SSESSMENT OF UNDISCOVERED GOLD, SILVER, COPPER, LEAD, AND ZINC, U.S.—U.S. Geological Survey Circular 1178

in the United States 

U.S. GEOLOGICAL SURVEY CIRCULAR 1178 
Region 1, Northern Appalachian Mountains

Region 2, Adirondack Mountains

Region 3, Southern Appalachian Mountains

Region 4, East-central

Region 5, Great Plains

Region 6, Lake Superior

Region 7, Nothern Rocky Mountains

Region 8, Central and Southern Rocky Mountains

Region 9, Colorado Plateau

Region 10, Pacific Coast

Region 11, Great Basin

Region 12, Southern Basin and Range

Region 13, Brooks Range

Region 14, East-central Alaska

Region 15, West-central Alaska

Region 16, Southeastern Alaska

Region 17, South-central Alaska

Region 18, Southwestern Alaska

Region 19, Alaska Peninsula and Aleutian Islands

Cover. Geographic regions assessed in this report. See figure 3 for region numbers.
U.S. Department of the Interior 

U.S. Geological Survey 

1998 Assessment of Undiscovered 

Deposits of Gold, Silver, Copper, Lead, and 

Zinc in the United States 

By the U.S. Geological Survey National Mineral Resource Assessment Team 

U.S. GEOLOGICAL SURVEY CIRCULAR 1178 

Overview of the 1998 National Mineral Resource Assessment 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

CHARLES G. GROAT, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 2000 


Published in the Eastern Region, Reston, Va. 


Manuscript approved for publication February 17, 1999. 


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.
 

Free on application to
 

U.S. Geological Survey
 

Information Services
 

Box 25286, Federal Center
 

Denver, CO 80225
 

or call the toll-free telephone number
 

1–888–ASK–USGS
 

Library of Congress Cataloging in Publication Data 

U.S. Geological Survey National Mineral Resource Assessment Team. 1998 assessment of undiscovered deposits of gold, 

silver, copper, lead, and zinc in the United States/ by the U.S. Geological Survey National Mineral Resource Assessment 

Team. 

p. cm.—(U.S. Geological Survey circular; 1178) 

Includes bibliographical references. 

1. Mines and mineral resources—United States. 2. Ore-deposits—United States. I. Series
 

TN23.U57 1999
 

553.4’0973—dc21 99–29062
 

CIP 
CONTENTS
 

Summary of Results........................................................................................................ 1
 

Introduction..................................................................................................................... 1
 

Acknowledgments................................................................................................... 1
 

Commodities Assessed.................................................................................................... 1
 

Assessment Categories.................................................................................................... 1
 

Terminology .................................................................................................................... 3
 

Data Base ........................................................................................................................ 4
 

Areas of Study................................................................................................................. 5
 

Method of Assessment .................................................................................................... 5
 

Undiscovered Deposits............................................................................................ 5
 

Probability of No Deposits...................................................................................... 7
 

Monte Carlo Simulation.......................................................................................... 7
 

Aggregation and Dependency ................................................................................. 7
 

Significant Deposits ................................................................................................ 7
 

Results of the Assessment............................................................................................... 7
 

Gold......................................................................................................................... 10
 

Silver ....................................................................................................................... 10
 

Copper..................................................................................................................... 10
 

Lead......................................................................................................................... 10
 

Zinc ......................................................................................................................... 10
 

Discussion and Comparison of Results with the 1973 National 

Mineral Resource Assessment ............................................................................ 13
 

Gold......................................................................................................................... 13
 

Silver ....................................................................................................................... 13
 

Copper..................................................................................................................... 14
 

Lead......................................................................................................................... 14
 

Zinc ......................................................................................................................... 15
 

Some Considerations for Other Uses of the Assessment ................................................ 15
 

Extensions to Identified Resources ................................................................................. 15
 

Implications..................................................................................................................... 16
 

References Cited ............................................................................................................. 16
 

Members of the U.S. Geological Survey 1998 National
 

Mineral Resource Assessment Team................................................................... 18
 

COMPACT DISC 

[In pocket] 

1998 Assessment of Undiscovered Deposits of Gold, Silver, Copper, Lead, and Zinc in 

the United States—CD-ROM Containing Circular Report, Data Base, and Graphical 

Display Program 

III 
IV CONTENTS 

FIGURES 

1.	 Pie charts showing estimated resources and past production of gold, silver, copper, lead, and zinc in 

the United States.................................................................................................................................................... 2 

2. Diagram showing mineral resource classification used in this report................................................................... 3 

3. Map showing geographic regions assessed in this report ..................................................................................... 6 

4–8. Pie charts showing— 

4. Gold in undiscovered mineral deposits ......................................................................................................... 12 

5. Silver in undiscovered mineral deposits........................................................................................................ 12 

6. Copper in undiscovered mineral deposits ..................................................................................................... 12 

7. Lead in undiscovered mineral deposits ......................................................................................................... 12 

8. Zinc in undiscovered mineral deposits.......................................................................................................... 12 

TABLES 

1.	 Estimates of undiscovered deposits, identified resources, past production, and discovered resources of 

gold, silver, copper, lead, and zinc in the United States .............................................................................................. 2 

2.	 Deposit models and their descriptions and major metals used in quantitative assessment of 305 of the 

447 permissive tracts delineated in the 1998 National Mineral Resource Assessment ............................................... 8 

3. Estimates of contained metal in undiscovered deposits in the United States .............................................................. 11 

4. Summary of identified and hypothetical copper resources in the United States, 1973 ............................................... 14 

5. Summary of speculative copper resources in the United States, 1973 ........................................................................ 14 

METRIC CONVERSION FACTORS 

For readers who wish to convert measurements from the metric system of units to the 

inch-pound system of units, the conversion factors are listed below: 

Multiply By To obtain 

gram (g) 0.03215 ounce (troy) 

metric ton (t) 1.102 short ton 

kilometer (km) 0.6214 miles 

Note: Throughout this report, the amounts of gold and silver are expressed in metric tons; 

the amounts of copper, lead, and zinc are expressed in thousand metric tons (kilotons). 
1998 Assessment of Undiscovered Deposits of Gold, Silver, 

Copper, Lead, and Zinc in the United States 

By the U.S. Geological Survey National Mineral Resource Assessment Team 

SUMMARY OF RESULTS 

This report summarizes the results of the 1998 

National Mineral Resource Assessment that estimated the 

gold, silver, copper, lead, and zinc in undiscovered deposits 

in the United States. This project also estimated the identified resources and past production of these five metals. 

Assessment results include the following: 

•	 It is estimated that 18,000 metric tons (t) of gold, 

460,000 t of silver, 290,000 kilotons (kt) of copper, 

85,000 kt of lead, and 210,000 kt of zinc are in undiscovered deposits minable with existing technology. 

•	 In addition, it is estimated that 15,000 t of gold, 

160,000 t of silver, 260,000 kt of copper, 51,000 kt of 

lead, and 55,000 kt of zinc remain in identified 

resources. 

•	 Past production from the largest identified resources 

of gold, silver, copper, lead, and zinc is estimated to 

be 12,000 t of gold, 170,000 t of silver, 91,000 kt of 

copper, 41,000 kt of lead, and 44,000 kt of zinc. 

These deposits account for about 99 percent of cumulative domestic production in the United States. 

Estimated total resources of gold, silver, copper, lead, 

and zinc in the United States are listed in table 1 and shown 

in figure 1. 

INTRODUCTION 

How much of our Nation’s total mineral wealth has 

already been discovered? How much is left? These are 

important questions, and discovering the answers to them is 

an ongoing part of the mission of the U.S. Geological Survey (USGS). To address these questions, the USGS undertook to estimate, for the first time in probabilistic terms, the 

amounts of gold, silver, copper, lead, and zinc in undiscovered deposits believed to exist 1 kilometer (km) or less 

below the surface of the ground in the United States. This 

effort has resulted in the 1998 National Mineral Resource 

Assessment. 

As the Nation’s population grows and its economy 

matures, greater attention is being paid to such issues as 

land use and environmental quality, as well as to 

sustainability of mineral supplies to provide for the needs of 

future generations. National Mineral Resource Assessments 

provide a framework for addressing these issues by 

monitoring the continuing depletion of the Nation’s 

nonrenewable mineral wealth and by contributing 

information required to manage resource extraction while 

minimizing consequent environmental effects. For 

responsible stewardship of the Nation’s lands and resources, 

it is important to know in which areas future mineral 

resources may be located and how much metal such 

resources might contain (McCammon and Briskey, 1992). 

ACKNOWLEDGMENTS 

The USGS wishes to thank numerous individuals in 

industry, academia, and State geological organizations for 

their help. The State Geologists of many of the States were 

particularly helpful in reviewing material that related to the 

significant deposits in their States. 

COMMODITIES ASSESSED 

The commodities gold, silver, copper, lead, and zinc 

were chosen as the subject for this National Mineral 

Resource Assessment because, after iron and aluminum, 

they are the most valuable metals in our economy. These 

commodities have been produced widely and extensively. 

They also tend to occur together in nature, thereby introducing efficiencies in the estimation of undiscovered resources. 

Other mineral commodities may be the subject of future 

assessments. 

ASSESSMENT CATEGORIES 

The resource classification used in this report is 

illustrated in figure 2, a modified “McKelvey diagram” 

(McKelvey, 1972). Resources are classified along two 

axes—geologic certainty that the resource exists and 

economic feasibility of their development. The degree of 

geologic certainty decreases to the right, and the degree of 

economic feasibility decreases downward. Cumulative past 

1 
2 1998 ASSESSMENT OF UNDISCOVERED DEPOSITS IN THE UNITED STATES 

Table 1. Estimates of undiscovered deposits, identified resources, past production, and discovered resources of gold, silver, copper, lead, 

and zinc in the United States. 

