Mineralogy, hydrothermal alteration, fluid inclusion, and O-H stable isotopes of the Siah Jangal-Sar Kahno epithermal gold deposit, SE Iran

摘要

Siah Jangal-Sar Kahno deposit is located in the northern part of Taftan volcano in the Sistan and Balouchestan provinces, southeast of Iran, and is an example of a vein-type epithermal gold system with related stockwork and shear zone mineralization. The veins are hosted by flysch facies rocks of the Makran zone and subvolcanic dioritic porphyry. The NE-SW fault systems played an essential role in the genesis of alteration and mineralization in the area. Diorite porphyry ranges from alkali (Na2O + K2O) content of 4.1-6.66 wt%, and plot in the calc-alkaline to high-K calc-alkaline fields. The chondrite-normalized REE patterns for least altered porphyry diorite rocks, altered host rocks, and silica-carbonate vein show a LREE/HREE ratios of 10.40, 4.03, 4.00, and 2.66, respectively. Positive Eu anomalies in the silica-carbonate veins indicate the Ca-bearing minerals, such as plagioclase decomposition causing release of Eu into hydrothermal fluid and subsequent precipitation in silica-carbonate veins. Mineralization consist of quartz, adularia, calcite, pyrite, arsenopyrite, marcasite, Fe-poor sphalerite, galena, chalcopyrite, and native gold. Later supergene mineral assemblages consists of Fe-oxide and hydroxides, covellite, cerussite, and scorodite. Hydrothermal alteration is generally well developed enveloping the ore-bearing hydrothermal breccia and subdivided into prograde inner quartz-chlorite-pyrite-muscovite, and outer argillic alteration zones, with a regional barren, widespread near-surface supergene (blanket) advanced argillic alteration zone. Chemical discrimination by means of cationic R1-R2 diagram indicates that R1 increases, whereas R2 decreases with increasing argillic alteration. Microthermometry of fluid inclusions in quartz crystals from silica-carbonate vein show the homogenization temperatures (Thv-l) of 167 degrees-332 degrees C with salinities in the range of 0.21-4.24 wt% NaCl eq. The O and H isotopic compositions of quartz from hydrothermal vein and argillic alteration simply show a low sulfidation epithermal environment with meteoric waters as the main fluid. Due to a narrow range in salinity of the fluids, very low sulfide diversity, sulfide volume, and presence of adularia suggest dominantly meteoric water in the deposit, supporting classification as a low-sulfidation epithermal system.