سنگ‌نگاری، سنگ‌شناسی و زمین‌شیمی توده‌های نیمه‌آتشفشانی در کانسار مس- مولیبدن پورفیری دوگان (کمربند ترود- چاه‌شیرین)

محدوده معدنی دوگان در حاشیه شمالی دشت کویر مرکزی ایران و در شمال‌باختری روستای ترود از توابع شهرستان شاهرود جای دارد. واحدهای سنگیِ منطقه شامل سه توده مجزای نیمه‌آتشفشانی و به‌ترتیب سنی از قدیم به جدید شامل کوارتزمونزونیت تا مونزودیوریت پورفیری، دیوریت تا میکرودیوریت پورفیری، کوارتزدیوریت تا گرانودیوریت پورفیری و سنگ‌های درونگیر آتشفشانی و آذرآواری ائوسن هستند. بیشترین گسترش این توده­ها در بخش­های شمال­خاوری، مرکز و جنوب‌خاوری محدوده است و توده کوارتزمونزونیت تا مونزودیوریت پورفیری نسبت به دیگر توده­ها گسترش بیشتری دارد. میزان sio2 این توده‌ها برابر با 52.15 تا 66.85 درصدوزنی است. بر پایه شواهد زمین‌شیمیایی، این توده­ها سرشت متاآلومین تا پرآلومین و کالک‌آلکالن تا کالک‌آلکالن با پتاسیم بالا دارند. در نمودار بهنجارشده به ترکیب گوشته اولیه و کندریت کمابیش همه نمونه‌ها، از عنصرهای خاکی کمیاب سبک (lree) نسبت به عنصرهای خاکی کمیاب سنگین (hree) غنی‌شدگی نشان می‌دهند. در این نمونه­ها مقدار (la/ybn) برابر با 1.83 تا 20.02 است. غنی­شدگی از عنصرهای لیتوفیل با یون بزرگ (lile) مانند rb، cs و k در همه نمونه­های نیمه‌آتشفشانی منطقه، ویژگی‌های زمین‌شیمیایی یک محیط ژئودینامیکی مرتبط با فرورانش را نشان می‌دهد. با توجه به نمودارy در برابر sr/y و و ybn در برابر (la/yb)n، بیشتر نمونه‌های نیمه‌آتشفشانی منطقه شاخص شبه‌آداکیتی دارند. نسبت sr/y در توده کوارتزمونزونیت تا مونزودیوریت پورفیری برابر با 14.13-50.28، در توده دیوریت تا میکرودیوریت پورفیری برابر با 47.77 -74.49 و در توده کوارتزدیوریت تا گرانودیوریت پورفیری برابر با 58.51- 137.03 است.

petrography, petrology, and geochemistry of subvolcanic bodies in dogan porphyry copper-molybdenum deposit (toroud-chah shirin belt)

