USGS Research on Mineral Resources-1991 

Program and Abstracts 

Seventh Annual V.E. McKelvey Forum on 

Mineral and Energy Resources 

U.S. GEOLOGICAL SURVEY CIRCULAR 1062 

.r • 
Cover: San jose tin~silver mine at Oruro, Bolivia, where high-grade 

ores are hosted by a volcanic complex consisting of quartz latite 

domes in coeval lavas and breccias of Miocene age. One of the 

world's largest silver deposits, it has been worked almost continuously since its discovery in the 16th century. Photograph by George E. 

Ericksen. 
USGS Research on Mineral Resources-1991 

Program and Abstracts 

Edited by ELIZABETH E. GOOD, JOHN f. SLACK, and 

RAMA K. KOTRA 

Seventh Annual V.E. McKelvey Forum on Mineral and Energy 

Resources, held in Reno, Nevada, February 11-14, 1991 

U.S. GEOLOGICAL SURVEY CIRCULAR 1062 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

ORGANIZING COMMITTEE FOR THE 1991 MCKELVEY FORUM 

Rama K. Kotra, Chairman 

Robert A. Ayuso 

Harvey E. Belkin 

James P. Calzia 

Richard J. Goldfarb 

William F. Hanna 

Richard F. Hardyman 

Thelma F. Harms 

Carroll Ann Hodges 

Thomas D. Light 

Melvin H. Podwysocki 

William I. Ridley 

Michael G. Sawlan 

John F. Slack 

Bruce D. Smith 

Holly J. Stein 

Gary R. Winkler 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991 

Free on application to the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

ISSN 1054-3899 

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

does not constitute endorsement by the U.S. Government. 
A society's wealth depends on the use it makes of raw 

materials, energy, and especially ingenuity. 

- V.E. McKelvey 

Ill 
The Siberian city of Noril'sk (above the Arctic Circle in the 

U.S.S.R.) is near one of the world's largest concentrations 

of magmatically accumulated nickel, copper, cobalt, and 

platinum-group metals . This east-looking view shows 

downtown Noril 'sk, smelters that process sulfide ores, 

and the highland (right background) where ores were first 

mined. As part of a recently establ ished agreement 

between the U.S. Geological Survey and the Soviet Ministry of Geology, joint U.S.-Soviet research on mineral 

deposits in the Noril'sk district is underway. 

IV Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
FOREWORD 

The V. E. McKelvey Forum is named for Vincent E. McKelvey in recognition of his 

lifelong contributions in mineral and energy resources as a research scientist, as Chief 

Geologist of the U.S. Geological Survey (USGS), and as Director of the USGS. The 

Forum is an annual event, and its purpose is to encourage direct communication between 

USGS scientists and members of the private sector, academia, and other government 

agencies. The first six Forums have been well attended and have been very successful in 

fostering better communication. The subject of the Forum has alternated between mineral 

and energy resources. This year the focus is again on minerals and has as its general theme 

domestic and international mineral resource perspectives. 

This 1991 V.E. McKelvey Forum presents results from a broad spectrum of 

mineral-resource studies by USGS scientists that apply new concepts and techniques in 

the earth sciences. Much of the interdisciplinary research covered at the Forum stresses 

novel methodologies and approaches that can aid in the discovery of new mineral deposits 

and the assessment of mineral resources. Once again, we have selected Reno, Nevada, as 

the site for the Forum, as the city represents an important center for the United States 

mining industry. Such an environment should enhance the invaluable interchange between 

USGS and industry personnel that is essential for shaping future USGS program policies 

and for communication of program results to our intended users. 

As in past years, our program emphasizes both basic and applied research activities. 

The former includes the examination of various geologic characteristics and exploration 

guides for a variety of mineral deposit types. Topics include discussion of significant 

mineral deposits from a variety of different tectonic settings, including the anorogenic 

mid-continent region, the Basin and Range, the continental margin orogenic belts, and 

mid-oceanic ridges. Results of more applied studies, integrating geological, geochemical, 

and geophysical investigations, are used to assess the mineral resources of quadrangles in 

Alaska and the lower 48, National Forest Roadless Areas of Idaho, and Alaska's Tongass 

National Forest, our Nation's largest National Forest. A number of papers also assess the 

United States resources of industrial and agricultural minerals, commodities certainly of 

growing concern in the 1990's. 

A significant addition to the 1991 McKelvey Forum is the special daylong program 

emphasizing international studies. The morning session covers the role of Federal and 

commercial sectors in international minerals development. The afternoon program of 

talks, as well as various poster presentations, discusses mineral-resource studies either 

completed or in progress in Canada, Mexico, Venezuela, Ecuador, Bolivia, Peru, Chile, 

Argentina, Saudi Arabia, Indonesia, China, and the U.S.S.R. Much of this work has been 

carried out in collaboration with representatives of academia and government from those 

countries. The resulting dissemination of new information will hopefully provide industry 

with additional overseas exploration targets and with new data to refine exploration 

models in this country. 

I am confident that once more the McKelvey Forum will result in productive 

exchanges between government, academia, and the minerals industry. It is our hope that 

this unique meeting will continue to provide the USGS with the necessary feedback that 

will help guide our future policy directions and will help us serve the mineral-resource 

needs of the Nation. 

Dallas L. Peck 

Director 

Foreword V 
PROGRAM OF LECTURES AND DISCUSSIONS 

MONDAY, FEBRUARY 11, 1991 

4:00-9:00 p.m. Registration 

7:00-10:00 p.m. POSTER SESSION/Reception 

TUESDAY, FEBRUARY 12, 1991 

7:30a.m. 

8:30 

8:40 

9:00 

9:30 

10:00 

10:30 

11:00 

12:00 p.m. 

1:00 

1:30 

2:00 

2:30 

3:00 

3:30 

4:00 

4:30 

5:00 

Registration 

Welcome and opening remarks 

Introduction to the research programs of the Office of 

Mineral Resources, U.S. Geological Survey -

Michael P. Foose 

Preview of U.S. Geological Survey mineral resources 

research: Seventh Annual V.E. McKelvey Forum 

- William C. Bagby 

Exploration guides for precious-metal deposits in 

volcanic domes - Charles G. Cunningham and 

George E. Ericksen 

Farah Garan, Saudi Arabia-Bimodal-volcanogenic 

Zn-Cu-Au mineralization in a Late Proterozoic rift 

basin, southeastern Arabian Shield-Jeff L. Doebrick, Arthur A. Bookstrom, and Richard B. 

Carten 

COFFEE BREAK AND POSTER SESSION 

KEYNOTE LECTURE: Transitions between epithermal and mesothermal environments - Richard H. 

Sil/itoe 

LUNCH 

Tectonomagmatic settings of Proterozoic metallogenic provinces in the Southwestern United States 

- Clay M. Conway 

Origin of iron, rare-earth elements, copper, and gold 

in Middle Proterozoic deposits of the midcontinent 

region, U.S.A. - W.C. Day, G.B. Sidder, A.E. 

McCafferty, L.E. Cordell, E.B. Kisvarsanyi, R.O. 

Rye, and L.M. Nuelle 

Concentration and zonation of copper and palladium 

in the Mesozoic Belmont diabase sheet, Culpeper 

basin, northern Virginia-An exploration guide to 

enrichment of precious metals in mafic magmatic 

rocks - A.J. Froelich, Laurel G. Woodruff, 

Harvey E. Belkin, and David Gottfried 

Advances in plumbic prospecting -Bruce R. Doe 

COFFEE BREAK AND POSTER SESSION 

The use of lead-isotope geochemistry to evaluate 

economic potential for gold-silver mineralization 

-An example from the North Amethyst vein 

system and the Creede mining district, Colorado -

Nora K. Foley and Robert A. Ayuso 

The Re-Os isotope system as a tracer in the study of 

the origin of platinum-group-element and gold 

deposits - R.I. Walker, J. W. Morgan, D.D. 

Lambert, A.J. Naldrett, J.K. Bohlke, and V. 

Rajamani 

Synorogenic, auriferous fluids of the Juneau gold belt, 

southeast Alaska-Stable-isotope evidence for a 

deep crustal origin - R .J. Goldfarb and W.J. 

Pickthorn 40 Ar/ 39 Ar thermochronology of fracture-controlled 

mineral deposits of the Idaho batholith-Age, thermal history, and origin - L. W. Snee, Karen Lund, 

K. V. Evans, C.H. Gammons, and M.J. Kunk 

TUESDAY EVENING, FEBRUARY 12, 1991 

7:30 

8:30-10:00 

DIRECTOR'S LECTURE: Tectonic control on the 

migration of crustal fluids and the formation of 

regional metallogenic provinces - David L. Leach 

POSTER SESSION/Reception 

WEDNESDAY, FEBRUARY 13, 1991 

8:30a.m. 

9:00 

9:30 

10:00 

10:40 

11:00 

11:30 

12:00 p.m. 

2:00 

2:30 

3:00 

3:30 

Regional structural setting of gold deposits in the 

Carolina slate belt of North Carolina - Terry W. 

Offield and Terry L. Klein 

Black shales as hosts for unconventional platinumgroup-element resources? Examples from southern 

China and the Yukon, Canada, and implications for 

U.S. resources- Richard I. Grauch, Raymond M. 

Coveney, Jr., James B. Murowchick, and Chen 

Nansheng 

Assessment of undiscovered mineral resources, Tongass National Forest, southeastern Alaska - David 

A. Brew, Lawrence J. Drew, Jeanine M. Schmidt, 

David H. Root, and Donald F. Huber 

Detailed profiles of southeastern Arizona obtained by 

a truck-mounted magnetometer-A step toward 

better utilization of the information content of 

geophysical data - Mark E. Gettings, Mark W. 

Bultman, and FrederickS. Fisher 

AND 

Geology and mineral-resource assessment of part of 

the northern Safford basin, southeastern Arizona -

Brenda B. Houser, J.M. Kruger, and Roy A. 

Johnson 

COFFEE BREAK AND POSTER SESSION 

Subsurface structure and lithology near the Getchell 

gold trend, Osgood Mountains, Nevada-Geophysical insights - V.J.S. Grauch, Donald B. 

Hoover, and Wayne S. Wojniak 

Mapping minerals with imaging spectroscopy near 

Canon City, Colorado - Roger N. Clark, Andrea 

J. Gallagher, and Gregg A. Swayze 

McKELVEY FORUM LUNCHEON 

LUNCHEON ADDRESS: Making high grade -

Stephen E. Kesler 

Large sulfide-sulfate mounds, hydrothermal fluids, 

and altered sediment in Escanaba Trough, southern 

Gorda Ridge-Elements of a mature hydrothermal 

system at a sediment-covered spreading axis -

Randolph A. Koski, Janet L. Morton, Robert A. 

Zierenberg, Wayne C. Shanks III, Andrew C. 

Campbell, and Keith A. Kvenvolden 

Marine ferromanganese deposits near the Marshall 

Islands -James R. Hein, Jung-Keuk Kang, Marjorie S. Schulz, Suk-Hoon Yoon, and Virginia K. 

Smith 

COFFEE BREAK AND POSTER SESSION 

U.S. Geological Survey research on agricultural 

industrial minerals in the United States-Implications for exploration - James R. Herring, David 

z. Piper, Sherilyn Willliams-Stroud, Charles S. 

Spirakis, and Richard A. Sheppard 

VI Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
WEDNESDAY, FEBRUARY 13, 1991-Continued 

4:00 

4:30 

Mercury-sulfur-gypsum mineralization at Crater, California-A suggestion of gold mineralization at 

depth - Sherman P. Marsh, Chester T. Wrucke, 

and Kevin P. Corbett 

Geology and mineral potential of Precambrian basement rocks of the Trans-Hudson orogen, U.S.A.-

P.K. Sims, Zell E. Peterman, and T.G. Hildenbrand 

WEDNESDAY EVENING, FEBRUARY 13, 1991 

5:30-7:00 

8:00-9:30 

OPEN HOUSE - Mackay School of Mines, University of Nevada, Reno, and new USGS field office 

in the Laxalt Strategic Materials Building 

DIRECTOR'S SPECIAL BRIEFING: US-USSR 

cooperative studies in mineral resources 

THURSDAY, FEBRUARY 14, 1991 

Morning Session: The role of Federal and commercial sectors in international minerals development 

Session Chairman: Dallas L. Peck, Director, U.S. Geological Survey 

Opening remarks and the role of the USGS in international mineral resources - Dallas L. Peck, 

Director, USGS 

The role of the Bureau of Mines in international 

mineral resources - TS Ary, Director, U.S. 

Bureau of Mines 

Mellon Bank's role in stimulating international minerals development - Peter H. Conze, Jr. , Senior 

Vice-President, Mellon Bank 

The Inter-American Development Bank's role in stimulating minerals development - Juan Proaiio, 

Inter-American Development Bank 

Criteria for investment in international exploration -

Jack Parry, Senior Vice-President, Newmont Mining Corporation and Newmont Gold 

Criteria for investment in international exploration -

John I. Sharpe, Vice-President for Exploration, 

Battle Mountain Gold Company 

Opportunities for mineral exploration in Bolivia -

lng. Marcelo Claure Zapata, Director, Servicio 

Geologico de Bolivia (GEOBOL) 

Opportunities for mineral exploration in Chile - Juan 

Carlos Parra Espinosa, Subdirector for Geology, 

Servicio Nacional de Geologia y Mineria (SERNAGEOMIN), Chile 

Afternoon Session: Examples of selected USGS international mineral 

resource investigations 

Volcanic-hosted epithermal precious-metal desposits 

in the central Andes-Geology, resources, and 

exploration strategy - George E. Ericksen, 

Charles G. Cunningham, and Barbara A. Eiswerth 

An overview of the mineral resources and USGS 

acti vities in Venezuela - Jeffrey C. Wynn 

Mineral resources of the Bolivian Altiplano - Results 

of the GEOBOL-USGS-TDP Mineral Resource 

Evaluation Project - Keith Long, S. Ludington, 

and R. Carrasco 

Mineral-resource assessment of Puerto Rico - Gregory E. McKelvey and Walter J . Bawiec 

Platinum-group-mineral investigations, East and 

South Kalimantan, Indonesia- Michael L. Zientek 

and Bambang Pardiarto 

Mineral-resource investigations in Saudi Arabia by 

the U.S. Geological Survey - PaulL. Williams 

Small-scale hydraulic mm1ng adjacent to the Chicanan River near Kilometer 88, 

Venezuela. Gold is being extracted from the Uairen Formation of the Roraima 

Group. Photograph by Jeffrey C. Wynn in january 1989. 

Program of Lectures and Discussions VII 
LIST OF POSTERS 

I. INTERNATIONAL STUDIES 

Mineral-resource assessment of Puerto Rico - by Walter J. Bawiec, 

Andrew Griscom, Richard D. Krushensky, Sherman P. Marsh, Gregory E. 

McKelvey, and Kathryn M. Scanlon 

Copper deposits in Tertiary red beds in Bolivia - by Dennis Cox, Raul 

Carrasco, Orlando Andre, Alberto Hinojosa, and Keith Long 

Use of Landsat Thematic Mapper images for studying volcanic-rockhosted precious-metal deposits in the central Andean region - by B.A. 

Eiswerth, T.L. Bowers, Lucia Cuitino, Felipe Diaz, Hugo Gumucio, Aldo 

Gutarra, Nestor Jimenez, J.L. Lizeca, Ramon Moscoso, Fernando 

Murillo, Luis Quispesivana, Jaime Rodriguez, L.C. Rowan, Rubin 

Tejada, and Cisar Vilca 

Comparative metallogeny of the Soviet Far East and Alaska - by Donald 

J. Grybeck, Warren J. Nokleberg, and Thomas K. Bundtzen 

Geology and genesis of the Baid Al Jimalah and Silsilah tungsten-tin 

deposits, Kingdom of Saudi Arabia - by Robert J. Kamilli 

Mineral-resource assessment of the Altiplano and Cordillera Occidental, 

Bolivia-A progress report - by Sherman P. Marsh, Dan H. Knepper, 

Jr., Keith R. Long, and John W. Cady 

A reconnaissance study of gold mineralization associated with garnet skarn 

at Nambija, Zamora Province, Ecuador - by Gregory E. McKelvey and 

Jane M. Hammarstrom 

Reconnaissance geologic mapping using digital aeromagnetic data and 

space-shuttle radar data for a heavily forested area of the Guayana Shield, 

northwestern Brazil -by Fernando P. Miranda, Anne E. McCafferty, and 

James V. Taranik 

Latin American mineral-resource data available through the U.S. Geological Survey - by Frances Wahl Pierce and Karen Sue Bolm 

Early to Middle Proterozoic supracrustal rocks and mineralization of the 

southern Guayana Shield, Venezuela - by Gary B. Sidder, William E. 

Brooks, Yasmin Estanga, Fernando Nunez, and Andres Garcia 

Evolution of an Early Proterozoic rift basin in the La Esmeralda area, 

Guayana Shield, Venezuela - by G.B. Sidder, W.C. Day, R.M. Tosdal, 

S.D. Olmore, Luis Guzman, and Freddy Prieto 

Gold-bearing quartz veins along the Mojave-Sonora megashear zone, 

northern Sonora, Mexico - by Miles L. Silberman, Anita Moore-Nail, 

and Brian M. Smith 

SOPAC-A decade of research on mineral and hydrocarbon resources in 

the South Pacific - by F.L. Wong, B.A. Richmond, H.G. Greene, J.R. 

Dingler, J.R. Hein, K.A. Kvenvolden, M.S. Marlow, J.L. Morton, D.M. 

Rubin, D. W. Scholl, and J.G. Vedder 

U.S. Geological Survey mineral-resource and tectonic studies in Venezuela- by J.C. Wynn, S.D. Qlmore, Floyd Gray, W.C. Day, G.B. Sidder, 

and N.J Page 

II. MINERAL-RESOURCE ASSESSMENTS 

Geophysical applications in mineral-resource assessments, Cedar City 

1° X 2° quadrangle, southwestern Utah - by H. Richard Blank 

Preliminary assessment of the mineral resources of the Cedar City 1° x 2° 

quadrangle, Utah- by Theresa M. Cookro, Michael A. Shubat, and Janet 

L. Jones 

Mineral-resource potential of the Sitka 1° X 3° quadrangle, southeastern 

Alaska - by S.M. Karl, R.J. Goldfarb, K.D. Kelley, D.M. Sutphin, C.A. 

Finn, A.B. Ford, and D.A. Brew 

Mineral-resource assessment of the Goodnews 1 o x 3° quadrangle and 

parts of the Hagemeister Island and Nushagak Bay quadrangles, southwestern Alaska - by J.E. Kilburn, S.E. Box, R.J. Goldfarb, J.E. Gray, 

and J.L. Jones 

Areas of mineral-resource favorability (with emphasis on gold and chromite) in the Anchorage 1 o x 3° quadrangle, southern Alaska - by D .J. 

Madden-McGuire and G.R. Winkler 

Resource assessment of the Mount Katmai 1 o x 2° quadrangle and adjacent 

parts of the Naknek and Afognak quadrangles, Alaska Peninsula - by J.R. 

Riehle, S.E. Church, and L.B. Magoon 

Ill. MINERALIZING PROCESSES AND SYSTEMS 

Mineral deposits of the Midcontinent Rift, Lake Superior region, United 

States and Canada- by William F. Cannon and Teresa A. McGervey 

Geologic map of the Sierrita-Mogollon Corridor (Arizona-New Mexico) 

and implications for mineral resources - by Leslie J. Cox, Brenda B. 

Houser, Eric R. Force, Mark E. Gettings, Alison Burchell, and Frederick 

S. Fisher 

Anomalous concentrations of fine-grained native gold in stream sediments 

of east-central Lemhi County, Idaho - by George A. Desborough, 

William H. Raymond, and Karl V. Evans 

Element dispersion in alluvium covering gold deposits in the Kelley Creek 

valley, Getchell gold trend, Humboldt County, Nevada- by D.E. Detra, 

S.M. Smith, P.K. Theobald, and P.M. Theodorakos 

Triassic-Jurassic magmatic arc of western Nevada and eastern CaliforniaPart III: Mineral deposits - by J.L. Doebrich, L.J. Garside, D.R. Shawe, 

J.H. McCarthy, Jr., R.L. Turner, R.F. Hardyman, J.A. Erdman, H.F. 

Bonham, and J. V. Tingley 

The Bisbee Group of the Tombstone Hills, southeastern Arizona-Stratigraphy, structure, metamorphism, and mineralization - by Eric R. Force 

Lithologic and tectonic controls on mercury mineralization in the Bethel 

1° x 3° quadrangle, southwestern Alaska - by Thomas P. Frost and 

Stephen E. Box 

Geochemical investigation of ground water associated with disseminated 

gold deposits along the Getchell trend, northern Nevada - by D.J. 

Grimes, W.H. Ficklin, J.B. McHugh, and A.L. Meier 

The Mogollon mining district, southwestern New Mexico-Classic epithermal silver-gold vein deposits revisited - by Robert J. Kamilli 

Recognition of distinct terranes in lower Paleozoic host rocks in the 

Getchell gold trend, Humboldt County, Nevada- by Dawn J. MaddenMcGuire and Sherman P. Marsh 

Tourmalinite and iron-formation in the Yellowjacket Formation, Idaho 

cobalt belt, Lemhi County, Idaho - by Peter J. Modreski and Jon J. 

Connor 

Phosphorite deposits of the northern Blake Plateau as observed from the 

NR-1 submarine and Delta minisubmersible - by Peter Popenoe and 

Frank T. Manheim 

Silt-fraction mineralogy of unconsolidated sediment samples from the 

continental shelf, slope, and rise off the northeastern United States - by 

L.J. Poppe and J.A. Commeau 

Triassic-Jurassic magmatic arc of western Nevada and eastern CaliforniaPart IV: A model for the evaluation of Cenozoic basins for concealed 

mineral systems - by D.L. Sawatzky, G.L. Raines, J.L. Doebrich, R.L. 

Turner, L.J. Garside, and J.H. McCarthy, Jr. 

VIII Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Triassic-Jurassic magmatic arc of western Nevada and eastern CaliforniaPart 1: Geology - by R.A. Schweickert, J.H. Stewart, J.H. Dilles, L.J. 

Garside, R.C. Greene, R.F. Hardyman, D.S. Harwood, and N.J. 

Silberling 

Triassic-Jurassic magmatic arc of western Nevada and eastern CaliforniaPart II: Audio-magnetotelluric survey near Reno and Carson City, Nevada 

- by R.M. Senterfit and D.P. Klein 

The Getchell gold trend, northwestern Nevada-Geologic structure delineated by further processing of electromagnetic data collected during a 

helicopter survey - by W.S. Wojniak and D.B. Hoover 

IV. GEOCHEMICAL AND GEOPHYSICAL 

INVESTIGATIONS 

Geochemical evidence for near-surface precious- and base-metal disseminated and vein deposits in the west-central Vermilion district, northeastern 

Minnesota - by H. V. A/minas, J.B. McHugh, and E. C. Perry, Jr. 

Chemistry of natural waters in the St. Helens mining district, Washington-A test of hydrogeochemical prospecting methods in the Cascade 

Range - by Roger P. Ashley, Russell C. Evarts, and William R. Miller 

Geochemistry and metallogeny of granitic rocks from the Appalachian 

Mountains-Examples from the Northeast Kingdom batholith, Vermont, 

and the Lucerne and Deblois batholiths, Maine - by Robert A. Ayuso and 

Joseph G. Arth 

Applications of gravity data to studies of framework geology, evaluation of 

mineral deposits, and mineral prospecting in northwestern Alaska - by 

David F. Barnes and John S. Kelley 

Synthesis of geophysical and geological data centered on the Idaho 

batholith and the Challis volcanic field, Idaho - by John W. Cady, Viki 

Bankey, Keenan Lee, A.E. McCafferty, James A. Pitkin, and W.D. Stanley 

Can trace and rare-earth elements in fluorite be useful in geochemical 

exploration? Encouraging evidence from the Sierra Cuchillo area, New 

Mexico - by R.G. Eppinger, L.G. Closs, A.L. Meier, and J.M. Matoaka 

Biogeochemical and geochemical expression of poorly exposed epithermal 

mineralization in the southeastern Challis volcanic field, Idaho - by J.A. 

Erdman, Falma Moye, and P.K. Theobald 

Geochemical exploration criteria for epithermal cinnabar and stibnite 

deposits, southwestern Alaska - by J.E. Gray, D.E. Detra, R.J. 

Goldfarb, and K.E. Slaughter 

Seasonal variation in the silver concentration in mesquite (Prosopis 

juliflora) collected near Globe, Arizona - by Thelma F. Harms 

Low-level gold determinations by use of flow injection analysis-atomic 

absorption spectrophotometry-An application to precious-metal-resource 

assessment in the Iditarod 1° x 3° quadrangle, southwestern Alaska - by 

D.M. Hopkins, J.E. Gray, and K.E. Slaughter 

Geophysical and geological studies of the Roseau 1 o x 2° quadrangle, 

Minnesota and Ontario - by R.J. Horton, W.C. Day, T.L. Klein, and 

B.D. Smith 

Interpretation of new geophysical data obtained by airborne instruments for 

the Effie-Coon Lake area, Minnesota - by Robert J. Horton, Bruce D. 

Smith, Victor F. Labson, and Robert J. Bisdorf 

Regional aeromagnetic and gravity data bases for studies centered on the 

Idaho batholith and Challis volcanic field - by Anne E. McCafferty and 

Viki Bankey 

Hydrogeochemical exploration in areas of thin glacial overburden, northeastern Minnesota - by W.R. Miller, W.H. Ficklin, and J.B. McHugh 

Preliminary results of geochemical investigations of aerial gamma-ray 

anomalies in the Getchell gold trend, Humboldt County, Nevada - by 

James A. Pitkin 

Development and testing of the CHIM electrogeochemical exploration 

method - by David B. Smith, Donald B. Hoover, and Richard F. 

Sanzo lone 

V. GENERAL TOPICS AND NEW TECHNIQUES 

Geologic settings of barite deposits as indicators of the potential for other 

types of mineral deposits - by Sandra H.B. Clark 

Laser-microprobe studies of sulfur isotopes in stockwork and massive 

sulfide ores, Rua Cove mine, south-central Alaska - by Douglas E. 

Crowe, Wayne C. Shanks III, and John W. Valley 

An overview of industrial minerals of the U.S. Atlantic and Gulf of 

Mexico Coastal Plains - by John W. Hosterman 

Natural aggregates-Mining challenge of the 1990's - by William H. 

Langer 

Heavy-mineral placers at the Fall Zone - by Curtis E. Larsen 

Application of grade and tonnage models to the development of strategies 

for mineral deposit exploration- by Gregory E. McKelvey and James D. 

Bliss 

Geophysical logs in an oxide copper deposit near CasaGrande, Arizonaby Philip H. Nelson, David J. Johnston, Henry G. Kreis, and Jon L. 

Mikesell 

Basin evolution and petroleum studies in the U.S. eastern Great BasinRelevance to mineral investigations - by Christopher J. Potter, John A. 

Grow, Charles H. Thorman, Harry E. Cook, and James A. Peterson 

The origin of evaporite deposits-A model based on a hydrologically open 

system- by Warren W. Wood and Ward E. Sanford 

List of Posters IX 
CONVERSION FACTORS 

Conversion factors and abbreviations for units used in this report are given below. 

Multiply 

millimeter (mm) 

meter (m) 

kilometer (km) 

inch (in.) 

foot (ft) 

mile (mi) 

square centimeter ( cm2

) 

square kilometer (km2

) 

liter (L) 

gram (g) 

kilogram (kg) 

metric ton (t) 

bar 

degree Celsius (°C) 

Abbreviations 

A, ampere 

A, angstrom 

Ga, giga-annum (109 years ago) 

h, hour 

Hz, hertz 

kHz, kilohertz 

Ma, mega-annum (106 years ago) 

By 

Length 

0.03937 

3.281 

0.6214 

25.4 

0.3048 

1.609 

Area 

0.1550 

0.3861 

Volume 

1.057 

Mass 

0.03527 

2.205 

1.102 

Pressure 

100 

Temperature 

1.8 and add 32 

m. y., million years ( 106 years duration) 

ng, nanogram 

nT, nanotesla 

ohm-m, ohm-meter 

ppb, parts per billion 

ppm, parts per million 

JJ.g, microgram 

JJ.m, micrometer 

V, volt 

yr, year 

To obtain 

inch (in.) 

foot (ft) 

mile (mi) 

millimeter ( mm) 

meter (m) 

kilometer (km) 

square inch (in2d 

square mile (mi ) 

quart (qt) 

ounce avoirdupois ( oz avdp) 

pound avoirdupois (lb avdp) 

short ton (2,000 lb) 

kilopascal (kPa) 

degree Fahrenheit (°F) 

X Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources 

USGS Research on Mineral Resources-1991 

Program and Abstracts 

Edited by Elizabeth E. Good, john F. Slack, and Ram a K. Kotra 

Geochemical Evidence for Near-Surface 

Precious- and Base-Metal Disseminated and Vein 

Deposits in the West-Central Vermilion District, 

Northeastern Minnesota 

By H.V. Alminas, j.B. McHugh, and E.C. Perry, Jr. 

The study area encompasses some 568 km2 within St. 

Louis, Lake, and Cook Counties, Minn., and is centered on 

the Archean greenstone belt of the west -central Vermilion 

district. This belt is composed predominantly of mafic 

metavolcanic and associated rocks that were intruded on the 

north and south by major granitoid bodies (Sims, 1976). 

A-horizon soils, developed from glacial till, outwash, 

and lacustrine deposits of Wisconsin age that cover this area 

to a depth of 0-17 m, were sampled at 754 sites in 

cooperation with the Minnesota Geological Survey. These 

soils were analyzed for 31 elements by spectrographic 

methods and for gold by atomic-absorption spectroscopy. 

This study indicates that two major areas contain 

anomalous concentrations of gold, silver, and base metals. 

A third area is characterized by anomalous concentrations 

of Cr, Mg, Ni, Co, and platinum-group elements. The more 

intensely mineralized gold-bearing area is approximately 89 

km2 in areal extent and is centered on Lake Vermilion. Here 

the gold contents of the soils are as high as 1.1 ppm, and the 

gold is associated with Ag, Cu, Ti, Co, Pb, Zn, Mo, Cr, Ni, 

La, V, and Y. The mineralization is restricted primarily to 

the felsic volcaniclastic units, and a pronounced metal 

zonation is evident. A strong positive aeromagnetic 

anomaly, apparently related to that of the Lost Lake 

intrusive bodies, shows a good spatial correlation with the 

mineralization. 

Manuscript approved for publication November 5, 1990. 

The second gold-bearing area is in the eastern end of 

the belt between Fall and Jasper Lakes and encompasses 

some 77 km2

• Here the soils contain up to 68 ppb gold. The 

total geochemical association in this area includes Cu, Ti, 

Mn, Ag, Co, Pb, Mo, Ni, Sc, Zn, and V. Zonation in this 

anomaly is less evident, and Ag is less abundant. This area 

has greater Pb, Zn, and Mo concentrations than does the 

area at Lake Vermilion. Lamprophyric and syenitic intrusions are present in the central part of this area, and a 

pronounced aeromagnetic anomaly is essentially coextensive with it. 

The two geochemical anomalies indicate widespread 

precious- and base-metal mineralization in the district. This 

mineralization is apparently related to the buried epizonal 

granite and syenite in the Lake Vermilion area as well as to 

lamprophyric rocks intruded into the Lake Vermilion Formation and Ely Greenstone during the Algoman orogeny. 

Northeast-trending faults, probably associated with the late 

phase of the Algoman orogeny (Sims and Mudrey, 1972), 

may have controlled the mineralization. These geochemical 

anomalies probably indicate the presence of near-surface, 

gold-bearing disseminated and (or) vein deposits. Striking 

similarities exist between the geologic, structural, and 

geochemical features of the Vermilion district and comparable features described by Col vine and others ( 1988) for 

the Archean lode gold deposits in Ontario. 

A Cr-Mg-Ni-Co soil anomaly centered on Little Long 

Lake encompasses some 61 km2

. The anomalous area is 

associated with mafic and ultramafic intrusions and is 

characterized by the presence of abundant sulfides and 

narrow, linear, northeast-trending positive aeromagnetic 

anomalies. Some outcrop samples collected within the area 

have platinum and palladium concentrations as high as 0. 15 

and 0.10 ppm, respectively. This area is believed to hold 

some potential for economic concentrations of the platinumgroup elements. 

Alminas and others 
REFERENCES CITED 

Colvine, A.C., Fyon, J.A., Heather, K.B., Marmont, Soussan, Smith, 

P.M., and Troop, D.G., 1988, Archean lode gold deposits in 

Ontario: Ontario Geological Survey Miscellaneous Paper 139, 

136 p. 

Sims, P.K., 1976, Early Precambrian tectonic-igneous evolution in the 

Vermilion district, northeastern Minnesota: Geological Society of 

America Bulletin, v. 87, no. 3, p. 379-389. 

Sims, P.K., and Mudrey, M.G., Jr., 1972, Syenitic plutons and associated 

lamprophyres, in Geology of Minnesota; A centennial volume: St. 

Paul, Minn., Minnesota Geological Survey, p. 140--152. 

Chemistry of Natural Waters in the St. Helens 

Mining District, Washington-A Test of 

Hydrogeochemical Prospecting Methods in the 

Cascade Range 

By Roger P. Ashley, Russell C. Evarts, and William 

R. Miller 

The main part of the St. Helens mmmg district, 

Washington, is defined by a cluster of small mines and 

prospects that occupy an area about 12 km in diameter 

centered about 17 km north-northeast of Mount St. Helen~ (fig. 1). Most mineral occurrences in the district are small 

polymetallic veins, but the district also includes an unmined 

porphyry copper deposit, and several hydrothermally 

altered areas without known mineral prospects. 

Streams, springs, and drill holes throughout the 

district were sampled in three successive years (1977-79); 

samples were collected during periods of both high and low 

surface flow. Constituents determined included copper, 

zinc, molybdenum, arsenic, uranium, calcium, magnesium, 

sodium, potassium, sulfate, silica, chloride, and fluoride; 

alkalinity was also measured. Cation/chloride ratios, which 

tend to normalize the dilution effects associated with high 

flow levels, show stronger anomaly contrast than do unadjusted cation values. 

The Earl porphyry copper deposit is poorly exposed 

over an area of about 1 km2

; it is drained mainly by two 

second-order streams. An adjacent area of mineralization of 

similar size extends under glacial deposits and alluvium in 

the Green River, a third-order stream. Drill holes, springs, 

and surface waters in the vicinity of the Earl deposit show 

anomalous amounts of copper, zinc, molybdenum, and 

sulfate. 

Five second-order streams drain most of a 5-km2 

hydrothermally altered area on a ridge in the northern part 

of the district. This area is underlain by intensely leached 

and pyritized plutonic rock and hornfels. The alteration is 

mainly advanced argillic; in addition to quartz and pyrite, 

mineral assemblages include pyrophyllite, diaspore, alunite, kaolinite, corundum, topaz, and rutile. All stream 

waters contain anomalous concentrations of sulfate, produced by the oxidation of abundant pyrite. Two drainages 

show anomalous amounts of copper and zinc, two other 

drainages show anomalous amounts of molybdenum and 

fluoride, and one drainage shows both these associations. 

A hydrothermally altered area covering 8 km2 spans 

the lower part of the valley of Quartz Creek at the northeast 

edge of the district. It is drained by numerous high-gradient, 

first- and second-order streams that empty into Quartz 

Creek. The alteration is mainly argillic and is characterized 

by mineral assemblages dominated by kaolinite or smectite, 

but advanced argillic assemblages occur locally. All 

streams sampled in this area show anomalous amounts of 

one or more of the components sulfate, copper, molybdenum, and arsenic. 

Four areas of patchy clay-carbonate alteration associated with dikes and sills occur around the periphery of the 

St. Helens district. Surface waters from two of these areas 

lack anomalous components, and surface waters from one 

area show anomalous concentrations of sulfate only. The 

fourth area includes zones of carbonate-free argillic alteration and contains abundant pyrite; streams draining this 

area have anomalous concentrations of sulfate, copper, 

zinc, and arsenic. In all the hydrothermally altered areas 

that produce sulfate anomalies, the components magnesium, sodium, potassium, and silica are also enriched in 

surface waters because of the destruction of feldspars and 

mafic minerals in adjacent unaltered rocks by acidic waters 

derived from oxidizing pyrite. 

The polymetallic veins in the district are small quartz 

veins containing pyrite, chalcopyrite, sphalerite, and 

galena. Some have significant gold and silver contents. 

Associated propylitic alteration envelopes surround inner 

phyllic alteration zones and are no more than a few meters 

wide. Eight of these veins occur within the Spirit Lake 

pluton in drainages where we saw no other alteration or 

mineralization. All these drainages show anomalous 

amounts of zinc, molybdenum, or arsenic, and a few also 

show copper or sulfate anomalies. Only one of five veins in 

hornfels around the Spirit Lake pluton, however, produced 

anomalies (zinc and arsenic) in surface waters. 

Our results show that surface-water sampling for base 

metals and sulfate together can reliably detect significant 

areas of porphyry copper mineralization and intense hydrothermal alteration in the Cascade Range. Areas of such 

intense alteration may host shallow precious-metal deposits 

and deeper base-metal deposits. 

Figure 1 (Ashley and others). Location of the St. Helens 

mining district, Washington. 

2 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Geochemistry and Metallogeny of Granitic 

Rocks from the Appalachian MountainsExamples from the Northeast Kingdom Batholith, 

Vermont, and the Lucerne and Deblois 

Batholiths, Maine 

By Robert A. Ayuso and joseph G. Arth 

Detailed petrographic and chemical studies of six 

calc-alkalic Devonian plutons of the Northeast Kingdom, 

Vermont, and of the Devonian, generally alkali-calcic 

Lucerne and Deblois batholiths in coastal Maine characterize the metallogenic potential of granitic rocks in the 

Appalachian Mountains. The Northeast Kingdom batholith 

is probably underlain by Proterozoic Grenville-type basement; in contrast, the Lucerne and Deblois batholiths may 

be underlain by Proterozoic Avalonian-type basement. 

The Northeast Kingdom batholith ranges from metaluminous to strongly peraluminous and from quartz-biotiteamphibole gabbro and diorite to biotite granodiorite and 

muscovite-garnet leucogranite; the Si02 content of the 

batholith ranges from 48 to 77 weight percent. The rocks 

are characterized by euhedral sphene and magnetite. Low 

molar values of K20/Na20 ratios ( < 1) are also characteristic, except in leucogranitic rocks that are potassic. The 

batholith generally has low Rb ( <200 ppm) and high Sr 

(> 100 ppm) contents except in the muscovite-bearing, 

peraluminous leucogranites. Rare-earth-element (REE) 

contents, light REE fractionation (chondrite-normalized 

La/Yb ratios = 2-60), and the general depletion of incompatible elements are similar to those in calc-alkalic suites. 

Leucogranitic rocks that have a high potential to be associated with granitophile ore deposits have low to moderate 

REE contents (30-131 ppm), large negative Eu anomalies, 

numerous aplites and pegmatites, obvious hydrothermal 

alteration and stockworks, relatively high Rb/Sr ratios (up 

to 20), high F(Li+Rb)/(Sr+Ba) ratios (up to 3,000), high 

Sn, W, and Mo contents, and abundant sulfide minerals. 

Initial 87 Sr/86Sr ratios range from 0. 7045 to 0. 7071 , 206PbP04Pb ratios range from 18.13 to 18.46, and 143Nd/ 144Nd ratios range from 0.51187 to 0.51200. Isotopic 

results and range of abundance and ratios of incompatible 

cations (for example, Ta < 1 ppm, Th < 20 ppm, and U < 

10 ppm) are consistent with derivation of the batholith 

from sources that included the lower and mid-continental 

crust (Grenville-type rocks) in combination with subcontinental mantle. 

The Lucerne and Deblois batholiths are metaluminous and have Si02 contents ranging from 70 to 76 weight 

percent. They are mostly biotite bearing, commonly are 

enriched in alkali feldspar, and contain ilmenite, tourmaline, and minor hornblende. K20/Na20 ratios are > 1. 

Aplites and pegmatites are not abundant, but a fluid phase 

existed in the porphyritic facies of the Lucerne batholith, 

which also contains anomalously high concentrations of Sn 

(up to 45 ppm), W (up to 55 ppm), and As (up to 6,500 

ppm) and quartz and greisen veins containing pyrite, 

arsenopyrite, and fine-grained cassiterite. The batholiths 

have high Rb contents (>200 ppm) and low Sr contents 

( < 100 ppm). REE contents are generally high (up to 320 

ppm), and REE patterns have large negative Eu anomalies, 

are fractionated in the light REE, and are relatively flat in 

the heavy REE (chondrite-normalized La/Yb ratios = 

4-15). Incompatible cations are more abundant than in the 

Northeast Kingdom batholith (for example, Ta is > 1 ppm, 

Th is up to 60 ppm; U is up to 30 ppm). Initial 87 Sr/86Sr 

ratios in the Lucerne and Deblois batholiths range from 

0.7040 to 0.7110 (Andrew and others, 1983), and 206PbP04Pb ratios range from 18.38 to 18.89. Results are 

consistent with an origin from the middle continental crust 

by melting of metasedimentary and metavolcanic rocks of 

the Avalonian basement. 

The calc-alkalic Northeast Kingdom batholith is 

chemically distinct from the more alkalic rocks of the 

Lucerne and Deblois batholiths. However, granitic rocks in 

both areas contain specialized plutons likely to be associated with granitophile ore deposits, especially those containing tin, tungsten, and molybdenum. 

REFERENCE CITED 

Andrew, A.S., Loiselle, M.C., and Wones, D.R., 1983, Granitic plutonism as an indicator of microplates in the Paleozoic of central and 

eastern Maine: Earth and Planetary Science Letters, v. 66, 

p. 151-165. 

Applications of Gravity Data to Studies of 

Framework Geology, Evaluation of Mineral 

Deposits, and Mineral Prospecting in 

Northwestern Alaska 

By David F. Barnes and john S. Kelley 

More than 5,000 gravity measurements in the western 

Brooks Range, Alaska, aid in the interpretation of thrustbelt structures, estimation of the mass of two known 

mineral deposits, and possible identification of other undiscovered mineral deposits. The data set includes reconnaissance measurements that use river gradients and altimetry 

for elevation control and have an accuracy of about 15 m, 

very detailed traverses over two ore deposits, where surveyed elevations are accurate to about 0.3 m, and 3,500 

measurements with similar surveyed elevation control along 

seismic lines in the National Petroleum Reserve in Alaska 

(NPRA). 

Geologic structures are reflected in an arcuate band of 

strongly positive isostatic anomalies that strike northeastward approximately parallel to geologic grain across the 

western end of the Brooks Range. These anomalies are 

Barnes and Kelley 3 
Figure 1 (Barnes and Kelley). Oblique aerial photograph of 

the Nimiuktuk barite deposit, which forms a small hill (10 

m high, 50 m wide, and 80 m long) of almost pure barite in 

the western Brooks Range, Alaska. Nimiuktuk and the 

giant Red Dog massive sulfide deposit were used to define 

the gravitational signature searched for among the 3,500 

gravity measurements of the National Petroleum Reserve 

in Alaska. The photograph was taken from the westnorthwest by I.L. Tailleur in 1978. 

coincident with a belt of mafic volcanic and plutonic rocks, 

which many workers believe to be klippen of an extensive 

thrust sheet rooted in the Angayucham terrane along the 

south flank of the Brooks Range. Magnitudes of the 

isostatic anomalies are, however, much larger than those 

associated with rocks of the Angayucham terrane. The 

sources of positive gravity anomalies are better modeled as 

inclined stocks or, possibly, stacked slabs of igneous rocks 

rather than as simple klippen. The postulated stocks are 

probably still close to their emplacement sites. 

Detailed gravity surveys of the Nimiuktuk (fig. l) and 

Red Dog mineral deposits are part of the western Brooks 

Range data set. Two gravity profiles across the Nimiuktuk 

barite body indicated a 2-mgal anomaly having horizontal 

dimensions of more than l 00 m. The model that best fits the 

Nimiuktuk deposit is a cylinder having a density contrast of 

1. 70 g/cm3

; this model implies that the deposit is almost 

pure barite and has a radius and depth of about 50 m and a 

total mass of about 1.5 Mt. The Red Dog massive sulfide 

deposit, nearing production in 1990, causes a gravity 

anomaly of about 2 mgal with horizontal dimensions of 

about 200 x 500 m; an associated barite-rich body has an 

anomaly of about 4 mgal with horizontal dimensions of 

about 500X700 m. Preliminary estimates of ore quantities 

based on densities from surface sampling and possible 

mathematical models are comparable to those provided by 

early developmental drilling. The agreement among the 

estimates probably is partly due to the shallowness of the 

deposit. 

The NPRA investigation within the mountainous part 

of northwestern Alaska included more than 3,500 gravity 

measurements with surveyed elevation control and station 

separations of about 400 m. Anomalies similar to those at 

Nimiuktuk and Red Dog appear in this data set as singlestation spikes having amplitudes of 2-4 mgal; initially, such 

spikes were deleted from the data set because they were 

considered either errors or anomalies too narrow to represent deep structures. However, in 1981, the entire data set 

was reviewed, and 12 single-station anomalies in areas 

geologically favorable for mineralization and having associated geochemical anomalies were selected for possible 

verification and reexamination. In the ensuing 9 yr, only 

one anomaly has been reinvestigated. Here the gravity and 

elevation data were verified, but an adjacent bedrock 

outcrop indicated that basement topography beneath Quaternary sediments might explain the anomaly. The remaining promising gravity anomalies are yet to be field checked. 

Mineral-Resource Assessment of Puerto Rico 

By Walter j. Bawiec, Andrew Griscom, 

Richard D. Krushensky, Sherman P. Marsh, 

Gregory E. McKelvey, and Kathryn M. Scanlon 

The U.S. Geological Survey is currently involved in 

a mineral-resource assessment of the Commonwealth of 

Puerto Rico as part of the National Mineral Resources 

Assessment Program (NAMRAP). The assessment will aid 

the Government of Puerto Rico and the mining community 

in land-use planning, revisions of tax codes and commercial 

law, investment planning, and mineral exploration. This 

mineral-resource assessment, which will be completed in 

1992, will incorporate all available geological information 

for Puerto Rico in a readily understood format. Much of this 

information has never been published. 

Mineral resources, including metals, nonmetals, and 

construction materials, were assessed both onshore and 

offshore. Puerto Rico consists of a core of Early Cretaceous 

to Tertiary volcanic and volcaniclastic rocks, associated 

intrusive rocks, and limestone, transgressed by postvolcanic 

sedimentary rocks. Among these volcanic and intrusive 

rocks are primitive and calc-alkalic island-arc suites. Such 

settings host a wide variety of mineral deposits. More than 

200 mineral occurrences known in Puerto Rico have been 

classified by deposit types according to models of Cox and 

Singer ( 1986). 

Nickel is known in laterite deposits, copper and gold 

are in porphyry deposits, iron and copper are in skarns, gold 

is in placers and epithermal deposits, phosphate is derived 

from guano in caves, and bauxite is present in karst 

terranes. Clay, limestone, and glass sand are presently 

being extracted, and past production of gold, manganese, 

and iron is documented. 

4 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
For this project, an unpublished geologic map of the 

island has been digitized at a scale of 1: 100,000 as an 

information base. Individual lithologic units from the geologic map can be displayed as metallogenic terranes of 

similar or related environments on the digitized map. 

The geochemical data available for Puerto Rico have 

been derived mainly from a regional stream-sediment sampling program. Data from soil and rock samples are also 

available. The most detailed sampling has been in the 

central one-third of the island, where porphyry copper 

systems have been discovered. Regional geophysical data 

consist of aeromagnetic maps for nearly 60 percent of the 

island and gravity coverage of the entire island. 

Information on offshore areas comes partly from 

approximately 2,000 sediment samples from the insular 

shelf. Data from these samples have been used to map 

surficial sediment types and, in conjunction with seismicprofile data, to estimate offshore sand and gravel resources. 

REFERENCE CITED 

Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. 

Geological Survey Bulletin 1693, 379 p. 

Geophysical Applications in Mineral-Resource 

Assessment, Cedar City 1 o x 2° Quadrangle, 

Southwestern Utah 

By H. Richard Blank 

The Cedar City 1 o x 2° quadrangle in southwestern 

Utah encompasses the structural transition from Basin and 

Range to Colorado Plateau and was a locus of extensive late 

Cenozoic magmatism and mineralization. It contains 

diverse mineral and energy resources, including precious 

and base metals, rare earths, uranium, iron, oil, coal, and 

geothermal deposits, along with various industrial materials. Geophysical data contribute to knowledge of the 

geologic environment of these deposits and facilitate assessment of the potential for discovery of additional resources in 

the region. 

Geophysical coverage presently available consists 

mainly of regional aeromagnetic and gravity surveys and 

NURE (National Uranium Resource Evaluation) aeroradiometric surveys. A detailed aeromagnetic survey has been 

made of the so-called iron axis of southwestern Utah in 

support of iron exploration. The principal sources of aeromagnetic anomalies are not iron deposits but rather volcanic 

rocks, hypabyssal intrusive rocks, and Precambrian crystalline basement. 

Aeromagnetic data delineate the subsurface extent of 

the partly exposed Mineral Mountain, Bull Valley-Big 

Mountain, Iron Mountain, and Granite Mountain-Three 

Peaks Miocene monzonitic intrusions, all of which have 

associated mineral deposits. These data indicate the presence of additional hypabyssal intrusions beneath Sevier 

Valley, Parowan Valley, and the west flank of the Red 

Hills. A concealed extension of the Mineral Mountain body 

may have controlled hydrothermal systems responsible for 

mineralization in the Goldstrike district. 

Buried intrusions revealed by aeromagnetic anomalies in the northwest comer of the quadrangle are probably 

related to the Indian Peak and Caliente caldera complexes, 

whose ring structures are delineated by gravity data. 

The Gold Springs and State Line gold districts are possible manifestations of mineralization controlled by such 

structures. 

Bouguer gravity anomalies, mainly a reflection of 

contrasts between low-density sedimentary and volcanic 

rocks and much denser carbonate sequences and Precambrian basement, delineate a basin-range fabric beneath 

alluvium of the Escalante Valley. Some of the deeper basins 

in the valley may contain evaporite deposits, and, in at least 

two places (Newcastle and Zane), concealed basin-andrange faults control the circulation of geothermal fluids. 

Gravity data are used to distinguish pediment-covered 

bedrock from valley-fill areas and, thus, to identify areas 

accessible for mining. 

Aeromagnetic and gravity data have been used to 

address problems related to the regional geologic framework of southwestern Utah, which bear only indirectly on 

resource assessment yet are important for planning exploration. The Cedar City quadrangle straddles an easttrending gravity discontinuity that passes through the Basin 

and Range-Colorado Plateau transition and elsewhere 

appears to mark an abrupt northward deepening of the 

Precambrian basement accompanied by a drastic increase in 

volume of silicic igneous rocks. The presence of this 

discontinuity suggests that pervasive silicic magmatism 

beneath this part of the Colorado Plateau was responsible 

for the basement uplifts interpreted from signatures seen on 

the potential-field maps. The basement highs are closely 

associated with east-tilted fault blocks and are possible 

sources of weak hydrothermal systems. 

NURE radiometric data are of limited value in the 

Cedar City quadrangle because of excessive terrain clearances and wide traverse spacing. However, a more tightly 

controlled survey would have obvious utility in mapping 

uraniferous zones on the plateau. Spectroradiometric data 

and Landsat Thematic Mapper data are expected to be 

valuable in mapping hydrothermally altered zones throughout the region. 

Blank 5 
Assessment of Undiscovered Mineral Resources, 

Tongass National Forest, Southeastern Alaska 

By David A. Brew, Lawrence j. Drew, jeanine M. 

Schmidt, David H. Root, and Donald F. Huber 

The Tongass National Forest is the Nation's largest 

national forest; it covers about 70,000 km2

, which is more 

than 80 percent of southeastern Alaska (fig. 1). The U.S. 

