The gold-rich porphyries of the Kharmagtai district, Mongolia; A case study in exploration under cover.

摘要

The Kharmagtai porphyry deposit is located within the Omnogovi Province of southern Mongolia, approximately 420 km south-east of Ulaanbaatar (Figure 1). The project is strategically located 120 kilometres north of the giant Oyu Tolgoi, which contains 35.9Moz gold and 51 Bib copper. Outcropping mineralisation at Kharmagtai was initially identified by joint Mongolian and Russian exploration teams between 1960 and 1974. In the early 1990's Japanese groups were invited to explore in the Southern Gobi and Kharmagtai was re-identified as porphyry target. In 1996 Canadian explorer Quincunx picked up the Kharmagtai district initially exploring for sediment-hosted (replacement) style gold. In 2002 Ivanhoe Mines earned into the Kharmagtai project and undertook a significant resource style drilling and trenching program, drilling out the three existing porphyry deposits. In 2012, Turquoise Hill (owned by Rio Tinto) acquired Ivanhoe Mines and sold off all non-core assets. Kharmagtai was purchased by Xanadu Mines in late 2014. Xanadu conducted a JORC compliant resource estimation on the existing resources and set about expanding this resource by targeting additional porphyry deposits under shallow cover. To date four new confirmed porphyry systems have been discovered (Golden Eagle, Zaraa, Sandstorm and Zephyr) and the high gold (4:1 gold to copper) bornite zone of the Stockwork Hill Deposit has been found as a faulted offset to the main deposit. An additional eight potential systems have been identified and are yet to be tested. This high success rate can be attributed to the application of a systematic and pragmatic exploration approach based in geological observation and geochemical patterns. The previous 14 years of exploration in the same district had been driven mainly by geophysical targeting methods. A change to basing exploration targeting on geology and geochemistry, supported by geophysics has resulted in a remarkable change in exploration success. This paper discusses the different targeting paradigms used at Kharmagtai and implications for future exploration.