Äîêëàäè íà Áúëãàðñêàòà àêàäåìèÿ íà íàóêèòåComptes rendus de l’Academie bulgare des SciencesTome 61, No 3, 2008

GEOLOGIE

Mineralogie et petrologie

MAGMATIC MINERALS OF SULFUR IN VOLCANIC AND

PLUTONIC ROCKS IN BULGARIA AND REPUBLIC

MACEDONIA

Rossen Nedialkov

(Submitted by Corresponding Member I. Velinov on January 21, 2008)

Abstract

Sulfur as a volatile component in magma is of crucial importance for for-mation of orthomagmatic ore-bearing hydrothermal fluids and its minerals areindicative of the oxidation state of the magma. In reduction conditions sulfideminerals (pyrrhotite, chalcopyrite, pentlandite and others) are stables. Theyare established in different magmatic centres in Bulgaria and Macedonia (Zi-darovo, Zvezdel, Assarel, Kozuf, Kratovo-Zletovo). At oxygen fugacity aboveNNO+1 buffer in the magma, sulfur is present mainly as SO2 and magmaticanhydrite is formed (Zidarovo). The sulfide minerals are mainly incorporated inrock-forming minerals during their presence in the magma as sulfur melt. Thatis why ramifications of the dendritic inclusions are outward oriented. In theplutonic rocks of Zidarovo, anhydrite is interstitial (late magmatic) or comesin veins (hydrothermal), indicating the transition from the evolved fluid richmagma to the postmagmatic ore bearing hydrotherms derived from it.

Key words: magmatic sulfur minerals, pyrrhotite, anhydrite, sulfur fu-gacity, ore-generating capability

Introduction. Sulfur is one of the most important fluid components of themagma. Investigation of sulfur minerals in igneous rocks gives information about

The investigations are made with the financial support of the SCOPES project –7BUPJ02276.00/1 and the project of the Bulgarian Ministry of Education and Science NZ-1403/2004.

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the physical characteristics of the magma and its ore generating potential. Mag-matic sulfur has paramount importance for formation of orthomagmatic ore bear-ing hydrothermal fluids and formation of hydrothermal deposits.

This paper attempts to present information on sulfur minerals as indicatorsof the physical parameters of the magma with significance for the ore generatingpotential of different magmatic centres in Bulgaria and Republic Macedonia.

There are numerous facts showing the participation of sulfur in the magmaticrocks and processes. Huge amounts of expulsed sulfur gases are measured duringvolcanic eruptions. During the eruption of El Chichon (Chile) in 1982 the volcanoejects 25 000 t of SO2 per day and the total amount of erupted SO2 is 8 Mt [1].During the eruption of Pinatubo (Philippines) in 1991, the volcanic emission inthe atmosphere comprises 20 Mt of SO2 [2].

In many monographs and publications magmatic minerals of sulfur are de-scribed [3–7]. In igneous rocks are established sulfides (pyrrhotite Fe1−XS), pyrite(FeS2), chalcopyrite (CuFeS2), pentlandite (Fe,Ni)8S9, millerite (NiS), bornite(Cu5FeS4) and others, as well as sulfates (anhydrite (CaSO4) and barite (BaSO4)).

Considerable quantities of sulfide minerals were established in some basic toultrabasic rocks sometimes accumulated up to the formation of magmatic sulfideore deposits [4,5]. Sulfides are also established in more evolved magmas and evenin acid crustal rocks as small blebs (up to several tens of µm) trapped in rock-forming minerals or rarely in the groundmass [8–10].

In Bulgarian scientific papers magmatic sulfide minerals (heazlewoodite, pent-landite, millerite, pirrhotite) were described in ultrabasic rocks and metabasitesfrom the Svetulka area, Ardino region (Central Rhodopes) [11]. Sulfide micromin-erals as inclusions in rock forming minerals are also established in different rocksfrom the Zvezdel-Pcheloyad area and the Panagurishte region [12,13]. Pyrrhotite,cubanite, pyrite and chalcopyrite in cumulate xenoliths in Oligocene alkalinebasaltic and lamprophyric dikes from Eastern Rhodopes were also recently dis-cribed [14]. Magmatic sulfide minerals were also mentioned in the area of Elatzite[15]. The first sulfate magmatic mineral in Bulgaria (barite) was described inhigh-K dacite from the Dumbuluk volcano in Eastern Rhodopes [7].

