Epithermal systems of the Torud–Chah Shirin district, northern Iran: Ore-fluid evolution and geodynamic setting

Ore Geology Reviews 109 (2019) 253–275

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Epithermal systems of the Torud–Chah Shirin district, northern Iran: Orefluid evolution and geodynamic setting

T

Ebrahim TaleFazela, , Behzad Mehrabib, Majid GhasemiSianib ⁎

a b

Department of Geology, Faculty of Science, Bu-Ali Sina University, Hamadan, Iran Department of Geochemistry, Faculty of Earth Sciences, Kharazmi University, Tehran, Iran

ARTICLE INFO

ABSTRACT

Keywords: Epithermal systems Plate convergence Fluid evolution Torud–Chah Shirin district Iran

The Torud–Chah Shirin (TCS) ore district in northern Iran is defined by an NE alignment of Tertiary Ag-Au and base metal-rich epithermal systems, and it is part of the eastern Alborz orogenic belt of Iran. Intermediate- to high-sulfidation mineralization occurs as veins hosted by the Eocene−Oligocene volcanic, subvolcanic and volcaniclastic rocks. The TCS district is characterized by three fault system populations including ∼70°, ∼270°, and ∼340°−trending faults, and detailed structural mapping show that overall strike of the TCS vein system is 320°–340° but varies from ∼290° to ∼350°. The ∼N70°−trending faults are parallel to the Anjilow and Torud regional faults in the TCS ore district. Green- to grey-schist and metamorphosed dolomite and limestone are the oldest units (Ordovician–Silurian) in the TCS. Sedimentary rocks were initiated by limestone, dolomite and green shale in the Cretaceous and continued with conglomerate into the Paleocene (Fajan Formation). Magmatism mainly occurred sporadically from the Eocene to Oligocene (ca. 55–24 Ma), with two major episodes between early to middle Eocene (ca. 55 and 37 Ma, EME) and early to late Oligocene (ca. 34 and 24 Ma, ELO). Whole rock geochemical data of EME and ELO rocks of the TCS district shows a range from basalts to rhyolites with low-K calc-alkaline and shoshonitic affinity. Their rare earth elements (REEs) and high field strength elements (HFSE) signatures indicate the occurrence of a supra-subduction zone magmatism and all rocks have been sourced from the same parent melt. Samples from ELO display higher alkali contents compare with EME but have a similar trace element characteristics. Hydrothermal alteration is pervasive in volcanic and subvolcanic rocks but is mainly localized near the veins and ore zones. Alteration assemblages include quartz-sericite ( ± pyrite ± adularia), quartz-illite (kaolinite ± pyrite), carbonate-quartz and chlorite + calcite + epidote (propylitic). Homogenization temperatures of fluid inclusions in the epithermal prospects is in range of 125–375 °C, and ore-forming fluids were mainly of magmatic-hydrothermal origin (e.g., Gandy and Abolhassani prospects), with some contributions from meteoric water. The epithermal prospects have more or less similar salinities, ranging from 2 to 18 wt% NaCl equiv., with a distinct cluster between 6 and 15 wt% NaCl equiv. Our shreds of evidence suggest that the EME and ELO calc-alkaline volcanic activity in TCS occurs occurred in tension fractures related to orthogonal plate convergence that postdates the NE-trending strike-slip regional faults, and plays an important role in the development of the epithermal prospects. Simultaneous with, or soon after crustal heating related to the magmatism, strike-slip faults movement may also have been critical in the ore-forming process, leading to trans-tension of local compressive forces, enhancement of crust-scale permeability, and promotion of mixing of ore-forming fluids.

1. Introduction The Mesozoic to Cenozoic Alpine-Himalayan Orogenic Belt (Tethyan collage),stretches from the Alps, through the Balkan Peninsula, Turkey, Iran, Pakistan, Tibet, Indo-china and ultimately into the southwest Pacific (Jolivet et al., 1994; Stampfli et al., 1998;

⁎

Stampfli, 2000; Blundell et al., 2005; Schettino and Turco, 2011). This is also known as the Tethyan-Eurasian Metallogenic Belt (Jankovic, 1997; Jankovic and Petrascheck, 1987; Richards, 2015; Searle et al., 2016). Many deposits along the belt formed during short-lived magmatic events related to tectonic processes (Kouzmanov et al., 2005; von Quadt et al., 2005; Zimmerman et al., 2008; Kuscu et al., 2010;

Corresponding author. E-mail address: [email protected] (E. TaleFazel).

https://doi.org/10.1016/j.oregeorev.2019.04.014 Received 2 September 2018; Received in revised form 24 March 2019; Accepted 18 April 2019 Available online 20 April 2019 0169-1368/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Distribution of Tethyan ophiolitic and major epithermal deposits in Alpine–Himalayan orogenic belt (modified from Dilek and Furnes, 2009). Abbreviations: UDMA: Urumieh-Dokhtar Magmatic Arc, AMA: Alborz Magmatic Arc, KCB: Kerman Copper Belt.

and the development of the ore-forming fluid. The origin of deposits is discussed within the context of the regional geological evolution, with special reference to the northeastward subduction of the Neotethys beneath the Eurasian plate margin and the associated magmatic activity.

Richards et al., 2012). In the Iranian parts of the Tethyan-Eurasian Metallogenic Belt (Fig. 1), some epithermal- and Carlin-type gold deposits, occur, primarily, in the NW-SE trending Urumieh-Dokhtar Magmatic Arc (e.g., the Zarshuran and Agh Darreh at Takab Metallogenic Province; Mehrabi et al., 1999; Asadi et al., 2000; Daliran, 2008), the E-W trending Alborz Magmatic Arc (e.g., the Gandy and Abolhassani Au-epithermal prospects of the Torud-Chah Shirin (TCS) belt; Shamanian et al., 2004), and eastern Iran (e.g., the Sheikh Abad and Hired Au-epithermal deposits in the Lut Block; Richards et al., 2012). This study is focused on the E-W TCS belt in northern Iran (Fig. 1). We present a review of the main tectonic units and the geodynamic evolution of the TCS belt, an area that has received little attention in the western literature. The Cenozoic TCS magmatic district is a major ore province in the north of central Iran, eastern part of the Alborz mountain chain (Fig. 1), linked to subduction–related magmatism during the convergence of Africa–Arabia and Eurasia (Jankovic, 1997; Axen et al., 2001; Heinrich and Neubauer 2002; Richards et al., 2006). A regional scale stream sediment geochemical survey in the first half of the 1990s, by the Geological Survey of Iran (GSI), located a number of precious metal anomalies that subsequent work identified as epithermal-type prospects (Shamanian et al., 2004). The TCS exposes Eocene–Oligocene volcano–plutonic rocks that host a number of epithermal vein systems, some which are in various stages of exploration (i.e., Darestan, Cheshme Hafez, Pousideh, and Ghole Kaftaran) and some are abandoned mines, such as Gandy and Abolhassani. The nature and genesis of these prospects were not studied in detail and has only been reported in some preliminary studies (e.g., Hushmandzadeh et al., 1978; Valizadeh and Jafarian, 1991; Eshraghi, 1995), but are presently classified as epithermal deposits. The eastern part of the Alborz arc is currently a major mineral exploration target in Iran. Understanding the favorable geologic environments that produced the epithermal deposits in the TCS is a key for successful mineral exploration in a geologic setting that has experienced successive deformation events. We present new whole-rock geochemical, structural geology coupled with fluid inclusion data from the epithermal systems of the TCS district to characterize the Tertiary volcanic, sub-volcanic and volcaniclastic host rocks, tectonic evolution

2. Regional Tethyan tectonic evolution and geodynamic setting of the TCS district The Alborz mountain chain in northern Iran is part of the collisional Alpine–Himalayan orogenic belt which extends from Western Europe to Turkey, through Iran, and into western Afghanistan. This orogen resulted from the convergence of Africa, Arabia, and Europe and the closure of the Tethys Ocean in the past 100 million years (Dabovski et al., 1991; Ricou et al., 1998; Axen et al., 2001). Two significant geodynamic episodes have been recognized in the Tethys belt, i.e., the Paleo–Tethys and Neo–Tethys (e.g., Sengor, 1987). The Paleo–Tethys Ocean opened during the Ordovician, and its subduction to the north caused spreading in the south, the formation of Neo–Tethys I and rifting of continental fragments including Turkey, Iran, and Tibet from Gondwana (Hooper et al., 1994; Stampfli, 2000; Richards et al., 2006). These rifted fragments formed the Cimmerian super-terrane and included the Sanandaj–Sirjan zone, Lut microplate, and the Alborz magmatic arc of present-day Iran (Fig. 2). By the late Triassic (210 to 222 Ma), the Paleo–Tethys Ocean had been eliminated by subduction beneath the Eurasian continent, to which the Cimmerian super-terrane became attached (Glennie, 2006; Golonka, 2004). The Neo–Tethys I Ocean was subducted northward beneath the accreted margin of Eurasia in response to the opening of the Atlantic Ocean (Hooper et al., 1994; Glennie, 2006). By the late Cretaceous, southward subduction beneath Arabia and northward subduction beneath Eurasia contributed to the progressive destruction of this section of the Tethys Ocean and caused the late Cretaceous calc–alkaline magmatism and related porphyry and epithermal deposits in the Balkan Peninsula, Turkey, and Azerbaijan (Hou et al., 2011; Searle et al., 2016) but little mineralization in the Sanandaj–Sirjan zone. Glennie (2006) and Richards et al. (2006), consider that a small oceanic basin, Neo–Tethys II, opened between the Cimmerian super-terrane and Eurasia in 254

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a

Late Jurassic-early Cretaceous (ca. 140 Ma) Turan Plate

SCM

Middle Eocene (ca. 45 Ma)

Turan Plate TCS district

SP

SCM HM

Neo-Tethys

SS

0

1000 km Turan Plate

c

TCS district

MK 0

Middle Miocene (ca. 15 Ma)

Lut Microplate

SS

TCS district

+

HE

ez ar s l B ion ba u s Je Intr

++ ++

K In uh tr -p

HM Arabian Plate

0

Indian Ocean

Calc-alkaline volcanic center Calc-alkaline Plutonic rocks Torud-Chah Shirin

Lut

++usionasnj +

Persian Gulf

1000 km

HE

AL

SS

flysch basin MK

Trajectory of maximum horizontal principal stress Convergence direction

d

Turan Plate

SCM

+AL+

+

1000 km

+

+

SCM

HM

flysch basin

Arabian Plate

Oligocene-Miocene (ca. 23 Ma)

