Genesis of hydrothermal silver-antimony-sulfide veins of the Braunsdorf sector as part of the classic Freiberg silver mining district, Germany

The peripheral regions of the Freiberg vein-type silver mining district comprise several sub districts of which Bräunsdorf was among the richest in terms of Ag grade. 114 t (ca. 3.9 million ounces) of Ag were historically produced from the Neue Hoffnung Gottes Mine near Bräunsdorf. Mining there exploited basically just one single hydrothermal vein, the Neuer Segen Gottes Stehender. The vein infill is marked by a polymetallic sulphide-quartz assemblage (known as kb stage across the Freiberg District) and abundant Ag-Sb-sulphide-carbonate-quartz mineralization, which is limited to the peripheral areas of the Freiberg District. Although Ag-Sb- and Sb-sulphides seem to be spatially and paragenetically closely related to each other, they typically do not occur together in the veins on the scale of meters. Instead, specific zones are dominated by Sb-sulphides without Ag or vice versa. Generally, the abundance of Ag-Sb-sulphides increases with depth.
To develop a sound genetic understanding of polymetallic sulphide-quartz and Ag-Sb-sulphide-carbonate-quartz mineralization in the Bräunsdorf sub district we conducted detailed textural analyses of ore and gangue minerals, fluid inclusion analyses, electron microprobe analyses and thermodynamic computations in order to characterize the ore fluids and ore-forming processes related to the Ag-Sb mineralization. The early-stage polymetallic-sulphide mineralization (stage 1) is related to fluids with low salinities (0.5 - 4 % eq. wt(NaCl)) and formed at temperatures ≥ 300 °C. Microthermometric data related to minerals of the slightly younger Ag-Sb-sulphide assemblage (stage 2) show a range in salinity similar to ore stage 1, but have significantly lower homogenisation temperatures of 280-180 °C. Dissolution textures of previous ore stage 1 minerals and qualitative fluid constraints based on mineral chemistry imply that the composition of the ore fluid changed significantly from Cu-, Pb-, Zn- and As -rich fluids present during ore stage 1 to Ag-Sb-rich fluids prevailing during ore stage 2. Based on fluid inclusion data cooling can be regarded as the major ore-forming process. Reaction path model calculations for cooling of fluids with different initial pH values (5, 6 and 7) reproduce the observed mineral assemblages very well and predict spatial zonation of the Ag-Sb- and Sb-sulphide minerals that are in excellent agreement with field observations. We conclude that Ag-rich zones may well occur below Sb-rich zones in hydrothermal vein-type systems similar to those of the Freiberg District. This relationship may be of potential use for exploration targeting.