Copper and Uranium in Pennsylvanian and 

Permian Sedimentary Rocks, 

Northern Sangre de Cristo Range, Colorado 

U.S. GEOLOGICAL SURVEY BULLETIN 2116 
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Copper and Uranium in Pennsylvanian and 

Permian Sedimentary Rocks, 

Northern Sangre de Cristo Range, Colorado 

By David A. Lindsey and Reino F. Clark 

U.S. GEOLOGICAL SURVEY BULLETIN 2116 

A description of tire stratigraphy and depositional environments of host 

rocks and the chemical composition and possible origin of sedimenthosted copper and uranium occurrences in a redbed sequence 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government 

Library of Congress Cataloging-in-Publication Data 

Lindsey, David A. 

Copper and uranium in Pennsylvanian and Permian sedimentary rocks, 

northern Sangre de Cristo Range, Colorado I by David A. Lindsey and 

Reino F. Oark. 

p. em.-(U.S. Geological Survey bulletin; 2116) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:B2116 

1. Copper ores-Colorado. 2. Uranium ores-Colorado. 3. Geology, 

Stratigraphic-Pennsylvanian. 4. Geology, Stratigraphic-Permian. 

I. Oark, Reino F. II. Title. III. Series. 

QE75.B9 no. 2116 

[QE390.2.C6] 

557.3 s-dc20 94-41507 

[553.4'3'09788] CIP 
CONTENTS 

Abstract........................................................................................................................... 1 

Introc:luction .................................................................................................................... 1 

Ackllowledgments .................................................................................................. 3 

Paleoenvironmental Setting............................................................................................ 3 

Paleogeography ...................................................................................................... 3 

Paleohydrology and Redbed Formation................................................................. 4 

Sa.Dlpling and Analytical Methods ................................................................................. 5 

Descriptioo of Copper-Urailium Occurrences................................................................ 6 

King Midas Claims................................................................................................. 7 

Mt. Adams to Mt. Owen......................................................................................... 7 

Copper in Veins...................................................................................................... 10 

Chemical Compositioo ................................................................................................... 12 

Composition and Classification of Mineralized Rocks .......................................... 12 

Relationships Among Elements.............................................................................. 16 

Proposed Moc:lel for Mineralization................................................................................ 18 

Copper .................................................................................................................... 18 

Urailium.................................................................................................................. 19 

Conclusions .................................................................................. .......... ........................ 20 

References Cited............................................................................................................. 21 

FIGURES 

1. Map showing paleogeographic setting of the Ancestral Rocky Mountains and Ouachita orogenic belt in Middle 

Pennsylvanian to Early Permian time, and distribution of copper occurrences in rocks of that age......................... 2 

2. Restored cross section of Minturn and Sangre de Cristo Formations in northern Sangre de Cristo Range, 

Colorado..................................................................................................................................................................... 4 

3. Map showing generalized geologic setting and location of sediment-hosted copper-uranium occurrences in the 

northern Sangre de Cristo Range, Colorado ............................................................................... ............................... 5 

4. Measured sectioos showing distribution of mineralized rock, general depositional environments, and rock color at 

the King Midas claims, Crestone Needle, Colorado.................................................................................................. 8 

5. Stratigraphic sectioo showing distribution of mineralized rock and general depositional environments in the upper 

part of the Minturn Formation and lowermost Sangre de Cristo Formation, Mt. Adams to Mt. Owen, Colorado.... 9 

6. Photograph showing sequence of environments for mineralized shale of the upper part of the Minturn Formation 11 

7-9. Charts showing: 

7. Frequency distributions for organic carbon and total sulphur, and scattergram showing total sulphur versus 

total iron in mineralized rocks of the Minturn and Sangre de Cristo Formations ... ............ ......... ...................... 12 

8. Frequency distributions for copper in mineralized rocks, copper in unmineralized sandstones, 

uranium in mineralized rocks, and uranium in unmineralized sandstones of the Minturn and 

Sangre de Cristo Formations ................................. ............ .. ..... .......................... ...................... ............ .............. 13 

9. Frequency distributions for ThiU in mineralized rocks, Th/U in unmineralized sandstones, Fe/Cu in 

mineralized rocks, and Fe/Cu in unmineralized sandstones of the Minturn and Sangre de Cristo Formations. 14 

10. Scattergrams showing ThiU versus Fe/Cu and ThiU versus Ni/Cu for mineralized rocks and unmineralized 

sandstones of the Minturn and Sangre de Cristo Formations .. ........ ........................ ....... ....... ............ ........................ 15 

11. Plots showing eigenvalues and cumulative proportion of total variance explained by first ten principal factors...... 18 

12. Proposed moc:lel for formation of redbed copper-uranium deposits in basin fill of the central Colorado trough...... 20 

lli 
IV CONTENTS 

TABLES 

1. Chemical composition of sampled mineralized rocks compared to that of unmineralized sandstones, 

Minturn aild Sailgre de Cristo Formations................................................................................................................. 6 

2. Correlation coefficients for selected elements in mineralized rocks of Minturn and Sailgre de Cristo 

Formations................................................................................................................................................................. 16 

3. A six-factor geochemical model for mineralized rocks of Minturn aild Sangre de Cristo Formations..................... 17 
COPPER AND URANIUM IN PENNSYLVANIAN AND 

PERMIAN SEDIMENTARY ROCKS, NORTHERN 

SANGRE DE CRISTO RANGE, COLORADO 

By David A. Lindsey andReino F. Clark1 

ABSTRACT 

Redbed copper and uranium deposits of low grade 

occur mainly in beds and lenses of gray siltstone beneath a 

major red-gray color boundary in Pennsylvanian and Permian formations of the northern Sangre de Cristo Range, 

south-central Colorado. The deposits also contain anomalous amounts of silver, molybdenum, lead, vanadium, and 

zinc. All of the deposits are uneconomic, having low tonnages (1,000-2,000 to as much as 500,000 metric tons) and 

grades (about 0.025-0.085 percent Cu and 0.01-0.025 

percent U30g). Mineralized rocks contain small amounts of 

organic carbon preserved as plant remains. Uranium is disseminated in plant remains, probably as a urano-organic 

complex. Copper occurs as sulfides and as secondary malachite and azurite. 

Gray siltstone host rocks containing organic matter 

were deposited in both alluvial and marine environments. 

The lateral extent of some alluvial and nearshore-marine 

siltstone lenses, typically 100-300 m, was restricted by torrential deposition and erosion in alluvial-fan environments. 

In contrast, siltstone and shale lenses extending as much as 

11 km were deposited in more stable nearshore-marine 

environments. 

During Pennsylvanian and Permian time, extensive 

redbeds were formed when oxidizing ground water altered 

alluvial sandstone and siltstone around basin margins. Gray 

siltstone containing organic carbon from plant remains was 

preserved locally in the Minturn Formation. The present-day 

distribution of reduced siltstone is generally restricted to 

strata below the red-gray color boundary that marks the limit 

of redbed formation in the Sangre de Cristo and Minturn 

Formations. 

Copper occurs separately from uranium at the scale of 

sampling, which consisted of 0.3- to 1-m intervals representing the thickness of mineralized beds. Uranium and 

1

Present address of Oark: UNOCAL Corporation, Beaumont Terminal, P.O. Box 237, Nederland, TX 77626. 

vanadium are correlated, and lead and chromium are correlated with vanadium. R-mode factor analysis of data from 

mineralized rocks indicates (1) concentration of iron, cobalt, 

and nickel by sedimentation and diagenesis, (2) four separate 

modes of mineralization for vanadium-chromium-leaduranium, molydenum-uranium, zinc, and copper, and (3) 

concentration (or preservation) of sulphur (probably as sulfides) and organic carbon. 

Copper and uranium were probably transported by 

dilute mineralizing chloride-sulfate brines that originated in 

evaporite-bearing strata of the central Colorado trough. 

Brines probably equilibrated with abundant hematite and 

carbonate as they encountered oxidized sandstone and redbeds. Upon equilibration, brines became mildly oxidizing 

and alkaline, allowing transport of copper as cuprous chloride and uranium as uranyl phosphate or carbonate complexes. The brines leached copper from oxidized rocks and 

deposited it later as sulfide minerals in chemically reducing 

carbonaceous siltstone. Uranium was probably fiXed as a 

urano-organic complex on insoluble organic matter. Only 

small, low-grade deposits were formed because the mineralizing brines were probably too dilute and (or) too alkaline to 

transport sufficient copper. 

INTRODUCTION 

Investigations of so-called "redbed" copper-uranium 

occurrences in the northern Sangre de Cristo Range of 

south-central Colorado were begun as part of a mineral 

resource assessment of the proposed Sangre de Cristo Wilderness (Johnson and others, 1984). Although these redbed 

occurrences and others like them share some characteristics 

with much larger, economically important shale-hosted copper (copper-shale) deposits in other parts of the world, economically important redbed deposits are rarely found. Using 

samples and data collected for the mineral resource assessment of the wilderness, this investigation was pursued further to identify geologic controls that can affect the size and 

distribution of sediment-hosted copper deposits. 
2 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRISID RANGE, COLO. 

":"'-_/ 

/ (""'- ;--.........,__ """ r- '\_..' '" .> 

EXPLANATION 

X 

0 

Copper occurrences in 

Middle Pennsy 1 vanian to 

Lower Permian clastic rocks 

Rocky Mountain region 

Midcontinent region 

0 250 500 

KILOMETERS 

0 

[2] 

D 

-

Uplands and positive areas 

Foreland basins-Heavy line 

is limit of orogenic belt 

Arkosic sand 

Sand, mud, and carbonates 

Salt 

Figure 1. Paleogeographic setting of the Ancestral Rocky Mountains and Ouachita orogenic belt in Middle Pennsylvanian to Early Permian time, and distribution of copper occurrences in rocks of that age. Modified from McKee and others ( 1975, plates 15B and 15C); 

paleoequator from Smith and others (1981); distribution of arkosic sand from Mallory (1972) and Dutton (1982). Occurrences of copper in 

Middle Pennsylvanian to Lower Permian rocks in Arizona and New Mexico as follows: C, Coyote district; E, Estey district; N, general area 

of occurrences in Naco Formation; S, Scholle district. 

The designation "redbed copper" (redbed-hosted copper deposits of Eugster, 1989) is somewhat misleading 

because, although redbed deposits occur in redbed sandstone 

sequences, the host beds themselves are mostly gray. Redbed 

copper deposits are typically found in alluvial or mixed 

alluvial-marine strata. Sandstone and shale are both common 

host rocks, and siltstone is a common host in the Sangre de 

Cristo Range. 

Redbed copper deposits tend to be small, local deposits. 

In contrast, copper-shale (Kupferschiefer-type) deposits, 

such as those in the Permian Flowerpot Shale (Johnson, 

1976) of southwestern Oklahoma and northern Texas and the 
PALEOENN1RONMENTALSETTING 3 

Permian Kupferschiefer of Germany and Poland (Vaughan 

and others, 1989), are laterally extensive and economically 

important despite the fact that the mineralized interval may 

be less than a meter thick. Copper-shale deposits tend to 

occur in rocks that were deposited in restricted-circulation 

marine settings; they directly overlie redbeds and are closely 

associated with evaporites (Kirkham, 1989). Both redbed 

and copper-shale deposits appear to have been localized by 

a reducing host, and both exhibit features compatible with an 

epigenetic (early or late diagenetic) origin (see, for example, 

Jowett and others, 1987, for the Kupferschiefer of Poland; 

and Ensign and others, 1968, for the White Pine shale-hosted 

deposit of northern Michigan). 

