Controle estrutural do depósito aurífero de São Francisco, município de Currais Novos (RN).

Abstract

In the area of the former São Francisco Mine, now the Aura Borborema mine, metasedimentary units of mica schists of the Seridó Formation host a mineralized gold body. In this area, a structural analysis work was developed to determine the structural control of the main mineralized body, as well as the deformational context involved during its formation. The mine is located on the São Francisco farm, on the banks of BR 226, in the municipality of Currais Novos (RN). Regionally, the area is part of the Seridó folding system (Seridó Belt), Rio Grande do Norte Domain, Borborema Province. Three progressive deformation events, D1, D2 and D3, were recognized in the mica schists of the Seridó Formation (Seridó Group). Tectono-metamorphic evolution is considered to be restricted to the Paleo-Neoproterozoic. D1 was responsible for the formation of a locally folded S1 foliation. D2 was responsible for the formation of an S2 axial plane foliation of recumbent to inclined F2 folds, ranging from closed to isoclinal. These folds occur only on the outcrop scale. In the area, the S2 foliation shows a northeast-southwest direction (trend) with dips ranging from 30-60º to the northwest and southeast to sub-horizontal quadrants. The last recognized ductile deformation event in the area, D3, was responsible for the folding of the S2 foliation and the formation of an S3 foliation, which is locally parallel to the axial surface of F3 folds. They are antiform and shapeless folds with dimensions of tens of meters, strongly inclined and with a weak drape (10-15º) to the southwest. The S3 foliation shows a northeast-southwest direction and dips at a high angle (70-88°) to the southeast quadrant. The formation of a linear fabric, represented by intersecting lineations, fold hinges and quartz rods are related to the action of this event and share the same general orientation with the hinges of F3 folds. The main mineralized body of this deposit has an elongated shape in a northeast southwest direction with a weak drop (±10°) to the south-southwest, occurring embedded in the hinge zone of the F3 antiformal fold. The analysis of the three dimensional strain showed that the deformation associated with the last event (D3) occurred under a strain geometry that ranged from flattening (ellipsoids with oblate geometry) to constriction (ellipsoids with prolate geometry), also involving a plane strain. In general, the X-axis of the strain ellipsoid has a direction close to that shown by the elements of the linear fabric. It is concluded that the installation of the mineralized bodies (oreshoots) was, to some extent, controlled by the geometry of the ellipsoid strain during the D3 event. The local X-axis orientation of the finite strain ellipsoid was critical to enforce the current oreshoot orientation. Therefore, constituting a classic saddle reef type deposit.