Earth Science Frontiers ›› 2023, Vol. 30 ›› Issue (2): 384-400.DOI: 10.13745/j.esf.sf.2021.10.4

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Discriminating characteristics of hydrothermal fluids using epidote mineral chemistry and strontium isotopes: A case study of the Duotoushan Fe-Cu deposit, eastern Tianshan

ZHANG Weifeng1()(), CHEN Huayong2,3,*(), DENG Xin1, JIN Xinbiao1, LIU Shuzhan1, TAN Juanjuan1   

  1. 1. Wuhan Center of Geological Survey, China Geological Survey, Wuhan 430205, China
    2. CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
    3. Center of Excellence for Deep Earth Sciences, Chinese Academy of Sciences, Guangzhou 510640, China
  • Received:2021-04-13 Revised:2021-07-21 Online:2023-03-25 Published:2023-01-05
  • Contact: CHEN Huayong

Abstract:

Epidote, a common Ca-Al-bearing silicate mineral in many types of hydrothermal deposits, usually has high concentrations of trace elements, thus its mineral chemistry as well as Sr isotopic characteristics can reveal the physico-chemical conditions and sources of the ore-forming fluid. In this contribution we present a study on the epidote-group minerals precipitated during Fe mineralization at the Duotoushan deposit. With high Al-coordination numbers (1.95-2.24), and enriched in Fe3+ but depleted in Fe2+ and rare earth elements (REE), the studied mineral grains are identified as the epidote end member. The mineral grains are further characterized by MREE enrichment and LREE depletion with moderate negative Eu anomalies, suggesting the complexation of hard acids at neutral to basic pH may have caused the REE fractionation. Correlation analysis indicates REEs are mainly incorporated into the Duotoushan epidote through isomorphous substitution. At Duotoushan, the epidote-group minerals coexist with quartz, titanite and magnetite, exhibit negative Ce anomalies (0.64-0.91), and have high U/Th ratios, indicating they are formed under relatively high oxygen fugacity. Since the in-situ (87Sr/86Sr)i ratios (0.70462-0.70495) lie between the Bailingshan granitic complex and hosted rocks or Carboniferous seawater, we propose that the initial fluids are magmatic in origin and external basinal fluid may have added into the hydrothermal system during water-rock interaction. Furthermore, our study indicates that epidote is a robust indicator for tracing fluid evolution in magmatic hydrothermal deposits.

Key words: epidote, mineral chemistry, Sr isotope, fluid evolution, eastern Tianshan

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