Criteria for Outlining

Areas Favorable for

Uranium Deposits in

Parts of Colorado and

Utah

GEOLOGICAL SURVEY BULLETIN 1009-J

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with permission of the 

Commission

Criteria for Outlining 

i Areas Favorable for 

! Uranium Deposits in \

Parts of Colorado and 

Utah

,».

ByE. J. McKAY

i A CONTRIBUTION TO THE GEOLOGY OF URANIUM i

«

GEOLOGICAL SURVEY BULLETIN 1009-J

investigations in the Uravan and Gateway districts, Montrose and Mesa 

Counties, Colo., and Grand County, 

Utah. This report concerns work done 

on behalf of the U. S. Atomic Energy 

» Commission and is published with 

permission of the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1955
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E, Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. G. - Price 55 cents (paper cover)
CONTENTS

Page

Abstract_. .._...._........__......._..-._.--.___.._.___ 265

Introduction._____________________________________________________ 265

Acknowledgments. -_.-_______________.-___.__.___-_______-__-_____ 267

Geology___---___-_-_-----_--__--________-__-__----_-_---_---._--_ 268

Geologic setting.______________________________________________ 268

Salt Wash member._--.-._._.__....____......_.._--_---.__ 268

General features.________________________________________ 268

Principal ore-bearing sandstone zone.__-____-_-__-_-__-______ 269

Geologic guides to ore__--__--_-_--__-----__--_-__-____.____________ 270

Small-scale guides._-_----____-____-__-_--____---_.---__-_____- 270

Large-scale guides______-_____--_________-__.____-_-__--_-_-___ 272

Distribution of large-scale guides to ore deposits.______________________ 272

Southeast of Red Fox mine area.._______________________________ 272

Northwest of Uravan._________________________________________ 273

Northeast of Dolores group.____________________________________ 273

North and east of Blue Mesa.______-_________--__-___-__-_____- 274

Northwest of Blue Mesa.__.________-__-____--____--___-_-__-- 275

Distribution of other ore-bearing zones.__-_________---____-_-_-_____- 276

Localization and control of ore deposits._____________________________ 276

Relation of regional structure to ore deposits.___-____--____---______ 279

Origin of ore deposits.__-_-_-______-___.-_-________-__----__---_-__- 281

Literature cited._-_________-_-____-___-____._-______-_----__---_-- 282

ILLUSTRATIONS

Page

PLATE 12. Photodiagrams of the Salt Wash member of Morrison formation. __--_-_--_-.-__-__---------_---_--------.Facing 268

13. Geologic map of Uravan and Gateway districts. _________ In pocket

FIGURE 43. Index map of part of the Colorado Plateau showing Uravan

and Gateway mining districts._________________________ 266

m

A CONTRIBUTION TO THE GEOLOGY OF URANIUM

CRITERIA FOR OUTLINING AREAS FAVORABLE FOR 

URANIUM DEPOSITS IN PARTS OF COLORADO AND UTAH

By E. J. McKAT

ABSTRACT

Most of the uranium deposits in the Uravan and Gateway mining districts are 

in the persistent upper sandstone stratum of the Salt Wash member of the 

Morrison formation.

Areas in which this stratum is predominantly lenticular have been differentiated from areas in which the stratum is predominantly nonlenticular. The 

most fav.orable ground for uranium deposits is in areas of lenticular sandstone 

where the stratum is underlain by continuous altered greenish-gray mudstone. 

Ore is localized in scour-and-flll sandstone beds within favorable areas of lenticular sandstone.

Regional control of the movement of ore-bearing solutions in the principal 

ore-bearing sandstone zone is indicated by belts of discontinuously altered mudstone transitional in a northerly and southerly direction from an area of un« 

altered mudstone to areas of continuously altered mudstone; and an area of 

unaltered mudstone in which no ore deposits are found and an increase in size, 

number, and grade of ore deposits from areas of discontinuously altered to 

continuously altered mudstone.

Discrete regional patterns of ore deposits and altered mudstone are associated 

with Tertiary structures; where these structures and favorable host rocks 

occur in juxtaposition, regional controls appear to have localized ore deposits.

INTRODUCTION

Uranium deposits in sandstone beds of the Morrison formation have 

a wide and scattered distribution on the Colorado Plateau. Some of 

the most productive areas in Colorado have been studied and drilled 

intensively by the Geological Survey to obtain guides in prospecting 

for ore and to aid in the development of mineral resources. This work 

has resulted in the recognition that several lithologic features of the 

ore-bearing beds can be useful in guiding exploration and in appraising the favorableness for ore deposits. Those features that can be 

readily observed in drill core, and their application in exploration, 

have been described by Weir (1952), and they are summarized in thi?

265
266 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

k SAL ^^. 

-- - i-tf/r- -i - - _

-:v.'- : -v;^ :MONTROSE~CO

Natunta 

MONTROSE CO 

SAN MIGUEL CO

ABAJO

MOUNTAINS

FIGURE 43, Index map of part of the Colorado Plateau showiDg Uravan andi Gateway

mining districts.

report, They are particularly useful in guiding diamond drilling, as

they have limited areal distribution and offer relatively small-scale

targets. Other features can be observed better at the outcrop and can

be used as large-scale guides. , 

A special detailed study was made of the ore-bearing sandstone

zones in the Uravan and Gateway districts, Montrose and Mesa Counties, Colo., and the adjoining part of Grand County, Utah (fig. 43). 

Individual sandstone units of the Salt Wash member were mapped 

and examined in detail and compilation was made on topographic base
CRITERIA FOR FAVORABLE AREAS IN PARTS OP COLO.-TJTAH 267

maps (scale 1:24,000) in the Uravan district; elsewhere the orebearing zone was examined and mapped and most of the mines in 

this mapped area were inspected.

