- Email: [email protected]
Lower Eocene phosphorites of the western desert, Iraq
Sedimentary Geology, 33 (1983) 295-316 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
295
LOWER EOCENE PHOSPHORITES OF THE WESTERN DESERT, IRAQ
KHALDOUN S. AL-BASSAMand DIKRAN HAGOPIAN State Organization for Minerals, Directorate General of Geological Survey and Mineral Investigation, P. 0. Box 986 Alwiya, Baghdad (lraq)
(Received April 10, 1981; revised and accepted July 20, 1982)
ABSTRACT AI-Bassam, K.S. and Hagopian, D., 1983. Lower Eocene phosphorites of the Western Desert, Iraq. Sediment. Geol., 33: 295-316. Marine sedimentary phosphorites of Eocene age (Upper Ypresian) are exposed in the extreme west of Iraq within the Dammam Formation. They are associated with limestone and chert, and their deposition seems to have taken place in a shallow marine environment within a structurally controlled basin open to the sea from the northern and western sides only. The studied phosphorites are granular in texture, coarse-grained and cemented by calcite which is occasionally silicified. Bone fragments are present in small amounts. Carbonate-fluorapatite is the only phosphate mineral detected in these phosphorites, with relatively high amounts of the components SO4 2, COf 2, F-I, HaOl and Na I substituting in the crystal structure. The Lower Eocene phosphorites of Iraq are part of the Tethyan phosphorite province, and are comparable in many aspects with those of Paleocene and Upper Cretaceous age in the Western Desert of the country.
INTRODUCTION Iraq lies in the b o r d e r area between the two m a i n Phanerozoic units of the Middle East, i.e. b e t w e e n the A r a b i a n part of the N u b i o - A r a b i a n Platform a n d the Asian b r a n c h e s of the A l p i n e G e o s y n c l i n e (Buday, 1980). T h e p l a t f o r m part of Iraq is divided into two basic units: the Stable a n d the U n s t a b l e Shelf. The western part of the Iraqi Stable Shelf is characterized by the presence of the R u t b a uplift which had a n o r t h - s o u t h t r e n d d u r i n g the P a l e o c e n e - L o w e r Eocene, a n d has been a n area of n o n - d e p o s i t i o n since that time. The Lower Eocene phosphorites of Iraq are developed in the extreme western part of the c o u n t r y within the Stable Shelf, a n d cover a n area of a b o u t 1000 kin:. Similar to the other phosphorites of Iraq, the Lower Eocene deposits are m a i n l y developed along the western a n d n o r t h w e s t e r n sides of the R u t b a uplift. The phosphorites of Iraq are part of the T e t h y a n phosphorite province of U p p e r C r e t a c e o u s - E o c e n e age 0037-0738/83/0000-0000/$03.00
© 1983 Elsevier Scientific Publishing Company
296
which is known to extend from Morocco in the west to Iran and Turkey in the east. Within the Middle East area, Eocene phosphorites are known in lraq, Syria, Jordan and Saudi Arabia, occurring as thin phosphatic beds interbedded with limestone and chert. The Paleocene and Upper Cretaceous phosphorites of Iraq underlie the Lower Eocene deposit in the investigated area, and are exposed about 150 km to the east and northeast. The Iraqi phosphorites have many similarities with each other, being mostly of pelletal texture, cemented with calcite, and associated with chert to different degrees. Unlike many phosphorite deposits of the world, those of Iraq are not associated with black shale. The Paleocene deposit is the richest and best developed, the Upper Cretaceous phosphorites are thinly bedded with wide areal extension, and contain more clastics whereas those of the Lower Eocene have more chert interbeds. The mineralogy, geochemistry and genesis of the Upper Cretaceous and Paleocene phosphorites of Iraq have been discussed by A1-Bassam (1976) and AI-Bassam et al. (1980). The previous investigations have suggested that the mode of formation of these phosphorites included deposition of phosphate under reducing and alkaline conditions within the lime mud of bottom sediments. It was suggested that the dissociation of organic phosphatic matter in these sediments was the immediate source of phosphorus, and that upwelling currents of unknown direction and origin contributed to the regional enrichment of the southern and eastern coasts of the Tethys with phosphorus. The Upper Cretaceous-Eocene tectonic development of the region and the associated submarine volcanic activity were suggested by A1-Bassam et al. (1980) to have had some influence in the formation of these phosphorites. More than 100 samples have been examined and analyzed in the present study, covering most of the rock types identified during field work and regional mapping of the area (Hagopian, 1980). STRATIGRAPHY
The Eocene sediments in this part of the country belong to the Dammam and Jaddala Formations (Figs. 1 and 2). The Dammam Formation is a shallow marine facies, differentiated in this area into several lithostratigraphic units, and is composed mostly of limestone with occasional phosphorite and chert. The Jaddala Formation is an offshore limestone facies interfingering with the Dammam Formation in the investigated area. The Dammam Formation is differentiated into three lithostratigraphic units in this part of Iraq. These are from older to younger: Nhadain UnitmLower Ypresian. It is developed in the southern part of the area only where its thickness reaches about 38 m. It is composed of white limestone, Coarsely crystalline, cavernous, porous and highly weathered. The identified fauna are Nummulites deserti (Dela Harpe), Assilina placentula (Deshays), Operculina
297
/"
j./"~
d
- IIIIIN
u"
o..
-°,-~-2..~111111L~
-a2" o
u.ax.
I
'
~S A U D I A R A B I A
SCALE
Fig. 1. Geological sketch map of the extreme western part of Iraq.
lypica, Rotalia trochidiformis, Gastropoda, Bryozoa, Pelecypoda and Algae. According to fossil evidence this unit was deposited most probably in a sublittoral environment within a shoal zone. ~ Umm Chaimin Unit--Upper Ypresian. It is developed in the southern and middle parts only, covering a considerable area and reaching about 18 m in thickness. Lithologically it is composed of successive layers of phosphatic limestone, phosphorite, and chert in the west, which pass laterally to phosphatic limestone, calcareous limestone, and chert interbeds in the eastern part of the area. The phosphorite layers are 2-4 m thick each and are occasionally silicified. The following fauna were identified within the phosphorite-bearing horizons: Nummulites globulus (Leymeri), Nummulites atacicus, Globorotalia aspensis, Globorotalia linaperta, Peiecypoda, Ostracoda, Algae and fish scales. Accordingly, it seems that the deposition of these
298 NORTHERN
:
SECTION
lll
. i
,.i[,
SOUTHERN
i I
I , II
ii
e~
Z 0
SECTION
I~DI~w,1 ~ 1 I ~ I I
o o
T
i
,,
,
i
,
,J
.~..~~o i; ,~ii. = ~~.'ol .... ~
~ E E 5 •
.~
I--
0 E
56-
= i ii11_ o e4! i [ i I ,
LEGEND
,/ '~
Limestone
Dolomite
[ ~
1
80
,
/
"
e'~ z
Phosphatic limestone
phosphorite
72 t
,
, --,---
i L-
Chert E
96
~
,
I:
0
W .-I ~
Unconformity
Fig. 2. Columnar stratigraphicsections of the Eocene sequence in the extreme western part of Iraq.
rocks has taken place in a shallow sublittoral environment. The presence of Globorotalia, however, may indicate some contribution from deeper parts of the sea, which is also suggested by the extensive chert beds of this unit. In the eastern parts the following fauna were identified in the limestone of this unit: Nummulites planulatus (Lamarck), Numrnulites globulus (Leymeri), Numrnulites atacicus (Leymeri), Pelecypoda, Algae and fish scales, indicating again the shallow environment of deposition. Anaza Un/t--Lower to Upper Lutetian. This unit is exposed in the southern and northern parts overlying the other Eocene rock units in the area. It is best developed in the south, where it reaches a maximum thickness of about 28 m, compared with the northern area where only the upper part of this unit was found. Lithologically it
299 is not uniform; the first 15 m is nummulitic coarsely crystalline, white, cavernous limestone, whereas the upper 10 m is limestone with chert nodules. In the middle there is phosphatic limestone (Fig. 2). The faunal assemblages are Nurnmulites lavegatus, Nummulites gizehensis (Forskal), Nummulites discorbinus (Schlothein), Nummulites atacicus (Joly), Nummulites aturicus (Leymeri), Lockhartia alveolate (Silvester) and Alveolina elliptica (Van Flosculina). The faunal assemblage suggests a facies of a shallow marine environment. The Jaddala Formation is an offshore facies of late Lower Eocene-Upper Eocene age. The maximum thickness of the formation does not exceed 30 m in the northwestern parts of the country (Ditmar, 1971). This reduction in the formation thickness is obviously due to the close proximity of the Arabian Shield where the Eocene rocks are represented by neritic limestone facies of the Dammam Formation in Iraq and the upper parts of the Hibr Formation in Saudi Arabia (Powers et al., 1966). The Jaddala Formation is composed of chalky radiolarian limestone, occasionally dolomitic and contains chert nodules at the upper parts. It is developed only in the northern part of the investigated area, and the following fauna were identified: Globorotalia bullbrooki, Globorotalia interposita, Globorotalia linaperta, Radiolaria, Ostracoda and sponge spicules. The contact between the Dammam and the Jaddala Formations is conformable without break in deposition. The Eocene rocks in the investigated area are underlain by the Umm Er Radhuma Formation of Paleocene age unconformably. PETROGRAPHY AND MINERALOGY OF THE PHOSPHORITE All the samples examined petrographically came from the Lower Eocene Umm Chaimin Unit. These phosphorites are relatively coarse grained compared with the other phosphorites of Iraq, and contain various amounts of phosphatic material present mostly as rounded to subrounded, grains, detrital in appearance, and occasionally angular to subangular (Figs. 3 and 4). They range in diameter from 1 mm to about 3 cm, but mostly have the size of 3-7 mm. A fine-grained (1-3 mm), well-sorted variety of phosphorite was noticed to occur in the northern extremity of the deposit as thin phosphorite layers associated mostly with limestone hardgrounds (Fig. 5). The southern part is characterized by much coarser grained phosphorite and better development of phosphatic layers. The phosphate grains are white to creamy in color, usually covered with a dark-brown to black thin film of iron oxides and organic(?) stains. They show evidence of rolling of phosphate mud and development of pellets by wave action and bioerosion (Figs. 6 and 7). Coprolites are common (Fig. 8), and some of the phosphate pellets appear to have formed in small cavities within the associated limestone (Fig. 7). Phosphatic bone fragments are present in some samples and may reach several centimeters in length (Fig. 4). Most of the Lower Eocene phosphorites have a calcareous cement, b u t silicification is common in some restricted horizons.
