Assessment of grid-based whole-rock delta D surveys in exploration: Boulder County epithermal tungsten deposit, Colorado

Important epithermal tungsten mineralization in the Boulder County district is mainly hosted by quartz-ferberite (FeWO4) veins. Isotopic studies based on an extensive grid (deltaD) and individual samples of wall-rock alterati on and mineralization (deltaD, deltaO) have demonstrated the dominance of m eteoric fluids during alteration processes. However, the involvement of mag matic fluids during the deposition of the ferberite veins cannot be ruled o ut. The fluids responsible for alteration exchanged with the country rocks to produce a large deltaD anomaly but no concomitant delta O-18 anomaly, in dicating that water/rock ratios were small. The deltaD anomaly coincides cl osely with those produced by H2O+ and Rb/Sz used as a proxy for hydrotherma l alteration, but does not correlate closely with either lithochemical W an omalies or areas of greatest W production. It is concluded that the fluids responsible for district-wide hydrothermal alteration are distinct from tho se that produced the W mineralization and that ED anomalies, while a sensit ive technique for highlighting areas of significant fluid-rock interact ion , must be used with caution for identifying drilling targets. The study has provided an estimate of the stable isotope composition of early Tertiary m eteoric water in the Front Range (ca. deltaD = -140%, deltaO = 18.7%)