Mineralogia dos greisens da área Grota Rica, Plúton Água Boa, Pitinga, Amazonas

The topaz-alkali-feldspar-granite, the most evolved facies of the Água Boa pluton, was affected by hydrothermal alteration, represented by greisens and quartz veins, the main host for Sn- and subordinated Zn mineralization. The greisens are classified as quartz-topaz-siderophyllite-greisen, topaz-siderophyllite-greisen and quartz-topaz-quartz-greisen. They are composed essentially of quartz, topaz and siderophyllite, accompanied by variable amounts of fluorite, zinnwaldite, sphalerite, cassiterite, zircon and anatase and locally Ce-monazite, galena, pyrite, chalcopyrite and native bismuth. EMPA studies allowed identifying three types of micas: (1) brown siderophyllite from topaz-granite; (2) the green siderophyllite of greisens and (3) zinnwaldite, weakly colored, found as thin and discontinuous rims around green siderophyllite, and quartz vein. The siderophyllite composition of the granite towards greisens shows an evolution of siderophyllite to Li-siderophyllite with increase of VI Al, Li and Si contents. On the other hand, the siderophyllite of the greisen was partially replaced by zinnwaldite, with increase of VI Al, Li and Si contents. The cassiterite in the greisens forms euhedral to subeuhedral, twin and zoned crystals, with strong pleochroism. It occurs as aggregates together with topaz, quart and fluorite. The pure composition and the low content of the Nb and Ta of cassiterite indicate hydrothermal conditions.