Epithermal Intermediate-Sulfidation Veins in the Low-Grade Metamorphic

Home , Mooihoekite

СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, год. 78, кн. 1–3, 2017, с. 41–54 REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY, vol. 78, part 1–3, 2017, p. 41–54

Epithermal intermediate-sulfidation veins in the low-grade metamorphic rocks in the upper levels of the Elatsite porphyry copper deposit, Bulgaria Vassilka Mladenova, Rositza Apostolova, Zhivko Ivanov

Sofia University “St. Kliment Ohridski“, Faculty of Geology and Geography, 1504 Sofia, Bulgaria; E-mail: [email protected]

Епитермални умереносулфидни жили в нискостепенните метаморфити в горните нива на медно-порфирно находище Елаците, България Василка Младенова, Росица Апостолова, Живко Иванов

Софийски университет „Св. Климент Охридски“, Геолого-географски факултет, 1504 София, България

Abstract. Three types of ore veins are observed in the low-grade metamorphic host rocks (phyllites, quartz-sericitic schists) of the Vezhen pluton in the upper levels of the open pit of the Elatsite porphyry copper deposit, differing in their strikes, dips and mineral composition. The low-grade metamorphic rocks are not affected by the granitoids of the Vezhen pluton. The hydrothermal alterations style is sericitic. The quartz veins are subparallel and located everywhere conform to the general foliation trend in N-S direction. Their thicknesses vary from 1–2 to 25–30 cm. They are often flexed with the foliation. The main gangue mineral is quartz; barite, carbonates and fluorite occur rarely. The main ore minerals are pyrite, chalcopyrite and pyrrhotite; galena and sphalerite occur seldom and arsenopyrite, idaite, native bismuth and aschamalmite are scarce. The pyrite veins are N-S and NE-SW striking, steeply dipping 60–90° to E and have width from several centimeters to 40–50 cm. They crosscut the quartz veins. The main minerals are pyrite, chalcopyrite and quartz; arsenopyrite, native gold, tellurobismuthite(?) and unspecified Bi-Te and Ag-Te minerals occur in trace amounts. The base metal veins crosscut the first two vein types. They are striking 60–90° and dipping 50–70° SE and S with thicknesses from 0.5 to 7 cm. At places they are conform to the foliation but crosscutting is also common. The main ore minerals are sphalerite, galena, native silver and members of argentian tetrahedrite-freibergite series. Stephanite and pyrargyrite are also widespread. Chalcopyrite, marcasite and pyrite are rare. Mooihoekite, cupropolybasite and allargentum are in trace amounts. The geological setting, alteration style of the host rocks, main textures, mineral assemblages and composition of some minerals indicate that all three vein types are of intermediate-sulfidation style. The present results are crucial to develop a valid prospecting model and allow for the better planning of field, geochemical and geophysical explorations in the area of Elatsite deposit.

Keywords: Elatsite, hydrothermal veins, intermediate-sulfidation style, ore minerals.

Резюме. В горните нива на меднопорфирно находище Елаците в нискостепенните метаморфити (филити, кварц-серицитови шисти) от вместващите скали на Веженския плутон са установени 3 типа жили, различаващи се по посока, наклон и минерален състав. Нискостепенните метаморфити са хидротермално променени в серицитов фациес и не са засегнати от контактното въз- действие на гранитоидите на Веженския плутон. Кварцовите жили са субпаралелни и навсякъде са конформни на общия тренд на фолиация с посока С-Ю. Дебелината им варира от 1–2 до 25–30 cm. Често те са огънати заедно с фолиацията. Главният жилен минерал е кварцът, а баритът, карбонатите и флуоритът се срещат рядко. Главните рудни минерали са пирит, халкопирит и пиротин, по-рядко се срещат галенит и сфалерит, а арсенопиритът, идаитът, самородният бисмут и ашамалмитът са много по-редки. Пиритовите жили са с посока С-Ю до СИ-ЮЗ, стръмно залягащи на изток под ъгъл 60–90° и дебелина от няколко до 40–50 cm. Те пресичат кварцовите жили. Главните минерали са пирит, халкопирит и кварц. В малки количества се срещат арсенопирит, електрум, телуробисмутит(?) и неопределени Bi-Te и Ag-Te минерали. Полиметалните жили пресичат първите два типа жили. Те са с посока 60–90° и затъват под ъгъл 50–70° на ЮИ до Ю, а дебелината им е от 0,5 до 7 cm. На някои места те са конформни на фолиацията, на други я пресичат. Главните рудни минерали са сфалерит, галенит, самородно сребро и членове на редицата сребърен тетраедрит-фрайбергит. По-рядко се срещат стефанит и пираргирит. Халкопирит, марказит и пирит се срещат рядко. В малки количества са наблюдавани мойхокит, купрополибазит и аларгентум. Според геоложкото положение, типа хидротермална промяна на вместващите скали, главните текстури, минералните асоци- ации и състава на някои минерали, и трите типа новоустановени жили могат да бъдат определени като умереносулфидни. Тези резултати са важни за построяване на адекватен модел за проучване и за планиране на полевите, геохимичните и геофизичните изследвания в района на находище Елаците.

Ключови думи: Елаците, хидротермални жили, умереносулфиден тип, рудни минерали.

