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 delta O-18 values for samples from the Upper Zone and Main Zone of the northern limb indicate crystallization from a well-mixed, alre ady contaminated, magma having a delta O-18 value of 7.5 parts per thousand . Pyroxenes from the pyroxenites facies of the Platreef at Sandsloot Mine h ave delta O-18 values that are up to 2.4 parts per thousand higher than pyr oxenes from the Upper and Main Zones. These differences can abe explained b y additional assimilation of up to 18% dolomite, which is in contact with t he intrusion at this locality. Samples from the Platreef that have plagiocl ase and pyroxene not in oxygen isotope equilibrium appear to have interacte d with fluids during slow cooling. Quartz veins with granophyric margins ha ve delta O-18 values between 10.1 and 12.2 parts per thousand, which sugges t that the fluid that passed through the cooling Platreef had a delta O-18 value of 7-9 parts per thousand. These data, together with hydrogen isotope data from minerals and whole-rock samples (deltaD of biotite - 60 to -88 p arts per thousand) of the Platreef suggest interaction wit magmatic fluid a t low water/rock ratios. Interaction of this fluid with the calcsilicate fo otwall rocks lowered their delta O-18 values.