Original paper

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

Ledésert, Beatrice; Berger, Gilles; Meunier, Alain; Genter, Albert; Bouchet, Alain

European Journal of Mineralogy Volume 11 Number 4 (1999), p. 731 - 742

34 references

published: Jul 16, 1999
manuscript accepted: Mar 17, 1999
manuscript received: Oct 19, 1998

DOI: 10.1127/ejm/11/4/0731

BibTeX file

ArtNo. ESP147051104012, Price: 29.00 €

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Abstract

Abstract 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 ‹ 400°C) to the illite-calcite-quartz stage (140°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.

Keywords

Soultz-sous-Foretsgranitehydrothermal alterationtosuditesedimentary brinesfluid inclusions. composition that occur either at the metric