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On the albite-enriched granitoids at Um Ara area, Southeastern Desert, Egypt. 1. Geochemical, ore potentiality and fluid inclusion studies
ELSEVIER
Journal of Geochemical
Exploration
57 (1996) 127-138
On the albite-enriched granitoids at Urn Ara area, Southeastern Desert, Egypt. 1. Geochemical, ore potentiality and fluid inclusion studies H.M. Abdalla a, S. Ishihara b, H. Matsueda b, A.A. Abdel Monem a a Nuclear Materials Authority, Cairo, Egypt b Hokkaido Uniuersiv, Sapporo 060, Japan Received 9 May 1995; accepted
14 May 1996
Abstract A radioactive and metasomatically albite-enriched microgranite (apogranite, 556 Ma) stock was emplaced at the northern contact of Urn Ara monzogranite pluton (589 Ma). Rock/fluid interaction within the apogranite is well displayed as petrographical and geochemical zonal patterns in response to K- and Na-metasomatism. followed by Naf The metasomatic reactions commenced with addition of K+, Rb+ and Pbzf (amazonitization) (albitization) and finally with Mn- and Fe-oxide alteration. A gradual and gross enrichment of Nb, Y, Zr, Li, Zn, U, Th and F occurs from the amazonitized to albitized zone. Microthermometric study of fluid inclusions in quartz of the amazonitized and albitized microgranites indicates that high to moderate temperature, saline fluids interacted with the solidified microgranite cupola in the early metasomatic processes. Inclusions of the K-enriched zone homogenize in the range of 285-425” C, with salinities of 7-22.1 wt.% NaCl equivalent. However, inclusions in the later fracture and joint-filling quartz veins and fluorite veinlets homogenize in the range of 190-310°C with salinities of 2-5.1 wt.% NaCl equivalent. It is suggested that the presence of K+, Naf and F- in the ore fluids was essential to stablize complexes of the rare metal elements during extraction and transportation. In contrast, contemporaneous decrease of temperature and salinity, loss of CO, and increasing pH due to decreasing pressure are considered the essential factors for localization of disseminated mineralization of Zr, Y, Nb, Zn and Th and the fracture-filling with U in the apical parts of the Urn Ara apogranite stock. Keywords: amazonitization:
albitization;
rare metals; apogranite;
fluid inclusions
1. Introduction One of the pronounced radiometric anomalies in the Southeastern Desert of Egypt is associated with a metasomatically albite-enriched granite stock (Le., Urn Ara apogranite, 556 Ma) emplaced at the northem contact of the Urn Ara monzogranite pluton (589 Ma) (Fig. 1). The metasomatic nature of the apogranite was characterized according to the criteria of 03756742/96/$15.00 Copyright PII SO375-6742(96)00029-5
(1983) and Schwartz (1992). Although the time span between the emplacement of the two bodies is about 33 Ma (K-Ar age of mica separates, present study), both of the granitic masses represent post-erogenic, younger granitic activity that intruded the Egyptian Shield between 620 and 530 Ma during cratonization and sialification (Hassan and Hashad, 1990). The present study aims at characterizing the spe-
Pollard
0 1996 Elsevier Science B.V. All rights reserved.
Stream sediments. Uranium mineralization. Quartz breccia d&e. Zinnwaldite albitized microgranite. Zionwaldite Alkali-feldspar microgranite. Monzogrtite. Dokhan volcanics. Gtleissose granite. Melange rocks. Metavolcanics. Serpentmites. Fault. Fig.
C
I. Geological map ol lJtn Ara
cialized paragenetic type of the apogranite body and associated mineralization, assessing the role of postmagmatic alteration in the distribution of rare metal mineralization, depicting the evolution of the postmagmatic fluids, and elucidating the mechanism by which mineralization was formed.
2. Petrography The monzogranite has a generally homogeneous, pinkish, equigranular, coarse-grained texture, although in places a porphyritic variety is present. It is
apogranite
composed of intermediate microcline and microcline perthite. plagioclase (An ,j Zx1. quartz, and biotite. Rare flakes of muscovite are also observed. Sphene and magnetite are the common accessory minerals indicating that the granite belongs to the magnetite series granitoids of Ishihara (1977). Scarce mafic xenoliths (approx. 10 cm in diameter) are also present in these granites. Emplacement of the Urn Ara apogranite was primarily controlled by a system of faults and fractures. Rock/fluid interaction within the apogranite is well displayed by a petrographic zonal pattern including a lower unaltered miarolitic pink alkali-feldspar micro-
H.M. Abdalla
et al./.Ioumal
ofGeochemica1
granite, a middle zinnwaldite amazonitized microgranite (i.e. zone of microclinization or amazonitization), and a roof zone of zinnwaldite albitized microgranite (i.e. zone of albitization). The alkali-feldspar microgranite (grain size < 2 mm) is characterized by well developed miarolitic vugs 3-10 cm in diameter, filled with fragmented quartz in a felsitic matrix, and degassing breccias (fragmented microgranites cemented by quartz and fluorite). The mineral constituents are microcline, microperthite, albite (An,_ ,*), graphic quartz and minor white mica. The accessory minerals are fluorite, ilmenite, pyrite and rare columbite. Thus, the original magma of the apogranite may belong to the ilmenite series grantoids of Ishihara (1977). The zone of amazonitization is characterized by the development of highly- to fully-ordered green microcline (imparting to the rock a pale to deep green color), zinnwaldite and spessartine garnets replacing pre-existing minerals. A detailed description of the paragenetic sequence of formation of minerals during the magmatic and postmagmatic stages is given in Abdalla et al. (1994) and Abdalla et al. (1996). The amazonite exhibits the cross-hatched twinning and the pericline twins of the replaced albite. Accessory minerals are fluorite, columbite, Zn-Mn-Nb-rich ilmenites, zircon, xenotime, thorite, monazite and acicular uranophane. Violet and colorless fluorite veinlets are locally abundant. Local degassing breccias and pegmatite nests are common. The breccia is composed of fragmented euhedral crystals of fluorite (approx. 2 cm) embedded in a matrix of fine-grained felsic minerals and fluorite. The pegmatite nests are composed of large crystals of amazonite (approx. 3 cm>, pseudohexagonal crystals of zinnwaldite (approx. 2 cm) and massive quartz. The zone of albitization is characterized by abundantly developed subsolidus clusters of fine-grained albites (An,_,) replacing early formed microcline along cleavage and twinning planes. In addition to the development of zinnwaldite and spessartite, the same accessory minerals of the amazonitization zone increase in abundance. Many quartz veins and quartz breccia dikes (up to 10 m wide) traverse the apogranite and strike E-W. In places with extreme Na-metasomatism, the albitite rock has a patchy and erratic distribution of limited area1 extent. It is white, fine-grained, and
Exploration
57 (1996) 127-138
129
made up of > 80% albite with an average grain size < 200 Km, corroded quartz grains 1 mm in diameter and a few small relicts of microcline that are replaced in part by the fine albite laths. The alteration sequence concluded with Fe- (hematitization) and Mn-oxide alteration along later fractures and joints and as local impregnations. Field observations and microscopic (in reflected and transmitted light) examinations revealed the presence of three modes of occurrence for the rare metals mineralization associated with the Urn Ara apogranite stock. These modes are: (1) disseminated mineralization of manganocolumbite, Nb-Mn-Znrich ilmenite, Hf-rich zircon, xenotime, thorite, monazite; (2) uranium mineralization (essentially uranophane) occurring as disseminated acicular crystals and most commonly as fracture-filling; and (3) minor placer occurrences of columbite, zircon, xenotime, and thorite. Two modes of occurrences are exhibited by the uranophane in the Urn Ara apogranite; disseminated in the albitized microgranite and fracture-fillings in the amazonitized and albitized microgranites. The fracture-filling uranophane occurs as crystalline aggregates intimately intergrown with fine calcite crystals along joint and fracture surfaces. The fracture-filling uranophane is closely associated with late-stage Fe- and Mn-oxide alteration. Disseminated uranophane occurs as bright lemon-yellow acicular crystals associated with clots of secondary albite laths in the albitized microgranites. Detailed mineralogical and chemical characteristics of the rare metals mineralization have been addressed by Abdalla et al. (1994).
3. Sampling and analytical techniques A detailed geological map was constructed by N-S traverses. The mapping was accompanied by a systematic sampling (250 samples) at 50-500 m intervals. Major and trace elements were detected by XRF using the fused bead and pressed pellet techniques, respectively. Fluorine contents were determined by selective ion electrode, and Li was determined by atomic absorption spectroscopy. Fluid inclusion studies were conducted using 30 doublypolished wafers, 100-500 pm thick of samples from the alteration zones, later fracture-filling quartz veins,
130 Table
I
Averages
and ranges
of major
Monzogranite
and trace element
compositiona
of Urn Am granitoids
Alkali-feldspar
Amazonitized
Albitized
microgranite
microgranite
microgranite
Abitite
N
7 (7)
8 (8)
6 (26)
9 (27)
6 (7)
SiO,
71.62
75.60
74.68
7.5.28
68.77
(70.79-72.45)
(74.05-77.50)
(7 I .98-76.75)
(73.2X--76.Y9)
(66.2-72.00)
0.09
0.04
u.04
0.01
(0.03-0.05,
(0.02-0.05)
(0.01-0.02)
TiO,
0.27
1)
(0.22-0.3 Al?03
(0.03-O.
16)
13.72
12.76
13.34
Ii.34
18.26
(13.45-13.99)
(12.55-13.41)
(12.93-14.08)
(13.03-13.75)
(14.73-19.35)
1.71
0.99
0.68
0.75
(1.30-1.87)
(0.61-1.27)
(0.5-0.9)
(0.44-I
MnO
0.12
0.10
0.16
MgO
0.35
0.09
(0.24-0.42)
(0.06-o.
