Genesis of the Lakhshak orogenic Au–Sb deposit, SE Iran: insights from geology, microthermometry and stable isotope geochemistry

doi:10.21203/rs.3.rs-4138263/v1

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Genesis of the Lakhshak orogenic Au–Sb deposit, SE Iran: insights from geology, microthermometry and stable isotope geochemistry

https://doi.org/10.21203/rs.3.rs-4138263/v1

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The Lakhshak Au–Sb deposit in the SE Iran, is closely associated with Eocene calc–schist and quartz schist rocks intruded by Oligocene dikes and mylonitic granitoid. The main mineralization is characterized by a NE–SW shear zone structure and hydrothermal alterations have mainly developed in the contact zones of granitoid and calc–schist units. Quartz veins and veinlets are associated with Au and Sb–bearing minerals consisting of pyrite, arsenopyrite, stibnite, pyrrhotite, chalcopyrite, sphalerite, gold and electrum. The ore genesis, metallogenic processes, and the origin of ore fluids in the Lakhshak deposit are unknown, hence we report for the first time the geological, petrographic, microthermometry of fluid inclusions, Raman spectroscopy and sulfur and oxygen isotopes studies. In this study, four types of fluid inclusion assemblages were recognized in the mineralized quartz veins. The results of fluid inclusion microthermometric measurments yield homogenization temperatures of two–phase LV (type III) and ternary carbonic–aqueous phase (LCO₂ + LH₂ O + VCO₂; type IV) fluid inclusions vary between 200 to 280°C and 300 to 330°C, whereas their salinity range from 9 to 11% and 8 to 13% wt. % NaCl, respectively. This implies that gold precipitation is derived from low–temperature and low–salinity ore fluids. The calculated δ¹⁸O_fluid values of quartz vary from 7.5 to 9.8‰, implying that the ore fluids may have a metamorphic origin that reacted repeatedly with the volcanic rocks along the conduits as well as the granitoid units adjacent to the ores and subsequently experienced sporadic equilibrium in oxygen fractionation during formation. Therefore, due to the significant oxygen isotopic exchange in the fluid–rock reaction, the metamorphic fluids show the isotopic properties of specific fluids of magmatic origin. Moreover, the δ³⁴S values of pyrite (3.0 to 4.1‰) and stibnite (–0.9 to 0.8‰) from the main satge of mineralization show a magmatic origin for sulfur. Accordingly, it is suggested that the mixing and dilution of metamorphic fluids with meteoric water plausibly had a substantial effect on the evolution of ore–forming system at Lakhshak, similar to orogenic gold deposits worldwide.

Lakhshak Au–Sb deposit

Sistan suture zone

fluid inclusion

stable isotopes

orogenic gold deposit

The major tectonic units of Iran are consisted of Alborz, Central Iran block, Urumieh–Dokhtar magmatic belt, Kopeh–Dagh, Zagros fold–thrust belt, Sanandaj–Sirjan zone, Sistan suture zone and Makran (Berberian, 2014; Stern et al., 2021; Fig. 1). According to Moores et al. (2000) and Shafaii Moghadam and Stern (2015), the Iranian ophiolites partially comprise the Tethyan ophiolitic belt stretching from eastern Europe, along the Mediterranean and the Middle East to eastern Asia. They are collocated as pivotal suture zones, including the Bitlis–Zagros suture zone, the collisional suture of Paleo–tethys accompined by the Alborz to Kopeh-Dagh cordilleras and the Sistan suture zone (Şengör et al., 1987 and McCall, 1997). Ghazi et al. (2004) suggested that these suture zones and related ophiolites encompassing the central Iranian microcontinent probably formed in the Cretaceous period. According to Shafaii Moghadam and Stern (2015), Iranian ophiolites involve Paleozoic (Paleo–Tethys remnants) and Mesozoic (Neo–Tethys remnants) ophiolites. Most Neo–Tethys ophiolite remnants in Cyprus, Turkey, Iran, Oman, Afghanistan, Pakistan and Tibetan Plateau belong to Late Cretaceous, whereas few cases possess older ages (e.g., ophiolites in Makran and Sistan suture zone in Iran). Correspondingly, the Sistan suture zone and associated ophiolites in the SE Iran constitute a principal and extensive (700 × 200 km²) northern–southern trending part of the Neo–Tethyan ocean (e.g., the Sistan Ocean; Zarrinkoub et al., 2012). In general, Camp and Griffis (1982), Tirrul et al. (1983) and Agard et al. (2011) suggested that the Sistan suture zone is mainly constituted of rock units enlarging between the Gondwana–derived blocks of Lut in the west and the Afghan block in the east (Fig. 1). The tectonic reworking of Sistan suture zone was governed by the splitting and successive closure of various lodges of the Neo–Tethyan ocean (Tirrul et al., 1983). It is highly reported that the Sistan suture zone is an immensely enriched metallogenic zone which hosts several Sb and Au–Sb ore deposits including Tuzgi and Shurchah (Moradi et al., 2012), Sefidabeh (Eliaspour et al., 2010), Sefidsang and Dargiaban (Boomeri, 2014), Baout (Mojadadi Moghadam and Boomeri, 2021) and Lakhshak (Fig. 2; IMPASCO, 2016; Mazloom et al., 2017).

