ORIGINAL PAPER

Preliminary results of a first record of gold and uraniumin marble from Central Eastern Desert, Egypt: a witnessfor (syn- and post-?) metamorphic mineralizationin metasediments

M. M. Hamdy & G. A. Aly

Received: 22 October 2008 /Accepted: 19 April 2009 /Published online: 13 May 2009# Saudi Society for Geosciences 2009

Abstract This paper records, for the first time, themineralization of gold (0.98–2.76 ppm) and uranium(133–640 ppm) in marbles from the Arabian-Nubian Shieldof the Eastern Desert of Egypt. These auriferous anduraniferous marbles are hosted by sheared and alteredophiolitic serpentinized ultramafic rocks of Gebel El-Rukham (ER), Wadi Daghbag (DG), and Wadi Al Barra-miyah (BM). They occur as massive or banded in pod-likeor bedded shapes. The ER and BM-mineralized marbles areimpure calcitic, whereas the DG marble is impure calcitic toimpure dolomitic. Their protolith are pure limestones anddolomitic limestones with probable argillaceous compo-nents (BM marble), and their metamorphism (Pan-African)was retrograde. Peaks of metamorphism were at granulite-amphibolite facies for the ER and BM marbles, formingdiopside (Al2O3=0.17–1.07 wt.%) at 600–900°C andaugite (Al2O3=2.45–9.40 wt.%) at 825–975°C, and at theamphibolite facies for DG marble, recrystallising thecarbonate minerals and forming tremolite. The lowesttemperatures of metamorphism were at the upper sub-greenschist facies as chlorite (ER and BM marbles) andkaolinite (DG marble) were formed. Metamorphic fluidswere, most probably, essentially binary H2O–CO2 mixtureswith low NaCl and HF concentrations. Gold in the studiedmineralized marbles occurs as native nuggets (10–35 μm)having globule, rod, crescent, and streak shapes, in pores,

vugs, and fissures. The source of gold in all marbles ismostly the country ultramafic rocks. Timing of goldmineralization relative to the marblization and metamor-phism of the country source ultramafic rocks was both syn-and post-metamorphic. Concerning the ER and DGmarbles, it was syn-metamorphic, where Au liberation andtransportation were mostly by the metamorphic fluids. Thecomposition and temperature of these fluids were mostprobably inappropriate for formation of the sulfide com-plexes of gold. The gold mineralization of BM marble, onthe other hand, was mostly post-metamorphic. The miner-alising fluid was of surficial origin under oxidizingconditions. The encountered uranium minerals are ofsecondary origin such as autunite, uranophane, and carno-tite. These minerals occur as fine oval aggregates andirregular grains (10–50 μm) usually filling fissures andvugs. The uranium mineralization can be classified assurficial of ages <1.5 Ma. It is proposed that the U wastransported from its source (might be flesite and trachytedikes for the ER and DG marbles and granite rocks for BMmarble) to the marble rocks by surface and/or undergroundwater related to the pluvial periods in Egypt. In BM marble,U and Au have mutual mineralizing fluid but differentparagenesis.

Keywords Mineralization records . Auriferous-uraniferousmarble . Serpentinite . Metamorphism . Central EasternDesert . Egypt

Introduction

Gold deposits in Egypt are known to occur in the EasternDesert within the Arabian-Nubian Shield (ANS) rocks of950–550 Ma (Vail 1985). They occur either in volcanic and

Arab J Geosci (2011) 4:25–43DOI 10.1007/s12517-009-0054-0

M. M. Hamdy (*)Geology Department, Faculty of Science, Tanta University,31527 Tanta, Egypte-mail: mhamdyeg@yahoo.ca

G. A. AlyNuclear Materials Authority,P.O. Box 530, El Maadi,Cairo, Egypt

volcaniclastic rocks (e.g., Um Samiuki: Rasmay et al. 1983;Abu Marawat: Botros 1995; Dungash: Khalil et al. 2003;Sukkari: Helmy 2000), Algoma-type Banded Iron Formation(e.g., Abu Marawat: Botros 2004; Um Nar: Dardir andElshimi 1992), sheared ophiolitic ultramafic rocks (e.g., ElSid: Harraz 2000; Hutite: Takla et al. 1995; Um El Tuyor:Zoheir 2008), or at contact between mafic and granitic rocks(e.g., Umm Rus: Kamel et al. 1998; Atud: Harraz 2002).

Most uranium deposits in Egypt, on the other hand,are surficial and related to weathered granites (e.g.,Gattar; Esmail 2005; El-Missikat, El Shazly et al. 1982),weathered Hammamat sedimentary rocks (e.g., UmTawat: Shalaby and Moharem 2001), laterites (El-Aassyet al. 2000), and sedimentary phosphorites (El-Kammarand El-Kammar 2002).

In this paper, mineralization of gold (0.98–2.76 ppm)and uranium (133–640 ppm) in marble is recorded in threelocalities in the Eastern Desert of Egypt. These localitiesare Gebel El-Rukham (ER), Wadi Daghbag (DG), and WadiAl Barramiyah (BM; Fig. 1). Marbles which are coarse-grained metamorphosed calcitic or dolomitic rock are the

well representatives for the metasediments in the ANS.These metasediments belong to the Pan-African rockassemblage (suprastructure). Together with the island-arcvolcanic rocks and the ophiolitic rocks, these metasedi-ments were formed in the back-arc during the island-arcstage in the Upper Proterozoic (850 Ma, Stern 2002).However, the formation of these metasediments was mostlyprior to the vulcanicity (Abu El Ela 1985). Thus, themetasediments might be subjected to infiltration of thevolcanic-related fluids which, in turn, precipitate oreminerals. During the Pan-African orogenic stage (550-650 Ma, Clifford 1970), all back-arc rocks were sweptthrusting on the Pre-Pan-African continental rocks (infra-structure). The recycling of the subducted oceanic slabcaused melt generation (giving rise to the older granitoidsand the Dokhan volcanics), devoaltilization, and metamor-phism (Hamdy and Lebda 2007). Carbonates have markedchemical favorability for infiltration of hydrothermal fluids(Liu et al. 1999). Thus, the metacarbonates (marble) areprobably the host for many ore minerals. In addition, thepost-collision magmatism and even surficial alteration and

Fig. 1 Geologic maps of theauriferous and uraniferous mar-bles areas. Maps are modifiedafter the geologic map of WadiAl Barramiyah quadrangle,Egypt (1992)

26 Arab J Geosci (2011) 4:25–43

weathering could also be agents of enrichment andmineralizing the marble rocks.

This work documents, first time, gold and uraniummineralization related to syn- and post-metamorphic infil-tration of fluids in marble. The genesis of this mineraliza-tion is discussed from the petrological, mineralogical, andgeochemical points of view.

Field observations

The areas of Gebel El-Rukham, Wadi Daghbag, and WadiAl Barramiyah lie in the Central Eastern Desert of Egypt(Fig. 1). These areas comprise serpentinized ophioliticultramafics, island-arc metavolcanics and metasediments,and older granitoids. Pre-Pan-African schist is also found inthe vicinity of the ER and DG marbles, while metagabbrosare only recorded in the ER area. The suprastructureophiolitic and island-arc rocks were obducted over theinfrastructure rocks during the Pan-African collision stage(El Gaby et al. 1988). The metamorphic event took place

between 650 and 620 Ma during the collision stage (Fingerand Helmy 1998).

The ERmarble occurs in pod-like or bedded shapes (1–3 mthick and up to 60 m long) along high-angle faults (with NW–SE and N–S strikes) in highly deformed serpentinite rocks(Fig. 2a). The contact between the marble and the countryserpentinite rocks is usually irregular. Serpentinite insome places occurs as fragments within marble rocks.Marble is usually coarse-grained and white but gainsbrownish and reddish hues near contact with serpentinites.These brownish and reddish marbles are rich in silicateminerals which might be observed by the naked eye.Serpentinite rocks close to and at the contact with marbleare also imparted by brownish shades (Fig. 2b). In thesealtered serpentinites, minerals of carbonates, chlorite, andaltered chromite have frequently been encountered. Theseare indications that metamorphic reactions at the contactbetween marble and serpentinite rocks might occur at arelatively higher grade.

The DG marble is usually bedded (2–7 m thick and up to60 m long) with a nearly NW–SE strike and dip about 20°.

Fig. 2 Field photographs of themineralized marble rocks (a, bfor the ER marble; c–e for theDG marble, and f for the BMmarble). a Pods of marble (M)within the altered serpentinite(S). b Altered brown serpentin-ites at the contact (irregular)with the mineralized marble. c,d Bedded marble within schist(Sh) and altered serpentinite (S)rocks. e Dike-like bodies (Di) oftonalite to granodiorite intrudethe serpentinite (S) host rocks. fPods of marble (M) within thealtered serpentinite (S)

Arab J Geosci (2011) 4:25–43 27

Marble is fine-grained and dark black. It usually has latecross-cutting veinlets of coarse-grained calcite. This reflectslower grades of metamorphism to marble and post-metamorphic precipitation of calcite. The DG marbleoccurs usually with altered serpentinites and mylonitizedgraphite and chlorite schist (Fig. 2c, d). The contactbetween marble and the enclosing rocks is tectonic.Interlayer detachment fractures developed between marbleand their country rocks. Small dike-like bodies of tonaliteto granodiorite intrude the serpentinite in places (Fig. 2e),and some quartz veinlets traverse the felsic bodies. Alongthrust and shear zones, the country serpentinites show highalterations with the development of a range of talc andyellowish brown cavernous talc-carbonate rocks, andmarble on the other hand become richer in silicate minerals.

