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Potassium isotopes as a tracer of hydrothermal alteration in ore systems

Authors

Qiu,  Kun-Feng
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/persons/resource/romer

Romer,  R. L.
3.1 Inorganic and Isotope Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

Long,  Zheng-Yu
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Yu,  Hao-Cheng
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Turner,  Simon
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Wan,  Ruo-Qi
External Organizations;

Li,  Xiao-Qiang
External Organizations;

Gao,  Zi-Yue
External Organizations;

Deng,  Jun
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Citation

Qiu, K.-F., Romer, R. L., Long, Z.-Y., Yu, H.-C., Turner, S., Wan, R.-Q., Li, X.-Q., Gao, Z.-Y., Deng, J. (2024): Potassium isotopes as a tracer of hydrothermal alteration in ore systems. - Geochimica et Cosmochimica Acta, 368, 185-196.
https://doi.org/10.1016/j.gca.2024.01.005


Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5025581
Abstract
Hydrothermal alteration is crucial in the formation of many ore deposits, with potassium (K) mobilization and cycling being prevalent. Potassic metasomatism of wall rocks generally forms K-bearing minerals, such as hydrothermal feldspar and mica. However, determining the source and redistribution of K (and other elements transported by the same fluid) in hydrothermal systems is challenging. K isotopes offer a potential solution to this problem. This study presents new K isotope data from two K-rich alteration assemblages — K-feldspar and sericite-quartz-pyrite — in the Jiaodong gold province of China. The data covers a compositional range from unaltered granites to syn-magmatic potassic alteration (formation of K-feldspar) and post-magmatic syn-mineralization phyllic alteration (formation of sericite). Potassic alteration in granite correlates with significant K addition, whereas phyllic alteration of earlier phases of magmatic and hydrothermal K-feldspar resulted in K loss. K-feldspar altered granites display similar δ41K values (–0.55 to –0.42 ‰ for whole-rocks and –0.56 to –0.48 ‰ for K-feldspar separates) as unaltered granite (–0.52 to –0.47 ‰). The narrow δ41K range suggests that magmatic fluid exsolution and magmatic-hydrothermal alteration have a minor effect on δ41K of the altered rock. Phyllic alteration of K-feldspar altered precursor rock leads to K loss and elevated δ41K values ranging from –0.36 to –0.19 ‰ for whole-rocks and –0.34 to –0.17 ‰ for sericite mineral separates. As sericite preferentially incorporates 41K, sericite will have higher δ41K values than the precursor K-feldspar, whereas the fluids will have lower δ41K values. Our study demonstrates that hydrothermal alteration may affect the K isotope composition of altered rocks in several ways, contingent on the nature of the involved phases, making K isotopes a promising tool for studying hydrothermal alteration and associated mineralization.