Geochemical Anomalies and 

Alteration in the 

Moenkopi Formation, 

Skull Creek, 

Moffat County, Colorado

GEOLOGICAL SURVEY PROFESSIONAL PAPER 761
Geochemical Anomalies and 

Alteration in the 

Moenkopi Formation, 

Skull Creek, 

Moffat County, Colorado

By R. A. CADIGAN

GEOLOGICAL SURVEY PROFESSIONAL PAPER 761

Geologic and geochemical reconnaissance of a 

formation and an area containing significantly 

higher background metal values than are 

commonly found elsewhere in the Colorado 

Plateau region

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 72-600255

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

Washington, D.C. 20402 - Price 70 cents (paper cover)

Stock Number 2401-00221
CONTENTS

Page

Abstract. ________________________________

Introduction. _____________________________

Geology._ ______________________________

Stratigraphy. __ _______________________

Structure^ ___________________________

Mineral occurrences and mining activity _ 

Alteration in the Moenkopi Formation_______

Geochemical studies,______________________

Analytical methods.___________________

Sampling methods.____________________

Distribution of elements. ______________

Factor analysis. ______________________

Summary and recommendations. ___________

References cited.. ______________________

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ILLUSTRATIONS

FIGURE 1. Index map of the northern part of the Colorado Plateau region,______________________________________________

2. Photograph of part of the interior of the Skull Creek anticline.-___________________-__----__-__-______----__--

3. Geologic sketch map of the Skull Creek anticline area _______________________________________________________

4-6. Photographs of Moenkopi strata:

4. Altered strata, southern scarp.____________________________________________________________________

5. Interbedded altered and unaltered strata, western scarp_ _____________________________________________

6. Altered strata, northern and western scarps.________________________________________________________

7. Photograph of the southeastern arc of the erosion scarp______________________________________________________

8. Map showing sampling traverses at localities A and B_______________________________________________________

9. Map showing sampling traverse at locality E___________________________________________»____________________

10. Photograph of altered Moenkopi strata west of Miller Creek water gap._______________________________________

11. Map showing sampling traverse at locality C, malachite showings at locality D, and sampled area at the Gartra 

prospect ___________________________________________________________________________________________

12. Map showing sampling traverse at Lone Mountain__________________________________________________________

13. Diagram showing comparison of metal content of altered rocks with Moenkopi average for the Colorado Plateau 

region.___________________________________________________________________________________________

14. Diagram showing comparison of metal content of the Skull Creek cupriferous zone samples with Moenkopi average. _

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TABLES

TABLE 1. Stratigraphic units present in the Skull Creek anticline area___________________________________________________

2. Results of geochemical analyses of 20 samples collected from locality A_________________________________________

3. Coefficients extracted from the matrix of correlation coefficients of values for elements sampled from the Skull Creek 

geochemical halo_______________________________________________-__________---_------_-----_------.----

4. Proportion of positive covariance of each metallic element with each factor. ____________________________________

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GEOCHEMICAL ANOMALIES AND ALTERATION IN THE MOENKOPI 

FORMATION AT SKULL CREEK, MOFFAT COUNTY, COLORADO

By ROBERT A. CADIGAN

ABSTRACT

The Moenkopi Formation of Triassic age exposed in the scarp 

encircling Skull Creek anticline in Moffat County, Colo., contains 

a thick altered layer of greenish-gray siltstone which is overlain 

by reddish-brown siltstone in most localities. Metal content of 

both the red and the altered rocks in the Skull Creek area is significantly higher than the geometric mean of metal content of 

Moenkopi strata in the Colorado Plateau region as a whole.

A very thin zone of enrichment at the upper contact of the 

green altered rock with the overlying red rock contains anomalous 

amounts of copper (100-5,000 parts per million) and other metals. 

Samples from the enriched zone are at least four times higher than 

the Moenkopi averages in contents of vanadium, chromium, 

copper, nickel, cobalt, silver, lanthanum, and boron.

Mercury is present in anomalously high amounts (geometric 

mean of all analyses, 2 ppm; maximum, >10 ppm) in the altered 

Moenkopi exposed in the southeastern erosion scarp area, the only 

area where the Moenkopi is completely altered. No enriched zone 

was found there.

Other anomalous occurrences of mineralization and alteration 

include a copper-uranium deposit in the base of the Jurassic Curtis 

Formation, conspicuous alteration in the Triassic Gartra Member 

of the Chinle, sulfide minerals containing anomalous amounts of 

arsenic, lead, and zinc in the top of the Pennsylvanian and Permian Weber Sandstone and in joints in the Triassic and Jurassic 

Glen Canyon Sandstone and the Chinle Formation, and anomalously high values of chromium, vanadium, gold, and silver at one 

locality in the Gartra Member.

Factor analysis of measurements of metal content of the enrichment zone suggests two major events: (1) Invasion of the rocks by 

metal-bearing solutions and alteration of part of the Moenkopi, 

and (2) an interaction at the contact between the red rocks which 

represent an oxidized environment and the green rocks which 

represent an invading reducing environment. This interaction results in a thin zone of enrichment of leached and redeposited 

metals mercury, copper, silver, uranium, and gold at the geochemical interface.

The area is recommended for further geochemical exploration.

INTRODUCTION

During investigation of the distribution of metallic 

elements in the Moenkopi Formation in the Colorado 

Plateau region (Cadigan, 1971a), anomalous concentrations of some metals, particularly copper and 

mercury, were found in samples from the Skull Creek 

anticline in the extreme northwestern part of Colorado. 

Field studies and further sampling in 1969-70 established the presence of geochemical anomalies in the

area as a whole, of which the most conspicuous was a 

copper anomaly associated with bodies of pale-greenishgray rocks within the normally red strata of the 

Moenkopi in the scarp surrounding the anticline. 

The metal anomalies and abnormal color relationships 

suggest epigenetic alteration of the formation in this 

area.

Skull Creek anticline (fig. 1) is in the northern part 

of the Colorado Plateau region, north of the settlement 

of Skull Creek, Colo. The area is shown on the U.S. 

Geological Survey's 7^-minute quadrangle maps, 

Lazy Y Point and Skull Creek, Colo., and occupies all 

of T. 4 N., R. 101 W., and parts of adjacent townships. It is approximately 11 miles (18 kilometers) 

northeast of the Rangely oil field. Unimproved roads, 

passable only in dry weather, lead into the central 

part of the anticline from U.S. Highway 40. Fourwheel-drive vehicles are recommended for off-highway 

use in the area.

GEOLOGY

STRATIGRAPHY

Rocks exposed in Skull Creek anticline range in 

age from Pennsylvanian to Cretaceous. Table 1 summarizes the stratigraphic column. Thickness and 

descriptive data are adapted from Thomas, McCann, 

and Raman (1945); those data for the Moenkopi 

Formation are somewhat modified on the basis of the

41°

FIGURE 1. Index map of the northern part of the Colorado 

Plateau region.

1
GEOCHEMICAL ANOMALIES AND ALTERATION, MOENKOPI FORMATION, SKULL CREEK, COLORADO

work by Schell and Yochelson (1966), who redefined 

the contact between the Moenkopi and Park City 

Formations, and by F. G. Poole (oral and written commun., 1969), who described and measured a section 

through the Moenkopi at Skull Creek.

The uppermost unit of the Park City Formation is 

exposed in the base of the southern scarp face west of 

Miller Creek water gap where the upper contact crosses 

the section line between sees. 28 and 29, T. 4 N., R. 

101 W. This unit is a limestone 4 inches (10 centimeters) 

thick which lies at the top of the tawny beds of Schell 

and Yochelson (1966) and which is overlain by reddishbrown siltstone strata assigned to the Moenkopi 

Formation. Light-colored strata above this contact are 

related to color changes within the Moenkopi Formation.

The base of the Moenkopi is covered by alluvial 

wash in most of the anticline area, but the rest of the 

formation is well exposed. The upper contact of the 

Moenkopi with the Gartra Member of the Chinle 

Formation is marked by the conspicuous coarsegrained, resistant, ledge-forming sandstone strata of 

the Gartra. The Moenkopi contains some resistant, 

thin calcareous very fine grained sandstone beds,

but poorly resistant, coarse-grained siltstone strata 

predominate, as suggested by the exposures shown in 

figure 2.

STRUCTURE

Skull Creek anticline is an asymmetrical structural 

feature almost monoclinal on the southeastern flank 

of the Uinta Mountains structural system (Ritzma, 

1956). Its southern limb dips more steeply (20°-45°) 

than the northern limb (2°-5°). The anticlinal axis 

arcs from a west-east orientation to a northwestsoutheast orientation in the area of the study with the 

concave side of the axis to the south. This arc produces 

a structural feature which has been called Skull Creek 

dome (Rocky Mountain Association of Petroleum 

Geologists, 1941).

