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< or =400 degrees C) to the illite-calcite-quartz stage (140 degrees C). Assuming tosudite is a geochemical analog of kaolinite, the nature and chronology of the alteration sequence show similarities with the diagenesis of some kaolinitic sandstones: clay enrichment in plagioclase by acidic organic fluids, metastable coexistence of aluminous clays (tosudite here instead of kaolinite in sandstones) with K-feldspar and quartz, and post-illitization of the clay fraction promoted by the circulation of brines. The preservation of orthoclase during these alteration stages reinforces the assumption that the reaction K-feldspar+Al-clays-->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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