Journal of Microscopy and Ultrastructure 1 (2013) 96–110

Contents lists available at ScienceDirect

Journal of Microscopy and Ultrastructure

jou rn al hom ep age : www.elsev ier .com/ locate / jmau

Original Article

Microfabrics and microchemistry of sulfide ores from the 640 FW-Elevel at the Al Amar gold mine, Saudi Arabia

Adel A. Suroura,b,∗, Rami Bakhsha

a Department of Mineral Resources and Rocks, Faculty of Earth Science, King Abdulaziz University, B.O. Box 80206, 21589 Jeddah, Saudi Arabiab Department of Geology, Faculty of Science, Cairo University, Giza, Egypt

a r t i c l e i n f o

Article history:Received 15 November 2013Received in revised form 4 December 2013Accepted 19 December 2013

Keywords:Al AmarSaudi ArabiaVMSEpithermalSulfide deformation fabrics

a b s t r a c t

In a VMS ore at Al Amar gold mine (level 640 FW-E), sulfide minerals are parageneti-cally ordered as follows: pyrite(I)–sphalerite–chalcopyrite–galena–pyrite(II), deformationsvary from brittle to ductile deformation fabrics. Microscopically, the massive sulfides havepyrite porphyoblasts (up to ∼80%) that show evidence of creep dislocation as a result oflow-temperature plastic deformation rather than brittle failure, whereas high-temperatureannealment is completely lacking. Softer minerals such as chalcopyrite fill into fractures inpyrite as narrow slivers. Needle-shaped or lamellar morphology of chalcopyrite, togetherwith the chemical composition of Fe-poor sphalerite (with maximum 0.99 wt% Fe) suggesta combined replacement–coprecipitation mechanism of chalcopyrite disease formationrather than an exsolution texture. Greenschist facies metamorphism produces an ore withdistinct chalcopyrite disease into a stratified ore with microbands of chalcopyrite andsphalerite. Ore microfabrics and uncommon occurrence of epithermal stringers suggestnoticeable effect of the Najd tectonics in the studied level. The EMPA analyses indicatethat all sulfide minerals in the VMS ore are auriferous and the Au contents are consid-erable (up to 0.94, 1.31, 0.16 and 1.20 wt%; in sphalerite, chalcopyrite, galena and pyrite,respectively). Gold in pyrite is “invisible” whereas it occurs as submicroscopic inclusions in

sphalerite, chalcopyrite and galena. The VMS ore of Al Amar deep horizons are characterizedby the occurrence of “invisible gold”, Ag-free galena, Fe- and Ni-poor sphalerite, negligi-ble hydrothermal alteration, plastic deformation of pyrite and non-exsolution origin of thechalcopyrite disease intergrowth which are together strong indicators of low-temperature(250–300 ◦C).

© 2014 Saudi Society of Microscopes. Published by Elsevier Ltd.

1. Introduction

Exploration and exploitation of gold in Saudi Arabia

attracted many researchers and mining companies in orderto fit models of ore genesis and processing during the lastfour decades [1,2]. In the Neoproterozoic Arabian Shield

∗ Corresponding author at: Department of Mineral Resources andRocks, Faculty of Earth Science, King Abdulaziz University, B.O. Box 80206,21589 Jeddah, Saudi Arabia. Tel.: +966 506061416; fax: +966 2 69520295.

E-mail address: aasurour63@hotmail.com (A.A. Surour).

2213-879X © 2014 Saudi Society of Microscopes. Published by Elsevier Ltd.

http://dx.doi.org/10.1016/j.jmau.2013.12.003Open

of Saudi Arabia, some major gold mines are in produc-tion such as Mahd Adh Dhahab, Al Amar, Al Hajar, Bulghahand Suhkaybarat (Fig. 1). In addition to these major pro-ductive prospects or goldfields, some mining companies(e.g. Ma’aden, Arabian Gold and Citadel) currently conductfeasibility studies for the ore exploitation in some otherprospects such as Ar Rjum, Umm Matierah, Bi’r Tawilah, AlAmar south and Shayban [1].

Open access under CC BY-NC-ND license.

Primary gold mineralization in the Arabian shield canbe classified into three main types based on the tectonicsettings and host rocks [2]. The first type is associatedwith volcano-sedimentary sequences (hosted by the Siham

access under CC BY-NC-ND license.

A.A. Surour, R. Bakhsh / Journal of Microscopy

Fig. 1. General geologic map of the Arabian Shield showing majortectonostratigraphic terranes, ophiolite belts, sutures, fault zones, post-accretionary basins in the Arabian Shield of western Saudi Arabia (from[23] with modifications by Johnson and Woldehaimanot [20] and Sternand Johnson [60]). Numbers represent locations of the major gold depositsas follows: (1) Hamdah, (2) Al Hajar & Jadmah, (3) Mahd adh Dhahab, (4)Jabal Shayban, (5) Humaymah, (6) Bulghah, (7) Sukhaybarat, (8) Al Amar,((M

adaTs[idtsopLw3idoafWarta

9) Ad Duwayhi, (10) Um Matierah, (11) Ar Rjum, (12) Ash Shakhtaliya,13) Zalm, (14) Bi’r Tawilah (including Jabal Ghadarah, Al Mansourah and

asarah). The Najd faults are the NW-trending ones in red.

nd Hulayfah Groups, arc metavolcanics), which wereeposited in juvenile oceanic environments and were laterccreted to form crustal blocks before final amalgamation.he class includes gold-rich volcanogenic massive Cu–Znulfide deposits at Jabal Sayid and Wadi Bidah shear zone35], and the epithermal base and precious metal depositsn both Mahd Adh Dhahab and Al Amar. Gold in someeposits of this class may have been remobilized duringhe Najd Fault System to form epithermal mineralizationsuperimposed on the massive sulfide deposits [3,4]. Somef these deposits were subjected to supergene enrichmentrocesses due to chemical weathering processes. Mercier-angevin et al. [5] reviewed 513 VMS deposits in theorld and assigned high-grade auriferous ones as higher as

.46 g/t and added that some of the Nuqrah VMS depositsn Saudi Arabia assay 3.80 g/t gold. The second class of goldeposits is spatially associated with carbonatized ophi-litic serpentinites rocks (listvenites), which were formedlong suture zones during the amalgamation of the dif-erent terranes, e.g. at Bi’r Tawilah, Bi’r Umq and Jabal Al

ask [6,7]. The third class of gold deposits is particularly

bundant in the western parts of the so-called “Afif ter-ane” to the east of the Nabitah suture zone, where syn-o late-tectonic diorite to granite plutons intruded therc metamorphosed volcaniclastics (Siham and Hulayfah

and Ultrastructure 1 (2013) 96–110 97

Groups) and post-amalgamation slightly metamorphosedrocks of the Murdama and Bani Ghayy Groups, e.g. Sukhay-barat, Zalm, Bulghah, Bi’r Tawilah, Ad Duwayhi and JabalGhadarah deposits [8,9]. These deposits are mesothermaland are characterized by low sulfide content (typicallyarsenopyrite, pyrite and/or pyrrhotite) that deposit fromCO2-rich ore fluids as indicated by the detailed miner-alogical and fluid inclusion studies in the Zalm mine area.In addition to secondary oxidation zones with commonauriferous gossans, placers in some goldfields representpromising targets for low-cost gold mining, e.g. Mahd AdhDhahab, Jabal Mokhyat and Hamdah or Hajr [10,11] asanother type of secondary gold mineralization in Saudi Ara-bia.

