The Sulphur Bank Mercury Mine (SBMM) hydrothermal system offers insights into active degassing processes in the Clear Lake volcanic field (CLVF), a high-threat region based on its record of Holocene eruptions and proximity to populated areas. Here we present chemical and isotopic analyses of gas samples collected between 2015 and 2023, along with the first comprehensive CO₂ flux survey of the SBMM area conducted in 2023. Sampled gases are CO₂- and CH₄-rich (≥84 and 6 mol% in dry gas, respectively) with high mantle-derived helium contributions (³He/⁴He = 6.54–7.86 R_C/R_A). Carbon isotopic compositions of CO₂ (δ¹³C = −10.0 to −9.5 ‰) and CH₄ (δ¹³C = −35.8 ‰) indicate mixed sources, with significant contributions from metamorphism of organic-rich Franciscan Complex rocks hosting the hydrothermal system. Modeling of gas compositions shows that scrubbing by interaction with air-saturated groundwater strongly influences observed compositional variability. From our CO₂ flux measurements, we estimate the deeply derived CO₂ emission rate from the SBMM hydrothermal area (0.2 km²) at 240 t d⁻¹, comparable to many quiescently degassing volcanoes worldwide. We also provide a first-order estimate of CH₄ emissions at approximately 0.5 t d⁻¹. Our findings establish crucial baseline data for future volcanic monitoring efforts, enhancing detection capabilities for potential changes in this active hydrothermal system. This work contributes to the broader understanding of volatile contributions from volcanic and metamorphic sources to the global carbon budget, while highlighting the strong influence of bedrock geology on gas compositions in the CLVF.