Crustal Contamination and Fluid–Rock Interaction during the Formation of the Platreef, Northern Limb of the Bushveld Complex, South Africa

The Platreef is the main platinum group element (PGE)-bearing facies of the northern limb of the Bushveld complex, but unlike the Merensky Reef of the eastern and western limbs, it is in direct contact with the country rock. Mineral separate δ18O values for samples from the Upper Zone and Main Zone of the northern limb indicate crystallization from a well-mixed, already contaminated, magma having a δ18O value of 7·5‰. Pyroxenes from the pyroxenites facies of the Platreef at Sandsloot Mine have δ18O values that are up to 2·4‰ higher than pyroxenes from the Upper and Main Zones. These differences can be explained by additional assimilation of up to 18% dolomite, which is in contact with the intrusion at this locality. Samples from the Platreef that have plagioclase and pyroxene not in oxygen isotope equilibrium appear to have interacted with fluids during slow cooling. Quartz veins with granophyric margins have δ18O values between 10·1 and 12·2‰, which suggest that the fluid that passed through the cooling Platreef had a δ18O value of 7–9‰. These data, together with hydrogen isotope data from minerals and whole-rock samples (δD of biotite −60 to −88‰) of the Platreef suggest interaction with magmatic fluid at low water/rock ratios. Interaction of this fluid with the calcsilicate footwall rocks lowered their δ18O values.