introductiondogan mining area is located on the northern edge of iran’s central desert plain and northwest of toroud village. structurally, the area is a part of central alborz, eastern alborz, and central iran zones, which gave rise to the formation of deposits and a diverse metallogenic environment. geologically, this area is a part of toroud‐chah shirin (tcs) belt, a tertiary base metal and gold-silver mineral region in northern iran. the tcs belt consists mainly of eocene volcanic and pyroclastic rocks, equivalent to subvolcanic and intrusive bodies, although there are scattered outcrops of metamorphosed paleozoic and mesozoic rocks. structural patterns are controlled by two principal strike-slip faults, anjilow in the north and toroud in the south, both trending ne. so far, no detail study has been carried out on the study area in terms of separation and investigation of intrusive bodies, and only one subvolcanic microdiorite body has been introduced in the previous studies in this area. while, the overall data obtained from detailed field surveys, drilling core as well as petrographic studies indicate that at least three subvolcanic bodies have been identified injected in the area in a telescopic form, and all three are the carriers of copper mineralization as disseminated and vein-veined forms. therefore, the petrographic and geochemical characteristics of, these bodies are the purpose of the present study.regional geologythe main volume of rock units in the study area, based on 1:1000 geological map of dogan area, includes andesitic lavas, pyroclastic tuffs, and various subvolcanic bodies. the subvolcanic bodies are relatively diverse in composition, and the greatest areal extent of these bodies is in the northeastern, central, and southeastern parts of the region. the nature of these subvolcanic bodies are as follows:porphyritic quartz monzonite to quartz monzodiorite (qmz): a large part of the central part of the study area is surrounded by porphyritic body, which is older than the other bodies and is injected into eocene units.porphyritic diorite to microdiorite (dr): this unit lies in the central part of the area with a less extensive distribution than that of the porphyritic quartz monzonite body. this body in the central part cuts the porphyritic quartz monzonite body and is, therefore, younger in age, but its cutting by the dykes of the porphyritic quartz diorite body indicating it is older than this body.porphyric quartz diorite to granodiorite (qdr): as the youngest body as well as less extensive area than that of the existing bodies, is exposed in the central part of the area.analytical methods in this study, after field investigations and sampling of drill cores, about 200 thin sections were prepared from the collected samples for petrographic studies and about 60 polished sections for mineralogy studies. after microscopic studies and separation of intrusive units, 15 samples with minimal alteration were analyzed in the laboratory of zar azma company to measure the main oxides and determine the abundance of trace and rare earth elements by xrf and icp-ms methods.petrographyporphyritic quartz monzonite to quartz monzodiorite (qmz): microscopically, this porphyry body ranges from quartz monzonite to quartz monzodiorite, exhibiting a porphyritic texture with a fine-grained matrix. main minerals include plagioclase, k-feldspar, and quartz, with accessory amphibole, biotite, apatite, zircon, and opaque minerals. plagioclase and k-feldspar phenocrysts are pseudomorphed by sericite and carbonate, while ferromagnesian minerals (amphibole, biotite) are altered to chlorite and magnetite. secondary minerals including sericite, chlorite, carbonate, and iron oxides result from alteration.porphyritic diorite to microdiorite (dr): these rocks dominated by the presence of plagioclase and amphibole and small volume of quartz, apatite, zircon, and opaque minerals. they display a microlithic porphyritic texture, with subhedral plagioclase phenocrysts (showing mixed zoning) in a microlithic matrix of plagioclase and quartz. plagioclase is altered to sericite and clay minerals, while amphibole phenocrysts are pseudomorphed by chlorite and iron oxides. secondary biotite, actinolite, and ore accumulations occur, alongside potassic alteration.porphyritic quartz diorite to granodiorite (qdr): the essential minerals of this body’ are plagioclase, k-feldspar, and quartz, with accessory zircon and apatite as well as porphyritic texture with a fine-grained matrix. plagioclase exhibits mixed zoning; amphibole is partly altered to chlorite. biotite appears as primary (magmatic, altered to chlorite and iron oxides) and thesecondary (hydrothermal, flaky, brown). chloritization and iron oxide precipitation are common.discussionas the sio2 versus zr/tio2 diagram display the subvolcanic bodies in the dogan mining area are dominated by granodiorite, tonalite, and diorite/. these i-type granitoids are calc-alkaline to high-k calc-alkaline and metaluminous to peraluminous nature formed in a continental arc setting. geochemical data show depletion in sm, nd, ti, and y and enrichment in liles (ba, rb, cs, k), consistent with subduction-related magmatism. the ree patterns display lree enrichment over hree, further supporting a subduction origin. low th/nb and ba/th ratios point out to subduction zone fluids/melts and crustal contamination. the elevated y/rb ratios may reflect subduction enrichment or crustal input.the parental magma likely originated from a metasomatized mantle wedge, influenced by fluids from subducting oceanic lithosphere, and underwent fractionation and crustal contamination during ascent. on y versus sr/y and ybn versus (la/yb)n diagrams, the rocks show adakite-like signatures. given the association of high sr/y magmas with porphyry systems, these subvolcanic rocks may be linked to porphyry mineralization.acknowledgmentsthis article is part of the studies carried out for the first author’s ph.d. thesis, which is being conducted at the university of tabriz, and part of its costs were covered by the university of tabriz and the shahid arefi complex. therefore, i would like to express my deepest gratitude and appreciation to the respected management of the graduate studies department of the university of tabriz and the management of the shahid arefi complex.