Hydrothermal. Alteration of a Variscian Granite, Magmatic Autometasomatism and Fault Related Vein Metasomatism
Two types of hydrothermal alteration can be recognized in a biotite granite of Variscan age in Northern Switzerland. The first is related to the intrusion of the granite and lead to the replacement of K-spar and biotite by muscovite, of plagioclase by sericite and calcite and to a chloritization of biotite. This process affected the granite as a whole. Mass transfer calculations using analysed mineral compositions indicated locally mass balanced reactions without transport beyond neighbouring grains. Using the H2O and CO2 (from C?) already present in the granitic melt, this alteration process is an autometasomatism during cooling leading to sericitization/chloritization taking place at 350°C by a fluid with 6.5 weight aeq. NaCl. The second process, the main hydrothermal alteration is connected with young Palaeozoic faulting and locally anomalous heat flow. This process has lead to a vein parallel zonation shown by clay minerals. During this stage of alteration process the granite was depleted in the major elements Si and Na and in the trace elements Sr, Ba and U. This depletion lead to an increased porosity. Fluids evolved from 7 weight% NaCl aeq. at T 350°C down to 0,1 weight% NaCl at T 90°C. The composition of this fluid was calculated, assuming equilibrium with the fissure minerals quartz, albite, K-spar and illite. Because of the low water/rock ratio indicated by stable isotope data, a model of repeated water recycling between the fissure and the rock is proposed.
KeywordsCountry Rock Hydrothermal Alteration Fluid Composition Biotite Granite White Mica
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