Geology, petrography and mineral chemistry of iron oxide-apatite occurrences (IOA type), western sector of the neoproterozoic Santa Quiteria magmatic arc, Ceará northeast, Brazil

Abstract

Iron oxide-apatite (IOA) occurrences have recently been identified in the western sector of the Neoproterozoic Santa Quitéria magmatic arc (Ceará state, northeast Brazil), close to the boundary with the Paleozoic Parnaíba Basin. The IOA mineralization is hosted by albitized metadiorites and metavolcano-sedimentary rocks, which are crosscut by a late- to post-Brasiliano/Pan-African biotite granite intrusion with a U-Pb age approximately 548 ± 4 Ma. Within the metavolcano-sedimentary sequence, metavolcanic rocks are bimodal and consist of albitized metabasalt-andesite and metarhyolites with a U-Pb age of 554 ± 6 Ma, whereas the metasedimentary units are represented by calc-silicate, marble, and pelitic gneisses. The iron oxide-apatite mineralization occurs as (i) banded–stratabound lenses/layers composed of magnetite (50–70%), with or without ilmenite exsolution; apatite (≤8%); monazite; and locally subordinate copper sulfides (~2%); (ii) massive magnetite-(specular hematite)-apatite bodies; (iii) disseminated and vein magnetite-apatite within albitized metadiorite; (iv) massive rhombohedral hematite bodies; and (v) garnet-magnetite type. The host rocks also display marialite, diopside, albite, and epidote (sodic-calcic alteration), biotite and K feldspar (potassic alteration), chlorite and epidote in lower-T hydrothermal alteration assemblages. Mineral chemistry data reveal that (i) except for vein types, apatite in all the other occurrences is F rich (2.4 ≤ F ≤ 4.4%) and low in Cl (<0.5%), and (ii) magnetite contains variable concentrations of Ti, V, Cr and Ni attributed to both igneous and hydrothermal environments. In addition, the magnetite of the stratabound occurrence is low in TiO2 (≤1.6%); however, its oxy-exsolutions are rich in TiO2 (14–52%), indicating a titanium-rich original iron oxide. The low sulfide content and high concentration of apatite indicate that the investigated Fe-P occurrences are of the IOA type, similar to those of IOA provinces elsewhere, such as Kiruna, El Laco, Gushan, Pea Ridge, Pilot Knob, and Bafq. Hence, the discovery of these occurrences in the Neoproterozoic Santa Quitéria magmatic arc has a twofold metallogenic significance: (i) these are the first records of IOA-type deposits in Brazil; and (ii) open a favorability potential for the exploration of IOCG-type deposits in this tectonic domain.