TM-D- 1-18 U. S. ATOMIC ENERGY COMMISSION DIVISION OF RAW MATERIALS DENVER AREA OFFICE PRELIMINARY REPORT ON THE GEOLOGY OF URANIUM DEPOSITS IN THE BROWNS PARK FORMATION IN MOFFAT COUNTY, COLORADO, AND CARBON COUNTY, WYOMING By Allen Ormond "This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic 3aergy Ocmission, nor amy of their employees, nor any of their contractors, subcontractors, or their iaployees, make any warrantyr, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of ay infor- nation apparatus, product or process disclosed, or represents that its use would not infringe privately- owned rights." Casper, Wyoming June 1957 metadc1 393321 CONTENTS ABSTRACT.......................... .... 1 INTRODUCTION....... .......... .. .. 1 Location and Accessibility... ... .. ... 1 Climate, Topography, and Drainage . .. 3 History of Uranium Operations.... .. ... 3 Land Ownership.......... .. .. .... 3 Previous Investigations.... .. .. ..... 4 Map Coverage. .......... .. .. .. ... 4 Purpose and Scope........ ... ..... 4 Acknowledgments...... .. .. .. ... 5 GENERAL GEOLOGY.................... .. 5 Geologic History ............ 5 Stratigraphy . 5 Precambrian .............. 5 Paleozoic............... ... ... 6 Mesozoic............... .. .. .... 6 Tertiary . 6 Structure.................. .. .. ... 8 URANIUM DEPOSITS OF THE MAYBELL AREA . 10 Marge and Gertrude Depos:its...... 10 Location ....... 10 Extent . 10 Mine Workings . .. 10 Structure.... .. ..... 11 Host Rock... .. .. ... 11 Mineralogy.... .. ... 13 Equilibrium Factors .. 18 Sugar Loaf Mine . .. 18 Three Sisters Prospect . 18 White Star Prospect . .. 21 Buffalo Head Prospect .. 21 Cedars Prospect.. .... 21 Leon Prospect.... ..... 21 Agnes Prospect... ..... 21 September Morn Prospect 22 URANIUM DEPOSITS OF THE POISON BASIN AREA Cedar Hills Deposit........ .. .. ..... 22 Location . 22 Extent . 22 Page Mine Workings................ .. .. .. ... 22 Structure..................... .. .. .. ... 22 Host Rock.. ................... .. .. .. ... 22 Mineralogy...................... .. .. ... 23 Poison Basin Deposit ............. .. .. .... 23 Location....................... .. .. .... 23 Extent.. ..................... .. .. ..... 23 Mine Workings.. ............. .. .. .. ... 23 Structure..................... .. .. .. ... 23 Host Rock. ...................... .. .. ..... 23 Mineralogy...................... .. .. ... 27 Teton Prospect .................... .. ..... 27 Section 36 Prospect................ .. .. .. ... 27 PARAGENESIS........................... ... ... 27 ORIGIN OF THE URANIUM................ ... .. ... 28 SUMMARY................................... 29 REFERENCES ........................... .. .. ... 30 - ii - ILLUSTRATIONS Plate Pg 1. Geologic map showing the distribution of the Browns Park Formation in northwest Colorado and south-central Wyoming......... .. .. ..... 31 Figure 1. Index map of south-central Wyoming and northwest Colorado. ... ..................... 2 2. Geological map of the Maybell re, Moffat County, Colorado... .. .. .......................... 9 3. Geological map of the Baggs area, Carbon County, Wyoming... ... .......................... 9 4. Geological map of the open pit at the Marge mine as of March, 1957, Moffat County, Colorado . 12 5. Generalized geologic section A-A' Marge open pit, Marge No. 1 and Maybell No. 1 claims, Moffat County, Colorado................. .. .. .. ..... 14 6. Generalized geologic section B-B' Marge open pit Marge No. 1 and Maybell No. 1 claims, Moffat County, Colorado................. .. .. ..... 15 7. Association of uranium, limonite and calcium carbonate in Browns Park sandstone in the Marge mine , Marge claim No. 1, section 24, T . 7 N. , R. 95 W., Moffat County, Colorado.... ... ..... 16 8. Concentric rings of visible uranium mineral in Browns Park sandstone in the Gertrude mine, Gertrude No. 6 claim, section 17, T. 7 N., R. 94 W., Moffat County, Colorado.... .. ..... 17 9. Geological cross section C-C' Marge open pit Marge No. 1 claim, section 24, T. 7 N., R. 95 W. , Moffat County, Colorado.... ... .... 19 10. Geological cross section D-D' in the Marge mine, Maybell No. 1 claim, sec. 19, T. 7 N., R. 94 W., Moffat County , Colorado........... .. .. ..... 20 - iii - 11. Geological cross section A-A' No. 1 open pit Cedar Hills No. 6 claim, section 32, T . 13 N. , R . 92 W ., Carbon County Wyoming.... .. .. ... 24 12. Geological cross section A-A' No. 2 open pit Cedar Hills No. 10 claim, section 32, T . 13 N. , R . 92 W ., Carbon County Wyoming.... .. .. ... 25 13. Geological cross section B-B' No. 2 open pit Cedar Hills No. 10 claim, section 32, T . 13 N. , R . 92 W ., Carbon County , Wyoming.... .. .. ... 