ABSTRACT: The Boston Peak Fen: a Multidisciplinary Case Study of a Uraniferous Mountain Wetland
D. E. Owen, J. K. Otton, R. R. Schumann, R. A. Zielinski, J. P. McKee, E. I. Robbins, F. A. Hills
Uranium (U) enrichments as high as several thousand ppm occur in surficial, organic-rich sediments of mountain wetlands, which provide natural laboratories for studying the transport and fixation of U. Boston Peak fen covers about 8 ac in the glaciated upper Laramie River valley of north-central Colorado. The valley follows a north-trending fault zone that was active as late as the Oligocene. The fen is bounded primarily by Precambrian rocks along the sides and upper end and by a postglacial landslide deposit on the lower end. Ground-water hydrology is characterized by a basal, confined, gravelly, locally artesian aquifer overlain by lacustrine clay and an unconfined peat aquifer. Faults and fractures in the bedrock are conduits for uraniferous waters that enter the fen t rough seeps and springs. Present-day vegetation communities are controlled by peat thickness and seasonal groundwater fluctuations.
Uranium mineralization in the fen is closely related to its depositional history. Immediately after the landslide dammed the valley floor, a pond formed and began filling with silt and clay. About 10,000 years
ago, pond weeds became established and organic productivity increased. During this time, U was sorbed from the pond water by organic matter in the sediment, forming layers containing 10 to 150 ppm U (dry weight). About 3000 years ago, downcutting of the spillway and sediment infilling resulted in depths shallow enough for the establishment of peat-forming sedges. Since then, up to 3.5 m of peat has accumulated. During this time, ground-water recharge zones in the fen became point sources of metal input, and as much as 3300 ppm U (dry weight) accumulated locally in the peat.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990