Traverse of Mvurwi Section of the Great Dyke to examine the Upper and Lower African Surfaces
Grid Ref (WGS84 Lat/Long in decimal degrees)
A full-day, circular traverse of part of the Mvurwi section, principally to examine the heavily-eroded Upper African Surface towards the east and the better-preserved mesa terrain of the Lower African Surface towards the west. The route is ca. 6-7km long with a moderate ascent of ca. 300m at the start.
After parking the vehicles, (Park - WGS84 - S 16.63883, E 30.86755) the traverse proceeds east northeastwards up to Hill 1724, (WGS84 - S 16.63005, E 30.88949) then north westwards to Shear Zone Hill (WGS84 - S 16.62727, E 30.87665) via a series of ridges and finally down to Hill B (WGS84 - S 16.62507, E 30.87079) and then back to the vehicles. It may be possible to locate discrete, fracture-related, green, ‘garnierite-type’ hydrated Ni-Mg silicate minerals.
From several high points during the traverse, the rugged terrain of the Snake’s Head section (Musengezi Subchamber and Mvuradona Chamber) may be seen to the north and north east.
The eastern part of the Mvurwi section comprises a 6 km long, rugged massif, including Mvurwi Peak (1738 m) (WGS84 - S 16.63461, E 30.89885), with a total relief of ca. 300 m. Westwards, the massif gives way quite sharply to relatively subdued topography at a lower level. This area, extending about 5 km further northwest, is characterised by a dissected table-land comprising discrete, flat-topped hills (or mesas and a few small buttes), the largest up to almost 100 000 m2 in total area, with roughly accordant summits at a common elevation of ca. 1525m; these represent the Lower African Surface. The western mesas (e.g., Hills A & B and Shear Zone Hill) are characterised by a well-developed regolith with a distinctive layered stratigraphy. The regolith sequence typically displays (1) a cliff-like upper slope and a concave lower slope made up variously of massive serpentinite and horizontally-fractured serpentinite, and (2) an overlying silica cap of variably-silicified serpentinite in two parts: a lower zone of sheeted silica veins within horizontally-fractured serpentinite (sheeted silica vein zone), its base roughly coincident with the cliff-tops, and an upper (red-brown) ferruginous silicified zone.
The higher summits in the east (e.g., Hill 1724) are mostly rounded to pyramidal rather than flat-topped and their regolith sequence is heavily eroded and less systematic than that of the western mesas; these represent the Upper African Surface. The Upper and Lower African Surfaces are estimated to be ca. 200m apart vertically, with no evidence to explain their vertical separation by late faulting.
The granitic terrain either side of the Mvurwi section lies at ca. 100 m lower elevation to the north than to the south where a rolling Post-African plain at ca. 1350 m carries occasional inselbergs up to ca. 1500 m. Significantly, the summit elevation of the highest inselberg (Nyambari) is only slightly less than that of the western mesa terrain. Accompanying this elevation difference, the north-facing slopes (at least as far east as Mvurwi Peak) have steeper gradients and narrower, deeper valleys than the south-facing slopes. Because of the more recent rejuvenation evident on the north side, the mesas and many of the highest eastern peaks are eccentrically disposed towards the south or southwest.
The eastern massif is divided into two structurally distinct parts by the prominent, Camsasa fault, a curvilinear, sub-vertical, south- to southwest-trending structure of probable Proterozoic age located directly east of Mvurwi Peak. Further east, the contact zone with the north northeast-trending Horseshoe section is marked by two similar, related faults. Other important structural features of the Mvurwi section are (1) the marginal shear zones along the southern and northern contacts (indicated by a zone of variably sheared and altered serpentinites up to 200 m wide), and (2) silicified early fractures. Visible mainly on central hill-tops and their southern upper slopes, the ferruginous silicification associated with the early fractures is very common west of the Camsasa fault but quite rare on the rejuvenated northern slopes, even as detached blocks.
Bed-rock stratigraphy and lithologies
To the west of the Camsasa fault, on both the massif and in the mesa terrain further west, all the ultramafic rocks between the marginal shear zones consist of variably serpentinised dunite with occasional chromitites marked by surface rubble. The serpentinites are mostly massive, pale yellow-green and, particularly on elevated sites, are characteristically sculpted in outcrop; these serpentinites can contain ca. 50% fresh olivine. Elsewhere, they display closely-spaced fractures that, in general, lie parallel to the local slope and are interpreted as rebound or ‘release of load’ structures (e.g., the horizontally-fractured serpentinites of the mesa regolith).
Except for a large, 2 km-long, oval enclave of massive serpentinite (similar to that west of the Camsasa fault) on the upper part of Hill 1668, the ultramafic rocks east of the Camsasa fault are almost entirely represented by talc carbonate schists and variably-sheared, grey carbonated serpentinites. In the easternmost fault-block are two linear masses of distinctive megacrystic pyroxenite, and several chromitites have been exposed by limited mining. The pyroxenite (probably the P6 Pyroxenite at the base of the Pyroxenite Succession) is associated with a very coarse-grained chromitite (probably the C5 Chromitite overlying the P6 Pyroxenite). Other chromitites towards the southern granite contact are most likely the C6 to C9 Chromitites located stratigraphically beneath the P6 Pyroxenite. The serpentinites exposed in the Mvurwi section most probably represent the upper levels of the Dunite Succession and lowermost Pyroxenite Succession. Because of deformation, rare exposure of chromitite structural markers, and contradictory younging directions, the shape of the transverse synclinal layered structure here is uncertain.
Note the following:
1. Hill 1724. This hill (part of the eroded Upper African Surface) is capped by rubble lag deposits of the ferruginous silicified zone (with possible contributions from silicified early shear fractures). Portions of the sheeted silica vein zone are visible at lower levels and serpentinites from pits excavated nearby are Ni-enriched.
2. To the south east is Mvurwi Peak (1738m) – also part of the eroded Upper African Surface – with similar lag deposits of silicified rubble. (Unfortunately, a visit there would add 2km to the traverse.)
3. In the distance to the east northeast is Hill 1668 located within the Camsasa fault zone. This hill, with cliff-top elevations of ca. 1650m and a relatively well preserved regolith profile, is interpreted to be an Upper African Surface remnant now at a lower elevation than Hill 1724 and Mvurwi Peak due to re-activation of the Camsasa faults.
4. Shear Zone Hill. This gently-sloping remnant of the Lower African Surface is located on the lower slopes of the eastern massif and overlooks the mesa terrain of the western part of Mvurwi section. Some 33 rotary air blast holes were drilled here; geochemical profiles of two holes (with shallow and deep regoliths) are provided; the Ni-enriched profile is composite and up to 13m thick.
5. Hill B. This is one of the largest and best-preserved mesas of the Lower African Surface and displays a complete regolith stratigraphy.