ARTICLE

Ivan K. Bonev Æ Thomas Kerestedjian

Radostina Atanassova Æ Colin J. Andrew

Morphogenesis and composition of native gold in the Chelopechvolcanic-hosted Au–Cu epithermal deposit, Srednogorie zone, Bulgaria

Received: 18 December 2000 /Accepted: 20 November 2001 / Published online: 24 April 2002� Springer-Verlag 2002

Abstract Native gold is an important economic com-ponent of the complex ores of the Chelopech high-sul-phidation volcanic-hosted epithermal Au–Cu deposit(Bulgaria). The ore consists of pyrite, chalcedonic silica,chalcopyrite, enargite, luzonite, tennantite, bornite,sphalerite, galena, and numerous other sulphide, arse-nide and telluride minerals. Gold is paragenetically as-sociated with most of the arsenic-bearing and base metalsulphide minerals. The chemistry and morphology ofgold grains, which were separated from disintegratedores, were systematically studied by scanning electronmicroscopy (SEM) and electron probe microanalysis(EPMA). The gold is characterised by high fineness(�950) and a mean composition of 94.1 wt% Au,5.27 wt% Ag, 0.53 wt% Cu and 0.10 wt% Fe. Goldgrains show variable morphology: subhedral flakes, ir-regular grains, euhedral isometric crystals, elongatedrods, wires and fine-fibrous crystals, {111} twins, finedendrite-like formations, spongy gold, and polycrystal-line grains. The densest faces {111} are morphologicallythe most important. Morphometric measurements showa pronounced flatness of the gold particles. The meanvalue of the Corey factor (a measure of flatness from0 to 1 = spherical) is 0.14 and the mean length to widthvalue is 1.64. It is suggested that crystal growth of goldtook place in small voids, fine cracks and intergranularspace, the geometry of which have controlled the highlyvariable grain shapes. No direct correlation between thesize and composition of gold grains exists, but largergrains tend to be of higher fineness.

Keywords Chelopech Æ Epithermal Æ Gold ÆMorphology Æ Srednogorie

Introduction

The morphology of grains and their chemical composi-tion are two characteristics of native gold that are ofgreat importance for understanding mineral relation-ships and the origin of gold in deposits of different ge-netic types. In numerous publications on placer deposits,these characteristics are used to explain physical andchemical changes to gold grains during fluvial transport(Petrovskaya 1973; Hallbauer and Utter 1977; Giusti1986; Groen et al. 1990; Knight et al. 1999; Youngsonand Craw 1999) and the possible role of microorganisms(Watterson 1992). The relatively easy sampling of goldparticles from placers facilitates such studies. Goldcrystals of various shapes are also often described forlaterites (Lawrance and Griffin 1994; Bonev and Vess-elinov 1996; Colin et al. 1997) and other supergene de-posits.

In most hard-rock deposits, gold grains are highlydispersed in massive or disseminated ores and theirstudy is possible by microscopic and microprobe tech-niques in two-dimensional polished sections, when theirsize and concentration is large enough. However, espe-cially for characterising grain morphology in three di-mensions, such information gives only rough and ofteninadequate information.

Separation of gold particles from ores, using chemicaland physical methods, followed by detailed morpho-logical SEM study, is the most direct way to examinegold in its primary environment. The investigations ofgold in the Witwatersrand conglomerates (Utter 1979;Minter et al. 1993) and of gold in quartz–sulphide veinsin Russia, Kazakhstan and Uzbekistan (Abdulin et al.2000) indicate a variety of dendritic, spherical, disk-likeand other particle shapes from hard-rock hosted golddeposits of variable and partly disputed origin. How-ever, such data in the literature are very scarce.

Mineralium Deposita (2002) 37: 614–629DOI 10.1007/s00126-002-0273-8

I.K. Bonev (&) Æ T. Kerestedjian Æ R. AtanassovaGeological Institute,Bulgarian Academy of Sciences,1113 Sofia, BulgariaE-mail: bonev@geology.bas.bg

C.J. AndrewNavan Resources plc,Navan, Ireland

This approach was applied in the present study ofnative gold from the Chelopech Au–Cu high-sulphida-tion epithermal deposit from the Srednogorie metallo-genic zone in Bulgaria. The study was undertaken toobtain information about the morphogenesis of goldand ore crystallisation process in this deposit.

The Chelopech deposit

The Chelopech gold–copper deposit in the Srednogoriezone is one of the largest and economically most im-portant ore deposits in Bulgaria, and is Europe’s largestcurrently mined gold resource (Andrew 1997; MiningJournal 1997). Together with other epithermal deposits(Radka, Elshitsa and Krassen) and porphyry-copperdeposits (Elatsite, Medet, Assarel, Vlaykov Vruh, TsarAssen) of the Srednogorie zone, it is located in a centralsector of the extensive Tethyan–Eurasian copper beltformed during Mesozoic–Cenozoic time along thesouthern margin of Eurasia (Vassileff and Stanisheva-Vassileva 1981; Jankovic and Petrascheck 1987; Jank-ovic 1996). The Srednogorie zone has the features of asubduction-related, Late Cretaceous island arc, withdevelopment of calc-alkaline to subalkaline volcano-plutonic magmatism and associated hydrothermal oredeposits.

