Сomposition and U-Pb age of igneous rocks of the Evotinsky massif (Southern Yakutia, Aldan-Stanovoy shield)

Determining the metallogenic specialization of igneous rocks in ore districts remains a pressing and often controversial challenge. Addressing this issue requires a comprehensive approach, that integrates petrographic and petrogeochemical data with geochronological studies of individual formations. Such integrated analysis, when contextualized within specific formation conditions and metallogenic assessments, provides a robust framework for interpretation. The Evotinsky massif, located within the eponymous Evotinsky placer ore region in the central Aldan-Stanovoy shield, represents one such critical formation. This study investigates its petrogenesis based on rock samples collected 
during fieldwork from 2018 to 2020, focusing on the massif’s main phases. Subsequent processing and major element analysis were performed using silicate and multielement techniques at the Department of Physicochemical Analysis Methods of the Diamond and Precious Metals Geology Institute SB RAS (Yakutsk). Trace element concentrations were determined by mass spectrometry at the САС Plasma LLC laboratory (Tomsk). Finally, geochronological U-Pb dating of zircons was conducted using SIMS (SHRIMP-II) at the Center for Isotope Research of the Karpinsky Russian Geological Research Institute (VSEGEI). The Evotinsky massif displays an antidromous structure and comprises two intrusive phases: an earlier phase of amphibole quartz syenites and a later phase of amphibole-pyroxene quartz monzonites. The rocks are classified within the high potassium calc-alkaline series, corresponding to derivatives of the shoshonite-latite series. Geochemical data indicate a mixed mantle and crustal source for the magmas. Petrochemical parameters and trace element ratios (Rb, Y, Yb, Ta) show compositions analogous to volcanic rocks from continental margins and island arcs. U-Pb zircon geochronology (SHRIMP II) constrains the emplacement of the amphibole quartz syenites to 119 ± 1 Ma and the amphibole-pyroxene quartz monzonites to 117 ± 1 Ma (Aptian, Early Cretaceous). In conclusion, the Evotinsky massif formed under back-arc extensional conditions during the final stages of the Mongol-Okhotsk Ocean closure. Furthermore, these findings demonstrate the massif’s significant potential for gold mineralization.

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