USGS RESEARCH ON MINERAL RESOURCES-1994 

PART A-PROGRAM AND ABSTRACTS 

NINTH V.E. McKELVEY FORUM ON MINERAL AND ENERGY RESOURCES 

Cosponsored by 

THE UNIVERSITY OF 

•

ARIZONA 

GEOLOGICAL 

SOCIETY ARIZONA 

U.S. GEOLOGICAL SURVEY 

CIRCULAR 11 03-A 

TucsoN AruzoNA 

Arizona 

Geological 

Survey 
Cover. Drawings on the front and back covers are by Peter F. Corrao, Arizona Geological Survey. Front 

cover drawing© 1993 by Peter F. Corrao; back cover drawing© 1989 by Peter F. Corrao. 
USGS RESEARCH ON MINERAL RESOURCES-1994 

PART A-PROGRAM AND ABSTRACTS 

Arizona Geological Society 

Edited by L.M.H. Carter, M.I. Toth, and W.C. Day 

NINTH V.E. MCKELVEY FORUM ON 

MINERAL AND ENERGY RESOURCES 

U.S. GEOLOGICAL SURVEY 

Cosponsored by 

University of Arizona 

9th V.E. McKelvey Forum 

Mineral Resources 

Arizona Geological Survey 

U.S. GEOLOGICAL SURVEY CIRCULAR 1103-A 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Robert M. Hirsch, Acting Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994 

Free on application to U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

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

does not imply endorsement by the U.S. Government 

Library of Congress No. 9140066 

Organizing Committee for the 1994 V.E. McKelvey Forum 

Warren C. Day, General Chair 

Eric R. Force, Chair, Field Trip Committee 

Frances W. Pierce, Chair, Logistics Committee 

John G. Viets, Chair, Program Committee 

USGS Committee Members 

John H. DeYoung, Jr. 

Sandra H. Clark 

Frederick S. Fisher 

Bruce M. Gamble 

Jeffery N. Grossman 

Cosponsoring Agencies 

Marguerite J. Kingston 

Edwin H. McKee 

Suzanne W. Nicholson 

Norman J Page 

Martha S. Power 

Larry D. Fellows, State Geologist, Arizona Geological Survey 

John-Mark G. Staude 

Charles H. Thorman 

Margo I. Toth 

Kenneth Watson 

Warren E. Yeend 

Clem G. Chase, Chairman, Department of Geosciences, University of Arizona 

Brenda B. Houser, Past-President, Arizona Geological Society 

Frances W. Pierce, President, Arizona Geological Society 

Industry Representative Committee Members 

John E. Larson, BHP-Utab, S.A. de C.V., Hermosillo, Sonora, Mexico 

Greg E. McKelvey, Phelps Dodge Mining Company 

Anna M. Domitrovic, Field Trip Coordinator, Arizona Geological Sociey 
A society's wealth depends on the use it makes of raw materials, 

energy, and especially ingenuity. 

-V.E. McKelvey 

III 
FOREWORD 

The abstracts in this volume summarize talks and posters at the U.S. Geological Survey's 

Ninth V.E. McKelvey Forum on Mineral and Energy Resources. This year's Forum includes several innovations, chief among them being our cosponsors, the University of Arizona Department of 

Geosciences, the Arizona Geological Survey, and the Arizona Geological Society. Combining our 

efforts allows us to present a truly comprehensive program. Tucson was chosen as the site of the 

1994 Forum in part to highlight activities of the U.S. Geological Survey (USGS) Tucson field 

office, our cosponsors, and the Center for Inter-American Mineral Resource Investigations 

(CIMRI). Tucson is also an excellent staging area for field trips, another innovation of this year's 

Forum. The field trips provide opportunities to explore the mining history and mineral deposits of 

one of the country's most important mining regions; further, they allow interaction with researchers 

who are reinterpreting the igneous geology of the Tucson Mountains and studying the relation of 

mineralization, rock type, and structural features of southeastern Arizona. 

The McKelvey Forum was created to enhance communication between USGS scientists and 

members of the private sector, academia, and other government agencies; it is one of the main outreach activities of the USGS Geologic Division. Topics for the Forums have alternated between mineral and energy resources; this Forum concentrates on mineral resource issues. 

Our program covers a broad spectrum of concerns-from the assessment and genesis of mineral 

resources to mineral-related environmental studies. A symposium on geology and mineral deposits 

of the southwestern United States and Mexico explores new ideas on the tectonic development and 

mineral endowment of this important region. Results are also presented from a USGS effort to reconstruct the tectonic history of the mineral-rich terrane of the Tucson-Mogollon corridor of eastern Arizona and western New Mexico. Another study examines the location, availability, and economics of 

industrial and metallic mineral deposits along the United States-Mexico border. 

Mineral resource and environmental assessments are an integral part of wise and responsible 

land and resource management plans. Representatives from the U.S. Bureau of Mines, U.S. Forest 

Service, Bureau of Land Management, and Bureau of Indian Affairs will discuss their use of such 

information, along with presentations of the results of mineral resource assessment of the Coronado 

National Forest, Arizona and New Mexico; the Bureau of Land Management Roswell Resource Area, 

New Mexico; the Kaibab National Forest, Arizona; and the East Mojave National Scenic Area, California. 

Summitville, Colorado, has been the focus of an intense effort to understand the geochemistry 

and environmental effects of cyanide and toxic metals released from the Summitville mine on downstream water supplies and crops. The distribution and movement of metals from mining districts of 

Coeur D'Alene, Idaho, and Rio Tinto, Spain are also ongoing environment-oriented projects for 

which results are presented here. 

In light of the fact that in the private sector, emphasis for mineral exploration and development 

has shifted from a domestic to a global focus, this year's Forum also includes presentations of the 

results of mineral resource assessments of the Venezuelan Guayana shield; aeromagnetic data from 

the Brazilian part of the shield; new digital geologic information for Puerto Rico and Costa Rica; and 

studies of the regional framework and ore genesis from Bolivia, Brazil, Chile, Mexico, Peru, and the 

Venezuelan Andes. 

Our cosponsors and I welcome you to the Ninth V.E. McKelvey Forum on Mineral and Energy 

Resources. I look forward to the lively exchange of ideas and information during the Forum, and I 

anticipate receiving valuable feedback that will help guide our program development within the 

USGS to continue to meet the needs of the Nation. 

Robert M. Hirsch, Acting Director 

U.S. Geological Survey 

IV 
NINTH V.E. MCKELVEY FORUM ON MINERAL AND 

ENERGY RESOURCES 

TUCSON, ARIZONA, FEBRUARY 22-25, 1993 

PROGRAM OF LECTURES, DISCUSSIONS, AND FIELD TRIPS 

All McKelvey talks and posters will be held in the 

Tucson Convention Center Grand Ballroom 

TUESDAY, FEBRUARY 22, 1994 

Pre-meeting Field Trips 

field and laboratory spectroscopic analysis to investigate 

the environmental impact of mining in the southeastern San 

Juan Mountains and adjacent San Luis Valley, Colorado 

Trip 

2 

3 

4:00-9:00 pm 

6:00-8:00 

6:00-9:00 

Historic mining camps of southeastern Arizona 

(Brenda B. Houser, USGS) 

Relationship of mineralization to key rock types and 

structural features of southeastern Arizona-Rincon and 

Santa Rita Mountains 

(Harald D. Drewes, USGS, and 

Stephen J. Reynolds, Arizona State University) 

Silver Bell porphyry copper deposit, Silver Bell 

Mountains, Arizona 

(Spencer R. Tilley, University of Arizona) 

Registration-Tucson Convention Center Lobby 

Open House-CIMRI, MIO, and ESIC 

Posters and Reception 

WEDNESDAY, FEBRUARY 23, 1994 

7:30am 

8:00 

v 8:30-11:40 

Session Chairs: 

8:30 

8:50 

9:10 

9:30 

Registration 

Welcoming Comments and Overview 

Officials of the U.S. Geological Survey 

Elizabeth Ann Rieke (Assistant Secretary for Water and 

Science, U.S. Department of the Interior) 

Minerals Related Environmental Studies 

Kathleen S. Smith (USGS) and Larry D. Fellows 

(Director and State Geologist, Arizona Geological Survey) 

Gilpin R. Robinson, Jr., and Larry P. Gough (USGS), 

Overview of environmental mineral resources and related 

investigations in the Geologic Division, U.S. Geological 

Survey 

WilliamR. Miller, John B. McHugh, and Walter H. Ficklin* 

(USGS), The generation of natural acid drainage and the 

formation of iron bogs, Rocky Mountains, Colorado 

Walter H. Ficklin*, Geoffrey S. Plumlee, and Kathleen S. 

Smith (USGS), Geologic and geochemical controls on the 

composition of water draining from diverse mineral 

deposits 

Trude V. V. King, Cathy Ager, Roger N. Clark, Gregg A. 

Swayze, and Andrea J. Gallagher (USGS), Application of 

*Walter H. Ficklin, deceased 10/11/93. 

v 

9:50 

10:10-10:30 

10:30 

10:50 

11:10 

11:30 

S.E. Church (USGS), Use of lead isotopes to imgerprint 

sources of heavy-metal contamination in the environment 

Coffee Break and Poster Session 

James R. Herring and Larry P. Gough (USGS) Agricultural 

mineral mining, processing, and use-Environmental 

research opportunities 

C.H. Nelson, A. Van Geen, P.J. Lamothe (USGS), and A. 

Palanques (Ciencias del Mar, Barcelona, Spain), Heavymetal contamination. in river, estuary, and marine shelf 

sediment from the Rio Tinto mines, Spain 

William P. Blacutt (Univ. of Arizona), Recent 

environmental awareness in Latin America-The Bolivian 

case 

Carroll Ann Hodges (USGS), Mineral resources, 

environmental concerns, and land-use 

12:00 Luncheon (optional, preregistration requested) 

12:30 Willis H. White (USGS), Introductory remarks 

Welcoming remarks from Cosponsoring Organizations and 

Organizing Committee 

Larry D. Fellows (Director and State Geologist, Arizona 

Geological Survey) 

Clement G. Chase (Chairman, Department of Geosciences, 

Univ. of Arizona) 

Frances Wahl Pierce (President, Arizona Geological 

Society) 

Warren C. Day (General Chair, Organizing Committee, 

USGS) 

12:40 Luncheon Speaker 

Stephen D. Hoffman (Chief, Mining Section, Office of 

Solid Waste, U.S. Environmental Protection Agency), 

Developing a rational approach to regulating 

mining-Integrating the reauthorization of the Clean Water 

Act, CERCLA, and the reforming of the mining law of1872 

V 1:30-5:10 Symposium on Geology and Mineral Deposits of the 

Southwestern United States and Mexico 

Session Chairs: I.eslie J. Cox (USGS) and Clement G. Chase 

(Univ. of Arizona) 
1:30 

1:55 

2:20 

2:45 

3:10-3:30 

3:30 

3:55 

4:20 

4:45 

5:10-7:00 

Stephen J. Reynolds (Arizona State Univ.) and Ed DeWitt 

(USGS), Tectonic evolution of the southwestern United 

States 

Forrest G. Poole (USGS), Geologic setting of mineral 

deposits in the Ouachita-Marathon-Sonora orogenic 

system along the southern margin of North America 

J. Ruiz. E. Centeno-Garcia, P.J. Coney, R. Torres-Vargas, 

P.J. Patchett, and P. Yanez (Univ. of Arizona), Paleozoic 

and Mesozoic tectonic evolution of Mexico based on 

geochemistry of basement rocks 

Mark D. Barton (Univ. of Arizona), John-Mark G. Staude 

(USGS), and Lukas Ziircher (Univ. of Arizona), Igneousrelated mineralization in Mexico 

Coffee Break and Poster Session 

Jaime Roldan Quintana (Univ. of Mexico-Hermosillo), 

Brief description of the geology and mineral deposits of 

southern Sonora, Mexico 

John-Mark G. Staude (USGS), Tectonic reconstruction of 

northwestern Mexico with implications for mineral 

resources-Anew regional ore deposit synthesis and model 

Karen J. Wenrich and Miles L. Silberman (USGS), Au-Ag 

polymetallic mineralization in the Music Mountain area, 

Arizona, along the southwestern edge of the Colorado 

Plateau 

Robert J. Kamilli (USGS), The Mogollon mining 

district-Results from mapping, paragenesis, and fluid 

inclusion studies 

Poster Session and Refreshments 

THURSDAY, FEBRUARY 24, 1994 

V 8:0~10:00 am Mineral Resource and Mineral Environmental 

Assessments for Land and Resource Management 

Session Chairs: Martha S. Power and 

8:00 

9:00 

9:20 

9:40 

Joseph A. Briskey, Jr. (USGS) 

Panel Discussion on Land-Use PlanningUse of Minerals lnfomUJtion in Managing the Nation's 

Lands and Resources 

Introduction: Joseph A. Briskey, Jr. (USGS) 

Panel Members: 

Ransom F. Read Ill (U.S. Bureau of Mines, Chief, Branch 

of Mineral Land Assessment) 

G. Lynn Sprague (U.S. Forest Service, Director, Minerals 

and Geology Management) 

Adam A. Sokoloski (Bureau of Land Management, Deputy 

Assistant Director, Energy and Mineral Resources) 

Richard N. Wilson (Bureau of Indian Affairs, Chief, 

Division of Energy and Mineral Resources) 

Summary: Martha S. Power (USGS) 

Geoffrey S. Plumlee, Steven M. Smith, Margo I. Toth, and 

Sherman P. Marsh (USGS), Integrated mineral resource 

and environmental assessments of public 

lands-Applications in land-use management 

Dennis P. Cox, Joseph A. Briskey, Jr., Sandra H. D. Clark, 

Michael F. Diggles, Thomas D. Ught, and Alan R. Wallace 

(USGS), The national assessment of undiscovered deposits 

of copper, zinc, lead, silver, and gold-A progress report 

P. B. Barton, R. A. Ayuso, D. A. Brew, E. R. Force, B. M. 

Gamble, R. J. Goldfarb, D. A. John, K M. Johnson, D. A. 

Undsey, and S. D. Ludington (USGS), Quantitative 

assessment of undiscovered, non-fuel mineral resources 

VI 

10:00-10:20 Coffee Break and Poster Session 

V 10:2~11 :40 Results and Impact of Mineral Resource Assessments 

10:20 

10:40 

11:00 

11:20 

11:40 

12:20-1:30 

J.M. Hammarstrom, M.L Zientek, J.E. Elliott, R.R. 

Carlson, G.K. Lee, B.S. Van Gosen, and D.M. Kulik 

(USGS), Mineral resource assessment of the AbsarokaBeartooth study area, Custer and Gallatin National Forests, 

Montana 

James P. Shelden and Shemwn A. Sollid (U.S. Forest 

Service), Mineral resource assessment of the 

Absaroka/Beartooth study area-A USFS perspective 

Jean Juilland and Jerry Dutchover (Bureau of Land 

Management), Regional and Resource Area uses of mineral 

resource information 

Stephen A. Manydeeds (Bureau of Indian Affairs), 

UtilizationofU .S. Geological Survey mineral resource data 

by the Bureau of Indian Affairs 

Director's Lecture: John F. Slack (USGS), Tourmalinites 

and strata-bound ore deposits-Geology, genesis, and 

exploration applications 

No Host Deli Lunch and Poster Session (optional, 

preregistration requested) 

V 1:3~5:00 Mineral Resource Activities in Latin America 

Session Chairs: Barbara A. Eiswerth and Floyd Gray (USGS) 

1:30 

2:00 

2:20 

2:40 

3:00-3:20 

3:20 

3:40 

4:00 

4:20 

4:40 

Douglas B. Silver (Balfour Holdings), Trends in mineral 

exploration in Latin America 

Barbara A. Eiswerth (USGS), Geological remote 

sensing-History and development in Latin America 

C.G. Cunningham, R.E. Zartman, E.H. McKee, R.O. Rye, 

C. W. Naeser, and G.E. Ericksen (USGS), Timing and 

origin of ore deposition at the Cerro Rico de Potosi Ag-Sn 

and Porco Ag-Zn-Pb-Sn deposits, Bolivia 

Albert Hofstra1

, Luis Barrera2, Richard Hardyman1, 

Edwin H. McKee1, and Orlando Sanjinel (

1

USGS and 2Servicio Geologico de Bolivia), La Espanola prospect-A 

multiple intrusive porphyry complex with low-grade gold 

potential, northwestern Altiplano, Bolivia 

Coffee Break and Poster Session 

Richard I. Grauch (USGS), Gold-bearing exhalativehydrothermal systems in the Venezuelan Andes 

Richard M. Tosdal (USGS), Lead sources in Oligocene and 

Miocene precious-metal deposits of the western Andean 

cordillera, southern Peru, western Bolivia, and northern 

Chile 

Edwin H. McKee (USGS), Peter Craig Gibson, Donald C. 

Noble, and Kirk E. Swanson (Mackay School of Mines, 

Univ. of Nevada-Reno), Timing of igneous activity, 

alteration, and mineralization at the Orcopampa Ag-Au 

district, southern Peru 

EdDeWitt1, G. P. Landis1

, R. E. Zartman1, EnzioGaraypel, 

Strgio Luiz Martins Pereira3

, Milton Guimariles Bueno do 

Prada3, Frederico Wallace Reis Vieira', and C. H. 

Thorman1, (1USGS, S~o Bento Minerao S.A., Santa 

Barbara, Minas Gerais, 3Unamgen Minerao e Metalurgia 

S.A., Santa Barbara, Minas Gerais, and 4Minerao Morro 

Velho, Nova Lima, Minas Gerais), Isotopic and fluid 

inclusion data on the age and origin of the S~o Bento and 

Morro Velho gold deposits, Minas Gerais, Brazil 

Willis H. White (USGS), Summary and concluding remarks 
6:00 Cocktails-7:00 Dinner-Ramada fun. Here's your 

chance to unwind and enjoy a casual review with your 

colleagues (optional, preregistration required) 

FlUDAY, FEBRUARY 25, 1994 

Post-meeting Field Trips 

Trip 4 

5 

6 

7 

The Tucson Mountains caldera (Peter W. Lipman, USGS) 

New work in the San Manuel and Mammoth districtsAn introduction via Tucson Wash (Eric R. Force, USGS, 

and William R. Dickinson, Univ. of Arizona) 

Silver Bell porphyry copper deposit, Silver Bell Mountains, Arizona-repeat of Trip 3 

(Spencer R. Titley, Univ. of Arizona) 

Kitt Peak National Optical Astronomy Observatory 

(Floyd Gray, USGS) 

V.E. MCKELVEY FORUM-POSTER SESSIONS 

[Some of these abstracts share poster space] 

Aleinikoff, J.N., Peterman, ZE., Widmann, B.L, Walter, M., and Futa, 

Kiyoto--Isotopic tracers of gold mineralization in Late Proterozoic and 

Paleozoic limestones of southern Nevada 

Armstrong, Augustus K.-Jurassic Todilto Limestone Member, facies, diagenesis, and mineralogy, Grants uranium district, New Mexico 

Bartsch-Winkler, Susan-Mineral resource assessment of the Roswell 

Resource Area, New Mexico-A cooperative study by three Interior 

Department agencies 

Bawiec, Walter J., and Ambroziak, Russell A.-Digital Geologic 

Information and its application in the mineral resource assessment of 

Puerto Rico 

Bliss, James D.-Development of predictive models for sand and gravel 

deposits in the southwest United States 

Bookstrom, Arthur, Box, Stephen, Smith, Cole, and Lindsay, JamesDistribution and movement of metals derived from mining and mineral 

processing in the Coeur d'Alene River valley 

Bouse, Robin M., Wooden, Joseph L., Titley, Spencer R., Ruiz. Joaquin, and 

Tosdal, Richard M.-ThiU ratios as a potential exploration tool for Agor Au-enriched crustal provinces 

Bultman, Mark W.-A summary of the mineral resource assessment of 

Coronado National Forest, Arizona and New Mexico 

Bultman, Mark W., and Gettings, Mark E.-New techniques of geophysical 

data analysis; an investigation of the geometry, structure, and bedrock 

lithology of the San Rafael basin, Arizona 

Calderone, Gary J., Peterson, Donald W., Faulds, James E., and Butler, 

Robert F.-Paleomagnetism and magnetic fabric of the Apache Leap 

Tuff, central Arizona 

Chaffee, M.A., Carlson, R.R., and Theodorakos, P.M.-Regional 

geochemical studies in the Kaibab National Forest, Arizona 

Church, S.E., and Raines, G.L.-Geochemical maps of Arizona and the 

Great Basin 

Clark, Roger N., and Swayze. Gregg A.-Mapping minerals with imaging 

spectroscopy-Examples illustrating the beginning of a new mapping era 

VII 

Conway, Clay M.-Precambrian massive sulfide deposits in amphibolite 

and granulite-facies rocks, western Arizona and southeastern California 

Cox, Leslie ].-Porphyry copper deposits in Arizona-An example of 

resource evaluation for the U.S. Geological Survey's national assessment 

of copper, lead, zinc, gold, and silver 

Cunningham, Charles G., Fournier, Robert 0., Vikre, Peter G., and Rytuba, 

James J.-Hot-spring textures in epithermal environments 

Cygan, G.L., Hemley, J.J., and Doughten, M. W.-Fe, Pb, Zn, Cu, Au, and 

HCl partitioning between vapor and brine in hydrothermal fluidsImplications for porphyry copper deposits 

Desborough, George A.-Capture of copper, lead, zinc by the zeoliteclinoptilolite in metal-polluted drainages of Colorado 

Drewes, Harald-Geologic contributions to the mineral resource 

assessment of the Coronado National Forest, Arizona and New Mexico 

du Bray, Edward A., and Pallister, John S.-Magmatic evolution of the 

Turkey Creek caldera, Chiricahua Mountains, Arizona 

Erdman, J.A., and Smith, K.S.-lmpact of the Summitville mine on 

irrigation water, agricultural soils, and alfalfa in the southwestern San 

Luis Valley, Colorado 

Erickson, B.M., Briggs, P.H., and Peacock, T.R.-Metal concentrations in 

the wetland vegetation receiving acid mine drainage from St. Kevin 

Gulch, Leadville, Colorado 

Folger, Helen, Hofstra, Albert, and Nowlan, Gary -A progress report on 

the southern California geochemical database and map 

Force, Eric R.-Santa Catalina Mountains, southeastern ArizonaLaramide to Eocene paleodepths from field evidence, and implications 

for mineral potential 

Gallagher, Andrea J., Watson, Ken, Knepper, Daniel H., Jr. and Ager, 

Cathy-Mapping alteration in hot spring environments using imaging 

spectrometry 

Gettings, Mark E.-Constraints on mineral resource potential from analyses 

of geophysical data from the Coronado National Forest, southeastern 

Arizona 

Gettings, Mark E.-Some structural features along the Tucson-Mogollon 

Corridor inferred from gravity and magnetic anomaly data 

Gray, Floyd, Bliss, James D., Page, Norman J, Pierce, Herbert A., and 

Eiswerth, Barbara-Redefmition of lateritic bauxite deposit types, 

northern South America Shield 

Gray, Floyd, Wynn, Jeffrey C., Orris, Greta J., Page, Norman J, Brooks, 

William E., Sidder, Gary B., Day, Warren C., Bliss, J.D., Cox, Dennis P., 

Detra, David, Unkefer, Jason, and Gutierrez. John-Mineral resource 

assessment of the Venezuelan Guayana Shield 

Hoffman, J.D., and Marsh, S.P.-National Geochemical Data Base 

Hon, Ken, and Lipman, Peter-Jurassic calderas in southeastern 

Arizona-The surface manifestation of a composite batholith 

Houser, Brenda B.-Santa Ana-Reserve extension zone-A proposed zone 

of interactive northeast- and northwest-directed extension during the 

Cenozoic, New Mexico, Arizona, and Sonora 
Klein, Douglas P.-Geoelectrical contributions to the mineral resource 

assessment, Coronado National Forest, southeastern Arizona-Buried 

structure and alteration in magmatic centers 

Kover, Alan N., and Schoonmaker, James W., Jr.-U.S. Geological Survey 

side-looking airborne radar (SLAR) mosaics-A regional view for 

mineral exploration 

Kulik, Dolores M.-Gravity and magnetic interpretations applied to mineral 

resource assessment of the Bureau of Land Management Roswell 

Resource Area, east-central New Mexico 

Lichte, Frederick E., Erdman, James A., Gough, Larry P., Yanosky, Thomas 

M., and Balistrieri, Laurie S.-Spatial and temporal variability in the 

chemistry of tree rings downstream from the Summitville mine 

Luedke, Robert G.-Distribution, composition, and age maps of early and 

middle Cenozoic volcanic centers, Arizona, New Mexico, and west 

Texas 

McLean, Hugh-Geology and mineral resources of the Loreto-San Javier 

area of northern Baja California Sur, Mexico-Results of reconnaissance 

field mapping 

Meier, Allen L, Grimes, David J., and Ficklin, Walter H.-Inductively 

coupled plasma mass spectrometry-A powerful analytical tool for 

mineral resource and environmental studies 

Miranda, Fernando P., and McCafferty, Anne E.-Assessment of mineral 

resources and tectonic setting derived from digital aeromagnetic data in 

the Guayana shield, northwestern Brazil 

Nowlan, Gary A.-Geochemistry contributions to the mineral resource 

assessment of the Coronado National Forest, Arizona and New Mexico 

Orris, Greta J., Gray, Floyd, Long, Keith R., Page, Norman J, Staude, JohnMark G., and Bolm, Karen S.-Mineral resources and economics of the 

United States-Mexico border region 

Pierce, Herbert A.-Preliminary interpretation of aeromagnetic data in 

northeastern Nicaragua-Possible relation between structure and 

economic mineral deposits 

Pitkin, James A., Conway, Clay M., and Haxel, Gordon B.-Thorium, 

uranium, and potassium aeroradioactivity maps of Arizona-Geologic 

interpretation and metallogenic significance 

Plumlee, Geoffrey S., Ficklin, Walter H., Montour, Maria, Smith, Kathleen 

S., Meier, Allen L, and Briggs, Paul H.-The geochemistry and 

environmental degradation of cyanide at the Summitville mine, Colorado 

Plumlee, Geoffrey S., Ficklin, Walter H., Smith, Kathleen S., Montour, 

Maria, Gray, John, Hageman, Philip, Briggs, Paul H., and Meier, Allen 

L-Geologic and geochemical controls on the composition of acid 

waters draining the Summitville mine, Colorado 

Plumlee, Geoffrey S., and Severson, R.C.-USGS environmental 

geoscience studies of the Summitville mine and its effects on agriculture 

and wildlife ecosystems in the San Luis Valley, Colorado-Project 

overview 

Vlll 

Poppe, LJ., Commeau, J.A., and Luepke, G.-Silt heavy-mineral placer 

deposits in a lateritic environment-Rivers and insular shelf of northcentral Puerto Rico 

Ratt~. James C., Brooks, William E., and Bove, Dana I.-Preliminary 

geologic map of the Big Lue Mountains 15-minute quadrangle, Greenlee 

County, Arizona, and Catron County, New Mexico 

Rytuba, James J., and Miller, William R.-Environmental geochemistry of 

active and extinct hot spring mercury deposits in the California Coast 

Ranges 

Sawyer, David A.-Geologic map of a Late Cretaceous caldera and related 

porphyry copper-molybdenum ore deposits in the Silver Bell mining 

district, Arizona, and their subsequent Tertiary structural history 

Schruben, Paul G., and Swierk, Robert-Conversion of the mineral 

resource assessment portfolio of the Republic of Costa Rica to a digital 

geographic infonnation system 

Senterjit, R.M., Kamilli, R.J., Abrams, G.A., Klein, D.P., and Ratti, 

].C.-Audio-magnetotelluric and gravity study of the Mogollon mining 

district, southwest New Mexico 

Smith, Bruce D., and Merewether, E.A.-U.S. Geological Survey mineral 

resource studies on Indian lands 

Stewart, Kathleen C., and McKown, David M.-Prospecting for gold with 

sagebrush in the Great Basin-Results of greenhouse studies 

Stout, P.R., Erdman, J.A., and Emerick, J. C.-Impact of acid mine drainage 

from Summitville mine on barley fields in the San Luis Valley, southcentral Colorado 

Sutphin, David M.-Three-part quantitative mineral resource assessment 

for selected undiscovered mineral deposit types, BLM Roswell Resource 

Area, east -central New Mexico 

Sylvester, Marc A.-National Water Quality Assessment program 

Theodore, Ted G., Miller, Robert J., Bishop, Kenneth R., Conway, Clay M., 

Dohrenwend, John C., Duval, Joseph S., Haxel, Gordon B., Hendricks, 

John D., Hodges, Carroll A., Jachens, Robert C., Kingston, Marguerite 

J., Miller, David M., Nowlan, Gary A., Rytuba, James J., and Tosdal, 

Richard M.-Evaluation of metallic mineral resources in the East Mojave 

National Scenic Area, San Bernardino County, California 

Tidball, R.R .• and Bartsch-Winkler, Susan-Interactive. computer display 

on the geology and mineral and energy resources of the Roswell Resource 

Area, New Mexico 

Tidball, Ronald R., Smith, Steven M., and Stewart, Kathleen C.-

Geochemical mapping in the San Luis Valley, ColoradoHydrogeochemical and stream sediment data 

Walton-Day, Katherine, Ortiz. Roderick F., and von Guerard, 

Paul-Preliminary results of a synoptic water-quality study of the upper 

Alamosa River to the outlet of Terrace Reservoir 

NOTE: Due to limitations of space and constraints on topics, several 

abstracts are included in this volume that are not presented in the Forum. 
NINm V.E. McKELVEY FORUM ON MINERAL AND ENERGY RESOURCES 

USGS RESEARCH ON MINERAL RESOURCES-1994 

PROGRAM AND ABSTRACTS 

Edited by L.M.H. Carter, M.I. Toth, and W.C. Day 

ISOTOPIC TRACERS OF 

GOLD MINERALIZATION IN 

LATE PROTEROZOIC AND PALEOZOIC 

LIMESTONES OF SOUTHERN NEVADA 

J.N. Aleinikoff, Z.E. Peterman, B.L. Widmann, 

M. Walter, and Kiyoto Futa 

Disseminated gold deposits (Carlin-type) are important mineral exploration targets in the Great Basin. Many 

deposits occur in Late Proterozoic and Paleozoic limestones and dolomites that were mineralized in the Cretaceous and Tertiary by hydrothermal fluids which deposited 

micrometer- to submicrometer-sized gold particles. Commonly, ore deposition was structurally controlled, and in 

some areas shallow-dipping thrust or detachment faults 

were pathways for the mineralizing solutions. 

Late Proterozoic and Paleozoic shelf carbonate rocks 

crop out extensively in the mountain ranges of southern 

Nevada. Bare Mountain, located about 100 miles (160 km) 

northwest of Las Vegas (immediately west of Yucca Mountain), is composed of heavily mineralized carbonate rock. 

Other ranges, such as the Striped Hills, Specter Range, and 

Spring Mountains, contain the same stratigraphic section, 

but the rocks apparently are mostly barren. We have measured Sr and Pb isotopic compositions and Sr, Rb, Pb, U, 

and Th concentrations in fresh (unaltered) and mineralized 

(altered) rocks, including samples collected along traverses 

that cross contacts between unaltered rocks and ore zones 

in an operating underground mine. The objectives of this 

study are to determine (1) if mineralized rock has Sr and Pb 

isotopic signatures indicative of fluid alteration and Carlintype gold deposition, and (2) if these isotopic tracres can be 

used in exploration. 

1 

Marine limestones incorporate the 87 Srf86Sr value of 

sea water from which they are deposited. The oceans are 

isotopically uniform in strontium at any particular time, but 

87Srf86Sr values have varied systematically throughout Late 

Proterozoic and Phanerozoic time. However, if strontium of 

a different isotopic composition were added to or 

exchanged with strontium in the limestone, then the 

87Srf86Sr in these limestones might be significantly modified from its primary depositional value. Lead isotopic 

ratios (206pbJ204Pb, 207pbJ204Pb, and 208pbJ204Pb) in carbonate are a function of the age of the rock and the concentrations of Pb, U, and Th in the rock. Closed-system, 

modern-day Pb isotopic ratios in Paleozoic carbonate rocks 

are fairly high (such as 206pbJ204Pb >20). 

Cretaceous and Tertiary alteration of Paleozoic carbonate rocks of southern Nevada produced an increase in 

Sr, Pb, U, and Th concentrations, an increase in 87SrJ86Sr, 

and a decrease in 206pbJ204Pb, 207pbJ204Pb, and 

208pbJ204Pb. Although these modifications of the primary 

isotopic compositions and concentrations could have been 

caused by dolomitization or exchange among units during 

metamorphism, the changes were most likely caused by 

introduction of Sr, Pb, U, and Th by hydrothermal fluids 

that had acquired these elements from underlying Precambrian basement rocks. Thus, anomalously high 87Srf86Sr 

and Pb (ppm), and low 206pbJ204Pb, 207pbf204Pb, and 

208pbJ204pb could indicate hydrothermally altered and mineralized carbonate. For convenience and clarity, 87Srf86Sr 

values are expressed as the per mil deviation (()87Sr) from 

modern sea water 87Sr/86Sr. 

Both mineralized and barren Late Proterozoic and Paleozoic carbonates were collected from Bare Mountain. For 

comparison, suites of unmineralized samples were also collected from the Striped Hills, Spring Mountains, and various 

ranges in the vicinity of Indian Springs Valley. ()87 Sr values 

for Paleozoic carbonates from the Striped Hills, Spring 
Mountains, and mountain ranges near Indian Springs Valley 

have a limited range of -1 to +2%o. Some of these values are 

slightly higher than would be expected for primary marine 

values, but we attribute this to relatively minor local 

exchanfe during dolomitization or metamorphism. In contrast, S 7 Sr values for Bare Mountain samples range widely 

and have values as high as +25%o. Low values (<+2%o) are 

from areas on Bare Mountain that do not appear to be mineralized or altered. S87 Sr values of + 3%o or higher are from 

known mineralized areas such as the Sterling mine, the Gold 

Ace mine, Tungsten Canyon, Fluorspar Canyon, Secret Pass, 

and the Telluride mine. Similarly, unaltered rocks have very 

low Pb (0.81 ppm), U (0.35 ppm), and Th (0.78 ppm) and 

high Pb isotopic ratios (for example, 206pbJ204Pb=21.87), 

whereas altered rock has high Pb (51.3 ppm) and Th ( 4.46 

ppm) and low 206pbf204Pb (19.08). 

We conclude that hydrothermal alteration of gold-bearing Late Proterozoic and Paleozoic carbonate rocks is indicated by modifications of the Sr and Pb isotopic systematics. 

The isotopic ratios in the altered rocks presumably reflect the 

isotopic signature of underlying Precambrian basement 

through which the mineralizing fluids passed. Thus, we suggest that Sr and Pb isotopic tracers have considerable potential for identifying hydrothermal alteration in carbonate 

rocks of southern Nevada that may contain cryptic gold 

deposits. 

JURASSIC TODILTO LIMESTONE 

MEMBER, FACIES, DIAGENESIS AND 

MINERALOGY, GRANTS URANIUM 

DISTRICT, NEW MEXICO 

Augustus K. Armstrong 

The Todilto Limestone Member, basal member of the 

Wanakah Formation (Middle Jurassic) of the Grants uranium district, is 1-30 ft (0.3-9.1 m) thick and records the 

change in depositional environments from a restricted 

marine embayment with an ephemeral connection to the 

Curtis-Summerville sea to a completely enclosed and shrinking body of gypsiferous water. The salina measured 300 

miles ( 483 km) from east to west and 250 miles ( 402 km) 

from north to south, and was fringed by an extensive limestone-gypsum sabkha. The arenaceous-lime mudstone 

records a shoaling-upwards sequence deposited in subtidal 

to supratidal sabkha environments, in alternating brackish to 

hypersaline marine waters. Dolomite is absent in the study 

area. The calcite lime mudstones were derived primarily 

from an aragonite mud precursor and were subjected to 

extensive neomorphism. The aragonite-to-calcite diagenetic 

history is evident in the poorly preserved ooids. 

The salina waters did not support a normal marine 

invertebrate fauna or flora. Bioclasts of the marine 

2 

calcareous algae, dasyclads of the Tribe Salpingoporelleae, 

indicate short periods of near-normal marine water as a 

result of sea water influx into the Todilto embayment. 

Ostracodes lived in ephemeral-gypsiferous ponds and are 

abundant in the sabkha facies. The salinity of marine 

waters was influenced by seasonal influx from streams, 

by rainfall, by periods of drought, and by intermittent connections to the Curtis-Summerville sea. The overlying 

0-110 ft (0-33.5 m)-thick gypsum unit found to the east 

of the study area was deposited in the center of the basin 

during the final salina phase. Lacustrine alkaline evaporites, such as trona and shortite, are not known from the 

Todilto Limestone Member. 

Megapolygon mounds to intraformational folds of early 

diagenetic origin, in a vertical stack up to 10 ft (3 m) high 

and 45 ft ( 14 m) wide, occur in the supratidal facies and are 

associated with calcite pseudomorphs of anhydrite-gypsum 

and anhydrite ptygmatic-enterolithic layers. The fractures of 

the megapolygons may have acted as sites of preferential 

saline ground-water outflow. The intraformational folds are 

overlapped and buried by thick-bedded microbial mat, 

supratidal ostracode-lime mudstone. The Todilto Limestone 

Member was subjected to vadose solution that formed sinkholes, rundkarrens, and solution pipes that are filled with 

limestone breccias and ferruginous sands from the overlying 

Beclabito Member of the Wanakah. The smaller vadose 

solution cavities are partially filled with multiple cycles of 

spar calcite and iron-stained quartz sands. Commercial bodies of uranium ore are found in the Todilto intraformational 

folds and associated solution cavities. Isotopic ages 

(155-150 Ma) indicate that the uraninite is nearly syngenetic 

with the carbonates. 

INTRUSION-RELATED 

MINERALIZATION IN MEXICO 

Mark D. Barton, John-Mark G. Staude, and 

Lukas Zurcher 

Igneous-related mineralization in Mexico can be 

broadly divided between (1) epithermal deposits that have 

broad temporal and spatial correlations with volcanism, and 

(2) mineralized deposits clearly associated with intrusive 

centers and exhibiting close relationships in composition, 

time, and space. We review herein patterns of intrusionrelated mineralization and possible controls, based in part on 

continuing research and database compilation by the University of Arizona-mining industry-U.S. Geological Survey 

consortium on the mineral resources of Mexico. 

In Mexico, intrusion-related mineral deposits are primarily Mesozoic to middle Tertiary in age. Although complex in detail, three broad periods are prominent in the 

mineralization record: the late Mesozoic, the Laramide, and 
Selected Intrusion-related Deposits 

Mesozoic and 

Tertiary Magmatism 

0 Middle Tertiary (40-20 Ma); r 

17o D Laramide (80-40 Ma); L 

D pre-Laramide (160-80 Ma); M 

115° 

!Y 

Aguachile 

I • --more felsic _____.. 

Fe(-Cu) 0 W(-Mo) 

(dior-syen) (grd-gr) 

a... disseminated 

~ • Cu(-Mo) ~ 6'1.., t' (grd-gr porph) 

~ "'~""'~ ~ 

0 Mo(-Cu-W) 

(gr porph) 

0 Zn-Pb-Ag(-Cu) 

(gr-rhy porph) 

l 

I 

~ Ag-Au(-Cu-Zn-Pb)" T (dior-grd) V 

Fe 

(rhyolite) 

Au(-Cu) 

(dacite porph) 

F(-Be) 6 (rhyolite) 

(age of mineralization in italics) 

oL_ ____ ~s~oo~----~tooo 

kilometers 

Figure 1 (M. Barton and others). Generalized distribution and timing of selected intrusion-related ore deposits in Mexico. 

the middle Tertiary (fig. 1). Jurassic to Late Cretaceous 

calc-alkalic batholiths with sparse volcanic rocks occur 

along the Pacific margin mainly on eugeoclinal crust, 

although locally on continental crust (for example, in 

Sonora). Latest Cretaceous to early Tertiary ("Laramide") 

calc-alkalic batholithic, subvolcanic, and volcanic centers 

occur in an overlapping but somewhat more easterly band 

that extends with diminished intensity (and somewhat 

younger ages) into the Sierra Madre Oriental. Mid-Tertiary 

volcanism and local intrusive centers are widely developed, 

with a volume maximum of calc-alkalic felsic volcanics in 

the Sierra Madre Occidental, but extending as a more mafic 

middle to late Tertiary arc in the Sierra Madre del Sur in 

southern Mexico, and as a fringe of alkalic volcanic and 

subvolcanic centers in northeastern Mexico. 

Mexico is characterized by multiple styles of intrusionrelated mineralization. These can be divided by geologic 

associations, metal contents, and styles of alteration (table 

1). Although more than 1,000 intrusion-associated mineral 

deposits are known, the scarcity of data requires a simplified 

approach focusing on major districts. The broad continuum 

3 

that exists in the types of mineralization products is a function of exposure level and igneous compositions, which in 

turn mainly reflect age and province. 

Some inferences can be drawn from examination of 

these patterns: 

• Igneous compositions vary in time and space in Mexico, but multiple compositions commonly were emplaced at 

different times in the same region. Temporal variations (as 

in Sonora) are as important as differences in province (as 

between Sonora and southern Mexico). 

• Alteration and metal differences between alkaline and 

subalkaline, felsic and mafic magma suites (table 1) can be 

partly rationalized from equilibria among igneous minerals 

(see fig. 2, aAI203 vs aCaO [vs aSi02D and magmatic 

metal contents (reflecting province and process). 

• Exposure and preservation filter observed Mexican 

metallogeny. Erosion of the Mesozoic arc superstructure in 

the west leaves mainly tungsten skarns, burial of the 

Laramide arc in central Mexico interrupts porphyry copper 

patterns, and minimal exhumation of mid-Tertiary intrusive 

centers preserves distal vein/replacement systems. 
Table 1 (M. Barton and others). Generalized characteristics of intrusion-related mineralization. 

Deposit Type I 

(example) 

Porphyry Cu(-Mo) 

(Cananea, SON) 

Porphyry Cu(-MoAu) (El Arco, BC) 

Fe( -Cu) (Peiia 

Colorada, JAL) 

Ag-Au(-Cu-Zn-Pb) 

(Batopilas, CHI) 

Au(-Cu) (Mulatos, 

SON) 

W(-Mo) (San 

Nicolas, SON) 

Mo(-Cu-W) 

(Cumobabi, SON) 

Zn-Pb-Ag(-Cu) 

(San Martin, ZAC) 

Fe (Cerro de 

Mercado, ZAC) 

F(-Be-Sn) 

(Aguachile, COA) 

-5.50 

-6.00 

0 

«:! 

u 

tj -6.50 

OJJ 

0 

- -7.00 

-7.50 

Associated Igneous 

Rocks (lUGS) 

bi-hbl-mt granodiorite to bi 

monzogranite porphyry 

px-hbl-qz diorite to bi-hbl 

granodiorite porphyry 

px-hbl diorite to (bi) qz 

monzonite 

px-bi-qz diorite to bi 

granodiorite porphyry 

bi-mt(-hbl) dacite to bi 

rhyolite 

\ 

bi(-hbl) granodiorite to mu-bi 

granite 

bi granodiorite to bi granite 

porphyry 

bi granodiorite to (bi) 

rhyolite porphyry 

px-mt qz latite to hbl-mt 

rhyolite 

rieb syenite or bi qz latite to 

(bi-) alkali rhyolite 

Mineralization 

& Alteration 

stockwork Cu w Kf-qz/ ser-py; 

Zn skarn 

stockwork Cu w bi-Kf-mt/chl-pyser; ± Fe skarn 

Fe± Cu skarn w Na- (±K-) 

silicate alteration 

Ag-Au(-Pb-Zn) qz-carb vein & 

replacement; ± stockwork Cu 

Au(-Cu-Ag) advanced argillic & 

sericitic zones 

W(-Mo) skarns, pegmatite & 

greisen (mu-qz) W(-Mo) 

stockwork/pegmatitic Mow Kfqz-mu-py 

Zn-Pb-Ag(-Cu-W-Mo) skarn & 

replacement; ± stockwork qz 

massive hm(after mt) (volcanic?) 

w Fe-pyroxene to Qz-clay-apat 

carbonate replacement fl-qz±Be, 

(± igneous Sn, Mo, topaz) 

Time-space distribution 

(see also Figure 1) 

Laramide, minor pre-Laramide & 

Tertiary; inboard western 

pre-Laramide & Tertiary (SW 

Mexico); outboard western 

pre-Laramide and Laramide; 

outboard western 

pre-Laramide & Tertiary; 

inboard western & north-central 

Tertiary; inboard western 

pre-Laramide & Laramide; 

western 

Laramide; inboard western 

Tertiary, minor Laramide; 

north-central 

Tertiary; north-central 

Tertiary; north-central & east 

• The superimposed metalJogenic patterns in Mexico 

have parallels with metallogenic patterns in the western 

United States in terms of the effects of preservation, process, 

and province. 

Future work should focus on increasing the basic 

geologic data on mineral deposits and igneous rocks. 

Geochronology, petrology, and geochemistry studies 

would help better define the temporal, spatial, and compositional interrelations between tectonism, magmatism, and 

mineralization. 

QUANTITATIVE ASSESSMENT OF 

UNDISCOVERED, NON-FUEL 

MINERAL RESOURCES 

-8.00 +-......&...-......... -+------l--f----'..___~.....>1.4 P.B. Barton, R.A. Ayuso, D.A. Brew, E.R. Force, 

B.M. Gamble, R.J. Goldfarb, D.A. John, K.M. -3.00 -2.00 -1.00 0.00 

log aAiz03 

Figure 2 (M. Barton and others). Calculated aCaO vs. aA120 3 

diagram (at quartz saturation) showing simplified phase relations 

among igneous minerals and hydrothermal fluid. These relationships help rationalize differences between mineralization types associated with different kinds of quartz-bearing igneous rocks. 

Short dashed lines, limits of hornblende (logaparg = -2.7, appropriate for a plausible igneous composition) stability for potassiumbearing and potassium-lacking assemblages. 

4 

Johnson, D.A. Lindsey, and S.D. Ludington 

Since its establishment in 1879, it has been the responsibility of the U.S. Geological Survey to study and report on 

the mineral resources of the public domain. A wide variety 

of mineral deposits and identified resources have been 

described, and the circumstances and processes by which 

they formed have been extensively investigated. In recent 

years, there has been an expanded role in society for land-use 
planning. Because mineral extraction is one of the activities 

competing for access to land, a need has grown for assessments of undiscovered mineral resources. As early as 1975, 

the USGS began using techniques to quantify its assessments, combining grade and tonnage models with estimates 

of the number of deposits to arrive at an estimate of the 

amounts of metals. In 1984, the first assessment was made 

in which these estimates were combined by means of a 

Monte Carlo simulation to provide probabilistic estimates of 

tonnages of metals. 

These protocols have been and will continue to be subject to evolution and improvement. In response to internal 

dissent, the USGS commissioned an extramural panel, 

chaired by Dr. DeVerle Harris of the University of Arizona, 

to review the assessment methods. The report, released in 

March 1993 (USGS Open-file Report 93-258), examines 

and largely vindicates the methodology; it also contains an 

extensive list of suggested improvements. A series of inhouse forums, held at six USGS facilities during fall 1993, 

were designed to evaluate the suggested improvements, seek 

out others, prioritize all suggestions, and analyze their implementation. The outcome of these forums will inform and 

determine future improvements in the assessment program. 

MINERAL RESOURCE ASSESSMENT OF 

THE ROSWELL RESOURCE AREA, 

NEW MEXICO-A COOPERATIVE STUDY 

BY THREE INTERIOR 

DEPARTMENT AGENCIES 

Susan Bartsch-Winkler 

Comprehensive mineral resource reports on the 

Roswell Resource Atea, N.Mex., were prepared by the U.S. 

Bureau of Mines (USBM) and the U.S. Geological Survey 

(USGS) for use by the U.S. Bureau of Land Management 

(BLM) (Bartsch-Winkler, 1992; Bartsch-Winkler and 

Donatich, in press; Korzeb and Kness, 1993). The work was 

conducted in order to assist the BLM in preparing the 

Resource Management Plan for the resource area, which 

encompasses 14 million acres of private, State, and Federal 

land in seven counties in the east-central part of the State 

(fig. 1). The reports result from two separate studies (a mineral inventory by the USBM and an assessment of mineral 

resources by the USGS) which are compilations of published 

and unpublished data, and integrate new findings on the 

geology, geochemistry, geophysics, mineral, industrial, and 

energy commodities, and resources. Both reports were 

guided in the data-gathering processes, field operations, and 

writing stages by the BLM staff in Roswell, who were developing the Management Plan for the Roswell Resource Area. 

In addition, the reports will be used in future nationwide 

5 

mineral and energy resource inventories and assessments, 

and as reference and training documents; they can also be 

used as public-information tools by the BLM, USBM, and 

USGS. The USGS also provided BLM with digital maps on 

the geology (LaRock and Moore, 1992), geochemistry (Tidball and Erdman, 1992), mine and prospect locations, and oil 

and gas well locations and fields and an interactive computer 

display on the geology and resources of the study area prepared for use in the Public Information Office of the BLM in 

Roswell (see Tidball and Bartsch-Winkler, this volume). A 

valuable link was established between the staffs of these 

agencies as a result of this study, and a working relationship 

continues. 

The resource area is not now a major mineral producing area in the State; however, many metals, industrial mineral commodities, and energy resources are, or have been, 

produced or prospected in the past, and future exploration 

potential is discussed. Copper, gold, silver, thorium, uranium and (or) vanadium, rare-earth elements, iron, manganese, tungsten, lead, zinc, and molybdenum, as well as 

many industrial minerals (the most important being fluorspar and gypsum) and fuels and associated resources are 

evaluated in the assessment study. Important commodities 

that have yet to be identified in economic concentrations 

(have no past production) but are permissive in the area 

include potash, halite, polyhalite, anhydrite, sulfur, feldspar, brines, various gases associated with oil and gas 

exploration, and carbon dioxide. 

REFERENCES 

Bartsch-Winkler, Susan, ed., 1992, Mineral and energy resources 

of the BLM Roswell Resource Area, east-central New Mexico: 

U.S. Geological Survey Open-File Report 92-261, 153 p. 

Bartsch-Winkler, Susan, and Donatich, A.J., eds., in press, Mineral 

and energy resources of the Roswell Resource Area, east-central New Mexico: U.S. Geological Survey Bulletin 2063. [Supersedes Open-File Report 92-261.] 

Korzeb, S.L., and Kness, R.F., 1993, Mineral resource investigation of the Roswell Resource Area, Chaves, Curry, DeBaca, 

Guadalupe, Lincoln, Quay, and Roosevelt Counties, New 

Mexico: U.S. Bureau of Mines Open-File ReportMLA 12-92, 

229p. 

LaRock, E.J., and Moore, S.L., 1992, Digital geologic map of the 

Roswell Resource Area, New Mexico: U.S. Geological Survey 

Open-File Report 92-0328-A Discussion (paper copy), 

92-{)328-B Database (diskette), and 92-{)328-C Command 

Files (diskette). 

Tidball, E.R., and Erdman, J.A., 1992, NURE data for groundwater 

and stream sediments, BLM Roswell Resource Management 

Area, east-central New Mexico: U.S. Geological Survey OpenFile Report 92-365-A Discussion (paper copy), 92-365-B 

Database (diskette). 
ROSWELL 

RESOURCE 

AREA 

0 

I 

0 

10 

I I I 10 20 

~---

r· ___. 

r·---' 

r---

----y CURRY 

20 30 40 MILES 

I I I 

Jo ~ sb KILOMETERS 

Figure 1 (Bartsch-Winkler). Index map of the Roswell Resource Area, east-central New Mexico. 

6 
DIGITAL GEOLOGIC INFORMATION 

AND ITS APPLICATION IN THE MINERAL 

RESOURCE ASSESSMENT OF 

PUERTO RICO 

Walter J. Bawiec and Russell A. Ambroziak 

The availability of new and diverse modes of digital 

information, particularly in the field of earth science, has led 

to a new cycle of innovative applications to earth-sciencerelated issues. Included within these applications are mineral 

resource assessments, where digital databases of site-specific information and thematic layers of spatial data can be 

combined and manipulated to serve as an analytical tool. In 

~e mineral resource assessment of Puerto Rico, we used digItal data to produce both integrated and comprehensive 

derivative products. The digital thematic data will be available to the public on CD-ROM. 

The USGS (U.S. Geological Survey), in cooperation 

with the Puerto Rico Department of Natural Resources and 

the University of Puerto Rico at Mayaguez, recently completed a mineral resource assessment of the Commonwealth 

of Puerto Rico as part of the USGS National Mineral 

Resource Assessment Program (NAMRAP). The objectives 

in conducting this assessment were to aid government officials in land-use planning, to inform the community of 

potential new areas of interest, and to compile Puerto Rican 

earth-science-related information, much of which previously 

had not been released. 

This assessment is an example of the use and integration of digital data in the assessment process. The variety of 

digital data utilized includes geology, which permits selection of specific and (or) combinations of map units based on 

rock type or age; geophysics (including gravity and magnetic 

signatures), which infers subsurface structure and the extent 

oflithologies; geochemistry, which indicates surficial anomalous values for metallic and pathfinder elements; mineral 

o~cu.rre~ces, which provide information about the spatial 

~stnb~ttons of known commodities; remote sensing 

(mcludmg SLAR (Side Looking Airborne Radar)), which 

represents surficial geomorphic features; and GLORIA 

~Geological Long Range Inclined Asdic) system, whose 

Images of offshore landforms can be traced onshore. 

Puerto Rico, the easternmost island of the Greater 

Antilles, is a volcanic island-arc whose geologic record 

spans approximately 150 million years. The island consists 

?f volcaniclastic and epiclastic rocks of volcanic origin, sedImentary rocks of Late Jurassic to Paleocene-Eocene age, 

and intrusive mafic and felsic plutonic rocks of Late Cretaceous and early Tertiary age. These rocks are overlain 

unconformably by Oligocene and younger sedimentary 

rocks and sediments. The geology of Puerto Rico is represented by approximately 150 stratigraphic units. Twelve 

7 

terranes based primarily on lithologic affinity and age (fig. 1) 

were the initial starting point for delineation of permissive 

mineral deposit-type terranes. 

The rock types and depositional environments encountered in Puerto Rico are permissive for a wide variety of 

metallic and industrial mineral deposits associated with 

island-arc environments. (Table 1 lists our mineral deposit 

models and the probabilistic estimates of the number of 

undiscovered metallic deposits.) Porphyry copper deposits 

consist of a stockwork of veinlets of quartz, chalcopyrite, 

and molybdenite in or near a porphyritic intrusion. Copper 

skarn and iron skarn deposits are irregular or tabular-shaped 

ore bodies formed in carbonate or calcareous rocks near 

igneous contacts or in xenoliths in igneous stocks. Polymetallic veins consist of quartz-carbonate veins containing gold 

and (or) silver and associated with base-metal sulfides 

related to hypabyssal intrusions in sedimentary and metamorphic terranes. Volcanogenic manganese deposits include 

lenses and stratiform bodies of manganese oxide, carbonate, 

and silicate in volcanic-sedimentary sequences. Epithermal 

quartz-alunite gold deposits occur in island-arc and back-arc 

spreading centers and within vuggy veins and breccias in 

zones of high-alumina alteration. Copper- and zinc-bearing 

Kuroko massive-sulfide deposits occur in marine volcanic 

rocks of intermediate to felsic composition. Placer gold-PGE 

(platinum-group-element) deposits include detrital elemental gold and PGE alloys in grains in gravel, sand, silt, clay, 

and their consolidated equivalents. 

Development of CD-ROM technology enhances the 

use, distribution, and availability of layers of digital data to 

all segments of the earth-science community. The USGS has 

been an innovator in the application of this rapidly expanding technology to earth-science-related activities. In studies 

of Puerto Rico, the interactive layering and manipulation of 

these digital layers have produced new insights and relationships, capable of being observed and interpreted readily on a 

computer screen. 

IlEA VY -METAL CONTENTS OF 

SEDIMENT FROM THE KUSKOKWIM 

RIVER, BETHEL, ALASKAA BASELINE STUDY* 

Harvey E. Belkin and Harold M. Sparck 

The Kuskokwim River, at Bethel, Alaska, drains a 

major mercury-antimony metallogenic province in its upper 

reaches and tributaries. Gray and others (1991) and Nelson 

and others (1977) have reported anomalously high heavymetal values (up to =1,800 ppm) for stream sediments in 

the river above Bethel that are invariably associated with 

*Not presented at Forum. 
Table 1 (Bawiec and Ambroziak). Estimates of undiscovered mineral deposits at specified probability levels, for Puerto Rico. 

[MARK3 Input] 

Known mineral Known 

Model deposits occurrences 

Porphyry Cu 2 8 

Copper skarn 2 10 

Fe skarn 1 16 

Porphyry Cu-Au 5 2 

Polymetallic veins 2 59 

Volcanogenic manganese 3 11 

Epithermal qtz-alu 

Au (low grade) 2 3 

Kuroko massive sulfide 0 0 

Placer Au-PGE Ancient placer sites 3 

mercury-mineralized lode deposits. However, downstream 

from these lode deposits, approximately 10-25 km, they 

described a decrease of the heavy-metal content in the sediments to essentially background levels. This decrease is 

mainly due to dilution with relatively clean bed, bank, and 

hillslope material. 

Bethel (population 4,000) is situated on the Kuskokwim River floodplain and draws its water supply from wells 

located in river-deposited sediment These sediments may be 

secondary sources of containants to ground-water quality. 

These potential contaminants are not necessarily fixed as 

inorganic particulates but may be mobilized via biological 

and chemical processes within the sediment column and 

ground water, although biological processes may be minimal 

in this high-latitude environment. The Bethel area, typical of 

arctic tundra environments, is underlain by permafrost, and 

the ground is frozen for extended periods of time. Nevertheless, examining samples of the Kuskokwim River sediment 

is important in order to establish a baseline datum for heavymetal concentration. 

A boring on the riverbank down to and into permafrost 

(===8 m) provided 11 samples that we analyzed for mercury, 

arsenic, antimony, and selenium. These elements are present 

in anomalously high concentrations in the upstream deposits 

(Nelson and others, 1977; Gray and others, 1991). All the 

sediment passed through a 150-j..lm sieve and was therefore 

mostly very fine sand and silt; we noted little visible organic 

matter. Splits of the dried sediment were analyzed as follows: total mercury by cold vapor atomic absorption spectrometry, arsenic and selenium by hydride generation atomic 

absorption spectrometry, and antimony by graphite furnace 

atomic absorption spectrometry. Aluminum was analyzed by 

inductively coupled plasma atomic emission spectrometry 

and was used as a normalizing factor. 

The values of all the elements did not vary systematically with depth and ranged from (all ppm dry weight) 

[mean, 1 sigma]; Hg=0.08-0.29 [0.16, 0.06], As=2.7-4.2 

8 

Probability Level of 

.90 .50 .10 .05 .01 prospecting 

1 4 8 Low. 

0 0 1 2 3 Mod/High. 

2 3 5 8 10 High? 

1 4 15 Moderate. 

1 3 8 Moderate. 

1 2 4 Low. 

0 0 1 Very low. 

0 0 Very high. 

[3.5, 0.4], Sb=0.53-1.5 [0.87, 0.27], and Se=0.2-0.3 [0.2, 

0.0]. Aluminum (ppmx104) ranged from 5.91 to 6.27 [6.07, 

0.12]. 

The values determined at Bethel represent the normal 

products of weathering and show no indication of contamination from mercury-antimony mineralized areas. This study 

helps to establish a useful baseline datum at Bethel for these 

elements and will be valuable if mining activities increase 

upstream. 

