A Model for Fluids in Metamorphosed Ultramafic Rocks: IV. Metasomatic Veins in Metaharzburgites of Cima di Gagnone, Valle Verzasca, Switzerland

  • Hans-Rudolf Pfeifer
Part of the NATO ASI Series book series (ASIC, volume 218)


Most of the over one-hundred known metaperidotite lenses of the high grade metamorphic part of the Central Alps show metasomatic features like concentric zoning and composite veining. These indicate that an important hydrothermal event took place during an early stage of the uplift which followed the pressure peak of the regional metamorphism, at approximately 580–650 deg C and 4–6 kbar. Some of the most conspicuous veins occur in the Cima di Gagnone region in the North-Eastern Verzasca Valley in Switzerland. They are usually no longer than 2 to 5 meters and vary in width from 1 to 50 centimeters. Geometrically two types can be distinguished: The first type starts in the mafic marginal (“black wall”) zone of a lens or in mafic layers cross-cutting a lens. The second one has a tensioncrack shape and begins and ends anywhere in the ultramafic host rock. The veins are usually symmetrically zoned, composed of a central zone with several replacement zones on both sides. Based on their dominant mineral phases, 4 vein types can be distinguished: (A) Mg-amphibole dominated, (B) tremolite- chlorite dominated, (C) chlorite- talc dominated, and (D) talc- carbonate dominated veins. The veins are interpreted to represent the second stage of a hydrothermal metamorphism during which rapid deformation led to brittle behaviour of the ultramafic host rock. Fluid compositions deduced from mineral data with the aid of activity- activity-X( CO2) — diagrams indicate locally steep gradients in CO2, SiO2, iron, calcium and aluminium. A tentative interpretation of the changing mineral compositions as a function of the distance from the vein center indicates a diffusion-dominated mass transfer mechanism. In contrast to more acid rocks, aluminium seems to be easily transported in the high pH- and low fO2-conditions of the fluids present in these ultramafic rocks.


Wall Rock Ultramafic Rock Vein Center Activity Diagram Invariant Point 
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© D. Reidel Publishing Company 1987

Authors and Affiliations

  • Hans-Rudolf Pfeifer
    • 1
  1. 1.Section des Sciences de la Terre, Centre d’Analyse MinéraleUniversité de LausanneLausanneSwitzerland

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