میان‌ بارهای سیال، کانی شناسی و شیمی‌ کانی ها در کانسار پورفیری-اپی ترمال ساری‌ گونی، استان کردستان

کانسار طلای اپی ترمال ساری گونی (داشکسن) در شمال خاور شهرستان قروه و میزبان سنگ های آتشفشانی میوسن میانی قرار دارد. این کانسار بین کمان ماگمایی ارومیه-دختر و پهنه دگرگونی سنندجسیرجان واقع‌شده است. کانی سازی به صورت رگه ای و برشی و به ترتیب شامل پنج مرحله: 1) رگه-رگچه های کوارتز-سولفید-مگنتیت، 2) رگه های برشی کوارتزتورمالین، 3 و 4) رگه های اپی ترمال کوارتز-پیریت-استیبنیت-سولفید آرسنیک و 5) رگه های کوارتز-کلسیت-پیریت-گالن-اسفالریت-تتراهدریت است. شیمی تورمالین ها نشان می دهد که این کانی ها دارای منشا گرمابی بوده و از نوع دراویت هستند. مرحله اول کانه زایی در بازه دمایی 320 تا 380 درجه سانتی گراد و شوری 35 تا 45 درصد وزنی معادل نمک طعام تشکیل‌ شده است. رگه های کوارتزتورمالین برشی در یک بازه دمایی 203 تا 398 درجه سانتی گراد و شوری 31.43 تا 45.01 درصد وزنی معادل نمک طعام تشکیل شده اند. رگه های مرحله سوم و پنجم به ترتیب در یک بازه دمایی 200 تا 339 و 165 تا 230 درجه سانتی گراد و شوری 1.70 تا 11.74 و 1 تا 7.20 درصد وزنی معادل نمک طعام تشکیل شده اند. نتایج شیمی کانی های سولفیدی نشان می دهد که در اثر کاهش دما و فشار طی اختلاط با سیالات جوی و رقیق شدگی، تغییرات شیمی سیال به طور سریع رخ‌داده و هم‌زمان با جانشینی های آرسنیکآنتیموان در کانی های سولفید آرسنیک (رالگار و اورپیمنت)استیبنیت، جانشینی طلا با آهن در پیریت نیز رخ‌داده و طلا در این رگه ها به صورت محلول جامد و انکلوزیون های ریز در  سوتی پیریت (پیریت دوده ای) تشکیل‌شده است.

Fluid inclusions, mineralogy and mineral chemistry of the porphyry-epithermal Sari Gunay epithermal ore deposit - the Kurdistan province

