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. The Elkhorn thrust fault separates the Cretaceous and Tertiary rocks from Precambrian rocks along the eastern boundary. Twenty miles to the west, the Mosquito-Weston.reverse fault displaces Paleozoic and
Precambrian rocks in the Mosquito Range along the western boundary of the basin.
Tertiary Geochronology
Stark, et al. (1949, p. 137-146) presented the first comprehensive review of the Tertiary and Quaternary geologic history of the South Park basin. More recent work has been done by De Voto (1964) in the southwestern part of the basin, Ettinger (1964) in the south central part, and Sawatsky (1964) in the southeastern part.
During early stages of the Laramide orogeny, the Mosquito and Front Ranges were uplifted and as much as 8,000 feet of detritus was deposited to form the
Denver Formation of Paleocene age in the South Park basin. The configuration of the basin was modified by pronounced thrust and reverse faulting in the late
Eocene. Thin late Eocene (?) arkosic sediments then accumulated in local shallow subbasins; a thick section malf underlie and intertongue with the lake beds in the Antero syncline in southern South Park. The prevailing drainage of South Park, southward to the Arkansas River, was disrupted by the ext~sion of as much as 3,000 feet of volcanic flows and pyroclastics of the Thirtynine
Mile volcanic field (Epis and Chapin, 1968) during the Oligocene and ~arly Miocene. .De er 106°30' EXPLANATION COL~RA 1m Trump-Wogontongue Formations >-~ ELKHORN~~ Antero-Fiorissant lake beds THRUST~~ Andesitic Flows Arkose and volcanic sediments !-~ ti:f:}:d. . r,;r; CRIPPLE CREEKt. High~ ark Basin -· Tallahassee CreekJ -cJl - Cottonwood Creek Basin F.jg.5 0 2 4 Fig. 4. Generalized Geologic Map of the Southern South Park Thirtynine Mile Volcanic-Field Region Showing Distribution of Deposits and Early Tertiary Palecdrainage Systems, Central Colorado - 7 - Oligocene lacustrine sediments of the Antero Formation accumulated to great depths in a lake in the Antero syncline and to shallower depths in small basins near Florissant and northeast of Hartsel (Fig. 4) (Stark, et al., 1949). Miocene fluvial sandstone and conglomerai;;e of the Wagontongue and Trump Formations were deposited in the Antero syncline by streams again flowing south to the Arkansas River. Uplift of the area between South Park and the Arkansas River in the late Tertiary radically changed the drainage of South Park (Stark, et al., 1949, p. 139). The present South Platte River originated during the canyon-cutting cycle which incised the drainage northeastward across the Front Range during this period. Uranium Occurrences The three known occurrences of uranium in South Park are in highly tuffaceous, friable lake beds of the Antero Formation which crops out north east and southwest of Hartsel (Fig. 4). Minor production is credited to one of these occurrences where autunite, the predominant uranium mineral, is most concentrated in thin, carbonaceous laminations along bedding planes. One prospect near Florissant is in the Florissant Lake Beds Formation, correlative with the Antero Formation (Fig. 4). Deeper exploration in selected areas southwest of Hartsel was commenced in 1968. The target is the more favorable late Eocene ( ?) fluvial sediments subjacent to the tuffaceous lake beds. UPLAND BASINS OF THE THIRTYNINE MILE VOLCANIC FIELD REGION The Thirtynine Mile volcanic field is a hi~h, dissected tableland in central Colorado, west of the southern Front Range and south of the South Park basin (Fig. 4). ... 8 - Tertiary Geochronology In the region of the Thirtynine Mile volcanic field and the southern Front Range, nearly all the Paleozoic and Mesozoic rocks were eroded away, following major Laramide arching in the Paleocene. A widespread erosional surface that extended west under the present volcanic field to the Arkansas River graben near Salida and south into the Wet Mountains was developed on Precambrian rocks. In the late Eocene, streams draining this surface flowed . through small structural basins which gradually filled with arkosic sandstone, conglomerate and some shale. The Cottonwood Creek basin, a portion of the Tallahassee Creek basin, and Devils Hole are components of one of these basinal trends of probable late Eocene age that