Identification of weathering and hydrothermal alteration in acidic rocks : petrography and mineralogy of clay minerals

A major problem in ore prospecting on granitic terrains is to make a distinction between clay mineral assemblages due to weathering and those due to hydrothermal alteration. The most outstanding features of these two types of transformation are : ; 1. Potassic beidellite is typical of hydrothermal alteration. ; 2. Low iron content mineral assemblages in the presence of large amounts of kao-linite is typical of hydrothermal alteration. ; 3. Argillan veins and pore coatings are typical of weathered rocks. ; The control of microprobe analyses by X-ray diffraction on small quantities of matter is very useful for the identification of clay assemblages.