Stratigraphic and Structural Setting of Selected Gold and Base Metal Deposits in the Natal Thrust Front, South Africa

The Namaqua-Natal Metamorphic Province is a Mesoproterozoic
mobile belt formed by plate collision against the southern margin
of the Archean Kaapvaal Craton. The Natal Metamorphic
Province is regionally metamorphosed and incorporates obducted
ophiolites, marginal bimodal metasedimentary/metavolcanic
sequences, accreted island arcs and para-autochthonous granitegneiss
basement. The leading deformation edge, a 2-12 km wide
south- dipping imbricate complex of low-grade high-pressure metamorphic
rocks, comprises the Ntingwe and Mfongosi Groups transposed
along the basal Mfongosi Thrust. The northernmost Ntingwe
Group directly overlying the Kaapvaal Craton Margin comprises
metamorphosed dolomite, limestone, shale, mudstone, conglomerate
and breccia while the overlying Mfongosi Group consists of
phyllitic quartzite, quartz-chlorite schist, carbonate-rich quartz-chlorite
schist, epidote-actinolite schist, and garnet-staurolite-mica schist
representing interlayered schistose meta-argillites and metalavas.
Assemblages occur in the chlorite-garnet zone (upper greenschist),
to high- pressure, intermediate facies (500-550¡C at 12-16 Kb) with
epidote, chlorite and albite being prevalent. East- trending tight isoclinal
folds with south-dipping axial planes, disrupted by northwardverging
overthrusting, are ubiquitous.
The westernmost portion of the Mfongosi Group, the
ÒNgubevuÓ Area, hosts enigmatic quartzofeldspathic blows (largescale
augen structures) and stringers containing Au and base-metal
sulfide mineralization. It comprises two main lithological groups: a)
positively-weathering, volumetrically dominant epidote – actinolite
– quartz – talc ± calcite and chloritic schist/ metabasites and b) negatively-
weathering banded units, essentially crenulated graphite –
sericite – quartz – albite – tourmaline ± analcite ± chlorite ± calcite
schist, classified as metapelites to metacalcsilicates. Mineralized
quartzofeldspathic blows and intrafolial stringers are products of a
complex interaction between deformation and pressure solution.
Two constants of mineralization apply: a) blows and stringers occur
exclusively in competent metabasites containing minor metapelitic/
metacalcsilicate layers; and b) major mineralized blows formed
in early- to syn-tectonic times. The dominant planar foliation (S01) is
oriented at 090¡/60¡S, parallel to the craton margin. S01 exhibits
cylindrical and non-cylindrical folding, with axes trending between
110¡®39¡ and 133¡®48¡. Quartz/calcite tension gash formation
parallelling s1 (hinterland or para-autochthonous nappe zone origin)
accompanied folding and transpressional left-lateral movement
with finite strain ratios (Rf) of up to 15. Poles to late-tectonic tension
veins parallel the long axes of antitaxial quartz crystals on vein margins
(contoured maxima of 192¡®66¡) implying orthogonal vein
dilation. Mineralization appears spatially non-systematic but is
explained by intense and repeated deformation of the intersections
between early – syn-tectonic 190¡-trending tension gashes and fine
metapelite / metacalsilicate layers, in which deposition of Au, pyrite
and chalcopyrite occurred. Mineralized blows are the thickened
limbs of ptygmatically- or fish-hook-folded early- syn-tectonic tension
gashes, while stringers are residual, thinned limbs of these
folds. High strain rates and superimposed pure and simple shear
destroyed continuity between mineralized blows and stringers,
resulting in complex, yet resolvable, mineralization patterns.