The role of sulfidation of Fe-carbonate rocks in increasing gold contents at the Zarshuran deposit (northern Takab), Takab-Angouran metallogenic district

زمین شناسی اقتصادی

Volume 14, Issue 4 - Serial Number 35, December 2022, Pages 89-114   

Document Type: Research Article

DOI: 10.22067/econg.2022.75417.1042

Authors

Sharareh Heshmatnia¹; Ebrahim Tale Fazel^{* 2}; Abbas Oroji³

¹M.Sc., Department of Geology, Faculty of Science, Bu-Ali Sina University, Hamedan, Iran

²Assistant Professor, Department of Geology, Faculty of Science, Bu-Ali Sina University, Hamedan, Iran

³Ph.D. Student, Department of Geology, Faculty of Science, Bu-Ali Sina University, Hamedan, Iran

Abstract

Iron and arsenic sulfides are considered as the most important gold hosts in the sediment-hosted disseminated invisible gold deposits. The Zarshuran gold deposit (155 tons Au with average grade of 2.63 g/t) is formed in the Lower Cambrian black shale and siltstone (Zarshuran unit) and Fe-rich carbonates (Chaldagh unit) host rocks. As-sulfide (e.g., realgar and orpiment) and arsenian pyrites are the most important host minerals of gold in this deposit. Based on EPMA data, pyrite with As content below the detection limit to 3.99 wt% occurs in six different types, respectively, Py0 (gold content of 0.01 ppm), Py1 (gold content of 0.02 ppm), Py2 (gold content of 0.03 ppm), Py3 (gold content of 0.02 ppm), Py4 (gold content of 0.04 ppm), and Py5 (gold content of 0.01 ppm). According to the evidences, gold can be present as participating in chemical bounded (Au⁺ and Au⁺³) or nanoparticle inclusions (Au⁰). The weak geochemical correlation (R² = −0.6) between As and S elements in pyrites indicates that there is pyrite with a complex composition [Fe(S,As)₂Au₂S⁰], which As⁻ has replaced S²⁻. Mineralogy and the abundance of Fe and S in the rock units suggest that gold mineralization in the Zarshuran deposit is well occurs in response to sulfidation process. Sulfidation occurs when H₂S-rich ore-forming fluids react with Fe-bearing carbonate host rock to form pyrite, marcasite, and pyrrhotite minerals.
 
Introduction
Sediment-hosted disseminated gold deposits with high gold grade (> 1000 g/t) are known to contain gold as participating in chemical bounded (Au⁺ and Au⁺³) or nanoparticle inclusions (Au⁰)in the pyrite composition (Deditius et al., 2014). The Zarshuran gold deposit (155 t @ 2.63 g/t Au; Madan-Zamin Company, 2020) is situated 35 km north of Takab in the Takab-Angouran metallogenic district, NW Iran. According to Mehrabi et al. (1999) and Asadi et al. (2000), gold mineralization in the Zarshuran deposit is in many respects like the Carlin-style and Carlin-like gold deposits, respectively. Gold substitution has been reported to be below detection limit to a maximum of 0.64 wt.% and 3 wt.% in arsenian pyrite and arsenopyrite lattice, respectively (Voute et al., 2019). Here, we examine the relationship between geochemistry and textures of pyrite, with a view to constraining the mechanism of gold precipitation. We use electron probe microanalysis (EPMA) to study the composition of different pyrite types. The results show that distribution of ferrous iron (Fe²⁺) is controlled by the initial diagenesis of Fe-C-S systems.
 
Materials and methods
During field studies, mineralization outcrops and their associated host rocks were identified. After preparing thin and thin-polished sections, mineralogical studies were carried out by transmitted and reflected polarizing ZIESS Axioplan2 microscope. To identify and classify different pyrite types and investigate the possibility of  presence of gold and trace minerals, eight thin-polished sections were prepared, and carbon coated. This study was carried out by scanning electron microscope (SEM) with EVO MA15 model in the Central Laboratory of Kharazmi University (Tehran, Iran). Then, to achieve the chemical composition of different types of pyrite, 104 points were examined by EPMA (model JEOL JXA-8530F) in the Laboratory of Geo Forschungs Zentrum (GFZ). Spot analysis was performed with a voltage of 20 kV, electron beam current of 10 nA, X-ray diameter of 2 μ and radiation time of 5 to 20 seconds. The standard examples used to calibrate the various elements in this experiment were as follows: FeS₂ (for Fe and S), CoAsS (for As and Co), and free gold (for Au). The detection limits of the elements are Fe (300 ppm), As (200 ppm), S (300 ppm) and Au (200 ppm). To identify rare minerals such as mackinawite (Fe₉S₈), HR microscopic confocal Raman technique was used in the Central Laboratory of Shiraz University (Shiraz, Iran). The experiment was performed with an X50 laser, a wavelength of 785 nm with a power of 100 mw and a radiation time of 20s.
 
Results and discussion
Based on SEM and optical-microscopic studies, six types of pyrite have been identified in the Zarshuran deposit. They are (1) framboidal pyrite (Py0) (avg. = 30 μm in diameter), (2) fine-grained disseminated pyrite (Py1) (< 20 μm in diameter), (3) coarse-grained euhedral pyrite (Py2) (avg. = 100 μm in diameter), (4) porous/sponge pyrite (Py3) (avg. = 300 μm in diameter), (5) colloform pyrite (Py4) (avg. = 550 μm in diameter), (6) vein pyrites (Py5) (avg. = 55 μm in thickness). Decalcification and sulfidation of host rocks are two important mechanisms in genesis of sediment-hosted gold deposits, and the importance of sulfidation depends on the reactivity of Fe²⁺ and H₂S rocks (Voute et al., 2019). The interaction of H₂S-rich hydrothermal fluids with reactive iron originating from the host rocks (Cail and Cline, 2001) or through hydrothermal fluids added to the environment (Reich et al., 2005) which results in formation of Au(-As)-rich pyrite. Arsenic in pyrite can occur in various oxidation states that correspond to different crystallographic sites in the lattice and different substitution mechanisms. As⁻ substitution for S²⁻ is found in reducing conditions and often in sediment-hosted gold deposits; while As cations (As²⁺, As³⁺, As⁵⁺) replace Fe²⁺ under oxidation conditions (Reich et al., 2005; Deditius et al., 2014). The diagram of arsenic versus sulfur in the Zarshuran deposit shows that the concentration of As in the formed pyrites is strongly related to the decrease in concentration of S. Based on this, it can be concluded that As replaces S in the pyrite structure and in the form of As^−..; Moreover, As species are common in pyrites of gold deposits with sedimentary host rocks. In the Zarshuran deposit the occurrence of gold in As-Hg-Sb sulfides in late drusy quartz veins (Daliran et al., 2018) is more important than the presence of gold in various pyrites. Finally, all evidence suggest that gold mineralization in the Zarshuran deposit occurs well in response to sulfidation process of Fe-bearing carbonate rocks. The correlation between gold content and degree of sulfidation in the Zarshuran deposit indicates that sulfidation process is a much more important mechanism to gold precipitation relative to addition of pyrite (pyritization).

Keywords

Pyrite; Disseminated gold; Sulfidation process; Fe-carbonate; Zarshuran

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