Accuracy of Ore-Reserve 

Estimates for UraniumVanadium Deposits 

on the Colorado Plateau

GEOLOGICAL SURVEY BULLETIN 1030-D

\ Thts report concerns work done on behalf

\ of the U. S. Atomic Energy Commission

\ and is published with the permission of

\ the Commission

Accuracy of Ore-Reserve 

Estimates for UraniumVanadium Deposits 

on the Colorado Plateau

By A. L. BUSH and H. K. STAGER

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

GEOLOGICAL SURVEY BULLETIN 1030-D

This report concerns work done on behalf 

of the U. £ Atomic Energy Commission 

and is published with the permission of 

the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
UNITED STATES DEPARTMENT OF THE INTERIOR 

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY

Thomas B. Nolan, Director '

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

Washington 25, D. C. - ^Price 45 cents (paper cover
CONTENTS

	Page 

Abstract___________.__________________________ 131

Introduction. ______-_____--_-__-_----_-__.__--______-_-___________ 132

Method of study..________________________________ 133

Definition of reserves____________________-_______-___------__ 135

Method of calculation______________-______________--_------__ 135

Character of data_______________________________________________ 138

Accuracy of original reserve estimates_-___-___-___-__---------------- 139

Tonnage__________________________________._ 139

Grade ...._._____.__._.__-_-___._________________ 140

Variation with size of deposit__________----_______-_---__------- 141

Variation with density of drillmg__________________________-_-___ 143

Causes of inaccuracy______________________-_______________-------__ 145

Original reserve estimates____________________________________ 145

Revised reserve estimates_____________________________________ 146

Conclusions.--.___-_______________-_______-_______-____-_----_--__ 147

Literature cited___________________________________________________ 148

ILLUSTRATIONS

PLATE 7. Sketch maps of reserve blocks, showing location of original Page 

and revised reserve blocks and mine workings, Uravan 

mineral belt, Colorado Plateau____________________ In pocket

FIGUEE 28. Map of part of the Colorado Plateau showing location of the

Uravan mineral belt and of mineral claim groups studied- _ 133 

2P Graph showing variation in accuracy of original tonnage

reserve estimates-___-____-___--_--_-_--_--_---_----__ 140

30. Graph showing variation in accuracy of original reserve estimates of grade of UaOa, classed by size of deposit_______ 141

31. Graph showing variation in accuracy of original reserve estimates of grade of V2Os, classed by size of deposit.._______ 142

32. Graph showing variation in accuracy of original tonnage reserve estimates, for deposits classed by density of drilling.. 145

m

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

ACCURACY OF ORE-RESERVE ESTIMATES FOR

URANIUM-VANADIUM DEPOSITS ON THE

COLORADO PLATEAU

By A. L. BUSH and H. K. STAGER

ABSTRACT

The U. S. Geological Survey has made estimates of the reserves of uranium 

and vanadium in the carnotite deposits explored by Geological Survey drilling 

on the Colorado Plateau. This report presents an appraisal of the accuracy 

of the reserve estimates for carnotite deposits in the Uravan mineral belt, the 

causes of inaccuracy, and the significance of the estimates in terms of the total 

known reserves of the region.

The appraisal was confined to carnotite deposits that contain ore reserves of 

1,000 short tons or more. These reserves are classed as indicated and inferred 

reserves. Reestimates of the amount of material in the ground at the time of the 

original estimate were made for more than 30 deposits. This was done where 

information on size and grade, additional to that used at the time of the original 

estimate, was available. The reestimated reserves (revised reserves) include 

production since the original reserve estimates, plus the estimate of reserves 

remaining in the ground, and, therefore, can be compared with the original estimates, to furnish a measure of the accuracy of the original estimates.

The reappraisal indicates that for individual deposits the original reserve estimates range from large overestimates to very large underestimates (for both 

tonnage and grade). However, the original reserve estimate for the total reserves 

of all the carnotite deposits studied in the Uravan mineral belt is a very small 

underestimate. For the carnotite deposits studied in the mineral belt, the revised tonnage of ore reserves is about 15 percent greater than originally estimated. 

In terms of grade, the original and revised estimates are the same for UjOa and 

differ by less than 2 pounds per ton for V2 Os. Primarily as a function of the 

increased tonnage of reserves, the amount of contained UaOg is about 15 percent 

greater, and the amount of contained V2 Os is about 20 percent greater than 

originally estimated.

All of the individual deposits contained more than half the amount of ore 

originally estimated. A little more than 25 percent of the deposits contain over 

twice as much ore as originally estimated. The errors in estimation for individual 

deposits are mutually compensating; as more deposits are grouped into a single 

estimate, the amount of error decreases both in tonnage and grade.

131
132 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

The general source of error in the original estimates is the restricted number 

of points of observation (mostly drill holes) on which the estimates are based. 

This is complicated by the deviation of the ore deposits from an idealized uniformly tapering layer both in plan and in section, and by the erratic distribution 

of metal values within the deposit. No mathematical correction can be applied 

to the reserve estimate to compensate for this sampling, error.

As the deposits studied in the mineral belt represent a fair sample of all the 

uranium-vanadium deposits that the Geological Survey is finding in the Morrison 

formation, the realization ratios (the "accuracy of prediction" figures) given 

above can be applied with reasonable confidence, to the total of the estimates of 

known reserves for all of the deposits found by the Geological Survey. This 

total estimate is one of the major objectives .in the exploration work. As the 

error in the original estimate of total reserves for the deposits studied in the 

mineral belt is small, the methods of exploration and estimation being used are 

adequate to achieve a realistic appraisal of the total uranium-vanadium reserves 

of the region. ' "

INTRODUCTION

Since November 1947, the U. S. Geological Survey on behalf of the 

Division of Raw Materials of the U. S. Atomic Energy Commission 

has been conducting diamond- and wagon-drill exploration for uranium-vanadium deposits on the Colorado Plateau. Much of this 

exploration has been in southwestern Colorado, within the area described by R. P. Fischer and L. S. Hilpert (1952) as the Uravan 

mineral belt (fig. 28). In this belt, most of the deposits are in lenticular 

sandstone beds in the upper part of the Salt Wash sandstone member 

of the Morrison formation of Jurassic age. Uranium and vanadium 

minerals impregnate the sandstone and form roughly tabular bodies 

that generally parallel the bedding of the sandstone. In places the 

deposits cut across the bedding at a sharp angle. The deposits are 

irregular in plan, and they range in thickness from a knife edge to a 

few tens of feet, averaging a few feet thick. They range in content 

from only a few tons to many thousand tons. A more complete description of these deposits is given by Fischer (1942).

An integral part of the Geological Survey's exploration program has 

been the estimation of the reserves of uranium and vanadium in the 

explored deposits. The individual reserve estimates serve as a basis 

for the leasing of deposits on Government-controlled land, and collectively are useful as a measure of expectable production.

The uranium-vanadium deposits of the Colorado Plateau are of the 

type classed by McLaughlin (1939, p. 589-621) as "extramensurate 

those difficult to explore and measure much in advance of mining." 

It must be expected, therefore, that reserve estimates that are based 

mainly on samples from only a few drill holes would not be highly 

accurate for individual deposits. The present study has been undertaken to determine the accuracy of the reserve estimates, to investigate the causes of inaccuracy and the possibility of correction of these
ACCURACY OF URANIUM-VANDIUM RESERVE ESTIMATES 133

SAN JUAN

FIGURE 28. Map of part of the Colorado Plateau showing location of the Uravan mineral belt and of mineral

claim groups studied.

inaccuracies, and to determine the significance of the estimates in 

terms of total known reserves of the region.

METHOD OF STUDY

It is obvious that the accuracy of a reserve estimate can best be 

determined when the reserve block (the ore body) has been completely 

mined out. Only then can direct comparison be made between the 

estimated content and the actual content, as to both tonnage and 

grade. Very few of the deposits on the Colorado Plateau, for which
134 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

the Geological Survey has made reserve estimates, have been entirely, 

or even significantly depleted. In this sense this study is premature. 

However, interest is in the present content of the deposits, as well as 

the accuracy of the original estimate, not in a post-mortem knowledge 

of what the content or accuracy was. The study must take the form, 

therefore, of a reestimate of the total reserves of the deposit that 

existed at the tune of the original estimate. This reestimate (the revised reserve estimate) includes the production from the deposit since 

the original reserve estimate plus the estimate of the reserve remaining 

in the ground. By comparing the revised reserve estimate with the 

original reserve estimate, an expression of the accuracy of the original 

estimate is obtained.

Information additional to that available at the time of the original 

reserve estimate is necessary for a meaningful reappraisal of the 

reserves in these deposits. For this reason, only those deposits that 

have yielded, since the original reserve estimate, about 25 percent 

or more of the tonnage of the original reserve estimate were selected 

for study. Additional information resulting from drilling done by 

the mine operator in conjunction with the mining was also available 

for many deposits. Furthermore, because the production potential 

of small deposits is not very important, only deposits estimated to 

contain about 1,000 tons or more of ore were included in this study. 

These factors resulted in the selection of 30 deposits for reappraisal. 

These deposits, however, are thought to be typical and to represent a 

good sample of all deposits for which reserves have been estimated 

by the Geological Survey.

These deposits were mapped, generally at scales of 1 inch to 40 or 

50 feet, to obtain information on extent of mining, habits, thicknesses, 

and grades of the deposits and ore bodies, and their general trends. 

This information, plus the additional drilling information, was compiled on maps showing the original information available (drill holes, 

mine workings, ore-deposit boundaries, and reserve-block locations). 

On the basis of the additional information, the boundaries of the 

deposits and the location and size of the original reserve blocks were 

redefined. Thus the revised reserve blocks include the material in 

the ground at the time of the original reserve estimate.

