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New Insights on West African Tectonics and Iron Mineralisation Based on Ziron U-Pb SHRIMP Geochronology and Structural Mapping

Bert De Waele, Mathieu Lacord, Fabio Vergara, Gavin Chan
Friday, September 27, 2013
First presented: 
Whistler 2013: Geoscience for Discovery Conference
Three main types of iron mineralisation occur in West Africa: Palaeoproterozoic BIF-style on the Leo-Man Shield (e.g. Simandou), Neoproterozoic specularite of hydrothermal origin (e.g. in the Marampa Group), and metamorphic magnetite (e.g. in the Kasila Group). Understanding the distribution of major tectonic units and reconstruction of the geological evolution of the region are keys to assessing the prospectivity for iron mineralisation in underexplored areas.
Mapping, combined with zircon U-Pb geochronology of key units in Guinea and Sierra Leone, confirmed the presence of Archaean units with low metamorphic grade, ascribed to the Leo-Man Shield, structurally verlain by allochthonous units of the Marampa Group, which includes metavolcanics. To the west, an upperamphibolite to granulite-facies grade metasedimentary/metavolcanic succession occurs (Kasila Group). To the east of the Marampa Group, a succession of largely undeformed coarse clastics and volcanics occurs (Rokel River Group).
New zircon U-Pb geochronology of gneisses from the basement to the Marampa Group (Kenema Assemblage) in Sierra Leone indicated ages between ~3.17 and 2.85 Ga, within the range of published U-Pb data on rocks from the Leo-Man Shield farther east (3.54 – 2.71 Ga). One sample from a magnetite-rich unit in the Kasila Group, interpreted to be of volcanic origin, gave a crystallisation age of 1.94 Ga. Detrital zircons from two samples of the Marampa Group, one chloritic schist and one quartzite, yielded a broad range of between 3.2 and 1.1 Ga, including main populations at 3.05, 2.85, 2.2 – 1.8, 1.5 and 1.1 Ga. The youngest concordant detrital zircon, dated at 1030 ± 40 Ma, provides a maximum age of deposition. Source regions for some of these populations in the Marampa samples are not present within the West African Craton, but are present in the Amazonia Craton. Most samples analysed show evidence of Pb-loss, with regressions towards 0.5 Ga. Four concordant analyses in one sample provide an age of 588 ± 7 Ma, interpreted to reflect peak metamorphism in the Rokel-Kasila Belt. This metamorphic age falls within cooling age ranges on biotite, muscovite and hornblende across the region, ranging from 585 to 510 Ma. 
Based on the new data, a new tectonic model is proposed for the Rokel-Kasila Belt, in which the Kasila Group is interpreted to form a remnant part of the Amazonia Craton, left attached to the Leo-Man Shield after Neoproterozoic collision. The Marampa Group represents the thrusted nappes of intervening oceanic sedimentary successions, which show derivation from both the Amazonian and West African margins, while the Kenema Assemblage forms part of the Leo-Man Shield.
This interpretation facilitates reclassification of the known iron mineralisation, and enables regional-scale prospectivity analysis. Palaeoproterozoic BIFs are widespread over the Archaean shield in greenstone belts and are the most extensive and highest in grade. Specularite deposits are regionally restricted to the Marampa Group, and are controlled by the basal thrust contact with the underlying Kenema-Man Shield. These deposits are generally smaller and have lower grades. Metamorphic magnetite-rich units occur only in
the Kasila Group, but are sub-economic because of their low grade and small size.
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