Advances in Research

on Mineral Resources, 1994
Cover. Part of a color shaded-relief aeromagnetic map of north-central Minnesota. 

Color changes represent broad changes in magnetic field strength. The warm colors (red, 

orange, and yellow) represent areas of magnetic field highs; the cool colors (green, blue, 

and purple) represent magnetic field lows.

[From Horton, R.J., Day, W.C., and Bracken, R.E., Aeromagnetic survey of north-central 

Minnesota, chap. T of Scott, R.W., Jr., Detra, P.S., and Berger, B.R., eds., Advances Related 

to United States and International Mineral Resources: Developing Frameworks and 

Exploration Technologies: U.S. Geological Survey Bulletin 2039, p. 203]
Advances in Research

on Mineral Resources, 1994

Edited by Byron R. Berger

U.S. GEOLOGICAL SURVEY BULLETIN 2081

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Published in the Central Region, Denver, Colorado

Manuscript approved for publication June 17,1994

Edited by Richard W. Scott, Jr.

Photocomposition by Shelly A. Fields

Cover design by Art Isom

Additional assistance from Branch of Geochemistry:

Additional proofing and review by Pamela S. Detra

Graphics prepared by Dick Walker

Assistance with manuscript preparation by Margo Johnson 

and Barbara Ramsey

For sale by U.S. Geological Survey, Map Distribution

Box 25286, MS 306, Federal Center

Denver, CO 80225

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

does not imply endorsement by the U.S. Government.

Library of Congress Cataloging-in-Publication Data

Advances in research on mineral resources, 1994 / edited by Byron R. Berger. 

p. cm. (U.S. Geological Survey bulletin; 2081)

Includes bibliographical references (p. ).

1. Prospecting Geophysical methods. 2. Mines and mineral resources 

Research United States. I. Berger, Byron R. II. Series. 

QE75.B9 2081 

[TN270]

557.3 s dc20 94-35233 

[622'.1] Cff
CONTENTS

[Letters designate chapters]

Introduction: USGS An International Perspective on Mineral Resources and

Resource Technologies 

By Byron R. Berger............................................................................................... 1

A. Pb Isotope Compositions of the Bandelier Tuff, Valles Caldera, Jemez

Mountains, New Mexico An Active Geothermal System Associated with 

Mo Mineralization 

By Robert A. Ayuso and Robert L. Smith............................................................ 3

B. Implications of Eocene Through Miocene Ages for Volcanic Rocks,

Snowstorm Mountains and Vicinity, Northern Nevada 

By Alan R. Wallace and E.H. McKee................................................................... 13

C. 40Ar/39Ar and U-Th-Pb Ages Associated with Base- and Precious-Metal

Deposits in Central Idaho 

By Karl V. Evans, Lawrence W. Snee, and Robert E. Zartman............................ 19

D. Geology and Mineral Deposits in the Terrill Mountains Area, Holy Cross

District, Churchill County, Nevada 

By Stephen G. Peters, Richard F. Hardyman, and Katherine A. Connors............ 25

E. Time-pH-f^ Paths of Hydrothermal Fluids and the Origin of

Quartz-Alunite-Gold Deposits 

By George V. Albino............................................................................................. 33

F. Remote-Sensing Spectral Properties of Selected Industrial

Minerals and Ores 

By James K. Crowley............................................................................................ 43

G. Geochemical Exploration Studies in the Portuguese Pyrite Belt

By David J. Grimes and Delfim de Carvalho........................................................ 53

ill

INTRODUCTION

USGS AN INTERNATIONAL PERSPECTIVE ON 

MINERAL RESOURCES AND RESOURCE TECHNOLOGIES

By Byron R. Berger 1

Geographic landscapes change only slowly, but political and economic landscapes can change abruptly and unexpectedly The geological sciences are contributors to changes 

in these landscapes, but they also serve as a source of information on which an informed polity may base decisions on 

behalf of a society. The U.S. Geological Survey (USGS) is 

dedicated to providing information for the public good in all 

aspects of the geological sciences, including mineral 

resources.

Mineral resources are a part of the geosciences that 

impact all aspects of a society from national economic security to national defense to land-use decisions to health and 

welfare these impacts affect not just humans but all other 

animals as well as all flora. Minerals are an essential part of 

the well-being of both a society and the Earth. They serve as 

the basis for infrastructure, technological innovation and 

development, economic policies, national defense, and 

national economic security; they are also essential to 

informed decisions on environmental risk and environmental tradeoffs in any decision-making process.

The USGS provides mineral resource information, 

knowledge, and technical analyses from highly trained scientists with broad-ranging technical skills to Federal, State, 

and local governments; industries; schools and universities; 

and the general public of the United States. It also engages 

in cooperative studies with other countries and their institutions to provide training, exchange information and understanding, and attack important scientific problems. This 

Bulletin, Advances in research on mineral resources, 1994, 

reports progress on some of the current domestic and international research activities of the Office of Mineral 

Resources, Geologic Division of the USGS.

'U.S. Geological Survey, Mail Stop 973, P.O. Box 25046, Denver 

Federal Center, Denver, CO 80225.

ESTABLISHING A GEOLOGIC

FRAMEWORK 

FOR MINERAL DEPOSITS

Since the emergence of economic geology as a recognized subdiscipline within the earth-science research community within the last century, there has been heated debate 

about the role of igneous processes in the genesis of deposits that are spatially associated with intrusions and volcanic 

rocks. Do the metals in most deposits come from magmas, 

or is the magma merely a partial contributor or simply a 

heat engine? Even the use of sophisticated new technologies 

in the post-World War II era has not caused the issue to 

wither. Although there are stable-isotope methodologies to 

fingerprint probable water sources and radiogenic methodologies that reveal the source of some metals (and most 

deposits and rocks can now be dated with high precision), 

the debate continues. Lead, for example, in some epizonal, 

polymetallic, mineral deposits is isotopically indistinguishable from coeval igneous rocks, whereas in other, ostensibly 

identical, rock types, it more closely reflects the composition of basement crystalline rocks than do coeval igneous 

rocks.

In the first chapter in this volume, Ayuso and Smith 

(chapter A) use lead isotopes to examine the behavior of 

heavy metals in a shallow, siliceous magma chamber. Their 

results show that the lead isotopic composition of different 

magma batches from a presumed single reservoir may vary 

considerably a fact that has important implications for the 

use of isotopes in ore-genesis studies.

The timing of mineralization is of considerable importance in exploration as well as mineral-resource-appraisal 

studies. Wallace and McKee (chapter B) and Evans and 

others (chapter C) both present data of considerable import 

to two areas in the Western United States. Wallace and 

McKee show that both late Eocene and middle Miocene 

volcanism in northern Nevada blanketed larger areas than 

has been previously recognized. Base- and precious-metal
ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

mineralization is associated regionally with both periods of 

volcanic activity therefore, their work substantially 

enlarges the terrain that is permissive for mineral occurrences. Similarly, Evans and others have shown there to be 

a period of Late Cretaceous mineralization in Idaho in a 

region thought previously to contain only Tertiary 

mineralization.

Geologic mapping by Peters and others (chapter D) in 

western Nevada has established a geologic and structural 

context for epithermal mineralization in the Holy Cross 

mining district. This work permits an evaluation of the mineral resource potential at Holy Cross relative to nearby producing mines in the Rawhide and Santa Fe districts and 

contributes to the general volcanic stratigraphy of the 

region.

DEVELOPING A CONCEPTUAL

FRAMEWORK FOR 

ORE-DEPOSITING PROCESSES

Descriptive, geochemical, and geophysical data are 

compiled from numerous mineral deposits and mining districts and assembled into models that serve as backbones for 

exploration and resource-assessment activities. Marrying 

experimental data and descriptive data from actual deposits 

is a difficult task in modeling. Albino (chapter E) attacks 

this problem by examining the temporal relations between 

different deposits of a certain style of gold mineralization 

and postulates that these deposits are derived through the 

mobilization of metals from a previously existing deposit. If

this conceptual model is correct, then there are important 

implications for both exploration and assessment.

DEVELOPING AND

TESTING TECHNOLOGIES FOR

MINERAL RESOURCE ASSESSMENT

AND EXPLORATION

The development and use of appropriate and state-ofthe-art technologies is integral to mineral resource assessments and exploration. In chapter F, Crowley shows the 

results of laboratory experiments designed to evaluate the 

utility of remotely sensed spectral data in detecting industrial minerals and ores. His results are encouraging for the 

use of new airborne and satellite technologies in the search 

for and assessment of important industrial commodities.

When technologies have been developed that appear to 

be useful, performance testing under normal field conditions 

is valuable both to document the conditions under which a 

technology is useful and to transfer that technology to other 

users. Grimes and de Carvalho (chapter G) tested a variety of 

exploration geochemical techniques over wholly unexposed, 

massive sulfide mineralization in the Portuguese Pyrite Belt. 

They not only successfully found indications of unexposed 

mineralization with some techniques, but they also delimited 

the optimum conditions under which to apply these techniques. Their findings will be of considerable value to additional exploration in this part of Portugal and to future work 

in similar climatological and geologic environments.
CHAPTER A

Pb ISOTOPE COMPOSITIONS OF THE BANDELffiR TUFF,

VALLES CALDERA, JEMEZ MOUNTAINS,

NEW MEXICO AN ACTIVE GEOTHERMAL SYSTEM

ASSOCIATED WITH Mo MINERALIZATION

By Robert A. Ayuso1 and Robert L. Smith2

ABSTRACT

Two Pleistocene eruptions, approximately 350,000 

years apart, deposited the Otowi and Tshirege Members of 

the Bandelier Tuff. These members were associated with the 

collapse of the Toledo and Valles calderas, and each caldera 

had a complex post-collapse history of rhyolitic volcanism, 

which included the Cerro Toledo Rhyolite and the Valles 

Rhyolite. Pb isotope determinations of 78 whole rocks and 

K-feldspars from the Bandelier Tuff and associated rocks 

indicate that Pb isotope compositions changed significantly 

during evolution of the silicic rocks. The Otowi Member of 

the Bandelier Tuff is less radiogenic than the Tshirege (e.g., 

206pb/204pb _ 17.816-17.853 for the Otowi and 

17.938-18.003 for the Tshirege), and the Cerro Toledo Rhyolite has intermediate values (206Pb/204Pb = 

17.823-18.003). In the Valles Rhyolite, older domes 

(206pb/204pb _ i7.925_i8.oiO) overlap the Tshirege Member, but the younger Valles domes (206Pb/204Pb = 

17.686-17.745) are the least radiogenic in the BandelierToledo-Valles magma system. Pb isotope values of pre-Bandelier Quaternary basalts and intra-Bandelier basaltic andesites (206Pb/204Pb = 17.549-17.891) overlap the Bandelier 

Tuff; pre-Bandelier andesites, dacites, rhyodacites, and rhyolites, which include potential parents or contaminants of the 

Bandelier Tuff, have Pb isotope compositions that bracket 

the entire magmatic system. The Bandelier Tuff originated 

as a mixture of mantle-derived rocks and crustally derived 

melts; the magma chamber grew by replenishment or mixing 

of chemically variable magma at the bottom of the chamber

'U.S. Geological Survey, Mail Stop 954, National Center, 12201 

Sunrise Valley Drive, Reston, VA 22092-0001.

2U.S. Geological Survey, 2929 Fulton Avenue, Sacramento, CA 

95821.

(open system) and was influenced by fractional crystallization coupled with assimilation of isotopically different wallrocks. Volcanic eruptions in such periodically discharged 

systems may partially remove magmas from the roof of the 

magma chamber whose isotopic and other geochemical signatures differ from those stored at lower depths this process may give rise to isotopically zoned plutonic rocks.

INTRODUCTION

This paper reports preliminary results of an ongoing 

isotopic study of the Bandelier Tuff and associated igneous 

rocks in the Jemez Mountains (fig. 1) in an effort to better 

understand the genesis of compositionally zoned magmatic 

systems and to improve our knowledge of isotopic variations 

in an active hydrothermal system. We have established the 

range of Pb isotopic compositions of rhyolitic ash-flow 

sheets of the Bandelier Tuff (Otowi Member and Tshirege 

Member); Cerro Toledo Rhyolite; Valles Rhyolite; older and 

contemporaneous mafic, intermediate, and rhyolitic rocks; 

and Proterozoic basement (fig. 2; table 1).

The processes associated with caldera formation and 

zoned magma chambers have been documented in the Jemez 

Mountains, and the Valles caldera represents a classic example of an intracontinental resurgent caldera (Smith and 

Bailey, 1966, 1968; Smith and others, 1970; Gardner and 

others, 1986; Heiken and others, 1986; Self and others, 1986, 

1988). Various attempts have also been made to refine the 

geochemical constraints associated with the evolution of 

Bandelier magmas (e.g., Smith, 1979; Macdonald and 

Smith, 1988; Turbeville and Self, 1988; Loeffler and others, 

1988; Stix and others, 1988; Spell and Kyle, 1989; Skuba 

and Wolff, 1989; Ayuso and Smith, 1989; Stix and Gorton, 

1990; Hervig and Dunbar, 1992). The Valles caldera was
ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

106° 30' 106° 15'

| | Alluvium Bearhead Rhyolite and 

Canovas Canyon Rhyolite

Valles Rhyolite and Cerro I _ I Tschicoma Formation (north)

TnlRrin Toledo Rhvnlitp Rhyolite I I Palbra Fnrmatinn fcnuthl

Bandelier Tuff

Basalts of El Alto, Cerros 

del Rio, and Santa Ana 

Mesa

Paliza Formation (south)

Labato Basalt (north) Basalt 

of Chamisa Mesa (south)

~^l Santa Fe Group and 

' ' ' AbiquiuTuff

Puye Formation (north) 

Cochito Formation (south) L^

Mesozoic, Paleozoic, and 

Precambrian rocks 

undifferentiated

Figure 1. Generalized map of the Valles caldera, New Mexico 

(from Smith and others, 1970).

also selected as a site for deep drilling for the continental scientific drilling program (Goff and Gardner, 1988). Moreover, the Valles caldera is an active hydrothermal system 

associated with sub-ore-grade molybdenum mineralization, 

as well as Cu, Pb, Zn, Mn, Bi, Te, and Ag minerals (Hulen 

and others, 1987). A better understanding of the isotope variations in active hydrothermal systems and their ore deposits 

could well result in an increased ability to identify fossil ore 

deposits associated with silicic calderas.

CHEMICAL ZONING IN SILICIC SYSTEMS

Isotopic studies have been particularly useful in helping 

to constrain mechanisms leading to chemical zoning in 

silicic magma chambers. For example, variations in 

87Sr/86Sr values within individual ash-flow sheets were 

ascribed to assimilation of country rock (Noble and Hedge, 

1969), and variations in O isotopes were used to detect a 

gradually increasing component of meteoric ground water

during evolution of a volcanic complex (Lipman and Friedman, 1975). In order to explain a trend of increasing 

87Sr/86Sr values toward the roof of a magma chamber, a convection-driven, liquid-state differentiation mechanism has 

also been proposed (Hildreth, 1979). In many ash-flow tuffs, 

isotopic zonation is monotonic, and significant assimilation 

may occur in the roof zones of magma chambers (e.g., Tegtmeyer and Fanner, 1989; Johnson, 1989; Johnson and others, 1990). However, in some cases Sr, O, and Nd isotopic 

data were interpreted to indicate negligible contamination of 

magma (Halliday and others, 1984; 1989). Our results on the 

Bandelier Tuff system demonstrate that Pb isotopic variations are cyclic, probably reflecting the effects of magma 

recharge, mixing, fractionation, and assimilation of wallrocks (Ayuso and Smith, 1989).

THE BANDELIER TUFF AND 

RELATED ROCKS

The Bandelier Tuff consists of a lower member (Otowi, 

1.45 Ma, about 300 km3 in volume) and an upper member 

(Tshirege, 1.12 Ma, about 300 km3 in volume) (Smith and 

Bailey, 1966,1968; Doell and others, 1968; Izett and others, 

1981). The beginning of magmatic activity in the area, however, predates the Bandelier Tuff perhaps by at least as much 

as 10 m.y. (Bailey and others, 1969; Self and others, 1986; 

Gardner and others, 1986). The Otowi Member was erupted 

to form the Toledo caldera, followed by eruption of the 

Tshirege Member, which formed the Valles caldera. The 

Cerro Toledo Rhyolite, consisting of high-silica rocks, was 

erupted from the Toledo caldera and stratigraphically occurs 

between the Otowi Member and the Tshirege Member of the 

Bandelier Tuff. The Valles Rhyolite is also a high-silica rhyolite and was erupted after the collapse of the Valles caldera. 

The older Valles Rhyolite domes span from about 1.18 Ma 

to 0.75 Ma (Izett and others, 1981), and their age generally 

depends on the location of individual domes (from older to 

younger, Cerro del Medio, Abrigo, Santa Rosa, and Seco); 

the younger Valles domes and flows (San Antonio, South 

Mountain, Battleship Rock, and Banco Bonito) are < 0.73 

Ma (Izett and others, 1981). The boundary between the 

resurgent dome (Redondo Peak) and the western ring-fracture marks the strongest hydrothermal activity at the surface, 

and it is where investigation of a core hole documented fractured and quartz serialized tuff containing molybdenite, as 

well as anomalous enrichment in lead, zinc, and silver 

(Hulen and others, 1987).

The Otowi Member consists of a basal air-fall tuff that 

originated from the top of the magma chamber and ash flows 

that originated from deeper in the magma chamber (Smith 

and Bailey, 1966). Older volcanic rocks, Proterozoic granitic 

basement, and Proterozoic metamorphic rocks are the predominant xenoliths. Five subunits in the Tshirege Member
Pb ISOTOPE COMPOSITIONS, BANDELIER TUFF, VALLES CALDERA, JEMEZ MOUNTAINS, NEW MEXICO 5

15.66

15.58-

15.50-

15.42-

15.34

39.5

 Tertiary andesite 

Tertiary basalt

Quaternary basalt 

v Proterozoic granite

Bandelier Tuff

South of Valles 

Tertiary rhyolite

Tertiary rhyodacite

.North of Valles

Tertiary and Quaternary rhyolite

Proterozoic granite

£

38.5.

37.5-

36.5"

35.5

Bandelier Tuff

Quaternary basalt

P~ x Tertiary basalt

Tertiary andesite

Proterozoic granite

Bandelier Tuff

Tertiary rhyodacite

North of Valles 

Tertiary and 

Quaternary 

rhyolite

South of Valles 

Tertiary rhyolite

Proterozoic granite

16

B

17 18 

206 Rb/204 Rb

19 20 16 17

D

18

206 Rb/204 Pb

19 20

Figure 2. Pb isotope compositions in the Jemez Mountains, New Mexico. Data for Proterozoic granites, Tertiary basalt and 

andesite, Quaternary basalt, and Bandelier Tuff magma system: A, 206Pb/*"Pb vs. ^Pb/^Pb; B, ^Pb/^Pb vs. 208Pb/204Pb. 

Data for Tertiary rhyodacite, Quaternary rhyolites, and Bandelier Tuff magma system: C, ^Pb/^Pb vs. ^Pb/^Pb; D, 

206Pb/204Pb vs. ^Pb/^Pb. Two-stage Pb evolution curve (solid line) of Stacey and Kramers (1975) is shown for reference and 

is graduated at 250-Ma intervals.

are recognized. Quartz is found throughout Tshirege, but it 

is rare in the late, erupted ash flows; abundances of sanidine, 

clinopyroxene, fayalite, orthopyroxene, magnetite, and 

ilmenite vary consistently from the bottom to the top. Trace 

amounts of zircon, apatite, chevkinite, and allanite are also 

present.

The Bandelier Tuff consists predominantly of high-silica (76-77 percent) rhyolite except for the late eruptives of 

the Tshirege Member, which have SiC>2 = 72 percent (Smith, 

1979). Trace element contents change systematically within 

each member of the Bandelier Tuff. Lithophile trace elements (e.g., Nb, Ta, Th, etc.) decrease up section, and compatible and ferromagnesian elements (e.g., Eu, Co, Cr, Sc, 

etc.) increase up section. These variations were interpreted 

as evidence that the chamber was chemically zoned and that 

the chemical gradients existed in bulk rock, crystals, and 

glass in the liquid prior to the formation of the erupted phenocryst gradient (Smith, 1979).

ANALYTICAL TECHNIQUES

A total of 78 samples of the Bandelier Tuff and associated rocks were analyzed in this study; table 1 summarizes 

the results. The majority of whole-rock samples from the 

Bandelier Tuff were obtained from nonwelded vitric pumice 

blocks. Samples of basalts, andesites, dacites, and rhyolites 

included pumice, obsidian, perlite, and crystallized rocks. Kfeldspar separates were made from pumice fragments in the 

Bandelier Tuff and from Proterozoic granitic rocks. Pb was 

concentrated using standard HBr chemistry, and the Pb isotope values were measured using a 12-inch NBS-type mass 

spectrometer and a Finnigan MAT 262 multicollector mass 

spectrometer at the U.S. Geological Survey, Reston, Va. 

Procedural blanks during the course of this study were < 0.5 

ng and thus are negligible relative to the measured data. The 

Pb determinations have a maximum total analytical uncertainty of about 0.1 percent at the 95 percent level on the basis
ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Table 1. Pb isotopic ranges of 78 samples from the Bandelier Tuff and associated rocks, Jemez 

Mountains, New Mexico.

[n, number]

Unit 206pb/204pb

Pb isotopic ratios

207pb/204pb 208pb/204pb

Proterozoic granites (n = 18)

Whole rock

K-feldspar

17.074-18.589

16.261-16.749

15.477-15.578

15.411-15.430

36.030-37.028

35.801-36.040

Pre-Bandelier volcanic rocks (n = 20)

Tertiary 

Basalt1 

Andesite2 

Dacite3 

Rhyolite4 

Quaternary 

Basalt5 

Rhyolite6

18.027-18.340 

17.767-18.099 

17.124-18.075 

17.897-18.197

17.549-17.891 

17.192-17.862

15.460-15.527 

15.524-15.528 

15.435-15.546 

15.529-15.531

15.464-15.512 

15.440-15.546

37.754-37.860 

37.613-37.972 

36.988-37.737 

37.616-37.935

37.375-37.690 

37.019-37.695

Bandelier Tuff-Cerro Toledo-Valles magmatic system (n = 40)

Precursor7

Otowi

Early 

Late

Cerro Toledo

Early 

Late

Tshirege 

Early 

Late

Valles

Early 

Late

17.805-17.862

17.816-17.837 

17.831-17.853

17.823-17.828 

17.980-18.003

17.938-17.957 

17.969-18.003

17.925-18.010 

17.686-17.745

15.520-15.546

15.497-15.504 

15.516-15.534

15.511-15.512 

15.598-15.627

15.538-15.570 

15.545-15.577

15.557-15.570 

15.528-15.529

37.639-37.695

37.560-37.601 

37.648-37.699

37.604-37.607 

37.993-38.092

37.807-37.866 

37.838-37.930

37.920-37.990 

37.626-37.728

1 Basalts from Lobato and Chamisa Mesa.

2 Andesites from Paliza Canyon and Tschicoma Formations.

3 Dacites and rhyodacites from Tschicoma Formation.

4 Rhyolites from Canovas Canyon and Bearhead Formations.

5 Basalts from Totavi and El Alto.

6 Rhyolites from El Rechuelos Formation.

7 San Diego Canyon ash flows from precursor Bandelier Tuff (Smith, 1979).

of replicate analyses and accepted values of NBS 981 and 

NBS 982 standards. A more detailed description of the analytical procedures is given in Ayuso and others (1988). The 

Pb data were treated according to the scheme outlined by 

Ludwig (1988).

RESULTS

BANDELIER TUFF SYSTEM

The Pb isotopic range of the Bandelier Tuff magmatic 

system is relatively restricted compared to other volcanic 

rocks from the Jemez Mountains (Ayuso and Smith, 1989) 

(fig. 2). Pb isotopic values in the Bandelier Tuff plot for the 

most part below the average crust Pb evolution curve of Stacey and Kramers (1975) for uranogenic Pb; the values

straddle the model curve for thorogenic Pb in most cases, 

although a few of the samples plot above the curve. Isotopic 

compositions are thus significantly less radiogenic than 

present-day Pb, indicating a time-integrated evolution in a 

reservoir with relatively low U/Pb and Th/Pb values.

The Bandelier-Toledo-Valles magmatic system fluctuates in isotopic composition from relatively unradiogenic in 

Otowi (and the early units from the Cerro Toledo Rhyolite) 

to slightly more radiogenic in younger units of the Cerro 

Toledo Rhyolite, Tshirege Member, and early Valles Rhyolite domes (fig. 3). The younger Valles domes are significantly less radiogenic than the Tshirege Member and older 

Valles domes. The overall zigzag pattern from less to more 

radiogenic and back again to less radiogenic compositions 

seems to be an important feature that characterizes the entire 

magmatic system from pre-Bandelier rhyolites to the most 

recent Valles rhyolites.
Pb ISOTOPE COMPOSITIONS, BANDELIER TUFF, VALLES CALDERA, JEMEZ MOUNTAINS, NEW MEXICO

15.65

15.60-

15.55-

15.50-

15.45

Cerro Toledo 

late

"shirege

Valles Rhyolite 

early (eastern)

Cerro Toledo 

early \

38.2

38.0-

0. 37.8-

37.6-

37.4

Cerro Toledo />> 

Mi ^/V

Tshirege

Otowi

San Diego1 

Canyon

Valles Rhyolite 

late (eastern)

Valles Rhyolite 

early (eastern)

17.65 

B

17.75 17.85 

206 Pb/204 Rb

17.95 18.05

Figure 3. Range of Pb isotope compositions in the Bandelier Tuff 

magma system. A, 206Pb/204Pb vs. ^Pb/^Pb. B, ^Pb/^Pb vs. 

208Pb/204Pb. Analytical uncertainty at 2 sigma is shown by crosses 

in lower right portion of each part of figure.

A small but significant Pb isotopic difference distinguishes the two major members of the Bandelier Tuff 

(Ayuso and Smith, 1989) The Otowi Member is less radiogenic than the Tshirege Member (fig. 3). More importantly, 

each member plots as a coherent isotopic group. Pb isotope 

compositions of sanidines are indistinguishable from the 

whole-rock compositions, indicating that phenocrysts and 

matrix were in isotopic equilibrium.

Early and late ash flows within individual members of 

the Bandelier Tuff do not have significant differences in 

their Pb isotope compositions (table 1); in contrast, there is a 

strong zonation in abundances of the trace elements (Smith, 

1979). For example, early Otowi ash flows are higher in Ta 

(16 ppm) and lower in Eu (0.03 ppm) relative to late ash 

flows, which are lower in Ta (4.2 ppm) and higher in Eu 

(0.097 ppm); the Pb isotope compositions, however, are virtually the same between the early ash flows (206Pb/204Pb = 

17.816) and the late ash flows (206Pb/204Pb = 17.853).

Similarly, no isotopic gradients are evident in the Tshirege 

Member because the total variation in 206Pb/204Pb is from 

17.957 to 18.003; again, this is in contrast to an obvious 

zonation in Ta, from 12.5 to 3 ppm, and in Eu, from 0.1 to 

0.9 ppm, between early and late ash flows.

Pb isotope compositions of the Cerro Toledo Rhyolite 

overlap the Otowi and Tshirege Members of the Bandelier 

Tuff (Ayuso and Smith, 1989) (table 1). Stratigraphically 

lower samples of Cerro Toledo Rhyolite match the isotopic 

composition of the Otowi Member; up section, the Cerro 

Toledo Rhyolite is more radiogenic resembling the values in 

the Tshirege Member (fig. 3). The overall Pb isotope evolution of the Cerro Toledo Rhyolite thus provides direct evidence for recharge of the Bandelier Tuff magma chamber 

prior to eruption of the Tshirege Member, consistent with 

suggestions made on the basis of major- and trace-element 

variations (Smith, 1979; Stix and others, 1988).

The Valles Rhyolite domes have a wide range of Pb isotope compositions (table 1; fig. 3) that can be grouped 

according to age and generally according to location of individual domes. Redondo Creek Rhyolite and rhyolites from 

the older Valles domes (Cerros del Medio, Abrigo, Santa 

Rosa, and Seco) overlap the relatively radiogenic compositions and trace-element contents of the Tshirege Member of 

the Bandelier Tuff. In contrast, the Pb isotopic compositions 

of the younger Valles domes and flows (San Antonio, South 

Mountain, Battleship Rock, and Banco Bonito) are even less 

radiogenic than the Otowi Member of the Bandelier Tuff 

(fig. 3).

PRE-BANDELIER VOLCANIC ROCKS

Quaternary basalts that underlie the Bandelier Tuff at 

Totavi and at El Alto (table 1) and intra-Bandelier basaltic 

andesites in the Rio Grande gorge (Doe and others, 1969) 

overlap the Pb isotopic values of the Bandelier Tuff (figs. 

2B, 1C). The Pb isotopic composition from Tertiary basalt 

(Doe and others, 1969) and from Tertiary andesites from the 

Tschicoma Formation and Paliza Canyon Formation (Keres 

Group) overlap the Bandelier Tuff (fig. 2).

The pre-Bandelier rhyolite ash flows that chronologically and spatially(?) represent the closest precursor to the 

Bandelier Tuff (erupted about 0.3 m.y. prior to the Otowi 

eruption and collapse of the Toledo caldera) match the isotopic compositions of the Otowi Member (table 1; fig. 3). The 

pre-Bandelier ash flows may represent early leaks from the 

Bandelier Tuff magma chamber (Smith, 1979; Turbeville 

and Self, 1988). Other pre-Bandelier rhyolites and rhyodacites have Pb isotopic values that can be grouped on the basis 

of their relative location north and south of the Valles 

caldera. Tertiary and Quaternary rhyolites north of the 

caldera (El Rechuelos Rhyolite) are less radiogenic than the 

Bandelier Tuff; the Canovas Canyon Rhyolite south of the 

Valles caldera is more radiogenic; and the Bearhead
8 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Rhyolite has ^^Pb/^Pb values between those of the Otowi 

and the Tshirege Members of the Bandelier Tuff.

Tertiary dacites, rhyodacites, and quartz latites from 

most of the Tschicoma Formation (Polvadera Group) are 

generally slightly less radiogenic than the Bandelier Tuff 

(fig. 2). Our Pb isotopic data for Tschicoma dacitic rocks 

project generally toward the composition of K-feldspars 

from Proterozoic granite; thus, they are consistent with an 

origin by melting of a crustal source and mixing with basalt 

that had previously assimilated Proterozoic upper crustal 

rocks, Hpwever, the lower 207Ph/204Pb values of Tertiary 

rhyodacite and Quaternary rhyolites (table 1; fig. 2A), and 

the trend toward compositions less radiogenic than analyzed 

Proterozoic granites suggest involvement of an old, unradiogenic (low U/Pb) source. In general, the data overlap the 

unradiogenic portion of the extensive isotopic field for 1.7- 

Ga plutonic rocks from the Taos Range in northern New 

Mexico (J. Wooden, J. Aleinikoff, and J. Reed, unpub. data, 

1989) and trend toward the field of massive sulfide deposits 

(1.7-1.8 Ga) (Doe and others, 1979).

K-feldspars from Proterozoic granitic rocks southwest 

of the Valles caldera (e.g., Guadalupe Box, Soda Dam) and 

from the Sangre de Cristo Mountains have relatively

unradiogenic Pb initial ratios, plotting below the Pb model 

curve (table 1; fig. 2). K-feldspars have model ages of up to 

1.45 Ga and ^U/^Pb of about 9.5-9.9. The range in Pb 

isotope compositions overlaps the field for Proterozoic (1.7- 

Ga) granitic rocks in northern New Mexico and Colorado (J. 

Wooden and others, unpub. data). Whole-rock granite values 

near the Valles caldera are widely variable, however, and 

span a broad band from about 1.45 Ga toward the present.

DISCUSSION

Otowi ash flows that have the lowest incompatible element content represent the lowest level erupted from the 

Bandelier magma chamber; these constitute the dominant 

volume of magma tapped by the eruption (fig. 4) (Smith, 

1979). Such samples best characterize the starting compositions of the Otowi subsystem and correspond generally to 

subunits IV and V in the Tshirege Member, which likewise 

represent the lowest level tapped and the most likely starting 

composition for the Tshirege subsystem. In both cases, however, estimated starting compositions could be related to 

even more mafic magmas.

Bandelier depleted liquid

Volatiles increase leading to halogen-controlled trace 

element enrichments and density stratification

Zone of roof and wall melting, 

assimilation, and Pb mixing.

Zone of Bandelier magma fractionation

Dominant volume, zone of 

"homogeneous" mixed 

magma fractionates to 

^ Bandelier liquid

of Pb mixing * 

crust-mantle melts 

(variable over time and 

depth of melting)

Principal focus 

of mantle melt 

I injection and 

2 km I crustal melting

2km

Figure 4. Hypothetical magma chamber model for the Bandelier Tuff.
Pb ISOTOPE COMPOSITIONS, BANDELffiR TUFF, VALLES CALDERA, JEMEZ MOUNTAINS, NEW MEXICO

If the Bandelier Tuff had been produced exclusively by 

fusion of the analyzed Proterozoic granites, its values for 

uranogenic and thorogenic Pb ideally would be within the 

granite field (fig. 2). Although there is a substantial overlap, 

there is no exact isotopic match. Also, 208Pb/204Pb values of 

the Bandelier Tuff are higher relative to Proterozoic granites; 

therefore, the total fusion of these rocks cannot produce the 

entire spectrum of Pb isotope compositions in the Bandelier 

Tuff.

The Bandelier Tuff has Pb isotopic differences that cannot be related to fractional crystallization or melting of a 

basaltic parent. Differentiation of basalt is incapable of producing isotopic fractionation of radiogenic heavy isotopes 

within a comagmatic sequence (Leeman and Hawkesworth, 

1986) and is incapable of producing the elevated values of 

incompatible elements found in the Bandelier Tuff. 

Although we accept the idea that fractionation operated in 

the Bandelier magma chamber (Warshaw and Smith, 1988), 

we have concluded that fractionation alone is not a viable 

mechanism to explain the small but consistent isotopic differences among the different members of the Bandelier Tuff 

magmatic system. In addition, melting of a mafic source 

similar to the basalts analyzed in this study is not likely to 

account for the observed range of isotopic values or for the 

major- and trace-element variations in the high-silica rhyolites.

Because the uranogenic Pb isotopic composition of the 

Bandelier Tuff is intermediate between that of Proterozoic 

granites near the Valles caldera and Quaternary basalts, we 

think that the dominant volume of the Bandelier chamber 

may have been generated by mixing mantle-derived mafic 

magma and crustally derived sialic magma. In fact, a close 

association between mantle-derived and rhyolitic magmas 

has been shown in many studies of ash-flow tuffs (e.g., Doe 

and others, 1982; Hildreth and others, 1984; Bacon and others, 1984). Totavi basalt or intra-Bandelier basaltic andesites 

may represent mafic magmas that provided the heat necessary for melting in the mid-crust starting about 2.5 m.y. ago 

(Smith and Shaw, 1975; Luedke and Smith, 1978) and that 

probably constituted an intermittent source of material during the evolution of the Bandelier magma system. The 

basalts near the Valles caldera might have been previously 

contaminated by old, unradiogenic Proterozoic granitic 

rocks, consistent with suggestions that apply to adjacent 

regions (Lipman and others, 1978; Johnson and Lipman, 

1988). The felsic magma invoked in this mixing scenario 

could have been produced from Proterozoic basement.

After eruption of the Otowi Member, a large volume of 

magma remained that still contained remnants of the less 

radiogenic upper parts of the Otowi magma chamber, as 

indicated by the Pb isotope composition of the early units of 

the Cerro Toledo Rhyolite, which represent "leaks" of this 

magma. Pb isotope zonation in the Cerro Toledo Rhyolite 

cannot be explained by liquid-state diffusive processes or by 

fractional crystallization, although the latter process apparently explains the trace-element variations (Stix and others,

1988). After eruption of the Otowi Member and before eruption of the Tshirege, there must have been renewed input of 

magmas with more radiogenic Pb isotope compositions. 

Continued fractionation, assimilation of roof rocks (Smith, 

1979; Heiken and others, 1986; Stix and others, 1988) as 

suggested by xenoliths of older volcanic rocks, Proterozoic 

granites, and Proterozoic metamorphic rocks and mixing 

with new increments of more radiogenic magma ultimately 

shifted the composition of the Cerro Toledo Rhyolite toward 

the composition of the Tshirege Member. This shift produced the observed continuum of Pb isotopic values from the 

Cerro Toledo Rhyolite to the Tshirege (Ayuso and Smith,

1989) and the progressively higher abundances of incompatible elements in successively younger tephras of the Cerro 

Toledo Rhyolite (Smith, 1979; Stix and others, 1988; Stix 

and Gorton, 1990).

The higher and distinct Pb isotopic values in the 

Tshirege Member cannot be explained by fractional crystallization of Otowi-like magma. The Tshirege chamber may 

well have been roofed by consolidated remnants of pre-Bandelier plutons or by partly consolidated Otowi magma. 

Alternatively, the Otowi Member and the Tshirege Member 

of the Bandelier Tuff could represent separate evolutionary 

events, but this does not account for the intermediate Pb isotopic compositions of the Cerro Toledo Rhyolite.

Isotopic variations in the Valles Rhyolite domes appear 

to be influenced by their location and age. Following collapse of the Valles caldera, fresh inputs of magma produced 

(1) early rhyolitic leaks (Redondo Creek Member) on the 

western side of the Valles caldera and (2) relatively radiogenic compositions in Valle Grande Member domes (Cerro 

Medio, Abrigo, Santa Rosa, San Luis, Seco) on the eastern 

and northern side of the caldera that match the isotopic composition of Tshirege Member. This isotopic resemblance 

suggests that Tshirege magma was not completely drained 

and that it continued to evolve toward a chemically more 

fractionated and less radiogenic composition, perhaps by 

wallrock assimilation and (or) mixing with new unradiogenic magma. In order to explain the geochemical trend, 

Smith (1979) suggested that, after eruption of this early postcaldera magma, there was a thermal hiatus and a change 

from convective to conductive cooling. If the chamber solidified at this time, the later magmatic activity that produced 

the youngest and least radiogenic Valles Rhyolite domes and 

flows (San Antonio I and II, South Mountain, Battleship 

Rock, and Banco Bonito) in the western half of the caldera 

must represent a separate event, possibly involving melting 

and mixing of larger increments of unradiogenic magma 

from depth.