The Chelopech deposit, named after an adjacent vil-lage, is located about 60 km to the east of Sofia. Pros-pecting and mapping started in the area in 1956,exploration began in 1960, and it intensified after 1975when an ore-dressing plant was built. Since 1992, thedeposit has been exploited by a Bulgarian-IrelandMining Company (BIMAC), and after 1998, by theNavan-Chelopech Mining Company. Increased pro-duction in the last few years reflects reorganisation andimprovement of the ore processing. The undergroundworkings are now situated at about 300 m below thesurface (mainly on level 405 m).About 30 tonne (t) ofgold have been extracted from the mine to date (24.8 tuntil 1995, Milev et al. 1996). The ore reserves and re-sources now include about 50 Mt of copper–gold oresgrading 1.40% Cu and 3.30 g/t Au, at a 4 g/t goldequivalent cut-off (considering the value of copper in theore; Andrew 1997). The total metal tonnage of gold(mined + reserves) is about 195 t. Other estimates bythe Navan-Chelopech Mining Company (Mining Jour-nal 1997), at a 3-g/t cut-off, give about twice the metalreserves. Based upon its geological and mineralogicalcharacteristics, Chelopech is a high-sulphidation (acid-sulphate) epithermal type of gold deposit (Heald et al.1987; Sillitoe 1989, 1995; Hedenquist and Lowenstern1994; Arribas 1995; and others), and is one of the largestof this type. The geology, stratigraphy, structure, min-eralisation and zoning of the deposit have been a subjectof numerous studies during the last decades (Terziev1968; Tzonev 1982; Kovalenker et al. 1986; Petrunov1994, 1995; Popov et al. 2000, and others). This infor-mation is summarised in the subsequent two sections.

Geological setting

The Chelopech deposit is located in the northern part ofthe Panagyurishte region in the central sector of theSrednogorie metallogenic zone (Fig. 1). The basement inthe area consists of gneisses and amphibolites of the Pre-Rhodopian Supergroup (Precambrian ?), and lowerPalaeozoic phyllites and diabases overlain by Turonianconglomerates, sandstones and coal-bearing shale. Vol-canic rocks of the Coniacian to Campanian ChelopechFormation include early subvolcanic dacitic–andesiticrocks, younger main stage andesitic lavas, agglomeratesand tuffs of an interpreted former stratovolcano, andlatest dykes and subvolcanic bodies of andesite, daciteand porphyritic rocks. The Chelopech Au–Cu deposit islocated in the central part of the Chelopech volcanicstructure and is controlled by a system of radial andconcentric faults related to the caldera (Popov andKovachev 1996; Popov et al. 2000). Other, smaller oreoccurrences in the area include the Vozdol polymetallicvein deposit and the low-grade Karlievo copper por-phyry deposit (Fig. 1). A large porphyry copper deposit(Elatsite) is located about 7–8 km north-west ofChelopech (out of the map in Fig. 1) and is related tosimilar subvolcanic dykes and intrusive bodies (Strash-imirov et al. 2002, this volume).

Upper Senonian sedimentary rocks of the Mirkovoand Chougovitsa Formations (sandstones, marlstonesand terrigenous flysch) and thick Pliocene and Quater-nary clays, sands and gravels overlie the volcanic rocksand associated orebodies, thus marking the end of themagmatic and hydrothermal activity. The K–Ar deter-minations on ore-related micas cluster at 78–74 Ma(Lilov and Chipchakova 1999), which coincides wellwith the geologically constrained Late Cretaceous agebut differs from U–Pb zircon ages of dykes bracketingCu–Au mineralisation in the nearby Elatsite depositnear 92 Ma (Fanger et al. 2001).

Ore mineralisation

The mineralisation is represented by massive sulphidebodies, stockwork zones of fine sulphide veinlets, dis-continuous sulphide veins and disseminated ore in al-tered wall rock. The massive bodies have a complexmorphology of steeply-dipping (65–90�) lenses and pipesextending to depths of more than 600 m (Fig. 1). Mostof the orebodies have no outcrop on the surface and thatis why no significant placers in the area are known.

The andesitic and tuffaceous host rocks show intensehydrothermal alteration ranging from an outer propy-litic zone, through quartz–adularia, quartz–sericitic andadvanced argillic assemblages, to intensive silificationadjacent to ore mineralisation. The main geochemicalsignature of the ore includes Cu, Au, Fe, S, As and Ba,with less consistent Sb, Bi, Se, Te, Ag, Pb, Zn, Sn, In,Ga, Ge and Tl.

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Pyrite is the dominant sulphide mineral. Chalcopy-rite, tennantite, enargite, luzonite, bornite, sphaleriteand galena are also common, with subordinate tetrahe-drite, goldfieldite, famatinite, digenite and covellite. Alot of minor ore minerals also occur. The main gangueminerals are quartz, barite, anhydrite, kaolinite, dickite,alunite, ankerite and calcite. The ore-forming process iscomplicated because of the overlapping mineral associ-ations of three successive main ore stages (Petrunov1994, 1995): early pyrite, main Cu–As sulphide, and latepolymetallic stages.

During the first stage, massive pyrite lenses, veinlets,stockworks and disseminations were formed. They arecharacterised by fine-grained, colloform, globular andfine-layered textures. The pyrite is accompanied bychalcedonic silica and, especially at depth, by anhydrite.Petrunov (1995) hypothesised that this early pyrite

formed in a submarine environment, but no strong evi-dence for this was given. The economically most im-portant stage is the complex copper- and arsenic-richstage that reflects formation of a high-sulphidation epi-thermal system in subaerial conditions and associatedwith intense fracturing and brecciation. Advanced arg-illic alteration and silification included formation of al-unite, dickite, diaspore, andalusite, quartz, anhydriteand barite, which overprinted earlier propylitisation andargillisation. The chalcopyrite–tennantite, enargite–luz-onite, quartz–pyrite or bornite–digenite dominated as-semblages that form the massive orebodies, stockworksand veinlets, show distinct spatial distributions. Highlyoxidised sulphide and telluride minerals occur in theupper levels of the deposit, followed at gradually in-creasing depth by enargite–luzonite, tennantite andchalcopyrite, and bornite–digenite–anilite. Late-stagesphalerite–galena ores, which are associated with barite,are predominantly present in the higher and outer partsof the deposit. The orebodies in the neighbouring Voz-dol deposit are represented by steep mineralisedsphalerite–galena veins (Fig. 1).

