Mineralogy, mineral compositions and fluid evolution at the Wenzel hydrothermal deposit, southern Germany: implications for the formation of Kongsberg-type silver deposits

The post-Variscan Wenzel vein-type deposit near Wolfach, in the Black Forest, Germany, the type locality of the Ag-Sb alloy dyscrasite, was investigated by ore microscopy, electron-microprobe analysis, stable isotope and fluid-inclusion analysis. Three stages of mineralization could be distinguished. Whereas the first stage is a typical sulfide mineralization including galena and tetrahedrite, the second and third stage show a sulfide-poor association of Ag-Sb alloys, as well as Fe, Co and Ni arsenides and sulfarsenides, in a calcite matrix. The main ore minerals of this stage are allargentum and dyscrasite. The microprobe data for the diarsenides show extensive, and partly hitherto undocumented, solid solution in Fe-Co-Ni space. Seven distinct generations of calcite were distinguished. The delta C-13 (V-PDB) and delta O-18 (V-SMOW) values of these generations show a positively correlated trend that evolves from -13.0 to -4.0 parts per thousand and from 12.3 to 23.6%, respectively. Fluid-inclusion data of stage I show homogenization temperatures of 100-180 degrees C at salinities of 17-26 wt.% NaCl eq. Fluid inclusions in stage-II calcite display similar, but more restricted values, 110-150 degrees C and 25-28 wt.% NaCl eq., respectively. The stage-Ill fluid inclusions show similar temperatures of homogenization, but different salinities. Earlier calcite of this stage contains inclusions with salinities of 27-30 wt.% NaCl eq., whereas later calcite has lower salinities, 3-10 wt.% NaCl eq. The initial temperatures of ice melting of most fluid inclusions range between -45 and -60 degrees C and are typical of an H2O-NaCI-CaCl2 fluid. On the basis of all available geochemical data and phase-equilibrium constraints, we favor a model in which basement-derived near-neutral-pH hydrothermal fluids remobilized older products of mineralization. Mixing of these fluids with more alkaline formation-waters from the Mesozoic cover rocks resulted in the precipitation of the silver alloys in an enrichment zone at P-T conditions of 120-150 degrees C and approximately 200 bars. A significant shift in pH from near-neutral to alkaline can explain the abundant association of silver alloys with calcite gangue and the general absence of quartz in the enriched ore zone. This conceptual model can be applied to similar ore deposits worldwide, where rich silver ores are hosted by calcite-rich and quartz-poor assemblages of gangue minerals.