HIGH METAL AND RARE EARTH ELEMENT CONTENTS IN SURFACE WATERS AT RED MOUNTAIN, AN UNMINED VOLCANOGENIC MASSIVE SULFIDE DEPOSIT IN THE BONNIFIELD MINING DISTRICT, ALASKA RANGE, EAST-CENTRAL ALASKA

The unmined Red Mountain (Dry Creek) deposit, located 80 km south of Fairbanks, displays a remarkable environmental footprint of natural acid generation, high metal, and exceedingly high rare-earth-element (REE) concentrations in surface waters. The volcanogenic massive sulfide (VMS) deposit is a well-constrained example of acid-generation, metal-leaching, metal-precipitation, and self-mitigation (via dilution and neutralization) processes that occur in an undisturbed natural setting. Pyrite dissolution and associated secondary reactions under near-surface, oxidizing conditions are the primary causes for the acid generation and metal leaching that occur in felsic metavolcanic rocks of the Devonian to Mississippian Mystic Creek Member of the Totatlanika Schist. The well-exposed deposit and its associated alteration halo are dissected by three drainages, allowing for sample collection above, within, and downstream of the alteration zone.

Springs and streams within the quartz-sericite-pyrite alteration zone have the lowest pH values (many below pH 3.5), highest specific conductance (commonly above 2,500 µS/cm), and highest major- and trace-element concentrations. Aluminum, Cd, Co, Cu, Fe, Ni, Pb, and Y, are found in high concentrations, ranging across four orders of magnitude, and extremely high concentrations are found for Zn (median, 13,000 µg/l; maximum 240,000 µg/l), Mn (median, 4,200 µg/l; maximum 49,000 µg/l), and total REE (median 3,200 µg/l; maximum 59,000 µg/l). Waters collected upstream of the alteration zone have near-neutral pH values, lower specific conductance (370 to 830 µS/cm), lower metal contents, and measurable alkalinities. Downstream of the alteration zone, waters have pH values and metal concentrations intermediate between these two extremes. Stream sediments are anomalous in Zn, Pb, S, Fe, Cu, As, Co, Sb, and Cd relative to local and regional background values. Red Mountain Creek and its tributaries do not and probably never have supported significant aquatic macrofauna.

This study establishes a premining geochemical baseline of an exposed, unmined VMS deposit, a rare occurrence in North America. These data and interpretations may be useful as natural analog data to previously developed VMS deposits in similar climatic settings where premining data do not exist.