Fluid mixing during ore deposition at the Tynagh base-metal deposit, Ireland

Abstract

The Tynagh base-metal deposit is hosted by upper Tournaisian to lower Visean age Waulsortian carbonates in the west of Ireland. Epigenetic galena, sphalerite and baryte are located in the hanging wall of a major normal fault. Fluid inclusion in quartz, sphalerite and baryte show that two distinct fluids were responsible for the mineralisation. A high temperature moderate salinity fluid (240 o C, 12 Wt.%), derived from convection of Carboniferous seawater in the crust, carried most of the ore metals. A low temperature high salinity fluid (<70 o C, 21 Wt.%), residing on or close to the seafloor, provided most of the reduced sulphur and sulphate. These fluids mixed to a minor extent on the seafloor as well as in the epigenetic ore zones and the fault zone which acted as a major conduit for both ascending and descending fluids. Post-ore fluid inclusions in calcite show no evidence of the high temperature fluid but record that the high salinity was being progressively diluted by a low salinity (c. 1 Wt.%) SO 4 2− or HCO 3 − meteoric fluid. The results are similar to those from the Silvermines deposit and suggest that other Irish base-metal deposits may have been derived from similar fluids