Journal of Asian Earth Sciences 27 (2006) 819–834 www.elsevier.com/locate/jaes

Geology and geochemistry of the Mendejin plutonic rocks, Mianeh, Iran A. Karimzadeh Somarin * Department of Geology, Faculty of Natural Sciences, Tabriz University, Tabriz, Iran Received 23 July 2004; accepted 28 August 2005

Abstract The Mendejin pluton is located in the Mianeh area, NW Iran, 550 km from Tehran. This pluton is probably of Oligo-Miocene age and is the result of extensive magmatism which occurred during and after the Alpine Orogeny. Similar plutons are common in the Alborz–Azarbaijan structural zone of Iran, and it is likely that there are concealed plutons related to this extensive Cenozoic magmatism, but due to their youth and low rates of erosion they have not yet been exposed. The Mendejin pluton is a composite body made up of four types of plutonic rocks: pink tonalite, grey tonalite, diorite and aplite. The pink tonalite is porphyritic and contains phenocrysts of plagioclase, K-feldspar and hornblende in a groundmass consisting of quartz, plagioclase, K-feldspar, hornblende, zircon, monazite, leucoxene, apatite and hematite. The grey porphyritic tonalite has more biotite, pyroxene and pyrite and less accessory phases compared with the pink tonalite. The diorite has a microporphyritic texture with phenocrysts of plagioclase, hornblende and augite. This rock also occurs as xenoliths in the Mendejin pluton. The aplitic dykes are the youngest magmatic products at Mendejin. The Mendejin tonalite contains more Cl, As, S, Cu, Ni and Zn than the global granite. These rocks are of I-type, peraluminous and calc-alkaline, with medium to high potassium, and were formed as part of a volcanic arc. The Mendejin pluton contains up to 8 ppb gold and could potentially have been the source of an economic gold deposit by leaching of Au from wall rocks and deposition in extensive hydrothermally altered marginal zones. q 2005 Elsevier Ltd. All rights reserved. Keywords: Mendejin; Iran; Plutonic rocks; Geochemistry

1. Introduction The Mendejin pluton is located at 550 km from Tehran in the Mianeh area, NW Iran (Fig. 1). This area has attracted exploration activity due to the presence of high Au contents in the rocks (Karimzadeh Somarin, 2004a). In the structural classification of Iran (Nabavi, 1976), the Mendejin area is located in the western part of the Alborz–Azarbaijan zone (Fig. 1). The Mendejin volcanic and plutonic rocks were intruded as part of an extensive magmatic event which occurred in the Cenozoic, during and after the Alpine Orogeny. The earliest significant volcanism in the Alborz–Azarbaijan zone occurred during the Cretaceous (Didon and Gemain, 1976; Babaie et al., 2001), but there was intense magmatic activity between the Eocene and Miocene. The most extensive volcanic activity, * Tel.: C98 411 3357267; fax: C98 411 3356027. E-mail address: [email protected].

1367-9120/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2005.08.004

mainly submarine, took place during the Eocene (Eftekharnezhad, 1975) when andesitic–dacitic lavas, pyroclastic rocks and tuffs, up to 4000 m thick, were deposited (Hrayama et al., 1966). It is believed that this volcanism was the result of a phase of extension which followed a Late Cretaceous compressional phase (Darvishzadeh, 1991). SSW–NNE compression again affected Azarbaijan during the Upper Oligocene (Didon and Gemain, 1976) and continued into the Miocene, with volcanism localized along regional faults. Volcanism continued during the Middle Miocene, the Pliocene and into the Quaternary. Plutons ranging in composition from gabbro to granite (Karimzadeh Somarin and Moayyed, 2002) were intruded into the Cretaceous sedimentary and Eocene volcanic rocks of the western Alborz– Azarbaijan zone during the Alpine Orogeny, from the Early Oligocene to the Miocene. Many of the plutons of the Alborz–Azarbaijan zone are associated with porphyry and skarn mineralization (Hezarkhani and Williams-Jones, 1998; Karimzadeh Somarin and Moayyed, 2002), but the mineralization associated with the Mendejin

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A.K. Somarin / Journal of Asian Earth Sciences 27 (2006) 819–834

Fig. 1. The location of the Mendejin area in the Alborz–Azarbaijan Zone of northern Iran.

