Michael S. Indorf. URANIUM-PHOSPHORUS DETERMINATIONS FOR
SELECTED PHOSPHATE GRAINS FROM THE MIOCENE PUNGO RIVER
FORMATION, NORTH CAROLINA. (Under the direction of Dr.
Stanley R. Riggs) Department of Geology, April 1982.
The Aurora Phosphate District is located in the
Central Coastal Plain of North Carolina. Phosphate
production in the area is derived from the mining of the
phosphorite sediments of the Pungo River Formation. This
thesis evaluates the relationship between uranium content
and phosphorus content within the central facies phospho-
rite of the Formation in terms of regional location,
stratigraphic position, grain size, and grain type.
A total of 154 subsamples representing five core hole
locations were examined for this study. They were obtained
from 19 sediment samples representing units A, B, C, and
D/DD of the Pungo River Formation and 3 sediment samples
representing the Yorktown Formation. Subsamples consisted
of phosphate grains selected from the 00, 20, and 40 size
ranges. Grains in the 00 and 20 size ranges were further
separated into intraclast, skeletal fragment, pellet, and
disc grain type groups.
Uranium and phosphorus contents have been determined
by fluorometric and spectrophotometric methods, respec-
tively. Uranium contents ranged from 5.1 to 285.9 ppm U.
7, y. ,h; V'JKit .
TîAST CAUC.wij's A Uî'vit ¿¿iCSITX
Phosphorus contents ranged from 23.25 to 38.22 % P205- The
combined mean uranium content was calculated to be
92.5+27.4 ppm U. The combined mean phosphorus content was
calculated to be 30.64+0.84% ^2^5'
There are no significant trends in the regional
distribution of uranium and phosphorus in the study area.
Phosphate grains from unit D/DD may be slightly depleted in
uranium and phosphorus relative to underlying and overlying
units. There is an apparent inverse relationship between
grain size and mean uranium content. The mean uranium
content was 71.6 ppm U for Ojó size phosphate grains; 82.3
ppm U for 2(0 size phosphate grains; and 123.5 ppm U for 4)ó
size phosphate grains. The mean phosphorus and uranium
contents of skeletal grains were slightly higher than those
of the intraclast, pellet, and disc grains.
The apparent differences which have been identified
among the data as grouped by core hole, unit, grain size,
and grain type are very slight. The differences exist
within an overall context of extreme variance. Statistical
comparisons show that the mean values for the particular
subgroups are essentially the same.
URANIUM-PHOSPHORUS DETERMINATIONS
FOR SELECTED PHOSPHATE GRAINS
FROM THE MIOCENE PUNGO RIVER FORMATION,
NORTH CAROLINA
A Thesis
Presented to
the Faculty of the Department of Geology
East Carolina University
In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Geology
by
Michael S. Indorf
April 1982
URANIUM-P BOSPHORUS DETERMINATIONS
FOR SELECTED PHOSPHATE GRAINS
FROM THE MIOCENE PUNGO RIVER FORMATION,
NORTH CAROLINA
by
Michael S. Indorf
APPROVED BY:
r ^
Comm ittee
: À :?
Dr'. Stanley R. Riggs \ \
/i^L¿
D r ./L oh n TI B ray
Dr . Richard L. auger
Dr. Richard K. Sprui 1 I
CHAIRMAN OF THE DEPARTMENT OF GEOLOGY
Dr. Char") es Brown
DEAN OF THE GRADUATE SCHOOL
ÏÏr r/jose'f/h G. Boyette’^
ACKNOWLEDGEMENTS
Dr. Stanley Riggs sparked my initial interest in this
project. His zeal for the geological profession and his
high standards for performance have set a good example.
Dr. John Bray has lea me by the hand through the jungle of
analytical trace element geochemistry. His patient
guidance in the lab phase of this thesis has been surpassed
in quality only by his chili. Analytical work was
conducted entirely within the domain of the Trace Element
Laboratory, Department of Surgery, ECU School of Medicine.
Dr. Richard Mauger and Dr. Richard Spruill served as
members of the thesis committee. Their comments have been
helpful and to the point. Dr. Charles Brown helped to
underwrite some of the equipment expenses for the project.
The International Minerals and Chemical Corporation
and Texasgulf, Inc. supplied the cores and samples used in
this study. The NC Department of Natural Resources and
Community Development provided grant support for this
project. Richard Dayvault and Nicholas Korte of the Bendix
Field Engineering Corporation, as well as Robert Jaroszeski
of the International Minerals and Chemical Corporation,
made helpful suggestions with respect to uranium analysis.
John Johnson of Olsen Associates, Inc., was of tremendous
help in the computer statistical analysis. Olsen
Associates also provided employment and access to their
drafting facilities at various times during the course of
this research. Andra Deadwyler did an excellent job of
French translation. The Indorfs, Lowrys, and many other
friends have supplied tremendous moral and financial
support; Doug and Beth Gomes, Ken Knott, Alan and Judy
Larkins, Steve and Anna McKinzie, as well as Jeff and Sarah
Pierce, oeserve special mention.
Maureen Fox has worked long and hard hours at typing
this manuscript to perfection. I thank her, and all those
mentioned above, for the part which each of them has had in
the completion of this thesis.
My deepest love and gratitude is reserved for my sweet
wife Bonnie, who has faithfully and patiently waited for me
to finish this project and spend some time at home, for a
change!
TABLE OF CONTENTS
Page
INTRODUCTION 1
GLOBAL SIGNIFICANCE OF PHOSPHATE 1
OBJECTIVE OF THIS THESIS 4
PREVIOUS WORK 6
URANIUM IN PHOSPHATE DEPOSITS 6
a. Sedimentary Marine Phosphorite Nomenciature...6
b. Uranium as a Phosphorite Component 6
C. U/P^Og Relationships 12
URANIUM IN THE PHOSPHATE MINERAL 15
GEOLOGY OF THE PUNGO RIVER FORMATION 18
a. Structural-Stratigraphic Framework 19
b. Petrology 22
c. Uranium as a Phosphorite Component 26
d. U/PgOg Relationships 28
PROCEDURES 31
SAMPLING 31
FLUOROMETRIC DETERMINATION OF URANIUM 33
a. Summary of Method 33
b. Apparatus 37
c. Reagents 39
d. Uranium Standard 40
e. Working Standards 40
f. Control Sampl es . 41
g. Protocol ~ 41
SPECTROPHOTOMETRIC DETERMINATION OF PHOSPHORUS 43
a. Summary of Method 43
b. Apparatus 44
c. Reagents 44
d. Mixed Reagent 44
e. Phosphorus Standard 45
f. Working Standards . 45
g. Control Sampl es . 45
h. Protocol 45
CALCULATIONS 47
URANIUM ANALYSIS 47
a. Working Standards 47
b. Control Sampl es . 47
c. Duplicate Sam"^es 49
Table of Contents (Continued)
PHOSPHORUS ANALYSIS 49
a. Working Standards .49
b. Control Samp les.. 51
RESULTS 52
DISCUSSION 59
CONCLUSIONS 68
REFERENCES CITED 70
APPENDIX A 75
PRECISION AND ACCURACY DETERMINATIONS 75
LIST OF FIGURES
Page
Figure 1. Phosphate districts of the Southeast
Atlantic Coastal Plain 2
Figure 2. Structural framework of the Aurora
Embayment 5
Figure 3. Summary description and correlation
of major lithologies 21
Figure 4. Forms of occurrence of macroscopic
phosphorites 23
Figure 5. Typical phosphate grain types 24
Figure 6. Apparatus for uranium analysis 38
Figure 7. Uranium calibration curve 48
Figure 8. Phosphorus calibration curve 5Û
Figure 9. Scatter diagram of % ^2^5
ppm U, for all samples. 60
Figure 10. Scatter diagrams of % P2O5 vs.
ppm U, by core hole 62
Figure 11. Scatter diagrams of % P2O5
ppm U , by unit 63
Figure 12. Scatter diagram of % ^2^5
ppm U, for all samples; By grain size 65
LIST OF TABLES
Page
Table 1 . Summary of general characteristics
of uraniferous phosphate resources,
Southeastern United States .. .3
Tab! e 2. Uranium and PoOc concentrations of
phosphorites from Africa and
Middle East . . .8
Tab! e 3. P2OJ- and U concentrations for samples
taken from sea floor phosphate
accumulations off the western margin
of South Africa . . .9
Table 4. Miscellaneous occurrences of high U
contents in phosphate deposits . .10
Tabl e 5. Uraniferous phosphate resources of the
Aurora Phosphate District . . 29
Table 6 . Summary of sample mass, U content, and
P^Og content based on analysis of 134
samples from the Pungo River Formation,
North Carolina . . 53
Table 7 . Summary of mean ppm U, by unit
and grain size . . 54
Table 8. Summary of mean ppm U, by core hole
and grain size . . 54
Table 9. Summary of mean % ^9^5’ unit
and grain size . . 55
Table 10. Summary of mean % by core
hole and grain size. . .55
Tabl e 11. Summary of mean ppm U, by unit
and grain type . . 56
Tabl e 12. Summary of mean ppm U, by core
hole and grain type . .56
Table 13. Summary of mean % P205> unit
and grain type . .57
Table 14. Summary of mean % P205> core
hole and grain typeT . . 57
INTRODUCTION
GLOBAL SIGNIFICANCE OF PHOSPHATE
The current world population explosion is demanding
tremendously increased food supplies. More than ever
before, there is an urgent need for greater agricultural
production on a global scale. Therefore, concern has
focused on the need for phosphorus. Phosphorus is one of
the three basic plant nutrients found in the chemical
fertilizers used to enhance quality and quantity in agri-
cultural production. U.S. General Accounting Office
reports (GAO, 1979) indicate that agriculture consumes
roughly 90% of the 120 million metric tons of annual world
phosphate production. A large proportion of this phosphate
product is derived from the strip mining and processing of
marine sedimentary phosphate.
