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The Bi’r Tawilah deposit, central western Saudi Arabia: Supergeneenrichment of a Pan-African epithermal gold mineralization

Adel A. Surour a,b,⇑, Hesham M. Harbi a, Ahmed H. Ahmed a,c

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

a r t i c l e i n f o

Article history:Available online 3 July 2013

Keywords:Bi’r Tawilah prospectSaudi ArabiaNajd shearingEpigenetic goldSupergene gold

a b s t r a c t

The Bi’r Tawilah gold deposit in central western Saudi Arabia represents a Pan-African example of goldmineralization in which both hypogene and supergene ores are recorded. The sulphidic gold ore is hostedin intermediate to felsic intrusions that occur along the N–S trending thrust-fault zone within the so-called ‘‘Nabitah orogenic zone’’. There are four rock units present (from oldest to youngest): serpentinitesand related listwaenites, diorites, granitic rocks and porphyries. Hydrothermal alteration consists of chl-oritization, sericitization, carbonatization and silicification and affects all rock types. Chloritization of bio-tite results in abundant rutile, whereas sulphidization coincides with carbonatization. The Bi’r Tawilahore is confined to NW-trending shears (Riedel fractures) related to N–S slip of the pre-existing Tawilahthrust due to activation within the Najd fault system. Samples from the boreholes show macro- andmicroscopic evidence of shearing such as micro-shear planes and strain shadows of pyrite. Sulphidesand gold are present in most rock types. Paragenetically, the sulphides consist of abundant pyrite and rel-atively lesser amounts of arsenopyrite, in addition to very minor chalcopyrite, sphalerite and galena. In allboreholes, it was noticed that the abundance of arsenopyrite increases with depth.

The elevated silver content of electrum (�13–22 wt%) at Bi’r Tawilah is typical of gold deposits andlow-sulphidation epithermal deposits. The early mineralization stage took place in proximity to hydro-thermally altered intermediate to felsic intrusions. The aerially restricted hydrothermal alteration by car-bon-aqueous fluids led to ore remobilization in which gold amounts up to 4.3 g/t. Finally, goldenrichment (up to 5.4 g/t) resulted from supergene alteration that took place during weathering abovethe water table at a depth of �20–25 m.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Gold mineralization in the late Proterozoic Pan-African Ara-bian–Nubian Shield is of great interest for understanding the rela-tionship of ore genesis and tectonics of the orogeny. In the NubianShield in Egypt and northern Sudan, gold mineralization in thePan-African belt has different genetic types where the noble metalis encountered in different lithologies (e.g. carbonated ultramafics,gabbro-granite contacts, exhalative volcanic and associated VMS)that are all characterized by multiple metal sources (e.g. Surouret al., 2001; Kusky and Ramadan, 2002; Zoheir, 2008, 2011; Gabret al., 2010). Recently, gold has been of particular interest forexploration and mining in the Arabian Shield of Saudi Arabia. Goldoccurrences in the Arabian shield are widespread and most of

these occurrences are known as sites of old workings developedon gold-bearing quartz veins, gossans and, rarely, placers (Botros,2004). Some of these gold occurrences are ranked as ore depositsof economic significance and have been mined or are under evalu-ation, e.g. Mahd Ad Dahab, Sukhaybarat, Bulghah, Al Hajar, Jadmah,Hamdah, Ad Duwayhi and Bi’r Tawilah by the Saudi Mining Com-pany (Ma’aden) and the Shayban prospect (CITADEL, 2008, 2009)(Fig. 1).

Primary gold mineralization in the Arabian shield can begrouped into three main types based on tectonic settings and hostrocks (Harbi et al., 2006). The first type is associated with volcano-sedimentary sequences (e.g. the Siham Group or the early HulyfahGroup), including volcanogenic massive sulphide deposits (VMS)and epithermal base- and precious-metal deposits. Gold in someof the VMS deposits may have been re-worked by the Najd FaultSystem (transcurrent deformation, Fleck et al., 1976) where epi-thermal mineralizations are associated with some massivesulphide deposits (e.g. Mahd Ad Dhahab and Al Amar). Thegold-bearing zones at Al Hajar gold mine were subjected to some

1464-343X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jafrearsci.2013.06.007

⇑ Corresponding author at: Department of Mineral Resources and Rocks, Facultyof Earth Sciences, King Abdulaziz University, B.O. Box 80206, 21589 Jeddah, SaudiArabia. Tel.: +966 506061416; fax: +966 2 69520295.

E-mail address: [email protected] (A.A. Surour).

Journal of African Earth Sciences 89 (2014) 149–163

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supergene enrichment processes and gold was enriched in the oxi-dation zones that almost lack any sulphides. Dubé et al. (2007) dis-tinguished two genetic models for Au-rich VMS deposits: (1)conventional syngenetic volcanic-hosted Au-poor VMS mineraliza-tion overprinted during regional deformation by Au mineraliza-tion; and (2) syngenetic VMS deposits characterized by ananomalous fluid chemistry (with magmatic input) and/or deposi-tion within a shallow-water to subaerial volcanic setting equiva-lent to epithermal conditions. Lack of detailed mineralogical andmicrofabric studies for the Saudi VMS-related gold deposits does

not allow researchers to affiliate them to one or both of the geneticmodels of Dubé et al. (2007). Recently, Mercier-Langevin et al.(2010) defined in their review a geometric mean gold grade for513 VMS deposits worldwide of 0.76 g/t, and deposits with morethan 3.46 g/t Au (geometric mean plus one geometric standarddeviation) are considered auriferous. This latter category also in-cludes some examples from Saudi Arabia such as the Nuqrah de-posit with a gold grade of 3.80 g/t Au.

The second type of gold deposits is spatially associated withcarbonatized ophiolitic ultramafic rocks (listwaenites), which

Fig. 1. General geologic map of the Arabian Shield showing major tectonostratigraphic terranes, ophiolite belts, sutures, fault zones, post-accretionary basins in the ArabianShield of western Saudi Arabia (from Nehlig et al., 2002 with modifications by Johnson and Woldehaimanot, 2003; Stern and Johnson, 2010). Numbers represent locations ofthe major gold deposits as follows: (1) Hamdah, (2) Al Hajar and Jadmah, (3) Mahd ad Dhahab, (4) Jabal Shayban, (5) Humaymah, (6) Bulghah, (7) Sukhaybarat, (8) Al Amar,(9) Ad Duwayhi, (10) Um Matierah, (11) Ar Rjum, (12) Ash Shakhtaliya, (13) Zalm, (14) Bi’r Tawilah (including Jabal Ghadarah, Al Mansourah and Masarah).

