Geological And Geochemical Characteristics Of The Banded-iron-formation-hosted Nabeba Iron Deposit In The Ivindo Basement Complex,Republic Of The Congo

The Nabeba iron deposit of northwestern Republic of the Congo is one of the world's largest iron deposits in Central Africa.It is mostly hosted within the banded iron formation(BIF)sequences of the Archean greenstone belt,which is surrounded by tonalite-trondhjemite-granodiorite(TTG)and granitic rocks suites and/or associated shear zones in the Ivindo Basement Complex(IBC).However,the Algoma-type Nabeba BIF is deformed/dismembered and metamorphosed,thus making reconstruction of their depositional and overall geologic settings difficult.Despite being the archetype,very limited research in the context of recent ideas about the Archean has been conducted on the Nabeba BIF,which also host the Nabeba iron deposit.Therefore,this thesis aims to understand the petrogenesis of the IBC TTG suite,elucidate the nature and environment of the Nabeba BIF deposition and then,establish a genetic model for hypogene iron enrichment in the Nabeba BIF-hosting iron ore deposit.To achieve this aim,an integration of geology,petrology,zircon U-Pb geochronology and Hf-Lu isotopic analysis,whole rock and magnetite geochemistry,and fluid inclusions on ore-related quartz veins was employed.The geochemical,chronological and Zircon Hf isotopic results show that the TTG suite were emplaced at ca.2897 to 2889Ma in the setting which is likely to have been syn-to post-collisional.Zircon Hf isotopic(?Hf(t))values of+1.22 to+4.55 and Th/La>0.12 for the tonalite/granodiorite and trondhjemite indicates that these rocks were derived from partial melting of an older mafic reservoir(TDM2 ages range from 3028 to 3230Ma),possibly triggered by mantle upwelling or 583plume activity.The geochemical characteristics of the Nabeba BIF suggest that these BIFs are pure chemical precipitates.Low content of Al₂O₃+TiO₂(<0.5%),high-field strength elements(HFSEs),transition metals(e.g.,Zn,Cr V,Sr,and Pb)and REEs in the Nabeba BIFs indicate insignificant detrital input.In addition,the high Ti,V,Ni,Cr,and Mg contents of the Nabeba magnetite can be ascribed to the involvement of external moderate-to high temperature hydrothermal fluids during their deposition in a relatively reduced environment.The Post-Archean Australian Shale(PAAS)-normalized REY patterns of the Nabeba BIF are characterized by LREE depleted partition pattern and varied degree of La and Y enrichments,and positive Eu anomalies.This suggests that the Nabeba magnetite has retained the initial geochemical signature of the primary Fe-oxyhydroxides,which precipitated on the paleoseafloor from fluids derived from the mixture of seawater and hightemperature(>250?)hydrothermal fluids.Based on microthermometric and laser Raman spectroscopic measurements of the fluid inclusions,the hydrothermal fluids responsible for the enrichment of hypogene iron ore were of two types,starting from H₂O-NaCl±CO₂ fluids which were high-to-moderate temperatures(203-405?)and varying salinities(low-tomoderate:1-15wt.%NaCl equiv.;high:30-35wt.%NaCl equiv.),which may have evolved into H₂O-NaCl fluids of moderate-to-low temperatures(150-290?)and salinities(1-11wt.%NaCl equiv.).This process involved reactions between the hydrothermal fluids and the magnetite BIF,and gave rise to the hypogene magnetite-rich ore.The analysis of the oxygen isotope compositions of quartz conducted on quartz vein samples associated with the hypogene ores from both stages,indicate that the decrease in the quartz ¹⁸O values is due to the interactions of among the magmatic ± metamorphic fluids,meteoric water,and the original BIF.This fluid mixing and the subsequent phase separation was likely responsible for the hypogene iron enrichment during the ore formation.