GEOLOGICAL SURVEY CIRCULAR 831 

Principles of a 

Resource/Reserve Classification 

For Minerals 

L 

Principles of a 

Resource/Reserve Classification 

For Minerals 

By the U.S. Bureau of Mines and 

the U.S. Geological Survey 

GEOLOGICAL SURVEY CIRCULAR 831 

A revision of the classification system 

published as U.S. Geological Survey Bulletin 1450-A 

1980 
United States Department of the Interior 

CECIL D. ANDRUS, Secretary 

Geological Survey 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

United States. Bureau of Mines. 

Principles of a resource/reserve classification for minerals. 

(Geological Survey circular ; 831) 

Edition for 1976 published under title: Principles of the mineral resource classification system of the U.S. 

Bureau of Mines and U.S. Geological Survey. 

Bibliography: p. 

1. Mines and mineral resources- United States-Classification. I. United States. Geological Survey. 

II. Title. III. Series: United States. Geological Survey. Circular; 831. 

QE75.C5 no. 831 [TN153] 5fi7.3s [553'.01'2] 80-607901. 

Free on application to Branch of Distribution, U.S. Geological Survey 

1200 South Eads Street, Arlington, VA 22202 
CONTENTS 

Page 

Introduction____________________________________________________________________ 1 

Resource/reserve definitions ------------------------------------------------------- 1 

Guidelines for classification of mineral resources --------------------------------------- 3 

ILLUSTRATIONS 

Page 

FIGURE 1. Diagrammatic illustration of major elements of mineral-resource classification ----------------------------- 5 

2. Diagrammatic illustration of reserve base and inferred reserve base classification categories ------------------- 5 

iii 
Principles of a Resource/Reserve Classification 

for Minerals 

By the U.S. BUREAU OF MINES and the U.S. GEOLOGICAL SURVEY 

INTRODUCTION 

Through the years, geologists, mining engineers, 

and others operating in the minerals field have 

used various terms to describe and classify mineral 

resources, which as. defined herein include energy 

materials. Some of these terms have gained wide 

use and acceptance, although they are not always 

used with precisely the same meaning. 

Staff members of the U.S. Bureau of Mines and 

the U.S. Geological Survey collect information 

about the quantity and quality of all mineral 

resources, but from different perspectives and with 

different purposes. In 1976, a team of staff 

members from both agencies developed a common 

classification and nomenclature, which was 

'published as U.S. Geological Survey Bulletin 

1450-A- "Principles of the Mineral Resource 

Classification System of the U.S. Bureau of Mines 

and U.S. Geological Survey." Experience with this 

resource classification system showed that some 

changes were necessary in order to make it more 

workable in practice and more useful in long-term 

planning. Therefore, representatives of the U.S. 

Geological Survey and the U.S. Bureau of Mines 

collaborated to revise Bulletin 1450-A. 

Long-term public and commercial planning must 

be based on the probability of discovering new 

deposits, on developing economic extraction processes for currently unworkable deposits, and on 

knowing which resources are immediately 

available. Thus, resources must be continuously 

reassessed in the light of new geologic knowledge, 

of progress in science and technology, and of shifts 

in economic and political conditions. To best serve 

these planning needs, known resources should be 

classified from two standpoints: (1) purely geologic 

or physical/chemical characteristics-such as 

grade, quality, tonnage, thickness, and depth- of 

1 

the material in place: and (2) profitability analyses 

based on costs of extracting and marketing the 

material in a given economy at a given time. The 

former constitutes important objective scientific 

information of the resource and a relatively unchanging foundation upon which the latter more 

variable economic delineation can be based. 

The revised classification system, designed 

generally for all mineral materials, is shown 

graphically in figures 1 and 2 (see page 5); its components and their usage are described in the text. 

The classification of mineral and energy resources 

is necessarily arbitrary, because definitional 

criteria do not always coincide with natural boundaries. The system can be used to report the status 

of mineral and energy-fuel resources for the Nation or for specific areas. 

