Evidence for differential mobility of platinum-group elements in the secondary environment in Shetland ophiolite complex

Pt and Pd anomalies were intersected in three 50-m drill-holes in crustal dunites north of Baltasound in the Shetland ophiolite complex. The selection of targets for drilling was based, in part, on the identification of Pt and Pd anomalies in the overlying soils. Data collected during the surface exploration phase, which located platinum-group elements (PGE) in the soils and streams north of Baltasound, are used to assess the mobility of PGE and pathfinder elements in the secondary environment. The anomaly that is associated with one target area, where the drill core intersected an unexposed, vertically dipping mineralized horizon 1 m thick in the dunites, is examined in more detail. A systematic upward increase in Pt in the soils over the mineralized horizon indicates that Pt remains in the soils and that Pd is more mobile. Further evidence for this is the higher Pt/Pd ratios in the soils (average, 1.3) than in the underlying dunite (average, 0.7). The mineralogy of the PGE in the mineralized dunites controls the geochemistry of these elements in the soils. Os, Ir and Ru concentrate with chromite in stream sediments and Pt and Pd are correlated with Cu and Ni, respectively, in the soils, reflecting the presence of Pt-Cu alloys the Pd-bearing Ni antimonides in the mineralized dunite rocks.