Role of fluid mixing and wallrock sulfidation in gold mineralization at the Semna mine area, central...

Ore Geology Reviews 34 (2008) 580–596

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Role of fluid mixing and wallrock sulfidation in gold mineralization at the Semnamine area, central Eastern Desert of Egypt: Evidence from hydrothermal alteration,fluid inclusions and stable isotope data

Basem Zoheir a,⁎, Ahmed Akawy b, Imbarak Hassan c

a Department of Geology, Faculty of Science, Benha University, 13518 Benha, Egyptb Department of Geology, Faculty of Science, South Valley University, Egyptc Department of Geology, Faculty of Science, Suez Canal University, Egypt

⁎ Corresponding author.E-mail address: [email protected] (B. Zoheir)

0169-1368/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.oregeorev.2008.09.007

a b s t r a c t
a r t i c l e i n f o
Article history:
The Semna gold deposit is o Received 6 March 2008Received in revised form 22 September 2008Accepted 26 September 2008Available online 17 October 2008
Keywords:Semna gold depositStructural controlsHydrothermal alterationFluid mixingδ34SDokhan VolcanicsEgypt

ne of several vein-type gold occurrences in the central Eastern Desert of Egypt,where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks.The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dippingWNW-trendingshear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit acomplex structure of massive and brecciated quartz consistent with a change of the paleostress field fromtensional to simple shear regimes along the pre-existing fault segments. Textural, structural andmineralogicalevidence, including open space structures, quartz stockwork and alteration assemblages, constrain on veindevelopment during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite,subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyriteand kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunitecharacterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminatedsulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization hasrendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed atmoderately shallow depths in the transition zone between mesothermal and epithermal veins.Petrographic andmicrothermometric studies aided recognition of CO2-rich, H2O-rich andmixedH2O–CO2fluidinclusions in the gold-bearing quartz veins. TheH2O–CO2 inclusions are dominant over the other two types andare characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixingof two immiscible carbonic and aqueousfluids. The generally light δ34S of pyrite and chalcopyritemay suggest amagmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities arelikely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditionsof gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and350–1100 bar, under conditions transitional between mesothermal and epithermal systems.The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?),fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported asAu-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, andprecipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters andvariations in pH and fO2.

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1. Introduction

In the Eastern Desert of Egypt, more than a hundred goldoccurrences are mostly confined to the Precambrian basement rocks(El Ramly et al., 1970; Sabet and Bordonosov, 1984; Pohl, 1988; AbdelTawab,1992). At least 40 of these occurrences show previousworkingsof the ancient Egyptians (Pharoans), who likely worked out the richestparts of the auriferous quartz veins (Azer, 1966). Although mining

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activity was extensive during the Pre-Dynastic, Roman, Ptolemaic andIslamic times, gold productionwas episodic with several hiatus spreadover time. Gold production from the large gold deposits, mostly in thecentral Eastern Desert: e.g., E1 Sid, Um Rus, Atud, Barramiya andSukari, lasted until 1958 (Pohl, 1988; Abdel Tawab, 1992). The AbuMarawat Au–Fe ore prospect wasworked extensively in Pharaonic andRoman times over an area of 1 km2 and to a depth of 40 m. The formerEgyptian Geological Survey and Mining Authority (EGSMA), inassociationwith Russian geological teams, mapped the area, collectedsamples and excavated trenches. In 1987, MinexMinerals, a subsidiaryof Greenwich Resources Plc, carried out a percussion and diamond

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581B. Zoheir et al. / Ore Geology Reviews 34 (2008) 580–596

drilling program. In the last few years, the Egyptian Government hasissued several companies (e.g., Hamash, Centamin) the rights todevelop some of the Egyptian gold deposits. In 2006, Alexander NUBIAInc., a Canadian mining company, was awarded the license for goldexploration and exploitation of two blocks in the central EasternDesert including Semna mine.

Most of the important gold deposits in the Egyptian Eastern Desertrelate to vein-type mineralization and are linked, at least spatially, toquartz-mineralized shear zones formed during compressional/trans-pressional stages or regional shear and extension tectonics late in thePan-African Orogeny (Harraz and Ashmawy, 1994; Hassaan and El-Mezayen, 1995; Loizenbauer and Neumayr, 1996; Harraz, 2000; Kuskyand Ramadan, 2002; Botros, 2004; Helmy et al., 2004; Zoheir, 2004,2008; Zoheir and Klemm, 2007). These shear zones cut sequences ofophiolitic andmetasedimentary rocks or occur along their contacts withsmall granitoids stocks (Almond et al., 1984; Gabra,1986). Botros (2004)suggested a genetic link between the vein-typemineralization hosted insheared ophiolitic rocks in the Eastern Desert and the carbonatized-silicified serpentinite (listwaenite) along the thrust zones (e.g., Barramiyaand Hutit gold deposits; Botros, 1991; Takla et al., 1995; Botros, 2002).Basta et al. (1996) studied the Atalla and Semna deposits and suggestedthat the exposed gabbro as a favorable source rock for gold because of itsintrinsically elevated gold concentrations (360 to 710 ppb Au).

The mineral assemblage in gold-bearing quartz veins from theEgyptian deposits commonly includes pyrite, arsenopyrite, subordinatechalcopyrite, sphalerite, galena, tetrahedrite and rare stibnite. Theseveins are interpreted collectively as products of hydrothermal activityinduced either by metamorphic or cooling effects of Lower Palaeozoicmagmatism or during an Early Cambrian subduction-related calc-alkalinemagmatic event (Garson and Shalaby,1976; Almondet al.,1984;El Gaby et al.,1988; Pohl,1988). Other authors relate goldmineralizationto the emplacement of granitoid rocks that intrude mafic/ultramaficrocks (Hume, 1937; Amin, 1955; El Shazly, 1957). Almond et al. (1984)suggested that gold deposition was related to an episode of shearingafter the emplacement of all batholithic intrusions and coeval with theregional cooling.Hussein (1990)argued thatmostof thesehydrothermalvein deposits are epithermal rather than mesothermal. A geneticrelationship between the granite–gabbro association and gold miner-alization is suggested by Takla et al. (1990). Two of the Egyptian golddeposits, HamashandUmGarayiat,were suggested to be porphyry-typesystems (Hussein, 1990). Hilmy and Osman (1989) described aremobilization process of gold from a high temperature chalcopyriteandpyrite assemblage in theHamashdeposit. Osman(1995) suggestedasimilar mechanism of gold deposition at the Um Balad mine in northEastern Desert, in which gold was remobilized from early sulfides.

Although commonly variable from one deposit to another, the wallrock alteration around the hydrothermal Au-bearing quartz veins ofthe Egyptian Eastern Desert shows distinct mineralogical changessuggestive of progressive fluid chemistry buffering through interac-tionwith wallrock (e.g., Osman and Dardir, 1989; Harraz, 1991; Harrazand El-Dahhar, 1994). The metasomatic styles were strongly influ-enced by variations in the CO2 and alkali contents of the mineralizingfluid (Harraz et al., 1992; Harraz and El-Dahhar, 1994). Botros (1993)reviewed the common types of hydrothermal alteration associatedwith the Egyptian vein-type gold deposits and suggested thatsericitization, argillic alteration and silicification are common in theacid rocks, whereas chloritization, carbonatization, sericitization,pyritization and propylitization prevail in the intermediate and basichost rocks. Several authors reported that many of these alterationzones contain significant gold contents (e.g., Osman and Dardir, 1989;Botros, 1991; Takla et al., 1995).

The Semna gold deposit, located in the central Eastern Desert,~5 km east of Gabal Semna and 3 km SW of Bir Semna (26°26′43″N,33°35′32″E) is one of several gold occurrences in the area linked, atleast spatially, to small granitoid stocks and massifs. In the presentcontribution, studies comprising: petrographic and textural investiga-

tion, structural controls, patterns of wallrock alteration, fluid inclu-sions and sulfur isotope systematics of the Semna deposit were aimedat assessing the nature and evolution of the ore fluids. Moreover, anattempt to constrain on the ambient temperature, fO2 and pH duringgold deposition-hydrothermal alteration is based on fluid inclusion,hydrothermal alteration assemblages and isotopic data.

