Gold-bismuth mineralization in the southeastern part of the Eastern Sayan

Abstract

In the southeastern part of the Eastern Sayan, thereare numerous gold deposits and occurrences, amongwhich quartz-vein (Pionerskii, Granitnii, Dinamitnii)and gold–quartz–sulfide polygenic deposits and occur-rences (Zun-Kholba, Zun-Ospinskii) are the most abun-dant [7, 10]. Recently, in the studied region, gold occur-rences and deposits of new geochemical and formation-genetic types were found and investigated: Au–Hg andAu–Ag types hosted in terrigenous–carbonate sequences;and porphyry gold type hosted in diorite–plagiograniteintrusions [1, 4, 6, 8]. The absence of Bi and As, or onlya trace, in ore is a common feature of these objects. Sig-nificant arsenopyrite is known only at the Verkh-nekhonchenskii occurrence related to listvenitized vol-canic rocks [7].In the southern Bokson–Gargan structural–forma-tion zone of the Eastern Sayan, we have discovered agold occurrence different in many features from theaforementioned types. This occurrence is distinguishedby abundant arsenopyrite and bismuth minerals, miner-alization hosting in granite pluton, and specificgeochemical specialization of ore. The article describesthis object.The Pogranichnii occurrence is located within theDibinskii ore field near the border with Mongolia andincludes the upper reaches of the Khore and Ukha-Gol.The area of Pogranichnii comprises granitoids of theSarkhoi Complex intruding volcaniclastic sequences ofthe Dibinskii Formation and volcanic rocks of theSarkhoi Series (Fig. 1). The eastern contact of the plu-ton with terrigenous–carbonate rocks of the MantagolFormation is tectonic. Ore zones screened by hornfelsafter rocks of the Dibinskii Formation and mylonitizedand foliated zones are located within the granite plutonand concentrated in the eastern near-contact igneousrocks.Host granitoids are medium-grained two-feldsparbiotite granite. Megascopally, the melanocratic and leu-cocratic gray and light gray varieties are characteristicof granite. In the near-contact parts of the pluton, typi-cal medium-grained granite is replaced by granite por-phyry with porphyritic quartz.Chemically, granitoids correspond to leucogranite ofnormal alkalinity of K–Na series (75.5–76.5 wt % SiO