رخساره‌های کانسنگ، کانی‌ شناسی، دگرسانی، ژئوشیمی و الگوی تشکیل کانسار باریت- روی-سرب-مس ونکان (سوکان)، شمال شرق سمنان

کانسار باریت‌روی‌سرب‌مس ونکان (سوکان) با سنگ میزبان توف، توف شیلی، گدازه آندزیتی و تراکی آندزیت به سن ائوسن در شمالی‌ترین بخش از کمربند ماگمایی شمال ایران مرکزی در شمال‌شرق سمنان واقع‌شده است. توالی میزبان از پایین به بالا شامل سه واحد سنگی است: 1) واحد غنی از سنگ‌های رسوبی شامل کنگلومرا، سنگ آهک و ماسه‌سنگ، 2) واحد غنی از گدازه حاوی سنگ هایی با ترکیب حدواسط تا اسیدی، آندزیت و تراکی آندزیت به همراه میان‌لایه هایی از شیل و 3) واحد غنی از توف شامل توف‌های داسیتی، توف ریولیتی و توف شیلی. منطقه معدنی ونکان در ناودیسی با روند محوری شمال‌شرقیجنوب‌غربی قرار دارد. کانه زایی در کانسار ونکان به‌صورت سه افق کانه زایی دیده می شود. افق اول و اصلی کانه زایی که در یال شمال غربی ناودیس قرار دارد، شامل کانسار ونکان 1 بوده که از پایین به بالا از پنج رخساره کانه دار تشکیل شده است: 1) رخساره رگه‌رگچه ای، 2) کانسنگ سولفید توده ای، 3) کانسنگ باریت لایه ای، 4) کانسنگ لایه ای نواری سولفیدی و 5) رخساره نواری رسوبی‌برون‌دمی. کانسار ونکان 2 واقع در یال جنوب شرقی دارای رخساره های 1) رگه‌رگچه ای و برشی و 2) نواری رسوبی‌برون‌دمی است. بافت ماده معدنی اغلب شامل رگه‌رگچه ای، توده ای، برشی،  نواری‌لامینه و دانه پراکنده است. کانی‌های اولیه و اصلی در ماده معدنی به ترتیب شامل باریت، اسفالریت، گالن، پیریت، کالکوپیریت و مارکاسیت و کانی‌های ثانویه مالاکیت، کریزوکولا، گوتیت، لیمونیت، اسمیت زونیت و سروزیت هستند. دگرسانی عمده در کانسار ونکان در سنگ دیواره کمرپایین از نوع کلریتی و سریسیتی و به مقدار کمتر اپیدوتی، آرژیلیکی و سیلیسی است. طبق پژوهش‌های انجام‌شده، به‌نظر می رسد کانسارهای باریت‌فلزات پایه ونکان بر اساس مقایسه با انواع مختلف کانسارهای سولفید توده ای آتشفشان‌زاد، بیشترین شباهت را با کانسارهای نوع کوروکو نشان می دهند.

Ore facies, mineralogy, alteration, geochemistry and genesis of the Vanakan (Sokan) barite-zinc-lead-copper deposit, north east of Semnan