2. Geologic setting

2.1. Regional geology

The Egyptian Eastern Desert is divided into three distinct lithotec-tonic terranes— the northern, central and southern terranes (Stern andHedge, 1985). Massive and gneissic granitic rocks are abundant in thenorthern and southern sectors, respectively, whereas the highestconcentration of ophiolitic rocks occurs in the central Eastern Desertexposes (Stern et al.,1984). The basement complex in the central EasternDesert displays strong ensimatic affinities and consists mainly of anisland arc complex (with abundant banded Fe formations; Habib, 1987;Abdel Karim, 1994) overthrust by dismembered ophiolitic sequencesand intruded by syn-tectonic grey granitoids (Fig. 1; Stern, 1981; Rieset al., 1983; Sturchio et al., 1983; El Ramly et al., 1984). These rocks areoverlain in places by anchizonal to low-grademetamorphosedmolasse-type sediments (the Hammamat Group: Akaad and Noweir, 1969;Grothaus et al., 1979; El Kalioubi, 1996; Messner, 1996; Kamal El Dinet al., 1996), calc-alkaline silicic to basic volcanic, namely Dokhan Vol-canics (Ressetar and Monrad, 1983; Stern and Gottfried, 1986), andintruded by post-orogenic younger granites (Grothaus et al., 1979;Greenberg, 1981). The ophiolitic rocks, Hammamat sediments andDokhan Volcanics are assembled as thrust slices and dissected folds(Ries et al., 1983). Thrust transport directions toward the NNW, SWand/or NE have been reported in several studies (e.g., Ries et al., 1983;Greiling,1987;ElGabyet al.,1988; Shackleton,1994;Abdeenet al.,1996).The thrusts are deformed by NW-trending sinistral transcurrent ductileshears, which possess several kinematic features analogous to the NajdFault System(NFS) in theArabianShield (Abuzeid, inpress;Davies,1984;Stern, 1985). In some areas, however, the prevailing structural fabricstrikes in an E–W direction (Fritz et al., 1996, 2002).

The Semna mine district is underlain mainly by ophioliticserpentinite, metagabbro and basalt, arc-related metavolcanic rocks(i.e., metabasalt and andesite), different granitoids and subordinatemetasedimentary rocks. Sequences of conglomerate and siltstone(Hammamat Group sediments) form discrete NW-elongate exposuresin the eastern part of the study area. Themetavolcanic rocks have beenstudied by Aly et al. (1991), who classified them as Semnametavolcanics and Abu Kalb metavolcanics. The Semna metavolcanicsare composed of intercalated lapilli tuffs and breccias with andesiticcomposition in lower part, metabasalt sheets in the middle part andupper part mainly of alternating metabasalt flows (Aly et al., 1991).These rocks are tectonically overlain by a thin strip of peliticmetasedimentary rocks (ca. 200 m-wide). Bedding and laminationin these rocks trend generally in an E–W direction. The Abu Kalbmetavolcanics consist mainly of metatuffs and subordinate sulfide-bearing lava flows, sills and dykes with andesitic and less commonbasalt, diabase and dacite composition. The metabasalt unit of theSemna metavolcanics shows tholeiitic affinities and is attributed to aprimitive intra-oceanic island arc remnant, whereas the Abu Kalbmetavolcanics have generally calc-alkaline geochemistry and likelyhave formed during an advanced stage of an island arc development(Aly et al., 1991). The Abu Kalb metavolcanics are known to hostAlgoma-type banded iron ores occurring as N1.5 m-wide bandsalternating with sequences of andesitic tuffs and flows.

The ophiolitic suite occurs as dismembered components includingserpentinite and tectonized/carbonatized derivatives, metabasalt,metagabbro and intermingled mélange rocks. Serpentinites are themost abundant ophiolitic rocks in the area, likely originated through

Fig. 1. Geologicalmapof theSemna–KabAmiri area, central EasternDesert (modified fromAkaad andNoweir,1980;Aly et al.,1991;Moghazi, 2002). Inset shows the location of the studyarea.

582 B. Zoheir et al. / Ore Geology Reviews 34 (2008) 580–596

fore-arc seafloor spreading (Azer and Stern, 2007). The island arc andophiolitic assemblages are intruded by older calc-alkaline granitoidsand younger alkali granite. The older (syn-orogenic) granitoids have

Fig. 2. Field photographs of Semnamine showing: (a) Ore crusher andmineworkings close tosheared wallrock adjacent to rhyolitic dike (looking south).

been further subdivided into trondhjemite–tonalite and granodioritesuites (Akaad and Noweir, 1980; El Gaby and Habib, 1982; Akaad,1997). The younger granites are best expressed in the area by the Kab

themain adit north of themine area (looking south), and (b) Quartz vein and associated

Fig. 3. Sketch drawing, based on field exposure, showing the shear-hosted quartz lensesand associated altered and brecciated wallrocks.


Amiri leucogranite pluton, which has sharp contacts with the countryvolcano–sedimentary rocks and older granitoids.

The Semna mine area and its surroundings experienced multipledeformation and a metamorphic history induced by complex tectonicmovements during the Pan-African orogeny. The volcano–sedimen-tary and ophiolitic rock associations are deformed and metamor-phosed under greenschist and lower amphibolite facies (Moghazi,2002). The trondhjemite–tonalite suite is significantly deformed andfoliated, whereas the granodiorite unit is weakly deformed at theintrusion peripheries. The younger leucogranite rocks are unde-formed, except weak brecciation along fault planes.

2.2. Geology of the mine area

The Semna mine area is underlain by slightly metamorphosed, butlocally intensely sheared, gabbroic rocks, granodiorite and lessabundant siliceous metasediments (sericite–chlorite–biotite schist).

Fig. 4. (a) Rose diagram showing the distribution of fault trends at the Semna mine area. (graphically determined according to the P and T dihedra method of Angelier and Goguel (197developed by Etchecopar et al. (1981).

Abundant felsic to intermediate dykes, generally sulfide-bearing, anddyke-like bodies of pink granite are common in the area, mostlytrending in a roughly E–W direction (Fig. 2). Two main generations ofmineral foliation are observed in the mine area, including older ~E–Wfoliation and younger less pervasive N–S crenulation foliation bestdeveloped in the metasediments and mélange matrix rocks. Themetagabbro rocks exhibit a weak E–W foliation but intense NW-SEand WNW-ESE joint/fracture sets. The area is densely dissected bynumerous faults trending in different directions — NE-SW, NNW-SSE,NW-SE, WNW-ESE and ENE-WSW. The ENE-WSW and WNW-ESEfaults accommodate slight right-lateral displacement; the NNW-SSWand NW-SE faults are commonly associated with considerable left-lateral displacement. The gold-bearing quartz veins are confined toWNW-ESE-trending shear zones dissected by conjugate NE-SW faultsets and associated with abundant felsic dykes. Deflection of thehost rock foliation into the shear planes adjacent to the quartz-mineralized shear zones is distinct next to the quartz veins (Fig. 3).These zones are composed of bleached wallrock in which dissemi-nated pyrite is abundant.

Our field observations and satellite/aerial photograph interpreta-tions allow us to attribute the structural evolution of the mine area toepisodic development of conjugate fault systems through four majordeformational stages, including: (1) nearly N–S shortening, expressedby south-verging thrust segments and development of E–W mineralfoliations in the ophiolitic and volcano–sedimentary rock associations,(2) simple shear led to the development of km-scale NW and NEsinistral and ENE and WNW dextral faults as two component sys-tems of a major NW strike-slip sinistral fault system (likely affiliatingto the Najd system), (3) terrain uplift accompanied by extensionand emplacement of various dykes and barren quartz veins, and(4) transcurrent shear system superimposed on the pre-existingdiscontinuities, and led to reactivated WNW faults.

