Les paragenèses réduites ou oxydées de concentrations manganésifères des «schistes lustrés» de Haute-Maurienne (Alpes françaises)

Abstract

A phengite quartzite interbedded in calcschists overlying an ultramafic body near Bonneval-sur-Arc (Savoie) contains Mn-deposits having undergone HP-LT metamorphism and displaying following assemblages : — Rhodochrosite - pyroxmangite - rhodonite - tirodite - spessartine - tephroite - sonolite - alleghanyite - friedelite. — Kutnahorite - alleghanyite - sonolite - galaxite - jacobsite - Mn-thuringite. Apatite, pyrophanite, siegenite, pentlandite, millerite, chalcocite, bornite are there accessory minerals ; neotocite, kellyite (?) and digenite secondary ones. — Quartz - piemontite - ardennite - spessartine - braunite - hematite - phyllosilicates - richterite. The juxtaposition of these oxidized and reduced assemblages dates back to the sedimentary origin of the rocks. This implies very low oxygen mobility during metamorphism, which is evidenced through the whole metamorphic pile. Pyroxmangite in equilibrium with rhodonite is abnormally Ca-rich. The composition of Mn-humites is shown to be stoichiometric and Mn(OH, F)2 ⇄ TiO2 to be the most important substitution involved in them. Maximum Al-substitution in piemontite is shown not to be dependent from the physical conditions but essentially from the chemical parameters. An attempt is made to delineate the galaxite-jacobsite solvus. The noticeable Ni and Co content of the phyllosilicates and the highly magnesian chemistry of the most oxidized assemblages are shown to be mere consequences of the high fO2. We discuss the chemistry of the enclosing rocks and the possible «oceanic » origin of these deposits which are equivalent to those described in Piémont (e. g. Saint-Marcel).