REFERENCES 

Gray, I.E., Goldfarb, R.I., Detra, D.E., and Slaughter, K.E., 1991, 

Geochemistry and exploration criteria for epithermal cinnabar 

and stibnite vein deposits in the Kuskokwim river region, 

southwestern Alaska: Journal of Geochemical Exploration, v. 

41, p. 363-386. 

Nelson, H .• Larsen, B.R., Jenne, E.A., and Sorg, D.H .• 1977, Mercury dispersal from lode sources in the Kuskokwim River 

drainage, Alaska: Science, v. 198( 4319), p. 820-824. 

RECENT ENVIRONMENTAL 

AWARENESS IN LATIN AMERICATHE BOLIVIAN CASE 

William P. Blacutt 

The Bolivian national legislature recently passed a general environmental law to protect the environment and to 

promote sustainable development. This law delineates general principles of environmental policy; actual implementation procedures will be specified in future regulations. Much 

of the controversy surrounding this legislation may disappear when it is seen as a guide rather than an operational part 

of Bolivia's industrial policy. 
\C) 

67° 

~ 

66° 

I 

r~ 

~ 

~ 

Ill 

Podiform CITomite 

Copper / ... on Skan 

Volcalogenic ._.,. 

l<!roko Massive StJfide 

Figure 1 (Bawiec and Ambroziak). Permissive terranes for metallic mineral deposits of Puerto Rico. 

~ 

[I 

D 

I 

66° 

Epitherrna QtzAkrlite Au 

Bwxitic aay Belt 

Backg-OU'ld Areas 
66° 

I 

-0 

18°-

m Placer Au - PGE ~ Porphyry Copper 

~ Cu - Ag Malto 0 BackgrOLnd Areas 

I2J Possible Gold P1ocer Areas 

Figure 1 (Bawiec and Ambroziak) (Continued). Permissive terranes for metallic mineral deposits of Puerto Rico. 
Bolivia's legislation illustrates the recent jump from 

practically nonexistent environmental awareness to an_ allencompassing framework for rules and regulations. The law 

is in this respect an "encyclical" or "bill of rights" announcing that the society and the State are now ready to "guarantee 

everyone the right*** to enjoy a healthy and pleasant environment in the development and performance of all their 

activities." How this is done is left to subsequent regulations. 

The unfinished regulatory work can raise risk and apprehension for potential investors. For example, economic evaluation of the development of a hypothetical 500 t (metric 

ton)/day underground zinc mining and milling operation, 

located in the Bolivian Altiplano, indicates that excessive 

environmental regulation would render the project unprofitable at current commodity market prices. 

Missing in much of the risk assessment is an appreciation for the compromises implied in Bolivia's multiple 

objective planning procedures. Trade-offs are to be expected 

given the nature of environmental and economic costs and 

benefits. Although major improvements in environmental 

quality are always possible at moderate implementation 

costs, efforts to eliminate hazardous waste altogether will 

founder on exponentially rising costs. Thus, limited economic resources available for environmental quality 

improvements will tend to limit mitigation costs to levels 

that will not threaten investment in industrial activities. 

Under national and international pressure, all 

countries will ultimately find themselves compelled to 

implement adequate environmental standards that will permit and encourage rational resource development. That 

Bolivia has acted on this issue relatively early on may 

prove to be advantageous. 

DEVELOPMENT OF PREDICTIVE 

MODELS OF SAND AND 

GRAVEL DEPOSITS IN THE 

SOUTHWEST UNITED STATES 

James D. Bliss 

Modeling of sand and gravel deposits needs to 

address (1) deposit size and geometry, (2) size distribution of rocks and mineral fragments within the deposit, 

(3) physical characteristics of the material, and (4) chemical composition and chemical reactivity of the material. 

Development of models to characterize size and geometry 

of some deposits was accomplished based on data from 

California and the United Kingdom (Bliss, 1993). With 

the exception of sand and gravel deposits in alluvial fans, 

nearly all significant sand and gravel deposits (for example, a minimum thickness of 1-2m) can be described by a 

single volume model, and this model is valid for fluvial, 

11 

glacial-fluvial, and other kinds of sand and gravel deposits found in California or the United Kingdom. The applicability of this model to the sand and gravel deposits in 

the Southwest United States has yet to be tested, but it is 

expected to be appropriate here as well. 

Preliminary modeling of size distribution of clasts is 

currently underway for sand and gravel pits and prospects in 

New Mexico. The definition of deposits includes all sand 

and gravel bodies within 1.5 km of each other, but in the case 

of the 193 New Mexico sites, it is not always clear if "sites" 

are different deposits, which may bias the study in ways not 

recognized. 

A model for size distribution of clasts in sand and 

gravel deposits must consider what percent of each deposit 

consists of sand and gravel. The definition of a sand and 

gravel deposit is that it must contain at least 25 percent 

gravel by weight (Langer, 1988). Sand and gravel deposits 

contain not only sand and gravel, but also cobbles and boulders, as well as silt. In practice, those searching for surficial 

sand and gravel deposits find and describe other types of 

deposits which fail to meet the 25-percent-gravel criterion. 

For these, several other subtypes of deposits are identified. 

Thus six subtypes can be distinguished using clast sizes: (1) 

sand deposits, (2) sand deposits with gravel, (3) sand deposits with gravel and boulders, (4) sand and gravel deposits, (5) 

sand and gravel deposits with boulders, and ( 6) boulder 

deposits with sand and gravel. 

An outline of the standard classification rules (fig. 1) 

shows how the distributions of these subtypes of deposit 

(based on percent of clast sizes) for undiscovered sand and 

gravel deposits for a given population can be predicted. 

Three models are needed-the distributions of sand, silt, and 

gravel within deposits (fig. 1, models I, II, III). The values 

in brackets give the probabilities that a particular classification (fig. 1) occurred in the New Mexico data. In brief, all 

sand and gravel deposits contain sand and silt. Rarely was 

the sand subtype (without some gravel-sized clasts) 

described [p=0.005], not a surprising result given that sand 

and gravel deposits were sought._ The most common outcome [p=0.76] was a sand and gravel deposit with boulders. 

Sand and gravel deposits without boulders [p=O.ll], sand 

deposits with gravel and boulders [p=0.063], sand deposits 

with gravel but no boulders [p=0.039], and boulder deposits 

with gravel and sand [p=0.021] occurred less frequently. If 

the boulder deposits are suitable for crushing, 89 percent of 

the New Mexico sites are suitable for extraction of sand- and 

gravel-sized clasts and 11 percent for sand-sized clasts. 

These probabilities are not likely applicable to sand and 

gravel deposits strictly defined or to deposits found elsewhere. The standard classification scheme is sufficiently 

general in nature that it and the appropriate probability could 

be used in Monte Carlo simulation to predict the nature of 

undiscovered sand and gravel deposits anywhere in the 

world given models I, II, and III (fig. 1). 
I. 

ITJ 

II. 

sand, percent of deposit 

(with boulders) 

silt, 

D 

percent of deposit 

SAND DEPOSIT 

SAND DEPOSIT 

(with gravel and 

boulders) 

NO 

YES 

BOULDER DEPOSIT 

(with gravel and sand) 

Figure 1 (Bliss). General outline for classifying subtypes of surficial deposit using clast sizes. 

12 

REFERENCES 

Bliss, J.D., 1993, Modeling sand and gravel deposits-Initial strategy and preliminary examples: U.S. Geological Survey OpenFile Report 93-200, 31 p. 

Langer, W., 1988, Natural aggregates of the conterminous United 

States: U.S. Geological Survey Bulletin 1594, 33 p. 

DISTRIBUTION AND MOVEMENT OF 

METALS DERIVED FROM MINING AND 

MINERAL PROCESSING IN THE COEUR 

D'ALENE RIVER VALLEY 

Environmental Research Team (contact: Arthur 

Bookstrom, Stephen Box, Cole Smith, and 

James Lindsay, Spokane Field Office), 

USGS Office of Mineral Resources 

The Coeur d'Alene River valley has been contaminated 

by metals from mine tailings flushed downriver during the 

past 1 00 years of mining in the Coeur d'Alene mining district. Large portions of the flood plain of the Coeur d'Alene 

River have been blanketed with tailings-contaminated sediments. Geochemical processes involving the interaction of 

sediment, surface and ground water, air, and plants modify 

the contaminants in the flood plain. The hazardous character 

of these contaminants was recognized as early as 1904, when 

the first lawsuits were filed by valley landowners against the 

mine operators. 

The Environmental Protection Agency declared 21 

square miles of the mining district as a Superfund site in 

1986, and numerous Federal and local agencies are interested in evaluating the extent of contamination downstream 

of the Superfund site. The area is extensively used for farms, 

residences, recreational activities, and wildlife habitat, so the 

potential for human and wildlife health risks is significant. 

A multidisciplinary USGS team, consisting of geologists, geochemists, and geophysicists in the Office of Mineral Resources, has begun a study of the distribution and 

movement of mining-derived metals in the valley of the 

Coeur d'Alene River between the mining district and Coeur 

d'Alene Lake. Surficial geologic mapping of flood deposits, 

together with reconnaissance characterization sampling, is 

being used to design a systematic geochemical sediment 

sampling program. Concentrations ofPb, Zn, Cd, Cu, Sb and 

As, as well as the mineral carriers of these elements, are 

being determined. Natural stream-cut banks and cores are 

being used to determine lateral thickness variations of the 

contaminated sediments, mostly limonite-cemented sand 

and silt. Heavy-metal content of gray clay, which underlies 

the tailings-contaminated sediments, also is being determined to indicate natural background concentrations of 
metals in sediments derived by weathering and erosion of the 

veins and altered rocks of the Coeur D'Alene district. 

Multispectral satellite and hyperspectral aircraft images 

are being analyzed and will be used to map surficial variations in the character of the flood-plain sediments. Geophysical methods will be tested to aid in estimating thickness of 

the sandy-to-silty tailings-derived sediments, particularly in 

areas where thickness of contaminated sediment exceeds 1 

meter. Geophysical studies combined with studies of porewater chemistry will help us to understand the chemical 

mobility of heavy metals in the flood-plain environment. 

Proposed biogeochemical studies can indicate metal-uptake 

and fixation by plants. We are coordinating our studies with 

those of other agencies concerned with bioavailability and 

health risks of the contaminants to human and wildlife populations. Data will be made available in a geographic information system (GIS) database (Arc/Info). Results will be 

used to generate recommendations of remedial actions for 

present problem areas and preventive actions to lessen future 

problems. 

Th/U RATIOS AS A POTENTIAL 

EXPLORATION TOOL FOR 

Ag- Or Au-ENRICHED 

CRUSTAL PROVINCES 

Robin M. Bouse, Joseph L. Wooden, 

Spencer R. Titley, Joaquin Ruiz, and 

Richard M. Tosdal 

Lead isotopic compositions and model Th/U ratios 

(based upon a 1. 7 Ga formation age for Arizona crust) for 

ores from 50 mineral districts throughout Arizona fall into 

three groups on a 208Pbf204pb versus 206pbf204pb diagram 

that mimic the regional lead-isotopic trends defined by Early 

Proterozoic whole rocks in the southwestern United States. 

The Early Proterozoic whole rocks (and the ores) define the 

crustal provinces: western Arizona and eastern Mojave 

Th/U>4; central Arizona Th/U:2; and southeastern Arizona 

Th/U=4. 

Ore deposits in western Arizona and central Arizona 

have Ag/Au ratios that predominantly range between 0.5 and 

10 and average about 4.5 (n=120), whereas ore deposits in 

southeastern Arizona have Ag/ Au ratios that predominantly 

range between 30 and 300 and average about 90 (n=116). 

Low Ag/ Au ratios correspond to crustal provinces with both 

the highest (western Arizona/eastern Mojave) and lowest 

(central Arizona) Th/U ratios. Thus, Au enrichment correlates with differential mobility in U and Th. 

Silver enrichment and high Ag/ Au ratios are found in 

the southeastern Arizona province, in which Th/U ratios and 

Th and U concentrations are similar to those of average 

13 

crust. (Th!U=4 is approximately the crustal average.) This 

suggests that provinces with Th/U ratios either higher or 

lower than 4 have a higher potential for relatively gold rich 

deposits, whereas provinces with Th/U=4 have a higher 

potential for relatively silver rich deposits. The hypothesis 

does not imply that gold- or silver-rich deposits are exclusive 

to a crustal province but that a predominance of silver-rich 

deposits will occur in crustal provinces with Th/U=4 and that 

gold-rich deposits will predominate in crustal provinces 

depleted or enriched in Th or U. 

For example, the Mojave province (Th/U>4) encompasses western Arizona, southeastern California, and a portion of the Great Basin, and is known to contain many goldrich deposits. Island arcs also contain many gold-rich deposits and they commonly have Th/U ratios around 2. Southeastern Arizona and western New Mexico (Th/U=4) have 

long been known for their silver production. Northern Chile 

(lat 23°-28° S.) also has Th/U around 4 and contains many 

silver-rich deposits. The Wyoming province has high Th/U 

ratios and is a possible gold-enriched province. 

These observations strongly suggest that Th/U ratios 

and Th and U concentrations may be important indicators of 

Ag- and Au-enriched provinces and they should be considered as a general geochemical/isotopic exploration tool for 

evaluating crustal province-scale potential. 

A SUMMARY OF THE MINERAL 

RESOURCE ASSESSMENT OF 

CORONADO NATIONAL FOREST, 

ARIZONA AND NEW MEXICO 

Mark W. Bultman 

The area in and around Coronado National Forest, Arizona and New Mexico, has been a major supplier of mineral 

resources for the Nation: its total metal production is valued 

at approximately 20 billion dollars. The U.S. Geological 

Survey was asked to provide a mineral resource assessment 

of Coronado National Forest, to assist the U.S. Forest Service in complying with regulations requiring them to provide 

information and interpretations so that the mineral resources 

can be considered with other types of resources in land-use 

planning. This presentation is a summary of the results of 

that assessment. 

The procedure used to obtain the mineral resource 

assessment follows the concept of a mineral deposit model, 

which is systematically arranged information that describes 

the essential earth science attributes of a particular group or 

class of mineral deposits and their accompanying grade and 

tonnage models. First, all known and potential mineral 

deposits in the region are identified and classified into a mineral deposit model. Next, regions that are favorable for the 

occurrence of these mineral deposits are delineated on 
mineral resource assessment tract maps. In order to provide 

land-use planners with the most useful information, the parts 

of the favorable tracts that have high potential are also delineated where possible. Finally, subjectively based team judgments of the number of deposits likely to occur are integrated 

with estimates of the likely grades and tonnages of ore 

deposits as they exist in and around the forest. This last step 

produces a probabilistic estimate of the number of metric 

tons of ore and metal that may occur in the forest. 

The metallogeny of Coronado National Forest consists 

of a porphyry copper zone and a gold- and silver-bearing 

vein zone. Each zone contains mineral deposits unique to 

that zone in addition to deposits common to both zones. The 

porphyry copper zone is coincident with a northwest-striking 

belt of world-class porphyry copper deposits and passes 

through the portion of the forest southwest of Tucson. The 

porphyry copper zone is characterized by large, multiphase, 

and dispersed deposits in which the value of produced copper far outweighs the value of other metals. Besides typical 

porphyry deposits, the zone contains stockwork deposits, 

breccia pipes, and large, irregular, composite ore deposits. 

The gold- and silver-bearing vein zone covers the eastern 

portion of the forest, extending to the east into New Mexico 

and to the south into Chihuahua. These small to (rarely) 

medium sized deposits are mostly vein deposits but do 

include some small pipes and contact metasomatic deposits. 

Lead, zinc, gold, and silver, mostly in polymetallic vein 

deposits, may be present throughout the forest; the more 

complex and dispersed types of these deposits are more typical of its southwestern part. Fluorite and manganese are 

present in small but minable concentrations in the eastern 

portion of the forest. Minable tungsten deposits are most 

likely to be present in its southwestern part. Although these 

generalizations represent an oversimplification of the metallic mineral deposits in the forest, they offer a useful framework for consideration of metallic mineral resource 

distribution therein. 

Initially, delineation of favorable mineral resource 

tracts was based solely on geologic evidence. The boundaries established were then modified through consideration 

of extensive geochemical, geophysical, remote sensing, and 

mineral resource data. The earth science database and the 

interpreted mineral resource tracts are the major product of 

the assessment; they will benefit future land-use planning, 

mineral resource assessment, and mineral resource exploration in and around the forest. 

Finally, consensus subjective estimates of the number of 

undiscovered deposits for each mineral deposit model in each 

tract were reached by team members. These estimates were 

combined with estimates of the grades and tonnages of mineral deposits from their respective mineral deposit models. 

The result is a series of estimates for the contained producible 

metal in each tract, which are summed to create a combined 

contained metal estimate for the entire Coronado National 

Forest. Users of contained metal estimates should realize that 

14 

these estimates contain large variances and a low level of precision, and incorporate a variety of possible errors. 

NEW TECHNIQUES OF GEOPHYSICAL 

DATAANALYSIS; AN INVESTIGATION OF 

THE GEOMETRY, STRUCTURE, AND 

BEDROCK LITHOLOGY OF THE 

SAN RAFAEL BASIN, ARIZONA 

Mark W. Bultman and Mark E. Gettings 

Large undiscovered conventional mineral deposits may 

be concealed by alluvium in basins in productive terranes. 

Research on assessment and exploration for such deposits 

has focused on the acquisition and interpretation of high-spatial-resolution total intensity magnetic field data collected at 

ground level through a truck-mounted proton precession 

magnetometer. Previous investigations recognized a strong 

similarity between these magnetic field profiles and certain 

profiles of fractal behavior (especially those known as selfaffine fractals). These high-quality magnetic field profile 

data are integrated with more conventional geophysical data 

and interpreted so that basin structure and geometry can be 

precisely modelled. Also, the fractal character of these magnetic field profiles is used to assign a textural "fingerprint" 

to the magnetic field acquired from specific lithologies. 

These "fingerprints" can be used to help identify the lithology of some concealed magnetic field source rocks. 

In order to identify concealed lithologies, we compare 

magnetic field profile data from exposed candidate lithologies with data from covered areas. It is necessary to account 

for the attenuation in high spatial frequencies in the component of the data from the concealed lithology due to depth of 

burial. We have done this by upward continuing data 

acquired from exposed candidate rocks by an amount equal 

to the depth of burial of the bedrock whose lithology is 

unknown. The depth estimates and the upward continuation 

are iterated until a self-consistent model is achieved. An estimate of the magnetic field due to the cover itself is then 

added to the continued data. This estimate is generally determined from simulations or by high pass filtering of data 

acquired over thick basin fill. The field resulting from the 

sum of the upward-continued field (upward continuation of 

the exposed candidate lithology) and the estimated field due 

to cover is then compared with the observed field acquired 

over the covered area. Comparisons are based on various 

statistical measures of the data, including the fractal dimension of the profiles. If appropriate, cluster analysis is applied 

to the data. 

An analytic approach to geoscience data integration and 

visualization is an aid in developing models of basin geometry and structure. Multivariate statistical methods are 

applied to reduce the dimensionality of the many layers of 
geoscience data sets. Image processing tools are used to analyze patterns and textures in an attempt to correlate these 

data to exposed and subbasinal features. Scientific visualization tools are used to display the resulting three-dimensional 

model. The product of the investigation is a three-dimensional model incorporating basin geometry, structural geology, and interpretations of concealed lithologies where 

possible. 

We selected the San Rafael basin, Arizona, for the application of these techniques due to its high resource potential, 

a shallow accumulation of sediment, and proximity to the 

Coronado National Forest, the subject of a recent mineral 

resource assessment. The east margin of the basin lies along 

the Sawmill Canyon fault complex, a northwest-trending system that may control the placement of Laramide intrusive 

bodies elsewhere along its length. A complex assemblage of 

volcanic rocks, with some mineral production history, forms 

the north margin of the basin. The Patagonia Mountains and 

the Harshaw Creek fault, which form the west margin, define 

an area with a rich history of ore production. Our investigation 

shows that many of the structural features and lithologies necessary but not sufficient for ore deposit formation can be 

found concealed under the San Rafael basin. One large anomaly, 15 km east -southeast of the Red Mountain porphyry copper deposit, can be modelled as a small stock or cupola. This 

anomaly may be an attractive target. 

PALEOMAGNETISM AND MAGNETIC 

FABRIC OF THE APACHE LEAP TUFF, 

CENTRAL ARIZONA 

Gary J. Calderone, Donald W. Peterson, 

James E. Faulds, and Robert F. Butler 

The 20-Ma Apache Leap Tuff is a zoned ash-flow tuff 

deposited on a surface of considerable topographic relief 

over an area of at least 1,000 km2 near the mining districts of 

Superior, Miami, and Globe in central Arizona. Analyses of 

paleomagnetic remanence directions from 353 samples from 

39 sites at 16 localities in the Apache Leap Tuff agree with 

conclusions made earlier, based on geologic evidence, that 

the tuff is a single cooling unit. The tuff records an essentially instantaneous direction of the Miocene geomagnetic 

field at D=5.3°, I::40.9° (t:lgs=6.7°, k=31.0, R=15.5164, 

n=16), which corresponds to a virtual geomagnetic pole 

(VGP) at lat 79.6° N., long 42.3° E. (A9s=6.7°, K=31.7, 

R=15.5273, N=16). Site mean anisotropy of magnetic susceptibility (AMS) typically deviates only a few percent from 

an isotropic state and varies considerably in the shape of the 

fabric ellipsoid. AMS ellipsoids range from prolate uniaxial 

forms to oblate uniaxial forms, and most ellipsoids exhibit 

either prolate or oblate triaxial forms. The AMS shape 

parameters do not correlate with stratigraphic zones in the 

Apache Leap Tuff. Bulk susceptibility, however, decreases 

15 

up-section by an order of magnitude from the lower densely 

welded brown zone to the upper nonwelded white zone, most 

probably the result of increased deuteric oxidation in the 

white zone. 

At each locality, the mean minimum AMS axis is subvertical whereas the intermediate and maximum AMS axes 

define a subhorizontal plane of magnetic foliation. The maximum axis defines a magnetic lineation in the plane of the 

magnetic foliation. Both the magnetic lineation and the magnetic "imbrication" (as defined by the azimuths of the minimum AMS axes) exhibit a crudely radial geographic pattern, 

although the pattern of magnetic lineations is much better 

defined. The convergence of these radial patterns, in association with other geologic and geophysical constraints, is 

consistent with a source in the vicinity of the Haunted Canyon cauldron (about 12 km north of Superior) near the westcentral margin of the known exposures of the tuff. 

REGIONAL GEOCHEMICAL STUDIES IN 

THE KAIBAB NATIONAL FOREST, 

ARIZONA 

M.A. Chaffee, R.R. Carlson, and 

P.M. Theodorakos 

The Kaibab National Forest, northern Arizona, consists 

of three separate areas-the North Kaibab area, located 

north of the Grand Canyon; the Tusayan area, located just 

south of the Grand Canyon; and the Chalendar area, located 

farther south near the Mogollon Rim. The regional geology 

of the three areas is relatively simple and consists of a 

sequence of generally flat lying Paleozoic sedimentary rocks 

that are locally folded and (or) faulted and locally covered by 

volcanic rocks. In the North Kaibab area, most outcrops are 

of the carbonate-dominant Lower Permian Kaibab Formation. Outcrops in the Tusayan area mostly include the Kaibab 

Formation and Tertiary and Quaternary volcanic flows, 

tuffs, and cinder deposits. Rocks in the Chalendar area consist predominantly of Tertiary and Quaternary volcanic 

flows. The region is semiarid, and the forest contains no 

perennial streams. 

The major known mineral resources of the Kaibab 

National Forest are in breccia pipes, formed in collapse 

structures in Paleozoic sedimentary rocks in and near the 

Tusayan area and just west of the North Kaibab area. Hundreds of pipes have been identified; many of these pipes are 

mineralized and some contain economic deposits of copper 

and (or) uranium. Associated elements commonly include 

Ag, As, Bi, Cd, Co, Fe, Mo, Ni, Pb, S, Sb, V, and Zn. 

For our investigations, 478 samples of bulk sediment 

were collected rom dry first- and second-order stream channels in the forest. The samples included organic-rich to clean 

mixtures of sand, silt, and (or) clay-sized material. In order 

to create a uniform sample medium, all samples were wet 
sieved and two fractions (0.5 to 2 mm and <0.25 mm) were 

saved. Samples from both fractions were analyzed for 39 elements (Ag, AI, As, Au, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cu, 

Eu, Fe, Ga, K, La, Li, Mg, Mn, Mo, Na, Nb, Nd, Ni, P, Pb, 

Sb, Sc, Sn, Sr, Th, Ti, U, V, Y, Yb, and Zn). The analyses of 

the coarser fraction were found to be the most useful because 

more analytical results of the mineral deposit-related elements were in the range of determination. 

On a regional scale, the areal extents of the different 

major lithologies can be delineated on the basis of their 

chemistry. Of the 39 elements determined, Ba, Ce, Cr, La, 

and Na are particularly good lithologic discriminators. The 

regional chemical changes within a single formation can also 

be seen. In the North Kaibab and Tusayan areas, Ca and Mg 

show a distinct regional zoning within the areas containing 

the Kaibab Formation, with low concentrations in the topographically higher core areas and higher concentrations in 

the lower perimeters. This zoning may be related to differences in annual precipitation at different elevations. This 

variation can affect the rate at which these two elements are 

leached from the relatively soluble material forming the 

stream alluvium. 

Of the ore-related elements listed, three (As, Mo, and 

Sb) were derived solely from mineralized breccia pipe 

deposits and showed the best spatial correlation with known 

mineralization. In the Chalendar area, with limited exposure 

of Paleozoic sedimentary rocks, no significant geochemical 

anomalies were found that could be related to Cu/U-rich 

breccia pipe mineralization. The North Kaibab area contains 

hundreds of sinkholes; however, none has yet been found to 

form a breccia pipe at depth or to be mineralized. Mineralized pipes are known only to the west of this part of the forest. In the Tusayan area, widespread, significant anomalies 

for many ore-related elements extend well beyond the areas 

of known deposits, suggesting that additional mineralized 

breccia pipes may be present in that part of the forest. 

USE OF LEAD ISOTOPES TO 

FINGERPRINT SOURCES OF 

HEAVY-METAL CONTAMINATION 

IN THE ENVIRONMENT 

S.E. Church 

The isotopic composition of lead in rocks varies as a 

direct result of the decay of uranium and thorium. During 

the formation of ore deposits, lead is isolated from its parent 

isotopes, uranium and thorium, and has a specific lead-isotopic composition of limited range that is preserved at the time 

of ore formation. In many ore deposits, lead is a major constituent, and is present in either galena or some other leadbearing phase. Thus, many ore deposits, especially those 

associated with magmatic hydrothermal systems, have a 

16 

characteristic lead-isotopic composition. As a consequence, 

ore deposits can be "fingerprinted" by their lead-isotopic signatures. 

Weathering of hydrothermal deposits containing sulfide minerals results in the oxidation of sulfide minerals. In 

the oxygenated surface waters of streams, secondary iron 

and manganese oxides form as a result of this weathering 

process. Metals released are sorbed by the iron and manganese oxides that coat sand grains in the stream. By analyzing 

the lead-isotopic variation in the HCl-H202 extractable fraction in stream sediments, we can maximize the signal caused 

by the weathering of sulfide minerals in the ore deposits 

exposed in a drainage basin. This lead-isotope signature and 

the concomitant trace-metal signature of individual deposits 

can then be traced into larger drainage basins and the effect 

of the heavy metals from a specific source can be traced 

downstream. This procedure has application in the study of 

environmental consequences of mining and industrial activity on the geochemical landscape. 

This lead-isotopic tracer technique has been used to distinguish between the base-metal veins that surround the 

Mount Emmons, Colo., porphyry molybdenum deposit and 

the deposit itself. The base-metal veins have variable leadisotopic compositions (206pbJ204Pb generally > 17.9 and 

208pbJ204Pb generally >38.1) that differ from that of the porphyry molybdenum deposit (206PbJ204Pb=17.3-17.5 and 

208pbJ204Pb=37 .9-38.1 ). Furthermore, the Ph-isotopic signatures of both deposit types differ greatly from that of the 

surrounding country rock (206pbJ204Pb generally >19.0 and 

208pbJ204Pb generally >38.8). Isotopic studies of the composition of lead sorbed by the iron and manganese oxide coatings on pebbles from the streams showed that the leadisotopic composition in the leaches from the pebbles varies 

over short lateral distances. This indicates rapid sorption of 

lead from the stream by iron and manganese oxide coatings 

and validates the leaching procedure as a valuable tool in 

environmental geochemistry. 

Application of the method in the upper Arkansas 

River drainage basin, Colorado, has demonstrated that lead 

from tributaries to the river above Leadville, Colo., has a 

variable, but radiogenic lead-isotopic signature 

(206pbJ204Pb generally > 18.4 and 208pbJ204Pb generally 

>38.6), indicating a crustal rock source. In contrast, the 

high concentrations of lead from the ores from Leadville 

totally overwhelm the lead occurring in the sediments. 

Stream-sediment samples from all the drainages within 

the Leadville mining district, as well as the samples from 

the Arkansas River collected immediately below the town 

of Leadville, contain substantial amounts of lead and 

other base metals, and have the lead-isotopic "fingerprint" 

of the ores from the Leadville mining district 

(206PbJ204Pb from 17.95 to 18.25 and 208pbJ204pb from 

38.4 to 38.6). Cores taken in the Arkansas River below 

Leadville show that the baseline concentrations for heavy 

metals prior to mining were similar to those in the upper 

Arkansas River basin above Leadville. These data 
demonstrate that mining, rather than natural processes, 

has resulted in the present distribution of heavy metals in 

the Arkansas River at Leadville, Colo. 

GEOCHEMICAL MAPS OF 

ARIZONA AND THE GREAT BASIN 

S.E. Church and G.L. Raines 

Stream-sediment geochemical data for the elements Cu, 

Mo, Pb, Zn, As, Sb, Ag, and Au have been used to produce 

a series of maps for Arizona and the Great Basin. The data 

were collected during the NURE and various USGS regional 

sampling programs over the last 30 years. Evaluation of the 

quality of the data from the four different NURE laboratories 

and the USGS laboratories has been made in terms of the 

detection limit (DL) and the analytical resolution (AR) of the 

analytical methods used. When element concentrations are 

expressed in terms of the crustal abundance (CA) of the element, that is in clarke units, useful regional geochemical 

maps can be made. Copper is the only element that has been 

satisfactorily determined by all methods used in these laboratories over this time period; nevertheless, useful data 

showing the character of the geochemical landscape can be 

derived from the data of lower quality. 

Approximately 50,000 stream-sediment samples from 

these two regions were collected at variable sampling densities appropriate for sampling programs at map scales ranging 

from 1 :500,000 to 1 :50,000. In the present study, several 

approaches have been used to effectively evaluate geochemical variations resulting from variable sampling densities, 

analytical noise, interlaboratory variability, and analytical 

resolution. To reduce the effect of individual samples and 

variable sample densities on the geochemical data base for 

Arizona, the data were gridded into 25 kffi2 cells and the 

highest value in the cell was assigned to the cell. This procedure resulted in about 7,500 cells for the map of Arizona. 

Analytical resolution of the gridded data is sufficient to 

assign five concentration intervals for several elements, 

based on the clarke values, to make individual geochemical 

maps: DL to 7/3 xCA, 7/3 to 1th xCA, 11/2 to 21/2XCA, 21/2 to 

4xCA, and >4xCA. Experiments with more than five intervals resulted in maps that displayed various types of noise 

and were difficult to interpret. 

Comparison of composite geochemical maps with distribution of ore deposits and districts shows a good correspondence with known base-metal districts. The correlation 

of the precious metals, As, and Sb with epithermal metal 

districts is severely limited by the analytical quality, detection limits, and analytical gaps in the geochemical data base. 

The geochemical distribution of Au, determined during the 

NURE program, appears to be random. We conclude that, in 

general, these data are useful to define regional geochemical 

17 

baselines and to outline areas of elevated metal concentrations for a number of metals where analyses are complete. 

These geochemical maps are a valuable resource for landuse planning, mineral resource assessment, and environmental geochemical evaluations. Significant and cost-effective 

improvements in data quality could be achieved by reanalysis of samples collected during these previous studies. 

MAPPING MINERALS WITH IMAGING 

SPECTROSCOPY-EXAMPLES 

ILLUSTRATING THE BEGINNING OF A 

NEW MAPPING ERA 

Roger N. Clark and Gregg A. Swayze 

Imaging spectroscopy is a new mapping tool and the 

next generation in remote sensing technology. The narrow 

spectral channels of an imaging spectrometer form a continuous reflectance spectrum of the Earth's surface, contrasting 

with the 4-7 channels of the previous generation of imaging 

instruments such as the MSS (Landsat Multispectral Scanner) and TM (Thematic Mapper) instruments. Systems like 

Landsat can distinguish general brightness and slope differences in the reflectance spectra of the surface; imaging spectroscopy not only does that, but also resolves absorption 

bands in the spectrum that can be used to identify specific 

materials. Spectroscopic analysis of imaging spectroscopy 

data allows any material (mineral, vegetation, man-made, 

water, snow) to be mapped if it has unique absorption features in the measured spectral region. 

Imaging spectrometers have improved remarkably over 

the last 3 years. The NASA A VIRIS (Advanced Visual and 

Infra-Red Imaging Spectrometer) instrument is now producing excellent data with a signal:noise ratio in the hundreds. 

A VIRIS acquires data in the spectral range from 0.4 to 2.45 

J.lm in 224 spectral channels. The instrument is flown in an 

ER-2 aircraft (a modified U-2 reconnaissance plane) at 

19,800 m (65,000 ft). The ground resolution is 20 m, the 

swath width about 11 km (614 pixels), and the flightline 

length as much as 1 ,000 km. 

Spectral analysis algorithms have matured to the point 

that identification and discrimination of even similar minerals 

are possible, producing astonishingly detailed maps. For 

example, not only can clays be mapped, but also specific identification of alteration minerals (such as kaolinite, montmorillonite, or muscovite) can be achieved. The relative 

crystallinity of clays can be determined spectroscopically 

(for example, kaolinite, dickite, and halloysite can be 

distinguished from each other). Iron-bearing minerals are 

another class of minerals that can be spectrally distinguished. 

For instance, hematite, goethite, jarosite, chlorite, and nontronite have been identified at Cuprite, Nev., with AVIRIS 

data. In addition, A VIRIS data may allow us to differentiate 
pyroxenes, amphiboles, and olivines at Barstow, Calif. In the 

case of sulfates, like alunite, the solid solution series (from 

sodium to potassium) has been spectrally identified showing 

chemical zonation in a hydrothermal alteration zone. Swayze 

and others (work in progress) have also found that the spectra 

of alunites appear different depending on their hydrothermal 

formation temperature. Thus paleotemperature mapping is 

possible and is being applied to decipher tectonic movements 

in the Basin and Range province. 

Many rocks are composed of mineral assemblages 

which are spectrally detectable (for example, clays, carbonates, micas, evaporites, and fe-hydroxides) over the 

spectral range of the A VIRIS instrument. We have found 

that simple combinations of these minerals in proper proportions serve to uniquely identify rock types. Experimental mapping in Arches and Canyonlands National Parks in 

Utah, using A VIRIS data, shows that we can produce 

spectrolithologic maps that correspond remarkably well 

with geologic maps. In fact, these spectrolithologic maps 

show mineralogical changes so subtle (normally undetectable by human eye or in airphotos) that we have subdivided previously undivided formations. We believe that 

this capability has direct applications for sedimentary 

provenance studies. 

Imaging spectroscopy can also address global change 

and environmental problems. For example, the continuous 

spectral coverage and fine spatial resolution of the 

A VIRIS instrument make it unique among remote sensing 

instruments, in determining accurate albedos of earth 

materials. Albedo information is of critical importance to 

Global Circulation Models. Snow and ice grain sizes have 

been remotely mapped in the Sierra Nevada of California 

using A VIRIS data. Vegetation communities have also 

been mapped, even in low vegetation cover terrains such 

as those in the Colorado Plateau. We also believe that 

imaging spectroscopy analysis has great potential for 

mapping surface contamination in areas experiencing 

environmental problems. 

PRECAMBRIAN MASSIVE SULFIDE 

DEPOSITS IN AMPIDBOLITE- AND 

GRANULITE-FACIES ROCKS, 

WESTERN ARIZONA AND 

SOUTHEASTERN CALIFORNIA 

Clay M. Conway 

Volcanogenic massive sulfide deposits have long been 

known in amphibolite-grade metamorphic rocks in the Early 

Proterozoic volcanic belts at Bagdad and at Boriana Canyon 

(Hualapai Mountains) in western Arizona. However, the 

metamorphic character of these deposits and especially of 

18 

associated footwall alteration zones has not been appreciated. In particular, recent recognition of metamorphosed 

altered rocks at Bagdad has shown the altered zones to be far 

more extensive than previously thought and thus to indicate 

new exploration potential in the Bagdad volcanic belt. Additionally, reconnaissance mapping in western Arizona has 

revealed similar metamorphosed alteration zones in new 

localities (for example, northern Hualapai Mountains, Grand 

Wash cliffs, lower Grand Canyon), suggesting further exploration potential. 

Detailed work by the USGS at Bagdad indicates at 

least four alteration types in the silicic to mafic volcanic 

rocks that host the massive sulfide deposits: albitized, 

epidotized, silicified, and chloritized rocks. Of these, the 

chloritized rocks are easiest to recognize and are most 

proximate to the massive sulfide deposits. They form 

"chlorite pipes" immediately subjacent to the massive 

sulfide deposits. Chloritized rocks provide an excellent 

regional exploration tool. The chloritized areas at 

Bagdad have a distinctive yellow tinge on infrared aerial 

photographs. 

Chloritized rocks at Bagdad and elsewhere in amphibolite-facies rocks in western Arizona exist primarily as a chlorite-garnet-magnetite assemblage or a higher grade 

assemblage containing orthoamphibole (anthophyllite)+ 

cordierite±garnet. Cordierite-anthophyllite rocks have long 

been recognized in Canada and elsewhere as metamorphosed chlorite pipes in association with massive sulfide 

deposits; however, this association has sometimes been 

overlooked in the southwestern United States. Some cordierite-anthophyllite exposures at Bagdad were previously 

mapped as gabbro. 

Preliminary work in the Turtle Mountains, eastern 

Mojave Desert, southeastern California, suggests that CuZn deposits in the Hom Spring area are volcanogenic massive sulfide deposits metamorphosed to granulite facies. 

A regional granulite-facies metamorphism occurred during the I vanpah orogeny at about 1. 7 Ga and may be 

responsible for the metamorphism of these deposits. Metamorphosed hydrothermally altered rocks associated with 

gossans contain combinations of chlorite, quartz, orthoamphibole, garnet, phlogopite, spinel (pleonaste?), and cordierite(?). These magnesian metamorphic rocks occur as 

layers and pods immediately adjacent to the gossan layers, suggesting the familiar deposit-footwall relationship of 

massive sulfide deposits. This is apparently the first documentation of massive sulfide deposits in the high-grade 

metamorphic terrane of southeastern California. However, 

reports do exist of pods (or thin layers) of orthoamphibole-cordierite in the Old Woman Mountains, gedrite 

schist in the Halloran Hills, and chlorite-talc or orthoamphibole-cordierite in the eastern Transverse Ranges. 

These magnesian assemblages hint at volcanogenic metasomatism and thus invite exploration for massive sulfide 

deposits in the Mojave Desert country. 
THE NATIONAL ASSESSMENT OF 

UNDISCOVERED DEPOSITS OF COPPER, 

ZINC, LEAD, SILVER, AND GOLDA PROGRESS REPORT 

Dennis P. Cox, Joseph A. Briskey, Jr., 

Sandra H.D. Clark, Michael F. Diggles, 

Thomas D. Light, Barry C. Moring, and 

Alan R. Wallace 

The mineral resource assessment of the United States, 

proposed by McCammon and Briskey (1992), focuses on 

resources in deposits as yet undiscovered (as opposed to 

those in extensions of known deposits) and is limited to 

deposits of the five most commonly explored for metals. The 

goal of the assessment is to decrease the time required to 

respond to requests for mineral resource information on the 

public lands, requests that have come with increasingly 

shorter deadlines and which involve increasingly larger 

areas, some with uncertain boundaries. The assessment will 

create a large digital database to be published at a small mapscale (1 :2.5-3.5 million), but available also at intermediate 

scales (1:750,000 to 1:1 million), that will provide mineral 

information for the whole country. 

The assessment follows procedures outlined by Singer 

(1993): (1) teams of experts on mineral deposits, regional 

geology, geochemistry, and geophysics delineate tracts permissive for undiscovered deposits of one or more deposit 

types using known relationships between deposit types and 

geologic environments; (2) the teams use grade-tonnage 

models based on worldwide reserve and production data to 

represent the probable tonnage and metal grades of undiscovered deposits of each type; and (3) the teams estimate the 

number of undiscovered deposits of each type within the permissive tracts that are consistent with the grade-tonnage 

models, using a depth range of 1,000 m. These estimates, 

presented as three or more probability distributions (number 

of deposits, tonnage, and grade of one or more metals), have 

been combined by Monte Carlo simulation (Root and others, 

1992) to produce probabilistic estimates of tons of undiscovered metal. 

The teams, based in Reston, Va., Denver, Colo., Menlo 

Park, Calif., Anchorage, Alaska, Spokane, Wash., Reno, 

Nev., and Tucson, Ariz., also have compiled estimates of 

past production and remaining resources of the five metals in 

the important known deposits of each type in the regions 

being assessed. These estimates can be compared conveniently with the tonnages of metals estimated for undiscovered deposits. 

At this writing (November 1993) digital tract maps and 

resource estimates from each region are being reviewed 

internally, and an external review process is planned. 

19 

REFERENCES 

McCammon, R.B., and Briskey, J.A., Jr., 1992, A proposed national mineral-resource assessment: Nonrenewable Resources, v.1, 

no. 4, p. 259-266. 

Root, D.H., Menzie, W.D., and Scott, W.A., 1992, Computer Monte Carlo simulation in quantitative resource estimation: Nonrenewable Resources, v. 1, no. 2, p. 125-138. 

Singer, D.A.,1993, Basic concepts in three-part quantitative assessments of undiscovered mineral resources: Nonrenewable Resources, v. 2, no. 2, p. 69-81. 

PORPHYRY COPPER DEPOSITS IN 

ARIZONA-AN EXAMPLE OF RESOURCE 

EVALUATION FOR THE U.S. 

GEOLOGICAL SURVEY'S NATIONAL 

MINERAL ASSESSMENT OF COPPER, 

LEAD, ZINC, GOLD, AND SILVER 

Leslie J. Cox 

Porphyry copper deposits in Arizona contain 53 percent 

of the copper in all known deposits in the United States as 

well as significant amounts of lead, zinc, gold, and silver. 

The U.S. Geological Survey's numerical assessment of the 

potential for new porphyry copper deposits took the following steps: selection of a grade and tonnage model that 

describes the probable characteristics of undiscovered 

deposits, delineation of a tract permissive for porphyry copper deposits, and estimation of the number of undiscovered 

deposits in the permissive tract. 

A general porphyry copper deposit model was used to 

guide the delineation of tracts permissive for the occurrence 

of this deposit type and related porphyry Cu-Au, porphyry 

Cu-Mo, and porphyry Cu skarn-related deposits. This model 

was based on deposits worldwide and incorporates the 

known characteristics of the geologic and tectonic setting, 

rock type and ages, ore (including grades and tonnages), 

alteration, association with other deposit types, and 

geochemical and geophysical expression. Permissive tracts 

were defined as the areas remaining after exclusion of terrains where expecting a deposit of the type in question to be 

present is unreasonable, based on our present understanding 

of the characteristics of this deposit type and the geology of 

Arizona. Tracts permissive for the occurrence of porphyry 

copper deposits were drawn for Arizona at the scale of 

1:1,000,000 (fig. 1). 

Estimates of the number of undiscovered porphyry copper deposits in Arizona were obtained by the following process: the geologist who led a team of scientists in the tract 

delineation made a short presentation about the permissive 

tract and the favorable features contained within it. Additional information was offered by the regional geologic 
50 KILOMETERS 

EXPLANATION 

e Porphyry copper deposits that contain at least 50,000 

metric tons of copper metal, pre-production 

Boundary separating geologic basement-defined 

terranes (1-V) of Titley and Anthony (1989, p. 489) 

ARIZONA 

0 Aagstaff 

Boundary separating Terrane I into east and west sections 

C=:J Tract permissive for the occurrence of porphyry copper deposits 

Figure 1 (L. Cox). Tracts permissive for the occurrence of porphyry copper deposits in Arizona. 

20 

• 
experts, the team geochemist, and the team geophysicist. 

This presentation was followed by group discussion of the 

data, analog areas, perceived history of exploration, the 

appropriateness to Arizona of the deposit model and the 

associated grade and tonnage distributions, and the ratio of 

the area covered by 0-1-km-thick strata of Eocene and 

younger age to the area covered by exposed pre-Eocene 

rocks. 

We then decided to make separate probability estimates 

for each of five geologic terranes (fig. 1) rather than for the 

State as a whole. Terrane I was divided into east and west 

sections (fig. 1), based on the differences in the amounts of 

Eocene and younger cover (not shown in fig. 1 ). The estimation method stipulates that the distribution of the deposits 

estimated in each area considered must have the same distribution of tonnages and grades as the worldwide porphyry 

copper model. The initial, individually made assessments 

were followed by discussion of geologic and informational 

issues identified by each assessor as most sensitive to his or 

her estimate. A second assessment was made and the scores 

compiled. A consensus score, close to the mode, was 

attached to each probability distribution. 

REFERENCE 

Titley, S.R., and Anthony, E.Y., 1989, Laramide mineral deposits 

in Arizona, in Jenney, J.P., and Reynolds, S.J., eds., Geologic 

evolution of Arizona: Tucson, Ariz., Arizona Geological Society Digest, v. 17, p. 485-514. 

SPECTRAL PROPERTIES OF SELECTED 

INDUSTRIAL MINERALS AND ORESPOTENTIAL FOR REMOTE SENSING 

DETECTION AND EXPLORATION* 

James K. Crowley 

Visible and near-infrared (0.4-2.5 J.tm; VNIR) reflectance spectra and thermal infrared (8.0-13.5 J.lm; TIR) emission spectra were measured for 19 industrial minerals and 

ores to evaluate prospects for mineral detection with spectroscopic remote sensing data. The minerals and ores selected 

for study (table 1) occur in surficial exposures and have 

moderately high intrinsic value, making them potentially 

viable targets for remote sensing exploration programs. 

High-purity quartz sand and calcitic carbonate rock samples 

were also examined, because remote sensing exploration for 

these less valuable commodities might be practical in some 

situations. The study is not comprehensive, as a number of 

industrial minerals and ores are not included, and minerals 

21 

represented here by only one or two samples may exhibit 

considerable spectral variation. 

The laboratory spectra show that many industrial minerals and ores exhibit VNIR and (or) TIR spectral features 

that might be useful for detecting the minerals remotely. The 

two different wavelength ranges are complementary, in that 

minerals lacking absorption bands in one part of the spectrum commonly exhibit spectral features in the other. VNIR 

spectral data may be especially useful for discerning borates, 

pyrophyllite, talc, and vermiculite (fig. 1 ), as well as carbonates, diatomite, and gypsum. Spectral discrimination in the 

VNIR is generally based on the occurrence of narrow, diagnostic, absorption bands attributable to hydroxyl groups and 

hydrate water molecules, and (or) the borate and carbonate 

anion groups. Such features can be observed remotely with 

suitable instruments, such as the NASA Airborne Visible/Infrared Imaging Spectrometer (A VIRIS). Thermal-infrared 

spectral data may be particularly useful for locating barite, 

quartz, perlite, phosphorite, and thenardite (fig. 2). TIR 

remote sensing prospects for the industrial minerals and ores 

were evaluated by convolving the sample spectra to the six 

spectral bandpasses of the NASA Thermal Infrared Multispectral Scanner (TIMS). A spectral library of 43 well-characterized igneous and sedimentary rocks was also convolved 

to the TIMS bandpasses, permitting the rock spectral 

responses and the industrial mineral spectral responses to be 

compared. Diagnostic TIR spectral features are produced by 

vibration modes of the borate, phosphate, silicate, and sulfate molecular groups. Some important industrial minerals, 

such as zeolites, may not be amenable to remote sensing 

detection because of near-infrared absorption interference 

by atmospheric water vapor and the minerals' occurrence in 

fine-grained samples that have low TIR spectral contrast. In 

practice, the remote sensing detection of industrial minerals 

is also likely to be affected by soil, rock, and vegetation mixtures, as well as by weathering processes that may degrade 

the spectral signatures. 

Table 1 (Crowley). Industrial minerals and ores examined in this 

study. 

Barite (2) 

Calcite 

Colemanite 

Diatomite 

Erionite 

Graphite schist 

Gypsum 

Kimberlite 

Perlite (2) 

*Not presented at Forum. 

Phillipsite 

Phosphorite 

Pyrophy llite 

Quartz sand 

Talc 

Thenardite 

Ulexite 

Vermiculite 
Talc 

(1) 

0 

c Vermiculite C'O 

.... 

0 

(1) 

-.::: 

(1) 

0: 

Ulexite 

Colemanite 

0.5 1.0 1.5 2.0 2.5 

Wavelength (J.Lm) 

Figure 1 (Crowley). Visible and near-infrared reflectance spectra of selected industrial minerals. Spectra are offset vertically for 

viewing purposes. Arrowheads, positions of diagnostic VNIR absorption features. Dot pattern, wavelength region obscured by atmospheric water-vapor absorption. Tidemarks on vertical axis 

show reflectance intervals of 10 percent. 

HOT -SPRING TEXTURES IN 

EPITHERMAL ENVIRONMENTS 

Charles G. Cunningham, Robert 0. Fournier, 

Peter G. Vikre, and James J. Rytuba 

Hot-spring systems are the modem analogs of many 

Tertiary precious-metal-bearing epithermal systems. 

22 

Thenardite 

c 

0 

·u; 

Cl) .E 

w 

TIMS 2 3 4 5 6 

8.0 9.0 10.0 11.0 12.0 

Wavelength {J.tm) 

Figure 2 (Crowley). Thermal emission spectra of selected industrial minerals and ores. Spectra are offset vertically for viewing 

purposes, and TIMS scanner bandpasses are shown on the lower 

part of the figure. Arrowheads, positions of diagnostic emission 

features. Tickmarks on vertical axis indicate emission intervals of 

5 percent. 

Several distinctive textural features in modem hot springs 

are commonly also preserved in fossil systems, and they 

can help to differentiate sinter from similar-appearing silicified sediments and airfall tuff. These textural features 

include mud cracks, bubble tubes, geyserite, terraces, filamentous and conophyton bacteria, and reeds. Where textures are absent to definitively identify sinter from silicified 

sediments or tuff, a very low Al203 content and lack of 

Ti02 and Zr02 can be diagnostic of sinter. 
ESTIMATED PALEOSURFACE 

PARTS PER MILLION PERCENT 

Figure 1 (Cunningham and others). Abundance of vertically zoned hydrothermal phases and components in the Buckskin Mountain 

precious-metal deposit. M, muscovite; I, illite. Modified from Vikre (1985). 

Hydrothermal explosion features are also near-surface 

manifestations of epithermal systems. These features include 

breccia aprons, craters, and hot-spring pools as indicated by 

fossil conophyton bacteria, and fossil filamentous bacteria 

marking the outflow of the pools. Hydrothermal breccias 

often mark the location of the upflowing part of a hydrothermal system, where pressure and temperature changes are 

greatest, and where precious-metal deposits tend to form. 

Hot-spring systems have distinctive physical, chemical, 

and mineralogical vertical variations that reflect the results 

of active processes and the consequent thermochemical 

environments. The Miocene Esperanza silver-gold deposit in 

the Maricunga district of Chile is one of the best documented 

examples of hot springs superimposed on acid-sulfate deposits in the Andes. The deposit formed in a dacitic volcanic 

dome environment, and much of the known precious-metal 

mineralization was contained within vuggy silica formed by 

volcanic degassing and acid-sulfate alteration. The deposit, 

however, contains many hot-spring features preserved at the 

paleoground surface in the extremely arid environment of 

this part of the Andes. The tops of some hydrothermal systems, at the paleoground surface, are marked by explosion 

craters with rims of ejecta. Breccia fragments in these rims 

include fragments of vuggy silica that are indicators of the 

acid-sulfate alteration processes that occurred at depth; this 

vuggy silica marks the zones of higher permeability that 

localized later mineralizing fluids. Some of the outflow 

channels of the former pools in explosion craters contain fossil filamentous bacteria. 

Well-documented examples of fossil hot-spring systems in which most of the upper part is preserved and the 

subsurface is exposed by mining and drilling exist throughout the world. Vertical variations in minerals and selected 

elements at the Buckskin, Nev., precious-metal deposit are 

illustrated in figure 1. The distribution of silica, as sinter with 

23 

interbedded hydrothermal explosion breccias, at the paleoground surface, results from the large temperature gradient 

near the ground surface. Mercury is commonly concentrated 

in the sinter because it can travel through the lower part of 

the system as a vapor and is condensed at the ground surface. 

The zonation of alunite immediately beneath the silica, and 

kaolinite mostly beneath the alunite, reflects the interaction 

of the hydrothermal system with atmospheric oxygen, the 

oxidation of hydrogen sulfide at a fluctuating water table, 

and the resulting distribution of temperature, pH, and sulfate 

activity. The distribution of precious metals reflects the 

changes in gold and silver solubility as the upwelling nearneutral-pH alkali-chloride fluids encounter the low-pH, oxygenated environment near the ground surface. 

REFERENCE 

Vikre, P.G., 1985, Precious metal vein systems in the National 

District, Humboldt County, Neyada: Economic Geology, v. 

80, p. 360-393. 

TIMING AND ORIGIN OF 

ORE DEPOSITION AT THE 

CERRO RICO DE POTOSI Ag-Sn AND 

PORCO Ag-Zn-Pb-Sn DEPOSITS, BOLIVIA 

C.G. Cunningham, R.E. Zartman, E.H. McKee, 

R.O. Rye, C.W. Naeser, and G.E. Ericksen 

Cerro Rico de Potosi is the world's largest silver 

deposit, having produced an estimated 30,000 to 60,000 t 

(metric tons) of fine silver from high-grade Ag and Ag-Sn 
Mining zinc and silver, Porco, Bolivia. 

veins that cut a dacitic volcanic dome. Recent dating by a 

variety of isotopic techniques indicates that the deposit is 

large because it was mineralized several times. Interpretation of U-Th-Pb isotope systematics on zircon from the 

Potosi dacite dome is complicated by the presence of inherited components, but extrapolation to the time of dome 

extrusion constrains the timing of extrusion of the dome to 

about 14 Ma. The main episode of alteration and mineralization took place shortly after dome extrusion at 12.4±0.4 Ma. 

A second alteration-mineralization episode is temporally 

bracketed by K-Ar age determinations on widespread sericite of 11.1±0.4 Ma and 10.5±0.4 Ma and a fission-track age 

on zircon of 12.5±1.1 Ma. The mineralization resulted in a 

core enriched in Sn-W-Bi with a peripheral zone of Pb-ZnAg. As the hydrothermal system collapsed, Pb-Zn-Ag minerals were overprinted on the core zone. A third significant 

episode of mineralization occurred "'6-8 Ma and is documented by K-Ar ages on sericite (7.3±0.6 Ma) and vein 

alunite (5.7±0.2 Ma, 6.3±0.1 Ma, 6.4±0.2 Ma, and 8.3±0.5 

Ma). The vein alunite was deposited contemporaneously 

with sulfides; 180 partitioning between S04 and OH in the 

alunite indicates deposition temperatures of 240-290 oc for 

this event. The BD values (-69 to -74 per mil) of alunite 

require a magmatic fluid origin for the alunite, and by inference, for the related sulfides. The B34S values of coexisting 

24 

alunite and pyrite are out of equilibrium and probably 

indicate rapid ascent of alternating liquid and vapor-phase 

fluids. This third mineralizing event occurred at about the 

same time as most of the Los Frailes ash-flow tuffs were 

erupted from multiple caldera sources northwest of Potosi. 

This event is part of a regionally extensive 6-9 Ma magmatic, tectonic, hydrothermal episode in southern Bolivia. 

The Porco Ag-Zn-Pb-Sn deposit, ,35 km southwest of 

Potosi, was a major silver producer in the 16th century and 

is currently the largest zinc producing mine in Bolivia. It is 

located within a 3x5 km resurgent caldera that formed in 

response to the eruption of the dacitic Porco tuff that extends 

northward beneath the "'6-9 Ma Los Frailes tuffs. K-Ar ages 

on biotite from the Porco tuff indicate the age of eruption at 

12.0±0.4 Ma and 12.1±0.4 Ma. Mineralization appears to be 

genetically related to the central Huayna Porco stock, which 

is at the center of a system of radial dikes and zoned metal 

and alteration mineral patterns. A K-Ar age on sanidine from 

Tomas Vila (left), Chief Geologist of Minero Anglo American 

Chile, and Skip Cunningham at the volcanic-hosted, zoned, Aldebaran porphyry gold-copper and epithermal precious-metal deposit, 

Maricunga district, northern Chile. Andesitic volcanic host rocks in 

background show transition from the porphyry to the epithermal 

hot-spring environment. 
La Joya district, Bolivia; the only volcanic-hosted disseminated gold producer in Bolivia. George Ericksen at left. 

25 
the Huayna Porco stock indicates an age of intrusion of 

8.6±0.3 Ma. The known ore deposits occur in steeply dipping 

irregular and curvilinear veins that cut the intracaldera Porco 

tuff about 1 krn east of the Huayna Porco stock. The distribution of metals is zoned, with Sn concentrated nearer the 

stock and Pb-Zn-Ag farther away; additionally, Ag increases 

upwards in the base-metal veins. Geologic mapping has indicated that the known Pb-Zn-Ag veins are localized along the 

east side of the ring fracture of the caldera; the west side of 

the ring fracture has not been explored. 

Fe, Pb, Zn, Cu, Au, AND HCI 

PARTITIONING BETWEEN VAPOR AND 

BRINE IN HYDROTHERMAL 

FLUIDS-IMPLICATIONS FOR 

PORPHYRY COPPER DEPOSITS 

G.L. Cygan, J.J. Remley, and M.W. Doughten 

Hydrothermal fluids experiencing decompression will 

eventually split into two phases, one high density and one 

low density, each with very different chemistry. Base metals 

are partitioned between these two conjugate fluids. We have 

measured the KCl, FeCl2, PbCl2, ZnCl2, CuCl, HCl, and 

AuCl partitioning coefficients and phase separation systematics of the KC1-H20 system. Two devices are used: (1) a 

flexible-volume Au bag of approximately 20 mL capacity at 

500 oc in a large-volume extraction vessel, and (2) a fixedvolume extraction vessel. Pressures were monitored with 

calibrated transducers (accuracy of ±0.2 percent). Measurements were made at isothermal conditions and decreasing 

pressure to determine the characteristics of a single-phase 

KCl fluid as it splits into two fluids of differing density. 

Results are similar to but more detailed than those given earlier on liquid-vapor partitioning in a quartz monzonite-buffered, sulfide-bearing system at 500 °C, 500 bars (Hernley 

and others, 1992; Hernley and Hunt, 1992). 

