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Platinum-group element mineralisation in the Unst ophiolite, Shetland

By A.G. Gunn, R.C. Leake and M.T. Styles


The ophiolitic basic and ultrabasic rocks of the island of Unst, Shetland\ud comprise a sequence of harzburgites, dunites, clinopyroxene-rich cumulates, and\ud gabbro, within tectonic blocks that have been thrust over a migmatite complex\ud during the Laxer Palaeozoic. Concentrations of chromite are found in the\ud harzburgite and dunite, and to a small extent in the pyroxene cumulate rocks.\ud They occur as disseminations, sometimes forming millimetre scale layers, and as\ud more massive schlieren and pods of chromitite. Five alteration or hydrothermal\ud events have been recognised in the ultrabasic rocks. These comprise early\ud pervasive serpentinisation, later fracture controlled serpentinisation, veining\ud and pervasive carbonation, minor late serpentine veining and talc-carbonate\ud alteration controlled by fault zones.\ud Exploration for platinum group element (PGE) mineralisation uas carried out\ud using a combination of drainage, overburden and rock sampling. Analyses of PGE\ud were obtained by fire assay followed by either neutron activation analysis or\ud flameless atomic absorption spectrometry, and up to 20 other elements Here\ud determined by X-ray fluoresence analysis.\ud Panned concentrate samples were taken from 73 drainage sites distributed\ud throughout the complex. Ir, the only PGE determined in all samples, showed a\ud greater concentration in samples derived from the harzburgite unit than those\ud from other units. Lox amplitude anomalies are present in three discrete areas in\ud the harzburgite but the maximum level of 210 ppb Ir is associated with a sample\ud derived from a prominent N-S zone of faulting and hydrothermal activity markedly\ud discordant to the regional trend of layering in the harzburgite and dunite. This\ud discordant zone, which extends for at least 7 km, is also marked by samples\ud containing enrichments in Fe, Co, Ni, Cu and As. The highest Cr levels are\ud associated with an area in the north of the harzburgite with no previous history\ud of chromite working but where many locally derived pieces of chromitite float\ud have been discovered. Relatively high Cr levels are also associated nith the\ud area of dunite containing the greatest concentration of visible chromite and old\ud norkings.\ud A technique of collecting panned heavy mineral concentrates from overburden\ud samples was adopted as a reconnaissance exploration technique after orientation\ud sampling in the harzburgite unit at Cliff, an area with high PGE levels in\ud chromitite and associated dunite. Systematic sampling in the Cliff area outlined\ud a zone of coincident Pd, Pt and Rh enrichment near to but separate from the\ud chromite workings knorrn to be enriched in PGE. In contrast the distribution of\ud Ru was entirely different with scattered lon amplitude anomalous zones and a\ud maximum anomaly 300m from the chromite-rich zone. Reconnaissance lines were\ud sampled at other locations within the harzburgite, dunite and cumulate units.\ud Lore amplitude Pd and Pt anomalies were detected xithin the dunite unit,\ud especially in 'a traverse across the trace of the prominent N-S fault zone at\ud Helliers Uater, adjacent to the outcrop of the cumulate unit. In general the\ud overburden data suggest some association between PGE enrichment and enhanced\ud levels of Ni relative to typical silicate levels apparent when expressed as the\ud ratio Ni/MgC.\ud Rock samples Here collected from all parts of the complex, including most of the\ud main chromitite workings. Very high levels of all PGE occur in samples of\ud chromitite, chromite-rich dunite and dunite from the Cliff area, with a strong positive intercorrelation between all PGE. The proportions of the various PGE\ud are very similar to those present in deposits in major layered basic/ultrabasic\ud complexes like Bushveld and Stillwater, irith strong relative enrichment in Pd\ud and Pt. These PGE proportions are completely different from the Ru-Ir-0s\ud dominant assemblage typical of ophiolitic rocks. Associated with high levels of\ud PGE are enrichments in Ni, Cu, As, Sb and Te. There is no correlation rrith Cr\ud and some samples of chromitite from the Cliff area contain only background\ud levels of PGE.\ud High to moderate levels of PGE with the same proportions of elements as the\ud Cliff samples also occur in samples of chromitite and serpentinised dunite from\ud the dunite unit and in samples of pyroxenite from the cumulate unit, In contrast\ud PGE-rich samples of chromitite from the harzburgite unit near Harold's Grave\ud have entirely different proportions of PGE with Ru and Ir in greatest abundance.\ud This PGE distribution is similar to that in some background samples of\ud harzburgite and closely resembles the pattern found in typical ophiolites. The\ud PGE in the Harold's Grave samples do not exhibit the Ni enhancement noted in the\ud Cliff PGE mineralisation.\ud In samples from the Cliff area the platinum-group minerals (PGM) sperrylite,\ud stibiopalladinite, hollingnorthite, laurite and possibly irarsite have been\ud identified, mostly as grains less than 10 microns in size. In chromite-rich\ud rocks these minerals occur Rithin chlorite haloes around chromite, in the\ud blackened altered rims of chromite grains and in interstitial Ni-rich\ud serpentine/carbonate intergroxths in association with pentlandite, orcellite and\ud other Ni sulphides and arsenides, sometimes spatially related to\ud chlorite-carbonate-magnetite veins. They also occur as fine grains Rithin\ud magnetite rims around chromite and in magnetite or carbonate veins in dunite.\ud The Ni sulphide/arsenide assemblage associated Rith the PGH is characteristic of\ud serpentinisation at temperatures less than 500'C, Rell belox the range of\ud magmatic conditions. A hydrothermal origin for the PGE mineralisation is\ud proposed, probably related to the second phase of serpentinisation. This\ud involved the redistribution of Ni accompanied by the introduction of As, Sb and\ud Te probably with a StrUCtUral Control. Pre-existing concentrations of chromite\ud may have acted as a precipitation barrier causing rich PGH deposition in the\ud alteration haloes around chromite grains.\ud Continuous borehole or trench sections through mineralised zones are required to\ud assess the economic significance of the PGE mineralisation. Nevertheless the\ud high levels of PGE attained and the evidence of xidespread occurrence of the\ud Cliff-type PGE enrichment are favourable indications. The PGE enrichments found\ud in the cumulate complex are of potential interest as they may originally have\ud been of magmatic origin. Larger tonnage targets may therefore be present in this\ud unit compared Rith the likely size of structurally-controlled mineralisation\ud elswhere in the complex

Topics: Earth Sciences
Publisher: British Geological Survey
Year: 1984
OAI identifier:

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