Like the ER marble, the BM marble occurs in pod-likeand bedded shapes (5–8 m thick and up to 100 m long). It isusually gray to grayish white. Their country rocks are madeprincipally of altered serpentinites. Serpentinite rockssometimes occur as fragments within marble. The contactbetween marble and serpentinite is usually not sharp. Atthis contact, the serpentinites are usually highly sheared,foliated, sometimes folded, and become rich in carbonates,graphite, and chlorite (Fig. 2f).

Methodologies

Petrographic examination of the mineralized marbles wascarried out using both polarized and scanning electronmicroscope (SEM at the Nuclear Materials Authority-NMA-Cairo). The SEM is equipped with Link AnalyticalAN-1000/855 energy dispersive X-ray spectrometer (EDX)calibrated using natural standards to identify elements anddetect, semi-quantitatively, the chemical composition ofaccessory minerals of very small size. SEM-EDX waspreferred than the electron microprobe because of its largerelectron spot, which allows safe and easy detections ofmineral grains of very small size. The accelerating voltageused during analysis was 25–30 kV. The analyticalprecisions ranges from 2% to 5% for elements with Z>9and from 5% and 10% for lighter elements. Identification ofminerals and their relative abundances were confirmedusing X-ray diffraction (XRD) at the NMA (results of XRDare not in the manuscript). Electron microprobe analysis ofminerals have been done by a JEOL-ISM 6310 scanningelectron microscope linked with OXFORD-energy disper-sive detector (EDX), model 6687, at the Institute of EarthSciences (Mineralogy and Petrology), Karl Franzens Uni-versity of Graz, Austria. The analyses were done at 15 kVacceleration voltage and 5 nA beam current. Natural andsynthetic minerals were used as standards. The matrixcorrection was calculated with ZAF-correction program.

Concentrations of whole-rock major elements weredetermined at the NMA by conventional wet chemicalanalysis methods according to Shapiro and Brannock(1962). The trace elements (Sr, Rb, Cr, Au, Ag, Ni, Cu,Zn, Pb, Cd and Cl) were detected by atomic absorptionspectrophotometer. Before digestion, samples were heatedto 1,100°C to determine loss on ignition (LOI%). Digestionof samples for Au and Ag analyses was carried out by aquaregia. The analytical precision is ±5%.

Contents of Uchemical and Thchemical were determinedspectrophotometricaly (Colormetric method) after HCldigestion at the NMA. Laboratory gamma-ray spectromet-ric determination of equivalent U (eU) was based on themeasurement of gamma-rays emitted by its daughtersbecause U itself is not a gamma emitter.

Results

Petrography and mineral chemistry

In the studied auriferous and uraniferous marbles, the maincarbonate mineral is usually calcite, except in the DG marblewhere it is sometimes dolomite. In addition to gold anduranium minerals, autunite, uranophane, carnotite and urano-thorite, an assemblage of accessory minerals such as chromite,hematite, goethite, bunsenite (NiO), danbaite [(Cu-Zn) O],quartz, apatite, rare earth elements (REE) minerals (monaziteand allanite), zircon, baddeleyite, halide minerals (halite andsylvite), clinopyroxene, amphibole, talc, chlorite, pyrophyl-lite, kaolinite, and graphite have been encountered. Accordingto the classification of Rosen et al. (2004) (Fig. 3), all studiedmarbles are impure. The mineralogical composition classi-fies the ER and BM mineralized marbles as impure calcitic,with those from DG as impure calcitic to impure dolomitic.

All marbles are massive; however, banded structure(Fig. 4a) is sometimes observed in the ER marble (0.5–2 cm wide) that lies close to the contact with the countryserpentinite. All the samples commonly show the petro-graphic feature that silicate and graphite minerals are in-terspersed in carbonate matrix, whereas other accessoryminerals occur in vugs and fissures. Different textures areobserved in marbles (the terms used are of Winter 2001).The ER and BM marbles exhibit coarser carbonate grains(3–10 mm) with slightly curved to straight grain bound-aries. In the ER marble, grain junctions are sometimestriple, meeting at about 120° angles pointing to well re-crystallization (Moens et al. 1988). The DG marble, on theother hand, is “microgranular” characterized by a smallergrain size (0.05–0.3 mm) and predominantly irregular grainboundaries (Fig. 4b). Sometimes, carbonate crystals in theDG marble are granulated along their borders and contouredby a fine-grained matrix, showing mortar texture.

28 Arab J Geosci (2011) 4:25–43

Concentrations of major components in calcite anddolomite (Table 1) are rather homogenous while minorelements, particularly MnO, FeO, and SrO, are not(Fig. 5a). The highest concentrations of MnO, FeO, andSrO are detected in the calcite and dolomite of the BMmarble. However, dolomite contains always less SrO thanthe corresponding calcite. Opposite relation are encounteredfor FeO and MnO. When calcite and dolomite are equili-brated, Sr prefers to enter calcite to replace Ca, whereas Mnand Fe tend to substitute Mg in dolomite (Kretz 1982;Dickson 1990). The estimated partition coefficients of Fe,Mn, and Sr between calcite and dolomite in the studiedmarbles (KDFe<1, KDMn<1, KDSr>1) suggest that thechemical equilibrium might have been achieved or pre-served between carbonate minerals (Lentz 1994).

Gold mineral occurring in pores, vugs and fissures in thecarbonate matrix is obviously found as nuggets and notincluded in or associated with any sulfide minerals. Thegranularity of these native gold nuggets is generally 10–30 μm for gold in ER, 10–20 μm for DG, and 20–35 μmfor BM. The shape of gold varies from globules or rods(Fig. 4c) in the ER and DG marbles, but it occurs ascrescents or irregular streaks in the BM marble (Fig. 4e). Thecommon trace element found in gold grains is copper. Usingthe semiquantitative results of EDX (Fig. 4d, f), the contentof Cu ranges from 7.46 wt.% in gold from DG marble to8.98 wt.% in that of BM marble. The silver content withingold of ER and BM marbles is negligible. On the contrary,the DG marble contains from 7.95 to 9.63 wt.% Ag.

Uranium minerals detected in the studied marbles aresecondary in origin, except for uranothorite. They vary widelyin abundance and shape occurring usually within kaolinite,hematite, and goethite and sometimes within quartz. In adecreasing order, they can be arranged as follows; autunite >uranophane > carnotite > uranothorite. Autunite occursusually as cluster and chain-shapes (20–50 μm long;Fig. 4g, h) and infrequently as disseminated globules (10–20 μm). Uranophane occurs as fine oval-shaped grains (10–15 μm), while carnotite occurs as irregular spots (30–50 μm;Fig. 4i, j). In most samples, uranothorite occurs as finedisseminated subhedral grains (3–7 μm; Fig. 4k, l) or asirregular relict in other uranium mineral grains. This impliesthat uranothorite might be a primary mineral from whichsecondary uranium minerals have been formed.

Oxide minerals are encountered with all mineral assemb-lages but with various abundances. In the ER and DGmarbles, bunsenite and danbaite are the main oxide minerals,whereas in BMmarble, these oxides are mainly chromite andbunsenite. However, hematite and goethite are present in allmarbles but more concentrated in the BM marble. Quartz isusually anhedral occurring interspersed within the carbon-ate matrix and sometimes in fissures and vugs enclosinguranium and REE minerals. Zircon is detected only in BMmarble occurring as small anhedral grains and sometimesencloses baddeleyite. Zircon, sometimes, contains signifi-cant concentration of U. Apatite occurs in all marbles assubhedral to anhedral grains in vugs. In the DG and BMmarbles, the interspaces between apatite grains are usuallyfilled with graphite. Apatite in all marbles is characterizedby its high REE content. The detected REE mineralsinclude monazite and allanite which occur as anhedralsingle crystals or as aggregates in carbonate matrix. TheEDX of monazite (Fig. 4m) shows that the mineral containsconsiderable amounts of Th and U. Halite and sylvite areusually euhedral occurring in fissures and vugs.

