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 is a descriptive term used to account for the variation a single mineralized horizon exhibits within the layered ultramafic-mafic sequence. One such horizon is that of the Merensky Reef, which has been laterally subdivided into different facies based on 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.)

Examples of different types of Merensky Reef.

Examples of different types of Merensky Reef.

When considering different facies of the Merensky Reef (defined here as the economically exploitable zone containing platinum group elements), mineralization is neither located at a consistent stratigraphic level within the Merensky Cyclic Unit nor consistently associated with a specific lithological layer. For instance, in a facies referred to as the RPM-Pegmatoidal Merensky Reef, 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 the Marikana Merensky Reef, a different 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 (where three such layers are present). For two other facies, the 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 facies are analogous to “contact reef” at Rustenburg Section and “pyroxenite reef” at Impala.

To better understand the process (or processes) that led to development of all the different facies of the Merensky Reef, it will be necessary to study each facies in great detail. One approach might be to then consider each facies within the context of the three types of stratification often found in layered intrusions. Modal layering is any 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 rocks and/or minerals with stratigraphic height in a layered sequence. All three types of stratification are found within the ultramafic-mafic layered sequence of the Bushveld Complex (i.e., Rustenburg Layered Suite). Relative to these types of layering, is the PGE mineralization strataform or stratabound?