The Rio Itapicuru greenstone belt, Bahia, Brazil: Structure and stratigraphical outline

Abstract

The Rio Itapicuru greenstone belt is situated within the São Francisco craton, and is the largest of a series of N-S trending volcanosedimentary sequences surrounded by gneisses and granitoids. Pb-Pb and Rb-Sr dating of andesitic lavas within the supracrustal sequence yielded ages of 2.1 Ga, with late tectonic granitoids emplaced about the same time. The succession appears to be developed on a sialic basement represented by the Santa Luz gneiss complex. Although the actual contact between basement and supracrustals has not been observed, there is a sharp contrast in strain and metamorphic conditions near to the contact zone. A 2.9 Ga U-Pb age obtained from a gneissic raft within the intrusive Ambrosio dome is interpreted as a basement to the greenstone sequence and was plucked from a lower crustal level by the ascending magma. The Itapicuru River section provides good exposures along an E-W profile and has been used to attempt a reconstruction of the lithostratigraphy in this area, where the supracrustals reach a maximum thickness of 9.5 km. Rare stratigraphic facing and structural data indicate that the base of the sequence is marked by an intrusive granitoid. Above this contact is a 5.0 km thick succession of tholeiitic subaqueous basalts with rare cherts, banded iron formation, pelites, andesitic lavas and tuffs. This mafic domain is overlain by a 3.5 km thick sequence of andesites, tuffs and volcanogenic sediments, with rare dacites and laminated cherts. A silica gap between 55% and 60% SiO 2 characterises the lavas as a bimodal sequence. Two small ultramafic bodies with komatiitic chemistry also occur in the andesitic volcanic domain. The uppermost 1.0 km consists of volcanogenic siltstones and shales with rare arkoses, tuffs and basalts. Localised bedding-parallel shear zones are the earliest deformation features observed. The shear zones may be up to 200 m thick and were important in controlling subsequent lode gold mineralisation. In the north central sector of the greenstone belt these shear zones were deformed by N-S trending, east verging folds, with ∼ 20 km wavelength. The main greenstone sequence in the Itapicuru River section lies on the overturned limb of a syncline, and the Ambrosio dome occupies the adjoining anticline. In the southern part of the greenstone belt, the initial deformation produced a major shear zone striking E-W with a 40° S dip and a subhorizontal E-W stretching lineation. North verging folds and thrusts produced the present E-W trend. A late, upright, NE-SW trending fold phase terminated the ductile deformation. Greenschist facies metamorphism affected the major part of the belt during deformation, with amphibolite facies assemblages developed around the margins, and thermal aureoles developed around intrusions. Metamorphic fluids transported gold to higher crustal levels via pre-existing, steeply dipping shear zones and faults and produced the large Fazenda Brasileiro deposit, along with many smaller deposits.