Forest Service, which is revising the Tongass Land Management Plan, has requested resource information from 

many sources, has analyzed and interpreted the information, and has prepared a Draft Environmental Impact 

Statement. It is now reviewing the comments thereon and 

revising the alternatives prior to releasing the Final Environmental Impact Statement. The Forest Service is attempting to include mineral-resource data in the plan. 

The U.S. Geological Survey (USGS) has been continuously active in southeastern Alaska since 1956 and is 

currently conducting a probabilistic assessment of the 

undiscovered mineral resources of the region. This regional 

assessment is based largely on previously completed assessments of individual Wilderness Areas, Wilderness Study 

Areas, and quadrangles that make up almost all of southeastern Alaska and that have been mapped at the scale of 

1:250,000 as part of the Alaska Mineral Resources Assessment Project. These previously completed assessments 

were based on USGS reconnaissance geologic, geochemical, and geophysical mapping that has been reported in 

USGS publications. A few assessments were joint USGSU.S. Bureau of Mines (USBM) efforts, and one is primarily 

a joint effort by the USBM and the State of Alaska Division 

of Geological and Geophysical Surveys. Only 4 of the 15 

quadrangles in southeastern Alaska are not included in a 

preexisting assessment; however, 2 of the 4 are already 

studied. 

The available data on regional geology, economic 

geology, stream-sediment and panned-concentrate geoMiles 

Figure 1 (Brew and others). Location of the Tongass 

National Forest in southeastern Alaska. 

chemistry, bedrock geochemistry, and mineral resources 

were reviewed at the 1:250,000 scale along with aeromagnetic, gravity, aeroradiometric, and telegeologic information. One hundred twenty-seven mineral-resource tracts, 

ranging in area from 16 to 2,920 km2

, were identified as 

likely to contain undiscovered metallic mineral resources in 

34 different types of deposits. Each tract was judged to 

contain one or more different types of mineral deposits. The 

number of as-yet-undiscovered deposits of each type discoverable by conventional mineral exploration methods was 

estimated for each tract at the 0.95, 0.90, 0.50, 0.10, and 

0. 05 probability levels. Many small tracts were combined to 

allow probabilistic assessment, but, because such assessments were not made of the smallest tracts or of tracts 

judged to be already well explored, we made final probabilistic assessments of 96 tracts containing 21 different 

types of deposits. Areas not assigned to tracts are interpreted to lack undiscovered mineral deposits of the types 

specified. 

Estimates of the number of deposits in each tract were 

used in combination with the worldwide tonnage and grade 

distributions for each deposit type to calculate a probabilistic undiscovered mineral-resource endowment for each 

tract by means of the USGS MARK3 mineral-resourceendowment simulator (Root and Scott, 1988). By Monte 

Carlo simulation, this computer program produces a distribution of tonnages for each metal contained in each deposit 

type in a given tract. When aggregated over all deposit 

types, these distributions yield a probabilistic estimate of 

the undiscovered mineral resources in each tract. The 

estimates of mineral-resource endowment for all the individual tracts are combined to provide an aggregated estimate of the undiscovered mineral-resource endowment of 

the Tongass National Forest and adjacent lands in southeastern Alaska. 

REFERENCE CITED 

Root, D.H., and Scott, W.A., 1988, User manual for mineral simulation 

program: U.S. Geological Survey Open-File Report 88-15, 63 p. 

Synthesis of Geophysical and Geological Data 

Centered on the Idaho Batholith and the Challis 

Volcanic Field, Idaho 

By john W. Cady, Viki Bankey, Keenan Lee, A.E. 

McCafferty, james A. Pitkin, and W.D. Stanley 

A digital geophysical data base, assembled for 

mineral-resource assessments of Wilderness Study Areas in 

Idaho, can be used for geophysical and geological interpretations beyond the scope of the mineral-resource assessments. The study area is bounded by lats 42° and 47° N. and 

longs 110° and 118° W. 

6 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Maps being prepared at a scale of 1: 1 ,000,000 

include topographic, gravity, and aeromagnetic maps and 

terrace-density and terrace-magnetization maps derived 

from the gravity and magnetic data (see McCafferty and 

Bankey, this volume). Data used to create the maps are 

available on magnetic tape from the EROS (Earth Resources Observation Satellite) Data Center, Sioux Falls, SD 

57198. Other maps in preparation as of August 1990 

include radioelement maps derived from aerial gamma-ray 

spectrometry and linear-feature and lineament maps derived 

from interpretation of Landsat multispectral scanner 

imagery. 

Density, magnetic susceptibility, and remanent magnetization of about 700 rock samples collected from the 

study area will be included in the regional data base to 

calibrate geophysical interpretations. Seven thousand mine 

and prospect records have been obtained from the U.S. 

Geological Survey's Mineral Resources Data System 

(MRDS) for evaluating the relations between geophysical 

anomalies and mineral deposits. Geophysical, rock physical 

property, and MRDS data and a digital geologic map are 

being transferred to the GRASS geographic information 

system developed by the U.S. Army Corps of Engineers. 

GRASS will be used to determine characteristic geophysical 

parameters and physical properties of geologic map units to 

aid geophysical interpretation and correlation with known 

mineral deposits. 

An east-west magnetotelluric (MT) profile was made 

across the west side of the Idaho batholith and the Idaho 

suture zone near McCall in 1989. In a separate study, an 

east-west MT profile was made near Leesburg. MT measurements were made in 1990 across the trans-Challis 

lineament zone northwest of Ketchum. The MT studies are 

designed to determine subsurface structure and to evaluate 

the potential for ore bodies in the contact areas between 

electrically resistive rocks of the batholith and conductive 

sedimentary and volcanic rocks. 

Preliminary interpretations include the following: 

(1) Magnetic highs show that Tertiary intrusive rocks are 

extensive in the shallow subsurface beneath the Challis 

Volcanics and the Idaho batholith of Cretaceous age. 

Gravity lows show that the Tertiary intrusions coalesce 

at depth into a batholith comparable in size to the Idaho 

batholith. 

(2) A northwest-trending lineament defined by the aeromagnetic, radioelement, and topographic data crosses 

the Salmon River Mountains midway between Challis 

and Salmon along the alignment of Big Creek. The 

lineament is defined by linear magnetic highs caused by 

magnetic Precambrian quartzite and schist, by Ordovician mafic to ultramafic intrusive rocks, and, possibly, 

by Tertiary intrusive rocks. 

(3) Ovoid belts of magnetic highs 40-70 km in diameter 

occur in the southwestern Snake River Plain and Owyhee Plateau in association with calderalike structures in 

felsic volcanic rocks. 

Mineral Deposits of the Midcontinent Rift, Lake 

Superior Region, United States and Canada 

By William F. Cannon and Teresa A. McGervey 

About 450 mineral deposits are known in Middle 

Proterozoic rocks of the Midcontinent Rift system in Michigan, Wisconsin, Minnesota, and Ontario. Data on these 

deposits provide clues for further exploration of the rift, 

both where it is exposed in the Lake Superior region and 

along its nearly 2,000-km-long extension in the Precambrian basement beneath Paleozoic cover in the midcontinent 

region. The 1:500,000-scale map (Cannon and McGervey, 

in press) of the Lake Superior region shows the locations, 

size classifications, and geologic types for these deposits on 

a geologic base map. 

The principal mineral produced from the rift has been 

copper. It has been obtained mostly from native copper 

deposits hosted in basalt flows and interflow sedimentary 

rocks and from stratabound deposits in lacustrine shales. 

Lesser copper production has come from sulfide veins in 

basalt and from breccia pipes. Silver has been produced 

from epithermal veins in Ontario, which also host polymetallic deposits of lead, zinc, copper, barite, and fluorite. 

Very large resources of copper-nickel-cobalt mineralization are proven in the Duluth Complex and other 

rift-related mafic intrusive bodies. Lesser known but potentially significant resources include uranium in pitchblende 

veins, uranium and rare-earth elements in alkalic rocks, 

copper sulfide (chalcocite) deposits in volcanic rocks, 

titanium and vanadium in mafic intrusive rocks, and 

platinum-group elements in mafic intrusive rocks. 

All the mineral deposits and resources have been 

discovered along about 15 percent of the strike length of the 

rift. This part of the rift is well exposed in the Lake Superior 

region. Subsurface extensions of the rift in the midcontinent 

region to the south, well known from geophysical studies 

and a limited number of basement-penetrating wells, constitute a major exploration frontier. If the buried parts of the 

rift are as thoroughly mineralized as the exposed parts, 

several thousand mineral deposits should be present at the 

Precambrian surface beneath Paleozoic cover rocks. 

REFERENCE CITED 

Cannon, W.F., and McGervey, T.A., in press, Map showing mineral 

deposits of the Midcontinent Rift, Lake Superior region, United 

States and Canada: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-2153, scale 1:500,000. 

Cannon and McGervey 7 
Mapping Minerals with Imaging Spectroscopy 

near Canon City, Colorado 

By Roger N. Clark, Andrea j. Gallagher, and Gregg 

A. Swayze 

Imaging spectroscopy data were obtained in 1989 

over Canon City, Colo., by using the U.S. National 

Aeronautics and Space Administration's Advanced Visual/ 

Infrared Imaging Spectrometer (AVIRIS), which has a 

ground resolution of 20 m. Spectral analysis of imaging 

spectroscopy data allows mapping of any material (such as 

minerals, vegetation, manmade materials, and water) having unique absorption features in the measured spectral 

region. AVIRIS acquires data in the spectral range from 0.4 

to 2.45 J.Lm in 224 spectral channels. The narrow spectral 

channels of A VIRIS form a continuous reflectance spectrum of the surface, in contrast to the four to seven channels 

of Landsat data. Landsat data can be used to distinguish 

general brightness and slope differences in the reflectance 

spectrum of the surface; imaging spectroscopy provides the 

same information and also resolves absorption bands in the 

spectrum that can be used to identify specific minerals. 

A new method of spectral analysis has been developed to map minerals in the Canon City test area. First, an 

absorption feature in the spectrum is isolated by removing 

the continuum in the reflectance spectrum (which also 

removes topographic effects from the analysis). Then a 

least -squares fit is performed on the absorption feature 

profile by using the reference-library spectrum of a known 

material. The method can be used to map minerals (and 

other materials) at very low concentrations as well as to 

distinguish between minerals traditionally difficult to differentiate in the field. 

At Canon City, dolomite, limestone, arkose, sandstone, and shale units form hogbacks dipping gently southwest on the east limb of a broad syncline. These rock units 

are truncated by northwest-trending faults, which juxtapose 

them against meta-igneous and gneissic rock units to the 

northeast. We have distinguished and mapped calcite, 

dolomite, illite, montmorillonite, hematite, goethite, kaolinite, and other minerals in these units and subtle compositional variations within individual rock units. These 

mineral-based map images correspond well with the geologic maps of the region, and field checks of the results 

prove that they are highly accurate. 

Clays and other OR-bearing minerals, iron oxides, 

and carbonates are just some of the many minerals that have 

diagnostic absorption features that can be used for mapping. 

Spectral analysis shows that kaolinite can be mapped where 

its concentration is only a few weight percent and that even 

crystalline order and disorder in kaolinite can be distinguished. The images for the Canon City area show more 

detail in some places than do standard geologic maps. 

Mineral maps for hundreds of square kilometers can 

be computed from images in only a few minutes of 

computer time. Such maps could be used to create geologic 

maps for petrologic, tectonic, and mineral-exploration studies. Color images produced from combinations of mineral 

maps yield a pseudogeologic map. Such images could 

provide valuable information before or during fieldwork 

and, thus, could greatly enhance the productivity of field 

studies. 

Geologic Settings of Barite Deposits as 

Indicators of the Potential for Other Types of 

Mineral Deposits 

By Sandra H.B. Clark 

Different types of barite deposits that form under 

different physical and chemical conditions have distinctive 

mineral associations (Clark and others, 1990). Some 

sediment-hosted, stratiform barite deposits, such as those of 

the Nevada barite belt, are predominantly barite, but others 

are associated with Zn-Pb-Ag deposits, such as those of the 

Selwyn basin and Kechika trough of western Canada, the 

Red Dog area of Alaska, and the Meggen deposits of 

Germany. Barite nodules in shales or mudstones may occur 

peripheral to stratiform deposits of barite and (or) basemetal sulfides; therefore, occurrences of barite nodules may 

indicate proximity to undiscovered stratiform base-metal 

sulfide and (or) barite deposits. However, barite nodules in 

shales or mudstones are not always indicators of associated 

stratiform deposits, because they may form by redox 

precipitation at anoxic-dysoxic interfaces in unconsolidated 

sediments during diagenesis and do not necessarily indicate 

the involvement of barium-enriched fluids. 

Volcanic-associated stratiform barite deposits may 

occur with polymetallic (Cu-Pb-Zn±Au±Ag) massive sulfide deposits, such as the Kuroko ore bodies of Japan 

and the Buchans ore bodies of Newfoundland. Barite also 

is spatially related to volcanic-associated Au-Ag deposits in 

the Hemlo area of Ontario and the Barite Hill area of 

South Carolina. The relations between barite and Au-Ag 

mineralization in these areas are uncertain; some of the 

barite may be stratiform, but later hydrothermal barite 

mineralization is more closely associated spatially with 

Au-Ag mineralization. 

Some stratiform syngenetic to diagenetic barite 

deposits are associated with economic concentrations of 

evaporite minerals that formed in supratidal zones. Examples are barite-celestite deposits of northeastern Mexico and 

western Argentina. 

Carbonate-rock-hosted stratabound deposits of barite 

and residual deposits that overlie them are common in areas 

that contain deposits or occurrences of sphalerite, galena, 

and (or) fluorite. In some barite deposits, such as the 

residual deposits of the Cartersville district of Georgia, 

barite in the bedrock is not associated with sulfide minerals 

8 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Figure 1 (Sandra Clark). Stratabound barite (white material 

in fractures above and behind Travis A. Paris) in crackle 

breccia zone in Lower Ordovician Mascot Dolomite, 

Thompson mine , Sweetwater district, Tennessee. Photograph by Sandra H.B. Clark, 1988. 

Figure 2 (Sandra Clark). Stratabound fluorite (gray) and 

barite (white) in fractures in collapse breccia zone in 

Lower Ordovician Kin gspo rt Formation , Ballard mine, 

Sweetwater distri ct, Tennessee . Photograph by Sandra 

H.B. Clark, 1988. 

or fluorite, but, in others, stratabound barite in the bedrock 

is associated with fluorite (Sweetwater district, Tennessee; 

figs. 1 and 2) and (or) base-metal sulfide minerals (Southeast Missouri district). In the Central Tennessee district, 

barite occurs both within major stratabound Zn-Pb-F deposits and within veins overlying them. 

As the above examples suggest, study of the geologic 

setting of barite occurrences and deposits can provide 

information about the potential for spatially associated 

deposits of other mineral resources, including base-metal 

sulfides, precious metals, fluorite, and celestite. 

REFERENCE CITED 

Clark, S.H.B., Gallagher, M.J. , and Poole, F.G. , 1990, World barite 

resources-A review of recent production patterns and a genetic 

classification , in Minerals, materials, and industry: London, The 

Institution of Mining and Metallurgy, p. 175-184. 

Tectonomagmatic Settings of Proterozoic 

Metallogenic Provinces in the Southwestern 

United States 

By Clay M. Conway 

Tholeiitic, calc-alkalic, and calcic, bimodal to unimodal, largely marine , subduction-arc assemblages dated at 

1, 790 to 1, 720 Ma are exposed from the Cheyenne belt in 

southern Wyoming to northern New Mexico and central 

Arizona. Rocks older than 1, 760 Ma are present only near 

the Cheyenne belt. The arcs were accreted to the North 

American craton between about I, 750 Ma and 1, 700 Ma, 

probably as numerous tectonostratigraphic terranes. They 

constitute a metallogenic province characterized primarily 

by submarine exhalative deposits. Most of the scores of 

massive sulfide deposits throughout this province are rich in 

Zn and Cu and contain lesser and variable amounts of Pb, 

Au, and Ag. Several Au deposits are exhalative, and areas 

having potential for exhalative Au have been identified. 

Exhalative W-Cu deposits in Colorado may belong to this 

province . Precious- and base-metal veins and porphyry-type 

Cu prospects are locally associated with plutonic rocks of 

the province. Ultramafic rocks or sheeted dikes known in 

several localities suggest the presence of oceanic crust. A 

Co-Ni-Cr-Cu prospect is known at one locality of ultramafic 

rocks. 

By 1, 700 Ma, the subduction-arc terrane formed a 

belt of incompletely stabilized continental crust more than 

600 km wide. Strata and intrusions of continental character 

were emplaced within and marginal to this new crust largely 

in the interval 1, 700 to 1 ,650 Ma, possibly during a rifting 

event that resulted in a passive continental margin in 

Arizona and New Mexico. Paleocurrent data from thick 

fluvial to tidal mature sandstone sequences in central 

Conway 9 
Arizona, northern New Mexico, and the Needle Mountains 

of Colorado indicate transport toward deep basins in southem Arizona and New Mexico. The mature sandstones at 

most localities overlie and are locally interbedded with 

high-silica ash-flow rhyolite and minor basalt; rhyolite and 

associated hypabyssal rocks are products of subaerial caldera magmatism. Deep-seated granites are present in southeastern California, central to northern Arizona, northern 

New Mexico, and as far north as the Boulder Creek 

batholith in Colorado. The igneous rocks dated at 1, 700 to 

1,650 Ma are generally alkalic-calcic and subaluminous to 

peraluminous; they are similar to anorogenic granites of the 

region that are dated at 1 ,480 to 1 ,400 Ma. 

The 1 , 700- to 1 , 650-Ma rocks constitute a metallogenic province dominated by elements enriched in evolved 

granitic magmas. There has been little production of metals 

from this province, however. The Boriana tungsten deposit 

of northwestern Arizona, which is genetically related to a 

1 ,690-Ma two-mica granite, is possibly the best-known 

deposit of this type. Caldera-related hypabyssal rocks in 

central Arizona are enriched in Sn, Be, Nb, La, and Y; 

associated sedimentary rocks contain small Be deposits. 

Early subaqueous volcanic sequences contain a few small 

massive sulfide deposits. In northern New Mexico, W 

prospects and Sn anomalies in drainage sediments are 

associated with felsic volcanic and hypabyssal rocks. One 

stratiform W deposit and a regionally extensive sediment 

layer enriched in Mn, Nb, Ta, W, Sn, and Sb are between 

rhyolite and an overlying quartz sandstone and may be 

exhalative. Fe-rich beds are also associated with rhyolite 

and quartzite in this province. 

Granites dated at 1 ,480 to 1 ,400 Ma occur throughout 

the regions discussed above and constitute a third distinct 

metallogenic province. Like the 1,700- to 1 ,650-Ma igneous rocks, they are not likely to be subduction related, and 

they have potential for metals characteristically enriched in 

evolved granite magmas. For example, the granites locally 

are highly enriched in U and Th; near Bagdad, Ariz., they 

produced W vein deposits. Petrographically and metallogenically similar plutons dated at 1 ,370 to 1 ,360 Ma and 

1,060 Ma occur in limited areas in Colorado. 

In southeastern California, igneous rocks of the three 

provinces discussed above are superimposed on crust 

formed between 2.3 and 2.0 Ga. This region of anomalous 

crust has apparently unique metallogenic characteristics. It 

contains 1, 720-Ma massive sulfide deposits that have 

extremely low contents of Au and Ag and 1 ,400-Ma syenite 

complexes, one of which hosts the Mountain Pass rareearth-element deposit. 

A region in southern California west of the MojaveSonora megashear differs from the provinces discussed 

above. It contains, for example, the 1.2-Ga anorthosite 

complex in the San Gabriel Mountains, which has resource 

potential forTi, V, and rare-earth elements. 

Preliminary Assessment of the Mineral Resources 

of the Cedar City 1 o x 2° Quadrangle, Utah 

By Theresa M. Cookro, Michael A. Shu bat, and 

janet L. Jones 

Mineral resources of the Cedar City 1 o X 2° quadrangle, Utah, were assessed in 1989-90 during a cooperative study by the U.S. Geological Survey (USGS) and the 

Utah Geological and Mineral Survey (Eppinger and others, 

1990). This preliminary study, which was part of the 

National Mineral Resources Assessment Program, was used 

to aid Federal land-use planning and to provide data for 

future mineral-resource studies. During the assessment, the 

Utah Geological and Mineral Survey added new information to the Mineral Resources Data System (MRDS) of the 

USGS. 

Principal commodities of the 13 metal mining districts in the quadrangle include iron, silver, gold, gallium, 

germanium, and base metals. Nationally, the Iron Springs 

and Pinto districts were the greatest producers of iron ore 

west of the Mississippi until the mines closed in the 1980's; 

mines have reopened, and the districts currently supply 

about 550,000 tons of ore per year to the Geneva steel plant 

in Orem, Utah. The Escalante mine, in the Escalante 

district, was one of the largest silver mines in Utah until it 

closed in 1989. Gold has been produced as a byproduct at 

base-metal and silver mines in the quadrangle, and a new 

bulk-minable gold deposit is now in production in the 

Goldstrike district. The Apex mine, in the Tutsagubet 

district, produced gallium (used for integrated circuits) and 

germanium (used in infrared optics and fiber optics) from a 

collapse breccia pipe. The pipe originally was mined for 

highly oxidized ores of copper, lead, and silver that 

contained minor gold and zinc. In the Silver Reef district, 

silver was produced from a roll-front environment, similar 

to that of the uranium roll-front deposits. Exploration is in 

progress in the Goldstrike, Tutsagubet, and Antelope Range 

districts and in the Washington Dome area. 

Industrial mineral resources within the quadrangle 

include volcanic rock products, bentonite clay (produced 

for the Glen Canyon Dam project), kaolin, petrified wood, 

agate, dimensional sandstone, limestone, gypsum, marble, 

and sand and gravel. Oil and coal resources are also 

abundant within the Cedar City quadrangle. The Virgin oil 

field, Utah's oldest oil field, produced 25 million barrels. 

Three coal fields (Kolob, Harmony, and Alton) and part of 

a fourth (Kaiparowits) lie within the quadrangle and contain 

approximately 6 billion tons of reserves. 

REFERENCE CITED 

Eppinger, R.G., Winkler, G.R., Cookro, T.M., Shubat, M.A., Blank, 

H.R., Crowley, J.K., Kucks, R.P., and Jones, J.L., 1990, Preliminary assessment of the mineral resources of the Cedar City 1 o x 2° 

quadrangle, Utah: U.S. Geological Survey Open-File Report 90-34, 

146 p. 

10 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Copper Deposits in Tertiary Red Beds in Bolivia 

By Dennis Cox, Raul Carrasco, Orlando Andre, 

Alberto Hinojosa, and Keith Long 

Stratabound copper deposits are found in red to 

brown sandstone, conglomerate, and siltstone of Miocene 

and Pliocene age in the Altiplano of southwestern Bolivia. 

Chalcocite and native copper replace carbonate cement and 

locally replace fossil plant materials. Ore minerals are 

associated with zones of bleaching and chloritization of the 

host rocks. More than 70 deposits have been found from 

Lake Titicaca to near the border with Argentina. More than 

half of these deposits contain less than 10,000 t of copper, 

but two larger districts are known: Corocoro and Chacarilla, 

which together contain more than 1,000,000 t of copper. 

These large deposits are associated with diapirs of gypsum, 

and the Corocoro deposit, with a possible salt intrusion. 

These diapirs originated from evaporites in the lower part of 

the Tertiary section. The Corocoro fault controlled most of 

the mineralization at Corocoro and is unique in that strata on 

both sides dip away from the breccia-filled fault zone. 

Massive halite was encountered near the fault in a shaft 

sunk in 1974. 

Replacement of carbonate in the sandstone matrix by 

native copper, formation of native copper pseudomorphs 

after aragonite, and bleaching and chloritization of host 

rock near depositional sites can be described by the following two equations: 

8 CuCl~- + 12 CaC03 + 16 H+ + CH4 

and 

=8 Cu0 + 12 CaC12 + 13 C02 + 10 H20 

limonite kaolinite 

=4 (Fe5Al)(Si3Al)010(0H)8 + 5 C02 + 2 H20 

chlorite 

The ore solutions are assumed to have been brines of 

low pH that mixed with a hydrocarbon reductant to reduce 

iron and deposit copper. Conditions favoring copper precipitation were widespread in the Altiplano, but large 

deposits formed only near evaporite diapirs. Elevated temperatures near these diapirs accelerated convective flow of 

brines that leached copper from large volumes of sedimentary rock. Flow was focused along the flanks of the diapirs. 

Mixing of brines with reducing fluids in the sedimentary 

wall rocks caused copper deposition. 

Geologic Map of the Sierrita-Mogollon Corridor 

(Arizona-New Mexico) and Implications for 

Mineral Resources 

By Leslie j. Cox, Brenda B. Houser, Eric R. Force, 

Mark E. Gettings, Alison Burchell, and Frederick S. 

Fisher 

The U.S. Geological Survey is conducting a multidisciplinary study of a geologic transect across the southern 

Basin and Range province (fig. 1). The Sierrita-Mogollon 

transect corridor contains abundant known mineral resources that include copper, precious metals, diatomite, and 

zeolites. Undiscovered mineral deposits may be concealed 

beneath a thin cover of basin fill, Tertiary lava flows, or the 

upper plates of detachment faults. Highlights of recent and 

(or) unpublished data used to compile the geologic map of 

the transect corridor are summarized below. 

Sierrita and Tucson Mountains.-Recognition of a 

Cretaceous caldera in the Sierrita Mountains that was tilted 

to the southwest during middle Tertiary extensional tectonism led to reinterpretation of the positions of ore deposits 

relative to what may be a subcaldera batholith. The deposits 

formed in and around the roof of the batholith rather than 

down along its side (Lipman and Fridrich, 1990). Reinterpretation of movement along faults between precaldera and 

intracaldera volcanic units indicates that some present-day 

reverse faults were steep normal faults prior to southwest 

rotation. 

Santa Catalina Mountains. -Previous interpretations 

of the Catalina detachment fault in the Santa Catalina 

Mountains assigned the entire mountain mass to the lower 

plate. New work suggests that the forerange is in the lower 

plate but that the main range occupies a structurally intermediate position above an older, deformed mylonite and 

below a younger mylonite, most of which has been eroded 

away. Within this intermediate plate are several newly 

mapped granitic intrusions that apparently postdate the 

emplacement of Eocene two-mica granites. A sequence of 

Precambrian to Upper Cretaceous supracrustal stratified 

rocks in the intermediate plate is host to several small 

mineral deposits and, potentially, could host large deposits. 

San Manuel-Mammoth area.-Three generations of 

extensional faulting have affected the Mammoth vein set 

and other middle Tertiary mineralized and altered zones of 

the San Manuel-Mammoth area. From youngest to oldest, 

these are (1) middle Miocene or younger, steep Basin-Range 

faulting on the Mammoth fault zone, (2) early Miocene 

low-angle normal faulting and rotation on the San Manuel 

fault system, and (3) late Oligocene detachment on the 

Cloudburst fault and its side ramp, the Turtle fault. For the 

early Miocene and younger faults that offset the Mammoth 

vein set, precise vectors have been calculated that permit 

vein reconstruction. The late Oligocene detachment faulting 

apparently is genetically related to the vein mineralization. 

Cox and others 11 
Galiuro Mountains. -Reconnaissance and detailed 

geologic mapping in the Galiuro Mountains shows that the 

central part of the range consists of intermediatecomposition lava flows overlain by local pyroclastic flow 

and air-fall deposits and by a previously unrecognized, 

areally extensive complex of dacitic to rhyolitic flows and 

domes. Possibly compatible hypotheses as to the source of 

the Galiuro volcanic field include (1) a volcano-tectonic 

depression inferred from a pattern of gravity lows that 

straddles the flow and dome complex and part of the San 

32° 

Pedro basin and (2) a large eruptive center that may have 

formed along an east-northeast-trending zone of crustal 

weakness that developed within an extensional accommodation zone. 

The Galiuro Mountains are flanked by northwesttrending normal faults and are between the Catalina and 

Pinaleno metamorphic core complexes. Tectonic interpretations inferred for the structure of the Galiuro Mountains 

suggest the possibility of detachment faults at shallow 

depths. If correct, this interpretation has implications for the 

I 

• 

I 

• 

I 

• 

I 

• 

I 

• 

I 

• 

Mogollon 

Mts. 

ARIZONA I NEW MEXICO 

• 

I 

• 

I 

• 

I 

• 

Figure 

~~--------------------------------------~·-------- 1 (Leslie Cox and others). Mountains and basins (stippled) in the Sierrita-Mogollon corridor, Arizona and New 

Mexico. 

12 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
potential for detachment-fault-related precious metals in the 

range. 

Alma basin, New Mexico. -Oblique-slip normal 

faults around the margins of the Alma basin in basin-fill 

units as young as early Pliocene suggest that the basin may 

have been rotated slightly counterclockwise during Basin 

and Range extension. The Alma basin is near the edge of 

the Colorado Plateau, between the Morenci-Reserve fault 

zone on the northwest and the Mogollon Mountains on the 

east. This tectonic setting suggests that the inferred rotation 

of the basin block may have been in response to interactions 

between the northeast-trending Morenci-Reserve fault zone , 

the relatively stationary Colorado Plateau, and east-westdirected Basin and Range extension. Stratigraphic evidence 

from Miocene conglomerates indicates that an early stage of 

the inferred rotation may have been coeval with mineralization in the Mogollon mining district. 

REFERENCE CITED 

Lipman , P.W., and Fridrich, C.J., 1990, Cretaceous caldera systems: 

Tucson and Sierrita Mountains, in Gehrels, G.E., and Spencer, J.E. , 

eds., Geologic excursions through the Sonoran Desert region , 

Arizona and Sonora: Arizona Geological Survey Special Paper 7, 

p. 51-65. 

Laser-Microprobe Studies of Sulfur Isotopes in 

Stockwork and Massive Sulfide Ores, Rua Cove 

Mine, South-Central Alaska 

By Douglas E. Crowe, Wayne C. Shanks Ill, and 

John W. Valley 

Many of the massive sulfide deposits in the Prince 

William Sound district of south-central Alaska (fig. 1), 

which historically were interpreted as epigenetic vein and 

replacement deposits, recently have been reinterpreted as 

volcanogenic massive sulfide (VMS) deposits formed syngenetically on or near the sea floor (Wiltse, 1973; Koski and 

others, 1985 ; Crowe and others, 1988). Understanding the 

origin of the deposits is critical for defining this region as a 

potential exploration target for VMS deposits. The morphologies and geologic settings of the deposits are consistent with Cyprus- and Besshi-type VMS models. The host 

rocks are predominantly flysch and dismembered ophiolite 

of the Chugach (Late Cretaceous) and Prince William (early 

Tertiary) terranes, which constitute the bulk of the rocks 

exposed in southern Alaska. Conventional analyses of 

stable sulfur isotopes show o34S ranging from +3.5 to +4.5 

per mil; these values are consistent with a VMS model for 

the deposits but are not definitive. The major limitation to 

obtaining meaningful isotope data is the fine-grained, intergrown nature of the sulfide ores that typify the deposits. 

A new laser-microprobe technique for performing 

micro-scale stable-isotope analyses (Crowe and others, 

km 100 > 

I 

\o 

140° 

Figure 1 (Crowe and others). Tectonostratigraphic terranes 

and locations of mines in selected historically significant 

massive sulfide deposits in south-central Alaska. 1, Rua 

Cove mine; 2, Lynx Creek mine; 3, Standard Copper 

mine; 4, Ellamar mine; 5, Port Fidalgo mine; 6, Beatson 

mine. 

1990) allows us to measure sulfur-isotope ratios of individual minerals in fine-grained slabbed samples. Sulfur-isotope 

data were collected at the University of Wisconsin, Madison, by using a Nd-Y AG (yttrium-aluminum-garnet) laser 

connected to an isotope-ratio mass spectrometer. A similar 

system (which uses a C02 laser) at the U.S. Geological 

Survey in Reston , Va. , is shown in figure 2. Lasermicroprobe sulfur-isotope data for samples from the Rua 

Cove mine, Prince William Sound, provide new evidence 

that many of these deposits are syngenetic. 

Conventional combustion-extraction measurements 

of o34S from hand-picked coexisting chalcopyrite-pyrrhotite 

(cp-po) pairs (fig. 3) from both the massive sulfide blanket 

and the underlying stockwork feeder zone indicate isotopic 

disequilibrium; pyrrhotite is slightly depleted in 34S, in 

contrast to the expected equilibrium enrichment with 

respect to chalcopyrite. The conventional data are inconsistent with ore formation by epigenetic replacement and vein 

mineralization or by syngenetic precipitation in the stockwork zone of a VMS deposit. Physical separation of the 

cp-po pairs is difficult because the samples are fine grained; 

thus, the results obtained conventionally are suspect. 

Laser-microprobe analyses of the same samples (fig. 

3) reveal that the stockwork sulfide pairs are in isotopic 

equilibrium if Llcp-po = 1.4 per mil at the 300 °C temperature indicated by fluid-inclusion homogenization data . 

However, the massive sulfide sample analyzed by laser 

microprobe shows an average Llcp-po = 2. I per mil; this 

value suggests either lower temperature precipitation or 

rapid quenching . These data are inconsistent with epigeCrowe and others 13 
Figure 2 (Crowe and others). C02 laser microprobe connected to an isotope-ratio mass spectrometer at 

the U.S. Geological Survey in Reston, Va. The equipment is used to determine sulfur-isotope ratios for 

individual minerals in fine-grained slabbed rocks. Photograph by W.C. Shanks Ill in 1990. 

7 conventional laser microprobe 

"' <) 

E "' > '0 I '0 ·:;. ~ ~ 

~ "' l 1-< ] ~ ~ I I (]) 0 E 

0.. "' 3: 

·~ -"' - u 

5 "' 0 

"' ;r. I [/). E 

~ : I -"' r<"l I 0 

3: 

I -"' u 

~ 3 g 

• = po 

• = cp 

Figure 3 (Crowe and others). Conventional and lasermicroprobe data for su lfur isotopes from the Rua Cove 

mine. The fine-grained, intergrown nature of the 

chalcopyrite-pyrrhotite (cp-po) pairs made conventional 

mineral separation difficult; this difficulty is reflected in 

the nearly identical 834S values for these sulfides. The 

laser-microprobe data indicate equilibrium 834S values for 

the stockwork pairs and lower temperature or disequilibrium 834S values for the massive sulfide pairs . 

netic vein or replacement formation under equilibrium 

conditions at similar temperatures, where spatial variations 

in temperature required to produce llco- po variations of this 

magnitude are uncommon . The laser-microprobe data are 

consistent with the predicted isotopic values for sulfide 

mineral pairs from the physicochemically distinct stockwork and massive sulfide environments. 

REFERENCES CITED 

Crowe, D.E., Nelson, S.W., Brown, P.E. , and Shanks, W.C. , III, 1988, 

Stable isotope and fluid inclusion investigation of massive sulfide 

deposits, Orca and Valdez Groups, Alaska-Evidence for syngenetic sulfide deposition [abs.]: Geological Society of America 

Abstracts with Programs, v. 20, no. 7, p. 153. 

Crowe, D.E. , Valley , J.W., and Baker, K.B. , 1990, Micro-analysis of 

sulfur-isotope ratios and zonation by laser microprobe: Geochimica 

et Cosmochimica Acta , v. 54, p. 2075-2092. 

Koski, R. , Silberman , M.L. , Nelson , S.W., and Dumoulin, J.A., 1985 , 

Rua Cove-Anatomy of volcanogenic Fe-Cu sulfide deposit in 

ophiolite on Knight Island , Alaska [abs.]: American Association of 

Petroleum Geologists Bulletin , v. 69 , p. 667. 

Wiltse, M.A. , 1973 , Fe-Cu massive sulfide deposits in ancient outer-arc 

ridge-trench slope environments [abs.]: Geological Society of America Abstracts with Programs, v. 5, no. 1, p. 122-123. 

14 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Exploration Guides for Precious-Metal Deposits 

in Volcanic Domes 

By Charles G. Cunningham and George E. Ericksen 

Volcanic domes are common in intermediate to silicic 

volcanic fields and are exploration targets for epithermal 

precious-metal deposits. Domes occur throughout volcanic 

fields, but they are most common along structural margins 

and radial faults associated with calderas, within and on the 

flanks of stratovolcanoes (fig. 1), and along regional faults, 

especially where an inflection occurs along the fault. 

Domes generally form late in the magmatic evolution of an 

eruptive center and typically are cupolas above larger 

magma bodies at depth that were sources of much of the 

metals and fluids in addition to the magma. 

The Neogene to Quaternary volcanic complex in the 

Andean Highlands of Peru, Bolivia, and Chile contains 

many excellent examples of mineralized volcanic domes. 

These domes have well-preserved features, commonly 

exposed during mining, that record the evolution of the 

domes and provide insight into the controls on the distribuFigure 1 (Cunningham and Ericksen). Volcan Guallatiri, an ice-covered andesitic stratovolcano in northernmost Chile, near the Bolivian border, showing 

fumarolic activity (tiny white plume at highest point) 

and active deposition of native sulfur. The distribution of ore deposits in this region indicates that a 

tion of associated hydrothermal ore deposits. Some of these 

controls are described below and are illustrated in figure 2. 

Forceful emplacement of silicic magma may cause 

development of outward-flaring cone fractures above a 

magma chamber that serve as natural conduits for hydrothermal fluids. As magma rises toward the surface, interaction with ground water may result in phreatomagmatic 

explosions that open a conical vent and deposit a surrounding apron of crudely bedded breccia fragments. This explosion breccia generally consists of fragments of the country 

rock and the carapace of the intrusive body in a tuffaceous 

matrix. This heterogeneous mixture can be highly porous 

and, consequently, is a favorable site for mineralization. 

The breccia commonly is overlain by bedded air-fall tuff 

resulting from subsequent explosive activity in the vent. 

Where it is well preserved, the tuff ring is draped over the 

mound of breccia and dips outward at the periphery as well 

as inward into the vent. 

Viscous, flow-banded magma may then be extruded 

and expand over, and shoulder aside, the breccia and tuff; 

these magma movements cause the formation of both radial 

and concentric faults and fractures. Obsidian rings may 

newly formed or presently forming gold-silver 

deposit may exist deep within or beneath this volcano. Hydrothermal alteration zones in deeply 

eroded volcanoes of this type are prime exploration 

targets for new gold-silver deposits in this part of the 

Andes. Photograph by George E. Ericksen in 1979. 

Cunningham and Ericksen 1 S 
Todos 

MINERALIZATION 

CONTROL 

Figure 2 (Cunningham and Ericksen). Exploration guides 

for precious-metal deposits in volcanic domes, showing 

examples from Bolivia. Faults, fractures, and breccias are 

permeable features that are favorable sites for ore deposition . 

form at the base of the dome where the magma is quenched 

as it overrides the cooler tuff ring. Continued extrusion of 

magma causes the dome to expand and break its solidified 

carapace to form mantle breccias. Hydrothermal and intrusive breccia pipes may form; they range from those less than 

a meter in diameter to those that encompass most of the 

dome . Eroded fragmental material accumulates as porous 

volcaniclastic sediments and talus along the flanks and base 

of the dome . Reactivation of faults that initially guided 

magma emplacement can cause development of fracture 

systems within the dome in which crosscutting epithermal 

veins may form. 

Origin of Iron, Rare-Earth Elements, Copper, and 

Gold in Middle Proterozoic Deposits of the 

Midcontinent Region, U.S.A. 

By W.C. Day, G.B. Sidder, A. E. McCafferty, L.E. 

Cordell, E.B. Kisvarsanyi, R.O. Rye, and L.M. 

Nuelle 

Middle Proterozoic anorogenic rocks of the midcontinent region of the United States host several magnetitehematite deposits rich in rare-earth elements (REE) and 

locally rich in copper and gold. The anorogenic rocks 

exposed in Missouri represent the root zone of an eroded 

volcano-plutonic terrane that developed on rocks of the 

Early Proterozoic Central Plains orogen. Subvolcanic biotite granite massifs are overlain by comagmatic high-silica 

rhyolite, which erupted from caldera complexes that locally 

underwent resurgence. The resurgent cauldrons are cored 

by plutons of high-silica, two-mica granite that have characteristic magnetic lows; amphibole granite and a 

magnetite-trachyte suite form ring intrusions along the 

margins of the calderas. The magnetite-hematite ore deposits are cut by aplite and postvolcanic mafic dikes indicating 

ore emplacement during the waning stages of igneous 

activity . 

A 20-mgal, northwest-trending steep gravity gradient, as well as several smaller (5-10 mgal) en echelon 

gradients, transects the anorogenic terrane. Most of the 

magnetite-hematite ore deposits are spatially associated 

with the gravity gradients. Although the depth and precise 

geologic cause of the gravity gradients are unknown, the 

gradients require fairly abrupt density discontinuities over a 

vertical zone of more than 4 km and may represent a 

deep-seated fracture system that provided structural conduits for the magmatic and mineralizing systems. 

The Pea Ridge magnetite deposit in Missouri is the 

best studied ore deposit of this class in the midcontinent 

region. The volcanic-rock-hosted ore body is steeply dipping, lenticular, and zoned outward from a magnetite-rich 

core through a footwall hematite zone to a quartz-potassium 

feldspar zone. The hanging wall is an amphibole-quartzmagnetite skarn. REE-rich breccia pipes composed of 

barite, potassium feldspar, monazite, xenotime, and local 

gold cut the footwall margins of the magnetite ore body. 

Total REE oxide and gold contents of the breccia pipes 

reach 20 percent and several parts per million, respectively; 

however, gold distribution is erratic. 

New major- and trace-element chemical data indicate 

that the iron-REE±copper±gold deposits are genetically 

related to the host anorogenic terrane. The rhyolitic and 

granitic rocks represent lower crustal melts generated in 

response to mantle upwelling during the Middle Proterozoic. The magnetite-trachyte suite represents either a high 

degree of partial melting of the lower crust or crustcontaminated mantle melts. The rhyolitic and granitic 

magmas ascended into the upper crust and formed volcanoplutonic complexes. One scenario allows that the 

magnetite-trachyte suite underwent fractionation and liquid 

immiscibility and generated an Fe-P-REE-rich phase and a 

Si-Al-K-rich phase. The Fe-P-REE-rich phase was the 

progenitor of the ore deposits, whereas the Si-Al-K-rich 

phase was the precursor of the quartz-potassium feldspar 

alteration zone at Pea Ridge. These fluids may have been 

channeled along the deep-crustal discontinuities and 

emplaced into the volcano-plutonic complexes. 

Anomalous Concentrations of Fine-Grained 

Native Gold in Stream Sediments of East-Central 

Lemhi County, Idaho 

By George A. Desborough , William H. Raymond, 

and Karl V. Evans 

Preliminary reconnaissance studies of heavy minerals 

in stream sediments show that anomalous amounts of 

fine-grained (microscopic) native gold in concentrations 

>0.05 ppm are present in a large area around Salmon, 

Idaho (fig. 1). Native gold in anomalous amounts was 

16 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
IDAHO 

·Boise 

EXPLANATION 

Gold Anomalies 

Moccasin 

Creek 

/ 

114' 

North Fork 

X >30 native gold grains/kg (>0.19 ppm) 

Creek 

10-20 native gold grains/kg (0.06-0.12 ppm) 

10 MILES 

;------,--~-'"'---,.-,15--' LOMETER S 

(, 

J~,~~k 

_

4

~ 

. Tendoy\ 

~ ~ 

45' 

Figure 1 (Desborough and others). Locations of stream-sediment gold anomalies in 

east-central Lemhi County, Idaho. 

Desborough and others 17 
recovered from samples of bulk stream sediment by using 

laboratory panning methods that effectively recover gold 

grains in the size range of 50-200 J.Lm , as counted and 

measured by using a microscope. Many of the gold anomalies are in areas that have not been recognized for potential 

gold resources. Nine localities have what we consider 

highly anomalous concentrations of native gold (> 30 

grains/kg or 0.2-0.3 ppm). Ten other localities are considered to have anomalous gold concentrations (10-20 grains/ 

kg or 0.06-0 .12 ppm). 

More than 100 sites were sampled. Nine samples 

from streams that drain the Leesburg area of lode-gold 

mineralization average 13 grains/kg (range of 8-22 grains/ 

kg). Twenty percent of the other sample sites have anomalous amounts of gold (that is, >0.06 ppm); in contrast, 25 

percent of the sample sites lack detectable gold. 

Results of these studies suggest that fine-grained 

native gold is widely dispersed by streams because of small 

grain size, but that other heavy minerals or metals that 

locally are associated with native gold in lode deposits 

elsewhere are absent from the Salmon area. 

Element Dispersion in Alluvium Covering Gold 

Deposits in the Kelly Creek Valley, Getchell 

Gold Trend, Humboldt County, Nevada 

By D.E. Detra, S.M. Smith, P.K. Theobald, and 

P.M. Theodorakos 

The Chimney Creek and Rabbit Creek sedimentaryrock-hosted gold deposits are covered by 15-185 m of 

basin-fill alluvium in the Kelly Creek valley in Humboldt 

County, north-central Nevada (fig. 1). These are two of the 

deposits that define the Getchell gold trend. The exploration 

and development of these two deposits provide a unique 

opportunity to define three-dimensional element-dispersion 

patterns in the alluvium away from the ore bodies and to 

study the geochemical and physical processes that produced 

these patterns. 

In this multidisciplinary study, reverse-circulation 

rotary drill cuttings of alluvium were obtained from 25 

vertical exploration and development holes drilled by Gold 

Fields Mining Corporation and Santa Fe Pacific Mining, 

Inc. Cuttings generally were sampled at 6-m intervals 

within the alluvial sequence, were sieved to obtain the 

fraction from -20 to + 100 mesh, and then were ground to 

-100 mesh for analysis. Total and oxalic-acid partial-leach 

analyses for 35 elements were performed by direct-currentarc atomic-emission spectrography. Results of these analyses show a distinct vertical distribution of elements or 

element suites within the alluvium in a 21-km2 study area 

around and over the Rabbit Creek and Chimney Creek 

deposits. Anomalous element concentrations were detected 

by both total and partial analyses; however, these anomaFigure 1 (Detra and others). The Rabbit Creek open-pit 

gold mine of Santa Fe Pacific Mining, Inc., in Kelly Creek 

valley, Humboldt County, Nev. The pit exposes 120 m of 

Tertiary basin-fill sediments; each bench is 12 m high. 

View to the southwest; Osgood Mountains in background. Photograph by Steven M. Smith, August 25, 1990. 

lous values were significantly enhanced in results from the 

partial analyses. This enhancement suggests that the metals 

may be concentrated in clast coatings. 

Vertical variations in the concentration of major and 

minor elements are related to lithologic changes within the 

alluvial sequence and (or) to later diagenetic changes and 

(or) water-rock reactions within the alluvium. For the most 

part, gold and base metals occur in low concentrations and 

lack a systematic distribution with depth. Of the trace 

elements determined, As, Ge, Sb, and W show the greatest 

contrast in concentration relative to position within the 

alluvial sequence. Anomalous concentrations of As, Ge, 

Sb, and W in the alluvium delineate a cohesive, laterally 

continuous blanket near the base of the alluvium. A 

three-dimensional model of the data for this suite shows that 

the blanket has a fairly flat upper surface and a lower 

surface that conforms to the irregular surface topography of 

the basement. The blanket is apparently offset by normal 

faults within the alluvium and lacks a spatial relationship to 

the present-day water table. These observations suggest that 

the As, Ge, Sb, and W suite may be a fossil geochemical 

anomaly that is largely unrelated to currently active 

element-dispersion processes in the basin. Three possible 

mechanisms could have created the blanket: (1) deposition 

early in the basin's history of mineralized or metal-enriched 

clastic detritus derived from the Osgood Mountains to the 

west; (2) percolation of fluids from a low-temperature 

hydrothermal system through the alluvium early in the 

basin's history; or (3) migration downgradient along the 

bedrock-alluvium interface of a plume of ground water 

slightly enriched in trace elements. 

18 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Anomalous trace-element concentration patterns in 

the deeper parts of the alluvial sequence do not appear to be 

reflected in surface soil and plant geochemical anomalies 

and do not coincide vertically with any ore body at depth. In 

addition, there is no evidence that surface geochemical 

anomalies in the Kelly Creek valley are a direct reflection of 

individual ore bodies beneath the alluvium. 

Advances in Plumbic Prospecting 

By Bruce R. Doe 

Since publication of the original paper on plumbic 

prospecting and the application of lead isotopes to evaluating mineral prospects in Cretaceous and Tertiary magmatothermal ore deposits in the Western United States (Doe, 

1979), effort has concentrated on these types of deposits in 

the Northwestern United States. Emphasis is herein placed 

on the Butte and Dillon 1 o X 2° quadrangles in western 

Montana (Doe and others, 1986) and the current study of 

the Challis and Hailey 1 o x 2° quadrangles in central Idaho 

(Doe and Delevaux, 1985). These studies include an initial 

evaluation of the benefits of adding sulfur-isotope data in 

prospect evaluation. The original lead-isotope paper (Doe, 

1979) was based largely on data for ore deposits where the 

oldest basement is 1, 800-1,200 Ma. Previously, the only 

known Cretaceous and Tertiary supergiant deposit (that is, 

one having production worth more than $1.5 billion) in the 

Northwestern United States was the Butte, Mont., copper 

deposit, which may be partly underlain by Archean basement. Since 1979, the Thompson Creek porphyrymolybdenum deposit has opened in central Idaho; it also 

may be underlain by basement like that of the Basin and 

Range province. 

The largest magmatothermal ore bodies (such as the 

Butte and Thompson Creek deposits) generally have leadisotope ratios like those of igneous rocks and nonradiogenic 206PbP04Pb ratios that are less than 18.1; this isotopic range 

undoubtedly characterizes many small prospects as well. 

Giant deposits (such as those at Tintic and Park City, Utah) 

that have produced ore worth $150 million to $ 1. 5 billion 

may be somewhat more radiogenic e06Pb/ 204Pb ratios are 

less than 18.6). Moderate-sized deposits (for example, 

Ag-Au deposits at Creede, Colo.) may show values of 206Pb/ 204Pb ratios up to 19.1. The largest deposits having 206Pb/ 204Pb ratios greater than 19.1 are in the Wood 

River district, Idaho, and at the Nancy Hanks mine, Montana; these deposits have total production worth more than 

$15 million but much less than $150 million. Although such 

radiogenic lead-isotope values are not encouraging for 

discovery of major magmatothermal ores, consideration 

might be given to Mississippi Valley-type deposits (such as 

those at the Pacific Mountain mine, Idaho). 

At Liver Peak, Idaho, lead isotopes were used by 

Kleinkopf and others ( 1972) to define the cupola of a buried 

Tertiary stock. More examples are now known where 

lead-isotope signatures of igneous rocks are found in 

galena-bearing deposits within sedimentary rocks and suggest a cupola at depth (for example, the Iron Mask mine, 

Bannock area, Montana). The galenas also may contain 

o34S values typical of igneous rocks (-3 to + 3 per mil). In 

some deposits (for example, the Cannivan Gulch porphyrymolybdenum prospect, Montana), a halo of more radiogenic lead-isotope ratios is suggested away from the cupola, 

and sulfides may show enrichment in o34S. 

Use of sulfur-isotope analyses can improve a plumbic 

prospecting evaluation by providing another indication of 

the magmatothermal origin for a deposit. Care must be 

taken, however, for o34S values can be negative in deposits 

formed under oxidizing conditions (for example, gold ores 

of the Alder district, Montana). A radiogenic lead-isotope 

composition with a very negative o34S value, however, 

would not be encouraging evidence for an economic 

deposit. Normally nonradiogenic lead with a heavy o34S 

value would not be encouraging, but, in central Idaho, o34S 

values can be unusually heavy (for example, Thompson 

Creek samples have o34S = +8 to + 12); thus, the largest 

deposits there may result from more assimilation of country 

rock than is typical of igneous terranes. Studies during the 

last decade reinforce the value of plumbic prospecting in the 

Western United States, but it should be extended to new 

areas with care. 

