Brokeoff volcano, a Quaternary stratocone located in the Lassen volcanic center in northern California, has been deeply eroded, exposing a 10-km2 meteoric hydrothermal alteration zone at the core of the volcano. Portions of the former volcanic edifice are sufficiently well preserved that an unusual opportunity exists wherein the alteration pattern can be correlated with the position of the volcanic cone. The Δ18O analyses of more than 100 whole rock samples, consisting primarily of andesitic lavas, vary from +9.8 to +0.6 per mil. The highest Δ18O values occur in bleached, solfatarically altered rocks that have interacted with low-pH, fumarolic hot springs associated with the present-day hydrothermal system. Low Δ18O values are found in propylitically altered rocks that underwent isotopic exchange with meteoric hydrothermal fluids at elevated temperatures, mostly during the stratovolcanic stage (650--400 ka) of the hydrothermal system, but probably continuing today at depth.

Electron microprobe analyses of secondary layer silicate minerals in strongly propylitized samples (Δ18O<+5.0) revealed the presence of discrete chlorite, suggesting that temperatures up to 200 to 250 ¿C were attained in the shallow levels of the system. Two zones of pervasive meteoric hydrothermal alteration, defined by concentric 18O contours that are probably interconnected at depth, are located within the original topographic edifice of the volcano. The most intensely altered rocks within these equant zones of alteration define NNW trends that coincide with stream valleys and with regional structural patterns. A comparison of the characteristics of the 18O-depleted zone at Brokeoff with those of more deeply eroded volcanic centers, such as the Comstock Lode mining district (Criss and Champion, 1991), permits the construction of composite 18O cross sections through a hypothetical intact stratovolcano. At both Brokeoff and Comstock, hydrothermal fluids were strongly focused into plumelike zones of intense 18O depletion. At Comstock, these low-18O plumes are associated with faults. Although major fault displacements are not observed at Brokeoff, the topographic and alteration patterns are consistent with the presence of a linear array of faults that acted as conduits for fluid flow up into the shallow levels of the volcano. ¿ American Geophysical Union 1994