زمین شناسی و خاستگاه کانه زایی مس در کانسار دهنه، شمال خاور زنجان

کانسار مس دهنه در فاصله 84 کیلومتری شمال خاور زنجان، در پهنه البرز باختری‌آذربایجان و زیرپهنه طارم قرارگرفته است. سنگ های رخنمون‌ یافته در منطقه دهنه، مجموعه‌ا ی از سنگ های آتشفشانی و آتشفشانی‌رسوبی ائوسن (معادل سازند کرج) شامل واحدهای توف و گدازه‌های بازالت پورفیری و بازالت آمیگدالوئیدال هستند. بررسی‌های ژئوشیمیایی نشان می دهد که سنگ های آتشفشانی منطقه دهنه دارای ترکیب بازالتی با ماهیت شوشونیتی بوده و در محیط زمین‌ساختی مرتبط با فرورانش تشکیل شده‌ اند. کانه‌ زایی مس در منطقه دهنه به‌صورت رگه‌رگچه ای در درون واحدهای گدازه بازالتی رخ‌ داده است. بر اساس بررسی‌های میکروسکوپی انجام‌شده، کانه های موجود در منطقه مورد بررسی شامل مس طبیعی، نقره طبیعی، کوپریت و به مقدار کمتر مالاکیت و آزوریت است. عمده ترین دگرسانی های موجود در این منطقه شامل کربناتی ، کلریتی‌اپیدوتی، سریسیتی و زئولیتی است. بافت کانه‌ها شامل رگه رگچه ای، پرکننده فضای خالی، بازماندی (جزیره‌ ای) و جانشینی است. بررسی‌های انجام‌شده در محدوده دهنه نشان می دهد که فرایند دیاژنز و دگرگونی تدفینی در توالی آتشفشانی میزبان کانه زایی، سبب ایجاد سیال های نسبتاً داغ شده است که با چرخش این سیالات، مس و نقره از سنگ های آتشفشانی و آتشفشانی‌رسوبی ائوسن شسته‌شده و تحت‌ شرایط اکسیدی در واحد های بازالتی ته‌ نشست شده‌ اند. در ادامه و در اثر فرایندهای سوپرژن و هوازدگی، کانی های معدنی اولیه (مس طبیعی) به کانی های ثانویه از جمله کوپریت، مالاکیت و آزوریت تبدیل شده‌اند. با توجه به نوع سنگ میزبان، کانی‌ شناسی و دگرسانی می توان اظهار داشت که کانه‌ زایی مس در کانسار دهنه شباهت زیادی با کانسارهای مس مرتبط با سنگ های بازالتی (نوع میشیگان) دارد.