0.96
0.49
0.32
(0.73-1.12)
(0.39-0.79)
(O.I2-0.8
FezO,
(0.05-O.
CaO Na,O K,O P*@ L.O.I.
F Li Rb Ba
Zr Y Nb Sn
0.20
13.70
13.60
0.3 I
0.36
0.81
(Fe,O,)
(0.25-0.46)
0.50
1.10
(FeO)
0.16
0.09
0.06
0.05
0. I I
0.27
0.30
0.7 I
4.62
3.48
4.26
5.06 0.14
.Ol )
(0. I-0.27)
(0. I-0.2)
(0.03-O.
0.05
0.05
0.02
(O.OI-0.12)
(0.01 -0. IS)
(0.01-0.02~
I)
0.04
0.27
0.07
(0.17-0.32)
(0.04-O.
I I)
12)
4.42
3.97
5.42
10.06
(3.78-4.07)
(5.05-5.75)
(7.76-10.65)
4.43
4.29
5.63
3.54
0.4 I
(4.37-4.49)
(4.07-4.68)
(4.86-6.52)
(3.03-3.87)
(0.12-1.75)
0.08
0.02
0.02
0.02
< 0.01
0.02
(0.05-0.09)
(0.01-0.03)
(0.0 I-0.03)
0.89
0.64
0.30
OS>4
0.46
_
(0.18~0.65)
(O.-GO.Y)
(0.35-0.59)
99.29
YY.5 I
98.47
I.001
98.69
(0.49-I
.Ol )
99.49
293
1851
3496
33.3 I
480
(250-320)
(I 150-2480)
(273X-4352)
(2050-X450)
(420-5
305
369
42
(150-580)
( 170-660)
(32-52)
27
IO1
(22-30)
(65-
186)
209
582
1095
(167-314)
(443-909)
(884-
379 I
)
1456)
573
1 I4
(305-757)
(60-39
32
23
22
I8
(22-48)
( < 15-40)
( < 15-66)
(15-32)
I87
I3
II
II
11
(88-214)
(9-31)
(8-25)
(X-30)
(10-12)
127
88
(X2-148)
(70-l
I IO
203
57
13)
(72-168)
(93 -338)
(45-125)
157
I71
12
19)
(125-191)
(8X-350)
(30-60)
.?I
79
(25-48)
(56-l
IO
49
I I2
(g-12)
(33-64)
(34-261
J
206
31
(35-459)
(20-57)
9
9
I6
15
I0
(7-10)
(5-13)
( I O-30)
(I l-2.5)
(8-16)
:;3-16)
22
38
(21-25)
(33-J
;:o-75,
21 (12-33)
Zn
19)
74.20
(4.10-4.72)
Ga Pb
18)
75.80
(4.44-4.84)
(238-45 Sr
(0.06-O.
2
4.54
(0.75Total
15)
I
47
98
(14-66)
(42-
186)
3’)
44
(33-463
(40-52)
220
I57
48
(88-397)
(34-274)
(42-64)
146
l5Y
42
(65-220)
(53-213)
(32-58)
IJ
2385
850
251
40
557
170
49
840
26
100
92
175
96
40
42
21
IO)
I)
40 _
_
64
I9
_
_
H.M. Abdalla et al./Joumal
of Geochemical Exploration 57 (1996) 127-138
131
Table 1 (continued) Monzogranite N U
Alkali-feldspar
Amazonitized microgranite
Albitized microgranite
Abitite
microgranite
1
2
7 (7)
8 (8)
6 (26)
9 (27)
6 (7)
6 (5-8)
12 (g-20)
23 (12-1385)
7iP2-,095,
12 (8-18)
-
-
;;7-28)
:1$-52)
&-115)
(6253-95)
$3-38)
-
Th
N: number of samples analyzed for major and trace elements (in brackets). 1: The Arabian specialized-granites, Du Bray et al. (1988). 2: The low-Ca granites, Turekian and Wedepohl (1961). ’ Total iron as Fe,O, except in columns 1 and 2.
and fluorite veinlets. Microthermometric analyses were performed utilizing a Linkham TH 600 heating/freezing stage.
4. Geochemistry 4.1. Urn Ara monzogranites
and alkali-feldspar
mi-
crogranites
Compared with the low-Ca granites (Table l), it is clear that the alkali-feldspar microgranite of the Urn Ara apogranite exhibits a specialized geochemical signature of evolved felsic magma. This is reflected by low contents of the compatible elements Ti, total Fe, Mg, Ca, Sr and Ba and high to enhanced contents of the incompatible elements Rb, Li, Nb, Ga, Y, Pb, Zn, U, Th and F. These elements are commonly enhanced in rare metal granites (Stemprok, 1979). The geochemical signature of the alkali-feldspar microgranite was confirmed when compared with the specialized granites of the Arabian Shield which are known to be associated with rare metal mineralization (Du Bray et al., 1988). The monzogranite has high concentrations of total Fe, Ca, Ti, Mg, P, Zr, Ba and Sr and less silica. Both the alkali-feldspar microgranite and the monzogranite are marginally metaluminous, but with evolved characteristics for microgranite and calcic nature for the monzogranite. 4.2. Geochemistry
of alteration
zones
Each of the alteration zones has a distinctive alkali ratio and a specific trace element signature.
Major oxides are in wt.% and trace elements are in ppm.
Amazonitization proceedd through replacement of K for Na and Ca of plagioclase and is distinguished by considerable gain of Rb and Pb and enhanced concentrations of Nb, Zn, Li and F. Loss of Na and desilicification occurs when the process becomes extremely pervasive. As K-metasomatism progressed, the compositions plot away from the alkalifeldspar microgranite field toward the Or apex (Fig. 2). Albitization proceeded through the replacement by Na for K and Ca of preexisting feldspars. The elements Mn, Li, F, Ga, Al, Na and the rare metals Nb, Zr, Y, Th and U were significantly increased whereas Mg, Ca, Ba, Fe were lost during this process. An accompanying decrease of Rb with K is also con%-rned. Fig. 3 depicts well the main chemiQ?
o Monzogradtes
b Alkali-feldsparmicrogranites . Zinnwldite + Zinnwaldite x Albitites
Ab /
amamnltied microgranites albidzed microgranites
1
or
Fig. 2. Normative quartz-albite-orthcclase composition of Urn Ara granitoids. The ternary minima for 1 kb H,O pressure are from Tuttle and Bowen (1958) and the stars represent the ternary minima for granite system with 0 and 4% F (from Manning, 1981). Vector A shows the migration of ternary minima as F content increases, whereas B shows the migration as K content decreases. The shown trends of granitic alteration types are from Stemprok (1979).
132
z
0
A
L
i
Log K*Mg*SriRh2’l,i Fig. 3. The lOOOo~Ga/A1
versus log K.
Mg Sr/Rh’
Li diagram
of Urn Ara granitoids. The arrow indicate\ the K and Na mctasomatic trends in the Urn Am apogranite. Symbols as in Fig. 2
cal changes during the albitization and shows that both Rb and Ga are highly sensitive to the process of albitization and can be used as a good discriminators for distinguishing granitic rocks associated with postmagmatic alteration and the cogenetic rare metal mineralization of Zr, Y. Nb and U. The plot of U versus Th content (Fig. 4) of Urn Ara granitoids shows four populations of data points. The monzogranite shows the lowest U and Th contents whereas the alkali-feldspar microgranite shows enhanced contents of both elements. with average Th/U ratio = 3.0. The amazonitized microgranitc exhibits an expanded enrichment of Th relative to L;. The Th and U of the amazonitized and some of the albitized microgranites may be included within the structure of the refractory accessory minerals. It is worthy to note that besides the occurrence of‘ thorite and monazite (with 0.53-10.5% ThO,) the manganocolumbite and Hf-bearing zircon contain LIP to I .6% UO, (Abdalla et al., 1994). On the other
l-a @ volume% z-95
2
l-b
hand. most of the albitized microgranite samples (population III) and population IV display distinctive enrichment of U relative to Th. High content of U as much as 1385 ppm is essentially due to the incorporation of U mineralization occurring as disseminations and fracture-fillings. In other words. although the high levels of U are evidently related to the albitization process (the average U content is 76 ppm in the Na-metasomatized zone). the occurrence of anomalously high U contents as fracture-filling in both the K- and Na-metasomatized zones indicates the structure-controlled nature for the localization of uranium mineralization within the apogranite. When the albitization process was pervasively overwhelming (i.e. in albitites) there was considerable leaching of Ti, Ca. K and Si accompanied by a drastically sharp decrease in Li. Rb and rare metals. Only Ga, Al and Na were significantly enriched in the albitites. Fig. 2 shows that the plots of albitized microgranite shift toward the albite pole and extend to higher Na content in the albitites (vector B) and show no correlation with the experimental pseudo-
3
~5 vol me%
Fig. 5. Sketch showing the fluid inclusion types encountered !n Um Ara apogranitr facics. aqueous type, (3) multi-phase aqueous type. (4) two-phase CO,( &CH, 5 commonly show a separation of CO,
phase into inner vapour CO,
I-H,0
(NaCI),
and an outer CO,
( iah)
monophase aqueous type, (2) two-phase
(5) monophase CO,( +CH,)-rich liquid phase.
type. Types 4 and
2
Primary
PS.
F’S, S
HOSI
mmeral
Quartz
Quartz
Quartz
Rock
type/zone
Amazonitized
mlcrogranite?
and nlbitited
A Amazonitized
2)
2)
2)
? 31
2.3)
1s
IS
IO
9
IO
36
K
16
35
which
Clath..
