Chemical Changes and Genesis of Secondary Minerals during the Alteration of Biotites from Ignimbrites in the Tazzeka Mountain (Morocco)

Jean Dejou1, Christian De Kimpe2, Jean-Jacques Macaire3 and Alain Perruchot4
1 1, rue des Raux, 15250 Jussac, France
2 Agriculture and Agri-Food Canada, Research Branch, Sir John Carling Building, 930 Carling Avenue, Ottawa, Ontario, Canada K1A 0C5
3 EA 2100—Laboratoire de Géologie des Environnements Aquatiques Continentaux, Université François Rabelais, Parc de Grandmont, 37200 Tours, France
4 EP CNRS 1748, Université de Paris-Sud, Bâtiment 504, 91045 Orsay Cedex, France

Abstract: The Tazzeka Mountain, located approximately 20 km south of Taza, eastern Morocco, is composed of a Westphalian volcano-sedimentary complex. It contains rhyolitic ignimbrites with the following minerals: quartz, potassium feldspar, oligoclase-andesine, and biotite. The ignimbrites are extensively altered because of a dense network of fractures in the massif. Alteration has resulted in the formation of spheroidal rocks and saprolite, the thickness of which depends on local topography. The evolution of the biotites in the ignimbrites was investigated by microprobe analysis of the mica crystals. This technique provides data that are not accessible through classical analytical methods. Biotites are transformed into secondary clay minerals, mainly chlorites and illites; intermediate stages are related to the degree of alteration of biotite, the latter being expressed by the K2O content which decreases progressively from 7.3 to 1.3%. Next come protochlorites and chlorites sensu stricto, in which the K2O content is 0.3%. Several processes including retrodiagenesis, hydrothermal activity, fumarolic activity, and geochemical weathering contributed to the transformation of the biotites at Tazzeka.

Key Words: Alteration • Biotite • Chlorite • Ignimbrite • Illite • Structural Formula

Clays and Clay Minerals; April 1999 v. 47; no. 2; p. 144-155; DOI: 10.1346/CCMN.1999.0470204
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)