The United States accounts for 41% (GAO, 1979) of
worldwide phosphate production. The major producing
districts of the Southeast Atlantic Coastal Plain include
Florida and North Carolina (Figure 1) which contribute 86%
of domestic phosphate, and therefore an impressive 35% of
total global production of phosphate (GAO, 1979). Table 1
summarizes current estimates of the phosphate resource base
for the southeastern United States. Considering the impor-
tance of these Coastal Plain deposits, and the fact that
2
Figure 1. Phosphate districts of the Southeast Atlantic Coastal Plain
(adapted from Riggs, 1979a)
Phosphate District
Southern North East
Extension Florida- Ocala Georgia- Aurora
Central of Central South East "Hard South (North Central
Characteristic Florida Florida Georgia Florida Rock" Carolina Carolina) Tennessee
Geologic Type
1
m m m ni m, k m m r, k
Average Overburden 20-30' 20-30' 30-40' 70-100' 20-90' 20-90' 80-110' 10-20'
Thickness
Average Phosphorite 15-25' 20'30' 15-30' 30-45' 5-15' 15-35' 35-60' 5-20'
Thickness
Average P„0g Content 15-18Ï 10-15Ï 8-10% 8-16% 20-23% 4-15% 8-18% 16-20%
of Phosphorite
Average U Content of 65 ppm 36 ppm 55-82 ppm 47-57 ppm 30-35 ppm 30-60 ppm 20-40 ppm 20 ppm
Phosphorite
Total Phosphate Resource 9,760 25,854 19,110 25,590 114 18,179 71,761 158
(millions of tons)
Estimated Recoverable 1,129 3,050 1,764 2,662 9 1,787 9,429 60
Phosphate Product
(millions of tons)
Estimated Recoverable 124,000 336,000 123,000 272,000 500 142,000 566,000 1,200
Uraniun (tons)
= marine phosphorite, with marine and fluvial reworking in seme cases
r = weathered residual material
k = karst-filling aspect
Table 1. Suimary of general characteristics of uraniferous phosphate resources, Southeastern United States (Based on an assumed
mineable grade of 4.0 percent P^Og and thickness of 5 feet; Fountain and Hayes, 1979.)
4
they represent a depleting resource, it is not surprising
that they are the subject of intense governmental interest,
industrial exploitation, and academic research.
OBJECTIVE OF THIS THESIS
The intent of this study is to add to the present
understanding of the detailed geochemical relationships
between uranium, phosphorus and the phosphate sediments
within the economically important Aurora Phosphate District
in the Central Coastal Plain of North Carolina. Specifi-
cally, the objective of this thesis is to evaluate the
relationship between uranium content and phosphorus content
within the central facies phosphorite of the Pungo River
Formation (Figure 2). The evaluation of this relationship
will include the consideration of the following controls:
1. regional location;
2. stratigraphic position;
3. grain size; and
4. grain type.
Placing this study in its proper context requires a brief
review of the geologic nature of 1) uranium in phosphate
deposits; 2) uranium in the phosphate mineral; and 3) the
Pungo River phosphorite of North Carolina.
Figure 2. Structural framework of the Aurora Embayment (adapted from Scarborough, 1981)
6
PREVIOUS WORK
URANIUM IN PHOSPHATE DEPOSITS
a. Sedimentary Marine Phosphorite Nomenclature
Reports on phosphate deposits often present nomen-
clature which is not consistent from one author to the
next. The terminology used by Riggs (1979a) to describe
material containing phosphate as a minéralogie component
has been adopted for this report:
Phosphorite - a rock term applied to all sediments or
rocks containing at least 10 percent
(volumetrically) individual phosphate
grains;
Phosphatic - a rock term applied to all sediments or
rocks containing at least 1 percent but
less than 10 percent (volumetrically)
individual phosphate grains; and
Phosphate - a general term applied to a class of
chemical compounds, a group of minerals,
a type of mineral deposit or a type of
sedimentary grain or particle.
b. Uranium as a Phosphorite Component
Cathcart (1978) stated that marine phosphorites
worldwide normally contain 50 to 300 ppm U. He has
tabulated uranium contents along with phosphate reserve and
/
production volumes, for the major producing countries.
Altschuler (1980) has tabulated the regional or formational
average trace element abundances for rich phosphorites from
around the world. He cited an average concentration of 120
ppm U in phosphorite, based on the analysis of seven phos-
phorites. The overall range of 30-260 ppm U reported by
Altschuler compares favorably with that of Cathcart noted
above. A more extensive review of the free world's urani-
ferous phosphate resources is found in DeVoto and Stevens
(1979). Slansky (1977) has compiled the published data on
the U and 82^5 contents of prominent phosphate deposits in
the African and Asian areas; his findings are condensed in
Tables 2-4.
There are several important observations concerning
these data.
1. Syrian phosphorite does not indicate a consistent
relationship between high U content and high P2O5
content (Table 2) .
2. Israeli phosphorite does indicate a variation in
samples from two different areas (Table 2).
3. Moroccan phosphorite does indicate that U content
varies with the age of the unit. Also U content
is independent of 82^5 content (Table 2).
Uranium (in ppm) (PoOj (in %)
Number of Average Range; Mise. Average Range
Source Samples Cone. Min-Max Values Cone. Min-Max ppm Ur^P^Oc
Egypt — — 30-100 — — —
(upper Cret.)
Red Sea — — 98-190 — — — —
Upper Nile — — — 82, 97, — — ---
68, 150,
130
,+
Jordan 26 — 14-156 — — 5.0-34.3 3
(upper Cret.)
Syria — — — 130, 35.0 — 3.7
(upper Cret.) 60 35.0 1.7
Israel : Zefa — — — 115-126 — — — _ - 20-36 6.0
: Saraf 254 — 67-132 — 21.2-29.4 3.5
Tunisia 50-75 — 27-32 —
Morocco
1. Ypresien 130 highest
2. Thanetien 160
3. Maestrichtien 190
4. Maestrichtien 250 lowest
Table 2. Uranium and P^Or concentrations ot phosphorites from Africa and Middle East (compiled from Slansky,
1977) 00
9
Average Values
Kati 0 Number of
Sample Type % PqOc ppm U ppm U:%P^0c Specimens
Phosphate/Diatom
Mud 8.32 5 0.5 3
Concretions
- soft 25.20 14 0.6 3
- compact 29.37 35 1 .4 6
- hard 33.09 53 1 .6 5
Coprolites
- soft 25.85 28 1 .0 2
- compact 31 .64 68 2.2 1
- hard 32.06 77 2.4 1
Table 3. P^0(- and U concentrations for samples taken from sea
floor phosphate accumulations off the western margin of
South Africa (from Slansky, 1977)
Uranium (in ppm) P205 (in %)
Number of Average Range: Mi sc. Average Range: Ratio
Source Sanipl es Cone. Min-Max Values Cone. Min-Max ppm U/%P.,0j;
Eocene-Bakouma 15 1660-5600 20.37 8.75-32.58 155.7
(central Africa) (81.7-392.9)
Jurassic-Mussouri 24 408 25-831 15.34 9.19-23.62 29.0
(India) (2.2 - 50.5)
Paleozoic 100-4000 1-24
(Siberia)
Eocene 1200
Lacustrine deposits
(Wyoming-Utah)
Phosphatic Limestone 700
(north England)
Table 4. Miscellaneous occurrences of high U contents in phosphate deposits (compiled from Slansky, 1977)
o
11
4. Sea floor samples from the western margin of South
Africa ^ indicate an increase in U content with
progressive lithification (Table 3).
5. Samples from certain phosphorites indicate
extremely high and variable concentrations of U
(Table 4).
Data such as those presented by Slansky (1977),
Cathcart (1978), and DeVoto and Stevens (1979) must be
evaluated with caution and a number of assumptions made if
the data are to be used in making comparisons. For
example, the authors seldom indicate the following
information:
1. The type of the sample analyzed (e.g. concentrated
mine products vs. total sediment; weathered vs.
unweathered).
2. The amount or size of sample analyzed.
3. The number of samples analyzed (if reported, the
number is often low).
4. The method and reliability of the analysis.
5. The relationships between analytical values and
basic geological parameters such as the petrology,
associated lithologies, or the structural-strati-
graphic framework (if reported, the degree of
control for these parameters is poor).
12
For the most part, the data published to date is intended
to show economic potential, with grand tonnage estimates
given for U and P2O5 production based upon assumed values
of concentration. Such broad generalizations can be mis-
interpreted, due to the factors mentioned, when applied at
a more detailed or specific level.
c. U/P^Ocb Relationships
It is generally accepted that uranium is associated
with the apatite component of phosphate deposits. The
relationship between uranium and apatite is usually
reported as a linear correlation value for U vs. or as
a ratio of However, the analysis of ratios
demonstrates conflicting trends. Altschuler et al. (1958)
analyzed 18 samples from various beds and locations within
the Moroccan Oulad-Abdoun phosphorites. They noted an
overall positive U vs. P2^5 correlation, in spite of a
marked scatter (100 to 160 ppm U) among the higher grade
(20 to 25% ^2^5^ samples. Slansky (1977) referred to a
study of Jordanian phosphorite in which U/P^Og ratios were
found to stay between 2 and 3 when the uranium concentra-
tion was less than 50 ppm. U/P20g ratios appeared to vary
randomly, from 3.6 to 12, when uranium concentrations were
greater than 50 ppm. In contrast, an analysis of 123 phos-
phate samples from 5 locations within the Phosphor i a
13
Formation of Idaho (Thompson, 1953, 1954) indicated that
when the uranium content was higher (average U = 310 ppm
for 12 samples), the U vs. P2®5 correlation was the strong-
est (+0.9). Altschuler et al. (1958) postulated that this
strong correlation could be associated with a relative lack
of post-depositional change. McKelvey and Carswell (1956)
also showed that on a regional scale, uranium concentra-
tions were highest in the portions of the Phosphoria Forma-
tion having the richest and thickest phosphorite accumula-
tion. Samples from the Bone Valley Formation of Florida
(Cathcart, 1956) exhibited an increase in uranium content
with a corresponding increase from 10 to 20% PoOr.
c b
Cathcart ( 1 956 ) has also linked grain size to
ratios. He reported a -0.64 correlation coefficient for U
vs. P2O5 samples of +150 mesh "pellets and pebbles" from
the Bone Valley Formation. However, testing of the
Moroccan Khouribga pelletai phosphorites (Altschuler et
ai., 1958) indicated the same basic uranium content in all
grain sizes. Likewise, Thompson (1953, 1954) concluded
that, in her analysis of samples from the Phosphoria
Formation, there was "little reason for believing that
pellet size has an influence upon, or reflects [the uranium
concentration]..."
Further observational notes pertaining to the
phosphate deposits of the southeast Atlantic Coastal Plain
14
have been made by various authors. McKelvey (1956) stated
that U content increased with P2O5 content in Florida
pellets. Concentrations above 100 ppm U were associated
with P2O5 contents above 30%.
Cathcart (1956) explained an inverse relationship
between grain size and U content in the Central Florida
Phosphate District as proof of the syngenetic marine origin
(rather than groundwater percolation emplacement) of the
uranium in the Miocene Hawthorn Formation. Marine re-
working caused U enrichment of the Pliocene Bone Valley
Formation of the same district.
