U-Pb rutile dating of the host metagranite and later gold mineralization (Hamzeh- Qharanien) in a ductile shear zone, southwest Saqqez, Iran

The study area is part of Kasnazan- Qolqoleh shear zone that is located in southwest of Saqqez in west of Iran. It consists of a part of an orogenic belt in western part of Iran. It is the result of closure of Neo-Tethys during Cretaceous- Tertiary continental collision between the Afro-Arabian plate beneath central Iran. This zone hosted several orogenic gold deposits and occurrences, including the Qolqoleh, Kervian, Qabaglojeh, Hamzeh- Gharanein, Gharechar, and Kasnazan mineralization, which are related to the major Saqqez- Sardasht thrust.According to the geological maps of Saqqez, the main units belong to the Percambrian- Paleozoic rocks and included chlorite schist, marble and mylonitized granite. Host rocks of gold mineralization are mostly chlorite schist, metagranite, and mylonitic granite in across Kasnazan- Qolqoleh shear zone. Based on microscopic studies, chlorite mica schist in Qolqoleh and Qabaghlojeh is mainly included chlorite around 20-60%, quartz (30-35%), plagioclase (20- 25%), biotite (5- 0%), sericite (15- 20%), and carbonate (5%). Metagranite in Hamzeh-Gharanein mainly includes quartz, alkali feldspar, plagioclase, and biotite with accessory minerals zircon, rutile, apatite, sericite, and iron oxide.In this study, we have evaluated in situ dating of rutile by LA- ICPMS. The U-Pb analyses were carried out at the University of New Brunswick in Canada, using a Resonetics 193 nm Excimer laser coupled to an Agilent 7700X quadrupole ICP411 MS. Rutile were analyzed in standard 30- m- thick polished thin sections. Only minerals free of inclusions were used. Despite its common occurrence, rutile has so far received less attention as a geochronometer than zircon and monazite, because of the low U concentration in many rutile (as low as 0.01 ppm). To solve this problem, we chose grains with sufficient U concentrations for dating. One sample of metagranite from Hamzeh-Gharanein and two samples of chlorite- schist in Qolqoleh and Qabaglojeh were selected for U-Pb dating. Rutile were surveyed by cathodoluminescence (CL) to detect their internal structures at University of New Brunswick in Canada. Most of them are based on morphology are subhedral and no zoning is observed. For this study, 65 single spot U-Pb analyses of 3 samples were measured by LA-ICP-MS. There are 2 main populations, which included 506.3 ± 6.8 Ma (MSWD=0.0119) gained, which are related to inherent rutile in Hamzeh-Gharanein prospect and 123.7 ± 8.0 Ma (MSWD=1.8) in chlorite schist in Qolqoleh and Qabaglojeh, which show the hydrothermal age event in study area. Also, one of major geochemical characteristic of rutile is low Th content. Most of rutile in chlorite schist formed from chlorite and muscovite breakdown and some hydrothermal rutile most likely precipitated directly out of a fluid. Based on availability of Th and U during crystal growth can be one of the main reasons for low Th/U ratios. Based on microscopic studies, coexisting of monazite as a Th-rich accessory phase can cause the low Th/U ratio. Also, as the lattice strain models should affect Th and U incorporation in rutile to an even greater degree when compared to zircon.