Otechestvennoe Tin Deposit (Magadan Region, North-East Russia): Genesis and Bismuth Mineralization

The geology and mineralogy of the Otechestvennoe tin deposit and host Khenikandzha intrusion (Magadan region, NE Russia) have been described. Ore composition has been studied and detailed characteristics of bismuth mineralization, wall-rock alterations, and fluid inclusions are reported. Granitoids of the Khenikandzha intrusion form a continuous rock series with SiO2 content from 55 to 75% documented as a series of successive phases. According to the composition and age, the intrusion has been interpreted as formed in an intraplate extensional setting. The tin deposit includes two types of mineralization: (1) metasomatically altered feldspathic (albitite) bodies with cassiterite dissemination and (2) feldspar–chlorite–quartz vein/veinlets with cassiterite and bismuth minerals. Analysis of wall-rock alterations indicates that chlorite–quartz veins were formed after pervasive albitization of the granites and were accompanied by potassium alteration. The shape, structure, and composition of ores and thermobarogeochemical features of quartz from the ore bodies indicate the primary molten state of bismuth “droplets” in hydrothermal chloride solutions with a temperature of 334–378°C (i.e., higher than the melting point of native bismuth), and the later formation of bismuth sulfotellurides and sulfides. Gold from the hydrothermal solutions was probably concentrated in the primary bismuth melt. During subsequent solidification of the latter and its replacement by bismuth sulfotellurides and sulfides, gold was released as native gold mainly along the boundaries of mineral phases.