Olson ( 1966 ) noted that in South Georgia the white
phosphate pebbles had the highest P2O5 content (34.1%) and
U content (90 ppm). The black phosphate pebbles had the
lowest P2O5 content (26.0%) and U content (60 ppm).
Cathcart (1978) reported 1) an overall range of 30 to
300 ppm U for Central Florida Bone Valley phosphates; 2) an
average of 110 ppm U for the high P2O5 sand size pellets;
3) an average of 150 ppm U for the low P2®5 Pebbles; and 4)
an overall range of 40 to 110 ppm U (average 60 ppm) for
the North Florida-South Georgia District.
Altschuler (1980) cited a concentration of 140 ppm U
for chemically analyzed pebbly and pelletai phosphorites in
the Bone Valley Formation. He specified that the reported
value was based on the average of "eight composites: four
15
pebble and four pellet concentrates composited from one
week's production at each of four mining localities in
[the] Land Pebble Field, representative of approximately
100,000 tons, P205:20-35%" (Altschuler, 1980).
URANIUM IN THE PHOSPHATE MINERAL
The minerals which make up sedimentary phosphorites
are related to the theoretical f1uorapatite: Ca^^(PO^ ) .
Due to substitutions, especially of PO^ by CO^, the
chemistry is actually more complex:
francolite - Ca^^íPO^, C02)g(F,0H2)
carbonate fl uorapati te - C aq ( P 0 ^ , C 0 ^ ) g F ^ ^ » o
Ca^Q(PO^)g_^(CO^F)^(F,OH ) 2 where x varies from
0-1.5 (Fountain and Hayes, 1 979 ; Altschuler et
al . , 1 958 ; SIansky , 1 977 ) .
Other notable substitutions include: SO4, SÍO4, or H^O^
for PO^; H^O, OH or Cl for F; and Sr , U, Th, or rare earths
for Ca.
Altschuler et al. (1958) made a detailed examination
of evidence supporting two modes of occurrence of uranium
within the apatite mineral. The first mode involves the
+ 4 +2
substitution of U for Ca within the apatite structure.
They suggested four major lines of support for this lattice
replacement:
16
1. U and Ca contents are parallel through sections of
leached and altered phosphorite.
2. Ionic radii of Ca^^ (0.99A) and (0.97A)are
comparabl e.
3. Uranium in apatite has a cosmopolitan, rather than
a provincial nature. This was demonstrated by the
petrographic fluorescence analysis of phosphate,
as well as by chemical and nuclear emulsion
analyses.
4. Uranium minerals are generally scarce in phosphat-
ic sediments.
The second mode involves the adsorption onto the crystal
surface of U as (UO^) , which is too large to replace
+ 2
Ca within the lattice.
According to Altschuler et al. (1958) both U^^ and U^^
+ 4
occur as primary constituents of the apatite. U is the
dominant form, although it oxidizes easily to the U^^ form
in weathered phosphate deposits, thereby increasing the
relative abundance of hexavalent uranium in some
phosphorites.
Through "regenerative capture" the marine apatite
+ 4
removes "the small amounts of U produced in sea water by
the reduction of (UÛ2)^^ [and] causes more U^^ to be
produced for its own uptake... it thus interferes with the
attainment of equilibrium while fixing an unusual quantity
17
of (Altschuler et al., 1 958, p. 45). According to
Gony (1971), this uptake of uranium by phosphate is limited
_ 3
mainly by the significance of substitution of (PO^) and
Ca"^^ by (003)"^ and Na'^.
Thus, the ultimate quantity of primary uranium in a
given depositional environment, as pointed out by Slansky
(1977), is a function of:
1. the minute amount of uranium initially present in
the sea water [1 to 2 ppb, according to
Altschuler, et al. (1958 ); approximately 3 ppb.,
according to Ku, et al. (1977)];
2. the amount of organic matter accumulation;
3. the abundance of particulate surface area avail-
able; and
4. the rate and duration of actual phosphate sedi-
mentation [which is in turn a function of
structural limitations, physicochemical conditions
and regional tectonic influences (Riggs, 1979b,
1980, 1981)].
The effect of these limiting factors upon the concentration
of primary uranium is such that levels in unmodified marine
apatite seldom exceed 300 ppm (Cathcart, 1978).
According to Altschuler, et al. (1958), secondary
concentration or removal of uranium in phosphate sediments
can also be quite significant. They described a number of
18
the uraniferous phosphorites in the southeast Atlantic
Coastal Plain (Fig. 1; Table 1) which have been affected by
secondary processes. Postdepositional enrichment of
uranium in residual phosphates characterizes the Central
Tennessee District phosphates. Uranium enrichment by
ground water has influenced phosphate deposits in South
Carolina. Enrichment in the Bone Valley aluminum phosphate
mineral grains has resulted from extreme lateritic
weathering processes. More moderate weathering has
produced surficial enrichment in South Carolina's Cooper
Marl .
GEOLOGY OF THE PUNGO RIVER FORMATION
The Miocene Pungo River Formation is a sedimentary
phosphorite which underlies the northeastern half of the
Atlantic Coastal Plain in North Carolina (Figure 2), and
forms part of a southeast dipping sedimentary sequence.
According to Miller (1971) the unit increases in thickness
from its feather-edged western limit to over 35 meters in
eastern Beaufort County, beneath 12 to 70 meters of over-
burden. The extremely fossiliferous, unconsolidated,
interbedded sands and clays of the Pliocene Yorktown
Formation unconformab 1 y overlie the phosphorite. The
phosphorite is uncon form ably underlain by sandy and
fossi1iferous moldic carbonates of pre-Miocene age.
19
Brown (1958) first noted the occurrence of a
phosphorite unit in Beaufort County in water well cuttings.
He recognized the unit as Miocene on the basis of a fora-
mini feral correlation with the Middle Miocene Calvert
Formation in Maryland. Kimrey (1964) named this unit the
Pungo River Formation, and later described the unit and its
distribution (Kimrey, 1965). Gibson (1967) subdivided the
section at the Lee Creek Mine into zones, based upon lith-
ology and microfossils. Rooney and Kerr (1967) identified
the phosphate at the Texasgulf mine as a carbonate fluora-
patite ( Ca-j Q ( PO^ , CO^ )gF2 Regional stratigraphic evalu-
ations.of the unit have been made by Riggs (1967), Miller
(1971) and Brown et al. (1972). A suite of papers is
presently in press which synthesizes the regional structur-
a1 , stratigraphic, petrologic, seismic and paléontologie
aspects of the Pungo River Formation within the Aurora
Embayment and off the North Carolina coast in Onslow Bay
(Riggs et al., in press; Scarborough et ai., in press;
Katrosh and Snyder, in press; Lewis et ai., in press;
Snyder, S.W. et al., in press; Snyder, S.W.P. et al., in
press) .
a . Structural-Strati graphic Framework
Variations in the volume and 82^5 content of a given
district's resources are linked to the relative position of
20
the individual district within the regional structural and
stratigraphic framework (Riggs, 1979b, 1980, 1981). In
North Carolina (Figure 2), maximum development of the
phosphates occurred during the Miocene in the southwestern
portion of the Aurora Embayment (Riggs, et ai., in press;
Scarborough et al., in press), which is separated from the
equivalent stratigraphic sequence in the Chesapeake Bay
region to the north by the Norfolk Arch (Gibson, 1967).
The Aurora Embayment is bounded on the south by the Cape
Lookout High (Snyder, S.W.P., et al., in press), a
pre-Miocene positive topographic structure upon which the
Pungo River sediments thin to approximately 15 meters. The
depositional basin is outlined on the west by a north-south
trending structural hingeline (Brown, et al., 1972), which
defines the updip erosional truncation of the phosphorite.
The Chowan Arch, an east-west trending structure, marks the
northern limit of the basin.
Scarborough (1981) has identified seven major
lithologic units within the Pungo River Formation: units A,
B, C, D, BB, CC, and DD (Figure 3). He recognized three
regional facies among these units in the southwestern
Aurora Embayment. The central facies is characterized by
the following well defined phosphorite units: A, a dolomi-
tic, muddy phosphorite quartz sand; B, a muddy phosphorite
quartz sand with a dolomitic cap rock; and C, a quartz
CENTRAL FACIES: COMPOSITE SECTION S O U T H E R N FACIES: EASTERN FACIES:
AURORA EMBAYMENT
UNIT THICKNESS LITHOLOGY JNIT THICKNESS LITHOLOGY UNIT THICKNESS LITHOLOGY
WMltt. lUgMljr photphttU «Al «liirti lAAdjr.
ciUirtoui, AlAcUtttc intU h<th IbarnAClti,
E lr)oio«Ai) «ItA <201 catcltc auA Va llowl sh'graoA, oH|ht1y phosphatlc aad
0/ r*» 6 ouartaABSENT D laAdy, dolosllty btoclasttc shall hathr*I (brveaoant, baroaclti, oaaoIIé tubtt) to
DD O I shall/ dolfioMa ouds
M 1) OOui »A> grt t A , iMghtly phospAittC «Ad
guarti (and|, ooUsIU/ oloclatClc shall hath
Ibrvcioafli. oarnaclti. aanalld tubas) to
ihailjr doloaltc aiuos
Craai colorad, aonlnduratad ta lAduratad,
f« 11s Itferews aad ?oldic, AMOftphattc and
Obirti ctlcarceut «ud or ll?tltOAt
Iclarocds «hlCA dccrasia doonoard
Inttrbadded, vary dark |raan(th |ri/, alight)/
B thtlly, qvarii phcsphorlta saod «alch bacoaas
c (7Y acra aaislva doanaird
t
O
lO Vary dark grtanlih gray, aatslva, burrowad to
I aottlad. BOdarataly auody quarts phoiphertta
O land «IIA aiAor shell aatarla)
Whitt to light gray to light olivo graan. Light gray Ish>grttA, allghtly calcaraeus.
calcareous
cc silty auds to vtry ihcllv,
slightly photphatic and quarts laAdy,
ctlcaraeut auddy, soattlaet gravally, slightly dlatoaacioui aud; diataa fragatatt coapesa op
O photphatic (OOl) quarts sands to fût of thi sadlaant
C9
light oM«t graan, Induratld to iCBt-tndur>
z atad, highly burroatd and locally slltciflad,
e slightly foss111farous and aolotc, photphatic
r** and quarts sindy doloalta auo
a. I
o
E Modarata oliva grian, burrOMad to aottlad,
evi doloalta Buddy, phosphorlit quarts stnd
6 Carl ollia graan, aatslva and aottlad, auddy
r» phosphorite quarts sand which It locally Dark graan, gravally (phoipharlit granulas),
t gravai)/ (photphorlta granulas) bast auddy, phosphorite quarts landnear
o
Light alive green. Indurated to nanlnduratad,
highly burrouad and locally alltclfltd,
iMgntly fasti 11 f aroui and aaldtc, pnoiphatlc
and qwarts land/ doloalta aud
A NOT RECOVERED
A
Mcdtraia olive graan, Ourrowtd to Bottled,
doloBitic, BjOdy pnoiphorita quarts tana ahlch
Is locilly gravelly Ipnetphorlta and quarts
gravels) near tata.