Development of the NW sinistral strike slip fault system occurredwhen the paleostress field rotated so that σ1 in the E–Wdirection, σ2perpendicular to the earth surface and σ3 in N–S direction. After-wards, as the terrain uplifted σ1 was oriented normal to the earthsurface and σ2 in an E–W direction. Normal faulting took place andaccompanied by the development of a conjugate joint system andtension gashes. Continuous rotation of the paleostress field led to σ1in the NNW-SSE direction, σ3 perpendicular to the earth surface andσ2 in ENE-WSWdirection, where emplacement of Au–Cu quartz veinstook place along WNW dextral faults (Fig. 4).

b) Equal area projection of fault planes and their slip striations. Strain directions were9), and a paleostress tensor was comparatively calculated using the inversion algorithm


3. Local structures, mineralization style and vein mineralogy

Gold mineralization and hydrothermal alteration at the Semnamine area are closely associated with a system of sub-parallel WNW-ESE to NW-SE quartz and quartz-carbonate veins and lenses confinedto moderately or steeply dipping shear zones (up to 6 m-wide) at thecontact between a granodiorite intrusion and foliated metagabbro(Fig. 5). These shear zones are macroscopic zones of high-densityfaulting and shearing relative to adjacent areas. Strain distribution isheterogeneous, reflected by narrow, tabular high-strain zones (brit-tle–ductile faults) separating lithons of essentially undeformed rockand deflection of the host rock foliation into the shear direction. In themain lode, vein thickness varies considerably from less than half ameter to 3 m with 0.6 to 1.2 m being the average, whereas the strikelength is up to ~350 m. In addition to the main lode, two othermineralized veins (south of the main lode) measure 300 and 220 malong strike, respectively, and rarely exceed 0.4 m in thickness.Abundant small scale milky quartz veins/splays (b2 m-long and 0.3 mthick) form tension gashes along fault segments commonly dippingnormal to the mineralized quartz veins. These tension gashes are goldbarren and contain no wallrock material but abundant vuggy silica.Fine brecciated quartz intermingled with sericite–pyrite–kaolinite–alunitemixtures forms the vein terminations. Themain lode and othersmaller veins were worked out through three steeply inclined shaftsand an adit entrance (Fig. 5).

The main lode is composed of two distinct sheets of quartz, i.e.,marginal barren comb-textured prismatic quartz and inner sheet of

Fig. 5. Geologic and structural setting of the Au–Cu quartz veins at Semna mine ar

sheared quartz intermixed with wallrock material, i.e., carbonate anddisseminated sulfide minerals (Fig. 6a). This complex structure caneasily be distinguished in the field as the ancient miners mined theinterior of themain lode and left the peripheral parts untouched. In themarginal parts of the main lode, prismatic quartz crystals are orientednormal to the vein boundaries, whereas elongate or stretched quartzforms the internal parts. Calcite veinlets form stockwork in themineralized quartz veins and adjacent wallrock, especially in zoneswhere metagabbro juxtaposes the metasedimentary rocks (in thenorthern part of the mine area). Disseminated pyrite and chalcopyriteare also observed in zones in which quartz veins are missing, butabundant calcite stockwork and pervasive alteration are notable(quartz, chlorite, kaolinite, and alunite mixture; Fig. 6b). Field andpetrographic observations indicated that the hydrothermal alterationassemblage (chlorite, sulfides and sericite) is partially replaced by alate assemblage of drussy silica, alunite and kaolinite, which mightrepresent either an advanced phase of hydrothermal alteration as thecirculating hydrothermal solutions have been totally buffered/dilutedor even represent a late weathering overprint.

The mineralogy of the mineralized quartz veins is simple anduniform, although the proportions of the vein-forming minerals varysignificantly. The mineralized veins are mainly composed of quartz(70 to 90 vol.%), carbonate (10 to 30 vol.%) and sulfides (~1 to 5 vol.%).Pyrite, chalcopyrite and arsenopyrite comprise more than 90% of thetotal sulfides. Pyrite is ubiquitous as scattered euhedral grains in thealtered wallrocks, yet most abundant within and adjacent to the veins.In some samples, pyrite has been replaced along grain boundaries by

ea. Insets showing a vein sample and detailed view of the main lode texture.

Fig. 6. Field photographs of Semna mine showing: (a) The main lode with early marginal barren quartz left and more or less completely stopped out mineralized quartz in the centralparts (looking west), and (b) Malachite, kaolinite and iron oxides/hydroxides mixture replacing metagabbro adjacent to the mineralized veins (looking west).


goethite. Mineral inclusions of sphalerite, pyrrhotite, galena andfeldspar are common in pyrite grains from the mineralized quartzveins and wallrocks. Chalcopyrite is next to pyrite in abundance. Itoccurs as disseminated subhedral to anhedral grains associated withpyrite (Fig. 7a), as thin veinlets and as cavity fillings within pyrites andis often replaced along cracks and grain boundaries by covellite. Arse-nopyrite is less abundant, but occurs as subhedral grains commonlysmall in size than the associated pyrite crystals. Galena, sphalerite andgold are subordinate constituents in the veins, commonly occurring asfine inclusions in pyrite (Fig. 7b, c). Gold occurs as native Au blebs,electrum and as intergrowths with Bi-bearing tellurides, commonlyas inclusions in pyrite and chalcopyrite. Visible gold occurs as fineinclusions, 20 to 330 μm long, or as veinlets and stringers in pyrite and

Fig. 7. Reflected light photomicrographs of polished sections from Semna deposit showing: (lode, (b) Sphalerite (sph) inclusions within pyrite, (c) Gold (Au) and galena (gn) inclusions i

as dispersed blebs associated with carbonate and chlorite in the inten-sively altered wallrocks (Fig. 7c, d).

4. Quartz textures and hydrothermal alteration

The orebodies are manifested by generally narrow quartz veins andlenses along dilatant parts in the shear zones, commonly wheredragging or mashing of the foliated metagabbro and metasediments isobserved, and/or along the contact betweenmetagabbro and the granitestock. Early barren comb-textured quartz dominates the marginal partsof the main lode, whereas, stretched and ribbon-shaped quartz con-stitutes themineralized central vein portions. The ribbon-shaped quartzdisplays serrate grain boundaries, stretched in a dextral sense and

a) Euhedral pyrite (py) and anhedral chalcopyrite (cpy) grains disseminated in the mainn pyrite, and (d) Gold inclusions along growth planes (surfaces; dashed lines) in pyrite.


commonly intermingledwith alterationmaterial. Late drussy quartzfillsspaces among the pre-existing quartz. Late calcite forms fracture-controlled veinlets parallel to the shear direction (Fig. 8a). Calcite occursalso as intergrowths with chlorite along dilation interspaces parallel tothe vein quartz veins (Fig. 8b). Late alteration assemblage of alunite,kaolinite and fine crystalline silica replaces early sericite and dissemi-nated sulfide grains (Fig. 8c). Sericite and kaolinite are common wherethe vein quartz is heterogeneous in terms of grain size (Fig. 8d).

Quartz veining and associated hydrothermal alteration have beenimposed on the metagabbro and granodiorite, and to lesser extent onthe metasedimentary rocks. Signs of hydrothermal alteration are evi-dent in zones adjacent to the quartz veins and extending several tensof meters away from the quartz vein boundaries. Alteration is typicallypervasive and texture-destructive, and the relationships betweenalteration assemblages are complex. Disseminated quartz, calcite andchlorite are common in the altered metagabbro and in the vein body(Fig. 9a), whereas, pyrite, sericite, quartz, kaolinite and alunite arecommon next to quartz veins cutting through granodiorite (Fig. 9b).Gold blebs are common in the mineralized quartz veins, especially inzones rich in wallrock selvages and mixtures of chlorite, calcite andsericite (Fig. 9c, d).

Themain types of hydrothermal alteration in themetagabbro rocksinclude pervasive chloritization, carbonatization and selective silicifi-cation and sulfidation, whereas intense sericitization, sulfidation andsubordinate chloritization and hematitization prevail in granodiorite.Subtle sericitization and chloritization overprint the metamorphicmineralogy of the metasedimentary rocks in zones of high fractureintersections in the northernpart of themine area. Kaolinite, hematite,alunite and vuggy silica form veinlets and nodules in the upper partsof the mineralized veins especially next to altered granodiorite.Hydrothermal alteration progresses from chloritization to sericitiza-

Fig. 8. Characteristic features of the Au–Cu quartz veins from Semnamine: (a) Sheared vein qu(b) Chlorite–calcite mixtures alternating with elongate quartz in central part of a mineralizedvein quartz, (d) Pervasive sericitic alteration in interstitial spaces in a mineralized vein.

tion approaching the Au-bearing quartz veins. Chlorite is common inwallrock selvages in the mineralized veins. Chlorite forms olive greenaggregates and flakes filling in spaces between the feldspar and quartzcrystals. In granodiorite, chlorite replaces biotite and hornblende(Fig. 10a), whereas chlorite shreds and plates replace hornblende andrelict pyroxene more or less completely in the altered metagabbro(Fig. 10b).

