Manganese oxyhydroxide mineralization is widespread in the Constantiaberg Massif. It is largely hosted by west-northwest – east-southeast trending brittle structures in the competent Ordovician arenites of the Peninsula Formation of the Cape Supergroup. Manganese is also found impregnating more porous Peninsula Formation arenites and Quaternary scree. This study proposes that the more significant deposits at Hout Bay and Constantiaberg differ from most of the mineralization in that they are spatially associated with saprolithic dolerite dykes belonging to the 132 Ma False Bay dolerite dyke swarm. It is suggested that this deep weathering may be related to a Miocene palaeo-landsurface, yielding a maximum age for these more important Mn deposits. However, there is evidence for mineralization of different ages through the Quaternary Period. Apart from Mn-mineralization hosted by rare breccias containing hydrothermal quartz, most of the mineralization is supergene, having been leached and transported in reduced acidic groundwater and precipitated at or near a redox front in a near-surface environment.

The Muizenberg Block, is an outlier of the Table Mountain Group (Cape Supergroup) and lies to the west of the main occurrence of the Cape Fold Belt. Structural mapping of this outlier has identified a spaced cleavage in Peninsula Formation quartz arenites, and an exposure of dolerite that have not been previously documented. Monoclinal folding, an unrelated spaced cleavage, low-angle faults, and steeply dipping faults, that strike eastnortheast-west-southwest, are similar to structures found in the syntaxis domain of the Cape Fold Belt and are therefore thought to have formed during the Late Permian-Early Triassic Cape Orogeny. However, as the Muizenberg Block lies outside the presently accepted boundaries of the syntaxis domain, those boundaries may deserve re-examination. Sub-vertical dilational quartz veining, jointing, cataclastic faulting and the intrusion of a sub-parallel dolerite dyke, that all strike northwest-southeast, are thought to represent a younger tectonic event. They are provisionally attributed to the Upper Jurassic to Cretaceous taphrogenesis that preceded the drifting apart of South America and Africa.

An exploration programme over the Blaauwbank group of gold deposits in the Witwatersberg Goldfield, which lies to the south and in the footwall of the Bushveld Complex, South Africa, has shown that these deposits possess many similarities and some differences to turbidite-hosted gold deposits.The dominant control to this hydrothermal mineralization is structural, with the associated quartz veins being localized along bedding planes, fold axes and a variety of semi-ductile to brittle faults. The deposits of the Witwatersberg Goldfield are also preferentially located in the Klapperkop Quartzite Member and overlying shales of the Timeball Hill Formation, Transvaal Supergroup. The rocks do not appear to form a particularly favourable protore but rather represent a favourable depositional site. The competency contrast of the shales, siltstones and sandstones in this stratigraphic interval is regarded as being important in facilitating vein formation, and the zone is also characterized by a high Fe2+/Fe3+ ratio and high C content, which may have reduced the mineralizing fluids and caused gold to precipitate. Fluid inclusion data show the quartz veins formed at 250-330 degrees C and pressures of 1.3-1.75 kb, from CaCl2-rich brines.Comparison with structural studies to the north and with the Sabie-Pilgrim's Rest Goldfield to the cast suggest that the mineralization probably predates the peak of metamorphism related to the intrusion of the Bushveld Complex. (c) 2005 Elsevier Ltd. All rights reserved.

Faults are important to the economic geologist in that they can either control the formation of some classes of ore deposits or modify, the position and geometry of ore bodies. Fault rock classification is an important part of understanding the structural geology of an area. In this paper practical modifications are suggested to the most commonly used classifications of fault rock, to make it more applicable for the economic geologist. The system recommended is based in the first instance on the classification according to cohesiveness of the rock. The second defining attribute is whether the matrix is foliated or has a random fabric, providing the subdivision of cohesive fault rocks into the mylonitic and cataclastic series. The third attribute used to categorize fault rocks is the proportion of matrix to clasts, and differs in detail from those previously published. The terminology relating to crush breccias used by earlier systems is considered confusing and Superfluous and has been eliminated.Although of lesser importance to the economic geologist, the terminology evolving for mylonites and mylonitc gneisses is included and is based on internal fabrics and textures.Application of this simplified classification to geological mapping in underground exploration development and logging of faults in core will enhance the geologist's ability to interpret the structure of ore deposits.

