Abstract:

Carbon and oxygen isotopic values provide criteria for the evaluation of depositional environment. Sixteen siderite samples from the Dajing ore deposit, Inner Mongolia, China, were analyzed in order to determine depositional environments. Siderites at the Dajing ore deposit are characterized by δ¹³C_PDB from -1.8 ‰ to -6.6 ‰ and δ¹⁸O_VPDB from -23.1 ‰ to -17.3 ‰, and their values lie in the hydrothermal sedimentary area on a δ¹³C versus δ¹⁸O diagram, showing hydrothermal sedimentary genesis of siderite. The palaeotemperature is estimated, based on the following equation: T=16.998-4.52(δ¹⁸_cO-δ¹⁸_wO) + 0.03(δ¹⁸_cO-δ¹⁸_wO)². A higher palaeotemperature, averaging 117°C, suggests that the mineralizing fluids are from hydrothermal sedimentary fluids and deep lacustrine water. Depositional settings of limestone can be inferred from equation: Z=2.048(δ¹³C+50)+0.498(δ¹⁸O+50). Samples with Z value above 120 would be classified as marine, while those with Z below 120 as fresh-water ones. The synthetic Z parameter (between 100 and 114) in the Dajing ore deposit, also suggests lacustrine environments. Together with tectonic setting, deposit geology, and rare-earth element composition, the hydrothermal systems are recognized in a lacustrine setting in the early mineralization, which is important not only for a better understanding and interpretation of the genesis of the Dajing deposit but also for exploration of hydrothermal events in the lacustrine environment.

Key words: carbon and oxygen isotope, sedimentary hydrothermal mineralization, lacustrine environment, Dajing ore deposit