RESOURCE/RESERVE DEFINITIONS 

A dictionary definition of resource, "something 

in reserve or ready if needed," has been adapted 

for mineral and energy resources to comprise all 

materials, including those only surmised to exist, 

that have present or anticipated future value. 

Resource. -A concentration of naturally occurring 

solid, liquid, or gaseous material in or on the 

Earth's crust in such form and amount that 

economic extraction of a commodity from the 

concentration is currently or potentially 

feasible. 

Original Resource.- The amount of a resource 

before production. 

Identified Resources.-Resources whose location, 

grade, quality, and quantity are known or 

estimated from specific geologic evidence. 

Identified resources include economic, 

marginally economic, and subeconomic components. To reflect varying degrees of geologic 
(Identified Resources- Continued) 

certainty, these economic divisions can be subdivided into measured, indicated, and 

inferred. 1 

Demonstrated. -A term for the sum of measured plus indicated. 

Measured.- Quantity is computed from 

dimensions revealed in outcrops, 

trenches, workings, or drill holes; 

grade and( or) quality are 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.- Quantity and grade and( or) 

quality are computed from information 

similar to that used for measured 

resources, but 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. 

Inferred.- Estimates 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. 

Reserve Base. -That part of an identified resource 

that meets specified minimum physical and 

chemical criteria related to current mining and 

production practices, including those for 

grade, quality, thickness, and depth. The 

reserve base is the in-place demonstrated 

(measured plus indicated) resource from which 

reserves are estimated. It may encompass 

those parts of the resources that have a 

reasonable potential for becoming economically available within planning horizons beyond 

those that assume proven technology and current economics. The reserve base includes those 

tThe terms "proved," "probable," and "possible'', which are commonly used by in· 

dustry in economic evaluations of ore or mineral fuels in specific deposits or districts, 

have been loosely interchanged with the terms measured, indicated, and inferred. 

The former terms are not a part of this classification system. 

2 

(Reserve Base-Continued) 

resources that are currently economic 

(reserves), marginally economic (marginal 

reserves), and some of those that are currently 

subeconomic (subeconomic resources). The 

term "geologic reserve" has been applied by 

others generally to the reserve-ba.'.;e category, 

but it also may include the inferred-reservebase category; it is not a part of this classification system. 

Inferred Reserve Base.-The in-place part of an 

identified resource from which inferred 

reserves are estimated. Quantitative estimates 

are based largely on knowledge of the geologic 

character of a deposit and for which there may 

be no samples or measurements. The estimates 

are based on an assumed continuity beyond the 

reserve base, for which there is geologic 

evidence. 

Reserves. -That part of the reserve base which 

could be economically extracted or produced at 

the time of determination. The term reserves 

need not signify that extraction facilities are in 

place and operative. Reserves include only 

recoverable materials; thus, terms such as "extractable reserves" and "recoverable reserves" 

are redundant and are not a part of this classification system. 

Marginal Reserves.- That part of the reserve base 

which, at the time of determination, borders on 

being economically producible. Its essential 

characteristic is economic uncertainty. Included are resources that would be producible, 

given postulated changes in economic or technologic factors. 

Economic. -This term implies that profitable 

extraction or production under defined investment assumptions has been established, analytically demonstrated, or assumed with 

reasonable certainty. 

Subeconomic Resources. -The part of identified resources that does not meet the economic 

criteria of reserves and marginal reserves. 

Undiscovered Resources. -Resources, the existence 

of which are only postulated, comprising 

deposits that are separate from identified 

resources. Undiscovered resources may be 

postulated in deposits of such grade and 

physical location as to render them economic, 

marginally economic, or subeconomic. To 

reflect vazying degrees of geologic certainty, 
(Undiscovered Resources-Continued) 

undiscovered resources may be divided into 

two parts: 

Hypothetical Resources. -Undiscovered resources that are similar to known mineral 

bodies and that may be reasonably expected to exist in the same producing 

district or region under analogous geologic 

conditions. If exploration confirms their 

existence and reveals enough information 

about their quality, grade, and quantity, 

they will be reclassified as identified 

resources. 

