Geotectonic signature and hydrothermal alteration of metabasalts under- and overlying the giant Serra Norte iron deposits, Carajás mineral Province

Abstract

Archean metabasalts enclose the Serra Norte iron deposits in the Carajás Mineral Province, Brazil. They experiencedseafloor hydrothermal alteration, regional metamorphism under low greenschist facies conditions, and hematite-associated hydrothermal alteration, characterizing (i)least-altered, (ii) chlorite, and (iii) hematite-chlorite basalts inthe N4 and N5 deposits. Their petrographic and geochemical changes were caused by both chlorite and hematite hydrothermal alteration. Nevertheless, part of the original, igneous textures and some of their primary minerals (plagioclase and clinopyroxene) are commonly preserved. Geochemical investigation of the least-altered basalt indicates a calc-alkaline affinity. Trace element behavior (e.g., Nb and Th) is consistent with an arc-related setting and continental crustal contamination. Where hydrothermal alteration affects basalts the most,distal, intermediate, and proximalalteration zones are distinguished based on the amount of chlorite ± hematite (less magnetite). Chlorite composition in (i), (ii) and (iii) marks these three mafic lithotypes,with variations in theFet/(Fe + Mg)ratios that clearly reflect buffered through unbuffered conditions.Mineralogical and textural features, such asrupturedamygdales, indicatefluid interaction with hydrothermal minerals forming veins and breccias caused by hydraulic fracturing.Bothchlorite and hematite-chlorite basalts experienced renewed isotopic equilibrium due to hydrothermal alteration, showing a decrease inδ18O (7.4 to 1.9 ‰) in comparison to the least-altered basalt reflecting an increase in fluid-rock ratio.On most discrimination diagrams and MORB-normalized spidergram, the investigated rocks are comparable to a volcanic arc environment associated with subduction zone. These and other evidence such as (a) a seafloor-associated volcanogenic sequence; (b) arc-type sedimentation of the Grão Pará Group indicated by hyaloclastic tufts, breccias and volcaniclastic rocks; (c) jaspilites in the Serra Norte; and (d) a large subduction signature expressed by enrichment in Th, all suggest that the Grão Pará basaltsoverflowed on the continental crust in a back-arc environment, with theprimary magma having experienced significant upper crustal contamination. The arc-association evolution and the variability of lithological, sedimentological, igneous volcanic, structural, metamorphic, and other evolutionary aspects of the Itacaiúnas Supergoup is suggestive of a greenstone-belt-type setting.An arc-associated evolution and the variability of lithological, sedimentological, igneous volcanic, structural, metamorphic, and other aspects of the Itacaiúnas Supergoup supports a greenstone-belt-type setting.