Sulphur and sulphate-oxygen isotopes and the origin of the silvermines deposits, Ireland

Abstract

New sulphur and sulphate-oxygen isotope measurements for the main discordant and stratiform lead-zinc-barite orebodies at Silvermines Co. Tipperary, allow reappraisal of previously offered differing interpretations (Graham, 1970; Greig et al., 1971) of the bearing of sulphur isotopes on the genesis of this important Irish deposit. The following aspects of the data are confirmed: barite δ³⁴ S-values range from 17–21‰, similar to lower Carboniferous seawater sulphate: stratiform sulphide lens pyrites have δ³⁴ S-values ranging from −13 to −36‰; vein sulphide δ³⁴ S-values range from −8 to 4‰; sulphide δ³⁴ S-values increase upwards and outwards respectively in the related discordant and stratiform “G” orebodies; galena-sphalerite isotope palaeotemperatures are not too consistent, ranging from 40 to 430°C (using the calibration of Czamanske and Rye (1974). New facts are as follows: barite δ¹⁸O-values range from −13 to −17‰, stratiform barites ranging from 13 to 14.5‰; sulphides separated from a single stratiform ore lens hand specimen usually have δ³⁴ S_sl > δ³⁴ S_ga > δ³⁴ S_py; the outward decrease in δ³⁴ S-values in the stratiform “G” orebody is confined to the first few hundred feet only; pyrite δ³⁴ S-values progressively increase downwards through one stratiform sulphide orebody; yet variations of 13‰ occur within a single colloform pyrite structure from another stratiform orebody. It is concluded that there were at least two sources of sulphur, seawater sulphate and deep-seated sulphur. The former was the dominant source of all sulphate and, via biogenic reduction, of the sulphur in the bulk of the stratiform sulphide. The latter was the source of the sulphur in the vein sulphides. There was minimal isotopic interaction between the cool seawater sulphate and the warm unwelling ore fluid sulphur species, even though the latter precipitated under near isotopic equilibrium conditions when the temperature dropped and/or the pH and Eh increased. The lack of isotopic equilibrium between pyrite and ore sulphides in the stratiform ore lenses may result from the latter having precipitated slightly later than the former because of solubility relationships. Overall the present isotopic evidence supports considerable geological evidence favoring a syngenetic origin for the stratiform Silvermines orebodies.