Geology, geochemistry, and genesis of the Alvand sapphire-bearing syenitic pegmatite, Sanandaj-Sirjan Zone, Hamedan Province, northwest Iran

doi:10.21203/rs.3.rs-1530821/v1

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Geology, geochemistry, and genesis of the Alvand sapphire-bearing syenitic pegmatite, Sanandaj-Sirjan Zone, Hamedan Province, northwest Iran

https://doi.org/10.21203/rs.3.rs-1530821/v1

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Alvand blue sapphire is related to the igneous rocks of Alvand pluton, located in the Sanandaj–Sirjan zone, Hamadan area. The main geological unit is the Alvand batholith including gabbro, granite, and granitoid. Sapphire occurs within the desilicated pegmatite dykes intruding magmatic-metamorphic lithologies, closely to metasomatic zones. Sapphire-bearing rocks are situated in the central zone of aplitic-pegmatitic composite dykes. In addition to sapphire, the rocks contain albite, microcline, and orthoclase as major minerals, the absence of quartz is noticeable. The whole-rock geochemical characteristics show a relatively silica-poor and alumina-rich composition of syenitic sapphire-bearing rocks. According to peraluminous and S-type signatures, aplite and pegmatites indicate crustal magma origin. The slight increase of Al₂O₃/TiO₂ ratio in the sapphire-bearing pegmatites may suggest mobilization of Al, which either by melt mobilization through alkaline hydroxide-complexation or by alkali-bearing high-temperature fluids such as Na-Al-Si-O polymers. Also based on strong enrichment in Th, sapphire may be formed by late-stage fluids. The high Ti, Fe, and Ga parallel to the low Cr and V contents indicate a magmatic-metasomatic origin of the Alvand sapphire.

Sapphire

Syenite

Metasomatic

Pegmatite

Alvand batholith

Corundum is a relatively rare mineral that formed under special geochemical conditions of low silica activity combined with high aluminum contents of the host rock (Yakymchuk and Szilas 2017). Corundum, which consists of more than 98% of Al₂O₃, in its purest form is colorless, but some coloring agents such as Fe, Cr, V, Ti, Mg, and Ga enter the structure, causing forming ruby (red) or sapphire (other colored) gem varieties of the mineral corundum (Peucat et al. 2007; Yakymchuk and Szilas 2017).

In recent years, the most important concern of mineralogists and gemologists identified the process of formation and origin of gem-quality sapphire for exploration purposes and also control the international trade of the sapphire market. Therefore, the different petrological and geochemical aspects of corundum deposits (ruby and sapphire variety) have been recently investigated in detail (e.g., Graham et al. 2008; Simonet et al. 2008; Sutherland et al. 2008; Giuliani et al. 2009; 2014; Uher et al. 2012; Rakotosamizanany et al. 2014).

Based on the genetic processes, gem corundum (ruby and sapphire) deposits are generally classified into primary (igneous and metamorphic deposits) and secondary (sedimentary and volcanic-xenoclastic) deposits. Aplitic and pegmatitic dykes are common in magmatic intrusions and metamorphic assemblages (Azizi et al. 2016), but the ones containing sapphire have attracted much attention.

The genesis of the sapphire-bearing syenitic pegmatite is practically unknown since similar pegmatites have not been reported previously from Iran and seem to be very rare in other parts of the world (Sorokina et al. 2019). Some sapphire-bearing pegmatites are formed by both direct crystallization of alkali magma and late-stage metasomatism processes (Simonet et al. 2008; Alehashem et al. 2021). Metasomatic reactions and fluid interaction are the main processes of primary deposit. Small-scale metasomatic most involve desilicification reactions between silico-aluminous rocks (pegmatites, aplite, gneisses, etc.) and silica-poor rocks (ultramafics, meta-carbonates, residual alkali fluids in magma), which usually lead to the formation of local limited-size corundum deposits (Simonet et al. 2008).

The study area is located 10 km south of Hamadan city which is situated on the Sanandaj–Sirjan magmatic-metamorphic belt in west Iran (34° 41' 51.65"; 48° 31' 9.79"). The Alvand pluton with more than 400 km² in width is one of the most important plutonic complexes, comprising various plutonic rocks from mafic to felsic, which occurs in the Hamedan region (Fig. 1), and has attracted much attention from many researchers. The geochemical characteristics of some pegmatites and associated granitoid of the region are studied by Sepahi (2008), Shahbazi et al. (2010), Aliani et al. (2012), and Sepahi et al. (2018).

Although based on a few recent pieces of research on the Alvand pegmatites, the researcher demonstrated that the sapphire-bearing pegmatite formed from different origins such as metamorphic events before the emplacement of the Alvand pluton (Valizadeh and Torkian 1999), the combination of metamorphic and magmatic fluids (Masoudi et al. 2009), formation from kyanite during the metamorphism event (Sadeghi et al. 2019) and generation from direct crystallization of magma (Sepahi et al. 2020); but it is not clear. The goal of this research is to investigate the magma nature, tectonic setting, and genesis of sapphire in an aplitic-pegmatitic dykes situated in the Alvand plutonic complex, by using mineralogical and geochemical major, minor, and trace elements data, as well as electron probe micro analysis (EPMA), results on the pegmatite syenite bearing sapphire.

