Swiss Journal of Geosciences

, Volume 111, Issue 1–2, pp 317–340 | Cite as

Bristen granite: a highly differentiated, fluorite-bearing A-type granite from the Aar massif, Central Alps, Switzerland

  • Kurt Bucher
  • Ulrike Seelig


Bristen granite is a body of fine-grained leucogranite occurring in the Gotthard rail base tunnel in the Central Alps. During construction of the tunnel, Bristen granite (Brgr) has been drilled along a 600 m long section. The aplite-granite belongs to the suite of Variscan granitoid intrusions of the Aar massif and contains a variety of accessory minerals typical of highly differentiated granites. Rock forming fluorite, partly enriched in yttrium (Y) and rare earth elements (REE), is intergrown with the late Y- and REE-bearing carbonate mineral synchysite. The granite contains a variety of Ti- and Y-REE-niobates, thorite, and zircon. Compared with the calc-alkaline central Aar granite (cAgr), Bristen granite is strongly depleted in Ti, P, Mg, Sr, and Ba and shows a remarkable enrichment in incompatible elements such as Rb, Th, U, Nb, Y, HREE and F. Bristen granite is the most evolved granitoid rock of the Aar massif. The composition of Brgr is typical of post-collisional reduced (ferroan) A-type granites. The Brgr melt formed in the lower crust and crystallized from a highly differentiated melt at the cotectic point in the quartz-feldspar system close to 100 MPa and 700 °C. The Brgr intruded as a small isolated stock pre-Variscan gneisses with sharply discordant contacts. The primary igneous host of Nb, Ta, Y, U, Th and REE is biotite in addition to minor amounts of allanite, and zircon. The presence of Y-REE-fluorite, synchysite, parisite and Y- and Ti-niobates and other REE-minerals can be related to reaction of igneous biotite and primary fluorite with hydrothermal fluids. The reaction is associated with alpine metamorphism, because Y-bearing fluorite and synchysite have been reported from Alpine fissures. The transformation of primary biotite to chlorite and muscovite released the heavy metal oxides under lower greenschist facies conditions that formed the Alpine diagnostic mineral stilpnomelane at about 300 °C.


Bristen granite Aar massif Fluorite Synchysite REE niobates 



We would like to thank AlpTransit Gotthard AG for providing access to the tunnel. The generous support by the tunnel geologists Beat Frei and Thomas Breitenmoser providing rock and water samples is gratefully acknowledged. We are grateful to Peter Amacher for contributing data on the Alpine fissure minerals from the tunnel to the present study. Special thanks go to all technicians of the Mineralogy and Geochemistry laboratories of the University of Freiburg. Peter Hayoz from Swisstopo, Wabern, supported this study by providing fresh core samples of the granites. We gratefully acknowledge the constructive reviews by Christian Gisler and an anonymous reviewer and the efficient editorial handling by Edwin Gnos.


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© Swiss Geological Society 2018

Authors and Affiliations

  1. 1.Institute of Mineralogy and PetrologyUniversity of FreiburgFreiburgGermany
  2. 2.Projektträger Jülich (ptj)Forschungszentrum Jülich GmbHBerlinGermany

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