300
Fig. 3. Phosphorite sample showing mixture of coarse and fine phosphate grains cemented with calcite.
Fig. 4. Phosphorite sample composed of fine, well-sorted phosphate grains mixed with very coarse angular and rounded intraclasts of phosphate and limestone, and bone fragment.
301
Fig. 5. Fine-grained, well-sorted phosphorite.
Fig. 6. Weathered surface of a phosphorite sample showing phosphate grains covered with a thin film of dark-colored iron oxide.
302
Fig. 7. Phosphate grains overlying limestone hardground. One of the grains is a warped phosphate sheet. A few phosphate grains were developed in cavities.
Fig. 8. Phosphate grain showing convolutionary texture and is probably a coprolite.
303
Fig. 9. Phosphate pellet showing primitive internal symmetrical distribution of impurities, and encasing an older phosphate grain. Polarized light.
Fig. 10. Phosphate grain (coprolite?), contains calcite inclusions, fragments of bones and fossil shells. Polarized light.
304
Fig. 11. Same as in Fig. 10 under crossed nicols.
Fig. 12. Rounded phosphate grain (black) partly replaced by calcite (gray). Crossed nicols.
305
Fig. 13. Reworked phosphate grain encases a bone fragment and shows evidence of wear and tear. Polarized light.
The microscopic examination showed that the composition of the Lower Eocene phosphorites is relatively simple; it is dominated by phosphate as grains, pellets and bones, and carbonate and silicate as cementing materials. The phosphatic grains are mostly intraclasts, but coprolites and faecal pellets are common, whereas oolites are rare. The phosphatic grains are brown to pale brown in color under plain polarized light, mostly without any internal symmetry, rich in mineral pigments and organic(?) residues, and occasionally contain calcite inclusions, bone fragments and fossil shells (Figs. 9, 10 and 11). The contact between the phosphatic grains and the cementing material is sharp and well defined. Partial replacement of some phosphatic pellets by the cementing calcite occur occasionally (Fig. 12). Evidence of wear and tear due to local reworking are also shown as broken and corroded pellets (Fig. 13). Under crossed nicols, the phosphatic grains are isotropic, similar in this respect to most marine sedimentary phosphorites, and this is probably due to the very minute crystallite size of the phosphate phase. On the other hand, the bone fragments are anisotropic and were identified as apatite. Shelly fossils are few within the phosphorite-bearing rocks, and when present they are composed of calcite. The cementing material is composed of sparry calcite. Replacement of the cementing calcite by chalcedony is a common phenomenon, however, the phosphatic pellets appear less affected by this process (Fig. 14). There is no sign of phosphate cement (microsphorite). Traces of clay minerals were observed in a few phosphatic
306
Fig. 14. Silicified phosphorite. The cementing material is completelyaltered to chalcedony, whereas, the phosphate grains are only affected at outer margins.
grains, but never in the cementing material. Detrital quartz was not found in any of the examined samples. Almost all of the examined phosphorite samples are dominated by clastic phosphate grains which appear to have formed elsewhere in the basin during the same sedimentary cycle and subsequently were transported as clastic particles and redeposited together with the chemically precipitating calcite in favorable traps. X-ray diffraction analysis of several phosphorite samples showed that the main minerals are apatite, calcite and impure silica (chalcedony). For accurate determination of the type of apatite present in these rocks, two representative concentrates of coarse and fine phosphatic grains were carefully analyzed by a diffractometer using internal standard. The results showed that both varieties of phosphatic grains are composed of carbonate-fluorapatite with lattice parameters as follows: Coarse a0-parameter (,~) c0-parameter (~,)
co/ao Volume (~,)3
9.325 6.898 0.7397 519.5
Fine 9.346 6.906 0.7380 522.4
The variation in the lattice parameters of these carbonate-fluorapatites can be
307
mainly due to the various amounts of CO~-z and F - ~ substituting in their structure. Both components result in a significant contraction of the a0-parameter and eventually the cell size (McConnell, 1942, 1952, 1970; LeGeros, 1965; Gulbrandsen, 1970). The lattice parameters of the Lower Eocene apatites generally fall within the range reported for apatites from the other marine sedimentary phosphorite deposits of Iraq. However, a general trend of decreasing a0-parameter value from older (Campanian) to younger (Eocene) exists and seems to be due to the higher amounts of carbonate in the structure of the younger apatites according to the data obtained by A1-Bassam (1976) and Jamil et al. (1979). GEOCHEMISTRY OF T H E PHOSPHORITE A N D ASSOCIATED ROCKS
Among the rock units of the Dammam Formation, Umm Chaimin is remarkable for being phosphorite-bearing. Lithologically this unit is composed of alternating layers of phosphorite, limestone and chert. About 100 samples representing the various types of these rocks were analyzed for their major, minor and trace-element constituents. In addition, phosphate grains representing the majority of the examined phosphorite samples were concentrated by hand picking, crushed, and apatite was purified by heavy liquid separation (bromoform), and triammonium citrate extraction (Silverman et al., 1952). The samples were analyzed by several techniques; Ca by titration, P, A1 and SO3 by colorimetry, Na and K by flame photometry, Si and L.O.I. by gravimetry, Mg, Cu, Cr, Ni, Sr, Mn and Fe by atomic absorption spectrophotometry, Y and V by emission spectrography, F by ion selective electrode. The results of the chemical analysis are presented in Tables I, II and III. The calcareous phosphorite represents the main part of the phosphorite-rich beds, it is characterized by lower magnesium content relative to the other phosphorites of Iraq, indicating the near absence of dolomite and palygorskite, both minerals are common associates of the Paleocene and Upper Cretaceous phosphorites (A1-Bassam, 1976; A1-Bassam et al., 1980). The very low contents of Si and AI in the calcareous phosphorites indicate that aluminosilicates are present in trace amounts. Similarly, iron oxides and hydroxides seem to be lower in the Lower Eocene phosphorites relative to the older phosphorites of Iraq. The analyzed major and trace elements are generally shared between carbonatefluorapatite and the cementing calcite. The geochemical association of elements, as shown by their correlation coefficients, indicate that F, Na, S, U, V, K and Cu form one geochemical group of elements, and they follow phosphorus in the apatite phase. Their concentration in the analyzed phosphorites is controlled mainly by the apatite content. Fluoride is an important constituent of all marine sedimentary apatites, and it plays an important role in its deposition (Mansfield, 1940; McConnell, 1962). This is supported by the similarity of the F/P205 ratio in all the analyzed phosphate rocks and in the purified apatite fraction; the latter contains 4. 1%F and 33.5 % P205.
308 TABLE 1 Chemical composition of the Lower Eocene phosphorite-bearing rocks Calcareous phosphorite
Siliceous phosphorite
Phosphatic limestone
Phosphatic chert
% SiO2 CaO MgO A1203 Fe2Oa SO t P205 K 20 Na20 F L.O.I. Total O= F Total -
0.39 54.37 0.35 0.10 0.19 1.06 20.84 0.03 0.51 2.40 20.54 100.78 1.01 99.77
40.96 31.30 0.23 0.10 0.50 1.14 18.68 0.20 0.43 2.12 5.40 101.08 0.89 100.19
I. 16 53.83 0.40 0.08 0.15 0.35 7.91 0.02 0.12 1.00 34.50 99.52 0.42 99. l0
71.85 13.82 0.17 0.01 t.17 I).47 4.21 0.02 0.15 0.50 6.88 99.25 0.21 99.04
21 8 137 15 832 8 45 2.16 0.12 16
32 10 160 25 1026 25 38 2.03 0.11 7
20 6 24 12 269 8 20 2.53 0.13 29
91 12 23 58 470 69 I0 2.38 0.12 17
pprn
Cr Ni V Cu Sr Mn U U/P205 F/P205 No. of samples
O n the o t h e r h a n d , s o d i u m a n d sulfur (as sulfate) c a n e n t e r the a p a t i t e s t r u c t u r e in c o u p l e d s u b s t i t u t i o n for c a l c i u m a n d p h o s p h o r u s r e s p e c t i v e l y w i t h o u t d i s t u r b i n g the electrical n e u t r a l i t y
o f the m i n e r a l
(A1-Bassam,
1976).
Minor
amounts
of
p o t a s s i u m m a y a c c o m p a n y s o d i u m in the c a l c i u m site; d e h r n i t e a n d l e w i s t o n i t e are species n a m e s g i v e n to a p a t i t e s w h i c h c o n t a i n s u b s t a n t i a l a m o u n t s of these t w o e l e m e n t s . P a r t of s o d i u m , h o w e v e r , m a y b e a c c o m o d a t e d in the calcite s t r u c t u r e ( V i z e r et al., 1977), w h e r e a s s o m e sulfate m a y be p r e s e n t as traces of g y p s u m a n d a n h y d r i t e in these p h o s p h o r i t e s . T r a c e s o f u r a n i u m (as U 4) are k n o w n to s u b s t i t u t e for c a l c i u m in the a p a t i t e s t r u c t u r e ( A l t s c h u l e r et al., 1958). T h e L o w e r E o c e n e p h o s p h o r i t e s
o f I r a q are
g e n e r a l l y d e p l e t e d in u r a n i u m ; an a v e r a g e of 45 p p m was f o u n d in the unsilicified
309
TABLE II
Chemical composition of the Lower Eocene non-phosphatic rocks , Dammam formation
Jaddala formation
U m m Chaimin unit
Nhadain Unit
Anaza Unit
Limestone
Limestone
Limestone
Chert
Limestone
% SiO 2 CaO MgO
1.17 53.33 1.29
69.35 14.68 0.47
1.06 53.42 1.12
0.77 54.89 0.19
1.09 54.85 0.29
A1203 Fe203 SO 3 P205 K20
0.11 0.14 0.15 0.41 0.07
0.09 1.95 0.61 0.79 0.02
0.09 0.11 0.21 0.23 0.06
0.07 0.10 0.29 0.28 0.02
0.15 0.12 0.35 0.31 0.02
Na20 L.O,I.