contain uranium (Fig. 4). The first volcanic activity in the Thirtynine Mile field is recorded by rhyolitic ash flow tuffs, less than 100 feet thick, that accumulated over large portions of the southeast quadrant of the volcanic field in the late Eocene about 40 million years ago (Epis and Chapin, 1968, p. 52). Tuffaceous arkosic and volcanic conglomerate accumulated in basinal trends in Tallahassee Creek and in High Park (Fig. 4) during erosion of the welded tuffs, the under- lying Eocene arkosic basin fill, and Precambrian rocks. A reworked, carbonaceous, bentonitic tuff bed in the upper part of these sediments is host to over half the ore in Tallahassee Creek. The High Park deposits are in locally tuffaceous, arkosic sandstone channels that dissected the underlying welded tuff. During the Oligocene and early Miocene, as much as 3,000 feet of andesitic flows, ignimbrites, and ash fall tuffs covered the sediments and welded rhyolitic tuffs (Epis and Chapin, 1968). The basal portion of this volcanic pile laterally grades into unsorted a~desitic boulder conglomerate gravity slide masses in Tallahassee Creek at the southeast margin of the volcanic pile. .;. 9 - Erosion during the Quaternary has dissected the margins of the volcsnic field and exhumed the favorable fluvial uranium host rocks at the base of the pile. Uranium Occurrences The Devils Hole, Tallahassee Creek, 9ottonwood Creek, and High Park basinal areas along the southern and eastern margins of the Thirtynine Mile volcanic field all contain stratiform uranium deposits qr prospects in fluvial arkosic and volcanic detritus of late Eocene age. Tallahassee Creek, with 15 deposits ranging up to 25,000 tons in size, has accounted for nearly all the production from these basins. The High Park basin west of Cripple Creek contains a few undeveloped deposits. Prospects in the Cottonwood Creek basin north of Tallahassee Creek and in Devils Hole south of Tallahassee Creek have not produced ore. Deposits in all these upland basins are less than 200 feet deep. The results of deeper exploration in the Devils Hole, Tallahassee Creek and Cottonwood Creek basins during 1967 and 1968 have not been announced. Uranium deposits in Tallahassee and Cottonwood Creeks are in each of the two late Eocene sedimentary basinal systems (Fig. 5). These include a pre volcanic, arkose-filled, north- to northwest-trending system and a stratigraphi cally higher, volcaniclastic-filled, west- to northwest-trending system: In Tallahassee Creek (Fig. 5), ore bodies 1 to 10 feet thick are tabular~ lenticular and concordant to bedding. Their outlines are generally elongate in the direction of fluvial sedimentary trends. Uraninite, the predominant ore mineral, with accessory pyrite, hematite, and limonite is concentrated in the vicinity of carbonaceous debris. Autunite is locally abundant in small oxidized near-surface deposits. Nearly all of the ore is in the reduced zone, below the oxidized section. A discontinuous band of hematite that coats grains and fills small fractures immediately overlies the ore zone in many mines. TYPically, the hematite-stained rock grades upward into limonite-stained sediments. EX PLANATION 0 ~ ~Andesi~ic flows eu u::!:~ .~·p:·\'·':'·'·} ...... ,.,, Volcanic.~erote \Trc) cqnglom- 0 41:?: ~Rhyolite flow !?. :; r:-=-:-'l Arkose and shale ~41 ~ (To) c=JPrecombrion .,.,.,.... .,..Fault .::= Trc basinal trend -·- To basinal treni:l -·- Uranium occurrence Miles A' 8 8' ...... _~---~-~------~·' E: ~...... ::::=] 0 I 1::::= ... ===l- 10000' ~ wa... d;;;;~iii!IIMI!IIIili.liilililllllllaill!!ll!lllllll~-~~~c·f a o oo · 0 Precambrian 0 2 Mi. Fig. 5. Generalized Geologic Mop and Sections of the To II ohossee Creek -Cottonwood Creek Basins, Central Co lorado - 10 - In all of these upland basins, the carbonaceouS material in the fluvial host rocks is the most important chemical control on the localization of uranium. During erosion of the silicic pyroclastic volcanic rocks of Eocene age, the contained trace amounts of uranium may have been solubilized and transported by ground water to favorable reducing environments created by the carbonaceous detritus in the river channels. RA'IDN BASIN The Raton Basin of Colorado and New Mexico is a north-trending asymmetrical depression bounded by the Sangre de Cristo Range on .the west, by