The Skull Creek anticline is an excellent example of 

a breached anticline. Throughout an area (fig. 3) 12 

miles (19 km) long and 6 miles (10 km) wide, it has 

been eroded down to the Weber Sandstone which forms 

a broad slightly to moderately dissected domed surface. An erosional scarp which has a perimeter of 

approximately 30 miles (48 km) and which encircles 

the exposed Weber Sandstone is cut in the Park City,

TABLE 1. Stratigraphic units present in the Skull Creek anticline area

Period and unit Thickness 

(ft)

General description

Cretaceous:

Mancos Shale.___________________ 5,000 Gray to black carbonaceous marine siltstone with minor calcareous beds and lenticular sandstone strata.

Frontier Sandstone Member. __ 300 Thick- and thin-bedded gray sandstone interbedded with gray to black carbonaceous siltstone. 

Mowry Member._____________ 50 Hard dark-gray to black siliceous shale weathering to light ash-gray chips.

Dakota Sandstone.----_-----_____ 50 Resistant thick-bedded brownish-gray-weathering sandstone; characteristically

forms prominent massive beds. 

Burro Canyon (?) Formation. ______ 50 Green to varigated gray-green and purple siltstone upper unit and a thick-bedded

light-gray conglomeratic sandstone lower unit. 

Jurassic:

Morrison Formation, _.____._____. 675 Variegated green-gray, and maroon siltstone upper part, and a thick crossbedded,

fine-grained, white sandstone lower part.

Curtis Formation__________^_ . _. 115 Thin-bedded gray shale, platy glauconitic brownish-gray very fine grained sandstone; abundant Belemnite fragments. 

Entrada Sandstone. ______________ 175 Massive thick-bedded light-gray very fine to fine-grained sandstone; forms high

rounded cliffs: glauconite in upper part. 

Carmel(?) Formation____ ___._._ 25 Pinches out in area; reddish-brown silty sandstone.

Jurassic and Triassic:

Glen Canyon Sandstone 1 _-______._ 540 Spectacularly crossbedded massive thick-bedded fine-grained sandstone; forms

high, bare dome-shaped buttes. 

Triassic:

Chinle Formation...-._---________ 250 Red calcareous siltstone, very fine grained red sandstone and lime-pellet conglomerate. 

Gartra Member _____________ 20 Basal conglomerate of the Chinle, white conspicuous ledge, medium-grained

conglomeratic sandstone; hematite-impregnated. 

Moenkopi Formation. ____________ 525 Reddish-brown to grayish-green regularly bedded micaceous coarse siltstone and

very fine grained sandstone; red color absent locally. 

Permian:

Park City Formation.. ___________ 220 Grayish-orange and yellowish-brown siltstone (the tawny beds of Schell and

Yochelson, 1966) underlain by gray very fine grained calcareous sandstone. 

Permian and Pennsylvanian:

Weber Sandstone.--______________ 975 Massive crossbedded thick-bedded gray fine-grained sandstone; forms steepwalled canyons, rocky tree-covered slopes.

1 Called the Navajo Sandstone by Thomas, McCann, and Ram an (1945); renamed by Poole and Stewart (1964) on the basis of regional Stratigraphic relationships.
GEOLOGY

Jcu

FIGURE 2. Interior valley of the Skull Creek breached anticline, 

viewed northwestward. Jcu, basal sandstone of Curtis Formation; 

Je, Entrada Sandstone; J"fcg, Glen Canyon Sandstone; "fcc, 

Chinle Formation; "fcm, Moenkopi Formation; Ppc, Park City 

Formation; PPw, Weber Sandstone. White strata in the left 

foreground and in the distant scarp are altered rocks in the

Moenkopi Formation. Skull Creek is in the center foreground 

in the arroyo which is cut in thick valley alluvium. TheCarmel(?) 

Formation, which occurs discontinuously between the Entrada 

and Glen Canyon Sandstones, was observed in lenses as much as 

10 feet thick in two exposures in the south scarp. Neither exposure is visible in this photograph.

Moenkopi, and Chinle Formations, and in the rimforming Glen Canyon Sandstone. Succeeding concentric hogbacks which are present only on the southern 

steeply dipping limb are (1) the Entrada Sandstone 

rimmed by the resistant basal sandstone of the Curtis 

Formation, (2) the persistent ledge-forming Dakota 

Sandstone, and (3) the Frontier Sandstone Member 

of the Mancos Shale.

The east-striking Wolf Creek fault (fig. 1), a steeply 

dipping thrust fault related to the Uinta Mountains 

structural complex, borders the northern limb of the 

anticline (W. R. Hansen, oral commun., 1970). Numerous fault and joint systems striking generally from 

north to east may be observed within the anticline, 

but only minor displacement is evident. Displacement 

of hogback ridges south of Miller Creek water gap, 

apparent in aerial photographs, suggests the presence 

of an important fault occurring in conjunction with a 

sharp fold. Some fault traces in the western part of 

the anticline converge on a broad center near Red 

Wash water gap; this convergence suggests fracturing 

in response to localized stress. Major open faults, some 

of which are shown in figure 3, and major systems of 

parallel joints trending between northeast and east cut 

the Weber Sandstone in the core of the anticline. 

Erosion along the northeast-trending zones of weakness 

has produced fairly straight narrow deep box canyons 

in the Weber Sandstone and in the rocks forming the 

south slope of the southern limb of the anticline. All 

drainage from the anticline is to the south. Subsequent

interior drainage parallels the scarp surrounding the 

core, particularly on the southern and eastern edges 

where deposits of reddish-brown alluvial sediment and 

loess up to 30 feet in thickness are deeply incised. The 

alluvial fill extends southward through the three major 

water gaps in the southern scarp, Skull Creek, Miller 

Creek, and Red Wash, and into the flatlands south 

of the anticline.

MINERAL OCCURRENCES AND MINING ACTIVITY

The Skull Creek anticline area and the region to the 

north have been prospected extensively since the 1870's 

and have been the source of some mineral production. 

The Douglas Mountain district that is approximately 

20 miles north of the Skull Creek area has a record of 

some copper and silver ore production (200 ± tons) 

from 1873 to 1947 (Vanderwilt, 1947, p. 144). Records 

in the Moffat County Clerk's office describe a considerable amount of prospecting and claim staking in 

the Blue Mountain area between Skull Creek and 

Douglas Mountain from 1870 until much of the Blue 

Mountain area was incorporated into the Dinosaur 

National Monument in 1938.

During the early 1900's, high-grade radium ore was 

mined from the base of the Curtis Formation in the 

south limb of the Skull Creek anticline (N3^ sec. 35, 

T. 4 N., R. 101 W.; fig. 3). The deposit was mined for 

vanadium from 1917 to 1920 and finally, for uranium 

in the 1950's. According to Isachsen (1855), the ore 

occurred in carbonized plant-bearing mudstone and
4 GEOCHEMICAL ANOMALIES AND ALTERATION, MOENKOPI FORMATION, SKULL CREEK, COLORADO

108-50' R. 102 W. 47'30" R. 101 W. 45'

40°| I J ( II 

15 R. 102 W. R. 101 W. 

3 MILES

3 KILOMETERS

FIGURE 3. Geologic sketch map of the Skull Creek anticline area showing outcrop of altered strata in the Moenkopi Formation.

contained copper, uranium, and vanadium minerals. 

Production is believed to have been less than 1,000 tons. 

According to Stanley (Bud) Biles of Dinosaur, Colo., 

during the uranium boom of the 1950's, the ownership 

and the name of the mine changed many times.

The AEC (Atomic Energy Commission) conducted 

an exploration drilling program for uranium in the hogback area east and west of Miller Creek

sec. 35, T. 4 N., R. 101 W., and SEM sec. 33, T. 4 N., 

R. 101 W.) in 1953 (McDougald, 1955). Drill cores 

were taken through the basal sandstone ledge of the 

Curtis Formation, and holes were bottomed in the top 

of the Entrada Sandstone. According to McDougald, 

the mine in the Curtis was known as The Blue Mountain 

Group mine during 1953 operations. 

