Sulfur isotope variations from orebody to hand-specimen scale at the Mežica lead–zinc deposit, Slovenia: a predominantly biogenic pattern

• Published in: Mineralium deposita 2010-08, Vol.45 (6), p.531-547
• Main Authors: Herlec, Uroš, Spangenberg, Jorge E., Lavrič, Jošt V.
• Format: Article
• Language: English
• Subjects: Carbonate rocks; Chemical analysis; Diagenesis; Earth and Environmental Science; Earth Sciences; Geology; Isotopes; Mineral Resources; Mineralization; Mineralogy; Organic compounds; Petrology; Pyrite; Seawater; Stable isotopes; Sulfate reduction; Sulfates; Sulfides; Sulfur; Thermodynamics; Triassic; Water analysis
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-010-0290-y

The Mississippi Valley-type (MVT) Pb–Zn ore district at Mežica is hosted by Middle to Upper Triassic platform carbonate rocks in the Northern Karavanke/Drau Range geotectonic units of the Eastern Alps, northeastern Slovenia. The mineralization at Mežica covers an area of 64 km 2 with more than 350 orebodies and numerous galena and sphalerite occurrences, which formed epigenetically, both conformable and discordant to bedding. While knowledge on the style of mineralization has grown considerably, the origin of discordant mineralization is still debated. Sulfur stable isotope analyses of 149 sulfide samples from the different types of orebodies provide new insights on the genesis of these mineralizations and their relationship. Over the whole mining district, sphalerite and galena have δ 34 S values in the range of –24.7 to –1.5‰ VCDT (–13.5 ± 5.0‰) and –24.7 to –1.4‰ (–10.7 ± 5.9‰), respectively. These values are in the range of the main MVT deposits of the Drau Range. All sulfide δ 34 S values are negative within a broad range, with δ 34 S pyrite < δ 34 S sphalerite < δ 34 S galena for both conformable and discordant orebodies, indicating isotopically heterogeneous H 2 S in the ore-forming fluids and precipitation of the sulfides at thermodynamic disequilibrium. This clearly supports that the main sulfide sulfur originates from bacterially mediated reduction (BSR) of Middle to Upper Triassic seawater sulfate or evaporite sulfate. Thermochemical sulfate reduction (TSR) by organic compounds contributed a minor amount of 34 S-enriched H 2 S to the ore fluid. The variations of δ 34 S values of galena and coarse-grained sphalerite at orefield scale are generally larger than the differences observed in single hand specimens. The progressively more negative δ 34 S values with time along the different sphalerite generations are consistent with mixing of different H 2 S sources, with a decreasing contribution of H 2 S from regional TSR, and an increase from a local H 2 S reservoir produced by BSR (i.e., sedimentary biogenic pyrite, organo-sulfur compounds). Galena in discordant ore (–11.9 to –1.7‰; –7.0 ± 2.7‰, n  = 12) tends to be depleted in 34 S compared with conformable ore (–24.7 to –2.8‰, –11.7 ± 6.2‰, n  = 39). A similar trend is observed from fine-crystalline sphalerite I to coarse open-space filling sphalerite II. Some variation of the sulfide δ 34 S values is attributed to the inherent variability of bacterial sulfate reduction, including metabolic recycl