Geological setting

Generally, the west of Iran is divided into three parallel structural zones with NW–SE trending (Alavi 1994; Mohajjel et al. 2003), namely, the Zagros Fold and Thrust Belt, the Sanandaj–Sirjan Zone (SSZ), and the Tertiary–Quaternary Urumieh–Dokhtar magmatic arc (Shahbazi et al., 2010) (Fig. 1). The SSZ is a part of northeast Gondwanaland during the Paleozoic period, which separated from it in the Late Paleozoic by intra-continental rifting that formed the Neotethys along the Zagros suture zone (Golonka 2004; Sears et al. 2005; Shahabpour 2005; Ahmadi Khalaji et al. 2007; Fazlnia et al. 2009). The subduction of Neo-Tethyan oceanic crust beneath the central Iranian micro-continent south of the Eurasian supercontinent caused the SSZ has been considered as a typical active continental margin with significant magmatism in the Mesozoic period (e.g., Ghasemi and Talbot 2006; Mehdipour Ghazi and Moazzen 2015; Hassanzadeh and Wernicke 2016). The SSZ contains abundant Mesozoic arc-related plutonic and plutono-metamorphic complexes, such as Alvand, Almogholagh, Samen, Ghorveh, Borojerd, Urumieh, Arak, Astaneh, Qori, and Siah-Kuh (Saki 2011) (Fig. 1). These plutons occur in the oldest metamorphic rocks, including the medium and high-grade metamorphic rocks, such as garnet schist, staurolite schist, andalusite schist, and sillimanite schist, and low- to very low-grade metamorphic rocks such as slates and phyllites (Baharifar et al. 2004). Most plutons are metaluminous to slightly peraluminous and belong to the high-K calc-alkaline series (Shahbazi et al. 2010).

Alvand batholith consists of a wide range of plutonic rocks, mainly gabbro, diorite, granodiorite, granite, and leucogranites that were intruded by aplitic and pegmatitic dykes (Sepahi et al. 2018). Based on U-Pb zircon dating, the average age of three major rock associations in the Alvand batholith including gabbros, granites, and leucocratic granitoid is 166.5 ± 1.8 Ma, 161.7 ± 0.6 to 163.9 ± 0.9 and 153.3 ± 2.7 to 154.4 ± 1.3 Ma; respectively (Shahbazi et al. 2010). The U-Pb allanite, zircon, and monazite dating of sapphire-free pegmatite-aplite show 154.1 ± 3.7 to 172.2 ± 0.9 Ma (Sepahi et al. 2018), and sapphire-bearing pegmatite-aplite with 164 ± 1 to 168 ± 1 Ma in age (Sepahi et al. 2020). This dating indicates pegmatites, aplites, gabbros, granites, and leucocratic granitoid in the Alvand pluton are formed simultaneously in the Jurassic.

Local Geology and sampling

The study area contains outcrops of pegmatitic, aplitic, and composite aplitic and pegmatitic dykes of the Alvand complex associated with surrounding metamorphic rocks (Fig. 1). In this area, aplites and pegmatites occur in the interior and margin of the Alvand pluton. In some places aplitic-pegmatite dykes crosscut the plutonic (granites) and metamorphic (cordierite- hornfelses, andalusite-hornfelses, andalusite-sillimanite schists) lithologies (Fig. 1).

The thickness of dykes is in a range of a few centimeters to more than 3 m, mostly simple granitic composition and comparatively small size of sapphire-bearing syenitic rocks. The aplites are observed in two different forms including simple dykes (aplite) and complex dykes that are closely associated with pegmatite. The latter sapphire-bearing pegmatite in composite dykes, commonly occurs in the central zone surrounded by aplites (Fig. 2a). They contain blue sapphire with diameters of a few millimeters to more than 1 cm (Fig. 2b). The aplitic simple dykes are sapphire-free (barren) and have a lower thickness than composite dykes (Fig. 2c).

The current research discriminates between productive and barren dykes by geochemical analysis and the genesis of sapphire-bearing rocks. The samples were taken from sapphire-bearing pegmatite (SBP) and simple aplitic rocks consisting of barren composite aplite (BCA) which are associated with sapphire-bearing pegmatite and barren aplite (BSA) (Fig. 2a, c).

