Uranium Ore in the Mariano LakeLake Valley Cores and the Genesis of 

Coffinite in the Grants Uranium Region, 

New Mexico

U.S. GEOLOGICAL SURVEY BULLETIN 1808-A
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Chapter A

Uranium Ore in the Mariano LakeLake Valley Cores and the Genesis of 

Coffinite in the Grants Uranium Region, 

New Mexico

By PAULA L HANSLEY

U.S. GEOLOGICAL SURVEY BULLETIN 1808 

EVOLUTION OF SEDIMENTARY BASINS-SAN JUAN BASIN
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988

For sale by the

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Any use of trade names in this report is for descriptive purposes only and does not 

imply endorsement by the U.S. Geological Survey and U.S. Bureau of Mines

Library of Congress Cataloging-in-Publication Data

Hansley, Paula L.

Uranium ore in the Mariano Lake Lake Valley cores and the genesis of 

coffinite in the Grants uranium region, New Mexico.

(Evolution of sedimentary basins San Juan Basin ; ch. A) (U.S. Geological 

Survey bulletin ; 1808-A)

Bibliography: p.

Supt. of Docs, no.: I 19.3:1808-A

1. Uranium ores New Mexico Mariano Lake Region. 2. Uranium 

ores New Mexico Lake Valley Region. 3. Coffinite New Mexico Grants 

Region. 4. Uranium ores New Mexico Grants Region. 4. Uranium 

ores New Mexico Grants Region. I. Title. II. Series. III. Series: U.S. 

Geological Survey bulletin ; 1808-A.

QE75.B9 no. 1808-A 557.3s [553.4'932'097898] 87-600323 

[QE390.2.U7]
CONTENTS

Abstract Al 

Introduction Al

Geologic setting of the Morrison Formation in the San Juan basin A3 

Methods of sample preparation A4

Description of uranium ore in the Mariano Lake-Lake Valley cores A4 

Primary ore A4

Diagenetically altered primary ore A9

Ore formation evidence from Mariano Lake-Lake Valley cores A13 

Origin of epigenetic amorphous organic matter A13 

Primary ore A15

Influence of the Brushy Basin Member of the Morrison Formation A17 

Alteration of primary ore A19

Early Cretaceous(?) alteration A19 

Mid-Tertiary alteration A19 

Late Tertiary alteration A20 

Genesis of coffinite All 

Comparison of deposits in the Westwater Canyon Member with deposits in the Salt

Wash Member of the Morrison Formation A22 

Summary and conclusions A24 

References cited A24

FIGURES

1. Grants uranium region, southern San Juan basin, northwestern New 

Mexico A2

2. Generalized cross section of the Morrison Formation in the southern San 

Juan basin A3

3. Major areas of uranium production from the Morrison Formation on the 

Colorado Plateau A5

4. Cross section of Mariano Lake-Lake Valley cores A6

5. Photomicrograph of fission tracks showing concentrations of uranium on 

grain rims and in primary pores A7

6. Photomicrograph of primary uranium ore from the Ambrosia Lake 

district A8

7. X-ray fluorescence micrographs depicting the distribution of uranium, 

silica, and carbon in primary ore A9

8. Photomicrograph of altered iron-titanium oxide grain containing primary 

ore A12

9. Photomicrograph of hematite spheres in skeletal plagioclase grain in 

oxidized ore A13

10. Scanning electron micrograph of diagenetically altered primary ore with 

microbotryoidal morphology A14

11. Scanning electron micrograph of diagenetically altered ore intergrown with 

primary ore A15

12. Scanning electron micrograph of radiating coffinite crystals A16

Contents III
FIGURES

13. Photomicrograph showing bands of coffinite and organic carbon in the 

Nose Rock deposit A17

14. Photomicrograph showing bands of coffinite and pyrite in the Ambrosia 

Lake district A18

15. Photomicrograph of coffinite replacement of organic carbon A19

16. Scanning electron micrograph of coffinite replacement of primary ore A20

17. Scanning electron micrograph of microbotryoidal ore within skeletal 

plagioclase All

18. Photograph of reduced, organic-rich mudstone in lower Brushy Basin 

Member A22

19. Pie diagram showing depositional and diagenetic facies of the Brushy Basin 

Member and diagenetic facies of the Westwater Canyon Member A23

IV Contents
Uranium Ore in the Mariano LakeLake Valley Cores and the Genesis of 

Coffinite in the Grants Uranium Region, 

New Mexico

By Paula L. Hansley

Abstract

Petrographic and geochemical research on uraniumbearing sandstones in the Upper Jurassic Morrison Formation 

in the Grants uranium region, northwestern New Mexico, indicate that compactional fluids, which migrated from finegrained fluvial and lacustrine units into adjacent sandstone intervals during early diagenesis, were instrumental in primary 

ore formation. These pore fluids carried uranium and other 

elements derived from the alteration of abundant volcanic 

material in the Brushy Basin Member of the Morrison Formation. Where these fluids reacted with concentrations of organic 

matter in sandstones of the Westwater Canyon Member, 

uranium was adsorbed, reduced, and concentrated into 

primary ore deposits.

Primary (carbonaceous) ore has undergone a significant 

amount of diagenesis due to the bacterial and thermal maturation of amorphous organic matter. Progressive diagenesis 

decreased the organic carbon:uranium ratio in primary ore, 

transforming it to various mixtures of microbotryoidal ore and 

coffinite. Coffinite formed where silica activity was high due 

to dissolution of aluminosilicate minerals by organic-acidbearing solutions. Organic acids were responsible for a wide 

variety of water-rock reactions, which were catalyzed by a 

warm (at least 100°C) deep-basin fluid that migrated updip 

through permeable ore-bearing sandstones in the mid-Tertiary. 

Regularly interstratified illite-smectite, chlorite, adularia, quartz, 

and ankerite precipitated at this time. From the late Tertiary 

to the present(?) time, downdip migration of meteoric waters 

from outcrops along the southern margin of the San Juan basin 

caused widespread oxidation of organic matter and redistribution of uranium into orebodies along a regional oxidationreduction front.

Recognition that diagenesis can transform organic-rich ore 

into organic-poor ore and that organic acids may facilitate lowtemperature formation of coffinite has important implications 

for interpreting the genesis of sediment-hosted ore deposits. 

Many organic-poor uranium deposits in the Westwater Canyon 

Member of the Morrison Formation in the Grants uranium 

region and in the Salt Wash Member of the Morrison Formation on the northern part of the Colorado Plateau may have 

formerly been organic-rich (primary) ore.

INTRODUCTION

In the Grants uranium region of the southern San 

Juan basin, the Upper Jurassic Morrison Formation contains some of the World's largest known sedimentary

uranium deposits (fig. 1). Despite intense interest in this 

region during the uranium boom of the 1950s and again 

during a flurry of mining activity in the 1970s, the genesis 

of these deposits is still not clearly understood. Two 

genetic types of uranium ore traditionally are recognized 

in the Grants uranium region: primary (pre-fault) and 

secondary (post-fault or redistributed) ore (Granger and 

others, 1961). Primary ore occurs as a matrix that is commonly a 1:1 mixture of carbonaceous material and 

uranium in bulk samples (Leventhal, 1980). Most 

uranium is present as cryptocrystalline coffinite, although 

some may reside in uranium-organic complexes. Uranium 

was probably derived from the hydrolysis of abundant 

volcanic glass during early diagenesis (Waters and 

Granger, 1953). Blanket-like bodies of primary ore as 

much as 1000 m long and 5 m thick are suspended in 

sandstone units without apparent relation to tectonic 

structures. Many orebodies are oriented with long axes 

parallel to current directions in fluvial channels. Kirk and 

Condon (1986) noted that ore tends to occur in thickened 

sandstone intervals that are related to syndepositional 

growth faults.

Radiometric age determinations (U/Pb) indicate 

that primary ore was deposited in Late Jurassic to Early 

Cretaceous time (Ludwig and others, 1984). This time 

span leaves room for debate as to the nature of oreforming processes, particularly the origin of the epigenetic 

noncellular organic material (amorphous organic 

material) that served as a concentrating agent for 

uranium. Most agree that the organic material was 

epigenetic, although some models promote an extrinsic 

origin, and others an intrinsic origin (Granger, 1968).

