[SCG60-P04] Geology and ore mineralogical study of Undurnaran gold deposit, South Mongolia
Keywords:Undurnaran, gold, sulfide mineralization, carboniferous diorite, andesite
Mineralization of the Undurnaran deposit is related to Upper Carboniferous Gunbayan formation volcanogenic-sedimentary rocks and Late Carboniferous Mandakh complex diorite. Microquartzite, quartzite and carbonate veins contain gold micrograins. Sometimes gold is included in metaandesite and diorite. Alteration zone with a little arched form directs from southeast to northward. Size of the alteration zone is ranging from 20 meters up to 30 meters in wide and its length continues approximately 500 meters. Gold grade reaches approximately 3.5-4.5 g/t within mineralization zone. Gold mineralization is mainly controlled by Saikhandulaan abyssal fault which continues from northeast to southwest.
Host rocks such as diorite, andesite and their tuff are suffered to phyllic and argillic alterations. Ore minerals of gold-bearing rocks are represented by martitizated magnetite (hematite), goethite, and hydrogeothite, arsenopyrite, pyrite, pyrrhotite, chalcopyrite, sphalerite, galena, covellite, chalcosite, malachite, azurite and plumbojarosite. Mineral assemblages are pyrite+chalcopyrite+sphalerite+galena. Oxidation mineral assemblages is represented by goethite+ secondary pyrite+ plumbojarosite.
According to the results of fluid inclusion study, quartz veins origin of gold-bearing deposit is mesothermal (308℃ up 335℃).Gold mineralization is originated from H2O-NaCl-CO2 composition solution under low pressure. Second stage fluid inclusions found in re-crystallized quartz grains temperature is 183° up 197℃. Ice melting temperature is ranging from -4.20C up to -10C and salting 1.7-6.7. Gold mineralization of the Undurnaran deposit is from epithermal to mesothermal. Aging data analysis of the host rocks is 303±6 MA Rb-Sr and 303± 5 MA by U-Pb methods.