Our results for KCl-H20 relations agree with Tkachenko 

and Shmulovich (1992) at 500 oc although the liquid phase 

is shifted slightly to higher concentrations. Addition of base 

metals has an insignificant effect upon the pure KC1-H20 

system. Partition coefficients K' d=ppmivapor/ppmiliquid, for 

Fe, Pb, Zn, and Cu decrease with decreasing P. Values at 500 

°C, 550 bars are 0.17, 0.09, O.o7, and 0.19 for Fe, Pb, Zn, and 

Cu, respectively; at 400 °C, 250 bars, they are 0.024, 0.008, 

0.006, and O.ot4. The effect of pressure upon the distribution • V/L V V L L coefficient, D M/ = (eM •• leKCI) I ( eM.· ;eKCI) is 

small to insignificant, where CMe+ refers to individual metal 

concentrations. Adding 0.1 molal HCl to the system, similarly, did not significantly alter the pure KC1-H20 system 

relations. The vapor tended to be significantly more acid than 

the liquid-by a factor of two. Total molal quench acidity 

26 

600 

EXPLANATION 

0 Fe 

580 • Pb 

.A. Zn 

• Cu 

560 (/) 

a: 

<I: 

co 

~ 

uJ 540 a: 

:::::> 

(/) 

(/) 

LU 

a: 

CJ._ 

520 

500 

102 103 

METAL. IN PARTS PER MILLION 

Figure 1 (Cygan and others). The vapor-liquid relations at 

500° C plotted versus pressure. Initial base-metal concentrations 

used in this experiment for Fe, Pb, Zn, and Cu are, in ppm, 5,600, 

2,000, 2,000, and 1,000, respectively. These values represent the 

approximate fluid composition in equilibrium with a quartzmonzonite-buffered Single-phase hydrothermal solution at 500 °C, 

1,000 bars (Hemley and others, 1992). The Fe curve is displaced 

to higher concentrations as expected, but the other metals show 

their own individuality. Copper shows the smallest difference between liquid and vapor concentrations; zinc shows the greatest. In 

this sense, copper is favored in vapor phase partitioning relative to 

zinc and lead, but the difference between vapor and liquid concentrations is still a factor of 16 at 500 bars. 

values at 500 °C, 550 bars are 0.074 and 0.037 for vapor and 

liquid phases (~=2.0). Figure 1 shows the relative concentrations of base metals measured in both brine and vapor 

phases. Note that whereas a11 metals are more concentrated 

in the liquid phase, copper concentrations are increased relative to lead and zinc in the vapor. Gold measurements, which 

ranged as high as 10 ppm, indicate behavior similar to base 

metals-that is, partitioning favors the liquid phase. 

These data demonstrate the potential for a rising, 

decompressing hydrothermal fluid to undergo phase separation and partition and transport metals through a fractional 

vaporization process. The concentrations of metals, especially iron, and the relatively high mobility of the low-density vapor suggest that the mineralizing potential of the 

vapor may be significant in hydrostatic pressure regimes 

where vapor may dominate. Furthermore, HCl is strongly 

partitioned into the low-density vapor phase compared with 

the liquid phase at these P, T conditions, and this process 

results in a highly reactive mobile fluid that may migrate 
through the host rock. This fluid would produce intense 

hydrolytic alteration characterized by an advanced argillic 

assemblage, with accompanying pyrite and minor copper 

mineralization, as is seen in acid-sulfate precious-metal systems and in the upper levels of the porphyry copper deposits. 

REFERENCES 

Hemley, J.J., Cygan, G.L., Fein, J.B., Robinson, G.R., and d' Angelo, W.M., 1992, Hydrothermal ore-forming processes in the 

light of studies in rock-buffered systems-!, Iron-copper-zinclead sulfide solubility relations: Economic Geology, v. 87, 

p. 1-22. 

Hemley, J.J., and Hunt, J.P., 1992, Hydrothermal ore-forming processes in the light of studies in rock-buffered systems-11, 

Some general geologic applications: Economic Geology, v. 87, 

p. 23-43. 

Tkachenko, S.I., and Shmulovich, K.I., 1992, Liquid-vapor equilibria in the systems water salt (NaCl, KCl, CaCl2, MgCl2) at 

400-600°C: Doklady Akademii Nauk, v. 326, p. 1055-1059. 

CAPTURE OF COPPER, LEAD, AND ZINC 

BY THE ZEOLITE-CLINOPTILOLITE IN 

METAL-POLLUTED DRAINAGES OF 

COLORADO 

George A. Desborough 

Clinoptilolite-rich rocks (CRR) from 10 deposits in the 

western United States and one from British Columbia were 

tested for ion-exchange capture of copper, lead, and zinc in 

several Colorado mine and stream drainages with pH ranging from 4 to 9. Spring and summer 1993 tests evaluated element capture by CRR samples from Buckhorn, N.Mex. 

(BNM); Castle Creek, Idaho (CCI); Creede, Colo. (CCO); 

Fish Creek, Nev. (FCN); Ft. LaClede, Wyo. (FL W); Harney 

Lake, Ore. (HLO); Princeton, B.C. (PBC); South Dakota 

(SDI, SDA, SDC); St. Cloud mine, N. Mex. (SNM); 

Sheaville, Ore. (SVO); and Tilden, Texas (TT). Sized 20-g 

packets of CRR (1.7-4.7 mm) in 2x3-mm-mesh nylon bags 

were immersed in the drainages, with a maximum of eight 

side-by-side tested simultaneously. Test periods were from 

19 hours to 28 days. After testing, the samples were dried, 

sieved to remove sand-sized particles and Fe-hydroxide precipitates, pulverized, and analyzed forK, Ca, Fe, Cu, Zn, Rb, 

Sr, Cd, Ba, and Pb using three radioisotope X-ray sources 

and aLi-drifted detector. The drainage waters, all containing 

significant heavy metals, were anlyzed for nine elements at 

a commercial laboratory. 

Mine-tunnel or adit drainages that were tested vary 

only slightly in pH, temperature, flow rate, and composition, temporally, and thus "appear" to be point sources (for 

27 

example, Argo drain, Idaho Springs; Rawley adit, Bonanza; 

Wellington mine drain, French Creek, Breckenridge). Other 

larger, metal-polluted drainages, such as the Alamosa 

River, California Gulch, Leadville, Mineral Creek, Silverton, and St. Kevin Gulch (northwest of Leadville), vary significantly in pH, temperature, flow rate and composition. 

Thus, remediation treatment of water in each drainage must 

be tested prior to selection of remediation methods. Some 

possible complexities of water treatment for heavy-metal 

remediation are shown by the variations in CRR metal capture at the lower California Gulch test site; pH varied from 

4.3 on 5/29/93 during highest flow rates, to a pH of 9 on 

8/3/93 during the low summer flow (table 1). 

Data in table 1 show that large amounts of Zn were captured by FLW (Fort LaClede) at low pH, but much smaller 

amounts were captured at high pH. At the highest pH, SDA 

(South Dakota) and SVO (Sheaville, Ore.) captured significantly more Zn than the other CRR samples. For the water, 

between ·6/4/93 and 8/3/93, the amount of Cu decreased by 

one-fourth, the amount of Zn decreased by one-half, Cd 

decreased by one-fifth, and Pb decreased by one-third. The 

greatest concentrations of heavy metals of interest here were 

present during peak spring runoff. 

Based on 1993 testing of 138 samples in seven drainages in Colorado, Cu was readily captured by the best performing CRR at aqueous concentrations as low as 0.06 

mg/L. Significant amounts of Pb were captured by the best 

performing CRR at dissolved concentrations of0.1 mg/L. At 

Zn concentrations of 1 mg/L, or more, the most efficient 

CRR captured significant amounts of this metal. Only minor 

amounts of Cd were captured at concentrations of 0.2 mg/L. 

ISOTOPIC AND FLUID-INCLUSION DATA 

ON THE AGE AND ORIGIN OF THE SAO 

BENTO AND MORRO VELHO GOLD 

DEPOSITS, MINAS GERAIS, BRAZIL 

Ed DeWitt, G.P. Landis, R.E. Zartman, 

Enzio Garaype, Sergio Luiz Martins Pereira, 

Milton Guimaraes Bueno do Prado, Frederico 

Wallace Reis Vieira, and C.H. Thorman 

The Sao Bento and Morro Velho mines in the state of 

Minas Gerais, Brazil, exploit stratabound gold-arsenic 

deposits within stratified rocks of the Late Archean Nova 

Lima Group. Models ranging from syngenetic sea-floor hot 

springs to epigenetic quartz veins in shear zones have been 

proposed for these and other similar deposits in Minas 

Gerais. In order to more precisely determine the origin of 

each deposit, we applied techniques of thin- and polishedsection petrography, U-Th-Pb geochronology, Pb-Pb 
Table 1 (Desborough). Concentrations of elements in the water and elements gained or lost(-) by 

CRR during four test periods at the lower California Gulch test site, west of Stringtown, Colo. 

[n.d., not detennined. FLW, Ft. LaClede, Wyo; SDl, SDA, SOC, South Dakota; IT, Tilden, Texas; CCO, Creede, Colo.; FCN, 

Fish Creek, Nev.; HLO, Harney Lake, Ore.; PBC, Princeton, B.C.; SVO, Sbeaville, Ore.] 

K Ca Cu ZN Rb Sr Cd Ba Pb 

ppmx 103 arts per millio 

S/29/93-6/4193 (6 days) pH=4.3-4.9, T=12-14 °C 

FLW 0.2 1.1 10 1,625 0 55 -230 640 

SD1 -0.2 -1.1 30 680 85 60 -285 790 

SDA -0.3 -0.1 55 1,180 0 45 <1 -55 525 

soc -0.1 0.4 50 995 -10 35 1 25 660 

TI -0.3 1.2 50 810 0 20 1 -335 790 

6/4193-6/13193 (9 days) pH=4.9-6.1, T=14.5-17 °C 

water 

(mg/L) 2.6 145 0.4 21 n.d. n.d 0.11 0.4 0.3 

FLW 0.3 1.5 0 1,335 0 70 <1 -105 570 

SD1 -0.2 -0.5 40 790 75 50 <1 -200 530 

SDA -0.3 0.6 30 1,095 -10 45 <1 -50 810 

soc -0.2 0.3 40 910 -10 60 <1 -20 785 

TI -0.6 -0.1 0 565 -10 -15 <1 -155 535 

6/13193-712193 (19 days) pH=6.1-7.4, T=15-17 °C 

FLW 0.3 1.9 0 1,300 -10 115 <1 -160 210 

SD1 -0.3 0.1 55 1,360 -10 85 <1 -400 165 

SDA -0.2 -0.3 0 1,020 0 65 <1 -315 155 

soc -0.4 0.7 0 1,190 -15 80 1 -155 175 

TI -1.3 0.4 0 485 -10 70 <1 -160 200 

7120/93-8/3193 (14 days) pH=8.2-9.2, T=15-17 °C 

water 

(mg/L) 2.5 218 <0.1 10.3 

ceo -0.2 0.4 0 1,400 

FLW 0.4 2.1 0 1,185 

FCN 0 0.6 0 1,320 

HLO 0 0.5 0 915 

PBC 0.2 0.5 0 645 

SDA -0.7 0.9 0 2,130 

svo -0.8 0.6 0 1,945 

analyses, and in-place mass spectrometry of gases in fluid 

inclusions to selected samples. 

The Sao Bento deposit is hosted by siderite-rich ironformation interbedded with carbonaceous phyllite and minor 

volcaniclastic material. Primary arsenopyrite-pyrite-pyrrhotite ore has a distinctly bedded appearance, with the greatest 

concentration of sulfide grains in siderite- and albite-rich 

beds. Pretectonic quartz veins are common and most numerous in the siderite-rich ore, but are also present in carbonaceous phyllite and surrounding rock units. Remobilization 

n.d. n.d. 0.02 0.04 0.11 

-45 10 <1 160 40 

-10 80 <1 -255 0 

-45 55 <1 45 45 

15 45 <1 75 35 

-20 -65 <1 -80 50 

0 45 <1 55 50 

-20 60 <1 65 110 

28 

textures and recrystallization of primary pyrrhotite and 

pyrite adjacent to quartz veins are well documented. Chert 

beds within iron-formation have variably recrystallized to 

form quartz veins parallel to bedding. The nonsulfide mineralogy of veins that cut beds is controlled by host rock type: 

veins that cut siderite beds are only quartz; veins that cut 

siderite-rich iron-formation are quartz-siderite; veins that cut 

magnetite iron-formation are quartz-magnetite. 

Gas chemistry of fluid inclusions from a variety of 

quartz veins, Pb-Pb analysis of arsenopyrite and pyrite from 
primary ore, and thin-section petrography indicate that most 

veins at Sao Bento formed in place from siderite-rich ironformation during regional metamorphism. Inclusions from 

veins in phyllite are water rich; all other veins are water poor. 

Veins in siderite-rich iron-formation are methane rich and 

have variable to high carbon dioxide concentration. Veins in 

magnetite iron-formation are the only ones to contain significant nitrogen. Veins that cross from phyllite to siderite-rich 

iron-formation have the gas composition of those in sideriterich iron-formation. Pb-Pb analyses of refractory arsenopyrite and pyrite from bedded and remobilized ore plot on a 

single-stage growth curve at 2.65 Ga. Pyrrhotite, which is 

more easily recrystallized than the other sulfide minerals, 

plots slightly off the growth curve but within the field of Late 

Archean remobilized lead. 

The Morro Velho deposit is hosted by the Lapa Seca, an 

iron-rich dolomite containing volcanic(?) grains of albite to 

oligoclase. The Lapa Seca is interbedded with slate and 

greenschist-facies shale-rich volcaniclastic strata. Ore 

ranges from disseminated arsenopyrite-pyrrhotite-pyrite 

within bedded, laminated Lapa Seca to massive ore of the 

same minerals within a minor siderite-dolomite gangue. 

Quartz veins are very minor, and remobilization features that 

disrupt the primary ore are minimal. Pb-Pb data from arsenopyrite, pyrite, and pyrrhotite from the 26-level of the Mand X-orebodies contain such low concentrations of lead 

(6-14 ppm) that meaningful interpretations are difficultother than to say that Late Archean or Early Proterozoic lead 

originally present in the sulfide grains has been modified significantly during Early Proterozoic metamorphism and 

deformation. 

GEOLOGIC CONTRIBUTIONS TO THE 

MINERAL RESOURCE ASSESSMENT OF 

THE CORONADO NATIONAL FOREST, 

ARIZONA AND NEW MEXICO 

Harald Drewes 

A mineral resource assessment of the Coronado 

National Forest of southeastern Arizona and southwestern 

New Mexico was recently completed by six U.S. Geological 

Survey scientists. The forest comprises 12 separate units 

which total about 7,500 km2 (fig. 1). 

Metallic mineral production from the forest areas was 

mainly copper, lead, zinc, molybdenum, silver, and gold, 

obtained from vein, stockwork, replacement, porphyry, and 

placer deposits. Production of nonmetallic commodities was 

minor. Current value of metals produced from the immediate 

forest region (forest units plus tracts between them) is $0.5-1 

billion; about $20 billion has been produced from an 

extended region (50 mi; 80 km) around the forest. Mineral 

29 

resource assessment is a key element to land-use planning 

for this forest. 

A complex geologic history of deposition, intrusion, 

and tectonism is apparent in many of these areas within the 

forest and belies tlie apparent surface simplicity of others. 

This study focuses on five partly interdependent geologic 

factors that contributed to the final geologic assessment map; 

geochemistry and geophysics are covered separately. 

A porphyry copper metallogenic belt extends through 

the western part of the forest region and contains most of the 

large mineral deposits of the extended region. Location of a 

forest area within this metallogenic belt is one of the favorable geologic factors. This belt is associated with a particular 

magmatic-tectonic zone (western intermediate zone) of the 

Cordilleran orogen and is favorable for the occurrence of differentiated granodioritic plutons and their hydrothermal systems bearing base and precious metals. Under present 

market conditions, deposits east of the porphyry copper belt 

would not as likely be economically viable without having a 

substantial amount of precious metals. 

A second favorable geologic factor is the presence of a 

hydrothermal system. A tract having all other favorable factors but no sign of hydrothermal alteration or scattered weak 

signs of mineral accumulation does not have a particularly 

high mineral potential. In many of the forest units, observations have been made on the distribution of mobile metals, 

silicified rock, and clay-mineral alteration; these indicators 

augment the geochemical and geophysical studies, which are 

more concerned with quantitative factors. 

A third geologic factor is proximity to plutonic rocks of 

favorable composition and age. Plutons of granite to diorite 

composition were emplaced in rocks of the forest during (1) 

Early and Middle Proterozoic (1,650-1,450 Ma), (2) Middle 

Proterozoic (1,100 Ma), (3) Jurassic (190-140 Ma), (4) Late 

Cretaceous and Paleocene (70-55 Ma), (5) Paleocene-Oligocene (55-30 Ma), and (6) Oligocene (30-25 Ma). Of 

prime importance are plutons of group (4), particularly those 

having multiple compositional phases from diorite to alkali 

granite, and in several mining districts, to a quartz latite porphyry, known as the "ore porphyry." Such compositionally 

diverse plutons occur mainly in the western part of the forest 

region; plutons of group (4) are fewer and smaller to the east. 

Of secondary importance are plutons of groups (3) and (6): 

Two of five Jurassic stocks are associated with mineral 

deposits, one a major mining district; many Oligocene stocks 

are associated with small mines and prospects but in places 

have concentrated large amounts of precious metals. Plutons 

of group (5) may have had a negative influence on ore accumulation. These peraluminous muscovite-biotite-garnet 

granites, which are associated with gneiss-cored domes 

(core complexes), may have come from partially melted 

lower crust that was impoverished in metals and whose late 

thermal history and high topographic and structural position 

favored the presence of outward-flowing hydrothermal 

systems prone to flush out, rather than accumulate, metals. 
X 

X 

Porphyry-Cu 

(large deposit) 

metallogenic 

-4---beh--~ 

I 

I 

.. ~-.... :. 

e Douglas u.s.:-:·(·: ------- 0 20 KILOMETERS 

I I 

Figure 1 (Drewes). Tracts of high mineral resource potential, based on geologic factors 

alone (solid black), in Coronado National Forest area (stipple pattern). x, major mine or 

mining district 

A fourth favorable factor is proximity to northwesttrending steep faults. Faults of many orientations, ages, and 

origins cut rocks of the forest, and most have the potential for 

channeling ore fluids. Northwest-trending structures, which 

may have begun in the Proterozoic as left-slip faults, are 

somewhat sinuous in plan but always steep, and they are 

widely spaced through the region, as if they controlled major 

crustal block movements. These faults appear to have been 

the dominant control of fluid movement at deep levels; faults 

of other trends and inclinations dispersed fluids at shallower 

levels. Neogene low-angle normal faults (glide faults) seem 

to be of little consequence to ore fluids because they postdate 

most known ore deposits of the region. 

A fifth favorable geologic factor is the presence of a 

receptive host rock. The forest region is underlain by more 

than 10 km of sedimentary, volcanic, and metamorphic rocks 

that are potential hosts for mineral deposits. Although some 

mineral deposits are found in most rocks, only a few are particularly good hosts, because of either their chemical or 

physical properties, their age, or their spatial distribution relative to other geologic factors. The most favorable host rocks 

are the Middle and Upper Cambrian Abrigo Formation and 

overlying Upper Devonian Martin Formation, both mixed 

clastic and carbonate rocks widespread in the western part of 

the forest region. Andesite volcanic rocks of Late Cretaceous 

or Paleocene age are also good host rocks in many places, for 

they are brittle, are intruded by late orogenic plutons, and are 

abundant to the west. In some places, the Mississippian 

Escabrosa Limestone, Pennsylvanian and Lower Permian 

Horquilla Limestone, and Mural Limestone-equivalent beds 

of the Lower Cretaceous Bisbee Group, and other Mesozoic 

volcanic rocks are also favorable hosts. In covered or only 

slightly mineralized areas, in which other factors are favorable, these host rocks have potential for subsurface mineral 

deposits. 

Although the five geologically favorable factors have 

been considered separately, all typically act together to 

define the geologic characteristics used in evaluating mineral resource potential. The resulting tracts of high potential 

(fig. 1) and of lower levels of potential (given in full report) 

were then collated with evaluations derived from the other 

studies to provide a final assessment of resource potential. 

MAGMATIC EVOLUTION OF THE 

TURKEY CREEK CALDERA, 

CHIRICAHUA MOUNTAINS, ARIZONA 

Edward A. du Bray and John S. Pallister 

The Turkey Creek caldera is a 20-km-diameter volcanic 

collapse structure in the Chiricahua Mountains, Ariz. 

Caldera magmatism began with development of a shallow, 

30 
compositionally stratified magma reservoir in which rhyolitic magma overlay dacitic magma across a sharp interface. 

Stratigraphic relations and overlapping ages indicate that 

both magmas were present simultaneously in the source reservoir. Caldera formation was initiated by eruption of 

500-1,000 km3 of the Rhyolite Canyon Tuff. Lava flows and 

tuff erupted after caldera formation record the compositional 

evolution of the underlying magma reservoir. The caldera 

floor is not exposed, and zones of megabreccia are rare in the 

Turkey Creek caldera. The caldera's deeper levels have been 

exhumed by faulting and erosion, exposing the roof of the 

crystallized magma reservoir. 

The caldera evolved through three principal stages: 

1. The rapid, voluminous eruption of Rhyolite Canyon 

Tuff caused magma withdrawal and concomitant caldera 

collapse into the partially evacuated reservoir. The outflow 

facies of the tuff, composed of lower, middle, and upper 

members, is as much as 400 m thick and consists of multiple 

ash flows that represent a series of eruption pulses; ash flows 

spread laterally from the caldera and accumulated in topographic lows during the paroxysmal eruptions. The intracaldera facies of the tuff, which ponded to a thickness of 

more than 1.2 km inside the caldera, is compositionally and 

petrographically akin to only the upper member of the outflow facies. Equivalents of lower and middle members of the 

tuff either were not deposited within the caldera or are concealed therein. 

2. Following eruption of the Rhyolite Canyon Tuff, 

dacitic magma from beneath the compositional interface 

formed an intrusion, and associated lava flows were erupted 

on the floor of the caldera. Emplacement of the dacite intrusion, a > 1-km-thick laccolith, within the collapsed floor of 

the caldera bowed up the caldera's central region and caused 

formation of a dome-encircling moat within the collapse crater. Interpretation of the intrusion as an intracaldera laccolith, rather than a core pluton, explains the lack of exposed 

floor rocks, scarcity of megabreccia within the caldera, and 

the absence of intracaldera equivalents of most of the outflow tuff: each is presumed to be concealed beneath the laccolith. Dacite lava flows were erupted from caldera-collapse 

ring faults, vents that previously fed ash-flow eruptions. A 

sequence of locally glassy, vesicular, and flow-ramped dacite lava flows are >0.5 km thick in the moat. Unerupted dacite was trapped in and beneath the vents and solidified to 

form a ring dike at depth. 

3. A period of volcanic quiescence and erosion is represented by a sequence of sedimentary rocks deposited in the 

caldera moat; these rocks are in tum overlain by = 135 km3 

of mainly crystal poor rhyolite lava that represents the final 

phase of caldera-related volcanism. The sedimentary rocks 

are rich in Rhyolite Canyon Tuff debris and are mainly 

poorly sorted breccias and ash-rich sandstone that were 

deposited within the caldera moat on or near the eroded 

caldera wall. Rhyolite erupted following moat sedimentation 

records generation of a voluminous new batch of mainly 

31 

high silica rhyolite with a distinct geochemical signature. 

Initial, volumetrically minor low-silica rhyolite is overlain 

by three sequences of high-silica rhyolite lava and minor 

associated tuff. Despite complex internal flow folding, lavaflow map units are remarkably planiform sheets. Rhyolite 

dikes and feeder zones for the moat rhyolite are rare; the 

moat rhyolite units may overlie their own vents. 

40 Arf39 Ar data are consistent with all of the Turkey 

Creek caldera rocks being broadly cogenetic; ages on sanidine for the Rhyolite Canyon Tuff and the dacite are 

26.9±0.1 Ma, whereas ages for two samples of moat rhyolite 

are 26.9±0.1 and 26.6±0.1 Ma. These ages constrain the erosional interval between emplacement of the dacitic rocks and 

eruption of the moat rhyolite to < 300,000 years. 

GEOLOGICAL REMOTE SENSINGIDSTORY AND DEVELOPMENT IN 

LATIN AMERICA 

Barbara A. Eiswerth 

The use of remote sensing technology in geologic mapping and mineral exploration programs in Latin America and 

the Caribbean region has increased dramatically in the last 5 

years. Factors such as the need for reliable assessments of a 

country's mineral endowment, the current favorable investment terms for multinational companies, accelerated technology transfer, improved data availability, and expanded 

variety of data types and uses have all contributed to this 

increase. 

Latin America and the Caribbean region are ideal for 

the application of remote sensing technology because many 

areas are remote and isolated, accurate base maps are not 

always available, and infrastructure is often limited. Analysis of multispectral and radar images contributes to mineral 

resource studies by providing a rapid and efficient means of 

delineating major tectonic and volcanic features, lithologies, 

and hydrothermally altered rocks. Interpretation of remotely 

sensed data, coupled with conventional mapping, is used to 

enhance and augment exploration efforts. 

The shift in mining exploration from the United States 

and Canada toward Latin America results in part from favorable changes in investment laws throughout Latin America. 

In 1992, 203 Canadian and 42 United States companies had 

mining interests in Latin America and the Caribbean region 

(Don E. Howell, Mining Record, oral common., 1993). 

Accompanying this shift is the export of modem tools and 

technology including innovative image processing, and the 

use of remote sensing and GIS (geographic information 

systems). 

Specialized centers exist to train Latin American geoscientists in the application of remote sensing techniques. 
More universities offer courses in remote sensing and GIS 

for geoscience applications than in the past. Foreign governments facilitate transfer of technology through cooperative 

research programs involving equipment acquisition, intensive training, and practical application of methods learned. 

Many national geological surveys and other government 

agencies in Latin America and the Caribbean have developed remote sensing laboratories that specialize in earth science investigations and provide information for mineral 

development in their respective countries. 

The variety of digital data types available has increased 

rapidly over the last 10 years. Landsat MSS (Multispectral 

Scanner) data have been collected since 1972 and TM (Thematic Mapper) data since 1982. TM data have been useful in 

exploring for gold placer deposits in heavily vegetated areas 

in Central and South America. Data collected by the French 

SPOT (Satellite Probatoire pour !'Observation de Ia Terre) 

have been available since 1986. Analyses of SPOT data have 

been used to identify potential bauxite deposits in Venezuela. Imaging radar data from Seasat and the SIR (Shuttle 

Imaging Radar) systems have been used extensively since 

the 1970's for geologic mapping, especially in areas 

obscured by cloud cover or dense vegetation. Radar images 

are the sole source of areal coverage for parts of the Brazilian 

Amazon Basin and are often used for regional structural 

interpretation as well as lithologic mapping. 

The 1992launch of Japan's JERS-1 and the European 

Space Agency's ERS-1 satellites will produce unprecedented continent-wide radar coverage. The JERS-1 satellite 

also is equipped with visible and near-infrared spectral 

bands. Landsat 6 was unfortunately lost during launch in 

1993, but Landsat 5 is expected to continue functioning until 

Landsat 7 is launched in 1997. The sensor aboard Landsat 7 

will provide higher spatial and spectral resolution. RADARSAT, a Canadian satellite scheduled for launch in 1994, will 

provide improvements on past systems such as variable and 

high spatial resolution and changeable incidence angles. 

Several hyperspectral airborne imaging spectrometers, 

both commercial and government sponsored, are actively 

collecting data in areas in Latin America. These systems 

include A VIRIS (Airborne Visible and Infrared Imaging 

Spectrometer), TIMS (Thermal Infrared Imaging Spectrometer), Geophysical Environmental Resources' GER-63 channel sensor, and GEOSCAN' s 24 channel sensor. Since 1980, 

Intera Information Technologies' airborne radar system, 

STAR-I, has been providing very high spatial resolution 

data with stereoscopic capabilities. Commercial data from 

both the GER-63 sensor and the GEOSCAN sensor are being 

used by mining companies in Chile to map distinct zones of 

hydrothermal alteration associated with porphyry copper 

and volcanic-hosted epithermal precious-metal deposits. 

The variety and utility of remote sensing data will have great 

impact on the systematic exploration and geologic mapping 

of Latin America and the Caribbean region. 

32 

IMPACT OF THE SUMMITVILLE MINE 

ON IRRIGATION WATER, 

AGRICULTURAL SOILS, AND 

ALFALFA IN THE SOUTHWESTERN 

SAN LUIS VALLEY, COLORADO 

J.A. Erdman and K.S. Smith 

Contamination from the Summitville gold mine in the 

San Juan Mountains has raised concerns over the effects of 

low pH and metal-laden surface waters carried down the 

Alamosa River. These waters enter the Terrace Reservoir, 

which provides irrigation water to the southwestern part of 

the San Luis Valley. The purpose of this study was to assess 

whether significant differences exist between the effects of 

two source waters on the compositions of alfalfa and the 

associated soils, respectively. The two source waters are Terrace Reservoir water and Rio Grande River water and (or) 

confined ground water. 

Sampling was conducted June 3-6, 1993, just prior to 

the first cutting of alfalfa. Irrigation water, soils, and alfalfa 

were collected from four sprinkler-irrigated Terrace Reservoir fields and from similarly irrigated "control" fields using 

a balanced one-way analysis-of-variance design. Four sites 

were selected randomly within each of the eight fields. 

The water samples were taken from the irrigation system of each field. The pH values of the Terrace Reservoir 

waters ranged from 5.6 to 6.8 and the samples contained no 

measurable alkalinity. Preliminary chemical data indicate 

that these waters contained anomalous concentrations of 

copper (60-350 J..Lg/L), zinc (150-190 J..LgiL), and manganese 

(360-520 J..Lg/L) in unfiltered samples. In contrast, the pH of 

irrigation waters originating from sources other than Terrace 

Reservoir ranged from 7.6 to 9.2, and the concentration of 

dissolved metals was less than 2 J..Lg/L, 25 J..Lg/L, and 20 J..Lg/L 

for copper, zinc, and manganese, respectively. It appears 

that fields irrigated with waters originating from the Terrace 

Reservoir receive more acidic water and higher concentrations of dissolved metals than do fields using irrigation 

waters from another source. 

Results of total analyses of soils from sites irrigated 

with Terrace Reservoir water are similar to those of the soils 

irrigated with other sources of water. Differences in element 

concentrations were not significant at the 0.01 probability 

level, but were significant at the 0.05 level for copper and 

phosphorus. When compared with geometric means for soils 

from the western United States (Shacklette and Boerngen, 

1984), soils we collected from the southwestern part of the 

San Luis Valley contain higher concentrations of aluminum, 

barium, beryllium, cobalt, copper, iron, manganese, potassium, sodium, strontium, titanium, vanadium, and zinc. 

These higher concentrations probably reflect the composition of the alluvial parent material from which the soils were 

derived. This fan material is detritus shed from weathering of 
Figure 1 (Erdman and Smith). View of one of eight alfalfa fields sampled, all of which were irrigated by a center-pivot sprinkler 

system similar to the one shown here. Photograph taken June 4, 1993. 

volcanic rocks and mineralized areas in the Alamosa River 

drainage basin. All of our soil data from the southwestern 

part of the San Luis Valley are within geochemical baselines 

for soils from the western United States (Shacklette and 

Boemgen, 1984). 

Stem-and-leaf samples of alfalfa were collected from 

10 points within a 1-m radius of each soil pit. An analysisof-variance for 15 elements showed no significant differences in alfalfa irrigated by the two source waters when 

tested at the 0.01 probability level. However, concentrations 

of copper and manganese between the two sources were significantly different at the 0.05level. More importantly, concentrations of these metals in alfalfa affected by both water 

sources (1) meet published nutritive requirements for ruminants, (2) are far below concentrations reported to be toxic to 

cattle, and (3) are comparable to concentrations in alfalfa 

found in other parts of the country. 

In conclusion, preliminary results indicate that the 

metal concentrations of the irrigation water affected by the 

Summitville mine may be anomalous. Yet these waters have 

no measurable effects on the total soil chemistry and seem to 

have had a minimal impact on the nutritional value of the 

associated alfalfa. 

REFERENCE 

Shacklette, H.T., and Boemgen, J.G., 1984, Element concentrations in soils and other surficial materials of the conterminous 

United States: U.S. Geological Survey Professional Paper 

1270, 105 p. 

33 

METAL CONCENTRATIONS IN THE 

WETLAND VEGETATION RECEIVING 

ACID MINE DRAINAGE FROM 

ST. KEVIN GULCH, LEADVILLE, 

COLORADO 

B.M. Erickson, P.H. Briggs, and T.R. Peacock 

Acid mine drainage (AMD) from St. Kevin Gulch, 

Leadville, Colo., drains into a 26-ha (hectare) subalpine wetland. The wetland is a sedge meadow with two ecosystem 

components: (1) sedges growing on top of hummocks-the 

drier component ( Carex canescens L.); and (2) sedges growing between or off hummocks-the aquatic component 

( Carex utriculata Boott.). The purpose of this study was to 

evaluate the affects of the AMD on the wetland sedges. Two 

main tasks were identified. The first task was to establish the 

expected baseline trace-element concentrations in the sedges 

to delineate areas of possible effects. The second task was to 

examine seasonal variation and establish biomass production estimates to determine the amount of metals being 

removed from the AMD by the wetland. 

As of the 1989 growing season, no signs of metal stress 

were visible on the wetland vegetation. However, the AMD 

has affected the metal concentrations of the sedges growing 

on the wetland. Sedge concentrations of Cd, Cu, Fe, Pb, Mn, 

and Zn, known to be high in the AMD waters, are higher in 

the area of direct inflow than the sedge baseline geometric 
means (GM) reported for the wetland. Concentrations higher 

than the GM in other areas of the wetland are evidence for 

contamination from other sources. Maximum cattle dietary 

levels reported in the literature for Cd (0.5 ppm) and Zn (500 

ppm) are exceeded in some areas of the wetland and could 

present a nutritional problem for the cattle grazing this wetland each year. Vegetation toxicity levels for Cu (20-100 

ppm) and Pb (30-300 ppm) are not exceeded, but Pb concentrations are higher in the inflow area and may indicate that 

accumulation is occurring. Iron deficiency, common in vegetation and foodstuffs, was not detected in the sedges of this 

wetland. Manganese concentrations above the 500 parts per 

million level, reported in the literature as resulting in signs of 

visible stress in vegetation, are present in most areas of the 

wetland. The Fe/Mn ratio necessary for healthy vegetation 

(1.5 to 2.5), occurs only in the areas with the higher Fe concentrations. Presently the only element being removed from 

the AMD waters by the wetland is Fe. If the sedges accumulate higher levels of Fe, the ratio of Fe/Mn will not necessarily be improved. Iron and manganese are physiological 

antagonists, and at toxic Mn levels, iron absorption could 

reverse, resulting in more stress and eventual loss of 

biomass. 

GEOLOGIC AND GEOCHEMICAL 

CONTROLS ON THE COMPOSITION OF 

WATER DRAINING FROM DIVERSE 

MINERAL DEPOSITS 

Walter H. Ficklin, Geoffrey S. Plumlee, and 

Kathleen S. Smith 

Pyrite may be the most abundant of all the sulfide minerals. It occurs in some form in almost all mineral deposits. 

The oxidation of pyrite initiated by atmospheric oxygen is 

responsible for one of the most widespread environmental 

problems that will be encountered in the utilization of mineral deposits: acid rock drainage. The reaction is catalyzed 

by bacteria, and the main products are acid, sulfate, and dissolved and solid forms of iron. Many other minerals are dissolved in the process. Once the reaction is established, it 

does not stop until one or more of the reactants is exhausted. 

The overall reaction is slow enough that pyrite deposits may 

not be completely exhausted for several thousand years. Yet 

it is fast enough that a mineral deposit undergoing oxidation 

may seriously affect the quality of water draining that 

deposit. 

The composition of water samples draining from mines 

located throughout the Colorado Mineral Belt is quite variable. Out of approximately 40 sample locations tested, the 

pH varies from 1.7 to 7.8. Iron concentrations range from 

not detectable to 8,000 mg/L; zinc concentrations range from 

34 

20 to 700,000 Jlg/L; and copper concentrations range from 

not detectable to 500,000 J..Lg/L. Uranium, thorium, the rare 

earth elements, and several other metals are present in concentrations that are far greater than would be found in suitable drinking water. 

The Colorado Mineral Belt hosts a broad spectrum of 

mineral deposits, each having characteristic structural features, host rocks, wallrock alteration, gangue minerals, and 

trace metals. The composition of waters draining diverse 

deposit types is a function of several factors: (1) the acidbuffering capacity of mineral-deposit host rocks and gangue 

material; (2) the types and abundance of metal-bearing sulfide minerals in the deposits; (3) the exposure of the sulfides 

to weathering; and (4) the availability of dissolved oxygen 

for the sulfide oxidation process. The mining method used in 

exploitation of mineral deposits has a bearing on the composition of water. As a generalization, open-pit mines produce 

water having low pH and high concentrations of metals, 

whereas underground mines produce water of intermediate 

to low pH and relatively lower concentrations of metals. 

Tunnels draining several mines often produce low-pH and 

high-metal waters. 

In light of the observed range of pH and metals, we 

have developed an empirical metal-pH classification scheme 

for mine drainage. This scheme uses pH as a master variable 

that conveniently summarizes large amounts of data and provides a means for comparing mine water composition from 

different geologic settings. A plot of the sum of copper, zinc, 

lead, cadmium, cobalt, and nickel concentrations from mine 

drainage samples collected throughout Colorado as a function of pH is presented in figure 1. These base metals are of 

environmental concern and are present in nearly all drainage 

water. Samples of similar mineral deposit type and geology 

are outlined. Sulfide-rich deposits oflow buffering capacity 

plot as a group in the upper left comer. On the other hand, 

deposits situated where the acid is neutralized by carbonates 

or other water-rock reactions plot in the lower right comer. 

Even though (as indicated in fig. 1) the most environmentally damaging water is derived from sulfide-rich-low-buffering deposits with pH values of less than 3, many of the 

mines with pH values greater than 5.5 still contain significant quantities of metals. Our interpretation of mine drainage chemistry in a geologic context is enabling us to develop 

a model that can be used to predict drainage chemistry from 

analogous undeveloped mineral deposits. 

REFERENCES 

Ficklin, W.H., Plumlee, G.S., Smith, K.S., and McHugh, J.B, 1992, 

Geochemical classification of mine drainages and natural 

drainages in mineralized areas, in Kharaka, Y.K., and Maest, 

A.S., eds., Proceedings of the 7th International Symposium on 

Water-Rock Interaction-WRI-7, Park City. Utah, July 13-18, 

1992: Rotterdam, A.A. Balkema, p. 381-384. 
10,000 

1,000 ... .c 

c.. c:: + 0 100 ·-z ·-- + - 0 E 

0 ... 

(. ...... , \ ' 

Increasing pyrite 

content and decreasing 

acid-buffering capacity 

----.-..... ,.. ' 

I \ 

+ (I) 10 "CC.. 

0 fn 

~~--!_ e) --- ~----- .... 

+ ... :::J ... 

ns 0 c. 1 \ \. ., ' 

-, 

1/ ,. / ~· '\ ,_ ., 

+ c:: c:: 

N 

" ,.. ~'\I 

Increasing heavy-met;l-- ,'~]' 

sulfide and pyrite content'~ ..... ,.. / 

0.1 

0.01 

1 2 3 4 5 

pH 

6 7 8 9 

Figure 1 (Ficklin and others). Geologic controls on the pH and dissolved metal content (black dots) of diverse 

mine drainage waters from Colorado. Data from Ficklin and others (1992) and Plumlee and others (1993). Mineral 

deposit types are defined by differences in their geologic characteristics, which largely control the composition of 

mine draining waters. The dashed lines enclose waters that drain mineral deposits with similar geologic characteristics. These characteristics can be used to predict, plan for, and remediate potential environmental effects caused 

by natural weathering or mining of these deposit types. 

Plumlee, G.S., Smith, K.S., Ficklin, W .H., Briggs, P.H., and 

McHugh, J.B., 1993, Empirical studies of diverse mine drainages in Colorado-Implications for the prediction of minedrainage chemistry, in Planning, Rehabilitation and Treatment 

of Disturbed Lands, Sixth Billings Symposium, Billings, 

Mont., March 21-27, 1992: Reclamation Research Unit, Montana State University, p 176-186. 

APROGRESSREPORTONTHE 

SOUTHERN CALIFORNIA 

GEOCHEMICAL DATABASE AND MAP 

Helen Folger, Albert Hofstra, and Gary Nowlan 

A 5-year mineral resource assessment of southern California was initiated in 1991, as part ofNAMRAP (National 

Mineral Resource Assessment Program). Using existing 

data, we have compiled a preliminary regional geochemical 

database and constructed a distribution map of selected elements for southern California. Review of the data showed 

35 

good correlation between the geochemical anomalies exhibited in the map and known mining districts and metallogenic 

provinces. We present here an element-distribution map 

based on the existing database and outline future plans for 

the project. 

The study focused on the regfon between lat 37° N. and 

the Mexican border. The southern California study area 

includes parts of the following physiographic provinces: 

Basin and Range, Mojave Desert, Sonoran Desert, Salton 

Trough, and Transverse and Peninsular Ranges~ The study 

area includes the Monterey, Fresno, Death Valley, San Luis 

Obispo, Bakersfield, Trona, Kingman, Santa Maria, Los 

Angeles, San Bernardino, Needles, Long Beach, Santa Anna, 

Salton Sea, El Centro, and San Diego 1 °X2° quadrangles. 

Our data were compiled from three principal databases: 

the U.S. Geological Survey RASS and PLUTO databases, 

and the Department of Energy NURE database. Because 

these databases contain samples from many different studies 

collected over a long period of time, sample media, sample 

density, analyzed elements, and analytical methods vary 

both within and among the three databases. Nevertheless, 
we have successfully constructed a coherent regional 

geochemical database for the study area. 

The compiled regional geochemical database is based 

on the analyses of minus-80 mesh and minus-100 mesh 

stream-sediment and soil samples. We evaluated a total of 

9,650 NURE samples and 9,797 RASS and PLUTO samples. The geochemical map we constructed uses vector symbols to represent relative concentrations for lead, zinc, 

copper, gold, silver, arsenic, antimony, and molybdenum, 

"pathfinder" elements selected for their common association 

with precious- and base-metal deposits. All data plotted on 

the geochemical map were gridded at a scale of 4 km2. By 

gridding the samples, we generated a composite sample-by 

selecting the highest concentration value for each element 

from samples located within the grid. Only values considered anomalous are depicted by vectors. 

Where adequate sample coverage exists, the geochemistry reflects areas of known mineralization. However, not 

all quadrangles have sufficient coverage to adequately evaluate the resource potential. A preliminary interpretation of 

this database indicates that it was inadequate for mineral 

resource assessment; additional samples and analyses were 

needed. Only the Fresno, San Luis Obispo, Bakersfield, 

Kingman, and Salton Sea quadrangles have samples that 

have been analyzed for some of the eight pathfinder elements shown on the geochemical map. In all quadrangles 

within the study area, low-level gold, arsenic, and antimony 

analyses are almost completely lacking. 

In order to adequately evaluate the resource potential of 

this region, we needed an improved, updated geochemical 

database. Thus several thousand NURE samples are being 

reanalyzed for 40 elements by inductively coupled plasmaatomic absorption spectroscopy (ICP-AES) methods as well 

as by graphite-furnace atomic absorption (AA) for low-level 

gold (0.002 ppm detection limit). Additional stream-sediment samples collected in areas of sparse existing coverage 

are being analyzed by the same methods. These new data 

will provide greater sample coverage for a suite of elements 

with lower detection limits. We plan to complete and update 

the database and map of the regional geochemistry for 

Southern California by the end of 1996. 

SANTA CATALINA MOUNTAINS, 

SOUTHEASTERN ARIZONA-LARAMIDE 

TO EOCENE PALEO DEPTHS FROM FIELD 

EVIDENCE, AND IMPLICATIONS FOR 

MINERAL POTENTIAL 

Eric R. Force 

Reconstruction of paleosurfaces in the Santa Catalina 

Mountains suggests that Laramide (Late Cretaceous-Paleocene) paleodepths vary from zero in the Stratton-Peppersauce area in the northern part of the range, through about 2 

km at the Geesaman fault on the north slope, to about 4 km 

36 

at the range crest. These figures are based on the following 

chain of reasoning: 

1. Sedimentary rocks of the Upper Cretaceous American Rag Formation and a subaerial-vent facies of the older 

subunit of Rice Peak porphyry record a Laramide paleosurface in the Stratton-Peppersauce area; 

2. Supracrustal rocks dipping gently east and northeast 

as a result of post-Laramide deformation can be used to 

extrapolate this paleosurface southward to the Geesaman 

fault; 

3. The Geesaman, an earlier Laramide fault, was 

intruded by Laramide Leatherwood quartz diorite. Thus 

Laramide paleodepths north and south of the fault are the 

same; and 

4. The Leatherwood quartz diorite forms a nearly concordant sheet that cuts almost all structures including thrust 

faults between the Geesaman fault and the range crest. The 

Marble peak syncline which deforms all these features is the 

exception, but its effect can be calculated. The angle 

between the Leatherwood sheet and sedimentary host rocks 

is constant; extrapolation of the paleosurface using this angle 

and dip of supracrustal rocks gives a 4.1±0.5-km paleodepth 

for the range crest. 

In the Eocene, seven leucogranite sills of the Wilderness suite were intruded below this pile. The sills total 3.9 

km thickness in only 0.7 km of host-rock screens. Sill intrusion caused Eocene paleodepths to increase to 8.8 km at the 

structural base of the forerange at the south end of the range 

(fig. 1). However, Eocene paleodepths of exposed rocks 

northward from the range crest remained about the same 

because these rocks overlie the sills. 

These field-controlled paleodepths imply that Laramide 

phyllitic fabrics and tight sheath folds in the range-crest area 

formed as shallow as about 4 km, consistent with low grades 

of Laramide metamorphism except for contact metamorphism. Previously published Laramide paleodepths based on 

"magmatic" epidote in the Leatherwood quartz diorite are 

incorrect; those depths are geologically impossible and the 

epidote is not magmatic. Later mid-Tertiary mylonitization 

apparently occurred at depth ranges of about 8.7 to 4.5 km in 

the forerange (assuming little late Eocene-early Oligocene 

erosion); these are the most deep-seated events recorded in 

the Catalinas. 

The Catalinas have been considered a mid-crustal metamorphic core complex, and this view has implied minimal 

resource potential for metallic minerals in deep-seated mylonitic rocks. Actually, only the forerange should be considered such a complex, and the newly calculated Laramide 

paleodepths are permissive of a variety of upper-crustal mineral-deposit types in the rest of the range. Paleodepths represented by the few already-known Laramide deposits are 

about 2 km or less. 

In addition, many more of the northern parts of the 

range than previously supposed consist of favorable host 

rocks. For example, the Abrigo Formation forms extensive 
Locality in forerange 

Locality along range crest 

Figure 1 (Force). Paleodepths versus age from field evidence in 

the Santa Catalina Mountains, southeastern Arizona. 

dip-slopes south of Edgar-type thrust faults, with Laramide 

paleodepths of about 3 km, and contains previously unreported Laramide skarns. The Geesaman fault, a deformed 

left-oblique fault with about 7 km displacement, is the locus 

of a previously unrecognized zoned population of midTertiary deposits. 

MAPPING ALTERATION IN 

HOT SPRING ENVIRONMENTS USING 

IMAGING SPECTROMETRY 

Andrea J. Gallagher, Ken Watson, 

Daniel H. Knepper, Jr., and Cathy Ager 

In August 1992, TMS (Thematic Mapper Simulator) 

and A VIRIS (Airborne Visible and Infrared Imaging Spectrometer) data were acquired over a portion of the Rio 

Grande Rift in south-central Colorado containing four hot 

springs areas. The primary objective of this study is to determine the distribution of epithermal minerals at each hot 

springs site and to relate them to differences in the geologic 

settings. A secondary task is to evaluate each site's vegetation characteristics as a function of the epithermal mineral 

distribution. 

TMS has spatial resolution of 20 m per pixel and has 10 

channels in the 0.4 to 2.5 J.lm region. A VIRIS acquired data 

with a spatial resolution of 20 m per pixel, and measures 224 

channels from 0.4 to 2.5 J.lm. These narrow, contiguous 

37 

bands form reflectance spectra for each pixel measured. 

Characteristic spectral features of minerals, and thence the 

minerals, can be mapped by comparing the A VIRIS data 

with reference mineral spectra measured in the laboratory. 

Using TMS data we can detect the presence of clay or carbonate minerals, but with A VIRIS data we are able to identify the specific mineral. 

_ Many epithermal minerals associated with hot springs 

deposits and hydrothermal alteration are ideal for mapping 

with imaging spectrometry data because they have characteristic absorption features in the 0.4 to 2.5 J.lm region. Such 

minerals include iron oxides, clays, carbonates, micas, 

jarosites, and ammonium minerals such as buddingtonite. 

Subtle differences in the crystallinity or chemistry of these 

minerals may be detected in reflectance spectra. 

The four hot springs areas in the study area occupy different geologic settings. Valley View Hot Springs is on the 

east edge of the San Luis Valley along the Neogene fault that 

bounds the west edge of the Sangre de Cristo Mountains. 

Mineral Hot Springs lies in the Tertiary and Quaternary valley-fill sediments of the San Luis Valley, at the projected 

intersection of two fault zones. Poncha Hot Springs is in Precambrian rocks along the northern boundary fault of the 

west-trending horst block separating the Arkansas River 

Valley and the San Luis Valley. Waunita Hot Springs is 

located on a fault in Cretaceous sedimentary rocks intruded 

by Tertiary rocks. 

Preliminary mineral mapping results using the A VIRIS 

data show a calcite signature associated with the travertine 

mound at Mineral Hot Springs. A halo of amorphous iron 

oxide or combination of iron oxides extends up to a mile 

( = 1.6 km) from the hot springs. This suggests that the areal 

extent of alteration from the hot springs is much greater than 

that determined from field observations. Iron oxide and illite 

map together in an area east of Mineral Hot Springs, which 

may indicate alteration along another fault set that extends 

into the San Luis Valley. In progress are clay mineral maps 

that should help further characterize the epithermal alteration. At the other hot springs locations, we expect to detect 

similar alteration patterns, although the mineralogy may 

vary with the geologic setting. 

CONSTRAINTS ON MINERAL RESOURCE 

POTENTIAL FROM ANALYSES OF 

GEOPHYSICAL DATA FROM THE 

CORONADO NATIONAL FOREST, 

SOUTHEASTERN ARIZONA 

Mark E. Gettings 

Regional gravity and aeromagnetic anomaly data, 

NURE (National Uranium Resource Evaluation) low-level 

aeromagnetic and aeroradiometric flightline data, and 
detailed ground magnetic data have been analyzed to assist 

in evaluation of the mineral resource potential of the Coronado National Forest, southeastern Arizona. Regional-scale 

structures derived from interpretation of complete Bouguer 

gravity anomaly and regional aeromagnetic data define a 

rectangular pattern of elongate-to-the-northwest structural 

blocks that define the style of post-Laramide extension. The 

northwest and southeast ends of the individual blocks are terminated by northeast- or north-northeast-trending structures 

across which differential extension has occurred. If we 

assume that the northeast- and northwest-trending block 

boundaries correspond to deep fractures of crustal scale, then 

we can reasonably assume that the movement of both heat 

and mass during and following extension may be concentrated near the block boundaries. In southeastern Arizona, 

many of the post-Laramide plutons are proximal to these 

geophysically defined boundaries. Geologic and geophysical studies have shown that within the regional blocks, 

extension has also occurred, and at least some instances of 

post-Laramide mineralization can be shown to be localized 

to extensional faults. Thus proximity to extensional domain 

boundaries is one criterion for favorability of post-Laramide 

mineralization. Many of the block boundaries may be determined by preexisting structure; if so, proximity to these 

boundaries becomes a favorability criterion for older mineralization as well, and it may explain why many known 

deposits are located on the edges of mountain ranges, which 

often are block boundaries. 

In some areas of the forest, several plutons or plutonic 

complexes are of large enough volume (as determined by 

geophysical modeling) and near enough in age to the times 

of crustal extension to be heat sources for detachmentrelated precious-metal deposits. This definition fits several 

areas of favorability in the Santa Catalina, Galiuro, and 

Santa Teresa Mountains. Analysis of the aeromagnetic data 

in the Whetstone Mountains, supported by gravity anomaly 

and physical property data, suggests that the mountain range 

may be entirely underlain by intrusive rocks, probably of 

Tertiary age. The intrusives have "rafted" the range up and 

by their own thickness constitute a significant proportion of 

the uplift of the range. Geologic work by E.R. Force 

(described elsewhere in this volume) has demonstrated the 

same effect in the Santa Catalina Mountains. The intrusives 

constitute significant heat and possibly metal sources and 

render areas of the northern Whetstone Mountains favorable 

for replacement-type deposits where favorable host rocks are 

present. This type of model applies to several areas in the 

Coronado National Forest, notably the Chiricahua, Dragoon, 

and Santa Rita Mountains, in addition to those already listed. 

Analysis of the gravity anomaly map of the Santa Rita 

Mountains and surrounding vicinity suggests that extension 

in at least the southern part of the range contains a strong 

rotational component in a clockwise sense. This component 

is presumably due to differential rotation across a boundary 

to the east with the Patagonia Mountains and the Canelo 

38 

Hills. The rotation is the apparent source of the tensional 

regime resulting in the east-west-trending zone of dikes that 

underwent silver mineralization. On the basis of this analysis, the favorable area for mineralization can be extended 

into areas of cover. 

Although flightlines of the NURE study are too widely 

spaced (5 km) to be contoured onto maps for resource evaluation purposes, the flightline data can be projected and plotted onto maps that overlay geologic maps. In the Chiricahua 

Mountains, the gamma ray spectrometer data were used to 

show that the rhyolitic rocks in the southeastern part of the 

range were distinct from the Turkey Creek rhyolites in the 

rest of the range, even though no definitive mapping had 

been done in much of the southeastern region. Throughout 

the forest study area, the best data available for depth-tosource estimates came from NURE aeromagnetic profiles, 

because the flightline profiles preserve the observed gradients of the magnetic field rather than a smoothed version 

produced by a map. In the northern Santa Catalina Mountains, a consistent set of sources with depth-to-top of about 2 

km below the surface was observed. These sources are probably magnetic contrasts in the host rocks below the Wilderness granite, and they provide additional evidence for a silllike overall structure of the Wilderness suite. In the Patagonia Mountains, gamma ray spectrometer data from the 

NURE survey defined several areas of potassic metasomatism, one of which is mineralized. As all the areas of potassium metasomatism are somewhat favorable for 

mineralization by other criteria, the radiometric data provide 

another constraint on favorability for mineralization. 

SOMESTRUCTURALFEATURESALONG 

THE TUCSON-MOGOLLON CORRIDOR 

INFERRED FROM GRAVITY AND 

MAGNETIC ANOMALY DATA 

Mark E. Gettings 

Interpretation of a complete Bouguer gravity anomaly 

map and a regional aeromagnetic anomaly map has delineated several structural patterns, suggesting that in southeastern Arizona, regional-scale extension has occurred 

transverse to elongate, northwest-trending, generally tilted, 

structural blocks about 30 km in width (northeast-southwest) 

and with length:width ratios of about 4:1 for the major 

blocks. Structural blocks are cut off by northeast- or northnortheast-trending wrench zones that offset blocks by as 

much as a block width. The gravity anomaly map generally 

exhibits this regional-scale pattern of rectangular blocks 

more clearly, whereas on the aeromagnetic map, the pattern 

is overprinted by many lithologic variation anomalies; 

locally the aeromagnetic patterns define the northeasterly 

trending block-terminating structures. Pre1iminary estimates 
of southwest -northeast extension suggest that it is of the 

order of 50--100 percent across the map area. 

The Santa Catalina Mountains and Pinaleno Mountains 

metamorphic core complexes are included in the area of 

study, and both have similar gravity and magnetic anomaly 

patterns. These patterns are general gravity anomaly highs 

relative to the surrounding basins, but they include large 

superimposed anomalies caused by plutons and other 

sources in and adjacent to the complexes. Magnetic anomaly 

patterns over the core complexes are intricate; large lows 

correspond to the Eocene "Wilderness granite" (Shakel, 

1978) sills of the Santa Catalina and Rincon Mountains and 

some of the gneisses and granites of the Pinaleno Mountains. 

A consistent relationship is gravity anomaly highs wherever 

there are mapped detachment faults and fragments of the 

upper plate exposed; the maximum of the high is slightly displaced from the detachment fault and over the upper plate. In 

the case of the Santa Catalina core complex, the anomaly 

maximum falls over the northern Tucson basin, a fact which 

has led to difficulties in regional-residual field separation in 

other gravity anomaly studies in the area. In the Pinaleno 

core complex, the gravity anomaly shows that the upper 

plate-lower plate boundary trends nearly south from the 

Santa Teresa Mountains across the range at Eagle Pass. The 

entire Mount Graham portion of the Pinaleno Mountains 

from Eagle Pass to the eastern terminus of the range appears 

to be a wrench or accommodation zone, with the upper plate 

absent. The gravity anomaly map suggests (but does not 

require) an upper plate-lower plate boundary at the east end 

of the range. Neither the gravity nor the magnetic data indicate the presence of a detachment fault along the northwesttrending edge of the range where it bounds the San Simon 

Valley. The upper-lower plate boundary also has a consistent magnetic high anomaly, which is displaced to magnetic 

south from the gravity anomaly maximum, as would be 

expected for a magnetic dipolar response. Thus, interpretation of the magnetic anomalies corroborates that of the gravity anomalies with regard to the location and trend of the 

detachment structures. Models of the correlative gravity and 

magnetic highs suggest that the Santa Catalina core complex 

actually has its axis of uplift approximately along the south 

edge of the Santa Catalina Mountains, as suggested on geologic evidence by E.R. Force (this volume). The presence of 

both gravity and magnetic highs requires a mafic composition for the material in the core of the core complex. Model 

calculations, together with measurements of magnetic susceptibility and bulk density, strongly suggest that the Pinaleno Mountains also are underlain by a similar mafic 

intrusive mass. 

Constraints on extension in a structural block have been 

obtained in the Galiuro Mountains area. Gravity survey data 

in the Galiuro Mountains and surrounding San Pedro and 

Sulphur Springs Valleys have delineated a large circular 

gravity minimum about 40 km in diameter, interpreted to 

represent the gravity effect of the volcanotectonic depression 

39 

of the Galiuro Volcanics (Oligocene and Miocene). The circular gravity anomaly contains numerous northwest-trending gradients that correlate with faults exposed in the Galiuro 

Mountains and indicate the location of faults beneath the valley fill. The inference of buried faults in the San Pedro Valley has been corroborated by interpretation of detailed 

magnetic profiles collected with a truck-mounted magnetometer. These faults are the result of syn- and post-volcanic 

extension of the area along a northeast-southwest trend. A 

minimum extension in the mountain range of 15 percent can 

be demonstrated from exposed offsets of volcanic units on 

faults. Assuming similar dips for the faults beneath cover, a 

minimum extension of 15 percent since approximately 26 

Ma (the oldest faulting dated by field relations in the Galiuro 

Volcanics) can be established for the area from the easternmost Santa Catalina Mountains bedrock in the Peppersauce 

Wash area to the westernmost outcrops of bedrock of the 

Pinaleno Mountains at the foot of Eagle Pass. Spacing 

between faults in this domain is of the order of 5 km with 

length:width ratios of about 4: 1. 

REFERENCE 

Shakel, D.W., 1978, Supplemental road log number 2-Santa Catalina Mountains via Catalina Highway: New Mexico Geological Society 29th Field Conference Guidebook, p. 105-111. 