Fig. 1 Geologic map of the Chelopech ore deposit, and crosssection along line A–B, representing the geological position oforebodies in the Chelopech (right side) and Vozdol (left side)deposits (simplified after Popov et al. 2000)

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Methods

Mineral relationships of gold in the different type ores were studiedin more than 100 polished sections by routine reflected light mi-croscopy. The characteristic relations are summarised in the nextsection and are presented in Fig. 2. In addition, polished sections ofore concentrate particles cemented in epoxy were microscopicallyexamined.

Samples of both, the gravity and the flotation sulphide con-centrates of disintegrated mineral grains were widely used. Theore processing circuit at the processing plant of the Chelopechmine uses fresh water and includes conventional crushing, grind-ing in ball mills and separation in Knelson concentrators. Itproduces a gravity concentrate – ‘black sand’, which is enriched infree gold and has not been treated chemically. By additionalflotation at the plant, copper-sulphide and pyrite concentrates are

Fig. 2A–I Mineralogical relationships between native gold and themain sulphide minerals in the Chelopech ores. Reflected light(crossed nicols in E and H); the scale bar is valid for all figures.A An interstitial gold grain included together with chalcopyrite,tennantite and quartz, in early fine-grained pyrite. B Gold withchalcopyrite infilling interstitials in fine-grained early pyrite.C Bornite–chalcopyrite veinlet with large gold grain, cutting theearly pyrite aggregate.DGold in tennantite. E Interstitial gold in anenargite–tennantite–barite aggregate. F Gold in tennantite masspenetrating fine-grained pyrite. G Gold (fineness 950, anal. nos. 22and 23 in Table 1) and galena veinlets in iron-poor sphalerite.H Large gold grains in a bornite–chalcocite mass. I A branchedveinlet of electrum [Au1(Ag, Cu)1, anal. nos. 28 and 29 in Table 2]and galena, crosscutting iron-free sphalerite. AU Native gold (alsoin the circles); PY pyrite; CP chalcopyrite; EN enargite; TENtennantite; BN bornite; SP sphalerite; GA galena; Q quartz; BAbarite

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also produced. For this study, nearly pure gold fractions werederived from the gravity concentrates, after additional laboratorymagnetic separations and gravity separation in a micropanner.Systematic microscopic and SEM examination of concentratesestablished that the number of mechanically deformed particlescaused by milling is relatively small, not more than a few per cent,and they are clearly distinguishable. Their characteristic featuresare the presence of groups of subparallel bent or chaoticallyoverlapped surface scratches, which are obvious artefacts oftechnogenic origin (Fig. 3A). Thus, excluding them, we were ableto obtain a large number (thousands) of not distorted gold par-ticles of variable size, shape and composition, with preservedoriginal morphology and state, suitable for morphological andanalytical studies. It was not possible to distinguish directly thegold grains belonging to the different mineralisation stages. Usingthe mean concentrate of the annual output gave us an integratedand representative picture for the main workings levels of themine (�405–415 m). Some gold was obtained, for comparison, bychemical treatment and selective dissolution of crushed, high-grade ore and of flotation copper-sulphide concentrate, althoughonly a small gold amount was recovered in such a way.

After a preliminary stereomicroscopic examination, selectedindividual gold particles were studied under a scanning electronmicroscope to determine their detailed morphology. A JEOLSuperprobe 733 SEM device with an ORTEC EDS system and aJEOL SEM T-300 with a Link EDS analytical system were usedin these studies. Several hundred particles were examined, andphotomicrographs of several tens of these were obtained at dif-ferent magnifications, from 40· to 10,000·. Before photography,the identification of each particle as gold was confirmed by EDSanalysis. Associated intergrown minerals, when available, wereidentified by the same methods. Systematic measurements of thethree dimensions of single gold particles were made under anoptical ore microscope using a micrometer eyepiece grid and thefocusing screw micrometer of the microscope.

The chemical composition of single gold grains was studiedby electron microprobe analyses in polished sections of concen-trate particles, and also in polished ore samples. Pure metalswere used as standards for the quantitative microanalyses. Thesize of gold grains in these arbitrary two-dimensional intersec-tions was also measured. The data for gold of the late poly-metallic stage were obtained mainly from ore samples.

Native gold in the ore

Native gold, the main gold-bearing phase in the deposit,is irregularly distributed in the orebodies. Several gold–telluride minerals have been also identified: kostovite(type locality), sylvanite, krennerite, calaverite, petzite,nagyagite and montbrayite. However, as determined bychemical analysis of ore concentrates, their overallcontribution to the balance of gold in the ores is insig-nificant (<1%), and they will not be a subject of dis-cussion here.

The relationship of gold with the early fine-grainedand colloform pyrite of the first ore stage is not very clear.Todorov (1991) reported 6.7 ppm Au in large samples ofsuch pyrite. But, in the pure monomineral pyrite bodies(according to mine production assays), the gold contentis commonly much lower. We cannot discuss here thepossible presence and nature of submicroscopic ‘invisi-ble’ gold in the early pyrite, which is only a minor com-ponent of the gold resource of the deposit, and the studyof which needs of special methods. Microscopicallyvisible gold grains of micrometre size observed in the

intergranular space of the massive pyrite aggregates areusually associated with other sulphide minerals (Fig. 2A,B) and thus belong to the later mineralisation stages.

Much of the gold at the Chelopech deposit is asso-ciated with the copper- and arsenic-rich minerals. Inchalcopyrite, gold occasionally occurs as irregular orrounded inclusions of different size (Fig. 2B). Some-times, several tiny gold blebs are enclosed in a singlechalcopyrite grain, whereas in the neighbouring grains,they are completely absent.

