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Methane origin and oxygen-fugacity evolution of the Baogutu reduced porphyry Cu deposit in the West Junggar terrain, China

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Abstract

Most porphyry copper deposits worldwide contain magnetite, hematite, and anhydrite in equilibrium with hypogene copper-iron sulfides (chalcopyrite, bornite) and have fluid inclusions with CO2 >> CH4 that are indicative of high fO2. In contrast, the Baogutu porphyry Cu deposit in the West Junggar terrain (Xinjiang, China) lacks hematite and anhydrite, contains abundant pyrrhotite and ilmenite in equilibrium with copper-iron sulfides (chalcopyrite), and has fluid inclusions with CH4 >> CO2 that are indicative of low fO2. The mineralized intrusive phases at Baogutu include the main-stage diorite stock and minor late-stage diorite porphyry dikes. The main-stage stock underwent fractional crystallization and country-rock assimilation-contamination, and consists of dominant diorite and minor gabbro and tonalite porphyry. The country rocks contain organic carbons (0.21–0.79 wt.%). The δ13CvPDB values of the whole rocks (−23.1 to −25.8 ‰) in the wall rocks suggest a sedimentary organic carbon source. The δ13CvPDB values of CH4 (−28.2 to −36.0 ‰) and CO2 (−6.8 to −20.0 ‰) in fluid inclusions require an organic source of external carbon and equilibration of their Δ13CCO2-CH4 values (8.2–25.0 ‰) at elevated temperatures (294–830 °C) suggesting a significant contribution of thermogenic CH4. Mineral composition data on the main-stage intrusions, such as clinopyroxene, hornblende, biotite, magnetite, ilmenite, sphene, apatite, and pyrrhotite, suggest that the primary magma at Baogutu was oxidized and became reduced after emplacement by contamination with country rocks. Mineral compositions and fluid inclusion gas compositions suggest that the redox state of the system evolved from logfO2 > FMQ + 1 in the magma stage, to logfO2 < FMQ as a consequence of country rocks assimilation-contamination, to logfO2 > FMQ in the hydrothermal stage. Though oxidized magma was emplaced initially, assimilation-contamination of carbonaceous country rocks decreased its fO2 such that exsolved fluids contained abundant CH4 and deposited a reduced assemblage of minerals.

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Acknowledgments

We are very grateful to Editor-in-chief Georges Beaudoin, associate editor T. Wagner, and two reviewers for constructive comments and improvement of the manuscript. Editor-in-chief Georges Beaudoin and one reviewer have also carefully provided detailed corrections which substantially improved the presentation. We are indebted to H.P. Zhu for help during the gas compositions analysis, Q. Mao for help during the mineral composition analysis, and L.J. Feng for help during the carbon isotope analysis. This paper was financially supported by the National Natural Science Foundation of China (Grant No. U1303293, 41390442, 41272109, 40972064), Key Deployment Project of the Chinese Academy of Sciences (Grant No. KJZD-EW-TZ-G07), National International Cooperation in Science and Technology Project (Grant No. 2010DFB23390), and National 305 Project (Grant No. 2011BAB06B01).

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  1. Key Laboratory of Mineral Resources, Institute of Geology & Geophysics, Chinese Academy of Sciences, Beijing, 100029, China

    Ping Shen

  2. College of Earth Sciences, Chang’an University, Xi’an, 710054, China

    HongDi Pan

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Shen, P., Pan, H. Methane origin and oxygen-fugacity evolution of the Baogutu reduced porphyry Cu deposit in the West Junggar terrain, China. Miner Deposita 50, 967–986 (2015). https://doi.org/10.1007/s00126-015-0580-5

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