the Wet Mountains on the northeast, and by the broad Apishapa Arch and the Sierra Grande Arch on the east and southeast (Fig. 6). The axis is along the west side of the basin, marginal to the Sangre de Cristo Range. This basin is large and deep, but it is dissimilar to the North and Middle Parks basin and the South Park basin in that it is not bounded by high ranges on both sides. Tertiary Geochronology During the Laramide, the Raton Basin formed east of the rapidly rising Sangre de Cristo Range (Fig. 6). As much as 11,000 feet of terrestrial sediments, derived largely from the uplifted Sangre de Cristo Range, accumulated in the basin (Johnson, 1959). This section includes the Raton and Poison Canyon Formations of the Paleocene and the CUchara and Huerfano Formations of the Eocene. Silicic and basic plutonic activity, including the Spanish Peaks plutonism, probably took place in the late Eocene. The Wet Mountains, east of the northern portion.. of the basin, were stJ;'ongly uplifted in the late Eocene (Johnson, 1959). All units from the Precambrian through the Eocene Huerfano Formation were erosionally truncated west of the range in Huerfano Park (Johnson, 1959). During the late Eocene, detritus from the range overlapped the older formations above an angular unconformity and Denver COL.. AOJ Area of stratiform prospects and small depostts in Terttory sediments 0 I 10 I I :==i Miles a&~~~~M. _____.:~ Hue r fa n o Park Loco I i ty Fig. 7 COLORADO NEW MEXICO Fig. 6. Index Map of Raton Basin- Huerfano Park- Wet Mountain Valley Basinal Structure Lineament, South Central Colora do - 11- formed the 1,200 feet of arkosic sandstone, conglomerate and thin beds of carbonaceous mudstone of the Farisita Conglomerate. The only exposure of this formation is in the Huerfano Park locality at the far north end of the Raton Basin (Fig. 7). The Farisita is the only Tertiary formation in the Raton Basin known to be uraniferous. In the northern portion of Huerfano Park, the Farisita is overlapped by as much as 1,300 feet of volcaniclastic debris and water-laid tuff of the Devils Hole Formation (Steven and Epis, 1968). The inferred source of this volcanic detritus was late Eocene silicic. volcanic rocks extending southeast ward from Westcliffe along the west side of the Wet Mountains (Siems, 1968; Steven and Epis, 1968). Quaternary alluvium and gravel mask older units for 30 miles northwestward from Huerfano Park along the projection of the Raton Basin structural depression that is known as the Wet Mountain Valley. Uranium Occurrences In 1955 and 1956, seven uranium prospects were discovered in the upper middle Farisita Conglomerate in the Huerfano Park locality of the Raton Basin, about eight miles northwest of Gardner (Fig. 7). No economically minable deposits were delineated by the shal~ow dozer cuts and·a few shallow drill holes. Autunite and probably uraninite are concentrated in lenses of carbona ceous shale or shaley arkose in a predominantly buff-colored, conglomeratic, arkosic sandstone with orange-colored bedding planes. The abundant iron oxide staining in the shallow dozer cuts at some of the prospects may have been derived from oxidation of pyrite. The reducing environment of the carbonaceous shale is the only estab lished control for the localizations of uranium in t~e Farisita Conglomerate. A EXPLANATION Oligocene volcantc sediments, tuff. andesite flows D Late Eocene Farisita Conglomerate Eocene intrusives 2 3 Paleocene ond Eocene Poison Canyon, I I ~ C u ch a r a , H u e rf a no F o r m a t i o n s Males Upper Paleozoic and Mesozoic ~ Fault, barbs on uprer plate t:----g-=------undivided of tnrust faults Precambrian undivided Uranium occurrenc.es Figure 7. Generalized Geologic Map and Section of the Hue r fa no Pork L o c a I i t y, Colorado - 12 - Briggs and Goddard. (1956) concluded that sands of the lower part of the Farisita Conglomerate intertongue with red mudstone of the upper part of the Huerfano Formation along a northwesterly trend through Huerfano Park. At depth, this zone of intertonguing would project under the area of uranium occurrences in the middle part of the Farisita Conglomerate. This inferred facies change in the basal Farisita might be an attractive exploration target. The entire area underlain by the Farisita was relocated in 1966 and 1967. The