Biogeochemistry of a Hyperacidic and Ultraconcentrated Pyrite Leachate in San Telmo mine (Iberian Pyrite Belt, Spain)

This work describes recent research carried out in an extremely acidic (pH 0.61–0.82) and hypersaline (e.g., 134 g/L SO₄ ²⁻, 74 g/L Fe, 7.5 g/L Al, 3 g/L Mg, 2 g/L Cu, 1 g/L Zn) leachate which seeps from a pyrite pile in San Telmo mine (Huelva, SW Spain) and forms evaporative pools of ultra-concentrated water in which attractive crystals of Zn-rich melanterite (FeᴵᴵSO₄ 7H₂O) are formed. Geochemical modeling with the Pitzer method indicates that the acidic brine was near saturation with respect to melanterite (SIMₑₗ = 0 ± 0.2). The microbiological investigation has revealed a surprisingly high biomass (1.4 × 10⁶ cells mL⁻¹) and an exotic ecosystem composed of acidophilic, Fe-oxidizing archaea (mainly Ferroplasma spp., representing 52% of the microbial population), and minor numbers of acidophilic bacteria (including Leptospirillum spp. (3.2%), Acidithiobacillus spp. (1.6%), and Alphaproteobacteria (2.8%)). The microbial production of Feᴵᴵᴵ allows the oxidative dissolution of pyrite and other sulphides, which results in additional inputs of Feᴵᴵ, SO₄ ²⁻ and acidity to the system. The surfaces of the pyrite crystals show a typical etch-pitted texture, as well as blobs of elemental sulphur, which are both compatible with this indirect, microbially mediated oxidation mechanism. The composition of the acidic leachate seems to result from the combination of several processes which include: (1) formation of melanterite within the pile during relatively dry seasons, (2) subsequent dissolution of melanterite during rainy episodes, (3) microbial oxidation of Feᴵᴵ, (4) sulphide oxidation mediated by Feᴵᴵᴵ, (5) dissolution of chlorite and other aluminosilicates present in the pile, and (6) cooling and/or evaporation of seepage from the pile and consequent melanterite precipitation.