Platinum-group element occurrences in Britain: magmatic, hydrothermal and supergene

摘要

In-situ platinum-group element-bearing mineralization of magmatic, hydrothermal and near-surface origin occurs widely in Britain. In addition, grains of platinum-group minerals and other phases that contain platinum-group elements (PGE) have been found in overburden and stream sediments without any identified bedrock source. The Caledonian mafic-ultramafic intrusions of northeast Scotland have been a major target of investigations by the British Geological Survey. Magmatic copper-nickel sulphide deposits, with up to 500 ppb Pt + Pd, have been identified in the Knock intrusion and at Arthrath, near Ellon, in contaminated and xenolithic gabbroic rocks. A magmatic origin, subsequently modified by hydrothermal reworking at high temperatures in shear zones, is proposed for the mineralization in these areas. PGE enrichments, up to 280 ppb Pt + Pd, also occur in clinopyroxene-bearing ultramafic rocks in deformed mafic-ultramafic intrusions along a major shear zone in the upper Deveron valley near Handy. At Loch Borralan, on the northwestern margin of the Scottish Caledonides, PGE mineralization, up to 878 ppb Pt + Pd, occurs over a 2-km strike length. Two phases of mineralization are evident: a late magmatic phase comprising base-metal sulphides and rare PGE sulphides; and a later, low-temperature phase with PGE tellurides and antimonides associated with shearing, brittle fracturing and veining. A hydrothermal origin that involved the introduction of As and Sb and the redistribution of early formed sulphides is proposed for this mineralization. High PGE concentrations occur at several stratigraphic levels in the Unst ophiolite, Shetland. Pt and Pd both exceed 100 ppm close to the basal thrust of the ophiolite in a zone of talc-carbonate alteration. Lower-grade Pt-Pd-dominant mineralization has also been reported in the cumulate sequence of the complex. Elsewhere on Unst PGE mineralization more typical of ophiolites, dominated by Os, Ir and Ru, occurs in association with chromitites. Recent studies have identified platinum-group minerals (PGM) in layered ultramafic rocks in the Tertiary igneous centres on Rum, Mull and Skye in northwest Scotland. In each case abundant and diverse PGM assemblages have been recorded and a combined orthomagmatic-hydromagmatic origin has been proposed. In several parts of Britain a gold-PGE association has been identified at the boundary between Permian red-bed basins and underlying reduced strata. Complex Au-Pt-Pd grains and discrete PGM are widespread in drainage sediments and overburden derived from the Lower Devonian and Permo-Triassic rocks in the South Hams area and the Crediton Trough in Devon, and the Mauchline and Thornhill basins in southern Scotland. The genesis of the precious-metal grains is attributed to metal transportation in saline, oxidizing fluids with deposition along faults and in favourable host rocks due to mixing with reduced rocks or fluids.