Mineralogical and geochemical constraints on high-sulfidation epithermal alteration in the Zamharir kaolin deposit, northwestern Iran

Abstract

The Zamharir kaolin deposit in northwestern Iran represents hydrothermal alteration of the Oligocene andesitic rocks within a high-sulfidation epithermal system. Mineralogical assemblages display conspicuous spatial zonation, with kaolinite, alunite, pyrite, and rutile concentrated in the central part of the deposit, whereas peripheral zones are characterized by smectite, illite, and goethite. Mass balance calculations indicate extensive depletion of major, trace, and rare earth elements, including Si, Fe, Ti, Ca, Mg, Na, K, P, Rb, Cs, Ni, U, Cr, Co, and heavy rare earth elements (HREE), throughout the hydrothermal system. These geochemical patterns reflect pervasive leaching under the influence of acidic sulfate-rich hydrothermal fluids. Geochemical proxies further suggest dominance of hypogene oxidizing acidic conditions, with only limited contribution from supergene processes. Elemental behavior during kaolinization is controlled by variations in alteration intensity, fluid pH, complexation of ions in solution, adsorption onto secondary minerals, and the development of mineralogical zonation. The integrated evaluation of mineralogical zonation, elemental mobility, mass-balance constraints, and REE behavior provides a comprehensive understanding of the hydrothermal kaolinization processes and highlights the role of acidic hydrothermal fluids in mobilization and redistribution of the elements. These findings may serve as a useful framework for assessing similar volcanic-hosted kaolin systems worldwide.