In general, experience has shown that after 1,000 to 1,500 tons of 

ore has been produced from a deposit, or in the case of small (less 

than 5,000 tons) deposits, after about 25 percent of the ore has been 

mined, the cumulative grade of the production is a fair representation 

of the overall grade of the deposit. For this reason, the grade of ore 

produced was used in many cases as the grade of the reserves in the 

revised reserve estimate. The validity of this approach appears to 

borne out by an extremely close correspondence of the average U3O8
ACCURACY OF URANIUM-VANDIUM RESERVE ESTIMATES 135

and V2O6 grades of the original reserve estimate for all the deposits 

studied with the average grade for the production from those deposits. 

The mines were mapped in their extent as of December 31, 1952. 

To correlate with this extent of the mining, figures of production to 

the same date were used.

DEFINITION OF RESERVES

In 1949, the Geological Survey staff on the Colorado Plateau 

prepared a statement to define the general philosophy and the 

rules for the calculation of reserves for the uranium-vanadium 

("carnotite") deposits of the Colorado Plateau, in order to obtain a 

reasonable consistency of reserve calculations made by different 

geologists and to assist those who would use these figures in interpreting their significance.

The terms "indicated" and "inferred" reserves are applied to the 

uranium- and vanadium-bearing materials in the deposits that are 

known from exposures in natural outcrops, mine workings, or drill 

holes. These reserves are subdivided by thickness and grade cutoffs 

and the method used in calculating them is explained on pages 136-137.

Known reserves are classed as indicated and inferred. Reserves 

that might be classed as measured are included with indicated reserves, 

because the amount of measured reserves that could be calculated 

within a small limit of error is so small as to be nearly negligible. 

This is due to the general lack of abundant sample data for individual 

reserve blocks and the erratic deviations, within short distances, in 

the thickness and grade of the carnotite ore.

Indicated reserves are those for which the grade is computed from 

drill-hole samples, exposures in mine workings and natural outcrops, 

gamma-ray logs, and production data, and for which the tonnage is 

computed by projection for a reasonable distance on geologic evidence 

from points of exposure (drill holes, mine workings, and natural outcrops). Inferred reserves are those for which quantitative estimates 

are based largely on broad knowledge of the geologic character of the 

deposits, and for which there are few, if any, samples or measurements.

METHOD OF CALCULATION

Reserves are not classified according to their availability for mining, 

although consideration is given to current mining and milling practices 

in selecting the higher grade and thickness cutoffs. This is done to 

obtain figures for a category of reserves that will express as nearly as 

possible the tonnage and grade of the material that actually might be 

mined from these deposits under present conditions.

379289 56 2
136 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Both chemical data from assay of drill cores and gamma-ray data 

from probing of drill holes are used in classification of the ore-bearing 

rock as to thickness and grade.

Thickness cutoff. Mining practices differ from place to place in the 

region as well as with individual operators, but under present conditions most ore bodies of average grade are being mined to where 

they pinch to a layer about 1 foot thick. Reserves, therefore, are 

calculated with a thickness cutoff of 1 foot. Layers of material less 

than 1 foot thick are mined in places if the grade is high. The tonnage 

of minable material less than 1 foot thick is small with respect to the 

total reserves, and for that reason reserves less than 1 foot thick are 

calculated in a separate category.

Grade cutoffs: The deposits contain two metals of economic value, 

uranium and vanadium. Within the deposits, the two metals are so 

erratically distributed that a single sample, such as obtained from a 

drill hole, is not necessarily representative of the grade or metal ratio 

of the material near the point sampled. Knowing this by experience, 

the miner will drive to a drill hole that shows a good value in vanadium 

even though the uranium content of the sample might be negligible. 

Thus, the material in the vicinity of this sample must be classed as a 

reserve, even though the sample shows a value for only one metal. 

Under 1953-54 price schedules, the vanadium content of the ore 

represents an appreciable part of the ore's market value. Both metals, 

therefore, must be considered in reserve appraisals and in selecting 

grade cutoffs.

Reserves 1 foot or more thick are classified by two grade cutoffs. 

The higher cutoff used 0.10 percent U3O8 or 1.0 percent V2OS 

corresponds to the AEC purchase cutoff for uranium and the approximate cutoff used at the government-owned mill at Monticello, Utah, 

in selecting ore for treatment to extract vanadium. Reserves also are 

figured on a lower cutoff 0.05 percent U308 or 0.50 percent VjOg on 

the possibility that conditions in the future might demand or permit 

the mills to accept low-grade ore.

Calculation of tonnage. The method used for calculating the volume, 

and hence the tonnage, of a reserve unit 1 foot or more thick differs 

with the geologic interpretation of the form of the deposit. Some 

deposits are roughly tabular or lenticular, so that projections can 

reasonably be made between drill holes, and the average thickness of the 

drill-hole samples that can be combined within a specified grade class 

can be assumed to be the average thickness of the reserve unit. Other 

deposits consist of small bodies of ore of minable thickness, connected 

by layers of mineralized sandstone that are too thin to mine. Reserves 

are estimated by assuming that each drill hole in ore indicates a single 

minable body that is comparable in tonnage to the average size of the
ACCURACY OF URANIUM-VANDITJM RESERVE ESTIMATES 137

ore bodies that have been mined nearby, or that the deposit contains 

a comparable tonnage per unit area to that of a deposit that has been 

mined nearby.

