KOMI

GEOLOGICAL SURVEY CIRCULAR 882

Sedimentary Phosphate 

Resource Classification System 

of the U.S. Bureau of Mines 

and the U.S. Geological Survey

 v
Sedimentary Phosphate 

Resource Classification System 

of the U.S. Bureau of Mines 

and the U.S. Geological Survey

GEOLOGICAL SURVEY CIRCULAR 882

1982
United States Department of the Interior

JAMES G. WATT, Secretary

Geological Survey

Dallas L. Peck, Director

Library of Congress catalog-card No. 82-600634

Free on application to Distribution Branch, Text Products Section, 

U. S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page

Introduction __________________________________________ 1

Reserve/resource definitions ___________________________________ 2

Degree of geologic assurance ________________________________ 2

Degree of economic feasibility of use ____________________________ 4

Dynamics of phosphate resources ________________________________ 9

References cited _________________________________________ 9

ILLUSTRATIONS

Page

FIGURE 1. Diagram showing classification of mineral reserves and resources _______________ _ 3

2. Diagrams showing

A, Major elements of mineral-resource classification, excluding reserve base and inferred reserve base 4 

B, Reserve base and inferred reserve base classification categories ___________________ 5

3. Diagram showing southeastern phosphate resource category definitions: strippable resources, present major

industry economics and technology _____________________________ _ 6

4. Diagram showing northwestern phosphate resource category definitions: strippable resources, present major

industry economics and technology _________________________________ 7

5. Diagram showing northwestern phosphate resource category definitions: underground resources ___ _ 8

in
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 working group developed a standardized, definitive, 

broadly applicable classification system to derive uniform, coordinated resource 

estimates. The principles of the system are given in U.S. Geological Survey Circular 831, and this supplementary report presents the criteria for classification 

of phosphate resources. The subcommittee preparing this report consisted of 

William B. Hall, Cochairman, William F. Stowasser, and Gary A. Kingston of 

the U.S. Bureau of Mines, and Richard P. Sheldon, Cochairman, David F. Davidson, and Thomas Leshendok of the U.S. Geological Survey. Many other geologists and engineers participated in the work, and particularly helpful were 

James B. Cathcart and Robert A. Gulbrandsen.

Approved:

Director, Bureau of Mines Director, Geological Survey
Sedimentary Phosphate Resource Classification System

of the 

U.S. Bureau of Mines and the U.S. Geological Survey

INTRODUCTION

The availability of phosphate resources to support domestic phosphate fertilizer supply and the 

U.S. phosphate export industry has been questioned for a number of years. Discussion of this 

problem has tended to center mistakenly around 

"optimistic" and "pessimistic" resource estimates. 

However, on detailed analysis of the various estimates of phosphate, the differences between estimates are commonly due more to differences in 

what is being estimated than in how much is there. 

One resource scientist/engineer may give a large 

figure but include rock that is only potentially 

available along with the rock currently available, 

while another scientist/engineer may limit his estimate to only the currently available rock. Differences in estimates may relate to differences in 

areas treated, differences in price assumptions or 

mining technology assumptions, differences in 

quality of phosphate rock for use in different ore 

processing or fertilizer manufacturing technology, 

differences in geologic assurance of existence 

or measurement, differences between the sets of 

data used in making the estimates, and finally 

differences in the methodology of assessing the 

resource.

Difficulties in producing compatible phosphate 

resource estimates occur because of imprecise 

technical language, which has been confusing to 

the specialist as well as the nonspecialist. A sedimentary phosphate resource classification system 

is presented here to provide a pathway out of the 

jungle of confused terminology and to permit real 

differences between scientific/engineering estimates to be identified and analyzed.

The principles of this classification system have 

been presented in U.S. Geological Survey Circular 

831, published in 1980 and jointly prepared by the 

U.S. Bureau of Mines and the U.S. Geological 

Survey. It should be made clear that the purpose of 

this sedimentary phosphate resource classification

system is to facilitate the assessing of national 

phosphate rock resources.

The phosphate reserve and inferred reserve on 

federally owned lands must be assessed and evaluated by the U.S. Department of the Interior for the 

purpose of leasing to private developers. This activity may seem to be similar to the activity of 

assessing the national phosphate rock resource, 

but it is much more restrictive. First, only federally owned lands are studied for the leasing program; phosphate rock resources on other lands are 

ignored. Second, the evaluation of federally owned 

lands requires establishing the reserves to at least 

an inferred level. Thus, an economic analysis of the 

lease tracts must be made. National phosphate 

resource assessment, on the other hand, requires 

an estimate of the reserve base and inferred reserve base, that is, the physically determined body 

of rock from which reserves and inferred reserves 

are calculated. Thus, the reserve base must always 

be larger than the reserve, and the inferred reserve base must always be larger than the inferred 

reserve. Therefore, this set of resource definitions, 

which deals with the national phosphate rock resources, is largely irrelevant to the lease tract 

evaluation of phosphate lands.

In the following section, resource/reserve definitions relative to phosphate rock are presented, 

along with criteria for each resource class. The 

criteria were made to relate to current industrial 

practice as much as possible. It should be emphasized that the criteria for the resource classes are 

arrived at by using parameter limits currently in 

use by industry. However, industry practices vary 

somewhat from mine to mine and from district to 

district. Also, trade-offs between parameters are 

possible; for example, richer ore may sometimes be 

mined in thinner beds than stipulated in the 

criteria, or blending of two ores, each with one 

parameter below the criteria, may give a mixture 

that is within the criteria. Thus, these criteria are 

only guidelines that may be replaced for specific
deposits by parameter limits reached through detailed studies. As the high-grade phosphate rock is 

depleted, technology will change to allow the mining and processing of lower quality rock, if favorable economics permit. This would result in changing the classification criteria for the resource 

classes.

The two main phosphate provinces in the United 

States are the southeastern province, from Florida 

to North Carolina, and the northwest province of 

southeastern Idaho and the adjacent States of 

Montana, Wyoming, and Utah. These two provinces accounted for 97 percent of the U.S. 1980 

phosphate rock production and contain the overwhelming share of U.S. phosphate resources. Only 

these two provinces have been treated in this 

study. Due to differences in the physical characteristics of the deposits, somewhat different technologies of mining and processing have been developed for the two districts, making it necessary to 

establish different sets of classification criteria for 

the respective districts. Also, the criteria for 

identification of resource classes are made considering only major industry economics and technology. Thus, small mining operations that operate 

on different criteria are excluded.

RESERVE/RESOURCE DEFINITIONS

The classification system is based on two fundamental aspects of mineral resources (fig. 1; 

McKelvey, 1972): first, how well known and well 

measured the deposit is (degree of geologic assurance), and, second, how feasibly a deposit can be 

brought into production with existing mining and 

processing technology at existing prices (feasibility 

of economic recovery). The criteria for classifying 

resources according to those two aspects, of 

course, differ and are treated below, with reference to figures 2A and 2B. Summary diagrams 

showing the resource class criteria for the southeast and northwest provinces are given in figures 3 

through 5.

DEGREE OF GEOLOGIC ASSURANCE

Phosphate resources are broken into two groups, 

identified and undiscovered.

Phosphate Identified Resources. Phosphate resources whose location, grade, quality, and 

quantity are known or estimated from specific 

geologic evidence. To reflect varying degrees 

of geologic certainty, phosphate identified resources can be subdivided into demonstrated 

and inferred resources.

(Phosphate Identified Resources Continued) 

Phosphate Demonstrated Resources. A

term for the sum of measured plus indicated resources.

Phosphate Measured Resources. Quantity is computed from dimensions 

revealed by 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 phosphate content of the resource are well 

established.

Criteria for classification: The delineation of measured resources is the function 

of industry and often is proprietary. No 

criteria have been set in this paper for this 

resource class. The criteria generally used 

in industry are a sampling density of more 

than 64 boreholes per square mile.

Phosphate Indicated Resources. 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.

Criteria for classification: At least four 

boreholes or measured stratigraphic sections per square mile or no more than 0.5 

mile between holes.

Phosphate Inferred Resources. 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.

Criteria for classification: At least one 

hole or measured stratigraphic section per 

square mile or no more than 1 mile between boreholes. A greater distance between holes may be used if, in the considered judgment of the resource geologist, 

geologic inference allows.
IDENTIFIED

Proved Probable Possible

Reserves

UNDISCOVERED

Resources

A

Degree of certainty1 

The "degree of uncertainty" was replaced by "degree of geologic assurance" in USGS Circular 831.

FIGURE 1.-Classification of mineral reserves and resources. Degree of uncertainty increases from right to left, and feasibility of 

economic recovery increases from bottom to top. From McKelvey, 1972.

Phosphate Undiscovered Resources. Phosphate 

resources whose existence is only postulated. 

Two categories of phosphate undiscovered resources exist, hypothetical and speculative.

Phosphate Hypothetical Resources. Phosphate undiscovered resources that are 

similar to and may be hypothesized extensions of known phosphate 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.

Criteria for classification: That part of a 

deposit more than 1 mile away from a borephosphate Undiscovered Resources-Continued) 

(Phosphate Hypothetical Resources -

Continued)

hole or a measured stratigraphic section of 

that deposit, or a farther distance away 

from such a point source if, in the considered judgment of the resource geologist, 

geologic inference allows.

Phosphate Speculative Resources. Undiscovered phosphate resources that may occur either in known types of deposits in 

favorable geologic settings where phosphate 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,
Cumulative

Production

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RES

Demonstrated

Measured Indicated

Reserves

Marginal Reserves

Demonstrated 

Subeconomic Resources

OURCES

Inferred

Inferred Reserves

Inferred 

Marginal Reserves

Inferred 

Subeconomic 

Resources

UNDISCOVERED RESOURCES

Probability Range

 (or) 

I

1

1

1

FIGURE 2A.- Major elements of mineral-resource classification, excluding reserve base and inferred reserve base. From U.S.

Geological Survey Circular 831.

(Phosphate Undiscovered Resources-Continued) 

(Phosphate Speculative Resources -

Continued)

grade, and quality, they will be reclassified 

as identified resources.

Criteria for classification: No identification by borehole or surface outcrop.

Phosphate 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 criteria for reserves except that 

they are restricted from extraction by laws or 

regulations.

DEGREE OF ECONOMIC FEASIBILITY OF USE

Resources can be broken down into three classes 

according to their degree of economic feasibility of 

use. These classes include economic, marginally 

economic, and Subeconomic resources, which are 

discussed below. The criteria for identifying economic and marginally economic resources are 

deposit and time specific and are not based on 

physical parameters alone. Thus, they lie outside 

the scope of this report.

Phosphate Economic Resources. Those phosphate resources for which profitable extraction 

or production under defined investment assumptions have been established, analytically 

demonstrated, or assumed with reasonable

(Phosphate Economic Resources-Continued) 

certainty. Economic resources include reserves, inferred reserves, and economic undiscovered resources discussed below.

Phosphate Reserves.-That part of the demonstrated resources that can be economically extracted or produced at the time of 

determination. The term "reserves" need 

not signify that the extraction facilities are 

in place and operative. Reserves include 

only recoverable materials.

Phosphate Inferred Reserves. That part of 

the inferred resources that can be economically extracted or produced at the 

time of determination. Otherwise similar 

to phosphate reserves.

Phosphate Economic Undiscovered Resources. In general, it is not possible to 

subdivide undiscovered resources into 

classes based on feasibility of economic recovery because of lack of information on 

the physical characteristics to do so. However, it is possible to make statistical estimates of such classes by analogy with identified deposits.

Phosphate Marginally Economic Resources. 

Those phosphate resources that, at the time of 

determination, border on being economically 

producible. Their essential characteristic is

4
Cumulative 

Production

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

Measured Indicated

Reserve

Base

Inferred

Inferred

Reserve 

Base

UNDISCOVERED RESOURCES

Probability Range

 (or) 

Hypothetical Speculative

1-

1

FIGURE 2J3.- Reserve base and inferred reserve base classification categories. From U.S. Geological Survey Circular 831.

(Phosphate Marginally Economic ResourcesContinued)

economic uncertainty. Included are phosphate 

resources that would be producible given 

postulated changes in economic or technologic 

factors. Marginally economic resources include 

marginal reserves, inferred marginal reserves, 

and marginal undiscovered resources.

Phosphate Marginal Reserves.-That part of 

demonstrated resources that, 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.

Phosphate Inferred Marginal Reserves. 

That part of inferred resources that, 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.

