Geology, mineralogy and stable isotope geochemistry of the Dzuunmod area in northern Mongolia: Constraints for gold ore genesis and sources

Abstract

The Dzuunmod area located in the North Khentii Gold belt (NKGB) of northern Mongolia includes lode gold deposits such as Gatsuurt, Sujigtei and Boroo with several minor gold deposits and occurrences. They show similar hydrothermal alteration assemblages (sericitic, siliceous and potassic) and ore mineral assemblages (pyrite and arsenopyrite with minor amount of galena, sphalerite and chalcopyrite). Gold occurs as native form and invisible gold in pyrite and arsenopyrite. The major sulfide minerals are separated into earlier non-auriferous stage and later auriferous grains containing invisible gold. Native gold postdates the major sulfide mineralization. Gold and arsenic content of pyrite grains indicates that gold exists mainly as solid solution form (Au+1) in the Gatsuurt and Boroo deposit whereas gold nanoparticle (Au0) is present in the Sujigtei deposit. High Co/Ni and Mo/Ni ratios of pyrite grain suggest a post-sedimentary or hydrothermal origin and the ore-forming fluid was significantly affected by fluid-host rock interactions during mineralization processes. Large variation of δ34S values of pyrite and arsenopyrite from 2.6‰ to 17.2‰ indicates that sulfur seems to be mainly derived from a source with heterogeneous sulfur isotope composition, even though the role of magmatic sulfur as one of possible sulfur sources cannot be ruled out. Consistent with geological evidence, relatively positive δ34S values suggest that sulfidation plays an important role for gold and sulfide precipitation. The calculated δ18O values of hydrothermal fluid from the measured δ18O values of quartz samples (from 14.7‰ to 17.7‰) indicate a metamorphic derivation of ore-forming fluid. Gold mineralization processes in the Dzuunmod area seem to occur several times by multiple input of hydrothermal fluid and fluid-host rock interactions. The gold deposits in the Dzuunmod area are considered to be orogenic gold type influenced by fluid-host rock interactions in the deposit area.