Analytical methods

Fifty samples were collected through the lateral and vertical surveys in order to obtain representative samples from different lithology including the sapphire-bearing pegmatite (SBP), barren composite aplite (BCA), and barren aplite (BSA) in the study area. Twenty thin sections were prepared and examined using conventional techniques under the reflected optical microscope for petrography studies. X-ray diffraction (XRD) was performed on 6 samples using an X-ray diffractometer PW1800 model equipped with a copper X-ray tube (radiation length Cukα1 = 1.54 Å) with graphite monochromator, 40 kV, 30 mA, an angle of 2θ between 3 and 60 degrees, BRUKER, GADDS bi-dimensional detector Histar at the Binaloud laboratory in Tehran, Iran.

Whole-rock of major element abundances were determined by X-ray fluorescence spectrometry (XRF) at the Zarazma laboratory. This method was carried out on fused lithium tetraborate glass disks using X-ray fluorescence spectrometer PANalytical AxiosmAX fitted with a 4.0 KW Rh X-ray tube. For further geochemical studies of these elements, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) of the PERKIN 9000DRCE model at Zarazma was used. International standard Certified Reference Material (CRM) samples were used to verify the samples. Major and trace element analyses of sapphires were also carried out by electron probe microanalysis (EPMA) on 52 points of nine thin-polished sections on Cameca SX100 instrument in the 20 kV, 4 × 10 − 7 Torr pressure, and 20 nA beam current and 1 to 5 mm beam long with maximum 30 s counting times. The Cameca PAP correction software was used for data reductions.

Petrography and mineralogy

Based on XRD results, mineralogical components, presence of corundum (sapphire) and quartz minerals: igneous rocks of the Alvand area can be classified into two types (Fig. 3, 4; Table 1):

The first one is sapphire-bearing pegmatite (SBP) with the sapphire; albite, microcline, and orthoclase as major mineralogical phases, whereas, muscovite and biotite are accessory minerals. The coarse-grained sapphire shows a fracture system of rhombohedral, crosscutting, or parallel (Fig. 4a). Their diameters vary from a few millimeters to more than 2 cm. The sapphire was incompletely altered to biotite, muscovite, chlorite, and sericite (Fig. 4a, b). The main textural characteristics of the K-feldspar and plagioclase are the subhedral and granular textures. Also, feldspars (microcline) show the perthite texture (Fig. 4c).

The second group is quartz-bearing rocks including barren composite aplite (BCA) and barren simple aplite (BSA) that have albite, muscovite, and biotite as accessory minerals. The occurrence of aplite is less than the pegmatites in the study area. The first one (BCA) with medium to fine grains occurs in sharp contact with the sapphire-bearing pegmatite (Fig. 4d), which the medium-grained aplite is composed of quartz, plagioclase, and biotite as the main minerals (Fig. 4e); and very fine-grained simple aplite consist of mostly quartz and mica (Fig. 4f). Additionally, Muscovite is rarely replaced by the minerals of the chlorite group.
Locally, muscovite forms pseudomorphs after the alkali feldspar (Fig. 4e).

Whole-rock geochemistry

The result of major and trace elements of bulk rocks that were carried out by XRF and ICP-MS methods are given in Table 2. Using the major and trace element contents is necessary for determining the genesis of the sapphire-bearing rocks and the role of silica depletion on the stability of sapphire for three sample groups taken from the study area (sapphire-bearing pegmatite (SBP), barren composite aplite (BCA), and barren simple aplite (BSA)).

The whole-rock geochemical characteristics of the sapphire-bearing rocks show the significant depletion of SiO₂ accompanied by enrichment of Al₂O₃ and K₂O (Fig. 5a, b). The Al₂O₃ content varies from 21.19 to 23.09 wt% and 19.16 to 19.87 wt% in sapphire-bearing pegmatite (SBP) and barren composite aplite (BCA), respectively (Table 2). The contents of Al₂O₃ increase from the barren simple aplite (BSA) towards the sapphire-bearing pegmatite (SBP). The SiO₂ content in sapphire-bearing rocks is up to 63.45 wt%, whereas the contents of SiO₂ are higher in barren aplite rocks (up to 69.02 wt%). Potassium is strongly enriched in sapphire-bearing rocks (up to 8.55 wt%). Also, the average content of some alkali elements (Na₂O+K₂O) in the sapphire-bearing pegmatite is relatively high (11.81 wt%) but in the aplite is typically low (10.08 wt%). The SiO₂ values show correlate negatively with Al₂O₃ and K₂O contents (Fig. 5a, b), indicating that SiO₂ decreases in sapphire-bearing dyke with distance from barren rocks.

The content of TiO₂ is not generally much different between the barren rocks and sapphire-bearing rocks (Table 2), but the sapphire-bearing rocks show an increase in Al₂O₃/TiO₂ ratio (Fig. 5c). Also, Th versus Th/TiO₂ indicates a strong enrichment in Th for the sapphire host rock (Fig. 5d).

