issn: 2354-2268 submitted: 19/11/2015 accepted: 25/11/2015...

ISSN: 2354-2268

Submitted: 19/11/2015

Accepted: 25/11/2015

Published: 11/12/2015

DOI: http://doi.org/10.15580/GJGES.2015.2.111915161

Geochemistry and a composite M-type with W-type of REE tetrad effect in

altered granites of Abu Furad area, Central Eastern Desert, Egypt

By

Ahmed Mohamed El-Mezayen

Mohamed Galal El-Feky

Sayed Ahmed Omar

Sherif Abd El Aziz Ibrahim

Greener Journal of Geology and Earth Sciences ISSN: 2354-2314 Vol. 3 (2), pp. 013-029, December 2015.

www.gjournals.org 13

Research Article (DOI http://doi.org/10.15580/GJGES.2015.2.111915161)

Geochemistry and a composite M-type with W-type of REE tetrad effect in altered granites of Abu Furad

area, Central Eastern Desert, Egypt

Ahmed Mohamed El-Mezayen1, Mohamed Galal El-Feky2, Sayed Ahmed Omar2, Sherif Abd El Aziz Ibrahim1

1Geology Department, Faculty of Science, Al Azher University, Egypt.

2Nuclear Material Authority, Egypt.

Corresponding Author’s Email: aelmezayen50@ hotmail. com

ABSTRACT The Abu Furad area is located in Central Eastern Desert, Egypt. It comprises metasediments, metavolcanics, metagabbros, older and younger granitoids, in addition to numerous dykes and veins of different shapes and composition intruded and invading all the older rocks cropping out in the study area. Field, petrographic, mineralogic and chemical investigations indicated that Abu Furad granites are classified as quartz diorite, tonalite, granodiorite and syenogranites. Altered syenogranite at Abu Furad area is affected by multi-stages of hydrothermal alteration processes along brittle structures, this alteration comprises silicification, hematitization, sericitization, saussuritization, chloritization and muscovitization. Hydrothermal alterations in silicified syenogranite samples show an increase in SiO2, MnO , Na2O, Cr, Nb, Zr, Ga, Hf, Th and U and a decrease in TiO2, Al2O3, Fe2O3, MgO, CaO, K2O, P2O5, L.O.I., Ni, Co, V,Cu, Pb, Zn, Rb, Ba, Sr,Ta, Y, Sn, Tl, Mo and REEs. On the other hand, desilicified and hematitized granites exhibit increase in TiO2 , Fe2O3, MnO, MgO, CaO, P2O5, L.O.I., in Cr, Ni, Co, V,Cu, Pb, Zn, Cd, Sr, Ta, Nb, Zr, Y, Cs, Ga, Hf, Sn, Th, Tl, U, W, Mo, Ag, Bi and REEs and a prominent decrease in SiO2, Al2O3, Na2O, K2O, Rb and Ba. The studied altered syenogranite revealed that the chondrite normalized REE patterns are different from the normal M- and W-type of tetrad effects and has complex characteristics of the two types. The first four elements (La, Ce, Pr, Nd) and the fourth (Er, Tm, Yb, Lu) exhibit a clear convex curve (M-type) while the third set (Gd, Tb, Dy, Ho) define distinct concave curves (W-type), on chondrite-normalized plots. The convex (M-type) T1 and T4 accompanying with concave (W-type) T3 may be related to the physico-chemical conditions that prevailed during the alteration processes. The unusual MW-type tetrad effect could be considered as geochemical exploration method for Au mineralization of reworked plutons. Keywords: geochemistry, isovalents, REE-tetrad effect, altered granite, silicification and desilicification, Egypt. 1. Introduction The study area represents a part of Central Eastern Desert of Egypt. It covers about 350 km² of crystalline basement rocks. It is bounded by Latitudes 26º 35´ 20" - 26º 43 ´32" N and Longitudes 33º 36´ 35" - and 33º 47 ´ 27" E (Fig.1). The area was previously studied by several authors. El-Gaby (1975) concluded that the granitic rock of Abu Furad is granodiorite, while Habib (1982) mentioned that Abu Furad younger granite is a composite pluton representing the passage from the syn-orogenic granitoids to late-orogenic granites. Ghobrial and Girgis (1982) mentioned that the granitoid rocks of Wadi El-Bulah area are mainly quartz diorite, tonalite and granodiorite. Habib (1987) showed that, the ultramaftc-mafic rocks exposed in the Pan African basement between Gabal Meatiq and Gabal Abu Furad, represent arc ophiolites. Mahmoud (1995) concluded that, the older granitoids and younger granites originated under compressional and extensional forces respectively. Esmail and Moharem (2009) described unzoned and zoned pegmatite pockets as the most important rock types from the radioactive point of view. Azab (2011) studied the petrography, geochemistry, mineralogy and radioactivity of Wadi Safaga area to the north of Gabal Um El-Huwitat. The lithotectonic rock units exposed in the area are arranged starting with the youngest as: syenogranites (youngest), felsite sheets, monzogranites, Dokhan volcanics and granodiorite-tonalite rocks (oldest). The principal objectives of the current study are to understand briefly the geochemical behaviors of REEs, uranium, thorium and some isovalents during mineralization.


