Drainage and overburden geochemistry in exploration for platinum-group element mineralisation in the Unst ophiolite, Shetland, U.K.

Abstract

Abstract The Unst ophiolite comprises 60 km 2 of basic and ultrabasic rocks thrust over a migmatite basement. The main nappe consists of four major units, a basal harzburgite overlain in turn by dunite, clinopyroxene-rich cumulates and gabbro. Podiform chromite mineralisation was worked periodically, up to 1945, from shallow surface excavations. Recent work has led to the recognition of two platinum-group mineral (PGM) assemblages in association with chromite. One of these occurs at several localities in the complex, is distinctly different from that typical of ophiolites worldwide, and owes its origin to hydrothermal processes. Studies of the mineralogy and chemistry of the platinum-group element (PGE)-bearing rocks provided the basis for orientation drainage surveys. Ni, especially when expressed as Ni/MgO, correlated with PGE in chromities and was therefore identified as a potential pathfinder for the PGE. Partially panned, heavy-mineral concentrates and suspended fraction samples were collected at a number of test sites. A preferential concentration of Pt in the panned samples was revealed by chemical analysis. Both types of drainage sample have higher Pt/Pd and Pt/Rh ratios than rocks and chromitites, with values for the panned samples generally exceeding those in the suspended fraction. Panned-concentrate drainage samples were collected from sites throughout the complex. Individual units within the complex have characteristic chemical signatures and, in addition, a major discordant fault zone is delineated by the drainage data. This zone is characterised by enrichments in Fe, Cu, Ni, As, Pt, and Pd indicating its potential for the occurrence of hydrothermal PGE-bearing mineralisation similar to that recognised elsewhere in the complex. The distribution of Ir is more sporadic. The highest Ir value occurs in the fault zone, but most high values cluster in the harzburgite unit associated with enhanced Cr levels. In contrast, the dunite unit is characterised by enhanced levels of Pt, Pd and Cu. Following orientation studies at a mineralised locality in the basal harzburgite unit, drainage sampling methods were adapted for use in reconnaissance overburden surveys elsewhere in the complex. A traverse within the lower part of the dunite unit revealed Cu enrichment in the overburden which, when tested by a shallow borehole at one site, was found to relate to enhanced PGE levels in dunite bedrock.