Diagenetic-type reactions related to hydrothermal alteration in the Soultz-sous-Forets granite, France

This paper describes a hydrothermal alteration sequence in the Soultz-sous- Forets granite (Alsace, France) and presents an interpretation of its origi n, based upon petrographic observations, microthermometric measurements and mass-balance calculations. The alteration results from interactions betwee n the granitic basement and sedimentary fluids which have circulated throug h the fracture network. The main stages of alteration observed in the studi ed zone are: total dissolution of primary quartz on both sides of a fault; complete transformation of oligoclase into tosudite in the presence of orga nic matter; precipitation of hairy illite, calcite and quartz in veinlets. The temperature during these events appears to have decreased from the tosu dite stage (T less than or equal to 400 degrees C) to the illite-calcite-qu artz stage (140 degrees C). Assuming tosudite is a geochemical analog of ka olinite, the nature and chronology of the alteration sequence show similari ties with the diagenesis of some kaolinitic sandstones: clay enrichment in plagioclase by acidic organic fluids, metastable coexistence of aluminous c lays (tosudite here instead of kaolinite in sandstones) with K-feldspar and quartz, and post-illitization of the clay fraction promoted by the circula tion of brines. The preservation of orthoclase during these alteration stag es reinforces the assumption that the reaction K-feldspar + Al-clays --> il lite +. quartz can be inhibited in natural environments, depending on the n ature 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.