Evolution of the gold (copper) mineralization in the porphyry stock and the related skarn zones and epithermal veins in the Astarghan area, NW Iran: Evidence from fluid inclusion, mineral chemistry and sulfur isotope analyses

Highlights

• Astarghan deposit includes skarn, porphyry and epithermal mineralizations (Cu–Au).

• Epithermal mineralization of Au in Astarghan shows low-sulfidation characteristics.

• Tetrahedrite and stibnite are the main hosts for gold in the Astarghan deposit.

• Native gold also occurs in epithermal quartz veins as tiny particles of 50–200 μm.

• The δ³⁴S values of pyrites in all mineralization zones show magmatic origin.

Abstract

The Astarghan area in northwest Iran (~50 km north of Tabriz) represents an excellent example of coexisting and associated skarn, porphyry and epithermal mineralizations of copper and gold. This contribution aims to study the geology, alteration and mineralization in these deposits, as well as the occurrence of gold and its host minerals, and present the physico-chemical characteristics of ore-forming fluids and the sulfur origin, in order to get a better understanding of development and evolution of these mineralizations. The Astarghan porphyry stock (Oligo–Miocene) intruded the upper Cretaceous flysch-type sedimentary sequence (calcareous sandstone and limestone with intercalations of siltstone, shale and marl) and brought about metamorphism and metasomatism along the contact. The Cu–Au mineralization in the Astarghan area occurred mainly in three forms: (1) skarn patches along the contact (Zone A), (2) stockwork-type and disseminated porphyry mineralization within the potassic (at Kaghdara and Boiokwari areas) and phyllic (at Sildirimdara area) alteration zones (Zone B), and (3) native gold-bearing low-sulfidation epithermal veins within the argillic alteration zone at Nowrozkala area (Zone C). The micro-thermometric studies on the porphyry-type Kaghdara potassic alteration zone showed that homogenization temperatures for liquid-rich and vapor-rich 2-phase, as well as halite-bearing multiphase inclusions range about 160–420 °C, 220–460 °C, and 320–580 °C, respectively. Salinities of the 2-phase and multiphase inclusions have ranges of 5–25 and 40–70 wt% NaCl_eq., respectively. However, fluid inclusions present in the epithermal veins of Nowrozkala are mainly liquid-rich 2-phase, and their homogenization temperature and salinity values lie within the ranges of 118–325 °C and 0.6–7.4 wt% NaCl_eq., respectively. The approximate estimated depth of the sulfide and gold mineralization in the porphyry system ranges from 1.6 to 2.5 km, and in the epithermal system is < 0.9 km. The sulfur isotopic analysis of pyrite from the porphyry and epithermal mineralizations in the potassic alteration zone at Kaghdara and argillic alteration zone at Nowrozkala indicate that the δ³⁴S values (–0.1 to –0.3 and –0.5 to –1.1‰, respectively) are close to the range of magmatic sources. Finally, electron probe micro-analyses indicate that besides independent particles of 50–200 μm in the epithermal veins, gold is mainly hosted by tetrahedrite within the porphyry-related phyllic alteration zone, but especially stibnite within the epithermal veins; these minerals, as well as epithermal polyhedral pyrites contain native gold inclusions.

Introduction

Mineral deposits of the porphyry–epithermal system are generally associated with magmatic arcs in convergent geodynamic settings and display spatial and temporal relationship with intermediate to felsic sub-volcanic intrusions (Seedorff et al., 2005, Simmons et al., 2005). In all cases, these deposits are thought to have formed at shallow crustal levels (<1.5 km for epithermal and < 6 km for porphyry deposits [Seedorff et al., 2005, Simmons et al., 2005]). One of the most significant metallogenic belts hosting large metallic mineral resources of these types is the Alpine–Himalayan metallogenic belt, extending from Eastern Europe through Central Asia to the Pacific region, which was formed as the result of a very complex tectonic evolution during the history of Paleo- and Neo-Tethys oceans, involving subduction of the oceanic crust. Thus, various types of mineralization are related to a variety of tectonic settings such as intracontinental rifting, ocean-floor spreading, subduction-related rifting, and continent–continent collision (Singer et al., 2005, Richards, 2015).