41 Introduction Popov et al., 2000; von Quadt et al., 2002; Tarkian et al., 2003; Strashimirov et al., 2003). Porphyry deposits are the world’s most important Four faults of different orientation have been dis- source of Cu, Au and Mo, and often contain signifi- tinguished in the area of Elatsite deposit as a result cant quantity of Ag, Sn, Pt, Pd and W. from deformation events within a broad kinematic The porphyry copper systems develop at the con- right shifting fault system with regional character: vergent plate boundaries and include porphyry deposits NW-SE, E-W, NE-SW, N-S (Ivanov et al., 2004f1). centered on intrusions, skarn, carbonate-replacement, The porphyry Cu-Au-PGE mineralization is con- sediment-hosted gold deposits, high- and intermedi- trolled by NW-SE and NE-SW trending faults, and ate-sulfidation epithermal deposits. High-sulfidation covers an area of about 1 km2 and can be traced to a epithermal deposits may occur in lithocaps above depth of about 800 m. porphyry copper deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au±Ag-rich, disseminated deposits approximately Materials and analytical techniques within the uppermost 500 m. Less commonly, intermediate-sulfidation epithermal mineralization, chiefly The 1:500 scale mapping of the high levels of the veins, may develop on the peripheries of the lithocaps open pit (over 1270 m) and sampling of drill core (Sillitoe, 2010). Commonly these deposits occur in was undertaken in the period 2010–2013. More than continental margin and island-arc settings (Sillitoe, 100 samples were collected and about 40 polished 1972, 2010; Cooke et al., 2005; Richards, 2011). sections were prepared. The optical studies and The Elatsite porphyry copper deposit is one of microphotographs were performed on microscope the two operating Bulgarian Cu and Au deposits. Opton-Universal pol-U. Numerous papers discuss its structure, mineralization, The chemical composition of minerals was deter- depositional conditions, source and timing of magma- mined in two electron microscopy laboratories. The tism and mineralisation. analyzing system at the laboratory in Eurotest-Control In the last years the open pit of the deposit is en- EAD is JEOL JSM 35 CF with Tracor Northern TN- larged to S and SW and unexplored areas were re- 2000, accelerating voltage of 25 kV and sample cur- vealed. In 2010–2013, a team lead by Prof. Zhivko rent of 1.10–9 A. The standards and lines used for anal- Ivanov undertook a detailed mappping and sampling ysis were PbS – PbLα, ZnS – ZnKα, CuFeS2 – CuKα of the high levels of the open pit. Three vein types and SK ; PbTe – TeL ; pure Ag and Co – AgL , CoK ; with hydrothermal mineralization are observed in the α α α α Bi2S3 – BiLα; Sb2S3 – SbLα. Acquisition times for all low-grade metamorphic rocks which are outside the elements were 100 s. The analytical device at the zone of contact impact of the Vezhen pluton. laboratory to the University of Mining and Geology, The aim of this study is to present the new data Sofia, is JEOL JSM-6010 PLUS/LA, and the analyti- concerning the ore mineralizations in the low-grade cal conditions are 20 kV accelerating voltage, 30 Pa metamorphic host rocks of the Vezhen pluton in the pressure, and low vacuum. The Ag and Ag-bearing, area of Elatsite deposit. Bi and Te-bearing minerals were analyzed at Eurotest- Control EAD. Pyrite, marcisite, pyrrhotite, galena and sphalerite were analyzed at University of Mining and Geological setting of the Elatsite deposit Geology. Some analyses were duplicated on both de- vices. The results show satisfactory matching. The Elatsite deposit is located on the northern slope of the Chelopeshka Baba Peak, about 55–60 km East of Sofia and 6 km South of the town of Etropole. Results It is situated in the Upper Cretaceous Banat- Srednogore tectonic zone. The deposit is part of the Elatsite-Chelopech ore field and occupies the Many researchers have studied the mineral deposition northernmost parts of the Panagyurishte ore district in the Elatsite deposit (Bogdanov, 1987; Petrunov et (Fig. 1). Over 150 mainly porphyry and epithermal al., 1992; Dragov, Petrunov, 1996; Strashimirov et al., copper ore deposits and occurrences are known in 2002, 2003; Kehayov et al., 2003; Georgiev, 2009). this district. They are genetically connected with the The mineral succession is divided in different mineral Late Cretaceous magmatic activity (Bogdanov, 1987; parageneses or mineral associations. Мost succession Popov et al., 2000, 2003). schemes include quartz-magnetite, magnetite-bornite- The ore deposits are hosted mainly in the Upper Cretaceous subvolcanic quartz-monzonitic to granodi- oritic intrusions (U/Pb age 91.5–92 Ma) and partly in 1 Ivanov, Zh., N. Petrov, A. Lazarova, K. Nedkova. 2004f. Paleozoic low-grade metamorphic rocks and granodi- Structural Characteristic of Elatsite Deposit – Geological Setting and Regional Position. Geodynamical Condition of Formation of Upper orites of the Vezhen pluton (314±4.8 Ma) (Bogdanov, Cretaceous Magmatic Bodies in the Region and Assessment of Their 1987; Petrunov et al., 1992; Popov, Kovachev, 1996; Ore Capability. Unpubl. Report for Ellatzite-Med AD (in Bulgarian).

42 Fig. 1. a, location of Panagyurishte mineral district in the Upper Cretaceous Banat-Srednogorie tectonic zone (after Berza et al., 1998; Ciobanu et al., 2002; Heinrich, Neubauer, 2002); b, simplified tectonic map of Bulgaria (after Ivanov, 1998) with location of the Panagyurishte mineral district in the Srednogorie zone; c, geological map of the Panagyurishte ore district (after Cheshitev et al., 1995) Фиг. 1. a – разположение на Панагюрския руден район в горнокредната Банат-Средногорска тектонска зона (по Berza et al., 1998; Ciobanu et al., 2002; Heinrich, Neubauer, 2002); b – опростена тектонска карта на България (по Ivanov, 1998) с разположение на Панагюрския руден район в Средногорската зона; с – геоложка карта на Панагюрския руден район (по Cheshitev et al., 1995)

