Rare metal resources in polymetallic nodules from the Eastern Equatorial Pacific Ocean
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The Clarion-Clipperton fracture zone (CCFZ) in the eastern equatorial Pacific is the area with the greatest interest in terms of metal resources in polymetallic nodules. The nodules are enriched (relative to crustal composition) in economically important metals such as Mn, Ni, Cu, Zn, and so-called critical metals such as Co, Rare Earth Elements and Y (REYs). This project involves the geochemical and mineralogical investigation of nodules from five box cores from the UK claim area in the CCFZ. Bulk geochemical analysis shows that the nodules formed from both hydrogenetic (precipitation from bottom oxic seawater) and diagenetic (precipitation from oxic/suboxic sediment porewaters) processes. Bulk analysis revealed high contents of Mn (29%), Ni (1.3%), Cu (1.0%), Co (0.17%) and REY (700 – 800 ppm). Analysis of (micro-drilled) individual layers in two nodule samples using ICP-MS shows that REYs are more enriched in the hydrogenetic layers (up to 1200 ppm) compared to the diagenetic layers (up to 600 ppm). Similarly, Co is also concentrated in the hydrogenetic layers, while Ni and Cu are enriched in the diagenetic layers. Mineralogical analysis using XRD shows the 10 Å and 7 Å phyllomanganates as the dominant mineral phases in the nodules. Sequential leaching experiments using dilute acetic acid, HCl, oxalic acid and concentrated nitric acid revealed that ~60% of REYs are hosted in the interlayer spacings of the phyllomanganates while less than 7% are hosted in the amorphous FeOOH phase compared to 30% of Co that is associated with the FeOOH phase. The association of the REYs with the phyllomangnates was further investigated using mineral synthesis approaches which shows that REY can occupy interlayer spacings. The Nd and Sr isotopic compositions of the individual growth layers of a nodule sample revealed that the hydrogenetic and diagenetic layers source their metals from different sources, with North American and east Asian dust the likely source for hydrogenetic layers while seamounts and oceanic basalt as the source for diagenetic layers.