150 A.A. Surour et al. / Journal of African Earth Sciences 89 (2014) 149–163


decorated the suture zones during the amalgamation of the differ-ent terranes. Listwaenites are widespread and have been identifiedand described from different suture zones: Nabitah, Al Amar, Bi’rUmq and Yanbu (Al Shanti, 1993). Currently, the Saudi Ma’adenmining company explores the Bi’r Tawilah area in the central partof the Arabian shield; e.g. Al Mansourah and Masarah. Gold miner-alization associated with listwaenite is a promising target for cur-rent and future exploration along the Nabitah suture zone in thewestern provinces in the Afif composite terrane (Harbi et al., 2006).

The third type of the gold deposits is particularly abundant inthe western parts of the Afif terrane to the east of the Nabitah su-ture zone, where late- to post-tectonic (640–610 Ma) diorite–gran-ite plutons and/or their subvolcanic equivalents intruded theMurdama and Bani Ghayy groups, e.g. Sukhaybarat, Zalm, Bulghah,Bi’r Tawilah, Ad Duwayhi and Jabal Ghadarah (Collenette and Gra-inger, 1994; Nehlig et al., 1999; Leistel et al., 1999; Al Jahdli, 2004;Doebrich et al., 2004). These deposits have been described by Agaret al. (1992) as structurally and/or magmatically controlled meso-

thermal vein-type gold mineralization formed in intraplate set-tings. In this case, the gold mineralization is quartz veins andstringers where gold mainly occurs as free microscopic specksalong fractures of pyrite, arsenopyrite and occasionally chalcopy-rite. These deposits are characterized by low sulphide mineralscontent, typically associated with arsenopyrite, pyrite and/ or pyr-rhotite, and were deposited from CO2-rich ore fluids (Al Jahdli,2004; Harbi et al., 2006). The role of these intermediate to felsicintrusions whether they were the source of the mineralizing fluidsor the source of heat which initiated meteoric water convectionand leaching of gold from the country rocks or both is still debat-able. The intrusions were emplaced after terrane amalgamation(�670 Ma). Gold was mined from the oxidation zone in some ofthese deposits, e.g. Bulgah and Sukhaybarat.

During the 1980s, gold mineralization in carbonated ultramaficoutcrops or listwaenites in the Bi’r Tawilah (Figs. 1 and 2) receivedsome attention, e.g. Labbé (1984), Billa (1987) and Couturier(1988) especially for the outcrops at Jabal Ghadarah, some 4 km

Fig. 2. General geologic map of the Zalm quadrangle and location of the Bi’r Tawilah gold prospect (after Agar, 1988).

A.A. Surour et al. / Journal of African Earth Sciences 89 (2014) 149–163 151


west of the Bi’r Tawilah main prospect. Current exploration anddrilling programs are focused, however, on granitoids and porphy-ries. The present work aims to distinguish the different stages ofmineralization that possibly range from magmatic to hydrother-mal activities and finally to supergene alteration. Our study alsotries to establish a paragenetic sequence of ore and gangue miner-als, and finally discusses the genetic interpretations of the Bi’rTawilah gold mineralization.

2. Geologic set up of Bi’r Tawilah area

The Arabian–Nubian Shield (ANS) consists of a juvenile Neopro-terozoic crust that represents an area of suturing between East andWest Gondwana before the Paleozoic (Johnson et al., 2011). Itformed through accretion of numerous inter-oceanic island arcsalong ophiolite-decorated suture and gneissic fault zones between900 Ma and 550 Ma when the Mozambique ocean closed (Stern,1994). This took place much earlier than the opening of the RedSea, 30–25 Ma ago (Camp and Roobol, 1992) during which theANS was divided into the Arabian and Nubian Shields. In theframework of the terrane model given by Johnson (1999, 2003),the Bi’r Tawilah area is located along the so-called ‘‘Ad Dafinahthrust’’ that separates the Afif terrane in the east from the Jeddahterrane in the west. The Bi’r Tawilah area (Figs. 2 and 3) is locatedon the eastern side of the Nabitah suture zone west of the Afif ter-rane. The latter was defined by Stoeser and Camp (1985) as a com-plex assemblage consisting of volcanic-arc sequences and amicroplate of older crust that are overlain by �710 Ma volcanosed-imentary sequences in deep basins (Doebrich et al., 2004). Initially,the Afif terrane was termed as Zalm terrane (Johnson and Vranas,1984). Schmidt et al. (1979) and Stoeser and Camp (1985)indicated that the orogenic belt of the Nabitah suture defines a100–200 km wide zone of crustal deformation, remobilizationand plutonism. The belt is characterized by linear complexes ofsynorogenic gneissic to massive granitic rocks, separated by re-gions of amphibolite- to middle greenschist-grade metamorphicrocks mostly of the volcanosedimentary sequence of the Hulyfahor Siham Group, the younger metasedimentary rocks of Bani GhayyGroup and the ophiolitic mafic–ultramafic rocks which mark the

suture zone between the Afif terrane and the western arc terranes(Stoeser and Camp, 1985).

In the Bi’r Tawilah area (Figs. 2 and 3), the early and late Hul-yfah volcanosedimentary sequences, namely the Siham and BaniGhayy Groups in chronological order are located to the east andwest of a major thrust fault. They are flysch-type sediments whilethe younger molasse-type volcanosedimentary rocks are known asthe Murdamah Group (Viland, 1986). Doebrich et al. (2004) studiedthe younger syn-orogenic rocks (granodiorite and rhyodacite por-phyry) of the Haml batholith and assigned a 710–685 Ma SHRIMPU/Pb zircon age for them.

The Bi’r Tawilah area contains ancient gold and tungsten mineworkings (Fig. 3) close to a N-striking thrust fault which is partof the Nabitah suture zone. This thrust fault is decorated by thelistwaenized serpentinites of the Arabian Peninsula (Buission andLeblanc, 1985, 1987) and separates the Bani Ghayy Group in thewest from the Siham Group in the east (Saber and Labbé, 1986).However, Al Jahdli (2004) in accordance with Viland (1986) usedthe term distal facies (early Hulyfah or the Siham Group) for therocks exposed to the east of the fault. The Siham Group consistsof highly foliated and deformed chlorite, sericite and hornblendeortho-schists while the Bani Ghayy Group consists of conglomer-ate, greywacke and siltstone intercalated with pyroclastics andmetabasalts. Both the Siham and Bani Ghayy Groups were sub-jected to greenschist-lower amphibolite facies metamorphism (AlJahdli, 2004). The rocks exposed at Jabal Bani Ghayy were consid-ered by Agar (1988) to consist of ortho-schists and a part of themolasse-type sediments.

