As a kind of hobby Th’ Gaussling has been surveying the literature on uranium occurrences in North America. Uranium is found in many interesting locations and as a result of several distinct kinds of ore forming processes.

Prospector with Geiger Counter

From Ballard &Conklin, Uranium Prospectors Guide, 1955 Harper & Brothers

For the most part, uranium ore body formation is the result of aqueous transport and deposition.  Uranium is found as a lode in vein formations in precambrian  igneous/metamorphic structures as in the case of the Schwartzwalder mine near Denver. In fact, there are many lode occurrences that contain a variety of uranium minerals in the Colorado mineral belt.

What seemed counterintuitive to me was the extent to which uranium is found in sandstone. Evidently I had developed a bias for connecting heavy metal occurrences with igneous/metamorphic formations.

Uranium occurrences in sandstone take on certain characteristics as a result of ore forming processes. Uranium is often found in concentrated bodies called “roll fronts” or “ore rolls”. A roll front is a body of concentrated mineral with a lenticular cross section and is found in confined strata sandwiched between impermeable clays, shales, or mudstones.

Roll Front Cross Section

Adler & Sharp, Guidebook to the Geology of Utah, No. 21, Utah Geological Society, 1967, p. 59.

The action of oxygenated meteoric water (i.e., rain and surface water) migrating through a porous sandstone stratum will selectively mobilize mineral species that are soluble. In the case of uranium, the relatively insoluble U4+ compounds are oxidized to more soluble U6+ species which are then mobilized and flow in the formation.

Eventually, as the water flow encounters reducing conditions, U6+ gets reduced to U4+ and deposition occurs. Sandstone with organic material may be a net reducing environment and provide the necessary carbonaceous reductants to do the deed.

As the U6+ enriched aqueous flows encounter reducing conditions, deposition of U4+ insolubles occurs in a manner determined by fluid mechanical forces. The result is an elongated and tapered ore body confined to a narrow stratum.

Uranium roll fronts are common in many uranium districts. The Uravan uranium belt in the Colorado Plateau is a good example. Uranium is found concentrated in tuffaceous formations as well. An example of this is the uranium occurrence found in the 39 Mile Volcanic Field in the central Colorado mountains.

What is interesting to ponder is the geological effect of plant metabolic byproducts like oxygen. Oxygen directly contributes to a natural process that lead to the concentration of a scarce element like uranium. Plant life facilitating nuclear power. Hmmm.

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