Manganese deposits of Africa

Nicolas J. Beukes, Edward P.W. Swindell, Herve Wabo

Research output: Contribution to journalArticlepeer-review

53 Citations (Scopus)

Abstract

known land-based ore resources of manganese metal and produced some 41.1% of the 18 million tonnes (Mt) of manganese metal in ores, that was mined during 2014. The deposits are mainly of sedimentary and supergene origin comprising four major types, namely banded iron formation (BIF)-hosted, black shale-hosted, oolitic and supergene/karst-hosted deposits. The BIF-hosted Kalahari Manganese Field (KMF) is by far the largest of these deposits holding some 4,200 Mt of manganese metal that represents about 77% of the world's known land-based resource. Other major deposits are present in the Francevillian of Gabon, represented by high-grade supergene manganese oxide ores derived from weathering of rhodochrosite-bearing black shale source rock, and manganese carbonates interbedded with black shale and greywackes associated with volcanicdominated orogenic belts in the Birimian of West Africa and Lukoshi Complex at Kisenge in the Democratic Republic of the Congo (DRC). High-grade supergene manganese oxide ores cap the manganese carbonate beds. All these major ore deposits formed in the interval 2.0-2.2 Ga. Additional deposits that contain only relatively small resources but that hold important scientific information about the Earth's history include the ∼ 2.4 Ga BIF-hosted Rooinekke deposit of the Transvaal Supergroup, which formed prior to the ∼ 2.32 Ga Great Oxidation Event (GOE), karst-hosted deposits of the Postmasburg area that formed along a 2.0 Ga palaeoweathering profile below red beds of the Gamagara/Mapedi succession in South Africa, the ∼1.9 Ga Tolwe deposit that represents the oldest known example of oolitic manganese ore in the world and the BIF-hosted Otjosondu deposit in Namibia, related to the Neoproterozoic Sturtian Snowball Earth Event (an extremely cold period of time in geologic history when Earth was covered by a virtual global ice cap). The manganese carbonates present in most of these deposits are all highly enriched in light organic carbon isotopes indicating that they were derived from reduction of original manganese oxide precipitates by organic carbon during diagenesis. This implies that the water columns from which the sedimentary manganese beds originally precipitated, as well as the floors of the depositories, were oxygenated. Post Archaean Australian Shale (PAAS)-normalized cerium (Ce) anomalies in Rare Earth Element (REE) data, support the presence of oxic conditions in the basins. It is thus not surprising that all the major 2.0-2.2 Ga deposits formed after the GOE. In this regard the relatively small BIF-hosted Rooinekke manganese deposit is very significant in that it formed prior to the GOE suggesting that at least some free oxygen was present in ocean basins before that time. There also appears to have been a major tectonosedimentary control on the distribution of the major 2.0- 2.2 Ga deposits. All of them, including the KMF, apparently formed in back- Arc basinal settings immediately prior to amalgamation of cratonic blocks along Eburnian orogenic belts and formation of the supercontinent Columbia.

Original languageEnglish
Pages (from-to)285-317
Number of pages33
JournalEpisodes
Volume39
Issue number2
DOIs
Publication statusPublished - Jun 2016

ASJC Scopus subject areas

  • General Earth and Planetary Sciences

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