-
phase
impregnated
of Urn
(0.76-O
(0.55wI.78)
The
87)
85)
T,
( - 5h.h
in
(-
(-
Fig.
56.9
56.6
to
- 58.3)
- 5M
I)
Volume
melting
temperature
and mode of homogenrzatio”
of Heyen
5.
to - fitI 4)
to
I to - 60.1)
CO?
uamg the method
ure given
Ice is the final
TN CO,
alteratto”.
using
(1979).
by Fe- and Mn-oxidc
typea
(0 K-0.25)
Ii I‘,
18) ( ~ 58.
10)
((1.0 10 0.08)
(0.08-O.
(0 o-o.
X CH,
10
apogranire
wab eslimdred
inclusion
(0.5~0
+ CO?): of CoIlin\
of
Ara
phase
@m/cm’)
CO,
Density
9
(0.50-0.75)
facies
is estimated
respectively.
the method
of the CO,
X CH,
(0.04~0.22)
(0.03-0.22)
= CH,/(CH,
too
Inclusiona.
applying
Drrlstty
fraction.
arc intcnaely
T,
temperature
molar
IO0
(20-50)
-
(20-50)
co,
x CO?
8
in the different
Vol.?,
7
incluhicmb
Number
6
p~ctv.lusecondary
temperature.
microgranites
melting
find
(fh.
(lb.
(lb.
(5)
(41
(lb.
(5)
(4)
(lb.
tYPC
Inclu>wn
s
the en,~mined
is the CH,
sec~“dav
X CH,
homoyenlzatlo”
the cldthrate
estimated
1s the totnl
total
usng
T,
phase
C H,O).
(0 pr~rn~y.
= CO,/(COz
refer
s
X CO,
I’, S and PS
’
(Ssl5)
oxtdes
Quart?
(5 to
(5-30)
Fe & M”
P. s. PS
P, S, PS
(3-S)
(S-25)
(S-25)
(3-8)
(S-20)
(5%?5)
for
> SO)
Fluwtc
Quartz
s
s
s
S
S
(pm)
Sk,
4
analyais
veinlets
Fluorite
veans
PS,
QUdfll
Quartz
PS,
Quartz
microgranite\
PS.
Quartz
Albirirrd
PS.
XCOdWy
pseudo-
/secondary
3
2
of mlcrothermometric
I
A summary
Table
12
to -3.2) I 2 to - 3.4)
I2
CO,
of CO, of froxn
13 14
I.6-30.8)
phase.
CO,
15
of the C02(+CH,)-H,O
to 425)
total
16
(NaTI)
wt.%
fraction. inclusions
was
of the CO,
molar
(I-6)
(2-5.1)
1.5)
-2fl.X)
(2-5.0)
(6-1
(6.3
(7 to 10.5)
I)
equiv.)
(7 to 22.
(N&I
Salinity.
tcmpcrkre
L\ rhe CO,
(145-240)
(IYO-300)
(195-310)
(275-350)
(240-365)
(295-420)
(285
T,
homogeniratwn
at 40°C.
XCOL
to 30.91 (9.5-30.4)
(22.4
(loto22.1)
(2
Tt,
i\ the partial Saliniry aqueous
phawz
T, CO2
ehtimatc
(J-k-7OJ
2)
Clrth
(4.2-b
T,
is a wstill et al. (1982).
‘i
t - 0.5 to - 4.0)
(~
(p
-18
4.6 to - 20)
ice
p.JlO
(-
T,
ternary minima for the granite melt-HzO-F (vector A). The monomineralic albitites with albite of An = 0 (EPMA analysis) can not be considered as a magmatic product.
5. Fluid inclusion
study
The fluid inclusions were studied in quartz from the amazonitized and albitized microgranites as well as in quartz and fluorite of the later fracture-filling quartz veins and fluorite veinlets. Genetic classification of these inclusions (primary. secondary and pseudosecondary) is very subjective especially in alteration zones like those in the Urn Ara apogranite. Therefore, an alternative scheme based on phase proportions at room temperature has been used.Fig. 5 shows a sketch of inclusion types encountered in Urn Ara apogranite facies, and the results of microthermometric analyses are summarized in Table i. The large compositional and volume variation of the COZ( f CH,) phase in the inclusions of the same population suggests a heterogeneous entrapment of fluids that had unmixed into H,O(NaCl)-rich fluid and CO,-rich vapour at approximately I .S kbar 1
60
r~ Inclusions in quartz of the Amazonitized mirrogran~te.~ +Inclusions in quartz of the Nbitized microgranites. l Inclusions in ouarn of the quark veins and fluorite Of the fluorite veinlets. Alnclusions in quartz of the Amazonitized and Albitired microgranites which are intensively impregnated h? Fe- and Mn-oxide alteration. .W?‘/
(Bowers and Helgeson. 1983). The type 4 inclusions, CO,( + CH d)-H 20(NaCl), of the amazonitized and albitized microgranites plot mainly between the HIO-CO, solvi for 6 and 12% NaCl at 1.5 kbar (Bowers and Helgeson, 1983). indicating the reliability of salinities estimated by the clathrate melting method. The spread of values of temperature of total homogenization and salinity in the aqueous inclusions of the amazonitized and albitized microgranites (Fig. 6) indicates that the subsolidus alteration processes have proceeded as the temperature (and to less extent the salinity) of the fluids decreased. Inclusions in the amazonitized microgranites homogenized in the range of 285-425” C with salinities of 7-22.1 wt.% NaCl cquiv.. indicating high to moderate temperature and saline fluids in the early stage. However, the microgranite samples which are intensively impregnated by later Fe- and Mn-oxide (most commonly developed along fractures) contain voluminous trails of small secondary aqueous inclusions. The homogenization temperatures and salinities of these inclusions suggest that the Fe- and Mn-oxides alteration might prevail at lower temperatures (14%240°C) and salinities (I-6 wt.% NaCl equiv.) during the waning hydrothermal stage contemporaneous with the release of pressure and fracturing of the solidified microgranite cupola. However, some of the aqueous inclusions show temperatures of first melting below the cotectic temperature of H,O-NaCl ( - 30.8” C> in the range of - 24.0 to - 26.8” C indicating the presence of other cations besides Na’. The presence of vapour- and liquid-rich inclusions (type I) may indicate that a local boiling had been occurred.
6. Discussion
100
200 flomogenlratlon
300 temprature.’
400
500
C
Fig. 6. Salinity versus total homogenization temperature for aqurous fluid inclusions in Urn Ara apogranite facies. The halite saturation curve in H20-NaCI system is taken from Sourirajan and Kennedy ( 1962).
The following experimental review is of considerable importance for understanding the postmagmatic metasomatic processes affecting the Urn Ara apogranite. Balitskiy and Komova (1971) found that solutions with a molar K/(Na + K) ratio less than 0.13 at 300°C and 784 bars. can albitize microcline. The rate of the process increases with increasing pH from 5.5 to 10 whereas, microclinization proceeds at higher ratios. According to Holland (1972) signifi-
H.M. Abdalla et al. /Journal
ofGeochemica1 Exploration 57 (1996) 127-138
cant concentrations of alkalis can partition into an aqueous fluid of relatively high chloride content. Potassium and Na have a fluid/melt partition coefficient > 1.0 for fluids with > 3.0 molal Cl (for K) and with > 2.2 molal Cl (for Na). Chlorine molalities of l-4 are expected within a highly evolved magmatic system because Cl partitions almost exclusively into aqueous fluid relative to silicate melt (Burnham, 1979). However, Foumier (1976), concluded that shallow intrusive magmas can yield a chloride solution which due to low pressure unmixes into gas plus brine. Reaction of that gas or brine will result in potassic feldspathization (see Foumier, 1976). The geologic setting, petrographical criteria and evolved geochemical signature all strongly indicate
the anorogenic origin for the high-level alkali-feldspar microgranite cupola. The lower unaltered alkalifeldspar microgranite plots in the within-plate granite field (WPG) of the petro-tectonic scheme of Pearce et al. (1984). The evolved nature of the alkali-feldspar microgranite suggests melting of older anhydrous crust by ascending mafic magma underplating the base of extended stabilized Late Precambrian crust of the Nubian Shield. However, the extraction of a monzogranite protolith which was emplaced during the post-erogenic phase led to a residual melt depleted in Ca, Ba, Sr and enriched in Zn, Pb, Ga, Li, Rb, high charge cations and an anhydrous volatile phase (e.g., F, Cl and CO,). The presence of F, Cl and CO, and the alkalis Na, K, Li and Rb in the melt allowed
/ ,Q ,F
Fracture. Quartz brecclas and veins. Fluorite veinlets. Q DegassLug breccias. CXPMiarolitlc cavities. < Residual uranium-bearing fluid “‘RCMn’(Assldated with Fe- and Mn-oxide alteration). Cl Zone of Na-metasomatism (Albitizatfon). CZJZone of K-metasomatism (Amazonitization). IBl Zone of maximum exsolved fluid. C!Z!iAlkali-feldspar rnlcrogranite. lloB Urn Ara monzogranite. E!J Urn Dubr Dokhan volcanks. Fig. 7. An autometaosomatic
alteration model of Urn Ara apogranite.