Chemical compositions of chlorites were obtained using a superJEOL JXA-8600 microprobe at the University of Lausanne. The appliedanalytical conditions were, acceleration voltage=15 kV, currentintensity=30 nA, counting time=10 s per element, and defocusedelectron beam diameter=1–5 μm. Calibration was done with availablenatural and synthetic standards. Structural formulae of chlorite werecalculated on the basis of O20(OH)8. The sum of octahedral cations (T) isvery close to 12 in all samples (Table 1). Chlorite compositions showsubstantial variations in the concentration of several components. Thelargest proportional variations occur for FeO (16.68 to 25.95 wt.%), MgO(16.12 to 21.58 wt.%), with FeO increase and MgO decrease in chloritefrommetagabbro to granodiorite. Very slight variations in A12O3, MnO,and K2O occur with no obvious differences between the different hostrocks. The contents of TiO2, MnO, CaO, K2O and Na2O are uniformly low.The SiO2 content and Fe/(Fe+Mg) ratios have been used in classificationandnomenclature of chlorite.ΣSi ions in the analyzed chlorite, generallywith pycnochlorite compositions, vary from 5.96 to 6.31 a.p.f.u., andFe/(Fe+Mg) ratios range between 0.30 and 0.47 (Fig. 11a).

The electron microprobe data show that chlorites, althoughgenerally showing similar composition, are characterized by higherMgO contents in the altered metagabbro relative to granodiorite. Therelationship between chlorite composition and parent rock compo-sition has been documented by several investigators. Laird (1988)introduced an Al/(AI+Mg+Fe)−Mg/(Mg+Fe) diagram to show the

artz with calcite veinlets parallel to the shear direction, notice the dextral sense of shear,vein, (c) Alunite in associationwith pyrite, chalcopyrite and sericite intermixed with the

Fig. 9. Characteristic features of the gold–copper quartz veins from Semna mine. (a) Zone of pervasive silicification with abundant altered metagabbro components, i.e., chlorite andribbon quartz (Q1), (b) Silicified granodioritewith high contents of sulfideminerals and disseminated alunite replacing primary feldspar and ribbon quartz (Q1) as dispersed patches;whereas, late drussy, microcrystalline quartz (Q2) occurs as fine veinlets traverse the early quartz and alteration assemblage, (c) Sericite–calcite–alunite mixtures alternatingwith ribbon quartz (Q1), and microfractures filled with late buck quartz, (d) Milky quartz veins in metagabbro, rich in alteration material including sulfides, chlorite and Fe-oxides.cc = calcite, chl = chlorite, kao = kaolinite, py = pyrite, cpy = chalcopyrite, ser = sericite, aln = alanite.


relationship, from which parent rock can be identified using chloritechemistry. Fig. 11b shows that chlorite in granodiorite has relativelylowMg/(Mg+Fe) ratios (0.52–0.57) than chlorite inmetagabbro (0.58–0.62). Consideration of the locations of the analyzed samples of relativeto the Au–Cu-bearing quartz veins shows that theMg/(Mg+Fe) ratio inchlorite increases with proximity to the mineralization zone. Similarobservation of the increase in Mg/(Mg+Fe) ratios in chlorite withapproaching the auriferous quartz veins have been documented inwallrocks of several deposits elsewhere (e.g. Large, 1975; Nesbitt,1982). The experimental studies have shown that the Mg/(Mg+Fe)ratio in chlorite is sensitive to fO2, fS2. Based on the compositionaluniformity of the analyzed chlorite, low concentration of alkalineoxides (Na2O+K2O+CaOb0.5 wt.%), and lack of correlation betweenthe Fe/(Fe+Mg) and the Si/Al (Fig. 11c), Alt ratios (Fig. 11d), inter-growths of smectite and/or illite with chlorite are limited (c.f. Foster,1962). The calculated temperatures of chlorite formation using the Si–

Fig. 10. Chlorite replacing biotite, hornblende and occasional augite in: (a) Altered gr

Alvi substitution (geothermometer of Cathelineau, 1988, and Aliv

correction after Kranidiotis and McLean, 1987), range from 226 to267 °C (Table 1), with noticeable increase in parts with abundantdisseminated sulfide minerals next to the mineralized quartz veins.

5. Fluid inclusion study

5.1. Fluid inclusion types and distribution

A number of 183 fluid inclusions in 6 samples of the gold-bearingquartz and carbonate-rich quartz veins were microthermometri-cally studied. The samples were prepared as 100- to 200-μm thickdoubly polishedwafers.Measurementswere carried out, following theprocedures outlined by Shepherd et al. (1985), using a LinkhamTHMSG-600 heating/freezing stage at the fluid inclusion laboratory ofthe Earth Sciences Institute, University of Geneva. The accuracy of the

anodiorite and (b) Altered metagabbro adjacent to the mineralized quartz veins.

Table 1Microprobe analyses of chlorites in metagabbro and granodiorite selvages in gold-bearing quartz veins from the Semna deposit

Sample Metagabbro Granodiorite

s12_1 s12_2 s12_3 s9_1 s9_2 s9_3 s13_1 s13_2 s13_3 s13_4 s23_1 s23_2 s23_3 s21_1 s8_1 s8_2 s8_3 s8_4 s6_1 s6_2 s6_3 s10_1 s10_2 s10_3

SiO2 28.82 30.86 30.84 29.66 30.13 29.06 30.15 29.52 30.23 29.20 29.25 28.83 29.42 29.88 28.71 28.54 29.52 29.25 30.27 29.48 29.20 30.18 29.15 30.03TiO2 0.15 0.00 0.00 0.13 0.21 0.24 0.17 0.21 0.22 0.13 0.27 0.05 0.37 0.19 0.17 0.01 0.29 0.21 0.22 0.13 0.33 0.24 0.26 0.00Al2O3 17.32 16.37 17.09 17.19 16.39 17.58 16.23 16.39 16.75 16.14 16.91 17.75 16.90 16.75 16.51 15.90 16.07 16.93 16.24 16.71 15.92 16.56 16.49 16.88FeO 22.25 21.32 21.79 21.45 21.52 21.68 22.82 22.79 22.43 23.76 22.74 22.54 22.96 21.87 24.78 25.95 25.86 23.94 23.95 24.66 24.93 23.18 24.46 23.75MnO 0.41 0.49 0.62 0.46 0.54 0.61 0.42 0.43 0.48 0.48 0.51 0.50 0.48 0.29 0.22 0.36 0.36 0.49 0.53 0.47 0.45 0.50 0.41 0.34MgO 18.92 18.69 18.02 19.01 19.07 18.58 18.39 18.60 17.95 18.59 18.29 18.01 17.86 18.39 17.62 16.98 16.12 16.61 16.69 16.51 16.89 16.94 16.94 17.15CaO 0.18 0.18 0.12 0.15 0.23 0.15 0.11 0.23 0.14 0.09 0.23 0.16 0.20 0.20 0.21 0.19 0.12 0.23 0.14 0.23 0.12 0.13 0.19 0.12Na2O 0.20 0.01 0.01 0.02 0.05 0.16 0.16 0.12 0.11 0.00 0.08 0.07 0.11 0.12 0.21 0.23 0.13 0.11 0.00 0.00 0.16 0.04 0.07 0.00K2O 0.08 0.02 0.02 0.03 0.02 0.02 0.04 0.03 0.06 0.01 0.02 0.04 0.03 0.03 0.02 0.04 0.07 0.30 0.00 0.04 0.08 0.01 0.02 0.00Total 88.33 87.94 88.51 87.45 88.16 87.43 88.49 88.32 88.37 88.40 88.30 87.95 88.33 87.72 88.45 88.20 88.54 88.07 88.04 88.23 88.08 87.78 87.99 88.27T2 12.36 12.28 12.22 12.30 12.30 12.35 12.33 12.38 12.31 12.44 12.38 12.41 12.38 12.37 12.51 12.65 12.54 12.51 12.45 12.50 12.58 12.44 12.54 12.39