0

Lut Microplate

SS

Neo-Tethys

Arabian Plate

HE

AL man and-O Nayb ult fa

Lut Microplate

b

Persian Gulf

1000 km

HM flysch basin MK

Indian Ocean

Normal fault Transform fault Subduction/collision/thrusting

Fig. 2. a–d) Evolutionary sketches of the closure of the Neo–Tethys Ocean and Alpine–Himalayan collision in Iran during the, a) Late Jurassic-early Cretaceous, b) Middle Eocene, c) Oligocene-Miocene, d) Middle Miocene (Golonka, 2004; Shafiei et al., 2009). Note the Eocene development of the main TCS district and the South Caspian Microcontinent (SCM) due to subduction of the northern branch of Neo–Tethys Ocean beneath the Turan plate. Abbreviations: SCM–South Caspian Microcontinent, SS–Sanandaj-Sirjan, HE–Herat, HM–Helmand, MK–Makran, AL–Alborz, SP–South Pamir.

the late Triassic and this super-terrane began to break up to form an archipelago consisting of the Sanandaj–Sirjan zone in southwest and the Lut microplate in the northeast (Fig. 2). Closure of Neo–Tethys II was completed by the Late Cretaceous (Maastrichtian) (Glennie, 2006; Richards, 2003) or late Oligocene (Hooper et al., 1994; Agard et al., 2005), resulting in accretion of the Sanandaj–Sirjan and Lut microplate onto the margin of Arabian plate along the Zagros suture zone. Calc–alkaline to alkaline magmatism in the TCS occurred in the early Eocene to late Oligocene, which has been generally related to north to northeast subduction of the Lut microplate beneath the Eurasian plate (Turan plate) and development of numerous polymetallic epithermal prospects (Eshraghi, 1995; Shamanian et al., 2004) (Fig. 3). Northward subduction of Neo–Tethys II beneath of the Urumieh–Dokhtar Magmatic Arc (UDMA) in the late Cretaceous or Paleocene, resulted in extensive arc magmatism along the belt in the Eocene through to the Miocene (Glennie, 2006; Mohajjel et al., 2003; Richards et al., 2006). Porphyry Cu–Au related calc–alkaline intrusions in the UDMB (e.g., Sarcheshmeh and Sungun) occurred toward the end of this period mainly in the Oligocene–middle Miocene (McInnes et al.,

2003; Yaghubpur, 2003; Calagari, 2003, 2004; Zarasvandi et al., 2005). This appears to be related to the final phase of arc magmatism due to collision and crustal thickening (Shafiei et al., 2009). Final closure of the Neo–Tethys Ocean is recorded by numerous oceanic relicts (ophiolites and ophiolite mèlanges) along a series of the Mesozoic sutures; from west to east, the Bitlis suture in Turkey, the Zagros suture in Iran, the Bela–Waziristan–Quetta suture in Pakistan and the Indus–Yarlung–Zangbo suture in Tibet (Sengor, 1987; Sengor and Natalin, 1996; Sorkhabi and Heydari, 2008; Fig. 3). Several economically significant porphyry Cu belts (after Clark, 1993) and associated epithermal polymetallic deposits have been discovered along the Alpine–Himalayan orogenic system, spatially related with these suture zones (Fig. 3). 3. Geology and metallogeny of the TCS district The NE-trending TCS belt, lying in the central to the eastern portion of the Alborz mountain system is approximately 300 km2 in length, with a complex tectonic, magmatic, and stratigraphic history (summarized by Alavi, 1996). The pre–Mesozoic basement rocks of the TCS 255

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10E E Alps

30E

Budapest

Zagreb POD

40N

Istanbul

Mediterranean Sea

40E

50E

60E

Banat-Timok-Srednogrie (BTS) belt

70E

Black Sea B

Tehran T

Delhi CPCB

KPCB: Kerman Porphyry Cu Belt CPCB: Chagai Porphyry Cu Belt

ARABIA

Karachi Muscat

0

30N

GPCB

Shiraz

GPCB: Gangdese Porphyry Cu Belt

Lhasa

TCS

Baghdad

20N

40N

N

Kouhistan Island Arc

Izmir

AFRICA

100E

Samarghand

Taurides

30N

90E

EURASIA

Ankara

Agean Sea

80E

Gulf of Oman

600 km

APCB: Arasbaran Porphyry Cu Belt YPCB: Yulong Porphyry Cu Belt POD: Panagyurishte Ore District

TCS: Torud-Chah Shirin porphyry deposits/prospects epithermal deposits/prospects

INDIA

Bay of Bengal

20N

Indian Ocean

volcanic arc/metallogenic belt Tethyan ophiolites Tethyan suture zones

Fig. 3. Distribution of collision-related Cenozoic epithermal- and porphyry-type deposits in the Alpine–Himalayan orogenic system (modified from Singer et al., 2005; Dilek and Furnes, 2009; Hou et al., 2011).

(Figs. 4 and 5) consist of amphibolites, schists, and gneisses of uncertain Precambrian age (Crawford, 1977; Hushmandzadeh et al., 1978), Cambrian crystallized cherty dolomite, Silurian to Devonian units shale, limestone and trachyandesite (Niur Formation), sandstone and shale (Padeha Formation), dolomite, limestone and gypsum (Bahram

Formation) and Permian limestone and dolomite (Hushmandzadeh et al., 1978; GSI-Geological Survey of Iran (1978). All these units were subjected to ductile deformation and associated low-grade metamorphism at ∼100 Ma (Hassanzadeh et al., 2002) and are unconformably overlain by early to late Cretaceous conglomerate and

Fig. 4. Simplified geology map of the TCS ore district, showing the main rock units, main areas of mineralization, and related structures (based on 1:250,000 geologic map of Torud, GSI-Geological Survey of Iran, 1978).

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Fig. 5. Schematic stratigraphic column of the TCS district, showing the main sequences, dominant rock type, magmatic events (EME and ELO) and mineralization (modified from Hushmandzadeh et al., 1978; Shamanian et al., 2004). Abbreviations: rrhyolite, da-dacite, pd-porphyric dacite, qd-quartz diorite, mz-monzodiorite, EME-early to middle Eocene, ELO-early to late Oligocene.

limestone and Paleocene polymictic conglomerate (Fig. 5). These early sedimentary and metamorphic rocks are in turn covered by Eocene–Oligocene magmatic rocks, subvolcanic and effusive rocks, over the whole area. However, intrusive rocks are more abundant in the north (e.g., Baghu area) and southwest of the TCS district (e.g., Kuh-e Doshakh, north of Gandy and Zereshk Kuh) (Fig. 6). Andesite and trachyandesite predominate in the northeast and central TCS, whereas dacite and rhyodacite domes are more abundant in the southwest sector (Amidi et al., 1984; Eshraghi and Jalali, 2006). Subvolcanic dacite and rhyodacite, quartz monzodiorite, and alkali granite intrusions are comagmatic with the Eocene–Oligocene volcanic rocks (Fig. 6). Detrital limestone, nummulitic conglomerate and sandstone with abundant volcanic rock fragments are interbedded with the lower to middle Eocene volcanic rocks (Fig. 5). These sedimentary rocks are Ypresian to Lutetian in age, based on paleontological dating by Partoazar et al. (2006). According to Berberian and King (1981) and Hassanzadeh et al. (2002), this sedimentation could have occurred in an intra-arc basin, related to the destabilization of the volcanic edifice. The intra–arc spreading formed sedimentary basins between the Alborz mountain chain and Central Iran, which is characterized by Oligocene alkaline magmatism (Alavi, 1996; Axen et al., 2001; Hassanzadeh et al., 2002). The major ore deposits of the TCS district (Fig. 6) include many mineral occurrences and abandoned mines, particularly epithermal precious and base metal veins, hosted by volcanic and subvolcanic alkaline rocks, such as Gandy (Au–Ag ± Pb–Zn), Abolhassani (Pb–Zn ± Ag ± Au), Cheshmeh Hafez (Pb–Zn–Cu ± Ag), Ghole Kaftaran (Pb–Zn–Cu ± Ag), Pousideh (Cu ± Au), Darestan (Au–Ag ± Cu) and Chahmessi (Cu) (Shamanian et al., 2004; Fard et al., 2006; Mehrabi and Ghasemi, 2012). In addition, other types of deposit in the district include turquoise and placer gold at Baghu (Au ± Cu), Fe–skarn deposits at Chalu, and Pb–Zn (Ag) carbonate-hosted deposits

(MVT) at Reshm, Khanjar and Anarou (Shamanian et al., 2004; Niroomand et al., 2018) (Fig. 6). The Baghu Au ± Cu deposit in the north of the TCS is hosted by subvolcanic to hypabyssal intrusions (quartz diorite and quartz monzodiorite), and locally by volcanic and crystalline basement rocks indicating proximity to a porphyry intrusion-related Au ± Cu deposit (Rashidnejad, 1992; Shamanian et al., 2004; Niroomand et al., 2018). Our data and previous studies (e.g. Hushmandzadeh et al., 1978; Eshraghi, 1995; Hassanzadeh et al., 2002; Fard et al., 2006; Mehrabi and Ghasemi, 2012) indicate that most of the Tertiary intrusion-related gold and epithermal prospects of the TCS district are linked with early to middle Eocene (ca. 55 and 37 Ma, EME) calc-alkaline and early to late Oligocene (ca. 34 and 24 Ma, ELO) alkaline magmatic rocks that have evolved through assimilation and fractional crystallization (AFC) processes at shallow crustal levels (< 20 km). 4. Epithermal systems in the TCS district Epithermal base- and precious-metal deposits are hosted by EoceneOligocene volcanic rocks (Table 1) in the NE-SW trending TCS magmatic district. This represents the Eocene–Oligocene island-arc of the Alborz mountain belt, developed as a consequence of a short period of northwestward subduction of the Neotethys beneath the Eurasian plate (Berberian, 1974; Alavi, 1996; Hassanzadeh et al., 2002; Allen et al., 2004). Calc-alkaline volcanic and volcaniclastic units, dated as Ypresian to Chattian (i.e., Early Eocene to Late Oligocene), developed in the island-arc of the TCS district (Amidi et al., 1984; Axen et al., 2001; Davidson et al., 2004) whereas economic mineralization occurs in Eocene rocks of Ypresian and Lutetian age. The available literature on the geology of these deposits is scant as only general descriptions can be found in Hushmandzadeh et al. (1978) and Eshraghi and Jalali (2006). 257

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Fig. 6. Simplified geological map of the TCS ore district with A-B cross section (modified after GSI-Geological Survey of Iran, 2006), showing the location of various ore deposits and prospects of major precious- and base-metal epithermal systems.