The volcanosedimentary rocks of the Siham and Bani GhayyGroups (�620 Ma and P690 Ma, respectively; Stacey and Agar,1985) were intruded by syn- to late-orogenic and post-orogenicintrusive rocks. Accordingly, the Bi’r Tawilah thrust fault is youn-ger than 620 Ma, and therefore coincides or post-dates the thirdmajor event of the Nabitah suture that consists of three episodes:710–680–640 Ma (Quick, 1991). The syn- to late-orogenic intrusiverocks that mostly occur to the east of the Bi’r Tawilah fault and lo-cally in the central part range in composition from diorite to grano-diorite. According to Al Jahdli (2004) the syn-orogenic intrusiverocks are represented by highly weathered, fractured and locally

Fig. 3. Geological map of the Bi’r Tawilah area (after Saber and Labbé (1986)). Locations of gold and tungsten prospects are shown.



sheared diorite and granodiorite that are exposed to the east of Bi’rTawilah at Jabal Ghadarah along the contact between listwaeni-tized serpentinite and the late Hulyfah Group rock varieties. Itwas found that the granodiorite contains pyrite and arsenopyritedisseminations that are gold-bearing (Al Jahdli, 2004). Severaltypes of syn- to late-orogenic intrusive rocks including porphyriticand fine-grained granite and porphyritic microgranodiorite weredescribed also by Saber and Labbé (1986).

The post-orogenic intrusive rocks (Haml suite) occur as largebatholiths (Fig. 2), mostly to the north and east of Bi’r Tawilahand are represented by Jabal Al Awjah monzogranite truncatingthe northern extension of the Bi’r Tawilah fault. The age of the plu-tons belonging to the Haml batholith ranges from 659 ± 7 Ma(Doebrich et al., 2004) to 651 ± 12 Ma (Agar et al., 1992). LateNW-trending aplitic microgranite dykes cross-cut the volcanosed-imentary rocks of the Siham Group and the syn- to late-orogenicintrusive rocks. These dykes host the Bi’r Tawilah W mineralization(Saber and Labbé, 1986).

Previous studies believed that the so-called ‘‘Najd Fault System’’played an important role in the genesis of gold in the Bi’r Tawilahand other deposits in the Arabian Shield. According to Johnsonet al. (2011), the extent of Late Cryogenian–Ediacaran Najd–north-west-trending transcurrent fault system in the Arabo-NubianShield and surrounding areas makes it one of the largest shear sys-tems known on Earth. In their review, Johnson et al. (2011) statedthat the Najd shears range from single, linear faults to broader setsof shears. Movement was predominantly sinistral, although theoffset of some plutons and inferences about the origin of some Jiba-lah Group pull-apart basins suggest local dextral slip (Cole andHedge, 1986; Agar, 1987; Matsah and Kusky, 2001; Kusky andMatsah, 2003). The shears chiefly trend NW to NNW, but a smallnumber of NE-trending dextral shears such as the Ad Damm faultzone are believed to be conjugate shears for the Najd system(Davies, 1984).

3. Methods and techniques

The studied core samples were collected from several boreholesof a current diamond drilling program by the Saudi Ma’aden Min-ing Company at the Bi’r Tawilah. The ore minerals, as well as thegangue minerals and mutual relationships, were first studied usingboth transmitted and reflected light microscopy. Prior to the stageof electron microprobe, the well representative samples wereinvestigated by using the SEM-EDS technique for more detailedidentification of minerals, textures and microstructures. A Philipsscanning electron microscope with energy dispersive X-ray attach-ment (SEM-EDX) Model XL 30 was used at the Central Laboratoriesof the Egyptian Mineral Resources Authority of Egypt. Workingconditions are 30 kV accelerating voltage, 15–20-nA probe currentand 5 lm beam diameter.

Chemical composition of the sulphide minerals and native goldwas determined using a JEOL electron-probe microanalyzer JXA-8200 at the Department of Mineral Resources and Rocks, Facultyof Earth Sciences, King Abdulaziz University, Saudi Arabia. Oreminerals were analyzed for Fe, S, Cu, Zn, Pb, Te, Sb, Bi, As, Au andAg. Analytical conditions were 20-kV accelerating voltage, 20-nAprobe current, 3 lm probe diameter and the counting time was10 s for each element except for Au and Ag which were 50 and30 s, respectively. Standards used were pyrite for Fe and S, puremetals for Cu, Au, Ag, Zn, Pb, Te, Sb, and Bi, and gallium arsenidefor As. At, such conditions, detection limit of gold in the analyzedsulphides was 400 ppm which is equivalent to 0.04%. Gold contentin the whole-rock samples (g/t) was determined by the fire assaytechnique at Al Amri Laboratories, Jeddah, Saudi Arabia; wherethe detection limit was 0.01 ppm.

4. Gold at the Bi’r Tawilah deposit

Exploration for gold is currently in progress by the Saudi MiningCompany (Ma’aden) in three sites. The first is close to the Tawilahwater well (Bi’r Tawilah), whereas the other two localities (Al Man-sourah and Masarah) are located farther south. The main litholog-ical units underlying the Bi’r Tawilah area are mafic volcaniclasticsof the Siham group, conglomerate and sandstone. These rocks aredeformed by N–S thrust structure, namely the Bi’r Tawilah faultzone. There are limited exposures of granodiorite, and much lesserdiorite, at the surface. The exposures of granodiorite increasenorthwards until being well represented at Jabal Ghadarh. The solegood exposure in the area is 55–120 cm thick barren quartz veinsthat trend NW, NE and E–W which are the main three shear com-ponents of the regional fault system. This goes in harmony withstructural data obtained by Agar (1988) who analyzed the move-ment vectors of the Najd fault system at the Zalm quadrangleincluding the Bi’r Tawilah deposit area.

For the last five years, the Saudi Ma’aden Mining Companylaunched an exploration program for gold and associating noblemetals at the area to the south of Bi’r Tawilah. The program con-centrated exploration at two locations, namely Al Mansourah andMasarah. The main aim of the extensive diamond drilling was goldin carbonatized ultramafics or the listwaenite. Assay results re-vealed gold anomalies in the listwaenite. The present study focuseson the area to the south of the Bi’r Tawilah where the boreholesindicate that the main lithologies are little ophiolitic serpentinite(particularly at depth), diorite, granitic and porphyry intrusions.

The granitic rocks include massive and slightly deformed grano-diorite and monzogranite. The late quartz-feldspar porphyry intru-sions are identified at some boreholes (e.g. BTWC-028 and BTWC-031) at depths of �58 and 197 m below surface. These rocks con-tain 4–10% sulphide minerals, but locally attain up to �20% in por-phyries, with the exception of some quartz-feldspar porphyry fromthe drillhole BTWC-031 that has no sulphides but with fluorite andradioactive mineralization. This variety of porphyries (at depth of�197 m) could be related to the W-bearing post-orogenic granitesin the area of study that have been identified earlier by Saber andLabbé (1986). At the surface outcrops as well as in borehole BTWC-031, Sn–W mineralization is confined to a series of east-trendingradioactive fluorite-bearing quartz veins and stringers (Saber,1983). The cross-cutting relationships suggest a younger age ofSn–W mineralization as it overprints the gold-bearing zones inhydrothermally altered diorite and garnodiorite. Feybesse and LeBel (1984) concluded that the Bi’r Tawilah Sn–W mineralizeddykes and veins have typical REE patterns of specialized granitesand considered the Awjah microgranite as the probable parent plu-ton of such mineralization. It can be added here that the radioac-tive fluorite-bearing quartz-feldspar porphyries in the boreholesare equivalent to the microgranite at the surface.

