Aspectos litoestruturais e mineralizações Salobo 3A (Serra dos Carajás-PA)

Abstract

The Salobo district is located at the Carajás Province in the Cinzento strike-slip system, and is formed of supracrustal rocks of the Salobo Group and gneisses of Xingu Complex, which were assembled by several generations of shear zones. The Salobo Group includes magnetite-fayalite schist, biotite-almandine-magnetite-fayalite-grϋnerite schist, biotite schist, anfibol schist, chlorite schist, banded iron formation and quartzite. The Xingu Complex includes banded tonalitic gneisses, trondhjemitc and granodiritic gneisses, partialy migmatized. The older shear zones correspond to ductile thrusts to which the generalized imbrication of the lithological units and the tectonic layering, defined by strips and lenses of supracrustais rocks alternated with gneisse is due. Their movements is synchronous to that of the Itacaiúnas belt; minerals transformations under thermal conditions of amphibolite facies, as well as important modifications in the stratigraphic relationships of several lithological units, are related to therm. The second generation of shear zones formed the transtensive Salobo-Mirim duplex. They are sinistral strike-slip shear zones, linked by normal shear zones along which minerals transformations of greenschist facies occurred. The development of the duplex was controled by the former tectonic layering, and its assymetric shape was influenced mainly by the presence of a basement gneiss megalense. The third generation of shear zones is of transcurrent type, with NW-SE and NNW-SSE trends, which modified the Salobo-Mirim duplex geometry, and are interpreted as X features linked to the sinistral moviments. The shear zones of the west corner of the area represent the propagation of a horse tail structure at the west end of the Carajás fault; in these case, they could be oblique thrust. The Salobo 3A deposit is situated in the central part of a normal oblique shear zone, which belongs to a release bend along the main strike-slip shear zone of the Salobo-Mirim duplex. The copper and gold mineralizations are hosted in extension structures, as single and composite pull-apart structures, transtensive sigmoide stringers, tension gashes, pressure shadows and star strucutures in the cross-cutting zones of descontinuities. The Salobo 3 A deposit is an example of concentracion/reconcentracion of cooper and gold mineralizations in s transtensive shear zone due to deformational, hydrothermal and metamorphic processes.