The six main epithermal deposit/prospects of the TCS district, from northeast to southwest include Ghole Kaftaran (Pb–Zn–Cu ± Ag), Darestan (Au–Ag ± Cu), Pousideh (Cu ± Au), Cheshmeh Hafez (Pb–Zn–Cu ± Ag), Abolhassani (Pb–Zn ± Ag ± Au), and Gandy (Au–Ag ± Pb–Zn) deposit (Table 1), which all are hosted by EoceneOligocene volcanic rocks (Fig. 6). Exploitation of the TCS ore deposits dates back to mid 19th century (Hushmandzadeh et al., 1978), with a number of different state and privately owned companies operating the mines since then. Accurate production statistics are difficult to estimate, but taking all available data into account, at least 100,000 tons of copper, lead and zinc were mined in the district since 1960 (Borna and Eshghabadi, 1998). Today, metal extraction from these mineral prospects has ceased, but streamsurvey geochemical exploration has been carried out in this area as a prelude for future exploration.

flows of basaltic andesite to rhyodacite composition (Fig. 7). Mineralization in Darestan occurs in NNW trending structurally-controlled veins and preferentially as replacement of the host rock matrix in clastic rocks (Fig. 7). Textures within individual veins can be variable and hydraulic breccia textures with fragmented pyrite have been observed in places. The ore-bearing volcano-sedimentary sequence hosting the Darestan mineralization is pervasively altered to chlorite, illite/muscovite, carbonate, and epidote, with more abundant sericite close to the ore and in the upper parts of the deposit. Chalcopyrite and pyrite with minor chalcocite, covellite, Ag–rich tetrahedrite and electrum in a gangue of quartz and carbonate are the dominant minerals. Trace minerals such as emplectite (CuBiS2), wittichenite (Cu3BiS3), tellurobismuthite (Bi2Te3), tetradymite (Bi2Te2S) and native gold have also been observed in the deposit. The Pousideh Cu ± Au deposit, located about 5 km north of Cheshme Hafez, with mineralization hosted by Middle to Late Eocene andesitic to dacitic porphyritic lava flows (Fig. 6). The host rocks show argillic and residual quartz alteration with local developments of dickite/narcite, alunite, and pyrophyllite. Vein, veinlets and open-space filling mineralization dominate in the Pousideh deposit whereas most of the veins and veinlets show roughly an east-west orientation with 50° to 70° dip to the south. The Pousideh deposit is characterized by a mineral assemblage that is indicative of high sulfidation and includes pyrite,

4.1. Mineralization 4.1.1. Darestan and Pousideh The high sulfidation Darestan Au–Ag ± Cu prospect is situated in the central part of the TCS district about 4 km north of Pousideh (Fig. 6). Mineralization is hosted by Early to Middle Eocene volcanic and volcaniclastic rocks, brecciated lava, bedded hyaloclastite and lava 258

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200

Mehrabi and Ghasemi (2012) 40 (Pb) 23 (Zn) 15 (Cu)

4.1.2. Cheshme Hafez The intermediate sulfidation polymetallic breccia vein system of the Cheshme Hafez Pb − Zn − Cu ± Ag prospect is located in the southcentral part of the TCS district (Fig. 6). The mineralized veins of this deposit are hosted by subvolcanic porphyritic andesite, dacite, basaltic andesite and trachyandesite (Fig. 8), and extend for 1000 m to the NNW with a dip 50° to 60° to the southwest. The veins are extensional with a phyllic/argillic alteration halo. Sericite and dickite are commonly associated with mineralization in the central parts of the deposit, whereas chlorite, carbonate, pyrite ± epidote alteration is dominant in the more distal parts. Pyrite, chalcopyrite, sphalerite, Ag–rich tetrahedrite and galena are the main ore minerals, with minor electrum associated with the late stage carbonate. Pyrite occasionally contains up to 40 μm sized inclusions of digenite, bornite, chalcocite, and enargite. Quartz, carbonates, rhodonite, and barite are the main gangue minerals. In the northeastern part of the deposit, in the Gardaneh Toto area (Fig. 8), quartz/illite (kaolinite ± pyrite) alteration assemblages appear within the Lutetian dacite and rhyodacite host rock. With increasing distance from these veins, kaolinite, dickite/nacrite, and sericite occur with minor hematite, goethite and supergene limonite. According to Mehrabi and Ghasemi (2012), mineralization in the Cheshme Hafez deposit has a higher grade near the basaltic andesite dikes.

Galena, pyrite, chalcopyrite, rhodochrosite

Quartz, carbonate, sericite, barite Quartz, calcite, barite Sericite, dickite, chlorite, carbonate, pyrite ± epidote

4.1.3. Abolhassani and Gandy Mineralization in the Abolhassani Pb–Zn ± Ag ± Au and Gandy Au–Ag ± Pb–Zn abandoned mines occurs as shallow, massive lensshaped replacement bodies and as east-west striking and steeply dipping veins and breccias. Mineralization increases with depth and the massive ore bodies occur between tuff and andesitic flows, whereas in Gandy the ore lenses are found in the volcaniclastic and intermediate lava flows (Fig. 9). In the immediate vicinity of the mineralization, the host rocks are altered to an assemblage of kaolinite–sericite–carbonate ± chlorite. Mineralization in the lower part of the Abolhassani and Gandy deposits is enriched in lead and zinc ( ± silver). The Abolhassani and Gandy deposits have been classified as intermediate sulfidation based on their mineral assemblage (Shamanian et al., 2004; Fard et al., 2006). The main ore minerals in the deposits are chalcopyrite, pyrite, sphalerite, galena, and chalcocite. Trace minerals include bornite, tennantite, emplectite (CuBiS2), argentite, and native gold. The major gangue minerals are quartz, chlorite, and sericite, with barite, calcite, dolomite, chalcedony, and epidote present in minor amounts. Three mineralization stages were distinguished by Shamanian et al. (2004); an early quartz–pyrite–chalcopyrite stage, followed by the economic polymetallic stage (quartz–galena–sphalerite–chalcopyrite) and late quartz–pyrite–calcite mineralization. The Abolhassani mine area has two old tunnels, named Abolhassani and Cheshmeh Sefid, which provide direct access to the expose mineralized veins and breccias.

NNW- and NEtrending shear zone

Pb–Zn–Cu ± Ag

Pb-Zn-Cu ± Ag

Ghole Kaftaran

Cheshme Hafez

Spilite basalt, andesite breccia tuff, basalt

Pb-Zn ± Ag ± Au Abolhassani

Porphyric andesite, andesitic basalt

Au-Ag ± Pb-Zn Gandy

Trachyandesite, dacite, rhyolite

Cu ± Au Pousideh

Dacite, rhyodacite, and trachyandesite

NNW-trending faults

Sericite, dickite, pyrophyllite and quartz

Galena, sphalerite, chalcopyrite, pyrite, tennantite

20–50 Quartz, epidote, chlorite Galena, sphalerite, pyrite Carbonate, kaolinite, epidote EW- and NWtrending faults Andesitic flows and subvolcanic rocks

tennantite, tetrahedrite, chalcopyrite, specular hematite and minor galena and lautite (CuAsS) as main ore minerals. Minor amounts of bornite, digenite, covellite, enargite, native silver, and tellurides have also been observed. Quartz is the dominant gangue mineral, with minor barite and calcite occurring sporadically.

20–100

Mousavi (2009)

Shamanian et al., (2004)

Shamanian et al. (2004); Fard et al. (2006)

14.5 (Au) 31 (Pb) 8.4 (Zn) 64 (Pb) 12 (Zn) 8.3 (Cu) 0.85 (Au) 44 (Pb) 21 (Zn) 9.2 (Cu) 20–100 Quartz, calcite, barite Pyrite, sphalerite, galena, native gold NE-trending breccia zone

Au–Ag ± Cu Darestan

Volcaniclastic, lapilli tuff, volcanic breccia Andesitic and trachyandesitic flows

NNW-trending faults

Trachyandesite, trachyandesite basalt, volcanic breccia Rhyolite to rhyodacite domes

Kaolinite–sericite–carbonate ± chlorite

30–50 Quartz, barite, calcite

32 (Cu) 7.6 (Au)

Tajeddin (1999), IMRCIranian Mineral Research Corporation (2007) IMRC-Iranian Mineral Research Corporation (2007) 7.2 (Cu) 2.3 (Au) 400 Quartz, calcite

Chalcopyrite, pyrite, chalcocite, electrum, Agtetrahedrite Pyrite, tennantite–tetrahedrite, chalcopyrite, hematite NNW-trending faults Volcaniclastic rocks

Andesite, andesitic-dacite breccia Andesitic to dacitic porphyritic lava flows

Chlorite, illite/muscovite, carbonate, epidote, and abundant sericite Argillic, residual quartz, dickite/ narcite, alunite and pyrophyllite

Gangue minerals Ore controlling structure Associated magmatism Host rock Metals Ore deposit

Table 1 Characteristics of the Tertiary epithermal gold-base metal prospects in TCS ore district.

