Journal of Economic Geology (Dec 2022)
Petrogenesis of the Khaku pegmatites corundum-bearing in the margin of Alvand granite complex, and its comparison with important worldwide ruby and sapphire deposits
Abstract
Blue-colored corundum (sapphire) is found in syenitic pegmatites in Alvand igneous complex in the Khako area. From the petrographic point of view, the host rock is composed of feldspar and small amounts of quartz, which includes corundum crystals of transparent to translucent blue color, 1 to 6 cm long, covered with a sheath of mica with a composition of muscovite to phengite. It seems that the core of corundum crystals crystallized in the magmatic stage in equilibrium with mica + aluminosilicate + feldspar ± perthite, and in the next metasomatic stage, corundum crystals grew from the reaction of plagioclase and mica. The chemical composition of the main minerals in pegmatite rocks indicates the magmatic origin of these rocks, which were formed during the magmatic differentiation process. In the continuation of the metasomatic cycle, the geochemical conditions for formation of new corundum crystals and regrowth of primary corundum have been prepared by reducing the silica content (de-silicification) and increasing the Al/Si ratio. These processes during post-magmatic cycles in granite masses, are known as Plumasite processes. The classifications done in pegmatites, based on abundance of rare elements and phosphorus content in feldspar crystals, indicate fertility of these rocks from rare elements. These pegmatites are included in the group rich in lithium and beryllium. Moreover, comparison of the chemical composition of corundum crystals with famous corundum deposits in other parts of the world shows that these rocks have a special similarity with magmatic corundum deposits in connection with synthetic intrusive masses. In addition, these rocks show a composition similar to magmatic deposits created by metasomatic processes. Introduction The corundum crystals found in the southern part of Alvand's intrusive mass have been introduced as the most important sapphire gem potential in Iran (Sadeghi et al., 2022; Sajjadi Alehashem et al., 2021). The corundum crystals in this area have been reported in preliminary studies conducted by different researchers (Beharifar, 2012). Although it is not possible to determine certain criteria for the gem value of a mineral., characteristics such as transparency, color, hardness, refractive index, reflection, and absorption of light are among the most important factors in determining the quality of colored gems. The crystallized oxide of pure corundum is colorless However, corundum has five main coloring elements, including Cr3+, Fe3+, Fe2+, Ti4+ and V3+. Geochemical factors in the formation of corundum are usually magmatic or high-grade metamorphic processes that cause formation of aluminum oxide crystals from a magma with a low silica content (Černý, 1991; Černý and Ercit, 2005) or a differentiated pegmatite magma (Sánchez-Muñoz et al., 2017). These rocks can provide special conditions to produce corundum with the help of fluids originating from magmatic fractionation and metamorphic processes (Zwaan et al., 2015; Peucat et al., 2007). Many studies have been done on classification and genesis of corundum deposits (Sutherland et al., 2009, Schmetzer and Peretti, 1999; Simonet et al., 2004). Classification of corundum reserves is essential in choosing detailed prospecting and exploration methods and prospecting for new reserves in similar structural zones. Materials and methods 25 thin sections were prepared from the collected samples. 5 thin polished sections have been used in electron microscope studies and electron probe microanalysis (EPMA). 5 samples of pegmatites containing corundum crystals found in the Khaku deposit Index and 10 single crystals of separated corundum crystals, have been chemically analyzed by XRF and ICP-MS methods in Zarazma Company of Tehran under standard conditions for major elements and trace elements. Also, EPMA analysis of accompanying minerals and corundum crystals was performed by a Cameca-SX-100 device with a beam diameter of 5 microns and accelerating voltage of 2400 mA at the Iranian Mineral Materials Processing Research Center in Karaj. Results In the newest pegmatite classification (Černý, 1991; Černý and Ercit, 2005), which is most widely used by researchers nowadays, a combination of factors, including depth of