Differential Geochemical Mobility of Uranium, Thorium, and Potassium Used as Geologic Environment Indicators and Explorations Tools
Philip C. Goodell
Uranium, thorium, and potassium have significantly varied geochemical properties, so their relationships can be used to identify geologic environments. Data used to quantify the relationships included (1) uranium-thorium-potassium contents in rocks, determined during heat-generation studies; (2) uranium-thorium-potassium contents of ore deposits and mineralized occurrences generated during exploration programs, including the NURE program; and (3) uranium-thorium-potassium contents reported here.
Uranium and thorium are small, highly charged actinide elements that behave sympathetically in the +4, or reduced, state. They are concentrated in felsic magmas and syenites, and often enter the same accessory minerals. Magmatic trends are plotted on log uranium-log thorium graphs, where they show linear trends and diverge toward the high silica end. The accessory minerals are generally resistates that carry uranium and thorium into the sedimentary cycle.
Uranium also has a +6, or oxidized, state in which it is mobile. The high uranium branch at the evolved end of the igneous trend does have oxidizable uranium, and extrusives of this chemical character form good regional source rocks. Potassium in the magmatic system closely follows the progressive enrichment of uranium and thorium; however, relative enrichment varies with the chemistry and magmatic processes involved. Uranium is concentrated in albitites in the intrusive stage. During caldera eruption, the elements are dispersed throughout the volcanic pile, where crystallization and leaching may remove uranium and potassium differentially. Remanent vitrophyres must be tested for regional uranium source rock studies.
Sedimentologically, much uranium and thorium are tied up in resistates, and their ratio in specific minerals is a clue for provenance studies. Sandstones contain varying amounts of uranium. Typical shales lie on the thorium-rich side of the magmatic trend and are diluted from granite, indicating that some uranium was leached from the magmatic trend.
Uranium and potassium are easily leached from tuffaceous rocks, although thorium is not. Transport efficiency is higher for potassium than uranium. Uranium is immobilized by reduction or complexing. Potassium tends to be more thoroughly flushed, although it may be retained on clays and stabilized during diagenesis. Uranium and potassium tend to be separated during uranium precipitation. Organic-rich shales and uraniferous sandstones can be identified.
Uranium-thorium-potassium contents determined by chemical analysis will differ from values obtained by gamma-ray spectroscopy, but these differences are informative with respect to the timing of geochemical mobility.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.