Introduction The Sari Gunay veining and breccia epithermal gold mineralization is situated between the UrumiehDokhtar magmatic belts and the SanandajSirjan metamorphic zone in centralNW Iran. The Sari Gunay gold deposit is hosted by a middle Miocene volcanic complex that has been formed in the two Sari Gunay and Agh Dagh hills with ~2 km distance. The Sari Gunay volcanic complex consists of dacite to rhyolite volcanics and its coeval volcaniclastic rocks. There are some published data on the Sary Gunay ore deposit (e.g. Richards et al., 2006), while mineral chemistry of silicate and sulfide minerals have not been studied previously. The main goal of the present investigation is to determine type of mineralization based on detailed mineralogy, mineral chemistry, and fluid inclusion evidence and previously published data by Richards et al. (2006).   Materials and methods A total of 300 samples were collected systematically from 25 drill cores and outcrops. A total of 100 samples from different mineralization veins were selected for optical microscopy and after comprehensive study by stereomicroscope that was carried out at the Kharazmi University and Iranian Mineral Processing Research Center (IMPRC). The selected mineral phases were analyzed by an Electron Microprobe Analysis (EPMA) Cameca X100 with 20 kV and 20 nA, with a beam diameter of 5 μm at the IMPRC. Micro thermometric analyses were carried out on 10 doubly polished thin sections from breccia quartztourmaline and quartzpyritearsenic sulfidesstibnite and quartztourmaline veins using a Linkam THMS 600 freezingheating stage, mounted on a ZEISS Axioplan2 research microscope at the IMPRC.   Results Field geology and petrographic observations indicate that veining and breccia ore mineralization in the Sary Gunay ore deposit have occurred in deferent levels including quartzmagnetitesulfide veinlet in the deeper levels and brecciated quartztourmalinesulfide veins in the shallow levels. Several highgrade goldbearing veins and veinlets of quartzpyritestibniterealgarorpiment with diverse abundance ratio have formed within, and finally silverbearing quartzbase metals veins have been formed outward of the hydrothermal system. EPMA data indicate that gold has occurred in arsenian pyrite as solid solution and very fine inclusions. Stibnite, realgar and orpiment exhibits extensive range in As/Sb substitution. Hgbearing minerals have been detected in stibnite and arsenian sulfide minerals and also rutile has been detected in pyrite by EPMA. According to EPMA evidence, all tourmalines are alkaline belonging to dravitetype which show hydrothermal origin of quartztourmaline breccia veins. Fluid inclusions in the first stage have homogenization to a liquid in the range of 320° to 380°C, corresponding to salinities of 35 to 45 wt. % NaCl equivalent. Moreover, fluid inclusions in quartztourmaline veins show homogenization to a liquid in the range of 203° to 398°C, corresponding to salinities of 31.43 to 45.01 wt. % NaCl equivalent based on Sterner et al. (1988). Fluid inclusions in quartzpyritestibnite veins homogenized to a liquid between 200° and 339°C, with salinities of 1.70 and 11.74 wt. % NaCl equivalent, and finally base metal veins were formed by fluid with 165° and 230°C, with salinities of 1 and 7.20 wt. % NaCl equivalent based on Bodnar (1993).   Discussion Textural relationships and microscopic features allowed us to recognize five stages of veining; (1) quartzmagnetitesulfide, followed by (2) quartztourmaline breccia, (3) quartzpyritegoldstibnite, (4) quartzpyritestibniterealgarorpimentgold and (5) late Agbearing quartzcalcitepyritegalenasphalerite. There is evidence of As/Sb substitution in stibniterealgarorpiment minerals. Moderate temperature and salinity features, presence of V and L rich in association with L+V fluid inclusion types, variation in fluid composition, and pressure fluctuation of the mineralizing fluid during the main stage of gold mineralization are the main highlights of the Sari Gunay epithermal deposit, whereas high salinity and temperatures with first quartzsulfidemagnetite veins are consistent with porphyry ore mineralization in depth. Possibly rapid variations in the fluid chemistry and availability of enough As and Sb in the solution are responsible for As/Sb substitution, indicating that gold mineralization has occurred approximately at 250°C, which is supported by fluid inclusion data. A large As/Sb substitution range has also been reported by Mehrabi et al. (1999) in the Zarshuran ore deposit. In this condition, gold has occurred in mineral structure defected in arsenian pyrite due to substitution of Fe with large As ion. There are differences in core and rims of pyrite crystals on BSE images, reflecting lower As and higher S contents in the core of pyrite grains. Compositional zoning that has been found in pyrite represents rapid evolving conditions during ore mineral precipitation, probably due to episodic hydrothermal fluid degassing. The correlation between gold content and degree of Asenrichment in arsenian pyrite could indicate that gold has precipitated from hydrothermal fluids on to the Asrich growth surfaces of pyrite (e.g. Cepedal et al., 2008). Decrease of temperature and salinity during paragenitic sequences are consistent with fluid mixing with meteoric water and following fluid dilution. We can then conclude that the occurrence of porphyryepithermal veins in the Sary Gunay deposit is due to the presence of a fault system under the aquifer causing sudden depressurization and gradual mixing with shallow water. During temperature and pressure decrease gold was precipitated in the main stage of epithermal gold mineralization evidenced by extensive AuAsSbFe substitution in stibniterealgarorpimentpyrite minerals.   References Bodnar, R.J., 1993. Revised equation and table for determining the freezing point depression of H2ONaCl solutions. Geochimica et Cosmochimica Acta, 57(3): 683–684. Cepedal, A., Fuente, M.F. and MartinIzard, A., 2008. Goldbearing Asrich pyrite and arsenopyrite from the El Valle gold deposit, Asturias, Northwestern Spain. The Canadian Mineralogist, 46(1): 233–247. Mehrabi, B., Yardley, B.W.D. and Cann, J.R., 1999. Sedimenthosted disseminated gold mineralization at Zarshuran, NW Iran. Mineralium Deposita, 34(7): 673–696. Richards, J.P, Wilkinson, D. and Ullrich, T., 2006. Geology of the Sari Gunay epithermal deposit. Economic Geology, 101(8): 1455–1496. Sterner, S.M., Hall, D.L. and Bodnar, R.J., 1988. Synthetic fluid inclusions. V. Solubility relations in the system NaClKClH2O under vaporsaturated conditions: Geochimica et Cosmochimica Acta, 52(5): 989–1005.