Remobilization of both uranium and carbon during an Early Cretaceous erosional interval may account 

for younger (Early Cretaceous) ages on primary ore and 

the roll-front features of tabular (organic-rich) ore. Oxidation and removal of uranium from primary deposits 

and the subsequent reprecipitation of reduced uranium 

at an oxidation-reduction boundary or along a Laramide 

geologic structure resulted in formation of redistributed 

orebodies (Granger and others, 1961). Redistribution 

more than a few hundred meters was rare (Adams and 

Saucier, 1981). Radiogenic ages indicate that most 

redistribution occurred in the late Tertiary (Ludwig and

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A1
Outcrop of Morrison Formation

Drill hole

Town

DISTRICT B Crownpoint 

^Mariano Lake

Figure 1. Map of Grants uranium region, southern San Juan basin, northwestern New Mexico showing the locations of 

uranium districts and drill holes discussed in the text.

others, 1982; Ludwig and others, 1984), although some 

Pleistocene ages have been reported (Rosenberg and 

Hooper, 1982), and younger ages suggest that redistribution is occurring at the present time (Ludwig and others, 

1982).

A unique opportunity to examine a suite of core 

samples from drill holes that transected the Grants 

uranium region from Mariano Lake to Lake Valley (MLLV) was afforded by a drilling program designed by the 

U.S. Geological Survey (Kirk and others, 1986). Within 

the Grants uranium region core was recovered from eight 

drill holes; four contained uranium ore (defined as >1000

ppm in this study). The detrital and authigenic petrology 

of ML-LV core samples has been described previously 

and will not be repeated in this report (Steele, 1984; 

Hansley, 1986a, 1986b, 1986c); and Reynolds and others, 

1986). The most northern samples analyzed were from 

a hole (CC-12) drilled by the U.S. Department of Energy 

located just north of core 8. Ore samples were also collected from various mines in the Ambrosia Lake district, 

the Nose Rock deposit, and numerous barren and 

mineralized cores in the Grants uranium region; the 

petrology of these samples was compared with that of 

ore in the ML-LV cores.

A2 Evolution of Sedimentary Basins San Juan Basin
The present study was part of a multidisciplinary 

effort to understand the provenance, sedimentology, clay 

mineralogy, geochemistry, and geophysics of uranium 

ore-bearing rocks in the Morrison Formation. The ore 

mineralogy was characterized using modern techniques 

such as the scanning electron microscope and the electron microprobe. (The fine-grained nature of the ore and 

the intimate mixture of uranium with organic material 

have precluded its characterization in the past). Early 

diagenetic alterations and primary uranium ore were 

found to be related in part to depositional and diagenetic 

alteration patterns in overlying units.

As a result of this investigation, a new ore type, 

diagenetically altered (carbon-deficient) primary ore was 

identified. Diagenetically altered ore formed during thermal and bacterial maturation of primary ore. As the 

amorphous organic matter in primary ore broke down 

into organic acids and carbon dioxide, the primary ore 

evolved from mixtures of organic matter and uranium 

to mixtures of clay minerals and coffinite. At the same 

time, a complex assemblage of authigenic minerals 

precipitated. During the mid-Tertiary when the Morrison 

Formation was most deeply buried, migration of warm 

deep-basin fluids through ore-bearing units (Northrop 

and Whitney, 1985) facilitated the alteration of primary 

ore.

Diagenetically altered primary ore includes ore 

previously described as primary and as redistributed. 

Recognition that amorphous organic matter can be 

broken down into soluble components during diagenesis

has profound implications for the re-interpretation of 

organic-poor sedimentary uranium deposits.

GEOLOGIC SETTING OF THE MORRISON 

FORMATION IN THE SAN JUAN BASIN

The sedimentology and stratigraphy of the Morrison Formation in the San Juan basin is summarized 

below. For a complete discussion of the regional geology 

and tectonics in the San Juan basin, see Hilpert (1969), 

Kelley (1957), and Santos and Turner-Peterson (1986). 

The ML-LV drillholes lie on the Chaco slope, a homocline 

formed during the Laramide uplift, that dips gently (<5 °) 

northward from the Zuni Mountains along the southern 

edge of the San Juan basin. In this area, the Morrison 

Formation is composed of three members, in ascending 

order the Recapture, Westwater Canyon, and Brushy 

Basin Members (fig. 2). The Morrison Formation was 

deposited primarily by braided streams on a broad alluvial 

floodplain during the Late Jurassic (Turner-Peterson, 

1985, 1986), although Galloway (1980) interpreted the 

depositional environment of the Westwater Canyon 

Member to have been an alluvial fan complex deposited 

by laterally migrating streams. Source areas for these 

sediments lay to the west and southwest of the depositional basin in the vicinity of the ancient Mogollon 

highlands (Cooley and Davidson, 1963) and possibly farther to the west where continental-margin volcanism occurred during the Late Jurassic (Silver and Williams,

sw 

Core 1 CROWN POINT

NE 

CC-12

CRET -v^' DAKOTA" sAN b sib NE

:VV.; ;:£:'wESf WATER' CANYON 'MEMEJER;

' RE CAPTURE.>it M$E R;

(HORIZONTAL DISTANCE NOT TO SCALE)

EXPLANATION

Conglomeratic sandstone

Sandstone, siltstone

Mudstone, claystone 

Bentonite

VERTICAL

SCALE

(meters)

r 0

L-50

Figure 2. Generalized cross section of the Morrison Formation in the southwestern San Juan basin, New Mexico.

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A3
1981). Nearby positive areas such as the Defiance and 

Zuni uplifts shed minor amounts of debris into the San 

Juan basin.

Thinly bedded sandstone and interbedded red and 

green mottled mudstone units of the Recapture Member 

overlie the Middle Jurassic Todilto Limestone Member 

of the Wanakah Formation in the Crownpoint area. The 

Recapture was deposited primarily by low-energy 

meandering streams, but some Recapture units were 

formed in local sahbka, playa-lake, and eolian environments (Condon and Peterson, 1986). Sandstones in the 

Recapture are typically moderately to well sorted and very 

fine to fine grained.

The Westwater Canyon Member consists of moderately well-sorted and fine- to coarse-grained feldspathic 

litharenites and lithic arkoses which were deposited 

primarily by braided streams. In general, sandstone units 

in the upper part contain fewer mudstone breaks and are 

coarser and more massive than sandstone units in the 

lower part of the member and in the Recapture.

The Brushy Basin Member is composed of an 

alluvial facies nearest the basin margin, a mudflat facies 

farther into the basin, and, finally, a lacustrine (playalake) facies in the center of the San Juan basin (Bell, 

1986). The concentric pattern formed by these facies is 

characteristic of sediments deposited in a closed salinealkaline lake basin (Sheppard and Gude, 1968). The 

alluvial facies is predominantly smectitic sandstone and 

mudstone; the mudflat facies consists of smectitic mudstone and claystone, and the lacustrine facies is characterized by smectitic claystone interbedded with discrete 

tuff beds, which have been silicified and/or altered to 

zeolites.

Nonmarine deposits of the Upper Cretaceous 

Dakota Sandstone unconformably overlie the Brushy 

Basin Member. A regional erosion surface that represents 

all or part of Early Cretaceous time truncates the Brushy 

Basin Member from south to north in the southwestern 

part of the San Juan basin. In places, the Dakota rests 

directly on intensely kaolinitized sandstones of the Westwater Canyon Member. In Late Cretaceous to early Tertiary time, tectonism associated with the Laramide 

orogeny tilted the Morrison Formation, exposing it in 

outcrops around the periphery of the San Juan Basin.

Uranium ore in the ML-LV cores occurs only in the 

Westwater Canyon Member, although it is occasionally 

found in the Brushy Basin Member in other ore deposits. 

Ore-bearing zones consist of thick sandstone channels interbedded with thin bentonitic mudstone overbank 

deposits.

On the north-central part of the Colorado Plateau, 

tabular uranium deposits are found in fluvial quartzose 

sandstones of the Salt Wash Member, the oldest member 

of the Morrison Formation. Major uranium deposits in 

the Salt Wash Member are found in the Henry basin of

south-central Utah and the Uravan mineral belt in eastern 

Utah and western Colorado (fig. 3). Primary uranium ore 

in these deposits, unlike primary ore in the Westwater 

Canyon Member, is not generally associated with amorphous organic matter.