Pennsylvanian and Lower Permian clastic sedimentary 

rocks in the southern Rocky Mountain and Midcontinent 

regions contain many occurrences of redbed copper (fig. 1). 

In the northern Sangre de Cristo Range of Colorado, copper 

occurs with uranium in the Middle Pennsylvanian Minturn 

Formation and in the lower part of the Middle Pennsylvanian 

to Lower Permian Sangre de Cristo Formation (Clark, 1982; 

Clark and Waltz, 1985). In the southern Sangre de Cristo 

Range of New Mexico, copper occurs with uranium in the 

lower part of the Sangre de Cristo Formation in the Coyote 

district (C, fig. 1; Tschanz and others, 1958). Elsewhere in 

New Mexico, copper occurs at numerous localities in Pennsylvanian and Lower Permian rocks. Occurrences that 

appear to resemble those of the Sangre de Cristo Range are 

located in the Lower Permian Abo Formation in the Scholle 

district (S, fig. 1; LaPoint, 1976; Hatchell and others, 1982) 

and in the Lower Permian Bursum Formation in the Estey 

district (E, fig. 1; Bachman, 1968, p. J38-J41). Uranium is 

reported with copper at some mines and prospects in both the 

Scholle and Estey districts (Gibson, 1952). In Arizona, uranium and copper occur together in the uppermost part of the 

Middle and Upper Pennsylvanian Naco Formation (N, fig. 1) 

and the lower part of the Upper Pennsylvanian and Lower 

Permian Supai Formation (Peirce and others, 1977). 

Known redbed copper deposits in clastic sedimentary 

rocks of Pennsylvanian and Early Permian age are not currently (1995) economic. Mining has been attempted at scattered localities, with the greatest production (12,340 metric 

tons of ore at 3.85 percent Cu, and 21.5 grams Ag per metric 

ton) from the Scholle district in New Mexico (Hatchell and 

others, 1982), but deposit sizes are generally too small to be 

economic. The occurrences in the northern Sangre de Cristo 

Range, described in this report, have had virtually no production (Nelson-Moore and others, 1978). 

ACKNOWLEDGMENTS 

We thank D.M. Walz for his assistance in measuring 

sections and collecting samples in the field and for identifying opaque minerals in polished sections. Mineralized samples were analyzed for major oxides by J.E. Taggart, J.S. 

Wahlberg, A. Bartel, and J. Baker; for total sulfur and 

organic carbon by G.T. Mason, Jr., J. Karasik, and V.E. 

Shaw; for Ag, Co, Cu, Mo, Ni, Pb, V, and Zn by L.A. Bradley, P. Briggs, J.G. Crock, M.J. Malcolm, V .M. Merritt, and 

G.D. Shipley; and forTh and U by H.T. Millard, Jr., M. 

Coughlin, S. Danahey, B.A. Keaton, S. Lester, F.M. Lumun, 

S. Storey, and B. Vaughn. Sandstone samples were analyzed 

for major oxides by J.S. Mee and D.F. Siems; for Cu, Mo, 

Pb, and Zn by J .M. Motooka; forTh and U by RJ. Knight; 

and for other trace elements by E.F. Cooley. R.E. Phillips 

determined modal mineralogy in thin sections of sandstones. 

R.A. Zielinski prepared fission-track maps of five thin sections and assisted in interpretation. D.P. Cox and R.A. Zielinski provided comments on drafts of the manuscript. All 

are, or were at the time work was performed, employees of 

the U.S. Geological Survey. 

PALEOENVIRONMENTAL SETTING 

PALEOGEOGRAPHY 

The host rocks of copper and uranium in the northern 

Sangre de Cristo Range-the Minturn and Sangre de Cristo 

Formations--were deposited in coastal areas and shallow 

seas in the central Colorado trough (fig. 1). The central Colorado trough lay between the Ancestral Uncompahgre uplift 

and the Front Range and Apishipa uplifts of the Ancestral 

Rocky Mountains (fig. 1). The Minturn and Sangre de Cristo 

Formations comprise a sequence of about 4,000 meters (m) 

of mostly clastic sediment deposited by prograding fan deltas and alluvial fans (fig. 2) (Lindsey, Clark, and Soulliere, 

1986). Subsidence of the trough was sufficient for fan deltas 

to prograde onto the sea floor during Middle Pennsylvanian 

time, depositing coarse alluvial sediments and complex 

intertongued marine limestones and shales in cyclic 

sequences. Afterward, during Late Pennsylvanian and Permian time, extensive erosion of the Ancestral Rocky Mountains produced great fans of coarse alluvial sediment that 

spread over the mixed marine-continental fill of the central 

Colorado trough. 

Environmental conditions were essentially similar 

throughout the Ancestral Rocky Mountains, so that the 

stratigraphic setting of host rocks for copper and uranium is 

more or less the same in all basins (fig. 1). The uplift history 

of most ranges of the Ancestral Rocky Mountains is more or 

less the same (Kluth, 1986). Most of the region lay within 

10° south of the equator during Pennsylvanian and Permian 

time (Smith and others, 1981; Ziegler, 1989). The climate 

was probably monsoonal, with arid conditions prevailing on 

the west sides of ranges (Mack and others, 1979), and it 

probably became increasingly arid in Permian time (Suttner, 

1984). East of the Ancestral Rocky Mountains, a vast epicontinental sea extended from the Ouachita orogenic belt 

north onto the craton (Heckel, 1977). On the craton, the 
4 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRISID RANGE, COLO. 

s 

METERS 

4000 --- ::::;:;:-::~~-

GRAY 

EXPLANATION 

BEDs· 

N 

METERS 

4000 

2000 

0 

m 

Sangre de Cristo Formation 

Alluvial-fan deposits 

Upper fan D 

Minturn Formation 

Fan-delta deposits 

0 

I 

5 10 

I 

I Prodelta above wave base KILOMETERS I 

Vertical exaggeration, 2X 

Lower fan ~ 

~ Prodelta below wave base 

Figure 2. Restored cross section of the Minturn and Sangre de Cristo Formations in northern Sangre de Cristo Range, Colorado (from 

Lindsey, Clark, and Soulliere, 1986). Section, located as line S-N in figure 3, is parallel to present structural strike and approximately 30° 

northwest of depositional strike. 

red-on-gray sequence of clastic rocks in l;>asins of the Ancestral Rocky Mountains is replaced by cyclic sequences of 

limestone, shale, and evaporites. 

PALEOHYDROLOGY AND 

REDBED FORMATION 

By the end of the Pennsylvanian Period an extensive 

series of alluvial deposits fllled the basins and mantled the 

eroded uplifts of the Ancestral Rocky Mountains (fig. 1) 

(Mallory, 1972; McKee and others, 1975, pl. 15). During and 

after deposition, these sediments must have served as 

regional aquifers. Oxygenated, relatively fresh ground water 

flowed through permeable fan deposits while reducing 

conditions prevailed in saline water derived from compaction of basin sediment. The shape and location of 

oxidizing-reducing interfaces in intrastratal waters would 

have been dynamic, depending upon the recharge rate of 

fresh ground water, the location of permeable zones and 

reduced sediment, and the thickness of basin fill, among 

other factors. 

Water flowing through fan deposits altered arkosic 

detritus to a deep red color as it oxidized iron in mafic minerals (Walker, 1967, 1989). Alteration was most intense 

around the margins of basins, where oxidizing conditions 

prevailed in intrastratal water. Iron from detrital ferromagnesian minerals such as hornblende and biotite was redeposited 

as grain coatings and pore fillings of secondary iron oxides 

or hydroxides. IDtimately, these amorphous iron compounds 

were converted to finely crystallized hematite that colored 

the rock red (Walker, 1967, 1989). 

The basinward flow of oxidizing water produced redbeds near the highlands of the Ancestral Rocky Mountains. 

As conditions became more continental during Pennsylvanian and Permian time, oxidizing water reached the centers 

of basins, extending the zone of reddening through much of 

the upper basin flll. In the central Colorado trough, the 

former extent of complete oxidation is marked by a laterally 

extensive color boundary that in many places corresponds 

approximately, but not in detail, to the nonmarine Sangre de 

Cristo Formation-marine Minturn Formation boundary (fig. 

2) (Lindsey, Clark, and Soulliere, 1986). Above the red-gray 

boundary, the Sangre de Cristo was oxidized to redbeds; 

below the red-gray boundary, the Minturn was oxidized 

incompletely, so that gray beds predominate over redbeds. 

Even these gray Minturn beds contain considerable hematite, 

indicating partial oxidation; and near the basin margin, the 

Minturn was oxidized completely to redbeds. Most importantly, some carbonaceous beds in the gray Minturn survived 

oxidation; these beds provided reducing traps for later fixation of metals from mineralizing fluids. 

The red-gray color distinction is an important control 

for copper-uranium deposits in the Pennsylvanian and Permian rocks of the Sangre de Cristo Range. Nearly all of the 

stratiform occurrences in the northern Sangre de Cristo 

Range lie within an interval of about 500 m below the major 
SAMPUNG AND ANALYTICAL METIIODS 5 

color boundary, and within that interval mineralized lenses 

are nearly all in gray, carbonaceous beds and lenses. In the 

s?u~ern Sangre de Cristo Range of New Mexico (Coyote 

district, fig. 1), copper and uranium occur mostly within a 

300- to 400-m-thick interval of gray and interbedded red and 

gray beds (Tschanz and others, 1958). Additional examples 

of copper and uranium in predominantly gray intervals of 

Pennsylvanian and Permian rocks are described in the literature of New Mexico and Arizona In east-central Arizona 

(~g. 1), copper and uranium are confmed to an interval spannmg the red-gray transition from the uppermost part of the 

Middle and Upper Pennsylvanian Naco Formation to the 

lower part of the Upper Pennsylvanian and Lower Permian 

Supai Formation (Peirce and others, 1977). In the Scholle 

district of New Mexico (fig. 1), copper occurs with small 

amounts of uranium and organic matter in gray shale, siltstone, and sandstone interbedded with redbeds of the lower 

part of the Lower Permian Abo Formation (LaPoint, 1976). 

In the ~s~ey district (fig. 1), copper occurs mostly in gray 

beds wtthm the Abo and the underlying Lower Permian Bursum Formation (Bachman, 1968). 

