OROGENIC-TYPE GOLD MINERALIZATION AND GENETIC CONDITIONS: EXAMPLES FROM THE BOROO, GATSUURT AND ULAANBULAG GOLD DEPOSITS IN THE NORTH KHENTEI GOLD BELT, CENTRAL NORTHERN MONGOLIA

Gold mineralization and crystallization processes of ore-forming minerals in representative deposits of the North Khentei gold belt of Mongolia were investigated to characterize the ore genesis and its evolution. The gold mineralization in representative deposits occurs as two ore types: I) disseminated and stockwork ores; and II) auriferous quartz-vein ores. 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 alterations. 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 bothore types range from 3–6 wt% (NaCl equiv.). The U-Pb age of zircons from Boroohost granites of 472.5 ± 6.9 Ma is much older than previously defined alteration minerals ages. By applying arsenopyrite geothermometer to arsenopyrite-II in the Type-II ores, crystallization temperature and logfs₂ are estimated to be 300-365ºC and -7.5 to -10.1, respectively. The results indicate that these gold deposits formed from the same source of the hydrothermal activity in this region. Thus, 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