شیمی‌ کانی و کانه زایی کرومیت در منطقه قره‌ناز-‌‌علم کندی، غرب زنجان

کانسار کرومیت قره‌ناز-علم‌کندی  در باختر استان زنجان و پهنه سنندج-سیرجان  واقع‌شده است. از نظر سنگ‌نگاری، واحدهای سنگی موجود در منطقه مورد بررسی شامل آمفیبولیت، آمفیبول شیست، گارنت میکا شیست و گرانیت ‌گنایسی به سن پرکامبرین به همراه واحدهای اولترامافیک با ترکیب هارزبورژیت سرپانتینیتی شده، لرزولیت سرپانتینیتی شده، دونیت سرپانتینیتی شده و سرپانتینیت است. کانه‌زایی کرومیت در بخش‌های اولترامافیک و درون سنگ میزبان هارزبورژیت های سرپانتینیتی شده و سرپانتینیت ها به صورت دانه پراکنده، عدسی شکل و رگه‌رگچه ای مشاهده می شود. به علت محدود‌بودن گسترش سنگ میزبان پریدوتیتی، کانه زایی نیز گسترش زیادی ندارد و به‌صورت عدسی‌هایی با طول نهایت تا دو متر و عرض چند سانتی متر تا کمتر از یک متر مشاهده می شود. بررسی‌های شیمی‌ کانی کروم‌اسپینل های منطقه مورد بررسی بیانگر آن است که کانه زایی کرومیت در این منطقه از نوع، highcr و غنی از mg و فقیر از ti است. نتیجه بررسی‌ها نشان می دهد که کانه زایی کرومیت در این منطقه از نوع کرومیت های افیولیتی نوع پادیفرم بوده و از یک ماگمای بونینیتی منشا گرفته اند. بنابراین می توان اظهار‌داشت که کرومیت های منطقه قره‌ناز‌علم کندی در محیط زمین‌ساختی سوپراسابداکشن و در ارتباط با فرورانش اقیانوس پروتوتتیس به زیر بلوک ایران در زمان پرکامبرین‌کامبرین تشکیل‌شده است.

Mineral chemistry and chromite mineralization in the Qaranaz-Alamkandi area, west Zanjan

Introduction Malek Ghasemi and Karimzadeh Somarin (2005) reported that Chromite deposits in Iran occur in Paleozoic and Mesozoic ophiolite complexes in association with serpentinite and serpentinized peridotites and dunites (Ghazi et al., 2004; Shafaii Moghadam and Stern, 2014). There are more than 74 chromite deposits that have been explored in these complexes and they are mainly of alpinetype (Ghorbani, 2013). These ophiolite complexes are part of the Tethyan belts which link to other Asian ophiolite belts such as Pakistan and Tibet in the east as well as ophiolites in the Mediterranean region such as Turkey, Troodos, Greek, and east Europe in the west (Yaghoubpur and Hassannejhad, 2006; Hassanipak and Ghazi, 2000). New data in the current research study are used to infer the geology, mineralization, mineralogy, mineral chemistry and origin of the QaranazAlamkandi chromite.   Materials and methods After preparing 72 samples from the study area, microscopic studies were carried out on 18 thin sections and 23 polishedthin sections for recognition of the microscopic features of the host rock as well as the mineralogy and texture of the ore body. Then, two chromite samples were analyzed at the Iran Mineral Processing Research Center, Karaj, Iran using electron microprobe and scanning electron microscope (SEM) methods.    Discussion and Results The QaranazAlamkandi chromite occurrence is located in the west of the Zanjan province and in the northern part of the SanandajSirjan zone. This area is composed of ultramafic sequences associated with Precambrian metamorphosed rocks such as amphibolite, marble, granitic gneiss and schist. According to petrographic studies rock units in the QranazAlamkandi area consist of serpentinized harzburgite, serpentinized lherzolite, serpentinized dunite, serpentinite, amphibolite, amphibole schist, gneissic granite and mica shcist. This study show that the peridotitic rocks in this region include dunite, harzburgite and lherzolite. Olivine, orthopyroxene and lesser amounts of clinopyroxene associated with secondary minerals (such as serpentine, chlorite and calcite) and opaque minerals (chromite and magnetite) are the main minerals in peridotites. Mineral chemistry of olivines in the peridotites shows magnesium rich olivine with forsterite composition, slightly tending to chrysolite. The composition of olivines falls in the olivine spinel mantle array. Moreover, the olivines of dunites are comparable with those from the oceanic suprasubduction zone peridotites. Clinopyroxenes and orthopyroxenes are FeMgCa rich. Furthermore, clinopyroxenes show augite composition and are mainly of the calcium magnesium type. Orthopyroxenes show mainly bronzite and minor samples showing hypersthene composition. The composition of clinopyroxenes is similar to those of boninites and arc related magmas. This result and the given fact of the low contents of TiO2 and high contents of SiO2 in the structural formula of the pyroxenes suggest that the pyroxenes of the study area are comparable with those from the subduction tectonic settings. Chromite mineralization in the QaranazAlamkandi area has occurred within the ultramafic rocks with serpentinized harzburgite and serpentinite composition. Due to the limited expansion of the peridotitic host rocks, chromite mineralization is also limited and it has occurred as lenses with maximum length up to two meters and one meter width. Chromite mineralization has occurred as massive, disseminated, lenzoid and vein veinlets form in this area. Mineral chemistry of Chrome spinels from the QaranazAlamkandi area indicate that the chromite samples plot within the ophiolite complexes and high magnesium chromite field (Leblanc and Nicolas, 1992), which classifies them as podiform chromite deposits in terms of mineralization type (Arai et al., 2004). Chromite mineralization in the QaranazAlamkandi area indicates an Alpine type deposit which is enriched in Cr and Mg and depleted in Ti. The QaranazAlamkandi chromite mineralization has been formed from boninitic magmas which were derived from the subduction process in a suprasubduction zone and forearc tectonic settings (Ahrabian, 2018).   Acknowledgements The authors are thankful to the Research Council of University of Zanjan and Iranian Mines Mining Industries Development Renovation Organization for financially supporting this study. We also acknowledge valuable comments from editor of Journal of Economic Geology and anonymous reviewers which improved the quality of the manuscript.   References Ahrabian, P., 2018. Geology, Geochemistry and Genesis of chromite mineralization of Gharenaz Alamkandi area, west of Zanjan, Iran. M.Sc. Thesis, University of Zanjan, Zanjan, Iran, 215 pp. (in Persian with English abstract) Arai, S., Uesugi, J. and Ahmed, A.H., 2004. Upper crustal podiform chromitite from the northern Oman ophiolite as the stratigraphically shallowest chromitite in ophiolite and its implication for Cr concentration. Contributions to Mineralogy and Petrology, 147(2): 145–154. Ghazi, A., Hassanipak, A., Mahoney, J. and Duncan, R., 2004. Geochemical characteristics, 40Ar39Ar ages and original tectonic setting of the BandeZeyarat/Dar Anar ophiolite, Makran accretionary prism, SE Iran. Tectonophysics, 393(1–4): 175–196. Ghorbani, M., 2013. The Economic Geology of Iran: Mineral Deposits and Natural Resources. 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