Pb isotopic signatures of younger domes in the Valle 

Grande Member (San Antonio I and II) are less radiogenic 

than those of older domes and form two distinct Pb isotopic 

groups (Ayuso and Smith, 1989) that also have distinct initial Sr isotopic ratios (Spell and others, 1989). In fact, the Pb
10 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

isotopic data establish a genetic link between the youngest 

domes of the Valle Grande Member (San Antonio I and II) 

and the youngest Valles Rhyolite units (Battleship Rock and 

Banco Bonito members). Molybdenum mineralization and 

intense alteration effects are concentrated in tuffs of the 

Valles Rhyolite, for the most part, and in the upper part of the 

Tshirege Member of the Bandelier Tuff. This type of mineralization conceivably could represent a shallow expression 

of a deeper, Climax- or granite-type (White and others, 

1981) stockwork molybdenite mineralization (Hulen and 

others, 1987). The Pb isotope composition of the mineral 

assemblage associated with the molybdenum mineralization 

is unknown, but that of the host Valles Rhyolite, as indicated 

above, spans a wide spectrum. If the mineralization were 

related specifically to the Battleship Rock Member of the 

Valles Rhyolite (J. Hulen, oral commun., 1993), the mineralized rocks would resemble the least radiogenic units in the 

magmatic system. In the Creede district Colorado, a classic 

example of a mineralized fossil hydrothermal system that 

shares many geological attributes with the Valles caldera, Pb 

isotope compositions evolved toward more radiogenic values as a function of age of the mineralized rocks. For example, the older hydrothermal assemblages in the Creede 

district associated with gold mineralization contain the least 

radiogenic Pb; in contrast, the younger assemblages associated with silver and base metals are the most radiogenic 

(Foley and Ayuso, in press).

The evolving Bandelier Tuff magmatic system thus 

grew by incremental replenishment with mantle-derived 

mafic magma into the bottom of the chamber, by mixing 

with silicic crustal melts, and by assimilation of relatively 

unradiogenic silicic wallrocks near the top of the chamber 

(fig. 4) (Smith, 1979). The magmatic system was open (e.g., 

Smith, 1979; Lipman, 1984, 1988; Hildreth and others, 

1984) and dynamic in the sense that several isotopically distinct magma batches were added to the Bandelier magmatic 

system during its evolution. Despite the isotopic differences 

between the Otowi Member and the Tshirege Member, it 

seems clear that major members of the Bandelier Tuff are 

linked in a chemical continuum by tephras from the Cerro 

Toledo Rhyolite, which represent recharging and continued 

fractionation of the system. This interpretation is in agreement with the long-lived nature of the magmatic system 

associated with the Valles and Toledo calderas (Smith and 

Bailey, 1966, 1968; Self and others, 1986; Heiken and others, 1986).

Some other pre-Bandelier rocks may also have been 

part of the Bandelier system, although their isotopic heterogeneity clearly identifies them as "separate" magma batches. 

Isotopic changes from less radiogenic Pb to more radiogenic 

Pb and back again to less radiogenic compositions after 

caldera-forming eruptions are an important feature of the 

Bandelier magmatic system. In such periodically discharged 

volcanic systems, magmas whose isotopic signatures differ 

greatly from magmas deeper in the chamber may be entirely

or partially removed (Smith and Macdonald, 1979; Lipman, 

1984; Druitt and Bacon, 1988; Ayuso and Smith, 1989). 

When the magma chamber ultimately solidifies to plutonic 

rock, crystallization after complete removal of the upper volcanic members or convective homogenization may obliterate the initial isotopic heterogeneity in the magmatic system. 

However, where the upper volcanic members are not eroded 

entirely, or if homogenization does not occur, the initial isotopic zonation may be preserved.

ACKNOWLEDGMENTS

We thank R.A. Bailey, B.R. Doe, N.K. Foley, M.F. 

Koran, and R.P. Koeppen for their many helpful suggestions, 

comments, and reviews of the paper.

REFERENCES CITED

Ayuso, R.A., Horan, M.F., and Criss, R.E., 1988, Pb and O isotopic 

geochemistry of granitic plutons in northern Maine: American 

Journal of Science, v. 288 A, p. 421-460.

Ayuso, R.A., and Smith, R.L., 1989, Lead isotopic diversity in the 

Bandelier Tuff, Valles caldera, and related rocks of the Jemez 

Mountains, New Mexico [abs.]: International Association of 

Volcanology and Chemistry of the Earth's Interior, Santa Fe, 

New Mexico Bureau of Mines and Mineral Resources, Bulletin 

131, p. 10.

Bacon, C.R., Kurazawa, H., Delevaux, M., Kistler, R.W., and Doe, 

B.R., 1984, Lead and strontium isotopic evidence for crustal 

interaction and compositional zonation in the source regions of 

Pleistocene basaltic and rhyolitic magmas of the Coso volcanic 

field, California: Contributions to Mineralogy and Petrology, 

v. 85, p. 366-375.

Bailey, R.A., Smith, R.L., and Ross, C.S., 1969, Stratigraphic nomenclature of volcanic rocks in the Jemez Mountains, New 

Mexico: U.S. Geological Survey Bulletin 1274-P, p. 1-19.

Doe, B.R., Leeman, W.P., Christiansen, R.L., and Hedge, C.E., 

1982, Lead and strontium isotopes and related trace elements 

as genetic tracers in the upper Cenozoic rhyolite basalt association of the Yellowstone plateau volcanic field: Journal of 

Geophysical Research, v. 87, p. 4786-4806.

Doe, B.R., Lipman, P.W., Hedge, C.E., and Kurazawa, H., 1969, 

Primitive and contaminated basalts from the southern Rocky 

Mountains: Contributions to Mineralogy and Petrology, v. 21, 

p. 142-156.

Doe, B.R., Steven, T.A., Delevaux, M.H., Stacey, J.S., Lipman, 

P.W., and Fisher, F.S., 1979, Genesis of ore deposits in the San 

Juan volcanic field, southwestern Colorado Lead isotope evidence: Economic Geology, v. 74, p. 1-26.

Doell, R.R., Dalrymple, G., Smith, R.L., and Bailey, R.A., 1968, 

Paleomagnetism, potassium-argon ages, and geology of rhyolites and associated rocks of the Valles caldera, New Mexico: 

Geological Society of America Memoir 116, p. 211-248.

Druitt, T.H., and Bacon, C.R., 1988, Petrology of the zoned calcalkaline magma chamber of Mount Mazama, Crater Lake,
Pb ISOTOPE COMPOSITIONS, BANDELffiR TUFF, VALLES CALDERA, JEMEZ MOUNTAINS, NEW MEXICO 11

Oregon: Contributions to Mineralogy and Petrology, v. 101, 

p. 245-259.

Foley, N.K., and Ayuso, R.A., in press, Paragenetic constraints on 

Pb isotopic evolution of the north amethysts Au-Ag vein system, Creede mining district, San Juan volcanic field: Economic 

Geology.

Gardner, J.N., Goff, F., Garcia, S., and Hagan, R.C., 1986, Stratigraphic relations and lithologic variations in the Jemez volcanic field, New Mexico: Journal of Geophysical Research, v. 

91, p. 1763-1778.

Goff, F., and Gardner, J.N., 1988, Valles caldera region, New Mexico, and the emerging continental scientific drilling program: 

Journal of Geophysical Research, v. 93, p. 5997-5999.

Halliday, A.N., Fallick, A.E., Hutchinson, J., and Hildreth, W., 

1984, A Nd, Sr and O isotopic investigation into the causes of 

chemical and isotopic zonation in the Bishop Tuff, California: 

Earth and Planetary Science Letters, v. 68, p. 379-391.

Halliday, A.N., Mahood, G.A., Holden, P., Metz, J.M., Dempster, 

T.J., and Davidson, J.P., 1989, Evidence for long residence 

times of rhyolitic magma in the Long Valley magmatic system The isotopic record in precaldera lavas of Glass Mountain: Earth and Planetary Sciences Letters, v. 94, p. 274-290.

Heiken, G., Goff, F., Stix, J., Tamanyu, S., Shafiqullah, M., Garcia, 

S., and Hagan, R., 1986, Intracaldera volcanic activity, Toledo 

caldera and embayment, Jemez Mountains, New Mexico: Journal of Geophysical Research, v. 91, p. 1799-1815.

Hervig, R.L., and Dunbar, N.W., 1992, Cause of chemical zoning 

in the Bishop (California) and Bandelier (New Mexico) magma chambers: Earth and Planetary Science Letters, v. Ill, 

p. 97-108.

Hildreth, W., 1979, The Bishop Tuff Evidence for the origin of 

compositional zonation in silicic magma chambers: Geological 

Society of America Special Paper 180, p. 43-75.

Hildreth, W., Christiansen R.L., and O'Neil, J.R., 1984, Catastrophic isotopic modification of rhyolitic magma at times of caldera 

subsidence, Yellowstone plateau volcanic field: Journal of 

Geophysical Research, v. 89, p. 8339-8370.

Hulen, J.B., Nielson, D.L., Goff, F., Gardner, J.N., and Charles, 

R.W., 1987, Molybdenum mineralization in an active geothermal system, Valles caldera, New Mexico: Geology, v. 15, 

p. 748-752.

Izett, G.J., Obradovich, J., Naeser, C., Cebula, G., 1981, Potassiumargon and fission-track zircon ages of Cerro Toledo rhyolite 

and tephra units in the Jemez Mountains, New Mexico: U.S. 

Geological Survey Professional Paper 1199-D, p. 37-43.

Johnson, C.M., 1989, Isotopic zonations in silicic magma chambers: Geology, v. 17, p. 1136-1139.

Johnson, C.M., and Lipman, P.W., 1988, Origin of metaluminous 

and alkaline volcanic rocks of the Latir volcanic field, northern 

Rio Grande rift, New Mexico: Contributions to Mineralogy 

and Petrology, v. 100, p. 107-128.

Johnson, C.M., Lipman, P.W., and Czamanske, G.K., 1990, H, O, 

Sr, Nd, and Pb isotope geochemistry of the Latir volcanic field 

and cogenetic intrusions, New Mexico, and relations between 

evolution of a continental magmatic center and modifications 

of the lithosphere: Contributions to Mineralogy and Petrology, 

v. 104, p. 99-124.

Leeman, W.P., and Hawkesworth, C.J., 1986, Open magma systems Trace element and isotopic constraints: Journal of Geophysical Research, v. 91, p. 5901-5912.

Lipman, P.W., 1984, The roots of ash flow calderas in Western 

North America Windows into the tops of granitic batholiths: 

Journal of Geophysical Research, v. 89, p. 8801-8841.

 1988, Evolution of silicic magma in the upper crust The 

mid-Tertiary Latir volcanic field and its cogenetic granitic 

batholith, northern New Mexico, U.S.A.: Transactions of the 

Royal Society of Edinburgh, Earth Sciences, v. 79. p. 265-288.

Lipman, P.W., Doe, B.R., Hedge, C.E., and Steve, T.A., 1978, Petrologic evolution of the San Juan volcanic field, southwestern 

Colorado Pb and Sr isotopic evidence: Bulletin of the Geological Society of America, v. 89, p. 59-82.

Lipman, P.W., and Friedman, I., 1975, Interaction of meteoric water with magma An oxygen isotope study of ash-flow sheets 

from southern Nevada: Geological Society of America Bulletin, v. 86, p. 695-702.

Loeffier, B.M., Vaniman, D.T., Baldridge, W.S. and Shafiqullah, 

M., 1988, Neogene rhyolites of the Northern Jemez volcanic 

field, New Mexico: Journal of Geophysical Research, v. 93, 

p. 6157-6167.

Ludwig, K., 1988, A plotting and regression program for radiogenic isotope data: U.S. Geological Survey Open-File Report 

88-557,41 p.

Luedke, R.G, and Smith, R.L., 1978, Map showing distribution, 

composition, and age of Late Cenozoic volcanic centers in Arizona and New Mexico: U.S. Geological Survey Miscellaneous Investigations Series Map I-1091-A, scale 1:1,000,000.

Macdonald, R., and Smith, R.L., 1988, Relationships between silicic plutonism and volcanism: Geochemical evidence: Transactions of the Royal Society of Edinburgh, Earth Sciences, v. 

79, p. 257-263.

Noble, D.C., and Hedge, C.E., 1969, 87Sr/86Sr variations within individual ash-flow sheets: U.S. Geological Survey Professional 

Paper 650-C, p. 133-139.

Self, S., Goff, F., Gardner, J.N., Wright, J.V., and Kite, W.M., 

1986, Explosive rhyolitic volcanism in the Jemez Mountains Vent locations, caldera development and relation to regional structure: Journal of Geophysical Research, v. 91, 

p. 1779-1798.

Self, S., Kircher, D.E., and Wolff, J.A., 1988, The El Cajete series, 

Valles caldera, New Mexico: Journal of Geophysical Research, v. 93, p. 6113-6127.

Skuba, C.E., and Wolff, J.A., 1989, Trace element and radiogenic 

isotope constraints on the genesis and evolution of rhyolitic 

magmas beneath Valles caldera [abs.]: International Association of Volcanology and Chemistry of the Earth's Interior, Santa Fe, New Mexico Bureau of Mines and Mineral Resources, 

Bulletin 131, p. 245.

Smith, R.L., 1979, Ash-flow magmatism, in Chapin, C.E., and Elston, W.E., eds., Ash-Flow Tuffs: Geological Society of 

America Special Paper 180, p. 5-27.

Smith, R.L., and Bailey, R.A., 1966, The Bandelier Tuff A study 

of ash-flow eruption cycles from zoned magma chambers: Bulletin of Volcanology, v. 29, p. 83-104.

 1968, Resurgent cauldrons: Geological Society of America, Memoir 116, p. 613-662.

Smith, R.L., Bailey, R.A., and Ross, C.S., 1970, Geologic map of 

the Jemez Mountains, New Mexico: U.S. Geological Survey 

Miscellaneous Investigations Series Map 1-571, scale 

1:125,000.
12 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Smith, R.L., and Macdonald, R., 1979, Rhyolitic volcanism and its 

relation to granitic plutonism [abs]: Geological Society of 

America Abstracts with Programs, v. 11, p. 520.

Smith, R.L., and Shaw, H.R., 1975, Igneous-related geothermal 

phenomena: U.S. Geological Survey Circular 726, p. 58-83.

Spell, T.L., and Kyle, P.R., 1989, Petrogenesis of Valle Grande 

Member rhyolites, Valle caldera, New Mexico Implications 

for evolution of the Jemez Mountain magmatic system: Journal 

of Geophysical Research, v. 94, p. 10379-10396.

Spell, T.L., Kyle, P.R., and Thirlwell, M., 1989, Geochemistry and 

Sr isotopic compositions of post-Bandelier Tuff rhyolites, 

Valles caldera, New Mexico Evidence for multiple magma 

systems in the Jemez Mountains volcanic field [abs.]: International Association of Volcanology and Chemistry of the 

Earth's Interior, Santa Fe, New Mexico Bureau of Mines and 

Mineral Resources, Bulletin 131, p. 251.

Stacey, J.D., and Kramers, J.N., 1975, Approximation of terrestrial 

lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 36, p. 207-222.

Stix, J., Goff, F., Gorton, M.P., Heiden, G., and Garcia, S.R., 1988, 

Restoration of compositional zonation in the Bandelier silicic 

magma chamber between two caldera-forming eruptions 

Geochemistry and origin of the Cerro Toledo Rhyolite, Jemez

Mountains, New Mexico: Journal of Geophysical Research, v. 

93, p. 6129-6147.

Stix, J., and Gorton, M.P., 1990, Variations in trace element partition coefficients in sanidine in the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico Effects of composition, 

temperature and volatiles: Geochimica et Cosmochimica Acta, 

v. 54, no. 10, p. 2697-2708.

Tegtmeyer, K., and Farmer, G.L., 1989, Isotopic evidence for the 

origin of late Tertiary metaluminous and peralkaline rhyolites 

from the Great Basin, Western U.S.: International Association 

of Volcanology and Chemistry of the Earth's Interior, Santa 

Fe, New Mexico Bureau of Mines and Mineral Resources, 

Bulletin 131, p. 266.

Turbeville, B.N., and Self, S., 1988, San Diego Canyon 

ignimbrites Pre-Bandelier tuff explosive rhyolitic volcanism 

in the Jemez Mountains, New Mexico: Journal of Geophysical 

Research, v. 93, p. 6148-6156.

Warshaw, C.M., and Smith, R.L., 1988, Pyroxenes and fayalites in 

the Bandelier Tuff, New Mexico Temperatures and comparison with other rhyolites: American Mineralogist, v. 73, 

p. 1025-1037.

White, W.H., Bookstrom, A.A., Kamilli, R.J., Ganster, M.W., 

Smith, R.P., Ranta, D.R., and Steininger, R.C., 1981, Character 

and origin of Climax-type molybdenum deposits: Economic 

Geology, 75th Anniversary Volume, p. 270-316.
CHAPTER B

IMPLICATIONS OF EOCENE THROUGH MIOCENE AGES FOR

VOLCANIC ROCKS, SNOWSTORM MOUNTAINS

AND VICINITY, NORTHERN NEVADA

By Alan R. Wallace1 and E.H. McKee2

ABSTRACT

New potassium-argon dates on volcanic rocks from the 

Snowstorm Mountains area in northern Nevada, coupled 

with existing geochronology and new field mapping, shed 

new light on three major volcanologic topics in that area. 

First, new dates show that the hiatus between the eruption of 

two major middle Miocene bimodal suites was less than 1 

million years. Second, a new date on a welded tuff confirms 

field evidence for a previously unrecognized late Eocene 

volcanic field west of the Snowstorm Mountains. Third, field 

relations and additional dates on pre-middle Miocene volcanic rocks define a complex eruption history and distribution 

of late Eocene and younger volcanic rocks.

INTRODUCTION

The Snowstorm Mountains of western Elko and eastern 

Humboldt Counties in northern Nevada are underlain by a 

complex middle Miocene volcanic field erupted onto Paleozoic sedimentary rocks and Eocene, Oligocene, and lower 

Miocene volcanic rocks (Wallace, 1993; A.R. Wallace, 

unpub. mapping, 1987-89). Geochronologic studies of the 

middle Miocene field showed that the volcanics were 

erupted between about 15 and 6 Ma (Wallace and others, 

1990), but the wide range in ages of the volcanic rocks 

beneath these middle Miocene units was previously 

unknown.

The new K-AT dates reported in table 1 are used to evaluate three topics related to Tertiary volcanism and tectonism 

in north-central Nevada: (1) confirmation and refinement of

'U.S. Geological Survey, Mail Stop 905, P.O. Box 25046, Denver 

Federal Center, Denver, CO 80225.

2U.S. Geological Survey, Mail Stop 901,345 Middlefield Road, Menlo Park, CA 94025.

the age of early bimodal volcanism in the middle Miocene 

volcanic field, (2) identification of a previously unrecognized middle Tertiary volcanic field west Of the Snowstorm 

Mountains, and (3) analysis of pre-middle Miocene Tertiary 

volcanism and tectonism. The purpose of this paper is to 

elaborate on these topics and make suggestions for additional studies.

MIDDLE MIOCENE VOLCANISM

Geologic mapping (Wallace, 1990,1993) in the SnOwstorm Mountains has defined two middle Miocene bimodal 

volcanic sequences. The older sequence consists of predominantly mafic flows and volcaniclastic sedimentary rocks 

with subordinate amounts of pyroclastic rhyolite tuffs and 

rhyolitic domes; these units were erupted and emplaced, 

respectively, between about 16.7 and 14.5 Ma (Wallace and 

others, 1990; Stewart and others, 1975; Zoback and others, 

1994). The younger sequence, composed of anhydrous rhyolites and rhyodacites with minor basalt, formed between 

about 14.3 and 9.8 Ma, with minor basaltic volcanism at 6.4 

Ma (Wallace and others, 1990). The older bimodal assemblage is related to the north-northwest-trending northern 

Nevada rift, which formed during incipient basin-and-range 

extension (Zoback and Thompson, 1978; Stewart and others, 

1975). The younger bimodal assemblage mostly is a product 

of early volcanism interpreted to be related to the Yellowstone hot spot (Bonnichsen and Kauffman, 1987; Honjo and 

Leeman, 1987).

In the Midas mining district in the southeastern part of 

the Snowstorm Mountains, basaltic andesite forms two 

sequences that are locally separated by a variably thick pyroclastic unit and lacustrine sedimentary rocks. A dike that fed 

flows in the lower sequence was dated at 15.1 ±0.5 Ma (sample 9LH24; see comment in Appendix regarding possible

13
14 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Table 1. Summary of dates for volcanic rocks units, Snowstorm Mountains and vicinity, northern Nevada.

[rad, radiogenic]

Sample no.

9LH25

9LH24

9LH11

9LH22

9LH8

Rock type K2O 

(weight percent)

Basaltic andesite

Basaltic andesite

Basaltic andesite

Andesite

Andesite

Dacite

0.575

6.43

6.43

2.97

0.492

0.928

"A* 

(mol/g)

1.21855xlO-n

1.401607xlO-10

1.399958xlO-10

9.53078xlO-H

2.52043xlO-H

5.1823xlO-n

"WAibtf 

(percent)

33.6

84.4

85.4

42.8

39.2

19.8

Age 

(Ma)

14.7±0.5

15.1±0.5

15.1±0.5

22.1±0.7

35.2±1.1

38.4±1.6

alteration), and a flow in the upper sequence was dated at 

14.7±0.5 Ma (sample 9LH25). These dates are consistent 

with other dates reported along the northern Nevada rift 

(Stewart and others, 1975; Zoback and others, 1994) and 

with a date of 15.2±1.6 Ma on a basaltic andesite lava flow 

west of Midas (Wallace and others, 1990). The mafic volcanic rocks and associated pyroclastic and sedimentary rocks 

in the Midas district were classified as middle Tertiary by 

Coats (1987), an age that is too old in light of these new 

dates.

In Midas, the 14.7-Ma upper basaltic andesite flow 

underlies by only a few tens of meters the oldest rhyolite unit 

of the younger bimodal sequence; to the west of Midas, this 

rhyolitic unit is cut by a rhyolite dike that has been dated at 

14.310.8 Ma (Zoback and Thompson, 1978; Wallace, 1990; 

Wallace and others, 1990). Throughout the Snowstorm 

Mountains, the basaltic andesites that directly underlie the 

younger rhyolite show no evidence of paleosol development 

or lag deposits related to surficial processes, despite evidence (paper birch leaves and conifer plant debris in the 

intervening lacustrine sedimentary rocks; Wallace, 1993) of 

temperate, nonarid climatic conditions. Combined with the 

geochronologic data, this evidence indicates relatively little 

time between the cessation of older bimodal volcanism and 

inception of younger volcanic activity.

POSSIBLE EOCENE VOLCANIC FIELD

Along the west to northwest margin of the Snowstorm 

Mountains, biotite-hornblende rhyolitic to dacitic flows and 

welded tuffs underlie middle Miocene volcanic and sedimentary rocks (Wallace, 1993; A.R. Wallace, unpub. mapping, 1988-91). A late Eocene date of 38.4±1.6 Ma (sample 

9LH8) was obtained from a dacitic welded tuff in this 

sequence.

The presence of hydrous mafic minerals petrographically distinguishes the Eocene rocks from the middle 

Miocene felsic volcanics of the Snowstorm Mountains. The 

middle Tertiary felsic volcanic rocks elsewhere in northern 

Nevada typically contain a biotite-hornblende assemblage of

mafic minerals (Coats, 1987). In contrast, the middle 

Miocene volcanic rocks are anhydrous, and the mafic 

minerals include pyroxenes and olivine (Wallace, 1993). 

This anhydrous mineralogy typifies felsic volcanic rocks of 

this age throughout a large area that includes northern 

Nevada, southeastern Oregon, and southwestern Idaho 

(Ekren and others, 1984; Rytuba and McKee, 1984), and it 

probably is the product of partial melting of a granulitic 

lower crust (Honjo and Leeman, 1987).

Reconnaissance mapping (A.R. Wallace, unpub. mapping, 1988-91) indicates that the biotite-hornblende-bearing 

volcanic rocks are exposed over a widespread area of at least 

several tens of square kilometers, generally west and northwest of Chimney Reservoir (fig. 1). The total area! extent of 

this volcanic field is undoubtedly larger than is shown in figure 1, but parts of it are covered by younger volcanic and 

sedimentary rocks, so its total size is not known. Volcanic 

rocks in this area were designated as Miocene (Stewart and 

Carlson, 1978) and Miocene or Pliocene in age (Willden, 

1964). At the spillway for Chimney Reservoir, a biotite rhyodacite forms a north-northwest-trending elongate ridge 

with a contorted basal vitrophyre and breccia. Thin sections 

show no evidence of fiamme or phenocryst breakage, indicating that the rock is not a welded tuff. The ridge most likely 

is an elongate, exogenous dome. To the west, other biotitebearing volcanic rocks are pumiceous welded tuffs with broken phenocrysts.

The nearest documented middle Tertiary felsic volcanic rocks are exposed approximately 60 km east-southeast 

of Chimney Reservoir in the vicinity of Willow Creek Reservoir (Stewart and Carlson, 1978; Coats, 1987). These 

rocks include thick ash-flow tuffs, air-fall tuffs, and tuffaceous sedimentary rocks; the ash-flow tuffs have been 

dated at 32.9±1.1 Ma (recalculated from McKee and others, 

1976). Combined with the late Eocene date on the dacite 

west of the Snowstorm Mountains, field relations and 

petrography indicate the presence of a late Eocene igneous 

center in the general vicinity of Chimney Reservoir. Additional field, petrologic, and geochronologic studies are necessary to better define the volcanic rocks and the age of 

volcanic activity in this area west and northwest of the 

Snowstorm Mountains.
VOLCANIC ROCKS, SNOWSTORM MOUNTIANS AND VICINITY, NORTHERN NEVADA 15

117°30' 117°00' 116°30'

10 MILES

10 20 KILOMETERS

Possible 

Eocene-volcanic .-.-.Held'. .'.

Figure 1. Map showing locations of dated samples (triangles), as 

well as pertinent geographic features. Location of late Eocene volcanic field (stippled) is conjectural but is shown to provide general 

location of rocks that might be part of that field.

PRE-MIDDLE MIOCENE VOLCANISM 

AND TECTONISM

In addition to the Eocene date on the dacite described 

above, new K-Ar dates have been obtained from two other 

pre-middle Miocene volcanic sequences to the west and east 

of the Snowstorm Mountains. These dates, coupled with 

field relations and previous geochronologic studies, provide 

insight into pre-middle Miocene volcanism and tectonism.

The Dry Hills, at the northeast end of the Osgood 

Mountains, approximately 10 km south of Chimney Reservoir and 10 km west of the Snowstorm Mountains (fig. 1), 

are underlain by a series of andesite flows that blanket Paleozoic sedimentary rocks (Stewart and Carlson, 1978). A date 

of 22.1±0.7 Ma was obtained from an andesite flow in the 

central part of the Dry Hills (sample 9LH11). Similar andesitic flows are exposed in two large areas along the western 

and southern flanks of the Osgood Mountains (Hotz and 

Willden, 1964); andesites in the southern area of outcrop 

were dated at 22.6±0.7 Ma (recalculated from Erickson and 

Marsh, 1974). These three relatively large areas of exposure 

suggest that the Osgood Mountains-Dry Hills area was covered at least partially by andesitic flows at about 22 Ma.

Four major Cenozoic volcanic groups are exposed 

along the western margin of the Snowstorm Mountains and 

areas to the northwest and west: the Eocene sequence, the 

Dry Hills andesites, and the two middle Miocene bimodal 

assemblages. The distribution of these groups is erratic, due 

either to nondeposition in certain areas or removal by

erosion. In areas where the base of the Dry Hills andesites is 

exposed, flows clearly were erupted directly onto Paleozoic 

sedimentary rocks, and the Eocene volcanic rocks are absent. 

The Eocene volcanic rocks, as exposed at Chimney Reservoir spillway, are capped by lacustrine sedimentary rocks 

that are related to the younger sequence of middle Miocene 

volcanism (Wallace, 1993), with no evidence of the intervening Dry Hills andesite or early bimodal-related basalt. 

Along the western flanks of the Snowstorm Mountains, the 

Dry Hills-type andesite is not present, and the Paleozoic sedimentary rocks are overlain by Eocene volcanics, basalts of 

the older bimodal suite, and rhyolites and lacustrine sedimentary rocks of the younger bimodal suite, either individually or in all combinations. Additional work is needed to 

determine the distributions of these volcanic rocks and their 

stratigraphic relations west and northwest of the Snowstorm 

Mountains.

The volcanic stratigraphy on the eastern and southeastern margins of the Snowstorm Mountains is equally complex. Near Scraper Springs, rhyolites of the younger middle 

Miocene bimodal sequence were emplaced directly on pumice-rich air-fall tuff and tuffaceous sedimentary rocks, 

which, in turn, overlie andesitic flows and dome-collapse 

breccias. One of these andesite flows was dated at 35.211.1 

Ma (sample 9LH22). JTie andesites are part of a larger 

group of late Eocene to early Oligocene andesites that are 

exposed to the east and northeast of Scraper Springs; they 

overlie Paleozoic sedimentary rocks 5 km north of Scraper 

Springs. The overlying air-fall tuffs and related tuffaceous 

sedimentary rocks can be traced southeastward some 15 km 

to Willow Creek Reservoir, where they directly underlie the 

32.9-Ma sequence of welded tuffs.

Twenty kilometers to the south of Scraper Springs, in 

the Ivanhoe mining district, middle Miocene volcanic rocks, 

including those of the lower bimodal sequence, directly 

overlie Paleozoic sedimentary rocks; the early Oligocene 

tuffaceous sedimentary rocks are absent, and the 32.9-Ma 

ignimbrites are exposed only locally at the very north end of 

the district (Bartlett and others, 1990). The extent of premiddle Miocene volcanic rocks west of Scraper Springs is 

unknown. Volcanic-rock xenoliths of many compositions 

are present in inter-basaltic andesite pyroclastic rocks in the 

Midas mining district, but none can be attributed to a specific 

older unit. No Paleozoic rocks are exposed in the core of the 

Snowstorm Mountains.

In summary, the volcanic rocks on the margins of the 

Snowstorm Mountains reveal a complex relationship 

between volcanism, tectonism, and erosion. The distribution 

of late Eocene and Oligocene pyroclastic rocks, which 

would have formed blankets over a subdued paleotopography, is erratic, indicating either substantial post-eruption 

erosion or rugged paleotopography. Andesitic flows and 

breccias, such as in the Dry Hills and at Scraper Springs, 

may have had much greater areal distribution, forming 

locally thick accumulations.
16 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

The older middle Miocene bimodal volcanism related 

to the north-northwest-trending northern Nevada rift may 

have been preceded by thermal doming and attendant erosion. Deep drilling in the Ivanhoe district has revealed a 

north-northwest-trending paleohorst in the Paleozoic basement upon which middle Miocene volcanic rocks were 

erupted (Bartiett and others, 1990). Limited evidence suggests that a similar situation may have occurred in the Snowstorm Mountains. In both the Snowstorm Mountains and 

areas to the south, including the Ivanhoe district, the older 

bimodal sequence has a limited extent to the east and west. 

This elongate dome may have foundered during volcanism, 

permitting the preservation of thousands of meters of 

Miocene volcanic rocks along its axis.

Although the general volcanic stratigraphy in the 

Snowstorm Mountains is fairly well understood (Wallace, 

1993), additional work clearly is needed to more fully understand relationships in the volcanic rocks to the west and east. 

This additional work would add to the understanding of middle Tertiary volcanism and tectonism in northern Nevada if 

coupled with new field and geochronologic studies of middle 

Tertiary volcanism and related tectonics in eastern Elko 

County by Thorman and others (1993).

ECONOMIC GEOLOGY

A belt of bulk-minable gold deposits, known as the 

Getchell trend, is located along the eastern base of the 

Osgood Mountains west of the Snowstorm Mountains (fig. 

1). The deposits occur in Paleozoic sedimentary rocks and 

Cretaceous igneous rocks. The age of mineralization is a 

point of controversy, with one camp arguing for a Cretaceous age (Silberman and others, 1974) and the other for a 

middle Tertiary or Miocene age (Joraleman, 1975). Andesite 

flows in the Dry Hills extend to within a few kilometers of 

the northern exposed limit of the gold belt. Establishing the 

temporal relationship between the volcanic rocks and the 

gold deposits would demonstrate that the deposits are either 

younger or older than 22 Ma. If they are older, the area 

beneath the volcanic rocks would have significant potential 

for undiscovered, concealed gold deposits; if they are 

younger, then altered andesites might provide a guide to 

undiscovered mineral deposits.

Late Eocene volcanic rocks host precious metal deposits in several places east of the Snowstorm Mountains, 

including the Tuscarora, Burner Hills, and Good Hope districts (LaPointe and others, 1991). Mineralization at these 

locations was related genetically to the volcanic systems. 

Although mineralized rocks were not observed in the late 

Eocene volcanic rocks west of the Snowstorm Mountains, 

evidence for an eruptive center in that area raises the possibility that mineralization may have accompanied volcanic 

activity.

The middle Miocene volcanic rocks host epithermal 

precious metal deposits at Midas and in the Ivanhoe district, 

and these volcanic rocks contain mineral deposits to the 

north-northwest and south-southeast. Geologic and geochronologic evidence indicates that the mineral deposits were 

formed during the waning stages of the older middle 

Miocene bimodal volcanic activity or the very earliest stages 

of the younger bimodal volcanism (Wallace, 1993; Bartiett 

and others, 1990). In the vicinity of the Snowstorm Mountains, middle Miocene volcanic rocks do not extend to the 

east or west, and related mineral deposits therefore would 

not be expected beyond the limits of these volcanic rocks.

REFERENCES CITED

Bartlett, M.W., Enders, M.S., and Hruska, D.C., 1990, Geology of 

the Hollister gold deposit, Ivanhoe district, Elko County, Nevada, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and Ore Deposits of the Great Basin, 

Symposium Proceedings: Reno, Geological Society of Nevada, p. 957-978.

Bonnichsen, Bill, and Kauffman, D.F., 1987, Physical features of 

rhyolite lava flows in the Snake River Plain volcanic province, 

southwestern Idaho: Geological Society of America Special 

Paper 212, p. 119-145.

Coats, R.R., 1987, Geology of Elko County, Nevada: Nevada Bureau of Mines and Geology Bulletin 101,112 p.

Cox, Allan, and Dalrymple, G.B., 1967, Statistical analysis of geomagnetic reversal data and the precision of potassium-argon 

dating: Journal of Geophysical Research, v. 72, no. 10, 

p. 2603-2614.

Dalrymple, G.B., and Lanphere, M.D., 1969, Potassium-Argon 

Dating: San Francisco, W.H. Freeman and Co., 258 p.

Ekren, E.B., Mclntyre, D.H., and Bennett, E.H., 1984, High-temperature, large-volume, lavalike ash-flow tuffs without 

calderas in southwestern Idaho: U.S. Geological Survey Professional Paper 1272,76 p.

Erickson, R.L., and Marsh, S.P., 1974, Geologic map of the Golconda quadrangle, Humboldt County, Nevada: U.S. Geological Survey Geologic Quadrangle Map GQ-1174, scale 

1:24,000.

Honjo, Norio, and Leeman, W.P., 1987, Origin of hybrid ferrolatite 

lavas from Magic Reservoir eruptive center, Snake River Plain, 

Idaho: Contributions to Mineralogy and Petrology, v. 96, 

p. 163-177.

Hotz, P.E., and Willden, Ronald, 1964, Geology and mineral deposits of the Osgood Mountains quadrangle, Humboldt County, Nevada: U.S. Geological Survey Professional Paper 431, 

128 p.

Ingamells, C.O., 1970, Lithium metaborate flux in silicate analysis: 

Analytica Chimica Acta, v. 52, p. 323-334.

Joraleman, Peter, 1975, K-Ar relations of granodiorite emplacement and tungsten and gold mineralization near the Getchell 

mine, Humboldt County, Nevada A discussion: Economic 

Geology, v. 70, p. 405-409.
VOLCANIC ROCKS, SNOWSTORM MOUNTIANS AND VICINITY, NORTHERN NEVADA 17

LaPointe, D.D., Tingley, J.V., and Jones, R.B., 1991, Mineral resources of Elko County, Nevada: Nevada Bureau of Mines and 

Geology Bulletin 106,236 p.

McKee, E.H., Tarshis, A.L., and Marvin, R.F., 1976, Summary of 

radiometric ages of Tertiary volcanic and selected plutonic 

rocks in Nevada. Part V: Northeastern Nevada: Isochron/West, 

no. 16, p. 15-27.

Rytuba, J.J., and McKee, E.H., 1984, Peralkaline ash flow tuffs and 

calderas of the McDermitt volcanic field, southeast Oregon 

and north central Nevada: Journal of Geophysical Research, v. 

89, p. 8616-8628.

Silberman, M.L., Berger, B.R., and Koski, R.A., 1974, K-Ar relations of granodiorite emplacement and tungsten and gold mineralization near the Getchell mine, Humboldt County, Nevada: 

Economic Geology, v. 69, no. 5, p. 646-656.

Stacey, J.S., Sherrill, N.D., Dalrymple, G.B., Lanphere, M.A., and 

Carpenter, N.V., 1981, A five-collector system for the simultaneous measurement of argon isotopic ratios in a static mass 

spectrometer: International Mass Spectrometry Ion Physics, v. 

39, p. 167-180.

Steiger, R.H., and Jager, E., 1977, Subcommission on Geochronology: Conventions on the use of decay constants in geocosmochronology: Earth and Planetary Science Letters, v. 36, 

p. 359-362.

Stewart, J.H., and Carlson, J.E., 1978, Geologic map of Nevada: 

U.S. Geological Survey, scale 1:500,000.

Stewart, J.H., Walker G.W., and Kleinhampl, F.J., 1975, OregonNevada lineament: Geology, v. 3, p. 265-268.

Thorman, C.H., Brooks, W.E., Snee, L.W., Potter, C.J., Dubiel, 

R.J., and Ketner, K.B., 1993, Late middle Eocene Nanny Creek 

calc-alkaline volcanic field, NE Nevada and NW Utah: Age, 

extent, and implications for Eocene tectonics [abs.]: Geological Society of America Abstracts with Programs, v. 25, no. 5, 

p. 155.

Wallace, A.R., 1990, Geologic map of the Jake Creek Mountain 

quadrangle, Elko County, Nevada: U.S. Geological Survey 

Geologic Quadrangle Map GQ-1672, scale 1:24,000.

 1993, Geologic map of the Snowstorm Mountains and vicinity, Elko and Humboldt Counties, Nevada: U.S. Geological 

Survey Miscellaneous Investigations Series Map 1-2394, scale 

1:50,000.

Wallace, A.R., McKee, E.H., Zoback, M.L., and Zimmermann, 

R.A., 1990, New ages for volcanic rocks, western Elko County, Nevada: Isochron/West, no. 55, p. 3-5.

Willden, Ronald, 1964, Geology and mineral deposits of Humboldt 

County, Nevada: Nevada Bureau of Mines and Geology Bulletin 59,154 p.

Zoback, M.L., McKee, E.H., Blakely, R.J., and Thompson, G.A., 

1994, The northern Nevada rift: Regional tectono-magmatic 

relations and middle Miocene stress direction: Geological Society of America Bulletin, v. 106, p. 371-382.

