Sulphur and lead isotope geochemistry of the Rubiales Zn-Pb ore deposit (NW Spain)

This ore deposit originated by replacement of Lower Cambrian limestones due to hydrothermal fluids circulating along a fault zone during the Hercynian orogeny. Its Pb and S isotopic composition is consistent with that of other carbonate-hosted Zn- Pb deposits in the Hercynian belt in this area. Sulphur isotope signatures in ore minerals are very uniform, with delta (super 34) S 25.6 +- 0.5 per mille in sphalerites and 22.6 +- 0.4 per mille in galenas. These values are interpreted in terms of the S having been derived from abiogenic reduction of sulphates equilibrated with Cambrian marine waters. Ore deposition took place at approx 200 degrees C. High (super 238) U/ (super 204) Pb and (super 232) Th/ (super 204) Pb ratios in galena support a crustal origin for lead in the ore deposit. The linear array of lead isotope ratios can be interpreted as due to mixing of two different types of lead or selective leaching of the heavier lead isotopes of a primary ore. Sulphur and most of the metals in the deposit may be derived from remobilization of diagenetic, stratabound Zn-Pb mineralizations occurring in Cambrian limestones.