The Al Amar goldfield (23◦47′ and 45◦03′ E) is one of themajor prospects for gold in the Precambrian shield rocksof Saudi Arabia (Fig. 1). According to [12], some impor-tant mineralized sites are located in the Al Amar Groupwith close confinement to the Al-Amar-Idsas fault (Fig. 2).They include sites for base metals (Al Amar, Khnaiguiyahand Marjan) and precious metals (Al Amar, Umm Ash Sha-lahib, At Taybi and Umm Ad Dabah). Early explorationprograms at Al Amar were in the scope of zinc prospectionfrom 1950s until 1980s, and then a special attention wasgiven to the presence of precious metals in the prospect.Due to the common presence of ancient workings, theDirectorate General of Mineral Resources of Saudi Arabia(DGMR) started to examine and sample the old pits. TheDGMR continued investigation for zinc and executed anunderground exploration program, with a total of 909 mintersections from 1974 to 1976. Extensive explorationby the Riofinex Mining Company indicated resources of1.077 Mt grading 33.1 g/t Au and 7.79% Zn in the North VeinZone (Fig. 3). This was followed by intensive drilling of 49boreholes with a total depth of 13,338 m by Petromin Mid-dle East in the same vein zone [9]. Recent exploration byArabian Gold Corporation [13] some 35 km to the south ofAl Amar gold mine indicates promising extension of eco-nomic VMS deposits with 1.55 Mt@12.3 g/t Au, 21.1 g/t Ag,6.16% Zn and 0.92% Cu.

The present work presents the first detailed ore fab-rics, sulfide mineral chemistry and possible genesis of anewly explored deep horizon in the Al Amar gold mine atthe 640 footwall level below the eastern part of the mine,particularly beneath the Northern Vein Zone. This includesdetailed ore microscopic investigation for accurate spec-ification of ore mineral paragenesis, their parageneticsequence and style of deformation on the microscopicscale. Also, the study aims investigate if the shear effect ofthe Najd fault system extends deep enough to deform theVMS ores in deep levels, or its effect is localized to shallowerlevels.

2. Regional geology

The late Proterozoic Arabian Shield is dominatedby rocks and ores that are contemporaneous to the

Pan-African orogeny, and the shield consists mostly ofjuvenile crust that originated by transpressive suturingbetween East and West Gondwana [14,15] that culmi-nated ∼550 Ma. It was formed through a prolonged history

98 A.A. Surour, R. Bakhsh / Journal of Microscopy and Ultrastructure 1 (2013) 96–110

ine alon

Fig. 2. Geological map of the gold belt that include Al Amar gold m

of accretion of mainly interoceanic island arcs alongsutures now marked by ophiolites [16–18]. These accre-tions episodically occurred between 900 and 550 Ma ago,as the Mozambique Ocean was closed. Accretion may alsohave included an oceanic plateau formed by the head ofan upwelling mantle plume. The Arabian Shield in SaudiArabia is only slightly metamorphosed (except for someareas of gneissic rocks) and it constitutes one of the bestpreserved and well exposed Neoproterozoic assemblagesresulting from the accretion of volcanic arcs. It is over-lain to the east, north, and south by a thick succession ofPhanerozoic sedimentary rocks, and is bound to the west bythe Red Sea that now separates the individual Arabian andNubian Shields in western Arabian Peninsula and NE Africa,respectively. According to [19], the Arabian Shield rocks areexposed at present time as a result of Late Mesozoic upliftin the west-central part of Saudi Arabia resulting in a struc-tural high referred to as the Ha’il arch, and Cenozoic upliftalong the western margin of Saudi Arabia, in associationwith Red Sea spreading. According to the tectonic terrane

concept, the Wadi Ar Rayn terrane, that includes the AlAmar goldfield, is of probable continental origin in contrastto other terranes in the Arabian Shield (AS) that pertainintraoceanic island arc terranes, for example Midyan, Hijaz

g Al Amar-Idsas Fault in the Wadi Ar Rayn quadrangle. From [28].

and Asir [20,21]. Detailed survey of the volcanosedimen-tary basins in the Arabo-Nubian Shield reveals that orogenywas closely linked with subsidence and deposition, and thisrepresents an excellent example of crustal growth at theend of the Precambrian and a prime target for calibratingNeoproterozoic Earth history [22].

Al Amar village and its mineralized environs is located atthe eastern extremity of the AS crystalline rocks near thecontact with the Phanerozoic sedimentary cover (Fig. 2),some 200 km to the southwest of Ar Riyad (Riyadh) cityalong the highway that connects the capital with the har-bor city of Jeddah on the Red Sea, at the so-called “WadiAr Rayn quadrangle”. This quadrangle contains some ofthe oldest gneisses in the Arabian shield, in addition toophiolitic rocks, a thick pile of arc metavolcanics in associ-ation with volcanogenic carbonates, chert and jasper, and ayounger association of intrusive rocks that are representedby granitoids, granites and dyke swarms of different ages[23].

About 1/4 of the Wadi Ar Rayn quadrangle area (lati-

tudes 23◦00′–24◦00′ and longitudes 45◦00′–45◦30′) isoccupied by shield rocks whereas the 3/4 is occupied bygently dipping Phanerozoic sandstone and limestone. Thelate Proterozoic AS in the quadrangle is distinguished into

A.A. Surour, R. Bakhsh / Journal of Microscopy and Ultrastructure 1 (2013) 96–110 99

gold min

tAcmscntbflmas

Fig. 3. The detailed geology of Al Amar

wo main lithostratigraphic units that are separated by thel Amar-Isdas mega fault (Fig. 2). To the east of the fault,arbonatized serpentinite, metamorphosed mafic to inter-ediate volcanic rock with pillow structures, chert and

mall intrusion of plagiogranite, in addition to arc metavol-anics outcrop and fill a large synclinorium. Further to theorth (e.g. Halaban and Ad Dawadmi quadrangles) outsidehe area shown in Fig. 2, this ophiolitic and arc assem-lage are unconformably overlain by a thick succession of

yshoid sediments, mainly greywacke and shales as theajor components of the so called “Abt Formation” [24,25]

t the western side of the Al Amar-Isdas Fault. From thetructural point of view, both the ophiolitic assemblage

e area by Calvez et al. [27], cited in [9].

and the Abt Formation were folded and metamorphosedin the greenschist facies during at least two major phasesof deformation including the Idsas phase that producedthe Al Amar-Idsas Fault as a mega thrust zone [26]. Thenorthern segment of the Al Amar-Idsas Fault is a singlehigh-angle thrust outlined by a thin band of serpentiniteand listvenite whereas the southern segment is dividedinto sub-segments in which the ophiolitic rocks, a part ofthe Abt Formation and younger units such as the Al Amar

and Hamir Groups (arc metavolcanics) were subjectedby small scale and high angle thrust fault system [25].According to the latter workers, both the ophiolitic assem-blage and the arc metavolcanics were intruded by ∼630 Ma

roscopy

100 A.A. Surour, R. Bakhsh / Journal of Mic

syn-orogenic Pan-African calc-alkaline granitoids (tonaliteand trondjhemite), followed by post-orogenic calc-alkalineand alkaline microgabbro and microgranite that have beenaffected partially by the epi-orogenic movement of themajor NW Najd fault system that dominates in the AS withextension further to the west in the Nubian Shield of Egyptand the Sudan.

3. Geology of Al Amar goldfield area

In the Wadi Ar Rayn terrane, the layered successionsthat unconformably overly the ophiolitic rocks are called“the Urd Group” (Ar Ridayniah and Abt formations) whichis equivalent to the Baish and Bahah Groups in the southernparts of the AS [40]. The Abt Formation covers a vast areaof the map (Fig. 2) and consists of a thick alternating foldedsuccession of conglomerate, greywacke, siltstone, shaleand subordinate volcanic flows. The Abt formation showsdifferent metamorphic imprints in terms of greenschistfacies regional metamorphism (quartz–chlorite–sericiteschist) and superimposed contact metamorphism thatproduced garnet-biotite hornfelsic schist. Age of the AbtFormation can be much old as 800 Ma as it is intrudedby 724 Ma trondjhemite [27]. To the east of the Al Amar-Idsas Fault, the area is occupied by a thick successionof volcanosedimentary rocks that belong to the Al AmarGroup (Fig. 2). Calvez et al. [27] determined 667 Ma ageof the Al Amar Group. Lofts [4] summarized that Al AmarGroup is intruded by syn-orogenic 700–600 Ma gabbro,quartz-diorite, granodiorite and trondjhemite, as well as bypost-orogenic 600–550 Ma granite. Al Amar Group (equiv-alent to Halaban group in northern terranes) itself issub-divided into lower massive metandesite and upper sili-cic volcanic flows, pyroclastic and epiclastic rocks [25].

