The effects of iron-rich ultramafic pegmatite on the composition and mineralogy of the UG2 chromitite: a case study in the western Bushveld Complex, South Africa

Abstract

AbstractDiscordant iron-rich ultramafic pegmatites (IRUPs) intersect the UG2 chromitite at many places in the Bushveld Complex. The effects of IRUP interactions on the UG2 ore mineralogy and PGE grade are assessed at the Thaba mine, north-west Bushveld, based on a borehole profile through the UG2 layer and detailed analysis of mineral textures and compositional variations across the UG2-IRUP contacts using micro-XRF element mapping. The UG2-IRUP interaction operated at different scales and probably by different mechanisms. At the local scale (< 10 cm), a thin layer of Fe–Ti–Cr spinel and ilmenite formed on the IRUP side of the contact with UG2, whereas the UG2 chromitite developed grain coarsening, loss of interstitial silicates, and chemical gradients in Cr, Al, Fe, and Ti that extend a few centimeters from the contact into the seam. These local effects are attributed to the intrusion of IRUP melt into the solidified UG2 layer, followed by re-equilibration of the oxide minerals across the contact during cooling. On a larger scale, changes in the ore and gangue mineral assemblages in UG2 took place throughout the entire meter-thick main seam. Compared with regional UG2 compositions, chromite has higher TiO2 and lower Mg#, and there is an anomalously low abundance of interstitial plagioclase. The IRUP-affected UG2 shows relatively abundant secondary hydrous silicates, replacement of PGE sulfides by PGE alloys and PGE-As–Sb–Bi–Te–Pb phases, and formation of secondary Ni–Cu–Fe sulfides after pentlandite and chalcopyrite. These large-scale effects are attributed to hydrothermal fluids derived from IRUP melts. The IRUP bodies at the Thaba mine caused redistribution of PGE within the UG2 layer but did not significantly reduce the overall grade. However, significant changes in the ore mineral assemblage and an increased abundance of secondary silicates can reduce the efficiency of PGE recovery.