The structural, metamorphic and tectonic context of selected sub-economic veining in the Natal thrust front and Natal Nappe zone, Northern KwaZulu-Natal.
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Date
2000
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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.
Description
Thesis (Ph.D.)-University of Natal, Durban, 2000.
Keywords
Geology--KwaZulu-Natal., Geochemistry--KwaZulu-Natal., Gold ores--KwaZulu-Natal., Theses--Geology., Geology, Stratigraphic--Precambrian.