Uranium deposits of the El'kon ore district in the Aldan Shield

The paper presents the characteristics of the main uranium deposits of the El'kon ore district in the Aldan Shield, which is regarded as the primary r eserve of uranium resources in Russia. The deposits are localized in ore-be aring structures of four types: (1) rejuvenated ancient regional faults, (2 ) large Mesozoic zones partly inheriting ancient structural elements, (3) e xocontacts of preore Mesozoic dikes, and (4) variously oriented Mesozoic fr acture zones. Around 80% of uranium resources are confined to rejuvenated a ncient faults. In the major Yuzhnaya zone, the ore mineralization was trace d for nearly 25 km along strike and for more than 2 km along dip without re aching its lower limit. These parameters are unique for hydrothermal deposi ts of uranium and other metals. Different types of uranium-bearing zones va ry in geological structures, combination of tectonic elements of different ages, morphology of ore bodies and their vertical extent. At the same time, mineral associations of uranium ores reveal no considerable lateral change s or vertical zoning. Uranium ores are closely related to pyrite-carbonate- K-feldspar metasomatites, which are rich in K2O, CO2, and sulfide sulfur an d contain an Au admixture of a few g/t. In the northwestern sector of the o re district, the primary brannerite ores were transformed into uraninite or es during the emplacement of postore Mesozoic alkaline intrusions. There ar e no analogs to the El'kon uranium deposits in the world. To a certain degr ee, they resemble some fault-related deposits in sodic metasomatites of the Ukrainian Shield. The latter, however, were formed during the Early Proter ozoic protoactivization and granitization. In central Aldan, uranium deposi ts originated during the Mesozoic tectonomagmatic activization and ate 1.5 Ga younger than the crystalline basement and ancient faults. These factors define the specific features of the deposits.