135
s 5 8. 9
The Roman numbers refer to the evolution stages of the apogranite.
stable complexing of the high charge elements. The elements (Na, Li, Rb. and U) and (Zr. Nb, and Y) are efficiently stablized by F- and Cl-rich residual fluid phase. respectively (Hildreth. 198 1). Continued extension of the continental crust permitted this evolved and less dense magma to rise and produce shallow magma chambers. where it solidified into alkali-feldspar microgranite cupola co-existing with an exsolved supercritical fluid. The rare-metal granite magma (due to the depressed granitic melt minima caused by the presence of H,O, CO,. F. Cl and rare alkalis which result in fuzzy transition from magmatic to postmagmatic condition) evolve fluid phase at a late stage in its crystallization history. This fluid phase was initially confined to grain boundaries, microstructures and vugs (Pollard and Taylor, 1986). The fluid phase was further separated into a highly mobile volatile phase and a low denisty aqueous fluid. Textural characteristics e.g., miarolitic cavities, degassing breccias and the pegmatite nests are good evidence for early separation of a volatile phase. It is worthy of note that the Fe/Mn ratio decreases in mica, spessartite, columbite and NbZn-Mn-ilmenite from the amazonitized to the roof albitized microgranite, whereas, the F and Li of the mica increase upward, reflecting the role of upward volatile diffusion of the rare elements aided by their tluorophile affinity (AbdaIla et al., 1994). According to the available experimental data, the early high temperature reactions proceeded through K-metasomatism where K’, Rb’ and Pb’+ in the fluid replaced Na and Ca of early existing feldspars. The high contents of Rb and Pb of the amazonitized microgranites are found to be incorporated within the K-feldspar (green amazonite) structure (Abdalla, 1996: Abdalla et al., 1996). The released Na and Ca during the K-metasomatism were stablized in the albite and the common accessory and veinlet tluorite in the roof zone of albitization under high CITY environment. These reactions may proceed towards increasing Al-O, and Na,O with contemporaneous desilicification when the aibitization becomes extensively overwhelming. However, Alexandrov et al. ( IOtiS) concluded that when the pH of the fluid changes from 5.7 to 7.5. uNh/uTa increases from 0.8 to 14 and accordingly the Nb/Ta ratios in tantaloniohates will vary. The average Nb/(Nb + Ta) atomic ratio in columbites of
Urn Ara apogranites is 0.99. so a basic and relatively high temperature ( > 425” C, the maximum measured homogenization temperature) medium can be assumed for their deposition, as the stability of Ta-complexes is favored by low temperature conditions (Wang et al.. 1982). The evolution of postmagmatic metasomatic processes is summarized in Fig. 7. This model is closely consistent with the apogranite concept of Beus et al. ( 1962) and Taylor and Pollard (1988). It is thus thought that a large amount of volatiles were exsolved at the completion of crystallization of the alkali-feldspar microgranite cupola and concentrated (aided by their hydrophile to fluorophile nature) at the apical parts of the microgranite cupola (stage Il. Fig. 7). This fluid phase is initially confined to grain boundaries. microstructures and miarolitic vugs. However. the composition deduced from the fluid inclusions indicate that the emerged mineralizing fluids were moderately saline (daughter halide crystals were not observed), but were rich in CO,. The early encountered high temperature reactions proceeded through amazonitization (K-metasomatism) followed by albitization (Na-metasomatism) at the roof zone (stage III, Fig. 7). According to Pichavant (1983). the F-rich fluid phase in equilibrium with two alkali feldspars is strongly enriched in Na compared with Cl-rich fluid. Thus. any process leading to reduction of F content of the fluid (such as stablizing it into F-rich minerals, e.g. fluorite and zinnwaldite) will result in the albitization of the wall rocks. It seems that the loss of CO, (and other volatiles. stage V, Fig. 7) by local boiling and effervescence (unmixing of CO,-H,O-salt tluids) related to pressure release and-decreasing solubility ofCO, concurrently with falling temperature. decreasing salinities and the increased pH of the fluids led to destablization of rare element complexes in the tluids. favoring their deposition; Y as xenotime, Zr as rircon. Nb as columbite, Th as thorite. and Zn into NbMn-rich ilmenite. The close association of the fracture-filling uranophane and calcite in the amazonitized and albitized microgranites may indicate that the deposition of carbonates and uranium mineralization occurred during such a period of CO, loss in the waning hydrothermal stage (Abdalla et al.. 1994). However. the development of the fracture system the
H.M. Abdalla et al./ Journal of Geochemical Explomtion 57 (19961 127-138
(stage IV, Fig. 7) appears to be late in the crystallization history of Urn Ara apogranite. This is evidenced by the pervasive alteration of the stock, the predominance of the disseminated type of rare metal mineralization, and the absence of the greisenized and mineralized veins. It is probable that the marked decreasing of salinity and homogenization temperatures detected in the later fracture-filling quartz and fluorite and the Fe (M&impregnated microgranites is related to minimal contribution of meteoric water encountered during the late fracturing event.
7. Conclusions The geologic setting, textural, geochemical and fluid inclusion studies indicate that the zonal pattern of Urn Ara apogranite has been developed as the result of subsolidus reactions of an exsolved postmagmatic fluids with the already consolidated alkali-feldspar microgranite cupola. The gross enrichment of the rare elements towards the roof Na-rich zone indicates that these elements have been stablized by a highly mobile diffusing volatile phase (i.e., F, Cl, Na, K, Rb and Li) during their transportation. However, the decreasing temperature and salinity and the loss of CO, (and the other volatiles) and increasing pH are considered the essential factors for rare metal localization at Urn Ara apogranite. Some of the delineated geochemical anomalies deserve a further exploratory programme. It is suggested that the multiplicative ratios 10000 . Ga/Al and log{(K . Mg . Sr)/(Rb’ Li)) can be used as good discriminators for distinguishing granitic rocks associated with postmagmatic alteration and the cogenetic rare metal mineralization of Zr, Y, Nb and U. The greater the Ga/Al and the negativity of the log ratios, the greater the rare metals potentiality of the concerned granitoids. From the exploration point of veiw. the present study draws attention to the necessity of assessment of the uranium potentiality of the metasomatically albite- and (Nb, Y, Zr)-enriched granitoids. Compared with the other post-orogenie granitoids (including the magmatically albiteenriched ones) in the basement complex of Egypt, the metasomatically albite-enriched granitoids exhibit distinctive airborne radiometric anomalies (e.g., Hassan. 1973). Besides the petrographical and geo-
137
chemical criteria discussed in the present study, the very high Nb/Ta ratio may assisst in recognizing these granitoids.
Acknowledgements H.M.A. is indebted to Prof. Nabil El-Hazek and the group of Aswan Camp of Nucl. Mater. Authority, Egypt. Special thanks are due to Prof. S. Kanisawa of Tohoku University for his kind help during the F and Li analyses. Drs. H. Miura, H. Nakagawa and Mr. S. Terada of Hokkaido University are also acknowledged for help during XRF and EPMA analyses. An earlier version of the manuscript was benefitted greatly from suggestions of Drs. R. Taylor and P. Pollard of James Cook University and Dr. G. Taylor of CSIRO, Australia.
References Abdalla, H.M., 1996. Geochemical and mineralogical studies at Urn Ara rare metals prospect, Southeastern Desert, Egypt. Unpub. Ph.D. thesis, Hokkaido University, Sapporo, 178 pp. Abdalla, H.M., Matsueda, H., Ishihara, S. and Miura. H.. 1994. Mineral chemistry of albite-enriched granitoids at Urn Ara. Southeastern Desert, Egypt. Int. Geol. Rev., 36: 1067-1077. Abdalla, H.M., Ishihara, S. and Matsueda, H., 1996. On the albite-enriched granitoids at Urn Ara area, Southeastern Desert, Egypt. 2. Feldspar chemistry, microstructures and mineralogy. In preparation. Alexandrov. I.V.. Krasov, A.M. and Kochnova. L.N., 1985. The effects of K. Na and F on rock-forming mineral assemblages and the formation of tantaloniobate mineralisation in rare-element granite pegmatites. Geoch. Int., 22: 85-94. Balitskiy. V.S. and Komova, V.V.. 1971. Influence of physicochemical factors upon the intensity and rate of the process of microcline replacement by albite. Geokhimiya, 3: 332-338. Beua, A.A., Severov, E.A.. Sitnin, A.A., and Subbotin, K.D.. 1962. Albitized and Greisenized Granites (Apogranites). Acad. Sci. Press, Moscow, 196 pp. (in Russian). Bowers. T.S. and Helgeson, H.C., 1983. Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H,O-CO,-NaCI on phase relations in geologic systems. Equation of state for H20-CO,-NaCI fluids at high pressures and temperatures. Geochem. Cosmochim. Acta, 47: 1247-1275. Burnham, C.W., 1979. Magmas and hydrothermal fluids. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits. 2nd ed.. Wiley, New York, pp. 71-136. Collins, P.L., 1979. Gas hydrates in CO,-bearing fluid inclusions and the use of freezing data for estimation of salinity. Econ. Geol., 74: 1435- 1444.
