An oxygen isotope study was made on hydrothermal quartz and altered volcanic rocks from six areas of advanced argillic alteration in the San Juan volcanic field, southwest Colorado. These include the historically productive mining districts at Summitville (delta 18 O quartz = 13.0-15.5ppm) and the National Belle pipe at Red Mountain Pass (delta 18 O quartz = 10.0-12.3ppm). Other areas studied are Engineer Pass (delta 18 O quartz = 6.4ppm, delta 18 O (sub whole rock) = 1.9-3.4ppm), Carson Camp (delta 18 O quartz = 11.9-13.6ppm), and Calico Peak near Rico (delta 18 O quartz = 6.9-9.0ppm, delta 18 O (sub whole rock) = 4.7-6.9ppm). Samples were also analyzed from surface outcrops (delta 18 O quartz = 4.9-11.2ppm) and from an 837-m cored drill hole in the Red Mountain quartz latite lava dome on the eastern edge of the 23-m.y.-old Lake City caldera. At this locality the altered volcanic rocks (delta 18 O (sub whole rock) = 1.7-8.2ppm) and the hydrothermal quartz (delta 18 O quartz = -1.4-+11.2ppm) exhibit a distinct vertical 18 O/ 16 O gradient of about 0.8 per mil/100 m, with the highest delta 18 O values at the highest elevations. These data are incorporated into a general model for solfataric alteration in these mid- to late Tertiary hot spring systems; this involves a convecting meteoric-hydrothermal fluid, which at shallow depths rises along the hydrostatic boiling curve. The delta 18 O variation in solfataric quartz in the uppermost 300 m of the hot spring system (from 5-11ppm) is mainly a function of a temperature variation from about 225 degrees to 125 degrees C along the boiling curve, and to a lesser extent a result of the delta 18 O effects that accompany vapor phase separating during boiling.