Fluid mixing during ore deposition at the Tynagh base-metal deposit, Ireland
Banks, David A.; Russell, Michael J.
European Journal of Mineralogy Volume 4 Number 5 (1992), p. 921 - 932
published: Oct 14, 1992
manuscript accepted: Jan 20, 1992
manuscript received: Jun 6, 1991
ArtNo. ESP147050405016, Price: 29.00 €
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 inclusions in quartz, sphalerite and baryte show that two distinct fluids were responsible for the mineralisation. A high temperature moderate salinity fluid (240 °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 °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.%) SO42- or HCO3- 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.