Phosphate Reserve Base. -That part of an identified phosphate 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

(Phosphate Reserve Base Continued)

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 resources that are currently 

economic (reserves), marginally economic 

(marginal reserves), and some of those that are 

currently subeconomic (subeconomic resources), but the latter is included only if it is 

the unrecoverable part of an economic or 

marginally economic resource and should not 

be included if it is a separate deposit. The purpose of distinguishing between the reserve 

base and the combined reserves and marginal 

reserves (defined above) is that the reserve 

base is a body of rock identified by its physical 

parameters (thickness, grade, quality, depth) 

alone, whereas reserves and marginal reserves 

are identified by economic factors in addition 

to the physical parameters. Thus, the reserve 

base is independent of short-term variations of 

price or other short-term economic factors and 

changes only by losses from production and increases from discovery and technologic 

improvements.

Criteria for classification: At the present 

time, only strippable phosphate rock is 

economic or marginally economic. An exception to this is an underground mine in Montana 

in the northwestern phosphate province. This 

mine is a part of an integrated Canadian opera-
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5UBECONOIV

IDENTIFIED RESOURCES

Demonstrated

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Product: P2O5 >28.4%, CaO/ 

MgO < 1.0%, Fe2( 

Matrix: Cu. yd. overburden 

ton product <30,

Inferred

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'2O5 <1.55 

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+ matrix per 

thickness >2'

Minimum size >40x 106 tons product 

Mine depth <150'

Product: P2OS >29% 

Matrix: Cu. yd. overburden+matrix 

per ton product <40, 

thickness >2'

Minimum size >500 tons product per acre

Mine depth <200 '

UNDISCOVERED RESOURCES

Hypothetical Speculative

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01 HI

OD 

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FIGURE 3.-Southeastern phosphate resource category definitions: strippable resources, present major industry economics and

technology.

(Phosphate Reserve Base-Continued)

tion producing sulfuric acid from metallic 

sulfide deposits with overall acceptable economics. Mining and processing practices differ 

between the southeastern and northwestern 

fields, and the minimum criteria for the resource differ. These two provinces are dealt 

with separately below.

Southeastern Phosphate Province. - The 

product1 must have more than 28.4 per1 Product is "pebble" or concentrate, which is made of the fraction greater than 1 

nun in diameter and the concentrate between 0.1 and 1.0 mm, unless one of these 

fractions is below acceptable grade.

(Phosphate Reserve Base-Continued)

(Southeastern Phosphate Province-Continued) 

cent P205 [62.0 percent BPL (bone phosphate of lime)], a ratio of CaO to P205 of 

less than 1.55, an MgO content of less than 

1.0 percent, and a combined Fe208 and 

A1208 analysis of less than 4 percent. The 

matrix (phosphate ore bed) must have a 

thickness of more than 2 feet and a stripping ratio of cubic yards of overburden and 

matrix per ton of product of less than 30. 

The minimum size of the deposit must be 

greater than 40 million tons of recoverable
IDENTIFIED RESOURCES UNDISCOVERED RESOURCES

Demonstrated Inferred Hypothetical Speculative

.11

^ '5»

c "o .2 *

8 o

11

Product: P205 > 18%, CaO/P205 <1.55, 

MgO <1.0%, Fe203+AI203

Cu. yd. overburden per ton of 

rock <3.5, thickness >5'

Minimum size >20xioe, unconsolidated

3 C

151

^ "c 

Eo

~c ~O

Product: P205 >15% 

Cu. yd. overburden per ton of 

rock <9, thickness >3'

(Aui

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FIGURE 4.-Northwestern phosphate resource category definitions: strippable resources, present major industry economics and

technology.

(Phosphate Reserve Base Continued)

(Southeastern Phosphate Province - Continued) 

product, and the maximum mine depth 

must be less than 150 feet.

Northwestern Phosphate Province.-The 

minable unit of phosphate rock must be 

weathered or oxidized and must average 

greater than 18 percent P205 ; the rock 

must have a ratio of CaO to P2O5 of less 

than 1.55, an MgO content of less than 1.0 

percent, and a combined Fe2O3 and A1203 

analysis of less than 3 percent; the thickness of the bed must be more than 5 feet;

(Phosphate Reserve Base Continued)

(Northwestern Phosphate Province-Continued) 

the stripping ratio of cubic yards of overburden per ton of phosphate rock must be 

less than 3.5; and the size of the deposit 

must be greater than 20 million tons of 

rock.

Phosphate 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, for which 

there may be no samples or measurements.
W)

RESERVE B/

o

SUBECONOM

IDENTIFIED RESOURCES

Demonstrated

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Product: P2O6 >24°/ 

Thickness >3'

<1000' below entry

Inferred

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level

UNDISCOVERED RESOURCES

Hypothetical Speculative

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FIGURE 5.-Northwestern phosphate resource category definitions: underground resources.

