Petrogenetic and mineralization processes in Paleo- to Mesoproterozoic rapakivi granites: examples from Pitinga and Goiás, Brazil

Abstract

Listen

Translate article

The 1.8 Ga Pitinga granites—Água Boa and Madeira massifs—in Amazonas (northern Brazil) are within-plate, shallow-level rapakivi granites associated with an extensional fracture system. They comprise an early facies of pyterlitic to wiborgitic rapakivi granite, a fine- to medium-grained biotite granite, as well as topaz granite (Água Boa massif) and albite granite (Madeira massif). The granites are usually metaluminous to peraluminous, the albite granite, however, is peralkaline. They are enriched in SiO 2, K 2O, Na 2O, F, Rb, Th, Nb, Y, Zr and the rare-earth element (REE) and impoverished in MgO, TiO 2, P 2O 5 and Sr, as the majority of Sn-mineralized granites. Sn contents range from ∼1 ppm in the rapakivi facies to ∼3400 ppm in the albite granite. ε Nd (at 1.8 Ga) values vary from −2.2 to +0.4 and Nd model ages lie between 2.4 and 2.1 Ga. Mineralization in the Madeira massif includes disseminated magmatic cryolite, zircon, cassiterite, pyrochlore, columbite-tantalite and xenotime and massive cryolite bodies in the F-rich peralkaline albite granite. In the Água Boa massif, Sn mineralization is associated with greisens and episyenite. The 1.77 Ga (g1) and 1.58–1.57 Ga (g2) rapakivi granites of Goiás (central Brazil) are coeval with the Araı́ rift basin. Granites of the g1 suite are metaluminous and alkaline, while the g2 suite is metaluminous to peraluminous. Both are enriched in F, Sn, Rb, Y, Th, Nb, Ga and the REE. Primary micas range from Fe-rich biotite to zinnwaldite. The micas of the more evolved granites and the metasomatic micas are strongly enriched in F, with F/Li between 2 and 10. The initial ε Nd values of both suites show a considerable range (∼−14 to 0) and indicate substantial compositional variation in source. Sn deposits in Goiás are hosted mainly by greisens. Indium is concentrated in quartz–topaz rock and albitized g2d granite of the Mangabeira massif and is always related to a cassiterite-sulfide association. This quartz–topaz rock is of metasomatic origin, probably generated by a hydrothermal fluid derived from the topaz–albite granite. Mineralization in the studied deposits was essentially associated with F enrichment. In the peralkaline Madeira albite granite, extremely high F contents favored disseminated mineralization, while in the Goiás Tin Province (GTP) and Água Boa massif greisenization was related to early fluid saturation. The GTP and Pitinga granites display tectonic, petrogenetic, geochemical, isotopic and metallogenic similarities that can be applied in search of Sn and rare-metal deposits.