Results and interpretation. The data in this study cover several Bulgar-ian magmatic centres (Zvezdel, Zidarovo and Panagurishte volcanic area; Asarel,Krasen-Petelovo and Pesovetz volcanic stripes) and two Macedonian volcanic re-gions (Kozuf and Kratovo-Zletovo). Most of the established sulfur minerals aresulfides (pyrrhotite, chalcopyrite, bornite and Co-pentlandite) trapped in phe-nocrysts of volcanic rocks and occasionally in mafic minerals of plutonic rocks.The sulfide minerals are rounded or dendritic up to several tens of microns. Thereare individual inclusions or groups of small ones concentrated in some rock form-ing minerals. The morphology of the sulfide minerals suggests that they weretrapped during the period when the sulfide substance was in melt state. Theindividual rounded sulfide melt inclusions (with dimensions up to 35 µm) were

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T a b l e 1

Selected analyses from sulfide minerals – composition (in wt. %) mineral formulae and fS2. Tempera-tures are from pyroxen [18] and plagioclase-amphibole [19] geothermometers

Sample 30 30 30 BKX1 28 103(2) Alsh-1 Alsh-1 26 1111 1211 ZIS-057 ZIS-057

Rock Lat Lat Lat Ban Ban Trach Subv Lat Subv Lat An An Gb MD MD

Host min. Hb Hb Hb CPx Hb CPx Pl CPx CPx CPx CPx CPx CPx

Region Asarel Asarel Asarel Asarel Kras-Petel Pesovetz Kozuf Kozuf Krat-Zlet Pcheloyad Pcheloyad Zidarovo Zidarovo

Sulf. Min. Po Po Po Po Po Po bornite Po Po Co-Pent Cp Po Po

Wt. %

Fe 61.09 61.77 60.92 57.81 57.54 62.88 5.12 60.84 60.30 16.53 30.78 59.35 57.54

S 38.36 35.60 39.08 38.27 41.77 36.29 24.96 38.57 37.95 34.90 34.21 39.28 40.47

Cu 0.55 2.43 – 3.55 0.37 – 69.92 0.21 1.64 4.59 34.83 1.02 1.98

Ni 0.00 0.20 – 0.37 0.33 0.14 0.00 0.38 0.11 7.94 0.18 0.00 0.00

Co 0.00 0.00 – 0.00 – 0.44 0.00 0.00 0.00 36.04 0.00 0.35 0.00

Total 100.00 100.00 100.00 100.00 100.01 99.75 100.00 100.00 100.00 100.00 100.00 100.00 99.99

Formulaunits

Fe 0.914 0.996 0.895 0.867 0.791 0.995 0.471 0.906 0.912 2.175 1.033 0.867 0.816

S 1.000 1.000 1.000 1.000 1.000 1.000 4.000 1.000 1.000 8.000 2.000 1.000 1.000

Cu 0.007 0.034 0.047 0.004 5.654 0.003 0.022 0.531 1.027 0.013 0.025

Ni 0.000 0.003 0.005 0.004 0.002 0.000 0.005 0.002 0.994 0.006 0.000 0.000

Co 0.000 0.000 0.000 0.007 0.000 0.000 0.000 4.494 0.000 0.005 0.000

T(C◦)= 850-880 850-880 850-880 1100 850-880 1130 1120-1130 1130 1120-1160 1140 1140 1115 1120

log f S2= 0.63 −7.25 1.67 3.69 5.89 −4.19 2.48 2.18 3.72 5.01

Abreviations:

Po – Pyrrhotite; Cp – Chalcopyrite; Co-Pent – cobalt pentlandite;

Kras-Petel – Krasen Petelovo volcanic stripe; Krat-Zlet – Kratovo-Zletovo region in Republic Macedonia;

Lat – latite; Ban – basaltic andesite; Trach – trachite; Subv Lat – subvolcanic latite; An – andesite; Gb – gabbro; MD – monzodiorite

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incorporated in the minerals as the sulfide melt bleb contacts the mineral surfaceduring a relatively slow crystallization. The shape of a single sulfide inclusioncould be spherical, ellipsoidal or stick like (Fig. 1 A, C, E, F, G, H). Dendriticsulfide inclusions are oriented with their ramifications toward the periphery ofthe host mineral. The dendrites are developed in one (crystallographic) plane orin the 3D volume of the mineral. The dendrites were probably formed from rela-tively bigger sulfide melt segregations gradually incorporated by the host mineral,and hence the ramification is outward oriented (Fig. 1 B and D). The establishedgroups of sulfide melt inclusions (Fig. 1 A, C, E) actually represent a transversalcrosscut of a dendritic inclusion. The groups of sulfide inclusions are also disposedin a line (crystallographic plane) or without orientation in a small area (volume)in the mineral.

