Chemical changes and genesis of secondary minerals during the alteration of biotites from ignimbrites in the Tazzeka Mountain (Morocco)

The Tazzeka Mountain, located approximately 20 kin south of Tate, eastern M orocco, is composed of a Westphalian volcano-sedimentary complex. It contai ns rhyolitic ignimbrites with the following minerals: quartz, potassium fel dspar, oligoclase-andesine, and biotite. The ignimbrites are extensively al tered 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 anal ytical methods. Biotites are trans formed into secondary clay minerals, mai nly 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 chl orites sensu stricto, in which the K2O content is 0.3%. Several processes i ncluding retrodiagenesis, hydrothermal activity, fumarolic activity, and ge ochemical weathering contributed to the transformation of the biotites at T azzeka.