[Undiscovered deposits, deposits believed to exist 1 kilometer or less below the surface of the ground within a geologically defined area; identified 

resources, resources whose location, grade, quality, and quantity are known or estimated from specific geologic evidence. Totals rounded to two significant 

figures] 

Category 

Gold 

(tons) 

Silver 

(tons) 

Copper 

(kilotons) 

Lead 

(kilotons) 

Zinc 

(kilotons) 

Undiscovered U.S. deposits, 

estimated --------------------- 18,000 460,000 290,000 85,000 210,000 

Identified U.S. resources1------- 15,000 160,000 260,000 51,000 55,000 

Past U.S. production------------- 12,000 170,000 91,000 41,000 44,000 

Discovered U.S. resources ---- 1 27,000 330,000 350,000 92,000 99,000 

1

Estimates from Long and others (1998). Discovered U.S. resources are the sum of identified U.S. resources and past U.S. production. 

Undis Prod Prod 

40% 27% 

Iden 

20% 

22% 

Undis 

Iden 58% A 33% B

Prod 

14% 

Iden 

41% 

Prod 

14% 

Prod 

23% 

Iden 

29% 

Undis 

45% 

Undis 

48% 

Iden 

18% 

Undis C D 68% E
 

Figure 1. Estimated resources and past production (as percentage of estimated total resources) of gold, silver, copper, lead, and zinc in 

the United States. A, gold; B, silver; C, copper; D, lead; E, zinc. Prod, past production; Iden, identified resources; Undis, undiscovered 

resources. 
TERMINOLOGY 3

ECONOMIC FEASIBILITY 

Cumulative 

production 

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES 

ECONOMIC 

MARGINALLY 

ECONOMIC 

SUBECONOMIC 

Demonstrated 

Measured Indicated 

Inferred 

Reserve 

base 

Inferred 

reserve 

base 

This 

report 

Probability range 

Hypothetical Speculative 

GEOLOGIC CERTAINTY 

Figure 2. Mineral resource classification used in this report (modified from McKelvey, 1972; U.S. Bureau of Mines and U.S. 

Geological Survey, 1980; U.S. Geological Survey, 1998). The degree of geologic certainty decreases to the right; the degree of economic 

feasibility decreases downward. 

production is not part of identified resources and thus is not 

indicated in figure 2. The undiscovered resources estimated 

in this report include the hypothetical category and part of 

the speculative category. Identified resources include the 

demonstrated and inferred categories. 

TERMINOLOGY 

The terminology used in this report is intended to represent standard definitions and usage by the minerals industry and the resource assessment community (U.S. Bureau of 

Mines and U.S. Geological Survey, 1980; American Geological Institute, 1997). No attempt has been made to 

include a detailed listing of all the definitions; rather, those 

definitions that are essential to the proper understanding of 

this report are presented. Some of the definitions are 

intended to be generally explanatory rather than strictly 

technical. A few of the definitions are specific to this report. 

Mineral deposit. A mineral concentration of sufficient size 

and grade richness that it might, under the most favorable of 

circumstances, be considered to have potential for economic 

development. 

Undiscovered mineral deposit. A mineral deposit 

believed to exist 1 km or less below the surface of the 

ground, or an incompletely explored mineral occurrence 

or prospect that could have sufficient size and grade to be 

classed as a deposit. 

Significant mineral deposit. A mineral deposit known 

or believed to contain at least 2 t of gold, or 85 t of silver, 

or 50,000 t of copper, or 35,000 t of lead, or 50,000 t of 

zinc (Singer, 1995). 

Descriptive mineral deposit model. A set of data in a convenient form that describes a group of mineral deposits having similar characteristics. 

Grade and tonnage model. Frequency distributions of the 

grade and size of well-explored individual mineral deposits. 

Permissive tract. A geographic area delineated such that 

the probability of deposits of the type delineated occurring 

outside the boundary is negligible. 

Resource. A mineral concentration of sufficient size and 

grade and in such form and amount that economic extrac-
4 1998 ASSESSMENT OF UNDISCOVERED DEPOSITS IN THE UNITED STATES 

tion of a commodity from the concentration is currently or 

potentially feasible. 

Identified resources. Resources whose location, grade, 

quality, and quantity are known or are estimatable from specific geologic evidence. 

Demonstrated resource. The sum of measured plus 

indicated resources. 

Measured resource. Quantity computed from dimensions revealed in outcrops, trenches, workings, or drill 

holes; grade and (or) quality computed from the 

results of detailed sampling. The sites for inspection, 

sampling, and measurement are spaced so closely and 

the geologic character is so well defined that size, 

shape, depth, and mineral content of the resource are 

well established. 

Indicated resource. Quantity and grade and (or) 

quality computed from information similar to that 

used for measured resources. However, the sites for 

inspection, sampling, and measurement are farther 

apart or are otherwise less adequately spaced. The 

degree of assurance, although lower than that for measured resources, is high enough to assume continuity 

between points of observation. 

Reserve base. That part of an identified resource that 

meets specified minimum physical and chemical criteria related to current mining and production practices. The reserve base is the in-place demonstrated 

(measured plus indicated) resource from which 

reserves are estimated and includes those resources 

that are currently economic (reserves), those that are 

marginally economic, and some of those that are currently subeconomic. 

Reserves. That part of the reserve base that can be 

economically extracted or produced at the time of 

determination. 

Inferred resource. Estimates of resource that are based 

on an assumed continuity beyond measured and (or) indicated resources for which there is geologic evidence. 

Inferred resources may or may not be supported by samples or measurements. 

Inferred reserve base. The postulated extension of 

the reserve base. The inferred reserve base is an estimate based on an assumed continuity beyond identified resources for which there is geologic evidence. 

The inferred reserve base is the in-place part of an 

identified resource from which inferred reserves are 

estimated. 

Inferred reserves. The postulated extension of 

reserves. Inferred reserves are estimates that are 

based on an assumed continuity beyond measured 

and (or) indicated reserves for which there is geologic evidence. 

Undiscovered resources. Resources in undiscovered mineral deposits whose existence is postulated on the basis of 

indirect geologic evidence. 

Hypothetical resources. Undiscovered resources in 

known types of mineral deposits postulated to exist in 

favorable geologic settings where other deposits of the 

same types have been mined. 

Speculative resources. Undiscovered resources that may 

occur either in known types of deposits in favorable geologic settings where mineral discoveries have not been 

made or in types of deposits as yet unrecognized for their 

economic potential. 

Cumulative past production. The total amount of all past 

production. 


Discovered resources. The total amount of identified 


resources and cumulative past production. 


DATA BASE 

The National Mineral Resource Assessment is based 

mainly on data collected and reviewed from 1993 to 1998. 

The data were either published, commercially available, or 

from unpublished USGS studies. Data sources included the 

following: 

•	 Published and unpublished USGS geologic, 

geochemical, geophysical, mineral deposit, and mineral-occurrence data at scales of 1:500,000 and 

1:1,000,000 that were used to define the boundaries 

of tracts of land permissive for the occurrence of 

undiscovered mineral deposits. 

•	 USGS data on mineral deposit models that included 

descriptive and grade and tonnage models. 

•	 Data from the USGS and the former U.S. Bureau of 

Mines on past production. 

•	 Production and resource data from Securities and 

Exchange Commission 10K forms. 

•	 Geologic, geochemical, geophysical, mineral deposit, 

and mineral-occurrence data maintained by State geological surveys. 

• Published articles in the academic literature. 

• 	 Periodicals and newsletters that serve the mining 

industry. 

The data base of the tracts that were used to generate 

the resource estimates obtained in the National Mineral 

Resource Assessment is stored on a Compact Disc-Read 

Only Memory (CD-ROM), which is included in a pocket at 

the end of this report. The CD-ROM also contains this circular and a browser that can be used to examine the tracts by 

region and by major deposit model type and the significant 

mineral deposits that occur in the tracts. The data base consists of the following: 

•	 Estimates of the tonnages and amounts of contained 

metal in undiscovered deposits, 
METHOD OF ASSESSMENT 5 

•	 A table that lists the mineral deposit model types used 

in the National Mineral Resource Assessment, 

•	 A table that lists the tonnages and amounts of contained metal in the significant mineral deposits in the 

United States, 

•	 The rationale that was used to assess each of the tracts 

in the National Mineral Resource Assessment, and 

• A list of references for each tract in the National Mineral Resource Assessment. 

The browser is a map-based, stand-alone Windows 

application that can be installed from the CD-ROM. The 

browser allows a user to click on a tract for a selected mineral deposit model type and to display either the tabular data 

associated with the tract or the rationale that was used to 

assess the tract. The rationale contains hypertext links to the 

tonnage and amounts of contained metal in undiscovered 

deposits, the mineral deposit model type used to assess the 

particular tract, and the significant mineral deposits associated with the tract. A user can also choose to display the 

assessment regions, mining districts, and the significant 

mineral deposits, as well as several standard thematic layers 

such as political boundaries, cities, airports, rivers, and 

color-shaded relief. 

AREAS OF STUDY 

The undiscovered resources of gold, silver, copper, 

lead, and zinc in the United States were evaluated by interpreting the known geology of the 19 assessment regions 

shown in figure 3. The regions were constructed to provide 

broad geologic groupings of the Nation’s mineral-producing 

regions. The regions are named for structural or geographic 

features within their boundaries. 