chalcopyrite, quartz-molybdenite, quartz-chalcopy- Quartz veins rite-pyrite, quartz-pyrite and quartz-galena-sphalerite parageneses. The first four parageneses occur in the These are subparallel veins and lenses located con- central part of the deposit and are associated with per- form to the general foliation trend with N-S direction vasive potassic alteration (quartz-K-feldspar-biotite). (Ivanov et al., 2011f2, 2012f3). The vein thicknesses The last two mineral parageneses are deposited in the vary from 1–2 to 25–30 cm. These are often flexed upper and marginal parts and are associated with pro- with the foliation. The main gangue mineral is quartz; pylitic and phyllic-argillic alterations. barite, carbonates and fluorite occur rarely. Well- The operation of the open pit to S and SW ex- shaped pyrite, chalcopyrite and pyrrhotite, rare galena posed unexplored areas with 3 types of ore veins. The and sphalerite and the gangue minerals up to 3–4 cm veins are hosted in the low-grade metamorphic rocks (phyllites and quartz-sericitic schists), have different strikes, dips and mineral composition. The low-grade 2 Ivanov, Zh., Zh. Yalamov, N. Hadzhieva, R. Vassilkyova, P. metamorphic rocks are not affected by the contact im- Apostolov. 2011f. Detailed Geologic-structural and Geotechnical Studies of the Rock Massif in the Southern and South-western Sectors pact of the granitoid of the Vezhen pluton. The hydro- of the Elatsite Open Pit and of the Adjoining Parts of the Mine Heaps. thermal alterations style is sericitic. The three types Unpubl. Report for Geotehmin AD (in Bulgarian). of veins are quartz veins, pyrite veins and base metal 3 Ivanov, Zh., N. Hadzhieva, R. Vassilkyova, P. Apostolov, О. veins. The quartz and pyrite veins occur very often Pavlov. 2012f. Geologic-structural and Geotechnical Studies in the Southern Sectors of the Elatsite Open Pit at Levels 1420 to 1300 and whereas the base metal veins are found only in a few Some Part from the Western and Eastern Highwalls. Unpubl. Report parts of the open pit. for Geotehmin AD (in Bulgarian).

43 in size are observed in the cavities. Arsenopyrite, 1980). Idaite in the quartz veins is probably a result idaite, native bismuth and aschamalmite are found from decomposition of bornite. This assumption is due only by optical microscope and electron microprobe. to the chalcopyrite lamellas in idaite. In other inclu- All ore minerals were analyzed using electron micro sions chalcopyrite and idaite do not show intergrowths scopy. The results are shown in Table 1. relations and in this case it is likely to be directly de- Pyrite is the most common mineral and is deposit- posited from the hydrothermal solution. ed as euhedral to anhedral grains from several microns Idaite was the subject of numerous controversies to massive aggregates. Most of them are solid and are about its chemical composition and crystallographic rarely cracked. Some grains contain chalcopyrite in- properties. Yund (1963) proposed a general formu- clusions and mixed chalcopyrite and idaite inclusions la Cu5.5xFexS6.5x for the synthetic idaite-like phase, (Fig. 2a, b). The microprobe analyses established only whereas Lévy (1965) suggested an ideal composition

Fe and S. Cu3FeS4. Ottemann and Frenzel (1971) concluded that Pyrrhotite is deposited in cavities together with its composition varies from Cu3FeS4 to Cu5FeS6. The pyrite and chalcopyrite as latest mineral. It forms platy chemical compositions of idaite from the Elatsite de- hexagonal crystals up to 0.5 сm. It contains rarely in- posit fit better to the formula proposed by Lévy (1965). clusions from arsenopyrite (Fig. 2c). The chemical Arsenopyrite is deposited as inclusions in pyr- composition is calculated to the formula Fe0.94S1.06. rhotite in the form of single subhedral crystals up to Kehayov (2005) has analyzed pyrrhotite from the 100 µm in size (Fig. 2c). The calculated chemical early quartz-pyrrhotite paragenesis in the porphyry compositions were Fe1.08As0.93S0.99 and Fe1.07As0.82S1.11. copper ore and applying the Co content of the coex- Irregular grains with similar colors and sizes from 2 to isting pyrite and pyrrhotite he calculated depositional 50 µm, but most often around 15–20 µm, were found temperatures in the range 251–505 °C with maximum in calcite by observation in back scattered electrons. between 245 to 284 °C. These values put the initial The microprobe analyses revealed that all grains con- temperatures of hydrothermal deposition, whereas the tain Bi. Based on microprobe data, native bismuth and analyzed in this study pyrrhotite from the quartz veins aschamalmite are distinguished. is one of the latest minerals. Native bismuth is deposited as single grains up to Chalcopyrite is a rare mineral in the veins. It oc- 50 µm, the most of them around 20 µm, in calcite in as- curs as subhedral grains from 50 to 150 µm in pyrite sociation with aschamalmite (Fig. 2d). The microprobe and in cavities in the quartz (Fig. 2a, b). The chemical analyses revealed Fe (3.79 wt%) and Mn (0.63 wt%). composition of the mineral is similar to the theoretical. Aschamalmite was found as 10–15 µm grains in Idaite occurs predominately in close intergrowths calcite (Fig. 2d). Besides Pb, Bi and S, the electron with chalcopyrite as numerous oval or irregular inclu- microprobe analyses indicated also some Fe and Mn. sions in pyrite up to 50 µm (Fig. 2b). These inclusions Both native Bi and aschamalmite contain some Fe and are rarely set up by idaite only. Mn and it is very likely that these elements had been Idaite is often a lamellar product of decomposi- trapped from the host carbonate. Therefore the recal- tion of bornite; the color of the both minerals is very culation of the chemical composition did not fit well to similar and they could easily be confused (Ramdohr, the theoretical formula (Table 1).

