رخ‌ داد معدنی آهن لولک‌ آباد، شمال‌ باختر زنجان: کانه زایی تیپ آتشفشانی- رسوبی دگرگون و دگرشکل شده در زون ایران مرکزی

رخ‌داد معدنی آهن لولک آباد، در گوشه شمال باختری زون ایران مرکزی و در فاصله 55 کیلومتری شمال باختر زنجان قرار دارد. کانه زایی آهن در این منطقه، به صورت رگه ای رگه‌چه ای و عدسی شکل، درون واحدهای دگرشکل شده شیستی، متاولکانیکی (معادل سازند کهر) و گرانیتی رخ داده است. بر اساس بررسیهای میکروسکپی، کانیهای معدنی در لولک آباد، شامل اولیژیست، اندکی مگنتیت و پیریت کانیهای باطله، شامل کوارتز، کلسیت و کلریت است. بافت کانه ها شامل دانه‌‌پراکنده، رگه رگه‌چه ای، بِرشی و پرکننده فضای خالی است. دگرسانی شامل سیلیسی، کلسیتی و کلریتی شدن بخشهای کانه دار است. دو مرحله کانه‌زایی در رخ‌داد معدنی آهن لولک‌آباد قابل تفکیک است. کانه زایی مرحله اول، به صورت مگنتیت های دانه‌پراکنده و عدسی‌شکل چینه‌سان و چینه‌کران هم‌زمان با واحدهای آتشفشانی رسوبی میزبان دیده می‌شود. مرحله دوم کانه زایی، به صورت رگه و رگه‌چه های اولیژیستی پیریتی کلسیتی رخ داده و برگ‌وارگی غالب میلونیتی سنگها را قطع کرده است. مقایسه الگوی عناصر نادر خاکی در سنگهای میزبان فاقد کانه زایی و بخشهای کانه دار، بیانگر غنی شدگی عناصر نادر خاکی سنگین نسبت به عناصر نادر خاکی سبک در بخشهای کانه دار است. این امر بیانگر نسبت بالای واکنش سیال به سنگ در لولک آباد است. مقایسه ویژگیهای زمین شناسی، کانی شناسی، زمین شیمیایی و ساخت و بافت ماده معدنی در رخ‌داد معدنی لولک‌آباد با نوعهای مختلف کانسارهای آهن نشان می دهد؛ که کانه زایی آهن در لولک‌آباد، در ابتدا به‌صورت آتشفشانی رسوبی تشکیل شده و بعد به صورت رگه ای تمرکز دوباره یافته است.

Lulak Abad Iron Occurrence, Northwest of Zanjan: Metamorphosed and Deformed Volcano-Sedimentary Type of Mineralization in Central Iran

