Mineralogy and geochemistry of ferromanganese crusts from Johnston Island EEZ

A total of 1360 kg of ferromanganese crust bearing samples were collected from 10 sites in the Karin Seamount Range and Kell Ridge of Johnston Island exclusive economic zone on a recent cruise of University of Hawaii research vessel Kaimikai-O-Kanaloa. Sixty eight ferromanganese crust samples and subsamples were analyzed mineralogically and chemically for 36 elements. The crusts are dominantly composed of vernadite (manganese dioxide). They are rich in Co (0.88%) and rare earth elements and poor in Cu (0.1%). They show a significant Ce anomaly and have a low Mn/Fe ratio. This is characteristic of marine hydrogenous Mn-Fe oxides. The rare earth element content of Johnston Island ferromanganese crusts is very close to the grades of land-based rare earth element deposits currently mined in China. There are two growth styles of crust. The first is as a spherical crust around loose debris. The second is as a sheet-like crust on the outcropping rock of the seamount. There are chemical and mineralogical differences between these two growth styles. The spherical crusts exhibit a smooth surface structure in the upper Mn-Fe layers. This upper area is chemically richer in Mn, Co, Ni, Ca and P than the lower part of the crust which is richer in Si and Al. The lower part has a rough surface structure. The sheet-like crust growth style exhibits a Co content which increases from the substrate to the crust surface. The compositional variation is mainly controlled by reaction between substrate and seawater. R-mode factor analysis was performed on the samples. Eighty percent of variance can be described by four factors. Factor 1, the trivalent rare earth element factor, with 36% of the variance is assumed to represent hydrogenous mineralization of Mn-Fe oxides, particularly incorporating rare earths. Factor 2 with 23% of the variance has a strong positive lending associated with Mn, Fe, Co, Ti and a strong negative loading associated with Ca and P. It is also a hydrogenetic meter and indicates an inverse relationship between ferromanganese oxides and calcium phosphate growth. Factor 3 with 13% of the variance has negative loadings on Ni, Cu, Cd, Mg and Zn. It may be a submarine weathering factor or indicate a hydrothermal input. Factor 4 with 7% of variance has strong positive loading for Si and Al. It is an aluminosilicate detrital mixing factor. Apart from scientific interest, the flat topography and high cobalt and rare earth content of this area make it an excellent candidate for potential mining in the next century.