pluton is less well known, as this pluton has not been systematically studied. The occurrence of both porphyry- and epithermal-type mineralization in NW Iran, related to the OligoMiocene magmatism, may indicate a transition between these two types of deposits in response to changes in the tectonic setting during the evolution of a volcanic arc. A similar transition is associated with economic gold deposits in the southwest Pacific (e.g. Carlile and Mitchell, 1994; White et al., 1995). Further research on the geology and geochemistry of this type of pluton and its associated hydrothermal alteration and mineralization may indicate the level to which the system is currently exposed and may assist our understanding of the genesis of Au mineralization and provide new exploration targets in magmatically active zones. This paper examines the geological and geochemical characteristics of the poorly exposed Mendejin pluton in a geologically active zone of Iran. The behavior of gold in the magma and possibility of Au mineralization around the Mendejin pluton will be discussed. 2. Geology of Mendejin The greater part of the Mendejin area is covered by Paleogene volcanic rocks (Fig. 2). The volcanic sequence is composed of tuffs and volcanoclastic rocks of dacite, andesite, basaltic andesite and basalt. Each volcanic lithology is found at

several levels in the stratigraphic column, suggesting multistage extrusion and possibly intrusion. Based on lithostratigraphic evidence and similarity of these rocks to the well-known Karaj Formation (Middle Eocene in age; Darvishzadeh, 1991), the Mendejin volcanic rocks are considered to be of Eocene–Oligocene in age. The Mendejin pluton cuts these Eocene–Oligocene volcanic rocks and was, therefore, intruded in post-Oligocene time (Fig. 3A). Similar Oligo-Miocene plutons, are common in Azarbaijan, Iran (Karimzadeh Somarin and Moayyed, 2002). The largest (w50 km2) outcrops 15 km to the south of Mendejin. It is probable that there are more plutons related to this extensive Cenozoic magmatism, but due to their youth and a low rate of erosion, they are not yet exposed at the surface. The pluton at Mendejin is 250 m in width, and dykes and apophyses cut the volcanic rocks, cropping out in the valley and in the surrounding hills to the west of the Mendejin village (Fig. 3B). Forceful emplacement of the Mendejin pluton is inferred from changes in the dip of the adjacent volcanic layers which range from 10–208 remote in the pluton, to 60–708 in the contact zone. The pluton at Mendejin is composed of four rock types distinguished by their color and mineralogy: pink tonalite, grey tonalite, diorite and aplite. The pink and grey tonalites have a gradational boundary. The contact between the diorite and tonalite was not seen. No distinct cross-cutting relations are

A.K. Somarin / Journal of Asian Earth Sciences 27 (2006) 819–834

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Fig. 2. Geological map of the Mendejin area, Mianeh, Iran.

seen between the various components of the Mendejin pluton, they are, therefore, considered to be broadly contemporaneous. However, aplite dykes cut the other members of the pluton and are considered to be the youngest component of the complex.

3. Petrography 3.1. Pink tonalite The pink color of the tonalite is due to a high proportion of pink K-feldspar. The pink tonalite is porphyritic,

composed of phenocrysts of plagioclase, K-feldspar and hornblende (up to 5 mm) in a medium-grained (2 mm) groundmass consisting of quartz, plagioclase, K-feldspar, with hornblende, zircon, monazite, leucoxene, apatite and hematite as accessory minerals (Fig. 4A). Clay minerals, sericite, calcite, chlorite and epidote are secondary phases. Some feldspar phenocrysts are perthitic. Quartz-Kfeldspar intergrowth is rarely found. Zircon and monazite occur either as aggregates or as single crystals. Apatite is found as elongate crystals, commonly associated with plagioclase. Pale to dark green hornblende (up to 5 vol.%) occurs both as phenocrysts and as a groundmass mineral. In

Fig. 3. A. The Mendejin pluton (light color) cuts the Eocene–Oligocene volcanic rocks. The contact is shown by line. B. Outcrop of the Mendejin pluton in the hills of the southwestern (arrow 1) and bottom of the western (arrow 2) valleys.

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Fig. 4. A. Photomicrograph of pink tonalite. B. Photomicrograph of grey tonalite. C. Uralitization of pyroxene grains in grey tonalite. D. Replacement of biotite by chlorite, sericite and magnetite. E. Replacement of pyroxene and hornblende by calcite and chlorite, respectively. F. Aplitic dykes cut the Mendejin pluton.

intensely altered samples feldspar phenocrysts are altered to clay minerals and sericite. Some xenomorphic quartz grains accompany sericite in altered feldspars suggesting the following reaction: 3NaAlSi3 O8 C 2HC C KC / KAl2 AlSi3 O10 ðOHÞ2 sericite ðmuscoviteÞ

albite

C 3NaC C 6SiO2 quartz

Epidote and calcite replace plagioclase. Ferromagnesian minerals, especially pyroxene, are replaced by chlorite, mainly penninite. 3.2. Grey tonalite Greenish to pinkish grey tonalite shows a granular, hypidiomorphic, porphyritic texture, with phenocrysts (up to 5 mm, rarely 12 mm) of plagioclase, K-feldspar, hornblende, biotite and pyroxene in a fine-grained groundmass (!1 mm) composed of plagioclase, K-feldspar, hornblende, pyrite, magnetite, hematite and ilmenite (Fig. 4B). Secondary minerals include epidote, chlorite, sericite, magnetite, hematite

