The Tonkolili Iron Occurrence of Sierrra Leone: A Petrological Enigma?*

Abstract

This paper endeavours to resolve, by petrological and mineralogical interrogations, the apparent puzzle being caused by the different names and classifications that have been assigned to the Tonkolili iron occurrence of Sierra Leone by different authors. The mineralogy and petrography of drill core samples of iron occurrences from Tonkolili district of Sierra Leone are used to investigate the type of iron deposit, its environment of formation and classification. Three main rock types that host the Fe mineralisation have been identified as amphibolites, tuffs and mafic volcanic rocks. The amphibolites which crop out and are mapped on the surface are underlain by tuffs. Two types of tuffs were identified, namely as rhyolitic and crystal tuffs. The mafic volcanic rocks lie underneath the tuffs. The Fe mineralisations occur as bands and lenses in the tuff and are encountered from depths of 120 to 160 m in the drill holes. The Fe mineralisations which are mainly of Precambrian age are chemical sediments, contain about 55 % Fe and occur as mappable bands and lenses that have ironstone layers defining their top and bottom and thus satisfy Kimberly (1978)’s classification as “iron formation”. It is therefore the conclusion of this paper that with the realisation that the primary Fe mineral is magnetite rather than haematite and since there is an intimate association between the volcanic rocks and the Fe mineralisations, the Tonkolili Fe deposits can be classified as an Algoma type of Banded Iron Formation according to the classifications of James (1954) and definition of Cannon et al., (1986).