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Contrôles géologiques, structuraux, minéralogiques et géochimiques sur la formation des minerais riches dans le district minéralisé de Streltsovsky, Russie

Abstract : The ambiguity in the genetic interpretations in the formation of the Mo&U deposits of the Streltsovsky ore field led us to perform additional studies which have established that closely associated U and F were progressively gained in the Late Mesozoic volcanic rocks from the older basic (170 Ma) to the younger silicic ones (140 Ma). The Early Cretaceous postmagmatic hydrothermal epoch (140-125 Ma) is subdivided into preore, U ore, and first and second postore stages. The primary brannerite-pitchblende ore was formed associated with fluorite. During the first postore stage, it was replaced by a U-Si metagel, previously identified as coffinite. The preore metasomatic alteration and related veined mineralization were formed under the effect of Na (bicarbonate) chloride solution at temperatures of 250-200°C. Uranium ore formation began with albitization and hematization of rocks affected by a supercritical fluid at 530-500°C; brannerite and pitchblende precipitated at 350?300°C. The chondrite-normalized REE patterns of pitchblende hosted in trachybasalt, trachydacite, and granite demonstrate a pronounced Sm-Nd discontinuity and a statistically significant tetrad effect of W type. These attributes were not established in REE patterns of rhyolites derived from the upper crustal magma chamber. These data and the gap of 5 Ma between silicic volcanism and ore formation do not allow us to suggest that U was derived from this magma chamber. According to the new proposed model, an evolved silicic Li-F magma was a source of U. U4+, together with REE were fractionated into the fluids as complex fluoride compounds. The U mineralization was deposited at a temperature barrier.
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Alexei Aleshin. Contrôles géologiques, structuraux, minéralogiques et géochimiques sur la formation des minerais riches dans le district minéralisé de Streltsovsky, Russie. Earth Sciences. Université Henri Poincaré - Nancy 1, 2008. English. ⟨NNT : 2008NAN10152⟩. ⟨tel-01746149⟩

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