Geology, geochemistry, fluid inclusion and genesis of the Arabshah magnetite-apatite mineralization, SE Takab

Document Type : Research Article

Authors

• Mir Ali Asghar Mokhtari ¹
• Hosein Kouhestani ¹
• Soheila Aghajani Marsa ²

¹ Associate Profesor, Department of Geology, Faculty of Sciences, University of Zanjan, Zanjan, Iran

² M.Sc., Mineralogy Lab., Iran Mineral Processing Research Center (IMPRC), Tehran, Iran

10.22067/econg.2021.69859.1015

Abstract

The Arabshah Fe mineralization is the only known magnetite-apatite mineralization at the Takab–Takht-e-Soleyman–Angouran subzone in southeast of Takab. The oldest rock units in the mineralization area include sedimentary succession of the Qom Formation that was intruded by the Pliocene Ayoub Ansar volcanic dome. Magnetite- apatite mineralization at the Arabshah occurs as vein-veinlets with E-W stright within the Ayoub Ansar dacitic dome. Brecciated zones containing narrow magnetite vein- veinlets occur at footwall and hanging wall of the main vein. Hydrothermal alterations include sodic-calcic, silicification and argillic. Magnetite is the only ore mineral in this mineralization which is accompanied with apatite, clinopyroxene, albite and quartz as gangue minerals. Mineralization textures in the Arabshah deposit include vein-veinlet, brecciated, disseminated, and replacement. REEs concentration within apatite crystals are more than 1%, and demonstrate LREE enrichment with high LREE/HREE ratio and distinctive negative Eu anomalies which is indicative for Kiruna- type iron ores. The result of fluid inclusion studies indicates the presence of two-phase and poly-phase inclusions include LV, VL, LVH, LVS and LVHS fluid inclusions with homogenization between 230-550 °C. The salinity of halite bearing poly-phase fluid vary between 35-60 wt.% NaCl equiv. Fluid inclusion data indicates that Arabshah magnetite-apatite mineralization originated from magmatic fluids. Evidences like mineral assemblages, hydrothermal alteration, ore structure and textures, geochemical characteristics and fluid inclusion data, indicate that the Arabshah magnetite-apatite mineralization can be classified as Kiruna-type iron ores.
 
 
Introduction
Iron oxide-apatite deposits (IOA) are considered to be Kiruna-type iron ores which have formed between Protrozoic to Tertiary eras in different parts of the world. Apatite occurs as a major constituent of these deposits which is accompanied by magnetite and some actinolite. Higher concentration of REEs is one of the important features of these deposits (Frietsch and Perdahl, 1995). The Arabshah Fe mineralization is the only known magnetite-apatite mineralization at the Takab–Takht-e-Soleyman–Angouran subzone within the Sanandaj-Sirjan zone which is located about 15 km southeast of Takab. During the past years, some exploration works were done on the Arabshah Fe mineralization, but its geological characteristics, mineralogy, texture, geochemistry, characteristics of mineralized fluids and genesis have not been studied yet. Recognition of characteristics of the Arabshah magnetite-apatite deposit as the first explored deposit of the Kiruna type mineralization in the Takab area is useful for exploration of this type of mineralization in NW Iran.
 
Materials and methods
This research study can be divided into two parts including field and laboratory studies. Field work includes recognition of different lithological units and ore veins along with sampling for laboratory studies. During field work, 34 samples were selected for petrographical, mineralogical and analytical studies. 10 thin sections and 5 thin-polished sections were used for petrographical and mineralogical studies. For geochemical studies, 6 samples from ore vein were analyzed by ICP–MS methods at the Geological Research Center, Karaj, Iran. Microthermometric measurements were performed on 2 samples using a Linkam THMS-600 heating–freezing stage attached to a ZIESS microscope in the fluid inclusion laboratory of the Iran Minerals Processing Research Center.
 
Results
The oldest rock units in the Arabshah area include Oligo-Miocene sedimentary succession of the Qom Formation that was intruded by the E–W-trending Pliocene Ayoub Ansar volcanic dome. Based on petrographic studies, the Ayoub Ansar volcanic dome has porphyritic, felsophyric and glomeroporphyritic textures and it is composed of plagioclase, amphibole and some quartz and K-feldspar phenocrysts set in a quartz-felspathic groundmass, and it is compositionally classified as dacite-rhyodacite. These rocks have medium-K calc-alkaline affinity and are classified as metaluminous I-type granitoids. They have been formed in an active continental margin to post-collisional tectonic setting and demonstrate geochemical characteristics similar to high silica adakites (Sabzi et al., 2018).
Fe mineralization at the Arabshah mineralization occurs as vein-veinlets of magnetite-apatite within the Ayoub Ansar dacitic dome. Brecciated zones occur at footwall and hanging wall of the main vein. The ore vein has east- west trend and crops out in 50 m length and maximum 1 m width. Coarse-grained euhedral apatite crystals are mainly present at the margins of the main vein. Hydrothermal alterations around the mineralized veins include sodic-calcic, silicification and argillic alterations. Mineralogically, the ore minerals include magnetite along with apatite, clinopyroxene, albite and quartz as gangue minerals. Goethite was formed during supergene alteration. Mineralization textures in the Arabshah deposit include vein-veinlet, brecciated, disseminated, and replacement form. Apatite crystals have high concentrations of REEs (about 1%). Condrite-normalized REE patterns for apatite crystals, magnetite-apatite ores and magnetite ore without or with minor apatite demonstrate LREE enrichment with high LREE/HREE ratio and distinctive negative Eu anomalies.
Based on phase relationships at room temperature, three types of fluid inclusion including two-phase (LV and VL), three-phase (LVH and LVS) and polyphase (LVHS) are present within the apatite crystals at the Arabshah mineralization. Microthermometric measurements indicate that LV and VL fluid inclusions have homogenized between 253-550 °C and 363-490 °C, respectively. Tree-phase LVH fluid inclusions have been homogenized between 278-508 °C and have salinities between 35-59.8 wt.% NaCl equiv. Three-phase LVS fluid inclusions have been homogenized between 240-520 °C. Polyphase LVHS fluid inclusions have been homogenized between 230-520 °C and have salinities between 36-59 wt.% NaCl equiv.
 
Discussion
Similar REE patterns of apatite crystals and mineralized samples with samples from host dacitic dome demonstrate a genetic link between magnetite-apatite mineralization and dacites. Furthermore, REE patterns of the Arabshah mineralization is similar to other iron oxide-apatite deposits from the Tarom–Hashtjin metallogenic belt (Mokhtari et al., 2018), and those of Central Iranian iron ores (Mokhtari et al., 2013). Moreover, REE patterns of the Arabshah deposit are similar to REE patterns of the Kiruna-type iron ores (Frietsch and Perdahle, 1995).
Fluid inclusion data indicates that Arabshah magnetite-apatite mineralization originated from magmatic fluids. Positive correlations between salinity and homogenization temperatures indicate that mineralization at the Arabshah deposit involved mixing of magmatic fluids and a dilute and cooler meteoric fluid.
Totally, based on mineral assemblages, hydrothermal alteration, textures, geochemical characteristics and fluid inclusion data, the Arabshah magnetite-apatite mineralization can be classified to be of the Kiruna-type iron ores.
 
Acknowledgment
This research study was made possible by a grant from the office of vice-chancellor of research and technology, University of Zanjan. We hereby acknowledge their generous support. The Journal of Economic Geology reviewers and editor are also thanked for their constructive comments.

Keywords

• Magnetite-apatite mineralization
• Rare earth elements
• Kiruna-type
• Arabshah
• Takab

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