Geologic, Fluid Inclusion and Stable Isotope Constraints on Mechanisms of Ore Deposition at the Datuanshan Copper Deposit, Middle–Lower Yangtze Valley, Eastern China

Abstract

The Datuanshan deposit is one of the largest and most representative stratabound copper deposits in the Tongling area, the largest ore district in the Middle–Lower Yangtze River metallogenic belt. The location of the orebodies is controlled by the interlayer–slipping faults between the Triassic and Permian strata, and all the orebodies are distributed in stratiform shape around the Mesozoic quartz monzodiorite dikes. Based on field evidence and petrographic observations, four mineralization stages in the Datuanshan deposit have been identified: the skarn, early quartz–sulfide, late quartz–sulfide and carbonate stages. Chalcopytite is the main copper mineral and mainly formed at the late quartz–sulfide stage. Fluid inclusions at different stages were studied for petrography, microthermometry, laser Raman spectrometry and stable isotopes. Four types of fluid inclusions, including three‐phase fluid inclusions (type 1), liquid‐rich fluid inclusions (type 2), vapour–rich fluid inclusions (type 3) and pure vapour fluid inclusions (type 4), were observed. The minerals from the skarn, early and late quartz–sulfide stages contain all fluid inclusion types, but only type 2 fluid inclusions were observed at the carbonate stage. Petrographic observations suggest that most of the inclusions studied in this paper are likely primary. The coexistence of different types of fluid inclusions with contrasting homogenization characteristics (to the liquid and vapour phase, respectively) and similar homogenization temperatures (the modes are 440–480°C, 380–400°C and 280–320°C for the skarn, early and late quartz–sulfide stages, respectively) in the first three stages, strongly suggests that three episodes of fluid boiling occurred during these stages, which is supported by the hydrogen isotope data. Laser Raman spectra identified CH4 at the skarn and early quartz–sulfide stages. Combined with other geological features, the early ore–forming fluids were inferred to be under a relatively reduced environment. The CO2 component has been identified at the late quartz–sulfide and carbonate stages, indicating that the late ore‐forming fluids were under a relatively oxidized environment, probably as a result of inflow of and mixing with meteoric water. In addition, microthermometric results of fluid inclusions and H–O isotope data indicate that the ore forming fluids were dominated by magmatic water in the early stages (skarn and early quartz–sulfide stages) and mixed with meteoric water in the late stages (late quartz–sulfide and carbonate stages). The evidence listed above suggests that the chalcopyrite deposition in the Datuanshan deposit probably resulted from the combination of multi‐episode fluid boiling and mixing of magmatic and meteoric water.