زمین شناسی، کانی شناسی، ساخت و بافت، زمین شیمی و منشا کانسار اکسید‌ آهن-‌آپاتیت گلستان آباد (خاور زنجان)

کانسار اکسید آهن‌آپاتیت گلستان ‌آباد یکی از کانه‌ زایی ‌های آهن در کمربند فلززایی طارم‌هشتجین است که در فاصله حدود 30 کیلومتری خاور شهر زنجان قرار‌گرفته است. واحدهای سنگی موجود در این منطقه عبارت از توالی آتشفشانی‌رسوبی مربوط به زیرعضو آمند سازند کرج به همراه توده‌ های نفوذی با ترکیب کوارتز مونزودیوریت، پیروکسن کوارتز مونزودیوریت و کوارتز دیوریت پورفیری است. توده ‌های نفوذی کوارتز مونزودیوریت و پیروکسن کوارتز مونزودیوریت دارای ماهیت کالک‌ آلکالن پتاسیم بالا بوده و از نوع متآلومین و itype هستند. این توده‌ ها در محیط تکتونوماگمایی حاشیه فعال قاره ای تا پس از برخورد تشکیل شده ‌اند. کانه‌ زایی اکسید آهن‌آپاتیت در کانسار گلستان‌ آباد به‌ صورت عدسی ‌ها و رگه‌رگچه‌های اکسید آهن‌آپاتیت در داخل توده نفوذی کوارتز مونزودیوریتی و پیروکسن کوارتز مونزودیوریتی و به مقدار کم در داخل سنگ ‌های آتشفشانی‌رسوبی ائوسن مجاور توده نفوذی تشکیل‌شده است. کانه ‌های اصلی در این کانه‌ زایی شامل مگنتیت، آپاتیت و اکتینولیت است. ساخت و بافت ‌های موجود شامل رگه‌رگچه ‌ای، نواری، توده‌ای، بِرشی، دانه ‌پراکنده، استوک ورک، جانشینی، بازماندی و پُرکننده فضاهای خالی است. کانسار اکسید آهن‌‌‌آپاتیت گلستان‌ آباد شباهت‌ های زیادی با ذخایر آهن نوع کایرونا از نظر مجموعه کانی‌‌ شناسی، ساخت و بافت ماده معدنی، دگرسانی سنگ دیواره و زمین ‌شیمی نشان می‌ دهد.

Geology, mineralogy, structure and texture, geochemistry and genesis of the Golestan Abad iron oxide- apatite deposit (East of Zanjan)