The tonnage of indicated reserves is computed by projection for a 

reasonable distance on geologic evidence. The distance that indicated 

reserves are projected, both between and beyond drill-holes and other 

sample points differs with the geologic habits of the deposits; the 

limits of projection used are specified. Reserves are classed as inferred 

rather than indicated if they are projected farther than the limits 

chosen for the indicated class. Inferred reserves are projected to the 

assumed limits of the deposit, as assumed by geologic evidence and 

interpretation; no arbitrary limit is placed on the distance of projection.

Although a single drill hole in ore obviously permits the designation 

of some tonnage of indicated reserves, there is no reasonable basis 

for projecting an indicated reserve block more than a few feet from 

a single hole. Rather than calculate such a separate indicated reserve 

block, or assign a small arbitrarily selected amount of indicated reserves to a single hole, the reserve block is projected to its assumed 

limits and the ore calculated and classed as inferred.

A constant of 14 cubic feet per ton is used to calculate tonnage.

Calculation of grade. The grade of indicated reserves in a single 

block is calculated by weighting the assay grades by lengths of the 

samples. If the deposit has been mined in part, the grade of the ore 

produced is also considered in establishing the grade of the adjoining 

reserve block. In reserve blocks containing only one or two drill 

holes, however, if the core assays are appreciably higher than the 

average grade of the ore mined nearby, it may be assumed that the 

drill penetrated abnormally high-grade parts of the body, and an 

estimated grade may be assigned. The grade of the samples in an 

inferred block is considered, but is used in conjunction with the grade 

of production from nearby mines, or from the area as a whole.

Strict grade cutoffs are used in calculating reserves 1 foot or more 

thick. Except as noted in the foUowing paragraph, no material 

belonging to a class with a lower grade cutoff is included with material 

of a higher cutoff class, even though the weighted average grade of 

the whole is above the cutoff grade of the higher class.

In mining, layers of waste, or of low-grade mineralized material, 

less than 1 foot thick, would probably be blasted with the enclosing 

layers of ore. Some of this material might be picked out by hand, 

but most of it would go to the mill with the ore. If the aggregate 

thickness of this interbedded ore and waste is 1 foot or less thick, it is 

included in calculating reserves, thereby increasing the thickness of 

ore, but decreasing the grade proportionately. This ore is classed 

according to the grade of the weighted average. If the waste is more
1.38 CONTRIBUTIONS TO THE GEOLOGY' OF URANIUM

than 1 foot thick, it probably would be blasted separately from the 

ore layers in mining, and thus ore layers more than 1 foot apart, with 

waste or low-grade material between, are calculated as separate ore

bodies.

CHARACTER OF DATA

The original reserve estimates were based largely on drill-hole data, 

supplemented in some cases by exposures in mine workings and natural 

outcrops. In general, the amount of information available for the 

reserve estimate is a function of the drill-hole spacing. A major objective of the Geological Survey's exploration work is the discovery of 

ore deposits, and the estimation of their size, in terms of reserves that 

are not more precise than the indicated class. At this level of precision, 

the spacing of drill holes is designed to outline the limits and to indicate 

the order of magnitude of the deposits with a minimum of drilling. 

The drill holes were spaced mostly at 50- to 100-foot intervals; in a 

few places holes were as much as 200 feet apart.

Chemical assays for U3O8 and V2O6 are available for all the mineralized rock cored in the drilling. Although core recovery differs from 

hole to hole, as well as from one lithologic unit to another within a 

single hole, the average core recovery for mineralized rock is probably 

about 80 percent. The core recovery for barren sandstone is usually 

better and in many units is 95 to 100 percent. The position and thickness of the mineralized rock, therefore, can often be determined with 

considerable accuracy, despite core loss within the unit. The grade 

can be seriously distorted by core loss, however, as the richer material 

is commonly fine and friable. A partial correction for the loss of the 

richer material may be made through the use of gamma-ray data obtained by probing the drill holes.

The revised reserve estimates are invariably based on additional 

information. The habits of the deposits can be studied in new or expanded mine workings, the boundaries of the deposit are exposed in 

places, additional drilling in many cases is available so that the extent 

and average thickness of the deposit are better known, and the production records give a strong basis for the assignment of grades to the 

unmined ore.

Generally, the data available from additional drilling done by the 

mine operator is less reliable than that from Geological Survey drilling. 