METHODS OF SAMPLE PREPARATION

Uranium ore samples were examined from each 

Westwater Canyon Member ore zone in MV-LV cores 3, 

4, 7, and 7a (fig. 4). Ore samples from various mines in 

the Ambrosia Lake district and more than one hundred 

samples from other mineralized and barren localities in 

the Grants uranium region were also analyzed. Ore from 

core CC-12 was not available.

Polished sections from all ore samples were examined in oil under direct, reflected light with a petrographic 

microscope. A Cambridge 250 Mark 2 scanning electron 

microscope1 (SEM) with an attached energy dispersive 

X-ray analyzer (EDX) was used to study textural and 

paragenetic relations among ore zone phases. The EDX 

system was also used for qualitative and semi-quantitative 

elemental determinations. In order to study the spatial 

relationships among elements and phases in ore samples, 

an ARL electron microprobe was used to generate X-ray 

fluorescence elemental maps. Samples were coated with 

aluminum so that carbon could be detected with the 

wavelength dispersive system (WDS) of the electron 

microprobe. Quantitative analyses were conducted on 

coffinite-bearing samples. Because of their fine-grained 

nature, uranium-bearing phases in ore samples were identified by X-ray diffraction and X-ray powder camera 

methods.

DESCRIPTION OF URANIUM ORE

IN THE MARIANO LAKE-LAKE VALLEY CORES

Both primary and secondary ore and a new type 

of secondary ore, diagenetically altered primary ore, are 

present in the Westwater Canyon Member in the ML-LV 

cores. Gradations between primary and secondary ore 

suggest that they are genetically related through the processes of diagenesis.

Primary Ore

In all ore zones, uranium occurs in amorphous carbonaceous matrix that shows no visible morphology 

under the SEM. The matrix is medium-gray and has a

'Any use of trade names is for descriptive purposes only and does 

not imply endorsement by the U.S. Geological Survey.

A4 Evolution of Sedimentary Basins San Juan Basin
URAVAN

MINERAL

BELT

ESCALANTE

MHENRY MOUNTAINS 

MINING DISTRICT

UTAH 

ARIZONA

COLORADO 

NEW MEXICO

'Beclabito 

dome

IGALLUP 

I

GRANTS

URANIUM

REGION

Figure 3. Major areas of uranium production from the Morrison Formation on the Colorado Plateau.

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A5
Cores 1 7a CC-1 2 Dakota Sandstone

171

262

2300 

ppm

1015

1000 ppm 

3000 ppm

1 100 ppm 

1300 ppm

1338

Brushy

Basin

Member

Westwater 

Canyon 

Member

Recapture 

Member

EXPLANATION

- Above this line Fe-Ti oxides are

strongly altered (Reynolds and others, 1986)

 Uranium ore (> 1000 ppm U)

Figure 4. Cross section of Mariano Lake-Lake Valley cores showing locations of uranium ore zones and iron-titanium oxide 

alteration zone. Datum is base of Dakota Sandstone.

low reflectivity in direct reflected light. This ore exhibits 

all characteristics of primary ore (Adams and Saucier, 

1981; Fishman and others, 1985; Hansley 1986a, 1986c). 

For instance, autoradiographs and fission-track maps 

(fig. 5) revealed that uranium is concentrated in detritalgrain coatings and at grain contacts and also occurs within 

altered iron-titanium (Fe-Ti) oxide grains. In high-grade 

deposits, ore filled primary pores (fig. 6) and corroded 

detrital quartz margins causing them to fluoresce dull red 

due to radiation damage (Don Marshall, Nuclide Corporation, written commun., 1978). Uranium concentrations 

are highest near pyrite grains in primary ore (Hansley,

1986a, 1986c; Webster, 1983). The presence of cryptocrystalline coffinite is suggested by the close association 

of uranium and silicon as seen in elemental X-ray 

fluorescence maps (fig. 7) and was confirmed by weak 

coffinite peaks on X-ray diffractograms. Where coffinite 

does not appear on X-ray traces, crystals may be too small 

to be detected. Uranium may also be present in uranoorganic complexes.

Qualitative SEM and EM energy dispersive spectra showed that primary ore has a relatively high carbon:uranium ratio (usually >1), major but variable silicon 

and minor yttrium, aluminum, titanium, iron, potassium,

A6 Evolution of Sedimentary Basins San Juan Basin
Figure 5. Photomicrograph of fission tracks in a mica detector showing concentrations of uranium on grain rims and in primary 

pores. Neutron flux of 100 kilowatts for 4 hours. Plane-polarized light. Core 3, sample 1864 (568 m).

calcium, sulfur, vanadium, magnesium, manganese, zinc, 

chromium, phosphorus, selenium, molybdenum, and lead 

(radiogenic?). The silicon peak varies from half to twice 

the intensity of the uranium peak.

Primary ore occurs in relict Fe-Ti oxide grains, 

which have been altered to titanium dioxide (anatase?) 

and pyrite. Some grain-sized areas of ore were interpreted 

to be altered Fe-Ti oxide grains, because of the presence 

of iron and titanium. Qualitative electron microprobe 

analyses showed that ore within altered Fe-Ti oxide grains 

contained less uranium than did grain margins; variable 

amounts of titanium, silicon, and carbon; and also minor 

but variable iron, aluminum, potassium, calcium, 

magnesium, phosphorus, yttrium, and manganese. The 

highest concentrations of organic carbon generally correlate with lower silicon and titanium and minor iron, 

sulfur, magnesium, calcium, potassium, and phosphorus.

A light-gray, highly reflective phase containing 

uranium, titanium, and silicon replaced ilmenite lamellae 

in altered Fe-Ti oxide grains. Similar alterations in Fe-Ti 

oxides from the Ambrosia Lake district were thought to 

be a uranium-titanium silicate phase (Simova, 1982). The

widespread uranium-silicon association and SEM observations of tiny anatase crystals and blade-like coffinite 

crystals (fig. 8) occupying former ilmenite lamellae in 

altered Fe-Ti oxide grains, however, suggest that two 

phases, coffinite and titanium dioxide, are present. This 

close association of uranium and titanium in altered FeTi oxides has been noted by others in the ML-LV cores 

(Reynolds and others, 1986), in ore from the Nose Rock 

uranium deposit (Rhett, 1980), and in south Texas rolltype uranium deposits (Reynolds and others, 1977).

The main mineralized interval in core 3 is interpreted to be remnant primary ore, because isolated 

pockets of carbonaceous, uraniferous matrix are commonly surrounded by large open, secondary pores filled 

with kaolinite containing inclusions of ore. Uranium ore 

is intergrown with euhedral pyrite and, locally, with partly 

oxidized pyrite grains and feathery needles containing 

iron and selenium (ferroselite?). Primary ore that remains 

within Fe-Ti oxide grains was probably insulated from 

later oxidizing, ore-destructive fluids. Ore from mines in 

the nearby Smith Lake district has been interpreted to 

be primary (Fishman and others, 1985); nevertheless,

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A7
Figure 6. Photomicrograph of primary uranium ore (interstitial black material) from the Ambrosia Lake district. Plane-polarized light.

others contend that it is redistributed (Place and others, 

1980). Both Smith Lake and core 3 ores exhibit a variety 

of diagenetic alterations such as skeletal plagioclase, and 

contain potassium feldspar overgrowths that Fishman 

and others have interpreted to be pre-primary-ore alterations. Other petrographic evidence, however, has shown 

that most feldspar alterations were post-primary ore 

(Hansley, 1986a). Furthermore, no primary ore samples 

from the Ambrosia Lake district contain pre-ore feldspar 

alterations. Ore-stage chlorite rims skeletal feldspar 

grains; therefore, feldspar dissolution postdated precipitation of chlorite and ore. Carbonaceous ore within skeletal 

feldspar grains is probably primary ore that was redistributed during early diagenesis (See "Early Cretaceous(?) 

Alteration"). This interpretation may explain why "primary" ore from the Smith Lake district is consistently 

younger than primary ore from the Ambrosia Lake 

district (Ludwig and others, 1984).

