『Abstract
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.』
Introduction
General
Nsuta manganese deposit
Geological setting
regional
Local
The matasedimentary host-rocks
Geochemical traits
Analytical technique
Discussion
Conclusions
Acknowledgements
References