Drill-hole locations may be less accurately recoverable, chemical assays 

on the core are usually lacking, and data on the position and thickness 

of the mineralized material, both in core and noncore drilling, may be 

somewhat less reliable. Many of these disadvantages, however, are at 

least partially compensated by the greater density of drilling. In 

some cases a major weakness exists in the production record. Frequently the ore from several deposits on a claim, or a group of claims,
ACCURACY OF URANITTM-VANDIUM RESERVE ESTIMATES 139

is grouped and attributed to a general area, rather than to a specific 

deposit. It thus becomes impossible to assign the production accurately to the individual deposit, and the reliability of the revised reserve estimate is weakened thereby.

ACCURACY OF ORIGINAL, ORE RESERVE ESTIMATES

The basic tool in the assessment of the accuracy of the original 

estimate is the realization ratio, the ratio of the revised estimate to the 

original estimate. Realization ratios have been calculated for each 

deposit (or each case studied) in terms of tonnage ratio, grade ratio 

of both U3O8 and V2O5 , and content ratio of both U3O8 and V2 O5 . 

Ratios have also been calculated for each large group of deposits, for 

each district, and for all 30 deposits studied in the mineral belt.

Errors in the original reserve estimates decrease generally, as the 

number of deposits included in the estimate is increased, due to compensation of individual errors. Estimates on individual deposits 

range widely in accuracy, from large overestimates to very large 

underestimates. Estimates for the mineral belt, however, are well 

within the limits of accuracy expected, constituting a small underestimate for tonnage, contained U3Os, and contained V2O5 . The 

progression is orderly, the error is greatest on individual deposits, less 

on large groups of deposits, still less by districts, least for the entire 

mineral belt.

For the entire mineral belt, this study indicates that the tonnage 

of the revised estimate is about 15 percent more than originally 

estimated. There is also about 15 percent more U3O8 and about 20 

percent more V20S in the revised reserves than in the reserves originally 

estimated. Expressed in terms of grade, in the revised reserves the 

average U3O8 grade is the same as the original estimate and the 

average V2O5 grade is slightly higher than the original estimate, 

differing by less than 2 pounds per ton.

The Geological Survey's reports to the Atomic Energy Commission 

on carnotite reserves have included the following statement 

Because of the variations in thickness and grade of ore and the scarcity of sample 

data, the indicated reserves in any single reserve block might actually amount 

to as much as twice or as little as one-half the calculated tonnage. The limit 

of error of the total tonnage for several blocks, however, is apt to be considerably 

lower, perhaps not more than 25 percent of the calculated tonnage. The limit of 

error in the tonnage figures for inferred reserves, of course^ is apt to be higher 

than for the indicated reserves. The possible limit of error in the calculated or 

estimated grade for both indicated and inferred reserves probably is somewhat 

smaller than the possible limit of error in the tonnage figures.

TONNAGE

The data presented in figure 29 show that the quoted statement 

above is substantially correct. The comparison shows that all of the 

deposits studied actually contained (according to the revised esti-
140 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

GROUP AND DISTRICT

V\

A

A

\A

EXPLANATION

 

MMdml deposits

A 

CUhn group

 

District

 Uravati mineral tott

REVISED ESTIMATE, IN THOUSANDS OF SHORT TONS

FIOXJEE 29. Graph showing variation in accuracy of original tonnage reserve estimates.

mate) more than one-half the originally calculated tonnage of both 

indicated and inferred reserves. The only deviation from the statement is that a little over one-quarter of the deposits appear to contain 

more than twice the amount of indicated and inferred reserves.

The limits of error are smaller for groups of deposits than they are 

for single deposits. In four of the eight large groups, the realization 

ratios for tonnage of indicated and inferred reserves fall between 0.8 

and 1.3. The other four all contain from 1% to a little less than 3 tunes 

the originally calculated tonnage of indicated and inferred reserves.

Considered by districts, the tonnage realization ratios for two of the 

three districts are 0.8 and 0.9, the third contains over twice as much 

reserves as originally calculated. For all the deposits studied in the 

mineral belt the tonnage realization ratio is about 1.15.

GRADE

In general, the range of error and the limit of error in U3O8 and V2OS 

grades (figs. 30 and 31) are considerably smaller than those for the
ACCURACY OF URANIUM-VANDlUM RESERVE ESTIMATES 141

tonnage of ore (fig. 29). On an individual basis, almost three-quarters 

of the deposits have U308 grade realization ratios between 0.5 and 1.5, 

and seven-tenths of the deposits have V20S grade realization ratios 

between 0.75 and 1.25. By groups, the U308 realization ratios of six 

of the eight groups are between 0.5 and 1.5 (a seventh has a ratio of 

1.6) and five of the eight groups are between 0.75 and 1.25. The V2OS 

realization ratios of all eight groups are between 0.85 and 1.1, and for 

six of the eight groups are between 0.95 and 1.05. By districts, both 

the U3O8 and V2O5 realization ratios are between 0.85 and 1.1. For 

all the deposits studied in the mineral belt, the U3O8 and V2O5 realization ratios are 1.0 and 1.05 respectively.