REFERENCES CITED 

Doe. B.R .. 1979, The application of lead isotopes to mineral prospect 

evaluation of Cretaceous-Tertiary magmatothermal ore deposits in 

the Western United States, in Watterson, J.R., and Theobald, P.K., 

eds., Geochemical exploration 1978, Proceedings of the Seventh 

International Geochemical Exploration Symposium, Golden, Colorado: Association of Exploration Geochemists, p. 227-232. 

Doe, B.R., Berger, B.R., and Elliott, J.E., 1986, Lead-isotope evaluation 

of selected ores and mineral prospects in the Butte and Dillon 1° X 

2° quadrangles, Montana: U.S. Geological Survey Open-File Report 

86-111' 40 p. 

Doe, B.R., and Delevaux, M.H., 1985, Lead isotope characteristics of ore 

systems in central Idaho [abs.], Chapter 0 qfMcintyre, D.H., ed., 

Symposium on the Geology and Mineral Deposits of the Challis 1° 

x 2° Quadrangle, Idaho: U.S. Geological Survey Bulletin 1658, 

p. 181. 

Kleinkopf, M.D., Harrison, J.E., and Zartman, R.E., 1972, Aeromagnetic and geologic map of part of northwestern Montana and 

northern Idaho: U.S. Geological Survey Geophysical Investigations 

Map GP-830, scale 1:250,000. 

Doe 19 
Farah Garan, Saudi Arabia-BimodalVolcanogenic Zn-Cu-Au Mineralization in a Late 

Proterozoic Rift Basin, Southeastern Arabian 

Shield 

By jeff L. Doebrich, Arthur A. Bookstrom, and 

Richard B. Carten 

The Farah Garan ancient mine site is in the Late 

Proterozoic Malahah rift basin in the southeastern Arabian 

Shield. Several small ( <0.25 Mt) volcanogenic Zn-Cu-Au 

deposits at the site formed during the emplacement of a 

rhyolite dome into comagmatic sea-floor basalt. These 

deposits are among numerous volcanogenic Zn-Cu sulfide 

deposits (five containing more than 1 Mt of ore) that formed 

during the evolution of the Malahah rift basin. Metavolcanic rocks of the Malahah basin are mildly alkalic. 

Geochemistry of sulfide ores from the basin indicates high 

concentrations of components (for example, Te, Se, Mo, 

and C02) that are typically associated with alkalic magmatism. Field and geochemical evidence suggests that the 

Malahah basin evolved either as a rift marginal to a 

subduction-related volcanic arc or as a mid-ocean rift near 

plume-related oceanic islands. 

At Farah Garan, a north-trending, west-facing 

sequence of intercalated metabasalt and metarhyolite is 

conformably overlain by graphitic, calcareous, and micaceous metasedimentary rocks. Mineralization is confined to 

three stratabound lenticular exhalative units (east, south, 

and west units) that are individually as much as 40 m thick 

and 300 m long. The east and south units were deposited 

immediately above a rhyolite dome and occur at the base of 

the metasedimentary section. The west unit was deposited 

farther above the dome after almost 200 m of sediment had 

accumulated. The exhalative units were deposited as dolomite mounds and are composed of intercalated layers of 

dolomite, talc, chlorite, chert, and massive and disseminated sulfides, sulfosalts, and tellurides. Though not 

present at the Farah Garan deposits, a dolomite-talc-breccia 

feeder zone is present below a dolomite mound at the Farah 

Garan East prospect (1 km to the east) and indicates that the 

mounds formed proximal to hydrothermal vents. Observations at Farah Garan and Farah Garan East indicate that a 

broad zone of footwall quartz-sericite-pyrite (QSP) alteration was overprinted by talc-chlorite alteration immediately below the stratabound exhalative units and along the 

margins of the feeder zones. Following exhalite deposition, 

accumulating sediments were altered to form a narrow QSP 

alteration zone in the hanging wall. These proximal alteration zones are enveloped by a carbonate-sericite alteration 

assemblage that extends 4 km north and south of the Farah 

Garan area. 

Anomalous surface concentrations of Au, Ag, Zn, 

As, Sb, and Te in rock and electromagnetic geophysical 

anomalies aided in siting seven cored drill holes totaling 

1 ,203 m. Significant mineralization was encountered only 

in the south exhalative unit. On the basis of two drill holes, 

the deposit is estimated to contain approximately 225,000 t 

averaging 2.5 percent Zn, 0.9 percent Cu, 2.8 ppm Au, and 

33 ppm Ag. 

Mineralization in the south unit exhibits distinct metal 

zonation. The Zn/Cu ratio increases gradually upsection. 

Au and Ag concentrations increase dramatically at the top 

of the mineralized section. Ore-mineral paragenesis 

occurred in three stages. Precipitation of base-metal sulfides 

was followed by the precipitation of As-Sb sulfosalts, 

which, in tum, was followed by the precipitation of 

tellurides. Metal zoning and mineral parageneses indicate 

that Cu sulfides (chalcopyrite and covellite) were precipitated proximal to hydrothermal vents during an early 

high-temperature phase of fumarolic activity; Zn and Pb 

sulfides (sphalerite and galena)~ followed by sulfosalts 

(tetrahedrite, tennantite, arsenopyrite, jamesonite, and boulangerite) and tellurides (altaite, hessite, and tellurobismuth), were deposited distally as hydrothermal fluids 

cooled. 

Triassic-Jurassic Magmatic Arc of Western 

Nevada and Eastern California-Part Ill: Mineral 

Deposits 

By j.l. Doebrich, L.j. Garside, D.R. Shawe, j.H. 

McCarthy, Jr., R.l. Turner, R.F. Hardyman, 

j.A. Erdman, H.F. Bonham, and j.V. Tingley 

Mineral deposit studies in the Middle Triassic to 

Jurassic magmatic arc of western Nevada and eastern 

California were conducted ( 1) to characterize mineral systems hosted by the arc complex, (2) to evaluate the arc 

complex for volcanogenic massive sulfide (VMS) potential, 

and (3) to evaluate Neogene basins for concealed arc-hosted 

mineral systems. 

Jurassic arc-related mineral deposits within the complex include porphyry-Cu. Cu-skarn, Fe-skarn, polymetallic replacement, epithermal Au-vein, polymetallic vein, and 

Al-silicate deposits. Most of these Jurassic deposits are 

related to the Yerington supersystem, which comprises at 

least three individual mineral systems (Yerington-Pumpkin 

Hollow(?), Ann Mason-Buckskin, and MacArthur-BearLagomarsino-Airport), each of which produced porphyryCu deposits and related skarn, replacement, and vein 

deposits. Cretaceous and Tertiary deposits hosted by the arc 

complex include W skarns (Gardnerville district), W veins, 

a low-F porphyry-Mo deposit (Pine Nut deposit, Gardnerville district), Au-Cu quartz-tourmaline veins (Meadow 

Lake district), Au-Ag adularia-sericite and quartz-alunite 

veins (Ramsey and Pea vine districts), Au-bearing polymetallic veins (Lucky Boy and Pamlico districts), barite veins, 

volcanogenic U deposits, and base-metal skarns. 

The apparent absence of VMS deposits from the 

Triassic and Jurassic arc rocks is a consequence of the 

20 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
subaerial to shallow-marine depositional environment that 

existed in the study area. Volcanic rocks deposited in 

subaerial and shallow subaqueous environments dominate 

the volcanic sections. Such shallow, near-shore, highenergy environments are not conducive to the formation and 

preservation of VMS deposits. Although thick sedimentary 

sections within the arc complex (Gardnerville Formation of 

Noble (1962) and Sailor Canyon Formation) provide evidence for local deep restricted basins, there is no evidence 

that they were sites of significant coeval magmatism, which 

would have been needed to generate sea-floor fumarolic 

activity. 

Rock and stream-sediment geochemistry (from new 

data, data obtained by National Uranium Resource Evaluation studies, and data in the U.S. Geological Survey's 

Rock Analysis Storage System) was used to characterize 

deposit types and mineral systems as well as to identify new 

anomalous areas within exposed blocks. Reconnaissance of 

three Neogene basins (Smith Valley, Mason Valley, and 

Antelope Valley) for concealed mineral systems was conducted by using biogeochemical analysis of basin flora, soil 

geochemical sampling, a soil-gas geochemical survey, and 

a geobotanical survey of indicator plants. Anomalous areas 

in the basins were delineated for further assessment. 

REFERENCE CITED 

Noble, D.C., 1962, Mesozoic geology of the southern Pine Nut Range, 

Douglas County, Nevada: Stanford, Calif., Stanford University. 

Ph.D. thesis, 200 p. 

Use of Landsat Thematic Mapper Images for 

Studying Volcanic-Rock-Hosted Precious-Metal 

Deposits in the Central Andean Region 

By B.A. Eiswerth, T.L. Bowers, Lucia Cuitino, Felipe 

Diaz, Hugo Gumucio, Aldo Gutarra, Nestor 

jimenez, j.L. Lizeca, Ramon Moscoso, Fernando 

Murillo, Luis Quispesivana, jaime Rodriguez, L.C. 

Rowan, Ruben Tejada, and Cesar Vilca 

Landsat Thematic Mapper (TM) images are being 

analyzed to augment conventional methods of exploration 

for volcanic-rock-hosted epithermal precious-metal deposits 

in the Neogene-Quaternary volcanic complexes of the 

central Andean region. TM data were digitally processed 

for the regions of Berenguela, Bolivia; Maricunga, Chile; 

and Orcopampa, Peru; processing enabled us to distinguish 

lithologic units, especially hydrothermally altered rocks, 

and to delineate major volcanic and tectonic features. 

Spectra of hydrothermally altered rocks typically 

exhibit more intense absorption features than do those of 

unaltered volcanic rocks (fig. 1). In the short-wavelength 

region, Fe3

+ absorption may result from the oxidation of 

pyrite and ferromagnesian minerals. In the region from 2.0 

TM CHANNELS 

100~------------------------------------~ I 2 3 4 5 

1-

z 

w 

(J 

0:: 

w 

a.. 

~50 

z 

<t 

1-

(J 

w 

_j 

lL 

w 

0:: 

I I I I I 1...--..-.J 

WAVELENGTH (fLm) 

Figure 1 (Eiswerth and others). Reflectance spectra of 

hydrothermally altered rock and unaltered lithic tuff 

showing spectral contrast in Landsat Thematic Mapper 

(TM) channels (shown at the top). 

to 2.5 J.Lm, intense OR-absorption features are due to clays 

and other OR-bearing minerals produced by hydrothermal 

alteration of feldspars and ferromagnesian minerals. Spectra 

of carbonate minerals also may exhibit an absorption feature 

in this wavelength region. 

The TM system records reflected solar radiation in six 

channels in the region from 0.4 to 2.4 J.Lm and has a spatial 

resolution of 30 m. For each of the above-mentioned study 

areas, a single-channel black-and-white image and a colorcomposite image of three selected channels were used to 

distinguish volcanic and tectonic features. Color composites 

of selected ratio images were used to distinguish lithologies. 

The TM 517 ratio was included in the color-ratio composite 

images to display the spectral contrast between OR-bearing 

hydrothermally altered rocks and unaltered volcanic rocks 

in the region from 1.5 to 2.5 J.Lm (fig. 1). The TM 3/1, TM 

511, and (or) TM 5/4 ratios were used to display variations 

in Fe3 

+ absorption intensity. High values of these ratios are 

suggestive of alteration, but field evaluation is needed to 

resolve possible ambiguities, such as the presence of 

carbonate rocks. Where vegetation is an important component, TM 4/3 ratos were included to distinguish vegetation 

from hydrothermally altered rocks. 

The false-color composite and black-and-white 

images outline numerous eruptive centers, which exhibit 

varying degrees of erosion. The color-ratio composite 

images delineate many zones of probable hydrothermal 

alteration in all three study areas. The most intense and 

widespread alteration is related to eroded stratovolcanoes 

and calderas. Variations in the color-ratio composite images 

suggest mineralogical differences that may reflect the type 

and intensity of alteration. Although a few of these altered 

areas have been explored by using modern techniques, most 

have not been studied in detail. 

Eiswerth and others 21 
Can Trace and Rare-Earth Elements in Fluorite 

be Useful in Geochemical Exploration? Encouraging Evidence from the Sierra Cuchillo Area, 

New Mexico 

By R.G. Eppinger, L.G. Closs, A.L. Meier, and j.M. 

Motooka 

Fluorite was collected in the Sierra Cuchillo region of 

south-central New Mexico from four fluorite-bearing 

deposit types: Au-Ag veins, Ba-Pb veins, W-Be-Fe skarn, 

and barren quartz-calcite-fluorite veins apparently devoid of 

ore metals. The fluorite was analyzed for trace elements by 

inductively coupled plasma (ICP) atomic-emission spectrometry and for rare-earth elements (REE's) by ICP mass 

spectrometry. 

Trace-element and REE patterns in fluorite samples 

are distinctive for the different deposit types. Fluorite from 

Au-Ag veins has positive Eu anomalies, a high Sr content, 

and low Be, Ba, Ti, U, andY contents. Fluorite from Ba-Pb 

veins has negative Eu anomalies, high Ba and Pb contents, 

and low Sr and U contents. Fluorite from W-Be-Fe skarn 

has negative Eu anomalies, high Be, Ti, U, andY contents, 

and low Sr and Ba contents. Fluorite from barren veins 

contains variable amounts of the above trace elements and 

exhibits variable Eu anomalies. The wide chemical variation in the fluorite studied may be due in part to changes in 

fluid chemistry resulting from reactions with feldspar in 

wall rock. 

Fluorite from barren veins is more abundant than 

fluorite from other deposits and generally shows the most 

chemical variation. One group of fluorite samples from 

barren veins has chemical characteristics similar to those of 

fluorite from W-Be-Fe skarn. A second group of fluorite 

samples from barren veins exhibits chemical affinities with 

fluorite from Au-Ag veins. These similar geochemical 

signatures in the fluorite, coupled with favorable geologic 

settings, suggest the possibility of concealed skarn deposits 

underlying one barren vein group and the possibility of 

precious-metal deposits underlying the other barren vein 

group. Thus, the geochemistry of fluorite delineates potential concealed mineral deposits in the Sierra Cuchillo area 

and may be a useful tool for mineral exploration elsewhere. 

Biogeochemical and Geochemical Expression of 

Poorly Exposed Epithermal Mineralization in the 

Southeastern Challis Volcanic Field, Idaho 

By j.A. Erdman, Falma Moye, and P.K. Theobald 

Biogeochemical and other geochemical exploration 

techniques were used in the Lava Creek mining district, 

north of Craters of the Moon National Monument, and in 

the Baker Creek area near Ketchum, Idaho, to assess the 

potential for new metallic mineral deposits. Stream sediments, soils, heavy-mineral concentrates, and several species of plants, principally big sagebrush (Artemisia tridentata), were collected in reconnaissance drainage-basin 

surveys (Adrian and others, in press; Erdman and others, 

1990). Biogeochemical methods can be especially effective 

where surface expression of mineralization is poor. 

The Lava Creek area was selected for study because 

of its historical importance as a producer of silver and 

byproduct gold. Most of this area is underlain by Eocene 

Challis Volcanics that have been so hydrothermally altered 

that good outcrop is extremely limited. At Lava Creek, 

initial results (Erdman and others, 1988) showed geochemical anomalies in sagebrush which suggested that a possible 

host for epithermal precious metals was the Lower Mississippian McGowan Creek Formation (chiefly turbidites), 

peripheral to the formerly active mines. Followup sampling 

of sagebrush, stream sediments, and soil defined the source 

of a Hg-Ag-Au-As-Sb anomaly in a fault breccia near a 

poorly exposed dacite porphyry intrusion. 

Hydrothermal alteration east of Baker Creek chiefly 

affected dacitic lava; a central zone of intense argillic 

alteration is surrounded by a broader zone of propylitic 

alteration. The Baker Creek area lacked evidence of mineral 

prospecting when the survey was made, but it has geological characteristics indicating that an epithermal preciousmetal system may be present. 

In the Baker Creek area, heavy-mineral concentrates 

all contain abundant barite, even beyond the zone of 

propylitic alteration. Botryoidal pyrite was also found, but 

it is restricted to the alteration zones. Visible gold and 

pyromorphite, a lead phosphate, were found in heavymineral concentrates from a small drainage near a 

stockwork-veined outcrop in the central zone of argillic 

alteration. A grab sample of the outcrop contains 50 ppb 

Au, 1.5 ppm Ag, and 50 ppm Mo. The maximum concentration of Au in nearby stream sediments and upland soils is 

only 2 ppb, but aquatic mosses from within the argillically 

altered zone contain as much as 64 ppb Au in the ash. This 

intensely altered zone is characterized by anomalous levels 

of Ag, Mo, Pb, Cd, and Zn and, to a lesser extent, Au, Bi, 

Mn, As, Sb, and Tl in the various media sampled. The 

geochemical signature suggests a Ag- and Mo-rich, lowtemperature, epithermal type of mineralization. 

REFERENCES CITED 

Adrian, B.M., Erdman, J.A., Bullock, J.H., and Hageman, P.L., in 

press, Analytical results and sample locality map of heavy-mineralconcentrate, stream-sediment, soil, and big sagebrush samples from 

the Lava Creek mining district, within the Idaho Falls 

1° x 2° quadrangle, Butte County, Idaho: U.S. Geological Survey 

Open-File Report 90-545. 

Erdman, J.A., Moye, Palma, Bailey, Betty, O'Leary, R.M., and Roemer, 

T.A., 1990, Analytical results and sample locality maps of soil, 

plant (Douglas-fir, sagebrush, sedge, aquatic moss), stream22 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
sediment, and heavy-mineral-concentrate samples from the Baker 

Creek study area , Blaine County, Idaho: U.S. Geological Survey 

Open-File Report 90-57, 16 p. 

Erdman , J.A. , Skipp, Betty, Theobald, P.K. , and Moye, Falma, 1988, 

Drainage-basin geochemical survey of Lava Creek district in central 

Idaho using sagebrush and heavy-mineral concentrates-Preliminary evidence for sediment-hosted gold in McGowan Creek Formation [abs.]: Geological Society of America Abstracts with Programs, 

v. 20, no. 6, p. 414. 

Volcanic-Hosted Epithermal Precious-Metal 

Deposits in the Central Andes-Geology, 

Resources, and Exploration Strategy 

By George E. Ericksen , Charles G. Cunningham, 

and Barbara A. Eiswerth 

The Neogene-Quaternary volcanic complex of the 

central Andes (fig. 1) contains hundreds of hydrothermally 

altered areas, many of which are associated with worldclass silver-gold deposits; others are prime exploration 

targets. This complex, which constitutes a widespread 

volcanic cover in southern Peru, northern Chile, western 

Bolivia, and northwestern Argentina , is characterized by 

hundreds of andesitic to dacitic stratovolcanoes, at least 30 

calderas related to widespread rhyolitic ash-flow tuffs, and 

many volcanic domes. 

Three types of volcanic-hosted, epithermal preciousmetal deposits are present: (1) silver-rich, locally gold-rich, 

polymetallic base-metal veins; (2) silver-rich, gold-poor, 

polymetallic tin veins and stockworks; and (3) low-grade 

gold-bearing stockwork or disseminated deposits, with or 

without silver and base metals. Many veins and districts 

show mineral zoning; in the central zones, base metals and, 

in some deposits, gold and tungsten are abundant, and, in 

the outer zones, silver, arsenic, and antimony are abundant. 

In the vertical zoning present at some places, base metals 

are most abundant at depth , and silver is abundant near the 

surface. Both adularia-sericite and acid-sulfate types of 

epithermal deposits are present; the former type is by far the 

more abundant and is characterized by intense sericitic and 

(or) argillic alteration, with or without adularia, and the 

latter is characterized by hypogene alunite and enargite. 

Grade-tonnage data for both high-grade, multivein 

deposits and low-grade stockwork deposits show that most 

of the known epithermal deposits contain between 500 and 

6,000 t of silver and less than 50 t of gold. Cerro Rico de 

Potosi, the world's largest silver deposit, has produced an 

estimated 30,000 to 60,000 t of silver from ores averaging 

about 0.5 kg/t Ag (fig. 2). A similar amount of silver is 

estimated to remain in Cerro Rico stockwork ores and waste 

dumps having grades ranging from 150 to 250 g/t Ag. A 

single vein at Pulacayo, Bolivia (about 135 km southwest of 

Cerro Rico de Potosi), which is 2.7 km long and was mined 

to a depth of 1, 100 m below the surface, yielded about 

12' 

,.. 

20' 

76' 12' 

PERU 

1 Julcani 

2 Nevado Portuguesa 

3 Ccarhuaraso 

4 Arcata 

5 Orcopampa 

6 Sucuitambo, San Miguel, 

Paco Paco 

7 Cailloma 

8 Berenguela 

9 La Joya 

10 Oruro 

11 Choquellmpie 

,.. 12 Carangas, Todos Santos 

13 Cerro Rico de Potosi 

,.. 

14 Porco 

15 Chocaya 

16 Maricunga 

17 Farallon Negro 

18 El Indio 

0 200 400 Km 

76' 

... \ 12' 

' ) 

' l 

BOLIVIA ,.. •La Paz 

,.. 

,.. 

ARGENTINA 

... 

Figure 1 (Ericksen and others). Distribution of principal 

silver and gold deposits in the Neogene-Quaternary volcanic complex (stippled areas) of the central Andes. Most 

of the hundreds of andesitic to dacitic stratovolcanoes, at 

least 30 calderas, and many volcanic domes have associated hydrothermal alteration zones that are prime exploration targets for precious-metal deposits. 

5,000 t of silver from ores averaging between 0 .5 and 1 kg/t 

Ag. Gold grades in silver-rich veins generally are less than 

1 g/t Au, but ore shoots in some veins may contain 100 or 

more g/t Au. Total gold production from silver-rich veins or 

districts is estimated to range from a few metric tons to 

about 30 t. The richest gold vein now being mined is the 

3,500 vein in the El Indio district, Chile, which originally 

contained an estimated 37 t gold in ore that averaged about 

0.25 kg/t Au. Low-grade ore (9 g/t Au) in this district is 

estimated to contain an additional 67 t of gold. Other 

low-grade stockwork deposits in Bolivia and Chile that are 

amenable to heap leaching have ores averaging between 1.2 

and 2.1 g/t Au and containing up to 90 .g/t Ag. Estimated 

resources of such deposits range from about 25 to 60 t gold 

and from little or no silver to a maximum of about 3,000 t 

Ag. 

Digitally enhanced Landsat Thematic Mapper images 

of selected areas in Peru, Bolivia, and Chile show that most 

Ericksen and others 23 
Figure 2 (Ericksen and others). Cerro Rico de Potosi, 

Bolivia, a volcanic dome that hosts the famous Potosi 

tin-silver deposit. Since its discovery in the mid-16th century, it has yielded an estimated 30,000-60,000 t of fine 

silver from ore averaging 0.5 kg/tAg; thus, this hill is the 

world's largest silver deposit. Similar amounts of silver 

remain in low-grade ores in Cerro Rico and in waste 

dumps from ea rlier mining. Photograph by George E. 

Ericksen in 1984. 

Figure 3 (Ericksen and others). Aerial view of the 

Arcata mining district, one of the major silver producers in the Andean Highlands (4,000-5,000 m altitude) 

of southern Peru. The concentration plant and townsite are in the foreground, a 10- to 30-m-wide kaolinitic 

hydrothermal alteration zone (the linear light-colored 

alteration zones considered to be favorable exploration 

targets are within or near deeply eroded stratovolcanoes and 

calderas. Analyses of geochemical samples of the alteration 

zones over ores may or may not detect gold and silver but 

generally do show anomalous values for arsenic. Copper, 

lead, and zinc, even though present in the ores, tend to 

show only background values in the surficial alteration 

zones. Some alteration zones are visible in aerial photographs (fig. 3). Additional study should provide new data 

about these zones and identify many as favorable exploration targets. 

The Use of lead-Isotope Geochemistry to 

Evaluate Economic Potential for Gold-Silver 

Mineralization-An Example from the North 

Amethyst Vein System and the Creede Mining 

District, Colorado 

By Nora K. Foley and Robert A. Ayuso 

Lead-isotope compositions of galenas were determined for ore deposits and volcanic wall rocks from the 

band in the middle ground) marks the outcrop of one 

of the major veins in andesitic lavas of Pliocene age, 

and light-colored ash-flow tuffs and young basaltic 

(dark) lavas can be seen in the distance . Photograph 

by George E. Ericksen in 1987. 

24 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
central San Juan Mountains, Colo., in an effort to evaluate 

the use of lead-isotope compositions for mineral prospecting (fig. 1). The study area includes the North Amethyst 

area and the central and southern parts of the Creede mining 

district. The North Amethyst vein contains two crosscutting 

mineral associations with contrasting mineral parageneses 

and metal associations (I, II in fig. 1) that are separated 

locally by breccias and sedimentary rock. The earlier, 

fine-grained association (I) has rocks from two stages that 

contain different mineral assemblages: Ia contains quartz, 

rhodonite, Mn-carbonates, and minor base-metal sulfides; 

Ib contains hematite, magnetite, electrum, Au-Ag-sulfides, 

Ag-sulfosalts, and base-metal sulfides. The second association (II) consists of coarser grained quartz, calcite, sericite, 

chlorite, hematite, adularia, fluorite, base-metal sulfides, 

and Ag-tetrahedrite. Veins of the central and southern parts 

of the Creede district (III, IV) consist primarily of coarsegrained quartz and base-metal sulfides, sericite, hematite, 

chlorite, adularia, fluorite, and barite and contain minor 

Ag-sulfides and Ag-sulfosalts. 

Our detailed studies show that paragenetically early 

galenas from the Au-rich North Amethyst mineralization 

(Ib) have Ph-isotope compositions that are less radiogenic 

than those of galenas formed later in the North Amethyst 

area, during Ag and base-metal mineralization (II). Galenas 

from association Ia have lower 207Pb/ 204Pb ratios but are 

otherwise similar to galenas from association II. Their 

paragenesis is uncertain as they formed in a reaction zone 

involving associations I and II. New lead-isotope data for 

the central and southern parts of the Creede district agree 

with the lead-isotope data from a regional study by Doe and 

others (1979) that included five samples from the Creede 

district. Galenas and adularia of the central and southern 

Creede district (Ill) and galenas from the Bondholder 

district (V), north of Creede, are isotopically similar to 

North Amethyst II galenas. Galenas of the Alpha-Corsair 

vein (IV), which was mined for silver prior to 1910, are 

isotopically similar to North Amethyst Ib galenas. Unexplored segments of the Alpha-Corsair vein thus may be 

mineralogically similar to North Amethyst Ib assemblages; 

therefore, the Alpha-Corsair vein has the potential for high 

gold contents. 

Lead-isotope data for the North Amethyst vein system and the rest of the Creede mining district generally 

define two distinct groups (fig. 1). Both groups are isotopically more radiogenic than the enclosing volcanic rocks of 

the central part of the San Juan volcanic field as defined by 

Lipman and others (1978) and Matty and others (1987). Our 

lead-isotope results indicate that Au mineralization in the 

North Amethyst area may represent the product of an older 

and relatively local hydrothermal system distinct from that 

of the younger base-metal and Ag mineralization found 

throughout the region. Fluids that deposited the Au may 

have derived their lead-isotope composition by a greater 

degree of interaction with shallow, relatively less radio15.7 

.0 

J0 

C\1 

:0 

~ 0 

C\1 

15.5 

I North Amethyst Mn-Au association 

II North Amethyst Ag-base-metal association 

Ill Central and southern Creede district 

IV Alpha - Corsair vein 

V Bondholder district 

* CRT K-feldspar 

18.6 206 Pb/204pb 19.0 

Figure 1 (Foley and Ayuso). Comparison of lead-isotope 

compositions of ore with those of volcanic rocks of the 

central part of the San Juan volcanic field (CSJVF); data are 

from Lipman and others (1978), Doe and others (1979), and 

Matty and others (1987). Asterisks identify additional 

analyses (this study) of the Carpenter Ridge Tuff (CRT) that 

hosts the ore. Dashed line is orogene reference curve of 

Doe and Zartman (1979). 

genic volcanic wall rocks. The Ag and base-metal-rich 

mineralization that overprints the Au mineralization in the 

North Amethyst area clearly has a more radiogenic isotopic 

signature, which implies that the later mineralization 

derived a greater component of its lead from Precambrian 

source rocks or sediments derived from them at depth. 

Paragenetically early vein assemblages thus have the 

least radiogenic galenas and generally also have the highest 

gold contents. These characteristics may be related to 

temperature, as the early association formed from fluids 

hotter than those that deposited the later association. Identification of paragenetically early vein assemblages with 

relatively unradiogenic lead-isotope compositions similar to 

those of the North Amethyst area provides an additional 

exploration tool for gold in the central San Juan Mountains 

area. 

REFERENCES CITED 

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. 

Doe, B.R., and Zartman, R.E., 1979, Plumbotectonics, the Phanerozoic, 

in Barnes, H.L., ed., Geochemistry of hydrothermal ore deposits 

(2d ed.): New York, Wiley, p. 22-70. 

Lipman, P.W., Doe, B.R., Hedge, C.E., and Steven, T.A., 1978, 

Petrologic evolution of the San Juan volcanic field, southwestern 

Colorado-Ph and Sr isotope evidence: Geological Society of 

America Bulletin, v. 89, p. 59-82. 

Foley and Ayuso 25 
Matty, D.J., Lipman, P.W., and Stormer, J.C., Jr., 1987, Common-Ph 

isotopic characteristics of central San Juan ash flow tuffs [abs.]: 

Geological Society of America Abstracts with Programs, v. 19, 

no. 5, p. 319-320. 

The Bisbee Group of the Tombstone Hills, 

Southeastern Arizona-Stratigraphy, Structure, 

Metamorphism, and Mineralization 

By Eric R. Force 

The Lower Cretaceous Bisbee Group (of Butler and 

others, 1938) is the host of most silver ores mined in the 

Tombstone district, southeastern Arizona. The group was 

locally subdivided by Butler and others (1938) only in the 

Tombstone basin, which forms the eastern of two main 

outcrop areas in the northern Tombstone Hills. In this 

study, I divided and mapped the Bisbee Group in both 

outcrop areas, which total about 7 km2 and are about 1.5 km 

apart. Stratigraphic subdivision has allowed delineation of 

new structural features and has provided information on 

their histories. Contact-metamorphic isograds were also 

mapped; they reflect the shape and emplacement history of 

intrusions such as the Schieffelin Granodiorite. 

The Bisbee Group can be divided into two informal 

units- an upper formation consisting mostly of argillite and 

having an upper part dominated by sandstone and a lower 

formation characterized by conglomerate and limestone. 

The lower formation includes the Glance Conglomerate and 

the "novaculite" and "blue limestone" of Butler and others 

(1938). 

The two discrete outcrop areas of the Bisbee Group 

show reasonably similar stratigraphic sequences in the 

upper formation, which is more than 250 m thick. The 

argillite contains a thin cherty limestone bed, minor tuff, 

and petrified-wood horizons. 

The lower formation shows a great transition in grain 

size, composition, and sequence from west to east. In the 

westernmost outcrop area, the lower formation is a sedimentary megabreccia of Paleozoic blocks, whereas in 

the easternmost Tombstone basin, 4.5 km away, it consists 

of a basal siliceous unit overlain by several limestone beds 

and intercalated argillite. A provenance difference between 

the two outcrop areas and a maximum thickness (300 m) 

along the western margin of the Tombstone basin may have 

been controlled by syndepositional movement on the intervening Ajax Hill fault, known to be pre-Schieffelin in age. 

The transition as a whole must have been controlled by a 

more important syndepositional fault farther west. 

Folding of the Bisbee Group is of the dome-and-basin 

type. In the western outcrop area, folding is open and 

upright and is accentuated by steep faults. In the Tombstone 

basin, folding is of the same type, but some northnorthwest-striking structures are tight and overturned to the 

west-southwest. The Lucky Cuss fault and related arcuate 

faults of the Tombstone basin appear to cut northnorthwest-trending tight folds but are deformed by later 

west-trending open folds. 

The Late Cretaceous augitic Schieffelin Granodiorite 

forms a flat-topped intrusion, probably a sill, underlying the 

Bisbee Group, which had been folded and extensively 

faulted by the time of intrusion. This intrusion fingers out 

toward the east, but a probable extrusive equivalent, the 

tuffaceous Uncle Sam unit, overlies the Bisbee Group 

toward the west. The Bisbee sequence shows three zones of 

contact metamorphism characterized by wollastonite, grossularite, and epidote; the zones have a total thickness of 

more than 200 m where the Schieffelin was sufficiently 

thick. 

The east -trending Prompter fault has been thought to 

cut the Bisbee Group in the western outcrop area but 

previously could not be traced through it. Identification of 

separate stratigraphic units and metamorphic grades indicates its trace and suggests a two-stage movement historythe first and main movement was pre-Schieffelin and up on 

the south, and post-Schieffelin movement was down on the 

south. 

Mineralization is mostly contained within the lower 

formation of the Bisbee Group, against its contact with the 

upper formation. It consists of veins within faults and 

replacements along certain beds and (or) anticlinal crests. 

The mineralization is apparently Laramide but probably is 

post-Schieffelin in age. Mined ore was at least partly 

supergene; it produced mainly silver and smaller amounts of 

gold, manganese, and base metals. 

REFERENCE CITED 

Butler, B.S., Wilson, E.D., and Rasor, C.A., 1938, Geology and ore 

deposits of the Tombstone district, Arizona: Arizona Bureau of 

Mines Bulletin 143, Geology Series 10, 114 p. 

Concentration and Zonation of Copper and 

Palladium in the Mesozoic Belmont Diabase 

Sheet, Culpeper Basin, Northern Virginia-An 

Exploration Guide to Enrichment of Precious 

Metals in Mafic Magmatic Rocks 

By A.j. Froelich, Laurel G. Woodruff, Harvey E. 

Belkin, and David Gottfried 

Chemical zonation marked by intervals enriched in 

copper (Cu) and platinum-group elements (PGE) is present 

in a lensoid body of ferrogabbro, ferrodiorite, and granophyre about 150 m thick in the upper part of the Belmont 

diabase sheet in the Culpeper basin, Virginia. Five subhorizontal geochemical zones that thicken and thin reciprocally 

are best defined by variations in Cu abundance (table 1). 

26 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Zone 2 has the highest Cu content, but zone 3 shows the 

greatest enrichment in Pd and, to a lesser extent, Pt and Au. 

Other metals, such as Co and Ni, show little variation 

across the zones enriched in Cu (table 1). Various field, 

petrographic, geochemical, electron microprobe, SEMEDXRF (scanning electron microscope-energy dispersive 

X-ray fluorescence spectroscopy), sulfur- and oxygenisotope, and fluid-inclusion studies were conducted to 

understand the transport, alteration, and mineralization 

processes that occurred here. 

The Belmont diabase sheet is an irregularly shaped 

Mesozoic intrusion as much as 600 m thick in Upper 

Triassic sedimentary rocks (fig. 1). The intrusive rock is 

composed of layered units resulting from differentiation of 

a high-Ti02 (about 1.1 weight percent), quartz-normative 

(Si02 ~ 51.5 weight percent; MgO ~ 7.5 weight percent), 

tholeiitic magma (HTQ, high-titanium quartz-normative, 

type). The lower few hundred meters of the sheet (MgO = 

9-11 weight percent) is depleted in incompatible elements 

relative to the chilled margins and contains cumulus MgOrich orthopyroxene (bronzite). The upper part of the sheet 

contains segregations of evolved rocks (granophyre and 

ferrodiorite, in which Si02 ~ 53 weight percent; MgO ~ 

3.5 weight percent; Ti02 ~ 2.5 weight percent) that are 

enriched in Cu and have incompatible element abundances 

several times greater than those of the enclosing chilled 

margins; this element distribution indicates that these 

evolved rocks are complementary fractions to the cumulate 

rocks. As the orthopyroxene-rich cumulate rocks are about 

10 km north of the evolved rocks in the upper part of the 

same sheet (fig. 1), considerable fractionation of the original magma by lateral and vertical migration and flow 

differentiation is indicated. 

As shown in table 1, the ferrogabbro in zone 2 has the 

highest average Cu content (650 ppm); it overlies ferrogabbro in zone 3 having the greatest Pd enrichment (average 

100 ppb, locally 500 ppb) and having an average Cu 

abundance of 220 ppm. Zone 3 also is locally enriched in 

FeOTotai (to 21.5 weight percent), Ti02 (to 4.8 weight 

percent), Cl (to 2,900 ppm), Ba (to 1,550 ppm), S (to 760 

ppm), V (to 350 ppm), As (to 19.4 ppm), Se (to 6.4 ppm), 

Mo (to 5.2 ppm), U (to 2.15 ppm), Ag (to 0.3 ppm), Au (to 

137 ppb), Pt (to 41 ppb), and Rh (to 11 ppb). This 

enrichment suggests late- or post-magmatic concentration 

of some precious metals. 

The degree and mode of alteration vary systematically with the zones defined by Cu and Pd abundances and 

ratios (table 1). During alteration, primary clinopyroxene 

was replaced by iron- and halogen-rich amphiboles and 

chlorite, and plagioclase was converted to fine-grained 

sheet silicates, such as sericite. Chalcopyrite is the most 

abundant sulfide in the altered rock, and its texture suggests 

that it is a very late phase. Chalcopyrite occurs with bornite, 

galena, sphalerite, chlorine-rich amphibole, quartz, apatite, 

and ilmenite. Also occurring with this late-stage alteration 

assemblage are micrometer-sized grains of electrum (Au > 

Ag) and phases containing Pd+Bi+Sb and Co+Fe+As 

(fig. 2). Sulfides in zones 2 and 3 show a restricted range in 

834S of -0.47 to + 1.26 per mil. Oxygen-isotope ratios 

from whole-rock core samples collected from the same 

zones indicate a narrow range of 8180 from +7.1 to +7.7 

per mil, whereas 8180 in the pyroxene mineral separates 

ranges from +6.12 to +6.66 per mil. Both the oxygen- and 

sulfur-isotope data suggest a magmatic source. Multiphase 

fluid inclusions (containing N aCl +other chlorides+ liquid 

+vapor) occur in quartz throughout the entire range of 

studied samples (fig. 3). The common occurrence of coeval 

vapor-rich and high-salinity multiphase fluid inclusions 

suggests pervasive boiling conditions, at least during some 

stages of alteration. 

The zones of Cu and PGE enrichment at Belmont are 

believed to have formed from late magmatic, chlorine- and 

sulfur-rich aqueous solutions that hydrothermally altered 

the original ferrogabbro, granophyre, and diorite. PetroTable 1 (Froelich and others). Variations in average abundance and ratios of selected metals in the upper part of the 

Belmont diabase sheet, northern Virginia 

[n = number of core samples analyzed from five coreholes 60-150 m deep. Analyses by U.S. Geological Survey analytical laboratories in Reston, Va., 

and Lakewood, Colo. Pd and Pt determined by graphite-furnace atomic-absorption spectrometry (GFAAS); Co, Ni, and Cu determined by flame atomicabsorption spectrometry and GFAAS] 

Lithology Zone 

Ferrodiorite .. .. .. . .. .. . . . .. .. . 1 (n=5) 

Ferrogabbro and minor 

granophyre .. . . .. .. . .. .. .. .. 2 (n=35) 

Ferrogabbro and minor 

granophyre .. .. . .. .. .. .. .. .. 3 (n=66) 

Ferrodiorite and granophyre... 4 (n=29) 

Diorite and diabase . . . . . . . . . . . 5 (n=3) 

Approximate 

distance below 

upper contact 

(meters) 

450 

500 

510-550 

530-570 

560-600 

Cu Pd Pt Pd/Pt 

(ppm) (ppb) (ppb) Cu/Pd Ni Co Cu/Ni (ppm) (ppm) 

420 1.6 1.0 1.6 400,000 8 38 75 

650 15 

220 100 

150 17 

120 6 

5 

12 

4 

7 

3 

8 

4 

1.0 

45,000 15 

2,000 15 

8,000 20 

20,000 65 

50 

50 

40 

50 

43 

15 

7.5 

2.0 

Froelich and others 27 
I 

i 

\ 

1 

f 

i 

I 

~ QUADRANGLE LOCATION 

EXPLANATION 

~ m l TO DIABASE (DIKES) 

< 

a: Ei . · · Granophyre and Ferrodiorite 

~ : :. · ··~: HTO DIABASE (SHEET) a: ... 

~ · ~: · ~ Orthopyroxene Cumulate 

g r-! TRIASSIC-JURASSIC 

L______j SEDIMENTARY ROCKS 

-PRE-TRIASSIC ROCKS 

/ORMAL FAULT, TICK ON DOWN SIDE 

;/ ::;::N~:::·:OD~::CTION , 

GEOCHEMICAL SAMPLE 

W-~B] CORE HOLE 

1 MILES 

1 KILOMETERS 

Figure 1 (Froelich and others). Simplified geology of the Belmont diabase sheet in the early Mesozoic Culpeper 

basin , northern Virginia. LTQ, low-titanium quartz-normative diabase; HTQ, high-titanium quartz-normative 

diabase. 

28 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Figure 2 (Froelich and others). Palladium mineral (P) along 

a silicate-oxide grain boundary in a backscattered electron 

image taken with a scanning electron microscope (I = 

ilmenite, A = chlorine-rich amphiboles, M = titanomagnetite). Arrow indicates grain shown in the enlargement 

inset; P = Pd + Bi + Sb, and C = Co + Fe + As according 

to SEM-EDXRF analysis. Belmont corehole W-886, sample 

depth 55.5 m below land surface, zone 3. 

4pm 

Figure 3 (F roel ich and others). Primary multiphase fluid 

inclusion in quartz, plane-polarized transmitted light photomic rograph (H = halite, B =vapor bubble, F = birefringent daughter crystal(s)). Belmont corehole W-890, sample depth 38.7 m below land surface, zone 1. 

graphic and fluid-inclusion evidence also suggest that the 

alteration was a multistep, nonisothermal process during 

which metals were deposited by a chemically evolving, 

cooling, magmatic hydrothermal fluid . More than two 

dozen thick Mesozoic diabase sheets of the HTQ magma 

type are known in the Eastern United States from Connecticut to Virginia, and most of these contain large bodies of 

ferrodiorite and ferrogabbro. To date , four (Palisades sheet, 

New York and New Jersey; Reesers Summit and Orrtanna 

sheets, Pennsylvania; and Belmont sheet, Virginia) have 

shown 10- to 50-fold enrichment in Pd. The evidence 

presented here indicates that exploration for precious metals 

in the HTQ-type diabase sheets should focus on the latestage ferrogabbro and ferrodiorite differentiates enriched in 

Cu. 

Lithologic and Tectonic Controls on Mercury 

Mineralization in the Bethel 1° x 3° Quadrangle, 

Southwestern Alaska 

By Thomas P. Frost and Stephen E. Box 

As part of the Alaska Mineral Resources Assessment 

Project (AMRAP), 1 ,486 stream-sediment and 1,104 

heavy-mineral-concentrate samples were collected from 

first- and second-order streams from 1987 to 1989 in the 

Bethel 1 o x 3° quadrangle and in the southern part of the 

Russian Mission quadrangle in southwestern Alaska. In 

addition, I ,375 bedrock samples were analyzed. Mercury 

and, to a lesser extent, arsenic contents are closely linked to 

locally subtle variations in bedrock lithology and to major 

structural elements in the area. 

The region is underlain by at least five pre-midCretaceous tectonostratigraphic terranes (fig. 1). Proterozoic orthogneisses and granitic rocks make up the Kilbuck 

terrane; Paleozoic and Mesozoic arc- and oceanic-affinity 

metabasalts, metacherts, clastic rocks, and minor marble 

are present in the Goodnews, Togiak, and Tikchik terranes; 

and Mesozoic arc volcanic rocks make up the Nyac terrane . 

Unconformably overlying all but the Nyac terrane is the 

Lower and Upper Cretaceous Kuskokwim Group, which 

consists predominantly of marine turbidites. Shallowmarine to deep-marine conglomerates and sandstones are 

present above a basal unconformity in the central part of the 

Kuskokwim basin. 

The Kuskokwim Group can be divided into three 

belts, each having a distinct provenance. The western belt 

consists of an eastward-prograding deltaic sequence derived 

from an enigmatic chert-rich source area not presently 

exposed in the Bethel region. The basement on which the 

westernmost chert-rich facies was deposited is also not 

exposed. The eastern belt rests unconformably on the 

Togiak terrane and consists of turbiditic base-of-slope 

deposits derived from an eroded volcanic source. The 

central belt was deposited in a northeastward-flowing submarine fan system, which received sediment from both 

flanks and from a southern high-grade metamorphic source. 

The Kuskokwim Group was complexly deformed prior to 

the Late Cretaceous to early Tertiary magmatic episode. 

Frost and Box 29 
Russian 

6

o MI SSiOn 

~---2 1< m 

Qs 

EXPLANATION 

~-- - Contact- Dashed where inferred 

.,.....-......... ·· Fault - Dashed where approximate, dotted where inferred 

~ Thrust fault- Teeth on upper plate 

Overlap sequences 

Surficial deposits (Quaternary) 

Volcanic rocks (early Teniary and Late Cretaceous) --

Basalt to rhyolite flows and domes, includes some 

hypabyssal rocks. 

j-r+ T~g: +j Granitic to granodioritic plutons (early Tertiary and Late 

Cretaceous) 

~ I ' 

~ 

~ 

Granitic to granodioritic plutons (Early Cretaceous) 

Kuskokwim Group (Late and Early Cretaceous)--

Consists of: 

Undivided pan 

Chert-clast facies 

Volcaniclastic facies 

Pre-mid-Cretaceous terranes 

~ 

~ 

Togiak terrane (Early Cretaceous to Late Triassic) --

Andesitic volcanic rocks, volcaniclastic rocks, and 

tuffaceous argillite 

Nyac terrane (Late and Middle Jurassic)--

Andesitic volcanic rocks and minor volcaniclastic 

sedimentary rocks 

Goodnews terrane (Mesozoic and Paleozoic)--· 

Tuffaceous argillite, chen, basalt, metabasite, 

metachen, and marble 

Tik.chik. terrane (Mesozoic and Paleozoic) --

Metavolcanic rocks, metachen, graywacke, and 

limestone 

~ 

~ Kilbuck terrane (Proterowic) --

Amphibolite facies orthogneiss with minor amphibolite and quartz-mica schist 

as 

Figure 1 (Frost and Box). Regional geology of the Bethel 

quadrangle and parts of the Russian Mission and Goodnews quadrangles, southwestern Alaska. The Rainy Creek 

locality is indicated by a star. 

Granitic to granodioritic rocks of Early Cretaceous 

age were emplaced only in the Nyac terrane. Similar 

plutons (73-55 Ma) were emplaced in the older terranes to 

the east and in the overlying Kuskokwim Group. Wall rocks 

generally are metamorphosed to biotite and cordierite hornfels within a kilometer of the intrusive contacts. 

Latest Cretaceous and early Tertiary intermediate to 

silicic volcanic fields and hypabyssal intrusions were 

erupted through both the eastern volcaniclastic and western 

chert-rich facies of the Kuskokwim Group. Rhyolite domes 

and ash-flow tuffs, which may be part of the western 

volcanic field, also were erupted through the andesitic 

volCanic rocks of the Nyac terrane, the westernmost of the 

exposed basement terranes. Dikes of andesitic to dacitic 

composition are common throughout the Kuskokwim 

Group and older rocks, although their relationship to the 

volcanic fields is uncertain . 

Anomalous mercury contents are present in sedimentary rocks of the Kuskokwim Group (0.5-1.1 ppm Hg), in 

overlying volcanic rocks (0.5-11 ppm Hg), and in stream 

sediments (1-> 36 ppm Hg) wherever the eastern volcaniclastic facies of the Kuskokwim Group is exposed in the 

region. The epithermal Rainy Creek mercury prospect 

(Sainsbury and MacKevett, 1965) at the southern edge of 

the Bethel quadrangle is hosted by the eastern volcaniclastic 

facies of the Kuskokwim Group. Anomalous mercury 

concentrations (1-10 ppm) also are present in streamsediment samples from just west of the eastern volcaniclastic facies of the Kuskokwim Group, where it is strongly 

deformed by east-dipping thrust faults in the southern part 

of the map area. 

Despite similarities in age, lithology, and bulk chemistry, the western and eastern Late Cretaceous to early 

Tertiary volcanic fields have significant differences in their 

potential for mercury mineralization. The volcanic rocks 

emplaced through the eastern volcaniclastic facies of the 

Kuskokwim contain anomalous mercury, whereas those 

emplaced through the western chert-rich facies contain 

significantly less mercury. In addition to many streamsediment samples that have anomalous mercury concentrations and to heavy-mineral concentrates that contain cinnabar, some rhyolite samples from the eastern field contain 

more than 36 ppm mercury. The western volcanic field has 

few mercury anomalies in any sample media that are clearly 

related to the volcanic rocks or to underlying Kuskokwim 

sedimentary rocks. Arsenic anomalies show some overlap 

with the mercury anomalies but are more heavily concentrated in the Togiak terrane beneath the eastern edge of the 

Kuskokwim Group. The difference in mercury content 

between the eastern and western volcanic fields appears to 

reflect variations in the composition of the underlying 

Kuskokwim Group and its basement. 

Mercury anomalies also are present in sediments from 

streams that drain serpentinite having silica-carbonate alteration along the Golden Gate and related faults that form the 

30 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
western boundary between the Kuskokwim Group and the 

Jurassic volcanic rocks of the Nyac terrane. Samples 

collected near the active Togiak fault in the southeastern 

comer of the area also contain anomalous mercury. Sediment samples from streams draining several of the Cretaceous granitic plutons also have anomalous mercury concentrations, which suggest that the plutons may have 

mobilized mercury from the Kuskokwim Group or the 

underlying basement. 

Rocks having the greatest potential for undiscovered 

mercury deposits in the Bethel region are the eastern 

volcaniclastic facies of the Kuskokwim Group and volcanic 

rocks that were emplaced through the eastern volcaniclastic 

facies. Additional mercury deposits may be found in rocks 

associated with the major fault zones that form terrane 

boundaries. 

RE:=ERENCE CITED 

Sainsbury, C.L., and MacKevett, E.M., Jr., 1965, Quicksilver deposits of 

southwestern Alaska: U.S. Geological Survey Bulletin 1187, 89 p. 

Detailed Profiles of Southeastern Arizona 

Obtained by a Truck-Mounted 

Magnetometer-A Step Toward Better 

Utilization of the Information Content of 

Geophysical Data 

By Mark E. Gettings, Mark W. Bultman, and 

Frederick S. Fisher 

Computerized methods of data acquisition, processing, and analysis have made it possible to move geophysical 

surveying into a realm of very high data density, where the 

data are nearly continuous. Although significant problems 

remain for truly high density map coverage, profile data at 

very closely spaced intervals can now be accommodated. 

The U.S. Geological Survey has embarked on a program to 

develop better methods of extending bedrock mapping 

beneath cover, mainly in basins. Part of this program is the 

collection of detailed geophysical data from a truckmounted platform. A proton precession magnetometer and a 

computer-controlled data-acquisition system have been 

installed on a four-wheel-drive vehicle. An electromagnetic 

system utilizing 60-Hz powerlines as a source is being 

constructed for installation into the truck, and a gamma-ray 

spectrometer will be incorporated so that the profiles will be 

essentially the horizontal equivalent of borehole logging. 

The system is capable of sampling at intervals from 25 em 

to 25 m; work to date has used an interval of about 5 m. 

Because closely spaced data are used, problems such as 

aliasing of spectra and loss of signal from small local 

sources are eliminated. Profiles collected over several 

porphyry copper deposits and several replacement deposits 

strongly suggest that alteration zonation is detectable in the 

magnetic signatures of some deposits. The major objectives 

of the research program are to devise new methods of 

inferring the history of geochemical transport of metals in 

and around ore deposits and to find new ways to visualize 

and interpret data to define geologic structure. 

