Petrological significance of manganese carbonate inclusions in spessartine garnet and relation to the stability of spessartine in metamorphosed manganese-rich rocks

摘要

Idioblastic spessartine garnet pervasively developed in Mn-rich rocks and impure manganese carbonate ore at the Lower Proterozoic Nsuta manganese deposit, Western Ghana, contains abundant inclusions of micritic and microconcretionary carbonates and, to a lesser extent, quartz. Detailed mineralogical and microprobe studies indicate all the carbonate phases (i.e. carbonate inclusions in garnet, carbonates coexisting with garnet and carbonates not directly in contact with garnet, the latter hereafter referred to as matrix carbonates) lie within the rhodochrosite–kutnahorite solid solution series, i.e. ~Mn55–80(Ca + Mg)20–45CO3 to Ca42(Mn + Mg)58(CO3)2. Minor compositional differences occur in the various carbonate phases, but partition of major elements among coexisting phases indicate most carbonate minerals strongly fractionate Ca and Mg over coexisting spessartine. The nature, composition and textural relationship of coexisting minerals and inclusions in porphyroblastic spessartine indicate that the latter formed from metamorphic reactions in which rhodochrosite and/or kutnahorite and quartz were consumed, in part corroborating earlier observations on a rhodochrosite precursor for spessartine. Spessartine formation is thus envisaged to have taken place when the predominantly Mn carbonate–quartz assemblage became unstable in the presence of minor amounts of an unknown aluminous phase. Because all the carbonates appear to be low-temperature phases with no indications of significant recrystallisation or homogenisation, it could be argued that the spessartine + rhodochrosite ± kutnahorite ± quartz ± pyrite assemblage stabilised during very low-grade greenschist facies metamorphism under relatively low but uniform f O2 conditions. These observations also suggest the stability field of spessartine could extend to relatively lower temperatures than currently envisaged.