Clinopyroxene occurs only in the ER and BM marbles. Itoccurs as neoblasts and idiomorphic crystals in amphiboleand sometimes as interstitial grains among carbonateminerals. Clinopyroxene in the ER marble is diopside(Al2O3=0.17–1.07 wt.%), whereas that in the BM marble isaugite (Al2O3=2.45–9.40 wt.%; Table 2, Fig. 5b). In someBM samples, augite is the only metamorphic silicatemineral (Fig. 4n) with the assemblage:

augite þ calciteþ dolomite

This may represent a metamorphic assemblage atgranulite facies of metamorphism because of the absenceof hydrous phases. Amphibole is the most abundantaccessory silicate mineral. It ranges from Al2O3-rich(1.06–8.07 wt.%) in BM marble to Al2O3-poor (0.48–0.75 wt.%) in DG marble and from FeO-rich (1.83–6.5 wt.%) in BM marble to FeO-poor (0.69–0.95 wt %) in ER

Fig. 3 Plotting the mineralized marbles on the classification diagramof metacarbonate and related rocks based on modal content ofcarbonate and silicate minerals (Rosen et al. 2004). C carbonateminerals (calcite, dolomite, and aragonite); CS Ca-rich silicateminerals (e.g., diopside-hedenbergite, epidote group minerals, tremo-lite, wollastonite); S Ca-poor silicate minerals

Arab J Geosci (2011) 4:25–43 29

marble (Table 3). On the International MineralogicalAssociation (IMA) amphibole classification of Leake et al.(1997) (Fig. 5c), amphibole in the ER marble usually hasthe composition of tremolite; in the BM marble, it ranges in

composition from the tremolite to the magnesiohornblende,whereas in the DG marble it is magnesiohornblende.Chlorine and fluorine have not been detected in most casesby EDX. Therefore, the hydroxyl site of amphiboles is

Fig. 4 a Banded structure inmarble (sample ER-22). bBack-scattered electron image(BSEI) of the carbonate miner-als (Cc calcite and Dol dolo-mite) and anthophyllite (Anth)which is grown within the do-lomite (sample DG-02). c, d)BSEI and EDX of gold (G) rod(sample DG-01). e, f BSEI andEDX of gold (G) crescent(sample BM-04). g, h BSEI andEDX of the autunite (Aut) clus-ters and chains (sample DG-08).i, j BSEI and EDX of irregularspots of carnotite (Crn) betweencalcite (Cc) and quartz (Qz)(sample BM-08). k, l BSEI andEDX of uranothorite (Urt)subhedral fine grains (sampleDG-08). m EDX of monazite(sample ER-03). n BSEI ofinterstitial pyroxene (Px) grainsamong calcite (sample ER-22).o BSEI of xenoblastic magne-siohornblende (Mgh) and ser-pentine (Serp) fragments withincarbonate matrix (sample DG-01). p BSEI of tremolite (Tr)surrounds magnesiohornblende(Mgh), and chlorite (Chl) xeno-blast coexists with dolomite(Dol) and calcite (Cc) (sampleBM-06)

30 Arab J Geosci (2011) 4:25–43

assumed to be filled principally with OH molecule. It occursas slender subhedral prisms (up to 2 mm long) or as acicularcrystals (Fig. 4o, p). The amphibole sometimes includesrelict clinopyroxenes, and this texture may show retrograderehydration of clinopyroxene. Xenoblastic magnesiohorn-blende with large amounts of Al content usually replacesaugite with the assemblage:

augiteþmagnesiohornblendeþ calciteþ dolomite

Some magnesiohornblendes are heterogeneous in com-position with an Al-rich interior and a tremolitic exterior(Fig. 4p). Prismatic tremolites with relatively homogenouscomposition surround these magnesiohornblendes, coexist-ing with calcite and dolomite. This implies that tremolitemay be formed later than magnesiohornblende. Chlorite ispresent sporadically in the DG and BM marbles. On the

classification of Hey (1954), its composition (Table 4) isusually clinochlore in BM marble and clinochlore andpenninite in DG marble. It is sometimes xenoblastic andlocally coexists with clinopyroxenes, calcite, and dolomite(Fig. 4p). In some places, it is intergrown as lamellae alter-nating with amphibole. Textural relations show that chlorite isa secondary product and may have been formed later thanamphibole. Talc [(Mg# Mg/(Mg+Fe2+)≈1.0, Table 4] isrecorded only in DG marble. The xenoblastic talc withirregular rims is enclosed within or at the rims of pyroxene.This texture implies that the talc may be formed on theexpanse of pyroxene. Pyrophyllite has been detected only inDG marble. In places, it is in the form of individual anhedralcrystals setting in carbonates or in bundles within kaolinite.Kaolinite occurs sporadically in DG marble and forms verysmall crystals, either discrete or finely intergrown withpyrophyllite or as minute flaky aggregates. Textural relations

Fig. 4 (continued)

Arab J Geosci (2011) 4:25–43 31

Tab

le1

Representativeelectron

microprob

eanalyses

ofcalcite

anddo

lomite

inthemineralized

marbles

Mineral

Calcite

Dolom

ite

Locality

ER

DG

BM

ER

DG

BM

Sam

ple

ER-04

ER-16

ER-22

DG-01

DG-08

DG-15

BM-02

BM-06

BM-10

ER-04

ER-16

ER-16

DG-01

DG-02

DG-08

BM-02

BM-06

BM-10

Analysis

4-2

16-5

22-3

1-6

8-19

15-8

2-9

6-2

10-1

4-1

16-6

16-10

1-16

2-7

8-15

2-1

6-7

10-2

SiO

20.14

0.05

0.03

0.21

0.09

0.23

0.05

0.02

0.06

0.07

0.02

0.04

70.27

0.12

0.09

10.12

0.14

0.15

TiO

20.06

0.04

0.00

0.01

0.02

0.06

0.61

0.03

0.28

0.01

0.02

0.03

20.08

0.05

0.02

0.01

0.01

0.02

Al 2O3

0.01

0.04

0.05

0.06

0.09

0.09

0.02

0.09

0.08

0.02

0.06

0.02

50.65

0.62

0.21

0.35

0.1

0.11

FeO

0.08

0.07

0.09

0.08

0.09

0.09

0.65

0.44

0.31

0.4

0.1

0.23

80.19

0.29

0.20

50.82

0.15

1.22

Cr 2O3

0.13

0.07

0.01

0.02

0.03

0.02

0.06

0.00

0.00

0.17

0.09

0.08

0.07

0.01

0.04

0.12

0.14

0.11

MnO

0.05

0.07

0.06

0.07

0.08

0.07

0.19

0.13

0.14

0.11

0.03

0.08

70.09

0.21

0.12

0.3

0.43

0.43

MgO

2.35

2.68

2.94

1.77

2.72

1.97

2.72

1.57

2.56

20.6

2222

.54

21.2

21.2

21.81

21.6

22.8

22

CaO

53.59

52.91

52.67

52.99

53.57

53.26

51.92

53.88

52.52

31.8

29.3

29.98

30.8

30.6

30.64

3028

.228

.4

P2O5

0.29

0.28

0.33

0.33

0.26

0.52

0.01

0.01

0.03

0.05

0.26

0.28

50.29

0.21

0.32

0.07

0.05

0.01

SrO

0.04

0.05

0.02

0.05

0.01

0.06

0.08

0.06

0.10

0.01

0.01

0.01

40.02

0.01

0.02

10.04

0.03

0.04

Na 2O

0.54

0.25

0.50

0.24

0.09

0.18

0.17

0.01

0.00

0.25

0.07

0.15

40.47

0.9

0.61

0.26

0.01

0.01

Total

57.3

56.5

56.7

55.8

57.1

56.55

56.5

56.2

56.08

53.5

52.1

53.48

54.2

54.2

54.09

53.7

5252

.4

No.

ofcatio

nson

thebasisof

6ox

ygen

atom

s

Si

0.01

30.00

50.00

30.02

10.00

90.02

20.00

50.00

20.00

60.00

60.00

20.00

40.02

40.011

0.00

80.011

0.01

20.011

Al

0.00

10.00

50.00

60.00

70.01

00.01

00.00

20.01

00.00

90.00

30.00

60.00

30.06

80.06

50.02

20.03

80.011

0.00

1

Ti

0.00

40.00

30.00

00.00

10.00

10.00

40.04

50.00

20.02

10.00

00.00

20.00

20.00

50.00

30.00

10.00

10.00

00.09

4

Fe2

0.00

70.00

60.00

70.00

70.00

70.00

70.05

30.03

70.02

50.03

10.00

80.01

80.01

40.02

20.01

50.06

20.00

10.00

8

Cr

0.01

00.00

50.00

10.00

20.00

20.00

10.00

50.00

00.00

00.01

20.00

70.00

60.00

50.00

10.00

30.00

90.01

00.03

3

Mn

0.00

40.00

60.00

50.00

60.00

60.00

60.01

60.011

0.01

20.00

90.00

20.00

70.00

70.01

60.00

90.02

30.03

33.01

5

Mg

0.33

70.24

40.42

30.25

90.38

80.55

30.39

60.14

40.37

42.79

83.01

83.00

72.79

62.80

72.88

92.89

53.12

32.79

5

Ca

5.47

95.62

85.43

25.57

65.49

65.23

95.40

35.77

65.51

03.09

62.88

82.87

52.92

02.911

2.91

72.88

92.77

40.00

1

P0.02

40.02

30.02

70.02

70.02

10.04

10.00

00.00

10.00

20.00

30.02

00.02

20.02

20.01

60.02

40.00

50.00

40.00

1

Sr

0.00

20.00

30.00

10.00

30.00

10.00

30.00

40.00

40.00

50.00

00.00

00.00

10.00

10.00

00.00

10.00

20.00

10.00

2

Na

0.10

10.04

70.09

40.04

60.01

70.03

30.03

30.00

30.00

00.04

40.01

30.02

70.08

10.15

50.10

50.04

50.00

20.00

0

Sum

6.00

5.98

6.00

5.96

5.96

5.93

5.96

5.99

5.96

6.00

5.97

5.97

5.94

6.01

5.99

5.98

5.97

5.96

XCaC

O3

95.70

94.48

94.05

94.63

95.66

95.11

92.71

96.21

93.79

56.7

52.4

53.54

5554

.654

.71

53.5

50.3

50.6

XMgCO3

4.94

5.63

6.17

3.72

5.71

4.14

5.71

3.29

5.38

43.3

46.3

47.33

44.5

44.5

45.8

45.3

47.8

46.2

XFeC

O3

0.13

0.11

0.15

0.13

0.15

0.15

1.05

0.71

0.50

0.65

0.16

0.38

30.31

0.47

0.33

1.32

0.24

1.96

XMnCO3

0.08

0.11

0.10

0.12

0.13

0.11

0.31

0.21

0.23

0.18

0.05

0.14

10.15

0.34

0.19

40.48

0.69

0.69

XSrCO3

0.07

0.08

0.04

0.09

0.02

0.10

0.13

0.10

0.16

0.01

0.01

0.02

40.04

0.01

0.03

50.07

0.04

0.07

32 Arab J Geosci (2011) 4:25–43

show that kaolinite may be a secondary product and formedlater than pyrophyllite. Graphite is quite frequent, relativelypredominant in black and gray marbles from DG and BM,where it is dispersed uniformly between carbonates.

