Geochemistry and Genesis of Beryl Crystals in the LCT Pegmatite Type, Ebrahim-Attar Mountain, Western Iran

Narges Daneshvar,Motohiro Tsuboi,Masayo Minami,Hossein Azizi,Yousif O Mohammad,Yoshihiro Asahara

doi:10.3390/min11070717

Abstract

Ebrahim-Attar granitic pegmatite, which is distributed in southwest Ghorveh, western Iran, is strongly peraluminous and contains minor beryl crystals. Pale-green to white beryl grains are crystallized in the rim and central parts of the granite body. The beryl grains are characterized by low contents of alkali oxides (Na2O = 0.24–0.41 wt.%, K2O = 0.05–0.17 wt.%, Li2O = 0.03–0.04 wt.%, and Cs2O = 0.01–0.03 wt.%) and high contents of Be2O oxide (10.0 to 11.9 wt.%). The low contents of alkali elements (oxides), low Na/Li (apfu) ratios (2.94 to 5.75), and variations in iron oxide (FeO= 0.28–1.18 wt.%) reveal a poorly evolved magmatic source of the beryl grains. Low abundances of rare earth elements (ΣREE = 0.8–4.9 ppm) with high 87Sr/86Sr(i) ratios of 0.739 ±0.036 for the beryl grains and 0.7081 for the host granites infer that the primary magma was directly produced by partial melting of the upper continental crust (UCC). The crystallization temperature of the Ebrahim-Attar granitic pegmatite changes from 586 to 755 °C (average = 629 °C), as calculated based on the zircon saturation index. Furthermore, the quartz geobarometer calculation shows that crystallization occurred at pressures of approximately 233–246 MPa. This pressure range is a promising condition for saturation of Be in magma. During granitic magma crystallization, the melt was gradually saturated with Be, and then beryl crystallized in the assemblage of the main minerals such as quartz and feldspar. Likewise, the host granite is characterized by high ratios of Nb/Ta (4.79–16.3) and Zr/Hf (12.2–19.1), and peraluminous signatures are compatible with Be-bearing LCT (Li-Ce and Ta) pegmatites.

Highlights

Beryl (,Na,Cs,H2 O)(Al,Sc,Fe,Mg)2 (Be, Li)3 (Si6 O18 ) is the main beryllium mineral produced in a few geological settings: (1) highly evolved S-type granites and granitic pegmatite [3]; (2) hydrothermal deposits related to granite; (3) volcanogenic hosted beryllium deposits [3]; (4) metamorphic rock, emerald-bearing schist [4]; (5) carbonate-hosted beryllium deposit [3,5]
We report the chemical signatures of beryl crystals by measuring the major elements by applying XRF, trace elements including rare earth elements (REEs), and Sr isotope ratios using inductively coupled plasma mass spectrometry (ICP-MS) and thermal ionization mass spectrometry (TIMS) for the first time and provide more accurate and precise data to evaluate the geochemical features of host granitic pegmatite and beryl mineralization, respectively
Be-bearing host rocks based on the alkali, silica, and alumina components on total-alkali (Na2 O + K2 O) vs. silica (SiO2 ) and CaO/(Na2 O + K2 O + CaO) vs. Al2 O3 /(Na2 O + K2 O

Summary

Introduction

0.27 Å, is the lightest alkaline-earth element on Earth [1] and belongs to the rare lithophile elements group (RLE) [2] It is mainly concentrated in the upper continental crust (1.9–3.1 ppm; [3]), especially in peraluminous granites, alkaline rocks, and granitic pegmatites [2,3]. Beryllium (Be2+ ) has a radius that is too small in tetrahedral coordination and cannot enter the crystal lattice of the main rock forming minerals such as quartz, feldspar, biotite, and muscovite [2]. It tends to produce distinct minerals such as beryl [3]. Beryl (,Na,Cs,H2 O)(Al,Sc,Fe,Mg) (Be, Li) (Si6 O18 ) is the main beryllium mineral produced in a few geological settings: (1) highly evolved S-type granites and granitic

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