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Schlagwörter:
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Zusammenfassung:
Discordant iron-rich ultramafic pegmatites (IRUPs) intersect the UG2 chromitite at many places in the Bushveld Complex but the effects on ore grade and mineralogy have rarely been studied in detail. We investigated a drillcore profile through the UG2 layer affected by IRUP intrusions at Tha-ba mine, NW Bushveld. The work involved quantitative assessment of the ore mineralogy from mineral liberation analysis (MLA), chemical analyses of the main silicate and oxide minerals by elec-tron microprobe (EPMA) and micro-X-ray fluorescence (micro-XRF) element maps of the UG2-IRUP contacts. The data reveal features of UG2-IRUP interaction at different scales. The micro-XRF study of the contacts shows that a thin layer of Fe-Ti-Cr spinel and ilmenite formed on the IRUP side, while chromite on the UG2 side underwent grain coarsening with loss of interstitial space, and developed chemical gradients in Cr, Al, Fe and Ti. On a larger scale, the MLA data document changes in ore and gangue mineral assemblages through most of the meter-thick UG2 layer. This includes formation of 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.
