Journal of Economic Geology (Sep 2023)
Ore Zoning, Geochemistry and Genesis of Iron Mineralization in Hormuz Volcano-Sedimentary Sequence, Larak Island, South of Bandar Abbas
Abstract
Iron mineralization has occurred in Larak Island, located 30 km south of Bandar Abbas, within the Persian Gulf, in the Zagros structural zone. Stratigraphically, Larak Island dominantly consists of the Late Proterozoic Hormuz volcanic-sedimentary series, including rhyolitic lava and tuff, shale, and evaporative sediments. Iron mineralization has occurred in a specific stratigraphic horizon within the volcanic-sedimentary sequence. The orebodies involve four ore facies from bottom to top: vein-veinlets, brecciated, massive, and banded. The ore minerals are dominated by magnetite, oligiste, and hematite with gangue minerals including pyrite, apatite and secondary goethite and limonite, associated calcite, quartz, and anhydrite. Wall rock alterations in this area include silicic-sericitic, carbonatic, chloritic, and secondary argillic. The samples obtained from iron ores in the geochemical diagrams including the Fe/Ti versus Al/(Al+Fe+Mn+Na+K+Ca) diagram and Mg versus V/Ti diagram, were plotted in the range of Banded Iron Formation (BIF) deposits. Due to the volcanic-sedimentary nature of the host sequence, lack of observation of glacial sediments, ore facies, ore textures, structures and mineralogy, alteration and geochemical features, the iron mineralization in Larak Island shows the most similarities with the Algoma-type BIF deposits. Introduction The study area is located in Hormozgan province and southeast of Qeshm Island, on Larak Island. Based on structural-geological divisions of Iran, Larak Island is located in the southeastern part of the Zagros zone (Figure 1). The major rock outcrops of the region are part of the Hormuz Formation (HF), which is a collection of evaporites, volcanic, pyroclastic, and sedimentary units related to the Precambrian. According to Aghanabati (2004), at the southeastern part of the Zagros, especially in the area between Kazeroon and Minab faults, following the extensional phases of Katanga orogeny, evaporitic basins have been formed accompanied by igneous activities related to Katangan orogenic phase. Age measurements done by Ramezani and Tucker (2003) show that the igneous rocks of HF dates back to Early Cambrian. The HF rock units were exhumed as salt domes in southern Iran. The distribution of salt domes in the south of Iran is not uniform. They are gathered in two areas: One is Bandar Abbas-Sarvestan, including 101 domes and diapers, and the other is the southern part of Kazeroon, including only 14 domes and diapers. The total of 115 salt domes host reserves of salt, gypsum, potash, uranium, phosphate and iron (often hematite and in the form of red soil). Larak Island is one of the salt domes in southern Iran that has been less studied in terms of mineralization. The type and manner of formation of Larak Island rocks are very similar to those of Hormuz Island. Acidic magmatic activities in Larak Island and other salt domes in southern Iran are the result of melting of the upper crust by a magma derived from the upper mantle. Based on recent research, the iron mineralization in the Hormuz Formation is introduced as Rapitan-type Banded Iron Formation (Atapour and Aftabi, 2017). Moreover, the origin of iron and copper ores within the Hormuz volcano-sedimentary sequence in the Zendan salt dome located in Bandar Lengeh is inferred to be related to VMS type mineralization (see Biabangard et al., 2018). The Tang-e-Zagh Fe deposit in Bandar Abbas has been introduced as sedimentary-type (Tavakoli et al., 2014), and a sedimentary and magmatic-hydrothermal origin has been proposed for the genesis of iron and iron-apatite ores of Hormuz Island (Fakhri Dodoei and Alipour Asl, 2020). This study tries to introduce the evidence for genesis of iron-apatite mineralization in Larak Island of Persian Gulf. Materials and methods Various sedimentary and igneous units were taken as samples from Larak Island. About 10 thin sections were prepared and petrographic examinations were performed on them. Moreover, 3 polished sections and 10 thin sections were prepared and studied for mineralization, mineral structures and textures, paragenesis sequence of minerals, and distribution of ore minerals by reflecting microscope. Four samples from alteration zones were selected and studied by X-ray diffraction (XRD) method, and 13 samples of ores were analyzed to determine the amount of major and trace elements by ICP-OES/MS methods in the laboratory of Iran's Minerals Procurement and Production Company (IMPASCO). Results and discussion The Hormuz series is the major host of iron mineralization in Larak Island. The main rock outcrops of the study area belong to the Late Precambrian-Lower Paleozoic Hormuz Formation, involving rhyolite and rhyodacite lavas, rhyolite tuff, tuff shale, sandstone, marl, sandy limestone, and evaporitic layers. The HF rock units are covered by Cenozoic sequences consisting of sandstone, marl, and fossiliferous limestone. From bottom to top the HF sequence involves four major rock units (Faramarzi et al., 2015): H1 includes marl and limestone, H2 includes tuff, rhyolite, trachyte, diabase, anhydrite, and limestone, H3 includes algal black limestone, and H4 includes sandstone and tuff. The stratabound and stratiform iron mineralization occurred within specific stratigraphic horizons in the H2 unit, as lenticular and layered orebodies underlay by vein-veinlet and brecciated ores hosted by tuff and lavas. The major ore facies involve vein-veinlets (stringer), brecciated, massive, and banded. The ore minerals are dominated by oligiste (hematite), magnetite, and secondary goethite, and limonite. The main gangue minerals are calcite, quartz, and clay minerals. Wall rock alterations in this area generally include sericitic, chloritic, carbonatic, and secondary argillic. The geochemical nature of the volcanic rocks of the region varies from calc-alkaline series to high-grade calc-alkaline and shoshonitic. To classify the ores, in addition to geological and mineralogical studies, geochemical behavior of major and trace elements can be used (Dill, 2010). For this purpose, geochemical charts based on the elements of iron, titanium, aluminum, manganese, sodium, potassium, calcium, magnesium, and vanadium were used. The samples obtained from iron ores in the geochemical diagrams including the Fe/Ti versus Al/(Al+Fe+Mn+Na+K+Ca) diagram (Boström, 1975) and Mg versus V/Ti diagram (Nyström and Henríquez, 1994), were plotted in the range of Banded Iron Formation deposits. Various features of the iron mineralization in Larak Island, including tectonic setting, volcanic host and associated rock-types, geometry of orebodies, different ore facies (from bottom to top including vein-veinlets, brecciated, massive and banded-laminated), ore mineralogy, alteration zones and geochemistry of ores showed that the mineralization is very similar to the Algoma-type BIF deposits (e.g., Gross, 1996; Taner and Chemam, 2015). Conclusion The features of iron mineralization in Larak Island, involving tectonic setting, host and associated rock-types, geometry of orebodies, different ore facies (from bottom to top including vein-veinlets, brecciated, massive, and banded-laminated), ore mineralogy, alteration zones, and geochemistry of ores show that the mineralization is very similar to the Algoma-type Banded Iron Formation deposits, formed due to seafloor exhalations and subsea-floor replacement of the host rocks at Late Proterozoic-Early Paleozoic time. Due to diagenesis, rare amounts of iron oxides were remobilized and deposited within the overlaying young Asmari-Jahrom limestone units.
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