Platinum-group minerals (PGM) typically occur in such low concentrations that quantitative mineralogical evaluation is difficult or impossible. The FEI Mineral Liberation Analyzer (MLA) system used by Vale Base Metals Technology Development has been successfully employed to produce large PGM datasets that are statistically robust. The MLA Sparse Phase Liberation mapping routine produces false-color images that accurately document mineral textures, including intergrowths between various platinum-group minerals and between PGM and other tellurides and sulfides. The MLA PGM search results can be used in conjunction with mineral compositions to calculate the distribution of platinum-group elements among minerals; the MLA PGM images can be used to produce model grade-recovery curves that can predict platinum and palladium recovery in mineral concentrator circuits.

The composition and Ar systematics of biotite (710-355 µm) from MAC-83, a glassy, rhyodacitic crystal-lapilli tuff, have been characterized by electron microprobe and laser 40Ar-39Ar analysis. Fifty electron microprobe analyses demonstrate that the biotites are homogeneous in composition. Twenty-six laser total-fusion, 40Ar-39Ar analyses of the biotite, calibrated against Taylor Creek Rhyolite sanidine (28.34 Ma) and Fish Canyon Tuff sanidine (28.02 Ma), yielded a weighted mean age (based on the respective errors) of 24.36 ± 0.17 Ma. Mass spectrometric determinations of Ca/K and Cl/K were reproducible yielding Cl/K = 0.050 ± 0.002, Ca/K = 0.025 ± 0.040 (errors at 2s). Both conventional furnace and laser step-heating analysis of MAC-83 biotite reveals that it is characterized by minor excess argon, but that overall it yields a well-defined, internally concordant spectrum. These data indicate chemical homogeneity and regular', reproducible argon-release behaviour, implying suitability as a reference material for 40Ar-39Ar laser geochronology.