VARIATION WITH SIZE OF DEPOSIT

A study was made of the accuracy of the original reserve estimates 

grouped by size of the ore deposits. (See table below and figs. 30 and

X 2X 3X 4X 

U3O8 GRADE (ARBITRARY UNITS), REVISED ESTIMATE

EXPLANATION

Less than 5000 tons 10,000-50,000 tons

A X 

5000-10,000 tons More than 50,000 tons

FIGUBX 30. Qraph showing variation in accuracy of original reserve estimates of grade of TTjOs, classed by

size of deposit.
142 CONTRIBUTIONS TO THE GEOLOGY OF TJRANITJM

X 2X 3X 4X 

V205 GRADE (ARBITRARY UNITS), REVISED ESTIMATE

5X

EXPLANATION

Less than 5000 tons

D 

10,000-50,000 tons

5000-10,000 tons More than 50,000 tons

FIOUKE 31. Graph showing variation in accuracy of original reserve estimates of grade of VsOs, classed by

size of deposit.

31.) The deposits were classed into four categories less than 5,000 

tons, 5,000-10,000 tons, 10,000-50,000 tons, and more than 50,000 tons. 

The distribution of deposits in these classes is such that there are 

probably sufficient cases in the first three categories to warrant some 

generalizations. With only two cases in the fourth category, it is not 

possible to make significant generalizations.

On the basis of the size of the reserve block as originally estimated, 

deposits thought to be in the less than 5,000 ton class are underestimated in tonnage in about nine-tenths of the cases, and underestimated in both U3O8 and V205 grade in about two-thirds to threequarters of the cases. In the 5,000-10,000 ton class, tonnage is underestimated in four-fifths, and both U3O8, and V2O5 grades are underestimated in three-fifths of the cases. In the 10,000-50,000 ton class
ACCURACY OF URANTOM-VANDIUM RESERVE ESTIMATES 143

tonnage is underestimated in two-fifths, U3O8 grade in three-fifths, 

and V2O5 grade in two-fifths of the cases. The relative amount of 

error represented by the average realization ratios for each class 

decreases from the less than 5,000 ton class to the 10,000-50,000 ton 

class.

Variation in accuracy of original estimates of reserves^ grouped by size of deposit.

Deposit class

Less than 5,000 tons.. .............

5,000-10,000 tons.. ....... .......

10,000-50,000 tons... ............

Number 

of 

cases

IS 

5 

5 

2

Original 

reserve 

estimate, 

in short 

tons '

50,740 

42,160 

91, 610

169,875

Revised 

reserve 

estimate, 

hi short 

tons l

103,900 

80,300 

76,200 

164, 100

Realization ratios >

Tonnage

2.05 

1.90 

.85 

.90

Content

U3 0s

2.45 

2.05 

.90

.75

V,0t

2.25 

1.85 

.80 

.85

Grade

U»0g

1.20 

1.05 

1.10 

.80

V»0s

1.10 

.95 

.95 

.95

1 Includes indicated and inferred reserves.

! Ratio of revised estimate to original estimate.

VARIATION WITH DENSITY OP DRILLING

An analysis was made of the accuracy of the original reserve estimates grouped by the density of drilling. (See table below and fig. 32.) 

As is to be expected, denser drilling permitted more accurate reserve 

estimates. The density of drilling by the Geological Survey has been 

described briefly on page 138. Most of the deposits drilled by the 

Geological Survey in the mineral belt were explored under a drilling 

plan consisting of four drill holes at a spacing of 50 feet around the 

discovery hole, followed by other drill holes at a spacing of 100 feet 

(pi. 7A). Other deposits were explored by six drill holes at a spacing 

of 50 feet around the discovery hole, followed by other drill holes at 

the same spacing, with additional drilling at a spacing of 100 feet on 

some deposits (pi. 7B).

Of the 30 cases studied, 25 fall into these two classes of density of 

drilling; there are 18 cases of Type B drilling to only 7 cases of Type A 

drilling. The Geological Survey has used three other drilling plans, 

one of which is illustrated in plate 7(7, but with only 5 deposits in 

these 3 classes, generalizations are not significant. In 4 of the 7 cases 

where Type A drilling information was used, the revised reserve estimate for tonnage is more than twice the original, and the range is up 

to 8 times the original. Where Type B information was available, the 

revised reserve estimate was more than twice the original in only 2 of 

the 18 cases, and the range was up to only 3 times. The data also 

show a general increase in accuracy for both U308 and VjOg grades in 

the cases for which more information is available.
144 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Variation in accuracy of original estimates of reserves, grouped by density of drilling

and size of deposit

[See also figure 32]

Drilling plan and deposit class

Type A:' 

Less than 5,000 tons ...........

5,000-10,000 tons...... .........

Totals and weighted

Type B: » 

Less than 5,000 tons.. .........

5,000-10,000 tons.. .............

10,000-50,000 tons ...........

Totals and weighted

Type O:>

10,000-50,000 tons ........

Totals and weighted

TypeD:< 

Less than 5,000 tons ___ .....

