Zircon petrochronology of volcano-sedimentary sequences unraveling the timing of epithermal gold mineralization at the Axi deposit, Tianshan, NW China

Abstract

Epithermal gold deposits are prime exploration objectives in subaerial volcano-sedimentary basins. However, the ages of epithermal gold deposits in ancient orogenic belts have long been difficult to obtain, largely due to the general lack of minerals unequivocally linked to gold deposition that can be accurately dated by radiogenic isotope techniques. The well-preserved Devono-Carboniferous volcano-sedimentary sequences and angular unconformity directly overlap the orebodies of the Axi epithermal gold deposit in Chinese Tianshan and provide a rare opportunity to unravel the timing of gold mineralization and post-mineralization processes through combined U–Pb zircon dates with zircon geochemistry investigations from volcanic host rocks, basal breccias and its sedimentary cover. Two weakly-mineralized host andesite samples yielded zircon U–Pb ages of 361.2 ± 1.8 Ma and 349.7 ± 2.6 Ma, respectively. The basal breccia of the sedimentary cover immediately above the angular unconformity contains a mineralized andesite clast (348.2 ± 2.0 Ma) derived from the erosion of the underlying epithermal deposit. Combined with a maximum depositional age (MDA) of ca. 344.0 ± 3.8 Ma for the clastic rocks directly overlain the basal breccia, suggesting that the Axi angular unconformity was formed in the early Visean (ca. 348.2–344.0 Ma). The youngest detrital zircons from clastic rocks in the upper section of the sedimentary cover constrain its deposition to ca. 344.0–305.3 Ma. These data together indicate that epithermal gold mineralization at Axi predates formation of the angular unconformity at no later than ca. 344.0 Ma. Rapid burial of the gold mineralization by the sedimentary cover (i.e., Aqialehe Fm; ca. 344.0–305.3 Ma) soon after exhumation (ca. 348.2–344.0 Ma) was crucial for the preservation of Early Carboniferous epithermal gold mineralization at Axi.