Manganiferous phyllosilicate assemblages: occurrences, compositions and phase relations in metamorphosed Mn deposits

Spessartine Garnets in a Manganiferous Carbonate Formation from Nsuta, Ghana

“…It can be assumed, therefore, that the manganese deposits in the Guiana Shield may have been subjected to a higher grade of metamorphism than the Nsuta deposit. Further, it is noted that garnets in lower greenschist facies manganese deposits from Kodnitzal, Austria, similar in metamorphic grade to the Nsuta deposit, are also enriched in the spessartine endmember (Abrecht, 1989). These comparisons suggest that the composition of garnet is affected by metamorphic grade, a fact which may, in turn, support the inference given above that metamorphic conditions at Nsuta may have been such that garnet appeared first in favourable rocks with restrictively high spessartine-rich compositions.…”

“…It could be inferred from these observations that restrictions imposed by local chemical composition may have resulted in the formation of spessartine garnet or chlorite, but not both, in rocks containing these two minerals. In contrast to petrologic evidence for garnet formation from chlorite and quartz in some manganiferous formations (Brown, 1969;Abrecht, 1989), chlorite may not have participated in garnet forming reactions at Nsuta but was possibly produced instead of spessartine garnet in manganese carbonate-poor and magnesium-rich environments during metamorphic reactions. Silica-poor compositions may have been favourable for chlorite formation in high grade manganese carbonate ores.…”

“…8). Our data are in fair 20 RESOURCE GEOLOGY: F. K. Nyame, K. Kase and M. Yamamoto (Holtrop, 1965); open squares: Mn deposit, Nsuta, Ghana (Kleinschrot et al, 1994); open circles: gondites and Mn-rich rocks, northern Ghana (Melcher, 1995); stippled: Mn deposits, Kodnitzal, Austria (Abrecht, 1989). Abbreviations as in Fig.…”

“…In many of these spessartine garnet occurrences, reported manganese carbonate-silicate assemblages are thought to have been derived from reactions involving manganese carbonates admixed with clay and silica during regional metamorphism (Roy, 1981). In some cases, Mn-containing silicates have themselves been suggested as prime candidates for spessartine garnet formation (Hsu, 1968;Abrecht, 1989).…”

“…Garnets in metamorphosed manganese bearing assemblages commonly contain spessartine as a principal component in solid solution. The mineral, however, exhibits wide variation in chemical composition in many manganese deposits (Holtrop, 1965;Buhn et al, 1995), even though fairly pure spessartines have been reported (Abrecht, 1989;Theye et al, 1996). It can be gleaned from these literature, among others, that in weakly metamorphosed manganese deposits spessartine contents of garnets are usually high.…”