A sulfur isotope study of iron sulfides in the late Precambrian Dalradian Easdale slate formation, Argyll, Scotland

Hall, A.J., Boyce, A.J. and Fallick, A.E. (1988) A sulfur isotope study of iron sulfides in the late Precambrian Dalradian Easdale slate formation, Argyll, Scotland. Mineralogical Magazine, 52, pp. 483-490.

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Pyritic slates from the late Precambrian, Middle Dalradian Argyll Group Easdale Slate Formation, contain mainly quartz, muscovite and chlorite with variable amounts of dolomite, albite and paragonite. Slates from Easdale Island and Cuan Ferry contain pyrite porphyroblasts with 634S = + 12 to + 16~o. The pyrite grew during a post-tectonic retrogressive event at the expense ofpyrrhotine which formed during the main regional metamorphism of the Grampian orogeny by reduction of diagenetic pyrite. Slate from Oban contains abundant diagenetic framboidal pyrite and small syn-tectonic pyrite porphyroblasts with 6348 = +22%o. This pyrite was not all reduced to pyrrhotine on metamorphism so there was little retrogressive growth of pyrite. Metamorphism appears to have homogenized local (cm scale at least) isotopic inhomogeneities and preserved an average seawater-sulphate-sulphide isotopic fractionation value. Middle Dalradian seawater-sulphate had a fia4S value of about + 35~o, so the small fractionations are appropriate for bacteriogenic reduction in bituminous sediments, the heavier sulphide in the case of the Oban slate indicating more rapid reduction of sulphate. Lower Dalradian Appin Group, Ballachulish slate contains pyrite with 634S = + 15_+2%o and is best interpreted as forming in the same manner as the Easdale slates of Easdale Island and Cuan Ferry; the sharp increase in late Precambrian ocean-sulphate sulphur isotope signature from + 15 to > + 30%o therefore occurred by Lower Dalradian Appin Group times.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Boyce, Professor Adrian and Fallick, Professor Anthony
Authors: Hall, A.J., Boyce, A.J., and Fallick, A.E.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Mineralogical Magazine
Publisher:Mineralogical Society
ISSN (Online):1471-8022
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