Igneous sulfate minerals (anhydrite) were established in Pinatubo volcano(Philippines) [6] as phenocrysts in dacite pumice fragments. Interstitial late-magmatic anhydrite was established in quartz-diorites and quartz-monzodioritesfrom the Zidarovo pluton (Fig. 2). It was formed simultaneously with potassicfeldspar and quartz. Quartz is with equal or lesser idiomorphism, K-feldspar iswith higher idiomorphism than the anhydrite (Fig. 2 A – D). Adjacent minerals(plagioclases, biotites and amphiboles) are not hydrothermaly altered. Anhydritewas formed at the end of the crystallization process, in oxidized conditions, frommelts relatively rich in volatiles, especially in SO2. Anhydrite sometimes corrodesplagioclases extracting Ca necessary for its own formation (Fig. 2 E). Anhydrite isalso established as a hydrothermal mineral in veins, veinlets, or in patches, whereall of the adjacent minerals are hydrothermaly altered (Fig. 2 F) transformed inchlorite, calcite, sericite. Anhedral crystals of magmatic anhydrite are up to 0.5mm, colorless, transparent, with a refraction index higher than that of plagio-clase, with parallel extinction and interference colours of third order, biaxial –positive.

Fig. 1. Sulfide melt inclusions in phenocrysts from volcanic rocks of the Panagurishteregion. A) Two groups of sulfide inclusions in amphibole in transmitted light, // N (thescale bar in all photographs in Fig. 1 is 100 µm). The two groups probably representoblique (G1) and perpendicular (G2) cross sections of dendritic sulfide inclusions. B)Dendritic sulfide melt inclusions in clinopyroxene with ramifications oriented outwardthe host mineral (transmitted light, // N). C) A group of sulfide inclusions, essentiallyisometric and stick like, disposed in two main crystallographic plans of the host amphibole(transmitted light, // N). D) Dendritic sulfide melt inclusion with fish tale morphologydisposed in one crystallographic plain with ramifications oriented toward the peripheryof the mineral (transmitted light, // N). E) A group of pyrrhotite sulfide inclusions inamphibole with rounded and stick like morphology (reflected light, // N). F) individualmelt inclusion of pyrrhotite in clinopyroxene with inhomogeneities at the periphery andlamellae in the centre (reflected light, // N). G) Rounded melt inclusion of pyrrhotite inamphibole (reflected light, // N). H) Rounded individual melt inclusion of chalcopyrite in

amphibole (reflected light, // N)

→

374 R. Nedialkov

The chemical composition of sulfide minerals (Table 1) was obtained by elec-tron microprobe analysis (JEOL–JMS 033 EL; acquisition of the spectrum 100 s,at 25 KEV – “Eurotest”, analysed by Hristo Stanchev).

The presence of sulfide or sulfate minerals in igneous rocks depends mainlyon the oxygen fugacity. The sulfides are stable at fO2 below the NNO + 1 buffer,and above NNO + 1 the sulfates are stable [6, 16] (Fig. 3). The sulfur fugacity inthe magma is calculated from the pyrrhotite composition [17] (one of the mostfrequent sulfide minerals in igneous rocks) and the temperature (calculated forthe crystallization of the host mineral). The determination of fS2 depends on theaccuracy of determination of the composition of the small pyrrhotite minerals andthe temperature of the incorporation of the sulfide bleb. Our results show that ina single sample the values of log fS2 could vary in several units (several kbars).Natural pyrrhotites are not pure and show isomorphic replacement of Fe by Cu,Ni, Co that diminishes Fe in the crystal formula and leads to increasing of the cal-culated log fS2, based on NFeS (mol fraction in pyrrhotite). The incorporation ofsulfide melt inclusions in clinopyroxenes occur at higher temperature (determinedwith the pyroxene geothermometer of Lindsley [18] and the determined values oflog fS2 are systematically higher. The crystallization temperature of amphibolesand plagioclases is determined with the amphibole-plagioclase geothermometerof Blundy and Holland [19].

The values of the log fS2 from the studied samples are plotted on the di-agram log fS2 vs. 1000/T (Fig. 4). Compared to typical andesitic arc magmas,the basalts from Zidarovo show higher sulfur fugacity, and the magmas of Assarel(with composition from basaltic andesites to dacites) exhibit normal but also bothhigher and lower values. This is probably due to changes in sulfur fugacity relatedto periodic eruptions and release of S-bearing volatiles and periodic replenishmentof the intermediate magmatic chamber with more primitive basaltic to pyroxeniticmagma (established mixing [13]). The magmas from Krassen-Petelovo, Kozuf andKratovo-Zletovo have higher log fS2 values. The Pessovetz volcanic stripe ex-hibits typical arc characteristics for log fS2. The estimation of the sulfur fugacitybased on temperature and oxygen fugacity [20] gives more coherent approximative

