| چکیده انگلیسی مقاله |
Introduction Schist and gneiss metamorphic rocks, especially in orogenic belts, mainly host microcrystalline, amorphous, and flaky graphite deposits in the Earth’s crust. Due to its modern technological use as graphene source and commercial lithium-ion batteries, graphite has a growing economic value and known as critical minerals (International Energy Agency, 2021). Graphite can be formed through maturation and metamorphism of biogenic carbonaceous material; as precipitation from C-O-H fluids; mantle-derived; and through reduction of carbonates (Simandl et al., 2015). Important graphite deposits are found in carbonaceous sedimentary rocks subjected to regional or contact metamorphism and in veins precipitated from fluids. The Band-e-Cherk district located in the Kuh-e-Dom metallogenic area and consist of graphite-bearing schists that superposed contact metamorphism and are associated with Eocene volcanoplutonic suits. However, the microscopic characteristics, geochemistry, ore genesis, carbon source, and other features of graphite-bearing schists in the district have not been thoroughly explored. This study determined the geochemical features of graphite-bearing schists from the Band-e-Cherk district to reconstruct the metamorphic protoliths and Palaeo-sedimentary environment. Geology The study area lies in the Kuh-e-Dom metallogenic area at the central Iran zone. This region forms an important part of the Anarak metallogenic belt. Magmatic rocks in the Kuh-e-Dom metallogenic area are Eocene volcanic and granites, whose extension is controlled by west–east trending faults. The Kuh-e-Dom metamorphic complex consists of phyllite and various schist units in contact with magmatic rocks and with the same trending. The Kuh-e-Dom metallogenic area is characterized by the Fe–Cu–Au–Mn–Pb–Zn–Au–Ag mineralization. Lithologically, the Band-e-Cherk district are dominated by Permo-Triassic metamorphic units (including muscovite schist, epidote-hornblende-schist, muscovite-chlorite schist, biotite-graphite-schist), and Lower Cretaceous limestones as the oldest unit, as well as Eocene andesite, andesite-basalt volcanic rocks and equivalent tuff. Graphite-bearing schists are mostly in contact with marl, limestone and small amount of diorite intrusion as well. Analytical methods For this research, sampling was carried out on graphite-bearing schists, and twenty-five thin-polished sections were prepared and studied with a Zeiss Axioplan 2 transmission-reflection optical microscope at the Kharazmi University. Twenty-six samples of graphite-bearing schists were analyzed for major elements by XRF and trace elements by ICP-MS method at Zarazma Mineral Processing Research Center and Iranian Mineral Processing Research Center (IMPRC). The analytical uncertainties were determined based on several internal certified reference samples. For the XRF these are <1% for SiO2 and Al2O3, and <5% for other major and minor elements. The detection limits were 0.1% for SiO2 and 0.01% for other major elements. Results The results show that the SiO2 content of the metamorphic rocks is high (52.98% to 80.68%), while Na2O is 0.07% to 5.32%, K2O is 0.33% to 5.61%, K2O > Na2O, and K2O/Na2O + K2O > 0.5. SiO2 is significantly negatively correlated with Al2O3. P2O5 ranges from 0.01% to 4.86% (with average 0.54%, which is generally low, and MnO is between 0.001% and 0.61%, with a small variation range. On Harker diagrams, SiO2 is negatively correlated with Al2O3, CaO, K2O, MnO, Fe2O3, and TiO2, reveal that chemical differentiation of the rocks is constrained by sedimentary differentiation (Cheng et al., 2023). The fractionation degree of light rare earth elements (LREEs) is greater than that of heavy rare earth elements (HREEs), with LREE/HREE ratios of 3.33 to 34.2; LaN/YbN is 3.58 to 24.98, with a mean value of 10.44. The rocks have moderate negative Eu anomalies (δEu = 0.42 to 1.93, mean = 0.91). Ionic lithophile elements (e.g., Rb, and K) are relatively enriched, but Sr is fairly depleted. Discussion As the Zr/TiO2–Ni diagram displays (Winchester et al., 1980; Renmin et al., 1986), graphite-bearing schists samples were projected into the zone of sedimentary rocks (Figure 1A). Figure 1. A) Ni vs. Zr/TiO2 (Winchester et al., 1980; Renmin et al., 1986); B) Composition diagram of sedimentary-metamorphic rocks in different climatic zones of (Renmin et al., 1986) (1: Terrestrial facies clay compositions in humid and hot climatic zones; 2: Marine facies, lacustrine and lagoon facies clay compositions in dry climatic zones; 3: Terrestrial facies clay compositions in cold or moderately cold climatic zones); C) La/Th versus Hf diagram (Floyd and Leveridge, 1987), which shows that the studied samples are in the range of acidic-intermediate states. Based on Simonen’s diagram (Simonen, 1953) (Al+fm−C+alk−Si), all samples fall into the argillaceous sedimentary rock zone, confirming that the protoliths of the metamorphic rocks were sedimentary and that the metamorphic rocks are para-metamorphic. For determining of Palaeo-sedimentary environment, we used a ternary diagram of claystone composition in different climatic zones, indicating a shallow depth terrestrial facies zone of a cold or moderately cold climate for the samples under study (Figure 1B). Based on Ni–TiO2 discriminating diagram (Floyd et al., 1989), the protoliths of metamorphic samples plot in the sandstone zone and felsic rocks of the magmatic zone. All samples, on La/Th–Hf diagram (Figure 1C; Floyd and Leveridge, 1987), fall into within the mixed felsic–intermediate source zone, and on Th–Hf–Co ternary diagram (Taylor and McLennan, 1985), plot within the upper crust district. Conclusion The lithogeochemical analysis of graphite-bearing schists shows that the protolith are sedimentary rocks are of feldspathic sandstone and clay-rich materials, which are in a paleo-sedimentary environment of fresh to saline water. It was formed in a continental environment in a cold or relatively cold climate. Sediments originated from the upper crust and the main components of their origin were argillaceous rock and sandstone with an acidic-intermediate mixture composition. The tectonic discrimination diagrams show that the precursors of metamorphic rocks were probably deposited in an organic-rich river-flood facies environment in the continental margin. Feldspathic sandstone (possibly arkose), clay, and organic-rich graywacke were deposited over a long period and metamorphosed during regional metamorphism, and graphite formed by organic carbon metamorphism. However, the degree of metamorphism was not sufficient to obtain flake graphite, and the graphite is the microcrystalline amorphous type, which has undergone a weak degree of graphitization, and the temperatures have not exceeded 450 ºC. |