سنگ‌نگاری، زمین‌شیمی، کانی‌شناسی و ارزیابی نوع کانه‌زایی مس (نقره) در کانسار رحیم‌آباد، جنوب‌باختری اردستان

کانسار مس (نقره) رحیم‌آباد در 21 کیلومتری جنوب‌باختری اردستان و 80 کیلومتری شمال‌خاوری اصفهان، در بخش میانی کمربند ماگمایی ارومیه- دختر جای دارد. سنگ‌های آتشفشانی- رسوبی ائوسن و تودة آذرین درونی دوروجین به سن میوسن از مهم‌ترین سنگ‌های رخنمون‌یافته در این ناحیه به‌شمار می‌روند. کانه‌زایی مس (نقره) در این منطقه در گدازه‌های آندزیتی، آندزیت بازالتی و بازالتی تیره‌رنگ گسترش یافته است. این سنگ‌ها سرشت کالک‌آلکالن دارند و از نوع متاآلومین هستند و ویژگی‌های کمان آتشفشانی وابسته به پهنة فرورانش را نشان می‌دهند. دگرسانی‌های اصلی منطقه شامل سیلیسی، پروپیلیتیک، آرژیلیک، سریسیتی و زئولیتی هستند. شکل هندسی مادة معدنی در این کانسار چینه‌کران و ساخت و بافت مادة معدنی به‌صورت پرکنندة فضای خالی، دانه‌پراکنده، رگه – رگچه‌ای و جانشینی است. بررسی کانی‌ها با روش‌های میکروسکوپی و تجزیة ریزکاوالکترونی نشان می‌دهد مهم‌ترین کانه‌های مس و نقره شامل کالکوپیریت، بورنیت، کانی‌های گروه کالکوسیت- کولیت، مالاکیت، آزوریت، آکانتیت و جالپایت هستند که با مگنتیت و هماتیت همراهی می‌شوند. پیریت بیشتر به‌صورت کانی جداگانه در سنگ میزبان دیده می‌شود. مقایسة ویژگی‌های کانه‌زایی مس رحیم‌آباد از دیدگاه پهنة زمین‌ساختی، نوع سنگ میزبان، هندسه، بافت و ساخت، کانی‌شناسی و کانی‌های همایندِ مس و دگرسانی، با کانسارهای گوناگون مس نشان می‌دهد کانسار مس رحیم‌آباد، بیشترین شباهت و همخوانی را با کانسارهای مسِ مانتو دارد.

petrography, geochemistry, mineralogyand type of cu (ag) mineralization study of rahimabad ore deposit, southwest of ardestan

