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A Sulphur Isotope Study of Iron Sulphides in the Late Precambrian Dalradian Easdale Slate Formation, Argyll, Scotland

Published online by Cambridge University Press:  05 July 2018

A. J. Hall ,
A. J. Boyce  and
A. E. Fallick
A. J. Hall
Affiliation:
Department of Applied Geology, University of Strathclyde, Glasgow G1 1XJ
A. J. Boyce
Affiliation:
Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU
A. E. Fallick
Affiliation:
Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU
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Abstract

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 δ34S = + 12 to + 16‰. 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 δ34S = +22‰. 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 δ34S value of about + 35‰, 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 δ34S = +15±2‰ 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‰ therefore occurred by Lower Dalradian Appin Group times.

Keywords

sulphur isotopesiron sulphideDalradianArgyllScotland
Type
Petrology and Geochemistry
Information
Mineralogical Magazine , Volume 52 , Issue 367 , September 1988 , pp. 483 - 490
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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