Figure 3. Summary description and correlation of major lithologies (after Scarborough, 1981)
22
phosphorite sand with a calcareous cap rock. These three
units are overlain by unit DD, a bioclastic hash in a
calcite matrix. Unit D, a bioclastic hash in a dolos i It
matrix, occurs in place of unit DD in the Aurora Area. The
eastern facies is distinguished by the presence of unit BB,
a slightly phosphatic and quartz-bearing diatomaceous mud.
This 11 meter thick unit is the downdip equivalent of units
B and C. Also present in the eastern facies is the
dolomitic unit D. The southern facies is associated with
the "shoaling environment" of the Cape Lookout High. Units
D and DD are absent in the southern facies; units A, B, and
C grade into unit CC, a slightly phosphatic, calcareous,
shelly quartz sand.
b. Petrology
Sedimentary phosphate grains have been classed by
Riggs (1979a) into four basic groups on the basis of their
petrology. These are the intraclastic, pelletai, oolitic,
and skeletal grain types (see Figure 4). Figure 5 shows
grains typical of those used in this thesis.
Scarborough (1981) has examined phosphate grain types
as they occur in the Pungo River Formation. He stated that
phosphate intraclasts (Fig. 5a) are the dominant grain type
within the formation. Granule size intraclasts are usually
dark brown to black; those of sand size are usually light
23
FIGURE 4 Forms of occurrence of macroscopic phosphorites (adapted
from Riggs, 1979a)
24
a. Intraclasts 0.5 mm b. Skeletal
, 0.5 mm
fragments
0.5 mm
Figure 5. Typical phosphate grains
25
to dark brown. Abundant phosphate pellets (Fig. 5c)
distinguish the very fine to fine sand size fraction of
unit A. Pellets within the formation are moderate to dark
brown in color. Phosphatic skeletal fragments (Fig. 5b)
are plentiful within the formation and phosphate discs
(Fig. 5d) are also present. The overall proportion of
phosphate grains to terrigenous or carbonate sediments
according to Scarborough (1981), is the highest in the
mid-slope region within the central facies. He suggested
that the relative volume of phosphate sediments decreases
updip to the west of the mid-slope. A decrease in the
volume of phosphatic sediments also reflects the downdip
transition from the central facies to the southern and
eastern facies.
Scarborough (1981) defines four significant
sedimento!ogical trends with respect to the Pungo River
Formation:
1 . The phosphate content of the total sediment
increases upsection from unit A through unit C.
2. Mean grain size decreases from unit A through unit
C (a "fining upward" trend).
3. The phosphate content of the total sediment
increases from the southern embayment margin to
the Aurora Area (central facies).
26
4. Periods of increased carbonate sedimentation and
decreased phosphate deposition separate successive
phosphorite units in the Aurora Area (central
facies) .
Interpretation of the Pungo River lithologies by
Scarborough (1981) suggests the following depositional
scenario. Marine transgression first led to the accumu-
lation of units A, B, and C. Phosphogenic conditions
prevailed throughout most of this transgression; intermit-
tent carbonate sedimentation, non-deposition, and perhaps
erosion also took place. Deposition of the unit C phos-
phorite, the unit BB diatomite, and the unit CC quartz
sands marked the maximum transgression. Subsequent regres-
Sion produced units D and DD. Finally, erosion caused
truncation of Pungo River sediments across the western and
southern margins of the Aurora Embayment. As a result of
this truncation, the Pungo River sediments appear to repre-
sent a regressive (offlap) episode; however, he concluded
on the basis of the lithologic interpretation that the
phosphate units of the formation actually indicate a pre-
dominantly transgressive sequence.
c. Uranium as a Phosphorite Component
The estimated composite ^2^5 content for the entire
vertical section (units A through D/DD) of the Pungo River
27
Formation is from 2 to 12+%, according to Fountain and
Hayes ( 1979 ). They reported 8.8 to 18.5+% P2*^5 ^
"middle member" [interpreted to be the unit B phosphorite]
currently being mined in the Aurora area. Average
concentrations have been variously reported as 10.3% P2O5
for the entire Pungo River section (Gibson, 1967); and
13.9% ^2^5 (Fountain and Hayes , 1979) or 15.3% P ro 0 cn
(Redeker, 1966) for the [unit B ] phosphorite ore 0 f
Texasgulf , Inc. Sand si ze grains (concentrate) make up
90-95% of the phosphate in the beneficiated product; only a
small fraction of the total available P„0cb is discarded inc
mud and cl ay wastes.
The uranium concentration of the [unit B] phosphorite
ore has been estimated at 20-40 ppm U (Fountain and Hayes,
1979). Cathcart (1978) has indicated an average content of
60 ppm U, with a range of 40-110 ppm U for the North
Carolina phosphate concentrate. Average contents of 70 ppm
U for the beneficiated product have recently been reported
for two analyses by the Tennessee Valley Authority (TVA,
1979). According to Fountain and Hayes (1979) the
Texasgulf phosphoric acid product has a concentration of 80
ug/ml U^Og (or approximately 60 ppm U for the beneficiated
product); the clay waste has a concentration of 17 ppm UgOg
(based on one sample). Altschuler (1980) cited a 65 ppm U
content for "pelletai phosphorites" from the Pungo River
28
Formation. This uranium value was based on the chemical
analysis of two samples which were "concentrates from pro-
specting composites of the entire mined zone in two areas"
(Altschuler, 1980). Estimates of the uraniferous resources
of the Aurora phosphate district are summarized in Table 5.
d. U/P^Og Relationships
Although there have been a number of reports citing
contents and even U contents for the Pungo River
Formation, there have been virtually no attempts to relate
the two chemical components to one another in terms of
petrologic, stratigraphic, and regional structural
controls. Rooney and Kerr (1967) did distinguish between
phosphate grains in the Aurora Area on the basis of color.
Their "dark" grains were dark green to black in color,
hard, and polished. Their "light" grains were brown to
white in color, soft, and somewhat dull (pitted). The
results of chemical analyses performed on samples of their
"light" and "dark" grains indicated concentrations of: 1)
28.65% ^2^5 0.0002% U^Og (1.7 ppm U) for "dark" grains;
and 2) 30.97% P^O^ ana 0.0002% U^Og (1.7 ppm U) for "light"
grains. These values seem to indicate variation in
phosphorus content. The uranium concentrations suggest
uniformity, but at a level much lower than reported by
Cathcart (1978), Fountain and Hayes (1979) and the TVA
29
Phosphate
6
Total Phosphate Resource 71 ,761 X 10 tons
Typical Pá-Ojb- Content of Phosphorite 8-18%1
Total Potential Phosphate Product 20,876 X 10® tons
2
Excluded Phosphate Resource 19,902 X 10® tons
Potentially Mineable Phosphate Resource 51 ,859 X 10® tons
Potentially Mineable Phosphate Product^ 15,086 X 10® tons
3
Estimated Recoverable Phosphate Product 9,429 X 10® tons
Typical P20g Content of Phosphate Product 29-32%
Uranium
Average Uranium Content of Phosphorite 30 ppm
Uranium in Total Phosphate Resource 2,153,000 tons
Uranium in Potentially Mineable Phosphate
Resource 1 ,556,000 tons
Average Uranium Content of Phosphate
Product 60 ppm
4
Estimated Recoverable Uranium 566,000 tons
^Includes 100 percent of in-place phosphate pebble (+14
mesh) and phosphate sand (-14 to 200 mesh) of the phosphorite.
2
Excluded phosphate resource includes that underlying
municipalities, principal roads, areas of high population
density, large lakes, and other environmentally sensitive areas.
3
A distnct-wide average recovery factor of 62.5 percent
has been applied to the "Potentially Mineable Phosphate Product"
to determine this estimated recoverable phosphate product.
4
The total uranium contained in the estimated recoverable
phosphate product.
Table 5. Uraniferous phosphate resources of the Aurora
Phosphate District (Fountain and Hayes, 1979)
3Q
(1979). It may be significant that the distinction between
light and dark grains which formed the basis for the Rooney
and Kerr analysis has not been substantiated by the detail-
eo petrologic work of Scarborough (1981).
Tobiassen (1981) recently completed a trace element
analysis of "whole rock" and grain type (skeletal fragment,
intraclast, and pellet) subsamples obtained from sediment
samples of units A, B, and C in a single core from the
Aurora Area. He distinguished between light and dark
grains, after the manner of Rooney and Kerr (1967), in the
selection of his samples. Tobiassen measured phosphorus
by spectrophotometry and uranium by alpha spectrometry.
His analysis of three sediment samples indicated 1) 6.78%
and 18.2 ppm U for unit A; 2) 13.04% P2O5 23.4 ppm
U for unit B; and 3) 19.21% P2^5 45.0 ppm U for unit C.
He concluded that both phosphorus and uranium content
increase upsection from unit A to unit C. Tobiassen's
analysis of eleven subsamples (separated by grain type and
color) indicated that 1) phosphorus content ranged from
28.15 to 31.49% P205> 2) uranium content ranged from 44.3
to 63.3 ppm U; 3) the skeletal fragments contained slightly
higher concentrations of U and P205'> intraclast and
pellet subsamples were not consistently different from each
other; and 5) the light and dark subsamples were not
consistently different from each other.
PROCEDURES
SAMPLING
The samples analyzed in this study were obtained from
five cores from the central facies (Scarborough, 1981) of
the Pungo River Formation (Fig. 2). Four of these cores
(BTN-9, BTN-11, PON-3, and PON-2) were drilled by the
International Minerals and Chemical Corporation (IMC) in
1966. The fifth core (TGC) was drilled by Texasgulf, Inc.
in the active mine area in 1979. These five cores are in
storage at the Department of Geology, East Carolina
University. The 1 i thostratigraphi c framework set up by
Scarborough (1981) provided the basis for sample selection
in this study. He included core holes BTN-11, BTN-9 and
PON-3 in his petrologic and lithostratigraphic study of the
Pungo River Formation. He also evaluated the petrology and
lithostratigraphy of the remaining two cores used in this
study (holes PON-2 and TGC), although they were not in-
eluded in his report (Scarborough, pers. comm., 1980).
