© 2012 Society of Economic Geologists, Inc. Special Publication 16, pp. 187–213 Chapter 9 Geologic Overview of the Oyu Tolgoi Porphyry Cu-Au-Mo Deposits, Mongolia DAVID CRANE† AND IMANTS KAVALIERIS Asia Gold Mongolia LLC, Shuren Bldg., Olympic Street-8, Sukhbaatar District, Ulaanbaatar-13, Mongolia Abstract The Oyu Tolgoi porphyry Cu-Au deposits in southern Mongolia constitute the largest high-grade group of Paleozoic porphyry deposits known in the world. Exploration by Ivanhoe Mines has thus far defined total con- tained metal exceeding 92 billion pounds (41.7 million metric tons) of copper and 49.8 million troy ounces (1,549 metric tons) of gold. The deposits are related to multiple intrusions of Late Devonian (~372 Ma) quartz monzodiorite, emplaced within juvenile calc-alkaline basalts that belong to the Gurvansayhan island-arc terrane. The tectonic setting is part of the Central Asian orogenic belt, a zone of arc-continent collision, active from the Silurian to Early Car- boniferous. Porphyry Cu-Au deposits and exploration targets occur along a 26 km-long, north-northeast belt termed the Oyu Tolgoi trend. All deposits are related to phenocryst-crowded quartz monzodiorite intrusions and contorted anastomosing A-type quartz veins. The individual deposits have varied characteristics in regard to host rock, quartz monzodiorite morphology, alteration assemblages, sulfide mineralogy, grade, and Au/Cu ratios. The pre-Carboniferous stratigraphy of Oyu Tolgoi consists of the Oyu Tolgoi sequence, consisting of massive augite basalt, conglomerate, dacitic tuffs, and siltstones, which is overthrust by the Heruga sequence, com- prising basaltic flows, volcaniclastic rocks, and siltstones. Only the lower parts of the Oyu Tolgoi sequence host porphyry mineralization and associated alteration. The Carboniferous Sainshandhudag Formation uncon- formably overlies the older rocks. Major Carboniferous or younger faults disrupt the Oyu Tolgoi trend and bound the western side of the Hugo Dummett deposits. Early sodic-calcic alteration overprinted by younger wall-rock biotite-magnetite alteration along with K- feldspar alteration, the latter two largely restricted to the host quartz monzodiorite porphyry, dominate the deeper parts of the system, especially in the southern parts of the trend. Gold-rich chalcopyrite mineralization is directly related to the biotite-magnetite and K-feldspar alteration. Carapace-style quartz-sericite alteration with associated chalcopyrite-molybdenite mineralization overprints the quartz monzodiorite and wall rocks in the upper parts of the system and uncommonly at depth. In the central and northern parts of the trend, ad- vanced argillic alteration occurs at the top of the system and is telescoped onto earlier alteration. Parts of the telescoped system are characterized by high-grade, bornite-rich mineralization, especially where a series of prograde and retrograde alteration events overlap. High sulfidation-style hypergene pyrite-enargite ± covellite and chalcocite mineralization occurs on the fringes of the alteration system, with broad zones of hypergene cov- ellite-pyrite mineralization in the central part of the trend weathering to form a supergene chalcocite blanket close to the surface. Introduction Mongolia and is related to granodiorite of Triassic age. Oyu THE OYU TOLGOI deposits (106.85° E and 43.01° N) are lo- Tolgoi comprises the largest group of Paleozoic porphyry de- cated in the South Gobi Desert, approximately 650 km south posits in the world, with a total Proven and Probable reserve of Ulaanbaatar. The average elevation in the area of the de- of 1,392 Mt at 0.93 wt % Cu and 0.38 g/t Au and a total Indi- posits is 1,160 m and relief is less than 50 m. The deposits are cated and Inferred Resources of 3,093 Mt at 0.84% Cu and being developed under a joint venture agreement between 0.33 g/t Au (at 0.6% Cu equiv cut-off) in six main deposits the Government of Mongolia (34%) and Ivanhoe Mines Ltd. (Rio Tinto, 2011 Annual Report). Construction of the Oyu (66%; Rio Tinto took a majority stake in Ivanhoe Mines on Tolgoi mine commenced in late 2009, and first production is January 24th, 2012). Oyu Tolgoi is a newly discovered por- expected in late 2012. phyry Cu-Au mineral district within the early to mid-Paleo- Other significant porphyry and related alteration systems in zoic Gurvansayhan arc terrane, which extends through south- the Gurvansayhan terrane include Tsagaan Suvarga, Khar- ern Mongolia as an arcuate belt (Fig. 1). magtai, and Shuteen (Fig. 1). At Tsagaan Suvarga, chalcopy- Prior to the discovery of Oyu Tolgoi, the potential for is- rite and molybdenite mineralization (92 Mt at 0.71% Cu and land-arc gold-rich porphyry deposits in Mongolia was unrec- 0.025% Mo; Kirwin et al., 2005) of Late Devonian age (370.4 ognized. The only well-known porphyry deposit at the time ± 0.8 Ma; Watanabe and Stein, 2000) is hosted by syenogran- was the Erdenet mine (1.78 billion metric tons (Bt) at 0.62 wt ite and granodiorite porphyry intrusions