METAL-SULFIDE OXIDATION IN COASTAL ENVIRONMENTS OF EASTERN PRINCE WILLIAM SOUND, ALASKA

Sulfide-rich rock debris related to mining activities during the early twentieth century is distributed along shorelines in Prince William Sound, Alaska. Copper ores were recovered from lenses of massive sulfide (chalcopyrite + pyrrhotite + pyrite + quartz + chlorite) enclosed by steeply dipping and deformed strata of the Orca Group, a continental-margin sequence of flysch and basaltic lavas of early Tertiary age. Today, mineralized rocks are exposed in mine workings and dumps, storage bunkers, stockpiles, and outcrops that extend approximately 500 vertical meters from the steep slopes surrounding the fiords into the marine environment. Ellamar, the largest copper deposit in eastern Prince William Sound, was mined offshore behind a protecting cofferdam, and massive sulfide is exposed on the margins of the glory hole. At the Ellamar and Threeman mines, massive sulfide talus extends into the littoral zone from remnant ore and waste piles onshore.

To determine the extent of metal-sulfide oxidation, acid generation, and metal fluxes in this transitional, onshore to marine environment, field studies were conducted at five sites in eastern Prince William Sound during June, 2003. Samples of ore, mine waste, surface water, sediment, and pore water were collected from mine workings, drainages below mines and mine dumps, and the mixing zone between ground water and seawater in subsurface beach gravels. Variable degrees of Fe and S oxidation and altered fluid compositions were observed at each collection site.

Preliminary observations suggest that (1) acidic (pH=3.2-5.5) waters are present in mine workings; (2) sulfide oxidation is producing low-pH fluids within subaerial mine dumps, especially where pyrite is abundant; (3) acidic to neutral (pH=3.5-7.2) ground water resulting from oxidizing sulfides extends into the intertidal zone at the Ellamar and Threeman sites; and (4) Al- and Fe-rich precipitates are forming from acidic (pH=3.3-5.5) fluids passing through gravels into the intertidal zone at the Ellamar site. The rate and extent of sulfide-mineral oxidation in massive-sulfide detritus appears to be favored by coarser grain size of sulfide minerals, high pyrite/pyrrhotite ratios, porphyryoblastic and foliated textures, and high aggregate density of sulfide-rich fragments.