Age of the Baula PGE mineralization (Orissa, India) and its implications concerning the Singhbhum Archaean nucleus

Abstract

Two types of platinum group element (PGE) mineralization are identified in the mafic facies of the Baula Complex intruding the Gabbro-Anorthosite Unit of the Singhbhum Archaean nucleus. The first, Pt-dominated, is magmatic - the PGM are included in the magmatic silicates of a gabbro-norite intrusion (the Bangur Gabbro) cutting the Complex's ultramafic facies and are found preferentially where the gabbro incorporates clasts of dunite and chromitite near the gabbro/ultramafics contact. The second, Pd-dominated, is hydrothermal - the PGM are associated with hydrous silicates located in the gabbro matrix of a magmatic breccia in which the blocks are from the Baula ultramafic facies. The originating fluids for the second type of mineralization were exsolved from the magma and concentrated PGE and volatile elements; we see no evidence of remobilization or external fluid contamination. A structural study has demonstrated that (a) the breccia was provoked by the intrusion of the upper part of the Bangur Gabbro, and (b) the breccia matrix corresponds to the Bangur Gabbro. Consequently, both types of PGE mineralization are associated with the same magmatic event. A zircon age of 3122 ± 5 Ma has been obtained for the main Bangur Gabbro intrusion, and similar ages (3123 ± 7 and 3119 ± 6 Ma) have been obtained on the PGE-mineralized gabbro matrix of the breccia. This supports the hypothesis of a genetic link between the Bangur Gabbro with its Pt-dominated PGE and the breccia with its Pd-dominated PGE, giving a mean weighted age of 3121 ± 3 Ma for the mineralization. Sm-Nd data on eight gabbro samples of the Baula Complex yielded a similar age of 3205 ± 280 Ma. The dating of these events places them close to the broad-scale stabilization of the Singhbhum Archaean nucleus, estimated at 3.1 Ga, and defines one of the oldest PGE mineralizing events ever recorded. The Bangur Gabbro shows many affinities with the Gabbro-Anorthosite Unit of the Singhbhum Archaean nucleus in terms of age of emplacement and isotopic composition. It may be, therefore, that the Bangur Gabbro actually forms part of the Gabbro-Anorthosite Unit and that a fundamental control of its two types of PGE mineralization is the presence of ultramafic clasts.