and pyrite. Pyroxene grains (up to 2 mm) are idiomorphic to xenomorphic and are commonly fresh, although some show uralitization (Fig. 4C). Pyroxene is replaced by calcite along the cleavages and around the margins suggesting that the pyroxene is augite. Locally, chlorite replaces pyroxene as well. Biotite occurs as phenocrysts, commonly replaced by chlorite, sericite and magnetite, with only relics of primary biotite to be seen (Fig. 4D). Idiomorphic plagioclase phenocrysts, up to 3 mm in size, show compositional zoning. Locally, plagioclase is replaced by epidote and sericite along the cleavages. Idiomorphic to xenomorphic green hornblende grains are partially replaced by chlorite. K-feldspar grains (up to 1.5 mm, rarely 12 mm) are xenomorphic and unaltered. Some feldspar grains are perthitic. Quartz occurs as intergranular xenomorphic grains. Xenomorphic hematite, up to 0.5 mm, is disseminated in the groundmass. In contrast, idiomorphic to subidiomorphic magnetite, up to 0.5 mm, occurs mainly as an exsolution product within ilmenite. Pyrite crystals are sub-idiomorphic to xenomorphic up to 1.5 mm in size, and forming 1 vol.% of the rock, is replaced by hematite along the fractures and around the margins.

A.K. Somarin / Journal of Asian Earth Sciences 27 (2006) 819–834

3.3. Diorite Dark grey diorite shows a microporphyritic texture with phenocrysts (up to 3 mm) of plagioclase, hornblende and augite. Plagioclase is the most abundant phenocryst. The groundmass is composed of plagioclase, augite and hornblende as the main minerals, with apatite, biotite, quartz and K-feldspars as accessory phases. In hydrothermally altered samples, pyroxene and hornblende have been replaced by calcite and chlorite, respectively (Fig. 4E). In addition, calciteGsericite replace plagioclase. Some of the samples from the dioritic unit are granodioritic in chemical composition. 3.4. Aplite The Mendejin pluton is cut by aplitic dykes up to 15 cm thick (Fig. 4F). These dykes can be traced in outcrop for distances of up to 10 m. There is a sharp contact between the aplite dykes and the other rocks of the pluton. The aplite is composed of quartz and feldspar. In thin section, grain size varies from 120 mm in the centre of the dykes to 15 mm at the contact, suggesting rapid cooling of magma at the contact. The dykes show hydrothermal alteration similar to that in the coarser grained rocks of the Mendejin pluton, suggesting that they were emplaced before or during hydrothermal activity. A pinkish grey aplite with dark spots (hematite up to 0.5 mm) in hand specimen, occurs at the margin of the Mendejin pluton. This aplite shows a granular allotriomorphic texture and is composed of xenomorphic quartz, subidiomorphic plagioclase and xenomorphic to sub-idiomorphic K-feldspar. Zircon is the only accessory mineral. Plagioclase occurs rarely as phenocrysts. Some of the plagioclase shows compositional zoning, with calcite replacing the core, suggesting that there was an increase in Ca towards the centre of the crystal. Secondary pyrite, hematite and leucoxene are found as disseminated grains in the groundmass. 4. Xenoliths in the Mendejin pluton Dark-colored xenoliths of two types are common in the Mendejin pluton: (1) Andesitic xenoliths, rounded and up to 10 cm in diameter, occur in the marginal parts of the pluton. Hydrothermal alteration has resulted in the development of abundant hematite in the xenoliths, and carbonation (replacement of Ca-bearing minerals such as plagioclase and pyroxene by calcite) is seen in both xenoliths and the host rocks. The xenoliths usually have sharp outlines, but locally the outlines are irregular. In places, the andesitic xenoliths are partially assimilated and intruded by quartz – feldspar veins. (2) Dioritic xenoliths show a porphyritic texture, with phenocrysts of plagioclase, ortho- and clinopyroxene, hornblende and rare magnetite, up to 3 mm in size, in a groundmass of plagioclase (up to 1 mm), hornblende and

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pyroxene. Some idiomorphic pyroxene crystals are uralitized and sub-idiomorphic hornblende crystals are replaced by chlorite and hematite. Some plagioclase phenocrysts show poikilitic texture, with inclusions of pyroxene, magnetite and hematite.

