Notes on Lateritization in Sierra Leone

Abstract

Summary1. Lateritization has been found to affect a variety of rocks in Sierra Leone, including norite, granitic rocks, and detrital deposits.2. The norite of Sierra Leone is not deeply lateritized except in highly jointed areas or in certain bands readily susceptible to the alteration. It yields a gibbsitic laterite. A core of unaltered norite is enclosed in a light-coloured halo (“zone of leaching” of Professor Lacroix), which passes gradually into a red-brown laterite (“zone of concretion”). The transition from core to laterite is a rapid one. The minerals of the norite are attacked in the following order: felspar, diallage, hypersthene, magnetite (titanomagnetite).3. Norite containing only a small quantity of disseminated magnetite is less readily lateritized than norite containing a moderate amount. Where, however, the norite includes an unusually high proportion, as in the streaks of magnetite, lateritization proceeds very slowly.4. The granitic rocks of the Protectorate are frequently lateritized to a depth of 15 feet, and sometimes even to 30 feet. Nevertheless laterite is only slightly developed in the inselberg mountain districts of the northern part of the country. Lateritization of granitic rocks proceeds as follows: the rocks decompose into a white kaolinitic product, which passes into.brown lateritic clay (“zone of leaching”); this lateritic clay then develops into laterite (“zone of concretion”). There is a progressive change from the unaltered rock to the laterite crust, the granitic rocks in this respect presenting a marked contrast to the basic rocks.5. Lateritization affects detrital deposits also, particularly the Pleistocene sands and clays of the coastal plain. The phenomenon is most pronounced in the felspathic sands and diminishes in the more argillaceous beds until it is practically non-existent in pure clay. The typical lateritite is a red-brown scoriaceous rock, moderately hard, and in the hand specimen coarsely porous. It forms a crust about 10 feet thick at the top of the Pleistocene beds. This lateritite is indistinguishable from the laterite derived from other rocks.6. The scoriaceous laterite (lateritite) was produced by the growth and ultimate coalescence of numerous small ferruginous concretions, followed by the removal of the soft material between the concretions.7. The laterite formed from detrital deposits passes locally into lateritic iron-ore by increase in the percentage of iron oxide. Such iron-ores are well developed along the inner margin of the coastal plain encircling the colony, and occur principally in the neighbourhood of Devil Hole, near Waterloo. They were derived originally from the titanomagnetite which enters largely into the composition of the norite, and they are consequently very rich in titanium.8. Lateritization is frequently an important factor in the production of caves and subterranean channels, owing to the ease with which the unconsolidated material is removed from beneath the thick hard crust of laterite.