A small surface prospect in the base of the Gartra
42'30"

GEOLOGY

40' R. 100 W. 108°37'30"

EXPLANATION

Entrada Sandstone, 

Carmel(?) Formation, 

and Glen Canyon 

Sandstone

Chinle Formation

Moenkopi Formation 

"Em, reddish-brown part 

"fcma, greenish-gray altered 

part

Park City Formation 

and Weber Sandstone

Contact

Dashed where approximately located and 

covered by thick alluvium in stream gaps

Major joint or fault

trace 

Not all are shown

E

General locality referred 

to in text

T. 25

3 

N Strike and dip of beds

R. 101 W. R. 100 W.

The map was compiled by the author from field observations and aerial photographs on topographic base maps that are cited in the text.

Member of the Chinle Formation in the northeast 

corner of the area (S^ sec. 20, T. 4 N., R. 100 W.; 

fig. 3) was opened in the 1950's, according, to Mr. 

Biles. Small amounts of uranium-bearing clay were 

found around mineralized logs, and several sacks of 

this ore were mined and sold. No copper or vanadium 

minerals are visible in the prospect excavations. The 

sandstone bed below the mineralized wood layer contains 1-inch pyrite concretions, but no other visible 

evidence of mineralization is apparent.

A small uranium deposit in the Weber Sandstone 

just north of Skull Creek anticline is reported (Isachsen, 

1955) to have been discovered and mined in 1954. 

Except for the drilling program conducted by the AEC 

in 1953, no coordinated modern surface or subsurface 

exploration has been attempted in the area.
GEOCHEMICAL ANOMALIES AND ALTERATION, MOENKOPI FORMATION, SKULL CREEK, COLORADO

ALTERATION IN THE MOENKOPI 

FORMATION

The Moenkopi Formation is a red-bed formation 

typical of several formations in the Colorado Plateau 

region. Petrologic evidence (Cadigan, 1971b) suggests 

that the red color is diagenetic and resulted from the 

oxidation of iron in detrital minerals composing the 

rock strata. The pigmentation is in the form of megascopic to microlitic crystals of hematite impregnating 

the clayey and calcareous fractions of the coarse red 

siltstone and very fine grained sandstone, the dominant 

lithology of the Moenkopi.

In the Skull Creek anticline area and adjoining areas 

to the east and west, the Moenkopi Formation contains 

lens-shaped light-greenish-gray to greenish-yellow 

bodies of rock with, in most localities, reddish-brown 

strata both above and below.

An almost continuous zone about 1 cm thick that 

contains 100-5,000 ppm (parts per million), of copper 

coincides with the upper boundary of the grayish-green 

strata. The color boundary and the cupriferous zone 

are at many localities parallel to the bedding planes, 

but in others they cross bedding planes (fig. 4). At 

most localities the color change is not abrupt, but 

observed in detail the red beds are separated from the 

green beds by a narrow transition interval of mottled 

strata, or by an interval of interstratified thin beds or 

laminae of green and red siltstone (fig. 5). Viewed from 

a distance the color change is conspicuous and unmistakable, as shown in figure 6A, B. The thickness

FIGURE 4. Altered grayish-green Moenkopi strata overlain by 

unaltered red strata. Note that the color boundary rises in the 

section away from the camera and crosses sedimentary beds. 

Exposure at locality A (N^SEM sec. 27, T. 4 N., R. 101 W.), 

southern scarp. "Etna, green Moenkopi; "fim, red Moenkopi; 

ft eg, Gartra Member forming the base of the Chinle Formation. 

The view is toward the east.

of the greenish strata ranges from a few inches to 

approximately 500 feet in different parts of the area. 

Along part of the eastern scarp (area C in fig. 3) the 

entire formation is greenish yellow and there are no 

red strata to be found (fig. 7) in the Moenkopi Formation except possibly one or two thin beds just below 

the Moenkopi-Chinle contact.

The cupriferous zone may be detected at the upper 

color boundary of the greenish strata where these 

strata are 50-200 feet thick. It could not be found in 

the eastern scarp where there is no typical green-red 

color change and the entire formation is light greenish 

yellow or gray. At some localities (fig. 3A, B, and D) 

1-millimeter-thick lenses of malachite (Cu2CO3 (OH) 2 ) 

are found along the color boundary. Some unoxidized 

pyrite nodules occurring below the color boundary in 

the southern scarp are high in copper (200 ppm) and 

mercury (9 ppm).

Where red strata interlayed with green strata divide 

the main green zone into two or more green zones, 

rock samples from each of the red-over-green contacts 

may show anomalous copper content, but, in most 

instances, samples from the highest such contact in 

the section contain the largest concentration of copper. 

The light-green strata were originally thought to be 

related to conditions of deposition or to ordinary 

diagenesis. Several factors suggest that rock in the green 

zone was altered from red to green as the result of an 

extraneous postdepositional event unrelated to normal 

diagenesis as it is observed in the Moenkopi; these are 

the restriction of the green strata to the Skull Creek 

and adjacent areas, the observed extreme variation 

in thickness of the greenish strata from locality to 

locality, the crossing of sedimentary structures and 

textures by the color boundary, and the coincidence of 

the cupriferous zone with the green-red interface. 

This alteration evidently occurred before the breaching 

of the Triassic rocks that formerly covered the Skull 

Creek anticline and may have been penecontemporaneous with the structural deformation that produced the 

anticline and adjacent structural features.

The Gartra Member of the Chinle Formation is 

also highly altered in some outcrops along the southern 

scarp; the altered rock, a coarse-grained sandstone, is 

variegated white, red, and purple, and contains clinkerlike concretions of purple hematite-cemented grains and 

areally restricted 10- to 20-cm-thick lenses of discolored 

chert or jasperoid.

Metal values other than those of copper are anomalously high. Of particular interest are the mercury values 

found in Moenkopi strata in the eastern and southern 

scarps, some of which are greater than 10 ppm. The 

presence of the high content of mercury in the rock 

suggests the dispersion patterns of mercury that were
GEOCHEMICAL STUDIES

FIGURE 5. Interbedded altered green and unaltered red strata in the Moenkopi Formation. The cupriferous zone is present in the uppermost part of the green strata. The exposure is on the western scarp at the approximate center of sec. 22, T. 4 N., R. 102 W. Thickness of Moenkopi shown here is 200-250 feet. ~6ma, green zone; "6m, red Moenkopi; H, upper cupriferous zone.

found related to hydrothermal mineralization by 

Saukov (1946), Williston (1964), and Erickson, Marranzino, Oda, and Janes (1964).

GEOCHEMICAL STUDIES 

ANALYTICAL METHODS

The general metal content of the rocks was determined by six-step spectrographic analysis for 30

elements, supplemented by atomic-absorption analyses 

for mercury, gold, silver, and copper. Analyses for 

uranium by a colorimetric method were done on a 

few selected samples. All samples were checked for 

radioactivity with a scintillometer. All analyses were 

made by the Denver laboratories of the U.S. Geological 

Survey. Analysts were R. N. Babcock, K. J. Curry,
8 GEOCHEMICAL ANOMALIES AND ALTERATION, MOENKOPI FORMATION, SKULL CREEK, COLORADO

FIGURE 6. Conspicuous altered greenish-gray strata in the Moenkopi Formation seen exposed on A, the northern scarp near 

Lone Mountain (sees. 11, 14, T. 4 N., R. 101 W.), and B, the 

western scarp near locality E (NE^ sec. 23, T. 4 N., R. 102 W.). 

In A, reddish-brown strata occur in the middle of the altered

B

strata; in B, they occur near the top of the altered strata. The 

major cupriferous zone is at the top of the upper part of the 

altered strata. J"R g, Glen Canyon Sandstone; ~fcc, Chinle Formation; ft eg, Gartra Member of the Chinle; ~fc ma, altered strata 

of Moenkopi; "fcrn, unaltered reddish-brown strata of Moenkopi.

FIGURE 7. The southeastern arc of the erosion scarp encircling 

Skull Creek dome as viewed looking eastward from the highest 

point on the south rim between Skull Creek and Miller Creek 

water gaps (SWM sec. 26, T. 4 N., R. 101 W.). The Moenkopi 

Formation forming the lower half of the eastern scarp is altered 

from base to top. The group of houses in the right medium 

distance is the settlement of Skull Creek. The road was U.S.

Highway 40 in 1970. The low grass-covered hogback is formed 

by the resistant Frontier Sandstone Member of the Mancos 

Shale (Kmf). Kd, Dakota Sandstone; Jcu, Curtis Formation 

(basal ledge); Je, Entrada Sandstone; J~fcg, Glen Canyon 

Sandstone; ~fcc, Chinle Formation; "Reg, Gartra Member of 

the Chinle Formation; "fcma, greenish-gray and greenishyellow (altered) Moenkopi Formation.
GEOCHEMICAL STUDIES

J. V. Desmond, M. S. Erickson, C. L. Forn, J. G. 

Frisken, D. J. Grimes, J. R. Hassemer, R. T. Hopkins, 

H. D. King, R. W. Leinz, D. G. Murrey, D. F. Siems, 

J. G. Viets, L. A. Vinnola, K. C. Watts, Jr., and A. 

W. Wells. Assistance in the field studies was provided 

by Laurette N. Bates, Norma L. Noble, and A. J. 