5. Hydrothermal alteration zones Hydrothermally altered zones appearing as bleached patches on aerial photos, cover an extensive part of the Mendejin area. Six zones of alteration are seen in the volcanic wall rocks: phyllic, propylitic, carbonatized, sulfidized, silicified and argillic. The rocks of the pluton are also carbonatized and silicified, and show argillic alteration. Phyllic alteration is characterized by sericite, quartz, pyrite and green tourmaline (dravite) as an openspace filling or replacing magmatic minerals. Propylitic alteration is characterized by calcite, chlorite, epidote, kaolinite, illite, quartz, pyrite and sericite replacing pyroxene, hornblende, and plagioclase. Carbonatization is the most common and widespread mode of hydrothermal alteration at Mendejin. Calcite, as replacement mineral or filling open-spaces, is the main product of this alteration. Carbonatization is more intense in basic wall rocks than in felsic rocks, suggesting a lithological control on this process. Sulfidation is indicated by pyrite as disseminated crystals or locally in veins. The argillic alteration zone is composed mainly of kaolinite, illite and jarosite. Hydrothermal mineralization has produced veins with various mineral assemblages including calcite–pyrite, quartz–hematite–calcite–gypsum, quartz–pyrite, quartz–pyrite–epidote and chlorite–calcite–pyrite. Fluid inclusion studies show that the hydrothermal minerals were precipitated due to a reaction between a hydrothermal solution and the wall rocks, and during the mixing of a relatively saline fluid (4.34 wt% equivalent NaCl) with low-salinity ground water (0.35 wt% equivalent NaCl) at temperatures from 385 to 150 8C (Karimzadeh Somarin and Lentz, in preparation). Open-space filling sulfide mineralization, locally as pyrite stockwork veins, occurs mainly in the bottom of the western valley. Hypogene hematite is rare, whereas the supergene variety is very common. Detailed field studies show that sulfide mineralization is associated with the zones of hydrothermal alteration. Also, there is a close spatial relationship between the zones of mineralization and the outcrop of the Mendejin pluton, suggesting a genetic link. Gold is not found as a discrete mineral at Mendejin. The maximum gold value is 15 ppb in the alteration halo of quartz–calcite–pyrite–chlorite veins. However, microprobe analyses of hydrothermal pyrite grains shows up to 0.02% Au (Karimzadeh Somarin and Lentz, in preparation). In contrast to the other hydrothermal prospects in the region (Hajalilu, 1999), there is no Cu or Zn and Pb mineralization at Mendejin.

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Table 1 Chemical composition of the representative Mendejin plutonic rocks Rock type

Tonalite

SiO2 Al2O3 TiO2 Fe2O3 MgO K2O Na2O MnO CaO P2O5 LOI Total U (0.5) Th (0.5) Cl (5) S (2) Sn (0.05) Ba (5) Ce (3) Co (2) Cr (1) Cu (1) La (1) Ni (1) Pb (1) Rb (2) Sr (5) V (2) Y (2) Zr (2) Zn (1) Mo (3) As (2) Nd (1) Ga (1) Sb (0.2) Nb (1) Au (5) Cs (2) Hf (1) Sc (0.1) Ta (1) Sm (0.1) Eu (0.2) Tb (0.5) Yb (0.2) Lu (0.05)

60.83 13.66 0.62 5.77 3.62 3.65 2.25 0.22 4.89 0.22 3.80 99.52 5 13 121 144 36 690 67 15 92 62 33 26 15 144 334 97 24 218 62 7 32 22 20 6 17 !5 11 5 17 !1 5.2 1.4 0.9 3.3 0.51

Rock type SiO2 Al2O3 TiO2 Fe2O3 MgO K2O Na2O MnO CaO P2O5 LOI Total U Th

Altered aplite 63.56 13.48 0.59 5.16 4.21 3.96 2.29 0.11 4.05 0.19 2.32 99.91 5 15 57 89 32 800 72 16 113 33 34 35 14 98 291 86 25 235 66 5 14 30 19 2 14 6 10 7 16 3 5.7 1.3 0.5 3.7 0.57

61.53 13.86 0.61 6.03 5.13 3.66 2.36 0.17 3.94 0.21 2.16 99.66 3 12 168 196 2 480 57 17 87 22 27 33 12 88 328 108 24 211 79 7 42 20 18 5 12 8 12 5 15 !1 4.7 1.2 0.6 2.9 0.45

62.80 13.09 0.50 4.81 3.96 3.55 2.22 0.10 4.34 0.19 3.16 98.72 4 13 102 !2 3 760 69 16 81 59 32 32 28 132 311 74 24 222 87 5 12 22 18 2 13 !5 8 6 15 !1 5.1 1.2 !0.5 2.8 0.4

60.31 13.54 0.62 6.76 4.64 3.29 2.50 0.11 4.59 0.21 2.63 99.19 4 10 138 8 !2 950 48 18 85 7 23 33 10 92 427 116 20 197 36 6 41 21 18 3 8 !5 11 5 18 !1 4 1.2 !0.5 2.7 0.42

62.54 13.75 0.66 5.21 4.73 3.71 2.25 0.1 4.25 0.2 1.70 99.10 4 13 92 98 54 430 12 7

98

23 213 63

11 5

1

3.1

Diorite 56.88 13.71 0.82 8.63 3.52 3.00 2.09 0.09 7.14 0.20 3.49 99.57 3 8

64.73 13.94 0.49 3.70 2.67 4.18 2.25 0.15 3.29 0.14 3.99 99.53 5 16 64 !2 16 630 83 7 28 7 41 9 17 163 257 62 28 286 58 3 26 29 24 6 15 !5 8 7 10 2 5.6 1.2 0.7 3.3 0.5