METHOD OF ASSESSMENT 

The method used to estimate the quantity and quality 

of undiscovered deposits of gold, silver, copper, lead, and 

zinc was based on the three-part form of quantitative assessment described by Singer (1993). In three-part assessments, 

areas are delineated according to the types of deposits permitted by the geology, the amount of metal in typical deposits is estimated by using grade and tonnage models, and the 

number of undiscovered deposits of each type is estimated 

by using a variety of subjective methods. Estimates of the 

number of undiscovered deposits are consistent in that the 

geologic settings of the delineated areas are consistent with 

the geologic settings for the descriptive models, as well as 

for the identified resources in the area and the deposits that 

constitute the grade and tonnage models. Every effort is 

made to incorporate the available information in the estimates, and the uncertainty is explicitly represented. The 

three-part form of quantitative assessment has been applied 

by the USGS since 1975. 

UNDISCOVERED DEPOSITS 

The national assessment was conducted by 19 regional 

assessment teams of scientists from the USGS. Each team 

comprised geologists, geochemists, geophysicists, and 

resource analysts. To begin an assessment, existing data for 

large (Alaska) and multi-State assessment regions were 

compiled into maps at scales of 1:500,000 and 1:1,000,000. 

For each region, a team reviewed the geology and decided 

upon the appropriate mineral deposit models to be used in 

making the assessment. On the basis of all the available 

information, permissive tracts then were delineated for each 

deposit model type such that the probability was negligible 

that undiscovered deposits for a particular deposit model 

type would occur outside the tract boundary. 

Maps showing the geology and the locations and types 

of mineral deposits and occurrences were used in outlining 

these permissive tracts. Geophysical and geochemical maps 

also were useful, as well as knowledge of the exploration 

history of the areas. 

Estimates of the undiscovered resources were made to 

a depth of 1 km beneath the surface. If an area of otherwise 

permissive rock was covered by more than 1 km of rock 

known or was inferred to be barren, then the area was 

excluded from the tract. The somewhat arbitrary depth of 1 

km was chosen as the limit below which the existence of 

undiscovered deposits would not be considered because of 

difficulties in acquiring and (or) extrapolating geologic, 

geochemical, and geophysical data to greater depths. 

Because some mineral deposits have vertical extents of 

more than 1 km, the rule was adopted such that if any part of 

a deposit was judged to be within 1 km of the surface, then 

the whole deposit was counted. This rationale is consistent 

with mining practice. Although direct mineral exploration is 

seldom conducted below 1 km, deposits may be explored 

and developed up to 3 km deep once they have been discovered at lesser depths. 

The regional assessment teams reviewed grade and 

tonnage data for all identified resources in a tract, then 

decided whether worldwide models were applicable, and 

modified the models if necessary. In most cases, the models 

used were those described in Cox and Singer (1986) and 

Bliss (1992). In cases for which it was not possible to use 

these worldwide models, local models were used. The 

undiscovered deposits estimated to exist in the tract were 

treated as if they would be similar in grade and tonnage to 

the deposits in the deposit model (either accepted or modified). 

Where available information warranted making 

quantitative estimates, the teams then estimated the number 

of undiscovered deposits of each deposit model type that 

might exist in each permissive tract. These estimates were 

made by subjective interpretation and extrapolation of 

available earth science information. The number of 

undiscovered deposits was estimated at different levels of 
6 1998 ASSESSMENT OF UNDISCOVERED DEPOSITS IN THE UNITED STATES

16

13

14

17

18

2

8

9 4

5

12

10

11

1

3

7

6

 1 

 2 

 3 

 4 

 5 

 6 

 7 

 8 

 9 

10 

11 

12 

0

0

500

500

1000 MILES

1000 KILOMETERS

15

19

13 

14 

15 

16 

17 

18 

19 

 Aleutian Islands

Figure 3. Geographic regions assessed in this report. Heavy lines, region boundaries; light lines, State boundaries.

Northern Appalachian Mountains

Adirondack Mountains

Southern Appalachian Mountains

East-central

Great Plains

Lake Superior

Northern Rocky Mountains

Central and Southern Rocky Mountains

Colorado Plateau

Pacific Coast

Great Basin

Southern Basin and Range

Brooks Range

East-central Alaska

West-central Alaska

Southeastern Alaska

South-central Alaska

Southwestern Alaska

Alaska Peninsula and
RESULTS OF THE ASSESSMENT 7 

certainty. For example, on the basis of all the available 

information, a team might estimate that there was a 

90-percent chance or better that one or more undiscovered 

deposits existed; a 50-percent chance or better that three or 

more existed; and a 10-percent chance or better that five or 

more existed. Commonly, estimators made estimates for 

five levels of certainty—90th, 50th, 10th, 5th, and 1st 

percentiles. 

The estimates were made by scientists who had 

detailed knowledge of an area and (or) the selected deposit 

model. The teams used a variety of methods to arrive at a 

consensus. The most common method was simply to 

continue their discussion until general agreement was 

reached. Decision-aiding tools, such as deposit spatial 

density models and assumptions about the extent of 

exploration, also were used to guide the final estimates 

(Singer, 1993). The result of the estimation process was a 

probability distribution of the number of undiscovered 

deposits. In cases where the grade and tonnage model was 

based on data for a mineral district rather than for individual 

deposits, the result was a probability distribution of the 

number of undiscovered districts. 

PROBABILITY OF NO DEPOSITS 

In some cases, it was necessary to estimate the probability of no deposits. This was done primarily when estimates of the numbers of undiscovered deposits at all levels 

of certainty were small and the estimators wished to further 

constrain the probability density estimates. 

MONTE CARLO SIMULATION 

A Monte Carlo simulation computer program was used 

to combine the probability distribution of the number of 

undiscovered deposits with the grade and tonnage data sets 

associated with each deposit model to obtain the probability 

distribution for undiscovered metal in each tract (Root, 

Menzie, and Scott, 1992; Root, Scott, and Selner, 1997). 

Estimates of the undiscovered metal in each tract were represented as a range of values corresponding to probabilities 

of occurrence to express the uncertainty inherent in the 

assessment of unknown quantities. The input variables—the 

number of undiscovered deposits and the tonnages and 

grades of the deposits in the deposit models—were 

expressed as cumulative probability distributions of uncertain values. The resulting cumulative probability distributions represented the estimated quantity of gold, silver, 

copper, lead, and zinc. From these distributions, various 

fractiles, including the low (F90), the high (F10), and the 

mean estimates, were obtained. 

Unrealistically large values for grades and tonnages of 

undiscovered deposits can result from the highly skewed 

cumulative probability distributions, which are often characteristic of deposit data. To avoid such unrealistic values, 

piecewise linear approximations of the tonnage and grades 

of metals in the deposit data were used in the simulations. 

To preserve the dependencies of grades in the deposit data, 

the deposits within each model were grouped into suites 

according to the metals they contained, and each suite was 

sampled proportionally to its frequency of occurrence for 

that deposit model. Also, the tonnages of the deposits in 

each suite had different distributions depending upon the 

suite of metals present. 

AGGREGATION AND DEPENDENCY 

To arrive at the estimated quantity of undiscovered 

deposits of gold, silver, copper, lead, and zinc for large 

areas, such as geographic regions or the Nation as a whole, 

distributions estimated for individual tracts were progressively aggregated, with geologic dependency incorporated 

at each level of aggregation. To aggregate tracts within 

regions, tracts assessed by the same team of estimators were 

assumed to be perfectly correlated, whereas tracts assessed 

by different teams were assumed to be uncorrelated. In 

aggregating regions to obtain a national total, regions were 

assumed to be statistically independent. 

SIGNIFICANT DEPOSITS 

As a part of the 1998 National Mineral Resource 

Assessment, a data base was compiled for the largest identified resources of gold, silver, copper, lead, and zinc in the 

United States (Long and others, 1998). The resources in 

this data base accounted for about 99 percent of domestic 

production of these metals and probably a similar share of 

identified resources. 

Production data for deposits in the data base were 

stated in terms of metals recovered. Generally, between 40 

and 97 percent of the metal originally contained in the ores 

was recovered, depending on the efficiency of the extractive 

processes used and the quality of labor and management. 

The data included those situations in which the initial mining operations recovered so little of the contained metal that 

the resulting waste heaps were reprocessed later. Resource 

data for deposits in the data base were stated in terms of 

metals contained in remaining material. The sum of the past 

production and the remaining (identified) resources was the 

total discovered resources reported herein. 

RESULTS OF THE ASSESSMENT 

A total of 55 major deposit model and submodel types 

were used to delineate 447 permissive tracts for the 1998 

assessment. Quantitative assessments were made for 305 of 

these tracts (CD-ROM, in pocket). The 30 major deposit 

model types used in the quantitative assessments are listed 

in table 2. 
-------------

------------

------------------------

-------

-----

--------------------------------

-----------------------

------------

Table 2. Deposit models and their descriptions and major metals used in quantitative assessment of 305 of the 447 permissive tracts delineated in 

Resource Assessment. 

[Deposit models based on Cox and Singer (1986) and Bliss (1992). The plutonic porphyry gold deposit model is based on Hollister (1992). For grade and tonnage data for 

Scott, and Selner (1997)] 

Deposit model Description 

Alaskan platinum group elements Crosscutting ultramafic to felsic intrusive rocks with approximately concentric zoning of rock types that contain 

(PGE) ----------------------------- chromite, platinum, and titanium-vanadium-magnetite. 

Alkaline gold-tellurium ----------- Veins or breccias associated with alkalic igneous rocks that contain gold ± gold tellurides ± vanadian micas. Gold skarn--------------------------- Gold in skarns near intrusive igneous contacts. Includes copper, zinc, lead, and iron skarns with gold as a major 

commodity. 

