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.
Abstract Porphyry, epithermal, and volcanogenic massive sulfide (VMS) deposits can form together in the same mineral district in convergent margin environments. Their spatial association and superposition indicate evolving tectonic settings. The Artvin Au-Cu district is one of the major clusters of VMS bimodal-felsic, porphyry, and epithermal deposits in the Eastern Pontides belt in northeast Turkey. Whereas ore-forming processes, timing, and tectonic setting of VMS mineralization are well defined in Artvin, those for porphyry and epithermal mineralization remain less constrained. Our district-scale field study focused on the Hod gold corridor in the Artvin district, which is defined by the NE-trending alignment of the recent Au-Cu mineral discoveries (~205 t Au; ~0.33 Mt Cu; e.g., Hod Maden, Ardala-Salinbaş, and Taç-Çorak) that include Au-rich porphyry, highand intermediate-sulfidation epithermal, carbonate-replacement, and hybrid VMS-epithermal mineralization styles. Our new U-Pb, 40Ar/39Ar, and Re-Os geochronological results interpreted with previously compiled data show that magmatism in the Artvin district formed in the Carboniferous (358–325 Ma), Jurassic (182–174 Ma), Late Cretaceous (92–78 Ma), Eocene (51–40 Ma), and Oligocene (30 Ma). Porphyry and epithermal mineralization along the Hod gold corridor peaked in the Early (~113 Ma; Berta prospect) and Late Cretaceous (~86.5-82 Ma; e.g., Taç and Çorak deposits) and Eocene (~50 Ma; e.g., Ardala deposit), whereas VMS bimodal-felsic mineralization only formed in the Late Cretaceous (~91–85 Ma). Therefore, we interpret that the Hod gold corridor was a long-lived, deep crustal-scale structural feature along which the successive magmatic and mineralization events were emplaced. In addition, the timing of porphyry Cu-Mo mineralization can significantly (>20 Ma) postdate the crystallization age of the intrusive host rocks in the Artvin district, such as at Berta and Balcılı camp, which emphasizes the importance of dating mineralization directly to correctly attribute the tectonic setting. The distribution of Late Cretaceous mineral occurrences suggests a possible eastward temporal evolution from VMS (~91–85 Ma) to epithermal-porphyry systems (~86.5–82 Ma), transitioning from back-arc to arc settings at the onset of the Northern Neotethyan oceanic slab rollback and accompanied by increasingly elevated gold content eastwards across the Artvin district.