GOLD-BEARING EXHALATIVEHYDROTHERMAL SYSTEMS IN THE 

VENEZUELAN ANDES 

Richard I. Grauch 

Three regions (Mucuchies, Burbusay, Canagmi) of previously unrecognized, submarine exhalative-hydrothermal 

activity have been recognized in the Venezuelan Andes. Coticule (spessartine-quartz rock) and tourmalinite interlayered 

with pelitic schist, amphibolite, and felsic gneiss are indicators of pre metamorphic, submarine exhalative-hydrothermal 

activity. Protoliths of the coticule were probably manganiferous chert and silt, whereas that of the tourmalinite was 

most likely boron-metasomatized sediment. At least one 

region, Mucuchies, contains gold in upper amphibolitefacies metamorphites, specifically in coticule, tourmalinite, 

and sulfide-rich gneiss. 

In the Mucuchies region, coticule occurs as thin (as 

much as 1 em thick) interlayers in quartz-biotite-two-feldspar schist that is part of a Paleozoic(?) sequence of sillimanite-grade schist, amphibolite, and felsic gneiss. Peridotite is 

a minor component of the sequence and is associated with an 

abnormal amount of amphibolite. Gamet in the coticule is 

fine grained ( 1-3 mm in diameter), optically unzoned, and 
compositionally homogeneous (approximately 30 percent 

spessartine, 60 percent almandine, 7 percent pyrope, and 3 

percent grossular). The predominant opaque phase is MnZn-bearing ilmenite. Trace amounts of a Fe-Cu-Zn-S phase, 

a REE-P (rare-earth-element-phosphorus) phase, and a AuTe phase with minor Ag are also present. The Au-Te phase 

occurs as small ( <1 J..Lm long) grains on the rims of garnet and 

as inclusions in biotite and both feldspars. The tourmalinite 

(>80 volume percent tourmaline) has been found only as 

float but must be part of the same lithologic sequence as the 

coticule. Translucent phases are dravite (Mg0/Fe0=0.6 to 

1.0), quartz, biotite, two feldspars, and trace amounts of a 

REE-P phase. The opaque assemblage consists of graphite, 

Mn-bearing ilmenite, chalcopyrite, and trace amounts of 

phases composed of Fe-S, Pb-S, and Au-Te with minor Ag. 

The Au-Te phase occurs as small inclusions in tourmaline 

and quartz. The sulfide-rich gneiss occurs about 10 km along 

strike from the coticule. It is composed of quartz, two feldspars, biotite, muscovite, almandine garnet, and a REE-P 

phase. Minor amounts ofMn-bearing ilmenite and Fe-S, FeCu-S, Fe-Cu-Zn-S, and Au-Te phases make up the opaque 

assemblage. The Au-Te phase has minor Ag and occurs as 

small inclusions in biotite. Cordierite-bearing gneiss in the 

vicinity of the sulfide-rich gneiss may be a further indication 

ofpremetamorphic hydrothermal activity (magnesium metasomatism) in the region. 

Tourmalinite in the Burbusay region occurs in a middle 

to late Paleozoic sequence of staurolite-bearing pelitic 

schist, quartzite, and amphibolite. Fine-grained garnetiferous quartzite (possibly coticule) is a minor but widely distributed portion of the sequence. The tourmalinite is a twomica-tourmaline (approximately 30 volume percent)-quartz 

schist. Apatite, a REE-P phase, and a Y -P (yttrium-phosphorus) phase are minor components of the rock. Tourmaline 

compositions are more magnesian than those in the 

Mucuchies region; MgO/FeO values exceed 1. Opaque 

phases include ilmenite and chalcopyrite; the latter phase 

occurs as inclusions in tourmaline and quartz. 

Tourmalinite in the Canagmi region occurs within the 

same stratigraphic unit as that in the Burbusay region. 

Although the exact nature of the surrounding lithologies is 

not known, they are, in general, greenschist-facies phyllite 

(in part, graphitic) and amphibolite. Forty kilometers across 

strike from Canagua, the same unit hosts the small, highgrade Bailadores massive Zn-Pb-Cu sulfide deposit. 

The lithologic packages, especially in the Mucuchies 

region, that host the coticule and tourmalinite are indicative 

of ocean floor deposition. Graphite in the tourmalinite indicates that at least some of the original sediments were deposited in a restricted basin. Base metals within the coticule and 

tourmalinite are strong indications of metalliferous, exhalative-hydrothermal activity in a marine environment, and their 

presence suggests the possibility of sediment-hosted basemetal deposits in regions far removed from the Bailadores 

deposit. The gold and tellurium in the Mucuchies region 

40 

represent unusual, but not unprecedented, metal accumulations in exhalative, marine environments. Their economic and 

petrogenetic significance remains to be examined. 

REDEFINITION OF LATERITIC BAUXITE 

DEPOSIT TYPES, NORTHERN SOUTH 

AMERICA SffiELD 

Floyd Gray, James D. Bliss, Norman J Page, 

Herbert A. Pierce, and Barbara Eiswerth 

Bauxite deposits are widely variable in their origin and 

geological settings, and various classification schemes can 

be found in the literature (Harder, 1952; Patterson, 1967, 

1986; Hill and Ostojic, 1984 ). Many of the classifications for 

the important deposits of northern South America combine 

genesis, elevation, age, parent rock, and tectonic setting, 

although they generally recognize two important groupings: 

bauxite developed in upland areas and those deposits formed 

on lowland peneplains in sedimentary settings (Van Kersen, 

1956; Moses and Michell, 1963; Bleackley, 1964; Grubb, 

1973a, b; Aleva, 1981; Aleva and Hilversum, 1984; Menendez and Sarmentero, 1984). We propose to redefine the 

known and potential bauxite resources of the northern margin of the Guayana Shield as being of two broadly defined 

types: remnant and continentally derived detrital (fig. 1). 

These two types are distinguished by their stratigraphic profile and geomorphologic settings. Other characteristics such 

as age, thickness, elevation, chemistry, textural variation, 

and cover sequences cannot be used to discriminate between 

these deposits. 

Remnant deposits result from weathering of underlying 

bedrock. The deposits grade downward from the bauxite 

horizon into kaolinized bedrock, weathered bedrock, and 

unweathered bedrock material (fig. 1, profile A-B). Bedrock 

can include various aluminum-bearing igneous, sedimentary, and metamorphic rocks, although unmetamorphosed 

sedimentary rocks are rare. Bauxitization is a complex, multistage process that may have begun as early as Late Cretaceous in South America, although the majority of the 

deposits studied are Eocene to recent in age. These deposits 

Figure 1 (Gray, Bliss, and others) (facing page). Generalized 

profiles of lateritic bauxite types, northern South America. Three 

types of exposures for remnant-type deposits are presented in cross 

section A-B. Two types of exposures for continentally derived detrital deposil'i are represented by cross section C-D. The three types of 

outcrops for remnant deposits shown are (1) topographically prominent deposit'i subject to dissection, (2) topographically indistinct deposits subject to continued bauxitization (may be partly buried with 

younger sediments), and (3) blind deposits totally covered with sediments. The two types of continentally derived detrital deposits are 

(4) topographically prominent deposits, and (5) blind deposits covered by recent sediments. Modified from Van Kersen (1956). 
A 1 

c 

NOT TO SCALE 

Bauxite with carbonaceous clay lens 

Transgressive cover rocks 

Bauxite 

Kaolinite-bauxite 

Weathered bedrock 

Unweathered bedrock 

Sedimentary kaolinite with carbonaceous clay lens (bauxitized) 

Unweathered bedrock 

EXPLANATION 

11!!11111 Sediments 

-Bauxite 

E2J Kaolinite 

CJ Weathered bedrock 

~ Crystalline bedrock 

W~:':\~);}~j Arkosic sand 

41 

B 

D 
can be at high elevations or at sea level but are predominantly residual in origin. Pakaraima Mountains, Guyana, 

Nassau Mountains, Suriname, and Los Pijiguaos, Venezuela 

are examples of this deposit type. 

Continentally derived detrital deposits are found in sediments shed off the margin of the Precambrian crystalline 

basement complex (fig. 1, profile C-D) as typified in the 

coastal plain of Suriname. Arkosic sandstone is the dominant 

lithology with variable minor occurrences of intercalated 

carbonaceous sandy clay lenses, kaolin, and shale. The sedimentary sequence hosting this deposit type is part of deltaic 

and (or) large alluvial basin systems and unconformably 

overlies crystalline basement rocks that may include remnant-type bauxite horizons. Bauxitization of the transported 

sedimentary beds has resulted in bauxite and kaolin-rich 

beds resting unconformably above weathered bedrock. 

The sedimentary deposits historically have formed the 

bulk of the production of low-iron refractory-grade bauxite 

in northern South America since the turn of the century, principally from Guyana (ltuni, Mackenzie) and eastern Suriname (Onverdacht, Moengo). A high-grade (refractory) 

exception is the large Venezuelan Los Pijiguaos deposit, 

which overlies alkaline leucogranite. Remnant deposits 

found on the shield have been exploited but generally produce a nonrefractory-grade, high-iron bauxite with variable 

and lower ore grades (Aleva and Hilversum, 1984). 

Redefinition of the classification of bauxite deposits 

may affect (1) delineation of areas with potential for undiscovered deposits; (2) development of grade, tonnage, and 

exploration models; and (3) environmental characterization. 

Exploration for remnant deposits is best facilitated by analysis of regional erosion surfaces using elevation, geobotanical 

mapping by Thematic Mapper and radar, and field sampling 

(shallow core). Exploration for continentally derived detrital 

deposits requires profiles of edges of basin structures using 

gravity or seismic data, age and subsidence rate analysis, 

core drilling, and sampling. Regional tectonic analysis is 

important in the assessment for both deposit types. For 

example, the rate of uplift of the Imataca terrane (Venezuela) 

and the Bakhius Mountains horst (Suriname) affected the 

isostatic equilibrium of the shield and the subsidence rates of 

adjoining basins in northern South America, thereby affecting the shape of potential domains for bauxite resources. 

Using these newly defined deposit types to assess lateritic 

bauxite potential may change the calculated bauxite reserves 

for the area, and these newly defined models may be applied 

to other regions of South America as well as to other continental regions where lateritic processes occur. 

REFERENCES 

Aleva, G.J.J., 1981, Essential differences between the bauxite deposits along the Southern and Northern edges of the Guiana 

Shield, South America: Economic Geology, v. 76, 

p. 1142-1152. 

42 

Aleva, G.J.J .• and Hilversum, A.H., 1984, Bauxite in West Suriname-Known deposits and potential. in Leonard, Jacob, Jr., 

ed., Bauxite; Proceedings of the 1984 Bauxite Symposium, 

Los Angeles, Calif., Feb. 27-March 1: New York, Society of 

Mining Engineers AIME, p. 319-348. 

Bleackley, 0.,1964, Bauxites and laterites of British Guiana: British Guiana Geological Survey Bulletin 34. 154 p. 

Grubb, P.L.C., 1973a, Genesis of bauxite deposits in the Lower 

Amazon Basin and Guianas Coastal Plain: Economic Geology, 

v. 74. p. 735-750. 

__ 1973b. High-level and low-level bauxitization-A criterion 

for classification: Mineral Science and Engineering, v. 5, 

p. 213-231. 

Harder, E. C., 1952, Examples of bauxite deposits illustrating variations in origin: Proceedings from Problems of Clay and Laterite Genesis Symposium, Society of Mining Engineers AIME, 

p. 35-64. 

Hill, V.G., and Ostojic, Slavko, 1984, The characteristics and classification of bauxites, in Leonard, Jacob, Jr., ed., Bauxite; Proceedings of the 1984 Bauxite Symposium, Los Angeles, Calif., 

Feb. 27-March 1: Society of Mining Engineers AIME, 

p. 31-48. 

Menendez. Alfredo, and Sarmentero, Alberto, 1984, Geology of the 

Los Pijiguaos bauxite deposit, Venezuela, in Leonard, Jacob, 

Jr., ed .. Bauxite; Proceedings of the 1984 Bauxite Symposium, 

Los Angeles, Calif., Feb. 27-March 1: Society of Mining Engineers AIME, p. 387-407. 

Moses, J.H .• and Michell, W.O., 1963, Bauxite deposits of British 

Guiana and Suriname in relation to underlying unconsolidated 

sediments suggesting two-step origin: Economic Geology, v. 

58, p. 250-262. 

Patterson, S.H., 1967, Bauxite reserves and potential aluminum resources of the world: U.S. Geological Survey Bulletin 1228, 

176 p. 

--1986. Descriptive model of laterite type bauxite deposits, in 

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

U.S. Geological Survey Bulletin 1693, p. 255. 

Van Kersen, J.F., 1956, Bauxite deposits in Surinam and Demerara 

(British Guiana): Liedse Geologische Mededelingen, v. 21, 

p. 249-373. 

MINERAL RESOURCE ASSESSMENT OF 

THE VENEZUELAN GUAYANA SIDELD 

Floyd Gray, Jeffrey C. Wynn, Greta J. Orris, 

Norman J Page, William E. Brooks, 

Gary B. Sidder, Warren C. Day, J.D. Bliss, 

Dennis P. Cox, David Detra, Jason Unkefer, and 

John Gutierrez 

A quantitative mineral resource assessment of the Venezuelan Guayana Shield is the result of a 5-year cooperative 

scientific program sponsored by U.S. Geological Survey and 

the Corporacion Venezolana de Guayana, Tecnica Minera 

C.A. Products of the assessment include a series of 
Table 1 (Gray, Wynn, and others). Probability estimates of undiscovered deposits in the Venezuelan 

Guayana Shield. 

Deposit type 90percent 

Algoma iron 9 or more 

Low sulfide gold- 20 or more 

quartz veins 

Carbonate 2 or more 

Kuroko massive 

sulfide 

Synorogenic-synvolcanic nickelcopper 

70° 

SEA 

65° 

Chance of occurrence 

50 percent 10 percent 5 percent 1 percent 

26 or more 90 or more 

40ormore 50 or more 

5 or more 12 or more 

1 or more 

1 or more 

60° 55° 

,A/~ 

~" 

/ ' EORGETOWN 

(\ \ '- PARAMARIBO 

VENEZUELA 

')-. GUYANAr-' ""....._0 CAY£NNE 

C"-. _./""' ~" ( \ ' 

,---~ ( '- SURINAME , FRENCH 

\ I "" I GUIANA( \ 

\~ \ \ <'~ I ' ,.... \ .1"">--.,) ........ ,--/ ~ 

"""w,-r ) 

COLOMBIA 

~ 

BRAZIL /~ ___ _, ,.------/ .. 

500 1000 KILOMETERS 

50° 

Figure 1 (Gray, Wynn, and others). Index map showing location of Venezuelan Guayana Shield (dashed outline) in northern South 

America. 

1: 1 ,000,000-scale maps and accompanying text, which 

characterize the mineral potential of the Venezuelan Guayana Shield, an area of Precambrian terrane in the south and 

east half of Venezuela, encompassing more than 415,000 

km2. These are supported by a geographic map, geologic 

43 

and tectonic map, simple Bouguer gravity anomaly map, 

side-looking airborne radar (SLAR) image, two mineral 

occurrence maps, and two permissive domain maps. 

U.S. Geological Survey mineral resource assessment 

methodology comprises three parts: (1) selection of 
appropriate descriptive deposit models and delineation of 

areas permissive for each deposit type; (2) construction of a 

grade-tonnage model for each deposit model; and (3) probabilistic estimation of the number of undiscovered deposits 

for model types with sufficient data. These estimates are 

made using the deposit density and the target counting 

methods. Thirteen known deposit types are identified in the 

Venezuelan Guayana Shield, and nine other deposit types 

are possible. Quantitative estimates of undiscovered deposits were made for five deposit types: Algoma iron, low-sulfide gold-quartz veins, carbonatite, kuroko massive sulfide, 

and synorogenic-synvolcanic nickel-copper (table 1); data 

are insufficient to estimate the number of undiscovered 

deposits for the remainder. 

Deposits in deeply eroded regions, such as the Imataca 

and Pastora-Supamo terranes, contain mineral deposits that 

(1) were formed in the Archean and Early Proterozoic, 

infolded with their host rocks, and metamorphosed; or (2) 

were formed during, or soon after, regional metamorphism 

associated with the Trans-Amazonian orogeny. These 

deposits include stratiform iron and manganese at Cerro 

Bolivar and El Pao, and deposits of gold in quartz veins from 

the El Callao, Kilometer 88, and Lo Increible districts, and 

elsewhere. 

Sedimentary rocks of the Roraima Group, volcanic 

rocks of the Cuchivero Group, and epizonal plutonic rocks 

that intrude Cuchivero Group rocks form a supracrustal terrane that is a potential host for deposits formed near, or 

within a few kilometers of, the surface during the Early to 

Middle Proterozoic. These deposits include carbonatite and 

diamond pipes, .. as well as yet to be discovered deposit types 

such as tin greisen, porphyry copper, volcanic-hosted iron 

deposits, and polymetallic or epithermal veins. 

Surficial processes such as erosion and chemical weathering, followed by uplift of the Venezuelan Guayana Shield, 

have created many permissive environments for mineral 

deposition. Deposits formed by leaching and residual enrichment of metals and nonmetals include bauxite, iron, kaolinite, and possibly nickel laterite. Transport and deposition of 

metals and minerals produced placer deposits of gold, diamonds, tin, and titanium. In the northern part of the shield, 

most placer deposits were formed during Miocene to Quaternary time, coinciding with the time of maximum uplift. 

Some deposit types, for example, sedimentary kaolin, are 

produced by a combination of these processes. 

This study provides information for mineral-related 

land-management decisions affecting future mining activity 

and a framework for identifying areas affected by past and 

present mining activity needing environmental mitigation 

and concurrent reclamation. A careful review of the data 

obtained during the course of this cooperative agreement 

will form the basis for future studies. Results of these investigations will serve, in part, to guide the growth and development of the Venezuelan mineral sector in the future. 

44 

MINERAL RESOURCE ASSESSMENT OF 

THE ABSAROKA-BEARTOOTH STUDY 

AREA, CUSTER AND GALLATIN 

NATIONAL FORESTS, MONTANA 

J.M. Hammarstrom, M.L. Zientek, J.E. Elliott, 

R.R. Carlson, G .K. Lee, 

B.S. Van Gosen, and D.M. Kulik 

The Absaroka-Beartooth study area encompasses 

approximately 1.4 million acres in the Custer and Gallatin 

National Forests of southwestern Montana. The area contains most of the identified lode resources of platinumgroup elements and the largest identified chromium 

resource in the United States. Mines have operated in the 

area since the 1860's and have produced gold, silver, copper, lead, zinc, arsenic, tungsten, chromium, platinumgroup elements, and travertine. Exploration by industry 

since 1970 led to the discovery and development of platinum-group element deposits of the Stillwater mine and 

gold deposits of the Mineral Hill mine and the discovery of 

gold-copper-silver deposits in the New World district. 

Recent exploration efforts have focused on gold, chromium, platinum-group elements, and oil and gas. 

A 2-year study of the geology, geochemistry, geophysics, exploration history, and nature and distribution of mineral occurrences enabled a USGS team to delineate nine 

tracts of land as geologic environments deemed permissive 

for the occurrence of 20 different types of mineral deposits. 

Favorable areas for undiscovered deposits are delineated 

within some permissive tracts. Important geologic controls 

on the occurrence of mineral deposits in the study area 

include the following: (1) an Archean metasedimentary 

package of rocks that includes banded iron-formation and 

hosts a lode gold ( +arsenic+tungsten) deposit at the Mineral 

Hill mine; (2) the Stillwater Complex, an Archean layered 

mafic intrusion that hosts disseminated to massive coppernickel sulfide deposits near its base, chromite layers, and a 

thin, laterally persistent horizon enriched in platinum-group 

elements (the J-M Reet), which is partially exploited by the 

Stillwater mine; and (3) at least five deeply eroded Cretaceous to Eocene stratovolcano centers that have associated 

porphyry copper-gold-molybdenum and related copper-gold 

skams and polymetallic vein deposits. 

Estimates of undiscovered resources for selected commodities are made by combining probabilistic estimates of 

numbers of undiscovered mineral deposits with grade and 

tonnage models (Cox and Singer, 1986; Theodore and others, 1991) in a computer simulation (Root and others, 1992) 

to describe expected distributions of commodities contained in-place in undiscovered mineral deposits. Results of 

the simulation for four types of mineral deposits (Archean 

lode gold, banded iron-formation, porphyry copper, and 
gold-bearing skarn) indicate a 50 percent chance that undiscovered deposits contain at least 90 t (metric tons) of gold, 

700 t of silver, 3.1 million t of copper, 35,000 t of molybdenum, 62 million t of iron, and 3,000 t of phosphorus (as a 

byproduct of potential iron ore). These results provide a 

quantitative basis for including mineral information in 

land-use planning; however, an economic analysis must be 

conducted to estimate any potential net worth of minable 

resources. 

Evaluation of exploration data for three additional types 

of mineral deposits in the Stillwater Complex indicates that 

approximately 225,000 t each of copper and nickel and 6.5 

million t of chromium may be present in undiscovered 

deposits. Extensions of identified resources within the Stillwater Complex may contain additional resources of these 

commodities as well as gold and platinum-group elements. 

REFERENCES 

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

U.S. Geological Survey Bulletin 1693,379 p. 

Root, D.H., Menzie, W.O., and Scott, W.A., 1992, Computer Monte Carlo simulation in quantitative resource estimation: Nonrenewable Resources, v. 1, p. 125-138. 

Theodore, T.G., Orris, G.J., Hammarstrom, J.M., and Bliss, J.D., 

1991, Gold-bearing skarns: U.S. Geological Survey Bulletin 

1930, 61 p. 

AGRICULTURAL MINERAL MINING, 

PROCESSING, AND USEENVIRONMENTAL RESEARCH 

OPPORTUNITIES 

James R. Herring and Larry P. Gough 

The agricultural minerals (nitrogen, phosphate, potash, 

sulfur, zeolites) are responsible for providing or moderating 

release of the principal agricultural nutrients (N, P, and K) 

and are vital to the United States agricultural industry and 

economy. Their value to the national economy is the sum of 

(1) their intrinsic value as mined commodities (equal to 10 

percent of nonconstruction mineral value to the U.S. economy); (2) their additional contribution because of manufacturing-added value (about 5 billion dollars); (3) export value 

and contribution to the balance of payments and foreign 

trade; and (4) their use as required nutrients to grow food in 

the Nation, 40 percent of which is exported and further contributes to trade (about 15 billion dollars export value). Continued production and best use of these minerals depend 

greatly on understanding their environmental interactions; 

consequently, our goals are twofold: (I) to use these minerals in ways most beneficial and least harmful to the 

45 

ecosystem and (2) to understand the optimal rates and methods of mining use that will allow their sustained availability 

to a growing society. 

Zeolites offer possibilities for environmental remediation principally by their ion exchange properties. This technology may be useful to many industries, for example, 

metal mining, where waste-water streams must be Cleaned 

of metallic and other impurities to meet certain discharge 

criteria. 

In the United States, most sulfur is used to produce sulfuric acid; in turn, most of the sulfuric acid is used to produce 

phosphoric acid for fertilizers. About half the sulfur used in 

the United States is obtained through the frasch process, 

which produces a clean, reagent-grade product. However, 

much of the remainder of the sulfur--obtained from secondary recovery from mineral and fossil fuel processinginvolves association between sulfur and trace elements of 

environmental concern, such as zinc, cadmium, and arsenic, 

that are released during cleaning, processing, or use of the 

fuel. Residual sulfur compounds left in coal, both sulfides 

and organosulfur compounds, also contain noxious trace elements that are released during combustion. USGS efforts 

address these considerations in the coal-quality program. 

Nitrogen fertilizer is not a rock-sourced nutrient but 

subsequently enters the geosphere because of application, 

plant uptake, and soil interaction. The environmental concerns associated with agricultural nitrogen involve its release 

to surface, vadose, or ground waters after application as a 

crop nutrient. Current interest in the USGS is in nitrogen 

interaction with these waters and is focused in the toxic-substances and water-quality programs. 

In the United States, the bioessential nutrient phosphorus is obtained from world-class deposits of phosphorite. 

Because of inequities in world resource distribution and 

interplay of politics and economics in use of global reserves, 

phosphate is regarded as the most critical agronutrient and 

the key to world food supply. U.S. production supplies its 

internal needs as well as providing about 30 percent of world 

phosphate trade (about 3 billion dollars in trade revenue). 

Major environmental concerns result from the complete 

phosphate use cycle, from mining and manufacturing to use 

and disposal. Current USGS research involves characterization of clay type (important to settling characteristics of the 

fine-grained waste), other waste products (acidic gypsum 

heaps), radioactivity, and trace element assemblages in presently mined and potential future reserves of phosphorite. 

Also being studied are depletion models of known reserves 

and scenarios for best future use that will lead to optimal 

resource demand for a sustainable global food supply. 

The United States produces only 30 percent of the potash that it uses, all from evaporite deposits. The principal 

environmental consideration associated with the production 

of potash is disposal of waste salt and clay. Waste salt is produced at approximately 2 tons per ton of mined potash. 
Significant environmental considerations are involved 

with the mining, pressing, manufacturing, and use of these 

agricultural minerals. Some of these considerations are 

being addressed by present USGS research. However, many 

additional environmental concerns offer future research 

opportunity. 

MINERAL RESOURCES, 

ENVIRONMENTAL CONCERNS, AND 

LAND-USE 

Carroll Ann Hodges 

Among the most vigorously debated issues in U.S. economic geology circles is the extent to which nonfuel minerals industries, and thereby the Nation, are adversely affected 

by land restrictions and environmental regulations, and public advocacy thereof (Ward, 1992; Hale, 1992). A presumed 

grave consequence is the departure of mining companies for 

more hospitable shores. (See, for example, Cameron, 1986.) 

The relative importance of factors such as labor costs, ore 

tonnage and grade, political stability, investment incentives, 

and exploration potential-as well as U.S. foreign policy 

interests-is seldom addressed in public commentary. The 

global political and economic cataclysms that introduced the 

1990s presage far more extensive international interdependence in world minerals trade as centrally planned economies strive to become market-driven. The producer 

oligopoly that prevailed in the decades of strong economic 

growth following World War II is dramatically diminished 

in today's global metal markets (Bomsel, 1990). 

Contrary to predictions from the 1950s through the 

early 1980s, persistent, widespread shortages of nonfuel 

minerals have never occurred, partly because exploration has 

been remarkably successful. Despite increased rates of consumption, present estimates suggest that world mineral supplies are adequate for the foreseeable future (McLaren and 

Skinner, 1987; Pearse, 1991; Vogely, 1993). Stable or 

declining prices, the potential availability of new and presumed large ore resources abroad (due in part to the demise 

of the Soviet economy as well as to new discoveries elsewhere), increased recycling, technological innovation and 

substitution-all affect supplies of raw materials and thus 

are relevant to policy decisions regarding mineral development, use of U.S. public lands, and attendant environmental 

concerns. 

The 1991 "Earth Summit" in Rio de Janeiro demonstrated that environmental consciousness is not unique to 

the industrialized world; prevention and mitigation of 

environmental problems are increasingly required elsewhere, partly as a result of United Nations encouragement 

and recent dictates from the World Bank. The Berlin 

46 

Guidelines, established at a United Nations conference in 

June 1991, mandate attention to environmental impacts, 

while at the same time ensuring the long-term economic 

and social benefits that mineral development can provide, 

specifically to resource-rich countries of the Third World. 

Social (external) costs of mining historically have not 

been incorporated in commodity prices, but that may 

change on world markets; developing nations have made it 

clear that environmental protection is of major concern 

internally and not merely "a luxury item peddled by first 

world participants" (United Nations, 1993). The newly 

created (March 1991) International Council on Metals 

and the Environment (ICME) was established specifically 

to promote sound environmental policy for the industry 

worldwide. Rigorous adherence to high standards for environmental protection may be as applicable abroad as in the 

United States sooner rather than later; the "writing is on 

the wall" (Mining Journal, 1991). 

Since World War II, U.S. policy objectives have 

included acquisition of necessary raw materials at lowest 

possible prices, consistent with the interests of our trading 

partners and with other foreign policy goals (President's 

Materials Policy Commission, 1952). Self-sufficiency is 

no longer even remotely possible, and thus a major goal 

must be to ensure adequate and dependable foreign 

sources of supply. Interests of the Nation may now 

include (in addition to domestic jobs) economic development in those countries dependent on mineral rents, without which improvement in education and living standards 

and a consequent reduction in birth rates are unlikely. 

Continued population growth at rates between 2 and 4 percent in much of the less developed Southern Hemisphere 

(World Resources Institute, 1992) poses dire implications 

not only for those nations striving to improve economic 

status, but also for the industrialized North and for the 

planet's entire ecosystem. It may behoove the United 

States to take the less parochial view that investment in 

mineral deposits overseas is advantageous politically and 

economically, abetting improved education and living 

standards in developing countries, while maintaining 

favorable consumer prices at home. Transfer of efficient 

"green" technology abroad can ensure that the pollution 

we now attempt to clean up will be avoided altogether in 

new projects overseas. The issues are complex, but the 

United States may, like Japan, become increasingly dependent on foreign raw materials, irrespective of domestic 

environmental regulations-or mining law reform. 

As world tensions diminish and trade expands, concerns about U.S. self-sufficiency and military security no 

longer seem overriding. Of greater importance may be the 

contribution of mineral resources to the economic rescue 

of some emerging nations-and a consequent reduction 

of fertility rates, without which environmental concerns 

are ultimately irrelevant. 
REFERENCES 

Bomsel, Olivier, 1990, Mining and metallurgy investment in the 

third world-The end of large projects?: Development Centre 

Studies, OECD, 221 p. 

Cameron, E.N., 1986, At the crossroads-The mineral problems of 

the United States: New York, John Wiley, 320 p. 

Hale, Alma, 1992, Public lands legislation-Drowning in a sea of 

bills: AMC Journal, August 1992, p. 5-6. 

McLaren, D.J., and Skinner, B.J., eds., 1987, Resources and world 

development: Chichester, U.K., John Wiley, 940 p. 

Mining Journal, 1991, Writing is on the wall: Environment Supplement, v. 317, no. 8140, p. 1. 

Pearse, P.H., 1991, Scarcity of natural resources and the implications for sustainable development: Natural Resources Forum, 

February 1991, p. 74-79. 

President's Materials Policy Commission, 1952, Resources for freedom, Summary of Volume 1: 82 p. 

United Nations, 1993, Interregional seminar on guidelines for the 

development of smalVmedium scale mining; Seminar Report, 

U.N. Department of Economic and Social Development (UNDESD) and Government of Zimbabwe, 15-19 February 1993, 

Harare, Zimbabwe: 38 p., 7 annexes. 

Vogely, W.A., 1993, An economist looks at resource assessment: 

Non-Renewable Resources, v. 2, no. 2, p. 67-68. 

Ward, Milton H., 1992, Mining law repeal effort is 'wrong-headed' 

AMC Journal, February 1992, p. 26-27. 

World Resources Institute, 1992, World resources, 1992-'93, A guide 

to the global environment: New York, Oxford, p. 246-247. 

NATIONAL GEOCHEMICAL DATA BASE 

J.D. Hoffman and S.P. Marsh 

The National Geochemical Data Base (NGDB) contains data from geochemical surveys conducted by the U.S. 

Geological Survey and other Federal Government agencies, 

including data from the NURE (National Uranium Resource 

Evaluation) HSSR (Hydrogeochemical and Stream Sediment Reconnaissance) program (1976-1980) from the 

National Wilderness Program (1965-present) and from 

numerous mineral resource assessment and geologic 

projects. The NGDB is maintained and controlled by the 

geochemistry staff of the U.S. Geological Survey. Administrators, land-use planners, and scientists can use this multipurpose data base to influence their decisions concerning 

health, the environment, and mineral resources. Because 

large portions of the data were collected in the 1960's and 

1970's, they can be used in "baseline" studies to establish 

background values for many metals in areas that have since 

been developed. 

The portion of the NGDB that most represents regional 

coverage for the United States is the data collected during the 

NURE HSSR. These data represent the only regional 

geochemical data base for the conterminous United States 

and Alaska. Most of these data were compiled from analyses 

of stream sediments, soils, and ground water and surficial 

water. Each sample was analyzed utilizing various chemical 

47 

methods including INA (induced neutron activation), AA 

(atomic absorption), and ES (emission spectrography) for 

uranium and as many as 58 other elements, plus sulfate. The 

USGS has undertaken the reanalysis of the NURE HSSR 

samples using multi-element inductive coupled plasma 

(ICP) and low-level (parts per billion) gold techniques. 

The history of the NURE HSSR program is complex. 

Starting in 197 4, NURE program personnel acquired and 

compiled geologic and other information required to evaluate the extent and distribution of uranium resources and to 

determine areas favorable for the occurrence of uranium in 

the United States. The HSSR survey sampling component of 

the program began in 1976 and ended in 1980. Initially, the 

samples were analyzed only for uranium. In 1977, analyses 

for other elements were added. 

Four DOE (Department of Energy) national laboratories were involved in the survey: Lawrence Livermore Laboratory, Los Alamos Scientific Laboratory, Oak Ridge 

Gaseous Diffusion Plant, and Savannah River Laboratory. 

Each developed its own procedures for sample collection, 

analyses, and data management; these procedures were 

sometimes altered during the life of the program. In 1977, 

the laboratories were instructed to conform all sampling and 

data management to the 1 °X2° NTMS grid system. This 

decision resulted in incomplete coverage in some States, 

mainly along their borders; also, some quadrangles were 

sampled by more than one laboratory. An inevitable consequence of using the analytical capabilities of all four laboratories and the multiple procedural changes was the 

generation of 47 different data formats. In addition, some of 

the national laboratories subcontracted analytical work to 

one or more of the other national laboratories and to private 

laboratories. 

Under contract to the DOE, the Information Systems 

Programs of the Energy Resources Institute, University of 

Oklahoma, Norman, reorganized substantial portions of the 

data into one format (1983-1985). In 1985, a memorandum 

of understanding between the DOE and the Department of 

the Interior transferred responsibilities for maintenance of 

HSSR samples and HSSR data collected under the NURE 

program to the U.S. Geological Survey. 

The NURE HSSR data for the western United States, 

using the reformatted data, have been published as USGS 

Publication DDS-0001 using Compact Disc-Read Only 

Memory (CD-ROM) technology. A CD-ROM for the 

Alaska NURE HSSR data and a CD-ROM containing all the 

NURE HSSR data for the conterminous United States are 

also being developed. A fourth CD-ROM publication is to 

contain the NURE HSSR data in the original formats, plus 

indexes and a user's guide. In all, the NURE HSSR data 

encompass 428 1 °X2° quadrangles in the conterminous 

United States and Alaska, totaling about 850,000 records. 

Data may be retrieved from Publication DDS-0001 

according to sample type, quadrangle, latitude and longitude, 

element, element concentration range, or any combination of 

these criteria. Once the requested data are found, they may 
be written in dBASE format or several different ASCII formats to the user's hard disk. Context-sensitive help files are 

available. The prototype Alaska and conterminous United 

States CO-ROM's have similar structures and capabilities. 

LA ESPANOLA PROSPECT-A MULTIPLE 

INTRUSIVE PORPHYRY COMPLEX WITH 

LOW-GRADE GOLD POTENTIAL, 

NORTHWESTERN ALTIPLANO, BOLIVIA 

Albert Hofstra, Luis Barrera, Richard Hardyman, 

Edwin Mckee, and Orlando Sanjines 

La Espanola prospect lies in the Bolivian portion of the 

Neogene to Quaternary magmatic arc of the central Andes 

(lat 17°14' S., long 69°31' W.). La Espanola was selected for 

study because it is the most intense color anomaly on remote 

sensing imagery of this part of the arc and because gold was 

found during recent prospecting. Field mapping, geochronology, exploration geochemistry, and fluid-inclusion studies were conducted by geologists from GEOBOL (Servicio 

Geologico de Bolivia) and the USGS in an effort to determine the origin of the gold-anomalous rocks at La Espanola. 

The results suggest that La Espanola can be characterized as 

a multiple intrusive porphyry complex with epithermal alteration/mineralization assemblages superimposed on deeper 

porphyry-style assemblages. 

The hydrothermal system is centered on a 1.5x3 km 

composite andesite to dacite porphyry stock that intrudes 

Oligocene to middle Miocene andesite to dacite flows, 

lahars, and volcaniclastic sedimentary rocks. The stock consists of at least four early intrusive phases, including one distinguished by an abundance of miaroliti·c cavities. In most 

places these intrusions are pervasively altered to an assemblage of quartz, sericite, and pyrite, although potassic and 

propylitic alteration is also present. These altered porphyries 

are intruded by several small unaltered dacite porphyry dikes 

and plugs. K-Ar determinations on hornblende in premineralization dacite flows (11.2±0.7 Ma) and on biotite from a 

premineralization intrusive phase in the stock (11.8±0.8 Ma) 

indicate the stock was emplaced between about 12.6 and 

10.5 Ma. Alunite, associated with vuggy silica alteration that 

crosscuts the early altered porphyries, is dated at 1 0.3±0.3 

Ma, indicating that alteration and mineralization soon followed emplacement. The volcanic edifice inferred to have 

overlain La Espanola has been completely stripped away. 

Nearby volcanoes, 1-2 km higher in elevation than La 

Espanola, are dated at 4-8 Ma. 

Four types of alteration/mineralization are recognized 

at La Espanola: (1) black, porphyry-style stockwork veinlets, in potassically altered portions of the stock, consist of 

quartz, ±pyrite, ±bornite, ±chalcopyrite, ±magnetite, ±gold; 

(2) stockwork veinlets, in phyllically altered portions of the 

48 

stock, consist of pyrite, sericite, quartz, ±gold; (3) quartzbase-metal veins with sericitic alteration, in fractures marginal to the stock, contain pyrite, chalcopyrite, sphalerite, 

±galena; (4) acid-sulfate alteration, along structures within 

and marginal to the stock, is characterized by an early stage 

of vuggy silica and quartz-alunite alteration and a second 

stage of silicification with pyrite, barite, ±enargite, ±silver 

sulfosalts, ±galena, ±sphalerite, ±gold. 

Altered rocks contain low but anomalous concentrations of Au, Ag, As, Cu, Hg, Mo, Pb, Sb, and Zn. Rock samples from the stockwork zones show that gold is correlated 

with elevated copper values whereas the acid-sulfate zones 

are characterized by elevated Ag, As, Sb, ±Au and base metals. Gold potential is primarily in stockwork zones in the 

altered porphyries. Bulk samples from stockwork zones at 

the surface have gold values of0.05 to 1.7 ppm. Samples of 

stream sediment collected from drainages surrounding the 

stock contained low parts per billion-level gold values below 

each of the known mineralized areas. Heavy-mineral concentrates and moss were less effective sample media. 

Secondary fluid inclusions in quartz lining miarolitic 

cavities are either hypersaline, containing halite daughter 

minerals, or undersaturated with halite. Preliminary homogenization temperature data vary widely from 400 to 200 °C. 

Fluid inclusions in quartz-base-metal veins and acid-sulfate 

mineralization are undersaturated with halite and have 

homogenization temperatures from 300 to 200 °C. 

La Espanola records multiple intrusive episodes and 

hydrothermal activity over a period of about 1 million years. 

The early stockwork potassic and phyllic alteration/mineralization assemblages have the earmarks of the porphyry copper environment, with the exception that the copper grade is 

subeconomic and the quartz and associated alteration minerals are fine grained (~1mm). The acid-sulfate alteration/mineralization is typical of the high sulfidation epithermal 

environment that is thought to occur at shallow levels above 

porphyry intrusions. The fact that acid-sulfate-altered rock 

crosscuts potassic- and phyllic-altered porphyries suggests 

that rapid erosional unroofing of the porphyry intrusive complex occurred during the life span of the system. The acidsulfate alteration at the surface suggests the presence of a 

deeper, unexposed porphyry intrusion. Undiscovered stockwork mineralization may therefore exist at depth within the 

system. 

JURASSIC CALDERAS IN 

SOUTHEASTERN ARIZONA-THE 

SURFACE MANIFESTATION OF A 

COMPOSITE BATHOLITH 

Ken Hon and Peter Lipman 

Volcanic deposits from three overlapping calderas of 

Jurassic age, recently identified in the Canelo Hills and 

Huachuca Mountains of southern Arizona, chronicle the 
emplacement of a large composite batholith. Smaller plutonic complexes were also emplaced near Bisbee and Gleeson at approximately the same time. 

The Montezuma caldera, the oldest of the three, erupted 

within a cluster of andesitic to dacitic volcanoes. Although 

no sections of these early rocks are preserved intact, large 

blocks and shattered masses of andesite and dacite form 

extensive megabreccia lenses within the Montezuma 

caldera, indicating the presence of a thick sequence of lavas 

on the rim during caldera collapse. Intracaldera accumulations of crystal-rich dacite tuff (68 percent Si02) also interfinger with massive landslide lenses consisting of blocks of 

Paleozoic limestone, dolomite, and sandstone, and Jurassic(?) sandstone and siltstone. The intracaldera crystal-rich 

dacite is overlain by thin rhyolitic outflow sheets from the 

Turkey Canyon and Parker Canyon calderas. A similar outflow sequence is found in the Mustang Mountains some 

20-30 km to the north. 

Following caldera collapse, a large composite granitic 

intrusion was emplaced into the intracaldera dacite tuff. The 

intrusion consists of a medium- to coarse-grained, porphyritic granite (70 percent Si02) containing 2-3 em phenocrysts of orthoclase and a medium- to coarse-grained 

equigranular granite that ranges from hornblende granite (70 

percent Si02) to biotite granite (76 percent Si02). Aplite 

dikes cut both the porphyritic and equigranular granites but 

are more common in the former. Cuspate blebs (10-20 em) 

of dark andesite-dacite are found in both granites but are 

more plentiful in the porphyritic granite. Similarly, discontinuous basalt dikes (50 percent Si02) are more common 

within the porphyritic granite. 

After a short hiatus, crystal-poor rhyolite tuff erupted 

from the Turkey Canyon caldera. This welded tuff is 

strongly rheomorphic within the caldera and in the Mustang 

Mountains. High volatile contents are indicated by the presence of abundant lithophysae and secondary potassium feldspar related to pervasive vapor-phase alteration of this unit. 

Similar strongly rheomorphic, crystal-poor rhyolite tuffs are 

known to have erupted at relatively high temperatures 

(800-900 oq in mid-Tertiary western United States. 

The youngest of the three calderas, the Parker Canyon 

caldera, erupted a crystal-rich rhyolite tuff consisting of .:40 

percent crystals of quartz, potassium feldspar, and biotite. 

The high crystal content of this unit suggests that it records 

the eruption of a cooling pluton. The intracaldera tuff is 

intruded by a rhyolite porphyry of similar composition but 

does not appear to be structurally domed. 

Sediments interbedded between the three ash-flow tuffs 

indicate a time break between caldera-forming events. The 

amount of sedimentation within the Montezuma caldera is 

similar to that found within Tertiary calderas, which take 

tens of thousands to hundreds of thousands of years to accumulate. 

The nested nature of the three Jurassic calderas suggests 

that they formed during emplacement of successive cupolas 

49 

Resurgent granite (lower, light-colored cliffs) intruding homfelsed 

intracaldera dacite tuff (upper, slope-forming exposures) within the 

Montezuma caldera, southern Huachuca Mountains. By Ken Hon. 

of a composite batholith. The general progression from early 

andesitic volcanism to more silicic ash-flow tuff eruptions 

mimics well-documented patterns seen in many of the midTertiary volcanic fields of the western United States. 

Important porphyry copper mineralization of Jurassic 

age is associated with emplacement of the Juniper Flat Granite and the associated Sacremento Hill stock at Bisbee. Weak 

copper skarn mineralization is widespread near contacts of 

limestone megablocks and limestone pendants with the 

resurgent pluton in the Montezuma caldera. The two systems 

are approximately coeval, but it appears that development of 

a porphyry copper system in the Huachuca Mountains was 

thwarted by eruption of the Montezuma caldera. Hydrothermal activity associated with emplacement of the resurgent 

intrusion and a later dike swarm appears to have scavenged 

enough copper and other metals from the intracaldera dacite 

tuff to make the small skarn deposits. 

SANTA ANA-RESERVE EXTENSION 

ZONE-A PROPOSED ZONE OF 

INTERACTIVE NORTHEAST- AND 

NORTHWEST -DIRECTED EXTENSION 

DURING THE CENOZOIC, NEW MEXICO, 

ARIZONA, AND SONORA 

Brenda B. Houser 

Studies of the tectonic setting of the Sierrita-Mogollon 

transect project indicate that the transect coincides with a 

proposed system of Cenozoic northeast-trending faults and 

grabens (fig. 1). This system can be traced from northern 

Sonora, Mexico, to north of Reserve, N.Mex., where it was 

recognized earlier as the Morenci-Reserve fault zone (Ratte, 

1989). The entire system is named the Santa Ana-Reserve 

extension zone (SAREZ) for towns near the southwest and 

northeast ends of the zone, respectively. As currently 
Explanation 

Northeast-trending normal fault Metamorphic core complex 

• Mid-Tertiary sedimentary deposit • Porphyry copper deposit 

Figure 1 (Houser). Approximate location of proposed Santa Ana-Reserve fault zone in relation to exposures of mid-Tertiary sedimentary 

units (older than 18 Ma), porphyry copper deposits, and to pattern of faults, basins, ranges, and other tectonic elements of the southern Basin 

and Range province. 

50 
~ Pa ntan o f ormation 

m In fe r r ed ba sin depocen ter 

[<<-:·.1 Bedroc k 

Fa ult s 

~ Ca tal ina detachment fault 

Norma l fault , dash ed where inferredbar and ball on downthrown side 

Inferr ed stri ke slip fau lt 

Gr avity , con t o ur i nterva l 4 m i ll igals 

- -8- Co mputed r es i du al g ravity c ontour 

Drill ho les 

Gy ps um enc o un tered 

No g yp su m encount ered 

Figure 2 (Houser). Tectonic map of Tucson basin showing offset of basin depocenters that may be related to inherited displacement along an inferred northwest-trending fault. 

defined, it is about 500 km long and widens from 60 km at 

the northeast end to 200 km at the southwest end. 

The SAREZ is characterized by horsts and grabens 

bounded by chiefly high angle normal faults trending N. 

30°-60° E., by local occurrences of thick mid-Tertiary sedimentary deposits, by metamorphic core complexes, and by 

porphyry copper deposits. Mid-Tertiary sedimentary deposits (about 30 to 18 Ma) such as the conglomerate of Bonita 

Creek and Pantano, Mineta, and lower Cloudburst Formations in Arizona, probably the Magdalena Formation in 

51 

Sonora, Mexico, and various unnamed pre-18 Ma sedimentary units throughout the zone are inferred to have been 

deposited in northeast-trending grabens of the SAREZ that 

have since been disrupted by oppositely directed basin-andrange extension (post-18 Ma) and by core complex uplift. 

Many of these deposits have not been studied in detail, but 

those plotted in figure 1 have well-constrained ages between 

30 and 18 Ma, are as thick as 3,000 m, are complexly faulted 

and tilted, and have two other significant features in common: (1) their facies bear no relationship to the present 

topography, and (2) most contain clasts that have no known 

source nearby. Similar deposits are uncommon in the region 

outside the SAREZ. 

Tectonic activity in the SAREZ continued through the 

Miocene and Pliocene as evidenced by ages of basin-fill sediments in northeast-trending grabens such as the Patagonia, 

Tucson, and Alma basins. Pleistocene activity in the zone is 

demonstrated by northeast-trending fault scarps that cut 

Pleistocene pediment gravel along the Santa Rita fault on the 

southeast side of the Tucson basin (fig. 2) and along 

unnamed faults on the northwest side of the Alma basin, 

New Mexico. 

In addition to northeast-trending faults and grabens, the 

SAREZ is the locus of a large number of metamorphic core 

complexes (fig. I). Uplift of these core complexes may have 

been localized along the SAREZ by virtue of greater tectonic 

extension within the zone-that is, the interactive effects of 

the northwest-directed extension of the SAREZ (1) with the 

northeast-directed midcrustal extension at about 24 Ma that 

produced the mylonite fabric of the core complexes, and (2) 

with the regional northeast-directed basin-and-range extension after about 18 Ma. The two directions of extension may 

have had the effect of producing a northeast-trending zone of 

greater attenuation in the upper crust, which in tum resulted 

in locally greater crustal buoyancy and concentration of core 

complex uplift along the zone. 

The chief barriers to recognizing and tracing the 

SAREZ have been that southwest of Morenci, the zone was 

disrupted by uplift of the core complexes, and southwest of 

Tucson, it was obscured further by the fragmented terrane of 

the Jurassic magnatic arc and by more core complexes. The 

map pattern shown in figure 1 suggests that south of the Tucson basin, northeast-trending grabens and fault segments 

have been progressively offset to the southeast, possibly by 

an inherited (but not necessarily reactivated) pattern of leftlateral offset on northwest-trending strike-slip faults in the 

Jurassic terrane. This southeast offset is also indicated in figure 2, a gravity and tectonic map of the northeast-trending 

Tucson basin, that shows apparent offset of Pliocene(?) depocenters along an inferred northwest-trending strike-slip 

fault. The depocenters are defined by the presence of evaporite minerals (chiefly gypsum) at depths of about 150 m. 

Because there is no surface evidence for Neogene reactivation of this fault, the segmented shape of the Tucson basin 

may have been predetermined by earlier offset by the fault. 
Figure 1 shows that Laramide porphyry copper deposits 

tend to be clustered along the SAREZ but are less common 

outside the zone; this northeast-trending distribution has 

been noted previously by many investigators. Spatial coincidence of the copper deposits and the proposed SAREZ 

suggests the possibility of a genetic relationship, but the 

coincidence may simply be a function of exposure. Late 

Cenozoic uplift associated with the many core complexes in 

the zone could have resulted in exposure of more porphyry 

copper deposits relative to areas outside the zone. 

REFERENCE 

Ratte, J.C., 1989, Geologic map of the Bull Basin quadrangle, Catron County, New Mexico: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1651, scale 1 :24,000. 

THE MOGOLLON MINING 

DISTRICT-RESULTS FROM 

MAPPING, PARAGENESIS, AND 

FLUID-INCLUSION STUDIES 

Robert J. Kamilli 

The Mogollon district in southwestern New Mexico 

comprises numerous silver-copper-gold epithermal veins of 

Miocene age. The district is situated along the Queen fault, a 

reactivated north-northeast-trending segment of the structural wall of the Oligocene Bursum caldera (fig. I). 

The hypogene paragenesis is complex, owing to 

repeated episodes of hydrothermal brecciation, mineralization, and faulting. Any individual episode generally commenced with hydrothermal brecciation, followed by 

deposition of quartz-rich (commonly chalcedonic) veins 

containing chlorite, adularia, pyrite, electrum, copper sulfides, and copper-silver sulfides. This ore-forming event 

commonly was followed by a period of calcite±fluorite deposition, accompanied by dissolution of calcite and concomitant replacement by quartz. 

Auid-inclusion petrography and filling temperatures 

confirm that boiling of the hydrothermal fluids was a common phenomenon throughout the district. The restriction of 

the most productive ore horizons to an elevation interval of 

about 200 m, regardless of depth below the current land surface, supports the hypothesis that fluid boiling was an integral part of the ore-deposition process, as do anomalously 

high filling temperatures in quartz, up to 461 °C. However, 

the abundance of chalcedony and other forms of recrystallized silica in many specimens precludes the rigorous demonstration that boiling was the primary ore-deposition 

process because fluid inclusions in such material are notoriously untrustworthy. Most filling temperatures for quartz are 

210-290 °C, for calcite 180-260 °C, for sphalerite 230-260 

52 

1os•so· 

33"28'00 

33°22'30" 

108°46' 

Figure 1 (Kamilli). Generalized map of the Mogollon 

district. QTg, Quaternary and Tertiary gravels; Tv, Tertiary volcanics. Bar and ball symbol denotes dip of fault. 

Crossed pick and hammer, principal mine. 

°C, and for fluorite 120-200 °C. Most salinities are less than 

2 weight percent NaCl equivalent. A few values are as high 

as 5 weight percent NaCI equivalent; this is probably due to 

the concentrating effects of boiling rather than to the presence of a second fluid. 

Several lines of evidence are consistent with the presence of a buried porphyry-type intrusion in the district. 

Although all dated igneous rocks in the district are at least 

23.2 Ma, an 40Arf39Ar date of 16.7 Ma on vein adularia 

suggests that the thermal event represented by the mineralization at Mogollon could not have occurred during the formation of the Bursum caldera (28 Ma), but must be related 

to a buried intrusion or to increased heat flow caused by 

basin-and-range extensional faulting (Ratte, 1989). The 

presence of an intrusion is suggested by the existence of the 

box or ladder mosaic patterns of the Silver Creek graben in 

the central part of the district. This pattern may have been 

caused by doming above the hypothesized intrusion. Other 

suggestive evidence includes the presence of a major porphyry molybdenum prospect II km south of the district 

along the trace of the Queen fault and the presence of21-16 

Ma rhyolite intrusions southwest of the district (Ratte and 

Brooks, 1991). 

Underground mapping has demonstrated that ore-grade 

mineralization extends to the range-front fault zone, which 

separates the Mogollon Mountains from the Alma basin on 
the west. It seems likely therefore that mineralization may 

extend beneath the late Cenozoic basin fill at a qepth 

between 1,000 and 350m. However, these depth estimates 

assume that movement along the range-front fault is entirely 

post-mineralization. If some displacement along the basinand-range faults predates mineralization, as is likely, the 

depth to the buried mineralized horizon could be much less 

and basin fill itself could be mineralized. 

Audio-magnetotelluric resistivity profiles through the 

district indicate that as much alteration occurs east of the 

Queen fault as occurs to the west, where all historical production occurred (Senterfit and others, this volume). The 

veins east of the Queen fault are predominantly calcite±chalcedonic quartz. The character of the veins, the 

geophysical data, and probable offset along the Queen 

fault of several hundred meters are seen as permissive evidence that the part of the district east of the Queen fault, 

where there has been no production, may contain significant mineral deposits. 

REFERENCES 

Ratte, J.C., 1989, Days 3 and 4-Selected volcanic features of the 

western Mogollon-Datil volcanic field, in Ratte, J.C., Cather, 

S.M., Chapin, C.E., Duffield, W.A., Elston, W.E., and Mcintosh, W .C., Excursion 6A-Eocene-Miocene Mogollon-Datil 

volcanic field, New Mexico, in Chapin, C.E., and Zidek, J., 

eds., Field excursions to volcanic terranes in the western United States, volume 1, Southern Rocky Mountain region: New 

Mexico Bureau of Mines and Mineral Resources Memoir 46, 

p. 68-91. 

Ratte, J.C., and Brooks, W.E., 1991, Rhyolite rift zone in southwestern New Mexico--A possible target for precious metals 

exploration: Geological Society of America Abstracts with 

PTograms,v.23,no.4,p.58. 

APPLICATION OF FIELD AND 

LABORATORY SPECTROSCOPIC 

ANALYSIS TO INVESTIGATE THE 

ENVIRONMENTAL IMPACT OF 

MINING IN THE SOUTHEASTERN 

SAN JUAN MOUNTAINS AND ADJACENT 

SAN LUIS VALLEY, COLORADO 

Trude V.V. King, Cathy Ager, Roger N. Clark, 

Gregg A. Swayze, and Andrea J. Gallagher 

A series of laboratory, field, and theoretical studies 

have recently been completed to support the interpretation 

of airborne imaging spectroscopy data to be collected by 

53 

NASA's AVIRIS (Airborne Visible and Infrared Imaging 

Spectrometer) in August/September 1993. This investigation is part of an integrated environmental study of the 

southeastern San Juan Mountains and adjacent San Luis 

Valley in Colorado. The Alamosa River and its tributaries 

drain several highly mineralized areas that are sources for 

both natural and mining-related contamination. This contamination is of concern due to the extensive downstream 

use of the Alamosa River for irrigation in the San Luis Valley, for domestic use, and as a source for water in the 

Monte Vista Wildlife Refuge. 

The A VIRIS data are obtained over the wavelength 

interval of 0.4 to 2.5 Jlm at a spatial resolution of 20 m and a 

spectral resolution previously attained only by laboratory 

instruments (9-15 nm). AVIRIS data will be obtained for the 

Summitville mine (a possible source of contamination from 

mining-related activities) and downstream areas in the San 

Luis Valley, thus providing a synoptic view not available 

through other techniques. 

The primary emphasis for the preflight portion of the 

investigation has been to determine if absorption features 

related to materials introduced by mine drainage can be 

detected in vegetation and mineral spectra. To examine the 

influence of water chemistry on alfalfa, a field investigation 

was undertaken to determine if, and to what extent, variations in irrigation sources (some possibly high in metals and 

other contaminants) would affect the spectral signature. In 

addition, a series of measurements was made to determine 

the spectral effects of liming (an agricultural method to control the pH of soil) in fields irrigated with Alamosa River 

water. These data will provide the calibrations necessary to 

map species distribution, and potentially their vigor, from 

the A VIRIS data. 

Laboratory spectroscopic studies and previous studies using AVIRIS data (Clark and others, 1993) show that 

it is possible to detect and map the distribution of minerals 

based on slight changes in chemistry and crystal structure. Laboratory studies indic~te that it is possible to 

identify mixtures of matrix (soil constituents) and cyanide 

compounds based on diagnostic absorption features that 

should allow their identification in A VIRIS data. The laboratory calibration data suggest that it will be possible to 

use A VIRIS data to produce mineral, soil, and contamination maps for a large area. 

Preliminary spectroscopic examination of soil samples taken from a drainage ditch in the San Luis Valley 

indicate that material is being deposited on the banks that 

is not a common mineral phase associated with agricultural activity. The spectral signature of this material is 

diagnostic and could readily be mapped with AVIRIS. 

However, the full extent to which the AVIRIS data will be 

effective for mapping potential environmental contaminants in the region must be determined. 
REFERENCE 

Clark, R.N., Swayze, G.A., and Gallagher, Andrea, 1993, Mapping 

minerals with imaging spectroscopy, in Scott, R.W., Jr., and 

others, eds., 1993, Advances related to United States and international mineral resources-Developing frameworks and exploration techniques: U.S. Geological Survey Bu11etin 2039, 

p. 141-150. 

GEOELECTRIC CONTRIBUTIONS TO 

THE MINERAL RESOURCE ASSESSMENT, 

CORONADO NATIONAL FOREST, 

SOUTHEASTERN ARIZONA-BURIED 

STRUCTURE AND ALTERATION IN 

MAGMATIC CENTERS 

Douglas P. Klein 

Audiomagnetotelluric (AMT) traverses within and 

adjacent to the Coronado National Forest, southeast Arizona, 

provide electrical resistivity information at depths from 

about 10 to 3,000 m. The data provide information on the 

location and extent of buried low-resistivity features, including geothermal fluid, hydrothermal alteration, and fault breccia, as well as contrasts in resistivity from lithologic changes 

across faults, and high-resistivity intrusions. These geologic 

features delineate potential targets for ore deposition, provided other geologic and geophysical conditions are favorable. Four specific areas studied within the Coronado 

National Forest (fig. 1) are west-central Winchester Mountains, northern Dragoon Mountains, northern Chiricahua 

Mountains, and central Chiricahua Mountains. These areas 

are all underlain by volcanic or plutonic rocks either in outcrop or at shallow depth. 

In the Winchester Mountains area (Martin, 1986; Martin and others, 1982), AMT data sense the location of a narrow low-resistivity zone along the western margin of the 

mountains; this zone is interpreted to represent conductive 

gouge of a range-front fault. Farther west into the basin, a 

low-resistivity layer at an average depth of about 400 m is 

inferred to represent basin brine or thermal fluid; hot springs 

above and possibly emanating from this layer are found 

about 5 km west of the mountain range. 