Concerning samples of the weathered (supergene zone) ob-tained from the Bi’r Tawilah boreholes it is evident that diorite ishighly weathered and has barren quartz stringers (Fig. 4a). Somediorite samples are hydrothermally altered and then subjected toweathering (Fig. 4b). The granodiorite shows different degrees ofweathering (Fig. 4c and d). Even the porphyries are weathered asindicated by oxidized pyrite (Fig. 4e).

5. Petrography

At Bi’r Tawilah gold prospect, the encountered rock types ob-tained from the boreholes are serpentinite, diorite, granitoids andfelsic porphyry intrusions.

The ophiolitic serpentinites are traversed by �3 cm thickbanded carbonate veins. The vein displays sharp contact with the



ultramafic rock. The diorites are mostly microdiorite or medium-grained variety that are sheared, silicified and hydrothermally al-tered at depth up to �200 m. On the other hand, diorites in theweathered horizon show superimposed supergene alterations onthe earlier hydrothermal alteration. Carbonate minerals in the veininclude coarse calcite crystals that show banding, interlocking andidiomorphism towards the vein centre. Coarse comb-structuredcalcite crystals cut diorite and show distinct growth zoning andassociate idiomorphic pyrite at their rims. The coarse carbonatecaptures ‘‘fish-like’’ microscopic enclaves of metasomatized ser-pentinite. K-metasomatism in the serpentinites is represented byalteration of actinolite and tremolite to phlogopite due to soakingof the ultramafic lithology by magmatic-hydrothermal fluids com-ing from beneath that are related to the post-orogenic granites inthe area such as the monzogranite.

The diorite is melanocratic and consists of amphiboles and pla-gioclase, in addition to chlorite as the common alteration (second-ary) mineral. The amphiboles are represented by hornblende andmuch finer actinolite. Some hornblende crystals have dense blackcore and some others exhibit both simple and lamellar twinning.Actinolite occurs as short rods that are altered to chlorite especiallyin the sheared parts. Along micro-shear planes, the diorite is highlysericitized. Locally, the rock is silicified where secondary quartz iscoarse and anhedral. This quartz is strained, fractured and tra-versed by very thin veinlets of silica and carbonates. Where altereddiorite is weathered, the rock becomes darker and Fe-oxides andhydroxides are common. Hydrothermal biotite is very commonas interstitial phase and not as a primary mineral. Also, coarse sil-ica veinlets are present containing elongated layers of quartz per-pendicular to the wall of fractures. Along micro-shear planes,carbonatization is more pronounced.

On textural basis, the granodiorite is either non-porphyritic orporphyritic. It shows distinct shearing, breciation, hydrothermal

alteration and finally weathered products rich in sericite, kaoliniteand Fe-oxyhydroxide. In the granodiorite, plagioclase is altered tosericite flakes oriented along cleavage planes. Sometimes, the ser-icite shows perfect alignment along the plagioclase twining lamel-lae. The plagioclase rim is almost sericite-free as sericitization isselective at the core. Carbonates fill zigzagged fractures and thecarbonate veinlets sometimes show bifurcation and invade alteredplagioclase and quartz with fluid inclusions. Biotite is altered tochlorite and rutile. All of the investigated granodiorite samplescontain zoned zircon crystals that are rimmed with radioactivepleochroic haloes against the biotite flakes.

The monzogranite contains two generations of plagioclase thatis commonly sericitized. Biotite is highly kinked due to deforma-tion, altered to rutile and chlorite. Biotite occurs as aggregates thatcontain different kinds of inclusions such as apatite, rutile andradioactive zircon.

In the late porphyry intrusions, euhedral plagioclase occurs asphenocrysts that contain biotite inclusions and altered to sericiteparticularly at the core. Coarse biotite is altered to sericite and ru-tile. Inclusions in biotite are apatite and zircon. The porphyries aretraversed by 0.3–0.7 cm thick quartz stringers that are rich in idi-omorphic pyrite and arsenopyrite.

Fig. 5 illustrates some distinct hydrothermal alterations in thedeep horizons such as chloritization of biotite and rutile formation(Fig. 5a and b). Carbonatization is common in the form of sub-par-allel veinlets (Fig. 5c and d).

6. Ore paragenesis and paragenetic sequence

A summary for ore mineral paragenesis in the main five rocktypes (serpentinites, diorite, granodiorite, monzogranite and por-phyries) is given in Fig. 6.

Fig. 4. Some representative core samples from the weathered (supergene zone) obtained from the Bi’r Tawilah boreholes. Number of borehole is given while depth and goldcontent are given between brackets. (a) Weathered diorite with barren quartz stringers (Qtz). (b) Highly sheared diorite that is extensively hydrothermally altered, and thenweathered. (c and d) Two core samples of granodiorite showing different degrees of weathering and gold content. (e) Slightly weathered quartz-feldspar porphyry containingoxidized pyrite (Py).



The oldest rock type at the deep levels (�190–205 m) is repre-sented by serpentinites as the sole ophiolitic component. Ore min-erals in the serpentinite are either orthomagmatic (chromite andpyrrhotite) or hydrothermal (pyrite and native gold). The serpent-inite is rich in highly brecciated ferrichromite which is much coar-ser than pyrrhotite. Both fine pyrite and gold dust in theserpentinite are only confined to the silicified parts of the rockwhere the silica veinlets cross-cut magmatic pyrrhotite.

In the diorite, magmatic Fe–Ti oxides are represented by ilmen-ite and lesser amounts of magnetite. The percentage of ilmenite isapproximately equal to that of the hydrothermal sulphide ore min-erals. Ilmenite is either homogeneous (altered to hematite, rutileand coarse titanite) or homogeneous in the form of hemo-ilmeniteexsolution intergrowth.

Two distinct episodes of pyritization are suggested, namely dis-seminated and hydrothermal pyrite. The first one produced dis-seminated pyrite in non-deformed diorite and granodiorite at Bi’rTawilah as well as at Jabal Ghadarah (Al Jahdli, 2004). Dioriteand granodiorite at depth (up to 155 m) are intensively shearedand hydrothermally altered. Some of the coarse hydrothermal pyr-ite shows distinct strain effects as evidence of shearing. Thisstrained pyrite is distorted and develops ‘‘pressure shadow’’ madeup of strained quartz. In few cases, deformation of the host rocksleads to limited brecciation of the sulphide as well. Upon weather-ing, the altered sheared diorite becomes a reddish rock character-ized by schistose-like texture and common occurrence of ferricoxyhydroxides where invisible gold in the pyrite structure is con-verted into visible gold. In few instances, chalcopyrite inclusionsare encountered in both pyrite and arsenopyrite in the sheareddiorite.