Totals and weighted

TypeE:« 

More than 50,000 tons .........

Totals and weighted

Number 

of

cases

5 

2

7

11 

3 

4

IS

1 

1

2

1 

1

2

1

1

Original 

reserve 

estimate, 

in short 

tons

11,400 

18,080

29,480

34,110

24,080 

71,610

129,800

830 

20,000

20,830

4,400 

71,500

75,900

98,375

98,375

Revised 

reserve 

estimate, 

in short 

tons

31,100 

55,400

86,500

55,300 

24,900 

63,200

143,400

5,900 

13,000

18,900

11,600 

72,200

83,800

81,900

81,900

Realization ratios

Tonnage

2.75 

3.05

2.95

1.60 

1.05 

.90

1.10

7.10 

.65

.90

2.65 

1.00

1.10

.85

.85

Content

U3 0«

4.05 

2.35

2.70

2.00 

1.75 

1.00

1.35

10.20 

.65

.75

2.80 

.70

.80

.75

.75

V80t

2.55 

2.75

2.70

1.95 

1.15

.85

1.20

9.00 

.60

.85

3.10 

.80

.90

.95

.95

Grade

UiOs

1.45

.75

.90

1.20 

1.70 

1.10

1.20

1.45 

.95

.85

1.05 

.70

.75

.90

.90

VjOj

0.95 

.90

.90

1.20 

1.10 

.95

1.10

1.25 

.90

.95

1.15 

.80

.90

1.10

1.10

i One cycle of four drill holes around discovery hole, at 50-foot distance; additional holes at 100- 

foot intervals.

» One or more cycles of six drill holes around discovery hole, at 50-foot distance: additional holes at 100-foot 

intervals.

* One or more cycles of drill holes around discovery hole at 100-foot distance; additional holes at 100-foot 

intervals.

* One or more cycles of drill holes around discovery hole at 100-foot distance; additional holes at 150- to 

200-foot intervals. 

«Drill holes 50 to 100 feet apart, along lines 200 feet apart
ACCUEACY OF URANIUM-VANDIUM; RESERVE ESTIMATES 145

"B 9 10 20 30 40 50 .60 70 80 90100

REVISED ESTIMATE, IN THOUSANDS OF SHORT TONS

FIOTJKE 32. Graph showing variation in accuracy of original tonnage reserve estimates, for deposits classed 

by density of drilling. (See table on page 144 for explanation of letters.)

CAUSES OF INACCURACY

Both the original and the revised reserve estimates are subject to 

error. In general, the causes of error differ for the type of estimate, 

but one major cause of error is common to both; the restricted number 

of point of observation (sample points) on which the estimates are 

based.

ORIGINAL RESERVE ESTIMATES

The density of sampling is the basic source of error in the original 

reserve estimates. In general, the drilling is sufficient to indicate the 

approximate outline and extent of the ore deposit, but in specific cases, 

the ore bodies may extend between barren or weakly mineralized drill 

holes for a significant distance (pi. 7A and Z>). The drill holes may 

not adequately sample, however, or may not even intercept the parts 

of the deposit that constitute the ore bodies (pi. 7-4).
146 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

The drill-core samples, in addition, may not be representative of 

the average thickness of the ore body or of its average grade. Core 

loss in drilling may result in significant misinterpretations of the thickness of the mineralized rock. In addition, the more richly mineralized 

rock is generally softer and more friable and hence is more subject to 

grinding and disintegration. Core loss, therefore, is frequently concentrated in the more richly mineralized rock, so that the apparent 

grade of the sample is less than the true grade.

In most cases, the concept of the deposits (and ore bodies) as uniformly tapering tabular layers, essentially parallel to the bedding of 

the host sandstone, is used in calculating the reserves (pi. 7B). Other 

limiting conditions are also assumed; for example the edge of the 

deposit is assumed to be midway between a sample point hi barren 

rock and a sample point in mineralized rock. Deviations in form, 

shape, and habit from these idealized conditions introduce errors in 

the reserve estimate (pi. 7A and O).

REVISED RESERVE ESTIMATES

The revised reserve estimates also have their basic source of error 

in the restricted number of points of observation. The influence of 

this distribution is less pronounced than in the case of the original 

estimate, as the prerequisite to the revised estimate is additional information. Additional drilling by the operator frequently outlines 

both the deposit and the ore bodies far better (pi. 7D). The mine 

workings permit a more accurate assessment of the habits of the deposit, and the concentration and size of the ore bodies (pi. 7B and O). 

In many cases, however, the thickness and grade data available from 

the operator's drilling is less reliable than data from the Geological 

Survey's drilling, because of difference in objectives, technique, and a 

common lack of chemical assay data.

