Fluid–rock interaction processes related to hydrothermal vein-type mineralization in the Siegerland district, Germany: implications from inorganic and organic alteration patterns

Altered wallrocks of vein-type Pb–Zn–Sb mineralization, Siegerland district, Rheinisches Schiefergebirge, have been investigated by a combination of inorganic and organic geochemical methods, including major and trace element analysis, vitrinite reflectance measurements, C isotope and elemental analysis of kerogen. Alteration features of the siliciclastic pelitic-psammitic Lower Devonian wallrocks are increased K/Na ratios, significant desilicification and relative immobility of a number of elements, notably Al, Ti, Zr, Cr, V. Wallrock kerogens display elevated vitrinite reflectance values, decrease in H/C atomic ratios coupled with increase in S/C atomic ratios and heavier C isotope compositions, compared to the unaltered precursor sedimentary rocks. Interaction processes between the hydrothermal fluids and the respective wallrocks, related to injection of high-temperature silica-undersaturated solutions, are dominated by quartz dissolution coupled with sericitization reactions. Heat transfer due to fluid infiltration/convection and wallrock reactions caused fluid cooling, which promoted the sequential deposition of quartz and stibnite/sulphosalts within the vein systems. Hydrocarbons, detected in ore assemblages of Pb–Zn and Sb mineralization, were most probably derived from the Lower Devonian very low-grade (meta)sedimentary rocks. High maturity levels and pronounced, typical organic alteration patterns indicate that thermochemical SO2−4 reduction (TSR) played an important role in precipitation of metal sulphides. The present study demonstrates that a combination of inorganic and organic investigations on fluid–rock interaction processes is particularly useful for deciphering precipitation mechanisms of base metal sulphides.