results of exploratory drilling in 1968 have not been announced. During 1968, nearly 200,000 additional acres northward to Westcliffe and southward to Trinidad were acquired by various groups. This additional ground is under- lain by Tertiary and Quaternary sediments along the northward projection of the Raton Basin structural element, the Wet Mountain Valley, and by pre-Farisita, early Tertiary formations along the southward projection of the basin to Trinidad. The Farisita may underlie Tertiary volcanic detritus and Quaternary alluvium northward through the Wet Mountain Valley as far as Westcliffe or beyond. INTERBASIN CORRELATIONS OF URANIFEROUS STRATIGRAPHIC UNITS ·Figure 8 represents a preliminary attempt at gross interbasin correlations of the major Tertiary sedimentary and volcanic units in the intermontane basins of Colorado that are reviewed in this report. The most recent results of separate studies related to this problem are included in the July 1968 Colorado School of Mines Quarterly entitled Cenozoic Volcanism in the Southern Rocky Mountains. Recent isotopic age determinations of the volcanic units have clarified Tertiary geochronology in some areas, but additional dating and mapping are needed in many others. ·Upland Basins of Thirtynine Mile Volcanic Field Northern Southern Tallahassee Creek High Park Raton Basin Middle Park South Park Cottonwood Creek Florissant Huerfano Park Tweto,l'957 Stork, 19<49 Mo<:P1\enon, 195$ Wolon, 196.$ Johnson.l959 Toyfor, Thtol:lold, Izatt, 1968 Et1in~r, 1964 Chopin~ 19G4 Siems, 1968 SowottkJ, 1964 Molen, 196$ Steven ond Epts, IK8 Epls and Chopin, 1968 Epit end Chcptn,l963 -Andesite flowa 1200' Vok:anie eon;lomerato 200' Andnitic flOws I I· Jevilt Hole Frn.: I I tuHocrou• volc:onk: detritus I I 1300' Trump /,m.; I I I ForilttO ConQIOft'lltOft t sorur ond ;ravel / Ottotic u. and C0"91otMroto 9-soo'/ II I I 1200' TtoubiHomt Fm.; tuffo~Yt loko btda, 1 /I with tower fluvial Wogonton9u1 Fm.; I I MCfion volean•c dtftlfUt 0-100o' / I Mioecne / ;oo-;o· Huorfono Fm.; I I Oholty red btds / F IOflttontI o-400' Lokt 2000' 1/ Cu<:horo Fm.; red to troy n and &halo :!..,~,!_-J,:;.;....,.I------..!:14~0~0:_'--ootum horu:on, Eocfftt -~·Eocene contact Middlt· Pork Fm.; Poitort Conyon Fm ; low.,. flow brtceio hff to red st. Ond oqolomerofe, 2000' middle conolomuale, O.nver ''"·( ond upper oril, lhole, Arllotic tondttono. Polo:oce-nt ond eon9lomtrate con-,lomtrott, 0-3000' and sfw:llt '?8000' I . I 4118 Strotlvraptlic polihon of ltt'Onium min.,olitotton Note: YilMrt more ftlon ont tourco rohrenc. it oinn, columft 11 o eoct~ttom1•• of intorpr•tationt. Figure 8. Stratigraphic Correlations in the Tertiary Intermontane Basins of Colorado - 13 - With the exception of a small deposit at the base of the onlap of the upper Middle Park Formation onto Precambrian rocks near Hot Sulphur Springs in Middle Park, no uranium occurrences are knmm in the thick pre-Eocene Tertiary sections of the deeper basins. Most of the prospects in Middle Park are in favorable fluvial sediments subjacent to tuffaceous lake beds of the Troublesome Formation of Miocene age. In South Park, uranium prospects are in Oligocene tuffaceous lake beds of the ~tero Formation, but the more promising targets are in arkosic or volcaniclastic fluvial sediments subjacent to the lake beds. In the Raton Basin, the fluvial arkosic Farisita Conglomerate subjacent to volcaniclastic sediments and flows appears to be the most favorable Tertiar.y formation. In the upland basins marginal to the Thirtynine Mile volcanic field, late Eocene fluvial sediments subjacent to volcanic rocks are productive, but their areal extent is much more limited than that of the inferred targets in the deep basins. If large deposits exist in the Tertiary .intermontane basins of Colorado, they are probably in the more widespread, lithologically favorable Eocene sediments in the deeper basins. SOURCE OF THE URANIUM Most of the established uranium resource in the Colorado Rockies is in Tertiary vein deposits (Fig. 9). These deposits including those in the Front Range (Sims and Sheridan, 1964), in the Cochetopa district near ~unnison (Malan and Ranspot, 1959), and at Marshall Pass in the Sawatch Range (unpublished AEC file report) are spatially and probably genetically related to plutonic sources of early