Based on their lithological and geochemical character-istics, Pouit et al. [28] distinguished the Al Amar Group into4 volcanic units as follows from oldest to youngest (Fig. 3).Thickness of each unit is ∼100 m in average with the excep-tion of the 1st unit that is 300 m [28] and up to 600–700 m[29].

Volcanic Unit 1: Common intermediate and subordinatemafic pyroclastics that are intercalated with ash-falls, andan upper effusive andesitic to rhyolitic lava flow.

Volcanic Unit 2: Felsic pyroclastics are dominant, withsubordinate intercalations of jasper and chert bands. Thesebands represent Fe- and silica-rich submarine volcanicproducts as diagnostic features of the Al Amar group [30].

Volcanic Unit 3: Intermediate to felsic pyroclastics (ash-fall, pumice and lapilli tuff), volcanic massive sulfides(VMS), barite and Fe–Mn carbonates.

Volcanic Unit 4: Volcanic siltstone, sandstone, crystaltuff and volcanogenic carbonate.

In addition to these four units, another 5th volcanic unitbelonging to the Al Amar Group outcrops outside the areashown in Fig. 3. The Volcanic Unit 5 is dominated by inter-mediate to felsic pyroclastics [4].

Structurally, the first 3 units were subjected to com-

pressive forces and the first unit represents the footwallof Al Amar deposit. Accordingly, the data revealed fromthe boreholes indicate horst-dominated paleorelief in units1, 2 and 3 over which the units 4 and 5 were deposited.

and Ultrastructure 1 (2013) 96–110

According to extensive drilling programs, Al Amar depositsare classified into (1) volcanic massive to sub-massive sul-fides that are confined to unit 3 and was defined by Ferrandet al. [12] as “stratiform” VMS deposits and as layered VMSdeposits by Idris [26] and (2) vein-type deposits travers-ing units 1, 2 and 3 [4]. In the upper 250 m succession, themineralized quartz veins (∼0.5–0.7 mm stringers and up to4.5–5 m lodes) are represented by crustified milky quartzwith native gold [31,32].

4. Megascopic characteristics of the footwall VMSore

Beneath the northern vein zone, the present authors col-lected the VMS samples from a deep level (640 FW-E) inthe underground mine. These VMS hand specimens weresampled from Zn-rich zones that dip 72–76◦ to the NW,and are confined to the unit 3 that is less deformed thanits counterpart in shallower depths up to 250–300 m. Dueto the rarity of silicic injections and shearing, the Zn-richore at the 640 FW-E level shows very minor hydrothermalalteration that are only represented by detached batches ofminor sericite and chlorite, if present. The VMS orebody andmetapyroclastics of unit 3 are traversed by basaltic dykes.

On the megascopic scale, the massive sulfide oresamples from the investigated level are dominated by spha-lerite and relatively lesser amounts of chalcopyrite. Whenthe VMS samples are slightly brecciated or fragmented,they bear some quartz stingers and boudins with commonoccurrence of megascopic idioblastic pyrite (∼0.2–0.5 mmin dimension). Pyritization can be seen in the fragmentedbasaltic to andesitic metapyroclastics and sometimes theirhand specimens contain up to ∼75% pyrite. In contrast tothe shallower horizons, the present authors identified nomegascopic galena in the VMS of the 640 FW-E level thatcould be identified by the naked eye. Another characteristicfeature of this deep level is the complete absence of exhala-tive barite, gypsum and anhydrite that are common in unit3 of the shallower levels [4]. There is no talc alteration inthe 640 FW-E level and in one sample only, a ∼0.9 mm widespecular-quartz stringer traverse the studied VMS.

5. Procedures and techniques

The authors collected samples from the undergroundmine at Al Amar using a major vertical shaft at groundsurface level of ∼900 m a.m.s.l, then the newly explored640 FW-E level was accessed at depth of ∼310 m from thesurface. In this underground mine, the vertical distancebetween nearly horizontal levels is 15 m. Samples weretaken in 10 sections representing both floor and ceilingof the level and horizontal interval between each sectionwas 20 m. A complete set of polished mounts and polishedthin-sections were prepared for the detailed ore petro-graphic study using transmitted and ore microscope. Theset includes 15 polished mounts and 22 polished thin-sections. Also, 23 thin-sections were studied for the mutual

relationship between the ore and the host rocks.

Backscatter imaging and chemical composition of thesulfide minerals and native gold was determined usinga JEOL electron-probe microanalyzer JXA-8200 at the

roscopy

DEmAvcAwTt4

6

gmagtohtVItl

r(orsmsltwoocfonot

ssiilhtrtsc(n(

A.A. Surour, R. Bakhsh / Journal of Mic

epartment of Mineral Resources and Rocks, Faculty ofarth Sciences, King Abdulaziz University, Saudi Arabia. Oreinerals were analyzed for Fe, S, Cu, Zn, Pb, Te, Sb, Bi, As,u and Ag. Analytical conditions were 20-kV acceleratingoltage, 20-nA probe current, 3 �m probe diameter and theounting time was 10 s for each element except for Au andg which were 50 and 30 s, respectively. Standards usedere pyrite for Fe and S, pure metals for Cu, Au, Ag, Zn, Pb,

e, Sb, and Bi, and gallium arsenide for As. At, such condi-ions, detection limit of gold in the analyzed sulfides was00 ppm which is equivalent to 0.04%.

. Paragenesis and ore microfabrics

The modal percentages of sulfide or minerals sug-est distinct lateral variation in the contents of the fourajor minerals; namely sphalerite, chalcopyrite, galena

nd pyrite that are arranged here with respect to the para-enetic sequence. On scale of 20–30 cm, the percentages ofhe three minerals vary greatly and each of them dominatesver the rest two. Generally, the percentage of pyrite inighly pyritized samples amounts 70–80%, and it is evidenthat this percentage of pyrite includes the mineral in bothMS sample and its associated quartz veins and stringers.

n the silica-poor samples, the VMS ore samples contain upo 65% sphalerite and up to ∼50% chalcopyrite. Pyrite in theatter case is a minor component and never exceeds ∼10%.

Ore minerals in the host rocks lacking sulfides areepresented by high percentage of idioblastic magnetite60–200 �m wide) that sometimes forms microbands withccasional jasper in few samples. The sulfide-rich hostocks contains up to ∼70% ore minerals. In the quartztringers traversing these sulfide-rich samples, sulfide oreinerals amount 30–35%. The current detailed ore micro-

copic investigation revealed that the Al Amar 640 FW-Eevel is rich in shpalerite with chalcopyrite that some-imes reaches a percentage of ∼83% in the VMS samples,hereas homogeneous sphalerite amounts as low as 2–3%

f the bulk sulfides. Fig. 4 shows the progressive stagesf transformation of chalcopyrite disease into bandedhalcopyrite–sphalerite ore. This figure documents the per-ect alignment of chalcopyrite needles along the two setsf sphalerite cleavages [octahedral (1 1 1) planes] and theseeedles start to transform into irregular chalcopyrite blebsf different sizes. Finally, microbanding of chalcopyrite inhe host sphalerite is resulted.