The middle section of this belt passes through the Iranian territory, represented by the Alborz and Zagros mountain ranges. Therefore, Iran documents the closure of at least two Tethyan oceans, (1) Paleo-Tethys during the Paleozoic and (2) Neo-Tethys during the Cenozoic (Sengör and Natal’in, 1996, Richards et al., 2006, Richards, 2015). Although a small number of mineral deposits and the related igneous rocks are associated with the Paleo-Tethys ocean in Iran (e.g., early Cambrian Kiruna-type iron oxide–apatite deposits in the Bafq area of Central Iran), it is the Cenozoic Urumieh–Dokhtar magmatic arc (UDMA) that played a significant role in the formation and distribution of many porphyry–epithermal deposits throughout Iran (Fig. 1a). This arc is over 2000 km long and ~ 50 km wide and is associated with the Late Mesozoic–Early Cenozoic northeastward subduction of the Neo-Tethyan oceanic crust beneath the Central Iranian plate, as a result of collision of the African and Eurasian plates (Alavi, 1994, Stampfli, 2000, Omrani et al., 2008). The UDMA also coincides with the porphyry copper metallogenic belt of Iran, which is in turn divided into three main metallogenic zones located in the northwest (Arasbaran zone), center, and southeast (Kerman zone) of the arc (Ghorbani, 2013). The most prosperous zone is the Kerman zone, where is located the nation’s largest porphyry copper deposit (PCD) (Sarcheshmeh [Shahbpour, 1982], as well as Miduk [Taghipour et al., 2008], Sarkuh and Iju [Mirnejad et al., 2013] and many other large and small deposits and prospects). The central metallogenic zone for Cu is represented by PCDs such as Dalli [Ayati et al., 2013, Fatehi and Asadi, 2017, Fatehi and Asadi, 2019] and Kahang [Komeili et al., 2017]), as well as epithermal deposits such as Shanegh (Sakhdari et al., 2011). Some porphyry and epithermal deposits also occur in East Iran (e.g., Shadan [Richards et al., 2012] and Sharaf Abad [Karimpour et al., 2011] PCDs and Chah Shalghami epithermal deposit [Karimpour et al., 2011]).

The northwestern part of the UDMA is known as the Arasbaran or Ahar–Jolfa metallogenic zone, which extends from south Armenia to the northwest of Iran, and is regarded as part of the lesser Caucasus in the Alpine–Himalayan metallogenic belt. The Cenozoic magmatism and accompanying porphyry–skarn–epithermal mineralizations in this zone were the result of post-collisional, slab breakoff-induced heat and fluid flow from upwelling asthenosphere beneath the young orogenic belt (Jamali et al., 2010, Simmonds, 2013). Both Cu ± Mo and Cu ± Au porphyry and epithermal types of deposits of Late Eocene, Middle Oligocene and Early Miocene exist in the Arasbaran zone, mainly associated with the emplacement of Middle Eocene to Miocene magmatic pulses in the Qaradagh batholith and the Shaivar-Dagh plutonic complex, as well as numerous satellite stocks within the Cretaceous–Cenozoic sedimentary-volcanic sequence (e.g., Aghazadeh et al., 2011, Ghorbani, 2013). Some of these well-known deposits are the porphyry Cu–Mo deposits at Sungun (Calagari, 2003, Calagari, 2004a, Calagari, 2004b, Aghazadeh et al., 2015, Simmonds et al., 2017), Haft-Cheshmeh (Aghazadeh et al., 2015), Kighal (Simmonds, 2013, Simmonds, 2019, Simmonds et al., 2015), Ali-Javad (Hajalilou and Aghazadeh, 2016), Sonajil (Hosseinzadeh et al., 2010), vein-type and possible porphyry Cu–Mo mineralization at Qarachilar (Simmonds and Moazzen, 2015, Simmonds et al., 2016), porphyry Cu–Au mineralization in the Mirkuh Ali Mirza deposit (Maghsoudi et al., 2014), the Masjed Daghi Cu–Au porphyry–epithermal mineralization (Atalou et al., 2017), the epithermal gold deposits of Mazraeh-Shadi (Radmard et al., 2017), Nabijan (e.g., Jamali et al., 2017), Zailic–Sarilar (Miranvari et al., 2020), and finally the Cu–Fe skarns of Mazraeh (Mollai et al., 2009) and Sungun (Calagari and Hosseinzadeh, 2006) (Fig. 1b).

Also, various types of mineralization were formed within the northward extension of the Qaradagh batholith in South Armenian Block (Meghri–Ordubad pluton) including the Agarak, Kadjaran (Zvezdov et al., 1993, Moritz et al., 2016a, Moritz et al., 2016b, Rezeau et al., 2016) and Paragachay (Babazadeh et al., 1990, Moritz et al., 2016a) porphyry Cu–Mo deposits, Zod epithermal gold deposit (Konstantinov et al., 2010, Moritz et al., 2016a), and the Kapan, Alaverdi and Mehmana deposits with a hybrid VMS–epithermal–porphyry genesis (Mederer et al., 2014, Moritz et al., 2016a).

The Astarghan deposit also lies on the UDMA belt at NW Iran, where skarn, porphyry and epithermal mineralizations of Cu and Au have occurred consecutively. As a result, studying this mineral deposit may elucidate the evolution of magmatic–hydrothermal ore-forming fluids upon emplacement of the intrusive body and formation of contact metamorphism and metasomatites, through hydro–fracturing and generation of porphyry-type stockwork and disseminated mineralization, up until the occurrence of epithermal mineralization. In this regard, the present research focuses mainly on the geology, alteration, mineralization and fluid characteristics, including temperature, salinity and sulfur origin, in order to get a better understanding of the development and evolution of the deposits, as well as to identify the occurrence of gold and its host minerals.