38° 

07' 

30" 

37° 

52' 

30" 

EXPLANATION 

~ ~, ::,1 Minturn FormationContains Cu-U 

occurrences 

Contact 

........_ Thrust fault-Sawteeth 

on upthrown side 

.Ll..l. Normal fault-Hachures on 

downthrown side 

~ Mines and prospects 

0 10 

KILOMETERS 

COLORADO 

Area~ffig 3 

Figure 3. Generalized geologic setting and location of sedim~nt-hosted copper-uranium occurrences in the northern Sangre de 

Cnsto Range, Colorado. Line S-N is cross section of figure 2; numbered localities 2-10 are measured sections of figure 5. Peaks referenced in text are: CN, Crestone Needle; EM, Eureka Mountain; 

HP, Hermit Peak; MA, Mt. Adams; MM, Marble Mountain; MO, 

Mt. Owen. Mines and prospects are: KM, King Midas claims; TC, 

Middle Fork Taylor Creek prospects; RM, Rita Alta mine. RAS, 

Rito Alto stock. 

SAMPLING AND ANALYTICAL 

METHODS 

Mineralized and potentially mineralized beds in the 

Minturn and Sangre de Cristo Formations were sampled in 

measured sections (Clark and Walz, 1985). Using a 

hand-held gamma-ray scintillometer to guide sampling, 

composite chip samples weighing about 400-500 grams (g) 

were collected from approximately 0.3- to 1-m intervals representing individual beds or lenses. Most samples were from 

outcrops, but a few prospects were sampled. A total of 120 

samples were collected and analyzed for major oxides, C, S, 

metals, Th, and U (table 1). Of these, 98 samples contained 

at least 50 parts per million (ppm) Cu or 30 ppm U and, 

based on inspection of the data, were considered mineralized; 22 samples were determined to be unmineralized. Of 

the 98 mineralized ~pies, 65 are from the King Midas 

claims and the area between Mt. Adams and Mt. Owen (fig. 

3). Data from these samples are the basis for this report. In 

addition, some copper prospects in veins are described but 

not sampled. 

Arkosic sandstones from measured sections (Lindsey 

and Schaefer, 1984; Lindsey and others, 1985; and D.A. 

Lindsey, unpub. data) across the entire thickness of the Minturn and Sangre de Cristo Formations were also sampled to 

determine geochemical background. All samples are single 

hand specimens weighing about 200-300 g and were collected from outcrops. A total of 50 samples of sandstone was 

collected, of which 42 were accepted for determination of 

background. The other eight samples were rejected because 

they were collected from a section within 2 kilometers (km) 

of the Rito Alto stock and contained anomalous concentrations of trace metals. Of the 42 samples, 18 are distinctly red 

and 24 are pale pink to gray in color. All samples were analyzed for major oxides, metals, Th, and U (table 1). Thin sections of all but four samples were examined. Typically, 

sandstones of the Minturn and Sangre de Cristo Formations 

contain abundant feldspar and a chloritic matrix. 

Although most analytical methods used for the two 

sample sets are the same, some differ somewhat because the 

unminef(lJized samples were analyzed about 10 years later 

than the mineralized samples. Mineralized samples only 

were analyzed for total sulfur by the combustion method and 

for organic carbon. Organic carbon was determined by difference between total carbon (analyzed by combustion) and 

carbonate carbon (method of coulimetric cell collection and 

colorimetric titration) (Jackson and others, 1987). Mineralized samples were analyzed for Ag, Co, Cu, Mo, Ni, Pb, V, 

and Zn by inductively coupled plasma-atomic emission 

spectroscopy (ICP-AES) (Lichte and others, 1987). Unmineralized sandstones were analyzed for Cu, Mo, Pb, and Zn by 

the ICP-AES method also, but other trace metals, including 

Ag, Co, Ni, and V, were determined by the six -step 

semi-quantitative spectrographic method (Golightly and 
6 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRISTO RANGE, COLO. 

Table 1. Chemical composition of sampled mineralized 

rocks ( 40 siltstones, 14 sandstones, 8 shales, and 3 silty 

limestones) compared to that of unmineralized sandstones, 

Minturn and Sangre de Cristo Formations 

[Sample localities shown on figs. 3, 4, and 5. Fey03, total Fe as Fe20 3; 

LOI, loss on ignition; Cc0 3' carbonate carbon; Corg• organic carbon; ST, 

total sulphur; leaders (- -), not analyzed; <, less than] 

Oxide or Mineralized rocks Unmineralized sandstone 

element Median Range 1 Median Range 

Concentration in weight per cent 

Si0 2 58.6 51.8-68.7 66.8 59.2-80.1 

Al20 3 16.5 13.6-19.6 13.7 9.35-16.2 

FeT03 5.95 3.87-8.61 5.13 1.91-7.95 

MgO 4.07 2.45-6.20 2.73 0.27-4.64 

CaO 0.86 0.43-5.55 0.69 0.08-6.95 

Na20 2.28 0.37-3.04 2.88 0.58-4.27 

K20 3.91 2.51-5.73 3.32 2.37-4.70 

Ti02 0.84 0.46-1.14 0.57 0.18-1.06 

P205 0.28 0.15-0.46 0.16 0.05-0.51 

MnO 0.09 0.04-0.21 0.08 0.02-0.22 

LOI 4.32 2.76-7.60 2.67 0. 75-6 .07 

Cco3 <0.01 <0.01-0.63 

COrg 0.40 <0.02-1.77 

ST 0.04 <0.01-0.48 

Concentration in parts per million 

Ag2 <1 <1-16 

As 5.7 0 . 7-26 

B 52 10-150 100 10-700 

Ba 957 196-3,400 1,000 300-5,000 

Co 16 6-34 15 3-50 

Cr 61 21-679 70 7-300 

Cu 161 2-8,500 6.2 0.4-34 

La 50 30-200 

Mo 3 5 <2-43 0.13 <0.05-0.68 

Ni 30 7-61 20 5-50 

Pb 51 5-7,480 5 1-21 

Sc 20 7-30 

Sr 100 70-500 

Th4 14.3 7.3-30.1 7.3 3.8-15.6 

u 52.7 3.55-1 ,080 2.68 0.83-7.85 

v 110 38-1,730 100 50-200 

y 30 10-300 

Zn 100 45-520 55 13-130 

1 Three li mestone samples omitted from range of major 

oxides. 

20nly 26 mineralized rocks analyzed for silver. 3 Detection limits differ for mineralized rocks (<2 ppm) 

and unmineralized sandstone (< 0.05 ppm) . 4 0nly 17 valid measurements of thorium concentration 

among mineralized rocks because of high values for uranium. 

All other mineralized rocks determined as "less than" values. 

others, 1987). Detection levels and reporting intervals are 

different for the two methods. V aloes determined by the 

six-step method are reported in six logarithmic classes per 

decimal place in the series 1, 1.5, 2, 3, 5, 7, 10, and so forth. 

Thorium and uranium were determined by the delayed neutron method (McKown and Millard, 1987) for both mineralized and unmineralized rocks. Finally, all samples were 

analyzed for major oxides by wavelength-dispersive X-ray 

fluorescence spectrometry (Taggart and others, 1987). 

No investigation of analytical or sampling error was 

made. Previous investigations (for example, Lindsey, 1985, 

table 3) show that, for mineralized samples, analytical error 

(including errors due to preparing and splitting the sample 

for analysis) for metals accounts for only a small part of total 

variation. Sampling error (local variation) may account for a 

large proportion of the total variation in mineralized samples. Although local variation is a handicap to precise estimation of average values at individual sample sites, such 

variation is useful in studying covariation among elements. 

Lower limits of detection, where encountered during analysis, are indicated in table 1. 

DESCRIPTION OF COPPER-URANIUM 

OCCURRENCES 

All of the occurrences of copper and uranium in the 

northern Sangre de Cristo Range described here are stratiform deposits in the Minturn Formation and the lowermost 

Sangre de Cristo Formation; most are within the upper 500 

m of the Minturn Formation. Many occurrences have been 

explored by shallow prospect pits. Two groups of occurrences are described to illustrate typical sedimentary facies 

of host rocks for stratiform copper and uranium (figs. 2 and 

3). These groups are located in the upper part of the Minturn 

Formation at the King Midas claims (at the foot of Crestone 

Needle, east side) and in the upper part of the Minturn from 

Mt. Adams to Mt. Owen. Small, structurally controlled veins 

in the Sangre de Cristo Formation are discussed separately at 

the end of this section. 

Most stratiform copper-uranium deposits in the northern Sangre de Cristo Range are in dark-gray siltstone, and 

less commonly in fine-grained gray sandstone, silty limestone, and shale. Most of the host rocks contain carbonaceous plant remains; some contain marine fossils but many 

do not. Mineralized rock commonly contains tiny pyrite tetrahedra, rusty spots of oxidized pyrite, and, less commonly, 

yellow secondary uranium minerals (radioactive, but 

unidentified), malachite, and azurite. The principal copper 

minerals, visible only under the microscope, are disseminated chalcopyrite and chalcocite; bornite and covellite were 

identified in a few samples. The host of uranium is organic 

matter, although the presence of primary uranium minerals 

cannot be excluded. Fission-track maps conflriD the tendency for uranium to concentrate in carbonaceous matter and 

reveal no obvious point sources of radiation that might indicate discrete grains of uranium minerals. Radioactivity, as 

measured by scintillometer, is commonly 100-200 counts 

per second (cps) but almost never more than 2,000 cps; background is about 50 cps. 
DESCRIPTION OF COPPER-URANIUM OCCURRENCES 7 

KING MIDAS CLAIMS 

Lenses of mineralized siltstone in the upper part of the 

Minturn Formation have been explored by numerous prospects at the King Midas claims (KM, fig. 3), located on the 

east side of the foot of Crestone Needle. The King Midas 

claims produced only 0.22 short tons (0.20 metric tons) of 

ore at an average grade of0.39 percent U30 8 (Nelson-Moore 

and others, 1978, p. 122); copper is present but evidently was 

not recovered. The main adit, located on the lower east slope 

of the ridge running northwest to Crestone Needle, intersects 

and follows gray-black radioactive siltstone and small quartz 

veins (Ellis and others, 1983, p. 96). At the surface, the mineralized siltstone is seen to consist of a number of individual 

beds separated by thin beds of sandstone (fig. 4, left section). 

The thickest mineralized siltstone bed is as thick as 3 m at the 

surface and is correlated with a thinner, similar bed 500 m 

northwest (fig. 4, right section). A small tear fault of probable Laramide age offsets the bed. Prospect pits have been 

dug in radioactive siltstone beds located along strike to the 

northwest 

Most mineralized beds on the claims are 1-3m thick 

and extend for 100--300 m along strike; these dimensions 

indicate that each mineralized bed contains about 

1,000-2,000 metric tons of rock or less. Metal values in siltstone exclusive of vein quartz range from 28 to 5,400 ppm 

Cu and 20 to 227 ppm U among 10 analyses from 8 mineralized beds; the average grade of these beds is only about 

0.085 percent Cu and 0.010 percent U30 8. 

The main mineralized bed above the King Midas adit is 

composed of 3 m of shaly siltstone containing plant remains 

and rusty spots; a middle interval, 1 m thick, contains calcareous nodules having lower metal values than the surrounding 

rock. No marine fossils were found. The mineralized bed 

overlies red conglomeratic sandstone of braided alluvial origin that, together with the siltstone, forms a fming-upward 

cycle of sediment deposited on an alluvial fan. The siltstone 

is interpreted to have been deposited in a wetland depression 

on the fan surface. Two overlying fining-upward cycles of 

conglomeratic sandstone and radioactive siltstone are also 

interpreted as alluvial cycles ending in wetland deposition. 

