GOLD, IRIDIUM, PALLADIUM AND PLATINUM IN REFERENCE-STANDARD PCC-1 AND A KOMATIITIC PERIDOTITE - IMPLICATIONS FOR SELECTION OF IGNEOUS ROCK REFERENCE MATERIALS FOR PPB-LEVEL NOBLE-METAL ANALYSIS

New analyses for Au, Ir, Pd and Pt in the USGS rock standard PCC-1, carried out by two different radiochemical neutron activation procedures, are compared with similar analyses of a spinifex-textured komatiitic peridotite. Better precision obtained for most of these elements on the komatiitic peridotite, in conjunction with consideration of literature values for PCC-1, suggest that the volcanic peridotite is characterized by a more homogeneous distribution of noble metals than PCC-1, a plutonic peridotite. These results suggest that reference materials for the analysis of sulfur-poor mafic and ultramafic rocks with background levels of noble metals will be more satisfactory if volcanic rocks are selected.