Chemical Fingerprints of an Orogenic Gold System: Fluid–Rock Interaction and Metal Mobilization in the Akbaştepe Au–Ag Deposit (Bilecik, NW Türkiye)

The Akbaştepe Au–Ag mineralization is located in the Söğüt district (Bilecik, NW Türkiye), within the Sakarya Zone on the northern margin of the İzmir–Ankara–Erzincan Suture Zone. The deposit is hosted by greenschist-facies schists of the Nilüfer Formation and occurs as a steeply dipping, N70W-trending quartz vein system extending for approximately 2 km with an average thickness of ∼5 m. The geometry and orientation of the vein system indicate strong structural control related to late-stage orogenic deformation.

This study integrates ore petrography, alteration mineralogy, whole-rock geochemistry, and sulfur isotope data to constrain the role of fluid–rock interaction and metal mobilization during the formation of the Akbaştepe mineralization. Reflected-light microscopy reveals a multi-stage paragenesis dominated by pyrite, arsenopyrite, scheelite, Hg–Te minerals (coloradoite), and Au–Te phases, with native gold occurring both as free grains (15–95 µm) and as inclusions within fractured and oxidized pyrite. Multiple generations of pyrite indicate episodic fluid flow and repeated mineralizing events within structurally prepared zones.

Hydrothermal alteration is characterized by silicification, carbonatization, and Fe-oxidation, overprinting the primary greenschist assemblage. Alteration mineralogy records progressive fluid–rock interaction, marked by a systematic transition from chlorite-dominated greenschist facies toward smectite-, kaolinite-, and illite-bearing assemblages localized along mineralized and shear zones. The inverse relationship between chlorite and smectite reflects increasing chemical re-equilibration between hydrothermal fluids and reactive host rocks, emphasizing the role of alteration processes in controlling metal precipitation.

Whole-rock geochemical data show Au contents up to 10 ppm and consistently high Au/Ag ratios, reflecting gold-dominated mineralization. Gold exhibits strong positive correlations with As, Hg, W, and Sb, whereas base metal concentrations remain low. This elemental association is characteristic of orogenic gold systems and indicates efficient metal mobilization controlled by fluid chemistry and wall-rock interaction. The presence of scheelite and Hg–Te phases further supports a chemically reactive ore-forming system.

Sulfur isotope compositions of pyrite and arsenopyrite range between –2.1‰ and –8.8‰ (δ³⁴S), suggesting a sedimentary sulfur source, most likely related to devolatilization of subducted marine sediments. The close mineralogical association between gold and Fe-sulfides highlights the key role of sulfide precipitation during fluid–rock interaction.

The integration of ore petrography, geochemical signatures, and sulfur isotope data indicates that the Akbaştepe Au–Ag mineralization represents a structurally controlled orogenic gold system formed during crustal-scale fluid flow. Gold deposition was governed by chemical reactions between metamorphic fluids and reactive host rocks, leading to efficient metal precipitation along shear zones. These results provide new insights into orogenic gold metallogeny within the Sakarya Zone and contribute to a broader understanding of fluid–rock interaction processes in convergent tectonic environments.