『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
± ilmenite. The Mn amphiboles are identified as manganoan actinolite,
tirodite, and dannemorite. Sulphides are widespread as premetamorphic
inclusions in Mn garnet frains. In the basin centre, chlorite
schist containing garnet with 50-60 mo.% 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 2000Ma), 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゜C at 2-3 kbar.
Keywords: gondite, manganese phyllite, hydrothermal activity,
Nangodi belt, Lawra belt, Ghana, Birimian Superfroup』
Introduction
Geologic setting
Occurrence of manganese-bearing rocks
Mineralogy of manganese-bearing rocks
Manganese oxide-bearing phyllites
Gondites
Garnet-bearing chlorite schists
Geochemistry of manganese-bearing rocks
Discussion
Perological implications and metamorphism
Precursor sediments
Geological environment
Conclusion
Acknoeledgments
References