tsintsov ivanov 2012

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Comptes rendus de lAcademie bulgare des Sciences

Tome 65, No 11, 2012

GEOLOGIE

Mineralogie

FEATURES OF AuAg ALLOYS IN THE EPITHERMAL

LOW-SULFIDATION AuAg KHAN KRUM DEPOSIT,EASTERN RHODOPES

Zdravko L. Tsintsov, Ivan P. Ivanov

(Submitted by Academician T. Nikolov on June 4, 2012)

Abstract

The paper presents the results from the study of AuAg alloys and blue-green (amazonite type) adularia observed as surface finds in a veinlet with

bonanza AuAg mineralization (Au content of up to 7.899 kg/t) in the so-called upper zone of Ada Tepe district of Khan Krum deposit. Separategrains of the natural series of AuAg solid solutions are represented by nativesilver, native gold, and electrum. Most of them are characterized by homo-geneous distribution of the compositional elements, but rarely representativeswith chemical inhomogeneity are also observed. Most often the grains are com-bined in aggregates with different morphology and sizes reaching up to 15 mmalong the longest axis, which makes them observable with a naked eye. Acoloured low-temperature variety of orthoclase was found, which was provi-sionally defined as blue-green (amazonite type) adularia. It is proposed thatsupergene processes have played an important role during the formation of thepresent outlook of the studied mineralizations.

Key words: electrum, low-sulfidation, epithermal, Ada Tepe, Khan Krumdeposit, Eastern Rhodopes

Introduction. Khan Krum deposit (Ada Tepe district) is considered asepithermal to low-sulfide (adularia-sericite) type of gold deposit intruded in sed-imentary rocks [1,2]. All tectonic ruptures in the sediments are represented bynormal slip faults and separation faults grouped in two main systems with com-paratively steep slopes. These systems have controlled the hydrothermal activityin the area [3]. The latter author has pointed out that within the range of the

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separate knots low-temperature metasomatites with gold-bearing mineralizationhave been formed.

Due to the great variation in the morphology of the mineralization zones andthe lack of precise criteria for marking the ore bodies, the massif around Ada Tepepeak is looked upon as one big ore body in which, according to morphologicalfeatures, two types of ore zones have been separated [3]:

Lower zone (The Wall) is a sloping, layer-like ore zone dipping to north.The gold mineralization is incorporated in massive quartz body with a lens-like morphology and approximate sizes of 150350 m with a mean thicknessof 17 m and 7.3 g/t content of gold.

Upper zone represents a series of a high-angle vein bundles with predom-inating E-W orientation, incorporated in breccia, breccia-conglomerates,and sandstones. Their thickness varies in the range from 0.10 to 0.80 mand the content of Au reaches 638 g/t.

Four overlapping stages of mineral formation have been separated based onthe cross-cutting specificity, variations in the mineral composition, the structures,and the content of Au [2]. The second stage has been considered as most impor-tant from economic point of view.

The ore mineralization in Ada Tepe section is not much variable. It is rep-resented mainly by electrum and subordinate quantities of pyrite and rare finds

of AuAg tellurides (hessite and petzite), greenockite, gersdorffite, marcasite,galena, and sphalerite [2,3]. According to these authors, the gangue minerals arealso reduced with respect to phase variation in the deposit and are restrictedto quartz, adularia, carbonates (calcite, dolomite, ankerite, siderite), chlorite,sericite, and clay minerals (kaolinite, illite).

Only electrum is of economic value and it is represented mainly by micro-scopic grains (according to [4]) in quartz and adularia and rarely in goethite.The mineralogical studies have proved that about 90% of the electrum grainsin Ada Tepe section are with dimensions from 312 m to 6575 m, rarely inthe range 50180 m (or represent chains with length up to 650 m) [3]. Theelectrum grains form impregnations, micro-clusters, colloform stripes, aggregates

in interstices of quartz and adularia, or are spread among them in the form ofsingle individuals [3,5]. The SEM studies have showed that the aggregates havegrape-like, globular, dendrite, or wire-like morphology and the grains that buildthese aggregates are characterized by an irregular lens-like form, which is oftenbroken by plenty of small caverns [5]. Sometimes electrum is deposited in nestsand fine veinlets with very high concentrations and forms rich in gold ore, formingthe so-called bonanzas. All samples with content of Au higher than 1 g/t havealmost constant Au/Ag ratios ( 3), reflecting the composition of the electrum 7673 Au wt % and because of that the ore from Ada Tepe section has been

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named in the same way [2

]. The separate AuAg individuals have fineness from637 to 828 with a tendency for lowering its values from the low towards the highparts, where the element admixtures (Ag Cu, Fe, Te, Sb, etc.) are in greaterquantities [3].