Based on the classification diagram of Cox et al. (1979) and Middlemost (1994), the samples were plotted in the fields of syenite, granite, and quartz monzonite (Fig. 6). Therefore, the sapphire-bearing samples lie in the syenite field of the diagram while the other aplitic rocks lie in the granite and quartz monzonite field. The chemical compositions of sapphire-bearing samples differ from the barren aplites, due to lowering the content of SiO₂, but other aplitic rocks show some similarities with granite (Fig. 6a).

The contents of molar A/CNK (molar Al₂O₃/(CaO+Na₂O+K₂O)) ratio of the samples vary from 1.20 to 1.41. Based on the discrimination diagram by Shand (1943), all aplite and pegmatites are peraluminous (Fig. 7a). Plotted geochemical data of I- and S-type granites discriminating diagram illustrated in Figure 7b, confirmed the S-type field for all samples (Chappell and White 1992). The S-type affinity of these granites and pegmatites of the Alvand plutonic complex has been also confirmed earlier by Sepahi (2008), Aliani et al. (2012), and Sepahi et al. (2018). The spider diagrams of all samples normalized to primitive mantle show a similar trend of distribution patterns together with positive Rb, K, U, Th, and Pb anomalies (Fig. 7c).

Chemical characteristics of the Alvand sapphire

The electron probe microanalysis of the Alvand sapphire data is shown in Table 3. The analyses of Ti, Fe, Cr, V, and Ga, were carried out and show the blue sapphire crystals are rich in Ti and Fe. TiO₂ content in the studied sapphire is up to 0.29 wt% with an average of 0.16 wt%. The FeO content of the analyzed samples varies from 0.08 to 0.32 wt%. Based on these results, Ga₂O₃ content is up to 0.13 wt%, but Cr₂O₃ and V₂O₃ contents are very low (up to 0.01 and up to 0.04 wt%, respectively).

The chemical classification diagram (the FeO-Cr₂O₃-MgO-V₂O₃ versus FeO+TiO₂+Ga₂O₃diagram) by using the database of 2000 microprobe analyses of sapphire samples from primary worldwide deposits represents the different fields of deposits such as sapphire in mafic rocks, alkali rocks, marble, and metasomatites (Giuliani et al. 2010; Uher et al. 2012; Rakotosamizanany et al. 2014). The plotting of analytical points on this diagram indicates the alkali rocks and metasomatic origin of Hamedan sapphire (Fig. 8).

Evaluation of mineralogy

Field observations, mineral paragenesis, and geochemical studies suggest that the main processes in the sapphire formation are the intrusion of the granitic magma and partial melting of crustal rocks (Saki et al. 2012). Based on this study, the formation of minerals associated with the sapphire in the syenitic pegmatite of the Alvand batholith occurred at least in two stages including magmatic and metasomatic. In the first magmatic stage, the formation of the sapphire syenitic pegmatites is accompanied by the intrusion of mafic magma and the formation of alkaline rocks. As a result, the sapphire, alkali feldspar, and microcline are major mineral assemblages, observed in the microscopic studies. The sapphire crystallized in the magmatic stage in equilibrium with K-Na-feldspar and mica.

In the second stage (later metasomatic stage), the mica group of minerals especially muscovite is formed by the reaction between sapphire and orthoclase. Then, the intergrowth of feldspar in perthite texture indicates a metasomatic phase. Also, feldspar-group minerals are strongly altered (serizitized) with the remanent of the Carlsbad twinning (Sorokina et al. 2017).

Biotite is commonly observed around sapphire crystals as a result of sapphire reaction with alkali feldspar (Fig. 4). Biotite may be formed by the water/fluid saturated melt when it originated from the slab of mantle melting. Thus, the parental melts related to subduction are considered the origin of the igneous rocks (Shahbazi et al. 2010). Also, the presence of quartz and K-feldspar in the granite as well as the geochemical characteristic such as S-type granite composition are in accordance with the crustal origin (Ghalamghash et al. 2009). During these two stages, the aplites are usually formed due to the intrusion of mafic dykes into crystallizing granitic plutons and the composite dykes have formed by the intrusion of the alkali magma in the fractures accompanied by the dykes is confirmed by Sepahi (2008).

Geochemical variations

Different classifications of gem corundum (sapphire) deposits have been suggested by different researchers based on the geological settings, the nature, and lithology of host rocks, and the genetic processes of sapphire formation (Simonet et al. 2008; Giuliani et al. 2010; Uher et al. 2012; Rakotosamizanany et al. 2014).

The genesis of the rare sapphire deposit in the syenitic pegmatite is not well-known. According to geochemical data, the syenitic nature of the host rock of sapphire has been reported in Russia, Canada, India, and Norway (Giuliani et al. 2014) and this study has investigated the origin and the formation of this rare kind of deposit.

