Le Spèctre des Terres Rares de la Fluorine en Tant Que Marqueur des Propriétés du Milieu de Dépôt et des Interactions Entre Solutions Minéralisantes et Roches Sources. Exemple Pris dans le District de la Marche Occidentale (France)

Partitioning of lanthanides between solution and fluorspar is characterised by high partition coefficients in favor of fluorite, the coefficients growing slightly from La to Lu. On the example of the fluorite's district of the Western Marche France, we show how this property may be used as a geochemical criterion, to study the genesis of fluorspar. We find that two geochemical systems with different compositions and physicochemical properties have coexisted in the district. The first system deposited Chatenet and Charbonnière fluorite, the other one Rossignol fluorite. The chondrite normalised R. E. E. distribution of Rossignol fluorite shows a positive anomaly in europium, the origin of which is discussed. We propose that this anomaly (as well as the pattern of the various R. E. E. distributions analysed) are inherited from surrounding rocks by alteration processes rather than being the result of oxydoreduction phenomena. We suggest that, in general, coexistence of barite and of over-abundant europium fluorite is due to alteration of feldspars (or feldspar-rich rocks) which provides a possible source of barium.