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

Abstract

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 finecrystalline 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 recycling in a locally partially closed system and contribution of H 2 S from hydrolysis of biogenic pyrite and thermal cracking of organo-sulfur compounds. The results suggest that the conformable orebodies originated by mixing of hydrothermal saline metal-rich fluid with H 2 Srich pore waters during late burial diagenesis, while the Editorial handling: B. Lehmann Electronic supplementary material The online version of this article

Assessment of contribution of metal pollution sources to attic and household dust in Pb‐polluted area

“…All this indicates that metallic sulfides and sulfates in attic dust do not originate directly from mine‐waste material and past mining activities or their weathering products. Since metallic sulfides and sulfates usually become somewhat enriched in 34 S during high‐temperature smelting due to isotopic fractionation, it is more likely that they result from primary Pb‐smelting of original local sulfide ore. Gypsum and anhydrite in attic dust (LF and IF) have 29 times higher δ 34 S values than natural gypsum from local ore deposit . Such enrichment in 34 S is consistent with enrichment in anthropogenic gypsum formed during flue‐gas desulfurization .…”

“…The δ 34 S values and Pb isotope ratios in metallic sulfides and sulfates and non-metallic sulfates in attic and household dust, minewaste material, Pb-acid battery material, and local ore minerals are presented in Table 1 34 S during high-temperature smelting due to isotopic fractionation, 39,40 it is more likely that they result from primary Pb-smelting of original local sulfide ore. Gypsum and anhydrite in attic dust (LF and IF) have 29 times higher δ 34 S values than natural gypsum from local ore deposit. 37 Such enrichment in 34 S is consistent with enrichment in anthropogenic gypsum formed during flue-gas desulfurization. 41 Thus, it can be assumed that gypsum and anhydrite in attic dust formed as a solid product during primary smelting of local sulfide ore, which was subsequently transported by air.…”

“…Mean δ 34 S values of non‐metallic sulfates are −0.6‰ (gypsum and anhydrite) in attic dust and 3.8‰ in household dust. Metallic sulfates and sulfides in attic dust are enriched in 34 S by 5.9 times, 4.5 times, and 2.6 times as compared with local mine‐waste material, sulfide minerals from local ore deposit and local galena ore, respectively. They are depleted with respect to used Pb‐acid battery.…”

Nature and Evolution of the Ore-Forming Fluids from Nanmushu Carbonate-Hosted Zn-Pb Deposit in the Mayuan District, Shaanxi Province, Southwest China

“…In general, the CH 4 -rich hydrothermal fluids may come from three ultimate sources: (1) abiogenic origin derived from the mantle [33,34]; (2) abiogenic origin derived from postmagmatic alteration by Fischer-Tropsch type synthesis [35][36][37]; (3) incorporation of thermally decomposed organic material (thermogenesis) or/and products from microbial processes (bacteriogenesis) [38][39][40][41]. The deposit characteristics (without postmagmatic alteration) and sulfur isotope characteristics excluded the abiogenic origin.…”

“…In consideration of temperatures of FIs in the Sp(a) and Sp(b) at the range of 183 ∘ -314 ∘ C, the possible mechanism for reduced sulfur is thermochemical sulfate reduction (TSR), because TSR can occur under high temperatures while bacterial sulfate reduction (BSR) can only take place under low temperatures (<127 ∘ C) [38,46]. In addition, TSR would like to produce a series of organic matter (e.g., bitumen, C 3 H 8 , C 2 H 6 , and CH 4 ) [38,40,46,47]. Ore-forming fluids show an increase in carbonaceous contents over time.…”