Pegmatitos mineralizados em água-marinha e topázio do ponto do marambaia, Minas Gerais: tipologia e relações com o granito caladão

Aquamarine deposits from Ponto do Marambaia, part of the Eastern Pegmatite Province of Minas Gerais, are inserted in the pegmatite field of Padre Paraiso-Catugi, Padre Paraiso Pegmatite district. The pegmatites contain aquamarine (blue and green), beryl and goshenite, blue topaz, decimetric to metric crystals of hyaline quartz and murion, and are hosted in the Caladao Granite. The aquamarine presents significant economic importance in the gemological world scenario, in respect to its historical production and potentiality, to the amount and quality of the recovered gems. Records of famous aquamarine findings, including the “Papamel” and “Marta Rocha”, respectively with 74 and 33,9 kg in weight are internationally known. Pegmatites exhibit irregular zoning, contain K feldspar, quartz, biotite, aquamarine and topaz and do not present any structural control, making sharp to gradational contacts with the granite. Pegmatites display steep to gentle dips, decimetric to metric thicknesses, decametric extension, irregular and ameboid shapes and relative differentiation. Pegmatites evolved from a K feldsparquartz graphic fraction in the magmatic liquid, toward the generation of K feldspar mega-crystals and interstitial and massive milky quartz, biotite and the gem minerals, topaz and aquamarine, under brittle conditions. At the end of the evolution, residual silica-rich liquids propitiated the crystallization of pyramid-shaped hyaline quartz and murion, after fracturing ceased. The Caladao Granite is porphyritic, garnet-free, with no ductile deformation and exhibits clear magmatic flow and occasional inclusions of Padre Paraiso Charnockite, in gradational contacts. It was derived by the partial re-melting of the surrounding crustal granitoids, known by Carai, Itaipe, Faisca and Wolf, in intergradational contacts and with the Caladao Granite. Regionally is part of the Aimore Intrusive Suite, considered as post-collisional, I-type of the Aracuai Belt with ages comprised between 490 Ma and 520 Ma. Pegmatites from the Ponto do Marambaia may be classified in Rare Element group, Nb/Ta-free, beryl-type. The intimate spatial relationships between the aquamarine with the K feldspar mega-crystals allow to suppose a metasomatic process by the activity of a residual, aqueous, Be-rich fluids, resulting in aquamarine, muscovite and quartz crystallization: 5KAlSi 3 O 8 + 2H 2 O + 3 Be 2+ = Be 3 Al 2 Si 6 O 18 + KAl 3 Si 3 O 10 (OH) 2 + 6SiO 2 + 5 K + + 2H + (K feldspar) (fluid) (aquamarine) (muscovite) (quartz)