Mapping technique involved drawing the outline of individual sandstone units as they appeared from adjacent canyon walls; the relation 

in space of these units and mudstone lenses to one another was shown 

and where mines occur, the position of ore bodies within individual 

sandstone units or the relation of ore bodies to several sandstone units 

was also shown. Next, the mapped strip, commonly a quarter of a mile 

to half a mile long, was "rim walked" to determine details of lithology 

and bedding, the thickness of altered greenish-gray mudstone beneath 

the ore-bearing sandstone zone as well as the alteration thickness in 

mudstone lenses separating the sandstone units, and the orientation of 

the thicker portion of the individual sandstone lenses.

The purpose of the work was to determine the gross lithologic 

characteristics of these beds, both in the vicinity of and away from 

uranium deposits, in order to evaluate certain geologic features as ore 

guides, and to aid in planning exploration and appraising large areas 

from the standpoint of favorableness for ore deposits. The pertinent 

results of this work, prediction of favorable and unfavorable ground 

for ore deposits, have been supported by drilling done subsequently by 

the Atomic Energy Commission and the Geological Survey.

In the Uravan and Gateway districts, the general character of the 

bedding in the ore zone and the amount of alteration in the underlying 

mudstone, and the relation of these to one another, are the features 

most useful as large-scale ore guides. These features and their distribution, as well as regional geologic structures, are either associated 

with deposits or have influenced ore deposition. Observed relations 

indicate that ore deposits in the main ore-bearing zone are localized by 

scour-and-fill sandstone bedding in a persistent, dominantly lenticular 

upper sandstone stratum of the Salt Wash member that is underlain by 

continuously altered greenish-gray mudstone.

The patterns of alteration and ore deposits probably reflect regional 

control of ore deposition by Tertiary structure. If this is true, the use 

of regional structure guides in conjunction with small- and large-scale 

guides to ore should aid the search for future ore reserves. This study 

suggests that there are at least two discrete northwest-trending belts 

instead of a single belt as outlined by Fischer and Hilpert (1952).

ACKNOWLEDGMENTS

This report concerns work done in 1950 by the U. S. Geological Survey on behalf of the Division of Raw Materials of the U. S. Atomic 

Energy Commission. It is impossible to give individual recognition 

to the many persons who contributed time and knowledge to this study,
268 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

but special thanks are given R. P. Fischer, F. W. Cater, L. B. Riley, 

E. M. Shoemaker, and R. L. Boardman for their many suggestions.

GEOLOGY

GEOLOGIC SETTING

The Uravan and Gateway mining districts are in a broad, shallow 

syncline between the northwestward-trending Uncompahgre Plateau 

on the northeast and the northwestward-trending Paradox Valley and 

Sinbad Valley anticlines on the southwest. The area is a deeply dissected, rugged highland ranging from 5,000 to 7,000 feet in altitude. 

Rocks ranging in age from Permian to Cretaceous are exposed. The 

Morrison formation of Late Jurassic age is about TOO feet thick and 

is composed of two members, the upper Brushy Basin member 

and the lower Salt Wash member. Uranium-bearing deposits are 

found mostly in the Salt Wash member (pi. 12A).

SALT WASH MEMBER 

GENERAL, FEATURES

The Salt Wash member ranges from 240 to 400 feet in thickness and is composed of about equal parts of sandstone and 

rnudstone strata. On fresh exposures, the color of sandstone ranges 

from white to gray, yellow, and reddish brown. Limonite occurs in 

places as specks in the sandstone, and limonite diffusion bands are 

prominent in some exposures of the sandstone. The sandstone is 

composed dominantly of fine- to medium-grained clear quartz with 

minor amounts of white and pink chert, clear feldspar, and interstitial clay. The quartz grains are subrounded to rounded and are 

well sorted, though in a few places, the textural range is from fine to 

coarse. Chert grains are angular and their modal diameter is larger 

than the modal diameter of the quartz grains. Calcite is the principal cementing material. Fossil logs occur in places and are composed of one or more of the following substances: Carbon, silica, 

limonite, calcite, dolomite, gypsum, and carnotite.

The sandstones of the Salt Wash member are broadly lenticular. 

Lenses are numerous, forming fairly continuous sandstone strata, and 

are generally thicker near the top and base than near the middle of 

the member (pi. 12A). Scour-and-fill bedding is present in places 

(pi. 12B).

A mudstone sequence with interbedded sandstone lenses occurs between the upper and lower sandstone units. Mudstone also occurs 

in the sandstone as thin lenses, films, and reworked fragments ranging in size from pebble to cobble. The mudstones are dominantly
GEOLOGICAL SURVEY Hl'I.I.KTIN 1 (Mill PLATE 12

- ' '"*£

A. VIEW LOOKING TOWARD DOLORES GROUP OF MINES

Salt Wash member of Morrison formation (It) overlain by Brushy Basin member (a), underlain by 

Summerville formation (c) and Entrada sandstone (d). Caprock of mesa is Burro Canyon formation of 

Early Cretaceous age.

400 feet

B. VIEW LOOKING TOWARD EAST SIDE OF CLUB MESA

Lenticular upper sandstone stratum of the Salt Wash member composed of scour-aml-hll sandstone beils («) 

that interfinger with mudstone at (b).

PHOTODIAGRAMS OF THE SALT WASH MEMBER OF MORRISON FORMATION
CRITERIA FOR FAVORABLE AREAS IN PARTS OF COLO.-UTAH 269

red and are composed largely of argillaceous material in which minor 

amounts of fine- to medium-sized quartz grains and other minerals 

of silt size or slightly larger are mixed. Fossil logs and remnants of 

vegetal material are almost absent in mudstone.

Over wide areas red mudstone has been altered to a greenish-gray 

color within and immediately beneath the upper sandstone stratum 

of the Salt Wash. Altered greenish-gray mudstone is also found 

locally beneath the lower sandstone stratum of the Salt Wash and 

beneath individual sandstone lenses in the mudstone stratum. Although the phenomenon of alteration of the clay from red to greenish 

gray has been established by field evidence the chemical nature of 

this change and its significance in relation to ore deposition is only 

partly understood. Results of clay studies by Weeks (written communication, 1952) show that the total iron and ferric-ferrous iron ratio 

is higher in the red than in the greenish-gray clay and suggest that 

the change was effected under reducing conditions.

