GJO-l~SC c. Grand Junction Office (AEC), Colorado URANIUM IN THE TERTIARY INTERMONTANE BASINS OF COLORADO C Malan 1969 TECHNICAL I I URANIUM IN THE TERTIARY INTERMONTANE BASINS OF COLORADO~ ROGER C.~ Grand Junction, Colorado .ABSTRACT The Tertiary intermontane basins in Colorado contain numerous, shallow prospects and several small deposits of uranium in geologic environments similar to those in the prolific uranium districts of central Wyoming. The occurrences in Colorado are in the deep Middle Park~ South Park, and Raton basins and in smaller upland basins .along .the eastern margin of the Thirtynine Mile volcanic field northwest of Canon City. Only the Tallahassee Creek locality in the upland basinal system has produced a significant amount of uranium ore. The uranium occUrrences in the Colorado basins are spatially related to volcanic fields, principally of Oligocene age, where lithologically favorable fluvial host sediments are (1} subjacent to the volcanic piles in the upland basins or (2) subjacent to volcaniclastic outwash and tuffaceous lake beds in deeper basins near the volcanic piles. 1 Rea.d before the National Western Mining Conference of the Colorado Mining Association at Denver, Colorado, January 30, 1969. ~blished by permission of the U. S. Atomic Energy Commission, Grand Junction, Colorado. 2 U. S. Atomic Energy Commission, ·Grand Junction, Colorado. ~e critical review of this paper and the helpful suggestions offered by E. W. Grutt, Jr. and R. T. Russell, U. S. Atomic Energy Commission, Grand Junct~on, Colorado, aided materially in its preparation. - 1 - - 2 - During the past two years, nearly one-half million acres have been acquired for uranium exploration and several deep exploratory drilling efforts have been started. INTRODUCTION The major stratiform uranium deposits in sedimentary roCks in the Tertiary basins of Wyoming account for approximately one-fourth of the total uranium reserves plus production in the Western United States. The current exploration boom has generated renewed interest in most other areas of uranium mineralization in Tertiary sediments throughout the· Western United States. Uranium has been known to exist in most of these outlying areas since the 1950•s, but only the Texas gulf coast and the Meybell area in northwest Colorado have been important producers. In Colorado, concentrations of uranium occur in Tertiary sediments within the discontinuous chain of basins between the two major range systems (Fig. 1). The major basins in this chain are the North Park-Middle Park basin, South Park basin, and the Raton Basin. They were formed through major downwarping in the early Tertiary and contain thick sections of Tertiary and pre-Tertiary sediments. Uranium deposits also occur in smaller upland basins southeast of South Park in the region of the Thirtynine Mile volcanic field. These basins contain relatively thin sections of Tertiary sediments and superjacent volcanics that were deposited on a widespread erosional surface on Precambrian rocks. Tallahassee Creek, one of these small intermontane basins of Colorado, is . the ollly area in which significant amounts of uranium have been produced from ~rtiary stratiform deposits. 25 0 25 50 75 100 lolilll SCALE =- £X PLAN AT ION c::J Prtcombr~on boumtnl rockt upoatd . = town « C•ly Ito I: ltd Escori)ment mountain or pou ··y---·-~--------- 1 ARIZIONA j l Fig. 1. Index Map of the Major Tartlary Intermontane Basins in Colorado - 3 - Prior to 1967, only superficial exploration was done at most prospects in the Tertiary basins of Colorado; all discoveries were less than 200 feet deep. Since then, however, about 500,000 acres have been acquired for uranium exploration, and nearly a dozen deep exploratory drilling programs have been started. The past efforts of many geologists of the U. S. Atomic Energy Commission form an important part of this report. In the mid-1950's, most of the areas of uranium occurrences in the Tertiary basfns of Colorado were studied. From the time of the first discoveries in 1954 to the present, we have routinely followed exploration and mining activities in these areas. From 1964 to 1967, the author studied the potential of the uranium veins and stratiform deposits of the Colorado Rockies as part of the resource appraisal program of the U. S. Atomic Energy Commission. NORTH PARK AND MIDDLE PARK BASIN The North Park and Middle Park basin is a large intermontane structural depression situated between tilted Mesozoic strata and Precambrian crystalline rocks of the Front and Park Ranges (Fig. 2). Volcanic rocks of the Rabbit Ears Range form a highland that divides the basin into the North Park and Middle Park components. The basin is underlain by Tertiary and Mesozoic sedimentary rocks. Tertiary Geochronology The basin developed between the rapidly rising Park Range and the Front Range during the first stage of the Laramide orogeny. North- to northeast­ trending folds in the southern part of Middle P.ark (Tweto, 1957, p. 30) and a northwest-trending uplift in the southern central part of the Park also developed during this stage. This uplift is now expressed as a northwest­ trendi.ng highland through Hot Sulphur Springs that is underlain by Precambrian and Mesozoic rocks (Fig. 2). EXPLANATION C=:J Tertiary sedimentary rocks ~ Tertiary volcanic rocks I Mesozoic, Paleozoic, and WALDEN Precambrian Major faults • Uranium occurrence in Tertiary sediments 10 20 North Park Miles Fig. 2. Generalized Geologic. Index Map of the North and Middle Parks Basin, Colorado ;.. 