Genesis and 40Ar/39Ar dating of K-Mn oxides from the Stavelot Massif (Ardenne, Belgium): Insights into Oligocene to Pliocene weathering periods in Western Europe

The occurrence of Ordovician Fe-Mn-rich sediments (Ottré Formation, Salm Group) in the Stavelot inlier (NE Ardenne, Rhenish Massif, Belgium) has drawn the attention of geoscientists for more than one century, as they contain natural “coticule” whetstones and numerous weathered Mn deposits. Two locations expose the Fe-Mn-bearing rocks: the Chevron area to the north is located at an altitude of 230–380 m, and the Salmchâteau at 480–570 m. The Mn-bearing sediments have undergone greenschist facies metamorphism in relation to the Caledonian and Variscan orogenesis, and more recently weathering processes, transforming the early mineral assemblages (mainly rhodochrosite and/or kutnohorite, spessartine, muscovite, chloritoïd, andalusite, hematite) into secondary Mn oxides,—hydroxides and—oxyhydroxides (cryptomelane, nsutite, lithiophorite, manganite, pyrolusite), clays (mainly kaolinite) and Fe oxides (goethite). In this paper, we investigate the weathering processes of these Mn-rich rocks by i) new mineralogical, petrographical and geochemical data, and ii) new 40Ar/39Ar geochronology of K-Mn oxides. Petrographical and geochemical data indicate that rhodochrosite (and/or kutnohorite) is the main source of Mn, from which Mn oxides have precipitated. Spessartine and other Mn-silicates have poorly contributed to the Mn mobilization. Pure Mn oxides, namely manganite, nsutite, and pyrolusite, successively replace rhodochrosite under increasing O2 conditions due to the weathering fluids, while the formation of cryptomelane and then lithiophorite in veins is associated to the recombination of K+, Li+ and Al3+ released from the phyllosilicate matrix of the host slate, probably in relation with sligh pH changes. This multistage character of weathering processes is well identified in the Salmchâteau area, where weathering conditions are stronger than in the Chevron area because of the elevation. This also positions the Mn levels close to the roll front within the lower saprolite and the fissured horizon, respectivelly. Such differences in the weathering grade of these areas determined a weathering-type deposit in the Salmchâteau area and a carbonate-hosted sedimentary deposit in the Chevron area. Weathering deposits are accompanied by an enrichment in Mn and associated transition metals (As, Co, Cu, Ni). Dating of pure cryptomelane samples corroborate the multistage character of the ore and yield at least four weathering stages: Chattian (26.33 ± 0.17 Ma), Aquitanian (22.2 ± 0.6 and 20.0 ± 0.2 Ma), Serravalian-Tortonian (11.79 ± 0.10 to 10.12 ± 0.18 Ma) and Pliocene or younger (5.30 ± 0.76 and 4.11 ± 0.57 Ma). A relatively continuous weathering of the Massif since the late Oligocene, or older weathering phases, cannot be excluded as the upper part of the saprolite (and laterite) have been removed. As climatic conditions evolve from tropical to temperate by the Oligocene, the uplift of the Ardenne is probably the main factor triggering the development of weathering mantle, together with the increasing seasonality and high precipitation. These ages with others obtained in supergene ores of Western European massifs define a long-lasting weathering wave since the late Oligocene until the uppermost Neogene. The absence of older periods in the 40Ar/39Ar record of these deposits indicates that the upper part of the weathering mantle was recently stripped and limit their economic interest.