Hydrothermal natrolite formation in the Fohberg phonolite, Kaiserstuhl Volcanic Complex, Germany Tobias Björn Weisenberger Department of Geosciences, University of Oulu, Oulu, Finland Simon Spürgin Hans G. Hauri Mineralstoffwerke, Bötzingen, Germany Abstract. The subvolcanic Fohberg phonolite (Kaiserstuhl volcanic complex, Germany) is an economic zeolite deposit, formed by hydrothermal alteration of primary magmatic minerals. The Fohberg phonolite is mined due to the high (>40 wt%) natrolite content, which account for remarkable zeolitic physicochemical properties of the ground rock. It is used in the concrete industry as well as in many other applications. Natrolite formed during hydrothermal alteration under alkaline conditions and completely replaces felspathoids in the matrix of the rock. In addition, veins and open fissures cut the intrusive body, wherein natrolite is precipitated, followed by apophyllite and calcite as younger generations. New mineralogical and geochemical studies are carried out a) to evaluate the manifestation of hydrothermal alteration, b) to constrain the physical and chemical conditions of the fluids, which promoted hydrothermal replacement, and c) to relate the hydrothermal overprint to the chronology and volcanic history of the Kaiserstuhl volcanic complex. Keywords. Natrolite – Zeolite - Apophyllite - Hydrothermal alteration - Kaiserstuhl volcanic complex 1 Introduction Zeolites are among the most common products of chemical interaction between groundwater and the Earth’s crust during diagenesis, hydrothermal mineral alteration, and low-grade metamorphism. Natural zeolites form by reactions of aqueous solutions with volcanic rocks, and volcanogenic sediments. They occur in low-temperature (<250°C), low-pressure (<200 MPa), water saturated or fluid rich environments. The copious amounts of silica, aluminium, and alkali and alkaline-earth cations necessary for the formation of most zeolites are commonly derived from dissolution of volcanic glass and from the alteration of feldspar and feldspathoids. Since the early days of its geological and mineralogical exploration, the Kaiserstuhl volcanic complex (KVC) is well known for its wealth in zeolites. A detailed description of zeolite distribution in relation to the host rock is given in Weisenberger and Spürgin (2009). Nevertheless, phonolite intrusions in the eastern KVC, namely the Fohberg phonolite, are the only local zeolite occurrences of economic interest. Therein natrolite is the predominant zeolite-group mineral. Today this rock is known for its excellent qualities in concrete and other applications, which are directly related to the high content of zeolites and their physicochemical properties, e.g. cation exchange (Hauri 2006). In this study we present new geochemical and mineralogical data and provide a genetic model for the hydrothermal replacement of primary magmatic phases by zeolite minerals. 2 Geological setting The KVC is located in the central-southern segment of the Upper Rhine Graben, south-western Germany (Fig. 1), where it is the only larger volcanic edifice. Alkaline and carbonatitic rocks erupted along deep- rooting faults in a disrupted crustal segment (Bourgeois et al. 2007) at the intersection of two prominent fault zones (Hüttner 1996; Schreiner 1996). The KVC covers an area of 16 x 12 km northwest of Freiburg. A north- south oriented pre-volcanic horst structure in the eastern Kaiserstuhl, mainly comprising Paleogene marls, sandstones, and limestones, is partly overlain by effusive and explosive volcanics, and penetrated by subvolcanic intrusions (Fig. 1). Extrusive rocks of the KVC erupted from various volcanic centres and formed a complex stratocone or a volcanic field. Figure 1. a Simplified geologic map of the Kaiserstuhl volcanic complex (after Wimmenauer 1963). The phonolite- hosted Fohberg zeolite deposit is shown with an arrow. b Outline of Germany Phonolitic rocks of the KVC are derived from a hypothetical, initially fractionated K-basanite. This fractionates to rocks of a so-called essexitic family (essexites, tephrites), and a phonolitic family, e.g. phonolites, syenites, and evolved leucocratic dykes (Wimmenauer 1957, 1959, 1962).