A magnetometer profile consisting of about 20,000 

observations over a distance of 111 km was used for an 

initial data set. This profile begins in the Tucson basin, 

crosses the Santa Catalina metamorphic core complex, 

passes through the copper deposits at the base of the 

sedimentary section, continues onto the gravels of the San 

Pedro Valley, and ends in the volcanic andesites and 

rhyolites of the Galiuro Mountains. A comparison of the 

observed magnetic data with the surface lithologies shows 

distinct correlations of the textures or "noisiness" of the 

magnetic field with various rock units. Two approaches 

were attempted to create a quantitative measure of this 

relation. The first used traditional Fourier analysis, which 

resulted in a spectrum over each rock unit. The spectra were 

successful in demonstrating some relations. For example, 

the spectrum of the central part of the San Pedro basin is 

quite different from that of the gravel sheets on the edges of 

the basin, presumably because allochthonous material has 

been transported by the river and lake system in the central 

basin. Finally, the gravels northeast of the basin center have 

a spectrum similar in shape to that of the Galiuro volcanic 

rocks and different from that of the gravels on the southwest 

side of the basin. However, the spectra are ambiguous and 

must be used with other data if lithologies are to be 

predicted reliably. 

The second approach used some of the ideas of fractal 

geometry to describe the various "magnetic textures." The 

fractal dimension is commonly used to describe roughness. 

In a magnetometer profile, the coordinates are in different 

units, and, providing the profile can be described by a 

fractal, common methods of determining the dimension are 

dependent on the relative scaling between the distance and 

magnetic field coordinates; these methods require computations at several scaling factors to determine the fractal 

dimension. Two other measures have been used to reduce 

computation. They are computationally efficient and are 

fractal measures in the sense that they describe the tendency 

of the profile to fill the two-dimensional space. The two 

measures are applied within a window moving along the 

profile and consist of calculating the total "length," S, of the 

data (in mixed units of kilometers and nanoteslas) and 

counting the number of peaks and troughs, N. Both numbers measure the relative roughness of the data, and the 

parameter S/(2N) describes the average anomaly amplitude 

relative to the regional variation within the window and thus 

characterizes the short-wavelength sources. The mean, 

minimum, maximum, and standard deviation are also computed. For the profile studied, N appears to be a useful 

Gettings and others 31 
discriminant for shallow sources and may be especially 

useful for helping to distinguish between alluvial gravel 

units; S appears to be most useful in discrimination of 

lithologies. 

Multidimensional plots where each measure is plotted 

along an axis have proved useful in studying the information contained in the various measures in a combined way. 

In this method, clustering among the measures is sought by 

viewing different projections of the plot ("cloud plot"), and 

lithologies are characterized on the basis of their cluster 

response to several measures. Efforts to find and apply 

other methods of plotting the data have been undertaken. 

New techniques have been developed to combine multiple 

data layers into one composite map in order to reduce the 

dimensionality of the data. In one technique to integrate the 

magnetometer profile data with geophysical and geologic 

map data, the magnetometer profile data are displayed as 

gray-scale or color stripes or as projected profiles that are 

superimposed on a traditional or composite map. In this 

context, the use of the profile data often simplifies or 

highlights the images of the geophysical or geologic data 

sets and can clarify relations in regional data. 

Multidimensional plots of magnetometer and other 

data are also used to study structural relations in basins. The 

three-dimensional models of the basins can be viewed from 

any angle, can be cut in any location, and can have 

individual lithologies, such as basin fill, removed. These 

techniques enable one to explore the consequences of the 

model and often lead to the discovery of new relations. 

Synorogenic Auriferous Fluids of the Juneau 

Gold Belt, Southeast Alaska-Stable-Isotope 

Evidence for a Deep Crustal Origin 

By R.J. Goldfarb and W.j. Pickthorn 

Mesothermal gold deposits of the Juneau gold belt 

extend for 200 km along both sides of the Coast Range 

megalineament, a major steeply dipping shear zone about 

125 km inland of the continental margin. Gold-bearing 

fissure veins, stockworks, and stringers were deposited in a 

variety of igneous and metasedimentary host rocks at 55 

Ma; deposition took place at depths of at least 5-8 km 

during the late stages of activity in the mid-Cretaceous to 

Eocene southeast Alaska-British Columbia orogen. Fluid 

migration and vein emplacement were coeval with rapid 

uplift of host rocks along the western edge of the orogen. 

The source of the fluids responsible for mesothermal gold 

veins throughout the accreted terranes of the northern part 

of the cordillera is controversial. New stable-isotope data 

for minerals from auriferous veins and country rocks along 

the Juneau gold belt provide new information on the source 

of these ore fluids. 

The 8180 values of gold-bearing quartz from mines of 

the gold belt range from + 15.2 to + 20.8 per mil and thus 

indicate ore fluid values of+ 10.0±2.8 per mil at about 300 

°C, as determined from fluid-inclusion studies. Similar 

8180 values for quartz veins hosted by igneous and metasedimentary rocks indicate little local wall-rock control on 

oxygen-isotope compositions. The 8180 values are best 

interpreted as representative of a fluid in isotopic equilibrium with pelitic metasedimentary rocks. The narrow range 

in oxygen-isotope compositions precludes the use of any 

fluid-mixing model for gold deposition along the gold belt. 

Calculated~ 180qtz-kspar values of -0.2 to + 1.1 per mil for 

igneous rocks in the vicinity of the gold belt are suggestive 

of postcrystallization exchange between an isotopically 

heavy metamorphic fluid and the igneous rocks. 

Hydrothermal micas from many of the gold deposits 

are characterized by 8D values of -75 to -53 per mil. Ore 

fluids calculated to be in equilibrium with the micas have 

values of-35 to -15 per mil. These data, as well as the gas 

geochemistry of the fluids determined by fluid-inclusion 

investigations, are most consistent with a metamorphically 

derived ore fluid (fig. 1). Extracted fluid-inclusion waters 

from gold-bearing quartz have a broader range of 8D, from 

-48 per mil in relatively undeformed veins to about -110 

per mil for more deformed veins. This range of greater than 

60 per mil reflects various mixtures of fluids from inclusions containing isotopically heavy primary ore solutions 

10 

-10 

-30 

1.0 

-15 -10 -5 0 

6 tao (%o) 

Metamorphic H20 

10 

Fluid-inclusion 

waters 

15 20 25 

Figure 1 (Goldfarb and Pickthorn). Plot of &180 versus &D 

showing the composition for the ore fluids calculated on 

the basis of isotopic measurements of quartz and micas. 

The field for &D values determined from fluid-inclusion 

waters in quartz is significantly lighter. Specific deposits: 

J, jualin; K, Kensington; Scsi glassy quartz in calc-silicates 

at Sumdum Chief-figure shows a shift to an anomalously 

heavy oxygen value because of low water: rock ratios in 

calc-silicates; C, Crystal; R, Reagan; E, EPU; A, AlaskaJuneau; T, Treadwell; S, Sumdum Chief. A meteoric water 

evolution path, calculated for Tertiary waters at 300 oc, 

indicates water: rock ratios required to reach various isotopic compositions. In the calculations for the path, metasedimentary rocks are assumed to have an average &180 

value of +16 per mil and &D of -60 per mil. 

32 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
and those contammg isotopically light meteoric waters 

trapped during uplift of the veins to shallow crustal levels. 

The large isotopic shifts from the meteoric water line 

required to attain the &180 and &D values of the ore fluids 

preclude significant contributions from meteoric sources in 

the ore-forming process. Required oxygen shifts of about 20 

per mil would necessitate water:rock ratios of less than 0.1; 

required hydrogen shifts demand ratios of even less than 

0.01 (fig. 1). Such rock-dominated conditions are unlikely 

for any type of channelized, downward flow of meteoric 

waters into the deep crust. Rather, isotopically heavy fluids 

produced through prograde metamorphic reactions are more 

likely to have migrated upward during periods of failure 

along the Coast Range megalineament. 

Black Shales as Hosts for Unconventional 

Platinum-Group-Element Resources? Examples 

from Southern China and the Yukon, Canada, 

and Implications for U.S. Resources 

By Richard I. Grauch, Raymond M. Coveney, Jr., 

James B. Murowchick, and Chen Nansheng 

Black shales in China and Canada host Mo- and 

Ni-sulfide deposits and occurrences that contain significant 

and potentially byproduct amounts of platinum-group elements (PGE's) and precious metals. The ores contain 

similar suites of metals (although the metal ratios differ), 

have very similar and unusual textures, and are hosted by 

'black shales that were probably deposited in similar tectonic 

and metallogenic environments. The presence of analogous 

black shale sequences in Alaska and in the conterminous 

United States suggests a strong possibility of discovering 

comparable mineral resources in the United States. 

Lower Cambrian black shales in nine provinces of 

southern China host Mo- and Ni-rich sulfide deposits and 

occurrences that contain up to several hundred parts per 

billion each of PGE's and Au. The mineralization at any 

specific locality is a single stratiform, metal-rich sulfide 

layer. The layer (=2-15 em thick, reported up to 35 em 

thick) occurs within 10 m of the Proterozoic-Cambrian 

unconformity and crops out discontinuously along an 

approximately linear belt for 2,000 km. The Mo cutoff 

grade in the Zunyi area (the only active mining area), in 

Guizhou Province, is 4.1 percent. Annual production (from 

hand-driven adits) within the area during the last several 

years has been between 500 and 1 ,000 t having grades 

between 4.1 and 7.1 percent Mo. The ores also contain as 

much as 7 percent Ni, 2 percent Zn, and 2.5 percent As. 

The sulfide layer commonly overlies thin (up to a 

meter thick) phosphorite rich in rare-earth elements. In 

some locations, the sulfide layer is separated from the 

phosphorite by thin layers of black shale that contain 

calcareous concretions. The sulfide layer is apparently 

lithified clastic sediment (up to 90 percent sulfide clasts) 

comprising interlayered conglomeratic, granule to very fine 

sand-sized, well to poorly sorted black sandstones and 

black, finely laminated siltstones and claystones. Clast 

types include, in order of abundance, (1) sulfide clasts and 

nodules, some of which are finely laminated, (2) phosphatic 

organic-carbon-rich nodules, some of which are sulfide 

rich; and (3) organic-carbon-rich clasts, either layered or 

massive. One sample, from near Dayong, Hunan Province, 

contains what appear to be fossilized worm tubes. The 

sediments are cemented by sulfides, calcium phosphate, 

quartz, and (or) carbonate. Ore mineralogy is complex; the 

major phases include pyrite, millerite, bravoite, and gersdorffite. Sphalerite and chalcopyrite are minor ore phases. 

Molybdenum occurs as a Mo sulfide (probably jordisite) 

and in organic matter. 

Middle Devonian black shales in the central Yukon 

Territory, Canada, host a stratiform Ni-rich sulfide layer 

that is very similar to the metal-rich layer in China. The 

sulfide layer in the Yukon contains an average of 5. 7 

percent Ni, 1.2 percent Zn, and 0.24 percent Mo and 

contains up to 700 ppb Pt, 900 ppb Pd, 46 ppm Re, and 6 

ppm Ag (L. Hulbert, Geological Survey of Canada, oral 

commun., 1990; Hulbert and others, 1990). The layer is up 

to 10 em thick, crops out discontinuously for approximately 

14 km along the strike of a doubly plunging synclinal 

structure, and has been intersected by several drill holes 

within the syncline (R. Carne, Archer-Cathro and Associates, oral commun., 1990). The clastic texture of the sulfide 

layer is remarkably similar to that of the Chinese layer. 

Significantly, it also contains fossils, which are sulfide 

replacements of plant material. However, the mineralogy of 

the layer (pyrite, vaesite, wurtzite, and organic matter, 

according to R. Carne, oral commun., 1990) is apparently 

less complex than that of the Chinese layer. The shales and 

metals were deposited in an anoxic basin related to an 

Atlantic-type passive margin (Goodfellow, 1990). Within 

the same environment and generally in the same time frame, 

barite and sedimentary-exhalative (SEDEX) base-metal 

deposits formed elsewhere in the Selwyn basin. The basin 

can be traced into Alaska, and the possibility exists, 

therefore, that similar occurrences are present in the United 

States. 

REFERENCES CITED 

Goodfellow, W.D., 1990, Processes of metal accumulation in Paleozoic 

basinal shales, North American passive margin, Canadian Cordillera 

(Yukon) [abs.]: International Association for the Genesis of Ore 

Deposits (IAGOD) Symposium, 8th, Ottawa, Canada, Program with 

Abstracts, p. A109-All0. 

Hulbert, L., Came, R., Gregoire, C., and Paktunc, D., 1990, Sedimenthosted Ni-Zn-PGE mineralization in the Selwyn basin, Yukon-A 

new environment for nickel and platinum-group element mineralization [abs.]: IAGOD Symposium, 8th, Ottawa, Canada, Program 

with Abstracts, p. Alii. 

Grauch and others 33 
Subsurface Structure and lithology near the 

Getchell Gold Trend, Osgood Mountains, 

Nevada-Geophysical Insights 

By V.).S. Grauch, Donald B. Hoover, and Wayne S. 

Wojniak 

A complicated picture of subsurface structure and 

lithology is emerging from the interpretation of aeromagnetic, airborne electromagnetic (AEM), and gravity data on 

the eastern side of the Osgood Mountains, Nevada, where 

six sedimentary-rock-hosted disseminated-gold deposits 

define the Getchell gold trend. This integrated interpretation 

has provided new information on the three-dimensional 

geologic setting of the Getchell trend, although the details 

of the picture are still unclear. 

The three geophysical data sets were collected as part 

of a U.S. Geological Survey program to evaluate applications in mineral-deposit studies of geophysical data collected by various airborne geophysical instruments. Each 

data set represents the response to different physical properties at different depths within the subsurface and provides 

unique information for geophysical interpretation. The 

AEM data, collected at three different frequencies (900, 

7 ,200, and 56,000 Hz), best define lithologic and structural 

features in the shallow subsurface and can be used to 

distinguish some areas of hydrothermal alteration. The 

aeromagnetic data best delineate the subsurface configuration of igneous rocks and associated faults and altered areas. 

The gravity data, acquired along three profiles, best distinguish major normal faults that juxtapose bedrock against 

basin fill. 

Interpretation of the geophysical data involved application of mathematical filters to gridded data, digital 

comparison of data grids, and modeling of profiles and 

isolated anomalies. Many poorly defined or previously 

unknown geologic features were better defined or discovered during the interpretation; several are of particular 

interest. The Osgood Mountains pluton, which is adjacent 

to several of the gold deposits, extends laterally east and 

west to some depth; previous geologic interpretations suggested that the buried part of the pluton did not extend west 

of the exposed west side. Shallow dikelike apophyses may 

be common on the west side. Knowledge of the subsurface 

extension of the pluton is important if gold mineralization is 

genetically related to the pluton (the origin of the gold 

deposits is still controversial). The overall fault pattern 

delineated by the geophysical data is presumably related to 

Basin and Range structure and consists of four sets of 

approximately parallel faults that follow different trends in 

different parts of the study area. The fault sets trend 

north-northeast to the northeast of the Osgood Mountains 

pluton, north-south next to the pluton, northeast just south 

and southeast of the pluton, and north-south further south, 

east of the Preble gold deposit. The AEM data suggest that 

Figure 1 (V.).S. Grauch and others). Looking north across 

the Mag pit, which is along the Getchell gold trend, at a 

normal fault that cuts bedrock, basin fill, and gold ore. 

Photograph by James A. Pitkin in 1989. 

hydrothermal alteration occurred along many of the faults, 

especially those related to gold mineralization. Modeling 

results suggest that episodic faulting, basin filling, and 

volcanism have resulted in step-down faults or grabens 

where bedrock is covered by a variable amount of alluvium 

intermixed with volcanic flows. A fault cutting both bedrock and alluvium is shown in figure 1. 

Drill-hole information has revealed a few unexpected 

sources for certain local geophysical anomalies. North of 

the Chimney Creek gold deposit, an anomalous aeromagnetic high is probably produced by pyrrhotite interspersed in 

pillow lavas, rather than by an intrusion as previously 

thought. North of the Mag open pit, an aeromagnetic high 

over an area of low resistivity may be produced by 

secondary magnetite in carbonaceous argillite. These 

unusual results suggest that the details of the subsurface 

picture are far from being established and indicate the 

direction for future work. 

Geochemical Exploration Criteria for Epithermal 

Cinnabar and Stibnite Deposits, Southwestern 

Alaska 

By ).E. Gray, D.E. Detra, R.j . Goldfarb, and K.E. 

Slaughter 

Epithermal cinnabar- and stibnite-bearing vein systems are scattered over several thousand square kilometers 

in southwestern Alaska. More than 40,000 flasks of mercury have been produced in the region, mostly from the Red 

Devil mine. Mineralized quartz-carbonate veins and stockworks are found within Cretaceous basin-fill sedimentary 

rocks of the Kuskokwim Group and adjacent accretionary 

terranes. Many of the deposits are in or adjacent to brittle 

rocks, largely mafic dikes, rhyolite dikes and sills, and 

34 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
intermediate to felsic stocks, that intrude sedimentary rock. 

Mineralized veins are commonly localized where beddingplane faults in surrounding sedimentary rocks intersect 

these more competent units. Cinnabar is the dominant ore 

mineral; it typically is accompanied by stibnite. Realgar, 

orpiment, pyrite, limonite, hematite, and native mercury 

are less common. Dickite is a typical alteration mineral at 

many of these occurrences. 

In 1989, a geochemical orientation survey was conducted around several cinnabar and stibnite vein deposits. 

Stream-sediment, heavy-mineral-concentrate, stream-water 

and vegetation samples were collected in drainages 

surrounding the Red Devil, Cinnabar Creek, White Mountain, Rhyolite, and Mountain Top deposits. Three size 

fractions of the stream-sediment samples were selected for 

analysis. The heavy-mineral-concentrate samples were split 

into nonmagnetic, paramagnetic, and magnetic fractions. 

These samples and the stream-water and vegetation samples 

were analyzed for multielement suites by different chemical procedures. Nonmagnetic heavy-mineral concentrates 

were also examined microscopically to identify their 

mineralogy. 

Results confirm that Hg, Sb, and As in minus80-mesh stream sediments are effective pathfinder elements 

in exploration for epithermal cinnabar and stibnite occurrences. Concentrations greater than 3 ppm Hg, 1 ppm Sb, 

and 15 ppm As in the minus-80-mesh stream sediments, 

regardless of the host lithology, are indicative of upstream 

cinnabar and stibnite occurrences in the Kuskokwim River 

region. Stream sediments coarser grained than minus80-mesh are less useful in the exploration for these mineral 

occurrences. Base metals in stream sediments and heavymineral concentrates are ineffective pathfinders for this 

epithermal type of occurrence. 

Heavy-mineral concentrates provide little additional 

advantage in the exploration for these mineral systems. Sb 

and As dispersion patterns downstream from mineralized 

areas are generally more restricted in heavy-mineral concentrates than they are in the stream sediments. Distribution 

patterns of placer cinnabar observed in heavy-mineral 

concentrates are similar to those of anomalous Hg and Sb in 

corresponding stream sediments. Stream waters are a less 

effective geochemical sample medium than stream sediments or heavy-mineral concentrates, and vegetation samples are ineffective geochemical sample media in exploration for this type of mineral occurrence. 

Several of the epithermal cinnabar and stibnite vein 

deposits in the Kuskokwim River region contain anomalous 

gold concentrations. Gold contents in vein samples containing cinnabar and stibnite range from 0.05 to 6.9 ppm. These 

concentrations indicate that gold may be a byproduct of 

some of these epithermal systems, which were previously 

recognized only for their Hg and Sb contents. The gold 

anomalies suggest the potential for significant gold concentrations deeper in some of these cinnabar- and stibnitebearing deposits. 

Geochemical Investigation of Ground Water 

Associated with Disseminated Gold Deposits 

along the Getchell Trend, Northern Nevada 

By D.j. Grimes, W.H. Ficklin, j.B. McHugh, and 

A.L. Meier 

Ground-water samples were collected from wells and 

a drill hole near the Chimney Creek and Rabbit Creek 

disseminated gold deposits along the Getchell trend in 

northern Nevada to study the chemistry, distribution, and 

mobility of ore-associated elements such as Au, As, Sb, and 

W. A custom-built double-check valve bailer was used to 

obtain ground-water samples from various depths in the 

drill hole at and below the water table. The deepest sample 

was from 700 ft below the ground surface. 

The samples were analyzed for a number of constituents, including Au, at the 1-part-per-trillion (ppt) level by 

anion-exchange graphite-furnace atomic-absorption spectrometry (GFAAS). As, Sb, and W contents were determined by inductively coupled plasma-mass spectrometry 

(ICP-MS), and concentrations of As species (arsenite or 

As(III) and arsenate or As(V)) were determined by ionexchange GF AAS. 

Water samples from nine observation wells around 

the perimeter of the Rabbit Creek deposit contain variable 

concentrations of As, Sb, and W. In these waters, As 

concentrations range from 19 to 540 J.Lg/L (ppb); Sb, from 

0.4 to 140 J.Lg/L; and W, from 2 to 120 J.Lg/L. High 

concentrations appear to be very local. In general, the 

higher concentrations of these constituents occur where 

measured Eh values indicate reducing conditions. Thus, it 

appears that the increased concentrations of these elements 

in water are derived from contact with the sulfidic parts of 

the gold deposit. Ordinarily, Sb and W contents are not 

determined in water samples because their abundances are 

so low that spectrophotometric methods are not effective; 

however, given the ease of determination provided by 

ICP-MS, we were able to establish background concentrations and confirm that anomalous concentrations of Sb and 

W occur in ground water near sedimentary-rock-hosted gold 

deposits. 

As, Sb, W, and Au concentrations were determined 

from ground-water samples taken at various depths in a drill 

hole between the Rabbit Creek and Chimney Creek deposits 

(fig. 1). Concentrations of As range from 600 to 710 J.Lg/L 

at five different depths in the water column; Sb concentrations range from 110 to 140 J.Lg/L; W concentrations range 

from 130 to 150 J.Lg/L; and Au concentrations range from 1 

to 11 ng/L (parts per trillion). The values suggest that these 

elements are derived from mineralized rock. 

Grimes and others 35 
~ 800 11 

i 700 ···········-··-·-··--···-·-·· _, .• ,""~---":·:c.::~:::::----............................... ~t~:-~:;·-.. -· 10 ~ 

t :: :~:=~= ~E?=~=====7~= : I 

§ ,rtl 6 .5 

g 400 ..... ·--·-·-------·--·-·---.. ----·--------.. -·-·--·-------·-·-·--,L··-·----

~ .. ·-·-·-·---... ui ' 5 § 8 b~ ,/ ~ 8 300 ----·-·-·-·-----... ·-· ................. - .............................................. ___ _, __ /~~~----------....... _ .. __ , .. ______ .. 4 ~ 

$': / g 

:200 -~-,- -~- :~t'~~ -- );!-=:--:] "'100 !=- ___ ..-- ' 1 

<( 0 50 1 00 150 200 250 300 350 

Depth below water table, in feet 

Figure 1 (Grimes and others). Depth below water table 

versus concentrations of As, Sb, W, and Au in groundwater samples from a drill hole between the Rabbit Creek 

and Chimney Creek gold deposits in northern Nevada. 

Results for the determination of arsenic species, 

ferrous iron, and oxidation-reduction potential are presented 

in figure 2. The data suggest that vertical ground-water 

"redox zones," which may affect the mobility of some of 

the ore-associated elements, can occur in the water column 

of a drill hole. The highest redox readings occur near the 

water table, where all the As is As(V) and ferrous iron 

concentrations are low. In contrast, the reduced zone of the 

water column is characterized by the predominance of 

As(III), the reduced form, and by greater concentrations of 

ferrous iron. 

Our data indicate that many chemical and physical 

parameters affect the mobility of ore-associated elements in 

ground water. A better understanding of these parameters 

will enable us to use water geochemistry more effectively as 

an exploration tool. 

~ Cii 800 a. 

E "' 

~ 

Cl 

e 

u .E 

.5 ~ 

ui 0 500 c > 

g 

u; 

<( 

50 

........ 1As1(111) I 

' 

100 150 200 250 

Depth below water table, in feet 

300 350 

Figure 2 (Grimes and others). Depth below water table 

versus concentrations of arsenic species and ferrous iron 

and redox potential for ground-water samples from the 

same drill hole for which data are plotted in figure 1. 

Comparative Metallogeny of the Soviet Far East 

and Alaska 

By Donald j. Grybeck, Warren j. Nokleberg, and 

Thomas K. Bundtzen 

For the last year and a half, we have been engaged in 

a systematic study with Soviet geologists from the Academy 

of Science and the Ministry of Geology of the metallogeny 

of the Soviet Far East and Alaska. In 1989, we spent more 

than a month visiting mineral deposits throughout the Soviet 

Far East and participated in extended discussions with 

Soviet geologists at numerous mines and research institutes. 

In return, last summer, two groups of Soviet geologists 

visited Alaska to examine important mineral deposits and to 

participate in workshops with American geologists. 

The work to date has been productive. Numerous 

maps, publications, and data have already been exchanged 

and are in the process of being integrated into a consistent 

description of the geology and metallogeny of the Soviet 

Far East and Alaska. Differences remain, partly because of 

our different approaches to geology, but more often because 

both sides of the exchange have been introduced to new 

facets of the geology of the northern Pacific and to new 

types of mineral deposits. For instance, we visited the 

"Bor" deposit at Dalnegorsk in Primorye, which is a unique 

skarn deposit that is the major source of boron in the Soviet 

Union. It was certainly unknown to us and apparently was 

unmentioned in the Western literature. As the work has 

progressed, we feel that there has been marked convergence 

in our thinking. We are confident that the result will be a 

joint publication (in English and Russian versions) describing the mineral deposits of Alaska and the Soviet Far East 

in mutually agreeable terms and relating them to the 

geologic history of the two areas. Plans are underway to 

extend the work to the Canadian Cordillera. 

This study is useful to both Soviet and American 

geologists because new knowledge and ideas are being 

exchanged across the northern Pacific. In addition, it is 

particularly germane at this time because of the opportunities that seem to be opening up for foreign investment in 

Soviet mineral deposits. 

Seasonal Variation in the Silver Concentration in 

Mesquite (Prosopis juliflora) Collected near 

Globe, Arizona 

By Thelma F. Harms 

Biogeochemical prospecting, which depends on 

chemical analyses of vegetation, can be an effective tool in 

the search for buried mineral deposits. The extensive root 

systems of plants accumulate aqueous fluids from ground 

36 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
water or pore water and transport these fluids along with the 

metal ions in solution to the upper parts of plants. In this 

way, plants concentrate metals from a large volume of 

ground. In areas containing relatively shallow layers of 

transported overburden, vegetation may give a better reflection of buried mineral deposits than the soils derived from 

the overburden. 

Mesquite (Prosopis juliflora), a common tree species 

in the Southwest, ranges from Texas to California and 

southern Utah but grows best at elevations below 1 ,500 m. 

It is a phreatophyte; that is, a species whose roots tap a zone 

of water saturation or its capillary fringe. Well-established 

mesquite trees have taproots that penetrate vertically 12-15 

m; mesquites also have an extensive lateral root system that 

may extend 15m beyond the base of the tree. Mesquite has 

been widely used in biogeochemical surveys in the arid 

Southwest, especially in studies of porphyry copper 

deposits. 

To determine the feasibility of using mesquite for 

biogeochemical studies of silver and to determine the 

optimum sampling time, a study of the seasonal variation in 

the silver content of mesquite was undertaken in an area 

northwest of Globe, Ariz., where the ground water may 

contain as much as 1-3 ~g/L (ppb) of silver (J.H. 

Eychaner, U.S. Geological Survey, oral commun.). Samples of velvet mesquite were collected at monthly intervals 

from individual marked trees at three sites. Mesquite leaves 

were collected from late April1990 until late autumn, when 

the leaves fell from the trees; mesquite twigs were collected 

for a full year. Mesquite inflorescences (flowers) and beans, 

though of less use for biogeochemical studies because of 

their ephemeral nature, were collected when available. The 

samples were washed to remove surface dust and were 

ground and ashed. The silver content of the ash was 

determined by atomic-absorption spectroscopy. The data 

collected through mid-August show silver accumulations of 

up to 3. 5 ppm in the ash. With the results of this study, 

sampling plans can be designed to maximize the contrast 

between background and anomalous values where silver is 

an element of interest. 

Marine Ferromanganese Deposits near the 

Marshall Islands 

By james R. Hein, jung-Keuk Kang, Marjorie S. 

Schulz, Suk-Hoon Yoon, and Virginia K. Smith 

In September and October 1989, the U.S. Geological 

Survey and the Korea Ocean Research and Development 

Institute (KORDI) cooperated in two joint oceanographic 

cruises to study marine mineral deposits. The first cruise 

was dedicated to the study of Co-rich and Pt-bearing Fe-Mn 

crusts within the Exclusive Economic Zone (EEZ) of the 

Republic of the Marshall Islands. The second cruise was 

dedicated to the study of Fe-Mn nodules at the western 

margin of the Clarion-Clipperton Fe-Mn nodule belt. In 

September and October 1990, two additional joint cruises 

were undertaken. The first cruise was dedicated to the study 

of Fe-Mn nodules in an area adjacent to that studied in 

1989, and the second cruise was dedicated to the study of 

Fe-Mn crusts, island-arc hydrothermal deposits, and phosphorites in the EEZ of the Federated States of Micronesia. 

Following is a summary of the results obtained from the 

1989 cruise to the Marshall Islands. 

The two main objectives of the cruise were to 

determine the geological, oceanographic, and geochemical 

controls on the origin and evolution of Co-rich Fe-Mn crusts 

that occur on seamounts in the Marshall Islands EEZ and to 

determine the origin and evolution of the seamounts and 

atolls in the region. Shipboard operations included collecting CTD (conductivity -temperature-density )-oxygen profiles of the water column, dredge hauling, and conducting 

video-camera surveys of the ocean bottom and bathymetric, 

seismic-reflection, gravity, and magnetic surveys. With 

completion of this work, the Marshall Islands became the 

most extensively studied area for Co-rich Fe-Mn crusts. 

All dated seamounts in the Marshall Islands are of 

Cretaceous age; most are Late Cretaceous. Volcanic rocks 

are chiefly alkalic basalt, and some are strongly alkalic. 

Seamounts lack simple or consistent morphologies and 

structures; even adjacent seamounts differ significantly. 

The summit platforms have complex, rugged surfaces that 

may be tilted. Summits may have a sediment cap; where 

one is present, the sediment shows evidence of erosion and 

slumping. The flanks of some seamounts have multiple 

terraces that vary in water depth and width. Seamount 

flanks are sites of extensive slumping and mass failures. 

Oxygen and temperature profiles in the water column 

vary with latitude and longitude and show greater depletion 

in oxygen to the east and south. Oxygen content of the 

seawater influences the metal contents of the Fe-Mn crusts. 

A direct correlation exists between low oxygen contents in 

seawater and high concentrations of Co, Cd, Mn, K, and Ti 

(manganophile elements) in the crusts and between low 

oxygen contents in seawater and low concentrations of Y, 

Cu, As, and Fe (siderophile elements) in the crusts. Oxygen 

content controls the Fe/Mn ratio and, thereby, the minor 

elements associated with each of the major phases. 

The thickest Co-rich crusts known in the world were 

collected on the 1989 cruise; they were as thick as 180 mm. 

Inferred growth rates range from 1. 5 to 16 mrnlm. y. and 

average 4. 7 mrnlm.y. The thickest crusts began to form 

between about 30 and 16 Ma, long after the formation of the 

Cretaceous seamounts. The crusts average 16.6 percent Fe, 

25.6 percent Mn, 2. 06 percent P, 0. 70 percent Co, 0.11 

percent Cu, 0.57 percent Ni, and 0.67 ppm Pt. Crusts from 

the Marshall Islands have higher mean Pt contents than do 

crusts from other central Pacific areas. Co content (grade) 

varies inversely with crust thickness (tonnage). The 

platinum-group elements (PGE's = Pt, Rh, Pd, Ru, and Ir) 

Hein and others 37 
in bulk crusts show significant enrichments over their 

lithospheric averages and seawater abundances. PGE ratios 

indicate that Pt, Ir, and Rh are derived from seawater and 

that Pd and Ru are derived from seawater and clastic debris. 

Contents of all rare-earth elements (REE' s) range from 

0.09 to 0.28 percent for bulk crusts and up to 0.37 percent 

for individual crust layers. Many chondrite-normalized 

REE patterns show either a small positive Ce anomaly or no 

anomaly; several have a negative anomaly. These patterns 

differ from those of other central Pacific crusts. The REE 

compositions and chondrite-normalized patterns change 

with depth in the crusts, but the changes are not consistent 

from one crust to another nor are the REE's consistently 

associated with the same crust phases. The geographic 

distribution of the REE's, particularly Ce and Eu, in the 

crusts may reflect the regional oxidation potential of 

seawater. 

U.S. Geological Survey Research on Agricultural 

Industrial Minerals in the United StatesImplications for Exploration 

By James R. Herring, David Z. Piper, Sherilyn 

Williams-Stroud, Charles S. Spirakis, and Richard 

A. Sheppard 

The indisputable need for the agricultural minerals, 

used principally as fertilizers, will parallel the increasing 

need for food to support population growth. U.S. Geological Survey (USGS) research on the agricultural mineral 

commodities includes studies of phosphate, sulfur, potash, 

and zeolites. The agromineral industries provide 41 percent 

of U.S. industrial mineral revenue and 7 percent of all U.S. 

nonfuel mineral revenue. USGS research on deposits of 

these commodities principally focuses on their geology, 

geochemistry, and mineralogy; production figures are provided by the U.S. Bureau of Mines. Overall research goals 

of the USGS are to understand the nature of existing 

deposits and to be able to use this information to benefit 

future exploration. A change in the classic exploration 

attitude, which considered mainly ore grade and tonnage, 

has been forced by modem societal values. Consequently, 

current USGS research also focuses on additional aspects 

important to the mining and use of the agromineral commodities. These aspects include, for example, determination of potentially useful byproducts or of material that 

adversely affects mining, processing, and eventual use of 

the commodity. 

Phosphate rock is the most valuable nonenergy industrial mineral in the United States; it accounts for slightly 

over a quarter of the annual revenue from all industrial 

minerals, which are collectively valued at about $4 billion. 

The United States is the world's largest producer of phosphate, supplying its own needs and about one-quarter of 

world exports. Of domestic phosphate production, 90 percent is used for fertilizer and other agrochemicals. Phosphate is produced from two principal phosphorite deposits: 

(1) the western field, which includes the Permian Phosphoria Formation in Idaho, Montana, Wyoming, and Utah, and 

(2) Neogene deposits of the Atlantic Coastal Plain, principally in Florida and North Carolina. Tennessee has minor 

production, which is included with production in the westem field for U.S. production statistics. The Neogene 

deposits account for 88 percent of the U.S. annual production of 49 Mt of phosphate rock. Three countries, the 

United States, the U.S.S.R., and Morocco, account for 

about two-thirds of world production, which totaled 163 Mt 

in 1989. The United States has sufficient phosphate resources to last well into the next century, but availability of this 

phosphate will be determined by international pricing and 

mining costs. Many of these deposits can be mined only at 

much greater cost than the presently developed deposits and 

may require new, as yet unproved, technologies for ore 

processing. Specific USGS studies focus on the use of new 

analytical techniques to provide geochemical traverses of 

deposits showing, in addition to phosphate, the distributions 

of major and trace elements that are of concern to mining or 

the environment. The goal is to understand the controls 

affecting these distributions and to predict how these 

elements vary in the deposits that may become future 

exploration targets. 

U.S. sulfur production in 1989 was 12 Mt (worth $1 

billion) while consumption was 13 Mt. About 70 percent of 

sulfur is used for agricultural chemicals, mostly for phosphate processing, but also as a soil amendment. Total world 

production in 1989 was 59 Mt. About 57 percent of the 

sulfur produced in the United States was recovered from oil 

and gas processing, 33 percent was from Frasch mining of 

sulfur deposits in west Texas and the Gulf of Mexico Coast, 

and 10 percent was S02 recovery from smelters. To satisfy 

domestic sulfur requirements, about 8 percent was 

imported, principally from Canada and Mexico. As environmental regulations further limit atmospheric sulfur emissions from industrial processes, increasing amounts of 

sulfur will be recovered. Nevertheless, native sulfur deposits in salt domes of the Gulf of Mexico Coast and in 

sedimentary basins of west Texas will continue to be 

attractive exploration targets. USGS studies of sulfur deposits in caves of the Guadalupe Mountains of New Mexico 

may provide clues to the genesis of nearby, similar deposits 

of west Texas and may help to identify additional commercial deposits. 

The U.S. potash production in 1989 of 1.5 Mt 

(valued at $280 million) supplied only 27 percent of 

domestic consumption. Major imports were from Canada, 

Israel, the U.S.S.R., and the German Democratic Republic. 

Most U.S. potash comes from salt deposits in New Mexico, 

where it is produced by conventional underground mining 

techniques at depths of 100 to 200m. The limiting depth for 

38 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
underground mining, around 1,100 m, is constrained by the 

low strength of the salt. The United States has estimated 

potash reserves of 90 Mt, while total estimated resources 

are about 6,000 Mt. Most of the potash resources are in the 

Williston Basin in Montana and North Dakota and in the 

Paradox Basin in Utah, but the deposits occur between 2 

and 3 km depth. These two basins present important 

research possibilities for deposit characterization and new 

production methods, such as solution mining. Because there 

are no substitutes for potash as an essential plant nutrient, a 

continued supply is necessary for the United States to meet 

present agricultural needs and to become less dependent on 

imports. Current USGS research focuses on creating a new 

model for the deposition of sylvite (KCl, the principal ore 

mineral of potassium compounds), which may shift 

approaches in exploration. 

Zeolites are important agricultural minerals for the 

future. Presently, they are used in agriculture as slowrelease fertilizers, carriers for herbicides and pesticides, and 

traps for heavy metals in soils. Commercial use of natural 

zeolites in the United States is in its infancy; production and 

use in Japan, eastern Europe, the U.S.S.R., and Cuba 

exceed those in the United States. Most U.S. zeolites occur 

in Cenozoic tuffaceous rocks west of the Mississippi River. 

Major objectives of USGS investigations are to locate new 

high-grade deposits and characterize their grade, tonnage, 

and thickness of overburden as well as their mineralogy and 

chemical composition. A long-term goal is to develop and 

refine models of zeolite deposits as an aid in future 

exploration. As new markets open, the United States should 

become a major exporter of zeolites. 

Low-Level Gold Determinations by Use of Flow 

Injection Analysis-Atomic Absorption Spectrophotometry-An Application to Precious-MetalResource Assessment in the lditarod 1 o x 3° 

Quadrangle, Southwestern Alaska 

By D.M. Hopkins, j.E. Gray, and K.E. Slaughter 

Low-level gold determinations were performed by the 

U.S. Geological Survey (USGS) to assess the preciousmetal resources in the Iditarod 1 o x 3° quadrangle, Alaska, 

as part of the ongoing Alaska Mineral Resources Assessment Project. Flow injection analysis-atomic absorption 

spectrophotometry (FIA-AAS) was used to determine gold 

concentrations in stream-sediment samples collected during 

a reconnaissance geochemical survey of the quadrangle. 

The most common method for gold determination is 

separation from hydrochloric or hydrobromic acid solution 

with methyl isobutyl ketone (MIBK) and detection by 

means of flame atomic-absorption spectrophotometry 

(FAAS) or graphite-furnace atomic-absorption spectrophotometry (GFAAS). Although FAAS is less sensitive than 

GF AAS, some advantages of F AAS are ( 1) fewer problems 

with interference, (2) continuous monitoring of the analyte 

in a liquid stream, (3) suitability for online automated 

analysis, and (4) more rapid determinations. Furthermore, 

F AAS procedures can be transformed conveniently into a 

FIA-AAS technique that uses solvent extraction and a 

10-fold analyte preconcentration to produce a lower limit of 

determination, one that approaches the crustal abundance of 

gold (3-5 ppb). Thus, the analysis of gold at concentrations 

that formerly required GFAAS is accomplished more effectively with the FIA-AAS technique. 

The FIA-AAS technique uses a sample weight of 10 

g for digestion with a hydrobromic acid-bromine solution. 

Gold is extracted from the acid mixture with 1 mL of 

MIBK; a 10-fold preconcentration and a detection limit of 5 

ppb can be achieved. The flow injection device introduces 

0.1 mL of MIBK extractant per determination into the 

spectrometer; more than 50 determinations per hour can be 

completed by this technique. Precision of the technique at 

the 100-ppb level is 14.3 percent relative standard deviation 

based on a 5-g aliquot of USGS reference GXR-2 in 1 mL 

MIBK. Results f~r reference standards by this FIA-AAS 

technique agree well with values reported in the literature. 

Approximately 12 percent of the stream-sediment 

samples from the Iditarod quadrangle contain greater than 5 

ppb gold. Most of these samples cluster into 13 distinct 

areas. Five of these areas are associated spatially with 

Cretaceous-Tertiary volcano-plutonic complexes that 

intrude sedimentary • rocks of the Cretaceous Kuskokwim 

Group: Six other anomalous areas are near CretaceousTertiary dikes or small stocks that intrude rocks of the 

Kuskokwim Group. One gold anomaly is found in an area 

underlain by Cretaceous-Tertiary volcanic rocks of the 

Y etna field; another occurs in an area underlain by rocks of 

the Kuskokwim Group. 

Samples that contain anomalous gold cluster in areas 

of known placer districts such as Flat, Moore Creek, and 

Donlin Creek. Gold anomalies are observed also in areas in 

which little or no gold had been reported previously, such as 

the Beaver Mountains, Mount Joaquin, and Granite Mountain. These anomalies probably are related to (1) vein 

deposits containing quartz, carbonate, fluorite, stibnite, 

cinnabar, gold, scheelite, and arsenopyrite similar to those 

near Fiat or (2) veins containing chalcopyrite, galena, 

sphalerite, cassiterite, tourmaline, and fluorite such as those 

in the Beaver Mountains. These gold anomalies correlate 

well with anomalous concentrations of Ag, Sb, As, Cu, 

Pb, Zn, Sn, or W in stream-sediment or heavy-mineralconcentrate samples. However, the low-level gold concentrations in the stream-sediment samples most effectively 

define areas of precious-metal favorability in the Iditarod 

quadrangle. 

Hopkins and others 39 
Geophysical and Geological Studies of the 

Roseau 1 o x 2° Quadrangle, Minnesota and 

Ontario 

By R.J . Horton, W.C. Day, T.L. Klein, and B.D. 

Smith 

The U.S. Geological Survey (USGS), in cooperation 

with the Minnesota Geological Survey (MGS), the Minnesota Department of Natural Resources, and the Geological 

Survey of Canada (GSC), has compiled geologic, aeromagnetic, and gravity maps for the Roseau I 0 X 2° quadrangle , 

Minnesota and Ontario. This compilation was done as part 

of the Conterminous United States Mineral Assessment 

Project (CUSMAP). More than 95 percent of the bedrock in 

the Roseau quadrangle is covered by westward-thickening 

Quaternary glacial deposits. Therefore, the geologic map is 

based on the interpretation of geophysical data supported by 

4SO 00' 

drill-core studies and studies of a limited number of 

outcrops. 

The aeromagnetic map was produced by compiling 

existing data from the MGS and the GSC with new data 

collected by the USGS. The magnetic data were processed 

with a reduction-to-the-pole program to compensate for 

inclination and declination of the induced magnetic field 

that shifts the magnetic anomalies away from their source 

rocks. Shaded-relief (fig . 1) and first-vertical-derivative 

maps were also produced; they enhance such subtle bedrock 

features as dikes and faults. Other derivative maps were 

generated that define the boundaries of magnetic bodies 

from magnetic gradients and that outline terranes based on 

bedrock units having different magnetic susceptibilities. 

The complete Bouguer anomaly map was produced 

by compiling existing data from the MGS , the GSC, and the 

U.S. Department of Defense . The USGS collected new 

gravity data to help fill the regional coverage and to better 

94° 00' 

Figure 1 (Horton , Day, and others). Shaded-relief aeromagnetic map of the Roseau 1° x 2° 

quadrangle , Minnesota and Ontario. This map is a compilation of data from the U.S. Geological 

Survey, the Minnesota Geological Survey, and the Geological Survey of Canada. The lighter areas 

represent magnetic-field highs; the darker areas represent magnetic lows. The apparent relief is 

produced by computer-generated southwest illumination. The Vermilion fault can be seen striking 

west-northwest across the center of the map. A major northwest-trending dike swarm, readily seen 

in the southern part of the image, cuts metasedimentary and migmatite terrane of the Quetico 

subprovince. The oval feature in the southwest quarter of the map is a granitoid pluton surrounded 

by metavolcanic rocks of the Wabigoon subprovince. The area mapped is approximately 100 by 150 

km. 

40 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
define contacts. The combined data set was processed to 

produce a terrain-corrected Bouguer anomaly map. In 

addition, derivative maps were generated that enhance 

boundaries and define terranes based on bedrock units 

having different densities. 

The above-mentioned geophysical data were combined with new geologic data to produce a comprehensive 

synthesis of the regional geology and mineral potential of 

the area. The Roseau quadrangle is underlain by Archean 

rocks of the following subprovinces: the Wabigoon in the 

north and west, the Quetico in the central part, and the 

Wawa-Shebandowan in the southeast. Rocks of the WawaShebandowan subprovince are a continuation of the western 

Vermilion district greenstone belt, which crops out east and 

south of the quadrangle. The Wabigoon and WawaShebandowan subprovinces are made up of rocks typical 

of greenstone-granite terranes, whereas the Quetico is 

dominantly composed of amphibolite-grade schists and 

migmatites. 

Major high-angle transpressive fault zones delineate 

the boundary between the Wabigoon and the Quetico 

subprovinces. These faults, which are up to 2 km wide and 

more than 100 km long, are zones of intense ductile 

deformation that have pronounced aeromagnetic and gravity 

signatures. The latest ductile deformation, which had a 

dextral sense of shear, produced S-C mylonites, phyllonites, and mica schists that are variably altered to quartz, 

calcite, pyrite, and chlorite. Secondary ductile shear zones, 

which parallel the major bounding fault zones, locally have 

anomalous gold concentrations. 

Although poorly exposed in the map area, the boundary between the Quetico and Wawa-Shebandowan subprovinces is interpreted to be gradational and without any abrupt 

change in lithology. The metamorphic grade gradually 

increases from low to medium grade northward into the 

metasedimentary rocks of the Quetico subprovince. No 

mineralization has been discovered to date in the subprovince within the Roseau quadrangle. However, Algoma-type 

iron deposits and shear-zone-hosted lode-gold prospects are 

present to the east in the Vermilion district. 

Interpretation of New Geophysical Data 

Obtained by Airborne Instruments for the EffieCoon Lake Area, Minnesota 

By Robert j. Horton, Bruce D. Smith, Victor F. 

Labson, and Robert j. Bisdorf 

New airborne geophysical instruments and dataprocessing methods developed by the U.S. Geological 

Survey (USGS) have been used to supplement standard 

aerial magnetic methods commonly used in regional subsurface mapping. The purpose of this study is to apply 

emerging technology in aerial geophysics and advanced 

data-processing methods to geologic mapping of an area 

covered by glacial overburden. 

The study area is the Effie-Coon Lake area, Minnesota. It is underlain by a Late Archean granite-greenstone 

bedrock which is more than 95 percent concealed by 

Quaternary glacial deposits as much as 100m thick. Within 

the study area, three large felsic plutons and a mafic sill 

complex (Deer Lake Complex) have intruded the steeply 

dipping metasedimentary and metavolcanic country rocks 

that also host several thin iron-formations. A northwesttrending Early Proterozoic mafic dike swarm cuts the 

Archean terrane. Records of the Minnesota Department of 

Natural Resources show that the Deer Lake Complex was 

previously explored for base-metal deposits. This exploration used detailed commercial aerial electrical and magnetic 

surveys, in which flightlines were spaced at 1/s-mi intervals. 

Recent mineral exploration just east of the study area has 

identified gold mineralization associated with ironformations, such as those in the southeastern part of the 

study area. 

New USGS aerial data presented here include standard total magnetic field data and data from USGSdeveloped electromagnetic (EM) very low frequency (VLF) 

and powerline-source instruments (fig. 1). The sources for 

VLF signals are distant radio transmitters operating at about 

20 kHz. The powerline system measures the electromagnetic fields radiating from powerlines at 60 Hz and harmonic frequencies of 180 and 300 Hz, which penetrate 

much more deeply than do VLF signals. 

The new aerial magnetic data were processed to 

produce color shaded-relief and first-vertical-derivative 

maps that enhance subtle bedrock geologic features. Other 

analysis techniques were used to generate "maxspot" and 

terrace maps, which can be used to interpret boundaries of 

subsurface magnetic sources on the basis of interpreted 

variations in magnetic susceptibility. This type of processing can enhance the variations of magnetic properties of 

iron-formations, which could indicate areas having structures or alteration zones favorable for gold mineralization. 

USGS data from the VLF and powerline EM systems 

have been reduced to make apparent-resistivity maps of the 

survey area. In this glaciated terrain, the VLF data generally 

reflect near-surface (tens of meters) features, such as glacial 

deposits and shallow high-resistivity bedrock, and are 

useful in interpretation of more deeply penetrating powerline aerial EM data. In the south-central part of the sur\rey 

area, eskers are associated with sinuous apparent-resistivity 

highs. Low apparent resistivities in the northwestern part of 

the survey area reflect thick glaciolacustrine clay deposits 

that can attenuate the aerial powerline EM anomalies. 

In contrast to the VLF signals, the powerline EM 

system penetrates hundreds of meters as a result of its lower 

frequency and is less sensitive to the effects of glacial 

Horton and others 41 
Figure 1 (Horton, Smith, and others). The Fairchild HeliPorter research aircraft operated by the U.S. Geological 

Survey. The turbo-prop, STOL (short takeoff and landing) 

aircraft is equipped with magnetic and electromagnetic 

instruments for airborne geophysica l mapping. A very low 

frequency electromagnetic receiver is mounted in the 

boom or "stinger." A magnetometer and a powerli nesource electromagnetic sensor are mounted in wing-tip 

instrument pods. Photograph by Herbert A. Pierce in 1989. 

overburden. Individual anomalies from the powerline system compare favorably with those mapped by previous 

commercial aerial EM systems. However, the anomalies 

detected by the powerline system suggest that the lowresistivity zones in the Deer Lake Complex extend fa.Jther to 

the southwest than those mapped by the commercial EM 

systems. 

Integrated interpretation of the geophysical, geological, and geographical data is facilitated by using 

geographic-information-system (GIS) software. GIS software can easily display overlays of various data sets to 

enhance their subtle features; for example, the magnetic and 

resistivity signatures of the intrusive complexes can be 

combined to enhance geologic features that may be associated with various types of mineral deposits. 

An Overview of Industrial Minerals of the U.S. 

Atlantic and Gulf of Mexico Coastal Plains 

By john W. Hosterman 

Industrial minerals are an economically important, 

though often overlooked, part of the resource base of the 

United States. Recent advances in deposit models for 

industrial mineral commodities that include grade and 

tonnage data now allow attempts at quantitative assessments 

of industrial mineral resources. This report presents an 

overview of the industrial mineral resources of the U.S. 

Atlantic and Gulf of Mexico Coastal Plains States and 

represents the initial phase of a new effort to assess these 

resources of the Nation . 

Industrial minerals are the principal mineral commodities produced in the following 16 States of the U.S. 

Atlantic and Gulf of Mexico Coastal Plains: New York, 

New Jersey , Delaware, Maryland , Virginia , North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Tennessee, Kentucky, Arkansas, Louisiana, and 

Texas. Approximately $5.4 billion worth of industrial 

minerals were produced by these States in 1987. Florida 

was the leading producer, with more than $1.3 billion worth 

of industrial minerals, and was followed by Georgia and 

Texas, each with about $800 million. 

