Building the Las Cruces Gossan (SW Spain), Part II – Could Microbes have formed Sulfides in the shallow Subsurface?

Microbial sulfate reduction (MSR) may have played a major role in the formation of secondary copper sulfides such as covellite (CuS) and chalcocite (Cu₂S) in the cementation zone of the Las Cruces gossan (Seville, Iberian Pyrite Belt, Spain)^[1,2]. However, the formation of copper sulfides by MSR under conditions relevant to the Las Cruces gossan (e.g. hydrochemistry, temperature, pH) has not been experimentally evaluated yet. Using an established geomicrobiological model system for the Las Cruces gossan (see abstract by Schlund et al.), we explored MSR by tracking electron acceptor turnover (e.g. sulfate, phosphate) in incubation experiments over five weeks and by analysing yielded solid phases with SEM-EDS. Our data demonstrate significant MSR between 30–50 °C and even at high Cu concentrations (38 mg/L). Sulfate turnover increased with temperature (10 mmol/L at 30 °C, 17.3 mmol/L at 50 °C) and was highest in high-Cu experiments (20.1 mmol/L). Decreasing sulfate concentrations were accompanied by increasing phosphate concentrations and rising pH (4.5 to 7–8.5). SEM-EDS revealed the presence of copper sulfides in the experimental products, partially associated with C_org – similar to findings from the Las Cruces gossan^[2]. Taken together, our results support the hypothesis that Cu sulfides in the cementation zone of the Las Cruces gossan formed secondarily, driven by MSR. Moreover, our study contributes to a better understanding of the deep biosphere – including potential ore forming geomicrobiological processes in the shallow subsurface.

References

[1] Tornos et al. (2017) Mineralium Deposita 52: 1-34.

[2] Tornos et al. (2018) Geology 47 (2): 143-146.