Zn-Pb mineralisation in the Silvermines district, Ireland: a product of burial diagenesis

Abstract

Carbonate-sulphide cement stratigraphic relationships in the host rock and ore have been used to constrain the age of mineralisation at the Silvermines zinc-lead-barium deposit. The base-metal sulphides post-date planar dolomite and replace stylolites. Furthermore, the pre-mineralisation planar dolomites also replace stylolites. These and other diagenetic observations indicate that the base-metal sulphides formed at burial depths greater than 800 m, but probably predate the Variscan deformation (since pressure shadows overgrow base metal sulphides). This indicates that the sulphides are of epigenetic origin, constraining the age of mineralisation to between the late Chadian (≈347 Ma) and the late Westphalian (≈307 Ma). However, the most likely age for mineralisation, (based on widespread macro-stylolite development) is Asbian (≈339 Ma) or younger. No evidence of synsedimentary sulphides (in the form of hydrothermal chimneys, vent faunas, or sulphides intergrown with marine cements) was observed at Silvermines. Mineralised breccias (black matrix breccias), late-stage internal sediments, and dissolution zones within the carbonate cements all appear to be produced by hydrothermal karsting that occurred during the mineralisation process. Fluid inclusion homogenisation temperatures for ore-stage calcites (up to 300 °C) approach the peak temperature estimates derived from regional maturation parameters (270 to 310 °C from conodont alteration indices and vitrinite reflectance). This suggests that homogenisation temperatures represent maximum heating temperatures (probably during Variscan time) rather than mineralisation temperatures.