and middle Tertiary metallization in the Colorado mineral belt (Sims and Sheridan, 1964;' Malan, 1968). r N North and Middle Park Basin ----4/ Rabbit ~ " Ears x DENVER X 0 0 0 c 0 A Thirtynine Mile I /,/" • I / ,.o / afr~~- Wet Mts. X Tertiary vein deposits Raton Basin--~ Fig. 9. Distribution of Tertiary Stratiform and Vein Uranium Deposits in the Colorado Rockies - 14 - The distribution of stratiform uranium deposits in Tertiary sediments in Colorado is spatially unrelated to the mineral belt; these deposits are, however, closely related to late Eocene to early Miocene volcanic fields where lithologically favorable, fluvial host sediments are subjacent to the volcanic piles in upland basins or to tuffaceous lake beds and fluvial volcaniclastic outwash in deeper basins. The erosion of Precambrian granitic terrain in surrounding highlands often provided either part or all of the clastic material in the fluvial host rocks. The high fluid transmissivity of such sediments probably was an important control on the circulation of mineralizing meteoric waters. Some of the granites in the Front Range (Phair and Gottfried, 1964) are highly enriched in thorium and moderately enriched in uranium, but the uranium content of the Tertiary silicic pyroclastic volcanics is not established. The availability of trace amounts of uranium through solution in migrating meteoric waters appears to be much greater in the pyroclastic rocks than in the granitic rocks or in arkose derived from the granites. The amount of uranium in present-d~ ground and surface waters draining tuffaceous sedimentary environments is nearly eight times greater than the amount of uranium in waters draining granitic or arkosic environments in Colorado (AEC files) and in·Wyoming (Denson, et al., 1956). Trace amounts of uranium in both the granite and the pyroclastic volcanics m~ be sources for the concentrations of uranium in the Tertiary basins of Colorado, but the pyroclastics probably are the more important contributors. Since these Tertiary pyroclastics originated through syntexis in Precambrian rocks, the original source of the uranium. they contain was in the basement (Malan, 1968, p. 191). The U. S. Atomic Energy Commission is ctirrently analyzing the distribution of uranium and thorium in the various age groups and facies of the Precambrian throughout the Western United States. - 15 - PROSPECTING AND EXPLORATION GUIDES In addition to standard airborne and ground radiometric surveys, at least two other ore search techniques may be applicable in the Tertiary basins of Colorado. 1. Color Changes - In some areas, color changes may indicate the path of the mineralizing solutions. In the Kremmling basin, the greatest concentration of near-surface occurrences is in predominately....__ gray to buff sediments near an irregular transition to predominately pink ---to red sediments. Supergene oxidation of pyritic zones in radioactive, more highly carbonaceous sediments produces locally prominent orange ' coloration in Huerfano Park. This coloration indicates lithologically favorable sections that may contain economic deposits at depth. Insufficient data are available to demonstrate the existence of solution fronts, the dominant ore control in W,yoming. 2. Water Sampling - Water sampling of streams and wells is an excellent rapid method of initially evaiuating Tertiary basins for uranium. Water samples ane.J..Yzed by the U. S. Atomic Energy Connnission indicate that all known areas of uranium mineralization in the Tertiary inter- montane basins of Colorado could have been recognized by a limited amount of sampling of readily accessible streams and water wells. This prospecting method is more applicable to the high altitude basins with numerous perennial streams as in the Colorado Rockies than to basins in arid regions. - 16 - CONCLUSIONS 1. T.he continental sedimentary facies in the Tertiary intermontane basins in Colorado are geologically similar to those in wyoming. 2. Prospecting and limited, shallow exploration efforts of the middle 1950's resulted in the discovery of numerous prospects in most of the basins and several economically minable small deposits in the Tallahassee Creek basin. More systematic deeper drilling programs now underw~ m~ result in the discovery of larger deposits in deeper.portions of the Tertiary basins in Colorado. 