Hydrothermal alteration and zeolitization of the Fohberg phonolite, Kaiserstuhl Volcanic Complex, Germany

Research Collaboratores:
Dipl. Geol. Simon Spürgin, Hauri, Mineralstoffwerk, Bötzingen, Germany
Dr. Yann Lahaye, Geological Survey of Finland

The subvolcanic Fohberg phonolite (Kaiserstuhl Volcanic Complex, Germany) is an economic zeolite 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 mineralogical 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 mineralogy: feldspathoid minerals, K-feldspar, aegirine–augite, wollastonite, and andradite. The rare-earth elements-phase götzenite is formed during the late-stage magmatic crystallization. Fluid-induced re-equilibration of feldspathoid minerals and wollastonite caused breakdown to a set of secondary phases. Feldspathoid minerals are totally replaced by various zeolite species, calcite, and barite. Wollastonite breakdown results in the formation of various zeolites, calcite, 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 followed by natrolite. The sequence reflects an increase in log[(aNa+)/(aH+)] and decrease in log[(aCa2+)/(a2H+)] of the precipitating fluid. Low radiogenic 87Sr/86Sr values indicate a local origin of the elements necessary for secondary 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.

Reference:
Weisenberger T.B., Spürgin S & Lahaye Y. (2014) Hydrothermal alteration of the Fohberg phonolite, Kaiserstuhl Volcanic Complex, Germany.  International Journal of Earth Sciences. 103 (8). 2273-2300. doi: 10.1007/s00531-014-1046-1 (pdf)

Zeolites in alkaline rocks of the Kaiserstuhl volcanic complex, SW Germany - new micropobe investigation and their relationship to the host rock

Low-grade zeolite facies mineralisation in the Kaiserstuhl volcanic complex results from alteration of alkaline volcanic rocks. Nine different zeolite species are known from the volcanic complex: analcime, chabazite-Ca, faujasite-Na, faujasite-Mg, natrolite, offretite, phillipsite-K, phillipsite-Ca and thomsonite. Their occurrence as well as their chemical composition depends on the chemical composition of the host rock, local hydrological features and porosity of the rock. New chemical analyses are presented, in order to give chemical compositions of Kaiserstuhl zeolite species, which were never analysed before, e.g. thomsonite from the Strümpfekopf-Totenkopf area, as well as a re-evaluation of zeolite species from classic localities. New microprobe analyses of barrel-shaped offretite from the Limberg area clearly show that the postulated epitaxial intergrowth of offretite and erionite does not occur. Textural and chemical observations require that the elements for zeolite formation derived from the hydration and palagonitisation of basaltic glass, and alteration of primary phases (e.g. sodalite, leucite). The zeolite species depends on the primary mineralogy and chemical composition of the host rock: Analcime occurs as pseudomorphs after leucite, which is a rock forming mineral in most lavas and tuffs of tephritic composition, and natrolite is derived from hydrothermal alteration of sodalite-group minerals in phonolitic intrusions. Reference: Weisenberger T. & Spürgin S. (2009) Zeolites in alkaline rocks of the Kaiserstuhl volcanic complex, SW Germany - new micropobe investigation and their relationship to the host rock. Geolgica Belgica 12/1-2, 75-91 (pdf)

Fig. 1: Scanning electron microscopy images showing the morphology of zeolite species from the Limberg quarry I. A Faujasite  B Barrel-shaped offretite Fig. 2: Backscattered electron and element distribution images of offretite in a limburgite vesicle, collected in the Limberg quarry I.

Research Collaboratores:
Dipl. Geol. Simon Spürgin, Hauri Mineralstoffwerk, Bötzingen, Germany
Joachim Hörth, Bühl, Germany

A new leucitophyre dike (phonolitic leucitite), which are very rare in the Kaiserstuhl, was found at Strümpfekopf near the village of Bickensohl. Beside this locality, only four leucitophyre descriptions are confirmed during the last 200 years of research; one of them the well-known dike from Eichberg (Oberrotweil). The dike at Strümpfekopf is hosted by a 16,2 million years old, medium-grained essexite and therefore belongs to the younger  products of volcanism in the Kaiserstuhl. The Mg-value (0.2) indicates considerable fractionation. The dike contains 42.4 vol-% phenocrysts in very fine-grained to vitreous matrix, essentially leucite >1.4 mm and a second leucite population of 0.2-0.6 mm. Additional phenoctyst phases are sanidine, aegirine-augite, melanite, apatite and accessories (magnetite, biotite, calcite). Leucite is completely decomposed to analcime, sanidine is also heavily altered. Different observations (e.g. shape and inclusions in analcime, mineral chemistry, cracking, rock alteration) confirm the secondary origin of analcime pseudomorphs after primay magmatic leucite. Reference: Spürgin S., Weisenberger T. & Hörth J. (2008) Das Leucitophyrvorkommen vom Strümpfekopf im Kaiserstuhl – eine historische und mineralogische Betrachtung. Berichte der Naturforschenden Gesellschaft zu Freiburg i. Br. 98, 221-244

Fig. 1: Leucitophyr dike Fig. 2: Thin section potograph under plane polarized light. Euhedral analcime and augite crystals. Image width 3.7 mm