Abstract

The hilly northern part of the north-northeast-trending Great Dyke – the Mvukwe Range – comprises a 300 km2, elongate mass of exposed serpentinite displaying remnants of two principal erosion surfaces: an arealy-dominant upper surface and a subordinate lower surface confined to a northerly location. Both surfaces, assigned to successive phases of the composite, continent-wide, mid-Cretaceous to end-Oligocene, African Surface, are represented by variably-preserved table-lands of plateaus, mesas, buttes, and accordant summits. To the east and west, inselberg-bearing, granitic plains form a composite, Miocene, Post-African etch surface at contrasting lower elevations, resulting in eccentric dispositions of African erosion surfaces and Post-African internal valleys, all attributed to the varying maturity of the main drainage systems either side of the Mvukwe Range, a probable regional watershed since pre-Karoo times.

The lower and upper African surfaces have present-day elevations of ca. 1525 m and (mostly) ca. 1620 m, respectively, although an original vertical separation of ca. 200 m is estimated in the north. Complex northward variations in preservation and elevation of the upper surface, plus a general northward increase in modal olivine in the serpentinised dunite protolith, point to significant regional uplift, probably associated with Tertiary displacement of the Zambezi Escarpment horst active since the Triassic at the northern extremity of the Great Dyke, as well as possible re-activation of Zambezi belt-related Proterozoic faults.

Preserved African Surface regoliths comprise (cliff-forming) horizontally-fractured serpentinite overlain by a composite silica cap of horizontally-fractured serpentinite with sheeted silica veins below ferruginous silicified serpentinite. The predominantly, goethite-chrysotile African regoliths carry nickel enrichments of 1 to 2% Ni (or more) to depths of up to 10 m (or more) in the fractured serpentinite and sheeted silica vein zones; other nickel enrichments occur more sporadically within the protolith. Three principal mineralogical associations are postulated for the contained nickel: (1) discrete, fracture-related, ‘garnieritic’ minerals (in both regolith and protolith), (2) nickeloan serpentine within drusy vugs associated with silica veins, and, probably, (3) pervasive enrichments associated with goethite and/or chrysotile. Cobalt, concentrated towards the top of the preserved regolith profile, is most likely linked to goethite via a primary association with Mn.

The geomorphological and geological features of Great Dyke nickel laterites are closely analogous to those of classic, saprolite-type nickel laterite deposits in Brazil. Both groups probably formed in fundamentally similar ways, the lower grade/thickness of the Great Dyke deposits attributable to the slightly differing climatological and geomorphological histories of northern Zimbabwe and of equatorial and tropical Brazil.

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