REE ENRICHMENT AND DEPOSITION CONDITION OF PHANEROZOIC STRATIFORM FERROMANGANESE DEPOSITS IN JAPAN

To investigate candidacy for future heavy rare earth production, 69 ore samples from 54 of stratiform manganese and ferromanganese deposits within Phanerozoic accretionary complexes in Japan were examined. The manganese deposits mainly occur within chert and/or shale layers, and are composed of braunite, hausmannite, tephroite, rhodochrosite or rhodonite. The ferromanganese deposits, which are composed of hematite and bementite, occur between basalts and chert layers. The average ∑REE contents of the manganese ores are 61 ppm with 3 ppm of Dy. In contrast, average ∑REE contents of the ferromanganese ores are 1800 ppm with 80 ppm of Dy. Some ferromanganese ores, which have over 2200 ppm of ∑REE, contain a REE vanadate mineral associated with bementite. PAAS (post-Achaean average shale)-normalized REE patterns of these ores show that the manganese ores are depleted in LREE (La_N/Yb_N = ~0.6), while the ferromanganese ores are relatively enriched in HREE and show negative Ce anomalies (La_N/Yb_N = 0.8, Ce* = 0.24). The results indicate that the ferromanganese deposits contain elevated REE values relative to the manganese deposits. As decreasing Fe₂O₃ contents of the ferromanganese ores from 70 wt.% to 20 wt.%, the REE contents increase (24 – 110 ppm of Dy), and transitional elements such as V and Cu and Ce* values decrease. The REE, V and Cu contents are low in the manganese ores that have less than 20 wt.% of Fe₂O₃. These suggest that the pH increase and oxidation of hydrothermal solution by mixing with seawater triggered REE precipitation prior to Mn precipitation due to scavenge of REE on iron hydroxide. These ferromanganese ores are higher in HREE contents than those of the "ion-adsorption" type deposits (weathered granite) in southern China, which is the present major HREE producer.