This study focuses on the Lakhshak Au–Sb deposit within the SW section of the Sistan suture zone (Fig. 1). The Lakhshak deposit has been described as an epithermal and orogenic deposit by Mazloom et al. (2017) and Niroomand (2015), respectively; thus, however, the type and mechanism of ore genesis of this deposit remains argued. Moreover, the metallogenic processes involved, the source and the possible ore fluids evolution at Lakhshak are unknown. This study provide geological, petrographic, microthermometry of fluid inclusion, Raman spectroscopy and stable isotopes (S–O) studies to constrain the fluids origin and sulfur sources related to the gold mineralization. Hence, it may help us to better understand the formation of these deposits and implies an exploration basis for other mineralization potentials in the Lakhshak district.

2.1. Sistan suture zone

The Lakhshak Au–Sb deposit, located in SE Iran, lies along the SW margin of the Sistan suture zone. Tirrul et al. (1983) and Agard et al. (2011) porposed that the Sistan suture zone is an extensive collisional episode between Eurasia and India and the succeeding combination of Lut and Afghan blocks, including the accretionary complexes of Neo–Tethys besides accompanying island arcs. Tirrul et al. (1983) believe that this suture zone was formed due to the Sistan ocean closure and the separation of Afghan and Lut blocks resulting in the formation of the volcano–sedimentary rocks. Nabiei and Bagheri (2013) and Bagheri and Damani Gol (2020) suggested eight tectonic phases for this region, including the formation of the mélange unit in the active margin of the Neo–Tethys accretionary prism, collision of Lut and Afghan blocks, development of SW–trending reverse folds, deposition of a huge sequence of molasse sediments mixed with the Lut block during ongoing collision, the occurrence of a post–collision event, reactivation of the Nosratabad fault and deactivation of the Kahurak fault.

It is reported that the Sistan suture zone is bounded by Afghan and Lut blocks and divided into three tectonic units, including Sefidabeh basin, Neh and Ratuk accretionary prisms (Fig. 3; Tirrul et al., 1983; Fotoohi Rad et al., 2005). The Ratuk accretionary prism with NW–SE trending is predominantly composed of metamorphosed ophiolitic complexes and the Neh is constituted of sedimentary rocks of Senonian to Early Eocene ages and ophiolitic terraines. The Sefidabeh basin is also primarily composed of clastic sediments sequences followed by carbonate deposition events and volcanism with calc–alkaline affinities.

