The Profitis Ilias deposit, Milos island, Greece : a case study of boiling in an epithermal system recorded by stable isotope and fluid inclusion data

Stable isotope (δD and δ18O) and microthermometric data are presented for the Profitis Ilias adularia-sericite-type epithermal gold deposit on Milos Island, Greece. The deposit was discovered by Midas Enterprise SA, and estimated reserves are 5 million tonnes grading at 4.4 g/t Au and 43 ppm Ag. Microther-mometric data show that the system boiled. Stable isotope ratios were determined on inclusion-fluids (δD and δ18O) and vein quartz (δ18O). The inclusion-fluid data show fractionation trends consistent with single–stage steam separation and are broadly comparable to the currently active Milos geothermal system. The trends and comparisons in the fluid-mineral data are more difficult to interpret. This suggests that data derived by ana-lysing both δD and δ18O on inclusion-fluids can provide a better framework for interpreting fluid processes in epithermal systems than current methodologies based on calculating δ18O from fluid-quartz equilibria.