A sulphur isotope study of iron sulphides in the late Precambrian Dalradian Ardrishaig Phyllite Formation, Knapdale, Argyll

Abstract

Synopsis The origin of sulphur in iron sulphides occurring in siliceous and chloritic metasediments of the Ardrishaig Phyllite Formation of Knapdale, Argyllshire, was assessed using sulphur isotope analysis. The formation lies within the Argyll Group of the late Precambrian Dalradian Supergroup. Iron sulphide textures indicate an early metamorphic generation of pyrrhotite, while pyrite porphyroblasts grew at a late stage of regional metamorphism. Preferential association of iron sulphides with certain lithologies indicates that the sulphide accumulations are of original synsedimentary origin, even though there have been several stages of remobilization. δ 34 S values range from − 14.8 to − 6.3‰ ( n = 18; x = − 11.7‰). The values are rather low for the sulphur to be of magmatic or hydrothermal origin which is also unlikely from geological considerations. Since the sulphur isotopic ratio of seawater sulphate during Ardrishaig Phyllite times would have been between + 30 and +40‰, the isotopic fractionation of about 40–50‰ is too large to be explained by abiogenic reduction of original evaporitic sulphates on metamorphism. This isotopic fractionation is consistent with the sulphides originating by bacterial reduction of sulphate within marine sediments of general high permeability with ample replenishment of seawater.