Apofenitic albitites of the Ukrainian Shield
Authors: Kryvdik S.G.
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Apofenitic albitites were discovered in the Azov Sea area (the Chernigivka carbonatite massif, the fenites of Dmitrovka village, Tunikova ravine), in the western part of the Ukrainian shield (Proskurivska massif of alkaline rocks, Beryozovaya Gat’s fenites) and in the Middle Dnieper region (Mala Tersa alkaline massif). These albitites form on pyroxene fenites (more often as vein-like bodies), whereas in the Chernigivka massif they occur in the endocontact of carbonatite bodies. During formation of apofenitic albitites process, such metasomatic alteration of fenites occur most often: instead of alkaline perthite feldspars a granoblastic aggregate predominantly with albite and subordinate amount of microcline and calcite is formed, and alkaline pyroxene is replaced by Fe-Mg biotite. Sometimes the Ca-Al-amphibole of the edenite-hastingsite composition crystallize. In comparison with initial fenites these albitites have lower alkalinity ((Na + K)/Al), because the alkaline pyroxene is replaced by biotite, and instead of aluminumless and alkaline amphiboles (richterites, riebeckites, arfvedsonites) Ca-Al-amphiboles (edenite-hastingsite series) are crystallized. The most interesting feature of apofenitic albites is increased in or high Zr, Nb and U concentration (in comparison with fenites or even carbonatites) which are concentrated in zircon, columbite, pyrochlore group minerals the formation of apofenitic albitites is due to the increasing acidity (and water enrichment) of the primary ultralkalinity fluids at the early stages the fenitization of granitoids caused, and at the final stages (during cooling, lowering of alkalinity and water enrichment) led the alteration of the fenites with the formation of ore-bearing albitites. The enriched in Zr and Nb apofenitic albitites in the geochemical aspect are similar to the mariupolites, but they differ significantly from the latter. The nepheline as main mineral of mariupolites is absent in albitites, but the latter have Fe-Mg-micas and amphiboles. The secondary micas from mariupolites are presented by strongly enriched in Fe and Mn biotites. Marioupolites in comparison with albitites are enriched in REE and depleted in Sr. Apofenitic albitites are the original metasomatites for Ukraine and they can be considered as new type of ore-bearing rocks for such rare metals as Nb, Ta and Zr.