The granodiorite is characterized by frequent aggregates of ru-tile that displays two habits, needle-shaped and massive and re-sults from alteration of magmatic ilmenite. Subordinate flakyspecularite develops at the expense of magnetite. Pyrite occurs asinclusions in arsenopyrite or as euhedral pyrite crystals with rutileinclusions. Arsenopyrite starts to crystallize at the late stages ofhydrothermal pyritization and it continues until the formercross-cut the latter. Fine-grained gold occurs at the contact be-tween pyrite and arsenopyrite or as inclusion in the earlier pyriteonly. Similar to the case of diorite, supergene ferric oxyhydroxidesdevelop visible gold due to the oxidation of pyrite uponweathering.

In the monzogranite, pyrite occurs as idiomorphic crystals thatdisplay distinct growth zoning by zonal arrangement of rutile andsilicate inclusions. Very few visible gold inclusions occur in pyriteat the depth of �55 m. The highly weathered rocks near the surfacebear also some gold specks.

Primary Fe–Ti oxide minerals in the feldspar and quartz-feld-spar porphyry intrusions are lacking, but hydrothermal mineralsare common instead. The latter comprises pyrite and fluorite. At

Fig. 5. Photomicrographs of some hydrothermal alterations in unweathered deep samples from some boreholes. Number of borehole is given between brackets. (a)Decomposition of biotite (Bt) to chlorite (Chl) and rutile needles (Rt), porphyritic monzogranite, polarized light (BTWC-028). (b) Rutile needles (Rt) in interstitial quartz (Qtz),brecciated granodiorite, cosssed-nicols (BTWC-032). (c) Sub-parallel calcite veining (Cal) in porphyritic momzogranite consisting of plagioclase (Pl), microcline perthite (Mc)and quartz (Qtz), crossed-nicols (BTWC-028). (d) Calcite veinlet (Cal) showing colloform texture, brecciated granodiorite, crossed-nicols (BTWC-6).

Fig. 6. Paragenetic sequence of hydrothermal ores, supergene ores and gangueminerals. FCA = Ferric Ca-arsenate and FOH = ferric oxyhydroxide.



depth greater than 175 m, the porphyry contains high amount ofpyrite (up to 20%). Flourite occurs as violet interstitial phase. Inthe weathered samples, pyrite in porphyries is commonly oxidizedto goethite comparable with ferric oxyhydroxides in the other pre-viously described weathered rock types.

Fig. 7 illustrates some of the common ore minerals and texturesin the Bi’r Tawilah mineralized rocks at depths. Pyrite is commonas idiomorphic crystals (Fig. 7a). Idiomorphic pyrite crystals con-tain some inclusions of microcline and other silicates (Fig. 7b).Strained pyrite with pressure shadow is common in the shearedrocks (Fig. 7c). Pyrite is commonly developed along fractures asso-ciating colloform calcite (Fig. 7d). Arsenopyrite occurs in its typical

rosette-like habit associating coarse banded calcite veining, even inthe oldest rock type or the serpentinites (Fig. 7e). Cross-sections ofrhombic arsenopyriteare common and post-date rutile (Fig. 7f).Arsenopyrite post-dates the event of partial to complete conver-sion of pyrite to arsenopyrite (Fig. 7g). Finally, some idiomorphicarsenopyrite contains inclusions of rutile, chalcopyrite and pyritesupporting the observation that arsenopyrite is the latest sulphidephase in the paragenetic sequence (Fig. 7h).

Also, the BSE images support the observations obtained fromthe ore microscopic investigation. In the weathered samples, goldoccurs along fractures and growth zones of colloform-structuredferric oxyhdroxide (Fig. 8a and b). Chalcopyrite is earlier than

Fig. 7. Photomicrographs showing the ore fabrics at Bi’r Tawilah mineralized rocks in the unweathered deep horizon. Number of borehole is given between brackets. (a)Population of idiomorphic pyrite (Py), feldspar porphyry, polarized light (BTWC-028). (b) Idiomorphic pyrite cube (Py) with irregular microcline inclusions (Mc), slightlydeformed granodiorite, crossed-nicols (BTWC-006). (c) Quartz strain shadow (Qtz) of pyrite embedded in a groundmass rich in quartz and sericite (Qtz-Ser), moderately tohighly sheared diorite, crossed-nicols (BTWRC-007). (d) Pyrite veining (Py) in brecciated granodiorite, polarized reflected light (BTWC-006). (e) Arsenopyrite rosette (Apy)associating coarse calcite (Cal) invading metasomatized serpentinite, polarized light (BTWC-004). (f) Idiomorphic arsenopyrite (Apy) enclosing earlier rutile (Rt), brecciatedgranodiorite, polarized reflected light (BTWC-006). (g) Transformation of pyrite (Py) to pyrrhotite (Po), quartz-feldspar porphyry, polarized reflected light (BTWC-028). (h)Idiomorphic arsenopyrite (Apy) enclosing inclusions of rutile, polarized reflected light (Rt), chalcopyrite (Ccp) and pyrite (Py), Brecciated granodiorite, polarized reflectedlight (BTWC-006).



pyrite, pyrrhotite and arsenopyrite. Chalcopyrite veinlets are over-printed by idiomorphic pyrite and pyrrhotite (Fig. 8c). In the deepzones (65–135 m), fresh samples contain visible gold inclusions inpyrrhotite (Fig. 8d).

7. Gold geochemistry and mineral chemistry

During the early phases of exploration at Bi’r Tawilah, the Ma’a-den Mining Co. carried out some ICP analyses of some trace ele-ments in addition to the fire assay data of gold. Fig. 9, which is acourtesy of the Ma’aden Co., shows the variation of such elementsin both weathered zone (�40 m thick) with supergene alterationsand the deep zone with hydrothermal alteration as well. It is evi-dent that gold increases in the weathered zone which goes in har-mony with the ore microscopic investigation supported by the BSEimages and mineral spectra. It is important to notice that regard-less of the degree of weathering, hydrothermal gold stands as theresponsible cause of the precious element enrichment. This canbe easily noticed in Fig. 3c and d where the less weathered grano-diorite bears much more gold (2.52 g/t) than the least weatheredgraodiorite (0.90 g/t) at the same borehole. Accordingly, amountof hydrothermal gold is an important controlling factor beforebeing more concentrated by weathering. This figure shows someenrichment of Cu, Cr and Ni in the weathered zone whereas noobvious variations in the contents of Pb and Zn can be noticed. Ar-senic defines an irregular trend due to its erratic distribution inboth weathered and fresh deep zones which is controlled by intro-duction of arsenopyrite almost in all rock varieties.

