PLEISTOCENE MAGMATIC-HYDROTHERMAL SYSTEMS IN THE LASSEN REGION, NORTHEASTERN CALIFORNIA

The eroded cores of Maidu Volcanic Center (MVC) and Brokeoff Volcano expose multiple Pleistocene magmatic-hydrothermal centers that were active between and after major edifice-building events. Rocks in the >3 km² center on the NE flank of MVC were pervasively altered to alunite and residual silica. ⁴⁰Ar/³⁹Ar ages for alunite (~1445 ka) shortly predate the onset of the second stage of Maidu volcanism (~1414 ka).

Andesitic Brokeoff Volcano consists of the Mill Canyon (~600 to 450 ka) and Diller (~373 ka) sequences. Two centers of acidic magmatic-hydrothermal alteration, Brokeoff Mountain (BM) and Mt. Diller (MD), and a larger area of propylitic and intermediate argillic alteration in Little Hot Springs Valley (LHSV) are exposed in the volcano's core. These alteration zones are partly obscured by steam-heated alunite-kaolinite-opal alteration near modern hot springs.

BM alteration extends away from small andesite plugs (~420 ka) in the Mill Canyon sequence. A core of residual silica and alunite-rich advanced argillic alteration (~400 ka) is flanked by intermediate argillic ± propylitic alteration. Alteration was controlled by W-trending fractures and their intersection with permeable breccia units.

The MD center is in both the Mill Canyon and Diller sequences and has a pyrite-rich kaolinite/dickite core surrounded by montmorillonite-pyrite; alunite and residual silica are uncommon. The age of MD is constrained between ~373 and 260 ka by the age of altered and unaltered rocks. The LHSV center is in both the Diller and Mill Canyon sequences and has the same age constraints as MD. Alteration was controlled by primary rock permeability near intrusions and consists of montmorillonite-pyrite alteration that becomes propylitic downward.

S-O-H isotope data indicate multiple origins for alunite. Maidu and deeper BM alunites have S-O-H values consistent with magmatic SO₂ disproportionation and condensation in meteoric water. Shallow BM alunites have S and O values indicating formation in near-surface SO₂-rich fumaroles. Alunites around modern hot springs have S values consistent with H₂S oxidation in a steam-heated environment.

Variable styles of alteration developed during construction of volcanic centers reflect depth of exposure, proximity to intrusions, primary rock permeability, fractures, and topography.