MANGANESE-RICH ASSEMBLAGES IN THE BARRHORN UNIT, TURTMANNTAL, CENTRAL ALPS, SWITZERLAND

Abstract

The Triassic carbonates of the Briançonnais Domain in Turtmanntal, Western Swiss Alps, contain lenses up to 3 m across filled with Mn- and Fe-rich sediments. These rocks display a fine compositional banding at a millimetric scale, and consist of alternating carbonate layers (kutnohorite, dolomite, rhodochrosite and calcite), oxide layers (jacobsite; reduced horizons with manganosite ± hausmannite; Fe-rich zones with magnetite and hematite) and silicate layers (spessartine, manganocummingtonite, ± tephroite, fluorapatite). Minerals of vanadium, including pyrobelonite, turtmannnite, reppaite and sarkinite occor in the main schistosity and in discordant veinlets. Other accessory minerals include crednerite, pennantite, barite, pyrite, chalcopyrite, bornite(?), native copper and algodonite. Quaternary weathering resulted in the formation of birnessite, takanelite, nsutite-ramsdellite, hollandite, manganite, pyrolusite, cuprite and malachite. The Pipji Mn-rich sediments were originally interpreted as residual sediments deposited in karstic cavities during Middle Jurassic. However, a hydrothermal input is indicated by the low REE contents and MREE enrichment of the hydrothermal end-member composition relative to PAAS, a large range in V:As ratios, and low U and Th contents. Low silica contents suggest a low-temperature hydrothermal fluid (≤60°C). The Pipji occurrence hence is related to Mesozoic exhalative Mn-rich sediments from the Briançonnais, Helvetic and Piemont domains of the Alps. Comparison of the Pipji Mn-rich rocks with Alpine occurrences of exhalative, residual and hydrogenous origins shows that discrimination diagrams based upon transition metals are useful for interpreting the petrology of Mn-rich rocks within particular depositional environments, but cannot be generalized to different settings. The Pipji Mn-rich lenses developed their present mineralogy during Tertiary metamorphism under upper greenschist-facies conditions. They are characterized by relatively reduced parageneses, including the manganosite-jacobsite association. This association is stable under conditions for which CO2 and CH4 coexist in the gas phase, suggesting that the redox state of the Mn-rich mineral assemblage during metamorphism was buffered by the granite-bearing dolomitic marble hosting the lenses.