ALBERT

Description: The hilly northern part of the north-northeast-trending Great Dyke – the Mvukwe Range – comprises a 300 km 2 , 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.

Description: Set in ca. 74 km 2 of rugged terrain close to the Zambezi Escarpment, the Snake’s Head Platinum Project covers the northeastern half of the Musengezi Subchamber in the northern part of the Great Dyke. Here, the linear Great Dyke straddles the boundary between the Zimbabwe craton and the late Archaean Migmatitic Gneiss Terrain and is folded into a prominent S-shape adjacent to the Neoproterozoic-Phanerozoic Zambezi orogenic belt. Snake’s Head contains the northernmost remnant of the Great Dyke’s P1 Pyroxenite Layer, which hosts the economically-important, stratabound, PGE-rich Main Sulphide Zone (MSZ), as well as the lower grade (but petrogenetically very similar) Lower Sulphide Zone (LSZ), together containing one of the Great Dyke’s last undeveloped platinum resources (〉80 m oz). The original, gently-plunging, synclinal layered structure of the Musengezi Subchamber is preserved in the western part of Snake’s Head but is replaced in the east and north by several, contiguous, kilometre-scale structural blocks in different orientations and separated by curvilinear, brittle-ductile thrust zones suggestive of sequential stacking towards the west. Shearing and associated metasomatism increase from the generally pristine western blocks to the northern and eastern blocks where the structure is overlain above another major, south-directed thrust by the Marginal Gneiss Terrain of the Zambezi belt and the underlying, well-developed magnetite gabbro unit at the top of the Great Dyke sequence is mostly converted to mafic schist. The P1 Pyroxenite displays layer-thicknesses, mineral associations and textures, and other layering features, and the MSZ shows slight systematic variations in thickness, and in sulphide and metals contents, that all indicate (1) primary magma chamber locations varying from the axis (in the case of the western blocks) to midway between the axis and margins (eastern/northern blocks), and (2) an original magma chamber width that varied along its length but was significantly greater than in other parts of the Great Dyke. The MSZ and LSZ preserved at Snake’s Head are, unlike their correlatives elsewhere in the Great Dyke, broadly similar in thickness and metals content, probably because they formed along the axial to mid-axial/marginal facies of a very wide magma chamber where the horizontal rather than the vertical gradient dominated the cooling regime to a far greater extent than elsewhere in the Great Dyke. Emplacement of the 2.58 Ga year-old Great Dyke took place towards the end of late Archaean greenstone formation, granite intrusion and deformation along the northern edge of the Zimbabwe craton at ca. 2.57 to 2.62 Ga. Thrust deformation at Snake’s Head fits only the earliest (late Archaean) stage of deformation along the northern margin of the craton and so may have occurred soon after Great Dyke emplacement during the waning phase of Migmatitic Gneiss Terrain development and the final stabilisation of the craton. Folding of the Snake’s Head thrusts and thrust blocks may reflect cross-folding during extension of the Marginal Gneiss Terrain during the earliest stage of Zambezi belt orogenesis at 0.75 to 0.85 Ma. The marked north-south shortening of the northernmost part of the Great Dyke is likely the product of south-directed thrust movements during Zambezi contractional events at 0.85 to 1.10 Ga and/or 0.50 to 0.60 Ga.