Modeling the Merensky Reef, Bushveld Complex, Republic of South Africa

The Merensky pegmatoid (normal reef) in the western Bushveld Complex is commonly characterized as a pyroxene-rich pegmatoidal unit with a base that is enriched in chromite and platinum-group element-bearing sulfides overlying a leuconorite footwall. Models for its formation have ranged from those that view it as entirely a magmatic cumulate succession to those that have suggested that it is a zone of volatile-induced remelting. The consequences of the latter interpretation are investigated using the numerical modeling program IRIDIUM, which links diffusive and advective mass and heat transport with a phase equilibration routine based on the MELTS program. The initial system consists of a simple stratigraphic succession of a partially molten leuconorite overlain by a partially molten pyroxenite, both initially at 1,190°C and 2 kbar. 2 wt% of a volatile fluid composed of 75 mol% H2O, 20 mol% CO2 and 5 mol% H2S is then added to the lower 20 cm of the pyroxenite. The system is then allowed to evolve under conditions of chemical diffusion in the liquid. The addition of the volatile components results in a modest increase in the amount of melt in the pyroxenite. However, chemical diffusion across the leuconorite–pyroxenite boundary leads to more extensive melting at and below the boundary with preferential loss of opx from the underlying leuconorite, preferential re-precipitation of sulfide and chromite and concentration of the PGE at this boundary. These results mimic actual mineral and compositional profiles across the Merensky pegmatoid and illustrate that long-term diffusion process can effectively produce mineralogical and compositional layering not present in the original assemblage.