Weathering of the meade peak phosphatic shale member, phosphoria formation: Observations based on uranium and its decay products

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Abstract

Variably weathered outcrop samples of the Meade Peak Phosphatic Shale Member of the Phosphoria Formation have 5-10% of the contained uranium (U) in a form readily extractable by 0.1 M sodium bicarbonate. Fission track radiography of outcrop samples and other less-weathered channel and core samples indicate that this mobile fraction of U is likely hosted by organic matter, secondary iron oxides and clay minerals, trace uraninite, and very fine-grained apatite cement. During weathering, this extractable U fraction is especially susceptible to redistribution, which produces small but measurable departures (1-15%) from radioactive (secular) equilibrium in the 238U decay-series. The most weath- ered samples show the strongest isotopic evidence for redistribution of U during the last 350 ka, but sequestration of U by alteration products limits open-system losses of U at the whole-rock scale. In less-weathered samples, isotopic evidence for minor U loss (or gain) over longer time periods (1 Ma) is consistent with relatively non-aggressive attack of phosphatic rock during weathering. Comparative extractability of selenium (Se) suggests that a larger fraction of Se (19%) is readily available for mobilization during the earliest stages of weathering.

Additional publication details

Publication type Book chapter
Publication Subtype Book Chapter
Title Weathering of the meade peak phosphatic shale member, phosphoria formation: Observations based on uranium and its decay products
Chapter 9
DOI 10.1016/S1874-2734(04)80011-1
Volume 8
Year Published 2004
Language English
Publisher Elsevier
Contributing office(s) Central Energy Resources Science Center, Central Mineral and Environmental Resources Science Center
Description 24 p.
Larger Work Type Book
Larger Work Subtype Monograph
Larger Work Title Handbook of Exploration and Environmental Geochemistry
First page 227
Last page 250