Mineralogical and geochemical features of Co-rich crust on Caiwei Guyot, Northwest Pacific Ocean

Co-rich crusts are a kind of marine mineral resources rich in Mn, Co, Ni and rare earth elements (lanthanide and yttrium, abbreviated as REY). The Co-rich crust sample studied in this paper was collected on the mountaintop edge of Caiwei Guyot in the Northwest Pacific Ocean onboard research vessel “Kexue” (Science) with a TV grab during the HOBAB5 cruise expedition in 2018. The microstructure, mineralogy, and geochemistry of the Co-rich crust were analyzed by scanning electron microscope, X-ray diffraction (XRD), inductively coupled plasma atomic emission spectrometer (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS), and its genetic type and formation mechanism were discussed. The textures of the Co-rich crust could be divided into yellowish loose layer (C8-5), black dense ferromanganese layers (C8-2, C8-3 and C8-4), and rough surface in black botryoidal shape (C8-1) from inside to outside. The yellowish loose layer is composed of Fe-vernadite, quartz, albite, anorthite, todorokite, and phillipsite. It has high porosity with a high content of Al and low content of Mn. The black dense ferromanganese layers have low porosity and are mainly columnar in structure, and the main component minerals are Fe-vernadite, quartz, albite, and anorthite. Compared with C8-5, the content of Al decreases but the content of Mn increases, indicating that the supply of terrigenous materials gradually decreased. In the later growth process of the Co-rich crust, its microstructure changed mostly from columnar structure to mottled structure, and the transition area is in layered structure and composed of ferromanganese oxides and Si-rich clastic materials. The Mn/Fe ratios of layers of the Co-rich crust range from 1.16 to 1.85, and each layer shows positive Ce anomaly and negative Y anomaly. The above characteristics indicate that the Co-rich crust in the study area shows its hydrogenetic origin and is not affected by hydrothermal activities, whose metal elements are derived from oxidizing seawater. According to the chronological data, we conclude that the growth process of the Co-rich crust was controlled by Pacific Deep Water from the late Oligocene to the middle Pliocene. The continuous increases of Co/(Fe+Mn) and Co/(Ni+Cu) indicate that the Co-rich crust has been growing in a highly oxidizing marine environment. Compared with other oceans and seas, the Co-rich crust on Caiwei Guyot is enriched in Co, Ni and REY, and shall has very high economic value and mining prosperity.