Formula on basis of 28 anionsSi 5.93 6.31 6.27 6.07 6.17 5.97 6.19 6.08 6.20 6.05 6.03 5.96 6.06 6.15 5.98 6.01 6.16 6.09 6.27 6.13 6.11 6.25 6.08 6.19Ti 0.02 0.00 0.00 0.02 0.03 0.04 0.03 0.03 0.03 0.02 0.04 0.01 0.06 0.03 0.03 0.00 0.05 0.03 0.03 0.02 0.05 0.04 0.04 0.00Al(iv) 2.07 1.69 1.73 1.93 1.83 2.03 1.81 1.92 1.80 1.95 1.97 2.04 1.94 1.85 2.02 1.99 1.84 1.91 1.73 1.87 1.89 1.75 1.92 1.81Al(vi) 2.13 2.25 2.37 2.22 2.12 2.23 2.11 2.06 2.24 1.98 2.14 2.28 2.17 2.22 2.03 1.96 2.12 2.24 2.24 2.23 2.04 2.29 2.14 2.30Fe(ii) 3.83 3.64 3.71 3.67 3.68 3.73 3.92 3.93 3.84 4.11 3.92 3.89 3.96 3.77 4.31 4.57 4.51 4.17 4.15 4.29 4.36 4.01 4.27 4.10Mn 0.07 0.08 0.11 0.08 0.09 0.11 0.07 0.08 0.08 0.08 0.09 0.09 0.08 0.05 0.04 0.06 0.06 0.09 0.09 0.08 0.08 0.09 0.07 0.06Mg 5.80 5.70 5.47 5.80 5.82 5.69 5.63 5.72 5.49 5.74 5.62 5.55 5.49 5.65 5.47 5.33 5.02 5.16 5.16 5.12 5.27 5.23 5.27 5.27Ca 0.04 0.04 0.03 0.03 0.05 0.03 0.02 0.05 0.03 0.02 0.05 0.04 0.04 0.04 0.05 0.04 0.03 0.05 0.03 0.05 0.03 0.03 0.04 0.03Na 0.08 0.01 0.00 0.01 0.02 0.06 0.06 0.05 0.04 0.00 0.03 0.03 0.04 0.05 0.08 0.09 0.05 0.04 0.00 0.00 0.06 0.02 0.03 0.00K 0.02 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.02 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.08 0.00 0.01 0.02 0.00 0.01 0.00Total 20.00 19.72 19.69 19.84 19.83 19.90 19.85 19.93 19.77 19.95 19.90 19.90 19.85 19.81 20.02 20.06 19.86 19.86 19.70 19.80 19.91 19.71 19.87 19.76Alivc 2.35 1.96 2.01 2.20 2.10 2.30 2.10 2.20 2.09 2.25 2.26 2.33 2.23 2.13 2.33 2.31 2.17 2.22 2.04 2.19 2.21 2.06 2.23 2.11T (°C)a 267 226 231 251 241 262 240 251 240 256 257 265 254 243 265 263 248 254 234 250 252 236 254 242Si/Al 1.41 1.60 1.53 1.46 1.56 1.40 1.58 1.53 1.53 1.54 1.47 1.38 1.47 1.51 1.48 1.52 1.56 1.47 1.58 1.50 1.55 1.55 1.50 1.51Mg/(Mg+Fe) 0.60 0.61 0.60 0.61 0.61 0.60 0.59 0.59 0.59 0.58 0.59 0.59 0.58 0.60 0.56 0.54 0.53 0.55 0.55 0.54 0.55 0.57 0.55 0.56Fe/(Fe+Mg) 0.40 0.39 0.40 0.39 0.39 0.40 0.41 0.41 0.41 0.42 0.41 0.41 0.42 0.40 0.44 0.46 0.47 0.45 0.45 0.46 0.45 0.43 0.45 0.44Al/(Al+Mg+Fe) 0.30 0.30 0.31 0.30 0.29 0.31 0.29 0.29 0.30 0.29 0.30 0.31 0.30 0.30 0.29 0.29 0.29 0.31 0.30 0.30 0.29 0.30 0.30 0.30Na+K+2Ca 0.18 0.09 0.06 0.08 0.13 0.14 0.11 0.16 0.12 0.04 0.14 0.12 0.13 0.14 0.19 0.18 0.13 0.22 0.06 0.11 0.14 0.08 0.12 0.06

a Temperature estimates calculated according to Cathelineau (1988); Alivc=Al in tetrahedral sites according to Kranidiotis and McLean (1987).

588B.Zoheir

etal./

Ore

Geology

Reviews34

(2008)580

–596

Fig. 11. (a) Plot of Fe/(Fe+Mg) ratios vs. Si contents of chlorites from Semna Au-deposit. Nomenclature and boundaries are after Hey (1954), (b) Plot of Al/(Al+Mg+Fe) vs.Mg/(Mg+Fe)ratios of the studied chlorites, (c) Si/Al vs. Fe/(Fe+Mg) plot of chlorites from Semna deposits, and (d) Al vs. Fe/(Fe+Mg) plot of chlorites from Semna gold deposits. Notice the distinctgrouping of chlorites from gabbro and granodiorite in plots a–c.


microthermometric results was estimated at ±0.3 °C between −60 and−20 °C, ±0.2 °C between −20 and +50 °C and ±1 °C between 50and 350 °C. Reproducibility is of the order of ±0.2 °C between −100 and−20 °C, ±0.1 °C between −20 and +30 °C and ±0.5 °C between 30 and350 °C. Analytical errors are thus insignificant with regard to anygeological interpretation. The volumetric proportions of the aqueousand CO2-rich phases in mixed inclusions were visually estimated(typical error approximately ±10%) at room temperature by referenceto the volumetric charts of Shepherd et al. (1985).

Based on their relative phase proportions at and below room tem-perature, and phase changes during freezing runs, three compositionaltypes of fluid inclusions were identified, i.e., CO2-rich, H2O-rich andmixed H2O–CO2 fluid inclusions. Inclusions of these three types arecoexistent in randomly distributed clusters, or occur along three-dimensional arrays and fractures in quartz. TheH2O-bearing inclusionslack any daughter minerals. The mixed inclusions (H2O–CO2–NaCl)are dominant in all investigated samples, typically with diametersranging from ~5 to 20 μm. These inclusions occur either as two-phase(H2Oliq–CO2 liq) or as three-phase (H2Oliq–CO2 liq–CO2 vap) inclusions atroom temperature, with estimated degree of filling (volume propor-tion of liquid in inclusions) varying from40 to 85%, although clusteringaround 70%. These inclusions exhibit variable shapes includingirregular, ellipsoidal, polygonal, negative crystals and spindle-shaped.In some of these inclusions, a dark CO2 bubble represents the totalcarbonic fraction, whereas no vapor but only a liquid carbonic phase isobserved in others.

The CO2-rich inclusions, the least common of the three types inboth quartz and quartz-carbonate veins, occur commonly as mono-phase or two-phase (CO2 liq+CO2 vap) inclusions, at room temperature.These inclusions vary in diameter from 5 to 12 μm and in shape fromirregular to oval and negative crystals. Some of these inclusionscontain small amounts of H2O, which is petrographically undetected.This assumption is based on the observation of clathrate meltingduring heating to reach the CO2 homogenization in some of the largeinclusions. Aqueous (H2O liq–H2O vap) inclusions occur commonly astwo-phase inclusions with diameters ranging typically from 6 to14 μm. Filling degrees are rather constant and high, ~95%. Aqueousinclusions that are associated with CO2-rich and mixed aqueous-carbonic inclusions in single trails are commonly elongated in shapeand less abundant compared to the carbonic inclusions. Inclusionsoccurring preferentially along intragranular trails together with themixed aqueous-carbonic inclusions are relatively smaller in size thanthe isolated ones. The isolated inclusions are oblate, oval, negativecrystal or have irregular shapes.

