Mineralogy And Geochemistry Of Ferromanganese Crusts From The CM4 Guyot,Caroline Ridge In The Western Pacific

In August 2017,the Institute of Oceanology of the Chinese Academy of Sciences conducted a multidisciplinary comprehensive survey of the Caroline ridge CM4 guyot in the low latitude of the Western Pacific,founding that a large number of ferromanganese crusts are distributed on the mountain surface.In the study area,types of ferromanganese crusts are single,which are basically in the shape of single-thin layer with the thickness of less than 1 mm.The substrate attached by ferromanganese crusts is carbonatite,most of which is hard limestone and dolomite,a little of which is hermatypic coral.Nine ferromanganese crusts collected from Caroline ridge CM4 guyot have been studied for Mineralogy and geochemistry.By using XRD,ICP-OES,ICP-MS and EPMA,mineral composition and contents of major elements and trace elements of samples were analyzed,and the geochemical characteristics and genesis were preliminarily discussed.Based on macro and micro analyses,the resources potential of Caroline ridge CM4 guyot ferromanganese crusts was evaluated.Mineralogical analysis shows that ferromanganese crusts are mainly composed of manganese mineral phase,iron mineral phase,detrital mineral phase and biogenic mineral phase.Manganese minerals are dominated by vernadite with a small amount of todorokite and birnessite.The crystallinity of iron minerals is low,and some minerals are difficult to be recognized.The iron minerals that can be identified are goethite and lepidocrocite.Other minerals include quartz and calcite.According to the thickness of the ferromanganese crusts,samples are divided into two categories:(1)thickness < 0.5 mm;(2)thickness about 1 mm.Compared with samples with the thin thickness,contents of elements,such as Ca,Mg,Mn,Fe and other elements in the carbonatite with the large thickness are relatively higher,while contents of Sr,U and B are relatively lower.Element analysis shows that the average concentration of Mn,Fe,Co,Ni of the samples is 24.24 %,15.14 %,0.16 %,0.34 %,respectively.Compared with these of ferromanganese crusts in Pacific,Atlantic and Indian Ocean,Cu contents of this seamount samples are very low,with the average value is only 0.01 %,which may be related to the low dissolved Cu contents or the majority of Cu in organic form in the seawater of the area.Compared with these around the world,?REE contents of CM4 guyot samples are relatively lower.?REE contents range from 925 to 1511 ?g/g.?LREE contents range from 826 to 1314 ?g/g.LREE is generally enriched.According to the north American shale–normalized,CM4 guyot samples show a relatively flat REE distribution model,with obvious positive Ce anomalies,negative Y anomalies and positive Ho anomalies.Electron probe microanalysis shows that metal elements contents in the carbonatite are very low.On the other hand,Mn,Fe,Co,Ni contents of ferromanganese crusts reduce from the surface to the bottom.Therefore,carbonatite substrate does not directly provide ore-forming elements for ferromanganese crusts.Mineral composition,element ratios and element groups indicate that samples are hydrogenetic,without obvious diagenesis.The element analysis and electron probe analysis of ferromanganese crusts and substrate indicate that carbonatite does not directly provide ore-forming elements for ferromanganese crusts.The overlying seawater is the direct source of contents of ferromanganese crusts.Hydrothermal activity is not found near the CM4 guyot.Therefore,the main sources of ore-forming elements of ferromanganese crusts are the continental sources of river,wind-sand input or elements produced by water-rock reactions in the seamounts.The ore-forming elements contents of CM4 guyot samples are equivalent to these of ferromanganese crusts in the global oceans,but the thickness of ferromanganese crusts is smaller.Due to the small number of stations and the lack of drilling data for seamounts,the resources potential cannot be estimated for the time being.