Oxygen and Hydrogen Isotopic Study of the Iron Ore Group (IOG), a Mid Archean Eastern Indian Greenstone Belt

The mid-Archean Iron Ore Group (IOG) in the type-area of the eastern Indian Singbhum-Orissa craton forms a NNE-SSW belt as an overturned synclinorium, sandwiched between the Singhbhum Granite to the east and the Bonai Granite to the west. The IOG sequence consists of lower basaltic-andesite followed successively upwards by lower shale, tuffs, banded iron ore, upper shale and upper lava. The metamorphic condition for the lower lava of the western limb is higher (lower amphibolite) than the lower lava of the eastern limb and the upper lava, where conditions range from zeolite to greenschist facies. Oxygen isotope analysis of 23 metaigneous rocks (0 ≤d¹⁸O≤ 10‰; mean 6.8‰) shows the lower lava and a single gabbro (-0.2‰) of the western limb (3.7 ≤d¹⁸O≤ 6.4‰; 7 lavas) to be depleted and the lower lava of the eastern limb (6.7 ≤d¹⁸O≤ 8.3‰; 6 samples) and the upper lava (6.9 ≤d¹⁸O≤ 10.1‰; 7 samples) are both enriched in ¹⁸O compared to the average mantle d¹⁸O value (5.7‰). The depletion in ¹⁸O of the western limb may be due to regional thermal metamorphism from the Bonai granite, and may not represent a complementary ¹⁸O-depleted oceanic crust as in the Phanerozoic. The d¹⁸O of clinopyroxene (1.6 ≤d¹⁸O ≤ 8‰) and feldspar (3.8 ≤d¹⁸O≤ 11.8‰) show a wide range of values. Assuming alteration temperature of 550°C, the calculated fluid d¹⁸O ranges between 3 to 11‰. This is either ¹⁸O-shifted ocean water or metamorphic water. Hydrogen isotopes of 20 samples (-80 ≤dD≤ -59‰) lie within the Phanerozoic dD ranges of similar rocks. Hydrogen and oxygen isotopic ratios show that the alteration of the greenstones was due to interaction with a modern seawater-type fluid, as seen in other Archean greenstone belts.