In core 4, remnant uranium ore is contained in 

medium- to dark-gray, fine- to medium-grained feldspathic sandstone characterized by abundant kaolinite 

nests visible in hand specimen. A low correlation between 

organic carbon and uranium and the association of

uranium with titanium and silicon (also noted by 

Reynolds and others, 1986) indicates that the ore has been 

oxidized. Petrographic evidence for oxidation includes 

hematite on post-ore chlorite rims, on kaolinite, and 

within altered grains on the peripheries of the ore zone 

(fig. 9) (Hansley, 1986a, 1986c). Below the ore zone, an 

interval of hematitic alteration contains uraniferous 

overgrowths (oxidized ore?) on anatase and pyrite. Both 

ferroan carbonate and kaolinite cements locally contain 

remnants of unoxidized primary ore (Hansley, 1986c). 

Drillhole 4 may intercept a limb of Conoco's Section 29 uranium deposit near Crownpoint (Wentworth 

and others, 1980). The Section 29 deposit has been interpreted to be primary ore, 100-139 m.y. (Lee and 

Brookins, 1978; Ludwig and others, 1984), that was oxidized in the middle to late Tertiary (Wentworth and 

others, 1980). A regional (late Tertiary?) oxidationreduction front that has been mapped throughout the 

Grants uranium region crosses through the Crownpoint 

area (Saucier, 1980). Uranium that was originally 

deposited in east-southeast trending fluvial channels was 

remobilized by oxidizing groundwaters to locations downdip and north of former sites of accumulation.

A8 Evolution of Sedimentary Basins San Juan Basin
Figure 7. X-ray fluorescence micrographs that depict the distribution of uranium (A) silicon (8), and carbon (C) in primary ore. 

Core 7, sample 3179 (969 m).

Typical 634S values on ore-stage euhedral pyrite in 

cores 3 and 4 range from -21.4 to -9.9 per mil. These 

light values are similar to 634S values from other primary 

uranium orebodies in the Grants uranium region. The 

634S values on ore-stage pyrite from the Smith Lake 

district just south of the ML-LV core fence range from 

-29 to -42 per mil (Fishman and others, 1985). These light 

values reflect the large isotopic fractionation characteristic of biogenic pyrite produced by bacterial sulfate 

reduction in the presence of organic matter (Kaplan, 

1983). Sulfur isotopes from primary Ambrosia Lake ore, 

however, are generally heavier (Jensen, 1963). Heavier 

values may reflect a mixture of pyrite generations in the 

ore sample. Early diagenetic (pre-ore) pyrite cement in 

the ML-LV cores and in other Morrison deposits has consistently heavy 634S values of near + 20 per mil; mixing 

heavy pre-ore and light ore-stage pyrite would result in 

intermediate values depending on the relative proportions 

of each generation of pyrite.

Rhett (1980) and Clark (1980) interpreted the Nose 

Rock orebodies to be Late Jurassic to Early Cretaceous

roll-front deposits. The data gathered in this study from 

cores 7 and 7a and the Nose Rock samples, however, 

suggest that this ore was originally a primary deposit 

that was transformed during later (Tertiary?) diagenesis 

into partly oxidized orebodies with roll-front features. 

A strong organic carbon-uranium correlation indicates 

that primary ore was originally deposited in the Nose 

Rock area. The highest uranium concentration in the 

ML-LV cores is at 969.2 m in core 7. Here a poorly 

sorted, medium-grained sandstone is impregnated with 

uraniferous organic matter in a manner identical to that 

of unaltered primary ore from the Ambrosia Lake 

district.

Diagenetically Altered Primary Ore

Most ore with a low carbon content in the cores 

was interpreted to be diagenetically altered primary ore. 

Diagenetically altered ore may have different morphologies ranging from microbotryoidal to amorphous

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A9
Figure 7. Continued.

with <1 micrometer coffinite crystals to coarsely 

crystalline cof finite with carbon inclusions.

Under direct reflected light in oil, microbotryoidal 

ore appears amorphous, but SEM observation revealed 

that it is composed of irregularly shaped microspheres 

<2 micrometers in diameter (fig. 10). It has a higher 

reflectivity than primary ore due to a lower carbon: 

uranium ratio. The compositional of microbotryoidal ore 

is variable. In addition to uranium, other elements present in the microspheres include silicon, carbon, 

aluminum, vanadium, copper, titanium, potassium, iron, 

phosphorus, calcium, yttrium, ytterbium, and sulfur. The 

presence of only uranium and silicon peaks in SEM spectra of some microspheres implies that they contain cryptocrystalline coffinite. When associated with coffinite, 

microbotryoidal ore commonly forms perfectly smooth 

spheres containing mostly aluminum, silicon, and 

uranium with traces of iron and vanadium. Some spheres 

are almost pure aluminum, while others contain only 

uranium (uraninite?).

Similar encrusting, microbotryoidal material from 

other ore deposits in the Morrison Formation in the

Grants uranium region has been identified as coffinite 

(Brookins, 1979, figs. 16 and 41). Nevertheless, 

microspheres from sandstone-type uranium deposits in 

Pakistan, which are morphologically similar to the ones 

described here, are uraninite (Basham and Shilston, 

1978). No uraninite was detected in analyses conducted 

during this study, but this does not rule out the presence 

of X-ray-amorphous uraninite. Minor uraninite has been 

reported from the Grants uranium region, but virtually 

all occurs above the water table and is, therefore, interpreted to be a product of the oxidation (and subsequent 

reduction) of primary ore (Granger and others, 1961).

In the core 4 ore zone, small (<1 micrometer) coffinite crystals are embedded in uraniferous, carbonaceous 

matrix (fig. 11). Clay minerals, particularly ferroan 

chlorite, are abundant in this ore zone. This ore is interpreted to be altered primary ore that may be an intermediate stage of alteration between amorphous 

primary ore and microbotryoidal cof finite-rich ore.

Coarse-grained coffinite, a uranous silicate 

[U(SiO4)4_x(OH)4x] was discovered by Stieff and others 

(1956). It commonly occurs with microbotryoidal ore as

A10 Evolution of Sedimentary Basins San Juan Basin
single, micron-size crystals or as large void-filling clusters 

of radiating colloform crystals that commonly contain 

shrinkage(?) cracks (fig. 12). It is brownish gray in direct 

reflected light and has a low reflectivity. Qualitative 

SEM-EDS and quantitative EM-WDS analyses of coffinite showed a consistent uranium:silicon ratio of 4:1 to 

3:1; minor calcium, yttrium, phosphorus, radiogenic lead, 

and heavy rare earth elements are also present in the coffinite structure (Hansley and Fitzpatrick, in press). 

Because quantitative electron microprobe analyses do not 

total 100 percent, water of hydration, carbon, and 

uranium in the +6 (oxidized) valence state are also 

thought to be present. Micrometer-size inclusions of 

titanium dioxide and organic carbon are common within 

the large coffinite crystals. Electron microprobe analyses 

of coarsely crystalline cof finite from the Woodrow mine 

one mile east of the Jackpile mine in the Laguna district 

(fig. 1) found calcium, phosphorus, vanadium, and 

aluminum to be minor constituents (Kim, 1978). The 

aluminum is either a contaminant from associated clay 

minerals or may actually be one or more rare earth 

elements as some rare earth elements emit X-rays in the 

same wavelength range as aluminum.

A light-gray-red sandstone interval (990.2-996.3 m) 

in core 7 near the base of the Westwater Canyon Member 

contains discrete areas of interstitial ore composed of coffinite and microbotryoidal ore. In coffinite-rich ore, colloform coffinite crystals coat quartz grains without 

evidence of having replaced the quartz and are finely 

mixed with both primary and microbotryoidal ores. 

Sphalerite and areas of nickel and cobalt enrichment were 

also detected. Interestingly, cobaltite (CoAsS) is present 

in coffinite-rich ore from the Woodrow mine (Moench, 

1962); arsenic was not detected, however, in EDX 

analyses of cobalt-rich spots in the Nose Rock ore. Some 

highly reflective material mixed with coffinite contained 

only nickel and sulfur.

Although most amorphous organic material in the 

cores was mineralized, curiously, some epigenetic organic 

material in the Nose Rock ore did not contain detectable 

uranium. For instance, nearly pure carbonaceous bands 

alternate with coffinite bands on detrital grains (fig. 13). 

