This study presents a new high-precision 40Ar/39Ar age for the Devonian hot-spring system at Rhynie. Hydrothermal K-feldspar sampled from two veins that represent feeder conduits and a hydrothermally altered andesite wall rock, date the hydrothermal activity, the fossilised biota, and syn - K-feldspar gold mineralization at 403.9±2.1Ma (2σ). Oxygen isotope data for the parent fluid (-4‰ to 2‰) show that the K-feldspar was precipitated from a dominantly meteoric fluid, which mixed with magmatic fluids from a degassing magma chamber. The 40Ar/39Ar age (403.9±2.1Ma [2σ]) when recalculated (407.1±2.2Ma [2σ]) with respect to the astronomically tuned age for Fish Canyon sanidine (28.201±0.023Ma [1σ]), also provides a robust marker for the polygonalis-emsiensis Spore Assemblage Biozone within the Pragian-?earliest Emsian. Furthermore, the age identifies the Devonian pull-apart volcano-sedimentary basins of the British and Irish Caledonides (and their root zones), as specific targets for future gold exploration.