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The chromite deposits of the Paleoarchean Bacuri Mafic-Ultramafic Complex in the Guyana Shield-Amazonian craton, Brazil, were recently identified as the oldest economic orthomagmatic mineralization deposits on Earth and provide an exceptional glimpse into early Earth processes. Despite its economic and geological significance, the knowledge about the Bacuri Complex is still limited. In this study, we report zircon U-Pb and Lu-Hf isotopic data for chromitite and metanorite from the chromite-mineralized layered sequence and present hallmark textural and microstructural evidence for the precipitation of zircon from late-stage, evolved mafic melt. The investigated samples consist of metamorphosed chromitite from the > 25 m-thick Main Chromitite Layer (3 samples) and metanorite of the Lower Mafic Zone (3 samples). We found that the mode of zircon is higher in metanorite than in chromitite samples, and zircon grains exhibit a range of morphologies, textures, grain sizes, and mineral inclusions, reflecting three zircon types investigated: (i) > 3400 Ma inherited zircon xenocrysts, (ii) magmatic zircon precipitated at c. 3340 Ma from late-stage, evolved melt, in pockets of the cumulate pile, and (iii) micrometric zircon neoblasts or very thin overgrowth rims formed from the break-down of Zr-bearing minerals during metamorphic events. Additionally, we show that xenocrystic and magmatic zircon types were modified by coupled dissolution-precipitation processes at varied degrees in the magmatic system and during younger tectonic and metamorphic events, resulting in a > 800 Myr span in near concordant spot ages. We hypothesize that xenocrystic zircon grains were entrained from the magmatic pathways or wall rocks of the mafic–ultramafic melts and suggest that crustal contamination was a major triggering factor in the genesis of the massive chromitite layers of the Bacuri Complex. This conclusion is supported by the positive and near chondritic εHf(3343 Ma) values (+2.2 – 0.0) obtained on magmatic zircon domains. The elevated Sc/Yb ratio (0.61 – 1.24) in magmatic zircon indicates an extensive fractionation of the interstitial melts in the Bacuri cumulate pile. In addition, the 0.82 U/Yb ratio indicates a plume-related origin for the mafic–ultramafic melts. We suggest these plume-related melts have mixed with crustal materials during continental rifting in within-plate settings, forming the Bacuri chromite deposits at ∼ 3340 Ma. Significantly, the U-Pb concordant ages on inherited cores (3563 ± 47 Ma to 3407 ± 38 Ma) overlap with the Hf-TDMC model ages of magmatic zircon domains, suggesting that they represent actual events of crust formation in the Paleoarchean of the Guyana Shield.
Spier et al. (Thu,) studied this question.