Mineral chemical investigation on sulfide mineralization of the I stala deposit, Gümüşhane, NE-Turkey

Ore Geology Reviews 53 (2013) 306–317

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Mineral chemical investigation on sulfide mineralization of the Istala deposit, Gümüşhane, NE-Turkey Yılmaz Demir a, İbrahim Uysal b,⁎, M.B. Sadıklar b a b

Recep Tayyip Erdoğan University, Department of Geological Engineering, TR-53100, Rize, Turkey Karadeniz Technical University, Department of Geological Engineering, TR-61080, Trabzon, Turkey

a r t i c l e

i n f o

Article history: Received 15 January 2012 Received in revised form 17 January 2013 Accepted 25 January 2013 Available online 8 February 2013 Keywords: Istala mine Ore mineralogy Silver minerals Mineral chemistry Northeastern Turkey Gümüşhane

a b s t r a c t The mineralogy of the Istala deposit, Gümüşhane, northeastern Turkey, was studied in detail, and a geochemical investigation was carried out using electron probe micro-analysis (EPMA). Sphalerite, galena, chalcopyrite and pyrite are the major sulfide minerals found in the Istala deposit, with minor amounts of bornite, idaite, tetrahedrite–tennantite, anilite, yarrowite, mckinstryite, covellite and chalcocite. In addition to these, barite and a small quantity of quartz occur as gangue minerals. Based on the textural relations and mineral assemblages, five different stages of crystallization have been recognized. Mineral paragenesis of the first four stages has been found to be similar, whereas clear enrichment has been observed in the modal abundance of the copper sulfide mineral assemblage at the fifth-stage ore formation. Whole-rock geochemical analyses of the Istala ore show an enrichment of Ag content up to 3328 ppm. Optical observations and EPMA study indicated that abundant silver mineralization was found in the Istala ore, especially during the later-stage ore deposition. Repetition to the presence of native silver in the samples, a significant amount of silver was incorporated in bornite, idaite, tetrahedrite–tennantite, anilite, yarrowite, mckinstryite, covellite and chalcocite, whereas a trace amount of silver has been detected in sphalerite, galena, chalcopyrite and pyrite. The homogenization temperatures (Th) of the primary fluid inclusions were measured between 98 and 284 °C, with frequency peaks around 140 °C, 190 °C and 240 °C. All data obtained support the theory that later stage copper-rich sulfides, formed under the low temperature conditions, are responsible for the large amounts of silver content in the Istala mine. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The geological setting of northeastern Turkey has been considered to be a magmatic arc by many authors (Bektaş et al., 1999; Boztuğ et al., 2004; Dokuz et al., 2006; Karslı et al., 2007, 2010; Okay and Şahintürk, 1997; Şengör and Yılmaz, 1981; Şengör et al., 2003; Topuz et al., 2005; Yılmaz et al., 1997). Depending on the nature of the magmatic arc, different types of tectonic units, containing various types of ore deposits, developed in the region (Çamur et al., 1996; Güven, 1993; Özgür, 1993; Tüysüz, 2000). The distribution of these deposits demonstrates a marked metallogenic zonation from north to south, with massive sulfide-type in the northern part, and skarns, porphyry systems, U and F, and Cu and Cr deposits in the southern part (Fig. 1). Sixty-two volcanogenic massive sulfide (VMS) deposits have been identified in the region (Pejatoviç, 1979), with the most widely known deposits extending from east to west, including Cerattepe, Murgul, Çayeli, Kutlular, Kanköy, Harköy, Lahanos and Istala. The Istala deposit is located 30 km north of the Gümüşhane province (in the northeastern part of Turkey), and mining activity of the ⁎ Corresponding author. Tel.: +90 532 3024578; fax: +90 462 3257405. E-mail addresses: [email protected], [email protected] (İ. Uysal). 0169-1368/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.oregeorev.2013.01.014

Istala deposit ceased in the middle of the 20th century, due to the economic and technical difficulties at that time. However, geochemical studies have shown that the Istala ore is locally rich in silver content (up to 3328 ppm). Although the main geological characteristics and genesis of the VMS deposits in the northeastern Turkey were systematically studied by various authors (e.g., Akçay et al., 1998; Akıncı, 1984; Craig, 1981; Japan International Cooperation Agency-JICA, 1985; Pejatoviç, 1979; Schneider et al., 1988; Tüysüz, 2000), there has been no comprehensive study regarding the silver mineralogy of the Istala ore. In this paper, the compositions of the silver-bearing and silver-free minerals are investigated in detail, in addition to the systematic mineralogical investigation of the Istala ore deposit. Furthermore, the genetic and chemical characteristics of the hydrothermal fluids and ore-deposition conditions are discussed, taking into account, mineral chemistry analyses and fluid inclusion studies. The results may help to better understand the occurrence of silver in the VMS deposits found in northeastern Turkey. 2. Geological setting The metallogenic province of the eastern Black Sea is located along the Alpine metallogenic belt, which is related to an island arc formed

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307

Fig. 1. Distribution of the different types of ore deposits in northeastern Turkey. The most widely known VMS deposits are indicated as 1 — Cerattepe, 2 — Murgul, 3 — Çayeli, 4 — Kutlular, 5 — Kanköy, 6 — Harköy, 7 — Lahanos and 8 — Istala.

