Contribuições à exploração de cassiterita na Província Estanífera de Rondônia: a evolução magmática e metalogenética do Maciço Massangana

Abstract

The Rondônia Intrusive Suite (Younger Granites of Rondônia) represents the latest granitic magmatism that reached the SW of the Amazon Craton, with associated mineralizations in Sn, Nb, Ta, W, topaz and beryl. The Massangana Massif constitutes an elliposidal intrusion limited by large regional fractures with a NNE/SW direction; and smaller granite stock (São Domingos) that occurs in the southeastern portion of the Massif. The Massangana Massif and Stock São Domingos are mostly composed of coarse to porphyritic granites with rapakivi texture (pitherlites) or not, medium-grained granites and locally of fine biotite-granites in the form of dikes and processes. Rocks of syenitic composition with REEs and sulfides (galena and sphalerite) appear punctually. Geochemically plotted in the field of type-A, ferroan, metaluminous to peraluminous granites, they present accentuated negative anomalies in Ba, Sr and Ti, enrichment in Rb and Th and in Ce, Sm. The crystallization ages obtained were between 995.7 ± 9.5 Ma at 1026±16 Ma for the rocks of the Massangana massif and at 1060±35 Ma, 1081 ± 28 Ma, 1114±30 Ma for the São Domingos stock. The Massangana Massif showed εHf values varying between negative, related to predominantly crustal sources with TDM Ages related to emplacements, and positive, related to sources of juvenile magma. The ԐHf values of Stock São Domingos are predominantly negative with TDM ages between 2.4 and 1.9Ga, not related to any of the sites discovered so far in the region. The chemistry and petrography of the mineralized rocks show the action of comagmatic hydrothermal fluids enriched in H2O and F. The geometries of the mineralized bodies are defined by the contact between the housings and the dome in the magma chamber and can be attributed to residual accumulation through processes of fractional crystallization.The geological evolution proposed for the Massangana Massif and São Domingos stock is marked by the following stages: (1) granitic intrusion at the apical levels of biotite-granites with medium to coarse texture variation; (2) intrusion of fine biotite-granite related to an acidic residual magma represented by zinwaldite-granite rich in volatiles (F, Cl and H2O), (3) formation of pegmatite veins (4) greisenization of granites and beds and formation of quartz veins with cassiterite in the areas of contact with the beds. The rocks related to the mineralized zones have an isotopic signature of Hf that indicates an older source in relation to the non-mineralized rocks of the Massangana granite. Rocks with syenitic composition appear to have a different magmatic evolution from other granitoids, presenting the highest concentrations of Nb, ETRL and disseminated sulfides in this study. In contact with these rocks, the biotite-granite and alkali-granite show zones with disseminated sulfides (galena and sphalerite) in addition to beryl and cassiterite.