène à la rapporter plutôt à l'Éocène. Les plis P1 de type Prégentil, décrits par Lameyre (1958), sont attribués ici à la phase 1 ; aussi la surrection tectonique du socle cristallin, dont ils sont contemporains, est-elle à rapporter à cette phase et non à la phase 2

Reaction textures in Proterozoic calcsilicates from northern Guinea: a record of the fluid evolution

International audienceProterozoic calcsilicate rocks in contact with the different types of granite from the granitic belt of northern Guinea show particular mineral assemblages, recording different steps of the tectono-metamorphic and magmatic evolution of the area. Petrological study provides evidence of a clockwise metamorphic P–T path with a metamorphic peak at temperature around 800 °C and pressure of 4–6 kb, corresponding to the emplacement of both generations of massive granite between 2115 and 2075 Ma. Retrograde metamorphism is characterized by decompression to 2–3 kb, associated with the emplacement of late small granite stocks and followed by cooling until 450–600 °C. Hydrothermal alteration involved by late fluid circulation is only weakly developed and limited to calcsilicate/granite contact (specially small stocks) and shear zones. Early fluids were essentially metamorphic and magmatic fluids, largely buffered by calcsilicate mineral assemblages, whereas surface-derived fluids were introduced at the end of the tectonic evolution via shear-zones