Role of Chamber Replenishment in the Formation of the Merensky Reef and its Footwall Anorthosite

Abstract The Merensky Reef of the Bushveld Complex represents a magmatic unconformity that some researchers attribute to chamber replenishment by relatively primitive magma. It is propounded that cumulate rocks in this chamber reacted with replenishing melt, as part of the process that ultimately produced chromitite stringers and reef-style platinum-group element mineralization. This study investigates as to whether chamber replenishment contributed to the formation of the Merensky Reef and its underlying anorthosite at the Rustenburg Platinum Mine in the western lobe of the Bushveld Complex. At this location, the Merensky Reef is a coarse-grained pyroxenite bracketed by millimeter-scale chromitite stringers. This sequence is underlain by a centimeter-scale anorthosite which in turn is underlain by leuconorite. The leuconorite comprises normally zoned cumulus orthopyroxene with poikilitic rims (Mg80-79) and cumulus plagioclase (An80-58), where the latter defines a magmatic fabric indicative of gravitational settling of tabular crystals in a quiescent melt. The contact between leuconorite and anorthosite is marked by an increased abundance of late-stage accessory minerals, and the composition of poikilitic orthopyroxene at this horizon is consistent with trapped liquid shift. Plagioclase crystals in the anorthosite are variably zoned (An79-64) and record a magmatic fabric that strengthens with proximity to the reef. This unit is traversed by sinuous networks of sulfides, pyroxenes, quartz, and very fine-grained chromite that terminate at the contact with the leuconorite. The lower chromitite hosts both amoeboidal and blocky chromite crystals that are enclosed by complexly zoned plagioclase oikocrysts in the lower two-thirds and by orthopyroxene oikocrysts in the upper third. The upper chromitite hosts only blocky crystals, similar to those in the upper portion of the lower chromitite. Microtextural characteristics of the amoeboidal crystals coupled with their propensity to host polymineralic inclusions, suggests that these were initially skeletal crystals that subsequently underwent dissolution-reprecipitation. There is no discernible chemical difference between amoeboidal and blocky crystals; however, accessory mineralogy and chromite chemistry imply that the upper portion of the lower chromitite and the upper chromitite experienced post-cumulus re-equilibration with evolved intercumulus silicate melt. Our observations are consistent with the anorthosite being a restite of partially molten leuconoritic cumulates. This theory is supported by thermodynamic modelling that demonstrates that under certain conditions, replenishing melts can reconstitute noritic cumulates to anorthosite, troctolite, or feldspathic orthopyroxenite restites. The porosity generated during this process was exploited by downward percolating sulfide melt that displaced a proportionate amount of intercumulus silicate melt upward to the level of the nascent reef. Initially, these partial melts were likely relatively volatile-rich, triggering Cr-supersaturation at the cumulate-melt interface, and later became Cr-bearing with the consumption of poikilitic orthopyroxene and very fine-grained chromite.