As mentioned before, the common parageneticequence of the studied VMS ore is pyrite(I)–phalerite–chalcopyrite–galena–pyrite–pyrite(II). Pyrite(I)s the earliest and cracked whereas pyrite(II) is not craked,dioblastic (50–160 �m wide) and post-dates both spha-erite and chalcopyrite (Fig. 5a). Sometimes, pyrite(I) takesomogeneous sphalerite as inclusions or engulfs crystalshat show chalcopyrite disease (Fig. 5b). Massive chalcopy-ite fills into the cracks of coarse pyrite (Fig. 5c) whereasiny chalcopyrite fills the micro-fractures in quartztringers (Fig. 5d). Galena is microscopic and encloses

halcopyrite and sphalerite but cut by idioblastic pyriteFig. 5e and f). Extensive idioblastic pyrite(II) dissemi-ation is recorded in some metabasaltic pyroclastic hostFig. 6a) where this pyrite is common in the silicified part

and Ultrastructure 1 (2013) 96–110 101

of this host (Fig. 6b and c). Sometimes, the metabasalticfragments themselves contain few sporadic pyrite crystals(Fig. 6d). Occasionally, the metapyroclastic host showsdistinct two generations of pyrite (Fig. 6e). When stringersbecome more wide (∼0.8–0.9 mm), the sulfide ore min-erals can be easily seen inside, e.g. chalcopyrite (Fig. 7aand b), translucent homogeneous sphalerite (Fig. 7c), andpyrite (Fig. 7d–f).

The specular-quartz stringers contain typical feather-like hematite (Fig. 8a). The specular hematite invadesmassive chalcopyrite (Fig. 8b and c) and crystals with chal-copyrite disease (Fig. 8d). Sulfide-bearing quartz stringersand specularite-quartz stringers also invade the maficdykes. In very rare cases, the stringer becomes wide(∼1.7 cm) and quartz shows crustification, comb structureand growth zoning.

As a direct indication of compressive deformation,which is lacking on the megascopic scale, few samplesshow oriented chalcopyrite needles in the chalcopyrite dis-ease. This results in open-folding as seen by curvature ofthe intergrown chalcopyrite needles either along two setsof cleavage or a single cleavage plane (Fig. 8e and f). Localshearing or uplift could cause such slight deformation.

7. Mineral chemistry of sulfide ores minerals

Three representative samples were analyzed by theelectron microprobe for their base and precious metalsin addition to sulfur. The obtained data are given inTables 1–4.

Table 1 shows that chalcopyrite has the highest con-centration of gold (up to 1.31 wt%) among the four sulfideminerals in the paragenesis at Al-Amar deep footwall (640FW-E). The studied sphalerite is Fe-poor and this wasnoticed microscopically because the mineral was translu-cent in the thin-section. The amount of Fe in the sphaleritelies in the range of 0.17–0.99 wt% (Table 2). Very low con-tent of Co in sphalerite ranges from 0 to 0.02 wt%. Galena isAg-free but a single grain contains 0.16 wt% Au (SB-13-15)(Table 3). Accordingly, this galena is not argentiferous butslightly auriferous occasionally.

The content of gold in pyrite is also considerable andit amounts up to 1.203 wt% (Table 4). The obtained analy-ses suggest that there is little substitution for Fe2+ by As3+

and Ni2+. The maximum contents of As and Ni in the ana-lyzed pyrite are 1.35 and 0.131 wt%, respectively. Pyrite(I) isarsenian in which As ranges from 0.71 t0 1.35 wt% whereaspyrite(II) is almost free of As, i.e. non-arsenian. In the restof sulfide minerals, As is also low or absent. It is com-pletely absent in galena whereas it amounts up to 0.029and 0.02 wt%, respectively in chalcopyrite and sphalerite.Some submicroscopic native gold inclusions have an aver-age composition of Au93Ag7.

8. Discussion

The ore assemblage, nature of host metapyroclastic

rocks and style of mineralization indicate that two VMS oretypes can be distinguished: (1) VMS rich in chalcopyrite andsphalerite where both minerals are intergrown and exhibittypical chalcopyrite disease texture and (2) “stringer” ore

102 A.A. Surour, R. Bakhsh / Journal of Microscopy and Ultrastructure 1 (2013) 96–110

Fig. 4. Progressives stages for the transformation of chalcopyrite disease intergrowth into micorbands of chalcopyrite (yellow) and sphalerite (gray).All microphotographs are in polarized reflected light. (a) Perfect orientation of chalcopyrite needles and fine blebs along the (1 1 1) octahedral cleavageof sphalerite. (b) Coalescence of fine blebs and needles into coarser blebs and lamellae. (c) Increase of bleb-sized chalcopyrite and confinement of thechalcopyrite needles to the core. (d) Demolishing of chalcopyrite needles and dominancy of irregular blebs. (e) Advanced stage of irregular chalcopyriteblebs formation. (f) Alternating microbands of chalcopyrite and sphalerite.

Table 1Electron microprobe analyses of auriferous chalcopyrite.

Element, wt% SB11-1 SB11-2 SB11-3 SB11-4 SB11-5 SB11-6 SB11-7 SB11-8 SB11-12 SB11-13 SB11-14 SB11-15 SB11-16 SB11-17

S 33.77 33.9 34.58 34.28 34.19 34.72 34.8 34.97 34.59 34.54 34.47 34.46 34.24 34.29Fe 29.64 29.72 29.87 30.91 29.81 29.76 29.69 29.83 29.69 29.42 29.03 30.12 29.6 29.51Cu 35.41 34.42 34.61 32.89 34.3 33.22 34.5 34.1 33.48 33.9 33.82 32.99 34.48 34.89Ni 0.02 0 0 0 0 0 0 0.03 0 0 0 0.06 0 0Co 0 0.01 0 0 0 0.01 0 0.01 0 0 0 0.02 0.04 0.04Zn 0.07 0.04 0.05 0.07 0.03 0.02 0.08 0.05 0.01 0.04 0.06 0.11 0 0.06Pb 0 0 0 0 0 0 0 0 0.06 0.02 0 0.01 0.01 0As 0 0 0.03 0 0.03 0 0 0 0 0 0 0 0 0.03Au 0.96 0.78 0.34 0.82 0.99 1.31 0.11 0.72 1.21 0.72 1.17 0.49 0.72 0.57Ag 0.01 0.05 0 0.04 0 0 0 0.04 0 0 0 0 0 0.02Total 99.88 98.92 99.48 99.01 99.35 99.04 99.18 99.75 99.04 98.64 98.55 98.26 99.09 99.41

A.A. Surour, R. Bakhsh / Journal of Microscopy and Ultrastructure 1 (2013) 96–110 103

Fig. 5. Sulfide microfabrics in VMS and some epithermal quartz stringers. Microphotographs from (a) to (e) are in polarized reflected light. (a) Euhedralpyrite(II) porphyroblasts (Py) disrupting chalcopyrite (Cp) and partially encloses sphalerite (Sph). (b) Sub-idioblastic pyrite(II) (Py) with some sphaleriteinclusions (Sph) and engulfment of chalcopyrite–sphalerite (Cp–Sph). (c) Soft slivers of chalcopyrite (Cp) that behaves ductile and protrudes into fracturesi (d) Rems (Py) thag rite (Cp)

tmccs(bpdisao

n coarse pyrite porphyroblast (I) (Py) that practiced plastic deformation.

tringers. (e) BSE image showing euhedrality of pyrite(II) porphyroblastsalena (Gn) at the contact of pyrite(II) porphyroblasts (Py) with chalcopy

hat is characterized by homogeneous sphalerite and com-on occurrence of pyrite porphyroblasts in crustified and

omb-textured milky quartz stringers and few veins. Theurrent data suggest that the remobilized part of the ore intringers has been formed by open-space filling of fracturestensile fissures) due to metal redistribution that enhancedy the mobilization of base metals in the VMS ore contem-oraneous to greenschist metamorphism and formation ofuctile shear planes at the eastern part of the studied level

n analogy to similar world examples (e.g. [33]). This levelhows complete absence of supergene sulfides that occurt the surface or at shallower levels of Al Amar mine areanly [34].

obilized chalcopyrite (Cp) ore filling fractures in quartz of the epithermalt traverse into chalcopyrite (Cp) galena (Gn). (f) BSE image showing fine.