Basaltic copper --------------------- Disseminated native copper and copper sulfides in the upper parts of thick sequences of subaerial basalt. Besshi massive sulfide Thin, sheetlike bodies of massive to well-laminated pyrite, pyrrhotite, and chalcopyrite within thinly laminated 

clastic sediments associated with basalt flows and tuffs. 

Comstock epithermal vein -------- Gold, electrum, silver sulfosalts, and argentite in vuggy quartz-adularia veins hosted by felsic to intermediate vol- 

canic rocks that overlie predominantly clastic sedimentary rocks and their metamorphic equivalents. 

Creede epithermal vein Galena, sphalerite, chalcopyrite, sulfosalts, ± tellurides, ± gold in quartz-carbonate veins hosted by felsic to inter- 

mediate volcanics. 

Copper skarn Chalcopyrite in calc-silicate metasomatic rocks near contacts with weakly mineralized igneous intrusives. Distal disseminated silver-gold--- Disseminated silver and gold mainly in sedimentary rocks distal to porphyry copper, skarns, and polymetallic 

veins. 

Homestake, stratiform gold Gold in massive persistent quartz veins mainly in shear zones in regionally metamorphosed Archean and Protero- 

zoic volcanic rocks and volcanic sediments. 

Hot spring gold-silver ------------- Fine-grained silica and quartz in silicified breccia with gold, pyrite, and antimony and arsenic sulfides. 

Kuroko massive sulfide------------ Copper- and zinc-bearing massive sulfide deposits in marine volcanic rocks of intermediate to felsic composition. 

Low-sulfide gold-quartz vein Gold in massive persistent quartz veins mainly in shear zones in regionally metamorphosed volcanic rocks and vol- 

canic sediments. 

Mississippi Valley/southeast Mis- Stratabound, carbonate-hosted deposits of galena, sphalerite, and chalcopyrite in rocks having primary and second­ 

souri lead-zinc ------------------- ary porosity, commonly related to reefs on paleotopographic highs. 

Mississippi Valley/Appalachian Stratabound deposits of sphalerite and minor galena in primary and secondary voids in favorable beds or horizons 

zinc in thick platform dolostone and limestone. 

Native copper Native copper filling vesicles, breccias, and fractures in subaerial basalt. Placer gold -------------------------- Elemental gold and platinum-group alloys in grains and (rarely) nuggets in gravel, sand, silt, and clay and their 

consolidated equivalents, in alluvial, beach, eolian, and (rarely) glacial deposits. 

Plutonic porphyry gold Gold in stockwork veinlets in hydrothermally altered granodiorite and quartz monzonite porphyry. 
Deposit model Description 

Polymetallic replacement --------- Hydrothermal, epigenetic, silver, lead, zinc, and copper minerals in massive lenses, pipes, and veins in limestone, 

dolomite, or other soluble rock near igneous intrusive contacts. 

Polymetallic vein------------------- Quartz-carbonate veins with gold and silver associated with base metal sulfides related to hypabyssal intrusions in 

sedimentary and metamorphic terranes. 

Porphyry copper-------------------- Generalized model includes various subtypes, all of which contain chalcopyrite in stockwork veinlets in hydrother- 

mally altered porphyry and adjacent country rock. 

Porphyry copper, skarn-related--- Chalcopyrite in stockwork veinlets in hydrothermally altered and mineralized porphyry and in calc-silicate contact 

metasomatic rocks with extensive retrograde alteration. 

Porphyry copper-gold ------------- Stockwork veinlets of chalcopyrite, bornite, and magnetite in porphyritic intrusions and coeval volcanic rocks. 

Ratio of gold (parts per million) to molybdenum (percent) is greater than 30. 

Porphyry copper-molybdenum --- Stockwork veinlets of quartz, chalcopyrite, and molybdenite in or near a porphyritic intrusion. Ratio of gold (in 

parts per million) to molybdenum (percent) is less than 3. 

Quartz-adularia epithermal vein - Gold, electrum, silver sulfosalts, and argentite in vuggy quartz-adularia veins hosted by felsic-to-intermediate vol- 

canic rocks that overlie unspecified basement. 

Sediment-hosted gold ------------- Very fine grained gold, pyrite, and arsenic-antimony sulfides disseminated in carbonaceous calcareous and sili- 

ceous sedimentary rocks and associated jasperoids. 

Sediment-hosted copper----------- Stratabound, disseminated copper sulfides in reduced beds of red-bed sequences. 

Sedimentary exhalative zinc-lead Stratiform basinal accumulations of sulfides and barite interbedded with euxinic marine sediments form sheet- or 

lenslike ore bodies tens of meters thick, distributed through a stratigraphic interval of more than 1,000 m. 

Synorogenic-synvolcanic Massive lenses, matrix and disseminated sulfide in small to medium-sized gabbroic intrusions in greenstone belts. 

nickel-copper --------------------

Zinc-lead skarn --------------------- Sphalerite and galena in calc-silicate rocks near igneous intrusive contacts. 
10 1998 ASSESSMENT OF UNDISCOVERED DEPOSITS IN THE UNITED STATES 

Examples of submodels that were used include a British Columbia/Alaska porphyry copper submodel, a North 

America porphyry copper submodel, a modified gold-skarn 

deposit model for the Western United States, a 

reduced-facies submodel for the sediment-hosted copper 

deposit model, a Precambrian-only submodel for the kuroko 

massive sulfide deposit model, and an Archean submodel 

for the low-sulfide gold-quartz vein deposit model. In one 

case, a single deposit, Chicken Mountain in Alaska, was the 

sole representative of a plutonic porphyry gold submodel. 

Submodels were constructed and used when the teams’ 

detailed knowledge of the local geology suggested that the 

general models did not adequately represent identified mineral deposits in a permissive tract for which undiscovered 

resources were being estimated. 

It is easiest to discuss the results of the 1998 National 

Mineral Resource Assessment one metal at a time. 

GOLD 

The estimate of the amount of gold in undiscovered 

mineral deposits in the United States ranged from 13,000 t 

at a 90-percent probability to 22,000 t at a 10-percent probability. The mean estimate of gold in undiscovered deposits 

was 18,000 t. Estimated amounts of gold are listed by region 

in table 3 and shown by major deposit type in figure 4. 

Nearly one-quarter of the gold was estimated to be 

contained in undiscovered porphyry copper deposits. Other 

major deposit types were hot spring gold, epithermal vein, 

plutonic porphyry gold, sediment-hosted gold, Au-Ag-Te 

veins, and low-sulfide gold-quartz vein deposits. The identified gold resource was estimated to be 15,000 t. Past production was estimated to be 12,000 t. The total discovered 

gold resource in the United States was estimated to be 

27,000 t. 

SILVER 

The estimate of the amount of silver in undiscovered 

mineral deposits ranged from 290,000 t at a 90-percent 

probability to 660,000 t at a 10-percent probability. The 

mean estimate of silver in undiscovered deposits was 

460,000 t. Estimated amounts of silver are listed by region 

in table 3 and shown by major type in figure 5. 

Nearly one-half of the silver was thought to be contained in undiscovered sediment-hosted copper and epithermal vein deposits. Other major deposit types were 

polymetallic replacement, sedimentary exhalative, porphyry 

copper, and volcanogenic massive sulfide deposits. The 

identified silver resource was estimated to be 160,000 t. Past 

production was estimated to be 170,000 t. The total discovered silver resource in the United States was estimated to be 

330,000 t. 

COPPER 

The amount of copper in undiscovered mineral deposits was estimated to range from 170,000 kt at a 90-percent 

probability to 440,000 kt at a 10-percent probability. The 

mean estimate of copper in undiscovered deposits was 

290,000 kt. Estimated amounts of copper are listed by 

region in table 3 and shown by major type in figure 6. 

More than two-thirds of the copper was thought to be 

contained in undiscovered porphyry copper deposits. Other 

major deposit types were sediment-hosted copper and volcanogenic massive sulfide deposits. The identified copper 

resource was estimated to be 260,000 kt. Past production 

was estimated to be 91,000 kt. The total discovered copper 

resource in the United States was estimated to be 

350,000 kt. 

LEAD 

The estimate of the amount of lead in undiscovered 

mineral deposits ranged from 47,000 kt at a 90-percent 

probability to 130,000 kt at a 10-percent probability. The 

mean estimate of lead in undiscovered deposits was 

85,000 kt. Estimated amounts of lead are listed by region in 

table 3 and shown by major type in figure 7. 

Nearly one-half of the lead was thought to be contained 

in undiscovered sedimentary exhalative deposits. Other 

major deposit types were Mississippi Valley and polymetallic replacement deposit types. The identified lead resource 

was estimated to be 51,000 kt. Past production was estimated to be 41,000 kt. The total discovered lead resource in 

the United States was estimated to be 92,000 kt. 

ZINC 

The estimate of the amount of zinc in undiscovered 

deposits ranged from 130,000 kt at a 90-percent probability 

to 290,000 kt at a 10-percent probability. The mean estimate 

of zinc in undiscovered deposits was 210,000 kt. Estimated 

amounts of zinc are listed by region in table 3 and shown by 

major type in figure 8. 

Nearly 40 percent of the zinc was thought to be contained in undiscovered Mississippi Valley-type deposits. 

Other major deposit types were sedimentary exhalative, volcanogenic massive sulfide, and polymetallic replacement 

deposit types. The identified zinc resource was estimated to 

be 55,000 kt. Past production was estimated at 44,000 kt. 