Alteration assemblages

Ore minerals

Vein length (m)

Average grade (%) (Au: g/t)

References

E. TaleFazel, et al.

4.1.4. Ghole Kaftaran Mineralization in the Ghole Kaftaran intermediate sulfidation Pb–Zn–Cu ± Ag vein system is hosted by Ypresian−Lutetian dacite, rhyodacite, and trachyandesite. Subvolcanic quartz dacite domes (Early Oligocene) intrude the rocks hosting the mineralization (Fig. 10), and the brecciated contact between these is in an N10–15°W direction dipping steeply to the northeast (up to 60°). Mineralized normal faults structurally control the Ghole Kaftaran deposit (Fig. 10) acting as a major channel for the mineralizing fluids. The NNW-striking veins contain galena, sphalerite, chalcopyrite, pyrite, tennantite and tetrahedrite as the main minerals, with lesser amounts of bornite, chalcocite, 259

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F1 faults have a ∼70° orientation, and an average dip of 75° N to NW (Fig. 11) which is parallel to the NE–alignment of the Gandy vein deposit in the TCS (Fig. 11). The Reshm–Pirmardan thrust fault is parallel to F1 faults and is associated with displacement of the Paleozoic and Mesozoic units in the south and southwest of the TCS (Fig. 6). This fault system is also characterized in some places by sinistral strikeslip movement along the major Reshm–Pirmardan thrust, as shown by the thrust slices in the sandstone and calc–schist metamorphosed host rocks of the Abolhassani deposit in the southwest. The Au (Cu) quartz–sulfide vein mineralization in the Baghu mine area (north of the TCS) is 700 m long and 10 to 30 cm wide, with a ∼50° orientation parallel to the NE alignment of the major Baghu fault, and dipping ∼70° to the SE (Fig. 11). In the Gandy gold deposit, mineralization breccia zone is economically important with gold averaging 14.5 g/t, and base metal sulfides. Mineralization is tabular shape with a strike ∼60°, and dipping 60° to 90° SE, parallel to the Gandy strike-slip reverse fault. The breccia zone is 20 to 100 m long with a variable thickness of 10 cm to 1 m (Shamanian et al., 2004). 4.2.2. ∼340° faults Faults termed F2 in this study (highlighted in blue in Fig. 11), are parallel to the regional NNW alignment of the TCS district ore deposits (e.g., Cheshme Hafez and Ghole Kaftaran sinistral strike-slip faults). The Darestan and Pousideh fault-vein sets are parallel to the F2 system faults. Our detailed data has separated the F2 fault set into two, with slightly different orientations; one has a strike and dip averaging 340°/ 50°, the other 300°/63° (Fig. 12). The Cheshme Hafez Pb−Zn−Cu ± Ag quartz–sulfide breccia has a strike of 330 to 350° and a dip of ∼40 to 60° to the SW corresponding to the NNW alignment of the Cheshme Hafez fault. The Au–Ag ± Cu vein mineralization of the Darestan deposit corresponds with the Darestan strike-slip fault which has a strike and dip of 340°/∼60°. Mineralization occurs in a brecciated tuff which has a 400 m length and a 30 cm to 1.5 m thickness. The Ghole Kaftaran Pb–Zn–Cu ± Ag vein occurs over a strike length of 200 m with an average width of 1.0 to 1.5 m and has an average strike of ∼ 330° with dips between 60 and 70° to the NE and SW.

Fig. 7. Simplified geological map of the Baghu-Darestan area (modified after Tajeddin, 1999).

electrum, stomeyerite (AgCuS) and jalpaite (Ag3CuS2). Quartz, carbonates, sericite, and barite are main gangue minerals. Compared with the other intermediate sulfidation deposits of the TCS, Ghole Kaftaran has a more acidic alteration style with the development of sericite, dickite, pyrophyllite, and quartz.

4.2.3. ∼280° faults Faults termed F3 in this study (highlighted in brown in Fig. 11), are essentially sub-vertical and parallel to the regional E–W trend, and include the Astaneh–Amru (Chalu) dextral strike–slip fault. F3 faults have a strike and dip of ∼280°/70° S to SW. These generally caused displacement of the Cretaceous carbonaceous rocks and the middle Eocene volcanic rocks (trachyandesite) in the southwest of TCS (Figs. 6 and 11). The orientation of precious and base metal vein mineralization in the Abolhassani and Cheshmeh Sefid deposits is parallel to the EW to NW trending faults in the TCS district. The Abolhassani veins have a strike of ∼310° and dip of ∼50° NE with a length of 20 to 50 m. The Cheshmeh Sefid veins have a strike of ∼290° and dip of ∼40° NE, with lengths of 20 to 200 m (Shamanian et al., 2004). Equal area stereographs of the three principal fault generations, identified during surface and underground mapping, are presented in Fig. 12, and indicate that F1 is the most abundant fault system.

4.2. Structural geology and mineralization Alavi (1991, 1996) suggested that the TCS district and related volcanic rocks in the adjacent areas are related to the Eocene magmatism in the Central Iran magmatic zone and not related to the volcanic rocks of the Alborz Magmatic Arc. Hassanzadeh et al. (2002) suggested that the Eocene–Oligocene volcanism in the eastern part of Alborz range and associated TCS district may have formed in pull-apart basins along an NE–trending fault segment. In this study the part of the TCS discussed is restricted, to the north and south, by the Anjilow and Torud N60–70E–trending strike–slip faults (Fig. 4). Movement of these faults caused the north–south trending related fault systems in the TCS district such as the Darestan, Cheshme Hafez and Pousideh secondary faults. Three principal fault orientations have been recognized in the TCS, which are present in the various ore deposits in the area (Hushmandzadeh et al., 1978; Khademi, 2007; Keynejad et al., 2011; this study):

5. Geochemistry of the igneous rocks

4.2.1. ∼70° faults Faults termed F1 in this study (highlighted in green in Fig. 11), include the Torud and Anjilow sinistral strike–slip faults in the south and north, respectively and the Baghu and Deh Now normal faults in the northern part of TCS district, which are parallel to the regional N–E trend (Fig. 6).

Tertiary continental arc magmatic activity in the TCS belt is dominated by various volcanic and intrusive rocks (Fig. 6), with major magmatic pulses in the EME at ca. 55 and 37 Ma., and in the ELO at ca. 34 and 24 Ma. (Hushmandzadeh et al., 1978; Jafarian, 1988; Valizadeh and Jafarian, 1991).

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Fig. 8. Simplified geologic map of the Cheshme Hafez deposit with A-B cross section (modified after Mehrabi and Ghasemi, 2012).

The EME succession consists of andesitic lavas, volcanic breccia, tuffaceous sandstones, rhyolites and rhyodacites, mainly outcropping in the southwest of the TCS. Whereas, intermediate to basic submarine lavas and volcaniclastic rocks, spilitic basalt, andesite lava of middle to late Eocene age and conglomerates with volcanic clasts outcrop in the central and northeast part of the TCS belt. The ELO subvolcanic and intrusive rocks are extensively exposed throughout the district, consisting of granite, granodiorite, trachyandesite, dacitic, dacitic–andesite, and mafic (gabbro) to intermediate (dacite-rhyodacite) composition dikes. The EME and ELO groups (Fig. 5) are located in different parts of the TCS belt as shown in Fig. 6.

studies of Hushmandzadeh et al. (1978), Valizadeh and Jafarian (1991); Hassanzadeh et al. (2002) and Eshraghi and Jalali (2006). 5.2. Major and trace elements geochemistry In a total alkali vs. silica diagram (TAS; Fig. 13a) samples from the TCS igneous belt range from a basaltic to rhyolitic in composition with a small number of samples, mainly from the EME rocks, plotting within the basaltic trachyandesite to trachyandesite fields. In contrast, the ELO samples are characterized by elevated alkali contents and lie clearly within the trachybasalt to trachyte fields (Fig. 13a). Similarly, in the K2O vs. SiO2 plot (Fig. 13b) samples from the EME and ELO rocks lie within the medium and high K fields. The K2O/Na2O ratios from the EME and ELO varies between 0.2 and 1.5 (in most cases is > 1.0) and 0.1 to 1.2 (in most cases is < 1.0), respectively. Samples from the ELO plot as medium and high K calc-alkaline rocks (Fig. 13b) and lie within the high K to shoshonitic fields in the P2O5/Al2O3 vs. K2O/Al2O3 diagram (Crawford et al., 2007; Fig. 13c). Furthermore, intrusive and subvolcanic rocks from the ELO show a pronounced enrichment in P2O5 relative to EME rocks (Table 2). Igneous rocks of the TCS district define a typical calc-alkaline trend on the AFM diagram (Fig. 13d). Primitive mantle-normalized trace element diagrams from both the EME and ELO have similar incompatible trace element patterns (Fig. 14a) with more enrichment in large ion lithophile elements (LILE: K, Rb, Sr, Cs, Pb) and depletion of high field strength elements (HFSE: Nb and Ti) in the ELO rocks compare with the EME, which are typical features of magmas from convergent tectonic settings (Saunders et al., 1980). Chondrite-normalized REE patterns are presented in Fig. 14b and both EME and ELO have very similar REE patterns, with a moderate

5.1. Sampling and analytical method Twelve samples from EME volcanic rocks and nine samples from ELO intrusive and subvolcanic rocks were collected from surface exposures and drill core for whole rock geochemical analysis. Particular care was taken to collect unaltered samples for microscopic examination. Major oxide, trace and REEs analyses of the Eocene-Oligocene volcanic and intrusive rocks of TCS district are presented in Table 2. Major elements were analyzed by inductively coupled plasma-atomic emission spectrometer (ICP-AES) method with a detection limit of 0.01%. Trace elements were analyzed by an inductively coupled plasma-mass spectrometer (ICP-MS) with a detection limit of 0.01 ppm to 0.1 ppm, after Li metaborate fusions and HNO3 total digestion in the ALS Chemex Laboratory, Canada. The analytical error for most elements is < 2%. Details of the chemical procedures are accessible at www.alschemex.com. The ages of several volcanic and all intrusive rocks collected in this study (Table 2) are constrained by previous

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Fig. 9. Simplified geological map of the Gandy deposit (modified after Shamanian et al., 2004).

light REE enrichment, and a flat to slightly concave heavy REE. Almost all samples of the TCS igneous belt are characterized by the absence of an Eu anomaly. Variations of major and minor oxides with silica in the least altered igneous rocks from EME and ELO in the TCS (Fig. 15) display normal

fractionation trends, consistent with them being broadly co-magmatic. Most oxides (TiO2, Fe2O3, MgO, CaO, and P2O5) decrease with increasing silica except K2O and Na2O which show a slight increase. Extensive plagioclase fractionation most probably did not occur, as indicated by the absence of any Eu anomaly and negative correlation of

Fig. 10. Simplified geological map of the Ghole Kaftaran deposit with A-B cross section (modified after Mousavi, 2009).

