The Oyu Tolgoi copper-gold porphyry deposits, South Gobi, Mongolia / Медно-золотопорфировое месторождение Оюу-Толгой, Южное Гоби, Монголия

The Oyu Tolgoi copper-gold porphyry deposits arc located 6S0 kilometres south of Ulaan Baa tar in the south Gobi region of Mongolia. Copper mineralisation was first discovered in the Bronze age. During the late 1990’s extensive exploration including drilling was conducted by ВНР. Ivanhoe acquired the property in 2000 and subsequent exploration at four cogenetic porphyry copper-gold deposits (South West, South, Central and Hugo Dummctt) has defined very large resources of gold and copper amenable to open pit and underground mining. Gold-copper mineralisation at South West Oyu is characterised by quartz stockwotk veins, chalcopyritc. gold and minor bomite mineralisation mainly hosted in biotite altered basaltic volcanics which have been intruded by albitised monzodiorite stocks and dykes. Mineralisation at South Oyu is similar but more bomite rich and a marked decrease in the gold:copper ratios observed in South West Oyu. In contrast both Central Oyu and South Hugo Dummctt feature substantial high sulphidation covellite. chakocitc and minor enargite zones with associated alunite, muscovite, pyrophyllite, topaz and dickite alteration assemblages. These overprint stockwork quartz chalcopyritc and bomite mineralisation. North Hugo Dummctt which has the highest primary copper grades is characterized by an intense quartz stockwork zone which has a bomite rich core zoning outwards into to chalcopyritc and pyrite. The copper and gold grades and the gold to copper ratios appear to increase towards the northern part of the deposit. The main hosts for this mineralisation are an intense silica-sericite altered quartz monzodiorite and pervasively sericitised augite basalt. Geophysics has played an important role in exploration at Oyu Tolgoi. South West Oyu is clearly defined by magnetic and coincident changeability highs whereas the Central Oyu high sulphidation zone is a well defined chargeability high within a magnetic low anomaly. The deep blind and very high grade copper mineralisation at Hugo Dummctt was discovered by systematic drilling of a north east trending chargeability high under an flat area of extensive cover.