between the Jurassic and the Miocene age, during the subduction of the Tethyan oceanic crust (Çamur et al., 1996; Okay and Şahintürk, 1997; Okay et al., 1997; Yılmaz et al., 2003). The geological setting of northeastern Turkey provides insight into the paleo-island arc magmatism and long-term crustal evolution from pre-subduction rifting, through arc volcanism and plutonism, to post-subduction alkaline volcanism (Akın, 1979; Karslı et al., 2008; Okay and Şahintürk, 1997; Robinson et al., 1995; Şengör and Yılmaz, 1981; Şengör et al., 2003; Yılmaz et al., 1997). Northeastern Turkey is divided into three main zones, the northern, southern, and axial zones, based on - lithological characteristics (Bektaş et al., 1995). These zones are separated by faults orientated along E–W, NE–SW and NW–SE directions, thus defining a block-fault tectonic style in northeastern Turkey (Bektaş and Çapkınoğlu, 1997). The northern zone, containing massive sulfide ores, includes extensive Upper Cretaceous and Tertiary volcanic rocks. The northern volcanic series comprises Late Cretaceous mafic and felsic volcanic rocks interbedded with sedimentary layers. The Liassic basaltic rocks in the region overlie the Hercynian basement (Boztuğ et al., 2004; Kandemir and Yılmaz, 2009; Schneider et al., 1988). Early mafic rocks were followed by andesite, basalts and their pyroclastites during the Late Cretaceous period, and a large number of granitic intrusions intruded into the region during the Late Cretaceous to Eocene period (Boztuğ et al., 2007; Karslı et al., 2004; Topuz et al., 2005; Yılmaz and Boztuğ, 1996). Andesitic and basaltic Eocene volcanic rocks formed the final phase of the subduction in the region (Karslı et al., 2010). In the northeastern area, the VMS deposits are situated in the intra-arc rift zone of the northeastern island arc, which is characterized by a bimodal volcanic rock suite of calc–alkaline dacites and basalts. All the massive sulfide occurrences are associated with the dacitic domes of the Upper Cretaceous felsic volcanic rocks (Tüysüz, 2000). Akçay et al. (1998), Akçay and Arar (1999), and Tüysüz (2000) have also shown that the geological and petrographical features of the host rocks, around the VMS deposits, imply submarine conditions, as revealed by the submarine sediments that are associated with the massive sulfide mineralization. Most of the massive sulfide deposits located in northeastern Turkey, associated with the dacitic series of the Upper Cretaceous,

have a geological environment similar to that of the Kuroko deposits in Japan (Pejatoviç, 1979).

3. Geology of the Istala Mine The area around the Istala mine consists mainly of volcanic and, to a lesser extent, sedimentary rocks. Volcanic rocks are divided into three main groups: lower mafic sequence (LMS), lower felsic sequence (LFS) and upper mafic sequence (UMS). The facies of the volcanic rocks in the lower part of the LMS gradually change upwards into andesite, agglomerate, lapilli tuff and medium- to fine-grained tuff. The Istala mine is located in the LFS, and is composed of dacite, dacitic tuff, dacitic agglomerate and breccias commonly interbedded with red biomicrite layers. The LFS overlies the LMS, and is overlain by the UMS, which is composed of andesite, basalt and associated pyroclastites. All volcanics are cut by andesitic dykes and porphyr dacites (Fig. 2). Dacitic rocks, including those in the Istala mine, are divided into two different groups: Dacite-I and Dacite-II, according to their trace-element contents. Based on the K–Ar method, the age of the illites in Dacite-I and Dacite-II was calculated at 78.7 ± 2.3 and 75.3 ± 2.4 Ma (Campanian–Danian), respectively (Sipahi, 2005). In addition, the age of the red biomicrite layers, interbedded with the dacitic rocks, is considered to be Late Cretaceous (Turonian to Santonian) on the basis of the paleontological evidence (Marginotruncana pseudolinniana, Marginotruncana sp., and Ticinella sp.; JICA, 1985). Two faults, striking N20E and N50E, have developed after the mineralization (Fig. 3a), because of the intensive tectonic activity in northeastern Turkey. The distinct lack of mineralization along these faults indicates that they are post-ore formation. The uplifted NW block of the main fault (N50E strike) has probably led to the erosion of some sections of the ore. However, the levels of brecciated ore, massive barite and massive black ore can be observed in the mine shaft. The mine waste, scattered over a wide area near the adits, indicates that large-scale mining was carried out in the lower part of the ore body.

308

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Fig. 2. Geological map of the Istala mine area (modified after Güven, 1993).

4. Characteristics of the Istala Ore From bottom to top, three different ore levels, that are massive black ore, massive barite and brecciated ore, can be observed in the Istala ore system (Fig. 3b). Unfortunately, because the bottom of the mine shaft is filled up with collapsing host rock, the deeper part of the section could not be studied. The massive black ore, up to 2 m in thickness, is mostly composed of galena and sphalerite (Fig. 4a), in addition to a lesser amount of pyrite and chalcopyrite. The percentage of sulfides in this level reaches up to 90%. A small amount of alteration products of copper minerals (covellite and chalcocite) has also been observed along with gangue minerals of quartz and barite. The massive black ore, containing fine grained sulfide minerals (b1 mm), are usually separated from the overlying massive barite layers through a sharp contact. The barite zone, which is approximately 5 m in thickness, occurs stratigraphically above the massive black ore (Fig. 3b). Sulfide minerals in this zone are generally distributed in massive barite layers (Fig. 4b), and can only be observed under a microscope because of smaller grain size (b 500 μm) and lower sulfide percentages (less than 2%). Mineral paragenesis of the massive barite layers, in decreasing order of abundance, is as follows: sphalerite, galena, pyrite, chalcopyrite and tetrahedrite–tennantite. Compared to the massive black ore zone, the mineralization in the massive barite zone is characterized by an enrichment of galena and tetrahedrite–tennantite, and a marked depletion of pyrite and chalcopyrite.

