Abstract: Age Relationships of Acid Drainage Water Quality at the New World Mining District, Park County, Montana

G. Furniss

Natural acid drainage from oxidation of sulfide minerals occurs at the alpine headwaters area, New World mining district, northeast of Yellowstone Park. Pleistocene glaciers eroded and exposed sulfide-bearing rocks distributing acid-generating minerals in till. Post-glacial mass wasting on steep, high elevation terrain between 8,000 and 10,338 feet MSL, continues the exposure of sulfide minerals to oxygen in talus and debris flows and along fracture systems.

Chemical sediment of iron oxide containing heavy metals have precipitated from surface runoff and spring waters cementing layered aprons and flood plain sediments. Iron oxide minerals precipitate at high rates that maximize the preservation of plant and bacteria debris cemented into stratified deposits that are several feet thick. Radiometric age dating of wood cemented within the iron oxide layers range from 1,670 +/-40 to 8,620 +/-60 years B.P..

Analysis of ancient stream terrace and recent stream bottom iron oxide cements show nearly identical chemistry. Metal ratio trends with distance downstream are identical in both ancient and modern iron oxide cements, due to stream pH increases from headwater cirques to valley floors. Elevated metal concentrations in acid springs and soils, as well as abundant sulfide minerals in shallow bedrock and talus occur in headwater areas undisturbed by historic mining. The research demonstrates that the headwater stream systems have been impacted by naturally occurring acid drainage with ensuing release of heavy metals for thousands of years.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana