The 2.7 Ga Komatiite Event in the South Central Zimbabwe Craton and asociated Nickel and Chromite Mineralisation.

The 2.7 Ga Upper Bulawayan Group, comprising a several-kilometre-thick conformable sequence of chemical and clastic sediments, komatiite lavas and high Mg and then tholeitic basalts, is traceable throughout the greenstone terrain of the south-central Zimbabwe craton with a minimum unfolded lateral extent of many hundreds of kilometres. The lower sedimentary rocks generally overlie widely developed 2.8 Ga Lower Bulawayan felsic volcanic rocks as well as older gneisses and folded greenstones. Intruded at various (but generally very high) structural levels beneath the Upper Bulawayan volcanic rocks is an almost ubiquitous series of large dunite sills (with variably developed upper pyroxenite-gabbro units) that are thought to represent feeders to the overlying komatiite flows. The komatiite flow and sill phases account, respectively, for a significant part of Zimbabwe’s nickel resource and for some of the country’s highest quality chromite.

Despite variable deformation and metamorphism, the principal nickel deposits can be shown to occupy anomalous magmatic structures, each assignable to specific facies of the volcanic feeder system and flow field: in (Epoch Mine) and overlying (Shangani) fissure vents, and Mount Keith-type proximal lava rivers (Hunters Road and , possibly, Damba-Silwane). No distal Kambalda-type deposits have yet been recognised. Striking features of the komatiite flow field is its lateral variability and the prominence of komatiite basalts (Epoch and Shangani) - as opposed to true komatiites (Hunters Road) - as hosts to nickel mineralisation.

Although chromite mineralisation is sporadic in the sill phase as a whole, chromitites are widespread in the Prince Ultramafic Sill (PUS). At the Prince Mine, eleven massive and disseminated olivine chromitites, mostly 1-10 m thick, make up a well layered sequence more than 70 m thick positioned towards the middle of the sill. The PUS contains one of the best preserved of all known komatiite-associated chromitite sequences.

The 2.7 Ga komatiite event in the south-central Zimbabwe craton has important implications for (1) the structure of komatiite volcanic edifices and their associated flow fields, (2) the nature and development of, and thus exploration for, komatiite-associated nickel and chromite mineralisation, and (3) current controversies regarding the oceanic versus continental tectonic setting of the Upper Bulawayan volcanic sequence.