مطالعه كاني شناسي و فرآيندهاي اسكارني شدن در اسكارن مس آهن آوان، شمال‌خاور خاروانا، شمال باختر ايران

Mineralogy and skarnification processes at the Avan CuFe Skarn, northeast of Kharvana, NW Iran

اسكارن مس آهن آوان در نتيجه نفوذ باتوليت گرانيتوئيدي قره داغ به‌ داخل سنگهاي كربناته ناخالص كرتاسه بالايي تشكيل شده است. اسكارن آوان متشكل از پهنه هاي مرمر، اگزواسكارن و اندواسكارن بوده و بخش اصلي آن پهنه اگزواسكارن، است. بررسيهاي كاني شناسي بيانگر آن است كه پهنه اگزواسكارن شامل زيرپهنه هاي گارنت اسكارن، پيروكسن گارنت اسكارن و كانه اسكارن است. مهمترين كاني كالك سيليكاته بي آب در پهنه اگزواسكارن، گارنت و تركيب آن اوگرانديتي (Ad5389) است. برخي بلورهاي گارنت، منطقه‌بندي دارد و تركيب آنها به طرف حاشيه‌ها به آندراديت خالص (Ad99) ميل مي‌كند. كلينوپيروكسن موجود در زيرپهنه پيروكسن گارنت اسكارن از نوع ديوپسيد (Di7596) است. زيرپهنه كانه اسكارن از كانه‌هاي مگنتيت، هماتيت، پيريت، كالكوپيريت، بورنيت، مالاكيت و گوتيت تشكيل شده است. وجود ميزان محدود ولاستونيت در مجموعه كاني شناسي اسكارن آوان، هم‌رشدي بلورهاي گارنت و كلينوپيروكسن و نبود هاله واكنشي بين اين دو كاني و نبود بافتهاي جانشيني نشان‌دهنده آن است كه اين كانيها به‌صورت هم‌زمان در محدوده دمايي C°600 430 و فوگاسيته اكسيژن بيشتر از 1026 تشكيل شده‌اند.

