Mineralogical and Geochemical Characteristics of Placer and Ore Gold in the Upper Reaches of the Amazar River (Mogochinsky Gold Mining District, Eastern Transbaikalia)

A detailed study of gold from placers and eluvium of the ore zones of the Amazarkan deposit, located in the basin of the three upper tributaries of the Amazar River, was carried out. The placers are not very far from the primary sources. The supergene transformation of gold is weak and is expressed in the presence of high-fineness rims, petrovskaite rims, and newly formed gold on Au particles. Gold from eluvium is characterized by a fineness of 650–800 %. Endogenous features of the chemical composition are inherited by placer gold, but higher grade (above 850 %) gold predominates or is significantly present in placers. In gold ores (1,2) and placers (3–5), the following associations of mineral inclusions have been established: 1) chalcopyrite, sphalerite, Bi sulfotellurides (with Pb, Se); 2) galena, tetradymite, tsumoite, minerals of the composition BiPbTeS, SbPbBiFeTe; 3) arsenopyrite, pyrrhotite, gersdorffite, alloclasite, tetradymite; 4) galena, tetradymite, Bi-containing sulfosalts (with Pb, Sb, sometimes Se), tsumoite, aurostibite; 5) arsenopyrite, gersdorffite, cobaltite, galena, tetradymite, bismuthin. Nonmetallic minerals are represented by quartz, tourmaline, muscovite, siderite, potassium feldspar, and epidote. There is no relationship between the fineness of gold and the presence of inclusions of a particular mineral in it. Gold mineralization of different stages is spatially combined, and on the upper horizons of the ore bodies, gold was probably redeposited with coarsening and increasing fineness. Such gold is typical for placers of the Verkhny-Amazar ore cluster (Amazarkan, Bolshoy Amazar, Vasilievka rivers). Among the inclusions in gold, galena and Bi minerals predominate (association 4), association 3 is less common. The primary sources of gold from the Maly Amazar River placer probably largely include mineralization at a relatively deep level (mesothermal). Inclusions of Bi minerals in gold are rare; galena is associated not with them, but with arsenopyrite (association 5). The association of galena and Bi minerals in inclusions in gold is also typical for placers in other areas of eastern Transbaikalia, where gold mineralization is associated with granitoids of the Amudzhikan or Shakhtama complexes.