The total discovered zinc resource in the United States was 

estimated to be 99,000 kt. 
------------

---------------

-------------

----

Table 3. Estimates of contained metal in undiscovered deposits in the United States. [Mean value totals may not be equal to the sums of the component means given elsewhere because numbers have been independently rounded. Fractile values (F90, F10) 

probability distributions of the contained metals for each area or of the total area and are nonadditive. F90 represents a 9-in-10 chance, and F10 represents a 1-in-10 chance, 

amount tabulated. NM, no metal] 

Gold Silver Copper Lead 

Assessment re (tons) (tons) (kilotons) (kilotons) 

gion 

F90 F10 Mean F90 F10 Mean F90 F10 Mean F90 F10 Mean 

Alaska 

Alaska Peninsula and 

Aleutian Islands---------- 110 1,400 680 1,900 26,000 12,000 2,000 20,000 10,000 60 1,100 500 

Brooks Range 0 140 54 660 34,000 14,000 32 1,400 580 580 23,000 9,500 1

East-central 120 1,300 660 0 2,000 710 58 4,500 1,800 NM NM NM South-central ------------- 320 3,100 1,700 1,200 2,200 10,000 3,600 36,000 19,000 5 1,500 580 Southeastern-------------- 0 280 100 78 11,000 3,900 10 1,800 5,700 8 1,200 510 Southwestern 360 2,000 1,100 14 14,000 5,200 120 6,000 2,500 1 9,900 3,600 West-central -------------- 0 37 18 0 1 2 NM NM NM NM NM NM 

Statewide------------------ 280 700 480 1,500 56,000 24,000 2 860 310 2,100 50,000 22,000 5

TOTAL, Alaska --------------- 3,400 6,200 4,800 36,000 110,000 72,000 23,000 57,000 39,000 13,000 69,000 38,000 30United States exclusive of Alaska 

Adirondack Mountains NM NM NM NM NM NM NM NM NM NM NM NM 

Central and Southern Rocky 

Mountains----------------- 180 2,700 1,200 1,200 40,000 17,000 1,300 14,000 6,700 97 5,600 2,300 

Colorado Plateau ---------- NM NM NM 0 1 0 0 350 100 NM NM NM East-central----------------- NM NM NM 110 15,000 5,400 NM NM NM 900 14,000 7,100 

Great Basin ----------------- 1,600 6,400 3,800 31,000 170,000 92,000 3,700 86,000 39,000 1,800 14,000 7,300 

Great Plains ---------------- NM NM NM 0 47,000 19,000 180 42,000 17,000 29 19,000 7,200 

Lake Superior-------------- 260 1,700 880 3,800 170,000 61,000 6,800 70,000 35,000 130 1,600 900 

Northern Appalachians --- 32 460 220 2,400 35,000 16,000 1,300 15,000 8,100 500 20,000 7,900 Northern Rocky 

Mountains ---------------- 220 4,100 1,900 11,000 130,000 65,000 4,900 52,000 24,000 120 18,000 6,700 

Pacific Coast --------------- 1,000 3,300 2,100 11,000 66,000 33,000 2,400 19,000 9,800 1 710 340 Southern Appalachians --­ 350 1,900 1,100 2,500 29,000 16,000 1,900 20,000 9,600 140 4,300 1,800 

Southern Basin and Range 770 3,200 1,800 25,000 110,000 64,000 18,000 220,000 110,000 1,100 12,000 5,800 

TOTAL, United States includ­
ing Alaska---------------------- 13,000 22,000 18,000 290,000 660,000 460,000 170,000 440,000 290,000 47,000 130,000 85,000 13
12 1998 ASSESSMENT OF UNDISCOVERED DEPOSITS IN THE UNITED STATES 

copper 

24% 

Plutonic 

Sedimenthosted Au 

LSGQ vein 

7% 6% 

VMS Other Porphyry 

7% 

Au-Ag-Te 

veins 

8% 

Hot 

Epithermal spring 

8% porphry vein gold 

gold 12% 18% 

10% 

Figure 4. Gold in undiscovered mineral deposits, by major 

deposit type. LSGQ vein, low-sulfide gold-quartz vein; VMS, 

volcanogenic massive sulfide (includes Besshi-, Cyprus-, and 

kuroko-type deposits); epithermal vein (includes Comstock-, 

Creede-, and quartz-adularia-type deposits). 

VMS Other 

6% 

Sedimenthosted 

copper 

18% 

7% 

Porphyry 

copper 

69% 

Figure 6. Copper in undiscovered mineral deposits, by major 

deposit type. VMS, volcanogenic massive sulfide (includes 

Besshi-, Cyprus-, and kuroko-type deposits). 

Polymetallic Other 

8% 

replacement 7% Mississippi 

Valley-type 

39% 

VMS 

15% 

Sedex 

31% 

VMS Other Sediment-hosted 

9% 5% 

Porphyry 

copper 

11% 

copper 

25% 

Sedex Polymetallic Epithermal 

12% replacement vein 

13% 25% 

Figure 5. Silver in undiscovered mineral deposits, by major 

deposit type. Sedex, sedimentary exhalative; VMS, volcanogenic 

massive sulfide (includes Besshi-, Cyprus-, and kuroko-type 

deposits); epithermal vein (includes Comstock-, Creede-, and 

quartz-adularia-type deposits). 

Other 

9% 

Sedex Polymetallic 

replacement 46% 

19% 

Mississippi 

Valley-type 

26% 

Figure 7. Lead in undiscovered mineral deposits, by major 

deposit type. Sedex, sedimentary exhalative. 

Figure 8. Zinc in undiscovered mineral deposits, by major 

deposit type. Sedex, sedimentary exhalative; VMS, volcanogenic 

massive sulfide (includes Besshi-, Cyprus-, and kuroko-type 

deposits). 
DISCUSSION AND COMPARISON OF RESULTS WITH THE 1973 NATIONAL MINERAL RESOURCE ASSESSMENT 13 

DISCUSSION AND COMPARISON OF 

RESULTS WITH THE 1973 

NATIONAL MINERAL RESOURCE 

ASSESSMENT 

As a part of the response to the Mining and Minerals 

Policy Act of 1970, the USGS published Professional Paper 

820 (Brobst and Pratt, 1973), which included the first overall assessment of mineral resources in the United States 

since the Paley Commission report in 1952 (President’s 

Materials Policy Commission, 1952). Prior to Professional 

Paper 820, traditional resource appraisals mainly inventoried measured reserves. In the professional paper, the geologic availability of resources that would be needed by 

future generations was considered for the first time. Each 

chapter of the professional paper was devoted to a single 

commodity and was written by geologists, most of whom 

had many years of experience studying the geology of mineral deposits and the commodity about which they were 

writing. Because of the large number and varied nature of 

the commodities that were examined, Brobst and Pratt provided the authors with a suggested outline of general topics 

to be covered and some guidelines as to scope and philosophy of approach. Beyond that, however, no attempt was 

made to impose a fixed format. Consequently, the chapters 

ranged from comprehensive summary reports to general 

essays. Each chapter reflected the individuality of the 

authors, as well as such variations among commodities as 

geologic occurrence, physical and chemical properties, use, 

and economics. One of the difficulties in interpreting the 

results of the assessment was the lack of a common method 

for estimating the amounts of undiscovered resources. Consequently, it was difficult, if not impossible, to disaggregate 

the estimates provided according to geographic area or by 

deposit type. Moreover, it was not always possible to gain 

insight into how the authors arrived at their estimates or to 

evaluate the level of confidence in the estimates they 

reported. 

Most of the chapters in Professional Paper 820, including those for gold, silver, copper, lead, and zinc, contained 

summaries of the geologic environment, types of deposits, 

resources, prospecting techniques, and problems for future 

research. The authors divided the resources into two categories—identified and undiscovered. Whenever possible, they 

also divided the undiscovered resources into two categories—hypothetical and speculative. In some cases, the 

authors further categorized resources according to type of 

deposit and geographic region of occurrence. Because of the 

nonuniform and (or) uncertain methods of assessment used 

by the different authors, it is easiest to compare their results 

with the 1998 National Mineral Resource Assessment one 

metal at a time. 

GOLD 

According to Simon and Prinz (1973), hypothetical 

and speculative gold resources, although they could not be 

estimated as accurately as reserves, were probably large 

compared with then-current U.S. gold production or consumption but small compared with total world reserves. If 

one correlates their words with figures for U.S. gold production and total world reserves for 1972, then an estimate of 

undiscovered gold resources can be obtained that ranges 

from 56 t to 31,000 t. The largest potential resources of gold 

were considered to be contained in Tertiary or Cretaceous 

placer deposits or auriferous conglomerates, followed, in 

decreasing order of magnitude, by disseminated, lode, 

“bonanza,” and other placer deposits (Simon and Prinz, 

1973). 

The USGS 1998 mean estimate of 18,000 t of gold in 

undiscovered deposits, with a range of 13,000 to 22,000 t, 

was well within the range of the 1973 estimate. Even though 

significant undiscovered gold placer deposits remain in Tertiary or Cretaceous placer deposits in the Northern Rocky 

Mountain region, the 1998 team was unable to estimate the 

quantity of gold because of difficulties in applying grade 

and tonnage concepts. As a result, these potential resources 

in the Northern Rocky Mountains have been supplanted by 

potential resources for undiscovered sediment-hosted and 

hot spring gold deposits in the Great Basin and undiscovered plutonic porphyry gold deposits in Alaska. In addition, 

it was estimated that the amounts of gold contained in 

undiscovered porphyry copper deposits in the Basin and 

Range province and in epithermal vein and low-sulfide 

gold-quartz vein deposits in the Western United States and 

Alaska were significant. 

