Geochemistry and genesis of Fe-Mn deposits, West Sewell Ridge, Andaman Sea

Introduction: The layer-wise morphology, mineralogy and geochemistry of Fe-Mn crusts and nodules from the West Sewell Ridge (WSR) in the Andaman Sea were analyzed. Materials and methods: We have investigated exploration and geochemical methodologies, statistical input, and cobalt chronometry to understand the genesis of Fe-Mn crusts and nodules. Results: The dominant hydrogenetic input during the inner layer development with a slower growth rate (GR) has enriched the trace elements in the inner layers, in comparison to those in the outer layers. The mineralogy and geochemistry of nodules and crusts reveal that these precipitates are dominantly hydrogenetic in origin; however, diagenesis also contributes to their growth, especially for the nodules. Cobalt chronometry showed that the heterogenetic growth of the crusts and nodules started from ~10 Ma and continued up to ~2.5 Ma with an average GR of ~7 mm/Ma. However, from 2.5 Ma to the present time, a higher diagenetic input and an increase in GR (~11 mm/Ma) were noticed due to the increased supply of sediments in the WSR. The hydrogenetic growth is more dominant in the inner layers, and is progressively altered to both hydrogenetic and diagenetic in the outer layers. Conclusions: We infer that the WSR crusts and nodules developed on the ridge top in a continental margin environment. A detailed internal morphological study establishes the presence of an intensified oxygenated deepwater flow which is responsible for the deposition of oxyhydroxides. Due to the increased GR and development in a continental margin environment with continental sediments supplied primarily from the continental margin of the Andaman Accretionary Prism, the bulk trace metal concentrations are depleted in the crusts and nodules in comparison to the open ocean crusts and nodules. However, WSR crust/nodules are economically viable due to the high contents of nickel and rare earth elements present in the inner layers.