0.05 42.47 99.19
0.06 11.32 99.43
0.05 43.16 99.51
0.04 42.98 99.63
0.04 42.73 99.95
Total ppm
Cr Ni V Cu Sr Mn No. of samples
13 7 11 5 112 20 28
52 20 12 50 169 101 17
10 3 4 2 146 21 35
8 3 7 2 160 20 47
~23 12 10 6 202 22 55
Graf (1960) 9 12 15 14 475 500
phosphorite. The fine-grained variety is relatively richer in uranium; containing up to 100 ppm in some samples. The U/P205 ratio in most of the analyzed phosphorites is about 2, but the rare presence of secondary uranium vanadates of the carnotiie type may increase this ratio in some samples. Copper is probably attached to some organic residues which are common impurities in the phosphatic pellets. Part of Cu 2, however, may be incorporated in the apatite lattice in substitution for Ca 2 (Klement and Haselbeck, 1965). Strontium is shared between calcite and apatite, but it seems to favor,the latter as a host since Sr concentration in the phosphorite-bearing rocks is several times higher than the associated limestone (Tables I and II). The apatite lattice can accomodate relatively high amounts of strontium without distortion (Larsen et al., 1952). Vanadium may be introduced to the apatite structure as VO4-3 replacing minor amounts of PO4 -3, and can be also attached to the organic residues within the phosphatic pellets. The concentration of vanadium in the phosphorites and the separated apatite fraction of
310 TABLE 111 Chemical composition of the Lower Eocene apatite and calculation of the structural formula i Analysis of the apatite 2
SiO2 CaO MgO A120~ Fe203 SO3 P2Os K20 Na20 CO: H20 ~ F Total -O~F Total Cr
Ni V Cu Sr Mn U Y Zn
Structural components
Atomic Preliminary propor, of formula oxygen (ions/unit cell)
(%) (~) 0.75 CaO 53.45 0.9531 53.45 Na20 0.40 0.0065 0.25 0.06 P205 33.45 1.1786 0.05 CO 2 5.40 0.2455 0.95 SO3 0.95 0.0356 33.45 0.02 F 4.10 0.2158 0.40 H20 2.65 0.1472 5.40 Total 2.7823 2.65 - - O - - F 0.1079 4.10 Total 2.6744 101.54 26/2.6744= 9.7218 1.72 99.82 (+ charges 49.45: charges 49.88)
Ca Na
9.27 0.13
P C S
4.58 1.20 0.12
F H
2.10 2.86
Last refinement of the formula (ions/unit cell)
Ca Na H ~O P 3/4C S [t 4 F O
9.37 0.13 0.50 4.63 0.91 0.12 0.34 2.12 23.88
10
} 26
(ppm) 65 16 200 20 980 20 6O 32 100
Calculated according to the model proposed by McConnell (1970). 2 The analysis includes 0.2% organic carbon.
the U m m C h a i ' m i n U n i t is m u c h h i g h e r t h a n that in the n o n - p h o s p h a t i c l i m e s t o n e a n d chert o f the s a m e unit. In a d d i t i o n to the e l e m e n t s d i s c u s s e d a b o v e , the c h e m i c a l analysis of the sepa r a t e d a p a t i t e f r a c t i o n s h o w s the p r e s e n c e of c o n s i d e r a b l e a m o u n t s of C O 2 a n d H2 O + . T h e f o r m e r (as C O l 2) is k n o w n
to r e p l a c e p h o s p h a t e in m o s t m a r i n e
s e d i m e n t a r y a p a t i t e s ( M c C o n n e l l , 1959, 1965, 1970; M c C l e l l a n a n d Lehr, 1969). T h e s t r u c t u r a l f o r m u l a o f the L o w e r E o c e n e c a r b o n a t e - f l u o r a p a t i t e ( c a l c u l a t e d a c c o r d i n g to the m o d e l p r o p o s e d by M c C o n n e l l , 1970) is b a s i c a l l y similar, b u t w i t h m i n o r v a r i a t i o n , to t h o s e of U p p e r C r e t a c e o u s a n d P a l e o c e n e (A1-Bassam, 1976; J a m i l et
311
al., 1979). The similarity is in the wide range of isomorphic substitutions in their structure dominated by carbonate and sulfate replacement of phosphate, sodium of calcium, and fluoride of oxygen, not forgetting the considerable hydration of these apatites in various structural sites. Several other minor and trace-chemical elements which were detected in the analysis of the Lower Eocene phosphorites are not related to the phosphate phase; Fe, Cr and Ni are generally low and seem to be attached to the traces of aluminosilicates in these rocks, as suggested by the strong positive correlation with alumina and silica. On the other hand, magnesium and manganese seem to be associated with the cementing calcite. The associated non-phosphatic limestone is calcareous in composition and is similar in its trace-element composition to average carbonates (Graf, 1960), but it is significantly depleted in Sr and Mn. It is also similar to the other carbonate rock units of the Dammam Formation in the area (Table II). The silicification process, which influenced the carbonate cement mostly, has partly altered the geochemical character of the original calcareous phosphorite and influenced the distribution and association of some elements. The concentrations of Fe, Mn, Cr, Ni, Cu and SO 3 are higher in the silicified rocks. Iron and manganese concentrations in particular are higher by about ten times. On the other hand, U, Sr, V, Na, and to some extent Cu retained their original positive correlation with phosphorus, whereas, Mg, K and Ni are associated with silica. The sulfate seems to be present as traces of anhydrite within the silicified matrix in view of the positive correlation between calcium and sulfate in these rocks. The phenomena of anhydrite relation to the formation of chert layers and chert nodules have been discussed by several authors (Folk and Pittman, 1971; Siedlecka, 1972; Chowns and Elkins, 1974). Relics of anhydrite were found in most of the geodes and chert nodules of Upper Cretaceous-Eocene phosphorite-bearing carbonate rocks of the Western Desert, Iraq (Petranek et al., 1978). The original content of anhydrite in the Lower Eocene phosphorites can not be estimated sinc~ most of the primary material was altered to chalcedony. Iron and organic(?) impurities apparently cause most of the coloration of these cherts which vary from dark-brown to white. GENESIS
The mechanism of phosphorite formation and the suitable marine conditions for its precipitation are still a matter of argument. The work of the last 40 years or so has by no means solved the problem of phosphorite formation. However, since the publication of Kazakov's theory in 1937, it has been generally accepted that oceanic upwelling of cold phosphate-rich water is an important factor in the formation of marine sedimentary phosphorites. Ames (1959), Bushinsky (1963), Sheldon (1964, 1981), McKelvey (1967), Gulbrandsen (1969), Cook and McElhinny (1979) and Riggs (1979) have significantly contributed, among others, to this subject.
312
The local conditions suitable for the formation of marine phosphorites can be summarized as relatively higher than average phosphate concentration in solution that should be kept from dilution by detritals and by (normal) seawater, a relatively high pH value (higher than 7.0) should be maintained, a relatively high concentration of calcium should be available either in solution or as lime mud, and a high C a / M g (higher than 5.2) is considered important (Ames, 1959; D'Anglejan, 1967; Gulbrandsen, 1969; Martens and Harriss, 1970). Several authors have found that the conditions prevailing in the interstitial environment of soft sediments are more suitable for the deposition of phosphate in early diagenesis than those of the overlying open marine environment (Bushinsky, 1963; Pevear, 1966; Summerhays, 1970), The anaerobic decay of the organic remnants starts directly after burial and this process results in increasing the concentration of dissolved phosphorus in pore solution and elevating the pH value due to the formation of ammonia. Furthermore, the C a / M g ratio in such environments can be suitable for the deposition of phosphate. The extensive deposition of phosphorite in Iraq and other areas in the Middle East and North Africa began during the Upper Cretaceous (Campanian) and continued to the end of the Cretaceous in this region. This was followed by the early Tertiary phosphogenic epoch which continued in successive cycles from the Paleocene to the end of the Eocene. The genesis of the Upper Cretaceous and Paleocene phosphorites of Iraq was discussed by A1-Bassam (1976) and AI-Bassam et al. (1980). It has been suggested that the immediate source of phosphorus appears to have been biogenic, and that phosphate was most probably precipitated in the shallow, warm and phosphorus-rich semi-restricted environment of near-shore lagoons, near the sediment-water interface under high pH and reducing conditions. Later diagenetic processes have played an important role in developing many petrographic and geochemical characteristics of the phosphatic grains, the majority of which appear to have been subjected to several stages of reworking by currents and waves before they were finally deposited mechanically in favorable traps. It has been also suggested that the Upper Cretaceous-Eocene paleographic setting of the Tethys and the associated submarine volcanism at that time have contributed to the formation and development of the Tethyan phosphogenic system (A1-Bassam et al., 1980). The Lower Eocene phosphate deposit of Iraq is part of the Tethyan phosphogenic province, it was deposited in a shallow marine environment within a basin bordered from the east and south by the Hail Arch and its northern extension the Rutba Uplift (Fig. 15). The phosphate basin extended from northeast to southwest inside Iraq, Syria, Jordan and Saudi Arabia. Based upon paleontological evidence, A1Hashimi (I 972) suggested that the Eocene basin in the western parts of Iraq has had more free communication with the Mediterranean side of the Tethys than it had with the Indian side. The petrographic characteristics of the Lower Eocene phosphorites discussed
313
3'13°
3~7°
) /
4',1°
4~ °
4~ °
oo~.~"
•. , . ~. ; ~ . ~ .
Phosphatic
~
Molosse
Marly- chalky facies
~
Internal geosynclinal
[TT~TIq facies Neritic- shoal
~
Lagoonol evtAoorItlc facies
p-------1 facies Open sea
~']
Area of non deposition or erosion
facies
....
