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

The Sari Gunay veining and breccia epithermal gold mineralization is situated between the Urumieh-Dokhtar magmatic belts and the Sanandaj-Sirjan metamorphic zone in central-NW 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).

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 X-100 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 quartz-tourmaline and quartz-pyrite-arsenic sulfides-stibnite and quartz-tourmaline veins using a Linkam THMS 600 freezing-heating stage, mounted on a ZEISS Axioplan2 research microscope at the IMPRC.

Field geology and petrographic observations indicate that veining and breccia ore mineralization in the Sary Gunay ore deposit have occurred in deferent levels including quartz-magnetite-sulfide veinlet in the deeper levels and brecciated quartz-tourmaline-sulfide veins in the shallow levels. Several high-grade gold-bearing veins and veinlets of quartz-pyrite-stibnite-realgar-orpiment with diverse abundance ratio have formed within, and finally silver-bearing quartz-base 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. Hg-bearing 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 dravite-type which show hydrothermal origin of quartz-tourmaline 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 quartz-tourmaline 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 quartz-pyrite-stibnite 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).

Textural relationships and microscopic features allowed us to recognize five stages of veining; (1) quartz-magnetite-sulfide, followed by (2) quartz-tourmaline breccia, (3) quartz-pyrite-gold-stibnite, (4) quartz-pyrite-stibnite-realgar-orpiment-gold and (5) late Ag-bearing quartz-calcite-pyrite-galena-sphalerite. There is evidence of As/Sb substitution in stibnite-realgar-orpiment 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 quartz-sulfide-magnetite 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 As-enrichment in arsenian pyrite could indicate that gold has precipitated from hydrothermal fluids on to the As-rich 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 porphyry-epithermal 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 Au-As-Sb-Fe substitution in stibnite-realgar-orpiment-pyrite minerals.