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Michael Komarov, All Russian Scientific Research Institute Economics of Mineral Resources and Subsoil Use (Russian Federation)
Yuri Kiperman, All Russian Scientific Research Institute Economics of Mineral Resources and Subsoil Use (Russian Federation)
Andrei Il'in, Institut of Geology (Russian Federation)
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Keywords: old, in situ and reworked phosphorites, trace elements Old or Ediacaran (540-630 Ma) phosphorites represent the giant epoch of phosphogenesis in the Earth history. The another giant epoch occurred in the Late Mesozoic-Recent. Mineralogically old phosphorites are dominated by francolite (carbonate florapatite) changing in some phosphate basins to carbonate-fluorhydroxyapatite. Lithologically they are usually granular (recycled) phosphorites but in situ structureless varieties are also present. The latter are characteristic for the Khubsugul basin in Mongolia.
About 200 samples of granular and structureless phosphorites representing both giant epochs were collected around the world in a framework of IGCP project Phosphorites and analyzed by ICP MC technics for trace element content. Granular phosphorites are up to 1-1.5 order richer in sum REE and Y and twice-thrice richer in Mo, Co, Ni, Cr, Ag. This is explained by corresponding difference in sea-water residence time. The conspicuous feature of old phosphorites as related to the younger one is their low U (4-7 ppm), Zn (20 ppm), Cd (2-3 ppm) content and low La/Yb ratio (5-6ppm). They are also appreciably poorer in Mo, Co, Ni, Cr, Ag. The low U content is believed to be caused by specific mode of the isomorphism called compensatory and expressed by low CO2/P2O5 and F/ P2O5 ratios.
Impoverishment in some heavy metals is probably related to their usage by living organism in a construction of metallo-enzymes?ferments catalyzing biochemical reactions. Participation of Zn, Mo, Ag and other enzymes in such catalyzing biochemical reactions is generally attributed to the Phanerozoic biota but not yet to the Ediacaran. Generally Ediacaran phosphorites are poor in many harmful trace-element as compared with Mezo Cenozoic ones. It makes them ecologically preferable.
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