Sequence and time of mineralization in certain hydrothermal uranium deposits

An examination of the mineral composition of ores as well as the isotopic composition of lead from galenites and that of uranium from pitchblendes in the ores of fields confined to the graphitized zones of large old linear faults in the edge portion of the hard crystalline bedding of a median mass has been made. 8 mineral complexes of different ages have been discriminated, corresponding to 8 mineralization stages. Horizontal mineralogical zoning has been established for the mass as a whole: prevalent in the central part are early mineral complexes, while in the edge portion, middle-aged and younger ones. Reverse zoning has been revealed within the fields, the amount of calcite-pitchblende ores decreases appreciably with depth, while the areal extent of chloritecoffinite and pitchblende-zeolite mineral complexes increases. The absolute age of uraniferrous minerals from different mineral associations has been established. Analysis of 206Pb/238U, 207Pb/235U and 206Pb/207Pb ratios indicated that the most accurately determined age, on the basis of the 207Pb/235U ratio for pitchblendes from calcite-pitchblende veins at the field edges is 276 million years. The isotopic composition of lead in galenite corresponds to the model age of 130 to 260 million years. The discriminated mineral complexes developed in three stages. The uraniferrous ores were developed during the last two stages, which took 230 to 270 million years