SILVER 

Heyl and others (1973) gave an estimate of 68,000 t for 

the identified resources of silver and 99,000 t for the 

hypothetical resources of silver. Roughly one-half of their 

identified resources and less than one-tenth of their 

hypothetical resources were in deposits in which silver was 

the main product. With respect to speculative resources, 

they indicated that the best possibilities for new discoveries 

were in disseminated copper, porphyry copper, and massive 

sulfide deposits. 

The 1998 mean estimate of 460,000 t of silver in undiscovered deposits and the estimate of 160,000 t of silver in 

identified resources represent a substantial increase over the 

1973 estimates. The increase is due, in large part, to the 

greater present-day knowledge about the geologic occurrence and distribution of silver-rich sediment-hosted copper 

deposits and also of epithermal vein and polymetallic 

replacement deposits. Possibilities of new discoveries that 

contain significant silver also exist for porphyry copper, 

sedimentary-exhalative, and volcanogenic massive sulfide 

deposits. 
------------

---------------

14 1998 ASSESSMENT OF UNDISCOVERED DEPOSITS IN THE UNITED STATES 

COPPER 

Cox and others (1973) gave an estimate of 69,000 kt 

for the identified resources of copper, 90,500 kt for the 

hypothetical resources of copper, and 109,000 kt for the 

speculative resources of copper. Because of the large 

amount of information available on the copper resources in 

the United States, they were able to further estimate the 

amount of copper in the different resource categories 

according to geographic region and by major deposit type. 

Table 4 lists the identified and hypothetical copper resources 

reported by Cox and others (1973). 

The hypothetical resources in the Western United 

States comprised concealed porphyry copper deposits in the 

Basin and Range province and the Absaroka Range of Wyoming and sedimentary copper deposits in the Belt Supergroup of Idaho and Montana. Alaskan hypothetical 

resources were considered to include porphyry copper 

deposits in eastern Alaska and replacement deposits in the 

Brooks Range. 

Table 5 lists the speculative copper resources as 

reported by Cox and others (1973). They estimated that the 

greatest likelihood for new discoveries of copper resources 

was in the American Southwest, primarily in the southwestern, western, and northwestern parts of the Basin and Range 

province. These areas had not been evaluated as intensively 

as those in southeastern Arizona where many known porphyry copper deposits in a similar geologic setting have 

been found. Undiscovered porphyry copper deposits of 

Paleozoic age were believed to exist in the Appalachian 

Mountains. Finally, in southwestern Alaska and the Aleutian Islands, geologic conditions were considered to be similar to those of the southwest Pacific, which permitted the 

occurrence and possible discovery of porphyry copper 

deposits. Together, the hypothetical and speculative copper 

resources estimated by Cox and others (1973) totaled 

199,500 kt in the United States. 

The 1998 mean estimate of the amount of copper in 

undiscovered deposits of 290,000 kt was roughly one-half 

again as much as the 1973 estimate. The increase bore out 

the earlier estimators’ optimism for the existence of 

then-undiscovered porphyry copper deposits in the Southwestern United States. Continuing discoveries of new 

deposits and extensions of known ones, together with 

increasingly detailed mapping and exploration, support continuing expectations for further discoveries of these deposits 

in the region. Additional, more detailed mapping of the 

Rocky Mountain region also has resulted in expectations of 

new discoveries of sediment-hosted copper deposits. For the 

same reasons, British Columbia/Alaska-type porphyry copper deposits probably will be discovered in the Alaskan 

Range. The discovery of the Crandon massive sulfide 

deposit in the Lake Superior region in 1976, together with 

other subsequent discoveries in the region, led to expectations for further discoveries of volcanogenic massive sulfide 

deposits there. 

LEAD 

Morris and others (1973) estimated that hypothetical 

world lead resources could be equal to one-half or more of 

world reserves and that the speculative world lead resources 

could equal or exceed the world reserves. Using world 

reserves for 1972 as a basis yielded an estimate of undiscovered lead resources for the world of 190,000 kt. Assuming 

that roughly 30 percent of the world reserves for 1972 were 

in the United States and that a similar percentage held for 

undiscovered lead resources in the United States, Morris 

and others’ estimate of the undiscovered lead resources in 

the United States would translate into 57,000 kt. Morris and 

others concluded that the areas that held the best promise 

for new discoveries were located in active or formerly active 

mining districts, namely, central Tennessee, central and 

south-central Missouri, northern Arkansas, central Texas, 

northeastern Washington, and central Kentucky. Deposits 

have remained undiscovered in these areas either because 

they are concealed by barren rocks or because insignificant 

exposures do not indicate their true size or grade. 

The 1998 mean estimate of the amount of lead in 

undiscovered deposits of 85,000 kt was roughly one-half 

again as much as the estimate of Morris and others (1973). 

In large part, this increase was due to the discovery of the 

Red Dog deposit in the Brooks Range, Alaska, and the 

resulting expectation for further undiscovered sedimentary 

exhalative deposits throughout Alaska. The rest of the 

increase was due to the probable existence of undiscovered 

Mississippi Valley-type deposits in the east-central and 

northern Appalachian regions. 

Table 4. Summary of identified and hypothetical copper 

resources in the United States, 1973. 

[In kilotons. Modified from Cox and others (1973, table 39)] 

Area Identified Hypothetical 

Eastern -------------- 9,000 4,500 

Western, except 

Alaska 58,000 68,000 

Alaska 2,000 18,000 

Total ------------- 69,000 90,500 

Table 5. Summary of speculative copper resources in the United 

States, 1973. 

[In kilotons. Modified from Cox and others (1973, table 40)] 

Area Speculative 

Basin and Range porphyry copper -------- 91,000 

Alaska porphyry copper -------------------- 9,000 

Appalachian Mountains--------------------- 9,000 

Total --------------------------------------- 109,000 
EXTENSIONS TO IDENTIFIED RESOURCES 15 

ZINC 

Wedow and others (1973) estimated the recoverable 

identified zinc resources of the United States to be 45,000 kt 

and the undiscovered recoverable resources to be 60,000 kt. 

They postulated that the undiscovered or potential resources 

would be dominated by the massive sulfide ores in metamorphic rocks, in which zinc occurs chiefly with copper and 

lead, and by the stratabound Mississippi Valley-type deposits in carbonate rocks. 

The 1998 mean estimate of the amount of zinc in 

undiscovered deposits was 210,000 kt, which is more than 

three times the estimate of Wedow and others (1973). In 

large part, this difference was due to the increased expectation of large resources of zinc that may occur in major 

undiscovered zinc districts of the Mississippi Valley/Appalachian-type in an area that extends from Tennessee to the 

Canadian border. The discovery of the Red Dog deposit in 

the Brooks Range, Alaska, combined with increased exploration in the area, also resulted in increased expectations for 

future discoveries of sedimentary exhalative deposits 

throughout Alaska and in parts of the conterminous United 

States. The balance of the larger estimate was due to the 

likelihood of further discoveries of volcanogenic massive 

sulfide deposits in the Lake Superior region, the northern 

Appalachians, and the Western United States. 

SOME CONSIDERATIONS FOR OTHER 

USES OF THE ASSESSMENT 

Care must be exercised when using the results of the 

1998 National Mineral Resource Assessment to answer 

questions that involve the potential for economic development of the estimated undiscovered resources. These estimates were based on grade and tonnage models, and the 

present-day economic viability of the deposits used to construct the models varies widely from deposit to deposit, 

from model to model, and from submodel to submodel. In 

addition, large parts of the areas delineated as permissive 

are unavailable for mineral development. Some areas are 

already being developed as urban areas, transportation corridors, and so forth. Mining and exploration are prohibited by 

law in the public interest in areas such as Wilderness and 

Scenic Areas and National Parks. Some areas are owned by 

those not interested in mineral development. 

Estimated amounts of metals contained in undiscovered deposits are derived from estimates of numbers of 

deposits that are likely to exist, not necessarily from those 

likely to be discovered. For example, although concealed 

deposits were included in the assessment, they may be so 

expensive to search for that they are unlikely to be discovered in the near term. 

It also is important to understand the distinctions 

among identified resources and districts, undiscovered 

deposits and districts, and extensions to identified resources 

and districts. An identified deposit or district is one for 

which an estimate of the metal contained in the deposit can 

be made directly. The total consists of the sum of past production, if any, and any identified resource remaining in the 

ground. In some cases, the teams were aware of prospects, 

revealed by past or current exploration efforts, that were 

believed to be significant deposits but that did not yet have a 

documented grade and tonnage. Such deposits are treated 

herein as undiscovered deposits, albeit ones with a high 

degree of certainty of existence. Finally, it should be 

remembered that initial exploration leading to an announced 

discovery almost never identifies the total resource that 

exists and (or) ultimately may be mined. As exploration 

continues and (or) mining begins, an accompanying 

increase in knowledge about the deposit geology almost 

always results in increasing estimates of the size of the 

deposit. 

One further note of caution should be given to users of 

the 1998 National Mineral Resource Assessment. Because 

the assessment methodology depended on deposit models, 

the teams were unable to estimate resources in types of 

deposits that were too poorly characterized for a deposit 

model to be constructed. An example is the copper-gold 

deposits in Washington. To better appreciate lack of understanding as a potential source of error in the assessment, 

imagine such an assessment being completed in 1905 before 

the recognition of porphyry copper-type deposits or in 1960 

before the discoveries of sediment-hosted gold deposits. 

The consequent underestimation of copper and gold 

resources would have been so large as to undermine the conclusions about undiscovered resources at the time. Although 

the United States is much better explored and geologically 

known now than it was in 1905, new deposit types unimagined today can be expected to contribute substantially to the 

mineral resources of the United States in the future. Such 

considerations make it all the more imperative to conduct 

national assessments on a recurring basis. 