Samples were selected from at least three distinct
stratigraphic positions (i.e. from units A, B, C, D or DD
or the Pungo River Formation) within each of the five drill
holes. Comparison samples of the Yorktown overburden
sediments were also used in three cores.
32
The 0|6 (-2.0 mm, +1.0 mm), 2(4 (-0.52 mm, +0.25 mm),
and 4|ó (-0.13 mm, +0.063 mm) grain size fractions of each
core sample were provided by Scarborough. Each of these
fractions was examined through a binocular microscope. A
subsample of the skeletal, intraclast, pellet, or disc
grains was hand picked if the type comprised at least 5% by
volume of the size fraction being examined. Where pos-
sible, 1 to 2 mg subsamples of each grain type were hand
picked from the 0)6 ana 2)6 size fractions. Samples of this
quantity were estimated by trial and error to contain
adequate concentrations for the uranium analytical
procedures. Composite subsamples of phosphate grains were
picked from the 4)6 grain size fraction. Each size/type
subsample was then photographed and stored in a labelled
grain-mount microscope slide prior to chemical analysis.
Subsamples were labelled using a name having three
parts (separated by slashes). The first part consisted of
the letters and numbers from the original drillers log,
locating the particular sediment sample by core hole and
depth. The second part of the name represented the grain
size of the subsample; either 0|4, 2jó or 4)6. The third part
of the name identified the grain type of the subsamples of
the 0)6 and 2)6 grain sizes. The four grain categories that
were used were abbreviated by the letter "C" (intraclast).
33
"S" (skeletal fragment), "P" (pellet), or "D" (disc). The
subsample of intraclasts in the '¿t> size range from sample 7
of hole BTN-11 would be identified, using this scheme, as
BTN ll-7/2j6/C.
A total of 154 subsamples were examined for this
study. They were obtained from 19 sediment samples repre-
senting units A, B, C, and D/DD of the Pungo River
Formation and from 3 sediment samples representing the
Yorktown Formation.
FLUOROMETRIC DETERMINATION OF URANIUM
a. Summary of Method
Low level uranium concentrations in geological samples
have been determined fluorometrically for over thirty
years. The methoo is based on the measurement of the
fluorescence of a fused tablet (of mixed fluoride flux and
uranium compound) exposed to ultraviolet light. The
uranium concentration is proportional to the fluorescence
intensity; sample fluorescence is compared to that of known
standards. Two primary variations of this method are
currently in use: 1) the "direct" method; and 2) the
"extraction" method. They are so named because of differ-
enees in sample preparation.
Price et al. (1953), in a comprehensive review of
fluorometric technique, proposed the direct method as the
34
solution to interference problems in uranium analysis.
Certain ions were shown to quench or enhance the flúores-
cence of a fused uraniurn-fluoride tablet. These authors
pointed out that interference effects could be reduced or
eliminated by diluting the sample and withdrawing extremely
small aliquots for analysis. They found that the effect of
sample impurities was a function of their concentration in
the fused flux tablet; the proportion of impurities to
uranium in the tablet was not a factor.
The range of concentrations tested by Price et al.
(1953) was from 0.0001 to 10 ug U per flux tablet (0.3 g
NaF). They reported a 21+% coefficient of variation in
their analyses at the 0.0001 ug U (per flux tablet) level;
this value incorporated variation in both the blanks and
samples. For the 0.001 to 10 ug U (per flux tablet) range,
they indicated a 5+% coefficient of variation.
The advantage of the direct method is that it elimin-
ates time consuming chemical preparation of the samples.
The two main problems associated with this (and other)
fluorometric analyses are: 1) fluorescence ("noise") in
the blanks; and 2) variation in the optical properties of
the fused flux tablet. Sample spiking and tight control of
the procedure can lessen the impact of these sources of
error (Price et al., 1953).
35
The extraction method was developed by Grimaldi and
Levine (1950). This technique involved the mixing of the
sample digestate with aluminum nitrate. The "salted"
uranium was then extracted into ethyl acetate. An aliquot
of the extracted uranium was fused with a flux tablet for
analysis.
Grimaldi et al. (1954) reviewed several techniques of
uranium analysis, including direct and extraction fluorom-
etry. They stated that with routine analysis using these
methods, errors usually amounted to 8-15% of the uranium
content being measured. "When errors occur, the results
are generally low (Grimaldi et al., 1954)." In other
words, the uranium fluorescence is generally quenched
rather than enhanced.
Grimaldi and Guttag (1954) described a direct fluoro-
metric method for measuring the uranium content of
phosphate rock. They digested 150 mg of sample in 50 ml
(18+82) HNO^, and fused a 0.6 ml aliquot (=1.8 mg of
sample) with 3 g of flux. A 2 minute digestion was
reported to give excellent dissolution.
In an effort to reduce interference and quenching
errors, Centanni et al., (1956) employed a combination of
sample dilution and extraction techniques. Previously,
Price et al. (1953), had pipetted each aliquot of solution
onto the fusion dish, evaporated the solution, and then
36
added the NaF flux tablet. However, Centanni et al. (1956)
transferred each solution aliquot directly onto the flux
tablet in the fusion dish prior to evaporation. In this
revised method, the 0.4 g flux tablets consisted of a 98%
NaF-2% LiF mixture. Fusion was accomplished by using a
propane-air burner assembly monitored by a thermocouple to
heat the flux tablets to 900°C for 2 minutes, then at 850°
for 1 minute. The fluorescence of the flux tablets was
measured after a 30 minute cooling at room temperature .
This cooling step allowed for an initial increase in the
fluorescence intensity, after which the fluorescence was
stable for about one hour. Centanni et al. (1956) analyzed
solutions containing approximately 0.01 mg (10 ug) U^Og per
ml of 5% HNOg. They reported a 0.7% coefficient of
variation in their results.
A brief fluorometric analytical note by Jaroszeski and
Gregg (1965) stressed the need for a "uniform" flux mixture
having a low blank. They mixed the 98% NaF-2% LiF flux for
8 hours at 32 rpm using a modified Patterson-Kelley
blender. Jaroszeski (pers. comm., 1980) pointed out that
the NaF-LiF fl ux wa s generally accepted as superior to
other mixtures. Their procedure involved pipetting the
aliquot into the fusion dish, evaporating the liquid. and
then adding the flux tablet. Samples were fused over a
propane burner at 950°C for 1 minute after the flux was
37
completely melted. The fused tablets were cooled in a
dessicator for 30 minutes prior to being analyzed.
The ASTM (American Society for Testing and Materials, 1978)
and the Bendix Field Engineering Corporation (Korte, pers.
comm., 1980) use fluorometric routines derived from those
mentioned above, particularly from that of Centanni et al.
( 1 956 ) .
The method used for uranium analysis in this thesis
was based on the direct method of Price et al. ( 1 953 ) as
modified by ASTM (1978). Extensive testing of other
techniques and variations proved the following adaptation
to be the most effective.
b. Apparatus
Air/acetyl ene burner (Figure 6b). The torch assembly
in an atomic absorption spectrometer was chosen
as the heat source because of its controlled
flame, protection from drafts, and proximity to a
ventilation hood.
FIuorometer (Figure 6c). Turner Model 111 equipped
with a Uranium Pellet Holder Door, 7-60 optical
filter (primary, 360 nm), and 2A-12 optical
filter (secondary, above 510 nm), measured
uranium fluorescence in the flux tablets.
38
Figure 6. Apparatus for uranium analysis
39
Glasses (didymium). These facilitated observation of
molten flux.
Platinum fusion dishes (Figure 6a). Ten dishes were
made to order by Engelhard Industries Division,
Iselin, NJ to the specifications of the Turner
110-804 Uranium Fusion Dish. The interior
diameter of each dish was 14 mm at the top and 9
mm at the bottom. Each dish was 3 mm deep, and
had a 0.5 g flux capacity. Dishes were cleaned
with 20% HNO^ wash acid and rinsed thoroughly
after every use.
Tablet maker (Figure 6a). This simple press was made
to order locally for compatibility with platinum
fusion dishes. The base was machined from a 15
mm section of 63 mm diameter stainless steel rod.
The top was machined from a 50 mm section of 57
mm diameter aluminum rod. The top was compatible
with a 2 mm deep recess machined into the base.
A 9 mm diameter hole through the center of the
aluminum top accepted a 130 mm long stainless
steel plunger.
c. Reagents
Flux Mixture. 98% by weight reagent grade sodium
fluoride (NaF) and 2% by weight reagent grade
40
lithium fluoride (Li F) were tumbled for 48 hrs at
20 rpm on a modified tube rotator to ensure
homogeneity.
Nitric Acid. 18 volumes of double distilled
concentrated nitric acid (HNO^) were mixed with
82 volumes of water.
Water. Deionized distilled water was used throughout.
d. Uranium Standard
Primary. Alfa product number 88115., AAS Standard
Solution had a concentration of 1000 ug U/ml at
20°C when packaged by Alfa Products, Danvers, MA.
Secondary. A 0.5 ml portion of the 1000 U/ml stock
solution was pipetted into a 50 ml volumetric
flask, and diluted to mark with nitric acid.
This made a 10 ug U/ml solution.
e. Working Standards
0.01, 0.05, 0.1, and 0.2 ml volumes of the 10 ug U/ml
solution were pipetted into 10 ml volumetric flasks,
and diluted to mark with nitric acid. These made
0.01, 0.05, 0.1 and 0.2 ug U/ml solutions.
respectively.
41
f. Control Samples
A control sample of either National Bureau of
Standards (NBS) Standard Reference Material 120b or
Association of Florida Phosphate Chemists (AFPC) Standard
Check Sample No. 20 was run in each set of 10 samples. One
duplicate sample was also run in each of 3 consecutive sets
of 10 samples. The order of analysis was thus (B = blank;
S = sample; C = control; 0 = duplicate; and standards are
indicated by concentration of U in ug/ml ) :
Position: 123456789 10
Set 1 B .01 C .05 S S S S S D
Set 2 B .10 C .20 S S S S S D
Set 3 B .05 C .20 S S S S S D
g. Protocol
1. Control samples and grain size/grain type sub-
samples were weighed, with an estimated error of +
0.001 mg, on a Cahn Model 26 Electrobalance.
Subsamples of Ob grain size were ground to a fine
powder with a mortar and pestle prior to weighing;
those of 2b and 4b size were weighed as grains.
Weighed subsamples and control samples were placed
into 16 X 150 mm glass culture tubes.
2. Two ml of nitric acid were added and the solutions
were then heated for 2 hours at 140°C on a block
digester. After cooling, the solutions were
42
brought back to volume with nitric acid and trans-
ferred to 14.5 x 45 mm (1 dram) stoppered glass
vials.
