Application of a Field Emission Mineral Liberation Analyser to the in Situ Study of Platinum-Group Element Mineralisation in the Merensky Reef of the Bushveld Complex, South Africa

Abstract

Quantifying spatial relationships of minerals of the platinum-group elements in ores in situ is difficult, as petrologists are faced with issues of representative sampling, together with laborious and time-consuming microscopic examination. Although automated mineralogy on SEM-based platforms does provide a solution (Fandrich et al., 2007), this is a time-consuming process on conventional instruments equipped with a thermionic tungsten source and SiLi detectors, when applied to the in-situ examination of rock’s thin sections. The advent of the field emission mineral liberation analyser allows for very rapid, automated acquisition of mineralogical data on ores and successor products. FEI’s 600F mineral liberation analyser (MLA) combines a very bright Schottky field emission source with two high-speed silicon drift (Bruker) energy dispersive X-ray spectrometers, with the result that analysis time for MLA applications is significantly lowered. We present procedures followed, and the results of, platinum-group mineral (PGM) searches, identification and size frequency distributions, as well as the quantification of host rock phases occurring in close association with the PGMs encountered, utilising an FEI 600F MLA. Samples of chromitite stringers in Merensky Reef from the Two Rivers and Marikana platinum mines examined for the present study contain a full suite of PGMs consisting mainly of ferroplatinum (PtFe), cooperite (PtS), braggite (PtPdNiS), maslovite (PtTeBi) and laurite (RuS), as well as other Pt and Pd sulfides, arsenides, tellurides, bismuthides and alloys, which range in size from <4 micron to ~100 micron. Gold is present as electrum (AuAg) and also as a rare AuBiPdTe compound.