Structural control of REE-pegmatites associated with the world-class Sn-Nb-Ta-cryolite deposit at the pitinga mine, amazonas, brazil [Controle estrutural dos pegmatitos ricos em etr associados ao depósito madeira (Sn-nb-ta), mina pitinga, amazonas, brasil.]

Abstract

We have studied the structural control of pegmatites (F-REE-Li vein-type granite pegmatite) associated with the albite-enriched granite facies (AEG) of the Madeira A-type granite (~1.83 Ga). This facies corresponds to the Madeira world-class Sn-Nb-Ta-F (cryolite) deposit at the Pitinga mine. These REE-rich pegmatites [xenotime-(Y) and gagarinite-(Y)], presently exploited together with the disseminated ore, have potential to exploitation by selective mining. They have a common geometric arrangement and share the same mineralogy, therefore they all originated from the same source. They have the same mineralogical composition of the host rock and their emplacement occurred in the parental rock itself. The geometric arrangement of the pegmatites is settled by contractional brittle structures (reverse faults, imbrication fans and horses). The reverse fault planes (~N320/60SW) were essentially the conduits for the fluid. The preferential sites for the pegmatites bodies were horizontal tensile fractures. The well-marked geometric arrangement of the tectonic structures and the fact that there are also reverse faults planes without pegmatites show that the fractures that host the pegmatites were not formed by the fluid pressure. The orientation of the contractional structures in the AEG indicates a transport from SW to NE. With reduced dimensions, the AEG cooled fast, however its location in the cold upper crust and the low solidus temperature allowed pegmatites formation. The pegmatites were formed with the AEG emplaced in a structural level above the critical crustal depth, where minimum normal stress is vertical.