2. Methodology Seven samples were chemically analyzed from altered granites and pegmatites (Table1) for their major oxides, trace and rare earth elements in ACME analytical Laboratories of Vandcouver, Canada. 3. Geology of the study area Based on aerial photographs, land sat images and field measurements in Abu Furad area, the studied rocks cropping out in the area are classified from the oldest to the youngest as follows: 1) metasediments, 2) metavolcanics, 3) metagabbros, 4) older granitoids, and 5) younger granites, in addition to numerous dykes and veins of different shapes and composition invading all the older rocks cropping out in the study area.

The mapped area is dissected by a number of faults which are either concomitant with wadies and drainage lines or cutting through the country rocks. These faults are of variable length with some of them being of regional scale and extend beyond the border of the mapped area for several tens of kilometres. They are mainly of strike-slip type (sinistral or dextral).

The metasedimentary rocks are represented by small belt of hornblend schist and amphibole schists cropping out at the southeastern corner and the southwestern parts of the mapped area forming circular outcrop around El-bulah granodiorite.

Fig. : Geologic map of Gabal Abu Furad area (modified after Fowler et al., (2006)



The metavolcanics occupy the southeastern part of the map area. They are trending approximately NE – SE, and traversed by Wadi El Bulah and Wadi Abu Furad. The rocks are fine to medium grained and are of greenish black to reddish black in colour. They form an alternating sequence of mataandesites and metabasalt. They are highly altered, sheared and traversed by some basic and acidic dykes, they are stained with reddish spots of iron oxides and black dendrites of manganese oxides. The metagabbros are found as one elongate belt trending in the ENE-WSW direction. This mass crops out at the southeastern side of the studied area (south Gabal Abu Furad). These rocks are intruded by Abu Furad younger granites from the north. The relationship between metagabbros and metavolcanics is hardly to be traced sharp, while the metagabbros show well defined sharp contact with the older and younger granitoids. The metagabbros show medium to low topography).

The older granitoids are the most common rocks, dominated at the northern side of the mapped area. They show low resistance to weathering such as exfoliation and cavernous features and form scattered low hills on vast lands. These older granitoids are usually friable and show well developed foliation with gneissose texture, especially along and near contacts. They are fine to medium grained, high relief, highly fractured and occasionally foliated. They are intruded by numerous basic, acidic dykes and invaded by pegmatite. They are grey in colour, but become darker near the contacts with metavolcanics (southwestward) and metagabbros (southeastward). Their colour change to faint pink or whitish pink, especially at the area between Gabal Um Tagher and Gabal Abu Furad, due to the presence of several offshoots of younger granites and feldspars veinlets and pockets.

The younger granites are of limited distribution in the study area and are represented by considerable pluton of G.Abu Furad at the middle part of eastern corner of the study area. The studied younger granites are medium to coarse grained and distinguished by pink to red colour forming high topographic relief, intruding the older rocks such as the metavolcanics, metagabbros and older granitoidse. In addition, they are characterized by their occurrence in the form of small outcrop of semicircular to oval outline that rises up to 1032 meters above sea level. The younger granites contain xenoliths of different shapes and sizes from the pre-existing rocks. These xenoliths are mostly oval to elongated and vary in size from 5 cm diameter to large masses. The younger granites are highly fractured, jointed and altered in some parts due to secondary processes especially along the fault planes and contacts. These granites usually show exfoliation cavernous weathering. They are invaded by pegmatite and intruded by basic dykes. The micro fractures along these younger granites are sometimes filled with quartz and feldspar veinlets. 4. Petrography According to the petrographic examination, the studied younger granites could be categorized as syenogranites.These rocks are generally equigranular, medium to coarse-grained with hypidiomorphic texture. They are mainly composed of potash feldspars, quartz, plagioclase, biotite and muscovite as essential minerals. Zircon, sphene and iron oxides are the main accessory minerals. Epidote and clay minerals are found as secondary minerals. Potash feldspars are represented by microcline and microcline perthites. They form intergrowth with quartz to give micrographic texture (Fig.2-A). Quartz occurs as subhedral to anhedral crystals. Sometimes, quartz occurs as inclusions of variable sizes and shapes within other minerals such as skeletal forms in perthite. Some quartz crystals show distinct cracking and undulatory extinction, indicating high strain effects. Plagioclase occurs as subhedral to euhedral crystals and sometimes, they are cracked and show lamellar and simple twinning (Fig.2-B). Biotite and muscovite are seen as irregular flakes. Zircon is recorded as prismatic euhedral to subhedral crystals. It is usually observed with very high relief and high interference colour. Sphene occurs in its characteristic sphenoid shape as euhedral to subhedral crystal with brown or yellowish-brown colour (Fig.2-C).

The granitic mass shows different degrees of hydrothermal alterations as silicification, hematitization, sericitization, saussuritization, chloritization and muscovitization. Silicification is identified by the polycrystalline silica at the expense of plagioclase and feldspars accompanied with clay minerals. (Fig.2-D). Sericitization, saussuritization, muscovitization and chloritization are manifested by partially sericitized, muscovitized, chloritized plagioclase and k-feldspars (Figs.2-E and F). Sometimes, biotite is altered to chlorite with iron oxides along cleavage planes (Fig.2-G).



Fig. 2-A: Photomicrograph showing micrographic texture in syenogranite; B. Lamellar twining of plagioclase associated microcline crystal (PL) associating microcline (Micro), syenogranites; C. Sphene (Sph) crystal associated with microcline (Micro), quartz (Qz) and iron oxide in syenogranite; D. Silicification of plagioclase and k-feldspars associating clay minerals; E. Saussuritization of plagioclase; F. Chloritized biotite; G. Desilicification of plagioclase and k-feldspar with secondary muscovite (Ms) and vug filling opaque minerals (Fe-OX); H. Fracture filling by iron oxide, secondary silica, radioactive and REE minerals with micro-displacement.