IntroductionThe Vanakan (Sokan) baritezincleadcopper deposit is located at 23 km northeast of Semnan, in the North Central Iran magmatic belt. It has occurred within the Eocene volcanicsedimentary sequence. The host rocks of the ores mainly consist of tuff, shale and shaly tuff. Volcanic rocks in the district at the Ahovan region involve both mafic and felsic compositions including basalt, andesite, dacite, rhyolite and tuff. Many studies have been conducted on ore deposits in the Semnan region including Poshteh barite base metals volcanogenic massive sulfide (VMS) deposit (Ghaffari, 2017), Hamyard (HajiBahrami, 2012) and northeast Semnan (e.g., Ghiasvand et al., 2009; Shahri, 2011) iron skarn deposits. Therefore, studying the baritemetal deposits in the Central Iran magmatic belt such as the Vanakan deposit, can provide exploratory keys to discover new reserves, which is one of the main goals of this research study. In this work, study on ore facies,mineralogy, alteration, geochemistry and genesis of the Vanakan baritezincleadcopper deposit are considered. Materials and methodsFirst, regional and local geology, alteration, ore textures and structures and mineralogy of ore horizons in the Vanakan ore deposit were carefully checked out and studied during field studies. Then, the samples were systematically collected from trenches and open pit of the mine. Mineralogical studies were conducted on 24 thin sections and 8 polished samples in the microscopic laboratory at the Shahrood University of Technology. For geochemical studies, about 16 systematic samples from different ore facies and ore horizons were collected. Then, the samples were analyzed by inductively coupled plasmaatomic emission spectroscopy (ICPAES) method, and a few samples were studied by Xray diffraction (XRD) method in the Aria Sharif Laboratories Company. ResultsThe host sequence in the Vanakan deposit involves three units, from bottom to top: Unit1: conglomerate, limestone, sandstone; Unit2: andesitic to dacitic lavarich, and unit3: acidic tuffrich. Mineralization as the Vanakan 1 and 2 deposits occurred at top of unit 2 and within unit 3. The entire Vanakan area involves a local syncline with northeastsouthwest axial trend, in which the Vanakan 1 and Vanakan 1 deposits are located in the northern and southern limbs of the syncline, respectively. Based on structural,  textural and mineralogical studies, five different ore facies were distinguished  in Vanakan 1, from bottom to: 1) veinveinlet and breccia: involving baritepyritequartz veinveinlets, 2) massive sulfide: composed of massive sphalerite, galena, barite, chalcopyrite and pyrite, 3) layeredbanded sulfide ore: involving alternations of ore and sericite altered tuffrich bands, 4) baritic ore: comprising of mainly barite and little sulfides, and 5) bandedexhalative cherty sediments. The ore facies in the Vanakan 2 from bottom to top are 1) barite (galena)rich veinveinlets and 2) banded cherty iron oxidehydroxides rich red exhalative sediment. From a mineralogical point of view, the ores in the Vanakn 1 mainly consist of barite, sphalerite, galena, pyrite, chalcopyrite and marcasite accompanied with secondary minerals such as malachite, chrysocolla, smithsonite, cerussite, hematite, limonite, goethite. DiscussionBased on different characteristics of mineralization in the Vanakan district, such as geometry of ore bodies, textures and structures, ore facies, wall rock alterations, mineralogy, metal zonation and geochemical features, the Vanakan deposit can be classified as a bimodal felsic or Kurokotype volcanogenic massive sulfide (VMS) deposit, similar to those of the Mount Read volcanic deposits of Tasmanian Australia such as Rosebery (Large, 1992; Large et al., 2001) and Hokuroko basin in Japan (Huston et al., 2011; Ohmoto and Skinner, 1983). ReferencesGhaffari, G., 2017. Mineralogy, geochemistry and genesis of the Poshteh baritekaolinecopper deposit, east of Semnan. M.Sc. thesis, Shahrood University of Technology, Shahrood, Iran, 186 pp. (in Persian with English abstract)Ghiasvand, A., Ghaderi, M. and Rashidnejad, N., 2009. Mineralogy, geochemistry and origin of iron deposits in north of Semnan. Geosciences, 18(72): 33–44. https://doi.org/10.22071/GSJ.2010.57133HajiBahrami, M., 2012. Petrography, geochemistry and genesis of the Hamyard iron deposit, northeast of Semnan. M.Sc. Thesis, Damghan University, Damghan, Iran, 175 pp. (in Persian with English abstract).Huston, D.L., Relvas, J.M.R.S., Gemmell, J.B. and Drieberg, S., 2011. The role of granites in volcanichosted massive sulphide oreforming systems: an assessment of magmatic–hydrothermal contributions. Mineralium Deposita, 46(5–6), 473–507. https://doi.org/10.1007/s0012601003227Large, R.R., 1992. Australian volcanichosted massive sulfide deposits; features, styles, and genetic models. Economic Geology, 87(3): 471–510. https://doi.org/10.2113/gsecongeo.87.3.471Large, R.R., McPhie, J., Gemmell, J.B., Herrmann, W. and Davidson, G.J., 2001. The spectrum of ore deposit types, volcanic environments, alteration halos, and related exploration vectors in submarine volcanic successions: Some examples from Australia. Economic Geology, 96(5): 913–938. https://doi.org/10.2113/gsecongeo.96.5.913Ohmoto, H. and Skinner, B.L., 1983. The Kuroko and related volcanogenic massive sulphide deposits: Introduction and summary of new findings. In: H. Ohmoto and B.J. Skinner (Editors), Kuroko and related volcanogenic massive sulphide deposits. Economic Geology, Canada, pp. 18. https://doi.org/10.5382/Mono.05.01Shahri, M., 2011. Investigation of skarnization, metasomatism and related to mineralization in Zartul area (Northeast Semnan). M.Sc. thesis, University of Technology, Shahrood, Iran, 144 pp. (in Persian with English abstract)