Important epithermal tungsten mineralization in the Boulder County district is mainly hosted by quartz-ferberite (FeWO4) veins. Isotopic studies based on an extensive grid (δD) and individual samples of wall-rock alteration and mineralization (δD, δO) have demonstrated the dominance of meteoric fluids during alteration processes. However, the involvement of magmatic fluids during the deposition of the ferberite veins cannot be ruled out. The fluids responsible for alteration exchanged with the country rocks to produce a large δD anomaly but no concomitant δ18O anomaly, indicating that water/rock ratios were small. The δD anomaly coincides closely with those produced by H2O+ and Rb/Sr, used as a proxy for hydrothermal alteration, but does not correlate closely with either lithochemical W anomalies or areas of greatest W production. It is concluded that the fluids responsible for district-wide hydrothermal alteration are distinct from those that produced the W mineralization and that δD anomalies, while a sensitive technique for highlighting areas of significant fluid-rock interaction, must be used with caution for identifying drilling targets. The study has provided an estimate of the stable isotope composition of early Tertiary meteoric water in the Front Range (ca. δD = –140‰, δ18O = –18.7‰).