Figure F2. Based on differences in mineralogical and geochemical composition, several different types of alteration have been recognized. In these diagrams, the variable normative abundances of the most important minerals are illustrated, excluding SiO2 polymorphs. The averages for pyrite and anhydrite are shown separately for each group of samples. Unaltered and weakly altered samples are rich in normative plagioclase and contain only traces of pyrite and anhydrite. Hydrothermal minerals such as chlorite, smectite, illite, and K-feldspar are abundant in chlorite-pyrite and K-feldspar-illite alterations. Anhydrite-pyrite-pyrophyllite alteration and pyrite-anhydrite alteration represent the most strongly altered samples, which contain no more primary plagioclase but abundant pyrite, anhydrite, illite, pyrophyllite, paragonite, and chlorite + smectite. There is considerable compositional variability within every group.