دانشگاه فردوسی مشهد زمین شناسی اقتصادی 2008-7306 18 2 2026 08 23 Mineral Chemistry and Crystal Size Distribution of Plagioclases in the Monzonitic Intrusive Body of the Seridune Porphyry Copper Deposit, Iran: Emphasis on Processes Controlling Magma Evolution شیمی کانی و توزیع اندازه بلور پلاژیوکلازها در توده نفوذی مونزونیتی کانسار مس پورفیری سریدون، ایران: با تأکید بر فرایندهای کنترل‌ کننده تکامل ماگما 33 59 48536 10.22067/econg.2026.1165 FA مهدیه بامروت دانشجوی دکتری، گروه زمین‌شناسی، دانشکده علوم، دانشگاه شهید باهنر کرمان، کرمان، ایران 0009-0001-3917-3334 سارا درگاهی دانشیار، گروه زمین‌شناسی، دانشکده علوم، دانشگاه شهید باهنر کرمان، کرمان، ایران 0000-0002-6882-2276 محسن آروین استاد، گروه زمین‌شناسی، دانشکده علوم، دانشگاه شهید باهنر کرمان، کرمان، ایران 0000-0003-4928-1176 علیرضا شاکر استادیار، گروه زمین‌‌شناسی، دانشکده علوم، دانشگاه شهید باهنر کرمان، کرمان، ایران 0009-0004-4775-9011 جیمی جی ویلکینسون استاد، گروه علوم‌زمین و مهندسی، دانشکده مهندسی، دانشگاه امپریال کالج لندن، لندن، بریتانیا 0000-0002-7706-6003 ادگار ای کورتس-کالدرون پژوهشگر، مرکز تصویربرداری و تحلیل، موزه تاریخ طبیعی، لندن، بریتانیا 0000-0002-5690-6907 مجید امین زاده رئیس بخش زمین‌شناسی و زهکشی، مجتمع مس سرچشمه، شرکت صنایع ملی مس ایران 0009-0006-7857-9755 Journal Article 2025 11 21 The Seridune porphyry copper deposit is located in the central part of the Kerman porphyry copper belt, Iran, three kilometers northeast of the Sarcheshmeh copper mine. This research focuses on reconstructing magmatic crystallization processes through a qualitative and quantitative study of plagioclase crystals in the monzonitic intrusive body of the Seridune porphyry copper deposit using petrology, mineral chemistry, and crystal size distribution studies. This unit rock, which ranges in composition from monzonite to quartz monzonite, comprises plagioclase, quartz, alkali feldspar, biotite, amphibole, as well as accessory minerals such as magnetite, apatite and zircon. The composition of the plagioclase crystals studied varies from labradorite to albite (An3.0-55.0Ab44.1-94.5Or0.0-17.7, n= 89). Based on crystal size distribution (CSD) studies, the semi-logarithmic patterns of the the studied plagioclase crystals indicate multi-stage crystallization of magma at varying depths. Also, the presence of sieve texture, oscillatory zoning, and CSD data indicate decompression and injection of mafic pulses in a dynamic system. Assuming a constant growth rate and nucleation density (n0), the nucleation rate of plagioclase crystals was determined to be between 64Î10-11mm-3/s-1 and 66Î10-11 mm-3/s-1. Also, based on the 3D crystal size distribution data and using a growth rate (G=10-10), the residence time of plagioclase crystals was calculated to be between 56 and 63 years. The calculated residence time, along with excess aluminum in the plagioclase crystals are consistent with the emplacement of the magma at shallow crustal levels and its high potential for the exsolution of fluids; although the monzonite body is not considered to be the responsible phase for mineralization in the Seridune porphyry copper deposit.IntroductionThe Urumieh–Dokhtar Magmatic Assemblage (UDMA) in the Kerman province contains numerous porphyry copper deposits, known collectively as the Kerman porphyry copper belt (KPCB; Fig. 1A) (Ghorbani, 2013). This belt, the primary source of copper ore in Iran, is primarily composed of folded and faulted early Tertiary volcano-sedimentary rocks (Aghazadeh et al., 2015(. The Seridune porphyry copper deposit (SPCD) is located in the central KPCB within the UDMA (Fig. 1A). It is hosted by Eocene volcanic rocks cut by hypabyssal felsic intrusions and all of the rock units have undergone variably intense hydrothermal alteration (Barzgar, 2007). Phyllic, argillic and advanced argillic alterations are widely exposed at the surface, whereas