کانه زایی، کانی شناسی، ساخت و بافت و ژنز کانسار سرب و روی اورتاسو، شمال غرب زنجان

کانسار سرب و روی اورتاسو، در شمال غرب زنجان و در پهنه ایران مرکزی قرار‌گرفته است. در این منطقه سازندهای کهر، شمشک، قرمز زیرین، قم و قرمز بالایی گسترش‌ دارند. سازند قرمز بالایی که میزبان کانه‌ زایی در این منطقه به شمار می‌ رود، شامل تناوبی از مارن، سیلت‌سنگ، ماسه سنگ های قرمز و خاکستری‌ رنگ و کنگلومراست. کانه‌زایی چینه‌ سان سرب و روی درون ماسه‌ سنگ های لیت آرنایتی (اغلب چرت آرنایت) خاکستری‌ رنگ دارای مواد آلی، رخ‌داده است. در کانسار اورتاسو دو افق کانه‌ زایی قابل‌شناسایی است که هر‌یک از آنها از دو پهنه سرخ اکسیدان و شسته‌شده و یک زیرپهنه کانه‌ زایی تشکیل‌شده است. کانه های اصلی تشکیل‌دهنده کانه زایی شامل گالن، اسفالریت، پیریت و کالکوپیریت هستند که کانی های ثانویه از جمله اکسیدهای آهن، کالکوسیت، کوولیت و سروزیت طی فرایندهای برون‌ زاد و هوازدگی جانشین آنها شده‌ اند. بافت های غالب در این کانسار شامل بافت های شبه لامینه ای، دانه پراکنده، پیریت فرامبوئیدال و سیمان بین بلوری هستند. بررسی‌های زمین شیمیایی انجام‌شده بر روی نمونه‌ های ماسه‌ سنگی، نشان دهنده آن است که ماسه‌سنگ های این منطقه در محیط های حاشیه فعال قاره ای ته‌ نشست کرده‌ اند. این ماسه‌ سنگ ها از سنگ های ماگمایی با ترکیب فلسیک منشا گرفته و در محیطی دارای شرایط آب و هوایی خشک ته‌ نشست کرده اند. بررسی‌های انجام‌ شده از جمله سنگ‌ شناسی، چینه شناسی، کانی شناسی، ساخت و بافت و عوامل کنترل کننده کانه‌ زایی (وجود آثار و بقایای گیاهی، وجود گنبدهای نمکی، نفوذپذیری مناسب سنگ میزبان، گسل ها و شکستگی ها)، بیانگر آن است که کانسار اورتاسو شباهت زیادی با بخش دیستال کانسارهای مس رسوبی نوع redbed دارد.

Mineralization, Mineralogy, Structure, Texture and Genesis of Ortasu PbZn Deposit, NW Zanjan

