Potassium isotopes as a tracer of hydrothermal alteration in ore systems

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.