Geology and Origin of the Dohneh Copper Mineralization, Northeast of Zanjan

Introduction;The Dohneh copper deposit is located northeast of Zanjan within the Tarom subzone and the western AlborzAzarbayjan magmatic belt. There are several reports of copper mineralization in the Tarom region such as the Aliabad MousaviKhanchay Cu deposit (Saeedi, 2015),the Lolan CuAu deposit (Zamanian et al., 2016), the Mari Cu deposit (Hosseinzadeh et al., 2016) and the Gheshlagh Cu deposit (Abbaspour, 2017) which make this subzone an important metallogenic zone in Iran. Prior to the present research, there was no detailed study done on the Dohneh Cu deposit. Thus, the aim of this research is to present detailed characteristics of geology, mineralogy and geochemistry of the host rock as well as the origin of Cu mineralization in the Dohneh area. The results of this study can be considered as an exploratory pattern in the TaromHashtjin metallogenic belt in terms of time and space.; ;Materials and methods ;Petrographic and mineralogical features of the Dohneh Cu mineralization and host volcanic rocks were determined by studying 52 thin and thinpolished sections. Major and trace element compositions of six volcanic samples (the host of mineralization) were determined by the Iran Mineral Processing Research Center and Zarazma Company using XRF and ICPMS methods, respectively. Furthermore, four samples were selected for electron microprobe and scanning electron microscope (SEM) analysis in the Iran Mineral Processing Research Center, Karaj.; ;Results and Discussion;The main rock units in the Dohneh area include Eocene tuffs and basalts which are related to the Kordkand member of the Karaj Formation. The tuff unit displays basic composition and is comprised of plagioclase, pyroxene, minor olivine and opaque minerals. There are also some basaltic fragments within the tuff unit. The Dohneh volcanic lavas can be divided into two lava flows varying in texture and mineralogy. The amygdaloidal basalt shows porphyritic and amygdaloidal textures and contains plagioclase, clinopyroxene, orthopyroxene, olivine and opaque minerals. The secondary minerals are carbonate, serpentine, epidote, chlorite, zeolite (filling the cavities), sericite and iron oxide. The second lava flow in the Dohneh area is the porphyritic basalt which shows specified porphyry texture which consists of pyroxene phenocrysts enveloped within the finegrained matrix including plagioclase, olivine and pyroxene. The geochemical features of the Dohneh basaltic lavas show calcalkaline nature with enrichment in LILE and LREE and depletion in HFSE and HREE. The Dohneh samples show negative anomaly of Nb, Ta and Ti in primitive mantle normalized spider diagrams. This geochemical evidence together with trace element data suggest that the Dohneh lavas have formed through partial melting of metasomatized lithospheric mantle. The status of the Dohneh samples in the tectonic discrimination diagrams shows subduction related magmatism analogous to those reported from the Tarom and Qazvin areas (Nabatian et al. 2014; Asiabanha and Foden, 2012).;The Cu mineralization occurred in both amygdaloidal and porphyritic basalt lavas. According to e mineralography studies, the Dohneh deposit includes native copper, native silver, cuprite, malachite and azurite minerals. The minerals occur in the forms of veinveinlet, open space filling, replacement and residual. The major alteration minerals in the Dohneh deposit include carbonate, chlorite, zeolite, and serpentinite and minor epidote, which have formed as replacement, veinveinlet and open space filling in the host rocks.;The field and microscopic observations, whole rock chemistry and mineral chemistry data from the Dohneh deposit suggest that the mineralization fluids and hot saline aqueous fluids have been generated during the late diagenesis and burial metamorphism in the volcanic sequence. During the ascending of fluids through the fractures and faults, the copper metal leached out of silicate minerals and turned into an elemental Cu2+ which is soluble in the fluid. Then, through the injection of mineralized fluids into the fractures and empty spaces of host rocks which was associated with decreasing pressure, copper and native silver minerals associated with zeolite formed at the end of the diagenetic stage, and in particular in the burial metamorphic phase. Moreover, during the circulation of fluids in the host rock, secondary minerals have formed. Consequently, mineralization of zeolite and part of copper mineralization occurred during burial metamorphism. In the final stages of mineralization and during the supergene and meteoric waters affected the minerals and caused formation of secondary minerals. In the final stages of mineralization and during the supergene and weathering activities secondary minerals have been generated. According to this study and comparing the characteristics of the Dohneh deposit to Michigan copper type deposits, it can be stated that the characteristics of the Dohneh copper deposit is the most similar to those of the Michigan copper deposits.; ;References; Abbaspour, N., 2017. Petrography and geochemistry of Gheshlagh Cu deposit and its host rocks, north of Zanjan. M.Sc. thesis, Zanjan University, Zanjan, Iran, 76 pp.; Asiabanha, A. and Foden, J., 2012. Postcollisional transition from an extensional volcanosedimentary Basin to a continental arc in the Alborz Ranges, NIran. Lithos, 148(1): 98–111.;Hosseinzadeh, M., Maghfouri, S., Maiid, M. and Farid Asl., V., 2016. Introduction of copper Mari copper deposit as a tropical cantaloupe in the Tarom zone, Northwest of Iran. Iranian Journal of Geology, 10(38): 17‌–‌37. (in Persian with English abstract) ;Nabatian, Gh., Ghaderi, M., Neubauer, F., Honarmand, M., Lui, X., Dong, Y., Jiang, SY and Bernroider, M., 2014. Petrogenesis of Tarom highpotassic granitoids in the AlborzAzarbaijan belt, Iran: Geochemical, UPb zircon and SrNdPb isotopic constraints. Lithos, 184–187(1): 324–345.;Saeedi, A., 2015. Investigation of copper mineralization in the Aliabad MousaviKhanchay ore occurrence, east of Zanjan, using by lithological, mineralogy and chemical data. M.Sc. thesis, University of Zanjan, Zanjan, Iran, 180 pp.; Zamanian, H., Rahmani, Sh., JanNessari, M., Zarei Sahamyeh, R. and Borna, B., 2016. Oregenests study of The CuAu veintype deposit in The Taron Granitoid (North Zanjan) based on mineralogical, geochemical and fluid inclusion evidences. Scientific Quarterly Journal, Geosciences, 25(98): 255–‌282.