This paper describes a hydrothermal alteration sequence in the Soultz-sous-Forets granite (Alsace, France) and presents an interpretation of its origin, based upon petrographic observations, microthermometric measurements and mass-balance calculations. The alteration results from interactions between the granitic basement and sedimentary fluids which have circulated through the fracture network. The main stages of alteration observed in the studied zone are: total dissolution of primary quartz on both sides of a fault; complete transformation of oligoclase into tosudite in the presence of organic matter; precipitation of hairy illite, calcite and quartz in veinlets. The temperature during these events appears to have decreased from the tosudite stage (T illite+quartz can be inhibited in natural environments, depending on the nature of the clay. A major difference between this alteration sequence and diagenetic reactions is that it proceeds with decreasing temperature. This implies that the fundamental parameter controlling the alteration sequence observed here is the chemistry of the fluids.
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Ledésert et al. (1999) studied this question.