Sand and gravel are the leading bulk commodities 

produced in each Coastal Plain State. They have an average 

market value of about $3.50 per ton. Because of relatively 

high transportation costs, they are used near mining sites 

and processing facilities. 

The second major commodity is clay, which is also 

produced in all the Coastal Plain States. The market value 

for clay ranges from about $4.50 per ton for common clay 

to $94.50 per ton for high-grade white kaolin. Consequently, some clay products are used locally, whereas 

high-grade clay products are transported to national or 

international consumers. Common clay in all the Coastal 

Plain States is used in the manufacture of face and structural 

bricks and lightweight aggregate. Georgia and South Carolina are the major producers of high-grade white kaolin, 

which is used for filling and coating paper and for white 

dinnerware. Georgia and Florida are the only producers of 

palygorskite (attapulgite), a type of fuller's earth, which is 

used chiefly to absorb grease, oil , and animal waste. 

Mississippi , Alabama, and Texas are the major producers of 

southern, or nonswelling, bentonite, which is used primarily for bonding foundry sand to make molds for metal 

castings. Kentucky and Tennessee are the major producers 

of ball clay, which is used in making pottery, sanitary ware , 

tile, and ceramics. 

About 95 percent of the phosphate rock (89 percent of 

the P20 5) produced in the United States in 1987 was mined 

in Florida and North Carolina. Other commodities produced 

from the Atlantic and Gulf of Mexico Coastal Plains were 

42 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
cement (limestone plus clay) from South Carolina, Florida, 

Mississippi, Louisiana, and Texas; peat from New Jersey, 

South Carolina, and Florida; salt from Alabama, Louisiana, 

and Texas; glauconite from Delaware and New Jersey; 

industrial sand from Tennessee and Texas; lime and aggregate from Florida and Texas; sulfur from Louisiana and 

Texas; staurolite, zircon, and rutile from placer deposits in 

Florida; and gypsum and bauxite from Arkansas. 

The future demand for the unique palygorskite-type 

fuller's earth from Georgia and Florida and for the highgrade white kaolin from Georgia and South Carolina probably will increase at a rate approximately equal to the 

growth of the Gross National Product. In Florida, however, 

the phosphate mining industry may decline because of 

competing land uses (retirement housing, agriculture, tourism) and foreign competition. 

Geology and Mineral-Resource Potential of Part 

of the Northern Safford Basin, Southeastern 

Arizona 

By Brenda B. Houser, J.M. Kruger, and Roy A. 

Johnson 

The 100-km-long part of the northern Safford basin 

between Fort Thomas and Interstate 10 in southeastern 

Arizona is within the Sierrita-Mogollon corridor being 

mapped by the U.S. Geological Survey (Leslie Cox and 

others, this volume). The stratigraphy and structure of the 

northern Safford basin are inferred to have been controlled 

in large part by late Cenozoic Basin and Range extension 

along preexisting low- to moderate-angle faults associated 

with the Pinaleno Mountains metamorphic core complex. 

Basin geometry is revealed by test wells, residual 

Bouguer gravity maps, seismic-reflection data, and 

magnetic-anomaly data. These data show that between Fort 

Thomas on the north and the Graham-Cochise County line 

on the south, the northwest-trending Safford basin is a half 

graben, deepest on the southwest side. South of the county 

line, a west-northwest-trending bedrock high, covered by 

probably less than 0.5 km of basin-fill sedimentary rocks, 

crosses the Safford basin. This bedrock high may be 

associated with either the Stockton Pass fault or the Oak 

Draw detachment fault. 

Seismic-reflection data indicate that the deep southwestern side of the Safford half-graben is bounded by a 

northwest-trending normal fault with a concave upward 

fault plane that dips northeast 25°-50° near the range front 

and has more than 7 km of structural relief. Well data show 

that, to the northeast across the basin near the Safford 

mining district, a low-relief surface on andesitic bedrock 

dips about 5° southwestward beneath basin-fill sedimentary 

rocks. Seismic-reflection and well data indicate that this 

surface has low to moderate relief farther to the southwest 

and dips about 10°-15° southwestward to the deep part of 

the basin, 10 km east of the Pinaleno range front. Bedrock 

is at a depth of 355 m in a well 4 km from the Gila 

Mountains on the northeast side of the basin. In contrast, a 

test well 4 km from the Pinalenos on the southwest side of 

the basin penetrated 2,590 m of semiconsolidated sand and 

gravel but did not reach bedrock. 

Seismic-sequence analysis shows that the basin contains two major sedimentary sequences. The older sequence 

is inferred to have been deposited between 19 and 8 Ma on 

the basis of tentative correlation with other older sedimentary sequences exposed in the area, including sedimentary 

rocks of the Black Rock and Oak Draw detachments. The 

older sequence is 3-4 km thick in the deep part of the basin 

and wedges out against the bedrock slope on the northeastern side of the basin. Analysis of stacking velocities 

suggests that the older sedimentary sequence may contain 

evaporite deposits as thick as 0. 5 km in the western half of 

the basin. 

Seismic profiles show that bedding-plane reflectors in 

the older sequence bend abruptly downward within a few 

kilometers of the southwestern side of the basin and dip 

toward the inferred detachment fault. The dip of the 

bedding planes decreases with decreasing depth and age, 

overlying beds clearly onlap older, steeper beds, and the 

thickest part of each successively younger unit in the 

sequence lies westward of the thickest part of the underlying 

unit. These relationships suggest episodic eastward displacement of the older sequence during deposition. This 

displacement may have resulted from periodic reactivation 

of the Pinaleno detachment fault system during Basin and 

Range extension. These relationships are not seen in the 

younger sedimentary sequence. 

The younger sedimentary sequence in the basin is 

probably late Miocene and Pliocene in age. It is as much as 

1.5 km thick in a narrow, northwest-trending linear downwarp about 5 km northeast of the Pinaleno Mountains but is 

probably less than 0. 6 km thick throughout most of the 

basin. The younger sequence overlies the older sequence 

unconformably in depositional onlap and appears to be 

relatively undeformed except for tectonic subsidence in the 

downwarp adjacent to the Pinalenos and minor but widespread normal faulting. Provenance studies of basin-fill 

sedimentary rocks indicate that the downwarped area was 

the depositional center of the Safford basin between 

Thatcher and the Graham-Cochise County line throughout 

the late Miocene and Pliocene. Fault scarps in Pleistocene 

and Holocene alluvium above the downwarped zone suggest that subsidence continues to the present. 

The entire northeastern margin of the northern Safford basin has high potential for the occurrence of porphyry 

copper and copper vein deposits similar to those in the 

Safford mining district. This assessment of a high potential 

is based on the apparent continuation of the volcanic host 

rocks beneath a relatively thin cover of basin fill. 

Houser and others 43 
Seismic data indicate that the older sedimentary 

sequence in the northern Safford basin contains thick 

evaporite deposits that may be as shallow as about 0.6 km 

near the middle of the basin. However, test wells show that 

the shallower evaporite deposits generally are interbedded 

with significant amounts of mudstone. Sparse evidence 

suggests that there may be ground-water circulation 

between deep faults and some of the evaporite units. Wells 

drilled southwest of the Butte fault near the Safford mining 

district have encountered artesian water as warm as 65 °C 

that contains more than 2,000 ppm Cl. Base- and preciousmetal deposits near detachment faults may have been 

deposited in the past by similar warm brines derived from 

evaporite sequences in overlying basin-fill sedimentary 

rocks. Thus, the Oak Draw and Black Rock detachment 

faults, which may be at fairly shallow depths along the 

southwest margin of the northern Safford basin, are proposed as target areas for geochemical study. 

Geology and Genesis of the Baid AI Jimalah and 

Silsilah Tungsten-Tin Deposits, Kingdom of Saudi 

Arabia 

By Robert J. Kamilli 

The Baid al Jimalah and Silsilah tungsten-tin deposits 

are in the northeast Arabian Shield in Saudi Arabia. Baid al 

Jimalah (569 Ma) is a sheeted greisen-vein tungsten deposit 

containing minor tin; Silsilah (587 Ma) is a disseminated, 

tin-greisen deposit containing minor tungsten. Both deposits are spatially, temporally, and genetically associated with 

highly differentiated, peraluminous granites that intrude 

Late Proterozoic immature sandstones. 

Most of the vein constituents at Baid al Jimalah came 

from hydrothermal fluids exsolved from a granitic magma 

during a single cycle of magma intrusion and hydrothermal 

activity. The hypogene mineralization can be divided into 

three main periods represented by three sets of veins: early 

quartz±molybdenite stockwork veins, wolframite- and 

scheelite-bearing greisen-bordered veins, and late quartzcarbonate-fluorite veins. Early stockwork mineralization 

occurred at near-magmatic temperatures (580-700 °C); a 

C02 -rich, low-density fluid and an H20-rich, higher density fluid were present. The greisen mineralization formed 

from fluids in the liquid state at temperatures mostly 

between 390 and 430 °C. Late quartz-carbonate-fluorite 

veins formed from liquids at temperatures at least as low as 

238 °C. These temperatures, corrected for pressure by 

assuming a depth of mineralization of 3.1 km, are 75-80 oc 

higher than fluid-inclusion filling temperatures. Important 

volatile constituents of the hydrothermal fluids were H20, 

C02 , and CH4 . 

At Silsilah, four distinct phases of peraluminous 

granite are recognized throughout a 16-km2 area. However, 

only the most differentiated, cupola-shaped intrusions host 

significant volumes of greisen and related mineralization. 

Also, a strong correlation exists between the geometry of a 

given cupola and the intensity of alteration: the most intense 

apical greisenization is associated with small-diameter 

cupolas that have steeply dipping contacts. 

The paragenetic sequence at Silsilah can be divided 

into five stages: pegmatite formation, pervasive albitization, pervasive greisenization with deposition of cassiterite, 

formation of quartz-wolframite veins, and formation of 

mostly barren quartz veins. Temperatures during albitization were higher than those during pervasive greisenization; 

mean filling temperatures of fluid inclusions are 412 and 

359 °C, respectively. During the first three stages, two 

aqueous phases were present. Fluid inclusions in wolframite 

are characterized by two immiscible fluids, one rich in H20 

and the other nearly pure C02 • The mean filling temperature of H20-rich fluid inclusions in the quartz-wolframite 

veins is 286 °C. Fluid-inclusion salinities at both deposits 

are generally less than 11 weight percent NaCl equivalent. 

The Baid al Jimalah and Silsilah deposits share 

numerous characteristics with Climax-type molybdenum 

deposits. Silsilah has aplitic caps and the multiple intrusions 

typical of Climax-type deposits, but the intrusions are 

spread out and are not stacked vertically like those at the 

Climax and Henderson deposits, where overlapping mineralized zones form minable ore. Although Baid al Jimalah 

has only one cycle of mineralization and lacks the multiple 

intrusions of Climax-type deposits, the mineral assemblages 

and sequence of mineralization are almost identical. Only 

the relative intensities of the processes that caused the 

various periods of mineralization are reversed. 

As of 1990, these deposits are economically marginal. Nevertheless, they do show that many of the processes needed to form an economically viable tin, tungsten, 

or molybdenum deposit occurred in the Late Proterozoic of 

the Arabian Shield and that there is significant potential for 

larger, higher grade deposits in the region. 

The Mogollon Mining District, Southwestern 

New Mexico-Classic Epithermal Silver-Gold 

Vein Deposits Revisited 

By Robert J. Kamilli 

The Mogollon mining district in southwestern New 

Mexico comprises a number of Ag-Au-Cu epithermal veins 

of early to middle Miocene age. The district is in and near 

a north-northeast-trending segment of the structural wall of 

the late Oligocene Bursum caldera known as the Queen 

fault zone. This fault zone, which itself is mineralized, 

divides the district into two parts. Most of the past production has,come from west-northwest-trending veins west of 

the Queen fault zone, in an area approximately 3 x 1.5 km. 

44 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Veins within the downthrown block east of the Queen fault 

zone lack recorded production. Wall rocks of the productive 

parts of veins are andesitic lava flows intruded by dikes and 

shallow, domal intrusions of rhyolite. Rhyolitic intrusion 

breccias are common in structures that later served as sites 

for vein mineralization. 

The district is remarkable for the widespread distribution of hydrothermal breccias that are located in fault 

zones and throughout the country rock. The breccias range 

between two end-member types. One type consists of 

intrusive hydrothermal pebble dikes that contain clasts of 

wall rock, exotic rock types, and vein material; the matrix 

typically is quartz rich and contains much comminuted wall 

rock. The second type of breccia comprises zones in which 

angular wall-rock fragments have been hydraulically shattered, with little or no translation or rotation; the matrix is 

composed of quartz, calcite, and chlorite. 

The hypogene paragenesis in the district is complex 

because of repeated episodes of hydrothermal brecciation, 

mineralization, and faulting; from oldest to youngest, the 

events were approximately as follows: (1) hydrothermal 

brecciation and injection of fluids along zones of weakness 

with concomitant deposition of minerals that form the 

breccia matrix; (2) deposition of finely banded, crustified, 

fine-grained quartz with chlorite, adularia, pyrite, electrum, 

and silver sulfides (principally argentite); (3) deposition of 

coarse quartz and white, lamellar calcite intergrowths; (4) 

partial to complete dissolution of calcite and concomitant 

replacement by quartz (commonly amethystine); (5) deposition of fluorite; (6) deposition of quartz, pyrite, chalcopyrite, and bornite; and (7) deposition of coarse, gray to 

black calcite. Events 1-5 occurred at least twice, and it is 

probable that this sequence of events (or parts of it) 

occurred many times in different parts of the hydrothermal 

system(s). 

Certain district-wide, mineral zoning patterns are 

apparent, although the overall character of mineralization is 

similar throughout the district. Copper-bearing mineralization is more common in the north and west parts of the 

district, and the ratio of calcite to quartz increases with 

elevation at any given location. 

The Mogollon district is currently being explored, 

and the probability is high for the discovery of additional 

silver-gold ore bodies with moderate grade and tonnage. 

Similar potential exists in the analogous Steeple Rock 

district to the southwest, where erosion was less extensive 

and a hot spring zone has been preserved above the veins. 

The presence of a complex, trellislike network of faults, 

abundant hydrothermal breccias, and copper mineralization 

at Mogollon suggests strongly that a porphyry-copper system is present at depth, although the age relations of 

mineralization and exposed igneous rocks need further 

study. 

Mineral-Resource Potential of the Sitka 1 o x 3° 

Quadrangle, Southeastern Alaska 

By S.M. Karl, R.j. Goldfarb, K.D. Kelley, D.M. 

Sutphin, C.A. Finn, A.B. Ford, and D.A. Brew 

The Sitka 1 o x 3° quadrangle encompasses roughly 

11,000 km2 in southeastern Alaska and contains sedimentary, volcanic, and plutonic rocks derived from Paleozoic 

and Mesozoic arc settings. Paleozoic rocks are assigned to 

the Alexander and Wrangellia terranes; three different 

Mesozoic arc assemblages are assigned to the Wrangellia, 

Alexander, and Chugach terranes, respectively. 

In the Sitka quadrangle, the Chichagof mining district, which has a history of activity that dates to the 1800's, 

was a leading producer of gold in Alaska until 1945. The 

opening of the Greens Creek mine, only a few kilometers 

north of the quadrangle, has renewed interest in the baseand precious-metal potential of the region. 

Nine tracts have been identified as having potential 

for certain types of mineral deposits because of the presence 

of known deposits, specific lithologic types, and geochemical and geophysical anomalies: 

(1) mafic-ultramafic synorogenic-synvolcanic Ni-Cu deposits in Tertiary norites on western Chichagof Island 

and eastern Yakobi Island; 

(2) polymetallic veins and porphyry Mo-Cu deposits in 

Jurassic to Tertiary intermediate to felsic plutons on 

Chichagof and Baranof Islands; 

(3) low-sulfide gold-quartz veins within major fault zones 

on western Chichagof Island and Baranof Island and 

along Lisianski Inlet-Hoonah Sound; 

( 4) U, Th, and rare-earth elements in Paleozoic alkalic 

plutons on eastern Chichagof Island; 

(5) marine evaporite gypsum and hydrothermal gypsum in 

Mississippian limestone on eastern Chichagof Island; 

(6) volcanogenic massive sulfide deposits in Permian and 

Triassic volcanic and sedimentary rocks on central and 

eastern Admiralty Island; 

(7) epithermal vein and hot-spring Au and Ag deposits in 

Tertiary volcanic rocks on southern Admiralty Island; 

(8) skarn, replacement, and polymetallic vein deposits in 

Paleozoic carbonate rocks intruded by late orogenic 

plutons on eastern Chichagof Island and Admiralty 

Island; and 

(9) coal and sandstone U deposits in Tertiary nonmarine 

clastic rocks on southern Admiralty Island. 

In addition, scattered low-temperature geothermal systems 

are active on Baranof and Chichagof Islands. 

The greatest resource potential for the area lies in 

tracts 3 (gold-quartz veins) and 6 (volcanogenic massive 

sulfide deposits). The gold-quartz veins, identified in 59 

deposits in the western Chichagof gold belt, have yielded 

800,000 oz of gold, mostly from the Chichagof and 

Hirst-Chichagof deposits. These deposits consist of ribbon 

Karl and others 45 
quartz filling northwest-trending shears. Gold is accompanied by minor amounts of pyrite, arsenopyrite, galena, 

sphalerite, pyrrhotite, and chalcopyrite. Fluid-inclusion 

analyses indicate that ore-forming fluids were low-salinity, 

aqueous-dominant solutions having trapping temperatures 

between 190 and 238 oc and trapping pressures in excess of 

500 bars; these characteristics suggest a metamorphic origin. The 8180 values of quartz are 15.7-16.0 per mil, 

which are typical of mesothermal hydrothermal systems. 

The 8D values of fluid inclusions are equivocal; they range 

from -55 to -110 per mil. 

The main volcanogenic massive sulfide (VMS) 

deposits are Pyrola and several deposits near Pybus and 

Gambier Bays. They occur within the Retreat Group in a 

setting similar to that of the Greens Creek and other Triassic 

VMS deposits in a 350-km-long belt in southeastern 

Alaska. At Pyrola, massive basalt with chlorite-carbonate 

alteration overprinted by sericite-quartz-pyrite alteration is 

in contact with fine-grained clastic and chemical sedimentary rocks. At this contact, pods of massive pyrite, sphalerite, and Ag-rich barite contain minor associated galena, 

chalcopyrite, jamesonite, and boulangerite. The deposit is 

underlain by a siliceous stockwork containing auriferous 

pyrite, and remobilized mineralization is present in later 

quartz veins. 

Mineral-Resource Assessment of the Goodnews 

1 o x 3° Quadrangle and Parts of the Hagemeister Island and Nushagak Bay Quadrangles, 

Southwestern Alaska 

By j.E. Kilburn, S.E. Box, R.j. Goldfarb, j.E. Gray, 

and j.L. jones 

Mineral resources were assessed in the Goodnews 1 o 

X 3° quadrangle and in parts of the Hagemeister Island and 

Nushagak Bay quadrangles, which are north of Bristol Bay 

in southwestern Alaska. The region is underlain largely by 

volcanic and sedimentary rocks of the. northeast-trending 

Togiak and Goodnews tectonostratigraphic terranes. Jurassic ultramafic rocks and associated gabbros are most common in the west near Goodnews Bay and Island Mountain. 

Late Cretaceous· to early Tertiary granitic stocks are widespread, though they are more prevalent in the central and 

eastern parts of the study area. The ultramafic rocks south 

of Goodnews Bay have yielded high-grade platinum placers 

in the Salmon River and its tributaries. A few minor gold 

placers, an isolated sphalerite-rich vein, and an epithermal 

cinnabar lode are the only other documented mineral 

occurrences in the study area. 

During the mineral assessment of the region in 

1975-77, stream-sediment and heavy-mineral-concentrate 

samples were collected and analyzed. Geochemical data 

from these media delineate areas characterized by large 

elemental anomalies. The samples are enriched in Ag, As, 

Au, Bi, Cr, Cu, Mo, Pb, Sn, W, or Zn (Cieutat and others, 

1988). 

During the summer of 1990, additional rock samples 

were collected from several of the areas having geochemical 

anomalies. In all the revisited drainage basins, strongly 

oxidized Cretaceous-Tertiary stocks of intermediate composition and associated hornfels zones were observed. 

During the followup study, geochemical anomalies were 

identified in the following areas: 

(1) the Mt. Waskey-Rainbow basin region-samples of 

quartz-arsenopyrite-pyrite veins and stockworks in 

granite and silicified argillite collectively contain max-

. imum concentrations of 1.5 ppm Ag, > 10,000 ppm As, 

1.4 ppm Au, 1,000 ppm Bi, 2,000 ppm Cu, 15 percent 

Fe, 50 ppm Mo, 200 ppm Sb, 20 ppm Sn, and 500 ppm 

Zn; 

(2) a drainage basin south of Atshichlut Mountain-samples of silicified, bleached, and Fe-stained argillite with 

minor pyrite veinlets collectively contain maximum 

concentrations of 100 ppm Ag, 0.40 ppm Au, >5,000 

ppm Ba, 200 ppm Cu, 10 percent Fe, 1.8 ppm Hg, 50 

ppm Mo, 300 ppm Pb, and 1,000 ppm Zn; 

(3) the Trail Creek-Mt. Oratia area-samples of silicified 

quartz vein breccia, Fe-stained hornfels containing 

quartz-tourmaline stockworks, and silicified graywacke 

with minor quartz veins contain maximum concentrations of 0. 7 ppm Ag, 2,000 ppm As, 0.15 ppm Au, 

>2,000 ppmB, >5,000 ppmMn, and 150 ppm Sb; and 

(4) the west bank of the Togiak River-samples from an 

oxidized sphalerite-rich vein hosted in pillow basalt 

contain maximum concentrations of 5 ppm Ag, 0.35 

ppm Au, 500 ppm Cd, 700 ppm Cu, 5 percent Fe, 10.4 

ppm Hg, >5,000 ppm Mn, and >10,000 ppm Zn. 

REFERENCE CITED 

Cieutat, B.A., Goldfarb, R.J., and Speckman, W.S., I988, Analytical 

results and sample locality map of stream-sediment, heavy-mineralconcentrate, and organic material samples from the Goodnews, 

Hagemeister Island, and Nushagak Bay quadrangles, southwest 

Alaska: U.S. Geological Survey Open-File Report 88-59I , I88 p., I 

oversize sheet, scale I :250,000. 

Large Sulfide-Sulfate Mounds, Hydrothermal 

Fluids, and Altered Sediment in Escanaba 

Trough, Southern Gorda Ridge-Elements of a 

Mature Hydrothermal System at a SedimentCovered Spreading Axis 

By Randolph A. Koski, janet L. Morton, Robert A. 

Zierenberg, Wayne C. Shanks Ill, Andrew C. 

Campbell, and Keith A. Kvenvolden 

Large sulfide-sulfate mounds (lengths measured in 

hundreds of meters and widths and heights in tens of 

46 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
meters) and hydrothermal vents are the surface manifestations of past and present hydrothermal activity in the 

sediment-covered axial valley of Escanaba Trough, southem Gorda Ridge. The slow-spreading (2.4 cm/yr) Escanaba 

Trough has a morphology like that of the Mid-Atlantic 

Ridge; a deep (3,300 m) and wide (5 to 15 km) axial valley 

is flanked by ridges that rise 1 ,000-1 ,500 m above the 

valley floor. The southern two-thirds of the Escanaba 

Trough is covered by 300-700 m of Quaternary sediment, 

including turbidites and hemipelagic muds derived from the 

continental margin of Washington, Oregon, and California. 

Seismic profiles reveal highly reflective horizons 2-10 m 

apart that probably represent coarse-grained intervals in the 

mud-rich uppermost sediment section. Within the axial 

valley, a series of volcanic edifices spaced 10-15 km apart 

penetrate the sediment cover; sill-like bodies of basaltic 

magmas have intruded and dilated the sediment and have 

resulted in uplift of 1-km-wide sediment hills (fig. 1). 

Where the edifices breach the sediment pile, pillow lavas 

and sheet flows are locally extruded onto the sea floor. 

Hydrothermal deposits are concentrated along the 

perimeters of several sediment-covered hills where bounding faults provide cross-stratal permeability (fig. 1). Sand 

layers between relatively impermeable silt and mud layers 

in the sediment pile also provide a favorable milieu for 

lateral fluid flow and subsurface mineralization. The rapid 

alteration of terrigenous organic matter in the sediment has 

resulted in the formation of asphaltic petroleum, that is, an 

admixture of products formed over a wide range of thermal 

regimes. The insulating effect of the sediment blanket has 

produced relatively long lived (hundreds to thousands of 

years) geothermal systems along the ridge axis and relatively large and mature hydrothermal deposits. 

The mineral deposits in the Escanaba Trough have 

been sampled by dredge, gravity core, and submersible; 

they include large pyrrhotite-rich mounds, polymetallic 

(Zn-Pb-Cu-As-Sb-Ag) sulfide structures, and barite chimneys. Sulfate (barite+ anhydrite) sinters are forming around 

active vents (maximum measured fluid temperature = 220 

°C). Compared to massive sulfide deposits formed at 

sediment-starved ocean ridges, sulfide samples from the 

Escanaba Trough sites are enriched in As (to 2.8 percent), 

Pb (to 14 percent), Ag (to 680 ppm), Bi (to 820 ppm), Sb 

(to 8,700 ppm), and Sn (to 1,500 ppm). Au occurs in 

amounts up to 10.1 ppm in Cu-rich samples. Pyrrhotite is 

the predominant mineral phase, but isocubanite, sphalerite, 

marcasite, and galena are abundant in some samples. Minor 

phases include arsenopyrite, lOllingite, stannite, boulangerite, native bismuth, and alabandite. Sulfur-isotope values 

for sulfides range from -0.7 to + 11.6 per mil; the lighter 

values indicate the presence of bacteriogenic sulfide 

whereas heavier values may reflect sulfide derived from 

reduction of seawater sulfate in the sediment pile. Compositional characteristics of the hydrothermal fluids (nearneutral pH; high alkalinity; high concentrations ofNH;, Li, 

C_ 

0 1km 

D mudandsilt sand • sulfide I :: ::I basalt 

I Y I fluid pathways [2J faults 

Figure 1 (Koski and others). Schematic cross section 

showing the relations among a sediment-covered volcanic 

edifice, an uplifted sediment hill, hydrothermal circulation, and location of sulfide deposits in Escanaba Trough. 

Faults bounding the sediment hill provide cross-stratal 

permeability for upwelling hydrothermal fluids and 

focused flow for deposition of sea-floor sulfide-sulfate 

mounds. Sand layers in the sediment pile provide favorable permeability and porosity for lateral fluid flow and 

subsurface sulfide deposition. 

Rb, Cs, B, I, and Tl; high 87Sr/86Sr ratios; low concentrations of H2S, Fe, Mn, Cu, Zn, Cd, and Ag; low values for 

o11B) are consistent with extensive interaction between 

hydrothermal fluid and terrigenous sediment that contains a 

small amount of organic matter. 

Sediment samples recovered in gravity cores are 

characterized by Mg metasomatism and the formation of 

smectite and chlorite. The alteration of fine-grained sediment to nearly monomineralic aggregates of clinochlore 

(Fe/(Fe+ Mg) = 0.21) was accompanied by an increase in 

Mg from 4 to 25 weight percent and the nearly complete 

removal of Na, K, Rb, Ca, and Sr. Oxygen-isotope fractionation temperatures calculated for smectite (181-235 °C) 

and chlorite (203-223 °C) are consistent with measured 

temperatures of vent fluids. The alteration of surficial 

sediment at temperatures near 200 °C took place during the 

mixing of upwelling hydrothermal fluid with shallowly 

circulating seawater. 

Natural Aggregates-Mining Challenge of the 

1990's 

By William H. Langer 

Crushed stone and sand and gravel are the two main 

sources of natural aggregates. These essential construction 

materials frequently are used interchangeably with one 

another throughout the United States. U.S. aggregate proLanger 47 
duction in 1989 had a value of approximately $8.5 billion 

(U.S. Bureau of Mines, 1990); thus, aggregates are among 

the leading nonfuel mineral commodities in the United 

States. Their value by far exceeds those of iron ore ($1. 7 

billion), copper ($4.4 billion), or the precious metals, 

including gold, silver, and the platinum-group metals (combined production in 1989 worth $4.3 billion). 

In the United States, natural aggregates are not 

universally available for use. In addition to the highly 

visible land-use and zoning conflicts, the aggregate industry 

is faced with a variety of source and utilization problems. 

Some areas are devoid of sand and gravel, and potential 

sources of crushed stone may be covered with sufficient 

unconsolidated material to make surface mining impractical. In other areas, aggregates do not meet the physicalproperty requirements for certain uses, or they contain 

mineral constituents that react adversely when used in 

concrete. Proper long-range planning based on an understanding of aggregate availability and physical properties 

can help assure adequate supplies. In order to obtain an 

overview of the availability of natural aggregates, two 

1:5,000,000-scale maps (Langer, 1988) were prepared for 

the conterminous United States: (1) a map showing the 

major potential sources of sand and gravel and (2) a map 

showing the distribution of selected types of bedrock 

normally considered for use as crushed stone. 

Natural aggregates are distributed throughout the 

United States in a variety of geologic environments. The 

map showing potential sources of sand and gravel is divided 

into geographic regions based, in general, on geologic 

history and physiography. The mode of distribution of 

sand and gravel is similar within a region and is fairly 

distinct from that of other regions. Within each of these 

regions, major areas of sand and gravel resources are 

shown. 

Crushed-stone aggregates are derived from a wide 

variety of parent bedrock materials. Limestone and other 

carbonate rocks account for approximately three-quarters of 

the rocks used for crushed stone; granite and other igneous 

rocks make up most of the remainder. Limestone deposits 

are widespread throughout the Central and Eastern United 

States and are scattered in the West. Granites are widely 

distributed in the Eastern and Western United States and in 

the Lake Superior region; they are exposed in only a few 

other places in the midcontinent. Igneous rocks (excluding 

granites) are largely concentrated in the Western United 

States and in a few isolated localities in the East. 

Aggregates are in short supply or of poor quality in 

approximately one-third of the conterminous United States. 

In the High Plains, aggregate is largely restricted to river 

channels and terraces and commonly lacks coarse fractions. 

Large areas of glaciolacustrine deposits in Michigan and 

North and South Dakota lack significant deposits of sand 

and gravel. The seaward sections of the Coastal Plain are 

primarily composed of sand that commonly lacks coarse 

fractions. Much of the Mississippi River alluvial plain and 

loessial upland lack significant deposits of sand and gravel. 

In some areas, aggregates contain mineral constituents that 

limit their use. When some aggregates are used in concrete, 

alkali-silica reactions or alkali-carbonate reactions cause the 

concrete to break up. When other aggregates are used in 

bituminous mixes, reactions between minerals and water 

may cause the bituminous film to be stripped. 

Identification of areas having adequate sources of 

aggregate represents only the first step in commercial 

exploitation. In areas having limited aggregate sources, 

geophysical or other remote-sensing techniques can be used 

to help locate deposits. Geochemical analyses and geologic 

mapping can identify areas where deleterious constituents 

are absent from the aggregate source. Research can be 

conducted on processing techniques to improve aggregate 

quality and on methods to improve concrete and bituminous 

products. Zoning, regulations, and competing land uses 

further restrict or preclude mining. The problems associated 

with aggregate availability, aggregate quality, and socioeconomic restrictions present a challenging environment for 

the development of new techniques and methodologies. 

REFERENCES CITED 

Langer, W.H., 1988, Natural aggregates of the conterminous United 

States: U.S. Geological Survey Bulletin 1594, 33 p., 2 pls. 

U.S. Bureau of Mines, 1990, Mineral commodity summaries, 1990: 

199 p. 

Heavy-Mineral Placers at the Fall Zone 

By Curtis E. larsen 

Placers are concentrations of heavy minerals sorted 

by density and shape through fluvial, littoral, and eolian 

processes. The specific gravity and hydraulic equivalence 

of a mineral relative to quartz determine the type of 

sedimentary deposit in which it is found; for example, a 

grain of gold or a titanium mineral, such as ilmenite or 

rutile, in the very fine sand range (0.06-0.12 mm) tends to 

concentrate in deposits having dissimilar grain sizes. Gold 

of this size occurs with very coarse quartz sand (1.0-2.0 

mm) or gravel. Very fine grained titanium minerals, as well 

as zircon and monazite, typically are deposited with fine 

quartz sand (0. 13-0.25 mm). 

In a river system, the size of the sediment it carries 

and subsequently deposits depends on its discharge, slope, 

depth, and velocity. Sediments with limiting grain sizes that 

host placers are deposited in predictable reaches of the 

system where slope and velocity decrease. In the Southeastern United States, rivers draining the eastern Appalachian 

Mountains commonly decrease in gradient and velocity 

until they reach the outer edge of the Piedmont Lowlands. 

48 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Here, gradients typically steepen abruptly at structural 

boundaries and then flatten at the Fall Zone before the rivers 

enter the Coastal Plain. The abrupt change in river dynamics preferentially deposits coarse-grained sediments near the 

Fall Zone and medium and fine sand at the edge of the 

Coastal Plain. Depending on the mineralogy of upstream 

source terranes, the Fall Zone may have potential for placer 

gold, and the inner edge of the Coastal Plain may have 

potential for monazite, zircon, rutile, and ilmenite. 

The effect of stream gradient changes at the Fall Zone 

has geologic longevity; as an example, placer gold is found 

in the basal Triassic gravels of the North Carolina Mesozoic 

basins in settings downstream from the Carolina slate belt. 

Similarly, small amounts of placer gold are reported from 

the basal units of Cretaceous and Cenozoic fluvial deposits 

near the Fall Zone and at the innermost edge of the Coastal 

Plain. Upper Cretaceous fluvial sediments near the Fall 

Zone in Georgia and South Carolina contain small concentrations of monazite, zircon, rutile, and ilmenite. Finer 

grained Cretaceous upper delta and delta facies deposited on 

the basement tend to concentrate less dense, but hydraulically equivalent, heavy-mineral suites characterized by 

staurolite, kyanite, amphiboles, pyroxenes, epidote, and 

rutile. Similar gradation of heavy minerals is present within 

the Cenozoic fluvial facies of the Coastal Plain. 

Although a qualitative relation between certain 

heavy-mineral suites and fluvial deposits of the inner 

Coastal Plain has been known for some time, the role of 

stream gradient has not been critically examined. Detailed 

geomorphologic and stratigraphic research in the Fall Zone 

region will allow more effective predictive models of placer 

mineral occurrence to be developed. Drainage-basin analysis utilizing stream discharge, gradient, channel width, and 

depth can be linked with geologic studies of source terranes 

to target river reaches that have sediment sizes conducive to 

concentration of specific heavy-mineral suites. Such studies 

will allow the extensive data base of streamflow characteristics in the Southeastern States to be combined with 

economic geologic knowledge so that heavy-mineral prospecting techniques in the Fall Zone region can be refined. 

Recognition of Distinct Terranes in Lower 

Paleozoic Host Rocks in the Getchell Gold 

Trend, Humboldt County, Nevada 

By Dawn j. Madden-McGuire and Sherman P. 

Marsh 

The lower Paleozoic rocks that host gold deposits 

along the Getchell gold trend in Humboldt County, Nev., 

represent several allochthonous blocks (fig. 1) rather than a 

simple sequence of continuous deposition. We are calling 

the allochthons "terranes," a term used in a descriptive 

sense. Evidence for allochthonous terranes in this area 

117° 30' 117°00' 

EXPLANATION 

• Terrane3 

• Terrane2 

~ Terrane1 

IJIIl Co~~::.~~. 41° typeloca~ty 

00' 

10km 

EAST 

Schematic cross section, showing the three terranes of 

lower Paleozoic rocks in the Getchell trend. 

Figure 1 (Madden-McGuire and Marsh). Simplified geologic map of the Osgood Mountains, Edna Mountain, and 

surrounding areas, showing outcrops of rocks that definitely fit into terrane 1 (Preble Formation and Osgood 

Mountain Quartzite), terrane 2 (Vinini and Valmy Formations), and terrane 3 (Comus Formation in its type locality) 

and rocks currently mapped as the Comus Formation in 

the Osgood Mountains, outside the type locality. Abbreviations: £, Upper Cambrian sedimentary rocks; Pv 

upper Paleozoic rocks; K, Late Cretaceous granodiorite; 

T, Tertiary volcanic rocks. 

includes fault boundaries and differences in age, rock type, 

and structural style among several rock packages. 

The two most widespread and distinct terranes in the 

area are ( 1) intensely deformed and uniformly regionally 

metamorphosed marine rocks (Lower Cambrian Osgood 

Mountain Quartzite, Lower Cambrian to Lower Ordovician 

Preble Formation, and some of what is currently mapped as 

the Comus Formation) and (2) generally less deformed 

chert, clastic sedimentary rocks, and volcanic rocks (Ordovician Valmy and Vinini Formations). Bedding and foliation in terrane 1 dip predominantly eastward and folds verge 

westward, whereas folds in terrane 2 verge southeastward. 

Lower Paleozoic rocks in this area and their contact 

relations are regionally significant. Exposures of terranes 1 

and 2 might preserve evidence for Paleozoic metamorphism 

and deformation. Terrane 1 is intensely folded and regionMadden-McGuire and Marsh 49 
ally metamorphosed at map and outcrop scales. The same 

rock type, structural style, and predominance of westverging folds (and east-dipping foliation) are consistently 

displayed along the strike of terrane 1 for 60 km from 

Anderson Canyon (on the northwestern flank of the Osgood 

Mountains) southward past Hot Springs Ranch (on the east 

side of the Sonoma Range). Regional deformation and 

metamorphism of terrane 1 might have been post-Early 

Ordovician and pre-Middle Pennsylvanian, because Lower 

Ordovician strata occur in terrane 1 , and the conglomerate 

subunit of the Middle Pennsylvanian Highway Limestone at 

Edna Mountain contains clasts of phyllite and reddish 

limestone of the Preble Formation. The Upper Permian 

Edna Mountain Formation contains lithic fragments thought 

to be Preble phyllite (terrane 1) and overlies the Preble 

Formation along a contact mapped as depositional at Edna 

Mountain; these features further suggest that deformation 

and metamorphism of terrane 1 occurred in Paleozoic time. 

In the northern Osgood Mountains, outcrops of terrane 2 are 

overlain depositionally by the Middle Pennsylvanian to 

Upper Pennsylvanian or Lower Permian Etchart Limestone. 

This contact suggests that terrane 2 was emplaced before 

Middle Pennsylvanian time. The age of the low-angle 

structure separating terranes is uncertain; the limited geologic evidence available suggests that it is Paleozoic, but no 

Mesozoic sedimentary rocks are preserved to overlie the 

structure and confirm a Paleozoic age. 

The Comus Formation, which is Middle Ordovician 

in age in its type locality at Edna Mountain (south of Iron 

Point), might represent a third terrane (fig. 1) situated 

structurally between terranes 1 and 2. Use of the name 

"Comus Formation" outside the type locality (particularly in 

the Osgood Mountains) has created confusion and needs 

reevaluation. Some of the rocks currently mapped as Comus 

Formation belong to the other terranes. 

Areas of Mineral-Resource Favorability (with 

Emphasis on Gold and Chromite) in the 

Anchorage 1° x 3° Quadrangle, 

Southern Alaska 

By D.j. Madden-McGuire and G.R. Winkler 

The U.S. Geological Survey conducted geologic, 

geochemical, and geophysical investigations to determine 

areas of mineral-resource favorability in the Anchorage 

1 ox 3° quadrangle. The quadrangle contains three major 

fault-bounded terranes that were accreted between midCretaceous and early Tertiary time. The Peninsular terrane, 

bounded on the south by the Border Ranges fault system, 

comprises intrusive and extrusive phases of an early Mesozoic intraoceanic magmatic arc and late Mesozoic marine 

sedimentary rocks deposited in forearc basins. The Peninsular terrane is intruded by voluminous composite plutons 

of Late Cretaceous and Paleocene age. Paleocene and 

younger nonmarine volcanic and coal-bearing sedimentary 

rocks overlap both the Peninsular terrane and the Chugach 

terrane to the south. The Chugach terrane consists of 

Mesozoic polygenetic melange and accreted wedges of 

Upper Cretaceous flysch and minor metatuff. South of the 

Contact fault, the Prince William terrane consists of an 

accreted Paleogene deep-sea fan complex interbedded with 

minor tholeiitic basalt. 

Gold, which is currently the commodity of greatest 

economic interest, has been produced from gold-quartz 

veins and related placer deposits in the Peninsular and 

Chugach terranes, primarily in the Willow Creek and 

Girdwood mining districts (fig. 1). The two areas that 

include the historic mining districts are assigned the highest 

and most certain levels of potential for gold-quartz veins 

and gold-placer deposits in the quadrangle. 

In the Willow Creek district, high-sulfide goldquartz-carbonate veins cut a pluton dated at 79-72 Ma 

(recording multiple stages of alteration) and adjacent pelitic 

schist. A fault contact between these host rocks might have 

served as a conduit for mineralizing fluids (mineralization 

occurred at 66 Ma and at 57-55 Ma). The extent of the 

high-potential area is defined by mineral deposits, geologic 

contacts, and panned concentrates that contain gold and are 

enriched in As, Ag, Sb, and Pb; stream sediments have 

anomalous As contents. 

In the Girdwood district, Au and Ag are concentrated 

in quartz-carbonate veins within joints and shears in 

greenschist-facies metasedimentary rocks of Cretaceous age 

and Eocene quartz diorite (53 Ma). Mineralization is 

younger than 53 Ma. Elsewhere in the Valdez Group (in 

adjacent quadrangles), gold-quartz veins are younger than 

34 Ma. Panned concentrates from Crow Creek and surrounding drainages contain gold and locally are enriched in 

Ag, As, and Sb. 

In Prince William Sound, areas around Harriman 

Fiord and the Coghill River have been assigned a moderate 

resource potential because gold-quartz veins are exposed in 

a mine, in prospects, and in outcrop. Additional areas of 

moderate potential for gold-quartz veins in the Chugach 

terrane have been identified on the basis of the presence of 

gold in panned concentrates and, locally, in mineralized 

outcrops. A particularly large area of moderate potential for 

gold-quartz veins is between Mount Marcus Baker and the 

Border Ranges fault. 

Podiform chromite occurs in the Peninsular terrane 

within two fault-bounded ultramafic complexes near 

Eklutna and in the upper part of Wolverine Creek on the 

north side of the Border Ranges fault (fig. 1). Cumulate 

chromitite, having a moderate potential for Cr20 3 , occurs 

within deformed cumulate dunite and wehrlite. Adjacent 

outcrops of ultramafic rocks have a moderate potential for 

Cr20 3 but a lower level of certainty. Chromitite has not 

been observed in the serpentinized ultramafic rocks that 

50 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
0 10 Ml 

OI-l --1 ,...b -KM...J' 

Willow 

Creek 

us'!<.f>-

w.r>-'r>-~~ WolverinV 

Palmer • com 1 

Gold mining 

districts 

EXPLANATION 

Gold 

.High 

Potential 

Harriman Fiord 

Chromite 

@jill Moderate 

0 

"U 

m 

z 

z 

(/) 

c 

r 

> 

JJ 

--i 

m 

::D 

::D 

)> 

z 

• Moderate 

I> I Low 

fm Low ... 

Figure 1 (Madden-McGuire and Winkler). Areas of favorability for gold-quartz veins and podiform chromite in the 

Anchorage 1° x 3° quadrangle, southern Alaska. Wolverine and Eklutna Complexes of Burns (1985). 

occur within pelitic schist in the Willow Creek district, and 

nearby stream sediments are not enriched in chromium; the 

area of exposed serpentinite is therefore assigned a low 

potential for Cr20 3 . 

The Peninsular terrane includes areas that have low 

potential for porphyry-copper deposits, copper-silver 

skams, volcanic-rock-hosted stratiform and stratabound 

mineralization, and uranium mineralization associated with 

granitic rocks. Coal has been mined from the Chickaloon 

and Tyonek Formations north of the Border Ranges fault . 

The Chugach and Prince William terranes include areas of 

moderate and low potential for base-metal veins, massive 

sulfide deposits, and uranium mineralization. A moderate 

potential for base metals has been assigned to a prospected 

area in the southeastern corner of the quadrangle between 

Kadin and Miners Lakes, where veins and massive sulfide 

occurrences in the Prince William terrane are locally 

enriched in Zn, Cu, Pb, and Ag, and panned concentrates 

contain chalcopyrite and sphalerite. 

REFERENCE CITED 

Burns, L.E., 1985 , The Border Ranges mafic and ultramafic complex, 

south-central Alaska-Cumulate fractionates of island-arc volcanics: 

Canadian Journal of Earth Sciences, v. 22, no. 7, p. 1020-1038. 

Madden-McGuire and Winkler 51 
Mineral-Resource Assessment of the Altiplano 

and Cordillera Occidental, Bolivia-A Progress 

Report 

By Sherman P. Marsh, Dan H. Knepper, Jr., Keith 

R. Long, and John W. Cady 

In early 1990, the U.S. Geological Survey (USGS) 

and the Geological Survey of Bolivia (GEOBOL) initiated a 

Trade Development Program (TDP), which is funding a 

collaborative mineral-resource assessment of the 150,000-

km2 Altiplano and Cordillera Occidental, Bolivia. This area 

of Bolivia ranges between 3,500 and 6,000 min elevation 

and consists primarily of Tertiary volcaniclastic sedimentary rocks and volcanic rocks. 

The goal of the joint project is to produce a mineralresource assessment of the Altiplano and Cordillera Occidental by using innovative digital techniques. Geologic 

maps are vital to the assessment. Geologic data were 

obtained from paper maps compiled by GEOBOL geologists at 1:250,000 scale; they were digitized and converted 

into ARC/INFO files. The ARC/INFO geographic information system (a product of Environmental Systems Research 

Institute (ESRI), Inc., Redlands, Calif.) was used to produce colored digital geologic maps at 1:250,000 scale and a 

regional colored geologic map at 1:500,000 scale. 

Data on mineral deposits, mines, alteration zones, 

salars, and other features related to mineralization were 

compiled from hundreds of unpublished Bolivian reports, 

from observations made during field investigations, and 

from the published literature. The area has a long history 

of mostly small-scale mining activity. Deposit locations 

were digitized from large-scale topographic maps by using 

the USGS GSDIG software. USGS GSMAP software was 

used to generate a map showing the locations of mineral 

deposits. 

Thirteen Landsat Thematic Mapper (TM) scenes have 

been digitally processed to assist in site identification for 

field examination and in preparing a map of the entire 

Altiplano showing areas of possible hydrothermal alteration. Images prepared at a scale of 1:250,000 include (1) a 

simulated natural color (SNC) composite image that shows 

the terrain much as it would appear on color air photographs 

and (2) a color infrared (CIR) composite image that depicts 

the sparse vegetation cover in shades of red and enhances 

the expression of geologic structure and geomorphic features. The SNC and CIR images are digital contrast 

stretched for color display and are enhanced by a nondirectional edge technique to sharpen the display of the highfrequency components of the image data. These images are 

useful as index maps of the region and for general geologic 

and structural mapping. 

Ratio images have been prepared by dividing the data 

numbers (DN) of each picture element (pixel) in one band 

of data by the DN of the corresponding pixels of another 

band of data. TM ratio images provide information about 

the mineralogic composition of exposed surface materials. 

A color-ratio composite (CRC) image shows the distribution of two classes of minerals: (1) iron oxides and 

hydroxides and (2) minerals that contain hydroxyls and 

carbonates. Both groups include minerals formed during 

hydrothermal alteration or derived from the weathering of 

hydrothermally altered rocks. Because vegetation produces 

a spectral response similar to those of the clays, hydrated 

sulfates, and carbonates, a version of the CRC image has 

been prepared with vegetation, water, shadows, clouds, and 

snow masked from the CRC image. This masked color-ratio 

composite image is the fundamental image used for targeting areas of possible hydrothermally altered rocks in the 

Altiplano. 

Aeromagnetic data, which were compiled from existing surveys and from a GEOBOL commercial survey, have 

been processed and analyzed. The resulting low-level, 

2-km-line-span data are available for over 60 percent of the 

Altiplano and Cordillera Occidental. 

Mercury-Sulfur-Gypsum Mineralization at 

Crater, California-A Suggestion of Gold 

Mineralization at Depth 

By Sherman P. Marsh, Chester T. Wrucke, and 

Kevin P. Corbett 

A large deposit of native sulfur and gypsum at Crater, 

Calif., in the Last Chance Range of the northern Death 

Valley region is the surface expression of a hot spring 

system that has potential for concentrations of gold at depth. 

Hydrothermal alteration characterized by the elemental 

suite of As, Sb, Hg, and Au and nearby concentrations of 

mercury at the El Capitan mine are indicators of possible 

gold mineralization at Crater. 

The sulfur deposit at Crater is mainly in limestone of 

the Carrara Formation of Cambrian age in a structurally 

complex setting in the upper plate of the Last Chance thrust. 

At Crater, upper plate strata are Cambrian carbonate and 

siliciclastic rocks that were emplaced above Mississippian 

phyllitic and coarse clastic rocks, probably in the Late 

Permian or Triassic. Probable renewed offset along 

northeast-trending faults in the Tertiary helped prepare the 

ground for a postulated igneous intrusion in the Pliocene. 

Sulfur-gypsum alteration is exposed intermittently for a 

distance of nearly 2 km along the northeast-trending faults. 

Exposures of jasperoid and quartz breccia show evidence of 

multiple stages of brecciation. Available evidence, including K-Ar ages from a nearby basalt-rhyolite suite, indicates 

that hot spring discharge to the surface may have begun in 

the Pliocene and continued into the Quaternary. 

52 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Native sulfur and gypsum deposits at Crater resulted 

from intense alteration in a vapor-dominated acid sulfate hot 

spring system. This system developed as the postulated 

igneous intrusion rose to shallow crustal levels in a drainage 

basin having a small capacity for ground-water recharge. 

Intensely altered rock high in the system was depleted in 

most metals but retained traces of the gold geochemical 

suite, as evidenced by gold values as high as 1. 7 ppm in a 

small quartz vein near the El Capitan mine. Much of the 

limestone host rock has been altered to silica minerals, 

sulfur, and gypsum. Samples of siliceous sinter give X-ray 

patterns for quartz and cristobalite. Most of the mercury 

was deposited in peripheral areas because of its high 

volatility; the mercury deposit at the El Capitan mine is 3.5 

km north of Crater. Significant concentrations of gold, if 

present, would be at depth in a zone where boiling took 

place in late Tertiary hydrothermal systems. 

Regional Aeromagnetic and Gravity Data Bases 

for Studies Centered on the Idaho Batholith and 

Challis Volcanic Field 

By Anne E. McCafferty and Viki Bankey 

Recently compiled aeromagnetic and gravity data for 

most of Idaho and adjoining parts of Montana, Wyoming, 

Oregon, and Washington (an area bounded by lats 42° and 

47° N. and longs l10° and 118° W.) were used to produce 

a series of maps in support of investigations of the mineralresource potential of National Forest Wilderness Study 

Areas in Idaho. An immediate objective of the project was 

to assemble all available magnetic and gravity data into 

digital composite data sets. The aeromagnetic compilation 

includes a synthesis of digital data acquired from more than 

60 separate surveys flown at different times with varied 

flight elevations, flight spacings, and data-reduction procedures. The complete Bouguer gravity anomaly data were 

compiled using data obtained from 32,152 stations 

extracted from State and Federal Government gravity data 

bases and from data collected as part of university theses 

and dissertations. The initial products include an aeromagnetic anomaly grid at a datum level of 305 m above terrain, 

a complete Bouguer gravity anomaly grid, and an isostatic 

gravity anomaly grid. All are being prepared in color at a 

scale of 1:1,000,000. 