Some fming-upward cycles at the King Midas claims 

are overlain by impure silty limestone containing marine fossils (fig. 4), indicating that these alluvial cycles ended with 

marine submergence. Some silty limestones are mineralized; 

others grade into mineralized (anomalously radioactive) siltstone above or below. 

Study of the two measured sections at the King Midas 

claims (fig. 4) reveals that typical alluvial cycles in the upper 

Minturn Formation at Crestone Needle consist of (1) a 

coarse channel-filling conglomerate deposited by floods and 

debris flows, (2) cross-bedded conglomeratic sandstone 

deposited in braided channels and on fan surfaces, (3) lenses 

of shaly siltstone deposited in local wetland depressions on 

the fan surface, and (4) sporadic beds of impure limestone, 

shale, and siltstone deposited in shallow-marine environments between fan distributaries. Rocks of environments (3) 

and (4) are typically reduced (gray-black in color) and contain fragmentary plant remains; many are mineralized. 

Rocks of facies (1) and (2) are partly to completely oxidized 

(pink to red color) and are not mineralized. Alternating torrential alluvial and shallow-marine conditions, represented 

by interbedded debris-flow conglomerate and marine limestone, respectively, indicate that alluvial-fan and marine 

environments were telescoped together. Such telescoped 

depositional environments yielded lenticular siltstone bodies 

capable of hosting small, local copper-uranium deposits. 

Local unconformities contribute further to the lenticular 

geometry of host rocks. 

MT. ADAMS TO MT. OWEN 

Copper and uranium occur in anomalous amounts for 

more than 18 km along the top of the Minturn Formation 

from Mt. Adams to Mt Owen (fig. 5) (Clark and Walz, 

1985). These occurrences have been prospected locally, such 

as at the now-caved Mercury prospect, located 300 m north 

of the summit of Hermit Peak (fig. 3). Mineralized rocks 

occur at three stratigraphic intervals, listed in descending 

order: (1) numerous thin lenses of mineralized siltstone and 

sandstone scattered throughout a distinctive interval of gray 

to pink cross-bedded sandstone and siltstone, 36--81 m thick, 

that extends for more than 18 km along strike, (2) variably 

mineralized siltstone and shale, 1-2 m thick, that extends 

along strike for about 11 km, and (3) weakly mineralized 

lenses of siltstone and shale in the upper 500 m of the 

Minturn Formation. The cross-bedded sandstone and siltstone (interval 1) is located directly above a widespread 

marine marker limestone bed and directly beneath the Sangre de Cristo Formation; the mineralized siltstone and shale 

(interval 2) is located immediately below the marker bed of 

marine limestone. 

Some estimates of tonnage and grade, based on reconnaissance sections and limited analytical data, are given to 

provide a general sense of the amount of mineralized rock. 

Average grades were estimated by calculating weighted 

means, dependent upon the tonnage of rock represented by 

adjacent analyses. Despite high local variation and the very 

limited number of analyses, estimates of the average grade of 

mineralized rock above and below the marker limestone are 

remarkably consistent. Both mineralized intervals have very 

low grades and low tonnages compared to economic deposits. No estimates of grades and tonnages were made for scattered mineralized lenses of interval 3, below the marker 

limestone. The range of values for copper and uranium in 

interval 3 is about the same as for intervals 1 and 2. 

Scattered lenses of mineralized siltstone and sandstone 

in interval! (fig. 5) are about 1-4m thick and less than 100 

m in strike length. They make up as much as 8 percent of 

nine measured sections along a strike length of 18 km; 
8 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRIS10 RANGE, COLO. 

UPPER FAN 

Mudflow deposits 

······················································ 

-M 

LOWER FAN 

Braided-stream deposits 

--- --- --- --- --- --- --- --- ---

Debris-flow and 

sheetflo~- ?~p-~s_i~~ ____ _ 

MOSTLY MARINE 

LOWER FAN 

Braided-stream deposits 

UPPER FAN 

--~ 

Debris-f!?Y: _c!~I?9~~t- _-- tiii!Jjj;J!F 

LOWER FAN 

Braided-stream deposits 

UPPER FAN 

Debris-flow deposits 

LOWER FAN 

Braided-stream deposits 

-M 

._ 

-M 

~ 

EXPLANATION 

LITHOLOGY 

(Keyed to width of box) 

Conglomerate 

Conglomeratic sandstone 

Sandstone 

Siltstone and shale 

Limestone--M, marine fossils 

COLOR 

Dark red 

Red 

Light red 

Pink 

Gray, gray-green 

Mineralized bed 

METERS 

100 

50 

0 
DESCRIPTION OF COPPER-URANIUM OCCURRENCES 9 

SOUTH NORTH 

2 3 4 5 6 7 8 9 10 

*~~*·~@·®~~**~~*·~@•®~~**~~®·@@•m~~**~~®·@@>®~~~IMETERS 

200 

0l 

<il 

;> 100 ... 

~ 

..s 

t:'") 

50 

e<:S 

;> 

.... 

~ 0 

..s 

... . ~ 

EXPLANATION 

Sangre de Cristo Formation Minturn Formation ltz!J Lower fan braided-stream deposits ~ Marine sand shoal deposits 0 

I 

2 

I 

3 

I 

4 

I 

5 

I 

Minturn Formation 

Lower fan braided-stream deposits 

Delta-front deposits 

Tidal-flat deposits 

Marine carbonate biostromeM, marker limestone 

Mineralized extent of interval 2 

Mineralized lens, intervals 1 and 3 

KILOMETERS 

Vertical exaggeration, 25X 

Figure ;5. Distribution of mineralized rock and general depositional environments in the upper part of the Minturn Formation and lowermost Sangre de Cristo Formation, Mt. Adams to Mt. Owen, Colorado. Mineralized lenses (arrows) shown only on lines of measured sections. 

Sections based on Clark and Walz (1985); numbers correspond to their sections, located on figure 3. 

however, no mineralized lenses were found in two sections. 

Analysiis of 10 composite samples, each from a different 

mineralized lens, yielded a range of 30-8,500 ppm Cu and 

4-951 ppm U; median values were about 100 ppm Cu and 

135 ppm U. Assuming a downdip extension of 1 km, the 

aggregate amount of rock in mineralized lenses is estimated 

at about 1 million metric tons and the estimated average 

grade (arithmetic mean value) is 0.026 percent Cu and 0.025 

percent U30 8. 

Figure 4 (facing page). Distribution of mineralized rock, general 

depositional environments, and rock color in measured sections at 

the King Midas claims, Crestone Needle, Colorado. Left section begins at main adit, end of jeep trail; right section located on ridge 

approximately 500 m north of adit. 

The distinctive gray to pink cross-bedded sandstone 

and siltstone of interval 1 was divided into three facies: 

marine sandstone (interpreted as sand-shoal deposits by 

Clark and Walz, 1985), delta-front and prodelta marine 

sandstones, and braided alluvial sandstones (fig. 5). Mineralized lenses of gray silty, limey, shaly, and, less commonly, 

sandy rock are found in each facies. The lenses are typified 

by two that occur about 5 and 30m below the top of the Minturn Formation on Hermit Peak. The upper lens is composed 

of calcareous siltstone in delta-front deposits; it contains 

abundant plant remains and molds of brachiopods and 

crinoid columnals. The lower lens is composed of sandstone 

within otherwise unmineralized pink arkosic sandstone beds 

interpreted as braided-stream deposits. A composite sample 

of the upper siltstone lens yielded 2,500 ppm Cu but only 14 
10 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRISTO RANGE, COLO. 

ppm U, even though the lens is anomalously radioactive (300 

cps). A composite sample of the mineralized sandstone lens 

contained 426 ppm Cu and 209 ppm U. 

Mineralized siltstone and shale of interval2 (fig. 5) may 

contain about 500,000 metric tons at a possible average 

grade of 0.025 percent Cu and 0.025 percent U30 8. Analyses 

of seven composite samples from six localities along the 

11-km strike length of the bed (sections 4-9, fig. 5) gave 

wide ranges of 86-537 ppm Cu and 6-728 ppm U, making 

estimates of grade error-prone. Although the mineralized 

interval is anomalously radioactive at many places and can 

be traced with a scintillometer, it may not be completely continuous. Median values are about 190 ppm Cu and 105 ppm 

U, well above those for some samples analyzed, and average 

values representing grade are even higher owing to the 

presence of individual high values. Tonnage was calculated 

assuming a conservative downdip extension of 1 km. Actual 

downdip extension might be 3-4 km, which would yield a 

tonnage three to four times that given. 

Dark-gray siltstone and shale of interval 2 contains 

carbonaceous plant remains and was probably deposited in a 

delta-front environment (Clark and Walz, 1985). It is 

overlain by a continuous marker limestone that contains 

shallow-marine fossils, including crinoid columnals, 

brachiopods, and stromatolites. Commonly, the mineralized 

interval extends into the lower, silty part of the marker limestone, but at some localities the mineralized inteiVal is 

separated from the marker limestone by as much .as 1 m of 

unmineralized gray-pink arkosic sandstone. At sections 4 

and 9 (fig. 5), the mineralized interval extends into sandstone 

below. At section 6, only the lower, silty part of the marker 

limestone is mineralized; the beds immediately below have 

been oxidized red and are unmineralized. Evidently, 

mineralized interval2 transects bedding slightly. The overall 

continuity of the mineralized interval is due to deposition of 

laterally extensive carbonaceous siltstone and shale in a 

marine environment and preservation from later erosion. 

Subsequent deposition by aggrading fan deltas and streams 

assured preservation. 

Below the marker limestone, braided alluvial sandstones and conglomerates of interval 3 are intertongued with 

delta-front, prodelta, and tidal-flat sandstones, siltstones, 

and shales and thin marine carbonate biostromes (fig. 5). 

These rocks were deposited in fining -upward cycles by prograding fan deltas in the Minturn Formation (Lindsey and 

others, 1985). Thin (1-2m) mineralized lenses of siltstone 

and some shale are scattered throughout the upper Minturn; 

21 such lenses were mapped below the marker limestone 

(fig. 5) and many others doubtless occur between sections. 

Within this interval, mineralized lenses occur in every 

depositional environment from braided alluvial to marine 

biostrome. The lenses themselves commonly extend only a 

few hundred meters, but they appear to be ahnost as 

numerous as those above the marker limestone, and in nearly 

the same variety of depositional environments. 

A mineralized lens in the upper part of the Minturn Formation at an elevation of 12,300 feet on the east spur of peak 

13,325, located about 1 km northeast ofMt. Adams, is noteworthy for a clearly interpretable sequence of depositional 

environments and for localization in carbonaceous host rock 

(fig. 6). The depositional sequence consists of delta-front 

sandstones containing marine invertebrate fossils and rare 

shark's teeth, gray stromatolitic calcareous shale deposited 

in the intertidal zone, the mineralized. lens of dark-gray to 

black carbonaceous shale deposited in nearshore wetlands, 

and red braided alluvial sandstones arranged in 

fming-upward cycles. Trunks of Calamites occur in place, 

still rooted in the wetland shale and enveloped by overlying 

alluvial sand that inundated them. The mineralized shale 

contains about 1 percent organic carbon and as much as 

1,470 ppm Cu. Malachite stains are visible on the outcrop. 