54.72 13.45 0.86 3.42 1.78 1.80 4.57 0.06 9.23 0.41 9.38 99.68 2 6 24 !2 9 660 67 12 27 8 31 12 8 53 444 100 20 164 40 5 15 25 22 3 16 !5 !2 3 17 !1 5.5 1.4 1.4 2.6 0.4

Granodiorite 58.53 14.21 0.75 7.31 5.12 3.10 2.21 0.21 4.91 0.22 3.28 99.85 3 9

56.41 13.65 0.85 8.02 2.71 2.56 1.98 0.13 7.98 0.19 4.92 99.40 3 7

58.73 14.08 0.71 7.11 5.04 2.76 2.06 0.11 6.28 0.23 2.04 99.15 3 7

57.82 13.27 0.75 7.24 4.10 4.07 1.97 0.33 5.74 0.21 3.80 99.30 3 9

57.13 14.59 0.97 8.84 5.30 1.78 2.22 0.14 8.27 0.20 0.48 99.92 !0.5 5

64.70 13.50 0.41 3.80 3.39 4.55 2.42 0.12 3.12 0.16 3.58 99.75 5 16

65.20 13.85 0.39 3.10 2.51 4.61 2.19 0.14 3.16 0.13 4.07 99.35 5 17

55.55 14.62 0.76 9.35 1.90 1.17 1.82 0.06 4.03 0.18 9.41 98.85 !0.5 5

(continued on next page)

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Table 1 (continued) Rock type

Diorite

Cl S Sn Ba Ce Co Cr Cu La Ni Pb Rb Sr V Y Zr Zn Mo As Nd Ga Sb Nb Au Cs Hf Sc Ta Sm Eu Tb Yb Lu

116 271 !2 750 51 24 90 41 24 26 19 91 317 130 20 166 191 8 7 14 17 1 12 8 !2 4 21 !1 4.3 1.2 !0.5 2.9 0.44

Granodiorite 56 70 !2 480

89 185 15 610

98 120 !2 387

19

28

21

9

52

38

76

91

85

21 165 72

22 136 56

20 160 46

9 8

8 !5

10 7

2

2

2

2.8

2.6

2.7

38 189 !2 910 52 21 85 39 29 33 15 185 295 116 25 172 98 7 14 22 18 6 9 !5 7 5 19 !1 4.8 1.3 !0.5 3 0.46

100 128 13 440 43 29 116 56 17 28 11 53 315 196 16 104 101 9 9 13 17 1 3 !5 !2 3 29 !1 4.1 1.4 !0.5 2.6 0.39

Rock type

Silicified diorite

Argillic tonalite

Hydrothermal vein

SiO2 Al2O3 TiO2 Fe2O3 MgO K2O Na2O MnO CaO P2O5 LOI Total U Th Cl S Sn Ba Ce Co Cr Cu La Ni Pb Rb Sr V Y

59.3 11.7 0.8 7.0 3.7 4.8 1.8 0.3 5.4 0.2 4.6 99.6 3 9 50 615 !2 1000 48 20 86 44 26 31 13 224 234 117 27

66.66 14.08 0.41 2.96 0.60 5.80 1.49 0.05 2.99 0.10 4.32 99.46 5 29 100 342 33 650 91 6 35 9 43 23 27 218 120 49 37

7.84 3.12 0.24 3.91 0.90 0.16 0.51 0.44 45.03 0.05 37.50 99.70 1 1 70 2463 !2 13 34 8 !10 5 17 2 !3 9 246 39 12

58 43 17 590

83 30 !2 620

5

4

18

43

110

120

26 254 83

28 280 66

12 !5

13 !5

1

!1

3.4

3.6

23 1554 !2 240 48 32 47 98 22 17 3 48 90 130 15 106 31 8 22 18 19 2 7 6 5 3 15 2 4.3 1.3 !0.5 1.9 0.31 Carbonate zone

70.83 2.90 0.16 1.71 0.63 1.84 0.17 0.32 13.88 0.03 7.97 100.44 !0.5 !0.5 21 !2 36 220 4 2 29 5 3 12 10 130 68 19 14

4.17 1.23 0.16 1.57 0.65 0.21 0.07 0.54 47.91 0.06 38.44 95.01 !0.5 !0.5 58 26611 !2 100 45 2 !10 5 21 !2 7 13 196 17 13 (continued on next page)

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Table 1 (continued) Rock type

Silicified diorite

Zr Zn Mo As Nd Ga Sb Nb Au Cs Hf Sc Ta Sm Eu Tb Yb Lu

166 95 8 51 18 17 11 14 !5 5 4 20 !1 4.7 1.1 !0.5 2.9 0.44

Argillic tonalite

Hydrothermal vein

230 105 7 25 39 22 13 23 7 8 8 8 !1 6.5 1 !0.5 4.7 0.71

Carbonate zone

21 22 11 43 13 4 3 2 6 !2 !1 6 !1 3.4 1.2 !0.5 1.4 0.21

17 21 7 18 !5 5 43 2 9 2 !1 2 !1 0.6 K0.2 !0.5 0.5 0.07

16 15 11 14 21 2 1 2 8 !2 !1 2 !1 5.6 2.7 0.9 0.9 0.14

Oxides in percent, trace elements is ppm and Au in ppb. Detection limits for trace elements and Au are shown in parentheses in the first column.