Introduction#13;The Lulak Abad iron occurrence is located in the northwestern part of the Central Iran, 55 km west of Zanjan. Mineralization at the Lulak Abad area was originally identified by Zamin Gostar Company (2007), during a geophysical exploration. The present paper provides an overview of the geological framework, the mineralization characteristics, and the results of a geochemical study of the Lulak Abad iron occurrence with an application to the ore genesis. Identification of these characteristics can be used as a model for exploration of this type of iron mineralization in the Central Iran and elsewhere.#13;#13;Materials and methods#13;Detailed field work was carried out at different scales (give scales in parentheses) in the Lulak Abad area. About 16 polished thin and thin sections from host rocks and mineralized and altered zones were studied by conventional petrographic and mineralogical methods at the Department of Geology, University of Zanjan. In addition, a total of 7 samples from ore zones at the Lulak Abad occurrence were analyzed by ICPOES for minor and trace elements and REE compositions at Geological Survey of Iran, Tehran, Iran. #13;#13;Result#13;Rock units exposed in the Lulak Abad area consist of schists and metavolcanic units the Kahar Formation; Lotfi, 2001) that were intruded by granite and microdiorite bodies. The schist units consist of chloritebiotitemuscovite schist and muscovite schist that show granolepidoblastic texture with foliationparallel disseminated magnetite. The metavolcanic units consist of metadacite, rhyolitic metatuff and metaandesite with porphyritic textures. They are marked by dominant mylonitic foliation surrounding feldspar and quartz porphyroclasts. Alkali feldspar and quartz are the principal minerals of the granite. The intrusion is characterized by intense deformation features and is highly mylonitized. Based on field and microscopic studies, the microdiorite postdated metamorphic and deformation events and shows neither schistosity nor mylonitic foliation. It is composed principally of plagioclase with minor disseminated magnetite and a microgranular texture. Two deformation events are recognized at the Lulak Abad area, one principally ductile, the other brittle. #13;Iron mineralization at Lulak Abad occurs as veins, veinlets and lensshaped bodies in schist units, mylonitic metavolvanic rocks and mylonitic granite. The main ore vein extends up to 100 m in length and averages 3 m in width, reaching a maximum of 6 m. It trends NE, dipping steeply SE. The ore lenses are parallel to the mylonitic foliation and variably boudinaged, about 10 m in length and vary in thickness up to 5 cm. Two stages of mineralization can be distinguished at Lulak Abad. Stage 1 mineralization is recognized as stratiform and stratabound lenses, laminated and disseminated crystals of magnetite in volcanosedimentary host rocks. Stage 2 is characterized as hematitepyritecalcite veins and veinlets cutting the mylonitic foliation of the host rocks. Hydrothermal alteration is restricted to silicified, calcitic and chloritic altered parts of the ore zones. #13;The ore minerals at Lulak Abad formed as vein and hydrothermal breccia cements, and show veinveinlet, brecciated, disseminated and open space filling vein and veinlet textures. Hematite is the main ore mineral, accompanied by minor magnetite and pyrite. Goethite occurs as a supergene mineral. Quartz, calcite and chlorite are present in the gangue minerals that represent veinveinlet and vug filling textures. #13;The Lulak Abad mineralized veins and breccias show lower concentrations of LREE and HREE (i.e., Pr, Er, Ho, Dy and Yb) relative to barren granitic host rocks but higher Tm, Gd, Eu and Lu concentrations. Chondritenormalized REE patterns (Sun and McDonough, 1989) of host barren granite and the mineralized samples at Lulak Abad indicate that mineralized samples are depleted in LREE (except Ce) but enriched in most HREE (beside depletion in Dy and Ho). These signatures indicate high wall rock interaction (e.g., Lottermoser, 1992; Liegeois et al., 2003).#13;Comparison of the geological, mineralogical, geochemical, textural and structural characteristics of the Lulak Abad occurrence with different types of iron deposits reveals that iron mineralization at Lulak Abad was originally formed as volcanosedimentary, and then reconcentrated as vein mineralization (Karami et al., 2012; Karami et al., 2013).#13;#13;Acknowledgements#13;The authors are grateful to the University of Zanjan Grant Commission for research funding. Journal of Economic Geology reviewers and editor are also thanked for their constructive suggestions on alterations to the manuscript.#13;#13;References#13;Karami, M., Ebrahimi, M. and Kouhestani, H., 2012. Geological and mineralization characteristics of Lulak Abad iron occurrence, east of Mahneshan. 31th Symposium of Geosciences, Geological Survey of Iran, Tehran, Iran (in Persian with English abstract).#13;Karami, M., Kouhestani, H. and Ebrahimi, M., 2013. Mineralogy, structure, texture and type of iron mineralization in Lulak Abad occurrence, east of Mahneshan. 1st International Conference on Mining, Mineral Processing, Metallurgical and Environmental Engineering. University of Zanjan, Zanjan, Iran (in Persian with English abstract).#13;Liegeois, J.P., Latouche, L., Boughrara, M., Navez, J. and Guiraud, M., 2003. The Latea metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the PanAfrican orogeny. Journal of African Earth Sciences, 37(3–4): 161–190.#13;Lotfi, M., 2001. Geological map of Mahneshan, scale 1:100,000. Geological Survey of Iran.#13;Lottermoser, B.G., 1992. Rare earth elements and hydrothermal ore formation processes. Ore Geology Reviews, 7(1): 25–41.#13;Sun, S.S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: A.D. Saunders and M.J. Norry (Editors), Magmatism in the Ocean Basins. Geological Society of London, London, pp. 313–345.#13;Zamin Gostar Company, 2007. Primary iron exploration project at Lulak Abad area, Industry, Mine and Trade Organization, Zanjan, 32 pp.