Brecciated ore, up to 3 m in thickness, occurs at the top of the ore sequence, and the size of the barite fragments in brecciated ore varies between a few centimeters and a half meter in diameter (Fig. 4c). Barite fragments also contain disseminated sulfides. Detailed ore microscopy indicates that the barite blocks, at this level, belong to the underlying massive barite layers, according to the mineral paragenesis and textural properties of the sulfides (Fig. 4d). At this level, the barite fragments are surrounded by fine-grained black ore, which is similar to massive black ore with their high volume percentages (up to 90%) and textural properties of the sulfides (Fig. 4c and d). Compared to the massive black ore, the brecciated ore zone is characterized by the enrichment of chalcopyrite, tetrahedrite–tennantite, bornite and idaite, in addition to a lack of sphalerite and galena. 5. Analytical techniques Ore samples from the Istala deposit were studied under the reflected light microscope and the paragenesis of the ore minerals was determined based on their textural relationship. The composition of the minerals in the selected samples was analyzed at the Department of Mineralogy and Petrology, University of Hamburg, Germany, using a Cameca SX–100 wavelength-dispersive electron probe micro-analyzer. The operating conditions were 20 kV at 20 nA. Corrections were applied using the PAP computer program developed by Pouchou and Pichoir (1984). Pure elements and minerals, including pyrite, chalcopyrite, sphalerite, galena and covellite

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309

a

b

Fig. 3. Geological map of the Istala mine and its surroundings (a) and schematic cross section of the ore in the mine shaft (b). Zones of silicification, brecciated ore, massive barite and massive black ore are indicated by A, B, C and D in Fig. 3b (modified after JICA, 1985).

a

b

c

d

Fig. 4. Field images of the Istala ore. (a) Sample of massive black ore contains fine-grained sphalerite and galena minerals; (b) sample of massive barite including disseminated sulfides; (c) brecciated ore level of the Istala mine and (d) barite fragment in the brecciated ore level, containing disseminated sulfides.

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were used as standards. The Lα X-ray lines were measured for As, Ag, Sb and Cd, Kα lines for S, Cu, Fe and Zn, and Mα lines for Pb. The detection limits of each element were calculated based on the 3σ statistical precision approach, and are given here in parentheses (in ppm): Fe (500), Zn (800), Cd (1300), Ag (1000), Cu (600), Pb (2500), As (1600), Sb (800) and S (300). Microthermometric measurements were carried out using 100to 200 μm thick double-polished sections at the Department of Mineralogy and Petrology, Friedrich Schiller University, Jena, Germany. The investigations were conducted within the temperature range from − 196 to 600 °C, with a measurement accuracy of ± 0.2 °C in the temperature interval from + 20 to − 20 °C; beyond this range, the accuracy was ± 1.5–2 °C. A total of 12 samples were analyzed for their major and trace elements. All analyses were carried out at the ACME Analytical Laboratories, Vancouver (Canada) using inductively coupled plasma-emission spectroscope. The analytical precision of the method for each element was within the limits expected for research-quality analysis. The detection limits ranged from 0.01 to 0.1 wt.% for the major elements, and from 0.1 to 10 ppm for the trace elements. Fig. 5. Paragenetic sequence of the Istala ores.

6. Mineral paragenesis and textural properties The ore minerals were investigated in 32 polished sections, collected from massive black ore (13), massive barite (10) and brecciated ore (9) levels. According to the detailed microscopic investigation of the Istala ore and the mineral chemical analyses, the mineral paragenesis of the Istala ore, in decreasing order of abundance, comprise sphalerite, galena, pyrite, chalcopyrite, tennantite–tetrahedrite, bornite, idaite, anilite, digenite, yarrowite and mckinstryite. Quartz and barite occur as gangue minerals. Covellite, chalcocite, malachite, azurite and digenite are common secondary minerals, occurring generally at the margins of bornite, idaite and chalcopyrite. Five different stages of crystallization are recognized based on the textural relations and the mineral assemblages. These are summarized in Fig. 5. In spite of the mineral paragenesis of the first four stages being similar, enrichment in the modal abundances of silver-bearing mineral assemblage at the fifth-stage ore generation is clearly observed. Replacement and exolutions are the most common ore texture in the massive black ore and brecciated ore level, whereas disseminated sulfides are dominant in the massive barite level. Replacement of sphalerite by galena (Fig. 6a) and pyrite by chalcopyrite has been generally observed, and replacement of different stage ore minerals by later stage galena was also observed. Different types of exsolutions, such as chalcopyrite in sphalerite, chalcopyrite in bornite and covellite in galena, were commonly found. Chalcopyrite exsolutions were distributed irregularly (dot-like) in the sphalerite-rich sections of all the ore levels (Fig. 6b). Chalcopyrite exsolutions, arranged along the two different crystallographic directions of bornite, occurred in the chalcopyrite-rich brecciated ore level (Fig. 6c). In many cases, covellite and chalcocites were found to occur along grain boundaries of bornite (Fig. 6c and d), occurring as secondary copper minerals. Although covellite was generally of secondary origin, myrmekitic ore textures occurred between galena and covellite at the brecciated ore level (Fig. 6e) indicate that the fifth-stage covellite was formed as a primary mineral. Irregularly distributed fine grained chalcopyrite, sphalerite and fahlore intergrowths are the most common feature of the massive barite level (Fig. 6f).

proposed relationship between silver and other base metals (Fig. 7) suggests that the distribution of silver in the ore strongly correlates with that of copper (r = 0.96, p b 0.05), whereas there is no significant correlation with those of zinc and lead. The existence of silver-rich samples, belonging to the brecciated ore level, also reveals that silver was deposited in large quantities during the late stages of mineralization. Hence, in this study, mineralogical observations and electron probe micro-analysis (EPMA) were undertaken to determine the mineralogical composition and distribution of silver and silver-containing phases in the ore. In the following pages, common ore minerals of sphalerite, galena, chalcopyrite and pyrite are described. Moreover, the compositions of the silver and silver-bearing phases are also investigated. 8. Silver-free minerals 8.1. Sphalerite Sphalerite is the most abundant mineral in the Istala mine, and is found in all stages of ore formation. Replacement of sphalerite by galena is a common feature found in the ore, whereas replacement by second stages fahlores is seldom observed in the sections. Chalcopyrite exsolutions are heavily scattered in the earliest generation sphalerite, but they are rarely found in the later stages of ore formation. In some cases, chalcopyrite exsolutions arranged in one or two different crystallographic directions of sphalerite have also been observed. Composition of sphalerite is presented in Table 2. According to the microchemical analyses, sphalerite contains less than 0.89 wt.% Fe, with the exception of one sample which had a higher value of 3.29 wt.%. Cadmium contents are variable, ranging between 0.35 and 0.75 wt.%, except in a two-point analysis that shows high amounts of Cd (1.49 and 1.60 wt.%). The calculated Zn/Cd ratios of the sphalerite range between 89 and 163. Arsenic concentration in sphalerite was found to be very low, being less than 0.11 wt.%.

