پترولوژی و زمین شیمی توده گرانیتوئیدی و کانسار آهن اسکارنی قوزلو (باختر زنجان)

کانسار آهن قوزلو در فاصله 65 کیلومتری باختر زنجان واقع‌شده و بخشی از کمان ماگمایی ارومیهدختر در پهنه ایران مرکزی است. در این منطقه، تناوب لایه های سنگ‌آهک میکرواسپارایتی، آهک مارنی، شیل و ماسه سنگ مربوط به کرتاسه بالایی توسط توده گرانیتوئیدی ائوسن بالایی مورد هجوم قرار‌گرفته و هاله دگرگونی مجاورتی و کانه زایی آهن تشکیل‌شده است. از نظر سنگ شناسی، توده گرانیتوئیدی متشکل از گرانیت‌گرانودیوریت پورفیری و کوارتزمونزودیوریت بوده و دارای ماهیت کالک آلکالن پتاسیم بالا و متعلق به گرانیتوئیدهای متاآلومینوس نوع i است. در نمودارهای تمایز محیط زمین‌ساختی، این توده در محیط حاشیه فعال قاره ای قرار می گیرد. بر اساس بررسی‌های میکروسکوپی، هاله دگرگونی مجاورتی متشکل از زیرپهنه های گارنت اسکارن، گارنتپیروکسن اسکارن، پیروکسن اسکارن، اپیدوت اسکارن، مرمر پیروکسن دار و اسکارن کانه دار است. مگنتیت کانی اصلی کانسار است که با کانی های فرعی پیریت، کالکوپیریت و پیروتیت همراهی می شود. گارنت، کلینوپیروکسن، اپیدوت، اکتینولیت، کلسیت و کوارتز به‌عنوان کانی های غیر‌ فلزی حضور دارند. شواهد بافتی در سنگ های هاله دگرگونی مجاورتی نشان‌دهنده تشکیل هم زمان گارنت و کلینوپیروکسن در محدوده دمایی 430 تا 550 درجه سانتی گراد و10^-26 10^-23=ƒo2است.

Petrology and Geochemistry of the Qozlou Granitoid and Related Fe skarn (west Zanjan)