Desilicification of the studied granites is evidenced by dissolution of plagioclase and quartz leaving vugs with pronounced hematitization (Fig.2-H). Dissolution of quartz is one of the most important alteration processes since it provides greatly enhanced porosity for circulation of the hydrothermal fluids (Chary and Pollard, 1989; Min et al., 1999)

H

Ms

Fe-Ox

G

Chl

F E

D

A B

Micro

PL

C

Micro

Sph

Fe-ox

Qz



5. Geochemical characteristics of the altered syenogranites: The distribution of the trace elements and REEs in the altered zones and pegmatites (Table 1) yielded useful information on rock/fluid interaction characteristics in addition to the physicochemical conditions of the system. During the hydrothermal alteration, nearly all the trace elements were mobilized due to dissolution or replacement of the main components and accessory minerals and new-formation of mineral phases. Six samples were collected from the fine grained sandstone lamina with 800m extension and about 120m intervals. The results of analyses by different techniques for the altered granites as fire-assay and ICP-MS showed that Au ranges between 0.42 and 0.64 ppm, Ag between 0.02 and 0.56 ppm, REEs between 38.79 and 4292.3 ppm, U between 1.3 and 33 ppm and Th between 5.3 and 231.1 ppm. The different types of hydrothermal alterations can be obtained by using the normative Qz-Ab-Or of Stemprok (1979) and Na2O-K2O variation diagram (Cuney et al., 1987). Creasy (1959) classified the hydrothermally altered rocks as argillic facies (characterized by any member of the kaolinite group) and K-silicate facies (characterized by muscovite-biotite and K-feldspar). The argillic facies was further classified by Meyer and Hemley (1967) as advanced argillic (kaolinite and montmorillonite replacing k- feldspar) and intermediate argillic (all the feldspars are converted to dickite and kaolinite) (Figs. 3, 4 and 5).

According to the normative Qz-Ab-Or composition, the altered granitic samples could be classified into sodic, potassic, silicic and greisen (Fig.3). In the studied altered syenogranites and pegmatites, samples having high SiO2 content are shifted towards quartz and also show imprints of greizenization as indicated by Manning (1981).

Table 1: Chemical composition of Abu Furad altered syenogranites and pegmatites, Eastern Desert, Egypt

Oxides Silicified granite Desilicified granite

Pegmatites

% 3 D 5 D 9 D 10 Da 10 Db 2 C 23 C SiO2 77.62 80.67 77.82 63.64 64.31 78.2 78.22 Al2O3 11.53 9.94 9.28 11.60 7.61 10.54 11.05 TiO2 0.04 0.08 0.09 1.64 4.28 0.05 0.09

Fe2O3t 0.71 1.16 4.13 9.97 10.07 0.78 1.14

MgO 0.10 0.10 0.10 2.57 2.12 0.08 0.15 CaO 0.25 0.18 0.28 2.31 3.26 0.67 1.08 Na2O 4.54 3.80 5.03 4.73 2.39 4.46 4.25 K2O 4.79 4.04 2.98 0.42 0.29 5.01 3.49 MnO 0.01 0.01 0.01 0.23 0.21 0.02 0.01 P2O5 0.00 0.00 0.00 0.45 0.47 0.00 0.01 L.O.I. 0.41 0.02 0.28 2.44 4.99 0.19 0.51

Ba 117 130 32 29 24 51 2829 Co 0.3 0.3 0.6 6.6 16.7 <0.2 0.5 Cs 0.4 0.3 0.1 0.3 0.2 1.3 0.2 Ga 28.59 19.18 24.37 36.60 27.37 18.68 15.16 Hf 22.28 3.03 1.77 6.28 9.33 4.30 1.77 Nb 21.29 6.12 4.20 42.16 386.68 3.18 2.63 Rb 51.6 41.7 24.37 5.0 4.7 117.8 32.4 Sn 0.2 1.0 0.8 3.9 51.3 0.4 0.2 Sr 32 21 16.00 76 81 36 302 Ta 0.8 0.5 0.3 6.3 39.9 0.8 0.5 Th 185.0 5.3 35.4 101.2 231.1 50.5 5.8 Tl 0.28 0.25 0.11 <0.05 <0.05 0.57 0.14 U 31.9 1.3 4.8 18.9 33.0 12 1.0 V 12 1.0 12 1.0 12 3 5 W <0.1 <0.1 <0.1 0.4 0.8 <0.1 <0.1 Zr 340.4 80.1 45.4 125.1 136.2 102.7 59.3 Mo 0.12 0.19 0.20 0.42 0.96 0.19 0.08 Ag 0.11 0.04 0.17 0.56 0.02 0.05 0.02 Au 0.48 0.42 0.46 0.54 0.64 - - Cu 2.87 2.57 6.02 12.26 31.49 1.35 2.40 Pb 11.89 7.29 6.59 15.62 23.59 29.94 12.30 Zn 6.5 13.8 13.0 286.4 228.3 15.1 16.5 Ni 1.5 0.8 1.0 2.3 10.1 1.7 1.1 Bi <0.04 <0.04 <0.04 0.22 0.42 0.06 <0.04



