There are many natural resources in the earth of Cape Town and the Western Cape area. Greater Cape Town is in close proximity to stone aggregates from Malmesbury Group quartzitic hornfels and fine-grained homogenous granite of the Cape Granite Suite. The main stone aggregate resources found in the Greater Cape Town area are; granite, Malmesburg slate, Table Mountain sandstone. There is also; brick clay, plastic clay, silica sand, gravel (ferricrete), and kaolin. The distribution of these resources are all shown in the image below (click to expand).
Building sand is another main resource located near the central urban hubs of Cape Town. These sand dune resources, however, are limited and future sources of sand will either be from crushed sandstone from the Table Mountain Group or sand transported from distant sites. Areas containing building sand, phosphate, and limestone are shown in the diagram below.
Course-grained, porphyritic granite is found in the Mamre region of Cape Town. This granite composes part of the Late Precambrian Darling Batholith. Some granite within the Batholith contain large alkali feldspar phenocrysts, which are used in concrete.
Melmesbury slate is the major source of stone aggregate in the Cape Town area. This resource contains hornfels within the zone of thermal metamorphism and quartzite sandstone from the Late Precambrian Malmesbury Group. These resources are currently being exploited from four quarries in the Tygerberg Hills and the Eerste River. These materials are also used for concrete and road construction.
Table Mountain Sandstone
Table Mountain sandstone used to be used for concrete aggregate, railway ballast, and road construction, however, the quarry is now part of the Cape Peninsula National Park, which disallows mining. Resources are, however, being exploited northeast, east, and southeast of Somerset West, via a shallow quarry and Table Mountain sandstone continues further east for another 20 km.
Most of the brick clay in Cape Town comes from the Malmesbury Group and can usually be found in the generally flat palaeo-erosion surfaces. These surfaces underwent weathering and were covered with ferricretes and silcretes in the pedogenic process. There are a total of 12 resource fields between Stellenbosch and Klipheuwel and most of these contain residual clay from the Malmesbury Group.
Plastic clay in Cape Town is Tertiary in age and consists of transported sedimentary clays, which are rich in kaolinite and contain small quantities of quartz, mica, illite, and montmorillonite. When fired, these clays are white or off-white in colour, so they have been used in the ceramic industry. However, their main use is as a plasticizer in the brick making industry.
Three resource fields occur for silica sand in the Cape Town area. This sand is used for glass manufacture. The silica sands in the Springfontyn Formation are clean, well sorted, and round, with a fine to medium grain size. The silica sand deposits in Cape Town are relatively high in grade and there is little known deposits outside of the area.
Ferricrete is the most valuable resource for gravel in Cape Town. It is used for asphalt and concrete. Surfaces containing ferricrete have undergone much weathering in the past and so contain thick residual clays. Many of the resource fields for ferricrete have alrady been exploited in Cape Town.
Kaolin was previously used for ceramic manufacure, paper filling, paint filling, pesticides, medicines, cosmetics, and pharmaceuticals. Due to the discovery of cheaper resources, kaolin lost it’s demand and the last mine was closed in 2007. All kaoline deposits in Cape Town are a result of weathering of course-grained poryphritic granite granite.
The building sand is course-grained and contains shell fragments, making it suitable for mortar sand. In the Philippi Horticultural Area, the dune sands overlie high-grade silica sand. High-grade silica sands could also be used for mortar or plaster sand in addition to glass manufacture. There are no major building sand resources outside of the Cape Town area and these resources will likely be depleted within the next 20 years. The best replacement will likely be crushed sand from Table Mountain sandstone.
Limestone in Cape Town comprises of quartz-bearing limestone, calcareous sandstone, and subordinate pedogenic calcrete. The original source came from beaches and was deposited in the north dunes on the Cape Flats. These sediments were calcified due to groundwater dissolution of calcium carbonate, leading to cemented aeolianite. Due to low grades, building use is low for this limestone.
There is one deposit of phosphate in Cape Town and it consists of quartz sand and phosphate shell fragments that formed five million years ago. This deposit is used for agricultural fertilizing.