A Study of Natural

Radioactive EquilibriumIn Selected Uranium Minerals

Erik HunterColorado School of Mines

GEOL 59010/31/05

Transmuting base elements into gold and silver has always been thedream of the ancient alchemists. Little did they know that some of the rocks andminerals that they were working with contained elements that were naturallytransmuting into different elements. Some rocks and minerals can containsignificant amounts of radioactive isotopes from the following primordial decaychains: Uranium 238, Uranium 235, Thorium 232, and Potassium 40. Theisotopes formed from the decay of Uranium 238 are Th 234, Pa 234, U 234, Th230, Ra 226, Rn 222, Po 218, Pb 214, At 218, Bi 214, Po 214, Tl 210, Pb 210, Bi210, Po 210, Tl 206, and stable Lead 206. The decay schemes for U238, U235,Th232, and K40 are included in this report in Appendix A. The proportion of

decay products relative to the parent isotope can vary. This phenomenon istermed “equilibrium”. Rosholt defines this concept by stating “Equilibrium isattained in a radioactive series when all the daughter products decay at the samerate that they are produced from the parent isotope. Thus, at equilibrium each ofthese daughter products would be present in a constant proportion to the parentisotope. The loss or gain, by geologic processes of any of certain importantisotopes during the more recent part of the existence of a mineral causesdisequilibrium in the proportions of the parent isotope to its daughter products”(1959, p.2).

Determining whether or not uranium minerals are in equilibrium can havesignificant economic impacts where uranium mining and yellowcake productionare concerned. There have been many instances where a rich “uranium deposit”has been discovered with the use of radiometric equipment such as geigercounters or scintillometers. Upon chemical assay, the “ore” sometimes turns outto be rich in radium and somewhat deficient in uranium. Since there is no longera market for radium, the “ore” could turn out to be subeconomic. In some cases,deposits containing some out-of-equilibrium ore may be still be economic, butmining techniques may have to be modified to work around it. An excellentexample of this is the Lucky Mc Mine in Fremont County, Wyoming. The deposit

was discovered in 1953 by Neil McNeice of Riverton, Wyoming (Anderson, p.275). Mr. McNeice was prospecting on foot with a scintillometer when hediscovered a showing of radioactivity at the surface. He brought in experts fromthe Atomic Energy Commission, who determined that the deposit did indeedcontain low grade uranium ore of economic value. One peculiar aspect of thisdeposit, and near-surface deposits in general, is that the uranium can beselectively leached from the deposit by oxidizing surface waters, leaving itsdecay products behind. This phenomenon can lead to a significant discrepancybetween radiometric measurements and the actual uranium content of the ore.Anderson reports that “The mining of commercial grade ore from surface

exposures was extremely difficult, since years of erosion and weathering hadcaused an imbalance of gamma radiation compared with uranium content,making chemical assaying mandatory for grade control” (p. 276-277).

Uranium is not only found in varying ratios with its decay products, butthere may be compositional variations between the isotopes of uranium in certaindeposits. Uranium 238 and Uranium 235 isotopes occur in the ratio of 99.3%U238 and 0.7% U235 in all known deposits except for the Oklo uranium depositin Gabon, Africa. Research has shown, however, that there can be alsoconsiderable variation between the ratios of Uranium 235 and Uranium 234 insandstone deposits. This variation has proven to be a problem in themanufacture of nuclear fuel, because U234 cannot be separated from thedesirable isotope, Uranium 235, by conventional methods. Uranium 234 is muchmore radioactive than U235 or U238, and poses a threat to workers who handlethe fuel. Capus reports that the specific activity of U234 is 300 times the specificactivity of U235 and 18,000 times the specific activity of U238 (P. 218). He alsoasserts that “In the past, almost all, if not all, fresh uranium produced at themines seems to have satisfactorily complied with the isotopic specification.Contrasting with the past situation, some uranium shipments in the recent years[sic] have shown excess content in 234U. All these shipments came from ISL

operations, or from mine water recovery plants linked to remediation programs.It seems that among the anomalous mines, those using a sulfuric acid processare representing a very large share, if not all” (P. 213). The shipments, whichcontained the excess U234 isotope were blended down with fuel of properspecifications and in some cases, had to be discarded (P. 219).

