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Geochemistry: Exploration, Environment, Analysis Don't get GSW? Talk to your librarian.
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Geochemistry: Exploration, Environment, Analysis; August 2007; v. 7; no. 3; p. 207-223; DOI: 10.1144/1467-7873/07-136
© 2007 Geological Society of London
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Original Article

Environmental geochemistry at Red Mountain, an unmined volcanogenic massive sulphide deposit in the Bonnifield district,Alaska Range, east-central Alaska

Robert G. Eppinger1, Paul H. Briggs1, Cynthia Dusel-Bacon2, Stuart A. Giles1, Larry P. Gough3, Jane M. Hammarstrom3 and Bernard E. Hubbard3

1 US Geological Survey, Denver Federal Center, PO Box 25046, MS 973, Denver, Colorado 80225, USA (e-mail: eppinger@usgs.gov, sgiles@usgs.gov)
2 US Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA (e-mail: cdusel@usgs.gov)
3 US Geological Survey, National Center, MS 954, Reston, Virginia 20192, USA (e-mail: lgough@usgs.gov, jhammars@usgs.gov, bhubbard@usgs.gov)

The unmined, pyrite-rich Red Mountain (Dry Creek) deposit 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 sulphide deposit exhibits well-constrained examples of acid-generating, metal-leaching, metal-precipitation and self-mitigation (via co-precipitation, dilution and neutralization) processes that occur in an undisturbed natural setting, a rare occurrence in North America. Oxidative dissolution of pyrite and associated secondary reactions under near-surface oxidizing conditions are the primary causes for the acid generation and metal leaching. The deposit is hosted in Devonian to Mississippian felsic metavolcanic rocks of the Mystic Creek Member of the Totatlanika Schist.

Water samples with the lowest pH (many below 3.5), highest specific conductance (commonly >2500 µS/cm) and highest major- and trace-element concentrations are from springs and streams within the quartz–sericite–pyrite alteration zone. Aluminum, Cd, Co, Cu, Fe, Mn, Ni, Pb, Y, Zn and, particularly, the REEs are found in high concentrations, ranging across four orders of magnitude. Waters collected upstream from the alteration zone have near-neutral pH, lower specific conductance (370 to 830 µS/cm), lower metal concentrations and measurable alkalinities. Water samples collected downstream of the alteration zone have pH 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 abundances. Red Mountain Creek and its tributaries do not, and probably never have, supported significant aquatic life.

KEYWORDS: acid rock drainage, volcanogenic massive sulphide, hydrogeochemistry, unmined, rare earth element






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