Lead (324 ppm) is also anomalous. 

Nearly all of the mineralized localities (in all three 

intervals) between Mt. Adams and Mt Owen are located in 

gray (or a color variant of gray) beds in the upper part of the 

Minturn Formation. A few occurrences were found in thin 

gray siltstone and shale interbedded with redbeds of the 

lower Sangre de Cristo Formation. Only near Mt Owen are 

the principal mineralized beds located in the lowermost part 

of the Sangre de Cristo Formation (above the level of section 

in fig. 5); there, gray marine strata interfmger with red fluvial 

strata of the Sangre de Cristo Formation. Where redbeds are 

locally abundant in the upper Minturn, as for example within 

a 50-m interval located immediately below the marker limestone at section 6, no mineralized beds were found. 

COPPER IN VEINS 

Quartz and metal sulfide veins containing copper minerals cut redbeds of the Sangre de Cristo Formation at a variety of locations in the northern Sangre de Cristo Range. 

Principal localities (fig. 3) are (1) quartz veins at the King 

Midas claims (KM), (2) quartz veins with malachite and 

azurite in the Middle Fork of Taylor Creek (TC), and (3) 

copper-iron sulfides in quartz-barite veins at the Rita Alta 

Mine (RM). All of these veins fill fractures; veins at the Rita 

Alta Mine occur in fractures located at the crests of folded 

beds. Many of the veins do not contain detectable uranium. 

The fracture-filling relation of copper-bearing veins 

indicates that their formation postdates Laramide deformation, and may even be contemporary with late Oligocene to 

recent Rio Grande rifting. Thus, their origin has little bearing 

on that of the redbed copper -uranium deposits described 

above. Although these vein deposits offer the possibility for 

collection of fluid-inclusion data on temperatures and salinities, such data would be irrelevant to the origin of the redbed 

deposits. The veins are described briefly here to substantiate 

their post-redbed-copper interpretation. 

Small quartz veins occupy fractures in the upper part of 

the Minturn Formation at the King Midas claims. A small 
DESCRIPTION OF COPPER-URANIUM OCCURRENCES 11 

Figure 6. Sequence of depositional environments for mineralized shale of the upper part of the Minturn Formation. From bottom to top 

of picture, marine sandstone (m), stromatolitic limestone (Is) and shale of tidal-flat environment, mineralized shale (sh) of nearshore 

wetland environment, and braided alluvial sandstone ( ss ). Man is pointing to Calamites trunk, rooted in coastal swamp shale. Note 

stromatolite (s) structure in sample, left foreground. Located at lowermost mineralized lens shown on section 2, figure 5. 

tear fault of probable Laramide age passes within a few 

meters of the main adit. Quartz veins collected underground 

contained small amounts of copper (as malachite) and uranium (Ellis and others, 1983, p. 96). 

Fractures ftlled with quartz, calcite, and malachite (as 

observed in adits and samples on prospect dumps) in the 

Sangre de Cristo Formation in the Middle Fork of Taylor 

Creek are typical of fracture-controlled copper mineralization observed at scattered localities in the northern Sangre de 

Cristo Range. No primary (sulfide) minerals and no anomalous radioactivity have been observed there. At the Taylor 

Creek prospects, sandstone beds of the Sangre de Cristo Formation have been rotated to near-vertical position and extensively fractured and sheared during emplacement of the 

supetjacent Laramide Spread Eagle Peak thrust plate. The 

Taylor Creek veins are located only ~ km south of the 

Verde Creek area, which contains many quartz-chalcopyrite-galena-barite veins of probable hydrothermal origin in 

fractured and faulted Precambrian rocks (Ellis and others, 

1983, p. 18, 80-84). 

Veins at the Rita Alta Mine (Bagg, 1908) occur in both 

steeply dipping and flat fractures in the hinge of a small anticline between two larger folds; clearly, the Rita Alta deposit 

postdates Laramide folding. The steep fractures in the anticlinal hinge may represent local extension in the crest of the 

anticline during folding; the flat fractures developed perpendicular to the minimum direction of stress during folding. 

Fractures of both orientations are filled with barite, quartz, 

chalcopyrite, pyrite, bornite, and sparse azurite (Ellis and 

others, 1983, p. 57). 
12 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRISTO RANGE, COLO. 

20 

(J) 

(l) 1 5 0.. 

E 0.0 ro 

(J) 

-0 10 I... -0.2 (l) 

..Q 

E 

::::::1 

z -0.4 5 

-0.6 0 

0 ..... 0 8 -2.0 -1.0 0.0 1.0 c 

(l) -0.8 

A u 

I... 0 000 

Log1 0 of organic C in percent (l) 

0... 

c 00 CO> 0 -1.0 (/) 8 0 0 ro 

..... 0 0 

20 0 

..... -1.2 0 0 

-0 00 

0 (J) 0 <IDO o) -1.4 (l) 1 5 0.. 0 

E _j CD 0 ro -1 .6 (J) 

-0 10 CDa:::D I... 

(l) 

-1.8 ..Q 

E 

::::::1 

z 5 

-2.0 CD OCD 

0 -2.2 

-3.0 -2.0 -1.0 0.0 0.0 .2 .4 .6 .8 1.0 

B Log1 0 of total S in percent c Log1 0 of total Fe in percent 

Figure 7. Frequency distributions for A, organic carbon and B, total sulphur, and C, scattergram showing total sulphur versus 

total iron in mineralized rocks of the Minturn and Sangre de Cristo Formations. Total iron calculated from total Fe as Fe20 3. 

CHEMICAL COMPOSITION 

COMPOSITION AND CLASSIFICATION OF 

MINERALIZED ROCKS 

The major-element composition of mineralized rocks 

sampled from the Minturn and Sangre de Cristo Formations 

is mostly comparable to that of unmineralized sandstones 

(table 1) even though the mineralized rocks (40 siltstones, 14 

sandstones, 8 shales, and 3 silty limestones) are finer 

grained. Both groups are chemically and texturally immature, contain abundant feldspar and chlorite, and contain 

comparable amounts of AI20 3, CaO, total Fe as F~0 , 

Na20, K20, and MnO. Although the median value of Si02 

is lower for mineralized rocks and medians for MgO, Ti02, 

and P205 are higher by roughly one and one-half to two 

times, the ranges of these oxides overlap. Large differences 

in pre-mineralization metal content between the two groups 

would not be expected. 

Mineralized rocks are not particularly rich in organic 

carbon (median value of 0.40 percent, range of <0.02-1.77 

percent) or total sulphur (median value of 0.04 percent, 

range of <0.01-0.48 percent) (table 1). Most organic carbon 

values are below 1 percent (log10 value of 0, fig. 7A). The 

frequency distribution of total sulphur is bimodal (fig. 7 B), 

with the lower mode below the limit of detection at <0.01 

percent (log10 value of -2) and the higher mode at 0.1 percent (log10 value of -1). Low values for sulphur and lack of 

correlation between sulphur and iron indicate that only a 

small part of total iron is present as pyrite (fig. 7 C). Also, 

sulphur probably occurs as sulfate in some mineralized rocks 

and thus is not in pyrite. Preservation of both organic carbon 
CHEMICAL COMPOSffiON 13 

and pyrite in some mineralized rocks, however, indicates 

local isolation from oxidizing fluids. 

Comparison of the metals content of mineralized rocks 

with that of unmineralized sandstone in the Minturn and 

Sangre de Cristo Formations (table 1) reveals anomalous 

values for Cu, Mo, Pb, Zn, V, and U. Silver values above 1 

ppm in mineralized rocks are probably also anomalous 

because silver is seldom detected at that level in unmineralized rocks; however, no data for comparison with unmineralized sandstone are available. Median values (and ranges) 

for mineralized rocks are 161 (2-8,500) ppm Cu, 5 (<2-43) 

ppm Mo, 51 (5-7,480) ppm Pb, 52.7 (3.55-1,080) ppm U, 

110 (38-1,730) ppm V, and 100 (45-520) ppm Zn (table 1). 

Although the median value for vanadium is near the background level of unmineralized sandstone (100 ppm), maximum metal values are clearly anomalous. Anomalous values 

for cobalt, which occurs in some sediment-hosted copper 

deposits (Kirkham, 1989), were not found in the Sangre de 

Cristo occurrences. 

(/') 

Q) 

0.. 

1 5 

~ 10 (/') 

4-

0 

!.... 

Q) 

..c 

E 

::I 

z 

A 

(/') 

5 

0 

20 

~ 15 

E 

C'O 

(/') 

4-

0 10 !.... 

Q) 

..c 

E 

::I 

z 5 

0 

B 

-1 

-1 

N=65 samples 

0 2 3 4 

Log1 0 of Cu in ppm 

N=42 samples 

50 ppm 

l 

0 2 

Log1 0 of Cu in ppm 

The values for copper and uranium in mineralized rocks 

(2-8,500 ppm Cu, 3.55-1,080 ppm U) are nearly all above 

those for unmineralized sandstone (0.4-34 ppm Cu, 

0.83-7.85 ppm U) (figs. 8A-D). The frequency distribution 

of copper in mineralized rocks is skewed toward high values 

approaching 10,000 ppm (log10 value of 4) Cu (fig. SA). The 

frequency distribution of uranium in mineralized rocks has 

modes at about 10, 100, and perhaps 1,000 ppm (log10 values 

of 1, 2, and 3, respectively, fig. 8C); all are above background. In contrast to mineralized rocks, frequency distributions for copper and uranium in unmineralized sandstone are 

leptokurtic and nearly symmetrical, with no anomalously 

high values (figs. 8B, D). As noted in the "Sampling and analytical methods" section, rocks were considered mineralized 

if they contained at least 50 ppm Cu or 30 ppm U, but not 

necessarily both. These threshold values are above the range 

of background values for unmineralized sandstone (figs. 8B, 

D) and roughly 10 times median background values, thus 

making inclusion of unmineralized rocks unlikely. 

(/') 

Q) 

0.. 

15 

~ 10 (/') 

4-

0 

!.... 

Q) 

..c 

E 

::I 

z 

c 

(/') 

5 

0 

-1 

20 -

~ 15 -

E 

C'O 

(/') 

4-

0 10 ~ !.... 

Q) 

..c 

E 

::I 

z 5 -

0 

-1 

D 

N=65 samples 

0 

-

r 

0 

2 

Log1 0 of U in ppm 

-

N=42 samples 

30 ppm 

l 

I I 

2 

Log1 0 of U in ppm 

3 4 

Figure 8. Frequency distributions for A, copper in mineralized rocks, B, copper in unmineralized sandstones, C, uranium in mineralized 

rocks, and D, uranium in unmineralized sandstones of the Minturn and Sangre de Cristo Formations. 
14 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRIS'IO RANGE, COLO. 

30 

25 

tJ) 

~ 20 

E 

ro 

tJ) 

4-

0 15 I.... 