The degree to which the U234 isotope is concentrated in roll front typedeposits has been reported by Rosholt, et al in their study of the Shirley Basin,Wyoming deposits. “Ratios of U234/U235 approaching radioactive equilibriumoccur in unaltered sand above and below the ore, whereas U234 deficiencies of

7 to 22 percent are found in the ore. U234 excess of as much as 70 percent isfound in the altered sand tongue near the ore” (1964, p. 570). Rosholt, Et Al.also offer an explanation for this phenomenon “U234 is contributed to theenvironment in two ways: (a) some of the U234 atoms are mixed, transported, orprecipitated with U238 and U235 and subsequent changes in their isotopic ratiosare caused primarily by radioactive decay of U234, and (b) the remaining U234atoms are generated in situ from the radioactive disintegration of precursors,Th234 and Pa234, and thus are subject to differential migration with respect tothe U238 from which they were derived” (1964, p. 570).

Study Preparation and Background

For this study, the author collected samples of uranium containingminerals from ten different locations for gamma ray analysis. Gamma rayanalysis is a method by which certain gamma-ray emitting isotopes are detectedusing a gamma ray spectrometer. The particular spectrometer used in this studywas a Berkeley Nucleonics SAM-905. This spectrometer uses a thallium dopedsodium iodide crystal coupled to a photomultiplier tube. The incoming gammarays cause the crystal to flash or “scintillate” and the photomultiplier tubeconverts the flashes into electronic pulses. The pulses are counted and sorted

by their gamma ray energy with the multichannel analyzer in the SAM-905.Gamma ray energies are measured in electron volts. The standard unit used is“keV”, or 1,000 electron volts. The SAM-905 features a reference library ofgamma ray energies that correspond to known gamma emitting isotopes. Likelyisotope-gamma ray matches are then automatically presented by the SAM-905 ina report, along with CPM (counts per minute). Since some gamma emittingisotopes emit rays at different energies, a correlation in terms of percent can beassigned to the identification. This correlation is a measure of the probability thatthe sample identification is accurate. For the purposes of this study, anycorrelation below 80% was not considered as a positive identification. More

sensitive types of gamma spectrometers exist, but were too costly to consider forthis study.

Sample and Locality Description

Grab samples of radioactive minerals were taken from the following locations bythe author between 1999 and 2003.

1. Foss, OklahomaA mineral identified by Fay and Hart as carnotite is present at a location

northeast of Foss, Oklahoma. Totten and Fay describe this location further bystating “The Red Rock Uranium Company of Foss mined 20 tons of ore from thebasal Doxley siltstone (Permian) north of Foss in 1955, with assays ranging from60 to 16,100 ppm uranium” (p. 10). This translates to a uranium concentration of0.006% to 1.6%.

2. Ascension Mine, Golden, ColoradoThe Ascension mine is located approximately 500 feet southwest of

Golden Gate Canyon Road in section 24, R 71W, T3S, Jefferson County,Colorado. “The Ascension Mine….has produced about 1,450 tons of crude ore

that contained an average of about 0.26 percent U3O8 … The breccia reef faultsand associated fractures belong to the Hurricane Hill fault system…Pitchblende,associated with varying amounts of carbonate minerals and quartz, occurs ascolliform coatings on breccia fragments along the faults and as thin stringersalong subsidiary fractures” (Sims and Sheridan, 1964). The host rocks are calc-silicate gneiss of the Precambrian Idaho Springs formation (Nelson-Moore, et al,1978).

