Mineralogical and geochemical constraints on the origin of alunite and APS minerals from advanced argillic alteration systems: A case study from Kuh-e-Lakht Au-Ag-Cu mineralization, NE Isfahan, Iran

Abstract

A significant association of alunite supergroup minerals commonly associated with quartz and pyrite is found in the Kuh-e-Lakht district of NE Isfahan, Iran. These minerals occur as pervasive alterations and within hydrothermal quartz veins. Based on field observations, microscopic studies, and geochemical analyses, the alunite subgroup exhibits noticeable variations in morphology, composition, and geochemical signatures between areas with moderate to high mineralization and those with poor mineralization. Poorly mineralized regions contain aluminum-phosphate-sulfate (APS)-free minerals of the alunite subgroup formed from steam-heated acid or supergene processes at an estimated temperature of about 200 °C, with a higher K₂O than Na₂O content. These crystals exhibit pseudocubic or acicular habits and zoned formations with rhythmic bands and lack APS minerals. Moderately to highly mineralized areas in Kuh-e-Lakht host tabular to platy and bladed alunite subgroup crystals with a hypogene origin, formed at around 250 °C, characterized by a higher or equal Na₂O content compared to K₂O. These areas also feature APS minerals with different modes of occurrence. In the southwestern, north-central, and northwestern parts, a strong genetic link with high-sulfidation minerals like pyrite, enargite, and chalcopyrite is observed. Alunite subgroup crystals with the highest Na₂O/K₂O ratio or equal values, as well as APS minerals with svanbergite-woodhouseite composition, occur within alunite crystals or as individual euhedral crystals exhibiting complex oscillatory zoning. These characteristics confirm a close association with the ore formation period and also the influence of higher-temperature fluids with magmatic-hydrothermal components, likely originating in closer proximity to intrusive sources. The estimated temperature for the alunite subgroup and associated ore minerals in these areas falls in the range of approximately 250 °C to 300 °C.