Abstract

The Ak-Sug porphyry Cu-Au-Mo deposit is situated within the Altai Sayan region that forms part of the Tuva-Mongol arc of the Central Asian orogenic belt. The Ak-Sug intrusive complex is made up of a series of nested intrusions, including diorite, feldspar porphyry, quartz-feldspar porphyry, tonalite I, and tonalite II. Cu-Au-Mo mineralization occurs mainly within quartz-carbonate ± K-feldspar sulfide veins and later sericite-sulfide veins and alteration. In the northern zone, sulfide zoning consists of a central bornite core flanked by chalcopyrite and pyrite-chalcopyrite zones, with a pyrite zone on the hanging-wall side.

Re-Os molybdenite dating of an early-stage quartz-carbonate ± K-feldspar sulfide vein and a late-stage molybdenite-only vein gives ages of 517.3 ± 3 and 517.4 ± 3 Ma, respectively. This indicates an Early Cambrian age for mineralization at Ak-Sug, making it one of the oldest porphyry copper deposits in the Central Asian orogenic belt. Published whole-rock geochemical data indicate that intrusive rocks at Ak-Sug and other Cambrian to Early Ordovician intrusive complexes in the Tuva-Mongol arc evolved by hornblende ± titanite fractionation due to high magmatic water contents, leading to adakite-like compositions with high Sr/Y and flat to listric-shaped rare earth element patterns that characterize many fertile arc magmas.

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