3. Yellow Chief Mine, Juab County, UtahThe Yellow Chief mine is located in the Thomas Range Mountains of Juab

County, Utah. The host rock is the beryllium tuff member of the 21 MYA SporMountain Formation. The uranium minerals present have been identified byLindsey as weeksite and Beta-Uranophane (p.95). Lindsey argues that these oreminerals “probably were precipitated from ground water as uranyl phases” (p. 97)

4. Two Sisters Mine, Gilpin County, ColoradoThis locality is unique in the Central City district due to the fact that the

dominant uranium mineral is metatorbernite. The genesis of this mineral wasdescribed by Sims, Osterwald, and Tooker as follows: “The metatorbernite wasprobably formed by oxidation, solution, and transportation of uranium fromprimary pitchblende, but it may be a primary mineral deposited directly frommineralizing fluids of different composition from those which depositedpitchblende” (p.19). The host rock is reported to be a Sillimantic Biotite QuartzGneiss of Precambrian age. Sims, et al. also report that “The uranium and othermetals are in or adjacent to steeply dipping mesothermal veins of Laramideage… Selected samples of metatorbernite bearing rock from one mine dumpcontain as much as 6.11 percent uranium” (p. 1).

5. Mi Vida Mine, La Sal District, UtahOne of the largest sandstone hosted uraninite deposits in history was

discovered near La Sal, Utah in 1952 by Charles Steen. Mr. Steen developedthis deposit into the "Mi Vida" mine. It was the first confirmed instance of asandstone deposit which contained a large amount of very high grade uraninite(up to 87% U3O8). The host rock is in the lower Triassic Chinle formation.Steen, et al. assert that the uranium was deposited after the Triassic sedimentswere laid down; “…about relations to the large late-Cretaceous to early Tertiaryintrusive masses of the plateau region. The writer (Charles A. Steen) believesthat the last mentioned concept accounts for the origin of the uranium deposits”

(p. 6).

6. Little Warrior Mine, Clear Creek County, ColoradoThe Little Warrior mine is an open pit mine which is located at an altitude of11,000 feet and 7.8 Miles NW of Mount Evans in Clear Creek County, Colorado.This mine is small, and is estimated by the author to be about 200 feet X 100 feetand 20 feet deep. The host rock is a early to middle proterozoic pegmatiteintrusion in the Precambrian Idaho Springs Formation. The host rock is furtherdescribed as “Pegmatite – Quartz and feldspar with various amounts of biotite,muscovite, magnetite, and hornblende. Granite Pegmatite is medium-grained topegmatitic” (Widmann, et al., Map) The Minerals found are described as“Uranium, Autunite” (Nelson-Moore, et al., p.104)

7. Mt. Pisgah Uranium Occurrence, Jim Thorpe, PennsylvaniaThis uranium occurrence was known to exist as long ago as 1874. In

1954, Klemic and Baker reported that; “The Mount Pisgah uranium deposit…is inthe basal sandstone and conglomerate member of the Pottsville Formation.Carnotite, tyuyamunite, leibegite, uranophane, and beta-uranophane have beenidentified in the Mount Pisgah deposit. An attempt is being made to identify theblack radioactive material that occurs in the matrix of the rock” (p.3-4).

8. Deer Park Mine, Yancey County, North CarolinaThe Deer Park mine is a pegmatite deposit located in the Spruce Pine

district. A sample of monazite from the mine collected by Professor AdolphKnopf was analyzed and was found to contain 4.81%-4.86% thorium, 0.131%-0.134% lead, and 0.01% uranium. The age of the monazite was estimated to be600 MY. (Bliss, p.215).

9. Zia Mines, Grants, New MexicoThe Zia Mines are located about 9 miles northeast of Grants, New Mexico

at an elevation of about 7,050 feet. The main pit is small, measuring about 250feet X 150 feet and 25 feet deep. The ore occurs in the top of the Entradasandstone formation and the basal parts of the overlying Todilto Limestone(Hilpert, p.10). The ore minerals are carnotite and possibly tyuyamunite. Thesamples obtained for this study were a mixture of uranium minerals in bothlimestone and sandstone.

