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Getahun, Aberra, 1994

Fluid-rock reaction and mineralization in two high-level volcanic settings: Augustine fumaroles and the Summitville acid-sulfate copper-gold deposit

Bibliographic Reference

Getahun, Aberra, 1994, Fluid-rock reaction and mineralization in two high-level volcanic settings: Augustine fumaroles and the Summitville acid-sulfate copper-gold deposit: University of Oregon, Eugene, Ph.D. dissertation, 331 p., illust., maps.


Altered wall rocks bordering 870 degree C fumaroles at Augustine Volcano, Alaska, exhibit distinct mineral zoning: (a) tridymite-anhydrite, (b) tridymite-hematite-Fe hydroxide-molysite with minor anhydrite and halite, (c) anhydrite-halite-sylvite-tridymite with minor molysite. The altered rocks exhibit enrichments in silica, base metals, halogens, and sulfur, and show very strong depletions in Al in all alteration zones. Thermochemical modeling of gas-rock reactions at 640 and 375 degrees C produce silicates, oxides, and sulfides. Cooling of the volcanic gas produces: (a) anhydrite, halite, sylvite; (b) Cu, Mo, Fe, and Zn sulfides; (c) chlorides, fluorides, and sulfates of Mn, Fe, Zn, Cu, and Al; and (d) hydrated sulfates, liquid sulfur, crystalline sulfur, and hydrated sulfuric acid. Mixing of volcanic gas with air produces Na and K sulfates, hematite, anhydrite, hydrated sulfates, and sulfuric acid. Comparison of results with the natural samples suggests that the alteration assemblages include: (1) minerals that precipitate from direct cooling of the volcanic gas, (2) phases that form by volcanic gases mixing with air, and (3) phases that form by volcanic gas-rock reaction. The Cu-As-Au mineralization at Summitville, Colorado, is formed at a high-level volcanic setting and is characterized by a central zone of vuggy silica, surrounded successively by zones of quartz-alunite, quartz-kaolinite, quartz-illite, quartz-montmorillonite-illite, and quartz-albite-chlorite-muscovite alteration. Quartz-sericite-pyrite alteration with local zunyite-pyrophylite-enargite and fault-controlled kaolinite-dickite assemblages occur at depth. At the top, the system is capped by a cristobalite, alunite, and kaolinite alteration assemblage. The vuggy silica zone is enriched in Si, Co, Ta, As, Sb, W, and locally Cu and depleted in Al, Fe, Ca, Mg, Mn, K, Na and trace and rare-earth elements. The downward tapering of the vuggy silica zone, wedging out of the quartz-alunite zone at depth, and enrichment of Al and Fe in the deeper zones suggest that condensation of magmatic vapor occurred at high level followed by a downward percolation of acidic fluid. Numerical models indicate that the alteration boundary between the alunite and kaolinite zones and the mineralization of gold, silver, covellite, and enargite are formed by mixing of meteoric water with hydrothermal fluids in contact with wall rock and by cooling and boiling of acidic, metal-rich brine.

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