Origin of the Naoniushan Cu–Au deposit in Inner Mongolia, NE China: Constraints from fluid inclusions and H‐O‐S‐Pb isotopes

Abstract

The Naoniushan Cu–Au deposit is located in the eastern Mongolia–Xing'an orogenic belt, NE China, which hosts abundant endogenous base metal and precious metal deposits. The mineralization mainly occurs in Lower Cretaceous continental volcanic rocks. This study aims to constrain the ore metal sources based on H‐O‐S‐Pb isotopes as well as geological characteristics. The δ34S values range are from 1.5‰ to 3.5‰, with a tower‐shaped distribution pattern, with a peak in the range of 2.0‰ to 2.5‰, indicating a magmatic source. The 206Pb/204Pb values show a range of 18.169–18.510, 207Pb/204Pb of 15.484–15.646, and 208Pb/204Pb of 37.877–38.431. Lead isotopic composition is markedly homogeneous, suggesting that the lead is derived from a Th‐rich, U‐poor source. In the Zartman diagram, the samples plot in the field between mantle and orogenic belt or near to the line of the evolution of orogenic belts. In Δβ–Δγ genetic classification diagram, 12 samples fall into the field of the upper crust and mantle mixed zone, whereas 4 samples fall in the mantle source, adjacent to the boundary of these two source areas. We therefore infer that the ore‐forming materials in the Naoniushan Cu–Au deposit were mainly derived from magmatic sources, with the contamination of upper crustal material. The ore‐forming fluids in this deposit were derived from magmatic fluids in the early stage, followed by addition of meteoric water. We correlate the tectonic setting to the transition from a compressional to extensional regime.