GEOLOGY AND MINERALIZATION OF THE KHARMAGTAI PORPHYRY CU-AU DEPOSIT, MONGOLIA

The Kharmagtai porphyry Cu-Au district is located in the Gobi Desert, Mongolia, 430 km south of the Mongolian capital Ulaanbaatar and 130 km north-northwest of the huge Oyu Tolgoi Cu-Au porphyry deposit being developed by Ivanhoe Mines Mongolia Inc (IMMI). The host rocks at Kharmagtai are a volcano-sedimentary sequence of massive to weakly bedded ash siltstones, volcaniclastic sandstones, laminated chert beds, weakly hornblende-phyric andesite lavas and dikes. IMMI geologists recognize six intrusive phases within the project area that are, in order of emplacement: diorite, diorite porphyry, monzodiorite, quartz-diorite, quartz-diorite dikes and andesite dikes. The quartz-diorite dikes are associated with intense hydrothermal alteration, quartz stockwork emplacement and porphyry copper-gold mineralization. Four hydrothermal stages of porphyry mineralization are recognized: Stage 1 albitization of feldspar and formation of magnetite±epidote and magnetite veins within the quartz diorite dikes. Stage 2 quartz-chalcopyrite-pyrite±gold stockwork emplacment. Stage 3 chlorite+epidote+chalcopyrite ±magnetite alteration and Stage 4 sericite+chlorite+pyrite-carbonate alteration. Late-stage copper-mineralized tourmaline breccia pipes are common throughout the Kharmagtai area and consist of commonly albite-altered diorite porphyry and quartz-diorite clasts set in a tourmaline, quartz, chalcopyrite, pyrite, epidote and calcite matrix. The deposit is unusually gold rich, with two trends in Au:Cu ratios that may indicate two separate mineralization events.