Fire assay data of the different host rocks varieties from theboreholes are given in Table 1. The table shows that gold in thesheared serpentinites at depths from 195 to 203 m averages0.127 g/t. This might be attributed to the presence of fine sulphidesin the silica–carbonate alteration of the ultramafics in analogy tosimilar Pan-African ophiolitic serpentinites in the Eastern Desertof Egypt (Zoheir and Lehmann, 2011). Al Jahdli (2004) observedK-metasomatism in some surface outcrops of serpentinites at the

Bi’r Tawilah and reported similar gold content comparable to thatwe had obtained from borehole samples. Our metsomatized serp-entinites from the borehole (BTWC-04) show evidence of biotiteformation at the expense of actinolite. As to the diorite, gold in-creases in the weathered samples which contains up to 5.401 g/t.Some hydrothermally altered diorite at depth of �155 m containsup to 4.285 g/t Au owing to the presence of native visible gold inpyrite and interstitial quartz, particularly in the silicified samples.In contrast to diorite, both granodiorite and monzogranite samplescontain lesser gold content in the weathered zone amounting1.881 and 2.517 g/t, respectively. Weathered porphyries containthe least gold content amounting 0.057 g/t.

Gold is relatively concentrated in the weathered rock zone (e.g.BTWC-006). Locally, gold is much higher in the deep zones of aneighbouring borehole, only 25 m apart (e.g. BTWC-079). On theother hand, gold is enriched at shallow depths in neighbouringboreholes (e.g. BTWC-004 and BTWC-073). In the former, gold var-ies in the range of 0.53–1.18 g/t for some �17.25 m thick intersec-tions. In the latter, there is a supergene zone following �1 m thickgravel and sand with an average gold content of 0.57 g/t followedby a narrow 2 m with 0.33 g/t Au. Details of gold content in theborehole BTWC-006 give some useful information where theweathered granodiorite bears 1.03–1.15 g/t Au with remarkableenrichment in the brecciated granodiorite in the supergene zone(1.33–2.46 g/t). In the deep zone, fresh granodiorite containsremarkably high gold content in the range of 2.92–12.78 g/t whenbeing invaded by �30 cm thick quartz veins. The quartz veins as-say maximum gold of 1.68 g/t suggesting that much of their goldis transported into the adjacent granodiorite.

Gold and different sulphides were analyzed using the electronmicroprobe technique and the averages of about 103 spot analysesare given in Table 2. The table shows that gold in both weatheredand deep samples contains considerable amount of Ag amounting12.73% and 21.99%, respectively. Such analyses suggest that goldinclusions in pyrite from fresh deep horizons are Ag-rich and canbe considered as ‘‘electrum’’.

Fig. 8. EDX backscatter images (BSE) of gold and some sulphides and their inclusions. All are taken for samples from borehole BTWC-006. (a) Coarse subhedral gold (Au) alongfractures and colloform growth planes of ferric Ca-arsenate (FCA) associating ferric oxyhydroxide (FOH), weathered granodiorite. (b) Details of gold (Au) along the growthplanes of ferric Ca-arsenate (FCA), weathered granodiorite. (c) Sulphide assemblage comprising early chalcopyrite veinlet (Ccp) interrupted by late idiomorphic pyrrhotite(Po) with detached chalcopyrite inclusions, fresh quartz-feldspar porphyry. (d) Coarse anhedral gold (Au) hosted by pyrrhotite (Po) juxtaposing arsenopyrite (Apy), freshgranodiorite.



Pyrite contains the highest gold content (0.17–0.34%) comparedto other sulphide minerals; e.g., arsenopyrite (0.18–0.25%) andpyrrhotite (0.14–0.16%) as given in Table 2. Also, sphalerite andchalcopyrite inclusions in the Fe–As sulphides are auriferous, andtheir gold content amount 0.20–0.31% and 0.18–0.21%, respec-

tively. On the other hand, galena is Au-free but it bears consider-able content of Ag (averaging 0.71%). Galena has some impuritiesof Te and Sb amounting up to 0.145% and 0.215%, respectively.Te and Sb in pyrite, pyrrhotite and arsenopyrite are negligible(up to 0.01% and 0.04%, respectively).

Fig. 9. Variation of gold and some trace elements in weathered and fresh horizons at borehole BTWC-001, Bi’r Tawilah prospect.

Table 1Gold content in the Bi’r Tawilah boreholes intersecting weathered and deep zones.

Sample no. Depth Rock name Horizon Au content (g/t)

(a) Serpentinite4-1 197.00–198.00 m Sheared serpentinite Deep 1.881

(b) Diorite4-2 194.00–200 m Micro-diorite Deep 0.0047-4 39.70–40.15 m Altered diorite Deep 0.3907-7 155.00–156.35 m Altered diorite Deep 4.2857-8 181.00–182.90 m Altered diorite Deep 0.0047-2 31.85–32.15 m Highly sheared diorite Deep 1.3107-6 145.90–146.90 m Highly sheared diorite Deep 0.2444-00 21.40–21.50 m Altered diorite Weathered 5.40161-1 8.00–8.10 m Altered diorite Weathered 0.81938-1 6.90–7.00 m Highly sheared diorite Weathered 0.662

(c) Granitic rocks7-0 14.00–14.15 m Granodiorite Weathered 1.88115-1 10.80–10.90 m Brecciated granodiorite Weathered 0.90031-2 9.30–9.40 m Granodiorite Weathered 0.3387-5 121.00–122.00 m Porphyritic granodiorite Deep 0.0047-1 191.00–193.00 m Monozgranite Deep 0.0244-0 7.00–7.15 m Brecciated monzogranite Weathered 0.7398-1 17.00–17.15 m Monzogranite Weathered 1.96715-2 20.00–20.10 m Monzogranite Weathered 2.51726-1 17.40–17.50 m Monzogranite Weathered 0.15328-3 197.40–197.50 Porphyritic monzogranite Deep 0.045

(d) Porphyry rocks28-1 58.10–58.30 Feldspar porphyry Deep 0.01131-1 197.40–197.50 Quartz-feldspar porphyry Deep 0.00934-1 9.90–10.00 m Quartz-feldspar porphyry Weathered 0.057



The element distribution map of some elements in pyrite trans-formed to pyrrhotite from fresh granodiorite (BTWC-006) showsenrichment of arsenic at the rim (Fig. 10). Element distributionmap of some elements in some sulphides from fresh granodiorite(BTWC-006) shows homogeneity of pyrrhotite and arsenopyrite(Fig. 11).

8. Discussion

In analogy to the Zalm, Bulghah and Sukhybarat prospects in theNubian Shield of Saudi Arabia, the Bi’r Tawilah gold mineralization issuggested to be intrusion-related (e.g. Nehlig et al., 1999; Sahl et al.,1999) where the host rocks comprise diorite, granodiorite and

Table 2Average EPMA composition of the analyzed ore minerals from the Bi’r Tawilah mineralized zones. (Concentrations are in wt%. n = number of points analyzed, n.d = not detected.).