The Jurassic magmatic activities in the Sanandaj-Sirjan zone (Sepahi et al. 2018) together with the geochemical characteristics of the Alvand pegmatitic-aplitic simple and composite dykes indicate that they originated from the same crustal sources. The primitive mantle normalized patterns of trace elements in the Alvand sapphire-bearing pegmatite are confirmed by the crustal melts from the source magma, which is marked by the enrichment of Rb, K, U, Th, and Pb (Harris et al. 1986; Searle and Fryer 1986; Chappell and White 1992) (Fig. 7c). In the study area, the crustal origin is supported by S-type and peraluminous signatures of the rocks and consists of the magmatic origin (Fig. 7a, b). In addition, the crustal origin of the Alvand sapphire is confirmed by Gheshlaghi et al. (2020) based on δ¹⁸O data. Also, the calc-alkaline characteristics of the pegmatites demonstrate the typical geochemical features of magmatic arc intrusions associated with an active continental margin (Aliani et al. 2012).

A genetic model for the formation of the sapphire-bearing pegmatite

The based on field observations, mineralogical, geochemical, and microprobe data we suggest the following genetic model (Fig. 9):

1. The subduction of Neo-Tethys oceanic crust is the main reason for the Jurassic magmatism of the Sanandaj-Sirjan zone (Shahbazi et al. 2010; Sepahi et al. 2020). Alvand complex was formed during this period according to Shahbazi et al. (2010) dating by the emplacement of originated mafic magma from the oceanic crust into the continental crust. The situation of the samples on the Na₂O+K₂O-SiO₂ diagram (Fig. 7d) gives an indication of partial melting processes (Wilson 2007). These results are also consistent with the S-type, peraluminous nature, and enrichment of Rb, K, U, and Pb that mention above and demonstrated partial melting of crust (Harris et al. 1986; Searle and Fryer 1986; Chappell and White 1992).

2. The melt loss process led to the enrichment of SiO₂ (Yakymchuk and Szilas 2017) and the simple fine-grained granitic-aplitic dyke was formed, which is barren and free of sapphire. At this stage, the remnant magma is depleted of silica. The results agree well with the dating of Sepahi et al. (2020) which confirms older simple dykes than sapphire-bearing composite dykes.

3. Fracture filling dyke which is formed from the remnant magma of stage two, causes the zonation in pegmatite and composite aplitic-pegmatitic dyke formation. The fracture fillings pegmatites may be useful as a prospecting tool for colored gemstone (Sepahi et al. 2018), which are present as composite dyke with zoning in the Alvand area. Internal zonation could be the most important feature in pegmatites (London 2014). In this research, the presence of the internal zonation is confirmed by the field observation, which is the center of attention in most pegmatite research. This zonation is formed within pegmatite bodies by the mineralogical and textural changes (London 2009). This is including the wall zone on the border and the core zone in the center. The aplitic wall zone of the fracture filling with the granitic composition is composed of mostly quartz minerals (London 2013). The mineral crystals into the aplite (wall zone) formed rapidly from the melt which is a very fine grain. The core zone shows the syenitic nature with very low SiO₂content. In the study area, aplites and composite aplitic-pegmatitic rocks occur as discordant dykes that intrude igneous and metamorphic rocks which are common in the interior and margin of the Alvand pluton. There is a reasonable genetic relation between granitic aplites and syenitic pegmatites.

4. The fractional crystallization is the main process in this stage, which is the result of silica depletion due to the entrance of SiO₂ to the aplitic wall and the formation of Al-hydroxide complexes in the fluid. The suggested mechanism for SiO₂ depletion in the core zone during the sapphire formation process is melted loss, in which silica was depleted because of silicate minerals formation in the wall zone such as quartz, plagioclase, and biotite during fractional crystallization (London 2009). The fluid equilibrated with the crystallized phase is an alkali-rich melt that becomes increasingly potassic, sodic, and aluminous, which formed a core zone (London 2014). The enrichment of silicate-rich minerals in the wall zone is evidence of the silica transfer during the formation of these minerals (London 2013). Also, the silica loss causes the enrichment in Al₂O_3, and this process is responsible for the formation and stabilizing of sapphire in the core zone of the composite pegmatite of the Alvand. The high content of Al in the sapphire formation is fed from Al that transform by hydrothermal addition of Al-hydroxide complexes (such as Na-Al-Si-O polymers) in highly alkaline conditions (Manning 2006; Kerrick 2009; Szilas et al. 2016; Yakymchuk and Szilas 2017). This is supported by the presence of the alkali-silicate minerals, such as orthoclase, and microcline, and the absence of quartz in the core zone together with the syenitic composition of sapphire bearing pegmatite.

