Sulphur and lead isotope geochemistry of the Rubiales Zn-Pb ore deposit (NW Spain)
Tornos, Fernando; Arias, Daniel
European Journal of Mineralogy Volume 5 Number 4 (1993), p. 763 - 774
published: Jul 22, 1993
manuscript accepted: Apr 19, 1993
manuscript received: Mar 3, 1992
ArtNo. ESP147050504006, Price: 29.00 €
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 δ34S values of 25.6 ± 0.5 %o in sphalerites and 22.6 ± 0.4 %o 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 238U/204Pb and 232Th/204Pb 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 206Pb/204Pb-207Pb/204Pb and 208Pb/204Pb- 207Pb/204Pb 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.