Initial in situ sulfur (S) isotope measurements of the Martian bedrock in Gale Crater have revealed an unexpectedly wide range of δ³⁴S values (−47 to +28%). Generally, it is unclear what processes could have contributed to these large isotope fractionations. Therefore, we studied S sources and aqueous SO²⁻₄ cycling in the Valles Caldera volcanic complex, New Mexico to better understand S isotope fractionations related to S degassing, hydrothermal activity, and low-temperature processes in aqueous environment. Overall, our study demonstrates that volcanic systems show large spatial heterogeneity in δ³⁴S. Magmatic S sources are obvious in steam-dominated H₂S degassing and precipitation of secondary minerals from hydrothermal fluids with low δ³⁴S values of +0.9 ±3%. Locally, however, hydrothermal processes have resulted in more negative δ³⁴S values in sulfide minerals (−18 to −4%) and more positive δ³⁴S values in sulfate minerals (−1 to +3%). Major aqueous SO²⁻₄ sources are oxidation of H₂S from modern hydrothermal gas emission, and oxidation and dissolution of sulfide and sulfate minerals present in the hydrothermally altered bedrock and crater-lake sediments. The δ³⁴S of aqueous SO²⁻₄ in surface water and groundwater varies widely (−8 to +5%) and is similar to major S endmembers that undergo oxidation and/or dissolution by active hydrological system. Minor SO²⁻₄ contributions with more positive δ³⁴S values (+9 to +14%) come from deeply circulating geothermal fluids and negligible amounts from atmospheric deposition (+5 to +7% in snow). Elevated SO²⁻₄contents are mainly associated with modern and past H₂S emissions and oxidations near the surface. On regional scale, however, most of the intracaldera bedrock is S-depleted, thus the SO²⁻₄contents are usually low in the surface aquatic system and younger sedimentary lake deposits formed at times of negligible near surface hydrothermal activity. In general, magmatic-hydrothermal processes apparently cause the largest δ³⁴S variation in S-bearing minerals on volcanic terrains. Therefore, we infer that the measured wide range of δ³⁴S values in the Gale sediments by the Curiosity rover on Mars can be explained by S isotope composition of magmatic-hydrothermal sulfide and sulfate minerals that were present in the initial igneous/volcanic rocks prior to crater formation. Later aqueous processes involved oxidation and dissolution of S minerals initially present in these rocks and led to subsequent formation of diagenetic fluids and alteration products enriched in SO²⁻₄ with relatively large δ³⁴S variation. Additionally, physical erosion, transport and deposition of detrital hydrothermal S minerals from igneous/volcanic rocks might be in part responsible for the measured wide range of δ³⁴S in Gale Crater. These unique S isotope results, measured in situ on another planet for the first time, imply the importance of magmatic-hydrothermal fluids in S transport on early Mars and their subsequent alteration in low-temperature aqueous environments.