Q) 

..0 

E 

::J 

z 10 

5 

0 

A 

tJ) 

~ 10 

E 

ro 

tJ) 

4-

0 5 I.... 

Q) 

..0 

E 

0 

<Th/U 

N=65 samples 

2 

Th/U 

3 

N=42 samples 

4 

::J 

z 0 rr .. or~~~~~~~~~~~~~~~~ 

0 2 3 4 5 6 

Th/U 

B 

tJ) 

Q) 

D.. 

E 

1 5 

~ 10 4-

0 

I.... 

Q) 

..0 

E 

::J 

z 

c 

tJ) 

Q) 

D.. 

E 

ro 

tJ) 

4-

0 

I.... 

Q) 

..0 

E 

::J 

z 

D 

5 

1 5 

10 

5 

0 

0 

0 

N=65 samples 

2 3 4 5 

Log1 0 of Fe/Cu 

N=42 samples 

2 3 4 5 

Log1 0 of Fe/Cu 

Figure 9. Frequency distributions for A, Th/U in mineralized rocks, B, Th/U in unmineralized sandstones, C, Fe/Cu in mineralized rocks, 

and D, Fe/Cu in unmineralized sandstones of the Minturn and Sangre de Cristo Formations. 

Evidence for enrichment (or depletion) of copper and 

uranium in mineralized rocks and Uillllineralized sandstones 

was examined by means of ratios. Ratios have advantages 

over values for individual elements in that they are less sensitive to variations in the abundance of individual minerals, 

such as might occur if rocks contain varying amounts of 

detrital iron minerals, carbonate, chlorite matrix, or alteration products. Ratios are also less affected by the tendency 

toward negative correlation of major constituents (closure 

effect ofChayes, 1960). Typically, continental crustal rocks 

have Th/U values of two to seven (Rogers and Adams, 

1972), whereas uranium-mineralized rocks have much lower 

Th/U. Values of Th!U above seven might suggest depletion. 

Although we lacked guidelines for metal ratios involving 

copper, we experimented with Fe/Cu and Ni/Cu. Judging 

from comparable values in mineralized and unmineralized 

rock, values for iron and nickel are not believed to have been 

affected by mineralization. Values of Th/U, Fe/Cu, and 

Ni/Cu are fairly uniform in unmineralized sandstones of the 

Minturn and Sangre de Cristo Formations, as would be 

expected if neither appreciable enrichment nor depletion had 

taken place. These metal ratios for unmineralized sandstones 

were used as a basis for comparison with those for mineralized rocks. 

The frequency distribution of ThiU in mineralized 

rocks is strongly skewed and appears to be bimodal or 
CHEMICAL COMPOSITION 15 

polymodal (fig. 9A). A major mode below 1 is strongly 

indicative of uranium enrichment. Values of Th/U above 

unity in mineralized rocks overlap the range of unmineralized sandstones, which have normal crustal values of 

Th!U=l--6 (fig. 9B). No uranium depletion was identified. 

The precise values of ThiU below unity could not be determined because delayed-neutron radiation counting of thorium is masked by high uranium. Only less-than ( <) values, 

higher than the probable actual values, were reported. These 

"<Th" values represent maximum values, and resulting 

<Th!U values are also maximum values. Thus, the ThiU 

mode below unity may be even lower than shown. 

. 

. 

In contrast to metals that appear to have been concentrated by mineralization, iron (total Fe as F~0 ) and nickel 

are present at levels comparable to those found in unmineralized sandstones of the Minturn and Sangre de Cristo Formations (table 1). Thus Fe/Cu and Ni/Cu should reflect 

enrichment (or depletion) of copper. The frequency distributions of log10 of Fe/Cu in mineralized rocks (fig. 9C) and 

unmineralized sandstones (fig. 9D) illustrate enrichment in 

copper in mineralized rocks. In contrast to Th/U, the frequency distribution for log10 of Fe/Cu in mineralized rocks 

appears to be unimodal and somewhat overlaps that of 

unmineralized sandstones, indicating a continuous range 

.... . 

. 

. .... .... ........... t ..- • .A........ 

. 

A 

::J 

......... 

..c 

IB 

. 1 

10 r · 

.1 

.001 

. 

........ • .... ...... W" .... ............ .......... . . ... • ' ,I • .I .. 10 100 1000 10000 100000 

Fe/Cu 

''' 'I I 'I , 'I . . 'I . . .. ~ 

0 

,I I ••••• I . .. I I 

.01 .1 10 100 

Ni/Cu 

EXPLANATION 

Mineralized rocks 

• Total S ~ 0.01 percent 0 Unmineralized sandstones 

..A.. Total S > 0.01 percent 

Figure 10. Scattergrams showing A, Th/U versus Fe/Cu and B, Th/U versus Ni/Cu for mineralized rocks and onmineralized sandstones of the Minturn and Sangre de Cristo Formations. 
16 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRIS10 RANGE, COLO. 

from unmineralized to copper-mineralized rock. No obvious 

copper depletion was identified from frequency distributions, but the possibility of small depletion of copper in individual samples cannot be excluded. The ratios ofFe/Cu were 

tran~formed to logarithms because untransformed ratios 

have a wide range. Skewness of Th/U, Fe/Cu, and Ni/Cu 

indicates that all should be transformed to logarithms, or 

plotted on logarithmic scales, for further analysis. Logarithmic scales were used to summarize ratio data (fig. 10). 

Relationships among mineralized and unmineralized 

rocks are well defmed by plots of Th/U versus Fe/Cu (fig. 

lOA). Mineralized rocks are divisible into two groups: (1) 

highly uranium mineralized (distinguished by Th!U<l) and 

(2) not appreciably uranium mineralized (Th!U=l-3). Both 

groups were subjected to varying degrees of copper mineralization. Neither copper nor uranium shows a preference 

for groups classified by sulphur content (>0.01 percent and 

s;O.Ol percent). Thus, copper, uranium, and total sulphur 

values are independent of one another. Unmineralized 

sandstones plot outside both groups; Fe/Cu values form a 

continuum with values for mineralized rocks (fig. lOA). A 

plot of 1biU versus Ni/Cu gave nearly identical results 

(fig. lOB). 

RELATIONSIDPS AMONG ELEMENTS 

Relationships among organic carbon, total sulphur, 

and metals in mineralized rocks were investigated by 

means of correlation coefficients (table 2) and R-mode factor analysis (Cooley and Lohnes, 1962, p. 151-172). The 

methodological approach has been described more completely elsewhere (Lindsey, 1985). Mter some experimentation, only 12 elements of interest in this investigation 

were included in the data matrix to simplify statistical analysis and interpretation. Values for all elements except 

cobalt and nickel were transformed to logarithms to reduce 

skewness of frequency distributions. The 12 by 12 matrix 

of correlation coefficients was solved for twelve roots 

(eigenvalues). Six eigenvalues are near unity or greater (fig. 

llA) and account for 80 percent of the total variance (fig. 

llB). Six uncorrelated principal components were defined 

by an initial matrix of factor loadings. A fmal matrix of 

loadings (table 3) was derived by the Varimax criterion 

(Harman, 1960) and used as a basis for factor interpretation 

(table 3). The six-factor model accounts for 70 percent or 

more of the variance of each element, as shown by the 

communalities (table 3). 

Criteria for selection of the factor model included: (1) 

eigenvalues must be near one or greater, (2) the total of all 

eigenvalues must account for more than one-half the total 

variance, (3) the model must account for more than one-half 

the variance of each element (communalities >0.5), and (4) 

the model must be amenable to interpretation. The six-factor 

model meets or exceeds these requirements. 

Inspection of the correlation coefficients (table 2) 

reveals that, in general, correlations are weak (r<0.58, 

commonly r=0.2-0.4), although even a coefficient of r=O.l 

is significant at the 0.05 level for N=65 (that is, the probability of noncorrelation is 1 in 20). Such low correlations may 

arise from (1) independent occurrence of elements at the 

scale of sampling and (2) a high proportion of analytical 

Table 2. Correlation coefficients for selected elements in mineralized rocks of Minturn and Sangre de Cristo Formations 

[Coefficients bas.e~ on 65 values for each element; all values except Co and Ni transformed to logarithms prior to calculation. pet, percent; 

ppm, parts per rmlhon] 

log COrg log ST Co log Cr log Cu log Mo Ni log Pb log U log V log Zn 

pet pet ppm ppm ppm ppm ppm ppm ppm ppm ppm 

log Fer 0 3 pet 0.29 -0.05 0.39 0.21 0.01 -0.03 0.54 -0.18 -0.31 0.07 -0.04 

log C0 rg pet 0.31 0.23 0.07 -0.07 -0.04 0.39 -0.02 -0.18 -0.10 0.20 

log ST pet -0.07 0.08 0.04 0.08 0.11 0.07 -0.16 -0.11 -0.07 

Co ppm 0.28 0.29 0.26 0.66 0.10 0.21 0.25 0.11 

log Cr ppm 0.22 -0.01 0.28 0.24 0.02 0.58 -0.09 

log Cu ppm 0.26 0.15 -0.16 0.02 0.17 0.18 

log Mo ppm -0.04 0.27 0.27 0.09 0.05 

Ni ppm -0.04 -0.02 0.14 0.19 

log Pb ppm 0.35 0.48 0.18 

log U ppm 0.57 -0.02 

log V ppm -0.04 
CHEMICAL COMPOSffiON 17 

Table 3. A six-factor geochemical model for mineralized rocks of Minturn and Sangre de Cristo Formations 

Varimax factor loadings on orthogonal axes 

Variable Factor 1 Factor 2 Factor 3 Factor 4 Factor 5 Factor 6 Communality 

log FeT 03 pet 0.80 -0.11 -0.18 -0.01 -0.18 -0.01 0.72 

log Corg pet 0.49 -0.08 -0.06 0.55 0.31 -0.23 0.70 

log ST pet -0.06 -0.01 0.08 0.91 0.09 0.06 0.85 

Co ppm 0.77 0.19 0.36 -0.10 0.08 0.18 0.80 

log Cr ppm 0.28 0.69 -0.31 0.18 -0.18 0.37 0.84 

log Cu ppm 0.08 0.02 0.17 0 0.13 0.93 0.91 

log Mo ppm 0.02 0.05 0.86 0.12 -0.02 0.20 0.80 

Ni ppm 0.86 0.10 -0.03 0.10 0.16 0.07 0.79 

log Pb ppm -0.14 0.71 0.26 0.16 0.26 -0.29 0.77 

log U ppm -0.09 0.56 0.53 -0.33 0.01 -0.14 0.73 

log V ppm 0.12 0.90 0.04 -0.16 -0.08 0.12 0.87 

log Zn ppm 0.06 0 -0.01 -0.03 0.96 0.12 0.93 

Interpretation of model 

Factor I (FeT03 , Co, and Ni}-Concentration of iron oxides by sedimentation and diagenesis. 

Factor II (V, Pb, Cr, and U)-Mineralization by vanadium, lead, chromium, and uranium. 

Factor Ill (Mo and U)-Mineralization by molybdenum and uranium. 

Factor IV (ST and C0 r

9

)-Concentration (or preservation from oxidation) of sulfides and organic matter. 