In tennantite and in enargite, gold can be found assmall, irregular grains (Fig. 2D, F). In more complex,fine-grained sulphide mineral aggregates of enargite,luzonite, chalcopyrite and tennantite, gold grains usuallyoccupy interstitial positions (Fig. 2E). Gold inclusionsoccurring in the bornite–chalcopyrite and in bornite–chalcocite–anilite assemblages (Fig. 2C, H) are oftenlarger, reaching ‡0.5–1 mm in diameter in some cases(see also Kovachev et al. 1988).

A second main association of gold in Chelopech iswith spheroidal, iron-poor but copper-bearing sphale-rite. Gold, accompanied by fine-grained galena, occursas elongated grains in the interstices of the radial-fibroussphalerite, or as fine crosscutting veinlets (Fig. 2G). Insome places, this sphalerite contains a large amount ofgold grains, whereas none are found in the surroundingpyrite. In rare cases, veinlets of galena and silver-richgold (electrum) were observed cutting such sphalerite(Fig. 2I). Within some orebodies with silica-rich Pb–Znores and high-grade chalcopyrite–enargite ores the goldcontent reaches 100 ppm (Andrew 1997).

Chemistry of native gold

A systematic study of the chemical composition of goldgrains established their high purity. The silver content ofthe grains is relatively low, rarely exceeding 7–8 wt%.The mean native gold composition from 230 microprobedeterminations is 94.14 wt% Au, 5.27 wt% Ag,0.53 wt% Cu and 0.10 wt% Fe. No other metalliccomponents were detected. The fineness (1,000· Au/(Au + Ag) generally varies in the range of 900–1,000,with a mean value of 947. The amount of contamination

Fig. 3A–H Micromorphology of gold particles with developmentof o{111} and a{100} faces (SEM photomicrographs). A One of thescratched gold grains, with intensive mechanical deformationsproduced during ore processing. B Isometric cubo-octahedralcrystal with rounded edges. C Highly-distorted, elongated along[110] gold crystal. D Gold crystal with distinct re-entrant angle(top) and striated faces (in the middle) caused by polysynthetictwinning along (111) planes parallel to the upper octahedral face(inset). E Wire-like and fine fibrous crystal formations. F A cubo-octahedral crystal with a rough surface, flattened along (111). G Anirregular particle, flattened in (111) plane and elongated in [110]direction. H Branched protrusions on a (111) flattened crystal with[110] outlines

c

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by Ag, Cu and Fe is shown on Fig. 4. Some typicalanalyses are given in Table 1.

Copper is a minor, but characteristic, component inthe Chelopech gold grains, as well as in most othersulphide minerals, such as pyrite and sphalerite. Itscontent in native gold is usually below 1 wt% and inrare cases reaches 2–3 wt%. Many gold grains also showlow (<0.77 wt%) iron contents.

In general, the gold grains are chemically homoge-neous. Etching with aqua regia and BSE images do notreveal any distinct zoning. The small compositionalvariations do not exceed 1–2%. In some grains, the goldcontent gradually decreases from core to rim because ofa slight increase in silver or copper (e.g. Table 1, anal.nos. 1–2, 7–8, 11–12, 15–16), but in others the trend isopposite. Probably this is an influence of the neigh-bouring minerals.

Only a few studied grains (Table 1, anal. nos. 26–29)are of electrum with more than 20 wt% Ag, as usuallydefined (Harris 1990). Electrum is closely associatedwith galena in branched veinlets which crosscut fibrous,iron-poor sphalerite (Fig. 2I). This electrum, withfineness 664 includes 33.5 wt% Ag and 0.50 wt% Cu(average of six analyses), thus approximating a quasi-stoichiometric formula Au1(Ag, Cu)1. Associated galenacontains 0.60 wt% Ag and no other admixtures. In theliterature, there are no convincing data that indicate inthe continuous Au–Ag solid–solution series such acomposition can form an ordered stoichiometric phaseof crystallochemical importance (Yushko-Zacharova

et al. 1986; Knight and Leitch 2001). However, as shownby Gammons and Williams-Jones (1995), the preferen-tial concentration of silver in electrum reduces its solu-bility. Other, relatively silver-rich gold grains (Table 1,anal. nos. 24–25, with about 13 wt%) are associatedwith galena as well, but silver-poor gold also occurs iniron-poor sphalerite (Table 1, anal. nos. 22 and 23; e.g.Fig. 2G). The close paragenetic contemporaneity ofelectrum and galena shows that silver is a more impor-tant component in the hydrothermal fluids of the latepolymetallic, Pb–Zn stage.

Because of the limited amount of other metals sub-stituting in the gold, the Au/Ag inverse correlation isvery strong (Fig. 5). No correlation between gold andcopper or iron is observed. Also, no clear correlationexists between the size and composition of the grains,but larger grains tend to have high fineness (Fig. 6). Noclear link between gold fineness and specific sulphidemineral phases could be established. For gold includedin chalcopyrite, variations in fineness are small, with arange of 960–980.

Morphology of gold grains

Various morphological types of gold particles werefound in the Chelopech ore, which include irregular orflattened grains with random or crystallographic out-lines, dendrite-like branched grains, elongated wire-like,filamentary and spongy formations, euhedral isometricor distorted crystals and clusters of crystals. The grainsmainly combine uneven and rounded surfaces with somebetter expressed crystallographic growth surfaces. Themorphological descriptions given below follow the orderof decreasing idiomorphism of particles, not their fre-quency in ore.

Euhedral isometric crystals

Gold grains with relatively well-presented crystal shapesare very rare, but they are interesting from a geneticpoint of view. The octahedral o{111} faces are mostcommon, followed by rarer cubic a{100} faces. Theprevailing octahedral faces have hexagonal [110] out-lines, following the crystallographic o:o and o:a edges.The crystal faces are not perfectly smooth, and theiredges and corners are usually rounded. Such an iso-metric, cubo-octahedral gold crystal is shown inFig. 3B. Sometimes, poorly-developed cubic crystalsalso occur.