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. 

If exploration confirms their existence and 

reveals enough information about their 

quantity, grade, and quality, they will be 

reclassified as identified resources. 

Restricted Resources/Reserves.-That part of any 

resource/reserve category that is restricted 

from extraction by laws or regulations. For example, restricted reserves meet all the requirements of reserves except that they are 

restricted from extraction by laws or regulations. 

GUIDELINES FOR CLASSIFICATION OF 

MINERAL RESOURCES 

1. All naturally occurring metals, nonmetals, 

and fossil fuels in sufficient concentration can be 

classified in one or more of the categories. 

2. Where the term reserves is used alone, 

without a modifying adjective such as indicated, 

marginal, or inferred, it is to be considered 

synonymous with the demonstrated-economic 

category, as shown in figure 1. 

3. Definitions of resource categories can be 

modified for a particular commodity in order to 

conform with accepted usage involving special 

geological and engineering characteristics. Such 

modified definitions for particular commodities will 

be given in forthcoming government publications. 

4. Quantities, qualities, and grades may be expressed in different terms and units to suit 

different purposes, but usage must be clearly 

stated and defined. 

3 

5. The geographic area to which any 

resource/reserve estimate refers must be defined. 

6. All estimates must show a date and author. 

7. The reserve base is an encompassing 

resource category delineated by physical and 

chemical criteria. A major purpose for its recognition and appraisal is to aid in long-range public and 

commercial planning. For most mineral commodities, different grades and tonnages, or other 

appropriate resource parameters, can be specified 

for any given deposit or area, or for the Nation, 

depending on the specific objectives of the 

estimators; therefore, the position of the lower 

boundary of the reserve base, which extends into 

the subeconomic category, is variable, depending 

on those objectives. The intention is to define a 

quantity of in-place material, any part of which 

may become economic, depending on the extraction plans and economic assumptions finally used. 

When those criteria are determined, the initial 

reserve-base estimate will be divided into three 

component parts: reserves, marginal reserves, and 

a remnant of subeconomic resources. For the purpose of Federal commodity assessment, criteria for 

the reserve base will be established for each commodity. 

8. Undiscovered resources may be divided in accordance with the definitions of hypothetical and 

speculative resources, or they may be divided in 

terms of relative probability of occurrence. 

9. Inferred reserves and the inferred reserve 

base are postulated extensions of reserves and of 

the reserve base. They are identified resources 

quantified with a relatively low degree of certainty. 

Postulated quantities of resources not based on 

reserve/reserve-base extensions, but rather on 

geologic inference alone, should be classified as undiscovered. 

10. Locally, limited quantities of materials may 

be produced, even though economic analysis has indicated that the deposit would be too thin, too low 

grade, or too deep to be classified as a reserve. This 

situation might arise when the production facilities 

are already established or when favorable local circumstances make it possible to produce material 

that elsewhere could not be extracted profitably. 

Where such production is taking place, the quantity of in-place material shall be included in the 

reserve base, and the quantity that is potentially 

producible shall be included as a reserve. The 

profitable production of such materials locally, 

however, should not be used as a rationale in other 
areas for classifying as reserves, those materials 

that are similar in thickness, quality, and depth. 

11. Resources classified as reserves must be considered economically producible at the time of 

classification. Conversely, material not currently 

producible at a profit cannot be classified as 

reserves. There are situations, however, in which 

mining plans are being made, lands are being acquired, or mines and plants are being constructed 

to produce materials that do not meet economic 

criteria for reserve classification under current 

costs and prices, but would do so under reasonable 

future expectations. For some other materials, 

I 

economic producibility is uncertain only for lack of 

detailed engineering assessment. The marginal-

!reserves category applies to both situations. When 

~economic production appears certain for all or 

some of a marginal reserve, it will be reclassified as 

reserves. 

