CHAPTER 5 Characteristics of U.S. Uranium Deposits

• Jurisdiction: United States

Mining Exploration Technology for Lawyers and Landmen
(Apr 1980)
CHAPTER 5
Characteristics of U.S. Uranium Deposits
Gene K. Ealy
Cyprus Exploration Company
Boulder, Colorado

Uranium is a lithophile element. A lithophile element is an element which is enriched in the earth's crust. Elements other than uranium are enriched in the earth's mantle or core. These would include copper, iron and nickel. Other lithophile elements would include aluminum and silica. As a lithophile element, uranium is ubiquitous in the earth's crust occurring at an average concentration of 2-4 parts per million in crustal rocks and 3-4 parts per billion in the ocean. However, only rarely is uranium concentrated sufficiently to constitute an ore deposit. By definition, ore is a mineral which can be economically exploited.

Like every other commodity known to man, the supply of uranium is dependent on the economics of producing it. Under present cost geometry if the price of U₃O₈ were $8.00 per pound as it was in the 1950's and 1960's, for all practical purposes uranium production in the United States would be nonexistant. If, on the other hand, the U₃O₈ price were three or four times as high as the current price of about $40.00 per pound, there is sufficient uranium in the ocean and in some shales and granites to make the U₃O₈ supply infinite for all practical purposes. Therefore, when considering uranium as an element, we can consider it to be very common, but when thinking of uranium in terms of an economic commodity it is much less common; indeed to most exploration geologists and companies uranium may appear very elusive in economic concentrations and proportions.

There are just two practical uses for uranium, both due to uranium's ability to produce energy. These two uses are as a fuel in nuclear power plants and as an explosive in nuclear weapons. Formerly, some of the uranium minerals which are bright yellow were mined and marketed for pigments in the glass industry. Needless to say, now that the dangers of radiation are understood, uranium minerals are no longer put to this use.

The largest uranium deposits in the free world are found in the United States, Canada, Australia, and to a lesser extent in Africa. The United States' deposits are unlike the deposits in the rest of the world, with one notable exception. This uniqueness is due to the fact that in the United States the major deposits occur in sandstone host rock. Only in the region of Niger in Africa have significant similar deposits been found to date. The huge deposits in Canada and Australia occur in metamorphic rocks that are plus two billion years old. The U.S. deposits occur in much younger rocks, primarily of Jurassic, Triassic and Eocene age, ranging in age from 190 million to 53 million years old.

There are two theories currently popular for the source of our domestic uranium deposits. Quite simply they are that the uranium came from: 1) the feldspar particles in the sandstone host rocks which were derived from granitic rocks, or 2) volcanic rocks or rocks derived from volcanics. The favored theory by most geologists is that the source of uranium in our domestic deposits was volcanic rocks or rocks derived from volcanic rocks. The largest deposits in the U.S. are located in New Mexico, Wyoming, Colorado, Utah, and Texas, and were all formed in much the same manner. Under the favored theory, sandstones which are or once were porous and permeable are either overlain or underlain by volcanic rocks or rocks derived from volcanic sources. These volcanics are anomalously

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high in uranium content. Uranium minerals are very soluable in oxygenated water. Water derived from rain or from snow melt percolated through the volcanic rocks, taking the uranium into solution. When these uranium enriched waters entered the adjoining permeable sandstones they migrated laterally until they encountered a reducing environment where they precipated as uranium minerals. Materials that can act as reducing agents are numerous, but those that are instrumental in siting our domestic uranium deposits are usually either carbon trash, in the form of carbonized leaves, twigs, or tree trunks, or in the case of the Texas deposits, natural gas emanating from underlying oil or gas deposits. The carbonized wood was deposited by the same stream systems that deposited the host sandstones. In the foregoing discussion we have identified the three things that every uranium geologist deems most important as factors identifying a good uranium exploration bet, that is — permeability of the host formation, a reducing environment, and favorable source rocks. Two of these, the permeability and a reductant are found in paleo river systems. The third, either volcanic or granitic source rocks, are surprisingly wide spread throughout the western United States.

A word about roll front deposits is in order here. Many of you have probably heard the term when used with uranium deposits. It is used to describe that phenomenon occurring after uranium has been precipated by a reducing environment, and additional oxygenated waters redissolve the fixed uranium, carrying it laterally to a site nearby, where reducing conditions again prevail and a second precipitation occurs. This is usually accompanied by insertion of additional uranium enriched waters, thereby causing upgrading of the system...