Factor V (Zn)-Unique factor for zinc mineralization. 

Factor VI (Cu)-Unique factor for copper mineralization. 

variance (error) relative to total variance. Because the mineralized rocks contain highly anomalous amounts of Cu, Pb, 

U, V, and Zn, and therefore have high natural variance for 

these elements, analytical error should not contribute greatly 

to their low correlation. Thus, for example, the low correlation of copper and zinc with other elements probably reflects 

their independent occurrence at the scale of sampling, which 

consisted of composite chips from mineralized intervals 

about 0.3-1 m thick. 

Six independent factors are defined in the model 

selected for interpretation (table 3). The six factors are interpreted as effects of sedimentation and diagenesis (factor 1), 

mineralization (factors IT, III, V, and VI), and concentration 

(or preservation) of sulfides and organic matter (factor IV). 

Factor I has high loadings for total iron, cobalt, and 

nickel and evidently reflects concentration of these metals, 

probably in iron oxide minerals and in chlorite matrix, by 

sedimentation and diagenesis. Iron, cobalt, and nickel were 

not affected by copper or uranium mineralization, and thus 

their use in metal ratios (Fe/Cu, Ni/Cu) to detect enrichment 

or depletion appears to be appropriate. 

Factors II, III, V, and VI represent effects of mineralization. Vanadium, lead, chromium, and uranium have high 

loadings on factor II, reflecting relatively high correlations 

for V-Cr, V-Pb, and V-U; chromium, lead, and uranium are 

common in various vanadium minerals, and their correlation 

with vanadium, but only weakly or not at all with each other, 

suggests that several vanadium minerals may be present in 

small amounts. Molybdenum and uranium have high loadings on factor lll; however, molybdenum is only weakly correlated (r=<0.27) with uranium (and with cobalt, copper, and 

lead) (table 2). Factor III may reflect minor enrichment in 

molybdenum during mineralization. Factors V and VI, 

termed "unique factors" because they have high loadings for 

only one element, represent zinc and copper mineralization, 

respectively. According to the model, mineralization by zinc 

and copper was spatially separate (at the scale of sampling) 

and separate also from vanadium-chromium-lead-uranium 

mineralization. This conclusion is in accord with evidence 

presented previously that copper and uranium have different 

patterns of enrichment (fig. 10). 

Factor IV represents concentration or preservation of 

sulfides and organic matter. Total sulphur and, to a lesser 

extent, organic carbon have high loadings on factor IV. 

Association with metals is nil as indicated by low factor 

loadings and by vanishingly small correlation coefficients 

with total sulphur. Noncorrelation of total sulphur and 

organic carbon with metals may be explained if metals have 

been redistributed from their original site of precipitation, 

even if redistribution was only local. Values from samples 

spanning the thickness of an entire mineralized bed (commonly, ~1 m) would mask metal redistribution by averaging 

values across the bed, and thus might yield low correlations 

between metal values and metal-ftxing elements such as 

sulphur and organic carbon. Because most of the enclosing 

strata, including gray beds in the Minturn Formation, contain 
18 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRISID RANGE, COLO. 

0 

0 

2 Q) 

::J 

-ro 

> 

c 

Q) 

Ol 0 0 LLi 

0 

0 

0 

0 

0 

0 

0 

0 2 3 4 5 6 7 8 9 10 1 1 

Factor 

A 

0 0 

Q) 0 u 0 c 

·~ .8 0 

ell 

> 0 ...... 

0 0 § .6 

·e 0 0 

Q. 

0 

0..4 0 Q) 

> ·.;; 

ell 

"3 

E .2 0 

::J 

u 

0 

0 2 3 4 5 6 7 8 9 10 11 

Factor 

B 

Figure 11. Plots showing A, eigenvalues and B, cumulative proportion of total variance explained by first ten principal factors. 

hematite and thus represent an overall oxidizing 

environment, factor IV is interpreted tentatively as an 

expression of preservation of sulfides (probably, as minor 

amounts of pyrite and copper sulfides) and organic matter 

from oxidation. 

PROPOSED MODEL FOR 

MINERALIZATION 

A model for copper and uranium mineralization (fig. 

12) is proposed that requires (1) deposition of sediment containing organic carbon and pyrite in reducing environments, 

(2) local preservation of some reducing environments during 

redbed formation, and (3) introduction of copper and uranium in the surviving reducing environments. 

Depositional environments played a broad role in determining the distribution of mineralized host rock (fig. 12A). 

Marine depositional environments, where plant debris accumulated in silt or mud, were relatively stable and extensive 

compared to nearby alluvial environments. Mineralized 

rocks of interval 2 (approximately 11 km long) were deposited in nearshore-marine environments and preserved by 

continued sedimentation. Euxinic lagoonal or marine environments did not develop, however, and these would be 

required for deposition of a host bed like the Kupferschiefer. 

In contrast, the small (approximately 100-300 m wide) 

lenses of mineralized rocks at the King Midas claims were 

deposited near a shoreline that was dominated by alluvial 

sedimentation near rugged uplands. Local marine and wetland environments, where organic-rich host sediments were 

deposited, were subject to erosion and inundation by alluvial 

fans. 

As deposition in the central Colorado trough proceeded, 

mildly alkaline, oxidizing ground water altered arkosic sediment to redbeds in alluvial fans along the margin of the central Colorado trough (fig. 12B). In later Pennsylvanian and 

Permian time, redbed alteration spread throughout most of 

the basin. Redbed alteration left remnants of unoxidized sediment buried in the lower and central parts of the basin fill, 

however. Organic matter and minor pyrite preserved in these 

sediments provided a reducing and adsorptive trap for metals 

that were introduced later by brines (fig. 12C). 

COPPER 

Brines rich in chloride are the most likely agent of copper transport in redbed sequences (Rose, 1989). Initial basin 

fluids may have evolved into oxidizing brines during passage through evaporites and redbeds (Eugster, 1989; Sverjensky, 1989) in basin fill of the Ancestral Rocky Mountains 

(fig. 1). If chloride ion is present, such brines are capable of 

transporting copper even if pH is neutral or weakly alkaline. 

Although no salt deposits are known near the Sangre de 

Cristo Range, the central Colorado trough does contain gypsum, anyhdrite, and halite. Chloride-sulfate brines might 

well have originated from contact with the Swissvale Gypsum Member of Brill (1952), located about 600 m below the 

top of the Minturn Formation and about 50 km north of the 

mineral occurrences described here, and from the Middle 

Pennsylvanian Eagle Valley Evaporite, located farther north 

(inset map, fig. 12). Other gypsum- and salt-bearing formations of Jurassic, Triassic, and Permian age were not deposited over Pennsylvanian-Permian rocks in much of 

south-central Colorado (MacLachlan, 1972; Peterson, 1972; 

Rascoe and Baars, 1972). 

Brines driven updip by burial compaction and lateral 

expulsion or by thermal convection (for example, Jowett, 

1986) from the central Colorado trough (fig. 12C) or its buried basin fill after Permian time, would necessarily have 
PROPOSED MODEL FOR MINERALIZATION 19 

equilibrated with hematite, carbonate, and silicate minerals 

in oxidized rocks as described by Rose (1989). After 

equilibration, the Eh and pH of brines would be within the 

hematite stability field, with pH additionally buffered by 

reaction with silicate and carbonate minerals. Equilibration 

would have occurred not only in redbedls but also in gray, 

hematite-bearing beds of the Minturn Formation. Reducing 

environments were confined to local beds and lenses containing organic matter and minor pyriite, mostly in the 

Minturn Formation. Carbonate was pres(~nt initially as lime 

mud in marine strata of the Minturn Formation and in paleocaliche deposits in the Sangre de Cristo Formation; 

eventually, it was redistributed widely as calcite cement in 

both the Minturn and Sangre de Cristo Formations. 

Hypothetically, chloride-sulfate brines from the central 

Colorado trough migrated into basin-margin fill, where they 

could have leached copper and zinc from sandstone. Copper 

is soluble as a cuprous chloride complex over a wide range 

of pH; zinc is soluble as a sulfate complex to a pH of 8.6 

(Rose, 1989). Where mineralizing brines encountered 

reducing conditions in beds containing organic matter and 

pyrite, copper and zinc were precipitated (probably as 

sulfides). After mineralization, sufficient: pyrite and organic 

matter remained to maintain a reducing environment in 

which sulfide minerals were preserved. Considering the 

apparent continued availability of reductants, the low level 

of mineralization suggests low metal concentrations in mineralizing brines. Perhaps the brines were too dilute or too 

alkaline to leach and transport large quantities of copper and 

other metals. 

The dilute nature of mineralizing lbrines is predicted 

from the sequence and geometry of s.trata deposited in 

southern Colorado prior to Late Cretaceous time. No Shale or 

evaporite that would provide an impetmeable barrier or 

source of soluble salts overlay the site of the mineralization 

(MacLachlan, 1972; Peterson, 1972; Rascoe and Baars, 

1972). Rising basinal brines could thus c:ontinue to the surface from Pennsylvanian to Early Cretaceous time, and convecting brine cells as proposed by Jowett (1986) for the 

central European Zechstein basin could not form. Temperature contrasts, required to drive brine convection, were probably smaller in the central Colorado trough than in the 

Zechstein basin because the pre-Laramid(~ geothermal gradient was probably much lower (<13 OC/km; Lindsey, 

Andriessen, and Wardlaw, 1986) than dwing Triassic rifting 

of the Zechstein. 

URANIUM 

Uranium occurs in residual organic matter in mineralized rocks but is only broadly associated with copper. At the 

scale of sampling, uranium is not correlated with copper, but 

with vanadium. The separate distribution of uranium and 

copper is probably related to different mechanisms of 

transport or precipitation. Whereas copp;r is most readily 

mobilized as complexes of cuprous (Cu+ ) chloride in brines 

(Rose, 1989), uranium is more generally mobile as uranyl 

(U+~ phosphate and carbonate complexes (Langmuir, 

1978). Copper is usually precipitated as sulfide minerals, 

whereas uranium may be adsorbed by organic matter 

(Nakashima and others, 1984). 

Uranium (as uranyl carbonate complexes) is soluble in 

alkaline, oxidizing waters such as those responsible for 

redbed formation. Redbed-fotming waters would have transported uranium downdip into local reducing environments 

where remnants of organic matter were preserved in the 

Minturn Formation (fig. 12B). There, uranium would have 

been fiXed by reaction with organic matter. Experimental 

evidence suggests that under the alkaline, oxidizing conditions of redbed formation, uranium is adsorbed by hematitic 

pigment; metals including uranium are held even more 

tightly in recrystallized hematite in older redbeds (Zielinski 

and others, 1983). Leaching and removal of uranium from 

redbeds might occur in fluids with low pH or high concentrations of metal-complexing anions (such as chloride or carbonate). If uranium was leached extensively by acid fluids, 

zones of bleaching in redbeds should be evident Bleached 

redbeds in the study area are not abundant, however. 