4 - Following these earliest Tertiary major structural dislocations, as·much as 5,000 feet of predominately clastic sediments with minor volcanics, comprising the Middle Park Formation, were deposited in the basin. This formation, which is continuous through Middle and North Parks, almost alweys unconformably over­ lies the Pierre Shale of Upper Cretaceous age. Renewed tectonism at the close of the Paleocene resulted in westward displacement of Precambrian rocks of the west margin of the Front Range over the Middle Park basin along the low-angle, eastward-dipping Williams Range .and Vasquez thrust faults (Tweto, _1957, p. 30). In the Oligocene (Teylor, Theobald, and Izett, 1968), the extrusion of 1,500 to 2,000 feet of andesitic breccia, the Rabbit Ears Breccia (Richards, 1941, p. 33), formed a drainage divide along the boundary of North and Middle Parks (Fig. 2). Streams in Middle Park mey have flowed northward through North Park prior to the breccia extrusion. Stream courses probably were blocked by the flows; then, thin late Oligocene fluvial sediments and thick Miocene lacustrine sediments accumulated in the Granby, Fraser, and Kremmling subbasins in southern Middle Park in the areas of the late Paleocene thrust fault dislocations. These basins were eventually filled, and subsequently Middle Park drainage mey have breached the Gore Range, a component of the Park Range, along the Gore Canyon fault on the southwest side of Middle Park. This Middle Park drainage system became the headwater area of the Colorado River. Uranium Occurrences All of the known uranium occurrences in Tertiary sedimentary rocks of North and Middle Parks basin are in Middle Park (Malan, 1957). Most of these occurrences are in the Kremmling basin north of the Colorado River (Fig. 3). SIW SOW SOW 79W EXPLANATION Oligo.-Miocene Troublesome Fm. Ot"i gocene vol conics Paleocene Middle Pork Fm Mesozoic Precambrian Fault, high angle Thrust Fault Uranium prospect Paleostream direction 0 2 3N 2N Miles 2N IN A e,ooo' Figure 3. Generalized Geologic Map and Section of the Kremmling Basin, Middle Park, Colorado. - 5 - Uranium was discovered in October 1953 through ground radiometric reconnaissance by the U. S. Atomic Energy Conunission (Schlottmann and Smith,. 1954). In 1954, about 200 holes less than 50 feet deep were drilled and many trenches and prospect pits were dug. Widespread mineralization was encountered, but rio economically minable deposits were discovered. There was no explorati~n in this locality from 1955 to 1967. Three exploration programs have been started since 1967, but the results have not been announced. Most of the 15 shallow, oxidized occurrences of uranium are in fluvial carbonaceous sandstone lenses in the lower part of the Troublesome Formation. This formation, as much as 1,000 feet thick, is comprised of predominately pastel-colored tuffaceous clays and silts of lacustrine origin (Richards, 1941). Scour trends and cross laminae in fluvial sediments in the lower part of the formation near prospects east of Troublesome Creek (Fig. 3) indicate streams were flowing west to southwest. Nearly half of the holes drilled in· 1954 were mineralized. Disequilibrium conditions in which radiometric assays of uranium exceed the chemical assays by an average of eight times were a feature of most of the samp~es of mineralized cuttings from these holes and in samples from prospect pits. The average vanadium content of numerous samples from several prospects was 0.77% VaOs. Significant amounts of vanadium in association with uranium are rare in Tertiary sediments in the Western United States. The shallow, small oxidized vanadiferous deposits in the Pumpkin Buttes area of the Powder River Basin, wyoming are an exception. Much of the uranium in the numerous shallow occurrences in the Troublesome Formation is not stable in the oxidized zone despite the abundance of vanadium. The susceptibility of the uranium to oxidation, solution and redistribution may have resulted in the formation of larger deposits, by downward migrating solutions, in the favorable basal facies at deeper levels in the basin. - 6 - SOUTH PARK South Park is the southernmost basin in the series of structural basins between the Front Range and the Park-Gore-MOsquito chain of ranges (Fig. 4). The synclinal structure of the basin is interrupted locally by folds and related reverse faults which are parallel to the north-northwest elongation of the basin. The faults with the largest displacements are along the east and west margins of the basin.