Calcareous lagoGmcll facies
trough
calcareous facies
4pO km
SCALE Fig. 15. Sketch map showing the Paleocene-Lower Eocene .facies distribution in Iraq and neighboring countries (after Buday, 1980).
before indicate that the majority of these phosphates were most probably deposited initially as thin layers of phosphorite mud which was later indurated and broken up by currents, waves, and probably by biological action. Mechanical transportation of the phosphatic intraclasts resulted in more abrasion and roundness of the grains which were finally deposited as angular, subrounded, and rounded grains together with the chemically precipitating carbonates. However, some pellets in a few samples appear to have formed in situ by an early diagenetic precipitation of phosphate within soft lime mud in a very shallow environment, as indicated by the association of phosphate pellets with cavities in limestone overlying hard grounds. The variation in the grain size of the phosphorite from south to north is mainly a function of distance from the source area where the phosphates were initially
314
precipitating. The grain size becomes finer and the phosphorite layers thinner towards the north as the basin becomes deeper where the shallow-water facies is interfingering with the basinal facies. The southern part is characterized by coarsegrained and thicker phosphorite. The enrichment of the phosphorite with the minor and trace elements such as F, U, Y, Zn, etc. appears to have taken place mostly during the precipitation of the phosphate mud since they generally show a uniform mode of distribution in the analyzed phosphate samples. Some elements were probably further concentrated in the phosphate phase after precipitation; probably during diagenesis, reworking and transportation which may explain to some extent the minor variation in the concentration of uranium which is more concentrated in the fine-grained phosphorite relative to the coarse-grained variety. The crystallization of the carbonate-fluorapatite, and the primitive redistribution of the impurities inside some of the grains and pellets may have taken place later during the various stages of diagenesis. The absence of nuclei, and the random intermixing of apatite with other minerals in the phosphate grains and pellets, and the angularity of a significant proportion of these grains together with the clear clastic texture of the phosphorite indicate that growth around nucleating centers had been rare. The deposition of a microcrystalline phosphorite mud seems more consistent with the available observations. The mechanical recycling, transportation and redeposition within the basin are indicated by the sharp contact between the phosphate grains and the cementing material, by the detrital appearance of the grains (including wear and tear), and by the absence of any phosphate cement in these rocks. Based upon the carbon isotopic composition of the Lower Eocene apatites (Al-Bassam, 1980), and in view of the abundance of organic matter in the phosphate grains, it seems that the dissociation of organic phosphatic matter was most probably the local source of phosphorus and carbon concentrations in solution required for the precipitation of carbonate-apatite. The phosphate was originally precipitated directly a n d / o r by replacement of lime mud in an environment characterized by very high biologic activity, rich in phosphorus and silica, probably reducing, and was occasionally affected by waves and currents, and by bioerosion. The silica may have been locally supplied from the diagenetic dissolution of siliceous organic detritus without the necessity of invoking direct chemical precipitation; some types of chalcedonic sponge spicules and diatoms may flourish in shallow-water carbonate environment (Chowns and Elkins, 1974), and have a long geologic record of contribution to bedded cherts in shelf carbonates (Hey, 1956; Rodda and Fisher, 1968). On the regional scale, upwelling currents were active during Upper CretaceousEocene times in the Tethyan seaway (Sheldon, 1964, 1981), and appear to have had served as transporting agents of phosphorus and silica from the deeper parts of the ocean to the shallow warmer parts, resulting in the flourishing of huge amounts of
315
marine fauna in these areas; a characteristic feature of present-day zones of upwelling. It is interesting to note that the phosphorites of Iraq were deposited along the western side of the Rutba uplift only, which indicates that this side was more suitable as a trap for the phosphorus and silica-rich waters, and for the precipitation of phosphate than the eastern side where carbonates have apparently diluted the sedimentary system. ACKNOWLEDGEMENTS
The authors express their appreciation to Dr. R.P. Sheldon of the U.S. Geological Survey, who read the manuscript and suggested improvements. This paper is a contribution to the I.G.C.P. Project 156 on phosphorite. REFERENCES Al-Bassam, K., 1976. The mineralogy, geochemistry, and genesis of the Akashat phosphorite deposit, western Iraq. J. Geol. Soc. Iraq, 9: 1-33. AI-Bassam, K., 1980. Carbon and oxygen isotopic composition of some marine sedimentary apatites from Iraq. Econ. Geol., 75: 1231-1233. Al-Bassam, K., Jamil, A. and AI-Dahan, A., 1980. Campanian-Maastrichtian phosphorites of Iraq. Petrology, geochemistry, and genesis. Geol. Surv. Iraq, Internal Rep., 30 pp. Al-Hashimi, H., 1972. Foraminiferida of the Dammam Formation (Eocene) in Iraq. Ph.D. Thesis, University of London, London, 350 pp. Altschuler, Z.S., Clarke, R.S. and Young, E.J., 1958. The geochemistry of uranium in apatite and phosphorite. U.S. Geol. Surv., Prof. Pap., 314-D: 45-90. Ames, L.L., 1959. The genesis of carbonate apatite. Econ. Geol., 54: 829-841. Buday, T., 1980. The Regional Geology of Iraq. Stratigraphy and Paleogeography. State Organization for Minerals, Baghdad, 445 pp. Bushinsky, G.I., 1963. On Shallow Water Origin of Phosphorite Sediments. 6th Int. Sedimentology Congress on Deltaic and Shallow Marine Deposits. Dev. Sedimentol., 1: 62-70. Chowns, T.M. and Elkins, J.E., 1974. The origin of quartz geodes and cauliflower cherts through the silicification of anhydrite nodules. J. Sediment. Petrol., 44: 885-903. Cook, P.J. and McElhinny, M., 1979. A reevaluation of the spatial and temporal distribution of sedimentary phosphate deposits in the light of plate tectonics. Econ. Geol., 74: 315-330. D'Anglejan, B.F., 1967. Origin of marine phosphates off Baja California, Mexico. Mar. Geol., 5: 15-44. Ditmar, V., 1971. Geological conditions and hydrocarbon prospects of the Republic of Iraq (northern and central parts). Iraqi National Oil Co., Internal Rep. Folk, R.L. and Pittman, J.S., 1971. Length-slow chalcedony: a new testament for vanished evaporites. J. Sediment. Petrol., 44: 1045-1048. Graf, D.L., 1960. Geochemistry of carbonate sediments and sedimentary carbonate rocks. Part Ili: Minor element distribution. Illinois State Geol. Surv. Circ., 301, 71 pp. Gulbrandsen, R.A., 1969. Physical and chemical factors in the formation of marine apatite. Econ. Geol., 64: 365-382. Gulbrandsen, R.A., 1970. Relation of carbon dioxide content of apatite of the Phosphoria Formation to regional facies. U.S. Geol. Surv., Prof. Pap., 700B: B9-BI3. Hagopian, D., 1980. Regional geological mapping of the Nhadain-Altinif area. Geol. Surv. Iraq, Internal Rep., 35 pp.
316
Hey, R.W., 1956. Cherts and limestones from the Crow Series near Richmond Yorkshire. Yorkshire Geol. Soc. Proc., 30: 289-299. ,lamil, A., Al-Dahan, A. and Al-Bassam, K., 1979. Mineralogy and crystal chemistry of apatite from Tayarat Formation of the Western Desert, lraq. Iraqi .1. Sci., 20:519-540. Kazakov, A.V., 1937. The phosphorite facies and the genesis of phosphorites in geological investigations of agricultural ores. Sci. Inst. Fertilizers and Insecto-Fungicides Trans., 142: 95- 113. Klement, R. and Haselbeck, H., 1965. Apatite und Wagnerite, zwei wertiger Metalle. Zeit. Anorg. Allg. Chem., 336:113 128. Larsen, Jr., E.S., Fletcher, M.H. and Cisney, E.A., 1952. Strontian apatite. Am. Mineral., 37: 656-658. LeGeros, R.Z., 1965. Effect of carbonate on the lattice parameters of apatite. Nature. 206:403 404. Mansfield, G.R., 1940. The role of fluorine in phosphate deposition. Am. ,i. Sci., 238: 863-879. Martens, C.S. and Harriss, R.C., 1970. Inhibition of apatite precipitation in the marine environment by magnesium ions. Geochim. Cosmochim. Acta, 34: 621-625. McClellan, G.H. and Lehr, ,I.R., 1969. Crystal chemical investigation of natural apatites. Am. Mineral.. 54: 1374-1391. McConnell, D., 1942. X-ray data on several phosphate minerals. Am. J. Sci., 240:649 -657. McConnell, D., 1952. The problem of carbonate-apatites. IV. Structural substitutions involving CO 3 and OH. Bull. Soc. Fr. Mineral. Cristallogr., 75: 428-445. McConnell, D., 1959. The problem of carbonate apatites. Econ. Geol., 54: 749-751. McConnell, D., 1962. Dating of fossil bones by the fluorine method. Science, 136:241 244. McConnell, D., 1965. Deficiency of phosphate ions in apatite. Naturwissenschaften, 52, p. 183. McConnell, D., 1970. Crystal chemistry of bone mineral hydrated carbonate apatite. Am. Mineral., 55: 1659-1669. McKelvey, V.E., 1967. Phosphate deposits. U.S. Geol. Surv. Bull., 1252-D: D1 [)21. Petranek, ,i., Jassim, S., Al-Bassam, K. and Hak, ,i., 1978. Quartz geodes from Western Desert, Iraq. Proc. Fifth Iraqi Geological Congress (in press). Pevear, D.R., 1966. The estuarine formation of United States Atlantic Coastal Plain phosphorite. Econ. Geol., 61: 251-256. Powers, R.W., Ramirez, L.F., Redmond, C.D. and Elberg Jr.. E.L., 1966. Geology of the Arabian Peninsula. Sedimentary Geology of Saudi Arabia. U.S. Geol. Surv., Prof. Pap. 560D: D1-D147. Riggs, S.R., 1979. Phosphorite sedimentation in Florida-A model phosphogenic system. Econ. Geol., 74: 285--315. Rodda, P.U. and Fisher, W.L., 1968. Dolomite depositional models, Edwards Formation (Lower Cretaceous) Texas. Geol. Soc. Am., Spec. Pap., 115, p. 187 (abstract). Sheldon, R.P., 1964. Exploration for phosphate in Turkey, a case history. Econ. Geol., 59:1159 1175. Sheldon, R.P., 1981. Ancient marine phosphorites. Annu. Rev. Earth Planet. Sci.. 9: 251-284. Siedlecka, A., 1972. Length-slow chalcedony and relicts of sulfates--evidences of evaporitic environments in the Upper Carboniferous and Permian beds of Bear Island Svalbard. ,i. Sediment. Petrol., 42: 812-..816. Silverman, S.R., Fuyat, R.K. and Weiser, J.D., 1952. Quantitative determination of calcite associated with carbonate-bearing apatites. Am. Mineral., 37:211-222. Summerhays, C.P., 1970. Phosphate deposits on the North-West African Continental Shelf and Slope. Ph.D. Thesis, London University, London, 213 pp. Vizer, J., Lemieux, J., Jones, B., Gibling, M. and Savelle, J., 1977. Sodium: Paleosalinity indicator in ancient carbonate rocks. Geology, 5: 177-179.