3. A 0.4 g flux tablet was added to each of the 10
platinum fusion dishes. NOTE: The fusion dishes
were cleaned, rinsed with water and dried under an
infrared heating lamp before addition of flux
tablets .
4. A 0.04 ml aliquot of nitric acid blank, standard,
control, or sample solution was pipetted onto each
flux tablet following the order of analysis
previously set forth.
5. The solution aliquots were evaporated by placing
the fusion dishes and flux tablets under the
heating lamp for 5 minutes.
6. The flux tablets were fused using the air/acety-
lene burner in the following manner:
- preheated, at a level 20 cm above flame head,
for 15 seconds
- heated, at a level 10 cm above flame head, for
15 seconds
- heated at melting point, predetermined by trial
and error to be at a level 5 cm above flame
head, for 15 seconds
43
- heated above melting point, at a level 3 cm
above flame head, until the flux had completely
melted (average time = 1 min. 15 sec.), then the
sample was heated for an additional 30 seconds
to ensure thorough mixing
- cooled at 5 cm level for 15 seconds
- cooled at 10 cm level for 15 seconds
- cooled at 20 cm level for 30 seconds
- removed from burner and cooled on an asbestos
pad for an additional 30 minutes
7. After cooling, the relative fluorescence of each
tablet was measured with the fluorometer.
SPECTROPHOTOMETRIC DETERMINATION OF PHOSPHORUS
a. Summary of Method
Phosphorus analysis in this study was by the single
solution, phospho-molybdenum blue method as modified by
Strickland and Parsons (1972). An aliquot of sample
di gesta te was diluted and allowed to react with a mixed
reagent. The absorbance of the resulting blue solution was
measured at 885 nm.
44
b. Apparatus
Absorbance measurements were made using the Beckman
Model 35 Spectrophotometer. Readings were recorded using
the Beckman Model 39 printer.
c. Reagents
Ammonium molybdate solution. 15 g of ammonium
paramolybdate ( ) gMo^O^^’4H2O (reagent grade)
was dissolved in 500 ml deionized distilled
water.
Ascorbic acid solution. 27 g of ascorbic acid was
dissolved in 500 ml deionized distilled water.
Potassium antimonyl-tartrate solution. 0.34 g of
potassium antimonyl-tartrate was dissolved in 250
ml deionized distilled water.
Sulfuric acid solution. 140 ml of sulfuric acid
(reagent grade) was added to 900 ml of deionized
distilled water.
Water. Deionized distilled water was used throughout.
d . Mixed Reagent
The reagents were mixed as follows:
5 parts potassium antimonyl-tartrate
10 parts ammonium molybdate
10 parts ascorbic acid
25 parts sulfuric acid
45
e. Phosphorus Standard
Spex Industries ICP Standard had a concentration of
1000 ug/ml phosphorus in 2% HNO^. The standard was obtain-
ed from Spex Industries, Inc., Metuchen, NJ.
f. Working Standards
0.25, 0.50, 0.75, 1.00, 1.25, and 1.50 ml volumes of
the 1000 ug P/ml stock solution were pipetted into 25 ml
volumetric flasks, and diluted to mark with water. These
made 1 0, 20, 30, 40, 50, and 60 ug P/ml solutions,
respectively.
g. Control Samples
Three control samples of either NBS 120b or AFPC No.
20 were run with each set of 30 samples. The order of
analysis for sets of 40 tubes was: 1 water blank; 6
working standards; 3 control samples; and 30 samples. The
instrument baseline was checked with water after every 10
analyses.
h. Protocol
1. A 0.02 ml aliquot of each control sample and
sample digestate (g-2, U r a niurn Protocol ) was
pipetted into a test tube and diluted with 10 ml
of water.
46
2. A ü.l ml aliquot of each phosphorus working stan-
dard was pipetted into a test tube and diluted
with 9.9 ml of water.
3. A 1.0 ml aliquot of mixed reagent was added to
each tube and mixed by inversion immediately.
4. After a minimum 5 minute period of color develop-
ment, the absorbance of each solution was measured
at 885 nm, in a 1 cm cell. The cell was rinsed
once with each new solution prior to filling.
CALCULATIONS
URANIUM ANALYSIS
a . Working Standards
A working (calibration) curve of fluorometer reading
vs. concentration was prepared using the four standards and
the blanks (Fig. 7). The correlation coefficient for 66
analyses of the standards and blanks (Table A-1) was Ü.944.
Uranium concentration (in ppm U) was determined for the
samples and control samples as follows:
2 ml
ppm U (cone from regression, X (digest.)( wt. 57 sampi e , gT
b. Control Samples
Recovery of the control samples (in %) was determined
as follows:
X recovery = PPm U X 100
ppm U ((bbyy ccaelrctuifliactaioten))
where NBS = 128.4 ppm U (by certificate)
and AFPC = 121 ppm U (by certificate)
A total of 18 NBS and 7 AFPC control samples were analyzed
(Table A-2). The mean percent recovery of these 25
43
FRLUEOLREASTCEIVNCEE
CONCENTRATION OF STANDARD, in ugU/ml.
Figure 7. Uranium calibration curve
49
controls was 81.9 + 32.5%. All samples were corrected by a
factor of 1.22 based on this recovery.
The effect of initial sample size on the percent
recovery of uranium was analyzed by linear regression. The
correlation coefficient for mass vs. percent recovery was
-0.063 for the 25 control samples, indicating no
significant mass effects for the 1 to 4 mg range which was
tested.
c. Duplicate Samples
Seven duplicate samples (Table A-3) were run through
the fluxing and fluorometric procedures. A + 18.1 ppm
pooled estimate of sample variance (Crow et al., 1960) was
determined for these seven sets.
PHOSPHORUS ANALYSIS
a. Working Standards
A working (calibration) curve of solution absorbance
vs. concentration was prepared from the six concentration
standards and the blanks (Fig. 8). The correlation
coefficient for 35 standards and blanks (Table A-4) was
1.000. Phosphorus concentration (in % ^2^5^ determined
for the samples and control samples as follows:
50
AIBSnONRBmAN,CE
Figure 8. Phosphorus calibration curve
51
ï P2^O5 = jco^c(weigh/tegof'-essasmioplne., mugg)P/nil) p^^tor.’ where
Factor = I——( d 1 g e s t a t e ) ^ -jq ( ¿-j i y-ti on factor)
0.02 ml(aliquot)
^^ 141.9 g P^Oc V6179" ^ TüÔÛ
= 2.292
b. Control Samples
Recovery of the control samples (in %) was determined
as follows:
%
% recovery Po0(- (by calculation) wr~F^Ü^' where'( by certi f icate) ^
NBS = 34.57% ^2^5 certificate), and
AFPC= 32.93 % Po¿O,b- (by certificate)
A total of 16 NBS and 6 AFPC control samples were
analyzed (Table A-5). The mean percent recovery of these
22 controls was 78.2 + 4.6%, a value consistent with the
recovery of uranium. All samples were corrected by a
factor of 1.28 based on this recovery.
The effect of initial sample size on the percent
recovery was analyzed by linear regression. The córrela-
tion coefficient for mass vs. percent recovery was -0.353
for the 22 control samples, indicating no significant mass
effects for the 1 to 4 mg range.
RESULTS
A total of 154 phosphate subsamples obtained from 22
sediment samples representing the five core locations shown
in Figure 2 were examined in this study. Table 6 summar-
izes the results of the uranium and phosphorus determina-
tions for the 134 subsamples which were analyzed. Uranium
was measured in 110 subsamples; phosphorus was measured in
all 134 subsamples.
Uranium contents ranged from 5.1 ppm U in subsample
BTN 1 1-9/0|ó/s kel . to 285.9 ppm U in subsample BTN 11-7/4(6.
The mean uranium content for 110 samples was 92.5 + 27.4
ppm U. Tables 7 and 8 summarize the mean uranium contents
for all samples when grouped by unit vs. grain size and
core hole vs. grain size, respectively. Tables 11 and 12
summarize the mean uranium contents for all samples when
grouped by unit vs. grain type and core hole vs. grain
type, respectively.