Oxides Silicified granite Desilicified granite

Pegmatites

Li 0.7 1.3 1.1 23.1 4.7 2.2 1.2 Cr 9 5 7 5 30 4 5

Y 26.5 13.1 9.0 759.5 1561.1 11.3 6.8 La 4.7 14.4 4.9 68.9 149.3 6.5 17.9 Ce 18.17 42.66 16.09 278.5 530.30 14.79 38.50 Pr 1.9 4.1 1.6 52.5 116.3 1.7 4.3 Nd 7.7 16.2 6.6 258.8 579.7 7.1 17.1 Sm 2.6 4.7 1.5 102.0 223.3 1.6 2.4 Eu 0.6 0.8 0.5 11.1 23.0 0.4 0.7 Gd 4.2 3.8 1.9 127.5 269.5 1.7 2.6 Tb 0.5 0.5 0.1 24.5 48.6 0.1 <0.1 Dy 4.8 3.2 2.0 148.1 295.1 1.6 1.2 Ho 1.2 0.7 0.4 33.1 65.3 0.4 0.2 Er 3.8 1.7 1.1 84.2 165.9 1.0 0.4 Tm 0.8 0.3 0.3 14.1 27.3 0.2 <0.1 Yb 6.6 1.6 1.5 91.4 1771.6 1.2 0.6 Lu 1.6 0.2 0.3 15.6 27.1 0.3 0.1

La/Ybn 0.48 6.08 2.21 0.51 0.06 3.66 20.16 La/Smn 1.14 1.93 2.06 0.43 0.42 2.56 4.69 La/Nb 0.2 2.4 1.2 1.7 0.4 2.1 6.8 La/Ta 6.9 28.8 16.3 10.9 3.7 16.3 59.7

Gd/Ybn 0.51 1.92 1.02 1.13 0.12 1.14 3.50 Sr/Eu 53.3 26.25 32.00 6.85 3.52 90.00 431.43 Eu/Sm 0.23 0.17 0.33 0.11 0.10 0.25 0.29 Y/Ho 22.08 18.71 22.50 22.95 23.91 28.25 34.00 ΣREE 59.17 94.86 38.79 1310.3 4292.3 38.59 86.00 LREE 35.7 82.86 31.2 771.8 1622 31.1 80.9 HREE 23.5 12 7.6 538.5 2670.4 6.5 5.3

LREE/HREE 1.5 6.9 4.1 1.4 0.6 4.9 15.3 Ce/Ce* 1.57 1.36 1.42 1.24 1.08 1.06 1.04 Eu/Eu* 0.6 0.6 0.9 0.3 0.3 0.7 0.9 Zr/Hf 15.28 26.44 25.65 19.92 14.60 23.88 33.50 Nb/Ta 26.61 12.24 14.00 6.69 9.69 3.98 5.26 U/Th 0.17 0.25 0.14 0.19 0.14 0.24 0.17 Ba/Sr 3.66 6.19 2 0.38 0.30 1.42 9.37 Ba/Rb 2.27 3.12 1.31 5.80 5.11 0.43 87.31 Rb/Sr 1.61 1.99 1.52 0.07 0.06 3.27 0.11

t1 1.38 1.22 1.26 1.28 1.19 1.04 1.04 t3 0.75 0.84 0.56 1.01 0.98 0.53 0.52

TE1..3 1.02 1.02 0.84 1.14 1.08 0.74 0.74 t4 0.97 1.23 1.21 1.03 3.40 0.93 1.27

TE1,4 1.15 1.23 1.23 1.15 2.01 0.98 1.15

t1, t3, t4 and t are calculated according to Irber (1999). -: not determined.


altered syenogranite pegmatite

Fig.3: Normative Qz-Ab-Or ternary diagram

The ternary minimum for 1 kb H2O pressure from Tuttle and Bowen (1958) and the stars represent the ternary

minima for a granite system with 0% and 4% F (Manning, 1981). Vector A shows the migration of ternary minima

as F content increases in the melt. The trends of granitic alteration types are from Stempork (1979).

However, other samples with high Na2O contents are characterized by Na-metasomatism (desilicified

samples) are shifted towards albite which is consistent with the enrichment direction of fluorine, while during Na-

metasomatism, albitization proceeds through the replacement of Na+ for K

+ and Ca

2+ of the pre-existing feldspars

but silicification results in an increase of SiO2 at the expense of other major oxides and accompanied with

increase of some trace elements such as Zr, Ba and Rb. By using the Na2O- K2O variation diagram (Cuney et al.,

1987) (Fig.4), it is evident that the studied altered granites and pegmatite samples fall in desilicification,

albitization and silicification fields.

Fig.4: K2O−Na2O variation diagram (Cuney, 1987). Symbols as in Fig.3


Meyer and Hemley (1967) classified the K-silicate facies as: (1) propylitic (containing epidote-chlorite alteration), (2) sericitic (containing plagioclases and K-feldspars, both of which were converted to sericite and (3) potassic (characterized by the alteration of plagioclase into K-feldspar or mafic minerals into muscovite) subtypes. Altered syenogranite and pegmatite samples are plotted on the diagram (Fig.5) of Meyer and Hemley (1967). It is evident that two samples fall in sericite facies (due to sericitization processes) and one sample lie between sericite and propylitic facies while the other samples are close to the Al2O3-Na2O+K2O line, because of their high contents of Al2O3+Na2O+K2O. On the (Na2O+CaO)−Al2O3−K2O ternary diagram of (Nasbitt and Young, 1989) shows that all altered syenogranites and pegmatite samples plot parallel weathering trend, which its initial trend is parallel to the (Na2O+K2O)−Al2O3 side-line of the diagram (Fig.6).

