Chemical evolution of ore-forming brines – Basement leaching, metal provenance, and the redox link between barren and ore-bearing hydrothermal veins. A case study from the Schwarzwald mining district in SW-Germany

Abstract

Six mineralogically exemplary barren and mineralized hydrothermal veins (with Pb and Zn ores) of Jurassic-Cretaceous and Cenozoic age in the Schwarzwald mining district, SW Germany were chosen to shed light on the origin of their mineralogical diversity. The selection of the veins was guided by the fact that they represent the largest number of veins in the region, are very well known mineralogically and geochemically, and they provide nice examples of barren and mineralized veins of similar age. Fluid inclusion data from the individual veins overlap implying their diverse mineralogy is not caused by different fluid compositions participating during fluid mixing. LA-ICPMS data of single fluid inclusions indicate no systematic variations in major elements, but significant changes in fluid mixing ratios which caused variable trace element concentrations of different fluid inclusion assemblages in one sample. We suggest that different ore-gangue-assemblages can be produced by mixing of identical fluid endmembers, but variable mixing ratios. LA-ICPMS analyses of single fluid inclusions in barren and mineralized veins record similar base metal and sulfur concentrations. Hence, sulfide mineralization in the veins appears not to be controlled by metal solubility. Thermodynamic modeling based on the fluid data indicates that the presence of a reducing phase during fluid mixing is required for sulfide mineralization to precipitate.LA-ICPMS trace element analyses of feldspars, biotites, chlorites and clay minerals in granites and paragneisses which are the source of the metal content in the ore-forming fluids demonstrate that the dominant provenance for Pb, Zn, As, Sb, Ba, Tl, Mo, Fe and Mn are probably paragneisses, while Co, Cu and Ni are probably sourced from S-type granite. A rough quantification indicates that <1 m3 paragneiss or granite has to be altered (10% alteration) to supply sufficient Zn to 1 l of hydrothermal fluid reaching a concentration of 2 g/l Zn. Hence, this study confirms that ore fluids can be produced from any lithology in the upper crust that contains at least some trace metals (μg/g level is sufficient).