6. Geochemistry 6.1. Methodology After macroscopic and microscopic study of the rocks from the Mendejin pluton, 17 fresh and 29 altered samples were selected and analysed for trace elements and major oxides by INAA and XRF at Actlabs (Canada) and Atomic Energy Organization (Iran), respectively. Samples which contained no or very few secondary minerals were considered to be fresh. The analytical results and the detection limits for representative samples are shown in Table 1. Br, Ag, Hg and Ir were below detection limit (1 ppm, 5 ppm, 1 ppm and 5 ppb, respectively) and are not shown in Table 1. 6.2. Results In Table 2 analyses from the Mendejin pluton are compared with two Cenozoic ore-associated plutons from NW Iran and average global granite (Krauskopf and Bird, 1995). The Mendejin tonalite (see below) contains more Cl, As, S, Cu, Ni and Zn than the global granite. It shows lower average Cl, Cu, Ni and Zn, but higher S compared to the Tikmeh Dash pluton. Also the Mendejin pluton contains lower Cu and Zn relative to the Mazraeh pluton. The Mendejin pluton shows

quantities of Au up to 8 ppb, which is higher than the Au content in similar lithologies in the gold-associated Pataz Batholith, Peru (up to 7.5 ppb; Schreiber et al., 1990) and goldassociated dioritic and granodioritic plutons at Burkina Faso, Africa (up to 1.9 ppb; Huot et al., 1987). The Au content of the volcanic wall rocks is up to 5 ppb (Karimzadeh Somarin and Lentz, in preparation). Cu, Pb and Zn are not strongly enriched in the Mendejin pluton. It is notable that the sample with highest Au (8 ppb) contains the highest Cl value (168 ppm). The mineralogical and chemical composition of the Mendejin pluton ranges from granodiorite to diorite and tonalite (Fig. 5). These rocks are I-type (Fig. 6a,b), peraluminous (Fig. 6c) and calc-alkaline, with a medium to high content of potassium (Fig. 6d). Although the Mendejin pluton shows the chemical signatures of crystal fractionation (see below), it is not highly fractionated (Fig. 7). Harker diagrams of the Mendejin plutonic rocks (Fig. 8) show that TiO2, Fe2O3, CaO S, Co, V, Mo and Sc decreased during crystallization, suggesting crystal fractionation of biotite, hornblende, pyroxene, plagioclase and titanomagnetite. Incompatible K2O, Th, U, Sn, Ce, La, Y, Zr, Nd, Nb, Lu, Yb, Sm and Hf were accumulated in the residual magma. These components formed K-feldspar, zircon, monazite and apatite at later stages in the crystallization of the magma. P2O5 shows incompatible behavior

Table 2 Chemical components of the Mendejin pluton compared with average global granite (after Krauskopf and Bird, 1995) and two plutons (Mazraeh and Tikmeh Dash) from NW Iran (data from Karimzadeh Somarin and Moayyed, 2002; Karimzadeh Somarin, 2004c) Location

Mendejin Mazraeh Tikmeh Dash Ave. global

Rock type

Tonalite Diorite Granodi-orite Grbbrod-iorite Granite

Au in ppb, others in ppm.

Au

Cl

As

S

Cu

Ni

Zn

Range

Ave.

Range

Ave.

Range

Ave.

Range

Ave.

Range

Ave.

Range

Ave.

!5–8 !5–8

57–168 23–116

106 79

12–42 7–22

28 13

!2–196 70–271

77 167

28 25

5–1333 70

585

–

2–260 58

44

32 59 246 46

9–35 17–33

– –

7–62 39–98 155–390 8–110 13

1–160 28

34

36–87 46–191 55–84 1–288 45

64 93 70 84

– 0.5

A.K. Somarin / Journal of Asian Earth Sciences 27 (2006) 819–834

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Fig. 5. R1–R2 diagram showing that chemical composition of the Mendejin pluton ranges from granodiorite to tonalite. One altered sample plots in the gabbro field (diagram after de la Roche et al., 1980).