Table 1 Microprobe analyses of the minerals from the quartz veins Таблица 1 Микросондови анализи на минерали от кварцовите жили

Elements (wt%) Nr. Mineral Formula Fe Cu Pb Mn As Bi S

1. Pyrite 46.76 53.24 Fe1.00S1.99

2. Chalcopyrite 33.57 28.53 37.90 Cu0.80Fe1.07S2.11

3. Chalcopyrite 33.71 28.14 38.15 Cu0.79Fe1.07S2.12

4. Chalcopyrite 31.26 32.38 36.36 Cu1.01Fe1.01S2.05

5. Pyrrhotite 60.56 39.44 Fe0.93S1.06

6. Arsenopyrite 37.42 42.93 19.65 Fe1.08As0.92S0.99

7. Arsenopyrite 38.16 39.16 22.68 Fe1.07As0.82S1.10

8. Idaite 18.79 48.72 32.49 Cu2.90Fe1.27S3.83

9. Idaite 17.34 51.25 31.41 Cu3.08Fe1.18S3.74

10. Native Bi 3.79 0.63 95.58 Bi0.85Fe0.12Mn0.02

11. Aschamalmite 3.41 64.04 0.67 23.76 8.12 Pb7.01Bi2.58Fe1.38Mn0.27S5.74

44 Fig. 2. Microphotographs of the minerals from the quartz veins a, chalcopyrite inclusions in pyrite; b, idaite-chalcopyrite inclusion in pyrite; c, arsenopyrite inclusions in pyrrhotite (SEM-BSE image); d, native bismuth and aschamalmite in calcite (SEM-BSE image) Abbr.: Asch – aschamalmite; Asp – arsenopyrite; Bi – native bismuth; Cal – calcite; Chp – chalcopyrite; Id – idaite; Py – pyrite; Po – pyrrhotite; Qtz – quartz Фиг. 2. Микрофотографии на минералите от кварцовите жили а – халкопиритови включения в пирит; b – идаит-халкопиритови включения в пирит; с – арсенопиритови включения в пиротин (SEM-BSE изображение); d – самороден бисмут и ашамалмит в калцит (SEM-BSE изображение) Съкр.: Asch – ашамалмит; Asp – арсенопирит; Bi – самороден бисмут; Cal – калцит; Chp – халкопирит; Id – идаит; Py – пирит; Po – пиротин; Qtz – кварц

Mladenova et al. (2001) reported the presence of The main minerals are pyrite, chalcopyrite and complex Bi mineralization in the Svishti Plaz gold quartz. Arsenopyrite, native gold, Bi-Те (tellurbis- deposit and divided 3 sub-associations. The earliest muthite?) and unspecified Bi-Te and Ag-Te minerals sub-association comprises native Bi, chalcopyrite, were observed by means of optical microscope and aschamalmite, Cu-Pb-Bi sulfosalts and bismuthinite- microprobe analyses. Microprobe analyses of the min- pekoite. A breakdown of an ISS or primary complex erals from the pyrite veins are shown in Table 2. bismuth sulfosalt(?) is suggested. Pyrite commonly occurs as large (0.5 cm or more across) euhedral grains with sharp crystal edges and aggregates as earliest mineral in the veins. The late Pyrite veins chalcopyrite infills the cavities in the pyrite aggregates and embraces the single pyrite grains (Fig. 3a). These are well differentiated with a 60–90° dip to the Chalcopyrite occurs in close intergrowths with east, N-S to NE-SW strike and width from several pyrite (Fig. 3a). Occasionally it contains single gold centimeters to 40–50 cm. The pyrite veins crosscut the grains (Fig. 3b, c). The microprobe analyses did not quartz veins. indicate any trace elements in its composition.

45 Table 2 Microprobe analyses of the sulfide minerals from the pyrite veins Таблица 2 Микросондови анализи на сулфидни минерали от пиритовите жили

Elements (wt%) Nr. Mineral Formula Fe Cu Ag Au Te Bi As S

1. Chalcopyrite 31.59 32.70 35.68 Cu0.94Fe1.03 S2.02

2. Chalcopyrite 31.72 32.60 35.67 Cu0.94 Fe1.03S2.02

3. Native gold 22.31 77.69 Au1.31Ag0.69

4. Native gold 46.22 53.78 Au0.78Ag1.22 5. Tellurbismuthite(?) 8.36 3.01 38.65 41.59 8.40

6. Arsenopyrite 34.44 6.68 36.13 22.74 Fe0.96As0.75Cu0.16S1.11

7. Arsenopyrite 34.02 7.49 34.92 23.57 Fe0.94As0.72Cu0.18S1.14 8. Unspecified mineral 28.93 9.72 7.20 6.70 28.14 14.88 9. Unspecified mineral 32.67 22.70 16.01 28.58 10. Unspecified mineral 15.19 5.69 32.86 24.92 8.35 13.00