Across the northern Dragoon Mountains (Baer and 

Klein, 1984), high resistivity is associated with intrusive 

rock of the mountain core, and moderate resistivity is 

associated with localized sedimentary overburden. The 

data allow estimates of the maximum thickness of sedimentary rock overlying the resistive intrusive. Beneath 

alluvium about 5 km west of outcrop and coincident with 

an aeromagnetic anomaly, a low-resistivity anomaly at 

about 350 m depth below basin fill may indicate altered 

intrusive rock. 

54 

Figure 1 (Klein). Location of AMT electrical surveys in southeastern Arizona (dashed lines). Segments of the Coronado National 

Forest are enclosed by solid lines. Areas discussed in this report are 

2, 3, 4A, and 4B. 

In the Emigrant Pass area of the northern Chiricahua 

Mountains (Nervick and Boler, 1981 ), inferences from AMT 

resistivity data indicate a possible intrusion buried under 

lower-resistivity volcanic rock. AMT stations here provide 

data consistent with the interpretation that the inferred intrusion is either altered at a depth of 300-500 m, or that the 

intrusion bottoms near this depth. 

Traverses on the northeast boundary of Turkey Creek 

caldera (Senterfit and Klein, 1991) reveal moderate to low 

resistivity peripheral to the inferred core of a highly resistive 

ring intrusion (fig. 2, between A and C'). This low-resistivity 

rock is inferred to represent hydrothermal alteration associated with intrusion. Mine prospects and also small abandoned mines are found in this area, but major ore deposits are 

Figure 2 (Klein) (facing page). Location map (top), resistivity 

cross section (middle), and interpretation (bottom) of part of an 

AMT electrical survey, Turkey Creek caldera, Coronado National 

Forest, southeastern Arizona. 
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DEPTH-RESISTIVITY SECTION 

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(resistivity values in ohm-m) 

55 

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EXPLANATION 

JJ.2 AMT station location 

A---A' AMT traverse 

Qls 

Trc 

Tfc 

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Tmp 

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Caldera structural margin 

Fault-Bar and ball on downthrown 

side; dashed where uncertain 

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MAP UNITS (top) AND INFERRED 

UNITS AT DEPTH IN THE 

RESISTIVITY SECTION (bottom) 

Landslide breccia (Quaternary) 

Welded tuffs (Rhyolite Canyon 

Tuff-Oligocene) 

Moat lavas and tuff of Fife Canyon 

(Tertiary) 

Welded tuffs and lava flows (inferred 

to be low in the section-Tertiary) 

Monzonite porphyry (core and 

ring-Tertiary) 

Intrusive rock (Tertiary, but may 

include older granitic rocks and 

Proterozoic basement that is 

indistinguishable from intrusive 

rock by resistivity values) 

Sedimentary rocks (mostly 

Cretaceous Bisbee Formation) 

Sedimentary rocks (mostly carbonate 

rocks of Cretaceous and (or) 

Paleozoic age) 

OTHER SYMBOLS 

Fault(s) 

Brecciation, faulting and alteration 

AMT station 
unknown. To the northwest where the AMT traverse crosses 

an area of outcropping tuff, the resistivity generally 

increases with depth, however; one area of buried lowered 

resistivity is anomalous (near station 20, fig. 2). Faults 

mapped in this area seem to indicate that the low resistivity 

represents breccia or gouge along the faults, or alteration 

associated with hydrothermal fluids guided by the faults. 

Along part of a traverse in the west-central part of Turkey Creek caldera, AMT soundings indicate anomalously 

low resistivity at a depth of about 300m (fig. 2, top, near 

station 26). This area is inside the inferred caldera boundary. 

The anomaly is believed to be related to faulting. Subsequent to the analysis of this profile, faults were identified in 

nearby outcrops. 

REFERENCES 

Baer, M.J. and Klein, D.P., 1984, Audio-magnetotelluric data in the 

Dragoon Mountains Roadless Area, Cochise County, Arizona: 

U.S. Geological Survey Open-File Report 84-417, 59 p. 

Martin, R.A., 1986, Geophysical maps of the Winchester Roadless 

Area, Cochise County, Arizona: U.S. Geological Survey, Miscellaneous Field Studies Map MF-1851, scale 1:24,000. 

Martin, R.A., Sherrard, M.S., and Tippens, C.L .• 1982. Station location map and audio-magnetotelluric data log for an area between Hooker's Hot Springs and the Winchester Mountains, 

Arizona: U.S. Geological Survey Open-File report 82-779, 

8p. 

Nervick. K.H., and Boler, F.M., 1981, Audio-magnetotelluric investigations of the North End Study Area, Cochise County, Arizona: U.S. Geological Survey Open-File Report 81-77 4, 6 p. 

Senterfit, R.M., and Klein, D.P., 1991, Audiomagnetotelluric investigation at Turkey Creek Caldera, Chiricahua Mountains, 

southeastern Arizona: U.S. Geological Survey Bulletin 2012, 

p. K1-K9. 

THE U.S. GEOLOGICAL SURVEY SIDELOOKING AIRBORNE RADAR (SLAR) 

MOSAICS-A REGIONAL VIEW FOR 

MINERAL EXPLORATION 

Allan N. Kover and James W. Schoonmaker, Jr. 

The U.S. Geological Survey has acquired SLAR (sidelooking airborne radar) image data for more than 40 percent 

of the conterminous United States and Alaska, plus Puerto 

Rico and the U.S. Virgin Islands (fig. 1). These data are an 

under-used resource for mineral exploration. Many of the 

mineralized areas of the Southwest are particularly well covered, as are other areas of potential discovery. For convenience, the image data are assembled into I 0 X2° mosaic 

56 

quadrangles (1 °X3° north oflat 59° in Alaska), and afford an 

unparalleled synoptic and cloud-free view of terrain. The 

radar system's imaging geometry provides enhancement of 

subtle structural geologic features, especially when acquired 

with optimum look direction and depression angle. The 

design of mission flight plans takes advantage of these special attributes of radar images. 

The SLAR image data used to prepare the mosaics have 

a swath width of 20-46 km, depending upon the acquisition 

system, and are generally available in unbroken strips of at 

least quadrangle length. Images are collected with 60 percent 

sidelap to provide stereoscopic models and to facilitate 

mosaicing. All post-1984 image strip data are available on 

computer-compatible tape (CCT), and many earlier data sets 

are now available in digital form. Since 1991, data have been 

routinely provided on CD-ROM. 

The SLAR image strips are assembled into 1:250,000-

scale mosaic quadrangles that conform to the national topographic map series. (For names of individual conterminous 

U.S. quadrangles see USGS Index of Small Scale Maps of the 

United States, April 1, 1992, and for Alaskan quadrangle 

names consult the State Index. Both are available, without 

charge, from the local USGS Earth Science Information 

Office (ESIC), or call 1-800-USA MAPS.) The mosaics 

can be ordered as photographic prints, positive or negative 

transparencies, or diazo copies from the USGS EROS Data 

Center (EDC), User Services Section, Sioux Falls, SD 

57198. Other USGS SLAR data, available from EDC, 

include indexes (microfiche, and I :250,000- and 

1:1 ,000,000-scale prints), image strips, nine-track tape 

(CCT), and CD-ROM. Custom laboratory products can be 

provided. 

A composite SLAR mosaic of the State of West Virginia and parts of five adjoining States was compiled for the 

USGS at 1 :500,000 and 1: 1,000,000 scale. These mosaics 

can be ordered from EDC. Twenty other regional composite 

mosaics were prepared as value-added products by a private 

contractor and are not available from the USGS. 

The USGS SLAR data have been an aid in improving 

geologic mapping and in selecting areas for resurvey. 

Several CUSMAP (Continental U.S. Mineral Assessment 

Program) quadrangles include analysis of SLAR mosaics. 

Lineament studies have been useful in identifying potential 

mineralized areas in a number of States, including New 

York-New Jersey (lead-zinc), California, Connecticut, 

Idaho, Utah, North Carolina, Montana, and Michigan. Uranium-rich breccia pipes in northern Arizona are more easily 

identified on SLAR images in both forested and desert locations. An adequate water supply is necessary for many mining operations, and lineaments located on SLAR mosaics 

have been particularly useful in finding new sources of 

ground water. 
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57 
TESTING TEMPORAL DATA SETS FOR 

REGIONALENVIRONMENTALANALYSIS 

OF WEST VIRGINIA* 

M. Dennis Krohn, Robert C. Clark, and 

Charles W. Tremper 

Two distinctive vegetation units (fig. 1) were mapped 

in western West Virginia from an analysis of temporal vegetation and hydrologic data sets from 1990. The analysis 

was conducted as part of a statewide mineral resource 

assessment to test whether analysis of temporal data would 

have any utility for regional environmental studies. Two 

new data sets published on CD-ROM now make such an 

analysis feasible in the conterminous United States and 

Alaska. Vegetation response is depicted on a mosaic of 

weather satellite images from the A VHRR (Advanced Visible High Resolution Radiometer) sensor. A ratio index of 

vegetation reflectance is measured at a resolution of I km for 

19 biweekly intervals. Hydrologic response is measured 

from comparison of modern stream gauge readings to historic records from the HCDN (Hydro-Climatic Data Network). Fifteen gauging stations in West Virginia meet the 

HCDN criteria that minimize cultural influences on streamflow readings. 

The most distinctive vegetation unit is located in the 

southwest part of the State (diagonal lines, fig. I) and corresponds mainly to an area of Pennsylvanian-age coal-bearing 

rocks of the Conemaugh Group. The unit appears in the 

A VHRR mosaics as a slightly less vigorous response during 

the greening of vegetation in late April. Land-use practices 

over the coal-mining areas coupled with the primarily siliceous bedrock of the area are the likely causes of the delay 

in vegetation greening. A second, less distinctive unit (stipple pattern, fig. I) is mapped in the north-central part of the 

State and corresponds in part to a north-south outcrop pattern 

of Permian sedimentary rocks. The eastern border of the 

southern area (solid lines, fig. I) and the western border of 

the northern area (dashed line, fig. 1) seem to be marked by 

north-south-trending linear features. The linear features 

align with part of the New River, but only partially correspond to previously mapped linear features from analysis of 

structures, such as lateral ramps, and features from other 

statewide geologic compilations. 

The hydrologic records at this time are quite useful 

at eliminating spurious patterns observed in the satellite 

mosaics related to local meteorological effects. No relation is yet observed between vegetation units mapped 

from the satellite and patterns from the hydrologic records. 

The gauging records do show, however, that stream flow 

within the same drainage basin exhibits sufficiently distinctive patterns in the same time interval to be a poten58 

82" 79" 

PENNSYLVANIA 

40" OHIO 

VIRGINIA 

50 100 MILES 

50 100 KILOMETERS 

Figure 1 (Krohn and others). Vegetation units and linear features derived from analysis of 1990 temporal data sets for West 

Virginia. 

tial source of variation for the vegetation response. The 

minimum biweekly flow values appear to be the best indicator of subsurface stream-flow conditions that would 

affect a regional area. 

Initial analysis of the A VHRR and HCDN data sets 

shows that candidate vegetation areas and unmapped linear 

features can be recognized and would likely be components 

in a regional environmental analysis as well as in other 

resource assessments. 

*Not presented at Forum. 

GRAVITY AND MAGNETIC 

INTERPRETATIONS APPLIED TO 

MINERAL RESOURCE ASSESSMENT OF 

THE BUREAU OF LAND MANAGEMENT 

ROSWELL RESOURCE AREA, 

EAST -CENTRAL NEW MEXICO 

Dolores M. Kulik 

Gravity and aeromagnetic data were evaluated in conjunction with geologic, geochemical, and aerial gammaray data in determining the mineral resource potential of 

the Bureau of Land Management Roswell Resource Area, 

east-central New Mexico . 
Three broad magnetic highs occur along the eastern 

boundary of the study area, and generally coincide with high 

gravity anomalies. They are interpreted to result from mafic 

intrusions within the basement complex of the Central Basin 

Platform. 

Broad, low-amplitude magnetic anomalies present in 

the east half of the study area are caused by differences in 

lithology of the rocks of the Delaware Basin shelf and differences in depth to basement beneath these sedimentary rocks. 

Quaternary basalts, which frequently have associated magnetic anomalies, have no characteristic associated anomalies 

in the study area, suggesting that the basalts are thin. 

The Lincoln County porphyry belt in the westernmost 

part of the study area, where Tertiary intrusive rocks crop 

out, is characterized by short-wavelength magnetic anomalies with magnitudes up to 1,000 nT. Similar anomalies 

extend to the northeast from the porphyry belt where only 

Permian sedimentary rocks are exposed. The presence of 

these magnetic anomalies suggests that intrusive rocks similar to those of the porphyry belt occur at shallow depth in this 

broad area. A magnetic high is associated with the syenite 

and syenogabbro of Baxter Mountain, which are hosts for the 

White Oaks ore deposits; this anomaly indicates that a large 

body of similar composition extends to the southeast and 

includes the west flank of Carrizo Mountain. Magnetic lows 

in the Gallinas Peak and Rough Mountain areas are caused 

by hydrothermal alteration of the intrusive rocks. The occurrence of magnetic lows in the Tecolote Hills area suggests 

that the rocks here are also altered. Magnetic anomalies of 

1,600-2,000 nT extending southeast of the porphyry belt are 

attributed to uplifted Precambrian crystalline rocks of the 

Pajarito Mountain area. 

A T -shaped gravity anomaly indicates that relatively 

high density rocks underlie (1) the Lincoln County porphyry 

belt, (2) most of the northeast extension of the belt interpreted from magnetic data, (3) the area just northwest of the 

exposed intrusive rocks where similar rocks are probably 

also present in the subsurface, and ( 4) the area of Precambrian rocks southeast of the porphyry belt at Pajarito 

Mountain. The T -shaped gravity anomaly appears to be 

caused by a composite source including both Tertiary intrusive and Precambrian rocks. The shape of the anomaly suggests that the Tertiary rocks were emplaced in a northeasttrending zone, orthogonal to the north-northwest trend of the 

Precambrian Pedemal Uplift. The magnetic data show no 

consistent correlation of the intrusive rocks with either magnetic highs or lows, and some of the larger bodies are spatially associated with both high and low anomalies. The lack 

of any consistent correlation indicates widely variable magnetite content in the intrusive rocks. 

In the south-central part of the study area, strong northeast-trending gravity gradients parallel wrench faults, 

termed buckles, with normal, reverse, and strike-slip components. The continuity of the gradients suggests that the buckles, which are prospective for oil and gas, extend to the 

59 

northeast in the subsurface for many miles beyond mapped 

exposures. Similar strong northeast-northwest-trending gradients extend in a zigzag pattern across northeastern New 

Mexico and southeastern Colorado, and may represent a 

crustal transition and (or) change in basement character associated with crust-penetrating tectonic zones. 

SPATIAL AND TEMPORAL VARIABILITY 

IN THE CHEMISTRY OF TREE RINGS 

DOWNSTREAM FROM THE 

SUMMITVILLE MINE 

Frederick E. Lichte, James A. Erdman, 

Larry P. Gough, Thomas M. Yanosky, and 

Laurie S. Balistrieri 

The Summitville mine, located near the old mining 

town of Summitville in Rio Grande County, Colo., began 

operating in July 1986 as a large-tonnage open-pit heapleach gold mine and ceased operating in December 1992. 

During the mine's 6 years of existence, the trace-metal levels 

in drainage water from the site were elevated over historical 

(pre-1986) levels (Moran and Wentz, 197 4) due to input 

from three sources-heap-leach water, seeps that occur 

throughout the mine workings, and an increase in the metal 

load of water coming from the old Reynolds Adit. Drainage 

waters flow into Wightman Fork, a small tributary of the 

Alamosa River, which in tum flows east into the San Luis 

Valley. The increase in the trace-metal burden of the Alamosa River watershed is of concern to farmers, fishermen, and 

Federal and State wildlife agencies. (Also see companion 

abstracts concerning Summitville in this volume.) 

Our purpose was to chart potential spatial and temporal 

trace-metal trends in individual tree rings from about 1978 to 

the present. Metals, absorbed through the roots and transported to the aerial portions of a tree (such as the cellulose 

tissue of which rings are composed), may reflect changes in 

the bioavailability of metals being transported by both surface and ground water (Y anosky and Vroblesky, 1992). Tree 

rings have been used successfully to document changes in 

ground-water chemistry down-slope from point-sources of 

metal contamination. The analysis of tree-ring chemistry can 

be difficult to interpret, however, due to (1) potential inhomogeneity within a ring, (2) translocation of metals laterally 

between rings, (3) difficulties in tree-ring crossdating, and 

(4) differentiating among various metal sources (such as 

local vs. transported metals in ground water). 

Samples of narrow-leaf cottonwood (Populus angustifolia) tree-ring cores were collected from riparian, mixed 

forest communities along the Alamosa River from just below 

its confluence with Wightman Fork to a point down river 

about 25 miles where the river water is extensively diverted 
into irrigation laterals on the San Luis Valley floor. Nine 

sites, spaced approximately 3 miles apart, were established. 

At each site cores were extracted from each of two mature 

trees and samples of river water and sediment were collected. Variability in the elemental concentrations of the tree 

rings was partitioned among various sources, including (1) 

analytical, (2) between cores within a tree, (3) between trees 

at a site, and (4) among sites. We obtained ring chemistry 

results for both spatial (across the watershed) and temporal 

(among dated rings) trends. 

This study utilizes a laser-ablation inductively coupled 

plasma mass spectrometer (ICP-MS) (Lichte, 1993), which 

has the capability of determining = 16 elements in plant 

material (including the metals Cu, Fe, Hg, Mn, Ni, Pb, and 

Zn) at the Jlg/g concentration level over an area of the tree 

ring with a diameter as small as = 0.5mm. The analysis of 

tree rings by laser-ablation ICP-MS utilizes both the technique's high sensitivity and its spatial resolution. Rings 

were analyzed in place, as part of an increment core, by suspending them in an enclosed sample cell and ablating the 

zone of interest with a neodymium/yttrium-aluminumgarnet (Nd/YAG) laser. 

REFERENCES 

Lichte, F.E., 1993, The analysis of small mineral grains by laser ablation inductively coupled plasma mass spectrometry; Abstract 

201 in Proceedings of 35th Rocky Mountain Conference on 

Analytical Chemistry, July 25-29, Denver, Colo.: Society for 

Applied Spectroscopy. 

Moran, R.E., and.Wentz, D.A., 1974, Effects of metal-mine drainage on water quality in selected areas of Colorado, 1972-73: 

Denver, Colo., Colorado Water Conservation Board, Colorado 

Water Resources Circular 25. 

Yanosky, T.M., and Vroblesky, D.A., 1992, Relation of nickel concentrations in tree rings to groundwater contamination: Water 

Resources Research, v. 28, p. 2077-2083. 

ECONOMIC FUNDAMENTALS OF 

MINING LAW* 

Keith R. Long 

Mineral-bearing lands are typically owned by persons 

other than those able and willing to explore and to develop 

such lands. Mining law establishes the terms under which 

mineral lands are traded for purposes of exploration and 

development. From an economic perspective, an efficient 

mining law creates the conditions for maximizing social 

gains from such trades. 

Gains from trading mineral lands depend on the prices 

of mineral commodities, costs of production, cost of capital, 

mineral endowment, exploration efficiency, and other 

60 

factors. Under competitive conditions, involving many 

landowners and many mining firms, market forces will efficiently sort out which trades will yield the largest private 

gains. In the absence of externalities, including adverse 

environmental impacts, social gains from these trades will 

also maximize. 

Private trading of mineral lands should be adequately 

regulated by commercial law that guarantees enforceability 

of contracts and compensation for damages. Where governments own mineral lands, a competitive market for mineral lands can be created using the principle of selfinitiation, limiting the size of claims, and granting at least 

de facto private property rights to claim holders. Under 

self-initiation, competing firms decide when, where, and 

how much exploration and development to undertake. Parceling out claims to many firms prevents monopolization of 

mineral lands. Property rights allow claim holders to trade 

claims among themselves, secure financing, enforce contracts, and obtain compensation if mineral lands are expropriated for public use. 

A conflict of interest arises between government as a 

landowner and government as a guardian of public welfare. 

As a landowner, government seeks to maximize the economic return from its lands. As a monopoly supplier of mineral lands, governments are able to gamer excess returns by 

limiting supply. Noncompetitive limitations on land availability, excess taxes or holding fees, and selective granting 

of concessions in return for special considerations are forms 

of profiteering that inhibit economic growth and set a poor 

example for the private sector. 

Governments that encourage competition, however, 

enhance public welfare by minimizing costs to the mineral 

consumer. Governments would then expect to receive no 

more, and no less, than fair market value for mineral rights. 

Yet fair market value cannot be determined in the absence of 

a competitive market for mineral rights. A government 

monopoly on mineral rights, regulated by government, may 

not be a convincing substitute. 

Mineral lands are traded in return for some compensation. Ideally, mineral lands are a commodity having an 

undisputed market value. In reality, that value depends on 

expectations as to the type, quantity, and quality of minerals 

that might be found and profitably developed on a particular 

mineral property. The considerable uncertainty about that 

value leads to the almost universal acceptance of some form 

of risk-sharing between landowner and mining firm. Revenue or profit sharing distributes the risk of variations in earnings between landowner and the mining firm. 

Tax payments as a percentage of profits are more equitable and efficient than payments based on gross receipts 

from mineral sales (gross revenues). In an internationally 

competitive mineral market, a country that adopts a tax on 

gross revenue forces its mineral industry to absorb the full 

cost of the tax. That tax can only be paid out of profits. A 
tax on gross revenues is then an implicit tax on net income. 

Two firms with identical gross revenues will pay a greater or 

lesser share out of their net income depending on their relative profitability. A tax on gross revenue is regressive 

because firms earning less profits will pay a greater percentage of net income than firms earning greater profits. Equity 

can only be restored by explicitly stating a percentage of net 

income that will be collected. 

Gross revenue taxes alter the production decisions of 

mining firms by raising cut-off grades and rendering marginally profitable operations uneconomic. Under a gross revenue tax, individual mines will produce less and close earlier 

than under a net income tax, shrinking the tax base and 

reducing employment of labor and capital. Lost production 

will be compensated by opening new mines elsewhere. 

Although the essence of mining law is the trade in mineral lands, other important considerations include resolution 

of land-use conflicts, provisions for due diligence, capture of 

exploration information, and internalization of external 

(principally environmental) costs. Competition generally 

insures that lands will be acquired for their highest-value 

use. Diligence is insured by setting holding costs for mining 

claims just high enough to exclude acquisition by persons 

with no real interest in exploring and developing mineral 

rights. Prudent landowners, public or private, will retain 

information generated from exploration of their lands, even 

if the minerals found are not of current economic interest. 

To maximize the social return from mineral development, mining fmns must account for and internalize environmental costs in their production decisions. When faced with 

these costs, firms are better able to select the most efficient 

mitigation approach. Cost-effective instruments for internalizing these costs include legal actions to obtain compensation for damages, taxes on pollutants, and tradable permits 

for allowable emissions. 

*Not presented at Forum. 

DISTRIBUTION, COMPOSITION, AND 

AGE MAPS OF EARLY AND MIDDLE 

CENOZOIC VOLCANIC CENTERS, 

ARIZONA, NEW MEXICO, AND 

WEST TEXAS 

Robert G. Luedke 

Two new maps (Distribution and composition; Mode of 

origin and age) of Arizona, New Mexico, and West Texas 

are the first of a series to show the distribution, composition, 

and age of early and middle Cenozoic volcanic rocks and 

eruptive centers in the western conterminous United States. 

This map series is primarily a data base of igneous centers, a 

61 

major geologic environment related to and host to mineral 

deposits throughout western North America. 

The volcanic and associated plutonic rocks that were 

emplaced within the early and middle Cenozoic, about 58 to 

16 Ma, are divided into three principal time increments. An 

earlier published map series (USGS 1-1091) designed as a 

guide for the evaluation of igneous-related geothermal 

resources had 16 Ma as its maximum cutoff age. The igneous 

rocks also are classified compositionally into five major 

groups using a nongenetic system based upon the known or 

inferred silica content. This rock classification emphasizes 

the dominant rock type within a designated area, although 

several rock types may be intermixed. 

Some Laramide magmatic and tectonic events that 

began in the Late Cretaceous or Paleocene continued into the 

middle Eocene, or at least served as precursors of similar 

events later in the Cenozoic. However, most of the early and 

middle Cenozoic igneous rocks shown on the maps are 

related principally to volcanotectonic activity that started in 

early or middle Eocene time. Following a quiescence of volcanic activity in the late Eocene, widespread volcanism commenced in the very latest Eocene and continued on a major 

scale through the Oligocene into the Miocene, to the onset of 

late Cenozoic bimodal volcanism and associated extensional 

tectonism characteristic of the Basin and Range province. 

Most volcanic rock units shown on the maps fit within 

the selected time intervals, but, in some localities, a few rock 

units were included with the dominant younger or older unit 

in order to maintain geologic, petrologic, and tectonic continuity. For example, the widespread andesitic units occupying much of west-central New Mexico and east-central 

Arizona include domal eruptive centers, extensive lava 

flows, and assemblages of stratovolcano deposits consisting 

of vent and alluvial outflow facies that range in age from Oligocene to early Miocene. 

The data base of igneous systems shown on these maps 

provides the basic information necessary for derivative 

regional and topical studies such as the development of geologic concepts to identify and assess igneous-related mineral 

deposits or mineralized systems. Not only do the widespread 

early and middle Cenozoic volcanic rocks, particularly 

throughout the south half of Arizona, southwest quarter of 

New Mexico, and the Big Bend region of West Texas, possibly cover undiscovered base- and precious-metal vein and 

replacement deposits, but also they may serve as hosts for 

ore deposits. Caldera source areas and related volcanic 

rocks, mostly siliceous ash-flow tuffs, are potential sites for 

mineral deposits. 

These new maps show the temporal and spatial relations of the igneous rocks and their geochemical associations, at a single common scale (1:1,000,000), within the 

many different igneous centers. The maps can also be used 

as a base for studies of volcanology, volcanotectonics, and 

the general geology of volcanic and related rocks. 
UTILIZATION OF U.S. GEOLOGICAL 

SURVEY MINERAL RESOURCE DATA BY 

THE BUREAU OF INDIAN AFFAIRS 

Stephen A. Manydeeds 

The economic return to Indian mineral owners from 

development of mineral resources historically has been 

between 75 and 85 percent of the total income generated on 

Indian Trust lands. On some Indian lands, the mineral 

income has exceeded 95 percent of the total income generated. In general, three categories of activities are carried out 

by the Division of Energy and Mineral Resources (DEMR) 

and Indian mineral owners: (1) energy and mineral assessments, (2) outreach programs, and (3) educational programs. 

The U.S. Geological Survey (USGS) plays a role in each of 

these activities, supplying their expertise in resource assessments and the mineral resource data bases they maintain. All 

agencies of the Department of the Interior (DOl) share a 

mandated trust responsibility for Indian Lands that includes 

mineral resources. 

1. The DEMR accomplishes in-house mineral 

ASSESSMENT activities through its own staff. This activity makes use of public domain mineral resource data generated from a variety of USGS internally funded programs. 

The DEMR also funds the USGS to provide expertise in 

cost-effective sharing of networked computer resources and 

mineral resource data bases. DEMR funding is provided 

directly to Tribal governments to carry out mineral assessment projects under the direction of their own staff with 

technical monitoring by the DEMR. Some of these funds go 

to USGS to perform reservation-scale mineral assessment 

activities. 

2. An OUTREACH program developed by the 

DEMR addresses several issues: (1) aiding Tribal governments for funding proposals to develop mineral resource 

assessment programs appropriate to their needs in land-use 

planning, (2) providing forums for public presentation of 

the results of mineral assessment and land-use programs 

on Indian lands, and (3) fostering communication 

between mineral resource development companies and 

Indian land-use planners. The DEMR has sponsored four 

national meetings in Denver, Colo., three poster sessions 

at the Northwest Mining Association meetings, and 

regional conferences on oil and gas resources of the 

Southern Ute, Uinta and Ouray, and Wind River reservations, and mineral resources of the Annette Islands 

Reserve. The USGS has been a major contributor of posters and papers to these meetings and associated publications. 

3. In the category of EDUCATION, the DEMR 

addresses the issue oflimited expertise within Tribal governments to effectively utilize energy and mineral resource data 

for land-use planning. A program has been developed with 

62 

the Colorado School of Mines to train Tribal natural resource 

managers in effective utilization of energy and mineral 

resource technical information. 

The Annette Islands Reserve (Alaska) and the Wind 

River Reservation (Wyoming) provide two case histories of 

how the BIA and USGS share mineral resource information 

and expertise. In the Wind River project, a Tribal government staff dedicated to energy resource development has 

examined unconventional oil and gas resources. In the 

Annette Islands Reserve project, a limited staff of resource 

specialists working closely with the Tribal government has 

produced a metallic mineral assessment. Both projects have 

resulted in public presentations of resource assessments, outreach, and educational activities that have benefited the 

Tribes. 

ARSENIC AND ANTIMONY ANOMALIES 

IN NURE SEDIMENTS SHOW 

MINERALIZED AREAS IN 

NORTHWESTERN NEVADA* 

Dawn J. McGuire and George V. Albino 

Sediment geochemical data from the NURE (National 

Uranium Resource Evaluation) program collected in northwestern Nevada in 1976-1979 show enrichments in arsenic 

and (or) antimony which correlate with known mineralized 

areas and suggest possible mineral deposits in less-explored 

areas (fig. 1). We examined these data as part of a mineralresource pre-evaluation of existing regional geochemical 

data in northwestern Nevada. 

Trace elements were analyzed by NURE contractors 

using instrumental neutron activation analysis on sediments of different size fractions in four 1 :250,000-scale 

quadrangles: Winnemucca and Lovelock (20-40 mesh), 

and Reno and McDermitt ( <1 00 mesh). The difference in 

size fractions probably contributed to differences in arithmetic means for As: 33 ppm and 27 ppm for the coarser 

size fractions analyzed in the Winnemucca and Lovelock 

quadrangles, and 7 ppm and 5 ppm for the finer size fractions analyzed in the Reno and McDermitt quadrangles, 

respectively. Although arsenic was analyzed by contractors in all four quadrangles, antimony was analyzed only 

in the Winnemucca and Lovelock quadrangles. Samples 

were collected at about one site per 18 km2, but not all 

samples were analyzed for arsenic and antimony. The 

number of analyzed samples for each element and the 

replaced minimum (min), maximum (max), and arithmetic mean values for each quadrangle are as follows: 

Winnemucca, As, 601 samples, min 2.2 ppm, max 587.3 

ppm, mean 32.6 ppm; Winnemucca, Sb, 1,018 samples, 

min 0.5 ppm, max 1,800.0 ppm, mean 10.0 ppm; Lovelock, As, 575 samples, min 1.7 ppm, max 827 ppm, mean 

27.1 ppm; Lovelock, Sb, 617 samples, min 0.3 ppm, max 
120° 119° 118° 117° 116° 42 r-------------~~------------~--------------~------------~ 

41 ° 

40° 

Reno 

0 

0As 

.Sb 

0 

M 

0 

0 BM 

1 

50KM 

Figure 1 (McGuire and Albino). Areas of As and Sb enrichment in sediments collected and analyzed in the NURE 

program. BM, Battle Mountain; G, Gerlach; L. Lovelock; M, Midas; PV, Paradise Valley; W. Winnemucca. I, Chimney Creek; 2, Rabbit Creek; 3, Getchell; 4, Pinson; 5, Dutch Flat; 6, Preble; 7, Kramer Hill; 8. Iron Point; 9, Lone Tree; 

10, Marigold; II, Copper Basin; 12, Copper Canyon; 13, Goldbanks; 14. Kennedy; 15, Sleeper: 16, Central; 17, Mill 

City; 18, Florida Canyon; 19, Starr; 20, Unionville; 21, Rochester; 22. Relief Canyon; 23. Muttlebcrry; 24, Wild 

Horse; 25, Lake; 26, Desert; 27, Majuba Hill; 28, Sulphur; 29, Rosebud. 

473.7 ppm, mean 7.3 ppm; Reno, As, 956 samples, min 1 

ppm, max 50 ppm, mean 6.9 ppm; McDermitt, As, 1,320 

samples, min 1 ppm, max 44 ppm, mean 4.7 ppm. 

Geochemical interpretations are based on single-element 

plots of the 75th, 90th, 95th, and 97.5th percentiles of 

arsenic and antimony in each quadrangle. 

Arsenic is present in a variety of minerals in the different mineral deposits in the area, including arsenopyrite 

and arsenian pyrite, and the arsenic sulfides orpiment and 

realgar. Antimony-bearing minerals in deposits include 

stibnite, tetrahedrite, and lead-antimony and arsenic-antimony sulfosalts. 

Many areas of known mineralization can be identified 

in single-element geochemical plots of both arsenic- and 

antimony-enriched sediments: Florida Canyon hot springs 

gold mine, Rochester mine (Ag-Au), Relief Canyon mine 

(Au), Mill City tungsten district, Central district (polymetallic veins), the Battle Mountain district including the 

63 

Marigold mine (Au) and Copper Basin (Cu-Mo-Au), 

among others. The larger, more productive gold mines 

such as Florida Canyon and Marigold had not yet been 

discovered in the late 1970's when the NURE samples 

were collected and analyzed. Antimony enrichments, without arsenic enrichments, surround hot-springs gold deposits of the Sulphur district, Rosebud district (Au-Ag), 

Unionville district (Ag), Kramer Hill mine (Au), and 

other deposits. Arsenic enrichments, without antimony 

enrichments, characterize areas near the arsenic-enriched 

Chimney Creek (Twin Creeks), Getchell, and Pinson sediment-hosted gold deposits and the Kelley Creek valley to 

the east, as well as other areas. 

Several areas lack known significant mineralized rock 

but are enriched in both arsenic and antimony: the southern 

Calico Hills, the east-central part of the East Range northwest of the hot springs gold-mineralized area of Goldbanks 

Hills, and the east side of the Tobin Range (east of the Big 
Mike base-metal mine). Areas showing arsenic-enriched 

sediments without antimony enrichment include basins 

north of Chimney Creek (northern Eden and Kelley Creek 

valleys and adjacent areas south of the Humboldt River), 

Spring Creek (north of Hot Springs Peak and the Humboldt 

River), the Quinn River valley area, and a small area at the 

northern tip of Slumbering Hills (northeast of the Sleeper 

mine). Areas containing antimony-enriched stream sediments without arsenic enrichment include the East Range 

near and southeast of Kyle Hot Springs, the southern East 

Range northeast of McKinney Pass and southeast of Granite 

Mountain, and other, smaller areas with single-sample 

anomalies such as Granite Point near Gerlach, an area west 

of Pahsupp Mountain, and two areas in the southern part of 

the Black Rock Desert. 

The significance of these results is that the distribution 

of arsenic- and antimony-enriched NURE sediment clearly 

shows some known districts and mines, including mines that 

only recently were discovered and became significant gold 

producers in the late 1980's. The correspondence of arsenicand antimony-enriched sediments with known mineralized 

rocks suggests that similar anomalies in the less-explored 

areas listed herein may indicate locations of undiscovered 

mineral deposits of various types. 

*Not presented at Forum. 

DIVERGENT FOLD ORIENTATIONS IN 

LOWER PALEOZOIC ROCK UNITS 

INDICATE MULTIPLE TECTONIC 

EVENTS IN NORTH-CENTRAL 

NEVADA-OSGOOD MOUNTAINS, 

EDNA MOUNTAIN, AND THE 

HOT SPRINGS RANGE* 

Dawn J. McGuire and A. Elizabeth Jones 

Our studies have documented divergent orientations of 

folding in lower Paleozoic rocks in the Osgood Mountains, 

Edna Mountain, and the Hot Springs Range of north-central 

Nevada. These various orientations require either a single 

mid-Paleozoic tectonic event with multiple vergence directions in the different rock units (a complicated pattern of 

deformation during the Antler orogeny), or more likely, multiple tectonic events with differing stress orientations affecting these units during both Paleozoic and Mesozoic time. 

We have recognized three lower Paleozoic lithotectonic 

units based on stratigraphic and structural characteristics: (1) 

the Proterozoic(?) and Lower Cambrian Osgood Mountain 

Quartzite, the Lower Cambrian to Lower Ordovician Preble 

Formation, and rocks mapped as Ordovician Comus Formation on the east side of the Osgood Mountains; (2) Lower 

Ordovician to Lower Silurian rocks mapped as Valmy and 

Vinini Formations and the Middle Ordovician Comus For64 

mation at Iron Point (on the northeastern flank of Edna 

Mountain); and (3) the Upper Cambrian Paradise Valley 

Chert and overlying Upper Cambrian and Lower Ordovician(?) Harmony Formation in the Hot Springs Range. 

The first lithotectonic unit includes the Osgood Mountain Quartzite and the gradationally overlying Preble Formation (Hotz and Wilden, 1964; Madden-McGuire and Marsh, 

1991). Folds in the Preble Formation are predominantly 

west-vergent and south-plunging as observed along the 

strike of the unit for 60 km from Anderson Canyon in the 

northern Osgood Mountains to Hot Springs Ranch on the 

east side of the Sonoma Range. Poles to foliation and bedding cluster together on stereonet plots, based on more than 

200 measurements. Rocks previously mapped as the Comus 

Formation, on the east side of the Osgood Mountains, have 

more scattered fold orientations. Plots of more than 200 

poles to foliation and bedding show a cluster resulting from 

west-vergent folds similar to those of the Preble Formation, 

along with additional, scattered points, suggesting refolding. 

Stratigraphic evidence suggests that metamorphism and 

west-vergent folding occurred in middle Paleozoic time, 

between Late Ordovician (the age of the youngest involved 

strata) and Middle Pennsylvanian, when the Battle Formation is thought to have been deposited on an unconformity 

above metamorphosed lower Paleozoic rocks. Later movement along the unconformable contact created a low-angle 

fault above the Preble Formation and associated units (the 

"Iron Point thrust" of Erickson and Marsh, 1974). Exposed 

metasedimentary rocks of the first lithotectonic unit are 

inferred to have contributed to late Paleozoic sedimentation, 

because coarser parts of the upper Paleozoic units exposed 

on Edna Mountain contain clasts of quartzite, phyllite, and 

limestone (Erickson and Marsh, 1974). 

Later deformation is evident in the northwestern 

Osgood Mountains, where a post-Pennsylvanian-Permian 

tectonic event locally folded and faulted the Middle Pennsylvanian to Lower Permian(?) Etchart Limestone and refolded 

the Preble Formation in a west-vergent orientation. The 

extent of this Mesozoic(?) deformation is unknown, but certain areas clearly have been affected by post-Paleozoic westvergent deformation. 

The second lithotectonic unit includes the Valmy Formation in the northern Osgood Mountains and the Vinini and 

Comus Formations to the south, at Iron Point. These units 

show east-vergent folding that is clearly different from folding in the Preble and Harmony Formations, based on more 

than 250 poles to bedding and foliation. East-vergent folding 

took place between Early Silurian (the age of the youngest 

involved rocks) and Middle Pennsylvanian deposition of the 

Etc hart Limestone over the lower Paleozoic rocks, as seen on 

"Etchart hill" near the Getchell mine (Madden-McGuire and 

Marsh, 1991). Original stratigraphic continuity between the 

first and second lithotectonic units would require a complex 

and divergent structural history, and cannot be proven 

because of their differing fold orientations. 
4F 

00' 

NEVADA 

co 

0 20 40 KILOMETERS 

Figure 1 (McGuire and Jones). Location map showing 

Osgood Mountains, Edna Mountain, and surrounding area, Humboldt County, Nev. 

The third unit contains the Paradise Valley Chert and 

overlying Upper Cambrian (Hotz and Willden, 1964) and 

Lower Ordovician(?) (Madden-McGuire and others, 1991) 

Harmony Formation. These units show west-vergent folding 

in the southern part of the Hot Springs Range (based on more 

than 1 ,600 poles to bedding and foliation in the Harmony 

Formation and 76 poles to bedding in the Paradise Valley 

Chert; Jones, 1993). Approximately 300 facing-known poles 

to bedding in the Harmony Formation show that the steeply 

east dipping beds are generally the overturned ones, indicative of west-vergent folding (Jones, 1993). Folding of the 

Harmony Formation in the Hot Springs Range predates the 

intrusion of a small Cretaceous(?) granitic plug at Dutch 

Flat. The pervasive west-vergent folding of the Harmony 

Formation and Paradise Valley Chert and the lack of evidence for multiple folding events suggest that these units 

were not affected by east-directed "Antler" deformation during the middle Paleozoic. The unusual arkosic lithology of 

the Harmony Formation suggests that it is allochthonous 

with respect to the partly coeval Preble Formation. It is not 

clear whether the west-vergent folding in the Harmony Formation is related to the west-vergent folding of the Preble 

Formation, or to later Mesozoic(?) deformation that affected 

both the Etchart Limestone and the Preble Formation in the 

northwest Osgood Mountains. This later deformation is 

inferred to correlate with the post-Triassic ''Willow Creek" 

65 

belt, which is exposed to the south in the East Range and 

Sonoma Range. 

The lower Paleozoic units show evidence for a combination of west- and east-directed Paleozoic and westdirected Mesozoic deformational events. Their structural 

history cannot be explained by a single east-directed tectonic 

event, commonly known as the Antler orogeny. 

REFERENCES 

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

1:24,000. 

Hotz, P.E., and Willden, R., 1964, Geology and mineral deposits of 

the Osgood Mountains quadrangle, Humboldt County, Nevada: U.S. Geological Survey Professional Paper 431, 128 p., 

scale 1:62,500. 

Jones, A.E., 1993, Northwest vergent folding in the Harmony Formation, north-central Nevada-Lower Paleozoic tectonics revisited: Geological Society of America Abstracts with 

Programs, Cordilleran Section Meeting, v. 25, no. 5, p. 59. 

Madden-McGuire, D.J., Hutter, T.J., and Suczek, C.A., 1991, Late 

Cambrian/Early Ordovician microfossils from the allochthonous Harmony Formation at its type locality, northern Sonoma 

Range, Humboldt County, Nevada: Geological Society of 

America Abstracts with Programs, Cordilleran Section Meeting,v.23,no.2,p. 75. 

Madden-McGuire, D.J., and Marsh, S.P., 1991, Lower Paleozoic 

host rocks in the Getchell gold belt-Several distinct allochthons or a sequence of continuous sedimentation?: Geology, v. 

19, p. 489-492. 

*Not presented at Forum. 

TIMING OF IGNEOUS ACTIVITY, 

ALTERATION, AND MINERALIZATION 

AT THE ORCOPAMPA Ag-Au DISTRICT, 

SOUTHERN PERU 

Edwin H. McKee, Peter Craig Gibson, 

Donald C. Noble, and Kirk E. Swanson 

The Orcopampa Ag-Au district is located in southern 

Peru about 160 km northwest of Arequipa. It is one of a 

group of epithermal precious-metal deposits, including 

Arcata, Cailloma, Shila, and Suyckutambo, of Miocene age 

in the central Andes. Orcopampa is in a belt of calcalkaline 

volcanic rocks of intermediate to silicic composition; common volcanic features include stratovolcanos, calderas, and 

dome complexes. The mineral deposits in the Orcopampa 

district are in a series of generally northeast striking veins 

within steeply dipping normal faults. Several distinct periods 

of hydrothermal alteration are recognized. Precious-metal 

mineralization is associated with the youngest period, which 
was characterized by potassium metasomatism and quartzadularia-sericite alteration. 

Potassium-argon and 40 Arf39 Ar dating of 10 samples 

of unaltered volcanic rock in the region and 9 samples of 

altered wallrock and vein material are the basis for the geochronology at Orcopampa. Volcanic activity began with the 

formation of a stratovolcano complex at about 21 Ma. 

Rocks in this complex are mostly pyroxene andesite breccias, lavas, and associated silicic tuffs that are intruded by 

dikes, sills, and irregular bodies of andesite. These rocks 

are propylitized. Overlying this complex with slight unconformity is a regionally widespread rhyolite ash-flow sheet 

named the Manto Tuff (Gibson and others, 1990). Outflow 

sheet tuffs and intracaldera tuffs of the Manto Tuff exposed 

near or within its source, the Chinch6n caldera about 15 km 

southeast of the Orcopampa district, are about 20 million 

years old. The Manto Tuff is not propylitically altered near 

the mines. Dacite lava flows, domes, and dikes of the Sarpane volcanics (Gibson and others, 1990) in the Orcapampa district were emplaced about 1.5 million years after 

eruption of the Manto Tuff. Three K-Ar dates on biotite 

and hornblende phenocrysts from dacite domes in the Sarpane volcanics are 18.3±0.6, 18.6±0.6, and 19.4±0.6 Ma. 

Hydrothermal breccia along veins in the Sarpane volcanics 

in the eastern part of the district contains quartz, alunite, 

and kaolinite and is associated with zones of silicification 

and argillic alteration. K-Ar age determinations on three 

samples of alunite from the Santiago Norte vein yielded 

ages of 18.4±0.5, 18.4±0.6, and 19.5±0.6 Ma, which is the 

same, within analytical uncertainty, as the Sarpane volcanics. The veins that contain precious-metal ore are associated with quartz-adularia-sericite alteration. K-Ar 

and 40 Arf39 Ar dates on wallrock and vein adularia from the 

Manto, Calera, Santiago Norte, and Tudela veins 

are 17.0±0.5, 17.0±0.5, 17.6±0.6, 17.4±0.4, 17.7±0.6, 

17.9±0.5, 17.9±0.5 Ma; the preferred age of mineralization 

is 17.7 Ma, about 1 m.y. younger than the alunite. Anumber of paragenetic substages within this low-sulfidation 

assemblage are recognized, but they are too close to the 

same age to be distinguished by our radiometric dates. 

The total span of igneous and hydrothermal activity in 

the Orcopampa district is no more than about 3.5 to 4 million 

years (21 to 17.5 Ma). Within this short interval, the evolution of the volcanic hydrothermal system produced first volcanic rocks, followed by quartz-alunite alteration, in tum 

followed by quartz-adularia-sericite alteration. None of the 

earlier magmatic and hydrothermal events can be unequivocally related genetically to the 17.7 Ma episode of Ag-Au 

mineralization. 

REFERENCE 

Gibson, P.C., Noble, D.C., and Larson, L. T., 1990, Multistage evolution of the Calera Epithermal Ag-Au vein system, Orcopampa district, southern Peru-First result: Economic Geology, v. 

85, p. 1505-1520. 

66 

GEOLOGY AND MINERAL RESOURCES 

OF THE LORETO-SAN JAVIER AREA OF 

NORTHERN BAJA CALIFORNIA SUR, 

MEXICO-RESULTS OF 

RECONNAISSANCE FIELD MAPPING 

Hugh McLean 

The Loreto and part of the San Javier 1 :50,000 quadrangles in the northeastern part of Baja California Sur, Mexico, 

were mapped in reconnaissance by U.S. Geological Survey 

personnel in the mid-1980's. Field studies focused on the 

distribution and structural relations between Mesozoic granitic and metamorphic rocks and overlying Tertiary and 

Quaternary sedimentary and volcanic rocks. 

The oldest rocks in the Loreto area are volcaniclastic 

sandstone and breccia that are metamorphosed to greenschist 

facies by the intrusion of Upper Cretaceous granitic rocks. 

The granitic rocks are locally exposed and are interpreted to 

be part of the peninsular batholith which crops out discontinuously from southern California to the southern tip of the 

Baja peninsula. Sedimentary lithofacies that unconformably 

overlie the Mesozoic basement rocks include red and white 

crossbedded sandstone, impure phosphatic sandstone, and 

volcaniclastic sandstone interbedded with welded tuffs that 

yield Oligocene radiometric ages. The locally exposed 

crossbedded sandstones may be as old as Eocene. 

A section of lower Miocene volcaniclastic/volcanogenic sandstone, conglomerate, and breccia that is as thick as 

1 ,500 m constitutes the bulk of the mountainous areas in the 

Loreto area. Along the coast of the Sea of Cortez, massive 

flows and breccia represent a near-vent volcanic facies. 

Coeval strata exposed in mountains to the west were deposited mainly by fluvial processes and are thinner and finer 

grained than the near-vent facies. Approximately 6 km west 

of the town of Loreto, swarms of dark-colored basaltic 

andesite dikes intrude the lower Miocene near-vent breccia. 

Light-colored felsic dikes also intrude the breccia in the 

same area. The felsic dikes are several tens of meters thick 

and are visible on aerial photographs for as much as 6 km. 

Alteration of the host breccia by the intrusion of the mafic 

and felsic dikes has produced a suite of clay and zeolite minerals. Geochemical surveys are needed to determine the 

extent of metallic mineralization. Hornblende from an 

andesitic stock associated with the silicic dikes yielded a KAr age of 19.4±0.9 Ma. 

Epidote- and pyrite-bearing prebatholithic greenschistfacies rocks have been explored by small prospects with no 

reported recovery of valuable metals. Detrital modes indicate that impure phosphatic sandstone that unconformably 

overlies granitic rock contains as much as 60 percent 

pelloidal phosphate. 
Extensional faulting that appears to be associated 

with the opening of the Gulf of California juxtaposes fossiliferous Pliocene marine strata with granitic basement 

rocks and pre-Pliocene strata. Rhyodacitic tuffs interbedded with the marine beds yield radiometric ages that 

range from 1 to 3 Ma. Quaternary rocks include basaltic 

lavas, cinder cones, and pyroclastic ejecta of local extent. 

Elevated Quaternary alluvial terrace deposits are in various stages of dissection. 

INDUCTIVELY COUPLED PLASMA MASS 

SPECTROMETRY -A POWERFUL 

ANALYTICAL TOOL FOR 

MINERAL RESOURCE AND 

ENVIRONMENTAL STUDIES 

Allen L. Meier, David J. Grimes, and 

Walter H. Ficklin 

The analytical technique of inductively coupled plasma 

mass spectrometry (ICP-MS) is being used for both mineral 

resource and environmental studies. Ground water associated with disseminated-gold deposits in Nevada is analyzed 

by ICP-MS to study elemental dispersion patterns around 

these deposits. The technique is also used to analyze surface 

water draining from mineralized and previously mined areas 

in Colorado to determine elemental mobility from these 

sources, for use in environmental studies. ICP-MS is 

extremely useful in these applications because of its ability 

to rapidly determine as many as 60 elements directly in the 

sample without the need for preconcentration or dilution and 

with detection limits in the sub-part-per-billion range and a 

linear range of six orders of magnitude or more. 

Calibration for this extensive elemental coverage is 

accomplished by using a standard containing known concentrations of easily ionized elements across the elemental 

mass range as well as a few elements that are less likely to 

be totally ionized by the ICP. A response curve of the 

observed intensity for easily ionized elements versus mass 

is constructed using a second-order equation for a best fit 

over the entire mass range. The ion temperature and electron number density of the plasma are estimated using the 

data from the harder-to-ionize elements. These are used to 

estimate the degree of ionization for all elements in the 

plasma. By using the response curve derived, the degree of 

ionization, and the natural isotopic abundance, semiquantitative estimates of concentration for all elements can be 

made in samples without the need of a calibration standard 

for every element. 

The main limitations of the technique come from drift 

due to clogging of sampling orifices; changes in ion transfer 

efficiencies due to sample matrix effects, plasma conditions, 

nebulizer, or electronics; and isobaric interference from 

polyatomic or doubly charged ions. Internal standards are 

used to correct for drift; interferences are minimized by 

Figure 1 (Meier and others). A ground-water sample being collected from a drill hole using a 

double-check valve bailer. Photograph by David J. Grimes. 

67 
100000 ..--------------------, 

10000 

1000 

I 

•• 

•• • 

100 

:c 

Q. 

E: 10 

::::1 

0.1 ' 

0.01 

1 2 3 

•• • 

•• 

• 

Uranium 

(0.11J.m filt.) 

• 

• • •• 

• • • .. • 

•• 

• 

4 5 6 7 8 

pH 

9 

Figure 2 (Meier and others). Uranium concentrations determined by ICP-MS in water samples collected throughout Colorado 

plotted as a function of pH. 

selection of the isotope used for determination or by mathematical correction. 

The ICP-MS technique has been applied to the analysis 

of ground-water samples collected from drill holes near 

known covered disseminated gold deposits in northern 

Nevada, to study the hydromorphic mobility of ore-associated elements such as arsenic, antimony, and tungsten. 

Ground-water samples were collected during active 

drilling and from old exploration drill holes. The water from 

the old drill holes was sampled at and below the water table 

using a custom-built double-check valve bailer (fig. 1). At 

each sample site, a 40 mL portion of water was filtered 

through a 0.45 J.lm membrane filter, acidified to a pH of 

approximately 2 with four drops of concentrated nitric acid, 

and stored in polyethylene bottles for ICP-MS analysis. 

Results of the ICP-MS analysis of drill-hole water samples indicate the presence ofhydromorphic dispersion anomalies for arsenic, antimony, and tungsten near the buried 

disseminated gold deposits. In these ground-water samples, 

the arsenic concentrations ranged from 14 to 130 J.lg/L; antimony, from 2.6 to 82.0 J.lg/L; and tungsten, from 15 to 99 

J.lg/L. The low determination limits provided by the ICP-MS 

technique also enabled local background levels for the oreassociated elements in ground water to be established. 

ICP-MS has been used in an environmental study of 

mine drainage in Colorado. Because of its capability to 

quantify a range of elements, it is useful in assessing the 

role of elements that are not ordinarily determined in mine 

water samples. The major cations in mine water are ordinarily calcium, magnesium, sodium, and potassium. In 

68 

many low-pH mine waters, the concentration of iron, manganese, copper, and zinc can equal or exceed the concentration of the major cations. All these elements are easily 

determined by either atomic absorption or ICP-AES. However, the rare-earth elements, uranium, and many other 

metals that occur in small concentrations are not detectable 

by the more conventional methods used. Figure 2 shows 

uranium concentrations plotted as a function of pH for 

mine water samples collected throughout Colorado. This 

example demonstrates the low limits and range of the ICPMS technique for uranium. 

GENERATION OF NATURAL ACID 

DRAINAGE AND FORMATION OF 

IRON BOGS, ROCKY MOUNTAINS, 

COLORADO 

William R. Miller, John B. McHugh, and 

Walter H. Ficklin 

Pyrite oxidation in altered and mineralized areas in the 

Colorado Mineral Belt has led to the generation of natural 

acid drainage (NAD) that has impacted surface waters in 

areas where mining is absent or limited. The naturally acidic 

waters are similar in composition to some of the more familiar acid mine drainage (AMD) waters. The concentrations of 

Fe, Mn, and AI in stream waters are usually less than 10 

mg/L but can reach several hundred mg/L; Cu, Ni, and Zn 

are usually less than 1 mg/L but can reach several mg/L, and 

other metals such as Pb are generally less than 0.01 mg/L. 

The metal contents oflow-flow seeps may exceed these concentrations, but their impact on stream-water chemistry is 

usually insignificant. 

Geneva Creek in the Front Range of Colorado is an 

example of a drainage impacted by NAD. The Geneva 

Creek drainage basin is underlain by Proterozoic metamorphic and igneous rocks, which have been intruded by Tertiary felsic stocks with associated pyritic alteration. 

Oxygen-charged waters from snow and rain strongly oxidize 

and dissolve pyrite, releasing H+, Fe2+, S04

2-, and trace 

metals to the waters of the study area. The dominant source 

of trace metals in the waters is trace-metal-bearing disseminated pyrite. The Fe2+ reacts with water, forming hydrous 

iron-oxide precipitates that accumulate along changes in 

slope and at the base of permeable zones. In these places 

iron bogs and iron-oxide zones can occur either as masses or 

as material cementing clastic and organic matter. Large 

accumulations of hydrous iron-oxide precipitates also occur 

locally along Geneva Creek. Substantial amounts of these 

precipitates are scoured out and removed from the upper 

basin each season during spring runoff, but are preserved in 

the lower gradient areas. Wetlands and terraces trap the 
precipitates and prevent the spring runoff from removing this 

material, thus establishing iron bogs along the drainages. 

For these NAD waters, metals are present in the stream 

waters as simple cations. Chemical modelling calculations 

suggest that the concentrations of Fe and AI in stream waters 

are controlled by the mineral phases ferihydrite and jurbanite. At most sites the Cu, Ni, and Zn concentrations are controlled by sorption onto precipitating hydrous metal oxides. 

The range of metal contents in waters and the processes 

controlling the concentrations of metals in water in the study 

area can probably be considered typical for other similarly 

altered and mineralized areas underlain by geochemically 

comparable rocks in the Colorado Mineral Belt. The results 

of this study can aid in establishing natural geochemical 

backgrounds for waters draining areas affected by AMD and 

in the understanding of processes controlling the concentrations of metals in surface waters. The study suggests that it 

may be difficult to impossible to remediate AMD sites to 

unnatural conditions that are more pristine than those present 

prior to mining. 

ASSESSMENT OF MINERAL RESOURCES 

AND TECTONIC SETTING DERIVED 

FROM DIGITAL AEROMAGNETIC DATA 

IN THE GUA YANA SHIELD, 

NORTHWESTERN BRAZIL 

Fernando P. Miranda and Anne E. McCafferty 

Reconnaissance geologic mapping is usually the first 

step of mineral exploration projects in tropical, remote 

regions of Latin America. The objective of such an enterprise is to quickly determine as much as possible about the 

geology of an unknown area. Reconnaissance maps at 

scales of 1 :250,000 or less are typically based on photo geologic and SLAR (Side Looking Airborne Radar) interpretations and on incorporation of geology from published 

reports; only a minimum amount of time can be spent on 

the ground to identify rock types. A reconnaissance geologic map can also be obtained using information derived 

from the digital manipulation of data acquired in regional 

aerogeophysical surveys. This is especially true in dense 

jungle of the Guayana Shield. 

Our study area is situated in the Pico da Neblina and 

lea quadrangles, northwest Brazil, and covers an area of 

approximately 54,500 km2. A geologic reconnaissance of 

the region was included in the RADAMBRASIL Project, a 

government enterprise that mapped the whole Brazilian territory from 1971 to 1976 (Correa, 1980). Final geologic 

maps were presented by the RADAMBRASIL Project at 

1:1,000,000 scale (Pineiro and others, 1976; Fernandes and 

others, 1977). The region is mostly underlain by Middle 

Proterozoic crystalline rocks of the Guianense Complex, 

69 

which includes a complex association of gneissic and granitoid rocks not mapped as separate units on the RADAMBRASIL geologic map. However, Dall' Agnol and Abreu 

(1976) recognized three main lithologic types in order of 

abundance: sphene- and amphibole-bearing biotite granitoids and gneisses; two-mica granites and gneisses; and 

amphibolites and metabasic rocks. 

Serious logistical difficulties exist for geologic studies 

in the area: (1) the regional scale (1 :1,000,000) of the only 

available geologic maps; (2) dense vegetation and thick soil 

cover (the area is nearly devoid of rock exposures); (3) poor 

accessibility; (4) flat topography (lithologic boundaries and 

geologic structures have no evident geomorphic expression). 

Therefore, the main objective of our research is to produce 

useful maps by digitally processing and analyzing the available aeromagnetic data in order to facilitate the assessment 

of mineral resources and the study of the tectonic setting of 

the region. 

The digital aeromagnetic data used in our research were 

obtained in 1987 and are part of a larger reconnaissance aeromagnetic survey that covered the border region with Venezuela, Colombia, and Brazil. A reduced-to-pole magnetic 

anomaly map was calculated in order to correct for the 

severe anomaly distortion caused by the low geomagnetic 

latitude of the area and to facilitate the interpretation of 

anomalies located over their causative sources. A terracemagnetization map was calculated that delineates induced 

magnetization boundaries and outlines geologic structures 

that have contrasting magnetic properties. Maxima of the 

horizontal gradient of pseudogravity were calculated to map 

abrupt lateral changes in magnetization interpreted to reflect 

steep structural or lithologic contacts. 

