Abstract
D.Phil. (Geology)
A specific geological event has been characterized with the aid
of an integrated metamorphic and fluid inclusion study of data
obtained from syn-tectonic vein-quartz associated with thrusting
and bedding-parallel shear along the northern margin of the
Witwatersrand Basin.
The vein-quartz associated with this event occurs as
boudin-shaped bodies with their long and intermediate axes
orientated within the foliation-, bedding- or fault-planes. The
length of the quartz lenses which are spatially confined to shear
zones often exceeds the thickness of the shear zones. These
phenomena and the fact that quartz-fibres are orientated parallel
to and not at right angles to the foliation confirms the
syn-tectonic nature of the quartz veins.
Heterogeneous P-T condition is indicative of imbrication, i.e.
crustal thickening which is also substantiated by the random
growth of pyrophylite and kyanite in shear zone assemblages,
indicating that metamorphism outlasted deformation. Metamorphic
studies of aluminous schists and vein-quartz with pyrophylliteand
pyrophyllite - kyanite selvages established the development
of two critical mineral assemblages:
1 Kaolinite + 2 Quartz = 1 Pyrophyllite + 1 H20 ... (1)
and at higher P-T conditions
1 Pyrophyllite = 1 Kyanite + 3 Quartz + 1 H20 ... (2).
The schists and quartz vein assemblages are quartz-oversaturated
in contrast to the study material of Wallmach and Meyer (1990)
which is quartz-undersaturated. Peak metamorphic conditions,
therefore, are closely constrained by the position of the
reaction curve (2) in P-T space, as is also substantiated by the
presence of coexisting kyanite and pyrophyllite which are closely
associated with syn-tectonic vein-quartz at the Florida Lake,
Monarch Shaft and Krugersdorp localities. The nature of and
circumstances under which the equilibrium aSsemblage pyrophyllite
+ kyanite + quartz has formed support an univariant situation,
i.e. this assemblage can only coexist along the pyrophyllite
kyanite isograd. The mineral assemblages that equilibrated during
peak metamorphism are still present in the rocks of the shear
zones, and show only incipient rehydration.
The quartz-oversaturated nature of the rocks in the shear zones
and the fact that kyanite formation is ascribed to reaction (2),
cannot explain the abundance of quartz veins. Accordingly it is
concluded that there must have been an external source from which
Si02 was imported into shear zone to give rise to the formation
of the large quantities of vein-quartz.