Effects Of Phosphatization On Composition Of Co-rich Crusts On Central Pacific Seamounts

Co-rich crusts are hydrogenous crustiform precipitates which grow on seamounts and enrich Mn,Fe,Co,Ni,Cu,Pt and REEs. Since 1980, they have been focused on by governments all over the world. Following ferromanganese nodules the Co-rich crusts have become a new important sort of oceanic mineral resources now.Central Pacific seamounts are mainly distribution areas of Co-rich crusts. The former researchers discovered that most Co-rich crusts in this area generally have been phosphatized. But the research reports are fewer related to effects of phosphatization on the geochemical and mineralolgical composition of Co-rich crusts. So it is of theoretical and practical significance to throughly and systemically research the effects of phospharization on composition of Co-rich crusts for our country surveying and finding high grade Co-crusts deposits in ocean.The Co-rich crusts studied in this paper were collected from Wake range, Marcus ridge and Central Pacific seamounts during "No.1 Dayang" DY105-12,14 cruise in 2003, China. By dividing the Co-crusts into different crust layers, synthetically using mineralogy, petrology, geochemistry method and various advanced test technology, the author has systemically studied the effects of phosphatization on composition of the Co-rich crust. The main achievements are as follow:1 Phosphatization has in certain degree changed the mineral composition of the Co-rich crusts. Non-phosphatized crust layers are mainly composed of vernadite; Phosphatized crust layers are mainly composed of vernadite, carbonate fluorapatite(CFA) and todorokite.2 The concentrations of usable elements in the Co-rich crusts have been changed by phosphatization. The Ni, Cu, Pt and rare earth elements are enriched and the Mn, Fe, Co are depleted in the phosphatized crust layers compared to the non-phosphatized crust layers. The enrichment of the elements such as Ni, Cu, Pt and rare earth elements genetically relates to fluorapatite(CFA). The depletion of the elements such as Mn, Fe, Co genetically relates to todorokite.3 The ratio and relationship between platinum group elements(GPEs) as well as Re-Os isotopes show that the PGEs in the Co-rich crusts mainly come from oceanic crust and mantle as well as the weathering products of PGEs-bearing rock on continent, and that cosmic dusts are not its main sources, the mechanism for platinum enrichment in the Co-rich crusts is deduced to be that the PtCl42- in the seawater was reduced to Pt metal by Mn2+ in the vicinity and interior of the oxygen minimum zone, and that the Pt metal was introduced from seawater and was enriched in Co-rich crusts.4 According to the study of the effects of phosphatization on the Co-rich crusts, the author analyzes the mechanism of forming Co-rich crusts and constructs a correspondent model in this paper..