CHEMICAL COMPOSITION OF PHLOGOPITE, TOURMALINE AND ILLITE FROM HYDROTHERMAL ALTERATIONS OF THE NISTRU DEPOSIT, BAIA MARE, ROMANIA

The chemical composition of phlogopite, tourmaline, and illite from altered rocks of the Nistru Cu-Au-Bi deposit has been examined. Phlogopite of potassic alteration is enriched in fluorine and is similar in chemical composition to phlogopite from potassic alteration at porphyry copper deposits. Tourmaline occurs as three generations. The first-generation tourmaline is classified as dravite or oxydravite; second generation is dravite or schorl; and third generation is classified as dravite. Tourmaline II is enriched in ferric iron. The chemical substitutions in tourmaline I are Al + O2- -> Mg2+ + OH- and Fe -> Mg; in tourmaline II, the substitution is Fe -> Mg; and in tourmaline III, assumed substitution is Ca + Mg + Fe2+ -> 2Al + X-site vacancy. Tourmaline I could crystallize with potassium feldspar in potassic alteration; tourmaline II enriched in ferric iron and tourmaline III are found from propylitic alteration that is not related to porphyry system. Illite corresponds in composition to that of epithermal volcanogenic Au-Ag deposits. Thus, the chemical composition of examined minerals indicates three different hydrothermal stages at the Nistru deposit: (1) early porphyry not mentioned so far, (2) later propylitic accompanying minor ore mineralization and not related to porphyry system and (3) epithermal volcanogenic accompanying the base and precious metal mineralization.