Digital manipulation of the aeromagnetic data allowed 

the recognition of induced magnetization domains possibly 

associated with unreported intrusive bodies in the crystalline 

rocks of the Guianense Complex. Magnetization boundary 

lines extending for tens of kilometers may represent shear 

zones not portrayed in the geologic maps of the RADAMBRASIL Project. These features are potential target areas for 

future mineral exploration (gold in particular). However, follow-up ground geologic work is necessary to determine if a 

connection between the inferred targets and mineral 

resources exists. Results of the research are an extension of 

a similar methodology carried out in the Guayana Shield by 

Miranda and others (in press). 

REFERENCES 

Correa, A. C., 1980, Geological mapping in the Amazon Jungle-A 

challenge to side looking radar: Jet Propulsion Laboratory Publication 80-61, p. 385-416. 

Fernandes, P.E.C.A., and others. 1977, Geologia, Chapter l ofProjccto RADAMBRASIL, folha SA.l9, lea: Rio de Janeiro, 

Brazil, Departamento Nacional da Producao Mineral, p. 

17-124, 5 plates, scale I: 1.000,000. 
Miranda, F.P., McCafferty, A.E., and Taranik, J.V., in press, Reconnaissance geologic mapping of a portion of the rain forest 

covered Guiana Shield, northwestern Brazil, using SIR-Band 

digital aeromagnetic data: Geophysics. 

Pineiro, S.S., Fernandes, P.E.C.A., Pereira, E.R., Vasconcelos, 

E.G., Pinto, A.C., Montalvao, R.M.G., Issler, R.S., Dall'Agnol, 

R., Teixeira, W., and Fernandes, C.A.C., 1976, Geologia, in 

MME Projecto RADAMBRASIL, Levantamento de Recursos 

Naturais, Folha NA.19-Pico da Neblina: Rio de Janeiro, 

Brazil, Departamento Nacional da Produdio Mineral, v. 11, 

p. 17-137. 

SLEEPER GOLD-SILVER DEPOSIT, 

HUMBOLDT COUNTY, NEVADAIMPORTANCE OF MIOCENE 

EXTENSIONAL TECTONICS* 

J. Thomas Nash and Wayne Trudel 

The world-class deposit of gold and silver at the Sleeper 

mine, Nevada, formed at about 15-16 Main a local volcanic 

field during the early stages of basin-and-range extension. 

Bonanza veins of opal-adularia-electrum contain more than 

1.5 million oz Au, and widespread, bulk-mineable Ag-Aubearing quartz-FeS2 stockworks and breccias contain more 

than I million oz Au. Both ore types exhibit strong structural 

controls in brittle, formerly glassy rhyolite flows or a flowdome complex. Ore-associated alteration is dominated by 

opaline and microcrystalline silica. Silicification decreases 

downward, over about 100m, and grades laterally into argillic- and perlitic-altered rhyolite. Adjacent andesitic rocks, 

broadly altered to quartz-sericite-pyrite, are bulk-mined for 

their 0.02-0.1 oz Au/t. The Sleeper deposit has many of the 

features of the adularia- sericite class of epithermal deposits, 

except that it has a high Au:Ag ratio and lacks chlorite and 

base metals at depth. 

Basin-and-range extensional tectonics played a major 

role in creating and protecting the ore. The basal Tertiary 

unit of clastic sedimentary rocks, about 200 m thick, formed 

in an extension-related lacustrine basin of probable Miocene 

age. Bimodal volcanic eruptions occurred from> 17 to 13 Ma 

along normal faults. Andesitic and basaltic lavas about 150 

m thick are sulfidized and altered over a broad area below 

ore. Lapilli tuff and tuff cones were deposited locally on the 

lavas. The glassy rhyolite of the Sleeper mine, here called 

Sleeper rhyolite (about 17 Ma), is the main ore host; this rhyolite unit is shattered by pre- and post-ore fractures and 

faults. Ash-flow tuffs (16.1 and 15.6 Ma, about 100m thick), 

erupted from the McDermitt caldera complex (80 km north), 

probably covered the Sleeper rhyolite at the time of ore formation. After a period of substantial erosion, rhyolite lavas 

erupted at 13.6 Ma and probably covered ore. Multiple 

stages of normal faulting crushed but did not dismember ore 

70 

zones; ore was down-dropped about 1,000 m, oxidized at 

several stages, and in part eroded into nearby placers. 

Reconstruction of district geology suggests a larger role 

for structure and pre-Tertiary rocks than volcanism in ore 

formation. Geologic, geochemical, and geochronologic data 

do not indicate a pluton-driven geothermal system. Rather, 

the system probably was driven by high regional heat flow 

and deep meteoric circulation in active normal fault and fracture zones. Gold-adularia veins ( 17 Ma) in Mesozoic phyllite 

at the Jumbo mine, 6 km to the southeast, and in phyllite 

below the Sleeper orebody are manifestations of the moderately hot ( <300 °C) waters in basement rocks. Reactions in 

metapelitic rocks would have produced fluid compositions 

similar to those advocated for Carlin-type gold deposits 

(high C02, CH4, HS-). Based on volumes of altered rocks 

and likely flow paths, we favor a source of gold in metapelitic rocks. 

Sleeper ores formed at very shallow depth, probably 

<200 m below the contemporary land surface, and at temperatures of <200 °C. Conditions fluctuated repeatedly from 

local hydrothermal explosion breccias to open-standing 

large veins filled by delicately banded ore and gangue. Deposition probably was initiated by boiling, fluid mixing, and 

cooling. The bonanza zones are small targets that are difficult to anticipate in exploration, but the postulated extension-related geothermal system, with variable low- to highgrade ore, could have developed in diverse rock packages in 

many parts of the Basin and Range province. 

*Not presented at Forum. 

HEAVY-METAL CONTAMINATION 

IN RIVER, ESTUARY, AND 

MARINE-SHELF SEDIMENT FROM THE 

RIO TINTO MINES, SPAIN 

C.H. Nelson, A. Van Geen, 

P .J. Lamothe, and A. Palanques 

The Rio Tinto mine in the eastern Iberian Pyrite Belt 

(fig. 1) is the world's largest sulfide deposit (> 170 million t 

(metric tons)) and longest continually operating mine 

(>5,000 years)(Schermerhorn, 1982; Wilson, 1981). Extensive mining activities began during Roman times and supported the conquests and currency of the Roman Empire. An 

initial 500-750 million tons of ore may have been removed 

by erosion prior to man's mining activities. The natural erosion and especially the later mining activities of this ore, 

which contains 0.7 percent copper, 2.9 percent zinc, 1.1 percent lead, and 0.6 percent arsenic, have supplied heavy metals to sediment, river water, and seawater in the Strait of 

Gibraltar region. 
6. 4" 

38. 

•GRANADA 

CADIZ MALAGA 

&"Y;' 

~Cj 

I 

~ R 1N $ 

GULF GRAPHIC SCALE of 

CADIZ 

GIBRALTAR 

0 100 200 km 

Figure 1 (Nelson and others). Location map of the Rio Tinto and Rio Odiel study area showing the region of the 

Iberian Pyrite Belt and the main Tinto copper sulfide deposit in relation to the river drainage systems. 

The Tin to and Odiel Rivers drain 100 km from the east 

and west sides of the Rio Tinto sulfide mining district, and 

join at a 20-km-long estuary entering the Atlantic Ocean. A 

reconnaissance study of heavy-metal anomalies in river and 

estuary bottom sediment by semiquantitative emission dearc spectrographic analysis shows the following upstream to 

downstream ranges, in ppm: As (3,000- <200), Cd 

(30-<0.1), Cu (1,500-10), Pb (2,000-<10), Sb (300-<150) 

and Zn (3,000-<200) (fig. 2). Organic-rich (1.3-2.6 percent 

TOC-total organic carbon) sandy-silty overbank clay has 

been analyzed to represent suspended load materials. The 

high content of heavy metals in the overbank clay throughout the river and estuary systems indicates the importance of 

suspended sediment transport for dispersing heavy metals 

from natural erosion and anthropogenic mining activities of 

the sulfide deposit. The organic-poor (0.21-0.37 percent 

TOC) river-bed sand has been analyzed to represent bedload 

transport of naturally occurring sulfide minerals. The sand 

exhibits high metal contents upstream, but these decrease an 

order of magnitude in the lower estuary. Although concentration in estuary-mouth beach sand has been diluted to 

background levels, estuary mud exhibits increased contamination of heavy metals that is apparently related to finer 

grain size, higher organic carbon content, and precipitation 

71 

of river-dissolved solids. The contaminated estuary mud disperses to the inner-shelf mud-belt (IGME, 1974) and offshore suspended sediment, which exhibit metal anomalies 

from river sulfide sources (for example, Cu, Zn; fig. 2). The 

heavy metal contamination of Tinto-Odiel river sediment 

reaches or exceeds the highest levels encountered in other 

river sediment of Spain and throughout Western Europe. 

The mud blanket of the middle to outer continental 

shelf in the Gulf of Cadiz contains a historical record of 

metal input to the ocean from the Rio Tinto mining 

region. Preliminary results show 3- to 15-fold Cu and Zn 

enrichments in surface sediment over the whole mud blanket relative to deeper intervals in 2-m gravity cores. Dating of these sediment cores currently underway will help 

to define the onset of metal enrichment in the mud blanket 

and provide a time frame for comparison with known 

activities in the Rio Tinto mining region. Extremely high 

dissolved Cu and Zn concentrations observed in the water 

column of the Gulf of Cadiz (Atlantic Ocean), the Strait of 

Gibraltar, and the Alboran Sea (Mediterranean Sea) are 

directly traceable to input from the Tinto estuary. The distribution of sediment and water-column enrichments demonstrates the impact of the Rio Tinto mining activities in 

late Holocene time. 
EXPLANATION 

e SAMPLE LOCATIONS 

c::> SAND SAMPLE 

c:::J MUD SAMPLE 

50-100 BACKGROUND 

1ooo PPM 

~ ~00 

l. 

'I. 

1500 

2000 

SPAIN 

"C• 0\ 

~/RIO 

C GUADIANA 

C) 

l> 

rCu 

-

~ 

c::> 

>1000 (<0.062mm) ppm IN MUD~ 

GULF OF CADIZ 

0 20 

N 

1 C) SAND SAMPLE 

c:::J MUD SAMPLE 

100-300 BACKGROUND 

NOT DETECTED 

SPAIN 

PPM 

Zn 

N 

1 

Figure 2 (Nelson and others). Content of copper and zinc in Tin to and Odie! river-bed sand and river-bank mud. Offshore data are modified from IGME (1974) and typical background values are from Levinson (1974, table 2-1, p. 44-45). 

REFERENCES 

Institute Geologico y Minero de Espana (IGME), 1974. Investigacion Minera Submarina en el Subsector "Huelva I" Golfo de 

Cadiz: Madrid, Servicio de Publicaciones Ministerio de Industria, 89 p. 

Levinson, A.A., 1974, Introduction to exploration geochemistry: 

Wilmette, TIL, Applied Publishing Limited, 924 p. 

Schermerhorn, L.J.G., 1982, Framework and evolution of Hercynian mineralization in the Iberian Meseta: Comunicacione Geologica Iceo, Portugal, t. 68, fasc. 1, p. 91-140. 

Wilson, A.J., 1981, Archaeologists find "Missing Link" in Rio Tinto Mining History: Queensland Government Mining Journal, 

p. 32- 33. 

GEOCHEMISTRY CONTRIBUTIONS TO 

THE MINERAL RESOURCE ASSESSMENT 

OF THE CORONADO NATIONAL FOREST, 

ARIZONA AND NEW MEXICO 

Gary A. Nowlan 

The metal geochemistry of the Coronado National Forest and adjacent areas, Arizona and New Mexico, was evaluated as part of a U.S. Geological Survey study of the 

mineral resource potential of the forest. The geochemical 

data base consisted of (1) data from existing U.S. Geological 

Survey data bases, (2) data from analyses of additional 

72 

samples from areas of sparse coverage, and (3) new data 

from analyses of the existing samples. The geochemical 

evaluation is based mainly on stream-sediment data, because 

the stream-sediment sample coverage is much more extensive than that of other sample media. 

A total of 3,874 stream-sediment samples in this study 

were analyzed by 31-element, semiquantitative, direct-current arc emission spectrography. In addition, 2,537 streamsediment samples were analyzed for Au by graphite-furnace 

atomic-absorption spectrophotometry and for Ag, As, Au, 

Bi, Cd, Cu, Mo, Pb, Sb, and Zn by inductively coupled 

plasma-atomic emission spectroscopy. The geochemical 

evaluation is based principally on abundances of Ag, As, Au, 

Ba, Bi, Cd, Cu, Mn, Mo, Pb, Sb, Sn, W, and Zn because they 

are commonly associated with hydrothermal ore deposits 

known or suspected to occur in Coronado National Forest; 

these include porphyry copper and molybdenum deposits, 

epithermal precious-metal deposits, veins and replacements, 

various types of skams, various types of tin, tungsten, and 

beryllium deposits, and pegmatites. 

Based on geology, the forest was subdivided into 35 

areas that were ranked on a geochemical intensity scale. 

Rankings are based on the number of elements present in 

anomalous concentrations, the percentage of samples with 

anomalous concentrations, and the magnitude of concentrations in individual samples. The most anomalous area was 

assigned a value of 10. Parts of the Patagonia Mountains are 

highest on the geochemical intensity scale and parts of the 
111" 110. 109. 

EXPLANATION 

10 High L__j 

10 mi 

33" 

·-·· [L :. Tore••• 

lledium 

4 

Geochemical 

Intensity 

Low 

l.ltn:s 

~ 

)\iJ 

Study area 

Tucson 

0 

Winchester ~tna 

Will cox 0 

lltna 

'

Dragoon 

Santa~, lltno Rita 0 : 0 

ldt.ns aa 

Whet111t.one 

lLtna 

San Rafael Valley 

Figure 1 (Nowlan). Geochemical intensity rankings of subdivisions of the Coronado National Forest (patterned areas), southeastern 

Arizona and southwestern New Mexico, based on analyses of stream-sediment samples. 

Winchester Mountains are the lowest. The areas and their 

geochemical intensity rankings are shown in figure I. 

MINERAL RESOURCES AND ECONOMICS 

OF THE UNITED STATES-MEXICO 

BORDER REGION 

Greta J. Orris, Floyd Gray, Keith R. Long, 

Norman J Page, John-Mark G. Staude, and 

Karen s. Bolm 

In anticipation of increasing mineral trade with Mexico, 

the Center for Inter-American Mineral Resource Investigations (CIMRI) initiated a project in 1992 to study the mineral 

73 

economics of nonfuel mineral resources in the United 

States-Mexico border region. The study area was defined to 

include much of the southern parts of the States of California, Arizona, New Mexico, and Texas in the United States 

and the northern portions of the States of Baja California 

Norte, Sonora, Chihuahua, Coahuila, Nuevo Leon, and 

Tamaulipas in Mexico (fig. 1). As a first step in the border 

project, CIMRI began to construct a database of the known 

mines and mineral occurrences in and adjacent to the study 

area. This information is stored in the Mineral Resources 

Data System (MRDS), the U.S. Geological Survey's publicly accessible computerized system for storage of mine and 

mineral occurrence data. The most current versions of the 

border region data are available from the Minerals Information Office (MIO) at 340 North 6th Avenue, Tucson, 

AZ 85705. 
122'30" 

36'30" 

CALIFORNIA NEW MEXICO 

• Santa Fe 

93'00" 

Albuquerque • 

Matamoros 

21 'oo· L _ _ ________________ ___ ~. ~__j _____ _____ __, _____ ~ 

Figure 1 (Orris and others). The CIMRI United States-Mexico border region study area (shaded). 

Prior to CIMRI's project, data in MRDS for the border 

region consisted of data sets for Arizona, New Mexico, California, Baja California Norte, and Sonora that were compiled by the USGS and contractors in the late 1970's and 

early 1980's. These data sets were incomplete with regard to 

both mineral discoveries of the last 10 years and industrial 

minerals. Essentially no data were stored in MRDS for the 

States of Texas, Tamaulipas, Nuevo Leon, Cohuila, or Chihuahua. As of summer 1993, CIMRI has made a first pass at 

completing and updating mine and mineral occurrence information for most of the study area and entered that infoqnation in MRDS. Data sets known to be incomplete after the 

first pass include the data set for southern California, industrial minerals for north-central Mexico, and sand and gravel 

for Arizona. 

In addition to mineral resource data, as part of the border region project, CIMRI is also compiling data related to 

geology, infrastructure, transport, mineral processing, tariffs, electric power sources, and other data related to the mineral economics of the area. These data will allow CIMRI to 

address such issues as the relationship of mineral deposits to 

geology and the adequacy of mineral resources in the border 

region for infrastructure construction and rebuilding, environmental mitigation, and agricultural support. 

74 

PRELIMINARY INTERPRETATION OF 

AEROMAGNETIC DATA IN 

NORTHEASTERN NICARAGUAPOSSIBLE RELATION BETWEEN 

STRUCTURE AND ECONOMIC 

MINERAL DEPOSITS 

Herbert A. Pierce 

In northeastern Nicaragua, Mesozoic rocks host economic mineral deposits, some of which are associated with 

structures identified through aeromagnetic surveys. The 

dominant aeromagnetic anomalies extend northeast-southwest for more than I I 0 km, and in places they are several 

kilometers wide. 

Aeromagnetic data are available for I I ,764 km2 located 

between !at 13°30'N. and 14°15' N. and long 85°00' W. and 

83°45' W. The original data were gathered under a program 

financed by the United Nations in 1964 and 1965. Data were 

collected along east-west flightlines spaced 500 m apart and 

plotted on eight I :50,000-scale maps. The data were digitized and subsequently merged and gridded to create two 
1 :200,000-scale maps for the interpretation of structure and 

geology of the upper crustal rocks. For one map, the International Geophysical Reference Field (IGRF) and a secondary 

residual field were removed and the residual data plotted. 

For the second, the IGRF was removed, data were reduced to 

the pole, and maximum horizontal gradient locations were 

calculated before plotting. 

The data show a northeast-southwest-trending series of 

curvilinear magnetic anomalies extending across and probably beyond the survey area. A second, less obvious linear set 

of magnetic features cuts orthogonally across the northeastsouthwest-trending features. Interpretation of two high-frequency, high-amplitude magnetic areas suggests two separate groups of volcanic-intrusive rock. An area of low 

magnetic response representative of basin-filling sediments 

suggests a small basin adjacent to and north of the predominant northeast-southwest-trending anomalies. A circular 

feature 60 km in diameter and near the center of the survey 

area is proximal to and southeast of the main anomalies and 

appears to contain a smaller, nested (though less distinct) circular anomaly of unknown origin. 

To the north, near the common border between Honduras and Guatemala, curvilinear structures of similar scale and 

trend have been identified by geologic mapping, topographic 

features, Landsat, and interpretation of SIR-A radar data 

(Short and Blair, 1986). One major structure, the Rio Motagua-Polochic fault system, separates the Chortis block on 

the south from the Mayan block to the north (Molnar and 

Sykes, 1969). This fracture system has multiple placer and 

vein gold deposits along its strike. In central Honduras, 

another feature of similar scale and trend, the Guayape fault, 

cuts across the central region of the country and has placer, 

stockwork, and vein gold deposits dotted along its strike 

(D.G.M.H., 1989). 

In the west half of the Nicaraguan study area, existing 

geologic maps show Tertiary arc-related volcanic rocks and 

Cretaceous plutons exposed by erosion; in the east half ofthe 

area, Quaternary platform sediments are dominant (Dengo 

and others 1969; Case and Holcomb, 1980; Muehlberger, 

1992). However, recent geologic work (M. Venerable, written common., 1993) suggests a more complex picture. Preliminary mapping in the southern portion of the northeastsouthwest-trending anomaly identified serpentinites and 

other ultramafic rocks containing podiform chromite deposits. These rocks may represent back-arc terrane, marking the 

contact between the continental Chortis block (nuclear Central America) to the north and the oceanic crust of southern 

Central America to the south. Gold districts are associated 

with the magnetic anomalies. Two districts, Bonanza and 

Siuna, are proximal to the main northeast-southwest-trending series of anomalies; a third district, Rosita, is located 

along the northern boundary of the large circular feature 

described previously. Cumulative production(::: 1901-1991) 

from these districts comprises 35,000 tons Pb; I 02,260 tons 

Cu; 217,170 tons Zn; 2,504,590 oz Ag; and 850,100 oz Au 

(USGS, 1993). 

75 

Rock type is not consistent along the trend of the magnetic anomalies. The only consistent relation is that the 

deposits tend to cluster along the magnetic linears. If the 

trend of anomalies delineates a boundary, fault, or set of 

faults, these breaks in the crust may have served as conduits 

and control of mineral-bearing fluids. Exploration geologists 

should consider these trends in their exploration programs. 

REFERENCES 

Case, J.E., and Holcomb, T.L .. 1980, Geologic-tectonic map of the 

Caribbean region: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1100, scale 1:2,500,000. 

Dengo, G., Levy, E., Bohnenberger, 0., and Caballeros, R., 1969, 

Metallogenic map of Central America: Instituto Centroamericano de Investigation y Technologia Industrial, scale 

1:2,000,000. 

Direccion General de Minas e Hidrocarburos (D.G.M.H.). 1989, 

Mapa Metalogenetico de Ia Republica de Honduras: Scale 

1:500,000. 

Molnar, P., and Sykes, L.R., 1969, Tectonics of the Caribbean and 

Middle America regions from local mechanisms and seismicity: Geological Society of America Bulletin, v. 87, p. 1639. 

Muehlberger, W.R., 1992. Tectonic map of North America: American Association of Petroleum Geologists, scale 1:5,000,000. 

Short, N.M., and Blair, R.W., Jr., eds., 1986, Geomorphology 

from space, a global overview of regional landforms, chapters 

3 and 6: National Aeronautics Space Administration, p. 

216-217;388-389. 

U.S. Geological Survey. 1993. Mineral Resources Data System. 

THORIUM, URANIUM, AND 

POTASSIUM AERORADIOACTIVITY 

MAPS OF ARIZONA-GEOLOGIC 

INTERPRETATION AND 

METALLOGENIC SIGNIFICANCE 

James A. Pitkin, Clay M. Conway, and 

Gordon B. Haxel 

We have prepared and interpreted maps (1 :1,000,000 

scale) of thorium (Th), uranium (U), and potassium (K) concentrations for Arizona from aerial gamma-ray spectrometer 

(aeroradioactivity) data acquired during the National Uranium Resource Evaluation program of the U.S. Department 

of Energy. The maps were made from 3-km square grids of 

Th, U, and K for the conterminous United States (Phillips 

and others, 1993). The grid values are in ppm (parts per million) for Th and U and in percent for K; these values represent the near-surface (to 50 em depth) concentrations of the 

natural radioelements. With a few important exceptions, the 

spatial patterns of Th, U, and K are similar. We emphasize 
the distribution of Th, for which the patterns are most 

pronounced and systematic, probably reflecting the generally greater mobility of U and K and the dual oxidation states 

of U in the upper crust. 

The Colorado Plateau as a region exhibits relatively low 

to moderate and homogeneous radioelement concentrations 

in the widespread Paleozoic and Mesozoic sandstones and 

limestones. Areas of elevated relative concentrations are 

underlain by Cretaceous strata that include shale and coal 

around Black Mesa, lacustrine beds of the Neogene Bidahochi Formation, ash-bearing mudstones of the Upper Triassic 

Chinle Formation, silicic and alkaline Tertiary volcanic 

rocks, and, locally, redbeds of the Lower and Middle Triassic Moenkopi Formation. Detailed features in the radioelement maps suggest areas of higher potential for U within 

these more radioactive formations. 

Aeroradioactivity data in the Basin and Range-Colorado Plateau Transition Zone and the Basin and Range province show greater range, spatial variability, and generally 

higher radioactivity than are evident for the Colorado Plateau. The more radioactive rocks include 1.4-Ga granite, 

some 1.7--Ga granites, Jurassic granitoids, Tertiary volcanic 

rocks, and basin detritus from these bedrock sources. 

An anomalous belt of low radioactivity, most pronounced in the Th data, extends from the Prescott-Jerome 

region in the Transition Zone southwestward to the region 

between Gila Bend and Yuma in the Basin and Range 

province. In the Transition Zone, this belt corresponds 

closely to the distribution of 1.75-1.72-Ga calc-alkaline 

volcanic and plutonic suites unique to this part of Arizona. 

This belt includes very little 1.4--Ga granite, in contrast to 

the rest of the Transition Zone. The metallogeny of this 

low-radioactivity region is characterized by unusually 

abundant Proterozoic massive sulfide deposits, and abundant Laramide and Proterozoic base- and precious-metal 

veins. The aeroradioactivity maps suggest that this metallogenic province may extend southward into the Basin 

and Range province. 

· Within the generally high radioactivity of the Basin and 

Range, a broad, irregular corridor of low radioactivity about 

50--80 km wide trends northwestward from Douglas in 

southeastern Arizona to Parker in west-central Arizona. This 

corridor parallels and includes the- Arizona metamorphic 

core complex belt. Individual metamorphic core complexes, 

both within this belt and south of it in the Baboquivari 

Mountains area, generally are characterized by pronounced 

radioactivity lows. The core complex belt is also a region of 

low heat flow, which may be explained by low radioelement 

abundances in the middle crustal rocks exposed within the 

core complexes (J.H. Sass, oral common., 1993). 

Areas of potassium metasomatism, related to hydrothermal activity that occurred along and above detachment 

faults during core complex uplift, are readily distinguished 

on the K map, especially near the Buckskin-Rawhide and 

Harcuvar core complexes in west-central Arizona. Potas76 

sium aeroradioactivity is thus a potential exploration guide 

for Au-Cu and Mn deposits that also formed during this 

hydrothermal activity. 

In the southwest quarter of Arizona, first-order features 

of the aeroradioactivity maps evidently are largely controlled by the distribution of Jurassic granitoids. Two regions 

in this part of the State have generally high radioelement 

abundances. One of these regions extends from the Nogales 

area northwest to Ajo and Organ Pipe Cactus National Monument; the other lies between the lower Gila River and the 

lower Colorado River. Both are characterized by widespread 

and voluminous Jurassic granitoids, and many individual 

aeroradioactivity highs coincide with exposed Jurassic plutons. An intervening region of markedly low radioactivity, in 

and west of Cabeza Prieta National Wildlife Refuge, coincides with an area where Jurassic granitoids are sparse or 

absent. This aeroradioactivity low may be related to widespread latest Cretaceous leucogranite, and it forms the southwest end of the Prescott-Jerome-to-Gila Bend-Yuma belt of 

low radioactivity described previously. 

Aeroradioactivity data have considerable potential in 

Arizona for exploration for several lithophile elements in 

addition to U and Th. The 1.4--Ga granites in Arizona commonly are highly radioactive; their potential for hosting rare 

metals may be underappreciated. Tungsten deposits near 

Bagdad are known to be genetically related to 1.4--Ga granites. About two-thirds of the nearly 30 tungsten districts in 

Arizona are within or close to 1.4--Ga granites or pegmatites. 

A few fissure-type U deposits are also spatially related to 

1.4-Ga granite. A "red granite" complex of 1.7--Ga hypabyssal and volcanic rocks in the region between Payson and the 

New River Mountains in the Transition Zone is also highly 

radioactive and has associated anomalies of Sn, Be, Nb, La, 

andY. Modest positive aeroradioactivity anomalies occur in 

1.7--Ga granites in northwestern Arizona, where the largest 

tungsten deposit in the State, the Boriana deposit, is related 

to a 1. 7--Ga two-mica granite. A half dozen tungsten districts 

in the southwestern part of the State occur within the highly 

radioactive Jurassic terranes. 

Large low-grade uranium deposits are known in the 

sedimentary rocks of several Tertiary basins in west-central 

Arizona. The aeroradioactivity maps suggest that the source 

for the uranium in these basins is in adjacent ranges of highly 

radioactive bedrock, mainly 1.4--Ga granite. The aeroradioactivity data also indicate high radioelement concentrations 

in other Tertiary basins. 

REFERENCE 

Phillips, J.D., Duval, J.S., and Ambroziak, R.A., 1993, National 

geophysical data grids-Gamma-ray, gravity. magnetic, and 

topographic data for the conterminous United States: U.S. 

Geological Survey Digital Data Series DDS-9, 1 CD-ROM. 
THE GEOCHEMISTRY AND 

ENVIRONMENTAL DEGRADATION OF 

CYANIDE AT THE SUMMITVILLE MINE, 

COLORADO 

Geoffrey S. Plumlee, Walter H. Ficklin, 

Maria Montour, Kathleen S. Smith, 

Allen L. Meier, and Paul H. Briggs 

Among the environmental concerns stemming from 

recent mining activities at Summitville are past accidental 

releases of cyanide-bearing heap-leach solutions into the 

Wightman Fork of the Alamosa River, and the ongoing 

efforts to remove cyanide from solutions remaining in the 

heap-leach pad. We have initiated a research project to 

understand the geochemical processes (1) that are currently 

active in the Summitville heap-leach pad and (2) that would 

occur during an accidental release of the heap-leach solutions into the Wightman Fork. Unfiltered heap-leach solutions that are currently being treated in remediation typically 

have pH values near 9.4 and total cyanide concentrations 

varying from 120 to 150 ppm (parts per million). Approximately 100-130 ppm of the cyanide is present in WAD 

(weak-acid dissociable) form as complexes primarily with 

copper; the remainder is primarily present as free cyanide 

(CN-, minor HCN), along with lesser amounts as strong 

complexes with iron, cobalt, and nickel. Copper is the predominant metal in the heap solutions (21 0 ppm), and reflects 

the copper-rich nature of the ores and the strength of the copper complexes. Other metals present include Fe, Co, and Ni 

(1-2 ppm); AI (0.5 ppm); and Ag, Mo, and Zn (0.1-0.2 

ppm). High thiocyanate (SCN-) concentrations (250 ppm) 

show that sulfides are present within the heap and are undergoing oxidation; the thiocyanate forms predominantly by the 

reactions of free cyanide with intermediate-valency sulfur 

species formed during sulfide oxidation. 

Any surface leaks from the cyanide heap-leach operations that occurred during mining would have flowed into 

the Wightman Fork. We have designed laboratory experiments to model the mixing of the current heap-leach solutions with the Wightman Fork water. In the experiments, 

samples of heap-leach solution are mixed with varying proportions of a Wightman Fork water mixture (pH=3.2) consisting of Wightman Fork water from above the Reynolds 

Tunnel inflow, Reynolds Tunnel water, and Cleveland Cliffs 

Pond water. The experiments are conducted both outdoors in 

open containers (to evaluate the effects of photolytic cyanide 

degradation) and indoors in closed containers (to allow 

retrieval of volatilized cyanide gas). Compressed air is 

bubbled through the mixtures to approximate the turbulence 

that would be encountered during flow along the Wightman 

Fork. Figure 1 presents results of one experiment that document the progressive loss of cyanide from the mixture, a 

77 

Q) 

"C 

c 

~ c 

~ j 

.5! 

-a 

Q) 

> 

0 

en 

en 

i5 

Dissolved 

copper (ppm) 

%copper in 

particulates 

160 

140 "-- WAD Cyanide in Heap Leach Solution 

120 WAD cyanide complexes broken 

100 down by acid: 

80 

Cu(CN)3- + 3H+ .:::::> Cu+ + 3 HCN aq 

60 Total WAD cyanide in mixture 

40 I Dissolved WAD cyanide in mixture 

20 

0 

9 

• Heap Leach pH 8 I pH lncn18Sos due to cyanide 7 volatilization: 

HCN aq .:::::> HCN gas 

:a_s 

s-. ,.. - '-... pH of "Wightman"-heap leach4 

a• 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

0 

mixture 

.¥ pH of "Wightman" prior to mixing 

200 

Copper and other metals are 

sorbed by iron-hydroxide 

particulates, which settle out. 

Dissolved copper (ppm) 

400 600 800 1000 1200 

Time after mixing (minutes) 

I 

Figure 1 (Plumlee, Ficklin, Montour, and others). Plots showing results of an experiment in which Summitville heap-leach solution is mixed with an equal volume of Wightman Fork water 

mixture. 

transient increase in cyanide associated with solid precipitates, and the formation of abundant iron hydroxide precipitates. We interpret these data to reflect the progressive 

destruction of the copper-cyanide complexes through acidification by the Wightman Fork waters, and the volatilization 

of the cyanide as hydrogen cyanide according to the following reaction: 

Cu(CN)J=+3H+=Cu++3HCN (g). 

The temporary shift of cyanide into particulates most 

likely reflects the precipitation of solid copper cyanide; 

however, this solid cyanide compound is also apparently 

degraded over time. The copper originally complexed 

with the cyanide is sorbed onto the abundant iron hydroxides precipitated as a result of the mixing. 

Study results to late 1993 indicate that mixing with 

acidic Wightman Fork waters would likely trigger extensive degradation of cyanide in the heap-leach solutions. 

In fact, we are exploring further the mixing of heap-leach 
solutions with acid-mine drainage as a lower cost remediation tool for treating both heap-leach solutions and minedrainage. 

GEOLOGIC AND GEOCHEMICAL 

CONTROLS ON THE COMPOSITION OF 

ACID WATERS DRAINING THE 

SUMMITVILLE MINE, COLORADO 

Geoffrey S. Plumlee, Walter H. Ficklin, 

Kathleen S. Smith, Maria Montour, John Gray, 

Philip Hageman, Paul H. Briggs, and 

Allen L. Meier 

The Summitville mine, located at an elevation of 

11 ,500 feet in the San Juan Mountains of southwestern Colorado, /generates highly acidic, metalliferous mine-drainage 

waters from underground mine workings, mine dumps, mill 

tailings, and seeps both within and outside the mine's open 

pit. The pH of drainage waters sampled to late 1993 ranges 

from 1.7 to 3.2. The waters carry extremely high concentrations of metals and other potentially toxic elements, including Cu and Zn (hundreds of parts per million [ppm]); As, Cd, 

Cr, Co, Li, Ni, rare-earth elements, Se, U, Th, V (hundreds 

of parts per billion [ppb] to tens of ppm); Be, Ga, Ge, Te (ppb 

to hundreds ofppb). In general, the acidity and metal content 

of the waters increase from waters draining underground 

workings (Reynolds Tunnel, the main drainage tunnel at the 

site), to waters draining mine dumps and mill tailings 

(Cropsy waste dump, Cleveland Cliffs Pond), to low-volume 

seeps and numerous rainwater-derived puddles within the 

open pit. 

The acidic, metalliferous Summitville mine drainage 

waters are a predictable consequence of the deposit's geologic characteristics. Prior to ore mineralization, the volcanic host rocks were intensely altered by magmatic gas 

condensates; wallrock alteration grading outward from 

fractures includes vuggy silica, quartz-alunite, quartzkaolinite, clay, and propylitic assemblages. Sulfide-rich 

mineralization then took place in the highly altered rock. 

Post-mineralization oxidation occurred primarily along 

the high-permeability vuggy silica zones, occurring to 

depths as great as 300 feet. In contrast, the low permeability of the clay alteration zones allowed sulfide-rich rock 

to persist very near the surface; the open-pit mining fractured and exposed large volumes of this sulfide-rich rock 

to weathering. Many of the ore minerals, including pyrite, 

marcasite, chalcopyrite, and enargite, generate large 

amounts of acid during aqueous weathering; the highly 

altered wallrock coupled with the lack of carbonates in 

the ores resulted in very little means for buffering the 

acid generated during sulfide oxidation. 

78 

Important geochemical controls on the drainage 

chemistry, in addition to sulfide oxidation and acid generation, include mineral dissolution/precipitation, evaporation, and, to only a limited extent, sorption. High Cu:Zn 

ratios in the waters reflect the Co-rich nature of the ore. 

The high As concentrations result from the presence of 

enargite and tennantite. The high concentrations of U, Th, 

and rare-earth elements in the waters suggest that the acid 

waters are reacting extensively with wallrocks. The rapid 

formation of wine-colored, acid puddles in the open pit 

after rain suggests that readily soluble metal-sulfate salts 

are being dissolved from the pit rocks; the minerals precipitate again as the puddles evaporate. Sharp increases in 

metal contents in the Reynolds Tunnel waters during 

spring runoff may also reflect dissolution of soluble metalsulfate salts in the old mine workings and fracture systems. Hydrogen isotopic data indicate that evaporation 

helps enhance the extreme metal contents and acidities of 

the pit waters; the Reynolds Tunnel waters have isotopic 

signatures characteristic of snowmelt, whereas the open-pit 

waters and, to a lesser extent the mine dump waters, 

show evidence of evaporation. Metal sorption appears to 

be limited due to the general lack of particulates in the 

waters. 

Geologic and geochemical considerations such as these 

should be of critical importance to the success of long-term 

remediation efforts at Summitville. 

USGS ENVIRONMENTAL GEOSCIENCE 

STUDIES OF THE SUMMITVILLE MINE 

AND ITS EFFECTS ON AGRICULTURE 

AND WILDLIFE ECOSYSTEMS 

IN THE SAN LUIS VALLEY, 

COLORADO-PROJECT OVERVIEW 

Geoffrey S. Plumlee and R.C. Severson 

The Alamosa River and its tributaries drain several 

highly mineralized, sulfide-rich areas in the San Juan Mountains of Colorado (fig. 1 ). These mineralized areas are 

sources for both natural and mining-related metal loading in 

the Alamosa River. This metal loading is of concern due to 

the extensive downstream use of Alamosa River water for 

agricultural irrigation in the San Luis Valley, for domestic 

purposes, and for water supply to the Alamosa National 

Wildlife Refuge and nearby wetlands (seasonal hosts to 

endangered migratory species such as the whooping crane). 

The Summitville mining district, located on the Wightman 

Fork of the Alamosa River, has received considerable public 

attention in the past year as a result of environmental 

problems attributed to recent open-pit mining activities. 

Matters of environmental concern include (1) recent 
San Juan ~ Monte Vista 

Mountains ~ o 

San Luis 

Valley 

~ 

~ Monte Vista 

National Wildlife 

Summitville ~ Refuge _________..·.:·:·.·:··:-:-::-:·:-· 

Mine ~ 

~ 

Source 

characterization 

studies 

0 10 20km 

Wildlife habitat 

potentially affected by 

Alamosa River water 

Mineralized areas 

Farmlands irrigated with 

Alamosa River water 

Figure 1 (Plumlee and Severson). U.S. Geological Survey Summitville and San Luis Valley Studies. Cooperators include U.S. Fish and 

Wildlife Service, U.S.D.A. Forest Service, EPA, State of Colorado, Colorado State University, local extension services, irrigation districts, 

and farmers. 

increases in concentrations of metals and decreases in pH of 

the Alamosa River waters, San Luis Valley irrigation waters, 

and irrigated soils; (2) increased corrosion and fouling of 

irrigation equipment due to the lower pH of the irrigation 

waters, and (3) reduction of aquatic life in the Alamosa 

River. 

The U.S. Geological Survey Geologic Division 

(USGS-GD) and Water Resources Divison (USGS-WRD) 

have initiated multidisciplinary geoscience research projects 

to provide objective scientific information on Summitville 

and its downstream environmental effects. Potential users of 

the results of these studies include valley residents, fanners, 

regulatory agencies, and industry. The work is being carried 

out in close cooperation with the State of Colorado, U.S. 

EPA, U.S. Fish and Wildlife Service, Bureau of Reclamation, U.S.D.A. Forest Service, local agencies (CSU Extension Service), and local fanners. We here present an 

overview of USGS Summitville activities; many of the 

projects are discussed in detail in companion abstracts in this 

volume. 

USGS-GD studies at the Summitville mine site 

include ongoing characterization of the site geology, struc79 

tore, mine drainage, and heap-leach geochemistry; these 

studies will provide information needed to develop effective environmental remediation plans. Geologic and 

geochemical characterization of other mineralized areas 

in the Alamosa River basin (GD), as well as geochemical 

studies of the Alamosa River and its tributaries (WRD) 

will document the origin and nature of metal loadings 

into the Alamosa River from natural and mining-related 

sources besides Summitville. The WRD Alamosa River 

study will also provide insight into processes controlling 

metal mobility and attenuation downstream from Summitville. WRD and GD geochemical studies of Terrace 

Reservoir evaluate metal cycling between the reservoir 

sediments and waters. A VIRIS (Advanced Visible and 

Infra-Red Imaging Spectrometer) remote sensing data provide a synoptic view of the surficial geology, mineralogy, 

environmental geochemistry, and vegetation stress in the 

southwestern San Luis Valley and Alamosa River basin 

(including the Summitville area) that is not easily 

obtained through traditional remote sensing or geologic 

methods. Regional geochemical mapping of soils throughout the San Luis Valley documents the natural distribution 
of elements in valley soils and any superposition of possible recent metal contamination; the establishment of baseline values serves as a benchmark against which crop 

studies can be interpreted and any future cleanup of soil 

contamination can be defined. The occurrence and distribution of metals in wetlands ecosystems are being documented to establish pre-mining baseline geochemical 

signatures in sediments and selected vegetation; additionally, biogeochemical cycling (including fate and transport) of metals through the wetlands and metal uptake by 

organisms are being assessed. Metal mobility between 

soils and major crop types is being assessed to determine 

the potential impact of the Alamosa River irrigation water 

on metal uptake by crops in the southwestern San Luis 

Valley. The results of the Summitville/San Luis Valley 

studies will have broad application to the understanding 

and remediation of existing and future mine sites. 

INTEGRATED MINERAL RESOURCE AND 

ENVIRONMENTAL ASSESSMENTS OF 

PUBLIC LANDS-APPLICATIONS IN 

LAND-USE MANAGEMENT 

Geoffrey S. Plumlee, Steven M. Smith, 

Margo Toth, and Sherman P. Marsh 

Mineral resource assessments identify the potential for 

the occurrence of different mineral deposit types based on 

their characteristic mineralogy, host rock, wallrock alteration, trace-element signature, and structural geology. These 

same geologic attributes also produce predictable environmental signatures prior to and resulting from mining and 

mineral processing. Knowledge of mineral deposit geology 

is therefore critical to effectively predict, mitigate, and 

remediate the environmental effects of mineral 

development. In order to facilitate the use of environmental 

geoscience information in mineral resource development 

and related land-use planning, the U.S. Geological Survey 

Office of Mineral Resources (USGS/ OMR) is incorporating 

environmental risk assessments into its mineral resource 

assessments of public lands. 

A mineral resource assessment of a land unit provides 

a crucial geologic, geochemical, and geophysical framework upon which an environmental assessment is based. 

This framework includes compilation of regional geochemical and geophysical data, classifying existing mines and 

prospects according to deposit type, and identifying terranes favorable for the occurrence of undiscovered mineral 

deposit types. 

The environmental assessment of a land unit has several 

components. First, regional geochemical and geophysical 

data are used to establish baselines and to identify natural 

80 

and anthropogenic sources of metal loadings into the environment. Second, likely environmental effects of mineral 

resources (mined, known but unmined, and undiscovered) in 

the land unit can be estimated; this is done using empirical 

environmental models of mineral deposit types currently 

under development at USGS/OMR. These models identify, 

for given mineral deposit types, likely environmental signatures (a) present in soils, waters, and sediments prior to mining, and (b) resulting from mining, including the signatures 

of mine wastes, mine waters, mineral processing solutions, 

tailings, and smelter emissions. The environmental signatures take into account data on the abundance, residence, and 

mobility of metals and other elements of potential environmental concern. 

The third component of the environmental assessment 

is the delineation of "litho-environmental terranes," which 

are rock units or groups of rock units that have a particular 

effect on the environment. For example, carbonate terranes 

generate surface and ground waters with high acid-buffering 

capacity, and such terranes have large capacity to mitigate 

the effects of acid mine drainage. The final component is the 

development of an environmental risk assessment that ranks 

the deposit types and mining districts identified within the 

land unit according to the severity of their potential environmental hazards. For example, although a given deposit type 

may generate extremely acidic, metalliferous acid mine 

drainage, occurrences of that deposit type in a carbonate-rich 

terrane or flow of acid waters through a carbonate terrane 

would lessen the potential environmental effects on the surrounding land unit. 

Prototype mineral resource-environmental assessments are currently being completed on a regional scale for 

the State of Colorado, and on a more detailed scale for the 

San Juan National Forest in southwestern Colorado. Mineral 

deposit types in the San Juan National Forest vary from polymetallic carbonate replacement deposits, which tend to generate near-neutral-pH mine drainage waters with high 

dissolved metal contents, to Summitville-type Au-Cu-Ag 

deposits, which tend to generate highly acidic waters with 

extreme concentrations of Cu, Zn, As, Co, Ni, U, Th, and 

other metals. 

Uses for environmental assessments are numerous. By 

defining baseline environmental signatures for deposit types 

prior to mining, we can develop geologically realistic remediation standards for post-mining cleanup. The potential 

environmental effects resulting from the development of different types of mineral deposits can be better predicted and 

planned for by industry, regulators, and land-use planners. 

Identifying and prioritizing study of hazardous mine sites on 

public lands can be accomplished more efficiently. Finally, 

land managers can better estimate the potential environmental effects resulting from the development of undiscovered 

mineral resources contained in public lands. 

Acknowledgments. The environmental models of mineral deposit types are an outgrowth of studies by W. Ficklin, 
K. Smith, and Plumlee exammmg geologic controls on 

mine-drainage chemistry. Geologic and mineral resource 

information used in the prototype environmental assessments was compiled by S. Ludington, A. Wallace, T. Nash, 

N. Foley, R. VanLoenen, B. Moring, and G. Green. 

GEOLOGIC SETTING OF 

MINERAL DEPOSITS IN THE 

OUACHITA-MARATHON-SONORA 

OROGENIC SYSTEM ALONG THE 

SOUTHERN MARGIN OF 

NORTH AMERICA 

Forrest G. Poole 

The Ouachita-Marathon-Sonora (OMS) orogenic system is a belt of deformed Paleozoic rocks that borders the 

present southern edge of the North American craton. The 

collisional-subductional orogenic system extends 3,000 km 

from central Mississippi in the southeastern United States 

westward and southward along a sinuous trace into central 

Sonora in northwestern Mexico. The belt is concealed for 80 

percent of its known length; those exposed parts form the 

Ouachita, Marathon, and Sonora structural salients along the 

frontal margin. The OMS system is composed of a thick, 

folded and faulted sedimentary sequence of distinctive, relatively thin, lower Paleozoic eugeoclinal dark-gray radiolarian-graptolitic and siliceous mudstone, chert, sandstone, and 

limestone, and a relatively thick, upper Paleozoic dominantly olive gray flysch sequence, which includes deepwater turbidites deposited in rapidly subsiding foredeeps. 

The eugeoclinal sequence of chert, mudstone, siltstone, 

limestone, and subordinate sandstone, conglomerate, dolostone, barite, tuff, and lava represents offshelf continental 

slope and rise and oceanic deposits that were deformed and 

abducted cratonward (northward) onto the once-continuous 

southern shelf of paleo-North America in late Paleozoic 

time. During allochthon development and emplacement, 

foreland basins were formed that received synorogenic 

flysch and molasse deposits. Rocks of the three orogenic 

salients have many similarities and differ only in stratigraphic detail, structural scale, and degree of metamorphism. They are separated from each other by the Texas and 

Chihuahua promontories of paleo-North America. 

Rocks within the salients host a variety of Paleozoic 

and Mesozoic mineral deposits although ore minerals of 

commercial grades and quantities are present in only a few 

of the numerous deposits (for example, barite and vanadium in Arkansas and barite in Sonora). Syngenetic stratiform barite and some metal deposits in fine-grained rocks 

(such as manganese, vanadium, molybdenum) are related to 

seafloor environments controlled by proximity to hydrothermal vents, salinity of seawater, type and amount of 

81 

organic matter, and rates of sedimentation. Other sedimentary mineral deposits formed during diagenesis. Many layers of Devonian chert (originally opaline silica) were 

converted to novaculite (microcrystalline quartz) during 

post-depositional episodes of high heat flow. Some deposits 

formed along epigenetic veins during late Paleozoic and 

Mesozoic hydrothermal activity. In the Ouachita Mountains, late Paleozoic quartz veins generally are associated 

with small deposits of copper, lead, zinc, silver, antimony, 

and mercury. Layers and disseminated grains of pyrite/marcasite are found in barite, chert, and most fine-grained 

eugeoclinal and foreland-basin rocks. Veins in Arkansas 

containing mercury, antimony, titanium, and niobium may 

be related to Mesozoic igneous activity. 

Petroleum source and reservoir rocks are present both 

in the allochthons and in the foreland basins. Although 

rocks within the three salients exhibit variable temperature 

histories, oil and solid bitumen/asphalt are more common 

in lower Paleozoic eugeoclinal rocks and gas is more common in upper Paleozoic foreland-basin rocks. Mississippi 

Valley-type lead-zinc-barium deposits and petroleum accumulations in the Midcontinent region are probably related 

to episodic sediment dewatering and expulsion of geopressured metalliferous and petroliferous warm brines from 

deeply buried basinal black shales of the orogenic system. 

Similar deposits may exist in margins of shallow basins 

hosted by outer carbonate-shelf rocks in Texas, Chihuahua, 

and Sonora. Recognition of hydrothermal activity in many 

parts of the exposed OMS orogenic system has important 

implications for regional development of mineral deposits 

throughout the system in the southern United States and 

northern Mexico. 

SILT HEAVY-MINERAL PLACER 

DEPOSITS IN A LATERITIC 

ENVIRONMENT-RIVERS AND 

INSULAR SHELF OF 

NORTH-CENTRAL PUERTO RICO 

L.J. Poppe, J .A. Commeau, and G. Luepke 

Mineralogical studies of sediments from the Rio de Ia 

Plata, Rio Grande de Manati, rivers of the Rio Cibuco system, and the insular shelf in north-central Puerto Rico were 

conducted (1) to examine the effects of lateritic weathering 

on the silt fraction, (2) to explore the silt fraction for heavymineral placer deposits, and (3) to elucidate the active fluvial 

and shelf processes. The silt fraction, which is enriched in 

heavy minerals relative to the sand fraction, is mainly detrital 

but contains a strong authigenic component (fig. 1). The 

detrital silt heavy-mineral fraction in the rivers is dominated 

by an amphibole-garnet-pyroxene-epidote assemblage. 

Amphiboles are more common in the Rio Cibuco; pyroxenes 
82 
are more common in the Rio de Ia Plata; epidote and ilmenite 

are more common in the Rio Grande de Manati. The authigenic silt heavy-mineral fraction, which is largely a product 

of the lateritic weathering, is dominated by iron oxides 

(hematite and goethite) and altered grains (cemented mixtures of Fe, AI, Si, and Ti oxides). Grains of bladed rutile and 

leached ilmenite are common. The lateritization has dramatically altered the relative percentages of the minerals originally present in the source rocks. 

Lateral variability in silt-fraction mineralogy is considerable. Within the Rio Cibuco system, the variability is 

related to the differences in composition of the rapidly eroding source rocks. On the high-energy, wave-dominated 

shelf, variability in the heavy-mineral distributions is related 

primarily to differences in the assemblages introduced by the 

individual source rivers and to differential transport by shelf 

sorting processes. Silt heavy-mineral abundances on the 

shelf are greatest near the river mouths (where they can 

exceed 40 percent by weight) and decrease seaward. 

Significant differences are also present between the silt 

heavy-mineral assemblages in the rivers and those on the 

adjacent insular shelf, and between the heavy-mineral 

assemblages in the silt- and sand-sized fractions from these 

areas. For example, with the possible exceptions of zircon 

and magnetite, the economically important heavy minerals 

are much more likely to occur in the silt fraction. 

We detected no minerals containing significant 

amounts ofCu, Ni, Sn, or Zn in any of the river samples, and 

only traces of Cu, Ni, and Zn on the insular shelf. However, 

elevated concentrations of chromite, gold, magnetite, and 

titanium-bearing minerals occur in both the river and shelf 

samples. Most of the gold occurs as very small crystals (0.6-

7.0 J.lm) encrusted with iron oxides. This close association 

of gold with iron oxides in lateritic environments has been 

shown to occur when gold is dissolved from the parent rocks 

and is reprecipitated in the weathering crusts (Mann, 1984 ). 

Cerargyrite, which probably forms during the weathering 

process from silver-bearing sulfides, is present in the rivers 

in concentrations of up to 1.9 percent by population, but it is 

absent in the samples from the insular shelf. Presumably, the 

abundance of available chloride in the marine environment 

causes the existing grains of cerargyrite to convert to the 

much more soluble complexes of AgCI2- or AgCl3 -. 

Figure 1 (Poppe and others) (facing page). Scanning electron 

microscope photomicrographs of silt-sized mineral grains representative of those found in the river sediments of north-central 

Puerto Rico. A, Zircon grain, sample 28, Rio Dos Bocas; B. Corroded epidote grain, sample 28, Rio Dos Bocas; C, Amphibole group 

mineral, sample 1, Rio de Ia Plata; D, Chromite grain, sample RP7, 

insular shelf near Rio de Ia Plata; E, Backscatter image of an anhedral cerargyrite grain, sample 28, Rio Dos Bocas; F, Extensively 

corroded and altered potassium feldspar grain, sample 5, Rio de 

Los Negros. 

83 

REFERENCE 

Mann, A.W., 1984, Mobility of gold and silver in lateritic weathering profiles-Some observations from western Australia: Economic Geology, v. 79, p. 38-49. 

BRIEF DESCRIPTION OF THE 

GEOLOGY AND MINERAL DEPOSITS OF 

SOUTHERN SONORA, MEXICO 

Jaime Roldan Quintana 

Recent compilation of the geology of Sonora at 

1: 1 ,000,000 scale has provided new insights into the understanding of the geology of Sonora. The State of Sonora is 

located south of Arizona, and southern Sonora is herein considered to extend south from lat 28° 30' N. This region lies 

south of the Precambrian craton of North America, and no 

Precambrian rocks are here exposed. Some lower Paleozoic 

rocks are reported east of Tonichi, representing eugeoclinal 

offshelfstrata of Ordovician age. To the south, scattered outcrops of Paleozoic rocks are located in the region of 

Navojoa-Alamos, but their lithology and fossil content are 

poorly preserved, because of younger intrusions. Among 

these localities, only in El Trigo, northeast ofNavojoa, were 

reported poorly preserved fossils of probable late Paleozoic 

age. 

Triassic rocks present large exposures southeast of 

Hermosillo, where they are referred to as the Barranca 

Group. These rocks consist of quartz-feldspathic sandstones, 

red beds and coal-bearing shales rich in fossil plants. The 

middle member of the Barranca Group, the Santa Clara Formation, has been dated paleontologically as Late Triassic. 

The age of the entire group is unknown. At the southern tip 

of the State of Sonora in Sierra Sonobari, a Late Triassic isotopic age has been obtained from gneisses and amphibolites. 

However, the age of the metamorphism seems to be younger. 

No Jurassic rocks have been i4_entified in southern Sonora. 

Lower Cretaceous marine rocks occur only east of Alamos. 

Igneous rocks are the most important rock type in southern 

Sonora. Early Tertiary (62-49 Ma) granitic batholiths are 

widespread and represent the most continuous rock unit. 

Volcanic rocks consist of andesites of Paleocene age 

unconformably covered by rhyolitic ignimbrites and basaltic 

andesites of Oligocene-Miocene age. Tertiary extension has 

produced the basin-and-range structure that characterizes 

this part of Sonora. Volcanic and clastic fillings of the Baucarit Formation are common in these grabens. Undifferentiated clastic deposits and alluvium cover most of the coastal 

plain. 

The mineral deposits of southern Sonora are concentrated in two areas. One, at lat 28°20' N ., follows a general 

east-west trend about 40 km wide; it coincides with some of 
the outcrops of the Triassic rocks and large exposures of the 

Late Cretaceous-early Tertiary batholiths. In this mineralized belt are present (1) mantos of coal and graphite, (2) disseminated copper and copper-molybdenum, and (3) veins of 

silver, lead, zinc, and copper as sulfides. 

The second mineralized area is located at lat 27° N. 

around Alamos in a radius of about 50 km. Most of the 

deposits are hosted in Tertiary volcanic rocks or in intrusives. The types of mineralization are (1) skarn of tungsten 

and polymetallic sulfides; (2) disseminated copper-molybdenum deposits, of silver, lead, zinc, copper; (3) veins of silver, lead, zinc, copper with gold values (in the mines near 

Alamos, silver is present as sulfo-salts); and (4) mantos of 

graphite. Outside these mineralized areas there are a few 

scattered mines. 

Other nonmetallic minerals that have been exploited 

include rhyolitic tuffs, limestones, gypsum, halite, and construction aggregates. 

PRELIMINARY GEOLOGIC MAP OF THE 

BIG LUE MOUNTAINS 15-MINUTE 

QUADRANGLE, GREENLEE COUNTY, 

ARIZONA, AND CATRON COUNTY, 

NEW MEXICO 

James C. Ratte, William E. Brooks, and 

DanaJ. Bove 

The Big Lue Mountains 15-minute quadrangle, in 

southeastern Arizona and southwestern New Mexico, is at 

the southwestern margins of the mid-Tertiary MogollonDatil volcanic field, just east of the Morenci-Metcalf mining 

district. It also is at the northwest end of the Laramide Burro 

uplift near its intersection with the north-northeast-eastnortheast-trending Morenci lineament and Morenci Reserve 

fault zone, at the southeastern margin of the Colorado 

Plateau. 

The Big Lue Mountains quadrangle is covered largely 

by volcanic and volcaniclastic rocks of Oligocene to 

Miocene age. Total exposed thickness of the volcanic rocks 

in the quadrangle is on the order of 1,000 m, but no single 

section contains all the various extrusive and intrusive units. 

The mid-Tertiary volcanic rocks unconformably overlie Precambrian and Paleozoic rocks in the Red Hill area in the 

southwest comer of the quadrangle (fig. 1 ), presenting the 

possibility that Laramide intrusive rocks, like those that host 

the world-class copper porphyry deposits of the nearby 

Morenci-Metcalf district, could be present beneath the Tertiary volcanic rocks in the Big Lue Mountains quadrangle. 

Although no signs of significant mineralization are evident 

in the quadrangle, there are several mineralized occurrences 

in the pre-Tertiary rocks in the Red Hill area, as well as 

quartz and carbonate veins as much as 10 m wide along the 

Dix Creek fault in the north-central part of the quadrangle 

84 

(fig. 2), where manganese, silver, copper, and gold 

(0.04--0.15 ppm) were found (Ratte and others, 1982). 

The Tertiary volcanic sequence in the quadrangle in 

general decreases in age from the southwest comer of the 

quadrangle to the north, and the most complete sequence is 

exposed in the southwest-facing slopes of the Big Lue 

Mountains. The oldest rocks in the sequence in the Big Lue 

Mountains include the distal outflow facies of several 

caldera-related ash-flow tuffs (Cooney Tuff, about 34 Ma; 

Davis Canyon Tuff, 29 Ma; and Bloodgood Canyon Tuff, 

28.1 Ma) from the Mogollon Mountains caldera complex to 

the northeast. The tuffs are overlain by various amygdaloidal 

and porphyritic andesite lava flows, as much as 500 m thick, 

and a sequence of dacitic to rhyolitic flows, flow breccias, 

and pyroclastic beds at least 250 m thick, which are here 

called the volcanic rocks of the Big Lue Mountains 

(VRBLM). The VRBLM are overlain by andesitic flows that 

probably correlate with the Bearwallow Mountain Andesite. 

