Sedimentary characteristics based on sub-bottom profiling and the implications for mineralization of cobalt-rich ferromanganese crusts at Weijia Guyot, Western Pacific Ocean (2023)

Citation Excerpt :

Most guyot summits are covered by ferromanganese crusts, with or without areas of thin layer of sediments due to the influence of strong bottom current. By contrast, the summit of the Magellan guyots are covered by reef limestone overlain by calcareous pelagic sediments that combined measure up to 180 m thick on Caiwei guyot, with ferromanganese crusts occurring around the periphery of the summit platform (Zhao et al., 2020a; Zhao et al., 2020b). Many guyots, such as Caiwei and Weijia (Fig. 1), have collapsed flanks and landslide topography, manifested by amphitheater-shaped headwalls on the upper flank, linear debris ridges and troughs along the middle flank, and a debris apron covering the lower flank (Yang et al., 2020; Zhao et al., 2020a; Zhao et al., 2020b).

A reconnaissance survey of Fe–Mn crusts from the 5000km long (~31°S to 10°N) Ninetyeast Ridge (NER) in the Indian Ocean shows their widespread occurrence along the ridge as well as with water depth on the ridge flanks. The crusts are hydrogenetic based in growth rates and discrimination plots. Twenty samples from 12 crusts from 9 locations along the ridge were analyzed for chemical and mineralogical compositions, growth rates, and statistical relationships (Q-mode factor analysis, correlation coefficients) were calculated. The crusts collected are relatively thin (maximum 40mm), and those analyzed varied from 4mm to 32mm. However, crusts as thick as 80mm can be expected to occur based on the age of rocks that comprise the NER and the growth rates calculated here. Growth rates of the crusts increase to the north along the NER and with water depth. The increase to the north resulted from an increased supply of Mn from the oxygen minimum zone (OMZ) to depths below the OMZ combined with an increased supply of Fe at depth from the dissolution of biogenic carbonate and from deep-sourced hydrothermal Fe. These increased supplies of Fe increased growth rates of the deeper-water crusts along the entire NER. Because of the huge terrigenous (rivers, eolian, pyroclastic) and hydrothermal (three spreading centers) inputs to the Indian Ocean, and the history of primary productivity, Fe–Mn crust compositions vary from those analyzed from open-ocean locations in the Pacific.

The sources of detrital material in the crusts change along the NER and reflect, from north to south, the decreasing influence of the Ganga River system and volcanic arcs located to the east, with increasing influence of sediment derived from Australia to the south. In addition, weathering of NER basalt likely contributed to the aluminosilicate fraction of the crusts. The southernmost sample has a relatively large detrital component compared to other southern NER crust samples, which was probably derived predominantly from weathering of local volcanic outcrops.

Fe–Mn crusts from a dredge haul at 3412m water depth, 2°S latitude, are pervasively phosphatized along with the substrate rocks (site D7). Phosphatization took place through replacement of carbonate, preferential replacement of Fe oxyhydroxide relative to Mn oxide in the crusts, preferential replacement of silica-rich phases relative to Al-rich phases in the crusts, and precipitation of carbonate fluorapatite in pore space. The preferentially replaced silica may have been Si adsorbed on the Fe oxyhydroxide. The enrichment of Ni, Zn, and Cu in the phosphatized crust reflects preferential adsorption into the tunnel structure of todorokite. The rare earth element plus yttrium (REY) patterns indicate a lower oxidation potential during phosphatization of the NER crusts compared to Pacific phosphatized crusts. NER phosphatization occurred in a deeper-water environment than typical for phosphatization of Pacific crusts, occurred post-middle Miocene, a younger age than phosphatization the Pacific crusts, and had in part a different set of chemical changes produced by the phosphatization than did the Pacific crusts.

The southern third of NER has Fe–Mn crusts with the highest Co (0.91%), Ni (0.43%), ΣREY (0.33%), Cu (0.22%), Te (146ppm), Pt (1.5ppm), Ru (52ppb), and Rh (99ppb) contents. These are among the highest Pt, Ru, and Rh concentrations measured in marine Fe–Mn deposits. Because of these high metal concentrations, exploration is warranted for the southern sector of the NER, especially at shallower-water sites where the platinum group elements (PGE) and Co are likely to be even more enriched.

