Original paper
Alabandite (MnS) in metamorphosed manganiferous rocks at Morro da Mina, Brazil: palaeoenvironmental significance
Cabral, Alexandre Raphael; Zeh, Armin; Viana, Nívea Cristina da Silva; de Castro, Marco Paulo; Laufek, František; Lehmann, Bernd; Queiroga, Gláucia
European Journal of Mineralogy Volume 31 Number 5-6 (2019), p. 973 - 982
56 references
published: Dec 20, 2019
published online: Sep 17, 2019
manuscript accepted: Apr 12, 2019
manuscript revision received: Apr 4, 2019
manuscript received: Dec 6, 2018
Abstract
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 manganese-oxide-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.
Keywords
alabandite • graphite • molybdenite • palaeoenvironment • redox conditions • manganese ore