Pilot study of melt inclusions in chromites of the Merensky Reef reflecting melt compositions at Karee Mine, western Bushveld Complex, South Africa

Various melt inclusions in chrome spinel crystals of tempered samples from the lower chromitite stringer of the 'Merensky Reef' at Karee Mine in the western Bushveld Complex, were investigated to obtain an indication of the initial melt composition of the upper Critical Zone. The material was heated to 1300°C in a furnace under a graphite buffered atmosphere for at least 12 h and was rapidly quenched cold air thereafter. High resolution microscopy and SEM-EDX reveals that melt inclusions consist essentially of a glass matrix, sulphide phases and silicate phases of predominantly olivine composition. Both minerals are embedded into an amorphous melt phase. The mineral inclusions are chemically highly variable but the composition of the quenched glass phase is homogeneous. The rock matrix surrounding the chrome spinel crystals consists of bytownitic plagioclase and melt of basaltic composition. During the rapid cooling by quenching 'blade textured' plagioclases were formed. Chrome spinels display chemical variability with rim positions having higher Al2O3 and lower Cr2O3 contents than grain centres as optically evident by change in colour. The reason could be a exchange of the chromite crystal with the silicate melt in the process of reaching a diffusive equilibrium, resulting in a substitution of Cr3+ by Al3+. The sulfide phase is very heterogeneous containing mineral phases such as pyrrhotite, pentlandite, bornite, trevorite, daubreelite, taenite with variable elemental proportions. The finding of sulfide next to sulfur-undersaturated phases in the silicate inclusions does not allow any conclusion on origin of the sulfur neither on the state of equilibrium of the melt. SEM-EDX analyses prove the existence of two compositionally distinct groups of melt inclusions. The first are part of a melt batch which exsolved sulfidic, silicate and oxidic melt phases, as evident from the co-existence of sulfidic next to silicate inclusions in chromites. The second group has a basaltic composition and constitutes the matrix to the chromite crystals; it probably formed from a homogenized melt after magma mixing. Both melt compositions show a chemical affinity to Bushveld B1 and B2 magmas respectively.