Toevs. Mrs. Bates also organized the sample collections and analytical data reports.

Element symbols Cu(A), Ag(A), Hg(A), and Au(A) 

used in tabular presentations indicate results of analyses 

made by the atomic-absorption methods described by 

Huffman, Mensik, and Rader (1966), Huffman (1968), 

and Vaughn (1967). The regular element symbols 

Cu, Cr, V, and so forth in tables and figures indicate 

results of analyses made by the semiquantitative sixstep spectrographic method, slightly modified from the 

three-step technique described by Myers, Havens, 

and Dunton (1961). Both spectrographic and atomicabsorption values for copper and silver are given in 

the report and treated statistically as separate variables.

The lowest concentration at which an element is 

detected and reported for quantitative geochemical 

purposes is called the detection limit. It varies according 

to element and analytical method used. The highest 

concentration that can be estimated and reported 

quantitatively is called the reporting limit, which also 

varies according to element and analytical method; 

for example, the reporting limit of most abundant 

elements is 10 percent or 100,000 ppm for the spectroscopic method. The detection limit in parts per million 

for the elements reported in this study are as follows:

Magnesium, Mg_ _____ 200

Iron, Fe_____________ 500

Calcium, Ca_----_-_- 500

Titanium, Ti_ ________ 20

Barium, Ba__________ 20

Manganese, Mn_ _____ 10

Strontium, Sr_ _______ 100

Zirconium, Zr________ 10

Vanadium, V_ ______ 10

Chromium, Cr_____ 5

Copper, Cu__________ 5

Yttrium, ¥__________ 10

Lead, Pb____________ 10

Silver, Ag___ _________ .5

Uranium, U________ 20

Nickel, Ni___________ 5

Cobalt, Co___________ 5

Beryllium, Be ________ 1

Scandium, Sc__ _____ 5

Lanthanum, La_____ 20

Molybdenum, Mo_____ 5

Niobium, Nb_______ 10

Boron, B____________ 10

Copper(A), Cu(A)____ 10

Silver (A), Ag(A)______ .2

Mercury(A), Hg(A)_ _ .01

Gold(A), Au(A)______ .02

Zinc, Zn, is also occasionally mentioned in the report, 

but it is not treated statistically; it has a detection 

limit of 200 ppm.

If the analytical data are to be treated statistically, 

certain properties of the frequency distributions of the 

concentrations of each of the elements must be considered (Miesch, 1967). One of the problems involves 

truncated or censored distributions those which contain values below the limits of detection or above the 

reporting limits. It is a common practice to assign 

arbitrary values where no values can be reported to 

avoid the use of zero which would also be an arbitrary

and probably incorrect value in most instances. In 

this study, values reported as less than the detection 

limit have been assigned a value equal to one-half of 

the detection limit. For example, gold concentrations 

that were reported as less than 0.02 ppm were assigned 

values of 0.01 ppm. The problem was discussed in 

greater detail in an earlier report (Cadigan, 1971a).

Elements discussed in this report for which censored 

statistical distributions of concentrations constitute a 

serious problem are uranium, gold (A), beryllium, 

molybdenum, niobium, and scandium. Absolute values 

(such as, means) computed for these elements are not 

reliable, but graphic comparisons of relative abundances 

and computed correlations are adequate for the purposes for which they are used.

SAMPLING METHODS

Detection and definition of the cupriferous zone 

were accomplished by the collection and analysis of 

four separate sets of samples. The first set consisted of 

stratified samples, as defined by Cochran (1953), of 

Triassic sedimentary formations of the Colorado 

Plateau region. Analytical results of Moenkopi samples 

collected from the south scarp at the Skull Creek 

locality (Cadigan, 1971a) suggested that the area was 

one of higher than average metal content.

The first sample collected from the cupriferous zone 

itself was one of a second set of samples collected during 

the subsequent and more intensive stratified sampling 

of the Moenkopi Formation at locality A (figs. 3, 8).

The character of the zone was determined from 

analyses of a third set of rock and soil samples collected 

along the strike of the bed represented by the sample 

collected in the second set which contained the highest 

copper values. Some of the analyses of the third set of 

samples were obtained from a Geological Survey 

mobile laboratory which was brought into the area 

for a few days to provide "instant" analytical results.

The extent of the cupriferous zone was determined 

by using a man-carried copper-test kit to confirm the 

presence of the zone along the 30 miles (48 km) of 

erosional scarp surrounding the core of the anticline. 

A fourth set of samples was collected as representative 

of this cupriferous zone.

For purposes of comparison the sample analytical 

results are separated into three different groups: (1) 

Those for stratigraphic samples selected as representative of altered and unaltered but noncupriferous rocks 

of the Moenkopi Formation and Gartra Member of 

the Chinle Formation, (2) those for samples selected as 

representative of the cupriferous zone, and (3) those 

for miscellaneous unique rock and mineral samples 

from the Moenkopi, Gartra, and other rock units in 

the area.

475-889 O - 72 - 2
10 GEOCHEMICAL ANOMALIES AND ALTERATION, MOENKOPI FORMATION, SKULL CREEK, COLORADO

R. 101 W.

EXPLANATION

Contact

Dashed where approximately 

located and covered by thick 

alluvium in stream gaps

Dirt road

FIGURE 8. Sampling traverses at localities A and B, sees. 27 and 28, T. 4 N., R. 101 W. The area is divided by 

Miller Creek water gap. KJbc, Burro Canyon, Morrison, and Curtis Formations; J"fieg, Entrada and Glen 

Canyon Sandstones; "fcc, Chinle Formation; "fcm, Moenkopi Formation; "fcma, altered Moenkopi; PPpw, 

Park City Formation and Weber Sandstone.

DISTRIBUTION OF ELEMENTS

The distribution-of-elements study of the Moenkopi 

Formation (Cadigan, 1971a) and the Navajo mercury 

study (Cadigan, 1969) suggested that the rocks of the 

Skull Creek anticline area are anomalously high in 

metal content. This suggestion is confirmed by spectrographic, and other instrumental analyses of samples 

collected during the present study.

Table 2 shows the analytical results from 20 samples 

collected at locality A (figs. 3, 8), the second suite of 

samples collected in the area. The samples consist of 

10 pairs of samples, a pair being replicates taken from 

the same 0.3-meter-square area.

The last four columns of table 2 shown for purposes 

of comparison are (1) the geometric mean metal values 

for 10 reddish-brown and reddish-yellow samples, (2) 

geometric means for eight greenish-gray and grayishyellow samples, (3) geometric means for 18 of the 20 

samples collected in the vertical traverses (fig. 8) at 

locality A, and (4) geometric means for 323 samples 

collected from the Moenkopi Formation throughout 

the Colorado Plateau region. The two samples from the

cupriferous zone at locality A were not used in the 

geometric mean calculations in (3) because of their 

anomalously high values (for example, copper, vanadium, lead, and zinc).

Comparison of the four columns indicates that red 

and green and gray samples are not significantly 

different in metal content and that the geometric-mean 

metal values of the 18 samples collected at locality A 

are significantly higher in metal content than the regional geometric-mean values.

Comparison of a similar suite of 20 samples collected 

from the bottom to the top of the Moenkopi at locality 

E (figs. 3, 9) showed similar results although the geometric-mean metal values for the samples from E were 

generally lower than the geometric means for those 

collected at A.

The suite of samples with the highest metal values 

was that collected from the cupriferous zone at approximately 75-foot (23-m) intervals on a 600-foot (183-m) 

east-to-west lateral traverse west of Miller Creek gap 

at locality B (figs. 1, 8,10). A partial listing of analytical 

results in parts per million for nine samples is given 

below, tabulated in the order collected.
GEOCHEMICAL STUDIES II

R. 102 W.

1 MILE

I

FIGURE 9. Sampling traverse at locality E, sec. 23, T. 4 N., 

R. 102 W. The cupriferous zone is present at the intersection of the sample traverse with the top of the altered 

zone ( ma). J"Sg, Glen Canyon Sandstone; "fie, Chinle 

Formation; "fc m, Moenkopi Formation; ^ma, altered 

zone in Moenkopi; PIPpw, Park City Formation and 

Weber Sandstone.