Du Bray, E.. Elliot, J.E. and Stuckless. J.S., 1988. Proterozoic peraluminous granites and associated Sn-W deposits. Kingdom of Saudia Arabia. In: R.P. Taylor and D.F. Strong (Editors). Recent Advances in the Geology of Granite-related Mineral Deposits. Can. Inst. Min. Metall.. 39: 112-156. Foumier, R.O.. 1976. Exchange of Na+ and K ’ between water vapor and feldspar phases at high temperature and low vapor pressure. Geochim. Cosmochim. Acta, 40: 155% 156 I. Hassan, M.A., 1973. Geology and geochemistry of radioactiic columbite- bearing psammitic gneiss of Wadi Abu Rush&d. Southeastern Desert. Egypt. Ann. Geol. Surv. Egypt. 3: 207775 ___ Hassan, M.A. and Hashad. A., 1990. Precambrian of Egypt. In: R. Said (Editor). The Geology of Egypt. Balkema. Rotterdam and Brookfield Pub., pp. 201-245. Heyen, G., Rambor, C. and Dubessay. J.. 1982. Simulatton des equilibres de phases dans le systeme CO, -CH, en dessous de 50°C et de 100 bar. Application aux inclusions fluides. C.R. Acad. Sci. (Paris) Ser. II. 294: 203-206. Hildreth. W., 1981. Gradients in silicic magma Chambers: Implications for lithospheric magmatism. J. Geophys. Res.. X6(8II): 10153-10192. Holland. H.D.. 1972. Granites. solutions and hase metal deposits. Econ. Geol.. 67: 28 I-301. Ishihara. S.. 1977. The magnetite-series and ilmcntte-eerie\ 01 granitic rocks. Min. Geol.. 27: 293-305. Manning. D.A.C.. 1981. The effect of fluorine on liquidus phase relationships in the system Qz-Ab-Or with exec\ water at I kb. Contrib. Mineral. Petrol.. 76: 206-2 IS. Pearce. J.A.. Harris, N.B.W. and Tindle, A.G.. 1984. Trace elcment discrimination diagrams for the tectonic interpretation ot granitic rocks. J. Petrol.. 25: 956-983. Pichavant. M.. 1983. (Na, K) exchange between alkali feldspar\ and aqueous solutions containing borate and fuoride anions.
experimental results at P = I kbar. In: 3rd NATO Adv. Stud. Lnst. Feldspars and Feldspathoids, Rennes, p, 102. Pollard. P.J.. 1983. Magmatic and postmagmatic processes in the formation of rocks associated with rare-element deposits. Trans. Inst. Min. Metall. (Sect. B: Appl. Earth Sci.), 92: 1-9. Pollard. P.J. and Taylor, R.G.. 1986. Progressive evolution of alteration and tin mineralisation: Controls by interstitial permeability and fracture-related trapping of magmatic fluid reservoirs in tin granites. Econ. Geol.. 81: 1795-1800. Schwartz, M. O., 1992. Geochemical criteria for distinguishing magmatic and metasomatic albite-enrichment in grdnitoids examples from the Ta-Li granite Yichun (China) and the Sn-W deposit Tikus (Indonesia). Mineral. Deposita, 27: IOI-
Iox. Stemproh. M., 1979. Mineralized granites and their origm Episodes. 3: 20-24. Sourira,jan. S. and Kennedy. G.C., 1962. The system H,O-NaCI XI elevated temperatures and pressures. Am. J. Sci.. 260: 11%131. Taylor, R.G. and Pollard, P.J.. 1988. Pervasive hydrothermal alteration in tin- bearing granites and implications for the evolution of ore-bearing magmatic fluids. In: R.P. Taylor and D.F. Strong (Editors), Recent Advances in the Geology of Granite-related Mineral Deposits. Can. Instn. Min. Metall. Spec. Vol.. 39: 86-95. Turekian. K.K. and Wedepohl, K.H.. 1961. Distribution of the elements in some major units of the Earth’s crust. Bull. Geol. Sot. Am., 72: 1755191. Tuttle. O.F. and Bowen. N.L., 195X. Origin of granite in the light of experimental studies in the system NaAlSi,O,KAlSi,O,-SiO?-H20. Geol. Sot. Am. Mem. 74. I53 pp. Wang, Y.. Jitian, L. and Fan, W., 1982. Geochemical mechanism of Nb-Ta mineralization during the late stage of granite crystallization. Geochem. Beijing, I: 175-185.
Journal of Geochemical
Exploration
57 (1996) 127-138
On the albite-enriched granitoids at Urn Ara area, Southeastern Desert, Egypt. 1. Geochemical, ore potentiality and fluid inclusion studies H.M. Abdalla a, S. Ishihara b, H. Matsueda b, A.A. Abdel Monem a a Nuclear Materials Authority, Cairo, Egypt b Hokkaido Uniuersiv, Sapporo 060, Japan Received 9 May 1995; accepted
14 May 1996
Abstract A radioactive and metasomatically albite-enriched microgranite (apogranite, 556 Ma) stock was emplaced at the northern contact of Urn Ara monzogranite pluton (589 Ma). Rock/fluid interaction within the apogranite is well displayed as petrographical and geochemical zonal patterns in response to K- and Na-metasomatism. followed by Naf The metasomatic reactions commenced with addition of K+, Rb+ and Pbzf (amazonitization) (albitization) and finally with Mn- and Fe-oxide alteration. A gradual and gross enrichment of Nb, Y, Zr, Li, Zn, U, Th and F occurs from the amazonitized to albitized zone. Microthermometric study of fluid inclusions in quartz of the amazonitized and albitized microgranites indicates that high to moderate temperature, saline fluids interacted with the solidified microgranite cupola in the early metasomatic processes. Inclusions of the K-enriched zone homogenize in the range of 285-425” C, with salinities of 7-22.1 wt.% NaCl equivalent. However, inclusions in the later fracture and joint-filling quartz veins and fluorite veinlets homogenize in the range of 190-310°C with salinities of 2-5.1 wt.% NaCl equivalent. It is suggested that the presence of K+, Naf and F- in the ore fluids was essential to stablize complexes of the rare metal elements during extraction and transportation. In contrast, contemporaneous decrease of temperature and salinity, loss of CO, and increasing pH due to decreasing pressure are considered the essential factors for localization of disseminated mineralization of Zr, Y, Nb, Zn and Th and the fracture-filling with U in the apical parts of the Urn Ara apogranite stock. Keywords: amazonitization:
albitization;
rare metals; apogranite;
fluid inclusions
1. Introduction One of the pronounced radiometric anomalies in the Southeastern Desert of Egypt is associated with a metasomatically albite-enriched granite stock (Le., Urn Ara apogranite, 556 Ma) emplaced at the northem contact of the Urn Ara monzogranite pluton (589 Ma) (Fig. 1). The metasomatic nature of the apogranite was characterized according to the criteria of 03756742/96/$15.00 Copyright PII SO375-6742(96)00029-5
(1983) and Schwartz (1992). Although the time span between the emplacement of the two bodies is about 33 Ma (K-Ar age of mica separates, present study), both of the granitic masses represent post-erogenic, younger granitic activity that intruded the Egyptian Shield between 620 and 530 Ma during cratonization and sialification (Hassan and Hashad, 1990). The present study aims at characterizing the spe-
Pollard
0 1996 Elsevier Science B.V. All rights reserved.
Stream sediments. Uranium mineralization. Quartz breccia d&e. Zinnwaldite albitized microgranite. Zionwaldite Alkali-feldspar microgranite. Monzogrtite. Dokhan volcanics. Gtleissose granite. Melange rocks. Metavolcanics. Serpentmites. Fault. Fig.
C
I. Geological map ol lJtn Ara
cialized paragenetic type of the apogranite body and associated mineralization, assessing the role of postmagmatic alteration in the distribution of rare metal mineralization, depicting the evolution of the postmagmatic fluids, and elucidating the mechanism by which mineralization was formed.
2. Petrography The monzogranite has a generally homogeneous, pinkish, equigranular, coarse-grained texture, although in places a porphyritic variety is present. It is
apogranite
composed of intermediate microcline and microcline perthite. plagioclase (An ,j Zx1. quartz, and biotite. Rare flakes of muscovite are also observed. Sphene and magnetite are the common accessory minerals indicating that the granite belongs to the magnetite series granitoids of Ishihara (1977). Scarce mafic xenoliths (approx. 10 cm in diameter) are also present in these granites. Emplacement of the Urn Ara apogranite was primarily controlled by a system of faults and fractures. Rock/fluid interaction within the apogranite is well displayed by a petrographic zonal pattern including a lower unaltered miarolitic pink alkali-feldspar micro-
H.M. Abdalla
et al./.Ioumal
ofGeochemica1
granite, a middle zinnwaldite amazonitized microgranite (i.e. zone of microclinization or amazonitization), and a roof zone of zinnwaldite albitized microgranite (i.e. zone of albitization). The alkali-feldspar microgranite (grain size < 2 mm) is characterized by well developed miarolitic vugs 3-10 cm in diameter, filled with fragmented quartz in a felsitic matrix, and degassing breccias (fragmented microgranites cemented by quartz and fluorite). The mineral constituents are microcline, microperthite, albite (An,_ ,*), graphic quartz and minor white mica. The accessory minerals are fluorite, ilmenite, pyrite and rare columbite. Thus, the original magma of the apogranite may belong to the ilmenite series grantoids of Ishihara (1977). The zone of amazonitization is characterized by the development of highly- to fully-ordered green microcline (imparting to the rock a pale to deep green color), zinnwaldite and spessartine garnets replacing pre-existing minerals. A detailed description of the paragenetic sequence of formation of minerals during the magmatic and postmagmatic stages is given in Abdalla et al. (1994) and Abdalla et al. (1996). The amazonite exhibits the cross-hatched twinning and the pericline twins of the replaced albite. Accessory minerals are fluorite, columbite, Zn-Mn-Nb-rich ilmenites, zircon, xenotime, thorite, monazite and acicular uranophane. Violet and colorless fluorite veinlets are locally abundant. Local degassing breccias and pegmatite nests are common. The breccia is composed of fragmented euhedral crystals of fluorite (approx. 2 cm) embedded in a matrix of fine-grained felsic minerals and fluorite. The pegmatite nests are composed of large crystals of amazonite (approx. 3 cm>, pseudohexagonal crystals of zinnwaldite (approx. 2 cm) and massive quartz. The zone of albitization is characterized by abundantly developed subsolidus clusters of fine-grained albites (An,_,) replacing early formed microcline along cleavage and twinning planes. In addition to the development of zinnwaldite and spessartite, the same accessory minerals of the amazonitization zone increase in abundance. Many quartz veins and quartz breccia dikes (up to 10 m wide) traverse the apogranite and strike E-W. In places with extreme Na-metasomatism, the albitite rock has a patchy and erratic distribution of limited area1 extent. It is white, fine-grained, and
Exploration
57 (1996) 127-138
129
made up of > 80% albite with an average grain size < 200 Km, corroded quartz grains 1 mm in diameter and a few small relicts of microcline that are replaced in part by the fine albite laths. The alteration sequence concluded with Fe- (hematitization) and Mn-oxide alteration along later fractures and joints and as local impregnations. Field observations and microscopic (in reflected and transmitted light) examinations revealed the presence of three modes of occurrence for the rare metals mineralization associated with the Urn Ara apogranite stock. These modes are: (1) disseminated mineralization of manganocolumbite, Nb-Mn-Znrich ilmenite, Hf-rich zircon, xenotime, thorite, monazite; (2) uranium mineralization (essentially uranophane) occurring as disseminated acicular crystals and most commonly as fracture-filling; and (3) minor placer occurrences of columbite, zircon, xenotime, and thorite. Two modes of occurrences are exhibited by the uranophane in the Urn Ara apogranite; disseminated in the albitized microgranite and fracture-fillings in the amazonitized and albitized microgranites. The fracture-filling uranophane occurs as crystalline aggregates intimately intergrown with fine calcite crystals along joint and fracture surfaces. The fracture-filling uranophane is closely associated with late-stage Fe- and Mn-oxide alteration. Disseminated uranophane occurs as bright lemon-yellow acicular crystals associated with clots of secondary albite laths in the albitized microgranites. Detailed mineralogical and chemical characteristics of the rare metals mineralization have been addressed by Abdalla et al. (1994).
