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Geological Society, London, Special Publications; 1995; v. 87; p. 207-222;
DOI: 10.1144/GSL.SP.1995.087.01.17
© 1995 Geological Society of London

Mineralogy and sulphur isotope characteristics of a massive sulphide boulder, Galapagos Rift, 85°55'W

Richard Knott1, Anthony E. Fallick2, David Rickard1 & Harald Bäcker3

1 Department of Earth Sciences, University of Wales, Cardiff CF1 3YE, UK
2 Isotope Geosciences Unit, SURRC, East Kilbride, Glasgow G75 0QU, UK
3 GEOMAR Technologie GmbH, Wischofstrasse 1-3, Geb. 11 24148 Kiel, Germany

The submarine hydrothermal sulphide deposits of the Galapagos Rift are characterized by being exceptionally Cu-rich. Fe- and Cu-Fe sulphides are extremely heterogeneous, display granular textures and evidence of open space growth. Core was examined from a 1.5 m diameter massive sulphide boulder collected by TV grab on the GARIMAS II cruise. The main growth stage is dominated by pyrite and chalcopyrite, forming granular aggregates from <1 mm to >5 cm. These sulphides have {delta}34S from +2.7 to + 5.5{per thousand}. The Fe-Cu sulphides are modified by minor seawater oxidation and a late stage marcasite-sphalerite-galena-barite assemblage. {delta}34S varies along the core in a systematic manner and there appears to be an isotopically heavy deviation from the modal +3.9{per thousand}. This is related to particular zones and suggests a second, isotopically distinct S source, which is interpreted to be a hydrothermal diagenetic solution derived from a shallow seawater-hydrothermal circulation system.