Geochemical and tectonic constraints on the genesis of iron formation-hosted magnetite-hematite deposits at the Guanhães Block (Brazil) by contact metasomatism with pegmatite intrusions

Abstract

The Horto-Baratinha (HBD) and Cuité (CTE) are two high-grade iron ore deposits (>60% wt. Fe) located in the Guanhães Tectonic Block (GB), at the eastern margin of the São Francisco Craton bordering the late Neoproterozoic–Cambrian Araçuaí-West Congo Orogenic belt. The orebodies are hosted by Orosirian to early Statherian metamorphic banded iron formation (IF) associated with quartzite and aluminous schists. The metasedimentary units are time-correlated with the Serra da Serpentina and Serra de São José Groups exposed in the Southern Espinhaço fold-thrust belt. They occur as tectonic slivers inserted in the pre-Rhyacian gneissic rocks and the granitic plutons of the Statherian Borrachudos Suite that comprise the GB. In the GB, the IF was intruded by anatectic pegmatites of Ediacaran to Cambrian crystallization age with the development of massive, granular magnetite-rich high-grade orebodies at the contact zone, contrasting with the hosting schistose quartz-hematite IF. The Fe-mineralization of the IF during the Brasiliano Orogeny developed in three distinct stages. (1) Syn- to late-collisional stage (ca. 580–560 Ma) with partial Fe enrichment along sheared zones, crystallization of oriented platy hematite crystals and development of discontinuous lens-shaped schistose high-grade orebodies. (2) Syn- to late-collisional stage (ca. 560–530 Ma) with the formation of massive magnetite bodies by contact metamorphic-metasomatism with the pegmatite. (3) Late- to post-tectonic stage with the oxidation of magnetite to kenomagnetite, martite and crystallization of granular hematite, probably by increased input of meteoric fluids during gravitational collapse of the Araçuaí-West Congo Orogen (ca. 530–490 Ma). In the HBD deposit, the first mineralization stage was unimportant for the enrichment of the IF that exhibit Fe contents of 32.45 to 43.50 wt%.Whole-rock data indicates that the massive hematite-magnetite high-grade iron ore pods exhibit Fe contents between 50.24 and 69.46 wt%, and increasing content of Al2O3, MgO and CaO and the trace elements Zr, Y, Zn, and Ni near the contact with pegmatite, associated with the presence of interstitial silicates and carbonates in their composition. Lamellar hematite exhibit very low contents of trace elements as in other specularitic IFs and orebodies from the eastern São Francisco Craton. A consistent increase in the content of trace elements, mainly Mg, V, Cr, Mn, Co, Ni, Zn, and Ga, is observed in kenomagnetite. Granular hematite grains are enriched an Al, Ti, Zr, Nb, Mo, Sn, Sb, Hf, Ta, and W. Notably, Mn and Ti may occur in relatively high concentrations in kenomagnetite and hematite with the crystallization of ilmenite inclusions, associated with the contact-metasomatic stages.