3. Sampling and analytical techniques A detailed geological map was constructed by N-S traverses. The mapping was accompanied by a systematic sampling (250 samples) at 50-500 m intervals. Major and trace elements were detected by XRF using the fused bead and pressed pellet techniques, respectively. Fluorine contents were determined by selective ion electrode, and Li was determined by atomic absorption spectroscopy. Fluid inclusion studies were conducted using 30 doublypolished wafers, 100-500 pm thick of samples from the alteration zones, later fracture-filling quartz veins,
130 Table
I
Averages
and ranges
of major
Monzogranite
and trace element
compositiona
of Urn Am granitoids
Alkali-feldspar
Amazonitized
Albitized
microgranite
microgranite
microgranite
Abitite
N
7 (7)
8 (8)
6 (26)
9 (27)
6 (7)
SiO,
71.62
75.60
74.68
7.5.28
68.77
(70.79-72.45)
(74.05-77.50)
(7 I .98-76.75)
(73.2X--76.Y9)
(66.2-72.00)
0.09
0.04
u.04
0.01
(0.03-0.05,
(0.02-0.05)
(0.01-0.02)
TiO,
0.27
1)
(0.22-0.3 Al?03
(0.03-O.
16)
13.72
12.76
13.34
Ii.34
18.26
(13.45-13.99)
(12.55-13.41)
(12.93-14.08)
(13.03-13.75)
(14.73-19.35)
1.71
0.99
0.68
0.75
(1.30-1.87)
(0.61-1.27)
(0.5-0.9)
(0.44-I
MnO
0.12
0.10
0.16
MgO
0.35
0.09
(0.24-0.42)
(0.06-o.
0.96
0.49
0.32
(0.73-1.12)
(0.39-0.79)
(O.I2-0.8
FezO,
(0.05-O.
CaO Na,O K,O P*@ L.O.I.
F Li Rb Ba
Zr Y Nb Sn
0.20
13.70
13.60
0.3 I
0.36
0.81
(Fe,O,)
(0.25-0.46)
0.50
1.10
(FeO)
0.16
0.09
0.06
0.05
0. I I
0.27
0.30
0.7 I
4.62
3.48
4.26
5.06 0.14
.Ol )
(0. I-0.27)
(0. I-0.2)
(0.03-O.
0.05
0.05
0.02
(O.OI-0.12)
(0.01 -0. IS)
(0.01-0.02~
I)
0.04
0.27
0.07
(0.17-0.32)
(0.04-O.
I I)
12)
4.42
3.97
5.42
10.06
(3.78-4.07)
(5.05-5.75)
(7.76-10.65)
4.43
4.29
5.63
3.54
0.4 I
(4.37-4.49)
(4.07-4.68)
(4.86-6.52)
(3.03-3.87)
(0.12-1.75)
0.08
0.02
0.02
0.02
< 0.01
0.02
(0.05-0.09)
(0.01-0.03)
(0.0 I-0.03)
0.89
0.64
0.30
OS>4
0.46
_
(0.18~0.65)
(O.-GO.Y)
(0.35-0.59)
99.29
YY.5 I
98.47
I.001
98.69
(0.49-I
.Ol )
99.49
293
1851
3496
33.3 I
480
(250-320)
(I 150-2480)
(273X-4352)
(2050-X450)
(420-5
305
369
42
(150-580)
( 170-660)
(32-52)
27
IO1
(22-30)
(65-
186)
209
582
1095
(167-314)
(443-909)
(884-
379 I
)
1456)
573
1 I4
(305-757)
(60-39
32
23
22
I8
(22-48)
( < 15-40)
( < 15-66)
(15-32)
I87
I3
II
II
11
(88-214)
(9-31)
(8-25)
(X-30)
(10-12)
127
88
(X2-148)
(70-l
I IO
203
57
13)
(72-168)
(93 -338)
(45-125)
157
I71
12
19)
(125-191)
(8X-350)
(30-60)
.?I
79
(25-48)
(56-l
IO
49
I I2
(g-12)
(33-64)
(34-261
J
206
31
(35-459)
(20-57)
9
9
I6
15
I0
(7-10)
(5-13)
( I O-30)
(I l-2.5)
(8-16)
:;3-16)
22
38
(21-25)
(33-J
;:o-75,
21 (12-33)
Zn
19)
74.20
(4.10-4.72)
Ga Pb
18)
75.80
(4.44-4.84)
(238-45 Sr
(0.06-O.
2
4.54
(0.75Total
15)
I
47
98
(14-66)
(42-
186)
3’)
44
(33-463
(40-52)
220
I57
48
(88-397)
(34-274)
(42-64)
146
l5Y
42
(65-220)
(53-213)
(32-58)
IJ
2385
850
251
40
557
170
49
840
26
100
92
175
96
40
42
21
IO)
I)
40 _
_
64
I9
_
_
H.M. Abdalla et al./Joumal
of Geochemical Exploration 57 (1996) 127-138
131
Table 1 (continued) Monzogranite N U
Alkali-feldspar
Amazonitized microgranite
Albitized microgranite
Abitite
microgranite
1
2
7 (7)
8 (8)
6 (26)
9 (27)
6 (7)
6 (5-8)
12 (g-20)
23 (12-1385)
7iP2-,095,
12 (8-18)
-
-
;;7-28)
:1$-52)
&-115)
(6253-95)
$3-38)
-
Th
N: number of samples analyzed for major and trace elements (in brackets). 1: The Arabian specialized-granites, Du Bray et al. (1988). 2: The low-Ca granites, Turekian and Wedepohl (1961). ’ Total iron as Fe,O, except in columns 1 and 2.
and fluorite veinlets. Microthermometric analyses were performed utilizing a Linkham TH 600 heating/freezing stage.
4. Geochemistry 4.1. Urn Ara monzogranites
and alkali-feldspar
mi-
crogranites
Compared with the low-Ca granites (Table l), it is clear that the alkali-feldspar microgranite of the Urn Ara apogranite exhibits a specialized geochemical signature of evolved felsic magma. This is reflected by low contents of the compatible elements Ti, total Fe, Mg, Ca, Sr and Ba and high to enhanced contents of the incompatible elements Rb, Li, Nb, Ga, Y, Pb, Zn, U, Th and F. These elements are commonly enhanced in rare metal granites (Stemprok, 1979). The geochemical signature of the alkali-feldspar microgranite was confirmed when compared with the specialized granites of the Arabian Shield which are known to be associated with rare metal mineralization (Du Bray et al., 1988). The monzogranite has high concentrations of total Fe, Ca, Ti, Mg, P, Zr, Ba and Sr and less silica. Both the alkali-feldspar microgranite and the monzogranite are marginally metaluminous, but with evolved characteristics for microgranite and calcic nature for the monzogranite. 4.2. Geochemistry
of alteration
zones
Each of the alteration zones has a distinctive alkali ratio and a specific trace element signature.
Major oxides are in wt.% and trace elements are in ppm.
Amazonitization proceedd through replacement of K for Na and Ca of plagioclase and is distinguished by considerable gain of Rb and Pb and enhanced concentrations of Nb, Zn, Li and F. Loss of Na and desilicification occurs when the process becomes extremely pervasive. As K-metasomatism progressed, the compositions plot away from the alkalifeldspar microgranite field toward the Or apex (Fig. 2). Albitization proceeded through the replacement by Na for K and Ca of preexisting feldspars. The elements Mn, Li, F, Ga, Al, Na and the rare metals Nb, Zr, Y, Th and U were significantly increased whereas Mg, Ca, Ba, Fe were lost during this process. An accompanying decrease of Rb with K is also con%-rned. Fig. 3 depicts well the main chemiQ?
o Monzogradtes
b Alkali-feldsparmicrogranites . Zinnwldite + Zinnwaldite x Albitites
Ab /
amamnltied microgranites albidzed microgranites
1
or
Fig. 2. Normative quartz-albite-orthcclase composition of Urn Ara granitoids. The ternary minima for 1 kb H,O pressure are from Tuttle and Bowen (1958) and the stars represent the ternary minima for granite system with 0 and 4% F (from Manning, 1981). Vector A shows the migration of ternary minima as F content increases, whereas B shows the migration as K content decreases. The shown trends of granitic alteration types are from Stemprok (1979).
