1 3 Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531-014-1046-1 ORIGINAL PAPER Hydrothermal alteration and zeolitization of the Fohberg phonolite, Kaiserstuhl Volcanic Complex, Germany Tobias Björn Weisenberger · Simon Spürgin · Yann Lahaye Received: 2 December 2013 / Accepted: 6 June 2014 © Springer-Verlag Berlin Heidelberg 2014 the precipitating fluid. Low radiogenic 87 Sr/ 86 Sr values indicate a local origin of the elements necessary for sec- ondary mineral formation from primary igneous phases. In addition, fractures cut the intrusive body, which contain zeolites, followed by calcite and a variety of other silicates, carbonates, and sulfates as younger generations. Stable isotope analysis of late-fracture calcite indicates very late circulation of meteoric fluids and mobilization of organic matter from surrounding sedimentary units. Keywords Zeolite · Götzenite · Pectolite · Sepiolite · Hydrothermal alteration · Kaiserstuhl Volcanic Complex Introduction Hydrothermally altered phonolitic rocks are of economic interest in the Kaiserstuhl Volcanic Complex (KVC), SW Germany (Fig. 1), due to the occurrence of zeolites, domi- nantly natrolite-group minerals and analcime. The pro- nounced cation exchange and molecular sieve capabilities of zeolites and zeolite-rich rocks make them an important economic target for mining (Pabalan and Bertetti 1999, 2001). These properties have widespread industrial appli- cation in water softening (Kallo 2001), catalysis (Weit- kamp 2000), remediation of soils and soil quality (Leggo and Ledesert 2001; Leggo et al. 2010; Mercurio et al. 2010, 2012), wastewater treatment (Kallo 2001; Leggo and Lede- sert 2001; Leggo et al. 2010), as additive in the cement industry (Kassautzki 1983; Colella et al. 2001; Hauri 2006), or as agent in solar energy heating and energy stor- age (Tchernev 2001). Zeolites are tectosilicates characterized by an open three-dimensional framework of (Si, Al)O 4 tetrahedra. The tetrahedra form a network of open channels containing Abstract The subvolcanic Fohberg phonolite (Kaiser- stuhl Volcanic Complex, Germany) is an economic zeo- lite deposit, formed by hydrothermal alteration of primary magmatic minerals. It is mined due to the high (>40 wt%) zeolite content, which accounts for the remarkable zeolitic physicochemical properties of the ground rock. New min- eralogical and geochemical studies are carried out (a) to evaluate the manifestation of hydrothermal alteration, and (b) to constrain the physical and chemical properties of the fluids, which promoted hydrothermal replacement. The alkaline intrusion is characterized by the primary miner- alogy: feldspathoid minerals, K-feldspar, aegirine–augite, wollastonite, and andradite. The rare-earth elements-phase götzenite is formed during the late-stage magmatic crys- tallization. Fluid-induced re-equilibration of feldspathoid minerals and wollastonite caused breakdown to a set of sec- ondary phases. Feldspathoid minerals are totally replaced by various zeolite species, calcite, and barite. Wollastonite breakdown results in the formation of various zeolites, cal- cite, pectolite, sepiolite, and quartz. Zeolites are formed during subsolidus hydrothermal alteration (<150 °C) under alkaline conditions. A sequence of Ca–Na-dominated zeolite species (gonnardite, thomsonite, mesolite) is fol- lowed by natrolite. The sequence reflects an increase in log[(a Na + )/(a H + )] and decrease in log[(a Ca 2+ )/(a 2 H + )] of T. B. Weisenberger (*) Department of Geosciences, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland e-mail: tobias.weisenberger@oulu.fi S. Spürgin Hans G. Hauri Mineralstoffwerke, Bötzingen, Germany Y. Lahaye Geological Survey of Finland, Espoo, Finland