Native gold occurs as oval or irregular grains up 50–70° SE and S. At some places they are conform to to 20 μm in chalcopyrite (Fig. 3b, c). Electron micro- the foliation but crosscutting is also common. Their probe analyses revealed that the gold was always al- thicknesses vary from 0.5 to 7 cm. The minerals in loyed with Ag. The silver content in the two analy- these veins are deposited symmetrically to the host ses is 22.31 wt% and 46.22 wt% respectively, what rock; the quartz is deposited on the contact with the defines these compositions as electrum (Petrovskaya, host rock whereas the central parts of the veins are 1973; Harris, 1990). The fineness 776.9 and 537.8‰ filled up mainly with sphalerite. Often the minerals was calculated using the formula proposed by Harris are cataclased. (1990). The main ore minerals are sphalerite, galena, na- Native gold is common mineral for quartz-pyrite tive silver and members of argentian tetrahedrite- and quartz-galena-sphalerite asemblages of the por- freibergite series. Stephanite and pyrargyrite occur phyry copper ore at the Elatsite deposit (Bogdanov et often also. Mooihoekite, cupropolybasite and allar- al., 2005; Kehayov, 2005). gentum are in trace amounts. Unspecified Bi-Te and Ag-Te minerals occur as Sphalerite is the most common mineral. It occurs as irregular fine inclusions in chalcopyrite (Fig. 3b, c, d). irregular grains and nests and it seems to be deposited The small sizes make correct analyses difficult and first (Fig. 4a–d). Besides Zn and S, it also contains Fe hence a correct diagnostic. They always contain sig- and Ag (Table 3). The sphalerite is FeS-poor (1.95 and nificant quantities of Cu, Fe and S apparently trapped 2 mole percent FeS in both analyses) with Ag 0.94 wt% from the host chalcopyrite. It is difficult to attribute and 2.15 wt% respectively. Cadmium was found in one with certainty their composition to any of the known analysis only, in concentration of 0.69 wt%. compounds (Table 2). Galena is deposited as irregular grains or veinlets We suppose very cautiously that some of them in quartz or embrace and cut the sphalerite (Fig. 4a, could be related to tellurbismuthite(?) with ideal com- b, d). The electron microprobe analyses indicate that position Bi2Te3 (Table 2, an. 5). Based on the opti- it contains systematically as trace elements Zn from cal properties and microprobe analyses, the irregular 3.12 to 6.37 wt%, Ag in 2 analyses and Fe in one grains in chalcopyrite could be attributed to arseno- analysis (Table 3). pyrite despite the Cu content which is probably due to Chalcopyrite is less abundant than galena and the host chalcopyrite (Table 2, an. 6, 7). It is difficult sphalerite. It is deposited as irregular grains up to 150– to refer the analyses from 8 to 10 on Table 2 to any 200 µm in fractures and cavities in sphalerite together certain mineral. with the associated minerals (Fig. 4b, c). Marcasite is scarce. It forms platy crystals up to 1–1.5 mm as entirely replacement of pyrrhotite Base metal veins (Fig. 4d). The lamellae, having a distinct cleavage par- allel to their length, are oriented along the basal plains The base metal veins crosscut the first two vein types of the primary pyrrhotite. Numerous studies indicate and hence are the latest veins in the low-grade meta- this replacement phenomenon of pyrrhotite to both py- morphic rocks. These are striking 60–90° and dipping rite and marcasite. The optical properties of the platy

46 Fig. 3. Microphotographs of the minerals from the pyrite veins a, grained aggregate from pyrite and chalcopyrite; b, arsenopyrite and single gold grain in chalcopyrite (SEM-BSE image); c, inclusions of gold and unspecified Ag-Te mineral phase in chalcopyrite (SEM-BSE image); d, inclusions of unspecified Bi-Te (tellurobismuthite?) mineral phase in chalcopyrite (SEM-BSE image) Abbr.: Asp – arsenopyrite; Au – gold; Bi-Te tellurobismuthite(?); Chp – chalcopyrite; Id – idaite; Py – pyrite; unspecified Ag-Te mineral Фиг. 3. Микрофотографии на минералите от пиритовите жили а – зърнест агрегат от пирит и халкопирит; b – арсенопирит и зърно от самородно злато в халкопирит (SEM-BSE изображение); с – включения от самородно злато и от неопределен Ag-Te минерал в халкопирит (SEM-BSE изображение); d – включения от неопределен Bi-Te минерал (телуробисмутит?) в халкопирит (SEM-BSE изображение) Съкр.: Asp – арсенопирит; Au – самородно злато; Bi-Te – телуробисмутит(?); Chp – халкопирит; Id – идаит; Py – пирит; неопределен Ag-Te минерал

crystals are sufficiently distinct from pyrite. The platy The Ag and Ag-bearing minerals in the base metal marcasite pseudomorphoses exhibits a large amount veins are native silver, stephanite, pyrargyrite, mem- of pore space which has preferred orientation relative bers of argentian tetrahedrite-freibergite series, cu- to the parent pyrrhotite, which suggests that the mech- propolybasite, allargentum and mooihoekite. They anism of the transformation is of Fe removal (Fleet, occur in close intergrowths in between and embrace or 1978; Murowchick, 1992). It is assumed that marca- crosscut the sphalerite. Because of the small sizes all site forms predominantly at low pH or S-2 deficient by they were identified by means of microprobe analyses the transformation of pyrrhotite than direct from the (Table 4). hydrothermal solution (Qian et al., 2011). Native silver is a new mineral for Elatsite deposit Pyrite is minor mineral in the base metal veins and and it is the most common silver mineral. Its identi- occurs in the quartz as grains up to 50 µm. fication was facilitated by its low hardness and easy

47 Fig. 4. Microphotographs of the minerals from the base metal veins a, close intergrowths between sphalerite, galena and quartz; b, galena, native silver, stephanite, tetrahedrite and chalcopyrite in cavities in sphalerite; c, native silver, tetrahedrite and chalcopyrite deposited in cracks in sphalerite; d, platy marcasite in cracks in sphalerite, galena and quartz Abbr.: Ag – native silver; Chp – chalcopyrite; Gal – galena; Mar – marcasite; Py – pyrite; Qtz – quartz; Sph – sphalerite; St – stephanite; Tt – tetrahedrite Фиг. 4. Микрофотографии на минералите от полиметалните жили а – тясно прорастване между сфалерит, галенит и кварц; b – галенит, самородно сребро, стефанит, тетраедрит и халкопирит в празнини в сфалерита; c – самородно сребро, тетраедрит и халкопирит, отложени в пукнатини в сфалерита; d – плочест марказит в пукнатини в сфалерита, галенита и кварца Съкр.: Ag – самородно сребро; Chp – халкопирит; Gal – галенит; Mar – марказит; Py – пирит; Qtz – кварц; Sph – сфалерит; St – стефанит; Tt – тетраедрит

