Alabandite (MnS) in metamorphosed manganiferous rocks at Morro da Mina, Brazil: palaeoenvironmental significance

The mineral alabandite, cubic MnS with variable amounts of FeS, occurs in a number of mineral assemblages in manganiferous metasedimentary rocks, some of which are mined as manganese ore. One of them is Morro da Mina, Mina Gerais, Brazil, where alabandite is a minor component of queluzite, a manganese-silicate-carbonate rock. The Morro da Mina alabandite contains 7.3 +/- 1.4 mol% FeS and makes up aggregates with graphite and molybdenite. A comparison of alabandite occurrences worldwide indicates four main types of mineral assemblages: (1) alabandite + graphite; (2) alabandite + graphite + molybdenite; (3) alabandite + rhodochrosite + molybdenite; (4) and alabandite + rhodochrosite. The different assemblages suggest that manganese sulfide formed in a range of marine palaeoenvironments, from highly reducing and sulfidic - i.e., euxinic - settings to manganeseoxide-precipitating conditions. Morro da Mina represents assemblage (2), which is characteristic of a euxinic water column in a stratified ocean. In general, our comparison of alabandite-bearing mineral assemblages indicates that palaeoenvironmental information can be retrieved from sedimentary rocks that experienced variable degrees of metamorphic overprint.