Zoback, M.L., and Thompson, G.A., 1978, Basin and Range rifting 

in northern Nevada: Clues from a mid-Miocene rift and its 

subsequent offsets: Geology, v. 6, p. 111-116.

APPENDIX: ANALYTICAL METHODS 

AND SAMPLE DESCRIPTIONS

ANALYTICAL METHODS

K-Ar sample preparation and analyses were done in the 

laboratories of the U.S. Geological Survey, Menlo Park, 

Calif. Analyses were by standard isotope dilution procedures 

as described by Dalrymple and Lanphere (1969). Plagioclase 

and hornblende concentrates were made by heavy-liquid, 

magnetic, electrostatic, and hand-picking procedures. The 

andesite and basaltic andesite samples were ground to 80 to 

100 mesh, leached in HNO3 and HF solutions, and washed 

and dried immediately before loading in the high-vacuum 

gas-extraction system. Potassium analyses were performed 

by lithium metaborate flux fusion flame photometry techniques, the lithium serving as an internal standard 

(Ingamells, 1970). Argon analyses were performed using a 

15.2-cm-radius, Neir-type mass spectrometer or a five-collector mass spectrometer (Stacey and others, 1981). The precision of the data, shown as the "± value," is the estimated 

analytical uncertainty at one standard deviation (Cox and 

Dalrymple, 1967). It represents uncertainties in the measurement of radiogenic 40Ar and K2O based on experience with 

hundreds of replicated analyses in the Menlo Park laboratories. Mass discrimination of the spectrometer is routinely 

determined on the basis of multiple analyses of purified air. 

The constants used in age determination are those from the 

Subcommission on Geochronology (Steiger and Jager, 

1977).

SAMPLE DESCRIPTIONS

9LH8: Dacite of Kelly Creek Spring (lat 41°19'51"N., long 

117°07'12"W.; Kenny Creek 7.5-minute quadrangle; 

Humboldt Co., Nevada); outcrop of dacite welded 

tuff. Contains broken, 1-mm-long phenocrysts of plagioclase, biotite, hornblende, and sparse opaque minerals, quartz, and sanidine; plagioclase » sanidine. 

Groundmass is partially devitrified and composed of 

weakly compacted angular shards.

9LH11: Andesite of Dry Hills (lat 41°13'30"N., long 

117°inO"W.; Dry Hills North 7.5-minute quadrangle; Humboldt Co., Nevada); fresh andesite excavated 

from trench for pipeline. Aphyric with a very fine 

grained groundmass of plagioclase, iron oxides, and 

pyroxene. Drill cuttings from nearby drill hole indicate andesite overlies argillites of presumably Paleozoic age, based upon exposures of contact 2 km to the 

southeast.

9LH22: Andesite of Scraper Springs (lat 41 ° 19'38"N., long 

116°40/01"W.; Scraper Springs 7.5-minute quadrangle, Elko Co., Nevada); andesite flow collected from 

roadcut at the mouth of Scraper Springs Creek.
18 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Contains phenocrysts of plagioclase (An45_55) and 

pyroxene; Coats (1987) also reports amphibole. 

Capped 1.5 km to the west by andesite breccia, which 

is overlain by a nonwelded pumice tuff of unknown 

age and middle Miocene rhyolite flows.

9LH24: Basaltic andesite dike (lat 41°15'34"N., long 

116°48'32"W.; Oregon Canyon 7.5-minute quadrangle; Elko Co., Nevada); outcrop in western part of 

Midas district. Dike fed flows in lower basaltic andesite sequence in the district. Petrographically similar to 

9LH25, but slightly finer grained. Influence of 15.5- 

Ma mineralization in the district is unknown, but the 

dike is more than 2 km from the nearest veins, and the 

date on the dike is consistent both with a 15.2-Ma 

date to the west (Wallace and others, 1990) and with 

the relative age of the dike and the 14.7-Ma flows in

the upper basaltic andesite sequence (sample 9LH25). 

However, the high K2O content (6.43 weight percent) 

suggests potassic alteration that is not evident in thin 

sections.

9LH25: Basaltic andesite flow (lat 41°16'49"N., long 

116°47'29"W.; Oregon Canyon 7.5-minute quadrangle; Elko Co., Nevada); exposure of massive flow unit 

in roadcut near northern margin of Midas Creek valley. Part of the upper of two basaltic andesite 

sequences in the Midas area. Contains sparse 1-mmlong plagioclase (An65_75), olivine, orthopyroxene, 

and clinopyroxene phenocrysts in fine-grained 

groundmass of the same minerals. Two km to south, 

unit overlies intervening pyroclastic rocks; less than 

500 m to west, unit overlain by basal rhyolite flow of 

the younger bimodal sequence.
CHAPTER C

-Th-Pb AGES ASSOCIATED WITH BASE- AND 

PRECIOUS-METAL DEPOSITS IN CENTRAL IDAHO

By Karl V. Evans, 1 Lawrence W. Snee,2 and Robert E. Zartman2

ABSTRACT

^Ar/AT analysis of muscovite from quartz fissurevein gold and base-metal deposits at the Mammoth mine in 

west-central Idaho yields a plateau age of 69 Ma. Other 

argon analyses of biotite and a whole-rock sample from the 

Bobcat Gulch porphyry copper-molybdenum prospect in 

east-central Idaho indicate an age of mineralization of about 

49 Ma. These results confirm a growing body of evidence 

that both the Late Cretaceous and middle Eocene were times 

of significant metallic mineral deposition in Idaho. U-Th-Pb 

analyses of zircon from a rhyolite sill within the Yellowjacket Formation at the West Fork Indian Creek gold prospect in east-central Idaho yield an essentially concordant age 

of 47 Ma, confirming previous suggestions that the rhyolite 

is part of the Challis volcanic-plutonic event; the age of gold 

deposition at the prospect remains unknown.

INTRODUCTION

In the course of studies by the U.S. Geological Survey 

(USGS), during which the geology and mineral resource 

potential of current and proposed Wilderness Areas were 

investigated, several mineral deposits were sampled to provide information regarding the age of base- and preciousmetals deposition. This report provides a brief description of 

the geologic setting, analytical data, and interpretations of 

the results. Isotopic techniques follow Snee (1982), Snee and 

others (1988), and Evans and Zartman (1990). Age calculations are based on the decay constants of Steiger and Jager 

(1977).

Acknowledgments. We would like to thank Paul de 

Renne (consulting geologist) for providing information and

'U.S. Geological Survey, Mail Stop 905, P.O. Box 25046, Denver 

Federal Center, Denver, CO 80225.

2U.S. Geological Survey, Mail Stop 963, P.O. Box 25046, Denver 

Federal Center, Denver, CO 80225.

access to the West Fork Indian Creek prospect and Cominco 

American for allowing Robert C. Pearson (USGS) to collect 

the core sample from Bobcat Gulch. Discussions with Norm 

Lavery (consulting geologist), Bob Pearson, and Karen Lund 

improved our understanding of the mineral deposits. Ed 

DeWitt and Bob Pearson reviewed the manuscript.

MAMMOTH MINE

The Mammoth mine is in the Dixie mining district, 

about 1.5 km from the boundary of the Cove Mountain addition to the Frank Church-River of No Return Wilderness 

(fig. 1). The mine operated intermittently from 1932 to 1963, 

producing as much as 1,000 oz each of gold and silver, up to 

500 Ibs each of copper and lead, and an unknown amount of 

zinc (Mitchell and others, 1991). The host rock for the 

deposit is Cretaceous biotite-muscovite granite of the Idaho 

batholith (Lund and others, 1986; Lund and Esparza, 1990). 

At the Mammoth mine, the granite shows intense sericitic 

alteration and silicification, with numerous gold- and basemetal-bearing quartz veins that range from several centimeters to nearly 1 m in width (Capps and Roberts, 1939).

Muscovite from the quartz veins is cogenetic with orebearing sulfide minerals; its age, therefore, should yield the 

time of metal deposition. A ^Ar/^Ar plateau date of 

69.5±0.5 Ma (1 sigma error) (table 1; fig. 2A) indicates that 

ore deposition at the Mammoth mine is Late Cretaceous, 

similar in setting and age to deposits in the nearby Buffalo 

Hump mining district (Lund and others, 1986) and numerous 

other mining districts in central Idaho (Snee and others, 

1991).

BOBCAT GULCH PROSPECT

The Bobcat Gulch prospect is a copper-molybdenum 

porphyry target located about 28 km northeast of the Special 

Mining Management Zone Clear Creek of the Frank

19
20

116°00'

ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994 

115°00' 114°00'

\ X X West Fork 

^ * Indian Creek 

prospect

Shoup XL

Blackbird 

XX mine

10 MILES

\ \

10 KILOMETERS

Figure 1. Index map of central Idaho showing sample localities and geographic features mentioned in text. CM, Cove Mountain addition to the Frank Church-River of No Return Wilderness; SMMZ-CC, Special Mining Management Zone Clear Creek (Lund and others, 

1983).

80

75

 DC 70 

20- '"

uJO 65

z«

Ill5 6Q

$Q-U. 

<0 55

60

-JILI

Plateau age = 69.5±0.5 Ma

uJO «;o mlL. OU <s

0-11. 

<0

40

B

Plateau age = 48.5±0.8 Ma

50 100

ArK RELEASED, IN CUMULATIVE PERCENT

Church-River of No Return Wilderness (Lund and others, 

1983). Cominco American (Running and Burnet, 1981) 

drilled several holes between 1977 and 1981; Nisbet and 

Scales (1990) later interpreted the data as defining 40 million 

tons of 0.40-percent copper, but the prospect has not been 

developed further. Copper and molybdenum minerals are 

associated with several intrusive phases, ranging from rhyolite to quartz diorite, that exhibit various stages of alteration 

(Bunning and Burnet, 1981; Loen and Pearson, 1989; Pearson and others, 1992).

Biotite and a whole-rock sample from a diamond drill 

core yield similar ages by the ^Ar/^Ar method. Biotite phenocrysts from fresh to very slightly altered rhyolite porphyry 

give a total-gas date of 49.1±0.5 Ma (table 1). A whole-rock 

sample from the same drill hole (1497-1538-ft interval) is an 

altered and mineralized mafic dike consisting of clinopyroxFigure 2 (facing column). ^Ar/^Ar age spectrum diagrams for 

samples from Mammoth mine and Bobcat Gulch prospect. Ages for 

each heating step are shown as horizontal lines with 1 sigma error 

bars. 39ArK indicates the 39Ar produced from 39K during irradiation. 

A, Muscovite from quartz veins at the Mammoth mine (sample 

IB20-83). B, Whole-rock sample of mafic dike from drill core at the 

Bobcat Gulch prospect (sample 80PO61).
AGES ASSOCIATED WITH BASE- AND PRECIOUS-METAL DEPOSITS IN CENTRAL IDAHO 21

Table 1. ^Ar/^Ar data for samples from Mammoth mine and Bobcat Gulch prospect.

[Sample locality, Mammoth mine: lat45°27'39"N.,long 115°29'58"W.; sample locality, Bobcat Gulch prospect: lat45°2r23"N., 

long 113°59'37'*W. See McDougall and Harrison (1988) for definition of "J." Sample IB20-83, muscovite, sample weight = 

0.0824 g, J = 0.006752; sample 80PO62, biotite, sample weight = 0.25 g, J = 0.005225; sample 80PO61, whole rock, sample 

weight = 2.5 g, J = 0.006068. 39ArK, 39Ar produced from 39K during irradiation; ^Ar^ radiogenic Ar]

Temperature 

(°C)

°Ar/3'Ar 37Ar/39Ar -AT/-AT

(% of total)

Arif

C ^*v

Apparent 

age (Ma)

Mammoth mine, sample IB20-83

550 

650 

800 

900 

1,000 

Fuse

6.783 

6.223 

6.010 

6.035 

5.979 

5.986

0.002281 

0.001209 

0.0002708 

0.0006911 

0.001034 

0.0006752

0.005190 

0.002047 

0.0007552 

0.0007195 

0.0005718 

0.0003419

5.0 

6.4 

25.9 

22.5 

20.0 

20.1 

Total-ga 

Platea

77.3 

90.2 

96.2 

96.4 

97.1 

98.2 

is age.. .......

uage.........

62.8±0.3 

67.1±0.3 

69.1±0.4 

69.5±0.4 

69.4±0.3 

70.2±0.4 

....69.0

....69.5±0.5

Bobcat Gulch prospect, sample 80PO62

Fuse 6.450 0.054 0.003957 100 81.85 

Total-gas age.........

49.1±0.5 

....49.1±0.5

Bobcat Gulch prospect, sample 80PO61

650 

700 

800 

1,000 

Fuse

10.984 

6.403 

6.571 

9.107 

8.027

0.201 

0.288 

0.341 

0.247 

1.127

0.01981 

0.005966 

0.007153 

0.01583 

0.01206

11.84 

16.80 

19.85 

25.41 

26.10 

Total-ga 

Platea

46.80 

72.74 

68.17 

48.81 

56.69 

is age. . .

uage.........

55.4±0.8 

50.3±0.6 

48.4±0.6 

48.0±0.7 

49.2±0.6 

....49.7

....48.5±0.8

ene, brown hornblende, biotite, and plagioclase (albite), with 

minor quartz completely replacing some phenocrysts (clinopyroxene?). Plagioclase laths are slightly ragged and 

irregularly shaped. Alteration phases include minor carbonate minerals, clays, and pyrite; much of the fine-grained 

material is unidentifiable. Because adequate mineral separates could not be obtained from the fine-grained altered 

core, the whole rock was analyzed. This sample yields a plateau date of 48.5±0.8 Ma; the total-gas date is 49.7 Ma. The 

shape of the age spectrum suggests minor excess argon (fig. 

25). Both plateau dates are identical within analytical error 

and indicate that the Cu and Mo mineralization is related to 

the Eocene Challis volcanic and plutonic event. Curiously, a 

very nearby gold deposit developed in sheared Proterozoic 

quartzite has been dated by conventional K-Ar methods on 

sericite at 69 Ma (Nisbet and Scales, 1990). The latter date is 

similar to the age of the Beartrack gold deposit at Leesburg 

(K.V. Evans and L.W. Snee, unpub. data) but is distinctly 

different from that at Bobcat Gulch.

WEST FORK 

INDIAN CREEK PROSPECT

The West Fork Indian Creek prospect is about 15 km 

northeast of Shoup, Idaho (fig. 1). Recent drilling by an 

independent exploration and management company has

demonstrated the presence of gold mineralization (P. de 

Renne, consulting geologist, oral commun., 1990); however, we are not aware of public information regarding the 

size and grade of the deposit. Rocks in the area of exploration are strata of the Middle Proterozoic Yellowjacket Formation, at least some of which is recognizable as the middle 

unit of that formation (Connor and Evans, 1986; Evans, in 

press; Evans and Connor, 1993). Reconnaissance mapping 

by Lopez (1982) characterizes an area of the West Fork 

drainage as being mixed Yellowjacket strata and Tertiary 

rhyolitic dikes. However, some of the "dikes" are strictly 

parallel to bedding, and core logging has indicated that they 

might be spherulitic, rhyolite ash-flow tuffs, apparently 

with alteration developed only on one side of the "tuff 

(N.G. Lavery, consulting geologist, oral and written commun., 1990). If the rhyolites are tuffs, they would provide 

an excellent means to date the sedimentary rocks of the Yellowjacket and perhaps relate the gold deposit to a Proterozoic mineralizing system such as those known in the region 

at the Blackbird (Nash and others, 1987) and Ulysses mines 

(Chevillon and Herberger, 1989). Because field, drill-core, 

and even thin-section data remained equivocal, rhyolite 

from a fresh roadcut through one of the bodies was sampled 

for U-Th-Pb analysis of zircon.

Two splits of hand-picked zircon demonstrate that the 

rhyolite is Eocene in age and corroborate Lopez's (1982) 

original field interpretation. One zircon split (split A, table 2
22 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Table 2. U-Th-Pb zircon data for rhyolite from West Fork Indian Creek prospect.

[Sample locality: lat 45°29'05"N., long 114°10'29"W. Isotopic composition of common lead is assumed to be 204Pb:206Pb:207Pb:208Pb = 1:18.63:15.63:38.54. Split A: elongate, 

euhedral crystals with average length-to-width ratio of 6:1; split B: nearly equant, euhedral crystals with average length-to-width ratio of 1.6:1. Crystal diameters are from 20 to 

SOujn]

Split A 

Split B

Sample 

weight

(wO

44.8 

145.7

Concentration 

(ppm)

U

925.6 

883.6

Th Pb

430.7 7.796 

415.2 8.147

Isotopic composition of 

lead (atom percent)

204pb

0.1117 

0.1826

206pb

77.01 

73.27

207pb

5.279 

6.203

208pb

17.60 

20.34

Age 

(millions of years before present")

206pb/238u

47.1±0.8 

48.1±0.4

207pb/235TJ

47.3±0.9 

49.0±0.5

207pb/206pb

57±13 

97±7

208Pb/232Th

54.6±0.8 

58.9±0.6

0.0092

0.0088

0.0084

-j 0.0080

^0.0076 

CL

~ 0.0072 

0.0068 

0.0064

0.0060

Intercepts at 

1,306±1,100 and 46.8±2.0 Ma

0.40 0.44 0.48 0.52 0.56 0.60

Figure 3. Concordia diagram for zircon from rhyolite at the 

West Fork Indian Creek prospect (sample 48-9KE-103B). Error 

ellipses plotted using procedure of Ludwig (1988). "A" indicates 

error ellipse for elongate, euhedral zircon crystals (split A). "B" 

indicates error ellipse for nearly equant, euhedral zircon crystals 

(split B).

 elongate, euhedral crystals with average length-to-width 

ratio of 6:1) is essentially concordant within analytical error, 

and the second split (split B, table 2 nearly equant, euhedral crystals with average length-to-width ratio of 1.6:1) is 

only slightly discordant. Taken together, both splits suggest 

the presence of a minor component of inherited Precambrian 

radiogenic lead in the zircon (fig. 3, table 2) with the greater 

proportion residing in the more equant crystals. This phenomenon is probably best explained as arising from the 

incorporation into the rhyolite of a small amount of xenocrystic zircon derived from the surrounding country rock. The 

xenocrystic zircon then served as nuclei for newly crystallizing overgrowths of the same mineral. A crystallization age of 

46.8±2 Ma (fig. 3) is considered the best time estimate for 

the emplacement and solidification of the rhyolite. At 

present, the age of the gold deposit remains undetermined.

DISCUSSION

Prior to the advent of modern isotopic geochronology, 

the ages of mineral deposits in Idaho were often extremely

difficult to ascertain. Because several deposits were hosted 

in volcanic rocks of the Tertiary Challis Volcanic Group, 

geologists tended to promote a very strong bias that most 

deposits were Tertiary (see Lund and others, 1986, for a brief 

historical account). Recent geochronologic studies have 

clearly identified a Late Cretaceous period of mineralization 

(Snee and others, 1991) in addition to the well-documented 

Tertiary events. The present study adds to this growing database and confirms the complex history of mineralization in 

Idaho. Data from the Mammoth mine are consistent with 

numerous other Late Cretaceous dates of mineralization 

throughout central Idaho. The Bobcat Gulch data confirm 

the importance of Eocene intrusive and volcanic rocks as 

ore-producing systems. The significance of the zircon date 

of the rhyolite at the West Fork Indian Creek is unclear with 

respect to the drilled mineral deposit. Dating of appropriate 

alteration minerals directly associated with the deposit will 

probably be needed to resolve the age of mineralization.

REFERENCES CITED

Bunning, B.B., and Burnet, F.W., 1981, Copper-molybdenum mineralization in the Bobcat Gulch porphyry system, Lemhi County, Idaho: Unpublished paper presented at the 87th Annual 

Northwest Mining Association Convention, Spokane, Washington, Dec. 5, 1981.

Capps, S.R., and Roberts, R.J., 1939, The Dixie placer district, Idaho, with notes on the lode mines: Idaho Bureau of Mines and 

Geology Pamphlet No. 48,35 p.

Chevillon, V., and Herberger, D.J., 1989, Syngenetic gold mineralization in the Proterozoic Yellowjacket Formation, Ulysses 

gold mine, Lemhi County, Idaho [abs.]: Geological Society of 

America, Abstracts with Programs, v. 21, no. 5, p. 65.

Connor, J.J., and Evans, K.V., 1986, Geologic map of the Leesburg 

quadrangle, Lemhi County, Idaho: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1880, scale 1:62,500.

Evans, K.V., in press, The Yellowjacket Formation of east-central 

Idaho, in Berg, R.B., ed., Proceedings of Belt Symposium III: 

Montana Bureau of Mines and Geology Special Publication.

Evans, K.V., and Connor, J.J., 1993, Geologic map of the Blackbird 

Mountain 15-minute quadrangle, Lemhi County, Idaho: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-2234, scale 1:62,500.
AGES ASSOCIATED WITH BASE- AND PRECIOUS-METAL DEPOSITS IN CENTRAL IDAHO 23

Evans, K.V., and Zartman, R.E., 1990, U-Th-Pb and Rb-Sr geochronology of Middle Proterozoic granite and augen gneiss, 

Salmon River Mountains, east-central Idaho: Geological Society of America Bulletin, v. 102, p. 63-73.

Loen, J.S., and Pearson, R.C., 1989, Map showing locations of 

mines and prospects in the Dillon I°x2° quadrangle, Idaho and 

Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-1803-C, scale 1:250,000.

Lopez, D.A., 1982, Reconnaissance geologic map of the Ulysses 

Mountain quadrangle, Lemhi County, Idaho: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1445, scale 

1:48,000.

Ludwig, K.R., 1988, ISOPLOT for MS-DOS: A plotting and regression program for radiogenic-isotope data, for IBM-PC 

compatible computers, version 2.0 [revision as of January 3, 

1990]: U.S. Geological Survey Open-File Report 88-557, 

35 p.

Lund, K., and Esparza, L.E., 1990, Mineral resources of the Gospel-Hump Wilderness, Idaho County, Idaho: U.S. Geological 

Survey Bulletin 1812,19 p.

Lund, K., Evans, K.V., and Esparza, L.E., 1983, Mineral resource 

potential map of the Special Mining Management 

Zone Clear Creek, Lemhi County, Idaho: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1576-A, scale 

1:50,000.

Lund, K., Snee, L.W., and Evans, K.V., 1986, Age and genesis of 

precious metals deposits, Buffalo Hump district, central Idaho: 

Implications for depth of emplacement of quartz veins: Economic Geology, v. 81, p. 990-996.

McDougall, lan, and Harrison, T.M., 1988, Geochronology and 

Thermochronology by the ^Ar/^Ar Method: New York, Oxford University Press, 212 p.

Mitchell, V.E., Vance, R.E., and Bennett, E.H., 1991, Mines and 

prospects of the Elk City quadrangle, Idaho (2d ed.): Idaho 

Geological Survey Mines and Prospects Map Series, 83 p., 

scale 1:250,000.

Nash, J.T., Hahn, G.A., and Saunders, J.A., 1987, The occurrence 

of gold in siliceous Co-Cu exhalite deposits of the Blackbird 

mining district, Lemhi County, Idaho: U.S. Geological Survey 

Open-File Report 87-410,14 p.

Nisbet, P., and Scales, W.G., 1990, Descriptive road log, northeastern Leesburg Basin-Bobcat project area, in Moye, F.J., ed., 

Geology and Ore Deposits of the Trans-Challis Fault System/Great Falls Tectonic Zone: Guidebook of the 15th Annual 

Tobacco Root Geological Society Field Conference, Salmon, 

Idaho, August 16-18,1990, p. 45-54.

Pearson, R.C., Trautwein, C.M., Moll, S.H., Berger, B.R., Hanna, 

W.F., Loen, J.S., Rowan, L.C., Ruppel, E.T., and Segal, D.B., 

1992, Maps showing mineral resource assessment for copper 

and molybdenum in porphyry and stockwork deposits and for 

tungsten, iron, gold, copper, and silver in skarn deposits, Dillon 

I°x2° quadrangle, Idaho and Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-1803-G, scale 

1:250,000.

Snee, L.W., 1982, Emplacement and cooling of the Pioneer 

batholith, southwestern Montana: Columbus, Ohio State University, unpub. Ph.D. thesis, 320 p.

Snee, L.W., Lund, K., Evans, K.V., Gammons, C.H., and Kunk, 

M.J., 1991, ^Ar/^Ar thermochronology of fracture-controlled 

mineral deposits of the Idaho batholith Age, thermal history, 

and origin: U.S. Geological Survey Circular 1062, p. 74-76.

Snee, L.W., Sutler, J.F., and Kelly, W.C., 1988, Thermochronology 

of economic mineral deposits: Dating the stages of mineralization at Panasqueira, Portugal, by high-precision ^Ar/^Ar age 

spectrum techniques on muscovite: Economic Geology, v. 83, 

p. 335-354.

Steiger, R.H., and Ja'ger, E., 1977, Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, 

p. 359-362.

CHAPTERD

GEOLOGY AND MINERAL DEPOSITS IN THE

TERRILL MOUNTAINS AREA, HOLY CROSS DISTRICT,

CHURCHILL COUNTY, NEVADA

By Stephen G. Peters, 1 Richard F. Hardyman, 1 and Katherine A. Connors 1

INTRODUCTION

The Terrill Mountains, between Fallen and Schurz in 

western Nevada (fig. 1), contain numerous silver-gold veins 

hosted in Tertiary volcanic, volcaniclastic, and hypabyssal 

rocks. These veins are spatially associated with copper, lead, 

zinc, and manganese deposits. Mineral deposits in the Terrill 

Mountains were discovered in 1910 (Lincoln, 1923; Vanderburg, 1940), and mining in the area has been intermittent 

from that time to the present. These mineral occurrences, 

which constitute the southern part of the Holy Cross mining 

district, are located in the central part of the Terrill Mountains. The northern part of the district, 7 km to the northeast 

in the Barnett Hills, is separated from the southern part by 

Rawhide Flats, a northwest-trending valley thought by us to 

conceal a major northwest-trending strike-slip fault (see Bingler, 1978; Hardyman, 1980). Early production records are 

incomplete (Schraeder, 1947), but reported production since 

1934 is 74,000 oz silver, 282 oz gold, and 34,000 Ibs lead, 

and minor zinc and copper, from approximately 2,000 tons 

of ore (Willden and Speed, 1974).

This study was initiated using l:5,000-scale geologic 

mapping and rock geochemistry in both the north and south 

parts of the Holy Cross district in order to investigate the 

geological, geochemical, and lode characteristics on each 

side of the inferred strike-slip fault. This study is part of a 

larger regional study of the central Walker Lane (Hardyman 

and Oldow, 1991; Hardyman, unpub. mapping, 1983-93; 

Peters, unpub. mapping, 1992). Comparison of geologic and 

mineralization characteristics allows speculation on the timing and genetic relationships between the two parts of the 

mining district. The results of this study also allow comparison of the Holy Cross district with the larger Rawhide gold-

'U.S. Geological Survey, Mackay School of Mines, Mail Stop 176, 

Reno, NV 89557-0047.

silver district, located 20 km to the southeast in a similar geologic setting (cf., Ross, 1961; Black and others, 1991). The 

purpose of this report is to summarize geologic relationships 

in the Terrill Mountains part of the Holy Cross mining district and to discuss the relationship of mineral deposits to 

Walker Lane-style strike-slip and other faulting and to Tertiary volcanism and plutonism.

REGIONAL SETTING

The Terrill Mountains area is within the central part of 

the Walker Lane structural zone of western Nevada (fig. 1). 

This zone contains a composite set of major northwest-striking faults that dissect 30- to 3-Ma (Tertiary) igneous and sedimentary sequences that unconformably overlie faulted and 

locally deformed and metamorphosed Paleozoic and Mesozoic sedimentary and volcanic basement rocks and intrusive 

Mesozoic plutons. Pre-Tertiary tectonism of these units 

involved several periods of thrusting from middle Paleozoic 

to middle Mesozoic time this tectonism, toward the end of 

thrusting, was accompanied by transpressional faulting 

(Oldow and others, 1989).

Tertiary tectonic evolution has been interpreted by Hardyman and Oldow (1991) to involve a change in the regional 

least principal stress direction from north-northeast to eastwest to northwest-southeast from 28 to 8 Ma. This extension 

was variously accompanied by normal, strike-slip, and associated detachment faulting. Tertiary faulting was partitioned 

differently throughout the central Walker Lane, resulting in 

several structural domains, which are defined by the relationship of tilting between basement rocks and overlying 

Tertiary sequences. Domains where basement rocks and 

overlying Tertiary sections were tilted together can be documented in some areas. In other areas associated with large 

strike-slip faulting, domains are typified by tilted Tertiary 

strata that lie in fault contact above non-tilted basement

25
26 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

118045'

10 KILOMETERS

^nrstudyarea

i=vza WALKER LANE 

STRUCTURAL 

ZONE

f**S<. ; ! '.Reservoir] 

.........^..

Schurz (1 mile)

Figure 1. Index map of western Nevada showing the Walker Lane structural zone (inset) and generalized regional geologic map of 

the Terrill Mountains. Map units: Qal, Quaternary alluvium; Tbsm, Tertiary ash-flow tuffs of the Blow Sand Mountains; Tt, Tertiary 

ash-flow tuffs of uncertain correlation; Tal, younger Tertiary andesite lavas and lahars and younger basaltic andesite and basalt lavas 

unconformably overlying the ash-flow tuff sequences; Tr, Tertiary rhyolite intrusions; Ti, Tertiary hypabyssal dacite intrusions in the 

eastern part of the Terrill Mountains and the southern Calico Hills; Jgd, Jurassic(?) basement rocks. NWR, northern Wassuk Range; 

solid heavy lines arc faults; heavy dotted lines are inferred concealed faults or extensions of mapped faults in bedrock. Area on map 

near Camp Terrill shown by diagonal lines is area shown in figures 2 and 3. Regional geologic map modified after Hardyman and 

others (1992).

rocks. The Terrill Mountains area is a fault-bounded structural block, which is interpreted by Hardyman and others 

(1992) as tilted Tertiary strata lying upon non-tilted pre-Tertiary basement rocks (see Dilles and others, 1993; Hardyman 

and others, 1992).

GEOLOGY

The Terrill Mountains rise approximately 600 m above 

the valley floor of Rawhide Flats and consist of widespread 

exposures of late Oligocene to earliest Miocene silicic ashflow tuffs and early Miocene hypabyssal dacite intrusions.

These rocks are unconformably overlain by middle Miocene 

hornblende-andesite lahars and pyroxene andesite and olivine-basalt lava flows. Except in local areas adjacent to some 

intrusive dacite contacts or faults where compaction foliation is steep to moderate, the Tertiary volcanic section 

exposed in the Terrill Mountains has a generally gentle 

(approximately 15°) west-southwest dip.

Limited exposures of Mesozoic (Jurassic?) gabbro and 

diorite are found along the eastern range front, approximately 2.8 km northwest of Camp Terrill (fig. 2). Some of 

these Mesozoic basement rocks are in contact with the southeastern part of a 4-km-long by 0.9-km-wide, northwesttrending, ash-flow tuff/fissure-vent complex that intrudes
GEOLOGY AND MINERAL DEPOSITS, TERRILL MOUNTAINS AREA, CHURCHILL COUNTY, NEVADA 27

118°42130'<

Qal 

Tal 

Td 

Ttb 

Tta 

TV 

Tto 

Tbt 

Jgd

EXPLANATION

Alluvium (Quaternary)

Andesite lavas and lahars (Tertiary)

Hypabyssal dacite (Tertiary)

Tuff B (Tertiary)

Tuff A (Tertiary)

Ash-flow tuff/ fissure-vent complex (Tertiary)

Tuff of Toiyabe (Tertiary)

Basal tuff (Tertiary)

Gabbro and dacite (Jurassic(?))

_ .. . Fault, dashed where approximately located, 

dotted where inferred

Detachment fault 

Study area boundary

1 MILE

39°05'00"

I I

0 1 KILOMETER

Figure 2. Simplified geologic map of the Terrill Mountains area, southern Holy Cross mining district, showing main geologic units and 

faults.

older, lower tuff units and may locally intrude the gabbro 

and diorite. Elsewhere, contacts between the gabbro and 

diorite unit and the Tertiary volcanic rocks are, for the most 

part, low-angle detachment faults. Steep faults trend northwest and north-northwest and further dismember the Mesozoic-Tertiary rock contact (fig. 2).

LITHOLOGY

Jurassic(?) gabbro and diorite (Jgd). These rocks 

comprise the oldest recognized unit in the Terrill Mountains 

(fig. 2). Much of the rock is a medium-grained hornblende 

gabbro with oligoclase and minor clinopyroxene, biotite,
28 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

lesser apatite, sphene, and opaques. The rock is massive and 

undeformed but contains local 1- to 10-m2 areas of metasomatism that are characterized by massive scapolite with 

lesser albite, brown garnet, and epidote. The nearest rocks of 

similar type are the 165- to 145-Ma (Middle Jurassic) 

lopolithic gabbros located 100 km to the northeast, in the 

Mopung Hills, Clan Alpine Mountains, and in the West 

Humboldt Mountains (Willden and Speed, 1974). No direct 

correlation has been made between these similar rocks.

Basal tuff (Tbt). The basal tuff hosts the majority of 

the mineral deposits in the Terrill Mountains and is the basal 

unit of the Tertiary section (fig. 2). A thickness of approximately 200 m of this unit is exposed in the east flank of the 

range. The base of the unit is either covered by Quaternary 

deposits or intruded by hypabyssal dacite. The unit is 

nowhere exposed in contact with the Jurassic(?) gabbro and 

diorite (Jgd) basement rocks. The basal tuff is unconformably overlain, in low-angle detachment-fault contact, by the 

tuff of Toiyabe (Tto) and tuff B (Ttb) (fig. 2). The basal tuff 

consists of two main cooling units of felsic ash-flow tuff 

with additional, truncated cooling units present at the base of 

and capping the exposed section. Mapped in detail, the basal 

tuff can be subdivided into seven individual 20- to 30-mthick horizons that are distinguishable by color, degree of 

welding, pumice and crystal content, and by specific ingredients such as local sedimentary rock lenses or wood shards. 

The central part of the basal tuff hosts the majority of the silver-gold mineral deposits, localized at the contact between 

porous, white, volcanic-pebble conglomerate and an overlying densely welded tuff member. The basal tuff unit is 

undated in the Terrill Mountains and has not been correlated 

with units outside the Terrill Mountains.

Tuff of Toiyabe (Tto). The 23- to 22-Ma tuff of Toiyabe is exposed in two areas south and southeast of Camp 

Terrill (fig. 2). The easternmost exposure overlies, and is in 

low-angle detachment-fault contact with the lower members 

of the basal tuff as described by Hardyman and others 

(1992). To the southwest, exposures of the tuff of Toiyabe 

are in low-angle detachment-fault contact with the upper 

members of the basal tuff (fig. 2). The tuff of Toiyabe is a 

gray- to brown-weathering, densely welded, devitrified, 

massive, and crystal-rich (quartz and feldspar) ash-flow tuff. 

The presence of relatively abundant sphene (up to 4 mm 

long) recognized in hand specimen is a distinguishing characteristic of the tuff. The upper part of the unit is eroded and 

is unconformably overlain by hornblende-andesite labors. 

The unit is locally hydrothermally altered and mineralized 

along the detachment fault (the Camp Terrill fault of Hardyman and others, 1992). The source of the tuff of Toiyabe is 

the Toiyabe caldera, located in the southern Toiyabe Range, 

approximately 110 km to the southeast along the Walker 

Lane (Bremm and others, 1985,1991).

Tuff A (Tta). This unit is exposed in the extreme northwestern part of the study area (fig. 2) and extends about 2.5 

km northwest along the range front, beyond the map area.

Tuff A is a relatively massive red to reddish-brown-weathering, crystal-rich, rhyolite ash-flow tuff that overlies pre-Tertiary gabbro and diorite in low-angle fault contact. It is 

intruded by crystal-poor fissure-vent/ash-flow tuff (unit TV, 

fig. 2) and is separated from tuff B to the west by a northwest-striking high-angle fault (fig. 2).

The stratigraphic position of tuff A is uncertain because 

it is nowhere in depositional contact with other Tertiary volcanic units within the Terrill Mountains and it has yet to be 

correlated with other ash-flow tuffs in the region. Mineralogically, it resembles a tuff in the northern Wassuk Range, 

approximately 30 km west-southwest of the Terrill Mountains, which has been dated at 28.6 Ma (Dilles and others, 

1993).

Tuff B (Ttb). This map unit is approximately 60-m 

thick and consists of four concordant and petrologically similar cooling units, which can be divided into seven distinct 

horizons. The rock types consist of light-brown- and lightgray-weathering, densely welded to moderately welded, 

devitrified, crystal-rich, moderately biotite rich, quartz-latite 

ash-flow tuffs. The lower four horizons are poorly welded, 

green-, purple-, and buff-colored pebble and cobble conglomerate and volcaniclastic rocks. The upper three horizons 

are black vitrophyre, light-brown, welded, crystal-rich tuff, 

and white, poorly welded, fine-grained, epiclastic tuff. The 

top of the unit is capped by intrusive dacite or is eroded. Tuff 

B is in fault contact with other ash-flow tuff units in the study 

area and is intruded by rocks of the ash-flow tuff/fissure-vent 

complex (Tv) and by hypabyssal dacite (Td); the unit was 

subsequently faulted. The unit dips about 15°-20° southwest 

and is unmineralized. No regional correlation of this tuff unit 

has been recognized.

Ash-flow tuff/fissure-vent complex (Tv). An ash-flow 

tuff/fissure-vent complex is exposed for approximately 4 

km along the east flank of the Terrill Mountains (Hardyman 

and others, 1992; Dilles and others, 1993). A 2-km-long 

exposure within the study area, approximately 2 km northwest of Camp Terrill (fig. 2), is composed of two, crystalpoor rhyolite units (< 5 volume percent crystals). Most of 

the western part of the fissure-vent complex within the map 

area (fig. 2) and that part of the fissure-vent complex north 

of the map area consists of lavender- to gray- and reddishbrown-weathering, devitrified, pyroclastic rhyolite containing recognizable pumice lapilli and "bubble-wall" glass 

shards. The pyroclastic rhyolite grades into red- and redbrown-weathering flow-laminated rhyolite that displays 

varied but generally steep (and locally internally contorted) 

flow foliation. Row-laminated rhyolite forms most of the 

southeastern part of the fissure-vent complex in the study 

area. North of the study area, flow-laminated rhyolite is 

scarce and the fissure-vent complex consists almost entirely 

of pyroclastic rhyolite. Both textural variants of the fissurevent complex rhyolite intrude older ash-flow tuff units, and 

locally they may intrude pre-Tertiary gabbro and diorite. 

