New data on the Mesozoic gold mineralization in western Aldan (Apsat graben)

Abstract

The Mesozoic epoch is characterized by the highest productivity in terms of the genesis of gold and uranium ores in the Aldan Shield. Commercial Mesozoic gold deposits are mainly confined to the central Aldan region. This region incorporates deposits of the Kuranakh, Lebedinsk, Ryabinovyi, El’kon, and other formation types associated with the Mesozoic alkaline magmatism. However, they are distinguished by geological settings of ore formation. Commercial mineralization of the Kuranakh and El’kon types is associated with the regional development of Mesozoic potassic metasomatites (gumbeites). The gumbeites represent preore rocks of the Kuranakh gold ore field [1]. They play the role of an autonomous essential source of gold and silver in complex deposits of the El’kon horst [2, 3]. In the western Aldan Shield, a similar mineralization has been found in the South Ugui auriferous zone (Ugui graben) at the tectonic contact of Lower Proterozoic rocks with the Archean framing. Goethite‐quartz‐ K feldspar metasomatites (with native gold) in this zone are considered an unconventional source of gold for the Aldan Shield. The Tabor deposit represents a standard of this independent type of gold mineralization [4‐6]. We carried out comprehensive prognostic‐mineragenic investigations in the western Aldan region and discovered a new productive setting of the Mesozoic commercial gold concentration in the Apsat graben. Samples taken in the course of detailed field works were analyzed by the ICP-MS (Elan 6100), electron probe (Cam Scan MW 2300), X-ray structural (DRON6), and other methods in laboratories of the Karpinskii All-Russia Research Institute of Geology, St. Petersburg. The present paper is based on the analytical data. The Apsat graben (Fig. 1) is located 25 km northwest of the Settlement of Chara in the Kalar area (Chita district). The graben is confined to the northeastern reactivated (Late Paleozoic‐Mesozoic) marginal part of the Early Proterozoic Kodar‐Udokan trough at the junction with long-lived near-meridional faults (Archean blastomylonite zones). The Apsat graben includes Upper Jurassic and Lower Cretaceous carbonaceous molasses. The Upper Jurassic rocks enclose the large Apsat coal deposit. The base of the Apsat graben is composed of Archean metamorphic granitoids, Early Proterozoic rapakivi-type granites of the Kodar pluton, and metaterrigenous rocks of the Udokan Complex