The role of sulfidation of Fe-carbonate rocks in increasing gold contents at the Zarshuran deposit (northern Takab), Takab-Angouran metallogenic district

Iron and arsenic sulfides are considered as the most important gold hosts in the sediment-hosted disseminated invisible gold deposits. The Zarshuran gold deposit (155 tons Au with average grade of 2.63 g/t) is formed in the Lower Cambrian black shale and siltstone (Zarshuran unit) and Fe-rich carbonates (Chaldagh unit) host rocks. As-sulfide (e.g., realgar and orpiment) and arsenian pyrites are the most important host minerals of gold in this deposit. Based on EPMA data, pyrite with As content below the detection limit to 3.99 wt% occurs in six different types, respectively, Py0 (gold content of 0.01 ppm), Py1 (gold content of 0.02 ppm), Py2 (gold content of 0.03 ppm), Py3 (gold content of 0.02 ppm), Py4 (gold content of 0.04 ppm), and Py5 (gold content of 0.01 ppm). According to the evidences, gold can be present as participating in chemical bounded (Au+ and Au+3) or nanoparticle inclusions (Au0). The weak geochemical correlation (R2 = −0.6) between As and S elements in pyrites indicates that there is pyrite with a complex composition [Fe(S,As)2Au2S0], which As− has replaced S2−. Mineralogy and the abundance of Fe and S in the rock units suggest that gold mineralization in the Zarshuran deposit is well occurs in response to sulfidation process. Sulfidation occurs when H2S-rich ore-forming fluids react with Fe-bearing carbonate host rock to form pyrite, marcasite, and pyrrhotite minerals.