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).
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 SiO2 content. 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 Al2O3, 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 TiO2 content 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, Al2O3, 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.