Re–Os and Sm–Nd Isotope and Trace Element Constraints on the Origin of the Chromite Deposit of the Ipueira–Medrado Sill, Bahia, Brazil

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The chromite deposit of the Paleoproterozoic Ipueira–Medrado sill is hosted in a single, thick (5–8 m), massive layer, which sets severe constraints for the origin of chromitites. It is divided from bottom to top into: (1) a Marginal Zone (5–20 m); (2) an Ultramafic Zone (<250 m) consisting of dunite and harzburgite that host the chromitite layer, in which intercumulus amphibole is important and more abundant toward the top; (3) a Mafic Zone (<40 m). The parental magma was large ion lithophile element and light rare earth element enriched and high field strength element depleted. Sm–Nd isotopic compositions are consistent with a 2 Ga age, but suggest a variable initial Nd isotopic composition that correlates with the abundance of amphibole. The more negative εNd (mean –6·5) of the amphibole-rich intervals argues for crustal contamination, although the εNd (mean –4·4) of the amphibole-free samples suggests an old, enriched, subcontinental lithospheric mantle source. Chromite separates have initial γOs values that range from –4·6 to +3. The negative γOs values are typical of old, Re-depleted, lithospheric peridotitic mantle and give Re-depletion model ages of up to 2·75 Ga. An integrated assessment suggests that the very high-Mg parental magma probably originated from Archean, subcontinental, metasomatized, peridotitic lithospheric mantle and was subsequently contaminated with up to 30% of crust, which triggered the chromitite crystallization.

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