Micro-fractures in the Ngubevu Gold and Base Metal Mines (Natal Thrust Front, Tugela Terrane, South Africa): Are they consistent with macro-scale deformation, and what do they tell us?

Abstract

In the Natal Thrust Front (NTF) of the Tugela Terrane, there is a lack of multi-scale characterisation of structural controls on shear-zone hosted precious and base metal mineralisation. This paper contributes to the current understanding of the structural controls on shear-zone-hosted precious and base metal mineralisation in the NTF with insight derived from the analyses of micro-fractures at and around the known shear-zone-hosted lode gold and base metal mineralisation in the Ngubevu area of the NTF. Based on the notion that the spatial distribution of stresses that control geological structures is fractal (i.e., possessing scale-invariance or self-similarity properties) across a range of geographic scales, this study (a) determines whether the stresses inferred from micro-fractures in the gold mines in Ngubevu are consistent with the stresses associated with regional- to local-scale geological structures in the NTF, and (b) interprets the mineralisation controls implied by these inferred and known stresses. From rose diagrams showing trends of micro-fractures in oriented vein material samples from Ngubevu, the relative micro-scale stress field per sample based on the maximum horizontal stress (SHmax) and the minimum horizontal stress (SHmin) were inferred. These were then compared to the local- to regional-scale stress field orientations derived from existing studies in the NTF. The north-northeast–south-southwest orientations of SHmax of micro-fractures in samples from Ngubevu are consistent with the north-northwest–south-southeast orientations of SHmax of regional- to district-scale thrust faults in the NTF of the Tugela Terrane. Likewise, the west-northwest–east-southeast orientations of micro-fractures in samples from the eastern part of Ngubevu are consistent with the general east–west orientations of local-scale S1 foliation. These orientations are associated with a D1 thrusting event, which was due to north to north-northeast-directed regional thrust tectonics. The clockwise change from the north-northwest–south-southeast orientations of SHmax of regional- to district-scale thrust faults to the north-northeast–south-southwest orientations of SHmax of micro-fractures in vein material attests to an earlier posited notion of clockwise rotation of regional- to district-scale thrust vergence. These suggest that V1 veins in Ngubevu were likely formed late in the D1 event. The orientations of micro-fractures in samples, mostly from the western part of Ngubevu, depict a strike-slip regime, which is consistent with D2 sinistral transpression that affected the earlier-formed east–west-trending structures. This suggests that some V1 veins in Ngubevu, particularly those in its western part, were affected by the D2 sinistral transpression (e.g., by remobilisation of gold in some of the V1 veins). The novel findings in this study are crucial to geological mapping, recognition of mineralisation controls and mineral prospecting in poorly-explored regions where the geology is permissive (i.e., represents processes that are favourable) for the formation of certain type(s) of mineralisation.