Facies of the Merensky Reef
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…
Mike on December 8th 2008 in Bushveld Complex









