Two generations of garnets and their relevance for the hydrothermal fluid evolution of the Hongyuntan deposit, NW China

•Two distinct generations (three types) of garnets were identified in the Hongyuntan deposit.•The temperature, salinity, oxygen fugacity (fO2), and pH of the Hongyuntan ore-forming fluid fluctuated.•Multiple episodic supplies of hydrothermal fluids occurred in the Hongyuntan deposit.•The Hongyuntan ore-forming fluid was likely formed through a magmatic–hydrothermal process.