فصلنامه زمین شناسی اقتصادی
سال چهاردهم شماره 3 (پیاپی 34، پاییز 1401)

Carbonatites are among the rare kind of rocks on this planet. These igneous rocks are included by share of more than 50% carbonate minerals; dolomite, calcite & ankerite. Specific look in petrography and mineralography studies is needed to investigate these rocks, especially for silicates and minor phases from macroscopic as well as to microscopic scales. On the last databases of carbonatites of the world, Iran is represented empty of carbonatites. While there are many unexplored carbonatitic outcrops exists in the Bafq Metallogenic Area, (hereafter BMA). BMA as a significant metallogenic province of Iran, has the largest cluster of riftogenic Kiruna-type Iron Oxide-Apatite (IOA) deposits in the world, its iron resources are estimated over 2 bt in 50 aeromagnetic anomalies. The geological story of BMA at the middle of Posht-e Badam Block (hereafter PBB) is magnificently beautiful. The BMA recorded the oldest major geological events in Central Iranian Microcontinent (hereafter CIM). From the early years of field surveys and published geological maps to recent works which are cited in thousands of papers and dissertations, subvolcanic mineralized carbonatite intrusions within metamorphic complexes and trondhjemites of before Middle Cambrian age in the area, mistaken for marble interlayers, limestone enclaves or just non-important sedimentary carbonate rocks. In spite of the brief hints that carbonatites are related to the huge (IOA) mineralization in the area which is available in the old papers, unfortunately these rocks still remained invisible, even in the new studies. In here, on the basis of provided field and petrography evidences as well as to depicted petro-chemical diagrams, the nature of carbonatites of the area is discussed from different geological aspects. In fact, carbonatites are the most important riftogenic units in lithological columns of PBB, specifically in BMA. These naked virgin multiple peaks should get under the detailed study in the aim for valuable exploration potential of REEs. Moreover, these rocks provide a magnificent case of geodynamic investigation about the Earth history which has occurred during the changing season of Proterozoic to Phanerozoic. CIM is located at the heart of the greatest orogen in the world; The Alp-Himalaya orogenic belt in between Gondwana & Eurasia Paleo-Supercontinents. During late Precambrian to Early Cambrian, the study area had suffered by complicated gigantic orogenic phases. The late units of the phenomena described here unconformably overlaied by the Middle Cambrian trilobite-bearing Mila Formation. Thereupon, the unique, and major geological incidents of BMA have occurred right before Biological Big Bang which is also known as the Cambrian Life Explosion, beneath the only great ice grip of the planet throughout its entire history.

Khorasanlu intrusive body with monzonite to granite composition is located 75 km south-east of Zanjan city in the Western Alborz. This intrusive body has intruded into the Eocene rocks of the region. Based on petrographic and geochemical studies, the Khorasanlo intrusive complex have alkali-calcic to alkaline nature and are peraluminum and ferroan. Enrichment of light rare earth elements, relatively high amounts of HFSE compared to other granitoids, high Ga/Al ratio and placement of samples in the A-type granitoids field on the discrimination tectonic setting diagrams, indicate that the Khorasanlu intrusion mass is A2-type granitoid and was generated in the post-collisional setting. It seems that the tension resulting from the rollback or increase of the subducted plate dip and asthenosphere uplifting, after the compressional tectonics regim, caused the melting of the crust and generation of the Khorasanlo young alkaline granitoids.

The Taktelar area is located in 140 Km northwest of Zahedan city in the Sistan Suture Zone. In this area Eocene flysch-like rocks such as shale, siltstone and sandstone are intruded and extruded by Oligocene-Miocene plutonic, extrusive and sub-volcanic rocks. The igneous rocks are dacite, andesite, and diorite porphyry and are mainly altered. The mineralization in the Taktelar area occurs as dissemination and vein types in sub-volcanic and sedimentary rocks. The disseminated mineralization is characterized with wide formation of pyrite with or without Cu minerals and vein mineralization includes malachite, galena and Au-bearing silicic veins. The hydrothermal alterations are potassic, phyllic, tourmalinitic, argillic, silicic and propylitic. The result of mass change calculation shows enrichments of SiO2 and depletion of other elements in phyllic alteration zone. In argillic alteration zone, contents of As, Mo, Zn, and Pb were enriched and other elements were depleted or were not changed. The alteration zones with quartz stockwork veins and enrichment of SiO2 and K2O have the most enrichment of Cu and Mo. The behavior of the elements in the propylitic alteration zone is different and generally increasing of the main elements is more significant. The ore-making elements have more enrichment in samples with silica veins so that the highest enrichment of copper and molybdenum occurs in rocks with higher SiO2 and K2O and quartz veins. Generally, in most altered zones, contents of copper, molybdenum, lead, zinc and arsenic show enrichment and rare earth elements show depletion. Light rare earth elements show more depletion than heavy rare earth elements, indicating their more mobility in hydrothermal systems.

