SULFIDE MINERAL OXIDATION PROCESSES AND METAL FLUXES IN THE INTERTIDAL ZONE: ELLAMAR, THREEMAN, AND BEATSON MINES, PRINCE WILLIAM SOUND, ALASKA

Volcanic and sedimentary rocks of Prince William Sound host volcanogenic massive sulfide deposits of early Tertiary age exposed on steep slopes that border the fiords of the region. Between 1897 and 1930, approximately 5,800,000 t of high-grade Cu-sulfide ore were produced and shipped from seven deposits, the largest of which was the Beatson deposit on Latouche Island. At these sites, massive sulfide debris in the shoreline environments shows a range of oxidation effects due to as much as 100 years of exposure to a combination of ground water and seawater. Oxidation of the principal ore minerals (chalcopyrite, pyrrhotite, pyrite, and sphalerite), generation of acidic fluids, and transport of metals in this natural laboratory provides an opportunity to test hypotheses and predictive models gained from ongoing experimental work. At the Ellamar mine in eastern Prince William Sound, Alaska, a rubbly pavement of sulfidic ore and waste material extends across the intertidal zone. Shallow groundwater samples have pH from 3 to 5.5, which generally decrease toward the shoreline where pit waters approach normal marine chloride concentrations. Jarosite is precipitated in shallow subsurface pore space between partially oxidized fragments of sulfide rubble. Contents of Al (16 mg/L), Cu (2.9 mg/L), Fe (48 mg/L), Hg (2000 ng/L), Pb (0.8 mg/L), Tl (9.2 ug/L), and Zn (2.2 mg/L) are enriched in the saline, acidic waters. In contrast, water flowing through the Threeman mine waste dump and through the extensive surface, underground, and open pit workings at Beatson mine have near neutral pH. Waters at the Threeman intertidal zone show only weak acidity, but mixing with marine waters produces iron oxide precipitates that were sampled offshore by submersible remotely operated vehicle. Uplift of about 4 m at the Beatson mine site during the 1964 Alaska earthquake has largely isolated sulfide mine waste there from oxidation in the marine environment., During high runoff, a large volume of meteoric water (minimum pH 7.0) exiting the mine workings appears to largely bypass the mine waste piles. More acidic waters seeping from the dumps (pH 5.3 to 5.8) precipitate Fe oxyhydroxides. Autosamplers and flow monitors have been deployed at Beatson to study changes in water chemistry with seasonal variations in stream discharge.