A novel approach for efficient identification and accurate chemical characterisation of platinum-group minerals by combining Electron Probe Microanalysis and Mineral Liberation Analysis

The identification and accurate characterisation of platinum-group minerals (PGMs) is usually a very cumbersome procedure due to their small grain size (typically below 10 µm) and inconspicuous appearance under reflected light. A novel strategy for finding PGMs and quantifying their composition was developed. It combines SEM-based automated mineralogy, in this study mineral liberation analyser (MLA) Quanta 650F by FEI, a point logging system (JEOL) and a FE-EPMA (JEOL JXA-8530F). Thin sections from a layered intrusion (UG2) in the Bushveld Complex and from two Uralian-Alaskan-type complexes in the Ural Mountains, Russia, were investigated as case studies.
As a first step, the PGMs are identified using the MLA. Grains identified as PGMs are then marked and coordinates recorded and transferred to the EPMA. Case studies illustrate that by introducing MLA for the efficient and largely automated identification of PGM grains in polished thin sections, up to 20 times more PGM grains were identified, whilst shortening time needed and avoiding the effects of human error invariably associated with reflected light microscopy. This is mainly due to the facts that (a) PGM with grain sizes < 5µm are reliably identified and (b) PGM and closely associated base metal sulfides and sulfosalts are well differentiated with the MLA. The analysis of PGMs by EPMA requires considerable effort due to the often significant overlaps between the X-ray spectra of almost all platinum-group and associated elements (e.g. OsMβ on IrMα and AuMβ on HgMα). X-ray lines suitable for quantitative analysis need to be carefully selected. As peak overlaps cannot be avoided completely, an offline overlap correction based on weight proportions has been developed. Considerable variations in the repeatedly determined overlap factors illustrate the need for a re-determination of the overlap factors with each calibration. Results obtained with the procedure proposed in this study attain acceptable totals and atomic proportions, indicating that the applied corrections are appropriate.