Chronology of Alkaline Magmatism and Gold Mineralization in the Central Aldan Ore District (Southern Yakutia)

Abstract

Abstract —The Central Aldan ore district (CAOD) is unique by the abundance of gold deposits associated with Mesozoic alkaline magmatism. Four types of gold deposits are established here: porphyry gold, gold–sulfide, gold–argillizite-K-feldspar–quartz, and gold–uranium. The available geochronological data on the age of igneous rocks and gold mineralization in the CAOD show that the latter formed in the period 151–120 Ma. These data also agree with the results of the U–Pb dating of the El’kon gold–uranium ore-magmatic system (143–125 Ma). Analysis of our and earlier published geochronological data showed two stages of magmatism evolution in this region. At the early stage (151–130 Ma), most of the alkali syenites, monzonites, and their analogues (sills, stocks, ring intrusions, and volcanic sequences) and ores formed. They are widespread in various deposits and massifs. In the Ryabinovyi massif, this stage is marked by the formation of most of the alkaline rocks (Aldan Complex) and ores: aegirine syenites, 151.4 ± 1.9 Ma; pyroxene–K-feldspar pegmatites, 144.8 ± 1.5 Ma; and Au–Cu ores, 137.5 ± 1.7—131.1 ± 16 Ma. The crystallization of amphibole in the syenite of the Lunnoe deposit (143.1 ± 2.0 Ma) and in the clinopyroxenite of the Inagli massif (142.4 ± 2.0 Ma) and the formation of most of the alkaline rocks of its ring framing also took place at this stage. At the Samolazovskoe deposit, this stage is marked by the formation of zircon in pseudoleucite syenite, 135.9 ± 1.9 Ma, and in different syenite porphyry phases, 141.39 ± 0.90—142.4 ± 5.0 and 134.25 ± 0.70—129.9 ± 2.6 Ma, as well as gold–skarn mineralization, 129.9 ± 2.6—134.9 ± 2.8 Ma. The same period included the formation of primary ores at the Kuranakh deposit, 136.2 ± 1.7 Ma, and the deposition of brannerite mineralization at the Lunnoe deposit, 132.4 ± 1.6 Ma. Early intrusive phases, such as potassic picrites, shonkinites, and lamprophyres, are scarce among the products of this stage of magmatism, which is possibly due to their burial beneath large volumes of later formed alkali syenites and monzonite–syenites. The second stage (128–120 Ma) was distinguished within the Ryabinovyi massif as small intrusions and dikes of olivine lamproites, shonkinite porphyry, minettes, and syenite porphyry. We revealed explosive breccias with an age of 127 Ma at the Samolazovskoe deposit. Magmatism of this stage was of limited occurrence in the CAOD and did not produce alkali syenites, monzonite–syenites, and ores. At the same time, rocks with an age of 121.1 ± 1.3—115.5 ± 1.6 Ma are widespread in the large (120 km2) Dzheltula ring massif of the Tyrkanda ore district, located east of the Central Aldan region.