Mineralogical and chemical studies of cassiterite and its solid inclusions: relationship with the mineral paragenesis from the São João del Rei Pegmatite Province, Minas Gerais, Brazil
The São João del Rei Pegmatite Province extends over 1400km2 in the southern portion of the Minas Gerais State, Brazil. This pegmatite province is characterized by Sn-Ta bearing granite pegmatite swarms which are distributed along the Nazareno - São Tiago (Volta Grande pegmatites), Ritápolis (Paiol mine and Penedo pegmatites), São João del Rei and Coronel Xavier Chaves (Garimpo Cascalho Preto) areas. The pegmatites have expressive cassiterite - tantalite modal contents and cut the sequences of the Barbacena Greenstone Belt being spatially associated with the paleoproterozoic Ritápolis Granitoid (2.121 ± 7 Ma). Semi quantitative SEM-EDS analyses of cassiterite grains from the pegmatites of four areas indicate wide compositional range: 94,5 – 99,0% wt. of SnO2; 0,6 - 4,4% wt. of Ta2O5 and 0,1 – 1,0% wt. of Nb2O5. Analysis of solid inclusions hosted in the cassiterite reveal the presence of columbite-tantalite, tantalite, manganotantalite, plumbotantalite, pyrochlore, microlite, bariomicrolite, zirconiferous hafnon, hafniferous zircon, zircon, sphalerite, biotite, albite, rutile with niobium and probable wodginite and ferrowodginite. The hafniferous zircon, bariomicrolite and microlite inclusions hosted in the cassiterite indicate that Sn mineralization process developed during a hydrothermal stage in the pegmatites bodies. The inclusions of columbite-tantalite, tantalite, manganotantalite and plumbotantalite in the cassiterite point to a late crystallization stage of this mineral in relations to the former niobates-tantalates.