Black-green to gray vitrophyre is locally present along the
GEOLOGY AND MINERAL DEPOSITS, TERRILL MOUNTAINS AREA, CHURCHILL COUNTY, NEVADA 29

margin of the fissure-vent-complex rocks. Silicified breccia 

and volcaniclastic conglomeratic rocks are locally present at 

the contact with the pre-Tertiary gabbro and diorite. Crosscutting millimeter-scale quartz veinlets and casts of weathered pyrite(?) cubes in the flow-laminated rhyolite indicate 

that the fissure-vent-complex rhyolite has been hydrothermally altered. The ash-flow tuff/fissure-vent complex has 

not been radiometrically dated, but cross-cutting relationships indicate that it is younger than the basal tuff and tuffs 

A and B. It is older than the hypabyssal dacite that intrudes 

rocks of the fissure-vent complex. The age relationship 

between the fissure-vent-complex rhyolite and the tuff of 

Toiyabe is not constrained by field relations.

Hypabyssal dacite (Td). Greenish-gray- and brownweathering porphyritic dacite forms extensive outcrops on 

the east flank of the Terrill Mountains (fig. 2). These exposures form stock-like intrusions a few square kilometers in 

area and dike, sill, and plug-like intrusions 2-200 m thick. 

The hypabyssal dacite consists predominantly of mediumand coarse-grained, porphyritic, biotite-hornblende-bearing 

dacite, but, in one stock in the western part of the study area, 

hornblende-rich dacite grades into hornblende-poor, biotiterich, medium-grained, equigranular rhyodacite. A sample of 

the hypabyssal dacite collected near the range front about 1.5 

km northwest of Camp Terrill yielded a K-Ar date of 21.8 

Ma (Silberman and McKee, 1972, recalculated). This unit 

intrudes all the ash-flow tuff units exposed in the east flank 

of the Terrill Mountains, as well as rocks of the ash-flow tuff 

fissure-vent complex.

Andesite lavas and lahars (Tal). Medium-gray, hornblende-andesite lahars and minor interbedded lenses of fluvial epiclastic sedimentary rocks form this unit, which 

unconformably overlies the ash-flow tuff units exposed on 

the east flank of the Terrill Mountains. This unit also caps the 

crest and much of the western slope of the range. Locally, in 

the study area and to the northwest in the northern part of the 

range, plagioclase-andesite porphyry lavas underlie the 

horablende-andesite lahars and unconformably overlie the 

sequence of ash-flow tuffs. The hornblende-andesite lahars 

characteristically form rubbly weathering outcrops of cobble, boulder, and occasional block size, rounded to subrounded clasts of hornblende-andesite in a coarse-grained 

clastic matrix. The exposed thickness of these rocks is about 

180 m, and they dip southwesterly at about 15°-20°. The 

andesite lavas and lahars unit in the Terrill Mountains has 

not been dated, but it is correlated with lithologically and 

texturally identical hornblende-andesite lahars to the east, in 

the Barnett Hills and southern Cocoon Mountains, where a 

K-Ar date of 18.1 Ma was determined on hornblende (Silberman and McKee, 1972, recalculated).

STRUCTURE

The Terrill Mountains are a fault-bounded structural 

block within the zone of northwest-trending strike-slip faults

of the central Walker Lane (Hardyman and Oldow, 1991; 

Hardyman and others, 1992). The main structural features in 

the Terrill Mountains are northwest-trending high-angle 

strike-slip faults, low-angle faults, and north- to north-northeast-trending high-angle faults.

Northwest-trending high-angle faults. The Tertiary 

volcanic rocks in the Terrill Mountains structural block are 

locally disrupted by high-angle faults that strike N. 40° W. 

The northwest-trending high-angle faults are Walker Lanestyle strike-slip faults and have exposed strike lengths of 1-2 

km (fig. 1). They are locally intruded and truncated by the 

hypabyssal dacite unit. Elongation of several dacite intrusive 

bodies and of the ash-flow tuff/fissure-vent complex suggests that emplacement of these intrusive units was controlled by northwest-striking structures that predate the 21.8- 

Ma dacite intrusions. Post-intrusion movement on some 

faults is indicated by slickensided fault surfaces on dacite 

dike margins and may indicate reactivation of some faults. 

Scarps in older alluvium occur along the trace of one of the 

high-angle strike-slip faults that bounds the range front 

southeast of Camp Terrill and extends beyond the study area. 

These scarps cut an alluvial surface of probable Pliocene age. 

Geomorphic evidence along this strike-slip fault indicates 

that fault movement produced significant right-lateral slip.

Low-angle faults. The ash-flow tuff units in the Terrill 

Mountains are locally decoupled from the pre-Tertiary basement rocks along a nearly flat detachment fault. At least two 

other major low-angle (< 10° to 15°) detachment faults occur 

within the ash-flow-tuff stratigraphic section (figs. 2 and 3). 

One of these, the Camp Terrill fault (Hardyman and others, 

1992), formed at about 22 Ma, after deposition of the tuff of 

Toiyabe (23-22 Ma) and prior to emplacement of the 

hypabyssal dacite (Td) (21.8 Ma). The Camp Terrill fault is 

cut by three northwest-striking high-angle faults, one of 

which has down-dropped the Camp Terrill fault and its upper 

plate about 200-250 m to the northeast. The fault is 0.5- to 

2-m wide and contains phyllonitic gouge and crushed rock 

that has been hydrothermally altered. The amount of lateral 

offset, if any, on these northwest-striking high-angle faults is 

unknown. The high-angle fault and the Camp Terrill detachment faults both are intruded by hypabyssal dacite (Td). The 

tuff of Toiyabe in the upper plate of the Camp Terrill fault 

generally dips 35°-45° east, except in a central panel of the 

fault where it dips 10°-20° west. A separate detachment 

fault, here termed the Waterfall fault, lies below (ash-flow) 

tuff B (figs. 2 and 3); the upper plate of this fault dips 

15°-20° west and is subparallel to the basal detachment 

fault. The Waterfall fault dips north and west in a bowl-like 

shape, has several anastomosing splays, contains a 0.5- to 1- 

m-thick gouge zone, and is locally filled with pods of mafic 

volcanic rocks. These features are accompanied by 50-100 

m of argillic alteration and bleaching that have affected wallrocks 5-100 m outward from the fault. The low-angle fault 

that dips 10° west beneath (ash-flow) tuff Tta is approximately 1 m wide and contains brecciated, hematitic, milky 

quartz.
30 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

EXPLANATION

Dacite-hosted veins 

Comstock-type veins 

Disseminated mineralization 

Shafts 

Prospects

...... Fault, dashed where approximately

located, dotted where inferred 

Detachment fault

Study area boundary

39°05'00"

1 MILE

I

1 KILOMETER

Figure 3. Mineralized areas in the Terrill Mountains portrayed with major faults, prospects, and mines. Each shaded area represents an 

area of altered rocks, mineral deposits and prospecting, or mining activity.

High-angle northeast-trending faults. Vertical faults, 

averaging N. 20° E. in strike form several composite systems 

in the study area. Two main fault systems occur in the central 

part of the study area (fig. 3). They are the structures that predominantly controlled the flow of hydrothermal solutions in 

the Terrill Mountains. These faults are commonly 0.5- to 

1-m wide and contain extensive gouge, crushed rock, and 

clay seams. Dips are both west and east, but they are seldom

less than 60°. Offset in plan view, interpreted from tuff 

stratigraphy, may have been as much as 600 m sinistrally.

Compaction foliation. Compaction foliation varies 

throughout the study area, but predominant attitudes are 

10°-20° west, particularly in the basal tuff, (ash-flow) tuff B 

(Ttb), and central occurrences of the tuff of Toiyabe (fig. 2). 

A central part of the basal tuff and tuffs in the southeast, 

which are separated by a northwest-striking fault, both dip
GEOLOGY AND MINERAL DEPOSITS, TERRILL MOUNTAINS AREA, CHURCHILL COUNTY, NEVADA 31

east. These distinct areas of east-dipping compaction foliation define two 1- by 0.5-km-long blocks that have been 

rotated separately by complex, high-angle and low-angle 

fault movement and hypabyssal dacite intrusion. Moderately 

steep tilts of the tuff of Toiyabe above the Camp Terrill fault 

were interpreted by Hardyman and others (1992) to be due to 

closely spaced domino-style normal faults (not shown on fig. 

2) and block rotation associated with the formation of the 

Camp Terrill detachment fault.

MINERAL DEPOSITS

Base and precious metals in the Terrill Mountains have 

been mined from oxidized and unoxidized zones of veins 

contained within the fault zones. The oxidized veins have 

been mined mostly for silver and manganese in vertical 

northeast-striking veins in hypabyssal dacite at the range 

front near Camp Terrill (fig. 3). At higher elevations on the 

east flank of the range, both oxidized and unoxidized ore 

zones in ash-flow tuff were mined for silver and gold (fig. 3). 

In addition, gold and silver were mined from veins in some 

flat-lying faults in the upper Camp Terrill fault and in veins 

within broad altered areas in and near the hypabyssal dacite 

in the west (fig. 3).

Minerals found in oxidized ores include cerargyrite, 

cerussite, gold, rhodochrosite, psilomelane, and pyrolusite. 

Minerals in unoxidized ores are sphalerite, tetrahedrite, chalcopyrite, galena, argentite, arsenopyrite, pyrite, native silver, 

and electrum. Veins range from 1 cm to 1 m in width and 

contain plunging oreshoots approximately 25 by 100 m in 

size. Oxide ore grades of vein material are typically 200 to 

20,000 g/t silver and 5 to 10 percent manganese; the veins 

locally yield gold and lead ores. Ore grades of unoxidized 

vein material are typically 8 to 16 g/t silver, 7 to 50 g/t gold, 

8 to 11 percent lead, and 13 to 17 percent zinc. Veins and 

oreshoots are most common in the tuffs at the faulted margins of dikes or along the contacts of stocks, plugs, and apophyses of the hypabyssal dacite. There is a distinct spatial 

association of mineralized rock and hypabyssal dacite intrusive bodies; however, there is no evidence to support a direct 

genetic relationship between mineralization and the 

emplacement of the hypabyssal dacite.

Preliminary geochemical studies of mineralized samples from the Terrill Mountains demonstrate a consistent 

ratio of gold to silver, suggesting possible deposition of both 

these elements from a single hydrothermal system. The ratio 

of gold to silver in mineralized rock samples taken throughout the Terrill Mountains is about 1:100 (fig. 4). This constant ratio suggests that the mineralizing system was 

generated from a single source and that it could have 

involved a single hydrothermal solution and process. If a single process was involved, local populations of samples with 

higher or lower gold-silver ratios could have resulted from 

supergene processes (fig. 4). In addition, a significant

o

1

10,000

Z 1,000

| 100

c

" 10 f)

r

£ 1

z

0.1

0.01

EXPLANATION

O Dacite and gabbro

-1- Ash-flow tuff 

O Mineralized rocks

D Wad

X Altered rocks

D

DVD v,

D ^

DD £

X v'-' or ^ A Y /\

- O 0 °

0 XO

- Y O x

A A

1 1

s.

O O

o ^ D

O Q^f) Q

^<X v X a

3 O

"^

<W^

».

® Au determination limit=0.002 ppm

Ag determination limit=0.06 ppm

0.001 0.01 0.1 10 100

GOLD CONCENTRATION, IN PARTS PER MILLION

Figure 4. Plot of gold versus silver from rock samples collected 

as part of this study. The samples represent grab and channel samples of mineralized rock in and around the veins, faults, and altered rock areas in the Terrill Mountains. Gold was determined by 

standard graphite-furnace analysis; silver was determined was by 

optical spectroscopy. Chemical analyses conducted by P.H. 

Briggs, J.M. Motooka, B.M. Adrian, P.L. Hageman, D.M. Hopkins, D.L. Fey, A.H. Love, and L.A. Bradley.

number of samples have gold and silver values of 0.5 to 3 g/t, 

which may indicate that these metals are disseminated 

through a large volume of rock in the area. This suggests that 

a mineralization event may have taken place in addition to 

that which formed the high-grade veins.

Alteration proximal to the veins commonly consists of 

sericite-adularia-quartz (and only locally carbonate minerals). Pyrophyllite is present locally as an alteration mineral 

in mineralized zones along some shallow-dipping faults. 

These minerals typically replaced the groundmass and feldspar phenocrysts in hypabyssal dacite without destroying the 

rock texture. Alteration in the tuffs was more pervasive, and, 

in some areas, wallrock alteration widened a vein width outward from the center of the host fissure. This relation suggests that fluid flow was focused along faults and fractures 

and rarely invaded the surrounding rock. Pervasive mineralization that occurred along highly permeable epiclastic sedimentary rock horizons between ash-flow-tuff cooling units, 

usually but not always adjacent to the main mineralized 

faults, is a striking exception.

Broad, pervasively altered areas in the western 

hypabyssal dacite (fig. 3), contain alteration assemblages of 

kaolinite, illite, quartz, and albite. These alteration zones are 

distinct in size, style, and mineralogy compared to the sericite-quartz-adularia alteration envelopes around veins to the 

east, and these alteration zones occur in four, large, northwesterly elongated areas in the western part of the study area 

(fig. 3). Porosity and permeability appear to have been high 

in these areas. These altered areas may represent a younger 

mineralizing event because, locally, the adjacent hornblende-andesite (18.1 Ma) is altered.
32 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

The silver-gold veins and associated altered rocks are 

similar to Comstock-type veins and associated altered rocks 

described by Cox and Singer (1986). Distribution of the mineral deposits, uniformity in their gold to silver ratios, and 

similarity of associated altered rocks suggest that a hydrothermal system of the Comstock type was active over at least 

a 16-km2 area in the Terrill Mountains. The zones of broad, 

pervasive, hypabyssal-dacite-hosted altered rocks can be 

interpreted to represent distinct mineralization that represents either a different level of a single hydrothermal system 

or, possibly, a separate younger event.

CONCLUSIONS

The Terrill Mountains area of the Holy Cross mining 

district contains Comstock-type silver-gold veins that, for 

the most part, are hosted in fault-hosted veins. Northwestand north-northeast-striking high-angle faults within the 

Terrill Mountains structural block were the predominant 

conduits for flow of hydrothermal fluids. Low-angle detachment faults locally trapped hydrothermal fluids where adjacent to, or above, high-angle fault conduits. Field 

relationships suggest that most of the faulting occurred 

within a short time frame, linked perhaps to one major tectonic event. Mineralization was limited to specific lithologic 

units in a discrete area, probably because of structural constraints on fluid flow and a focused source of hydrothermal 

fluids.

Although not precisely constrained, the age of mineralization and alteration in the Terrill Mountains area must 

postdate the 21.8-Ma intrusive hypabyssal dacite, with 

which it is associated spatially, but it must predate unaltered, 

unmineralized 18.1-Ma hornblende-andesite lahars that cap 

the Terrill Mountains. These constraints suggest that the age 

of silver-gold mineralization in the Terrill Mountains is 

essentially the same (about 19 Ma) as that of precious metal 

deposits farther to the southeast, in the central Walker Lane 

(for example at Paradise Peak, and the Santa Fe district), 

where structural settings are similar. However, it appears 

that the age of mineralization in the Terrill Mountains is 

older than mineralization at Rawhide (15-16 Ma), which is 

the nearest significant precious metal deposit. A possible 

exception to this is mineralization associated with pervasive 

alteration in the west part of the Terrill Mountains, which 

postdates the 18.1-Ma hornblende-andesite lahars.

REFERENCES CITED

Bingler, B.C., 1978, Geologic map of the Schurz quadrangle: Nevada Bureau of Mines and Geology, Map 60,1 pi.

Black, J.E., Mancuso, T.K., and Gant, J.L., 1991, Geology and mineralization at the Rawhide Au-Ag deposit, Mineral County, 

Nevada, in Raines, G.L., Lisle, R.F., Schaefer, R.W., and 

Wilkinson, W.H., eds., Geology and Ore Deposits of the Great

Basin: Reno, Nevada, Geological Society of Nevada, Symposium Proceedings, v. 2, p. 1123-1144.

Bremm, G.F., Purdy, T.L., and Snyder, D.B., 1985, Oligocene and 

Miocene volcanic-tectonic history of the southern Toiyabe and 

Shoshone Mountains [abs.]: Geological Society of America 

Abstracts with Programs, v. 17, p. 343.

Bremm, G.F., John, D.A., Nash, J.T., Poole, F.G., and Snyder, 

D.B., 1991, Mineral resources of the Arc Dome Wilderness 

Recommendation Area, Nye County, Nevada: U.S. Geological 

Survey Bulletin 1961,21 p.

Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. 

Geological Survey Bulletin 1693, 379 p.

Dilles, J.H., John, D.A., and Hardyman, R.F., 1993, Evolution of 

Cenozoic magmatism and tectonism along a northeast-southwest transect across the northern Walker Lane, west-central 

Nevada Part I, in Lahren, M.M., Trexler, Jr., J.H., and Spinosa, C., eds., Crustal Evolution of the Great Basin and the Sierra Nevada: Reno, Nevada, Department of Geological 

Sciences, Mackay School of Mines, University of Nevada, 

Reno, Geological Society of America Field Trip Guidebook, 

Cordilleran/Rocky Mountain Section meeting, p. 409-428.

Hardyman, R.F., 1980, Geological map of the Gillis Canyon quadrangle, Mineral County, Nevada: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1237, scale 1:48,000.

Hardyman, R.F., McKee, E.H., Snee, L.W., and Whitebread, D.H., 

1992, The Camp Terrill and Dicalite Summit faults Two contrasting examples of detachment faults in the central Walker 

Lane, Western Nevada, in Craig, S.D., ed., Geological Society 

of Nevada, Proceedings Volume, Walker Lane Symposium, 

p. 93-113.

Hardyman, R.F., and Oldow, J.S., 1991, Tertiary tectonic framework and Cenozoic history of the central Walker Lane, Nevada, in Raines, G.L., Lisle, R.F., Schafer, R.W., and Wilkinson, 

W.H., eds., Geology and Ore Deposits of the Great Basin: 

Reno, Nevada, Geological Society of Nevada, Symposium 

Proceedings, v. 1, p. 279-301.

Lincoln, F.C., 1923, Mining Districts and Mineral Resources of Nevada: Reno, Nevada, Nevada Newsletter Publishing Company, 295 p.

Oldow, J.S., Bally, A.W., Ave 'Lallemant, H.G., and Leeman, 

W.P., 1989, Phanerozoic evolution of the North American Cordillera United States and Canada, in Bally, A.W., and Palmer, A.R., eds, The Geology of North America An Overview: 

Boulder, Colorado, Geological Society of America, v. A, 

p. 139-232.

Ross, D.C., 1961, Geology and mineral deposits of Mineral County, Nevada: Nevada Bureau of Mines and Geology Bulletin 58, 

98 p.

Schraeder, F.C., 1947, Carson Sink area, Nevada: U.S. Geological 

Survey Open-File Report, 15 p.

Silberman, M.L., and McKee, E.H., 1972, A summary of radiometric age determinations on Tertiary volcanic rocks from Nevada 

and eastern California Part II, western Nevada: Isochron/ 

West, no. 4, p. 7-28.

Vanderburg, W.O., 1940, Reconnaissance of mining districts in 

Churchill County, Nevada: United States Bureau Mines, Information Circular 7093, p. 30-37.

Willden, R., and Speed, R.C., 1974, Geology and mineral deposits 

of Churchill County, Nevada: Nevada Bureau of Mines and 

Geology Bulletin 83,95 p.
CHAPTER E

TIME-pH-f02 PATHS OF HYDROTHERMAL FLUIDS 

AND THE ORIGIN OF QUARTZ-ALUNITE-GOLD DEPOSITS

By George V. Albino 1

ABSTRACT

Quartz-alunite-gold, massive enargite-pyrite, and porphyry copper-gold deposits constitute a related group of 

deposits associated with magmatic-hydrothermal systems. 

Alteration assemblages in porphyry deposits indicate mineralization by near-neutral fluids with elevated fQ2 (fugacity of 

oxygen) and low fg2 (fugacity of sulfur); those in enargitepyrite deposits show elevated fO2 and f§2 associated with 

very low pH; whereas gold-alunite-pyrite deposits were 

deposited from fluids with moderate pH, fS2, and fOr Copper-to-gold ratios and silver-to-gold ratios have similar 

ranges in porphyry and enargite-pyrite, whereas quartzalunite-gold deposits have much higher gold contents relative to other metals. Where several deposit types are found 

together, they apparently formed in the order porphyry copper, enargite-pyrite, quartz-alunite-gold. Spatial relations 

mimic temporal ones, with porphyry deposits formed deepest and quartz-alunite-gold deposits shallowest.

Time relations between different deposit types, combined with the progression of ore-fluid compositions, are 

consistent with high-grade quartz-alunite-gold deposits having formed by remobilization of gold, in the form of a bisulfide complex, from earlier formed copper-gold 

mineralization in which both copper and gold were transported as chloride complexes. The near-neutral, relatively 

reduced fluids responsible for deposition of the high-grade 

gold ores may have formed as a result of incorporation of 

meteoric or formation waters into a previously magmaticwater-dominated hydrothermal system.

INTRODUCTION

Temporal and spatial variations in compositions of oreforming fluids have been documented by many workers

'U.S. Geological Survey Mission, Unit 62101, APO AE 09811-2101.

from a variety of different types of ore deposits (Gustafson 

and Hunt, 1975; Bartos, 1989; Large and others, 1989). The 

formation of alteration and vein assemblages indicative of 

fluids with low pH and elevated fO2 and (or) fS2 is a common 

feature in a variety of precious- and base-metal ores. These 

include deposits considered to be epithermal (Ashley, 1990; 

John and others, 1989) as well as those inferred to have 

formed at relatively high temperatures (Sillitoe, 1983; Brimhall, 1980). In many cases, alteration by acid fluids occurs 

late in the history of hydrothermal activity (Gustafson and 

Hunt, 1975), whereas, in others, acid and oxidized fluids 

have been succeeded by those with higher pH and lower oxidation state (Beane and others, 1991). The origin of these 

acid fluids is a matter of debate, with a strong component of 

direct magmatic addition likely in at least some deposits. In 

a recent paper, Bartos (1989) proposed that two distinct evolutionary paths can be defined for magmatic-hydrothermal 

systems. In his terminology, where fluid pH decreases, and 

fO2 and f§2 increase with time, the system is said to follow a 

"prograding" path. For the opposite situation, in which later 

fluids are less acid and have lower fO2 and fs2, the evolution 

is defined as "retrograde." Note that this usage differs from 

that of Hemley and Hunt (1992), who use "prograde" and 

"retrograde" principally in a thermal sense. Bartos was concerned mainly with base-metal-enriched deposits, but the 

same ideas may have important implications in discussions 

of genesis of gold-silver deposits. This is true especially for 

acid-sulfate (Heald and others, 1987) or high-sulfur-type 

(Bonham, 1988) deposits, mainly enargite-bearing coppergold deposits, such as Goldfield (Ransome, 1909; Ashley, 

1974,1990; Tolman and Ambrose, 1934) or El Indio (Siddeley and Araneda, 1986; Jannas and others, 1990). Processes 

controlling formation of silver- and lead-rich acid-sulfate 

type deposits, such as Paradise Peak, Nevada (John and others, 1989), or La Coipa, Chile (Oviedo and others, 1991), 

may be different.

In this paper I would like to suggest that high-grade 

quartz-alunite-gold vein deposits form late in the

33
34 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Table 1. Districts containing Au-alunite veins and Au-rich, massive, enargite-pyrite deposits. 

[M, million, K, thousand]

Deposit Production/reserve Reference

A Quartz-alunite-gold veins

El Indio Chile

Goldfield, Nevada

Masonic, California 

Rodalquilar, Spain

Summitville, Colorado 

Chinkuashih, Taiwan

Nalesbitan, Philippines 

Wild Dog, Papua New Guinea 

Mt. Kasi, Fiji

Production (to 1986) 139,330 tons @ 225 g/t Au, 

104 g/t Ag, 2.4% Cu; Reserves (1986) 42,000 tons 

@ 197 g/t Au, 124 g/t Ag, 4.44% Cu

Production 4.19 M oz Au, 

1.45MozAu,7.67MlbCu

Production ca. 100 K oz Au

Production (to 1966) ca. 200 K oz Au; 

Reserves (1990) ca. 650 K tons @ 2.5 g/t Au

Production (to 1949) ca. 250 K oz Au, 

300KozAg, 300KlbCu

Chiufen area (high Au) prod. 800 k oz Au (official 

estimate may be much greater), Penshan (mixed 

high-Au and Cu-Au) 2 M oz Au, 120 M tons Cu 

(official estimate)

Ca. 150-300 oz Au, Cu in unoxidized zones < 0.3% 

No reported production or reserves 

Production (to 1946) 260 K tons @ 7.6 g/t Au

Siddeley and Araneda, 1986

Ashley, 1990

Johnson, 1951

Arribas and others, 1989; Rytuba and 

others, 1990; Hernandez and others, 

1989

Steven and Ratte, 1960; Stoffregen, 

1987

Tan, 1991;Huang, 1955,1960,1963

Sillitoe and others, 1990

Lindley, 1987

Taylor, 1987______

B Enargite-pyrite deposits

El Indio, Chile

Lepanto, Philippines 

Bor, Yugoslavia

Tintic, Utah

Chinkuashih, Taiwan 

Recsk, Hungary

Production (to 1986) 2.1 M tons @ 10-12 g/t Au, 

60-70 g/t Ag, 3.2% Cu, reserves (1986) 5.3 M tons 

@ 8.3 g/t Au, 101 g/t Ag, 4.77% Cu

27 M tons 1.5% Cu, 3.5 g/t Au

Ca. 90 M tons (several bodies) 1-2% Cu, up to 

3.5g/tAu

Production (to 1965) 2.15 M oz Au, 197 M oz Ag, 

220 M Ib Cu, 1.3 G Ib Pb, 138 M Ib Zn Note: this 

includes Cu-Au-Ag and Pb-Zn-Ag ores together

Penshan (mixed high-Au and Cu-Au) 2 M oz Au, 

120 M tons Cu (official estimate)

2 M tons 1-2% Cu, up to 3.5 g/t Au

Siddeley and Araneda, 1986

Sillitoe, 1983, oral commun., 1992

Representative grade quoted in 

Sillitoe, 1983

Morris, 1968

Tan, 1991

Representative grade quoted in 

Sillitoe, 1983

mineralization history of a plu ton-centered hyckothermal system, under conditions of moderate pH, f^, and fS2, following 

a period of acid leaching (i.e., they are "retrograde" in Bartos' 

(1989) terminology). The association of high-grade gold with 

these assemblages is a consequence of the high solubility of 

gold (as compared to copper) as gold-bisulfide complexes in 

moderately reduced, near-neutral solutions, which can result 

in large-scale remobilization of gold from previously formed 

copper-gold mineralization. Note that the two-stage process 

proposed below for pluton-related deposits is very similar to 

that documented by Large and others (1989) for the formation 

of late-stage, gold-enriched zones in volcanogenic massive 

sulfide deposits.

QUARTZ-ALUNITE-GOLD DEPOSITS

GENERAL CHARACTERISTICS

Quartz-alunite-gold (copper-silver) veins constitute a 

distinct class of ore deposits (Berger, 1986). Although relatively few deposits of this type are known, they have been 

important sources of gold production from a number of districts (table 1). They occur in some districts with temporally 

and spatially related massive enargite-pyrite mineralization 

of the type described below (El Indio, Chile Siddeley and 

Araneda, 1986; Chinkuashih, Taiwan Tan, 1991), and, in 

other districts, they occur without such associated deposits
TIME-pH-f^ PATHS OF HYDROTHERMAL FLUIDS AND ORIGIN OF QUARTZ-ALUNITE-GOLD DEPOSITS 35

(Masonic district, California Johnson, 1951). In some 

areas (Steven and Ratte*, 1960), deeper porphyry-style mineralization has been reported.

The characteristics of these deposits are somewhat variable, but important features in common include their association with mainly andesite, dacite, or quartz-latite tuffs, 

flows, and minor intrusions (Ashley and Silberman, 1976; 

Rytuba and others, 1989,1990; Siddeley andAraneda, 1986; 

Tan, 1991; Johnson, 1951). In all districts, there are large 

volumes (at least several cubic kilometers) of rocks that have 

been intensely altered to clays, quartz, and typically abundant alunite (Arribas and others, 1989; Johnson, 1951; Ashley, 1990). Within these large altered areas are small bodies 

of extremely high grade gold ores. In districts where copperpredominant deposits occur with quartz-alunite-gold veins, 

the gold-predominant deposits invariably formed later than 

the copper-rich deposits (Siddeley and Araneda, 1986; Jannas and others, 1990) and tend to occur above and (or) 

peripheral to the center of the copper-gold mineralization 

(Jannas and others, 1990; Arribas and others, 1989; Huang 

and Yeh, 1977). Early-formed low-grade disseminated gold 

mineralization (accompanied by copper sulfides) is reported 

in some districts (Stoffregen, 1987; Tan, 1991).

A further style of gold mineralization accompanying 

some quartz-alunite-gold deposits is typified by the El 

Tambo deposit, located approximately 5 km from El Indio, 

Chile (Siddeley and Araneda, 1986). Here, gold occurs 

mainly associated with largely sulfide-free barite-alunite 

veins that contain jarosite, interpreted as primary in origin 

(Beane and others, 1991). Apparently this style of mineralization lies above and in sharp contact with deeper enargitebearing veins. Similar very oxidized gold mineralization has 

also been described from Summitville (Stoffregen, 1987) 

and Goldfield (Keith and others, 1980; R.P. Ashley, oral 

commun., 1992).

METAL CONTENT OF 

QUARTZ-ALUNITE-GOLD VEINS

Metal ratios in high-grade gold ores are distinct from 

those in related deposit types, such as porphyry copper and 

massive enargite-pyrite veins. They are characterized by low 

Ag-to-Au ratios, typically less than 1 (table 1, part A), and 

Cu-to-Au ratios commonly between 103 and 102, and locally 

lower (see, for example, Johnson, 1951; Siddeley and 

Araneda, 1986; Vikre, 1989; Mosier and Menzie, 1986).

INTRUSION-RELATED 

COPPER-GOLD DEPOSITS

GENERAL CHARACTERISTICS

Gold and copper occur together in a suite of genetically 

related deposit types that includes porphyry copper-gold

EXPLANATION

Opal-cristobalitealunite-sulfur

Advanced argillic 

Phyllic

Propylitic 

Potassic 

Igneous intrusion

Figure 1. Generalized cross-section showing alteration zones 

developed in association with porphyry copper emplacement. 

Mineral assemblages characteristic of alteration zones are given 

in table 2. Modified from Lowell and Guilbert (1970), Gustafson 

and Hunt (1975), SiUitoe (1973).

deposits, and massive enargite-pyrite veins and replacements, in addition to the high-grade enargite- or tennantitebearing quartz-alunite-gold veins described above (Sillitoe, 

1988, 1989). In porphyry deposits there is a well-documented spatial pattern of alteration and ore and gangue minerals (fig. 1, table 2), in which deeper and more central zones 

have assemblages indicative of neutral to alkaline fluids with 

low f§2 and relatively high f^ (Rose and Burt, 1979; Beane 

and Titley, 1981). Peripheral to many porphyry deposits, and 

particularly at high levels, alteration and ore-mineral assemblages indicate that mineralizing fluids had higher H+ activities and f§2 (Gustafson and Hunt, 1975; Meyer and Hemley, 

1967). These conditions are indicated by phyllic, argillic, 

and advanced argillic zones (Meyer and Hemley, 1967). In 

the latter, copper sulfosalts are common, particularly those
36 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Table 2. Alteration zones and assemblages in porphyry copper deposits.

[Zone names after Lowell and Guilbert, 1970. Data from Lowell and Gnilbert, 1970; Beane, 1982; Beane and Titley, 1981; Sillitoe, 1990]

Zone

Potassic

Propylitic

Phyllic

Intermediate argillic

Advanced argillic

Silicate mineralogy

Orthoclase, quartz, phlogopite 

(amphibole, salite, chlorite)

Quartz, albite, chlorite, 

epidote (zeolites)

Quartz, muscovite

Quartz, albite, illite, 

smectite, chlorite

Quartz, pyrophyllite, kaolinite

Sulfides/oxides

Bornite ± chalcopyrite, magnetite ± 

hematite

Pyrite, hematite

Pyrite (chalcopyrite)

Pyrite (chalcopyrite)

Pyrite, enargite/luzonite, tennantite,

Sulfates/carbonates

Anhydrite

Calcite, dolomite

None

None

Alunite, Na-alunite;

covellite, chalcocite, diaspore, corundum Pb, Ca, Ba, Sr-Al 

sulfates and phosphates

Table 3. Comparative characteristics of different magmatic-related Cu-Au deposits.

[Mod, moderate; f(^, fugacity of oxygen; fs , fugacity of sulfur]

Porphyry copper deposits Massive enargite-pyrite deposits Quartz-alunite-gold veins

Alteration 

Opaque mineralogy

Cu/Au

Ag/Au

pH

Fugacities

Examples

K-feldspar, phlogopite, anhydrite

Bornite-chalcopyrite-magnetite 

(± hematite)

104 to>105

5:1 to 20:1

Alkaline/neutral

Mod/high fo2; low/very low fS2

Grasberg, Man Jaya; 

Bingham, Utah

Quartz, alunite, dickite/kaolinite, 

diaspore, pyrophyllite

Enargite, covellite, chalcocite, 

pyrite

1035-105 

5:1 to 20:1 

Very acid 

Very high fS2/fo2

Lepanto, Philippines; 

El Indio, Chile

Kaolinite, sericite, pyrophyllite

Tennantite, chalcopyrite, pyrite, 

sphalerite

< 102:1

<1:1

Acid to near neutral

Moderate to high fb2/fS2

Goldfield, Nevada; 

El Indio, Chile

belonging to the enargite-luzonite and tennantite-tetrahedrite 

solid solutions. These phases, as well as assemblages where 

the copper sulfides chalcocite and covellite coexist with 

pyrite, indicate crystallization at elevated f§2 (Barton and 

Skinner, 1979). These sulfur-rich ore-mineral assemblages 

may occur either as widespread disseminations or veinlet 

fillings in the lithocap above porphyry copper deposits 

(Corn, 1975; Bodnar and Beane, 1980), or, in some instances 

(Sillitoe, 1983), they form massive fracture and replacement 

enargite-pyrite deposits (table 3). A number of these deposits 

with high-sulfidation mineral assemblages have been important sources of gold as well as copper (table 1, part B). These 

deposits, in general, are accompanied by abundant primary 

hypogene alunite, indicating low pH and high f^ in addition 

to a high sulfidation state. The near-magmatic values of S^S 

seen in massive alunite interbanded with enargite at El Indio

(Jannas and others, 1990) suggest that nearly quantitative 

oxidation of magmatic sulfur may occur episodically in these 

systems under hypogene conditions.

METAL CONTENTS OF 

COPPER-GOLD DEPOSITS

The range of Cu:Au in porphyry deposits (Cox and 

Singer, 1988; Sillitoe, 1990) is from approximately 106 to 

104 (the latter corresponds to 1 g/t gold at a copper grade of 

1 percent). In massive enargite deposits, the range is similar 

(Morris, 1968; Siddeley and Araneda, 1986; Sillitoe, 1983), 

although more deposits may be somewhat gold rich (Sillitoe, 

1983) (table 3). Silver-to-gold ratio data are not available for 

some districts (for example, deep copper-sulfide-rich ores at 

Goldfield), but this ratio is apparently typically in excess of
TIME-pH-f^ PATHS OF HYDROTHERMAL FLUIDS AND ORIGIN OF QUARTZ-ALUNTTE-GOLD DEPOSITS 37

10:1 (table 3). Note that these ratios are quite similar to those 

estimated for the crust as a whole and for common igneous 

rocks (Krauskopf, 1979).

PARAGENETIC RELATIONSHIPS

BETWEEN QUARTZ-ALUNITE-GOLD

AND ASSOCIATED COPPER-GOLD

DEPOSITS

Changes in mineral assemblages with time are reported 

in well-studied deposits. At El Indio, for example, copperstage veins contain enargite, whereas later transitional and 

gold-stage veins contain tennantite and, locally, chalcopyrite 

(Jannas and others, 1991). An alunite-kaolinite assemblage 

is associated with the early copper-stage veins, and a sericite-kaolinite±pyrophyllite assemblage accompanies highgrade gold-stage veins. At Summitville, pre-ore alunite has 

been replaced by kaolinite (Stoffregen, 1987). Although 

unambiguous paragenetic data are not reported from other 

districts, workers studying other quartz-alunite-gold deposits (Arribas and others, 1989; Wang, 1973; Huang, 1955; 

Johnson, 1951) commonly report that deposition of enargiteluzonite and intense acid leaching and alunitization predate 

high-grade gold mineralization. Note that similar relationships have been described by other workers (Berger and 

Henley, 1989; Beane and others, 1991, Sillitoe, in press), 

wherein high-grade gold ore was deposited from fluids that 

do not fit the classic conception of "acid-sulfate" deposits.

TRANSPORT OF GOLD AND COPPER 

IN PORPHYRY COPPER, ENARGITEPYRITE, AND QUARTZ-ALUNITEGOLD DEPOSITS

The deposits discussed above* namely porphyry copper, massive enargite-pyrite, and quartz-alunite-gold veins 

(table 3), can be separated on the basis of their Ag-to-Au 

ratios and more particularly their Cu-to-Au ratios (cf., Cox 

and Singer, 1988). The similar ratios of copper and gold in 

porphyry and massive enargite-pyrite deposits and the sharp 

contrast between their relative abundances in quartz-alunitegold veins suggests that copper and gold transport and deposition occur by similar mechanisms in porphyry and enargite-pyrite deposits but that gold and copper are somehow 

separated in the formation of quartz-alunite-gold veins. 

Under the near-magmatic conditions typical of porphyry and 

related deposits, copper is transported as a chloride complex, 

most likely CuCl (Barnes, 1979). Gold, on the other hand, 

may be transported as either a bisulfide complex, most likely 

the neutral species HAu(HS)2 (Hayashi and Ohmoto, 1991), 

or as the chloride complex AuCl2~ (Seward, 1984). Recent 

experimental work (Hayashi and Ohmoto, 1991) has shown

Figure 2. Log f^ versus pH diagram illustrating gold solubility 

(shown in parts per billion) as Cl~ and HS~ complexes (AuCl2~ and 

HAu(HS)2, respectively). Note the dominance of gold complexed 

with Cl~ under acid and particularly oxidizing conditions, as compared to gold complexed with HS~ under relatively reduced conditions (from Hayashi and Ohmoto, 1991). At higher temperatures 

the solubility of AuCl2~ is substantially increased, whereas that for 

Au-bisulflde complexes is little changed. Heavy lines indicate 

boundaries of stability fields for solid mineral species. Dashed lines 

indicate boundaries of stability fields for aqueous species, hm, 

hematite; mt, magnetite; bn, bornite; py, pyrite; cp, chalcopyrite; 

po, pyrrhotite.

that, at temperatures between 250°C and 350°C, under most 

conditions where either pyrite or pyrrhotite is stable, bisulfide complexes predominate (fig. 2). Conditions under 

which chloride complexes are important in gold transport 

(fig. 2) are those in which pH is low and reduced sulfur is not 

abundant, either because it is subordinate to oxidized sulfur 

(i.e., an oxidized fluid) or because the total sulfur content is 

small. Highly saline fluids will also be more likely to carry 

gold as a chloride complex, both because of the increased 

complexing ability of Cl~ and because of pH effects (Rose 

and Burt, 1979). Note that the behavior in solution of silver 

is intermediate between those of copper and gold (for example, CI~ complexes probably play a dominant role in transport (Seward, 1976)) but, under some circumstances HS~ 

complexes are also important (Gammons and Barnes, 1989). 