Pseudotachylitic breccias, associated with either bedding-parallel or normal faults, are abundant in the northern and northwestern parts of the gold- and uranium-rich Witwatersrand Basin in South Africa. They are particularly abundant in a zone tangential to the Vredefort Dome, a structure which is now widely accepted to be the eroded remnant of the central uplift of the originally 250 to 300 km wide Vredefort impact structure.

Pseudotachylites have been documented in association with faults on the West Rand Goldfield. Several lines of equivocal evidence indicate that the pseudotachylites probably formed over a depth range of 1.9 to 6.6 km under relatively low differential stresses and near-hydrostatic pore fluid pressures. The factors that are considered important in the formation of these pseudotachylites are a high slip velocity and displacements of more than about 20 cm on very narrow faults. For a given set of conditions, frictional melting is more easily induced in Ventersdorp volcanic host rocks than in Witwatersrand quartzites.

The chemical composition of the pseudotachylyte in the West Rand Goldfield of the Witwatersrand Basin, South Africa, is closely related to the composition of the host rocks and this is reflected in the colour of the pseudotachylyte. Grey pseudotachylyte is generally hosted by and similar in composition to quartzites of the Witatersrand Supergroup, whereas maroon pseudotachylyte has a similar relationship to the mafic lava of the Ventersdorp Supergroup. In some instances, the composition of the pseudotachylyte is intermediate between these two host rock types and a mixing process is proposed. A study of the ferrous to ferric iron ratio provides limited evidence that pseudotachylyte is slightly more reduced than the rocks from which they have been derived. The only elements that are consistently enriched in the pseudotachylyte, irrespective of host rock composition, are S, Pb and Au. It is speculated that this indicates the existence of a sulphide-bearing fluid phase along the fault zone either prior to or during pseudotachylyte formation. Geochemical and petrographic evidence favour an origin by frictional fusion rather than ultracomminution for the pseudotachylyte from the West Rand Goldfield.

Several generations of pseudotachylite have been distinguished in both the Vredefort «Dome» and the Witwatersrand Basin and an enrichment in lead relative to their host rocks is shown by pseudotachylites from both settings. The planar microdeformations that characterize quartz clasts from Vredefort pseudotachylites have now also been found in quartz from Witwatersrand pseudotachylites, although they are not as abundant in samples from Witwatersrand localities. Experimental and field evidence show that these planar microdeformations are probably not diagnostic of shock metamorphism. Doubt is therefore cast on the validity of any theory postulating the origin of the Vredefort «Dome» and associated pseudotachylites as being due to a single central large-scale shock event, whether this be bolide impact or an internal gas explosion

Material similar to naturally occurring pseudotachylite was generated in Proterozoic lava as a result of frictional overheating of a diamond drill DH 5000 impregnated crown at a depth of 2.4 km below surface. Bit rotation equation to slip rates of about 1 m/s is thought to have caused severe overheating to last for 3–10 s. Estimates show that melting occurred in the temperature range 800°–1100°C under wet conditions and at comparatively low effective normal and shear stresses of about 1.9 × 107 and 9.7 × 106 Pa respectively. Associated breccia formed as a result of thermal decrepitation rather than dynamic comminution and, as a corollary, it is suggested that some cataclasites associated with pseudotachylite-bearing faults may also form by the mechanism. This implies that the presence of both cataclasite and pseudotachylite on a fault plane does not necessarily indicate reactivation.

A Proterozoic bedding-parallel fault zone is described from the Witwatersrand basin in South Africa. The fault zone is dominated by pseudotachylite, the youngest tectonite present, but also contains quartz veins and cataclasites which post-date the spatially associated mylonites. Both tectonic eliminations and duplications are caused by the fault zone which is dominated by a northerly to westerly overthrusting. The fault rocks probably did not form during a single event but as a result of a minimum of two periods of activation of the fault zone separated in time by at least the period required to deposit in excess of 4 km of sediments belonging to the lower part of the Transvaal Sequence.

Sporadic epigenetic antimony-gold mineralisation along the ENE-trending Antimony line of the Murchison schist belt is structurally controlled and spatially related to carbonated rock types with quartz-carbonate veins in tension fractures. The Antimony line is a discontinuous zone of brittle-ductile deformation and is locally oblique to lithological layering within a broad ductile shear zone. This tectonically early (D1) shear zone has an oblique-to dip-slip, reverse (north-side-up) movement sense with downward-facing fold structures in the hangingwall. Later (D2) fold structures deformed the Antimony line and its wallrocks into steep structures. The Antimony line is intruded by granitoids which post-date D1 and are not directly related to mineralisation, the source of which may have been metamorphic fluids.