Sheet 9 – Marico – 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: £125 (post-free in the UK)
Publication Date: 1910
Edition: 1st edition thus
Format: Colour printed lithograph
Condition: In very good condition
Sheet Size: 87.8cm x 54.9cm

Condition:

Sheet Size: 87.8cm x 54.9cm. Small Institutional stamp to upper margin. Creased, marked, dusty and rubbed but this is mainly confined to the margins. Colouration very bright and clean. A very good copy. Very scarce.

Location: Pocket RSAGEOL: SR: 002870

The Geology Of The Marico And Rustenburg Districts: A Brief Overview

1. Introduction and Geographic Context

The Marico and Rustenburg districts, located in the North West Province of South Africa, lie within one of the most geologically significant and economically important zones on the African continent. These districts straddle the south-western margin of the Bushveld Igneous Complex (BIC) and the surrounding Kaapvaal Craton, revealing a rich history of igneous activity, mineralisation, and sedimentary platform development.

Topographically, the region is defined by undulating bushveld terrain, low ridges, and platinum-rich geological structures, with the Marico River to the west and the town of Rustenburg forming the eastern anchor of the mining corridor.

2. Basement Geology: Kaapvaal Craton

Both districts are underlain by the ancient Kaapvaal Craton, one of the oldest preserved pieces of continental crust, dating back over 3 billion years. Although largely covered by younger formations, the craton forms the stable tectonic foundation upon which later geological events occurred.

The basement rocks include:

• Granitoid and tonalitic gneisses
• Remnants of Archaean greenstone belts
• Localised high-grade metamorphic zones

These rocks, while rarely exposed at the surface in this region, have strongly influenced the emplacement of younger igneous intrusions and the structural architecture of the surrounding crust.

3. The Bushveld Igneous Complex

The dominant geological feature of the Rustenburg and Marico districts is the Bushveld Igneous Complex, emplaced around 2.05 billion years ago during the Palaeoproterozoic. This vast, layered mafic intrusion forms a semi-circular structure across northern South Africa and is divided into:

• Rustenburg Layered Suite (mafic-ultramafic rocks)
• Lebowa Granite Suite (granitic upper portions)
• Rooiberg Group (felsic volcanic rocks)

In the Rustenburg and Marico sectors, the western limb of the Bushveld Complex is especially well developed and economically important.

1. Rustenburg Layered Suite

This suite consists of layered pyroxenites, norites, anorthosites, and chromitites, deposited in a magmatic layering sequence as part of a massive subvolcanic chamber. The most prominent mineralised layers include:

• The Merensky Reef, which hosts vast quantities of platinum-group elements (PGEs)
• The UG2 Chromitite Layer, rich in chromium and platinum
• The Main Magnetite Layer, a source of vanadium-bearing titanomagnetite

These layers were deposited through fractional crystallisation, and the Rustenburg area, in particular, is home to some of the world’s largest platinum mines.

4. Mineral Resources and Economic Geology

The Marico and Rustenburg districts form part of the western limb of the Bushveld Complex, home to extensive mining operations focused on:

• Platinum-group elements (platinum, palladium, rhodium)
• Chromite, used in ferrochrome production
• Vanadium, extracted from magnetite layers

Mining activity is particularly intense around Rustenburg, which has developed into a major industrial centre. In the Marico area, small- to medium-scale operations exist, alongside exploration activities for PGEs and base metals.

5. Structural Geology

The western Bushveld limb is broadly synclinal, with the layers dipping inward toward the centre of the complex. Structural features include:

• Gentle folding and flexures within the intrusion
• Faults and dyke intrusions, influencing orebody continuity
• Zones of igneous layering disruption, which can affect mining strategy

The structure of the Marico sub-basin is more subdued but still reflects the broader tectonic geometry associated with the western Bushveld.

6. Supracrustal Cover and Surface Geology

Although the Bushveld Complex dominates the bedrock geology, the surface across much of the Marico and Rustenburg area is covered by:

• Lateritic soils, developed from prolonged weathering of mafic rocks
• Colluvial and alluvial deposits, especially along seasonal rivers and drainage lines
• Karoo Supergroup remnants, preserved on the flanks of ridges or in faulted blocks

These deposits influence agricultural potential, surface water flow, and the ecological characteristics of the bushveld biome.

7. Hydrogeology and Water Resources

Groundwater in the region is hosted in:

• Fractured Bushveld rocks, with moderate to low yields
• Weathered granite and norite zones, where water accumulates in saprolite
• Alluvial aquifers along the Marico River and associated drainage basins

Water quality can be affected by mining activity, particularly where acid drainage or metal contamination from tailings and waste rock occurs. Nevertheless, groundwater remains a crucial source for rural communities and agricultural operations.

8. Environmental and Land Use Considerations

The geology of the Marico and Rustenburg districts has shaped both economic development and ecological constraints:

• Mining-induced land degradation and water stress are significant challenges
• Conservation areas in the Marico district protect dolomite springs, sandstone ridges, and rare fauna
• Land use is a mix of mining, cattle grazing, subsistence farming, and nature-based tourism

Balancing mineral extraction with long-term environmental sustainability remains a central issue in the region’s future.

9. Scientific and Educational Significance

This region is internationally significant for its:

• Role in understanding layered igneous intrusions
• Insights into platinum-group mineralisation and magmatic processes
• Geological training and research, with numerous field schools and academic studies

Both Marico and Rustenburg continue to be focal points for mining geology, igneous petrology, and economic geology research.

Conclusion

The geology of the Marico and Rustenburg districts tells a story of ancient crustal stability, colossal magmatic activity, and the concentration of the world’s most valuable mineral resources. From the deep-seated Archaean roots of the Kaapvaal Craton to the massive layered sequences of the Bushveld Complex, this region offers a near-complete cross-section of southern Africa’s geological evolution.

Economically pivotal, scientifically rich, and environmentally complex, the geology of this region continues to shape its identity and its global significance in the realm of mineral wealth and geological understanding.

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