Performance of commercial laboratories in analysis of geochemical samples for gold and the platinum group elements

Abstract

Twenty-six international geological certified reference materials (CRM) and two in-house soil control samples were gathered and submitted ‘blind’ in duplicate to five commercial geochemical laboratories for the determination of Au and the platinum group elements (PGEs). The methods employed comprise: Pb fire assay (PbFA) combined with inductively coupled plasma mass spectrometry (ICP-MS); NiS fire assay combined with ICP-MS or instrumental neutron activation analysis (INAA); and aqua regia ICP-MS, with and without prior roasting of the sample at 600 o C. The CRMs vary widely in their matrix and PGE concentrations, ranging from a background soil (e.g. GPt-1), sediment (e.g. GPt-2, JSd-2), and rock (e.g. WGB-1, CHR-Bkg) to altered rocks (e.g. WPR-1) and ore material (e.g. GPt-6, SARM-7b, WMS-1). The results of this ‘round-robin’ are provided and discussed in this paper. Results for Au showed the greatest variation across the laboratories, with one evidently encountering significant and spurious contamination. Comparison of the two fire assay techniques for Au, Pt and Pd was difficult as the number of data points was low and the variance within each technique across laboratories was too high. In general, PbFA-based methods for Au, Pt and Pd produced more accurate and precise results than those by NiS fusion and the data support PbFA detection limits for a 5–10 g sample of 1, 0.1 and 0.5 ppb for Au, Pt and Pd, respectively. A PbFA dataset for Rh demonstrated that this element is not recovered efficiently using an Ag inquart. Measurement of Rh by INAA rather than ICP-MS following NiS fusion facilitates detection below 1 ppb to c. 0.1–0.2 ppb. NiS/ICP-MS results for Ru, Os and Ir support detection limits of 1–2, 2–3 and 0.1 ppb, respectively; mean precision for these elements is in the range 10–15% RSD. Recovery of Os was very low by one laboratory, probably caused by its volatilization as OsO 4 during final digestion in the NiS procedure. As expected, recovery of the analytes by aqua regia was low and highly variable across the different matrices for Pt, Ru, Os and Ir but that for Au and Pd was often >80%; prior roasting of the samples had mixed effects.