GEOCHEMISTRY AND FORMATION STAGES OF ANOMALOUSLY COBALT-RICH FERROMANGANESE CRUST ON THE MIOCENE PETIT-SPOT VOLCANIC ROCKS OF ALBA GUYOT (MAGELLAN SEAMOUNTS, PACIFIC OCEAN)

The index-species of calcareous nannoplankton in two cored wells, penetrating the Miocene rocks of the Alba Guyot, have been used to identify time intervals for deposition of the young (Pleistocene-to-modern, 7–13 Ma) basanite tuff, tuffite sublayer (18–24 Ma). There have been discovered young Fe-Mn crust sublayers of contrasting composition and growth rate: early, enriched in Mn, Ni, Cu, Nb, and late, substantially ferruginous, with high REE, Y, Be, Sc, V, Zn, Rb, Cs, Sr, Zr, Hf, Mo, Sb, Pb, Th and U contents. Disregarding the duration of a probable break in the deposition of ore matter (Fe-Mn oxyhydroxides), the young crust with a Co-chronometry-based thickness of ~4 mm was formed over a period of ~2.3 Ma with an average growth rate from 1.6 mm/Ma in the early sublayer to 2.2 mm/Ma in the late sublayer. A young Fe-Mn crust 4–8 mm thick began to form on Miocene rocks 4.5 to 2.3 Myr, when the guyot surface was at depths below the oxygen minimum zone in the ocean water column (>600–700 m). A contrasting change in the composition of the late sublayer (a significant increase in Fe, REE, Y, V, Zn, Sr, Zr, Hf, Mo, Sb, Pb, Th and U contents) could have been caused by a 500000-year-old rapid subsidence of the guyot to depths similar to present-day (1250–1500 m at the plateau level). The average Co concentration (0.8–1.0 wt. %) in the young Fe-Mn crust on Miocene rocks is almost twice that of the bulk composition of thick (multilayered) crusts in other parts of the guyot. Relatively small-scale distribution of the young Fe-Mn crust, anomalously enriched in Co, will apparently have an insignificant impact on the increase in inferred ore reserves on the Alba Guyot for this strategically important metal.