کانی سازی و توزیع عناصر کمیاب پیریت به کمک تجزیه ریزکاوشگر الکترونی در چاههای اکتشافی محدوده طلای چشمه زرد (استان خراسان رضوی، ایران)

ناحیه مطالعاتی در شمال استان خراسان رضوی و 45 کیلومتری جنوب نیشابور قرار دارد. رگه های طلادار درون سنگهای گرانیت، گرانودیوریت، گرانودیوریت پورفیری، آندزیت، برش و توف نفوذ کرده اند. رگه ها از جنس کوارتز و کلسیت هستند. مناطق اصلی دگرسانی شامل منطقه سرسیتیک، سیلیسی، پروپلیتیک و منطقه کربنات است. کانی زایی به‌طور نزدیکی با دگرسانیهای سیلیسی و سرسیتیک به شکل رگه‌چه و با منطقه پروپلیتیک به شکل افشان همراه است. کانی شناسی کانیهای باطله کوارتز، کلسدونی، سرسیت، آدولاریا، کلسیت، دولومیت، ایلیت، دیکیت، آلبیت، کائولینیت و کلریت است. کانی زایی رگه ای عمدتاً با رگه‌چه ها، برش، افشان و استوک ورک همراه هستند. رگه‌چه ها از پیریت، مارکازیت، آرسنوپیریت و کمتر کالکوپیریت، اسفالریت، گالن، مگنتیت و هماتیت تشکیل شده اند. بیشترین عیار طلا در رگه های سیلیسی دیده می شود. پیریت کانی سولفیدی اصلی در کانی سازی اولیه است. سه نوع پیریت بر اساس ترکیب شیمیایی شناسایی شده است: پیریت خالص، پیریت غنی از آرسنیک و پیریت تیتانیوم وانادیوم دار. تجزیه های ریزکاوشگر الکترونی بیشترین غلظت آرسنیک، تیتانیوم و وانادیوم را به ترتیب 62/3، 91/3 و 53/0 درصد وزنی در پیریتهای رگه های طلادار نشان می دهند. طلا معمولاً همراه با پیریت آرسنیک دار و پیریت تیتانیوم وانادیوم دار است. براساس مطالعه بافت و ترکیب پیریتها، رگه‌چه‌های پیریت آرسنیک دار همراه با کانیهای سولفیدی آرسنوپیریت، کالکوپیریت، گالن، اسفالریت در دمای بالا و با کاهش دما و فشار، پیریتهای فرامبوئیدال با حاشیه حاوی آرسنیک و طلا شکل گرفته اند و در ادامه پیریتهای درشت تیتان دار به‌وجود آمده اند. پیریتهای خالص و تاخیری درشت بلور آخرین فاز کانی ساز در منطقه هستند.

Mineralization and trace element distribution in pyrite using EMPA in exploration drill holes from Cheshmeh Zard gold district, Khorasan Razavi Province, Iran

