Exploration and origin of stratigraphically controlled platinum-group element mineralization in crustal-sequence ultramafics, Shetland ophiolite complex

The Shetland complex exemplifies a class of olivine-rich, platiniferous ophiolite complexes generated by unusually high degrees of hydrous melting in a supra-subduction zone setting. In this environment the crystallization of abundant olivine from unusually picritic magmas has produced an exceptionally thick sequence of crustal dunite cumulates. Significant Pt + Pd concentrations (1-4 ppm) occur where such sulphide-bearing dunites are spatially associated with podiform chromite concentrations. In the area north of Balta Sound the chromitites appear to lie in distinct stratigraphic zones. A detailed programme of lithogeochemical and soil-rock interface geochemical exploration was undertaken, which culminated in diamond drilling and the successful intersection of three stratiform horizons of sulphide-bearing dunites and associated precious-metal mineralization. Cryptic geochemical layering is described at scales from 200 m to less than 1 cm. The patterns suggest open-system fractionation and are explained in terms of igneous concentration processes analogous to those operating in intracratonic layered complexes. The implications for platinum-group element exploration in other ophiolite complexes are assessed.