Major use is made of the production data hi revising the estimate 

of tonnage and grade given in the original reserve estimate. Several 

opportunities for error in the revised estimate may result from the 

character of the production data. In many cases the production 

record cannot be broken down with sumcient reliability to permit the 

accurate assignment of production to individual deposits. In such 

cases, the production must be estimated for each deposit, or the 

deposits must be grouped and a combined depletion estimate for the 

group must be made. It is obvious that additional uncertainties 

are introduced in either case, both for tonnage and for grade. Such 

uncertainties, however, are thought to be small generally, and certainly 

do not invalidate the overall picture presented here.

Nearly all the mine operators attempt to hold to a shipping grade 

above 0.20 percent U8O8, because the AEC pays bonuses for ore
ACCURACY OF URANIUM-VANDIUM RESERVE ESTIMATES 147

above this grade. Although the grade cutoff used by the Geological 

Survey in its estimates is 0.10 percent U808 (the lowest grade that is 

being purchased by the AEC), the average grade of the ore at this 

cutoff is generally above 0.20 percent U3O8 . Thus the material 

mined and that for which reserves are estimated are comparable. 

One other factor affects the use of the production data the dilution 

of ore by waste. The effect of this source of error is relatively minor, 

as the methods of reserve estimation used by the Geological Survey 

make allowance for a certain amount of dilution.

CONCLUSIONS

The accuracy of the Geological Survey's estimates of total indicated 

and inferred reserves, for the 30 cases studied in the Uravan mineral 

belt, fall well within the defined limits of error for indicated reserves 

alone, and indeed fall almost entirely within the defined limits of 

error (20 percent) for measured reserves. The limits of error, and 

the range in error, are greatest for reserves on individual deposits, 

less for reserves on groups of deposits, and still less for reserves of 

districts. The inaccuracies involved are largely compensating, to 

the point of very little error if a sufficient number of deposits are 

considered.

The 30 cases considered in this study represent a fair sample of all 

deposits on which the Geological Survey has made reserve estimates. 

They include deposits ranging from less than 1,000 tons of reserves 

to more than 50,000 tons; deposits where all the information came 

from drill-hole samples and deposits where a larger part of the information came from extensive mine workings; and deposits where the drillhole samples came from close-spaced, medium-spaced, and relatively 

wide-spaced holes, or any combination of these. As such, the realization ratios determined for the total group of cases studied can 

be applied with reasonable confidence as correction factors to the 

total of the indicated and inferred reserves estimated for all the 

deposits discovered or explored by the Geological Survey. These 

realization ratios are about 1.15 for the tonnage of ore, 1.15 for the 

amount of contained U8O8 , and 1.20 for the amount of contained 

VaOs. The original grade estimate for U3O8 is the same as the revised 

estimate, and the original grade estimate for V^Og differs from the 

revised estimate by less than 2 pounds per ton.

Correction factors cannot be applied to the individual reserve estimates. On a numerical basis, in about two out of every three cases 

the tonnage of reserves was underestimated; the range of the realization 

ratios is from about 1.0 to 8.0. However, in two-thirds of the cases 

where the tonnage of reserves was underestimated, the range of the 

realization ratios is from 1.0 to only 2.0. The range of the realization
148 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

ratios for overestimated reserves is from 0.6 to nearly 1.0. Thus any 

single correction factor designed to reduce the error of underestimate 

will increase the error of overestimate in one of every three cases. The 

range and distribution of error for the estimates of grade and amount 

of U3O8 and VzO5 are similar to those for total tonnage of ore, and 

these errors, as well, cannot be remedied by applying single correction 

factors to the individual deposits.

The realization ratios for each group of deposits studied can be applied with a measure of certainty to the total indicated and inferred 

reserves of all the deposits in that group. The reliability of the ratios 

increases with the number of cases used in determining the ratios. 

Still more reliance can be placed on the realization ratio for the reserves 

compiled by districts.

One of the Geological Survey's major objectives in the exploration 

work is the appraisal of the uranium-vanadium resources of the Morrison formation on the Colorado Plateau. The study indicates that 

the accuracy of the appraisal of the total known reserves is good. As 

such, the methods of exploration and estimation being used [are adequate to achieve the desired objectives.

Within the framework of the exploration done by the Geological 

Survey, increased accuracy in the estimates can only be obtained by 

increased knowledge of the geologic habits of the deposits, and this is 

largely dependent on the increase in the amount of information available at the individual deposit level supplied by additional and more 

closely spaced drilling. So long as the major interest is in the overall 

appraisal, the present methods are adequate. Increased accuracy on 

individual deposits will increase the accuracy for each succeeding 

grouping of deposits, but the significance of this increased accuracy 

decreases for each succeeding grouping as the amount of error involved 

under the present methods also decreases markedly.

LITERATURE CITED

Fischer, R. P., 1942, Vanadium deposits of Colorado and Utah, a preliminary

report: U. S. Geol. Survey Bull. 936-P. 

Fischer, R. P., and Hilpert, L. S., 1952, Geology of the Uravan mineral belt:

U. S. Geol. Survey Bull. 98S-A. 

McLaughlin, D. H., 1939, Geological factors in the valuation of mines: Econ.

Geology, v. 34, no. 6.

o