Petrografia e geoquímica dos greisens associados ao topázio-granito do pluton Água Boa, Província Estanífera de Pitinga (AM).

Abstract

The Pitinga Tin Province is situated 300 km north of the city of Manaus (state of Amazonas, Brazil) and is characterized by important world-class Sn, Nb, Ta and Zr deposits, related to the Madeira Suite, comprised by Paleoproterozoic (~1.82 Ga) A-type granites. The primary magmatic mineralization occurs in an albite-granite of the Madeira pluton, and the hydrotermal mineralization occurs in episyenites and greisens associated with the Água Boa pluton. Plutons of the Madeira Suite intrude the Paleoproterozoic acid volcanic rocks of the Iricoumé Group (~1.88 Ga). The Água Boa pluton has elliptic form, is alongated along NE-SW strike, covers near 350 km2 and is composed of four facies. The earlier facies is a metaluminous amphibole biotite alkali feldspar granite, locally showing rapakivi texture. It was followed, in order of emplacement, for porphiritic biotite granite and an equigranular to seriate biotite granite, both metaluminous to peraluminous; the later facies is a peraluminous, porphyritic topaz-bearing biotite alkali feldspar granite, named topaz granite. The tin mineralization in the Água Boa pluton occurs in altered metasomatic zones, formed by episyenites and greisens. The host rocks of the studied greisens are the topaz-bearing biotite alkali feldspar granite facies of the Água Boa pluton. Two textural variations were distinguished: a gray to pink fine- to medium-grained phase to pink porphyritic phase and albitized granite. Cassiterite-bearing leucogranite pegmatites, weakly albitized, occur transitionally between granite and greisens. Greisen formation is controlled by fractures and greisens occur as continuous zones up to 6 meters thick (F06Gr Grota Rica drill core), transitional to greisenized granites. Greisens are inequigranular light to dark grey, medium- to coarse-grained. They are composed essentially by quartz, green siderophyllite and topaz, with additional variable amounts of fluorite, zinnwaldite, sphalerite, cassiterite, zircon and anatase. Greisens are classified as quartz topaz siderophyllite greisen, topaz siderophyllite quartz greisen and topaz quartz greisen. Scanning electron microscopy studies indicate that these rocks also contain trace Ce-monazite, galena, pyrite, chalcopyrite and native bismuth. Coarsegrained quartz-only or sphalerite ± zinnwaldite bearing quartz veins cross-cut the greisens. Geochemistry data, including mass balance calculations, supported by petrographic observations, show that greisenization processes took place without changes in volume. These processes resulted in gain of Fe2O3t and more significant loss of Na2O, MgO, CO2 and K2O. Almost complete removal of Na2O and partial removal of K2O are related to the destabilization of feldspar and are the main characteristic of greisen formation. The distinct behavior of K2O is due to the retention of K in newly formed micas. Apparent immobility of Ca can be explained by low host granite Ca contents and by its retention in secondary fluorite. During greisenization S, F, Zn, Cu, Sn, Pb, Ta, Rb and U were enriched, while other elements declined. Increases in Fe, S, Zn, Cu and Pb are related to sulfide formation. Among the lithophile elements, Rb is strongly enriched in the greisen due to its retention in the siderophyllite structure, whilst Ba and Sr are removed during feldspar alteration. The rare earth elements (REE) reveal little mobility and patterns very similar to the granites. In general, they present similar patterns and slightly lower contents in greisens in comparison to the granites. REE depletion occurred during the formation of the quartz-topaz-siderophyllite-greisen and greisen relatively rich in quartz shows greater losses in light REE relative to heavy REE. Mineral chemistry allowed for classification of the brown micas within the topaz-bearing granite as annite transitional to siderophyllite, green micas from greisen as siderophyllite, and the late pale micas from greisens and quartz veins as zinnwaldite. The evolution of mica from the granite to greisen is given by annite → siderophylite, showing increasing content of VIAl, VI, Li and Si. The greisen siderophyllite was, in turn, partially replaced by zinnwaldite, also with increased content of VIAl, VI, Li and Si. The Kbearing feldspar phase analysed in granite and leucogranite pegmatite is orthoclase (Or93-98). Albite (Ab95-99) occurs as lamellae within perthites and intergranular growths. Cassiterite forms strongly pleochroic, twinned and zoned euhedral to subhedral crystals with low Nb and Ta contents. Topaz-bearing biotite alkali feldspar granite was postdated by localized formation of cassiterite-bearing leucogranite pegmatites and both were affected by post-magmatic alteration in form of intergranular albite. This process was followed by strong hydrotermal alteration represented by greisenization, and later local silicification that culminated in the formation of greisen and quartz veins, the main hosts for Sn-mineralization, and subordinated Zn-mineralization.