Geochemical constraints on argillic altered rocks in the Bashgol district, Tarom-Hashtjin metallogenic province, central-northern Iran

The Bashgol district constitutes a part of the Tarom-Hashtjin metallogenic province in central-northern Iran. The intrusion of granitic igneous bodies of Oligocene age into the middle Eocene dacitic rocks in this district is associated with the extensive hydrothermal alteration zones consisting mainly of argillic alteration zone (AAZ). The X-ray diffractometry (XRD) data featured that quartz and kaolinite are two major mineral assemblages in the AAZ, which are accompanied by lesser amounts of minerals such as halloysite, smectite, alunite, pyrite, rutile, goethite, illite, plagioclase, and svanbergite-woodhouseite. Depletion of elements like Zr, Nb, Th, Ta, and Y during the development of the AAZ is affiliated with the low-pH hydrothermal fluids, high water/rock ratios, and abundance of complexing ions. The significant enrichment of Mo, Sb, and As coupled with positive correlation of these three elements with sulfur, very low Rb/Sr ratio, mineral assemblages of the AAZ, and occurrence of vein-veinlet-type mineralization indicate that the Bashgol district possesses favorable conditions for the formation of epithermal deposits with high-sulfidation style. This study revealed that alunite, svanbergite-woodhouseite, kaolinite, smectite, halloysite, and goethite as alteration products controlled the geochemistry of rare earth elements (REE) in the AAZ. The occurrence of negative Ce anomaly in AAZ shows the destruction of zircon by oxidizing-acidic fluids. The occurrence of negative Eu anomaly during the development of the AAZ attests to destruction of plagioclase and hornblende by strongly acidic hydrothermal solutions and provides firm reasoning for high ƒO₂ of the environment.