Geology of the Sari Gunay epithermal gold deposit, Northwest Iran / Геология эпитермального золоторудного месторождения Сари Гунай, Северо-Западный Иран

The Sari Gunay epithermal gold deposit is located within a mildly alkaline latitic to trachytic volcanic complex in central-northwest Iran. Intrusive and volcanic rocks that host the deposit have been dated at between 11.7 and 11.0 Ma (with one younger sample at 8.0 Ma; 40Ar/39Ar dating of igneous biotite and hornblende), whereas sericitic alteration associated with an early stage of hydrothermal activity occurred between ~10.8 and ~10.3 Ma (the best age estimate is 10.7 Ma obtained by 40Ar/39Ar dating of sericite). The main hydrothermal system was structurally localized by a diatreme breccia complex centered on the Sari Gunay hill. Earliest hydrothermal activity, only encountered at depth (>300 m below surface), produced weak potassic alteration and quartz-sulfide-magnetite veining with low grades of Cu and Au mineralization, reminiscent of shallow-level (≤1.5-km depth) porphyry gold systems. Paragenetically later quartz-tourmaline breccias and veins followed the south-southwest–north-northeast trend of the diatreme breccias and produced strong sericitic alteration and silicification in igneous clasts and wall rocks. Fluid inclusions indicate temperatures of 246° to 360°C and salinities of 34.4 to 46.1 wt percent NaCl equiv in hypersaline inclusions coexisting with low-density, CO2-bearing vapor-phase inclusions. These fluids do not appear to have deposited gold, but the breccias provided further structural permeability for flow of later mineralizing epithermal fluids.

Invisible gold occurs in solid solution in fine-grained arsenical pyrite and minor arsenopyrite, deposited in the early stages of quartz-adularia-pyrite-stibnite veins. Late-stage minerals in these veins include realgar, orpiment, cinnabar, and extremely rare wire gold. Liquid-rich fluid inclusions in these veins have an average homogenization temperature (
trapping temperature) of 199° ± 24°C, salinities of 3.6 ± 1.1 wt percent NaCl equiv, and coexist with low-density CO2-bearing vapor-phase inclusions, suggesting low-pressure conditions. Phase separation may have been a contributory cause of gold deposition, but precipitation of sulfides in response to rapid cooling of the system may have been the main control on deposition of pyrite-hosted gold <...>