Fig.5: Al2O3− (Na2O+K2O)− (FeOT+MnO+MgO) ternary diagram (Meyer and Hemly, 1967). Symbols as in Fig.3

Fig.6: Al2O3− (Na2O+CaO)−K2O ternary diagram (Nasbitt and Young, 1989) for altered granitic rocks. Symbols as in Fig.3


6. Geochemistry of major oxides and trace elements of the altered syenogranites: The variations of the geochemical of the altered syenogranite are caused by the loss and gain of some elements. To understand the geochemical behavior of elements in the altered granites, it is recommended to normalize the pattern of such altered rocks to its corresponding fresh granite (syenogranite). After that, the reference granite pattern becomes flat at unity and the relative depletion or enrichment are given by the deviations on both sides of the reference line (Figs.7A, B and C & 8A, B, and C). Geochemistry of major oxide is discussed in terms of gains (positive) and losses (negative) of these elements during the alteration of granites. Altered samples which are affected by desilicification and hematitization exhibit increase in TiO2 , Fe2O3, MnO, MgO, CaO, P2O5 and L.O.I. and a prominent decrease in SiO2, Al2O3, Na2O, K2O (Fig.7-A). Enrichment of Fe2O3, MnO and MgO may be due to alteration of biotite (chloritization) and hematitization. P2O5 increased in the shear zone, reflecting that apatite continued to form during alteration processes. Formation of calcite is reflected by increased Ca contents. The enrichment of MnO reflects the formation of Mn minerals. High values of loss on ignition (L.O.I) in the altered samples are mainly due to saturation with intergranular water during formation of hydrous minerals (sericite and muscovite) and the formation of carbonates. However, trace elements in the desilicified samples show enrichment in Cr, Ni, Co, V,Cu, Pb, Zn, Cd, Sr, Ta, Nb, Zr, Y, Cs, Ga, Hf, Sn, Th, Tl, U, Mo, Ag, Bi and REEs whereas they are depleted in Rb and Ba. Enrichment in Cr is attributed to the mobility of the element from the adjacent ultramafic rocks and their metasomatic derivatives. These rocks are normally enriched in Cr, mostly in the form of chromite. Pb concentration increases as a result of the hydrothermal formation of creussite and to some extent with silver and gold mineralization as ascribed from mineralogical studies. Also, Na-metasomatism leads to Ba loss and Sr enrichment (Table.1). Zr, Hf and Y are relatively immobile and essentially concentrated in the accessory minerals (zircon and its alteration product branirite). The later minerals are rather resistant and remain in the residual products, so the elements Zr and Y are enriched (El-Feky et al., 2011). High contents of REEs, U, Th, Y, Nb and Ta may be related to the presence of Fergusonite (YNbO4), xenotime Y (PO4) and monazite (Ce, La, Nd, Th, Y) PO4 as indicated from the mineralogical studies.

Fig.7-A: Histogram showing the depletion and enrichment of major oxides of desilicified syenogranite

Fig.7-B: Histogram showing the depletion and enrichment of trace

elements of desilicified syenogranite

-12-11-10

-9-8-7-6-5-4-3-2-1012345

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 L.O.I

-500-400-300-200-100

0100200300400500600700800900

100011001200

Cr

Ni

Co V

Cu

Pb

Zn

Cd

Rb

Ba Sr

Ta

Nb Zr Y Cs

Ga

Hf

Sn

Th Tl U W

Mo

Ag Bi


Fig.7-C: Histogram showing the depletion and enrichment of REEs of desilicified syenogranite

On the other hand, silicified syenogranite samples show an increase in SiO2, MnO and Na2O and a decrease in TiO2, Al2O3, Fe2O3, MgO, CaO, K2O, P2O5 and L.O.I. Silicification process results in an increase of SiO2 at the expense of other major oxides. High contents of MnO reflects the formation of Mn minerals. The enhanced Na2O contents may be resulted from the fact that solutions causing alteration processes are rich in Na2O (Fig.8-A).

Abundances of trace elements in the hydrothermally altered silicified samples are either higher (Cr, Nb, Zr, Ga, Hf, Th and U) or they are lower (Ni, Co, V,Cu, Pb, Zn, Rb, Ba, Sr,Ta, Y, Sn, Tl, Mo and REEs) than the fresh granite. The chloritization of biotite can probably account for the depletion of Cu, Mo, Sn and Zn. Cr enrichment in the silicified samples (Fig.8-B) may result from the same circumstances valid for the desilicified samples. Sr depletion is mainly connected to the decrease of Ca. The depletion in P2O5 and REEs might be controlled by the probable dissolution of monaziten, xenotime REE minerals in the silicified zone and migration of these elements to the desilicified ones. Ni, Co, Cu, Pb, Cd, Cs, Sn, Tl, W, Mo, Ag and Bi do not show any changes during alteration processes of silicified granite.

Figures 7-A, B and C & 8-A, B and C, and (Table1) show that the altered (silicified and desilicified) samples of the present work are characterized by lower Ba/Rb, Ba/Sr and Rb/Sr ratios than the fresh syenogranite.

