Fossil Magmatic-Hydrothermal Systems in Pleistocene Brokeoff Volcano, Lassen Volcanic National Park, California

John, D; Breit, G; Lee, R; Dilles, J; Muffler, L; Clynne, M

The mineralogy, distribution, and isotopic composition of altered rocks exposed in the core of Brokeoff Volcano are attributed to two fossil magmatic-hydrothermal systems that are partly masked by younger alteration related to modern hot springs. Brokeoff Volcano was a large andesitic volcano (~600 to 400 ka) that preceded formation of Lassen Peak and the Lassen dome field. The two centers of fossil hydrothermal activity are about 1 km apart and are identified here as the Brokeoff Mountain (BM) and Mt. Diller (MD) systems. The BM system, centered about 1 km NE of Brokeoff Mountain, covers about 1.5 km2 extending 2.5 km west from Diamond Peak, through Sulphur Works, to west of the ridge between Brokeoff Mountain and Mt. Diller. Alteration affected mostly andesite lavas and breccias of the Mill Canyon sequence (~600-475 ka). Core alteration extends westward and upward from an altered andesite plug exposed west of Sulphur Works. It consists of narrow, west-trending, brecciated vuggy silica ledges as long as 600 m surrounded by zones of variable thickness (<1 to 30 m) composed of alunite, kaolinite, pyrophyllite, dickite, topaz, pyrite, and a range of silica minerals. Farther outward from the advanced argillic alteration are broader zones of propylitic (chlorite-calcite-illite-pyrite) and smectite-pyrite alteration. Initial S-O isotopic data indicate that alunite formed by high-temperature disproportionation of magmatic SO2. The ~3 km2 MD system, centered about 1 km SE of Mt. Diller, extends 3 km ESE to near Bumpass Hell. Lavas and breccias of the Mill Canyon sequence and the Mt. Diller sequence (ca. 400 ka) have been hydrothermally altered. Although the center of the MD system is largely obscured by landslides and by superimposed steam-heated acid leaching related to present-day hydrothermal activity, recognized core alteration consists of pyrite-rich quartz-dickite and quartz-kaolinite breccias; pyrite content locally exceeds 50%. Only minor amounts of alunite and vuggy silica alteration have been found. Core alteration grades laterally and upward to pyrite-rich smectite alteration. Alteration is asymmetric around the core of the system, with extensive areas of shallow smectite-pyrite and deeper chlorite-illite-calcite-pyrite alteration exposed to the southeast in the cliffs of Little Hot Springs Valley. Alteration in both systems was controlled by primary permeability, with breccia units more strongly altered. The most intense alteration in the BM system is structurally controlled by approximately west-striking high- angle fractures subparallel to dikes in the core of Brokeoff Volcano. Structural control of MD alteration is not apparent. The asymmetry and larger areal extent of the MD system may reflect a larger volume of hydrothermal fluid dominated by meteoric water as well as extensive lateral fluid flow through permeable breccia units. Contrasting alteration mineralogies in the systems are attributed to higher temperatures and more oxidized fluids with a larger component of magmatic volatiles (SO2, H2S, HF) in the BM system. Although core alteration assemblages and anomalous trace-metal concentrations (As, Bi, Cu, Hg, Mo, Sb, Sn, Te, and Zn) in both Brokeoff hydrothermal systems are similar to those in high-sulfidation epithermal Au-Ag deposits, the Brokeoff systems lack significant contents of precious metals.