Fingerprinting hydrothermal fluids in porphyry Cu deposits using K and Mg isotopes
- 15 Downloads
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.
Unable to display preview. Download preview PDF.
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).
- Bailey S W. 1984. Classification and structures of the micas. Rev Mineral Geochem, 13: 1–12Google Scholar
- Jin Z D. 1999. Geochemistry and evolution ore-forming fluids at Tongchang poprhyry copper deposit, Dexing county, Jiangxi province, Departaient of Earth Sciences (in Chinese). Dissertation for Doctoral Degree. Nanjing: Nanjing University. 114Google Scholar
- Johnson C M, Beard B L, Albarede F. 2004. Geochemistry of non-traditional stable isotopes. Mineralogical Society of America Geochemical SocietyGoogle Scholar
- Pan X F, Song Y C, Li Z Q, Hu B G, Zhu X Y, Wang Z K, Yang D, Zhang T F, Li Y. 2012. Restriction of H-O iostopes for alteration and mineralization ssytem of Tongchang Cu (-Mo-Au) porphyric deposit, Jiangxi Province (in Chinese). Mineral Deposits, 31: 850–860Google Scholar
- Reed M H. 1997. Hydrothermal alteration and its relationship to ore fluid composition. In: Barnes H L, ed. Geochemistry of Hydrothermal Ore Deposits. 3rd ed. New York: Wiley. 303–366Google Scholar
- Seedorff E, Dilles J H, Proffett J M, Einaudi M T, Zurcher L, Stavast W J A, Johnson D A, Darton M D. 2005. Porphyry deposits: Characteristics and origin of hypogene features. Econ Geol 100th Anniversary Volume. 251–298Google Scholar
- Weiss Z, Wiewiora A. 1986. Polytypism of micas. III. X-ray diffraction identification. Clays Clay Miner, 34: 53–68Google Scholar
- Zhu X, Huang C K, Rui Z Y, Zhou Y H, Zhu X J, Hu C S, Mei Z K. 1983. Dexing Porphyry Copper Deposit (in Chinese). Beijing: Geological Publishing HouseGoogle Scholar