PRINCIPAL ORE-BEARING SANDSTONE ZONE

The upper sandstone stratum, the principal ore-bearing zone, ranges 

from 15 to 80 feet in thickness. In places this stratum is rather thinly 

and evenly bedded; the beds range from a few feet to 60 feet in thickness and from several hundred to several thousand feet in horizontal 

extent (pi. 12A). Sandstone lenses are separated in many places by 

mudstone lenses that range from a fraction of an inch to 20 feet in 

thickness. As used in this report, areas of dominantly lenticular sandstones refer to the predominance of channel-type sandstone over 

sandstone of the flood-plain type. Conversely, areas of dominantly 

nonlenticular sandstone refers to the predominance of flood-plain 

sandstone.

Scour-and-fill bedding is common in areas of dominantly lenticular sandstone but is rare in areas of dominantly nonlenticular sandstone. The scour-and-fill bedding consists of truncated segments 

of sandstone lenses interbedded with thin discontinuous mudstone 

lenses and conglomerate consisting of mudstone, pebbles, and cobbles. 

Fragments and masses of fossil wood are abundant in many of these 

scour-and-fill beds and the sandstone is generally poorly sorted. 

Scour-and-fill sandstone bedding of the channel-fill type in some places 

occupies the entire stratigraphic interval of the upper sandstone zone 

of the Salt Wash but in most places only part of the interval is so 

occupied.

In places, sandstone lenses in the upper sandstone stratum have 

filled channels cut in the underlying mudstone, but the depth of these 

channels rarely exceeds 10 feet. The sandstone filling these channels

343911 55 2
270 CONTRIBUTIONS TO THE GEOLOGY OP URANIUM

generally contains more reworked interstitial and conglomeratic mudstone and fossil wood material than sandstone in scour-and-fill bedding at higher levels within the sandstone stratum.

The relatively slight scouring indicated at the contact between the 

upper sandstone stratum of the Salt Wash and the underlying mudstone over most of the area, together with relatively little redeposited pebble and cobble aggregates of mudstone in the sandstone, 

indicate a terrain of low relief and gradient upon which the upper 

sandstone stratum was deposited. Eelatively clean claystone and 

thin limestone lenses near or at the top of the mudstone stratum 

suggest local ponding prior to or simultaneous with initial stages of 

deposition of the upper sandstone. In places, the lower 2 or 3 feet 

of the sandstone lens contains much reworked fragments of claystone and limestone. The individual sandstone lenses within the 

sandstone stratum were deposited by many aggrading northeastwardflowing streams, which appear to have formed a braided rather than 

a dendritic pattern. Levee structures have not been recognized; conditions of intermittent flooding and aggradation were probably such 

that levees, if present, were low lying and continually broken and 

destroyed by shifting stream channels. Fossil plant material was 

not seen in place of growth; although stumps with roots in place of 

growth have been reported in some mines, most of the plant fragments 

were probably rafted into the positions they now occupy.

The sedimentary-structure trend of a sandstone lens is generally 

parallel to the long axis of the lens and represents the direction in which 

the stream that transported and deposited the sand was moving. The 

direction of movement is indicated by orientation of the thick channelfilled portion of the sandstone, crossbedding, current lineation, and 

ripple marks. Orientation readings of these sedimentary features

have different values and are variable, but where readings of similar

value are given equal weight the average of the readings are within 

20° of the long axis of the sandstone lens. Orientation of several

sandstone lenses in a sandstone stratum are generally confined to a

90° arc. Scour-and-fill bedding structures within a sandstone lens

or between two or more sandstone lenses have more erratic trend

directions than a single lens or two or more adjacent but discrete

sandstone lenses. Dominantly nonlenticular sandstones have relatively consistent trends.

GEOLOGIC GUIDES TO ORE

SMAIX-SCALE GUIDES

Geologic features observed in drill cores nave been analyzed statistically and have been shown by Weir (1952) to be useful small-scale 

guides in diamond-drill exploration for uranium ore in the Morrison
CRITERIA FOR FAVORABLE AREAS IN PARTS OF COLO.-TJTAH 271

formation in southwestern Colorado. These geologic features are as 

follows:

1. The ore-bearing sandstone is 40 feet thick or more in the vicinity 

of large ore deposits.

2. Sandstone near ore deposits is dominantly light brown, whereas 

away from ore deposits the sandstone has a reddish cast.

3. The mudstone in and immediately below the ore-bearing sandstone is altered from red to greenish gray in the vicinity of ore 

deposits, whereas away from ore the amount of altered mudstone 

decreases.

4. Carbonized woody material is more abundant in the sandstone 

near ore deposits than away from deposits.

In addition to these features which can be observed in diamonddrill core, geologists have observed that the ore deposits occur mainly 

in association with mudstone lenses in crossbedded, massive sandstones.

During this study of the Salt Wash member in the vicinity of Uravan 

the following geologic relations were observed:

1. The sandstones are dominantly lenticular and lithologically 

similar in the Salt Wash member. Sedimentary structures have a 

dominant east-northeasterly trend, though locally the trend diverges 

from the average as much as 45° (pi. 13).

2. Scour-and-fill bedding, fossil wood material, and greenish-gray 

mudstone occur together in the middle and lower sandstones of the 

Salt Wash member, but only in the upper sandstone stratum of the 

Salt Wash are these features associated with ore deposits.

3. Greenish-gray mudstone is continuous beneath lower and upper 

sandstone strata of the Salt Wash, ranges from 6 to 18 inches in 

thickness beneath lenticular sandstone strata, and is as much as 5 feet 

thick beneath channel-fill sandstone of the upper sandstone stratum.