295
LOWER EOCENE PHOSPHORITES OF THE WESTERN DESERT, IRAQ
KHALDOUN S. AL-BASSAMand DIKRAN HAGOPIAN State Organization for Minerals, Directorate General of Geological Survey and Mineral Investigation, P. 0. Box 986 Alwiya, Baghdad (lraq)
(Received April 10, 1981; revised and accepted July 20, 1982)
ABSTRACT AI-Bassam, K.S. and Hagopian, D., 1983. Lower Eocene phosphorites of the Western Desert, Iraq. Sediment. Geol., 33: 295-316. Marine sedimentary phosphorites of Eocene age (Upper Ypresian) are exposed in the extreme west of Iraq within the Dammam Formation. They are associated with limestone and chert, and their deposition seems to have taken place in a shallow marine environment within a structurally controlled basin open to the sea from the northern and western sides only. The studied phosphorites are granular in texture, coarse-grained and cemented by calcite which is occasionally silicified. Bone fragments are present in small amounts. Carbonate-fluorapatite is the only phosphate mineral detected in these phosphorites, with relatively high amounts of the components SO4 2, COf 2, F-I, HaOl and Na I substituting in the crystal structure. The Lower Eocene phosphorites of Iraq are part of the Tethyan phosphorite province, and are comparable in many aspects with those of Paleocene and Upper Cretaceous age in the Western Desert of the country.
INTRODUCTION Iraq lies in the b o r d e r area between the two m a i n Phanerozoic units of the Middle East, i.e. b e t w e e n the A r a b i a n part of the N u b i o - A r a b i a n Platform a n d the Asian b r a n c h e s of the A l p i n e G e o s y n c l i n e (Buday, 1980). T h e p l a t f o r m part of Iraq is divided into two basic units: the Stable a n d the U n s t a b l e Shelf. The western part of the Iraqi Stable Shelf is characterized by the presence of the R u t b a uplift which had a n o r t h - s o u t h t r e n d d u r i n g the P a l e o c e n e - L o w e r Eocene, a n d has been a n area of n o n - d e p o s i t i o n since that time. The Lower Eocene phosphorites of Iraq are developed in the extreme western part of the c o u n t r y within the Stable Shelf, a n d cover a n area of a b o u t 1000 kin:. Similar to the other phosphorites of Iraq, the Lower Eocene deposits are m a i n l y developed along the western a n d n o r t h w e s t e r n sides of the R u t b a uplift. The phosphorites of Iraq are part of the T e t h y a n phosphorite province of U p p e r C r e t a c e o u s - E o c e n e age 0037-0738/83/0000-0000/$03.00
© 1983 Elsevier Scientific Publishing Company
296
which is known to extend from Morocco in the west to Iran and Turkey in the east. Within the Middle East area, Eocene phosphorites are known in lraq, Syria, Jordan and Saudi Arabia, occurring as thin phosphatic beds interbedded with limestone and chert. The Paleocene and Upper Cretaceous phosphorites of Iraq underlie the Lower Eocene deposit in the investigated area, and are exposed about 150 km to the east and northeast. The Iraqi phosphorites have many similarities with each other, being mostly of pelletal texture, cemented with calcite, and associated with chert to different degrees. Unlike many phosphorite deposits of the world, those of Iraq are not associated with black shale. The Paleocene deposit is the richest and best developed, the Upper Cretaceous phosphorites are thinly bedded with wide areal extension, and contain more clastics whereas those of the Lower Eocene have more chert interbeds. The mineralogy, geochemistry and genesis of the Upper Cretaceous and Paleocene phosphorites of Iraq have been discussed by A1-Bassam (1976) and AI-Bassam et al. (1980). The previous investigations have suggested that the mode of formation of these phosphorites included deposition of phosphate under reducing and alkaline conditions within the lime mud of bottom sediments. It was suggested that the dissociation of organic phosphatic matter in these sediments was the immediate source of phosphorus, and that upwelling currents of unknown direction and origin contributed to the regional enrichment of the southern and eastern coasts of the Tethys with phosphorus. The Upper Cretaceous-Eocene tectonic development of the region and the associated submarine volcanic activity were suggested by A1-Bassam et al. (1980) to have had some influence in the formation of these phosphorites. More than 100 samples have been examined and analyzed in the present study, covering most of the rock types identified during field work and regional mapping of the area (Hagopian, 1980). STRATIGRAPHY
The Eocene sediments in this part of the country belong to the Dammam and Jaddala Formations (Figs. 1 and 2). The Dammam Formation is a shallow marine facies, differentiated in this area into several lithostratigraphic units, and is composed mostly of limestone with occasional phosphorite and chert. The Jaddala Formation is an offshore limestone facies interfingering with the Dammam Formation in the investigated area. The Dammam Formation is differentiated into three lithostratigraphic units in this part of Iraq. These are from older to younger: Nhadain UnitmLower Ypresian. It is developed in the southern part of the area only where its thickness reaches about 38 m. It is composed of white limestone, Coarsely crystalline, cavernous, porous and highly weathered. The identified fauna are Nummulites deserti (Dela Harpe), Assilina placentula (Deshays), Operculina
297
/"
j./"~
d
- IIIIIN
u"
o..
-°,-~-2..~111111L~
-a2" o
u.ax.
I
'
~S A U D I A R A B I A
SCALE
Fig. 1. Geological sketch map of the extreme western part of Iraq.
lypica, Rotalia trochidiformis, Gastropoda, Bryozoa, Pelecypoda and Algae. According to fossil evidence this unit was deposited most probably in a sublittoral environment within a shoal zone. ~ Umm Chaimin Unit--Upper Ypresian. It is developed in the southern and middle parts only, covering a considerable area and reaching about 18 m in thickness. Lithologically it is composed of successive layers of phosphatic limestone, phosphorite, and chert in the west, which pass laterally to phosphatic limestone, calcareous limestone, and chert interbeds in the eastern part of the area. The phosphorite layers are 2-4 m thick each and are occasionally silicified. The following fauna were identified within the phosphorite-bearing horizons: Nummulites globulus (Leymeri), Nummulites atacicus, Globorotalia aspensis, Globorotalia linaperta, Peiecypoda, Ostracoda, Algae and fish scales. Accordingly, it seems that the deposition of these
298 NORTHERN
:
SECTION
lll
. i
,.i[,
SOUTHERN
i I
I , II
ii
e~
Z 0
SECTION
I~DI~w,1 ~ 1 I ~ I I
o o
T
i
,,
,
i
,
,J
.~..~~o i; ,~ii. = ~~.'ol .... ~
~ E E 5 •
.~
I--
0 E
56-
= i ii11_ o e4! i [ i I ,
LEGEND
,/ '~
Limestone
Dolomite
[ ~
1
80
,
/
"
e'~ z
Phosphatic limestone
phosphorite
72 t
,
, --,---
i L-
Chert E
96
~
,
I:
0
W .-I ~
Unconformity
Fig. 2. Columnar stratigraphicsections of the Eocene sequence in the extreme western part of Iraq.
rocks has taken place in a shallow sublittoral environment. The presence of Globorotalia, however, may indicate some contribution from deeper parts of the sea, which is also suggested by the extensive chert beds of this unit. In the eastern parts the following fauna were identified in the limestone of this unit: Nummulites planulatus (Lamarck), Numrnulites globulus (Leymeri), Numrnulites atacicus (Leymeri), Pelecypoda, Algae and fish scales, indicating again the shallow environment of deposition. Anaza Un/t--Lower to Upper Lutetian. This unit is exposed in the southern and northern parts overlying the other Eocene rock units in the area. It is best developed in the south, where it reaches a maximum thickness of about 28 m, compared with the northern area where only the upper part of this unit was found. Lithologically it
299 is not uniform; the first 15 m is nummulitic coarsely crystalline, white, cavernous limestone, whereas the upper 10 m is limestone with chert nodules. In the middle there is phosphatic limestone (Fig. 2). The faunal assemblages are Nurnmulites lavegatus, Nummulites gizehensis (Forskal), Nummulites discorbinus (Schlothein), Nummulites atacicus (Joly), Nummulites aturicus (Leymeri), Lockhartia alveolate (Silvester) and Alveolina elliptica (Van Flosculina). The faunal assemblage suggests a facies of a shallow marine environment. The Jaddala Formation is an offshore facies of late Lower Eocene-Upper Eocene age. The maximum thickness of the formation does not exceed 30 m in the northwestern parts of the country (Ditmar, 1971). This reduction in the formation thickness is obviously due to the close proximity of the Arabian Shield where the Eocene rocks are represented by neritic limestone facies of the Dammam Formation in Iraq and the upper parts of the Hibr Formation in Saudi Arabia (Powers et al., 1966). The Jaddala Formation is composed of chalky radiolarian limestone, occasionally dolomitic and contains chert nodules at the upper parts. It is developed only in the northern part of the investigated area, and the following fauna were identified: Globorotalia bullbrooki, Globorotalia interposita, Globorotalia linaperta, Radiolaria, Ostracoda and sponge spicules. The contact between the Dammam and the Jaddala Formations is conformable without break in deposition. The Eocene rocks in the investigated area are underlain by the Umm Er Radhuma Formation of Paleocene age unconformably. PETROGRAPHY AND MINERALOGY OF THE PHOSPHORITE All the samples examined petrographically came from the Lower Eocene Umm Chaimin Unit. These phosphorites are relatively coarse grained compared with the other phosphorites of Iraq, and contain various amounts of phosphatic material present mostly as rounded to subrounded, grains, detrital in appearance, and occasionally angular to subangular (Figs. 3 and 4). They range in diameter from 1 mm to about 3 cm, but mostly have the size of 3-7 mm. A fine-grained (1-3 mm), well-sorted variety of phosphorite was noticed to occur in the northern extremity of the deposit as thin phosphorite layers associated mostly with limestone hardgrounds (Fig. 5). The southern part is characterized by much coarser grained phosphorite and better development of phosphatic layers. The phosphate grains are white to creamy in color, usually covered with a dark-brown to black thin film of iron oxides and organic(?) stains. They show evidence of rolling of phosphate mud and development of pellets by wave action and bioerosion (Figs. 6 and 7). Coprolites are common (Fig. 8), and some of the phosphate pellets appear to have formed in small cavities within the associated limestone (Fig. 7). Phosphatic bone fragments are present in some samples and may reach several centimeters in length (Fig. 4). Most of the Lower Eocene phosphorites have a calcareous cement, b u t silicification is common in some restricted horizons.
