PGE and Ag mineralization in a breccia zone of the Precambrian Nuasahi ultramafic-mafic complex, Orissa, India

The Nuasahi ultramafic-mafic complex within an Archaean belt of supracrustal rocks in Orissa contains high concentrations of PGE (Pt < or = 9.6 ppm, Pd < or = 17.9 ppm) and Ag (< or = 6.6 ppm) in a chromiferous sulphide-rich breccia zone. The ultramafic unit is in fault contact with and overlain by gabbros whose stratigraphic top is characterized by a zone of brecciation. Magnesiochromite compositions in the lode chromite deposits are Cr (sub 2) O (sub 3) approximately 60, Al (sub 2) O (sub 3) approximately 12 and MgO approximately 15%. Sudburyite members of the michenerite-testibiopalladite series and irarsite-hollingworthite are the common PGM in the chromite-sulphide assemblage of the breccia zone. Sudburyite is preferentially included within coarse-grained ferrian chromite, along with chalcopyrite, pyrrhotite, rutile, ilmenite and oriented lamellae of clinochlore. The distinctive noble-metal composition (high Pd, Pt and Ag with high Cu) and dominant Pd-Bi-Te-Se-bearing PGM in the mineralized portion indicate its evolved nature. Mineralization developed via a hybrid magma resulting from partial incorporation of serpentinized chromite-bearing ultramafic rocks and chromitite in the younger gabbroic magma was emplaced within the pre-existing breccia zone.