Petrogenesis of the mafic-ultramafic Canindé layered intrusion, Sergipano Belt, Brazil: Constraints on the metallogenesis of the associated Fe–Ti oxide ores

Abstract

The Canindé layered intrusion is hosted by a Neoproterozoic volcano-sedimentary sequence and contains Fe–Ti mineralization in the Canindé Domain of the Brazilian Sergipano Belt. The petrochemistry of this layered complex and the metallogenesis of the associated Fe–Tioxide mineralization are still enigmatic. The intrusion comprises three main outcropping rock units from north to south: Unit I contains melanotroctolite and olivine gabbro/gabbronorite, Unit II consists of gabbro and (leuco)troctolite, and the Fe–Timineralized Unit III contains leucogabbro with stringers or lenses of pegmatitic gabbro, apatite-bearing gabbro, hornblende-bearing gabbro, Fe-diorite, oxide gabbro, gabbro with Fe–Ticumulate layers, and magnetitite. The mineralized Unit III contains low-Cr magnetite and ilmenite ± hercynite occurring disseminated and as massive mineralization in layers and lenses within the leucogabbro, close to the contact aureoles with the surrounding calc-silicate and amphibolite rocks. LA-ICP-MS U-Pb zircon dating of a gabbro sample from Unit II yields an age of 703.5 ± 1.6 Ma. The age-corrected ƐNd (t = 703 Ma) in the Canindé intrusion range from + 1.3 to + 4.3. Together with the depleted 87Sr/86Srt ratios of 0.7032–0.7040, these data strongly indicate a relatively depleted mantle source, with limited assimilation of crustal material. The low Sr-values may indicate a contribution from the subcontinental lithospheric mantle (SCLM) and the observed isotopic heterogeneity can be accounted for by replenishment by distinct magma pulses and/or minor but varying crustal assimilation. Except for the magnetitites, all Canindé cumulates are enriched in LILE (Rb, K, Sr, Ba, and Pb). Their Nb–Ta and Zr–Hf depletions relative to the primitive mantle composition (Nb/Nb* 0.4–0.6; Zr/Zr* of 0.6–0.8) reflect their relative incompatibility in the early cumulates, and are at least partly compensated by Nb- and Zr-enriched magnetitites. Based on the observed petrochemical characteristics of the cumulates and comparison with similar layered intrusives, the inferred parental magma is similar to a hydrous tholeiitic basalt (Mg# 50–58) with intermediate Ti-content (1–2 wt% TiO2). Our petrogeochemical data further indicate that the Canindé rocks are predominantly related through cumulus-forming processes, with some assimilation in the border zones. Apatite saturation preceded magnetitite formation and was concentrated in apatite gabbro cumulates with ≤ 6 wt% MgO. The resulting phosphor-loss would have lowered Fe-solubility in the remaining liquid, and may thus have triggered formation of massive magnetitite Fe–Ti ore bodies at the top of the intrusion. Finally, no economic Ni–Cu resources were identified in the intermediate portions and drill core data are still needed to assess the potential for Cr–PGE resources in the unexposed lower portions.