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Journal of the Geological Society; 1995; v. 152; issue.2; p. 229-250;
DOI: 10.1144/gsjgs.152.2.0229
© 1995 Geological Society of London

Article

A Devonian auriferous hot spring system, Rhynie, Scotland

C. M. RICE1, W. A. ASHCROFT1, D. J. BATTEN2, A. J. BOYCE3, J. B. D. CAULFIELD1,6, A. E. FALLICK3, M.J. HOLE1, E. JONES4, M. J. PEARSON1, G. ROGERS3, J. M. SAXTON5, F. M. STUART5, N. H. TREWIN1 & G. TURNER5

1 Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, King’s College, Aberdeen AB9 2UE, UK
2 Institute of Earth Studies, University of Wales, Aberystwyth SY23 3DB, UK
3 Scottish Universities Research & Reactor Centre, East Kilbride, Glasgow G75 0QU, UK
4 North Horntowie, Cairnie, Huntly AB5 4TA, UK
5 Department of Geology, University of Manchester, Manchester M13 9PL, UK

The Early Devonian Rhynie hot spring system is the oldest known and is of the low sulphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radio-metric (396 ± 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin.

The Devonian rocks show intense hydrothermal alteration along the fault zone. The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and Mo. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks.

The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300–440°C) magmatic fluids occurred with {delta}D–65{per thousand} and {delta}18O around +8.5{per thousand}. {delta}34S (pyrite) and initial 87Sr/86Sr ratios (vein calcite) lie mainly within the ranges +3.4{per thousand} to +8.5{per thousand} and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. {delta}13C values for caliche and vein calcite imply derivation of carbon from non-organic sources.

The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin ({delta}18Ochert+ 13.1{per thousand} to +16.5{per thousand}) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/Cl and I/Cl ratios). Plant-bearing chert yielded an 40Ar/39Ar ratio (292.1± 0.6) significantly less than that of modem air and may be the first valid determination of a sample of ancient atmosphere.

Key Words: Hot springs • Devonian • Scotland • isotopes




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