My experiments proved that the radiation of uranium compounds … is an atomic property of the element of uranium. Its intensity is proportional to the quantity of uranium contained in the compound, and depends neither on conditions of chemical combination, nor on external circumstances, such as light or temperature — Marie Skłodowska Curie, Polish/French scientist who won 2 Nobel Prizes; her doctorate thesis was the first to show that radiation came from an atom, not from environmental conditions.
The theme of the 2015 Denver Gem and Mineral show is “Minerals of the American Southwest”. The theme evokes images of colorful copper minerals from Arizona, gold and silver from Colorado, red beryl and champagne topaz from Utah, and perfect fluorites from Bingham, New Mexico. The four states have a rich mining heritage with boom towns, barons, and villains. The influence of the American Southwest on the modern mineral collecting hobby is also outsized – from personalities of famous collectors and mineral dealers to mega mineral shows, perhaps no geographic region is more influential. The southwest was also the site of the greatest mineral rush in history, but is largely unremembered – the great uranium rush of the 1950s.
My father and I visited many of the uranium mines – mostly the abandoned ones in Utah – in the early 1970s, and I collected a boat load of “yellow smears”. I learned how to read the x-ray diffraction films from studying samples I prepared for identification. I was cautioned to store these treasures in the barn rather than my bedroom because of issues with radon or radioactive decay. I never really built a systematic collection, and my specimens were eventually disposed of for environmental reasons, but I was fascinated by the story of the uranium minerals. In 1974, I read Edward Abbey’s book Desert Solitaire, and was deeply affectedly by his descriptions of the joy of isolation and the beauty of the canyon country and mountains around Moab. I also saw in this book the tremendous loss that we experience when we destroy wilderness. I was asked to talk at the 2015 Denver show on minerals of the southwest, and it was assumed I would wax on and on about the silver minerals…but I decided to revisit uranium!
The launch of the Manhattan Project in 1939 suddenly made uranium a valuable commodity, but established mines were few globally; in the US only a few mines in western Colorado were producing any uranium (mainly as a byproduct of vanadium mining). Leslie Grooves, director of the Manhattan Project secretly purchased the entire stockpile of Vanadium Corporation of America, which was stored at Uravan, Colorado – but that was only 800 tons of ore. Once the war was over the US had less than 100 pounds of enriched uranium (U235), and development of domestic uranium mining became a government priority. The newly minted Atomic Energy Commission announced remarkable incentives for new uranium discoveries: a price of $3.50 per pound of uranium oxide, and a $10,000 bonus paid on the delivery of 20 tones of ore that assayed 20 percent uranium oxide. By the early 1950s there were more prospectors looking for uranium on the Colorado Plateau than ever mined gold in the history of California – in fact, there were 30 uranium “rushers” for every 1 ’49 rusher to the Sierra Nevada.
The AEC incentives worked — by the mid-1950s there were about 800 major uranium ore producers on the Colorado Plateau, and ore production was doubling every 18 months. The rush made more millionaires than the great Colorado silver rushes of the 1870s or the Arizona copper rushes of the 1880s. Moab, Utah was dubbed the “Uranium capital of the World”, and had 20 millionaires for every 250 citizens. The AEC cancelled the bonus for uranium production in the early 1970s, and eventually the mining industry declined to a subsistence level. However, the rush to the Colorado Plateau had a huge impact – from 1949 to 1971 the mines produced about 400 million tons of ore that yielded 200,000 tons metric tons of uranium metal.
Coincident with the great uranium rush, mineral hobbyist clubs and minerals shows exploded – the American Federation of Mineralogical Societies (AFMS) was founded in 1947. Although it is a stretch to directly connect the great uranium rush with the rise of mineral collecting as a hobby, it is obvious that the heavy promotion of uranium prospecting peaked the interest of many Americans, and rock hounding entered a golden age. Many collectors purchased or collected uranium minerals from the Plateau, and these prizes sat in places of honor in the collector’s cabinets. However, as the hazards of uranium mining became understood in the 1970s, collectors began to dispose many of their specimens. Today, it is almost impossible to find a fine Colorado Plateau uranium mineral specimen – and the heritage of an amazing American event has faded from the public memory.
A Brief History of the Colorado Plateau Uranium
The history of uranium on the Colorado Plateau begins a half century before the Great Rush when prospectors were looking for the more valuable commodities of radium and vanadium. Settlers in the Paradox Basin in southwest Colorado knew of a yellow, powdery material found in sandstones before 1880; it is likely that Ute and Navajo Indians collected this same material as a pigment for hundreds of years. By the late 1890s various prospectors had collected a few hundred pounds of the material, but did not know what it was, nor could they find a market. In 1881 Tom Talbert discovered the same yellow material in Montrose County, and eventually this material found its way to Gordon Kimball in Ouray. Kimball sent samples to Frenchman Charles Poulot (residing in Denver) in 1898, who determined it contained both uranium and vanadium. Poulot gave the material to M. M. C. Friedel and E. Cumenge (of Cumengite fame) who determined an approximate formula: K2(UO2)2(VO4)2·1-3H2O Also in the samples were significant amounts of radium – which is a decay product of U238. This coincided with Curie’s discovery of the element radium, and she began to purchase carnotite from Colorado for research. Radium became the vanguard material for worldwide research on radioactivity, and a number of mines were staked along the Colorado Plateau.