Introduction Iron oxideapatite deposits (IOA) are considered to be Kirunetype iron ores which have been formed during Proterozoic to Tertiary eras in different parts of the world. They usually have a connection with calcalkaline volcanic rocks (Hitzman, 2000). Apatite occurs as a major constituent of these deposits which is accompanied with magnetite and some actinolite. One of the most important features of these deposits (Frietsch and Perdahl, 1995) is higher concentration of REEs. There are some iron oxideapatite deposits in the TaromHashtjin magmaticmetallogenic belt, northwestern Iran. The Golestan Abad iron oxideapatite deposit is one of the IOA deposits at the TaromHashtjin belt which is located about 30 km east of Zanjan. The Golestan Abad deposit was studied during the exploration studies, but its geological characteristics, mineralogy, texture, geochemistry and genesis have not been studied yet.   Materials and methods This research study can be divided into two parts that include field and laboratory studies. Field studies include recognition of different lithological units and mineralization zones along with sampling for laboratory studies. During field studies, 60 samples were selected for petrographical, mineralogical and analytical studies. Moreover, 12 thin sections and 15 thinpolished sections were used for petrographical and mineralogical studies. For geochemical studies, 6 samples from intrusive host rocks and 7 samples from mineralized zones were analyzed by XRF and ICPMS methods at the Zarazma laboratory, Tehran.   Results The Golestan Abad area is composed of Eocene volcanosedimentary rocks of the Karaj Formation which have been intruded by quartz monzodiorite, pyroxene quartz monzodiorite and porphyritic quartz diorite intrusions. Based on petrographic studies, the pyroxene quartz monzodiorites have porphyritic and felsophyric textures and are composed of plagioclase, quartz, clinopyroxene, Kfeldspar and hornblende phenocrysts set in a quartzfeldspatic groundmass. Quartz monzodiorites show porphyritic and felsophyric textures and composed of plagioclase, hornblende, quartz, Kfeldspar and biotite. The quartz monzodiorite and pyroxene quartz monzodiorites have highK calcalkaline affinity and may be classified as metaluminous Itype granitoids. Primitive mantlenormalized (McDonough and Sun, 1995) trace elements diagrams for these granitoids indicate LILE enrichment along with negative HFSE and distinctive positive Pb anomalies. Chondritenormalized (McDonough and Sun, 1995) REE patterns for these granitoids demonstrate LREE enrichment (high LREE/HREE ratio) and weak negative anomalies in Eu. These granitoids were formed in an active continental margin to post collisional tectonic setting. Mineralization at the Golestan Abad occurs as lenses and veinveinlets of iron oxideapatite mainly within the quartz monzodiorite pyroxene quartz monzodiorite intrusions. Stockwork ores occur in the footwall of the main veins. Mineralized lenses and veins have up to 300m length and 20m width. Hydrothermal alterations around the mineralized veins include silicification, calcic (actinolitization), argillic and propylitic. From a mineralogical point of view, this deposit is composed of magnetite, apatite, actinolite, pyrite and chalcopyrite as primary minerals, while hematite, covellite, goethite and gypsum were formed during supergene alteration. Mineralization textures in the Golestan Abad deposit include veinveinlet, banded, massive, brecciated, disseminated, stockwork, replacement, relict and open space filling. Based on mineralogical and textural studies, 3 stages of apatite formation were distinguished which include: 1 coarsegrained idiomorphic apatite crystals within the magnetite matrix, 2 finegrained apatite crystals as matrix of brecciated magnetites, and 3 coarsegrained idiomorphic apatite crystals within the actinoliteapatite veins which have been cut in the previous stages. Apatite crystals of the 3 mentioned stages have high concentrations of REE that include 0.98, 0.92 and 0.95%, respectively. Condritenormalized (McDonough and Sun, 1995) REE patterns for 3 apatite generations demonstrate LREE enrichment with high LREE/HREE ratio and distinctive negative Eu anomalies.   Discussion Similar REE patterns of apatite crystals and mineralized samples with host quartz monzodioritepyroxene quartz monzodiorite samples demonstrate a genetic link between iron oxideapatite mineralization and granitoids. Furthermore, REE patterns of the Golestan Abad deposit are similar to other iron oxideapatite deposits of the TaromHashtjin metallogenic belt (Nabatian and Ghaderi, 2014; Mokhtari et al., 2017), and those of Central Iranian iron ores (Mokhtari et al., 2013). Finally, the REE patterns of the Golestan Abad deposit are similar with the REE patterns of the Kirunatype iron ores (Frietsch and Perdahle, 1995). Totally, based on mineralogical assemblages, hydrothermal alteration, mineralization textures and geochemical characteristics, the Golestan Abad iron oxide apatite deposit can be classified as the Kirunatype iron ores.   Acknowledgment This research was made possible by a grant from the office of vicechancellor for research and technology, University of Zanjan. We acknowledge their support. The respectable reviewers and editor of the Journal of Economic Geology are also thanked for their constructive comments.   References Frietsch, R. and Perdahl, J.A., 1995. Rare earth elements in apatite and magnetite in Kirunatype iron ores and some other iron ore types. Ore Geology Reviews, 9(6): 489–510. Hitzman, M.W., 2000. Iron oxideCuAu deposits: What, where, when and why? In: Porter, T.M., (Editor), Hydrothermal iron oxide coppergold and related deposits: A global perspective. Vol. 1.  Australian Mineral Foundation, Adelaide, pp. 9–25 Mokhtari, M.A.A., Hossein Zadeh, Gh. and Emami, M.H., 2013. Genesis of ironapatite ores in PoshteBadam Block (Central Iran) using REE geochemistry. Journal of Earth System Science, 122(3): 795–807. Mokhtari, M.A.A., Sadeghi, M. and Nabatian, Gh., 2017. Geochemistry and potential resource of rare earth element in the IOA deposits of Tarom area, NW Iran. Ore Geology Reveiws, 92: 529–541. Nabatian, Gh. and Ghaderi, M., 2014. Mineralogy and geochemistry of the rare earth elements in iron oxideapatite deposits of the Zanjan region. Scientific Quarterly Journal, Geosciences. 24(93): 157–170. (in Persian with English abstract)