The purpose of the present paper is to give additional data about the min-eralogy of the high-angle gold-bearing veins of the so-called upper zone of AdaTepe section. The terrain studies were realized during an archaeometallurgicalexperiment in the frame of the BulgarianGerman project Iron and gold. In thewake of the metallurgy of Ancient Thrace. This project was firstly financed byAlexander von Humboldt Fund, Germany, and since the autumn of 2010 byBalkan Mineral and Mining AD.

Materials and methods. The studied samples were isolated from a veinletbeing a part of a high-angle vein bundle with E-W orientation from the so-calledupper zone, which crops out on the surface in the highest part of Ada Tepepeak. The samples were collected from a chamber part of the veinlet and thosea with significant concentration of AuAg alloys (bonanza) on the surface wereseparated for analysis.

The optical investigations were realized under binocular magnifier (Olympus)and polarizing microscope Amplival. The composition of the separate grainswas analyzed on a Philips SEM515 apparatus equipped with energy-dispersiveelectron-probe micro-analyzer EDAX PV 9100 under the following conditions:U= 2025 kV, I = 0.5 mA, beam diameter is 5 m, time of spectrum scanning

is 5080 s. The powder XRD studies were performed on D2 Phaser (BrukerAXS) diffractometer using Ni-filtered Cu radiation (30 kV and 10 mA) and stepscanning in the interval 1080 2-theta. The powder XRD patterns were usedfor phase analyses by checking the data with the PDF database (ICDD). Thechemical analyses were performed by use of the apparatuses SPECTRO Analyticalinstruments (Germany) and VISTAMPX (Australia).

Results. The studied vein bundle is composed mainly of minerals fromthe first stage of hydrothermal mineralization, which are represented principallyby microcrystalline to fine-crystalline, white, massive and strong quartz. Later,products from the second stage have partially filled the free parts, thus formingveinlets with different morphology and sizes. Bonanza AuAg mineralization

was in one of these veinlets, which is the subject of the present study. Theveinlet is with lens-like form and with a length of 0.9 m and maximal thicknessof 0.12 m. Its contacts with the host sediments are sharp and clear. It isbuilt up mainly by quartz (represented by druses nets, etc.). In its central part,alternating thin quartz-adularia stripes are seen. A small chamber is formedamong them coated by several very thin fine-grained quartz-adularia layers.The latter form many bubble-like swellings resembling foam with oval form(spherical and ellipsoid) and with dimensions up to 10 mm (Fig. 1a, b). Thelayers are most often deposited upon thin crusts (up to 1 mm thick) of quartz

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from the second stage and rarely upon the quartz from the first stage. The layersare characterized by plenty of pores in the inner parts and reach a maximal totalthickness of 4 mm. They have pale beige, pale red, pale brown, pale yellowto dirty white colour on the surface. Rare depositions of goethite give darkbrown to black colour to some surface parts. The bubble-like swellings are veryoften fully or partially demolished, revealing in depth other similar bubble-likequartz-adularia layers. The bonanza AuAg mineralization has been depositedupon the surface of the quartz-adularia layers (Figs 2, 3).

The data from the chemical analyses show that the mean contents of Au inthe high-angle vein bundle are very high and usually display values is greater than0.5 kg/t. The quantity of Au in the veinlet with bonanza AuAg mineralization

in some cases reaches 7.899 kg/t.The aggregates of AuAg alloys built by many small grains have varying

morphology (globular, dendrite, grape-like, band-shaped, etc.) and display alarge range of sizes often reaching 15 mm along the longest axis (Figs 2, 3). Inthe latter case, the aggregates are observed with a naked eye.

