Formation of Spinel-Orthopyroxene Symplectites by Reactive Melt Flow: Examples from the Northern Bushveld Complex and Implications for Mineralization in Layered Intrusions

Abstract

Abstract Spinel-orthopyroxene symplectites are common throughout a >1,200-m-thick sequence of the Rustenburg Layered Suite in the Waterberg project area in the far north of the northern limb of the Bushveld Complex. The symplectites occur between primocrysts of olivine and spinel in the presence of plagioclase in the assemblage. The composition of spinel in the symplectites varies with host rock and increasing stratigraphic height: in the lower parts of the stratigraphy spinel is represented by Cr-poor chromite, the spinel is Cr magnetite in more evolved rocks, and in the most evolved rocks the spinel is magnetite. Olivine and clinopyroxene in the assemblage with the symplectites are partially resorbed and surrounded by an orthopyroxene rim in the lower part of the sequence, whereas a complex mantle of clinopyroxene-hornblende-phlogopite has developed around the orthopyroxene rims in evolved rocks. Preferential extension of orthopyroxene lamellae over host clinopyroxene results in common stepped boundaries toward either olivine or plagioclase. The symplectites and other disequilibrium microtextures are interpreted to be the result of the infiltration of a reactive melt, which destabilized the ferromagnesium minerals and spinel. The aureoles with symplectites are associated and spatially coincident with two zones of high-grade platinum group element (PGE) sulfide mineralization of the Waterberg project. Both mineralized intervals occur in lithologies and at stratigraphic intervals that are unique within the Bushveld Complex. Stratiform magmatic mineralization of the lower F zone was partly remobilized upward from ultramafic rocks by migrating melts, whereas PGE-rich sulfides of the upper T zone were likely derived during the reactive melt flow into the resident crystal mush.