3. Most of the uranium occurrences are in carbonaceous late Eocene fluvial sediments near late Eocene to early Miocene volcanic piles. The uranium m~ have been derived from silicic pyroclastic volcanic rocks that contain dispersed trace amounts of uranium readily solubilized during erosion. Uranium occurrences in lacustrine sediments in some of the basins are not economic, but subjacent fluvial sediments in paleodrainages are attractive exploration targets. - 17 - REFERENCES CITED Briggs, L. I., and Goddard, E. N., 1956, Geology of Huerfano Park, Colorado, in Guidebook to the geology of the Raton Basin, Colorado: Rocky Mtn. ~soc. Geologists, p. 40-45. Chapin, C. E., and Epis, R. c., 1964, Some stratigraphic and structural features of the Thirtynine Mile volcanic field, central Colorado: The MOuntain Geologist, v. 1, no. 3, p. 145-160. Denson, N. M., Zeller, H. D., and Stephens, J. G., 1956, Water sampling as a guide in the search for uranium deposits in evaluating widespread volcanic units as potential source beds for uranium, ,!!! Geology of uranium and thorium: United Nations, New York, Proc. Internat. Conf. Peaceful Uses Atomic Energy, 1955, v. 6,· paper 507, p. 794-800. De Voto, R. H. , 19q4, Stratigraphy· and structure of Tertiary rocks in south-· western South Park: The Mountain Geologist, v. 1, no. 3, p. 117-126. Epis, R. C., and Chapin, C. E., 1968, Geologic history of the Thirtynine,.Mile volcanic field, central Colorado: Colorado School Mines Quart:, v. 63, no. 3, p. 51-85. -'~Ettinger, Morris, 1964, Geology of the Hartsel area, South Park, Park County, Colorado: The Mountain Geologist, v. 1, no. 3, p. 127-132. Johnson, R. B. , 1959, Geology of the Huerfano Park area; Huerfano and Custer Counties, Colorado: U. S Geol. Survey Bull. 1071-D, 119 p. Malan, R. C. , 1957, Geology of uranium occurrences in North and Middle Parks, Colorado, in Guidebook to the geology of North and Middle Park basins,' Colorado: Rocky Mtn. Assoc. Geologists, p. 126-136. - 18 - Malan, R. c., 1968, Relationship of uranium in the Rocky Mountains of southwest Colorado to local and regional metallogenesis, ~ Guidebook of San Juan San Miguel-La Plata region, New Mexico and Colorado: 19th Field Conf., 1968, New Mexico Geol. Soc., p. 185-192. Malan, R. C., and Ranspot, H. W., 1959, Geology of the uranium deposits in the Cochetopa Mining District, Saguache and Gunnison Counties, Colorado: Econ. Geology, v. 54, p._l-19. MacPherson, B. A., 1959, Geology and uranium .deposits of the Tallahassee Creek district, Fremont County, Colorado: U. S. Atomic Energy Comm.. RME-1083, 64 p., unpub. Phair, G., and Gottfried, D., 1964, The Colorado Front Range, Colorado, U. S. A., as a uranium and thorium province, ~Adams, J. A. S., and Lowder, W. M., eds., The Natural Radiation Environment, Chicago, Univ. Chicago Press, · p. 7-38. Richards, Arthur, 1941, Geology of the Kremmling area, Grand County, Colorado: Univ. Michigan thesis,-68 p. Sawatsky, D. L., 1964, Structural geology of southeastern South Park, Park County, Colorado: The Mountain Geologist, v. 1, no. 3, p. 133-139. Schlottmann, J. D., and Smith, L. E., 1954, Preliminary report on uranium mineralization in the Troublesome Formation, Middle Park, Grand County, Colorado: U. S. Atomic Energy Comm. open-file rept. RME-1042, 14 p. Siems, P. L., 1968, Volcanic geology of the Rosita Hills and Silver Cliff district, Custer County, Colorado: Colorado School Mines Quart., v. '63, no. 3, p. 89-124. ..; 19 - Sims, P. K., and Sheridan, D. M., 1964, Geology of uranium deposits in the Front Range, Colorado: U. S. Geol. Survey Bull. 1159, 116 p. Stark, J. T., Johnson, J. H., Behre, C. H., Jr., Powers, W. E., Howland, A. L., Gould, D. B., and others, 1949, Geology and origin of South Park, Colorado: Geol. Soc. America Memoir 33, 188 P·. Steven, T. A., and Epis, R. C., 1968, Oligocene volcanism in South-Central Colorado: Colorado.School Mines Quart., v. 63, no. 3, p. 241-258. Teylor, R. B., Theobald, P. K., and Izett, G. A., 1968, Mid-Tertiary volcanism in the Central Front Range, Colorado: Colorado School. Mines Quart. , v. 63, no. 3, p. 39-50. Tweto, Ogden, 1957, Geologic sketch of southern Middle Park, Colorado, ~ Guidebook to the geology of North and Middle Park basins, Colorado: Rocky Mtn. Assoc. Geologists, p. 18-31.