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

Abstract

The Rubiales Zn-Pb ore deposit formed by replacement of Lower Cambrian limestones due to hydrothermal fluids that circulated along a fault zone during the Hercynian orogeny. Its lead and sulphur isotopic composition is consistent with that of Cambrian carbonate hosted Zn-Pb deposits of the Hercynian Belt in the CircumMediterranean domain. Sulphur isotope signatures in ore minerals are very uniform, with δ 34 S values of 25.6 ± 0.5 ‰ in sphalerites and 22.6 ± 0.4 ‰ in galenas. These heavy and monotonous values are interpreted in terms of hydrothermal remobilization of pre-existing sulphide mineral occurrences, the sulphur being derived from abiogenic reduction of sulphates equilibrated with Cambrian marine waters. Ore deposition took place at temperatures close to 200°C under reducing conditions and slightly acid to neutral pH; the process was promoted by fluid-carbonate rock interaction. The high 238 U/ 204 Pb and 232 Th/ 204 Pb ratios in galenas support a crustal origin for lead in the ore deposit, seemingly related ultimately to Palaeozoic detritic rocks derived from the erosion of a Precambrian basement. The linear array of lead isotope ratios in the 206 Pb/ 204 Pb- 207 Pb/ 204 Pb and 208 Pb/ 204 Pb- 207 Pb/ 204 Pb diagrams 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