Mucke(uの頭に¨) et al.(1999)による〔『Mineralogy, petrography, geochemistry and genesis of the Paleoproterozoic Birimian manganese-formation of Nsuta/Ghana』(297p)から〕


The manganese deposit of Nsuta, in the Ashanti Belt of Southern Ghana, is sandwiched between Birimian metasedimentary rocks. The metasedimentary rocks contain interbedded carbonate-rich layers, which exhibit a characteristic banded appearance near the contact with the orebody. The orebody is a carbonate-type manganese-formation and in terms of origin is considered here as a Mn-analogue of the volcanogenic-exhakative Alogoma type iron-formation. The protolith of the orebody (chemical sediment including Fe-bearing rhodochrosite and alabandite) is envisioned to have been formed in a marine basin with relatively high CO2 activity and Eh-pH conditions were extremely low (Eh 1 to -0.6 Volt and pH 8 to 11) during Birimian times (2170-2180 Ma). These conditions occurred immediately below the shelf break in a shallow-marine environment. Subsequent submarine weathering (halmyrolysis) followed later by metamorphism of Eburnian age (2100 Ma) led to the formation of Mg-Ca-Fe-bearing rhodochrosite, the dominant mineral in the orebody. Other minerals of the orebody are: sulfides (e.g. two generations of alabandite sphalerite, pyrite, millerite, niccolite, gersdorffite, and molybdenite), oxides and hydroxides (vanadium-bearing jacobsite, galaxite; brucite, Mn2+-todorokite), mn-silicates and an unknown boron mineral. Pyrochroite, possibly preceded by manganosite, occurs as a retrograde mineral. This mineral assemblage forms the protore of the Nsuta deposit. Opaque Mn4+-todorokite replacing Mn2+-todorokite, manganite, manganomelane, pyrolusite and nsutite which formed at the expense of rhodochrosite, are of supergene origin and represent the economic part of the deposit. The orebody is interleaved between the associated pelitic-psammitic metasedimentary rocks suggesting that its protoliths was deposited over a time interval during the sedimentation of the latter. Both units underwent subsequent processes (submarine weathering and metamorphism) together. The compositional differences between the orebody with high Mn and CO2 and low Si and Al contents relative to the metasedimentary rocks are explained by a model involving the continuous sedimentation of continent-derived materials (protolith of the metasedimentary rocks). During this time a pulsatory phase of submarine volcanism and consequent precipitation of materials of essentially volcanogenic-exhalative origin occurred (protolith of the orebody). From the exhalations, the carbonate minerals in both the manganese-rich sediments and the metasedimentary host-rocks (in the latter in the form of layers and disseminations leading to relatively high concentrations of Mn, Ca and CO2) were precipitated.』

Nsuta manganese deposit
Geological setting
The matasedimentary host-rocks
Geochemical traits
Analytical technique