This is in agreement with the Ag-to-Au ratio and Cu-to-Au 

ratio data because the variation in Ag-to-Au ratio between 

deposit types is less extreme than that of Cu-to-Au.
38 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Conditions of formation of porphyry copper deposits 

are indicated by alteration and ore mineral assemblages 

found in them. In the potassic zone (table 2), assemblages 

include bornite-rnagnetite and magnetite-orthoclase-phlogopite. The occurrence of hematite in some deposits, along 

with the common occurrence of anhydrite, indicates that the 

fluids were oxidized, with oxidation state close to that of the 

hematite-magnetite f^ buffer. This is consistent with the 

presence of magnetite+bornite in many (particularly goldrich) porphyry copper deposits (Sillitoe, 1979), which also 

indicates low fluid GH2S (activity of H2S). These conditions 

correspond to conditions under which, according to Hayashi 

and Ohmoto (1991), gold will be principally carried as 

AuCl2~. The stability of the AuCl2~ complex is much greater 

at high temperature, such that cooling will be an effective 

means of precipitating gold (Hayashi and Ohmoto, 1991, 

table 5). This is in accord with the situation in many goldrich porphyry copper deposits where gold is proportionately 

much higher in concentration (with respect to copper and silver) in the central part of the deposit (Cuddy and Kesler, 

1982; Sillitoe, 1990; Langton and Williams, 1982; Einaudi,

1990).

In highly acid fluids responsible for forming massive 

enargite-pyrite bodies, low pH and very oxidized conditions 

must also result in transport of gold as a chloride complex 

(Hayashi and Ohmoto, 1991). The similarity of Cu-to-Au 

ratios in these deposits to those in gold-rich porphyry copper 

deposits (and again, in turn, with likely compositions of 

parental magmas) is therefore a reflection of common processes operating in different geochemical environments.

In contrast, the much lower Cu-to-Au ratios typical of 

high grade quartz-alunite-gold veins probably indicates that 

some mechanism has operated that has served to separate 

gold and copper. This may involve either selective transportation or selective precipitation of gold, with selective transport of gold considered the more reasonable cause, given the 

absence of distal copper-rich zones that would exist if formation of these ores occurred via selective precipitation of gold. 

Gold-rich, relatively copper poor fluids almost certainly 

involve transport of gold as a bisulfide complex (Seward,

1991), which is consistent with the character of alteration 

assemblages associated with quartz-alunite-gold veins. A 

relatively low salinity is required to account for the low copper grades seen in the gold stage in most examples, in accord 

with fluid-inclusion data (Jannas and others, 1990).

REMOBILIZATION OF GOLD FROM

EARLIER FORMED COPPER-GOLD

MINERALIZATION

Quartz-alunite-gold vein deposits invariably are found 

within large areas of intensely argillized rock, many of 

which show some evidence of earlier formed mineralization 

with much higher Cu-to-Au and Ag-to-Au ratios. This

pattern, where main-stage gold deposition follows acid 

leaching and copper-gold mineralization, is consistent with 

formation of high-grade gold deposits by remobilization 

from preexisting, lower grade metal concentrations. The relative solubilities of gold and copper in reduced and nearneutral, as compared to oxidized and acid, fluids would favor 

preferential leaching of gold (in comparison to copper) in the 

former, and of copper (in comparison to gold) in the latter.

The situation documented in the Butte district, Montana, is contrary in terms of mobilized components but is 

conceptually similar to that proposed here: high-grade copper mineralization was formed from remobilization of lowgrade precursor mineralization. At Butte, the high-grade 

copper sulfosalt ores with advanced argillic alteration selvages cross-cut earlier formed porphyry mineralization that 

formed under K-silicate-stable conditions. Because copper 

minerals are volumetrically significant constituents of the 

early porphyry ores at Butte, Brimhall (1979,1980) was able 

to convincingly document a history of copper leaching and 

reconcentration with compositional and textural data. Note 

that Butte fits the "prograding" sequence of Bartos (1989), 

and, given the increased solubilities of copper (and other 

Cr-complexed metals, including gold under appropriate circumstances) in acid, oxidized fluids, (nonselective) remobilization of base and precious metals during hypogene acidleaching is an expected consequence of the hydrothermal 

history of the district.

The temporal relations between acid and near-neutral 

fluids in a retrograding sequence, such as described above 

for El Indio and Summitville, will not result in large-scale 

remobilization of copper. It may, however, cause selective 

mobilization of gold as a bisulfide species, resulting in formation of the high-grade ores typical of this deposit class.

ORIGIN OF LATE-STAGE NEARNEUTRAL REDUCED FLUIDS

Although quartz-alunite-gold deposits have ore-stage 

ore-mineral and associated alteration assemblages that are 

generally similar to those in adularia-sericite-type deposits, 

the deposit geochemistry (i.e., high concentrations of bismuth and tin Tolman and Ambrose, 1934; Johnson, 1951; 

Vikre, 1989; low Ag-to-Au ratio table 3) is different than 

most adularia-sericite deposits. This may suggest different 

mechanisms for the generation of ore-forming fluids in 

quartz-alunite-gold deposits.

A mechanism for generating these (relatively) reduced 

fluids is mixing of acidic, magmatic-derived waters with 

dilute, surface-derived or connate waters that were heated 

and reduced through interaction with country rocks. The 

large areas of propylitized rocks associated with argillization 

in quartz-alunite-gold districts have interacted with considerable volumes of meteoric and (or) formation water. The 

mineralogical changes associated with propylitization
I Tambo 

Barite-gold jGoldfield

El Indio

TIME-pH-f^ PATHS OF HYDROTHERMAL FLUIDS AND ORIGIN OF QUARTZ-ALUNITE-GOLD DEPOSITS 39

(namely formation of epidote, Fe-oxides, carbonates, and 

chlorite) comprise a set of reactions that result in oxidation 

of the original country rocks. This must lead to complementary reduction of the circulating fluid. An influx of such a 

reduced, Cl~-poor fluid into a system previously dominated 

by acid waters and copper-gold ore deposition could lead to 

reconcentration of gold into high-grade orebodies. The 

involvement of such formation waters or highly exchanged 

meteoric waters in porphyry copper systems has been documented in several areas by oxyjen and hydrogen isotope 

studies (Norman and others, 1991; Bowman and others, 

1987).

Quartz-alunite-gold

Massive 

enargite-pyrite

Porphyry copper-gold 

(K-silicate alteration)

Lepanto

RELATIONSHIP BETWEEN 

PORPHYRY COPPER, ENARGITEPYRITE AND QUARTZ-ALUNITEGOLD DEPOSITS

Figure 3. Schematic reconstructed section through mineralized 

system with porphyry Cu and related massive enargite-pyrite and 

quartz-alunite-gold deposits. Note change upward from porphyry 

copper to massive enargite-pyrite, quartz-alunite-gold veins (R. 

Araneda, oral commun., 1992), and barite-jarosite vein-hosted Au 

deposits (Beane and others, 1991). Shown at right are approximate vertical ranges seen in selected districts.

A general spatial and temporal framework for the 

development of these related ore types emerges from examination of districts where the different deposit types occur 

together (table 3). The main points of this model are that the 

sequence "porphyry copper -» enargite-pyrite massive sulfide (copper and gold bearing) > quartz-alunite-gold vein" 

represents different depths within the overall system, from 

deepest to shallowest (fig. 3). Quantitative estimates of depth 

of mineralization for any of these deposit types are scarce, 

but Vikre (1989) estimates a depth during mineralization at 

Goldfield of "a few tens of feet to a few hundreds of feet." 

The occurrence of (apparently) primary jarosite-alunite-barite assemblages at Tambo (Beane and others, 1991) likely 

represents an extremely shallow level where downwardmoving, oxidized surface waters mixed with ascending, 

moderately acid fluids. Such a process, which has been documented in more typical hot-spring deposits (Schoen and 

others, 1974), leads to local, extremely acid conditions and 

formation of these unusual mineral associations, as well as 

quantitative precipitation of any gold remaining in solution.

In terms of temporal evolution, field relations in several 

districts demonstrate that quartz-alunite-gold mineralization 

postdates deposition of enargite-rich ores (Jannas and others, 

1990). The relationship between porphyry-style (K-silicatestable) mineralization and massive enargite-pyrite deposits 

is less certain, but evidence from Lepanto (Sillitoe, 1983, 

1990) and other deposits (R.H. Sillitoe, written commun., 

1992) indicates that enargite-pyrite postdates porphyry-style 

stockwork mineralization. This is in accord with well-documented temporal relations at El Salvador (Gustafson and 

Hunt, 1975), where advanced argillic assemblages are superimposed on previously formed potassic alteration. The existence of an advanced argillic-enargite-pyrite association, 

manifested either as massive enargite-pyrite deposits or as an 

early copper-rich stage in gold veins, seems to be a

prerequisite for formation of high-grade quartz-alunite-gold 

deposits, possibly because well-developed structures are 

necessary for both deposit types.

SUMMARY

High-grade quartz-alunite-gold deposits are part of a 

family of magmatic-hydrothermal copper-gold deposits that 

also includes porphyry copper and massive enargite-pyrite 

deposits. Porphyry copper deposits generally formed at 

greatest depth, succeeded upward by enargite-pyrite and 

quartz-alunite-gold veins. The temporal relations between 

the different deposit types appears to mimic their spatial 

relations, with porphyry mineralization earliest, followed by 

massive enargite-pyrite deposits and finally quartz-alunitegold deposits. The large difference in Cu-to-Au ratios 

between quartz-alunite-gold and other deposit types indicates that some mechanism has concentrated, and presumably therefore leached and transported, gold preferentially to 

copper and silver. Gold solubility is high relative to that of 

copper in reduced, near-neutral fluids. Such fluids may result 

from a late influx of meteoric water that reacted with heated 

country rocks. A fluid of this type would be capable of leaching gold from earlier formed copper-gold concentrations 

this is conceptually similar to the mechanism for concentration of copper at Butte.

The observed and inferred history of alteration and fluid 

chemistry indicates that formation of quartz-alunite-gold 

deposits is related to the retrograde phase of evolution in systems that have initially followed a prograding path. Not all 

porphyry copper systems show prograde evolution, and 

probably fewer still had both well-developed prograde and 

retrograde stages. This may serve to explain the relative
40 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

scarcity of deposits of this type, in contrast to the abundance 

of porphyry copper deposits.

Acknowledgments. This paper is based in part on 

work done while employed by Corona Gold Inc., and support for research efforts, particularly by Chris Jennings, 

James Stewart, and Mike Fianacca, is gratefully acknowledged. Discussions with many geologists and comments on 

various versions of this manuscript by Roger Ashley, Barney 

Berger, Hal Bonham, Jeff Doebrich, Dick Sillitoe, and James 

Stewart helped clarify both content and presentation.

REFERENCES CITED

Arribas, A., Jr., Rytuba, J.J., Rye, R.O., Cunningham, C.G., Podwysocki, M.H., Kelly, W.C., Arribas, A., Sr., McKee, E.H., and 

Smith, J.G., 1989, Preliminary study of the ore deposits and 

hydrothermal alteration in the Rodalquilar caldera complex, 

southeastern Spain: U.S. Geological Survey Open-File Report 

89-327, 39 p.

Ashley, R.P., 1974, Goldfield mining district, in Guidebook to the 

Geology of Four Tertiary Volcanic Centers in Central Nevada: 

Nevada Bureau of Mines and Geology Report 19, p. 49-66.

 1990, The Goldfield gold district, Esmeralda and Nye 

Counties, Nevada, in Epithermal Gold Deposits Part I: U.S. 

Geological Survey Bulletin 1857-H, p. H1-H7.

Ashley, R.P., and Silberman, M.L., 1976, Direct dating of mineralization at Goldfield, Nevada, by potassium-argon and fissiontrack methods: Economic Geology, v. 71, p. 904-924.

Barnes, H.L., 1979, Solubilities of ore minerals, in Barnes, H.L., 

ed., Geochemistry of Hydrothermal Ore Deposits: New York, 

John Wiley and Sons, p. 404-460.

Barton, P.B., Jr., and Skinner, B.J., 1979, Sulfide mineral stabilities, in Barnes, H.L., ed., Geochemistry of Hydrothermal Ore 

Deposits: New York, John Wiley and Sons, p. 278-403.

Bartos, P.J., 1989, Prograde and retrograde base metal lode deposits 

and their relationship to underlying porphyry copper deposits: 

Economic Geology, v. 84, p. 1671-1683.

Beane, R.E., 1982, Hydrothermal alteration in silicate rocks: Southwestern North America, in Titley, S.R., ed., Advances in Geology of the Porphyry Copper Deposits, Southwestern North 

America: Tucson, University of Arizona Press, p. 117-138.

Beane, R.E., Brosnahan, David, Ahler, B., and Jannas, R., 1991, 

Oxidation of a gold-bearing quartz-sericite-pyrite solution 

Calculations and an example from the El Indio district, Chile 

[abs.]: Geological Society of America, Program with Abstracts, v. 23, p. A464.

Beane, R.E., and Titley, S.R., 1981, Porphyry copper deposits Part JJ. Hydrothermal alteration and mineralization, in 

Skinner, B.J., ed., Economic Geology, 75th Anniversary Volume, p. 235-269.

Berger, B.R., 1986, Descriptive model of epithermal quartz-alunite 

Au, in Cox, D.P., and Singer, D.A., eds., Mineral Deposit 

Models: U.S. Geological Survey Bulletin 1693, p. 158.

Berger, B.R., and Henley, R.W., 1989, Advances in the understanding of epithermal gold-silver deposits, with special reference to 

the Western United States, in Keays, R.R., Ramsay, W.R.H., 

and Groves, D.I., eds., The Geology of Gold Deposits The

Perspective in 1988: Economic Geology Monograph 6, 

p. 405^23.

Bodnar, R.J., and Beane, R.E., 1980, Temporal and spatial variations in hydrothermal fluid characteristics during vein filling in 

preore cover overlying deeply buried porphyry copper-type 

mineralization at Red Mountain, Arizona: Economic Geology, 

v. 75, p. 876-893.

Bonham, H.F., Jr., 1988, Models for volcanic-hosted epithermal 

precious metal deposits, in Schafer, R.W., Cooper, J.J., and 

Vikre, P.O., eds., Bulk Mineable Precious Metal Deposits of 

the Western United States: Geological Society of Nevada, 

p. 259-271.

Bowman, J.R., Parry, W.T., Kropp, W.P., and Kruer, S.A., 1987, 

Chemical and isotopic evolution of hydrothermal solutions at 

Bingham, Utah: Economic Geology, v. 82, p. 395-428.

Brimhall, G.H., Jr., 1979, Lithologic determination of mass transfer 

mechanisms of multiple-stage porphyry copper mineralization 

at Butte, Montana Vein formation by hypogene leaching and 

enrichment of potassium-silicate protore: Economic Geology, 

v. 74, p. 556-589.

 1980, Deep hypogene oxidation of porphyry copper potassium-silicate protore at Butte, Montana A theoretical evaluation of the copper remobilization hypothesis: Economic 

Geology, v. 75, p. 384-409.

Corn, R.M., 1975, Alteration-mineralization zoning at Red Mountain, Arizona: Economic Geology, v. 70, p. 1437-1447.

Cox, D.P., and Singer, D.A., 1988, Distribution of gold in porphyry 

copper deposits: U.S. Geological Survey Open-File Report 

88-46, 23 p.

Cuddy, A.S., and Kesler, S.E., 1982, Gold in the Granisle and Bell 

Copper porphyry copper deposits, British Columbia, in 

Levinson, A.A., ed., Precious Metals in the Northern Cordillera: Association of Exploration Geochemists, Special Publication 10, p. 139-155.

Einaudi, M.T., 1990, Zoning of gold and silver in central portions 

of porphyry copper districts, in Symposium on Geology and 

Ore Deposits of the Great Basin, Program with Abstracts: 

Geological Society of Nevada, p. 59-60.

Gammons, C.H., and Barnes, H.L., 1989, The solubility of AgS2 in 

near-neutral aqueous sulfide solutions at 25 to 300°C: 

Geochimica et Cosmochimica Acta, v. 53, p. 279-290.

Gustafson, L.B., and Hunt, J.P., 1975, The porphyry copper deposit 

at El Salvador, Chile: Economic Geology, v. 70, p. 857-912.

Hayashi, K-L, and Ohmoto, H., 1991, Solubility of gold in NaCland H2S-bearing aqueous solutions at 250-350°C: Geochimica 

et Cosmochimica Acta, v. 55, p. 2111-2126

Heald, P., Foley, N.K., and Hayba, D.O., 1987, Comparative anatomy of volcanic-hosted epithermal deposits Acid-sulfate and 

adularia-sericite types: Economic Geology, v. 82, p. 1-26.

Hemley, J.J., and Hunt, J.P., 1992, Hydrothermal ore-forming processes in light of studies in rock-buffered systems n. Some 

general geologic applications: Economic Geology, v. 87, 

p. 23^3.

Hernandez, P.A., Garcia-Estrada, P.A., and Cowley, P.N., 1989, 

Geological setting, alteration and lithogeochemistry of the 

Transaccion epithermal gold deposit, Rodalquilar mining district, southeast Spain: Transactions, Institute of Mining and 

Metallurgy, v. 98, p. B78-B80.
PATHS OF HYDROTHERMAL FLUIDS AND ORIGIN OF QUARTZ-ALUNITE-GOLD DEPOSITS 41

Huang, C.K., 1955, Gold-copper deposits of the Chinkuashih mine, 

Taiwan, with special reference to the mineralogy: Acta Geologica Taiwanica, v. 7, p. 1-20.

 1960, Gold deposits of the Wutanshan area, Taiwan, and 

their relation to the Chinkuashih deposits: Acta Geologica Taiwanica, v. 8, p. 13-25. 

-1963, Factors controlling the gold-copper deposits of the

Chinkuashih Mine, Taiwan: Acta Geologica Taiwanica, v. 10, 

p. 1-9.

Huang, C.K., and Yeh, K.S., 1977, Special features of the ore deposits of the Changjen Series, Chinkuashih mine, Taiwan: Acta 

Geologica Taiwanica, v. 19, p. 1-12

Jannas, R.R., Beane, R.E., Ahler, B.A., and Brosnahan, D.R., 1990, 

Gold and copper mineralization at the El Indio deposit, Chile: 

Journal of Geochemical Exploration, v. 36, p. 233-266.

Jannas, R., Bowers, T.S., and Petersen, U., 1991, Fluid chemistry 

modeling of gold and copper mineralization in acid-sulfate 

type gold deposits, El Indio mine, Chile [abs.]: Geological Society of America Program with Abstracts, v. 23, p. A464.

John, D.A., Thomason, R.E., and McKee, E.H., 1989, Geology and 

K-Ar geochronology of the Paradise Peak mine and the relationship of pre-Basin and Range extension to early Miocene 

precious metal mineralization in west-central Nevada: Economic Geology, v. 631-649.

Johnson, R.F., 1951, Geology of the Masonic mining district, Mono 

County, California: Berkeley, University of California, M.S. 

thesis, 51 p.

Keith, W.J., Calk, L., and Ashley, R.P., 1980, Crystals of coexisting 

alunite and jarosite, Goldfield, Nevada: U.S. Geological Survey Professional Paper 1124-A-F, p. C1-C5.

Krauskopf, K.B., 1979, Introduction to Geochemistry (2nd ed.): 

New York, McGraw-Hill, 617 p.

Langton, J.M., and Williams, S.A., 1982, Structural, petrological 

and mineralogical controls for the Dos Pobres orebody, in Titley, S.R., ed., Advances in Geology of the Porphyry Copper 

Deposits, Southwestern North America: Tucson, University of 

Arizona Press, p. 335-352

Large, R.R., Huston, D.L., McGoldrick, P.J., Ruxton, P.A., and 

Me Arthur, G., 1989, Gold distribution and genesis in Australian volcanogenic massive sulfide deposits and their significance for gold transport models, in Keays, R.R, Ramsay, 

W.R.H., and Groves, D.I., eds., The Geology of Gold Deposits The Perspective in 1988: Economic Geology Monograph 

6, p. 520-536.

Lindley, I.D., 1987, The discovery and exploration of the Wild Dog 

gold-silver-copper deposit, East New Britain, P.N.G.: Melbourne, Australasian Institute of Mining and Metallurgy, Proceedings, Pacific Rim Congress 87, p. 283-286.

Lowell, J.D., and Guilbert, J.M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: Economic Geology, v. 65, p. 373-408.

Meyer, C, and Hemley, J.J., 1967, Wall rock alteration, in Barnes, 

H.L., ed., Geochemistry of Hydrothermal Ore Deposits: New 

York, Holt, Rinehart, and Winston, p. 166-235.

Morris, H.T., 1968, The main Tintic mining district, Utah, in Ridge, 

J.D., ed., Ore Deposits of the United States, 1933-1967, The 

Graton-Sales Volume: American Institute of Mining and Metallurgy, v. II, p. 1043-1073.

Mosier, D.L., and Menzie, W.D., 1986, Grade and tonnage model 

of quartz-alunite-Au, in Cox, D.P., and Singer, D.A., eds.,

Mineral Deposit Models: U.S. Geological Survey Bulletin 

1693, p. 159-161.

Norman, D.K., Parry, W.T., and Bowman, J.R., 1991, Petrology 

and geochemistry of propylitic alteration at southwest Tintic, 

Utah: Economic Geology, v. 86, p. 13-28.

Oviedo, L., Fuster, N., Tschischow, N., Ribba, L., Zuccone, A., 

Grez, E., and Aguilar, A., 1991, General geology of La Coipa 

precious metal deposit, Atacama, Chile: Economic Geology, v. 

85, p. 1287-1300.

Rose, A.W., and Burt, D.M., 1979, Hydrothermal alteration, in Barnes, H.L., ed., Geochemistry of Hydrothermal Ore Deposits 

(2nd ed.): New York, John Wiley, p. 173-235.

Ransome, F.L., 1909, Geology and ore deposits of Goldfield, Nevada: U.S. Geological Survey Professional Paper 66,258 p.

Rytuba, J.J., Arribas, A., Jr., Cunningham, C.G., McKee, E.H., 

Podwysocki, M.H., Smith, J.G., Kelly, W.C., and Arribas, A., 

Sr., 1990, Mineralized and unmineralized calderas in Spain 

Part II, Evolution of the Rodalquilar caldera complex and associated gold-alunite deposits: Mineralium Deposita, v. 25, 

p. S29-S35.

Rytuba, J.J., Arribas, A., Jr., Cunningham, C.G., McKee, E.H., 

Smith, J.G., and Arribas, A., Sr., 1989, Evolution of the 

Rodalquilar caldera complex and associated gold-alunite deposits, Cabo de Gato volcanic field, southeastern Spain: U.S. 

Geological Survey Open File Report 89-326, 18 p.

Schoen, R., White, D.E., and Hemley, J.J., 1974, Argillization by 

descending acid at Steamboat Springs, Nevada: Clays and Clay 

Minerals, v. 22, p. 1-22.

Seward, T.M., 1976, The stability of chloride complexes of silver 

in hydrothermal solutions up to 350°C: Geochimica et Cosmochimica Acta, v. 40, p. 1329-1341.

 1984, The transport and deposition of gold in hydrothermal 

systems, in Foster, R.P., ed., Gold '82: Rotterdam, A.A. Balkema Publishing, p. 121-134.

-1991, The hydrothermal geochemistry of gold, in Foster,

R.P., ed., Gold Metallogeny and Exploration: Glasgow, Blackie, p. 37-62.

Siddeley, G., and Araneda, R., 1986, The El Indio-Tambo gold deposits, Chile, in MacDonald, A.J., ed., Proceedings of Gold 

'86, an International Symposium on the Geology of Gold: Toronto, p. 445-456.

Sillitoe, R.H., 1973, The tops and bottoms of porphyry copper deposits: Economic Geology, v. 68, p. 799-815.

 1979, Some thoughts on gold-rich porphyry copper deposits: Mineralium Deposita, v. 14, p. 161-174.

 1983, Enargite-bearing massive sulfide deposits high in 

porphyry copper systems: Economic Geology, v. 78, 

p. 348-352.

 1988, Gold and silver deposits in porphyry systems, in 

Schafer, R.W., Cooper, J.J., and Vikre, P.O., eds., Bulk Mineable Precious Metal Deposits of the Western United States: 

Geological Society of Nevada, p. 233-257.

 1989, Gold deposits in western Pacific island arcs The 

magmatic connection, in Keays, R.R, Ramsay, W.R.H., and 

Groves, D.I., eds., The Geology of Gold Deposits The Perspective in 1988: Economic Geology Monograph 6, 

p. 266-283.

 1990, Gold-rich porphyry copper deposits of the circum-Pacific region An updated overview, in Pacific Rim Congress 

'90, Gold Coast, Queensland 1990, Proceedings, v. 2:
42 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Melbourne, Australasian Institute of Mining and Metallurgy, 

p. 119-126. 

-in press, Epithermal models Genetic types, geometrical

controls, and shallow features, in Kirkham, R.V., ed., Mineral 

Deposit Modeling: Geological Association of Canada Special 

Paper.

Sillitoe, R.H., Angeles, C.A., Comia, G.M., Antioquia, E.C., and 

Abeya, R.B., 1990, An acid-sulfate-type lode gold deposit at 

Nalesbitan, Luzon, Philippines: Journal of Geochemical Exploration, v. 35, p. 387-411.

Steven, T.A., and Ratte", J.C., 1960, Geology and ore deposits of the 

Summitville district, San Juan Mountains, Colorado: U.S. 

Geological Survey Professional Paper 343,70 p.

Stoffregen, R., 1987, Genesis of acid-sulfate alteration and Au-CuAg mineralization at Summitville, Colorado: Economic Geology, v. 82, p. 1575-1591.

Tan, L-P., 1991, The Chinkuashih gold-copper deposits, Taiwan: 

Society of Economic Geologists Newsletter, no. 7, v. 1, 

p. 22-24.

Taylor, G., 1987, Breccia formation and its relation to gold mineralization at Mount Kasi, Fiji: Proceedings Pacific Rim Congress 87: Melbourne, Australasian Institute of Mining and 

Metallurgy, p. 597-601.

Tolman, C.F., and Ambrose, J.W., 1934, The rich ores of Goldfield, 

Nevada: Economic Geology, v. 29, p. 255-279.

Vikre, P.O., 1989, Ledge formation at the Sandstorm and Kendall 

gold mines, Goldfield, Nevada: Economic Geology, v. 84, 

p. 2115-2138.

Wang, Y., 1973, Wall rock alteration of late Cenozoic mineral deposits in Taiwan Mineralogical and physicochemical aspects: Acta Geologica Taiwanica, v. 16, p. 1-29.
CHAPTER F

REMOTE-SENSING SPECTRAL PROPERTIES OF 

SELECTED INDUSTRIAL MINERALS AND ORES

By James K. Crowley l

ABSTRACT

Spectral-reflectance and thermal-emission properties of 

19 selected industrial minerals and ores were examined to 

evaluate the possibility of mineral detection using spectroscopic remote-sensing data. The results show that use of visible and near-infrared data is potentially suitable for 

discerning borates, carbonates, diatomite, gypsum, pyrophyllite, talc, and vermiculite. Thermal-infrared data may be 

particularly useful for locating concentrations of barite, 

quartz, perlite, phosphorite, and thenardite. Some minerals, 

such as zeolites, may not be amenable to remote-sensing 

detection because of near-infrared absorption interference 

by atmospheric water vapor and the occurrence of these minerals in fine-grained samples that have low thermal-infrared 

spectral contrast. Further research is needed to study how 

weathering processes and mixtures of surficial materials 

may affect remote sensing of industrial minerals and ores 

under actual field conditions.

INTRODUCTION

Industrial minerals and ores include a wide variety of 

commodities, ranging from exotic substances, such as rare 

earth elements, to more commonplace materials, such as 

sand and gravel. Because of this diversity, industrial minerals and ores defy a simple definition. Industrial minerals are 

frequently referred to as "nonmetallics," even though this 

designation does not precisely fit materials like bauxite ore, 

lithium concentrates, and titanium oxides. Depending on 

how limiting a definition is applied, the annual dollar value 

of industrial minerals produced worldwide is as much as

'U.S. Geological Survey, Mail Stop 927,12201 Sunrise Valley Drive, 

Reston,VA 22092-0001.

twice the annual value of production from precious- and 

base-metal ore deposits (Bates, 1983).

The purpose of this study is to examine the spectralreflectance and thermal-emission properties of selected 

industrial minerals and ores and to evaluate prospects for 

locating mineral commodities with spectroscopic remotesensing data. Sophisticated airborne scanners, such as the 

NASA airborne visible/infrared imaging spectrometer 

(AVIRIS), can remotely measure spectral curves that compare favorably in quality and detail to curves obtainable with 

laboratory spectrometers (Vane and others, 1993). Such 

spectral curves can be used to observe characteristic absorption features produced by Earth-surface materials, including 

particular minerals, rocks, soils, and plant types. Also important is the recent development of portable spectrometers that 

permit real-time measurements of mineral and vegetation 

spectra in the field. Portable spectrometers are being used for 

mapping hydrothermally altered rocks and in studies of 

heavy metal concentrations in mine tailings (Munts and others, 1993; Ebel and others, 1993). Field spectral measurements may also have a useful role in ore quality assessment 

(J.K. Crowley, unpub. data). Although a number of general 

reviews dealing with the spectral properties of minerals and 

rocks are available (e.g., Hunt, 1977; Clark and others, 1990; 

Salisbury and others, 1991), none deals explicitly with 

industrial minerals. Similarly, the potential use of remotesensing data as an aid to exploration for industrial minerals 

has received only limited attention. In a spectral study of carbonatites and associated alkalic rocks, Rowan and others 

(1986) commented that rare-earth absorption bands might be 

observable with remote-sensing data, and such features subsequently have been detected (Kingston and Crowley, 1992). 

Crowley and others (1989) demonstrated that visible and 

near-infrared (VNIR; 0.4 to 2.5 jam) remote-sensing data 

might be useful for talc exploration, and Crowley (1993) 

reported on the use of high-spectral-resolution AVIRIS data 

for mapping saline evaporite minerals, including borates. 

Kingston (1989) has also reported on the VNIR spectral

43
44 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Table 1. Information on sample deposit types, localities, and mineralogical compositions as determined by X-ray diffraction analysis.

Sample Deposit type and locality Composition

Samples shown in figures 1 and 4

Thenardite

Baritel

Barite2

Phosphorite

Quartz

Perlitel

Perlite 2

Graphite

Playa; Saline Valley, California

Bedded; Slaven Canyon, Nevada

Bedded; Osgood Mtns., Nevada

Marine; Phosphoria Formation, Idaho

Sandstone; Tuscarora Sandstone,

West Virginia

Volcanic; Saguache Co., Colorado

Volcanic; Mohave Co., Arizona

Schist; Washington Co., New York

Thenardite

Barite

Barite

Quartz, calcite,

carbonate-fluorapatite

Quartz

Glass (amorphous)

Glass (amorphous)

Quartz, muscovite, calcite,

graphite

Samples shown in figures 2 and 5

Diatomite

Calcite

Kimberlite

Erionite

Phillipsite

Gypsum

Lacustrine; Stewart, Nevada

Limestone; Confusion Range, Utah

Volcanic pipe; Zeekoegat, S. Africa

Sedimentary tuff; Eureka Co., Nevada

Sedimentary tuff; Eureka Co., Nevada

Playa; Mesquite Playa, California

Amorphous silica

Calcite

Calcite, phlogopite, serpentine

Erionite, merlinoite

Phillipsite

Gypsum

Samples shown in figures 3 and 6

Pyrophyllite

Talc

Vermiculite

Ulexite

Colemanite

Hydrothermal; Pilot Mtn., North Carolina

Metasedimentary, Ruby Mtns., Montana

Unknown; Llano Co., Texas

Lacustrine; Boron, California

Lacustrine; Boron, California

Pyrophyllite

Talc

Vermiculite, amphibole

Ulexite

Colemanite

properties of kimberlites, with a view toward the use of 

remote-sensing data to explore for diamonds.

The present report will focus on selected industrial minerals and ores that generally meet two criteria: First, the minerals must occur in surficial exposures that potentially are 

amenable to remote-sensing detection. Second, most of the 

minerals included have relatively high intrinsic value. Highpurity quartz sand and calcitic carbonate rock samples are 

also included in the study because of their distinctive spectral properties and the possibility that remote-sensing exploration for these commodities may be economically practical 

in some situations. The study is not comprehensive because 

many industrial minerals are not included and the minerals 

represented here by only one or two samples may exhibit 

considerable natural variation. Further research is planned to 

examine additional samples and to investigate other factors, 

such as weathering stability, which may affect remote-sensing detection of the minerals and ores.

METHODS

A collection of industrial minerals and ores was 

obtained from U.S. Geological Survey commodity geologists and supplemented by samples on hand from previous

geologic remote-sensing studies (table 1). The samples generally represent the more important deposit types for each 

industrial mineral, according to deposit descriptions given in 

Lefond (1983). For most of the minerals and ores, multiple 

samples were examined spectrally, and representative specimens were then used for this report. Table 1 includes information on sample deposit types, localities, and mineralogical 

compositions based on X-ray diffraction analysis.

Visible and near-infrared (0.4-2.5 jam) reflectance 

spectra were recorded on a Beckman Instruments UV 5240 

spectrometer equipped with an integration sphere. The data 

were acquired digitally and corrected for errors in the 100 

percent line caused by the instrument and the white halon 

reference material (Crowley and others, 1986). Hemispherical reflectance spectra were measured in the thermal-infrared (2.5-13.5 Jim) by using a Nicolet 5DXB FTIR 

spectrometer equipped with a gold-coated integration 

sphere, a diffuse gold reference plate, and an infrared source 

(Salisbury and others, 1988). The hemispherical reflectance 

spectra were subsequently converted to emission spectra by 

using Kirchoff s law (Salisbury and others, 1988). Because 

the thermal-infrared (TIR) spectral signatures of principal 

interest from a remote-sensing standpoint are those within 

the 8-13.5-|im atmospheric transmission window, the 

present study focuses on this wavelength interval. Whenever
REMOTE-SENSING SPECTRAL PROPERTIES OF SELECTED INDUSTRIAL MINERALS AND ORES 45

practical, both VNIR and TIR spectra were measured on natural surfaces of whole-rock sample chips. In cases where the 

surfaces were too irregular, samples were ground to a coarse 

powder (approximately 250 (im) and poured into a sample 

cup for spectral measurements.

To evaluate TIR remote-sensing prospects for industrial minerals and ores, the sample spectra were convolved to 

the six spectral bandpasses of the NASA thermal-infrared 

multispectral scanner (TIMS). A spectral library of TIR data 

for 43 well-characterized igneous and sedimentary rocks 

(Salisbury and others, 1988; J.W. Salisbury, unpub. data) 

was also convolved to the TIMS bandpasses, permitting the 

rock spectral responses and industrial mineral spectral 

responses to be compared.

RESULTS

Figures 1 through 3 show VNIR spectra of the industrial minerals and ores examined in this study. Table 2 lists 

absorption band positions for each sample along with general band assignments. With a few notable exceptions, most 

of the samples exhibit relatively featureless VNIR spectra, or 

they show absorption bands that have limited utility from a 

remote-sensing standpoint because of interference by atmospheric absorption features. In particular, many of the spectra in figures 1 and 2 exhibit absorption bands near 1.4 and 

1.9 urn related to molecular water (Hunt and Salisbury, 

1970; Aines and Rossman, 1984). These water features cannot be observed remotely because the same wavelengths are 

obscured by atmospheric water-vapor absorption. In contrast, the 2.1-2.45-nm wavelength range has minimal atmospheric interference, and several of the figure 1 spectra show 

weak features in this wavelength region related to minor clay 

(present in weathered feldspar), K-mica, and carbonate mineral constituents (table 1). However, none of these spectral 

features in figure 1 are diagnostic of the contained industrial 

minerals. The figure 2 samples, in most cases, show greater 

spectral contrast (absorption band intensity) than the figure 

1 samples and do exhibit absorption features that are caused 

directly by the industrial mineral constituents. In the cases of 

diatomite and gypsum, diagnostic absorption features in the 

2.1-2.45-nm wavelength range produced by structural water 

molecules would probably permit identification of these 

minerals in a remote-sensing context. The calcitic carbonate 

rock shows a strong absorption feature near 2.335 ^m, which 

is related to vibrational absorption processes involving the 

CC>3 anion group (Hunt, 1977). On the other hand, the calcitic kimberlite sample lacks obvious absorption features, 

probably because of strong masking absorption by opaque 

carbonaceous impurities. Remote-sensing detection of the 

two zeolite minerals would also be problematic because they 

lack diagnostic features outside the 1.4- and 1.9-nm wavelength regions of atmospheric water-vapor absorption.

The minerals shown in figure 3 all have diagnostic 

spectral features between 2.1 and 2.45 urn that should permit

LU 

O 

DC 

LU 

Q_

LLI 

O

I

111

LL 

LLI 

QC

Barite

 -^

Barite 2 \

x

Phosphorite

0.5 1.0 1.5 2.0

WAVELENGTH, IN MICROMETERS

2.5

Figure 1. Visible and near-infrared (VNIR) reflectance spectra 

of industrial minerals and ores. Spectra are offset vertically for 

viewing purposes. Tickmarks on vertical axis show reflectance intervals of 10 percent. Stippling indicates wavelength regions obscured by atmospheric water-vapor absorption. Arrows with letter 

"c" indicate absorption features caused by contaminant clay minerals, K-mica, and calcite, Reflectance values of each spectrum at 2.5 

|im are (from top to bottom) 69 percent, 26 percent, 25 percent, 26 

percent, 54 percent, 8 percent, 7 percent, and 25 percent.

their remote-sensing identification with suitable VNIR data 

(table 2). In the case of pyrophyllite, the absorption features 

are caused by vibrational absorption processes involving 

aluminum and hydroxyl, whereas, in talc and vermiculite, 

the features are due to vibrational processes involving magnesium and hydroxyl (Hunt, 1977). The two borate minerals, 

ulexite and colemanite, exhibit bands due to hydrate water 

molecules and perhaps to vibration modes of the borate 

anion group (Hunt and others, 1972). Hunt and Salisbury 

(1970), Hunt (1977), Clark and others (1990), and Crowley 

(1991) provide additional information on the types of
46 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

111 

O 

DC 

LU 

CL

111 

O

P 

O 

LLI

LL 

LLI 

DC

0.5 1.0 1.5 2.0 

WAVELENGTH, IN MICROMETERS

2.5

Figure 2. Visible and near-infrared (VNIR) reflectance spectra of 

industrial minerals and ores. Spectra are offset vertically for 

viewing purposes. Tickmarks on vertical axis show reflectance intervals of 10 percent. Stippling indicates wavelength regions obscured by atmospheric water-vapor absorption. Plain arrows mark 

positions of diagnostic absorption features listed in table 2. Arrows with letter "c" indicate absorption features caused by contaminant iron oxide minerals. Reflectance values of each spectrum 

at 2.5 p,m are (from top to bottom) 58 percent, 31 percent, 7 percent, 32 percent, 22 percent, and 17 percent.

absorption features commonly observed in the VNIR reflectance spectra of minerals and rocks.

