Disseminated and faultbound autohydrothermal ferroan saponite in Late Paleozoic andesites of the Saar-Nahe Basin, SW Germany: Implications for the economic geology of intermediate (sub)volcanic rocks

Disseminated and faultbound argillic alteration is widespread in (sub)volcanic domes and dykes of the Late Variscan Saar-Nahe Basin, SW Germany. It is accompanied by small but now exhausted Cu, Hg and Ag deposits. Upper Carboniferous to early Permian basaltic andesites through trachyandesites host ferroan saponite, a product of autohydrothermal alteration . This alteration affected mafic minerals, e.g., pyroxene, amphibole, and biotite as well as feldspar and took place at a shallow depth underneath the zone of oxidation at temperatures below 100 °C–150 °C in the pH range from pH 7 to pH 5.5. Chlorite preceded saponitization under more alkaline conditions, while quartz and kaolinite post-dated it under more acidic and reducing conditions. Oxidizing conditions provoked hematite (plus jarosite) to develop. Saponite forms part of a slow-cooling argillic alteration system with a low potential to develop Cu–Au–Ag deposits of its own. Rapid cooling under near-ambient conditions under oxidizing and more acidic conditions prevented the wide range of Mg–Fe-bearing clay minerals from forming and might have ended up in the formation of kaolinite plus nontronite, APS and ore minerals. Ferroan saponite has some relevance in terms of economic geology as far as the industrial minerals and rocks are concerned. Autohydrothermal ferroan saponite in andesitic rocks is related in time and space to the creation of amygdules which are filled with amethyst and zeolites . Saponite is thus decisive for the rock strength of these magmatic rocks, which are quarried for aggregates and/or dimension stones. While a useful tool to assess the quantity of saponite in these rocks, measuring the CEC does not provide a full picture of the rock strength. The latter can only be determined when the structural types of saponitization are considered in context with the cations present in the lattice of saponite. Rocks containing isolated patches of saponite are less vulnerable than magmatic rocks with stockwork-like saponite. In the latter water may get in touch with saponite and induce a swelling which, in turn, creates a negative effect on the rock strength.