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,*Geology Department, Rand Afrikaans University, Auckland Park 2006, South Africa
Geology Department, Rhodes University, Grahamstown 6140, South Africa
Department of Geological Sciences, University of Cape Town, Rondenbosch 7700, South Africa
Corresponding author: email, harryt{at}earth.ox.ac.uk
The Kalahari manganese field in the Northern Cape province, South Africa, is a world-class manganese resource (ca. 8 billion tons at 20–48% Mn). It occurs
within the Hotazel Formation in the uppermost Paleoproterozoic (2.65–2.05 Ga)
Transvaal Supergroup and comprises three laminated Mn ore units interbedded with
iron formation. Currently, mining of manganese is concentrated in two areas of
the Kalahari manganese field, in the south and in the north of the field, which
respectively contain low-grade (<40 wt % Mn) carbonate-rich ore, and
high-grade, carbonate-free, oxide-rich ore (generally >44 wt % Mn). In the
southern Kalahari manganese field, the iron formation contains quartz,
magnetite, and carbonate (calcite, ankerite) as chief mineral constituents, and
exhibits bulk chemical and isotopic signatures comparable to other
Paleoproterozoic iron formations of the world. Carbonate isotope compositions in
the iron formation (
13C
= –18 to –4
vs. PDB; 18O
= 12–20 vs. SMOW) indicate diagenetic processes involving oxidation of
organic carbon, and reduction of Fe3+ sedimentary precursors. Values
from the inter-bedded manganiferous units (13C
= –12 to –8; 18O
= 14–22) are interpreted to reflect similar processes, with Mn4+
acting as the sole electron acceptor.
Over large parts of the northern Kalahari manganese field, pre-1.9 Ga shales
of the Olifantshoek Supergroup unconformably overlie the Hotazel Formation. In
these areas, three diverse iron formation types were identified across the
Hotazel stratigraphy (from bottom to top): (1) least-altered iron formation,
which is identical to that seen in the southern Kalahari manganese field; (2)
dolomitized iron formation, containing quartz, incipiently oxidized magnetite
(to hematite), and Ca-Mn–enriched dolomite; (3) enriched iron formation,
comprising exclusively SiO2 (as quartz) and Fe oxide (as hematite).
Mass balance calculations indicate that enriched iron formation formed through
carbonate leaching and residual Fe3+-enrichment of least-altered iron
formation, accompanied by compaction and mass loss of ca. 20 percent. On the
other hand, dolomitized iron formation resulted from partial Fe oxidation and
carbonate dissolution-reprecipitation at depth, in the form of Ca-Mn–enriched
dolomite. 18O
values of the latter (20–21) are higher than those of earlier diagenetic
carbonates by 2 per mil on average, whereas 18O
values of secondary hematite in enriched iron formation are lower than those of
precursor magnetite by approximately the same amount. This suggests the
involvement of an isotopically light fluid (either meteoric water or a
low-temperature hydrothermal fluid) in the oxidation, leaching, and enrichment
of the iron formation. The possibility emerges that extensive fluid flow in the
Kalahari manganese field was related to the Hotazel/Olifantshoek unconformity.
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