Investigation of the mantle source, magma evolution and sulphide mineralisation of the Phalaborwa Complex

Abstract

The economically significant Phalaborwa complex is well investigated but significant gaps remain in the petrogenetic model. The present study focuses on the mantle source characteristics, the genetic relationships between the lithologies and the nature of the sulphide mineralisation, and discusses the relative roles of mantle and crustal processes. Comprehensive textural and geochemical (SEM, EPMA) studies of carbonatites, phoscorite, and silicate lithologies were combined with in situ LA-ICPMS analyses of trace element (PGE, noble metal, metalloid, Bi, Cd) distributions. Sulphide assemblages and trace element compositions in the magmatic system were differentiated from those representing re- mobilisation by hydrothermal and supergene processes. The S/Se ratiosand S/Se vs. PGE ratios helped to separate (mantle-derived) magmatic and crustal signatures of the sulphides. Sulphide minerals with magmatic and magmatic-hydrothermal textures and compositions formed at the maximum of 600-700 oC. A later stage hydrothermal stage (≤ ~400) involving Cu-rich fluids formed abundant bornite-formation and secondary sulphides (e.g. valleriite), and altered non-sulphide phases. The cooling of the deposit is manifested in the formation of series of late stage, low temperature (<100 oC) Cu-S phases (e.g. djurleite, anilite). The in situ measurement of S/Se ratios showed mixed mantle-crustal ratios in the sulphides, with the majority showing the presence of crustal sulphur. The in situ trace element analyses showed a general depletion in PGE, noble metals and metalloids in solid solution and that these elements were remobilised and showed relatively elevated levels in secondary phases. PGM and metalloids readily formed microinclusions with other metals (Ag, Au, Bi, Zn, Co, Ni). PGM were found more frequently in secondary sulphides of serpentinised olivine phlogopitite suggesting remobilisation from either sulphides or silicates by high ƒO2, neutral fluids. Crustal processes have obscured the nature of primary mantle-derived processes so that modelling was used as to investigate the range of possible parental magmas. Natural xenoliths from the Kaapvaal craton (Phlogopite wehrlite, eclogite MARID) were used to approximate the composition of the possible Phalaborwa metasomatised/veined mantle source region. Possible parental melts were derived as a function of starting mantle composition and fractional melting plus mixing, and described in terms of their major and trace element characteristics. The results were interpreted in terms of possible genetic relationships between the Phalaborwa lithologies, particularly magma chamber processes (cumulate formation, immiscibility).