Chemical Geology, 76 (1989) 249-257 249 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Sulfur, oxygen and carbon isotope geochemistry of barite-iron oxide-pyrite deposits from the Apuane Alps (northern Tuscany, Italy) G. CORTECCI 1, P. LATTANZI 2 and G. TANELLI 3 1Dipartimento di Scienze della Terra, Universit& di Pisa, 1-56100 Pisa (Italy) 2Dipartimento di Scienze della Terra, Universit~ di Firenze, 1-50121 Firenze (Italy) 3Dipartimento di Scienze della Terra, Universit& di Napoli, 1-80138 Napoli (Italy) (Accepted for publication April 13, 1989) Abstract Cortecci, G., Lattanzi, P. and Tanelli, G., 1989. Sulfur, oxygen and carbon isotope geochemistry of barite-iron oxide- pyrite deposits from the Apuane Alps (northern Tuscany, Italy). In: K. GrSnvold (Guest-Editor), Water-Rock Interaction. Chem. Geol., 76: 249-257. 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 (~34S from -20.9 to + 18.9%o. Similarly, barites are quite variable in •348, with values from +4.3 to +30.8%o, with a clustering around + 19.4%o; on the other hand, their JlSO-values are fairly uniform around + 15.8%o. Calculated isotopic temperatures from barite-pyrite pairs are broadly distributed, even within a single deposit. Gangue carbon- ates show quite light J13C-values from -29.3 to -20.5%o, and JlSO-values closely around + 18.5%o. In vein ores, sulfide minerals and barite are much more uniform in J34S, with values closely around - 1.1 and + 18.0%o, respectively; barite JlS0-values are nearly equal to + 13.4%o. 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: (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 equilibra- tion with water under diagenetic conditions. (b) Vein ores formed from remobilization of stratiform ores by metamorphic fluids (Alpine orogeny ) at 380-400 ° C, highly enriched in 'sO probably by interaction with country schists. 1. Introduction In the Apuane Alps region (northwestern Tuscany, Italy), barite, iron oxide + pyrite are presently mined from the deposits of Monte Arsiccio, Pollone and Buca della Vena (Fig. 1 ). These deposits are currently the subject of a comprehensive research program. The main as- pects of geological setting, mineralogy and geo- chemistry have recently been described by Cor- tecci et al. (1984, 1985) and Benvenuti et al. (1986). The data collected so far allowed the aforementioned authors to hypothesize a sedi- mentary-metamorphic (Middle-Upper Trias- sic and Alpine) genetic model for the deposits. In this paper, the available S, C and O isotope data on minerals from these deposits are uti- lized in an attempt to clarify the interaction be- 0009-2541/89/$03.50 © 1989 Elsevier Science Publishers B.V.