Geological and metallurgical characteristics of banded iron formation associated detrital iron mineralisation in Central West Africa

Author(s): 
B De Waele, M Lacorde, K Bischoff, G Loveday and F Linares
Date: 
Monday, July 24, 2017
First presented: 
AusIMM Iron Ore Conference, 24-26 July 2017
Type: 
Published paper
Category: 
Geology
 
Recent exploration has identified the presence of detrital iron deposits (DIDs) associated with banded iron formation (BIF) units in the north-west of the Congo Craton, Gabon. The regional geology comprises a complex of granitoids and gneisses assigned to the Archaean Chaillu Complex, within which older slivers of lower amphibolite-grade greenstones are preserved including muscovite-biotite-garnet-bearing felsic schist, amphibolite and BIF. These units are typically overlain by several metres of residuum comprising colluvium, eluvium and duricrust. The residuum is overlain by a regionally extensive loess cover from 2m to 10 m thick.
 
The DIDs occur within the weathered residuum as mostly unconsolidated gravels with lesser canga (CAN) duricrust draped over deeply weathered hematite BIF. Head grades are in the range of 45 to 52 per cent Fe. Fresh BIF, located from 30 to 50 m below surface, is comprised of magnetite-quartz±amphibole. The DID gravels are comprised of rod- and plate-shaped clasts of hematite (martite)-maghemite-goethite composition, in a ferruginous sand to clay-sized matrix. The DIDs form ridges and plateaus that coincide with magnetic highs defined using high-resolution ground magnetic surveys.
 
The field relations and petrography indicate that the DIDs accumulations are the result of weathering (including enrichment) and erosion of primary BIF. This includes removal of quartz, further oxidation and re-cementation of BIF to form ferruginous caprock. This cap and the in situ oxidised BIF were subsequently disaggregated and liberated, and further weathered to form the detrital iron accumulations.
 
Metallurgical test work on bulk detrital iron samples has shown this material can be upgraded to lump and fines iron ore products with grades of 62 to 65 per cent Fe using simple scrubbing and wet screening, followed by dense media separation of the +1 mm fraction, with overall mass yields from 75 to 85 per cent. The test work indicates a high ratio of lump to fines products, sometimes exceeding 50:50.
 

Feature Author

Bert De Waele

Bert De Waele has over 23 years of structural mapping experience, mostly in African Precambrian terranes. In addition, he has consulted on base metal and gold prospects, setting up and running exploration programmes or interpreting regional geochemical data. With SRK, Bert has also conducted Independent Technical Assessments on various mineral assets including gold, porphyry copper, iron ore, manganese, diamond, mineral sands, phosphate, evaporite and uranium. His research interests focus on regional geology and tectonics, by integrating field and laboratory work, including U-Pb microprobe geochronology, isotope (147Sm-144Nd and 87Rb-86Sr) and whole-rock geochemistry and Geographic Information Systems. Bert has a strong commitment to transfer of knowledge, holding an Adjunct Research posting at The University of Western Australia. Bert has taught structural geology, computing in geoscience (GIS - Remote Sensing and DPA (Geosoft)) and supervised field classes, mapping projects and theses (BSc, MSc and PhD) both in French-speaking and English-speaking environments.

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PhD (Geology), RP Geo (Regional Geology, Mineral Exploration)
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