MINERALOGICAL COMPOSITION AND WEATHERING OF THE SULZBURG ANTIMONY DEPOSIT, BLACK FOREST

Concerns about antimony (Sb) have been raised in recent years because of increasing levels of Sb pollution, and because of its toxic and carcinogenic properties. The purpose of this study was to investigate various hydrothermal Sb deposits in the Black Forest, Germany, in an effort to characterize the weathering of Sb minerals and the concomitant transfer of Sb to water and soil. Here we report data for the Sulzburg deposit, located near the city of Freiburg and hosted by lower Carboniferous conglomerates (Kulmkonglomerat). Along with the main mineral quartz, the following phases have been identified in the primary ore: boulangerite (Pb₅Sb₄S₁₁), the main Sb mineral; stibnite (Sb₂S₃); plagionite (Pb₅Sb₈S₁₇); pyrite, arsenopyrite; and sphalerite containing ~ 1 wt% Cd. The ore is heterogeneous and thus varies considerably in composition: Sb₂O₃ (4-25 wt%); PbO (2-7 wt%); Fe₂O₃ (0.4-1 wt%); Al₂O₃ (0.2-2.5 wt%); and SiO₂ (46-73 wt%). Weathering of the ores in the mine dumps produces yellowish crusts, which contain the secondary Sb minerals stibiconite (Sb³⁺Sb⁵⁺₂O₆(OH)) and bindheimite (Pb₂Sb₂O₆(O,OH)).

Inside the abandoned mine shafts, fluid seepage leads to formation along the walls of orange-colored or black stalactites (several cm long), which are composed of quartz+ferrihydrite+clay minerals (with or without pyrite), or in some cases of calcite+kaolinite. The fluid accumulates on the floor of the adits, where an orange and black sediment is deposited. Both this sediment and the stalactites contain up to ~500 ppm of each Sb and Pb, and high amounts of As and Fe (up to 2.5 and 23 wt%, respectively). The water (pH ≈ 7) is relatively rich in Sb, As, and Pb (~30, 300, and 60 ppb, respectively) when exiting the mine. It then drains through vegetated mine dumps and discharges into a nearby creek. Downstream of the confluence of mine drainage and creek, the Sb concentration in the water is only 2 ppb, i.e. below the German drinking water limit, whereas the As and Pb concentrations exceed these limits. Our study indicates that relatively large amounts of the Sb released during weathering are not transferred to the creek, but rather are retained in the soil formed on the mine dumps (pH ≈ 6), where it is present as poorly soluble secondary minerals (~300 ppm) or bound to Fe-Mn oxides (~2.5 ppm), as demonstrated by sequential extraction methods.