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 contained 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 Carboniferous. 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, comprising 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 unconformably 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, advanced 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 covellite-pyrite mineralization in the central part of the trend weathering to form a supergene chalcocite blanket close to the surface.

Abstract In the Central Asian copper province seven copper belts each host at least one large (>5 million metric tons (Mt)) Cu deposit or deposit cluster; three other copper belts each host at least one medium-size (∼4 Mt) Cu deposit, with copper resources likely to increase during ongoing exploration. Of these, eight copper belts host porphyry deposits, including four giant (>10 Mt Cu) porphyries (Oyu Tolgoi, Almalyk, Aktogai, and Erdenet); one belt contains sediment-hosted deposits (including giant Dzhezkazgan); and one belt hosts volcanogenic massive sulfide (VMS) deposits. The deposits formed in seven periods between 510 and 240 Ma, with ∼30- to 50-m.y. intervals between porphyry emplacements occurring mostly between the major tectonic events, whereas formation of the sediment-hosted deposits was coeval with the major collisional tectonic event in the Tien Shan and Urals at 290 Ma. The greatest metal endowment and largest number of individual deposits fall into the period 320 to 340 Ma (Almalyk, Aktogai), followed by the second most important period at 385 to 370 Ma (Oyu Tolgoi and Magnitogorsk), and third most important event at 295 Ma (Dzhezkazgan). Individual copper belts are typically several hundreds of kilometers long, dominated by a single deposit type, and commonly have only one large to giant deposit in a belt (this may partially be a function of preservation but also of exploration maturity). Most copper belts were generated under transpressional tectonic regimes in either arc or backarc settings. Local structural controls include terrane boundaries and crustal-scale arc-oblique or arc-parallel faults. The immature arc terranes generally host deposits with 3- to 5-Mt Cu endowments in porphyry or VMS deposits. Deposits with giant, >10-Mt Cu endowments were discovered in regions of tectonic overlap; giant porphyry deposits occur in mature, overlapping magmatic arcs, and sediment-hosted deposits are present in 3- to 6-km-thick, overlapping sedimentary basins. Plate tectonic reconstructions suggest a strong correlation of higher grade (>0.6 wt % Cu) porphyry copper deposits with peri-oceanic magmatic arcs, whereas lower grade (0.35–0.5 wt % Cu) porphyry deposits occur in magmatic arcs that formed in relationship to subduction in backarc oceanic basins. Based on distance to the respective ophiolitic sutures, which indicate the traces of the former subduction zones, we have estimated the approximate dip of the paleosubduction zone. We propose a correlation between a low (∼30°) angle dip of the reconstructed subduction zone and the larger copper endowment of related porphyry deposits.

Abstract Porphyry Cu–Au ± Mo mineralization at Peschanka is hosted by monzodiorite and monzonite intrusions with high-K calc-alkaline to shoshonitic compositions and dated at about 144.1 ± 1.5 Ma, using U/Pb zircon ages. The Cretaceous intrusions are emplaced in a melange of Cretaceous island arcs, a tectonic setting comparable with other world-class porphyry Cu–Au deposits, such as Oyu Tolgoi, Mongolia and Pebble, Alaska. Abundant primary magnetite contents of the Peschanka intrusions, as well as numerous gypsum and anhydrite veins, reflect the high oxidation states of their parental magmas. This mineralogical interpretation is confirmed by high whole-rock Fe 2 O 3 /FeO ratios and high V/Sc ratios of the rocks of up to 1.27 and up to 21.9, respectively. The whole-rock Eu/Eu* ratios of the Peschanka intrusions are ≥1 which is also typical for potassic igneous rocks with high oxidation states. Abundant amphibole and biotite phenocrysts of the intrusions as well as their high whole-rock Sr/Y ratios of up to 225 document significantly high H 2 O contents of the high-K magmas. Peschanka contains a resource of >9.5 Mt of copper at an average grade of 0.43 wt% and 16.5 Moz of gold at a high average grade of 0.23 g/t and thus represents one of the largest undeveloped greenfield copper projects worldwide. The vicinity of Peschanka still offers significant brownfield exploration potential. The hypogene vein-related and disseminated Cu–Au ± Mo sulfide mineralization at Peschanka is structurally controlled by significant NE-trending strike-slips that acted as the conduits for the hydrothermal fluids. The central part of the orebody consists of high-grade north–south-trending sheeted quartz–bornite veining with unusually high vein densities. The highest Cu and Au grades are directly correlated with high vein densities. Peschanka is defined by distinct hydrothermal alteration zones including potassic, phyllic, propylitic and argillic assemblages, but a distinct lack of advanced argillic alteration. The mineralization itself is also zoned ranging from a central Mo–Cpy–Bn sulfide assemblage to a peripheral Py–Mt-dominated zone (‘pyrite-shell’). Late-stage polymetallic assemblages overprint and surround the main stockwork zone.