The Precambrian geology of the “Borborema”-Belt (States of Paraíba and Rio Grande do Norte; northeastern Brazil) and the origin of its mineral provinces

In the crystalline area of the “Borborema” in NE Brazil the final geologic event was a strong static recrystallization, which is manifest in regional granitization, hybride granite, or pegmatite veins. The manifestation varies within the mapped area, it depends on lithology, thickness and structure of the Precambrian metasediments. The age of this phase is between 550 and 450 M. years. Recrystallization is accompanied by weak folding of pre-existing anticlines as indicated by the structural control of the Borborema pegmatites. The principal phase of deformation and the accompanying regional metamorphism are considered to be older, approximately 850 M. years or more. This older regional metamorphism produced rocks of the almandine-staurolite subfacies of the amphibolite facies; locally higher (cordierite and sillimanite subfacies) or lower (greenschist facies) degree of metamorphism occurs. The pyroxene-hornfels facies, which is found in the contact aureoles of hybride granites, occurs exclusively in the tactites (metamorphic dolomitic marls). In clastic metasediments, high grade metamorphic facies is restricted to the contact aureole of a small hypersthene diorite stock. The structure is simple, and consists of synclines and anticlines the axial planes of which are vertical or, rarely, inclined. Faults are rare. The degree of lateral compression varies: low in the NW part of the area mapped, intermediate in die NE, and high in the southern part. The southern part belongs to the “Paraíba Transversal Zone”, an E-W striking fault zone along which the northern parts have been dislocated horizontally to the East. Stratigraphically the sequence is subdivided into three major groups by unconformities, which, however, are masked intensely by the principal tectonic events. These groups are: The oldest member occurs but locally in the cores of some anticlines; the youngest constitutes the cores of most synclines. The Caicó can be subdivided, in the most part, as follows: Florânia and Parelhas are essentially composed by unsorted clastic sediments (graywackes and arcoses). The Equador is a local lithofacies of the latter restricted to one anticline and a small part of another, and composed of well sorted clastics (muscovite quartzite). Another quartzite, locally developed at the top of the Florânia in the western part of the area, has been named “Member S. José do Seridó”. The Quixaba contains essentially carbonate rocks, but with intercalations of clastic sediments. The scheelite province of the area is generally connected with the Quixaba-Formation, the marbles of which served as transportation channels, whereas tactites were the receptors of the ore-bringing solutions, derived from some palingenetic granites and thus, indirectly, from the metasediments of the Parelhas. The deposition does not belong to the principal phase of contact metamorphism, but to a later-phase with hydrothermal paragenesis. Scheelite occurs also in common quartz-albite-veins. The famous pegmatite-province (Ta, Nb, Li, Be) cannot be considered as a product of magmatic differentiation, either. It must be the product of the statical recrystallization of the muscovite-rich Equador quartzite, by addition of thermal energy and K-rich solutions from the depth. Thus this province is only a special case of the general “granitization” with K-metasomatism: $$Quartz + Muscovite + K^ + to Microclin + H^ + + Water.$$ The resulting fluids formed pegmatites in the rocks overlying the quartzite, i. e., generally in the micaschists of the Seridó-Group. Productive dykes always fill longitudinal or transversal tension cracks. The Borborema province of zoned pegmatites is thus an excellent example for the recent genetic hypothesis ofGresens, all requirements of it being fulfilled.