Aside from staking claims, nothing of substance happened in carnotite mining until 1910 when a new medical market for radium developed — it appeared to have a dramatic effect on certain cancerous tumors — and southwestern Colorado became a major supplier. However, the outbreak of WWI completely quashed the demand. As the demand for radium dried up, the demand for vanadium increased rapidly. It was discovered early in the 20th century that adding a small amount of vanadium increased the strength of steel. In 1905 Henry Ford was introduced to a vanadium rich steel, and was so impressed with its characteristics, he used it in the chassis of his Model-T. By the end of WWI the carnotite mining shifted from the focus on recovering radium to vanadium, and by 1922 radium recovery ceased all together. Although the demand for vanadium was cyclic, by about 1935 it had become an important enough metal that Vanadium Corporation of America purchased most of the former carnotite mines, and founded the town of Uravan (contraction of uranium and vanadium).
After 1936 there was a steady increase in prospecting and development of properties along the Utah-Colorado border targeted the Morrison Formation (Jurassic age), in particular, a fluvial sandstone/mudstone unit called the Salt Wash Member. With the outbreak of WWII steel became essential; vanadium was declared a strategic metal, and more than 600,000 tons of vanadium ore with a 2% grade was produced.
The modern era of uranium exploration and mining on the Colorado Plateau began when the Atomic Energy Commission was established by the Atomic Energy Act of 1946. All functions of the Manhattan Project, including the acquisition of uranium, were transferred to the ACE at mid-night, December 31, 1946. The AEC set up a procurement shop in Grand Junction, Colorado and begin devising schemes for securing uranium. At the time there were only 15 mines operating on the Colorado Plateau, and uranium was still considered a lesser product than vanadium. The AEC was the only buyer of uranium, and thus, set a price they thought would incentivize production; it soon became apparent that the AEC also had to be involved in the milling of ore, and provided bonuses for uranium oxide concentrations. The very first procurement contract was signed with Vanadium Corporation of American on May 28, 1947. The AEC demanded a rapid expansion of exploration and mining efforts, and provided assistance by undertaking geologic surveys and providing free assay services.
Between 1948 and 1956 the AEC and the USGS tasked several hundred geologists to scourer the Colorado Plateau and make their maps and drilling core results freely available to prospectors. The AEC also built more than a thousand miles of roads across the plateau to promote access to the most remote regions. Prospectors and weekend treasure hunters flocked to Utah and Colorado, and later to New Mexico and Arizona. Some of these prospectors struck it rich, and lived life higher than the biggest copper or silver baron of the 19th century. One of these “lucky” prospectors was Charlie Steen.
For two years Steen roamed the area around Moab, Utah and “barely” subsisted with his family in a trailer and tar paper shack. Using his experience in the oil industry, he was certain the uranium would collect in anticlines – sort of like an oil trap – and he drilled haphazardly. On July 6, 1952, Steen drilled into an incredibly rich deposit of “pitchblende” (uraninite) at 200 feet depth, and over the next year developed the Mi Vida mine. The fact that the ore was uraninite – not carnotite- in a rock type previously not shown to carry uranium, and at a depth that similarly unexpected, caught the government geologists by surprise. Steen became a multi-millionaire, and his find electrified the country – and greatly accelerated the rush to the plateau!
Geologists began to understand the nature of the Colorado Plateau uranium deposits in the late 1950s. Garrels and Larsen (1959) published USGS professional paer 320, Geochemistry and Mineralogy of the Colorado Plateau Uranium Ores, and it became clear that hydrology was the most important factor in localizing the uranium in vast columns of sedimentary rock.
Uranium is found in more than a dozen sedimentary strata on the plateau but the Morrison formation of late Jurassic age and Chinle of Triassic age account for 95% of the produced uranium. The Morrison was formed from the erosional sediments derived from a highlands called the Elko that existed along what today is the Utah-Nevada border. These sediments were deposited in floodplains, river channels and swamps, not unlike the Mississippi delta today. The Chinle is dominated by eolian (wind-driven dunes) deposits with smaller river channels cut during intervals of more precipitation. The uranium appears to have mobilized by ground water; dissolving and moving sparsely concentrated uranium and precipitating and concentrating that uranium when structural or chemical boundaries are encountered. The deposition of the uranium occurred millions of years after the deposit of the sedimentary rocks — perhaps 100 million years later. The deposition is most signifiant where the sediments contained trapped organic materials – logs in river channels or organic ooze in swamps. In fact, there are many examples of petrified logs that have been completely replaced by uraninite or coffinite.
The real key to the Colorado Plateau deposits is the long term stability of the sediment column. This stability has allowed dilute solutions to build these diffuse deposits. This is not the best environment to grow beautiful crystals, but small variations in chemistry has allowed for a wide range of uranium minerals. Through 2010 the Colorado Plateau has produced more than 600,000 tons of uranium oxide – and today contains 15% of the worldwide uranium reserve.
