The Genesis of Sulphides in the Shetland Ophiolite

Maynard, Jonathan (1994). The Genesis of Sulphides in the Shetland Ophiolite. PhD thesis The Open University.



This thesis describes the sulphide, arsenide and antimonide mineral assemblages and stable isotope geochemistry of each magmatic and alteration lithology of the Shetland ophiolite complex and identifies a new assemblage of Pd and Pt-bearing nickel antimonides.

The assemblages have been characterized as follows. Cumulate rocks typically contain a variety of Ni- Fe- and Cu-bearing opaque (biases including relict sulphides, native metals, arsenides and antimonides. The sulphide assemblages show a strong correlation with the host silicate lithology. Dunite pods in harzburgite and cumulate dunites contain the assemblage pentlandite-heazlewoodite-millerite-godlevskite-chalcopyrite-chalcocite-native Cu ± awaruite ± troilite ± orcelite ± breithauptite/Ni- Cu antimonide. Cumulate wehrlites and pyroxenites contain pentlandite-pyrrhotite-pyrite-chalcopyrite-bornite-chalcocite-native copper ± orcellite and maucherite. Gabbros and dolerite dykes contain pyrrhotite-pyrite-marcasite-chalcopyrite ± pentlandite. These assemblages developed by the alteration of precursory magmatic sulphides.

A technique has been developed and used to measure whole-rock δ34S values to a precision of ±0.5‰. δ34S measurements suggest an igneous source for sulphur in sulphide-bearing dunite pods (+4.3‰), cumulate dunites (+3.1‰), cumulate pyroxenites (+2.9‰) and gabbros and dolerite dykes (+4.5‰). Talc-carbonate altered rocks (+0.9‰) from close to the basal thrust may have derived a portion of sulphur from the underlying metasediments (-4.2‰). Sub-massive pyrite mineralization from high-level gabbros (+9 to +14‰) requires a contribution of hydrothermal sulphur, this has been corroborated by He isotope analysis. Hydrogen and oxygen isotope analyses have shown that serpentine minerals in dunites have exchanged water with meteoric fluids, whereas pyroxenites and gabbros retain a hydrothermal seawater signature.

A diverse assemblage of PGM are known to be intergrown or intimately associated with the base metal suphides and nickel arsenides in dunite pods in harzburgite and cumulate dunites. A new assemblage of Pd ± Pt-bearing Ni and Ni- Cu antimonides are associated with these PGM and disseminated Ni- Fe-sulphides. The paragenesis of these antimonides is described for a sulphide-bearing dunite pod from the PGE-rich Cliff locality and from a PGE-enriched mineralized horizon in cumulate dunites. At both localities the paragenesis of the antimonides is closely linked to the alteration of the coexisting sulphide assemblage.

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