Constraining the parent magma composition of the mineralized deep Flatreef at Turfspruit, northern lobe, Bushveld Complex: insights from silicate mineral compositions

Abstract

Unlike platinum-group elements (PGEs) deposits from the western and eastern lobes of the Bushveld Complex which are generally thought to have been largely formed as a consequence of primary magmatic processes, PGEs from the northern lobe are generally thought to have originated by processes involving contamination of parental magmas by crustal sources. We investigated mineral compositions from the deep Flatreef at the Turfspruit farm where dolomitic calc-silicate rocks form part of the country rocks to determine the role of crustal sources in the formation of PGE mineralization. Orthopyroxene compositions vary widely from En77.9 to En91.7, and compositions of clinopyroxene display wider variations (Ca38.8−67.0Mg24.7−51.6Fe6.4−19.6). Plagioclase similarly varies widely in composition (An1.3−73.7) whereas compositions of olivine (Fo85.9−87.6) are very restricted. Temperatures ranging from ~ 500 to 1300 oC were obtained from two pyroxene geothermometers similarly likely indicating crystallization from contaminated magma and/or hydrothermal alteration. The wide compositional ranges of calcium-bearing phases such as clinopyroxene and feldspars suggest that the Platreef crystallized from parental magmas that were contaminated by dolomitic calc-silicate floor rocks. The very restricted and highly magnesian forsterite contents in olivines are inconsistent with expected primary magmatic compositions. Simple mixing models utilizing existing sulfur isotope data suggest that, in the Turfspruit area, contamination of Bushveld Complex parental magmas with 2–10% contamination by floor rock dolomitic calc-silicates with δ34S values of 32‰ would account for the observed data in the Turfspruit area.