THE KLIPKOOF GRANITES OF THE BUSHVELD GRANITE SUITE, SOUTH AFRICA: SIGNIFICANCE OF MAGMATIC AND HYDROTHERMAL TOURMALINE FOR POLYMETALLIC MINERALIZATION

The Paleoproterozoic Bushveld Igneous Province of South Africa includes several important igneous suites, some of which are associated with PGE, F, and polymetallic and possibly iron oxide-copper gold or carbonatite-related mineralization styles. The Bushveld granite suite comprises a series of sheeted intrusions with an aerial extent >30,000 km² and 1.5 to 3 km thick. Recent age dating has indicated that both extrusive and intrusive Bushveld magmatism occurred within a “quasi-continuous” event which took place between 2061 and 2054 Ma. The Klipkoof granite is the youngest member of the Bushveld granite suite and forms dykes and irregular sills. Three comagmatic facies have been differentiated as the coarse-grained, fine-grained, and albitized, largely reflecting magmatic evolution and possibly hydrothermal alteration. These granites have A-type granitic chemistry and field characteristics with geochemical trends from base to roof of the sheet of enrichment in Si, F, Rb, La, Y and Hf, with depletion in Ca, Mg, Ti, P, Sr and Ba. Common aplites, pegmatites and mariolitic cavities, and the presence of hydrothermal mineral infill and alteration is associated with innumerable polymetallic occurrences of Cu, Fe, Pb, Zn, and Ag associated with fluorite, REE and tourmaline. Tourmaline occurs as an abundant disseminated accessory and within “tourmaline spheroids” which typically vary from 2 to 12 cm in diameter but also form elongate shapes and may be associated with pegmatites, aplites and mariolitic cavities. The current study considers discrimination of the sources of boron and their petrogenetic implications on the source and evolution of the Klipkoof granite, and particularly the implications on potential metal sources for polymetallic mineralization. The interpreted magmatic and hydrothermal occurrence of tourmaline suggests derivation of boron from lower crustal rocks during formation of the A-type magma and hydrothermal influx from host sedimentary rocks during magma convection cell dynamics, consistent with the current understanding of the generation of A-type magmas and their evolution and associated pegmatites. Studies as to the potential metal sources for polymetallic mineralization are underway.