The structural, metamorphic and tectonic context of selected sub-economic veining in the Natal thrust front and Natal Nappe zone, Northern KwaZulu-Natal.

Abstract

The eastern portion of the Namaqua-Natal Mobile Belt, the Natal Metamorphic Province is divided into four main

tectonostratigraphic units. These units comprise two accreted island arcs: the Mzumbe and Margate Terranes; an imbricately

thrust nappe zone consisting of four ophiolitic nappes in a hinterland-dipping duplex; and the highly deformed

metavolcaniclastic/metagreywacke Mfongosi Group directly adjacent to the stable northern foreland of the Kaapvaal Craton.

Theories of late-tectonic left-lateral movement in the southern island arcs are extrapolated northwards of the southern margin

of the Kaapvaal Craton coincident with the Lilani-Matigulu Shear Zone. The relative timing and structural context of vein-hosted

mineralization with respect to major recognized tectonic events is resolved in five separate areas, two in the Natal Nappe

Zone and three in the Natal Thrust Front.

The Madidima Nappe of the Natal Nappe Zone contains several north-northeast- to northeast-trending and northeast- to east-northeast trending quartzofeldspathic veined reefs considered to have formed in a late-tectonic left-lateral shear system (main

shear and synthetic shear orientations, respectively). The northeast- to east-northeast-trending reef is duplicated due to infilling

of normally-faulted steep structures in the semi-brittle, incremental normal faulting of the banded amphibolite component of

the nappe. Later left-lateral movement has reactivated one of these steep structures along the southern margin of a regional

F2-folded band of granite-gneiss in that a southwest extension of this structure may be responsible for sub-economic veining

for a length of up to 9 km. The extensive flat-lying topography of the Mbongolwane Flats area, in which the reefs are situated,

is accounted for by the accelerated weathering of rocks which underwent sustained late-tectonic metamorphism in the epidoteactinolite facies, accompanied by pervasive shearing and block rotation to the south of the southern limb of the regional F2 fold in the granite-gneiss. A large, kilometer-scale, open advective fluid system which provided fluid-mediated exchange between co-existing rocks existed at the time of vein formation. The fluid system was driven by early-tectonic intrusion of a granite gneiss and amphibole-rich granite.

Two areas in the Mfongosi River valley, the northern and southern Mfongosi Valley areas, contain typical evidence of

deformation at the leading edge of collision in a mobile belt. The southern Mfongosi Valley area, at the confluence of the

Mfongosi and Tugela Rivers, contains veining which resulted from pressure solution of the host metavolcaniclastic/metagreywacke. Veining occupies predictable shear and tension fractures formed during the initial

deformation of a foreland margin sequence, in addition to occupying those fractures formed by buckling on the layer-scale.

The structural context of the northern Mfongosi Valley veining is defined by subsequent deformation and vein fragmentation

such that the metavolcaniclastic/metagreywacke was reduced to a melange in which vein segments acted as competent clasts;

a large-scale porphyroblast/matrix system. Formation of the Manyane Thrust to the south of the Mfongosi Group interrupted

the normal retrograde metamorphism of the remainder of the Tugela Nappe and initiated a "hot iron effect" whereby a short-lived

thermal pulse acted at the thrust plane, producing a reversed geothermal gradient in the underlying Mfongosi group. This

reversed gradient would have been counteracted by a steepened normal geothermal gradient in the Mfongosi Group caused by

overloading of the Natal Thrust Front by the Natal Nappe Zone. These geothermal gradients partly account for the

concentration of veining in the areas of the Mfongosi Group which are directly adjacent to the Manyane Thrust, and directly

adjacent to the Kaapvaal Craton, in the lower portions of the thrust front Stable isotope studies indicate fractionation between

vein and wall rock under a short-lived, mainly rock-buffered, layer-scale fluid-movement system.

