The Upper Proterozoic quartzites from the Taoudeni basin contain up to 27% quartz cement occurring as overgrowth on detrital grains. An oxygen isotope study of the quartz cement in the Upper Proterozoic quartzites of western Mali (Souroukoto Group, So 2 formation) permitted an assessment of the nature and origin of the cementing fluids as well as the source of cement. The delta 18 O values measured for the quartz overgrowths range from 15.9 to 17.7 per mil SMOW. These data, combined with petrological and geological evidence, suggest that quartz cementation took place relatively early after deposition, at shallow depth and low temperature from meteoric fluids. The markedly 18 O-depleted (with respect to seawater) character of the parent fluids (delta 18 O values between -25 and -11 per mil SMOW) is directly related to the cold climatic conditions prevailing on the West African craton during Late Proterozoic times. Mass transfer, paleogeographic, and paleoclimatologic considerations suggest that the bulk of the silica necessary to form the observed volume of quartz cement was internally derived.