Regional GeologyThe Great Dyke is a layered mafic and ultramafic intrusion that cut across thedominantly Achaean rocks of the Zimbabwe Craton,largely composed of granites and greenstones. The Zimbabwe Craton is bounded bythe Zambezi metamorphic belt to the north, the Mozambique belt to the east, andthe Limpopo belt to the south. The Great Dyke, aligned approximately NNE, isabout 550km long and between 4km and 11km wide. Parallel to it are a number ofgabbro and quartz gabbro satellite dykes. The Great Dyke is longitudinallysubdivided into a series of narrow contiguous layered chambers and sub chambers(Figure 4). The two main chambers have been recognized with furthersubdivisions totalling five subchambers on the basis of structure, style oflayering, and continuity of layers. The North Chamber is subdivided intothe Musengezi, Darwendale and Sebakwe subchambers, and the South Chamber intothe Selukwe and Wedza subchambers.
UnkiMining Company is located within the Selukwe Subchamber of the South Chamber ofthe Great Dyke in ZimbabweThe shape of theSelukwe Subchamber has to some extent been controlled by the proximity of theShurugwi Greenstone Belt, in that it has been deflected and constricted in places.Overall though, the broad structure of the Selukwe Subchamber is similar tothat of the other subchambers with an upper mafic sequence (280m thick)overlying an ultramafic sequence (exposed thickness of 1600m). In transversesection the subchamber is synclinal in shape, with essentially the same lithologicalsuccession being exposed on both sides of the longitudinal axis.
Asymmetry inthe layering pattern close to the walls is attributed to the physical shape ofthe chamber walls and the contrasting nature of the wall rocks, which aregreenstones on the west, and granite on the east. A number of prominenttransverse, steeply dipping faults interrupt the overall continuity of theSelukwe Subchamber and these, combined with the change in shallow plungedirection (from north to south for the northern zone and in the oppositedirection for the southern zone), result in a repetition of the lowergabbronorite on the longitudinal axis. This has implications for major groundchallenges which has been experienced at the Mine.