A second objective of the project was to apply 

analytical methods and digital filtering to enhance selected 

attributes of the anomaly fields and provide new interpretive 

information. One recently developed technique, called the 

"terrace method," has been applied to the aeromagnetic and 

isostatic gravity data in an effort to map geologic terranes 

and structures delineated by physical-property boundaries 

as calculated from the anomalous fields. The resulting 

products are a terrace-magnetization map and a terracedensity map. 

The terrace-magnetization map is roughly equivalent 

to a geologic map of magnetic rocks; in places, the edges of 

the magnetization units can be equated with mapped faults 

and other lithologic discontinuities. The magnetization 

domains themselves generally correspond to geologic bodies, such as buried or exposed intrusions or volcanic strata 

that contain enough magnetic minerals to produce discrete 

anomalies. 

The terrace-density map mimics a geologic map by 

theoretically outlining domains having sufficient density 

contrast to produce gravity anomalies. These domains may 

or may not correspond to real geologic formations. The 

terrace method is particularly helpful in delineating lowamplitude features not clearly visible on the original magnetic and gravity maps. 

The third and culminating objective of the project is 

to integrate the aeromagnetic and gravity data with geologic, geochemical, and other geophysical data in order to 

refine interpretations of the structural and lithologic setting 

of the Idaho batholith and Challis volcanic field and to 

assess resource potential. This phase of the project is 

currently in progress. 

Application of Grade and Tonnage Models to the 

Development of Strategies for Mineral Deposit 

Exploration 

By Gregory E. McKelvey and james D. Bliss 

Mineral deposit models developed by the U.S. Geological Survey (Cox and Singer, 1986) give the characteristics and the range in size and tenor of 71 types of mineral 

deposits. In order to construct these models and to compare 

tonnage and metal contents of the models, the following 

criteria must be met: ( 1) mineral deposits must be assigned 

specific deposit types by using geologic knowledge, (2) 

adequate data must be available to define grade and tonnage 

models, and (3) for each deposit type, a median grade and 

a median tonnage must be identified. An example of an 

application of these methods is presented by Bliss and 

others (1990). They identified exploration opportunities 

suitable for small-scale mining by using median deposit 

sizes for the 71 deposit types. 

The relative importance of each deposit type can be 

demonstrated by using the amount of contained metal of the 

median deposit. Contained metal is computed from the 

median tonnage and median grade, and grade and tonnage 

are assumed to be independent variables. For each deposit 

type, the amount of contained metal of the median deposit 

McKelvey and Bliss 53 
is compared to the estimated world annual production in 

1989 (U.S. Bureau of Mines, 1990) to gauge its relative 

importance. In our study, only base and precious metals are 

considered. For a particular metal to be considered, at least 

half the deposits of the specific deposit type must report a 

grade for that metal. This information can be used to adjust 

exploration efforts to search for the deposit type best suited 

to the investor. 

Twenty of the 71 deposit types under consideration 

have a gold grade reported for their median deposits. In two 

of the deposit types, the contained gold of the median 

deposits exceeds 1 percent of the world's annual gold 

production; they are the hot-spring Au-Ag deposit type (1.1 

percent of world production) and the porphyry Cu-Au 

deposit type (2 percent). Sixteen deposit types have silver 

grades in at least half their deposits. Four deposit types have 

median deposits with amounts of contained silver greater 

than 1 percent of the world's annual silver production (table 

1). Of these four, the median porphyry Cu-Mo deposit type 

contains the most silver-4.3 percent of the world's annual 

production. 

Fifteen deposit types have median deposits that contain copper, seven have median deposits that contain lead, 

and nine have median deposits that contain zinc. Median 

deposits of four of the deposit types with contained copper 

have copper contents greater than 5 percent of the annual 

world production (table 1 ); the greatest amount of contained 

copper is in the median deposit of the porphyry Cu-Mo 

deposit type (24 percent of world production). The median 

deposits of six of the seven deposit types with contained 

lead have amounts of lead exceeding 1 percent of the 

world's annual lead production (table 1); the largest amount 

of contained lead is in the median deposit of the sedimentary exhalative Zn-Pb deposit type, which contains 12 

percent of the world's annual production of lead. Three of 

the nine deposit types with contained zinc, Southeast 

Missouri Pb-Zn & Appalachian Zn (20 percent), sedimentary exhalative Zn-Pb (12 percent), and Pb-Zn skams (1.2 

percent), have amounts of zinc exceeding 1 percent of the 

world's annual zinc production. 

In general, the median deposits of the porphyry Cu, 

porphyry Cu-Au, and porphyry Cu-Mo deposit types have 

two orders of magnitude more copper, gold, and silver than 

the median deposits of other deposit types. Porphyry Cu 

deposits may also be sources of cobalt, platinum, molybdenum, and rhenium. Sedimentary-exhalative and 

carbonate-hosted deposits may also contain recoverable 

zinc, lead, and cadmium. 

Although the amount of contained metal of a median 

deposit can be used to rank the relative importance of its 

deposit type, exceptions exist. For example, there are 

several large deposits of the Kuroko massive sulfide deposit 

Table 1 (McKelvey and Bliss). Estimated contained metals 

in the median deposits for selected deposit types defined 

by Cox and Singer (1986) 

[Values are percentages of world annual production (1989) for each 

commodity as reported by the U.S. Bureau of Mines (1990). Only deposit 

types whose median deposits have amounts of contained metal greater than 

1 percent of world annual production are given] 

Deposit type 

Hot spring Au-Ag .......... 

Porphyry Cu-Au ............ 

Porphyry Cu-Mo ............ 

Distal disseminated Ag-Au . 

Polymetallic replacement ... 

Creede epithermal veins .... 

Porphyry Cu ................ 

Sedimentary-hosted Cu ..... 

Sedimentary exhalative 

Zn-Pb .................... 

Southeast Missouri Pb-Zn 

& Appalachian Zn ....... 

Sandstone Pb-Zn ............ 

Pb-Zn skams ............... 

Contained metal in median 

deposits as a percentage 

of 1989 world production 

Au Ag Cu Pb Zn 

1.1 

2.0 5.7 

4.3 24 

2.2 

1.9 2.7 

1.3 1.0 

8.6 

5.2 

12 12 

8.8 20 

3.4 

1.1 1.2 

type that produce zinc. These individual deposits account 

for a significant amount of the world's annual zinc production. However, the median deposit of the Kuroko massive 

sulfide deposit type does not contain zinc because fewer 

than half of the deposits have a reported zinc grade. Some 

deposit types of major importance (for example, the Witwatersrand gold type) do not yet have grade and tonnage 

models constructed for them; therefore, they cannot be 

evaluated. 

REFERENCES CITED 

Bliss, J.D., McKelvey, G.E., and Allen, M.S., 1990, Applications of 

grade and tonnage deposit models; the search for ore deposits 

possibly amenable to small-scale mining: U.S. Geological Survey 

Open-File Report 90-412, 24 p. 

Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. 

Geological Survey Bulletin 1693, 379 p. 

U.S. Bureau of Mines, 1990, Mineral commodity summaries, 1990: 

199 p. 

54 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
A Reconnaissance Study of Gold Mineralization 

Associated with Garnet Skarn at Nambija, 

Zamora Province, Ecuador 

By Gregory E. McKelvey and jane M . 

Hammarstrom 

A group of gold deposits in southeastern Ecuador, 

near the town of Nambija, has recently become the focus 

of intensive mining activity because of extremely high 

gold grades (unconfirmed reports of 50 g/t or more). The 

deposits at Nambija and at other prospects in the district 

are hosted by a sequence of sedimentary and volcanic rocks 

that form a xenolith within alkalic phases of the Jurassic 

Zamora batholith. Mineralization appears to be localized 

within a structural breccia zone that crosscuts a complex 

calcic skarn assemblage within the xenolith. The xenolith is 

composed of various lithologies (limestones, shales, sandstones, tuffs, and flows), all of which have been affected to 

some degree by skarn alteration. Although the nature of the 

host rocks has exerted some control on mineralization, 

high-grade ore is concentrated along north-northeasttrending breccias, faults, and dikes that crosscut the stratigraphy and skarn alteration. In addition, the strong linear 

trend of workings in the district suggests a major structural 

control on mineralization . 

Chemical analyses for a suite of 15 representative 

samples of skarn, porphyry endoskam, breccia, tuff, and 

intrusive rocks show the following: (1) highest gold values 

are observed for garnet-rich skarn samples (Au in skarn, 

< 0.002-47.0 ppm; Au in breccia, trace to 0.034 ppm; and 

Au in altered igneous rock, :s0.002 ppm); (2) gold/silver 

ratios are high; silver was detected (3 ppm Ag) in gold-rich 

massive garnetite skarn; all other samples contain < 2 ppm 

Ag; (3) copper contents are quite variable but are generally 

low (2-745 ppm Cu); samples that contain 1 ppm Au or 

more have extremely low Cu/Au ratios (0 .04-208); and (4) 

none of the samples is enriched in elements commonly 

associated with gold skarns, such as Bi, As, or Te . 

Coarse-grained veinlets and pods of electrum 

(Au77Ag23) are associated with quartz veinlets in massive 

garnet skarn (fig . l) that has undergone little retrograde 

alteration . In high-grade samples, garnet is much more 

abundant than pyroxene, and sulfides are rare . Chalcopyrite, pyrite, sphalerite , galena, arsenopyrite, hematite, and 

magnetite are recognized in the Nambija deposits; electrum 

does not appear to be intimately associated with sulfides 

in the samples studied so far. Gangue minerals include 

zoned anisotropic garnets (Ad34Sp4Gr62 to Ad62Sp3Gr35), 

pods of yellow, isotropic pure andradite, pyroxene 

(Hd22_28Di66_72Jo6), epidote, actinolite , potassium feldspar, apatite , chlorite, calcite, quartz, goethite, and clay. 

Feldspar phenocrysts in porphyry endoskam are altered to 

nearly pure albite and orthoclase. 

Figure 1 (McKelvey and Hammarstrom). Backscattered 

electron image taken with a scanning electron microscope of high-grade gold ore in garnet skarn from 

Nambija, Ecuador, showing electrum veinlets (white), 

quartz (black), and zoned garnets. Iron content of garnet 

increases from core (dark gray) to rim (light gray). 

The Nambija deposits share some characteristics of 

relatively oxidized gold skarns (for example, volcaniclastic 

host rock, mineralogy). More extensive studies are needed 

to determine the nature of these important deposits and to 

design exploration strategies for the region. 

Hydrogeochemical Exploration in Areas of Thin 

Glacial Overburden, Northeastern Minnesota 

By W.R. Miller, W.H. Ficklin, and ).B. McHugh 

Water can be used as a medium for geochemical 

exploration in areas of thin glacial overburden. In addition, 

modeling of the water chemistry contributes to an understanding of the chemical processes that are taking place in 

the surficial weathering environment. 

Hydrogeochemical exploration was used to detect a 

known but undisturbed Cu-Ni sulfide body along the basal 

zone of the Duluth Complex in an area in northeastern 

Minnesota covered by less than 3 m of glacial ground 

moraine. The most important pathfinders for detection of 

the sulfide body are Ni, Cu, and SO~ and, to a Jesser 

extent, Mg and Si02 • A map plot that uses the normalized 

sum of the concentrations of the pathfinders, 

Nil XN; + SiOzl Xs;o, + Cui Xcu + Mg/ XMg + so~ I X so~

5 

(X:= geometric mean concentration) defines the mineralized 

zone more consistently than a single-element plot, mainly 

because the absence of any one variable will not significantly influence the value of the normalized sum. 

Miller and others 55 
The study area is characterized by numerous hills and 

ridges interspersed with bogs and wetlands. Surficial mate~ 

rial is mainly ground moraine. Shallow and immature soils 

are developed on ridges, and gray~brown podzolic soils are 

developed on the lower slopes. Modeling the chemistry of 

waters collected from swamps, streams, springs, and a well 

indicates that the waters are oxidizing. The dominant 

trace~metal species in these waters are Cu2+, Ni2+, and 

so~-' which are derived from the oxidation and dissolution 

of sulfide minerals. During glaciation, rocks become fragmented and mechanically abraded, thus significantly reducing the grain size and making them particularly susceptible 

to chemical weathering. Most of the sulfide minerals in the 

glacial drift probably were destroyed by oxidation and 

dissolution during glacial melting. Therefore, most of the 

Ni2+, Cu2+, and SO~- in the waters of the study area are 

coming from the dissolution of sulfide minerals in nearby 

mineralized outcrops or shallowly buried bedrock. Areas 

containing sulfide-bearing rocks would have been particularly susceptible to intense chemical weathering and subsequent differential glacial ablation. As a result, these areas 

typically would be low lying and, in this environment, 

would tend to form wetlands. The possibility exists that 

sulfide-bearing zones may underlie some of the wetlands in 

this environment. Trace-metal concentrations in the water 

can be used to detect the underlying mineralization if 

the waters in contact with the mineralized bedrock are 

oxidizing. 

Plots of Cu and Ni contents in soils from the data 

reported by Alminas (1975) show that the most useful 

indicator elements for delineating the sulfide body are Cu 

and Ni. The ability of soil and water geochemical data to 

delineate the sulfide body supports the use of both media for 

geochemical exploration in this environment. In the wetlands, water is abundant, and soils are scarce or absent; 

conversely, where soils are abundant, waters are usually 

scarce or absent. The use of both media is recommended for 

geochemical exploration in areas covered by thin glacial 

overburden. 

REFERENCE CITED 

Alminas, H.V., 1975, Soil anomalies associated with a Cu-Ni mineralization in the South Kawishiwi area, northern Lake County, Minnesota: 

U.S. Geological Survey Open-File Report 75-158, 20 p., 6 pls. 

Reconnaissance Geologic Mapping Using Digital! 

Aeromagnetic Data and Space-Shuttle Radar 

Data for a Heavily Forested Area of the 

Guayana Shield, Northwestern Brazil 

By Fernando P. Miranda, Anne E. McCafferty, and 

james V. Taranik 

The part of the Guayana Shield in northwestern Brazil 

is one of the least geologically known regions of the 

country. The area is completely covered by rainforest, and 

lithologic boundaries and geologic structures lack obvious 

geomorphic expression. It is characterized by a complex 

association of gneissic and granitoid rocks of Middle 

Proterozoic age (Guianense Complex), which are not individually shown as distinct units on the geologic map 

published by the RADAMBRASIL Project at the scale of 

1:1,000,000 (Pinheiro and others, 1976). The project was 

sponsored by the Brazilian government to acquire X-band 

radar images and other data for the whole country. 

In 1984, the Shuttle Imaging Radar-B (SIR-B) 

acquired digital data over the Guayana Shield. This L-band 

(23 .4-cm wavelength) synthetic-aperture radar system operated at a fixed incidence angle of 35.7° over flat terrain. On 

the SIR-B image, brighter tones outline the flood plains of 

some streams. These tones are due to the presence of 

standing water beneath canopy layers, which strongly 

increases SIR-B backscatter. Linear geomorphic features 

were identified on the SIR-B image on the basis of (1) the 

spatial distribution of flooded vegetation indicative of subtle 

topographic relief of up to 8 m and (2) tonal linear features 

associated with drainage divides. 

The digital aeromagnetic data used in this research 

were obtained in 1987. Measurements were made by using 

a proton-precession magnetometer having a sensitivity of 

1.0 nT. Flightlines were north-south at a 2.0-km spacing. 

The survey was performed in a draped mode 150 m above 

relatively flat terrain. The following products were derived 

by digital data manipulation: (1) a gray-scale image of the 

reduced-to-pole magnetic anomalies, constructed in order to 

highlight zones of distinct magnetic signature; (2) a terracemagnetization map that delineates induced-magnetization 

boundaries and theoretically outlines geologic structures 

and terranes having different magnetic characteristics; and 

(3) a map showing the location of maximum values of the 

horizontal component of the pseudogravity gradient (boundary lines) that represent abrupt lateral changes in magnetization interpreted as steep structural or lithologic contacts. 

Comparison of the magnetic and radar results with the 

geologic field data acquired by the RADAMBRASIL 

Project allows two broad terrane categories to be delineated 

for the first time. Areas characterized by intermediate 

brightness levels on the gray-scale magnetic image and by 

low to intermediate magnetization values on the terrace map 

correspond to sphene- and amphibole-bearing biotite granitoids having a Rb-Sr age of 1 ,640±26 Ma and a mantle 

derivation (

87Sr/86Sr initial ratio = 0.703). Areas characterized by very bright signatures on the gray-scale image 

and by high terrace-magnetization values correspond to a 

granitic and quartz-monzonitic intrusive body of unknown 

age that crosscuts the mantle-derived granitoids. The limits 

of this intrusive body are inferred by magnetization boundary lines, which are coincident with linear geomorphic 

features identified on the SIR-B image. 

56 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
REFERENCE CITED 

Pinheiro, S.S., and others, 1976, Geologia, Chapter 1 of Projeto RADAMBRASIL, folha NA.l9, Pico da Neblina: Rio de Janeiro, Brazil 

Departamento Nacional da Prodw;ao Mineral, p. 17-137, 5 pls., 

scale 1:1,000,000. 

Tourmalinite and Iron-Formation in the Yellowjacket Formation, Idaho Cobalt Belt, Lemhi 

County, Idaho 

By Peter j. Modreski and jon j. Connor 

Stratabound concentrations of cobalt, copper, and 

iron exist at two stratigraphic levels within the Y ellowjacket 

Formation (Middle Proterozoic) in the Idaho cobalt belt, 

Lemhi County, Idaho. The principal cobalt-copper deposits 

are near the Blackbird mine; they occur in northwesttrending, biotite-rich strata within the middle unit (argillitesiltite-quartzite) of the> 13,000-m-thick Yellowjacket Formation (Modreski, 1985; Nash and Hahn, 1989; Connor, 

1990). Sulfide minerals within this 1 ,000-m-thick Blackbird zone include cobaltite, chalcopyrite, arsenopyrite, 

pyrrhotite, and pyrite. A second interval of mineralization, 

the Jackass zone, is stratigraphically lower, is a few 

hundred meters thick~ and contains abundant magnetite plus 

cobalt-bearing pyrite (Nash, 1989). 

Tourmaline occurs in veins and breccias that cut 

greenschist-facies metasedimentary rocks of the Yellowjacket Formation and in quartz-tourmaline pods and lenses 

within more strongly deformed rocks of higher metamorphic grade. In the vicinity of Iron Creek, in and near the 

Jackass zone, black, flinty veins and breccia matrices are 

composed of a fine-grained intergrowth of tourmaline and 

quartz, locally with orthoclase, albite, muscovite, biotite, 

apatite, magnetite, and rutile. Clasts in the breccias are 

angular and commonly consist of bleached, pale greenishwhite sericitic quartzite. In contrast to the silicified, tourmalinized breccias of the Blackbird district, which contain 

cobaltite at the Patty B prospect and the Haynes-Stellite 

mine, the tourmalinite veins and breccias near Iron and 

Jackass Creeks are essentially barren of cobalt ( <4 to 28 

ppm). Tourmaline throughout the district appears black in 

hand specimen but ranges from blue to olive-green and 

brown in thin section; some is color zoned. It is of 

intermediate schorl-dravite composition and is generally 

richer in iron than magnesium; atomic Fe/(Fe+ Mg) ratios 

range from about 0.4 to 0.7, and weight FeO/(FeO+MgO) 

ratios range from 0. 50 to 0. 80. Some tourmalinites in the 

Jackass zone are associated with bedded magnetite-rich 

iron-formation. 

The mineralized zones in the Y ellowjacket Formation 

are interpreted to have been loci of sea-floor hot-spring 

activity. The magnetite-rich iron-formation consists of 

exhalative deposits that formed from hydrothermal fluids 

that were more oxidized than those that precipitated the 

sulfide-arsenide deposits in the Blackbird zone; the more 

oxidizing nature of the fluid probably resulted from greater 

mixing with seawater. The tourmalinites appear to have 

formed by venting of hydrothermal fluids through the 

mostly lithified sediment column beneath and in the vicinity 

of the exhalative deposits. 

REFERENCES CITED 

Connor, J.J., 1990, Geochemical stratigraphy of the Yellowjacket Formation (Middle Proterozoic) in the area of the Idaho cobalt belt, Lemhi 

County, Idaho: U.S. Geological Survey Open-File Report 90-234 

(Part A, discussion, 3 8 p. ; Part B , diskette). 

Modreski, P.J., 1985, Stratabound cobalt-copper deposits in the Middle 

Proterozoic Y ellowjacket Formation in and near the Challis quadrangle, in Mcintyre, D.H., ed., Symposium on the Geology and 

Mineral Deposits of the Challis 1° x 2° Quadrangle, Idaho: U.S. 

Geological Survey Bulletin 1658-R, p. 203-221. 

Nash, J.T., 1989, Geology and geochemistry of synsedimentary 

cobaltiferous-pyrite deposits, Iron Creek, Lemhi County, Idaho: 

U.S. Geological Survey Bulletin 1882, 33 p. 

Nash, J. T., and Hahn, G .A., 1989, Stratabound Co-Cu deposits and mafic 

volcaniclastic rocks in the Blackbird mining district, Lemhi County, 

Idaho, in Boyle, R.W., Brown, A.C., Jefferson, C.W., Jowett, 

E.C., and Kirkham, R.V., eds., Sediment-hosted stratiform copper 

deposits: Geological Association of Canada Special Paper 36, p. 

339-356. 

Geophysical Logs in an Oxide Copper Deposit 

near CasaGrande, Arizona 

By Philip H. Nelson, David j. johnston, Henry G. 

Kreis, and jon L. Mikesell 

A joint industry-government project, cosponsored by 

the Santa Cruz Joint Venture (a joint venture between 

wholly owned subsidiaries of Asarco, Inc., and Freeport 

McMoRan, Inc.) and the U.S. Bureau of Mines, is investigating the use of in-place leaching to recover copper from 

an oxide ore deposit near Casa Grande, Ariz. Chemical 

solutions will be circulated through boreholes penetrating 

the deposit so that copper may be dissolved from chrysocolla and atacamite and brought to the surface in solution. 

At this stage, test holes have been drilled and logged, and 

some preliminary hydrologic tests have been completed. 

Objectives of the logging effort are to provide the following 

information: structure and fracturing, alteration mineralogy, 

and properties related to fluid flow. 

By using logging technology developed for the petroleum industry, Schlumberger has acquired a complete suite 

of logs in five 9.9-in. rotary boreholes, including caliper, 

density, neutron, electrical-resistivity, dipmeter, gammaspectrometry, sonic-waveform, and neutron-activationspectrometry logs. The U.S. Geological Survey acquired 

logs in a nearby 3.6-in. borehole, including caliper, 

Nelson and others 57 
neutron, resistivity, gamma-ray, sonic-velocity, inducedpolarization, and magnetic-susceptibility logs. Asarco has 

contributed geologic descriptions, copper analyses, data on 

fracture density, and mineralogic estimates. 

The data set is unique in that neutron activation 

spectrometry logs have been collected for the first time in a 

mineral deposit. These logs provide direct estimates of the 

concentrations of the elements Al, C, Ca, S, Si, Fe, and Ti. 

Natural gamma-ray spectrometry adds data on K, U, and Th 

abundances. Compensation for the effects of borehole fluid, 

cement, and fiberglass casing is required before the elemental concentrations of the rocks can be estimated. Mineral 

abundances are estimated by applying an element-tomineral transformation. Use of such transformations 

requires that the suite of permissible minerals be carefully 

restricted and that the results be checked against geologic 

and petrologic data. Results obtained thus far produce 

reasonable estimates of the proportions of quartz, Kfeldspar, and clay minerals within the granite host rock. 

Although copper has not been detected directly as an 

element, the estimated amounts of atacamite and chrysocolla are encouraging at this stage of the investigation. 

Porosity is best calculated as part of the procedure 

used to determine the mineralogy; alternatively and more 

conventionally, it may be estimated from the density and 

neutron logs. Because the host rock is granitic, fluid flow 

probably is controlled by fractures. A correlation between 

sonic velocity and rock quality (that is, fracture counts) 

indicates that the rock is so pervasively fractured that 

porosity is controlled by the fracture density. If so, permeability can be estimated directly from porosity. The hydrologic tests will be used to check and refine the permeability 

estimates. 

Regional Structural Setting of Gold Deposits in 

the Carolina Slate Belt of North Carolina 

By Terry W. Offield and Terry L. Klein 

Reconnaissance geologic mapping at the scale of 

1:250,000 and new interpretations of the lithotectonic 

framework and structural history of the Carolina slate belt in 

North Carolina offer insights useful in gold exploration. 

The slate belt is anomalous relative to other terranes of the 

Piedmont, not only for its low intensity of deformation and 

metamorphism, but also for its wrong-way vergence. Much 

of the belt is characterized by west-dipping cleavage axial 

planar to folds overturned eastward and by east-directed 

thrust faults; the principal deformation and accompanying 

greenschist-facies metamorphism have been dated as Late 

Ordovician (Taconian). 

Taconian thrusting produced shear zones in which 

tectonite fabric shows west-over-east dip slip as the dominant movement. These shear zones display phyllite and 

quartz veins where they occur in arc volcanic rocks that 

make up the northeastern structural slice of the slate belt. 

Large areas of these rocks are volcano genically altered, and 

some have anomalous gold values. The ductility contrast of 

the altered rocks served to focus and intensify shearing 

during thrust faulting; phyllites are almost continuous 

across a zone 24 km wide in one area of widespread 

alteration. West of the volcanic arc rocks, in a structural 

slice of felsic volcanic rocks and an arc-basin sequence, 

thrust-related shears (some at gold deposits) served as 

conduits for fluid flow during metamorphism. 

Late Paleozoic Alleghanian strike-slip faulting produced vertical shear zones that mark the east and west 

boundaries of the slate belt. The west (Gold Hill) shear zone 

has gold, silver, and base-metal deposits, some of which are 

exhalative deposits modified during subsequent structural 

events, whereas other deposits formed during phyllitization 

related to the Alleghanian wrench faulting. The east boundary, structurally analogous, also may be prospective but is 

largely covered by Triassic or Coastal Plain rocks. Other 

zones of anomalous, steeply dipping cleavage or phyllite 

found in many places within the slate belt typically trend 

north or east-northeast athwart the grain of the belt. All 

these zones, the boundary shears, and the rhombic shape of 

the slate belt are believed to be related to the wrenchfaulting event. In several areas (for example, Gold Hill 

shear zone, Sawyer/New Sawyer and Russell Au deposits, 

pyrophyllite deposits), wrench shearing overprinted earlier 

thrust shears. In such areas, tectonite fabrics record both 

dip-slip and strike-slip movement, and these fabrics commonly are most intense where the rocks earlier were 

prepared by volcanogenic alteration. 

Recent gold· occurrence models for the Carolina slate 

belt have focused on either an epithermal hot-spring model 

related to felsic volcanic centers or a mesothermal shearzone model requiring neither specific source nor specific 

host rocks. Our studies indicate that two episodes of 

shearing localized gold mineralization and that many 

intensely sheared areas were volcanogenically altered 

before deformation. Exploration efforts during the past 

decade have concentrated on areas of known gold occurrence or former production; however, a great deal of 

sheared ground (some previously altered) exists outside 

such areas, and so exploration targets may be abundant. The 

three producing gold deposits in South Carolina appear to 

be examples of ( 1) a volcanogenic deposit little modified 

structurally (Brewer), (2) a volcanogenic deposit modified 

by both thrust and wrench shearing (Haile), and (3) a 

deposit formed in a thrust shear zone (Ridgeway). If this 

interpretation is correct, it is reasonable to expect similar 

deposits in analogous settings in the North Carolina part of 

the slate belt. Structural targets are good starting places for 

exploration, but whether volcanogenic solutions deposited 

gold and whether shear-related processes enhanced or 

destroyed preexisting gold concentrations or formed 

58 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
entirely new deposits cannot be known without geochemical 

sampling or drilling; these questions remain to challenge 

future explorers. 

Latin American Mineral-Resource Data Available 

through the U.S. Geological Survey 

By Frances Wahl Pierce and Karen Sue Bolm 

The Min~rals Information Offices (MIO's) were 

established by the U.S. Geological Survey (USGS) to 

facilitate use of USGS mineralresource data and to encourage 

cooperation with other Federal 

and State agencies and with 

private organizations. The 

MIO's have access to several 

digital data bases and to the 

scientific expertise within the 

USGS. The four MIO's are in 

Washington, D.C.; Spokane, 

Wash.; Reno, Nev.; and Tucson, Ariz. 

The Center for InterAmerican Mineral Resource 

Investigations (CIMRI) in Tucson, Ariz., was established by 

the USGS to assess the mineral 

resources and to understand the 

metallogeny of Latin American 

countries. The center collects 

geological, geochemical, geophysical, and remote-sensing 

information relevant to mineral 

resources in South and Central 

America and the Caribbean. 

books, and papers on Latin American geology. The Tucson 

MIO helps CIMRI compile and distribute these collections. 

Figure 1 shows the Latin American and Caribbean 

locations for which the Mineral Resources Data System 

(MRDS) contains information. The MRDS is a digital data 

base maintained by the USGS. It includes data from the 

"Preliminary Deposit-Type Map of Northwestern Mexico" 

(Leonard, 1989) and the "Mineral Resource Assessment of 

the Republic of Costa Rica" (USGS and others, 1987). The 

data can be manipulated, combined, and displayed to help 

formulate exploration strategies for specific deposit types, 

including those potentially amenable to small-scale mining 

(Bliss and others, 1990). 

Recently, the Tucson 

MIO has collaborated with 

CIMRI to update, collect, and 

enter data on mineral deposits 

and occurrences in northern 

Mexico, the Guayana Shield of 

Venezuela, northern Chile, 

Honduras, and the Altiplano of 

Bolivia. CIMRI and the Tucson MIO have produced files of 

news clippings and mineralresource bibliographies for 

Bolivia and the Guayana Shield 

of Venezuela, partial bibliographies of geophysical data from 

the Caribbean and Latin America, and archives of maps, 

Figure 1 (Pierce and Bolm). Mineral sites in Latin America and the Caribbean for which 

the U.S. Geological Survey's Mineral Resources Data System contains information. 

Pierce and Bolm 59 
Companies interested in investing in foreign mineral 

projects may find that unpublished data on the mineral 

resources of a country are scattered among several agencies 

and are difficult to locate. Once these data are collected by 

CIMRI, the MIO's make them readily available in an easily 

transportable, digital format. 

We welcome contributions of mineral-resource data 

from all sources. Data can be entered into the MRDS on 

paper forms sent to an MIO, or they can be entered directly 

into a data base on computers compatible with Macintosh or 

IBM personal computers. Once these data have been 

checked for quality and completeness, they are ready to be 

accessed, manipulated, and interpreted by the general 

public. 

REFERENCES CITED 

Bliss, J.D., McKelvey, G.E., and Allen, M.S., 1990, Application of 

grade and tonnage deposit models; the search for ore deposits 

possibly amenable to small-scale mining: U.S. Geological Survey 

Open-File Report 90-412, 24 p. 

Leonard, K.R., 1989, Preliminary deposit-type map of northwestern 

Mexico: U.S. Geological Survey Open-File Report 89-158, 366 p., 

1 map. 

U.S. Geological Survey, Direcci6n General de Geologia, Minas e Hidrocarburos, and Universidad de Costa Rica, 1987, Mineral resource 

assessment of the Republic of Costa Rica: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-1865, 76 p., scale 

1:500,000. 

Preliminary Results of Geochemical Investigations of Aerial Gamma-Ray Anomalies in the 

Getchell Gold Trend, Humboldt County, Nevada 

By James A. Pitkin 

Selected anomalies in aerial gamma-ray data for the 

Getchell gold trend in Humboldt County, Nev., are being 

investigated for their significance in gold exploration. The 

aerial gamma-ray data were obtained by a contract helicopter survey (TerraSense, Inc., 1989) as part of the U.S. 

Geological Survey's Getchell trend airborne geophysics 

demonstration project (Hoover and others, in press). The 

demonstration area is on the east side of the Osgoood 

Mountains and includes six Carlin-type gold deposits. 

The aerial data portrayed as radioelement (U, K, Th) 

contour and color-composite maps include relatively anomalous areas in phyllitic shale and phyllite of the Preble 

Formation of Paleozoic age and in pediment detritus east of 

the mountain front. During October and November of 1989, 

anomalous sites along flight lines were located by using a 

calibrated, hand-portable gamma-ray spectrometer that 

measures radioelement concentrations; rock and soil samples were obtained. 

Geochemical analyses focused on Au, As, Hg, Sb, 

W, Ag, and F as major elements of interest for the gold 

deposits and oft> U, Th, and K for comparison of ground and 

aerial radioelement data. Analyses included Au by graphitefurnace atomic-absorption spectroscopy; Hg by cold-vapor 

atomic-absorption spectroscopy; F by specific ion electrode; 

W by visible spectrophotometry; As, Ag, Sb, and K by 

inductively coupled plasma-atomic emission spectroscopy, 

and U and Th by delayed neutron activation analysis. All 

the methods were described in Baedecker (1987). 

Comparison of analytical results for U, K, and Th 

with concentrations determined by ground and aerial measurements shows good agreement. The data indicate that 

anomaly sites were successfully located and sampled, 

despite the disparity between the 3-ft sample diameter of the 

portable detector and the 800-ft-wide swath of the aerial 

detector (defined as twice survey altitude of 400 ft above 

ground level). 

Analyses of 14 rock samples show that eight contain 

detectable Au; seven have <0.002 to 0.004 ppm, and one 

sample has 0.05 ppm. The highest Au value of 0.05 ppm is 

from a quartz vein in phyllite that has the highest As value 

of 210 ppm, 0.02 ppm Hg, probable background values of 

W, F, and Sb, and no detectable Ag. A sample of phyllite 

has a Au content of <0.002 ppm, the second highest As 

value of 140 ppm, background values ofW, F, and Sb, and 

no Hg or Ag. The other 12 samples, all of phyllite or 

phyllitic shale, include two with detectable Hg, five with 

Sb, and three with Ag (none thought significant) and have 

probable background values of As, W, and F. Three of the 

14 rock samples were collected in Felix Canyon just south 

of the granodiorite of the Osgood Mountains; they may have 

a geochemical character different from that of the other 11 

and may reflect the alteration halo surrounding the pluton. 

Eleven soil samples were obtained in the pediment 

south of the Pinson mine. Analyses show that eight samples 

contain Au in the range of <0.002 to 0.006 ppm and that 

one sample has 0.014 ppm. The sample with the highest Au 

value (0.014 ppm) is on the upthrown side of a fault that 

parallels the mountain front; except for Au, the geochemical 

character of this sample is similar to that of the other 10 

samples. The 11 soil samples all contain detectable As, W, 

F, Sb, and Ag, and Hg occurs in four of them. The 

concentrations determined are believed to represent background values for an area probably affected by hydrothermal alteration associated with formation of the gold deposits 

of the Pinson mine and the nearby Mag mine. 

Many of the 25 samples discussed here contain 

detectable Au, some have Hg, Sb, and Ag, and all have 

detectable As, W, and F, although not in any pattern 

thought significant in this preliminary study. These and the 

other samples, as well as additional geochemical data, are 

being evaluated to test the use of gamma-ray measurements 

in gold exploration. 

60 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
REFERENCES CITED 

Baedecker, P.A., ed., 1987, Methods for geochemical analysis: U.S. 

Geological Survey Bulletin 1770, 132 p. 

Hoover, D.B., Grauch, V.J.S., Pitkin, J.A., Krohn, M.D., and Pierce, 

H. A. , in press, An integrated airborne geophysical study along the 

Getchell trend of gold deposits, north-central Nevada, in Raines, 

G.L., Lisle, R.E. , Schafer, R.W., and Wilkinson, W.W. , eds. , 

Geology and ore deposits of the Great Basin-Symposium Proceedings: Reno, Geological Society of Nevada. 

TerraSense, Inc., 1989, Uranium, potassium, and thorium contour maps 

derived from a helicopter gamma-ray spectrometer survey of the 

Getchell trend, Humboldt County, Nevada: U.S. Geological Survey 

Open-File Report 89-287, 8 p., 12 sheets, scale 1:24,000. 

82"W 8l"W 80'·W 

34"N 

SOUTH CAROLINA 

~0 33"N + + + 

32"N 

3!"N 

30' N ,,J, + 

~ 29"N 

50 0 50 

13 H H H H 1--------------= 

Phosphorite Deposits of the Northern Blake 

Plateau as Observed From the NR-1 

Submarine and Delta Minisubmersible 

By Peter Popenoe and Frank T. Manheim 

The Charleston Bump is a bathymetric ramp on the 

northern part of Blake Plateau offshore of the Carol in~> that 

deflects the Gulf Stream (fig. 1). It is capped by a nearly 

continuous layer of cemented phosphorite gravel (pavement) winnowed from Miocene sediments by the intensification of Gulf Stream currents caused by constriction over 

79"W 78. 77"W 

34"N 

+ + 33"N 

32"N 

31"N 

BLAKE 

+ + 30' N 

PLATEAU 

29"N 

100 150 200 250 KILOMETERS 

1--------------- 1---------------

Albers Equal Area Conic ProJection 

Figure 1 (Popenoe and Manheim). Area capped by phosphorite pavement (pattern) on the northern Blake 

Plateau . EEZ, Exclusive Economic Zone. 

Popenoe and Manheim 61 
Figure 2 (Popenoe and Manheim). Phosphorite cobbles capping the sea floor of the northern Blake Plateau. The 

sponge near the right-center of the picture is about 18 em across. Photograph by Peter Popenoe. 

the shoal bathymetry. This pavement is overlain by a thin 

surficial layer of modern glauconitic carbonate sand and by 

residual phosphorite slabs, boulders, and gravel (fig. 2). 

The pavement is broken in many areas by large depressions 

associated with Gulf Stream scour. 

Two series of dives totaling about 12 days bottom 

time were made in 1989 and 1990 by U.S. Geological 

Survey and cooperating scientists to assess the thickness 

and extent of the phosphorite pavements and the presence 

and thickness of manganese rinds and to sample both the 

pavements and underlying sediments. The first dives to 

greater than 600 m depth were made in strong currents on 

the central plateau by the U.S. Navy nuclear submarine 

NR-1. These dives investigated two large scour depressions, 

which are more than 100 m deep and have near-vertical 

walls; the depressions expose well-indurated, layered siliceous limestone and chert of Paleocene and Eocene age . 

The upper edges of these scour depressions are rimmed by 

a 1-m-thick phosphorite pavement that overhangs in places 

by about 9 m; the top of this pavement is littered with 

broken phosphorite slabs and boulders. Farther down the 

Charleston Bump, the pavement is breached, and erosion 

has cut down to a second hard layer, presumably another 

phosphorite pavement. 

A second dive series was made as part of the U.S. 

National Oceanic and Atmospheric Administration's 

National Undersea Research Program. Scientists in the 

minisubmersible Delta investigated the shallower part of the 

Charleston Bump at depths of 300--400 m in an area of 

rugged, broken bottom. Here, a series of ridges about 

10-15 m high, whose sides and tops are capped by 

phosphorite boulders and slabs, alternate with depressions 

whose bottoms are composed mainly of carbonate sand. In 

some areas, phosphorite boulders and slabs exceeding 1.5 

min thickness are piled one upon another at varying angles. 

In other areas, a level bottom littered with phosphorite 

boulders, cobbles, and gravels is underlain by continuous, 

unbroken phosphorite pavement. 

The dive series documents an enormous resource of 

phosphorite on the northern Blake Plateau. Over 14,000 

km2 of ocean bottom in this area is covered by a phosphorite 

cap that exceeds 1 m in thickness. These dimensions 

indicate the presence of about 19 billion metric tons of 

equivalent phosphate rock concentrate. Because the phos62 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
phorite is at depths of 300 to 800 m, it cannot be considered 

a reserve base at the present time. However, the enormous 

mass and easy accessibility on the sea floor suggest that 

these phosphorite deposits should not be excluded from 

consideration as a possible resource for the future. 

Silt-Fraction Mineralogy of Unconsolidated Sediment Samples from the Continental Shelf, Slope, 

and Rise off the Northeastern United States 

By l.j. Poppe and j.A. Commeau 

Heavy-mineral analyses are useful as sedimentologic 

and stratigraphic tools and may provide information on the 

resource potential of sediments. However, studies involving 

heavy minerals have usually been conducted on the sand 

fraction because of difficulties in using organic heavy 

liquids on silts and the inability to study this finer size 

fraction under the petrographic microscope. A new technique allows the efficient examination of heavy minerals in 

the silt fraction ( 4-62 J.Lm) and, thus, broadens the scope of 

mineral studies of unconsolidated sediments. We separated 

the heavy minerals by centrifugation in sodium polytungstate and then identified them with a scanning electron 

microscope equipped with an automated image analyzer and 

an energy-dispersive X-ray spectrometer. Polymorphs and 

minerals having similar compositions were grouped or 

differentiated on the basis of X-ray diffractograms. 

Whereas heavy minerals from the continental shelf 

off southeastern New England average only 1.5 percent of 

the sand fraction, heavy minerals in the silt fraction (fig. 1) 

range from 11.8 percent in sandy, higher energy environments near Georges Bank to 3.4 percent in clayey silt south 

of Martha's Vineyard. Corundum and minerals of the Ti02 , 

epidote, and phosphate groups are enriched in the silt 

fraction; spinel, the amphiboles, gamet+staurolite, and 

kyanite-group minerals are relatively concentrated in the 

sand fraction. Opaque minerals in the silt heavy fraction 

average 27.8 percent ilmenite, 4.8 percent magnetite+ 

hematite+ goethite+ siderite, 1. 6 percent ferromanganesecoated grains, and 0. 9 percent pyrite. Lateral mineralogic 

variability within the silt-sized heavy-mineral assemblage is 

considerable. Zircon and ilmenite occur most commonly as 

coarse silt and progressively decrease in concentration to 

the west. Conversely, concentrations within the characteristically finer grained suites of amphibole- and epidotegroup minerals increase westward. 

Quartz dominates the silt-sized light mineral +phyllosilicate group on the shelf off southeastern New England; 

on average, it makes up 70.3 percent of that fraction. The 

potassium feldspar/plagioclase ratio and the 10AI14A ratio 

of phyllosilicate-group minerals average 0.38 and 2.61, 

respectively. The potassium feldspar/plagioclase ratio is 

higher in the sand fraction than in the silt fraction. Phyllosilicates are more concentrated in the finer grained sediments. 

Preliminary studies of surficial sediment samples 

collected from the U.S. mid-Atlantic continental slope and 

rise show that heavy minerals in the silt fraction average 

6.89 percent on the slope and 5.71 percent on the rise. 

However, unlike the silt fraction on the continental shelf off 

southeastern New England, sediments from these deeper 

water environments contain a more extensive authigenic 

mineral suite. 

This study has revealed several important relationships with regard to the composition, abundance, and areal 

distribution of silt- and sand-sized heavy minerals on the 

continental shelf off southeastern New England: (1) regardless of sediment texture, heavy minerals constitute a substantially greater weight percentage of the silt fraction than 

of the sand fraction; (2) the silt-sized heavy-mineral assemblage is diverse and predominantly detrital; (3) pronounced 

mineralogical differences exist between the silt and sand 

fractions; (4) Ti02 is enriched to an average of 0.67 weight 

percent in the silty sands southwest of Nantucket Shoals; 

and (5) the distribution trends observed in the sediments are 

largely due to sorting by westward-flowing bottom currents 

and are related to the specific gravity and characteristic 

shapes and sizes of the mineral grains. 

Basin Evolution and Petroleum Studies in the 

U.S. Eastern Great Basin-Relevance to Mineral 

Investigations 

By Christopher j. Potter, john A. Grow, Charles H. 

Thorman, Harry E. Cook, and james A. Peterson 

The U.S. Geological Survey is undertaking two 

complementary, multidisciplinary regional projects in the 

eastern Great Basin (eastern Nevada and western Utah), 

with the goal of providing a better definition of the geologic 

framework of hydrocarbon and mineral deposits. The two 

projects are part of the Evolution of Sedimentary Basins 

(ESB) and Onshore Oil and Gas (OOG) programs and have 

a duration of 5 yr (1990-95). In the eastern Great Basin, the 

ESB project involves systematic studies of Paleozoic 

through modem sedimentary basins and their tectonic settings, and the OOG project is concerned more specifically 

with the geologic controls on hydrocarbon occurrences. 

Both projects include several detailed investigations (site 

studies) that emphasize the geologic evolution of specific 

resource-bearing (or potentially resource-bearing) horizons 

or areas. The ESB project will also produce regional 

transects consisting of cross sections and palinspastic restorations. In the OOG project, organic geochemical studies 

associated with regional stratigraphic investigations of 

major petroleum source rocks will produce hundreds of new 

Potter and others 63 
Figure 1 (Poppe and Commeau). Scanning electron 

microscope photomicrographs of silt-sized heavymineral grains representative of those found on the 

southeastern New England outer continental shelf. A, 

Corroded grain of altered ilmenite (originally ilmenomagnetite?). B, Higher magnification photomicrograph of grain pi ctured in fig. 1A showing lattice of 

diagenetic rutile prisms (trell is structure). C, Subrounded detrital ilmenite grain; voids probably repreanalytical measurements. Together, the two projects provide an integrated approach to eastern Great Basin geology 

and related resources. Elements of the integrated study are 

summarized below. 

sent hematite exsolution lamellae removed during 

transport or diagenesis. D, Prismatic crystal of epidote. E, Slender, elongate grain of the kyanite group. 

F, Tetragonal , dipyramidal grain of zircon. G, Hexagonal calcium phosphate grain (apatite). H, Dodecahedral garnet (almandite?). /, Framboidal pyrite showing 

the spheroidal agglomeration of euhedral pyrite 

crystals. 

Detailed site studies are underway in the Adobe 

Range and Snake Mountains, the Pine Valley area, the 

Grant Range and Railroad Valley area, the Hot Creek 

Range , and the Mesquite Valley and Virgin Mountains area; 

64 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
they include stratigraphic, sedimentologic, structural, geophysical, geochemical, and thermal history investigations. 

East-west regional transects (from approximately long 

111°30' W. to 117° W.) will be constructed at lats 38°30' 

N., 40° N., and 41 °30' N. Several north-south transects will 

provide complementary data. The transects will incorporate 

regional geologic and geophysical information, including 

new geologic mapping and newly available seismic data, as 

well as data from the detailed site studies. As an example, 

work is currently underway with regional seismic lines from 

Railroad Valley to Lake Valley, along the transect at !at 

38°30' N. To establish fault geometries, we are using new 

geologic mapping in the Grant Range, well data, and 

gravity and magnetic data in conjunction with the reflection 

survey. New mapping is planned in the Egan and Schell 

Creek Ranges along this 91-km-long seismic line in order to 

provide new data on Basin and Range and older structures. 

Another broad-scale investigation is underway in the 

Toano-Goshute Range, Feber Flats, and the Deep Creek 

Range to evaluate multiple large-scale younger-over-older 

faulting events and their relation to mineralization and 

possible petroleum accumulation. 

Synthesis of the structural and geophysical framework with studies of the Paleozoic through Tertiary sedimentary rocks and with the thermal histories of the basins 

will allow construction of cross sections and palinspastic 

restorations that will provide useful contributions for mineral and hydrocarbon exploration in the eastern Great Basin. 

A better understanding of the depositional environments 

and structural evolution of the region is critical to determining the timing and paths of fluid migration, for both 

hydrocarbon and mineralizing fluids. This determination 

will lead to an understanding of how and when potential 

traps formed and of how and when mineral or hydrocarbon 

deposits became localized in the traps. 

Resource Assessment of the Mount Katmai 1 ° x 

2° Quadrangle and Adjacent Parts of the Naknek 

and Afognak Quadrangles, Alaska Peninsula 

By j.R. Riehle, S.E. Church, and L.B. Magoon 

Geologic and geochemical field studies have been 

completed in 15,000 km2 of the Mount Katmai 1° x 2° 

quadrangle and adjoining parts of the Naknek and Afognak 

quadrangles on the Alaska Peninsula as part of the Alaska 

Mineral Resources Assessment Project (AMRAP). Nearly 

the entire study area is within either the Katmai National 

Park and Preserve or the Becharof National Wildlife Refuge . The area is, in some ways, a geologic frontier: gravity 

data comprise only a few profiles or isolated measurements, 

aeromagnetic data are restricted to the Naknek quadrangle, 

and there are neither drill-hole data nor published reports on 

the several unpatented lode and placer claims. 

The study area is within the Peninsular terrane. The 

Bruin Bay reverse fault (fig. 1) separates Middle Jurassic 

plutonic rocks and roof pendants in the northwest part of the 

area from Upper Jurassic sandstones and siltstones of the 

Naknek Formation to the southeast. Upper Cretaceous 

flysch occurs near Cape Douglas. Early to middle Tertiary 

volcanic rocks of the Aleutian arc are exposed northwest of 

the Bruin Bay fault, and middle Tertiary plutonic rocks are 

intruded along and adjacent to the fault in the north-central 

part of the study area. Late Tertiary volcanic and hypabyssal rocks occur in the central and eastern parts of the 

area. Quaternary volcanoes (fig. 2) of the active part of the 

Aleutian arc are constructed on some of the late Tertiary 

hypabyssal and volcanic rocks along the crest of the 

Aleutian Range. 

Contributions of the Katmai AMRAP study to a fuller 

understanding of the geology and mineral potential of the 

Alaska Peninsula are (1) a better definition of the age and 

distribution of volcanic and plutonic rocks in the western, 

north-central, and northeastern parts of the area, (2) subdivision of the Naknek Formation, (3) discovery of a 50-

km-long belt of hypabyssal rocks adjacent to the Valley of 

Ten Thousand Smokes (fig. 1), (4) systematic reconnaissance geochemical coverage throughout the area, and (5) a 

mineral-resource assessment. 

Bedrock and stream-sediment geochemical data outline areas of anomalous concentrations of base and precious 

metals. The most important are (1) around Tertiary plutons 

in the north-central part of the study area; (2) within late 

Tertiary plutonic rocks and Tertiary sedimentary wall rocks 

near Cape Douglas; (3) in late Tertiary volcanic and 

hypabyssal intrusive rocks in the eastern part of the study 

Figure 1 (Riehle and others). The Katmai AMRAP (Alaska 

Mineral Resources Assessment Project) study area on the 

Alaska Peninsula. The study area consists of the Mount 

Katmai 1° x 2° quadrangle and adjacent parts of the 

Naknek 1° x 2° and Afognak 1° x 2S quadrangles. The 

trace of the Bruin Bay fault (BBf), the major structural 

feature of the Alaska Peninsula, is interrupted by middle 

Tertiary plutons. VTTS, the Valley of Ten Thousand 

Smokes. 

Riehle and others 65 
Figure 2 (Riehle and others). View to the north of Griggs 

Volcano, a Quaternary stratovolcano of the Aleutian arc. 

Griggs Volcano is built on Upper jurassic sandstones 

and siltstones of the Naknek Formation that have been 

intruded by Tertiary sills, dikes, and shallow porphyritic 

plutons. The head of the Valley of Ten Thousand 

Smokes, site of the 1912 eruption of Novarupta dome 

area; (4) in Mesozoic sedimentary wall rocks and Tertiary 

hypabyssal rocks in the central part of the study area; and 

(5) in volcanic rocks of the Aleutian arc and underlying 

Naknek Formation near the southern margin of the area. On 

the basis of element suites, followup studies of selected 

areas, and permissive geologic attributes, we infer, at the 10 

percent level of confidence, the existence of one or more 

porphyry Cu systems. Geochemical and geologic data 

suggest that polymetallic vein and epithern1al gold systems 

are also present; however, the Jack of subsurface data and 

the low density of our surface data preclude our making a 

quantitative estimate of the number of such deposits present 

where these deposits occur within an area of < 1 km2 . These 

inferred deposit types are similar to porphyry deposits or 

occurrences and to vein occurrences south of the Katmai 

(the "Katmai eruption"), is in the middle foreground. 

Two glaciers are visible in the lower right, and thick 

fallout deposits of the 1912 eruption can be seen mantling the lower slopes of Griggs Volcano. Griggs Volcano is about 8 km across at its base. Photograph by j.R. 

