Gold Mineralization and Genetic Evaluation in the North Khentei Gold Belt, Central Northern Mongolia

English Abstract:

Gold mineralization and crystallization sequences of ore-forming minerals in representative deposits of the North Khentei gold belt of Mongolia were investigated to characterize the ore genesis. The Au mineralization occurs as two ore types: I) disseminated and stockwork ores; and II) auriferous quartz-veins. Type-I mineralization in the ¹Boroo, ²Gatsuurt and ³Ulaanbulag deposits is composed of four distinct stages: (i) ^1,2,3pyrite-I + ^1,2,3arsenopyrite-I, (ii) ^1,2,3pyrite-II + ^1,2,3arsenopyrite-II, (iii) ^1,2,3sphalerite + ^1,2,3galena + ^1,2,3chalcopyrite + ^1,2tetrahedrite + ^1,2bournonite + ^1,2boulangerite + ²jamesonite + ²scheelite + ¹alloclasite + ¹native gold and (iv) ^1,2,3native gold. In Type-II mineralization, five, four and four crystallization stages are also recognized in these deposits, respectively: (i) ^1,2,3pyrite-I + ^1,3arsenopyrite, (ii) ^1,2,3pyrite-II + ^1,2,3arsenopyrite + ²galena + ²tetrahedrite-tennantite, + ²sphalerite + ²chalcopyrite + ^2,3bournonite, (iii) ^1,3sphalerite + ^1,3galena + ^1,3chalcopyrite + ²geocronite + ²geerite + ²native gold, (iv) ¹tetrahedrite-tennantite + ¹bournonite + ¹geerite + ^1,2,3native gold, and (v) ¹electrum. Both ore types are associated with sericitic and siliceous alteration. At Boroo and Gatsuurt both ore types contain coexisting CO₂-rich and aqueous fluid inclusions. The inclusions in the Type-I ores homogenized between 254–362ºC, whereas Boroo Type-II ore inclusions homogenized between 237–305ºC. Fluid salinities in both ore types range from 3–6 wt% (NaCl equiv.). The U-Pb age of zircons from Boroo host granites of 472.5 ± 6.9 Ma is much older than previously defined alteration minerals ages (Fig.1). The results indicate that these gold deposits can be classified as contiguous orogenic-type gold mineralization that developed in two stages. Type-I ores are early stage, formed by hydrothermal metasomatic alteration, and Type-II ores are later stage, deposited due to rapidly cooling temperature of the hydrothermal fluids.