Prospection for gold and new occurrences of gold-bearing mineralization in the eastern Mongolian Altay

Geological mapping and regional geochemical prospection on a scale of 1:50,000, covered an area of about 4,000 km(2) in the Zamtyn Nuruu region of the eastern Mongolian Altay. Northern part of the region is formed by the Lake Zone Terrane, the southern by the Gobi Altay Terrane; these two units are separated by the Bogd fault. The panned-concentrate heavy mineral survey was combined with the dry stream-sediment geochemistry, both being complemented by lithogeochemical sampling. The dry stream-sediment geochemistry proved to be a suitable method for regional prospection in semiarid to arid areas of Mongolia. The morphology and composition of placer gold grains were studied in order to estimate the distance from the primary source, and to constrain the nature of prospective primary mineral deposits. Based on the results of the regional geochemical survey and geological mapping, follow-up works were focused on prospective areas including the reconnaissance field works, detailed geochemical sampling, and geophysical measurements. New occurrences and signs of Au, Cu, Zn and (Ba)-As-Sb-Hg mineralizations were discovered. The ascertained goldbearing mineralizations are represented namely by Au and Au-(Ag) quartz veins in basic metavolcanic rocks of the Neoproterozoic Khan Taishir and volcanic rocks of the Permian Delger Khangay formations. Gold locally accompanies vein-disseminated base metal mineralization in some Proterozoic and Lower Palaeozoic formations. A prospective occurrence of clastic gold in Mesozoic conglomerates and sandstones was found at the northern edge of the Zamtyn Nuruu around the Samoandamba Uu1. The geochemical survey has revealed a contrast in occurrence of ore indices between the two terranes formig the studied area. While the Lake Terrane and, especially, the Neoproterozoic Khan Taishir Formation are rich in gold and base metal showings, the Gobi Altay Terrane is barren.