THE STRUCTURAL CONTROL OF THE GOLD DEPOSITS OF THE FAZENDA MARIA PRETA GOLD DISTRICT AT RIO ITAPICURU GREENSTONE BELT, NORTHEASTERN BRAZIL

Abstract

The Fazenda Maria Preta gold district comprises a group of individual shear-zone-hosted gold deposits located in the northern part of the Rio Itapicuru greenstone belt (RIGB), northeast Brazil. The deposits are limited to the Maria Preta unit, which is represented by rocks of andesitic composition with some interlayered pyroclastic and sedimentary rock lenses, and some dacitic and dioritic bodies, metamorphosed to the greenschist facies. The deformation in the area is characterized by a well developed NS-trending foliation dipping around 60 degrees W, with an associated stretching lineation shallowly plunging predominantly towards the north. The zones where this foliation is more intensively developed (lithologic contacts) were defined as shear zones (SZ-I to IV). In relation to the tectonic setting of the RIGB these zones correspond to 2 (super n) order shear zones, parallel to the NS striking Main Shear Zone. In these zones, the main structural fabrics observed were mylonitic foliation and stretching lineation. These elements define a sinistral strike-slip deformation generated by a dominant non-coaxial simple shear component, within ductile-brittle to brittle-ductile domains, with minor oblique-reverse transpressive or oblique-normal transtensive motions. The analysis of the kinematic indicators determine an oblate finite strain ellipsoid and weak to moderate tectonic deformation. The mineralized quartz veins, hosted in the shear zones, are mainly shear-type veins parallel to the mylonitic foliation, and minor extensional veins. The oreshoots present a tabular geometry, which constitute the primary control of the mineralization. A secondary control is the stretching lineation, which reorients the original tabular oreshoots parallel or perpendicular to its plunge. The quartz veins are generally massive or brecciated, and textures of open space filling are rare, indicating that the fractures jacked apart abruptly due to high fluid pressure. The field relations and the internal deformation features of the veins indicate that they were formed or emplaced at different stages during the deformation event. The model of emplacement of the veins proposed for the area, is analogous to the fault-valve model of Sibson et al. (1988).