Iron Ore Hypogene Mineralization in the São Francisco Craton

Abstract

Two large-scale mineralization events associated with two orogenies generated economically important iron formation-hosted hypogene high-grade iron orebodies in the Brazilian São Francisco craton each with distinctive fabric and chemical characteristics as genetic diagnostic features. During the 2.1–2.0 Ga Trans-Amazonian orogeny, magnetite–martite–hematite massive to banded orebodies from the Quadrilátero Ferrífero District in Minas Gerais were controlled by thin-skinned tectonic structures. Metamorphic fluids promoted leaching of gangue minerals and substitution of carbonates in the Paleoproterozoic Cauê Iron Formation of the Minas Supergroup to form the orebodies. The age of the Cauê IF is bracketed by the U–Pb SHRIMP date of 2580 ± 7 Ma for the youngest detrital zircons from the underlying quartzites and the Pb–Pb isochron date of 2420 ± 19 Ma for the overlying dolostones. During the Ediacaran to Cambrian Brasiliano orogeny, modified hydrothermal fluids of magmatic and metamorphic origin recurrently mineralized the Cauê IF and the younger, carbonate-free IFs of the Serra da Serpentina Group (1990 ± 16 Ma—MDA, U–Pb SHRIMP data), lower Espinhaço Supergroup along five events with ages of 497, 508, 518, 526, and 536 Ma related to granitic intrusions. Massive Ti- and Mn-rich magnetite orebodies from the Guanhães Tectonic Block formed at the contact zone of anatectic pegmatite intrusions in deep to intermediate crustal level. Schistose platy to granular hematite orebodies crystallized in thrust fault-related shear zones at the eastern border of the Espinhaço Fold and Thrust Belt by structurally controlled hydrothermal fluid flow. Mineralogical, chemical, and textural transformations of the Cauê Iron Formation during the first regional event were partially overprinted or obliterated by the far-reaching effects of the Brasiliano orogeny. The complex multistage evolution of schistose ore crystallized in shear zones has been tracked by micro-chemical, geochronological and EBSD petrofabric studies in the terrains eastern of the Espinhaço Range indicating a syn-deformational origin with the development of a crystallographic preferred orientation by crystal-plastic processes and hydrothermal fluid-assisted grain growth along thrust planes.