establishment, metamorphic degree, and rare element contents are considered. This classification has 4 main categories or classes including 2 Abyssal classes (high grade, high pressure to low), Muscovite (high pressure, low temperature), and Miarolitic (shallow level of rare element with low temperature and pressure). The class of pegmatites with rare elements is divided into LCT and NYF types based on abundance of rare elements: LCT Type is rich in lithium, cesium, and NYF type tantalum and rich in niobium, yttrium, and fluorine. Descriptions of sub-groups by Pezzotta Corundum-bearing pegmatites are included in the Abyssal sub-group. This group is of high pressure and temperature type and is formed at a temperature between 700 and 800 degrees Celsius and a pressure of 4 to 6 kbar (Černý and Ercit, 2005). The mineral abundance of muscovite and biotite shows that Alvand pegmatite shows few characteristics of the muscovite subgroup (high pressure, low temperature). This indicates formation of these rocks at temperatures lower than 700 degrees Celsius. A mineralogical classification is based on the latest classification done in pegmatites (Müller et al., 2022). The pegmatites of the Khaku area are in the corundum pegmatites, a product of magmatism, and are affected by the processes of subsolidus fluids. This group of pegmatites are mostly rich in rare elements and aluminum. Chlorite is the most abundant secondary mineral in corundum-bearing rocks. The chemical composition shows that chlorites of the region are rich in iron and poor in titanium and calcium, their XMg varies from 0.23 to 0.37 (Table 3), and in the classification of chlorites, they are in the range of pycnochlorite to diabenitite (Fig 4-A). The 8 samples of biotite and muscovite from the Khaku region were analyzed by EPMA (Tab 4). There were four biotite crystals in sample SH-1, and four other muscovite crystals in sample SH-3. The structural formula of micas has been calculated based on 11 oxygen atoms and 8 cations (Deer et al., 2013). The lowest amount of XMg in sample SH-1 is equal to 0.46, and the highest amount of XMg in sample SH-3 is equal to 0.56. According to Al versus Fe/Fe+Mg variations (Deer et al., 2013). These biotites are a compositional variety between polar siderophyllite and anite (Fig 3-B). The chemical composition of feldspars in the Khaku region shows a relatively uniform chemical composition rich in albite and a major part that is rich in an orthosis in pegmatites such that XAb is equal to 0.90 to 0.95. Most of the feldspars have a composition of orthosis type XOr 85 % to 90% (Tab 5). In the ternary diagram of albite-anorthite-orthoses (Deer et al., 2013), feldspars in pegmatites are located in the orthosis zone and in the zone of pure albite and oligoclase (Fig 3-C). The results obtained from chemical analysis of corundum crystals show that they all contain 86% to 97% aluminum oxide (Al2O3). In all the samples analyzed by theSiO2 method as the second main oxide, there is more than one percent in the crystal., and it varies in the range 1.01%-1.46% to. Discussion The geochemical and geotectonic characteristics of pegmatites in the Khaku area are like those of group II and III pegmatites of the classification of Sánchez-Muñoz et al., (2017), indicating that they are the result of primary crystallization from magmatic melts After those subsolidus reactions during exhumation and cooling rock mass has caused crystallization of corundum and condensation of rare elements (Fig 5). The samples of Khaku are in the range of corundums of magmatic origin and along the dividing line of the magmatic and metamorphic region. These crystals have significant similarities with blue sapphires from the Bo Phloi mine in Thailand (Bo Phloi/Thailand), which are magmatic-metasomatic corundums rich in titanium and magnesium (Fig 6-A). Chromium is also an important tracer in corundum with metamorphic genesis, especially corundum crystallized in alumina-rich marbles and under severe metasomatism. From this point of view, the corundums of the Hamedan Khaku deposit have relatively low amounts of chromium and vanadium and are among the metasomatic magmatic deposits. They are comparable to the sapphires of the Mogok mine in Myanmar (Mogok/Myanmar) (Fig 6-B). This mine is of the metamorphic magmatic type containing ruby in alumina-rich marbles with intense metasomatism (Peucat et al., 2007).
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