Origin of complex sulphide nodules related to diagenesis of lacustrine sediments of Middle Devonian age from the Shetland Islands

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Abstract
Synopsis: A laminated, carbonaceous-calcareous-siltstone facies (the Devonian Upper Melby Fish Bed, Shetland) of non-glacial varve origin deposited in a tropical, stratified lake, shows a complicated diagenesis. Sulphide nodules in this sequence, containing pyrite, chalcopyrite, covellite, bornite, sphalerite, galena and tennantite, show an equally complex development. The first period of growth produced a concentric zonation of pyrite and pyrite with sphalerite and galena. Following fracture of the nodule, Cu, Fe-sulphides replaced pyrite. Pb, Zn and As were remobilised and/or introduced at this time. It is probable that lacustrine regression and transgression greatly affected the chemical properties of groundwaters and led in part to the complicated nodule and host rock diagenesis. The nodules were later cut by small veins of tectonic origin.