Table 3 Microprobe analyses of sphalerite and galena from the base metal veins Таблица 3 Микросондови анализи на сфалерит и галенит от полиметалните жили

Elements (wt%) Nr. Mineral Formula Fe Ag S Zn Pb Cd

1. Sphalerite 1.95 0.94 33.82 62.60 0.69 Zn0.93Fe0.03Ag0.01Cd0.01S1.02

2. Sphalerite 2.00 2.15 33.43 62.42 Zn0.93Fe0.03Ag0.02S1.02

3. Galena 11.93 3.12 84.95 Pb0.99Zn0.12S0.90

4. Galena 3.00 11.85 3.25 81.89 Pb0.94 Zn0.12Ag0.07S0.88

5. Galena 0.41 4.44 12.37 6.37 76.40 Pb0.82 Zn0.22Ag0.09S0.86

48 ] 4 ] CuS 4 9 12.33 11.65 15.97 S S ) CuS [Ag 4.06 4.17 9 ) ) 12.46 15.81 6.10 S S S 0.36 0.22 [Ag 3.54 0.16 12.66 3.04 ) As As 5.94 ) S 2.80 S 1.40 (Sb 3.67 3.95 S 1.93 3.27 2.93 9.06 ) As 0.94 Sb ) Sb (Sb (Sb ) 1.49 2.14 1.11 0.10 13.07 12.65 13.19 3.52 ) ) ) Sb S Pb (Sb (As 0.46 0.05 0.16 1.44 0.76As0.18 1.03 0.92 Formula 5.85 13.00 3.25 15.94 ) Pb Pb ) Pb S S Sb (Sb (As Ag 0.19 0.31 0.31 0.42 0.46 0.98 0.28 0.10 3.19 6.12 8.04 ) ) 0.15 Zn Zn Zn Zn Zn Sb Sb Zn S 0.02 1.93 (Fe 1.68 1.82 1.50 1.82 1.59 0.02 0.01 0,02 0.02 2.81 0.11 Fe Fe 8.96 0.05 0.03 0.03 16.56 Fe Fe Fe Fe Fe ) Zn Zn Fe Fe S S S Sb Ag S 1.49 0.062 0.18 6.02 6.28 6.33 6.54 0.18 0.01 0.01 0.10 0.03 0.23 0.01 0.01 0.02 7.50 8.26 0.02 0.02 0.03 Ag Cu Zn Ag Ag Ag Ag Ag Fe Fe S S S Cu Cu Cu Cu Fe Sb Fe Fe 3.11 8.80 3.80 2.70 4.46 4.47 4.43 4.44 0.95 0.96 0.98 0.98 0.97 0.96 0.98 5.22 5.53 1.71 1.72 7.46 9.19 (Cu Ag Ag Ag Ag Ag Ag Ag Ag (Ag Cu (Cu (Cu (Cu (Cu (Cu Ag Ag (Cu Ag Cu S 0.98 0.62 0.46 0.84 2.35 0.70 1.01 15.30 13.84 12.10 16.64 29.36 30.27 15.84 20.31 21.49 18.61 20.70 33.49

As 5.19 2.54 4.07 1.37 5.64 0.81 0.24

Sb 8.71 9.79 1.97 1.18 6.95 15.40 17.04 11.51 22.95 14.03 23.94 13.02 20.27

Zn 0.67 0.83 0.18 0.82 0.70 0.62 1.16 1.49 1.36 1.05 Elements (wt%)

Cu 0.71 0.57 0.75 0.21 0.74 0.30 11.30 27.19 36.84 33.38 14.92 14.61 15.13 14.06 14.45 Ag 0.50 5.90 54.33 98.55 99.38 99.54 99.16 97.67 98.54 69.03 69.32 62.02 17.40 48.44 34.79 36.74 35.17 90.05 86.28 33.09 98.21

Fe 5.18 0.47 0.14 0.12 0.19 4.60 4.31 5.47 4.42 0.65 5.19 0.48 0.26 0.21 24.07 29.09 25.80 and Ag-bearing minerals from the base metal veins Cupropolybasite? Native silver Native silver Native silver Native silver Native silver Native silver Stefanite Stefanite Pyrargyrite Mooihoekite Mooihoekite Mooihoekite Cupropolybasite Tetrahedrite Tetrahedrite Tetrahedrite Allargentum? Allargentum? Tetrahedrite Mineral Native silver 2. 3. 4. 5. 6. 7. 8. 9. 1. 15. 10. 11. 12. 13. 14. 17. 18. 19. 20. 21. 16. Nr. Table 4 Microprobe analyses of Ag Таблица 4 Микросондови анализи на сребърни и Ag-съдържащи минерали от полиметалните жили Pb content (wt%) in analyses: 9 – 4.09; 13 1.20; 16 4.85; 17 0.50; 19 1.63 Pb съдържания (wt%) в анализи: 9 – 4.09; 13 1.20; 16 4.85; 17 0.50; 19 1.63

