تکامل ماگمایی - گرمابی سیال کانه‌ دار در کانسار مس - طلای پورفیری دالی با استفاده از شیمی‌ کانی‌ های آمفیبول و پلاژیوکلاز

کانسار مس طلای پورفیری دالی در غرب روستای راوه، شهرستان دلیجان و در بخش مرکزی کمربند ماگمایی ارومیه دختر قرار دارد و ناشی از نفوذ توده‌هایی با ترکیب دیوریت و کوارتز دیوریت به سن میوسن در سنگ‌های آتشفشانی با ترکیب آندزیت تا آندزیت بازالت پورفیری الیگو میوسن است. دگرسانی‌های پتاسیک، آرژیلیک، پروپلیتیک و به‌‌طور محلی فیلیک باعث افزایش شدت کانه‌زایی در بعضی مناطق شده‌اند. هدف از این پژوهش، بررسی ترکیب شیمیایی کانی‌های موثر بر کانه‌زایی و تکامل سیال کانه‌دار در ماگمای مولد و تاثیر آن در توان کانه‌زایی کانسار مورد نظر است. برای این منظور کانی‌های آمفیبول و پلاژیوکلاز در سنگ‌های کانه‌دار با استفاده از روش ریز‌پردازش الکترونی مورد بررسی قرار گرفتند و داده‌های به‌‌دست ‌آمده با بررسی میان‌بارهای سیال در رگه‌های دخیل در کانه‌زایی تلفیق شدند. نتایج حاصل از بررسی‌های ریز‌پردازش الکترونی نشان داد که آمفیبول‌های ماگمایی، از نوع کلسیک و در گستره منیزیو هورنبلند، ادنیت، منیزیو هستینگزیت و چرماکیت است. همچنین پلاژیوکلازها در گستره آندزین و لابرادوریت هستند. با توجه به دما– فشار سنجی، آمفیبول‌ها در فشار 5/3 تا 3/5 کیلوبار و دمای 818 تا 899 درجه سانتی‌گراد در محیطی با گریزندگی اکسیژن بالا و در شرایط اکسیدان شکل‌گرفته، این شرایط زمینه را برای حمل و انتقال مس و طلا فراهم می‌کند و بیانگر شرایط ماگمایی، ماگمای مادر پورفیری دالی است. نتایج به‌دست آمده از داده‌های ریز‌دماسنجی سیال درگیر مربوط به رگه‌های نابارور اولیه همراه با دگرسانی پتاسیک میزان دمای340-620 درجه سانتی‌گراد، شوری 35-75% درصد وزنی را نشان می دهد که با توجه به دمای آمفیبول‌ها این افت دما بیانگر تغییر از شرایط ماگمایی به گرمابی و آغاز فرآیندهای کانه‌زایی در این کانسار است.

Magmatichydrothermal fluid evolution of the Dalli porphyry CuAu deposit; using Amphibole and Plagioclas mineral chemistry