up to SiO2Z57% and then decreases due to apatite fractionation at SiO2O57%. Au does not show a clear trend in the Au–SiO2 diagram. However, there is a relatively positive trend in the Au–MgO diagram of the Mendejin plutonic and volcanic rocks (Fig. 9) suggesting that mafic lithologies tend to contain more gold than felsic rocks. There are two explanations for such a trend: (1) This is the real fractionation trend of gold during fractional crystallization (Goodwin, 1984). (2) The trend reflects the sulfide content of the fractionated rocks (Saager and Meyer, 1984; Ropchan et al., 2002). Commonly, early fractionated products contain more sulfide (and therefore gold) than late-stage rocks (Ropchan et al., 2002). In this case, a positive correlation between Au and S would be expected. Such a trend can be seen in most of the analysed rocks in Fig. 10, especially at S!1000 ppm, suggesting that a sulfide mineral (pyrite in Mendejin) was the carrier of the gold. This agrees with microprobe analyses of the Mendejin pyrite (Karimzadeh Somarin and Lentz, in preparation). Also, this is in agreement with experimental studies which show that the differentiation coefficient of Au (DAu) between sulfide and magma can reach up to 3000 (e.g. Stone et al., 1990). D between gold and sulfide mineral depends strongly on fS2 and fO2. DAu decreases with increasing fO2 and decreasing fS2 (Moss et al., 2001). DAu is also affected by the type of sulfide mineral. For example, Au differentiates to bornite more than to chalcopyrite (Simon et al., 2000). Titanomagnetite has

also been reported as an Au accumulator in tholeiitic basalts (Togashi and Terashima, 1997). However, the positive correlation between Au and S and also microprobe analyses indicate that pyrite is the main carrier of Au at Mendejin (Karimzadeh Somarin and Lentz, in preparation). In contrast to Mendejin, Huot et al. (1987) analysed goldassociated volcanic and plutonic rocks of the Burkina Faso Green Belt, West Africa and found out that mafic lithologies (basalt) contain less Au than felsic rocks (rhyodacite). It appears that the behavior of gold in magma is chiefly controlled by fO2. In the oxidized magmas, sulfur is dominantly present as sulfate and therefore, chalcophile elements such as Au are retained in the melt (Richards, 2003). In a reduced magma, such as that of the pyrite-bearing Mendejin pluton, fS2 is high and gold is scavenged by sulfide minerals from the early stages of sulfide differentiation. As a consequence, mafic lithologies contain a higher Au content than felsic rocks. The Mendejin plutonic rocks show a flat REE pattern with high LREE and low HREE (Fig. 11). It seems that pyroxene fractionation has controlled the partial differentiation of LREE from HREE. Zircon is also able to deplete HREE (Rollinson, 1993). An Eu anomaly is not developed, reflecting the contrasting effects of feldspar (which develops negative Eu anomaly) and other minerals such as hornblende and pyroxene (which develop a positive anomaly) on Eu differentiation. Tectonomagmatic diagrams (Fig. 12) show that the Mendejin pluton formed as part of a volcanic arc, consistent with the tectonomagmatic setting proposed for the western Alborz–Azarbaijan in the Oligo-Miocene (Darvishzadeh, 1991; Mohajjel and Fergusson, 2000).

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Fig. 6. Selected geochemical diagrams showing the I-type (A, B; after Newberry et al., 1990), peraluminous (C) and medium to high potassium calc-alkaline (D) features of the Mendejin plutonic rocks.

7. Conclusions and results Regional geology of NW Iran (e.g. Hezarkhani and Williams-Jones, 1998; Mohajjel and Fergusson, 2000; Karimzadeh Somarin and Moayyed, 2002; Karimzadeh Somarin, 2004b) indicates that Eocene–Oligocene volcanism, followed

by Oligo-Miocene plutonism, resulted in the intrusion of granitic, granodioritic and gabbrodioritic plutons into earlier volcanic sequences. Cross-cutting relationships and the increasing dip of the volcanic layers in the vicinity of the contacts indicate that Mendejin pluton was emplaced forcefully. It is believed that the volcanic activity occurred along

Fig. 7. The Mendejin data plot in the unfractionated granite field (after Whalen et al., 1987). FG, fractionated granite; OGT, unfractionated granite. Total Fe as FeO.

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Fig. 8. Chemical variation diagrams for major oxides and trace elements of the Mendejin plutonic rocks. Oxides in wt%, trace elements in ppm with Au in ppb. Half of Au detection limit (5 ppb) has been used in Au-bearing diagram.

the regional faults (Darvishzadeh, 1991), represented by largescale (O3 km length) andesitic dykes in the Mendejin area. The Mendejin pluton is a component of the extensive magmatism which occurred in a volcanic arc setting during

a short period in the Cenozoic in the west Alborz–Azarbaijan structural zone. This plutonism was associated with voluminous volcanism covering an extensive part of NW Iran. Large to medium size plutons outcrop in several places (Karimzadeh

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Fig. 8 (continued)

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Fig. 8 (continued)

