Limited availability of sulfur promotes copper-rich mineralization in hydrothermal Pb-Zn veins: A case study from the Schwarzwald, SW Germany

Mixing of compositionally contrasting fluids is widely accepted as the major ore-forming process in unconformity-related hydrothermal Pb-Zn-fluorite-barite veins. Although the general process is relatively well understood, the temporal evolution of such fluid systems which may result in distinct mineralogical changes of the ore and gangue minerals precipitating are insufficiently constrained. One specific example is the occurrence of late-stage siderite-chalcopyrite-gersdorffite mineralization in 28 hydrothermal veins of the Schwarzwald, SW Germany post-dating the major fluorite-barite-galena-sphalerite phase which is very common in the whole area. To investigate the underlying process for this late-stage Cu-Ni-stage, published fluid inclusion data have been complemented by detailed confocal Raman micro-spectroscopy of fluid inclusions, petrographic observations and electron microprobe analyses.
Petrography as well as mineral compositions of sphalerite, gersdorffite, pyrite, arsenopyrite, fahlore and galena record a gradual, but significant influx of Cu and Ni into a prevailing Pb-Zn hydrothermal system. This influx caused a distinct shift towards lower fO2 and Pb + Zn + Sb + Ag (+ S) in the hydrothermal system at the transition from the Pb-Zn to the Cu-Ni mineralization stage. This transition in mineralogy and the related gradual change of the hydrothermal fluid can be explained by changes in the relative proportion of involved fluid components. However, the transition to the Cu-Ni mineralization stage by fluid influx is significantly different compared to the common rapid fluid mixing from below (which is thought to be responsible for the typical Pb-Zn veins) and probably records the gradual transition from long-term stable deep fluid reservoirs with constant chemical compositions towards more shallow and perturbed fluid reservoirs during rifting and exhumation.