The noble metal mineralization additionally includes silver-white to dark-grey aggregate with a plastic form and approximate size of 25 60 m. Thisaggregate has a grape-like structure formed by plenty of small (from 3 to 10 m),isometric or slightly elongated grains, which are densely stuck to each other. Thesame aggregate has been deposited partially upon the surface of an aggregateof AuAg alloys with usual colouring (Fig. 4a). When separating it from its

basis, the aggregate disintegrates in many small particles, which, confirmed bythe analyses, are composed mainly of Ag.

Very often the AuAg aggregates are covered by a thin layer of Fe-oxides/hyd-roxides. This layer lowers the diffraction of the separate grains and during theoptical observations gives a mat lustre of the aggregates, thus making them al-most unobservable (Fig. 3a). In a regime of secondary electron emission, suchaggregates are hardly observed and practically in these cases only their peripherycontours can be followed. Having in mind the fact that the depth of penetrationof the X-rays in this case is about 10 m, it can be assumed that the thickness ofthis covering is close to this size. The layer of Fe-oxides/hydroxides is the reasonfor the rich colouring of the surface of the quartz-adularia layers. Sometimes upon

these aggregates single crystals and sponge groups of later deposited AuAg alloysdiffering with their surfaces glittering and non-polluted with Fe-oxides/hydroxidesare observed(Fig. 5a).

The grains of AuAg alloys from the finer fractions (< 50 m) have homo-geneous distribution of the elements composing them. However, a part of thelargest grains (those >80 m) possess chemical heterogeneity, which is expressedin a considerable fluctuation in the quantity of the separate compositional ele-ments across different parts. In these cases, the content of Au varies in the range65.6299.12 wt% and the admixtures are entirely from Ag. Upon polished sur-



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Fig. 1. Features of surfaces formed by bubble-like swellings of quartz-adularia layerswith distinctly observable aggregates of AuAg alloys, partially covered by:

a) Fe-oxides/hydroxides; b) goethite. Natural surfaces


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Fig. 2. Bonanza AuAg mineralization deposited upon destroyed quartz-adularia layers.Natural surfaces


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Fig. 3. Aggregates of AuAg alloys deposited upon destroyed quartz-adularia layers:a) partially covered by Fe-oxides/hydroxides; b) with clear surfaces of the grains. Natural

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Fig. 4. Formations of: a) aggregate of native silver; b) ellipsoid lamella of blue-green(amazonite type) adularia, partially covered by AuAg alloys. Natural surfaces


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Fig. 5. SEM images of AuAg alloys with: a) clean sur-faces deposited on earlier formed aggregates of AuAgalloys, covered by Fe-oxides/hydroxides; b)irregular dis-tribution of the compositional elements (the dark regions rich in Ag). Natural surfaces (a), polished surfaces (b)

faces, these fluctuations well mark the heterogeneous regions. The latter are withirregular character, sharp contours, varying form and are clearly differentiated

(the Ag rich parts are darker than the rest) both by optical microscopy and SEMobservations in secondary electron emission (Fig. 5b).

In a surface indentation partially filled with goethite, we found a thin (1 kg/t.

Irrespective of the many studies on different geological-mineralogical aspects

of Ada Tepe section, a series of genetic questions connected with the AuAgmineralization still remain not well explained. The genetic model, which mayexplain all available geological data, is connected with hydrothermal convectionin the metamorphic basement during which the fluids coming from depth to thesurface form low-temperature hydrothermal solutions [2]. The latter authors haveconcluded that the magmatism in the nearby Kessebir dome may be looked uponas a possible source of the metal-bearing fluids. According to them, the richAu stripes have been formed by colloids and part of the rest mineralization hasresulted from the boiling of the solutions.

The surface textural features of the quartz-adularia layers observed by ushave been formed as a result of boiling of ore-bearing solutions. The bonanza Au

Ag ore formation is with colloidal origin and has been favoured by this boiling.This conclusion corresponds with the statements of other authors [2,5,7].