Geochemical characteristics

Whole-rock major oxides composition (Table 5) suggests thatthere no marked differences between the studied threeoccurrences of the mineralized marbles. It is evident that,within each occurrence, analyzed major oxides are not mu-tually distributed. However, SiO2 and Fe2O3t (in the DG andBM marbles) display more systematic variation with sampleplace to the country serpentinites. These two oxides are higherin samples closer to the contact. The content of the traceelements, on the other hand, is more diverged (Table 5). The

normalization of the obtained trace elements data to the post-Achaean carbonates (PAC: Gao et al. 1998) and marine shale(MSh: Li 1991) suggests that the studied marbles are enrichedin most trace elements relative to the PAC and sometimes tothe MSh (Fig. 6). The most important and distinctiveenrichments are in Au (750- to more than 2,000-fold thePAC) and U (100- to 600-fold the PAC). This suggestspotential mineralization of these two economic metals (0.98–2.76 ppm Au and 133–641 ppm U). As far as the writers areaware, this significant mineralization has never been recordedbefore in the ANS of the Eastern Desert of Egypt. The studiedmarbles are also enriched in Cr, Ni, and Cl and sometimes inNa. The enrichment of marbles in Cl and Na are explained bythe presence of halite and sometimes to sylvite.

It is important to note that the uranium content in thestudied marbles is in a distinct disequilibrium state. The

Locality ER BM

Sample ER-04 ER-16 ER-22 BM-02 BM-06 BM-07Analysis 4-11 16-1 22-5 2-6 6-12 7-10

SiO2 55.06 55.28 55.09 51.90 55.19 53.41

TiO2 0.09 0.09 0.07 0.28 0.16 0.01

Al2O3 1.07 0.54 0.17 5.39 2.45 9.40

FeO 0.43 0.04 0.22 5.76 3.40 5.63

Cr2O3 0.15 0.05 0.03 0.06 0.09 0.03

MnO 0.01 0.02 0.01 0.06 0.01 0.03

MgO 20.78 20.16 17.94 23.63 26.72 21.92

CaO 20.21 22.45 25.42 12.68 11.49 9.35

Na2O 1.75 1.52 0.92 0.01 0.00 0.02

K2O 0.15 0.03 0.06 0.21 0.00 0.13

Total 99.69 100.18 99.93 99.97 99.51 99.92

No. of cations on the basis of 6 oxygen atoms

TSi 1.948 1.954 1.978 1.846 1.949 1.905

TAl 0.045 0.022 0.007 0.154 0.051 0.095

M1Ti 0.003 0.002 0.002 0.007 0.004 0.000

M1Al 0.000 0.000 0.000 0.154 0.051 0.095

M1Fe2+ 0.000 0.000 0.007 0.000 0.000 0.000

M1Cr 0.004 0.001 0.001 0.002 0.003 0.001

M1Mg 0.993 0.996 0.960 0.919 0.942 0.699

M2Mg 0.102 0.066 0.000 0.334 0.464 0.467

M2Fe2+ 0.013 0.001 0.000 0.171 0.101 0.168

M2Mn 0.000 0.001 0.000 0.002 0.000 0.001

M2Ca 0.766 0.850 0.978 0.483 0.435 0.357

M2Na 0.120 0.104 0.064 0.001 0.000 0.001

M2K 0.007 0.001 0.003 0.010 0.000 0.006

Sum 3.993 3.999 3.997 3.990 4.000 3.994

WO 40.86 44.42 50.28 25.30 22.39 21.13

EN 58.45 55.50 49.37 65.64 72.42 68.89

FS 0.69 0.09 0.36 9.06 5.19 9.98

Table 2 Representative electronmicroprobe analyses ofpyroxene in the mineralizedmarbles

Arab J Geosci (2011) 4:25–43 33

chemically analyzed uranium (Uchemical) is 50- to 300-foldthe radiometrically determined uranium (eU).

Discussion

Marble protolith

The chemistry of carbonate minerals can provide importantinformation about their origin. The contents of SrO, FeO, and

MnO in calcite and dolomite from the studied auriferous anduraniferous marbles are compared with those in carbonateminerals from marbles of sedimentary origin (Borra, India:Le Bas et al. 2002 and Sol Hamed, ED-Egypt: El-Shibiny etal. 2005) and carbonatite rocks (Grenville, Canada: Moecheret al. 1997 and Tamil Nadu, East Africa: Le Bas 1999)(Fig. 5a). This compilation shows that calcite and dolomitefrom the studied marbles are compositionally similar to thoseof sedimentary origin. The studied marbles have much lowerconcentration of SrO and higher concentration of FeO andMnO than carbonatites.

Table 3 Representative electron microprobe analyses of amphibole in the mineralized marbles

Locality ER DG BM

Sample ER-04 ER-22 ER-22 DG-01 DG-02 DG-08 DG-15 BM-02 BM-06 BM-07 BM-10Analysis 4-12 22-1 22-6 1-10 2-6 8-7 15-9 2-3 6-5 7-6 10-4

SiO2 52.77 52.55 51.96 53.64 54.43 53.86 53.15 54.97 54.51 52.34 51.37

TiO2 0.04 0.05 0.10 0.02 0.05 0.02 0.03 0.08 0.01 0.11 0.18

Al2O3 1.24 0.87 1.08 0.75 0.48 0.53 0.68 2.63 0.24 4.02 8.07

FeO 0.95 0.85 0.71 1.35 1.12 2.03 1.69 2.62 2.22 2.86 6.51

Cr2O3 0.03 0.08 0.09 0.00 0.01 0.01 0.01 0.01 0.04 0.04 0.19

MnO 0.02 0.03 0.04 0.03 0.01 0.03 0.01 0.08 0.02 0.07 0.06

MgO 22.14 23.04 22.97 27.58 28.17 27.40 26.45 22.95 23.59 23.96 17.59

CaO 20.05 19.78 20.07 13.70 12.08 13.13 15.05 14.02 14.95 12.96 12.39

Na2O 0.74 0.65 0.74 0.85 1.64 0.93 0.37 0.32 1.60 0.80 0.63

K2O 0.01 0.01 0.01 0.03 0.09 0.05 0.01 0.00 0.00 0.06 0.15

97.98 97.92 97.76 97.95 98.08 97.97 97.45 97.67 97.18 97.22 97.14

No. of cations on the basis of 23 oxygen atoms

TSi 7.401 7.386 7.324 7.413 7.482 7.451 7.413 7.555 7.639 7.302 7.190

TAl 0.204 0.143 0.179 0.121 0.078 0.086 0.111 0.426 0.040 0.661 0.810

TTi 0.004 0.005 0.010 0.002 0.005 0.002 0.003 0.000 0.001 0.011 0.000

CAl 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.521

CCr 0.003 0.008 0.010 0.000 0.001 0.001 0.001 0.001 0.004 0.004 0.021

CFe3+ 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.215 0.000 0.000 0.315

CTi 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.008 0.000 0.000 0.019

CMg 4.641 4.812 4.826 5.000 4.999 4.999 4.999 4.701 4.928 4.982 3.670

CFe2+ 0.112 0.100 0.084 0.000 0.000 0.000 0.000 0.066 0.067 0.013 0.447

CMn 0.002 0.004 0.004 0.000 0.000 0.000 0.000 0.010 0.000 0.000 0.007

CCa 0.242 0.076 0.076 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

BMg 0.000 0.000 0.000 0.683 0.774 0.652 0.501 0.000 0.000 0.000 0.000

BFe2+ 0.000 0.000 0.000 0.156 0.129 0.235 0.197 0.000 0.193 0.321 0.000

BMn 0.000 0.000 0.000 0.004 0.001 0.004 0.001 0.000 0.002 0.009 0.000

BCa 2.000 2.000 2.000 1.157 1.097 1.110 1.301 2.000 1.805 1.670 1.858

BNa 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.142

ACa 0.781 0.910 0.955 0.871 0.683 0.836 0.949 0.064 0.440 0.267 0.000

ANa 0.201 0.177 0.203 0.229 0.437 0.248 0.101 0.086 0.435 0.215 0.030

AK 0.001 0.002 0.002 0.005 0.016 0.009 0.002 0.000 0.000 0.011 0.027

Sum 15.59 15.62 15.67 15.64 15.70 15.63 15.58 15.15 15.56 15.47 15.06

34 Arab J Geosci (2011) 4:25–43

In the studied mineralized marbles, the presence ofaccessory silicate minerals, such as clinopyroxene, amphi-bole, chlorite, pyrophyllite, and kaolinite might be a markerfor a protolith of either limestones and dolomites withvariable argillaceous content (Le Bas et al. 2002) orlimestones and dolomites interacted with hydrothermalfluids before or during metamorphism. The formation ofthese silicate minerals can be attributed to decarbonationand hydration reactions during metamorphism.