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Fig. 11. Schematic structural map of the TCS district and distribution of various deposits and prospects (modified from Hushmandzadeh et al., 1978; GSI-Geological Survey of Iran, 2006).

Sr and SiO2. The decrease in Al2O3 and Rb, observed in the samples with more felsic compositions, is most probably due to K-feldspar fractionation at approximately 65–70 wt% SiO2. The curvilinear pattern of Zr is indicative of strong zircon fractionation at approximately 65–70 wt% SiO2, typical for felsic magmas (Fig. 16).

aqueous with no daughter minerals. All inclusions are two-phase at room temperature (liquid and vapor), with vapor to liquid ratios ranging between 0.1 and 0.5. Microthermometric measurements were carried out only on primary fluid inclusions in quartz and sphalerite interpreted to be trapped during the fluid inclusion assemblage (FIAs). Homogenization temperatures (Th) and salinities of fluid inclusions in various prospects of the TCS are summarized in Table 3 and Fig. 17. Homogenization temperatures of fluid inclusions in quartz and sphalerite vary from 125 to 375 °C, centering on peaks at approximately 160 and 240 °C. Homogenization temperatures of fluid inclusions in the Darestan and Abolhassani deposits respectively is 252 to 367 °C and 230 to 340 °C. Homogenization temperatures at these deposits are consistently higher than those of Cheshme Hafez (147 to 278 °C), Ghole Kaftaran (125 to 208 °C), and Gandy (234 to 285 °C). Salinities are in the range of 6.7 to 18.2 wt% NaCl equiv. in Darestan and Abolhassani which are consistently higher than those of Cheshme Hafez, Ghole Kaftaran, and Gandy (cluster between 2.2 and 17.6 wt% NaCl equiv.) (Fig. 17).

6. Fluid inclusions 6.1. Analytical methods Microthermometry studies were carried out on fluid inclusions in quartz and sphalerite using a Linkam THM600 heating-freezing stage, fitted with a TMS-93 thermal control unit and equipped with ZEISS microscope at the Kharazmi University of Iran, employing standard procedures (Shepherd et al., 1985). The stage enables measurements within the range of −190 °C and +600 °C. Freezing and heating runs were, respectively, undertaken using liquid nitrogen and a thermal resistor calibration of the stage was carried out by using standard natural and synthetic inclusions. Molar volumes, compositions, and density were calculated using the FLINCOR software (Brown, 1989). Salinity is expressed as equivalent wt% NaCl and was calculated from measurements of the ice melting temperature (Tmice) using the equations of Bodnar (2003) for aqueous inclusions.

7. Discussion 7.1. Type and style of epithermal mineralization The mineral deposits of the TCS district, with the exception of the Baghu deposit (Rashidnejad, 1992; Tajeddin, 1999; Niroomand et al., 2018), are classified as intermediate sulfidation epithermal vein deposits (i.e., Ghole Kaftaran, Jafarian, 1988; Valizadeh and Jafarian, 1991; Gandy and Abolhassan, Shamanian et al., 2004; Cheshme Hafez, Mehrabi and Ghasemi, 2012). Following the classification nomenclature for epithermal systems (e.g., Hedenquist et al., 2000; Einaudi et al., 2003; Sillitoe and Hedenquist 2003; Simmons et al., 2005), the

6.2. Petrography and microthermometry Fluid inclusions study was carried out on ore–bearing quartz veins of various ore prospects in the TCS district. The size of fluid inclusions ranges from 5 to 30 μm, with maximum sizes of up to ∼30 μm in elliptical, irregular, and rod shapes. Primary, pseudo-secondary and secondary inclusions were identified, which are mostly liquid rich and

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a

F1 fault set

±35°

b

c

Baghu/Gandy fault-vein

Summary veins, mine data

70

i ve

/75

ns

lt au 1f

t

se

F

n=56

d

F2 fault set

Cheshme Hafez breccia vein

e

50

0/

34

300/6

fault set, mine data

avg. 60°/74°

f

Summary

fault set1, mine data

fault set2, mine data

F2

3

l fau

veins, mine data

t se t

Conjugate two fault-set veins ±25° avg. 330°/60°

n=87

g

F3 fault set

Abolhassani fault-vein

h

i

Summary fault set, mine data

veins

280/70

F3 fault set

veins, mine data

±25°

n=103

avg. 285°/70°

Fig. 12. Lower-hemisphere stereonet plots of the TCS vein system. a-c) F1 fault set with average orientations of ∼70°/75° and strike scatter of ± 35°, corresponding to Baghu\Gandy fault-vein, d-f) F2 fault set with average orientations of 340°/50° and 300°/63°and strike scatter of ± 25°, corresponding to Cheshme Hafez breccia vein and Pousideh fault-vein, respectively, g-i) F3 fault set with average orientations of 280°/70° and strike scatter of ± 25°, corresponding to Abolhassani fault-vein. Note the faults- and veins-mine data in the summary stereonet plots (c, f, i) which are displayed for better clarity.

Darestan and Pousideh epithermal systems of the TCS belt are classified here as an high-sulfidation type. The intermediate and high sulfidation epithermal NNW striking veins, parallel to the F2 fault systems, are dominant in the northeast (Ghole Kaftaran) and center (Darestan and Pousideh) part of the TCS ore district, respectively. The Ghole Kaftaran veins are hosted mainly by the EME volcanic and volcano-sedimentary rocks, although Darestan and Pousideh vein prospects are hosted by the ELO subvolcanic and volcanic rocks. These deposits are mainly vein type, characterized by an ore mineral assemblage of chalcopyrite, pyrite with minor chalcocite, covellite, Ag–rich tetrahedrite and electrum in a gangue of quartz and carbonate (Table 1). In addition, minor amounts of bornite, enargite, native silver, and tellurides have also been reported. The host rocks have been argillically altered and locally show residual quartz alteration with dickite/narcite, alunite, and pyrophyllite. These ore and

gangue mineral assemblages have characteristics indicative of high sulfidation epithermal vein deposits (Hedenquist, 1987; Hedenquist et al., 2000). There are also intermediate-sulfidation epithermal base and precious metal mineralization at the TCS district, which are related to the distribution of the Eocene-Oligocene volcanic and volcaniclastic rocks (Fig. 6). The ore and gangue mineral assemblage of these intermediate sulfidation deposits is indicative of a sulfidation state between high and low sulfidation conditions. The crustiform banding veins filled by gangue and sulfides, including fairly coarse quartz with minor calcite and occasional rhodochrosite, together with an overall sulfide content of 5–25 vol%, are typical of intermediate sulfidation style at the Gandy, Abolhassani and Cheshme Hafez vein systems (Shamanian et al., 2004; Fard et al., 2006; Mehrabi and Ghasemi, 2012). Other characteristics such as the alteration assemblage (illite, illite/smectite) with minor

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265

68.02 0.20 15.24

Wt% SiO2 TiO2 Al2O3

202 89 806 79 135 5.5 2.1 23 54 178 40 2 2 74 62 8 45 4.5 1.4 2.3 0.5 1.2 0.2 1.8 0.1 1.1 0.3

79 90 253 28 155 7.5 3.9 45 46 130 87 2 1.7 65 56 7 25 2.2 1.1 1.2 0.4 1.1 0.4 1.2 0.2 1.4 0.1

GH0021 Subvolcanic rhyodacite EME CM

51.36 0.83 18.6 8.34 0.17 5.18 5.48 2.92 0.44 0.10 1.90 99.32

TS0032 Basalt EME PS

59.02 0.74 16.04 7.53 0.13 3.64 6.18 2.21 3.25 0.39 0.83 99.96

TS0096 Andesite EME BG

Sample no. Rock type Group Region

wt% SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2 O P2O5 LOI Total ppm Ba Rb Sr Y Zr Nb Th Pb Ni V Cr Hf Cs La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sample no. Rock type Group Region

74.17 0.20 14.08

GH034 Granite ELO BG

178 100 651 102 129 3.0 3.3 5 87 93 67 2 1.0 11 37 3 36 3.5 1.2 2.5 0.3 1.5 0.3 1.4 0.3 1.3 0.3

58.73 0.60 16.98 6.50 0.13 3.53 6.93 3.90 1.10 0.30 0.85 99.55

GY0043 Andesite EME GS

73.25 0.38 14.02

AB0061 Granite ELO DS

103 78 458 43 98 1.3 4.3 25 58 89 43 1 2 43 54 3 12 1.4 1.3 3.6 0.5 3.3 2.5 1.3 0.2 2.3 0.1

58.46 0.68 16.12 6.60 0.23 3.65 5.78 2.69 3.39 0.32 1.23 99.15

TCD0031 Basaltic-andesite EME DS

70.12 0.27 17.53

DS010 Granodiorite ELO DS

245 64 263 89 145 7.6 3.9 8 69 84 76 2 1.7 86 64 6 22 1.3 1.6 2.2 0.7 2.3 0.3 1.3 0.4 1.6 0.3

65.59 0.50 16.63 4.11 0.04 1.49 4.76 2.72 3.09 0.08 1.03 100.04

CH056 Dacite EME CH

Table 2 Major and trace element analyses of whole rock samples of the magmatic rocks investigated in this study.

