Petrology and ore content of magmatic formations of the Ukachilan ore field (northeast Yakutia)

The article concentrates on geology, petrology, crystallization conditions and geochemical features of granitoids and dykes of the Ukachilkani ore field with a complex cassiterite-silicate-sulfide mineralization. The ore field is localized on the border of Polousnyi synclinorium and buried structures of Primoskyi lowlands. The main ore bodies are confined to areas of intensive development of dikes and significantly divorced in time from the formation of granitoids of Ukachilkan massif. Geochemical specialization of all magmatic rocks on Bi, Sb, As, Ag is identified, that corresponds to the fact that crystalline basement rocks are enriched with these elements, while terrigenous strata are depleted in these elements and tin. The Sn content in granitoids of the massif is closey to clarke and its redistribution during differentiation of the melt and autometasomatism is not registered. Crystallization of granitoids took place in conditions of low activity of fluoride and high activity of oxygen, unfavorable for the generation of tin mineralization. Maximum concentrations of tin and associated elements characterize the rocks of the dikes crystallized from water-saturated melts at high activity of water, chlorine and sulfur, i.e. they could be sources of ore-bearing solutions. Dikes trachyandesite and trachydolerite are maximally enriched with ore and volatile elements; parental meltsof these dikes fre formed in metasomatized mantle. It is suggested that, melts that formed complexes of dikes, primarily of basic composition, were one of the main sources of tin and associated elements. It is shown that the main features of potential tin content – presence of minerals-concentrators of tin and associated elements and fluid regime of crystallization. Geochemical specialization of dikes on Au and high activity of chlorine with their crystallization resulted in formation of the related gold mineralization.

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