EXTENSIONS TO IDENTIFIED 

RESOURCES 

What are missing from the 1998 National Mineral 

Resource Assessment are estimates of the amounts of gold, 

silver, copper, lead, and zinc in extensions to identified 

resources. Experience shows that almost all estimates of 

mineral deposit sizes increase during production. Discovery 

and initial exploration almost never identify the total 

resource that exists. Mineral producers generally lack the 

financial incentive to prove reserves in excess of 10 to 20 

years production. The exceptions are those cases where pit 

and plant design or permitting requirements necessitate a 

complete inventory for the mine plan. Thus, most mining 

operations actually have lifetimes that are much longer than 
16 1998 ASSESSMENT OF UNDISCOVERED DEPOSITS IN THE UNITED STATES 

originally reported. Proved reserves increase with normal 

deposit development as boundaries of proved areas are 

extended by drilling, as extensions or new deposits are 

found and confirmed by drilling, as new infill drilling (vertical or horizontal) contacts previously inaccessible portions 

of the deposit, and with the introduction of improved recovery methods that allow lower grade material to be mined. 

Because a methodology that takes reserve growth into 

account is currently lacking, reserve growth is missing from 

the teams’ estimates. The teams, however, believe reserve 

growth is of the same order of magnitude as that of the identified resources. 

IMPLICATIONS 

The 1998 National Mineral Resource Assessment 

achieved its primary goal—to provide quantitative, probabilistic estimates of the gold, silver, copper, lead, and zinc in 

undiscovered deposits for the country as a whole. There is 

every reason to believe that, for conventional-type deposits 

that contain gold, silver, copper, lead, or zinc, about as much 

is left to be discovered in the United States as has already 

been discovered. Furthermore, the method that was used in 

carrying out the assessment can serve as a guide to assess 

other commodities of national interest. The data base that 

was created for the assessment can be useful to land-management agencies and resource-planning organizations for 

land- and resource-planning and decisionmaking. Such 

information permits, at least at some minimum level, evaluation of the impact of land-use decisions on the Nation’s 

undiscovered mineral resources. The availability of such a 

data base provides a unique opportunity for Federal, State, 

and local governments, industry, and the public to work 

cooperatively to plan the disposition of the Nation’s 

long-term mineral supply. Federal, State, and local 

land-management agencies can use the information provided by the assessment to estimate potential cumulative 

environmental impact of possible exploration and mining 

activities, to evaluate the potential economic benefits of 

mining in comparison with other possible land uses, to evaluate and plan for the potential impact of mining activities on 

other land uses, and to appraise the fair market value of land 

proposed for leasing, sale, exchange, or taking. The permissive tracts delineated as part of the 1998 National Mineral 

Resource Assessment permit industry to focus mineral-exploration programs on the most promising areas for 

discoveries. 

Looking to the future, the likelihood of finding new 

deposits presents new challenges, given that undiscovered 

deposits do not crop out, access is more difficult, environmental constraints are more severe, landownership is more 

fragmented, and foreign suppliers are cheaper. If the past 

can be used as a guide, deposits that are not yet recognized 

as economic targets, new technologies (for example, biomediated heap leaching) that might convert formerly discarded 

rock into ore, or some heretofore unrecognized source of 

metals (for example, gold in oil-field brines) that would 

define new targets for exploration are, without a doubt, in 

the Nation’s future. Carrying out national assessments on a 

recurring basis will ensure adequate mineral supplies and 

effective stewardship of resources in the future. 

REFERENCES CITED 

American Geological Institute, 1997, Dictionary of mining, mineral, and related terms (2d ed.): Alexandria, Va., American 

Geological Institute, 646 p. 

Bliss, J.D., ed., 1992, Developments in mineral deposit modeling: 

U.S. Geological Survey Bulletin 2004, 168 p. 

Brobst, D.A., and Pratt, W.P., eds., 1973, United States mineral 

resources: U.S. Geological Survey Professional Paper 820, 

722 p. 

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

U.S. Geological Survey Bulletin 1693, 379 p. 

Cox, D.P., Schmidt, R.G., Vine, J.D., Kirkemo, Harold, Tourtelot, 

E.B., and Fleischer, Michael, 1973, Copper, in Brobst, D.A., 

and Pratt, W.P., eds., United States mineral resources: U.S. 

Geological Survey Professional Paper 820, p. 163–190. 

Heyl, A.V., Hall, W.E., Weissenborn, A.E., Stager, H.K., Puffett, 

W.P., and Reed, B.L., 1973, Silver, in Brobst, D.A., and Pratt, 

W.P., eds., United States mineral resources: U.S. Geological 

Survey Professional Paper 820, p. 581–603. 

Hollister, V.F., 1992, On a proposed plutonic porphyry gold 

deposit model: Nonrenewable Resources, v. 1, no. 4, p. 293– 

302. 

Long, K.R., DeYoung, J.H., Jr., and Ludington, S.D., 1998, Data 

base of significant deposits of gold, silver, copper, lead, and 

zinc in the United States; pt. A, Data base description and 

analysis; pt. B, Digital data base: U.S. Geological Survey 

Open-File Report 98–206, 33 p., one 3.5-inch diskette. 

McCammon, R.B., and Briskey, J.A., Jr., 1992, A proposed 

national mineral-resource assessment: Nonrenewable 

Resources, v. 1, no. 4, p. 259–266. 

McKelvey, V.E., 1972, Mineral resource estimates and public policy: American Scientist, v. 60, no. 1, p. 32–40. 

Morris, H.T., Heyl, A.V., and Hall, R.B., 1973, Lead, in Brobst, 

D.A., and Pratt, W.P., eds., United States mineral resources: 

U.S. Geological Survey Professional Paper 820, p. 313–332. 

President’s Materials Policy Commission, 1952, Resources for 

freedom: Washington, U.S. Government Printing Office, 5 v., 

818 p. 

Root, D.H., Menzie, W.D., III, and Scott, W.A., 1992, Computer 

Monte Carlo simulation in quantitative resource assessment: 

Nonrenewable Resources, v. 1, no. 2, p. 125–138. 

Root, D.H., Scott, W.A., Jr., and Selner, G.I., 1997, Computer program for aggregation of probabilistic assessments of mineral 

resources: U.S. Geological Survey Open-File Report 96–94, 1 

diskette. 

Simon, F.S., and Prinz, W.C., 1973, Gold, in Brobst, D.A., and 

Pratt, W.P., eds., United States mineral resources: U.S. Geological Survey Professional Paper 820, p. 263–275. 
REFERENCES CITED 17 

Singer, D.A., 1993, Basic concepts in three-part quantitative 

assessments of undiscovered mineral resources: Nonrenewable Resources, v. 2, no. 2, p. 69–81. 

———1995, World class base- and precious-metal deposits—A 

quantitative analysis: Economic Geology, v. 90, no. 1, p. 88– 

104. 

U.S. Bureau of Mines and U.S. Geological Survey, 1980, Principles of a resource/reserve classification for minerals: U.S. 

Geological Survey Circular 831, 5 p. 

U.S. Geological Survey, 1998, Mineral Commodity Summaries: 

U.S. Geological Survey, 197 p. 

Wedow, H., Jr., Kiilsgaard, T.H., Heyl, A.V., and Hall, R.B., 1973, 

Zinc, in Brobst, D.A., and Pratt, W.P., eds., United States mineral resources: U.S. Geological Survey Professional Paper 

820, p. 697–711. 
18 1998 ASSESSMENT OF UNDISCOVERED DEPOSITS IN THE UNITED STATES 

MEMBERS OF THE U.S. GEOLOGICAL SURVEY 

1998 NATIONAL MINERAL RESOURCE ASSESSMENT TEAM 

Project Chief—Richard B. McCammon 

Editors 

Conterminous United States 

Steve Ludington 

Dennis P. Cox 

Alaska—Thomas D. Light
 

Eastern United States—Sandra H.B. Clark
 

Western United States—Michael F. Diggles
 

Central United States—Alan R. Wallace
 

Southern Basin and Range—Leslie J. Cox
 

Pacific Coast—Stephen E. Box
 

Methodology 

Donald A. Singer 

Lawrence J. Drew 

William D. Menzie, II 

Mathematician—David H. Root 

GIS Compilers 

Esther L. Castellanos 

Leslie J. Cox 

Pamela D. Derkey 

James Hall 

Garry E. Klein 

Devon Peterson 

Martin L. Sorenson 

GIS and CD-ROM Managers 

Paul G. Schruben 

Thomas D. Light 

Barry C. Moring 

David M. Sutphin 

Computing and Programming 

William A. Scott, Jr. 

Susan H. Garcia 

Significant Deposits Editors 

Keith R. Long 

John H. DeYoung, Jr. 

Steve Ludington 

Dan L. Mosier 

Significant Deposits Compilers 

James D. Bliss 

Arthur A. Bookstrom 

Sandra H.B. Clark 

Leslie J. Cox 

Carl A. Carlson 

Warren C. Day 

Russell C. Evarts 

David Frishman 

Robert J. Kamilli 

Lorre A. Moyer 

Chris Osterman 

Miles L. Silberman 

Significant Deposits Compilers—Continued 

Anna B. Wilson 

Region Teams 

Northern Appalachians 

Sandra H.B. Clark 

Robert A. Ayuso 

Joseph A. Briskey, Jr. 

Dennis P. Cox 

Michael P. Foose 

John J. Hemley 

Bruce R. Lipin 

John D. Peper 

Gilpin R. Robinson, Jr. 

John F. Slack 

Adirondack Mountains 

Sandra H.B. Clark 

Joseph A. Briskey, Jr. 

