نظریه جدید در خصوص زمان و گرادیان حرارتی پوسته اقیانوسی فرورانده شده با تاکید بر سنگ میزبان دولومیت و شیل در تشکیل کانسارهای سرب و روی کمربند ایرانکوه- آهنگران

کمربند ایرانکوه - آهنگران یکی از مهم‌ترین زو‌ن‌های کانی ‌سازی سرب و روی ایران است که با روند شمال‌غربی جنوب‌شرقی در مرکز زون ساختاری سنندج- سیرجان واقع‌شده است. سه منطقه ایرانکوه، آهنگران و حسین ‌آباد به‌عنوان مثال موردی در این مقاله بررسی شده ‌اند. بر پایه شواهد زمین‌شناسی، ساختاری، شکل و نوع کانی ‌سازی، شیمی محلول هیدروترمالی، آلتراسیون و پاراژنز مینرالی و مقایسه با انواع ذخایر سرب و روی، کلیه کانسارهای سرب و روی کمربند ایرانکوه آهنگران از نوع mvt هستند. این کانسارها با گسل ‌های تراستی مرتبط بوده و در زون ساختاری جلوی کمانی مربوط به پوسته اقیانوسی فرورونده جوان و داغ تشکیل شده ‌اند. در چنین پوسته ‌هایی قبل از رسیدن به اعماق 40 کیلومتری، بیش از 90 درصد آب اقیانوسی آزاد و سیلیکات‌های غنی از منیزیوم شکسته می ‌شوند. فلزات بخشی از پوسته اقیانوسی و بخشی از پوسته قاره‌ ای تامین شده است. از آنجایی‌که گسل ‌های تراستی سنگ ‌های ژوراسیک تا کرتاسه بالایی را قطع کرده ‌اند و با توجه به تعیین سن دقیق انجام‌شده در ایرانکوه، کانی‌سازی ‌های این کمربند در دامنه سنی بین 66 تا 56 میلیون سال پیش اغلب در پالئوسن شکل گرفته ‌اند.;شیمی محلول کانه‌دار ذخایر mvt فقیر از si و غنی از mg و fe است. چنین سیالی همراه با فلزات از طریق گسل‌های تراست بالا آمده است و در سنگ میزبان دولستون (ایرانکوه و آهنگران) یا شیل سیلتستون (حسین آباد و بخشی از ایرانکوه و آهنگران) کانی‌ سازی انجام‌شده است. تفاوت‌های فاحشی در نوع پاراژنزها، آلتراسیون، شکل، ابعاد، میزان ذخیره و عیار در کانسارهای این کمربند دیده می ‌شود که توسط نوع سنگ میزبان کنترل‌شده است.;با استناد به تمام شواهد سنگ‌شناسی، آلتراسیون، شکل و حالت کانی‌ سازی، وجود گسل ‌های تراست، پاراژنز مینرالی و موقعیت خاص زمین‌ شناسی و جغرافیایی ارائه‌شده، می‌توان برای اکتشاف ذخایر mvt پنهان در این کمربند اقدام کرد.

New hypothesis on time and thermal gradient of subducted slab with emphasis on dolomitic and shale host rocks in formation of PbZn deposits of IrankuhAhangaran belt