Phosphorus contents ranged from 23.25% ^2^5 ^ sub-
sample PON 3-11/4)6 to 38.22% *^2*^5 subsample PON
2 - 1 0/2 (6 / pe 1 1 e t. The mean phosphorus content for 134
samples was 30.64 + 0.84% P2^5* Tables 9 and 10 summarize
the mean phosphorus contents for all samples when grouped
by unit vs. grain size and core hole vs. grain size,
respectively. Tables 13 and 14 summarize the mean
Oil Size 20 Size 40 Size
Core Strat. C 1 Intraclast) S (skeletal) C (Intraclast) S (skeletal) P (pellet) 0 (disc) (conposlte)
Sample Unit/ mass U '?2°5 mass U '’2“5 mass U '’2'>5 mass U '’2O5 mass U '’2'>5 mass U '’2«5 mass U '’2‘>5
Number Subunit (mg)(ppm) (t) (mg) (ppm) (t) (mg) (ppm) (t) (mg) (ppm) (t) (mg) (ppm) (X) (mg) (ppm) (X) (mg) (ppm) (X)
BTN 9
9.10 Yorktown 2.664 60.6 28.80 3.298 100.6 34.09 4.010 108.0 31.91 3.017 132.9 35.91 2.323 112.5 32 16 1.169 104.8 32.34 0.698 — - 31.06
9.11 DO — -- -- — — — 7.552 54.1 29.97 1.144 56.1 31.15 0.140 — 31 65 0.473 — 28.47 0.657 18.3 26.74
9.14 C 1.251 76.3 30.46 2.321 75.6 31.98 3.843 42.9 30.12 3.609 78.8 32.97 1.799 56.9 31 00 1.423 33.70 2.656 88.3 32.90
9.16 B 2.603 51.4 30.15 2.830 50.9 31.16 1.494 166.2 31.28 2.179 69.3 43.63 0.259 31 38 1.414 50.3 32.17 0.137 --- 32.35
9.18 A 6.012 69.1 27.38 3.211 64.4 36.16 2.330 142.4 29.92 2.048 66.4 31.43 — — - - ... ... ... 0.158 76.1 28.04
BTN 11
11.7 C 3.326 55.1 31.09 3.020 138.6 30.29 5.625 71.3 30.79 3.328 125.0 33.20 0.487 24.7 33 55 — — 0.115 285.9 34.96
11.9 B 3.135 44.9 29.58 2.354 5.1 32.24 6.873 67.3 29.73 5.364 94.9 33.36 - 4.905 51.3 33.80 --- --- ---
11.10 B 5.524 89.8 28.98 4.737 116.2 31.79 6.509 — 28.57 6.679 — 31.06 — — .. _ 3.233 47.3 32.49 0.322 58.9 26.52
11.13 A 2.199 84.5 26.48 3.087 92.8 33.63 1.796 47.3 29.90 3.994 108.3 31.96 — — - - ... ... ... 0.189 ... 29.96
PON 3
2.10 Yorktown 3.624 92.5 27.39 — - — — 2.255 67.0 29.64 1.276 145.6 35.97 0.326 — 33 75 1.111 104.7 32.44 0.824 162.2 24.81
3.11 DO — — — — — — 0.731 78.3 30.22 0.818 108.2 32.02 0.301 16.9 31 10 0.391 28.13 0.067 179.5 23.25
3.15 C 3.006 14.4 24.42 2.061 117.1 32.63 6.439 55.1 32.62 2.794 21.7 32.44 — — . 1.087 32.40 1.458 79.8 32.89
3.16 B 5.001 10.7 29.37 6.734 54.4 23.39 2.895 160.6 30.75 4.026 101.4 33.53 1.722 67.5 31 43 1.038 28.40 0.653 --- 27.53
3.17 A 3.526 65.5 26.52 2.092 45.6 35.49 4.383 129.6 30.81 5.045 100.2 31.48 1.029 ... 30 24 0.518 9.8 29.96 0.460 116.8 30.17
PON 2
2.10 Yorktown 3.032 68.1 28.53 3.876 99.9 31.76 2.422 88.2 31.33 3.564 114.5 30.97 0.030 38 22 0.362 62.0 31.52 0.390 173.9 27.63
2.12 ? 2.466 56.1 30.52 1.523 94.6 32.30 3.418 58.4 32.17 2.236 31.8 33.39 — - — 0.173 - 30.36
2.16 B 3.280 57.3 26.01 2.140 41.3 30.66 2.209 41.6 29.14 1.906 126.6 32.38 0.303 108.5 29 54 1.012 121.8 27.92 0.554 46.8 32.46
2.17 A 3.233 19.8 27.66 1.847 63.0 30.41 2.764 113.7 30.42 2.031 112.0 31.29 0.930 27.9 31 70 0.982 --- 28.44 0.2793 — 29.10
TGC
34.1 C 2.495 99.5 32.71 2.188 138.9 33.55 5.932 79.4 31.64 4.401 142.5 32.92 1.716 32.97 0.091 30.65
34.2 C 3.332 61.0 29.68 2.569 66.9 28.41 3.819 45.0 30.83 3.012 111.3 33.91 --- - 1.624 71.6 30.54 0.452 --- 31.61
40.1 B 3.513 82.6 29.44 2.350 93.8 30.54 3.618 39.3 30.09 1.874 87.1 31.94 — — - — 0.159 35.63
41.3 A 4.802 82.8 26.75 5.121 89.2 31.82 4.470 88.2 27.81 3.908 83.1 33.29 0.317 --- 30 82 0.429 137.4 34.26 0.168 195.7 31.27
Table 6. Suimiary of sample mass, U content, and P 0 content, based on analysis of 134 samples from the Pungo River Formation, North Carolina
54
Grain Si ze
Uni t Oii n 20 n TS n Summary n
Yorktown 84.3 5 104.0 10 168.1 2 118.8 5
D/DD — 0 62.7 5 98.9 2 80.8 2
C 84.3 10 71 .2 13 151 .3 3 102.3 3
B 49.9 12 87.5 16 52.9 2 63.4 3
A 67.7 10 89.3 13 129.5 3 95.5 3
Summary 71 .6 39 82.3 59 123.5 12
Combined Mean n S.D.
92.5 3 27.4
Table 7. Summary of mean ppm U, by unit and grain1 size
Grain Size
Core Hole 00 n Ti n T3 n Summary n
BTN 9 68.6 8 88.7 14 60.9 3 72.7 3
BTN 11 78.4 8 70.2 9 172.4 2 107.0 3
PON 3 57.2 7 83.3 14 134.6 4 91 .7 3
PON 2 62.5 8 83.9 12 110.4 2 85.6 3
TGC 89.3 8 88.5 10 195.7 1 124.5 3
Summary 71.6 39 82.3 59 123.5 12
Combined Mean n S .0.
92.5 } 27.4
Table 8. Summary of mean ppm U, by core hole and grain size
55
Grai n Size
Uni t U0 n U n 40 n Summary n
Yorktown 30.11 5 33.01 12 27.83 3 30.32 3
D/DD — 0 30.34 8 25.00 2 27.67 2
C 30.52 10 32.23 16 32.60 5 31 .78 3
B 30.19 12 31.17 20 30.90 6 30.75 3
A 30.23 10 30.86 16 29.71 5 30.27 3
Summary 30.34 39 31.58 74 29.99 21
Combined Mean n S.D.
30.64 3 +0.84
Table 9. Summary of mean % P2°5* by unit and grain size
Grain Si ze
Core Hole “0? n '¿t n 40 n Summary n
BTN 9 31.27 8 31.79 18 30.22 5 31.09 3
BTN 11 30.51 8 31.67 11 30.48 3 30.89 3
PON 3 29.74 7 31 .44 19 27.73 5 29.64 3
PON 2 29.73 8 31.31 14 29.89 4 30.31 3
TGC 30.36 8 31.75 12 32.29 4 31 .47 3
Summary 30.34 39 31 .58 74 29.99 21
Combined Mean n S.D.
30.64 3 +0.84
Table 10. summary of mean % ^2^5’ core hole and grain size
56
4b Summary
Uni t C S C S P D
Yorktown 73.7 100.3 87.7 131 .0 112.5 90.5 168.1 109.1
D/DD -- -- 66.2 82.2 16.9 -- 98.9 66.1
C 61 .3 107.4 58.7 95.9 40.8 71 .6 151.3 83.9
B 56.1 60.3 95.0 95.7 88.0 67.7 52.9 73.7
A 64.3 71 .0 104.2 93.0 27.9 73.6 129.5 80.5
Summary 62.1 81 .5 83.0 96.1 59.3 76.1 123.5
Table 11. Summary of mean ppm U, by unit and grain type
o ro 4b Summary
Core Hole o S C </) P D
BTN 9 64.4 72.9 102.7 80.7 84.7 77.6 60.9 77.7
BTN 11 68.6 88.2 62.0 109.4 24.7 49.3 172.4 82.1
PON 3 45.8 72.4 98.1 95.4 42.2 57.3 134.6 80.0
PON 2 50.3 74.7 75.5 96.2 68.2 91 .9 110.4 81 .0
TGC 81 .5 97.2 63.0 106.1 -- 104.5 195.7 108.0
Summary 62.1 81.5 83.0 96.1 59.3 76.1 123.5
Table 12. Summary of mean ppm U, by core hole and grain type
57
zt> 4«1 Summary
Unit C S C S P D
Yorktown 28.24 32.93 30.96 34.28 34.71 32.10 27.83 31 .58
D/DD -- -- 30.10 31 .59 31 .38 28.30 25.00 29.27
C 29.67 31 .37 31 .20 33.09 32.28 32.40 32.60 31 .80
B 28.92 31.46 29.93 32.80 30.78 30.96 30.90 30.82
A 26.96 33.50 29.77 31 .89 30.92 30.89 29.71 30.52
Summary 28.60 32.17 30.44 32.78 32.04 31.17 29.99
Table 13. Summary of mean % P2O5. by unit and grain type
o«s 2Í 4b Summary
Core Hole C S C S P 0
BTN 9 29.20 33.35 30.64 33.22 31 .55 31 .67 30.22 31 .41
BTN 11 29.03 31 .99 29,75 32.40 33.55 33.15 30.48 31 .48
PON 3 26.93 33.50 30.81 33.09 31 .63 30.27 27.73 30.57
PON 2 28.18 31.28 30.77 31 .98 33.15 29.29 29.89 30.65
TGC 29.65 31 .08 30,09 33.02 30.82 32.59 32,29 31 .36
Summary 28.60 32.17 30.44 32.78 32.04 31 .17 29.99
Table 14. Summary of % ^2*^5* core hole and grain type
58
phosphorus contents for all samples when grouped by unit
vs. grain type and core hole vs. grain type, respectively.
A variety of statistical procedures were applied to
the data using the SAS computer system (Helwig and Council,
1 979 ) which is centered at the SAS Institute, Inc., Box
10Ü66, Raleigh, NC 27605. The GLM (General Linear Models)
procedure carried out a multiple regression analysis.
Uranium and phosphate concentrations within each grain
size/grain type subgroup were modeled against each other.
The uranium and phosphate concentrations of each size/type
subgroup were modeled against those of the other grain
size/grain type subgroups. The PLOT procedure generated
scatter diagrams for visual inspection of each model. The
CORR and RANK procedures listed in order from highest to
lowest the correlation coefficient for uranium vs. phos-
phate concentration within each size/type subgroup, as well
as those for uranium vs. uranium and phosphate vs. phos-
phate between all subgroups. The MEANS procedure tabulated
simple univariate descriptive statistics for the entire
data set.
DISCUSSION
Figure 9 is a scatter diagram relating P2O5 content to
U content for 110 subsamples of the Pungo River Formation
from the Aurora Embayment in the Central Coastal Plain of
North Carolina. U/P^Og ratios within this sample group
exhibit an apparently random distribution. However, as
Altschuler et al. (1958) have pointed out, "assemblies of
[uranium-phosphorus] data from different parts of the same
formation may represent a variety of different [physico-
chemical conditions] and an average of such varied groups
of data may have the effect of masking, rather than demon-
strati ng, a universal relation." On the basis of this
reasoning, the variance in the U/P20g values plotted in
Figure 9 does not necessarily reflect a simple, homogeneous
distribution of uranium and phosphorus throughout the
formation .
Recent and ongoing studies of the Miocene Aurora
Embayment and Onslow Bay phosphorites actually point to an
increasingly complex geological scenario for the region.
It follows that any valid interpretation of the U/P„0j- data
(Tables 6-14) must be made in terms of the complexities
characterizing the unit from which the data came. In an
effort to "unmask" the potential relationships between
uranium, phosphorus and the Pungo River phosphorite.
60
Figure 9. Scatter diagram of % P2OC vs. ppm U, for all
samples (excluding sample BIN ll-7/4d)
61
interpretations have been made on the basis of the controls
set forth in the Objectives.