←

Fig. 2. Magmatic (A – E) and hydrothermal (F) anhydrite in quartz diorite and quartzmonzodiorites from the Zidarovo pluton. All photographs are in transmitted light andcrossed polars and the scale bar is 0.3 mm. A) Anhydrite (Anh – with high interferencecolours) is anhedral disposed in the interstices between fresh euhedral plagioclases (Pl) andsubhedral potassic feldspar (KFs). Quartz (Q) included in the anhydrite is also anhedral(the same level of idiomorphism as ahnhydrite). B) Anhedral magmatic anhydrite (Anh)including small quartz and patassic feldspar grains is disposed in the interstices of freshplagioclases and biotites (Bi). C) and D) interstitial anhydrite disposed between the freshplagioclases and amphiboles (Hb). E) Plagioclase (grains in the centre) partly corroded byanhydrite during the magmatic stage. F) During the postmagmatic hydrothermal stageplagioclases and amphiboles are altered and replaces by chlorite (Chl), calcite (Cc) and

anhydrite (Anh). Plagioclase is intensively sericitized (Pl-S)

Compt. rend. Acad. bulg. Sci., 61, No 3, 2008 375

Fig. 3. S vs. f O2 diagram showing the sulfursolubility in dacitic melt at 900 ◦C [6] and theform of presence of the S minerals depending

on the oxygen fugacity

results for log fS2. The oxygen fugacity of magmas when igneous sulfide segre-gations are present is between NNO+1 and FQM buffers [6]. In an extrapolationof the diagram log fO2 vs. T(◦C) [20] approximative results for log fS2 for tem-peratures 850–880 ◦C (for amphiboles) are −3.5 to −0.5 and for temperatures1100–1150 ◦C (specific for pyroxenes) log fS2 are in the range −2 to +0.5. Onthe diagram log fS2 vs. 1000/T [21] (Fig. 4) for andesitic and latitic magmaswhere sulfide minerals are trapped in amphiboles (Asarel and Kratovo-Zletovo)the sulfur fugacity is typical of the andesitic arc magmas. Where pyrrhotites in-corporated in clinopyroxenes are present in the andesitic to basaltic magmas, thesulfur fugacity is equal to higher relative to andesitic arc magmas.

Basic magmas dissolve bigger quantities of S (up to 2000 – 3000 ppm) thanacid ones (up to 60 ppm) [22]. For explanation of the relation of sulfide hydrother-mal deposits to more evolved, more acid magmatic complexes the discreet con-nection of the magmatic chamber with more primitive magmas is established orsuggested [6]. For the Zidarovo magmatic centre the basic primitive magma isclearly dominant. For Assarel and Zvezdel mixing with more primitive basalticmagma is established [12,13]. Fore the other studied magmatic centres the moreprimitive magma support is not yet established.

For the ore generating capability of the magma, the importance of sulfur isnot only limited to its quantity as element building sulfide minerals. The formof presence of S is also of primary significance. In reduction conditions, sulfidemelts strongly concentrate ore elements. Elements as Ni, Co, Au, Cu, Pt, Ir, Mohave very high distribution coefficients (sulphide melt vs. silicate melt or sulphidemelt vs. silicate minerals) [23,24]. Thereby ore elements trapped in sulphide meltare isolated from the magmatic fluids giving the birth of the orthomagmatic hy-drothermal system and they could not be sufficiently enriched in ore components.In oxidized conditions (fO2 above the NNO buffer + 1) sulfide blebs are unstableand are destroyed, releasing sulfur and ore elements in the residual magma. Oreelements and new formed SO2 participate to the formation of the ore-bearing hy-drothermal fluids released from the magma. Enriched in Cl and SO2 fluids extractmore efficiently ore elements from the magma and transport them to the place ofthe ore deposit formation.

376 R. Nedialkov

Fig. 4. Log f S2 vs. 1000/T diagram showing sulfidation state of magmas and hydrothermalfluids with some mineral sulfidation reactions at 1 bar. The diagram is from [21], simplified.The elliptical fields in gray and dark gray are the estimated log f S2 data after [20] for sulfide

inclusion in amphiboles and clinopyroxenes respectively

Conclusions. Sulfur minerals are indicators for saturation of the magmawith S as well as for the oxygen fugacity. Based only on sulfur mineral studyof the Zidarovo magmatic rocks we could deduce the sulfur saturation of themagma and the increase of the oxygen fugacity in the evolving magma frombasaltic to monzonitoidic. The presence of sulfide minerals enclosed in phenocrystsindicate sulfur saturation and a relatively reduction state at the beginning of thecrystallization of the ferromagnesian minerals in the studied magmatic centres.Most probably the changes in sulfur fugacity in a single volcanic rock or a singlevolcanic succession was also influenced by the consecutive eruptions releasingsulfur volatiles from the intermediate magmatic chamber and it replenishmentwith more primitive magma richer in sulfur.

Acknowledgements. The author is grateful to Assoc. Prof. Dr. VioletaStefanova (Faculty of Mining and Geology, Stip) for the support and the samplescollected from Republic Macedonia.

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Faculty of Geology and Geography

St. Kliment Ohridski Sofia University

15, Tsar Osvoboditel Blvd

1504 Sofia, Bulgaria

378 R. Nedialkov