Epithermal gold mineralisation in the active Aegean volcanic arc: the Profitis Ilias deposit, Milos Island, Greece

The Profitis Ilias gold deposit, located on the western part of Miles Islan d, Greece, is the first epithermal gold deposit discovered in the Pliocene- Pleistocene Aegean volcanic are. Estimated ore reserves are 5 million tonne s grading 4.4 g/tonne Au and 43 g/tonne Ag. The deposit is closely associat ed with a horst and graben structure, and occurs in a series of steep inter connected crustiform-banded quartz veins up to 3 m wide, extending to depth s of at least 300 m. The mineralisation occurs in three stages and is hoste d by 3.5-2.5 Ma old silicified and sericitised rhyolitic lapilli-tuffs and ignimbrites. It consists of pyrite, galena, chalcopyrite, electrum and nati ve gold. Additionally, adularia occurs with quartz mainly in veins. Homogen isation temperatures of primary liquid-rich inclusions vary from 145 to 399 degreesC for the ore stage, and 112 to 263 degreesC for the post-ore stage . Salinities range between 0.1 and 11.4 wt% NaCl equiv. and 0.93 to 8.5 wt% NaCl equiv. for the ore stage and the pest-ore stage, respectively. Rare v apour-rich inclusions in ore stage quartz homogenise between 368 and 399 de greesC and estimates of eutectic melting (-25 to -38 degreesC) indicate the presence of Ca and Mg in the ore fluids. Sample elevation versus fluid inc lusion T-h-salinity relationships show (1) a high-salinity trend, where mod erate-temperature (300-250 degreesC) and moderate-salinity brines (similar to3 wt% NaCl equiv.) trend to high-salinity (up to 15 wt% NaCl equiv.) flui ds with lower (similar to 25-50 degreesC) homogenisation temperatures, and (2) a high-T-h trend where moderate-salinity and moderate-temperature brine s (200-250 degreesC; 3 wt% NaCl equiv.) develop into low-salinity(< 1 wt% N aCl equiv.), high-temperature(> 350 degreesC) fluids. These trends are best explained by extreme boiling and vapourisation phenomena between 200 and 2 50 degreesC. The 430-450 m asl (metres above sea level) level marks the tra nsition between a lower liquid-dominated segment of the system where only t he steep high-salinity trend is seen, and an upper vapour-dominated segment where the high-T-h trend or a combination of both are seen. There is a clo se spatial association between mineable gold grades and the upper segment o f the system. Depth-to-boiling curves suggest that the paleo-surface was si milar to 200 m above the present summit of Profitis Ilias. Comparison of th e mineralisation and fluid geochemistry at Profitis Ilias with that of the nearby modern geothermal system indicates that the processes of metal miner alisation have probably been continuous since the Late Pliocene.