12. Materials that are too low grade or for other 

reasons are not considered potentially economic, in 

the same sense as the defined resource, may be 

recognized and their magnitude estimated, but 

they are not classified as resources. A separate 

category, labeled other occurrences, is included in 

figures 1 and 2. 

13. In figure 1, the boundary between subeconomic and other occurrences is limited by the concept of current or potential feasibility of economic 

production, which is required by the definition of a 

resource. The boundary is obviously uncertain, but 

limits may be specified in terms of grade, quality, 

thickness, depth, percent extractable, or other 

economic-feasibility variables. 

14. Varieties of mineral or energy commodities, 

4 

such as bituminous coal as distinct from lignite, 

may be separately quantified when they have 

different characteristics or uses. 

15. The amount of past cumulative production is 

not, by definition, a part of the resource. Nevertheless, a knowledge of what has been produced is 

important to an understanding of current resources, in terms of both the amount of past production and the amount of residual or remaining 

in-place resource. A separate space for cumulative 

production is shown in figure 1. Residual material 

left in the ground during current or future extraction should be recorded in the resource category 

appropriate to its economic-recovery potential. 

16. In classifying reserves and resources, it is 

necessary to recognize that some minerals derive 

their economic viability from their coproduct or 

byproduct relationships with other minerals. Such 

relationships must be clearly explained in footnotes or in an accompanying text. 

17. Considerations other than economic and geologic, including legal, regulatory, environmental, 

and political, may restrict or prohibit the use of all 

or part of a deposit. Reserve and resource quantities known to be restricted should be recorded in 

the appropriate classification category; the quantity restricted and the reason for the restriction 

should be noted. 

18. The classification system includes more divisions than will commonly be reported or for which 

data are available. Where appropriate, divisions 

may be aggregated or omitted. 

19. The data upon which resource estimates are 

based and the methods by which they are derived 

are to be documented and preserved. 
RESOURCES OF (commodity name) 

[A part of reserves or any resource category may be restricted from extraction by laws or regulations (see text)] 

Cumulative 

Production 

ECONOMIC 

MARGINALLY 

ECONOMIC 

SUBECONOMIC 

Other 

Occurrences 

Author: 

AREA: (mine, district, field, State, etc.) UNITS: (tons, barrels, ounces, etc.) 

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES 

Demonstrated Probability Range 

I 

Inferred (ori 

Measured Indicated Hypothetical I Speculative 

I I 

Reserves Inferred Reserves 

!------+----1---------

+ 

Inferred 

Marginal Reserves Marginal Reserves 

~---+---1---------

Inferred + 

Demonstrated Subeconomic 

Subeconomic Resources Resources 

I I 

Includes nonconventional and low-grade materials 

Date: 

FIGURE 1.-Major elements of mineral-resource classification, excluding reserve base and inferred reserve base. 

RESOURCES OF (commodity name) 

[A part of reserves or any resource category may be restricted from extraction by laws or regulations (see text)] 

AREA: (mine, district, field, State, etc.) UNITS: (tons, barrels, ounces, etc.) 

Cumulative 

Production 

ECONOMIC 

MARGINALLY 

ECONOMIC 

SUBECONOMIC 

Other 

Occurrences 

Author: 

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES 

Demonstrated Probability Range 

Measured I Inferred (or}--

Indicated Hypothetical I Speculative 

I I 

Inferred 

Reserve 

- _:_ - - 1- + I 

Reserve 

Bas a Base - - - - -

+ ----~---t----- I 

Includes nonconventional and low-grade materials 

Date: 

FIGURE 2.-Reserve base and inferred reserve base classification categories. 

-

-

-

-

5 -k U.S. GOVERN/v'.ENT PRINTING OFFICE: 1980 0- 311-344/106 

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