7. Geochemistry of the Ore 8.2. Galena The average composition of the Istala ore was determined to be Pb–Zn dominated, according to the geochemical analyses of 12 ore samples collected from the massive black ore, massive barite and brecciated ore levels. Geochemical studies indicate that the Istala ore is rich in silver content, reaching up to 0.33 wt.% (Table 1). The

Galena is closely associated with sphalerite, and occurs in the massive black ore, in high proportions. According to optical microscopy, galena is found in all stages of ore formation, and the early stage minerals, such as pyrite, chalcopyrite, sphalerite and fahlores are generally

Y. Demir et al. / Ore Geology Reviews 53 (2013) 306–317

a

b

c

d

e

f

311

Fig. 6. Selected ore textures at the microscopic scale in the Istala ore. (a) Replacement of sphalerite by galena, (b) exsolution of irregularly distributed chalcopyrite in sphalerite, (c) chalcopyrite exsolutions along the crystallographic directions in bornite, (d) occurrence of covellite and chalcocite around the grain boundaries of bornites, (e) myrmekitic intergrowth of galena and chalcocite, and (f) fine grained chalcopyrite, tetrahedrite–tennantite, and sphalerite intergrowths at the brecciated ore level. Abbreviations: Sl — sphalerite, Gn — galena, Py — pyrite, Cp — chalcopyrite, Cv — covellite, Cc — chalcocite and Bn — bornite.

replaced by galena. In some cases, replacement relics of the other minerals were found in galena. Composition of galena is summarized in Table 2. The Ag content of galena is below the EPMA detection limit (0.05 wt.%), however, up to

0.3 wt.% of Ag was detected in a few grains. A significant amount of Cu (ranging between 0.07 and 4.77 wt.%) was detected in galena, whereas Zn (up to 0.61 wt.%) and Fe (up to 0.39 wt.%) were the minor components.

Table 1 Concentrations of the major (wt.%) and minor elements (ppm) of the Istala ore samples. Massive black ore

Cu Pb Zn Fe Ag Sb As Cd Mo Mn

wt.% wt.% wt.% wt.% ppm ppm ppm ppm ppm ppm

Massive barite

Brecciated ore

1

2

3

4

1

2

3

1

2

3

4

5

1.45 7.50 12.64 1.36 102 60 27 1126 192 44

2.44 8.84 8.54 1.60 625 7127 3309 654 22 56

4.03 13.99 19.48 0.69 292 10287 11128 1795 38 413

5.09 15.76 28.17 3.78 456 11060 10421 2574 54 61

0.83 1.83 2.71 0.72 108 1886 2034 211 9 16

4.83 14.97 1.01 1.18 330 3557 17587 1016 9 85

6.05 7.00 10.85 1.99 284 5401 17317 939 26 46

6.42 26.92 26.76 1.37 564 18198 13281 2039 55 55

8.29 6.40 17.94 1.84 750 26841 13832 2175 46 104

12.52 23.12 27.63 4.30 1336 27383 10204 2616 74 54

7.89 19.67 31.36 1.73 810 22382 17563 3122 93 44

31.87 6.57 5.37 6.17 3328 42350 3690 513 10 121

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a

b

c

Fig. 7. Ag contents versus Cu, Pb, and Zn contents in the whole-rock Istala ore. Tested correlation coefficients are statistically significant at the pb 0.05 level.

8.3. Chalcopyrite Chalcopyrite is a common ore mineral in Istala and has been observed in all the stages of ore formation. Chalcopyrite is a major constituent of the brecciated ore level, whereas it is less abundant in the sphalerite- and galena-rich massive black ore. A considerable amount of bornite also accompanies chalcopyrite in the chalcopyrite-rich section of the brecciated ore level. According to their textural relationships, chalcopyrite generally replaces pyrite, and is in turn replaced by later stage sphalerite and galena. Increase of the modal abundance of chalcopyrite in the brecciated ore is an indication that copper-rich hydrothermal solutions interacted with the system during the later stages of ore formation.

The EPMA result of chalcopyrite from the Istala mine is reported in Table 2. The chalcopyrite compositions were found to be nearly pure, although Zn and Ag were detected in very small quantities, i.e. up to 0.30 and 0.16 wt.%, respectively. These values indicate that the silver content of the chalcopyrite is low, although the ore in the Istala is generally rich in silver content. 8.4. Pyrite Optical microscopy shows that the modal abundance of pyrite is clearly less than that of chalcopyrite and it generally coexists with chalcopyrite. Pyrite occurs in three different textural relationships in the Istala ore: (1) as euhedral and anhedral coarse grains, (2) inclusions

Table 2 Statistical data (maximum, minimum, mean values and number of analyses) of electron probe micro analysis (EPMA) of sphalerite, pyrite, galena and chalcopyrite (MBO: massive black ore; MB: massive barite; BO: brecciated ore levels). wt.% Sphalerite

Pyrite

Galena

Chalcopyrite

MBO

N = 10

MB

N=7

BO

N = 11

MBO

N=9

MB

N=6

BO

N=6

MBO

N=9

MB

N = 11

BO

N=4

MBO

N=6

MB

N=5

BO

N = 13

Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean

S

Fe

32.87 31.10 32.51 32.69 32.17 32.53 33.38 31.84 32.58 53.87 53.32 53.58 53.45 52.60 53.12 53.29 53.06 53.13 13.79 13.29 13.54 13.69 13.34 13.53 13.56 13.18 13.36 34.88 34.61 34.73 35.13 34.72 34.92 34.94 34.29 34.56