IntroductionPyrite is the most abundant sulfide mineral in low sulfidation ore deposits. Experimental studies have shown that lowtemperature ( lt 150°C) pyrite that formed rapidly is more likely to be finegrained and framboidal in shape compared to pyrite crystals that formed more slowly and at a higher temperature ( gt 200°C) from hydrothermal or metamorphic fluids (Butler and Rickard, 2000). Framboidal pyrite mostly occurs in sedimentary environments, though it could also form during metamorphism and hydrothermal alteration (Scott et al., 2009). The pyrite formed tends to be enriched in various trace elements such as Au and As. For this study we have combined the geology, alteration, mineralization with recent studies of the description of the deposit from core logging and underground mapping and geochemistry in the CheshmehZard gold district and also investigated the compositional variation and textural differences between pyrite types. This study is based on the results of our alteration and mineralization mapping and detailed logging of 1937.8 m of drill core.Materials and Methods Geology, hydrothermal alteration and mineralization were examined in drill holes along several cross sections. Hostrock alteration minerals and veins were determined for 11 sles using standard Xray diffraction (XRD) and Xray fluorescence spectrometry (XRF) techniques. Polished sections were studied by reflected light microscopy and backscattered electron images (BSE). In this study, the traceelement composition of pyrite sles from the AuIII vein system was obtained using electron microprobe analyzer (EMPA) data. All analyseswere carried out at the department of Materials Engineering and Physics of the University of Salzburg in Austria. The EMPA measurements and BSE imaging were made using a JXA8600 electron microprobe. Spot analyses of 30 pyrite grains from CheshmehZard are given in Table 1.ResultsThe study area is located in the north of Khorasan Razavi Province 45 km to the south of Neyshabour. The area near CheshmehZard could become important as a site of economically significant gold mineralization. Six goldbearing vein systems were recognized east of Arghash. The estimated resources are about 2 million metric tons of potential ore with an average of 1.9 g/t Au (Samadi, 2001 Ashrafpour et al., 2012). Multiple intrusive events are recognized in the region including Precambrian to postOligoceneMiocene igneous rocks (Alaminia et al., 2013a). This includes the Arghash diorite pluton, upper Cretaceous granitoids (minor diorite, mainly quartz monzodiorite and granodiorite), early Eocene granite and several lrophyre and small intrusions of quartz monzodiorite porphyries. Volcanicsinclude andesite, dacite, pillow basalt and tuffs. Sedimentary rocks are conglomerate and minor limestone. Gold veins are hosted by intermediate to silicic volcanic rocks, tuffs, granite, granodiorite, and conglomerate. Veins consist of calcite and quartz. The main alteration zones mapped at the surface and underground are sericitequartzpyritecalcite, withsilicified, propylitic, argillic, and carbonate zones. The mineralization associated with sericiticalteration and silicificationoccurs asveinlets and disseminated in the propylitic zone. Gangue minerals are quartz, chalcedony, calcite, adularia, illite, and kaolinite. Mineralization occurs as veinlets, breccia filling and disseminated. The veinlets are comprised of pyrite, arsenopyrite, minor chalcopyrite, sphalerite, galena, magnetite and hematite. Pyrite is the main sulfide mineral in the hypogene ore. Sles were collected with the objective of studying the pyrite in the Au (III) vein systems. All sles were therefore pyrite rich. The paragenesiswas determined to show four stages of mineralization based on the following microscopic observations: 1. an initial pyrite veinlet stage with associated quartz, chlorite, epidote. Pyrite is fine to medium grained, anhedral and goldpoor. 2. a second pyritic stage (polymetallic sulfide stage) contains pyrite, chalcopyrite, galena, sphalerite, quartz and chalcedony, minor adularia and arsenopyrite. 3. An Asbearing pyrite stage with sericite, chalcedony and quartz. The pyrite isframboidal.. 4. Finally, a carbonatedominated stage. The pyrite is euhedral to anhedral and coarse grained. The Au concentration in Stages 2 and 3 pyrite is higher than that in Stage 4 pyrite. Conclusions The gangue mineral assemblages of carbonate, chlorite, quartz, and minor sericite and potassium feldspar in the oreforming process of the CheshmehZard gold district suggest that the pH value of the hydrothermal fluids was near neutral to slightly acid (approximately 4.5 to 5.3 under 250 to 300 °C and 1 kbar conditions) and that gold would be transported mainly as Au(HS)2 (Stefansson and Seward, 2004). Three types of pyrite based on the chemical composition have been investigated: As bearing pyrite, TiV bearing pyrite and pure or barren pyrite. EMPA analyses of the pyrite in gold veins show maximum concentrations of As (3.62 wt.%), Ti (3.91 wt.%) and V (0.53 wt.%) respectively. The occurrence of the gold is usually associated with arsenian pyrite and TiV bearing pyrite. Veinlets of the Py1 coexisting with arseno pyrite and gold Py2 implies the substitution of sulfur by arsenic. Gold precipitated under relatively reducing conditions in framboidal pyrite. Py3 formed prior to barren pyrite (IV). ReferencesAlaminia, Z., Karimpour, M.H., Homam, S.M. and Finger, F., 2013a. The magmatic record in the Arghash region, NE Iran, and tectonic implications. International Journal of Earth Sciences, 102(6):16031625.Ashrafpour, E., Ansdell, K.M. and Alirezaei, S., 2012. Hydrothermal fluid evolution and ore genesis in the Arghash epithermal gold prospect, northeastern Iran. Journal of Asian Earth Sciences, 51(1):30–44. Butler, I.B. and Rickard, D., 2000. Framboidal pyrite formation via the oxidation of iron (II) monosulfide by hydrogen sulphide. Geochimica et Cosmochimica Acta, 64(15): 2665–2672. Samadi, M., 2001. Exploration in Arghash Gold Prospect. Geological Survey of Iran, unpublished report, Tehran, 73 pp. (in Persian)Scott, R.J., Meffre, S., Woodhead, J., Gilbert, S.E., Berry, R.F. and Emsbo, P., 2009. Development of framboidal pyrite during diagenesis, lowgrade regional metamorphism, and hydrothermal alteration. Economic Geology, 104(8):1143–1168.Stefansson, A. and Seward, T.M., 2004. Gold (I) complexing in aqueous sulphide solutions to 500 °C at 500 bar. Geochimica et Cosmochimica Acta, 68(20):4121–4143.