2.2. Lakhshak area

The Lakhshak district, located in the SW margin of the Sistan suture zone (⁓28 km of NW of Zahedan; Fig. 2), is recongnized as major Au and Sb mineral resources in Iran. This deposit is consisted of two main lithological sequences, including Eocene volcano–sedimentary units (e.g., calc–schist and quartz schist) and Oligocene mylonitic granitoid and dikes (e.g., rhyolitic and dacitic; Fig. 4). The calc–schist units consist of quartz, feldspar, sericite, actinolite, chlorite, biotite and carbonate. The Eocene quartz schist units predominantly consist of quartz–chlorite, quartz–muscovite and sericite. The mylonitic granitoids intruded the Eocene volcano–sedimentary rocks and mineralized shear zones mostly contain quartz and feldspar. The Oligocene dikes are compositionally divided into rhyolitic and dacitic types and emplaced into the metamorphic units close to Lakhshak (Fig. 4). The rhyolitic dikes have NE–SW trending with a thickness of fewer than 1.5 m (Fig. 4) and mainly consist of quartz, plagioclase, biotite and hornblende phenocrysts. The dacitic dikes with NE–SW trending show a thickness of 10 m which consist of a granular texture dominated by phenocrysts of plagioclase and quartz in a fine–grained matrix composed of quartz, feldspar, biotite/sericite and hornblende. The Quaternary units consist of sand dunes, alluvial terraces and new days alluvium (Fig. 4).

The mineralization is constructionally governed by a ductile–brittle shear zone (with NE–SW trending) between calc–schist contacts and the granitoid intrusion. The mineralized quartz veins and veinlets within Eocene calc–schist (Fig. 4) are the major gold host which has experienced various deformation degrees, mainly as mylonitization, whereas barren quartz veins and veinlets from the late stages transect hydrothermal alteration zones. In general, the mean tonnage and grade of Au and Sb at Lakhshak were appraised as @5833 tonnes at 3.25 ppm and @8017 tonnes at 1.35%, respectively (IMPASCO, 2016).

Based on the field and microscopic observations, structures at Lakhshak comprise three principal events of ductile to brittle deformations, mainly defined as D1, D2 and D3 phases. The initial deformation phase (D1) is parallel to the bedding (S0) of the volcano–sedimentary rocks, characterized by a regional foliation (S1). The secondary deformation phase (D2) is further subdivided into D2a and D2b. The D2 phase deformed all rock assemblages and D1 structures. At Lakhshak, D2a is the significant phase characterized by the S2 foliation appearance, boudinated quartz veins and veinlets, sigma structure of quartz veins (sigmoid quartz veins), S–C fabric, mylonitization, mica fish–shaped structures and subgranulation/recrystallization (Fig. 5a–f). The D2b deformation includes NE–SW trending isoclinal folding resulting in cross–cutting mineralized quartz veins and veinlets (Fig. 5g). The third phase (D3) is subsequently subdivided into D3a and D3b, which is identified as oblique strike–slip or normal faults and brittle deformation with NE–SW or NW–SE trending. The D3a deformation constructs low slope extensional fractures filled with sulfide–bearing quartz veins and veinlets (Fig. 5h). D3b is the youngest deformation stage that involves strike–slip or normal faults, veins–veinlets, joints and fractures that are both parallel and transect to the mylonitic fabric.

At Lakhshak, 256 samples were selected from the outcrops and 18 drill cores. Overall, 130 thin–sections were provided for petrographical studies and 15 doubly polished thin–sections from outcrops and drill cores were selected from quartz segments of the ore–bearing quartz veins for fluid inclusions microthemometry studies and laser Raman micro–spectrometry analyses at Iran Mineral Processing Research Center. Microthermometric studies were carried out using a Linkam THMSG–600 heating/cooling stage with a temperature range of about − 196 to 600°C, and a Zeiss–Olympus petrographic microscope with a TMS93 heat controller and an LNP coolant at the Microthermometric Laboratory, University of Tehran. The device set–out was performed in a heating stage with the correctness of ± 0.6°C and cesium nitrate with a melting point of 424°C, and a cooling stage with a standard material of hexane. The salinity was measured as the wt.% NaCl Equiv. through the melting temperature of the last piece of ice (Tm_ice) and applying the formula and technique offered by Hall et al. (1988) and Bodnar et al. (1989, 2014), respectively. Five polished sections were selected for Raman studies using the confocal Jobin Yvon LabRAM Raman spectrometer with a wavelength of 532.02 nm and YAG–Nd and Olympus lenses in Leoben University to identify the types of phases in the fluid inclusions accurately and specify their composition. To determine the composition of the vapor phase in the quartz segments, laser Raman micro–spectrometry analysis was carried out to find out the fluid inclusions phases.

