The water cycle is an integral part of Earth surface dynamics, and water’s oxygen-isotope composition retains information about the forcing and response of Earth’s local and global climate. Water isotope signals of the recent geological past can be directly obtained from archives such as ice cores, groundwater, or pore fluid. For the more distant past, mineral proxies have to be used. Multiple episodes of global glaciation may have occurred in the Neoproterozoic Era, of which the record of oxygen-isotope composition of glacial meltwater is sparse; the few records that are derived from carbonate minerals are prone to late-burial and metamorphic alteration, and therefore subject to alternative explanations. Here we present a case in which meltwater δ18O is retrieved from barite (BaSO4) and malachite (Cu2CO3(OH)2)–associated sulfate (MAS) in a diamictite in Kaiyang, Guizhou, South China. The core of our argument is based on the lowest-ever-published sulfate δ18O values found in the barite and MAS, reaching as low as –20.3‰ (Vienna standard mean ocean water, VSMOW). These data suggest that the water involved in the oxidative weathering of these chalcocite clasts had a δ18O value of –34‰ ± 10‰, similar to that of polar glaciers today. Excluding the possibility of glacier meltwater alteration during the past 600 m.y. after the deposition of the diamictite, the sulfate mineral assemblage reported here provides an important constraint on the nature of the Neoproterozoic glaciation that the Kaiyang diamictite represents.