Sulfur, oxygen and carbon isotope geochemistry of barite-iron oxide-pyrite deposits from the Apuane Alps (northern Tuscany, Italy)

Abstract

The sulfur, oxygen and carbon isotopic compositions of a total of 64 samples from stratiform and vein barite-iron oxide-pyrite ore bodies of the Apuane Alps are presented. In stratiform ores, sulfide minerals display a wide range of δ 34S from − 20.9 to + 18.9‰. Similarly, barites are quite variable in δ 34S, with values from + 4.3 to + 30.8‰, with a clustering around + 19.4‰; on the other hand, their δ 18O-values are fairly uniform around + 15.8‰. Calculated isotopic temperatures from barite-pyrite pairs are broadly distributed, even within a single deposit. Gangue carbonates show quite light δ 13C-values from − 29.3 to − 20.5‰, and δ 18O-values closely around + 18.5‰. In vein ores, sulfide minerals and barite are much more uniform in δ 34S, with values closely around − 1.1 and + 18.0‰, respectively; barite δ 18O-values are nearly equal to + 13.4‰. Moreover, barite-pyrite and barite-galena pairs give concordant isotopic temperatures of ∼ 385°C. The isotopic data concur with previously acquired lines of evidence to support the following genetic model: 1. (a) Stratiform ores are synsedimentary in origin, the main source of sulfur having been Middle-Upper Triassic seawater sulfate which underwent bacterial reduction. Concurrent oxidation of organic matter supplied carbon in gangue carbonates. Probably, both barite and gangue carbonate suffered post-depositional oxygen isotope equilibration with water under diagenetic conditions. 2. (b) Vein ores formed from remobilization of stratiform ores by metamorphic fluids (Alpine orogeny) at 380–400°C, highly enriched in 18O probably by interaction with country schists.