As shown in the chapter of ore microscopy, the tex-tures vary and represent events of slight to moderatebrittle and ductile deformation that often characterizethe VMS of the Arabian Shield during the Neoproterozoicsubmarine–subaerial volcanism and formation of volcanicarcs [35]. Usually, textural data provide valuable informa-tion about the mineralization history and hence can serveeffectively successful exploitation and ore dressing [36].Ore textural and deformational fabrics of the mineral are

so helpful for understanding history of mineralization. Inthe VMS ore body, the pyrite(II) porphyroblasts are mostlyidioblastic (Figs. 5 and 6a and e) and show no evidence ofgrain annealment, triple junction or “foam texture” that

104 A.A. Surour, R. Bakhsh / Journal of Microscopy and Ultrastructure 1 (2013) 96–110

Fig. 6. Pyrite enrichment of the silicified (in vicinity of quartz stringers) intermediate metapyroclastic host. (a) Dusting by high percentage of idioblasticpyrite(II) (Py), plane polarized light. (b) Considerable amount of pyrite(II) (Py) associating quartz (Qtz) and chlorite-rich fragments (Chl), plane polarized

with tra) outsid

light. (c) Extensive silicification (Qtz) with pyrite(II) (Py) in the pyroclsticfragment with fine pyrite (Py) inside and some pyrite and chalcopyrite (Cpreflected light.

indicate directly the absence of any superimposed recrys-tallization by both regional and contact metamorphism[37]. The studied pyrite porphyroblasts from Al Amar arepartially brecciated and the fractures resulted from duc-

tile deformation at the time the surrounding softer sulfidesdeform by clastic flow mechanism and fill the fracturesin pyrite [38,39]. Then, it is evident that the Al Amarpyrite deforms by dislocation creep mechanism at low

chybasalt fragments, polarized reflected light. (d) Details of trachybasalte, polarized reflected light. (e) Two of idioblastic pyrite(II) (Py), polarized

temperature because if temperature was high, pyrite mightdeform by dislocation glide [40].

Texturally, the few sulfide-bearing stringers that trav-erse the VMS main ore body at the Al Amar 640 FW-E

level are characterized by homogeneous sphalerite. Onthe other hand, all of polished mounts from the VMSitself show common occurrence of chalcopyrite disease orchalcopyrite–sphalerite intergrowth. Barton [41] was the

A.A. Surour, R. Bakhsh / Journal of Microscopy and Ultrastructure 1 (2013) 96–110 105

Fig. 7. Sulfide ore minerals in epithermal quartz stringers. (a) Homogeneous chalcopyrite (Cp) in comb-structured quartz (Qtz), crossed nicols. (b) Same asin (a) but in polarized reflected light. (c) Translucent Fe-poor homogeneous sphalerite (Sph) in quartz (Qtz) stringer traversing the metatuff, plane polarizedlight. (d) Euhedral pyrite(II) (Py) in stringer traversing the metatuff, plane polarized light. (e) Silica microfracture (Qtz) in the metatuff with some pyrite(Py) and finer chalcopyrite (Cp), polarized reflected light. (f) Crustified silica stringer (Qtz) with nearly banded batches of pyrite (Py), chalcopyrite (Cp) andsphalerite (Sph), polarized reflected light.

Table 2Electron microprobe analyses of Fe-poor auriferous sphalerite.

Element, wt% SB13-11 SB13-12 SB13-14 SB13-18 SB14-8 SB14-9

S 33.19 32.87 32.96 32.55 33.41 33.4Fe 0.68 0.95 0.17 0.99 0.3 0.28Cu 0 0.16 0.07 0.8 0.12 0.09Ni 0 0.05 0.07 0 0 0Co 0 0 0 0.02 0 0Zn 64.03 64.8 64.64 64.82 64.41 64.75Pb 0 0.02 0 0 0.02 0As 0 0 0 0 0.02 0Au 0.94 0.19 0.6 0.5 0.92 0.91Ag 0 0 0 0 0 0Total 98.84 99.04 98.51 99.68 99.2 99.43

106 A.A. Surour, R. Bakhsh / Journal of Microscopy and Ultrastructure 1 (2013) 96–110

Fig. 8. Epithermal specular-quartz stringers and deformation of chalcopyrite disease in the VMS ore. All microphotographs are in polarized reflected light.(a) Specular hematite (Spec) with typical feather-like habit. (b) Quartz (Qtz) and specular hematite (Spec) protruding into microfracture in chalcopyrite(Cp) of the VMS ore. (c) Details of specular hematite (Spec) in chalcopyrite (Cp). (d) Chalcopyrite disease traversed by quartz (Qtz) and specular hematite(Spec). (e) Curvature of chalcopyrite needles and lamellae (yellow) oriented along two cleavage sets in sphalerite (gray). (f) Same as in (e) but curvilinearchalcopyrite is arranged along one set of cleavage.

Table 3Electron microprobe analyses of Ag-free galena.

Element, wt% SB13-10 SB13-13 SB13-15 SB13-19 SB13-20

S 13.81 13.02 13.76 13.72 12.99Fe 0.51 0.27 1.03 1.81 1.73Cu 0.14 0 0 0 0.05Ni 0 0 0 0 0Co 0 0.02 0 0 0.08Zn 0.05 1.09 1.55 0.99 1.82Pb 85.35 86.03 83.59 84.98 82.47As 0 0 0 0 0Au 0 0 0.16 0 0Ag 0 0 0 0 0Total 99.86 100.43 100.09 101.5 99.14

A.A. Surour, R. Bakhsh / Journal of Microscopy

Tab

le

4El

ectr

on

mic

rop

robe

anal

yses

of

auri

fero

us

pyr

ite.

Elem

ent,

wt%

SB11

-9b

SB11

-10b

SB11

-11b

SB13

-1a

SB13

-2a

SB13

-3a

SB13

-6a

SB14

-1b

SB14

-2b

SB14

-3b

SB14

-4b

SB14

-5b

SB14

-6b

SB14

-7b

SB14

-10b

SB14

-11b

S

52.5

3

53.2

3

52.7

4

52.9

7

53.6

4

53.8

53.3

1

52.6

2

52.8

8

52.6

7

53.0

4

53.7

53.6

9

52.9

9

53.3

3 53

.57

Fe

45.7

5

46.3

1

46.8

2

45.2

5

45.8

7

44.5

3

45.5

1

45.6

8

45.7

6

45.6

9

46.1

3

45.1

2

46.1

5

45.8

4

45.8

9

46.2

5C

u

0.04

0.03

0.07

0

0.06

0.09

0

0

0

0

0.01

0.02

0

0

0

0N

i

0.13

0.01

0

0.03

0

0.05

0.06

0

0

0

0

0

0.07

0

0.01

0.07

Co

0.02

0

0.02

0

0

0.01

0.02

0

0.03

0

0.05

0

0.02

0

0.02

0Zn

0.05

0

0.01

0.03

0.08

0.07

0

0

0.01

0.05

0

0.09

0

0.11

0.03

0.02

Pb

0

0

0

0

0

0

0

0

0

0

0

0

0

0.01

0

0A

s

0

0

0

1.35

0.71

1.01

1.17

0

0

0

0.02

0

0

0.01

0

0A

u

1.01

0.29

0

0.39

0.46

0.88

0.74

0.23

0.51

0.56

0.26

0.71

0.72

0.67

1.2

0.65

Ag

0

0

0

0

0

0.01

0

0

0.03

0

0.02

0

0 0

0.02

0To

tal

99.5

3

99.8

7

99.6

6

100.

