Volcanic arc stratigraphy and U-Pb geochronology of Upper Devonian to Carboniferous rock sequences; Oyu Tolgoi porphyry Cu-Au district, South Mongolia

CIM Vancouver 2006
Richard M. Tosdal, Charles Forster, Imants Kavalieris, David Crane, Douglas Kirwin,
Abstract The Oyu Tolgoi deposits in the South Gobi region of Mongolia form a recently discovered, giant porphyry copper-gold system situated in the Devonian Gurvansayhan island arc terrane. The Cu-Au deposits are hosted within Upper Devonian volcanic sequences and Late Devonian intrusions that are unconformably overlain by Carboniferous sedimentary and volcanic rocks. The Devonian sequence consists of basalt overlain by a complex subareal dacite pyroclastic sequence. An unmineralized sequence of sedimentary rocks interbedded with subaqueous basalt overlies the dacite unit. The age of the mineralized dacite unit is 365 ± 4 Ma (U-Pb, SHRIMP-RG; zircon) and the volcano-sedimentary unit immediately above the dacite sequence is cut by late-mineral granodiorite porphyry (362 ± 4 Ma). The Carboniferous rocks consist of polylithic andesite tuff (351 ± 2 Ma) overlain by sedimentary rocks and mafic volcaniclastic rocks. The volcaniclastic rocks are cut by dacite intrusions (346 ± 2 Ma). The youngest sequence detected in the district consists of andesites intercalated with fossiliferous clastic sedimentary rocks and dacitic to rhyolitic tuffs (336 ± 3 Ma).
The Oyu Tolgoi stratigraphic sequence records periods of sedimentation punctuated by basaltic to rhyolitic eruptive activity in a volcanic arc environment. Furthermore, the U-Pb geochronologic data suggests that the eruption of dacitic host rocks, deposition of overlying volcano-sedimentary rocks and the intrusion of late-mineral Cu-Au porphyries are close in age.
Keywords: volcanic arc, Geochronology, Porphyry, Mongolia, Stratigraphy, volcanology
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