FERRICRETE DEPOSITS-NATURAL ANALOGS OF ACID MINE-DRAINAGE PROCESSES, UPPER ANIMAS RIVER WATERSHED, COLO

Ferricrete, bog iron and iron spring deposits composed of goethite, ferrihydrite, and schwertmanite provide important data on release of trace metals by acid-rock weathering in the near-surface environment. Precipitation of iron-hydroxide minerals in anthropogenic mine drainage is well documented, although little attention has been given to their natural occurrence in highly mineralized systems. Precipitation of iron hydroxides requires an unlimited supply of dissolved iron and sulfate, and a favorable hydrologic environment in which reduced ground water comes into contact with the atmosphere or mixes with shallow oxygenated waters. In the upper Animas River watershed, Colorado, ferric hydroxides accrete down gradient from hydrothermally-altered volcanic rocks containing abundant pyrite. Sub basins having the highest percentages of acid sulfate and quartz-sericite-pyrite mineral assemblages are directly up gradient from the most abundant bog iron, iron spring, and ferricrete deposits. In a few drainages, areas that have been intensely altered by hydrothermal processes comprise as much as 35 to 60 percent of the total sub-basin area. Iron springs and iron bogs are fed by relatively deep ground water emerging from bedrock fractures. Iron-hydroxide deposits commonly occur near the base of steep slopes, such as the toes of alluvial fans and along stream channels. Colluvial ferricretes occur where ground water has discharged through talus, whereas fluvial ferricretes occur in stream channels fed by ground-water inflows. Fluvial ferricretes often are transitional with spring-fed wetland areas such as sedge grass bogs. Ferricretes and bog iron deposits may serve as solid-phase sinks for soluble sulfate, iron and base metals that are potentially toxic to aquatic life. Relative to streambed sediments collected from the same drainages, ferricretes are enriched in Ba, Fe, Ni, Ti, and V but contain similar amounts of Co, Cd, and Zn, and are relatively depleted in As, Pb, and Mn. Carbon-14 dating of inactive ferricretes shows a wide age distribution indicating that ferricretes have formed continuously throughout recent post-glacial geologic history in the upper Animas River watershed, providing geologic evidence of pre-mining acidic conditions.