Mineralogy and geochemistry of the Nabar hydrothermal kaolin deposit, South of the Kashan district, Isfahan province, West of Central Iran

摘要

The Nabar kaolin deposit is located 29 km south of Kashan in the Isfahan province and is part of the Iranian Central structural zone. Field evidence and geochemical study shows that the Nabar deposit is an alteration product of Eocene volcanic rocks. The major minerals are quartz, kaolinite, dickite, illite, montmorillonite, albite, muscovite, along with lesser amounts of anatase, diaspore, goethite and alunite. Element mass change calculations, assuming Al as the low to immobile index element, indicate that leaching and fixation are two prominent factors controlling the concentration of major, minor, trace, and rare earth elements. Geochemical studies show that variation of the negative to weak positive Eu anomalies (0.57-1.10) and negative to weak positive Ce anomalies (0.77-1.04) at Nabar were, respectively, controlled by the degree of feldspar alteration and changes in the rate of oxidation potential of the environment. From the geochemical data it can be inferred that hypogene alteration features were overprinted by supergene alteration in the course of deposit formation, indicating both deep fluid source and shallow chemical leaching took place. Element distribution modes in the deposit indicate that the element behavior during kaolinization of volcanic rocks was primarily controlled by porosity and permeability, but also by factors such as pH, redox potential, temperature variations, high fluid/rock ratios, scavenging and fixation by clays and iron oxides, discrepancies in the stability rate of minerals, abundance of complex-forming ions, and isomorphic substitution. Further geochemical considerations show that oxides and hydroxides of Fe and Mn along with secondary phosphates are the potential hosts for REE in the deposit.