Figures 4 through 6 show TIR emission spectra of the 

industrial mineral and ore samples. Examination of figure 4 

reveals that several samples, which were spectrally featureless in the VNIR wavelength range (fig. 1), exhibit strong 

TIR spectral features. In particular, the thenardite and barite 

spectra show features associated with fundamental molecular vibrations of the sulfate anion group, and the phosphorite

LLI -

LU 

CL

LLI 

O

§LL 

LLI 

DC

0.5 1.0 1.5 2.0 

WAVELENTGH, IN MICROMETERS

Figure 3. Visible and near-infrared (VNIR) reflectance spectra of 

industrial minerals and ores. Spectra are offset vertically for viewing purposes. Tickmarks on vertical axis show reflectance intervals 

of 10 percent. Stippling indicates wavelength regions obscured by 

atmospheric water-vapor absorption. Arrows mark positions of diagnostic absorption features listed in table 2. Reflectance values of 

each spectrum at 2.5 |im are (from top to bottom) 18 percent, 22 

percent, 16 percent, 11 percent, and 27 percent.

spectrum exhibits features attributable to the phosphate 

group (Farmer, 1974). The perlite spectra exhibit broad 

emission features due to silicon-oxygen stretching and bending modes, which evidently extend over a wide range of 

energies in these structureless silicate glasses. Silicon-oxygen vibrational modes also account for the well-defined features seen in the quartz-sand spectrum.

In contrast with figure 4, most of the TIR spectra shown 

in figure 5 are spectrally featureless. The reasons for this are
REMOTE-SENSING SPECTRAL PROPERTIES OF SELECTED INDUSTRIAL MINERALS AND ORES 47

Table 2. Diagnostic absorption features and band assignments for 

industrial minerals and ores.

[Leaders ( ) indicate no features present]

Mineral/ore Band positions (jam) Assignment

Samples shown in figures 1 and 4

Thenardite 

Barite 1 

Barite2 

Phosphorite 

Quartz 

Perlite 1 

Perlite 2 

Graphite

8.46, 8.82 

8.18,8.84 

8.18,8.85 

9.02, 9.57 

8.24,9.17 

9.45 

9.17

SO4 

SO4 

SO4 

P04 

SiO4 

SiO4 

SiO4

Samples shown in figures 2 and 5

Diatomite 

Calcite 

Kimberlite 

Erionite 

Phillipsite 

Gypsum

2.195,2.26 

2.335,11.24 

11.34

1.53,1.74,2.208,8.65

Si-OH 

CO3 

CO3

H20, S04

Samples shown in figures 3 and 6

Pyrophyllite 

Talc 

Vermiculite 

Ulexite 

Colemanite

2.08,2.165,2.32,8.73,9.12 

2.305, 2.38, 8.98, 9.62 

2.305,2.38,9.49 

2.168,11.59 

2.104,2.196,10.47,10.87

Al-OH, Si04 

Mg-OH, SiO4 

Mg-OH, SiO4 

H2O, BO3 

H2O, BO3

twofold: First, some of the figure 5 samples (diatomite, and 

the two zeolites) are naturally fine grained, a property that 

sharply reduces the TIR spectral contrast (see Salisbury and 

others, 1988). Second, whereas the VNIR spectra of these 

samples (fig. 2) showed features attributable to molecular 

water, the analogous TIR water-absorption features lie outside the wavelength range covered by the emission spectra. 

Note that the gypsum emission feature seen in figure 5, similar to the features noted above for thenardite and barite, is 

due to molecular vibration modes of the sulfate anion group 

and not to molecular water (Farmer, 1974). The calcite and 

kimberlite spectra show weak carbonate bands near 11.2 ^m, 

which in the case of kimberlite may be too subtle for remotesensing detection with TIMS data; however, this spectral 

feature might be stronger in other kimberlite samples.

Most of the TIR spectra in figure 6 show moderately 

strong spectral features. The ulexite spectrum is an exception, and its reduced spectral contrast again is a result of this 

sample's relatively fine particle size. The emission features 

seen in the spectra of pyrophyllite, talc, and vermiculite all 

involve internal vibration modes of the silicate anion group; 

the colemanite emission features observed near 10.6 jj,m are 

produced by molecular vibrations of the borate anion group 

(Farmer, 1974).

Table 3 compares TIMS band-ratio values, calculated 

from igneous and sedimentary rock spectra, to similar band 

ratios calculated from the industrial mineral TIR spectra. 

From the table it can be seen that certain industrial minerals

HI

O CC

HI

a.

O

CO 

CO

S

HI

8.0 9.0 10.0 11.0 12.0 

WAVELENGTH, IN MICROMETERS

Figure 4. Thermal emission spectra of industrial minerals and 

ores. Spectra are offset vertically for viewing purposes, and TIMS 

(thermal-infrared multispectral scanner) bandpasses are shown on 

the lower part of the figure. Tickmarks on vertical axis indicate 

emission intervals of 5 percent. Arrows mark positions of diagnostic absorption features listed in table 2. Emission values of 

each spectrum at 12.5 |im are (from top to bottom) 97 percent, 95 

percent, 94 percent, 95 percent, 96 percent, 96 percent, 95 percent, 

and 88 percent

are spectrally distinct from common rocks that might be 

encountered in a remote-sensing situation. For example, the 

thenardite and gypsum samples have relatively high 3/1 ratio 

values compared to typical sedimentary host rocks. Similarly, the two perlite samples have relatively low 3/1 ratios, 

making them spectrally distinct from most common igneous 

rocks. The colemanite sample has a high 4/6 ratio, and the 

two barites have low 2/1 ratio values, which may permit their
48 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

LLJ 

O 

DC 

LiJ 

0_

Z

o

HI

Diatomite

-Erionite

/ Phillipsite

Gypsum

'. TIMS . 1 .2 , 3 , , 4 t

8.0 9.0 10.0 11.0 12.0 

WAVELENGTH, IN MICROMETERS

Figure 5. Thermal emission spectra of industrial minerals and 

ores. Spectra are offset vertically for viewing purposes, and TIMS 

(thermal-infrared multispectral scanner) bandpasses are shown on 

the lower part of the figure. Tickmarks on vertical axis indicate 

emission intervals of 5 percent. Arrows mark positions of diagnostic absorption features listed in table 2. Emission values of 

each spectrum at 12.5 |im are (from top to bottom) 98 percent, 98 

percent, 98 percent, 98 percent, 98 percent, and 97 percent

remote-sensing discrimination. Quartz has a high 4/3 ratio 

value and phosphorite a relatively high 2/3 ratio compared to 

analogous ratio values for igneous and sedimentary rocks. 

The particular band-ratio combinations shown in table 3 

were selected by inspection of the 15 possible ratio combinations to find those that were most useful. It should be emphasized that other ratio combinations and other analytical 

techniques, such as spectral shape matching, may also be 

effective for distinguishing particular minerals. More importantly, it should be remembered that different samples of particular industrial minerals and ores are likely to exhibit some 

degree of spectral variation. Table 3 should therefore be 

regarded only as a preliminary guide to the use TIMS ratios 

for distinguishing industrial minerals.

DISCUSSION

The results of the study indicate that VNIR and TIR 

spectral data may permit remote-sensing discrimination of a 

variety of industrial minerals and ores. VNIR data may facilitate detection of borates, carbonates, diatomite, gypsum,

UJO

DC 

LLJ 

Q_

UJ

Vermiculite / 

,- 

Ulexite

_ -.

Colemanite

TIMS ,1.2.3

8.0 9.0 10.0 11.0 12.0 

WAVELENGTH, IN MICROMETERS

Figure 6. Thermal emission spectra of industrial minerals and 

ores. Spectra are offset vertically for viewing purposes, and TIMS 

(thermal-infrared multispectral scanner) bandpasses are shown on 

the lower part of the figure. Tickmarks on vertical axis indicate 

emission intervals of 5 percent. Arrows mark positions of diagnostic absorption features listed in table 2. Emission values of each 

spectrum at 12.5 |im are (from top to bottom) 97 percent, 97 percent, 97 percent, 97 percent, and 95 percent.

pyrophyllite, talc, and vermiculite. TIR data appear to be 

well suited for detecting barite, phosphorite, perlite, quartz 

sand, and thenardite, none of which display diagnostic VNIR 

spectral features. Of course, such discrimination would 

depend on other factors besides the spectral properties of the 

minerals as measured in a laboratory setting. Chief among 

these factors is dilution and masking of the industrial mineral 

spectral signatures by vegetation, soil, and other rock materials. Because remote-sensing observations typically involve 

mixtures of these surface materials, spectral interpretation of 

remote-sensing data can be much more complex than laboratory spectral measurements might suggest. Another factor 

affecting the remote-sensing detection of surface exposures 

is the weathering stability of particular industrial minerals. 

Some minerals, such as talc and vermiculite, are relatively 

stable and may become concentrated in soils under moderate 

weathering conditions. On the other hand, evaporite minerals and volcanic glasses (e.g., perlite) tend to weather rapidly, which may degrade their spectral response. Even 

though laboratory spectral data are useful for developing 

remote-sensing strategies for discerning industrial minerals, 

working experience with particular mineral deposit types 

under a variety of terrain and weathering conditions is still 

needed.
Table 3. Selected TIMS band-ratio values for industrial minerals and ores, igneous rocks, and sedimentary rocks.

[TIMS, thermal-infrared multispectral scanner. Diagnostic mineral-ratio values are marked by asterisks]

Mineral

Barite 1 .. ............................. ..

Barite2. .............................. ..

Calcite...................................

Colemanite............................

Diatomite. .............................

Erionite. ................................

Graphite................................

Gypsum.................................

Kimberlite. ............................

Perlitel.. .............................. .

Perlite 2............... ..................

Phillipsite...... ........................

Phosphorite ...........................

Pyrophyllite... ........................

Quartz...................................

Talc..... ................................. .

Thenardite.............................

Ulexite..................................

Vermiculite...........................

Igneous rocks (range). ............

Sedimentary rocks (range)

2/1

.....*0.879

.....*0.750

.......1.000

.......0.991

.......0.982

.......0.993

.......0.997

.......0.930

.......0.990

.......0.951

.......0.929

.......0.995

.......0.916

.......0.942

.......0.988

.......0.894

.......0.980

.......0.992

.......0.957

.......0.890-

1.006

........0.961-

1.028

2/3

0.968

*0.891

0.995

1.000

1.010

1.029

1.029

*0.874

1.027

1.090

*1.111

1.010

*1.151

0.998

*1.092

0.999

*0.784

1.019

1.054

0.963-

1.087

0.993-

1.069

3/1

0.908

*0.842

1.004

0.992

0.973

0.965

0.969

* 1.064

0.964

*0.872

*0.836

0.985

*0.796

0.943

0.905

0.895

* 1.250

0.974

0.908

0.888-

1.003

0.914-

1.019

4/1

1.032

1.082

1.013

0.959

0.978

0.974

0.966

1.100

0.923

0.872

0.944

0.983

0.889

1.110

* 1.429

0.962

*1.308

0.971

0.888

0.842-

1.155

0.941-

1.181

4/2

*1.174

*1.443

1.013

0.968

0.995

0.981

0.969

*1.184

0.932

0.917

1.017

0.988

0.970

*1.178

*1.445

1.076

*1.334

0.978

0.928

0.873-

1.128

0.979-

1.168

4/3

1.136

* 1.286

1.009

0.967

1.005

1.010

0.997

1.035

0.957

0.999

1.130

0.998

1.117

1.176

*1.578

1.075

1.046

0.997

0.978

0.912-

1.234

0.993-

1.184

4/6

0.954

0.958

1.015

"1.036

0.983

0.994

0.941

0.989

0.936

0.887

0.880

1.003

0.897

0.964

0.946

0.954

1.001

1.001

0.912

0.782-

1.005

0.907-

1.014

5/1

1.068

1.115

0.982

0.874

0.987

0.971

1.006

1.107

0.957

0.961

1.033

0.977

0.971

1.122

* 1.483

0.999

1.311

0.986

0.939

0.722-

1.333

0.976-

1.205

5/3

1.17

1.32

0.97

0.88

1.01

1.00

1.03

1.04

0.99

1.10

1.23

0.99

1.22

1.19

*1.63

1.11

1.04

1.01

1.03

0.80

1.42

0.98

1.24
50 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Sensor capabilities also have a determining role in the 

remote-sensing detection of industrial minerals. To identify 

minerals with VNIR data generally requires relatively high 

signal-to-noise and a narrow spectral sampling interval, such 

as the 11-15-nm interval of the 224-channel AVIRIS scanner (Vane and others, 1993). Passive thermal-infrared scanners, as a rule, have substantially fewer spectral bands, 

although there are plans to increase the number of bands 

beyond the six-band design of the current TIMS scanner 

(Kannari and others, 1992). Recently, an active TIR remotesensing system has been developed based on a rapidly tunable CC>2 laser that provides much narrower TIR sampling 

intervals than passive scanner designs (Whitbourn and 

Hausknecht, 1990). Continued improvements in the signalto-noise and sampling intervals of both TIR and VNIR 

remote-sensing systems will enhance the ability to 

deconvolve spectral components present in mixtures of surface materials and, thus, should facilitate remote-sensing 

detection of industrial minerals.

CONCLUSIONS

This laboratory study demonstrates that many industrial 

minerals and ores exhibit VNIR and (or) TIR spectral features that might be useful for detecting the minerals with 

spectroscopic remote-sensing data. The two different wavelength ranges are complementary insofar as minerals that 

lack absorption bands in one range commonly exhibit features in the other. In practice, the remote-sensing detection 

of industrial minerals is likely to be affected by soil, rock, 

and vegetation mixtures, as well as by weathering processes. 

Accordingly, the next phase of this research will examine 

remote-sensing prospects for particular industrial minerals 

under actual field conditions.

REFERENCES CITED

Aines, R.D., and Rossman, G.R., 1984, Water in minerals? A peak 

in the infrared: Journal of Geophysical Research, v. 89, 

p. 4059-4071.

Bates, R.L., 1983, Introduction, in Lefond, S.J., ed., Industrial Minerals and Rocks, Volume 1: American Institute of Mining, 

Metallurgical, and Petroleum Engineers, Inc., 722 p.

Clark, R.N., King, T.V.V., Klejwa, M., and Swayze, G.A., 1990, 

High spectral resolution reflectance spectroscopy of minerals: 

Journal of Geophysical Research, v. 95, p. 12653-12680.

Crowley, J.K., 1991, Visible and near-infrared (0.4-2.5 ^m) reflectance spectra of playa evaporite minerals: Journal of Geophysical Research, v. 96, p. 16231-16240.

 1993, Mapping playa evaporite minerals with AVIRIS 

data A first report from Death Valley, California: Remote 

Sensing of Environment, v. 44, p. 337-356.

Crowley, J.K., Brickey, D.W., and Rowan, L.C., 1989, Airborne 

imaging spectrometer data of the Ruby Mountains, Montana 

Mineral discrimination using relative absorption band-depth 

images: Remote Sensing of Environment, v. 29, p. 121-134.

Crowley, J.K., Sherman, D.M., and Bennett, I.J., 1986, A low-cost 

digital recording and display system for Beckman UV 5200 series spectrophotometers: U.S. Geological Survey Open File 

Report 86-27,23 p.

Ebel, P., Hauff, P., Hill, B., Nash, G., Ridd, M., and McGregor, L, 

1993, Integrating remote sensing techniques (AVIRIS, TM, 

TIMS, and field spectroscopy) for exploration: Case history 

Cuprite, Nevada, in Proceedings of the Ninth Thematic Conference on Geologic Remote Sensing, Feb. 8-11, 1993, Pasadena: Ann Arbor, Environmental Research Institute of 

Michigan, v. 2, p. 833-843.

Farmer, V.C., ed., 1974, Infrared Spectra of Minerals: London, Adlard and Son, Ltd., Mineralogical Society of London, Monograph No. 4,539 p.

Hunt, G.R., 1977, Spectral signatures of paniculate minerals in the 

visible and near-infrared: Geophysics, v. 42, p. 501-513.

Hunt, G.R., and Salisbury, J.W., 1970, Visible and near-infrared 

spectra of minerals and rocks I, Silicate minerals: Modern 

Geology, v. 1, p. 283-300

Hunt, G.R., Salisbury, J.W., and Lenhoff, C.J., 1972, Visible and 

near-infrared spectra of minerals and rocks V, Halides, phosphates, arsenates, vanadates and borates: Modern Geology, v. 

3, p. 121-134.

Kannari, Y., Mills, F., Watanabe, H., Ezaka, T., Narita, T., and 

Chang, S., 1992, Comparison of preliminary results from Airborne ASTER Simulator (AAS) with TIMS data, in Summaries of the Third Annual JPL Airborne Geoscience Workshop, 

June 1-5,1992, v. 2: Jet Propulsion Laboratory Publication 92- 

14, p. 13-15.

Kingston, M.J., 1989, Spectral reflectance features of kimberlites 

and carbonatites Implications for remote sensing for exploration, in Kimberlites and Related Rocks, v. 2, Proceedings of the 

Fourth International Kimberlite Conference, Perth, 1986: Geological Society of Australia Special Publication No. 14, 

p. 1135-1146.

Kingston, M.J., and Crowley, J.K., 1992, AVIRIS as a tool for carbonatite exploration A comparison of SPAM and Mbandmap 

data analysis methods, in Summaries of the Third Annual JPL 

Airborne Geoscience Workshop, June 1-5,1992, v. 1: Jet Propulsion Laboratory Publication 92-14, p. 116-118.

Lefond, S.J., ed., 1983, Industrial Minerals and Rocks: New York, 

American Institute of Mining, Metallurgical, and Petroleum 

Engineers, Inc., 1446 p.

Munts, S.R., Hauff, P.L., Seelos, A., McDonald, B., 1993, Reflectance spectroscopy of selected base-metal bearing tailings with 

implications for remote sensing, in Proceedings of the Ninth 

Thematic Conference on Geologic Remote Sensing, Feb. 

8-11, 1993, Pasadena: Ann Arbor, Environmental Research 

Institute of Michigan, v. 1, p. 567-578.

Rowan, L.C., Kingston, M.J., Crowley, J.K., 1986, Spectral reflectance of carbonatites and related alkalic igneous rocks: Selected samples from four North American localities: Economic 

Geology, v. 81, p. 857-871.
REMOTE-SENSING SPECTRAL PROPERTIES OF SELECTED INDUSTRIAL MINERALS AND ORES 51

Salisbury, J.W., Walter, L.S. and D'Aria, D., 1988, Mid-infrared spectrometer (AVIRIS): Remote Sensing of Environment, v.

(2.5-13.5 |im) spectra of igneous rocks: U.S. Geological Sur- 44, p. 127-143.

vey Open-File Report 88-686, 136 p. Whitbourn, L. B., and Hausknecht, P., 1990, Status of the CSIRO

Salisbury, J.W., Walter, L.S. Vergo, N., and D'Aria, D.M., 1991, airborne CO2 laser active raid-IR remote sensing project, in

Infrared (2.1-25 ^im) Spectra of Minerals: Baltimore, Johns Proceedings of the Fifth Australasian Remote Sensing ConferHopkins University Press, 267 p. ence, Perth, October 8-12,1990: Perth, Committee of the Fifth

Vane, G., Green, R.O., Chrien, T.G., Enmark, H.T., Hansen, E.G., Australasian Remote Sensing Conference, Inc., v. 1,

and Porter, W.M., 1993, The airborne visible/infrared imaging p. 391-398.

CHAPTER G

GEOCHEMICAL EXPLORATION STUDIES IN THE 

PORTUGUESE PYRITE BELT

By David J. Grimes 1 and Delfim de Carvalho2

This report was prepared with the support and cooperation of the Luso 

American Development Foundation, the Servifos Geologicos de 

Portugal, and the Services de Formento Mineiro

ABSTRACT

Geochemical exploration studies were conducted by 

the U.S. Geological Survey (USGS) and the Services 

Geol6gicos de Portugal (SGP) in the Portuguese Pyrite Belt 

(PPB) in southern Portugal during 1987 and 1988. Principal 

funding for this study was provided by the Luso American 

Development Foundation and was supplemented by funds 

from the USGS and the SGP.

The Portuguese Pyrite Belt is the part of the Iberian 

Pyrite Belt, which trends eastward from Seville, in southern 

Spain, across southern Portugal. The Iberian Pyrite Belt is 

about 230 km long and 30 km wide and is host to numerous 

copper-, zinc-, and lead-bearing massive sulfide deposits. 

The geology of this belt is structurally complex, and the 

rocks primarily consist of volcanic and sedimentary rocks of 

late Paleozoic age.

Rock, soil, plant, and water samples were collected near 

Neves-Corvo, in southern Portugal, where four, or more, 

clustered massive sulfide deposits, containing high amounts 

of copper, tin, and zinc and moderate amounts of several 

other metals, are covered by at least 200 m of sedimentary 

rock. In addition, a few plant and water samples were taken 

from other areas in the PPB where geological and geophysical conditions were considered favorable for massive sulfide 

deposits. The results of the geochemical surveys indicate 

that all of the methods tested may have application in the 

exploration for buried massive sulfide deposits.

^.S. Geological Survey, P.O. Box 25046, Mail Stop 973, Denver 

Federal Center, Denver, CO 80225.

2Servicos Geol6gicos de Portugal, Lisbon, Portugal.

INTRODUCTION

Geochemical exploration studies were conducted at 

selected localities of the Portuguese Pyrite Belt (PPB) as part 

of a U.S. Geological Survey/Servifos Geol<5gicos de Portugal cooperative program (fig. 1). These studies were 

designed to determine whether surface geochemical sampling methods combined with sensitive, multielement analytical techniques could be used to detect concealed massive 

sulfide deposits. A wide range of sample types, including 

rock, soil, vegetation, and water were collected and analyzed 

during these studies. Most of these samples were collected 

over the Neves-Corvo massive sulfide deposits (fig. 1).

NEVES-CORVO STUDY AREA

The Neves-Corvo area in southern Portugal was 

selected for these studies because it contains at least four 

large massive sulfide bodies that are buried by 200 m or 

more of weakly mineralized sedimentary and volcanic-sedimentary rocks. At the time of this study, the ground surface 

above the Neves-Corvo deposit was relatively undisturbed 

as compared to older mines in the PPB, and there was a lack 

of ore-treating and milling activities.

The Neves-Corvo study area is about 60 km south of the 

town of Beja and covers about 50 km2. The area has a gently 

rolling topography, and elevations range from about 170 m 

to 300 m. According to "Baedeker's Portugal" (1986), the 

climate in this part of Portugal is influenced by both maritime and continental air masses. The average temperature for 

January is 8.2°C (46.8°F), and for August, 23°C (73.4°F). 

Average annual rainfall is 561 mm (22 in). The lowest 

monthly average is 2 mm in August, and the highest is 82 

mm (3.2 in) in November.

53
54 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

9°W

39°N n

8°W

7°W

38°N

37°N 3

EXPLANATION - Study areas

1 Neves-Corvo 4 Cidrao

2 Garrochal-Almoleias 5 Mombeja

3 Salgadinho

Figure 1. Index map of southern Portugal showing geochemical 

study areas (crosses). Names of study areas are shown in the explanation and are keyed to the map by number. Open squares indicate towns or cities.

ORE DEPOSITS AND GEOLOGY

The ore deposits at Neves-Corvo contain over 200 million tons of polymetallic massive sulfides with large reserves 

of high-grade copper and tin ores; the deposits are the most 

important supply of these metals in western Europe (Carvalho, 1991). Known reserves are in four clustered deposits 

called Neves, Corvo, Gra9a, and Zambujal (fig. 2). Other 

metals occurring with the ore in these deposits include lead, 

zinc, and minor amounts of arsenic, antimony, bismuth, 

indium, cadmium, cobalt, selenium, silver, and gold.

The massive sulfide bodies in the Neves-Corvo area are 

syngenetic volcanogenic stratabound deposits that are associated with the culmination of felsic volcanism. The deposits 

are underlain by felsic volcanic rocks and overlain by sedimentary and volcanic-sedimentary rocks. The volcanic and 

volcanic-sedimentary rocks make up the Volcanic Siliceous 

Complex (VS), which comprise the host rocks for massive 

sulfide deposits throughout the Portuguese and Spanish parts 

of the Iberian Pyrite Belt (Schermerhorn, 1970; Carvalho 

and others, 1976). In the Neves-Corvo area, as in other parts 

of the PPB, the VS is overlain by the Culm Group, a flysch 

sequence of rocks that consists mostly of monotonous, cyclic 

sequences of graywacke and shale. Although the stratigraphic sequence in the Neves-Corvo area contains some 

differences as compared to other massive sulfide localities in 

the PPB, as discussed by Carvalho (1991), the lithologic 

similarities suggest that geochemical exploration methods

developed in the Neves-Corvo area would be applicable to 

other parts of the Iberian Pyrite Belt and perhaps to other 

similar metallogenic provinces.

GEOCHEMICAL STUDY ROCK AND 

SOIL LEACHATES

Ore and related elements that are released from primary 

and secondary minerals by chemical reactions and weathering processes may move in solution from buried deposits to 

the surface environment along hydrologic pathways, such as 

open pores, joints, bedding planes, fractures, and faults. At 

the surface, the mobilized metals may precipitate or be 

removed from solution at reaction sites on clay, organic 

material, carbonates, manganese oxides, iron oxides, and 

secondary sulfides. In exploration geochemistry it is important to determine the distribution of these pathfinder metals 

in various chemical phases in order to select the optimum 

dissolution technique to be used in an analytical procedure. 

This knowledge of the partitioning of ore-associated elements allows dissolution and analysis of only that sample 

fraction that contains the highest concentration of these elements. Selective dissolution often provides a greater 

geochemical contrast between anomalous and background 

values than does total dissolution. The use of partial-dissolution techniques has been shown to enhance geochemical 

anomalies in soils and sediments (see Bradshaw and others, 

1974; Chao and Theobald, 1976) and in rocks and joint coatings (Crone and others, 1984; Filipek and Theobald, 1981).

The geochemical study in the Neves-Corvo area was 

designed to determine if ore-associated metals were partitioned into one or more chemical phases in rocks and soils 

above and around the massive sulfide bodies and if this 

information could be used to delineate the location of the 

buried mineral deposit.

SAMPLE COLLECTION, 

PREPARATION, AND ANALYSIS

In July of 1987, rock and soil samples were collected 

along northeast to southwest traverses in the Neves-Corvo 

area (figs. 2 and 3). Traverse lines were perpendicular to the 

trend of the local, dominant, plunging anticline, the central 

part of which comprises the Volcanic Siliceous Complex, 

the host formation of the massive sulfide ore deposits. The 

lines were located approximately 0.5 km apart, where practical, and the rock and soil sample sites were spaced at intervals of about 0.5 km. Soil samples were collected at depths 

of 20 cm, air dried in the field, and then sieved to minus-80 

mesh (0.17 mm) in the laboratory. Rock samples containing 

visible iron or manganese oxide or other signs of alteration 

in the matrix, as coatings, or along fractures were collected 

at most sites. These were crushed to approximately minus-6 

mm in a jaw crusher and split with a riffle-type splitter. A
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRITE BELT 55

3 KILOMETERS

EXPLANATION

I I Culm Group

S Volcanic Siliceous Complex (VS)

1 * Buried massive sulfide deposits

A Neves

B Corvo

C Graca

D Zambujal

Figure 2. Rock sample localities, Neves-Corvo area, southern 

Portugal. Numbers indicate sites at which rock samples were collected; letters A-D indicate ore deposits; open squares indicate 

towns.

representative split was then ground to about 0.1 mm in a 

vertical pulverizer equipped with ceramic plates.

Rock and soil samples collected along the traverse containing samples 20-28 (figs. 2 and 3) were initially subjected 

to a four-step sequential dissolution scheme modified from a 

procedure described by Chao (1984). This scheme was 

designed to solubilize and separate chemical phases of the 

samples in the following order: (1) Mn-oxides and amorphous Fe-oxides, (2) crystalline Fe-oxides, (3) sulfides and 

organics, and (4) the residue ("silicate fraction"). The procedure for this four-step sequential dissolution technique is 

provided in Appendix 1 of this report. A separate total digestion was performed on each of these samples with a fresh 

0.5-g sample split using the same procedure as in step 4, 

Appendix 1.

Each chemical phase from the sequential dissolution 

procedure was analyzed for 18 elements at USGS laboratories in Denver, Colo., using induction coupled plasmaatomic emission spectrometry (ICP-AES) with operating 

parameters described by Motooka and Sutley (1982). Several of the rock samples on the traverse with samples 20-28 

were also analyzed by X-ray diffraction to determine the 

mineralogy of the fracture and surface coatings.

RESULTS OF SEQUENTIAL DISSOLUTION STUDY

The results of the sequential dissolution and analysis of 

rock and soil samples from the test traverse are summarized

3 KILOMETERS

EXPLANATION

[~1 Culm Group

HI Volcanic Siliceous Complex (VS)

1 2 Buried massive sulfide deposits

A Neves

B Corvo

C Graca

D Zambujal

Figure 3. Soil sample localities, Neves-Corvo area, southern 

Portugal. Numbers indicate sites at which soil samples were collected; letters A-D indicate ore deposits; open squares indicate 

towns

for selected elements in Appendixes 2 and 3. Analytical 

results of total digestions for the rock and soil samples are 

not listed in these appendixes, but the total concentrations 

were generally within 10 percent of the sum of the concentrations of steps 1-4, indicating no significant systematic 

errors due to any of the sequential dissolution steps. In both 

sample types, arsenic, iron, molybdenum, and zinc are selectively partitioned in the crystalline Fe-oxide fraction (Feox), copper and manganese in the Fe-ox and Mn-oxideamorphous Fe-oxide fractions, sodium primarily in the residue (silicate) fraction, and lead is partitioned into all of the 

above fractions at different sample sites. Arsenic, copper, 

and molybdenum show, in most cases, enhanced concentrations in the Fe-ox fraction for both rock and soil samples at 

sites 22, 23, 24, and 25, all of which are above or very close 

to the vertical projection to the surface of several of the buried massive sulfide bodies. Iron and zinc, even though selectively partitioned in the Fe-ox fractions, do not show the 

same degree of enhancement over the ore deposit as do 

arsenic, copper, and molybdenum. In fact, zinc concentrations for rock and soil samples from sites 22 and 23 indicate 

a depletion relative to the respective geometric means for 

zinc (tables 1 and 2). Manganese and lead show no pattern of 

enrichment over the deposit in either sample type, and 

sodium is selectively partitioned into the residual fraction, 

probably reflecting the occurrence of this element in the 

structure of aluminosilicate minerals.

X-ray diffraction analysis of coatings from several rock 

samples along the traverse for samples 20-28 confirms that
56 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Table 1. Statistical summary of chemical analyses for 4 N HC1 leachates of 80 rock samples, 

Neves-Corvo area, southern Portugal.

[Values preceded by 'Vindicate that the element was not detected at the lower limit of determination shown; these values were

replaced with values ec 

geometric deviations]

Variable

As (ppm)

Ba (ppm)

Ca (%)

Co (ppm)

Cr (ppm)

Cu (ppm)

Fe (%)

K (ppm)

Mg(%)

Mn (%)

Mo (ppm)

Na (ppm)

Ni (ppm)

P(ppm)

Pb(ppm)

Sr (ppm)

Ti (ppm)

Zn (ppm)

|ual to seven-tenths of

Minimum

<0.3

3.3

0.004

0.7

1

3

0.3

28

0.003

0.003

<1

34

1.8

10

<5

0.51

2.3

2

the lower determinate

Maximum

270

300

0.86

40

47

920

13

1,800

1.1

0.71

50

560

140

1,600

260

47

120

220

on limit (Miesch, 19 /

Geometric 

mean

10

31

0.051

6.5

15

29

3.5

420

0.24

0.063

1.4

97

25

130

15

5.7

15

39

'6) in determining gee

Geometric 

deviation

3.1

2.2

2.6

2.7

2.6

3.1

2.0

2.2

4.8

2.8

3.6

1.7

2.0

3.4

2.8

2.0

1.9

2.2

>metnc means an<

75th 

percentile

13

44

0.10

12

30

62

4.6

700

0.75

0.10

2.0

120

32

320

22

7.7

21

61

most of the iron present in the coatings is crystalline, with 

goethite (80-90 percent) and hematite (10-20 percent) identified as the principal iron-oxide minerals.

The results of the sequential dissolution study at NevesCorvo indicate that several ore-associated metals, such as 

arsenic, copper, and molybdenum, are selectively partitioned 

into the crystalline iron-oxide and manganese-oxide fractions of rock and soil samples and that analysis of these combined fractions may provide a high contrast between 

anomalous and background values. Thus, the chemical dissolution used for the 80 rock and 74 soil samples collected in 

the Neves-Corvo study area was that used in step 2, Appendix 1, of the sequential dissolution scheme. This dissolution 

solubilizes both the Fe-ox and the Mn-oxide-amorphous Feoxide fractions and is hereafter designated as the 4 N HC1 

leachate fraction.

RESULTS OF ANALYSIS OF 4 N HCL LEACHATES

Statistical summaries for selected elements from the 

ICP-AES analyses of 4 N HC1 leachates of rock and soil 

samples collected in the Neves-Corvo area are listed in 

tables 1 and 2. Other elements, such as antimony, bismuth, 

and tin, were detected in only a few samples and therefore 

are not listed. Concentration ranges for most of the elements 

listed in the tables fall within the analytical determination 

limits of the method used; however, some elements for several samples were reported as not detected at the lower limit 

of determination. For statistical computations, these 

qualified values were arbitrarily replaced with real values

equal to 0.7 times the lower determination limit, as described 

by Miesch (1976). Univariate analysis of these data was used 

to obtain basic element-distribution information. Inspection 

of frequency-distribution histograms and the general agreement between the median and geometric mean for most of 

the elements indicated lognormal distribution of the data. 

Cumulative frequency diagrams helped determine the 

threshold value between anomalous and background populations for each element. Where bimodal or mixed populations 

were found by this method, the threshold value commonly 

occurred between the 70th and 80th percentiles. Consequently, element threshold values were selected to be those 

concentrations at the 75th percentile as given in tables 1 and 

2. Trace-element concentrations at and above these threshold values are considered to be anomalous.

Trace-element-distribution maps for copper, arsenic, 

and molybdenum in 4 N HC1 leachates of rock samples are 

shown in figures 4, 5, and 6, and distribution maps for copper, lead, arsenic, and molybdenum in 4 Af HC1 leachates of 

soil samples are shown in figures 7, 8,9, and 10.

Copper. Anomalous amounts of copper were found 

in samples clustered in an area northwest of the town of A 

Do Neves (figs. 4 and 7). This anomaly may be related to 

surface and near-surface mineralized chert and sulfidebearing manganese occurrences, which are locally present. 

Other samples containing anomalous amounts of copper 

include rock and soil samples over the Gra9a and Zambujal 

ore bodies and samples from an area near a known nearsurface sulfide deposit northwest of the town of Monte dos 

Mestres.
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRITE BELT 57

Table 2. Statistical summary of chemical analyses for 4 N HC1 leachates of 74 soil samples, NevesCorvo area, southern Portugal.

[Values preceded by 'Vindicate that the element was not detected at the lower limit of detennination shown; these values were 

replaced with values equal to seven-tenths of the lower detennination limit (Miesch, 1976) in determining geometric means and 

geometric deviations]

Variable

As (ppm)

Ba (ppm)

Ca (%)

Co (ppm)

Cr (ppm)

Cu (ppm)

Fe (%)

K(ppm)

Mg(%)

Mn (%)

Mo (ppm)

Na (ppm)

Ni (ppm)

P(ppm)

Pb(ppm)

Sr (ppm)

Ti (ppm)

Zn (ppm)

Minimum

4

23

0.023

2.5

5.3

6

1.5

180

0.032

0.019

<1

55

5.8

110

7

3.1

16

<5

Maximum

55

320

2.5

31

28

140

4

2,000

0.74

0.43

11

340

48

810

110

120

87

110

Geometric 

mean

12

48

0.13

11

15

27

2.6

580

0.27

0.098

1.1

97

22

290

18

11

30

36

Geometric 

deviation

1.9

1.6

2.4

1.5

1.5

2.1

1.3

1.9

2.0

2.1

2.2

1.5

1.4

1.5

1.6

1.9

1.4

1.6

75th 

percentile

19

56

0.22

14

20

52

3.0

950

0.44

0.20

2.0

110

27

350

22

16

35

44

3 KILOMETERS

EXPLANATION

I | Culm Group Copper

[PH Volcanic Siliceous Complex (VS) (ppm)

^~j> Buried massive sulfide deposits < 62

 >62-130

 >130

Figure 4. Distribution of copper in rock leachates (4 N HC1), 

Neves-Corvo area, southern Portugal. Open squares indicate 

towns.

EXPLANATION

I I Culm Group

HI Volcanic Siliceous Complex (VS)

1 * Buried massive sulfide deposits

Arsenic 

(ppm) 

<13 

>13-67 

>67

Figure 5. Distribution of arsenic in rock leachates (4 N HC1), 

Neves-Corvo area, southern Portugal. Open squares indicate 

towns.
58 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

3 KILOMETERS 3 KILOMETERS

EXPLANATION

I I Culm Group

ili Volcanic Siliceous Complex (VS)

t"^ Buried massive sulfide deposits

Figure 6. Distribution of molybdenum in rock leachates (4 N 

HC1), Neves-Corvo area, southern Portugal. Open squares indicate towns.

Molybdenum

(ppm)

 <2

 >2-15

EXPLANATION

| | Culm Group

ID Volcanic Siliceous Complex (VS)

O Buried massive sulfide deposits

Lead 

(ppm)

 <22

 . >22-32

 >32

Figure 8. Distribution of lead in soil leachates (4 N HC1), 

Neves-Corvo area, southern Portugal. Open squares indicate

towns.