02

100.

82

100.

45

100.

81

98.5

3

99.2

2

98.9

7

99.5

3

99.6

4

100.

65

99.6

3

100.

5

100.

56

aPy

rite

(I).

bPy

rite

(II)

.

and Ultrastructure 1 (2013) 96–110 107

first to use the term chalcopyrite disease and stated that itis common in old and modern VMS deposits, black smok-ers at MOR spreading centers, skarns and epithermal veindeposits, but never occurs in Mississippi Valley-type sulfidedeposits. The origin of this intergrowth is greatly contro-versial and can be formed by a variety of processes such asexsolution crystallization [42], solid-state replacement anddiffusion induced segregation reactions [42–44], coprecip-itation mechanism [43,45] and in few cases by supergenealteration [46]. The morphology of chalcopyrite inclusionsin Al Amar sphalerite host (fine needles and lamellae) isin favor of replacement reaction and atom diffusion ratherthan an exsolution origin. In addition, the Fe-poor natureof the sphalerite host and presence of irregular bleb-likechalcopyrite inclusions indicate that coprecipitation mech-anism also played an important role in the developmentof Al Amar chalcopyrite disease [43]. Collectively, the fac-tors that influence uptake of Fe by sphalerite include sulfurfugacity and temperature [47]. With high sulfur fugacity,there is an increase of Fe3+ charge compensation for Znvacancies [48].

The mineral chemistry of sulfides in the Al Amar 640FW-E level indicates that sphalerite, chalcopyrite, galenaand pyrite Au-rich and free submicroscopic inclusions ofgold are responsible for high Au content in sphalerite,chalcopyrite and galena [49]. In most pyrite crystals, Auis invisible or lattice bound [50]. Similar world examplesof auriferous VMS are characterized by the occurrence of“invisible” gold where Au atoms is concentrated into solidsolution within the sulfide structures and do not form dis-crete or free “visible” gold due to absence or negligiblehydrothermal alterations or high-T metamorphism evenif Au is low as 0.02 wt% [51]. Translucent sphalerite inthin sections and the EMPA analysis of the mineral areindicative of its Fe-poor nature in which Fe never exceeds0.99 wt%. The analyzed sphalerite is Co-poor and this sug-gests that the atomic substitution of both Fe2+ and Co2+

for Zn2+ was very minor. Fig. 9a shows a misleading or anapparent negative correlation between contents of Au andZn in sphalerite. Actually, the Zn contains is almost con-stant within error for sphalerite, and then a vertical trendfor mechanical mixtures of sphalerite and sub-microscopicgold. Cd is detected spectrally in this Fe-poor sphalerite(not shown in Table 2) which is an indicator of low-temperature crystallization that never exceeds 250–300 ◦C[52]. Galena is Ag-free similar to that reported by Sharpand Buseck [53] and this suggests single ionic substitutionof 2Sb3+ for 3Pb2+ at low-temperature <300 ◦C instead ofthe common coupled substitution of Ag+ + Sb3+ for 2Pb2+ ifAg is present as documented by the latter workers. Fig. 9bis a plot of negatively correlated Zn vs. Pb in the galenastructure.

Tectonically, volcanogeneic massive sulfide deposits inthe Arabian Shield and nearby areas are confined eitherto ophiolite sequences such as the volcanic arcs thathave been developed during the Neoproterozoic Pan-African orogeny some 900–550 Ma ago (e.g. [35]). Dubé

et al. [54] distinguished two genetic models for Au-richVMS deposits: (1) conventional syngenetic volcanic-hostedAu-poor VMS mineralization overprinted during regionaldeformation by Au mineralization and (2) syngenetic

108 A.A. Surour, R. Bakhsh / Journal of Microscopy and Ultrastructure 1 (2013) 96–110

alerite a

Fig. 9. (a) Variation of Au vs. Zn in sph

VMS deposits characterized by an anomalous fluid chem-istry (with magmatic input) and/or deposition within ashallow-water to subaerial volcanic setting equivalentto epithermal conditions. Tectonic setting, occurrence ofepithermal veins and stringers, microfabrics and highly dis-persed gold in the Al Amar VMS suggest that the studiedauriferous sulfide ore belongs to the second genetic model.Similarly, Hakim and Chinkul [55] affiliated the Au-richVMS ore at Mahd Adh Dhahab to the second model and theyused the tools of fluid inclusions and stable oxygen isotopesto reach the conclusion that the ore has been deposited inshallow depth and represents a typical epithermal deposit.Based on the criteria materialized in the present paper, theauthors agree with [29] that Al Amar VMS has some similar-ities with the Kuroko-type VMS of Japan regardless the agedifference. The nature of Al Amar VMS ore and nature of itsAl Amar Group host volcanic favor an immature island arc

setting [28] which has been argued by [56] to be formed involcanic arcs that have been developed during the Neopro-terozoic Pan-African orogeny some 900–550 Ma ago, andthe magma is typically transitional to calc-alkaline [25] in

nd (b) variation of Zn vs. Pb in galena.

consistence with other world class VMS (e.g. [5,57]). Ourmegascopic observations, in collaboration with those of[12], support the setting of an immature island arc dueto the presence of intercalated chert and marble bandsas products of explosive subaerial silicic volcanism andcarbonate deposition. Finally, and based on the foregoingdiscussion, it is believed that studied Al Amar deep sulfidemineralization was affected in parts by structural controlsof mineralizations which appear to be confined to post-mineralization and slight to moderate deformation similarto the Wadi Bidah VMS in the Arabian Shield of Saudi Ara-bia taking in consideration that the latter example is muchmore deformed.

The megascopic and ore fabrics of the studied VMS andits associated sulfide-bearing epithermal stringers suggestmoderate effect of the Najd transcurrent faulting of [58]that appears more effective in the shallower levels in Al

Amar gold mine. Hydrothermal jasperoidal chert, extensiveshearing, pyrite brecciation, transformation of calcareousbands into talc are good indicators of the Najd effect in theshallower levels of the mine [4,12,23]. The negligible Najd

roscopy

sfao

9

asiIdta

eos

caefAof

cspictq

samaicF

awrrf

gamrlm

e(aL

[

[

[

[

[

[

[

[

A.A. Surour, R. Bakhsh / Journal of Mic

hear effect in the Al Amar 640 FW-E level was responsibleor the lacking of pervasive hydrothermal alterations thatre common in shallower levels as well as in the surfaceutcrops [26].

. Conclusions

(1) The Al Amar 640 FW-E deep level is characterized by stratabound VMS deposit with some younger epithermalulfide-bearing quartz stringers. The paragenetic sequences pyrite(I)–sphalerite–chalcopyrite–galena–pyrite(II).dioblastic pyrites reache up to ∼80% and it shows evi-ence of plastic deformation by dislocation creep ratherhan by brittle failure, and relatively softer chalcopyritelmost deforms similarly.

(2) There is a common occurrence of chalcopyrite dis-ase in all samples and galena is identified microscopicallynly. Owing to its deep horizon, supergene or oxidationulfide minerals are completely absent.

(3) The transformation of discrete coarse crystals withhalcopyrite disease into “banded” or “stratified” ore isttributed to “local” directive pressure as a deformationalxpression in low-temperature condition (greenschistacies). The morphology and chemistry of chalcopyrite in Almar chalcopyrite disease together suggest a replacementr a coprecipitation mechanism rather than an exsolutionrom a solid solution.