potassic alteration is concentrated at depth and is only accessible through drill core samples.Porphyry copper deposits form under the influence of dynamic processes governing a magma reservoir within the crust. These processes influence the magma's potential for metal enrichment and fluid dynamics (Park et al., 2021). Consequently, examining the physical and chemical evolution of minerals in igneous rocks is crucial for understanding the processes and dynamics of magmatic systems (Blundy and Cashman, 2008; Ellis et al., 2023) that lead to the formation of these deposits. Magmatic processes such as fractional melting, partial melting, fractional crystallization, and magma mixing (Higgins et al., 2015), which directly affect crystal nucleation and growth, are important petrological processes (Deb and Bhattacharyya, 2018). These processes do not change the bulk-rock chemical composition. Then, they cannot be constrained through chemical and isotopic studies of rocks (Higgins and Roberge, 2007). Instead, these processes have to be investigated through textural studies of rocks and crystal-scale mineral chemistry. Crystal size distribution (CSD) is a quantitative petrographic method which can provide important insights into the conditions within magma reservoirs or crystal mushes, including the rates of nucleation and growth, fractionation, coarsening, and mixing processes (Ngonge et al., 2013; Klein et al., 2017). This research aims to constrain the magmatic processes that governed the evolution of the monzonitic intrusive body in the Seridune porphyry copper deposit. This aim is achieved through the integration of petrographic observations with the determination of three-dimensional crystal shapes, Crystal Size Distribution analysis, and mineral chemistry of plagioclase crystals. This implications of the quantitative textural analyses on the studied samples are that they enable a more comprehensive understanding of the kinetic evolutionary history and the different processes—such as magma mixing and textural coarsening—that contributed to the formation of this intrusive body.Material and methodsTo identify the rock units and structures in the Seridune area, extensive field studies were conducted. For the present study, samples from the monzonite body of the Seridune porphyry copper deposit (Fig. 1B) exhibiting the least alteration were selected for preparation into polished thin sections. These sections were examined using a polarizing microscope to identify plagioclase crystals suitable for chemical microanalysis. Plagioclase compositions were determined using a JEOL 8530 F electron probe microanalyzer (EPMA) equipped with five tunable Wavelength Dispersive Spectrometers (WDS) hosted at the Imaging and Analysis Centre in the Natural History Museum, London. Operating conditions were 15 kV acceleration voltage, 20 nA beam current and a defocused beam of 10 μm diameter. On-peak counting times were set to 20 - 30 s for all elements and the background was acquired before and after the peak position for 10 - 15 s. Sodium, and potassium were analyzed first to reduce alkali migration due to prolonged electron dose. Standards were albite for Si kα and Na kα, corundum for Al kα, wollastonite for Ca kα, anorthoclase for K kα, fayalite for Fe kα, forsterite for Mg kα, celestine for Sr kα, barite for Ba kα, rutile for Ti kα and Mn-titanate for Mn kα. An internal secondary standard, plagioclase from the Isle of Rhum, was measured along with the unknowns to test the standardization and assess reproducibility. Oxygen was calculated by cation stoichiometry and included in the matrix correction. (Moy et al., 2023). Matrix correction was performed using the Phi-Rho-Z method, with mass absorption coefficients obtained from the FFAST (NIST version 2.1; Chantler et al., 2005). According to Armstrong (1998), the