For determination of δ¹⁸O values, four quartz samples were analyzed using a “conventional” vacuum extraction line employing ClF₃ as the reagent (e.g., Borthwick and Harmon, 1982) in the Stable Isotope Laboratory of Cape Town University, South Africa. Approximately 10 mg of quartz powder was loaded into externally heated Ni tubes after drying at 50°C overnight. The NBS–28 as quartz standard was used and analyzed in duplicate along with eight samples in each run. The samples were then degassed under vacuum at 200°C for two hours, and subsequently reacted with ClF₃ at 550°C for three hours. The following day the liberated oxygen was passed over a hot carbon rod, converting it to CO₂ gas and was frozen into “break seal” tubes. The oxygen isotope ratios were measured off–line using a Finnegan Delta–XP mass spectrometer (in dual inlet mode) and their data are reported in the δ notation relative to the Vienna Standard Mean Ocean Water (VSMOW) standard. For laser Raman micro–spectrometry analysis, oxygen isotope ratios were measured on O₂ gas and the isotope composition of the O₂ reference gas was determined by converting an aliquot of O₂ to CO₂ using the carbon convertor on the conventional extraction line. This value was used to calculate raw δ values of each sample relative to the VSMOW.

In addition, to detect for δ³⁴S compositions of sulfides, six pyrite and three stibnite samples were analyzed in the Stable Isotope Laboratory of Ottawa University, Canada. Samples are weighed into tin capsules with at least twice sample wt of tungstic oxide for inorganic and organic S by either the researcher or the lab. Samples are loaded into an Isotope Cube (Elementar, Germany) elemental analyzer to be flash combusted at 1800°C. Released gases were carried by helium through the EA to be cleaned, then separated. SO₂ gas is carried into the Delta Plus–XP isotope ratio mass spectrometer (ThermoFinnigan, Germany) via a conflo IV interface for δ³⁴S determination. Analytical precision is ± 0.2 per mil.

4.1. Mineralization and alteration

The main mineralization shows spatial and temporal associations with the ductile–brittle shear zone and the intense silicification–sulfidation in the contact zone between granitoid intrusion and calc–schist units (Fig. 6). With respect to the ore petrography and textural relationships, two types of sulfide–bearing quartz veins and veinlets were identified as concordant (parallel) and discordant (cross–cut). Both types of veins and veinlets (Fig. 7) with ⁓2 m thickness and > 1 km in length evidently emphasize on the ore–fluids physiochemical conditions related to the structural regime and hydrothermal alteraion events. Both types of veins and veinlets are accompanied by mineralization and predominantly consist of pyrite, stibnite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, gold, electrum, quartz ± carbonate ± ankerite, feldspar, chlorite and sericite. The high grade gold assemblages are associated with exceedingly deformed mylonitic calc–schists and occurred as sulfide–bearing quartz veins and veinlets. Based on mineral paragenesis and hydrothermal alteration assemblages, four groups of hydrothermal alteration zones were recognized: silicification, sulfidation, sericitization and carbonatization. Silicification and sulfidation are observed in the inner parts of the shear zone and are spatially associated with the mineralization. In general, the alteration zones containing mineralization are mostly composed of quartz ± carbonate–feldspar–sericite– chlorite and sulfide minerals.

4.2. Ore mineralogy

Microfractures, jointing and faulting provided suitable fluid migration pathways during the final deformation steps in the study area and the economic mineralization is mainly hosted in the contact zone of Oligocene granitoid and Eocene calc–schist rocks. The emplacement of sulfides in brittle fractures can be seen as mineralized quartz veins parallel and/or transsect to the mylonitic foliations. In general, the mineralization occurs as pyrite, stibnite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, gold and electrum (Fig. 8a–h). Pyrite is the most abundant sulfide mineral that is spatially accompanied by gold and electrum. Pyrite, pyrrhotite, arsenopyrite and sphalerite crystals (up to 200 µm) and stibnite and chalcopyrite crystals (up to 100 µm) show euhedral and subhedral shapes. Based on the textural relationships and microscopic studies, chalcopyrite, sphalerite, arsenopyrite and pyrrhotite were also formed synchronously with pyrite. The gangue minerals are mainly comprised of quartz, feldspar, sericite, muscovite and calcite. The secondary minerals are iron oxide–hydroxide (mainly hematite and goethite) and antimony hydroxide (stibiconite). Goethite was formed due to the decomposition of sulfide minerals such as pyrite and filled the cracks and joints. According to the mineral paragenetic associations (Fig. 9), three key episodes of mineralization were distinguished: (i) Stage I comprises a considerable precipitation of quartz and pyrite, (ii) Stage II involves the formation of quartz, pyrite, chalcopyrite ± calcite, gold and electrum, and (iii) Stage III comprise of quartz, calcite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, gold and electrum.