0.55 0.15 0.30 0.85 0.06 0.48 3.29 0.07 0.75 46.65 45.03 46.10 46.99 44.92 46.33 46.81 45.73 46.50

0.39 0.05 0.22 30.14 28.91 26.69 29.87 29.05 29.49 29.99 28.78 29.62

Cu

Zn

Cd

0.41 0.07 0.23 1.43 0.07 0.70 1.37 0.16 0.50 1.74 0.09 0.63 2.13 0.09 0.70 0.90 0.10 0.35 0.83 0.07 0.28

67.05 64.77 66.30 66.81 63.73 65.55 66.13 57.92 64.66 0.24 0.08 0.15 0.30 0.30 0.30

0.75 0.48 0.62 1.60 0.44 0.86 0.53 0.35 0.47

4.77 0.08 0.56 34.43 33.95 34.19 35.15 34.12 34.49 35.32 34.01 34.66

0.49 0.08 0.27 1.60 0.11 0.67 0.20 0.12 0.16 0.11 0.08 0.09

0.61 0.09 0.35

Pb

88.75 85.79 87.11 86.91 85.33 86.16 88.12 81.17 87.22

As

Ag

0.21 0.18 0.19 0.25 0.25 0.25 0.26 0.22 0.24

0.18 0.12 0.15 0.14 0.14 0.14 0.16 0.15 0.16 0.11 0.11 0.11

0.29 0.10 0.14 0.10 0.10 0.10

0.16 0.10 0.13

Y. Demir et al. / Ore Geology Reviews 53 (2013) 306–317

a

b

c

d

e

f

313

Fig. 8. Reflected light (a–c, e, f) and BSE (d) images showing the textural relations of the ore forming minerals. (a) Native silver grain formed around the bornite minerals at the brecciated ore level; (b) undefined silver bearing phases mostly observed at the grain boundaries of bornite; (c, d) anilite, yarrowite, mckinstryite and tetrahedrite associations from the bornite and chalcopyrite rich section of the ore; (e) first-generation fahlore included in the first-stage sphalerite and (f) association of second- and third generation fahlores with third-stage chalcopyrite. Abbreviations: Sl — sphalerite, Gn — galena, Py — pyrite, Cp — chalcopyrite, Bn — bornite, Cv — covellite, Cc — chalcocite, and Fah — fahlore group (tetrahedrite to tenantite).

in chalcopyrite represented as relics of replaced sulfides, and (3) disseminated in massive barite levels as medium- to fine-sized grains. An increase in the modal abundances of pyrite is observed in the brecciated

ore level, whereas these mineral is rarely found in the massive black ore. EPMA shows a nearly constant Cu content (up to 0.70 wt.%, Table 2), whereas analyses from three different points reveal a Cu contents as

Table 3 Representative EPMA results of native silver, bornite, covellite, anilite, yarrowite and mckinstryite. bdl: below detection limit. Native silver

