Petrography of the Mesozoic Alkaline Rocks of the Medvedev Massif (South Yakutia, Aldan-Stanovoy Shield, Leglier Ore Cluster)

The article considers petrographic compositions of the Mesozoic alkaline magmatic rocks of the Medvedey massif. The latter is a part of the Central Aldan ore district and is spatially located in the central part of the Nymnyr block. The massif is a multiphase structure, of the most productive stage of development of the territory - the Mesozoic tectonic-magmatic activation of the Aldan-Stanovoy shield. The main purpose of this work was to identify qualitative quantitative-mineralogical characteristics of the massif rocks, potential for gold mineralization, using crystal-optical methods. As a result of the field works and petrographic studies of the Mesozoic alkaline magmatic rocks, it was determined that, The Medvedev massif has a zonal structure and is represented by three phases of intrusion with distinct contacts between the latter, which rocks differ in structural and material features. The rocks of the first phase of the massif are composed considerably of leucocrate (the content of dark-colored minerals is up to 9%) syenite-porphyry with elements of the monzonite structure. Magmatic formations of the second phase of the massif differ from those of the first phase primarily in structural features and are represented by hypidiomorphic granular differences with an increased content of dark-colored minerals in the rocks up to 15%. The rocks of the final phase have a noticeable difference from the first two phases and are represented by mesocratic monzonite porphyry with a clearly expressed monzonite structure with a dark-colored mineral content up to 20%. Within the Central Aldan ore region, the large Samolazovskoye gold deposit is associated with similar multiphase massifs, in particular with the Yukhtin massif. During field works, it was found that gold-ore mineralization is associated with the massif, which formation is related to an intensively occurred contact-metasomatic effect on the host rocks. Hydrothermal-metasomatic transformations of the latter are the products of multi-stage alkali-silicate metasomatism, associated with the second and especially the first phases of intrusion of the massif, as well as with further weathering processes, disintegration of gold-ore metasomatites and formation of an oxidation zone. Based on the petrographic study of the material composition of the rocks of the massif, it is concluded that gold ore occurrences of commercial scale can be associated with such massifs of multi-phase intrusion.