4.3. Fluid inclusion studies

4.3.1. Fluid inclusion petrography

The microthermometry of fluid inclusions was conducted on the quartz segments of the ore–bearing quartz veins. According to the phase relationships at room temperature and the phase transitions during cooling/heating experiments, four types of fluid inclusions were recognized, including mono–phase gas (V) (type I; Fig. 10a), monophase aqueous (L) (type II; Fig. 10b), two–phase liquid–vapor (LV) (type III), and three–phase carbonic–aqueous (L_CO2+L_H2O+V_CO2) (type IV). During the microthermometry experiments, no data were collected from Types I and II, due to their small sizes and, therefore, types III (LV) and IV (L_CO2+L_H2O+V_CO2) of primary fluid inclusions were only studied in this study (Table 1 and Fig. 10). Type III (ranging from 5 to 16 µm) with spherical, elongated or irregular shapes, contains negative crystals and mainly composed of two phases (LV) at room temperature (Fig. 10c–e). Type IV (ranging from 4 to 18 µm) shows ellipsoidal, triangular and irregular shapes and contains three phases (L_CO2+L_H2O±V_CO2) at room temperature (Fig. 10f–h).

Table 1

Summary of the fluid inclusion data for quartz–sulfide veins from the Lakhshak deposit (*fluid inclusion assemblages)
FIAs*	Number		Size (µm)	Salinity (wt% NaCl)	Th (°C)	Tm_CO2 (°C)	Th_CO2 (°C)	Tm_clath (°C)	Tm_ice (°C)
Type III (L + V)	19		5–16	9–11	200–280	–	–	–	–4 to − 7
Type IV (carbonic–queous)		23	4–18	8–13	300–330	–58.8 to − 56.8	6.1 to 16.2	1.3 to 4.6	–

4.3.2. Fluid inclusion microthermometry and Raman spectroscopy

We studied about 200 fluid inclusions during microthermometry measurments and the following items were measured: initial melting of CO₂ phase (Tm_CO2), CO₂ homogenization (Th_CO2) and final melting of clathrate (Tm_clath) for CO₂–bearing inclusions, as well as homogenization temperature (Th) and final melting of ice (Tm_ice) for aqueous inclusions (Table 1). The obtained data were collected using the FLUIDS and CLATHRATES of Bakker's (1997, 1999) software packages. Moreover, the salinity (for aqueous and carbonic aqueous inclusions), fractions of compositions, the density of the carbonic liquid and bulk fluid were determined using the FLINCOR program (Brown, 1989).

The microthermometric measurements indicate that the homogenization temperature for type III (LV) fluid inclusions ranging from 200 to 280°C (avg. 270°C; Fig. 11a). The Tm_ice of type III ranges from − 4.0 to − 7.0°C (Fig. 11b), indicating a salinity between 9 and 11 wt.% NaCl equiv., with a sharp peak at ⁓9 wt.% NaCl equiv. (Fig. 11c). For type IV (L_CO2+L_H2O+V_CO2) fluid inclusions, the homogenization temperature varies between 300 and 330°C (avg. 320°C; Fig. 11a) and Tm_CO2 and Th_CO2 vary from − 58.8 to − 56.8°C (Fig. 11d) and 6.1 to 16.2°C, respectively. Moreover, the clathrate melting (Tm_clath) for these fluid inclusions ranging from 1.3 to 4.6°C (Fig. 11e) which futher suggests fluids with high salinity of about 8–13 wt. % NaCl equiv. (avg. 12 wt.% NaCl equiv.; Fig. 11c). Based on the microthermometric studies, the deficiency of variable liquid/vapor ratios in fluid inclusions, the low salinities and the lack of halite daughter–bearing inclusions at Lakhshak imply that the ore fluids did not undergo boiling mechanism during their entrapment. Raman spectroscopy studies are a non–destructive technique for analyzing fluid inclusions with a wide range of applications for the qualitative designation of solid, liquid and gaseous compounds (Cox, 2016). Based on the Raman spectroscopy results, type IV contains a high amount of CO₂ (98–100 mol %), CH₄ (70–80 mol %) and N₂ (92–100 mol %) and the liquid phase has a large amount of H₂O (Fig. 12a–d).