300
Fig. 3. Phosphorite sample showing mixture of coarse and fine phosphate grains cemented with calcite.
Fig. 4. Phosphorite sample composed of fine, well-sorted phosphate grains mixed with very coarse angular and rounded intraclasts of phosphate and limestone, and bone fragment.
301
Fig. 5. Fine-grained, well-sorted phosphorite.
Fig. 6. Weathered surface of a phosphorite sample showing phosphate grains covered with a thin film of dark-colored iron oxide.
302
Fig. 7. Phosphate grains overlying limestone hardground. One of the grains is a warped phosphate sheet. A few phosphate grains were developed in cavities.
Fig. 8. Phosphate grain showing convolutionary texture and is probably a coprolite.
303
Fig. 9. Phosphate pellet showing primitive internal symmetrical distribution of impurities, and encasing an older phosphate grain. Polarized light.
Fig. 10. Phosphate grain (coprolite?), contains calcite inclusions, fragments of bones and fossil shells. Polarized light.
304
Fig. 11. Same as in Fig. 10 under crossed nicols.
Fig. 12. Rounded phosphate grain (black) partly replaced by calcite (gray). Crossed nicols.
305
Fig. 13. Reworked phosphate grain encases a bone fragment and shows evidence of wear and tear. Polarized light.
The microscopic examination showed that the composition of the Lower Eocene phosphorites is relatively simple; it is dominated by phosphate as grains, pellets and bones, and carbonate and silicate as cementing materials. The phosphatic grains are mostly intraclasts, but coprolites and faecal pellets are common, whereas oolites are rare. The phosphatic grains are brown to pale brown in color under plain polarized light, mostly without any internal symmetry, rich in mineral pigments and organic(?) residues, and occasionally contain calcite inclusions, bone fragments and fossil shells (Figs. 9, 10 and 11). The contact between the phosphatic grains and the cementing material is sharp and well defined. Partial replacement of some phosphatic pellets by the cementing calcite occur occasionally (Fig. 12). Evidence of wear and tear due to local reworking are also shown as broken and corroded pellets (Fig. 13). Under crossed nicols, the phosphatic grains are isotropic, similar in this respect to most marine sedimentary phosphorites, and this is probably due to the very minute crystallite size of the phosphate phase. On the other hand, the bone fragments are anisotropic and were identified as apatite. Shelly fossils are few within the phosphorite-bearing rocks, and when present they are composed of calcite. The cementing material is composed of sparry calcite. Replacement of the cementing calcite by chalcedony is a common phenomenon, however, the phosphatic pellets appear less affected by this process (Fig. 14). There is no sign of phosphate cement (microsphorite). Traces of clay minerals were observed in a few phosphatic
306
Fig. 14. Silicified phosphorite. The cementing material is completelyaltered to chalcedony, whereas, the phosphate grains are only affected at outer margins.
grains, but never in the cementing material. Detrital quartz was not found in any of the examined samples. Almost all of the examined phosphorite samples are dominated by clastic phosphate grains which appear to have formed elsewhere in the basin during the same sedimentary cycle and subsequently were transported as clastic particles and redeposited together with the chemically precipitating calcite in favorable traps. X-ray diffraction analysis of several phosphorite samples showed that the main minerals are apatite, calcite and impure silica (chalcedony). For accurate determination of the type of apatite present in these rocks, two representative concentrates of coarse and fine phosphatic grains were carefully analyzed by a diffractometer using internal standard. The results showed that both varieties of phosphatic grains are composed of carbonate-fluorapatite with lattice parameters as follows: Coarse a0-parameter (,~) c0-parameter (~,)
co/ao Volume (~,)3
9.325 6.898 0.7397 519.5
Fine 9.346 6.906 0.7380 522.4
The variation in the lattice parameters of these carbonate-fluorapatites can be
307
mainly due to the various amounts of CO~-z and F - ~ substituting in their structure. Both components result in a significant contraction of the a0-parameter and eventually the cell size (McConnell, 1942, 1952, 1970; LeGeros, 1965; Gulbrandsen, 1970). The lattice parameters of the Lower Eocene apatites generally fall within the range reported for apatites from the other marine sedimentary phosphorite deposits of Iraq. However, a general trend of decreasing a0-parameter value from older (Campanian) to younger (Eocene) exists and seems to be due to the higher amounts of carbonate in the structure of the younger apatites according to the data obtained by A1-Bassam (1976) and Jamil et al. (1979). GEOCHEMISTRY OF T H E PHOSPHORITE A N D ASSOCIATED ROCKS
Among the rock units of the Dammam Formation, Umm Chaimin is remarkable for being phosphorite-bearing. Lithologically this unit is composed of alternating layers of phosphorite, limestone and chert. About 100 samples representing the various types of these rocks were analyzed for their major, minor and trace-element constituents. In addition, phosphate grains representing the majority of the examined phosphorite samples were concentrated by hand picking, crushed, and apatite was purified by heavy liquid separation (bromoform), and triammonium citrate extraction (Silverman et al., 1952). The samples were analyzed by several techniques; Ca by titration, P, A1 and SO3 by colorimetry, Na and K by flame photometry, Si and L.O.I. by gravimetry, Mg, Cu, Cr, Ni, Sr, Mn and Fe by atomic absorption spectrophotometry, Y and V by emission spectrography, F by ion selective electrode. The results of the chemical analysis are presented in Tables I, II and III. The calcareous phosphorite represents the main part of the phosphorite-rich beds, it is characterized by lower magnesium content relative to the other phosphorites of Iraq, indicating the near absence of dolomite and palygorskite, both minerals are common associates of the Paleocene and Upper Cretaceous phosphorites (A1-Bassam, 1976; A1-Bassam et al., 1980). The very low contents of Si and AI in the calcareous phosphorites indicate that aluminosilicates are present in trace amounts. Similarly, iron oxides and hydroxides seem to be lower in the Lower Eocene phosphorites relative to the older phosphorites of Iraq. The analyzed major and trace elements are generally shared between carbonatefluorapatite and the cementing calcite. The geochemical association of elements, as shown by their correlation coefficients, indicate that F, Na, S, U, V, K and Cu form one geochemical group of elements, and they follow phosphorus in the apatite phase. Their concentration in the analyzed phosphorites is controlled mainly by the apatite content. Fluoride is an important constituent of all marine sedimentary apatites, and it plays an important role in its deposition (Mansfield, 1940; McConnell, 1962). This is supported by the similarity of the F/P205 ratio in all the analyzed phosphate rocks and in the purified apatite fraction; the latter contains 4. 1%F and 33.5 % P205.
308 TABLE 1 Chemical composition of the Lower Eocene phosphorite-bearing rocks Calcareous phosphorite
Siliceous phosphorite
Phosphatic limestone
Phosphatic chert
% SiO2 CaO MgO A1203 Fe2Oa SO t P205 K 20 Na20 F L.O.I. Total O= F Total -
0.39 54.37 0.35 0.10 0.19 1.06 20.84 0.03 0.51 2.40 20.54 100.78 1.01 99.77
40.96 31.30 0.23 0.10 0.50 1.14 18.68 0.20 0.43 2.12 5.40 101.08 0.89 100.19
I. 16 53.83 0.40 0.08 0.15 0.35 7.91 0.02 0.12 1.00 34.50 99.52 0.42 99. l0
71.85 13.82 0.17 0.01 t.17 I).47 4.21 0.02 0.15 0.50 6.88 99.25 0.21 99.04
21 8 137 15 832 8 45 2.16 0.12 16
32 10 160 25 1026 25 38 2.03 0.11 7
20 6 24 12 269 8 20 2.53 0.13 29
91 12 23 58 470 69 I0 2.38 0.12 17
pprn
Cr Ni V Cu Sr Mn U U/P205 F/P205 No. of samples
O n the o t h e r h a n d , s o d i u m a n d sulfur (as sulfate) c a n e n t e r the a p a t i t e s t r u c t u r e in c o u p l e d s u b s t i t u t i o n for c a l c i u m a n d p h o s p h o r u s r e s p e c t i v e l y w i t h o u t d i s t u r b i n g the electrical n e u t r a l i t y
o f the m i n e r a l
(A1-Bassam,
1976).
Minor
amounts
of
p o t a s s i u m m a y a c c o m p a n y s o d i u m in the c a l c i u m site; d e h r n i t e a n d l e w i s t o n i t e are species n a m e s g i v e n to a p a t i t e s w h i c h c o n t a i n s u b s t a n t i a l a m o u n t s of these t w o e l e m e n t s . P a r t of s o d i u m , h o w e v e r , m a y b e a c c o m o d a t e d in the calcite s t r u c t u r e ( V i z e r et al., 1977), w h e r e a s s o m e sulfate m a y be p r e s e n t as traces of g y p s u m a n d a n h y d r i t e in these p h o s p h o r i t e s . T r a c e s o f u r a n i u m (as U 4) are k n o w n to s u b s t i t u t e for c a l c i u m in the a p a t i t e s t r u c t u r e ( A l t s c h u l e r et al., 1958). T h e L o w e r E o c e n e p h o s p h o r i t e s
o f I r a q are
g e n e r a l l y d e p l e t e d in u r a n i u m ; an a v e r a g e of 45 p p m was f o u n d in the unsilicified
309
TABLE II
Chemical composition of the Lower Eocene non-phosphatic rocks , Dammam formation
Jaddala formation
U m m Chaimin unit
Nhadain Unit
Anaza Unit
Limestone
Limestone
Limestone
Chert
Limestone
% SiO 2 CaO MgO
1.17 53.33 1.29
69.35 14.68 0.47
1.06 53.42 1.12
0.77 54.89 0.19
1.09 54.85 0.29
A1203 Fe203 SO 3 P205 K20
0.11 0.14 0.15 0.41 0.07
0.09 1.95 0.61 0.79 0.02
0.09 0.11 0.21 0.23 0.06
0.07 0.10 0.29 0.28 0.02
0.15 0.12 0.35 0.31 0.02
Na20 L.O,I.
