STABLE ISOTOPIC STUDIES OF THE CHROMITE- FE-CU-NI-SULFIDE AND PGE-MINERALIZED ARCHEAN NUASAHI ULTRAMAFIC-MAFIC COMPLEX, ORISSA, INDIA

The 3.2Ga old Nuasahi ultramafic-mafic complex of the Eastern Indian Shield occurs within an Archean terrain of the Iron Ore Group of metasedimentary and metavolcanic rocks. The serpentinized ultramafic component (~5km long, 0.4km wide) of the complex trends NNW with a high angle easterly dip, and contains fragmented lodes of chromitite. The gabbroic component is present in both the western and eastern side of the ultramafic component and shows intrusive relation with the former. A breccia zone, developed in the eastern part of the ultramafic component, is characterized by a heterogeneous occurrence of serpentinized ultramafic and chromitite fragments, embedded in a altered gabbroic matrix containing considerable amounts of ferritchromite and variable amounts of base metal sulfides. High PGE concentrations are found in the chromitite-sulfide assemblages and in the sulfide bearing gabbroic matrix. The stable isotopic compositions have been determined for sulfide rich and sulfide poor samples of the breccia zone as well as for ultramafic and gabbroic rocks of the complex. In general, the d³⁴S values for sulfide minerals from the breccia zone and from the gabbroic rocks in the eastern part are similar and close to an accepted mantle value of 0‰, and show no evidence for the involvement of crustally derived sulfur. Elevated whole rock d³⁴S values in the serpentinized dunite (2.9 to 5.0‰), in gabbroic rocks (2.3 to 3.6‰), and in secondary sulfide minerals in the breccia zone, indicate possible isotopic exchange with fluids during low temperature hydrothermal alteration. The d¹³C value for low temperature carbon in sulfide-rich samples of the breccia zone is close to -15‰, suggesting possible organic or biogenic carbon contribution. The d¹⁸O values for ferritchromite and kämmererite from the chromitite-sulfide assemblage are lower (0.8 to 3.7‰) than typical mantle values, which is attributed to isotopic exchange during hydrothermal alteration. The dD values of kämmererite from the same assemblage are –53 to –63‰, and similar to that of the gabbroic component and indicate that the fluids involved are of magmatic origin. The elevated and anomalous bulk rock dD values of sulfide-rich samples in the breccia zone (–65 to –117‰), and in serpentinized dunite (–67 to -101‰) indicate a contribution from non-magmatic fluids as well.