Bornite

Covellite

Anilite

Yarrowite

Mckinstryite

1

2

3

4

1

2

3

4

1

2

3

4

1

2

3

1

2

3

1

S Fe Cu Ag Zn Cd Total

wt.% wt.% wt.% wt.% wt.% wt.% wt.%

0.06 0.29 1.58 97.22 bdl 0.64 99.79

0.14 0.26 1.96 97.44 bdl 0.33 100.13

0.03 bdl 0.10 99.53 bdl 1.00 100.69

0.10 0.84 0.96 98.12 bdl 0.38 100.40

25.45 10.68 62.32 0.51 bdl bdl 99.09

25.40 10.33 60.88 2.08 0.15 bdl 98.93

25.22 10.97 63.30 0.53 bdl bdl 100.09

26.08 10.59 60.80 1.63 bdl bdl 99.25

22.82 3.08 73.98 0.64 0.10 bdl 100.65

20.51 bdl 77.98 0.87 0.09 bdl 99.52

21.80 bdl 77.22 0.44 bdl bdl 99.62

21.37 bdl 76.57 1.33 0.09 bdl 99.49

21.99 bdl 76.80 0.69 bdl bdl 99.67

22.11 bdl 77.54 0.46 bdl bdl 100.36

22.17 bdl 77.17 0.46 bdl bdl 99.90

30.63 1.12 63.98 3.28 0.13 bdl 99.273

30.21 0.83 63.79 3.79 0.19 bdl 99.092

30.52 0.05 63.24 5.65 0.16 bdl 99.621

15.88 bdl 35.68 48.67 bdl 0.49 100.73

S Fe Cu Ag Zn Cd

apfu apfu apfu apfu apfu apfu

0.002 0.005 0.027 0.960 – 0.006

0.005 0.005 0.033 0.955 – 0.003

0.001 – 0.002 0.987 – 0.009

0.003 0.016 0.016 0.962 – 0.004

4.016 0.963 4.997 0.024 – –

4.036 0.938 4.915 0.099 0.012 –

3.952 0.982 5.040 0.025 – –

4.136 0.960 4.822 0.078 – –

1.097 0.085 1.807 0.009 0.005 –

1.018 – 1.967 0.013 0.002 –

1.069 – 1.924 0.006 – –

1.056 – 1.922 0.020 0.002 –

3.951 – 7.012 0.037 – –

3.946 – 7.030 0.024 – –

3.965 – 7.011 0.024 – –

8.030 0.168 8.522 0.257 0.017 –

7.980 0.125 8.560 0.299 0.025 –

8.051 0.008 8.474 0.445 0.021 –

0.978 – 1.117 0.897 0.008

2.324 7.613 9.939 0.017 1.940 0.067 2.024 0.265 4.060 4.325 12.714 1.921 8.006 9.927 0.062 2.031 0.062 2.155 0.455 3.731 4.186 12.728 1.925 7.991 9.926 0.011 1.918 0.072 2.001 0.401 3.822 4.223 12.861 0.067 10.002 10.069 0.035 1.904 0.097 2.036 0.455 3.822 4.277 12.618 0.057 9.733 9.790 – 1.958 0.041 1.999 2.458 1.762 4.220 12.992 0.020 9.788 9.808 0.010 1.908 0.035 1.953 2.865 1.370 4.235 13.004 0.071 9.972 10.043 0.078 1.910 0.102 2.090 0.513 3.689 4.202 12.666 0.182 9.744 9.926 0.012 1.926 0.071 2.009 1.229 2.915 4.144 12.921 0.025 9.878 9.903 2.082 0.033 2.115 2.502 1.809 4.311 12.671 0.018 9.82 9.900 0.251 1.843 0.034 2.128 3.877 0.354 4.231 12.752 0.020 9.965 9.985 0.039 1.853 0.028 1.920 4.059 0.172 4.231 12.865 0.029 9.960 9.989 0.027 1.965 0.024 2.016 3.715 0.480 4.195 12.800 0.036 9.880 9.916 – 2.157 0.028 2.185 2.264 1.905 4.169 12.730 0.025 9.944 9.969 0.013 2.073 0.027 2.113 2.817 1.251 4.068 12.850 0.119 9.842 9.961 0.058 1.924 0.050 2.032 0.519 3.763 4.282 12.726 0.052 9.914 9.966 0.334 1.827 0.020 2.181 3.997 0.015 4.012 12.843

0.43 37.96 0.12 7.34 0.67 2.06 27.87 24.33 100.77 0.40 39.49 bdl 8.07 0.30 11.88 13.73 26.78 100.50 0.14 40.39 0.04 8.00 0.26 14.07 10.86 27.26 100.82 0.46 38.19 0.27 7.43 0.71 2.34 27.15 24.65 101.19 1.19 37.49 0.04 7.53 0.49 5.63 21.54 25.25 99.27 0.17 39.91 bdl 8.55 0.24 12.04 14.04 26.01 100.92 0.13 42.10 0.95 7.99 0.26 19.69 2.90 27.63 101.51 0.14 42.66 0.15 8.06 0.22 20.69 1.42 27.98 101.26 0.21 41.91 0.10 8.40 0.18 18.61 3.88 27.36 100.49 0.25 39.96 bdl 8.86 0.20 10.90 14.80 26.16 101.14 0.17 40.97 0.05 8.67 0.20 13.82 9.91 26.89 100.68 0.77 37.40 0.20 7.43 0.34 2.35 27.47 24.57 100.52 0.38 42.59 1.28 7.97 0.15 20.44 0.12 28.03 100.95 0.29 42.48 0.41 8.08 0.31 20.29 0.24 28.31 100.40

apfu apfu apfu apfu apfu apfu apfu apfu apfu apfu apfu apfu Ag Cu Total M+ Fe Zn Cd Total M++ As Sb Total SemiMe S

2 5 4 3 2

3

4

5

6

2

Massive barite

1 1

0.29 42.84 0.42 8.17 0.24 20.18 0.21 28.28 100.64 wt.% wt.% wt.% wt.% wt.% wt.% wt.% wt.% wt.%

9.2.4. Fahlore-group minerals The modal abundances of the tetrahedrite–tennantite series minerals are less than those of the common ore minerals such as sphalerite, galena, chalcopyrite and pyrite in the Istala ore. Three different generations of the fahlore-group minerals are identified based on their textural relationships. First generation fahlores are found in the first stage sphalerite (Fig. 8e) and represented by small sized grains (b 20 μm), whereas the second generation fahlores crystallized together with third stage chalcopyrite (Fig. 8f). Third generation

Massive black ore

9.2.3. Anilite, yarrowite and mckinstryite Considerable amounts of anilite, yarrowite and mckinstryite are observed in the brecciated ore level (Fig. 8c and d), which is the bornite- and chalcopyrite-rich section of the ore. Textural relationships and close association with bornite and chalcopyrite suggest that these minerals are the alteration products of bornite and/or chalcopyrite. Shalaby et al. (2004) also considered the mckinstryite as an alteration product of bornite and chalcopyrite, formed under 94 °C. The EPMA results are summarized in Table 3. The Ag content of anilite and yarrowite was found to be between 0.46 and 0.69 wt.%, and 3.28 and 5.65 wt.%, respectively. A single crystal of mckinstryite was also detected, with an Ag content of 48.67 wt.%.

Table 4 Representative EPMA results of tetrahedrite–tennantite serie minerals. bdl: below detection limit.

9.2.2. Covellite, chalcocite and digenite Although covellite, chalcocite and digenite are present in minor amounts in both the massive and brecciated ore levels, a clear enrichment of modal abundances of these minerals is observed in the brecciated ore compared to the massive black ore. These minerals are located in the fractures and the margins of copper sulfides, and are generally secondary in origin. However, myrmekitic ore textures between galena and covellite are the best indicators of the primary origin of covellite. The EPMA results indicate that Zn content is always less than 0.1 wt.%, whereas higher amounts of Ag, between 0.44 wt.% and 1.33 wt.%, are detected in covellite (Table 3).

6

9.2.1. Bornite Bornite is a dominant copper sulfide phase seen at the brecciated ore level, coexisting with chalcopyrite, tetrahedrite–tennantite and other forms of copper sulfides. Covellite, chalcocite and digenite are generally found around the bornite as alteration products. In some cases, bornite is partially or completely altered into covellite. Bornite is considered to be a hypogene due to the virtual absence of pyrite in bornite bearing ore samples and the presence of bornite exsolutions in chalcopyrite. According to the mineral chemistry, Zn and Ag are found in bornite as minor elements (Table 3). The Zn content is always constant, being less than 0.15 wt.%, whereas Ag content is higher and reaches up to 2.08 wt.%.

