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

Idioblastic spessartine garnet pervasively developed in Mn-rich rocks and i mpure manganese carbonate ore at the Lower Proterozoic Nsuta manganese depo sit, Western Ghana, contains abundant inclusions of micritic and microconcr etionary carbonates and, to a lesser extent, quartz. Detailed mineralogical and microprobe studies indicate all the carbonate phases (i.e. carbonate i nclusions in garnet, carbonates coexisting with garnet and carbonates not d irectly in contact with garnet, the latter hereafter referred to as matrix carbonates) lie within the rhodochrosite-kutnahorite solid solution series, i.e. similar to Mn55-80(Ca + Mg)(20-45)CO3 to Ca-42(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 n ature, composition and textural relationship of coexisting minerals and inc lusions 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 rhodochrosi te precursor for spessartine. Spessartine formation is thus envisaged to ha ve taken place when the predominantly Mn carbonate-quartz assemblage became unstable in the presence of minor amounts of an unknown aluminous phase. B ecause all the carbonates appear to be low-temperature phases with no indic ations of significant recrystallisation or homogenisation, it could be argu ed that the spessartine + rhodochrosite +/- kutnahorite +/- quartz +/- pyri te assemblage stabilised during very low-grade greenschist facies metamorph ism under relatively low but uniform f(O2) conditions. These observations a lso suggest the stability field of spessartine could extend to relatively l ower temperatures than currently envisaged.