Introduction;Mississippi ValleyType (MVT) deposits are epigenetic zinc and lead deposits with minor copper hosted by dolostone, limestone, and locally sandstone in platform carbonate sequences inboard of major orogenic belts (Leach and Sangster, 1993; Leach et al., 2010).;The IrankuhAhangaran Belt, which is the most important PbZn mineralized zone of Iran, is situated within the SanandajSirjan tectonic zone. This belt is 400 km in length and 100 km in width. Three deposits including Irankuh mininig district, Ahangaran and Hosseinabad deposits were studied in this article (Fig. 1).;The aim of this research is study of thermal gradient of subducted slab and age of formation of PbZn deposits at IrankuhAhangaran belt, which is contrary information has been published so far on the type and their formation. Also, chemistry of orefluid in MVT deposits and impact of dolomitic and shale host rock on paragenesis, alteration, style, reserves and grade of deposits were discussed.These parameters will certainly be useful for exploration of the hidden MVT type deposits in the IrankouAhangan belt.; ; Result and Discussion;The Irankuh mineralization is hosted by Cretaceous dolostone and minor Jurassic shale rocks as epigenetic. The constructive thrust fault, which has been cut the Jurassic and Cretaceous host rocks, has played a major role in the rising of fluid and formation of mineralization. Mineralization is occurred as replacement and open space filling (fault breccia, veinlets and cavity of rock) in dolostone and breccia, veinlet and open space filling in shale host rock. The mineral assembelages are Ferich sphalerite, Fe and Mnrich dolomite, ankrite, galena, minor pyrite, bituminous, calcite ± quartz ± barite within carbonate host rocks, whereas quartz, pyrite, Ferich sphalerite, galena, minor chalcopyrite, low Fedolomite, bituminous, ± barite ± calcite are important primary minerals at clastic hos rocks (Karimpour et al., 2018).;The Ahangaran deposit is very similar to Irankuh in host rock, alteration, paragenesis, and form of mineralization. Thrust fault has a constructive role for occurrence of mineralization and later destructive strike slip and normal faults have caused the displacement and destruction of mineralization.;The Hosseinabad deposit is hosted by Jurassic shale, siltstone, and sandstone rocks as veinveinlets, breccia and open space filling with structural control. Alteratin consists of silicification, chlorite, bituminous, and minor siderite, dolomite and ankerite similar to mineralization hosted by shale in Irankuh district. The mineral assemblages are galena, Ferich sphalerite, pyrite, chalcopyrite and minor phyrotite. Due to the lack of a proper dolostone unit in the Husseinabad deposit, mineralization is concentrated in particular areas with lowgrade and lowreserves.;Based on lithology, alteration, mineralization style, structural control by thrust faults, mineral paragenesis, and comparison with differnet types of PbZn deposits, all deposits of IrankuhAhangaran belt are MVTtype. Deepseated thrust faults formed during the early stages of subduction (~ 70 to 75 Ma), and played an important role in the upward migration of hydrothermal fluids from the basement to shallow depths. The geochronology of pyrite in Irankuh district based on ReOs method indicate age of Irankuh PbZn mineralization is 66.5 ± 1.6 Ma (Liu et al., 2019). Since the thrust faults have been cut the Jurassic to Upper Cretaceous rocks, and according to the absoulte age determined in Irankuh, the mineralization of this belt have been formed in the age range of 66 to 56 million years ago, mainly in the Paleocene (Fig. 15).;Karimpour and Sadeghi (2018) suggested the hydrothermal fluid originated from the dehydration of a hot and young oceanic subducted slab, which liberated Pb, Zn, and other metals, and may have removed metals from rocks and organic material of the continental crust. More than 90% of all the water within the oceanic slab was released in the depth zone of the forearc region (depth of 30 to 50 km) (Karimpour and Sadeghi, 2018). In the depth zone, Mgrich silicate minerals (such as antigorite, hornblende, chlorite, talc) have broken and the produced fluid is rich in Mg and Fe (Fig. 17).;The orefluid of MVT deposits is Sipoor and Fe and Mg rich. Such fluid is mineralized on the hosts of the dolstone (Irankuh and Ahangaran) or ShaleSiltstone (Hossein Abad, and part of Irankuh and Ahangaran). There are significant differences in the type of paragenesis, alteration, shape, dimensions, reserves and grade in the deposits of this belt, which is controlled by the host rock type.;Based on all lithological evidence, alteration, shape of mineralization, existence of thrust faults, mineral paragenesis and specific geological and geographic location, it can be used to exploration of the hidden MVT deposits in this belt.; ;References;Karimpour, M.H., Malekzadeh Shafaroudi, A., Esmaeili Sevieri, A., Shabani, S., Allaz, J.M. and Stern, C.R., 2018b. Geology, mineralization, mineral chemistry, and orefluid conditions of Irankuh PbZn mining district, south of Isfahan. Journal of Economic Geology, 9(2): 267–294. (in Persian with English abstract);Karimpour, M.H. and Sadeghi, M., 2018. Dehydration of hot oceanic slab at depth 30–50 km: KEY to formation of IrankuhEmarat PbZn MVT belt, Central Iran. Journal of Geochemical Exploration, 194: 88–103.;Leach, D.L. and Sangster, D., 1993. Mississippi Valleytype leadzinc deposits. Geological Association of Canadian. Specific Paper, 40: 289–314.;Leach, D.L, Taylor, R.D., Fey, D.L., Diehl, S.F. and Saltus, R.W., 2010. A Deposit Model for Mississippi ValleyType LeadZinc Ores, Chapter A of Mineral Deposit Models for Resource Assessment. U.S. Geological Survey, Reston, Virginia, Scientific Investigations Report 2010–5070–A., 64 pp.;Liu, Y., Song, Y., Fard, M., Zhou, L., Hou, Z. and Kendricke, M.A., 2019. Pyrite ReOs age constraints on the Irankuh ZnPb deposit, Iran, and regional implications. Ore Geology Reviews, 104: 148–159.