Ag Cu Fe Zn Cd As Sb S Total

9.2. Silver-bearing minerals

1

Brecciated ore

3

4

Some native silver grains, varying in size from 1 to 10 μm, were observed in the massive barite and brecciated ore levels. No clear relationship has been observed between the silver grains and other sulfides. However, the silver grains occur mostly in the copper-rich (near the bornite and chalcopyrite minerals) section of the ore (Fig. 8a). In addition to Ag, minor amounts of Cu (up to 0.35 wt.%) and Cd (up to 1.56 wt.%) were detected in some measured native silver grains (Table 3).

11.80 28.84 0.03 7.03 0.47 1.72 26.50 23.58 99.97

9.1. Native silver

0.039 9.924 9.963 0.108 1.858 0.039 2.005 3.983 0.030 4.013 13.019

5

9. Silver and silver-bearing minerals

0.039 9.987 10.026 0.110 1.875 0.031 2.016 3.952 0.025 3.977 12.979

6

high as 2.13 wt.%. The Zn, As and Ag contents never exceed 0.30, 0.26 and 0.18 wt.%, respectively. No distinct chemical variation has been detected in the different ore levels.

14.00 26.99 0.05 6.99 0.43 1.12 27.65 22.90 100.12

Y. Demir et al. / Ore Geology Reviews 53 (2013) 306–317

12.05 29.56 0.20 7.62 0.44 2.00 26.47 23.88 101.22

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Y. Demir et al. / Ore Geology Reviews 53 (2013) 306–317

fahlores are formed along with chalcopyrite, at the last stage of the ore forming process. They are volumetrically more abundant and coarser compared to those of first and second generation fahlores (Fig. 8f). Increasing amounts of tetrahedrite and tennantite have been observed in the brecciated ore level, especially chalcopyrite- and bornite-rich ore samples, compared to the galena- and sphalerite-rich massive black ore. Microchemical analyses results of the tetrahedrite–tennantite series minerals are given in Table 4. The Ag content of the fahlore group minerals is generally low (up to 1.19 wt.%), however, some tetrahedrite grains contain much higher Ag, reaching up to 14 wt.%. (Table 4, Fig. 9a). The measured Fe contents are lower than 0.81 wt.%. A wide range of Sb–As substitution has been determined in the fahlore minerals from the each ore levels (Fig. 9b). No compositional distinction has been observed, in terms of arsenic-rich, intermediate and antimony-rich endmembers of the fahlores, between the ore levels. 9.3. Undefined silver-bearing phases Four different undefined silver-bearing phases have been detected. These phases are observed at the margins of bornite (Fig. 8b), with grain sizes of 5–10 μm, at the brecciated ore level. The results of the EPMA studies of the undefined phases are given in Table 5. These data show that these phases contain high amounts of Ag, varying between 23.73 and 78.42 wt.%. Cu and Cd are the major components of the undefined phases (up to 51.51 wt.% and 0.68 wt.%, respectively). The calculated chemical formula of each phase is as follows: Un#1 = (Cu5.48Ag1.49)7S5; Un#2 = (Cu2.29Ag0.72)3S2; Un#3= (Cu4.52Ag1.48)6S5; and Un#4 = (Ag6.90Cd0.06Cu0.03)7S6. 10. Fluid-inclusion studies A total of 21 double-polished wafers for fluid inclusion study have been prepared from the ore samples collected from three different localities (B, C, and D level, Fig. 3b). Double polished wafers were prepared from quartz (5), sphalerite (4) and barite (12) containing samples to study the fluid composition and temperature ranges prevalent during the mineralization. We were not able to detect any primary fluid inclusions in 13 out of the 21 samples. Sphalerite minerals are almost devoid of visible fluid inclusions. Therefore, microthermometric measurements were carried out on only the remaining eight barite and quartz-containing samples. The fluid

a

315

Table 5 Electron probe micro analysis (EPMA) results of undefined silver-bearing phases. bdl: below detection limit. Silver-bearing undefined phases Un#1

Un#2

Un#3

Un#4

S Fe Cu Ag Cd Total

wt.% wt.% wt.% wt.% wt.% wt.%

23.91 bdl 51.51 23.73 0.27 99.51

22.24 bdl 50.76 27.22 0.33 100.61

26.46 bdl 47.24 26.32 0.30 100.34

20.43 bdl 0.19 78.42 0.68 99.81

S Fe Cu Ag Cd

apfu apfu apfu apfu apfu

5.014 – 5.484 1.488 0.008

1.978 – 2.292 0.722 0.014

4.987 – 4.518 1.483 0.022

6.008 – 0.027 6.904 0.056

inclusions examined were found to be liquid-rich, two-phase primary and secondary fluid inclusions at room temperature. This study is based primarily on fluid inclusions in barite and quartz, which appears as primary on textural grounds (Roedder, 1984; Van der Kerkhof and Hein, 2001). The primary inclusions are mostly 3–8 μm in diameter and occur as oval to elipsoidal shaped isolated inclusions. In contrast, the secondary inclusions occur as fracture-controlled arrays and are irregular in shape. All the observed inclusions contain an aqueous solution and a gas bubble that occupies ~10–20% of the total volume. The homogenization temperatures (Th) of the primary fluid inclusions were measured between 98 and 284 °C. Three different stages of mineralization have been recognized for the Istala mine with frequency peaks, at around 140, 190 and 240 °C (Fig. 10). Due to the small size of the two-phase L + V inclusions (less than 8 μm in size), no usable first melting (Tfm) and ice melting (Tm-ice) temperature determinations were completed. Hence, in this study, the initial melting of the frozen liquids and the ice-melting temperatures could not be determined precisely during the freezing experiments. Because the values were not corrected for pressure, the obtained Th values correspond to the minimum temperature of mineral formation. 11. Discussion The Istala ore, located in the lower part of the Late Cretaceous LFS, hosted by dacite, dacitic tuff, dacitic pyroclastic rocks and red biomicrite layers, has a marked similarity with the other VMS-type deposits found

b

Fig. 9. Compositional variations of fahlores on Ag versus Sb (a) and Sb versus As (b) plots. Diagonal line in Fig. 9b represents ideal (AsSb)4 stoichiometry.

