MERCURY IN EPITHERMAL SYSTEMS IN THE CASCADE VOLCANIC ARC

Mercury deposits and occurrences are present throughout the Cascade volcanic arc and are associated with Eocene to Miocene volcanic centers and Quaternary to Holocene volcanoes. Both hot-spring and quartz-alunite type mercury deposits are present. The Black Butte, Oregon, mercury deposit and nearby Hobart Butte kaolinite deposit (Hg to 180 ppm) are typical of the hot-spring type. At Black Butte, Eocene andesitic flows and tuffs are altered to kaolinite-quartz-pyrite, and Hg ores enriched in As (up to 950 ppm) occur in quartz-chalcedony-carbonate veins and breccias developed in an extensional environment within a NW-striking step-over in a NW striking right lateral fault zone. At Hobart Butte, Eocene volcaniclastic sedimentary rocks are composed of clasts of kaolinite and chalcedony. Beds of sedimentary realgar up to 4 cm thick formed in an environment similar to the Ozoresan hot-spring system in the Japan arc, where erosion of an active hot-spring system results in deposition of sedimentary arsenic sulfide, kaolinite, and barite in a caldera lake. Aeromagnetic lows are coincident with the alteration at Hobart and Black Buttes and are aligned along a NE-trending magnetic anomaly that reflects the projection of the contact between pre-Eocene volcanic and sedimentary rocks exposed west of the arc. Mercury deposits of the Bonanza-NonPareil district occur along this contact. In the Miocene volcanic centers, mercury in high sulfidation silica-gold occurrences is anomalously high, but limited production has occurred.

Elevated concentrations of Hg occur in kaolinite-quartz-chalcedony-pyrite alteration formed in the steam-heated environment in active geothermal systems, and in argillically altered rocks near the summits of Holocene volcanoes. At Lassen Peak volcanic center, elevated concentrations of Hg are present in opal-kaolinite alteration at Bumpass Hell (up to 60 ppm) and in kaolinite-chalcedony alteration at Sulphur Works (up to 20 ppm). Anomalous Au (0.9 ppm) present in silicified zones at Bumpass Hell was deposited from an early hydrothermal stage that is now being overprinted by kaolinite-alunite alteration formed in the steam-heated environment.