Petrografia e litoquímica das formações ferríferas bandadas da região de Quixeramobim - Boa Viagem, Ceará, Brasil

Abstract

Two new occurrences of banded iron formations (BIF’s) had been identified in the regions of Quixeramobim and Boa Viagem, Ceará State. The occurrences are associated to the Neoarchean to Paleoproterozoic rocks of the Cruzeta Complex, whose Sm/Nd - TDM model ages vary between 2,3 and 3,2 Ga. The chemical analyses of major elements in six samples show a relatively simple chemical composition, with SiO 2 and Fe2O3 representing more than 96% of their total composition. When normalized by NASC, the REE distribution shows two different groups: the first one correspond to the Quixeramobim BIF’s with REE total concentration very high (121,65 and 119,37) and characterized also by the gradual increase of the REE fractionation from La to Lu. In this group, two analyzed samples show moderately positive europium anomalies (Eun/Eu* = 1,31 and 1,49). The second group is the Boa Viagem one that has a REE total concentration very large (6,23 the 189,44) without europium anomalies (Eun/Eu*= 0,77 a 0,99). The REE patterns of the two groups are interpreted considering the local of iron deposition in relation to distance from the source. The REE patterns show by Quixemarobim BIF’s are very different from the patterns present by archean BIF’s Algoma type, though similar to Lago Superior type where the weak anomalies of europium (Eun/Eu* > 1 e < 2) are related to the little participation of hydrothermal fluids in the depositional basin.