Introducing Carbonatites of ‌Bafq Metallogenic Area, Central Iran, Economic Geology of the REEs based on petrology and geochemistry, Unique geodynamic model of continental arc changed to rift in Early Cambrian

Document Type : Research Article

Author

• Saeed Kolahdani

M.Sc., Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran

10.22067/econg.2022.78597.1054

Abstract

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.
 
Introduction
The broadly studied BMA at the middle of PBB, is the most important sub-zone of CIM. BMA is encompassing the highly mineralized part of PBB which its mineral resources is exploiting for decades. Numbers of world-class mines for multiple types of Fe-Mn/Fe/Fe-U-Th-Ti-REE/Fe-P/P and Pb-Zn deposits are active in the area. The carbonatites of BMA were emphasized generally in one paper by Samani (1988). In that country scale work, 3 carbonatite zones are outlined on the map of BMA but except for field descriptions, he didn’t offer valid geochemical data. Likely because lack of this information, still there is not even a single mark of carbonatites placed in Iran. This is while there are numerous outcrops of these REE-enriched rocks exist in BMA.
Except for Samani (1988), on all geological maps and researches published from the area since the beginning of surveys in the 70^th to recent publications, the important subvolcanic carbonatites were mistaken for sedimentary dolomites, minor marble interlayers of Precambrian metamorphic complexes and outcrop scale enclaves of limestone within trondhjemites, (Haghipour et al., 1977; Ramezani and Tucker, 2003; Rajabi et al., 2014). Here, carbonatites with their ultrabasics en suites described to be the riftogenic intrusives within all pre-Early Cambrian units that acted as the magmatic source of massive Kiruna-type Fe-P (IOA) deposits.
Along with the interpretation of geodynamic evolution of arc to rift setting having major IOA mineralization, the clarity of first-hand evidences provided for subvolcanic dikes indicate to possibilit of major carbonatite bodies close to the recent surface of BMA.
On the concept of carbonatite anatomic scheme, it is discussed how subvolcanic dikes in BMA may constitute uppermost level of magmatic chambers that could be connected to huge coarse grain intrusives as a high potential target for strategic REE resources.
 
Materials and methods
After presenting lithological evidence obtained from field surveys and petrography studies, geochemical analysis data are given for 5 selected crystalline samples. Plotting discriminative diagrams and REE normalization to chondrite has confirmed carbonatite genesis.
In trust to geochemical data and Zircon U-Pb geochronological ages given for granitoids of BMA available in the most cited paper of Ramezani and Tucker (2003), several petrological diagrams are discussed which are not presented in the original research. Achieved results have beautifully defined consecutive rapid change of the continental margin subduction system to a mature continental rift setting with carbonatite magmatism and massive IOA metallogenesis during the Early Cambrian at the Gondwana margin.
 
Results
In this research via interpretation of petrology and geochronology, it is debated how younger tholeiitic trondhjemites formed due to the metasomatic melting of continental crust by ascending and localization of carbonatite-ultrabasic plumes.
Overlapping to a zone of continental arc magmatism, the extreme riftogenic phase with its IOA mineralization happened short after the localization of arc-type granite and granodiorites. In such an environment, the thermal gradient in the basement should have overheated dramatically that led to originating of metasomatitic trondhjemites.
All published scenarios for the BMA are re-evaluated to assemble this theorey. The hypothesis of drastic change from subduction-related continental arc granitoid system to a back-arc basin that progressively evolved to a continental rift with fertile carbonatites which occurred from 533 to 529-525 Ma in Early Cambrian, is reasonably accountable for the area which seems to be a unique example of a tectono-magmatic phenomenon in the world.
Therefore, the presence of barren arc-type granitoids and mineralized carbonatite suites having major Fe-P deposits only across 50 km width, could be explained not only as a world-class untouched potential zone for REE prospecting, but also as an outstanding opportunity to study the geodynamic evolution of the planet Earth.
 
Discussion and conclusion
After all, it could be said that before the current study, Carbonatites of BMA not only yet remained unexplored, but have not even been introduced scientifically. Counting on the described facts, it is claimed that a new era of geological surveys and exploration with the aim for REEs needs to begin in Central Iran which also has a world-class unknown potential of these critical mineral resources.
In a 600km arcuate structural trend across PBB, numerous carbonatite suites already pointed within the geo-units of older than Middle-Cambrian. By reading geological maps matching to airborn geophysical data and RS studies, (ASTER and Geoeye images blended with DEM data), a database of prioritized prospects is provided in GIS-3D format.
By a little search on the web about the market status of REEs and the economical conflicts between China as the 90% holder of the world's carbonatitic reserves (Bayan Obo) with the United States as the main consumer of REEs in high-tech industries, and in the other hand, regarding to huge prices of REEs which in some cases reach to thousands of US$ per Kg, the importance of classified carbonatites of BMA becomes reveal more.
Few of the investigations planned to follow the study are listed below:
1. Providing stable (O, C, S) and all important radiogenic isotope data; for example, the ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios are essential to discuss the origin and evolution of carbothermal genesis, and in deeper view to study of upper mantle composition, (Mitchell, 2005). The role of subducted crustal materials in the origin of carbonatite melts can be investigated by boron isotopes, (Hulett et al., 2016)
2. Selecting the best situations and methods to determine isotopic ages for Carbonatites; Chemical U-Pb dating of primary prismatic apatite in carbonatites is one possible procedure. Sometimes carbonatites show large enough variations in Pb isotope ratios that yield decent isochrons using the ²⁰⁷Pb/²⁰⁶Pb method, (Tykot 1993)
3. Compare results with other occurrences in the Alp-Himalaya organic belt; Turkey for example, and to other provinces of the world, especially with Kiruna in Sweden, Fen in Norway and Aley Carbonatites, Canada 
4. Revise geological maps and determine the event of carbonatite magmatism on the litho-stratigraphic column of BMA by all updated databases
5. Incorporating carbonatite-ultrabasic phases in a detailed geodynamic model that generates such deep riftogenic suites with respect to calk-alkaline arc magmatism in a close time-space

Keywords

• Carbonatite
• REEs
• Trondhjemites
• Kiruna Type IOA deposits
• Bafq Metallogenic Area-(BMA)
• Posht-e Badam Block-(PBB)
• Central Iranian Microcontinent-(CIM)

20.1001.1.20087306.1401.14.3.1.6

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