3 KILOMETERS 3 KILOMETERS

EXPLANATION

I I Culm Group

S3 Volcanic Siliceous Complex (VS)

O Buried massive sulfide deposits

Copper 

(ppm) 

<52 

>52-69 

>69

EXPLANATION

\ | Culm Group

HU Volcanic Siliceous Complex (VS)

C.*^ Buried massive sulfide deposits

Arsenic 

(ppm) 

<19 

£ 19-32 

£32

Figure 7. Distribution of copper in soil leachates (4 N HC1), 

Neves-Corvo area, southern Portugal. Open squares indicate 

towns.

Figure 9. Distribution of arsenic in soil leachates (4 N HC1), 

Neves-Corvo area, southern Portugal. Open squares indicate 

towns.
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRITE BELT 59

3 KILOMETERS

EXPLANATION

f~i Culm Group

lH Volcanic Siliceous Complex (VS)

^* Buried massive sulfide deposits

Molybdenum

(ppm)

 <2

 >2-5

 >5

Figure 10. Distribution of molybdenum in soil leachates (4 N 

HC1), Neves-Corvo area, southern Portugal. Open squares indicate towns.

Lead. Soil leachates contain anomalous amounts of 

lead over the Grafa orebody, over the southern part of the 

Neves orebody, and northwest of A Do Neves (fig. 8).

Arsenic Figures 5 and 9 show that elevated levels of 

arsenic are present in the leachates northwest of A Do Neves, 

near a known near-surface sulfide deposit northwest of the 

town of Monte dos Mestres, and over the Gra^a orebody.

Molybdenum. High concentrations of molybdenum 

(figs. 6 and 10) also delineate anomalous areas northwest of 

A Do Neves and adjacent to the Gra^a orebody.

Other elements. Anomalous patterns for zinc were 

not found in rock and soil leachates; however, high zinc concentrations were found at scattered locations, such as at sample sites 71 (220 ppm in rock; 110 ppm in soil), 47 (160 ppm 

in rock), and 34 (83 ppm in soil). Antimony was detected in 

samples at sites 62 (36 ppm in rock; 8 ppm in soil), 42 (25 

ppm in rock), 70 (15 ppm in rock), and 22 (13 ppm in rock). 

Also, a rock sample collected at site 62 contained 7 ppm silver in the 4 N HC1 leachate fraction.

SUMMARY OF LEACHATE STUDY

Analytical results of the selective leaching of crystalline Fe-oxide and Mn-oxide-amorphous Fe-oxide fractions 

of rocks and soils in the Neves-Corvo study area appear to 

delineate anomalies spatially associated with a known nearsurface sulfide deposit, Cerro do Algare", northwest of the

town of Monte dos Mestres, and results also appear to delineate the concealed Gra9a and Zambujal massive sulfide bodies. An anomalous area northwest of the town of A Do 

Neves, probably related to nearby mineralized chert and sulfide-bearing manganese occurrences, but possibly related to 

unknown buried sulfide deposits, was also defined. Both 

sample media indicated similar element-distribution patterns; however, the anomalies appear to be more intense in 

rock leachates as compared to soil leachates, especially for 

arsenic, copper, and molybdenum. All of these anomalies 

occur within the boundaries of the Volcanic Siliceous Complex, and they may indicate lithologies favorable for massive 

sulfide deposits rather than delineating the deposits themselves.

Significant anomalies were not found over the deeper 

massive sulfide bodies of Neves and Corvo, particularly in 

those areas of outcropping shales and graywackes of the 

Culm Group. The lack of anomalies in these areas suggests 

that leachates may be most useful in the PPB for exploration 

for near-surface mineral deposits or in defining lithologies 

favorable for hosting massive sulfide deposits.

BIOGEOCHEMICAL STUDY

The use of biogeochemistry the collection and analysis of vegetation in the search for concealed ore deposits 

has been successfully applied as an exploration tool in many 

countries. The basic principles of biogeochemical prospecting and applications in various geological environments are 

discussed in detail by Levinson (1974), Brooks (1972), and 

Kovalevskii (1974). The biogeochemical study in the NevesCorvo area was designed to determine if analysis of surface 

vegetation would reflect the presence of buried massive sulfide bodies, and, if so, could this method be applied to other 

areas in the PPB in exploration for similar hidden ore deposits. This investigation, to our knowledge, is the first biogeochemical exploration study to be undertaken in the PPB.

The vegetation found in the Neves-Corvo study area is 

predominated by evergreens, most of which are Holm oak 

with a lesser amount of Cork oak. The Holm oak (Quercus 

ilex) was described by Humphries and others (1985) as a 

broadly domed evergreen tree as much as 25 m high with 

brownish-black bark cracked into shallow, square plates. 

The Cork oak (Quercus suber) was described as an evergreen tree as much as 20 m high with thick and corky bark 

that forms sinuous interconnecting ridges and is pale grayish-brown in color but reveals a smooth and red-brown trunk 

where the bark has been stripped. The undergrowth in the 

study area is covered by grasses and aromatic herbs and 

shrubs such as Esteva cistus. The oak, or azinheira, was 

selected for the biogeochemical investigation because of its 

widespread distribution in this area and in other parts of the 

PPB and also because of its deep root system, which gives 

this tree the potential for accumulating ore-associated
60 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

elements from deep below the ground surface. An example 

of the use of biogeochemical analysis of the twigs and leaves 

of oak trees to detect buried mineral deposits in the Appalachian Piedmont in Virginia was documented by Leavitt and 

Goodell (1979).

SAMPLE COLLECTION, 

PREPARATION, AND ANALYSIS

In July of 1987, samples of Holm oak trees (Quercus 

ilex) were collected at most of the soil sample localities 

along the same traverse lines previously described (fig. 3). 

At each oak tree sample site, the first-year growth (new 

leaves and stems), older stems (2- to 5-year growth), and, 

where available, the new acorns were collected. Tree loppers 

and hand pruners were used to collect composite samples 

from around the perimeter of individual trees. An attempt 

was made to sample trees of similar size and most vigorous 

growth. At several sites, samples were taken from individual 

Holm oak trees with trunk diameters ranging from 10 to 50 

cm, as measured from a point 1 m above ground level, to 

determine the influence of tree size on trace-element content. 

Trees from three sample sites were resampled in August, 

November, and December of 1987 to determine the effects 

of seasonal variation on trace-element concentrations. No 

Cork oak trees (Quercus suber) were found growing in the 

vicinity of the buried ore deposit; however, several Cork 

oaks were sampled near sample site 2 (fig. 3) for the purpose 

of comparing its biogeochemical response to that of Holm 

oak. In addition to samples of oak trees, samples of leaves 

and stems of Esteva cistus were collected at many of the 

same sample sites.

The plant samples were stored in cloth bags and air 

dried in the field. In the laboratory, the samples were washed 

with deionized water, dried in an oven at 35°C for 24 hours, 

ground in a Wiley mill, ashed in a muffle furnace at 450°C 

for 8 hours, and homogenized in a shaker.

In the vegetation-digestion procedure, 3 mL of concentrated HC1 was added to a centrifuge tube containing 0.36 g 

of plant ash. One mL of hydrogen peroxide was then added 

and the mixture was left to stand for 1 hour. The solutions 

were then heated in a water bath at 95°C for 20 minutes, 

cooled, and diluted with 5 mL of deionized water to give 4 

Af HC1 solutions with a dilution factor of 1:25. Finally, the 

sample solutions were agitated, centrifuged, and decanted.

Each solution was analyzed for 17 elements at USGS 

laboratories in Denver, Colo., using ICP-AES with operating 

parameters described by Motooka and Sutley (1982). Some 

of the plant samples collected along the traverse for samples 

20-28 (fig. 3) were also analyzed for several additional elements, including antimony, bismuth, indium, and tin, 

employing a very sensitive inductively coupled plasmamass spectrographic (ICP-MS) technique (Lichte and others, 

1987) using the same sample solutions as described above.

For this technique, cadmium was added to each sample solution as an internal standard to correct for sample matrix variation and instrumental fluctuations. The sample solutions 

were then analyzed directly using a Sciex Elan model 250 

ICP-MS system and compared to calibration standards 

which contained 100 \igfL of the elements of interest. All 

samples and standards were corrected for background matrix 

effects and blank effects.

RESULTS OF BIOGEOCHEMICAL STUDY

Preliminary evaluation of the analytical data from vegetation collected over the buried massive sulfide deposit at 

Neves-Corvo indicated a biogeochemical response for several ore-associated elements. The most favorable anomalyto-background ratios were found in parts of the Holm oak 

tree, which contained relatively high amounts of arsenic and 

copper. The first-year growth (leaves and stems) of the Holm 

oak contained the highest concentrations of these elements, 

with lower amounts in the older stems (2- to 5-year growth), 

the new acorns, and the leaves and stems of E. cistus. Consequently, the new growth of Holm oak, hereafter referred to 

as "oak leaves," was selected as the optimum biogeochemical sample medium for this study area.

At a given site, leaves from trees with trunk diameters 

ranging in size from 15 to 50 cm contained essentially the 

same trace-element content. However, at the same sites, 

trees with diameters less than 15 cm gave concentrations that 

were 20 to 40 percent lower. The higher concentration of 

trace elements found in trees with the larger trunk diameters 

may be due to their deeper root penetration. Also, samples of 

oak leaves that were collected in the months of July and 

August, during the active growing season, had 20 to 30 percent higher concentrations of arsenic and copper than did 

leaves from the same trees sampled in November and 

December. A visual comparison of the analytical results 

from the ash of Holm oak leaves to the ash of Cork oak 

leaves, which were collected from site 2 (fig. 3), indicated 

that both species had a similar biogeochemical response. 

Percent ash was measured for most samples and showed 

only minor variation for oak leaves, with values ranging 

from 2.8 to 3.1 percent.

Statistical summaries for the ICP-AES analysis of the 

ash from oak leaves collected in the Neves-Corvo area are 

listed in table 3. Concentration ranges for most of the elements listed in the tables fall within the analytical determination limits of the method used; however, some elements for 

several samples were reported as not detected at the lower 

limit of determination. For statistical computations, these 

qualified values were arbitrarily replaced with real values 

equal to 0.7 times the lower determination limit, as described 

by Miesch (1976). Univariate analysis of these data was used 

to obtain basic element-distribution information. Inspection 

of frequency-distribution histograms and the general
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRTTE BELT

Table 3. Statistical summary of chemical analyses for 73 samples of ash from oak leaves, NevesCorvo area, southern Portugal.

[Values preceded by "<" indicate that the element was not detected at the lower limit of determination shown; these values were

61

replaced with values equal to seven-tenths of the lower determination limit (Miesch, 1976) in determining geometric means and 

geometric deviations. See text for definition of "oak leaves'*]

Variable Minimum Maximum

As (ppm) < 3.5 81 

Ba(ppm) 64 310 

Ca(%) 10 28 

Co (ppm) 2.7 32 

Cr(ppm) 4.5 46 

Cu(ppm) 120 1,600 

Fe (%) 0.2 2.4 

K(%) 5.9 12 

Mg (%) 3.2 9.8 

Mn (%) 0.6 6.2 

Na(%) 0.5 3.3 

Ni(ppm) 19 240 

P(%) 1.2 3 

Pb(ppm) <6 65 

Sr(ppm) 280 1,000 

Ti (ppm) 21 97 

Zn(ppm) 310 1,300

agreement between the median and geometric mean for most 

of the elements indicated lognormal distribution of the data. 

Cumulative frequency plots helped determine the threshold 

value between anomalous and background populations for 

each element. Where bimodal or mixed populations were 

found by this method, the threshold value commonly 

occurred between the 70th and 80th percentiles. Consequently, the element-threshold values were selected to be 

those concentrations at the 75th percentile as given in table 

3. Trace-element concentrations at and above these threshold values are considered to be anomalous.

Trace-element distribution maps for copper and 

arsenic in the ash of oak leaves are shown in figures 1 1 and 

12, and the relationship of the element pairs arsenic and 

copper, antimony and indium, and bismuth and tin in the 

ash of oak leaves to buried massive sulfide deposits along 

the traverse for samples 20-28 at Neves-Corvo are shown in 

figures 13-15.

Copper. Anomalous concentrations of copper were 

found in samples of the ash of oak leaves at sites clustered 

over buried massive sulfide bodies at Neves-Corvo in locations south and west of the town of A Do Corvo (fig. 11). The 

highest concentrations of copper in these areas occur over 

the Corvo orebody, the main copper deposit. Figure 13 , 

shows the relationship of copper in oak leaves to the subsur- t 

face massive sulfide deposit at sample sites along the 

traverse for samples 20-28. The highest biogeochemical a 

response for copper is shown between sample sites 24 and 26 o 

over the Corvo orebody, in an area where near-surface faults n 

are known to penetrate the ore zone. These faults and u

Geometric Geometric 75th 

mean deviation percentile

9.4 1.8 14 

130 1.4 160 

18 1.2 20 

9.8 1.7 14 

11 1.4 13 

180 1.4 200 

0.49 1.6 0.68 

8.9 1.2 10 

5.7 1.3 6.4 

2.4 1.6 3.5 

1.3 1.6 1.7 

62 2.0 100 

2.0 1.2 2.3 

14 1.9 21 

560 1.3 680 

48 1.4 60 

500 1.2 560

n T ,

i 11 SambranaQ

^012 3 KILOMETERS

s

EXPLANATION

CH Culm Group Copper 

H Volcanic Siliceous Complex (VS) (PP ) 

1 * Buried massive sulfide deposits < 200 

 > 200-240 

 >240

"igure 11. Distribution of copper in the ash of oak leaves, 

feves-Corvo area, southern Portugal. Open squares indicate 

owns. See text for definition of oak leaves.

ssociated fractures may provide conduits for movement 01 

re-related metals from subsurface mineral deposits to the 

ear-surface environment, where they are available foi 

ptake by the oak trees.
62 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

EXPLANATION

| | Culm Group

(HI Volcanic Siliceous Complex (VS)

^ Buried massive sulfide deposits

Arsenic 

(ppm) 

<14 

>14-20 

>20

Figure 12. Distribution of arsenic in the ash of oak leaves, 

Neves-Corvo area, southern Portugal. Open squares indicate 

towns. See text for definition of oak leaves.

100

50

gj 2°

a. 10

SCALE

Culm Group

5,000

2,000*

1,000

500

200

100 8

22 23 24 25 26 

SAMPLE LOCALITY NUMBER

28

Figure 13. Transect along traverse containing sample localities 

20-28 (see fig. 2), Neves-Corvo area, southern Portugal. Graph in 

upper part of figure shows relationship of arsenic and copper concentrations in the ash of oak leaves to buried massive sulfide deposits that are shown in the cross section in the bottom part of 

figure. See text for definition of oak leaves. Vertical scale on cross 

section is diagrammatic.

o 10.0

5.0

I"

<

O 0.1

SCALE

Culm Group

Culm Group volcanic

Siliceous

Complex

20 21 22 23 24 25 26 

SAMPLE LOCALITY NUMBER

27 28

Figure 14. Transect along traverse containing sample localities 

20-28 (see fig. 2), Neves-Corvo area, southern Portugal. Graph in 

upper part of figure shows relationship of antimony and indium 

concentrations in the ash of oak leaves to buried massive sulfide 

deposits that are shown in the cross section in the bottom part of 

figure. See text for definition of oak leaves. Vertical scale on cross 

section is diagrammatic.

Other locations containing anomalous concentrations 

of copper include samples over the known shallow sulfide 

deposit northwest of the town of Monte dos Mestres and 

northwest of A Do Neves where the anomalies may possibly 

be associated with local mineralized chert and sulfide-bearing manganese occurrences.

Arsenic. Anomalous concentrations of arsenic were 

found in samples of oak leaves from sites clustered over buried massive sulfide bodies south and west of the town of A 

Do Corvo (fig. 12). The highest concentrations of arsenic in 

these areas occur over the Neves and Corvo orebodies. Figure 13 shows the relationship of arsenic in oak leaves to the 

subsurface massive sulfide deposit at samples sites along the 

traverse for samples 20-28. Arsenic exhibits a pattern similar to that of copper over the ore deposit; however, the biogeochemical response appears to be more widespread and 

intense for arsenic, with a greater anomaly-to-background 

ratio. This pattern may have resulted from a higher mobility 

for arsenic relative to copper from the ore zone to the nearsurface environment or from unique physiological mechanisms of arsenic uptake and translocation by the oak trees.

Antimony, bismuth, indium, and tin. The distribution 

and relationship of these elements in the ash of oak leaves to
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRTTE BELT 63

20.0

10.0

CC 5.0 

0.

g 2-0

W 1.0 

CD

0.5

0.2

0.1

Culm Group

20 21 22 23 24 25 26 27 20 

SAMPLE LOCALITY NUMBER

Figure 15. Transect along traverse containing sample localities 

20-28 (see fig. 2), Neves-Corvo area, southern Portugal. Graph in 

upper part of figure shows relationship of bismuth and tin concentrations in the ash of oak leaves to buried massive sulfide deposits 

that are shown in the cross section in the bottom part of figure. See 

text for definition of oak leaves. Vertical scale on cross section is 

diagrammatic.

the buried massive sulfide deposits are shown in figures 14 

and 15. Each of these ore-related elements shows a biogeochemical response above the buried ore zones along this 

traverse, with the highest concentrations located between 

sample sites 24 and 26 above the Corvo massive sulfide 

body. Bismuth and indium display the greatest response, 

with relatively higher anomaly-to-background ratios as compared to antimony and tin.

Other elements. Anomalous clusters for lead and zinc 

(not shown here) were not found in the ash of oak leaves; 

however, high lead and zinc concentrations were obtained at 

scattered locations such as at sample sites 14 (lead = 36 

ppm), 25 (lead = 65 ppm), 48 (lead = 37 ppm), 54 (zinc = 720 

ppm), 59 (zinc = 720 ppm), and over a known near-surface 

sulfide deposit at site 71 (zinc = 1,300 ppm).

SUMMARY OF BIOGEOCHEMICAL STUDY

Analytical results by ICP-AES and ICP-MS techniques 

on samples of vegetation collected in the Neves-Corvo study 

area delineate several anomalous areas, some of which are

associated with known, buried massive sulfide deposits. The 

highest concentrations of ore-related elements, such as antimony, arsenic, bismuth, copper, indium, and tin, were found 

in the first-year growth of leaves and stems of Holm oak 

(Quercus ilex). Optimum biogeochemical response for oreassociated elements occurred in samples collected during the 

active growing season (July-August) and from trees that had 

trunk diameters greater than 15 cm. The overall best biogeochemical indicator of sulfides in the Neves-Corvo study 

area seems to be arsenic in the ash of Holm oak leaves, possibly resulting from the high mobility of this element in the 

subsurface environment and unique translocation mechanisms in Holm oak trees. Additional elements that occur in 

anomalous concentrations in the oak leaves above the buried 

deposit include antimony, bismuth, copper, indium, and tin. 

Other trace elements that were determined, such as lead and 

zinc, did not show an obvious spatial relationship with the 

ore deposit.

The results of the biogeochemical study in the NevesCorvo area indicate that biogeochemical exploration methods, specifically the sampling and analysis of the first-year 

growth of oak trees during the active growing season, may 

be useful to exploration programs in the Portuguese Pyrite 

Belt in the search for concealed massive sulfide deposits.

HYDROGEOCHEMICAL STUDY

Circulating ground water in contact with buried ore 

deposits may dissolve and transport trace elements that are 

associated with the deposits and form aqueous dispersion 

patterns. The detection of these dispersion patterns, by collecting and analyzing ground water, forms the basis of 

hydrogeochemical exploration. Applications of hydrogeochemical exploration methods in various climates and 

geochemical environments have been discussed by Boyle 

and others (1971) and Miller (1979). The hydrogeochemical 

study in the Neves-Corvo area was undertaken to determine 

if the chemistry of ground water, collected from wells, 

would reveal the presence of the concealed massive sulfide 

deposit. If positive results were obtained from this study, 

then hydrogeochemical techniques could be used advantageously in exploration programs in the PPB, and possibly in 

other areas of Portugal because of the large number and general distribution of water wells available to sample.

SAMPLE COLLECTION, 

PREPARATION, AND ANALYSIS

In July of 1987 and 1988, samples of ground water were 

collected from open water wells and from pumped wells in 

the Neves-Corvo area (fig. 16). The water samples were collected from the wells by lowering a polyethylene bottle into 

the well with a nylon cord and filling the bottle with water.
64 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994 

Table 4. Analytical methods used of analysis of water samples, Portuguese Pyrite Belt, Portugal.

Variable Method Reference

Alkalinity

Cl, F, S04

Ca, K, Mg, Na, SiO2

Fe, Mn

As, Co, Cr, Cu, Mo, Ni, Pb, Sb, Zn

Au

Gran's plot potentiometric titration

Ion chromatography

Flame atomic absorption spectrophotometry

Flameless atomic absorption spectrophotometry

Inductively coupled plasma-mass spectrometry

Anion exchange-flameless atomic absorption

Orion Research Inc. (1978) 

Fishman and Pyen (1979) 

Perkin-Ehner Corp. (1976) 

Perkin-Elmer Corp. (1977) 

Lichte and others (1987) 

McHugh(1986)

3 KILOMETERS

EXPLANATION

Culm Group

Volcanic Siliceous Complex (VS) 

+ Buried massive sulfide deposits

A Neves

B Corvo

C Graca

D Zambujal

Figure 16. Water sample localities, Neves-Corvo area, southern 

Portugal. Numbers indicate sites at which water samples were collected; letters A-D indicate ore deposits; open squares indicate 

towns.

Where pumps were encountered (sample sites 8 and 12), the 

water was allowed to flow from the pump for at least 5 minutes before a sample was collected. At each sample site, a 

50-mL aliquot of water was filtered through a 0.45-j.im 

membrane filter and acidified to a pH of approximately 2 

with five drops of reagent-grade, concentrated nitric acid. A 

250-mL aliquot of water was collected at each site and left 

untreated. At two sample sites (8 and 24), 1-L aliquots of 

untreated water were also collected. All the water samples 

were stored in acid-rinsed polyethylene bottles.

Water temperature, pH, and specific conductivity were 

measured at each site. Antimony, As, Co, Cu, Cr, Fe, Mn, 

Mo, Ni, Pb, SiO2, and Zn were determined using the filtered 

and acidified samples. Alkalinity, chloride, sulfate, fluoride, 

calcium, magnesium, potassium, and sodium were determined on the untreated samples. Gold was determined on the

two 1-L samples after filtration and acidification in the 

laboratory. The analytical techniques used for each constituent measured in the water samples are listed in table 4.

RESULTS OF HYDROGEOCHEMICAL STUDY

Analytical results for water samples collected during 

orientation studies in July 1987 in the Neves-Corvo area 

showed that anomalous concentrations of several ore-associated elements were present in the ground-water system 

above the buried massive sulfide deposit. Additional sampling of ground water in the Neves-Corvo area and in other 

geochemical study areas in southern Portugal (discussed 

later in this report), was carried out in July 1988. Statistical 

summaries of the chemical analyses for water samples are 

listed in table 5. Concentration ranges for most of the variables listed in the tables fall within the analytical determination limits of the respective methods used; however, some 

variables for several samples were reported as not detected 

at the lower limit of determination. For the statistical computations, these qualified values were arbitrarily replaced with 

real values equal to 0.7 times the lower determination limit, 

as described by Miesch (1976). Univariate analysis of these 

data was used to obtain basic distribution information on the 

variables measured. Inspection of frequency-distribution 

histograms and the general agreement between the median 

and geometric mean for most of the variables indicated lognormal distribution of the data. Cumulative frequency plots 

helped determine the threshold value between anomalous 

and background populations for each variable. Where bimodal or mixed populations were found by this method, the 

threshold value commonly occurred between the 70th and 

80th percentiles. Consequently, the threshold values were 

selected to be those concentrations at the 75th percentile, as 

given in table 5. Concentrations at and above these threshold 

values are considered to be anomalous. Trace-element distribution maps for copper, arsenic, antimony, molybdenum, 

and sulfate in ground-water samples collected in the NevesCorvo area are shown in figures 17-21.

Copper. Anomalous concentrations of copper were 

found in samples of water from sites in the vicinity of the 

buried ore deposit at locations between the towns of A Do 

Neves and A Do Corvo (fig. 17). The highest concentrations 

of copper in this area are from samples collected from a
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRITE BELT 65

Table 5. Statistical summary of chemical analyses for 55 water samples, Neves-Corvo and other 

geochemical study areas, southern Portugal.

[Values preceded by 'Vindicate that the element was not detected at the lower limit of determination shown; these values were 

replaced with values equal to seven-tenths of the lower determination limit (Miesch, 1976) in determining geometric means and 

geometric deviations. Arithmetic mean and standard deviation are shown below for pH]

Variable

Alkalinity (mg/L)

As(ug/L)

Ca (mg/L)

Cl(mg/L)

Co(ug/L)

Cond. (us)

Cr(ug/L)

Cu(ng/L)

F(mg/L)

Fe(ug/L)

K(mg/L)

Mg (mg/L)

Mn(ug/L)

Mo (ug/L)

Na (mg/L)

Ni(ug/L)

Pb(ug/L)

pH

SO4 (mg/L)

Sb(ug/L)

SiO2 (mg/L)

Temp. (°C)

Zn(ug/L)

Minimum

5.0

1.0

5.0

6.0

<0.1

110

<0.1

1.0

<0.1

<10

0.1

3.0

<10

<0.1

13

<0.1

<0.1

4.7

4.0

<0.1

<1.5

16.7

<0.1

Maximum

620

90

120

560

8.6

2,300

14

16Q

2.0

690

23

75

13,000

16

420

18

29

9.1

270

120

47

28.9

400

Geometric 

mean

160

2.0

32

76

0.21

660

0.56

3.3

0.30

27

0.76

18

26

0.47

67

1.5

0.50

7.3

34

0.22

15

20.6

4.8

Geometric 

deviation

3.1

3.0

2.2

2.4

2.7

1.8

3.2

2.0

2.5

2.3

3.2

2.2

5.3

4.0

1.9

2.9

4.0

1.1

2.3

5.0

2.0

0.6

3.2

75th 

percentile

343

3

59

150

0.31

980

0.66

5

0.54

40

1.3

32.8

32.5

1

91.5

3

1.45

7.8

48.0

0.30

22

21.7

8.2

3 KILOMETERS

EXPLANATION

f~| Culm Group

HI Volcanic Siliceous Complex (VS)

O Buried massive sulfide deposits

Copper 

(pg/L)

 <5

 £5-7

3 KILOMETERS

EXPLANATION

I I Culm Group

S3 Volcanic Siliceous Complex (VS)

C. + Buried massive sulfide deposits

Arsenic 

(Pg/L) 

. <3 

 £3-8

Figure 17. Distribution of copper in water, Neves-Corvo area, Figure 18. Distribution of arsenic in water, Neves-Corvo area, 

southern Portugal. Open squares indicate towns. southern Portugal. Open squares indicate towns.
66 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

3 KILOMETERS

EXPLANATION

CH Culm Group

HI Volcanic Siliceous Complex (VS)

C. + Buried massive sulfide deposits

Antimony 

Gig/L)

 <0.3

 £0.3-3.0 

£3.0

Figure 19. Distribution of antimony in water, Neves-Corvo 

area, southern Portugal. Open squares indicate towns.

pump, near A Do Corvo, at site 8 (9 |ig/L), and from a well 

located adjacent to the Ribeira de Oeiras, at site 9 (8 |ig/L). 

By contrast, the geometric mean for copper in water is 3.3 

|ig/L (table 5). Background values for copper from wells 

located away from known massive sulfide deposits in areas 

dominated by rocks of the Culm Group, at sites 6,7,13, and 

14 (fig. 16), ranged from 3 to 4 |ig/L. Similarly, water collected from wells in areas dominated by rocks of the Volcanic Siliceous Complex, at sites 1,4,20, and 22, contained 

copper concentrations ranging from 2 to 4 |ig/L. A sample of 

mine water, originating from beneath the ore horizon at a 

depth of 600 m below ground level and pumped to the surface through steel pipes, was collected at site 24. The mine 

water was alkaline (pH > 7.0), with a measured pH of 9.1, 

and contained only a moderately anomalous concentration of 

copper (6 |ig/L). This relatively low concentration of copper 

probably results from the limited mobility of many metallic 

cations, including Cu"1"1", in alkaline waters. The pH of 

hydrolysis, which is the pH at which elements in solution 

begin to precipitate as hydroxides, is only 5.3 for Cu** (Britton, 1955). The highest concentration of copper in ground 

water at Neves-Corvo was from a well at site 23, northwest 

of the town of A Do Neves. That water contained 160 |ig/L 

copper and was acidic (pH < 7.0), with a measured pH of 6.7. 

The lower pH of the water in this well is more favorable for 

copper solubility and mobility. The chemistry of the water of 

this sample is likely affected by the exposed and near-surface mineralized chert and sulfide-bearing manganese 

occurrences that are locally present; however, the high

EXPLANATION

I I Culm Group

H3 Volcanic Siliceous Complex (VS)

C. + Buried massive sulfide deposits

Molybdenum 

(ug/L)

 <1

 £1-3.3

 £3.3

Figure 20. Distribution of molybdenum in water, Neves-Corvo 

area, southern Portugal. Open squares indicate towns.

3 KILOMETERS

EXPLANATION

| | Culm Group

lip Volcanic Siliceous Complex (VS)

^t Buried massive sulfide deposits

Sulfate 

(mg/L)

 <48

 £48-88

 £88

Figure 21. Distribution of sulfate in water, Neves-Corvo area, 

southern Portugal. Open squares indicate towns.

copper concentration and low pH of the water here could 

have resulted from contact with unknown buried sulfide 

minerals.
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRITE BELT 67

Arsenic. Anomalous concentrations of arsenic were 

found in water samples from sites over and peripheral to the 

buried sulfide deposit (fig. 18). Highest concentrations of 

arsenic in this area are from samples collected from a pump 

in the town of A Do Corvo, at site 8 (12 [ig/L); from a well 

near the Ribeira de Oeiras, at site 9 (36 Hg/L); from a well 

east of Monte dos Mestres, at site 11 (8 [ig/L); from a well 

northeast of A Do Corvo, at site 15(15 [ig/L); and from the 

sample of mine water originating from below the ore horizon 

at site 24 (90 [ig/L). By contrast, the geometric mean for 

arsenic in water is 2.0 [ig/L (table 5). Background values for 

arsenic from wells located away from known sulfide deposits in areas dominated by rocks of the Culm Group, at sites 6, 

7,13, and 14 (fig. 16), ranged from < 1.0 to 2.0 [ig/L. Similarly, water collected from wells in areas dominated by Volcanic Siliceous Complex rocks, at sites 1, 4, 20, and 22, 

contained arsenic concentrations ranging from < 1.0 to 2.0 

[ig/L. The pH of the water from the anomalous sample sites 

ranged from 7.2 to 9.1. This alkaline water favors the solubility and mobility of arsenic as complex anionic species and 

may be a controlling factor in the anomalous dispersion pattern of this element. The alkalinity may also have caused a 

higher anomaly-to-background ratio for arsenic than for copper above the buried massive sulfide deposit. A water sample 

collected from a well west of the town of Sambrana also contained high concentrations of arsenic (28 [ig/L). The cause of 

this anomaly is unknown; however, based on the topographic relief in this area, the ground-water source at mis 

well is likely from the south or southwest, suggesting that the 

area to the south could contain buried sulfide mineral 

deposits.

Antimony and Molybdenum. Anomalous concentrations of antimony and molybdenum were also found in water 

samples from the area around the buried sulfide deposit at 

Neves-Corvo (figs. 19 and 20). The highest concentrations 

of antimony and molybdenum are in samples of water collected from pumps, near A Do Corvo and A Do Neves, at 

sites 8 (6 [ig/L Sb and 8 [ig/L Mo) and 12 (18 and 5 [ig/L); 

from a well, near the Ribeira de Oeiras, at site 9 (11 and 7 

[ig/L); from a well, east of Monte dos Mestres, at site 11 (0.4 

and 3.3 [ig/L); from well water, northeast of A Do Corvo, at 

site 15 (0.4 and 7 [ig/L); and from the sample of mine water 

at site 24 (120 and 16 [ig/L). By contrast, the geometric 

means for antimony and molybdenum in water are 0.22 [ig/L 

and 0.47 [ig/L respectively (table 5). Background values for 

antimony and molybdenum from wells located away from 

known sulfide deposits in areas dominated by rocks of the 

Culm Group, at sites 6, 7, 13, and 14 (fig. 16), ranged from 

0.1 to 0.2 [ig/L for antimony and from 0.2 to 0.7 [ig/L for 

molybdenum. Similarly, water samples collected from wells 

in areas dominated by rocks of the Volcanic Siliceous Complex, at sites 1, 4, 20, and 22, contained concentrations of 

antimony ranging from < 0.1 to 0.2 [ig/L and concentrations 

of molybdenum ranging from 0.3 to 0.8 [ig/L. The distribution of anomalous antimony and molybdenum in the water 

samples is similar to that of arsenic and probably resulted

from the occurrence of these elements as complex anionic 

species, which are also relatively mobile in the alkaline 

ground-water environment.

Sulfate. Anomalous concentrations of sulfate were 

found in water samples in the vicinity of the buried ore 

deposit at locations between the towns of A Do Neves and A 

Do Corvo (fig. 21). The highest concentrations of sulfate in 

this area occurred in samples from pumps, near A Do Corvo 

and A Do Neves, at sites 8 (87 mg/L) and 12 (220 mg/L); 

from a well, near the Ribeira de Oeiras, at site 9 (130 mg/L); 

from well water collected south of A Do Neves, at site 10 (97 

mg/L); and from the mine water at site 24 (270 mg/L). By 

contrast, the geometric mean for sulfate is 34 mg/L (table 5). 

Background values for sulfate in water from wells located 

away from known sulfide deposits in areas dominated by 

rocks of the Culm Group, at sites 6, 7, and 14 (fig. 16), 

ranged from 31 to 37 mg/L. Similarly, water samples collected from wells in areas dominated by rocks of the Volcanic Siliceous Complex, at sites 20, and 22, contained 

sulfate concentrations ranging from 30 to 38 mg/L. The 

anomalous sulfate concentrations are probably derived from 

the dissolution of sulfide minerals associated with the buried 

ore deposit.

Other elements. Other variables that were measured 

in the water samples in the Neves-Corvo area did not show a 

spatial relationship with the buried ore deposit; however, 

several samples from individual sites did contain high concentrations of some variables. A water sample collected 

from a well northwest of A Do Neves, at site 23, near the 

mineralized chert and sulfide-bearing manganese occurrences, contained 29 [ig/L lead and 400 [ig/L zinc. The pH 

for this sample (6.7) favors the solubility and mobility of 

lead and zinc as cationic species. The sample of mine water 

from site 24, with the pH of 9.1, contained only 0.3 [ig/L lead 

and 2 [ig/L zinc, which demonstrates the limited mobility of 

these elements in alkaline waters.

Anomalous concentrations of gold were reported in the 

two 1-L samples collected from a pump, near A Do Corvo, 

at site 8 (2 ng/L), and from the sample of mine water at site 

24 (4 ng/L). These gold values probably reflect gold associated with the buried massive sulfide deposit at Neves-Corvo, 

and they suggest limited solubility and mobility of gold in 

high-pH waters around these types of deposits.

DISCUSSION OF HYDROGEOCHEMICAL RESULTS

Mine development activities prior to our hydrogeochemical study in the Neves-Corvo area included mine 

dewatering processes to reduce the inflow of water into 

active underground workings. Mine water is pumped to the 

surface through steel pipes from 600 m below the ground 

level and is discharged into the Ribeira de Oeiras at a point 

approximately 200 m east of water sample site 9 (fig. 16). 

Analysis of a sample of this mine water, collected at site 24, 

indicated anomalous amounts of chromium (8 [ig/L), a water
68 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

temperature of 28.8°C, and a high pH of 9.1. The high 

temperature and pH are normal for mine water from this 

depth (Fernandez-Rubio and others, 1988), and the high 

chromium may have resulted from the dissolution of this 

metal from contact of water with the 600 m of steel pipe. A 

water sample collected in a nearby well, adjacent to the 

Ribeira de Oeiras at site 9, contained slightly elevated levels 

of chromium (1.3 l-ig/L), a temperature measured at 26.7°C, 

and a relatively high pH of 7.8. The chemistry of this sample 

is similar in some respects to the chemistry of the sample 

from site 24, which suggests that it may be partly influenced 

by mine water pumped to the surface during mine dewatering. Other nearby water samples collected from sites 8 and 

12, at locations at least 1 km away from the point of discharge of the pumped mine water, have temperatures of 

19.4°C and 20.0°C and pH readings of 7.2 and 7.0. The chromium concentrations in the water samples from these two 

sites are 0.5 and 0.3 (Xg/L, respectively. These findings are 

considered normal, and it is deemed unlikely that the water 

at sites 8 and 12 has been contaminated to any degree by the 

pumped mine water.

In a hydrologic study at the Neves-Corvo deposit, 

Fernandez-Rubio and others (1988) defined three hydrologic 

units: (1) a surface-system aquifer including the alluvial terrace and the zone of weathering, (2) an intermediate-system 

aquifer with primary permeability provided by vertical 

faults, and (3) a deep-system aquifer with permeability supported by microfissures in the ore deposits. In more recent 

studies of the local hydrology, Nunes and others (1990) 

describe the surface system as including the zone of decompression of the rock mass to a depth of approximately 30 m, 

with the water table following local topography and flowing 

toward the Ribeira de Oeiras, with the exception of the zone 

of influence of the mine. Hanes and others (1990) stated that 

the intermediate hydrologic system acts purely as a water 

transfer system between the deep system, in contact with the 

ore deposit, and the surface system. They further noted that 

the deep system, with intense fracturing around the orebodies, would have a high fracture permeability. This would, in 

effect, provide a large surface area of contact between the 

ground water in the deep system and the ore and accessory 

minerals in the buried massive sulfide deposit. The large surface area of contact would favor dissolution of ore-related 

elements from the deposit into the deep ground-water system. Nunes and others (1990) concluded that, prior to mine 

development, vertical ascent of water would take place along 

decompression zones, such as faults and fractures. This 

ascent of water from the deep system to the surface system 

would provide a possible mechanism of vertical hydromorphic dispersion of ore-related elements to the near-surface 

ground-water environment.

The sample of mine water, collected from site 24 (fig. 

