زمین‌ شیمی، ایزوتوپ‏ های s و sr، و منشا کانسار باریت شاه‏ نشین‏، شمال‌ غرب استان کردستان، ایران

کانسار باریت شاه‌نشین (شمال‌شرق مریوان)، به‏گونه چینه‌سان و عدسی‌شکل، در میزبان سنگ ‏های ‏آتشفشانی کرتاسه‏ پسین جای‌دارد. باریت‏ ها به طور عمده تک کانی، درشت‌بلور و به شکل‌های صفحه ‏ای و تیغه ‏ای هستند. مقادیر δ34s بین 21.53-19.05‰ باریت‏ ها و مقایسه بین داده‌های 87sr/86sr نمونه ‏های باریت (0.70649 تا 0.70651) با نسبت‏ های آنها در آب دریای کرتاسه پسین (0.7075( و سنگ ‏های آتشفشانی میزبان (0.704 تا 0.705) بیانگر تشکیل کانسار شاه ‏نشین‏ توسط سیال‌های گرمابی زیردریایی است. این سیال دارای دو منشا ماگمایی و آب دریاست. تغییر در نسبت اختلاط این دو سیال به تغییر در فراوانی عناصر جزئی، (ppm 175.3-4.6) rees و نسبت‏ های lree/hree منجر‌شده است. نزدیکی مقادیر δ34s نمونه‏های باریت و آب دریای مرتبط با آنها‏ (2220‰)، گویای مقدار کم گوگرد سیال ماگمایی، تشکیل باریت در جریان آزاد آب و شرایط اکسیدان بستردریاست. داده ‏های باریت شاه ‏نشین همسان کانسار نوع فلسیک کوروکو، نهشته‌شده در حوضه حاشیه قاره‏ای بین پهنه فرورانش و حاشیه غیرفعال است.

Geochemistry, S and Sr isotopes and origin of the Shahneshin barite deposit, NW Kurdistan Province, Iran

IntroductionThe lenticular Shahneshin barite deposit (N: 35°39’36’’ and E: 46°36’11’’) is located 80 km  northeast of Marivan, Kurdistan Province; north SanandajSirjan Zone (Stöcklin, 1968). The deposit consists of stratiform ore and stringer zone. The stringer zone has occurred with alteration sericite quartz in the footwall dacitic unit (Kv) and evident in a series of veinveinlets under stratiform ore. The stratiform ore has been located concurrently on the dacite unit and below the Sanandaj Shale unit (Hasankhanloo, 2015).The study area mainly consists of Mesozoic succession dominated by the dacite tuff, andesiticbasaltic lava and pillow lava (Kv: the host rock), black slate and phyllite (Kss: Sanandaj Shale), dolomitic limestone with intercalation of sandstone (Kl), and black shale and slate (Ks: Sanandaj Shale). In this study, samples of the Shahneshin barite deposit have been analyzed for their 87Sr/86Sr and δ34S isotopes and trace elements (plus REE) geochemistry to assess the source of the deposit. Materials and methodsForty samples were collected from the Shahneshin barite deposit and country rocks. Polished blocks (6 samples) and thin sections (34 samples) were prepared for SEM images and petrographic examination at the University of Kurdistan. Trace elements (plus REE) were determined by ICPMS in the Geological Survey center of Iran (Karaj). The detection limits for elements are between 0.080.6 ppm. Three wholerock samples for Sr and S isotopes were analyzed at the Department of Earth and Space Sciences of the University of Science and Technology of China.Sulfur isotope analyses were measured by the use of a Delta V Plus Gas Isotope Ratio Mass Spectrometer. The analysis accuracy was determined to be ±0.2‰ (2σ) by using Canyon Diablo Troilite (CDT) as a standard (34S/32S=0.0450045). The strontium isotopic composition was completed on a Thermal Ionization Mass Spectrometry instrument by Phoenix. The measured strontium isotope normalized to 86Sr/88Sr=0.1194 and its NBS987 standard was 0.7102477±0.000014 (2σ). ResultsStrontium isotopes, sulfur isotopes, traceelement (plus REE) composition, fluid inclusion, and petrography of barites help to identify sources of mineralforming components and environments of precipitation (Baioumy, 2015). The Shahneshin barite deposit includes sulfate barium without valuable Pb and Zn elements. Barite crystals are mainly coarse (>20µ m) with euhedral, tabular, and bladed morphologies. Fluid inclusion microthermometry indicates that the barite formed from low salinity (2.08.5 wt.% NaCl eq.) fluids at low temperatures, between 115 to 215oC (Hasankhanloo, 2015).Chondritenormalized REE patterns for the barite samples reflect enrichment of the LREE/HREE ratios, as is shown by the high (Nd/Er)CN> 11 ratios. They also show Eu/Eu*CN (0.57.0), Ce/Ce*SN (0.11.16), La/Lu*CN >1, and ratios of the Ce/La (mostly>1), and Eu/Sm (0.12.83). The geochemistry of the barite samples represents variation in the abundance of trace elements and REECN patterns. The 87Sr/86Sr and Sisotopic values of Shahneshin barite samples are 0.706490.70651 and δ34S=19.0521.53‰, respectively. DiscussionBarite has very little Rb in its structure, and the isotopic composition of the original Sr is preserved in it (Martin et al., 1995). The 87Sr/86Sr values of the barite samples are consistent with 87Sr/86Sr=0.706490.70651, which itself is higher than those for host volcanic rocks (0.7040.705) but lower than that of the Cretaceous seawater (0.7075), the time of barites formation. We may conclude that strontium in the barites is a mixture of predominantly juvenile hydrothermal fluid of low Sr isotope and seawater that contains more radiogenic strontium. δ34S (=19.0521.53‰) of the samples indicate that much of the sulfur in barite was derived from seawater (δ34S=2022‰).Fluid inclusion salinities (Hasankhanloo, 2015), crystal sizes and morphology, abundance of sulfate mineral, lack of valuable Pb and Zn, distribution patterns of trace elements (plus REE) of barite reveal that mixture of magmatic fluid and seawater, with different ratios, were the source of the bariteforming fluid. That is supported by the δ34S values, the location of samples on the ThU diagram (seawater sources), variations in trace element (plus REEs), and Au anomaly (magmatic sources) in the samples. REEs, La enrichment, variations in abundance of trace elements, as well as δ34S and 87Sr/86Sr isotope values of the samples are consistent with the Kuroko massive sulfidetype (Marumo, 1989) and submarine hydrothermal volcanic model. In this model, barium has leached from the bedrocks by circulation of reduced hydrothermal fluid and transported along the synsedimentary normal faults to the seafloor. Then, fluid is mixed with SO42 of seawater and it causes the barite to precipitate. These barites have been formed in an open system, on or immediately below the seafloor.The Shahneshin barite deposit was, most probably, formed within caldera structures on top of the volcanic complexes in the latestage of submarine volcanic activity with andesitedacite in composition. It has occurred at continental margin tectonic setting between subduction zone and passive continental margin due to oblique collision along the SanandajSirjan Zone in the Late Cretaceous.