The Tengchong–Lianghe tin belt in western Yunnan Province, southwest China, is the most important tin mineralization belt in the Sanjiang Tethyan Metallogenic Domain (STMD). There are two large tin deposits in this belt: the Paleogene Lailishan and the Late Cretaceous Xiaolonghe deposits. The two deposits are spatially and temporally associated with the Lailishan and Xiaolonghe granitic plutons, respectively. Recent studies suggest that the Lailishan and Xiaolonghe granitic plutons exhibit most of the mineralogical and geochemical characteristics of A-type granites. In this study, we discuss the He, Ar, and S isotopes of pyrite samples from the two large tin deposits to trace ore-forming fluids and elements. We also review the relationship between A-type granites and tin deposits from a new perspective. The δ³⁴S_CDT values of the Lailishan tin deposit range from +4.9‰ to +6.7‰, with an average value of +5.53‰. The δ³⁴S_CDT values of the Xiaolonghe tin deposit range from +5.0‰ to +8.1‰, with an average value of +6.33‰. These values are slightly higher than those of the granites (0‰ to +5.7‰) in the Tengchong–Lianghe area, indicating mainly magmatic sources for the sulfur of ore-forming fluids with a small amount of S from the wall rocks. The Lailishan tin deposit has ³He/⁴He values of 1.57–3.46 Ra, with an average value of 2.078 Ra, and ⁴⁰Ar/³⁶Ar values of 382.00–622.99, with an average value of 459.67. The Xiaolonghe tin deposit has ³He/⁴He values of 0.53–0.88 Ra, with an average value of 0.686 Ra, and ⁴⁰Ar/³⁶Ar values of 301.06–348.43, with an average value of 322.04. These values suggest that ore-forming fluids of the Lailishan and Xiaolonghe tin deposits have mixed crustal, mantle, and a small volume of meteoric water sources in different proportions. The mantle ⁴He values and ³He/⁴He values of the Lailihsan tin deposit (26–44%, 1.57–3.46 Ra) are markedly higher than those of the Xiaolonghe tin deposit (8–15%, 0.53–0.88 Ra), implying increased crust–mantle interaction in the Paleogene relative to the Late Cretaceous in western Yunnan. This interaction may be attributed to the upwelling of the asthenosphere through the break-off of the Neo-Tethyan slab during the main collisional period (65–41 Ma) of India–Asia.