Fe-(oxyhydr)oxide participation in REE enrichment in early Cambrian phosphorites from South China: Evidence from in-situ geochemical analysis

Abstract

Marine phosphorites have shown extraordinary REE potential. Enrichment mechanisms have been investigated as yet, however, the roles of Fe/Mn (oxyhydr)oxides in REE enrichment were poorly known. In this paper, we report new data for early Cambrian phosphorites in South China with a particular focus on REE enrichment and Fe-redox cycling. Phosphorites from the Cambrian Gezhongwu occur in the upper and lower units. The phosphorites contain a large number of of Fe (oxyhydr)oxides, especially those in the upper unit. SEM and laser Raman show that Fe (oxyhydr)oxides have a gray rim and a bright core, attributed to transformation from hematite to goethite during the weathering. In-situ elemental analyses show Fe (oxyhydr)oxides have PAAS-normalized REE patterns depleted in LREE and enriched in HREE with negative to positive Eu anomalies and positive Y anomalies. However, two patterns are clearly distinguished according to Ce anomalies and these are related to the origins of Fe-(oxyhydr)oxides: hydrothermal Fe (oxyhydr)oxides with negative Ce anomalies and hydrogenic ones with positive Ce anomalies. The role of Fe (oxyhydr)oxides played in REE enrichment was controlled by Fe-redox pumping. In this process, Fe (oxyhydr)oxides adsorbed REEs in oxic water columns; after precipitating to suboxic/anoxic conditions, the absorbed REEs were released into porewater. Francolites subsequently formed and captured REEs during early diagenesis. Even though additional evidence is required, our investigations were significant in conforming the origins of Fe/Mn (oxyhydr)oxides and their role in REE enrichment in marine phosphorites.