Geologia, contexto Geotectônico e Potencial Metalogenético para depósitos de Ni-Cu- (EGP) do Complexo máfico-ultramáfico de Brejo Seco, Faixa Riacho do Pontal, Sudeste do Piauí

Abstract

The Mafic-Ultramafic Brejo Seco Complex occurs on the western end of the Neoproterozoic Riacho do Pontal fold belt, the northern limit of the São Francisco Craton, State of Piaui, Northeast of Brazil. In this work, cartographic, petrographic, geochemical and isotopic data enabled a new genetic model for the complex and an evaluation of its metallogenetic potential. The Brejo Seco Complex (BSC) consists of a layered igneous body, tectonically inverted, dipping on average 70° to north. It was considered here as intracontinental in origin, in contrast to an ophiolitic origin previously proposed. The stratigraphy of the complex can be divided into four major zones: Lower Mafic Zone (LMZ), Ultramafic Zone (UZ), Mafic Transitional Zone (MTZ) and Upper Mafic Zone (UMZ). Sm-Nd isotopic data indicate that the complex intruded the continental crust at c.a. 900 Ma. The magmatic crystallization sequence (Ol+Chr >> Pl+Ol+Chr >> Pl+Cpx+Ol >> Pl+Cpx+Ilm+Mag+Apt) defined for the BSC, together with geochemical fractionation and olivine chemistry, suggest an affinity for Mg-tholeiitic parental magma. Nd(T=900) values obtained along of Brejo Seco stratigraphy (-0.2 a +3.3), show the interaction of parental magma with sialic crust and together with crystallization sequence and mineral chemistry, record at least four major phases of magmatic chamber filling. The phase three has a special interest as it represents the sulphide mineralization, located in the contact between the UZ and the MTZ, and formed by the interaction between a primitive parental magma (Fo87-89) and contaminated (Nd < 0) and moderately a primitive magma (Fo82). The evaluation of the metallogenic potential of the BSC to host Ni-Cu-(EGP) sulphide deposits revealed no potential target specifically, but can be used as a guide for future exploration works. Chemical analyzes performed on mineralized zone indicated low levels of Ni-Cu-(EGP). The Tonian age (~ 900 Ma) obtained for the CBS allows to interpret it as a record of early rifting stages of the Riacho do Pontal fold belt, as well as correlate it to Monte Orebe basalts (~820 Ma), which represent the oceanic crust developed during the rifting evolution. The BSC can also be correlated to the Tonian extensional magmatic event responsible for fragmentation of the supercontinent Rodinia. Evidence of this magmatism are present in several fold belts as well as the São Francisco and Congo Cratons.