Comparison of epithermal base and precious metal deposits in Gandy and Abolhassani districts, Semnan Province, Iran

Gandy and Abolhassani epithermal precious-and base-metal deposits occur in the Tertiary calc-alkaline volcanic belt of northeast Iran. The exposed rock units consist of a volcaniclastic sequence of thin-bedded siltstones and sandstones, lapilli tuffs, volcanic breccias and intermediate lava flows at Gandy, and mostly andesitic flows at Abolhassani. Mineralization in Gandy occurs as veins and breccias and can be divided into three main stages. Stage I is economically important in terms of precious metal content.Stage II consists of four substages and contains the majority of base-metal mineralization. The final stage is dominated by quartz and calcite. Mineralization of the Abolhassani veins occurred in three main stages. The first two stages contain similar mineral assemblages, including quartz, calcite, galena, sphalerite, pyrite and chalcopyrite, which are economically important in terms of base-metal content, whereas the final stage is dominated by quartz and calcite. The average homogenization temperatures and salinities of fluid inclusion assemblages from Gandy range from 234o to 285 oC, and 4.2 to 5.4 wt% NaCl equiv. The homogenization temperatures are in a good agreement with isotopic temperatures from two sphalerite and galena pairs (236o and 245 oC). The average temperature and salinity of fluid inclusion assemblages from Abolhassani district range from 234o to 340 oC, and 6.7 to 18.7 wt% NaCl equiv. Sulfide pairs of sphalerite-galena do not give reasonable equilibration fractionation temperatures at Abolhassani. Comparison of Th versus freezing (TmI) values for the two deposits indicates the presence of a moderate salinity fluid (5-6 wt% NaCl) of similar temperature (250 oC) in each deposit, but with a higher salinity component also present at Abolhassani. The Abolhassani district shows higher average Ag/Au (34.7) and Pb + Zn contents (up to 7.6 wt%) than Gandy (Ag/Au 2.1 and 3.9 wt%), consistent with the nearly 4x higher maximum salinity at Abolhassani compared to Gandy.Precious and base-metal mineralization within hydrothermal breccias of Gandy may have deposited under boiling conditions, whereas base-metal mineralization in stage II most likely occurred due to dilution. Based on mixing trends, base-metal sulfides in Abolhassani veins were deposited during periodic injection and dilution of brines. The minimum depth of formation was at least 430 m and 600 m below the paleowater table for Gandy and Abolhassani, respectively.