Principles of the Mineral 

Resource Classification System 

of the U.S. Bureau of Mines 

and U.S. Geological Survey

GEOLOGICAL SURVEY BULLETIN 1450-A

A report published jointly by the 

U.S. Bureau of Mines and 

I \S. Geological Survey

c

Principles of the Mineral 

Resource Classification System 

of the U.S. Bureau of Mines 

and U.S. Geological Survey

MINERAL RESOURCE CLASSIFICATION SYSTEMS OF THE 

U.S. BUREAU OF MINES AND U.S. GEOLOGICAL SURVEY

GEOLOGICAL SURVEY BULLETIN 1450-A

A report published jointly by the 

U.S. Bureau of Mines and 

U.S. Geological Survey

Definitions of mineral resource classification 

terms used by the U.S. Bureau of Mines and 

U.S. Geological Survey

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 76-600025

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 

Stock Number 024-001-02813-1
FOREWORD

In order to use mineral resource terms with precision and common understanding and to compare resource data effectively, a 

joint U.S. Bureau of Mines and U.S. Geological Survey work 

group developed a standardized, definitive, broadly applicable 

classification system to derive uniform, coordinated resource estimates. This report presents the results of the work group that 

developed the basic terms of mineral resource classification. Other 

chapters in this series will present classification terms for specific 

commodities.

Thomas V. Falkie V. E. McKelvey

Director, Bureau of Mines Director, Geological Survey

III

CONTENTS

Foreword ___ ______________________ ____________

General definition of mineral and energy resources ________________ Al

Philosophic basis for a resource classification _______________ _____ 1

Glossary of resource terms _____________________ 2

Areas of responsibility and operational .procedures _ ___ 4

ILLUSTRATION

Page

FIGURE 1. Classification of mineral resources ____________________ A2

MINERAL RESOURCE CLASSIFICATION SYSTEMS OF THE 

U.S. BUREAU OF MINES AND U.S. GEOLOGICAL SURVEY

PRINCIPLES OF THE MINERAL

RESOURCES CLASSIFICATION SYSTEM

OF THE U.S. BUREAU OF MINES

AND U.S. GEOLOGICAL SURVEY

GENERAL DEFINITION OF MINERAL AND ENERGY 

RESOURCES

The dictionary definition of resource "something in reserve or 

ready if needed" has been extended for mineral and energy resources to comprise all materials surmised to exist having present 

or future values. In geologic terms a mineral or energy resource is 

a concentration of naturally occurring solid, liquid, or gaseous materials in or on the Earth's crust in such form that economic extraction of a commodity is currently or potentially feasible. Material classified as a reserve is that portion of an identified resource producible at a profit at the time of classification.

Total Resources are materials that have present or future value 

and comprise identified or known materials plus those not yet 

identified, but which on the basis of geologic evidence are presumed to exist.

PHILOSOPHIC BASIS FOR A RESOURCE CLASSIFICATION

Public attention usually is focused on current economic availability of mineral or energy materials (reserves). Long-term public and commercial planning, however, must be based on the 

probability of geologic identification of resources in as yet undiscovered deposits and of technologic development of economic 

extraction processes for presently unworkable deposits. Thus, all 

the components of Total Resources must be continuously reassessed in the light of new geologic knowledge, of progress in 

science, and of shifts in economic and political conditions.

Another requirement of long-term planning is the weighing of 

total or multi-commodity resource availability against a particular 

need. To achieve this the general classification system must be 

uniformly applicable to all commodities so that data for alternate 

or substitute commodities can be compared.

Al
A2 MINERAL RESOURCE CLASSIFICATION SYSTEMS

To serve these planning purposes Total Resoures are classified 

both in terms of economic feasibility and of the degree of geologic 

assurance. The factors involved are incorporated in figure 1 to 

provide a graphic classification of Total Resources.

General guides for the use of this classification system are as 

follows:

1. Resource categories and definitions in the classification, as 

specified in the glossary, should be applicable to all naturally occurring concentrations of metals, .nonmetals, and fossil fuels. The 

categories may be subdivided for special purposes.

2. Definitions may be amplified, where necessary, to make them 

more precise and conformable with accepted usage for particular 

commodities or types of resource evaluations.

3. Quantities and qualities may be expressed in a variety of 

terms and units to suit different purposes, but must be clearly 

stated and defined.

GLOSSARY OF RESOURCE TERMS

Resource. A concentration of naturally occurring solid, liquid, 

or gaseous materials in or on the Earth's crust in such form

TOTAL RESOURCES

Demonstrated

UNDISCOVERED

HYPOTHETICAL 

(In known districts)

SPECULATIVE

(In undiscovered

districts)

RESERVES

Increasing degree of geologic assurance 

FIGURE 1. Classification of mineral resources.
PRINCIPLES OF THE CLASSIFICATION SYSTEM A3

that economic extraction of a commodity is currently or 

potentially feasible.