4. The size of the ore bodies is proportional to the thickness of sandstone lenses in the ore-bearing sandstone stratum. Sandstone strata 

15 to 35 feet thick contain small deposits of ore, whereas the thicker 

strata contain both small and large deposits.

The most obvious examples of the association of ore deposits with 

scour-and-fill sandstone bedding can be seen in the Red Fox, Fortyfive 

Ninty, and Tramp mines, the Cue Ball and J. B. No. 1 mines in the 

Julian Group, the Grass Roots mine in the Wright Group, and the 

Little Dick and Ophir mines in the Dolores Group (pi. 13). Although 

the structure of the sandstone is not entirely exposed at any one of 

these mines, a composite section of exposed rocks suggests that the ore 

zone is composed of several sandstone lenses with relatively flat tops, 

thin margins, and thick, relatively narrow sections in the center. The 

long axes of these relict lenses are generally oriented within 45° of
272 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

one another. Some of the sandstone units are tilted slightly from one 

another so that underlying and adjacent lenses are beveled, scoured, 

and in some places, destroyed. Large-scale scour-and-fill bedding 

structures occur where the thicker part of these units have filled scours 

in the underlying sand lens or in the underlying mudstone stratum. 

Projection of these thicker sedimentary structures behind the outcrop 

is uncertain because of the unpredictable extent and varied trend of 

superimposed lenses within the ore-bearing sandstone stratum. Field 

observations suggest, however, that the various scour-and-fill beds are 

genetically related to one another and reflect the shifting of the braided 

streams that deposited the sands; therefore, scour-and-fill bedding 

structures are stratigraphically dispersed in the ore-bearing sandstone 

stratum, and clusters of these structures are elongated in the direction 

of depositional trend.

LARGE-SCALE GUIDES

The coincidence of two geologic features in favorable ground became 

apparent as the areal scope of field work increased away from the 

vicinity of Uravan. These features or guides are the character of the 

sandstone bedding, and the thickness and areal extent of altered greenish-gray mudstone immediately beneath the upper sandstone stratum 

of the Salt Wash. Other guides such as thickness and color of sandstone or presence of carbonaceous material were found to be related 

but mostly incidental to bedding and altered mudstone. (See 

plate 13.)

DISTRIBUTION OF LA.RGE-SC-AJL.E GUIDES TO 

ORE DEPOSITS

SOUTHEAST OF BED FOX MINE AREA 

Southeast of the Red Fox mine (pi. 13) sandstones composing the

principal ore zone become dominantly nonlenticular, the altered mudstone beneath the zone is continuous but thin, and few ore deposits are

±ouncL TJa© sandstones in tn© Salt AVash member as a whole become

dominantly nonlenticular and thin bedded in a distance of a few miles.

These sandstone layers are light brown or gray, in a few places reddish brown, uniformly fine grained, hard, and conspicuously limy.

The upper sandstone stratum of the Salt Wash is composed of 

several thin, relatively persistent sandstone lenses from 1 to 10 feet

thick. Total thickness of the stratum ranges from 15 to 25 feet in 

most places and only in a few places exceeds 30 feet. Scour-and-fill

bedding structures in the upper sandstone stratum occur only in g,

few places and are generally small and poorly developed. Sedimentary-structure trends of different sandstone lenses deviate no more 

than an average of 20° from a northeasterly direction. Greenish-gray
CRITERIA FOR FAVORABLE AREAS IN PARTS OF COLO.-UTAH 273

mudstone beneath the upper sandstone stratum is continuous, but only 

in the vicinity of seeps and springs and near small areas of scour-andfill bedding is it more than 6 inches thick. Fossil wood is not as 

abundant as in the Uravan area.

NORTHWEST OF URAVAN

Northwest of Uravan sandstone lenses in the upper sandstone zone 

of the Salt Wash are thick and dominantly lenticular, but continuously 

altered greenish-gray mudstone beneath the zone changes gradually to 

discontinuously altered mudstone where ore deposits are small and 

few. Discontinuously altered mudstone changes gradually to unaltered mudstone where no ore deposits are found.

Greenish-gray mudstone within, as well as beneath, the upper sandstone zone decreases in thickness northwestward from the Dolores 

Group of mines to Mesa Creek (pi. 13). Beneath lenticular sandstones at the Dolores Group the greenish-gray mudstone is generally 

from 6 to 18 inches thick, whereas beneath scour-and-fill sandstones 

it is from 1 to 5 feet thick. Along the outcrop between the Dolores 

Group of mines and Mesa Creek the mudstone is discontinuously altered and ranges from a thin film to 6 inches thick beneath lenticular 

sandstone and from 1 to 3 feet thick beneath scour-and-fill bedded 

sandstones. This partly altered mudstone commonly has a, mottled 

appearance or consists of alternately banded red and greenish-gray 

mudstone. Likewise, a transition from discontinuous greenish-gray to 

unaltered red mudstone occurs in the northwest end of Atkinson Mesa. 

On the north side of Mesa Creek and the southwest side of Blue Mesa 

the mudstone at the base of the upper sandstone stratum of the Salt 

Wash is dominantly red, and only in a few places are found thin discontinuous streaks and films of greenish-gray mudstone.

The transition in color of mudstone is accompanied by changes in 

color of the overlying sandstone. In the Dolores Group area, lenticular as well as scour-and-fill sandstones are predominantly light brown. 

In the transition area, an increasing proportion of lenticular sandstone 

is reddish brown whereas scour-and-fill sandstone is light brown. In 

the area of unaltered red mudstone, both lenticular and scour-and-fill 

sandstones are reddish brown.

NORTHEAST OF DOLORES GROUP

Northeast of the Dolores Group in the Blue Basin area the sandstones in the upper sandstone stratum of the Salt Wash are dominantly lenticular, but the underlying altered greenish-gray mudstone 

is discontinuous and ore deposits are small and few in number.