0.05 42.47 99.19
0.06 11.32 99.43
0.05 43.16 99.51
0.04 42.98 99.63
0.04 42.73 99.95
Total ppm
Cr Ni V Cu Sr Mn No. of samples
13 7 11 5 112 20 28
52 20 12 50 169 101 17
10 3 4 2 146 21 35
8 3 7 2 160 20 47
~23 12 10 6 202 22 55
Graf (1960) 9 12 15 14 475 500
phosphorite. The fine-grained variety is relatively richer in uranium; containing up to 100 ppm in some samples. The U/P205 ratio in most of the analyzed phosphorites is about 2, but the rare presence of secondary uranium vanadates of the carnotiie type may increase this ratio in some samples. Copper is probably attached to some organic residues which are common impurities in the phosphatic pellets. Part of Cu 2, however, may be incorporated in the apatite lattice in substitution for Ca 2 (Klement and Haselbeck, 1965). Strontium is shared between calcite and apatite, but it seems to favor,the latter as a host since Sr concentration in the phosphorite-bearing rocks is several times higher than the associated limestone (Tables I and II). The apatite lattice can accomodate relatively high amounts of strontium without distortion (Larsen et al., 1952). Vanadium may be introduced to the apatite structure as VO4-3 replacing minor amounts of PO4 -3, and can be also attached to the organic residues within the phosphatic pellets. The concentration of vanadium in the phosphorites and the separated apatite fraction of
310 TABLE 111 Chemical composition of the Lower Eocene apatite and calculation of the structural formula i Analysis of the apatite 2
SiO2 CaO MgO A120~ Fe203 SO3 P2Os K20 Na20 CO: H20 ~ F Total -O~F Total Cr
Ni V Cu Sr Mn U Y Zn
Structural components
Atomic Preliminary propor, of formula oxygen (ions/unit cell)
(%) (~) 0.75 CaO 53.45 0.9531 53.45 Na20 0.40 0.0065 0.25 0.06 P205 33.45 1.1786 0.05 CO 2 5.40 0.2455 0.95 SO3 0.95 0.0356 33.45 0.02 F 4.10 0.2158 0.40 H20 2.65 0.1472 5.40 Total 2.7823 2.65 - - O - - F 0.1079 4.10 Total 2.6744 101.54 26/2.6744= 9.7218 1.72 99.82 (+ charges 49.45: charges 49.88)
Ca Na
9.27 0.13
P C S
4.58 1.20 0.12
F H
2.10 2.86
Last refinement of the formula (ions/unit cell)
Ca Na H ~O P 3/4C S [t 4 F O
9.37 0.13 0.50 4.63 0.91 0.12 0.34 2.12 23.88
10
} 26
(ppm) 65 16 200 20 980 20 6O 32 100
Calculated according to the model proposed by McConnell (1970). 2 The analysis includes 0.2% organic carbon.
the U m m C h a i ' m i n U n i t is m u c h h i g h e r t h a n that in the n o n - p h o s p h a t i c l i m e s t o n e a n d chert o f the s a m e unit. In a d d i t i o n to the e l e m e n t s d i s c u s s e d a b o v e , the c h e m i c a l analysis of the sepa r a t e d a p a t i t e f r a c t i o n s h o w s the p r e s e n c e of c o n s i d e r a b l e a m o u n t s of C O 2 a n d H2 O + . T h e f o r m e r (as C O l 2) is k n o w n
to r e p l a c e p h o s p h a t e in m o s t m a r i n e
s e d i m e n t a r y a p a t i t e s ( M c C o n n e l l , 1959, 1965, 1970; M c C l e l l a n a n d Lehr, 1969). T h e s t r u c t u r a l f o r m u l a o f the L o w e r E o c e n e c a r b o n a t e - f l u o r a p a t i t e ( c a l c u l a t e d a c c o r d i n g to the m o d e l p r o p o s e d by M c C o n n e l l , 1970) is b a s i c a l l y similar, b u t w i t h m i n o r v a r i a t i o n , to t h o s e of U p p e r C r e t a c e o u s a n d P a l e o c e n e (A1-Bassam, 1976; J a m i l et
311
al., 1979). The similarity is in the wide range of isomorphic substitutions in their structure dominated by carbonate and sulfate replacement of phosphate, sodium of calcium, and fluoride of oxygen, not forgetting the considerable hydration of these apatites in various structural sites. Several other minor and trace-chemical elements which were detected in the analysis of the Lower Eocene phosphorites are not related to the phosphate phase; Fe, Cr and Ni are generally low and seem to be attached to the traces of aluminosilicates in these rocks, as suggested by the strong positive correlation with alumina and silica. On the other hand, magnesium and manganese seem to be associated with the cementing calcite. The associated non-phosphatic limestone is calcareous in composition and is similar in its trace-element composition to average carbonates (Graf, 1960), but it is significantly depleted in Sr and Mn. It is also similar to the other carbonate rock units of the Dammam Formation in the area (Table II). The silicification process, which influenced the carbonate cement mostly, has partly altered the geochemical character of the original calcareous phosphorite and influenced the distribution and association of some elements. The concentrations of Fe, Mn, Cr, Ni, Cu and SO 3 are higher in the silicified rocks. Iron and manganese concentrations in particular are higher by about ten times. On the other hand, U, Sr, V, Na, and to some extent Cu retained their original positive correlation with phosphorus, whereas, Mg, K and Ni are associated with silica. The sulfate seems to be present as traces of anhydrite within the silicified matrix in view of the positive correlation between calcium and sulfate in these rocks. The phenomena of anhydrite relation to the formation of chert layers and chert nodules have been discussed by several authors (Folk and Pittman, 1971; Siedlecka, 1972; Chowns and Elkins, 1974). Relics of anhydrite were found in most of the geodes and chert nodules of Upper Cretaceous-Eocene phosphorite-bearing carbonate rocks of the Western Desert, Iraq (Petranek et al., 1978). The original content of anhydrite in the Lower Eocene phosphorites can not be estimated sinc~ most of the primary material was altered to chalcedony. Iron and organic(?) impurities apparently cause most of the coloration of these cherts which vary from dark-brown to white. GENESIS
The mechanism of phosphorite formation and the suitable marine conditions for its precipitation are still a matter of argument. The work of the last 40 years or so has by no means solved the problem of phosphorite formation. However, since the publication of Kazakov's theory in 1937, it has been generally accepted that oceanic upwelling of cold phosphate-rich water is an important factor in the formation of marine sedimentary phosphorites. Ames (1959), Bushinsky (1963), Sheldon (1964, 1981), McKelvey (1967), Gulbrandsen (1969), Cook and McElhinny (1979) and Riggs (1979) have significantly contributed, among others, to this subject.
312
The local conditions suitable for the formation of marine phosphorites can be summarized as relatively higher than average phosphate concentration in solution that should be kept from dilution by detritals and by (normal) seawater, a relatively high pH value (higher than 7.0) should be maintained, a relatively high concentration of calcium should be available either in solution or as lime mud, and a high C a / M g (higher than 5.2) is considered important (Ames, 1959; D'Anglejan, 1967; Gulbrandsen, 1969; Martens and Harriss, 1970). Several authors have found that the conditions prevailing in the interstitial environment of soft sediments are more suitable for the deposition of phosphate in early diagenesis than those of the overlying open marine environment (Bushinsky, 1963; Pevear, 1966; Summerhays, 1970), The anaerobic decay of the organic remnants starts directly after burial and this process results in increasing the concentration of dissolved phosphorus in pore solution and elevating the pH value due to the formation of ammonia. Furthermore, the C a / M g ratio in such environments can be suitable for the deposition of phosphate. The extensive deposition of phosphorite in Iraq and other areas in the Middle East and North Africa began during the Upper Cretaceous (Campanian) and continued to the end of the Cretaceous in this region. This was followed by the early Tertiary phosphogenic epoch which continued in successive cycles from the Paleocene to the end of the Eocene. The genesis of the Upper Cretaceous and Paleocene phosphorites of Iraq was discussed by A1-Bassam (1976) and AI-Bassam et al. (1980). It has been suggested that the immediate source of phosphorus appears to have been biogenic, and that phosphate was most probably precipitated in the shallow, warm and phosphorus-rich semi-restricted environment of near-shore lagoons, near the sediment-water interface under high pH and reducing conditions. Later diagenetic processes have played an important role in developing many petrographic and geochemical characteristics of the phosphatic grains, the majority of which appear to have been subjected to several stages of reworking by currents and waves before they were finally deposited mechanically in favorable traps. It has been also suggested that the Upper Cretaceous-Eocene paleographic setting of the Tethys and the associated submarine volcanism at that time have contributed to the formation and development of the Tethyan phosphogenic system (A1-Bassam et al., 1980). The Lower Eocene phosphate deposit of Iraq is part of the Tethyan phosphogenic province, it was deposited in a shallow marine environment within a basin bordered from the east and south by the Hail Arch and its northern extension the Rutba Uplift (Fig. 15). The phosphate basin extended from northeast to southwest inside Iraq, Syria, Jordan and Saudi Arabia. Based upon paleontological evidence, A1Hashimi (I 972) suggested that the Eocene basin in the western parts of Iraq has had more free communication with the Mediterranean side of the Tethys than it had with the Indian side. The petrographic characteristics of the Lower Eocene phosphorites discussed
313
3'13°
3~7°
) /
4',1°
4~ °
4~ °
oo~.~"
•. , . ~. ; ~ . ~ .
Phosphatic
~
Molosse
Marly- chalky facies
~
Internal geosynclinal
[TT~TIq facies Neritic- shoal
~
Lagoonol evtAoorItlc facies
p-------1 facies Open sea
~']
Area of non deposition or erosion
facies
....