132
z
0
A
L
i
Log K*Mg*SriRh2’l,i Fig. 3. The lOOOo~Ga/A1
versus log K.
Mg Sr/Rh’
Li diagram
of Urn Ara granitoids. The arrow indicate\ the K and Na mctasomatic trends in the Urn Am apogranite. Symbols as in Fig. 2
cal changes during the albitization and shows that both Rb and Ga are highly sensitive to the process of albitization and can be used as a good discriminators for distinguishing granitic rocks associated with postmagmatic alteration and the cogenetic rare metal mineralization of Zr, Y. Nb and U. The plot of U versus Th content (Fig. 4) of Urn Ara granitoids shows four populations of data points. The monzogranite shows the lowest U and Th contents whereas the alkali-feldspar microgranite shows enhanced contents of both elements. with average Th/U ratio = 3.0. The amazonitized microgranitc exhibits an expanded enrichment of Th relative to L;. The Th and U of the amazonitized and some of the albitized microgranites may be included within the structure of the refractory accessory minerals. It is worthy to note that besides the occurrence of‘ thorite and monazite (with 0.53-10.5% ThO,) the manganocolumbite and Hf-bearing zircon contain LIP to I .6% UO, (Abdalla et al., 1994). On the other
l-a @ volume% z-95
2
l-b
hand. most of the albitized microgranite samples (population III) and population IV display distinctive enrichment of U relative to Th. High content of U as much as 1385 ppm is essentially due to the incorporation of U mineralization occurring as disseminations and fracture-fillings. In other words. although the high levels of U are evidently related to the albitization process (the average U content is 76 ppm in the Na-metasomatized zone). the occurrence of anomalously high U contents as fracture-filling in both the K- and Na-metasomatized zones indicates the structure-controlled nature for the localization of uranium mineralization within the apogranite. When the albitization process was pervasively overwhelming (i.e. in albitites) there was considerable leaching of Ti, Ca. K and Si accompanied by a drastically sharp decrease in Li. Rb and rare metals. Only Ga, Al and Na were significantly enriched in the albitites. Fig. 2 shows that the plots of albitized microgranite shift toward the albite pole and extend to higher Na content in the albitites (vector B) and show no correlation with the experimental pseudo-
3
~5 vol me%
Fig. 5. Sketch showing the fluid inclusion types encountered !n Um Ara apogranitr facics. aqueous type, (3) multi-phase aqueous type. (4) two-phase CO,( &CH, 5 commonly show a separation of CO,
phase into inner vapour CO,
I-H,0
(NaCI),
and an outer CO,
( iah)
monophase aqueous type, (2) two-phase
(5) monophase CO,( +CH,)-rich liquid phase.
type. Types 4 and
2
Primary
PS.
F’S, S
HOSI
mmeral
Quartz
Quartz
Quartz
Rock
type/zone
Amazonitized
mlcrogranite?
and nlbitited
A Amazonitized
2)
2)
2)
? 31
2.3)
1s
IS
IO
9
IO
36
K
16
35
which
Clath..
-
phase
impregnated
of Urn
(0.76-O
(0.55wI.78)
The
87)
85)
T,
( - 5h.h
in
(-
(-
Fig.
56.9
56.6
to
- 58.3)
- 5M
I)
Volume
melting
temperature
and mode of homogenrzatio”
of Heyen
5.
to - fitI 4)
to
I to - 60.1)
CO?
uamg the method
ure given
Ice is the final
TN CO,
alteratto”.
using
(1979).
by Fe- and Mn-oxidc
typea
(0 K-0.25)
Ii I‘,
18) ( ~ 58.
10)
((1.0 10 0.08)
(0.08-O.
(0 o-o.
X CH,
10
apogranire
wab eslimdred
inclusion
(0.5~0
+ CO?): of CoIlin\
of
Ara
phase
@m/cm’)
CO,
Density
9
(0.50-0.75)
facies
is estimated
respectively.
the method
of the CO,
X CH,
(0.04~0.22)
(0.03-0.22)
= CH,/(CH,
too
Inclusiona.
applying
Drrlstty
fraction.
arc intcnaely
T,
temperature
molar
IO0
(20-50)
-
(20-50)
co,
x CO?
8
in the different
Vol.?,
7
incluhicmb
Number
6
p~ctv.lusecondary
temperature.
microgranites
melting
find
(fh.
(lb.
(lb.
(5)
(41
(lb.
(5)
(4)
(lb.
tYPC
Inclu>wn
s
the en,~mined
is the CH,
sec~“dav
X CH,
homoyenlzatlo”
the cldthrate
estimated
1s the totnl
total
usng
T,
phase
C H,O).
(0 pr~rn~y.
= CO,/(COz
refer
s
X CO,
I’, S and PS
’
(Ssl5)
oxtdes
Quart?
(5 to
(5-30)
Fe & M”
P. s. PS
P, S, PS
(3-S)
(S-25)
(S-25)
(3-8)
(S-20)
(5%?5)
for
> SO)
Fluwtc
Quartz
s
s
s
S
S
(pm)
Sk,
4
analyais
veinlets
Fluorite
veans
PS,
QUdfll
Quartz
PS,
Quartz
microgranite\
PS.
Quartz
Albirirrd
PS.
XCOdWy
pseudo-
/secondary
3
2
of mlcrothermometric
I
A summary
Table
12
to -3.2) I 2 to - 3.4)
I2
CO,
of CO, of froxn
13 14
I.6-30.8)
phase.
CO,
15
of the C02(+CH,)-H,O
to 425)
total
16
(NaTI)
wt.%
fraction. inclusions
was
of the CO,
molar
(I-6)
(2-5.1)
1.5)
-2fl.X)
(2-5.0)
(6-1
(6.3
(7 to 10.5)
I)
equiv.)
(7 to 22.
(N&I
Salinity.
tcmpcrkre
L\ rhe CO,
(145-240)
(IYO-300)
(195-310)
(275-350)
(240-365)
(295-420)
(285
T,
homogeniratwn
at 40°C.
XCOL
to 30.91 (9.5-30.4)
(22.4
(loto22.1)
(2
Tt,
i\ the partial Saliniry aqueous
phawz
T, CO2
ehtimatc
(J-k-7OJ
2)
Clrth
(4.2-b
T,
is a wstill et al. (1982).
‘i
t - 0.5 to - 4.0)
(~
(p
-18
4.6 to - 20)
ice
p.JlO
(-
T,
ternary minima for the granite melt-HzO-F (vector A). The monomineralic albitites with albite of An = 0 (EPMA analysis) can not be considered as a magmatic product.
5. Fluid inclusion
study
The fluid inclusions were studied in quartz from the amazonitized and albitized microgranites as well as in quartz and fluorite of the later fracture-filling quartz veins and fluorite veinlets. Genetic classification of these inclusions (primary. secondary and pseudosecondary) is very subjective especially in alteration zones like those in the Urn Ara apogranite. Therefore, an alternative scheme based on phase proportions at room temperature has been used.Fig. 5 shows a sketch of inclusion types encountered in Urn Ara apogranite facies, and the results of microthermometric analyses are summarized in Table i. The large compositional and volume variation of the COZ( f CH,) phase in the inclusions of the same population suggests a heterogeneous entrapment of fluids that had unmixed into H,O(NaCl)-rich fluid and CO,-rich vapour at approximately I .S kbar 1
60
r~ Inclusions in quartz of the Amazonitized mirrogran~te.~ +Inclusions in quartz of the Nbitized microgranites. l Inclusions in ouarn of the quark veins and fluorite Of the fluorite veinlets. Alnclusions in quartz of the Amazonitized and Albitired microgranites which are intensively impregnated h? Fe- and Mn-oxide alteration. .W?‘/
(Bowers and Helgeson. 1983). The type 4 inclusions, CO,( + CH d)-H 20(NaCl), of the amazonitized and albitized microgranites plot mainly between the HIO-CO, solvi for 6 and 12% NaCl at 1.5 kbar (Bowers and Helgeson, 1983). indicating the reliability of salinities estimated by the clathrate melting method. The spread of values of temperature of total homogenization and salinity in the aqueous inclusions of the amazonitized and albitized microgranites (Fig. 6) indicates that the subsolidus alteration processes have proceeded as the temperature (and to less extent the salinity) of the fluids decreased. Inclusions in the amazonitized microgranites homogenized in the range of 285-425” C with salinities of 7-22.1 wt.% NaCl cquiv.. indicating high to moderate temperature and saline fluids in the early stage. However, the microgranite samples which are intensively impregnated by later Fe- and Mn-oxide (most commonly developed along fractures) contain voluminous trails of small secondary aqueous inclusions. The homogenization temperatures and salinities of these inclusions suggest that the Fe- and Mn-oxides alteration might prevail at lower temperatures (14%240°C) and salinities (I-6 wt.% NaCl equiv.) during the waning hydrothermal stage contemporaneous with the release of pressure and fracturing of the solidified microgranite cupola. However, some of the aqueous inclusions show temperatures of first melting below the cotectic temperature of H,O-NaCl ( - 30.8” C> in the range of - 24.0 to - 26.8” C indicating the presence of other cations besides Na’. The presence of vapour- and liquid-rich inclusions (type I) may indicate that a local boiling had been occurred.
6. Discussion
100
200 flomogenlratlon
300 temprature.’
400
500
C
Fig. 6. Salinity versus total homogenization temperature for aqurous fluid inclusions in Urn Ara apogranite facies. The halite saturation curve in H20-NaCI system is taken from Sourirajan and Kennedy ( 1962).