The formation of sapphire-bearing rocks is consistent with significant depletion of SiO₂ accompanied by the addition of Al₂O₃ and K₂O. The enrichment of alkali elements and depletion of SiO₂ contents of the sapphire-bearing pegmatite in comparison to the barren rocks may be attributed to the relatively higher degree of partial melting in the source magma (Wilson 2007). The silica content is depleted in the barren rocks through sapphire-bearing pegmatite, but the content of TiO₂ is not generally much different in the barren aplitic rocks and the sapphire host rocks. TiO₂ content is constant during metasomatism processes such as fractional crystallization and fluid interaction since both elements have high field strength (HFS) and are not easily mobilized (Yakymchuk and Szilas 2017). No significant difference in TiO₂content of the rocks, high Al₂O₃/TiO₂ ratio in the sapphire-bearing rocks and a strong enrichment in Th as an incompatible element in high temperature may suggest that Al was mobilized in these rocks; either by to melt mobilization and alkaline hydroxide-complexation or by alkali-bearing high-temperature fluids such as Na-Al-Si-O polymers (Manning 2006; Kerrick 2009; Szilas et al. 2016).

5. Sapphire and alkali feldspar crystallization (the formation of sapphire-bearing pegmatite) from the cooling core zone occurs in the last stage. Fluid circulation or small-scale metasomatic processes responsible for the formation of gem corundum (sapphire) deposits are usually desilication phenomena, which caused to form of a desilicated pegmatite (Simonet et al. 2008). The present study also suggests that fluid circulations during the late stage of magma crystallization have provided the conditions for the formation of the sapphire where metasomatic exchanges between residual fluid and acidic crystallized phase occurred. The geochemical characteristics (e.g., negative correlation between SiO₂, Al₂O_3, and K₂O) can be related to the contribution of late-stage fractional crystallization of alkali feldspar (London 2013). Based on mineralogical studies, the latest primary units are quartz-poor and alkali feldspar plus sapphire-rich. According to London (2009), the absence of quartz in this assemblage as a late-stage and last phase confirmed the magmatic origin (direct crystallization of alkali magma) and based on the EPMA data and Th enrichment, the metasomatic origin can be regarded as parallel to magmatic in the Alvand sapphire formation.

According to this model, we suggest some main indicators for the exploration of sapphire in this area: At first, the quartz-rich dyke should be ignored because quartz is present then sapphire is not expected to be stable, and; the second, pegmatites in composite dykes are suitable for prospecting goal. The composite pegmatite dykes with syenitic composition and without quartz are noticeable for a sapphire. This is confirmed in this research by geochemical data that barren rocks are silica-rich and alumina-poor, and sapphire-bearing rocks are silica-poor and alumina-rich with syenitic chemical composition.

The Alvand sapphire-bearing rocks are found in aplitic-pegmatitic composite dykes and their origin field is related to igneous environments of Alvand pluton in this area. Present study results suggest that fluid circulation during a late magmatic stage causes small-scale metasomatic exchanges between residual fluid and the acidic crystallized phase of magma, which is the major controlling factor in sapphire formation. Accordingly, we may suggest two main processes for sapphire formation in the Alvand area: (1) Pegmatite formation in composite dykes occur in two stages; in the first stage silicate minerals crystallized whereas in the barren composite aplite formed SiO₂ is depleted in fluid, and second stage sapphire crystallized (sapphire-bearing pegmatite) from residual Al₂O₃-rich and SiO₂-poor fluid and (2) sapphire formed from Al-hydroxide-complex, or alkali-bearing high-temperature fluids as Na-Al-Si-O polymers.

Acknowledgments

The authors would like to thank the members of the Department of Earth Science, Shiraz University. The authors acknowledge “Iran national science foundation”, for financial and logistic support for the realization of the Ph.D. thesis of Zahra Gholami.

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On behalf of my coauthor, I declare that all of the data and the text as well as all the documents of the manuscript are original. There are no business interests in this research work and no conflict of interests with other persons or organizations. The authors agree with the contents, the submission, and the sequence of authors listed. This manuscript was not submitted to other journals before, and it is not under review elsewhere. Submission is original and comprises previously unpublished data and interpretations.

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All authors participated in the writing and reviewed the manuscript.

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Authors Contributions

Zahra Gholami, Bijan Etemadi, and Soqra Rasti wrote the main manuscript text and Zahra Gholami prepared figures. All authors reviewed the manuscript.

Funding

Iran‘s national science foundation and Shiraz university have supported the financial and logistic support for the realization of the Ph.D. thesis of Zahra Gholami and this manuscript.

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Table 1 The XRD results of corundum bearing veins and barren aplite veins SBP: Sapphire-bearing pegmatite, BCA: Barren composite aplite, and BSA: Barren simple aplite.

Sample	Sapphire	Quartz	Albite	Microcline	Biotite	Muscovite	Orthoclase
SBP₁	*	-	*	*	-	*	*
SBP₂	*	-	-	*	-	*	*
SBP₃	*	-	*	*	*	-	*
BCA₁	-	*	*	-	*	-	-
BCA₂	-	*	*	-	-	-	-
BSA₁	-	*	*	-	*	*	-

Table 2 Chemical composition of major and trace elements of bulk rocks from Alvand samples by using XRF and ICP-Ms methods. Major contents have been measured by XRF in wt% and trace elements have been measured by ICP-MS in ppm. D.L. refers to the detection limit. SBP: Sapphire-bearing pegmatite, BCA: Barren composite aplite, and BSA: Barren simple aplite.

