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

A mechanical decrepitation device coupled with a gas chromatograph has been used to characterize the molecular composition of gaseous and liquid hydro carbons contained in minerals. Application of this technique allows the ide ntification of low-molecular-weight n-alkanes and some aromatic hydrocarbon s in sulfides and gangue minerals from epigenetic Variscan and post-Varisca n lead-zinc deposits in the Rhenish Massif, Germany. Based on the analysis of 200 samples, Variscan and post-Variscan mineralization can be distinguis hed by the composition of associated hydrocarbons. Variscan sulfides and ga ngue minerals contain high abundancies of methane. In contrast, n-alkanes i n the C-2-C-9 range and aromatic hydrocarbons (benzene, toluene) are domina nt in post-Variscan mineralization. The absence of high-molecular-weight hy drocarbons in ore minerals suggests highly mature gas associated with hydro thermal activity, during which hydrothermal fluids caused an increase in th ermal maturation of organic matter and the generation of low-molecular-weig ht hydrocarbons in the adjacent organic-rich rocks. The hydrocarbon composi tions 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 degrees C) generated predominan tly methane, whereas basement brines (T < 175 degrees C) expelled higher-mo lecular-weight hydrocarbons (wet gases, condensates, aromatic hydrocarbons) from adjacent rocks during the Mesozoic event. The specific role of hydroc arbons in :sulfide precipitation via thermochemical sulfate reduction is in dicated by geochemical characteristics of organic matter associated with th e Plombii res Pb-Zn deposit, in eastern Belgium. Intense alteration phenome na 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% R-o), a syste matic depletion in C-12 towards the vein-type mineralization, high atomic S /C ratios (0.49), and by low atomic H/C ratios (0.29). The data suggest tha t hydrothermal solutions caused a drastic increase in the thermal maturatio n of organic matter within the adjacent wall rock. Increased thermal matura tion resulted in increased delta(13)C-values of organic carbon due to the p referential release of C-12. Th, change in the organic matter to a H-deplet ed and S-enriched bulk composition in association with sulfide ores strongl y suggests that thermochemical sulfate reduction was responsible for organi c degradation. Thus, thermochemical sulfate reduction probably triggered ba se metal sulfide precipitation in Variscan and post-Variscan ore deposits o f the Rhenish Massif. Finally, based on data from this study and previous i nvestigations, new genetic models are presented for both Variscan and post- Variscan mineralization in the Rhenish Massif.