Riehle in july 1983. 

area and to vein occurrences associated with Tertiary 

plutonic rocks in the Iliamna quadrangle to the north. Other 

deposit types known elsewhere in southern Alaska (for 

example, tin-bearing granites, skarn deposits, syngenetic 

sea-floor gold in Jurassic volcanic rocks) are unlikely to be 

present in the Katmai area , where the geology is unfavorable for their occurrence. 

The potential for producible hydrocarbons in the 

study area is small, primarily because the most favorable 

reservoir rocks have been uplifted and dissected by erosion. 

Thermal springs near Quaternary volcanoes in the eastern 

part of the area indicate some low-temperature geothermal 

potential. Surficial sand and gravel deposits are abundant in 

the western half of the study area. 

66 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Triassic-Jurassic Magmatic Arc of Western 

Nevada and Eastern California-Part IV: A 

Model for the Evaluation of Cenozoic Basins for 

Concealed Mineral Systems 

By D.L. Sawatzky, G.L. Raines, j.L. Doebrich, R.L. 

Turner, L.J. Garside, and j.H. McCarthy, Jr. 

A computerized geologic information system based 

on several geographic-information-system (GIS) computer 

programs provides data management, digitizing and plotting, and powerful GIS functions. These functions provide 

the analytical tools for spatial analyses of map data bases for 

evaluation of mineral resources in basins. We used these 

tools to study Cenozoic basins in the Middle Triassic to 

Jurassic magmatic arc of western Nevada and eastern 

California. 

Data bases were compiled from new data and several 

existing sources of digital data. These data sets include ( 1) 

digital data on Nevada geology, (2) geologic maps at scales 

larger than 1:500,000, (3) mine and prospect data from the 

Mineral Resource Data System of the U.S. Geological 

Survey (USGS), (4) stream-sediment and soil geochemical 

data collected during National Uranium Resource Evaluation (NURE) studies and data from the Rock Analysis 

Storage System (RASS) of the USGS, (5) NURE aeroradioactivity data, (6) USGS aeromagnetic and gravity data, 

and (7) USGS digital-elevation-model (DEM) topographic 

data, audio-magnetotelluric profile data, biogeochemical 

data, and soil-gas data. 

The basin evaluation model is based on maps of 

pre-Cenozoic rocks, depth-to-basement maps, and maps of 

proximity to mines and prospects, geochemical anomalies, 

hydrothermal alteration, limonite anomalies, and radiometric anomalies. A digital subcrop map of pre-Cenozoic rocks 

was prepared by extrapolating basement geology beneath 

Cenozoic basins, utilizing known geologic relations in the 

ranges and structural data relating to folding and faulting, 

and interpreting magnetic, gravity, and electrical geophysical data. Depth to basement is estimated from complex 

physical relations among geologic, topographic, gravity, 

and magnetic data. It is used to estimate the thickness of 

Cenozoic fill on basement rocks. The proximity maps show 

prospective areas around mines and prospects and around 

geochemical and geophysical anomalies; they also show 

hydrothermal alteration zones. 

The geologic, depth-to-basement, and proximity 

maps are combined by using a linear weighting procedure to 

produce the evaluation model. The weights reflect the 

importance of each data type as a predictor of mineralization. Weights are determined intuitively or from statistical 

analysis of the distribution of the data. The result is a map 

of the areas favorable for mineralization at depths of less 

than 1 km under Cenozoic cover rocks in the basins. 

Triassic-Jurassic Magmatic Arc of Western 

Nevada and Eastern California-Part 1: Geology 

By R.A. Schweickert, j.H. Stewart, j.H. Dilles, L.j. 

Garside, R.C. Greene, R.F. Hardyman, D.S. Harwood, and N.j. Silberling 

Metavolcanic and metasedimentary rocks and plutons 

that represent a Middle Triassic to Jurassic magmatic arc 

crop out at scattered localities throughout a large region of 

western Nevada and eastern California. These rocks have 

recently been studied in an area extending northwestward 

from the Gillis Range near Walker Lake through Lake 

Tahoe to the northern Sierra Nevada near Bowman Lake 

and near Reno. Geologic relations of the arc have been 

complicated by the intrusion of younger Jurassic and 

Cretaceous plutons of the Sierra Nevada batholith, concealment beneath Cenozoic volcanic rocks, fragmentation by 

large-magnitude Cenozoic extension and strike-slip faulting, and burial beneath Neogene basin fill. 

The Mesozoic strata can be separated into at least four 

stratigraphic packages. The oldest package includes Middle 

to Late Triassic (Carnian) lava flows, breccias, and ashflow tuffs that range in composition from andesite to 

rhyolite. These rocks were deposited in both subaerial and 

marine environments and locally are intercalated with 

shallow-marine carbonate rocks. This package extends 

westward from the Gillis Range to the Carson City area but 

does not occur in the northern Sierra Nevada. 

Conformably overlying the oldest package are Upper 

Triassic (Carnian to Norian) platform to basinal marine 

limestones and local intercalated marine tuffs and pyritic 

argillites. This carbonate-rich package is overlain, apparently conformably, by a third package of uppermost Triassic to lower Middle Jurassic (Bajocian) basinal siltstone, 

argillite, and dark limestone. Shallow-marine to nonmarine 

quartz sandstone and evaporites occur near the top of the 

sequence. This general stratigraphy exists throughout much 

of the study area. The basinal units include the well-known 

Sailor Canyon Formation and the Gardnerville Formation of 

Noble (1962). 

The fourth, and youngest, stratigraphic package, 

which may include several unrelated sequences, consists of 

a diverse assortment of Middle and Middle(?) Jurassic 

volcanic units that include felsic ash-flow tuff, dacite and 

andesite porphyry, fluvial conglomerate, and hypabyssal 

intrusions. In some areas, rocks of the youngest package 

apparently rest unconformably upon the older units. Majorand trace-element analyses of volcanic rocks from all four 

packages suggest that they represent a calc-alkalic suite 

related to a magmatic arc developed along an active 

continental margin. 

Uncertainties about age(s) of stratigraphic units in 

some areas and of deformational events create problems of 

Schweickert and others 67 
interpretation. Geologic maps, geochronologic data for 

plutons, and structural data indicate that, in the central part 

of the study area, the oldest three packages were deformed, 

folded, and faulted prior to the intrusion of the Yerington 

batholith (169 Ma). However, the possibility that regional 

deformation occurred at different times in different parts of 

the region cannot be excluded. The intensity of deformation 

varied markedly. Rocks in some parts of the region are 

highly deformed tectonites, whereas those in other parts are 

folded but lack foliation. The extent, geometry, timing, and 

significance of the regional deformation are still being 

studied. 

The relation of the youngest stratigraphic package to 

the regional deformation varies across the study area. For 

example, Middle and Middle(?) Jurassic volcanic rocks in 

the Yerington area appear to postdate the regional deformation. In contrast, in the northern Sierra Nevada, the youngest Middle(?) Jurassic units were involved in regional 

deformation together with older units. 

The oldest Mesozoic rocks in the study area are 

interpreted to have formed in a low-relief, shallow-marine 

to subaerial volcanic terrane during the Middle to Late 

Triassic. As volcanism waned in the Late Triassic, subsidence occurred and one or more intra-arc or fringing basins 

developed. Continued subsidence and (or) sea-level rise 

deepened and enlarged the basin(s) during most of the Early 

Jurassic , as basinal marine siltstone, mudstone, and limestone were deposited. Late in the Early Jurassic, the region 

shoaled and nonmarine sedimentary deposits accumulated. 

Subsequently, Middle Triassic to Middle(?) Jurassic units 

were regionally deformed. At least locally, the deformed 

rocks were intruded by younger Middle Jurassic batholiths 

and were overlain by younger Middle(?) Jurassic volcanic 

sequences. An unresolved tectonic problem is whether 

major Mesozoic strike-slip or thrust faults proposed by 

various authors divide the Mesozoic units into different 

tectonostratigraphic terranes. 

REFERENCE CITED 

Noble, D.C. , 1962, Mesozoic geology of the southern Pine Nut Range , 

Douglas County, Nevada: Stanford, Calif. , Stanford University, 

Ph.D. thesis, 200 p. 

Triassic-Jurassic Magmatic Arc of Western 

Nevada and Eastern California-Part II: AudioMagnetotelluric Survey near Reno and Carson 

City, Nevada 

By R.M. Senterfit and D.P. Klein 

Audio-magnetotelluric (AMT) data in the frequency 

range of 4.5-27,000 Hz were used to investigate the 

resistivity structure of a Middle Triassic to Jurassic magmatic arc near Reno and Carson City, Nev. Sixty-one AMT 

stations were occupied. The data for each station consist of 

scalar electromagnetic measurements at 16 discrete frequencies for two orthogonal magnetic and electric field pairs. 

The fields are measured by electrodes and a magnetic coil 

at the ground surface (fig. 1). The goal was to provide 

information on lithologic or structurally controlled resistivity contrasts that may provide insight into the distribution 

of buried parts of rock units associated with the magmatic 

arc . Data were compiled as contoured maps showing the 

lateral variations in apparent resistivity and also as 10 

resistivity cross sections based on composited onedimensional modeling. 

The areas studied include (1) the Churchill ButteRamsey mine area, which is about 55 km east of Carson 

City, and the Carson River plain, (2) Carson Valley, 

including the western flank of the Pine Nut Mountains, and 

(3) the western side of Smith Valley, including the eastern 

edge of the Pine Nut Mountains; this last area is about 55 

km southeast of Carson City. Three exploratory stations 

were occupied on Peavine Peak near Reno. 

The cross sections show the approximate thickness of 

alluvial material and indicate locations of low-resistivity 

(3- 30 ohm-m) units believed to represent volcanic and 

sedimentary rocks. The locations of several alluviumcovered, north-trending faults are also suggested by the 

data, especially in the valley between Churchill Butte and 

the Ramsey mine area in the Virginia Range. The data 

suggest that the thickness of the alluvial fill in parts of 

Smith Valley may exceed 3 km. In conjunction with 

geologic mapping, the data appear to identify the basement 

structure, notably variable basement relief on the east edge 

of Carson Valley and various lateral resistivity contrasts 

within the bedrock that suggest lithologic and structural 

contacts. 

Figure 1 (Senterfit and Klein). Preparing to record an 

audio-magnetotelluric sounding. Photograph by Carl L. 

Long. 

68 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Early to Middle Proterozoic Supracrustal Rocks 

and Mineralization of the Southern Guayana 

Shield, Venezuela 

By Gary B. Sidder, William E. Brooks, Yasmin 

Estanga, Fernando Nunez, and Andres Garcia 

Recent mapping in southern Venezuela has provided 

new insights into the geology of the southern Guayana 

Shield. Felsic to mafic volcanic rocks of the Early Proterozoic (about 1,900 to 1,700 Ma) Caicara Formation and 

associated hypabyssal granitic intrusions are conformably 

and disconformably overlain by quartz sandstone and conglomerate of the Early to Middle Proterozoic Roraima 

Group. Intermediate to mafic dikes, sills, and small bodies 

of gabbro intrude the volcanic and sedimentary rocks. 

Field, petrographic, and chemical characteristics of the 

volcanic rocks indicate that they are correlative with the 

Early Proterozoic Surumu Formation in northern Brazil. 

The volcanic rocks are dominantly rhyolite and trachyte, with lesser amounts of andesite, basaltic andesite, 

and basalt. They are characteristically crystal-lithic ashflow tuff with minor flows and breccia. Whole-rock geochemical analyses demonstrate that the volcanic rocks 

are calc-alkalic. These rocks are not penetratively 

deformed. They lack index minerals indicative of regional 

metamorphism, although, adjacent to granitic and gabbroic 

intrusions, they contain contact metamorphic mineral 

assemblages. 

Mineralized volcanic rock is present in scattered areas 

of hydrothermal alteration and is characterized by propylitization, silicification, and as much as 10 modal percent 

pyrite and traces of associated chalcopyrite. Some quartz 

veins and altered rhyolite contain anomalous values of gold, 

silver, bismuth, and molybdenum and may constitute exploration targets. A deposit model for epithermal preciousmetal veins best represents these mineral occurrences in the 

Caicara Formation. In addition, cassiterite locally forms 

disseminated grains in rhyolite and occurs in panned concentrates of stream sediment. 

The gabbro bodies and intermediate to mafic dikes 

and sills that cut the volcanic rocks are typically deuterically 

altered, medium to coarse grained, and equigranular to 

porphyritic. Andesitic to basaltic ( diabasic) dikes and sills, 

some as much as 8 m thick, are moderately altered and 

contain sulfides. Some dikes have a distinctive chickentrack porphyritic texture. The mafic intrusive rocks, which 

represent different suites and ages of intrusion, are tholeiitic, alkalic, and calc-alkalic in affinity. The calc-alkalic 

basaltic andesite to basalt dikes and sills may be comagmatic with the volcanic rocks. However, the tholeiitic and 

alkalic basaltic dikes and sills and the gabbro bodies may be 

part of the Middle Proterozoic Avanavero Suite, or they 

may belong to the Mesozoic diabase dike swarms. 

Evolution of an Early Proterozoic Rift Basin in 

the La Esmeralda Area, Guayana Shield, 

Venezuela 

By G.B. Sidder, W.C. Day, R.M. Tosdal, S.D. 

Olmore, Luis Guzman, and Freddy Prieto 

New geologic mapping and geochemical and isotopic 

data help unravel the geologic history of an Early Proterozoic greenstone belt near La Esmeralda, 100 km southwest 

of Ciudad Bolivar, Venezuela. Supracrustal rocks within 

the area include basal feldspathic quartzite approximately 

100 m thick, which is overlain by tholeiitic metabasalt, 

microgabbro, and thin beds of metashale, chert, and quartzite. These supracrustal rocks form an east-trending synclinal 

structural basin 16 km wide and 40 km long, which is 

underlain both on the north and south by gneissic basement 

of the Guayana Shield. The basal supracrustal quartzite lies 

conformably on basement granitic gneiss, whereas thrust 

faults occur locally near the upper contact of the quartzite 

with the overlying metabasalt. The granitic gneiss was 

previously assigned to the Early to Middle Archean Imataca 

Group. However, new U-Pb zircon data indicate that this 

gneiss is 2,240±44 Ma and, therefore, that the basement in 

this area is in part Early Proterozoic. 

Two types of gold mineralization are known in the La 

Esmeralda area. An epigenetic type is represented by the 

Hueco Rico prospect, in which gold is localized within a 

shear-zone-hosted quartz-chlorite vein that cuts ductilely 

deformed mafic metavolcanic rocks; S-C mylonitic fabrics 

are preserved within the shear zone. A syngenetic type of 

mineralization is observed at the Cerro La Pinto mine, 

where gold occurs within a finely laminated, rhythmically 

bedded sequence of deeply weathered graphitic metashale, 

mafic metavolcanic rocks, and metachert. 

The supracrustal rocks formed in an Early Proterozoic 

intracratonic basin or along a continental margin during the 

early rifting phase of the Trans-Amazonian orogeny. Deposition of the basal quartzite within the incipient rift was 

followed by tholeiitic basaltic volcanism. Although tectonism has obscured the original texture of the mafic rocks, the 

presence of metashale and metachert interbedded with the 

basal part of the mafic metavolcanic rocks at Cerro La Pinto 

suggests that the volcanism was subaqueous. 

Closure of the basin was accompanied by regional 

compressive tectonism during the later phases of the TransAmazonian orogeny. The tectonism produced regional 

amphibolite-grade metamorphic rocks that show a penetrative structural fabric. Metabasalt has a moderately strong 

schistosity as well as a mineral lineation. The compressive 

regime deformed the basin into a synclinal structure and 

variably thrust the metabasaltic rocks over the basal quartzite; mylonites formed locally within the basal thrust zone. 

Sidder and others 69 
The U-Pb zircon data place a maximum age of about 

2,240 Ma for rifting during the early stages of the TransAmazonian orogeny. The penetrative fabric preserved in the 

rocks of the La Esmeralda area is associated with subsequent compressional stages of the Trans-Amazonian orogeny, which took place from 2,150 to 1,960 Ma. 

Gold-Bearing Quartz Veins Along the MojaveSonora Megashear Zone, Northern Sonora, 

Mexico 

By Miles L. Silberman, Anita Moore-Nail, and Brian 

M. Smith 

Gold-bearing quartz veins, quartz-vein stockworks, 

and silicified breccias occur in most of the gold deposits and 

prospects along the trend of the Mojave-Sonora megashear 

zone, northern Sonora, Mexico (fig. 1). Gold has been 

mined intermittently since the early 1700's, largely by 

primitive methods. An abandoned crusher at the Quitovac 

prospect is shown in figure 2. The deposits and prospects 

are hosted by sedimentary, metamorphic, and igneous rocks 

of Precambrian through Mesozoic age along or near lowangle structures such as thrust faults, shear zones, or 

detachment faults (Silberman and others, 1988). Relatively 

continuous, steeply dipping quartz veins several meters 

thick, such as those at Tajitos, are less common than 

stockworks and breccias. 

Reconnaissance petrographic data, fluid-inclusion 

data, and oxygen-isotope measurements were collected on 

gold-bearing quartz veins from seven deposits and prospective areas along the megashear trend from Sonoita through 

Santa Ana; they are Quitovac, Banco de Oro, Tajitos, 

Basura, Altar, Lluvia de Oro, and Llano. The rocks and 

quartz veins show evidence of both ductile and brittle 

deformation. Ductile deformation is indicated by ribbon 

texture in the quartz veins, foliation in the host rocks, 

rounding of porphyroblasts, augen of feldspar encased in 

mica, pressure shadows, and suturing of quartz-grain contacts. Brittle deformation is represented by fractures that cut 

all veins and host rocks and contain sericite and limonite 

(after pyrite). Sericitic alteration appears to have affected 

almost all the host rocks. Sericite replaces feldspars and 

occurs in highly fractured zones in many places with calcite 

and limonite. Adularia has crystallized in the groundmass of 

some sericitically altered rocks, but no quartz-adularia veins 

were seen. 

Three types of quartz were identified on the basis of 

petrographic and fluid-inclusion study of approximately 30 

quartz-vein samples: 

(1) Cloudy bull quartz-Milky quartz containing many 

healed microfractures arranged in sheetlike wispy textures. Crushing the quartz in oil indicates that the tiny 

inclusions defining the microfractures contain gases 

under pressure. 

(2) Clear bull quartz-White quartz similar to the cloudy 

bull quartz but having fewer and larger inclusions, 

which are mostly irregular in shape and contain two or 

three phases, including (H20)L + (C02 )v + (C02 )L. 

(3) Clear euhedral quartz-Clear quartz with euhedral terminations in vugs. Fluid inclusions in this quartz are 

irregular in shape and have inconsistent liquid/vapor 

ratios. Filling temperatures are less than 200 °C, and 

salinities are less than 1.5 equivalent weight percent 

NaCl. 

Quartz types 1 and 2 are dominant; typically one or 

the other makes up more than 90 percent of a sample. 

Where they occur together, type 2 quartz always replaces 

or, less commonly, cuts type 1. More detailed studies of 

other deposits indicate that these textural types of quartz are 

characteristic of relatively deep, magmatic or metamorphic 

environments. Type 3 quartz is always paragenetically late 

and cuts other types present. Low temperatures and salinities are suggestive of shallow, epithermal environments of 

deposition. Type 3 rarely makes up more than 10 percent of 

the quartz in any sample, but, where present, it appears to 

contain sulfide or oxide minerals. 

Oxygen-isotope compositions of 21 quartz samples 

from all seven deposits and prospective areas with one 

exception are in the range from + 10 to + 15.5 per mil 

relative to SMOW (standard mean ocean water). One 

prospect, Basura, has a lower o 180 value of + 8 per mil and 

has a larger proportion (20 percent) of the late, type 3 

quartz. Only one sample from a vein at Banco de Oro 

consists of a single type of quartz, type 2; fluid-inclusion 

homogenization temperatures average 265 °C. Water in 

equilibrium with this quartz would have had a minimum 

o180 value of approximately +4 per mil. A fluid of this 

composition could result from a mixture of magmatic or 

metamorphic water with meteoric water (at this latitude). If 

the actual temperature of formation were higher, on the 

order of 400 °C (due to depth of burial during vein 

formation), then the oxygen-isotope composition of the 

fluid would be in the magmatic or metamorphic range. 

The oxygen-isotope ratios for Sonoran quartz are 

approximately the same as those measured for gold-bearing 

type 1 quartz veins formed along low-angle shear zones at 

Cargo Muchacho, San Bernadino County, Calif. The geologic setting there is similar to that for many of the Sonoran 

gold occurrences. The ore fluid at Cargo Muchacho was 

largely magmatic in origin (Branham, 1987). 

Our reconnaissance results suggest ( 1) that most of 

the Sonoran gold-quartz deposits and prospects occur in 

host rocks that underwent complex deformation and (2) that 

gold-bearing quartz veins within them are complex and 

multistage. The veins formed or were recrystallized during 

both ductile and brittle deformation. The last stage of 

hydrothermal activity was evidently epithermal and may 

70 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
L E G E N D 

QUATERNARY AND D TERTIARY SEDIMENTS 

0 . 

[:2;_j ~ 

---

TERTIARY VOLCANIC AND 

INTRUSIVE ROCKS 

MESOZOIC SEDIMENTARY ROCKS 

PRECAMBRIAN AND PALEOZOIC 

METAMORPHIC AND SEDIMENTARY 

ROCKS 

MOJAVE-SONORA MEGASHEAR 

® MINE 

PROSPECT 

HIGHWAY 

0 -'0 100 Km 

GOLD DEPOSITS IN SONORA 

NORTHWEST MEXICO 

Figure 1 (Silberman and others). Gold mines and prospects and generalized geology of northern Sonora, Mexico. 

Modified from Silberman and others (1988). 

Silberman and others 71 
Figure 2 (Silberman and others). Abandoned arastra (crusher) at Quitovac, near old open pit. Diameter of circle is about 

2 m. Photograph by M.L. Silberman in 1984. 

have remobilized gold deposited by earlier magmatic or 

metamorphic fluids into brittle fractures. 

REFERENCES CITED 

Branham, A.D., 1987, Geology of the American Girl and Padre y Madre 

gold deposits, Imperial County, California: Pullman, Wash. , University of Washington , unpub. M.S. thesis, 144 p. 

Silberman, M.L. , Giles, D.A. , and Graubard, Cinda, 1988, Characteristics of gold deposits in northern Sonora, Mexico-A preliminary 

report: Economic Geology, v. 83, p. 1966-1974. 

Geology and Mineral Potential of Precambrian 

Basement Rocks of the Trans-Hudson Orogen, 

U.S.A. 

By P.K. Sims, Zell E. Peterman , and T.G. Hildenbrand 

Geological, geophysical, and well-log data were 

analyzed to map the subsurface geology and basement 

topography of the Early Proterozoic Trans-Hudson orogen 

and adjacent tectonostratigraphic terranes (fig. 1). The map 

(Sims and others, in press) was compiled as part of a 

cooperative project between the U.S . Geological Survey 

and the geological surveys of Minnesota, Montana, 

Nebraska, North Dakota, South Dakota, and Wyoming. 

The Trans-Hudson orogen is a north-trending orogenic belt of Early Proterozoic (1,910-1,800 Ma) age that 

extends from outcrop areas in Manitoba and Saskatchewan 

southward beneath Phanerozoic cover into the subsurface of 

the northern Great Plains. It mainly comprises Early Proterozoic arc-related rocks but locally includes Archean 

basement rocks. The western boundary of the TransHudson orogen is marked by two major Archean faults that 

displace the basement rocks-the Cedar Creek fault and the 

Hartville-Rawhide fault. The Hartville-Rawhide fault possibly merges with the much younger (about 1,700 Ma) 

Cheyenne belt. The eastern boundary of the orogen is 

marked by the Superior-Churchill boundary zone, a 55-

km-wide zone of low magnetization , which abruptly truncates the east-northeast-trending structural fabric of the 

Superior craton. 

72 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
46' 

44' 

Sioux Quartzite 

Early Proterozoic granite 

Penokean orogen 

~ Laramide uplift 

CENTRAL PLAINS 

OROGEN 

EXPLANATION 

Greenstonegranite terrane 

~ Normal fault-Bar and ball on downthrown side 

~ Thrust fau\l-Sawteeth on upper plate 

~ Transcurrent fault-Showing relative horizontal 

~ 

movement 

High-angle reverse fault-Hachures on 

downthrown side 

Figure 1 (Sims and others). Tectonic map of Precambrian basement, northern Great Plains, U.S.A. The Archean Superior and Wyoming cratons are 

separated by the Early Proterozoic Trans-Hudson orogen. The Superior 

craton is flanked on the southeast by the Early Proterozoic Penokean 

orogen. The Early Proterozoic Central Plains orogen truncates both cratons 

and older Proterozoic orogens on the south. BH , Black Hills; CB, Cheyenne 

belt; CCF, Cedar Creek fault; CMR, central magnetic region ; CHf, Chadron 

fault; Of, Douglas fault; GLTZ, Great Lakes tectonic zone; HRF, HartvilleRawhide fault; HU, Hartville uplift; LM, Laram ie Mountains; RF, Reservation fault; SCBZ, Superior-Churchill boundary zone; WMR, western magnetic region. 

Aside from the Black Hills uplift, mineral resources 

of the Trans-Hudson orogen must be sought in the subsurface. Stratabound gold deposits of the Homestake type 

could be present beneath a shallow cover of Phanerozoic 

rocks around the northern margin of the Black Hills uplift. 

Nickel and copper deposits similar to those in the Thompson nickel belt in Manitoba may exist in the ultramaficmafic rocks of the Superior-Churchill boundary zone. 

REFERENCE CITED 

Sims, P.K., Peterman, Z.E. , Hildenbrand , T.G., and Mahan, Shannon, 

compilers, in press, Precambrian basement map of the TransHudson orogen and adjacent terranes, northern Great Plains, 

U.S.A.: U.S . Geological Survey Miscellaneous Investigations 

Series Map 1-2214, scale I: I ,000,000 . 

Sims and others 73 
Development and Testing of the CHIM Electrogeochemical Exploration Method 

By David B. Smith, Donald B. Hoover, and Richard 

F. Sanzolone 

The CHIM electrogeochemical exploration technique 

was developed in the Soviet Union approximately 20 yr ago 

and has become a routine mineral exploration tool in that 

country. CHIM is an acronym for the Russian phrase 

"chastichnoe izvlechenniye mettalov ," which means "partial extraction of metals." The method is claimed to be 

particularly effective in the exploration for concealed mineral deposits that are not detectable by other geochemical or 

geophysical techniques. Soviet explorationists cite case 

histories in which concealed mineral deposits have been 

found through several hundred meters of barren cover in a 

variety of geologic settings. The People's Republic of 

China has been using a variation of this method for almost 

a decade , and the Geological Survey of India has recently 

begun to test the method . 

The basics of the CHIM method are relatively simple. 

A stable, high-voltage , direct-current electrical source provides current to an anode and to an array of special collector 

cathodes. Electric current to the individual collector cathodes is in the range of 0.1 to 0.5 A, and collection times 

range from 6 to 48 h. The anode may be an iron , steel, or 

carbon rod. The collector cathodes consist of a graphite or 

titanium rod inside a cylinder containing a solution of nitric 

acid. Electrical contact with the earth is made through a 

semipermeable membrane at the base of the cylinder (fig. 

1). Mobile cations from soil near the cylinder are collected 

in the nitric acid or on the inner rod during passage of the 

current. Upon cessation of the electric current, the cathodes 

are retrieved , and both the nitric acid solution and the 

graphite or titanium rod are analyzed for metallic elements 

that accumulated under the influence of the electric field . 

The U.S. Geological Survey is in the process of 

developing CHIM capabilities and testing the technique on 

various types of mineral deposits. Early work consisted of 

developing and testing electronics, testing electrode 

designs, and testing different types of semipermeable membranes in the laboratory . This work was successfully completed , and the first field tests were begun at the Kokomo 

mine near Central City, Colo. , in May 1990. 

The current field system obtains power from a trailermounted 15-kW, 400-Hz diesel generator and a Zonge 

Engineering induced-polarization transmitter using 50-

1,000 V and 0.2--4.3 A; the transmitter is mounted in a 

pickup truck. Direct-current output goes to a multichannel 

ampere-hour meter having 32 anode and 32 cathode positions to monitor the total charge transfer during a run. 

Electric current from the meter is adjusted for individual 

collector electrodes in an optional rheostat bank and is 

distributed through multiconductor cables to the electrodes. 

Figure 1 (Smith and others). Large (4-in. diameter) CHIM 

electrode emplaced in soil. Photograph by Misac Nabighian (Newmont Exploration) in September 1990. 

Cathodes for the field test consist of a titanium or graphite 

rod suspended in 4 N nitric acid housed in a polyvinyl 

chloride body . A parchment membrane , through which 

current passes, is held in place on the base of the unit with 

an 0-ring. 

The field tests at the Kokomo mine were conducted 

over a 2-m-wide vein containing Cu, Pb, and Zn sulfides 

and precious metals. The vein is covered by approximately 

3 m of colluvial material. Standard geochemical soil sampling failed to delineate the vein except that a weak Ag 

anomaly was detected (5 ppm compared to a background of 

1-2 ppm). The CHIM method successfully defined the vein 

on the basis of Cu , Ag, and Zn distribution . Figure 2 shows 

CHIM results for Zn obtained during a test in which 

samples of the nitric acid solution were collected 6 times 

from 12 sample sites positioned at 10-m intervals during a 

25-h run . 

40 ArP9 Ar Thermochronology of FractureControlled Mineral Deposits of the Idaho 

Batholith-Age, Thermal History, and Origin 

By L.W. Snee, Karen Lund, K.V. Evans, C.H . 

Gammons, and M.j. Kunk 

40 ArP 9 Ar age-spectrum dating is a highly precise 

( < 0.25 percent absolute analytical uncertainty) and accurate isotopic technique that is a powerful tool for solving 

some long-standing problems in economic geology, including the age , duration, number of stages and (or) episodes, 

and temperature of mineralization. This method offers the 

temporal resolution necessary for dating different stages of 

mineralization and alteration in hydrothermal systems. An 

age-spectrum or isochron analysis of the data from a sample 

74 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
14~----------------------~----------------~~----~ 

D 

4h 12 ..... -····-· ............ ,_ ... ________ ·------------.. ---·--------- -------·-______ .. ____________ _ 

- -cen 

~E o ro (1)~ 

0 0 (.) u 

c:::·- NE 

...... ...... 0 0 ....., en C-c 

::J c: 

o ro E en 

8h 10 ............................. _ .. _ .. ________________ ,_, ______ __ ~ 

14 h 

8 ..................................... _______ , ________ .. ________ .. 

-

19 h 

6 ________ ................ -·--·----------------- D 

23 h 

ro ~ -.c !9+-' 

0 c: 

4 -----·--·-----------·------.. -.. -------- _,._ M--"· 'H .. ----.. ----·--------·-- ~ 

25 h 

I-·-

60s50s40s30s20s10s0010n20n30n40n50n 

Stations 

Figure 2 (Smith and others). Total micrograms of zinc collected at CHIM collector 

electrodes spaced at 10-m intervals over a mineralized vein at the Kokomo mine, 

Colorado. Six samples were taken at each electrode during the 25-h run. The vein is 

approximately under station 00. 

provides information about the distribution of argon in the 

analyzed mineral or whole rock and can indicate whether 

the dated material formed before, during, or after alteration. 

Duration of mineralization and number of episodes can be 

evaluated by comparing mineral data from veins that 

formed during multiple stages with data from associated 

wall rocks. 40 ArP9 Ar age-spectrum dates are unique 

because the time when various minerals close to diffusion of 

argon can be related to temperature; thus, the dates provide 

a thermochronology. These time-temperature data are useful for understanding details of ore-forming processes and 

for planning mineral exploration. 40 Ar/39 Ar thermochronologic data have been used to provide valuable constraints on 

the age, thermal history, and origin of fracture-controlled 

mineral deposits of the Idaho batholith. 

The age and origin of gold-bearing quartz veins 

hosted in Cretaceous rocks of the Idaho batholith or older 

rocks in central Idaho were the subject of debate prior to 

detailed isotopic dating. Eocene plutonic and dike rocks 

crop out in many of the mining districts, and, locally, 

disseminated gold deposits are present in Eocene volcanic 

rocks such as in the Thunder Mountain district. It has been 

long argued that the numerous gold-bearing quartz veins 

distributed throughout the Idaho batholith were genetically 

related to the Eocene plutonic and volcanic event. 

Many quartz veins in the southern part of the Idaho 

batholith (within the Elk City, Challis, Hailey, and Twin 

Falls 1 o X 2° quadrangles) are near the roof zones of Late 

Cretaceous (78-70 Ma), marginally peraluminous, granitegranodiorite (primarily biotite granodiorite and muscovitebiotite granite) plutons; other quartz veins are in older 

metaluminous granodiorite-tonalite plutons (93-87 Ma) and 

metamorphic rocks that were intruded by the younger 

plutons. The deposits are both in discrete quartz-filled 

fissure veins and in disseminated deposits along silicified 

shear zones. All the deposits are characterized by quartz 

flooding, episodic brecciation, and open-space filling. 

Wall-rock alteration is minor. The fissure-vein and shearzone deposits contain sulfides and have produced gold, 

silver, copper, lead, zinc, molybdenum, tungsten, antimony, and mercury. Sulfide minerals of the veins vary from 

district to district, perhaps because of the depth of formation 

of the vein systems and possibly because of different 

sources of the metals. Although parallel Eocene dikes are 

common in several districts, the dikes cut mineralized rock 

and are neither silicified nor mineralized; thus, the Eocene 

dikes formed after the mineralization. 

High-precision 40 ArP9 Ar age-spectrum dates of muscovite and potassium feldspar from quartz veins and altered 

host rock from 15 districts range from 78 to 57 Ma. Ages of 

the deposits within any district tend to form clusters; 

locally, multiple mineralization events occurred in some 

districts (for example, Edwardsburg, Profile Gap, Yellow 

Pine, Atlanta). Ages of the deposits are slightly younger 

Snee and others 75 
than cooling ages of the peraluminous granites in most 

districts. Therefore, the vein deposits are Cretaceous or, 

less commonly, Paleocene and were not formed by hydrothermal activity associated with Eocene plutons. Furthermore, the 40 ArP9 Ar age spectra for minerals from the 

deposits and the cooling history of the region after emplacement of Cretaceous granites quantify the thermal and spatial 

effects of younger events. In particular, age spectra of 

quartz-vein muscovites commonly show 40 Ar loss that 

occurred during younger Cretaceous and Paleocene mineralization; only those samples collected within a few meters 

of Eocene dikes, plutons, and volcanic centers display 

partial 40 Ar loss incurred during Eocene events. In addition, 

microcline from nearby Cretaceous plutons yields Late 

Cretaceous to Paleocene 40 ArP9 Ar dates which indicate that 

the ambient temperature of host rocks and mineral deposits 

was already below about 150 oc (the argon closure temperature of microcline) before Eocene magmatic events. Thus, 

only Cretaceous or Paleocene mineral deposits that were cut 

by Eocene plutons or dike swarms show the effects of 

temperatures exceeding 150 °C. Mixed-layer illite/muscovite, which has an estimated argon closure temperature of 

less than 150 °C, was collected from two alteration zones 

adjacent to Eocene dikes and yielded 40 ArP9 Ar ages of 52 

Ma. Therefore, metals were remobilized locally during 

Eocene magmatic activity, but the importance of such an 

event has not been evaluated. 

In summary, most of the studied mineral deposits 

were episodically formed in fractures at or near the roof of 

marginally peraluminous, Cretaceous granite plutons, either 

in extensional fissures that opened during postemplacement 

cooling and uplift of the plutons or in preexisting and 

long-lived shear zones near the roof of the plutons. The 

Paleocene metallogenic activity may have been related to 

local slow cooling after the Cretaceous magmatic event or 

to undocumented Paleocene magmatism. Subsequently, 

most of these same fracture systems served as avenues for 

the emplacement of Eocene dikes and local remobilization 

of metals. 

The Re-Os Isotope System as a Tracer in the 

Study of Platinum-Croup-Element and Cold 

Deposits 

By R.j. Walker, j.W. Morgan, D.O. lambert, A.j. 

Naldrett, j.K. Bohlke, and V. Rajamani 

187Re decays to 1870s and has a half-life of approximately 43 b.y. The long half-life of 187Re and the large 

fractionations of Re from Os that take place during many 

magmatic and aqueous processes make the Re-Os system 

valuable for dating ores. The initial Os isotopic composition 

is a useful geochemical indicator for assessing the origin of 

the noble metals, of which Os is one. We have utilized the 

Re-Os system to study four ore occurrences. 

Re-Os isotopic analysis of rocks from the Late 

Archean Stillwater Complex, Montana, that have high 

Re/Os ratios yielded an isochron age of 2,710±20 Ma. This 

age indicates that the Re-Os system, unlike the Rb-Sr 

system, remained closed following crystallization. Analyses of sulfides and chromitites from the J-M reef revealed 

that the complex crystallized from melts having very 

different initial Os isotopic compositions ct 870s/1860s = 

0. 90-1.3). We infer from the isotopic evidence that magma 

mixing may have occurred throughout the crystallization of 

parts of the Stillwater Complex. The J-M reef contains 

platinum-group elements (PGE's) and has a much higher 

initial Os isotopic composition than that thought to prevail 

in the bulk mantle; ores in the reef were likely derived from 

melt containing a significant proportion (approximately 10 

percent) of crustal material. 

A Re-Os study of sublayer ores from the Sudbury 

Complex, Ontario, also revealed a significant contribution 

to the ores from a crustal source. Like those for Stillwater 

rocks, Re-Os isochrons for individual Sudbury ores (Levack 

West, Falconbridge, Strathcona) indicate closed-system 

behavior since the 1 ,850-Ma crystallization age of the 

complex. Very large initial Os isotopic variations exist for 

the different ores; 1870s/1860s ratios range from 4.7 to 8.7. 

These compositions indicate the presence of at least two 

isotopically distinct sources for the PGE's (and probably the 

Cu and Ni). It is likely that the chief source of the PGE's in 

at least some of the ores was predominantly the Late 

Archean Superior Province crust. Mixing calculations suggest that one ore (Strathcona) may contain >95 percent 

crustal Os. 

The Re-Os systematics of Au ores and related mafic 

and ultramafic rocks of the 2. 7-Ga Kolar schist belt, 

Karnataka, India, suggest a complex secondary history for 

many of the rocks. Several rocks and a sulfide lode ore have 

highly radiogenic initial 1870s/ 860s ratios of 33-39. Probably these rocks were equilibrated with fluids containing 

very radiogenic Os derived from ancient crust; some Au 

also may be derived from ancient crustal sources. In 

contrast, two quartz-carbonate vein ores have Re-Os systematics consistent with the derivation of Os from 2. 7-Ga 

mantle-derived rocks. Sources for this Os (and related Au) 

may have been the komatiites and basalts within the schist 

belt. 

To test the use of Os isotopes as tracers for fluiddeposited Au, we have begun an examination of the isotopic 

systematics of Au ores and related rocks from the Allegheny 

district of northern California. Initial results show that 

relatively unaltered samples of granite, basalt, and harzburgite have initial Os isotopic compositions that are similar 

to those of related rocks that have been altered by Aubearing fluids to quartz-carbonate schist and to sulfidic 

albitites. Presumably the Re-Os systematics of the altered 

76 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
rocks were affected only slightly by the alteration. Several 

ore samples, however, have Os isotopic compositions 

consistent with the incorporation of highly radiogenic Os 

from much older crustal sources. 

The Getchell Gold Trend, Northwestern 

Nevada-Geologic Structure Delineated by 

Further Processing of Electromagnetic Data 

Collected During a Helicopter Survey 

By W.S. Wojniak and D.B. Hoover 

The U.S. Geological Survey used a helicopter to 

conduct a low-altitude electromagnetic survey of the Getchell gold trend in the Osgood Mountains of northwestern 

Nevada as part of its Development of Assessment Techniques Program. The Getchell gold trend includes six 

Carlin-type gold deposits (fig. 1), tungsten and barite 

deposits, and potential for additional mineral deposits. The 

electromagnetic data were collected at three frequencies 

(56,000, 7 ,200, and 900 Hz) and were processed into 

apparent electrical resistivities by Dighem, Inc. The flightline data tapes are publicly available from the National 

Geophysical Data Center in Boulder, Colo. 

Faults and fractures associated with the gold deposits 

produce linear, low-resistivity anomalies of 1-10 ohm-m on 

the maps of Dighem's computed apparent resistivities. In an 

attempt to enhance anomalies that may indicate such structures, the data were further processed as digital images. 

First, the logarithmic electrical resistivities were processed 

into color shaded-relief images, and horizontal-gradient 

maps were calculated for the three logarithmic electromagnetic resistivity maps and the map of the total magnetic field 

reduced to the pole. Maps of the ratios between the 

resistivities at the three frequencies were prepared. Various 

multiplications of the linear and logarithmic horizontal 

gradients of the 900-Hz resistivity and total magnetic field 

were mapped, and the multiplications of horizontal gradient 

maps were multiplied by their correlation coefficients. A 

map of the two-layer Bostick inversion of the three apparent 

resistivities was also prepared. 

Of all the maps prepared, those showing (I) the 

horizontal gradient of the 900-Hz logarithmic resistivity and 

(2) the horizontal gradient of the 900-Hz logarithmic 

resistivity multiplied by the logarithm of the horizontal 

gradient of the total magnetic field were the most successful 

in enhancing and extending the representations of known 

faults, such as the Getchell fault zone. The horizontal 

gradient enhances areas where the physical properties of the 

rocks change sharply, such as boundaries between lithologies having different resistivities. When the horizontal 

gradients of the apparent resistivity and the magnetic field 

are multiplied together, the zones, such as faults, where 

Figure 1 (Wojniak and Hoover). The Rabbit Creek open-pit 

gold mine of Santa Fe Pacific Mining, Inc., along the 

Getchell trend, Nevada. The pit exposes 120m of Tertiary 

basin-fill sediments; each bench is 12 m high . View 

looking northwest toward the Dry Hills. Photograph by 

Steven M. Smith, August 25, 1990. 

both electrical and magnetic properties are changing are 

enhanced. 

The airborne resistivity data best detect the lowresistivity anomalies along the known faults over bedrock 

outcrops and over the upper pediment of the valley east of 

the Osgood Mountains where alluvial cover is the thinnest. 

The effective penetration of the electromagnetic fields into 

the electrically conductive alluvial cover is only tens of 

meters; nevertheless, linear structural trends were detected 

in the thick alluvium of the valley east of the Osgood 

Mountains. We therefore conclude that differences in the 

resistivity of the alluvial cover are present and that these 

differences reflect deep underlying structure. 

Wojniak and Hoover 77 
SOPAC-A Decade of Research on Mineral and 

Hydrocarbon Resources in the South Pacific 

By F.L. Wong, B.A. Richmond, H.G. Greene, j.R. 

Dingier, j .R. Hein, K.A. Kvenvolden, M .S. Marlow, 

j.L. Morton, D.M. Rubin, D.W. Scholl, and ].G. 

Vedder 

The U.S. Geological Survey (USGS) has been investigating mineral and hydrocarbon resources in the South 

Pacific for the past 10 yr through unilateral and cooperative 

programs with island nations and international agencies. 

The main program, referred to as SOPAC, was a cooperative effort among the USGS and agencies in Australia and 

New Zealand; it was coordinated by the multinational 

Committee for Coordination of Joint Prospecting for Mineral Resources in South Pacific Offshore Areas (CCOP/ 

SOPAC). The SOPAC program (which included major 

USGS cruises in 1982 and 1984) focused on the mineral 

resources and hydrocarbon potential of the island nations of 

Tonga, Vanuatu, Solomon Islands, Fiji, and Papua New 

Guinea (fig. 1). Additionally, as part of an ongoing cooperative program with CCOP/SOPAC, USGS personnel have 

EXP LAN ATIO N 

8 Hydrocarbo n 

\J Hydr othermal minerals 

EB Phosphorite 

IJ San d and gravel 

MARIANA I. 

CAROLI NE I. 

:. ::/:t:· : ... 

.·. 

Tropic ol Capricorn - -

16Q•E 

- Tropic of Cancercompleted and continue to produce economic mineral 

assessments in connection with studies of nearshore sedimentological processes for these and other South Pacific 

nations. 

Nearshore mineral resources of value to the island 

nations include sand and gravel, gold and other placers, 

phosphorite, and precious coral. Rapid economic growth in 

many South Pacific island countries has led to an increased 

demand for aggregate material used in the construction 

industry. In response to the gradual depletion of the most 

readily available source of aggregate , the surrounding 

beaches, and to the attendant coastal erosion, the USGS and 

CCOP/SOPAC have explored and identified alternate sources of sand and gravel in diverse island and reef environments in Tonga, the Cook Islands, Tuvalu, and Western 

Samoa. The USGS has done similar work in American 

Samoa. 

In 1986, a USGS-CCOP/SOPAC-Cook Islands mineral exploration program drilled for phosphorite in Aitutaki 

lagoon in the Cook Islands. No extensive deposits were 

found, but the mud deposits recovered were leached from 

mafic and ultramafic rocks that contained anomalous con180'" • 0 • It 1so•w 

. ~* - - - • • -'1!/..<j 

NECKER RIDGE_, ..-.--: .• , '! .. C 

·MARSHALL I. 

'.GIL BER T I 

7 20'"N 

C> 

·JOHNSTON I. 

KINGMAN-

·PALMYRA I. 

. .<~ 

HOWLAND- · ~ 

-BAKER I. -- __ (_ o· · ... ·.PHOENIX I. 

... D 

. :_TUVALU I. 

... 

2o•s 

Figure 1 (Wong and others). Sites of mineral and hydrocarbon exploration in the South 

Pacific. 

78 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
centrations of strategically and economically important 

metals including Al, Fe, Ti, Cr, Ni, Mn, U, and As. The 

metal concentrations are not of immediate economic interest 

but warrant further studies because similar processes have 

produced minable Ni-rich laterites in New Caledonia. 

Geophysical surveys of the Valu Fa Ridge region 

west of the Tonga arc conducted by the USGS in 1982 and 

1984 provided the basis for subsequent international exploration programs that led to the discovery of sea-floor sulfide 

deposits and active hydrothermal vents. USGS investigators 

used multichannel seismic-reflection data to locate the 

magma chamber under the Valu Fa Ridge. The presence of 

fresh andesite and dacite, warm bottom water and cored 

sediment, and abundant oxide deposits indicated extensive 

hydrothermal activity. Sites with high resource potential 

were outlined for future exploration. Later geophysical and 

photographic surveys and submersible exploration by 

French and German agencies have located polymetallic 

sulfide deposits along the ridge crest above the Valu Fa 

magma chamber. 

Hydrothermal mineralization has also been investigated in shallow-water and onshore environments. In conjunction with earlier work by the Australian Bureau of 

Mineral Resources (BMR), the USGS sampled volcanic 

deposits associated with active cones perched on Epi 

caldera offshore of Epi Island, Vanuatu. Anomalous Fe and 

Mo contents in two of the five samples collected by the 

USGS were similar to the metal enrichment determined by 

the BMR in this area. Volcanic rocks of quartz trachyte and 

trachyte composition were dredged from offshore of the 

Tabar-Lihir-Tanga-Feni island chain north of New Ireland, 

Papua New Guinea, during the 1984 USGS survey. Analysis of the dredge samples for precious metals has not yet 

detected any gold, but subsequent release of proprietary 

data by Papua New Guinea revealed that similar rocks do 

host epithermal gold deposits (18.4X 106 oz at 3.4 g/t) on 

Lihir Island. 

The USGS hydrocarbon resource assessment program 

in the South Pacific refined knowledge of the factors for 

hydrocarbon generation and accumulation in the southern 

Tonga platform in Tonga; the Central Basin of Vanuatu; the 

complex of basins in the Central Solomons Trough, Solomon Islands; and the New Ireland basin in Papua New 

Guinea. Adequate sediment thickness (>3 km), geothermal 

gradients (>30 °C/km), structural traps (anticlines, fault 

traps), and reservoir rocks of varying quality are present in 

each area. Hydrocarbon gases commonly found in nearsurface sediment from various offshore sites in the South 

Pacific appear to be mainly biogenic. Oil seeps, such as 

those reported from onshore Tonga, Fiji, and Vanuatu, 

indicate hydrocarbon generation in the arc regions, but 

onshore source-rock analyses suggest only a remote potential for significant quantities of oil and gas. However, the 

USGS is reevaluating old proprietary and public single- and 

multi-channel seismic-reflection data and those more 

recently acquired by the USGS to focus study on the most 

promising targets for future exploration. 

The Origin of Evaporite Deposits-A Model 

Based on a Hydrologically Open System 

By Warren W. Wood and Ward E. Sanford 

The origin of large thicknesses of commercially 

valuable evaporite minerals has, for many deposits, been 

difficult to explain on the basis of known or likely solute 

input to topographically closed basins. Our proposed leakage model suggests that a hydrologically open basin, having 

either seepage loss to surrounding ground-water systems or 

interchange with an adjacent ocean, is the dominant control 

on the thickness and suite of evaporite minerals forming 

within the basin. For a specific solute composition, the 

chemical divide determines the potential suite of minerals 

that can form in a chemically closed basin undergoing 

evaporation. However, the flux ratio (the ratio of water 

outflow to inflow) determines the minerals in the suite that 

actually precipitate, as well as their respective thicknesses. 

Attainment of hydrologic steady-state conditions permits 

large thicknesses of one or two evaporite minerals to form 

rather than the equilibrium sequence of several minerals that 

typically forms in a closed basin. 

The code PHRQPITZ (Plummer and others 1988) is a 

chemical-equilibrium model incorporating Pitzer's equations for calculating ion activity in a high-ionic-strength 

solution. By using the code with a lumped-parameter 

hydrologic model, we calculated ~the variation in thickness 

of gypsum, dolomite, glauberite, mirabilite, bloedite, polyhalite, halite, and hexahydrite deposits as a function of 

subtle changes in ground-water seepage rates (Wood and 

Sanford, 1990). These calculations utilized a typical mixed 

cation-anion ground water from the southern High Plains of 

Texas and New Mexico. We also calculated the mineral 

suite and thickness of variable leakage of seawater and a 

sodium carbonate ground water from southern Idaho typical 

of that found in many volcanic terrains. In addition to 

mineral thickness, the model provides the concentration of 

the individual solutes as a function of leakage and amount 

of evaporation and, thus, documents the evolution of the 

brine during evaporation. The model is general and is 

applicable for any solute composition and can be used to 

simulate ( 1) evaporite deposition in a topographically 

closed continental basin, (2) the interchange between a 

restricted evaporating basin and the ocean, and (3) reflux 

conditions, where certain solutes return to the ocean 

through seepage. Knowledge of the mechanism controlling 

evaporite thickness and mineral suites can aid in the 

exploration for evaporite resources by eliminating unnecesWood and Sanford 79 
sary exploratory drilling, explaining the absence of certain 

brines, and predicting the mineral suite of active systems 

from the results of inexpensive solute analyses of ground 

water surrounding the basin. 

REFERENCES CITED 

Plummer, L.N., Parkhurst, D.L., Fleming, G.W., and Dunkle, S.A., 

1988, A computer program incorporating Pitzer's equations for 

calculation of geochemical reactions in brines: U.S. Geological 

Survey Water-Resources Investigations Report WRI 88-4153, 

310 p. 

Wood, W.W., and Sanford, W.E., 1990, Ground-water control of 

evaporite deposition: Economic Geology, v. 85, p. 1226-1235. 