Introduction The Avan CuFe skarn is located at the southern margin of Qaradagh batholith, about 60 km north of Tabriz. The Skarntype metasomatic alteration is the result of Qaradagh batholith intrusion into the Upper Cretaceous impure carbonates. The studied area belongs to the Central Iranian structural zone. In regional scale, the studied area is a part of the Zangezour mineralization zone in the Lesser Caucasus. Several studies (Karimzadeh Somarin and Moayed, 2002; Calagari and Hosseinzadeh, 2005; Mokhtari, 2008; Baghban Asgharinezhad, 2012; Mokhtari, 2012) including master’s theses and research programs have been done on some skarns in the Azarbaijan area considering their petrologic and mineralization aspects. However, before this study, the Avan skarn aureole has not been studied in detail. In this paper, various geological aspects of the Avan skarn including mineralogy, bimetasomatic alteration, metasomatism and mineralization during the progressive and retrograde stages of the skarnification processes have been studied in detail. Research Method This research consists of field and laboratory studies. Field studies include preparation of the geological map, identifying the relationship between the intrusion and the skarn aureole, identifying the relationship between different parts of the skarn zone and also collecting samples for laboratory studies. Laboratory studies include petrography, mineralography and microprobe studies. Cameca SX100 Microprobe belonging to Geological Survey of the Czech Republic was used in order to determine the chemical composition of the calcsilicate minerals such as pyroxene and garnet in garnet skarn and pyroxene garnet skarn subzones. Discussion and conclusion Qaradagh batholith is composed of discrete acid to mafic phases including gabbro, diorite, quartz diorite, quartz monzonite, quartz monzodiorite, tonalite, granodiorite, monzogranite and granite porphyry which is dominated by granodioritequartz monzonite. Granitoids of this batholith are metaluminus, high K calcalkaline Itype granite (Mokhtari, 2008). The Avan CuFe skarn is related to the intrusion of granodioriticquartz monzonitic part of the Qaradagh batholith into the Upper Cretaceous flysch type rocks consisting of biomicrite, clay limestone, marl, siltstone and mudstone. The Avan skarn consists of three zones of endoskarn, exoskarn and marble. The main CuFe mineralized zone is related to the exoskarn zone, which has 600 meters of length and 50 meters of thickness, respectively. The Exoskarn zone consists of garnet skarn, pyroxenegarnet skarn and ore skarn subzones. Garnet, belonging to ugrandite series (Ad5389) with more than 50 percentage in volume, is the most important anhydrous calcsilicate mineral in the garnet skarn and the pyroxenegarnet skarn subzones. Some of the garnet crystals are zoned and their chemical composition changes toward the rim to almost pure andradite (Ad99). Clinopyroxene which has diopsidic composition (Di7596), is another anhydrous calcsilicate mineral in the exoskarn zone with an abundance that reaches up to 50 percent in volume in pyroxenegarnet skarn subzone. The ore skarn subzone is located toward the outer part of the exoskarn zone and close to the border of the marble zone. The abundance of ore minerals in this subzone reaches up to 50 percentage in volume and includes magnetite, hematite, pyrite, chalcopyrite, bornite, malachite and goethite among which pyrite is the most abundant. In this subzone, anhydrous calcsilicate minerals of garnet and clinopyroxene have undergone intensive alteration and are replaced with hydrous calcsilicate (epidote and tremolite actinolite), oxide (magnetite and hematite) and sulfide (pyrite, chalcopyrite and bornite) minerals. Based on the textural and mineralogical studies, the skarnification processes in the studied area can be categorized into two main stages: 1) prograde and 2) retrograde. During the prograde stage, the heat flow of the granitoid has caused isochemical metamorphism and changing more pure limestones to marble and marlly limestones to skarnoid (metamorphism and bimetasomatism). The high temperature magmatic fluids have caused prograde metamorphism during which anhydrous calcsilicate minerals including garnet and pyroxene have appeared. During the early retrograde stage, i.e. the mineralization substage, lower temperature hydrothermal fluids have caused hydrolysis and carbonization because of which anhydrous calcsilicate minerals along with their fractures and microfractures are changed to hydrous calcsilicate (epidote and tremoliteactinolite), oxide (magnetite and hematite), sulfide (pyrite, chalcopyrite and bornite) and carbonate (calcite) minerals. During the late retrograde stage, relatively low temperature fluids have altered anhydrous and hydrous calcsilicate mineral assemblage formed during the previous stages into a very fine grained mineral assemblage including clay minerals, chlorite and iron hydroxides. Presence of replacement textures in ore minerals and anhydrous calcsilicate minerals accompanied with open filling textures in the anhydrous calcsilicate minerals, for example oxide and sulphide veinlets within the garnet crystals, indicate that the mentioned ore minerals have been simultaneously generated with hydrous calcsilicate minerals (epidote and tremoliteactinolite) during the early prograde stage. The presence of minor amounts of wollastonite among the mineral assemblage of the Avan skarn, intergrowth of garnet and pyroxene, absence of reaction rim between garnet and clinopyroxene and absence of replacement textures indicate that these minerals have been simultaneously generated within the temperature ranges of 430–600 ºC and ƒO2 > 1026, respectively. Acknowledgements The authors are grateful to the Journal of Economic Geology reviewers and editors for their constructive suggestions to the manuscript. Reference Baghban Asgharinezhad, S., 2012. Investigation of genesis, mineralogy and geochemistry of FeCu skarn in Astamal area, NE Kharvana, Eastern Azarbaijan. MSc. Thesis, University of Tabriz, Tabriz, Iran, 185 pp. (in Persian with English abstract) Calagari, A.A. and Hosseinzadeh, G., 2005. The mineralogy of copperbearing skarn to the east of the SungunChay River, EastAzarbaijan, Iran. Journal of Asian Earth Sciences, 28(46): 423438. Karimzadeh Somarin, A. and Moayed, M., 2002. Granite and gabbrodiorite associated skarn deposits of NW Iran. Ore geology reviews, 20(34): 127138. Mokhtari, M.A.A., 2008. Petrology, geochemistry and petrogenesis of Qaradagh batholith (east of Syahrood, Eastern Azarbaijan) and related skarn with considering mineralization. Ph.D. Thesis, Tarbiat Modares University, Tehran, Iran, 347 pp. (in Persian with English abstract) Mokhtari, M.A.A., 2012. The mineralogy and petrology of the Pahnavar Fe skarn, in the Eastern Azarbaijan, NW Iran. Central European Journal of Geosciences, 4(4): 578591.