Exhumation and Preservation of Porphyry Cu-Au Deposits at Oyu Tolgoi, South Gobi Region, Mongolia

Abstract

Field relationships and U-Pb data (zircon) are used to constrain the timing of tectonomagmatic events that occurred shortly after the formation of Late Devonian (~372 Ma) porphyry Cu-Au deposits at Oyu Tolgoi (Mongolia). In the two northernmost of the several discrete porphyry centers (Hugo Dummett deposits), Upper Devonian rocks positioned stratigraphically above copper-gold mineralization preserve a record of uplift, exhumation, and subsequent burial shortly after mineralization. Strongly altered and mineralized quartz monzodiorite and volcanic rocks at Hugo Dummett are overlain by an unmineralized dacite sequence (369 ± 2 Ma). The basal breccia and conglomerate of the dacite sequence contain mineralized porphyry clasts derived from erosion of the porphyry deposits. Two zircon grains recovered from one of these mineralized porphyry clasts yielded chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) 206 Pb/ 238 U dates of 374.0 ± 1.9 and 371.4 ± 1.6 Ma, which are consistent with previously published ages for mineralized intrusions. This implies that Oyu Tolgoi or other nearby porphyry deposits were exhumed shortly after the main mineralization event. Weakly mineralized granodiorite dikes intruded the postmineralization dacite sequence, as well as Devonian sedimentary and volcaniclastic rocks that structurally overlie the dacite. A weighted mean 206 Pb/ 238 U age of 370.6 ± 0.5 Ma for this rock is based on the three youngest grains analyzed by the CA-TIMS technique (MSWD = 1.14). Burial of the Cu-Au deposits at Oyu Tolgoi soon after their exhumation was critical to their preservation. Exhumation and subsequent burial occurred at ~371 Ma, during a short time frame within the overlapping U-Pb zircon ages for the quartz monzodiorite, mineralized porphyry clast, unmineralized dacite sequence, and granodiorite unit.