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Fingerprinting hydrothermal fluids in porphyry Cu deposits using K and Mg isotopes

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Abstract

In this study, we performed an integrated investigation of K and Mg isotopes in hydrothermally altered rocks from the giant Dexing porphyry Cu deposit in China. Both the altered porphyry intrusion and the surrounding wall rocks exhibit large variations in K and Mg isotope compositions, with δ41K values ranging between −1.0296‰ and 0.38‰, and δ26Mg values ranging between −0.49‰ and 0.32‰. The δ41K and δ26Mg values of the majority of altered samples are higher than the isotopic baseline values for upper continental crust. We attribute the general increase in δ41K and δ26Mg in altered rocks to hydrothermal alteration, which caused preferential incorporation of heavy K and Mg isotopes in alteration products, particularly phyllosilicates. However, a few altered samples show anomalously low δ41K and δ26Mg values. The δ41K and δ26Mg values do not correlate with K and Mg concentrations, or mineralogy of altered samples. The variable K-Mg isotope data likely reflect fluids of different physical-chemical properties, or different isotopic compositions. Based on the combined K-Mg isotope data, at least three groups of hydrothermal fluids are distinguished from the Dexing porphyry deposit. Therefore, K-Mg isotopes are potentially a novel tracer for fingerprinting fluids in complex hydrothermal systems.

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Acknowledgements

Brian Beard helped in mass spectrometry for K isotope analysis. This manuscript benefited from constructive comments from two anonymous reviewers, as well as editorial handling by Prof. Fangzhen Teng. This study was supported by the National Key R & D Program of China (Grant No. 2018YFC0604106) and the National Natural Science Foundation of China (Grant Nos. 41622301, 41873004).

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Authors and Affiliations

  1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210046, China

    Weiqiang Li, Shugao Zhao, Xiaomin Wang, Shilei Li, Guoguang Wang & Tao Yang

  2. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, 710061, China

    Zhangdong Jin

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  1. Weiqiang Li
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  2. Shugao Zhao
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Correspondence to Weiqiang Li.

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Li, W., Zhao, S., Wang, X. et al. Fingerprinting hydrothermal fluids in porphyry Cu deposits using K and Mg isotopes. Sci. China Earth Sci. 63, 108–120 (2020). https://doi.org/10.1007/s11430-018-9387-2

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