U.S. Geological Survey Mineral-Resource and 

Tectonic Studies in Venezuela 

By j.C. Wynn, S.D. Olmore, Floyd Gray, W.C. Day, 

G.B. Sidder, and N.J Page 

A cooperative project was established in 1987 

between the U.S. Geological Survey (USGS) and C.V.G.-

Tecnica Minera, C.A. (TECMIN), a mining company 

owned by the Venezuelan Government. The project is 

centered in Puerto Ordaz and Ciudad Bolivar, Venezuela, 

and is staffed by 3 resident USGS scientists and up to 10 

visiting scientists per year. The 5-yr cooperative agreement 

encompasses training in geophysics, geochemistry, and 

economic geology, evaluating mineral sites, and upgrading 

laboratory and data-processing capabilities. A primary goal 

of the project is to complete a mineral-resource inventory of 

the Precambrian Venezuelan Guayana Shield (fig. 1) by the 

end of 1991. Intermediate steps in this effort include 

evaluation of known mineral deposits, exploration for new 

deposits, and a geologic inventory program. A major new 

Tectonic and Geologic Map Series will form an important 

milestone in this effort; maps in this series will incorporate 

available geophysical data and will be published at a scale 

of 1:500,000. 

The Puerto Ayacucho sheet (fig. 2) is representative 

of the new Tectonic and Geologic Map series. The map is 

being compiled from aeromagnetic data, radiometric data, 

gravity data, side-looking radar images, air photos, and 

reconnaissance field work. The mapped area is about the 

size of West Virginia. It contains a large granite pluton, 

which has potential for gold and rare-earth-element mineralization; the area also may contain possible kimberlites 

along Mesozoic fractures. Three previously unknown ringlike structures, one with a uranium association that may be 

a large carbonatite intrusion, have been identified during 

data interpretation and assembly of the map. Numerous 

small to medium-sized buried magnetic bodies are also 

being incorporated into the map representation, making it 

effectively a map of the top 10-15 km of the crust. This 

series of maps thus will become the foundation for a 

detailed mineral-resource appraisal of the Venezuelan 

Guayana Shield. 

The mineral-deposit map being compiled for the 

Guayana Shield shows several interesting associations. 

Shear-zone-hosted quartz veins within an Early Proterozoic 

greenstone belt are the primary source of gold in Venezuela. 

Districts such as El Callao, Lo Increible, and Kilometer 88 

currently produce gold from low-sulfide gold-quartz veins. 

Placer deposits are associated with rocks of the greenstone 

belt and with conglomerate lenses and beds of the Uairen 

Formation of the Early to Middle Proterozoic Roraima 

Group. Diamond-bearing kimberlite has been identified 

only in the Quebrada Grande district in the northeastern part 

of the Guayana Shield, but diamonds are mined in numerous areas in the central, eastern, and southern parts. 

World-class deposits of banded iron-formation in the 

Archean Imataca Complex are mined at Cerro Bolivar, San 

Isidro, El Pao, and several other places. Deposits of bauxite 

and aluminum-rich laterite are products of intense weathering of granitic (Los Pijiguaos) and mafic intrusive rocks 

(Upata, Nuria, and Gran Sabana). Placer, eluvial, and lode 

occurrences of tin are associated with the Middle Proterozoic Parguaza Granite along the western margin of the 

shield. 

The Lo Increible gold district is important because of 

its close spatial association with the El Callao mining 

district, which by 1906 was the largest gold producer in the 

world at over 7,000,000 oz. The Lo Increible district lies 

along a major ductile shear zone within the Early Proterozoic Pastora Group. Although the tropical weathering is 

intense, S-C mylonitic fabric is well preserved. The mineralization is typical of shear-zone-hosted lode-gold deposits in that native gold occurs in quartz-tourmaline veins 

that contain a quartz-calcite-chlorite-sericite alteration 

assemblage. 

Sierra Verdun is a long, linear structure associated 

with mafic and ultramafic bodies that stands out sharply in 

SLAR (side-looking airborne radar) imagery from an otherwise flat Proterozoic granitic terrane. Geologic mapping 

reveals a structurally disjointed terrane that includes a 

folded and multiply deformed greenstone belt dominated by 

volcanic flows and tuffs of intermediate composition that 

are intruded by gabbroic and ultramafic rocks, perhaps 

representing multiple intrusive phases. These gabbroic and 

ultramafic rocks include komatiitic lavas, a sequence of 

pyroxene-rich cumulus rocks, and diabase dikes. Substantial gold production is associated with a distinctive magnetic 

anomaly caused by a large pluton overlain by many shallow, east-trending apophyses. Other similar anomalies in 

the region, as well as the occurrence of sulfides in the 

mafic-ultramafic sequence, suggest additional potential target zones for hydrothermal gold and for magmatic deposits 

of platinum-group elements. 

80 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
69'VV 66"W 

~ 0 o. 

CIUDAD 

BOLIVAR TUCUPITA 

Figure 1 (Wynn and others). The Venezuelan Guayana Shield, showing 

locations of the Puerto Ayacucho area and the Sierra Verdun (SV) and Lo 

lncreible (LI) study areas. 

Wynn and others 81 
IN 

SCALE 1:1,250,000 

Figure 2 (Wynn and others). Geology and tectonic features in the Puerto Ayacucho area, Venezuela. 

Platinum-Group Mineral Investigations, East and 

South Kalimantan, Indonesia 

By Michael L. Zientek and Bambang Pardiarto 

Information was gathered by the U.S. Geological 

Survey (USGS) and the Directorate of Mineral Resources of 

the Republic of Indonesia on occurrences of platinumgroup-elements (PGE's) in Indonesia. The goal was to 

define the source of platinum-group minerals (PGM's) 

found in placers in East and South Kalimantan. USGS 

activities in PGM exploration in Indonesia were done in 

conjunction with the Geological and Mineral Survey 

Project, Asian Development Bank Loan No. 641 INO. 

For East Kalimantan, two new placer PGM localities 

(Tabang area and Sungai Marah area) were documented, as 

well as an additional area, the Pasir area, that has anomalous Pt contents (8 and 12 ppm) in panned concentrate 

samples. A detailed mineral-resource assessment of southeastern Kalimantan identified four permissive areas for the 

occurrence of primary PGE deposits and a large region 

where diamond, gold, and PGM placers are derived from 

82 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Correlation Chart 

}Cenozoic 

-3-Unconformity 

-2-Unconformity} Late Proterozoic 

I Ypr I r Middle Proterozoic 

ZXmr YXrp J 

Early Proterozoic 

? ? 

-1-Unconformity 

}Archean 

Figure 2 (Wynn and others).-Continued., 

Cretaceous conglomerates. The mineralogy and textures of 

the PGM' s from the placers and detailed investigations of 

one of the permissive areas indicate that the primary source 

of the PGM's is most likely chromitite schlieren associated 

with Alaskan-type ultramafic intrusions. 

In southeastern Kalimantan, pre-Tertiary rocks and 

foltled and faulted Eocene to Miocene sedimentary rocks 

are flanked by two late Cenozoic sedimentary basins. The 

pre-Tertiary rocks consist of an Early to Middle Cretaceous 

accretionary complex hosting deposits of podiform chromitite; iron±Au skarns; Fe-Cr oxide segregations in cumulate dunite, clinopyroxenite, and gabbro; epithermal Mn 

deposits; low-sulfide Au-quartz veins; and Au in sheared 

serpentinite. These rocks are unconformably overlain by the 

Late Cretaceous (Senonian) Manunggul Formation, a 

sequence of sedimentary and mafic volcanic rocks that are 

a source of placer diamond, gold, and PGM' s found in 

younger deposits. Eocene to Miocene sedimentary rocks of 

mixed shallow-marine, deltaic, and continental facies were 

EXPLANATION 

Ccs Cenozoic deposits and surfaces, including both 

depositional and erosional surfaces, alluvium, 

colluvium, and lag gravels. 

-3- Period of regional uplift and erosion. 

-2- Continental rifting in eastern Venezuela. 

Ypr Middle Proterozoic platform sedimentary rocks 

(Roraima Group). 

YXrp Middle Proterozoic Rapakivi-textured sadie granite 

to quartz monzonite (Parguaza Granite). 

ZXmr Alkalic mafic rocks (where hatched, visible in the 

magnetic data only). 

Xsr Middle Proterozoic calc-alkalic to alkalic granite 

(Santa Rosalia Granite). 

Xcv Early Proterozoic calc-alkalic volcanic rocks; extrusive manifestation of the Santa Rosalia Granite? 

-1- Early Proterozoic Trans-Amazonian orogeny; 

northwest-striking shear zones and fold axes. 

Xmq Early Proterozoic supracrustal quartzites, 

conglomerates, and volcanic rocks (highly magnetic 

unit: locally called the Moriche Formation). 

Wbc Archean soda-granodiorite basement complex (highly 

magnetic unit: equivalent to the lmataca Group?). 

deposited in stable shelf and epicontinental conditions over 

much of this area. After uplift of the Meratus Mountains, 

Pliocene and Pleistocene coarse clastic sediments were 

deposited on the western and southeastern flank of the 

mountains; the sediments contain Au-PGM and diamondAu-PGM placer deposits. 

Detailed investigations of one of the areas permissive 

for a bedrock source for PGM' s in South Kalimantan (Riam 

Pinang area) indicate at least two sources for the PGM's in 

the alluvial deposits in this area: ( 1) alluvial deposits on 

terraces 20-30 m above the present drainages and (2) 

dunitic bodies intrusive into clinopyroxene cumulates that 

are probably parts of Alaskan-type ultramafic complexes in 

the pre-Tertiary accretionary complex. A chromitite schlieren found during the trenching of one of these dunitic bodies 

contained an anomalous Pt concentration; analysis by NiS 

fire assay-inductively coupled plasma-mass spectroscopy 

yielded. the following results: Pt = 580 ppb; Pd = 3.4 

ppb; Rh = 10 ppb; Ru = 9 ppb; and Ir = 21 ppb. The 

Zientek and Pardiarto 83 
chondrite-normalized PGE pattern for this rock is characterized by elevated Pt and Ir contents and is similar to 

patterns for chromitites in Alaskan-type ultramafic intrusions. Chromitite schlieren made up approximately 11 

percent of the analyzed rock sample; therefore, concentrations of Pt in the chromitite alone may exceed 5 ppm. 

The two most valuable exploration methods for defining prospective targets in the Riam Pinang area are recording the PGM's seen in panned concentrate samples in the 

field and analyzing the panned concentrate samples for Pt, 

Pd , and Au. Up to 29 grains of PGM's were observed in 

panned concentrate samples in the field . Pt contents of the 

concentrates range from < 5 ppb to 85 .9 ppm. Fifty percent 

U.S. Geological Survey chemist William d'Angelo 

running samples on the inductively coupled 

plasma-atomic emission spectrometer (ICP-AES) 

in Reston, Va. By using a dedicated computer to 

of the samples have more than 107 ppb Pt; 25 percent have 

greater than 1,250 ppb Pt. Au contents range from 1.2 ppb 

to 291.3 ppm. Fifty percent of the samples have greater than 

1.43 ppm Au; 25 percent have greater than 13.4 ppm Au. 

High Pt and Au contents do not have a one-to-one correspondence (as would be expected because Au and PGM's 

are derived from different sources). Pd contents are all low, 

less than 33 ppb . In contrast, only three of the streamsediment samples have slightly anomalous PGE contents 

(their Pt contents are 394, 138, and 100 ppb). The rest of 

the samples have less than 44 ppb Pt and 23 ppb Pd. Au 

values are typically higher, averaging 90 ppb , with a high 

value of 130 ppb. 

record em1ss1on intensities, the ICP-AES can 

determine the concentrations of 51 elements in 

solution simultaneously. Photograph by Ph ilip j. 

Aruscavage. 

84 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

H. V. Alminas 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-9674 

Orlando Andre 

Servicio Geologico de Bolivia 

Casilla 2729 

La Paz, Bolivia 

011-591-237-4544 

Joseph G. Arth 

U.S. Geological Survey 

Mail Stop 937 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-4670 

Roger P. Ashley 

U.S. Geological Survey 

Mail Stop 901 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5416 

Robert A. Ayuso 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6347 

Viki Bankey 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1348 

David F. Barnes 

U.S. Geological Survey 

Mail Stop 989 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5315 

Walter J. Bawiec 

U.S. Geological Survey 

920 National Center 

Reston, VA 22092 

(703) 648-6148 

Harvey E. Belkin 

U.S. Geological Survey 

959 National Center 

Reston, VA 22092 

(703) 648-6162 

Robert J. Bisdorf 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1377 

H. Richard Blank 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1341 

James D. Bliss 

U.S. Geological Survey 

210 E. 7th Street 

Tucson, AZ 85705 

(602) 629-5502 

J.K. Bohlke 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6325 

Karen Sue Bolm 

U.S. Geological Survey 

210 E. 7th Street 

Tucson, AZ 85705 

(602) 670-5508 

H.F. Bonham 

Nevada Bureau of Mines and Geology 

University of Nevada-Reno 

Reno, NV 89557-0088 

(702) 784-6691 

Arthur A. Bookstrom 

U.S. Geological Survey 

Saudi Arabian Mission 

APO New York 09697-7002 

011-966-2-667-4188 

T.L. Bowers 

U.S. Geological Survey 

927 National Center 

Reston, VA 22092 

(703) 648-6354 

Stephen E. Box 

U.S. Geological Survey 

Room 656 U.S. Courthouse 

W. 920 Riverside Ave. 

Spokane, WA 99201 

(509) 353-2467 

David A. Brew 

U.S. Geological Survey 

Mail Stop 904 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5726 

William E. Brooks 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5627 

Mark W. Bultman 

U.S. Geological Survey 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

(602) 670-5503 

Thomas K. Bundtzen 

Alaska Division of Geological 

and Geophysical Surveys 

3700 Airport Way 

Fairbanks, AK 99709 

(907) 451-2771 

Alison Burchell 

U.S. Geological Survey 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

(602) 670-5572 

John W. Cady 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1330 

Andrew C. Campbell 

Department of Earth, Atmospheric, 

and Planetary Sciences 

Massachusetts Institute of Technology 

Cambridge, MA 02139 

(617) 253-5790 

William F. Cannon 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6345 

Addresses and Telephone Numbers of Authors 85 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

Raul Carrasco 

Servicio Geologico de Bolivia 

Casilla 2729 

La Paz, Bolivia 

011-591-237-4544 

Richard B. Carten 

U.S. Geological Survey 

Saudi Arabian Mission 

APO New York 09697-7002 

011-966-2-667-4188 

Chen Nansheng 

Institute of Geochemistry 

Academia Sinica 

Guiyang, Guizhou Province 

People's Republic of China 

S.E. Church 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1900 

Roger N. Clark 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1332 

Sandra H.B. Clark 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6331 

L.G. Closs 

Department of Geology and 

Geological Engineering 

Colorado School of Mines 

Golden, CO 80401 

(303) 273-3856 

J .A. Commeau 

U.S. Geological Survey 

Woods Hole, MA 02543 

(508) 548-8700 

Jon J. Connor 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5614 

Clay M. Conway 

U.S. Geological Survey 

2255 N. Gemini Drive 

Flagstaff, AZ 86001 

(602) 527-7199 

Harry E. Cook 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3007 

Theresa M. Cookro 

U.S. Geological Survey 

Mail Stop 937 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5706 

Kevin P. Corbett 

Department of Geology 

School of Natural Sciences 

Weber State University 

Ogden, UT 84408-2507 

(801) 626-7954 

L.E. Cordell 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1204 

Raymond M. Coveney, Jr. 

Department of Geosciences 

University of Missouri 

Kansas City, MO 64110-2499 

(816) 276-2980 

Dennis Cox 

U.S. Geological Survey 

Mail Stop 984 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5364 

Leslie J. Cox 

U.S. Geological Survey 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

(602) 670-5505 

Douglas E. Crowe 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6338 

86 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 

Lucia Cuitino 

Servicio Nacional de 

Geologia y Mineria 

Casilla 10465 

Santiago, Chile 

011-56-237-5050 

Charles G. Cunningham 

U.S. Geological Survey 

959 National Center 

Reston, VA 22092 

(703) 648-6121 

W.C. Day 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-6483 

George A. Desborough 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5611 

D.E. Detra 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-2449 

Felipe Diaz 

Servicio Nacional de 

Geologia y Mineria 

Cas ilia 10465 

Santiago, Chile 

011-56-237-5050 

J.H. Dilles 

Department of Geology 

Oregon State University 

Corvallis, OR 97331-5506 

(503) 737-1245 

John R. Dingler 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3109 

Bruce R. Doe 

U.S. Geological Survey 

923 National Center 

Reston, VA 22092 

(703) 648-6205 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

Jeff L. Doebrich 

U.S. Geological Survey 

Mackay School of Mines 

University of Nevada-Reno 

Reno, NV 89557-0047 

(702) 784-5596 

Lawrence J. Drew 

U.S. Geological Survey 

920 National Center 

Reston, VA 22092 

(703) 648-6483 

Barbara A. Eiswerth 

U.S. Geological Survey 

927 National Center 

Reston, VA 22092 

(703) 648-6379 

R. G. Eppinger 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-2468 

J.A. Erdman 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5512 

George E. Ericksen 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6323 

Yasmin Estanga 

Corporaci6n V enezolana de 

Guayana-Companfa Tecnica 

Minera, C.A. 

Edificio Sede CVG 

Ciudad Bolfvar, Venezuela 

011-58-85-25334 

Karl V. Evans 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5625 

Russell C. Evarts 

U.S. Geological Survey 

Mail Stop 901 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5415 

W.H. Ficklin 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1904 

C.A. Finn 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1345 

FrederickS. Fisher 

U.S. Geological Survey 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

(602) 670-5501 

Nora K. Foley 

U.S. Geological Survey 

959 National Center 

Reston, VA 22092 

(703) 648-6179 

Eric R. Force 

U.S. Geological Survey 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

(602) 670-5506 

A.B. Ford 

U.S. Geological Survey 

Mail Stop 904 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5723 

A.J. Froelich 

U.S. Geological Survey 

926 National Center 

Reston, VA 22092 

(703) 648-6950 

Thomas P. Frost 

U.S. Geological Survey 

Room 656 U.S. Courthouse 

W. 920 Riverside Ave. 

Spokane, WA 99201 

(509) 353-2165 

Andrea J. Gallagher 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1388 

C.H. Gammons 

Department of Earth Sciences 

Monash University 

Clayton, Victoria 3168 

Australia 

011-61-3-565-5772 

Andres Garcia 

Corporaci6n Venezolana de 

Guayana-Compania Tecnica 

Minera, C.A. 

Edificio Sede CVG 

Ciudad Bolivar, Venezuela 

011-58-85-25334 

L.J. Garside 

Nevada Bureau of Mines and Geology 

University of Nevada-Reno 

Reno, NV 89557-0088 

(702) 784-6691 

Mark E. Gettings 

U.S. Geological Survey 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

(602) 670-5507 

R.J. Goldfarb 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-2441 

David Gottfried 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6310 

Addresses and Telephone Numbers of Authors 87 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

Richard I. Grauch 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5551 

V.J.S. Grauch 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1393 

Floyd Gray 

U.S. Geological Survey 

c/o U.S. Embassy 

Caracas, Venezuela 

APO Miami 34037 

011-58-86-223-655 

J.E. Gray 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-2446 

H.G. Greene 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3078 

R.C. Greene 

U.S. Geological Survey 

Mail Stop 901 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5420 

D.J. Grimes 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5510 

Andrew Griscom 

U.S. Geological Survey 

Mail Stop 984 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5302 

John A. Grow 

U.S. Geological Survey 

Mail Stop 960 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5754 

Donald J. Grybeck 

U.S. Geological Survey 

4200 University Drive 

Anchorage, AK 99508-4667 

(907) 786-7403 

Hugo Gumucio 

Servicio Geologico de Bolivia 

Casilla 2729 

La Paz, Bolivia 

011-591-237-7310 

Aldo Gutarra 

Oficina Nacional de Evaluacion 

de Recursos Naturales 

Los Petirrojos 315 San Isidro 

Lima, Peru 

011-51-14-410425 A38 

Luis Guzman 

Corporacion Venezolana de 

Guayana-Companfa Tecnica 

Minera, C.A. 

C. C. Chilemex 

Puerto Ordaz, Venezuela 

011-58-86-228377 

Jane Hammarstrom 

U.S. Geological Survey 

959 National Center 

Reston, VA 22092 

(703) 648-6165 

R.F. Hardyman 

U.S. Geological Survey 

Mackay School of Mines 

University of Nevada-Reno 

Reno, NV 89557-0047 

(702) 784-5379 

Thelma F. Harms 

U.S. Geological Survey 

210 E. 7th Street 

Tucson, AZ 85705 

(602) 670-5514 

D.S. Harwood 

U.S. Geological Survey 

Mail Stop 975 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-4932 

88 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 

James R. Hein 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3150 

James R. Herring 

U.S. Geological Survey 

Mail Stop 939 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5559 

T .G. Hildenbrand 

U.S. Geological Survey 

Mail Stop 989 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5303 

Alberto Hinojosa 

Servicio Geologico de Bolivia 

Casilla 2729 

La Paz, Bolivia 

011-591-237-4544 

Donald B. Hoover 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1326 

D.M. Hopkins 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1908 

Robert J. Horton 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1338 

John W. Hosterman 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6316 

Brenda B. Houser 

U.S. Geological Survey 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

(602) 670-5509 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

Donald F. Huber 

U.S. Geological Survey 

Mail Stop 984 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5358 

Nestor Jimenez 

Servicio Geologico de Bolivia 

Casilla 2729 

La Paz, Bolivia 

011-591-237-7310 

Roy A. Johnson 

Department of Geosciences 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

(602) 621-4890 

David J. Johnston 

Schlumberger Well Services 

21250 E. 31st Circle 

Aurora, CO 80011 

(303) 375-8090 

Janet L. Jones 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1865 

Robert J. Kamilli 

U.S. Geological Survey 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

(602) 670-5576 

Jung-Keuk Kang 

Korea Ocean Research and 

Development Institute 

An Sun, P.O. Box 29 

Seoul425-600 Korea 

011-82-2-863-4770 

S.M. Karl 

U.S. Geological Survey 

4200 University Drive 

Anchorage, AK 99508-4667 

(907) 786-7428 

JohnS. Kelley 

U.S. Geological Survey 

4200 University Drive 

Anchorage, AK 99508-4667 

(907) 786-7414 

K.D. Kelley 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-2467 

J.E. Kilburn 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1865 

E.B. Kisvarsanyi 

Missouri Department of 

Natural Resources 

Division of Geology and 

Land Surveys 

P.O. Box 250 

Rolla, MO 65401 

(314) 364-1752 

D.P. Klein 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1313 

Terry L. Klein 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6339 

Dan H. Knepper, Jr. 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1385 

Randolph A. Koski 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3208 

Henry G. Kreis 

Asarco, Inc. 

P.O. Box 5747 

1150 North 7th Avenue 

Tucson, AZ 85703 

(602) 792-3010 

J.M. Kruger 

Department of Geosciences 

University of Arizona 

Gould-Simpson Building #77 

Tucson, AZ 85721 

( 602) 621-4890 

Richard D. Krushensky 

U.S. Geological Survey 

917 National Center 

Reston, VA 22092 

(703) 648-6060 

M.J. Kunk 

U.S. Geological Survey 

981 National Center 

Reston, VA 22092 

(703) 648-5332 

Keith A. K venvolden 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3213 

Victor F. Labson 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1312 

D.D. Lambert 

Department of Earth Sciences 

Monash University 

Clayton, Victoria 3168 

Australia 

William H. Langer 

U.S. Geological Survey 

Mail Stop 913 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1249 

Curtis E. Larsen 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6342 

Keenan Lee 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1375 

Addresses and Telephone Numbers of Authors 89 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

J.L. Lizeca 

Servicio Geologico de Bolivia 

Casilla 2729 

La Paz, Bolivia 

011-591-237-7310 

Keith R. Long 

U.S. Geological Survey 

210 E. 7th Street 

Tucson, AZ 85705 

(602) 629-5512 

Karen Lund 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5600 

Dawn J. Madden-McGuire 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1856 

L.B. Magoon 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 354-3010 

Frank T. Manheim 

U.S. Geological Survey 

Woods Hole, MA 02543 

(508) 548-8700 

M.S. Marlow 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3129 

Sherman P. Marsh 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5521 

Anne E. McCafferty 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1397 

J. Howard McCarthy, Jr. 

U.S. Geological Survey 

Mackay School of Mines 

University of Nevada-Reno 

Reno, NV 89557-0047 

(702) 784-5362 

Teresa A. McGervey 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6345 

J.B. McHugh 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1908 

Gregory E. McKelvey 

U.S. Geological Survey 

210 E. 7th Street 

Tucson, AZ 85705 

(602) 629-5581 

A.L. Meier 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1902 

Jon L. Mikesell 

U.S. Geological Survey 

990 National Center 

Reston, VA 22092 

(703) 648-6996 

William R. Miller 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5558 

Fernando P. Miranda 

Petrobras-CENPES 

Cidade Universitaria-Ilha do Fundao 

Rio de Janeiro RJ-21740 

Brazil 

90 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 

Peter J. Modreski 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5639 

Anita Moore-Nail 

Box 453 

Gardiner, MT 59030 

( 406) 848-7 408 

J.W. Morgan 

U.S. Geological Survey 

981 National Center 

Reston, VA 22092 

(703) 648-5334 

Janet L. Morton 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3209 

Ramon Moscoso 

Servicio Nacional de 

Geologia y Mineria 

Casilla 10465 

Santiago, Chile 

011-56-237-5050 

J .M. Motooka 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-2460 

Falma Moye 

Idaho State University 

Pocatello, ID 83209 

(208) 236-3549 

Fernando Murillo 

Servicio Geologico de Bolivia 

Casilla 2729 

La Paz, Bolivia 

011-591-237-7310 

James B. Murowchick 

Department of Geosciences 

University of Missouri 

Kansas City, MO 64110-2499 

(816) 276-2980 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

A.J. Naldrett 

Department of Geology 

University of Toronto 

Toronto, Ontario MSS 1A1 

Canada 

(416) 978-3022 

Philip H. Nelson 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1322 

Warren J. Nokleberg 

U.S. Geological Survey 

Mail Stop 904 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5732 

L.M. Noelle 

Missouri Department of 

Natural Resources 

Division of Geology and 

Land Surveys 

P.O. Box 250 

Rolla, MO 65401 

(314) 364-1752 

Fernando Nunez 

Corporaci6n Venezolana de 

Guayana-Companfa Tecnica 

Minera, C.A. 

Edificio Sede CVG 

Ciudad Bolfvar, Venezuela 

011-58-85-25334 

Terry W. Offield 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6135 

S.D. Olmore 

U.S. Geological Survey 

c/o U.S. Embassy 

Caracas, Venezuela 

APO Miami 34037 

011-58-85-25378 

N.J Page 

U.S. Geological Survey 

210 E. 7th Street 

Tucson, AZ 85705 

(602) 670-5580 

Bambang Pardiarto 

Directorate of Mineral Resources 

Jl. Diponegoro 57 

Bandung 40133 Indonesia 

011-62-22-73205 

E.C. Perry, Jr. 

Department of Geology 

Northern Illinois University 

De Kalb, IL 60115 

(815) 753-1944 

ZellE. Peterman 

U.S. Geological Survey 

Mail Stop 963 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-7887 

James A. Peterson 

U.S. Geological Survey 

301 Pattee Canyon Drive 

Missoula, MT 59812 

(406) 542-2087 

W .J. Pickthorn 

U.S. Geological Survey 

Mail Stop 901 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5446 

Frances Wahl Pierce 

U.S. Geological Survey 

210 E. 7th Street 

Tucson, AZ 85705 

(602) 670-5508 

David Z. Piper 

U.S. Geological Survey 

Mail Stop 902 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5187 

James A. Pitkin 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1387 

Peter Popenoe 

U.S. Geological Survey 

Woods Hole, MA 02543 

(508) 548-8700 

L.J. Poppe 

U.S. Geological Survey 

Woods Hole, MA 02543 

(508) 548-8700 

Christopher J. Potter 

U.S. Geological Survey 

Mail Stop 939 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-3282 

Freddy Prieto 

Corporaci6n V enezolana de 

Guayana-Companfa Tecnica 

Minera, C.A. 

C.C. Chilemex 

Puerto Ordaz, Venezuela 

011-58-86-228377 

Luis Quispesivana 

Instituto Geologico Minero 

y Metalurgico 

Pablo Bermudez 211 Jesus-Maria 

Lima, Peru 

011-51-14-316233 A41 

G.L. Raines 

U.S. Geological Survey 

Mackay School of Mines 

University of Nevada-Reno 

Reno, NV 89557-0047 

(702) 784-5574 

V. Rajamani 

J. Nehru University 

New Delhi 110067 

India 

William H. Raymond 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5622 

B.A. Richmond 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3144 

J.R. Riehle 

U.S. Geological Survey 

4200 University Drive 

Anchorage, AK 99508-4667 

(907) 786-7415 

Addresses and Telephone Numbers of Authors 91 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

Jaime Rodriguez 

Centro de Estudios Espaciales 

Universidad de Chile 

Arturo Prat 11 71 

Santiago, Chile 

011-56-2-5553400 

David H. Root 

U.S. Geological Survey 

920 National Center 

Reston, VA 22092 

(703) 648-6144 

L.C. Rowan 

U.S. Geological Survey 

927 National Center 

Reston, VA 22092 

(703) 648-6381 

D.M. Rubin 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3060 

R.O. Rye 

U.S. Geological Survey 

Mail Stop 963 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-7886 

Ward E. Sanford 

U.S. Geological Survey 

431 National Center 

Reston, VA 22092 

(703) 648-5882 

Richard F. Sanzolone 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-2451 

D.L. Sawatzky 

U.S. Geological Survey 

Mackay School of Mines 

University of Nevada-Reno 

Reno, NV 89557-0047 

(702) 784-5379 

Kathryn M. Scanlon 

U.S. Geological Survey 

Woods Hole, MA 02543 

(508) 548-8700 

Jeanine M. Schmidt 

U.S. Geological Survey 

4200 University Drive 

Anchorage, AK 99508-4667 

(907) 786-7 494 

D.W. Scholl 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3127 

Marjorie S. Schulz 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3151 

R.A. Schweickert 

Geology Department 

Mackay School of Mines 

University of Nevada-Reno 

Reno, NV 89557-0047 

(702) 784-6901 

R.M. Senterflt 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1355 

Wayne C. Shanks III 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6336 

D.R. Shawe 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5588 

Richard A. Sheppard 

U.S. Geological Survey 

Mail Stop 939 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5563 

Michael A. Shubat 

Utah Geological and Mineral Survey 

606 Blackhawk Way 

Salt Lake City, UT 84108-1280 

(801) 581-6831 

92 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 

Gary B. Sidder 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5607 

N.J. Silber ling 

U.S. Geological Survey 

Mail Stop 919 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5660 

Miles L. Silberman 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5511 

P.K. Sims 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5621 

K.E. Slaughter 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-2446 

Brian M. Smith 

Lawrence Berkeley Laboratory 

Mail Stop 70A-3363 

1 Cyclotron Road 

Berkeley, CA 94 720 

(415) 486-6508 

Bruce D. Smith 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1399 

David B. Smith 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1800 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

Steven M. Smith 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1191 

Virginia K. Smith 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3151 

L.W. Snee 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5619 

Charles S. Spirakis 

U.S. Geological Survey 

Mail Stop 939 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1558 

W.D. Stanley 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1328 

J.H. Stewart 

U.S. Geological Survey 

Mail Stop 901 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5412 

D.M. Sutphin 

U.S. Geological Survey 

920 National Center 

Reston, VA 22092 

(703) 648-6134 

Gregg A. Swayze 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1411 

James V. Taranik 

Desert Research Institute 

University of Nevada System 

P.O. Box 60220 

Reno, NV 89506-0220 

(702) 673-7312 

Ruben Tejada 

Instituto Geologico Minero 

y Metalurgico 

Pablo Bermudez 211 Jesus-Maria 

Lima, Peru 

011-51-14-316233 A39 

P.K. Theobald 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1853 

P.M. Theodorakos 

U.S. Geological Survey 

Mail Stop 973 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-2482 

Charles H. Thorman 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-5601 

Joseph V. Tingley 

Nevada Bureau of Mines and Geology 

University of Nevada-Reno 

Reno, NV 89557-0088 

(702) 784-6691 

R.M. Tosdal 

U.S. Geological Survey 

Mail Stop 901 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5423 

Robert L. Turner 

U.S. Geological Survey 

Mackay School of Mines 

University of Nevada-Reno 

Reno, NV 89557-0047 

(702) 784-5581 

John W. Valley 

Department of Geology and 

Geophysics 

University of Wisconsin 

Madison, WI 53706 

(608) 263-5659 

J.G. Vedder 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3096 

Cesar Vilca 

Instituto Geologico Minero 

y Metalurgico 

Pablo Bermudez 211 Jesus-Maria 

Lima, Peru 

011-51-14-316233 A39 

R.J. Walker 

Department of Geology 

University of Maryland 

College Park, MD 20742 

(301) 405-4089 

Sherilyn Williams-Stroud 

U.S. Geological Survey 

Mail Stop 939 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-8627 

G .R. Winkler 

U.S. Geological Survey 

Mail Stop 905 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-9232 

Wayne S. Wojniak 

U.S. Geological Survey 

Mail Stop 964 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303) 236-1351 

Florence L. Wong 

U.S. Geological Survey 

Mail Stop 999 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-3053 

Addresses and Telephone Numbers of Authors 93 
ADDRESSES AND TELEPHONE NUMBERS OF CONTRIBUTING AUTHORS 

Warren W. Wood 

U.S. Geological Survey 

431 National Center 

Reston, VA 22092 

(703) 648-5875 

Laurel G. Woodruff 

U.S. Geological Survey 

954 National Center 

Reston, VA 22092 

(703) 648-6327 

Chester T. Wrucke 

U.S. Geological Survey 

Mail Stop 901 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5413 

J.C. Wynn 

U.S. Geological Survey 

920 National Center 

Reston, VA 22092 

(703) 648-6463 

Suk-Hoon Yoon 

Korea Ocean Research and 

Development Institute 

An Sun, P.O. Box 29 

Seoul 425-600 Korea 

011-82-2-863-4770 

Michael L. Zientek 

U.S. Geological Survey 

Room 656 U.S. Courthouse 

W. 920 Riverside Ave . 

Spokane, WA 99201 

(509) 353-2166 

Robert A. Zierenberg 

U.S. Geological Survey 

Mail Stop 901 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5437 

U.S. Geological Survey geologists collecting samples below glaciers for a geochemical study, Chugach 

Mountains, south-central Alaska. Photograph by William R. Miller. 

94 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
ORGANIZATION OF THE U.S. GEOLOGICAL SURVEY 

Office Name Telephone City 

Office of the Director 

Director Dallas L. Peck 703/648-7411 Reston 

Associate Director Doyle G. Frederick 703/648-7412 Reston 

Assistant Director for Research Stephen E. Ragone 703/648-4450 Reston 

Assistant Director for Engineering Geology James F. Devine 703/648-4423 Reston 

Assistant Director for Administration Jack J. Stassi 703/648-7200 Reston 

Assistant Director for Programs Peter F. Bermel 703/648-4430 Reston 

Assistant Director for Intergovernmental Affairs John J. Dragonetti 703/648-4427 Reston 

Chief, Public Affairs Office Donovan B. Kelly 703/648-4459 Reston 

Assistant Director for Information Systems James E. Biesecker 703/648-7108 Reston 

National Mapping Division 

Chief Lowell E. Starr 703/648-5748 Reston 

Geologic Division 

Chief Geologist Benjamin A. Morgan 703/648-6600 Reston 

Water Resources Division 

Chief Hydrologist Philip Cohen 703/648-5215 Reston 

ORGANIZATION OF THE GEOLOGIC DIVISION 

Office of the Chief Geologist 

Chief Geologist Benjamin A. Morgan 703/648-6600 Reston 

Associate Chief Geologist William R. Greenwood 703/648-6601 Reston 

Assistant Chief Geologist for Program David P. Russ 703/648-6640 Reston 

Assistant Chief Geologist, Eastern Region Jack H. Medlin 703/648-6660 Reston 

Assistant Chief Geologist, Central Region Harry A. Tourtelot 303/236-5438 Denver 

Assistant Chief Geologist, Western Region William R. Normark 415/329-5101 Menlo Park 

Office of Mineral Resources 

Chief Glenn H. Allcott 703/648-6100 Reston 

Chief, Branch of Alaskan Geology Willis H. White 9071786-7403 Anchorage 

Chief, Branch of Eastern Mineral Resources Klaus J. Schulz 703/648-6327 Reston 

Chief, Branch of Central Mineral Resources David A. Lindsey 303/236-5568 Denver 

Chief, Branch of Western Mineral Resources William C. Bagby 415/329-5477 Menlo Park 

Chief, Branch of Geochemistry David B. Smith 303/236-1800 Denver 

Chief, Branch of Resource Analysis W. David Menzie II 703/648-6125 Reston 

Chief, Branch of Geophysics David L. Campbell 303/236-1212 Denver 

Office of Energy and Marine Geology 

Chief Gary W. Hill 703/648-6472 Reston 

Chief, Branch of Petroleum Geology Donald L. Gautier 303/236-5711 Denver 

Chief, Branch of Coal Geology Harold J. Gluskoter 703/648-6401 Reston 

Chief, Branch of Sedimentary Processes Walter E. Dean, Jr. 303/236-1644 Denver 

Chief, Branch of Pacific Marine Geology David A. Cacchione 415/329-3184 Menlo Park 

Chief, Branch of Atlantic Marine Geology Bradford Butman 617/837-4211 Woods Hole 

Organization of the U.S. Geological Survey 95 
Office 

Office of Regional Geology 

Chief 

Chief, Branch of Eastern Regional Geology 

Chief, Branch of Central Regional Geology 

Chief, Branch of Western Regional Geology 

Chief, Branch of Isotope Geology 

Chief, Branch of Astrogeology 

Chief, Branch of Paleontology and Stratigraphy 

Office of Earthquakes, Volcanoes, and Engineering 

Chief 

Chief, Branch of Engineering Seismology 

and Geology 

Chief, Branch of Global Seismology 

and Geomagnetism 

Acting Chief, Branch of Seismology 

Chief, Branch of Geologic Risk Assessment 

Chief, Branch of Tectonophysics 

Chief, Branch of Igneous and Geothermal Processes 

Office of Scientific Publications 

Chief 

Chief. Branch of Eastern Technical Reports 

Chief, Branch of Central Technical Reports 

Chief, Branch of Western Technical Reports 

Chief, Library and Information Services 

Chief, Branch of Visual Services 

Office of International Geology 

Chief 

Addresses 

U.S. Geological Survey 

National Center 

Reston, VA 22092 

U.S. Geological Survey 

4200 University Drive 

Anchorage, AK 99508-4667 

Minerals Information Offices (MIO's) 

MIO, U.S. Geological Survey 

210 East 7th Street 

Tucson, AZ 85705 

(602) 670-5508 

MIO, Department of the Interior 

MS 2647-MIB, Room 2647 

1849 C Street NW 

Washington, DC 20240 

(202) 208-5512 

Name 

Mitchell W. Reynolds 

Wayne L. Newell 

David L. Schleicher 

Rowland W. Tabor 

Carl E. Hedge 

Philip Davis 

John Pojeta, Jr. 

Robert L. Wesson 

Thomas L.Holzer 

Robert P. Masse 

John Van Schaack 

Kaye M. Shedlock 

William H. Prescott 

Robert L. Christiansen 

John M. Aaron 

Simon M. Cargill 

Lawrence F. Rooney 

James B. Pinkerton 

Barbara A. Chappell 

John R. Keith 

A. Thomas Ovenshine 

U.S. Geological Survey 

Box 25046, Denver Federal Center 

Denver, CO 80225 

U.S. Geological Survey 

2255 North Gemini Drive 

Flagstaff, AZ 86002 

MIO, U.S. Geological Survey 

Mackay School of Mines 

Telephone City 

703/648-6960 Reston 

703/648-6900 Reston 

303/236-1258 Denver 

415/329-4909 Menlo Park 

303/236-7880 Denver 

602/527-7015 Flagstaff 

703/648-5288 Reston 

703/648-6714 Reston 

415/329-5613 Menlo Park 

303/236-1510 Denver 

415/329-4780 Menlo Park 

303/236-1585 Denver 

415/329-4860 Menlo Park 

415/329-5228 Menlo Park 

703/648-6077 Reston 

703/648-6147 Reston 

303/236-5457 Denver 

415/329-5043 Menlo Park 

703/648-4305 Reston 

703/648-4357 Reston 

703/648-604 7 Reston 

U.S. Geological Survey 

345 Middlefield Road 

Menlo Park, CA 94025 

U.S. Geological Survey 

Quissett Campus, Building B 

Woods Hole, MA 02543 

Scrogham Engineering Mines Building 

University of Nevada-Reno 

MIO, U.S. Geological Survey 

Room 656 U.S. Courthouse 

W. 920 Riverside Ave. 

Spokane, W A 99201 

Reno, NV 89557-0047 (509) 353-2649 

(702) 784-5552 

96 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
INDEX OF AUTHORS 

A Dilles, J.H ............................................................... 67 

Alminas, H.V ............................................................. 1 Dingler, J.R ............................................................. 78 

Andre, Orlando ......................................................... 11 Doe, Bruce R ........................................................... 19 

Arth, Joseph G ........................................................... 3 Doebrich, Jeff L ................................................... 20, 67 

Ashley, Roger P .......................................................... 2 Drew, Lawrence J. . ..................................................... 6 

Ayuso, Robert A .................................................... 3, 24 

E 

B Eiswerth, Barbara A .............................................. 21, 23 

Bankey, Viki ......................................................... 6, 53 Eppinger, R. G. . ........................................................ 22 

Barnes, David F .......................................................... 3 Erdman, J .A ........................................................ 20, 22 

Bawiec, Walter J ......................................................... 4 Ericksen, George E ............................................... 15, 23 

Belkin, Harvey E ....................................................... 26 Estanga, Y asmin ........................................................ 69 

Bisdorf, Robert J ....................................................... 41 Evans, Karl V ...................................................... 16, 74 

Blank, H. Richard ....................................................... 5 Evarts, Russell C ......................................................... 2 

Bliss, James D .......................................................... 53 

Bohlke, J.K .............................................................. 76 F 

Bolm, Karen Sue ....................................................... 59 Ficklin, W.H ....................................................... 35, 55 

Bonham, H.F ............................................................ 20 Finn, C.A ................................................................ 45 

Bookstrom, Arthur A .................................................. 20 Fisher, Frederick S ................................................ 11 , 31 

Bowers, T.L ............................................................. 21 Foley, Nora K ........................................................... 24 

Box, Stephen E .................................................... 29, 46 Force, Eric R ....................................................... 11, 26 

Brew, David A ...................................................... 6, 45 Ford, A.B ................................................................ 45 

Brooks, William E ..................................................... 69 Froelich, A.J ............................................................ 26 

Bultman, Mark W ...................................................... 31 Frost, Thomas P ........................................................ 29 

Bundtzen, Thomas K .................................................. 36 

Burchell, Alison ........................................................ 11 G 

c 

Gallagher, Andrea J. . ................................................... 8 

Gammons, C.H ......................................................... 74 

Cady, John W ....................................................... 6, 52 Garcia, Andres .......................................................... 69 

Campbell, Andrew C ................................................. .46 Garside, L.J ........................................................ 20, 67 

Cannon, William F ...................................................... ? Gettings, Mark E .................................................. 11, 31 

Carrasco, Raul .......................................................... 11 Goldfarb, R.J ............................................. 32, 34, 45, 46 

Carten, Richard B ...................................................... 20 Gottfried, David ........................................................ 26 

Chen Nansheng ......................................................... 33 Grauch, Richard I. ..................................................... 33 

Church, S.E ............................................................. 65 Grauch, V.J.S ........................................................... 34 

Clark, Roger N ........................................................... 8 Gray, Floyd .............................................................. 80 

Clark, Sandra H.B ....................................................... 8 Gray, J.E ....................................................... 34, 39, 46 

Closs, L.G ............................................................... 22 Greene, H.G ............................................................. 78 

Commeau, J.A .......................................................... 63 Greene, R.C ............................................................. 67 

Connor, Jon J ........................................................... 57 Grimes, D.J ............................................................. 35 

Conway, Clay M ......................................................... 9 Griscom, Andrew ........................................................ 4 

Cook, Harry E .......................................................... 63 Grow, John A ........................................................... 63 

Cookro, Theresa M .................................................... 10 Grybeck, Donald J ..................................................... 36 

Corbett, Kevin P ........................................................ 52 Gumucio, Hugo ......................................................... 21 

Cordell, L.E ............................................................. 16 Gutarra, Aldo ........................................................... 21 

Coveney, Raymond M., Jr ........................................... 33 Guzman, Luis ........................................................... 69 

Cox, Dennis ............................................................. 11 

Cox, Leslie J ............................................................ 11 H 

Crowe, Douglas E ...................................................... 13 Hammarstrom, Jane M ................................................ 55 

Cuitino, Lucia ........................................................... 21 Hardyman, R.F .................................................... 20, 67 

Cunningham, Charles G .......................................... 15, 23 Harms, Thelma F ....................................................... 36 

Harwood, D.S ........................................................... 67 

D Hein, James R ..................................................... 37, 78 

Day, W.C ................................................. 16, 40, 69, 80 Herring, James R ....................................................... 38 

Desborough, George A ................................................ 16 Hildenbrand, T.G ....................................................... 72 

Detra, D.E .......................................................... 18, 34 Hinojosa, Alberto ....................................................... 11 

Diaz, Felipe ............................................................. 21 Hoover, Donald B ............................................ 34, 74, 77 

Index of Authors 97 
Hopkins, D.M ........................................................... 39 Morgan, J.W ............................................................ 76 

Horton, Robert J ................................................... 40, 41 Morton, Janet L .................................................... 46, 78 

Hosterman, John W .................................................... 42 Moscoso, Ramon ....................................................... 21 

Houser, Brenda B ................................................. 11, 43 Motooka, J.M ........................................................... 22 

Huber, Donald F ......................................................... 6 Moye, Falma ............................................................ 22 

Murillo, Fernando ...................................................... 21 

J Murowchick, James B ................................................. 33 

Jimenez, Nestor ......................................................... 21 

Johnson, Roy A ......................................................... 43 N 

Johnston, David J ...................................................... 57 Naldrett, A.J ............................. ; .............................. 76 

Jones, Janet L ...................................................... 10, 46 Nelson, Philip H ........................................................ 57 

Nokleberg, Warren J ............................ ~ ...................... 36 

K Nuelle, L.M ............................................................. 16 

Kamilli, Robert J ....................................................... 44 Nufiez, Fernando ....................................................... 69 

Kang, Jung-Keuk ....................................................... 37 

Karl, S.M ................................................................ 45 0 

Kelley, JohnS ............................................................ 3 Offield, ·Terry W ....................................................... 58 

Kelley, K.D ............................................................. 45 Olmore, S.D ......................................................... 69, 80 

Kilburn, J .E ............................................................. 46 

Kisvarsanyi, E.B ....................................................... 16 p 

Klein, D.P ............................................................... 68 Page, N.J ................................................................ 80 

Klein, Terry L ..................................................... 40, 58 Pardiarto, Bambang .................................................... 82 

Knepper, Dan H., Jr. .................................................. 52 Perry, E.C., Jr ............................................................ 1 

Koski, Randolph A ..................................................... 46 Peterman, Zell E ........................................................ 72 

Kreis, Henry G ......................................................... 57 Peterson, James A ...................................................... 63 

Kruger, J.M ............................................................. 43 Pickthorn, W.J .......................................................... 32 

Krushensky, Richard D ................................................. 4 Pierce, Frances Wahl .................................................. 59 

Kunk, M.J ............................................................... 74 Piper, David Z .......................................................... 38 

Kvenvolden, Keith A ............................................. 46, 78 Pitkin, James A ..................................................... 6, 60 

Popenoe, Peter .......................................................... 61 

L Poppe, L.J .......................... , .................................... 63 

Labson, Victor F ....................................................... 41 Potter, Christopher J ................................................... 63 

Lambert, D.D ........................................................... 76 Prieto, Freddy ........................................................... 69 

Langer, William H ..................................................... 47 

Larsen, Curtis E ........................................................ 48 Q 

Lee, Keenan .............................................................. 6 Quispesivana, Luis ..................................................... 21 

Lizeca, J.L ............................................................... 21 

Long, Keith R ...................................................... 11, 52 R 

Lund, Karen ............................................................. 74 Raines, G.L ............................................................. 67 

M R~amani, V ............................................................. 76 

Raymond, William H .................................................. 16 

Madden-McGuire, Dawn J ....................................... 49, 50 Richmond, B.A ......................................................... 78 

Magoon, L.B ............................................................ 65 Riehle, J.R ............................................................... 65 

Manheim, Frank T ..................................................... 61 Rodriguez, Jaime ....................................................... 21 

Marlow, M.S ............................................................ 78 Root, David H ............................................................ 6 

Marsh, Sherman P ............................................. 4, 49, 52 Rowan, L.C ............................................................. 21 

McCafferty, Anne E ...................................... 6, 16, 53, 56 Rubin, D.M ............................................................. 78 

McCarthy, J. Howard, Jr. ....................................... 20, 67 Rye, R.0 .......................... .' ...................................... 16 

McGervey, Teresa A .................................................... 7 

McHugh, J.B ................................................... 1, 35, 55 s 

McKelvey, Gregory E ......................................... 4, 53, 55 Sanford, Ward E ........................................................ 79 

Meier, A.L ......................................................... 22, 35 Sanzolone, Richard F .................................................. 74 

Mikesell, Jon L ......................................................... 57 Sawatzky, D.L .......................................................... 67 

Miller, William R ................................................... 2, 55 Scanlon, Kathryn M ..................................................... 4 

Miranda, Fernando P .................................................. 56 Schmidt, Jeanine M ..................................................... 6 

Modreski, Peter J ....................................................... 57 Scholl, D.W ............................................................. 78 

Moore-Nail, Anita ...................................................... 70 Schulz, Marjorie S ..................................................... 37 

98 Seventh Annual V.E. McKelvey Forum on Mineral and Energy Resources, 1991 
Schweickert, R.A ....................................................... 67 Thorman, Charles H ................................................... 63 

Senterfit, R.M ........................................................... 68 Tingley, Joseph V ...................................................... 20 

Shanks, Wayne C., III ........................................... 13, 46 Tosdal, R.M ............................................................. 69 

Shawe, D.R ............................................................. 20 Turner, Robert L .................................................. 20, 67 

Sheppard, Richard A ................................................... 38 

Shubat, Michael A ..................................................... 10 v 

Sidder, Gary B ................................................ 16, 69, 80 

Silberling, N.J ................. _ ......................................... 67 

Silberman, Miles L ..................................................... 70 

Valley, John W ......................................................... 13 

Vedder, J.G ............................................................. 78 

Vilca, Cesar ............................................................. 21 

Sims, P.K ................................................................ 72 

Slaughter, K.E ..................................................... 34, 39 

Smith, Brian M ......................................................... 70 

Smith, Bruce D .................................................... 40, 41 

Smith, David B ......................................................... 74 

Smith, Steven M ........................................................ 18 

Smith, Virginia K ...................................................... 37 

Snee, L.W ............................................................... 74 

Spirakis, Charles S ..................................................... 38 

Stanley, W.D ............................................................. 6 

Stewart, J. H ............................................................. 67 

Sutphin, D.M ........................................................... 45 

w 

Walker, R.J .............................................................. 76 

Williams-Stroud, Sherilyn ............................................ 38 

Winkler, G.R ............................................................ 50 

Wojniak, Wayne S ................................................ 34, 77 

Wong, Florence L ...................................................... 78 

Wood, Warren W ...................................................... 79 

Woodruff, Laurel G .................................................... 26 

Wrucke, Chester T ..................................................... 52 

Wynn, J.C ............................................................... 80 

Swayze, Gregg A ........................................................ 8 

y 

T Yoon, Suk-Hoon ........................................................ 37 

Taranik, James V ....................................................... 56 

Tejada, Ruben ............................................................ 21 z 

Theobald, P.K ..................................................... 18, 22 Zientek, Michael L ..................................................... 82 

Theodorakos, P.M ...................................................... 18 Zierenberg, Robert A ................................................. .46 

Index of Authors 99