On prospects of a large-scale gold deposit discovery in Ugakhan-Bokontya ore field

Abstract

The Ugakhan-Bokontya Ore Field occurs within the Galamo-Shantar megazone of the Mongol- Okhotsk fold-thrust system (1). In terms of metallog� eny, the ore field belongs to VerkhnyayaStoiba ore knot of the VerkhnyayaSelemdzha mineragenic zone of Dzhagda goldbearing province (2). A number of shows of ore (Kazanskii, Annenskii) and many mineral occurrences are known within the ore field. In addition, rich placer gold deposits occur within the ore field along the Ugakhan and Bagandzha rivers, and the following streams: Kazanskii, Elenin� skii, Utesnaya, Annenskii, Martyzhak Bol'shoi, Mar� tyzhak Malyi, Martyzhak Srednii, and numerous trib� utaries of those (figure). In this paper we do not con� sider placer deposits because they are too numerous and mention them just for the sake of comparison with primary deposits. Studies and exploration activities on the ore field area resulted in discovery of several shows of ore and mineral occurrences showing good prospects for largescale gold deposits. The most promising are described below. The Annenskii show of ore discovered in 1948 is sit� uated in headwaters of the stream of the same name, which is a left tributary of the Bagandzha River. The area consists of Proterozoic metamorphic rocks: phyl� lites, sericite, sericite-chlorite, chlorite schists, and graphitized phyllites. The metamorphic complex forms northwesttrending folds and is intruded by gra� nodiorite stock of Hercynian age. Dykes of quartz por� phyry occur widely. The latter rocks are poorly gold� bearing. Gold ore veins are confined to NWand NE� trending faults occurring on the northeastern flanks of anticline folds. Ten orebearing quartz veins are known, of which 7 are of NE strike (50°-80°). Three veins dip to the SE at angles of 75°-80°, four veins dip to the NW at angles of 40°-60°. Another three veins are of NW (290°-325°) strike dipping to the NE at 60°-80°. The vein thickness varies from 5 to 25 cm, rarely 60 cm. Only four veins were traced along the strike for a distance of 20-40 m. Seven veins are traced for 5-12 m down through the dip. The veins are com� posed largely of quartz with abundant impregnation of pyrite and arsenopyrite. Free gold is observed in vein 8. Quartz fragments with impregnation of pyrite, galena, chalcopyrite, and magnetite are found in the talus. Assay analysis data demonstrate gold contents from traces to 2.5-6.1 g/t. The gold content in vein 3 is 34 g/t, and in vein 1 it varies from traces to 14.4 g/t. In vein 1 the gold content in lumps of ore varies from 8 to 214 g/t. The predicted resources of gold ore on this site are estimated in category P 2 as 2 tons and in category P3 as 11 tons.