Calcareous lagoGmcll facies
trough
calcareous facies
4pO km
SCALE Fig. 15. Sketch map showing the Paleocene-Lower Eocene .facies distribution in Iraq and neighboring countries (after Buday, 1980).
before indicate that the majority of these phosphates were most probably deposited initially as thin layers of phosphorite mud which was later indurated and broken up by currents, waves, and probably by biological action. Mechanical transportation of the phosphatic intraclasts resulted in more abrasion and roundness of the grains which were finally deposited as angular, subrounded, and rounded grains together with the chemically precipitating carbonates. However, some pellets in a few samples appear to have formed in situ by an early diagenetic precipitation of phosphate within soft lime mud in a very shallow environment, as indicated by the association of phosphate pellets with cavities in limestone overlying hard grounds. The variation in the grain size of the phosphorite from south to north is mainly a function of distance from the source area where the phosphates were initially
314
precipitating. The grain size becomes finer and the phosphorite layers thinner towards the north as the basin becomes deeper where the shallow-water facies is interfingering with the basinal facies. The southern part is characterized by coarsegrained and thicker phosphorite. The enrichment of the phosphorite with the minor and trace elements such as F, U, Y, Zn, etc. appears to have taken place mostly during the precipitation of the phosphate mud since they generally show a uniform mode of distribution in the analyzed phosphate samples. Some elements were probably further concentrated in the phosphate phase after precipitation; probably during diagenesis, reworking and transportation which may explain to some extent the minor variation in the concentration of uranium which is more concentrated in the fine-grained phosphorite relative to the coarse-grained variety. The crystallization of the carbonate-fluorapatite, and the primitive redistribution of the impurities inside some of the grains and pellets may have taken place later during the various stages of diagenesis. The absence of nuclei, and the random intermixing of apatite with other minerals in the phosphate grains and pellets, and the angularity of a significant proportion of these grains together with the clear clastic texture of the phosphorite indicate that growth around nucleating centers had been rare. The deposition of a microcrystalline phosphorite mud seems more consistent with the available observations. The mechanical recycling, transportation and redeposition within the basin are indicated by the sharp contact between the phosphate grains and the cementing material, by the detrital appearance of the grains (including wear and tear), and by the absence of any phosphate cement in these rocks. Based upon the carbon isotopic composition of the Lower Eocene apatites (Al-Bassam, 1980), and in view of the abundance of organic matter in the phosphate grains, it seems that the dissociation of organic phosphatic matter was most probably the local source of phosphorus and carbon concentrations in solution required for the precipitation of carbonate-apatite. The phosphate was originally precipitated directly a n d / o r by replacement of lime mud in an environment characterized by very high biologic activity, rich in phosphorus and silica, probably reducing, and was occasionally affected by waves and currents, and by bioerosion. The silica may have been locally supplied from the diagenetic dissolution of siliceous organic detritus without the necessity of invoking direct chemical precipitation; some types of chalcedonic sponge spicules and diatoms may flourish in shallow-water carbonate environment (Chowns and Elkins, 1974), and have a long geologic record of contribution to bedded cherts in shelf carbonates (Hey, 1956; Rodda and Fisher, 1968). On the regional scale, upwelling currents were active during Upper CretaceousEocene times in the Tethyan seaway (Sheldon, 1964, 1981), and appear to have had served as transporting agents of phosphorus and silica from the deeper parts of the ocean to the shallow warmer parts, resulting in the flourishing of huge amounts of
315
marine fauna in these areas; a characteristic feature of present-day zones of upwelling. It is interesting to note that the phosphorites of Iraq were deposited along the western side of the Rutba uplift only, which indicates that this side was more suitable as a trap for the phosphorus and silica-rich waters, and for the precipitation of phosphate than the eastern side where carbonates have apparently diluted the sedimentary system. ACKNOWLEDGEMENTS
The authors express their appreciation to Dr. R.P. Sheldon of the U.S. Geological Survey, who read the manuscript and suggested improvements. This paper is a contribution to the I.G.C.P. Project 156 on phosphorite. REFERENCES Al-Bassam, K., 1976. The mineralogy, geochemistry, and genesis of the Akashat phosphorite deposit, western Iraq. J. Geol. Soc. Iraq, 9: 1-33. AI-Bassam, K., 1980. Carbon and oxygen isotopic composition of some marine sedimentary apatites from Iraq. Econ. Geol., 75: 1231-1233. Al-Bassam, K., Jamil, A. and AI-Dahan, A., 1980. Campanian-Maastrichtian phosphorites of Iraq. Petrology, geochemistry, and genesis. Geol. Surv. Iraq, Internal Rep., 30 pp. Al-Hashimi, H., 1972. Foraminiferida of the Dammam Formation (Eocene) in Iraq. Ph.D. Thesis, University of London, London, 350 pp. Altschuler, Z.S., Clarke, R.S. and Young, E.J., 1958. The geochemistry of uranium in apatite and phosphorite. U.S. Geol. Surv., Prof. Pap., 314-D: 45-90. Ames, L.L., 1959. The genesis of carbonate apatite. Econ. Geol., 54: 829-841. Buday, T., 1980. The Regional Geology of Iraq. Stratigraphy and Paleogeography. State Organization for Minerals, Baghdad, 445 pp. Bushinsky, G.I., 1963. On Shallow Water Origin of Phosphorite Sediments. 6th Int. Sedimentology Congress on Deltaic and Shallow Marine Deposits. Dev. Sedimentol., 1: 62-70. Chowns, T.M. and Elkins, J.E., 1974. The origin of quartz geodes and cauliflower cherts through the silicification of anhydrite nodules. J. Sediment. Petrol., 44: 885-903. Cook, P.J. and McElhinny, M., 1979. A reevaluation of the spatial and temporal distribution of sedimentary phosphate deposits in the light of plate tectonics. Econ. Geol., 74: 315-330. D'Anglejan, B.F., 1967. Origin of marine phosphates off Baja California, Mexico. Mar. Geol., 5: 15-44. Ditmar, V., 1971. Geological conditions and hydrocarbon prospects of the Republic of Iraq (northern and central parts). Iraqi National Oil Co., Internal Rep. Folk, R.L. and Pittman, J.S., 1971. Length-slow chalcedony: a new testament for vanished evaporites. J. Sediment. Petrol., 44: 1045-1048. Graf, D.L., 1960. Geochemistry of carbonate sediments and sedimentary carbonate rocks. Part Ili: Minor element distribution. Illinois State Geol. Surv. Circ., 301, 71 pp. Gulbrandsen, R.A., 1969. Physical and chemical factors in the formation of marine apatite. Econ. Geol., 64: 365-382. Gulbrandsen, R.A., 1970. Relation of carbon dioxide content of apatite of the Phosphoria Formation to regional facies. U.S. Geol. Surv., Prof. Pap., 700B: B9-BI3. Hagopian, D., 1980. Regional geological mapping of the Nhadain-Altinif area. Geol. Surv. Iraq, Internal Rep., 35 pp.
316
Hey, R.W., 1956. Cherts and limestones from the Crow Series near Richmond Yorkshire. Yorkshire Geol. Soc. Proc., 30: 289-299. ,lamil, A., Al-Dahan, A. and Al-Bassam, K., 1979. Mineralogy and crystal chemistry of apatite from Tayarat Formation of the Western Desert, lraq. Iraqi .1. Sci., 20:519-540. Kazakov, A.V., 1937. The phosphorite facies and the genesis of phosphorites in geological investigations of agricultural ores. Sci. Inst. Fertilizers and Insecto-Fungicides Trans., 142: 95- 113. Klement, R. and Haselbeck, H., 1965. Apatite und Wagnerite, zwei wertiger Metalle. Zeit. Anorg. Allg. Chem., 336:113 128. Larsen, Jr., E.S., Fletcher, M.H. and Cisney, E.A., 1952. Strontian apatite. Am. Mineral., 37: 656-658. LeGeros, R.Z., 1965. Effect of carbonate on the lattice parameters of apatite. Nature. 206:403 404. Mansfield, G.R., 1940. The role of fluorine in phosphate deposition. Am. ,i. Sci., 238: 863-879. Martens, C.S. and Harriss, R.C., 1970. Inhibition of apatite precipitation in the marine environment by magnesium ions. Geochim. Cosmochim. Acta, 34: 621-625. McClellan, G.H. and Lehr, ,I.R., 1969. Crystal chemical investigation of natural apatites. Am. Mineral.. 54: 1374-1391. McConnell, D., 1942. X-ray data on several phosphate minerals. Am. J. Sci., 240:649 -657. McConnell, D., 1952. The problem of carbonate-apatites. IV. Structural substitutions involving CO 3 and OH. Bull. Soc. Fr. Mineral. Cristallogr., 75: 428-445. McConnell, D., 1959. The problem of carbonate apatites. Econ. Geol., 54: 749-751. McConnell, D., 1962. Dating of fossil bones by the fluorine method. Science, 136:241 244. McConnell, D., 1965. Deficiency of phosphate ions in apatite. Naturwissenschaften, 52, p. 183. McConnell, D., 1970. Crystal chemistry of bone mineral hydrated carbonate apatite. Am. Mineral., 55: 1659-1669. McKelvey, V.E., 1967. Phosphate deposits. U.S. Geol. Surv. Bull., 1252-D: D1 [)21. Petranek, ,i., Jassim, S., Al-Bassam, K. and Hak, ,i., 1978. Quartz geodes from Western Desert, Iraq. Proc. Fifth Iraqi Geological Congress (in press). Pevear, D.R., 1966. The estuarine formation of United States Atlantic Coastal Plain phosphorite. Econ. Geol., 61: 251-256. Powers, R.W., Ramirez, L.F., Redmond, C.D. and Elberg Jr.. E.L., 1966. Geology of the Arabian Peninsula. Sedimentary Geology of Saudi Arabia. U.S. Geol. Surv., Prof. Pap. 560D: D1-D147. Riggs, S.R., 1979. Phosphorite sedimentation in Florida-A model phosphogenic system. Econ. Geol., 74: 285--315. Rodda, P.U. and Fisher, W.L., 1968. Dolomite depositional models, Edwards Formation (Lower Cretaceous) Texas. Geol. Soc. Am., Spec. Pap., 115, p. 187 (abstract). Sheldon, R.P., 1964. Exploration for phosphate in Turkey, a case history. Econ. Geol., 59:1159 1175. Sheldon, R.P., 1981. Ancient marine phosphorites. Annu. Rev. Earth Planet. Sci.. 9: 251-284. Siedlecka, A., 1972. Length-slow chalcedony and relicts of sulfates--evidences of evaporitic environments in the Upper Carboniferous and Permian beds of Bear Island Svalbard. ,i. Sediment. Petrol., 42: 812-..816. Silverman, S.R., Fuyat, R.K. and Weiser, J.D., 1952. Quantitative determination of calcite associated with carbonate-bearing apatites. Am. Mineral., 37:211-222. Summerhays, C.P., 1970. Phosphate deposits on the North-West African Continental Shelf and Slope. Ph.D. Thesis, London University, London, 213 pp. Vizer, J., Lemieux, J., Jones, B., Gibling, M. and Savelle, J., 1977. Sodium: Paleosalinity indicator in ancient carbonate rocks. Geology, 5: 177-179.