The presence of chromite and bunsenite as carrier of Crand Ni (up to 2,455 and 492 ppm, respectively in the BMmarble) refers to possible contribution of the countryserpentinite rocks in formation of marbles. Cr is mutuallydistributed with SiO2 and Al2O3 in the ER and DG marbles[correlation coefficient (r)=−0.969 and −0.916, respective-ly, for Cr versus SiO2 and =0.984 and 0.963, respectively,for Cr versus Al2O3; Fig. 7a, b]. This indicates that the

source of SiO2 and Al2O3 in these marbles is most probablythe country serpentinite rocks. In contrary, this relation isnot detected for the BM marble, since SiO2 and Al2O3 mayhave argillaceous precursor and reacted with the carbonateduring metamorphism.

Metamorphism

The formation temperatures of the metamorphic minerals inthe studied auriferous-uraniferous marbles are estimatedusing different thermometers (Table 6). As the interlockingcrystals of dolomite and calcite are interpreted to be inequilibrium, thus the geothermometer of Anovits andEssene (1987), depending on the XMgCO3 in calcite, isused. It gives range of 550–650°C for ER marble; 500–560°C for DG marble, and 450–560°C for BM marble.Pyroxene thermometry of Lindsley (1983) based on the

Table 4 Representative electron microprobe analyses of chlorite and talc in the mineralized marbles

Mineral Chlorite Talc

Locality DG BM DG

Sample DG-01 DG-08 DG-15 BM-06 BM-10 BM-13 DG-02 DG-08 DG-08 DG-15 DG-15Analysis 1-8 8-11 15-6 6-13 10-14 13-2 2-1 8-32 8-33 15-3 15-10

SiO2 33.15 31.95 30.05 28.21 28.29 31.64 61.46 60.96 60.72 64.88 60.93

TiO2 0.10 0.12 0.10 0.11 0.22 0.10 0.05 0.02 0.11 0.06 0.03

Al2O3 18.86 19.94 20.69 22.47 19.16 21.09 0.76 0.41 0.47 0.52 0.91

FeO 0.01 0.00 0.00 0.00 0.03 0.07 0.39 0.14 0.31 0.09 0.22

Cr2O3 12.85 13.56 10.24 9.25 5.55 5.83 0.02 0.04 0.08 0.01 0.08

MnO 0.00 0.01 0.01 0.01 0.00 0.10 0.02 0.06 0.01 0.02 0.06

MgO 21.74 22.12 25.54 24.14 25.53 29.19 35.23 34.96 35.98 32.51 35.56

CaO 0.15 0.10 0.17 0.35 1.04 0.19 0.06 0.66 0.15 0.17 0.04

Na2O 0.02 0.02 0.03 1.45 1.79 0.00 0.65 0.55 0.56 0.19 0.47

K2O 0.01 0.01 0.01 0.00 0.00 0.05 0.01 0.01 0.05 0.01 0.03

Total 86.88 87.83 86.8 85.99 81.61 88.26 98.67 98 98.8 98.5 98.5

O atoms 36 36 36 36 36 36 24 24 24 24 24

Si 6.485 6.221 5.855 5.569 5.808 5.925 3.799 3.794 3.754 3.969 3.773

Al 0.055 0.030 0.034 0.037 0.066

AlIV 1.515 1.779 2.145 2.431 2.192 2.075

AlVI 2.830 2.794 2.603 2.793 2.441 2.576

Ti 0.014 0.018 0.015 0.016 0.034 0.014 0.002 0.001 0.005 0.003 0.001

Fe2+ 2.102 2.208 1.669 1.527 0.953 0.913 0.020 0.007 0.016 0.005 0.011

Cr 0.001 0.000 0.000 0.000 0.005 0.010 0.001 0.002 0.004 0.000 0.004

Mn 0.000 0.001 0.001 0.001 0.000 0.016 0.001 0.003 0.001 0.001 0.003

Mg 6.340 6.421 7.419 7.104 7.814 8.149 3.246 3.244 3.316 2.965 3.283

Ca 0.031 0.020 0.035 0.074 0.229 0.038 0.004 0.044 0.010 0.011 0.003

Na 0.008 0.008 0.011 0.555 0.713 0.000 0.078 0.066 0.067 0.023 0.056

K 0.002 0.002 0.002 0.001 0.000 0.012 0.001 0.001 0.004 0.001 0.002

Sum 19.33 19.47 19.8 20.07 20.19 19.73 7.21 7.19 7.21 7.02 7.20

Mg# 0.75 0.74 0.82 0.82 0.89 0.90 0.994 1.00 0.995 1.00 1.00

Arab J Geosci (2011) 4:25–43 35

equilibrium of Di–En–Hd–Fs and that of Nimis and Taylor(2000) based on the enstatite content in clinopyroxeneminerals calculate temperatures ranging from 600°C to900°C for diopside in ER marble and from 825°C to 975°Cin augite from BM marble. In chlorite, the variations in siteoccupancy of AlIV was used as a geothermometer (Cath-elineau and Nieva 1985). The ranges of the estimatedtemperatures for chlorites in DG and BM marbles are 179–

245°C and 238–276°C, respectively. Judging from thepresence of tremolite, talc, pyrophyllite, and kaolinite, themetamorphic temperatures inferred for the mineral forma-tions would range from 500°C to 600°C, 400°C to 500°C,300°C to 400°C, and <260°C, respectively (Winter 2001).

Based on the textural relationships and the mineralassemblages of silicates and carbonates (described in“Petrography and mineral chemistry” section), metamor-

Fig. 5 a SrO–MnO–FeO wt.%plot of dolomite and calcitecompositions in the mineralizedmarbles and in other marbles.Data are from Kretz (1980);Moecher et al. (1997); Le Bas etal. (2002) and El-Shibiny et al.(2005). b Composition of theclinopyroxene in the ER andBM marbles according to theclassification of Morimoto et al.(1988). c Composition of am-phibole in the mineralized mar-bles according to theclassification of Leake et al.(1997). Symbols are as in Fig. 3

36 Arab J Geosci (2011) 4:25–43

Tab

le5

Con

centratio

nsof

major

andsometraceelem

entsandtheLOI%

ofthestud

iedmineralized

marbles

Locality

Gebel

El-Ruk

ham

WadiDaghb

agWadiAlBarramiyah

Sam

ple

ER-04

ER-07

ER-12

ER-16

ER-22

DG-01

DG-02

DG-08

DG-10

DG-15

BM-02

BM-04

BM-06

BM-07

BM-10

Major

oxides,ClandLOI(w

t.%)

SiO

28.20

5.25

3.22

2.20

3.80

4.00

5.00

2.96

8.40

7.00

2.30

3.21

7.60

4.79

8.00

TiO

20.08

0.07

0.06

n.d.

0.06

0.12

0.02

0.18

0.21

0.29

0.08

0.11

n.d.

0.34

0.46

Al 2O3

0.49

0.82

1.10

1.30

0.90

0.90

0.77

1.37

0.63

0.72

0.77

1.85

1.50

0.82

0.26

Fe 2O3

2.50

2.78

2.94

3.10

2.80

7.00

2.00

4.50

3.70

4.20

2.00

3.14

2.10

3.56

4.20

MnO

0.02

0.01

0.03

0.01

0.02

0.05

0.12

0.04

0.09

0.09

0.04

0.14

0.23

0.09

0.12

MgO

8.00

6.95

3.98

11.10

2.60

23.00

14.00

9.21

7.92

6.50

13.00

9.26

14.00

8.96

4.20

CaO

45.00

45.90

47.24

42.00

48.80

26.00

35.00

40.59

40.96

45.00

45.00

44.96

40.00

43.95

45.00

P2O5

1.10

0.09

0.05

0.03

0.01

0.27

0.20

0.03

0.12

0.02

0.64

0.59

0.02

0.63

0.26

Na 2O

1.40

0.87

0.64

0.50

0.40

0.40

0.30

0.66

0.37

0.45

0.50

0.53

0.60

0.47

0.60

K2O

0.12

0.05

0.09

0.12

0.06

0.03

0.03

0.03

0.02

0.03

0.06

0.06

0.06

0.03

0.06

SO3

0.02

0.09

0.02

0.12

0.10

0.04

0.20

0.08

0.12

0.07

0.03

0.07

0.03

0.08

0.10

Cl

0.90

0.38

0.78

0.20

0.80

0.20

0.50

0.50

0.52

0.60

0.40

0.58

0.70

0.67

0.20

LOI

32.00

38.00

39.00

39.00

39.00

38.00

41.00

39.00

39.00

35.00

35.00

35.00

33.00

35.00

36.00

Total

99.83

99.40

99.15

99.68

99.22

100.01

99.14

99.15

99.87

99.76

99.82

99.50

99.84

99.39

99.46

Trace

elem

entsin

ppm

Cr

3033

.88

39.62

43.65

37.75

125.89

8517

1.5

60.3

6524

54.7

149.2

130

105.9

1285

Ni

10.90

12.71

20.36

25.70

15.65

48.98

43.40

52.10

39.72

36.05

492.45

45.34

81.30

41.72

340.50

Cu

16.60

20.54

22.71

23.65

21.55

10.20

62.75

37.35

55.24

48.20

29.25

45.82

37.10

100.25

201.15

Zn

62.05

46.90

55.81

71.85

494.65

43.40

210.2

108.5

196.3

173.10

48.75

257.40

387.60

169.32

207.25

Rb

n.d.

n.d

n.d

n.d.

n.d.

n.d.