Introduction The formation of porphyry copper deposits is attributed to the shallow emplacement, and subsequent cooling of the hydrothermal system of porphyritic intrusive rocks (Titley and Bean, 1981). These deposits have usually been developed along the chain of subductionrelated volcanic and calcalkalin batholiths (Sillitoe, 2010). Nevertheless, it is now confirmed that porphyry copper systems can also form in collisional and post collisional settings (Zarasvandi et al., 2015b). Detailed studies on the geochemical features of orehosting porphyry CuMoAu intrusions indicate that they are generally adakitic, water and sulfur riched, and oxidized (Wang et al., 2014). For example, high oxygen fugacity of magma has decisive role in transmission of copper and gold to the porphyry systems as revealed in (Wang et al., 2014). In this regard, the present work deals with the mineral chemistry of amphibole and plagioclase in the Dalli porphyry CuAu deposit. The data is used to achieve the physical and chemical conditions of magma and its impact on mineralization. Moreover, the results of previous studies on the hydrothermal system of the Dalli deposit such as Raman laser spectroscopy and fluid inclusion studies are included for determination of the evolution from magmatic to hydrothermal conditions. Materials and methods In order to correctly characterize the physical and chemical conditions affecting the trend of mineralization, 20 least altered and fractured samples of diorite and quartzdiorite intrusions were chosen from boreholes. Subsequently, 20 thinpolished sections were prepared in the Shahid Chamran University of Ahvaz. Finally, mineral chemistry of amphibole and plagioclase were determined using electron micro probe analyses (EMPA) in the central lab of the Leoben University. Results Amphibole that is one of the the main rockforming minerals can form in a wide variety of igneous and metamorphic rocks. Accordingly, amphibole chemistry can be used as an indicator for characterizing the conditions involved during the evaluation of magma crystallization i.e., pressure, temperature, liquid water content and oxygen fugacity. Most recent studies on the porphyry copper intrusions in the Urumieh Dokhtar magmatic arc by (Zarasvandi et al., 2015a), indicate that all of the mineralized porphyry systems (Dalli porphyry is included) consistently show high levels of La/Sm and Sm/Yb, with concave upward patterns in the rare earth elements’ spider diagrams. Importantly, such features indicate high crustal assimilation in a relatively thickened crust and provide insight into the contribution of hornblende during the development of mineralized porphyry systems in the Urumieh Dokhtar belt. The results of this study indicate that amphiboles of Dalli intrusions belong to the calsic group and range in composition from magnesio hornblende, to edenite, magnesiohastingsite, and tschermakite. (Ridolfi et al., 2010), indicating that the alumina content of amphibole could be used for geobarometry. The calculations of geobarometry for quartz diorite intrusions of Dalli indicate that they formed in the pressure range of 136 to 287 (MPa). Also, calculation of magmatic water content using amphibole geochemistry indicates that the water content of quartz diorite intrusions in the Dalli were between 4.6 5.7 (wt. %). The results of plagioclase chemistry indicate that there is a little zoning in this mineral. Also, the plagioclase composition varies from Or0.01 to Ab 0.48, An 0.50, Or 0.018, Ab 0.62 and An 0.35. They mostly have Andesine and Labradorite compositions. Discussion Amphibole minerals of the Dalli intrusions are calcic type and exhibit geochemical signatures of subduction zones. Also, characterizing the source of orehosting intrusions with amphibole chemistry indicate that parental magma of Dalli intrusion were generated from mixing of mantle melts with crustal materials. It seems that in an ongoing process of closure of NeoTethys, during compression and crustal shortening favourable conditions were provided for mixing of mantle melts with crustal materials. The geothermobaromerty calculations using amphibole and plagioclase minerals indicate conditions of 777 850 oC and 14 Kbar, representing magmatic stage of Dalli intrusions. Also, amphibole minerals are characterized by Fetot/Fetot + Mg > 0.3 and AlIV > 0.7 which reveal the presence of primary oxidative magma in the Dalli porphyry CuAu deposit. Oxidative conditions seem to have prevailed during the onset of the hydrothermal stage of the Dalli porphyry deposit. This is because it has been confirmed by laser Raman spectroscopy analyses that the most primitive quartz veins in the potassic alteration of the Dalli deposit are characterized by the presence of anhydrite and hematite minerals (see Zarasvandi et al., 2015b). Also, microthermometry results on the most primitive barren quartz veins in potassic alteration represent temperatures as high as 620oC which indicate the beginning temperature of hydrothermal conditions. Acknowledgements Many thanks are due to the office of vicechancellor of the Shahid Chamran University of Ahvaz for valuable information concerning the field work and sampling. References Ridolfi, F., Renzulli, A. and Puerini, M., 2010. Stability and chemical equilibrium of amphibole in calcalkaline magmas: An overview, new thermobarometric formulations and application to subductionrelated volcanoes. Contributions to Mineralogy and Petrology, 160(1): 45–66. Sillitoe, R.H., 2010. Porphyry Copper Systems. Economic Geology, 105(1): 341. Titley, S.R. and Beane, R.E., 1981. Porphyry copper deposits. Part 1. Geologic Setting, Petrology, and Tectogenesis. In: J.W. Hedenquist, J.F.H. Thompson, R.J. Goldfarb and J.P. Richards (Editors), Economic Geologists, 75th Anniversary Volume. Society of Economic Geologists, U.S.A, pp. 214234. Wang, R., Richards, J.P., Hou, Z.Q., Yang, Z.M., Gou, Z.B. and DuFrane, A., 2014. Increasing Magmatic Oxidation State from Paleocene to Miocene in the Eastern Gangdese Belt, Tibet: Implication for CollisionRelated Porphyry CuMo ±Au Mineralization. Economic Geology, 109(7): 1943–1965. Zarasvandi, A., Rezaei, M., Sadeghi, M., Lentz, D., Adelpour, M. and Pourkaseb, H., 2015a. Rare earth element signatures of economic and subeconomic porphyry copper systems in Urumieh–Dokhtar Magmatic Arc (UDMA), Iran. Ore Geology Reviews, 70: 407423. Zarasvandi, A., Rezaei, M., Raith, J., Lentz, D., Azimzadeh, A.M. and Pourkaseb, H., 2015b. Geochemistry and fluid characteristics of the Dalli porphyry Cu–Au deposit, Central Iran. Journal of Asian Earth Sciences, 111: 175191.