Partially eroded stratovolcanoes worldwide, notably Mounts Rainier and Adams in the Cascades and several volcanoes in Japan, record episodic periods of eruption and geothermal activity that produce zones of hydrothermal alteration. The partly eroded core of late Pleistocene Brokeoff volcano on the south side of Lassen Peak exposes the upper 1 km of multiple ancient (ca. 410–300 ka) magmatic-hydrothermal alteration zones in a 3.5 by 5 km area that allows characterization of the three-dimensional hydrothermal evolution of the volcano. Both acid- and neutral-pH hydrothermal solutions produced distinctive alteration mineral assemblages in close proximity. Early hydrothermal activity is characterized by alunite-rich alteration that is temporally and spatially related to shallow intrusions in the center of the volcano. Younger acid alteration and a large area of neutral-pH alteration formed along the volcano’s flanks. The neutral-pH alteration is vertically zoned over 1000 m from shallow zeolite ± adularia through intermediate argillic (smectite-pyrite ± illite) to deep propylitic (chlorite-calcite-albite-illite) alteration. Pleistocene alteration is partly overprinted by surficial, steam-heated alteration related to Lassen’s modern hydrothermal activity. A large (~3.5 km2), shallow (≤>300 m), ca. 1.5 Ma alunite-rich magmatic-hydrothermal alteration zone is exposed on the northeast flank of the nearby Maidu volcanic center.

Hydrothermal activity occurred just prior to, or at the beginning of, major eruptive periods on both volcanoes. Fluid flow and hydrothermal alteration were controlled by primary permeability (lateral fluid flow driven by paleotopographic ground-water gradients in volcaniclastic rocks), steeply dipping fractures in more deeply exposed parts of alunite-rich alteration zones, and inferred basement structures.

Stable-isotope data indicate hydrothermal fluids were mixtures of variably exchanged meteoric and magmatic waters. Variations in types of hydrothermal alteration and fluid compositions may reflect a temporal increase in depth of magma emplacement and degassing that resulted in increased interaction with and neutralization by wall rocks during ascent of magmatic gases. Alteration zones on both volcanoes are not strongly mineralized, probably reflecting relatively low amounts of magmatic acid, sulfur, and chlorine consistent with degassing of small volumes of pyroxene andesite magma compared to large dacite and/or granodiorite batholiths that form significant epithermal and porphyry deposits.

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