Regional Geology

The Great Dyke is a layered mafic and ultramafic intrusion that cut across the
dominantly Achaean rocks of the Zimbabwe Craton,
largely composed of granites and greenstones. The Zimbabwe Craton is bounded by
the Zambezi metamorphic belt to the north, the Mozambique belt to the east, and
the Limpopo belt to the south. The Great Dyke, aligned approximately NNE, is
about 550km long and between 4km and 11km wide. Parallel to it are a number of
gabbro and quartz gabbro satellite dykes. The Great Dyke is longitudinally
subdivided into a series of narrow contiguous layered chambers and sub chambers
(Figure 4). The two main chambers have been recognized with further
subdivisions totalling five subchambers on the basis of structure, style of
layering, and continuity of layers. The North Chamber is subdivided into
the Musengezi, Darwendale and Sebakwe subchambers, and the South Chamber into
the Selukwe and Wedza subchambers. Unki
Mining Company is located within the Selukwe Subchamber of the South Chamber of
the Great Dyke in ZimbabweThe shape of the
Selukwe Subchamber has to some extent been controlled by the proximity of the
Shurugwi Greenstone Belt, in that it has been deflected and constricted in places.
Overall though, the broad structure of the Selukwe Subchamber is similar to
that of the other subchambers with an upper mafic sequence (280m thick)
overlying an ultramafic sequence (exposed thickness of 1600m). In transverse
section the subchamber is synclinal in shape, with essentially the same lithological
succession being exposed on both sides of the longitudinal axis. Asymmetry in
the layering pattern close to the walls is attributed to the physical shape of
the chamber walls and the contrasting nature of the wall rocks, which are
greenstones on the west, and granite on the east. A number of prominent
transverse, steeply dipping faults interrupt the overall continuity of the
Selukwe Subchamber and these, combined with the change in shallow plunge
direction (from north to south for the northern zone and in the opposite
direction for the southern zone), result in a repetition of the lower
gabbronorite on the longitudinal axis. This has implications for major ground
challenges which has been experienced at the Mine.