A correlation has been established between Zn productivity of volcanic-hosted massive sulfide districts and δ18O of quartz phenocrysts from cogenetic rhyolitic rocks. In highly productive districts, ores are associated with rhyolitic rocks, of possible S-type affinity, containing high δ18O quartz phenocrysts. In less-productive districts, ores are associated with I-type rhyolitic rocks with low δ18O phenocrysts. This correlation may be caused by (1) low-temperature isotopic alteration that produced 18O-rich overgrowths on phenocrysts, and (2) emplacement of transitional S-type intrusions at greater depths than I-type intrusions, resulting in larger hydrothermal cells and allowing leaching of more Zn. The latter hypothesis is favored.