Rockscape

Mention reef facies to most geologists and they might conjure up a mental image of a carbonate reef complex. But to geologists working in the Bushveld Complex, reef facies refers to the variation that a single mineralized horizon exhibits within the layered ultramafic-mafic sequence. One such example is that of the Merensky Reef, which has been laterally subdivided into different facies based on variations in the modal characteristics and textural appearance of the reef itself. The inherent complexities of these reef facies are revealed by detailed logging of numerous diamond drill cores through stratigraphic intervals. There are many different facies of the Merensky Reef; some examples in the western Bushveld Complex are shown in the corresponding illustration below. Note that various mines use different terminology to describe the same reef facies.

When considering the different reef facies, the Merensky Reef (defined as the economically exploitable zone containing platinum group elements) is neither located at a consistent stratigraphic level within the Merensky Cyclic Unit nor consistently associated with a specific lithological layer. In the RPM-Pegmatoidal Merensky Reef facies, the platinum group elements (PGE) are associated with the Merensky Pegmatoid and concentrated in the chromitite layers at the upper and lower contacts of the pegmatoid with the hanging and footwall units respectively. In contrast, in the Marikana Merensky Reef facies, PGE mineralization is not necessarily associated with the Merensky Pegmatoid, which may or may not be present. Instead PGE are found concentrated in the upper two of three chromitite layers, if three such layers are present. When considering Thin-Marikana Transitional Merensky Reef and Thin Merensky Reef, the Merensky Pegmatoid is absent and only a basal chromitite layer is present. These reef facies are bottom-loaded, that is, the PGE are largely concentrated about the basal chromitite layer with little or no mineralization stratigraphically higher in the Merensky Pyroxenite. The latter two reef facies are analogous to “contact reef” at Rustenburg Section and “pyroxenite reef” at Impala.

To better understand the process (or processes) that led to the formation of the different reef facies and the Merenksy Reef itself, it will be necessary to characterize in much finer detail any variation between the different reef facies. Such variation should be considered within the known context of the three types of stratification found in layered intrusions. Modal layering is characterized by variation in the relative proportions of constituent minerals. Phase layering is the appearance or disappearance of minerals in the crystallization sequence developed in modal layers. Cryptic layering is the systematic variation in the chemical (including isotopic) composition of minerals with stratigraphic height in a layered sequence. All three types of layering are known to be present in the ultramafic-mafic layered rocks of the Bushveld Complex, or more specifically, in the Rustenburg Layered Suite (RLS) of the Bushveld Complex.

Publications describing ore deposits of platinum…

What follows is a description of the brief mining history of the Mesoproterozoic-age Steenkampskraal monazite ore body located in southern Namaqualand, about 340 km north of Cape Town in South Africa. Steenkampskraal is a massive-lode ore body, possibly originating by igneous processes from an immiscible phosphate-sulphide-oxide magmatic liquid. The monazite ore occurs in a thin lenticular-shaped body surrounded by Mesoproterozoic granitic gneiss country rocks. The ore body is about 400 m in strike-length, extending about 450 m down-dip, with an average thickness of about 0.5 m.

The ore body was known to the early Bushman, who used the high-density monazite + apatite + magnetite ore for sling stones and arrowheads, and scattered artefacts such as stone flakes and core stones in the vicinity of the ore body. Samples from the ore body were first identified as monazite in 1949, and mining of monazite began in 1950 when a Vanrhynsdorp syndicate sunk three trenches about 5 m deep in the widest portions of the ore body. The first consignment of Steenkampskraal monazite ore was then exported from South Africa to the United Kingdom as a source of thorium. At about the time surface mining of monazite ore waned in early 1952, a crosscut from a vertical shaft located on the Steenkampskraal koppie intersected the ore body about 30 m below the surface.

In July 1952, a company named Monazite and Mineral Ventures Ltd. acquired the mineral rights from the Vanrhynsdorp mining syndicate, and undertook further development of the ore body. By July 1953, a residential area for the mine employees had been constructed at Steenkampskraal, and a specially designed processing plant was operating at the mine site. Full-scale underground mining also commenced at about this time, after an incline driven at an angle of 30 degrees for 140 m from the southern base of the Steenkampskraal koppie intersected the monazite ore body about 90 m beneath the surface. Mining continued until February 1959 when operations were suspended due to a lack of demand for thorium. The mine reopened in April 1962, and briefly operated for about a year, but operations finally ceased in 1963. Shortly after 1965, the processing plant and many of the surrounding buildings were dismantled, and Monazite and Mineral Ventures Ltd. ceased to exist.

Publications describing the occurrences and uses of thorium…