Fig.8-A: Histogram showing the depletion and enrichment of major

oxides of silicified syenogranite

0

50

100

150

200

250

300

350

400

450

500

550

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 L.O.I


Fig.8-B: Histogram showing the depletion and enrichment of trace

elements of silicified syenogranite

Fig.8-C: Histogram showing the depletion and enrichment of REEs of silicified syenogranite

6- REEs tetrad effect Fractionation of minerals alone cannot explain non-CHARAC behavior of Zr and Hf, Y and Ho (Bau, 1996, 1997). Therefore, it is suggested that the tetrad effect and highly fractionated trace element ratios of Y/Ho and Zr/Hf indicate a trace element behavior that is similar to that in aqueous system in which chemical complexation is significant. The term ‘tetrad effect’ in geochemistry refers to the subdivision of the 15 lanthanide elements into four groups in a chondrite normalized distribution pattern: (1) La–Ce–Pr–Nd, (2) Pm–Sm–Eu– Gd, (3) Tb–Dy–Ho, and (4) Er–Tm–Yb–Lu, and each group forms a smooth convex (M-type) or concave (W-type) pattern (Masuda et al., 1987). The values of tetrad effect were calculated according to the quantification method of Irber (1999): t1 = (Ce/Ce*× Pr/ Pr*), t3 = (Tb/Tb*×Dy/Dy*), t4 =(Tm/Tm* ×Yb/Yb*) Degree of the tetrad effect T1,3 = (t1 × t3)

0.5.

-320

-300

-280

-260

-240

-220

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

Cr

Ni

Co V

Cu

Pb

Zn

Cd

Rb

Ba Sr

Ta

Nb Zr Y

Cs

Ga

Hf

Sn

Th Tl U W

Mo

Ag Bi

-75

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu



A REE pattern that does not show a tetrad effect has values of TE1.3 < 1.1. The M-shaped pattern shows ТЕi <1.1 and the W-shaped TEi < 0.9.

The REE tetrad effect is most visible in late magmatic differentiates with strong hydrothermal interactions or deuteric alteration. This includes highly evolved leucogranites, pegmatites, and mineralized granites. Moreover, the tetrad effect is often accompanied by other modified geochemical behavior of many trace elements, which is termed by Bau (1996) as non-CHARAC behavior (CHARAC =Charge-and-Radius-Controlled). Such behavior occurs typically in highly evolved magmatic systems enriched in H2O, CO2 and elements such as Li, B, F and/or Cl, which suggests the increasing importance of an aqueous-like fluid system during the final stages of granite crystallization (Bau, 1996; Irber, 1999).

The kinked pattern, which is the characteristic REE tetrad effect, is well demonstrated in highly evolved granitic rocks. According to Masuda et al. (1987), extraction of a coexisting fluid from a peraluminous melt would result in both of the M-shaped and W-shaped REE tetrad effect, the former of which would be shown in the residual melt phase and the latter of which is shown in the fluid. However, this corresponding relationship for a magma system has not been observed in the natural environment. Recently, it has been argued that peraluminous magmatic systems represent the transition from a silicate melt to a high-temperature hydrothermal system, and thus, the geochemical behavior of the isovalent incompatible elements in highly evolved granitic rocks are controlled mainly by chemical complexation with a variety of ligands (Bau and Dulski, 1999; Bau, 1996, 1997; Dostal and Chatterjee, 2000). Therefore, the origin of the REE tetrad effect was ascribed to the interaction between fluorine bearing fluid and silicate melt phases (e.g., see Irber, 1999). Monecke et al., (2002) thought that the convex tetrad effect in the samples from the magmatic environment could not be explained by the removal of a respective complementary REE pattern by a coexisting hydrothermal fluid, as they found that the fluorite samples collected from hydrothermal vein within the endocontact of the Li-F granite of Zinnwald, Germany, obviously have the M-shaped REE tetrad effect instead of the W-shaped tetrad effect. Therefore, they proposed that the tetrad effect might have formed within the magma fluid system before emplacement in the subvolcanic environment where phase separation caused a split of this system into fluid and magma subsystems, or that the tetrad effect might also be inherited from an external fluid influencing the system during or after the emplacement of the magma. Takahashi et al., (2002) recently found both W- and M-type tetrad effect in REE patterns for the water-rock systems in the Tono uranium deposit, central Japan, which is interpreted as that the preference of the groundwater for a W-type tetrad effect produces an M-type tetrad effect in the granitic rocks during weathering processes. Cao et al., (2013) stated that, the tetrad effect observed in apatite from the muscovite granite and the pegmatite zones of Koktokay No. 3 pegmatite are most likely produced by the interaction of immiscible fluoride and silicate melts, rather than by weathering, fractional crystallization of individual mineral phases and/or fluid-melt interaction.

It is important to notice that the reported cases are M- or W-type tetrad effects occurring separately in natural systems. However, the composite M- and W-type REE tetrad effect were firstly reported by ZHAO et al., (2008). Thus, our study is an important step in understanding the REE tetrad effect. 7. Geochemistry of Isovalents: The tetrad effect is often accompanied by other modified geochemical behaviors of many trace elements, which is termed by Bau (1996) as non-CHARAC behavior. Such behavior occurs typically in highly evolved magmatic systems which are rich in H2O, CO2 and elements such as Li, B, F and/or Cl, and which may be regarded as transitional between a pure silicate melt and an aqueous fluid (e.g., London, 1987; Bau, 1996). Zr and Hf are known to have very similar geochemical behavior, which results in a small range of ratios in geological materials. In most igneous rocks, Zr/Hf ratios fall in a narrow range of 33–40. Deviation from the range is rare and usually attributed to metasomatism or intense fractionation of accessory minerals (Dostal and Chatterjee, 2000). The Zr/Hf ratio of the silicified granites (15.28-26.44), desilicified granites (14.60-19.92) and of pegmatites (23.88-33.50) is lower than the normal ratio of geological materials. According to Irber (1999), granites with Zr/Hf ratio (‹20) are affected by strong magmatic hydrothermal alteration. This ratio shifts towards smaller values with increasing evolution of silicate melt.

