Antimoniferous vein-type mineralization of the Berga Antiform, Eastern-Thuringia, Germany: A fluid inclusion study

Abstract

Abstract The antimoniferous vein-mineralization in the Schleiz and Greiz area, eastern Thuringia, are bound to NE-SW striking Variscan fault zones along the margins of the Berga Antiform. The Sb-Pb-Zn ores form massive sulfide-sulfosalt-quartz\u202f±\u202fcarbonate veins, breccias or stockworks within Ordovician and Silurian metasedimentary units, Devonian metabasalts and metagranitoids. Mineralogical investigations from several Sb mines and hard rock quarries near Schleiz and Greiz, Germany, show that the Sb-Pb-Zn-sulfides are texturally in close association with quartz and carbonates. Stibnite is the major Sb-bearing mineral. Minerals of the Sb-Pb-Zn-sulfide assemblage replace an earlier Zn-Fe-As-sulfide-quartz assemblage (quartz I), named the ore-stage I mineralization. Stibnite and quartz II overgrow these early sulfides accounting for the ore-stage II. Lead-dominated Sb-sulfides (e.g., zinkenite, plagionite, semseyite) represent the ore-stage III phase followed by sphalerite II, galena and quartz III accounting for ore-stage IV. Fluid inclusion studies on ore and gangue minerals of the different ore stages give insights into the composition and temperature of the mineralizing fluid(s). Primary fluid inclusion assemblages in ore-stage II stibnite and co-genetic quartz II show variable vapor fractions within fluid inclusion assemblages. Heterogeneous trapping of CO2 at constant salinities of fluid inclusions of the same assemblage indicates the influx of a buoyant pure CO2 fluid into an aqueous fluid phase. The presented data demonstrates the first example of CO2-influx into an aqueous fluid identified by fluid inclusion analyses. During this ore-stage II a H2O-CO2-NaCl fluid, with eq. w(NaCl) of 1 to 2.7% at 220–226\u202f°C prevails. In contrast, fluid inclusions in ore-stage III minerals (boulangerite) contain a homogeneously trapped H2O-NaCl-CaCl2 fluid, with eq. w(NaCl) up to 8.1% at temperatures ≥146\u202f°C. Ore-stage IV minerals (sphalerite II, quartz III) contain a homogeneously trapped H2O-NaCl-CaCl2 fluid with eq. w(NaCl) of >20% and homogenization into the liquid phase at around 120\u202f°C. Fluid inclusion data suggest P-T-conditions at the onset of mineralization at around 220\u202f°C at 0.70 to 0.75\u202fkbar (fluid pressure). The mechanisms of Sb-ore precipitation were induced by the influx of CO2, fluid cooling (quartz II – stibnite – boulangerite) and late stage mixing with a high salinity fluid resulting in the precipitation of sphalerite II, galena and quartz III. The unique opportunity to analyze fluid inclusions in many different, subsequently precipitating ore and gangue minerals enables the reconstruction of the temporal evolution of the Sb-veins. The distinct change of the ore-forming processes from the influx of CO2 into an aqueous fluid and cooling (ore-stage I, II) to mixing (ore-stage III, IV) as the dominant precipitation mechanisms are also accompanied by a drastic change in the mineralogy from Sb-sulfides to Pb-Zn-sulfides. Late-orogenic granitic intrusions served as a potential heat, metal and sulfur source. Variscan wrench and thrust tectonics and associated metamorphism account for the ore-stage I mineralization. Hydrothermal Sb-Pb-Zn assemblages (ore-stages II, III and IV) precipitated during the latest part of the Variscan orogeny most likely around the Carboniferous/Permian boundary. Although, fluid composition and formation temperatures of the Greiz and Schleiz mineralization are on the first view quite similar to those of other Variscan Sb mineralization, the observation of CO2-influx and fluid mixing with a high-salinity fluid is unique for European Sb-occurrences.