Somarin and Moayyed, 2002; Karimzadeh Somarin, 2004b), however, it appears that there are more concealed plutons beneath the volcanic sequences. Aplite dykes cutting the Mendejin pluton suggest multistage magmatism. Also, occurrence of the same volcanic lithologies at several levels in the stratigraphic column indicates multistage extrusion and possibly intrusion. However, due to their youth and the low rate of erosion many of these plutons have not yet been exposed. The I-type, peraluminous and medium- to high-potassium calc-alkaline granodiorite to tonalite and diorite in the Mendejin pluton resulted from the crystal fractionation of pyroxene, hornblende, biotite and plagioclase. The incompatible rare earth elements show a flat pattern indicating that fractional crystallization was the main magmatic mechanism. Lescuyer and Riou (1976) believe that in NW Iran partial melting of the upper crust at the end of Eocene formed a large volume of acidic magma, which erupted during the Late Oligocene. Similarly, it is suggested that the Au-associated volcanic and plutonic rocks of Indonesia originated from largescale crustal melting (e.g. Marcoux and Milesi, 1994). At Mendejin this suggestion is supported by the high Sn content (e.g. Taylor, 1979) in some samples, and the peraluminous

nature (Debon and Le Fort, 1983) of the plutonic rocks. In some parts of NW Iran, magma was emplaced at depths of 1– 3 km, forming granitic plutons, whereas in the faulted areas such as Mianeh, magma erupted at the surface as rhyolitic, dacitic, andesitic and basaltic lavas. Hydrothermal alteration, host rock geochemistry and age of the rocks at Mendejin are similar to those in the epithermal deposits of Indonesia (e.g. Kelian, Van Leeuwen et al., 1990; West Jawa, Marcoux and Miles, 1994; Masupa Ria, Thompson et al., 1994). However, in contrast, there is no strong Au mineralization at Mendejin. The presence of epithermal-type hydrothermal alteration zones at Mendejin (Karimzadeh Somarin and Lentz, in preparation) and of quartz-gold veins in the Sheikhdar Abad area (55 km west of Mendejin; Karimzadeh Somarin and Nokhodchi, 2004) within hydrothermally altered Eocene–Oligocene volcanic sequence, analogous to those at Mendejin, suggest a high potential for gold mineralization at Mendejin. However, field and geochemical studies show that there has been no strong gold mineralization at the level of the present erosion surface. It has been shown (e.g. Hedenquist et al., 1998; Hedenquist et al., 2000) that phyllic and argillic alteration (associated with hydrothermal veins) occur in the upper parts of hydrothermal

Fig. 9. Au–MgO diagram showing a relatively positive trend in the Mendejin plutonic and volcanic rocks.

Fig. 10. Au–S diagram showing positive correlation between Au and S especially at S!1000 ppm.

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Fig. 11. The Mendejin plutonic rocks show a flat REE pattern with high LREE and low HREE.

systems, surrounding a central pluton, such as that at Mendejin. It is, therefore, inferred that major, and possibly economic mineralization may occur at depths below the surface. This interpretation is supported by the occurrence of hydrothermal veins in the valley bottom at Mendejin, which is the most deeply eroded part of the Oligo-Miocene hydrothermal system. The Au content of the magma was controlled by sulfide differentiation. As a result, at Mendejin, basic rocks with a high sulfide content contain more Au than felsic rocks. This gold is of magmatic origin, and is accommodated in disseminated pyrite. Although the maximum content of Au at Mendejin (8 ppb) is lower than that in some plutonic rocks associated with gold mineralization (e.g. up to 284 ppb at Blueberry Hill, Canada; Grey and Hutchinson, 2001), an economic gold deposit could potentially have been formed by intense leaching of Au from the Mendejin rocks in an active hydrothermal system. This leaching

requires complete reaction between the hydrothermal solution and the wall rock. In structurally- and magmatically active zones such as Alborz–Azarbaijan (Shahabpour, 1999; Mohajjel and Fergusson, 2000; Babaie et al., 2001; Brunet and Cloetingh, 2003) and southwest Pacific (Carlile and Mitchell, 1994) where multistage plutonism and volcanism were common, it is likely that intense leaching of Au from the volcanic and plutonic rocks also took place. In such examples of successive volcanism and plutonism, as the magma cools, the reaction zone between the hydrothermal fluid and the country rocks moves downward (Moss et al., 2001). With the introduction of new batches of magma, the reaction zone migrates upward again. The vertical movement of successive hydrothermal systems causes a complete reaction between magmatic fluids and the wall rock, and each time more gold is leached from the rocks. Au may be carried as Cl and S complexes (Seward and Barnes, 1997) and

Fig. 12. Tectonomagmatic diagrams (after Pearce et al., 1984) suggesting a volcanic arc setting for the Mendejin pluton. All trace elements are in ppm. COLG, syncollision granite; VAG, volcanic arc granite; WPG, within plate granite; ORG, ocean ridge granite.

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