Geologic Overview of the Oyu Tolgoi Porphyry Cu-Au-Mo Deposits, Mongolia
David Crane, Imants Kavalieris, 2012. "Geologic Overview of the Oyu Tolgoi Porphyry Cu-Au-Mo Deposits, Mongolia", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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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.
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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.