Caractérisation minérale et datation des surfaces d’altération d’Europe de l’Ouest/
Mineral characterization and dating of weathering surfaces in Western Europe
Johan YANS – UNamur, Université de Namur, Belgique
Julien BAPTISTE – BRGM, Orléans, France
We have investigated four Mn districts on the margins of the Central Massif in order to (i) identify the composition and formation process of the Mn ores and (ii) perform 40Ar/39Ar dating on K-Mn oxides (in collaboration with Geosciences-Rennes). At this stage, two deposits have a supergene zone, while the other two probably follow a mixed epithermal-supergene model.
The long-term evolution of the continent is often less predictable than in sedimentary basins as remnants are scattered residuals rocks. So, the age of weathering is particularly indicative of the timing of transition of sediments between the source and the basin. The presence of K- bearing Mn oxides offer an incommensurable way to date (K-Ar and 40Ar/39Ar methods) and unravel the weathering timing of a deposit area. Therefore, establishing the pathway of their formation in Mn ores through their mineralogy, petrography and geochemistry is of primary order to identify (i) the type of deposit, (ii) the position of these minerals in the paragenetic sequence and (iii) the mineralization process responsible for the concentration of K-Mn oxides.
We have investigated four Mn deposits on the margins of the French Central Massif: (1) “Nontronais” (Haute-Dordogne; Mn-Fe; south-west), (2) Villerembert, La Matte and Vieussan (Montagne Noire; Mn; south); (3) Auxilhac (Lozère; Mn-Fe; south), and (4) Romanèche (Mn-F- Ba; east). The first two display a well-developed weathering zone, whereas the other two probably have a mixed origin between epithermal (upward) and supergene (downward) fluids. However, in each deposit, the K-bearing Mn oxides always crystallize in the late stages of the paragenetic sequence. Their age would then reflect the latest (?) meteoric fluid circulation, allowing us to better constrain the weathering events in the studied area.
Genetic types of the Mn deposits investigated in the project.