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

Sandeep Banerjee¹, Robert T. Gregory¹, and Asish R. Basu^2
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