Deciphering Au enrichment mechanisms in the Kalatag Cu–Zn volcanogenic massive sulfide (VMS) system, eastern Tianshan, NW China: Insights from the Huangtan Au–Cu–Zn deposit

• The mineralization of Huangtan deposit can be divided into the VMS mineralization stage and the hydrothermal mineralization stage. • The VMS mineralization and hydrothermal mineralization yield Re-Os ages of 436.2 ± 3.8 Ma and 436.3 ± 3.8 Ma, respectively • Invisible gold and petzite occur in the VMS stage, whereas electrum and native gold occur in the hydrothermal stage. • The enrichment of Au was mainly due to hydrothermal fluids rich in precious metals (Au, Ag) precipitating native Au and electrum along fracture structures. Gold-rich volcanogenic massive sulfide (VMS) deposits hold significant industrial value and research importance because their Au content can vary significantly. Despite this, the mechanisms responsible for the concentration of Au and the hosts of the Au in VMS systems remain poorly constrained. The Kalatag region in the eastern Tianshan of the Central Asian Orogenic Belt contains numerous economically important Cu deposits. The mineralizing processes in the Huangtan deposit can be divided into VMS and hydrothermal mineralization stages. The VMS stage can be further subdivided into the pyrite stage, chalcopyrite–sphalerite stage, and barite stage, whereas the hydrothermal mineralization stage can be subdivided into the quartz–sulfide stage and quartz–calcite stage. The Re–Os ages of pyrite in the Cu–Zn sulfide mineralization of the VMS stage (436.2 ± 3.8 Ma) and vein-type Au mineralization of the hydrothermal stage (436.3 ± 3.8 Ma) are indistinguishable. In the VMS stage, Au occurs as invisible Au in pyrite and as petzite along the margins of chalcopyrite. Economically significant metals (e.g., Au, Ag, Te, Sb, and Bi) predominantly occur within pyrite or as native metal inclusions concentrated along fractures that crosscut pyrite grains within the VMS stage. In contrast, Au largely occurs as electrum and native gold in quartz–sulfide veins of the hydrothermal stage. In the hydrothermal stage, the quartz within fractures between pyrite grains has elevated V, Au, and Ag contents, and low Te contents; the Ag, Au, and Te are likely present as nanoparticles or microinclusions. The contents of Co, Ni, As, Bi, and Te in pyrite in the hydrothermal mineralization stage are similar to those observed during the VMS mineralization stage, whereas Au and Ag concentrations are higher in pyrite in the hydrothermal stage than in the VMS stage. The Au-bearing ores occur as vein-like or stockwork-like structures along fractures that crosscut massive sulfide ores that formed concomitantly with sedimentary tuff, implying that vein-type Au mineralization formed slightly after the VMS mineralization. Driven by a submarine volcanic hydrothermal system, Cu–Zn-rich hydrothermal fluids ascended through volcanic channels to submarine slopes or depressions. During periods of volcanic quiescence, subvolcanic Au-bearing hydrothermal fluids infiltrated the intermediate–felsic tuff and sedimentary tuff. As temperature and pressure decreased, hydrothermal fluids precipitated precious metals (Au, Ag) along fractures, concentrating in the hanging wall fracture structures of the massive orebodies.