Research On Geochemistry And Mineralogy Of Co-rich Crust From The South West Indian Ridge

The Co-rich crust has been regarded as one of the most potential submarinemineral resources in both exploitation and scientific research for their abundance ofmetal and slow accumulation. Eight Co-rich crusts collected from the South WestIndian Ridge were analysed for systematic understanding of their mineralogy andgeochemistry composition. The XRD, ICP-MS （AES）and microprobe chemistrymeasurement were taken to determine the composition of their mineral, bulk andleaching distribution of major, trace and rare earth elements and the elementevolutions on the growth sections. By means of mathematical methods such asprincipal component analysis （PCA） and Empirical Mode Decomposition （EMD）, weinferred the genetic classification, componet resource and paleo-oceanographyinformation of the crusts based on the mineral and element data.The Co-rich crust in this study can be described as cauliflower shape ornipple-like shape shell with thickness of1～3mm. The microscope observationexhibits several kinds of microstructure, including parallel structure， columnarstructure, stromatolithic structure and taxitic structure. The co-existing of obviouslydifferent structures in the same crust may reflect the variation of hydrodynamiccondition in the local area.The mineral data show that the Vernadite consist the main phase while thetodorokite is much more less, except for two samples in which the todorokite takesthe visible component of manganese oxide. In view of the different formationcondition of those two, it’s reasonable to infer the exsistence of the sub-oxidationenvironment in some limitary region. The most abundant phase of the ferric mineral isX-ray amorphous FeOOH while weakly Goethite peak can be identified. Otherminerals such as quartz, plagioclase, clay and calcite exsist throughout all thesamples.The result of the bulk sample chemical analysis shows the value of Mn/Fe varies from0.802～2.340, with the average of1.247which suggests hydro-genesis. Thecharacteristic of detritus supply in this study is similar to the continental margin areaand differ from ocean seamount compared to crusts from other areas. As for the traceelement, the crust exhibit high concentration of Pb and Mo under the control of ferricmineral. The average concentration of REE is1261×10^-6and the separation of theLREE and HREE reaches up to8.3, presenting right-leaning in the partition curve.The positive δCe anomaly varies from1.031～1.998, with opposite to the negativeone in seawater, from which we can tell that the REE come from the oxydic seawater.The steady and lightly positive anomaly of δGd and δHo can also be observed in ourstudy.The element leaching results suggest the most of trace element absorption（except for Co and Ni）was largely controlled by the Fe-oxyhydroxide fraction, whilethe REE have a multi-phase resourse of which50%～60%comes from the Mn-oxideand Fe-oxyhydroxide fractions. The Fe-oxyhydroxide fraction riches in HREE whilethe Mn-oxide fraction riches in the light ones, and the positive anomaly of δCe, δGdand δHo is mainly depend on the latter. The elements can be classified based on theleaching results as follows:（1）Adsorbed cations（easily leachable） and carbonate fraction: Ca, Mg, Sr, U,Rb, Tl, Cd, Be;（2）Mn-oxide fraction: Mn, Co, Ni, Ba, Tl, Ga, La, Ce;（3）Fe-oxyhydroxide fraction: Fe, Ti, Mo, Al, As, Zr, Hf, W, Pb, Bi, Th, B, V,HREE;（4）Residual fraction: Al, Fe, Rb;The Co-rich crusts in our study were classified into hydrogenetic origin andfeatured by abundance of Vernadite and positive anomaly of δCe. The elementcomposition is controlled orderly by metallogenic adsorption of the Fe-oxyhydroxidefraction, Mn-oxide fraction, and physical mix of biogenic or continental debris. Thehydrothermal effect from local didn’t show up in this study.The Mn-Fe chronometer growth rates of the two targeted crusts were1.129mm/Ma and1.921mm/Ma based on the element contents from the microprobe analysis. Furthermore, the age of their accumulation were calculated to773.4Ka and650.4Ka, going back to mid-late Pleistocene.The distinct periodic fluctuations of the Al、Ca、Fe、Mn、P content records wereexihibited by the EMD analysis. The period of13～19Kyr、30～46Kyr、70～86Kyrand118～144Kyr correspond to the changes of earth’s orbital parameters meanwhilethe element records partly relate to the glacial-interglacial cycles. The reseach indicatethat the cumulation and evolution of Co-rich crust is a complex result of the oceanenvironmental change and atmospheric input variation triggered by the earth orbitalteration and climate cycle.