The Platreef ore horizon of the Bushveld Complex, South Africa, is the third largest platinum group element ore deposit in the world, but the origin of its ore remains enigmatic. A complex contact relationship between the igneous and footwall rocks of the Bushveld Complex, coupled with evidence for widespread late-stage hydrothermal processing, obscures the original mineralization history of the Platreef. We constrain the parental magmatic origin of the Platreef by exploiting multiple sulfur isotope contrasts across Bushveld Complex contact zones in order to see through the effects of postmineralization hydrothermal activity. We report S isotope measurements made on samples collected along two profiles through the Platreef into underlying metapelitic and metacarbonate footwall rocks. In both profiles, igneous rocks far from the contact have low Δ33S values (average Δ33S = 0.15‰), whereas metasedimentary rocks far from the contact have high Δ33S values (Δ33S to 5.04‰) with a smoothly varying profile between the two end members. The midpoint in both isotope profiles is displaced into the footwall, defining a classic advective-dispersive tracer geometry. This geometry is not present in the associated δ34S values. The displacement of the Δ33S front suggests fluid transport and advection of S into the country rocks; this was accompanied by back diffusion of the S isotope tracer into the Platreef. The Platreef magma was apparently S saturated prior to emplacement and, counterintuitively, lost S during the formation of the present Platreef ore horizon.