Platinum and palladium stream-sediment geochemistry downstream from PGE-bearing ultramafics, West Andriamena area, Madagascar

Several pilot studies were made in a PGE-mineralized area of central Madagascar in order to compare Pt,Pd halos in heavy mineral concentrates and to select the most suitable stream-sediment fractions, sampling densities and anomaly thresholds for regional PGE surveys. Results show low anomaly thresholds for Pt (30 ppb) and Pd (20 ppb) in the −63 μm fractions of the active sediment, with restricted halos of nearly 300 m for Pt and nearly 500 m for Pd. Using a slightly coarser fraction (−125 μm) increases the anomaly contrast. The Pt anomalies in heavy mineral pan concentrates are considerably enhanced (400–1,000 ppb) but occur further downstream in residual terraces. A regular increase in the weight of the heavy mineral concentrate for a given volume of sediment is noticed downstream. A simple weight correction of the raw Pt grade in the heavy mineral concentrate gives a better definition of the mineralized source upstream. Assessment of the corrected heavy mineral concentrate Pt anomalies together with Pt,Pd anomalies in the finest stream-sediment fraction produces the optimum definition of the target. Optical determination and scanning electron microscope studies of the PGM show sperrylite to be the major Pt-bearing mineral in the stream sediment, whereas the Pd mineralogy remains unresolved. Pt dispersion appears to be a predominantly mechanical process and Pd dispersion a chemical process with deposition controlled mainly by MnO scavenging.