Placers In A Deep-Water Environment-Myth Or Reality?

Typical coastal placers leave no doubt that placer accumulation can result in significant concentration of heavy minerals in marine shallow water environment. However, the same situation is quite predictable for guyot?s slopes at stages of submarine volcano formation and later submersion of guyots to a depth of up to 3-4 km. Where these placers could be found is also clear ? in coastal facies (as typical coastal placers), and in morphological traps down guyot slopes in zones of active submarine currents. The latter could be cracks in rocks, shallow depressions, spurs, and structures, such as coral reefs, stromatholiths, and even ferromanganese crusts. The last of these have been studied carefully by the author in the Magellan Seamounts and on the Mid-Pacific Rise: tens of minerals, including mineral forms of PGE, REE, Mo, W, Te, Cr, Co, Ni, Cu, &Zn were identified as a xenogenic (trash) minerals. The detailed study of polished epoxydized sections, showed that interstitial spaces of Fe-Mn botryoids are enriched in heavy minerals (predominantly titanomagnetite). Taking into account the irregular character of crust surfaces and their position in active water environment, we can definitely state that they are the ideal traps for placer minerals. Study of guyot basaltic matter showed the presence of similar REE, PGE minerals that are in the crusts, and the same (mostly) micron sizes of minerals. High content of PGE in some basaltic varieties (up to 1 ppm) suggests that this is a reasonable source of these elements in the crusts, rather than the water column (which is normally considered to be the case by most marine scientists). Predominance of Pt in hydrogenous (?) crusts is interpreted as due to the better stability of Pt mineral forms during weathering and transportation (these are isoferroplatinum, and silicides). Comparison of 143Nd/144Nd values in bulk crusts, heavy fraction of crusts, basalts, and water column supports the author?s idea, that a significant part of REE also comes from basaltic matter (in form of monazite, CeO2 etc.).