Formation and evolution stages of gold rich Barika massive sulfide deposit, east of Sardasht, northern Sanandaj-Sirjan zone: based on structural, textural and fluid inclusion studies

Barika gold (and silver) rich massive sulfide deposit is located 18 km east of Sardasht, north western part of Sanandaj-Sirjan metamorphic zone. Rock units outcropped in this area are Cretaceous metamorphosed volcano-sedimentary rocks in green schist facies, and include association of meta-andesite, meta-tuffite, phyllite and slate rocks, where the meta-andesitic unit (Kmv1) is host rock to the deposit. Barika deposit is composed of stratiform ore and stringer zone. Stratiform part contains massive and banded sulfide and barite associated with subordinary silica bands underlain by stringer silicic zone. The massive and banded ore is surrounded by an extensive zone of silicic stringer veins and disseminated mineralization within strongly altered and deformed andesitic volcanic rocks. Ore mineral assemblages in stratiform part of the deposit are quite variable and consist of pyrite, sphalerite, galena, stibnite, a variety of sulfosalt minerals and gold (electrum). Ore mineral assemblages in silicic veins in the stringer zone are simpler and consist of pyrite, galena, sphalerite, tetrahedrrite and trace chalcopyrite. Average gold and silver grades in stratiform ore are 8. 3 and 420 g/t respectively and in silicic stringer veins are 0. 7 and 30 g/t. Base metal content in both parts of the deposit are less than 1%. Based on our research, Barika is an immature Kuroko type massive sulfide deposit, which contains only black ore. The Barika deposit underwent low-grade metamorphism and high grade deformation after the ore deposition event. Geologic processes appear to control the mode of occurrence of gold in the deposit. In synvolcanism stage, deposition of a gold–bearing low temperature (140-200°C) and low salinity (1-9. 6 wt% NaCl) hydrothermal fluid formed a Kuroko black ore type deposit. In the stage of mineralization, invisible gold was concentrated in framboidal pyrite and other sulfide minerals. The main effect of progressive metamorphism (D1) on gold mineralization is recrystallization of the framboidal pyrite and migration of submicroscopic gold to the crystal boundary to form electrum under low to moderate strain. Highly strain and deformation (D2) on the deposit caused the remobilization of gold and accompanied َAs, Sb, Ag, and Pb minerals and continued to form coarse-grained electrum (up to 3 mm) in open space fractures. The formation of barren silica veins (D3) and high angle normal faults (D4) are the results of retrograde metamorphism.