ANALYSIS OF PLATINUM GROUP ELEMENTS AND GOLD IN GEOLOGICAL MATERIALS USING NIS FIRE ASSAY AND TE COPRECIPITATION; THE NIS DISSOLUTION STEP REVISITED

Abstract

The NiS fire assay-Te coprecipitation separation procedure for inductively coupled plasma mass spectrometry (ICP-MS) analyses of platinum-group elements (PGE) in silicate rocks has been revisited with the aim of reducing volatile PGE losses (Os, Pd). The NiS bead was dissolved in 20.2% HCl, in an open system allowing H2S to escape without loss of HCl. Yield, accuracy and reproducibility were tested by replicate analyses of a CANMET reference material (UMT1), a mantle lherzolite (FON B 93), prepared as in-house standard, and abyssal harzburgites, analysed as unknowns. The yields as estimated from UMT1 range from 97% (Ir) to 93-94% (Rh, Pt, Pd) and 91% (Au). The Os value (7.13+/-1.4 ppb) is equal within 1 sigma level, to the CANMET provisional value. The mean Os content of FON B 93 (3.42 ppb) fits the Os concentration inferred for the terrestrial mantle very well, as does the Os/Ir ratio (1.053 vs. 1.063) while Pd concentrations are increased by 18% compared to previous analyses after open-beaker dissolution steps. The abyssal harzburgites also yield consistent Os contents (3.28+/-0.19 ppb) and a perfectly chondritic Os/Ir ratio (1.07). Thus modified, the NiS fire-assay method allows Os to be nearly completely recovered, while greatly reducing the volatility of Pd. Moreover, the PGE analyses of coarse-grained rocks are highly reproducible (1% for Rh, Pt, Pd; 4% for Ir, 5% for Ru, 6% for Au), if performed from large-sized powder aliquots.