Also forming part of the Mfongosi Group of the Natal Thrust Front, the Ngubevu area contains an apparently enigmatic

distribution of veining accompanied by gold and base metal mineralization. The structural evolution of the Ngubevu area

occurred during consistent left-lateral transpression into which has intruded early-tectonic veins, formed by pressure solution

and having the same structural format as the early-tectonic veining in the southern Mfongosi Valley area. Subsequent

deformation of the system was accompanied by 1900 -trending tension gashes which were continually ptygmatically-folded,

sheared and offset to form occasionally mineralized quartzofeldspathic "blows" and along-strike stringers in the epidote-

actinolite schist. Where veining cross-cuts narrow calcite - graphite - sericite - quartz - albite - tourmaline ± chlorite schist

layers, gold mineralization occurred. The late-tectonic tension gashes, antitaxially filled by quartz and amorphous calcite,

cross-cut the entire range of lithologies. The fluid system during vein deposition varied: during infilling of early-tectonic

fractures a short-lived fluid-flow system dominated, with the emplacement of re crystallized wallrock occurring in a closed, non-advective regime under the influence of diffusion caused by pressure solution. The fluid system changed to a more open,

advective, greater than layer-scale rock-buffered one with a decreasing contribution of material from immediate host rocks.

An internal fluid source is implied for the entire period of vein emplacement, derived from structural analyses which indicates

negative dilation across the Mfongosi Group in this area and by comparison of vein:wallrock δ180 values which indicate a lack

of igneous-derived fluids.

The Phoenix Mine, in the central portion of the Tugela Nappe, and the Ayres Reef, hosted in Manyane amphibolite adjacent

to the Manyane Thrust, are grouped together on the basis of their cross-cutting nature and timing with respect to metamorphism

and deformation of the host rock, and also due to their similarity in isotopic plots. Both vein sets occur in approximately east-west to east-northeast-trending zones which show evidence of late-tectonic left-lateral movement. Phoenix Mine veining

occurs in weakly-metamorphosed meta-gabbro/meta-norite of the Tugela Rand Complex. The Manyane amphibolite

demonstrates the amphibolite facies of metamorphism due to the short-lived thermal pulse at the Manyane Thrust. Both sets

of veining display slickenlines which are indicative of their emplacement prior to the late-tectonic left-lateral movement. The

unusually thick quartz veins of both deposits are the results of late- to post-Tugela Rand Complex fluids or the tapping of late-tectonic metamorphic fluid reservoirs. This caused silica metasomatism and redeposition of material in post-thrusting collapse

features. A highly channelized, single-pass fluid system is proposed in the absence of intrusion-derived fluids.

Whole rock geochemical data allow a distinction to be made between the Natal Thrust Front and the Natal Nappe Zone: the

Foremost nappe of the nappe zone consists primarily of N-type mid-ocean ridge basalts/ocean-floor to within-plate basalts which

were intruded prior to nappe emplacement by metaluminous orogenic volcanic arc granitiods. The thrust front displays a lateral

variation in metabasite/metasediment ratio, with the ratio increasing from east to west in this inlier. In the east, in the

Nkandlha area, melanged metagreywackes dominate and there is a marked paucity of associated metabasites. In the central

portions of the thrust front, in the vicinity of the Mfongosi area, active continental margin/continental arc magmatogenic

greywackes and arkoses are interlayered with calk-alkaline volcanic arc basalts (volcaniclastics). The greywacke geochemistry

indicates little to no mafic/ultramafic influences in sediment contribution and the source of sediment is inferred to be the

southern portions of the Kaapvaal Craton. The Nkandlha and Mfongosi area Mfongosi Group segments are considered to be

in-situ or para-autochthonous. The western-most Ngubevu area predominantly hosts metabasites. The geochemistry of the

metabasites indicates that they are N-type mid-ocean ridge basalts/ocean floor basalts from a destructive plate margin setting.

The metabasites are interbanded with metapelitic/metacalcsilicate layers produced in a shallow water oxic environment, here

inferred as a spatially-restricted shallow, marginal basin. The metabasites in the Ngubevu area are notably similar to those

of the Madidima Nappe, indicating a similar provenance and pre-collisional mode of formation. It is proposed that the variation

in the Natal Thrust Front was due to a north-east/south-west distribution of lithological proportions or mixing, with greywackes

dominating in the northeast (in proximity to the Kaapvaal Craton) and metabasites dominating in the southwest. Left-lateral

transpressional movement within the Mfongosi Group of the Natal Thrust Front, and the Natal Nappe Zone, was continuous

throughout plate collision and obduction.