Fluid inclusion constraints on the origin of Au-Ag-base-metal mineralization in the Biga Peninsula, NW Turkey

The Turkish segment of the Tethyan Eurasian Metallogenic Belt, which extends from Western Europe through Anatolia to Iran, is currently being intensively prospected and contains a number of active mines. Western Anatolia and the Biga Peninsula in particular, contain a number of gold dominated deposits (Kısladag, Efemcukuru, Ovacık, Kartaldag, Muratdag, TV Tower) or basemetal deposits with lesser amounts of gold. There are a number of polymetallic deposits with different metal associations and ages in the Biga Peninsula such as epithermal Au-Ag, porphyry Au-Cu-Mo and epithermal PbZn-Cu-Au deposits. The main deposits in this area are Koru (0.5 Mt at 8% Pb, 2% Zn, 300 g/t Ag), Tesbihdere (2.8 Mt at 5.76 g/t Au), Arapucandere (4.0 Mt at 16.4% Pb, 12.1% Zn, 2.23% Cu, Au av. 4 g/t and Ag av. 260 g/t)Tepeoba (4.86 Mt at 0.32 % Cu, 0.046 % Mo), Kartaldag (153 Mt at 1.12 g/t Au), Halilaga (4.1 Mt, 11.01 g/t Au), Kirazli (4.2 Mt, 7.6 g/t Au), Madendag (8 Mt at 1.25 g/t Au) (MTA, 1993). The majority of the deposits, in northwest Turkey, are low to intermediate sulphidation Au + Ag deposits but there are a much smaller number that are exploited for Pb and Zn as galena and sphalerite. Epithermal deposits in western Turkey contain fluid inclusions with very large ranges of homogenization temperatures that cannot be explained by mixing with cooler meteoric fluids. Often salinity variations are at relatively constant temperature. L-V, V-L and boiling assemblages, in the ore veins, are observed to homogenize at temperatures c. 320-350°C thereby constraining the minimum depths to c. 2000m at hydrostatic pressure (which does not fit with the geological evidence) or c. 700m at lithostatic pressure. The range of homogenization temperatures for L-V inclusions, that are not obviously of a different generation, decrease to c. 250°C in most deposits. This temperature is exactly what would occur if the higher temperature, near lithostatic fluid, lost heat by adiabatic expansion as the pressure regime changed to near hydrostatic at the same depth of vein emplacement. In most instances boiling is not a major occurrence; therefore the pressure decrease from lithostatic to hydrostatic must have been gradual enough for cooling to occur more or less along the L-V curve. However, there are examples of where the pressure decrease was catastrophic as there are examples of “flashed fluids” that are present as inclusions that only contain a very low density vapour. This is only likely to occur when the pressure goes from lithostatic to sub-hydrostatic most likely due to movements caused by earthquakes. دومين همايش تخصصي و ملي كاربرد سيالات درگير در علوم زمين دانشگاه زنجان- 52 آبان ماه 6931 9 Mineralization in Biga Penincula primarily results from variations between lithostatic and hydrostatic pressure but there is evidence of “flashed fluids” from sudden sub-hydrostatic pressures resulting in adiabatic expansion and cooling of the ore fluid that is largely of magmatic origin, with little evidence supporting mixing with meteoric fluids.