A second possibility is that uranium and copper may 

have been precipitated from the same fluid. Uranium would 

have been mobile as soluble uranyl phosphate or carbonate 

species (Langmuir, 1978) in the weakly alkaline, oxidizing 

chloride-sulfate brine postulated for copper transport. Soluble uranyl species can be fixed as urano-organic compounds 

without reduction at temperatures consistent with moderate 

burial, as demonstrated by· experiment with lignite at 

45-120 oc (Nakashima and others, 1984). In the uranium 

deposits studied here, uranium in mineralizing brines could 

have been fixed by organic (plant) matter that survived oxidation in the Minturn Formation. 

Vanadium, which is correlated with uranium in mineralized rocks, may have coprecipitated with uranium. Uranium, vanadium, and possibly molybdenum and chromium 

may have been remobilized from their original residences by 

mildly alkaline, oxidizing fluids. Remobilization may have 

led to local dispersion and reprecipitation as secondary minerals such as carnotite. Vanadate (V+5

) and uranyl carbonate 

species are stable over much of the same range of Eh and pH 

and can coprecipitate as carnotite at intermediate pH values 

in oxidizing environments (Langmuir, 1978). If lead was 

also remobilized, the fluid could not have contained sulfate 

because PbS04 is insoluble. If more than one secondary mineral precipitated under slightly different conditions, various 

minerals containing uranium, vanadium, molybdenum, 

chromium, and perhaps lead might be dispersed locally 

throughout mineralized beds. Such dispersion might explain 

the correlations U-V, V -Cr, and V -Pb and the weak correlation U-Mo. 
20 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRISID RANGE, COLO. 

UNCOMPAHGRE UPLIFT 

CENTRAL COLORADO TROUGH 

BASEMENT 

2000 

EXPLANATION 

Alluvial fan conglomerates ~ Anhydrite 

0 10 20 30 

Alluvial fan sandstones XXXX Carbonaceous siltstone, 

silty limestone, and shale KILOMETERS 

A 

Sandstone containing 

interbedded marine limestones 

UNCOMPAHGRE UPLIFT 

BASEMENT 

EXPLANATION 

CENTRAL COLORADO TROUGH 

2000 

Redbeds xxxx Local reducing environment 

(completely oxidized) 

Gray beds Ground-water flow 

0 10 20 30 

(partly oxidized) KILOMETERS 

Anhydrite 

B 

Figure ll (above and facing page). Proposed model for formation of redbed copper-uranium deposits in basin fill of the central Colorado trough: A, principal lithofacies shortly after deposition, B, formation of redbeds by ground water flowing through fan deposits during 

and shortly after deposition, C, mineralization by through-flowing dilute brines after redbed formation. 

CONCLUSIONS 

Reasons for small tonnages and grades of redbed copper deposits, as represented by those studied, may include 

the following: 

1) The thickness and lateral extent of reducing host 

rocks were restricted by alluvial and nearshore-marine environments that received an abundant supply of detritus. 

2) The size of reducing environments was limited further by oxidation during redbed formation. 

3) Adsorption in abundant hematite pigment probably 

limited mobility of metals in redbeds. 

4) Mineralizing brines were too dilute and alkaline to 

leach and transport large quantities of copper. 

5) The basin fill and cover rocks did not contain an 

impermeable cap of shale or evaporite to force brine to convect in cells and leach source rocks repeatedly. 

6) The geothermal gradient was probably insufficient 

to produce the necessary thermal contrasts between basin fill 

and adjacent uplands to promote strong fluid flow. 
REFERENCES CITED 21 

c 

UNCOMPAHGRE UPLIFT 

BASEMENT 

EXPLANATION 

Redbeds XXXX Mineralized lens 

(completely oxidized) 

Gray beds 

(partly oxidized) -+- Mineralizing brine flow 

Anhydrite 

KILOMETERS 

Early Permian-Pennsylvanian 

paleogeographic setting 

REFERENCES CITED 

Bachman, G.O., 1968, Geology of the Mockingbird Gap 

quadrangle, Lincoln and Socorro Counties, New Mexico, in 

Shorter contributions to general geology 1967: U.S. Geological 

Swvey Professional Paper 594-J, 43 p. 

Bagg, R.M., Jr., 1908, Some copper deposits in the Sangre de Cristo 

Range, Colorado: Economic Geology, v. 3, no. 8, p. 739-749. 

Brill, K.G., Jr., 1952, Stratigraphy in the Permo-Pennsylvanian 

Zl~ugogeosyncline of Colorado and northern New Mexico: 

Geological Society of America Bulletin 63, no 8, p. 809-880. 

Chay(~s. F., 1960, On correlation between variables of a constant 

sum: Journal of Geophysical Research, v. 65, no. 12, p. 

4185-4193. 

CENTRAL COLORADO TROUGH 

2000 

0 10 20 30 

KILOMETERS 

EXPLANATION 

Conglomerate and sandstone 

Anhydrite 

Salt 

Line of section for figures 12 A-C 

Clark, R.F.,1982, Stratigraphy, sedimentology and copper-uranium 

occurrences of the upper part of the Middle Pennsylvanian 

Minturn formation, Sangre de Cristo Mountains, Colorado: 

Golden, Colo., Colorado School of Mines M.S. thesis, 150 p. 

Clark, R.F., and Walz, D.M., 1985, Stratigraphic sections, 

depositional environments, and metal content of the upper part 

of the Middle Pennsylvanian Minturn Formation, northern 

Sangre de Cristo Range, Custer and Saguache Counties, 

Colorado: U.S. Geological Survey Miscellaneous Field Studies 

Map MF-1622-D, 2 sheets. 

Cooley, W.W., and Lohnes, P.R., 1962, Multivariate procedures for 

the behaviorial sciences: New York, John Wiley, 211 p. 

Dutton, S.P., 1982, Pennsylvanian fan-delta and carbonate 

deposition, Mobeetie Field, Texas panhandle: American 
22 COPPER AND URANIUM IN PENNSYLVANIAN AND PERMIAN ROCKS, SANGRE DE CRIS10 RANGE, COLO. 

Association of Petroleum Geologists Bulletin, v. 66, no. 4, p. 

389-407. 

Ellis, C.E., Hannigan, B.J., and Thompson, J.R., 1983, Mineral 

investigation of the Sangre de Cristo Wilderness Study Area, 

Alamosa, Custer, Fremont, Huerfano, and Saguache Counties, 

Colorado: U.S. Bureau of Mines Mineral Lands Investigations 

Report MLA-65-83, 190 p., 2 pis. 

Ensign, C.O., White, W.S., Wright, J.C., Patrick, J.L., Leone, R.J., 

Hathaway, D.J., Trammell, J.W., Fritts, J.J., and Wright, T.L., 

1968, Copper deposits in the Nonesuch Shale, White Pine, 

Michigan, in Ridge, J.D., ed., Ore deposits of the United States, 

1933-1967; the Graton-Sales Volume: American Institute of 

Mining, Metallurgical and Petroleum Engineers, New York, p. 

460-488. 

Eugster, H.P., 1989, Geochemical environments of 

sediment-hosted Cu-Pb-Zn deposits, in Boyle, R.W., and 

others, eds., Sediment-hosted stratiform copper deposits: 

Geological Association of Canada Special Paper 36, p. 

111-126. 

Gibson, Russell, 1952, Reconnaissance of some red bed copper 

deposits in the southwestern Unitl(d States: U.S. Atomic 

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Golightly, D.W., Dorrzapf, A.F., Jr., Mays, R.E., Fries, T.L., and 

Conklin, N.M., 1987, Analysis of geologic materials by 

direct-current arc emission spectrography and spectrometry, in 

Baedecker, P.A., ed., Methods of geochemical analysis: U.S. 

Geological Survey Bulletin 1770, p. A1-A13. 

Harman, H.H., 1960, Modem factor analysis: Chicago, lli., 

University of Chicago Press, 2nd revised edition, 471 p. 

Hatchell, W.O., Blagbrough, J.W., and Hill, J.M., 1982, 

Stratigraphy and copper deposits of the Abo Formation, Abo 

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Wells, S.G., eds., Albuquerque Country II: New Mexico 

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Heckel, P.H., 1977, Origin of phosphatic black shale facies in 

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American Association of Petroleum Geologists Bulletin, v. 61, 

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Jackson, L.L., Brown, F.W., and Neil, S.T., 1987, Major and minor 

elements requiring individual determination, classical whole 

rock analysis, and rapid rock analysis, in Baedecker, P.A., ed., 

Methods of geochemical analysis: U.S. Geological Survey 

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Johnson, B.R., Lindsey, D.A., Ellis, C.E., Hannigan, B.J., and 

Thompson, J.R., 1984, Mineral resource potential map of the 

Sangre de Cristo Wilderness Study Area, south-central 

Colorado: U.S. Geological Survey Miscellaneous Field Studies 

Map MF-1635-A, scale 1:62,500, 2 pis., 13-p. pamphlet. 

Johnson, K.S., 1976, Permian copper shales of southwestern 

Oklahoma, in Johnson, K.S., and Croy, R.L., eds., Stratiform 

copper deposits of the Midcontinent region, a sympOsium: 

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Jowett, E.C., 1986, Genesis of Kupferschief Cu-Ag deposits by 

convective flow of Rotliegende brines during Triassic rifting: 

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Jowett, E.C., Rydzewski, A., and Jowett, R.J., 1987, The 

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Kirkham, R.V., 1989, Distribution, settings, and genesis of 

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Kluth, C.F., 1986, Plate tectonics of the Ancestral Rocky 

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Langmuir, D., 1978, Uranium solution-mineral equilibria at low 

temperatures with applications to sedimentary ore deposits: 

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deposits in New Mexico, in Johnson, K.S., and Croy, R.L., 

eds., Stratiform copper deposits of the Midcontinent region, a 

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Lichte, F.E., Golightly, D.W., and Lamothe, P.J., 1987, Inductively 

coupled plasma-atomic emission spectrometry, in Baedecker, 

P.A., ed., Methods of geochemical analysis: U.S. Geological 

Survey Bulletin 1770, p. B1-B10. 

Lindsey, D.A., 1985, A gold-mineralized breccia zone at Kendall, 

North Moccasin Mountains, Fergus County, Montana, in 

Studies of mineralized intrusive complexes in north-central 

Montana: U.S. Geological Survey Professional Paper 1301, p. 

45-56. 

Lindsey, D.A., Andriessen, P.A.M., and Wardlaw, B.R., 1986, 

Heating, cooling, and uplift during Tertiary time, northern 

Sangre de Cristo Range, Colorado: Geological Society of 

America Bulletin, v. 97, no. 9, p. 1133-1143. 

Lindsey, D.A., Clark, R.F., and Soulliere, S.J., 1985, Reference 

section for the Minturn Formation (Middle Pennsylvanian), 

northern Sangre de Cristo Range, Custer County, Colorado: 

U.S. Geological Survey Miscellaneous Field Studies Map 

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---1986, Minturn and Sangre de Cristo Formations of southern 

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Lindsey, D.A., and Schaefer, R.A., 1984, Principal reference 

section for the Sangre de Cristo Formation (Pennsylvanian and 

Permian), northern Sangre de Cristo Range, Saguache County, 

Colorado: U.S. Geological Survey Miscellaneous Field Studies 

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Mack, G.H., Suttner, L.J., and Jennings, J.R., 1979, 

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