Mineralogical and geochemical characteristics of AlFe granulites and banded iron formations in the Archean supracrustal unit of the In Ouzzal terrane (NW Hoggar, Algeria)

Abstract

In the northern In Ouzzal terrane, AlFe granulites and banded iron formations (BIFs) constitute a supracrustal unit, locally associated with AlMg granulites, marbles, calc-silicate granulites, and mafic or ultramafic rocks. The AlFe granulites display variable SiO₂ content (55.62 to 79.26 wt%) and X_Mg ratio (0.11–0.48) and are characterised by spinel–quartz parageneses, locally retrograded to garnet-sillimanite coronas. Cordierite is absent in some AlFe granulite samples, while others contain cordierite-spinel or cordierite-spinel-orthopyroxene symplectites that are a breakdown product of spinel-garnet-sillimanite-quartz assemblages. The BIFs display a unique corundum-quartz-magnetite paragenesis, characteristic of ultra-high-temperature (UHT) metamorphic conditions. Another exceptionally rare type of BIF in the region consists of orthoferrosilite (X_Mg = 0.14–0.29) in equilibrium with high chlorine-rich (3.88 to 5.12 wt%) hastingsite, surpassing values reported in international literature. The AlFe granulites exhibit geochemical characteristics comparable to post-Archean shales and are interpreted as the products of mixed protoliths. These protoliths consist of mature detrital material, primarily quartz, derived from trondhjemite and tonalite sources emplaced between 3300 and 2700 Ma. Additionally, they include immature predominantly chloritic components enriched in Al, Cr (100–300 ppm) and V (100–200 ppm). Banded iron formations (BIFs) are mainly composed of three key components: SiO₂, FeO, and Fe₂O₃, with their combined content ranging from 91.51 to 99.23 wt%. The concentrations of Al₂O₃, TiO₂, CaO, and rare earth elements (REEs) are notably low, consistent with those of classic Archean BIFs. This composition implies that their formation resulted from the direct precipitation of Fe-rich minerals in oxidizing marine exhalative environments, while silica-rich layers (mainly chert) formed during periods of low oxygen concentration, and with the absence of detrital materials.