Hydrothermal alteration in Maidu Volcano: A shallow fossil acid-sulfate magmatic-hydrothermal system in the Lassen Peak area, California

The upper parts of a latest Tertiary acid-sulfate magmatic-hydrothermal system are well preserved in the northeast part of Maidu Volcano, an eroded late Pliocene andesite-dacite stratocone near the south end of the High Cascades Range, northeastern California. Hydrothermally altered rocks are exposed over an area of about 3.5 square km and elevations from about 1500 to 1750 meters (4900 to 5800 feet). The
vertical exposure was produced by erosion of the original stratocone. Hydrothermal alteration is zoned vertically from alunite+silica+pyrite±kaolinite and montmorillonite+
pyrite+silica assemblages at the highest elevations, through alunite+quartz+pyrite±pyrophyllite at intermediate elevations, to alunite+quartz+pyrite±topaz and vuggy silica assemblages at the lowest elevations. Silica phases at high elevations include opal-C and opal-CT, as well as microcrystalline quartz. Huangite (Ca-alunite)
is widespread at moderate to low elevations. Other common alteration minerals are barite, Ba-Sr-Ca-rich aluminum-phosphate-sulfate (APS) minerals, and Ti-oxide minerals. Hydrothermal features also are zoned systematically downward from hydrothermal sediments and hydrothermal breccias at the paleosurface and very shallow subsurface, through silica-pyrite veins, to silica ledges formed at the deepest
exposed levels. The style of alteration changes downward from pervasive, blanket-like alteration near the paleosurface and at intermediate depths to more fracture controlled alteration at deeper levels. Reconnaissance geochemical studies show that altered rocks have anomalous concentrations of many “pathfinder” elements characteristic of shallow acid-sulfate magmatic-hydrothermal systems and high-sulfidation gold deposits, including As, Bi, Cu, Hg, Mo, Sb, and Te, but very low Au (≤34 ppb) and Ag (<1 ppm) contents. The concentration of pathfinder elements generally is much lower than in economic high-sulfidation gold deposits, however. Water samples collected from springs and streams draining the altered area also have low concentrations of base and precious metals, although the total concentration of base metals is comparable to concentrations reported for several unmined high-sulfidation systems in North America, and a ferricrete sample collected below two springs has elevated As and Cu contents. Preservation of the paleosurface and upper 250 meters of the acid sulfate magmatic-hydrothermal system at Maidu provides insight into the uppermost parts of a high-sulfidation system that are not commonly exposed. On the basis of a comparison to other high-sulfidation systems where the paleosurface is preserved, the structurally controlled vuggy silica ledges present at Maidu have the potential to contain enargite-gold mineralization at a relatively shallow depth below present levels of exposure.