26 - iv - TM-D- 1-18 GEOLOGY OF URANIUM DEPOSITS IN THE BROWNS PARK FORMATION IN MOFFAT COUNTY, COLORADO, AND CARBON COUNTY, WYOMING ABSTRACT Uranium was first discovered in the Browns Park Formation in 1951 in the Miller Hill area of south-central Wyoming . Since that time economical- ly important deposits in this formation have been discovered and developed in the Poison Basin of south-central Wyoming and in the Maybell area of northwest Colorado. The Browns Park is the youngest formation (Miocene) in the region and overlies older rocks with angular unconformity . The formation consists of a basal conglomerate, fluviatile, lacustrine, and eolian sandstones, and locally a few thin beds of clay, tuff, and algal limestone. The sandstones are predominantly fine- to medium-grained and consist of quartz grains, scattered black chert grains, and interstitial clay. The uranium deposits are of the sandstone-impregnation type and are not confined to specific stratigraphic horizons. The important ore minerals are autunite and uranophane in oxidized sandstones, and uraninite and coffinite in unoxidized sandstones. Uranium is often associated with limonite and calcium carbonate in concretionary forms. Woody material, thought to play an important part in the deposition of uranium in many sandstone-type deposits, is not present in the deposits of the Browns Park Formation. However, organic carbon in the form of petroleum and petroleum residues has been observed in association with uranium in both the Poison Basin and the Maybell areas. INTRODUCTION Location and Accessibility The Maybell area is in the east-central part of Moffat County, in the northwest corner of Colorado (fig. 1). The area is served by U. S. Highway 40, which crosses the county in an east-west direction. All parts of the area are accessible by state secondary roads or improved trails . Rail service is available from Craig, Colorado, to Denver and Grand Junction, Colorado. The Poison Basin area is in the southwest corner of Carbon County, in the south-central part of Wyoming (fig. 1). The area is accessible from state highway 789 by six miles of dirt road. The closest rail he ads are Craig , 45 miles to the south , and Rawlins , Wyoming , 80 miles to the northeast. - 1- Medicine Bow S W E E T W A T E RRawlins1 - RBA0LN I.A.N.Y C A -- 130 tAL B A Y 4789 /ooi POISON B ASIN A A3 ____ ____ Bogs _ __WYOMING 1 COLORADO UTAH JMAYBELL AREA 0 20 40 - ------ --- --- - 0MJL E S TM- 0-1-18 Figure I, Index map of south- central Wyoming andad nrhetnorthwest ColoradoClao Climate, Topography, and Drainage The climate is semiarid, the average precipitation being about 16 inches annually , principally as snow . The approximate temperature range is - 400 F to 1000 F. The topography of the region is characterized by rolling hills and a few prominent mountains and ridges. Altitudes range from about 6,000 to 8,000 feet and average about 6,300 feet. The Maybell area is drained by the Yampa River, a westward-flowing perennial stream. The Poison Basin area is drained by the Little Snake River, a southwestward-flowing perennial tributary which joins the Yampa west of Maybell, Colorado. Most of the tributary streams are dry during the summer months. Wells and reservoirs , the only other sources of water , are private, and water from them is usually not available for use in mining or drilling . History of Uranium Operations Uranium was first discovered in the Browns Park Formation in the Miller Hill area (fig. 1) of south-central Wyoming in October 1952. Discoveries were made in the Poison Basin area in October 1953, and in the Maybell area in March 1954. After the initial discoveries, exploration drilling and mining was begun. Economically important deposits were developed in the Poison Basin area by the Trace Elements Corporation, now wholly-owned subsidiary of the Union Carbide Nuclear Corporation, the King Oil Company , which now operates the Trace Elements Corporation property, and the Shawano Development Corporation. Economically important deposits were developed in the Maybell area by the Trace Elements Corporation, and further development was done by the Union Carbide Nuclear Corporation. Extensive exploration was also carried out by many other companies, including Utah Construction Company, Teton Exploration Drilling and Development Company, James Eskridge and Associates, Thunderbird Development Company, and American Leduc Corporation. In the Maybell area, Trace Elements Corporation began regular shipments of ore to the processing mill at Rifle, Colorado, from the Marge mine, in October 1955, and from the Gertrude mine in May 1956 . In the Poison Basin area, the Cedar Hills mine , operated by the King Oil Company, began ore shipments in May