Geology and Evaluation of the K2 Kimberlite, Koidu Mine, Sierra Leone, West Africa

Abstract

The K2 kimberlite is part of the Koidu Mine, located on the Man Craton in Sierra Leone, West Africa. The kimberlite is Jurassic in age, 0.5 ha in size at surface, and was emplaced along a pre-existing dyke system into extensively brecciated and locally altered granite country rock. The pipe is infilled by variably sorted, inhomogeneous volcaniclastic kimberlite (VK) and by less common coherent kimberlite (CK). A resource evaluation programme was conducted from 2008 to 2010 to advance the mine plan. This involved drill core logging, indicator mineral abundance and composition studies, bulk sampling, surface mapping, targeted mining, three-dimensional geology modelling and diamond grade estimation. The conduit-filling VK deposits are interpreted to represent several explosive eruptions. The internally complex and variable distribution of the VK rock types reveal both concentric deposition from the pipe margin towards the centre, and vertical stratification creating sub-horizontal domains. Post-pipe emplacement CK intrusions are common and frequently show mixing between the intruding kimberlite and the poorly to unconsolidated VK infill. Secondary alteration (serpentine, clay) has modified the kimberlite mineralogically and to a lesser degree texturally. The K2 pipe infill is characterised by many features consistent with the historical kimberlite classification ‘tuffisitic kimberlite’ (TK), but is more highly variable than typical large ‘TK’-infilled bodies. Diamond grade and value information obtained from large diameter drilling and targeted mining were integrated with simplified geological models to derive global estimates of diamond grade and value for geologically defined resource domains. These formed the basis of a feasibility study of the Koidu kimberlites and positive results led to expansion of the mining project.

An erratum to this chapter can be found at http://dx.doi.org/10.1007/978-81-322-1173-0_20