54.87 0.50 18.69

DS0042 Diorite ELO PS

135 74 557 70 160 4.5 3.3 9 87 139 68 1.8 2 65 87 6 27 3.3 2.1 2.9 0.3 2.2 0.5 2.1 0.2 2.3 0.2

58.21 0.55 16.89 7.40 0.01 4.11 4.50 3.65 3.75 0.30 0.92 100.29

CH0022 Trachy-andesite EME CH

69.02 0.35 16.1

PSS03 Dacite ELO PS

196 79 224 132 103 5.4 5.4 32 57 76 85 1 1 21 37 4 35 2.4 1.3 3.1 0.4 1.1 3.2 2.3 0.1 1.1 0.1

64.32 0.45 17.69 5.90 0.15 1.20 1.09 3.02 4.27 0.26 2.15 100.50

GY003 Dacite EME AB

66.23 0.48 18.17

GD031 Dacite ELO CM

102 29 712 76 87 8.3 4.3 23 87 48 58 1.6 6 9 23 9 25 1.6 1.1 2.1 0.6 1.1 0.7 1.4 0.2 1.1 0.2

54.3 0.60 15.70 5.80 0.10 4.12 8.10 3.45 3.50 0.29 3.30 99.26

TCD006 Andesite EME GD

59.76 0.64 17.37

GD0015 Dacitic-andesite ELO GH

170 56 576 112 132 1.4 2.1 15 46 43 98 2 5 32 42 7 43 2.2 1.2 1.4 0.5 2.6 0.3 3.1 0.2 2.1 0.3

54.48 0.68 17.02 7.49 0.18 5.21 5.38 2.34 2.80 0.30 3.73 99.61

GY045 Basalt EME GH

154 42 157 50 67 3.23 3.2 14 9 31 4 3 3 68 63 2 23 2.2 1.1 2.2 0.2 1.3 0.4 1.3 0.3 1.2 0.3

71.26 0.26 13.62 2.39 0.07 2.28 2.42 4.23 0.74 0.10 2.30 99.67

65.43 0.32 16.21

52.32 0.92 16.83

IG014 Gabbrodike ELO GS

236 23 71 67 38 5.8 2.3 12 98 189 89 2 3 17 45 4 34 1.2 0.9 1.2 0.6 2.2 0.2 1.9 0.3 1.7 0.2

75.34 0.15 13.03 1.27 0.20 0.69 2.43 3.40 2.29 0.06 1.60 100.46

CH0032 Rhyolite EME GD

(continued on next page)

IG001 Dacitedike ELO CM

TS0076 Subvolcanic rhyodacite EME PS

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266

465 43 897 14 18 1.1 1.4 6.4 23 56 6 2 2 54 12 4 15 2.4 1.8 3.3 0.3 3.2 0.5 2.3 0.2 2.1 0.1

187 51 308 86 78 1.9 1.6 13 16 45 12 4 3 32 46 3 25 4.6 1.2 1.2 0.3 1.1 0.4 1.2 0.2 1.1 0.2

422 52 1032 16 35 1.6 2.3 11 6 86 3 2 4 42 14 5 18 3.6 1.4 5.3 0.4 3.4 0.3 2.1 0.3 1.2 0.5

2.40 0.04 0.32 1.58 2.89 3.32 0.52 1.23 99.95

AB0061 Granite ELO DS

531 53 1132 19 32 1.7 3.2 13 6 23 16 3 3 23 34 6 19 5.2 1.0 4.3 0.2 2.1 0.2 1.4 0.3 1.3 0.2

1.81 0.06 0.29 2.26 3.70 3.09 0.78 0.33 100.24

DS010 Granodiorite ELO DS

557 43 1010 19 88 3.5 1.2 8.2 4 13 25 2 4 65 23 2 16 4.3 0.3 3.4 0.3 3.6 0.4 1.0 0.3 1.6 0.2

8.42 0.01 0.14 4.20 8.60 1.01 0.86 2.67 99.97

DS0042 Diorite ELO PS

403 61 986 20 46 1.4 1.4 21 7 43 32 4 2 11 16 3 26 4.4 1.2 1.2 0.3 4.2 0.3 0.9 0.4 2.2 0.3

2.92 0.05 0.22 1.60 4.78 3.63 0.59 1.05 100.31

PSS03 Dacite ELO PS

453 54 563 24 52 1.8 2.3 12 3 32 13 7 4 32 23 7 32 5.3 1.1 4.2 0.1 2.4 0.5 1.2 0.4 1.2 0.2

3.10 0.06 0.19 1.31 4.63 4.30 0.62 1.28 100.37

GD031 Dacite ELO CM

231 54 1013 22 47 2.3 3.5 7.5 4 11 3 3 3 8 43 4 16 6.2 1.0 3.2 0.5 2.3 0.2 2.1 0.5 3.2 0.3

6.07 0.16 3.01 3.98 4.04 3.26 0.86 1.03 100.18

GD0015 Dacitic-andesite ELO GH

432 67 1001 32 40 2.0 3.3 15 5 55 2 2 2 21 26 3 18 3.3 1.3 3.5 0.3 3.2 0.3 1.6 0.3 2.1 0.2

3.37 0.15 1.03 3.32 6.24 3.25 0.23 1.26 100.81

IG001 Dacitedike ELO CM

88 38 912 14 43 2.1 2.3 13 6 76 4 3 1 43 21 5 18 2.3 1.6 5.3 0.2 4.6 0.4 1.6 0.1 1.8 0.1

8.58 0.17 6.02 8.43 2.52 3.02 0.41 1.44 100.66

IG014 Gabbrodike ELO GS

Abbreviations: EME-early to middle Eocene, ELO-early to late Oligocene, BG-Baghu, PS-Pousideh, GS-Ghole Sukhteh, DS-Darestan, CH-Cheshme Hafez, AB-Abolhassani, GD-Gandy, GH-Ghole Kaftaran, CH-Chahmessi.

1.50 0.03 0.33 1.13 3.87 3.12 0.43 1.56 100.42

3.50 0.03 3.04 0.28 3.23 3.60 0.09 2.23 99.46

Fe2O3 MnO MgO CaO Na2O K2 O P2O5 LOI Total ppm Ba Rb Sr Y Zr Nb Th Pb Ni V Cr Hf Cs La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

GH034 Granite ELO BG

GH0021 Subvolcanic rhyodacite EME CM

Sample no. Rock type Group Region

Table 2 (continued)

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a

BTA- Basaltic trachyandesite

Na2O + K2O (wt%)

10

5

Tephriphonolite

Foidite

12

Trachyte

Phonotephrite

Rhyolite

Trachyandesite

8 BTA

Tephrite/

6

basanite

b

6

Phonolite

K2O (wt%)

14

TB- Trachybasalte

TB

High K

4 3

Medium K

2

4 Basaltic adesite

Basalt

Picrobasalt

2

Andesite

Dacite

1

Low K

0

0 40

45

50

55

0.1

60

SiO2 (wt%)

65

70

80

75

45

50

-ELO

P2O5/Al2O3

60

65

SiO2 (wt%)

c

Hi K Med K

55

F

70

75

d

-EME

Shosh

Tholeiitic

0.01

Low K

Calc-alkaline 0.001 0.01

0.1

K2O/Al2O3

1

A

M

Fig. 13. Major and trace element composition of unaltered Tertiary volcanic and intrusive rocks from the TCS district; a) (Na2O + K2O) vs. SiO2 (TAS diagram; division after Le Maitre et al., 1989); b) K2O vs. SiO2 diagram (division after Le Maitre et al., 1989). Most of the TCS igneous rocks plot within the medium-K and high-K fields; c) (P2O5/Al2O3) vs. (K2O/Al2O3) diagram (Crawford et al., 2007) showing samples lie within the high-K to shoshonitic fields; d) AFM diagram (division after Kuno, 1968). Data from Table 2 are used after normalization to 100% of the ten major oxides.

adularia, indicating the near-neutral pH of the hydrothermal fluids (Henley et al., 1984).

and, by inference, the hydrothermal fluids are between –7.1 (galena) and –3.3‰ (sphalerite) in the Gandy epithermal prospect and −6.1 (galena) and −2.8‰ (sphalerite) in the Abolhassani epithermal prospect consistent with a magmatic source for the sulfur (Shamanian et al., 2004). Albinson et al. (2001) compiled information for several epithermal deposits of Mexico and found that the higher salinity fluids were present in Ag- and base metal-rich deposits, comparable with the TCS ore district (e.g., Abolhassani, Cheshme Hafez and Ghole Kaftaran). Based on geochemical indicators such as salinities, ore geochemistry, and stable isotopic compositions (Shamanian et al., 2004), they proposed a magmatic to evaporitic source for the ore-forming fluids in those prospects. In Fig. 18, published microthermometric datasets from six ore prospects (except Pousideh for unmeasurable small size fluid inclusions) have been compiled on the same scale homogenization temperature-salinity diagram. Common to all prospects is an array of fluids along a more or less well-defined correlation trend (Fig. 18d) between essentially lower temperature and salinity (2–7 wt% NaCl equiv.; A) as meteoric water, and a higher-temperature liquid of low to medium salinity (5–12 wt% NaCl equiv.; B) with magmatic signature. Spatial zonations of prospects indicate that this trend is due to dilution of a hot magmatic fluid of relatively low salinity (B) by cooler meteoric water

7.2. Fluid evolution and metal precipitation Fluid inclusion studies show a wide range of salinities, from 4.7 to 17.6 wt% NaCl equiv. in quartz and 4.2 to 18.2 wt% NaCl equiv. in sphalerite. Consistently higher salinities in sphalerite compare with coexisting quartz have been described in other epithermal deposits that are either precious metal or base metal-rich, such as Fresnillo, Coneto de Comonfort, Durango, Real de Guadalupe, Guerrero, La Guitarra, Mexico and Caylloma (Simmons, 1991; Albinson et al., 2001; Echavarria et al., 2006). The high-salinity sphalerite-hosted fluid inclusions (e.g., Ghole Kaftaran) can be interpreted as being derived from the parental fluid and could be derived from either a magmatic source or interaction with evaporitic deposits. However, evaporative rocks are not known in the local stratigraphic sequence, and saline formation waters are unlikely in terrains with no evaporates or seawater circulation (Albinson et al., 2001). In volcanic-hosted systems, geothermal waters contain only up to 2 wt% NaCl equiv. (Simmons, 1995; Echavarria et al., 2006). Therefore, high salinity fluids in the TCS district most likely represent brines exsolved from a magmatic source. δ34SCDT values of the analyzed sulfides

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1000 -Andesite

EME

-Granite

ELO

-Basalt

-Diorite

-Dacite and rhyodacite

-Gabbro

100

Primitive mantle

100

Primitive mantle

-Dacite

-Rhyolite

10

1

10

1

0.1 Cs Rb Ba Th Nb La Ce Pb Pr Sr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y

a

0.1

Ho Er Tm Yb L u

Cs Rb Ba Th Nb La Ce Pb Pr Sr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y

1000

Ho Er Tm Yb L u

1000 -Andesite

EME

-Granite

ELO

-Basalt -Dacite and rhyodacite -Rhyolite

Chondrite

100

Chondrite

100

-Diorite -Dacite -Gabbro

10

10

1

b

1 La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

L

u

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

L

u

Fig. 14. a) Primitive mantle-normalized (after Sun and McDonough, 1989) and b) Chondrite-normalized (after McDonough and Sun, 1995) spider diagrams of the EME and ELO rocks.