316

Y. Demir et al. / Ore Geology Reviews 53 (2013) 306–317

Fig. 10. Frequency histogram showing the distribution of homogenization temperatures of the fluid inclusions in barite (cross lined area) and quartz (horizontal lined area) from different mineralization stages.

in northeastern Turkey, with reference to the host rock, ore-deposit shape, mineral paragenesis and intricate mineral composition, due to frequent replacement. All the VMS deposits in the region are associated with Upper Cretaceous dacitic rocks, and can be considered similar to the Kuroko deposits in Japan (Özgür and Schneider, 1988). A succession of massive black ore, massive barite layer and brecciated ore levels has been observed in the Istala ore. The brecciated ore level occurs stratigraphically above and is usually separated from the massive barite layers by sharp contacts. The barite fragments in the brecciated ore level have a distinct similarity with the underlying massive barite layers, considering the mineral paragenesis and textural relations of the included sulfide minerals. This may suggest that the barite fragments in the brecciated ore level originated from the massive barite layer because of the later stage hydrothermal fluids. Compared to massive black ore, the brecciated ore level is characterized by an enrichment of chalcopyrite, tetrahedrite–tennantite and bornite, in addition to a marked depletion in pyrite. These results also indicate that the hydrothermal fluids, from which the later stage ore formed, were responsible for the high amounts of silver deposition in the Istala mine. Sphalerite is considered to be the most refractory sulfide, potentially recording evidence for the conditions of its genesis (Grammatikopoulos et al., 2006). Iron contents in sphalerite in equilibrium with pyrite or pyrrhotite are known to depend on both the temperature and fugacity of sulfur (Czamanske, 1974; Lusk and Calder, 2004; Scott and Barnes, 1971). The later stage mineral assemblage at Istala consists of pyrite, bornite, idaite and covellite, corresponding to depositional temperature ranges from 200 to 568 °C and log fS2 values of ~−1 to −10 bar (Lusk and Bray, 2002). An extensively poor iron content of the sphalerite (b 0.89 wt.%) in the Istala ore can be interpreted as indicating low temperature and high sulfur fugacity conditions (Lusk and Calder, 2004; Scott and Barnes, 1971). The homogenization temperatures lower than 284 °C, as deduced from the fluid inclusions in barite and quartz support the theory of a lower formation temperature. The Zn/Cd ratios of sphalerite have been suggested as indicators of the classification of volcanosedimentary, hydrothermal, skarnhydrothermal and metamorphosed sedimentary deposits by several authors (Jonasson and Sangster, 1978; Viets et al., 1992; Xuexin, 1984; Zaw and Large, 1996). Gottesman and Kampe (2007) suggest that Zn/Cd ratios greater than 500 are inherited from basaltic source rocks, and moderate ratios (328–427) are from andesitic source rocks, whereas ratios lower than 250 are characteristically an indication of felsic sources. Taking into account the above classification, the lower Zn/Cd ratios of sphalerite (b163) in the Istala deposit may indicate the hydrothermal fluids that are linked to felsic source. The observed relationship between dacitic magmatism and ore deposition in the area

(Akçay and Arar, 1999; Akçay and Moon, 2001; Demir et al., 2008; Gökçe, 2001; Tüysüz, 2000) supports this hypothesis. Although the silver contents of galena in the Istala deposit are low (less than 0.29 wt.%), high amounts of silver have been detected in the other phases. The poor silver content of galena is suggested to be due to increasing antimony content of hydrothermal fluids that caused the crystallization of silver in the tetrahedrite–tennantite solid solution, prior to the crystallization of galena (Miller and Craig, 1983). However, Amcoff (1976, 1984) reported that silver and antimony are soluble in galena at temperatures above 390 °C, whereas these elements are more stable in sulfosalts at lower temperatures. The authors also state that the solubility of silver in galena is less than 2% below 200 °C. Similar findings have been reported by Sack and Goodell (2002) that the dissolution of silver in galena is quite difficult in conditions where there is a low Th. Based on these observations, the poor silver content of galena may be an indication of the lower temperature that existed at the time of ore formation. The measured Th (b284 °C) from the fluid-inclusion studies on barite and quartz minerals supports this hypothesis. 12. Conclusion In addition to the presence of large amounts of native silver grains, other phases such as tetrahedrite–tennantite, bornite, yarrowite, anilite, mckinstryite, covellite–chalcocite and digenite, in the Istala deposit were found to contain a significant amount of silver. Nevertheless, in the main ore forming minerals such as pyrite, chalcopyrite, sphalerite and galena, the silver content was below the detection limits of EPMA. Taking into account the modal abundances of tetrahedrite–tennantite (up to 10% in some samples) and its silver concentration (up to 14 wt.%), we conclude that tetrahedrite–tennantite is the main host for the silver present in the Istala ore. Interestingly, silver-bearing sulfides are more abundant in the brecciated ore level of the later stage ore. Therefore, we conclude that the reason for the high amount of silver content in the Istala deposit is due to the input of later stage copper-rich, low temperature hydrothermal fluids. Acknowledgments This study was financially supported by the Scientific Research Foundation of Karadeniz Technical University (Project# 2004.112.005.02). We thank Mahmud Tarkian greatly for giving us the opportunity to use the electron probe micro analysis laboratorie at the Institute of Mineralogy and Petrology of the Hamburg University, Germany. Oscar Thalhammer, and an anonymous referee reviewed this paper and provided valuable suggestions. Editorial comments by Dr. Franco Pirajno and Dr. Nigel

Y. Demir et al. / Ore Geology Reviews 53 (2013) 306–317

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