Sheet 12 – Pilandsberg – Union Of South Africa Department Of Mines And Industries – Geological Survey

Cartographer: Hall, A. L & Humphrey, W. A
Publisher: Union Of South Africa Department Of Mines And Industries – Geological Survey
Price: £145 (post-free in the UK)
Publication Date: 1911
Edition: 1st edition thus
Format: Colour printed lithograph
Condition: In good condition
Sheet Size: 86.2cm x 54.2cm

Condition:

Sheet Size: 86.2cm x 54.2cm. Small Institutional stamp to upper margin. Closed tears and chipping to the margins with a small amount of loss. Creased, marked, dusty and rubbed but this is mainly confined to the margins. Colouration very bright and clean. A good copy. Very scarce.

Location: Pocket RSAGEOL: SR: 002867

The Geology Of The Rustenberg District: A Brief Overview

1. Introduction and Geographic Context

The Rustenburg region, located in the North West Province of South Africa, lies on the southern flank of the Bushveld Igneous Complex (BIC)—the largest layered igneous intrusion on Earth. Rustenburg itself is situated near the western limb of this complex and has developed over time into one of the country’s key mining centres, particularly known for its platinum-group element (PGE) deposits.

This region’s geology reflects a deep and complex history of igneous activity, craton stability, and mineral concentration, making it of global scientific and economic interest.

2. Geological Framework: The Kaapvaal Craton

The Rustenburg area is underlain by the Kaapvaal Craton, a stable block of continental crust dating back over 3 billion years. This ancient craton provided the rigid tectonic platform upon which the Bushveld Complex was emplaced during the Palaeoproterozoic Era, around 2.05 billion years ago.

The underlying basement rocks include:

• Granitoids and tonalitic gneisses
• Remnants of greenstone belts
• Localised zones of shearing and metamorphism, related to earlier tectonic events

These basement rocks acted as a structural floor for the Bushveld magmatism and influenced the distribution and geometry of subsequent intrusions.

3. The Bushveld Igneous Complex

The dominant geological feature of the Rustenburg region is the Bushveld Igneous Complex, a vast, differentiated magmatic body composed of:

• Layered mafic and ultramafic rocks (in the Rustenburg Layered Suite)
• Overlain by granitic units (in the Lebowa Granite Suite)
• Associated with large-scale mineralisation, particularly of chromite, magnetite, and platinum-group elements

1. Rustenburg Layered Suite

This suite is the most economically important and geologically studied component of the region. It consists of:

• Pyroxenites, norites, gabbros, and anorthosites
• Stratiform layers of chromitite and sulphide-rich horizons
• The Merensky Reef, the primary source of platinum in South Africa
• The UG2 Chromitite Layer, rich in both chromium and PGEs

These layers were formed through fractional crystallisation within a large magma chamber, with sequential deposition of minerals in discrete bands over time.

4. Mineral Resources and Economic Geology

The Rustenburg region is one of the world’s most productive sources of platinum-group elements, which include:

• Platinum, palladium, rhodium, and associated metals
• Extracted primarily from the Merensky Reef and UG2 Layer
• Mining is conducted via underground shafts and open stopes

In addition to PGEs, other economically significant minerals include:

• Chromite (for ferrochrome production)
• Vanadium, especially in the magnetite-rich zones of the Main Magnetite Layer
• Nickel and copper, present as by-products in sulphide ores

The geology has directly shaped the development of Rustenburg into a major mining and industrial hub, supporting thousands of livelihoods and playing a key role in South Africa’s economy.

5. Structural and Tectonic Features

While the Bushveld Complex was emplaced under relatively stable tectonic conditions, post-emplacement deformation has influenced the geology of the Rustenburg area.

Key structural features include:

• Fault zones and dykes, especially the Rustenburg Fault
• Flexures and synclinal structures in the layered units
• Localised intrusions and cross-cutting relationships, especially in late-stage phases

These structures can affect mining operations, ore continuity, and hydrogeological conditions.

6. Surface Geology and Recent Deposits

Much of the region is covered by a thin mantle of Cenozoic soils, gravels, and residual clays. These are derived from the chemical and mechanical weathering of the Bushveld rocks, forming:

• Lateritic soils, rich in iron and aluminium oxides
• Alluvial and colluvial deposits, particularly in valley bottoms
• Occasional calcrete horizons, indicating semi-arid pedogenesis

These surface deposits influence land use, especially subsistence agriculture, grazing, and infrastructure development.

7. Hydrogeology

Groundwater resources are generally limited in the Rustenburg area, due to:

• Low primary porosity in the igneous rocks
• Reliance on fracture-controlled aquifers
• Variable water quality, with issues of salinity, hardness, and metal contamination in some mining-affected areas

Water is sourced from boreholes, mine dewatering systems, and nearby dams, and is critically important for both industrial and domestic supply.

8. Environmental and Sustainability Considerations

Intensive mining has raised a number of environmental challenges, including:

• Land degradation from open-cast and underground mining
• Water contamination from mine effluent and tailings
• Air quality issues, particularly from smelting operations

The regional geology plays a dual role—hosting rich ore bodies, but also posing complex rehabilitation demands due to the hardness and poor soil-forming capacity of the underlying rock.

9. Scientific Significance

The Rustenburg area is of international scientific importance due to:

• The Bushveld Complex being a type locality for layered mafic intrusions
• Extensive research into magmatic differentiation, ore genesis, and crustal evolution
• The role of the region in understanding mantle-crust interaction and large igneous provinces

It remains a key site for field education, mining geology, and advanced academic research.

Conclusion

The geology of the Rustenburg region encapsulates one of Earth’s greatest natural geological features: the Bushveld Igneous Complex. Formed through ancient magmatic processes on a stable craton, and enriched with the world’s largest reserves of platinum-group elements, this region represents a unique intersection of natural history, economic development, and ongoing scientific inquiry.

From the deeply ancient basement to the modern industrial landscape, Rustenburg’s geology continues to shape its past, present, and future.

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