Sample

CD 5539A-_._-_

5539B___.___

5540___---_-

LB 69-20___-_-_

69-22_______

69-23._____-

69-24. ______

69-25__--___

69-26_--_--_

Hg(A)

__ 2.1

__ 2.5

__ 2.4

_. 3.5

._ 10.0

__ 1.0

__ 3.5

.35

._ 10.0

Cu(A)

870

270

5,400

5,200

2,600

5,100

2,200

880

4,400

V

150

100

5,000

500

1,000

500

200

50

500

u

<20

<20

200

40

40

80

40

<20

<20

Ag(A)

1.5

1.7

2.3

1.8

2.0

12.0

4.0

2.4

2.4

Malachite-bearing laminae were observed in the 

cupriferous zone almost continuously along this lateral 

traverse at locality B (fig. 8).

A suite of 23 samples taken from bottom to top, 

normal to the strike of the altered strata in area B 

but excluding the cupriferous zone, yielded much 

lower metal values. A summary of values in parts per 

million follows:

Maximum _

Median

Minimum

Hg(A)

>10

2.4

.4

Cu(A)

34

14

V

100

50

30

u

<20

<20

<20

Ag(A)

1.8

.8

.4

A comparison of the analytical results for the two 

sets of samples from locality B shows that although the 

high copper (A), vanadium, uranium, and silver (A) 

values are confined to the cupriferous zone, high

mercury (A) values are dispersed vertically throughout 

the sampled strata.

Samples collected from the cupriferous zone on the 

east side of Miller Creek water gap at locality A 

(figs. 1, 3, 8) at 100- to 150-m intervals along a 1-km 

lateral traverse are lower in metal content than the 

samples from the cupriferous zone of locality B. A 

partial listing of values in parts per million is given 

below.

Sample Hg(A) Cu Ag(A)

CD 5527________

5529________

5530--------

5531_-------

5533_______-

5535A ____

553 5B

2.0

1.5

1.8

1.5

2.2

2.1

1.8

86

250

17

300

800

250

380

200

200

200

200

300

100

100

<20

<20

<20

<20

<20

<20

<20

1.1

1.1

1.7

1.6

2.7

1.2

1.5

Eleven samples were collected from the cupriferous 

zone at irregular intervals along the north scarp (figs. 

3, 6A, 11, 12) from locality D to Lone Mountain during 

field checking of the continuity of the cupriferous 

zone. These samples show values of mercury (A) 

lower by a factor of 100 and values of silver (A) generally

FIGURE 10. Altered Moenkopi strata (~fima) appear as lightcolored beds forming the bottom third of the scarp at Miller 

Creek water gap (locality B). Surface in the foreground is 

alluvial fill. Beds forming low white hillocks at the far end of 

the alluvial plain at the base of the scarp are in the Park City 

Formation. Rising slope to the right is Weber Sandstone. 

Rock in the immediate foreground, the Gartra Member ("fccg), 

also crops out on the scarp slope at locality B. J"Sg, Glen 

Canyon Sandstone; "fie, Chinle Formation; "5m, Moenkopi 

Formation.
12 GEOCHEMICAL ANOMALIES AND ALTERATION, MOENKOPI FORMATION, SKULL CREEK, COLORADO

TABLE 2. Results of chemical and spectrographic analyses, in parts per million, of 20 samples of the Moenkopi Formation from locality A

. the Colorado

[Numbers in parentheses beneath sample numbers in boxheads indicate distance above base in meters. .__., indicates data not available. Notation (A) as in Ag(A)

detected in only two samples, and is not treated statistically. R-Y, reddish yellow;

Chemical and spectrographic analyses, from locality A, of samples CD 

Ca... ......

Fe... .__.__

Mg.. ...

Ti______.__

Ba. ___ ...

Mn_.._

Sr.._ ..____

Zr. ......

V ._ ...

Cr ......

Cu(A)_____.

Cu. ___ ...

Y __ ......

Pb ___

Ni ......

Co... __ ..

Be... ......

Ao-CAl

Hg(A).....

Au(A)__ ...

Nb._......

B ...... .

La._ ....

Sc.. .......

Zn ___ ...

4748A 

(3.1) 

R-Y

----- 100,000

- ._ 10,000

. __ 20,000

. __ 2,000

_ __ 700

.- -. 300

_ __ 1,000

.- -- 100

.- - 50

-. -. 70

----- 18

. __ 20

. ... 15

..... 20

10

..... <5

----- <1

 .. 1.0

.04

----- <.02

..-- <10

...-- 20

..... 20

-..._ 5

4748B 

(3.1) 

R-Y

10,000

20,000

2,000

700

300

700

150

50

70

19

20

10

15

15

<5

<1

1.0

.04

<.02

<10

20

20

5

4749A 

(25.9) 

G-Y

30,000

30,000

5,000

700

700

200

500

100

100

24

30

30

20

50

15

<1

.6

.04

<.02

10

50

30

15

4749B 

(25.9) 

G-Y

30,000

5,000

700

700

300

200

100

100

25

20

20

15

30

10

<1

.5

<.01

<.02

10

50

30

10

4738A 

(51.8) 

GR-G

70 000

10,000

20,000

2,000

700

500

500

100

50

50

15

 15

15

15

15

5

<1

1.2

80

<.02

<10

20

20

5

4738B 

(51.8) 

GR-G

100,000

15,000

20,000

3,000

700

500

500

100

50

50

13

15

20

15

15

5

<1

1.1

.60

<.02

<10

20

20

5

4739A 

(74.7) 

GR-G

70,000

30,000

30,000

5.000

700

500

300

150

70

150

22

20

20

15

20

7

1

.6

.18

<.02

10

30

30

10

4739B 

(74.7) 

GR-G

50,000

30,000

30,000

5,000

700

700

150

150

100

100

19

20

20

20

20

7

<1

.7

.04

<.02

10

50

30

10

4740A 

(93.0) 

GR-G

150,000

10,000

15,000

3,000

700

1,000

1,000

200

100

30

40

30

20

20

10

5

<1

1.6

.03

<.02

<10

20

20

5

4740B 

(93.0) 

GR-G

150,000

15,000

15,000

3,000

1,000

1,500

1,000

200

100

30

58

70

20

30

15

7

<1

1.2

.07

<.02

<10

20

20

5

4742A 

(93.9) 

GR

2,000

50,000

20,000

5,000

700

500

200

100

700

100

1,700

1,500

20

300

50

15

1

3.4

.05

<.02

10

70

30

20

200

lower than the values shown by samples from the 

cupriferous zone in the south scarp at localities A and 

B. A statistical summary of analytical results is given 

below (values in parts per million).

Maximum

Median

Minimum .

Hg(A)

 _ 0.04

.02

.01

Cu(A)

1,800

320

100

V

700

150

50

u

20

<20

<20

Ag(A)

1.8

.8

.6

Uranium was detected in only one sample (fig. 12) 

from the north scarp near Lone Mountain. It was 

detected in five of the six samples collected along the 

lateral sample traverse in area B (fig. 8), from exposures 

just west of the gap. Three halo samples from the cupriferous zone in the west rim contain values for mercury- 

(A), copper (A), vanadium, and silver (A) in the same 

order of magnitude as the samples from the north rim, 

but none contains detectable uranium.

No cupriferous zone was found in the east scarp at 

locality C (figs. 3, 7, 11). Values for the five metals in 

the suite of 16 samples collected at irregular intervals 

from a 470-foot (143-m) vertical traverse of the grayishyellow and greenish-gray altered Moenkopi Formation 

strata of the east scarp are summarized below (in 

parts per million).

Hg(A) Cu(A) u Ag(A)

Maximum. 

Median___ 

Minimum.

>10 74

2.3 15

.5 <10

100

50

30

<20 

<20 

<20

1.2

.8

The geometric-mean values for 25 elements in four 

different sample suites of grayish and greenish rocks 

are compared graphically in figure 13 with the geometric 

means for 323 samples from the Moenkopi Formation 

as a whole in the Colorado Plateau region (Cadigan, 

1971a). The geometric means for the 323 samples are 

referred to for the sake of brevity as Moenkopi MG. 

The bottom scales of the bar graphs are multiples of 

the Moenkopi MG values shown in parts per million. 

Thus, the geometric mean for titanium in the 26 

stratigraphic samples from the altered strata at locality 

A (fig. 3) is between four and five times the Moenkopi 

MG for titanium, or 2,900-3,700 ppm.

For the Colorado Plateau region, usable analytical 

data are not available for the computation of Moenkopi 

MG values for scandium, lanthanum, molybdenum, 

niobium, and boron; scandium and lanthanum values 

were not sought; molybdenum, niobium, and boron 

values were sought but almost none of the values were 

high enough to be detected. These five elements,
GEOCHEMICAL STUDIES 13

(figs. 3, 8) and comparisons between geometric means of selected analyses of samples of the Moenkopi Formation from both locality A and 

Plateau region

indicates analysis by atomic absorption. Samples from cupriferous zone GR are not used in geometric mean comparisons. Zn has detection limit of 200 ppm, was 

G Y, grayish yellow; GR-G, greenish gray; GR, green; R-B, reddish brown]

Geometric means

4742B

(93.9)

GR

2,000

30,000

20,000

5,000

700

500

200

150

700

100

1,600

1,500

20

300

50

15

1

3.

<.

10

70

30

20

200

^ntMiiicai c

4741A

(94.5)

R-B

150,000

15,000

15,000

3,000

1,000

1,000

500

200

50

30

<10

15

20

100

15

5

<1

4 1.

05

02 <.

<10

20

20

5

uiu apei;uu£

4741B

(94.5)

R-B

150,000

15,000

15,000

3,000

1,000

1,000

300.

200

50

30

<10

30

20

20

15

5

<1

1 2.

06

02 <.

<10

30

20

5

jrapiuc ctiictj

4743A

(97.5)

R-B

100,000

20,000

20,000

5,000

700

700

300

200

100

50

38

50

20

20

20

7

<1

4 1.

05

02 <.

10

50

20

10

lyocOf iruni

4743B

(97.5)

R-B

100,000

20,000

15,000

3,000

700

700

300

300

100

70

<10

20

20

20

15

7

<1

2 1.

02

02 <.

<10

onOU

20

10

LUC2L11LO' A, Ul S

4744A

(120.4)

R-B

100,000 I

30,000

30,000

5,000

1,500

1,000

200

500

100

100

13

30

30

30

30

10

<1

2 1.2

13 .24

02 <.02

<10

50

30

10

aiiipies \^xj

4744B

(120.4)

R-B

LOO, 000

30,000

30,000

5,000

1,000

1,000

150

300

100

100

13

20

30

20

30

10

1

1.2

<.01

<.02

<10

50

30

10

 \^uii tmue

4745A

(132.6)

R-B

1,000

30,000

10,000

5,000

700

500

1,000

300

100

70

<10

30

150

20

30

15

2

.2

.04

<.02

10

50

150

10

u

4745B

(132.6)

R-B

1,000

20,000

7,000

5,000

1,000

500

1,000

300

100

70

<10

30

70

20

20

10

2

.2

.01

<.02

10

50

150

10

R-B, R-Y

(10 samples)

43,000

19,000

17,000

3,600

870

510

440

230

76

71

9.5

25

28

24

19

6.4

.71

1.3

.035

(.01)

7.0

34

33

7.6

Locality A

GR-G, G-Y

(8 samples)

73,000

19,000

22,000

3,700

730

710

400

180

80

66

24

24

20

18

19

7.1

.55

.86

.05

(.01)

7.1

30

24

7.5

R-B, R-Y,

GR-G, G-Y

(18 samples)

55,000

19,000

19,000

3,600

810

670

440

210

77

64

16

24

24

21

19

6.9

.65

1.1

.04

(.01)

7.0

32

29

7.6

Colorado Plateau

(323 samples;

Cadigan, 1971a)

22,000

8,900

5,100

740

410

190

100

76

31

22

13

10

11

11

6.2

3.8

.53

.26

.13

.016

however, were found in detectable amounts in the Skull 

Creek samples. To provide some standard of comparison, the numerical limits of detection are shown for 

scandium and lanthanum and the limits of detection 

times 0.5 are shown for molybdenum, niobium, and 

boron.

The geometric means of the metal values for samples 

of altered strata from four different localities, A, B, 

C, and E, are significantly higher than the Moenkopi 

MG values. In terms of the factor by which the metal 

values at each locality exceed, on the average, the 

Moenkopi MG, the localities are ranked as follows: 

locality C, 2.5; locality B, 2.1; locality A, 1.9; and locality E, 1.6.

Locality C ranks first in metal content of its samples, 

as shown above, and it is also the area where the color 

of all Moenkopi Formation strata is completely altered 

to greenish-yellow. Localities C and B have the highest 

mean mercury (A) values (2.0 ppm). Localities A and 

E, on the other hand, have mean mercury (A) values of 

only 0.05 ppm.

The geometric means of the analyses of 17 samples 

representative of the Gartra Member in outcrops 

between localities A and C are also compared graphically (fig. 13) with those of the Moenkopi MG. The 

Gartra as a whole contains neither significantly more 

nor significantly less of the metals than the Moenkopi 

MG with the exception of calcium content, which is 

significantly lower; iron, titanium, zirconium, vanadium, and chromium are notably (two to four times) 

higher; these differences suggest less calcite cement and 

more detrital minerals in the Gartra in the Skull Creek 

area than the average for the Moenkopi Formation 

in the Colorado Plateau. Mean mercury (A) content is 

significantly lower in the Gartra. Compared with 

samples of altered Moenkopi from localities A, B, C, 

and E, the Gartra tends to contain significantly lower 

calcium, magnesium, manganese, nickel, cobalt, and 

silver (A) but contains approximately the same average 

amounts of the other metals. Boron and lanthanum 

averages are well above their detection limit in all 

groups of Skull Creek samples.

The Gartra Member samples have maximum values 

of 20,000 ppm for iron, 5,000 ppm for chromium, and 

3,000 ppm for barium. Siliceous (jasperoid) concretions in the Gartra contain as much as 5,000 ppm 

barium and 1,000 ppm strontium.

Geometric mean values for 24 out of the 25 metals 

in the 36 samples from the cupriferous zone (fig. 14)
14 GEOCHEMICAL ANOMALIES AND ALTERATION, MOENKOPI FORMATION, SKULL CREEK, COLORADO

R. 101 W. R. 100 W.

Contact

Dashed where approximately 

located and covered by thick 

alluvium in stream gaps

FIGURE 11. Sampling traverse at locality C (sec. 31, T. 4 N-, R. 101 W.), malachite showings at locality D (sees. 18 

and 19), and sampled area at the Gartra prospect (sec. 20). J~R g, Glen Canyon Sandstone; "fie, Chinle Formation; "fcma, altered Moenkopi Formation (greenish gray or yellow); ~fcm, Moenkopi Formation (reddish brown); 

PPpw, Park City Formation and Weber Sandstone.
GEOCHEMICAL STUDIES 15

R. 101 W.

1 MILE

FIGURE 12. Sampling traverse (sec. 10, T. 4 N., R. 101 W.) 

at Lone Mountain. J~Rg, Glen Canyon Sandstone; "Re, 

Chinle Formation; "fim, Moenkopi Formation (reddish 

brown); "fima, altered Moenkopi Formation (greenish 

gray); PIP pw, Park City Formation and Weber Sandstone.

(all samples collected in previously described horizontal 

traverses) are higher than those of the Moenkopi MG . 

The geometric means for the cupriferous zone samples 

are significantly (at least four times) higher for copper- 

(A), copper, vanadium, nickel, silver(A), cobalt, and 

chromium. The 25 Gartra prospect samples (fig. 14) 

are, on the average, notably higher than the Moenkopi 

MG in nickel, beryllium, vanadium, cobalt, silver (A), 

and gold (A). The most remarkable aspect of the Gartra 

prospect samples is the high chromium, the geometric 

mean of which about 620 ppm is more than 27 times 

the Moenkopi MG . The maximum value for chromium 

is 10,000 ppm (1 percent); for vanadium, 2,000 ppm; 

for gold (A), 1.5 ppm; and for silver(A), 22 ppm. High 

chromium-bearing samples in the prospect area consist 

of green or fairly bright greenish-yellow soft clays.

An exploratory cross-country traverse (not shown on 

fig. 3) was made from south to north starting at the 

mouth, of East Rock Wall Draw (NEM sec. 5, T. 3 N., 

R. 101 W.) and ending on the eroded surface of the 

Weber Sandstone nearest the southern scarp (NWM 

sec. 28, T. 4 N., R. 101 W.). Grab samples collected 

on the traverse contain evidence of general mineralization; for example, partly altered sulfide joint fillings in 

the Glen Canyon Sandstone in the creek bottom contain as much as 2,000 ppm arsenic, and black sandy 

concretions in the canyon walls contain as much as 

5,000 ppm manganese. Calcareous veinlets in both red

and altered green siltstone in the top of the Chinle 

Formation exposed in a small window in the bottom of 

the draw show anomalously high metal values as 

much as 700 ppm lead, 200 ppm zinc, and 2,000 ppm 

manganese. Joint fillings of hematite 25 mm thick are 

abundant in the Glen Canyon Sandstone near where 

the East Rock Wall Draw joint (fault?) system intersects the scarp rim north of the draw. Samples of 

gouge, however, apparently from the same large fault 

in the Weber, north of the scarp rim, contain only 

background, or smaller, metal values.

Sulfide concretions in the top of the Weber Sandstone 

in southeastern outcrops, in the locality C area, contain as much as 700 ppm arsenic, 10 ppm silver (A), 50 

ppm molybdenum, and 300 ppm zinc.

FACTOR ANALYSIS

Many geologic factors or causes control the location 

and proportion of each metal, but it would be difficult 

to determine each geologic cause and its proportional 

effect on the mode of occurrence of each of 26 metals. 

If we can group the metals that have a relatively high 

correlation of occurrence, then we can begin to get some 

idea of the major geologic causes that control the occurrence of identifiable proportions of the metals. 

Even if grouping the metals does not account for all 

the variance of occurrence for each metal, general 

geologic interpretations may still be made on the basis 

of the determined groupings but with full realization 

that the groupings account for most of the variance 

present.

Factor analysis is a form of multivariate analysis 

that evaluates the covariance among a set of variables 

and groups the variables according to their mutual 

dependency and the strength of their relationship to 

common factors. Factor analysis was used in this study 

to divide the elements into related groups that, on the 

basis of geologic experience, could be interpreted as 

representing a series of geologic events.

The reader who is interested in gaining more than a 

cursory understanding of factor analysis is referred to 

the authoritative text by Harman (1967) and especially 

to an excellent, but brief, detailed introduction to the 

subject by Imbrie (1963).

Factor analysis (R-mode) Was applied to the analytical results for 26 elements in the 36 samples from 

the cupriferous zone. The first step was the arrangement of the values into a 26x36 data matrix. Logarithms of the element values were used in the data 

matrix to normalize the statistical distribution of values 

for each element. Next, correlation coefficients were 

computed for all possible pairs of elements to produce 

a 26X26 correlation matrix (table 3). The variance in
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GEOCHEMICAL STUDIES 17

COLORADO 

PLATEAU 

Moenkopi £ 

MG (ppm) uj 

323 5 

Samples -i

UJ

SKULL CREEK AREA

Cupriferous zone

36 samples

2,000 Ca 

8900 Fe 

5100 Mg 

740 Ti 

410 Ba 

190 Mn 

100 Sr 

76 Zr 

31 V 

22 Cr 

13 Cu(A)' 

10 Cu 

11 Y 

11 Pb 

6.2 Ni 

3.8 Co 

0.53 Be 

0.26 Ag(A)' 

0.13 Hg(AV 

0.016 Au(A) 1 

2 (5.0) Se 

'(10) La 

3 (2.5) Mo 

3 (5.0) Nb 

'(1.0) B

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MULTIPLE OF PARTS-PER-MILLION 

MOENKOPI-MG VALUE

1 Element symbols followed by (A) indicate analytical data obtained by atomicabsorption method.

2 Detection limit. Element not sought in the 323 Moenkopi Formation samples.

3 Half of detection limit. Element sought but not detected in the 323 samples.

FIGURE 14. Geometric-mean metal content of cupriferous zone 

samples and samples from a prospect in the Gartra Member of 

the Chinle Formation in the Skull Creek area compared with 

the geometric means (Moenkopi MG) of the same elements in 

323 samples representing the Moenkopi Formation as a whole 

in the Colorado Plateau region. Numbers at left are values in 

parts per million represented by Moenkopi MG line at multiple 1.

the correlation matrix formed the basis for the factor 

analysis which followed. Factors were taken from the 

computed reordered oblique factor matrix. Calculations 

were made by utilizing the U.S. Geological Survey's 

IBM 360-65 computer and available STATPAC 

programs.

Only the first six factors in order of importance were 

selected for this investigation. These six account for 

73 percent of the total variance in the correlation matrix. The six factors were selected on the basis of 

two conventional criteria; only factors with eigenvalues

(a measure derived from the factor loadings) of at 

least 1.0 would be used, and element groupings for the 

factors should include no single element "groups" in 

the reordered oblique projection matrix. In this investigation the first eight factors have eigenvalues of 

at least 1.0, but the seventh and eighth factor groupings 

contain one or more factors, each consisting of a single 

element. Beyond factor eight, individual factors increasingly are represented by individual metals until, 

for 26 factors, each is represented by a single metal.

The results of the factor analysis are the element 

groups listed in six columns below. The element at the 

top of each column coincides with the factor axis, and 

elements are listed in order of their degree of covariance 

with the axis element. The elements in brackets are 

minor elements of the group and occur as major elements, without brackets, in other groups; for example, 

[Ag], a minor element in the factor 1 group, is a major 

element, Ag, in the factor 2 group.

Factor 1 Factor 2 Factor 3 Factor 4 Factor 5 Factor 6

(Alteration (Enrichment (Cements) (Micas) (Sulfates) (Heavy

effects) effects) minerals)

Co 

Cr

Nb

Fe

Ni 

V

La

B

Pb

[Ag]

[U]

[Y]

[Cu(A)]

Hg(A)

Cu

Ag(A)

U

Ag

Cu(A)

Au(A)

Ca

Mn

Y

Mg

[Zr]

Be

Sc 

[La]

fMg]

Sr 

Ba

[Sc]

[Ag(A>] 

[Cu] 

[V]

Ti 

Zr

[B]

[Fe] 

[Mn]

With considerable dependence on the comprehensive 

work of Rankama and Sahama (1950) and aided by 

the minor previous experience of the author (Cadigan, 

1971a, 1972), each of the factors was interpreted as 

being related to a geologic cause or event, as indicated. 

Factor 1 is the most important geologic event, or 

cause, in terms of its effect on the covariance of the 

metals. The other factors represent other causes and 

are listed in order of the importance of their effects on 

the covariance of the metals. Their relative importance 

is suggested by the proportion of total variance in the 

correlation matrix accounted for by each factor and 

is shown following each interpretation. Interpretations 

are made on the basis of geochemical relations and mode 

of occurrence of the major elements in each factor group. 

Each of the six factors is assigned a title related to a 

geologic event. The factors are the result of objective 

mathematical analysis, but the titles are based on 

interpretations.

Factor 1: Alteration effects A group of elements 

similar in constitution and cobalt-nickel ratio to those
18 GEOCHEMICAL ANOMALIES AND ALTERATION, MOENKOPI FORMATION, SKULL CREEK, COLORADO

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GEOCHEMICAL STUDIES 19

found in metal-rich veins that occur in granite, pegmatite, or other silicic rocks was probably derived 

from a granitic source and transported by migrating 

solutions during an event, the final phase of which 

resulted in the widespread but stratigraphically confined alteration in the Moenkopi Formation in the 

Skull Creek and adjacent areas. Percentage of total 

variance accounted for by factor 1 is 25.

Factor 2: Enrichment effects. A group of metals 

typical of copper-uranium deposits in the area of study 

and in the Colorado Plateau region was probably the 

result of interaction between reducing solutions in 

the green Moenkopi altered zone and metal-bearing 

solutions (sulfates, bisulfates, bicarbonates) in equilibrium with the oxidized rocks of the red Moenkopi. 

The factor 2 group represents the heavy metals most 

susceptible to leaching and transport as salts; thus, 

segregation or grouping occurred during the leaching 

phase, not during the reduction-reaction phase which 

produced the metal-enriched cupriferous zone. The 

enriched zone then is composed of metals that were 

oxidized and separated from the metals introduced by 

circulating (hydrothermal?) solutions and then in small 

part recaptured by the reducing environment at the 

reduction-oxidation interface in the Moenkopi. Percentage of total variance accounted for by factor 2 is 18.

Factor 3: Sedimentary carbonate cements. A group of 

elements related to the occurrence of carbonate cements in the samples. The [Zr] covariance is probably 

the indirect effect of positive correlation between 

carbonate content and the proportion of fine sediment 

which is in turn positively related to zircon content. 

Percentage of total variance accounted for by factor 

3 is 11.

Factor 4- Sedimentary micas and clays. A group of 

elements related to the presence of micas and their 

hydrolyzate alteration products in the samples. Percentage of total variance accounted for by factor 4 is 8.