1. Regional Location
Samples analyzed in this thesis were taken from five
different core locations (Figure 2). Scatter diagrams
relating P20g content to U content for samples from these
cores are shown in Figure 10. Distinct differences in
U/P2O5 values between the different cores would appear as
tight clusters on such plots. Because of the high degree
of variance in the data for each core, there is in fact no
significant difference between the mean U/P^¿O^ values.D
This suggests that uranium and phosphorus are evenly
distributed laterally within the central facies area of the
Aurora Embayment.
2. Stratigraphic Position
Analyzing samples from different units of the Pungo
River Formation in each core hole provided additional
control. Figure 11 shows scatter diagrams of P2O5 vs. U,
by stratigraphic unit (compare to Tables 6, 7 and 9).
Although there is still a high degree of variance in the
values when grouped in this manner, several observations
can be made. Uranium and phosphorus concentrations in the
Pliocene Yorktown phosphate grains tend to be slightly
62
Figure 10. Scatter diagrams of % P^Or vs. ppm U, by core hole.
A) PON-3; B) BTM-11; C) POM-2; D) BTN-9; E) TGC
63
Figure 11. Scatter diagrams of % P2OC vs. ppm U, by unit.
A) Yorktown; B) unit D/DDT C) unit B; D) unit C;
E) unit A
64
higher than those in the underlying carbonate unit D/DD.
It is possible that the Miocene phosphate was enriched if
it was reworked into the Yorktown. Sedimentary evidence
contradicts this possibility. It is more likely that the
minor relative enrichment reflects different phosphogenic
conditions within the Yorktown. Bachelet et al. (1952)
+ 2
noted that carbonate tends to complex (UO^) and make it
more soluble; the generally low uranium content of unit
D/DD may be a result of such a process. Because unit D/DD
represents a non-phosphogen i c regressive period distinctly
different from the phosphogenic transgression that
culminated with the deposition of unit C (Scarborough,
1981), one would expect to see lower phosphorus
concentrations in unit D/DD. Since the Ll/P20g values for
the phosphorite unit C also tend to be slightly higher than
those of unit D/DD, it appears that the comparison of
U/PoOcb values for units C, D/DD, and the Yorktown agreesc
with the lithostratigraphic distinctions made by
Scarborough. The samples from units A, B and C do not
exhibit any statistically significant differences or
trends, due to the high degree of variance in their U/P^Og
values. The results presented here do not support the
suggestion of Tobiassen (1981) that P2O5 content increases
from unit A through unit C.
65
3. Grain Size
Figure 12 demonstrates the relationship of P2O5 vs. U
by grain size for 110 samples. Examination of this scatter
diagram in conjunction with Tables 6-10 suggests that
grains of lt> size tend to have slightly higher uranium and
phosphorus contents than the grains of 0?i size. Grains of
4)6 size appear to have the highest uranium contents ana the
lowest phosphorus contents. These phosphorus values do not
support the suggestion of Riggs (1979a) that the finest
grain sizes have the highest P2^5 content (due to less
included matter). These minor variations do not appear to
be related to differences in regional and/or stratigraphic
position. The large variance and low "n" values are such
that tests of significance indicate that the grain size
groups are statistically the same.
4. Grain Type
Due to the widely varying U and P2^5 concentrations
within the grain type subsamples (Tables 6; 11-14), no
statistically significant differences have been determined
for grain types when grouped by unit or core hole.
Comparison of the 00 intraclast to the 00 skeletal grain
analytical values indicates that there is a slight tendency
for skeletal grains to have higher P2^5 ^
66
34 -
Ci V,
% ?
Û Û ^
30 •bÛ* Û t .
lO
o ? m
CVJ ?
Q.
? ? £i
26 di
?
LEGEND
?
Û 0^
22 • Zi
? 4íí
I I « ? ? ‘ »
20 60 100 140 180
U IN PPM
Figure 12. Scatter diagram of %
samples (excluding samplB BIN 11-7/40), by
grain size
67
concentrations. Skeletal grains in the 2)ó size range also
appear to have a slightly higher average concentration of U
and P20g* This relative enrichment of skeletal grains has
also been observed by Tobiassen (1981). The Ü0 intraclast
and skeletal grains have a lower average P2O5 and U content
than the corresponding grains in the 2b size range.
Because of the general paucity of pellet and disc grain
data, these two types are not very useful in making direct
comparisons at this time.
CONCLUSIONS
Uranium and phosphorus contents have been determined
by fluorometric and spectrophotometric methods for 134
selected phosphate grain samples from the Pungo River
Formation. Interpretation of the results of these analyses
allows for several observations. It must be stressed that
the apparent differences which have been identified among
the data groups are very slight, and that the differences
exist within an overall context of extreme variance. When
compared statistically the mean values of the particular
subgroups are therefore essentially the same. In summary
then ,
1. Uranium contents ranged from 5.1 to 285.9 ppm U.
Phosphorus contents ranged from 23.25 to 38.22 %
P2O5 .
2. Based upon the evaluation of samples from five
different core holes there appear to be no statis-
tically significant lateral trends in the distri-
but ion of uranium and phosphorus in the central
facies area of the Aurora Embayment.
3. Phosphate grains from units A, B, C, and D/DD do
not exhibit any statistically significant or con-
si stent stratigraphic trends with respect to their
phosphorus and uranium contents. Grains from unit
69
D/DD may be slightly depleted in phosphorus and
uranium relative to underlying and overlying
sediments.
4. Although it is not statistically significant,
there is an apparent inverse relationship between
grain size and mean uranium content. The mean
uranium content was 71.6 ppm U for the 0|ó size
phosphate grains; 82.3 ppm U for 20 size grains;
and 123.5 ppm U for 40 size grains.
5. The mean phosphorus and uranium contents of skel-
etal phosphate grains were slightly higher than
those of the intraclast, pellet, and disc grains.
Further research at the grain level is recommended.
It is possible that trends do exist in the distribution of
uranium and phosphorus within the Pungo River Formation.
The results presented here neither confirm nor deny the
presence of such trends. Trace element studies of phos-
phate grains must be more tightly controlled with respect
to the complexities of the phosphate environment. It is
recommended that more expedient and precise analytical
methods be used in conjunction with more comprehensive
sampling schemes. In addition, studies of other trace
elements in the phosphate grains may be of help in
identifying and understanding not only the Pungo River
Formation but also the phosphogenic system in general.
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71
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APPENDIX A
PRECISION AND ACCURACY DETERMINATIONS
Table A-1 FIuorometer readings for uranium standards and
blanks.
Table A-2 Summary of control sample recovery (U).
Table A-3 Summary of duplicate sample analyses.
Table A-4 Absorbance measurements for phosphorus
standards and blanks.
Table A-5. Summary of control sample recovery
76
Concentration of Standard, i n ug U /ml
Blank .01 .05 .10 .20
5.0 14.5 21 .0 21 .0 56.5
n = 23 9.5 23.5 40.0 97.0
9.5 12.5 31 .0 55.0
23.5 27.0 53.0
26.0 51.0 73.0
23.5 66.0 76.0
31 . 5 56.0 83.5
Fl uorometer 39.5 36.5 86.5
Reading 13.0 39.5 97.5
39.0 90.5
27.0 69.5
25.0 83.0
15.5 71 . 5
25.0 72.0
31 . 5
25.0
40.0
Mean 5.0 11 .2 26.0 40.9 76.0
S.D. 0 2.9 8.4 14.5 14.6
n 23 3 17 9 14
Table A-1 Fluorometer readings for uranium standards and
blanks
77
Theory Found
Standard Mass (mg) U (ppm) U (ppm) % Recovery
NBS 1206 0.812 128.4 148.8 115.9
0.915 132.1 102.9
29.4 22.9
157.0 122.2
2.032 88.9 69.2
2.094 124.3 96.8
2.130 71 .4 55.6
2.338 169.8 132.2
2.358 73.0 56.8
51.3 39.9
2.385 67.4 52.5
67.4 52.5
2.518 72.9 56.7
111.3 86.7
3.372 98.3 76.6
3.580 112.5 87.6
3.674 119.7 93.2
122.8 95.6
AFPC #20 1 .388 121 .0 99.4 82.1
177.3 146.5
1 .669 94.6 78.7
144.0 119.0
26.4 21 .8
3.435 123.8 102.3
3.523 98.1 81 .1
Combined (n=25) 81.9+32.3
Mean Recovery ~
Correction Factor 1.22
Correlation Coefficient, Mass vs. % Recovery = -0.063
Table A-2. Summary of Control Sample Recovery (U)
78
U rani urn Concentration, i n ppm
Sample n Mean S.D.
BTN 9-14/2^5 54.4 87.1 94.8 3 78.8 + 21 .5
BTN 9-18/20 67.0 43.2 89.0 3 66.4 +22.9
BTN 11-9/20 63.9 62.4 75.6 3 67.3 + 7.2
BTN 11-10/00 77.7 102.8 89.0 3 89.8 + 12.6
PON 3-15/20 49.9 59.6 55.8 3 55.1 + 4.9
PON 3-16/20 155.4 178.2 148.2 3 160.6 + 15.7
PON 2-16/00 56.6 29.1 86.3 3 57.3 +28.6
Table A-3. Summary of duplicate sample analyses
79
Concentrât!on of Standard, in ug P/ml
Bl ank TD 20 3T) 50 60
0 76 148 217 284 362 429
1 78 148 216 284 360 431
1 76 147 217 284 361 434
Absorbance, 1 78 149 218 285 361 431
i n nm
3 76 149 217 285 365 435
Mean 1 .2 76.8 148. 2 217.0 284.4 361 .8 432.0
S.D. 1 .1 1 .1 0. 8 0.7 0.5 1 .9 2.4
n 5 5 5 5 5 5 5
Table A-4. Absorbance measurements for phosphorus standards and
blanks
80
Iheory Fôüricl
Standard Mass (mg) % PoOç % PoOc % Recovery
NBS 120b 3.580 34.57 25.83 74.7
3.372 26.94 77.9
2.094 27.28 78.9
0.915 27.68 80.1
2.338 28.28 81 .8
3.674 21.IS 79.1
4.293 26.93 77.9
4.408 27.61 79.9
5.146 28.13 81 .4
8.421 21 .92 63.4
3.934 28.14 81 .4
2.032 28.73 83.1
2.518 29.30 84.8
2.130 27.58 79.8
2.385 26.24 75.9
2.358 27.36 79.1
AFPC #20 1 .388 32.93 22.88 69.5
3.523 26.79 81 .4
1.669 IS.SI 77.6
0.520 24.62 74.8
3.435 26.62 80.8
5.334 25.69 78.0
Combined (n=22) 78.2+4.6
Mean Recovery
Correction Factor 1 .28
Correlation Coefficient, Mass vs. % Recovery = -0.353
Table A-5. Summary of control sample recovery (P)