Sample	SBP₁	SBP₂	SBP₃	SBP₄	SBP₅	SBP₆	SBP₇	BCA₁	BCA₂	BCA₃	BCA₄	BCA₅	BSA₁	BSA₂	BSA₃	D.L
Sapphire bearing pegmatite								Barren composite aplite					Barren simple aplite
SiO₂	61.92	62.22	63.45	59.23	60.34	62.45	60.27	63.64	62.74	69.02	67.96	65.24	64.26	65.73	65.79	0.01
TiO₂	0.37	0.73	0.35	0.00	0.00	0.73	0.00	0.35	0.35	0.26	0.64	0.35	0.6	0.47	0.00	0.01
Al₂O₃	22.97	23.09	21.19	21.75	22.45	21.96	22.07	19.16	19.56	19.45	19.87	19.78	13.83	18.92	17.25	0.01
Fe₂O₃	3.73	1.38	1.53	1.18	0.79	0.8	0.94	0.77	4.1	1.08	1.58	0.97	5.26	5.76	1.26	0.01
MnO	0.07	0.04	0.03	0.02	0.01	0.01	0.02	0.02	0.12	0.04	0.05	0.01	0.11	0.06	0.02	0.01
MgO	0.51	0.05	0.05	0.07	0.05	0.06	0.03	0.03	0.23	0.07	0.34	0.05	0.46	0.73	0.05	0.01
CaO	0.44	0.25	0.2	0.27	0.24	0.3	0.24	0.67	0.88	0.42	0.47	0.51	1.66	1.45	0.24	0.01
Na₂O	4.72	3.37	4.26	4.91	4.85	4.79	3.66	6.34	6.22	2.9	3.66	4.01	4.1	3.6	3.78	0.01
K₂O	5.94	6.55	8.55	8.07	7.65	8.09	7.27	3.86	3.9	5.85	5.28	5.94	5.78	5.35	4.18	0.01
Th	2.33	2.31	2.32	2.35	1.72	2.34	1.99	2.25	2.24	2.1	2.21	2.29	1.95	1.89	2.12	0.1
Nb	21	20	22.4	19.4	21.2	14.5	30.1	20.1	19.9	19.8	19.7	19.8	29.7	31.3	14.2	1
La	6	7	5	7	4	7	4	5	5	6	4	7	6	5	5	1
Ce	4	3	3	6	4	7	5	11	11	12	10	11	7	6	4	0.5
Pr	0.75	0.8	0.55	0.81	0.64	1.01	0.86	0.82	0.99	0.97	0.9	1.46	0.82	0.85	0.82	0.05
Nd	3.8	4.7	3.5	4.4	3.7	5.1	4.6	6.6	6.5	5.8	4.8	6.8	4.3	4.5	4.7	0.5
Sm	0.64	0.75	0.59	0.71	0.56	0.83	0.65	0.94	0.85	0.78	0.69	0.98	0.68	0.72	0.79	0.02
Eu	0.11	0.12	0.11	0.14	0.13	0.1	0.12	0.11	0.14	0.15	0.12	0.13	0.15	0.14	0.13	0.1
Dy	0.39	0.4	0.39	0.52	0.27	0.7	0.4	0.41	0.38	0.42	0.45	0.46	0.45	0.38	0.74	0.02
Er	0.2	0.18	0.19	0.27	0.11	0.26	0.4	0.21	0.23	0.18	0.2	0.19	0.19	0.25	0.61	0.05
U	0.85	0.95	0.8	1	0.9	1.8	0.96	0.74	0.88	0.75	0.98	1	0.74	0.75	0.73	0.01
Pb	150	171	145	147	164	154	148	105	115	117	116	98	97	80	109	1
Yb	0.27	0.25	0.3	0.3	0.2	0.3	0.5	0.34	0.31	0.29	0.2	0.3	0.3	0.4	0.8	0.05
Y	2.4	2.7	2.3	2.9	1.9	2.6	2.5	2.9	2.5	2.22	2.3	2.4	2.7	2.6	4.6	0.5
Cs	9.3	9.7	9.2	11.9	6.2	11.3	4.8	4.1	4.6	4.9	4.5	3.1	4.5	4.4	4	0.05
Ba	27	30	26	56	25	48	45	58	57	63	61	67	35	50	29	1
Rb	370	359	369	380	344	393	198	178	197	182	171	162	226	241	132	1
Sr	18	22.5	15.8	31.1	13.1	22.7	21.2	23	24.1	24.5	23.3	24.3	20.9	21.8	22.1	1
Zr	13	12	13	14	15	5	8	13	15	12	7	6	7	9	6	5

Table 3 EPMA analysis of the Alvand sapphires (wt %). bdl: below the detection limit.