The following experimental review is of considerable importance for understanding the postmagmatic metasomatic processes affecting the Urn Ara apogranite. Balitskiy and Komova (1971) found that solutions with a molar K/(Na + K) ratio less than 0.13 at 300°C and 784 bars. can albitize microcline. The rate of the process increases with increasing pH from 5.5 to 10 whereas, microclinization proceeds at higher ratios. According to Holland (1972) signifi-
H.M. Abdalla et al. /Journal
ofGeochemica1 Exploration 57 (1996) 127-138
cant concentrations of alkalis can partition into an aqueous fluid of relatively high chloride content. Potassium and Na have a fluid/melt partition coefficient > 1.0 for fluids with > 3.0 molal Cl (for K) and with > 2.2 molal Cl (for Na). Chlorine molalities of l-4 are expected within a highly evolved magmatic system because Cl partitions almost exclusively into aqueous fluid relative to silicate melt (Burnham, 1979). However, Foumier (1976), concluded that shallow intrusive magmas can yield a chloride solution which due to low pressure unmixes into gas plus brine. Reaction of that gas or brine will result in potassic feldspathization (see Foumier, 1976). The geologic setting, petrographical criteria and evolved geochemical signature all strongly indicate
the anorogenic origin for the high-level alkali-feldspar microgranite cupola. The lower unaltered alkalifeldspar microgranite plots in the within-plate granite field (WPG) of the petro-tectonic scheme of Pearce et al. (1984). The evolved nature of the alkali-feldspar microgranite suggests melting of older anhydrous crust by ascending mafic magma underplating the base of extended stabilized Late Precambrian crust of the Nubian Shield. However, the extraction of a monzogranite protolith which was emplaced during the post-erogenic phase led to a residual melt depleted in Ca, Ba, Sr and enriched in Zn, Pb, Ga, Li, Rb, high charge cations and an anhydrous volatile phase (e.g., F, Cl and CO,). The presence of F, Cl and CO, and the alkalis Na, K, Li and Rb in the melt allowed
/ ,Q ,F
Fracture. Quartz brecclas and veins. Fluorite veinlets. Q DegassLug breccias. CXPMiarolitlc cavities. < Residual uranium-bearing fluid “‘RCMn’(Assldated with Fe- and Mn-oxide alteration). Cl Zone of Na-metasomatism (Albitizatfon). CZJZone of K-metasomatism (Amazonitization). IBl Zone of maximum exsolved fluid. C!Z!iAlkali-feldspar rnlcrogranite. lloB Urn Ara monzogranite. E!J Urn Dubr Dokhan volcanks. Fig. 7. An autometaosomatic
alteration model of Urn Ara apogranite.
135
s 5 8. 9
The Roman numbers refer to the evolution stages of the apogranite.
stable complexing of the high charge elements. The elements (Na, Li, Rb. and U) and (Zr. Nb, and Y) are efficiently stablized by F- and Cl-rich residual fluid phase. respectively (Hildreth. 198 1). Continued extension of the continental crust permitted this evolved and less dense magma to rise and produce shallow magma chambers. where it solidified into alkali-feldspar microgranite cupola co-existing with an exsolved supercritical fluid. The rare-metal granite magma (due to the depressed granitic melt minima caused by the presence of H,O, CO,. F. Cl and rare alkalis which result in fuzzy transition from magmatic to postmagmatic condition) evolve fluid phase at a late stage in its crystallization history. This fluid phase was initially confined to grain boundaries, microstructures and vugs (Pollard and Taylor, 1986). The fluid phase was further separated into a highly mobile volatile phase and a low denisty aqueous fluid. Textural characteristics e.g., miarolitic cavities, degassing breccias and the pegmatite nests are good evidence for early separation of a volatile phase. It is worthy of note that the Fe/Mn ratio decreases in mica, spessartite, columbite and NbZn-Mn-ilmenite from the amazonitized to the roof albitized microgranite, whereas, the F and Li of the mica increase upward, reflecting the role of upward volatile diffusion of the rare elements aided by their tluorophile affinity (AbdaIla et al., 1994). According to the available experimental data, the early high temperature reactions proceeded through K-metasomatism where K’, Rb’ and Pb’+ in the fluid replaced Na and Ca of early existing feldspars. The high contents of Rb and Pb of the amazonitized microgranites are found to be incorporated within the K-feldspar (green amazonite) structure (Abdalla, 1996: Abdalla et al., 1996). The released Na and Ca during the K-metasomatism were stablized in the albite and the common accessory and veinlet tluorite in the roof zone of albitization under high CITY environment. These reactions may proceed towards increasing Al-O, and Na,O with contemporaneous desilicification when the aibitization becomes extensively overwhelming. However, Alexandrov et al. ( IOtiS) concluded that when the pH of the fluid changes from 5.7 to 7.5. uNh/uTa increases from 0.8 to 14 and accordingly the Nb/Ta ratios in tantaloniohates will vary. The average Nb/(Nb + Ta) atomic ratio in columbites of
Urn Ara apogranites is 0.99. so a basic and relatively high temperature ( > 425” C, the maximum measured homogenization temperature) medium can be assumed for their deposition, as the stability of Ta-complexes is favored by low temperature conditions (Wang et al.. 1982). The evolution of postmagmatic metasomatic processes is summarized in Fig. 7. This model is closely consistent with the apogranite concept of Beus et al. ( 1962) and Taylor and Pollard (1988). It is thus thought that a large amount of volatiles were exsolved at the completion of crystallization of the alkali-feldspar microgranite cupola and concentrated (aided by their hydrophile to fluorophile nature) at the apical parts of the microgranite cupola (stage Il. Fig. 7). This fluid phase is initially confined to grain boundaries. microstructures and miarolitic vugs. However. the composition deduced from the fluid inclusions indicate that the emerged mineralizing fluids were moderately saline (daughter halide crystals were not observed), but were rich in CO,. The early encountered high temperature reactions proceeded through amazonitization (K-metasomatism) followed by albitization (Na-metasomatism) at the roof zone (stage III, Fig. 7). According to Pichavant (1983). the F-rich fluid phase in equilibrium with two alkali feldspars is strongly enriched in Na compared with Cl-rich fluid. Thus. any process leading to reduction of F content of the fluid (such as stablizing it into F-rich minerals, e.g. fluorite and zinnwaldite) will result in the albitization of the wall rocks. It seems that the loss of CO, (and other volatiles. stage V, Fig. 7) by local boiling and effervescence (unmixing of CO,-H,O-salt tluids) related to pressure release and-decreasing solubility ofCO, concurrently with falling temperature. decreasing salinities and the increased pH of the fluids led to destablization of rare element complexes in the tluids. favoring their deposition; Y as xenotime, Zr as rircon. Nb as columbite, Th as thorite. and Zn into NbMn-rich ilmenite. The close association of the fracture-filling uranophane and calcite in the amazonitized and albitized microgranites may indicate that the deposition of carbonates and uranium mineralization occurred during such a period of CO, loss in the waning hydrothermal stage (Abdalla et al.. 1994). However. the development of the fracture system the
H.M. Abdalla et al./ Journal of Geochemical Explomtion 57 (19961 127-138
(stage IV, Fig. 7) appears to be late in the crystallization history of Urn Ara apogranite. This is evidenced by the pervasive alteration of the stock, the predominance of the disseminated type of rare metal mineralization, and the absence of the greisenized and mineralized veins. It is probable that the marked decreasing of salinity and homogenization temperatures detected in the later fracture-filling quartz and fluorite and the Fe (M&impregnated microgranites is related to minimal contribution of meteoric water encountered during the late fracturing event.
7. Conclusions The geologic setting, textural, geochemical and fluid inclusion studies indicate that the zonal pattern of Urn Ara apogranite has been developed as the result of subsolidus reactions of an exsolved postmagmatic fluids with the already consolidated alkali-feldspar microgranite cupola. The gross enrichment of the rare elements towards the roof Na-rich zone indicates that these elements have been stablized by a highly mobile diffusing volatile phase (i.e., F, Cl, Na, K, Rb and Li) during their transportation. However, the decreasing temperature and salinity and the loss of CO, (and the other volatiles) and increasing pH are considered the essential factors for rare metal localization at Urn Ara apogranite. Some of the delineated geochemical anomalies deserve a further exploratory programme. It is suggested that the multiplicative ratios 10000 . Ga/Al and log{(K . Mg . Sr)/(Rb’ Li)) can be used as good discriminators for distinguishing granitic rocks associated with postmagmatic alteration and the cogenetic rare metal mineralization of Zr, Y, Nb and U. The greater the Ga/Al and the negativity of the log ratios, the greater the rare metals potentiality of the concerned granitoids. From the exploration point of veiw. the present study draws attention to the necessity of assessment of the uranium potentiality of the metasomatically albite- and (Nb, Y, Zr)-enriched granitoids. Compared with the other post-orogenie granitoids (including the magmatically albiteenriched ones) in the basement complex of Egypt, the metasomatically albite-enriched granitoids exhibit distinctive airborne radiometric anomalies (e.g., Hassan. 1973). Besides the petrographical and geo-
137
chemical criteria discussed in the present study, the very high Nb/Ta ratio may assisst in recognizing these granitoids.
Acknowledgements H.M.A. is indebted to Prof. Nabil El-Hazek and the group of Aswan Camp of Nucl. Mater. Authority, Egypt. Special thanks are due to Prof. S. Kanisawa of Tohoku University for his kind help during the F and Li analyses. Drs. H. Miura, H. Nakagawa and Mr. S. Terada of Hokkaido University are also acknowledged for help during XRF and EPMA analyses. An earlier version of the manuscript was benefitted greatly from suggestions of Drs. R. Taylor and P. Pollard of James Cook University and Dr. G. Taylor of CSIRO, Australia.
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