The Kal-Baneh Mn deposit is located 56 km northwest of Jiroft city in the southeast of the Sanandaj-Sirjan Zone. The ore mineralization occurs as layered, banded, massive, and disseminated textures within radiolarian cherts of the upper parts of the Esfandaghe ophiolitic complex. The mineralogy is simple and ores consist mainly of pyrolusite and braunite, with trace amounts of hausmannite and hematite. The main gangue minerals are quartz and calcite. Geochemical evidence, including relatively high Mn/Fe (mean=89.39), Si/Al ratios (mean=20), Ba concentrations (mean=842.3 ppm), the low contents of Cu (mean=92.07 ppm), Ni (63.38 ppm), Co (mean=13.04 ppm), Zn (mean=68.15 ppm), total REE (28.1-96.13 ppm), negative Ce (Ce/Ce*: 0.3-0.7), slightly positive Eu (Eu/Eu*: 0.64-1.35) anomalies, the REE pattern of ore samples, and geochemical data reveal a distal hydrothermal-exhalative source and indicate the combination of hydrothermal solutions with seawater. Geochemical signatures, the geometry of orebodies, the nature of the host rock, mineralogy, and the textural features of the manganese ores are consistent with the submarine hydrothermal-hydrogenous Mn deposits.

Blue-colored corundum (sapphire) is found in syenitic pegmatites in Alvand igneous complex in the Khako area. From the petrographic point of view, the host rock is composed of feldspar and small amounts of quartz, which includes corundum crystals of transparent to translucent blue color, 1 to 6 cm long, covered with a sheath of mica with a composition of muscovite to phengite. It seems that the core of corundum crystals crystallized in the magmatic stage in equilibrium with mica + aluminosilicate + feldspar ± perthite, and in the next metasomatic stage, corundum crystals grew from the reaction of plagioclase and mica. The chemical composition of the main minerals in pegmatite rocks indicates the magmatic origin of these rocks, which were formed during the magmatic differentiation process. In the continuation of the metasomatic cycle, the geochemical conditions for formation of new corundum crystals and regrowth of primary corundum have been prepared by reducing the silica content (de-silicification) and increasing the Al/Si ratio. These processes during post-magmatic cycles in granite masses, are known as Plumasite processes. The classifications done in pegmatites, based on abundance of rare elements and phosphorus content in feldspar crystals, indicate fertility of these rocks from rare elements. These pegmatites are included in the group rich in lithium and beryllium. Moreover, comparison of the chemical composition of corundum crystals with famous corundum deposits in other parts of the world shows that these rocks have a special similarity with magmatic corundum deposits in connection with synthetic intrusive masses. In addition, these rocks show a composition similar to magmatic deposits created by metasomatic processes.

The study area is located 12 km south of Zahedan city, Sistan and Baluchistan province. Rock units in this area include a series of slightly altered flysch facies units with Eocene age, subvolcanic rocks and basic to intermediate dikes with probably Oligocene-Miocene age. Subvolcanic rocks in the form of stocks and domes intruded into the flysch unit and were cut by intermediate dykes in the later stages. The subvolcanic units of the region have two main compositions diorite-monzodiorite and quartzdiorite-quartzmonzodiorite with porphyry texture. The subvolcanic units of this area are in the range of metalalumin to slightly peralumin and I-type. This is consistent with the evidence such as the widespread presence of hornblende, apatite and sphene in the studied subvolcanic units, which indicates the oxidic conditions and high oxygen fugacity for these rocks at the time of formation. Spider diagrams drawn for trace and rare earth elements show the single origin and the role of fractional crystallization in the formation of these rocks. Based on the spider diagram of rare earth elements, the studied subvolcanic units of LREE elements show a enrichment toward to HREE elements, which is one of the prominent characteristics of calc-alkaline rocks in the subduction zones of the continental margin. These rocks are placed in the tectonomagmic environment diagrams in the adakitic range and in the category of shoshonite and high potassium calc-alkaline rocks, related to subduction environments.