Our observations gave reason to accept that supergene processes have playedmain role during the formation of the contemporary outlook of the discussed min-eralization. They have favoured both the surface deposition of Fe-oxides/hydroxi-des upon the grains of a considerable part of the earlier-formed AuAg alloys aswell as the formation of supergene ones. The coverings of Fe-oxides/hydroxidesupon AuAg alloys could exert unfavourable effect in the process of their dress-ing [4]. In this sense it is recommendable to take into account these features ofthe useful component during the processing of the deposit.

In supergene conditions, the circulation of the chloride-sulfate waters favours

the liberation of Au and Ag atoms and their transformation in cationic form,which form complex compounds (chlorides) weakly dissolvable in water, whichallow transportation to great distances. In suitable medium, they can be reducedto elementary metals and the reducers can be Fe2+, Mn2+, etc. Through themechanisms of cumulative crystallization, the metals Au and Ag are in conditionto form metal phases new in composition and particle sizes, which in our caseare represented by the grains and aggregates of Au and Ag and AuAg alloyswith sparkling surfaces. The supergene processes have also caused the chemicalinhomogeneities of the grains from the coarser fractions of the AuAg alloys.



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The chemical compositions of the grains are represented by a wide spec-trum of AuAg series of solid solutions from pure end member Au and Agrepresentatives to such with different intermediate composition. According tothe mineralogical classifications, the quantity of these elements determines theirmineral naming. In this sense it is recommendable to use during mineralogicalstudies the concrete mineral names. For the purposes of the metallogenic classi-fications, comparisons, etc. as well as in case of missing data for the compositionof the separate grains, one may use the generalized designations electrum andelectrum ore.

The colour of amazonite is formed during late metasomatic processes takingplace at elevated activity of Na and the intrusion of Pb and Rb [8]. Later, it

has been established that centres of [Pb-Pb]3+

couples have caused the typicalblue-green colour of the amazonite microcline [9]. According to the latter authors,other feldspars with such colouration with chromophores of the type [Pb-Pb]3+

couples are not known. In regard to these conclusions, the above authors proposethe name amazonite to be preserved only for the blue-green microcline, regardlessof the proposal of other authors [10] to expand this designation over all feldsparssimilar in colour. In accordance with the above opinions, we accept the designa-tion blue-green (amazonite) adularia for our blue-green find. It has been formedat late metasomatic processes in a crystallization medium with exceptionally lowfugasity of S and restricted income of Pb.

Acknowledgements. Thanks are due to Dr Chr. Popov for his agreement

to publish these data, to Dr V. Arnaudov for the helpful discussions on adularia,and to Dr B. Banushev for assisting us in the preparation of the manuscript.

REFERENCES

[1] Kunov A., V. Stamatova, P. Petrova. Mining and Geology, 2001, No 4, 1620(in Bulgarian).

[2] Marchev P., B. Singer, D. Jelev, S. Hasson, R. Moritz, N. Bonev.Schweiz. Mineral. und Petrogr. Mitt., 84, 2004, 5978.

[3] Jelev D. In: Gold deposits in Bulgaria (eds V. Milev, N. Obretenov, V. Georgiev,A. Arizanov, D. Jelev, I. Bonev, I. Baltov, V. Ivanov), Sofia, 2007, 104115 (inBulgarian).

[4] Harris D. C. Miner. Deposita, 25(Suppl), 1990, S3S7.[5] Marinova I. Geologica Macedonica, Special issue, 2008, No 2, 111120.[6] Hedenquist J., E. Izava, A. Arribas, N. C. With. Resource Geology Special

Publication, Published by the Society of Resource Geology, No 1, 1996.[7] Saunders J. A., P. A. Schoenly. Miner. Deposita, 30, 1995, 199210.

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[8

] Arnaudov V., M. Pavlova, S. Petrussenko. Bulletin of the Geological Insti-tute (Geochemistry, Mineralogy and Petrography), Bulgarian Academy of Sciences,16, 1967, 4144 (in Bulgarian).

[9] Petrov I., R. M. Mineeva, L. V. Bershov, A. Agel. American Mineralogist,78, Nos 78, 1993, 500510.

[10] Hofmeister A. M., G. R. Rossman.American Mineralogist, 70, 1985, Nos 56,794804.

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