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Oxygen and Hydrogen Isotope Study of the Iron Ore Group (IOG): Constraints on Fluid-Rock Interaction

Sandeep Banerjee1, Robert T. Gregory1, and Asish R. Basu2
1Huffington Department of Earth Sciences, Southern Methodist University, 3225 Daniel Avenue, Dallas, TX 75275
2Department of Earth and Environmental Sciences, University of Rochester, 227 Hutchison Hall, Rochester, NY 14627

The Iron Ore Group (IOG) of Singbhum-Orissa craton in eastern India is a mid-Archean greenstone belt with NNE-SSW trending asymmetric synclinorium, sandwiched between the Singbhum Granite Complex to the east and the Bonai Granite to the west. Oxygen isotope analysis of 23 meta-igneous rocks (0 ≤δ18O ≤ 10 ‰, mean 6.8‰) shows the lower lava and a single gabbro (-0.2‰) of the western limb (3.7 ≤δ18O ≤ 6.4‰; 7 lavas) to be depleted . The lower lava of the eastern limb (6.7 ≤δ18O ≤ 8.3‰; 6 samples) and the upper lava (6.9 ≤δ18O ≤ 10.1‰; 7 samples) are both enriched in 18O compared to the average mantle δ18O 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 δ18O of clinopyroxene (1.6 ≤δ18O ≤ 8‰) and feldspar (3.8 ≤ δ18O ≤ 11.8‰) show a wide range of values. Assuming an average mantle δ18O of 5.7‰ and alteration temperature of 550°C, the calculated fluid δ18O ranges between 3 to 11‰. Hydrogen isotopes of 22 samples (- 80 ≤δD ≤ -59‰, except for two values of -94.3 and -114.6‰) lie within the Phanerozoic δD 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 the Abitibi, Pilbara and Barberton greenstone belts.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas