COVER PHOTOGRAPHS

1

5

9

11

2

7

12

3

6

10

13

4

8

14

1. Asbestos ore

2. Lead ore-Balmat Mine, N. Y.

3. Chromite-chromium ore. Wash.

4. Zinc ore-Friedensville, Pa.

5. Banded iron formation,Palmer, 

Michigan

6. Ribbon asbestos ore, Quebec, Canada

7. Manganese ore, banded 

rhodochrosite

8. Aluminum ore, bauxite, Georgia

9. Native copper ore, Keweenawan 

Peninsula, Mich.

10. Porphyry molybdenum ore, Colo.

11. Zinc ore, Edwards, N. Y.

12. Manganese nodules, ocean floor

13. Botryoidal fluorite ore, 

Poncha Springs. Colo.

14. Tungsten ore. North Carolina
Grade and Tonnage Relationships 

Among Copper Deposits

By D. A. SINGER, DENNIS P. COX, and LAWRENCE J. DREW

and

Geochemical Exploration Techniques 

Applicable in the Search for 

Copper Deposits

By MAURICE A. CHAFFEE

GEOLOGY AND RESOURCES OF COPPER DEPOSITS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 907-A.B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

First printing 1975 

Second printing 1976

Library of Congress Cataloging in Publication Data

Singer, D. A.

Grade and tonnage relationships among copper deposits.

(Geology and resources of copper deposits)

(Geological Survey professional paper ; 907-A, 907-B)

Includes bibliographies.

Supt. of Docs, no.: 19.16:907-A.

1. Copper ores United States. 2. Copper Analysis. 3. Geochemical prospecting. I. Chaffee, Maurice A. Geochemical exploration techniques applicable in the search for copper deposits. 1975. II. Title. III. Series.

IV. Series: United States. Geological Survey. Professional paper; 907-A, 907-B.

TN443.A5S54 553'.43'0973 74-23594

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

Washington, D.C. 20402 

Stock Number 024-001-02669-4
APPRAISAL OF MINERAL RESOURCES

Continuing appraisal of the mineral resources of the United States is 

conducted by the U.S. Geological Survey in accordance with the provisions 

of the Mining and Minerals Policy Act of 1970 (Public Law 91-631, Dec. 

31, 1970). Total resources for purposes of these appraisal estimates includes currently minable resources (reserves) as well as those resources 

not yet discovered or not presently profitable to mine.

The mining of mineral deposits, once discovered, depends on geologic, 

economic, and technologic factors; however, identification of many deposits yet to be discovered, owing to incomplete knowledge of their distribution in the earth's crust, depends greatly on geologic availability and 

man's ingenuity. Consequently, appraisal of mineral resources results in 

approximations, subject to constant change as known deposits are depleted, 

new deposits are found, new extractive technology and uses are developed, 

and new geologic knowledge and theories indicate new areas favorable for 

exploration.

This professional paper discusses aspects of the geology of copper as 

a framework for appraising resources of this commodity in the light of 

today's technology, economics, and geologic knowledge.

Other Geological Survey publications relating to the appraisal of resources of specific mineral commodities include the following:

Professional Paper 820 "United States Mineral Resources" 

Professional Paper 926 "Geology and Resources of Vanadium" 

Professional Paper 933 "Geology and Resources of Fluorine"

Grade and Tonnage Relationships 

Among Copper Deposits

By D. A. SINGER, DENNIS P. COX, and LAWRENCE J. DREW

GEOLOGY AND RESOURCES OF COPPER DEPOSITS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 907-A

An analysis of the relationships between grades and 

tonnages of three types of copper deposits 

porphyry, massive sulfide, and strata-bound

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975

CONTENTS

Page

Abstract __________________ A1

Introdnction ____ _ 1

Acknowledgments _ _ - -- - -- - - 1

Porphyry copper deposits _____ __ _ _ _- 1

Strata-bound and stratiform deposits in sedimentary and metamorphic rocks 2

Massive sulfide deposits __________ _ __ - - 3

Geological considerations of tonnage and grade relationships 3

Nature of data ___________________________________ 4

Univariate statistical analysis __________________________________ 4

Correlations between grade and tonnage _________________________ 7

Discussion and conclusions ________________________________ 8

References cited ____________________________________________ 11

ILLUSTRATIONS

Page

FIGURE 1. Grades and tonnages for porphyry copper deposits _________________________________ A8

2. Grades and tonnages for African strata-bound copper deposits ______________________________ 9

3. Grades and tonnages for massive sulfide deposits ______________________________________ 10

TABLES

Page

TABLE 1. Summary statistics for the average grades of copper deposits ____________________________ A5

2. Summary statistics for the tonnage of copper deposits _________________________________ 5

3. Beta and chi square goodness-of-fit statistics __________________________________________ 6

4. Correlation coefficients between average copper grade and total tonnage _______________________ 7

VII

GEOLOGY AND RESOURCES OF COPPER DEPOSITS

GRADE AND TONNAGE RELATIONSHIPS AMONG COPPER DEPOSITS

By D. A. SINGER, DENNIS P. Cox, and LAWRENCE J. DREW

ABSTRACT

Three types of copper deposits porphyry, strata-bound, 

and massive sulfide are described, and the distributions of 

tonnages and grades for 267 deposits are compared with 

normal and lognormal frequency distributions. The relationships between grades and tonnages are analyzed by examining 

the correlation coefficients of these variables. Conclusions 

reached include the following: (1) Geologic factors influencing 

tonnage of a particular deposit type are probably distinct 

from those influencing grade; (2) frequency distributions of 

tonnages and grades approximate lognormality, making it 

possible to predict probability of various tonnage-grade classes 

and to test correlation between variables; (3) no significant 

correlation was found between tonnage and grade for porphyry 

or strata-bound deposits; (4) significant negative correlation 

between tonnage and grade was found for the massive sulfide 

subset, probably reflecting a mixture of high-grade low-tonnage massive ores, low-grade high-tonnage stockwork, and disseminated ores characteristic of some massive sulfide deposits; 

(5) significant negative correlation was found between tonnage and grade for the mixture of deposit types in the whole 

sample.

Extrapolation on the basis of the negative grade-tonnage 

correlation shown for the mixed population seems to imply the 

occurrence of large-tonnage very low grade deposits, which 

could be important in supplying future copper needs. This extrapolation is misleading, however, for both statistical and 

geological reasons. Furthermore, large-tonnage very low grade 

deposits in the porphyry class of the population are found to 

be very rare occurrences.

INTRODUCTION

It has become imperative to have reliable estimates 

of the resource potential of various areas and commodities so that alternatives may be considered in 

order to maintain an orderly supply of minerals. 

Estimates of the resource potential of an area or 

commodity must include both the unpublished ultimate reserves of known deposits and the undiscovered deposits that might be mined if found. In both 

cases it is useful to have estimates of the probabilities with which different grades and tonnages occur. 

This is often best accomplished by means of statistical models of the grades, tonnages, and the gradetonnage relationships of the different types of occurrences of the commodity of interest. An analysis 

within each type of occurrence is necessary because 

the types may occur in diverse geologic environments and may be characterized by distinct statistical models.

In this study, the grades and tonnages of three 

types of copper deposits are compared with theoretical statistical models in order to build models of 

future discoveries. The grades of copper only are 

considered because of the difficulty of equating the 

different metals found in copper deposits. The three 

types porphyry, strata-bound, and massive sulfide 

deposits represent the world's main sources of 

copper at present. The observed frequency distributions of grades and tonnages are compared with the 

normal and lognormal frequency distributions. The 

relationships among grades and tonnages of the 

three deposit types are analyzed by examining the 

correlation coefficients of these variables. These relationships are useful both in building models of 

future discoveries and in testing the frequently implied inverse relationship between grade and tonnage. This relationship is often a hidden assumption in large highly aggregated mineral resource 

models, implying that improvements in technology 

can enlarge the supply of minerals.

ACKNOWLEDGMENTS

This study was greatly helped by discussions with 

and data supplied by G. H. Goudarzi of the U.S. 

Geological Survey and various industry geologists.

PORPHYRY COPPER DEPOSITS

Disseminated copper deposits, genetically related 

to felsic or intermediate intrusive igneous rocks, are 

called porphyry copper deposits. Copper minerals

Al
A2 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

may occur as fine disseminated grains, in small 

crosscutting veinlets, or in thin coatings on joint 

surfaces. In most deposits, the copper ore body is 

mainly within the associated intrusive rock; but in 

some deposits, the surrounding rocks, particularly 

limestone, may be replaced by copper minerals. 

Copper minerals may also be deposited in breccia 

pipes or breccia veins, or in parallel or branching 

quartz veins sufficiently closely spaced to be extracted by mass-mining methods. All these ore types 

are believed to be closely related genetically and are 

commonly included in the porphyry copper classification.

Molybdenum, gold, and silver may be important 

byproducts, and lead, zinc, silver, and manganese deposits may be peripheral to the porphyry deposit. 

Replacement and vein deposits are usually found in 

a distinct zonal arrangement in the rocks surrounding a porphyry. Copper, in replacement ore bodies, 

may be within a few hundred metres of the intrusive lead-zinc silver deposits farther out, and manganese or gold-silver deposits may be on the outer 

fringes.

Porphyry copper deposits generally have a complex history involving several stages of igneous intrusion, mineral deposition, and hydrothermal alteration. In some deposits such as San Manuel, Ariz. 

(Lowell and Guilbert, 1970), the late magmatic and 

deuteric stages are well represented. In others such 

as Butte, Mont. (Meyer, Shea, and Goddard, 1968), 

hydrothermal activity that postdates cooling of the 

intrusive body predominates. In still others such as 

Morenci, Ariz. (Moolick and Durek, 1966), oxidation and enrichment by descending surface waters 

have obliterated most of the minerals that may have 

been deposited in earlier stages. In most deposits, 

evidence for all three stages is found.

Factors influencing tonnage. In deposits where 

mineralization is confined to an intrusive body, the 

tonnage of the deposit may be controlled mainly by 

the volume of the intrusion. Very few intrusions associated with porphyry copper mineralization are of 

batholithic proportions; in those that are, only a 

small part of the intrusion is mineralized or hydrothermally altered. In other deposits in which favorable wallrocks are mineralized, the size of the deposit may be dependent on the stratigraphic thickness of the favorable beds and on the width of the 

copper-rich mineral zones surrounding the porphyry 

deposit. This width is presumably dependent on the 

temperature, pressure, and hydrothermal conditions 

near the intrusion and on the physical character of 

adjacent host rocks.

Finally, in some deposits, postmineral faulting 

followed by erosion has had an obvious effect on the 

tonnage of ore. Ore bodies may be cut by major 

faults, and a large part may have been uplifted and 

destroyed by erosion.

Factors influencing grade. Conditions favoring 

oxidation and enrichment of porphyry deposits have 

an important influence on grade. The factors include 

climatic and ground-water conditions from the time 

of the original uplift and erosion of the deposit to 

that of the present erosion cycle. The degree of fracturing of the rock is important as well as an abundance of pyrite that oxidizes readily to provide acid 

ground water.

Pro to re grades of porphyry deposits (the copper 

content before enrichment by descending waters) 

seldom exceed 1 percent. An important factor influencing protore grade may be the availability of 

sites favorable for deposition of copper minerals. 

Such sites include (1) carbonate wallrocks and (2) 

dispersed iron oxide and iron-magnesium silicate 

minerals in both wallrocks and intrusive bodies. A 

close spacing of fractures also influences protore 

grade, presumably by providing access for copperbearing solutions to the available sites of deposition.

STRATA-BOUND AND STRATIFORM

DEPOSITS IN SEDIMENTARY AND

METAMORPHIC ROCKS

Strata-bound ore bodies locally cut across bedding 

but are confined to a single sequence of strata over 

large areas. Stratiform copper deposits have the 

form of a bed, concordant with strata of surrounding rocks, and are laterally continuous for a great 

distance relative to their thickness. Copper minerals 

commonly are disseminated in both these ore bodies, 

but, rarely, in some very high-grade deposits they 

form massive beds. Lead and zinc ore bodies may be 

zonally arranged outward from the copper-rich beds 

or may form separate beds in the same stratigraphic 

sequence. Cobalt, silver, and minor amounts of 

nickel and bismuth are found in some deposits; 

uranium, vanadium, and thorium are associated 

with copper in other types.

Origin of strata-bound deposits is the subject of 

great controversy. Some of the largest deposits are 

metamorphosed and show the effects of mobilization 

of copper and its redeposition in crosscutting fractures and noses of folds. Processes including direct 

sedimentary deposition, early stage diagenesis, 

ground-water mineralization, and hydrothermal ore 

deposition have been suggested to explain the origin 

of some deposits. Sulfur-reducing bacteria may have
GRADE AND TONNAGE RELATIONSHIPS A3

an important role in the precipitation of metal sulfides in sediments and sedimentary rocks. Metal-rich 

sediments underlying the hot brine pools in the Red 

Sea may be modern analogs of stratiform copper deposits, but the origin of these is also a matter of 

controversy.

Factors influencing tonnage. A thick and laterally extensive ore-bearing stratum is probably the result of many processes, beginning with sedimentation and ending with remobilization of copper during diagenesis or metamorphism. As in porphyry deposits, faulting and folding can cause uplift and destruction of large parts of a mineralized bed.

Factors influencing grade Original permeability 

of the sediment and abundance of organic matter, 

pyrite, or other possible copper-precipitating agents 

in the sediment are two factors that may influence 

grade. Abundance and activity of organisms may 

also be important, as well as many other factors 

now unknown. Downdip migration of copper during 

weathering may produce very high grade deposits 

by secondary enrichment.

MASSIVE SULFIDE DEPOSITS

Concordant lens-shaped deposits of copper-bearing massive pyrite are found in volcanic and sedimentary rocks. These deposits are generally more 

limited in areal extent than strata-bound disseminated deposits. Zinc, lead, and precious metals are 

found in massive sulfide ore bodies and may exceed 

copper in total value. Deposits may have discrete 

boundaries, passing from massive sulfide into barren 

wallrocks within a few centimetres. Low-grade ore 

bodies in stockworks of closely spaced quartz-chalcopyrite-pyrite veinlets are associated with some 

massive sulfide deposits, as are sedimentary beds of 

low-grade ore containing disseminated mineral fragments. Bedded deposits of manganese oxide, barite, 

or gypsum, and vein deposits of gold and silver may 

be closely associated in time and space with massive 

sulfide deposits.

Massive sulfide deposits are found in the eugeosynclinal or volcanogene parts of fold belts and in volcanic belts in Archean shield areas. Most deposits 

are strongly deformed by folds and faults clearly 

postdating the mineralization, and their origin is 

thus obscured. Some younger deposits, particularly 

the Kuroko deposits in the Miocene of Japan, show 

evidence of deposition on the sea floor by metal-rich 

exhalations from submarine volcanic eruptions (see, 

for example, Tatsumi and Watanabe, 1971). In 

these deposits, the low-grade ores in stockworks are 

believed to represent feeder channels in the underlying rock through which the exhalations passed, 

and the disseminated ores in sedimentary beds are 

believed to have been formed by the deposition of 

fine sulfide fragments scattered on the sea floor by 

the eruption activity.

Factors influencing tonnage. Massive sulfide deposits vary greatly in physical size and tonnage. 

In those deposits whose origin is clear, the strength 

and duration of submarine fumarolic activity was 

the most important factor in determining tonnage. 

If a massive sulfide ore body is closely associated 

with istockwork or disseminated mineralization of 

economic grade, then the tonnage of the combined 

ore bodies may be large.

Factors influencing grade. Grade is presumably 

controlled primarily by the copper content of the exhaled vapors and solutions relative to iron, zinc, and 

other metals. Copper content of these exhalates may 

be related to the type of associated volcanic rock. 

Anderson (1969) and others have shown that 

copper^bearing massive sulfide deposits are more 

commonly associated with mafic volcanic rocks and 

that lead-zinc varieties are associated with felsic 

volcanic rocks. The inclusion of large-tonnage disseminated and stockwork ores in a reserve estimate 

will greatly lower the average grade of the combined 

deposits. Secondary replacement of chalcopyrite and 

pyrite by chalcocite during weathering and enrichment is important in forming high-grade deposits 

under certain climatic and topographic conditions.

GEOLOGICAL CONSIDERATIONS OF 

TONNAGE AND GRADE RELATIONSHIPS

From the foregoing description of copper-deposit 

types, it becomes clear that these deposits are 

geologic entities having definite geologic boundaries. 

These boundaries represent the outer limits of the 

effects of unusual geologic processes that produce 

mineral deposits. Beyond these boundaries, "usual" 

varieties of rocks are found with copper content in 

the 5 to 150 parts per million (ppm) range. Copper 

deposits are not, therefore, merely high points or 

anomalies in a smoothly fluctuating curve of copper 

abundance in rocks.

The position of the deposit boundaries, and hence 

the tonnage of the deposit, is a function of the magnitude of the geologic systems involved, whether 

plutonic, volcanic, or sedimentary. To use a thermodynamic analogy, tonnage is an extensive variable. The abundance of copper minerals within these 

boundaries, that is, the grade, depends on a combination of factors relating to the intensity of the 

mineralization process, the concentration of the
A4 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

mineralizing solutions, the rate of change of those 

concentrations in the depositional process, and the 

availability of sites of deposition. Most importantly, 

grade is affected by repeated mineralization processes superimposed on each other (secondary enrichment) . In thermodynamic parlance, grade is an 

intensive variable. Thus, it can be concluded that, 

in general, the grade of a deposit and the tonnage of 

a deposit are controlled by separate and distinct 

geologic factors.

NATURE OF DATA

The data used include the historical production 

plus the estimated reserves of each of 267 deposits. 

Copper grades used are weighted in the appropriate 

amount by the production and reserves. Rather than 

use data from the operations of a single mining 

unit, each deposit is considered as a geologic unit. 

For example, Miami, Inspiration, and Miami East 

constitute the geologic entity at Miami, Ariz., and 

San Manuel and Kalamazoo constitute the entity at 

San Manuel, Ariz.

Grades and tonnages for the massive sulfide and 

strata-bound copper deposits were obtained from 

Manifile (Laznicka, 1973). The strata-bound deposits were selected by using the Manifile "similarity type" CPBT (Copperbelt deposits, AfricaU.S.S.R.). Only African strata-bound deposits were 

used because other known strata-bound deposits are 

few and are in different economic and possibly different geologic environments. Different economic 

conditions would alter the cutoff grades (lowest 

grade of blocks of ore considered in reserve estimates), which in turn would change the average 

grades and the tonnages. Other Manifile "similarity 

types" MSCP (massive copper pyrites), MSCZ 

(massive sulfide copper-zinc), and KRKO (Kuroko 

(Japan) deposits) were combined to provide the 

massive sulfide grades and tonnages.

Data from the porphyry copper deposits came 

from many sources. Among them are prospectus offerings of American Smelting and Refining Co. 

(1969), Kennecott Copper Corp. (1971), Newmont 

Mining Corp. (1969), Phelps Dodge Corp. (1970), 

Anaconda Co. (1968), and U.S. Smelting Refining 

and Mining Co. (1968); a book on porphyries by 

Sutulov (1974) ; and various issues of "Metals 

Sourcebook" (Metals Week, 1973-74). Additional 

grade and tonnage estimates were made by U.S. 

Geological Survey geologists familiar with individual deposits.

An attempt was made to obtain estimates of the 

grades and tonnages that ultimately might be recovered from each deposit. In general, this required 

using the tonnage and grade estimates associated 

with the lowest cutoff grade available. This effort 

was successful for the porphyries in the United 

States for which the authors have confidence in the 

estimates used. Most of the porphyries in Canada 

and in the southwest Pacific have only recently been 

drilled; a potential exists for higher tonnages as 

development proceeds. The earlier developed porphyries of South America have been operating with 

relatively high cutoff grades, which means that the 

average operating grades are high. Because of their 

very large size, some of the South American porphyries have not yet been drilled out, and estimates 

of grades and tonnages at low cutoff grades are not 

available at this time.

Although the difference is cutoff grades in diferent regions affects the average grades, reducing the 

cutoff grades to a common base would not completely remove the 'differences in average grades and 

tonnages. For example, if the average grades and 

tonnages of all the porphyry copper deposits of 

North and South America were estimated using a 

0.2 percent copper cutoff grade, the South American 

porphyries would probably still have the highest 

average grade and tonnage and the Canadian and 

Alaskan porphyries would still have the lowest average grade and tonnage. Evidence for this statement 

may be found in the relatively constant difference 

between the cutoff grade and the average grade 

within a given porphyry; the difference is relatively 

large for the South American porphyries and tends 

to become smaller for porphyries to the north.

UNIVARIATE STATISTICAL ANALYSIS

The purpose of univariate statistical analysis is to 

determine which theoretical distribution models 

best describe the individual distributions of the 

average grade and tonnage of the three types of 

copper deposits considered. In particular, the ability 

of normal and lognormal distribution models to describe distributions was examined. The adequacy 

with which these two theoretical frequency distributions fit the observed grade and tonnage frequency 

distributions was determined by using both beta 

(V&! and 62 ) and chi square goodness-of-fit statistics. 

The beta statistics describe the shape of the frequency distribution: For the normal distribution, 

V&i, « measure of asymmetry, has an expected value 

of 0.0, and b2 , a measure of peakedness, has an expected value of 3.0. The chi square statistic measures the class-to-class departure of observed distribution from the theoretical distribution model from
GRADE AND TONNAGE RELATIONSHIPS A5

which the observed distribution is hypothesized as 

being a 'sample. The beta statistics and the chi 

square statistics provide complementary information about the adequacy of fit, lesser weight generally being given to chi square statistics because of the 

arbitrary decisions that must be made when an observed frequency distribution is constructed (Shapiro and others, 1968).

Average grade and tonnage data were obtained 

from 267 copper deposits. Of these, 146 are massive 

sulfide deposits, 103 are porphyry deposits, and 18 

are strata-bound deposits. Frequency distributions 

of tonnage and grade were constructed, and summary and goodness-of-fit statistics were computed 

for each type of deposit. Within the 103 porphyry 

deposits, 4 subdivisions were recognized which were 

based upon the metallogenic province within which 

the deposits occurred. The summary statistics are 

shown in tables 1 and 2, and goodness-of-fit statistics, in table 3.

Examination of table 1 shows that the African 

strata-bound deposits have the highest average 

grade (3.78 percent Cu), followed closely by the 

massive sulfide deposits, which have an average 

grade of 2.92 percent copper. The porphyry deposits 

have an average grade of 0.63 percent Cu, which is 

nearly one-fifth that of the massive sulfide deposits. 

Within the porphyry deposits, the average grade 

ranges from 0.49 percent copper for the Canadian

deposits to 0.99 percent copper for the South American deposits. The average grades for the United 

States-Mexico deposits (0.59 percent copper) and 

the southwest Pacific deposits (0.52 percent copper) 

are close to the average grade of the Canadian 

deposits.

The massive sulfide deposits are far more variable 

in average grade than the other type of deposits; 

they have a standard deviation of 2.35 percent copper. This is a consequence of the large skewness in 

the distribution of grade of these deposits. The 

standard deviation of the average grades of the 

strata-bound deposits is 1.18 percent copper, which 

is only half that of the massive sulfide distribution. 

The standard deviations of three of the four subclasses of porphyry deposits range only from 0.12 

percent copper to 0.17 percent copper. The fourth 

member of this class of deposits (South American) 

has a standard deviation (0.36 percent copper) 

more than twice that of any of the other members of 

this class.

Porphyry copper deposits contain an average of 

548 million tons of ore (table 2), about six times as 

much ore as that contained in the strata-bound deposits, which average 91 million tons. Massive sulfide deposits contain an average of only 10.3 million 

tons of ore by comparison, or nearly an order of 

magnitude less than that contained in the average

TABLE 1. Summary statistics for the average grades of copper deposits

Arithmetic data

Type of deposit

Porphyry: 

Canada _ _ _

South America _ _ _ _

Southwest Pacific _ _ World _ __ __ ______

Massive sulfide _

All _____ __ __ ____ _ ________

Number 

of 

deposits

21

38

90

94

103

146

18

267

Mean 

grade

0.0049 

.0059 

.0099 

.0052 

.0063 

.0292 

.0378 

.0210

Standard 

deviation

0.0017 

.0014 

.0036 

.0012 

.0027 

.0235 

.0118 

.0213

Median 

grade

0.0047 

.0057 

.0093 

.0051 

.0059 

.0229 

.0362 

.0140

Logarithmic data

Mean, 

logio 

of grades

 2.331

 2.243 

 2.033

 2.296

 2.233

 1.640 

 1.442

 1.855

Standard 

deviation, 

logio 

of grades

0.147 

.101 

.160 

.114 

.162 

.303 

.132 

.392

TABLE 2. Summary statistics for the tonnage of copper deposits

Arithmetic data

Type of deposit

Porphyry:

United States and Mexico __

Southwest Pacific

World _

All _ _ _ __ _ __ __

Number 

of 

deposits

21

38 

20 

24 

103 

146 

18 

267

Mean 

(millions of 

metric 

tons 

of ore)

245 

815 

773 

203 

548 

10.3 

91 

223

Median 

(millions of 

metric 

tons 

of ore)

177 

338 

347 

120 

234 

2.26 

41.4 

16.5

Mean 

(thousands of 

metric 

tons 

of copper)

1,210 

4,781 

7,622 

1,058 

3,452 

301 

3,453 

4,679

Median 

(thousand of 

metric 

tons 

of copper)

824

1,932 

3,214 

608 

1,368 

52 

1,496 

230

Logarithmic data

Mean, 

logio of 

metric 

tons

8.247 

8.529 

8.540 

8.080 

8.369 

6.354 

7.617 

7.217

Standard 

deviation, 

logio of 

metric 

tons

0.354 

.629 

.610 

.436 

.565 

.828 

.690 

1.208
A6 GRADE AND TONNAGE RELATIONSHIPS

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GEOLOGY AND RESOURCES OF COPPER DEPOSITS A7

strata-bound deposit. Nearly the same relative differences in tonnages between deposit types were 

found when the median tonnage is used as a measure 

of the "center of gravity" of the distributions. 

Strata-bound deposits contain a median 41.4 million 

tons, and the massive sulfides have a median of 2.26 

million tons. When considering the distribution of 

the tonnage of ore in each type of deposit, the median is a more appropriate measure of the center 

of gravity because of the large skewness toward the 

high tonnage tail in each of these distributions.

African strata-bound deposits have the largest 

quantity of contained copper & median of 1,496,000 

tons. The porphyry deposits have a median that is 

slightly less 1,368,000 tons and the massive sulfide deposits contain even less copper by comparison a median of 52,000 tons. Within the four subclasses of porphyry deposits, the median metal 

content ranges from 608,000 tons for the southwest 

Pacific deposits to 3,214,000 tons for the South 

American deposits. The United States-Mexico deposits have a median of 1,932,000 tons of copper 

metal, which is more than twice as much as the 

median for the Canadian deposits but only a little 

more than half that for the South American deposits.

The process of selecting an appropriate theoretical 

distribution model to describe the observed grade 

and tonnage frequency distributions for each of the 

three types of copper deposits was based upon statistical tests of the beta and chi square goodness-offit statistics. The results of these tests are shown 

in table 3. With the exception of the southwest 

Pacific porphyry deposits, the lognormal distribution model was found to be superior to the normal 

distributions in describing the observed distribution 

of mean grade and tonnage.

Each of the observed distributions of tonnage of 

ore studied was found to be highly skewed toward 

high tonnages, and, as a consequence, the normal 

model does not provide an adequate fit to the observed data. The lognormal model, on the other hand, 

was found to fit each of the observed tonnage distributions rather well. For each category shown in 

table 3, column 1, the skewness of the observed tonnage distribution was within the expected range for 

a sample from a lognormal population, and in only 

one category did the peakedness depart significantly 

from the lognormal model. In four of the eight categories, the chi square goodness-of-fit statistics were 

significant. However, we give less weight to the chi 

square test statistic because the statistic will produce varying results depending upon the choice of 

class limits. The lognormal distribution is accepted 

as an adequate model for the distribution of tonnage 

of ore in deposits from each category of deposit.

As a result of the above analysis and the assumption that the observed data can be regarded as randomly drawn from their respective parent populations, the lognormal models provide not only an 

adequate description of tonnage of ore in yet-to-bediscovered deposits of the three types of deposits 

studied, but also offer an adequate description of 

their mean grades. An additional benefit of the above 

results is that the correlation between the logarithms of the mean grade and tonnage in these deposits can be tested statistically.

CORRELATIONS BETWEEN GRADE AND 

TONNAGE

Among the problems that must be resolved before 

resource models can be constructed is the relationship between the pairs of variables being examined. 

In this paper, the pairs of variables are the average grades and the tonnages of the copper deposits 

in each of the sets of deposit types under consideration; the degree to which these variables vary together is tested. The degree of association or interdependence of the pairs of variables is examined by 

means of the correlation coefficient for each set of 

samples. The values of the correlation coefficient can 

range between minus one and plus one, a perfect 

linear relationship being indicated by a minus one 

or plus one value, and complete linear independence 

indicated by zero. In this paper, the sample correlation coefficients are tested in table 4 against the 

null hypothesis that the population of grades and 

tonnages has zero correlation. The frequency distribution of the pairs of logarithmic variables examined in the last section closely approximate the 

bivariate normal distribution necessary for the significance tests of the null hypothesis.

TABLE 4. Correlation coefficients between average copper 

grade and total tonnage

[NS, not significant; **, significant at the 1-percent level]

Type of deposit

Porphyry:

United States and Mexico

World ________ -. 

All ____________-.____ - --

Number 

of 

deposits

21 

38 

20 

24 

103 

146 

18 

267

Correlation coefficient

Arithmetic 

data

-0.16NS 

.OONS 

 .07NS 

-.05NS 

.09NS 

 .13NS 

-.IONS 

 .22**

Logarithmic 

data

 0.22NS 

 .09NS 

-.17NS 

 .07NS 

.05NS 

 .42** 

 .19NS 

 .67**
A8 GRADE AND TONNAGE RELATIONSHIPS

MILLIONS OF METRIC TONS OF ORE

10.0 100.0 1000.0 10000.0

-1.7 ---

-1.8

-1.9

-2.0

-2.1

-2.2

-2.3

-2.4

-2.5

-2.6

-2.7

-2.8

1 1 1 I I 

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o 

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i_

z

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o

Ul

Q.

Z v

Ul

a

£

(3

COPPER

Ul

(3

£

Ul

>

LOG TONNAGE (METRIC) 

10

FIGURE 1. Grades and tonnages for porphyry copper deposits.

Sample correlation coefficients calculated for both 

the arithmetic data and the logarithmic data for 

each group of porphyry coppers and for all the 

porphyries together provide no basis for rejecting 

the null hypothesis of zero correlation between average grade and tonnage (fig. 1). In fact, less than 

5 percent of the variation of tonnage is explained 

by the average grade for these deposits. These results also hold for the strata-bound deposits (fig. 2).

The correlation coefficients for both the massive 

sulfide deposits (fig. 3) and the combination of all 

deposits are negative and highly significant (table 

4). This suggests a significant negative relationship 

between grade and tonnage for both groups, although it should be noted that the combination of all 

deposits represents not only a mixture of geologic 

types of deposits, but a statistical mixture as reflected in the significant departures of grade and 

tonnage from the- lognormal distribution. Additional 

statistical analyses were performed but not included

on two of the subgroups of massive sulfides; logarithms of the grades and tonnages from the Manifile "similarity types" massive copper pyrites and 

massive sulfide copper-zinc (see p. 1-2) each had 

significant negative correlations.

The negative relationship between grade and tonnage for the massive sulfide deposits may be accounted for by the fact that some deposits include a 

combination of low-tonnage high-grade massive ores 

and large-tonnage low-grade stockwork and disseminated ores. These ores, though genetically related, 

represent a mixed population with respect to the 

tonnage-grade relationship.

DISCUSSION AND CONCLUSIONS

The independence of grade and tonnage for porphyries is convenient for modeling of grades and 

tonnages of yet unmined deposits. As the grades and 

tonnages are essentially lognormally distributed for
GEOLOGY AND RESOURCES OF COPPER DEPOSITS 

MILLIONS OF METRIC TONS OF ORE

A9

i.o 10.0 100.0

-1.1

-1.2

-1.3

-1.4

-1.5

-1.6

O

O

O

O

O 

O

O

O

O

O oo

O O

O

O

O

-e7.9

6.3

5.0 g

O 

CK 

LU 

Q.

LU 

Q

o: 

4.0 CD

tr LU

Q. 

Q. 

O 

O

LU 

|

3.2 £

2.5

2.0

6.0 7.0 8.0 8.5

LOG 1Q TONNAGE (METRIC)

FIGURE 2. Grades and tonnages for African strata-bound copper deposits.

the porphyries, the calculation of the probability of 

the occurrence of any particular range of grades or 

tonnages, given that a porphyry exists, is an easy 

task. Because of the independence of grade and tonnage, the probability of a deposit falling in particular ranges of grades and tonnages is simply the 

product of the probabilities of the range of grades 

and of the range of tonnages. For example, suppose 

it is desirable to find the probability of a porphyry 

having a very low grade and a very large tonnage.

The probability of a very low grade, say less than 

two standard deviations below the mean (less than 

0.28 percent copper), is equal to 0.023 from tables of 

the areas of the normal curve (Dixon and Massey, 

1957). The probability of a very large tonnage, say 

more than three standard deviations above the mean 

(greater than 11.59 billion tons), is 0.0013 from the 

normal curve tables. Thus, the probability of a porphyry having a grade less than 0.28 percent and a 

tonnage greater than 11.59 billion tons is 0.023 X
A10 GRADE AND TONNAGE RELATIONSHIPS 

MILLIONS OF METRIC TONS

0.001 0.01 0.1 1.0 10.0 100.0 1000.0

n 0 , . , . __________ - __________ __________ , __________ , , , . . IK 0

 U.o ' i i 

-1.0

-1.2

uj -1.4 

Q

< 

CC

U

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a

cc.

Ul

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0

cf 

o

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-2.6

 O Q

I 1 0 1 1 1 i 1 1 1 1 1

o

o

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o o

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0 0

0 ° ° o a °

0 0 0 " 0 

rQ"o

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8 ° o 0

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(&) 

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S CD 0 0 ^Q

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0 ° OO O

o o o o 

OD° o

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0 0° ° CQ ° -

0 

0 00

0 0 0 

- 0 _

0

0 ~

-

o

1 1 1 1 1 1 1 1 I 1 1

i^>. O

10.0

6.3

4.0 Z

Ul 

0

cr

UJ

Q.

2.5 5

^

Ul

0

< 

1-6 g

(X 

UJ 

Q.

Q.

1.0 0

UJ 

0

<

0;63 S

^

0.40

0.25

n ie

£.O \j.s.\j

3.0 4.0 5.0 6.0 7.0 8.0 9.0

LOG 1Q TONNAGE (METRIC) 

FIGURE 3. Grades and tonnages for massive sulfide deposits.

0.0013 or 0.00003; that is, if 33,000 deposits were 

discovered, only one of them would be likely to have 

these characteristics a very unlikely event indeed. 

It is interesting to note that if such a deposit exists, 

it contains about 32 million tons of copper, which 

is about the amount contained in the largest known 

porphyries, and it would require the moving of 

about 4 cubic miles of rock to mine it.

If improvements in technology allow mining of 

lower and lower grade porphyry deposits, the most 

probable tonnage of these new deposits will be 

roughly the same as those presently being exploited; 

thus, each deposit will contain less copper. D. P. 

Harris (oral commun., 1973) has suggested that 

low-grade deposits of median tonnage are more 

abundant than those of higher grades. Although no

geologic evidence is presently available to support 

this assumption, it represents an important area 

for future investigation.

The lack of correlation of tonnage and grade for 

both the strata-bound and porphyry deposits has 

implications not only for quantitative modeling, such 

as mentioned above, but also qualitative modeling. 

Qualitative modeling is often used by those considering the future supply of natural resources, and 

usually an assumption of a negative correlation between grade and tonnage is implicit in their discussions. The above analysis does not support this view. 

Such untested beliefs could have arisen from several 

sources including the trend through time of lower 

average copper grades mined each year, which is 

related to improvements in technology, changing de-
GEOLOGY AND RESOURCES OF COPPER DEPOSITS All

posit types, and the tendency for porphyries to have 

richer grades in the earliest mined areas. Another 

possible source of these beliefs could come from 

misinterpretation of Lasky's (1950) work on the 

grade and tonnage relationship Lasky found that 

within porphyries the cumulative tonnage increases 

at a constant geometric rate as the grade decreases 

at an arithmetic rate. The method of using cumulative tonnage could be applied across porphyry deposits, and the results would be consistent with 

Lasky's results (John Whitney, written commun., 

1974). Lasky's cumulative contained copper curves 

become flat at a point that he called zero cutoff 

grade. Zero cutoff grade represents the outer limit 

of the effects of the special geologic conditions that 

gave rise to the ore deposit. Copper content does 

not fall to zero at this limit but tends to approach 

average abundance of copper in the Earth's crust.

In conclusion, an inverse relationship between 

grade and tonnage is often a hidden assumption in 

mineral-resource supply models. Use of such models 

for policy decisions could have a devastating effect 

on the orderly supply of minerals if large tonnagevery low grade deposits do not exist.

From the inverse relationship between average 

grade and tonnage shown by the mixture of three 

deposit types, it is tempting to extrapolate the existence of large-tonnage very low grade deposits, 

and it can be reasoned that such deposits can supply 

sufficient copper for our future needs. Such an extrapolation is misleading for statistical and geological reasons. It is statistically erroneous to extrapolate on the basis of a mixed population, especially 

because the distributions of tonnages and grades in 

the largest tonnage lowest grade (porphyry) subset 

of the population show that large-tonnage very low 

grade deposits are rare occurrences. In addition, discussion with industry geologists familiar with porphyry deposits indicates that a large number of such 

deposits is not being held in company inventories at 

present.

If large-tonnage very low grade deposits exist, 

they probably will not resemble any of the three 

important types presently exploited. Discovery of 

such unconceived resources will require continuing

research on a broadly based spectrum of geologic 

phenomena.

REFERENCES CITED

American Smelting and Refining,Co., 1969, [Prospectus proposing a merger with Pennzoil United, Inc., January 

1969] : 66 p.

Anaconda Co., 1968 [Prospectus offering debentures, Nov. 

1968]: 37 p

Andersen, C. A., 1969, Massive sulfide^ deposits and volcanism: 

Econ. Geology, v. 64, no. 2, p. 129-146.

Dixon, W. J., and Massey, F. J., Jr., 1957, Introduction to 

statistical analysis [2d ed.]: New York, McGraw-Hill, 

488 p.

Kennecott Copper Corp., 1971, [Prospectus offering debentures, 

April 1971] : 14 p.

Lasky, S. G., 1950, How tonnage and grade relations help predict ore reserves: Eng. and Mining Jour., v. 151, no. 4, p. 

81-85.

Lazinicka, Peter, compiler, 1973, Manifile; the University of 

Manitoba file of nonferrous metal deposits of the world: 

Manitoba Univ., Dept. Earth Sci., Centre Precambrian 

Studies Pub. 2, 3 v.: Pt. 1, 533 p.; Pt. 2, v. 1, 298 p.; Pt. 

2, v. 2, 767 p.

Lowell, J. D., and Guilbert, J. M., 1970, Lateral and vertical 

alteration-mineralization zoning in porphyry ore deposits: 

Econ. Geology, v. 65, no. 4, p. 373-408.

Metals Week, 1973-74, Metals sourcebook: New York, McGraw-Hill, [weekly pub.].

Meyer, Charles, Shea, E. P., and Goddard, C. C., Jr., 1968, 

Ore deposits at Butte, Montana, in Ridge, J. D., ed., Ore 

deposits of the United States, 1933-1967 (Graton-Sales 

Volume), v. 2: New York, Am. Inst. Mining. Metall., and 

Petroleum Engineers, p. 1373-1416.

Moolick, R. T., and Durek, J. J., 1966, The Morenci district, 

in Titley, S. R., and Hicks, C. L., eds., Geology of the 

porphyry copper deposits, southwestern North America: 

Tucson, Ariz., Arizona Univ. Press, p. 221-231.

Newmont Mining Corp., 1969, [Prospectus proposing a merger with Magma Copper Co., May 1969] : 59 p.

Phelps Dodge Corp., 1970, [Prospectus proposing a merger 

with Western Nuclear, Inc., October 1970] : 8 p.

Shapiro, S. S., Wilk, M. B., and Chen, H. J., 1968, A comparative study of various tests for normality: Am. Stat. Assoc. 

Jour., v. 63, no. 324, p. 1343-1372.

Sutulov, Alexander, 1974, Copper porphyries: Salt Lake City, 

Utah, Univ. Utah Printing Services, 200 p.

Tatsumi, Tatsuo, and Watanabe, Takeo, 1971, Geological environment of formation of the Kuroko-type deposits: Soc. 

Mining Geology Japan, Spec. Issue 3, p. 216-220.

U.S. Smelting Refining and Mining Co., 1968, [Prospectus offering debentures, Feb. 1968]: 8 p.

Geochemical Exploration Techniques 

Applicable in the Search for 

Copper Deposits

By MAURICE A. CHAFFEE

GEOLOGY AND RESOURCES OF COPPER DEPOSITS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 907-B

A compilation of proved and untried geochemical 

sampling media and techniques that may be useful 

in the search for new copper deposits

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975

CONTENTS

Abstract _______________

Introduction ___________

Geochemical sampling media _- 

Rocks and minerals _____

Soils __ -________________

Soil gas _________-__ 

Transported materials ____

Glacial debris ________

Stream sediments ____ 

Lake sediments ________

Water and related materials 

Water __________. 

Ice __________

Snow _____________

Vegetation __________

Geobotanical techniques

Page

Bl

1

2

2

4

6

11

11

11

13

13

14

14

Geochemical sampling media Continued 

Vegetation Continued

Biogeochemical techniques ____

Micro-organisms ________ -

Animals _ ______

Atmosphere _______ ____

Remote sensing ___________

Statistics ____________ 

Introduction ______________.

Correlation analysis ____ ___

Regression analysis _________

Discriminant analysis ________

Factor analysis __________

Cluster analysis ___________

Trend-surface analysis ________

Moving-average analysis ________

References cited __________________

Page

B14

16

17

18

19

21

21

21

22

22

22

22

23

23

23

ILLUSTRATIONS

FIGURES 1-4. Photographs:

1. Plastic hemisphere used in soil-gas sampling _.___________

2. Typical desert wash used in stream-sediment sampling ___

3. Mesquite roots used as a biogeochemical sampling medium __

4. Plastic bags used to collect exudates for geochemical analysis

Page

B7 

9

16 

20

in

GEOLOGY AND RESOURCES OF COPPER DEPOSITS

GEOCHEMICAL EXPLORATION TECHNIQUES APPLICABLE 

IN THE SEARCH FOR COPPER DEPOSITS

By MAURICE A. CHAFFEE

ABSTRACT

Geochemical exploration is an important part of copper-resource evaluation. A large number of geochemical exploration 

techniques, both proved and untried, are available to the geochemist to use in the search for new copper deposits.

Analyses of whole-rock samples have been used in both regional and local geochiemical exploration surveys in the search 

for copper. Analyses of mineral separates, such as biotite, 

magnetite, and sulfides, have also been used.

Analyses of soil samples are widely used in geochemical exploration, especially for localized surveys. It is important to 

distinguish between residual and transported soil types. Orientation studies should always be conducted prior to a geochemical investigation in a given area in order to determine the best 

soil horizon and the best size of soil material for sampling in 

that area. Silty frost boils, caliche, and desert varnish are 

specialized types of soil samples that might be useful sampling media.

Soil gas is a new and potentially valuable geochemical sampling medium, especially in exploring for buried mineral deposits in arid regions. Gaseous products in samples of soil may 

be related to base-metal deposits and include mercury vapor, 

sulfur dioxide, hydrogen sulfide, carbon oxysulfide, carbon 

dioxide, hydrogen, oxygen, nitrogen, the noble gases, the halogens, and many hydrocarbon compounds.

Transported materials that have been used in geochemical 

sampling programs include glacial float boulders, glacial till, 

esker gravels, stream sediments, stream-sediment concentrates, 

and lake sediments. Stream-sediment sampling is probably the 

most widely used and most successful geochemical exploration 

technique.

Hydrogeochemical exploration programs have utilized hotand cold-spring waters and their precipitates as well as waters 

from lakes, streams, and wells. Organic gel found in lakes and 

at stream mouths is an unproved sampling medium. Suspended 

material and dissolved gases in any type of water may also 

be useful media. Samples of ice and snow have been used for 

limited geochemical surveys.

Both geobotanical and biogeochemical surveys have been 

successful in locating copper deposits in many parts of the 

world. Micro-organisms, including bacteria and algae, are other 

unproved media that should be studied.

Animals can be used in geochemical-prospecting programs. 

Dogs have been used quite successfully to sniff out hidden 

and exposed sulfide minerals. Termite mounds are commonly 

composed of subsurface material, but have not as yet proved

to be useful in locating buried mineral deposits. Animal tissue 

and waste products are essentially unproved but potentially 

valuable sampling media. Knowledge of the location of areas 

where trace-element-associated diseases in animals and man 

are endemic, as well as a better understanding1 of these 

diseases, may aid in identifying regions that are enriched in 

or depleted of various elements, including copper.

Results of analyses of gases in the atmosphere are proving 

valuable in mineral-exploration surveys. Studies involvingmetallic compounds exhaled by plants into the atmosphere, and 

of particulate matter suspended in the atmosphere are reviewed; these methods may become important in the future.

Remote-sensing techniques are useful for making indirect 

measurements of geochemical responses. Two techniques applicable to geochemical exploration are neutron-activation 

analysis and gamma-ray spectrometry. Aerial photography is 

especially useful in vegetation surveys. Radar imagery is an 

unproved but potentially valuable method for use in studies 

of vegetation in perpetually clouded regions.

With the advent of modern computers, many new techniques, such as correlation analysis, regression analysis, discriminant analysis, factor analysis, cluster analysis, trendsurface analysis, and moving-average analysis can be applied 

to geochemical data sets. Selective use of these techniques can 

provide new insights into the interpretation and understanding 

of geochemical data that would not be possible without the 

use of computers.

INTRODUCTION

An important part of our fund of knowledge of 

copper resources must include techniques for the 

exploration for new copper deposits. Among the 

techniques, those using geochemistry are becoming 

increasingly important.

For this report I reviewed much of the geochemical literature. I have selected for discussion primarily those papers that deal with geochemical exploration for copper deposits in the widely differing geologic and climatic environments found on earth. In 

addition to published reports, I have also included, 

wherever pertinent, information on geochemical 

prospecting investigations currently being conducted 

by scientists in the U.S. Geological Survey. I have

Bl
B2 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

also suggested some unproved geochemical exploration techniques that might be useful in the search 

for new copper deposits.

No attempt has been made to compare and evaluate the various studies described in this report; too 

many variations exist to permit valid comparisons. 

Undoubtedly, some techniques have been overlooked; 

other methods, developed by private organizations, 

have not been documented in the literature, either 

because of their success or because of their lack of it.

Those readers interested in more complete discussions of the large geochemical prospecting bibliography should refer to the books by Ginzburg 

(1960), Hawkes and Webb (1962), and Levinson 

(1974). A review article dealing exclusively with 

porphyry copper deposits has also been published 

recently (Coope, 1973).

GEOCHEMICAL SAMPLING MEDIA 

ROCKS AND MINERALS

Geochemical sampling of rocks is the most basic 

type of sampling because, ultimately, it is the rock 

material of mineral deposits that must be studied 

and mined. Several types of rock samples are used 

in exploration. The two main types are (1) wholerock samples and (2) mineral-separate samples.

Whole-rock samples can be used to provide information about the abundance and distribution of 

elements or groups of elements. Variations in the 

concentrations of minor elements can be statistically 

interpreted to define mineralized areas on any scale. 

On a regional scale, geochemical studies alone, or in 

combination with geologic and (or) geophysical data, 

can be used to define copper-rich metallogenic provinces or districts. On a local scale, whole-rock analyses can be used for detailed studies to locate individual deposits or extensions of known deposits.

The use of bedrock geochemical surveys has generally not found favor among geochemists for several reasons. Rock bodies have normal variations in 

trace elements that can be ascribed to many causes 

other than mineral deposits. Thus, it is difficult to 

collect representative samples that can be economically evaluated purely on the basis of chemical 

abundances. Also, bedrock samples have not undergone mechanical dispersion that might enlarge a 

potential target.

Whole-rock analyses of bedrock samples can be 

useful in the search for large copper ore bodies such 

as porphyry-, magmatic segregation-, or beddedtype deposits. A comparison of the trace elements in 

samples of rock taken from intrusive rocks throughout a region can be used as a regional exploration 

guide. It is important, therefore, in this type of 

study that the samples collected from a given intrusive rock are as representative as possible of the 

character of the entire rock body.

Putnam and Burnham (1963) used whole-rock 

analyses in a regional study of a part of the Arizona 

copper province. They found that samples from 

plutonic bodies associated with known copper mineralization contained higher copper concentrations 

than did those from plutonic bodies not associated 

with known copper deposits. Most of the copper was 

found to occur in the ferromagnesian and sulfide 

minerals.

Other applications of whole-rock trace-element 

chemistry as a regional exploration technique in the 

search for copper were carried out in British Columbia by Warren and Delavault (1960, 1969). These 

authors used hot aqua regia for extraction, and 

found that copper and zinc concentrations in the 

samples from both plutonic and volcanic rock bodies 

indicated the presence of genetically associated copper mineralization nearby. High copper-zinc ratios 

(>0.35 for volcanic rocks and >0.20 for plutonie 

rocks) provided further information as to the favorability of certain areas.

In a study undertaken by the Geological Survey 

of Canada, Sakrison (1971) described another regional geochemical survey using whole-rock samples. 

In that survey, copper concentrations in bedrock 

samples from a large area of the Canadian Shield 

were determined. A number of areas anomalously 

high in copper were found, some of which correlated 

with known mineralized rock bodies.

Similar surveys using whole-rock samples may be 

useful for appraising the potential of any region 

thought to contain large strata-bound copper deposits. Many regions contain a thick stratigraphic 

section composed of many different rock types. 

Widely differing rock types can be expected to contain large normal variations in the abundances of 

many chemical elements. Ideally, it would be desirable to select rock material from only one formation or bed as a representative sample of the entire 

stratigraphic section at a given outcrop of the section. This restricted sampling scheme would significantly reduce the number of samples needed, and 

would also require sampling of only one lithology, 

thereby reducing or eliminating the complication of 

any chemical variability caused by sampling of different lithologies with different normal abundances 

of various trace elements. Results of sampling of 

selected beds or formations should be able to show
GEOCHEMICAL EXPLORATION TECHNIQUES B3

trends in trace-element content and also to indicate 

those geologic formations or beds that are most 

likely to contain copper deposits. My experience in 

the vicinity of the Vekol porphyry copper-molybdenum deposit near Casa Grande, Ariz., indicated 

that this technique of sampling a particular stratigraphic unit, when applied on a regional basis, could 

delineate localities containing epigenetic copper minerals in favorable sedimentary host formations. In 

my study, rock samples showing no visible signs of 

hydrothermal alteration were collected from formations in a Paleozoic sequence that cropped out over 

a large region in the vicinity of the Vekol deposit. 

Anomalous copper concentrations were detected in 

samples from several formations over 3 km (about 

2 mi) away from the deposit. Copper concentrations 

generally increased in samples taken progressively 

closer to the deposit; maximum concentrations were 

found in samples directly over the deposit. Anomalous molybdenum concentrations were detected in 

only two of the formations present in the region, 

and then only in those samples collected directly over 

the Vekol deposit; the same formations in other localities did not contain anomalous amounts of molybdenum.

At Jerome, Ariz., primary copper from Precambrian ore deposits was mobilized by weathering and 

deposited as syngenetic copper in the superjacent 

Cambrian sandstone that is present at the base of a 

thick Paleozoic sedimentary sequence. Huff (1955) 

collected and analyzed samples of this basal sandstone unit and demonstrated that this formation 

contained anomalous concentrations of copper as 

much as 3 km (about 2 mi) away from the known 

ore deposits.

Whole-rock analyses can also be used on a local 

scale to identify individual deposits in a district or 

to find extensions of known deposits. Most large ore 

bodies have boundaries that are economic rather 

than mineralogic; that is, their boundaries are based 

on a grade of ore that is economic and not on a sudden disappearance of a particular element. Looked 

at another way, most large ore bodies have dispersion aureoles of various elements. The geochemieal 

expression of a hypothetical porphyry deposit of the 

type common to the southwestern United States was 

reviewed by Jerome (1966). He noted that gold, 

silver, and molybdenum are commonly found enriched in the copper-ore zone, a fact generally 

known. On the basis of analyses of ores, gossans, 

leached cappings, smelter products, tailings, and 

concentrates from copper deposits, Jerome concluded 

that antimony, arsenic, bismuth, cobalt, indium,

nickel, rhenium, selenium, tellurium, tin, and vanadium may also be enriched in or around the copperore zone. My investigations confirm, at least in part, 

Jerome's findings. Most of the elements mentioned 

by Jerome have been detected in the ore zone of the 

Kalamazoo ore body near San Manuel, Ariz. Distributions of the trace-element concentrations relative 

to the known ore body indicate that many trace 

elements form either positive or negative primary 

dispersion aureoles around the copper deposit. Perhaps a statistical study of the concentrations of a 

group of trace elements in rock samples, together 

with their locations relative to this known porphyry 

copper ore body, can be used to predict the direction 

to the copper ore from any point within the hostrock body. If this type of study is successful and if 

a geochemieal model for the trace-element aureoles 

can be derived for this deposit, then it may be possible to use geochemieal data from sample sites near 

other similar but seemingly barren plutonic rocks 

to predict (1) the presence or absence of copper 

deposits at depth and (2) the possible location of a 

blind deposit.

Studies made of trace elements in rock samples 

near volcanogenic copper deposits in the Soviet 

Union revealed dispersion patterns of the ore metals, 

and such volatile elements as iodine, extending as 

far as 140 m (460 ft) from known ore bodies (Rubo, 

1969). This study, when considered in conjunction 

with previously described studies, indicates that the 

use of trace-element zoning patterns in bedrock may 

be valid as a geochemical-exploration technique for 

nearly any type of copper deposit.

Analysis of mineral separates is another approach 

to rock sampling. In their regional study of plutons 

in Arizona, Putman and Burnham (1963) noted that 

essentially all of the zinc and much of the copper in 

their rock samples came from the ferromagnesian 

minerals. Even more interesting is their observation 

that "in every sample in which chalcopyrite is presumed to be present, the coexisting biotite has a 

relatively high copper content, usually above the 

average copper content of biotites from other samples" (Putman and Burnham, 1963, p. 72). These 

authors concluded that the biotite and the chalcopyrite came from the same magma. Studies of the concentrations of copper in biotites from major copper 

districts in Nevada, Utah, Montana, and Arizona 

have since confirmed the close relationship between 

copper-rich biotite and copper-rich rock bodies 

(Parry and Nackowski, 1963; Al-Hashimi and 

Brownlow, 1970; Levering and others, 1970).
B4 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

A number of investigations of the trace-element 

content of magnetite have been made. Hamil and 

Nackowski (1971) demonstrated that the concentration of copper in magnetites was not a good criterion for locating porphyry-copper intrusive rocks. 

In contrast, they found that low concentrations of 

titanium and zinc in magnetite were better indicators of copper-related intrusive rocks. In my experience, the lack of correlation of concentrations of copper in magnetites with intrusive rock bodies enriched in copper-bearing minerals may be caused by 

the inability of the person making the study to interpret properly the age of the magnetite relative to 

the age of the copper-bearing minerals. The magnetite may not be cogenetic with the economic copper-bearing minerals. When this latter situation 

holds true, the trace-element content of the magnetite (or possibly of other mineral separates) probably bears no useful relationship to the presence or 

absence of copper in the intrusive body.

In another study, Smith (1970) determined the 

trace-element content of heavy-mineral fractions 

from samples collected from a group of plutpns in 

the Plumas copper belt in northern California. He 

found that the highest copper concentrations were 

restricted to samples collected from those plutons 

that contained the copper deposits that produced the 

largest tonnages of copper ore in the region.

The analysis of trace elements in metal sulfide 

concentrates derived from whole-rock samples has 

been used as a means of defining metallogenic provinces for several elements. Burnham (1959) analyzed more than 500 samples of chalcopyrite and 

sphalerite from 172 mining districts widely scattered 

throughout the southwestern United States. His data 

showed closely coincident zones or belts enriched in 

such elements as tin, silver, cobalt, nickel, indium, 

gallium, and germanium. The belts defined by the 

trace-element content of sphalerite are considered 

by Burnham to coincide with the trends of known 

copper belts.

The rhenium concentration in samples of molybdenite from porphyry copper deposits was shown to 

differ markedly from its concentration in samples 

of molybdenite from other types of deposits (Giles 

and Schilling, 1972). These differences in concentration suggest that the abundance levels of trace 

elements in ore minerals may be a useful guide in 

evaluating porphyry copper-molybdenum deposits 

and possibly other types of copper deposits.

Undoubtedly many other specialized types of rock 

samples could be used. Obvious ones include (1) the 

iron- or manganese-oxide materials found in gossans, leached cappings, and vein fillings, and (2)

fluid inclusions and geodes. Leakage halos may be 

present in the rocks cropping out above a mineral 

deposit that has not yet been exposed by erosion. In 

such an environment, the systematic sampling of any 

material suspected to be hydrothermally associated, 

such as iron- or manganese-stained fracture fillings 

or silica veinlets, could provide evidence of a concealed mineral deposit that would not be found by 

the sampling of only typical bedrock material. If 

veins are sampled, particular attention should be 

paid to the relationship, if any, between the vein 

orientation and the trace-metal content of the veins 

at each orientation, because veins of certain orientations may be of interest economically while those 

at other orientations may not (Rehrig and Heidrick, 

1972).

Fluid inclusions are commonly thought to represent samples of trapped hydrothermal ore fluids; 

consequently, the concentrations of various elements 

in the materia.1 found in fluid inclusions in rocks may 

also prove valuable in exploring for copper deposits. 

In their study of a porphyry copper prospect in 

Puerto Rico, D. P. Cox and J. T. Nash (written 

commun., 1973) found that the fluid inclusions in 

quartz from rocks collected over or near the copperrich exposures contain halite; similar inclusions 

from rocks collected outside the mineralized area do 

not contain halite. From a geochemical standpoint, 

then, a study of the trace-element content of fluid 

inclusions may provide a clue as to the location of 

copper deposits.

Geodes are another type of rock-sampling medium 

which might be useful for geochemical prospecting 

in the limited areas in which they occur. Because 

geodes crystallize from the outside to the center, the 

trace-element chemistry of the internal layers in a 

geode should reflect the chemistry of the environment surrounding the geode at the time the geode 

began to form. In this sense, geodes can be considered to be hand-specimen equivalents of fluid 

inclusions, and chemical analysis of geode interiors 

might be a useful means of studying some areas, 

especially those that have undergone deep weathering which might have leached more mobile elements 

such as copper from the surface rock and soil materials. The practical application of geode chemistry 

to geochemical exploration has not been demonstrated as yet, however.

SOILS

The use of soils in geochemical prospecting for 

copper has proved both popular and successful 

worldwide in many geochemical exploration programs. Soil sampling has been used mainly for detailed appraisals of limited areas rather than for
GEOCHEMICAL EXPLORATION TECHNIQUES B5

regional reconnaissance programs. A review of the 

various routine applications of soils to prospecting 

is beyond the scope of this report; the reader is 

referred to comprehensive discussions such as those 

by Hawkes and Webb (1962, p. 202-226) and Bradshaw, Clews, and Walker (1970a, 1970b; 1971; 

1972).

In most geochemical sampling techniques, it is 

important that persons collecting soil samples have 

an appreciation of the geology of the area being 

sampled and some idea of the characteristics of the 

soil being sampled. For example, it is important to 

know what soil horizon is being sampled and 

whether the soil is residual or transported.

A soil profile is commonly developed over bedrock 

except in most desert areas and in areas undergoing 

rapid erosion. It is important that orientation studies 

be made in any new area to determine information 

such as the best soil horizon for sampling as well as 

the best size of soil material to retain for analysis. 

Where present, the B horizon has most commonly been used for soil sampling because this horizon 

is normally enriched in metals relative to the A 

horizon. Under special conditions, the upper A horizon, which is rich in organic material, has also 

proved to be an effective sampling medium. Boyle 

and Dass (1967) reported that A-horizon soil samples from the Cobalt, Ontario, district contained 

higher concentrations of various elements including 

copper than did equivalent B-horizon samples. Their 

A-horizon samples also showed better anomaly contrast values. The authors attribute the higher metal 

values in the A horizon to biogeochemical enrichment processes. In another study, Curtin and 

others (1971) found that the ash of the forest 

humus layer (mull) from samples collected in the 

Empire district in Colorado was enriched in a 

number of base and precious metals as compared 

with concentrations of these metals in the underlying soils. The mull reflected known bedrock chemistry better than did the underlying soils, because 

the mull was derived from trees rooted in bedrock 

whereas the soils were derived mainly from transported glacial till.

The optimum grain size of soil material to be used 

for geochemical analysis should be determined for 

any new area being studied. The best size to use will 

depend on such factors as the original grain size of 

the minerals present, the degree to which these 

original grains have been eroded by weathering, and 

the anomaly contrast values obtained for each grain 

size. Hawkes and Webb (1962, p. 166) clearly demonstrated the effects of different grain sizes on concentrations for nine different elements.

In areas of transported cover, geochemical prospecting using soils has not been particularly useful. 

Hawkes and Webb (1962, p. 203) reported, however, 

that near-surface anomalies have been found over 

glacial till and alluvial deposits that were as much 

as 15 m (50 ft) thick. Apparently the metals causing the anomalies migrated to the surface either in 

the circulating ground water or through translocation from tree roots to leaves and then into the 

ground from the fallen leaves. Studies in humid 

areas (Curtin and others, 1971) and in arid areas 

(Chaffee and Hessin, 1971) have indicated that 

plants or plant materials are superior to soils as 

sampling media in areas containing soils derived 

from transported overburden.

Much better results have been obtained in soil 

sampling programs that utilized residual soil; however, the fact that the soil is residual does not assure 

that it will be a good sampling medium. Residual 

soil can form on top of transported material such 

as glacial till. In humid regions, deep residual soils 

are common, but they may have been so deeply 

leached by weathering that any copper minerals 

present would not be detected by analyzing samples 

for copper alone. Many of the circumpacific porphyry copper deposits, and some others as well, are 

enriched in gold as well as in copper. In the environment of deeply weathered residual soils in Puerto 

Rico, for example, the areal extent of gold-in-soil 

anomalies defined known porphyry copper deposits 

much more reliably than did the areal extent of 

copper-in-soil anomalies (Learned and Boissen, 

1973).

A thorough evaluation of relatively immobile trace 

elements in soils and hydrothermally associated trace 

elements in resistate minerals remaining in leached 

soils should be helpful in locating copper deposits 

hidden under such soils. Studies of hydrothermal 

quartz grains from residual soils over several porphyry copper deposits in Puerto Rico, for example, 

revealed that halite-bearing fluid inclusions, which 

are known to be present only where copper minerals 

are present, are preserved in the residual soils and 

could be used to locate the deposits (D. P. Cox, written commun., 1973). The trace-element chemistry of 

these inclusions has not been determined as yet.

Silty frost boils proved to be the best type of soilsampling material to use in the search for copper 

deposits in the Coppermine district of northern 

Canada where a perennial (continuous) permafrost 

zone exists. The boils acted as accumulators of fine 

inorganic soil material and were found to have a 

more uniform mineralogical composition than did
B6 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

the corresponding B-zone soils (Hornbrook and 

Allan, 1970).

Exploration geochemists conducting soil-sampling 

programs in arid areas should consider the possibility that eolian-transported silt- and clay-sized material may be intimately mixed with the residual 

soils of an area, thereby diluting metal concentrations in the soil. My investigations indicate that 

eolian dilution of metal concentrations should not be 

a problem in soil-sampling programs conducted in 

the southwestern United States; however, in desert 

areas of the Near East, wind-carried dust has been 

found to be an important dilutive factor which has 

to be considered (P. K. Theobald, oral commun., 

1971).

Two other specialized types of soil-related material suitable for geochemical sampling are found in 

arid areas caliche (calcrete) and desert varnish. 

Caliche is composed primarily of calcium carbonate 

that is deposited in warm arid climates as part of 

the soil-forming processes. Caliche often contains 

lithic impurities. In my experience, two types of 

caliche can be recognized; in sampling caliche it is 

important to distinguish between the two types. One 

type is commonly found interstratified in the sedimentary sequences of eroded gravels filling structural basins. These caliche layers represent old (now 

buried) and (or) modern soil horizons. Deposits of 

this type of caliche have never had any physical contact with bedrock and have no geochemical relationship to bedrock chemistry unless capillary action has 

moved mineral-rich ground water from areas of 

mineralized bedrock to the surface. Consequently, in 

most places, this first type of caliche is not suitable 

as a geochemical sampling medium. The second type 

of caliche is found at the present bedrock surface 

or along the bedrock-alluvium interface where bedrock is presently buried by alluvium. This second 

type is analogous to a residual soil horizon because 

it derives most of its trace-element concentrations 

from the underlying bedrock; consequently, this 

second type of caliche can be useful in geochemical 

prospecting. In his study of geochemical dispersion 

in the Twin Buttes, Ariz., porphyry copper district, 

Huff (1970) found anomalous amounts of copper 

in this second type of caliche in the vicinity of mineralized bedrock.

Desert varnish is an iron- and manganese-rich 

coating that forms on exposed surfaces of rocks in 

arid areas. It is probably the arid-environment 

equivalent of the iron and manganese staining found 

on stream sediments and float in the humid environment. In their study of desert varnish, Engel and

Sharp (1958) found that it contained a variety of 

trace elements, including copper. Most of these elements were derived from the rock under the coating. In another study including areas containing 

mineralized rock, Lakin, Hunt, Davidson, and Oda 

(1963) found in many of the areas a rough correlation of enriched trace metals in varnish with 

known ore deposits containing these trace metals. 

No detailed study has as yet been made within a 

single district to establish positively whether or not 

desert varnish might be a useful geochemical sampling medium in copper exploration. Unfortunately, 

the occurrence of varnish within a given area may 

be restricted; consequently, it may not be possible to 

obtain samples at all desired locations.

SOIL GAS

Research in recent years has demonstrated that 

many volatile elements and compounds are associated with mineral deposits. Weathering processes 

and the action of micro-organisms tend to decompose the metallic minerals of a mineral deposit. 

Under ideal conditions, gaseous products, such as 

mercury, sulfur compounds, carbon dioxide, and 

others described later, can form at depth and migrate upwards to the atmosphere. If these gases are 

present in near-surface soils, they can be collected 

in an apparatus analogous to a funnel, which is 

placed on top of the soil at a given site (fig. 1) or 

collected in a probe placed in a hole drilled into the 

soil. Any gaseous products that are present in the 

soil can then either (1) be passed directly, without 

concentration, into some type of instrument that 

can measure the concentration of one or more gaseous components that have accumulated in the funnel or probe during a given time span; or (2) be 

trapped and concentrated at the funnel neck or 

within the probe by some chemical reaction. The 

trap can then be removed from the collection site 

and the gas analyzed for its components in a separate instrument.

Although it is a relatively new technique, the use 

of analyses of soil gas has proved feasible for the 

detection of various types of mineral deposits including those of copper. Where gases are present, 

the method offers a great potential for locating deposits covered with thick layers of glacial debris or 

other transported overburden. It might also be 

possible to use soil gases to detect deposits overlain 

by postmineralization-age volcanic flows if the flows 

have sufficient permeability.

In humid areas, water will be a complicating factor in soil-gas surveys because it will tend to inhibit
GEOCHEMICAL EXPLORATION TECHNIQUES B7

FIGURE 1. Plastic hemisphere used in soil-gas sampling. The 

gases are collected either by using natural convection and a 

chemical trap attached to the top of the hemisphere, or by 

drawing the gases through a tube attached to the hemisphere and into an instrument designed to analyze gases. 

Scale is =30 cm long.

the upward migration of gases. In arid areas, however, where ground-water tables are either very deep 

or nonexistent, the use of soil gases to detect buried 

minerals should be an especially valuable technique. 

Mercury in air and in soil gas has received much 

attention as a possible pathfinder for base- and 

precious-metal deposits. In a review of volatile compounds as guides to ore, McCarthy (1972) noted 

that several workers have attempted unsuccessfully 

to find mercury anomalies in the air over porphyry 

copper deposits in the southwestern United States. 

Limited attempts to detect anomalous concentrations of mercury in soil gases over porphyry copper 

deposits have also proved unrewarding. The mercury 

concentrations may be too low to be detected with 

presently available instrumentation, or perhaps any 

anomalous concentrations of mercury have been 

dispersed well beyond the vicinity of a given deposit, and therefore were missed because sampling 

did not extend far enough beyond the deposit. In 

the final analysis, there simply may be no anomalous 

concentrations of mercury associated with porphyrytype copper deposits.

Gaseous products other than mercury vapor offer 

more promise in exploration for copper. Extensive

soil-gas surveys have been conducted over coppermolybdenum deposits in the Soviet Union (Shipulin 

and others, 1973). These authors reported concentrations of O2, N2, H2, CO2, H2S, SO2, COS, and CH4 

in the soils overlying these deposits. Anomalous concentrations of the sulfur-containing gases were detected over areas of buried copper-molybdenum mineralization known to occur no closer than 30-60 m 

(100-200 ft) to the surface. These gases were not 

found in barren areas. In contrast, a buried barren 

pyrite body did not produce significant sulfur-gas 

anomalies. Several of the nonsulf ur-containing gases 

were considered to be present because of atmospheric contamination, but their relation to the metal 

deposits, if any, was not described.

Rouse and Stevens (1971) detected anomalous 

concentrations of sulfur dioxide in soil gases collected from the Highland Valley porphyry copper 

district in British Columbia. Anomalous concentrations were found at the top of a glacial till layer 

that is as much as 600 m (200 ft) thick.

In their review of the use of volatile compounds in 

geochemical prospecting, Bristow and Jonasson 

(1972) indicated that the ratio of carbon dioxide to 

oxygen should be useful in soil-gas surveys. They 

noted that the gas-ratio method has been used successfully in the search for copper and other metals 

in various areas of the Soviet Union. Kravtsov and 

Fridman (1965) stated that oxidizing sulfides produce soil gases that contain less oxygen and more 

carbon dioxide and hydrogen sulfide than do the soil 

gases found in normal soils. These authors found 

increased amounts of hydrogen sulfide and sulfur 

dioxide in soils near chalcopyrite deposits.

Chitayeva, Miller, Grosse, and Christyakova 

(1971) described the presence of iodine in the supergene zone of the Gay chalcopyrite deposit in the 

Soviet Union. They found the iodine anomalies directly above the copper ore body and suggested that 

iodine should be a good pathfinder element to use 

in overburden surveys. Undoubtedly other soil gases 

will be found associated with copper deposits. Various authors have described the detection of different 

volatile compounds from mineral deposits other than 

copper deposits. For example, Ovchinnikov, Sokolov, 

Fridman, and Yanitskii (1973) detected helium, 

neon, argon, hydrogen, carbon dioxide, nitrogen, 

sulfur dioxide, hydrogen sulfide, methane, and the 

halogen gases in rock, soil-gas, and ground-water 

samples. In addition to many of the gases mentioned 

above, Kravtsov and Fridman (1965) found complex hydrocarbon gases ranging from methane to 

hexane associated with ore deposits.
B8 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

TRANSPORTED MATERIALS

The use of transported material as a geochemical 

sampling medium presents especially challenging 

problems. Because transported materials have often 

migrated great distances from their original source 

rocks, the geochemist using these media must have 

a knowledge of both the local and regional geological 

and geochemical environments in order to interpret 

his geochemical data properly. In spite of the obvious handicaps, transported material has been used 

successfully in many different exploration programs. 

Three types of transported material are discussed: 

these are glacial debris, stream sediments, and lake 

sediments.

GLACIAL DEBRIS

The use of glacial debris as a geochemical sampling medium has found wide application in reconnaissance surveys, especially in Canada, Scandinavia, 

and the Soviet Union. It is important that persons 

conducting and interpreting geochemical sampling 

programs involving glacial debris have a thorough 

understanding of the history, nature, and mode of 

formation of the glacial deposits present in the 

areas they examine. A sampler must, for example, 

be able to distinguish the origin of fine sediment that 

might be from till fines, former lake sediments, or 

glaciofluvial material. Various studies have indicated 

that basal till material is almost analogous to residual soil; lake-sediment deposits and glaciofluvial 

deposits, on the other hand, may be formed from 

material that has been transported long distances.

The distance that till fragments have been transported from their source is generally determined by 

recognizing rock fragments in the till that are 

unique to some known source rock upglacier. Although till fragments have been found as much as 

1,100 km (700 mi) from their source, probably more 

than 90 percent of all such material has travelled 

only about l^s km (1 mi) (Chamberlain, 1883). In 

Sweden, about 70 percent of all till is thought to be 

locally derived (Lundqvist, 1967).

The successful use of till samples in reconnaissance surveys for copper deposits has been widely 

documented in the literature (Hawkes and Webb, 

1962, p. 186-190; Bradshaw, Clews, and Walker, 

1971; Garrett, 1971) and will not be described in 

detail here.

In general, till is a very heterogenous type of material that ranges in size from clay-sized particles to 

large boulders. Different sizes of material have been 

used in different types of surveys. Trains composed 

of large rock fragments or boulders (float) in till

have been used successfully in geochemical prospecting for copper and other metals as well as in geological mapping (Dreimanis, 1958; Lee, 1971). From 

a purely geochemical standpoint, however, most 

sampling schemes have utilized fine material collected at the base of the till.

Many metallic elements, including copper, are 

preferentially enriched in the fine fraction of till 

sediments. Shilts (1971) stressed the importance of 

relating the amount of clay-size material in a sample 

to the relative concentrations of the elements under 

study. To properly evaluate till anomalies, Shilts 

suggested removing the clay-sized material from 

the silt-sized material before analysis of the sample. 

He also suggested using a heavy-mineral concentrate from the till sample as an alternate type of 

sample medium.

Esker gravels, a glaciofluvial type of deposit, have 

been used successfully as a sample medium in Canada (Lee, 1965; 1971). In his study of the distance 

of transport of material in esker gravels, Lee (1965) 

found that rock fragments had been carried an 

average distance of 5-13 km (3-8 mi) from known 

bedrock sources. Sampling of esker gravels is obviously limited by the areal extent of esker deposits; however, eskers are known that are several 

kilometres wide and as much as several hundred 

kilometres long, so that relatively large areas can 

be prospected using esker material. The technique 

should be especially useful where an esker crosses 

the structural and (or) stratigraphic grain of a 

region.

STREAM SEDIMENTS

The most universally accepted and most successful of all geochemical-prospecting techniques is 

stream-sediment sampling. Stream sediments probably reflect more accurately the upstream geology 

and chemistry of a large area than do any of the 

other types of sample media. As a consequence, the 

technique is extremely useful for rapid regionalreconnaissance geochemical surveys. The method is 

also very valuable for most detailed work in drainage basins found during reconnaissance work to contain anomalous metal concentrations. The technique 

has had wide application in the search for copper in 

many different geologic and climatic environments. 

Many examples are described in the geochemical 

literature.

In stream-sediment surveys in which copper is the 

element sought, the sampling environment must be 

carefully considered. For example, in humid climates where acid soil conditions and thorough chem-
GEOCHEMICAL EXPLORATION TECHNIQUES B9

ical decomposition of rocks generally prevail, stream 

sediments should be analyzed for cold-extractable 

copper rather than for total copper. Stream sediments have been analyzed for cold-extractable copper as part of surveys performed both in tropical 

humid areas such as the Philippine Islands (Govett 

and Hale, 1967), and in temperate humid areas such 

as Maine (Post and Kite, 1964).

Less chemical breakdown of rocks takes place in 

arid areas than in warm humid areas; consequently, 

copper may be dispersed in arid areas more by mechanical means than by chemical means (fig. 2). 

Also, in an arid environment, metal-rich ground 

water does not normally come in contact with stream 

sediments. Because of these facts, a total (hot-acidextr actable) analysis for copper has been used in 

many geochemical studies conducted in arid areas 

(Awald, 1971; Coolbaugh, 1971). It should be emphasized, however, that there is not as yet enough 

information relating the type of copper analysis to 

the type of environment. Orientation studies should 

be run in any new area to determine which type of

copper analysis gives the best anomaly contrast 

for that area.

A major concern in stream-sediment surveys has 

been the interpretation of the effects, if any, of 

secondary hydrous iron-manganese scavenging on 

the concentrations of other metals in the samples. 

Jenne (1968) made a comprehensive laboratory 

study and literature review of the role of hydrous 

iron and manganese oxides in scavenging (coprecipitating) heavy metals, including copper. He concluded 

that these two oxides (especially that of manganese) 

are the most important controls on the fixation of 

copper, cobalt, nickel, and zinc in soils and stream 

sediments. Chaffee, Botbol, and Hamilton (1972) 

applied a factor-analysis program to a data set 

containing analyses of more than 6,000 stream-sediment samples collected throughout central Maine. 

In contrast to Jenne's conclusions, the factor analysis clearly demonstrated that, at all factoring levels, 

the copper concentrations in the samples (both coldextractable and spectrographic copper) were not related to either the iron or manganese concentraFIGURE 2. Typical desert wash used in stream-sediment sampling in semiarid southern Arizona. Note the poor sorting of the

active sediment.
BIO GEOLOGY AND RESOURCES OF COPPER DEPOSITS

tions. Data from this study seemingly indicate that 

copper is not scavenged by secondary hydrous iron 

and manganese oxides. Insofar as copper is concerned, the relation of copper enrichment to the content of iron-manganese oxides in stream sediments 

is not yet completely denned. The study by Chaffee, 

Botbol, and Hamilton (1972) indicated that iron 

and manganese oxides do coprecipitate and concentrate such elements as cobalt, zinc, lead, and molybdenum. If any of these four elements is used as a 

pathfinder in the search for copper deposits, then the 

effect of secondary iron and manganese oxides 

should be considered in data interpretation.

The U.S. Geological Survey is currently making a 

study of the source of the secondary metals associated with stream sediments. Present information 

suggests that where metal-rich interstitial ground 

water flows into a stream from bank soil, there are 

abrupt changes in the physical and chemical environment. These changes cause the metals to precipitate 

in the stream, either as grain coatings or as small 

nodules, before they are transported very far downstream. Most of the cold-extractable metals found in 

stream sediments are thought to have been derived 

from these interstitial ground waters, even though 

stream waters usually constitute a much larger proportion of the total water volume at a given stream 

locality (G. A. Nowlan, oral commun., 1973).

As was mentioned in the section on soil surveys, 

the optimum size of material, in this case streamsediment material, used for analysis should be determined in any new area. Most, but not all, elements are concentrated in the sediment fines; however, the best grain size to retain for analysis should 

be the one that provides the best anomaly contrast. 

Hawkes and Webb (1962, p. 256-258) have given 

an excellent discussion of this problem.

Most stream-sediment surveys have used the finegrained ( 80 mesh) fraction for analysis; however, 

Fisher (1970) reported that in some areas of Australia, stream-sediment material as coarse as a 

 20 to +40 mesh fraction gave better results than 

did standard 80 mesh fractions. Different climatic, 

geologic, and terrain conditions were found to influence the metal content of different size fractions.

The importance of selecting the proper size of 

sample material to use in a stream-sediment survey 

was also emphasized by Erickson, Marranzino, Oda, 

and Janes (1966). Their study was conducted in 

an arid area of eastern Nevada not known to contain any significant metal occurrences. The authors 

concluded that the most successful method for finding new deposits in that terrain was analysis for

base metals and mercury of float cobbles and pebbles 

collected in the major stream channels. Chemical 

data from the 50 mesh fraction of stream-sediment samples failed to reveal any anomalous metal 

concentrations except in those samples collected 

from streambeds draining a previously known but 

abandoned small mining district.

Several types of concentrates, including magnetic 

and nonmagnetic heavy-mineral concentrates and 

grain-coating concentrates, can be made from standard stream-sediment samples. Analyses of these 

specialized samples are sometimes more useful in 

defining geochemical anomalies in stream channels 

than are the analyses of the original stream-sediment samples from which these specialized samples 

. are derived. Griffitts and Alminas (1968) collected 

I stream-sedimnt samples in an arid area of southern New Mexico and made heavy-mineral concentrates from these samples. They found that their 

analyses of the nonmagnetic heavy-mineral concentrates provided more information for locating mineralized areas than did the analyses of the corresponding samples of standard 80 mesh stream 

sediment. Huff (1971) experimented with different

 subsamples as possible guides to a known porphyrycopper district in southern Arizona. He sampled 

sediments from streams draining the copper de-

 posits present and found that the copper concentrations in (1) ultrasonic concentrates of sediment 

! grain coatings, (2) the nonmagnetic heavy-mineral 

, fraction, and (3) hydrothermally-altered stream 

I pebbles all located the known deposits as well as or 

i better than did the standard 80 mesh fraction of 

i the stream-sediment sample.

Heavy-mineral concentrates from old placer dis- 

, tricts may be useful in the search for copper deposits. In a study of placer minerals in Taiwan, Tan 

and Yu (1968) found that placer gold was present 

in the sediment from streams draining known 

copper-gold deposits. The worldwide association of 

gold with many copper deposits suggests that gold 

in stream-sediment concentrates should be a valuable pathfinder element in the search for copper 

, deposits.

The concentrations of camouflaged trace elements 

in heavy-mineral fractions of stream sedimnts can 

also be used in geochemical exploration for copper 

deposits. Tan and Yu (1968) detected anomalous 

concentrations of copper in pyrite concentrates collected from streams draining areas of known copper deposits in Taiwan. In another study, Bell and 

Hornig (1970) determined that iron-oxide pseudomorphs after pyrite may be a useful geochemical-
GEOCHEMICAL EXPLORATION TECHNIQUES Bll

sampling medium in the search for sulfide deposits 

in deeply weathered regions. These authors found 

iron-oxide pseudomorphs to be a common constituent in stream-sediment samples as well as in soil 

and saprolite samples that they collected in South 

Carolina. These pseudomorphs contained appreciable 

amounts of many trace elements. The authors suggested that the regional variation in trace-element 

content of the pseudomorphs may be a guide to areas 

of sulfide mineralization.

In a study of alluvial magnetites from streams 

in central Ecuador, de Grys (1970) found that the 

copper and zinc concentrations in the magnetites 

were useful in defining various base-metal deposits. 

She noted that it was important to determine 

whether the magnetite was fresh or weathered. In 

fresh magnetite, all the trace elements were considered to be primary; altered magnetite might contain secondary hydrous iron-oxide coatings that 

could coprecipitate other metals.

As noted in the section on rocks, my experience 

suggests that caution should be exercised in interpreting trace-element data from magnetites and 

possibly from other heavy minerals. In complex 

geologic terranes, different geologic formations may 

contain magnetites or other heavy minerals having 

different ages and geneses. These formations will, 

therefore, probably have different suites of trace 

elements. A knowledge of bedrock geology is thus 

essential for the proper interpretation of any alluvial heavy-mineral data.

Because of the large amount of information that 

can be generated from regional stream-sediment 

sampling programs, computer analysis of the analytical and other data is becoming more and more 

important for evaluating the data. Many computer 

programs are now available to treat large data sets. 

Some of these methods are discussed later in this 

report.

LAKE SEDIMENTS

In recent years, the collecting and analysis of lake 

sediments has been another useful geochemicalprospecting technique, especially for regional reconnaissance surveys. This method, which is especially applicable to remote areas where sampling 

can be done from pontoon-equipped helicopters or 

small fixed-wing aircraft, has been used successfully 

in regional-reconnaissance copper exploration programs in Canada. Field studies (Allan, 1971; Allan 

and Crook, 1972; and Allan and others, 1972, 1973) 

have demonstrated that standardized samples can 

be taken from lake sediments and that the chemical

analyses from these samples can define areas enriched in copper as well as can those from streamsediment samples. Best results were obtained from 

samples of sediment collected 5 to 15 cm (2 to 6 in.) 

below the lake-bottom surface at depths of about 90 

to 240 cm (3 to 8 ft) below the lake-water level. 

Although clay-sized material made the best sample, 

the use of clay- and silt-sized material together 

(about 50 micron and smaller-sized material) 

proved to be a useful sample that could be quickly 

obtained and prepared for analysis.

Factors such as Eh, pH, organic content, and 

secondary iron- and manganese-oxide content that 

can affect stream-sediment sample data can also 

affect lake-sediment sample data. Furthermore, it 

is important to recognize that the lake-sediment 

chemistry reflects that of the sediment source area 

and not that of the bedrock below the lake.

WATER AND RELATED MATERIALS

Marine and fresh water, as well as ice and snow, 

represent potential geochemical sampling media, but 

they have not been widely used because a great 

many parameters can influence the chemical data 

obtained and because only extreme low metal concentrations are normally found. With the development of more sensitive and accurate analytical techniques and new computer programs, these media 

are becoming more useful in geochemical studies.

WATER

Hydrogeochemical prospecting techniques have 

been reviewed in detail by Hawkes and Webb (1962, 

p. 227-246) and by Boyle and others (1971). Most 

hydrogeochemieal surveys have been conducted using 

lake water as the sampling medium; however, 

waters from streams, springs, seeps, wells, and the 

sea have also been used.

Water has not been a popular sampling medium 

because a great many physical and chemical factors must be considered in evaluating water analyses. Some of these factors are Eh, pH, the overall 

chemistry of the environment sampled, the distance 

from the source of metals to the sample site, the 

time of year, the duration, type, and annual rate of 

precipitation, the ambient temperature, the size and 

rate of flow or circulation of the water body, and the 

effect of organic matter. The form in which metals 

occur in water has also caused difficulties in data 

interpretation. Metallic elements such as copper can 

occur in natural water in many different forms, including free ions or undissociated molecules, ions 

adsorbed on suspended matter, and water-soluble 

metallo-organic complexes. Metals may also be found
B12 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

in organic gels, in waterborne micro-organisms, and 

in suspended matter such as colloids or finely divided inorganic sediment.

Kleinkopf (1960) sampled lake waters in Maine 

and demonstrated that clusters of lakes there contained anomalous amounts of copper, molybdenum, 

and other metals. In some areas he found an association between the anomalous concentrations of metals 

in the water samples and the known geology and 

mineralized rock bodies.

In a more recent study of lake waters conducted 

specifically for copper exploration, Allan (1971) and 

Allan, Lynch, and Lund (1972) analyzed lake-water 

and lake-sediment samples collected in a large region 

around the Coppermine district in Canada. Although 

the distribution of copper in lake-sediment samples 

gave the best indication of areas known to be enriched in copper, the distribution of copper in lakewater samples also successfully delineated the areas 

of highest copper concentration in the region. Seasonal and intra-lake chemical variations were not 

found to be serious problems for interpretation.

Boyle and others (1966) conducted a regional geochemical survey in New Brunswick, Canada, in which 

they collected samples from stream waters and 

stream sediments. Stream-water samples, even near 

known base-metal deposits, had very low concentrations of copper. Analyses of zinc and cold-extractable 

heavy metals in the water gave much better results; 

however, data for all elements from the stream-sediment samples provided better information than did 

the comparable water data.

In another study, Boyle and others (1971) showed 

that dispersion trains of metals in stream waters are 

usually more restricted than those in stream sediments. Consequently, these authors recommended 

using stream-sediment samples for regional reconnaissance surveys, and stream-water samples for detailed surveys. They recommended that all water 

samples be collected as near as possible to the headwaters of stream tributaries.

In some regions of the world the ground is frozen 

all year long (continuous permafrost) or at least 

part of the year (discontinuous permafrost). Until 

recently, the presence of continuous permafrost has 

suggested to most geochemists that metals would 

have little chance to migrate upwards to the surface ; however, thawed ground is now known to exist 

in areas of continuous permafrost under and around 

streams and large bodies of water such as lakes 

(Allan and Hornbrook, 1971). Shvartsev (1972a, 

1972b) successfully used samples of spring waters 

that have circulated through permafrost zones to

locate buried base-metal deposits in the Soviet 

Union.

Water from seeps and cold springs, and the precipitates that form around them, have been found 

useful in geochemical surveys (Boyle and others, 

1971; Shvartsev, 1972a, 1972b; Mehrtens, Tooms, 

and Troup, 1973). Water from hot springs, on the 

other hand, has not been successfully used as yet 

to locate copper deposits. Many perhaps most hot 

springs have no obvious relationship to potential 

copper deposits; however, White (1967) described 

the chemistry of some hot springs and hot-spring 

deposits that are associated with base-metal deposits 

in Japan and the United States. Weissberg (1969) 

made a similar study of some thermal areas in New 

Zealand. Both authors noted that base metals are 

not normally present in significant amounts in the 

waters issuing at the ground surface, but were found 

at depth in samples of drill core. The waters issuing 

at the surface commonly contained such ions as 

Ca+2 , Na+, Mg+2 , Li+, F~, Cl~, Br~, S04 -2, and 

HC03 ~ and the discharge precipitates contained such 

metals as Au, Ag, As, Sb, Hg, Tl, Fe, and Mn. Apparently, most of the heavy-metal ions, including copper, are precipitated below the surface; thus, geochemical exploration programs using samples of 

hot-spring waters will probably have to rely on some 

combination of pathfinder elements as guides to 

buried copper deposits.

The study of ground-water geochemistry can be 

valuable in geochemical prospecting. For most exploration purposes, wells, drill holes, and possibly old 

shafts or quarries are the only feasible places for 

sampling subsurface waters. Wells are especially 

useful sources in arid areas where there is very little 

surface water.

In arid areas, the ground water is normally alkaline except perhaps near oxidizing sulfides. In the 

alkaline environment, very few elements are mobile; 

however, in the absence of high concentrations of 

such ions as manganese, iron, carbonate, and sulfate, 

the metals chromium, molybdenum, and tungsten are 

fairly mobile. Copper is generally not very mobile at 

high pH levels. Huff (1970) was able to detect the 

buried copper-molybdenum deposits of the Pima district in Arizona by measuring the molybdenum concentrations in well-water samples. He found a molybdenum anomaly that extended as much as 13 km (8 

mi) downslope from the known deposits; copper was 

not present in anomalous concentrations in these 

samples.

Well waters have been sampled for molybdenum 

throughout much of the southwestern United States
GEOCHEMICAL EXPLORATION TECHNIQUES B13

by various private companies; however, little information has been made public. Anomalous molybdenum concentrations in well water are known to 

exist downslope from known copper-molybdenum 

deposits, but to date no new deposits have been 

found using well water as the sampling medium. No 

information about the content of tungsten or chromium in well waters is available.

The mineral potential of the world's oceans and 

ocean bottoms is gradually being recognized. The 

presence of copper in deep-sea manganese nodules, 

in manganese-rich encrustations on rock- and sediment-grain surfaces, and in bottom deposits such as 

those of the Red Sea, is already well known. At present we have little detailed knowledge of the movement of trace elements in ocean waters and how 

trace-element dispersions in marine waters might be 

used in geochemical prospecting for nearshore or 

deep-sea mineral deposits. Additional fundamental 

chemical and physical data concerning the oceans are 

needed before geochemists can begin to apply geochemical prospecting methods successfully to samples of ocean waters and bottom materials. In some 

deep-sea areas, the regional trends in the chemistry 

of near-bottom waters or of slowly upwelling waters 

may be useful in locating any deep-sea copper deposits that are not buried under thick layers of barren sediment. In many localities, submarine springs 

are known to be present. Analyses of samples of this 

type of spring water might be useful in determining 

the presence of anomalous concentrations of metals 

in the source aquifer.

In the future, deep-sea metal-rich bottom-sediment 

deposits, such as those found in the Red Sea and 

elsewhere, may well be an economic mineral resource. 

Holmes and Tooms (1973) made a reconnaissance 

geochemical survey of part of the Red Sea area. 

They collected samples of surface waters as well as 

samples of bottom sediment and near-bottom water 

from depths of about 2,000 m (6,500 ft). The dissolved metal species in the near-bottom water samples and the particulate matter in these samples 

were analyzed separately. Significant anomalies were 

found in all the sample media except those from the 

surface waters. This study suggested that reconnaissance geochemical surveys should be successful 

in locating deep-sea metal-rich bottom-sediment deposits as well as deep-sea brines that contain high 

concentrations of various metals.

Material suspended in fresh water has recently 

been considered as a possible geochemical sampling 

medium. Perhac and Whelan (1972) compared the 

metal content of stream water, of bottom sediment,

and of the suspended colloidal and coarser particulates in the water. The colloidal material was found 

to be greatly enriched in metals, including copper, 

as compared with the other types of samples. Because the separation of colloidal samples for analysis 

requires expensive equipment and an inordinate 

amount of time, the method is not very practical at 

present.

Timperley, Jonasson, and Allan (1973) found that 

most lakes in certain parts of Canada contained organic gels that may occur as much as 13 m (30 ft) 

above the inorganic sediment of lake bottoms. The 

gels are thought to include (1) organic precipitates 

of colloidal origin, (2) residual organic matter derived from decaying vegetation, (3) pollen, and (4) 

finely divided inorganic mineral matter. Preliminary 

data indicated that the gels contained concentrations 

of various elements, including copper, and that the 

trace-element concentrations of the gels can be related to the local geology. The use of organic gels 

from lakes as a regional geochemical sampling medium seems promising.

Gels also are found along shorelines where freshwater rivers discharge into bodies of saltwater (W. 

C. Overstreet, written commun., 1973). The extent 

of such gels along shorelines is not known, but if 

these gels are commonplace, then systematic sampling of them could be another effective reconnaissance geochemical exploration technique.

Gases are commonly dissolved in all types of natural waters. Analysis of radon gas in lake and stream 

waters has been used in Canada as a reconnaissance 

hydrogeochemical exploration method in the search 

for uranium deposits (Dyck and others, 1971) ; however, the analysis of gases in waters has not as yet 

been applied to copper exploration.

ICE

The use of samples of ice in a geochemical sampling survey is obviously quite limited but may be 

feasible. Shvartsev (1972a) has attempted to use 

naleds as a geochemical sampling medium. Naleds 

are ice sheets that form during the cold season in 

areas of discontinuous permafrost. They form where 

subsurface water is forced to the surface and flows 

out over the surface and is frozen. Shvartsev noted 

that there has not been sufficient work done on naled 

geochemistry to determine whether the sampling of 

naleds will be a useful technique for locating mineral 

deposits in cold regions.

SNOW

Traces of Cu, Hg, Zn, Pb, Ag, Mn, Cd, As, and Ni 

have been detected in snow samples collected from
B14 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

sites overlying several types of buried mineral deposits in Canada (Jonasson and Allan, 1973). Their 

study revealed that these metals were derived at the 

time of the first snowfalls from the weathered mineralized rocks below the soil horizons and not from 

particulate matter present in the air above these 

rocks. These authors concluded that snow may be a 

useful sampling medium but that its use is limited 

to detailed prospecting in areas of known mineral 

potential.

VEGETATION

Vegetation can be used both in geobotanical and 

biogeochemical prospecting. Geobotanical prospecting involves noting and mapping (1) the presence 

and abundance of, or (2) the absence of, a plant 

species or plant community with relation to soil and 

rock enriched in a given element such as copper. 

Geobotanical prospecting may also involve noting 

and mapping the presence of morphological and (or) 

mutational anomalies in plants. In contrast, biogeochemical prospecting involves analyzing plant 

materials for their trace-metal content in order to 

locate areas of high metal concentrations.

GEOBOTANICAL TECHNIQUES

Geobotanical prospecting has been discussed in 

detail by Brooks (1972). Where this technique is 

successful, it is a rapid and inexpensive prospecting 

method. Brooks (1972) noted that plant communities 

indicative of soils enriched in certain elements or 

indicative of certain rock types have been recognized 

for more than a century; however, no plant communities are known that uniquely define copperenriched soils. Several individual plant species, on 

the other hand, have been found to be associated 

with copper-rich soils in different parts of the world. 

Cannon (1971) listed 30 species of plants that have 

been used in various localities as indicators of copper. She noted that geobotanical prospecting has 

been especially successful in the search for copper 

in Australia, Katanga, China, and the Soviet Union. 

Cannon (1960) also stated that most copper-indicator plants belong to one of three plant groups; (1) 

the Caryophyllaceae, or pink family; (2) the Labiatae, or mint family; (3) the mosses.

My geobotanical studies in the porphyry copper 

region of the southwestern United States as well as 

those by Levering, Huff, and Almond (1950), Warren, Delavault, and Irish (1951), and Cannon (1960) 

indicate that abundant populations of the California 

poppy (Eschscholtzia mexicana) grow in close association with copper-rich soils near many porphyry

deposits. Unfortunately, the poppies are not found 

growing over all cupriferous soils of the region nor 

are all wild poppy populations associated with cupriferous soils. Evidently the conditions promoting 

vigorous poppy populations involve more than the 

mere presence of high copper concentrations in the 

soil. During the period of flowering, localities containing the poppy are easily recognized both from 

the ground and from the air at low altitudes. Areas 

containing the poppy certainly warrant close examination.

Despite studies by many workers, no other important copper-indicator plant species has been identified for the porphyry copper region of the United 

States. However, Cannon (1971) has described some 

plant species that are commonly associated with 

sulfur- or sulfate-rich soils; I have seen some of 

these species growing in soils formed from rocks 

containing oxidizing sulfides. Unfortunately, these 

species do not differentiate between biogenic-sedimentary sulfide and sulfate concentrations, and 

hydrothermally-introduced sulfide and sulfate concentrations.

A high content of copper in soils can cause morphologic or mutational changes in certain plant species or can cause the complete absence of vegetation. 

Cannon (1960, 1971) and Brooks (1972) have described areas in widely separated parts of the world 

in which copper-rich soils caused such effects.

BIOGEOCHEMICAL TECHNIQUES

Chemical analyses of plants or plant parts can be 

used as a geochemical-prospecting technique when 

it can be shown that the chemical content of the 

vegetation sampled has some predictable relationship to the chemical content of the nutrient soil or 

bedrock, or at least to the ground water moving 

through the soil or bedrock. Only some brief comments related to copper will be given here; for more 

details the reader is referred to Brooks (1972).

Biogeochemical prospecting requires specialized 

knowledge and equipment beyond that needed, for 

example, for rock or soil sampling, and these requirements generally determine where the method 

can be used most effectively. The primary advantage 

of biogeochemical sampling over the methods of 

surficial sampling is predicated on the fact that 

many plants are deep rooted. Chemical elements 

absorbed by a deep well-developed root system are 

commonly translocated to the leaves and stems of 

the plant. The leaves and (or) stems can then be 

collected for analysis. Roots growing to depths of 

as much as 66 m (200 ft) have been documented 

(Brooks, 1972, p. 107). With these facts in mind,
GEOCHEMICAL EXPLORATION TECHNIQUES B15

it can be seen that biogeochemical sampling should 

be a better technique than soil or rock sampling in 

regions of extensive but relatively thin postmineralization-age overburden. This overburden may include glacial deposits, talus, or landslide deposits, 

desert pediment gravels, or even fairly recent organic accumulations such as peat or deep humus 

soil layers found in areas of dense vegetation.

The mechanisms by which plants translocate and 

concentrate trace elements are not well understood. 

Many physical and chemical parameters such as soil 

Eh and pH, annual precipitation rate, mean annual 

temperature, aspect, soil type and texture, antagonistic effects of elements present, and physiology of 

each plant species, control the ultimate locations and 

concentrations of each element. As a result, sampling and analyses of plant material should be done 

on a consistent basis to minimize the effects of these 

parameters.

Probably the most common use of the technique 

of biogeochemical prospecting has been in areas covered with glacial deposits in Canada, Scandinavia, 

and the Soviet Union. Wolfe (1971) concluded that 

this method can be an effective technique under 

optimum conditions, but that if impermeable layers 

of clay and silt are present in the glacial substrate, 

such layers will prevent the upward migration of 

metals from mineralized bedrock below the layers, 

and no anomalous concentrations of metals will be 

present in plants, even in those growing directly 

over ore. This author also stated that, in contrast to 

other regions, most trees of the Canadian Shield 

probably do not have root systems that extend below 6-16 m (20-50 ft). This maximum depth of 

rooting reflects primarily the presence of shallow 

water tables and (or) impermeable clay or silt lenses. 

The maximum rooting depth of plants may also be 

controlled by frozen layers in the zone of discontinuous permafrost. Biogeochemical prospecting in 

glaciated regions will therefore be most successful 

in areas covered only by thin glacial debris composed mainly of unfrozen, coarse, porous sediment.

In a review of studies of vegetation conducted in 

the Soviet Union, Nesvetaylova (1961) described 

one case in which the iron content of the ash of 

birch leaves effectively outlined an area of copperrich ore deposits. In Canada, Warren and his colleagues (Warren and Howatson, 1947; Warren, 

Delavault, and Irish, 1949) collected samples of a 

variety of tree species and several different tree 

parts. The chemical data from these plant samples 

provided information that helped to delineate some 

copper-rich localities in western Canada. In these

studies Warren and his co-workers emphasized the 

use of copper-zinc ratios from the plant ash analyses 

as an aid in evaluating their data.

Peat, another type of biogeochemical sampling 

medium, is commonly found in humid regions. Samples of peat have been used successfully in both regional and local surveys in the search for copper 

deposits in many parts of the world. In Finland, 

Salmi (1967) discovered that anomalous concentrations of trace elements in peat ash could be used to 

locate copper and other types of mineral deposits 

buried under shallow layers of peat and glacial till. 

In Sweden, Brundin and Nairis (1972) found that 

the analytical data for copper from peat samples 

were superior to corresponding data from water and 

stream-sediment samples for delineating known mineral deposits. In the Soviet Union, anomalous concentrations of copper were found in samples of peat 

collected 300 to 600 m (985 to 1,970 ft) down drainage from a known copper deposit. (APbov and 

Kostarev, 1968).

Gleeson and Coope (1967) successfully located 

buried base-metal deposits in Canada by using peat 

as a sampling medium. They noted that the metals 

found in peat were probably derived from ground 

water circulating through the peat and that the 

metal content of the peat samples is related to the 

degree of humification of the peat and the pH of the 

associated waters.

Biogeochemical prospecting has also been applied 

to the search for copper in an arid environment, 

particularly in the porphyry copper province of the 

southwestern United States. In desert regions there 

is an important dichotomy of plant types caused 

principally by the depth of the permanent groundwater table in a given area. Riparian plant species 

those that grow almost exclusively in and along 

intermittent or permanent stream channels are 

mostly phreatophytes ; that is, species that have extensive root systems that reach to the permanent 

ground-water table (fig. 3). Nonriparian plant species, on the other hand, grow almost anywhere including in intermittent stream channels and are 

mostly xerophytes, that is, species that have shallow 

root systems and that obtain their water from intermittent rainfall and not from the permanent groundwater table. The sampling of riparian species is 

somewhat analogous to collecting stream-sediment 

samples, and the sampling of nonriparian species is 

analogous to collecting soil or rock samples. 

[ Because of the generally poor development of soils 

j in arid regions, most residual soil horizons are 

fairly thin; consequently, as long as any residual
B16 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

FIGURE 3. Mesquite (Prosopis sp.) roots are exposed in a 

wash near Tucson, Ariz. The deep, well-developed root system is typical of this phreatophyte and makes this species 

especially useful as a biogeochemical sampling medium in 

this semiarid region. From Huff (1970, fig. 6).

soil is available, there is rarely any reason to sample 

nonriparian plant species. In areas containing fairly 

thin layers of transported overburden, however, 

nonriparian species may give a better indication of 

buried mineral deposits than can the soils derived 

from the transported overburden. Plots of concentrations of copper, zinc, and molybdenum in plant 

ash from samples of plants growing in thin layers 

of transported pediment gravel deposits overlying 

the Vekol porphyry copper deposit in central Arizona showed limited but more widespread anomalies 

than did plots of the corresponding soil data (Chaffee and Hessin, 1971; and Chaffee, unpub. data).

Work by Huff (1970) and Brown (1970) and my 

studies clearly indicate that where phreatophytes 

are available they are an effective sampling medium 

for copper exploration. Mesquite (Prosopis juliflora) has been the most widely used phreatophyte 

in biogeochemical surveys in the arid regions of the 

western United States. Use of samples of this species has been successful partly because mesquite 

grows in warm areas over a wide region extending 

from southeastern California to southern Kansas 

and partly because its root system can penetrate to 

great depths. A live root tentatively identified as 

being from a mesquite plant was collected from an 

open-pit mine in southern Arizona at a depth of 

about 53 m (175 ft) (Phillips, 1963). This depth 

undoubtedly represents an extreme, but roots to 

depths of about 15 m (50 ft) are probably common.

In the Pima district in Arizona, Huff (1970) detected anomalous concentrations of molybdenum in 

the ash of mesquite stems collected as much as 13 km 

(8 mi) away from any outcrop of mineralized bedrock. Brown (1970) also found anomalous concentrations of copper in the stem ash of samples of 

mesquite collected over the Kalamazoo porphyry 

copper deposit near San Manuel, Ariz. This anomaly 

extended well beyond the exposed parts of the deposit into transported colluvial gravels. I have also 

been successful in locating partially buried porphyry 

copper deposits in Arizona by using analyses of 

copper, zinc, and molybdenum in the ash of leaves 

and stems of mesquite. Analyses of the ash of other 

species, including those of catclaw (Acacia, constricta), blue paloverde (Cercidum floridum), and a 

nonphreatophytic species, ironwood (Olneya tesota), 

have proved nearly as effective. Plants of these species have been found to contain anomalous concentrations of copper, zinc, and (or) molybdenum at 

sites at least 3 km (about 2 mi) downstream from 

any mineralized outcrop. Clearly, biogeochemical 

prospecting is an effective regional reconnaissance 

technique in the search for copper in an arid environment.

MICRO-ORGANISMS

Micro-organisms (mostly bacteria and algae) may 

be useful in geochemical prospecting. The role of 

micro-organisms in geological processes has been reviewed in detail by Kuznetsov, Ivanov, and Lyalikova 

(1963). Essentially all the data in the literature deal 

with the relationship of micro-organisms to fossilfuel, iron, or sulfur-sulfate deposits, or to the use 

of bacteria in the leaching of low-grade mine wastes 

for the recovery of base and ferrous metals.
GEOCHEMICAL EXPLORATION TECHNIQUES BIT

Apparently, very little research has been done on 

the subject of the use of micro-organisms in the exploration for ore deposits. The only practical application of micro-organisms in the search for ore deposits 

was done in the Soviet Union (Kuznetsov, Ivanov, 

and Lyalikova, 1963). This study showed that thiobacteria were present in material of an undescribed 

nature overlying a molybdenum ore deposit. Circulating waters above the deposit were distinguished 

from the regional ground water by the concentrations of certain types of bacteria.

In an analogy to vegetation surveys, micro-organisms could be used in geochemical prospecting in 

two ways. First, the presence or absence of, or the 

abundance of, certain types of micro-organisms may 

indicate the presence or absence of certain chemical 

species, such as sulfur or sulfate. Second, the organisms themselves can be collected and analyzed for 

their chemical content of any given element.

It should be feasible to study soil micro-organisms 

by making cultures of them in various culture media. 

If cultures can be found that will produce organism 

growth or death only in the presence of anomalous 

concentrations of copper, for example, then it should 

be possible to collect small amounts of soil over areas 

of interest and to run cultures to determine the content of copper or of some other element in those soils. 

This procedure has not been tested as yet; if successful, it might be more rapid than routine chemical 

analysis of the soils (H. W. Lakin, oral commun., 

1973).

Plankton are known to be concentrators of metals 

(Warren, Delavault, Fletcher, and Peterson, 1971). 

Sampling of plankton growing at the mouths of 

rivers might be a good reconnaissance geochemical 

prospecting method to use along marine or largelake shorelines. Samples of algae collected from the 

mouths of rivers in Puerto Rico have been found to 

contain anomalous amounts of copper. Whether the 

anomalous samples are from the mouths of rivers 

draining areas containing porphyry-copper-type minerals has not been established (R. E. Learned, oral 

commun., 1973).

ANIMALS

Studies in recent years have shown that animals 

can be used in two different ways in geochemical exploration. First, dogs can be trained to scent sulfide 

minerals. Second, animals in general tend to accumulate trace elements in their tissues and waste products ; thus, these materials can be used as a type of 

biogeochemical sample.

In the last 10 years, dogs have been used increasingly as an aid to geochemical prospecting in Canada 

and in Fennoscandia (Brock, 1972). Dogs have olfactory senses far superior to those of man; consequently, they can be trained to scent sulfide minerals. 

The widest application of dogs to geochemical prospecting has been in locating sulfides in glacial moraines (Nilsson, 1971). In field trials, dogs have been 

able to find sulfide minerals where no human could 

have found them by visual inspection alone. The dogs 

have found sulfide minerals buried under 30 cm (1 

ft) of overburden (Stirling, 1972). They have also 

found pieces of sulfide float in the fines of stream 

sediments (Brock, 1972). In Finland, a dog found 

boulders containing pyrite and chalcopyrite that 

were later found to be associated with a "copper ore 

body of economic significance" (Stirling, 1972).

Another animal that has been studied as a potential aid to geochemical prospecting is the termite. 

Termites are found throughout the tropical and subtropical regions of the world and in temperate regions to about lat 45° N. and S. All told, these regions constitute about two-thirds of the Earth's land 

surface (Lee and Wood, 1971). Many termite species 

build mounds above ground, most species construct 

extensive subterranean galleries. Although galleries 

have been reported to extend to depths of 70 m (230 

ft) (Yakushev, V. M., in Lee and Wood, 1971, p. 53), 

most termite galleries are at much shallower depths. 

Watson (1972) observed termite galleries at least 

23 m (about 75 ft) deep in mine workings. He believed that termites constructed these deep galleries 

in their search for the ground-water table. These 

observations suggest that the material present in 

termite mounds might represent deeply buried material and that these mounds could be used as a 

sampling medium for geochemical prospecting in 

overburden in termite-infested regions. However, in 

his studies of the Kalahari sand of Africa, Watson 

(1970, 1972) found that most of the material from 

termite mounds was derived from depths of less than 

3 m (10 ft), despite the fact that the termite galleries extended to much greater depths and into 

mineralized bedrock. If Watson's observations hold 

true for termite mounds in other regions, then the 

sampling of these mounds may be a useful geochemical prospecting technique for locating buried mineral deposits, but only in areas where the soil or overburden is less than 3 m (10 ft) thick. This technique 

should be investigated further.

Animals can be considered to be one of many steps 

in the natural food chain and therefore in the geochemical cycle of many elements present in the
B18 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

chain. Species of animals that move about on land or 

in streams only within a limited area may well collect 

or concentrate chemical elements available to them 

in their food. If the source of food for these animals 

is already anomalous in trace metals, then these elements might be concentrated to an unusual extent in 

the tissue or in the waste products of the animals. 

Techniques that utilize animal tissue or waste products as geochemical sampling media are described 

below. However, the idea of killing wildlife solely for 

use in geochemical prospecting cannot be recommended and would never be condoned by conservation authorities.

The base-metal content of trout livers from lakes 

and streams in British Columbia was studied by 

Warren, Delavault, Fletcher, and Peterson (1971). 

They noted that plankton can concentrate heavy 

metals and that trout feed on these plankton. The 

authors concluded that it may sometimes be possible 

to use the base-metal content of samples of trout 

livers to detect anomalous concentrations of metals 

in an area.

Worthington (1968) described a study by the 

Colorado State Game, Fish, and Parks Department 

in which samples of deer antlers were collected at 

hunting check stations and were analyzed for gold 

in an attempt to discover new areas of potential gold 

resources. The results of the survey were not given; 

the approach, however, could easily be applied to 

the search for copper or other metal resources. Unfortunately, from a geochemical exploration standpoint, most deer range over wide areas; consequently, even if metal anomalies were detected in 

antlers, it might be difficult to identify the source 

areas.

In the arid lands of the Near East, much of the 

finer grained clastic overburden material has been 

transported great distances by the wind and is 

therefore not suitable as a geochemical sampling 

medium. In such areas, vegetation, although sparse, 

should be a useful sampling medium; however, regional custom dictates that living plant matter be 

saved for forage. Animals of the region, including 

camels, sheep, and goats, graze on the vegetation; 

consequently, animal dung may be an acceptable and 

useful sampling medium for regional reconnaissance 

in that part of the world (W. C. Overstreet, written 

commun., 1969).

Many fish-eating birds live in large rookeries along 

the seacoasts of the world. If the fish have ingested 

metal-rich food, then analyses for copper or other 

metals in samples of guano from the bird rookeries

might indicate the locations of nearby metal-rich 

waters or seafloor deposits along coastlines.

Many endemic diseases of animals and man are 

now recognized as being related to excesses or deficiencies of trace elements in food or water. Webb 

(1971) described the use of stream-sediment chemical data to identify areas in England where livestock 

might be ingesting forage that contained excessive 

or insufficient concentrations of various trace elements known to affect animals. The reverse of 

Webb's approach might be a useful regional reconnaissance geochemical exploration technique; that 

is, it should be possible to locate areas containing 

anomalous concentrations of trace elements by locating areas high in incidence of trace-element associated endemic diseases of animals and man. Unfortunately, from an exploration standpoint, both man 

and animals are normally tolerant of anomalous 

amounts of copper (Scheinberg, 1970); however, 

excess amounts of other copper-related pathfinder 

elements do cause diseases. For example, the welldocumented cattle disease molybdenosis (teart) is 

known to be related to the ingestion of excessive 

amounts of molybdenum. High concentrations of 

fluorine in drinking water cause mottling of human 

teeth. Other copper-related pathfinder elements that 

are thought to be detrimental to animals and man 

when present in anomalous amounts in the food 

chain might include lead, arsenic, mercury, cadmium, 

selenium, and probably others. It should be stressed 

that anomalous concentrations of trace elements can 

be caused by many natural and man-made factors. 

Therefore, the presence of any trace-element-related 

diseases within a region would not necessarily mean 

that the area in question has any potential for copper deposits. However, the application of this technique, especially in the less developed countries of 

the world, might prove fruitful.

ATMOSPHERE

The results of analyses of the gases in the atmosphere, and of the suspended particulate matter in 

the atmosphere, are proving valuable in geochemical 

exploration surveys for new mineral deposits. The 

detection of gases in the atmosphere can be accomplished by using gas-analysis instruments mounted 

in aircraft or in motor vehicles. McCarthy (1972) 

and Bristow and Jonasson (1972) have recently reviewed the use of airborne gas-analysis instruments 

for detecting mineral deposits, including those of 

copper. These authors noted that there are gases 

emanating from mineral deposits, and that these
GEOCHEMICAL EXPLORATION TECHNIQUES B19

emanations can be detected in samples of the atmosphere collected above the deposits.

The application of chemical analyses of the element mercury in the search for mineral deposits has 

received the most attention in the literature to date, 

probably because mercury has a known close association with many types of sulfide mineral deposits, 

and because it is a very volatile element. Airborne 

gas analyzers have been used to detect mercury 

vapor in samples of the atmosphere collected above 

many different types of mineral deposits. McCarthy 

(1972) noted, however, that the concentrations of 

mercury in the atmosphere above porphyry copper 

deposits are apparently very low. He indicated that 

the measurement of mercury in the air above such 

deposits does not seem to be a useful means of detecting that type of copper deposit. The atmosphere 

above other types of copper deposits may contain 

higher levels of mercury. However, to date published 

data are not sufficient to indicate whether atmospheric mercury can be used to detect any type of 

copper deposit.

Anomalous amounts of sulfur dioxide, resulting 

from the oxidation of sulfide minerals, have been detected in samples of the atmosphere collected several 

metres above the Tyrone, N. Mex., porphyry copper 

deposit, as well as above other sulfide deposits (Rouse 

and Stevens, 1971). The chemical variations of sulfur 

dioxide concentrations in the atmosphere are therefore considered to be useful in the search for many 

types of sulfide deposits. Hydrogen sulfide should 

also be present wherever sulfide minerals are oxidizing and should be detectable in the atmosphere. 

Because the decomposition of organic matter can 

also produce gaseous sulfur species, anomalous concentrations of these compounds in the atmosphere 

must be carefully evaluated in order to determine 

the source of the gases.

Other gases have been found associated with 

various types of ore deposits. Fridman and Makhlova 

(1972) discussed the use of carbon dioxide as a geochemical indicator of mercury ores. In addition to 

CO2 , they detected nitrogen, hydrogen, helium, and 

argon in the vicinity of mercury ore. The work of 

Fridman and Makhlova was done underground; however, it does not seem unreasonable to expect that 

under the right conditions these same highly volatile 

species would reach the surface where they could 

escape and be detected in the atmosphere above the 

deposit.

Halogen gases are known to be associated with 

hydrothermal systems. The possibility of detecting 

anomalous amounts of these gases in the atmosphere

above copper deposits has not been adequately tested 

as yet. McCarthy (1972), without giving details, 

noted that bromine and iodine have been detected in 

the air over a porphyry copper deposit in Arizona. 

Bristow and Jonasson (1972) also noted that iodine 

is associated both with porphyry copper deposits and 

with other types of base-metal deposits.

The decay of 40K to 40Ar might produce anomalous 

concentrations of argon in the atmosphere above 

zones of potassium metasomatism. This argon could 

be analyzed with appropriate airborne instrumentation.

Other gases might be detected in geochemical 

surveys using samples of the atmosphere above copper deposits. It is possible that the oxides of nitrogen could form in the zone of oxidation. And, as 

noted earlier, hydrocarbons, including methane and 

other compounds, have been found associated with 

ore deposits (Bristow and Jonasson, 1972; Shipulin 

and others, 1973). However, because many hydrocarbon compounds are formed from the decomposition of organic matter, nonsignificant anomalies may 

also be present. Lastly, there may be cases in which 

the relatively high vapor pressure of such normally 

nonvolatile elements as arsenic, antimony, selenium, 

tellurium, and perhaps others, would make it possible to detect these elements in the atmosphere 

above mineral deposits.

Plants are known to give off water vapor during 

transpiration. Continuing studies by the U.S. Geological Survey indicate that plants also exhale metallic compounds during transpiration (G. C. Curtin, 

oral commun., 1973). To date, the method has not 

been fully evaluated as a geochemical-prospecting 

technique (fig. 4) ; however, measurement of concentrations of trace metals in the air around plants 

may prove to be a useful technique for detecting 

anomalous amounts of metals in the soil in which the 

plants are rooted.

The results of analyses of particulate aerosols collected from 30 to 60 m (100 to 200 ft) above the 

ground were used effectively as a rapid regional geochemical exploration technique for detecting copper 

and other types of mineral deposits (Weiss, 1971). 

The distribution of the size of the particulate matter 

was found to be related to the distance from the 

source of the material. The technique was found to 

be effective except over areas covered by water, 

snow, or thick tropical forest.

REMOTE SENSING

In this report the term "remote sensing" is defined as (1) the quantitative or semiquantitative
B20 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

FIGURE 4. Plastic bags are attached to conifer trees to 

collect exudates for geochemical analysis. Alpine environment west of Denver, Colo. Photograph by G. C. Curtin, 

U.S. Geological Survey.

measurement of the chemical concentrations of one 

or more elements or compounds in material in situ, 

or (2) the measurement of chemical effects on vegetation as seen using photographs or other imagery. 

The remote-sensing instrument may be placed directly against (or very close to) the sample to be 

analyzed, or the instrument may be located at a distance of as much as several hundred kilometres 

from the sample site, as in the case of a satellite, but 

more likely at several thousands of metres, as from 

an aircraft. Remote-sensing geochemistry can be 

performed on land, in the air, and on or under bodies 

of water.

Neutron-activation analysis is a remote-sensing 

technique that has not been widely used in geochemical prospecting, probably because the necessary 

equipment is expensive and somewhat bulky. The 

method involves bombarding a sample with neutrons 

and then measuring the resulting gamma-ray spectrum. The technique is applicable to the detection of

a number of elements, including copper. Because it 

measures the metal content in situ, this method 

offers two applications not readily available in other 

geochemical exploration techniques (Senftle, 1970). 

First, the apparatus can be adapted for down-thehole measurements of various elements. The exact 

location and concentration of a given element can be 

readily determined. Second, the method offers means 

of locating copper or other metals beneath bodies of 

water. This second application could be particularly 

valuable as a relatively inexpensive method of evaluating the mineral potential of ocean or large-lake 

bottoms. Areas containing manganese nodules or 

other metal-rich sea-bottom deposits could be detected without having to collect a bottom or core 

sample for analysis at the surface.

The remote detection of natural gamma radiation 

using a sensitive airborne gamma-ray spectrometer 

has been described by Bennett (1971) and by Moxham, Foote, and Bunker (1965). The airborne spectrometer can detect natural gamma radiation emitted 

by potassium, uranium, and thorium. Several different types of copper deposits in Australia have been 

detected by this method (Bennett, 1971). Delineation of the widespread zone of potassium metasomatism commonly associated with porphyry-type copper 

deposits was accomplished at several Arizona copper 

deposits by Moxham, Foote, and Bunker (1965). 

They also noted that potassium-thorium ratios 

should be valuable in delineating altered rock masses.

Aerial photography appears to be a useful remotesensing method for detecting and evaluating geobotanical and biogeochemical anomalies associated 

with base-metal mineral deposits, especially in areas 

with heavy vegetative growth. Ordinary color photography could be used to detect the presence or 

absence of, and the areal extent of, specific plants or 

plant communities. In addition, low-level color photography could be used to detect indicator plants 

during their flowering period. From altitudes of 

about 150 to 300 m (500 to 1,000 ft) above the 

ground, I have observed the colorful blooms of the 

California poppy (Eschscholtzia mexicana) growing 

in masses over known copper deposits in Arizona. 

Areas containing these flowers should be detectable 

on color aerial photographs taken at similar altitudes.

Infrared color film has been used to detect vegetation that is unhealthy because of disease or insect 

damage. Some types of vegetation, however, become 

stressed because of an abnormal enrichment or depletion of trace elements in the material in which 

they are rooted (F. C. Canney, oral commun., 1972).
GEOCHEMICAL EXPLORATION TECHNIQUES B21

Plants growing in metal-enriched soils have spectral 

properties that may be detectable from remote locations either by using infrared film or by using some 

sort of spectral scanning instrument (Marshall, 

1970a; Press and Norman, 1972). In regions containing deep-rooted plants, such instrumentation could 

be used to detect metal anomalies in bedrock covered 

by relatively thin layers of transported overburden. 

Studies of the spectral properties of plants would 

therefore be especially valuable in the search for 

copper deposits buried by glacial debris or by gravel 

deposits such as those commonly present on the 

desert pediments in the porphyry-copper province 

of the southwestern United States. The technique 

could also be used over areas of dense vegetation, 

such as tropical jungles, where bedrock cannot be 

readily sampled and little or no outcrop can be seen 

on aerial photographs.

The stunting or low density of vegetation in some 

areas may be caused by high metal concentrations in 

the associated soils. Areas showing these effects 

should be readily visible on either color or black-andwhite aerial photographs. Many humid tropical 

areas, however, are commonly covered with clouds 

for much of the year and are therefore not suitable 

for normal aerial photography. The effects of metal 

stress on vegetation in such areas might be evaluated 

by using airborne radar imagery. This imagery can 

be obtained through cloud cover and could be used 

to map the general height of the forest canopy and 

the density of growth in an area (F. C. Canney, oral 

commun., 1973).

STATISTICS 

INTRODUCTION

The most easily discovered metal deposits have 

already been found in most parts of the world. In the 

future, scientists will be looking for ways to distinguish subtle geochemical anomalies of possible economic significance from increasingly complex geochemical background data. Today we are trying not 

only to discriminate between natural metal anomalies and natural background populations, but also 

to distinguish between these natural populations and 

man-made populations caused by contamination and 

pollution. Clearly, new approaches to geochemical 

exploration are needed. The use of statistical techniques is one example. Various fields within the 

social, biological, and electrical engineering sciences 

have provided the basic statistical techniques that 

are now being adapted to problems in the interpretation of geochemical data.

It should be stressed that the ultimate evaluation 

of any computer output must be related back to the 

real world of natural systems; in other words, the 

data output is only as good as the data input. In 

short, the computer supplements but does not replace the geochemist.

Statistical treatment of geochemical data was 

necessarily limited before the advent of computers 

for data processing. Most precomputer statistical 

evaluations were limited to the determination of such 

parameters as mean, median, and standard deviation 

in order to establish the limits of background and 

anomalous populations. Graphical distributions of 

data sets were also studied and indicated that, in 

general, the analyses of a given population follow 

more closely a lognormal distribution than a normal 

one. As with any natural system, exceptions to this 

generalization are known. Ratios of elements to each 

other, to regional background, or to Clarke values 

have also been helpful for showing trends of elements both laterally and with depth. In the field or 

in localities where computers are not available, geochemists have determined threshold values and the 

ranges of background and anomalous values for even 

large data sets using cumulative frequency plots 

(Lepeltier, 1969).

Precomputer statistical analyses of geochemical 

data sets were generally limited to the use of only 

one variable at a time (univariate analysis). The 

advent of computer data processing means that several or many geochemical variables can be studied 

simultaneously (multivariate analysis). In addition 

to studying the chemical data of a sample, geochemists can now compare conditions of the physical and 

chemical environment of the sample site or region 

with the sample analytical data. In short, computer 

processing of data has opened up a wide range of 

possibilities for evaluating geochemical data.

In the following discussion, some examples are 

given of how statistical interpretation techniques 

might be applied to geochemical data. Several detailed reviews of this field have been published recently (Marshall, 1970b; Agterberg and Kelly, 1971; 

Rose, 1972; Nichol, 1973). The reader is referred to 

these papers for more specific applications of the 

techniques to geochemical exploration programs.

CORRELATION ANALYSIS

A simple correlation analysis measures the degree 

of association between two or more variables taken 

two at a time. A multiple correlation analysis measures the combined effect of several variables on a 

selected variable. Correlation analysis could be val-
B22 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

liable, for example, in determining which elements 

or other variables associate most closely with copper in any given sampling medium. Elements that 

are found to have a high degree of positive or negative correlation with copper might serve as pathfinder elements in an area under study.

REGRESSION ANALYSIS

Regression analysis attempts to equate mathematically one variable (the dependent variable) with 

a group of other variables (the independent variables). When only two variables are used, the analysis is called a simple regression analysis; when 

more than two variables are involved, the analysis 

is called a multiple regression analysis. The technique has been used in surveys to explain the variation of a given element in terms of other elements 

and of other physical and chemical factors. For example, the variation of the concentration of copper 

in a stream-sediment sample could be compared with 

such other variables as (1) the concentrations of 

other elements in the sample; (2) the grain size of 

the sample material; (3) the mineralogical composition of the sample; (4) the pH of the stream water; 

or (5) the width, depth, and velocity of the stream, 

in an attempt to determine which factors significantly influence the variability of copper in the 

sample and the relative importance of each factor. 

Once an equation for a given data set has been determined, regression analysis can be used to calculate, 

for example, the theoretical copper concentration at 

a given site based on the independent variables for 

that site. The differences between the calculated copper concentrations and the measured copper concentrations, called residual values, can then be computed. One can then study both the residual data 

and the theoretical data in an effort to find sites 

containing ore-related anomalies that could not be 

found using only the raw data.

DISCRIMINANT ANALYSIS

Discriminant analysis is a classification scheme 

for studying geochemical data. A commonly used 

type of discriminant analysis creates and uses a 

mathematical equation based on some combination 

of variables to maximize the differences between 

two populations. This technique requires data on two 

different populations to begin with. New data are 

mathematically compared with these two populations and then assigned to one or the other on the 

basis of all similarities and differences. An ideal 

example of application of this method would be one 

in which there were, for a given area, a data set

from a mineralized sample population and a data 

set from a background sample population. These 

data sets could be acquired in an orientation study. 

Equations derived from these two populations could 

then be compared with new data from samples collected for the entire survey to determine whether 

individual sites in the extended survey more closely 

represented anomalous or background conditions.

FACTOR ANALYSIS

Factor analysis is a method for identifying common factors in the variables of a data set. A factor 

analysis program starts with a matrix of values 

similar to a correlation matrix of geochemical data 

and assigns the various elements of the matrix to 

groups (factors) based on a common variability of 

these elements,

A Q-mode factor analysis investigates the interrelationships between samples. The analysis compares the set of variables at each sample site with 

those at every other site and groups the samples into 

a smaller number of "typical" samples, From this 

technique, the relationship between samples may be 

deduced. Q-mode factor analysis can be used, for 

example, to group data in terms of geologic parameters such as bedrock types.

An R-mode factor analysis investigates the interrelationships between variables. This type of analysis groups chemical data that vary sympathetically 

in all samples into a set of factors, each containing 

one or more of the original variables. R-mode factor 

analysis can be used to identify groups of elements 

that have common associations; for example, this 

technique can be used to find those elements that 

would be scavenged by the manganese and iron 

oxides found in stream sediments.

CLUSTER ANALYSIS

Multi-element data sets can also be classified into 

groups by the use of cluster analysis. A commonly 

used version of this technique, which can also be 

done in a Q- or R-mode, compares pairs of variables 

or samples, or pairs of pairs, and so on, and then arranges these pairs (clusters) into a hierarchical 

dendritic diagram (dendogram) in which the variables or samples are grouped according to levels 

of pairing. These levels are based on values for correlation coefficients between cluster pairs. As an 

example, the method could be applied to a data set in 

an attempt, on the basis of selected chemical associations, to separate clusters representing samples from 

mineralized areas and those representing samples 

from background areas.
GEOCHEMICAL EXPLORATION TECHNIQUES B23

TREND-SURFACE ANALYSIS

The preceding methods have dealt mainly with 

analysis of numerical geochemical data in order to 

find common associations as an aid to interpreting 

the data. Several techniques are also available for 

showing objectively, in the form of a map, the 

spatial distribution of geochemical data. These 

methods are especially applicable to large data sets 

from samples collected from large regions. Trendsurface analysis is similar to multiple-regression 

analysis but involves the fitting of mathematical 

surfaces to data. In trend-surface analysis the dependent variable, a chemical or geologic variable, 

is equated mathematically to geographic coordinates. 

The variation of this dependent variable over the 

region sampled defines a mathematical equation representing a plane or curved (trend) surface. This 

trend surface can be contoured for study in a manner similar to a topographic contour map. The 

mathematical equation for any trend surface can be 

expanded within practical limits by adding several 

more terms to the equation. This addition has the 

effect mathematically of more precisely defining the 

variations in the dependent variable. Visually, the 

effect of adding more terms is a more detailed contouring of the map.

Trend-surface analysis should be valuable in 

studying the changes in a given variable such as 

copper content, over an area of any size; however, 

adequate data-point distribution is imperative if the 

method is to succeed. As is the case with multipleregression analysis, trend-surface analysis can be 

used to compute theoretical values at each sample 

site, based on regional trends, to compare with the 

actual measured values at the site. The difference 

between the theoretical and measured values at a 

given site is the residual value for that site. Residual values can be plotted and contoured for data 

evaluation. In a manner similar to that used in regression analysis, residuals can be used to remove 

regional geochemical trends from local variations. 

Theoretically, the residuals should then delineate 

anomalous areas that might be related to mineral 

deposits. Unfortunately, in natural systems, chemical trends are seldom regular enough to make use 

of this method without taking into account other 

geological and chemical information.

MOVING-AVERAGE ANALYSIS

The moving-average analysis (also called a rolling mean analysis) is another popular technique for 

treating data spatially. In this technique, a search 

area large enough to include several data points on

a map is moved across the map. Values within the 

search area at any given time are averaged, and 

the mean value is plotted in the center of the search 

area. The degree of smoothing of the original data 

can thus be varied by changing the size or degree 

of overlap of the search area and by changing the 

manner of weighting and averaging the data within 

the search area. The technique can be used to smooth 

regional geochemical data in order to enhance regional geochemical trends because the technique 

tends to remove anticipated sampling and analytical 

noise from geochemical data.

In a manner similar to other computer analytical 

techniques, this technique can be used to calculate 

residuals, the differences between the smoothed 

mean value and the measured value at any point on 

the map of the study area. These residuals can then 

be contoured and used in evaluating the study area.

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B24 GEOLOGY AND RESOURCES OF COPPER DEPOSITS

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