The Salt Wash is not exposed between the Uravan area and Blue 

Basin. Between Blue Basin and Mesa Creek the upper sandstone
274 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

stratum of the Salt Wash is dominantly lenticular, but appears to 

have a greater range in thickness and probably has less continuity 

than the sandstone stratum on the west side of Atkinson Mesa and 

Blue Mesa. Discontinuously altered mudstone underlies exposures 

of the upper sandstone stratum in the area between Blue Basin and 

Mesa Creek. On the basis of amount of altered mudstone, this area 

was recognized as transitional, probably to more favorable ground 

to the east, and to unfavorable ground to the west. However, the 

Salt Wash has been lost to erosion east of the transition area and to 

the west it is covered.

A comparison of sedimentary features in the upper sandstone 

stratum of the Salt Wash between the unfavorable Blue Mesa and 

the favorable Uravan areas suggests the need for careful evaluation 

of ore guides. The sedimentary-structure trends of the lenticular 

sandstones are alike in the two areas. Fossil vegetal material is 

more abundant in scour-and-fill bedded sandstone than in surrounding sandstone in both areas. Light-colored sandstone underlain by altered greenish-gray mudstone in the Uravan area contain some carnotite-impregnated logs, but none of the reddish-brown sandstone underlain by red mudstone on Blue Mesa contains carnotite-impregnated 

logs. Limonite-speck stain occurs in the upper sandstone stratum in 

both areas, but limonite diffusion bands were observed only in the 

Uravan area.

The significance of locally thick altered mudstone zones can be misinterpreted because in some places altered mudstone is not associated 

with favorable ground. Greenish-gray mudstone is locally thicker 

in the vicinity of water seeps in the Uravan area. A water seep near 

the base of the upper sandstone stratum of the Salt Wash in a cutbank

above the North Fork of Mesa Creek has effected a color change in the 

sandstone from reddish brown to gray and has changed the color of

the underlying mudstone from red to greenish gray in a zone about 

3 feet thick and about 30 feet wide.

On the other hand, a locally thick altered mudstone zone may be 

related to favorable ground in a stratigraphically lower ore-bearing

zone. An altered mudstone and sandstone zone about 200 feet wide

occurs in the upper sandstone stratum of the Salt Wash on the southW6St side of Blue Mesa, where it is associated with a fault belonging

to the Hoc Creek fault system. The alteration normal to and along

this fault may be due to present-day water Seepage OP to Older Solutions, perhaps the same solutions responsible for ore deposition in the

lower sandstone stratum or tne Salt W aSh.

FOJITH AND EAST OF BLUE MESA

The sandstones composing the upper stratum of the Salt Wash are 

$pminantly lenticular north and east of Blue Mesa; unaltered mud-
CRITERIA FOR FAVORABLE AREAS IN PARTS OF COLO.-UTAH 275

stone beneath the stratum at Blue Mesa undergoes a change in a northward and eastward direction first to discontinuously altered mudstone 

where ore deposits are small and few in number, then to continuously 

altered mudstone where ore deposits are large and numerous.

The amount of altered greenish-gray mudstone increases to the 

northeast along the periphery of Blue Mesa and increases both north 

and south of Blue Mesa. The northeastern part of Blue Mesa and a 

belt through the central parts of Outlaw Mesa and Calamity Mesa 

are areas of discontinuously altered mudstone that are transitional in 

a northeasterly and northerly direction from an area of unaltered to 

an area of continuously altered mudstone. The area of continuously 

altered mudstone includes the northeast corner of Blue Mesa, the 

northern half of Outlaw Mesa and Calamity Mesa, and all of Tenderfoot Mesa.

Greenish-gray mudstone, from 1 to 5 feet thick, underlies the orebearing sandstone zone in the northern part of Outlaw Mesa and on the 

north-central part of Calamity Mesa. Greenish-gray mudstone is 

continuous on the northern part of Calamity Mesa and all of Tenderfoot Mesa but in few places is it as much as 1 foot thick.

Sedimentary structures and lithology of the ore-bearing sandstone 

zone on Outlaw Mesa and the central part of Calamity Mesa are similar 

to those in the Urava'n area, whereas on the northern part of Calamity 

Mesa and on Tenderfoot Mesa the upper sandstone stratum of the 

Salt Wash is persistent, dominantly nonlenticular, and is rarely more 

than 30 feet thick. This sandstone is light brown, fine grained to 

medium grained, and friable. Sedimentary-structure trends change 

from east on the northeastern end of Blue Mesa to northeast on the 

northern part of Outlaw Mesa to north on the central and northern 

parts of Calamity Mesa and on Flattop Mesa (pi. 13.) On Tenderfoot 

Mesa trends are dominantly to the east though locally they deviate 20°.

NORTHWEST OF BLUE MESA

On Flattop Mesa the upper sandstone stratum is reddish brown, 

persistent, lenticular, and more than 50 feet thick. It is underlain by 

red mudstone except in the vicinity of ore deposits along the northwestern side of the mesa where the upper 3 feet of the mudstone is 

greenish gray and the sandstone is light brown. The change from red 

to greenish-gray mudstone occurs in an interval of about 300 feet 

along the outcrop where greenish-gray mudstone underlies an outcrop 

of scour-and-fill sandstone; but, red mudstone underlies similiar sandstone structures exposed at other places on the mesa.

On Beaver Mesa the principal ore-bearing sandstone zone is not 

persistent but where present the sandstone beds are highly lenticular 

and, in places, unusually thick. The sandstone is generally reddish
276 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

brown, friable, and seems to contain more fossil wood material than 

the upper sandstone stratum of the Salt Wash studied elsewhere. 

Limonite and manganese oxide diffusion banding also seems to be more 

abundant and occurs in both reddish-brown and light-colored sandstone. The top of the mudstone underlying the upper sandstone zone 

on Beaver Mesa is red except in the immediate vicinity of ore deposits. 

On outcrop, no transition between areas of unaltered and altered mudstone is apparent and the change is similar to that on Flattop Mesa.

