Epithermal precious- and base-metal mineralization in the Eocene arc of Torud-Chah Shirin mountain range: Gandy and Abolhassani districts, Semnan, northern Iran

Abstract

Subduction of Tethyan oceanic crust produced voluminous arc rocks; the NW-trending Urumieh-Dokhtar belt in central Iran and the Alborz belt in northern Iran. The belts host many precious and base metal prospects. This paper documents the Gandy and Abolhassani prospects in the Alborz belt. The hosting Eocene volcanic rocks show a typical arc geochemical signature with high LILE/HFSE ratios. The veins and breccias in the Gandy area show three stages of mineralization; precious metal mineralization (Stage 1), base-metal mineralization (Stage 2) and the formation of quartz and calcite (Stage 3). The Abolhassani veins also show three stages. The first two stages forming quartz, calcite, galena, sphalerite, pyrite and chalcopyrite are followed by the formation of quartz and calcite. Fluid inclusion asseniblages from Gandy show the average homogenization temperatures ranging from 234 to 285 °C and salinities from 4.2 to 5.4 wt% NaCl equiv. They are similar to isotopic temperatures of sulfides (236 and 245 °C). The Abolhassani area shows higher average temperatures (234-340 °C) and salinities (6.7 - 18.7 wt% NaCl equiv.). The base metal-rich mineralization at Abolhassani is attributed to the periodic injections of this high salinity fluid. The mineralization occurred at least 430 m and 600 m below the paleowater table at Gandy and Abolhassani, respectively. Low Au contents and high salinity of fluids suggest low potential of precious-metal rich zones at greater depth at Abolhassani. By contrast, Gandy has a potential for extensions of the high-grade Au veins. Results of this study may aid in exploration of the numerous untested epithermal and related prospects in northern Iran.