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Geochemistry and Mineralogy of Ilmenite Exsolutions in Titanomagnetite and Their Implications for the Ore-Forming Process at the Damiao Deposit
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Abstract
The Damiao Fe-Ti-P deposit, located within the Damiao anorthosite complex in northeastern China, features Fe-Ti oxide ores and nelsonites that occur as irregularly inclined stratiform-like bodies, lenses, or veins with sharp contacts against anorthosite and gabbronorite. This deposit is characterized by abundant titanomagnetite that hosts diverse ilmenite exsolution textures, including blocky, lamellar, and cloth-like forms. In this study, we investigate the geochemistry and mineralogy of ilmenite exsolutions in titanomagnetite to understand their formation mechanisms and implications for the ore-forming process. Detailed petrographic observations and electron microprobe analyses reveal that the exsolution textures result from multiple mechanisms: oxy-exsolution due to titanomagnetite oxidation; subsolidus re-equilibration between magnetite and ilmenite involving elemental diffusion of Fe, Ti, Cr, Co, and Ni; and exsolution related to lattice defects caused by rapid cooling. Thermodynamic modeling using Gibbs free energy calculations and the QUILF program indicates that blocky, lamellar, and cloth-textured ilmenite exsolutions formed at temperatures above and below the solid solution solvus under decreasing oxygen fugacity. Additionally, our results indicate that the exsolution of zircon and pleonaste at ilmenite grain boundaries is attributed to the saturation and precipitation of elements like Zr and Al, due to the oxidation of titanomagnetite, rather than interactions between ilmenite and adjacent clinopyroxene. Reconstruction of the cooling history suggests that the oxygen fugacity of oxide-apatite gabbronorites was significantly higher than that of Fe-Ti-P ores. This confirms that increasing oxygen fugacity during magma evolution promoted immiscibility, leading to the formation of nelsonitic melts and ultimately the development of Fe-Ti-P ores.
DOI
https://doi.org/10.31223/X55M7T
Subjects
Geochemistry, Geology
Keywords
Ilmenite exsolution; Oxy-exsolution; Titanomagnetite; Subsolidus re-equilibration; Damiao Fe-Ti-P deposit
Dates
Published: 2025-02-15 08:41
Last Updated: 2025-02-15 16:41
License
CC BY Attribution 4.0 International
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Conflict of interest statement:
Funding：This work was funded by the National Natural Science Foundation of China (grant 42102094) and Natural Science Foundation of Hebei (grant D2022402028). Conflicts of interest：On behalf of all authors, the corresponding author states that there is no conflict of interest. Availability of data and samples: Data and samples used in this study are available from the corresponding author upon reasonable request. Consent for publication: All authors have read and approved the final version of the manuscript and agree to its submission to Acta Geochimica. Authors' contributions: 1. Kaiyuan Wang: Designed the research framework and study objectives. Conducted detailed petrographic observations of the Damiao Fe-Ti-P deposit, focusing on the diverse ilmenite exsolution textures within titanomagnetite. Performed electron microprobe analyses to determine the geochemistry and mineralogy of the exsolutions. Drafted the initial manuscript, particularly sections on the formation mechanisms and implications of the exsolutions for the ore-forming process. 2. Hongtao He: Assisted in the collection and analysis of geochemical data. Contributed to the thermodynamic modeling using Gibbs free energy calculations to understand the temperature and oxygen fugacity conditions under which the exsolutions formed. Participated in manuscript revisions, enhancing the discussion on the physicochemical conditions and cooling history of the deposit. 3. Wenjie Shi: Provided the geological context of the Damiao deposit, including insights into the Proterozoic anorthosite complexes and mafic-ultramafic intrusions. Helped interpret the complex ilmenite exsolution textures and their relevance to the ore-forming processes. Code availability: This study utilized publicly available software for the analyses. The software and its details are as follows: 1. Andersen, David J., Donald H. Lindsley, and Paula M. Davidson. "QUILF: A pascal program to assess equilibria among Fe-Mg-Mn-Ti oxides, pyroxenes, olivine, and quartz." Computers & Geosciences 19.9 (1993): 1333-1350.” 2. Yavuz, Fuat. "WinMIgob: A Windows program for magnetite-ilmenite geothermometer and oxygen barometer." Journal of Geosciences 66.1 (2021): 51-70. Ethics approval: N/A Consent to participate: N/A Acknowledgments：We sincerely thank Wenqin Zheng and Xiang Li for their assistance in EPMA laboratory work and data processing.
Data Availability (Reason not available):
The data will be made publicly available through a designated repository upon the formal publication of the paper.
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