DISTRIBUTION OF OTHER ORE-BEARING ZONES

Most of the other ore-bearing zones in the Uravan and Gateway 

districts occur in the Tenderfoot Mesa-Beaver Mesa area. These zones 

are scattered stratigraphically in conglomeratic sandstones lenses in 

the lower part of the Brushy Basin member, sandstone lenses between 

the upper and lower sandstone strata, and in the lower sandstone 

stratum of the Salt Wash. Altered greenish-gray mudstone underlying these zones is limited to the vicinity of the ore deposits.

An anomalous type of ore-bearing zone in the Koc Creek-Mesa Creek 

area is localized in the lower stratum of the Salt Wash by faults, fractures, and joints belonging to the Roc Creek fault system. The fault 

system is due to collapse of an intruded salt plug -in Eoc Creek, southeast of the Sinbad Valley salt anticline. The ore belongs to the copperuranium-vanadium type and upriver from Mesa Creek the deposits 

show a decrease in copper content in a southeastward direction.

LOCALIZATION AND CONTROL OF ORE DEPOSITS

The largest and richest known deposits in the Uravan and Gateway 

districts are in the vicinity of Long Park, Club Mesa, and the south 

part of Atkinson Mesa near Uravan; the northern part of Outlaw 

Mesa, and the central part of Calamity Mesa near Gateway (pi. 13). 

These deposits are restricted stratigraphically to the upper sandstone 

stratum of the Salt Wash. The sandstones composing this stratum 

in these areaS are dominantly lenticular and the top of the underlying 

mudstone is continuously altered. The OCCUrr6nC6 Of Stratigraphically 

Scattered depOSitS in the Tenderfoot Mesa and Beaver Mesa areas

probably reflects a lack of favorable Sedimentary Structure, the predominance of mudstone, Or a lack Of continuity of sandstone lenses in

the upper stratum of the Salt Wash. However, it must be assumed 

that definite regional Controls were active for ore deposition in the 

Gateway district, in spite of stratigraphic dispersion of deposits in 

certain areas. Similarly, the deposits localized by faults in the Roc 

Creek area and by favorable sedimentary structures in the Uravan 

district were subject to the same regional controls.
CRITERIA FOR FAVORABLE AREAS IN PARTS OF COLO.-TJTAH 277

Drilling in the principal ore-bearing sandstone zone has shown that 

ore bodies occur in elongate clusters, whose long axes are generally 

normal to the Uravaii mineral belt and parallel to the sedimentarystructure trend (Fischer and Hilpert, 1952, pi. 2). Individual ore 

bodies, however, may or may not be elongate parallel to the sedimentary-structure trend. No consistent interval between deposits could 

be seen in outcrop or on maps of deposits compiled from drilling data. 

Also, no consistent pattern of size, number, or spacing of ore bodies is 

apparent from one cluster to another. Ore deposits are erratically 

grouped in an elongated pattern probably because complex scourand-fill sandstone structures localized ore deposition; consequently the 

pattern of ore deposits reflects the complex distribution of the scourand-fill structures.

It has been generally conceded that carnotite or its unoxidized parent 

minerals were deposited by laterally migrating solutions. The present 

water-carrying capacity of the upper sandstone stratum of the Salt 

Wash indicates that the permeability and transmissibility of the beds 

are relatively high. Evidence suggests that the upper stratum of the 

Salt Wash which is overlain by impermeable mudstone of the Brushy 

Basin member, is limited in its present function as an aquifer by the 

small volume of water available in recharge areas rather than by 

physical nature of the sandstone. For example, one of the most favorable recharge areas for the upper sandstone stratum of the Salt Wash 

is on the flank of the Uncompahgre Plateau between Blue Basin and 

Mesa Creek. The discharge in seeps and in drill holes in a relatively 

discontinuous sandstone stratum downdip from this area is much 

greater than the discharge elsewhere.

Thus, the upper stratum of the Salt Wash can be likened to an 

aquifer within which are broad and thick lenticular sandstones of 

simple structure and lithology that allow the passage of a relatively 

large volume of circulating solutions. In contrast, the scour-and-fill 

sandstones within the aquifer are composed of interbedded sandstone 

and mudstone and mixed lithology in a structure which would either 

tend to inhibit movement of solutions, providing a locus for ore deposition, or control deposition as "rolls" in adjacent well-sorted sandstone. 

The ore shows a selectivity for only part of scour-and-fill sandstone 

and well-sorted sandstone in most places. The unpredictable nature 

of this selectivity is probably due to the concentration, volume, and 

pH of the ore-bearing solutions and the reaction of the solutions with 

clay minerals in the sandstone.

However, the movement of solutions as represented by the regional 

pattern of altered mudstone (pi. 13) does not correspond to the available regional capacity of the upper Salt Wash sandstones to transmit 

solutions during Morrison time. Instead, movement of solutions was
278 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

selective and the resulting pattern of altered mudstone suggests later 

regional structure control. Alteration effects and primary black 

uranium and vanadium ore minerals are intimately associated and 

both are probably the result of laterally migrating reducing solutions 

that predate oxidation of the metals to carnotite ore.

Ore deposits in or near the Uravan and Gateway districts are composed of carnotite ore, relict uraninite in carnotite ore, and black 

uranium and vanadium ore minerals in association with pyrite below 

present ground-water levels. The mineralogy of the black uraniumvanadium ore is largely unknown (Weeks, A. D., written communication, 1952) because the ore has only recently been recognized in deeper 

workings on Beaver Mesa, Tenderfoot Mesa, and Calamity Mesa, and 

in deep drill holes in the Uravan district. The occurrence of these 

ores with one another within the regional pattern of altered mudstone 

suggests that early ore-bearing solutions and later oxidizing solutions 

were subject to the same regional control. The associated pattern of 

altered mudstone and ore deposits also suggests that alteration may 

have taken place at the time of ore deposition, before or after ore 

deposition, but before oxidation of the primary black ore to carnotite 

ore.