Mineral Pyrite Arsenopyrite Pyrrhotite Chalcopyrite Sphalerite Galena Gold

Samplea 28-4 6-7 28-4 6-5 6-6 28-4 6-5 6-6 6-7 28-4 28-4 6-5 6-6 6-7 6-5 6-6 6-1 6-5No. (n = 1) (n = 13) (n = 4) (n = 3) (n = 22) (n = 9) (n = 10) (n = 4) (n = 23) (tail) (incl.) (n = 1) (n = 2) (n = 1) (n = 2) (n = 1) (n = 2) (n = 3)

As 0.06 0.01 40.78 39.89 42.21 0.09 0.48 0.47 0.10 0.01 0.01 0.03 0.04 0.03 n.d 7.98 0.02 0.01Fe 54.50 60.06 32.43 37.12 35.38 46.65 48.03 47.22 47.37 30.18 30.87 32.19 6.53 6.13 10.60 9.00 2.46 1.90S 41.02 39.49 22.36 22.95 21.31 50.83 51.36 51.68 52.06 35.61 36.08 34.15 33.82 33.44 20.88 13.94 n.d 0.25Zn 0.03 0.01 0.02 0.01 0.02 0.02 0.01 n.d 0.01 0.17 0.03 0.065 58.02 59.03 0.09 0.07 0.02 0.01Cu 0.02 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.02 31.14 30.45 31.54 0.03 n.d n.d n.d 0.02 0.03Te 0.02 0.01 n.d n.d 0.02 0.02 0.01 0.01 0.01 0.01 n.d 0.017 n.d 0.03 0.07 0.03 0.02 n.dPb 0.14 0.02 2.82 0.01 0.60 0.02 n.d 0.03 0.01 0.01 n.d 0.015 0.01 n.d 68.21 66.39 n.d n.dAu 0.34 0.17 0.25 0.24 0.18 0.14 0.16 0.15 0.14 0.18 0.19 0.21 0.31 0.20 n.d n.d 82.89 73.85Sb 0.03 0.01 n.d 0.03 0.01 0.01 0.01 n.d 0.01 0.02 n.d n.d n.d n.d 0.13 0.08 0.01 n.dAg 0.01 n.d 0.01 n.d n.d n.d n.d n.d n.d 0.05 0.04 n.d n.d n.d 0.71 0.93 12.73 21.99Total 96.18 99.79 98.77 100.27 99.72 97.84 100.06 99.58 99.72 97.37 97.69 98.22 98.76 98.86 100.69 98.42 98.94 98.05

a 6-1 = weathered granodiorite, 6-5 = fresh granodiorite, 6-6 = fresh brecciated granodiorite, 6-7 = fresh slightly deformed monzogranite and 28-4 = fresh quartz-feldsparporphyry.

Fig. 10. Element distribution map of some elements in pyrite transformed to pyrrhotite from fresh granodiorite (BTWC-006) showing enrichment of arsenic at rim. Noticethat the bright chalcopyrite inclusions are Au- and Ag-free.



granite. On the other hand, Al Jahdli (2004) and Harbi et al. (2006)suggested an epigenetic hydrothermal origin at the Bi’r Tawilahprospect, specifically at Jabal Ghadarah. The latter authors attrib-uted the enrichment of the late-orogenic granodiorite in gold andsulphides to leaching from listwaenites that decorate the Nabitahsuture zone along which the Bi’r Tawilah gold deposit is located.Based on ore microscopic investigation, the present work suggeststhat primary magmatic sulphides are negligible at the Bi’r Tawilahgranodiorite as well as associating diorite and monzogranite thatare not hydrothermally altered. In collaboration with the current de-tailed mineralogical, microstructural and fire assay data, we suggesthere the Riedel fractures of a N–S slip faulting system acted asimportant structural conduits for channeling the hydrothermal flu-ids. During the early phase of the Najd orogeny, there was initiationof pretectonic phase of fracture propogation, igneous emplacementand graben formation that was controlled by dextral shear in a NW-trend (Moore, 1979). Based on detailed structural analysis, Agar(1988) demonstrated that the volcanosedimentary rocks of the BaniGhayy Group at the Bi’r Tawilah deposit area were deformed bytranscurrent movements along faults of the same Najd trend but dis-placement becomes sinistral and not dextral, and finally sinistraldisplacements of all structures by the late phase of the Najd fracturesreactivation. The youngest fractures controlled the emplacement ofpost-tectonic plutons (e.g. Awjah granite and quartz-feldspar por-phyry) and dyke swarms as the final stage of the Najd strike-slipmovement (Agar, 1988). The latter author showed that the orienta-tion of the Riedel fractures at the Bi’r Tawilah area are very charac-teristic in a sinistral shear zone proximal to the Haml plutonicsuite to the east of the Tawilah Fault Zone.

In the fresh deep horizon, ore microscopic investigations sug-gest that rutile is the first mineral to be crystallized in the ore para-genesis which is then dominated by sulphides. The parageneticsequence of sulphide ore minerals in a single epigenetic minerali-zation stage: chalcopyrite (along with minor sphalerite andgalena) ? pyrite ? arsenopyrite, where arsenopyrite increaseswith depth. In the supergene zone at Bi’r Tawilah prospect, sulp-hides are decomposed and replaced by an assemblage of semi-crystalline to amorphous arsenates and Fe-hydroxides. Accordingto a recent mineralogical study on the arsenate mineralogy atBi’r Tawilah gold prospect, Surour et al. (2013) suggested that pyr-ite in the weathered horizon is pseudomorphed by ferric oxyhy-droxide, whereas arsenopyrite is oxidized to an intrastratifiedphase of arsenian ferricoxyhydroxide and Ca–Fe arsenate similarto yukonite. Sulphides are represented by nearly equal amountsof pyrite and arsenopyrite. Detailed investigation of microfabricssuggests that the sulphides were introduced to the host rocks afterthe late stages of solidification, i.e. connected to magmatic-hydro-thermal system characterizing the intrusion-related gold deposit.The history of mineralization suggests early crystallization ofpyrite that is subjected to brecciation as the strength of shearingincreases (Fig. 12a). Such deformation facilitates also the develop-ment of sub-parallel fractures along which pyrite and arsenopyritewere deposited associating calcite in veinlets (Fig. 12b and c)where the carbonate is colloform mostly. In some few instances,these sulphides form thin veinlets along irregular or intersectedfractures (Fig. 12d). If the Bi’r Tawilah sulphide-rich samples be-long to an intrusion-related deposit hosted by diorite and granodi-orite, presence of interstitial pyrite (Fig. 12e and f), as well as along

Fig. 11. Element distribution map of some elements in some sulphides from fresh granodiorite (BTWC-006) showing homogeneity of pyrrhotite (Po) and arsenopyrite (Apy).Notice that gold (Au) bears noticeable silver content.



fractures, favours an epigenetic rather than intrusion-related styleof mineralization at the Bi’r Tawilah gold deposit. The post-orogenic alkaline monzogranite and feldspar porphyry are youngerin age than the syn- to late-orogenic calc-alkaline diorite andgranodiorite in the area of study (Agar et al., 1992). Anomalousgold content in least altered granodiorite (up to 12.78 g/t) is attrib-uted to interstitial auriferous sulphides that are connected to near-by mineralized quartz stringers and stockworks that supportsagain an epigenetic origin. Gold content in the quartz veins them-selves never exceeds 1.68 g/tsuggesting that the precious metal istransported to the adjacent lithologies by post-injection leachingkinematics (Harbi et al., 2006).