4.4. Stable isotopes geochemistry

The δ¹⁸O_SMOW of quartz for four samples collected from the sulfide–bearing quartz veins and veinlets of the Lakhshak deposit ranges from 15.9 to 18.0‰ with an average of 16.74‰. The calcuñated δ¹⁸O_fluid values (using Zheng, 1993) approximately range from 7.5 to 9.8‰ (Table 2). The sulfur isotope compositions of six pyrite (from quartz veins and veinlets hosted in altered calc–schist, calc–schist and granitoid intrusion) and three stibnite samples vary from 3 to 4.1‰ and − 0.9 to 0.8‰, respectively (Table 3).

Table 2. Oxygen isotopic values Oof the quartz samples from the sulfide–bearing quartz veins and veinlets.

Sample No.	δ¹⁸O_SMOW (‰)	Th (°C)	δ¹⁸O_fluid (‰)
LK–34	18.00	270	9.8
LK–200	15.90	270	7.5
LK–208	16.01	320	7.9
LK–220	17.07	320	8.1

Table 3

Sulfur isotopic values of pyrite and stibnite samples from sulfide–bearing quartz veins and veinlets.
Sample No.	Rock type	Minerals	δ³⁴S (‰)
LK–6–1	Sulfide–bearing quartz veins and veinlets (hosted in altered calc–schist)	Pyrite	3.3
LK–208	Sulfide–bearing quartz veins and veinlets (hosted in altered calc–schist)	Pyrite	3.6
LK–192	Calc–schist	Pyrite	4.1
LK–198	Calc–schist	Pyrite	4.0
LK–200	Granitoid	Pyrite	3.1
LK–220	Granitoid	Pyrite	3.0
LK–34	Stibnite (± Pyrite )–quartz vein	Stibnite	0.8
LK–N3	Stibnite (± Pyrite )–quartz vein	Stibnite	0.5
LK–N1	Stibnite (± Pyrite )–quartz vein	Stibnite	–0.9

5.1. Evolution of the ore fluids

According to fluid inclusion microthemometry studies, the CO₂–rich inclusions imply that type IV of fluid inclusion was probably trapped during metamorphism, which means sourced from metamorphic fluids. Furthermore, the ubiquity of CO₂–rich fluid inclusions shows highly particiaption of the metamorphic fluids during the formation of the Lakhshak deposit. It seems that ore fluids in the study area are plausibly CO₂–rich under the greenschist–facies conditions. Hence, it is suggested that metamorphic dehydration of seafloor rocks produced numerous low–salinity aqueous–carbonic fluids at Lakhshak and gold seems to be transported in the form of bisulfide complexes in metamorphic fluids. The microthemometry results show that gold is derived from the deposition of low–salinity fluids (9 to 12% wt.% NaCl) at a temperature of 200 to 330°C. The homogenization temperature vs. salinity plot (Fig. 13) shows that the mineralization resulted from the deposition of a CO₂–rich hydrothermal fluid at a maximum temperature of 330°C, which has been gradually cooled. This plot shows the mixing of hydrothermal fluid (metamorphic fluid) with another fluid (meteoric fluid?) that have had lower salinity and temperature, which can be compatible with a combination of mixing and dilution processes (Wilkinson, 2001). Hence, it is suggested that the ore–forming system evolved from the mixing of a CO₂–rich metamorphic fluid with a CO₂–poor fluid as a result of influx of meteoric water. Therefore, it is implied that the fluid mixing and subsequent dilution processes were crucial prerequisites for gold precipitation at Lakhshak (Fig. 13). In addition, the δ¹⁸O values of quartz samples were measured between 7.5 and 9.8‰, which accordingly suggest that the ore fluids were metamorphic fluids that reacted repeatedly with the volcanic rocks along the conduits as well as the granitoid units adjacent to the ores and equilibrated with them in terms of isotopic composition. Therefore, due to the significant oxygen isotopic exchange in the fluid–rock reaction, the metamorphic fluids show the isotopic properties of specific fluids of magmatic origin. However, the possibility of entering magmatic fluids into the ore fluid system cannot be ignored.

