EVALUATION OF A RAPID TECHNIQUE FOR THE DETERMINATION OF PRECIOUS METALS IN GEOLOGICAL SAMPLES BASED ON A SELECTIVE AQUA-REGIA LEACH

Results are described for optimizing the rapid determination of precious metals in geological samples (principally of ophiolitic origin, including chromitites) using an aqua regia [HCl + HNO3 (3 + 1)] leach followed by inductively coupled plasma mass spectrometry. Little improvement in extraction efficiency is afforded by aqua regia attack for longer than 30 min and heating the extraction mixture appears to have a detrimental effect on the extraction efficiency of the precious metals. The procedure was evaluated by analysis of a suite of standard reference materials and some independently analysed ophiolitic rock samples. Generally, Au and Pd are quantitatively extracted, Pt, Rh, Ru and Os are extracted to a lower but significant extent (20-40% recovery) and Ir is poorly extracted (typically 1-10% recovery). The insolubility of selected platinum group element minerals in aqua regia is considered to be the dominant effect for the non-quantitative recoveries.