Polymetallic nodules and a single Fe-Mn crust from the Clarion and Clipperton Zone (CCZ) of the equatorial eastern Pacific have been analyzed to determine their chemical and mineralogical compositions. ICP-OES analyses of bulk nodules from the CCZ indicate that the nodules formed under oxic-diagenetic conditions with typical Mn/Fe ratios of 4–6, Ni+Cu values of 2–3wt.%, Co contents of 0.12–0.17wt.%. However, detail analyses of individual growth layers by electron microprobe analyses (EMPA) reveal much larger chemical heterogeneity of individual layers with Mn/Fe ratios ranging from <1 to >800. Two different genetic types of layers can be distinguished:

Layer type 1: dense layer with average Mn/Fe ratios of 1.80, average Ni+Cu of 0.81wt.%, Co contents of 0.30wt.% (hydrogenetic metal precipitation from oxic waters). These layers consist of Fe-vernadite (δ-MnO₂) epitaxially intergrown with feroxyhyte (δ-FeOOH) nanoparticles. Layer type 2: growth structures (dendritic growth structures and dense layers) with average Mn/Fe ratios of 96, Ni+Cu of 3.9wt.%, Co contents of 0.08wt.%, and a distinct Ni+Cu maximum of 6.51wt.% at a Mn/Fe ratio of 56 (suboxic-diagenetic, metal precipitation from suboxic pore waters). Type 2 layers mainly consist of turbostratic phyllomanganates such as 10Å vernadite, 7Å vernadite and todorokite are minor compounds.

Mixed layer type 3: These layers can occur as zones of low reflectivity in the transition from layer type 1 to 2 or build up inhomogeneous growth structures of nm-thin layers of low or high reflectivity. The Mn/Fe ratios of this material range between 3 and 11, Ni+Cu of 1–4.6wt.%, and Co contents are between 0.02 and 0.77wt.%. In most cases these growth structures represent a mixture of type 1 and 2 layers.

X-ray photoelectron spectroscopy (XPS) of the recent outermost nm layers of nodules (top, rim and bottom side) indicates hydrogenetic accretion processes under recent oxic conditions both from seawater and pore water. The mixed-type CCZ nodules are interpreted to consist of an oxic-hydrogenetic and a suboxic-diagenetic end member. Calculations of the proportions of the individual layer end members show that suboxic layers make up about 50–60% of the chemical inventory of the CCZ nodules whereas oxic-hydrogenetic layers comprise about 35–40%. The remaining part (5–10%) consists of incorporated sediment particles occurring along cracks and pores.

These results demonstrate that suboxic conditions alternate with oxic conditions during the growth of nodules in the eastern CCZ. This is probably due to fluctuating bioproductivity in the equatorial Pacific surface waters during glacial–interglacial periods which led to changing organic carbon flux to the sediment and changing oxygen consumption in near surface sediments. Furthermore, reduced ventilation of the deep ocean during glacial periods may have led to suboxic conditions in near-surface sediments.

Our investigations prove that bulk chemistry data of whole nodules only provides an average of individual layers that build up the nodules and is inappropriate for genetic interpretation.

This study examines Fe-Mn crusts that form on seamounts along the California continental-margin (CCM), within the United States 200 nautical mile exclusive economic zone. The study area extends from approximately 30° to 38° North latitudes and from 117° to 126° West longitudes. The area of study is a tectonically active northeast Pacific plate boundary region and is also part of the North Pacific Subtropical Gyre with currents dominated by the California Current System. Upwelling of nutrient-rich water results in high primary productivity that produces a pronounced oxygen minimum zone. Hydrogenetic Fe-Mn crusts forming along the CCM show distinct chemical and mineral compositions compared to open-ocean crusts. On average, CCM crusts contain more Fe relative to Mn than open-ocean Pacific crusts. The continental shelf and slope release both Fe and Mn under low-oxygen conditions. Silica is also enriched relative to Al compared to open-ocean crusts. This is due to the North Pacific silica plume and enrichment of Si along the path of deep-water circulation, resulting in Si enrichment in bottom and intermediate waters of the eastern Pacific.

The CCM Fe-Mn crusts have a higher percentage of birnessite than open-ocean crusts, reflecting lower dissolved seawater oxygen that results from the intense coastal upwelling and proximity to zones of continental slope pore-water anoxia. Carbonate fluorapatite (CFA) is not present and CCM crusts do not show evidence of phosphatization, even in the older sections. The mineralogy indicates a suboxic environment under which birnessite forms, but in which pH is not high enough to facilitate CFA deposition. Growth rates of CCM crusts generally increase with increasing water depth, likely due to deep-water Fe sources mobilized from reduced shelf and slope sediments.

Many elements of economic interest including Mn, Co, Ni, Cu, W, and Te have slightly or significantly lower concentrations in CCM crusts relative to crusts from the Pacific Prime Crust Zone and other open-ocean basins. However, concentrations of total rare earth elements and yttrium average only slightly lower contents and in the future may be a strategic resource for the U.S.