The concepts of localization and control of ore deposition have heretofore been based on the hypothesis of a penesyngenetic origin of ore 

in the ore-bearing sandstone. This has been given tentative support 

by the development of ore guides (Weir, 1952) which were believed to 

apply only to the vicinity of ore deposits. Other bases for the penesyngenetic hypothesis rest on local patterns established on the common 

orientation of ore deposits, trends of ore rolls, fossil logs, and cross 

beds in favorable ground. A regional pattern that includes parts of 

the Uravan and Gateway districts, normal to and including favorable 

ground, based on ore-production data and on closer spacing of deposits 

and "probably localized by geologic conditions extant during the time 

the ore-bearing Morrison formation was deposited" (Fischer and 

Hilpert, 1952, p. 1 and pi. 2) has been named the Uravan mineral belt.

Results of the author's study in the north half of the mineral belt

have established that ore deposits are related to one another in a 

regional pattern of altered greenish-gray mudstone. Sedimentary

structures and their trends are incidental to this pattern but within 

the pattern they are essential controls for localization of ore deposits.

In a general way, sandstones are dominantly lenticular northeast of 

Paradox Valley and Sinbad Valley but only within the pattern of

continuously altered mudstone do the ore deposits have a close spacing,

large size, and high grade. Ore deposits are small and scattered in

the adjoining transitional zone and are absent in the unaltered mudstone zone.
CRITERIA FOR FAVORABLE AREAS IN PARTS OF COLO.-UTAH 279

There is no evidence to, support projection of the pattern of continuously altered mudstone northeastward from Uravan around the 

east side of Blue Basin to Outlaw Mesa. The eastern limit of continuously altered mudstone and ore deposits on Spring Creek Mesa, 

determined by drilling, is from 1 to 1% miles southwest of the synclinal axis (pi. 13). Projection of the pattern of continuously altered 

mudstone to. the southwest side ,of Paradox Valley cannot be made 

because little, if any, work has been done there. However, the position 

of deposits on both sides of the yalley, downdip and roughly parallel 

to the long axis of the salt anticline, suggests control of ore deposition 

by that structure. , RELATION OF REGIONAL STRUCTURE TO 

ORE DEPOSITS

The major structural elements of the area (pi. 13) include a broad 

northwestward-trending syncline between the Paradox and Sinbad 

anticlinal'valleys on the southwest and the Uncompahgre Plateau uplift on the northeast. The en echelon pattern of these salt anticlines 

was developed parallel to and contemporaneously with the ancestral 

Uncompahgre uplift in late Pennsylvanian time. This part of the 

Uncompahgre was reduced to sea level by Late Triassic time but parts 

of the salt anticlines remained topographically high until covered by 

Salt Wash sediments during Late Jurassic time. Deposition continued near sea level during the remainder of Jurassic time and near 

or below sea level through most of Cretaceous time. Uplift of the 

Uncompahgre Plateau and compression of the salt anticlines to present 

regional structure probably began in Late Cretaceous or early Tertiary 

time. The broad folds or highlands formed by the Uncompahgre uplift and the salt anticlines were eroded to a region of low relief before 

middle Tertiary time. Broad uplift of the Colorado Plateau, re- 

. juvenation of streams and canyon cutting, and increase in groundwater circulation with breaching of the crests of the salt anticlines 

followed, probably in middle Tertiary time. The early stage of canyon 

cutting was interrupted and modified by doming resulting from igneous intrusion of the La Sal Mountains. Kenewed folding and 

uplift of the Uncompahgre Plateau evidenced by the present elevation 

of Unaweep Canyon probably occurred in late Tertiary or early 

Pleistocene time (Cater, 1954).

Modification of earlier Tertiary structure by doming of the La Sal 

Mountains of Utah and renewed late Tertiary uplift of the Uncompahgre Plateau disrupted the early synclinal structure parallel to the 

Uncompahgre Plateau and produced a high divide of the Beaver Mesa 

area. Northeast of Calamity Mesa renewed uplift of the Uncompahgre 

produced faults that have about 1,500 feet of stratigraphic throw and
280 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

northeast of Uravan faults that have about 500 feet of stratigraphic 

throw. The same movement probably shifted the synclinal axis to 

the southwest in the Uravan district. Abstraction of salt and collapse of a salt plug in Eoc Creek shifted the synclinal axis locally to 

the southwest in the lower Mesa Creek area.

Observed relations between the position of ore deposits and the patterns of altered and partly altered mudstone is interpreted as a reflection of fluctuating ground-water conditions in ore-bearing beds 

in both the Gateway and Uravan districts. However, the position of 

ore deposits and the related patterns of mudstone alteration show no 

apparent control by the thickness of the Salt Wash and Brushy Basin 

members (pi. 13), relative thickness of the members as drawn in a 

diagrammatic section (pi. 13), or by favorable sedimentary structure 

in the Blue Mesa area (pi. 13). If the apparent lack of evidence for 

regional control of ore deposition by geologic conditions extant during 

the time the Morrison formation was deposited is valid, then it must 

be assumed that the position of deposits and patterns of alteration 

were controlled by regional structures resulting from crustal movement in Late Cretaceous or early Tertiary time.