49 oxidation on the surface. The native silver occurs as The composition of cupropolybasite from Elatsite irregular grains and macroscopic nests up to 3–4 mm is similar to this of the type sample but there are some in close association and intergrowth with all Ag and differences also. Only the Cu content is almost in Ag-bearing minerals in cavities and is obviously later the same range, all other elements show deviations. than sphalerite. The nests have a completely concen- The chemical formulas of the two analyses from the tric-zoned structure of colloform type. The gaps be- Elatsite deposit are calculated on the basis of 29 atoms tween the zones are filled with members of argentian and according to the structural results and the revised tetrahedrite-freibergite series, mooihoekite and cu- chemical formula proposed by Bindi et al. (2007a, b) propolybasite (Fig. 5a, b, e, f). (Table 4). The Ag concentrations vary slightly (Table 4). Allargentum is a new mineral for Elatsite deposit. No Au is detected. The concentrations of S (0.46– It is observed as around 120 µm hexagonal crystals 1.01 wt%), Fe (0.21 to 0.48 wt%, in some analyses), in association with the other Ag minerals (Fig. 5b). Zn (0.67 wt%) and Cu (0.30–0.71 wt%) indicate im- Allargentum is relatively rare mineral and there are purities trapped from the associated minerals. two suggestions for its chemical formulae. Somanchi Stephanite and pyrargyrite occur in close relation- and Clark (1966) using X-ray powder diffraction con- ship as irregular grains and nests up to 150 µm in as- firmed the presence of the compound Ag6Sb; this com- sociation with native silver and the other Ag-bearing pound was verified by Cipriani et al. (1996). Petruk minerals (Fig. 5). They contain small amounts of Zn, et al. (1971) defined allargentum as Ag1–xSbx with Cu and Fe, likely due to sphalerite and mooihoekite im- x = 0.13–0.14. The two analyses of allargentum in this purities (Table 4). Stephanite is stable up to 197 °C and study indicate contents of Ag 86.28 and 90.05 wt% its presence defines the low deposition temperature of and of Sb 6.95 and 13.02 wt% (Table 4). The presence this silver association (Ushko-Zaharova et al., 1986). of Fe and S is probably due to impurities. Bogdanov et al. (2005) reported stephanite in the late Members of argentian tetrahedrite-freibergite se- quartz-galena-sphalerite assemblage in the copper ries are deposited in close association with mooihoe- porphyry ore of the Elatsite deposit. kite, stephanite and pyrargyrite in the space between Cupropolybasite is a new mineral for Elatsite de- the concentric zones of the colloform native silver; posit. It occurs as irregular grains up to 20 µm in as- thus it also forms concentric bands with width around sociation with native silver and silver-bearing miner- 10 µm (Fig. 5b). Argentian tetrahedrite-freibergite als (Fig. 5a, b). Although the cupropolybasite is rela- is a solid-solution series, which can be written (Cu, tively widespread in nature, it is defined recently as Ag)10(Fe, Zn)2Sb4S13 (Ebel, Sack, 1991; Sack, 1992). new mineral in samples from the Premier mine which The microprobe analyses reveal the typical elements is one of the British Columbia’s largest epithermal for tetrahedrite. Ag ranges from 33.09 to 36.74 wt%, Ag-Au deposits (Bindi et al., 2007a). All polytypes and Cu ranges from 14.06 to 15.13 wt%. The Fe and of polybasite are composed of 2 different layers Zn content in individual grains is homogeneous stacked along [001]: layer A, with general composi- and Fe content is generally higher than that of Zn. 2− tion [(Ag,Cu)6(As,Sb)2S7] , and layer B, with general Arsenic content is variable from absence to 5.64 wt% 2+ composition [Ag9CuS4] (Bindi et al., 2007b). (Table 4).

→ Fig. 5. Microphotographs of the association of silver minerals in the base metal veins a, intergrowth of native silver, stephanite, tetrahedrite and mooihoekite (SEM-BSE image); b, close intergrowth between native silver, stephanite, pyrargyrite, tetrahedrite, mooihoekite, polybasite and allargentum (SEM-BSE image); c, native silver deposited in cracks in sphalerite (SEM-BSE image); d, stephanite and pyrargyrite crosscut sphalerite and galena (SEM-BSE image); e, mooihoekite and tetrahedrite in space between the colloform shales of native silver and blebs from mooihoekite in native silver; f, relationship between sphalerite, native silver, tetrahedrite, stephanite and mooihoekite Abbr.: Ag – native silver; Alg – allargentum; Chp – chalcopyrite; Ga – galena; Mh – mooihoekite; Plb – cupropolybasite; Py – pyrite; Pyr – pyrargyrite; Qtz – quartz; Sph – sphalerite; St – stephanite; Tt – tetrahedrite Фиг. 5. Микрофотографии на асоциацията от сребърни минерали в полиметалните жили а – прорастване на самородно сребро, стефанит, тетраедрит и мойхокит (SEM-BSE изображение); b – тясно прорастване между самородно сребро, стефанит, пираргирит, тетраедрит, мойхокит, полибазит и алагрентум (SEM-BSE изображение); c – самородно сребро, отложено в пукнатини в сфалерита (SEM-BSE изображение); d – стефанит и пираргирит пресичат сфалерит и галенит (SEM-BSE изображение); e – мойхокит и тетраедрит, отложени в пространството между коломорфните обвивки на самородно сребро и зърна от мойхокит в самородното сребро; f – взаимоотношение между сфалерит, самородно сребро, тетраедрит, стефанит и мойхокит Съкр.: Ag – самородно сребро; Alg – аларгентум; Chp – халкопирит; Ga – галенит; Mh – мойхокит; Plb – купрополибазит; Py – пирит; Pyr – пираргирит; Qtz – кварц; Sph – сфалерит; St – стефанит; Tt – тетраедрит

50 Freibergite was found by Bogdanov et al. (2005) (Fig. 5a, b). It is deposited as fine-grained blebs in na- and Kehayov (2005) in the quartz-galena-sphalerite tive silver as well (Fig. 3e). Under reflected light it has assemblage in the porphyry ore of Elatsite magmatic- the color of chalcopyrite, but it tarnishes in air and is hydrothermal system. weakly anisotropic. Despite the significant differences Mooihoekite is deposited in the same spaces in the of the chemical composition from this of the mooihoe- colloform native silver as members of argentian tetra- kite from the type locality (Cabri, Hall, 1972), we sup- hedrite-freibergite series. Its form and size are restricted pose with some reservation that the observed mineral is by the space between the colloform bands but occasion- mooihoekite. The content 17.40 wt% Ag in one analysis ally irregular grains occur, approximately 20 µm in size could be attributed partly to the host native silver.

