Volcanic activity, magma-sediment-water interaction, hydrothermal alteration and vein mineralisation in south County Waterford

Abstract

The Ordovician volcanic sequence of the south Waterford coast is used to investigate magma-sediment-water interaction and its role in hydrothermal systems.

Field investigations and rock textures analysis have shown that relatively constant composition intrusions were emplaced in rapidly consolidating host sediments. Early intrusions into water-laden sediments generated peperite clasts, with morphology determined primarily by magma:sediment ratio. Hydrothermal alteration (chlorite + epidote + sericite ± pumpellyite) of the volcanic rocks is more pronounced within the more permeable and glassy sequences, such as peperite, hyaloclastite and pillow rims. The chemistry of chlorite (Fe# = 0.35-0.63), its distribution, and its association with epidote, pumpellyite, pyrite and Fe-oxides have been used to determine that an iron-rich hydrothermal fluid interacted with magnesium-rich seawater in varying ratios and that fluid convection was promoted in the permeable sequence. Later intrusions show no evidence of interaction with unconsolidated sediment and/or water and as a result the dominant alteration assemblage is representative of later diagenesis.

Chlorite geothermometry shows that alteration occurred between 300°C and 400°C and, adjacent to peperitic intrusions, Na, K, Si and some REE are locally mobile within fluidised sediment. Sulphide-bearing veins were formed at lower temperatures (200°C to 350°C) as a result of mixing between a high temperature, low salinity fluid and a lower temperature, high salinity (possibly meteoric) fluid. The overall drop in temperature of the hydrothermal fluid may have contributed to the precipitation of base metals. Extensive volcanic activity within the seafloor generated an elevated geothermal gradient that drove a hydrothermal convection system and formed veins with base metals sourced from early volcanic and volcano-sedimentary products.