Origin and growth of alpine fissure minerals in the western Swiss Alps: record of fluid flow during exhumation
Origin and growth of alpine fissure minerals in the western Swiss Alps: record of fluid flow during exhumation
Thesis defended by Eric May, 16 December 2016, Institut des dynamiques de la surface terrestre (IDYST), Lausanne, Switzerland.
This thesis focused on the study of late tectonic veins in the rocks of the western Swiss Alps. These veins, also known as 'alpine fissures', 'ovens' or 'pockets', contain perfectly shaped crystals that have been the subject of scientific research and the envy of crystal makers for centuries.
They form at depths of around 13 kilometres when the rocks that form the Alps are exhumed and undergo brittle deformation. This exhumation process began in the Miocene (around 23 million years ago) and continued until around 5 million years ago. The formation of these veins results from the coincidence of rock deformation and the presence of fluids in the rocks. The fluids contain, in solution, the ingredients necessary for the formation of the crystals that are deposited in the fractures produced by the deformation of the rocks. The orientations of the veins indicate contrasting behaviour between the different tectonic units studied, and their abundance often depends on the presence of shear zones accommodating shortening.
Some compartments have extended vertically (Mont Blanc and Aiguilles Rouges massifs) or perpendicularly to the Alpine arc (Salvan-Dorénaz and Sion-Courmayeur synclines). The tectonic stresses that formed these Alpine fissures affected the basement and cover rocks in different ways. A study of the textures of the veins and mineralogical assemblages reveals several opening phases. Early veins formed from metamorphic fluids in equilibrium with the surrounding rocks indicate a low-velocity opening when the quartz is highly supersaturated in the mineralising fluids. The result is a massive appearance.
Gradually, as the rocks are exhumed and therefore cool, the growth of quartz slows down, revealing perfectly shaped crystals. The appearance of chlorite generally corresponds to the moment when the quartz is depleted in the fluid and stops growing. Calcite generally appears at the end of mineralisation, as it covers the minerals already present in the vein.
In the second part, analysis of the stable isotope compositions (hydrogen, carbon and oxygen) of the total rocks and the various minerals forming the mineralogical assemblages was used to determine the apparent formation temperatures and the original isotopic compositions of the mineralising fluids. Apparent formation temperatures were calculated between 395°C and 100°C using several pairs of minerals whose texture indicates contemporary crystallisation. This cooling corresponds to hydrothermal activity of around 6 million years. Only the fluid compositions deduced from the calcite compositions clearly indicate the presence of a fluid percolating from the surface into the veins.
The third part discusses the record of the evolution of the system using stable isotope and trace element compositions in quartz crystals during their growth. During most of its growth, the crystal records a single fluid composition, but at the end of its growth, i.e. in colder conditions, fluids reach the vein from several origins as indicated by large variations in compositions over a short period. Following an initial generation of quartz, a new fluid with an isotopic composition that is not in equilibrium with the rock directly in contact may reach the vein.
Author
- Dr Eric May, GGTL Laboratories Switzerland. ResearchGate, Linkedin.
May E. (2016) Origine et croissance des minéraux des fissures alpines dans les Alpes de Suisse occidentale. Doctoral dissertation, Institut des Dynamiques de la Surface Terrestre (IDYST), Lausanne Suisse.