Identified resources. Specific bodies of mineral-bearing material 

whose location, quality, and quantity are known from geologic evidence supported by engineering measurements with 

respect to the demonstrated category.

Undiscovered resources. Unspecified bodies of mineral-bearing 

material surmised to exist on the basis of broad geologic 

knowledge and theory.

Reserve. That portion of the identified resource from which a 

usable mineral and energy commodity can be economically 

and legally extracted at the time of determination. The term 

ore is used for reserves of some minerals.

The following definitions for measured, indicated, and inferred

are applicable to both the Reserve and Identified-Subeconomic

resource components. 1

Measured. Reserves or resources for which tonnage is computed 

from dimensions revealed in outcrops, trenches, workings, and 

drill holes and for which the grade is 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, and mineral content are well established. The computed tonnage and grade 

are judged to be accurate within limits which are stated, and 

no such limit is judged to be different from the computed 

tonnage or grade by more than 20 percent.

Indicated. Reserves or resources for which tonnage and grade 

are computed partly from specific measurements, samples, or 

production data and partly from projection for a reasonable 

distance on geologic evidence. The sites available for inspecr 

tion, measurement, and sampling are too widely or otherwise 

inappropriately spaced to permit the mineral bodies to be 

outlined completely or the grade established throughout.

Demonstrated. A collective term for the sum of measured and 

indicated reserves or resources.

Inferred. Reserves or resources for which quantitative estimates 

are based largely on broad knowledge of the geologic charac1 The terms proved, probable, and possible (used by the industry and economic evaluations of ore in specific deposits or districts) commonly have been used loosely and interchangeably with the terms measured, indicated, or inferred (used by the Department of the 

Interior mainly for regional or national estimates). The terms "proved" and "measured" 

are essentially synonymous. The terms "pi-obable" and "possible," however, are not synonymous with "indicated" and "inferred." "Pi-obable" and "possible" describe estimates of 

partly sampled deposits in some definitions, for example, "probable" is used to describe 

deposits sampled on two or three sides, and "possible" for deposits sampled only on one 

side: in the Bureau-Survey definitions, both would be described by the term "indicated."
A4 MINERAL RESOURCE CLASSIFICATION SYSTEMS

ter of the deposit and for which there are few, if any, samples 

or measurements. The estimates are based on an assumed 

continuity or repetition, of which there is geologic evidence; 

this evidence may include comparison with deposits of similar 

type. Bodies that are completely concealed may be included if 

there is specific geologic evidence of their presence. Estimates 

of inferred reserves or resources should include a statement 

of the specific limits within which the inferred material may 

lie.

Identified-Subeconomic. Resources that are not Reserves, but 

may become so as a result of changes in economic and legal 

conditions.

Parantarginal. The portion of Subeconomic Resources that (1) 

borders on being economically producible or (2) is not commercially available solely because of legal or political 

circumstances.

Submarginal. The portion of Subeconomic Resources which 

would require a substantially higher price (more than 1.5 

times the price at the time of determination) or a major costreducing advance in technology.

Hypothetical resources. Undiscovered resources that may reasonably be expected to exist in a known mining district under 

known geologic conditions. Exploration that confirms their 

existence and reveals quantity and quality will permit their 

reclassification as a Reserve or Identified-Subeconomic 

resource.

Speculative resources. Undisovered resources that may occur 

either in known types of deposits in a favorable geologic 

setting where no discoveries have been made, or in as yet 

unknown types of deposits that remain to be recognized. 

Exploration that confirms their existence and reveals quantity and quality will permit their reclassification as Reserves 

or Identified-Subeconomic resources.

AREAS OF RESPONSIBILITY AND OPERATIONAL 

PROCEDURES

U.S. Bureau of Mines. The Bureau appraises, analyzes, and publishes reserve estimates from base data supplied by the minr 

eral and energy materials industry, the U.S. Geological 

Survey, and other governmental agencies. The Bureau judges 

commodity recoverability on existing economic and legal 

factors.
PRINCIPLES OF THE CLASSIFICATION SYSTEM A5

U.S. Geological Survey. The Survey appraises, analyzes, and, 

publishes estimates of Total Resources. It reports such measurable parameters of significance to resource evaluation as 

location, quality, quantity, and situation of Identified 

resources.

Annual Resource Summation. The U.S. Bureau of Mines and 

U.S. Geological Survey will confer and agree annually on 

estimates in all of the resource categories defined above. 

These data will be in Bureau or Survey publications and will 

be available for inclusion in the Secretary's Annual Report 

required by the Mining and Minerals Policy Act of 1970.

Ad Hoc Joint Conferences. The Directors will convene ad.hoc 

joint work groups to resolve problems in the resource area.