GENERATION AND TRANSPORT OF TOXIC METALS AT THE FORMOSA MINE SUPERFUND SITE, DOUGLAS COUNTY, OREGON

Volcanogenic massive sulfide (VMS) deposits are quite rare, but they can be very rich ore bodies. However, they also pose difficult environmental challenges. These challenges are being addressed at the Formosa Mine, an abandoned volocanogenic massive sulfide (VMS) mine located in the Coastal Range of southwestern Oregon. The mine generates acidic water containing toxic metals such as cadmium, copper, nickel and zinc. Mine water discharges through surface and subsurface pathways into area streams, severely impacting downstream aquatic ecosystems. Kuroko-type VMS ores produced chalcopyrite, sphalerite, pyrite and other sulfide minerals. Ore was processed by flotation, which produced copper and zinc concentrates, sulfide-rich mine tailings, and other mine wastes. Sulfide-rich mine wastes were consolidated within the underground mine and a near-surface mine waste repository. The acidic and metal-rich mine water is generated through microbially mediated oxidation and dissolution of sulfide minerals contained in the consolidated mine wastes.

Geologic and hydrogeologic factors contribute to generation and transport of toxic metals. Rocks in the area are subaqueously deposited volcanics including dacitic and basaltic tuffs and basalt flows. These rocks contain negligible neutralization potential to counteract acid generation. Groundwater flow, controlled by secondary porosity, is locally modified by pipe flow through underground workings. Silver Butte receives annual precipitation of approximately 140 cm, mostly occurring between November and March. This causes marked seasonal fluctuations in the water table and seasonal inundation of backfilled mine wastes. This results in development and rinsing of vestigial acidity and metals, and seasonal flushes of concentrated mine water into the environment. Mine water discharges from adit portals and fractures located in tributary drainages downgradient from the mine. In the downstream environment, dissolved metals attenuate through precipitation, adsorption, and dilution, causing accumulation of toxic metals in aquatic ecosystems. Understanding the interrelationships between VMS mineralization, mining and mineral processing methods, and site-specific geologic and hydrogeologic factors is the first step in remediating the abandoned mine.