About This Item
Share This Item
Chemical analyses of < 0.15-mm (< 100 mesh) fractions from Holocene stream sediment in Chimney and Spring Canyons in Logan County, northeastern Colorado, reveal values of uranium in excess of the regional background (3.8 ppm). These values reach a maxima of 60.2 ppm U. Chimney Canyon sediment ranges from 4.4 to 5.5 ppm U, and is composed of volcanic glass, quartz, and minor amounts of plagioclase, calcite, mica, and mixed-layer illite-smectite. Heavy minerals are rare and consist mainly of biotite and hornblende. Spring Canyon sediment ranges from 3.7 to 60.2 ppm U and is composed of quartz, potassium, and plagioclase feldspar, calcite, mica, and mixed-layer illite-smectite. Heavy minerals are common and include ilmenite, magnetite, hornblende, epidote, and zircon.
Content of uranium in samples of bedrock from the Brule, Arikaree, and Ogallala Formations ranges from 1.8 to 16.3 ppm U, and varies according to the composition of the detrital fraction and the amount and type of cement. Poorly cemented arkose and vitric siltstone contain between 1.8 to 2.9 ppm and 4.0 to 5.7 ppm U respectively. Bedrock cemented by micrite contains from 6.3 to 9.1 ppm U and silicified bedrock contains between 12.1 and 16.3 U.
Petrographic studies reveal that initial deposition of the bedrock was followed by: (1) mechanical infiltration of clay, bioturbation, and precipitation of mixed-layer illite-smectite; (2) precipitation of sparry calcite or micrite; (3) dissolution of framework grains, including volcanic glass; (4) destruction of mixed-layer illite-smectite and the development of caliche structures; and (5) precipitation of opal-CT and chalcedony.
Mica fission-track maps show that uranium is homogeneously distributed within micrite, opal-CT, and chalcedony cements. Fission-track density indicates that opal and chalcedony contain more uranium than micrite. Sparry calcite cement contains no uranium.
Four major conclusions result. (1) The content of uranium in Holocene stream sediment is related to the detrital composition of the parent bedrock. Uranium in the Chimney Canyon sediment occurs in volcanic glass, which is derived from tuffaceous siltstone of the Brule Formation, whereas the uranium in Spring Canyon sediment is contained in heavy minerals, which are derived from the Ogallala Formation. (2) Uranium in the bedrock is contained in volcanic glass, heavy minerals (mainly zircon, iron-titanium oxides, and monazite), and in cements of micrite, opal, and chalcedony. (3) Uranium in micrite, opal, and chalcedony is derived from intrastratal dissolution of volcanic glass. Volcanic glass is also the source for silica precipitated as opal or chalcedony. (4) The postdepositional alt rations observed in this study represent the earliest stages of mobilization of uranium required for the development of an epigenetic deposit.
End_of_Article - Last_Page 572------------