Ouille du Midi, Bonneval-sur-Arc, Saint-Jean-de-Maurienne, Savoie, Auvergne-Rhône-Alpes, Francei

Regional Level Types
Ouille du Midi	Mountain
Bonneval-sur-Arc	Commune
Saint-Jean-de-Maurienne	Arrondissement
Savoie	Department
Auvergne-Rhône-Alpes	Region
France	Country

Medium-grade metamorphic Mn mineralisations on the slope of Ouille du Midi.

This locality belongs to a small group of Mn mineralisations occurring in the upper Maurienne Valley (Chopin, 1978) [see also Le Coin, Bessans]. Manganese minerals occur in phengite quartzites interbedded in calcschists.

This Mesozoic sedimentary complex and the associated ophiolite assemblage, here mainly consisting of serpentinite, have undergone HP-LT metamorphism during Upper Cretaceous-Eocene.

The quartzite layers and lenses, containing spessartine, braunite, piemontite, ardennite/ardennite-(As), and phyllosilicates, are crosscut by veilets of quartz, albite, piemontite, talc, hematite, and calcite. These quartzites contain lenses/boudins of pink and brown colour: in the former abundant rhodochrosite is associated with pyroxmangite, rhodonite, spessartine, tephroite, and often "tirodite" and quartz; in the latter sonolite, spessartine, tephroite, alleghanyite, and friedelite are dominant minerals. Rarely the lenses described above are associated with brown-black and brown-orange masses, consisting of jacobsite + kutnohorite (or manganoan calcite) and galaxite + alleghanyite + sonolite + chlorite, respectively. Apatite, pyrophanite, siegenite, pentlandite, millerite, chalcocite, bornite, ilmenite, rutile, titanite, and dravite are accessory minerals; neotocite, possible kellyite, and digenite are secondary phases (Chopin, 1978).

The presence of calderite has been determined in more recent times (Chopin & Cenki-Tok, 2006). These authors observed the coexistence of a colourless and a yellow garnet in oxidised metachert (quartzite) with hematite and braunite (+ minor Mn-pyroxenoid and "tirodite"; with ardennite-(As) or piemontite in distinct layers), and in more reduced, carbonate-rich boudins included in it. The co-occurrence takes a variety of textural aspects, from coexisting euhedral garnets (10–100 µm in size for the calderite to mm-size for spessartine) to sharp overgrowths of yellow calderitic garnet on colourless spessartine, to yellow cauliflower-like masses (a few hundreds of µm in size) overgrowing colourless spessartine and showing evidence of oscillatory zoning, resorption stages and resumed growth. Sector zoning and anisotropy are common, although not consistent features.

Other Regions, Features and Areas containing this locality

References