Gold and Porphyry Cu–Mo Mineralization of the Bumbat Ore Cluster and Its Relationship with the Early Paleozoic Magmatism of the Lake Zone (Western Mongolia)

Abstract

Abstract ––Geological, mineralogical, and geochemical studies of copper and gold mineralization in the Bumbat ore district of Mongolia have shown its association with igneous rocks of different ages formed under different geodynamic, geologic, and geochemical conditions. Copper ore occurrences (site 98 and Altan Gadas site) with an age of 518.0 ± 4.9 Ma have similar mineralogical and geochemical features and seem to be related to plagiogranite-porphyry stocks formed at the late island arc stage (524.5 Ma). The ores were deposited from weakly concentrated solutions with a low content of СО2 at 240–230 °С under subsurface conditions. The association of mineralization with plagiogranites, its localization predominantly in veins, and its essentially copper composition, with high contents of Zn, Mn, Ba, and, in some samples, Ag and Bi, permit us to assign this mineralization to the vein quartz–sulfide type. Its commercial value has yet to be assessed. Gold mineralization of the Three Hills and Darvi sites formed later (455.9 ± 4.3 Ma), during the formation of the final granitoid phases at the accretion–collision stage (511–465 Ma). These sites are mineralized crushing zones composed of hydrothermally altered rocks of sericite–quartz composition with veinlet and disseminated (stockwork) sulfide mineralization and gold-bearing quartz veins. The content of gold in the ores varies from tenths to tens of ppm, and its fineness varies from 700 to 1000‰. The ores of both sites have high contents of Cu, Zn, Mn, Ba, and, in places, Mo. Mineralization formed from hydrothermal solutions with TDS = 9.5–12.0 wt.% NaCl eq. at medium temperatures (230–300 °C) under subsurface conditions. The above specific features of gold mineralization are typical of the flank zones of porphyry Cu–(Mo) deposits.