It has been reported that Y/Ho ratio would be discrepant from the chondritic value (Y/Ho = 28.8) for samples showing a tetrad effect (Zhang et al., 1994; Bau, 1996). The studied samples have Y/Ho ratios lower than the Chondritic value (18.71-22.5 in silicified granite, 22.95-23.91 in desilicified granite). In pegmatites, one sample has chondritic ratio while the other non-chondritic and larger than the chondritic ratio 28.8. The complexation with fluorine is interpreted as a major cause for Y/Ho>28, while the complexation with bicarbonate is assumed to generate Y/Ho values <28. Because REE accessory minerals commonly inherit the REE signature of the magma system, they are not the cause of REE tetrad effect (Irber, 1999). Y/Ho ratios of the studied altered granites reveal that complexation with fluorine is dominant. This could be assisted by the presence of fluorite and fluoritization accompanying U and REE mineralization, but the presence of calcite and malachite may clarify complexation with carbonate. Y/Ho ratio of the studied pegmatite indicate complexation with carbonate.


The studied altered granites show non-chondritic ratios for Nb/Ta (14- 26.61 for silicified granite, 6.69-9.69 for desilicified granite and 3.98-5.26 for pegmatites), La/ Nb (0.2-2.4, 0.4-1.7 and 2.1-6.8) and La/ Ta (6.9-28.8, 3.7-10.7 and 16.3-59.7) for silicified granite, desilicified granite and pegmatites, respectively with the exception of one pegmatite sample. The chondritic ratios are 17.6 ±1 for Nb/Ta, (0.96-1) for La/Nb and (16-18) for La/Ta (Jahn et al., 2001). These non-chondritic ratios are considered as another evidence of the highly differentiated nature of the studied altered granites.

Rb/Sr ratio increases with differentiation; this is due to the fact that Sr is depleted in the liquid magma as a result of crystallization of feldspar, while Rb is enriched in liquid phase. The studied altered granite occurrences have low Rb/Sr ratios (1.52-1.99, 0.06-0.07 and 0.11-3.27 for silicified granite, desilicified granite and pegmatites, respectively) indicating the effect of albitization in Sr enrichment due to the high Rb contents. Positive correlation between chemically analyzed uranium and thorium of the studied altered granites and pegmatites indicate uranium enrichment with differentiation in the studied altered granites and depletion in pegmatites (Fig.9). These uranium enrichment and depletion may suggest addition and leaching of uranium linked with alteration processes. Additionally, albitization promoted subsequent fluid circulation (for U, REE and HFSEs) by creating a more brittle and permeable rock assemblage (Alexandre, 2010).

Fig.9: Chemically analyzed uranium vs. chemically analyzed Th of the altered granites and pegmatites. Symbols as in Fig.3

8. Rare Earth Element geochemistry:

The average of the total REEs content of the studied silicified granite (av. ΣREE=64.27 ppm) is lower than that of the world-wide granite (ΣREE=250−270 ppm) as given by Hermann (1970), whereas the desilicified granite shows much higher contents. The depletion of REEs has been attributed to various processes including magmatic differentiation (Cuney and Friedrich, 1987), hydrothermal leaching (Cathelineau, 1987) and/or a combination of both.

In silicified graninite, there is a drastic decrease in the REE in general where ∑REE 38.79– 94.86 ppm (Tab.1), but ∑ LREE 31.2 – 82.86 ppm is higher than ∑ HREE 12-23.5 ppm and ∑ LREE / ∑ HREE 1.5 – 6.9. The fractionation of the REE in general is high La/Yb (0.9–9). It is known that the La/Yb ranges between (30 and 80) for metaluminous granites and between 1 and 9 for peralkaline granites (Harris and Marriner, 1980). In silicified granites, this ratio being typically related to the alkaline type. The LREE (La/Sm 1.81–3.06) and the HREE (Gd/Yb 0.64 to 2.38) are moderately fractionated. A negative to slightly or no Eu anomaly characterizes this granite where Eu/Eu* (0.6 - 0.9) and Eu/Sm (0.23 - 0.33). The fractionation among Eu and neighboring REEs (Sm and Gd) with the tetrad effect possibly leads to the decrease in the magnitude of the negative Eu anomaly (Feng, 2010; Ibrahim et al., 2015).