The Big Lue Mountains volcanic sequence (VRBLM) 

is well exposed in the southeastern part of the quadrangle 

along State Highway 78, beside Black Jack Canyon, where 

it is interlayered and intruded(?) by major masses of nearly 

aphyric rhyolite that have been tentatively dated at about 

28-25 Ma. The rhyolites include the rhyolite of Hells Hole, 

which forms an intrusive(?)-extrusive dome complex of at 

least 50 km2, mainly east of Black Jack Canyon (figs. 1, 2), 

and the north-northeast aligned rhyolite plug domes known 

as the White Peaks, in the south-central part of the quadrangle (fig. 2), as well as numerous other intrusive-extrusive 

bodies. A spectacular carapace breccia at the margin of the 

rhyolite of Hells Hole Canyon is well exposed at the viewpoint near the Needles Eye, a former narrow highway tunnel 

that has been eliminated in deference to modem traffic 

requirements on Highway 78. The carapace breccia is 

exposed intermittently beneath Bearwallow Mountain 

Andesite flows for about 8 km along Highway 78 between 

the Needles Eye and the Arizona-New Mexico State line. 

Outcrops along the highway south of the Big Lue Ranch road 

have the typical "honeycomb" vesicular texture of much of 

the rhyolite on top of the dome complex. The centimeter-size 

vesicles are lined with crystals of sanidine, niobium-rich 

pseudobrookite and ilmenite, and garnet. 

Except for some older volcanic rocks exposed in the Pat 

Mountain and Bird Canyon areas in the northwestern part of 

the quadrangle (fig. 2), the rocks in the north half of the 

quadrangle are mainly younger than those in the south half, 

and consist of Bearwallow Mountain Andesite, a bimodal 

basalt-rhyolite assemblage of Miocene age, and Gila Formation (Neogene) volcaniclastic sediments. 

Along the gorge of the San Francisco River in the northeastern part of the quadrangle, the stacked thin flows of 

Bearwallow Mountain Andesite total as much as 600 m 

thick. A major eruptive center for at least part of this flow 

sequence probably is represented by a partially exhumed cinder cone, 0.8-1.2 km in diameter, on the south canyon wall, 

on the New Mexico side of the State line (fig. 2). 
Sunset 

Peak& 

Red 

AHil 

Brushy 

Mtn& 

Chalk 

&Peak 

109'00' 

CATRON CO r<>RANTC:o0 SMILES 

f---,--1.-,--.-'-' .-''r----1---1 I I 

0 5 KILOMETERS 

1 • Big Lue Mtns quadrangle 

2 • Laramide Burro uplift Jt. 

3 • Morenci lineament NSSE §' 

4 • Morenci-Reserve raul zone N30E rJ 

~ 

NEW MEXICO 

Figure 1 (Ratte and others). Geographic index map and location map for the Big Lue Mountains 15-minute 

quadrangle, Arizona and New Mexico. 

85 
109'15' 

33'15' 

Tab 

0 

I 

0 

109'00' 

Tbg (5,.£ 

5 MILES 

I I 

5 KILOMETERS 

Figure 2 (Ratte and others). Big Lue Mountains 15-minute quadrangle showing generalized fault pattern, 

mafic dikes (crosshatched lines), silicic dikes (lines with solid dots), prospects and (or) calcite-quartz veins 

(crossed pick and hammer prospect symbol), selected eruptive centers (stars), and some major geologic bodies 

and eruptive centers, designated by numbers 1-8, as follows: 

I. Cold Spring Mountain eruptive center (uncertain mid-Tertiary age). 

2. Indian Creek eruptive center (uncertain mid-Tertiary age). 

3. Pat Mountain eruptive center (uncertain mid-Tertiary age). 

4. Bird Canyon eruptive center (uncertain mid-Tertiary age). 

5. Bearwallow Mountain Andesite eruptive center (~24 Ma). 

6. Bearwallow Mountain Andesite plug at Baldy Mountain (~24 Ma). 

7. Rhyolite plug at Grassy Mountain (~19 Ma). 

8. Prospects and (or) calcite-quartz veins. 

86 
Tbg 

EXPlANATION 

CORRELATION OF MAP UNITS 

Miocene 

TERTIARY 

Oligocene 

UNCONFORMITY 

II }PALEOZOIC 

} PRECAMBRIAN 

DESCRIPTION OF MAP UNITS 

Volcanic and volcanic:lastic rocks north of the Big Lue 

Mountains; mainly Bearwallow Mountain Andeaite and 

Gila Fonnation with interlayered basaltic lava flows 

(Miocene and Oligocene) 

Rhyolite of Mule Creek; rhyolite lava flows, domes and 

tuff rings (Miocene) 

Coarsely porphyritic andeaite intrusion (Miocene or 

Oligocene) 

Amygdaloidal, fine-grained and porphyritic andeaite lava 

flows, overlain by dacitic and rhyolitic flows and 

pyroclastic rocks of the Big Lue Mountains volcanic 

sequence (Miocene (?) and Oligocene) 

Rhyolite lava flows and intrusions and minor pyroclastic 

rocks of Hells Hole (Oligocene (?)) 

Andesitic to dacitic rocks of the Bird Canyon-Pat Mountain wlcanic center in northwest corner of quadrangle 

(Oligocene (?)) 

Oacilic to rhvoitic rocks of the Indian Creek erupliw c:erUr 

in nortiK:emal part of quadrangle (Oligocene (?)) 

Ash-flow tuffs; distal outflow of regional caldera-fonning 

tuffs. Include Bloodgood Canyon Tuff (28.0 Mal; Davis 

Canyon Tuff (29.1 Mal; and Cooney(?) Tuff (-34 Ma) 

Pre-Tertiary rocks, undivided; include Cambrian and 

Ordovician sedimentary rocks and Proterozoic granite 

(Paleozoic and Precambrian) 

* Volcanic eruptive center 

I II I I I I I I I I I I I Dike of mafic composition 

___ ..,... Dike of silicic composition 

;,;.....;....;.• ....;•;...,;,• Vein along fault 

----Contact 

_.....___.t8

_

7

_ Fault-Showing dip; bar and ball on down thrown side 

87 

The bimodal basalt-rhyolite assemblage consists 

mainly of the rhyolite domes and tuff rings of the rhyolite of 

Mule Creek in the Harden Cienega area near the northeast 

edge of the quadrangle, and "true" basalt flows and related 

dikes and small cinder cones that are interlayered with the 

volcaniclastic sediments of the Gila Formation throughout 

much of the northern part of the quadrangle. The rhyolite of 

Mule Creek is dated at about 18 Ma, and the basalts are about 

19-18 Ma. A dacite dome in the vicinity of Tennessee Creek 

in the east center of the quadrangle, near the State line, is 

about 21 Ma. 

Geologic structures in the southwestern part of the 

quadrangle are mainly faults and dikes that follow the northwest trend of the Burro uplift, which is represented by the 

small area of pre-Tertiary rocks in the southwest comer of 

the quadrangle. In the rest of the quadrangle, faults are 

mainly north-northeast- to east-northeast-trending, highangle normal faults that form a series of minor horsts and 

grabens that constitute the southwestern part of the MorenciReserve fault zone of basin-and-range age. 

REFERENCE 

Ratte, J.C., Hassemer, J.R., Martin, R.A., and Lane, M., 1982, Mineral resource potential of the Lower San Francisco Wilderness 

Study Area and contiguous roadless area, Greenlee County, 

Arizona, and Catron and Grant Counties, New Mexico: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-1463-C; scale 1 :62,500. 

TECTONIC EVOLUTION OF THE 

SOUTHWESTERN UNITED STATES 

Stephen J. Reynolds and Ed DeWitt 

The complex tectonic evolution of the southwestern 

United States resulted in a wide assortment of ore deposits 

having diverse origins. The oldest Proterozoic rocks in the 

region mostly represent island arcs and other oceanic terranes that were constructed or accreted against the Wyoming 

craton between 1.8 and 1.65 Ga. Proterozoic rocks in the 

Mojave isotopic province include an isotopic component of 

pre-2.0-Ga crust, whereas those in the Arizona isotopic province were derived directly from the mantle at about 1.78 to 

1.65 Ga. Arizona-type crust appears to overlie Mojave-type 

crust along an irregular boundary that is largely concealed. 

Another boundary separates older (pre-1.72 Ga) and younger 

(1.73 to 1.62 Ga) parts of the Arizona isotopic province, and 

influences the metallogenic signature of subsequent Phanerozoic ore deposits. Regional deformation and metamorphism culminated in major orogenies at 1.7 Ga and about 
1.65 Ga. Granites and rare-earth-element-rich alkalic plutons and carbonatite bodies were emplaced at 1.4 Ga. 

After regional uplift and erosion, Middle to Late Proterozoic sediments, mafic flows, and silicic pyroclastic rocks 

were deposited across the beveled landscape and were 

intruded by subhorizontal diabase sills at 1.1 Ga. Processes 

capable of producing mineral deposits are intrusion of sills, 

Keweenawan-age basaltic volcanism, intense chemical 

weathering of basalt flows and limestones, and widespread 

potassium metasomatism of the tuffaceous and sedimentary 

rocks. 

A cratonic setting persisted throughout Paleozoic and 

Early Triassic time, except in the western Mojave Desert 

where tectonism accompanied and succeeded truncation of 

the continental margin. Subduction beneath southwestern 

North America was underway by Late Triassic time and 

resulted in intermediate to felsic magmatism that was 

accompanied by uplift at some times and subsidence at others. A major pulse of felsic magmatism at about 160 Ma was 

accompanied by hydrothermal metasomatism that produced 

highly aluminous (quartz-kyanite) rocks. During or after this 

pulse, magmatism jumped westward, probably as an oceanic 

ridge approached the coast (analogous to mid-Tertiary orogeny?), and sinistral motion may have occurred on the 

Mojave-Sonoran megashear. In the latest Jurassic and Cretaceous, intracontinental sedimentation (McCoy Mountains 

Formation and Bisbee Group) was accompanied by 

emplacement of rift-related(?) alkalic mafic rocks. This was 

followed by regional Late Cretaceous folding, thrusting, and 

metamorphism (locally associated with synmetamorphic 

gold-bearing quartz veins) and by subsequent Laramide 

magmatism and porphyry copper mineralization related to 

low-angle subduction. Laramide compression in the Colorado Plateau and Transition Zone formed monoclines over 

reactivated basement faults. Uplift and erosion occurred 

both prior to and after Laramide magmatism, and may have 

been accompanied by Laramide tectonic denudation. 

Uplift may have continued until the middle Tertiary, 

when steepening and foundering of a low-angle subduction 

zone caused widespread magmatism and crustal extension in 

an east -northeast-west -southwest direction. Metamorphic 

core complexes and detachment faults represent gently dipping normal shear zones that penetrated to upper mid-crustal 

levels. Brines accumulating in detachment-related basins 

caused regional potassium metasomatism, leached metals 

from the affected rocks, and flowed up the detachment fault, 

forming detachment-related base- and precious-metal deposits. At 13 to 15 Ma, the mid-Tertiary style of tectonism was 

replaced by a basin-and-range style of tectonism characterized by generally east west, small-magnitude extension 

along high-angle normal faults, and fundamentally basaltic 

magmatism. 

88 

OVERVIEW OF ENVIRONMENTAL 

MINERAL RESOURCES AND RELATED 

INVESTIGATIONS IN THE GEOLOGIC 

DIVISION, U.S. GEOLOGICAL SURVEY 

Gilpin R. Robinson, Jr., and Larry P. Gough 

The importance of monitoring, characterizing, and 

remediating past and present-day mining sites in Western 

North America has led to an increase in environmental geoscience research in the USGS Geologic Division. One specific emphasis of this research is the environmental 

consequences of mineral resources exploration, development, and remediation. Much of the effort is in response to a 

growing public awareness of potential point-source contamination and land-use concerns, a need by Federal and State 

public-land trustees for published information and expert 

opinions upon which to base management decisions, and a 

need for sound scientific information to assist regulatory 

agencies in contaminated-site remediation and litigation. 

These efforts are being supported by agency funding, international cooperative funding, cooperative agreements 

between public and private agencies and organizations, and 

congressional initiatives. 

The environmental impacts associated with abandoned 

mine sites, hazardous waste locations, and natural sources of 

contamination on Federal lands are important considerations 

when developing Federal land management plans. These 

plans attempt to characterize the nature of the sites, minimize 

environmental impacts, identify appropriate remediation 

methods, and predict the effects of new mining ventures. For 

example, acid-rock drainage from natural and miningrelated sites, such as Summitville, Colo. (see Plumlee and 

others, for example, this volume), can create large, toxic 

metal-bearing water and sediment loads in streams, wetlands, and soils with widespread and long-lasting environmental impact. Such hazardous material sites on Federal 

lands must be inventoried and characterized in a comprehensive but safe manner before effective cleanup plans can be 

developed. The environmental geoscience studies in the 

Geologic Division address these issues by: 

1. characterizing the mobility of toxic elements from 

mineral sources into the soil, water, air, and vegetation; 

2. defining the natural dispersion and chemical reservoirs of potentially toxic compounds in unmined mineralized areas to establish baseline geochemical information for 

different mineral deposit types, regions, and climatic zones; 

3. investigating how the processes that control the 

mobility of toxic elements in the environment can be used in 

low-cost and low-maintenance remediation methods; 

4. developing models based on geologic setting, 

deposit type, mining method, and climate to predict the 

diverse environmental effects associated with mining 

development; 
5. developing and applying non-invasive geophysical 

and remote-sensing techniques to monitor and assess contamination; 

6. developing expert-system computer programs to 

identify the appropriate geochemical and geophysical techniques and technologies to characterize the degree and extent 

of contamination; 

7. developing geochemical and isotopic fingerprinting techniques to identify and segregate contamination 

sources. 

Examples of current USGS research in, and support 

facilities for, environmental studies include: (1) metal speciation, mobility, and rock/water interface geochemistry (with 

emphasis on As, B, Cd, Fe, Hg, Pb, S, Se, U, and Zn); (2) 

characterization of scale-related heterogeneity of geologic 

materials as they relate to hydrologic properties (using 

ground-penetrating radar and airborne geophysical techniques); (3) investigations on the relation of mine drainage 

chemistry to geology and geochemistry of ore deposits; (4) 

use of the Advanced Visible and Infrared Imaging Spectrometer (AVIRIS), and supporting laboratory techniques, to 

detect, identify, and map the distribution of hazardous materials; (5) provide geologic and mineral deposit information 

and expertise to EPA (Environmental Protection Agency) 

for use in the CERCLA (Superfund) site considerations and 

Department of Justice for use in CERCLA litigation; (6) 

studies that help quantify and characterize geologic substances perceived to have significance for human or animal 

health; (7) surficial geologic mapping studies that emphasize 

hydrogeologic features, transport corridors, and geologic 

conditions that affect ground stability and permeability; and 

(8) studies of metal mobility and environmental consequences downstream from mining operations. 

Two units established within the Geologic Division are 

designed to support, facilitate, and coordinate environmental 

studies: (1) the Analytical Chemistry Support Group, utilized by industry, government, and USGS personnel for 

technique development and for inorganic analysis of geologic and biologic materials, and (2) the Center for Environmental Geochemistry and Geophysics, which acts as a focal 

point and information transfer medium for environmental 

studies. 

PALEOZOIC AND MESOZOIC TECTONIC 

EVOLUTION OF MEXICO BASED ON 

GEOCHEMISTRY OF BASEMENT ROCKS 

J. Ruiz, E. Centeno-Garcia, P.J. Coney, 

R. Torres-Vargas, P.J. Patchett, and P. Yanez 

We have used most of the available exposures of basement rocks in Mexico, and lower crustal granulite facies 

xenoliths, to determine the isotopic and trace-element 

89 

characteristics of many of the tectonostratigraphic terranes 

that form Mexico. Basement rocks are exposed in the Sierra 

Madre, Chihuahua, Mixteca, and Guerrero terranes. Xenoliths are found in the Sierra Madre terrane, in particular. The 

samples thus represent the basement history of some of the 

largest terranes of Mexico. 

The exposed basement rocks in eastern, southern, and 

northern Mexico, found at Ciudad Victoria, Molango, Los 

Filtros, and Oaxaca, have Grenville ages of crystallization 

and present-day eNd values between -7 and -11. The samples 

represent the basement of the Oaxaca terrane, parts of the 

Sierra Madre terrane, and the Chihuahua terrane. Most of the 

samples collected, which include ortho- and paragneisses, 

show a remarkably consistent 1.6-1.3 Ga model Nd crustal 

extraction age from a depleted mantle. This model age is 

similar to other Grenville-age outcrops in Texas (Llano, Van 

Hom, and Franklin Mountains), in Virginia, and in New 

York (Adirondack Mountains). Farther north in the Grenville province of Quebec, the model Nd ages are older, demonstrating the involvement of Archean rocks in their 

evolution. South of Mexico, geochemical analysis of Grenville-age rocks in Colombia is insufficient to make significant comparisons. It is important to note, however, that none 

of the Grenville-age outcrops in Mexico could have come 

from much farther north than the Adirondacks because of 

their model Nd ages. 

To the west of the basement exposures in northern 

Mexico are numerous xenolith localities, where lower 

crustal granulite facies rocks have been brought to the surface by alkalic basalts of Cenozoic age. The xenoliths are 

generally similar to the exposed basement rocks in their isotopic chemistry, with the exception that many have Paleozoic model Nd ages. In the south, the polymetamorphic 

Acatlan Complex, which is the basement of the Mixteca terrane, has Devonian (380 Ma), Carboniferous (290 Ma), and 

Jurassic (160 Ma) ages. We believe that all the exposures 

just discussed were formed and deformed by Proto-Atlantic 

tectonic events. 

The westernmost basement rocks studied here are part 

of the Guerrero terrane. Briefly, the rocks are polymetamorphosed metasediments and basalts with MORB (middle oceanic ridge basalt) geochemistry. The age of sedimentation is 

thought to be Triassic, based on fossil data. The geochemistry of the sedimentary rocks indicates that they were formed 

from recycled sediments of Grenville age-type rocks, but 

they also contain chert. Unconformably overlying these 

rocks are Jurassic-Cretaceous arc rocks with positive eNd 

values, which indicate that the magmas did not sample old 

continental crust. The Guerrero terrane and its basement 

were formed by accretionary processes related to Cordilleran 

tectonics. 

It appears that the Paleozoic evolution of Mexico was 

dominated by tectonic events related to the opening and closing of the Proto-Atlantic Ocean. The Mesozoic evolution of 

Mexico was dominated by Cordilleran tectonic events. The 
flipover occurred after Devonian but before Permian times, 

based on the geochemistry of Permian-Triassic granitoids. 

ENVIRONMENTAL GEOCHEMISTRY OF 

ACTIVE AND EXTINCT HOT SPRING 

MERCURY DEPOSITS IN THE 

CALIFORNIA COAST RANGES 

James J. Rytuba and William R. Miller 

Numerous hot springs and fumaroles in the central 

Coast Ranges of California are actively depositing mercury 

and associated trace metals. The hot springs and fumaroles 

are present in and adjacent to mercury deposits in the north 

end of the 500-km-Iong Coast Range mercury belt. The mercury deposits in the northern part of the belt are generally 

younger than 0.5 Ma, and the hot spring and fumarolic activity reflects the waning stage of mercury mineralization. 

These geothermal systems are related to Pleistocene to 

Holocene volcanism associated with development of a slab 

window as the Mendocino triple junction migrated northward along the coast of California. The mines produced mercury primarily from 1870 to 1950; they are now inactive, 

except for the Manhattan mine, which produces gold and 

minor mercury (active workings now called McLaughlin 

mine). The Sulphur Bank mine (fig. 1), the fifth largest 

mercury producer in the United States (130,000 flasks), is an 

Environmental Protection Agency Superfund Site; and reclamation efforts are focused on limiting mine tailings and 

mine waters from entering Clear Lake, a major recreational 

area. These mitigation measures are complicated by the 

ongoing deposition of mercury and associated elements from 

hot springs (fig. 2) and fumaroles in and around the open pit 

excavated to mine the mercury deposit. Mercury vapor venting from the fumaroles is in part fixed as cinnabar by native 

sulfur generated from bacterial reduction of sulfate. The 

remainder of the mercury vapor is discharged into the atmosphere and forms a mercury plume extending from the 

deposit. Elevated levels of mercury in soils adjacent to the 

mine area reflect naturally occurring mercury derived from 

the fumaroles as well as mercury released during past 

mining. 

Hot springs in the Sulphur Creek mercury-gold district, 

located 20 km northeast of the Sulphur Bank mine, occur in 

and adjacent to previously mined mercury and gold deposits. 

Tailings and excavations associated with previous mining 

activity contribute metals to the drainage basin, but the major 

component of metals is derived from precipitates from the 

hot springs. The precipitates are fine black silt and clay composed of (in decreasing abundance) smectite, chalcedony, 

calcium-magnesian carbonate, pyrite, magnetite, rutile, and 

cinnabar. Anomalous concentrations of trace metals in the 

Figure 1 (Rytuba). Sulphur Bank Mercury mine with Mount Konocti, a 0.6 to 0.4 Ma composite 

volcano, on the skyline. The open pit mine is partially filled with water and fumaroles depositing cinnabar and native sulfur vent from the walls of the open pit (right side) above the water level. In the 

near foreground of the lake, fumarolic vents below the water level form disturbed gray areas on the 

water surface. 

90 
Figure 2 {Rytuba). Hot spring vent at the Sulphur Bank mine exposed on the floor of the open pit. 

Black precipitate is composed of pyrite and cinnabar. This vent area is now submerged by the lake 

that fills the open pit (see fig. 1). 

precipitate include gold (maximum value, 1.3 ppm), tungsten (50 ppm), cobalt (150), nickel (1,500), chromium 

(3,000), and titanium (3,000). Mercury content of the hot 

spring fluid is anomalously high, 10-20 ppb, indicating that 

only part of the mercury is deposited in the subsurface and 

that mercury in solution directly enters waters in the drainage basin. Cinnabar grains up to 8 Jlm in diameter in the precipitate were probably deposited at depth in the hot spring 

vent and transported to the surface as particulate grains. 

These enter the drainage basin and are dispersed downstream. Other elements with elevated contents in the highchloride (2.9 percent), alkaline, hot spring fluid include lithium (maximum content 16 ppm), titanium (600 ppb), bromine (210 ppm), antimony (100 ppb) and tellurium (25 ppb). 

Extremely high concentrations of tungsten (3,200-7 ,400 

ppb) are present in the hot spring fluids, the highest concentration being in fluid from the Elbow hot spring. These tungsten values are some of the highest known naturally 

occurring concentrations of tungsten in water. In these alkaline fluids, tungsten is probably present as tungstate anion. 

Only part of the tungsten is deposited near the hot spring 

vent; the remainder enters waters in the drainage basin, and 

elevated levels of tungsten (260-680 ppb) occur in the 

stream water up to several kilometers from the hot spring 

discharge site. In the Sulphur Creek district, active hot 

springs contribute the primary component of particulate 

mercury and solutes of mercury, tungsten, and other metals 

to waters in the drainage basin. Previous mining activity contributes only a minor component. Similar dispersion patterns 

91 

of metals can be expected adjacent to hot spring type mercury deposits elsewhere in the Coast Range mercury belt, 

and these patterns provide a base line to measure the effect 

of mining activity on the environment where natural processes contribute a major component of metals. 

GEOLOGIC MAP OF A LATE 

CRETACEOUS CALDERA AND RELATED 

PORPHYRY COPPER-MOLYBDENUM 

ORE DEPOSITS IN THE SILVER BELL 

MINING DISTRICT, ARIZONA, AND 

THEIR SUBSEQUENT TERTIARY 

STRUCTURAL HISTORY 

David A. Sawyer 

Geologic mapping at 1:24,000-scale in the Silver Bell 

and West Silver Bell Mountains provides evidence for a 

close temporal and spatial relationship between caldera volcanism, plutonism, and porphyry copper ore deposition. 

Intrusion of the precaldera biotite granite has been dated at 

72.7±1 Ma (concordant U-Pb age on zircon, E.W. James, 

written commun., 1984). This was followed by eruption of 

tuff of Confidence Peak (TCP-equivalent to dacite porphyry of the previous literature), concurrent subsidence 

along the ring-fracture zone of the Silver Bell caldera, and 
formation of caldera-collapse megabreccia composed of 

Paleozoic sedimentary rocks. The tuff of Confidence Peak 

accumulated to a intracaldera thickness of > 1.2 km in the 

central Silver Bell Mountains, while a thin outflow sequence 

of TCP was deposited in the West Silver Bell Mountains. 

Postcaldera volcanism produced andesite and dacite lava 

flows and domes, followed by deposition of the Cat Mountain Tuff (equivalent to Mount Lord Volcanics of previous 

literature), which caps the local volcanic sequence. The 

TCP, postcaldera andesites and dacites, and Cat Mountain 

Tuff in the Silver Bell Mountains are indistinguishable in 

age from the 73-Ma biotite granite. Quartz monzodiorite, 

granodiorite, and monzodiorite porphyries were intruded at 

69 Ma (K-Aron biotite) as stocks and dikes along the ringfracture zone of the Silver Bell caldera. Hypogene stockwork Cu-Mo mineralization took place in the apex of complex multiphase intrusions in the Oxide pit area, the El Tiro 

pit area, and at North Silver Bell, where ring-fracture intrusions intersect major northeast-trending quartz monzodiorite 

porphyry dike swarms. Significant Cu skarn ore formed in 

megabreccia blocks of Paleozoic sedimentary rocks in the 

caldera fill adjoining the ring-fracture intrusions. Later erosion, oxidation, and leaching of hypogene copper deposits 

formed secondary chalcocite ore that comprises most of the 

>90 Mt (million tons) of 0.8 percent Cu-0.022 percent Mo 

(average grade) ore mined in the Silver Bell district. 

Beginning in latest Cretaceous time and continuing 

through the Tertiary, regional faulting and tilting deformed 

the Cretaceous volcanic and plutonic rocks and their associated ore deposits, and sporadic volcanism took place during 

Oligocene-early Miocene time. Cretaceous plutons and volcanic rocks were cut by west-northwest-trending strike-slip 

faults north and south of the Silver Bell Mountains. To the 

north, the Ragged Top fault offset a Cretaceous granodiorite 

porphyry pluton and placed it in contact with late Precambrian granite and Apache Group (Middle Proterozoic) sedimentary rocks. Offset on the Ragged Top fault was dextraloblique having a minimum lateral separation of 13-26 km, as 

geologic histories cannot be matched across the exposed 

length of the fault. Movement on the fault occurred after 70 

Ma but predated the 26-Ma Ragged Top rhyolite lava dome 

which fills the fault zone. Several west-northwest-trending 

strike-slip faults also separate the Silver Bell Mountains 

from the Waterman Mountains to the south. Clockwise vertical-axis rotation of 30 degrees has been documented in the 

Cretaceous volcanic and plutonic rocks of the Silver Bell 

Mountains by paleomagnetic studies; these data are consistent with subregional dextral strain. Both Cretaceous volcanic rocks and ore deposits were also tilted 30 degrees to the 

east-northeast, affecting elevations of alteration and mineralization zoning in the ore deposits. Mid-Tertiary magmatism 

in the Silver Bell Mountains was limited to emplacement of 

the Ragged Top rhyolite dome and related north-northwesttrending dikes, and to eruption of basalt to andesite lava 

flows from 28 to about 20 Main the eastern part of the Silver 

92 

Bell Mountains, and in the West Silver Bell Mountains. 

Postvolcanic Neogene faulting cutting through the Silver 

Bell Mountains was minor in nature, but the faulting was 

more significant in uplift of the Roskruge Mountains-Silver 

Bell-Picacho Peak regional horst relative to subsidence of 

pre-Neogene rocks into the adjoining Avra Valley to the east 

and the Aguirre Valley to the west. 

CONVERSION OF THE 

MINERAL RESOURCE ASSESSMENT 

PORTFOLIO OF THE REPUBLIC OF 

COSTA RICA TO A DIGITAL 

GEOGRAPIDC INFORMATION SYSTEM 

Paul G. Schruben and Robert Swierk 

In 1986 and 1987, the U.S. Geological Survey, Direcci6n General de Geologfa, Minas e Hidrocarburos, and the 

Universidad de Costa Rica conducted a mineral resource 

assessment of the Republic of Costa Rica. The results were 

published as a large 80x50 em color portfolio (U.S. Geological Survey and others, 1987). The 75-page document consists of maps as well as descriptive and interpretive text in 

English and Spanish covering physiographic, geologic, 

geochemical, geophysical, and mineral site themes as well as 

a mineral resource assessment. The spatial data layers consist of the following: 

1. Physiographic base map at a scale of 1 :500,000 

with hypsography, place names, and drainage; 

2. Geologic map at a scale of 1 :500,000; 

3. Regional geophysics, including gravity, aeoromagnetic, and seismicity maps at various scales; 

4. Mineral sites map at a scale of 1 :500,000 showing 

mines, prospects, and occurrences; 

5. Volcanological framework of the area important 

for epithermal gold deposits, at a scale of 1: 100,000; 

6. Rock sample locations, mining areas, and vein 

locations for several parts of the country; 

7. Permissive areas delineated for selected deposit 

types. 

Conversion to a geographic information system (GIS) 

was undertaken to facilitate the presentation and analysis of 

the data. GIS techniques allow combining and overlaying of 

layers to analyze spatial relationships not readily apparent in 

the standard paper publication. For instance, themes such as 

aeromagnetics and rock type can be combined to help define 

permissive tracts for undiscovered deposits. As part of the 

original publication effort, many of the layers were compiled 

in a USGS-authored map-editing program called GSMAP 

(Selner and Taylor, 1987). Conversion from GSMAP to 

ARC/INFO was accomplished through the use of GSMARC 

(Green and Selner, 1988). The ARC/INFO "adjust" 
g Qt-...ry lllhMil 

m:ml Qt-...ry proximal- -

B ~--- ~ and Pliocene lgnlmbrttfB 

-~and Pliocene lntegmben vo-

~ Tertlory volamlc 

!limm Te!llary Intrusive 

- liccene to Qua!Bnary YO- oedlmenlary 

~ er.tuous to Pliocene ---oedlmenlary 

m:m er.tuous to Qual2mary deep-- oedlmenlary 

Pacific 

Ocean 

Can"bbean 

Sea 

O>rn!latlon of Map Unlls 

--- Conlact 

----------- -of mapping 

---- and pholoHneamenls 

~~~ lbru&tfoull 

~~~ Olldem I!agln 11111r& 

Figure 1 (Schruben and Swierk). Geologic map of Costa Rica. 

command was used to apply a rubber-sheet correction to a 

projection problem of unknown origin. The arcs and polygons were tagged using Alacarte (Wentworth and Fitzgibbon, 1991). The data are presented under Arc View. 

93 

The basement of Costa Rica is composed of Jurassic to 

Eocene oceanic crust, which is overlain by Cretaceous and 

Tertiary marine deposits that accumulated in a complex 

series of basins. Uplift during Miocene and later time formed 
the present isthmus and was accompanied by widespread 

igneous activity related to subduction along the Middle 

American trench. The resulting volcanic rocks cover more 

than half of present-day Costa Rica. The Tertiary volcanic 

rocks of the gold belt are the coalesced products of several 

discrete volcanic centers and consist of interlayered andesitic lava flows and fragmental rocks. 

The known deposit types of Costa Rica include precious-metal Sado-type quartz-adularia vein, placer gold, 

podiform chromite, volcanogenic manganese, Cyprus-type 

massive sulfide copper-zinc, and lateritic bauxite. The most 

important Sado-type deposit is the Bellavista deposit, with a 

geologic resource of more than 32 t (1.06 million oz) of gold. 

Undeveloped deposit types include hot-spring gold, 

polymetallic vein, porphyry copper, and porphyry-related 

skarn. Speculative deposit types include alkaline rockrelated Lihir Island type (Ludington and Bagby, in press), 

zinc-lead skarn, hot-spring mercury, and hematitic copper. 

REFERENCES 

Green, G.N., and Selner, G.I., 1988, GSMARC-A program and 

procedure to convert GSMAP data bases into ARCnNFO coverages: U.S. Geological Survey Open-File Report 88-403A 

and B, 16 p. 

Ludington, Steve, and Bagby, W.C., in press, The outlook for volcanic-hosted gold deposits in the Republic of Costa Rica, in 

Symposium on Energy and Mineral Potential of the Central 

American-Caribbean Region: New York, Springer-Verlag. 

Selner, G.I., and Taylor, R.B., 1987, GSDRAW and GSMAP version 4.0: U.S. Geological Survey Open-File Report 87-496A, 

B, C, and D, 90 p. 

U.S. Geological Survey, Direcci6n General de Geologia, Minas e 

Hidrocarburos, and the Universidad de Costa Rica, 1987, Mineral resource assessment of the Republic of Costa Rica: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

1-1865, 75 p. 

Wentworth, C.M., and Fitzgibbon, T.T., 1991, Alacarte user manual, version 1.0: U.S. Geological Survey Open-File Report 

91-587, 267 p. 

AUDIO-MAGNETOTELLURIC AND 

GRAVITY STUDY OF THE 

MOGOLLON MINING DISTRICT, 

SOUTHWEST NEW MEXICO 

R.M. Senterfit, R.J. Kamilli, G.A. Abrams, 

D.P. Klein, and J.C. Ratte 

Electromagnetic induction (AMT) and gravity data are 

used to infer subsurface structure and lithology of the Mogollon mining district in southwest New Mexico. The geoelectric AMT data were obtained using distant energy sources, 

94 

mostly of natural ongms in the audio-magnetotelluric 

frequency range of 4.5-27,000 Hz. The data for each station 

consist of scalar electromagnetic resistivity measurements at 

discrete frequencies for two orthogonal magnetic and electric 

field pairs. A gravity measurement with a Lacoste-Romberg 

gravimeter was made at each AMT station location, and incorporated with additional gravity data from the University of 

Texas at Austin and the U.S. Defense Mapping Agency. This 

work was done by USGS geophysicists, Denver, Colo., in 

cooperation with the Sierrita-Mogollon transect project, Tucson, Ariz., field office. 

The buried structural margin of the Oligocene resurgent 

Bursum caldera was imaged in three AMT profiles made in 

the gold-silver-copper-producing Mogollon mining district. 

The Queen vein zone in the Mogollon district is a high-resistivity feature, probably because it localizes massive quartz 

and calcite (±gold-silver-copper minerals) in veins as much 

as 30m wide, and massive, silicic rhyolite intrusions. To late 

1993, all mineral production in the mining district has been 

to the west of the Queen fault and vein zone. This zone, a reactivated segment of the caldera structural wall, is expressed as 

a high-resistivity anomaly ( 400 ohm-m or more) that is about 

500 m wide and possibly extends to a depth of2 km (fig. 1). 

Low-resistivity zones on either side of the Queen vein correlate with intense propylitic alteration, confirmed by coredrilling, related to the formation of the vein. At depths of 0.5 

to 1.5 km, the resistivity decreases to less than 100 ohm-m 

for a width of about 500 m on both the east and west sides of 

the vein. This relatively narrow symmetrical resistivity anomaly is centered in a 5-km-wide zone of moderate-resistivity 

material (250-400ohm-m) that is bounded by units with resistivities exceeding 400 ohm-m on the east and west. This broad 

250-400 ohm-m zone is interpreted to represent the extent of 

the hydrothermal system that gave rise to the Mogollon mining district. Because a low-resistivity zone is present on both 

sides of the fault, mineralization may be as likely on the east 

side of the Queen fault. This characteristic resistivity pattern 

across the mineralized area becomes weak and incomplete 3 

km north of the main area of mineralization, disappearing 

about 6 km to the north. 

Gravity measurements made along the AMT profiles 

show a dense body, approximately 10 km wide and 2 km 

below the surface, underlying the Queen fault zone about 5 

km to the east and west. This body may have driven the 

hydrothermal system that caused the mineralization in the 

Mogollon mining district. The gravity data show no discernable density contrast across the Queen fault zone, whereas 

the fault zone produces a significant AMT anomaly. 

Alongside geologic considerations, AMT and gravity 

measurements provide evidence that the part of the Mogollon mining district east of the Queen fault, although it has 

never had any mineral production, may have potential for 

mineralization. 
3 

WEST A 

Queen fault 

and vein zone A' EAST 

Range Front 30 29( 27 28 

en 

a: 

w 

tw 

2 

:I 1 

0 

...J 

~ 

! 

~ 

tIL. 

w 

c 

-1 

-2 

fault 

I 

0 2 3 4 5 6 7 8 

DISTANCE, IN KILOMETERS 

Figure 1 (Senterfit and others). An east-west resistivity section across the Mogollon mining area. Sections are based on the geometric 

average of the two scalar resistivity soundings taken at each site. Contours are in ohm-meters, with logarithmic contour intervals. Contour 

lines are dashed where depths are uncertain. Numbered solid triangle, AMT sounding location. 

TRENDS IN MINERAL EXPLORATION IN 

LATIN AMERICA 

Douglas B. Silver 

The emigration of United States and Canadian companies into Latin America is not a fad, but a trend which both 

offers the salvation for many mining companies and heralds 

the demise of industries fundamental to most industrial economies. The exodus is not caused by a single event but by the 

coalescence of several structural changes to the business fabric. Companies are moving from their original locations or 

even countries in response to evolving environmental regulations and reforms to the traditional Mining Law of 1872. 

Simultaneously, some Latin American countries are 

being led by politicians trained in many of the West's best 

business schools. These emerging countries realize that the 

answers to their economic problems depend on their abilities 

to build (or rebuild) their country's infrastructure, increase 

exports and build support for their currencies. An obvious 

way to achieve these ambitious tasks is to develop their natural resources, because these assets tend to be situated in 

95 

more remote areas of the country. In the course of developing a new oil field or mineral deposit, the operator will 

upgrade or create the infrastructure necessary for operating 

on an internationally competitive basis. Consequently, the 

country receives foreign investments, jobs, tax revenues, and 

economic stability, all provided by some of the most technologically advanced and profitable companies in the world. 

Approximately 10 percent of the 3,000 North American 

mining and exploration companies are currently operating in 

Latin America. However, this seemingly small representation includes most of the major and intermediate metal producers, and, therefore, most of the available funding. 

Although political boundaries continue to act as the first filter for country selection, these boundaries are slowly giving 

way to geologic potential. Curiously, the political deterrents 

to mineral exploration of a few short years ago in Latin 

America are now considered equal to the perceived and real 

costs of complying to the growing list of environmental 

regulations in the United States or Canada. 

Mexico and Chile are the dominant countries of choice. 

Mexico offers a southern extension to the bountiful Great 

Basin in the western United States. Companies such as 
Phelps Dodge are operating gold mines in this high-potential 

land. Kennecott and Placer Dome continue to expand 

reserves on their recent (and rumored) one million+ gold discoveries. Eldorado Gold Corporation, a newly formed Vancouver company, is pressing ahead with imminent 

production from its La Colorada gold project, as is Hecla 

Mining. Union Miniere acquired the enormous Cananea 

copper mine. International Curator snatched up the high 

grade (17 million short tons at 3 percent Cu) Kupferschieferstyle Boleo deposit. 

Porphyry copper and gold are also the metallic incentive for going to Chile and, to a lesser extent, to Argentina. 

MINORCO paid $190 million for a third interest in the Collahuasi copper project. Cominco and Teck are building Quebrada Blanca mine. Lac Minerals continues to find gaudy 

gold at El Indio. Bema Gold, Amax Gold, and Dayton Mining are trying to develop their large gold deposits. Placer 

Dome is everywhere as it develops the Zalvidar copper mine. 

The list of bidders for El Abra represents a who's who in 

world mining. Cyprus and Lac Minerals were the winners, 

having paid $404 million for a 51 percent interest, plus committing to their share of a $1 billion capital investment. 

The Maricunga gold belt, along the Chilean-Argentinian border, has been presently recognized to extend into 

Argentina. Previously known but unavailable porphyry copper-gold deposits are candidate targets. American Resource 

Corporation leads the pack in that it has acquired options on 

approximately one dozen known porphyry systems. International Musto is developing the massive Bajo El Alumbrera 

copper deposit with its 8 million ounces of gold and 6+ billion pounds of copper. Meanwhile, Crown Resources Corporation, one of the most successful United States exploration 

companies, has taken large concessions along the border 

zone. 

Copper is also being explored for in the Caribbean 

region and Central America. USMX acquired an interest in 

Cala Abajo, and Adrian Resources and Minnova are advancing the development of the Petaquilla copper porphyry. 

Miramar Mining, Joutel Resources, and Minnova are independently looking for copper and gold in Cuba, while Mallon Minerals develops the Rio Chiquito gold project in Costa 

Rica. Canyon Resources and Battle Mountain Gold Company are jointly developing the El Higo gold discovery in the 

Dominican Republic. 

Bolivia, Colombia, and Peru have experienced an 

influx of new capital for mineral exploration and development. Battle Mountain's success at Kori Kollo in Bolivia is 

as impressive as Newmont Mining's work in Peru. Less 

development is taking place in Ecuador, for reasons 

unknown; however, Armeno Mines and Kookaburra are 

presently exploring the Chaucha and Peggy porphyry 

systems. 

To the east, aluminum bauxites carry the day. However, 

Golden Star Resources periodically reminds investors of 

possible gold potential in Suriname. Cambior' s Omai gold 

96 

mine in Guyana has considerable promise on paper, and their 

technical people have performed like magicians, exceeding 

expectations in opening up this mine. 

Placer Dome's announcement of a major discovery in 

the Kilometer 88 area of Venezuela has sparked a land rush 

akin to those which developed California and the Yukon. 

But today's rush is not likely to create a world-class mining 

district because the players appear to have a preference for 

the performance of their share price rather than the technical 

quality of their properties. 

For the most part, Brazil is being ignored by the northem companies because its foreign repatriation of profits and 

its export policies are not competitive with those of its neighboring countries. The mineral potential of Paraguay and Belize is too speculative to interest most miners, but Uruguay 

has its first gold mine, developed by American Resource 

Corporation. 

As Latin American countries successfully vie for 

United States and Canadian technology and funding, the 

marketplace will become more competitive leading to 

resource acquisition prices comparable to those of North 

America. This will lessen the role of the United States and 

Canadian companies as primary metal producers while 

transferring wealth to Latin America. 

TOURMALINITES AND STRATA-BOUND 

MINERAL DEPOSITS-GEOLOGY, 

GENESIS, AND EXPLORATION 

APPLICATIONS 

John F. Slack 

Exploration and research over the past decade have 

documented a close association of tourmalinites with a variety of strata-bound and generally stratiform mineral deposits. Tourmalinites are strata-bound rocks that contain 20 

percent or more tourmaline by volume. They occur in 

diverse metamorphic terranes but are most common in clastic metasedimentary sequences of Proterozoic and early 

Paleozoic age. Principal host lithologies are pelitic schist, 

quartzite, and metagraywacke; metabasaltic amphibolite and 

metamorphosed felsic volcanic and (or) volcaniclastic rocks 

are present in some tourmalinite-bearing sequences. 

Tourmalinites locally are interbedded with coticule (finegrained spessartine-quartz rock), metachert, and (or) magnetite iron-formation. In weakly metamorphosed terranes 

(greenschist grade or lower), tourmalinites typically are fine 

grained ( <1 mm) and may be easily misidentified as chert or 

carbonaceous/graphitic siltstone. In terranes of higher metamorphic grade, the tourmaline generally is coarser grained 

(> 1 mm) as a result of metamorphic recrystallization, 

producing tourmalinites that may superficially resemble 

amphibolite. 
Most tourmalinites are bedded units <1 m thick that are 

traceable along strike for tens to hundreds of meters. In complexly deformed terranes, some tourmalinites have been 

mapped at the same stratigraphic horizon for as much as several kilometers. Structural studies in a few areas have identified metamorphic fabrics that document growth of 

tourmaline prior to the earliest deformation. Where present, 

synkinematic to postkinematic granites and pegmatitessome containing tourmaline--commonly postdate tourmalinite formation. Such field relations indicate that, in most 

cases, the formation of tourmalinites took place prior to 

deformation, metamorphism, and the emplacement of local 

granitic intrusions. However, because tourmalinites may 

also form during regional and contact metamorphism, care 

must be taken in determining their origin relative to hydrothermal, metamorphic, and granitic processes. 

Tourmalinites are metallogenically significant because 

of their spatial association with ore deposits containing leadzinc-silver (Broken Hill, Australia; Sullivan, British Columbia), copper-zinc (Prieska, South Africa; Elizabeth, Vermont), gold (Golden Dyke dome, Northern Territory), 

cobalt-copper-gold (Blackbird, Idaho), tungsten (Felbertal, 

Austria), and uranium (Rum Jungle, Northern Territory). In 

these and similar deposits, tourmalinites typically occur 

either in the immediate wall rocks, or as distal lateral equivalents to the ores. Concentrations of tourmaline are also 

present in the footwall feeder zones of some massive sulfide 

deposits (Kidd Creek, Ontario), although these deposits lack 

associated tourmalinites. 

Field relations and geochemical studies indicate that 

tourmalinites form by the replacement of aluminous sediments or volcanic rocks through chemical reaction with 

boron-rich hydrothermal fluids. The formation of most tourmalinites apparently takes place below the seafloor by the 

selective replacement of clays and feldspars; cross-stratal 

permeability is provided mainly by growth faults. Tourmalinites considered to be of exhalative origin are believed to 

form by reaction of a boron-rich brine pool with aluminous 

sediments at or near the sediment-seawater interface, in 

which the boron and some of the iron and manganese (and 

perhaps the silicon) in the tourmalinites comes from exhalative hydrothermal fluids. Initial growth of tourmaline occurs 

either on the seafloor or during early diagenesis. In 

manganese-garnet -bearing exhalative tourmalinites, some 

tourmaline may form from precursor manganese-borate 

minerals such as sussexite [MnB02(0H)] and (or) jimboite 

[Mn3B206], based on the observed tourmalinite-coticule 

connection in many deposits (Broken Hill, Sullivan) and on 

the occurrence of these minerals in bedded manganese 

deposits of Switzerland and Japan. 

Electron microprobe studies indicate that the compositions of ore-related tourmalines fall mainly along the schorl 

(iron-rich}-dravite (magnesium-rich) solid-solution join, 

with a tendency for magnesium-rich tourmalines to predominate. In hydrothermal systems, magnesium-rich tourmalines 

97 

may form either by premetamorphic replacement from seawater-derived fluids under high fluid/rock conditions, or by 

sulfide-silicate reactions during metamorphism. Tourmalinization under low fluid/rock conditions (for example, distal 

tourmalinites) generally produces tourmalinite bulk compositions that are similar to those of tourmaline-free (unreplaced) country rocks, except for major gain of boron and 

loss of potassium; in contrast, the compositions of tourmalinites that form under high fluid/rock conditions (for example, footwall feeder zones) are generally different from those 

of the host rocks, being controlled mainly by the chemistry 

of the hydrothermal fluid. Stable isotope analyses of tourmalines (BllB, atso, BD) in these types of systems suggest 

boron derivation from clays and feldspars in footwall sediments and (or) felsic volcanic rocks, and in some cases from 

evaporite borates, with evolved seawater being the major 

fluid agent. 

Tourmalinites may be useful prospecting guides for 

a variety of strata-bound mineral deposits. In the exploration for largely syngenetic deposits, tourmalinites with a 

major exhalative component must be distinguished from 

those that formed significantly below the seawatersediment (or -volcanic) interface. The former type commonly is closely associated (or in contact) with chemical 

sediments such as coticule, metachert, and iron-formation, 

and can be an effective time-stratigraphic marker in district exploration. The presence of sulfide minerals, gold, 

and anomalous metal concentrations (such as copper and 

zinc) in tourmalinites are obvious prospecting guides. Also 

important are abundant manganese-rich garnets, and possibly elevated contents of iron (nonsulfide) and phosphorus, 

as indicators of a brine-pool origin. Magnesium-rich tourmalines [::::Fe/(Fe+Mg)<0.3], many of which are easily 

identified by their brown color in hand specimen, are an 

additional guide; black tourmalines may also be magnesian, however, and electron microprobe analyses are necessary to accurately document tourmaline compositions. 

Proton microprobe studies, currently underway, suggest 

that the trace-element composition of the tourmalines can 

predict the metallogeny of associated mineralization, and 

may be useful in regional (and district) exploration. 

U.S. GEOLOGICAL SURVEY MINERAL 

RESOURCE STUDIES ON INDIAN LANDS 

Bruce D. Smith and E.A. Merewether 

The U.S. Geological Survey (USGS) has done both 

regional and site-specific energy and mineral studies of 

Indian lands. Regional studies are conducted mainly in 

USGS internally funded programs that cover a broad base of 

Federal, State, and Tribal government lands. Through a 

memorandum of understanding with the Bureau of Indian 
Affairs (BIA) Division of Energy and Mineral Resources 

(DE&MR), the USGS receives funds for three categories of 

activities in support of resource studies of Indian lands. 

First, BIA funds are provided for cost-effective sharing of 

computer assets that pertain to development and utilization 

of USGS digital energy- and mineral-resource data bases. 

Second, BIA funds support OUTREACH activities for 

USGS interaction with Tribal governments in organization 

and public presentation of resource data. Third, funds are 

provided for site-specific studies of Indian reservations. 

More recently, BIA-funded USGS projects have led to 

public presentations such as at the Northwest Mining Association (NWMA) annual meeting (Manydeeds and Smith, 

1991; Manydeeds, 1992). The DE&MR and USGS coordinate these presentations with Tribal governments, who often 

send representatives to the meetings. The following summarizes some of the results given at these public meetings. 

The Annette Islands Reserve: is located in southeast 

Alaska near the city of Ketchikan. The USGS mineral 

assessment project consisted of integrated geophysical (airborne and ground), geologic, and geochemical studies. 

Ground surveys were made at selected areas identified from 

the airborne geophysical surveys. A qualitative mineral 

resource assessment has been made that concludes a moderate potential exists for gold-quartz and polymetallic vein 

deposits. Detailed results of the project have been presented 

at the December 1993 NWMA meetings along with a publication of a monograph (Godwin and Smith, 1993). 

The Goshute Reservation: is located on the NevadaUtah border south of the town of Wendover, Nev. Geologic, 

geochemical, and geophysical studies have been conducted 

for the entire Reservation. Significant results of these studies 

reported by Nutt and others (1992) are as follows: 

1. Discovery of a jasperoid unit identified with alteration indicative of possible association with igneous rocks; 

2. Anomalous geochemical indicator elements for 

possible gold mineralization have been found to be associated with the jasperoid, and the anomalies extend outside of 

known prospects; 

3. Gravity and electromagnetic studies have been 

used to interpret the subsurface configuration of igneous 

rocks, thickness of pediments, and general nature of subsurface lithologies; 

4. Various methods to enhance satellite remote sensing data have been used to identify areas of anomalous alteration and to enhance linear features extending beyond 

known areas of mineralization. 

The Red Lake Indian Reservation: covers more than 

837,000 acres in north-central Minnesota. Most of the Reservation consists of discontinuous tracts of land in sparsely 

inhabited swampy areas. A mineral resource assessment of 

these scattered tracts of land has been completed based on 

existing USGS data, supplemented by new data and interpretations tailored to the needs of the Red Lake Tribal government for land-use planning purposes. These studies have 

98 

resulted in a qualitative assessment of mineral resources 

(base-metal massive sulfide and associated precious-metal 

deposits, iron-formation, copper-nickel-platinum group element (PGE) deposits, shear-zone gold) ranging from low to 

high for the Indian lands (Klein and others, 1992). 

The Rocky Boy's Reservation: is located in north-central 

Montana south of the city of Havre. This project entailed a 

geologic and geochemical study of a known area of mineralization in the southeast part of the Reservation, concentrating on possible gold deposits. Geologic studies have 

identified four major styles of gold mineralization that have 

not been considered in previous reports to the Tribe. 

Geochemical studies have used new analyses of low-level 

gold and other indicator elements from stream sediments, 

heavy-mineral pan concentrate, rock, and soil samples. 

Results from these analyses and from the geologic studies 

indicate possible gold mineralization in areas outside of the 

known prospects and mines (Armbrustmacher and others, 

1992). 

Although proposed BIA-funded USGS mineral 

resource assessment projects will focus on metallic minerals, 

the cooperative BIA-USGS program continues to evolve. A 

new subject in proposals for USGS resource assessments is 

industrial minerals, particularly the various natural materials 

considered aggregate. Other industrial minerals, such as 

phosphates, gypsum, vermiculite, and others, are also being 

considered in more recent USGS mineral assessment proposals. 

Another new component of the proposed USGS work 

on Indian lands is environmental analyses, for Reservations 

as well as larger regions which contain several Reservations. 

USGS data bases can be used to evaluate regions where rock 

and soil interact with water and air to create possible geologic hazards (for example, radon). Such environmental 

analyses, using new interpretations of geologic, geochemical, and geophysical data, could aid in prioritizing and 

designing additional environmental studies. 

REFERENCES 

Armbrustmacher, T.J., Modreski, P.J., and King, H.D., 1992, Mineral resources of the Black Mountain-Miners Gulch Area, 

Rocky Boy's Indian Reservation, Bearpaw Mountains, Montana, in Manydeeds, S.A., ed., 1992 Mineral frontiers on Indian 

lands: Bureau of Indian Affairs publication G-92-2, 

p. 107-116. 

Godwin, L.H., and Smith, B.D., 1993, Economic mineral resources 

of the Annette Islands Reserve Alaska: Spokane, Wash., 

Northwest Mining Association Monograph. 

Klein, T.L., Day, W.C., Clark, J.R., Horton, R.J., and Green, G.N., 

1992, Assessment of shear zone-hosted gold deposits for the 

Red Lake Indian Reservation, Northern Minnesota, in Manydeeds, S.A., ed., 1992 Mineral frontiers on Indian lands: Bureau of Indian Affairs publication G-92-2. p. 88-103. 
Manydeeds, S.A., ed., 1992, 1992 Mineral frontiers on Indian 

lands: Bureau of Indian Affairs General Publication, G-92-2, 

135 p. 

Manydeeds, S.A., and Smith, B.D., 1991, eds., Mineral frontiers on 

Indian lands: Bureau of Indian Affairs special publication for 

the Northwest Mining Association 1991 Annual Meeting, 

147 p. 

Nutt, C.J., Eppinger, R.G., Miller, S.H., Ponce, D.A., and Sampson, 

J.A., 1992, Geological, geochemical, and geophysical studies 

for the goshute reservation, Nevada and Utah, Precious-mineral assessment project, in Manydeeds, S.A., ed., 1992 Mineral 

frontiers on Indian lands: Bureau of Indian Affairs publication 

G-92-2, p. 50-69. 

TECTONIC RECONSTRUCTION OF 

NORTHWESTERN MEXICO WITH 

IMPLICATIONS FOR MINERAL 

RESOURCES-A NEW REGIONAL ORE 

DEPOSIT SYNTHESIS AND MODEL 

John-Mark G. Staude 

The metallogeny of northwestern Mexico is intimately 

related to the magmatic and tectonic histories of the region, 

and palinspastic reconstructions permit a new evaluation of 

the time-space development of metallic mineralization in the 

southwestern Cordillera. This study compiled and compared 

the host lithologies, age of mineralization, main productive 

metals, key structural controls, and other features of more 

than 2,000 metallic mineral occurrences in more than 600 

districts. Tectonic reconstructions were completed and mineral deposits restored to their position at the time they 

formed. These reconstructions and metallogenic model provide a more detailed analysis of mineralization as well as 

new data for understanding the geologic development of 

northwestern Mexico. 

Metallic mineralization has taken place throughout 

much of the past 150 m.y., and deposit types changed as 

magma composition and structural styles developed along 

the long-lived (>140 m.y.) magmatic arc. Mineralization 

events can be grouped into five periods corresponding to distinct magmatic and tectonic episodes. Late Jurassic to Early 

Cretaceous shear zones and mafic to intermediate plutonism 

generated gold, iron, and possibly copper deposits in a waning transpressional environment. Cretaceous tonalite to granodiorite batholith emplacement generated widespread W 

skarns and Au-Ag quartz veins in a compressional tectonic 

environment. Late Cretaceous--early Tertiary (Laramide) 

porphyry Cu-(Mo), W skarn, and Au-Ag veins are associated 

with intermediate composition, hypabyssal intrusions. These 

Laramide ore deposits are coeval with widespread thrust 

faults in Chihuahua and with local zones of extensional 

faulting in central Sonora. Late Eocene-Oligocene early 

intermediate, later felsic to bimodal volcanism is associated 

99 

Mineralization 

Abundances 

1 

Au-Ag 

t\o 

t5 o~~~~~~~~~~~~~~ 

·;:: 

+-' 

C/) 

03 

-0 

..... 

Q) 

..0 

E 

:::1 

z 

Figure 1 (Staude). Histogram showing number of mining districts in northwest Mexico arranged by age and deposit type. Shading in inset map shows area included for histogram. The number of 

districts for each commodity is denoted by N=. Abbreviations are 

L.Jur, Late Jurassic; Cret, Cretaceous; L.Cret-E.Tert, Late Cretaceous-early Tertiary (Laramide); Mid Tert, middle Tertiary; Late 

Tert, late Tertiary; SZ, shear zone vein districts; Sed, sedimentaryhosted; Acid, acid-sulfate; Ign. intrusion associated veins; Adul, 

adularia-sericite veins; Repl/Skam, replacement and skarn districts; 

and Porp, porphyry deposits. 

with many Au-Ag, Ag-Pb-Zn, and Au-Cu districts. Mid-Tertiary extensional features range from basin-bounding, highangle faults to low-angle detachment faults. From the late 

Tertiary to Holocene, hot-spring precious- and base-metal 

deposits were generated in association with rifting of the 

Gulf of California and mafic magmatism. 

Most metallogenic studies of Mexico and other parts 

of the world use the present geographic distribution of 

lithologies and ore deposits to interpret their formation conditions; however, in order to relate mineralization to coeval 

magmatism and tectonism at the time of their formation, 

palinspastic reconstructions are needed. Reconstructions of 

northwestern Mexico must account for opening of the Gulf 

of California, basin-and-range block faulting, and core 

complex extension to obtain the pre-late Oligocene 
relations. Integrating >50 geologic maps and >900 radiometric ages pennits restoration of extensional events to the 

pre-middle Tertiary configuration and provides new 

insights into the metallogenic history. These reconstructions show that Cretaceous batholithic rocks and associated 

skarns extend from Baja California into western Sonora, 

and that Laramide porphyry copper deposits and intrusive 

centers form a narrower belt than previously thought and 

are traceable for more than 400 km beneath the younger 

Sierra Madre Occidental volcanic province. These restorations also indicate that the mid-Tertiary Au-Ag deposits in 

Baja California represent the western part of the Sierra 

Madre Occidental metallogenic event and that the sediment-hosted, disseminated Au deposits are related to extension which is synchronous with magmatism in the area. 

PROSPECTING FOR GOLD WITH 

SAGEBRUSH IN THE GREAT 

BASIN-RESULTS OF 

GREENHOUSE STUDIES 

Kathleen C. Stewart and David M. McKown 

Sagebrush has been proposed for use as a biogeochemical prospecting medium for gold in the Great Basin of 

Nevada because of its widespread growth in areas where 

Carlin-type disseminated gold deposits are known to exist 

and because of its extensive root system which may be 

exposed to ions that migrate in water. Difficulties exist with 

the use of plants as a prospecting medium, however, where 

the possibility of aeolian contamination cannot be excluded 

in the interpretation of data. We designed a greenhouse study 

to detennine if sagebrush accumulates gold in a controlled 

setting, in order to define its limits for exploration purposes. 