(A) at the site of ore deposition. In some prospects, a slightly more saline fluid followed by a lower-salinity fluid of similar temperatures during high-grade ore deposition (e.g., Cheshme Hafez breccia veins: 18 decreasing to 4 wt% NaCl equiv., Mehrabi and Ghasemi, 2012). Fig. 18d shows that epithermal type prospects of the TCS, also display distinct but locally variable mixing lines along the steep grey arrows between the higher-salinity liquid (C array, Fig. 18d) and the lower-salinity array trending along the grey double-pointed arrow (Shamanian et al., 2004; Mehrabi and Ghasemi, 2012 and this study). Such steep mixing trends, if confirmed for assemblages of coeval inclusions, would indicate that higher-salinity liquid was intermittently introduced together with the low-salinity to medium-salinity fluid B. If both B and C are magmatic fluids introduced together, they must have coexisted as immiscible phases at depth and become miscible again in the epithermal domain (Heinrich et al., 2004; Heinrich, 2005). Alternatively, they may originate from entirely separate but simultaneously active magmatic sources at depth.

In the structural model, dextral west to northwest-striking faults (e.g., Astaneh-Amru dextral strike-slip fault, 280°/70°) and sinistral north to northwest-striking faults (e.g., Cheshme Hafez sinistral strikeslip faults, 345°/50°) have acted as a conjugate shear pair with an interplane angle of ∼70°. Fault kinematic analysis of the data (using Faultkin software of Allmendinger, 2001, http://www.geo.cornell.edu/geology/ faculty/RWA/maintext.html) suggests that the orientation of maximum horizontal stress (σH), modeled as the acute bisector of the conjugate pair, is ∼340° similar to majority of veins, so they are nearly pure extension fractures (F3, T-fractures) oriented roughly parallel to σH. During fault generation, extension fractures formed close to the plane containing the minimum and intermediate principal strain axes (the ε2ε3 plane). The overall strike of the TCS vein system is 320° to 340° but varies from ∼290° to ∼350°. The orientation of other veins in the district, such as at Baghu and Gandy, is 50° to 60° (Fig. 12). The structural model of the TCS ore district fits well with the Riedel shear model in which sinistral northeast striking faults and lineaments (Anjilow and Torud regional faults) would represent the main shear plane (M), and the north to northwest structures (Cheshme Hafez breccia vein) and west-northwest structures (Astaneh-Amru fault) represent R and R′, respectively (Fig. 19). The components of the model are at least partially recognized in other mineralized districts of the TCS district or are regionally recognized structural features. The majority of vein systems strike west north to northwest such as at South Chalu, Zereshk Kuh and Khorasani in the western TCS district. The Chahmessi and Ghole Sukhteh

7.3. Structural model Structural data indicate that veins and veinlets are associated with faults and fractures. Major veins are located in oblique normal faults and related extension fractures. We propose a structural model for the TCS district (Fig. 19), in which movement along first-order, northeast striking regional sinistral faults caused counterclockwise block rotation and development of second order, north to northwest striking sinistral faults, and third order extension fractures that host the mineralization.

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20

EME

EME

-TiO 2 -P 2 O 5 -MnO

0.8

EME

-Al 2 O 3 -MgO -Fe 2 O 3

-CaO -K2O -Na2O

8

15

Oxide (wt.%)

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Oxide (wt.%)

Oxide (wt.%)

0.6

0.4

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2

0.2

a 0.0

0 50

60

70

80

50

SiO 2 (wt.%)

1.0

60

50

60

70

SiO2 (wt.%)

ELO -Al 2 O 3 -MgO -Fe 2 O 3

-TiO 2 -P 2 O 5 -MnO

0.8

0

80

SiO 2 (wt.%)

20

ELO

70

80

ELO -CaO -K2O -Na2O

8

15

Oxide (wt.%)

6

Oxide (wt.%)

Oxide (wt.%)

0.6

0.4

10

5

4

2

0.2

b 0

0.0 50

60

70

80

0 50

60

70

80

SiO 2 (wt.%)

SiO 2 (wt.%)

50

60

70

80

SiO2 (wt.%)

Fig. 15. Harker diagrams showing variations of major and minor oxides with SiO2 for the EME (a) and ELO (b) of the least altered igneous rocks. Data from Table 2 recalculated to 100% free of volatiles.

300

600

300

400

200

400

200

200

100

200

100

0

0

0

0

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60

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-Rb -Zr -Sr

400

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Sr (ppm)

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Sr (ppm)

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500

EME

-Rb -Zr -Sr

400

800

1000

ELO

Rb and Zr (ppm)

500

1000

50

SiO 2 (wt.%)

60

70

80

SiO 2 (wt.%)

Fig. 16. Variation of Rb, Zr, and Sr vs. SiO2 for the EME and ELO volcanic, subvolcanic and intrusive rocks of the TCS district. 269

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Table 3 Summary of microthermometric data from epithermal deposits in the TCS district. Host mineral

Darestan Gandy Abolhassani Ghole Kaftaran Cheshme Hafez

FI type

Qz Sp Sp Qz Sp Qz

No.

P P P P P P

Tmice (°C)

65 27 45 20 32 65

Th (°C)

Salinity (wt% NaCl equiv.)

Range

Avg.

Range

Avg.

Range

Avg.

−20.3 to −7.6 −3.3 to −2.6 −15.6 to −4.2 −5.1 to −3.4 −9.3 to −6.2 −13.3 to −2.5

−13.2 −2.9 −9.9 −4.3 −8.5 −8.0

252–367 234–285 230–340 125–208 143–228 147–278

310 260 286 163 193 220

9.6–17.3 4.2–5.4 6.7–18.2 2.2–7.3 9.3–13.3 4.7–17.6

12.8 4.8 12.7 4.4 11.2 11.1

References

This study Shamanian et al. (2004) Shamanian et al. (2004) This study Mehrabi and Ghasemi (2012)

Abbreviations: Qz, quartz; Sp, sphalerite; P, primary; Tmice, final ice melting temperature; Th, homogenization temperature.

prospects in the northeast of TCS belt is characterized by two vein sets striking northeast and north northwest. Northeast-striking structures are a key part of the model as the master fault (M), are documented as representative of the major structural trend of the TCS (Fig. 19).

observation indicates that the ore deposits of the TCS district formed before the sedimentation of the Upper Oligocene Qom Formation, so it may be inferred that the age of mineralization is pre-late Oligocene. Extensive Eocene-Oligocene volcanism in the AMB generated the TCS district and other well-known epithermal deposits (e.g., Arasbaran zone and Tarom-Hashtjin district: Ghorbani, 2005; Calagari and Hosseinzadeh, 2006; Azizi and Jahangiri, 2008; Mederer et al., 2014). Widespread volcanism and mineralization, after a period of magmatic and tectonic quiescence during the late Eocene (Hushmandzadeh et al., 1978; Valizadeh and Jafarian, 1991; Emami, 2000), may be related to an increase in the Lut microplate convergence (Golonka et al., 2000; Golonka, 2004). It is coincident with a period of crustal shortening most likely related to underthrusting of the Lut microplate beneath the Turan plate and shift of the Sistan Ocean toward the Turan plate (Sengor and Natalin, 1996; Kopp, 1997; Golonka et al., 2000) (Fig. 2). At the district scale, the TCS igneous belt is not the only one in which the plutonic, subvolcanic, and extrusive rocks are preserved and

7.4. Tectonic events and magmatic evolution of the TCS belt To discuss and understand the tectonic setting and magmatic evolution of the TCS ore district, it is crucial to consider these prospects on a regional scale, with respect to both the E–W oriented Alborz Magmatic Arc (AMA) and the NE–SW ore deposit alignment of the TCS. The AMB is hosted by the Eocene-Oligocene magmatic rocks, which throughout the TCS, are covered by the limestone and shale of Qom Formation (Hushmandzadeh et al., 1978; Lam, 2002; Eshraghi and Jalali, 2006). The limestone of the Qom Formation is unaltered and contains at the bottom, a volcano-sedimentary conglomeratic layer with altered and mineralized fragments (Hushmandzadeh et al., 1978). This

20 Frequency

a

Darestan (n=65) Qz Ghole Kaftaran (n=52) Sp

10

0

Frequency

20

4 8 12 16 20 Gandy (n=27) Abolhassani (n=45) Cheshme Hafez (n=65)

24

4

24

120 160 200 240 280 320 360 400

b

10

0 0

8

12

16

20

120 160 200 240 280 320 360 400 Th ( C)

Salinity (wt.% NaCl eq.)

Fig. 17. a –b) Histograms of homogenization temperatures (Th) and fluid salinity (wt% NaCl eq.) of fluid inclusions from epithermal vein systems of the TCS. Abbreviations: Qz: quartz, Sp: sphalerite. Data are summarized in Table 3.

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25

Gandy (n=27)

Darestan (n=65)

Abolhassani (n=45)

20 15

Salinity (wt.% NaCl eq.)

10 5

a

0 25

b Summary (total points=254)

Cheshme Hafez (n=65) Ghole Kaftaran (n=52)

20

15

Fig. 18. Compilation of fluid inclusion data for epithermal prospects. Salinities vs. homogenization temperatures of liquid-rich inclusions (a–c), and a summary diagram of the data with a proposed interpretation of the fluid evolution and interaction processes (d). Most epithermal prospects record an interpreted mixing trend between a hot low-salinity to intermediate-salinity magmatic fluid ‘‘B’’ and cooler essentially salt-free meteoric water ‘‘A’’ (grey double-arrow in figure d). Inclusions above 15% salinity (e.g., Darestan, Abolhassani, and Cheshme Hafez) are subordinate but widespread and locally exhibit steep linear trends between higher-salinity liquid (brine ‘‘C’’) and the lower-salinity array ‘‘B–A’’. Microthermometric data in Gandy and Abolhassani deposits after Shamanian et al. (2004). Cheshme Hafez microthermometric data after Mehrabi and Ghasemi (2012). Data are summarized in Table 3.