10. Schwartzwalder Mine, Jefferson County, ColoradoThe Schwartzwalder mine is located near several other smaller vein-type

pitchblende deposits in "Precambrian gneisses and schists of the Idaho Springsformation" (Young 1977, p. 2). The ore zones are entirely fracture-controlled.(Young 1977, p. 9). The ore mineral, pitchblende is found exclusively in"Cymoid" or "S" shaped veins which were created by uranium-rich hydrothermalfluids about 60 million years ago (Wright, p. 84). The Illinois Fault System is theprimary zone of economic interest. Notably, though, during the late 1980's, asignificant amount of mining took place along the Precambrian West RogersReef, according to mine manager Tom Bucholz. A system of fractures within theIllinois Fault System, known as "Horsetail Fractures" are a major source of ore,with veins of pitchblende up to several feet thick. Horsetail fractures are knownto structural geologists as a unique feature of strike-slip faults. The force of the

fault is distributed through a large volume of rock relative to the volume that theprimary fault occupies. This is one of the reasons why this particular mine wasso successful. Uranium rich fluids were allowed to mineralize a great deal ofrock due to this unique aspect of rock mechanics.

The rock types that have been identified in the mine are as follows:hornblende gneiss, quartz-feldspar gneiss, quartzite, garnet-biotite gneiss, andmica schist. The host rocks for the ore are a "Proterozoic sequence of garnet-biotite gneiss and quartzite that forms a thin transition zone between regionallymore extensive units of hornblende gneiss and mica schist" (Wallace and

Karlson, p. 1844). The ultimate source of the uranium-rich fluids is not agreedupon. Some geologists claim that it is leached from the Precambrian igneousrocks. Other geologists assert that it was leached from the Paleozoicsedimentary rocks, which unconformably overlie the Idaho Springs formation atthe mine site. One issue that there is agreement on, however, is theextraordinary amount of uranium in the ore. The shipping grade of ore wasnearly 1% uranium oxide. Some individual samples have been assayed at 58%uranium oxide (Paschis, p. 126). To put this in perspective, in 1951, the A.E.Cspecified a cutoff grade of just 0.10 percent! The ore contains other valuableelements such as molybdenum, thorium, nickel, cobalt, silver, and arsenic(Paschis, p. 126).

Experimental Method

The samples were ground separately until at least 90% of the sample wasa fine powder (< 20 Mesh). Each powdered sample was placed in a 118 mLcontainer. Sample weights ranged from 25.3 to 25.9 grams. Each sample wasleft in its container for over two weeks to allow the radon to establish equilibriumwith the other radionuclides in the sample. Each sample was counted with the

SAM-905 gamma spectrometer for 5 minutes, except for the sample from Foss,which was counted for 10 minutes due to its low amount of radioactivity ascompared with the other samples. The scintillation probe was wrapped in a half-inch thick lead shield to reduce the background count. After counting, thespectra were printed out and the data was analyzed.

Analysis

The only two isotopes automatically identified by the SAM-905 in all of thesamples were Radium 226 and Bi 214. One interesting anomaly was that Ra224

was detected in the sample from the Deer Park Mine. Ra224 is a decay productof Thorium 232. The presence of Ra224 suggests that thorium minerals such asmonazite or thorite may have been present in the sample. Radium 226 and Bi214, in particular, are useful in uranium exploration because they are the mostprevalent emitters of gamma radiation. Other decay products undoubtedly existin the samples, but the decision was made to limit the scope of this paper to therelationship between Ra226 and Bi214. Attempts were made by the author todetermine if there were any correlations between the relative proportions of thesetwo isotopes in the samples. The calculated proportions were based on countsper minute (CPM) in each energy channel of interest. Since the samples wereof varying uranium content, a correction factor had to be used to standardize thedata. This standardization was accomplished for each individual sample bydividing the CPM in the 186.1 keV Radium channel by the total net CPM for allenergy channels. The CPM for the two Bismuth 214 lines (609.3 and 1120.3keV) were added together and divided by total net CPM. The Ra226 ratio wasthen divided by the Bi214 ratio and plotted against the estimated age ofmineralization in the samples.

Age Vs. Ra226/Bi 214

y = -0.0003x + 0.5283

R2 = 0.1694

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0 100 200 300 400 500 600 700

Age (MYA)

RA

22

6/

Bi2

14

Rati

o

Age Vs. Ra226/Bi214Linear (Age Vs. Ra226/Bi214)

The Ra226/Bi214 ratios were then plotted as a function of mineral type.

