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Lord, Richard Alastair
(1992).
DOI: https://doi.org/10.21954/ou.ro.0000dff1
Abstract
The Shetland ophiolite complex exhibits the lower part of the definitive Penrose sequence, but chromitites from the Cliff locality contain levels of Pt and Pd which are unusual in ophiolites. Aspects of this mineralisation suggest both magmatic and hydrothermal concentration processes, although neither individually explain the observed paragenesis. This study examines the distribution of PGE within a representative fresh vertical section of the ophiolite stratigraphy, to assess the relative roles of such processes in the concentration and fractionation of PGE.
In the field area around Balta Sound, tectonised mantle harzburgites are overlain by a layered ultramafic cumulate sequence. Accessory Ni-Cu sulphides occur throughout these cumulates. Two stratigraphically controlled zones of chromite enrichment have been identified which mark the bases of cyclic repetitions. Such chromite enrichments lack progressive changes in mineral chemistry so repetitions within the cumulate sequence are attributed to influxes of primitive magma during open system fractionation.
The distribution of PGE and other chalcophiles show primary lithological associations, indicating a magmatic origin. Os-Ir-Ru concentrations occur in cbromitites, whereas Pt-PdRh concentrations occur in sulphide-bearing dunites, chromite-rich dunites, or basal pyroxenites. Stratigraphically controlled zones of PGE-enriched sulphide-bearing cumulate dunites occur close to the base of each cyclic repetition and are continuous over a strikelength of 2 km. The fractionation between PGE and other cha1cophiles is consistent with magmatic processes.
The distribution and fractionation of PGE can be explained by an open system model. Negative slope patterns are produced by the precipitation of PGM from PGE saturated magmas during chromite crystallisation. This initial process buffers the PGE concentrations in each primitive magma pulse. On entering the axial chamber and mixing with the evolving resident magma, sulphide saturation occurs and the remaining PGE are partitioned into immiscible sulphide liquids. Multiple inputs result in repeated PGE enriched zones. This model explains why successive stratigraphic PGE-enriched horizons have similar positive slope patterns but progressively fractionated host silicate and sulphide compositions.
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