Pyrophyllite - Zunyite - Diaspore Mineralization at CHOUICHIAT, ANTI-ATLAS, MOROCCO
Résumé
The Chouichiat peraluminous structure, located in the northern domain of the Bou Azzer – El Graara (Central Anti-Atlas, Morocco), outcrops as a discontinuous whitish ridge 10 to 60 m wide extending E–W for more than 2 km. This structure coincides with a large reverse-sense steeply dipping shear corridor, marked by penetrative schistosities and folds. The Chouichiat structure displays a strong zoning due to the combined effects of metamorphism, hydrothermal alteration, and deformation of a rhyodacite protolith. It has 1) a central quartz–pyrophyllite zone, surrounded on both sides by 2) a pyrophyllite–quartz -rich proximal zone with small amounts of diaspore, kaolinite and zunyite, and spots of hematite, and 3) a distal pyrophyllite zone with quartz–pyrophyllite in smaller amounts, common hematite and diaspore. The zoning expresses the gradual transformation of rhyodacite to pyrophyllitic rocks, enhanced by the development of the shear zone, which has strongly increased the permeability owing to fluid–rock reaction. Such an interaction is represented by a set of reactions, the most important of which is the hydrolysis of white mica in the rhyodacite to produce pyrophyllite: 2KAl2AlSi3O10(OH)2 + 6 SiO2 + 2H+ ! 3Al2Si4O10 (OH)2 + 2K+. A decrease in temperature and an increase in silica activity led to replacement of early diaspore by pyrophyllite. Zunyite crystallized later, mostly at the interface between proximal and distal zones, together with spectacular rosettes of fibroradial hematite. The main kaolinite – pyrophyllite – quartz assemblage likely was generated around 273° ± 10°C and 1 kbar. Mass-balance calculations indicate a significant leaching of mobile elements, promoting the residual concentration of Al, and a substantial addition of silica, both due to hydrolysis of aluminum silicates. The pyrophyllite – diaspore – zunyite assemblage is characteristic of hydrothermally altered aluminous felsic volcanic rocks and very similar to rocks altered by acidic fluid associated with gold-bearing epithermal ore deposits.