Only organic carbon and traces of other elements were 

detected in the Nose Rock carbonaceous bands, unlike 

the uraniferous, carbonaceous ore from other parts of 

the Grants uranium region. This texture is similar to that

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A11
Figure 8. Photomicrograph of altered iron-titanium oxide grain (dashes outline grain) infilled by black and gray phases of primary 

uranium ore. Black areas consist of carbon, titanium, iron, silicon, uranium, aluminum, and magnesium; gray areas consist of 

carbon, titanium, iron, silicon, aluminum, magnesium, calcium, potassium, and a trace of uranium; and the light gray bladed 

areas are titanium, uranium, silicon, and calcium. The light gray blades are inferred to be former ilmenite lamellae replaced 

by a mixture of coffinite and anatase (titanium dioxide). Reflected light. Core 7, sample 3259, (994 m).

in some Ambrosia Lake samples in which coffinite alternates with pyrite bands (fig. 14). Carbonaceous and pyrite 

bands are always closest to detrital grain margins indicating that the organic matter and pyrite were precipitated 

before coffinite.

Some coffinite is interpreted to be a product of the 

diagenetic alteration of primary ore in situ as evidenced 

by inclusions of primary ore and anatase in the large coffinite crystals and coffinite embayment of primary ore 

(fig. 15). In addition, various stages in the replacement 

of primary ore by coffinite could be seen (fig. 16). The 

radiogenic lead content was measured in quantitative 

microprobe analyses of coffinite from cores 7, la, and 

the Nose Rock deposit. The relatively low lead content, 

which is small relative to that in primary Ambrosia Lake 

ore, suggests that the coffinite is fairly young, possibly 

mid-Tertiary in age.

Virtually all uranium ore with a low organic carbon content has traditionally been called redistributed

ore, implying movement and reprecipitation of uranium 

(Granger and others, 1961). The diagenetically altered ore 

of this report, however, is not a product of the movement of uranium away from a primary ore deposit, but 

rather it is the result of the alteration of primary ore in 

place. Diagenetically altered primary ore is characterized 

by a variety of alterations including moldic and intergranular porosity, chlorite, potassium feldspar, and a 

lower uranium:carbon ratio than unaltered primary ore. 

Where skeletal plagioclase feldspar and etched garnets 

(Hansley, in press) occur in primary ore zones, some 

alteration of ore can be inferred to have taken place, for 

pristine primary ore zones contain almost no evidence of 

previous diagenetic alteration. Microbotryoidal ore is 

commonly found within skeletal plagioclase grains (fig. 

17) only millimeters from primary ore. Low-grade, 

carbon-deficient primary ore that contains chlorite grain 

coatings may have originally been primary ore of a higher 

grade. Squyres (1963) suggested that brown ore from the

A12 Evolution of Sedimentary Basins San Juan Basin
Figure 9. Photomicrograph of late diagenetic hematite spheres (h) within dissolution cavity (v) in detrital plagioclase grain in 

oxidized ore zone. Chlorite (cl) coats grain rim. Plane-polarized light. Core 4, sample 1933 (589 m).

Ann Lee mine in the Ambrosia Lake district was leached 

primary ore.

Some microbotryoidal ore and coffmite may be 

redistributed uranium ore; however, in most cases, movement of uranium was no more than a few meters. Coffinite that is not intimately associated with primary ore and 

that does not have carbon inclusions may have formed 

during local redistribution and reprecipitation of uranium 

ore.

ORE FORMATION EVIDENCE FROM 

MARIANO LAKE-LAKE VALLEY CORES

Origin of Epigenetic Amorphous Organic Matter

Identification of the type of organic material in 

primary ore has been difficult. Radiation and diagenesis 

have altered the organic molecules to an insoluble amorphous carbonaceous residue deficient in hydrogen and

oxygen that cannot be identified by conventional infrared 

or chemical techniques (Leventhal, 1980). The organic 

matter is referred to as amorphous because it lacks 

observable morphology when observed petrographically 

(Tissot and Welte, 1984). Based on chemical tests and on 

the solubility of some of the organic matter in alkaline 

solutions, Granger and others (1961) concluded that 

degraded vegetal matter was probably the source of the 

carbonaceous material. Nuclear magnetic resonance data 

on the organic matter also indicate that it was derived 

from terrestrial plant material or humate (Hatcher and 

others, 1986).

The origin of the organic matter, however, remains 

a subject of controversy, although most agree that it is 

epigenetic. Major theories of its origin are, as follows: 

(1) organic matter was intrinsic to the uranium-bearing 

sandstones (Squyres, 1980); (2) organic matter originated 

in Early Cretaceous swamps that were eroded prior to 

deposition of the Upper Cretaceous Dakota Sandstone 

(Granger and Santos, 1982); and (3) organic matter was 

derived from solubilization of plant material during early

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A13
Figure 10. Scanning electron micrograph of diagenetically altered primary ore with microbotryoidal morphology. Poorly formed 

coffinite (c) is present. Core 7, sample 3259 (994 m).

diagenesis of mudflat sediments associated with salinealkaline lake deposits overlying ore-bearing units (TurnerPeterson and others, 1980; Turner-Peterson, 1985).

Abundant detrital plant material in the Westwater 

Canyon Member suggests that at least some of the humate 

was derived from intrinsic plant material (Squyres, 1980). 

However, Granger (1968) asserted that not enough 

detrital material is present to account for the amount of 

amorphous organic material present in the ore deposits. 

Abundant plant material must have been present in the 

Morrison Formation at the time of deposition, because 

of the great numbers of large animals such as dinosaurs 

that are known to have existed in the Late Jurassic (Dodson and others, 1980). Soluble organic acids may have 

also been derived from plant material in upstream fluvial 

facies of the Westwater Canyon Member.

Evidence for nearly simultaneous precipitation of 

organic matter and uranium indicate that organic acids 

precipitated sometime in the Late Jurassic to Early 

Cretaceous interval of ore formation. Thus, Late 

Cretaceous swamps could not have been the source of

the organic material as proposed by Granger and others 

(1961).

On the basis of sedimentological studies, others 

have suggested that humic and fulvic acids were derived 

from early compaction of mudstones in the lower part 

of the Brushy Basin Member and were subsequently carried downward by reducing, alkaline solutions into 

underlying sandstones (Turner-Peterson and others, 1980; 

Turner-Peterson, 1985). All known primary ore in the 

Grants uranium region lies under the mudflat facies of 

the Brushy Basin Member (Fishman and others, 1984). 

The downward movement of alkaline, humic-acidbearing pore fluids was driven by compaction and salinity 

gradients, a common phenomenon where fine-grained 

lacustrine sediments overlie permeable strata (Chilingarian, 1983) and which has been modeled in modern 

lakes by Winter (1981). This hypothesis is supported by 

Fe-Ti-oxide alteration patterns in the upper part of the 

Westwater Canyon Member in the ML-LV cores 

(Reynolds and others, 1986) and by regional Fe-Ti oxide 

alteration patterns in the Morrison Formation in the

A14 Evolution of Sedimentary Basins San Juan Basin
Figure 11. Scanning electron micrograph of diagenetically altered ore consisting of rod-shaped coffinite and microspheres intergrown with primary ore. Core 4, sample 1933 (589 m).

Grants uranium region (Adams and others, 1974). Irontitanium oxide alteration is thought to have been caused 

by reducing organic acid-bearing solutions.

A 7-10 m thick, organic-rich, pyritic green 

mudstone and claystone unit (fig. 18) in the lower part 

of Brushy Basin Member in all ML-LV cores lends 

credence to the theory that organic-acid-bearing waters 

may have been expelled from this unit during early 

diagenesis. Electron microprobe analyses of abundant 

black, irregular areas (<2mm) in this interval showed 

enrichment of organic carbon of 2-10 times background, 

significant iron, and minor aluminum and silicon. The 

presence of carbon and no detectable calcium imply that 

these carbon-bearing areas were originally fragments of 

plant material in a soil zone. Apparently, most organic 

material was removed as soluble organic acids from the 

green (reduced) mudstone before or during compaction. 

The hydrolysis of abundant volcanic ash that fell on the

mudflat formed basic solutions favorable for solubilization of organic matter.

