Genesis of the Silvermines zinc-lead-barite deposit, Ireland; fluid inclusion and stable isotope evidence

Abstract

The Silvermines zinc-lead-barite deposit occurs adjacent to a major normal fault system within a sequence of Mississippian (Tournaisian) sandstones, shales, limestones, and dolomites. It comprises large synsedimentary-syndiagenetic and smaller, underlying epigenetic orebodies.Fluid inclusions within the epigenetic minerals are dominated by a simple liquid-vapor type which in places contains one to five solid phases, the commonest of which is calcite. Single-phase liquid inclusions are commonest in barite; single-phase vapor and vapor-liquid inclusions are occasionally found in quartz, barite, and carbonates, although there is no indication of extensive boiling.Fluid inclusion salinities range from 8 to 28 equiv wt percent NaCl with modes at 14 and 19 wt percent. Leachate studies show the fluids to have high sodium concentrations with lesser, variable amounts of potassium and calcium and uniformly low magnesium concentrations (K/Na = 0.03-0.23; Ca/Na = 0.03-0.28; Mg/Ca = 0.1-0.67). Fluid inclusions within synsedimentary-syndiagenetic barite are highly saline, indicating the possible presence of a brine pool at the site of ore deposition.Homogenization temperatures range from 50 degrees to 260 degrees C, with most between 140 degrees and 220 degrees C. There is a mode at 190 degrees C and a subsidiary group at 140 degrees C. Required pressure corrections are less than 5 degrees C. If the pressure within the epigenetic ore zones was purely hydrostatic, as is reasonable, a minimum seawater depth of approximately 80 m is required to prevent boiling of some fluids.For the whole deposit there is a negative correlation between homogenization temperature and salinity for quartz-hosted inclusions, which is interpreted as the involvement of two (or possibly three) fluids in the mineralizing process. We suggest that high-temperature, 8 to 12 equiv wt percent NaCl fluids mixed with lower temperature, 18 to 22 equiv wt percent NaCl fluids, both within and below the ore zone during mineralization.Fluid inclusion waters have delta D values of -24 to -49 per mil for quartz, -23 to -29 per mil for dolomite, -41 to -55 per mil for barite, -46 per mil for sphalerite, and -58 per mil for galena. The delta 18 O values of the mineralizing fluids, calculated from mineral values, range from 1.1 to 7.7 per mil for quartz and 1.8 to 5.7 per mil for dolomite. These data best represent aqueous fluids that have equilibrated with the geosynclinal sequence and granites underlying the deposit. They also suggest that local Carboniferous surface and ground waters may have contained a deuterium-depleted (meteoric?) component.The fluid inclusion evidence presented refutes a model involving the expulsion of Carboniferous formation waters and indicates little, if any, involvement of magmatic waters. The geologic, geochemical, and fluid inclusion evidence best fits a model involving the convection of Carboniferous surface waters in the putative thick geosynclinal sequence and associated granitoids below the deposit, with a contribution from lower Paleozoic formation waters.Mineral precipitation resulted from fluid mixing in the ore zone with some contribution from carbonate wall-rock reactions and possibly boiling.