5.2. Microthermometric data

Microthermometric measurements were performed on selectedgroups or trails of fluid inclusions, which have constant liquid to vaporratios. The fluid inclusion groups with variable liquid/vapor ratios ordispersed homogenization temperatures were discarded to avoid theanalysis of inclusions, which have experienced leakage or decrepitation.


Salinity estimates are based on the equations of state of Diamond (1992)and Bodnar (1993). Although variable, the CO2 melting temperatures(Tm CO2) in CO2-rich inclusions cluster around −56.6 °C. Whereas, mostmeasurements of the mixed CO2–H2O inclusions (Fig. 12a, b) clusterbetween −56.6 and −59.2 °C, suggesting the presence of other dissolvedvolatile phase(s) in addition to CO2. A maximum of 14 mol% CH4 or20mol% N2may occur in the vapor-rich inclusions (sensu Burruss, 1981;

Fig. 12. Histograms of the microthermometric results for the three types of inclusions measuare given in the text.

Heyen et al., 1982; Thiéry et al., 1994). Homogenization of CO2 (Th CO2)in the CO2-bearing inclusions occur commonly into liquid, but rarecases gave homogenization through fading of the meniscus (criticalhomogenization). The measured inclusions yield Th CO2 between 16.9and 28.1 °C, although most measurements cluster between 21.6 and26.7 °C. Calculated CO2 densities for the carbonic phase in the mixedaqueous-carbonic and CO2-rich inclusions are similar, ranging from

red in vein quartz from the Semna deposit. Composition and characteristics of each type

Fig. 13. Photomicrograph in transmitted light of isolated fluid inclusions in quartz veins from Semna deposit showing: (a) Liquid-dominant CO2–H2O inclusions in association withaqueous and carbonic (CO2-rich) inclusions randomly dispersed in quartz, and (b) Vapor-dominant CO2–H2O inclusions associated with abundant carbonic inclusions.


0.65 to 0.81 g/cm3. However, slight tendency toward higher CO2 densityis observed in the mixed aqueous-carbonic inclusions relative to theCO2-rich inclusions (Fig.12c, d). Clathratemelting temperatures (Tm clath)in the aqueous-carbonic inclusions occur consistently below Th CO2,clustering between4.1 and 7.8 °C (Fig.12e), corresponding to salinities of10.4 and 4.3 wt.% NaCl equiv., respectively. The total homogenization ofthe mixed CO2–H2O inclusions (Th total) occurs mostly into the liquidphase over a wide range of temperatures between 224.3 and 263.2 °C,but clusters at 225 to 240 °C. Applying the equation of state of Bowersand Helgeson (1983), the calculated bulk densities of the mixed CO2–

H2O inclusions vary between 0.81 and 0.96 g/cm3.The H2O-rich inclusions show less variable phase proportions and

lower final homogenization temperatures compared to the associatedmixed CO2–H2O inclusions. The initial ice melting (eutectic tempera-ture, Teu) was observed around −28 °C, indicating the predominance ofNaCl as dissolved salts in the aqueous fraction. Final homogenizationoccurred into the liquid phase in all inclusions. In the quartz-carbonateveins, the H2O-rich inclusions, which are commonly associated withmixed CO2–H2O inclusions in a single trail, show ice melting values(Tm ice) of −1.9 to −6.7 °C and total homogenization temperatureranging between 198.7 and 248.4 °C. In the quartz veins, the H2O-richinclusions are more dilute compared to those observed in the quartz-carbonate veins (Fig.12f), but homogenization temperatures are rathersimilar in both vein types. Applying the equation of state by Bodnar(1993) indicate salinities of 3.2–12.1wt.%NaCl equiv. for aqueous (H2O-

Fig. 14. Isochores for highest and lowest density CO2–H2O inclusions in the auriferous quartzassociated with gold-bearing sulfides in the quartz veins. Also geothermal gradients of 50 anddensity of 2.6 g/cm3 for lithostatic overburden.

rich) inclusions in quartz veins and 4.2–13.3 wt.% NaCl equiv. aqueousinclusions in quartz-carbonate veins. The relatively low salinityaqueous inclusions occur commonly clustered with mixed CO2–H2Oinclusions, whereas isolated aqueous inclusions show relatively high-salinities. Applying the equation of state of Potter et al. (1978), the bulkdensities of the investigated aqueous inclusions vary between 1.01 and1.09 g/cm3.

5.3. Interpretation of the fluid inclusion data

The absence of major deformation increments affected the quartzveins after emplacement evokes that inclusions in primary sites,showing no necking down or stretching, may represent remnants ofearly entrapped fluids during vein formation and gold deposition(sensu Roedder, 1984; McCuaig and Kerrich, 1998). The variable CO2:H2O ratios in coexisting inclusions (Fig. 13a, b), spread in inclusiondensities, coexistence of the three types of fluid inclusions alongintragranular trails and the wide range of the total homogenizationtemperatures are typical criteria characteristic of heterogeneousentrapment, which could have induced by mixing or un-mixing oftwo immiscible fluids (Pichavant et al., 1982; Anderson et al., 1992) orthrough post-entrapment modifications. The lack of a continuum indegree of filling of the mixed CO2–H2O inclusions and absence ofhomogenization into vapor may rule out the immiscibility and phaseseparation as a potential mechanism in Semna gold deposit. Absence

veins from Semna mine. The hatched fields represent the temperature range of chlorite100 °C/km (dotted lines) are shown. Scale on the right side refers to depth in km at rock

Fig. 15. Histogram of H2S δ34S in equilibrium with each sulfide at a given temperature(using the average temperature of associated quartz fluid inclusions, or the equilibriumtemperature of chalcopyrite and pyrite pairs in the same hand specimen, applying theequation of Rye and Ohmoto, 1974). Mineral-H2S fractionations were calculated usingthe equations in Ohmoto and Rye (1979).


of a vertical trend in Th total vs. salinity correlation, which is typical fornecking-down and leakage during heating, and absence of any clearrelationship between inclusion size, phase ratio, salinity and densitydiscards the selective water loss via diffusion in the measuredinclusions. Mixing of fluids is suggested by the relation between Thtotal and salinity of aqueous and aqueous-carbonic inclusions (sensuCathelineau and Marignac, 1994), where salinities are likely attributedto the aqueous fluid tending to decrease asmuch asmixing proceeded.Evidence of mixing is also stipulated by the occurrence of discreteclusters of relatively low salinity aqueous inclusions close to clusteredand isolated high-salinity aqueous inclusions.

For calculating the fluid composition, the equation of sate ofBowers and Helgeson (1983) was used for mixed CO2–H2O inclusions.These inclusions have bulk compositions comprise 0.65 to 0.92 mol%H2O+0.07 to 0.33 mol% CO2+0.1 to 0.3 mol% NaCl±traces of CH4 and/or N2. Using the full range of isochors for the H2O–CO2 inclusions(highest and lowest bulk densities) combined with chlorite tempera-tures of intensively altered wallrock selvages in the gold-bearingquartz veins, the P–T boundary conditions outlined for Semna depositare 226–267 °C and 350–1100 bar (Fig. 14). This temperature intervalis coincident with the range of Th total of the aqueous andmixed CO2–

H2O inclusions. As the inclusions were trapped along a fault system,and given that the local host rock temperature is unlikely to varysignificantly during vein emplacement (e.g., Robert et al., 1995), thespread in the isochors may be caused by pressure fluctuation duringthe hydrothermal event, and/or switch between lithostatic andhydrostatic conditions. In this case the mean of the pressure data(~700 bar) likely represents average pressure conditions of golddeposition at Semna mine, assuming a lithostatic overburden, (e.g.,Hagemann and Brown, 1996). This mean pressure value correspondsroughly to 2 km overburden, which is typical for the ductile–brittle

Table 2Sulphur isotope composition of pyrite–chalcopyrite pairs and calculated δ34S (‰) for fluΔpy–cpy=0.67⁎ l03/T0.5)

Sample s25 s32 s27

Measured δ34S (‰) in pyrite 3.32 2.63 2.94Measured δ34S (‰) in chalcopyrite 1.35 1.41 1.77Calculatedδ34S (‰) H2S fluid (eq. with pyrite) 1.56 1.02 1.39δ34S (‰) H2S fluid (eq. with chalcopyrite) 1.57 1.61 1.96±a 0.35 0.32 0.31bT (°C): Ohmoto and Rye (1979) 204 225 235

a ± (error range).b Uncertainties due to equation accuracy ±35 °C.

transition zone. The corresponding geothermal gradient is, however,much higher than normal in the crustal rocks.

