The Akcal Epithermal Deposit: An Example of Low-Sulfidation Gold Mineralization in Western Turkey

Abstract

The Akcal epithermal Au-Ag deposit is located on the Biga Peninsula, southwestern Turkey and is estimated to contain 32.5 million tonnes of mineralization at an average grade of 1.03 g/t Au, equivalent to 33.5 tonnes of gold. Epigenetic mineralization is hosted by the Lower Miocene Sapci volcanic suite, a calc-alkaline subduction-related volcanic arc consisting of mainly 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 (SWIR) and X-ray diffraction (XRD) analyses have shown that kaolinite, illite-smectite, montmorillonite, illite, beidellite, gypsum, vermiculite, quartz and chlorite are the most common alteration assemblages at Akcal. The alteration mineralogy indicates that the temperature of hydrothermal fluids varied from 150 ? to 230 ?C, with a mildly acidic pH range of 5.0 to 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 Au-Ag-As-Sb?Pb?Zn signatures are dominant at Akcal throughout the volcanic-hosted epithermal mineralization and the carbonate-replacement occurrences in the basal limestone; the lack of Cu mineralization (< 92 ppm) is diagnostic for 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 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 paleodepths of greater than 250 meters (3.0-5.0 MPa) and model temperatures of 150 ? - 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 Akcal surface) and having formed near the paleo water table may indicate some involvement of CO2 - rich steam-heated waters, which would suggest some boiling at depth. Precipitation of gold during the fluid mixing process would have occurred through a shift to higher fO2 conditions and destabilization Au-thiosulphide complexes. This shift towards higher fO2 is supported by the occurrence of gypsum and hematite with gold mineralization at depth.