OXYGEN-ISOTOPE EVIDENCE FOR HYDROTHERMAL ALTERATION WITHIN A QUATERNARY STRATOVOLCANO, LASSEN VOLCANIC NATIONAL-PARK, CALIFORNIA

Brokeoff volcano, a Quaternary stratocone located in the Lassen volcan ic center in northern California, has been deeply eroded, exposing a 1 0-km(2) meteoric hydrothermal alteration zone at the core of the volca no. Portions of the former volcanic edifice are sufficiently well pres erved that an unusual opportunity exists wherein the alteration patter n can be correlated with the position of the volcanic cone. The delta( 18)O analyses of more than 100 whole rock samples, consisting primaril y of andesitic lavas, vary from +9.8 to +0.6 per mil. The highest delt a(18)O values occur in bleached, solfatarically altered rocks that hav e interacted with low-pH, fumarolic hot springs associated with the pr esent-day hydrothermal system. Low delta(18)O values are found in prop ylitically altered rocks that underwent isotopic exchange with meteori c hydrothermal fluids at elevated temperatures, mostly during the stra tovolcanic 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 (delta(18)O < +5.0) revealed the presence of discrete chlorite, suggesting that tem peratures up to 200 to 250 degrees C were attained in the shallow leve ls of the system. Two zones of pervasive meteoric hydrothermal alterat ion, defined by concentric O-18 contours that are probably interconnec ted at depth, are located within the original topographic edifice of t he 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 characteristic s of the O-18-depleted zone at Brokeoff with those of more deeply erod ed volcanic centers, such as the Comstock Lode mining district (Criss and Champion, 1991), permits the construction of composite O-18 cross sections through a hypothetical intact stratovolcano. At both Brokeoff and Comstock, hydrothermal fluids were strongly focused into plumelik e zones of intense O-18 depletion. At Comstock, these low-O-18 plumes are associated with faults. Although major fault displacements are not observed at Brokeoff, the topographic and alteration patterns are con sistent with the presence of a linear army of faults that acted as con duits for fluid flow up into the shallow levels of the volcano.