Evaluation of a rapid technique for the determination of precious metals in geological materials based on an aqua regia leach

Abstract

Analysis of aqua regia extracts have been made by both graphite furnace atomic absorption spectrometry (GFAAS) and inductively coupled plasma mass spectrometry (ICP-MS) with a view to developing a rapid, simple, and inexpensive technique for the determination of gold and the platinum-group elements (PGE) in exploration samples. Although selective extraction by aqua regia is expected to leave the sample matrix largely unattacked, resultant solutions still contain high levels of Fe, Ni, Cu, Zn, Mg, and As, which produce significant interferences when the solutions are analyzed directly by GFAAS. To overcome these difficulties a simple solvent extraction procedure was investigated in which Au and the PGE are selectively complexed with triphenyl phosphine, then extracted from the aqueous phase into 1,2-dichloroethane. Results will be presented to demonstrate residual interference effects, the optimization of the furnace heating programme for organic solutions and the recovery efficiency of the solvent extraction process. Routine direct analyses of diluted aqua regia solutions were found to be possible by ICP-MS. The procedures developed for the correction of drift effects and the calibration against aqueous standards will be described. These procedures have been applied to: a. a wide range of reference materials, b. a series of samples that had been preanalyzed by nickel sulphide-fire assay, c. two new chromitite reference samples, CHR-Pt+ and CHR-Bkg, in an appraisal of sample homogeneity. Results will be presented to characterize the overall recovery efficiency of the aqua regia leach procedure and to assess the realistic capabilities of the technique for samples of varying matrix and PGE mineralogy.