51 Discussions and conclusions fected by sericite alteration. All three types are conform to the general foliation trend with N-S direction, what is The understanding of porphyry-related ore systems is due to the facilitation of fluids flow. Unlike quartz veins the result of analysing many mining and exploration pyrite and base metal veins often crosscut the foliation cases around the world. Porphyry copper systems are also, what argue on their later deposition. defined as large volumes of hydrothermally altered The quartz veins are deposited first. They are con- rock centered on porphyry Cu stocks that may also conform to the general foliation trend with N-S direction. tain skarn, carbonate-replacement and sediment-host- The field observations indicate a synmetamorphic origin ed deposits. According to most authors the shallower of the quartz veins. Their main minerals are pyrite, pyr- parts of the porphyry copper systems may host high- rhotite and chalcopyrite. Arsenopyrite, galena and sphal- and intermediate-sulfidation epithermal Au±Ag±Cu erite are scarce. Native Bi, idaite and aschamalmite are orebodies (Kirkham, Sinclair, 1996; Corbett, Leach, occasional. The gangue minerals are quartz, calcite and 1998; Sillitoe, 1989, 2010; Hedenquist et al., 2000; fluorite. Probably the veins (mainly their gangue compo- Sillitoe, Hedenquist, 2003). nent) are deposited during the metamorphic events. The Comparison of the basic features of the two types sulfide minerals are deposited later in the same spaces. of epithermal deposits in the porphyry copper systems The pyrite veins crosscut the quartz veins and are shows considerable overlap in their characteristics. conform to the orientation of the generalised folia- There are, however, many distinctive features as well. tion trend and often they are flexed, boudinated and High-sulfidation epithermal Au, Ag, and/or Cu de- sheared. The main minerals are pyrite and chalcopy- posits are typically located in the lithocap environment rite, the gangue is quartz. Native gold occurs rare in above the porphyry copper body. They contain sulfide- associations with complex and not well defined Bi-Te rich assemblages of high-sulfidation state, typically and Ag-Te compounds. pyrite-enargite, pyrite-luzonite, pyrite-famatinite, and The base metal veins are deposited latest and occur pyrite-covellite, hosted by leached silicic rock with a only in some parts unlike the quartz and pyrite veins halo of advanced argillic minerals (Hedenquist et al., which are common. Most of them are conform to the 2000; Sillitoe, Hedenquist, 2003; Einaudi et al., 2003). foliation but the crosscutting is also frequent. Their Intermediate-sulfidation epithermal precious metal common minerals are sphalerite and galena, whereas deposits occur alongside lithocaps but typically spa- stephanite, pyrargyrite, members of argentian tetra- tially separate from the high-sulfidation orebodies and hedrite-freibergite series, native silver, chalcopyrite, commonly do not show such a close connection with pyrite and marcasite are less abundant. Allargentum, porphyry Cu deposits as do many of the high-sulfida- cupropolybasite and mooihoekite occur rarely. tion deposits (Hedenquist et al., 2000; Masterman et Based on the geological setting, alteration style of al., 2005). Intermediate-sulfidation epithermal deposits the host rocks, main textures, mineral assemblages and that are also sulfide-rich share many of the sulfide as- composition of some minerals it could be concluded semblages of high-sulfidation deposits, except that the that all three vein types are of epithermal intermediate- enargite-bearing assemblage is lacking and the Ag:Au sulfidation style. ratios are higher, at least 10:1, and typically >100:1. Some occurrences of apparently incompatible The major sulfide assemblage can be relatively sim- sulfide minerals are noted in the veins. Pyrrhotite is de- ple, including combinations of FeS-poor sphalerite posited as numerous crystals in cavities in the quartz (from <1 to 10 mole % FeS, locally up to 20 mole % veins occasionally with arsenopyrite. Pyrrhotite is not a FeS), galena, pyrite, chalcopyrite, and tetrahedrite but defining mineral for intermediate-sulfidation deposits. lacking appreciable arsenopyrite and pyrrhotite. Silver Such occurrences may indicate appreciable but local is present as Ag sulfosalts, and in some cases a large fluctuations in sulfidation state during the lives of the variety of these minerals occur in trace quantities. The system or, alternatively, that equilibrium was simply main gangue minerals are Mn-bearing carbonates, rho- not attained (Giggenbach, 1992; Einaudi et al., 2003). donite, and quartz (Hedenquist et al., 2000; Sillitoe, These results are crucial to develop a valid pros- Hedenquist, 2003; Einaudi et al., 2003; Masterman et pecting model and to allow planning of field, geo- al., 2005; Sillitoe, 2010). chemical and geophysical explorations in the area of Besides the mineral composition the two epithermal Elatsite deposit. deposits types differ by fluid compositions, salinity, pH, Eh, depositional temperature (White, Hedenquist, Acknowledgements: The authors gratefully ac- 1990; Arribas, 1995; Cooke, Simmons, 2000). knowledge Christo Stanchev from Eurotest-Control The three ore vein systems hosted by the low-met- EAD and Assoc. Prof. A. Zdravkov and Assoc. Prof. amorphic host rocks of the Elatsite porphyry copper K. Ruskov from University of Mining and Geology, deposit mark the low-temperature hydrothermal stage Sofia, for their technical help in microprobe analyses. of the magmatic-hydrothermal system. They are cavity Thanks аre due to the geological team supervised from filling veins with sharp boundaries with the host rocks. Prof. Zhivko Ivanov: N. Hadzhieva, P. Apostolov, O. The veins have distinct strikes, dips and mineral assem- Pavlov, А. Marinova and A. Gadzhev. All they assist- blages and all three types are hosted in rocks that are af- ed by the field work and sampling.

52 References

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(Постъпила на 28.03.2017 г., приета за печат на 06.06.2017 г.) Отговорен редактор Страшимир Страшимиров