An overview of the PGE concentrations in the Shetland ophiolite complex

Abstract

Extremely anomalous platinum-group element (PGE) concentrations are known at one site, Cliff, in the Shetland ophiolite complex. Systematic PGE analysis of all the mafic and ultramafic igneous and alteration lithologies in the ophiolite has revealed many lower grade anomalous PGE occurrences throughout the cumulate ultramafic sequence and their distribution is described here. More sites with PGE enrichments as anomalous as those at Cliff have not been found. At Cliff the anomalous values of over 60 ppm Pt + Pd are restricted to a chromite-rich sulphide-bearing dunite forming a small part of a 200 m dunite lens in mantle harzburgite close to the basal thrust of the ophiolite. Lower values of up to 6.4 ppm Pt + Pd occur in chromite-poor, sulphide-rich dunites also within this dunite lens. The only other chromite-rich dunite lens in the Cliff area also has detected Pt and Pd but with low Pt + Pd values of 200 ppb. In contrast, harzburgites, metasediments underlying the basal thrust, serpentinites from the basal thrust and internal faults are all barren of Pt and Pd (detection limit = 20 ppb). In the Cliff area, therefore, detected PGE values are restricted to magmatic chromite-rich dunite lenses. PGE analyses of drill core from beneath the disused chromite quarries at Cliff are low (227 ppb Pt + Pd) indicating that the extremely anomalous PGE values are very restricted. This supports the idea that PGE, concentrated magmatically within the dunite lens at Cliff, may have been remobilized locally to produce extremely anomalous PGE values in an alteration zone only a few metres in diameter. Elsewhere in the ophiolite, anomalous Pt + Pd values of up to 4 ppm occur in chromite-rich, sulphide-bearing dunite within the dunite unit, which stratigraphically overlies the harzburgite. Pt + Pd concentrations of up to 1 ppm are present in the pyroxenites and wehrlites of the upper part of the ultramafic sequences. High-level wehrlites and pyroxenites within the gabbro unit, representing late ultramafic differentiates, contain lower levels of PGE of up to 310 ppb Pt + Pd. The gabbros all have less than 20 ppb Pt and Pd. This distribution indicates that the PGE concentrations occur in specific primary magmatic lithologies and pathfinder analysis shows that they are associated with Ni, Cu, Au and Cr. PGE have not been found above detection limits in fault zones away from primary PGE mineralization sites and it is concluded that PGE are not significantly hydrothermally reconcentrated. At Cliff, remobilization may have occurred over short distances of only a few metres at most. © 1993 The Geological Society.