5.2. Source of sulfur

The δ³⁴S composition of pyrite and stibnite range from 3 to 4.1‰ and − 0.9 to 0.8‰, respectively (Table 3) which are analogous to the sulfur isotope composition of metamorphic fluids (i.e., − 25 to 20‰, Goldfarb et al., 2005). The results of sulfur isotopes are good agreement with the δ³⁴S values of the Lakhshak calc–schists. The positive sulfur isotopic composition (0.8 to 4.1‰) and the paragenesis of sulfide minerals indicate the regenerative nature of ores. According to the δ³⁴S isotopic ratios and the correlation of the sulfur isotopic range with a volcanic origin, Heydarian Dehkordi et al. (2020) suggested that the Lakhshak volcano–sedimentary rocks, which are rich in hydrous silicates, provided even more numerous fluids during transporting from the boundary of greenschist to amphibolite facies. However, the δ³⁴S values of sulfide ores approximately vary from − 0.9 to 4.1‰ comparable with the δ³⁴S values of the volcanic origin (i.e., 0 ± 5‰; Taylor and McLennan, 1995). Sulfur, gold and other ore minerals at Lakhshak, similar to other known orogenic gold deposits worldwide, appear to have originated from sulfur–rich volcanic–sedimentary units. Although the range of sulfur isotope compositions is close to the isotopic compositions of magmatic fluids (Taylor and McLennan, 1995), therefore, they can also be originated from the middle parts of the crust where sulfur was leached by the desulfidation process in volcanic rocks and involved in the ore fluids composition (Zheng, 1993).

5.3. Genetic model

Orogenic gold deposits form within the accretionary metamorphic belts of Archean to Tertiary age and quartz/quartz–carbonate veins (greenstone–hosted quartz–carbonate veins) in crustal–scale shear zones and related faults (Groves and Goldfarb, 2003; Bierlein et al., 2006; Pitcairn et al., 2021). Based on Groves et al. (2005) and Augustin and Gaboury (2017), these orogen–related deposits generate during compressional to transpressional deformation regims at convergent plate margins during accretionary processes, showing various degrees of deformation and metamorphism, although dominantly occur in greenschist facies sequences. Gold is predominantly related to sulfide–bearing assemblages (e.g., pyrite, pyrrhotite, arsenopyrite, chalcopyrite and other base–metal sulfides) in these deposits (Kerrich and Caldera, 1998; Goldfarb et al., 2001; Zhu, 2011).