of comparable age to % Cu and 0.025 wt % Mo; Kirwin et al., 2005), which occurs the Oyu Tolgoi deposits. At Kharmagtai, gold-bearing chal- within Precambrian and early Paleozoic rocks in northern copyrite mineralization is hosted by quartz diorite porphyry intrusions associated with tourmaline alteration; there are no † Corresponding author: e-mail, [email protected] published age data for the Kharmagtai complex. The Shuteen 187 Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/3812458/9781629490410_ch09.pdf by guest on 05 August 2020 188 CRANE AND KAVALIERIS a b Mongolia (before 1991), when the Erdenet Cu-Mo deposit Russia Ulaan was found and developed, despite these Bronze-age work- Drill proven Khuud deposit 650, 000 mE ings. Joint Mongolian and Russian geochemical surveys re- Cu-Au Mongolia prospect ported only a Mo geochemical anomaly in the vicinity of the Geological trend Central deposit (Kirwin et al., 2005). Korea Fault Identification of porphyry Cu mineralization at Oyu Tolgoi is Hugo Dummett North China accredited to BHP (Magma Copper Ltd.) geologists, who vis- Japan Hugo Dummett South ited the area in September 1996 and undertook the first explo- Central Oyu Southwest Oyu South Oyu ration between 1997 and 1999 (Perelló et al., 2001; Kirwin et India Oyu Tolgoi Heruga North al., 2003, 2005). BHP completed geochemical and geophysical 4,760,000 mN surveys (ground and airborne magnetics, induced polariza- Philippines Heruga Burma tion) and 23 diamond drill holes, culminating in the identifica- 1000 km Javkhlant 5 km tion of the North, Central, and South mineralized zones, for which Perelló et al. (2001) quoted a total in situ resource of c 90 96 102 108 114 438 Mt at 0.52% Cu and 0.25 g/t Au. The mineralized zones Russia are related to intermediate-composition porphyritic intrusions with advanced argillic, sericitic, and potassic alteration (Perelló 48 Erdenet Cu-Mo et al., 2001). BHP did not discover the largest deposit, Hugo Dummett, which is concealed beneath more than 800 m of Oyu Tolgoi unmineralized rock. In 1999, BHP offered the tenements for joint venture and a resulting agreement with Ivanhoe Mines 44 allowed acquisition of up to 100% of the properties. Gurvansayhan island arc terrane China Exploration by Ivanhoe Mines Mongolia Inc. (IMMI) began Island arc in May 2000, and exploration is still ongoing (exploration up Backarc/forearc basin to 2005 was summarized by Kirwin et al., 2005). More than 400km Cratonic basement 1,000 km of diamond core has been drilled to date in over Early Permian 2,400 holes. The exploration focus in 2000 to 2001 was a su- d Sodic-alkalic granite pergene chalcocite zone at the Central deposit, which en- Kharmagtai Shuteen N Tsagaan Early to tailed 8,000 m of reverse-circulation percussion drilling, suvarga Late Carboniferous which defined an indicated resource of 31.7 Mt at 0.8% Cu Granitoids Tavan Tolgoi and an inferred resource of 11.2 Mt at 0.78% Cu (Kirwin et Coal Cu-Au-Mo prospects al., 2005). A turning point in exploration came from the re- Advanced argillic sults of three diamond holes drilled at the end of the reverse Oyu Tolgoi alteration circulation program in late 2001; OTRCD150, drilled be- deposits Cu-Au-MoCu-Au-Mo oror coalcoal Tavan Tolgoi Permian coal mine tween two shallow BHP holes in the western side of South 50 kms ChinaChina 0 50km Dalanzadgad deposit, intersected 508 m of 0.81% Cu and 1.17 g/t Au, start- ing below 70 m, which was the discovery hole for the South- FIG. 1. (a). Location of the Oyu Tolgoi district in southern Mongolia. (b). west deposit. This hole essentially defined the Southwest de- The Oyu Tolgoi mineral trend. (c). Erdenet Cu-Mo deposit located in cra- tonic basement, distribution of the Gurvansayhan terrane and related island- posit in terms of grade, Au (g/t)/Cu (%) ratio, host rock, arc terranes (after Badarch et al., 2002). (d). Inset showing porphyry nature of quartz monzodiorite intrusions, and depth extent of prospects, alteration zones, and the Tavan Tolgoi Permian coal deposit. mineralization, despite a huge amount of later exploration. An extensive diamond drilling program ensued, using north- south gradient array induced polarization (IP) surveys to tar- prospect encompasses a large lithocap zone of unmineralized get alteration and sulfide minerals. Southwest Oyu and the quartz-alunite-pyrophyllite-topaz alteration. A microgranite mineralized zones already identified by the BHP program from Shuteen has a U-Pb zircon age of 330 ± 1.0 Ma (Blight were initially explored. A broad east-west−oriented IP target et al., 2010). in the northern part of North Oyu was drilled in mid-2002 along north-south lines and found to be mainly related to Exploration History pyrite in advanced argillic-altered rocks. However, a final hole Porphyry Cu-Au deposits and mineral occurrences in the at the east side of the IP target, OTD270 drilled to the north Oyu Tolgoi district occur in a 26-km-long, N-NWE−trending in September 2002, encountered 638 m at 1.6% Cu, starting belt.