Anomalously high fluorine values have been found in biotite, sericite, and spessartine garnet associated with the various intrusive and alteration sequences at the Henderson molybdenite deposit in Clear Creek County, Colorado. Twelve-element microprobe analyses of these minerals throughout this Tertiary rhyolite complex indicate that the highest fluorine values are found in vein-related, magnesium-rich biotite (up to 7.5 wt % F) and that the lowest fluorine values are found in sericite from the outer zone of pervasive sericitic alteration (<1.7 wt % F).Application of previously calibrated F-OH exchange relations (Munoz and Ludington, 1974; Ludington, 1974; Gunow, 1978) indicate equilibration of biotite-sericite pairs, high initial relative activity of fluorine, and substantial decreases in fluorine activity during evolution of the hydrothermal fluid. Log (f (sub H 2 O) /f HF ) values have been calculated at an assumed final temperature of exchange of 350 degrees C, and they indicate that, within the ore-body, the fugacity ratio averages about 3.9 for all varieties of mica. There is a progressive increase in the fugacity ratio with distance from the mineralization center, and values are >4.5 at a distance of 500 m from the 0.1 percent MoS 2 zone. These results provide evidence that fluorine was an essential component of the hydrothermal fluid at Henderson, that significant fluorine gradients are found within the sericitic alteration zone, and that the highest activity of fluorine was spatially associated with molybdenite mineralization.