If the fluid pressure was purely lithostatic, the estimated pressurerange (350–1100 bar) corresponds to depths of ~1 to 4 km, assumingan average density of 2.6 g/cm3 for the overlying rock overburden.Such a depth is proportional to geothermal gradients greater than100 °C/km. This high geothermal gradient might be accounted for anearby magmatic event (intrusion emplacement or volcanic erup-tion?). Similar high geothermal gradients were attributed to shallowconductive thermal anomalies along fault zones with moderate upliftrates in Archaean and Proterozoic terranes (Craw and Koons, 1989), inwhich hydrothermal fluid circulations and gold mineralization mightbe induced by the high geothermal gradients (e.g., Olivo and Marini,1988). In a strike-slip fault-controlled hydrothermal system, where σ3is sub-horizontal, the propagation of vertical extensional fracturesbreaks, allowing pressure to drop (Cox, 1995). Under these conditions,supralithostatic pressure regimes tend to be localized and short-lived,leading to small-scale gold deposits (e.g., Sibson, 1987; Cox, 1995).

6. δ34S values and sulfur source

The sulfur isotope compositions of 18 sulfide separates (6 pyritesamples from altered wallrock, 8 pyrites from vein quartz and 6chalcopyrite samples from vein quartz) hand-picked from powderedmineralizedwallrock and quartz vein sampleswere examined.Mineralseparates used for analysis were at least 99% pure. The conventionaltechnique described by Grinenko (1962) was used in order to extractSO2 for sulfur isotopic analysis. The stable isotopic compositions wereanalyzed with gas-isotope ratio mass spectrometers at the Stableisotope Laboratory at the University of Lausanne. Isotope data arereported in the conventional δ notation relative to the CDT standard.The standard error of each analysis is about ±0.1‰.

The δ34S values of disseminated pyrite from the altered wallrocksrange from 0.7 to 2.9‰, whereas, the 34S values of pyrite and chalco-pyrite from quartz veins vary between 1.5 and 4.1‰. Variations in theδ34S values can be attributed to different degrees of oxidation of anH2S-rich sulfur source during the ascent to the site of mineralization.Calculated sulfur isotope compositions of the hydrothermal fluids inequilibriumwith themeasured sulfides are generally light (δ34SΣS=0.6to 3.3‰), suggesting an igneous source of sulfur (Fig. 15; Ohmotoand Rye, 1979; Faure, 1986). Sulfur with light δ34S values might havebeen derived either directly from magmatic fluids or through dis-solution and leaching of pre-existing sulfide-bearing igneous sources(Ohmoto and Rye,1979). Except a likely out of equilibrium pair, pyrite–chalcopyrite pairs yield isotope equilibrium temperature of 145–240 °C (Table 2). Considering the uncertainties induced by the variablefractionation factors, sulfide deposition occurred likely at ~225 °C(Table 2).

6.1. Constraints on fO2–fS2–pH

The calculated P–T–XCO2 conditions of the ore fluid in Semnadeposit correspond to a range of oxygen fugacity (log fO2=−31.8 to

ids and temperatures of equilibrium on basis of equation of Ohmoto and Rye, 1979;

s28 s36 s41 s35 s29

3.10 2.72 2.11 2.57 3.200.53 1.44 0.38 1.41 2.05

−0.30 1.03 −0.18 1.04 1.680.95 1.65 0.67 1.60 2.240.68 0.34 0.46 0.31 0.3070 213 145 238 240

Fig. 17. Salinity vs. total homogenization temperatures of aqueous and CO2–H2O inclu-sions in Au-bearing vein quartz from Semna mine area.

Fig. 16. (a) Stability fields for selected mineral phases as a function of fO2 and fS2 (Fayek and Kyser, 1995 and references therein) showing the redox state conditions for themineralizing fluid of Semna deposit. fO2 is obtained from fluid inclusion composition and fS2 is estimated from stable isotope data in agreement with co-existing pyrite-chloritephases in the altered wallrock selvages in mineralized veins. (b) Plot of the Fe–C–O–S system in log fO2–log aH2S space (equilibrium constants used are fromHelgeson,1969; Helgesonet al., 1978; Bowers et al., 1984). Contour lines indicate the solubility of Au in ppb; where these contours are horizontal Au is predominantly complexed by Cl; Au-bisulfidepredominates where the contours are inclined (Seward, 1984).


−35.6) between the HM and QMF solid buffers (Ohmoto and Rye,1979;Huizenga, 2005 and references therein). Based on these values andassuming the pyrite–chlorite equilibrium, fS2 can be assessed from theactivity–activity diagram (Fig. 16a), which indicates a range of −10 to−11 for log fS2, implying a mildly reduced character of the ore fluid.For salinity values similar to that calculated for the Semna deposit,Mikucki and Ridley (1993) calculated a near neutral pH of ~5.2.Occurrence of iron ore at the mine district, and based on the abundantmagnetite and ferromagnesian minerals in the host rocks, sulfidationcould have played an important rule in gold deposition. Thisassumption is supported by dispersed gold globules and disseminatedpyrite and chalcopyrite in the intensively altered wallrocks. Thegenetic link between sulfidation and disseminated mineralization canbe discussed by considering a phase diagram of the Fe–C–O–S systemin the log fO2–log aH2S space under acid conditions (Fig. 16b). As thehydrothermal fluid percolated into the iron ore and iron-rich hostrocks, oxidants and S were consumed. Consequently, the fluid movedtoward equilibrium with Fe-carbonate, and crossed the Au solubilitycontours. This diagram suggests that gold was transported as Au-chloride rather than Au-disulfide complexes.

7. Discussion

Analyses of the structural data collected from the mine area andsurroundings indicate an early compressional stress regime, inwhichσ1(in an E–W direction) and σ3 (in a N–S direction) governed the NWsinistral strike slip faulting. Consequently,σ1wasorientednormal to theearth surface and σ2 in an E–W direction, likely during granodioriteemplacement and terrain uplift. The stress field was reflected by exten-sion structures that host the early quartz veins, and gently dippingrhyolite dykes. Continuous rotation of the paleo-stress field led to re-orientation of σ1 in the NNW-SSE direction, σ3 perpendicular to theearth surface and σ2 in the ENE-WSW direction. As a result, gold-bearing quartz veins emplacement took place alongWNW dextral faultsegments. Deformation was operating under low temperature, mostlybrittle conditions. Signs of fault valve system during vein formationinclude two texturally different types of quartz in a single vein. Barrenprismatic quartz occurs in the peripheral parts, whereas elongate,stretched quartz occupies the vein interior. Slaves and rafts of highlyaltered, pyritized wallrock are completely absent in the marginal parts,but common at the central parts of the vein. The quartz ribbons in thecentral parts of the auriferous veins are predominantly parallel to the

fault orientation. Undulose extinction and sub-grain formation aresuggestive of growth during slip movement on the fault planes. Finelaminations of intermixed carbonate, chlorite and sulfide minerals arecommon in the central parts of the auriferous veins. Features including:the comb texture of the early quartz, styloitized contacts between earlyand late quartz generations (Fig. 9), and interstitial sulfides andassimilated wallrock material in auriferous quartz veins, together witha heterogeneous fluid inclusion assemblage, may imply that the veinswere formed under conditions of episodic fluid over-pressuring (e.g.,Kreuzer, 2006). The mineralized ribbon quartz was likely derived byprogressive fracturing and recrystallization of the early barren combandbuck quartz sheet (e.g., Dowling and Morrison, 1989). Alunite andkaolinite concentrate in zones rich in disseminated sulfide minerals atfracture intersections (Fig. 9a–d).

