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

Abstract

The study shows chemical composition of the Mesozoic igneous rocks of the Medvedev massif. The massif is a part of the Central Aldan ore region. It is spatially located in the central part of the Nimnyr Block and is a part of the Leglier ore cluster. The massif is a multiphase structure of the most productive stage of the territory development – the Mesozoic tectonic-magmatic activation of the Aldan-Stanovoy shield. According to the materials of previous researchers, the rocks of the massif are represented by the augite hornblende and hornblende syenite porphyry. As a result of field work, we found that the massif has a zonal structure and is represented by three phases of intrusion. The rocks of the selected phases differ in their structural and compositional characteristics. The main objective of the work was to establish compositional characteristics by studying the chemical compositions of potential gold mineralization rocks of the Medvedev massif. In the course of studies and interpretation of chemical analysis of the massif Mesozoic magmatic rocks, we have identified the following characteristics. According to various classification data the following groups of rocks in Medvedev massif are identified – quartz syenites (Phase I), quartz monzodiorites of shoshonite series (Phase II) and monzonites of high-potassium calc-alkaline series (Phase III). The total alkali content (Na2O+K2O)>5% in them corresponds to alkaline rocks. All rocks are characterized by high Al2O3%>14 and low TiO2<1%. The potassium type of alkalinity Na2O/K2O<1 is typical for quartz syenites of the massif, while for quartz monzodiorites this indicator is intermediate, and in monzonites of the massif this parameter Na2O/K2O>1 corresponds to rocks of the alkaline potassium-sodium series. The massif rocks characteristic feature is the decrease of rocks alumina (al’) values from quartz syenites to quartz monzodiorites followed by the increase to monzonites, while the value of the rocks AG coefficient is significantly reduced in the series. Based on these facts, it was suggested that there are two sources of the mantle and mantle-crustal nature. Also, based on the study of the material composition of the Medvedev massif magmatic rocks and the similarity of the geological development with the Mesozoic magmatism of the Samolazovsky and Ryabinovy deposits, it was concluded that gold manifestations can be associated with such multiphase massifs within the Leglier ore cluster.