Fingerprinting hydrothermal fluids in porphyry Cu deposits using K and Mg isotopes

In this study, we performed an integrated investigation of K and Mg isotopes in hydrothermally altered rocks from the giant Dexing porphyry Cu deposit in China. Both the altered porphyry intrusion and the surrounding wall rocks exhibit large variations in K and Mg isotope compositions, with δ⁴¹K values ranging between −1.0296‰ and 0.38‰, and δ²⁶Mg values ranging between −0.49‰ and 0.32‰. The δ⁴¹K and δ²⁶Mg values of the majority of altered samples are higher than the isotopic baseline values for upper continental crust. We attribute the general increase in δ⁴¹K and δ²⁶Mg in altered rocks to hydrothermal alteration, which caused preferential incorporation of heavy K and Mg isotopes in alteration products, particularly phyllosilicates. However, a few altered samples show anomalously low δ⁴¹K and δ²⁶Mg values. The δ⁴¹K and δ²⁶Mg values do not correlate with K and Mg concentrations, or mineralogy of altered samples. The variable K-Mg isotope data likely reflect fluids of different physical-chemical properties, or different isotopic compositions. Based on the combined K-Mg isotope data, at least three groups of hydrothermal fluids are distinguished from the Dexing porphyry deposit. Therefore, K-Mg isotopes are potentially a novel tracer for fingerprinting fluids in complex hydrothermal systems.