Abstract
The addition of crustal sulphur to magma can trigger sulphide saturation, a process fundamental to the development of some Ni–Cu–PGE deposits. In the British Palaeogene Igneous Province, mafic and ultramafic magmas intrude a thick sedimentary sequence offering opportunities to elucidate mechanisms of magma–crust interaction in a setting with heterogeneous S isotope signatures. We present S-isotopic data from sills and dykes on the Isle of Skye. Sharp contrasts exist between variably light δ 34 S in Jurassic sedimentary sulphide (−35‰ to −10‰) and a local pristine magmatic δ 34 S signature of −2.3 ± 1.5‰. Flat-lying sills have restricted δ 34 S (−5‰ to 0‰) whereas steeply dipping dykes are more variable (−0‰ to −2‰). We suggest that the mechanism by which magma is intruded exerts a fundamental control on the degree of crustal contamination by volatile elements. Turbulent flow within narrow, steep magma conduits, discordant to sediments, and developed by brittle extension or dilation have maximum contamination potential. In contrast, sill-like conduits emplaced concordantly to sediments show little contamination by crustal S. The province is prospective for Ni–Cu–PGE mineralization analogous to the sill-hosted Noril’sk deposit, and Cu/Pd ratios of sills and dykes on Skye indicate that magmas had already reached S-saturation before reaching the present exposure level. Supplementary material: Details of the whole-rock chemical sulphur extraction method; simplified geological maps of the Isle of Skye, a geological cross-section of part of southern Skye, and a stratigraphic log of the Mesoproterozoic and Mesozoic sediments of western Scotland; trace element diagrams of sills and dykes used in this study; and tables for QA/QC of S-isotope results, sample location information, and whole-rock major and trace element results for sill and dyke samples and for Jurassic mudrocks are available at http://www.geolsoc.org.uk/SUP18834.
Highlights
We present the first published sulphur isotope compositions for the British Palaeogene Igneous Province, determining a ‘framework’ for the country rock δ34S of the Hebrides Basin compared with δ34S of the British Palaeogene Igneous Province upper-crustal intrusions on Skye
Together with the Archaean Lewisian gneisses and Moine metasediments, the Mesoproterozoic lithologies delineate a limited range of δ34S, from −1.4‰ to +4.7‰, with a mean of +2.0 ± 1.8‰. This is clearly distinct from the mean Jurassic sediment δ34S of −21.5 ± 8.0‰ (e.g. Fig. 6) and offers the potential to act as a tracer of upper shallow crustal contamination in the British Palaeogene Igneous Province intrusive suites
The results of this study provide the first comprehensive S-isotope framework for the British Palaeogene Igneous Province through determining the regional crustal signatures and those of the British Palaeogene Igneous Province magmas on Skye
Summary
In northern Skye, 23 Trotternish Sill Complex samples were collected, including a range of all rock types (picrites, picrodolerites and crinanites). Sample SK131 (from the centre of a 30 cm wide basalt dyke) gave the lightest δ34S, at −30.7‰, and has a measured whole-rock sulphur concentration of 1.339 wt%. Transposing all samples onto a NW–SE-oriented section (Figs 2 and 5) we observe that the δ34S composition of the dykes becomes significantly lighter from SE to NW This broadly coincides with an increase in the stratigraphic level of the Jurassic sediments that the dykes cross-cut, and into which they intrude. Of the Jurassic sediments collected from Robustan (Fig. 2) and analysed by whole-rock sulphur extraction and conventional analysis, the shale member of the Staffin Shale Formation and the Staffin Bay Formation Carn Mor Sandstone Member have the lightest sulphur isotopic signature, with δ34S = −32.6‰ and. This is clearly distinct from the mean Jurassic sediment δ34S of −21.5 ± 8.0‰ (e.g. Fig. 6) and offers the potential to act as a tracer of upper shallow crustal contamination in the British Palaeogene Igneous Province intrusive suites
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