Elongated rod-like crystals

Some gold crystals are elongated. Their length-to-widthratio reaches 3 or more. In such strongly distorted singlecrystals, the development of o and a faces is unequal.

Fig. 4 Summary histograms of the Ag, Cu and Fe content ofnative gold from Chelopech, as determined by electron microprobeanalyses (230 measurements)

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The elongation is along [110] or other directions. Theseparticles have no constant thickness. Some parts of thesecan be thicker, with more differentiated and better ex-pressed isometric shape (Fig. 3C, right). Their side faces

are not perfect. Often they have stepped surfaces,rounded bulges and small pits, as seen at higher mag-nification. Some elongated particles have arched-likeshape and rather rough surfaces, also with fine-steppedstructures outlining (111) growth layers. The striationsof equal orientation on the different parts of such dis-torted crystals prove their single crystal nature.

Table 1 Representative electron probe microanalyses of native gold (nos. 1–25) and electrum (nos. 26–29) from the copper–arsenic andpolymetallic stage, in wt%

No. Sample Au Ag Cu Fe Total Fineness Associated minerals

Copper–arsenic stage1 13/c 95.20 4.57 0.00 0.00 99.77 954 Pyrite2 13/r 93.65 4.93 0.38 0.50 99.46 9503 23 95.75 3.17 0.75 0.26 99.93 968 Pyrite4 105 97.18 1.96 0.48 0.23 99.85 980 Pyrite5 1/c 94.92 3.37 0.93 0.39 99.61 966 Chalcopyrite6 1/r 95.53 3.49 0.46 0.26 99.54 9657 25/c 95.76 3.79 0.00 0.39 99.94 962 Chalcopyrite8 25/r 95.35 3.00 0.83 0.21 99.39 9699 37/c 96.21 1.91 0.89 0.56 99.57 981 Enargite, tennantite10 37/r 95.63 2.84 0.84 0.61 99.92 97111 49/c 96.58 2.29 0.75 0.00 99.62 977 Enargite, barite12 49/r 94.85 4.82 0.00 0.23 99.90 95213 126 94.52 5.08 0.28 0.16 100.04 949 Tennantite14 137 94.21 4.73 0.58 0.31 99.83 952 Tennantite15 107/c 97.64 1.65 0.34 0.00 99.16 983 Bornite16 107/r 96.34 2.43 0.68 0.00 99.45 97517 37.3/c 93.18 5.68 0.27 0.00 99.13 942 –18 37.3/r 95.74 3.50 0.56 0.18 99.13 964 –19 37.42/c 97.15 3.11 0.28 0.03 100.57 968 –20 37.42/r 95.71 4.48 0.08 0.00 100.27 955 –21 CI.2 91.10 5.26 2.96 0.23 99.55 945 Tennantite, enargite

Polymetallic stage22 C3.8 94.68 5.05 0.40 0.00 100.13 949 Sphalerite23 C3.2 94.87 4.63 0.30 0.21 100.01 953 Sphalerite24 4.4/c 85.92 13.77 0.26 0.03 99.98 861 Galena25 4.3/r 86.62 13.08 0.18 0.18 100.06 86826 85/c 75.28 23.89 0.62 0.24 100.03 759 Galena, bornite27 85/r 75.70 23.78 0.32 0.22 100.02 76129 44.1/c 66.22 33.60 0.56 0.28 100.66 664 Galena, sphalerite28 44.1/r 65.32 33.83 0.51 0.02 99.68 659

r Rim; c core

Fig. 5 Correlation between gold and silver in the continuous solid-solution series in gold grains from the Chelopech deposit

Fig. 6 Plot of gold fineness versus equivalent circle diameter (meansize) of gold grains. Larger grains have higher fineness

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(111) twinned crystals

Twinning along {111} planes is characteristic for goldand is morphologically revealed on the crystal surfacesof some gold particles from Chelopech (Fig. 3D) . In thiswell-formed elongated crystal, octahedral bounding withvery small a faces prevails. The characteristic re-entrantangle in its upper end and the fine linear striation along aseries of parallel (111) twin planes, with [110] orientationparallel to crystal outlines, are manifestations of multi-ple polysynthetic twinning. This twinned crystal is at-tached to a larger, also undeformed crystal. In othercases, twinning is revealed by series of parallel steps andgrooves on the crystal surface.

Wire-like and fine fibrous crystals

Some euhedral isometric gold crystals are accompaniedby wire-like and fine fibrous formations (Fig. 3E). Goldwires grow out from protruded crystal parts and some-times connect opposite protrusions. The thickness ofwires is about 15–40 lm and their length is as much as150–200 lm. As seen at higher magnification, theirrather rough surfaces have a fine-stepped structure withsmall pits and grooves. The steps are terminal parts ofthick growth layers on {111} faces. The bending of thewires is caused by gradual changes in the growth direc-tion and, only in part, to plastic deformation.

The morphology of the fine gold fibres is similar, buttheir thickness (e.g. 3–4 lm) is one order lower than thatof the wires. Fibres also have a fine-stepped surfacebecause of layer growth of octahedral {111} faces. Theprevailing growth direction is [110], as seen in some partsof the fibres. Because of plastic deformation, some fibresobtain an irregular bent shape.

