OVERVIEW OF THE IMPACT OF MINING ON THE ... - IIED pubs

In its upper reaches, the sub-catchment is underlain by rocks of the Transvaal Sequence, which form the major
portion of the Drakensberg Mountain range. The lithology is dominated by quartzites, chert, hornfels, basic lava
and dolomite. The dolomite outcrops, in particular, are important sources of good quality water in the upper
catchment and contribute water throughout the year.
Further to the east and south, rocks of the Gravelotte Group (part of the Murchison Sequence) and Rooiwater
Complex outcrop in the vicinity of the town of Gravelotte. Quartzite, schists, basic lava and granitic rocks
dominate the Gravelotte Group lithology. These greenstone formations contain important deposits of antimony
and gold, with minor deposits of mercury and zinc. An extensive deposit of heavy mineral sands (illmenite, rutile
and zirconium) is located near the town of Gravelotte.
The felsites and gabbros of the Rooiwater Complex are easily eroded and contribute increased levels of
sediments in the valley depressions and river channels. Further to the east, the rocks of the crystalline
Basement Complex outcrop at various points across the Lowveld. These rocks consist mainly of potassium-rich
granites and gneisses with feldspar and mica outcrops. The exposed weathered granites are easily eroded,
particular where the vegetation cover is sparse or denuded. Accelerated rates of sediment production occur in
this region, with large amounts of coarse and fine sediments accumulating in river channels. The slightly harder
rocks of the Mashimale Suite are intrusive into the rocks of the Basement Complex and are visible as stack-like
outcrops of fine-grained, light-coloured granites.
The Phalaborwa area is underlain by granite and gneiss rocks of the Basement Complex. These were intruded
by a cyclical series of alkaline-rich magmas that have given rise to the highly mineralised carbonatite and
pyroxenite rocks of the Phalaborwa Complex. Plugs and dykes of fine-grained syenite that were intruded during
the alkaline phase of the Phalaborwa Complex are resistant to weathering and remain as conspicuous stack-like
hills and ridges.
North-east to south-west trending dykes and sills of undifferentiated mafic intrusives (mainly Karoo age dolerite)
occur over much of the Lowveld Basement Complex, with particularly well-developed examples of dyke swarms
in and around the Phalaborwa area. These rocks are less resistant to weathering than their granite and gneiss
host rocks and give rise to negatively weathered linear features in the landscape, often forming or delineating
drainage lines.
5.7.1.3 Pedology, agriculture and land use
Soils in the Selati sub-catchment can be divided into four broad groups or regions:
• Small areas of shallow, sandy or gravely soils the upper reaches;
• Areas of sandy colluvial soils at the foot of the escarpment, grading into red apedal soils in the western part
of the sub-catchment;
• Shallow, brownish to greyish-brown sandy soils overlying coarsely weathered rock in the eastern portion of
the sub-catchment; and
• Transported alluvial deposits of coarse to fine-grained sands and silts located along drainage lines.
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