Microbial Sphalerite Formation in Carbonate-Hosted Zn-Pb Ores, Bleiberg, Austria: Micro- to Nanotextural and Sulfur Isotope Evidence

Abstract

Microglobular sphalerite (90–180 μm) is a major form of sphalerite in the abandoned, Triassic carbonate-hosted Zn-Pb deposit at Bleiberg, Austria. It is common in ores from the Crest horizon and the first Cardita horizon, which contain about 0.5- to 3-mm-thick and up to 5-cm-long, wavy, discontinuous, sphalerite-rich bands composed of agglomerations of microglobular sphalerite. Based on comparisons with modern environments, these textures are interpreted as peloids and probably represent fossil microbial mats. These are inter-layered with carbonate- and marcasite-rich, as well as finely banded, sphalerite layers (±fibrous galena and relics of oxysulfides), resulting in the characteristic rhythmically banded macrotexture of these Zn-dominated ores. Microbial nanotextures, made visible by field emission scanning electron microscopy (FESEM) after etching, include sphalerite nanospheres (10–90 nm) and bacterial filaments. The sphalerite nanospheres are identical to those previously reported for mixed calcite-sphalerite peloids from Bleiberg and are very similar to nanotextures observed in recent bacterial biofilms made by Desulfobacteriaceae. The observed sphalerite nanospheres are interpreted as in situ metabolic products of sulfate-reducing bacteria. Micro- and nanotextures suggest that the larger, μm-sized sphalerite globules formed by agglomeration of sphalerite nanospheres, as well as by replacement of peloids representing former bacterial colonies; the latter are now composed of Zn-calcite cores surounded by serrated sphalerite rims. Most samples studied evidence recrystallization of sphalerite that is broadly coeval with formation of fluorite. Involvement of sulfate-reducing bacteria in the formation of this early sphalerite mineralization is supported by sulfur isotope data. Sphalerite is generally characterized by isotopic light sulfur with δ 34 S per mil values ranging from −30.5 to −20.3 per mil. The most negative sulfur isotope values are recorded from sphalerite (−30.5‰, −30.2‰) and galena with unusual fibrous to dendritic morphology (−31.9‰; −31.8‰); both are associated with oxysulfides. Microglobular sphalerite with well-preserved sphalerite globules has a δ 34 S value of −28.8 to −28.2 per mil; colloform sphalerite (schalenblende) ranges from −29.0 to −25.0 per mil. Microglobular sphalerite associated with fine-grained fluorite and euhedral sphalerite formed through recrystallization processes show a trend to less negative values; i.e., −26.3 to −22.2 per mil and −22.9 to −20.3 per mil, respectively. We suggest that the combined biogenic nano- to macrotextures and sulfur isotope data are evidence that microbes played a key role in formation of Alpine carbonate-hosted Zn-Pb deposits. Results of this study contradict the MVT model suggested for Alpine Pb-Zn deposits, which calls for formation of Pb-Zn mineralization during late diagenetic burial in the Late Triassic-Early Jurassic.