Isocon mass-balance constraints on element mobility in the Tepeköy high-sulfidation epithermal Au system within Melendiz volcanics, Central Türkiye

The Tepeköy Au prospect within the Melendiz volcanic suite (southern Central Anatolian Volcanic Province, central Türkiye) preserves a high-sulfidation (HS) epithermal alteration system hosted by andesite porphyry and basaltic–andesite volcanic rocks. Alteration is spatially zoned from an inner vuggy silica–Fe-rich domain outward to an alunite-rich halo and a distal kaolinite-rich zone, with gold concentrated mainly in the inner two zones. This framework and mineral association (dominantly pyrite ± arsenopyrite ± magnetite with supergene/oxidation products such as limonite, hematite, and goethite) are consistent with established HS epithermal model in which acid–sulfate fluids generate advanced argillic assemblages and residual/precipitated silica near the hydrothermal ascending core.

To quantify element mobility and bulk-rock modification across the alteration gradient, mass-balance calculations were evaluated using isocon methods, treating TiO₂–Nb–Zr as immobile indicators. The vuggy silica–Fe-rich zone records the most extreme open-system behavior, defined by substantial gains in SiO₂, Fe₂O₃, sulfur (S, SO₃) and LOI, accompanied by enrichment in As–Co–Ni–Mo–V–Cr (locally ±Sc). These increases occur alongside pronounced depletion of major base cations (Na₂O–K₂O–CaO–MgO) and marked loss of several chalcophile elements (Cu–Zn–Sn–Sb). Isocons indicate that both bulk mass and volume changes exceed the reference frame, implying high fluid/rock ratios and strong permeability focusing within the inner zone. The alunite-rich halo shows similarly robust additions of SiO₂–Fe₂O₃–SO₃–LOI and systematic pathfinder enrichment (notably As ± Mo ± V ± Pb ± Co–Ni), while maintaining persistent base-cation depletion; additional gains in Cl, Sr, and Ba and overall mass/volume increase suggest continued influx of sulfate-bearing fluids and deposition of hydrated sulfate phases. In contrast, the kaolinite-rich zone displays net mass/volume loss, relative SiO2 depletion, and more mixed gains (Fe₂O₃, SO₃, LOI, As ± Ba–Sr), consistent with distal buffering and/or dilution of the reactive acidic fluid. A coupled P₂O₅–Sr enrichment in the alunite halo supports stabilization of aluminum phosphate–sulfate (APS) minerals during apatite breakdown under advanced-argillic conditions, offering an additional geochemical vector toward the hydrothermal center.

Finally, the mass-balance results demonstrate that fluid-driven addition and leaching dominate within the vuggy silica core and alunite halo, while distal kaolinitization indicates reduced mass transfer. These patterns offer quantitative criteria for identifying High Sulfidation influx zones and their associated Au enrichment at Tepeköy.

Keywords: Mass-balance calculation; alteration geochemistry; Tepeköy high sulfidation epithermal Au mineralization; Melendis volcanic suite; Central Türkiye