Sample No.	MgO	FeO	Fe₂O₃	Al₂O₃	V₂O₃	Cr₂O₃	Ga₂O₃	TiO₂
1	bdl	0.15	0.17	98.69	bdl	bdl	0.12	0.16
2	bdl	0.15	0.17	98.89	0.01	bdl	0.1	0.16
3	bdl	0.23	0.25	94.28	bdl	bdl	0.13	0.12
4	bdl	0.19	0.21	95.21	0.01	bdl	0.1	0.11
5	bdl	0.21	0.23	98.54	bdl	bdl	0.1	0.11
6	bdl	0.13	0.14	97.56	bdl	bdl	0.04	0.17
7	bdl	0.17	0.19	98.53	bdl	bdl	0.01	0.17
8	bdl	0.2	0.22	97.71	bdl	bdl	0.07	0.23
9	bdl	0.08	0.09	98.44	bdl	bdl	0.06	bdl
10	bdl	0.18	0.2	98.83	bdl	bdl	bdl	0.17
11	bdl	0.13	0.14	98.79	bdl	bdl	0.09	0.17
12	bdl	0.18	0.2	97.34	bdl	bdl	0.09	0.2
13	bdl	0.17	0.19	96.47	bdl	bdl	0.12	0.2
14	bdl	0.26	0.3	97.84	bdl	bdl	0.06	0.26
15	bdl	0.17	0.19	96.48	bdl	bdl	0.09	0.17
16	bdl	0.1	0.11	99.73	0.01	bdl	0.1	0.19
17	bdl	0.14	0.15	97.76	bdl	bdl	0.08	0.08
18	bdl	0.15	0.17	98.14	bdl	0.04	0.02	0.01
19	bdl	0.23	0.25	99.29	bdl	0.01	0.03	0.03
20	bdl	0.14	0.15	98.67	bdl	bdl	0.05	0.11
21	bdl	0.27	0.30	96.82	bdl	bdl	bdl	0.28
22	bdl	0.3	0.33	97.89	bdl	bdl	0.05	0.25
23	bdl	0.3	0.33	97.71	bdl	bdl	0.02	0.21
24	bdl	0.23	0.25	97.97	bdl	bdl	bdl	0.28
25	bdl	0.19	0.21	97.53	bdl	bdl	0.1	0.22
26	bdl	0.26	0.3	97.59	bdl	bdl	0.05	bdl
27	bdl	0.26	0.31	96.82	bdl	bdl	0.07	0.24
28	bdl	0.2	0.22	96.11	bdl	bdl	0.02	0.23
29	bdl	0.16	0.18	96.73	bdl	bdl	bdl	0.23
30	bdl	0.2	0.22	97.12	bdl	bdl	0.09	0.22
31	bdl	0.25	0.28	96.76	bdl	bdl	0.05	0.17
32	bdl	0.22	0.24	96.85	bdl	bdl	bdl	0.23
33	bdl	0.24	0.26	95.77	bdl	bdl	bdl	0.22
34	bdl	0.19	0.21	96.92	bdl	bdl	0.08	0.24
35	bdl	0.25	0.28	96.72	bdl	0.01	0.06	0.18
36	bdl	0.22	0.24	97.92	bdl	bdl	0.03	0.11
37	0.03	0.16	0.17	95.01	bdl	bdl	0.09	0.18
38	bdl	0.25	0.27	98.29	bdl	bdl	0.02	0.29
39	bdl	0.27	0.30	98.32	bdl	bdl	0.03	0.2
40	bdl	0.27	0.30	96.6	bdl	bdl	0.07	0.03
41	bdl	0.17	0.19	95.74	bdl	bdl	0.03	0.27
42	bdl	0.13	0.14	98.82	bdl	bdl	0.08	0.16
43	bdl	0.17	0.19	96.16	bdl	bdl	0.03	0.17
44	bdl	0.16	0.17	97.52	bdl	bdl	0.13	0.03
45	bdl	0.22	0.24	98.74	bdl	bdl	0.07	0.04
46	bdl	0.16	0.18	97.04	bdl	bdl	0.09	0.16
47	bdl	0.22	0.24	96.17	bdl	bdl	0.05	0.14
48	bdl	0.27	0.30	96.3	bdl	bdl	0.1	0.13
49	bdl	0.32	0.35	95.77	bdl	bdl	0.11	bdl
50	bdl	0.19	0.21	96.61	bdl	bdl	0.02	0.01
51	Bdl	0.23	0.25	97.98	bdl	bdl	0.07	0.15
52	Bdl	0.25	0.28	96.15	bdl	bdl	0.1	0.17

No competing interests reported.

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Geology, geochemistry, and genesis of the Alvand sapphire-bearing syenitic pegmatite, Sanandaj-Sirjan Zone, Hamedan Province, northwest Iran

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Local Geology and sampling

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