the study area is situated 21 km sw of ardestan city and 80 km ne of esfahan (central iran). and according to the sedimentary structural divisions (fig. 1; aghanabati, 1998) in the central part of urumieh-dokhtar magmatic belt (udmb) the udmb in the alpine-himalayan orogenic belt, the most productive metallic belt of iran, composed of basic to acidic volcanic and plutonic rocks, tuff and agglomerate. the udmb represents geochemical characteristics of subduction zones with features of calc-alkaline locally toward alkaline (berberian and berberian, 1981; alavi, 1994; shahabpour, 2007; omrani et al., 2008; ghorbani and bezenjani, 2011; yeganehfar, 2013; rajabpour et al., 2017). the udmb hosts several porphyry cu±mo±au deposits including sungun, sarcheshmeh, kahang, darehzar, nowchun and meiduk (atapour and aftabi, 2007; zarasvandi et al., 2015; zamanian et al., 2016; alirezaei et al., 2017; jamali, 2017) and associated porphyry copper-gold, gold epithermal and manganese-iron deposits (rajabpour et al., 2017; ostadhosseini et al., 2018; alaminia et al., 2020; ostadhosseini et al., 2021). different stages of cenozoic magmatic activity in the middle segment of the udmb around the study area consist of different successions of volcanic and intrusive rocks (radfar, 1998). the eocene to miocene diorite- monzodiorite bodies were intruded the eocene volcanic and subvolcanic rocks. in the middle of the area, these intrusive units are juxtaposed with a fault boundary (marbin fault) adjacent to eocene volcanic units.  the eocene volcanic stage is dominated by basalt, andesitic basalt, andesite, tuffs and ignimbrites rocks. quaternary sediments are widespread in the northeastern and southern parts of the area. the oldest rock unit of this area is the shotori dolomite formation trending nw-sw and belonging to triassic age and located in the southwest of the study area. cu mineralization occurs within the eocene volcano-sedimentary sequence. the purpose of this study is to determine the type of cu mineralization based on the mineralization characteristics, geometry, texture, structure and alteration studies, as well as the geochemistry and tectonic environment of the host volcanic rock.materials and methodsfor the purposes of this study, 60 thin sections of volcanic rocks and 30 polished thin sections of ore samples were studied by a standard petrographic microscope under reflected and transmitted lights. 10 surface and drill–holes samples from volcanic rocks were crushed and powdered in tungsten carbide swing mill for whole-rock analysis. the chemical analyses were performed for the major elements using x-ray fluorescence (xrf) and trace elements using inductively coupled plasma-mass spectrometry (icp-ms). electron microprobe analyses were performed, using a jxa-8100 electron microprobe. operating conditions were 35 kv accelerating voltage, a beam current of 20 na, and a beam diameter of 2-10 μm.all the petrographic studies as well as chemical analyses including xrf, icp-ms and epma were carried out at the seoul national university laboratories, seoul, south korean.results and discussionthe dominant rocks of the area under study are basalt, basaltic andesite, andesite and a small volume of pyroclastic rocks which are metaluminous composition and calc-alkaline affinity. geochemically, they are enriched in lree relative to hree, enrichment of lile and negative anomalies of hfse (i.e., nb, ti), pointing to characteristics of subduction-related magmatic possibly generated by partial melting of metasomatized lithospheric mantle source. as the discrimination diagrams of tectonic setting display, the volcanic rocks are also classified as a subduction-related magmatic arc. alteration zones were developed in the rock types including silicic, propylitic, argillic, sericitic and zeolitic. the propylitic and silicic alterations were extended within the mineralized zones. the propylitic alteration is the dominant alteration consisting mainly epidote, chlorite, and calcite. the silicification zone consists of crystalline quartz-formation, which occurs as veins and veinlets including some copper minerals. the carbonate alteration is observed in basaltic andesite and andesite rocks. copper mineralization is mainly strata-bound, and occurs partially as veins, veinlets and disseminated in the andesite, basaltic andesites, and basaltic rocks. based on microscopic studies, three mineralization stages were recognized in the rahimabad deposit including pre-mineralization, mineralization, and post-mineralization stages. in the pre-mineralization stage, pyrite is formed in decreasing conditions in the host rock. in the main mineralization stage, pyrite is replaced by primary cu and ag sulfide minerals such as chalcopyrite, bornite, chalcocite, digenite, jalpaite and acanthite. finally, in the post mineralization stage, copper sulfide minerals are replaced by secondary copper sulfide minerals (chalcocite, covellite and digenite) and oxide minerals (malachite, azurite, goethite and hematite).conclusionsthe rahimabad cu (ag) deposit lies in the sw of the ardestan city in the urumieh-dokhtar magmatic belts, central iran. in this area, cu and ag mineralization is observed in the volcano-sedimentary rocks. the copper (ag) mineralization occurs in andesite, basaltic andesite and basaltic lavas. these rocks are meta-aluminous and have a calc-alkaline affinity and indicate a subduction-related magmatic arc. the main alterations zones are silicic, propylitic, argillic, sericitic and zeolitic. the geometry of mineralization is strata-bound and the texture and structure of mineralization is open space filling, disseminated, vein-veinlet and replacement. based on microscopic as well as epma data, the most important cu and ag minerals include chalcopyrite, bornite, chalcocite- covellite group minerals, malachite, azurite, acanthite and jalpaite, which are accompanied by magnetite and hematite. pyrite is mostly observed as a separate mineral in the host rock. the overall mineralization characteristics and tectonic setting, the type of host rock, geometry, texture and structure, mineralogy and the paragenetic of cu minerals and finally the alteration zones with different types of copper deposits document that the rahimabad copper deposit share many features with those of manto type copper deposits.acknowledgmentthe present study was financially supported by the cu-au company of ardestan. the authors are grateful to mr. sharif for providing sampling facilities and let us access to drill cores and exploration data.