Petrogenesis and geological history of a uranium source rock: a case study in northeastern Washington, U.S.A

A small (4 km2) drainage basin in northeastern Washington contains highly uraniferous groundwater and highly uraniferous peaty sediments of Holocene age. The U is derived from granitic bedrock that underlies the entire drainage basin and that contains 9-16 ppm U. This local bedrock was studied by petrographic, chemical and isotopic methods to determine conditions of its petrogenesis and post-emplacement history that may have contributed to its present high U content and source-rock capability. Locally intense fracturing promoted alteration, and fracturing and alteration probably continued during later regional uplift in the Eocene. Regional uplift was followed by low-temperature alteration and weathering in the middle to late Tertiary. The combined result of hydrothermal alteration and low-temperature alteration and weathering was the redistribution of U from primary mineral hosts such as allanite to new sites on fracture surfaces and in secondary minerals such as hematite. A model is proposed in which high-angle fractures beneath the drainage basin were the sites of Tertiary supergene enrichments of U. The chemistry, mineralogy, texture and geological history of this U source-rock suggest criteria for locating other granite terrain that may contain uraniferous waters and associated young surficial U deposits. The details of U distribution and mobility at this site also apply to the general topic of U mobility in granitic rocks. (author)