Introduction;According to Hitzman et al. (2005) and Hayes et al. (2015), the sedimenthosted copper deposits (SHC) are stratiform disseminated to veinlet copper mineralization within the reduced black shale, sandstone, and carbonate rocks. These deposits have formed during the middlelate Paleoproterozoic (e.g., Udokan, Russia: Volodin et al. 1994, Yinmin, South China: Zhao et al. 2013) to Tertiary (e.g., Corocoro, Bolivia: Flint 1989). The SHC deposits, frequently labeled as “stratiform” or “stratabound” and/or “diagenetic” deposits have been subdivided into three types (Cox 1986, Cox et al. 2003, Hitzman et al. 2005): 1 reducedfacies (RF), 2 Redbed (RB) and 3 Revett (RV).;There are several SHC deposits in the Zanjan region hosted by the Upper Red Formation. The Chehrabad, Cherlangoush, Ghezeljeh, Halab, Ortasou and Sarikand deposits are the most important ones and are welldeveloped in this district. These deposits consist predominantly of beddingparallel replacement and disseminated CuPbZn sulfides, roughly concordant with stratification. The average Cu and Pb values of these deposits are ~3 and 2 wt.%, respectively. These deposits are small but they are actively mined at present.;The aim of this work is to expand knowledge on the geological framework, mineralization features, geochemistry and genesis of the Ortasu PbZn deposit. The study of the Ortasu deposit can be used as an exploration model for similar deposits in the Zanjan district and other places.; ;Materials and methods ;Detailed field studies have been done at different scales in the Ortasou area. Polished thin and thin sections from mineralized zones and host rocks were studied by conventional petrographic and mineralogic methods at the University of Zanjan. Furthermore, a total of six samples from mineralized host rocks were analyzed by ICPMS for major and trace elements and Rare Earth Elements (REE) at Lab West Co., Australia.; ;Results and Discussion;The Ortasu PbZn deposit is located in the northwest of Zanjan and Central Iran zone. The outcrop formations in this region involve Lower Red, Qom and Upper Red Formations from which the Upper Red Formation hosts the mineralized zones. In this region, the Upper Red Formation consists of alternation of red to brown and grey to greenish marl associated with interbedded sandstone sequences. Based on petrographic studies, composition of the mineralized host rocks are litharenite which consist of sedimentary, metamorphic and igneous fragments, quartz and feldspar. In the Ortasu PbZn deposit, mineralization has occurred within two horizons of reducedgrey sandstone with about five to six m thickness and about 100 m length. These horizons contain both red oxidized zone and bleached zone with a mineralized reduced subzone which is located within the bleached zone.;The red oxidized zone consists of red marl and sandstone layers containing iron oxides which is located adjacent to the reduced horizons. The red color of this zone is caused by the presence of iron oxides around the grains. The oxidized pyrite crystals are the main important minerals in this zone. The bleached zone is part of sandstone sequences whose color has changed by the alteration processes. Grey and green colors in the bleached zone have occurred due to the presence of organic materials and diagenetic pyrites. The mineralization subzone has occurred within the organic materialsbearing bleached zones. Plant debris, plant fossils, diagenetic pyrites and permeability of host rock have the main important role for PbZn mineralization.;Galena, sphalerite, pyrite and chalcopyrite are the main important minerals in the Ortasu PbZn deposit in some parts. They have been replaced by secondary minerals such as cerussite, chalcocite, covellite and iron oxides due to the supergene and weathering processes. The most important textures in this deposit are disseminated and cemented textures. It should be mentioned that the laminatedlike, framboidal pyrite, replacement and relict textures are also observed in this deposit.;The presence of finingupward sequences is due to the sediment cycles of channels, layered structures, abundant organic materials in paleochannels and debris of plant fossil all of which indicate that the Ortasu PbZn deposit is formed in a continental to tidal environment. Geochemical studies show that the sandstones were deposited in an active continental margin. These sandstones originated from felsic magmatic rock and were deposited in an arid climate condition.;According to host rock, geometry, structure and texture, and mineralogy it can be concluded that the Ortasu PbZn deposit has more similarity with the distal parts of Redbed type of sedimenthosted copper deposits.; ;Acknowledgements ;All logistical supports during field studies came from the University of Zanjan, Zanjan, Iran.; ;References;Cox, D.P., 1986. Descriptive model of sedimenthosted Cu. In: D.P. Cox and D.A. Singer (Editors), Mineral deposit models. U.S. Geological Survey Bulletin. Canada, Pp. 101–104.;Cox, D.P., Lindsey, D.A., Singer, D.A., Moring, B.C. and Diggles, M.F., 2003. Sedimenthosted copper deposits of the worlddeposit models and database. U.S. Geological Survey, Canada, openfile report 03107, 53 pp.;Flint, S.S., 1989. Sedimenthosted stratabound copper deposits of the Central Andes. In: R.W. Boyle, A.C. Brown, C.W. Jefferson, E.C., Jowett and R.V. Kirkham (Editors), Sedimenthosted stratiform copper deposits. Geological Association of Canada, Special Paper, Canada. pp. 371–400.;Hayes, T.S., Cox, D.P., Piatak, N.M. and Seal, R.R., 2015. Sedimenthosted stratabound copper deposit model. U.S. Geological Survey, Canada, Scientific Investigations Report 20105070M, 147 pp.;Hitzman, M., Kirkham, R., Broughton, D., Thorson, J. and Selley, D., 2005. The sedimenthosted stratiform copper ore system. In: J.W. Hedenquist, J.F.H. Thompson, R.J. Goldfarb and J.P. Richards (Editors), One Hundredth Anniversary volume of Economic Geology. Society of Economic Geologists, Colorado, pp. 609–612.;Volodin, R.N., Chechetkin, V.S., Bogdanov, Y.V., Narkelyun, L.F. and Trubachev, A.I., 1994. The Udokan cupriferous sandstones deposit (eastern Siberia). Geology of Ore Deposits, 36(1): 1–25.;Zhao, X.F., Zhou, M.F., Li, J.W. and Qi, L., 2013. Late Paleoproterozoic sedimentary rockhosted stratiform copper deposits in South China: their possible link to the supercontinent cycle. Mineralium Deposita, 48(1): 129–136.