Soil collected from Humbolt County, Nevada, was disaggregated to <2 mm, homogenized, and mixed with perlite 

and disseminated gold ore in a V-blendor. The gold ores 

were obtained from the Getchell and Pinson mining districts 

and were ground to <100 mesh and homogenized before 

mixing. Gold is found in these ores both as metallic gold and 

associated with pyrite (FeS2). Other minerals present include 

realgar (AsS) and orpiment (As2S3). Soil mixture Ml, 

prepared to represent a nonmineralized soil from Humbolt 

County, contained <1 ppb gold, 6 ppm arsenic, 1 ppm antimony, and 2 ppm tungsten. Two additional mixtures (M2 

and M3) were prepared to contain anomalous levels of gold, 

arsenic, antimony, and tungsten: M2 contained 36 ppb gold, 

220 ppm arsenic, 13 ppm antimony, and 3 ppm tungsten; M3 

contained 190 ppb gold, 230 ppm arsenic, 12 ppm antimony, 

and 13 ppm tungsten. There was no significant difference in 

arsenic or antimony concentrations between M2 and M3, but 

the tungsten concentration was significantly higher in M3 

than in the other mixtures. Seedlings of basin big sagebrush 

.E 

Cl 

-~ 

c-

"0 

.0 

a. 

a. 

100 

0 

E 

a. 

a. 

~Ml-M2c::::JM3 

Figure 1 (Stewart and McKown). Mean concentrations of Carlin-indicator elements in above-ground parts of basin big sagebrush 

after 4 months growth in nonmineralized and mineralized soils under greenhouse conditions, n=6. M I is nonmineralized soil; M2 and 

M3 contain gold ore from the Getchell and Pinson mining districts 

in Nevada. 

(Artemisia tridentata ssp. tridentata) were planted in the soil 

mixtures, placed in a greenhouse in randomized blocks, and 

maintained at controlled temperature and moisture. After 4 

months, all above-ground tissue for each plant was harvested 

and analyzed as one composited sample by instrumental neutron activation analysis (INAA) for gold, arsenic, antimony, 

and tungsten. 

Sagebrush seedlings showed statistically significant 

uptake of arsenic, antimony, and tungsten, but not gold. Figure 1 presents the mean concentrations for six plants grown 

in each mixture. Duncan's F test for multiple ranges (Duncan, 1955) determined that means for sagebrush gold were 

not significantly different among the plants grown in the 

three soil mixtures (P=0.05), but mean arsenic and antimony 

concentrations in plants from the mineralized soils were significantly higher than in plants from the nonmineralized soil. 

Mean plant tungsten levels for the two lowest concentrations 

of soil tungsten (M 1 and M2) could not be distinguished at 

the 0.05 probability level, but its concentration in plants 

from the soil with elevated tungsten (M3) was significantly 

higher than in plants from the other two mixtures. Arsenic 

showed the greatest absolute uptake from the soils, but the 

relative uptake of antimony was higher by a factor of 2 compared to its concentration in the soils. Concentrations for all 

elements measured were above the determination limits, 

except tungsten in plants grown in the nonmineralized 

mixture. 

The results of the greenhouse study suggest that interpretation of gold data from sagebrush collected in the field 

cannot rule out the possibility of windblown contamination, 

because plants in this controlled environment did not 
accumulate significant gold, at least over a time period of 4 

months. On the other hand, the study indicates that sagebrush has good potential for use in prospecting for concealed 

Carlin-type deposits in the Great Basin using the pathfinder 

elements arsenic, antimony, and tungsten. Arsenic and antimony are especially sensitive indicators because sagebrush 

accumulates these elements in above-ground plant parts to 

levels that are well above INAA determination limits. 

REFERENCE 

Duncan, D.B., 1955, Multiple range and multiple F tests: Biometrics, v. 11, p. 1-42. 

IMPACT OF ACID MINE DRAINAGE 

FROM SUMMITVILLE MINE ON BARLEY 

FIELDS IN THE SAN LUIS VALLEY, 

SOUTH-CENTRAL COLORADO 

P.R. Stout, J.A. Erdman, and J.C. Emerick 

Possible contamination from the Summitville gold 

mine in the San Juan Mountains above Terrace Reservoir on 

the Alamosa River has raised concerns about using this 

water to grow barley in the San Luis Valley. Approximately 

45,000 acres of farmland with different crops in the San Luis 

Valley are irrigated with water from Terrace Reservoir. Copper, Mn, Zn, Cd, Fe and Ni have all shown increased concentrations in the reservoir since 1986 when the Summitville 

mine came into production (Agro Engineering, 1992). The 

objectives of this study were to examine whether the use of 

Terrace Reservoir water significantly increased metal uptake 

in malt barley (Hordeum vulgare cv. Moravian III) and to 

determine relationships between soil chemistry and plantmetal concentrations. 

Fields from six farms were selected as sample sites; 

three of the fields were irrigated with water from Terrace 

Reservoir and three were irrigated with water, unaffected by 

Summitville, either from the Rio Grande River or from deep 

wells (control fields). All the fields had similar physical soil 

types and were classified as well-drained sandy loams. Samples were collected three times over the summer of 1993 

(June 10--15, July 16-19, August 10--13). Three randomly 

selected, composited soil samples (0--20 em) were taken 

from each field on each date and three randomly selected, 

composited barley samples from each field were taken on the 

last date. Water samples were taken from the irrigation structures on the frrst two sampling dates. All samples were analyzed using inductively-coupled plasma atomic-emission 

spectroscopy (ICP-AES). 

The pH of water samples collected from the Terraceirrigated fields averaged 5.95 and ranged from 5.3 to 6.7. 

101 

The pH for control-field water averaged 8.25 with a range of 

7.3-9.2. These pH differences between source waters were 

significant at the 0.01 probability level. Fields irrigated with 

water from the Terrace Reservoir clearly receive more acidic 

water than do fields irrigated with water from other sources. 

No significant difference in pH was observed between the 

two sampling dates. 

Results of total analyses of soils show some differences 

between fields irrigated with Terrace Reservoir water and 

control fields. Concentrations of Co, Cu, Fe, Mn, Pb, V, and 

Zn all were significantly higher in soils from the Terraceirrigated fields than in soils from the control fields. On the 

other hand, concentrations of Ca, Li, Mg, Na, and Sr all were 

significantly higher in the soils from the control fields than 

in those from the Terrace-irrigated fields. Equally important, 

metal concentrations in all of the soils sampled were within 

ranges for Western U.S. soils given by Shacklette and 

Boemgen (1984). No significant temporal differences were 

observed among the three sample periods. 

DTPA (diethylene triamine pentaacetic acid)-extractable concentrations of La, Ni, Cu, Ce, Mn, Fe, and Zn were 

significantly greater in soils from the Terrace-irrigated fields 

than in soils from the control fields. DTP A extractions have 

been used to estimate the availability of minerals to plants. 

Significant negative correlations were observed between Fe, 

Mn, Ce, Cu, Ni, and Zn and soil pH. As with the pH of the 

irrigation waters, the soil pH of the Terrace fields was significantly lower than that of the control fields. The control-field 

soils yielded significantly greater DTPA-extractable Pb, K, 

Ca, Si, and Sr. No significant temporal differences were 

observed over the season sampled. These data suggest that 

certain metals become more available to the plants as soil pH 

decreases and that these metals may be more available to 

plants in the Terrace-irrigated fields than in the control 

fields. 

Water-extractable Ba was significantly higher in soils 

of the Terrace-irrigated fields compared to that of the control-field soils. Conversely, water-extractable Ca was significantly higher in the control fields than in the Terraceirrigated fields. A significant negative correlation between 

water-extractable Ba and soil pH was observed. No significant temporal changes were observed over the season sampled. These results indicate that few of the metals were 

extracted by water at concentrations detectable by ICP-AES 

and that no other detectable differences in water-extractable 

chemistry existed between the Terrace-irrigated fields and 

the control fields. 

Preliminary results suggest that irrigation with waters 

from the Terrace reservoir has a measurable impact on both 

the bulk-soil and DTPA-extractable chemistry. However, 

the total-metal concentrations of soils irrigated by these 

waters are well within the ranges for Western U.S. soils 

reported by Shacklette and Boerngen (1984 ). The effects on 

barley of higher metal values in the irrigation water and soils 
irrigated by this water are unknown because the analytical 

data for the barley-grain samples are still pending. 

REFERENCES 

Agro Engineering, 1992, Water, sediment and soil quality synopsis 

and literature review Alamosa River, Conejos County, Colorado: Unpublished report prepared for the Alamosa La Jara 

Water Conservancy District, December 29, 1992. 

Shacklette, H.T., and Boemgen, J.G., 1984, Element concentrations in the soils and other surficial materials of the conterminous United States: U.S. Geological Survey Professional Paper 

1270, 105 p. 

THREE-PART QUANTITATIVE MINERALRESOURCE ASSESSMENT FOR 

SELECTED UNDISCOVERED MINERAL 

DEPOSIT TYPES, BLM ROSWELL 

RESOURCE AREA, EAST -CENTRAL 

NEW MEXICO 

David M. Sutphin 

The U.S. Bureau of Land Management (BLM) Roswell 

Resource Area in east-central New Mexico contains approximately 14 million acres within seven counties. Within the 

area, mining produced several mineral commodities from 

the turn of the century through World War II; gold was the 

most important mineral product. As part of the mineralresource assessment of the Roswell Resource Area, the 

USGS estimated the undiscovered resources in several mineral deposit types that were permissible in the area and for 

which grade and tonnage models were available. The resulting mineral resource estimates have been incorporated by 

BLM into an overall land-use plan. 

The technique used in estimating the undiscovered mineral resources for selected deposit types is based upon the 

three-part type of assessment: (1) using known geologic, 

geochemical, and geophysical characteristics to delineate 

tracts that may contain specific deposit types; (2) estimating 

the probabilities that a certain number of undiscovered 

deposits exist in these tracts; and (3) estimating the amount 

of a given commodity contained in the undiscovered deposits by means of comparison with the grades and tonnages of 

known deposits of the same type. Parts (1) and (2) are conducted by a team of USGS specialists in economic geology, 

geochemistry, geophysics, and mineral resource assessment 

who are familiar with the area; part (3) is a computer simulation. This study uses the USGS' MARK-3 program that 

requires estimates of the number of undiscovered deposits of 

a given type stated in terms of likelihood of occurrence, 

resulting in a probability distribution. 

Table 1 (Sutphin). Estimates of pre-mining tonnages of commodities in selected undiscovered deposit types, Roswell Resource 

Area, east-central New Mexico. 

102 

Undiscovered resources (short tons) 

Probability range 

.90 .SO (median) .10 

Tract I: 

Barite ............... 520 140,000 1,900,000 

Copper .............. 0 8.3 330 

Gold .................. 23 290 1,700 

Iron .................... 3,400,200 51,000,000 530,000,000 

Lead .................. 58 4,400 53,000 

Manganese ........ 0 850 160,000 

Molybdenum ..... 0 55,000 550,000 

Rare-earth oxides 0 0 860 

Silver ................. 34 260 1,100 

Thoria ................ 0 190 8,800 

Zinc ................... 0 1,400 40,000 

Tract II: 

Cobalt. ............... 0 0 90,000 

Silver ................. 0 0 720,000 

Zinc ................... 0 0 4,200,000 

Tract III: 

Cobalt ................ 0 0 0 

Copper ............. 0 0 1,400,000 

Silver ................. 0 0 0 

Tract IV: 

Gypsum ............. 1.3x 108 2.0 X 109 2.6x1010 

Tracts favorable for the occurrence of undiscovered 

mineral resources in the Roswell Resource Area were delineated by comparing the geology, geochemistry, and geophysics of the area to the geologic environments and 

characteristics of deposit types having grade and tonnage 

models. A consensus by the team on tract borders, the types 

of deposits permissible in the tract, and the number of those 

deposits at the 90-, 50-, and 1 0-percent probability levels 

was reached after further discussion. By entering deposit 

estimates into MARK-3, probable quantities of undiscovered resources for selected deposit types in the four tracts 

within the Roswell Resource Area were calculated (table 1). 

Tract I includes areas where geophysical evidence 

suggests shallow Tertiary intrusive bodies; it includes several historical mining districts and encompasses the most 

significant identified metal resources in the study area. 

Using geologic criteria and historical production figures for 

the Roswell Resource Area, the team estimated that the 

tract contains undiscovered iron skarns, alkaline-associated 

gold-silver-tellurium veins, polymetallic veins, epigenetic 

barite veins, gold-platinum-group-element placers, thorium-rare-earth-element veins, and replacement manganese, 
porphyry-molybdenum low-fluorine, and fluorite-bastnaesite deposits. Tract II delineates an area permissible for sediment-hosted copper and southeast Missouri lead-zinc 

deposits on the basis of geologic and geochemical data and 

the presence of nearby mineralization outside the study 

area. Tract III encloses a 30-mi-diameter area, centered on 

the Stauber mine in Guadalupe County, that may contain 

undiscovered sediment-hosted copper deposits. The boundary of Tract IV connects 12 known gypsum occurrences 

and is likely to contain undiscovered marine-bedded gypsum deposits. 

NATIONAL WATER QUALITY 

ASSESSMENT PROGRAM 

Marc A. Sylvester 

Many complex water-quality issues face this Nation. 

Both point (for example, municipal and industrial effluents) 

and nonpoint (for example, runoff from agricultural and 

urban land) sources of water pollution have caused waterquality degradation. A clear and comprehensive understanding of water-quality conditions in the Nation does not exist, 

because most water-quality assessment programs have been 

local, short-term, independent, single-purpose, and (or) single-agency efforts. Basic questions about water-quality conditions remain unanswered. Is the Nation's water quality 

getting better or worse? What fraction of the Nation's surface water and ground water is uncontaminated and meets 

requirements for diverted and instream uses? What is the relative importance of agricultural versus urban runoff in causing water-quality degradation? How variable is water quality 

across the Nation? How do land alteration and changes in 

habitat affect water quality? A comprehensive, multiplescale, interdisciplinary, interagency, and long-term assessment of water-quality conditions using consistent 

approaches is needed to answer these questions and to provide reliable information for effective management of water 

resources. The National Water Quality Assessment Program 

(NA WQA) of the U.S. Geological Survey, which began in 

1991, meets these requirements and is designed to answer 

some of the basic questions regarding water-quality conditions and trends in the Nation. Information from NA WQA 

will help reduce uncertainty in water-resources planning, 

management, and regulation, and will be useful for the protection, use, and enhancement of the Nation's water 

resources. 

The goals of NA WQA are to describe the status and 

trends in the Nation's water quality, and to identify and 

explain, as possible, the major factors that affect observed 

conditions and trends. To accomplish these goals, NA WQA 

is composed of 60 Study Units covering a diversity of conditions across the Nation and 60-70 percent of the Nation's 

103 

water use. Comparable information from these 60 Study 

Units will be aggregated to obtain regional and national perspectives on water quality. Interdisciplinary and interagency 

teams of hydrologists, chemists, and aquatic biologists are 

doing studies in each Study Unit using consistent 

approaches. Streamflow and ground-water levels are measured, water-quality samples are collected, processed, and 

analyzed, and ecological evaluations are made of stream and 

floodplain habitats and aquatic communities of fish, benthic 

invertebrates, and algae. Data are stored in a consistent manner in a computerized relational data-base management system so that information is easily accessible and integrated for 

timely interpretation. 

EVALUATION OF METALLIC 

MINERAL RESOURCES IN THE EAST 

MOJAVE NATIONAL SCENIC AREA, 

SAN BERNARDINO COUNTY, 

CALIFORNIA 

Ted G. Theodore, Robert J. Miller, 

Kenneth R. Bishop, Clay M. Conway, 

John C. Dohrenwend, Joseph S. Duval, 

Gordon B. Haxel, John D. Hendricks, 

Carroll A. Hodges, Robert C. Jachens, 

Marguerite J. Kingston, David M. Miller, 

Gary A. Nowlan, James J. Rytuba, and 

Richard M. Tosdal 

Large areas of the East Mojave National Scenic Area 

(EMNSA) contain indications of various types of metallic 

mineralization at the surface. These metallic indications can 

be classified into approximately 20 specific types, or models, of metallic occurrences that are known to show 

extremely wide ranging concentrations of metals in variably sized accumulations, based on studies elsewhere of 

similar known deposits. Among metallic occurrences recognized are lead-zinc-silver-gold polymetallic vein; lowsulfide, gold-quartz vein; lead-zinc-silver polymetallic 

replacement; gold breccia pipe; gold-silver, quartz-pyrite 

vein; copper-lead-zinc-silver polymetallic fault and skarn; 

copper, zinc-lead, tungsten, tin-tungsten, and iron skarn; 

porphyry molybdenum-copper; and quartz-adularia 

(alunite) gold. Of these 20 types of mineral occurrence, 

enough geologic information is available only for 10 of 

them to make highly qualified estimates as to the numbers 

of additional mineral deposits that remain to be discovered 

in the EMNSA. Most of approximately 700 individual mineral occurrences known have been extensively prospected 

for more than a century, and at least 15 percent of them had 

some production, mostly of minor quantities of metals. 
Nonetheless, it is possible that economically significant 

concentrations of some metals remain to be discovered in 

the EMNSA, or that some known occurrences may become 

economic in the future if exploration were allowed. For 

example, gold ore bodies at three relatively large mineralized systems in the EMNSA (Castle Mountain, Colosseum, 

and Morning Star) have been brought into production and 

at another occurrence (Golden Quail) additional resources 

discovered. The combined gold production from these four 

deposits in the last 5 years and the gold reserves remaining 

in them as of 1993 are much greater than that of all the preceding discoveries of gold in the EMNSA. This is partly a 

reflection of the present availability of low-cost heap-leaching extraction methods. 

Some of the mountain ranges that have large numbers 

of metallic occurrences and that exhibit geochemical anomalies are the Providence Mountains, Clark Mountain Range, 

Ivanpah Mountains, and the New York Mountains. The general area of Hackberry Mountain lacks abundant metallic 

occurrences although it is included in a tract of land judged 

to be favorable for discovery of additional gold deposits similar to those present in the Castle Mountains. These five 

mountain ranges make up a broad, roughly north-south 

region in the central part of the EMNSA. Much less endowed 

with known metallic mineral occurrences are the Granite 

Mountains, central part of the Piute Range, Fenner Valley 

area, general area of Cima Dome, Old Dad Mountain area 

and areas west to Soda Lake, and the Cima volcanic field. 

These areas lie in the eastern and western parts of the 

EMNSA. 

We have made some judgments about the geology 

underlying the gravel-covered areas in the EMNSA, including the areal extent of shallow bedrock apparently covered 

only by thin veneers of gravel. These are areas that would be 

prime targets for exploration because many of the ores that 

were exposed at one time in the mountain ranges have been 

found during past periods of exploration, whereas the pediment areas have remained largely unexplored. But few data 

are available to us for most of these covered areas. The presence of mineralized rocks, the type of mineral occurrence, 

and the extent and intensity of the mineralized rocks in the 

covered areas are essentially unknown. Most covered mineral deposits are undetectable by standard aeromagnetic geophysical methods evaluated in this study, particularly at the 

broad spacing of our data-collection flightlines. 

Restricting estimates concerning presence of undiscovered metal resources, at reasonable levels of probability, 

only to currently known types of occurrences yields small 

estimates for volumes of many metals, particularly base 

and ferrous metals, that might be exploited there at some 

future date. However, this statement is true only if sizes of 

most previously discovered deposits in the EMNSA are in 

fact indicative of sizes of deposits still to be discovered 

there. Metals from any newly discovered copper, lead, and 

zinc deposit of the types presently known in the EMNSA 

104 

probably would be insignificant from the standpoint of 

national needs. For example, a newly discovered copper 

skarn deposit in the EMNSA would have roughly a 50 percent chance of containing in excess of approximately 

10,000 t (metric tons) contained copper. For comparison, in 

1989 domestic production of copper amounted to 1 ,500,000 

tofcopper. 

Based primarily on the geologic environments of large, 

recently discovered gold deposits in the EMNSA, some 

other parts of the EMNSA appear to represent geologic settings capable of hosting significant undiscovered gold and 

silver resources. In addition, widespread distribution in 

many parts of the EMNSA of geochemically anomalous 

samples and numerous mineral occurrences, many of which 

are associated with intrusion of igneous rock, indicates 

metal-bearing environments in those parts of the EMNSA 

that may be sites of additional future discoveries of new 

types of mineral deposits, if state-of-the-art exploration were 

allowed to take place in the EMNSA. 

INTERACTIVE COMPUTER DISPLAY ON 

THE GEOLOGY AND MINERAL AND 

ENERGY RESOURCES OF THE ROSWELL 

RESOURCE AREA, NEW MEXICO 

R.R. Tidball and Susan Bartsch-Winkler 

The U.S. Geological Survey (USGS) has assessed the 

mineral and energy resource potential of the Roswell 

Resource Area, a seven-county tract in east-central New 

Mexico administered by the Bureau of Land Management 

(BLM) (see Bartsch-Winkler, this volume, fig. 1). A comprehensive report (Bartsch-Winkler, 1992; Bartsch-Winkler 

and Donatich, in press) was provided to the BLM as timely 

input to a required land-use plan that the BLM must prepare 

for the Roswell Resource Area. Such land-use plans are 

required for each area under the jurisdiction of the BLM and 

other Federal agencies and are periodically reviewed. This 

report will also be used by the USGS in national mineral 

resource assessments. 

In addition to the published report, the USGS provided the BLM District Office in Roswell, N. Mex., with 

an interactive computer display that describes the general 

geology and the commodities present in the resource area. 

The display may be used to facilitate training of new 

employees and as an educational tool for use in the BLM 

Public Inquiries Office in Roswell. The interactive program, which runs on a Macintosh computer, enables a novice to choose alternative pathways through the graphical 

color displays to access topical information modules 

including units on the geology, geochemistry, geophysics, 

mining history, metallic and industrial mineral occurrence, 

hydrocarbons, and other gas resources. The program was 
developed with Aldus SuperCard, version 1.5, software to 

run on a Macintosh 11-series computer. Additional hardware requirements include color-graphics capability, 8 

megabytes of memory, and a minimum of 40-megabytes of 

disk storage. An executable version of the program in compressed form can be loaded from high-density diskettes 

using standard Macintosh decompression routines. 

REFERENCES 

Bartsch-Winkler, Susan, ed., 1992, Mineral and energy resources 

of the BLM Roswell Resource Area, east-central New Mexico: 

U.S. Geological Survey Open-File Report 92-261, 153 p. 

Bartsch-Winkler, Susan, and Donatich, A.J., eds., in press, Mineral 

and energy resources of the Roswell Resource Area, east-central New Mexico: U.S. Geological Survey Bulletin 2063 [Publication supersedes Open-File Report 92-261]. 

GEOCHEMICAL MAPPING IN THE 

SAN LUIS VALLEY, 

COLORADO-HYDROGEOCHEMICAL 

AND STREAM SEDIMENT DATA 

Ronald R. Tidball, Steven M. Smith, and 

Kathleen C. Stewart 

Prime agricultural land in south-central Colorado's San 

Luis Valley is at risk from metal contamination from several 

areas of mining activity. The Alamosa River receives acid 

mine water derived from the Summitville mine, located in 

the San Juan Mountains west of the valley (fig. 1 ); it also 

supplies water for an extensive irrigation system in the valley. The Summitville mine will soon be declared a U.S. 

Environmental Protection Agency (EPA) Superfund site. 

The Bonanza district near the north end of the valley is also 

currently under study by the State of Colorado, the U.S. Forest Service, and EPA as a potential Superfund site. Several 

creeks drain from this district into the valley. Background 

geochemical studies of soils and surficial materials in the 

valley are therefore needed to define the extent of metal dispersion into the valley from several mineralized areas and 

mining districts. 

Previous geochemical sampling in the valley has been 

sparse. Stream, spring, and pond sediment and water samples, in a project called Hydrogeochemical and Stream Sediment Reconnaissance (HSSR), were collected throughout 

the State of Colorado during the National Uranium Resource 

Evaluation (NURE) program in the 1970's. Although overall 

HSSR coverage of the State is extensive, coverage in the San 

Luis Valley is generally limited to a few samples in the 

southeastern and the far northeastern part of the valley. This 

Bonanza• N 

1 

20 40 KILOMETERS 

Figure 1 (Tidball and others). Map of San Luis Valley, southcentral Colorado, showing major drainages and Bonanza and Summitville mining districts. 

105 

sampling predates the recent active Summitville mining 

period of 1986-1992. 

The HSSR coverage is currently being supplemented 

by a soil sampling program by the USGS. The Alamosa 

River fan and adjacent fans in the southwestern part of the 

valley are being sampled intensively on a 1-mile (1.6 km) 

grid interval. A larger interval of 5 km (3.1 miles) is being 

used in the rest of the valley. 

We present here a view of the regional geochemistry 

based on multielement analyses of HSSR samples to estimate the occurrence of metal-rich source rocks near the valley margin. HSSR data from parts of the Montrose, Pueblo, 

Trinidad, Raton, Aztec, and Durango 1 °X2° quadrangles that 

surround the San Luis Valley are included. 

Intensive HSSR sampling in the eastern part of the 

Montrose quadrangle showed anomalies of Ag, Cu, Pb, Zn, 

As, Sb, and Sn bordering the northwest part of the valley. 

These anomalies are associated with ~he Bonanza mining 

district and Kerber and San Luis Creeks, which drain the 

district. Elsewhere, above-average amounts of Co, Cr, Ni, 

and V occur in the vicinity of outcrops of basalt in the San 

Luis Hil1s in the southern part of the valley and in the vicinity 

of the Summer Coon stratovolcano north of Del Norte. 
The data also show several anomalous sites in the Alamosa River watershed. Wightman Fork, the tributary which 

drains the Summitville mine area, has Cu values in the range 

of 100-900 ppm, Pb 150-250 ppm, and Zn about 200 ppm. 

Below Wightman Fork, Cu and Pb decrease to 50 ppm or 

less, and Zn decreases to 140-170 ppm. All these samples 

are from above Terrace Reservoir. The possibility exists that 

metal-laden sediments are trapped in Terrace Reservoir, but 

two samples on the upper part of the Alamosa River fan contain 150-170ppm Cu, 40-50 ppmPb, and 140-170 ppm Zn, 

thus suggesting that metals both can migrate and have 

migrated to the valley. HSSR data on pH show that Alamosa 

River water is the most acidic (pH 5-5.5) of any stream 

entering the valley, and corrosion observed in some sprinkler-irrigation systems using river water may be accelerated 

by its acidic condition. Scattered sites containing anomalous 

values of Bi, Sb, and Sn, plus above-average values of Cu, 

Pb, and Zn, are also found in a hydrothermally altered monazite area just north of Terrace Reservoir. 

Pb SOURCES IN OLIGOCENE AND 

MIOCENE PRECIOUS-METAL DEPOSITS 

OF THE WESTERN ANDEAN , CORDILLERA, SOUTHERN PERU, 

WESTERN BOLIVIA, AND 

NORTHERN CHILE 

Richard M. Tosdal 

A variety of precious-metal deposits characterize the 

western part of the Andean cordillera in southern Peru, westem Bolivia, and northern Chile. Deposits in this transect 

include polymetallic vein deposits in southern Pero (Orcopampa, Arcata, Cailloma; lat 15°-16° S.), quartz-adularia 

and quartz-alunite deposits in western Bolivia and northern 

Chile (La Espanola prospect, Choquelimpie; lat 17°-18° 

30' S.), and quartz-alunite and gold-rich porphyry deposits 

in north-central Chile (Maricunga and El Indio belts; 

between lat 26° and 28° S.). All deposits are associated with 

Oligocene and Miocene calc-alkaline volcanic or intrusive 

rocks erupted from, or emplaced in, stratovolcanos along the 

main arc. 

Lead isotopic compositions from sulfides and whole 

rocks indicate that three major crustal sources contributed 

lead to these deposits. The source regions include (1) old 

nonradiogenic and high-f.! (238UJ204Pb) and high-Th/U lithosphere; (2) high-J.! lithosphere with time-averaged Th/U ::::4; 

and (3) lower-J.! lithosphere with time-averaged Th/U ::::4. 

The nonradiogenic lithospheric source is characterized by retarded 206PbJ204Pb compositions mostly ~18.2. 

Thorogenic (208pbt204Pb versus 206pbJ204pb) and 

106 

uranogenic (207pbJ204Pb versus 206pbJ204Pb) Pb isotopic 

compositions lie above the average crustal growth curve of 

Stacey and Kramers (1975). The elevated 208pbJ204pb 

ratios demonstrate that Th/U values ~4 dominate this lithosphere, indicating the presence of granulitic rocks, or 

lower-crustal type Pb reservoirs, where U is depleted with 

respect to Th. This lower-crustal-like source is present in 

two adjoining areas. One area is underlain by the Early 

Proterozoic Arequipa massif in southern Peru, and is characterized by ::::2.0 Ga rocks, 206pb/204Pb between 16.0 

and 17.1, and 208pbJ204Pb between 37.0 and 41.0. The 

second area underlies western Bolivia and northern Chile 

where most 206pbJ204Pb ratios are between 17.0 and 18.2, 

208pbJ204Pb ratios are between 37.2 and 41.2, uranogenic 

Pb isotopic compositions scatter about 1.75-Ga reference 

isochrons, and U-Pb zircon ages are Middle Proterozoic 

(1.25 to 1.1 Ga). 

The high-f.! source is characterized by elevated 

207pbJ204pb with respect to a given 206PbJ204pb 

(206pbJ204Pb=18.5-19.9; 207pbJ204Pb=15.63-15.72) such 

that most of the uranogenic Pb isotopic compositions lie 

above and along the extension of the average crustal growth 

curve, and scatter about late Middle Proterozoic (::::1.0 Ga) 

reference isochrons. Thorogenic Pb isotopic compositions 

scatter about the average crustal growth curve. These characteristics indicate that the source had high f.! values, typical 

of upper-crustal environments, but average crustal Th/U. 

Carboniferous, Permian, and Triassic plutonic rocks in 

north-central Chile, representative of the leading edge of 

Gondwana, have these Pb isotopic characteristics, and are a 

product of this source region. Very limited data suggest that 

this source region also influenced Pb isotopic compositions 

of some early Paleozoic granitoids in the same region. 

The third source is characterized by lower J.! values and 

average crustal Th/U::::4. Uranogenic Pb isotopic compositions derived principally from this source generally lie along 

or below the average crustal growth curve 

(206pbJ204Pb<18.7; 207pbJ204Pb<15.62). This source is 

widely recognized in Cenozoic magmas in the Central 

Andes, and has been interpreted as subcontinental lithosphere that has been fluxed or enriched by material emanating 

from the subducting slab or as a zone of melting and assimilation at the crust-mantle boundary (Hildreth and Moorbath, 

1988). This source represents the least radiogenic isotopic 

compositions recognized in the deposits. 

Present-day Pb isotopic compositions of the deposits 

and their host volcanic rocks in this region of the Central 

Andes lie along mixing trends between two of the three 

sources. Deposits in the Maricunga and El Indio belts are 

influenced by the lower-f.!, subcontinental-type source and 

the high-J..l, upper crustal-type source. A similar mixing trend 

also appears to explain the lead-isotopic composition in the 

Orcopampa area, but a steep mixing trend in thorogenic Pb 

suggests that a third component with high Th/U may also 

have been involved. In western Bolivia and northern Chile, 
the Pb isotopic compositions of the ore deposits reflect mixing between the lower-Jl, subcontinental-type source and the 

Proterozoic lithospheric source. 

REFERENCES 

Hildreth, W., and Moorbath, S., 1988, Crustal contributions to arc 

magmatism in the Andes of central Chile: Contributions to 

Mineralogy and Petrology, v. 88, p. 435-489. 

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

lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207-221. 

PRELIMINARY RESULTS OF A 

SYNOPTIC WATER-QUALITY STUDY OF 

THE UPPER ALAMOSA RIVER TO THE 

OUTLET OF TERRACE RESERVOIR 

Katherine Walton-Day, Roderick F. Ortiz, and 

Paul von Guerard 

A synoptic study including pH, specific conductance, 

streamflow, and total copper loads was done from the headwaters of the Alamosa River downstream to the outlet of 

Terrace Reservoir to assess the source and fate of acid, 

metal-rich water in the river system. The study was done in 

cooperation with the Colorado Division of Minerals and 

Geology and the U.S. Environmental Protection Agency. 

The study area is located in the San Juan Mountains of southwest Colorado, and the rocks are mostly intrusive bodies and 

rhyolite ash flows and lava. The Summitville mine (which 

ceased operation in December 1992) lies within the study 

area. Acid-rock drainage and some drainage from cyanide 

heap-leach piles at the Summitville mine site enter the 

Wightman Fork, a tributary to the Alamosa River. 

Water-quality samples were collected every 2 to 4 

weeks beginning in late April through mid-August 1993 at 

eight sites along the Alamosa River and at three sites on tributaries. The Alamosa River sites, listed from upstream to 

downstream, are upstream and downstream from Iron Creek, 

downstream from Alum Creek and Bitter Creek, upstream 

and downstream from the Wightman Fork, and upstream 

from and at the outlet of Terrace Reservoir. The three tributary sites are Pipeline Creek (tributary to the Wightman 

Fork) upstream from the Summitville mine, the Wightman 

Fork directly downstream from the Summitville mine, and 

the Wightman Fork 5.5 miles (8.8 km) downstream from the 

mine and directly upstream from the confluence with the 

Alamosa River. 

Preliminary results indicate that (1) seasonal dynamics 

are complex; (2) multiple sources of water have low pH 

107 

values (pH<5) in addition to the water from the Summitville 

mine site; and (3) water quality improves somewhat downstream from the Summitville mine due to dilution and unidentified physical and chemical processes. 

V aloes of pH, specific conductance, and discharge 

exhibit large seasonal and spatial variation. For example, pH 

values range from 3.33 on the Wightman Fork downstream 

from the Summitville mine in late June to 7.59 on the Alamosa River upstream from Iron Creek in mid-May. Specific 

conductance ranges from 22 to 1 ,650 microsiemens per centimeter in Pipeline Creek in mid-July and in the Wightman 

Fork downstream from Summitville in late April, respectively. Streamflow varies from 1.5 ft3/s (cubic feet per second) in Pipeline Creek in early May to 858 ft3/s in the 

Alamosa River upstream from Terrace Reservoir in early 

June. 

Preliminary results indicate sources of acid water to the 

upper Alamosa River in addition to the Summitville mine. 

On the Alamosa River upstream from the Wightman Fork, 

pH values range from less than 4.0 during low flow (April) 

to about 6.4 during high flow (late June), whereas on the 

Wightman Fork upstream from the confluence with the Alamosa River, pH values range from about 7.5 during low flow 

to about 4.5 during high flow. These results indicate that 

sources for acid water to the Alamosa River exist upstream 

from the Wightman Fork. Specific sources, although not 

identified, are most likely located between the Wightman 

Fork and Iron Creek, as only pH values upstream from Iron 

Creek were circum-neutral throughout the study period. In 

addition, during a rainfall-runoff event, lower pH values 

were measured upstream from Terrace Reservoir than immediately downstream from the Wightman Fork, indicating 

sources of acid water to the Alamosa River between the 

Wightman Fork and Terrace Reservoir. 

Some increase in pH and decrease in specific-conductance values occur as water moves down the Wightman Fork 

from the Summitville mine to the confluence with the Alamosa River during and following runoff (late May to late 

June). For example, on the Wightman Fork directly below 

Summitville, pH values ranged from about 3.3 to about 3.7 

during and following runoff and specific conductance 

ranged from 932 to 1,242 microsiemens per centimeter. During that same period, on the Wightman Fork above the confluence with the Alamosa River, pH values ranged from 4.4 

to 5.0 and specific conductance ranged from 372 to 592 

microsiemens per centimeter. Streamflow increased as much 

as seven-fold between the two sites, indicating that dilution 

from other water sources contributed to these improvements. 

However, over the same reach of stream, instantaneous total 

copper loads (in kilograms per second) decreased as much as 

six-fold during early runoff. This decrease indicates that processes other than dilution, such as sorption and precipitation 

of solid phases, contribute to water-quality improvements. 

Evaluation of the complete data set will help identify these 

processes. 
Au-Ag POLYMETALLIC 

MINERALIZATION IN THE MUSIC 

MOUNTAIN AREA, ARIZONA, ALONG 

THESOUTH~TERNEDGEOFTHE 

COLORADO PLATEAU 

Karen J. Wenrich and Miles L. Silberman 

The Music Mountain mining district lies at the base of 

the Grand Wash Cliffs, a major fault-line scarp along the 

Grand Wash fault, which marks the southwestern margin of 

the Colorado Plateau (fig. 1 ). Nearly vertical Au-Ag polymetallic quartz veins are parallel to, and in contact with, altered 

diabase and granite porphyry dikes that cut Proterozoic granite, schist, and gneiss. The gold-bearing veins range in thickness from several centimeters to 1 m, and they contain 

significant amounts of sulfide minerals, principally galena, 

sphalerite, and pyrite, with minor chalcopyrite, pyrrhotite, 

arsenopyrite, and tennantite. Microprobe analyses of the sulfides showed the sphalerite to contain high Fe (Zn:Fe=4:1 

atomic proportion), and tennantite to be high in Zn and Fe 

(As:Zn:Fe=3: 1:1 atomic proportion). Bulk rock geochemical 

analyses yielded as much as 7.2 ppm Au, 1,000 ppm Ag, 2.4 

percent Pb, 1.7 percent Zn, 1.7 percent Cu, 500 ppm Cr, 460 

ppm Ni, 350 ppm Cd, 300 ppm Co, and 2.8 ppm Hg. Two 

types of quartz veins occur in the district and surrounding 

area: (1) veins, associated with pegmatites and some diabase 

dikes, that consist of coarse- to medium-grained bull quartz 

with occasional pyrite cubes-the pyrite has generally been 

altered to limonite or weathered out entirely leaving empty 

molds; (2) veins and stringers within diabase dikes, along the 

contact between diabase dikes and granite, and within shear 

zones in granite that are parallel to the dikes. Type-2 quartz 

is medium to fine grained with coxcomb crystals lining vug 

walls; the gold-bearing quartz veins are of this type. 

Stream-sediment and panned-concentrate samples were 

collected at 269 sites from all drainages in Precambrian rock 

within a 15 mi2 (40 km2) area directly north of the Music 

Mountain mines and eastward between the Grand Wash 

Cliffs and the Aubrey Cliffs, in an effort to locate gold-rich 

veins beyond the Music Mountain mines. Several samples 

had anomalous Au and Ag concentrations, including one 

panned concentrate with 528 ppm Au. In each case there 

were diabase dikes with adjacent quartz veins above the 

anomaly site. 

Diabase dikes and quartz veins in the district and to 

the north consistently strike N42°W to N57°W, which is 

one of the most prevalent Precambrian fracture orientations 

throughout northwestern Arizona (the other being approximately N45°E). Drifts and trenches of the mines in the district form striking N45°W arrays (fig. 2) that roughly 

parallel the Grand Wash fault in this area. In contrast, 

quartz veins associated with pegmatites generally strike 

N40°E to due east. In the Gold Basin-Lost Basin districts 

108 

to the north, the gold occurs in such pegmatite-quartz veins 

that strike northeast. Thirty miles ( 48 km) east along Diamond Creek, quartz veins and diabase dikes strike N45°E 

and have associated gold and silver anomalies in stream 

sediments and panned concentrates. To the west, major AuAg polymetallic quartz veins of the Wallapai mining district rarely show strikes outside of the range N30--60°W. 

Such northwest and northeast trends can also be seen in 

alignments of mineralized solution-collapse breccia pipes 

on this southwestern part of the Colorado Plateau. 

All diabase dikes were hydrothermally altered to some 

extent. K-Ar ages from four of the dikes with quartz veins 

along their margins range from 935±35 to 755±21 Ma. One 

vein was mineralized. Within the Music Mountain mines 

area, mineralized type-2 quartz veins occur at one location 

along the margins of a dacite dike where it contacts diabase 

intruded along a northwest-trending structure. Sericite from 

this dacite dike gave a K-Ar age of72±2 Ma. These data suggest that the northwest-trending diabase dikes, and one stage 

of mineralized quartz veins, are late Precambrian, and that a 

later stage of mineralized quartz veins is Late Cretaceous. 

Fluid-inclusion studies of 36 quartz veins in the Music 

Mountain area indicate that the vast majority of the quartz 

veins, most of which were of type 1, appear to have a 

"deep" mesothermal origin. Two localities containing type1 quartz veins are exceptions to this: (1) several quartz 

veins within the Music Mountain mines, and (2) quartz 

veins located 2 miles (3 .2 km) northeast of the mines. 

Quartz veins from these two areas contain clear cores surrounded by ratty, dendritic, outer growth zones. Fluidinclusion filling temperatures >350 oc were observed in 

quartz cores from both areas. Sulfides and sericite in the 

Music Mountain mines are associated with fluid inclusions 

in the outer growth zones with temperatures <200 oc and 

salinities commonly <7 wt. % NaCl eq. Exceptions to this 

include sphalerite inclusions adjacent to, and located in the 

same outer quartz growth zone with, primary inclusions; 

these inclusions yielded temperature <200 oc and variable 

compositions: (1) 22 wt.% NaCl eq., (2) 7 wt.% NaCl eq., 

and (3) vapor-C02-bearing. Such highly variable salinities 

adjacent to each other suggest a very saline brine mixed 

with a low-salinity fluid. In the area northeast of the Music 

Mountain mines are three types of inclusions similar to 

what might be expected from a porphyry system: (1) NaClsaturated brines (fig. 3-indicated by halite daughter minerals adjacent to vapor bubble within the liquid inclusion), 

(2) low-salinity inclusions, and (3) vapor-rich inclusions. 

Salinities as high as 38 wt. % NaCl and temperatures as 

high as 380 oc were observed in quartz cores from this 

area. The variable salinities in the outer quartz zones, and 

the high temperature-high salinities found within the cores 

of the quartz, suggest that initial fluids were high-temperature saline brines that were diluted by later low-salinity, 

low-temperature, sulfide-bearing fluids to produce the outer 

quartz growth zones. 
N 

A 

Gold 

Basin 

93 Mineral 

• Oatman 

Red 

Lake 

SHIVWITS 

PLATEAU 

• Gold mining district 

Yampai 

Cliffs 

0 

0 

10 20 MILES 

10 20 KILOMETERS 

Figure 1 (Wenrich and Silberman). The Music Mountain, Gold Basin-Lost Basin, and Hackberry mining districts 

lie at the edge of the Colorado Plateau at the base of the Grand Wash Cliffs. The Au-Ag polymetallic mineralization 

occurred within quartz veins associated with diabase dikes or pegmatites that commonly strike either northwest or 

northeast. 

All the geochemical sites (within a 1,000 mi2 (2,500 

km2) area) determined to be anomalous in gold during this 

study lie within 2 miles (3.2 km) of either the Grand Wash or 

the Hurricane faults. Although the overall trend of the Hurricane fault is north-south, it is composed of northwest- and 

northeast-trending segments. The Gold Basin and Lost 

Basin mining districts also lie close to the Grand Wash fault. 

Recurrent tectonism along northwest and northeast trends in 

109 

the region is reflected not only in the faulting but also in multiple episodes of mineralization. The Hurricane and Grand 

Wash faults, both major Precambrian fault zones that have 

probably been reactivated in the Phanerozoic (adjacent, 

approximately parallel faults, such as the Cremation, Bright 

Angel, Palisade, Butte, and Muav faults, have documentable 

Phanerozoic displacement), appear to be good exploration 

targets for gold-rich quartz veins associated with pegmatite 
Figure 2 (Wenrich and Silberman). Music Mountain mines. Note the four Jines of trenches striking N45°W. Hualapai Valley 

is in the background and the Grand Wash Cliffs, the southwestern edge of the Colorado Plateau in Arizona, lie along the upper right 

side of the figure. The Lost Basin district lies in the far distance on left side of photograph. View is to the north-northwest. 

or Precambrian diabase dikes, many of which may be buried 

beneath the thick alluvium of Hualapai Valley. The occurrence of NaCl-saturated brines, vapor-rich inclusions, and 

low-salinity inclusions in the quartz veins, along with 

110 

occasional filling temperatures in the range of 350-380 °C, 

suggests that Music Mountain gold mineralization may be 

peripherally related to a porphyry Cu-Mo system, perhaps 

similar to Mineral Park. 
Figure 3 (Wenrich and Silberman). Fluid inclusion from a quartz vein that lies adjacent to a 

northwest-trending diabase dike located 2 mi (3.2 km) northeast of the Music Mountain mines. Such 

an inclusion suggests that this quartz was deposited from a NaCl-saturated brine-note the cubic salt 

crystal and the vapor bubble. Salt crystal is 50 Jlm long. 

111 
View eastward from Hualapai Valley (Basin and Range) to the Grand Wash Cliffs. the southwestern boundary of the Colorado Plateau (northern Arizona). Note the flat-lying strata of the Cambrian Tapeats Sandstone and the Muav Limestone capping the top of 

the cliffs. Original slide by Karen J. Wenrich. 

112 
Northwest-trending, Precambrian diabase dike cutting a ridge 1 mile (1.6 km) north of the Music Mountain mine. This dike is similar to the diabase dikes in the Music Mountain mine area in that it too has a quartz vein following its margin (note darker, higher 

relief area on left edge of the dike). Original slide by Karen J. Wenrich. 

113 
AUTHOR INDEX 

A 

Abrams, G.A., USGS, Denver 94 

Ager, Cathy, USGS, Denver 37,53 

Albino, G., USGS, Mackay School of Mines, University of Nevada, Reno, 

NV 89557 62 

Aleinikoff, J.N., USGS, Denver 1 

Ambroziak, R.A., USGS, Reston 7 

Armstrong, A.K., USGS, New Mexico Bureau of Mines and Mineral 

Resources, Socorro, NM 87801 2 

Ayuso, R.A., USGS, Reston 4 

B 

Balistrieri, L.S., USGS, University of Washington School of 

Oceanography, Seattle, WA 98195 59 

Barrera, Luis, Servicio Geologico de Bolivia 48 

Barton, M.D., Department of Geosciences, University of Arizona, Tucson, 

AZ 85721 2 

Barton, P.B., USGS, Reston 4 

Bartsch-Winkler, Susan, USGS, Denver 5, 104 

Bawiec, W.J., USGS, Reston 7 

Belkin, H.E., USGS, Reston 7 

Bishop, K.R., USGS, Menlo Park 103 

Blacutt, W.P., Mining and Geological Engineering Department, 

Mineral Economics Program, University of Arizona, 

Tucson, AZ 85721 8 

Bliss, J.D., USGS, Tucson 11, 40, 42 

Bolm, K.S., USGS, CIMRI, Tucson 73 

Bookstrom, Arthur, USGS, Spokane 12 

Bouse, R.M., University of Arizona, Tucson, AZ 85721 and USGS, Menlo 

Park 13 

Bove, D.J., USGS, Denver 84 

Box, Stephen, USGS, Spokane 12 

Brew, D., USGS, Menlo Park 4 

Briggs, P.H., USGS, Denver 33, 77,78 

Briskey, J.A., Jr., USGS, Reston 19 

Brooks, W .E., USGS, Denver 42, 84 

Bueno do Prado, Milton Guimar~es, Unamgen Mineracao e Metalurgia 

S.A. Santa Barbara, Minas Gerais, Brazil 27 

Bultman, M.W., USGS, Gould-Simpson Building, University of 

Arizona, Tucson, AZ 85721 13,14 

Butler, R.F., Department of Geosciences, University of Arizona, Tucson, 

AZ 85721 butler@mag.geo.arizona.edu 15 

c 

Calderone, G.J., USGS, Denver calderon@greenwood.cr.usgs.gov 15 

Carlson, R.R., USGS, Denver 15, 44 

Centeno-Garcia, E., Department of Geosciences, University of Arizona, 

Tucson, AZ 85721 89 

Chaffee, M.A., USGS, Denver 15 

Church, S.E., USGS, Denver 16, 17 

Clark, R.C., USGS, Reston 58 

Clark, R.N., USGS, Denver 17, 53 

Clark, S.H.D., USGS, Reston 19 

Commeau, J.A., USGS, Woods Hole 81 

Coney, P .J., Department of Geosciences, University of Arizona, 

Tucson, AZ 85721 89 

Conway, C.M., USGS, Flagstaff 18, 75,103 

Cox, D.P., USGS, Menlo Park 19, 42 

114 

Cox, L.J., USGS, Gould-Simpson Building, University of Arizona, 

Tucson,AZ 85721 19 

Crowley, J.K., USGS, Reston 21 

Cunningham, C.G., USGS, Reston 22, 23 

Cygan, G.L., USGS, Reston 26 

D 

Day, W.C., USGS, Denver 42 

Desborough, G.A., USGS, Denver 27 

Detra, David, USGS, Denver 42 

DeWitt, Ed, USGS, Denver 27,87 

Diggles, M.F., USGS, Menlo Park 19 

Dohrenwend, J.C. USGS, Menlo Park 103 

Doughten, M.W., USGS, Reston 26 

Drewes, Harald, USGS, Denver 29 

du Bray, E.A., USGS, Denver 30 

Duval, J.S., USGS, Reston 103 

E 

Eiswerth, Barbara, USGS, 210 E. 7th St., Tucson, AZ 85705 31, 40 

Elliott, J.E., USGS, Denver 44 

Emerick, J.C., Division of Environmental Science and 

Engineering, Colorado School of Mines, Golden, to 80401 101 

Erdman, J.A., USGS, Denver 32, 59, 101 

Ericksen, G.E., USGS, Reston 23 

Erickson, B.M., USGS, Denver 33 

F 

Faulds, J.E., Department of Geology, University of Iowa, 

Iowa City, lA 52242jfaulds@vaxaweeg.uiowa.edu 15 

Ficklin, W.H., USGS, Denver (deceased) 34, 67, 68, 77,78 

Folger, Helen, USGS, Denver 35 

Force, E.R. USGS, Tucson 4, 36 

Fournier, R.O., USGS, Menlo Park 22 

Futa, Kiyoto, USGS, Denver 1 

G 

Gallagher, A.J., USGS, Denver 37, 53 

Gamble, B.M., USGS, Anchorage 4 

Garaype, Enzio, Sao Bento Mineracao S.A. Santa Barbara, 

Minas Gerais, Brazil 27 

Gettings, M.E., USGS, Gould-Simpson Building, University of 

Arizona, Tucson, AZ 85721 14, 37,38 

Gibson, P.C., Mackay School of Mines, University of Nevada-Reno, 

Reno,NV 65 

Goldfarb, R.J., USGS, Denver 4 

Gough, L.P., USGS, Denver 45, 59, 88 

Grauch, R.I., USGS, Denver 39 

Gray, Floyd, USGS, 210 East 7th St., Tucson, AZ 85705 40, 42, 73 

Gray, John, USGS, Denver 78 

Grimes, D.J., USGS, Denver 67 

Gutierrez, John, U.S. Forest Service, Puerto Ordaz, Venezuela 42 
H 

Hageman, Philip, USGS, Denver 78 

Hammarstrom, J.M., USGS, Reston 44 

Hardyman, Richard, USGS, Denver 48 

Haxel, G.B., USGS, Flagstaff 75,103 

Remley, J.J., USGS, Reston 26 

Hendricks, J.D., USGS, Flagstaff 103 

Herring, J.R., USGS, Denver 45 

Hodges, C.A., USGS, Menlo Park 46, 103 

Hoffman, J.D., USGS, Denver 47 

Hofstra, Albert, USGS, Denver 35, 48 

Hon, Ken, USGS, Denver 48 

Houser, B.B., USGS, Tucson 49 

J 

Jachens, R.C., USGS, Menlo Park 103 

John, D.A., USGS, Menlo Park 4 

Johnson, K.M., USGS, Spokane 4 

Jones, A.E., P.O. Box 7633, Reno, NV 89510 64 

K 

Kamilli, R.J., USGS, Tucson Field Office, University of Arizona, 

Gould-Simpson Rm 430, Tucson, AZ 85721 52,94 

King, T.V.V., USGS, Denver 53 

Kingston, M.J., USGS, Reston 103 

Klein, D.P., USGS, Denver 54, 94 

Knepper, D.H., Jr., USGS, Denver 37 

Kover, A.N., USGS, Reston 56 

Krohn, M.D., USGS, Reston 58 

Kulik, D.M., USGS, Denver 44, 58 

L 

Lamothe, P.J., USGS, Menlo Park 70 

Landis, G.P., USGS, Denver 27 

Lee, G.K., USGS, Denver 44 

Lichte, F.E., USGS, Denver 59 

Light, T.D., USGS, Anchorage 19 

Lindsay, James, USGS, Spokane 12 

Lindsey, D.A., USGS, Denver 4 

Lipman, Peter, USGS, Menlo Park 48 

Long, K.R., USGS, CIMRI, Tucson 60, 73 

Ludington, S.D., USGS, Menlo Park 4 

Luedke, R.G., USGS, Reston 61 

Luepke, G., USGS, Menlo Park 81 

M 

Manydeeds, S.A., Bureau of Indian Mfairs, Division of Energy and 

Mineral Resources, 730 Simms St., Lakewood, CO 62 

Marsh, S.P., USGS, Denver 47, 80 

Martins Pereira, Strgio Luiz, Unamgen Mineracao e Metalurgia 

S.A. Santa Barbara, Minas Gerais, Brazil 27 

McCafferty, A.E., USGS, Denver 69 

McGuire, D.J., USGS, Denver 62, 64 

McHugh, J.B., USGS, Denver 68 

McKee, E.H., USGS, Menlo Park 23, 48, 65 

McKown, D.M., USGS, Denver 100 

McLean, Hugh, USGS, Menlo Park 66 

Meier, A.L., USGS, Denver 67,78 

Merewether, E.A., USGS, Denver 97 

115 

Miller, D.M., USGS, Menlo Park 103 

Miller, R.J., USGS, Menlo Park 103 

Miller, W.R., USGS, Denver 68, 90 

Miranda, F.P., Petroleo Brasileiro S/A (Petrobras), Cidade 

Universitaria, Ilha do Fundao, Rio de Janeiro, RJ, 21949-900, 

Brazil 69 

Montour, Maria, USGS, Denver 77,78 

Moring, B.C., USGS, Menlo Park 19 

N,O 

Naeser, C.W., USGS, Reston 23 

Nash, J.T., USGS, Denver 70 

Nelson, C.H., USGS, Menlo Park 70 

Noble, D.C., Mackay School of Mines, University of Nevada-Reno, 

Reno,NV 65 

Nowlan, G.A., USGS, Denver 35, 72, 103 

Orris, G.J., USGS, Tucson 42, 73 

Ortiz, R.F., USGS, Norwest Bank Building, 8th and Main, 

Suite 200, Pueblo, CO 81003 107 

p 

Page, N.J, USGS, Tucson 40, 42, 73 

Palanques, A., Ciencias del Mar, Barcelona, Spain 70 

Pallister, J.S., USGS, Denver 30 

Patchett, P .J., Department of Geosciences, University of Arizona, Tucson, 

AZ 85721 89 

Peacock, T.R., USGS, Denver 33 

Peterman, Z.E., USGS, Denver 1 

Peterson, D.W., USGS, Menlo Park dpetersn@mojave.wr.usgs.gov 15 

Pierce, H.A., USGS, 210 E. 7th St., Tucson, AZ 85705 40, 74 

Pitkin, J.A., USGS, Denver 75 

Plumlee, G.S., USGS, Denver 34, 77, 78, 80 

Poole, F.G., USGS, Denver 81 

Poppe, L.J., USGS, Woods Hole 81 

Q,R 

Quintana, J. R., Estacion Regional del Noroeste, lnstituto de 

Geologia U.N.A.M., Apartado Posta11039, Hermosillo, 

Son., 83000 Mexico 83 

Raines, G.L., USGS, Mackay School of Mines, University of 

Nevada-Reno, Reno, NV 89557 17 

Rattt, J.C., USGS, Denver 84, 94 

Reynolds, S.J., Dept. of Geology, Arizona State University, 

Tempe, AZ 85287-1404 87 

Robinson, G.R., Jr., USGS, Reston 88 

Ruiz, Joaquin, Department of Geosciences, University of Arizona, 

Tucson, AZ 85721 13, 89 

Rye, R.O., USGS, Denver 23 

Rytuba, J.J., USGS, Menlo Park 22, 90,103 

s 

Sanjines, Orlando, Servicio Geologico de Bolivia 48 

Sawyer, D.A., USGS, Denver 91 

Schoonmaker, J.W., Jr., USGS, Reston 56 

Schruben, P.G., USGS, Reston 92 

Senterfit, R.M., USGS, Denver 94 

Severson, R.C., USGS, Denver 78 

Sidder, G.B., USGS, Denver 42 

Silberman, M.L., USGS, Denver 108 
Silver, D.B., Balfour Holdings, Inc., 3891 E. Irwin Place, 

Littleton, CO 80122 95 

Slack, J.F., USGS, Reston 96 

Smith, B.D., USGS, Denver 97 

Smith, Cole, USGS, Spokane 1l 

Smith, K.S., USGS, Denver 32, 34, 77, 78 

Smith, S.M., USGS, Denver 80, lOS 

Sparck, H.M., Harold Sparck and Associates, Box 267, 

Bethel, AK. 99559 7 

Staude, J.-M. G., USGS, C.I.M.R.I., Tucson, AZ 85719 and 

Department of Geosciences, University of Arizona, 

Tucson,AZ 85721 2,73,99 

Stewart, K.C., USGS, Denver 100, lOS 

Stout, P.R., Division of Environmental Science and Engineering, 

Colorado School of Mines, Golden, CO 80401 101 

Sutphin, D.M., USGS, Reston 102 

Swanson, K.E., Mackay School of Mines, University of 

Nevada-Reno, Reno, NV 65 

Swayze, G.A., USGS, Denver 17,53 

Swierk, Robert, Stanford University, Stanford, CA 92 

Sylvester, M.A., USGS, Menlo Park 103 

T 

Theodorakos, P.M., USGS, Denver 15 

Theodore, T.G., USGS, Menlo Park 103 

Thorman, C.H., USGS, Denver 27 

Tidball, R.R., USGS, Denver 104, lOS 

Tilley, S.R., Department of Geosciences, University of Arizona, 

Tucson, AZ 85721 13 

Torres-Vargas, R., Department of Geosciences, Universitiy of 

Arizona, Tucson, AZ 85721 89 

Tosdal, R.M., USGS, Menlo Park 13, 103, 106 

Toth, M.l., USGS, Denver 80 

Tremper, C.W., USGS, Reston 58 

Trudel, Wayne, AMAX Gold, Inc., Sleeper Mine, Winnemucca, NV 

89445 70 

116 

U,V 

Unkefer, Jason, Department of Geology, Vanderbilt University, 

Nashville, TN 42 

Van Geen, A., USGS, Menlo Park 70 

Van Gosen, B.S., USGS, Denver 44 

Vieira, F.W.R., Mineracao Morro Velho, Nova Lima, Minas Gerais, 

Brazil 27 

Vikre, P.T., ASARCO Inc., 510 East Plumb Lane, Reno NV 89502 22 

von Guerard, Paul, USGS, Norwest Bank Building, 8th and Main, 

Suite 200, Pueblo, CO 81003 107 

w 

Wallace, A.R., USGS, Denver 19 

Walter, M., USGS, Denver 1 

Walton-Day, Katherine, USGS, Denver 107 

Watson, Ken, USGS, Denver 37 

Wenrich, K.J., USGS, Denver 108 

Widmann, B., USGS, Denver 1 

Wooden, J.L., USGS, Menlo Park 13 

Wynn, J.C., USGS Mission in Saudi Arabia 42 

Y,Z 

Yanosky, T.M., USGS, Reston 59 

Yanez, P., Department of Geosciences, University of Arizona, 

Tucson,AZ 85721 89 

Zartman, R.E., USGS, Denver 23, 27 

Zientek, M.L., USGS, Spokane 44 

ZUrcher, Lukas, Department of Geosciences, University of Arizona, 

Tucson, AZ 85721 2 
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Manuscript approved for publication December 1, 1993 

Published in the Central Region, Denver, Colorado 

Photocomposition by Shelly A. Fields 

Final graphics preparation by Gayle M. Dumonceaux, Wayne 

Hawkins, and Carol A. Quesenberry 

119 

*U.S. ~ PRINl'IR'; OFFICE: 1993-573-038/86013 

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