Subhedral flake-shaped crystals

Many gold particles are flattened along a plane and havethe shape of thin or thicker platelets (Figs. 3F, G and7A, B). Some of these are also slightly elongated(Fig. 3G). This morphological type is common, asshown by the morphometric measurements discussedlater. Mostly flakes have uneven or rounded outlines(Fig. 7A). However, many of these, or their more sep-arated and protruded branches (Figs. 3H and 8B), aresurrounded by quasi-hexagonal contours (Fig. 3F).Following the [110] directions, they indicate that theflattening is usually along one octahedral (111) face.Oblique a and o side faces can also be seen. The surfacesof (111) flakes are usually rough and uneven, with wavyrelief. Fine-stepped structures (Fig. 7B, D) are presenton some sloped surface areas, as a visible consequence oftheir layer growth. Fine triangular terraces, [110] stria-tions, and small open pits are also sometimes seen ontheir surfaces. Some gold flakes have complicated,jagged or amoeba-like contours with differentiatedbranches of crystallographic or rounded contours

(Fig. 8A–C). Irregular internal voids occur in someparticles (Fig. 7E, F).

Subhedral and anhedral irregular particles

Many grains have irregular shapes and rugged surfaces,developed in three dimensions (Fig. 7C–F). They aresingle crystals or have polycrystalline nature.

Fine dendritic crystals

Some grains have fine dendrite-like surface structures(Figs. 7G, H and 8C, D). The single branches of mi-crometer size are either subparallel or randomly orient-ed. Some dendrite-like particles (Fig. 7G, H) have ahighly porous texture. The unequal orientation of thedendrite-like branches suggests the polycrystalline na-ture of some composite particles.

Spongy gold

The porous spongy structure of gold particles, knownfrom many ore deposits (Petrovskaya 1973), is rare atthe Chelopech deposit. A typical example is shown onFig. 8E, F, with spongy areas developed preferentiallyon the octahedral faces of cubo-octahedral gold crystals.At higher magnification, it is seen that these areas con-sist of short, tiny (lm-sized) fibre-like crystal forma-tions. The network of single fibres surrounding the smallopen pores has no equal elongation.

Polycrystals

Many particles consist of two or more, randomly at-tached single gold crystals. No regular relationshipsbetween these exist as indicated by the different orien-tation of their crystallographic elements. Polycrystallinegranular texture was revealed in some gold particles andby structural etching. No chemical differences betweenthe single grains were established.

Intergrowth of gold with other minerals

Because gold is associated with many sulphide, sulphateand silicate minerals, intergrowths between these are

Fig. 7A–H Micromorphology of gold particles at two differentmagnifications, with detailed views in right column by SEM.A, B A thin (111) gold flake with striated surface and outlines along[110]. C, D, and E, F Irregularly elongated gold particles withcomplex shape and finely stepped surfaces caused by (111) layeredgrowth. G, H A highly irregular porous particle with dendrite-likedevelopment and rounded edges

c

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common. Attached mineral grains are easily visible onthe surface of many of the separated gold particles, evenusing a stereo- and reflective microscope. More detailedstudy of the intergrowths is possible by SEM. Relation-ships of gold with barite are demonstrated in Fig. 9A,where a broken barite crystal is located between a mas-sive gold grain and its irregular wire-like branch. In an-other example, an irregular gold particle encloses severalgrains of aluminosilicate and copper sulphide minerals(Fig. 9B), which are not fully overgrown. In Fig. 9E,gold is intimately intergrown with covellite. Possibly,their relationships have epitactic character following themost closely packed (111) and (0001) planes.

Gold and pyrite often have mutual contacts. Becauselater pyrite of the second stage usually forms well-shaped crystals, the pyrite faces play a dominant role ingold–pyrite intergrowths. In Fig. 9C, a clear imprint of a

flat pyrite surface is displayed, with a fragment of thecracked pyrite crystal still attached. A similar case isshown in Fig. 9D, where the striated structure suggestsimprint of another crystal, probably again pyrite. Somepits are also seen on the rough gold surface, indicatingits imperfect crystallisation in a restricted space in con-tact with inert crystal faces of adjacent grains.

Morphometric data

The examination of polished sections of ores and of oreconcentrates shows that the gold particles usually haveflattened, elongated or irregular shapes and highlyvarying size. According to the first microscopic studiesof the Chelopech deposit by Terziev (1968), the size ofgold grains varies from 1 to 70 lm, with mean values of5–10 lm. Kovachev et al. (1988) and Strashimirov andKovachev (1994) have described coarser-grained goldparticles as large as several millimetre in diameter, whichthey considered mainly recrystallisation products.

For quantitative characterisation of the grain sizedistribution and geometrical proportions, we have sys-tematically studied the morphometric features of �200

Fig. 8A–F Micromorphology of irregular gold particles. A Acomplex amoeba-like crystal. B A branched protrusion boundedby o{111} and a{100} faces. C An irregular dendrite-like protrusionon a finely striated (111) grain. D Irregular surface micromorphol-ogy with fine jagged dendrite-like sprouts. E, F Gold grain withspongy, porous surfaces, consisting of thin wire-like sprouts jointedinto a skeleton

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non-deformed gold particles from the gravity concen-trates. A large number of unsorted grains of different sizeand shape were placed on their widest sides and mea-sured using a reflective microscope. Although the grainshape is to a high degree structurally controlled, we havetaken into consideration only its geometric features.

The main three dimensions were measured for eachparticle, as follows: length (L), along its longest axis;width (W), in perpendicular direction, and thickness (T),determined as the elevation of the highest point of theparticle over its base. The L, W and T values determinethe sides of the parallelepiped in which the particle,roughly considered as a three-axial ellipsoid, is inscribed.Its volume approximates (always with some excess) theparticle volume. Additionally, the following four metriccharacteristics were calculated:

Equivalent spherical diameter (ESD), was used torepresent the particle size by a single numeric value. It isdefined as the diameter of the sphere inscribed in a cube,

equal in volume to the measured parallelepiped LWT.Hence, ESD ¼3

ffiffiffiffiffiffiffiffiffiffiffiffiffiLWTð Þ

p. In a similar way, Giusti (1986)

used an equivalent diameter, ED ¼3ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi6=pð Þ LWTð Þ

p, de-

termined as the diameter of a sphere equal in volume tothat of the measured parallelepiped. However, 3

ffiffiffiffiffiffiffiffiffiffiffiffi6=pð Þ

p

is 1.24 times larger than the inscribed sphere. Becausethe real particle volume is always smaller than LWT, thechosen ESD parameter provides a closer approach to thereal particle size.

