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.