Zonality of uranium mineralization in low-temperature sodic metasomatites (by the example of ore fields in Northern Kazakhstan)

The article highlights the mineral zonality at the Grachevskoe, Kosachinskoe, and Shokpak-Kamyshovskoe uranium ore fields (Northern Kazakhstan) localized in low-temperature sodic metasomatites. The setting of these fields at different structural stages and erosional levels suggests that they were formed at different depth levels by ore-forming systems of one and the same type. This opinion is corroborated by the occurrence of a set of mineral assemblages that make up a pervasive uniform sequence. The series of productive mineral assemblages at individual deposits and ore fields partly overlap; this makes it possible to restore a general succession of the ore-forming processes within a single ore system. The analysis of available data showed that the earlier mineral assemblages were formed at higher temperatures than the later ones. In all deposits, the quartz-hematite metasomatites and chlorite-uraninite mineralization, the final members of the sequence of mineral assemblages, serve as indicators of the frontal parts of ore fields, being formed by the mixing of juvenile and vadose waters. The comparison of spatial mineralogical models showed that the ore fields could be interpreted as fragments of a vertical zonal column of hydrothermal. metasomatic rocks with an extension of about 3.5 km. The zonal column is made up of the following successive mineral assemblages: (1) the earlier coffinite-albite-apatite assemblage at the lower level; (2) the chlorite-coffinite and chlorite-brannerite assemblages with brannerite-dispersoid at the middle part of the column; and (3) the quartz-hematite metasomatites with crystalline uranium titanate and uraninite at the upper level. The studied ore fields occur at different depth levels of paleohydrothermal systems of an open type. The analysis of zonality at individual ore fields and deposits suggests that the localization of productive mineral assemblages was governed by the structural openness of each individual vidual mineralized locality. Each productive assemblage was deposited within a certain range of the structural openness. This tendency was established for the entire hydrothermal system as well as for ore fields and individual ore bodies. The degree of openness of an individual mineralized locality is accounted for by a combined influence of three factors: the depth of mineral formation, the spatial position of such a locality within the paleohydrothermal system, and geological-structural conditions.