(4) The VMS is traversed by some epithermal irregularomb-structured or crustified stringers that contain someulfides in which sphalerite is homogeneous and with com-lete absence of chalcopyrite disease. Epithermal veining

s also represented by few specularite-quartz stringer thatross cut the VMS ore and are possibly co-genetic withhe main event that led to the formation of sulfide-bearinguartz stringers.

(5) The EMPA analyses prove that all sulfides in thetudied level are Au-bearing and hence the ores are ofuriferous nature. Mostly, Au in pyrite is lattice-bound inost whereas high Au values in sphalerite, chalcopyrite

nd galena are attributed to presence of submicroscopicnclusions of native gold. Galena is abnormally Ag-free. Auontent is weakly correlated to Zn in the sphalerite that ise-poor where Fe never exceeds 0.99 wt%.

(6) Gold in the Al Amar 640 FW-E level is “invisible” andccordingly, the conventional cyanide leaching techniquesould not be feasible. For this reason, the present author

ecommend gold extraction from the Au-rich sulfides byoasting but this acquires enough energy and water supplyor the liberation of free “visible gold”.

(7) Common occurrence of “invisible gold”, Ag-freealena, Fe-poor sphalerite, negligible hydrothermal alter-tion, pyrite plastic deformation by dislocation creepechanism and non-exsolution origin of the chalcopy-

ite disease intergrowth are together strong indicators ofow-temperature crystallization (250–300 ◦C) of VMS or

inerals in the Al Amar deep horizons.(8) The auriferous sulfides in the quartz stringers are

pithermal and is not considered as an orogenic gold oree.g. [59]) because the fluids were produced after accretionnd the waning stage of the Pan-African orogeny during theate Proterozoic.

[

and Ultrastructure 1 (2013) 96–110 109

Conflict of interest

The authors confirm that no part of this work has beensubmitted or published elsewhere, and that there are noconflicts of interest.

Acknowledgements

The authors would like to thank the staff of geologistsin the Al Amar gold mine, the Saudi Mining Company(Ma’aden) for their kind field and logistic facilities. Dr.Ahmed Hassan Ahmed is acknowledged for his assistanceon the electron microprobe.

References

[1] Surour AA, Harbi HM, Ahmed AH. The Bi’r Tawilah deposit, cen-tral western Saudi Arabia: supergene enrichment of a Pan-Africanepithermal gold mineralization. J Afr Earth Sci 2014;89:149–63.

[2] Harbi HM, Eldougdoug AA, Al Jahdli NS. Evolution of the ArabianShield and associated mineralizations with emphasis on gold miner-alization. In: The 9th Arab conference for mineral resources. 2006.

[3] Lofts PG. Reappraisal of the Al Amar prospect and environs. Open-file report (RF-OF-04-33). Saudi Arabian Deputy Ministry of MineralResources; 1984.

[4] Lofts PG. Al Amar gold deposit. In: Colenette P, Grainger DJ, edit-ors. Mineral resources of Saudi Arabia, not including oil, naturalgas and sulfur. Jeddah, Saudi Arabia: Directorate General of MineralResources, Special Publication DGMR SP-2; 1994. p. 95–9.

[5] Mercier-Langevin P, Hannington MD, Dubé B, Bécu V. The goldcontent of volcanogenic massive sulfide deposits. Mineral Deposita2011;46:509–39.

[6] Al Shanti AMS. Mineral deposits of the Kingdom of Saudi Arabia.Jeddah, Saudi Arabia: Scientific Publication Center, King AbdulazizUniversity; 2009.

[7] Harbi HM, Eldougdoug AA, Madani AA, Ahmed AH. Listvenite rocksalong ophiolitic belts in the Arabian Shield as a future potentialsource of gold in Saudi Arabia: with a special emphasis on the appli-cation of remote sensing and GIS. Final report (project No. AT-27-73).Riyadh, Saudi Arabia: King Abdulaziz City for Science and Technology(KACST); 2011.

[8] Sabir H. Ancient mining and its impact on modern mineral explo-ration in Saudi Arabia. Technical report (BRGM-TR-11-3). SaudiArabian Directorate General of Mineral Resources; 1990.

[9] Collenette P, Grainger DJ. Mineral resources of Saudi Arabia, notincluding oil, natural gas and sulfur. Jeddah, Saudi Arabia: DirectorateGeneral of Mineral Resources, Special Publication DGMR SP-2; 1994.

10] Schmidt DL, Puffett J, Campell WL, Al Koulak ZH. Gold placer andquaternary stratigraphy of the Jabal Mokhyat area, southern NajdProvince, Kingdom of Saudi Arabia. Technical record (TR-19). SaudiArabian Deputy Ministry of Mineral Resources, U.S. Geological SurveySaudi Arabian Mission; 1981.

11] Hariri MM, Makkawi MH. Gold mineralization distribution withinrock units at the Hajr gold mine, southwest Saudi Arabia. Arab J SciEng 1994;29(2A):111–21.

12] Ferrand A, Milesi JP, Prévôt JC. Geologic and metallogenic assessmentof the Al Amar deposit. Open-file report (BRGM-OF-05-14). SaudiArabian Deputy Ministry of Mineral Resources; 1985.

13] AGS (Arabian Gold Corporation): exploring the Arabian Shield,Oxfordshire, UK Q1 212 Presentation; 2012. http://arabian-gold.com/i/./AGC%20ExecPres q1 2012.pps

14] Nehlig P, Genna A, Asfirane F. A review of the Pan-African evolutionof the Arabian Shield. GeoArabia 2002;7:103–24.

15] Genna A, Nehlig P, Le Goff E, Guerrot C, Shanti M. Proterozoic tecto-nism of the Arabian Shield. Precamb Res 2002;117:21–40.

16] Kröner A. Ophiolites and the evolution boundaries in the late Protero-zoic Arabian–Nubian Shield of Northeast Africa and Arabia. PrecambRes 1985;27:277–300.

17] Quick JE. Late proterozoic transpression on the Nabitah fault system– implications for the assembly of the Arabian Shield. Precamb Res

1991;53:119–47.

18] Al Saleh A, Boyle AP, Mussett AE. Metamorphism and 39Ar/40Ar datingof the Halaban ophiolite and associated units: evidence for two-stage orogenesis in the eastern Arabian Shield. J Geol Soc London1998;155:165–75.

roscopy

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

110 A.A. Surour, R. Bakhsh / Journal of Mic

19] Johnson PR. Proterozoic geology of Saudi Arabia: current conceptsand issues. In: The 6th meeting of the Saudi society for earth sciences.Riyadh, Saudi Arabia: King Abdulaziz City for Science and Technology(KACST); 2000.

20] Johnson PR, Woldehaimanot B. Development of the Arabian–NubianShield: perspectives on accretion and deformation in the northernEast African Orogen and the assembly of Gondwana. In: Yoshida M,editor. Proterozoic East Gondwana: supercontinent assembly andbreakup. Geological Society of London Special Publication No. 206;2003. p. 289–325.

21] Johnson PR, Andresen A, Collins AS, Fowler AR, Fritz H, GhebreabW, et al. Late Cryogenian-Ediacaran history of the Arabian–NubianShield: a review of depositional, plutonic, structural, and tectonicevents in the closing stages of the northern East African Orogen. J AfrEarth Sci 2011;61:167–232.

22] Johnson PR, Halverson GP, Kusky TM, Stern RJ, Pease V. Vol-canosedimentary basins in the Arabian–Nubian Shield: markers ofrepeated exhumation and denudation in a Neoproterozoic accre-tionary orogeny. Geosciences 2013;3:389–445.

23] Al Shanti AMS. Geology of the Arabian Shield of Saudi Arabia. Jeddah,Saudi Arabia: Scientific Publishing Center, King Abdulaziz University;2009.

24] Delfour J. Geologic map of the Halaban quadrangle, sheet 23G, King-dom of Saudi Arabia (1:250,000). Saudi Arabian Deputy Ministry forMineral Resources Geoscience Map, GM 46C 1979.

