Abstract

The Oyu Tolgoi porphyry Cu-Au system in the South Gobi desert, Mongolia, comprises five deposits that extend over 6 km in a north-northeast–oriented zone. They occur in a middle to late Paleozoic arc terrane and are related to Late Devonian quartz monzodiorite intrusions. The Hugo Dummett deposits are the northernmost and deepest, with up to 1,000 m of premineral sedimentary and volcanic cover rock remaining. They are the largest deposits discovered to date and characterized by high-grade copper (>2.5% Cu) and gold (0.5–2 g/t) mineralization associated with intense quartz veining and several phases of quartz monzodiorite intruded into basaltic volcanic host rocks. Sulfide minerals in these deposits are zoned outward from a bornite-dominated core to chalcopyrite, upward to pyrite ± enargite and covellite at shallower depth. The latter high-sulfidation–state sulfides are hosted by advanced argillic alteration mineral associations. This alteration is restricted mainly to dacitic ash-flow tuff that overlies the basaltic volcanic rock and includes ubiquitous quartz and pyrophyllite, kaolinite, plus late dickite veins, as well as K alunite, Al phosphate-sulfate minerals, zunyite, diaspore, topaz, corundum, and andalusite.

A reconnaissance oxygen-hydrogen and sulfur isotope study was undertaken to investigate the origin of several characteristic alteration minerals in the Oyu Tolgoi system, with particular emphasis on the Hugo Dummett deposits. Based on the isotopic composition of O, H, and S (δ18O(SO4) = 8.8–20.1‰, δD = −73 to −43‰, δ34S = 9.8–17.9‰), the alunite formed from condensation of magmatic vapor that ascended to the upper parts of the porphyry hydrothermal system, without involvement of significant amounts of meteoric water. The isotopic data indicate that pyrophyllite (δ18O = 6.5–10.9‰, δD = −90 to −106‰) formed from a magmatic fluid with a component of meteoric water. Muscovite associated with quartz monzodiorite intrusions occurs in the core of the Hugo Dummett deposits, and isotopic data (δ18O = 3.0–9.0‰, δD = −101 to −116‰) show it formed from a magmatic fluid with water similar in composition to that which formed the pyrophyllite. Mg chlorite (δ18O = 5.5‰, δD = −126‰) is a widespread mineral retrograde after hydrothermal biotite and may have formed from fluids similar to those related to the muscovite during cooling of the porphyry system. By contrast, paragenetically later and postmineralization alteration fluid, which produced dickite (δ18O = −4.1 to +3.3‰, δD = −130 to −140‰), shows clear evidence for mixing with substantial amounts of meteoric water. Relatively low δD values (−140‰) for this meteoric water component may indicate that its source was at high elevations.

The geologic structure, nature of alteration, styles of mineralization, and stable isotope data indicate that the Oyu Tolgoi deposits constitute a typical porphyry system formed in an island-arc setting. The outward zonation of sulfide minerals for the Hugo Dummett deposits, from a bornite-dominated core to chalcopyrite and pyrite-enargite, can be interpreted to be related to a cooling magmatic hydrothermal system which transgressed outward over enclosing advanced argillic alteration. This resulted in some unusual alteration and sulfide parageneses, such as topaz, or pyrite, enargite, and tennantite, entrained by high-grade bornite.

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