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

Autinite

Autinite

Carnotite

Carnotite

Metatorbenite

Uraninite

Uraninite

Uraninite

Uraninite

Weeksite

Series1

ConclusionsSeveral factors limit the usefulness of the data in this study. The primarylimitation is the fact that the study is limited to ten samples due to time andbudgetary constraints. A more comprehensive study that includes not only moresample localities, but multiple samples from each of the localities, would yieldmuch better information from a statistical standpoint. Another limitation is the

quality of the data from the gamma spectrometer. Gamma spectrometry alone

does not identify all of the daughter products present in the decay chains ofU238, U235, and Th232. A better funded research project could incorporate datafrom an alpha spectrometer or mass spectrometer to more accurately define theradionuclides present in the samples. The age of the samples in this study isalso somewhat uncertain. Except for the Deer Park sample, the ages of uraniummineralization were based on general geologic interpretation by geologists. Thisleaves considerable room for error. Isotopic dating techniques such as Pb-U, K-Ar, or Rb-Sr would yield much more accurate ages for the samples.

In spite of the limitations of the study, two general trends can be noticed in

the data. Although it has a low R2 value of 0.39, there seems to be a “peak” inthe Ra226/Bi214 ratio around 230 MYA. Also, the uraninite samples show ahigher Ra226/Bi214 ratio on average than the autunite samples. There existsthe possibility that other factors such as the geochemistry of mineralization mayhave a greater influence on the Ra226/Bi214 ratio than age or mineral type.

Appendix ADecay Chains

Glasstone p. 133

Glasstone p. 135

Glasstone p. 134

References

Anderson, D.C. “Lucky Mc Mine.” Case Studies of Surface Mining. TheAmerican Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. NewYork, 1969

Bliss, A.D., 1942, Analysis and age of monazite from Deer Park no. 5 mine,Spruce Pine, N.C.: Am Mineralogist, Vol 27, P. 215.

Bowyer, B., 1963, Yellow Chief uranium mine, Juab County, Utah, in Sharp, B.

J., and Williams, N. C., eds., Beryllium and uranium mineralization in westernJuab County, Utah: Utah Geological Society Guidebook to the Geology of Utah,no. 17, p. 15-22.

Capus, G., “Downstream constraints on product specification and ISL MiningMethods. Recent Developments in Uranium Resources and Production withEmphasis on In Situ Leach Mining. International Atomic Energy Agency, Vienna,2004.

Fay, R.O., and D.L. Hart. 1978. Geology and mineral resources of CusterCounty, Oklahoma. Oklahoma Geological Survey Bulletin 114.

Glasstone, Samuel. Sourcebook on Atomic Energy. Van Nostrand. 1958.

Hilpert, Lowell S. “Regional and Local Stratigraphy of Uranium Bearing Rocks.”Geology and Technology of the Grants Uranium Region. New Mexico Institute ofMining and Technology, (1963): 6-18

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Rosholt, J.N., Harshman, E.N., Shields, W.R., and Garner, E.L., “IsotopicFractionation of Uranium Related to Roll Features In Sandstone, Shirley Basin,Wyoming.”, Economic Geology. Volume 9, 1964

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Sims, P.K. and Sheridan, D.M. Geology of Uranium Deposits in the Front Range,Colorado, USGS Bulletin 1159, 1964

Steen, Charles and Dix, George. Uranium Mining Operations of the UtexExploration Co. in the Big Indian District, San Juan County, Utah. USBMInformation Circular 7669, 1953

Wallace, Alan and Karlson, Richard. "The Schwartzwalder Uranium Deposit, I:Geology and Structural Controls on Mineralization." Economic Geology v. 80(1985): pp. 1842-1857

Widmann, Beth, Kirkham, Robert, and Beach, Steven. Geologic Map of the IdahoSprings Quadrangle, Clear Creek County, Colorado. Colorado. Open FileReport 00-2, Colorado Geologic Survey, 2000

Wright, John "Economic Geology of the Schwartzwalder Mine." ColoradoSchool of Mines Uranium Resource Technology Seminar III. Colorado School of

Mines Press. (1980) pp. 73-92

Young, E.J. Geologic, Raidometric, and Mineralogic Maps and UndergroundWorkings of the Schwartzwalder Uranium Mine and Area, Jefferson County,Colorado. USGS Open File Report 77-725, 1977