60.25

32.70

56.34

52.55

97.45

91.20

n.d.

75.36

102.80

Sr

n.d.

n.d.

n.d.

n.d.

201.50

70n.d.

59.41

40n.d.

30.25

230

100.96

20

Ag

0.53

0.47

0.22

0.44

0.28

0.26

0.26

2.02

0.37

0.58

0.19

0.24

1.86

0.39

0.51

Cd

3.60

2.14

1.96

0.05

1.05

7.60

3.65

5.60

4.93

6.10

4.80

5.62

10.40

4.36

2.05

Au

2.76

2.23

1.78

1.37

2.09

1.22

1.32

0.98

2.22

1.80

1.54

1.65

1.28

1.35

1.44

Pb

16.30

3.85

1.52

34.45

1.80

21.55

12.40

121.9

18.4

11.10

29.70

36.90

40.35

44.36

58.10

Th

120

103

96110

9013

030

0.0

110.0

396.4

490

190

298

260

321

436

U14

125

619

433

213

317

564

1.0

191.0

536.0

521

458

395

591

576

508

LOI,loss

onignitio

nat

1,10

0°C;n.d.,no

tdetected

Arab J Geosci (2011) 4:25–43 37

phism in each studied marble was mostly retrograde. Thepeaks of metamorphism in the ER and BM marbles reachedthe granulite-amphibolite facies forming diopside andaugite, respectively. Metamorphism passed to the loweramphibolite facies and greenschist facies to form metamor-phic dolomite and calcite, and tremolite, then went down tothe upper subgreenschist facies forming chlorite. In DGmarble, the path of metamorphism started in amphibolitefacies, forming metamorphic calcite and dolomite andtremolite. It passed through greenschist facies to form talcgoing down to the upper subgreenschist facies formingpyrophyllite, chlorite, and kaolinite.

Deciphering the proper composition of the metamorphismfluids, fluid inclusion studies should be carried out. However,using the EDX results, the anions incorporated within thevolatile-bearing minerals are mainly hydroxide and carbonate,while chloride and fluoride are rare. This suggests that themetamorphism fluids most probably were essentially binary

H2O–CO2 mixtures with low NaCl and HF concentrations.The modal abundances of the anhydrous minerals that can beformed by decarbonation reactions in marbles from ER andBM, such as pyroxene, are low (~2–4%). This refers to low-XCO2 equilibrium fluids and, on the other hand, significantquantities of externally derived aqueous fluid during the passof the retrograde metamorphism.

Gold mineralization

The genetic models of the known gold deposits in the ANSrocks of the Eastern Desert of Egypt suggest their formationduring the island-arc and the Pan-African orogenic stages.Formation of gold in the island-arc stage most probablyhappened through exhalative hydrothermal processes dur-ing the submarine volcanic activity where the deposits arehosted in the island-arc volcanic and volcaniclastic rocks(Botros 2004). On the other hand, its formation in the Pan-

Fig. 6 Whole-rock multi-element diagrams normalized topost-Archean carbonate (PAC:Gao et al. 1998) and marine-shale (MSh: Li 1991)

38 Arab J Geosci (2011) 4:25–43

African orogenic stage occurred by hydrothermal alterationof ophiolitic ultramafics and mafics (e.g., Botros 2002;Khalil et al. 2003).

Whatever its genetic styles, gold always occurs asinclusions or associated with sulfide minerals. In contrast,gold occurs in the studied marbles as nuggets disseminatedin carbonate matrix and not associated with sulfideminerals. Actually, the genesis of this gold mineralizationis a disputed topic. Based on the relative timing tometamorphism (marblization), the authors propose threegenetic models for gold mineralization:

1. Mineralization occurred before the marblization (pre-metamorphic), where the source of gold and themineralizing fluids is the island-arc vulcanicity.

2. Mineralization occurred during marblization (syn-metamorphic).

3. Mineralization occurred after marblization (post-metamorphic), where post metamorphic magmatismand/or the surficial processes were responsible formineralization.

Source of gold

Up till now, no one has clearly identified a volumetricallysignificant group of rocks of sufficiently high gold contentto be source rocks (Keays 1984). The absolute gold contentin any lithology appears not to be the critical factor indetermining whether this lithology is potential source rock(Viljoen 1984) but the accessibility of gold for leachingwould seem to be the most important parameter (Keays andScott 1976; Viljoen 1984).

However, the island-arc vulcanicity which took placeduring the arc-stage is a well-known source for gold in itsmineralization in the ANS rocks in the Eastern Desert. Ifthe exhalative hydrothermal processes during the waningphases of island-arc volcanic activity were responsible fordeposition of gold in the back-arc carbonate rocks (i.e., theprotolith of the studied marbles), the present marble-hostedgold should show close temporal and spatial associationwith the ANS island-arc volcanism, and gold should behosted by either iron oxide minerals (like the auriferousbanded iron formation (BIF); e.g., Abu Marawat) or sulfide

ER marble DG marble BM marble

Geothermometry

Pyroxene (Lindsley 1983; Nimis and Taylor 2000) 600–900 825–975

Dolomite-calcite (Anovits and Essene 1987) 550–650 500–560 450–560

Chlorite (Cathelineau and Nieva 1985) 179–245

Minerals present

Tremolite 500–600

Talc 400–500

Pyrophyllite 300–400

Kaolinite <260

Table 6 Formation tempera-tures (°C) calculated for miner-als in the mineralized marblesbased on different geothermom-eters and mineral assemblages

Fig. 7 Variation diagrams of Cr(ppm) versus SiO2 (wt.%) andAl2O3 (wt.%) for the mineral-ized marble rocks. Symbols areas in Fig. 3

Arab J Geosci (2011) 4:25–43 39

minerals (such that in the volcanogenic-sulfide-hosted gold;e.g., Um Samuki). In case of the present new golddocumentation, the ANS island-arc volcanics are absent inthe vicinity of the mineralized marbles, while gold ispresent in carbonate matrix and is not associated withsulfide minerals. This declines the ANS island-arc vulca-nicity as a source of both gold and the mineralizing fluidsin the mineralized marbles.

The spatial relation of the studied mineralized marbleswith the serpentinite rocks points to their role as areasonable source of gold. The high contents of thetransition elements particularly Cr (up to ~2,455 ppm,Table 5) and their strong correlations with Au support thatthe serpentinites are probable sources of gold.

Liberation and transportation of gold

As the source of both SiO2 and Au, in the ER and DGmarbles, is most probably the serpentinite rocks and thecontents of these components are positively correlated [r=0.963 and 0.995 for ER and DG marble, respectively; Fig. 8],thus gold in these marbles most probably associated silicaduring liberation from the source serpentinite rocks andduring transportation to the carbonate rocks. Enrichment ofthe ER and DG marbles in metamorphic silicate minerals inparticular at the contacts with the country serpentinite rocksimplies that reactions that led to transportation of silica to thecarbonate rocks most probably occurred during metamor-phism. Hence, liberation of gold might have happened duringmetamorphism of the ultramafic rocks (syn-metamorphicmineralization). The peak of metamorphism of the ultramaficrocks was at the transitional greenschist-amphibolite faciesand took place mostly during the obduction (Hamdy and

Lebda 2007). At the P–T conditions of this metamorphism,brittle–ductile and brittle structures along thrusts weredeveloped (El Gaby et al. 1988), providing favorable channelways for fluid flow. Fluids moved along thrusts and shearzones reacted with the ultramafic rocks and liberated gold.

Yet, the absence of sulfides hosting or associating gold inthe ER and DG marbles, contrasting all the ANS ultramafic-related gold mineralizations, is debated. In known ANSultramafic-related auriferous mineralizations, it is suggestedthat both Au and S were released through breakdown ofsulfide minerals in the source rocks, and they were trans-ported to the mineralizing hydrothermal fluids as very stablecomplexes (Seward 1993). Here, in the ER and DG marbles,the authors suggest that the fluids of metamorphism, whichare proposed to be responsible for the liberation andtransportation of gold from the serpentinite rocks, hadinappropriate composition, temperature, and pressure toform the gold–sulfide complexes. Believing that these fluidswere essentially binary H2O–CO2 mixtures with low NaCland HF concentrations, the systematic of gold transportationfrom the serpentinites source and deposition in carbonaterocks during metamorphism were mainly in hydroxylcomplexes (AuOH0, Au (OH)2

−) and subordinately in halidecomplexes. Gold is present as AuOH0, Au(OH)2

− and Au0

in alkaline solutions up to pH=14 and 250°C in thepresence of a magnetite–hematite redox buffer assemblage(Seward 1993), and forms AuOH0 in alkaline solutions inthe absence of other ligands up to 750°C and 1.5 kbar(Ryabchikov et al. 1985). Also, gold is soluble in chloridesolutions over a wide range of temperatures and pressuresup to 800°C and 3 kbar (Glyuk and Khlebnikova 1982).

On the contrary, in the BM marble, gold associatinguranium (as indicated by the strong negative correlationbetween their contents, r=−0.994; Fig. 9) was mostprobably liberated, transported, and deposited after marbliza-tion (post-metamorphic mineralization) and metamorphismof the ultramafic source rock. The post-metamorphic age ofthe mineralization is based on the age of U mineralization (aswill be discussed in the next section) which is <1.5 Ma (aftermetamorphism). Occurrence of gold as irregular grains withstreak and crescent shapes, in fractures and pores of marble,not including in or associating sulfide minerals, and thedistinctive high contents of Cr and Ni in the hosting BMmarble, all indicate that leaching of gold took place mostlythrough high degree of ultramafic source rocks alteration andin the oxidation zone.

