Genesis of chemical sediments in Birimian greenstone belts: evidence from gondites and related manganese-bearing rocks from Northern Ghana

Abstract

Early Proterozoic chemical sediments of the Birimian Supergroup in northern Ghana host several types of metamorphosed manganese-bearing rocks. Differences in the mineralogy and geochemistry can be attributed to facies changes in a mixed volcanic-volcaniclastic depositional environment. Manganese oxide-bearing phyllite, which is enriched in transition metals (Cu, Ni, Co, Zn), formed on the flanks of submarine volcanic edifices above an oxidation boundary. Towards the deeper basin, manganese silicate-rich gondites occur. These consist either of spessartine + quartz + ilmenite, or of spessartine + quartz + Mn amphiboles __+ rhodonite __+ hyalophane __+ Mn stilpnomelane -t-ilmenite. The Mn amphiboles are identified as manganoan actinolite, tirodite, and dannemorite. Sulphides are widespread as premetamorphic inclusions in Mn garnet grains. In the basin centre, chlorite schist containing garnet with 50-60 mol.% spessartine represents the most distal manganese-bearing rock which is highly diluted by volcanogenic background sedimentation. The origin of Mn-rich rocks is explained by heat-driven seawater convection systems active in submarine volcanic centres that provided hydrothermal solutions. Precipitation of different minerals depended on the geochemical conditions prevailing on the seafloor. During the Eburnean event (around 2000 Ma), the chemical sediments were metamorphosed to upper greenschist or lower amphibolite facies. Mineral assemblages in the gondites point to metamorphic conditions in the range of 450-500~ at 2-3 kbar.

Heavy minerals from sedimentary rocks of the Malcov Formation and their palaeogeographic implications for evolution of the Magura Basin (Western Carpathians, Slovakia) during the Late Eocene – Late Oligocene

“…Spessartines with low amounts of grossular content are known to orig i nate from low-grade re gion ally meta morphosed rocks such as metapelites, metacherts, es pe cially those in ther mal au re oles (Miyashiro, 1955;Deer et al, 1982;Spišiak et al, 1989;Méres, 2008), or from Mn car bon ate-sil icate rocks (Matkovskyi, 1971;Deer et al, 1992;Mohapatra and Nayak, 2005;Matkovskyi et al, 2011;Kanungo et al, 2014), more over, from garnetites (of ten with high an dra dite com ponent, Kropáè, 2012) and gondites (Melcher, 1995;Matkovskyi et al, 2011;Vrána, 2011). They have rarely been found in granite pegmatites (Man ning, 1983;Baldwin and Von Knorring, 1983;Királi and Török, 2003), and in blueschists, with an absence of zon ing (Martínek and Štolfová, 2009).…”

Spessartine Garnets in a Manganiferous Carbonate Formation from Nsuta, Ghana

“…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.…”

“…agreement with data of Kleinschrot et al (1994) for the same locality, although their data do not contain garnets of more than 90% spessartine. Spessartine garnets from lithologically similar Birimian formations in northern Ghana have a wider range in composition than garnets in this study (Melcher, 1995). According to Melcher (1995), the host rocks were subjected to upper greenschist or lower amphibolite facies metamorphism, with rhodonite and Mn-amphiboles occurring in garnet-containing assemblages or "multimineral gondite".…”

“…Spessartine garnets from lithologically similar Birimian formations in northern Ghana have a wider range in composition than garnets in this study (Melcher, 1995). According to Melcher (1995), the host rocks were subjected to upper greenschist or lower amphibolite facies metamorphism, with rhodonite and Mn-amphiboles occurring in garnet-containing assemblages or "multimineral gondite". Garnets from manganese deposits in the Guiana Shield exhibit a much wider compositional range, although the analyses were done by conventional wet chemical method (Holtrop, 1965).…”

The mineralogy and petrography of the ferruginous manganese rocks at Mankwadzi, Ghana

“…Hydrothermal fluids from submarine volcanic activity, such as suggested by Kimberley (1989) for the exhalative formation of BIF, may be responsible for the formation of exhalative type manganese deposits. In such situations, Mn is discharged onto the sea floor by submarine volcanism from where it is either abstracted by seawater and circulated by heat-driven seawater convection systems (Melcher, 1995) or taken into the sediment pile by diagenetic pore fluids to be precipitated during metamorphism.…”