25] Delfour J, Dhellemmes R, Elsass P, Vaslet D, Brosse JM, Le NindreYM, et al. Geologic map of the Ad Dawadimi quadrangle, sheet 24G,Kingdom of Saudi Arabia (1:250,000). Saudi Arabian Deputy Ministryfor Mineral Resources Geoscience Map, GM 60A 1982.

26] Idris MH [M.Sc. thesis] A comparative study of assemblages and envi-ronment of deposition of Al Khnaiguiyah and Al Amar prospects,Kingdom of Saudi Arabia. Jeddah, Saudi Arabia: King Abdulaziz Uni-versity; 1988.

27] Calvez JY, Alsac C, Delfour J, Kemp J, Pellaton C. Geologic evolutionof western, central and eastern parts of the northern PrecambrianShield, Kingdom of Saudi Arabia. Open-file report (BRGM-OF-03-17).Saudi Arabian Deputy Ministry of Mineral Resources; 1983.

28] Pouit G, Elsass P, Ferrand A, Milesi JP, Sabir H. Metallogenic base metalstudies: review of activity during 1980–1983 A.D. (1400–1403 A.H.).Open-file report (BRGM-OF-04-11). Saudi Arabian Deputy Ministryof Mineral Resources; 1984.

29] Lofts PG, Bognar B, Howes DR, McHugh JJ, Saleh YT. Exploration ofthe Al Amar prospect. Open-file report (RF-OF-06-1). Saudi ArabianDeputy Ministry of Mineral Resources; 1986.

30] Maclean WH, Al Shanti AMS. Report on the ore deposits of the AlAmar mining district. Open-file report No. 167. Saudi Arabian DeputyMinistry of Mineral Resources; 1958.

31] Raguin M. Synthesis of work and results of the Al Amar zinc-goldoccurrence (1955–1976). Open-file report (BRGM-OF-015). SaudiArabian Deputy Ministry of Mineral Resources; 1981.

32] Al Sari AM, Al Shanti AM, El Mahdy OR. Composition of Al Amar oredeposits. J King Abdulaziz Univ Earth Sci Section 1982;6:579–91.

33] Castroviejo R, Quesada C, Soler M. Post-depositional tectonic modifi-cation of VMS deposit in Iberia and its economic significance. MineralDeposita 2011;46:615–37.

34] Charley MJ, Jeddah, Saudi Arabia A preliminary mineralogical andpetrological study of the Al Amar gold prospect. Riofinex open filereport (RF-OF-05); 1985.

35] Volesky JC, Stern RJ, Johnson PR. Geological control of massive sul-fide mineralization in the Neoproterozoic Wadi Bidah shear zone,southwestern Saudi Arabia, inferences from orbital remote sensing

and field studies. Precamb Res 2003;123:235–47.

36] Freitag K, Boyle AP, Nelson E, Hitzman M, Churchill J, Lopez-PedrosaM. The use of electron backscatter diffraction and orientation con-trast imaging as tools for sulfide textural studies: example fromGreens Creek deposit (Alaska). Mineral Deposita 2004;39:103–13.

[

and Ultrastructure 1 (2013) 96–110

37] Craig JR. Ore-mineral textures and the tales they tell. Can Mineral2001;39:937–56.

38] Cook NJ. Post-recrystallization phenomena in metamorphosedstratabound sulphide ores: a comment. Mineral Mag 2004;58:480–4.

39] Bailie RH, Reid DL. Ore textures and possible sulphide partial meltingat Broken Hill, Aggeneys, South Africa I: petrography. South Afr J Geol2005;108:51–70.

40] Barrie CD, Boyle AP, Cook NJ, Prior DJ. Pyrite deformation textures inthe massive sulfide deposits of the Norwegian Calodinides. Tectono-physics 2010;483:269–86.

41] Barton Jr PB. Some ore textures involving sphalerite from the Furu-tobe mine, Akita Perfecture, Japan. Mining Geol 1978;28:293–300.

42] Barton Jr PB, Bethke PM. Chalcopyrite disease in sphalerite: pathol-ogy and epidemiology. Am Mineral 1987;72:451–67.

43] Kojima S, Nagase T. An SEM examination of the chalcopyrite diseasetexture and its genetic implications. Mineral Mag 1997;61:89–97.

44] C iftc i E. Sphalerite associated with pyrrhotite–chalcopyrite oreoccurring in the Kotana Fe-skarn deposit (Giresun, NE Turkey): exso-lution or replacement. Turkish J Earth Sci 2011;20:307–20.

45] Eldridge CS, Bourcier WL, Ohmoto H, Barnes HL. Hydrothermal inoc-ulation and incubation of the chalcopyrite disease in sphalerite. EconGeol 1988;83:978–89.

46] Dickinson C, Pattrick RAD. Electron microscopic investigation ofsphalerite and sphalerite–chalcopyrite intergrowths. In: Intern Min-eral Assoc Abstracts with Programs 89. 1986.

47] Lepetit P, Bente K, Doering T, Luckhaus S. Crystal chemistry of Fe-containing sphalerite. Phys Chem Minerals 2003;30:185–91.

48] Di Benedetto F, Bernadini GP, Costagliola P, Plant D, VaughanDJ. Compositional zoning in sphalerite crystals. Am Mineral2005;90:1384–92.

49] Cook NJ, Ciobanu CL, Pring A, Skinner W, Shimizu M, DanyushevskyL, et al. Trace and minor elements in sphalerite. A LA-ICPMS study.Geochim Cosmochim Acta 2009;73:4761–91.

50] Cook NJ, Chrysoulis SL. Concentrations of “invisible” gold in the com-mon sulfides. Can Mineral 1990;28:1–16.

51] Vikentyev IV. Precious metal and telluride mineralogy of largevolcanic-hosted massive sulfide deposits in the Urals. Mineral Petrol2006;87:305–26.

52] Kucha H, Stumpfl EF. Thiosulphates as precursors of bandedsphalerite and pyrite at Bleiberg, Austria. Mineral Mag 1992;56:165–72.

53] Sharp G, Buseck PR. The distribution of Ag and Sb in galena: inclusionsversus solid solution. Am Mineral 1993;78:85–95.

54] Dubé B, Gosselin P, Mercier-Langevin P, Hannington M, Galley A.Gold-rich volcanogenic massive sulfide deposits. In: Goodfellow WD,editor. Mineral deposits of Canada: a synthesis of major deposit-types, district metallogeny, the evolution of geological provincesand exploration methods. Geological Association of Canada, MineralDeposits Division, Special Publication No. 5; 2007. p. 75–94.

55] Hakim HD, Chinkul MA. Fluid inclusion study on Mahd Adh Dhahabgold deposit, Saudi Arabia. J King Abdulaziz Univ Earth Sci Section1989;2:51–68.

56] Johnson PR, Vranas GJ. The geotectonic environment of Late Pro-terozoic mineralization in the southern Arabian Shield. Precamb Res2005;25:329–48.

57] Barrie CT, Taylor C, Ames DE. Geology and metal contents of theRuttan volcanogenenic massive sulfide deposit, northern Manitoba,Canada. Mineral Deposita 2005;39:795–812.

58] Moore JMM. Tectonics of the Najd transcurrent fault system, SaudiArabia. J Geol Soc London 1979;136:441–54.

59] Zoheir BA, Abdel-Fattah MG, El Alfy SM. Geochemistry and mineral

chemistry of lode gold mineralisation, SE Egypt: implications for oregenesis and exploration. Arab J Geosci 2013;5:4553–646.

60] Stern RJ, Johnson PR. Continental lithosphere of the Arabian plate:a geologic, petrologic, and geophysical synthesis. Earth Sci Rev2010;101:29–67.