Uranium mineralization

In discussing the genesis of uranium mineralization in thestudied marbles, a very important result should be analyzed,that the Uchemical is much higher than the radiometricallymeasured uranium (eU). Thus, the estimated eU/Uchemical

Fig. 8 Bivariate diagram of Au (in ppm) versus SiO2 (wt.%) for themineralized marble rocks. Symbols are as in Fig. 3

40 Arab J Geosci (2011) 4:25–43

ratio in the mineralized marbles is usually lower than unityindicating that it is recently added (i.e., the daughters whichemit gamma-ray are not yet produced, or at least the decayseries did not reach an equilibrium state). According toReeves and Brooks (1978) U attains equilibrium in about1.5 Ma. Therefore, the ages of uranium mineralization inthe studied marbles is <1.5 Ma (post-metamorphic), asthere was not enough time to restore equilibrium. The post-metamorphic mineralization of uranium in marbles advo-cate, undoubtedly, for it is secondary in origin. Moreover,the concentration of uranium minerals as fracture- and pore-filling and their mode of occurrence as phosphate, silicate,and vanadate endorse the secondary origin of uranium.Such secondary uptake of U imparts the studied marbleswith strong radiometric signature relative to other carbo-nates, e.g., the PAC contains 1.1 ppm U. Therefore, thisuranium mineralization can be considered as surficial typeaccording to the uranium deposits classification developedby the IAEA (1996). Such uranium mineralization mostlyformed due to weathering of primary source (not theprotolith carbonate rocks), transportation and deposition inmarble. This occurred either by underground or meteoricwater.

The source of uranium in the studied marbles is still amatter of controversy. As far as the authors are aware, thereis no possible supplier for uranium in the vicinity of thestudy area. However, the volcanic activity as represented byfelsite and trachyte dikes in ER and DG and the graniterocks in BM can be considered as possible sources. Theinconsistency between the age of these old igneousactivities and the suggested <1.5 Ma age of the Umineralization can be understood in terms of the U age

that expresses its latest mobilization. Osmond et al. (1999)reported that there was an episode of uranium migrationand secondary mineral formation in the Eastern Desert ofEgypt (ca. 100,000 years). This age is coincident withpluvial periods in Egypt, during which the Eastern Desertwas flooded by surface water. This means surface watercould be a favorable factor for the secondary uraniumconcentration in the studied marble rocks. During thesepluvial periods, the underground water level is raised, andconsequently, the underground water can also play a role insecondary uranium formation under suitable conditions oftemperature and pH (Osmond et al. 1999).

It seems eligible that collaborated oxidizing fluids haveliberated and drained U from its initial sources. Uponoxidation, U4+ became chemically unstable and readilycombines with two oxygen ions forming easily solubleuranyl ion (UO2)

2+ (Korzeb 1997; Kraemer and Genereux1998; Williams 2003). The drained quotient of U willsustain disequilibrium state for the source rock and themineralizing fluids as well. According to Weber et al.(2004), liberation of U from its source is mostly controlledby deformation that causes cracking and damaging in thecrystal structure of the U-bearing minerals.

The formed uranyl ion is not very soluble by itself, but itneeds complexing agent to become more soluble (Elevatorski1978). Such complexation takes place via phosphate, silicate,or vanadate groups (Levinson et al. 1982; Korzeb 1997).Precipitation of uranium as secondary minerals or in crystalstructure of existing minerals depends on the prevailingphysicochemical conditions. The evaporation, complexionwith liginds, or adsorption on iron oxy-hydroxides and clayminerals are possible scenarios for the precipitation ofsecondary U minerals (autunite, uranophane, and carnotite)along fractures and open voids.

The sympathetic negative relationship between U andAu contents in the BM marble (r=−0.994; Fig. 9) impliesthat the mineralizing fluid of uranium is the same for gold,and mineralizations took place within the last 1.5 Ma but indifferent timings.

Concluding remarks

The recorded mineralization of gold (0.98–2.76 ppm) anduranium (133–640 ppm) occur in the ANS marbles whichare hosted by the ophiolitic sheared and altered serpentin-ites at Gebel El-Rukham, Wadi Daghbag, and Wadi AlBarramiyah in the Central Eastern Desert of Egypt. Marblecompositions are impure calcitic (ER and BM) and impurecalcitic to impure dolomitic (DG), their protolith are purelimestones and dolomitic limestones with probable argilla-ceous components (BM), and their metamorphism (Pan-African) was retrograde. Peaks of metamorphism were at

Fig. 9 Bivariate diagram of Au versus U (in ppm) for the mineralizedmarble rocks. Symbols are as in Fig. 3

Arab J Geosci (2011) 4:25–43 41

granulite-amphibolite facies for the ER and BM marblesand at the amphibolite facies for the DG marble, whereasfor all marbles the lowest temperatures of metamorphismwere at the upper subgreenschist facies. The metamorphicfluids were, most probably, essentially binary H2O–CO2

mixtures with low NaCl and HF concentrations.The source of gold with its native nuggets is mostly the

country ultramafic rocks and the mineralization timing,relative to the marblization and metamorphism of the sourcerocks, was both syn- (ER and DG) and post-metamorphic(BM). We propose that for the syn-metamorphic mineraliza-tion in the ER and DG, liberation and transportation of Auwere by the metamorphic fluids and in hydroxo complexes.The post-metamorphic gold mineralization in the BMmarble, on the other hand, took place mostly by surficialfluids. Mineralizing fluid was also responsible for leachingAu from its serpentinite source. This occurred through highdegree of alteration and in the oxidation zone.

Uranium mineralization in marbles are of surficial typeand their ages <1.5 Ma. The possible source of uranium isfelsite and trachyte dikes for the ER and DG marbles andgranite rocks for the BM marble. The U was most probablytransported from its source to the marble rocks by pluvialperiods-related meteoric and/or underground water. In BMmarble, U and Au have mutual mineralizing fluid butdifferent in paragenesis.

Acknowledgments We acknowledge Profs. I. El-Aassy and M. A.Ibrahim, NMA of Egypt, for help with whole-rock chemical analysesand Mr. T. Abu Alam, Karl Franzens University of Graz, for help withEMP analyses. We express our gratitude to Profs. Hassan Harraz andAhmed El-Kammar and two anonymous reviewers for their criticalreviews and for suggestions on improvement of the manuscript. ChiefEditor Prof. A. Al-Amri is thanked for handling the manuscript.

References

Abu El Ela AM (1985) Geology of Wadi Mubarak District, EasternDesert, Egypt, Ph.D. Thesis, Tanta University, 359 p (unpublished)

Anovits LM, Essene EJ (1987) Phase equilibria in the system CaCO3-MgCO3-FeCO3. J Petrol 28:389–414

Botros NS (1995) Genesis of gold mineralization in the North EasternDesert, Egypt. Ann Geol Surv Egypt 20:381–409

Botros NS (2002) Metallogeny of gold in relation to the evolution ofthe Nubian Shield in Egypt. Ore Geol Rev 19:137–164

Botros NS (2004) A new classification of the gold deposits of Egypt.Ore Geol Rev 25:1–37

Cathelineau M, Nieva D (1985) A chlorite solid solution geo-thermometer. The Los Azufres (Mexico geothermal system).Contrib Mineral Petrol 91:235–244

Clifford TN (1970) The structural framework of Africa. In: CliffordTN, Gass IG (eds) African magmatism and tectonism. Oliver andBoyd, Edinburgh, pp 1–26

Dardir AA, Elshimi KA (1992) Geology and geochemical explorationfor gold in the banded iron formation of Um Nar area, CentralEastern Desert, Egypt. Ann Geol Surv Egypt 18:381–409

Dickson T (1990) Carbonate mineralogy and chemistry. In: TuckerME, Wright VP (eds) Carbonate sedimentology. BlackwellScientific Publications, Oxford, pp

El Gaby S, List FK, Tehrani R (1988) Geology, evolution andmetallogenesis of the Pan-African Belt in Egypt. In: El Gaby S,Greiling RO (eds) The Pan-African belt of Northeast Africa andAdjacent Area. Braunschweig/Wiesbaden, pp 17-68

El Shazly EM, El Kassas IA, El Taher MA (1982) Distribution andorientation of mafic dikes in wadi Abu Zawal area, EasternDesert. Egypt J Geol 16:95–105

El-Aassy IE, Ahmed FY, Afifi SY, El-Shamy AS (2000) Uranium inlaterites of south-western Sinai, Egypt. First Seminar on NRMand Their Technology

Elevatorski EA (1978) Uranium ores and minerals, 88p, Organized byIAEA

El-Kammar AM, El-Kammar MM (2002) On the trace elementscomposition of the Egyptian phosphates: a new approach.Proceedings of the 6th International Conference on the Geologyof the Arab World, Cairo University, Egypt, pp 227-244

El-Shibiny NH, Salem IA, Hamdy MM, Abu-Laban SA (2005)Mineralogy, geochemistry and petrogenetic implications ofmarbles from Sol Hamed Melange, South Eastern Desert, Egypt.Delta J Sci 29:185–211

