Hydrothermal and Hydrogenous Ferro-Manganese Deposits: Do They form a Continuum? The Rare Earth Element Evidence

Abstract

Ferro-manganese deposits in the deep-sea are not all likely to have just formed by either hydrothermal processes or hydrogenous ones. Some will contain material contributed by both these processes so that such deposits probably form a continuum.In this paper the rare earth element (REE) evidence for this supposition is appraised and the REE contents of marine ferro-manganese deposits are discussed.The hydrothermal and hydrogenous “endmembers” of the hypothetical continuum generally have distinct REE contents: the former being relatively depleted in cerium and the latter enriched.Ferro-manganese deposits with REE contents derived by both hydrogenous and hydrothermal processes apparently exist, but only those with predominantly “hydrothermal REE” can be easily recognised. This is because hydrogenous deposits generally contain higher REE abundances than hydrothermal ones so that even a small “hydrogenous REE” content swamps the “hydrothermal REE”. In the geologic record it should be possible to distinguish between ferro-manganese deposits formed by these different processes and ones formed in shallow water largely as a result of diagenesis. Hydrogenous deposits with apparently atypical REE contents (e.g.those of the Red Sea and Santorini) may also be identified.