Primary Ore

Organic acid-bearing solutions migrated through 

permeable, uncompacted sandstones in the Westwater 

Canyon Member and leached iron and possibly other cations such as vanadium and chromium from detrital FeTi oxides (Adams and others, 1974; Reynolds and others, 

1986; Delia Valle, 1981). These reducing solutions had 

a relatively high silica activity due to the dissolution of 

volcanic ash and probably carried some uranium in 

organic-acid complexes. Due to cation-loading of the 

humic acids (Fishman and Turner-Peterson, 1986) or to 

contact with a brackish or saline formation water in which 

organic acids were insoluble (Granger and Santos, 1982),

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A15
Figure 12. Scanning electron micrograph of radiating coffinite crystals. Core 7, sample 3260 (994 m).

metal-organic complexes in these solutions precipitated 

in tabular layers. Although cation-loading was probably 

a factor, the tabular form of the organic-rich layers and 

the fact that they are suspended in sandstone units 

without regard for bedding imply that organic acids 

precipitated at a hydrologic interface (Granger and 

others, 1961; Granger and Santos, 1982). The planar upper and lower surfaces of orebodies are very similar to 

those of humate layers that have precipitated at a fresh 

water-salt water interface in modern coastal Florida sands 

(Swanson and Palacas, 1965). The water below the interface may have been brackish sulfate-rich water, for 

sulfate is known to have dissolved at about that time in 

formations underlying the Morrison Formation (Hilpert 

and Moench, 1960; Ridgley, 1984).

Uranium enrichment on grain margins and on edges 

of pores infilled with primary ore indicates that most 

uranium mineralization occurred after organic matter 

precipitated (Webster, 1983; Hansley, 1986a). This conclusion is supported by chemical data that show organic 

material will carry only 10 weight percent chelated metals

(Swanson and others, 1966). In addition, the long span 

in radiometric ages of primary uranium ore (130-110 

m.y., Ludwig and others, 1984) indicates that uranium 

mineralization of humate layers continued for millions 

of years (although loss of radiogenic lead may account 

for some of the younger ages). Uranium was transported 

primarily as uranyl dicarbonate complexes (Langmuir, 

1978) in meteoric fluids that flowed downdip through 

Westwater Canyon sandstones, although a significant 

amount of uranium may have been expelled from the 

lower part of the Brushy Basin Member during compaction (see next section). Upon contact with gel-like masses 

of organic matter, uranium was adsorbed and then reduced to the uranous (+4) ion (Leventhal, 1980). Where 

silica activity exceeded the solubility of amorphous silica, 

cryptocrystalline coffinite formed within the organic 

layers, which were very porous (see "Genesis of Coffinite"). During the latter stages of mineralization, oxidizing solutions probably redistributed uranium locally 

and early anhedral pyrite was dissolved and reprecipitated 

as euhedral pyrite in the ore zones.

A16 Evolution of Sedimentary Basins San Juan Basin
Figure 13. Photomicrograph showing bands of coffinite (co) and organic carbon (oc). Organic carbon is always closest to detrital 

quartz grains (q). Direct reflected light, oil immersion. Nose Rock deposit.

Influence of the Brushy Basin Member 

of the Morrison Formation

The distribution of depositional facies and early 

diagenetic phases in the Brushy Basin Member, which 

overlies the Westwater Canyon Member, in the ML-LV 

cores suggest that alkalinity and salinity of interstitial 

fluids increased basinward. In cores 1, 3 and 4, the Brushy 

Basin is composed of tuffaceous mudstone, claystone, 

and minor sandstone containing abundant authigenic 

smectite matrix. Farther basinward in cores 5-7a, discrete 

altered tuff beds and abundant millimeter-size orange 

spots of clinoptilolite and calcite pseudomorphs after 

clinoptilolite in tuffaceous claystone and mudstone 

intervals reflect a distinct change in depositional and 

diagenetic facies. These lithologic and mineralogic 

changes are interpreted to represent a transition from 

mudflat to playa margin facies in the Brushy Basin

Member, similar to facies changes that Bell (1986) 

recognized in the southeastern part of the Grants uranium region. Rapid facies changes are typical of a 

saline-alkaline lake environment (Sheppard and Gude, 

1968).

The ML-LV cores do not extend to the playa facies 

of the Brushy Basin Member, but core CC-12 contains 

analcime and albite cements in the Westwater Canyon 

Member, indicating that the playa facies was present in 

this area. Hicks and others (1980) also noted the presence 

of analcime in the Westwater Canyon Member in the 

Chaco Canyon cores. Unfortunately, no core was recovered from the Brushy Basin interval in the CC-12 core. 

Analcime precipitated when sodium-rich pore waters, derived from early diagenetic reactions in the lacustrine 

facies of the Brushy Basin, moved downward into Westwater Canyon Member sandstones and reacted with 

earlier zeolites (clinoptilolite?) or detrital feldspars.

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A17
Figure 14. Photomicrograph showing bands of coffinite (co) and pyrite (py). Pyrite is always closest to detrital quartz grains 

(q). Direct reflected light, oil immersion. Ambrosia Lake district.

Authigenic albite probably formed during burial diagenetic alteration of the analcime.

Authigenic mineral phases in the upper Westwater 

Canyon Member show a progressive increase in salinity 

and silica activity basinward (Hansley 1984, 1986a, 

1986c), mimicking the documented chemical gradients in 

the Brushy Basin Member (fig. 19). These authigenic 

phases form bands that parallel margins of the San Juan 

basin. Progressing northward from the southern margin 

of the basin they are: a smectitic plagioclase dissolution 

zone, a siliceous potassium feldspar zone, and a silicarich analcime and albite zone (Hansley, 1984). The alteration bands are apparently not caused by varying depths 

of burial, as similar alteration zones occur in the upper 

Westwater Canyon Member along the western margin of 

the basin in sediments that have been buried to about the 

same depth (Hansley, 1984). The decrease in intensity of

each feldspar alteration zone downward from the Brushy 

Basin-Westwater Canyon contact implies that solutions 

migrated downward and outward into sandstones from 

overlying units. As many as four generations of authigenic silica in the same pore suggest that pulses of fluids, 

perhaps related to the dewatering of units in the Brushy 

Basin Member, moved into sandstones in the upper part 

of the Westwater Canyon Member. These fluids had 

relatively high silica, sodium, and potassium activities due 

to the alteration of rhyolitic volcanic ash. Feldspar alteration patterns in the Jackpile sandstone unit, an informal 

economic unit in the uppermost Brushy Basin Member 

in the Laguna district, are the mirror image of alteration 

patterns in the Westwater Canyon Member indicating that 

fluids moved upward into the Jackpile sandstone unit 

from underlying fine-grained Brushy Basin units (Adams 

and others, 1978).

A18 Evolution of Sedimentary Basins San Juan Basin
Figure 15. Photomicrograph of dark gray coffinite (co) that 

has replaced light gray organic carbon phase (oc) and pyrite 

(py). Matrix is almost pure carbon with traces of silicon, lead, 

molybdenum, and sulfur. Direct reflected light, oil immersion. 

Nose Rock deposit.

That pore water moved down is supported by 

geochemical studies showing that the whole-rock and the 

<2-micrometer fraction of the upper part of the Westwater Canyon Member are enriched in sodium, light rare 

earths, and uranium (Delia Valle, 1981). Delia Valle attributed the enrichment of these elements to solutions 

derived from compacting sediments in the overlying 

Brushy Basin Member. Dilute meteoric fluids that 

migrated downdip through permeable Westwater Canyon 

sandstone units also contributed elements needed to form 

authigenic phases and uranium ore, but the closed basin 

environment of the Brushy Basin Member allowed for 

higher concentrations of elements in the escaping pore 

fluids. These fluids may also have carried organic acids 

derived from decaying organic matter in Brushy Basin 

sediments (Fishman and others, 1984; Turner-Peterson 

and Fishman, 1986).

High contents of silica and cryptocrystalline coffinite in ore farthest into the basin (cores 4-8 and the Nose 

Rock deposit) may be related, in part, to chemical gradients in the Brushy Basin Member (Hansley and TurnerPeterson, 1984). At pH values greater than nine, not 

uncommon in lacustrine facies of a saline-alkaline lake 

environment (Sheppard and Gude, 1968; Hay, 1966), the 

dissolution rate of volcanic ash increases dramatically 

(Zielinski, 1982) as does the solubility of silica 

(Krauskopf, 1959). Thus, escaping Brushy Basin pore 

fluids may have had higher silica contents than fluids 

closer to the basin margin.

