Lithogeochemistry and zircon U-Pb geochronology of the Granjeiro Complex and associated units, Curral Novo do Piauí, SW-Borborema Province, Brazil: implications for Archean to Paleoproterozoic crustal evolution

Abstract

The Curral Novo Iron District, located in the center-east of the state of Piauí, is hosted in the Granjeiro Complex in its southwestern section, located in the Rio Piranhas-Seridó Domain, northern subprovince of Borborema Province. In this district, the banded iron rocks have an average content of approximately 27% Fe, with an estimated potential of more than one million tons of ore. Through the integration of petrographic, isotopic, geochemical and geochronological field data, a comprehensive characterization of the lithotypes present in the complex (orthogneisses, migmatites and a metavolcano-sedimentary unit) and associated units (syenogranitic orthogneisses and pegmatite granites) was carried out, focusing on the evolutionary discussion of this Archean complex, as well as the origin of metasomatism present in the iron district. Tonalitic and granodioritic orthogneisses include calcic to alkali-calcic rocks with low mafic mineral content. These rocks have low concentrations of lithophile elements and high field potential elements, significant fractionation of light rare earth elements and slightly negative Eu anomalies. U-Pb geochronology in tonalitic orthogneiss zircon from the Granjeiro Complex indicates the age of the protolith in the Paleoarchean at 3349 Ma. The metavolcano-sedimentary unit is made up of amphibolites, magnesian schists, serpentinites and banded iron rocks. Such rocks derived from mafic and ultramafic protoliths have a komatiitic and basaltic composition and have a genesis associated with the subduction phase of oceanic plates, presenting a geochemical affinity similar to that of modern island arc rocks. The iron rocks present sedimentary facies of the oxide and silicate types, with strong impoverishment of high field potential elements and rare earth elements and negative Ce anomalies. These units are intruded by syenogranitic plutons, collisional powders, type A2, aged 2651 Ma. Chemically, these granites are metaluminous to peraluminous, alkali-calcic and ferrous types, having a slight enrichment of light rare earth elements in relation to heavy rare earth elements and high contents of lithophile elements and high field potential elements. The rocks of the Granjeiro Complex show records of strong crustal reworking in the Paleoproterozoic, starting from the Riacian, with a metamorphic peak in metavolcanosedimentary rocks indicated by U-Pb dating in amphibolite zircon at 2200 Ma. An important magmatic event related to the global process of extension of continental masses during the Statherian period, aged 1759 Ma, represented by several intrusive plutons in all units of the area, promoted hydrothermal alteration in the surrounding amphibolites and iron rocks, causing silicification, albitization, potassification, carbonatization, sulfidation and reconcentration of iron. δ 34S values = -3.11 to +2.3 per mil in hydrothermal sulfides suggest magmatic sources for the sulfur associated with the hydrothermal system. The incorporation of the analysis of these diverse geological data, made it possible to obtain a detailed understanding of the geological evolution of the Granjeiro Complex in the study area, as well as the origin of metasomatism in the iron district.