Variscan and post-Variscan lead-zinc mineralization, Rhenish Massif, Germany: evidence for sulfide precipitation via thermochemical sulfate reduction

Abstract

A mechanical decrepitation device coupled with a gas chromatograph has been used to characterize the molecular composition of gaseous and liquid hydrocarbons contained in minerals. Application of this technique allows the identification of low-molecular-weight n-alkanes and some aromatic hydrocarbons in sulfides and gangue minerals from epigenetic Variscan and post-Variscan lead–zinc deposits in the Rhenish Massif, Germany. Based on the analysis of 200 samples, Variscan and post-Variscan mineralization can be distinguished by the composition of associated hydrocarbons. Variscan sulfides and gangue minerals contain high abundancies of methane. In contrast, n-alkanes in the C2–C9 range and aromatic hydrocarbons (benzene, toluene) are dominant in post-Variscan mineralization. The absence of high-molecular-weight hydrocarbons in ore minerals suggests highly mature gas associated with hydrothermal activity, during which hydrothermal fluids caused an increase in thermal maturation of organic matter and the generation of low-molecular-weight hydrocarbons in the adjacent organic-rich rocks. The hydrocarbon compositions contained in fluid inclusions of Variscan and post-Variscan minerals are probably governed by the maturation level of the potential source rocks. In Variscan time tectonic brines (T > 175 °C) generated predominantly methane, whereas basement brines (T < 175 °C) expelled higher-molecular-weight hydrocarbons (wet gases, condensates, aromatic hydrocarbons) from adjacent rocks during the Mesozoic event. The specific role of hydrocarbons in sulfide precipitation via thermochemical sulfate reduction is indicated by geochemical characteristics of organic matter associated with the Plombieres Pb–Zn deposit, in eastern Belgium. Intense alteration phenomena were observed in near-ore kerogens, compared with unaltered kerogens far from the ore body, as well as by a very high maturity (5.40% Ro), a systematic depletion in 12C towards the vein-type mineralization, high atomic S/C ratios (0.49), and by low atomic H/C ratios (0.29). The data suggest that hydrothermal solutions caused a drastic increase in the thermal maturation of organic matter within the adjacent wall rock. Increased thermal maturation resulted in increased δ13C-values of organic carbon due to the preferential release of 12C. The change in the organic matter to a H-depleted and S-enriched bulk composition in association with sulfide ores strongly suggests that thermochemical sulfate reduction was responsible for organic degradation. Thus, thermochemical sulfate reduction probably triggered base metal sulfide precipitation in Variscan and post-Variscan ore deposits of the Rhenish Massif. Finally, based on data from this study and previous investigations, new genetic models are presented for both Variscan and post-Variscan mineralization in the Rhenish Massif.