C Sp

10

B

5

c

Qz

0 100 150

200

250

300

350

A 150

d 200

250

300

350

Homogenization temperature ( C)

crystallization played a major role in the evolution of magmas in the TCS including: (1) increase in alkalis (K, Na) and LILE (Rb, K, Pb, Ba), and decrease in TiO2, Fe2O3 and MgO with increasing SiO2 (Figs. 15 and 16); (2) the close spatial and probably temporal association of the eruptive products of the andesitic and dacitic-rhyodacitic subvolcanic units; (3) overall increase of total REE contents with increasing SiO2; (4) variations of incompatible trace elements with a trend from low abundances in basic volcanic units to higher abundances in intermediate to felsic- subvolcanic rocks. Most probably fractionation of olivine, hornblende, Ti-bearing magnetite, apatite, clinopyroxene, zircon, and possibly K-feldspar has played an important role in the process of magmatic crystallization at the TCS (Spies et al., 1983; Valizadeh and Jafarian, 1991; Mehrabi and Ghasemi, 2012). The general absence of Eu anomalies suggests a reduced fractionation of plagioclase. The occurrence of related gabbroic, microdioritic and dioritic inclusions and lenses in the most evolved rocks (andesitic and dacitic porphyry) in the north and northeastern part of the TCS district (eg., Baghu, Chahmessi and Pousideh), indicates that fractional crystallization was accompanied, to some extent, by magma mixing and mingling at various stages of differentiation, as suggested by Jafarian (1988), Rashidnejad (1992) and Liaghat et al. (2008) for the Baghu pluton, located in north of the TCS district. Fig. 20 displays a schematic diagram illustrating the relationship among convergent margin tectonics, upper plate structure, and magmatism during the evolution of the TCS ore district. Epithermal mineralization in the TCS district took place over a period of approximately 50 to 25 Ma., contemporaneous with the Central Iran magmatic activity (Berberian and Berberian, 1981; Emami, 2000; Axen et al., 2001). The Tertiary precious and base-metal prospects are associated spatially with strike-slip movements or volcano-tectonic depressions

exposed at the present-day surface. Several intrusion-related epithermal pairs, described here, are mainly hosted by the EME volcanic strips, preserved along the margins of the northern and southern TCS plutons (e.g., Baghu intrusion-related system). The TCS igneous rocks shares geochemical characteristics of typical subduction-related magmatic series described in other parts of the AMB, such as the Arasbaran zone in western Alborz (Hezarkhani and Williams-Jones, 1998; Calagari and Hosseinzadeh, 2006; Jamali et al., 2012) and the Tarom-Hashtjin district in central Alborz (Moayyed, 2001; Yaghubpur, 2003) (Fig. 3). Petrographic and geochemical characteristics of the TCS igneous rocks, identified in the variation diagrams and trace element patterns (Figs. 14 and 15), are interpreted to indicate that all phases that outcrop in the TCS district are sourced from the same melt. The EME volcanic rocks in the TCS district have a similar large ion lithophile (LILE) and LREE-enriched composition in comparison with the ELO intrusive and subvolcanic rocks but are less enriched in REE and are significantly less alkaline. In contrast, the volcanic and subvolcanic rocks of the EME from the Cheshme Hafez and Pousideh prospects are significantly more LREE (e.g., La, Ce, Sm) enriched than those of volcanic rocks of the Gandy and Abolhassani deposits (Table 2). Consequently, the upper part of the EME rocks (middle- and late-Eocene) is interpreted to be a sub-alkaline precursor to the ELO alkaline intrusive and subvolcanic rocks. The gradual enrichment of LREE and LILE over time in the Eocene and Oligocene units indicates a source region that becomes progressively more LILE and LREE enriched during the Tertiary. The cause of this enrichment could be increased input of hydrous fluids from the descending slab over time (Hollings et al., 2011). Petrological and geochemical data suggest the existence of a continuous medium to high K, calc-alkaline magmatic series in the TCS igneous rocks. Many lines of evidence suggest that fractional

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Fig. 19. Landsat image, showing the structural interpretation and location of epithermal systems in the TCS ore district. The map shows NE-striking regional sinistral faults (F1, ∼70°), sinistral NNW-striking faults (F2, ∼340°) and dextral WNW- striking faults (F3, ∼280°). The orientation of σH-max, modeled as the acute bisector of the conjugate pair (∼340°) and the majority of veins are considered as nearly pure extension fractures (T) oriented roughly parallel to σH-max. The model fits well with the Riedel shear model in which sinistral NE-striking faults and lineaments would represent the main shear plane (M), and the NNW structures and WNW structures represent R and R′, respectively.

magmatic rocks of early to middle Eocene (EME, ∼55–37 Ma.) calcalkaline and early to late Oligocene (ELO, ∼34–24 Ma.) alkaline rocks that have continually evolved through assimilation and fractional crystallization (AFC) processes. It appears that magmas were emplaced at shallow crustal levels during a period of trans-compression ± transtension in the TCS district. The EME volcanic rocks, in the TCS region, have a similar LILE and LREE enriched composition comparable to the ELO intrusive and subvolcanic rocks, but are less enriched in REE and are significantly less alkaline. The gradual enrichment of LREE and LILE over time in the Eocene and Oligocene regional units indicates a source region that becomes progressively more LILE and LREE enriched during the Tertiary. The cause of this enrichment could be increased input of hydrous fluids from the descending slab over time and release of sulfur and chalcophile elements (e.g., Cu) from the melted source rocks and melt pathways during magma ascent. This process formed a hot oxidized and hydrous mafic melt, enriched in metals and sulfur, at the base of thickened arc crust which facilitated partial melting and assimilation of lowermost crustal rocks. Structural emplacement of the orebodies and the alteration zones of the epithermal deposits have been controlled by ∼N70 and ∼N340 trending faults. The structural model of the TCS district fits well with the Riedel shear model in which sinistral northeast striking faults and lineaments (Anjilow and Torud regional faults) would represent the main shear plane (M), and the north-northwest structures (Cheshme Hafez fault-vein) and west-northwest structures (Astaneh-Amru fault) represent R and R′, respectively. Orthogonal subduction of the Lut microplate beneath the Turan Plate may have played a significant role in sulfide and gangue mineralization in the TCS ore district during the Eocene-Oligocene.

together with the uplifted basement blocks of the Paleozoic metamorphic rocks. Lateral movements following trans-compression along both the Anjilow and Torud regional fault zones may represent a key factor for regional scale controls on mineralization. Simultaneously with, or soon after, magmatic heating, transformed movements along the major strike-slip fault zones, and may also have been a key component of the ore-forming process, leading to relaxation of local compressive forces and enhanced crustal-scale permeability. During extensional or trans-tensional stress periods, these structures provided enhanced permeability zones, therefore favoring magma emplacement in the crust (Choi et al., 2005; Chambefort and Moritz, 2006; Mederer et al., 2014) (Fig. 20). Orthogonal, northwestward, subduction following by oblique, northward, movement of the Lut microplate may also have played a significant role in mineralization of the TCS ore district during the Eocene-Oligocene. By analogy with the model of Tosdal and Richards (2001, 2002), the NE oriented F1 faults of the TCS mineralized belt are interpreted as major arc parallel faults during oblique subduction of the Lut microplate. The NNW oriented F2 faults are considered as strike-slip faults delineating the major arc parallel faults (Fig. 20), allowing the formation of hydrothermal alteration, mineralization and eventually resulting in emplacement of magmatic rocks in the TCS district (e.g., Baghu-Darestan and Pousideh intrusive and subvolcanic rocks). 8. Conclusions Tertiary evolution of the TCS ore district in the north of Iran, can be related to the location and intensity of the volcanic activity. The large majority of known intermediate to high-sulfidation epithermal prospects of the TCS district are spatially and temporally associated with 272

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Fig. 20. Plate-tectonic model for the TCS ore district, showing the relationship between convergent margin tectonics, upper plate structure and geochemical evolution of TCS magmas from the EME– to the ELO–type. a) Development of the TCS volcanic arc basin at Middle Eocene (ca. 48–37 Ma); b) emplacement of ELO magmas during orthogonal subduction, slab break-off and partial melting in sub-arc mantle wedge during the Oligocene-Miocene (ca. 32–20 Ma); c) sedimentation and slab detachment during a period of dominant transpression ± extension during the Middle Miocene (ca. 15–7 Ma).

Acknowledgments

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We acknowledge the financial support of the Iranian Mineral Research Corporation (IMRC) and Kharazmi University of Tehran. Ministry of Mines and Metals of Iran kindly provided unlimited access to the Torud-Chah Shirin prospects and the mine geologist A. Mohebbi, generously shared his knowledge on the Chahmessi and Ghole Kaftaran prospects with us. The manuscript has been significantly improved by the constructive comments by Dr. Majid Ghaderi and another anonymous reviewer. We would like to sincerely thank Dr. David Banks (Leeds University) for thorough reading and massive improvements of the manuscript. Dr. Asadi Harooni is also greatly appreciated for dedicated editorial handling. References Agard, P., Omrani, J., Jolivet, L., Mouthereau, F., 2005. Convergence history across Zagros (Iran): constraints from collisional and earlier deformation. Int. J. Earth Sci. 94, 401–419. Alavi M. Tectonic map of the Middle East, 1:5,000,000 (1 sheet). Geological Survey of Iran (GSI). 1991. Alavi, M., 1996. Tectonostratigraphic synthesis and structural style of the Alborz mountain system in northern Iran. J. Geodyn. 21, 1–33. Albinson, T., Norman, D.I., Cole, D., Chomiak, B., 2001. Controls on formation of lowsulfidation epithermal deposits in Mexico: Constraints from fluid inclusion and stable isotope data. Soc. Econ. Geol. Spec. Publ. 8, 1–32. Allen, M., Jackson, J., Walker, R., 2004. Late Cenozoic reorganization of the ArabiaEurasia collision and the comparison of short-term and long-term deformation rates. Tectonics 23, 16–32. Amidi, S.M., Emami, M.H., Michel, R., 1984. Alkaline character of Eocene volcanism in the middle part of central Iran and its geodynamic situation. Geol. Rundsch 73, 917–932. Asadi, H.H., Voncken, J.H.L., Kühnel, R.A., Hale, M., 2000. Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold deposit, northwest Iran. Miner.

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