Fig.10: Chondrite- normalized REE diagram (Boynton, 1984) for the

investigated silicified and average of syenogranite The LREE are depleted and the HREE (Gd to Lu) including Eu and Y are enriched. This general shift towards lower LREE/HREE is only observed in desilicified granites that underwent albitization. On the other hand, desilicified and Na-metasomatized granites are highly enriched in rare earth elements where ∑REE (1310.3 to 4292.3 ppm), moreover the ∑ HREE 538.5 to 2670.4 ppm are distinctly more enriched than the light rare earths (∑LREE 771.8 to 1622 ppm); and ∑LREE/∑HREE exhibits an average value of 1.0. The Chondrite- normalized patterns of the analyzed samples (Fig.10) exhibit very slight fractionation of the LREE, where (La/ Sm ~ 0.42 – 0.43), while the HREE are more fractionated (Gd/Yb 0.12 to 1.13). The altered granite show faint moderately fractionated pattern (La/Yb)n=0.06-0.51. The negative Eu anomaly is deep with a value of Eu/Eu 0.3 and Eu/Sm = 0.10-0.11. This type also belongs to the first group of Cullers and Graff (1984) with strong negative Eu anomaly. The studied altered syenogranites represent a particular pattern of the rare earth chondrite- normalized elements (Fig.10 and11). A composite M- with W-type of REE tetrad effect is recorded for the first time as aphenomena previously recorded in China (ZHAO et al., 2008; ZhenHua et al., 2010; Cao et al., 2013).

Fig.11: Chondrite- normalized REE diagram (Boynton, 1984) for the

investigated desilicified and average of syenogranite

The kinks in the REE patterns are camouflaged by prominent convex and concave tetrads and pronounced negative to slightly positive Eu anomalies. Visual inspection suggests that the first tetrad in most samples is more prominent than the third and fourth curved segments. The second tetrad is comparably difficult to recognize due to the anomalous behavior of Eu and the fact that Pm does not occur in nature. (Fig. 10 and 11) shows that samples of silicified and desilicified syenogranites have strong M-type tetrad effect in the first and fourth tetrad and strong W-type tetrad effect in the third tetrad only in silicified syenogranites. The index of tetrad effect

2

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100

200

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Ce

Pr

Nd Sm

Eu

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Tb

Dy

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Er

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Sam

ple

/C1 C

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1000

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20000

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Pr

Nd Sm

Eu

Gd

Tb

Dy

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mp

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1 C

ho

nd

rite


intensity, TE1,4, is higher than 1.10 ranging from 1.15 to 1.23 and from 1.15 to 2.01 in the silicified and desilicified syenogranite, respectively which implies that there was an interaction between melt and water-haloid-rich fluid when these granites are crystallized from magma. However, one of the pegmatite samples show clear convex (M-type) T4 with concave (W-type) T3 (Fig.12 and Table1).

ZHAO et al., (2008) suggest that the new MW-type of tetrad effect is likely to be caused mainly by the interaction of aqueous liquids with alkaline rocks. Mahdy and El Kammar, (2003) in Kab Amiri granite, CED, Egypt revised the convex (M-type) T1 accompanying with concave (W-type) T3 to the physico-chemical conditions that prevailed during the alkali-metasomatism of the Kab Amiri granitoids have always been changing and the alteration products vary from one place to another.

Fig.12: Chondrite- normalized REE diagram (Boynton, 1984) for the investigated pegmatite

ZhenHua et al., (2010) stated that the peculiar MW-type tetrad effect might be an indicator for Au mineralization of reworked plutons and this vision could be applied in the study area where there is high Ag and Au contents in the altered granite comparing with the fresh one. So, the gold mineralization was determined for the first time by this new geochemical exploration method. Conclusions Field, petrographic, mineralogical and geochemical investigations in the present work indicate that syenogranite at Abu Furad area is affected by multistages hydrothermal alteration processes along brittle structures, this alteration comprises silicification, hematitization, sericitization, saussuritization, chloritization and muscovitization. Hydrothermal alterations in silicified syenogranite samples show an increase in SiO2, MnO , Na2O, Cr, Nb, Zr, Ga, Hf, Th and U and a decrease in TiO2, Al2O3, Fe2O3, MgO, CaO, K2O, P2O5, L.O.I., Ni, Co, V,Cu, Pb, Zn, Rb, Ba, Sr,Ta, Y, Sn, Tl, Mo and REEs while desilicified and hematitized granites exhibit increase in TiO2 , Fe2O3, MnO, MgO, CaO, P2O5, L.O.I., in Cr, Ni, Co, V,Cu, Pb, Zn, Cd, Sr, Ta, Nb, Zr, Y, Cs, Ga, Hf, Sn, Th, Tl, U, W, Mo, Ag, Bi and REEs and a prominent decrease in SiO2, Al2O3, Na2O, K2O, Rb and Ba. Hydrothermal alteration is also indicated by a non-CHARAC behaviour of some trace elements (REE tetrad effect, Zr/Hf, Y/Ho, Rb/Sr, Nb/Ta, La/Nb and La/Ta fractionation). Silicified and desilicified syenogranites have strong M-type tetrad effect in the first and fourth tetrad and strong W-type tetrad effect in the third tetrad only in silicified syenogranites. However, one of the pegmatite samples show clear convex (M-type) T4 with concave (W-type) T3. The unusual MW-type tetrad effect may be related to the changes physico-chemical conditions that prevailed during the alkali-metasomatism. The complex MW-type tetrad effect could be considered as geochemical exploration tool for Au mineralization of reworked plutons.

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Cite this Article: Ahmed ME, Mohamed GE, Sayed AO, Sherif Abd El Aziz I (2015). Geochemistry and a composite M- with W-type of REE tetrad effect in altered granites of Abu Furad area, Central Eastern Desert, Egypt. Greener Journal of Geology and Earth Sciences, 3(2):013-029, http://doi.org/10.15580/GJGES.2015.2.111915161.

issn: 2354-2268 submitted: 19/11/2015 accepted: 25/11/2015...

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