Alteration of Primary Ore

Early Cretaceous(?) Alteration

Primary ore was remobilized locally during the latter stages of primary mineralization when mildly oxidizing uranyl-bearing solutions continued to react with 

earlier-formed primary orebodies. Tabular bodies of 

amorphous carbonaceous, uraniferous ore were re-shaped 

into S-shaped rolls. The two-dimensional nature of the 

line of cores, however, prohibited observation and interpretation of orebody morphology. Adams and others 

(1978) proposed that Jackpile ore was remobilized accounting for younger ages (115 ±9 m.y.), and Brookins 

(1980) thought that major diagenetic alteration took place 

during the Early Cretaceous. Light ore-zone pyrite characteristic of many primary orebodies may have formed 

when early pyrite cement was locally oxidized and the iron 

and sulfur reprecipitated downdip as isotopically light 

euhedral pyrite in the manner that Granger and Warren 

(1969) described for roll-type deposits. The close association of primary ore and diagenetically altered primary ore 

may account for numerous theories of origin for the same 

ore body, for depending on which part of the orebody 

was sampled, different conclusions as to the type of ore 

present could be reached.

Mid-Tertiary Alteration

Petrologic and stable isotope data indicate that 

most primary ore was altered during the mid-Tertiary 

when the Morrison Formation was most deeply buried. 

Regularly interstratified illite-smectite (Perry and Hower, 

1970), albitized plagioclase (Boles, 1982), and ankerite 

(Boles, 1978) indicate that formation temperatures within 

the Morrison Formation reached 100°C at this time. 

Deeply etched garnets occur only within the illite-smectite 

zone (Hansley, 1987). Etch-patterns on garnets identical 

to those these naturally etched Morrison garnets have 

been created experimentally by warm (80^) organic-acidbearing solutions (Hansley, 1985, 1987).

On the basis of oxygen and deuterium isotope data 

on illite-smectite and cogenetic chlorite, Northrop and 

Whitney (1985) suggested that a warm deep-basin fluid 

with the composition of an oil-field brine moved updip 

through the Westwater Canyon Member during burial of 

the Morrison Formation. Updip fluid migration may have 

occurred during the mid-Tertiary as oil was generated in 

the San Juan basin at this time (Rice, 1983). Warm fluids 

would have increased the solubility of silica (Siever, 1962) 

and, therefore, stabilized coffinite (Hemingway, 1982) 

and explain the presence of silicified Fe-Ti oxides containing traces of organic carbon in altered ore zones. Temperatures greater than 80 °C promoted the breakdown of

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A19
Figure 16. Scanning electron micrograph of coffinite (co) that has replaced primary ore (po). Core 7, sample 3260 (994 m).

organic molecules releasing soluble oxygenated functional 

groups and carbon dioxide; maximum generation of carboxylic acids occurs between 80°-100°C (Surdam and 

others, 1984). The resultant acid pore waters dissolved 

framework grains and carbonate cement thus producing 

abundant secondary porosity. Dicarboxylic acids derived 

from the oxidative degradation of immature type III kerogen are excellent solvents of plagioclase, because dicarboxylic acids have the ability to complex aluminum, 

which is insoluble in most natural inorganic solutions 

(Surdam and others; 1984; Surdam and Crossey, 1985). 

Binding of aluminum by organic complexes accounted 

for the lack of alteration products on skeletal grains. The 

maturation of type III kerogen would have resulted in 

the release of larger amounts of reactive oxygenated functional groups than either type I or type II kerogen (Tissot 

and Welte, 1984).

The fine-grained, idiomorphic nature of quartz, 

chlorite, and adularia, which are commonly intergrown 

in diagenetically altered ore, may also be the result of rapid 

precipitation in a zone of mixing between two formation 

waters (Heald and Renton, 1966; Engelhardt, 1977). 

Where this authigenic assemblage is now present, primary 

ore may once have existed (Spirakis and Hansley, 1986).

Isotopically light carbonate carbon in uranium ore zones 

(Leventhal, 1980; Hansley, 1986a) may have been derived 

from the diagenesis of organic material at this time.

It is curious that the migration of organic acids 

through permeable sandstones prior to and during 

primary ore formation did not cause widespread dissolution of framework grains. The hydrolysis of abundant 

silicic volcanic ash in the sandstones may have buffered 

the pH to moderately alkaline values (Zielinski, 1982) 

stablizing feldspars in the presence of organic acids.

Late Tertiary Alteration

During the late Tertiary, a tongue of oxidizing 

waters migrated downdip through the Crownpoint area 

northward to the Nose Rock vicinity (Saucier, 1980). Rollfront features and associated oxidized phases such as 

hematite and sulfates formed at this time. Corroded pink 

barite containing ferric oxide pseudomorphs of pyrite is 

overgrown by euhedral pyrite that represents a post-barite 

stage of sulfidization. Similar resulfidization textures have 

been observed in altered parts of the Mariano Lake ore 

(Place and others, 1980; Rhett, 1980; Hansley, 1986a, 

1986c).

A20 Evolution of Sedimentary Basins San Juan Basin
Figure 17. Scanning electron micrograph of microbotryoidal uranium ore within a skeletal plagioclase grain (p) rimmed by 

a potassium feldspar overgrowth (ka). Note ore on top of chlorite (cl). Core 7a, sample 3136 (956 m).

Abundant interstitial kaolinite in ore-bearing sandstones that show little alteration of detrital feldspar grains 

may have precipitated as a result of destabilization of the 

aluminum-organic complexes derived from the alteration 

of primary ore zones. Apparently anomalous mineral 

associations in deeper ore zones, such as (later) kaolinite 

and calcite, may be explained by the buffering capacity 

of organic acids. If carboxylic acids control the alkalinity of water calcite will become less soluble and precipitate 

as the partial pressure (P) of CO2 rises; if the alkalinity 

is buffered by the carbonate system, calcite will become 

more soluble and dissolve as Pco rises (Surdam and 

Crossey, 1985). 2

GENESIS OF COFFINITE

Coarsely crystalline coffinite is rare in sedimentary 

uranium deposits. Rhett (1979, 1980) noted abundant 

coarse-grained coffinite in radiating clusters in Nose Rock

ore, and Granger and others (1961, p. 1198) described 

coffinite as "microscopic jet black columnar crystals perched individually or in clusters on sand grains" in 

postfault (redistributed) ore from the Ambrosia Lake 

district. Large coffinite crystals having "radially fibrous 

internal structure" are visible in thin sections from the 

Woodrow mine, a collapsed pipe in the Laguna district 

(Moench, 1962). Vanadium-uranium deposits on the 

north-central part of the Colorado Plateau contain 

micrometer-sized tetragonal crystals of coffinite intergrown with chlorite (Northrop and others, in press(a); 

Goldhaber and others, 1987, in press). Wyoming rollfront uranium deposits contain colloform coffiniteuraninite mixtures (Ludwig and Grauch, 1980).

The origin of coffinite in low-temperature, 

sandstone-hosted uranium deposits is enigmatic, because 

it has not yet been synthesized at temperatures <200°C 

(Hemingway, 1982). Fuchs and Hoekstra (1959) synthesized coffinite in basic solutions (pH of 8.0 to 10.5) 

between 200°C and 360 °C. Using an estimated free

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A21
Figure 18. Photograph of altered plant(?) fragments in pyritic 

green mudstone from the lower part of the Brushy Basin 

Member. Light gray areas (Ig) are green (reduced) mudstone, 

medium gray (mg) areas are red mudstone, and black areas 

are inferred to be plant fragments (pf). Note light gray (reduced) 

halos around organic fragments. Core 4, 550 m.

energy of formation for coffinite of -1886 ±6 kilojoules 

per mole at 298.15 Kelvin, a figure derived from known 

free energy values of isostructural minerals (e.g. zircon), 

Hemingway (1982) determined that the activity of silica 

(as [H4SiO4]) necessary for coffinite to form is 10-3 - 59. 

