Iron sulphides in metasediments: isotopic support for a retrogressive pyrrhotite to pyrite reaction

Hall, A.J., Boyce, A.J. and Fallick, A.E. (1987) Iron sulphides in metasediments: isotopic support for a retrogressive pyrrhotite to pyrite reaction. Chemical Geology: Isotope Geoscience Section, 65(3-4), pp. 305-310. (doi: 10.1016/0168-9622(87)90010-8)

Abstract

The concepts and conclusions regarding the origin and significance of iron sulphides in sedimentary rocks cannot be simply extended to include metasedimentary rocks. This is because of the possible complex behaviour of iron sulphides during the metamorphic process. Textural and isotopic examination of Late Precambrian Lower Dalradian Ballachulish Slate Formation metasediments confirms that pyrite porphyroblasts can form at the expense of early pyrrhotite. The pyrite, δ34S = +15‰ (±2‰) has inherited the sulphur isotopic signature of the pyrrhotite, δ34S= +14‰ (±2‰). The pyrite formed at a late metamorphic retrogressive stage involving dissolution, remobilisation and oxidation. Sulphur isotope and trace-element analysis of pyrite from metasediments should therefore be interpreted with consideration given to a possible earlier pyrrhotite stage. The pyrrhotite in the Ballachuliah slates is of uncertain origin but has probably formed either: (1) after evaporitic sulphate with δ34S of contemporaneous seawater ∼+15‰, or after (2) bacteriogenic sedimentary pyrite with δ34S of contemporaneous sea-water ∼+30‰ and bacteriogenic fractionation, −15‰. The age of the Lower Dalradian is not well constrained and about this time ocean-water sulphur isotope value is known to have increased from ∼+15‰ to ∼+30‰. It is hoped that future research will resolve the origin of the pyrrhotite by establishing the δ34S-value of local seawater during Dalradian times.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• 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: Chemical Geology: Isotope Geoscience Section
• Publisher: Elsevier B.V.
• ISSN: 0168-9622
• ISSN (Online): 1872-6836