Kestoration of Tertiary regional structures that may have controlled ore disposition is complicated by episodes in the structural history such as doming of the La Sal Mountains, the extent and magnitude of early Tertiary uplift of the Uncompahgre Plateau, the difference in magnitude of salt folds, and the adjustment of salt anticline structure to abstractions of salt from middle Tertiary time to the 

present. The occurrence of ore deposits parallel to the long axis of 

the Dolores anticline and the Gypsum Valley and Paradox Valley salt

anticlines and the variable range in elevation of deposits from one 

anticline to another suggest early Tertiary control of mineralization

by .these structures, However, the position of deposits m the Gateway district is anomalous in that those on Tenderfoot, Calamity, and 

Outlaw Mesas have no apparent spatial relation to salt anticlinal

structure. If the hypothesis for Tertiary structural control 01 OrG

deposition is true it is probable that the early Tertiary Uncompahgre

uplift played a role similar to that of salt structures in controlling ore

solution movement in part of the Gateway district,

Thus, if the deposits in the Gateway district which now range between 6,400 and 7,000 feet in altitude were restored to the altitude

existing before Uplift Of the La Sal Mountains and the last uplift of

the Uncompahgre Plateau there is no assurance mat tn6 fftSlOFeu altitude would be that of deposits in the Uravan district which range 

from 5,300 to 6,500 feet. On the contrary, there is a strong suggestion in associated patterns of alteration and deposits in the UravanGateway districts that ore deposition occurred in discrete ground-
CRITERIA FOR FAVORABLE AREAS IN PARTS OF COLO.-UTAH 281

water systems localized by individual early Tertiary structural 

environments. *

ORIGIN OF ORE DEPOSITS

Coffin (1921), Hess (1933), and Fischer and Hilpert (1952) have 

defined mineral belts in southwestern Colorado and from detailed 

studies concluded that the geologic relations are best interpreted as 

the result of circulating ground waters leaching and redepositing 

uranium and vanadium minerals soon after deposition of the Salt 

Wash member. Butler (1920) and others believed that similar deposits in Utah may have been influenced by circulating ground waters 

resulting from folding and faulting in Tertiary time.

The geologic history of the Colorado Plateau permits the interpretation of ore deposition either soon after the ore-bearing Morrison 

formation was deposited, during Cretaceous time, or after an upset 

of Mesozoic structural conditions near the end of that era or in early 

Tertiary time. Results of this study in the Uravan and Gateway districts have established that a regional pattern of altered greenish gray 

mudstone is coincidental with a regional pattern of ore deposits in 

both of these districts. The patterns of ore deposits and altered mudstone responded to geologic conditions that were active either before 

or after Late Cretaceous time. This study has shown that the common orientation of sedimentary features such as cross beds, fossil 

logs, and ore "rolls" reflecting the influence of geologic conditions during Salt Wash time, are incidental to the pattern of altered mudstone 

but that within the pattern of altered mudstone scour-and-fill bedded 

sandstone structures are essential controls for localization of ore deposits. Also, the coincidental regional patterns of altered mudstone 

and ore deposits was probably not controlled by regional geologic 

structure extant before the end of Cretaceous time (pi. 13, diagrammatic section). On the other hand, the spatial relation of the common pattern of ore deposits and altered mudstone with Tertiary 

structures in the vicinity of or adjacent to salt anticlines suggests 

that ore deposition accompanied or followed folding in Late Cretaceous or early Tertiary time. The position of deposits and the pattern of mudstone alteration provide a geologic basis for delineating 

discrete mineral belts in the Uravan and Gateway districts.

If the suggested control of ore deposition by Tertiary structure is 

true then a hypogene source must be inferred for the ore-bearing solutions (Cater, 1954). Geochemical relations are necessarily generalized because little detailed geochemical work has been done on ore 

deposits on the Colorado Plateau and on the relation of salt bodies 

and ore deposits in particular. The presence of vein copper deposits 

and fault-controlled copper-uranium-vanadium deposits (pi. 13) in
282 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

close association with salt structures suggests that'uranium-bearing 

solutions could also move upward in channels provided by salt intrusions to the overlying Salt Wash member. Recent discovery of 

black uranium and vanadium ore minerals and uraninite in disseminated type ore deposits suggests that carnotite is secondary to these 

minerals in fault controlled deposits in the Roc Creek area.

The ores in the region of salt anticlines are predominantly potassium-uranyl-vanadates (carnotite) but outside this region, as in Monument Valley in Arizona, the Grants district in the Four Corners area, 

the Edgemont area near the Black Hills, and the Pumpkin Buttes area 

in northeastern Wyoming, the dominant minerals are calciumuranyl-vanadate (tyuyamunite) and hydrous calcium-uranyl-silicate 

(uranophane). It is reasonable to believe that the association of 

potassium with uranium and vanadium in the salt anticline region is 

due to the presence of potassium chloride and high potash shales in 

salt bodies. Solutions from which primary uranium and vanadium 

minerals were precipitated, and' other later solutions, in following 

the intrusive piath of salt bodies, took potash into solution from contact with black shales. These reducing, primary ore-bearing solutions were probably responsible for the color change of red to greenishgray mudstone in the vicinity of deposits. Later, aerated ground 

water oxidized the primary uranium and vanadium minerals to 

carnotite.

Recent work by Stieff and Stern (written communication, 1951) on 

age determinations of ore minerals in hypogene hydrothermal veins 

in the Front Range, in sediments of Triassic and Jurassic age on the 

Colorado Plateau, and in fault-controlled deposits on the Plateau, 

indicates a common, probably early Tertiary age for all these deposits.

LITERATURE CITED

Butler, B. S., and others, 1920, Ore deposits of Utah: U. S. Geol. Survey Prof,

Paper 111. 

Cater, F. W., Jr., 1954, Geology of the Bull Canyon quadrangle, Colorado: U. S.

Geol. Survey Geologic Quadrangle Map GQ 33.

Coffin, B. C., 1921, Radium, uranium, and vanadium deposits of southwestern

Colorado: Oolo. Geol. Siirvey Bull. 16. 

FiSCher, R. P., and Hilpert, D, S., 1952, Geology of the Uravan mineral Belt: u. S.

Geol. Survey Bull. 98&-A.,

Hess, F. L., Uranium, vanadium, radium, gold, silver, and molybdenum sedimentary deposits, in Ore deposits Of the Western States (Lindgren volume): 

Am. Inst. Mln. Metall. Eng., p. 467.

Weir, D. B., 1952, GeOlOgiC guides to prospecting for carnotite deposits on Colo- 

. rado Plateau: U. S. Geol. Survey Bull. 988-B.

0