C. Ervin Brown 

Dennis P. Cox 

Michael P. Foose 

John F. Slack 

Southern Appalachians 

Terry L. Klein 

Joseph A. Briskey, Jr. 

Sandra H.B. Clark 

Dennis P. Cox 

Jacob E. Gair 

Robert P. Koeppen 

Frank G. Lesure 

Bruce R. Lipin 

Terry W. Offield 

John D. Peper 

Gilpin R. Robinson, Jr. 

John F. Slack 

Lake Superior 

Klaus J. Schulz 

William F. Cannon 

Michael P. Foose 

Martin B. Goldhaber 

Terry L. Klein 

Gene L. LaBerge 

David L. Leach 

David A. Lindsey 

Suzanne W. Nicholson 

Walden P. Pratt 

Paul K. Sims 

Gregory T. Spanski 

John G. Viets 

Laurel G. Woodruff 
MEMBERS OF THE U.S. GEOLOGICAL SURVEY 1998 NATIONAL MINERAL RESOURCE ASSESSMENT TEAM 19 

Region Teams—Continued 

East-central 

Sandra H.B. Clark 

Joseph A. Briskey, Jr. 

Dennis P. Cox 

John D. Peper 

Howard A. Pohn 

John F. Slack 

Great Plains 

Walden P. Pratt 

Dennis P. Cox 

Warren C. Day 

Martin B. Goldhaber 

David L. Leach 

David A. Lindsey 

Gary B. Sidder 

Gregory T. Spanski 

John G. Viets 

Michael L. Zientek 

Northern Rocky Mountains 

Michael L. Zientek 

Byron R. Berger 

Arthur A. Bookstrom 

Stephen E. Box 

Dennis P. Cox 

Warren C. Day 

James E. Elliott 

David Frishman 

W. Dan Hausel 

David A. Lindsey 

Steve Ludington 

J. Thomas Nash 

Gregory T. Spanski 

Alan R. Wallace 

James W. Whipple 

Central and Southern Rocky Mountains 

Steve Ludington 

Susan B. Bartsch-Winkler 

Byron B. Berger 

Dennis P. Cox 

Warren C. Day 

W. Dan Hausel 

David A. Lindsey 

Virginia T. McLemore 

J. Thomas Nash 

Gary B. Sidder 

Miles L. Silberman 

Gregory T. Spanski 

Alan R. Wallace 

Michael L. Zientek 

Colorado Plateau 

David A. Lindsey 

Dennis P. Cox 

Michael L. Zientek 

Region Teams—Continued 

Pacific Coast 

Stephen E. Box 

George V. Albino 

Roger P. Ashley 

Arthur A. Bookstrom 

Stanley E. Church 

Dennis P. Cox 

Michael F. Diggles 

James G. Evans 

Russell C. Evarts 

Merlin D. Kleinkopf 

Steve Ludington 

Stephen G. Peters 

James A. Peterson 

William J. Pickthorn 

Gary Lee Raines 

James J. Rytuba 

Michael G. Sawlan 

Gregory T. Spanski 

Richard M. Tosdal 

Great Basin 

Dennis P. Cox 

George V. Albino 

Roger P. Ashley 

Byron R. Berger 

Stanley E. Church 

Ed H. DeWitt 

Michael F. Diggles 

Merlin D. Kleinkopf 

David A. Lindsey 

Steve Ludington 

J. Thomas Nash 

Constance J. Nutt 

James J. Rytuba 

Michael G. Sawlan 

Maureen G. Sherlock 

Donald A. Singer 

Gregory T. Spanski 

Douglas B. Stoeser 

Joseph V. Tingley 

Alan R. Wallace 

Michael L. Zientek 

Southern Basin and Range 

Leslie J. Cox 

William C. Bagby 

Susan B. Bartsch-Winkler 

Byron R. Berger 

Carl A. Carlson 

Stanley E. Church 

Dennis P. Cox 

Michael F. Diggles 

Frederick S. Fisher 

Eric R. Force 

James Guthrie 
20 1998 ASSESSMENT OF UNDISCOVERED DEPOSITS IN THE UNITED STATES 

Region Teams—Continued Region Teams—Continued 

Southern Basin and Range—Continued Alaska—Continued 

Robert C. Jachens 

Robert J. Kamilli 

Merlin D. Kleinkopf 

Steve Ludington 

Virginia T. McLemore 

J. Thomas Nash 

Stephen G. Peters 

James J. Rytuba 

Michael G. Sawlan 

Miles L. Silberman 

Donald A. Singer 

Gregory T. Spanski 

Ted G. Theodore 

Spencer R. Titley 

Richard M. Tosdal 

Alan R. Wallace 

Alaska 

Brooks Range 

Jeanine M. Schmidt 

Byron R. Berger 

John W. Cady 

Lawrence J. Drew 

Karen A. Duttweiler 

Richard I. Grauch 

Karen D. Kelley 

James E. Kilburn 

Gregory T. Spanski 

East-central 

Donald J. Grybeck 

Byron R. Berger 

Thomas K. Bundtzen 

John W. Cady 

Stanley E. Church 

Dennis P. Cox 

Robert G. Eppinger 

Bruce M. Gamble 

William J. Keith 

Gregory K. Lee 

Thomas D. Light 

Marti L. Miller 

Warren J. Nokleberg 

Jeffrey D. Phillips 

Jeanine M. Schmidt 

Gregory T. Spanski 

West-central 

Bruce M. Gamble 

Byron R. Berger 

Thomas K. Bundtzen 

John W. Cady 

Stanley E. Church 

Dennis P. Cox 

Robert G. Eppinger 

John E. Gray 

West-central—Continued 

Thomas D. Light 

Marti L. Miller 

Jeffrey D. Phillips 

Richard W. Saltus 

Jeanine M. Schmidt 

Gregory T. Spanski 

Frederic H. Wilson 

Southeastern 

David A. Brew 

David F. Barnes 

Lawrence J. Drew 

James L. Drinkwater 

Donald J. Grybeck 

Robert C. Jachens 

Richard D. Koch 

Cliff D. Taylor 

South-central 

Warren J. Nokleberg 

Thomas K. Bundtzen 

David L. Campbell 

Dennis P. Cox 

Bruce M. Gamble 

Richard J. Goldfarb 

Donald J. Grybeck 

William J. Keith 

Gregory K. Lee 

Thomas D. Light 

Marti L. Miller 

Jeffrey D. Phillips 

Jeanine M. Schmidt 

Warren E. Yeend 

Southwestern 

Marti L. Miller 

Thomas K. Bundtzen 

John W. Cady 

Stanley E. Church 

Dennis P. Cox 

Lawrence J. Drew 

Robert G. Eppinger 

Bruce M. Gamble 

John E. Gray 

William D. Menzie, II 

Warren J. Nokleberg 

Jeffrey D. Phillips 

Richard W. Saltus 

Jeanine M. Schmidt 

Frederic H. Wilson 

Alaska Peninsula and the Aleutians 

Frederic H. Wilson 
MEMBERS OF THE U.S. GEOLOGICAL SURVEY 1998 NATIONAL MINERAL RESOURCE ASSESSMENT TEAM 21 

Region Teams—Continued 

Alaska—Continued 

Alaska Peninsula and the Aleutians— 

Continued 

Stanley E. Church 

Lawrence J. Drew 

W. David Menzie, II 

Richard W. Saltus 

National reviewers 

Paul B. Barton 

Paul K. Sims 

Alaska reviewers 

Paul B. Barton 

Henry C. Berg 

Donald H. Richter 
About the Compact Disc 

1998 Assessment of Undiscovered Deposits of Gold, Silver, Copper, Lead, and Zinc 
 

in the United States—
 

CD-ROM Containing Circular Report, Data Base, and Graphical Display Program
 

The data base of the tracts that were used to gen- The browser is a map-based, stand-alone Winerate the resource estimates obtained in the National dows application that can be installed from the CDMineral Resource Assessment is stored on a Compact ROM. The browser allows a user to click on a tract for 

Disc-Read Only Memory (CD-ROM), which is a selected mineral deposit model type and to display included in a pocket at the end of this report. The CD- either the tabular data associated with the tract or the 

ROM also contains this circular and a browser that can rationale that was used to assess the tract. The rationale 

be used to examine the tracts by region and by major contains hypertext links to the tonnage and amounts of deposit model type and the significant mineral deposits 

that occur in the tracts. The data base consists of the contained metal in undiscovered deposits, the mineral 


following: deposit model type used to assess the particular tract, 


and the significant mineral deposits associated with the 


•	 Estimates of the tonnages and amounts of con- tract. A user can also choose to display the assessment
 tained metal in undiscovered deposits, regions, mining districts, and significant mineral 


• A table that lists the mineral deposit model types deposits, as well as several standard thematic layers used in the National Mineral Resource Assess- such as political boundaries, cities, airports, rivers, and ment, color-shaded relief. 

• A table that lists the tonnages and amounts of 

contained metal in the significant mineral depos- This CD-ROM was produced in accordance with 

its in the United States, the ISO 9660 Level 2 standard. The included browser 

•	 The rationale that was used to assess each of the installs only under Windows 95, 98, or NT operating 

tracts in the National Mineral Resource Assess- systems. 

ment, and To get started: Launch the “NASetup.exe” pro­

•	 A list of references for each tract in the National gram in the main directory of the disc. It installs a Start 

Mineral Resource Assessment. menu item called “National Assessment.”
in the United States

U.S. GEOLOGICAL SURVEY CIRCULAR 1178

SSESSMENT OF UNDISCOVERED GOLD, SILVER, COPPER, LEAD, AND ZINC, U.S.—U.S. Geological Survey Circular 1178