specific Phi-Rho-Z algorithm employed was the Armstrong/Brown/Scott-Love model.Quantitative measurements of crystal sizes were conducted on two representative, least-altered samples using a polarizing microscope. Digital images were captured, and image processing was performed using Adobe Photoshop (Adobe Creative Cloud version) to isolate plagioclase crystals in binary images. The long axis, short axis, area, and centroid of the crystals were measured using ImageJ software. The ShapeCalc spreadsheet program was then employed to convert two-dimensional data to three-dimensional data, allowing for the calculation of crystal dimensions based on the ratios of the short (S), intermediate (I), and long (L) axes. Finally, Crystal Size Distribution (CSD) plots were generated for each sample using CSD Correction 1.40 software.The residence time of the crystals for each sample was determined using the slopes from the CSD plots. This calculation was performed using the formula Tr = (-1/GÎm) / 31536000, where Tr is the residence time in years, m is the slope of the crystal population trend line, and G refers to the crystal growth rate in mm/s. The selected G value was based on the suggested value for intrusive magma (G = 10-10 mm/s, Cashman, 1993) and 31536000 is the conversion factor from seconds to years.ResultsSample petrographyThe Seridune Monzonite exhibits a porphyritic texture characterized by phenocrysts of plagioclase, alkali feldspar, quartz and biotite set in a microcrystalline groundmass. Secondary minerals resulting from hydrothermal alteration include biotite, alkali feldspar, chlorite, sericite and clay minerals.Plagioclase is the most abundant mineral in the Seridune Monzonite. It occurs both as subhedral to euhedral phenocrysts and microcrystals in the groundmass. Phenocrysts display polysynthetic and Carlsbad-albite twinning, along with oscillatory zoning. In some occurrences, these phenocrysts have even grown intergrown in a cruciform shape. It has commonly been replaced by sericite (Fig. 2B). This alteration is focused on the rim of the phenocrysts and the central parts of plagioclase microlites within the groundmass. Furthermore, these phenocrysts are occasionally crosscut by late-stage veinlets containing quartz, opaque minerals, and anhydrite. This crosscutting phenomenon is definitive evidence that the fluids bearing quartz, opaque minerals, and anhydrite are younger than the time of plagioclase crystallization, and it emphasizes the secondary permeability of the system (Sillitoe, 2000; Sillitoe, 2010).Plagioclase compositionThe results of 7 plagioclase EPMA analyses from two samples, together with the calculated (Wt.%) of anorthite, albite and orthoclase are presented in Table 1. The studied plagioclase crystals have a compositional range from labradorite to albite (An3.0-55.0Ab44.1-94.5Or0.0-17.7) (Fig. 3A). Some of these crystals show oscillatory zoning.Crystal size/shape analysisThe crystals shapes were determined by applying Zingg's diagram (Zingg, 1935). This classification scheme categorizes crystals into four categories: tabular, bladed, prolate (acicular) and spherical (equant), based on the ratios of their short, intermediate, and long axes. In the Seridune monzonite samples, plagioclase crystals are predominantly tabular and bladed in shape (Fig. 7B).DiscussionSome of these crystals exhibit oscillatory zoning, indicating that fractional crystallization and magma mixing play significant roles in influencing the evolving magmatic composition within the magma chamber (Kumar and Ashok, 2024; Ou et al., 2024). The plagioclase crystals are tabular and bladed in shape. This morphological variation is likely due to the thermodynamic conditions governing the crystallization process. The deformation of plagioclase crystals in the studied samples resulted from changes