Alteration and mineralization features of the Akcal epithermal gold deposit, Biga Peninsula, Western Anatolia, Turkey: evidence for low-sulfidation Au–Ag–Sb mineralization

The Akcal epithermal Au–Ag deposit, located on the Biga Peninsula, southwestern Turkey, is estimated to contain 32.5 million tonnes of mineralization at an average grade of 1.03 g t −1 Au, equivalent to 1.2 million ounces of gold. Epigenetic mineralization is hosted by the lower Miocene Sapci volcanic suite, a calc-alkaline subduction-related volcanic arc consisting mainly of andesitic and basaltic andesitic rocks, while the coeval Soma Formation is composed of sandstone, claystone, conglomerate, marl-limestone, siltstone and tuffite. Near the base of the volcanic pile, the Karakaya Complex is represented by limestone blocks of various sizes (from a few millimetres to several kilometres) within the Sapci volcanic suite. Short-wave infrared and X-ray diffraction analyses have shown that kaolinite, illite–smectite, montmorillonite, illite, beidellite, gypsum, vermiculite, quartz and chlorite are the most common alteration minerals at Akcal. The alteration mineralogy indicates that the temperature of hydrothermal fluids broadly varied from 150° to 230°C, with a mildly acidic pH range of 5.0–6.5; these conditions accompanied by a lack of alunite suggest low-sulfidation epithermal conditions for the formation of the Akcal epithermal Au–Ag deposit. Petrographic examination and microanalysis by scanning electron microscopy indicate that mineralization at Akcal consists of pyrite and arsenopyrite with lesser hematite, ilmenite, rutile and apatite. The occurrence of pyrite and hematite on cleavage planes of hydrothermal biotite is evidence of replacement of an earlier potassic alteration phase. Further petrographic examination of the basal limestone reveals replacement by sulfides displaying complex colloform intergrowths of pyrite and marcasite. The lack of skarn alteration and marble supports a carbonate-replacement origin for sulfides occurring in the basal limestone and is likely related to epithermal mineralization at Akcal. Geochemical results show that an Au–Ag–As–Sb ± Pb ± Zn signature is dominant across the volcanic-hosted epithermal mineralization and carbonate-replacement occurrences in the basal limestone; the lack of Cu mineralization (<92 ppm) is typical of low-sulfidation epithermal systems. Strong positive correlations between Au and Ag ( r ′ = 0.77), As (0.80) and Sb (0.86) indicate that gold is likely hosted as a sub-microscopic phase within pyrite, arsenian pyrite and arsenopyrite; visible gold (or free gold) and (or) electrum were not identified at Akcal. A model of fluid mixing involving shallow circulating meteoric waters with deeper magmatic-hydrothermal fluids is proposed for the genesis of gold mineralization at Akcal; a positive Au correlation with Mo ( r ′ = 0.74) and Mo concentrations of up to 582 ppm support the involvement of magmatic-hydrothermal fluids in the epithermal system. Boiling as a mechanism for gold precipitation is unlikely given palaeodepths of greater than 250 m (3.0–5.0 MPa) and model temperatures of 150° to 230°C. Mineralogical and textural evidence of boiling, such as adularia, bladed calcite and lattice-texture quartz, have not been observed at Akcal. However, the presence of a silica-rich blanket with chalcedony (at the Akcal surface) and having formed near the palaeo water table may indicate some involvement of CO 2 -rich steam-heated waters, which could suggest some influence from boiling at depth, although evidence of boiling has not been confirmed in samples from Akcal. Precipitation of gold during the fluid mixing process would have occurred through a shift to higher f O 2 conditions and destabilization Au–thiosulfide complexes. This shift towards higher f O 2 is supported by the occurrence of gypsum and hematite with gold mineralization at depth.