This value is similar to that (10~3) proposed by Langmuir (1978) but is considerably higher than that (1Q-69) 

suggested by Brookins (1975). Importantly, this latter 

value exceeds the equilibrium solubility of amorphous 

silica (10-368 ; Cuney, 1978). A value as high as lO'359 

would not have been unreasonable at the time of uranium 

mineralization (due to the hydrolysis of large volumes of 

silicic volcanic ash) or later in diagenesis (due to 

widespread dissolution of silicates), but it does not explain why coffinite is commonly associated with quartz 

rather than amorphous silica in Morrison deposits 

(Hansley, 1986c; Goldhaber and others, 1987). Perhaps 

the age of these deposits has allowed amorphous silica 

originally present in ore zones to recrystallize to quartz.

On the other hand Goldhaber and others (1987) proposed 

that high aluminum concentrations and acidic pH values 

generated by clay formation in ore zones inhibited the 

polymerization of silica and, thus, stabilized quartz 

relative to amorphous silica even at high concentrations 

of silica. Their conclusions are based on authigenic 

mineralogy, isotopic data on clay minerals in the ore 

zones in the Salt Wash Member of the Morrison Formation in the Henry basin, and on experimental data 

(Mohagheghi, 1985). Abundant ore-stage clay minerals, 

particularly vanadium-bearing chlorite, in Salt Wash ore 

zones attest to high concentrations of aluminum at the 

time of coffinite formation; clay mineral precipitation 

lowered the pH by consuming hydroxyl groups (Garrels 

and others, 1959; Byorkkle, 1982). Silica polymers were 

destabilized by high ionic strengths due to the presence 

of a brine in ore-forming horizons (Goldhaber and others, 

1987).

Isotopic data on ore-zone clay minerals and 

dolomite indicate that a solution interface between an 

overlying meteoric water and an underlying brine of 

seawater composition was present at the time of ore formation in the Salt Wash ores (Northrop, 1982; Northrop 

and others, in press(b)). In the mixing zone between the 

brine and meteoric water, a drop in pH of the saline fluid 

promoted adsorption of uranyl ion onto detrital grain surfaces. Subsequent two-step reduction of UO2 (uranyl) to 

UO2 + and then to UO2 + + by sulfide species such as 

hydrogen sulfide or HS~ in the presence of high silica activities promoted formation of coffinite plus quartz 

(Goldhaber and others, 1987).

In the Grants uranium region, early diagenetic clay 

minerals and bacterial metabolism of organic matter during sulfate reduction provided abundant hydrogen ions 

that favored the adsorption of uranium and, thus, promoted formation of cryptocrystalline coffinite in zones 

of organic matter accumulation. The later migration of 

a warm fluid through primary ore-bearing strata may 

have facilitated recrystallization of cryptocrystalline coffinite and precipitation of coarsely crystalline coffinite 

by causing breakdown of organic compounds and increasing the solubility of silica. Banded gel-like coffinite/ 

organic matter textures may have formed when organics 

were remobilized and uranium diffused out of the organic 

matter during redistribution in the warm fluids.

COMPARISON OF DEPOSITS IN THE 

WESTWATER CANYON MEMBER WITH 

DEPOSITS IN THE SALT WASH MEMBER 

OF THE MORRISON FORMATION

One of the major differences between primary 

uranium ore in the Salt Wash Member and primary ore 

in the Westwater Canyon Member is the relative amount 

of amorphous organic material: Westwater Canyon 

primary ore contains a large amount of amorphous

A22 Evolution of Sedimentary Basins San Juan Basin
(HORIZONTAL AND VERTICAL 

DISTANCE NOT TO SCALE)

EXPLANATION

if^^ Uranium ore 

 Drill hole

Figure 19. Pie diagram showing depositional and diagenetic facies of the Brushy Basin Member and corresponding diagenetic 

facies of the Westwater Canyon Member. Inferred directions of fluid movement during compaction are shown. Driil holes 

are located on southwest-northeast line.

organic matter; Salt Wash ore, almost none. In both 

members, orebodies have tabular shapes and planar upper surfaces, ore horizons rise stratigraphically basinward, and the ore forms a matrix that fills primary pore

spaces. Realization that organic matter can undergo 

diagenesis at low to moderate temperatures (<100°C) provides a mechanism whereby organic-rich ore can be converted into organic-poor ore; thus, Salt Wash deposits

Uranium Ore and Coffinite, Grants Uranium Region, New Mexico A23
might have originally contained more amorphous organic 

matter that was converted to organic acids during postore diagenesis. If this were the case, all tabular-type 

uranium deposits in the Morrison Formation may have 

had a common genesis (Spirakis and Hansley, 1986).

SUMMARY AND CONCLUSIONS

Chemical, sedimentologic, and mineralogic data 

suggest that mineralizing fluids were expelled downward 

and outward during compaction of tuffaceous Brushy 

Basin mudstones and claystones. These uranium-bearing 

solutions migrated through Westwater Canyon Member 

sandstones and precipitated uranium in and around concentrations of amorphous organic matter. Some organic 

matter was undoubtedly intrinsic to the ore-bearing 

fluvial sandstones, and some may have been derived from 

organic matter in overlying and interfingering fluvial and 

lacustrine mudstones of the Brushy Basin Member. The 

tabular nature of organic-rich primary orebodies suggest 

that organic acids precipitated at an interface between 

meteoric water and a brackish formation water.

Both primary and secondary ore are present in the 

ML-LV cores. Widespread diagenesis of primary ore 

resulted in the formation of diagenetically altered, 

carbon-poor ore containing coffinite. Bacterial and thermal alteration of primary ore resulted in the production 

of soluble organic acids and carbon dioxide increasing 

the acidity and complexing ability of pore fluids. Subsequent water-rock reactions resulted in dissolution of 

aluminosilicate minerals, formation of abundant secondary porosity and precipitation of chlorite, quartz, 

adularia, and ferroan carbonates.

Alteration of primary ore took place at several 

times during the postdepositional history of the Morrison 

Formation, but most diagenesis is inferred to have taken 

place in the mid-Tertiary. In contrast, late Tertiary oxidation of primary ore formed roll-front deposits characterized by oxidized minerals.

Petrologic similarities between carbonaceous 

primary ore in the Westwater Canyon Member and 

carbon-poor ore in the Salt Wash Member imply that ore 

in the Salt Wash Member may simply be diagenetically 

altered carbonaceous primary ore. Thus, lack of organic 

carbon in an ore zone may not be a valid criterion for 

dismissing the role of organic matter in the genesis of an 

ore deposit.

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Spirakis, C.S., and Hansley, P.L., 1986, The formation and 

alteration of tabular-type uranium-vanadium deposits as 

a variant of normal diagenetic processes in organic-rich 

sediments [abs.], in Carter, L.M.H., ed., USGS research 

on energy resources 1986, Program and Abstracts: U.S. 

Geological Survey Circular 974, p. 64-65.

Squyres, J.B., 1963, Geology and ore deposits of the Ann Lee 

mine, Ambrosia Lake area, in Kelley, V.C., compiler, 

Geology and Technology of the Grants uranium region: 

New Mexico Bureau of Mines and Mineral Resources 

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____1980, Origin and significance of organic matter in 

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New Mexico Bureau of Mines and Mineral Resources 

Memoir 38, p. 86-97.

Steele, B.A., 1984, Preliminary report on the petrography of 

the Upper Jurassic Morrison Formation, Mariano LakeLake Valley Drilling Project, McKinley County, New Mexico: U.S. Geological Survey Open-File Report 84-170,43 p.

Stieff, L.R., Stern, T.W., and Sherwood, A.M., 1956, Coffinite, a uranous silicate with hydroxyl substitution A new 

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p. 675-688.

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chemistry of secondary porosity, in McDonald, D.A., and 

Surdam, R.C., eds., Clastic diagenesis: American Association of Petroleum Geologists Memoir 37, p. 127-149.

Surdam, R.C., and Crossey, L.J., 1985, Mechanisms of 

organic/inorganic interactions in sandstone/shale sequences, in Relationship of organic matter and mineral 

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Swanson, V.E., Frost, I.C., Rader, L., and Huffman, Claude, 

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Swanson, V.E., and Palacas, J.G., 1965, Humate in coastal 

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Association of Petroleum Geologists Studies in Geology 

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13, p. 1-11.

A28 Evolution of Sedimentary Basins San Juan Basin

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