Heterogeneous trapping invalidates the use of the homogenizationtemperature for geologic thermometry, particularly as the total homog-enization temperatures are always shifted towards higher values(Ramboz et al., 1982). However, homogenization temperatures of theaqueous-carbonic inclusions intimately associatedwith gold particles inthe quartz veins are comparable to temperatures calculated usingchlorite thermometry for the adjacent wallrocks. Petrographic featuresand fluid inclusion data do not support boiling as a gold deposition


mechanism, but cannot completely rule it out either. The salinity of thehydrothermal fluid was variable, but generally greater than 4 wt.% NaClequiv. Cooling and mixing of moderately saline magmatic water (withsalinity in the range of 2–10 wt.% NaCl equiv.; e.g., Hedenquist et al.,1998) andmeteoricwater could explain the “inclined” trend observed inthe homogenization temperature vs. salinity diagram (Fig. 17). Golddeposition from such solutions can be effected by processes that reducethe activity of reduced S species, including oxidation (e.g., Seward,1973;Hofstra et al., 1988) or sulfidation of ferrous Fe (e.g., Phillips and Groves,1983; Hofstra et al., 1991). The importance of sulfidation to Auprecipitation depends on the reactivity of Fe from the host rock to H2Sin the fluid. The interaction between rock and the H2S in hydrothermalfluid would be significantly different if the Fe were present as pyriterather than as an oxide or carbonate. The Semna mine district is knownfor hosting disseminated and banded magnetite/hematite ores.

Gold mineralization associated with Algoma-type banded iron orewas reported at the Abu Marawat prospect, north of the Semna mine.At this locality, up to 2 ppmAuwere reported in the iron ores, either asinclusions in hematite or as fine specks in the gangue silicate andcarbonate minerals (Botros, 1991). Genesis of the auriferous ironbands at the Abu Marawat prospect was attributed to the interactionbetween hot brines and seawaters (Botros, 1991). A major shear zone,more than 15 km long and ~3 km-wide, extends from the AbuMarawat area to south of Gebel Semna in a N–S direction. This shearzone, which cuts through the Abu Kalb metavolcanics and associatediron ores and which is locally associated by sets of auriferous quartzveins, pockets and lenses, was, however, amissing factor in the geneticmodel suggested by Botros (1991). Our field observations in both AbuMarawat and Semna areas led to link the auriferous quartz veins andthis major shear zone spatially and temporally. Relationship betweengold mineralization and the shear zone was also suggested by Bridgeset al. (1990), who stated that gold and base metal occurrences in theAbuMarawat area are confined to auriferous quartz veins and silicifiedshear zones cutting highly silicified Precambrian metavolcanic andmetavolcaniclastic rocks. It is therefore suggested that iron ores andiron-rich host rocks in the Semna mine area favored gold depositionthrough sulfidation as sulfur from the percolating fluids was capturedby Fe to form pyrite leading to significant changes in fS2 and fO2.

Pressure–temperature conditions of several gold deposits from theEgyptian Eastern have been obtained from fluid inclusions and stableisotope studies (e.g., Harraz et al., 1992; Harraz and El-Dahhar,1993; ElKazzaz, 1996; Loizenbauer and Neumayr, 1996; Shazly et al., 1998;Harraz,1999, 2000, 2002;Helmyet al., 2004; Zoheir, 2004, 2008).Mostof these studies point toward temperatures between 235 and 430 °Cand pressures varying from b1 to ~2 kbar. Gold deposition in thesedeposits was attributed towallrock alteration and fluid unmixing (e.g.,Harraz, 2002; Zoheir, 2004), boiling (Harraz and El-Dahhar, 1993) andcomplex fluid mixing and unmixing through a cyclic crack-sealmechanism (e.g., Helmy et al., 2004; Zoheir, 2008). Based on the veinstructures and mineralogy, together with the hydrothermal alterationtypes, these deposits are referred to as orogenic-type deposits formedunder mesothermal conditions, consistent with the estimated pres-sure–temperature conditions. The present study concludes that Semnagold deposit was formed under lower temperatures and at shallowerdepths compared to the other Egyptian gold deposits which have beencomprehensively studied so far. An exception can be made for the UmRus gold deposit, for which Harraz and El-Dahhar (1993) estimated aP–T range of 250–300 °C and ~0.35 kbar, and suggested that boiling ofmagmatic CO2–H2O-richfluidsmixedwith shallower crustalfluidswasresponsible for gold deposition. Similarly, at Hamash mine in thesouthern Eastern Desert, Helmy and Kaindl (1999) studied a gold–copper mineralization showing geological, mineralogical and petrolo-gical characteristics identical for the transitional zone betweengranitoid-related porphyry type and epithermal vein type mineraliza-tion. There is, however, still a controversywith regard to the possibilityof porphyry systems in the Egyptian basement rocks.

Basta et al. (1996) suggested that gabbroic rocks exposed at theSemnamine areamight have been a favorable source for gold and basemetals. The lack of regional sulfur isotope data, however, makes ithard to drawa conclusion concerning the ultimate sources of sulfur forgold mineralization. Nevertheless, the generally light sulfur stableisotope values of sulfides associated with gold particles may suggest amagmatic source for sulfur. The abundant sulfide-bearing acid tointermediate dykes at Semnamine area may place a further constrainton a volcanic source for sulfur, likely linked to Dokhan volcanicity inthe region (?). A spatial and temporal link between auriferous quartzveins and Dokhan Volcanics is proposed by several authors (Botros,2002 and references therein). As the Dokhan Volcanics may representthe surface expression of calc-alkaline granites (e.g., El Gaby et al.,1988) and based on their calc-alkaline affinities and active continentalmargin tectonic environment (Basta et al., 1980), Hussein (1990) andBotros (2002) supposed that an ideal environment for the formationof porphyry gold–copper mineralization might exist in Egypt.

8. Conclusions

The main lode in Semna gold deposit exhibits a complex structureof two texturally-different quartz veins, suggestive of a change in thepaleostress field regimes from tensional to right lateral shearing alongthe pre-existing fault segments during the late stages of the tectonicevolution of the area. Textural, structural and mineralogical evidence,including open space structures, quartz stockwork and superimposedalteration assemblages, constrain on vein development during anactive fault system likely under ductile–brittle transitional conditions.The mineralization is expressed by pyrite, chalcopyrite and subordi-nate galena, sphalerite and gold disseminated in the quartz veins andadjacent wallrocks. Hydrothermal quartz–chlorite–sericite–calciteassemblage replaces the feldspar and ferromagnesian minerals inthe host rocks, implying a late metasomatic paragenesis un-related tothe host rocks. Distribution of the alteration assemblage is function ofthe host rock and distance from the mineralized quartz veins, withdistal quartz–chlorite and proximal sericite–pyrite zones. A likelysupergene assemblage manifested by mixtures of kaolinite, aluniteand vuggy silica overprints the host rock and hydrothermal alterationin zones of densely fracture intersections.

The Semna gold deposit can be attributed to a combination ofprotracted volcanic activity (Dokhan Volcanics?), fluid mixing, wall-rock sulfidation and a structural setting favoring gold deposition. Goldwas likely transported as Au(HS)2− under acid conditions concomitantwith the quartz–sericite–pyrite alteration. Gold precipitation mighthave induced by a decrease in gold solubility through fluid mixing,pressure fluctuation, and variations in pH and fO2. Conditions of golddeposition are estimated on basis of the fluid inclusions and sulfurisotope data as 226 to 267 °C and 350 to 1100 bar under lithostaticconditions. Mixing of originally heterogeneous CO2 and H2O–NaClfluids is probably the best explanation for the variable salinities ofCO2–H2O inclusions in a single and different inclusion trail(s). Thespread in final homogenization temperatures and densities can beregarded to trapping under pressure fluctuation during repeatedfracture opening and sealing. The combination of textural character-istics, fluid inclusion populations and pressure–temperature estimatessuggests formation under conditions transitional between mesother-mal and epithermal systems.

Acknowledgements

We acknowledge the immense help of Dr. R. Moritz and Prof. T.Vennemann during the analytical work at Geneva and LausanneUniversities. The authors are also grateful to Ore Geology ReviewsEditor in-Chief Nigel Cook, Associate Editor Greg Arehart and twoanonymous reviewers for valuable comments and criticism whichsignificantly improved the early version of this manuscript.


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Role of fluid mixing and wallrock sulfidation in gold mineralization at the Semna mine area, central...

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