Lithium and aluminum are both important metals. Recent studies have found that lithium–rich claystone and karst bauxite deposits are often associated with each other, however, the mechanisms of transformation of lithium–rich claystone to karst bauxite is unclear. In this study, two representative fresh profiles were collected from the Jiujialu Formation (C₁jj) in central Guizhou, and the pyrite and whole rock sulfur isotopes were studied. The results showed that the diameter of framboidal pyrite indicating poor homogeneity (standard deviation = 5.17), suggesting that the water column of the Jiujialu Formation was oxidized. The development of lithium–rich claystone/bauxite in the Jiujialu Formation occurred near the redox interface. The low δ³⁴S_–VCDT value of the whole rock (averaging –15.06 ‰, –33.72 ‰—1.64 ‰) and the presence of multiple generations of pyrite (eroded–annular pyrite) indicated the efficient biogeochemical cycling of sulfur in a fluctuating oxidizing environment. A small amount of samples exhibited heavier sulfur isotope characteristics, and the coexistence of tetrahedral framboidal pyrite and pentagonal octahedral framboidal pyrite indicated intermittent closed sedimentary environments in the Jiujialu Formation. The Si content of some clay minerals around the eroded–annular pyrite is significantly reduced, and the δ³⁴S_–VCDT value are significantly negatively correlated with inactive elements such as Al₂O₃, TiO₂, Ta and Nb, these indicate that the efficient biogeochemical cycling of sulfur resulted in the production and consumption of H⁺ around the pyrite, and the transformation of high silicon–bearing clay minerals around the pyrite to low silicon–bearing clay minerals and aluminum minerals (e.g., diaspore), ultimately leading to lithium–rich claystone transformed into the karst bauxite. The occurrence of this process requires an oxidizing atmospheric environment, which may elucidate why karst-type bauxite is exclusively formed during the Phanerozoic when atmospheric oxygen levels were high.