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

摘要

A mechanical decrepitation device coupled with a gas chromatograph has been used to characterize the molecular composition of gaseous and liquid hy-drocarbons 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 dis-tinguished by the composition of associated hydrocar-bons. Variscan sulfides and gangue minerals contain high abundancies of methane. In contrast, u-alkanes in the C_2-C_9 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 hy-drocarbons in the adjacent organic-rich rocks. The hy-drocarbon compositions contained in fluid inclusions of Variscan and post-Variscan minerals are probably gov-erned by the maturation level of the potential source rocks. In Variscan time tectonic brines (T > 175 deg C) generated predominantly methane, whereas basement brines (T < 175 deg C) expelled higher-molecular-weight hydrocarbons (wet gases, condensates, aromatic hydro-carbons) 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 percent R_o), a systematic depletion in ~(12)C 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 matu-ration resulted in increased #delta#~(13)C-values of organic car-bon due to the preferential release of ~(12)C. 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 re-sponsible for organic degradation. Thus, thermochemi-cal sulfate reduction probably triggered base metal sul-fide 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-Vari-scan mineralization in the Rhenish Massif.