Mineralogy, geochemistry and genesis of the polymetallic crusts from the Chagos-Laccadive Ridge, Arabian Sea, India.

Recent offshore exploration by the Geological Survey of India (GSI) located ferromanganese crusts (Fe Mn) along the Chagos-Laccadive Ridge (CLR) offshore western continental margin of India. CLR Fe Mn crusts grew on phosphorite substrate rock (phosphorite cemented sand/silt with moderate carbonate fluorapatite (CFA) cement and goethite). Phosphorite precipitation is the product of intense upwelling and phosphorus regeneration due to organic matter degradation and fish debris dissolution. CLR Fe Mn crusts are of hydrogenetic origin precipitated in an oxic environment. As a result of hydrogenetic processes under oxic conditions, Fe-vernadite dominated the mineral phase of Fe Mn crusts. CLR Fe Mn crusts formed when the North Indian deep-water layer mixed with oxygen-depleted waters from the Arabian Sea, Persian Gulf, and Red Sea. Three genetic layers are identified in the Fe Mn crusts; Type-I, Type-II, and Type-III developed in response to fluctuation in dissolved oxygen during periods of intensified suboxic bottom waters and monsoon maxima associated with past climatic events in the Arabian Sea. Type-I layers are columnar showing low Mn/Fe ratio (∼1), reflectivity, Mg (average 1.24%), and Ni (average 0.29%). Dendritic Type-II layers show high Mn/Fe ratios (average 12.29), reflectivity, Mg (average 3.51%) and Ni (average 1.69%). Type-III layers show very low Mn/Fe ratios (average 0.21), reflectivity, Mg (average 1.4%) and Ni (average 0.01%) and contain a substantial amount of terrigenous debris of aeolian origin. Elements of economic interest such as Mn, Co, Ni, Cu, Mo, Li, Te, and REY in CLR Fe Mn crusts are depleted owing to terrigenous contribution. • New occurrences of Fe Mn crusts found along Chagos-Laccadive Ridge. • Hydrogenetic Fe Mn crusts formed on phosphatized siltstone substrate. • Fe Mn crusts formed in response to mixing of North Indian deep water with Arabian Sea, Persian Gulf, and Red Sea waters. • Formation of phosphatized siltstone was triggered by intense upwelling and regeneration of phosphorus. • The elements of economic interest are depleted in Laccadive Fe Mn crusts. [ABSTRACT FROM AUTHOR]