16) and originating from the deep hydrologic system in contact with the massive sulfide orebodies, contained highly 

anomalous concentrations of arsenic (90

molybdenum (16 ng/L), and antimony (120 ng/L). This 

demonstrates that ore-related elements are being dissolved 

and dispersed into the lower ground-water system at NevesCorvo. Samples of water, collected from pumps at sites 8 and 

12 (fig. 16), contain moderately anomalous concentrations 

of arsenic (12 and 5 ng/L), antimony (6 and 18 Hg/L), and 

molybdenum (8 and 5 |Hg/L). The source of the water from 

these pumped wells is at a depth of 30 to 60 m below ground 

level, which corresponds to the intermediate hydrologic system of Fernandez-Rubio and others (1988) and the water 

transfer system of Nunes and others (1990). The analytical 

results of these pumped samples demonstrates that anomalous concentrations of ore-related elements are being hydromorphically dispersed from the deep hydrologic system, in 

contact with the orebodies, into die intermediate hydrologic 

system at Neves-Corvo. Samples of water collected from 

wells at sites 11 and 15 (fig. 16) at depths of 1.5 m and 3.5 m 

below ground level, contained anomalous concentrations of 

arsenic (8 and 15 ng/L), antimony (0.4 and 0.4 ng/L), and 

molybdenum (3.3 and 7 Hg/L). By contrast, background concentrations for arsenic, antimony, and molybdenum in water 

samples from wells located away from the ore deposit in 

areas dominated by rocks of the Volcanic Siliceous Complex, at sites 6, 7,13, and 14 (fig. 16), ranged from < 1.0 to 

2.0 ng/L for arsenic, from < 0.1 to 0.2 ng/L for antimony, 

and from 0.3 to 0.8 ng/L for molybdenum. The analysis of 

these water samples from wells in the surface hydrologic 

system demonstrates the hydromorphic dispersion of anomalous concentrations of ore-related elements from the deep 

hydrologic system to the surface ground-water environment.

SUMMARY OF HYDROGEOCHEMICAL STUDY

Analytical results by ICP-MS and other sensitive analytical techniques on samples of water collected from open 

wells and pumping wells in the Neves-Corvo area show 

anomalous hydrogeochemical dispersion patterns associated 

with the subsurface massive sulfide bodies. Strongly anomalous concentrations of antimony, arsenic, and molybdenum 

(10-100 times background) and weakly anomalous concentrations of copper, lead, sulfate, and zinc (2-10 times background) were found in the ground-water system above the 

buried deposit. Alkaline waters, which predominate in the 

Neves-Corvo area and in most of the Portuguese Pyrite Belt, 

favor the solubility and mobility of the ore-associated elements antimony, arsenic, and molybdenum as complex 

anionic species, but these waters greatly diminish the solubility and mobility of other ore-related metals that form cationic species, such as copper, lead, and zinc. This 

relationship is illustrated by the chemistry of the mine water 

originating from below the ore zone (site 24), which had a 

pH of 9.1 and contained 120 ng/L antimony, 90 ng/L 

arsenic, 16 \ig/L molybdenum, but only normal to slightly 

elevated levels of copper (6 ng/L), lead (0.3 Hg/L), and zinc
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRITE BELT 69

(2 ng/L). In contrast, the sample of water collected from a 

well in the area of near-surface mineralized chert and sulfide-bearing manganese occurrences (site 23) had a pH of 

6.7 and contained highly anomalous concentrations of copper (160 Hg/L), lead (29 ng/L), and zinc (400 ng/L), but only 

normal concentrations of antimony (0.1 ng/L), arsenic (< 1 

Hg/L), and molybdenum (0.1 ng/L). These facts underscore 

the importance in hydrogeochemical exploration studies of 

knowing the ore-associated elements, understanding the fundamental chemical controls that may affect trace-element 

solubility and mobility, and having the ability to measure 

these elements at very low concentrations by sensitive multielement analytical techniques.

The results of the hydrogeochemical study in the 

Neves-Corvo area indicate that ground water in contact with 

the buried massive sulfide mineral deposit is solubilizing and 

dispersing several of the ore-related elements, especially 

antimony, arsenic, and molybdenum, into the local groundwater system. The dispersion patterns of these elements 

define anomalous areas associated with the buried massive 

sulfide deposit at Neves-Corvo, and the patterns demonstrate 

the potential use of hydrogeochemical techniques in exploration programs for concealed massive sulfides in the Portuguese Pyrite Belt and in other geologically favorable areas.

D Cereal

A Castanheira EXPLANATION

Geodetic points 

 Water samples 

X Oak tree samples

Gravity-high area

1 KILOMETER

Figure 22. Water and oak tree sample localities, Salgadinho 

area, southern Portugal. Numbers (i.e., 8SAL) indicate sites at 

which water and (or) oak tree samples were collected. Open 

square indicates town.

GEOCHEMICAL STUDIES IN AREAS 

OF POSITIVE GRAVITY ANOMALIES

Biogeochemical and hydrogeochemical sampling was 

carried out in July and August of 1988 in several areas of 

southern Portugal where earlier geophysical studies had 

delineated strong, positive gravity anomalies. Samples of 

oak leaves and well water from these areas were collected, 

prepared, and analyzed by procedures previously described 

in this report. These studies were undertaken to determine if 

biogeochemical and hydrogeochemical data would help to 

evaluate and interpret these gravity anomalies in the search 

for concealed ore deposits in the Portuguese Pyrite Belt. 

Sampling time was limited to one day for each of these four 

anomalous areas, and, as a result, these studies are considered to be preliminary in nature.

SALGADINHO STUDY AREA

The Salgadinho study area is located in the Cercal-Salgadinho region of the PPB (fig. 1). This area contains buried 

volcanogenic disseminated sulfide mineral deposits in felsic 

volcanic rocks that have been investigated by diamond drilling in recent years and found to contain subeconomic 

amounts of copper, gold, lead, silver, and zinc. Narrow zones 

were found that contained as much as 16 ppm gold and 80 

ppm silver in drill-core samples. Sulfide minerals consist

primarily of pyrite and local concentrations of chalcopyrite, 

sphalerite, tetrahedrite, and galena. The geology, mineralogy, and alteration at Salgadinho is discussed in detail by 

Plimer and Carvalho (1982).

Samples of leaves from Cork oak trees (Quercus suber) 

at 3 sample sites and water from 8 wells were collected during August of 1988 at localities above and around the gravity 

high at Salgadinho (fig. 22). Analytical results for selected 

elements in the water samples and in the ash of the Cork oak 

leaves from this area are listed in tables 6 and 7.

Anomalous amounts of antimony, arsenic, and molybdenum were found in a water sample collected from a well at 

sample site 1SAL, approximately 130 m to the east of the 

Salgadinho geodetic point (fig. 22). The water from this site 

was alkaline (pH = 7.4), which favors the solubility and 

mobility of antimony, arsenic, and molybdenum as complex 

anions. Well water from site 2SAL was acidic (pH = 6.1) and 

contained anomalous concentrations of copper and zinc, and 

water from site 3S AL had a very low pH (4.7) and contained 

high concentrations of zinc and detectable gold. The low pH 

of these water samples, which probably resulted from the 

release of hydrogen ions (H"1") to the ground-water system as 

a result of oxidation of sulfides, favors the solubility and 

mobility of copper and zinc as cations. Dissolution of the 

sulfides may also cause the release of small amounts of gold 

to the ground water, resulting in the gold anomaly at site 

3SAL. Anomalous concentrations of zinc and low pH values 

were also found in water samples from wells at sites 7SAL 

and 8SAL in an area 1 km southeast of the Salgadinho
70 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Table 6. Selected analytical results for water samples from geochemical study areas, southern 

Portugal.

[Values preceded by "<" indicate that the element was not detected at the lower limit of determination shown]

Sample 

number

As Au Cr Cu Mo Pb pH Sb Zn 

(Hg/L) (ng/L) (ng/L) (ng/L) (ng/L) (ng/L)_______frig/L) (tig/L)

Salgadinho geochemical study area

1 SAL.. ............

2 SAL...... ........

3 SAL. .............

4 SAL..............

5 SAL.. ............

7 SAL... ...........

8 SAL... ...........

3 <1

<1 <1

..............1 1

..............1 <1

..........< 1 <1

0.4

0.4

0.3

0.4

0.5

0.5

0.4

3

3

3

2

3

1.1 0.1

0.1 0.2

<0.1 0.2

<0.1 0.4

<0.1 0.1

<0.1 0.2

<0.1 0.8

7.4

6.1

4.7

5.6

6.2

5.7

5.7

33

0.1

0.1

0.1

0.2

0.1

0.1

1

25

65

8

6

31

19

Garrochal-Almoleias geochemical study area

1W.... .............

2 W.... .............

3 W....... ..........

4W.... .............

5 W...... ...........

6W... ..............

..............2

..............1

..............2

..............7

........ ......1

..............4

0.5 

0.6 

0.5 

5.0 

<1 0.6 

<1 0.5

3 

3 

6 

1 

2 

2

0.7 

0.1

3.0 

0.4 

0.2

4.0 

3.8 

3.9 

0.1 

4.0 

3.8

7.0 

5.7 

6.3 

7.4 

7.5 

7.5

0.1 

0.2 

0.3 

0.4

3 

8 

10 

8 

3 

4

Cidrao geochemical study area

1CW.. ............

2 CW.. ............

3 CW.. ............

4 CW.. ............

5CW..............

..............3

..............4

..............2

..............3

..............2

0.4 

0.3 

<1 0.3 

1.3 

0.8

2 

3 

2 

3 

5

3.0 

1.1 

0.4 

2.0 

0.4

3.0 

3.3 

3.8 

3.7 

3.8

7.6 

8.4 

6.7 

8.3 

7.3

0.3 

1.5 

0.3 

3.0 

0.4

5 

2 

3 

7 

11

Mombeja geochemical study area

1ML................

2 ML................

3 ML................

4 ML................

5 ML.... ............

6 ML................

7ML................

8 ML................

9 ML.. ..............

10 ML..............

11 ML..............

.............. 1

.............. 1

.............. 1

. .. ....<!

..............2

..............1

......... ....1

.......... <1

............. 1

............. 1

.............2

<1 0.8

2 0.2

<1 1.4

<1 0.5

<1 0.6

<1 0.6

<1 6.0

< 1 12.0

8.0

14.0

<1 1.2

2

2

3

2

2

2

2

2

2

2

2

0.2

0.3

0.6

0.4

0.3

0.3

0.1

0.1

0.3

0.1

0.3

0.3

0.1

0.4

0.3

<0.1

0.1

2.5

0.1

1.4

0.8

0.5

7.4

7.3

7.9

7.2

7.5

7.2

7.4

7.3

7.9

7.3

8.1

<0.1

0.1

0.1

<0.1

0.1

0.1

0.1

0.1

0.1

0.7

0.2

3

1

3

1

2

1

1

<1

2

1

9

gravity high. These anomalies may be related to an extension 

of the Salgadinho mineralized zone.

Anomalous concentrations of copper were found in the 

ash of leaves from Cork oak trees at sample sites 6SAL and 

8SAL (fig. 22). High concentrations of lead were also found 

in oak leaves from sites 1SAL and 6SAL.

DISCUSSION OF RESULTS 

SALGADINHO STUDY AREA

Analytical results for samples of Cork oak leaves and 

well water collected in the Salgadinho study area show biogeochemical and hydrogeochemical anomalies in an area of 

known disseminated sulfide deposits and in an area 1 km 

southeast of the known deposits. Where the ground water is

acidic, the best geochemical pathfinder seems to be zinc in 

well water. Where the ground water is alkaline, antimony, 

arsenic, and molybdenum are found in anomalous 

concentrations associated with sulfide minerals. The zinc 

anomaly at site 8SAL may be related to ground water 

moving southeastward from the Salgadinho area. However, 

the zinc anomaly at site 7S AL, which is stronger than that at 

site 8SAL, suggests an unknown mineralized source in this 

area and may warrant further study.

GARROCHAL-ALMOLEIAS STUDY AREA

The Garrochal-Almoleias study area is located 10 km 

southwest of Castro Verde (fig. 1) and is part of the Estac.ab 

de Ourique-Neves volcanic lineament, along which occur
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRITE BELT 71

Table 7. Selected analytical results for samples of ash from oak 

leaves from geochemical study areas, southern Portugal.

[Values preceded by "<" indicate that the element was not detected at the lower limit 

of determination shown]

Sample 

number

As 

(ppm)

Cu 

(ppm)

Pb 

(ppm)

Zn 

(ppm)

Salgadinho geochemical study area

1SAL...

6 SAL...

8 SAL...

..... ....<!

.........< 1

...... ...<1

160 

250 

340

24 

22 

<6

480 

530 

470

Garrochal-Almoleias geochemical study area

1L........

2 L. .......

3L. .......

4L.. ......

5L........

6L. .......

1L. .......

8L........

9L........

10L......

.........<1

..... ....<!

.........<1

..... ....<!

..... ....<!

.........< 1

.........< 1

.........< 1

.........< 1

.........< 1

260 

190 

130 

160 

190 

170 

130 

170 

140 

150

12 

13 

28 

11 

14 

8 

8 

17 

9 

11

440 

310 

340 

320 

520 

420 

300 

300 

330 

350

Cidrao geochemical study area

1CL.....

2CL.....

3CL.....

4CL.....

.........< 1

.........< 1

.........< 1

..... ....<!

120 

150 

160 

150

10 

24 

36

17

370 

470 

570 

420

the major sulfide deposits of Neves-Corvo. The rocks in the 

study area belong to the Volcanic Siliceous Complex and 

include felsic volcanic rocks, spilites, diabase sills, and 

siliceous shales. Although the results of drilling did not, as 

yet, prove the existence of mineral deposits, the potential of 

this target remains to be fully evaluated.

Samples of leaves from Cork oak trees (Q. suber) at 

10 sample sites and water from six wells were collected 

during July of 1988 in the vicinity of the high gravity areas 

(fig. 23). Analytical results for selected elements in water 

samples and in the ash of Cork oak leaves from this area 

are listed in tables 6 and 7. Anomalous concentrations of 

copper, lead, and zinc were found in a water sample collected from a well at sample site 3W, approximately 1 km 

northwest of the Almoleias geodetic point (fig. 23). The 

water from this site had a pH of 6.3, which favors the dissolution and mobility of copper, lead, and zinc. A water sample from a well at site 2W also had a low pH (5.7) and 

contained 3.8 ng/L lead but only background concentrations of copper and zinc. Anomalous concentrations of antimony, arsenic, chromium, and molybdenum were measured 

in a water sample from a well at site 4W, approximately 0.5 

km west of a small gravity anomaly. The high chromium 

value in the water at this site may be due to the dissolution 

of this metal from weathering of exposed mafic sills that 

are present in the area. However, the reason for the antimony, arsenic, and molybdenum anomalies at this site is 

unknown.

EXPLANATION

Geodetic points 

Water samples 

Oak tree samples 

Gravity-high area

1 KILOMETER

Figure 23. Water and oak tree sample localities, Garrochal-Almoleias area, southern Portugal. Numbers (i.e., 9L, 6W) indicate 

sites at which water or oak tree samples were collected.

Anomalous concentrations of copper in the ash of Cork 

oak leaves was found at site 1L, and leaves from an oak tree 

at site 3L contained anomalous concentrations of lead. Both 

of these sample locations are on hills within the large gravity anomaly north of the Garrochal geodetic point. No 

vegetation samples in this study area contained detectable 

concentrations of arsenic.

DISCUSSION OF RESULTS 

GARROCHAL-ALMOLEIAS STUDY AREA

Analytical results on samples of Cork oak leaves and 

ground water collected in the Garrochal-Almoleias study 

area revealed two multielement hydrogeochemical anomalies. The cause of the copper, lead, and zinc anomaly in the 

water from site 3W is unknown; however, the low pH of 

the sample suggests that the oxidation of sulfides with subsequent release of hydrogen ions to the ground water may 

be responsible. Based on the topographic relief in this area, 

the direction of ground-water flow to this site is likely from 

the south or southwest, and detailed geochemical studies in 

this area may help in revealing the cause of this anomaly. 

The second multielement hydrogeochemical anomaly is 

from a well sample at site 4W. The water at this location 

contains relatively high concentrations of antimony, 

arsenic, and molybdenum and a pH of 7.4. The source of 

this anomaly is also unknown; however, the probable direction of ground-water flow to this site is from the east and 

southeast, from an area containing a positive gravity anomaly. Detailed geochemical studies around this gravity high 

may provide additional information on the source of this
72 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

A Geodetic points

 Water samples

X Oak tree samples

£& Gravity-high area

0 1 KILOMETER

Figure 24. Water and oak tree sample localities, Cidrao area, 

southern Portugal. Numbers (i.e., 2CL, 4CW) indicate sites at 

which water and (or) oak tree samples were collected.

antimony, arsenic, and molybdenum hydrogeochemical 

anomaly. Five of the six water samples collected in this 

area contained lead in concentrations greater than 3.8 ng/L. 

These high lead concentrations may be a result of higher 

local background for lead.

CIDRAO STUDY AREA

The Cidrao study area is located in the northwestern 

part of the Portuguese Pyrite Belt, near Grandola (fig. 1). 

The rocks in the study area correspond to the distal or terminal facies of the Volcanic Siliceous Complex and include 

tuffites, siliceous shales, and black shales. Shales and 

graywackes of the Culm Group are also present in the area. 

A gravity anomaly in this area was investigated by drilling in 

recent years without finding any evidence of mineral deposits. The explanation for the gravity anomaly in this area 

remains an open question.

Samples of leaves from Cork oak trees (Q. suber) at 

four sample sites and water samples from five wells were 

collected during August of 1988 in the vicinity of the gravity 

high, approximately 3 km south of the Cidrao geodetic point 

(fig. 24). Analytical results for selected elements in water 

samples and in the ash of oak leaves from this study area are 

listed in tables 6 and 7.

Anomalous concentrations of antimony, arsenic, and 

molybdenum were found in water samples collected from 

wells at sample sites 2CW and 4CW, and a water sample 

from site 1CW contained anomalous arsenic and molybdenum. These ground-water samples were all alkaline, with pH 

values ranging from 7.6 to 8.4. Water collected from a well 

at site 5CW, located 1 km southwest of the Cidrao geodetic 

point, contained 5 ng/L copper, 3.8 ng/L lead, and 11

zinc. Anomalous concentrations of lead and zinc were measured in the ash of oak leaves collected from site 3CL. A 

water sample collected from a well at site 3CW had a pH of 

6.7. All of the water samples collected in the Cidrao area 

contained lead in excess of 3.0 |ag/L, which may indicate a 

high local background concentration for lead.

DISCUSSION OF RESULTS CIDRAO STUDY AREA

Analytical results on samples of Cork oak leaves and 

well water collected in the Cidr5o study area indicate hydrogeochemical and biogeochemical anomalies in the area of a 

gravity high. Ground water in the gravity-high area contains 

anomalous concentrations of antimony, arsenic, and molybdenum at several sites, and water from a well located 1.5 km 

north of the gravity area contains high copper, lead, and zinc. 

High concentrations of lead and zinc were also measured in 

oak leaves from a site on the east edge of the gravity high. 

The source of these anomalies is unknown, but they could be 

related to water-rock reactions involving buried mineral 

deposits. Additional water sampling at lower elevations 

around this anomaly, where contributions from a deeper 

ground-water source are available, may help answer this 

question.

MOMBEJA STUDY AREA

An additional study was conducted in the Mombeja 

area, which is located immediately to the north of the PPB, 

approximately 25 km west of Beja in south-central Portugal 

(fig. 1). The Mombeja study area is underlain by mafic to 

ultramafic rocks consisting of amphibolite, gabbro, serpentinite, and listwaenite (altered serpentinite). These rocks are 

intruded locally by plagiogranite. The listwaenites consist 

primarily of quartz, ankerite, and chromium muscovite 

(fuchsite or mariposite); they are important hosts for gold 

deposits.

Samples of ground water were collected from 11 wells 

during August of 1988 along a northwest-trending alignment 

of listwaenite outcrops and around a gravity high located 4 

km east of the town of Mombeja (fig. 25). Analytical results 

for selected elements in the water samples from this study 

area are listed in table 6.

Anomalous concentrations of chromium, ranging from 

6 to 14 ng/L, were found in water samples from wells at sample sites 7ML, 8ML, 9ML, and 10ML in an area around this 

gravity high. At sample site 2ML, in an area with exposed 

listwaenite, the well water contained detectable amounts of 

gold.

DISCUSSION OF RESULTS MOMBEJA STUDY AREA

Analytical results on samples of well water collected in 

the Mombeja study area indicate that high concentrations of 

chromium are present in the ground-water system around the
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRITE BELT 73

0 ' 2 KILOMETERS

EXPLANATION

A Geodetic points

 Water samples

m Gravity-high area

Figure 25. Water sample localities, Mombeja area, southern 

Portugal. Numbers (i.e., 6ML) indicate sites at which water samples were collected. Open square indicates town.

gravity anomaly east of town. The chromium anomaly may 

be from water-rock reactions involving buried mafic and 

ultramafic rocks and suggests that the gravity high in this 

area may be related to unexposed rocks of similar composition. The cause of the anomalous gold concentration in the 

water sample collected from a well at location 2ML is not 

known, but it could be related to local occurrences of gold 

associated with listwaenites. Additional geochemical investigations in this area could help explain this high gold value.

CONCLUSIONS

The exploration geochemical studies conducted at 

Neves-Corvo and other selected localities in and near the 

Portuguese Pyrite Belt indicate that surface geochemical 

sampling methods combined with sensitive multielement 

analytical techniques can be used to detect concealed polymetallic massive sulfide deposits.

Analytical results of the selective leaching of the crystalline Fe-oxide and Mn-oxide-amorphous Fe-oxide fractions of rocks and soils in the Neves-Corvo area appear to 

delineate anomalies spatially associated with known nearsurface sulfide deposits. The anomalies are more intense in 

the rock leachates as compared to soil leachates, especially 

for arsenic, copper, and molybdenum. Significant leachate 

anomalies were not found in rock and soil samples collected 

over the deeper massive sulfide bodies of Neves and Corvo, 

particularly in areas covered by shales and graywackes. This 

suggests that leachate geochemistry may be most useful in 

the PPB for exploration for near-surface mineral deposits or 

in defining lithologies favorable for hosting massive sulfide 

deposits.

Analytical results on samples of vegetation collected in 

the Neves-Corvo area delineate several anomalous areas, 

some of which are associated with known buried massive 

sulfide deposits. The highest concentrations of ore-related 

elements, such as antimony, arsenic, bismuth, copper, 

indium, and tin, were found in the first-year growth of leaves 

and stems of Holm oak (Quercus ilex). Optimum biogeochemical response for ore-associated elements occurred 

in samples collected during the active growing season 

(July-August) and from trees that had trunk diameters 

greater than 15 cm. The best overall biogeochemical indicator of sulfides in the Neves-Corvo study area seems to be 

arsenic in the ash of Holm oak leaves, possibly resulting 

from the high mobility of this element in the subsurface environment and unique translocation mechanisms in Holm oak 

trees. The results of the biogeochemical study in the NevesCorvo area indicate that biogeochemical exploration methods, specifically the sampling and analysis of the first-year 

growth of oak trees during the active growing season, may 

be useful to exploration programs in the PPB in the search 

for concealed massive sulfide deposits.

Analytical results by ICP-MS and other sensitive analytical techniques on samples of water collected from open 

wells and pumping wells in the Neves-Corvo area show 

anomalous hydrogeochemical dispersion patterns associated 

with subsurface massive sulfide bodies. Strongly anomalous 

concentrations of antimony, arsenic, and molybdenum 

(10-100 times background) and weakly anomalous concentrations of copper, lead, sulfate, and zinc (2-10 times background) were found in the ground-water system above the 

buried deposit. Alkaline waters, which predominate in the 

Neves-Corvo area and in most of the PPB, favor the solubility and mobility of the ore-associated elements antimony, 

arsenic, and molybdenum as complex anionic species, but 

these waters greatly diminish the solubility and mobility of 

other ore-related metals that form cationic species, such as 

copper, lead, and zinc. The results of the hydrogeochemical 

study in the Neves-Corvo area indicate that ground water in 

contact with the buried massive sulfide mineral deposit is 

solubilizing and dispersing several of the ore-related elements, especially antimony, arsenic, and molybdenum, into 

the local ground-water system. The dispersion patterns of 

these elements define anomalous areas associated with the 

buried massive sulfide deposit at Neves-Corvo, and the patterns demonstrate the potential use of hydrogeochemical 

techniques in exploration programs for concealed massive 

sulfides in the Portuguese Pyrite Belt and in other geologically favorable areas.

Acknowledgments. This study was funded principally 

by the Luso American Development Foundation, who we 

gratefully acknowledge. Other financial, technical, and 

logistical support was provided by the U.S. Geological Survey (USGS), the Services Geol6gicos de Portugal (SGP), 

and the Servi?os de Formento Mineiro (SFM).
74 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

Special thanks go to Robert Earhart (USGS), Jose 

Oliveira (SGP), and Vitor Oliveira (SFM) for their assistance in Held orientation and sampling. Thanks are also 

extended to the mining company, Somincor, for allowing 

geochemical samples to be collected at Neves-Corvo. We 

especially acknowledge the assistance and hospitality provided by David Richards and Pedro Carvalho, geologists 

employed by Somincor.

The authors are grateful for the analytical assistance 

given by Walter Ficklin, Alien Meier, Jerry Motooka, and 

Richard Sanzolone of the Branch of Geochemistry, USGS.

REFERENCES CITED

Baedeker's Portugal, 1968, Englewood Cliffs, New Jersey, Prentice Hall Inc., 264 p.

Boyle, R.W., Hornbrook, E.H.W., Allan, R.J., Dyck, Willy, and 

Smith, A.V., 1971, Hydrogeochemical methods Applications in the Canadian shield: Bulletin of the Canadian Institute 

of Mining and Metallurgy, v. 64, p. 60-71.

Bradshaw, P.M.D., Thomson, lan, Smee, B.W., and Larsson, J.O., 

1974, The application of different analytical extractions and 

soil profile sampling in exploration geochemistry: Journal of 

Geochemical Exploration, v. 3, p. 209-225.

Britton, H.T.S., 1955, Hydrogen Ions (4th ed.): London, Chapman 

and Hall, 187 p.

Brooks, R.R., 1972, Geobotany and Biogeochemistry in Mineral 

Exploration: New York, Harper and Row, 290 p.

Carvalho, D., 1991, Case history of the Neves-Corvo massive sulfide deposit, Portugal, and implications for future discoveries, 

in Grauch, R.I., and Hutchinson, R.W., eds., Mineral Deposit 

Research, Symposium at the Geological Society of America 

Centennial Meeting: Monograph 8, Society of Economic Geolologists, p. 314-334.

Carvalho, D., Conde, L., Enrile, J.H., Oliveira, V., and Schermerhorn, L.J.G.S., 1976, Livro-Guia das excursoes geo!6gicas na 

Faixa Piritosa Ibe'rica: Comunicasoes dos Servigos Geol6gicos 

de Portugal, v. 60, p. 271-315.

Chao, T.T., 1984, Use of partial dissolution techniques in geochemical exploration: Journal of Geochemical Exploration, v. 20, 

p. 101-135.

Chao, T.T., and Theobald, P.K., 1976, The significance of secondary iron and manganese oxides in geochemical exploration: 

Economic Geology, v. 71, p. 1560-1569.

Crone, W.W., Larson, L.T., Carpenter, R.H., Chao, T.T., and Sanzolone, R.F., 1984, A comparison of iron oxide-rich joint coatings and rock chips as geochemical sampling media in 

exploration for disseminated gold deposits: Journal of 

Geochemical Exploration, v. 20, p. 161-178.

Fernandez-Rubio, Rafael, Carvalho, Pedro, and Real, Fernando, 

1988, Mining-hydrological characteristics of the underground 

copper mine of Neves Corvo, Portugal: Third International 

Mine Water Congress, Melbourne, Australia, 1988, 15 p.

Filipek, L.H., and Theobald, P.K. Jr., 1981, Sequential extraction 

techniques applied to a porphyry copper deposit in the Basin 

and Range Province: Journal of Geochemical Exploration, v. 

14, p. 155-174.

Fishman, M.J., and Pyen, Grace, 1979, Determination of selected 

onions in water by ion chromatography: U.S. Geological Survey Water Resources Investigations 79-101, 30 p.

Humphries, C.J., Press, J.R., and Sutton, D.A., 1985, Trees of Britain and Europe: Feltham, Middlesex, England, Hamlin Publishing Group Limited, 320 p.

Kovalevskii, A.L., 1974, Biogeochemical Exploration for Mineral 

Deposits: Moscow, Nedra Publishers, 136 p.

Leavitt, S.W., and Goodell, H.G., 1979, Evaluation of biogeochemical prospecting methods in the search for sulfide deposits in 

the Appalachian Piedmont, Virginia, USA: Journal of 

Geochemical Exploration, v. 11, p. 89-100.

Levinson, A.A., 1974, Introduction to Exploration Geochemistry: 

May wood, HI., Applied Publishing Ltd., 612 p.

Lichte, F.E., Meier, A.L., and Crock, J.G., 1987, Determination of 

the rare earth elements in geological materials by inductively 

coupled plasma mass spectrometry: Analytical Chemistry, v. 

59, no. 8, p. 1150-1157.

McHugh, J.B., 1986, Determining gold in water by ion-exchange 

batch extraction: Talanta, v. 33, p. 349-351.

Miesch, A.T., 1976, Geochemical survey of Missouri Methods of 

sampling, laboratory analysis, and statistical reduction of data: 

U.S. Geological Survey Professional Paper 954-A, 39 p.

Miller, W.R., 1979, Application of hydrogeochemistry to the 

search for base metals, in Hood P.J., ed., Special Volume, Papers Given at Exploration 77, Ottawa: Canada Economic Geology Report 31, Geological Survey of Canada, p. 479-487.

Motooka, J.M., and Sutley, S.J., 1982, Analysis of oxalic leachates 

of geologic materials by inductively coupled plasma-atomicemission spectroscopy: Applied Spectroscopy, v. 36, 

p. 524-533.

Nunes, J.F., Richards, D.G., and Gama, H.A., 1990, Mine water 

and environmental protection the Somincor case, in Norton, 

P.J., ed., International Symposium Lisboa 90 Acid Mine Water in Pyritic Environments, Lisbon, Portugal, p. 237-251.

Orion Research, Inc., 1978, Orion Research Analytical Guide (9th 

ed.): Cambridge, Mass., 48 p.

Perkin-Elmer Corporation, 1976, Analytical Methods for Atomic 

Absorption Spectrophotometry: Norwalk, Conn., Perkin-Elmer Corp., 586 p.

 1977, Analytical Methods for Atomic Absorption Spectrophotometry Using the HGA Graphite Furnace: Norwalk, 

Conn., Perkin-Elmer Corp., 208 p.

Plimer, I.R., and Carvalho, D., 1982, The geochemistry of hydrothermal alteration at the Salgadinho copper deposit, Portugal: 

Berlin, Mineralium Deposita, v. 17, no. 2, p. 193-211.

Schermerhorn, L.J.G., 1970, The deposition of volcanics and pyritite in the Iberian Pyrite Belt: Berlin, Mineralium Deposita, v. 

5, no. 3, 273-279.
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRTTE BELT 75

APPENDIX 1: SEQUENTIAL DISSOLUTION PROCEDURE 

USED IN THE NEVES-CORVO STUDY AREA

Rock and soil samples collected along the traverse containing sample sites 20-28 (figs. 2 and 3) were initially subjected to a four-step sequential dissolution scheme modified 

from a procedure described by Chao (1984). This scheme 

was designed to solubilize and separate chemical phases of 

the samples in the following order: (1) Mn-oxides and amorphous Fe-oxides, (2) crystalline Fe-oxides, (3) sulfides and 

organics, and (4) the residue ("silicate fraction").

In step 1, 25 mL of 0.25 M hydroxylamine hydrochloride in 0.25 N hydrochloric acid (HC1) were added to 1.0 g 

of sample and heated in a water bath at 50°C for 30 minutes. 

The samples were then centrifuged and decanted, with the 

leachate saved for analysis.

In step 2, 25 mL of 4 N HC1 were added to the residual 

material from step 1 and heated in a water bath at 95°C for 

30 minutes. The samples were centrifuged and decanted, 

with the leachate saved for analysis.

In step 3, 2 g of potassium chlorate and 10 mL of concentrated HC1 were added to the residual material from step

2. The samples were left to stand at room temperature for 45 

minutes and then diluted with 15 mL of water. The samples 

were centrifuged and decanted, with the leachate saved for 

analysis.

In step 4,10 mL of concentrated nitric acid were added 

to 50-mL Teflon beakers containing the residual material 

from step 3, and the resultant solutions were heated on a hot 

plate at 110'C for 10 minutes. Ten mL of perchloric acid 

and 10 mL of 48 percent hydrofluoric acid were added, and 

the sample solutions were heated on a hot plate at 220°C 

until a moist bead was obtained. Twenty-five mL of 4 NHCl 

were added and the samples heated at 100°C for 30 minutes. 

The solutions were transferred to test tubes, centrifuged, and 

decanted, with the liquid saved for analysis.

A separate total digestion was performed on each of 

these samples with a fresh 0.5-g sample split using the same 

procedure as in step 4 in order to compare the "total" metal 

concentrations to the sum of the sequential dissolutions of 

steps 1-4.
76 ADVANCES IN RESEARCH ON MINERAL RESOURCES, 1994

APPENDIX 2: ANALYTICAL RESULTS OF SEQUENTIAL EXTRACTIONS OF 

ROCK SAMPLES FROM THE NEVES-CORVO AREA (SAMPLES 20-28)

Appendix 2. Analytical results of sequential extractions of rock samples 20-28, Neves-Corvo area, southern Portugal.

[Analysis by inductively coupled plasma-emission spectFographic analysis of extracted fractions. See fig. 2 for sample locations. Values preceded by "<" indicate that the element 

was not detected at the lower limit of determination shown]

Sample

number

Mn-oxides

and amorphic

Fe-oxides

Crystalline

Fe-oxides

Sulfides and

organics

Residue

As(ppm)

20

21

22

23

24

25

26

27

28

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

11

390

28

9

10

6

5

6

<3

<3

130

8

<3

<3

<3

<3

<3

<3

<3

340

<3

<3

<3

<3

<3

<3

Cu(ppm)

20

21

22

23

24

25

26

27

28

1

2

14

3

29

2

2

1

2

3

15

180

67

100

33

16

10

4

<1

1

42

14

13

4

2

1

<1

<1

2

270

18

15

1

2

3

<1

Fe (%)

20

21

22

23

24

25

26

27

28

0.03

0.18

0.07

0.05

0.10

0.15

0.23

0.14

0.09

0.29

4.20

11.00

7.80

3.30

3.80

4.50

3.60

0.56

0.01

0.05

1.70

1.10

0.50

0.45

0.37

0.20

0.02

0.01

0.59

7.90

1.40

1.10

0.59

0.25

0.23

0.03

Mn(ppm)

20

21

22

23

24

25

26

27

28

14

590

25

<8

4,900

320

90

910

460

21

390

22

17

600

570

370

380

200

<8

47

16

<8

48

83

32

23

<8

<8

23

51

38

92

51

<8

<8

<8

Sample

number

Mn-oxides

and amorphic

Fe-oxides

Crystalline

Fe-oxides

Sulfides and

organics

Residue

Mo (ppm)

20

21

22

23

24

25

26

27

28

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

52

2

1

<1

<1

<1

<1

<1

< 1

19

<1

<1

<1

<1

<1

<1

<1

<1

47

<1

<1

<1

<1

<1

<1

Na (ppm)

20

21

22

23

24

25

26

27

28

<2

<2

<2

<2

<2

<2

<2

<2

<2

3

54

47

50

54

72

78

71

48

<2

<2

<2

<2

<2

<2

<2

<2

<2

330

20,000

1,100

3,300

1,300

12,000

17,000

16,000

150

Pb (ppm)

20

21

22

23

24

25

26

27

28

<5

<5

21

<5

<5

<5

8

7

<5

<5

7

150

10

<5

<5

8

<5

<5

<5

<5

18

<5

<5

<5

<5

<5

<5

<5

<5

49

65

<5

<5

<5

<5

<5

Zn(ppm)

20

21

22

23

24

25

26

27

28

<1

6

1

1

3

3

8

3

1

4

53

16

15

32

50

67

55

6

1

8

4

3

6

8

6

4

1

<1

9

29

4

12

8

3

3

1
GEOCHEMICAL EXPLORATION STUDIES IN THE PORTUGUESE PYRTTE BELT 77

APPENDIX 3: ANALYTICAL RESULTS OF SEQUENTIAL EXTRACTIONS OF SOIL 

SAMPLES FROM THE NEVES-CORVO AREA (SAMPLES 20-28)

Appendix 3. Analytical results of sequential extractions of soil samples 20-28, Neves-Corvo area, southern Portugal.

[Analysis by inductively coupled plasma-emission spectrographic analysis of extracted fractions. See fig. 3 for sample locations. Values preceded by "<" indicate that the element 

was not detected at the lower limit of determination shown]

Sample

number

Mn-oxides

and amorphic

Fe-oxides

Crystalline

Fe-oxides

Sulfides and

organics

Residue

As(ppm)

20

21

22

23

24

25

26

27

28

<3

<3

<3

4

<3

<3

<3

<3

<3

6

7

46

36

7

14

7

4

6

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

Cu(ppm)

20

21

22

23

24

25

26

27

28

2

13

29

6

5

10

1

3

2

9

17

67

28

22

43

14

6

16

1

4

7

2

3

52

2

<1

1

3

7

54

11

7

17

3

4

5

Fe (%)

20

21

22

23

24

25

26

27

28

0.10

0.09

0.16

0.04

0.12

0.11

0.06

0.33

0.11

2.30

2.20

1.60

2.30

3.30

3.30

1.40

1.80

2.80

0.15

0.14

0.12

0.11

0.22

0.15

0.14

0.07

0.12

0.41

0.42

0.58

0.41

0.50

0.50

0.38

0.32

0.37

Mn(ppm)

20

21

22

23

24

25

26

27

28

260

300

670

16

450

1,200

410

45

240

180

220

200

72

440

550

270

170

280

12

12

<8

<8

<8

<8

<8

<8

<8

<8

31

97

66

36

45

<8

<8

<8

Sample

number

Mn-oxides

and amorphic

Fe-oxides

Crystalline

Fe-oxides

Sulfides and

organics

Residue

Mo (ppm)

20

21

22

23

24

25

26

27

28

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

3

11

<1

<1

<1

<1

<1

<1

<1

< 1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

< 1

<1

< 1

<1

<1

Na (ppm)

20

21

22

23

24

25

26

27

28

50

<2

<2

<2

<2

<2

<2

<2

<2

31

18

13

16

23

28

24

26

28

<2

<2

<2

<2

<2

<2

<2

<2

<2

21,000

14,000

3,100

3,300

11,000

7,700

13,000

20,000

16,000

Pb (ppm)

20

21

22

23

24

25

26

27

28

<5

<5

8

<5

<5

10

8

9

6

<5

<5

<5

25

<5

7

8

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

28

<5

<5

<5

<5

<5

Zn(ppm)

20

21

22

23

24

25

26

27

28

3

3

3

<1

2

6

4

2

2

37

31

13

10

46

49

43

37

47

3

3

1

1

4

4

3

2

2

8

12

10

5

8

13

7

7

8

*U.S. GOVE30WENT PRINTING OFFICE: 1994-673-046/86063




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