Esmail EM (2005) Subsurface geological constraints controllinguranium mineralization at the northern part of Gabal Gattar,north Eastern Desert, Egypt. Ph.D. Thesis, Ain Shams University,194 p (unpublished)

Finger F, Helmy HM (1998) Composition and total-Pb model ages ofmonazite from high-grade paragenesis in the Abu Swayel area,south Eastern Desert, Egypt. Mineral Petrol 62:269–289

Gao S, Luo T-C, Zhang BR, Zhang HF, Han Y-W, Zhao ZD, Hu YK(1998) Chemical composition of the continental crust as revealed bystudies in East China. Geochim Cosmochim Acta 62:1959–1975

Glyuk DS, Khlebnikova AA (1982) Gold solubility in water, HCl, HFand sodium and potassium chloride, fluoride, carbonate andbicarbonate solutions at a pressure 1000 kg/cm2. Dokl AkadNauk SSSR 254:190–194

Hamdy MM, Lebda EM (2007) Metamorphism of ultramafic rocks atGebel Arais and Gebel Malo Grim, Eastern Desert, Egypt: mineral-ogical and O-H stable isotopic constraints. Egypt J Geol 51:105–124

Harraz HZ (2000) A genetic model for a mesothermal Au deposit:evidence from fluid inclusions and stable isotopic studies at El SidGold Mine, Eastern Desert, Egypt. J Afr Earth Sci 30:267–282

Harraz HZ (2002) Fluid inclusions in the mesothermal gold deposit atAtud mine, Eastern Desert, Egypt. J Afr Earth Sci 35:347–363

Helmy HM (2000) Pyrite zoning in the Sukkari gold mine, EasternDesert, Egypt: a possible mechanism of gold deposition. 13thAnnual Meeting of the Mineralogical Society of Egypt (Abstract)

Hey MH (1954) A new review of the chlorites. Mineral Mag 30:277IAEA International Atomic Energy Agency (1996) Uranium deposits

classification In: IAEA periodically report “Uranium, Resources,Production and Demand”

Kamel O, El Mahalallawi M, Niazy E, Helmy H (1998) Geochemistryof Umm Rus gold-quartz veins central Eastern Desert of Egypt.Egypt Mineral 10:31–50

Keays RR (1984) Archaean gold deposits and their source rocks: theupper mantle connection. In: Foster RP (ed) Gold 82, the geology,geochemistry and genesis of gold deposits. Balkema, Rotterdam

Keays RR, Scott R (1976) Precious metals in ocean ridge basalts assource for gold mineralization. Econ Geol 71:705–720

Khalil IK, Helba HA,Mücke A (2003) Genesis of the gold mineralizationat the Dungash gold mine area, Eastern Desert, Egypt: amineralogical-microchemical study. J Afr Earth Sci 37:111–122

Korzeb SL (1997) The chemical evolution and paragenesis of uraniumminerals from the Ruggles and Palermo granpegmatites, NewHampshire. Can Mineral 35:135–144

42 Arab J Geosci (2011) 4:25–43

Kraemer TF, Genereux DP (1998) Applications of uranium- andthorium-series radionuclides in catchment hydrology studies. In:Kendall C, McDonnell J (eds) Isotope tracers in catchmenthydrology. Elsevier, Amsterdam

Kretz P (1980) Occurrence, mineral chemistry, and metamorphism ofPrecambrian carbonate rocks in a portion of the GrenvilleProvince. J Petrol 21:573–620

Kretz R (1982) A model for distribution of trace elements betweencalcite and dolomite. Geochim Cosmochim Acta 46:1981–1999

Le Bas MJ (1999) Sovite and alvikite: two chemically distinctcalciocarbonatites C1 and C2. S Afr J Geol 102:109–121

Le Bas MJ, Subbarao KV, Walsh JN (2002) Metacarbonatite or marble?-the case of the carbonate, pyroxenite, calcite-apatite rock complex atBorra, Eastern Ghats, India. J Asi Earth Sci 20:127–140

Leake BW, Wooley AR, Arps CE, Birch WD, Gilbert MC, Grice JD,Hawthorne FC, Kato A, Kisch HJ, Krivovichev VG, Linthout K,Laird J, Mandarino J, Maresch WV, Nickel EH, Rock NM,Schumacher JC, Smith DC, Stephenson NC, Ungaretti L,Whittaker EJ, Youzhi G (1997) Nomenclature of amphiboles.Report of the subcommittee on amphiboles of the InternationalMineralogical Association Commission on New Minerals andMineral Names. Eur J Mineral 9:623–651

Lentz DR (1994) Exchange reactions in hydrothermally altered rocks:examples from biotite-bearing assemblages. In: Lentz DR (ed)Alteration and alteration processes associated with ore-formingsystems. Geol Assoc Can

Levinson AA, Bland CJ, Lively RS (1982) Exploration for U oredeposits. In: Ivanovich M, Harmon RS (ed) Uranium seriesdisequilibrium; applications to environmental problems

Li YH (1991) Distribution patterns of the elements in the ocean- asynthesis. Geochim Cosmochim Acta 55:3224–3240

Lindsley DH (1983) Pyroxene thermometry. AmerMineral 68:477–493Liu G, Ai Y, Feng K, Zhang Z (1999) Metamorphic rock-hosted

disseminated gold deposits: a case study of the Xiaotongjiapuzigold deposit of Eastern Liaoning. Acta Geol Sinica 73(4):429–437

Moecher DP, Anderson ED, Cook CA, Mezger K (1997) Thepetrogenesis of metamorphosed carbonatites in the Grenvilleprovince, Ontario. Can J Earth Sci 34:1185–1201

Moens L, Roos P, De Rudder J, De Paepe P, Van Hende J, Waelkens M(1988) A multi-method approach to the identification of whitemarbles used in antique artefacts. In: Herz N, Waelkens M (eds)Classical marble: geochemistry, technology, trade, NATOASI series,series E: applied sciences. Kluwer Academic Publishers, Dordrecht

Morimoto N, Fabries J, Ferguson AK, Ginzburg IV, Ross M, SeifertFA, Zussman J (1988) Nomenclature of pyroxenes. Mineral Mag52:535–550

Nimis P, Taylor WR (2000) Single clinopyroxene thermobarometryfor garnet peridotites. Part I. Calibration and testing of a Cr-in-

Cpx barometer and an enstatite-in-Cpx thermometer. ContribMineral Petrol 139:541–554

Osmond JK, Dabous AA, Dawood YH (1999) U series age and originof two secondary uranium deposits, Central Eastern Desert,Egypt. Econ Geol 94:273–280

Rasmay AH, Takla MA, Gad MA (1983) Alteration associated withore formation at Umm Samiuki, south Eastern Desert, Egypt.Ann Geol Surv Egypt 13:1–21

Reeves RD, Brooks R (1978) Trace elements analysis of geologicalmaterials. John and Sons, Inc, New York

Rosen O, Desmons J, Fettes D (2004) A systematic nomenclature formetamorphic rocks: 7. Metacarbonate and related rocks. Recom-mendations by the IUGS Subcommission on the systematics ofmetamorphic rocks. Available via SCMR http://www.bgs.ac.uk/SCMR. Accessed 31 May 2004

Ryabchikov ID, Barranova NN, ZotovAV, OrlovaGP (1985) The stabilityof Au(OH) 0 in supercritical water and the metal contents of fluids inequilibrium with a granitic magma. Geochem Int 22:116–117

Seward TM (1993) The hydrothermal geochemistry of gold. In: Foster RP(ed) Gold metallogeny and exploration. Chapman & Hall, pp 37–62

Shalaby MH, Moharem AF (2001) Geochemistry and radioelementdistribution in the fresh and altered Hammamat sedimentaryrocks along Wadi Belih, southern Gabal Um Tawat, north EasternDesert, Egypt. Egypt Sediment 9:123–134

Shapiro L, Brannock WW (1962) Rapid analysis of silicate, carbonateand phosphate rocks. US Geol Surv Bull 1144A, pp 56

Stern RJ (2002) Crustal evolution in the East African Orogen: aneodymium isotopic perspective. J Afr Earth Sci 34:109–117

Takla MA, El Dougdoug AA, Gad MA, Rasmay AH, El Tabbal HK(1995) Gold-bearing quartz veins in mafic and ultramafic rocks,Hutite and Um Tenedba, south Eastern Desert, Egypt. Ann GeolSurv Egypt 20:411–432

Vail JR (1985) Pan-African (late Precambrian) tectonic terrains and thereconstruction of the Arabian-Nubian Shield. Geology 13:839–842

Viljoen MJ (1984) Archaean gold mineralization and komatiites inSouthern Africa. In: Foster RP (ed) Gold 82, the geology,geochemistry and genesis of gold deposits. Balkema, Rotterdam

Weber FR, McCammon RB, Rinehart CD, Light TD, Wheeler KL(2004) Geology and mineral resources of the White MountainsNational Recreation Area, east-central Alaska: U.S. Geol SurvOpen-File Report 88-284, 120 p

Williams PA (2003) Oxide zone geochemistry. New York, London, TokyoWinter JW (2001) An introduction to igneous and metamorphic

petrology. Prentice-Hall Inc, Englewood Cloffs, NJZoheir B (2008) Characteristics and genesis of shear zone-related gold

mineralization in Egypt: a case study from the Um El Tuyormine, south Eastern Desert. Ore Geology Rev. doi:10.1016/j.oregeorev.2008.05.003

Arab J Geosci (2011) 4:25–43 43