Colorado Plateau Uranium Minerals
The mineralogy of uranium is a fascinating and complex topic – the nearly ubiquitous presence off U in the environment, its large atomic radius, strong affinity for oxygen (lithophile), and high solubility in certain valence states leads to large number of secondary uranium minerals. The lithophile nature of U also means that 4.5 billion years of Earth evolution has concentrated the element in the crust; the statistic abundance of U in the crust is 2.7 ppm, as compared to 0.075 pm for silver and .004 ppm for gold. In other words, there are 675 atoms of uranium for every atom of gold in crustal rocks!
The fact that uranium has been so strongly enriched in the crust is also one of the reasons it is so ubiquitous – traces can be found in every rock type, soils and water. Uranium is a “5f electron” element, which controls much of the way it behaves in the environment. 5f refers to the electron configuration, or the distribution of electrons about the atom; uranium is an electron donor and has four different valence states depending on environmental conditions. Two of these states, U4+ and U6+, are stable in geologic environments. In igneous environments the +4 valence state prevails, and uranium is one of the last elements to form minerals in a magmatic or hydrothermal fluid. By far, the most common mineral to form in igneous environments is uraninite (UO2). However, once that uraninite is exposed to a humid and oxidizing environment the surface of the uraninite undergoes oxidation and the U4+ → U6+; the higher valence state is incompatible with the uraninite structure, and the uraninite decomposes, releasing the U6+, which rapidly binds with two oxygen atoms to form the uranyl ion [UO2]2+
The uranyl ion has a linear structure, with very strong bonding between the uranium and oxygen, and it is highly soluble in ground water. This high solubility leads to extraordinary mobility of uranium, and a key contributor to Colorado Plateau deposits. The uranium mined today in Grants or Paradox Basin did not originate anywhere near those locations; it likely was deposited in minor uraninite deposits during the billions of years of mountain building in what has become the North American Plate. Decomposed, oxidize, and transported through great distances, the uranium was concentrated when subtle encounters with ground water chemistry changes.
The mineralogy of U6+ is very diverse because of the uranyl ion; it has a linear, dumbbell shaped structure that cannot be easily substituted by other high valence cations. The uranyl ion will most commonly attach to tetrahedral anion groups; SO4, PO4, AsO4, SiO4. Any charge balance is then accommodated by other cations. The figure above is a generalization of the paragenes of uranium minerals; at the top is the primary oxides and moving down the chart shows the minerals that form as uranyl migrates away from the primary source. Typically uranyl carbonates form first, and vanadates – like carnotite – form far from the original source and after many generations of mineral growth and decomposition.
There are about 160 different uranium minerals known, and 61 have been reported as coming from the Colorado Plateau, and 12 have the Plateau as their type locality (there are dozen uranium minerals yet to be characterized for the Plateau deposits). The names are mostly unfamiliar to collectors – things like Becquerelite Ca(UO2)6O4(OH)6 · 8H2O, Schrockingerite, NaCa3(UO2)(CO3)3(SO4)F · 10H2O and Rabbittite, Ca3Mg3(UO2)2(CO3)6(OH)4 · 18H2O. They have colors that span the rainbow, although bright yellows and deep greens are favored. Those minerals that contain the uranyl ion all fluoresce (although U4+ minerals do not), and differences in hydration – and loss of water with alteration – changes the fluorescence. Unfortunately, the sedimentary deposits of the Plateau do not lend themselves to environments in which large, distinct crystals grow. Below is a gallery of some of the more important uranium speciemens:
Carnotite K2(UO2)2(VO4)2 · 3H2O
Tyuyamunite Ca(UO2)2(VO4)2 · 5-8H2O
Zippeite K3(UO2)4(SO4)2O3(OH) · 3H2O
Unknown Uranium Carbonate
The uranium minerals were intimately associated with vanadium minerals – often these carried some amount of the uranyl complex. The Colorado Plateau vanadium minerals are as unique as their uranium counterparts, and often even more colorful and unusual. A few are featured below:
Schrockingerite NaCa3(UO2)(CO3)3(SO4)F · 10H2O
Metahewettite CaV6O16 · 3H2O
Pascoite Ca3(V10O28) · 17H2O
Metarossite Ca(V2O6) · 2H2O
Hewettite CaV6O16 · 9H2O
The Great Uranium rush was over by the early 1960s. Many lone prospectors roamed some of the most desolate and beautiful country in the world in search of radioactive treasure. Uranium mining in the 1960s was controlled by large corporations, and huge open pit mines like Jackpile east of Grants, New Mexico supplied tons of uranium. By 1971 the government had more uranium than it could possibly use, and cancelled the incentives. By 1980 the world market drove the prices for the silver-colored metal to prices that shut down even the largest producers. Today the prices occasionally spike, leading to much discussion about reviving mining on the Colorado Plateau. However, the heavy environmental toll has a very dark legacy. It is unlikely that there will ever be another uranium mine in the Triassic and Jurassic sandstones that make the plateau look like the landscape of Mars.