in undercooling, chemical potential gradients, and material diffusion. The non-liner trend observed in in the CSD plots for plagioclase crystals suggests multi-stage cooling process for the crystals (Fig. 8). The initial stage is marked by plagioclase phenocrysts with few nuclei, whereas in the following stage, they develop as microcrystalline formed under a high nucleation rate. The nonlinear CSD trends may show the multi-stage crystallization process of magma, magma mixing, and Ostwald Ripening (textural coarsening) processes, as well as modifications in the crystallization course due to changing environments and magma chambers at different depths. The observation of a porphyritic texture with a microlithic groundmass in the Seridune Monzonite, along with features such as oscillatory zoning and sieve texture, indicates multi-stage crystallization at varying depths. The process textural coarsening take place only when a crystal is kept at a temperature near to its liquidus for a prolonged period. Under such conditions, the nucleation rate approaches, while crystals larger than the critical size exhibit a high growth rate. According to Yang (2012), this does not necessarily imply that the temperature remains constant. The nucleation rate and residence time determined for the studied monzonitic intrusive body indicate a high nucleation rate and short residence time for the samples. Based on the Ca+Na+K versus Si+Al diagram, plagioclase crystals in the monzonitic body show a trend similar to those observed in the mineralizing magmatic systems and hydrous porphyries (Williamson et al., 2016). This trend, coupled with the calculated Al* values, reflects magmatic conditions favorable for porphyry-style mineralization; Although, despite these favorable indicators, this body itself is not considered the responsible phase for mineralization in the Seridune deposit. AcknowledgementThe authors gratefully acknowledge the financial and logistical support provided by the Vice Chancellor for Research at Shahid Bahonar University of Kerman and National Iranian Copper Industries Company (NICICO). Sincere thanks are also extended to the editor and anonymous reviewers of the Journal of Economic Geology for their constructive comments and valuable suggestions, which helped improve this manuscript. کانسار مس پورفیری سریدون، در بخش میانی کمربند مس پورفیری کرمان و سه کیلومتری شمال‌شرقی معدن مس سرچشمه واقع‌شده است. این پژوهش، با توجه به نقش پلاژیوکلاز در بازسازی فرایندهای تبلور ماگما، بررسی کیفی و کمی بلورهای پلاژیوکلاز توده مونزونیتی کانسار مس سریدون با بهره‌گیری از بررسی‌های سنگ‌شناسی، شیمی‌کانی و پراکندگی اندازه متمرکز است. این واحد سنگی که از نظر ترکیب از مونزونیت تا کوارتز مونزونیت متغیر است، شامل کانی‌های اصلی پلاژیوکلاز، کوارتز، فلدسپار پتاسیم، بیوتیت،آمفیبول و همچنین کانی‌های فرعی کدر اغلب از نوع مگنتیت، آپاتیت و زیرکن است. ترکیب بلورهای پلاژیوکلاز مورد بررسی از لابرادوریت تا آلبیت (An3.0-55.0Ab44.1-94.5Or0.0-17.7, n= 80) متغیر است. بر اساس بررسی‌های پراکندگی اندازه بلور، الگوهای نیمه لگاریتمی بلورهای پلاژیوکلاز مورد بررسی نشان‌دهنده تبلور چند مرحله‌ای ماگما در اعماق مختلف است. همچنین، وجود بافت غربالی، منطقه‌بندی نوسانی و نتایج پراکندگی اندازه بلور نشان‌دهنده کاهش فشار و تزریق پالس‌های مافیک در یک سامانه پویاست. با فرض نرخ رشد ثابت و چگالی هسته (n0)، نرخ هسته‌بندی بلورهای پلاژیوکلاز بین 1-/s3-mm 11- 10 Î 66 تا 1-/s3-mm 10-11 Î64 تعیین‌شد. همچنین، بر اساس داده‌های توزیع اندازه بلورهای سه‌بعدی و با استفاده از نرخ رشد 10- 10، زمان اقامت بلورهای پلاژیوکلازها بین 56 تا 63 سال برآورد شد. زمان اقامت محاسبه‌شده و آلومینیم اضافی بلورهای پلاژیوکلاز مورد بررسی با جای‌گیری ماگما در سطوح کم‌عمق‌تر و پتانسیل بالای آن برای آزادسازی سیالات همخوانی دارد؛ هرچند توده مونزونیتی فاز مسئول کانه‌زایی در کانسار مس پورفیری سریدون محسوب نمی‌شود. https://econg.um.ac.ir/article_48536_44b4034e6b33988e2b4539a0783215b9.pdf