Equivalent circle diameter, ECD ¼ffiffiffiffiffiffiffiffiffiffiffiLWð Þ

p, was used

for two-dimensional particle shapes (in projections orsections) by analogy with the ESD.

Anisometry factor, the L/W ratio, which gives a usefultwo-dimensional characteristic for planar particles, de-scribes a possible elongation in the plane of flattening.

Corey shape factor, CSF ¼ T=ffiffiffiffiffiffiffiffiffiffiffiLWð Þ

p, was used to

describe the flatness of the grains (e.g. see Giusti 1986)and varies between 0 and 1. Low CSF values are indi-cative of highly flattened grains, whereas high valuesdescribe grains with closer to spherical shape, with anextreme value of 1 for an ideal sphere. Other morpho-metric measures exist that describe flatness, e.g. thereciprocal in value coefficient of flatness, (L+W)/2T(Tishchenko and Tishchenko 1974), coefficient of shape(Shilo 1985) and width-to-thickness ratio (Groen et al.1990). However, when applied to flat forms, the Coreyvalues are easiest to compare and interpret.

The mean morphometric characteristics of gold par-ticles as determined statistically (200 measured grains)

Fig. 9A–E Intergrowths of gold with associated minerals. A Mas-sive (below) and wire-like (above) gold embracing the fragments of abroken barite crystal (BA). B Small grains of chalcopyrite (CP),pyrite (PY) and an aluminosilicate mineral (AL) enveloped by thebranches of a larger irregular gold grain. C Uneven part of a goldcrystal with two flat surfaces, imprints of a pyrite crystal (PY) afragment of which is partly preserved in the corner. D A goldsurface with notches and pits at the confining contact with astriated pyrite face (now missing). E Rounded gold intergrown witha stepped hexagonal covellite crystal (CV), probably in epitaxicrelationship

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are shown in Fig. 10 and Table 2. Morphometric mea-surements were made also on randomly cut grains ofconcentrate in polished sections, where only the length(l) and width (w) can be determined (Table 2). The mostsignificant results from the morphometric analysis are asfollows:

• The equivalent spherical diameter (which approxi-mates the size of the droplet the grain would melt to)differs noticeably from all three-dimensional parame-ters that are used to calculate it. According to themean size (67 lm), the particles of native gold arerelatively coarse.

• The very low value of the mean Corey factor, 0.14, isindicative of the substantial flatness of the goldparticles. The same is also indicated by the equivalentcircle diameter, which is nearly twice that of theequivalent sphere diameter.

• The 2-D projection anisometry factor (L/W=1.64),because it is greater than 1, also reveals a systematictwo-dimensional anisometry of particles in their habitplane. There is no evidence that this relative elonga-tion is crystallographically controlled.

• The similarly measured 2-D anisometry for arbitrarysections of particles in polished sections (l/w=2.11) issomewhat higher than their projection anisometrybecause of the larger difference in its two axes.

• The data show a strong positive correlation betweenthe metric characteristics of particles, with correlationcoefficients of 0.76 for the L–W pair, 0.50 for W–Tpair, and 0.80 for ESD to T (Fig. 11). This indicatesthat the shapes of smaller and larger particles arestatistically similar and the flatness of gold particles istheir common, size-independent characteristic.

• No clear correlation exists between the ESD and theCorey shape factor (Fig. 12).

• Distribution charts for all parameters show a rela-tively high skew. This deviation from normal Gauss-ian frequency distribution points to actual sizediversity with abundance of larger particles.

Discussion

Gold composition

Extensive studies and thermodynamic modelling on goldand silver solubility, transport and stability (Seward1984; Hayashi and Ohmoto 1991; Huston et al. 1992,Gammons and Williams-Jones 1995) show that at tem-peratures below 300 �C and near neutral pH, the dom-inating transporting complexes are Au(HS)2

– andAgCl2

–. We will not discuss in detail the physicochemicalparameters of ore formation in Chelopech because ore

Fig. 10 Histograms of themorphometric features of goldgrains, which display theirstriking anisometry (length>width >thickness; left sidediagrams), as well as some spe-cific morphometric characteris-tics, describing their volume(equivalent spheric diameter),size in the largest habit plane(equivalent circle diameter), andflatness (reciprocal to the Coreyshape factor)

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fluid P–T–X are not precisely known at the absence ofmeasurable fluid inclusions in the chalcedonic silicagangue. The widespread large-crystal enargite, as high-temperature Cu3AsS4 phase, indicates epithermal con-ditions (T �280–300 �C) and an oxidised environmentof the main copper–arsenic mineralisation stage, with ahigh H2S activity (pyrite–bornite–chalcopyrite assem-blage, iron-free sphalerite). Fluid inclusions observed inenargite by SEM and IR microscopy are most probablyof secondary origin and have low homogenisation tem-peratures (�120 �C) and low salinities (�4 wt. %NaCleqiv. or slightly higher; Bonev et al. 2000; Moritz et al.2001). It is suggested that metal precipitation in thedeposit was driven by fluid interaction with the wall rockin zones of advanced argillic alteration and possibly byfluid mixing.