(Phosphate Inferred Reserve Base-Continued) 

The estimates are based on an assumed continuity beyond the reserve base for which there 

is geologic evidence.

Criteria for classification: Same as minimum 

criteria for Reserve Base.

Phosphate Subeconomic Resources. That part 

of identified resources that does not meet the 

economic criteria of reserves and marginal reserves. In practice this class includes the re-

(Phosphate Subeconomic Resources-Continued) 

sources that do not meet the physical criteria 

of the reserve base and marginal reserve base 

and excludes those parts of the reserve base 

that are not economically recoverable to 

assure that no rock is double counted as a 

resource.

Criteria for classification:

Southeastern Phosphate Province. - The 

product must have a phosphate grade

8
(Phosphate Subeconomic Resources Continued)

(Southeastern Phosphate Province - Continued) 

greater than 28.4 percent P205 (62.0 percent BPL). The matrix must have a thickness of more than 2 feet, a stripping ratio 

of cubic yards of overburden and matrix 

per ton of product of less than 40, and a 

mine depth of less than 200 feet. The deposit must have the potential of greater 

than 500 tons of product per acre. Only 

strippable deposits are considered in this 

classification.

Northwestern Phosphate Province. - Subeconomic resources include both strippable 

deposits and underground deposits. Strippable subeconomic resources must be 

made up of a phosphate bed greater than 3 

feet thick, contain greater than 15 percent 

P205, and have a stripping ratio of cubic 

yards of overburden per ton of phosphate 

rock less than 9. Underground subeconomic resources must be made up of a 

phosphate bed greater than 3 feet thick, 

contain greater than 24 percent P205 , and 

occur in beds not more than 1,000 feet 

below entry level.

Other Occurrences. Materials not meeting the 

criteria for any of the resource/reserve classes 

but having sufficient magnitude of occurrence 

to indicate possible future interest can be 

recognized by this classification.

Criteria for classification: The general basis 

is characteristics below the minimum criteria 

of the subeconomic resources class. Also, the 

criterion of not having current or potential 

feasibility of economic production is applicable.

DYNAMICS OF PHOSPHATE RESOURCES

An assessment of any mineral resource is valid 

only at the time when the assessment is made, 

because all the factors that influence the magnitude of resources of the various classes are time 

dependent.

Changing economics has a clear effect on the 

magnitude of reserves, inferred reserves, marginal 

reserves, and inferred marginal reserves. A rise in 

price as other factors remain the same will cause 

some marginal resources to become economic and 

subeconomic resources to become marginal. On the

other hand, falling prices will have the opposite 

effect. The reserve base class was created to deal 

with fluctuation in price. The resources categorized by their physical parameters alone would remain stable in spite of short-term variations in 

price.

Governmental policies can affect the economics 

of mineral production and have a profound effect 

on resource classifications. Examples of these 

changing policies are policies on taxation, depletion allowance, railroad and shipping freight rates, 

environmental cost, and price controls.

The effects of changing mining and processing 

technology also can be profound and have the 

effect in extreme cases of making economic resources subeconomic by replacing them with what 

had been subeconomic resources. This happened, 

for example, when low-grade taconite was processed into high-quality pellets, replacing highgrade direct shipping iron ore and reducing the 

cost of hot metal from the blast furnace. Changes 

in technology of fertilizer manufacture can affect 

the quality specifications of phosphate rock, which 

in turn will affect the resource class boundaries.

The generation of geologic and engineering data 

relating to the assurance of existence of resources 

has the effect of moving resources to the left in the 

classification diagram. Basic research allows conception of speculative resources. Resource assessment activities create hypothetical resources from 

speculative resources; exploration creates inferred 

resources from the undiscovered classes; and development drilling creates demonstrated resources 

out of inferred resources.

We conclude that resource estimates are in a 

constant state of change, as basic science, resource 

assessment, exploration, development drilling, and 

research on mining and processing technology increase the availability of resources. It is necessary 

to continually update resource estimates of all 

classes. Resource estimates should be framed in 

precise technical terms and the dates of each estimate recorded.

REFERENCES CITED

McKelvey, V. E., 1972, Mineral resources estimates and public 

policy: American Scientist, v. 60, no. 1, p. 32-40.

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.