Introduction;Fe skarn deposits are one of the important Fe deposits in the Zanjan province which have been exploited in recent years. The Qozlou Fe deposit is one of these Fe skarn deposits which is located at 65 km west of Zanjan. In this area, alternation of microsparitic limestone, marly limestone, shale and sandstones of Upper Cretaceous were intruded by Late Eocene granitoids. This event caused to metamorphism contact and it caused the formation of Fe mineralization. Some of the Fe skarn deposits in the Zanjan province were studied during the past few years (e.g. Nabatian et al., 2017) and valuable information is present about their geological and mineralization characteristics. However, Qozlou granitoid and Fe deposit have not been studied yet. In this research, petrology and geochemistry of the Qozlou granitoid along with petrographic characteristics, mineralogy, structure and texture of Fe deposit and thermodynamic conditions for formation of contact metamorphic rocks have been studied.; ;Materials and methods;This research study can be divided into two parts including field and laboratory studies. Field studies include;The recognition of different parts of granitoid intrusion and skarn aureole along with sampling for laboratory studies. Thus, 50 samples were selected for petrographic and analytical studies. 16 thin sections and 16 thinpolish sections were used for petrographical and mineralogical studies. 13 samples from granitoid and ore skarn subzone were analyzed by XRF and ICPMS methods at the Zarazma laboratory, Tehran for geochemical studies. ; ;Results;Based on petrographic studies, the Qozlou granitoid is composed of porphyritic granitegranodiorite and quartz monzodiorite. Porphyritic granitegranodiorite have porphyritic to porphyroidic, micrographic and felsophyric textures and are composed of plagioclase, quartz, Kfeldspar, hornblende and biotite phenocrysts within quartzfeldspatic groundmass. Quartz monzodiorites indicate porphyroidic texture and they are composed of plagioclase, hornblende, quartz and Kfeldspar. The Qozlou granitoid demonstartes highK calcalkaline affinity and it is classified as metaluminous Itype granitoids. Trace elements normalized by primitive mantle (Sun and McDonough, 1989) for Qozlou granitoid indicate LILE and LREE enrichment along with negative HFSE anomalies and distinctive positive Pb anomaly. Chondritenormalized (Nakamura, 1974) REE patterns for the Qozlou granitoid demonstrate LREE enrichment (high LREE/HREE ratio). Based on tectonic setting discrimination diagrams, the Qozlou granitoid were formed in active continental margin.;Microscopic studies reveal that the skarn zone in Qozlou is composed of garnet skarn, garnetpyroxene skarn, pyroxene skarn, epidote skarn, and pyroxenebearing marble subzones. The Ore zone is present as massive and lensshaped with 300m length and up to 30m width. Magnetite is the main ore mineral along with some pyrite, chalcopyrite and pyrrhotite. Garnet, clinopyroxene, epidote, actinolite, calcite and quartz present in skarn zone. Based on field and microscopic studies, the Qozlou Fe deposit indicates massive, banded, disseminated, brecciated, veinveinlets, replacement and relict textures. Based on mineralogical and textural studies, skarnization processes in the Qozlou deposit can be divided into 3 stages including: (1) isocheimal metamorphic stage, (2) prograde metasomatic stage and (3) retrograde metasomatic stage. Chondritenormalized (Sun and McDonough, 1989) REE and trace element patterns for different skarn samples and porphyritic granite demonstrate similar patterns.; ;Discussion;Since all of minerals present in the Qozlou skarn aureole are located in CaFeSiCOH system, we used the temperature vs. logƒO2 diagram (Einaudi, 1982) to determine possible physicochemical conditions for skarn formation in the Qozlou. Based on this diagram and considering mineralogical and textural evidence, garnet and clinopyroxene were formed simultaneously in 430550°C and ƒO2 equal 1023 to 1026. In the temperature less than 430°C and increasing ƒO2, garnet and clinopyroxene replaced by epidote, actinolite, quartz and calcite, respectively. Furthermore, in temperature of less than 430°C, fluids in equilibrium with granitic intrusion and with relatively high sulfidation (ƒS2 gt;106), were not in equilibrium with andradite. Therefore, andradite was replaced by quartz, calcite and pyrite. With reducing ƒS2 ( lt;106), andradite was replaced by quartz, calcite and magnetite. During the early retrograde stage, magnetite and pyrite were formed along with quartz and calcite. Mineralogical studies indicate that pyrite was formed after magnetite. Based on this, it seems that metasomatic fluids probably had ƒS2≈106.5 and had less than 430°C temperature in the beginning of the retrograde stage. Presence of hematite lamellae within the magnetite demonstrates that ƒO2 probably was 1022 in the beginning of retrograde stage.; ;Acknowledgment;This research was made by the grant of the office of vicechancellor for research and technology, the University of Zanjan. We acknowledge their support. The reviewers and editors of the Journal of Economic G are also thanked for their constructive comments. For geochemical studies. For geochemical studies, For geochemical studies, For geochemical studies.; ;References;Einaudi, M.T., 1982b. General features and origin of skarns associated with porphyry copper plutons. In: S.R., Titley (Editor), Advances in geology of the porphyry copper deposits, southwestern North America. University of Arizona Press, Tucson, pp. 185–209.;Nabatian, Gh., Li, X.H., Honarmand, M. and Melgarejo, J.C., 2017 . Geology, mineralogy and evolution of iron skarn deposits in the Zanjan district, NW Iran: Constraints from UPb dating, Hf and O isotope analyses of zircons and stable isotope geochemistry. Ore Geology Reviews, 84(8):42–66.;Nakamura, N., 1974. Determination of REE, Ba, Mg, Na and K in carbonaceous and ordinary Chondrites. Geochemical et Cosmochemica Acta, 38(5): 75–775.;Sun, S.S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: Implication for mantle composition and processes. In: A.D. Saunders and M.J. Norry (Editors), Magmatism in the Ocean Basins, Geological Society of London Publications, Special Publication 42, London, pp. 313–345.