The studied grains of silver-rich gold and electrum inChelopech are associated with galena and sphalerite ofthe late polymetallic ore stage. Thus, there is a trendtowards increasing silver/gold ratio during the evolutionof the hydrothermal system. The irregular distributionof native gold observed in the orebodies and at micro-scopic scale suggests that metal deposition is probablycontrolled by local variations in the physicochemicalenvironment of the deposition site. Some workers(Kovachev et al. 1988) hypothesise that late recrystalli-sation and coarsening of earlier deposited fine-grainedgold is important, particularly around fault zones in thedeep levels of the deposit. However, we found that thecoarse-grained gold is more widely distributedthroughout the deposit.

Crystallisation of gold

The morphological analysis has defined some specificfeatures directly related to the mechanism of crystalli-sation of native gold in the Chelopech ores. It is sug-gested that the main morphological types of gold, whichinclude flattened, irregular, elongated and branchedcrystals, have nucleated in thin cracks, cleavages andintergranular fractures. During their growth, the goldcrystals have inherited the geometric configuration ofthese small open voids. Moreover, the crystallographic

Fig. 11 Plot, displaying the positive correlation between thespherical diameter (mean size) and thickness of individual goldgrains, proving that flatness is a size-independent characteristic

Fig. 12 Plot for equivalent spheric diameter (mean size) versusCorey shape factor, showing randomly dispersed points

Table 2 Main morphometriccharacteristics of gold particlesfrom the Chelopech deposit, inthree dimensions, and in ran-dom two-dimensional sections

Characteristic Definitions Mean value ±lm(lm)

In three-dimensional spaceLength L 160 6Width W 106 4Thickness T 18.4 0.7Equivalent sphere diameter ESD ¼3

ffiffiffiffiffiffiffiffiffiffiffiffiffiLWTð Þ

p67 2

Equivalent circle diameter ECD ¼ffiffiffiffiffiffiffiffiffiffiffiLWð Þ

p130 4

Corey shape factor CSF ¼ T=ffiffiffiffiffiffiffiffiffiffiffiLWð Þ

p0.14

2-D projection anisometry L/W 1.64

In two-dimensional sectionsLength l 131 9Width w 62 42-D anisometry l/w 2.11

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features of gold, with densely packed octahedral {111}faces, determine their own specific orientation. Thegrowth layers always follow the largest octahedral habitplane of the flakes. The rough crystal surfaces reflect, tosome extent, also the process of replacement of easilyreacting copper sulphide mineral hosts, or contain faceimprints of some earlier and non-reactive neighbouringmineral grains. Better-shaped isometric crystals havebeen rarely formed by crystallisation in some largervoids. The rarely observed rounding of some crystalsurfaces suggests a possible local dissolution of gold.

In contrast to other deposits (Petrovskaya 1973;Abdulin et al. 2000), hopper-like and skeletal-dendriticcrystals in Chelopech are rare. As it is known (Sunagawa1987), such crystals are indicative of free crystallisationin open space from highly supersaturated solutions in adiffusional regime. In the massive sulphide orebodies ofthe deposit such conditions are not common. However,in some isolated voids, the dendrite crystallisation hasbeen realised, thus forming the outer dendritic, wire-likeor fine-fibrous shape of some gold crystal formations.

The controversial origin of the ‘spongy’ gold is usu-ally explained (especially in placers) either by preferen-tial dissolution of silver or by direct precipitation ofhigh-purity gold (Petrovskaya 1973; Groen et al. 1990).Because gold in Chelopech is characterised by highfineness, the large volume of surface pores cannot be aresult of silver extracting. More likely, the fine fibrescomposing the spongy surface are formed during a pri-mary dendrite-like growth.

Primary gold and gold in placers

The morphology and size of gold particles play an im-portant role in the fluvial transport and origin of alluvialgold placers. It has been empirically shown for manyplacer deposits that the flatness of particles increaseswith increasing distance of transport (Tishchenko andTishchenko 1974; Shilo 1985; Knight et al. 1999;Youngson and Craw 1999). However, adequate infor-mation about the gold sources usually is absent or isvery scarce, and the tacit assumption that increasingflatness is related to reshaping during sediment transportis not generally proven. The morphological character-istic of gold from the Chelopech deposit suggests thatthe flatness of placer gold can be, to a large extent, in-herited from the primary source. Additionally, it may beincreased as a result of hydraulic sorting during trans-port and deposition of gold particles. Hence, the state-ment that placers close to primary gold deposits areunlikely to contain many flat gold particles (Tishchenkoand Tishchenko 1974) is not justified.

Conclusions

Native gold is the main gold carrier in the Chelopechhigh-sulphidation epithermal Cu–Au deposit from the

Srednogorie zone, Bulgaria. Mostly, it occurs in assem-blages of the main copper–arsenic sulphide stage and thelate polymetallic stage. Laboratory separation of a largenumber of gold particles from the gravity concentratesof the mine enables a systematic SEM study of theoriginal three-dimensional morphology of gold grains.Among the various particle shapes, subhedral flakes,wires, twins and irregular grains prevail, and usuallythey are of a considerable flatness. It is assumed that thegrain morphology is highly influenced by (1) the geom-etry of the small intergranular voids, cracks, and poresin which gold is deposited, and (2) the gold crystals own,densest and most predominant, octahedral growth faces.Native gold in Chelopech is characterised by high fine-ness (mean about 950), with a low content of silver(5.27 wt%), and minor copper (0.53 wt%) and Fe(0.10 wt%). An increase of silver content (up to33.8 wt%) in some gold and electrum grains is related tothe late polymetallic Pb–Zn stage.

Acknowledgements The authors are indebted to Navan BalkanResources AD for the financial support, to the administration ofthe Chelopech mine and to the colleagues Alex Arizanov, PlamenDoychev, Atanas Ignatov and Joe Crummy for their help duringthe investigations. The detailed reviews and useful and constructivecomments of Richard Goldfarb, Dave Craw, Claire Ramboz,Thomas Driesner and Chris Heinrich have greatly improved themanuscript, and are gratefully acknowledged.

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