Origin of Fe-Ti oxides in gabbro-magnetitites of the Mesoarchean Nuasahi Complex (India)

The Mesoarchean Nuasahi ultramafic-mafic complex in the Singhbhum Craton of eastern India comprises a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The rocks of the lower ultramafic unit are orthopyroxenite and serpentinized dunite with three interlayered chromitite bands. The gabbroic rocks are present on both the eastern and western sides of the ultramafic unit. The magnetitite orebodies are present as elongated bands at the upper part of the gabbroic unit on the east side of the igneous complex. Plagioclase in the eastern gabbro is andesine (An 32.3–47.8 ) to albite (An 1.0–3.0 ) with minor oligoclase (An 11.4 ), and clinopyroxene is diopside (En 27.9 Fs 24.6 Wo 43.9 ) to ferro-augite (En 16.8–38.3 Fs 35.4–54.8 Wo 25.5–27.8 ). The western gabbro has plagioclase (An 0.5–3.8, 40.2–46.5 ), clinopyroxene (En 32.9–33.9 Fs 20.8–24.2 Wo 41.7–43.6 ) and orthopyroxene (En 44.6–45.5 Fs 50.9–51.3 Wo 1.9–2.1 ). Fe-Ti oxide minerals are present as intercumulus disseminated phases within the gabbroic rocks and as a cumulus phase in the upper magnetitite orebodies. Contents of Cr 2 O 3 (up to 6.10 wt%), Al 2 O 3 (0.07–5.43 wt %) and NiO (up to 0.30 wt%) in the cumulus magnetite from the upper magnetitite orebodies are higher than in the accessory magnetites in the lower gabbroic rocks (Cr 2 O 3 up to 0.13 wt%; Al 2 O 3 = 0.08–3.69 wt%; NiO = up to 0.17 wt%). This suggests replenishment of the magma chamber by a new pulse of Fe-Ti-Cr-rich parental magma after the solidification of the gabbroic rocks, leading to oversaturation of the remaining magma with magnetite and the formation of the magnetitite layers. The presence of quartz, saussaritization of plagioclase, alteration of pyroxene to amphibole and chlorite, and alteration of magnetite to goethite suggest modification of primary igneous assemblages by hydrothermal activity and low-temperature weathering events. The presence of trellis intergrowth in magnetite near chlorite veins within the magnetitite ore also indicates hydrothermal alteration. The calculated temperatures ( T = 944°–349°C) and oxygen fugacity ( f O 2 = −41.0 to −11.4) of the co-existing magnetite/titanomagnetite and ilmenite minerals also indicate various stages of cooling history and re-equilibration from magmatic conditions to a low-temperature hydrothermal stage.