There are some world examples in which gold is associatedwith Sn–W–Mo–Sb–Cu–Bi mineralization. Genesis of some exam-ples is explained in terms of a porphyry model (Galley and Frank-lin, 1985) and in some other cases the origin of W is not clear withprobabilities of derivation from the magma directly or from theadjacent skarn and metasedimentary country rocks by the hydro-thermal fluids. In most of these examples, Sn–W-bearing quartzveins pre-date the gold precipitation in the same granitic pluton(Maloof et al., 2001; Kesler et al., 2003; Mikulski, 2011). This isan opposite model to the case of Bi’r Tawilah because our observa-tions document the cross-cutting relationships of much youngerSn–W-bearing quartz veins connected to the Awjah micrograitewhich was also accepted earlier by Feybesse and Le Bel(1984).Hence, the Au-bearing quartz veins at the Bi’r Tawilah traversethe hydrothermally altered diorite and porphyritic granodioritewhich are older than the albite microgranite and its Sn–W quartzveins.

9. Conclusions

The Bi’r Tawilah area is a part of the Neoproterozoic N–S trend-ing Nabitah belt (Greenwood et al., 1976; Gass, 1977; Roobol et al.,1983). According to Letalenet and Bounny (1977) and Agar (1988)both the Siham and Bani Ghayy Groups were formed at structuralgrabens during the Nabitah and Najd orogenies (D2 and D3),respectively. In a later stage, these successions were intruded bydiorites and post-orogenic granites that host some gold minerali-zation at the Zalm and Bi’r Tawilah prospects.

(1) Most rocks at Bi’r Tawilah boreholes are hydrothermallyaltered (chloritized–sericitized–carbonatized–silicified). Chl-oritization of biotite resulted in abundant rutile whereas thecrystallization of sulphide minerals is contemporaneous witha phase of carbonatization. The earliest pyrite generationoccurs in the form of accessory magmatic disseminations.

(2) Evidence of shearing that include conjugate fractures, micro-shear planes and strain shadows in pyrite. On the basis ofstructural analysis, all of them are resultants of prominentnorthwest-trending strike-slip faults of the so-called ‘‘theNajd fault system’’ that is dated back to 680 Ma (Johnson,1996) until 530 Ma (Stacey and Agar, 1985). The observedshear structures at the mineralized zone of Bi’r Tawilah aremerely fractures and do not seem to be parts of trough-goingshear zone.

(3) Based on the ore mineralogic investigation, it is concludedthat sulphides and gold are present in most rock types atthe Bi’r Tawilah deposit. Sulphides in the serpentinites are

Fig. 12. Ore fabrics of sulphides in the Bi’r Tawilah rocks at depth. Number of borehole is given between brackets. (a) Pyrite (Py) showing brecciation in brecciatedgranodiorite, polarized reflected light (BTWC-006). (b) Pyrite (Py) and calcite (Cal) filling fractures in brecciated granodiorite, polarized reflected light (BTWC-006). (c)Arsenopyrite (Apy) and calcite (Cal) filling fractures in brecciated granodiorite, polarized reflected light (BTWC-006). (d) Thin intersected veinlets of pyrite (Py) andarsenopyrite (Apy) in brecciated granodiorite, polarized light (BTWC-006). (e) Interstitial pyrite (Py) in porphyritic monzogranite, polarized light (BTWC-028). (f) Interstitialpyrite (Py) filling along silicate crystal boundaries in feldspar porphyry (BTWC-028).



pyrrhotite (magmatic) and pyrite (hydrothermal). On the otherhand, the diorite–granodiorite association contains both dissem-inated pyrite in non-deformed samples and remobilized pyrite inhydrothermally altered parts. In terms of abundance, pyrite isfollowed by arsenopyrite chronologically.

(4) Gold in the deep horizons occurs as inclusions in silica andpyrite (i.e. hydrothermal). In contrast, gold in the weatheredhorizon results from oxidation of pyrite to goethite (i.e. super-gene alteration viz weathering). Recently, Surour et al. (2013)presented a detailed mineralogic-chemical study on the Bi’rTawilah arsenates and documented intensive chemicalweathering that largely controlled the speciation of As3+ andAs5+. Surour et al. (2013) connected the formation of Ca–Fearsenate to pervasive chemical weathering to the extent thatthe associating marble bands were almost washed out.

(5) Free gold is present as inclusions either in interstitial quartzthat fills voids or in fresh and altered pyrite. It is interestingto distinguish gold inclusions in pyrite as ‘‘visible and invis-ible’’ gold where the visible one is only encountered in thedeep fresh horizon whereas invisible gold in the pyrite lat-tice is liberated and becomes visible due to supergene alter-ation of pyrite during weathering.

(6) Although silver-rich electrum is typical of intrusion-relateddistal disseminated gold deposits and low-sulphidation epi-thermal deposits, there is not enough evidence to be the caseof the Bi’r Tawilah.

(7) The mineralized fracture system at the prospect seems totrend NW that coincides with the main shear componentof the Najd fault system traversing great areas of the ArabianShield in western Saudi Arabia. Nevertheless, the presentauthors find that it is more acceptable to conclude that theNW-trending shears are Riedel fractures related to N–S slipon the pre-existing Tawilah thrust. All types of sulphideand gold mineralization at the Bi’r Tawilah prospect pre-datethrusting. Also, it is important here to summarize that gen-esis of gold at the Bi’r Tawilah as a whole should not belumped as one category only. Concerning the earliest cate-gory (i.e. intrusion-related), the mineralization develops inproximity to intrusions of intermediate to felsic composi-tion, but shows evidence of passage of carbonic hydrother-mal fluids and aerially restricted hydrothermal alteration.

(8) Gold content (up to 5.4 g/t) in weathered horizon is promisingfor future open pit mining by Ma’aden Company. To avoid clo-sure of the proposed open pit, mining in the future can con-tinue after consumption of the gold-fertile weathered zone.Gold in the deep horizons (4.3 g/t) is encouraging too forfuture gold exploitation and production at the Bi’r Tawilahprospect especially at current high price of gold in the interna-tional market (1608 USD/oz in February 18, 2013).

Acknowledgements

The authors would like to express their great gratitude to theexploration geologists at Al Mansourah (Bi’r Tawilah prospect),particularly Munir Noor, for their great efforts during our surveyfor their core boxes. They also provide us with useful fire assaydata of gold for several boreholes. We would like also to thank Te-wolde Woldeabzghi for facilitating our access to the core boxes andvisiting the prospect for investigating geology and selection ofsamples from the boreholes.

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