Factor 5: Interstitial precipitated sulfates. Elements 

which commonly occur in detrital sediments as sulfate 

precipitates from interstratal solution. The covariance 

with [Ag(A)], [Cu], and [V] suggests a similar method 

of deposition for fractions of these metals. Percentage 

of total variance accounted for by factor 5 is 6.

Factor 6: Sedimentary heavy mineral deposition. 

Sedimentary processes that control the occurrence of 

elements which are closely associated with heavy 

minerals. Percentage of total variance accounted for 

by factor 6 is 5.

Each of the elements listed in the correlation matrix 

(table 3) has a positive, negative, or zero relationship 

to each of the factors. The relationship may be stated 

in terms of the proportion of variance accounted for 

by the six factors combined. For example, the six factors account for 0.84 (84 percent) of the covariance of 

iron with other elements. This figure, 0.84, is called 

the communality. A list of communalities is given 

in table 4. If the 0.84 is assumed to be the total amount 

of positive association of iron among the six factors, 

and if we reduce it to proportions in percent in which 

0.84 is 100 percent, then the proportion of positive 

association (covariance) of iron with each of the six 

factors is as follows: factor 1, 81 percent; factor 2, 

negative; factor 3, negative; factor 4, zero; factor 5, 

2 percent; and factor 6, 17 percent. Proportions of 

negative covariance could also be computed but would 

have limited geologic use beyond indicating only an 

antipathetic relationship between a particular element 

and a particular factor. This indication can be made by 

merely showing that a negative relationship exists.

Continuing with the example for iron, 81 percent of 

its occurrence in the six factors is related to the alteration event, 17 percent is related to proportion of 

detrital sedimentary heavy minerals, and 2 percent 

is related to the proportions of interstitial sulfate 

minerals. The covariance of the other metals is similarly 

explained in table 4. Note that only 0.50 of, the covariance of gold appears to be related to the six factors. 

Sixty-one percent of the positive covariance of gold is 

related to the enrichment effects (factor 2), 25 percent 

is related to carbonate cements (factor 3), and 14 

percent to the detrital heavy minerals factor (factor 6).

TABLE 4. Proportion, in percent, of positive covariance of each 

metallic element with each factor

[Communalities indicate proportion of total covariance of each element 

accounted for by all six factors. Axis elements are in boldface type. 

( ) indicates that the covariance is negative between element and factor]

Factor 1 Factor 2 Factor 3 Factor 5 Factor 6

Element (Altera- (Enrich- (Ce- Factor 4 (Sul- (Heavy Commution merit ments) (Micas) fates) min- nality

effects) effects) erals)

Fe 

Mg

NL 

Pb. 

Sc

Au(A)_ 

Hg(A) 

Cu(A)_ 

Ag(A)_

81 (-) (-) 0 2 17 0.84

9 2 39 31 (-) 20 .73

(-) (-) 100 0 0 (-) .89

(-) 0 0 (-) (-) 100 .84

4 (-) 70 0 13 13 .75

43 57 (-) (-) (-) (-) .71

46 10 (-) (-) (-) 44 .59

12 1 (-) (-) 75 12 .73

0 (-) 0 100 (-) 0 .82

100 0 0 (-) (-) 0 .85

91 (-) 4 0 2 3 .82

3 84 (-) (-) 13 (-) .84

41 (-) 29 30 (-) (-) .54

89 (-) (-) (-) 11 (-) .58

81 (-) (-) 11 (-) 8 .82

41 31 (-) 6 13 9 .62

(-) (-) 9 50 35 6 .54

0 0 (-) 0 100 (-) .71

62 20 (-) (-) 17 1 .71

32 2 40 10 16 (-) .82

(-) (-) 41 (-) 2 57 .66

(-) 61 25 (-) (-) 14 .50

(-) 100 0 (-) (-) 0 .82

19 64 (-) (-) 17 (-) .82

4 74 (-) (-) 22 (-) .75

41 59 (-) (-) (-) (-) .74
20 GEOCHEMICAL ANOMALIES AND ALTERATION, MOENKOPI FORMATION, SKULL CREEK, COLORADO

Comparison of the communality for gold with those 

of the other metals indicates that gold has the lowest 

degree of involvement of any of the metals. To support 

this statement, when seven factors are selected instead 

of six, gold becomes a separate factor and increases 

its communality to 78 percent.

Each of the axis elements (p. 17), cobalt, mercury (A), 

calcium, beryllium, strontium, and titanium, by 

mathematical definition has positive covariance (100 

percent) with only one factor. Relationships to other 

factors either are zero or are negative.

The percentages of positive association in table 4 are 

of some interest. For example, 70 percent of the manganese is related to cements. Lanthanum with a communality of only 0.54 is split three ways 41 percent 

in alteration effects, 30 percent in micas, and 29 percent in cements. Boron is almost equally split between 

the extrinsic alteration-effects factor and the intrinsic 

heavy-minerals factor with a small proportion of association with the enrichment-effects factor.

SUMMARY AND RECOMMENDATIONS

The geochemical evidence derived from the brief 

investigation of anomalous metal occurrences in the 

Skull Creek anticline suggests that the anomalies are 

the effects of a regional alteration event of possible 

hydrothermal origin. The recognized effects, in summary, are:

1. The area-wide altered strata in the Moenkopi 

Formation with an accompanying persistent 

copper-enriched zone. Though the altered strata 

were studied only in the Skull Creek anticline, 

they were also observed in exposures immediately 

east and west of the anticline.

2. The significantly higher average metal content in 

altered and unaltered rocks of the Moenkopi in 

the vicinity of Skull Creek as compared with 

that of the Moenkopi Formation as a whole in 

the Colorado Plateau region.

3. The copper-uranium-vanadium ore deposit in the 

base of the Curtis Formation.

4. Anomalously high mercury values found in the 

Moenkopi Formation and Glen Canyon Sandstone in the southern (loc. B) and eastern (loc. C) 

parts of the scarp adjoining Skull Creek and 

Miller Creek water gaps. The highest average 

mercury values occur at locality C where the 

Moenkopi is completely altered. Mercury geochemical dispersion patterns are characteristic 

of hydrothermal mineralizing activity.

5. Anomalous values of gold, chromium, silver, and 

vanadium in the Gartra Member prospect in 

the northeast corner of the anticline area.

6. Anomalous values of arsenic, lead, zinc, manganese, 

and silver in observable sulfide minerals in joints 

in the Glen Canyon Sandstone and Chinle Formation, in concretions in the East Rock Wall 

Draw area, and in concretions in the Weber 

Sandstone in the southeast corner of the anticline 

area.

7. The two major factor groups in the Moenkopi

cupriferous or metal-enriched zone a cobaltchromium-niobium-iron-nickel-vanadium-lanthanum-boron-lead covarying suite of metals, and a

mercury-copper-silver-uranium-gold covarying

suite. These groups suggest (1) an invasion of the

Moenkopi by solution-borne metals and (2)

interaction between solutions in the reducing

(altering) environment characterized by green

rocks and solutions in the oxidizing environment

of unaltered red rocks above. This red-green

contact, the loci of the enriched zone, could have

been a mobile interface which moved upward in

the section to occupy different levels at different

times and to leave traces of previous positions as

indicated by isolated weak cupriferous zones.

Major questions left unanswered are: What was the

source of the metal-bearing solutions? Did this activity

produce only trace mineralization over a wide area, or

do the anomalies reflect the presence of economically

significant deposits at depth?

The samples collected in this reconnaissance study 

establish the area as anomalous in terms of metal occurrences, but they do not yield data adequate to 

define the regional dimensions of the anomalies. Trends 

are suggested but not defined. Mineralization occurred 

in both stratigraphically controlled and structurally 

controlled loci.

The following questions provide a starting point for 

any additional work that may be done in the area. Are 

the altered strata of the Moenkopi Formation a sulfide 

zone at depth, particularly southeast or east of locality 

C? Does sulfide mineralization in joints or faults in 

East Rock Wall Draw persist, disappear, or increase at 

depth? What happens to the unusual mineralization in 

the Gartra prospect beyond the face? What is the nature 

of the Curtis-Entrada contact to the east and south at 

depth, and does it appear to have a potential for additional ore deposits in one or both of these directions? 

Are there even more anomalous metal occurrences in 

similar horizons and structures exposed to the east 

and west of the reported anomalies?

These questions can best be answered by a program 

of detailed mapping and intensive geochemical exploration, perhaps coupled with drilling, in the Skull 

Creek anticline area as a whole and particularly in the 

areas along the scarp between localities B and D.
REFERENCES CITED 21

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U. S. GOVERNMENT PRINTING OFFICE : 1972 O - 475-889