Determination of the precious metals in geological materials by inductively coupled plasma-mass spectrometry (ICP-MS) with nickel sulphide fire-assay collection and tellurium coprecipitation

Abstract

A procedure has been developed for the routine determination of the precious metals (Ru, Rh, Pd, Os, Ir, Pt and Au) in geological materials using inductively coupled plasma-mass spectrometry (ICP-MS). The precious metals were isolated using a combination of the nickel sulphide fire-assay and Te coprecipitation procedures and quantified by ICP-MS using two internal standards to correct for matrix and drift effects; Cd for the light elements (Ru, Rh and Pd) and Tl for the heavy elements (Os, Ir, Pt and Au). Over 8 analytical runs, mean instrumental detection limits for a 15-g sample ranged from 0.07 ppb for Ir to 0.7 ppb for Au. Long-term mean reagent blank concentrations were <0.2ppb, except for Pd (0.6 ppb), Pt (0.4 ppb) and Au (1.2 ppb). For low-level samples, mean blank values were <0.15ppb, except Au (0.51 ppb), and overall method detection limits then approached instrumental detection limits. Analyses of the reference materials SARM-7, PTC-1 and SU-1a indicate that recoveries of most Pt-group elements (PGE) were 86–98%. Os and Au were less efficiently recovered. Relative standard deviations in the determinations of the more efficiently recovered elements ranged between 4% and 14%. Data from the analysis of Indian Ocean basalts demonstrate the ability of the procedure to make useful determinations at baseline levels in rocks for the full set of elements. Thus, the procedures described here extend the use of PGE geochemistry from mineral deposit studies to normal basaltic and mantle systems.