The Lakhshak deposit has generated in the subduction regime of accretionary–collisional orogenic belts by the opening and successive closure of the Sistan branch. It is marked by a series of ophiolites and volcano–sedimentary sequences. Based on Jentzer et al. (2020), the formation of the Sistan suture zone led to the setting up of main faults and shear zones that functioned as pathways for circulation of the ore fluids. In this deposit the mineralization is spatially and temporally related to the NE–SW trending ductile–brittle shear zone and the intense silicification and sulfidation occur in the contact of granitoid intrusion and calc–schist rocks. The shear zone contains structurally desirable conduits for the gold precipitation and acts as a pathway for transporting ore–forming fluids from the metamorphic dehydration zone to the deposition site in the upper crust (Heydarian Dehkordi et al., 2021). The gold occurance is accompanied by sulfide minerals such as pyrite, chalcopyrite, arsenopyrite and pyrrhotite in quartz veins and veinlets. The ductile deformation accelerated the pressure solution and release of ore fluids that generated the inclusions of electrum in pyrite. The ongoing deformation progress of primary gold in pyrite was accordingly liberated and precipitated as micro–inclusions within pyrite. Ascending metamorphic hydrothermal fluids were enriched in sulfur, gold and associated metals (i.e., Ag, As, Hg, Pb). According to oxygene and sulfur isotopes compositions, the role of diverse ore fluids is disclosed during the formation of mineralized quartz veins and veinlets through the evolution history of this deposit. Hence, it is suggested that by increasing geothermal gradients via the emplacement of garnitoid intrusion, the circulation of meteoric water and the ascension of metamorphic fluids initiated and subsequently led to the fluid mixing and gold deposition at Lakhshak. The high amount of CO₂ generated during the metamorphism of carbonate–rich sedimentary rocks shows the significant effect of metamorphic fluids during the formation of the deposit. The schematic model of gold deposition mechanism in the area is presented in Fig. 14. The Lakhshak deposit is of the orogenic type similar to those of Sanandaj–Sirjan zone such as Kervian, Kharapeh, Muteh and Chah Bagh, Ghabaqloujeh, Qolqoleh, Hamzeh–Gharanein and Zartorosht gold deposits (Rastgo Moghadam et al., 2003; Heidari, 2004; Niroomand, 2004; Kouhestani et al., 2006; Niroomand et al., 2011; Tajeddin et al., 2011; Aliyari et al., 2014; Maghsoudi et al., 2017).

The Lakhshak Au–Sb deposit (the SW margin of the Sistan suture zone) comprises Eocene volcano–sedimentary sequences intruded by Oligocene granitoid body and dikes. This deposit is an orogenic Au–Sb deposit consisting of sulfide–bearing quartz veins and veinlets mainly comprised of pyrite, pyrrhotite, chalcopyrite, arsenopyrite, sphalerite, stibnite, native gold and electrum. The mineralization is structurally governed by a NE–SW trending ductile–brittle shear zone in the granitoid intrusion and calc–schist contacts. The high–grade gold mineralization is spatially related to the intense hydrothermal alterations including silicification, sericitization and sulfidation in the inner sections of the shear zone. The mineral assemblage, fluid inclusion microthemometery and oxygen isotope compositions indicate that the deposit evolved from the mixing of a CO₂–rich metamorphic fluid with a CO₂–poor fluid due to an influx of meteoric water. Thus, it is suggested that the fluid mixing and dilution have played a significant role for gold precipitation at Lakhshak. Moreover, the sulfur isotopic compositions of sulfides imply that sulfur was predominately originated from the leaching of magmatic sulfides from the underlying volcano–sedimentary rocks. This study indicates that Lakhshak was formed during the compression–extension regime in a ductile–brittle shear zone which is normally similar to the global orogenic gold deposits.

Funding

The authors are gratefully acknowledged the Iran Minerals Production and Supply Company and the University of Tehran for their financially supports during this research.

Author Contribution

Shahrokh Rajabpour: writing, investigation, supervision, funding, final edition, Shojaeddin Niroomand: writing, investigation, funding, supervision, final edition, Nasim Heydarian Dehkordi: writing, investigation, supervision, final edition, Hossein Ali Tajeddin: writing, investigation, supervision, initial edition, Reza Nozaem: writing, investigation, supervision, first edition,

Acknowledgments

Not Applicable

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No competing interests reported.

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Genesis of the Lakhshak orogenic Au–Sb deposit, SE Iran: insights from geology, microthermometry and stable isotope geochemistry

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Abstract

Figures

1. Introduction

2. Geological setting

2.1. Sistan suture zone

2.2. Lakhshak area

3. Material and methods

4. Results

4.1. Mineralization and alteration

4.2. Ore mineralogy

4.3. Fluid inclusion studies

4.3.1. Fluid inclusion petrography

4.3.2. Fluid inclusion microthermometry and Raman spectroscopy

4.4. Stable isotopes geochemistry

5. Discussion

5.1. Evolution of the ore fluids

5.2. Source of sulfur

5.3. Genetic model

6. Conclusions

Declarations

Funding

Author Contribution

Acknowledgments

References

Additional Declarations

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