Química mineral de micas do maciço granítico Serra Branca, Goiás: processos de alteração hidrotermal como modificadores do reequilíbrio cristaloquímico

Abstract

In the Serra Branca Granite (SBG) at Goias State, Brazil, four petrographic facies are described according to the intensity of postmagmatic process forming greisen and cassiterite mineralization: a) pink to grey porphyroid granite (g1a); b) grey to pinkish, medium to coarse-grained inequigranular granite (g1b); c) grey, medium to coarsegrained inequigranular granite (g1c) and d) grey medium-grained inequigranular granite (g1d). Apical greisens and micaceous veins occur. The mineralogy includes quartz, biotite, white mica, perthitic microcline and albite. The acessory are zircon, apatite, allanite, monazite and opaques (ilmenite). Albitization, greisenization and microclinization are the postmagmatic phenomena. The greisens are composed of quartz and white mica; topaz, fluorite, cassiterite, beryl, monazite, scheelite, graphite, sphalerite, zoisite, zircon, apatite, phenakite and opaques. The biotite is the only ferromagnesian phase and is classified in the domain of biotite associated with muscovite. Muscovite occurs in varying quantities: as dominant or as the only mineral in the rock. Three types are defined: a) of (g1)a, (g1b) and (g1c) granites; b) of (g1d) granites; and c) of greisens and micaceous veins. The white micas are ferriferous, slightly magnesians and titaniferous, fluorous in median and not containing lithium. The phengitic (celadonitic) substitution is represented in a) percentages of celadonite due the excess of silica in tetrahedric position; b) Si + M 2 versus total Al diagram (Al VI + Al<supIV); c) Al VI + Al IV / Fet diagram (a.f.u.). The paragonitic substitution is between 2 and 5%. In the illitic substitution a decrease of interlayered cations is accompanied with the growth of tetrahedric Si.