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

Abstract

The Platreef is the main platinum group element (PGE)-bearing and contains some of the most important magmatic ore deposits yet discovered. The intrusion covers an area of facies of the northern limb of the Bushveld complex, but unlike the Merensky Reef of the eastern and western limbs, it is in direct roughly 65 000 km (e.g. Tankard et al., 1982) and lies almost entirely within the bounds of the Transvaal Basin contact with the country rock. Mineral separate O values for samples from the Upper Zone and Main Zone of the northern limb (Fig. 1). The layered igneous rocks (the Rustenburg Layered Suite, SACS, 1980) can be divided into eastern indicate crystallization from a well-mixed, already contaminated, magma having a O value of 7·5‰. Pyroxenes from the pyroxenites and western limbs of approximately the same size, and a smaller northern limb. The northern limb differs in facies of the Platreef at Sandsloot Mine have O values that are up to 2·4‰ higher than pyroxenes from the Upper and Main several important aspects from the more well-known eastern and western limbs (e.g. Van der Merwe, 1976; Zones. These differences can be explained by additional assimilation of up to 18% dolomite, which is in contact with the intrusion at Buchanan et al., 1981; Cawthorn et al., 1985; Eales & Cawthorn, 1996). First, the layered rocks show a this locality. Samples from the Platreef that have plagioclase and pyroxene not in oxygen isotope equilibrium appear to have interacted pronounced transgression across the floor rocks from a with fluids during slow cooling. Quartz veins with granophyric level above the Magaliesberg Quartzite Formation south margins have O values between 10·1 and 12·2‰, which of the town of Potgietersrus, across the whole Pretoria suggest that the fluid that passed through the cooling Platreef had Group, and the banded iron formations and dolomites a O value of 7–9‰. These data, together with hydrogen isotope of the older Chuniespoort Group, onto Archaean granitic data from minerals and whole-rock samples ( D of biotite −60 rocks (Fig. 1). Second, the igneous layering shows an to −88‰) of the Platreef suggest interaction with magmatic fluid apparent transgressive relationship with respect to the at low water/rock ratios. Interaction of this fluid with the calcsilicate contact, such that ultramafic rocks of the Lower Zone footwall rocks lowered their O values. abut against the contact south of Potgietersrus, whereas to the north, rocks forming the base of the intrusion are progressively higher in the stratigraphic succession of the