ARTICLE Stephanos P. Kilias á Jon Naden á Ioannis Cheliotis Thomas J. Shepherd á Heleni Constandinidou John Crossing á Ioannis Simos Epithermal gold mineralisation in the active Aegean Volcanic Arc: the Pro®tis Ilias deposit, Milos Island, Greece Received: 24 February 2000 /Accepted: 15 July 2000 Abstract The Pro®tis Ilias gold deposit, located on the western part of Milos Island, Greece, is the ®rst epi- thermal gold deposit discovered in the Pliocene±Pleisto- cene Aegean volcanic arc. Estimated ore reserves are 5 million tonnes grading 4.4 g/tonne Au and 43 g/tonne Ag. The deposit is closely associated 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 depths of at least 300 m. The miner- alisation occurs in three stages and is hosted by 3.5± 2.5 Ma old silici®ed and sericitised rhyolitic lapilli-tus and ignimbrites. It consists of pyrite, galena, chalcopy- rite, electrum and native gold. Additionally, adularia occurs with quartz mainly in veins. Homogenisation temperatures of primary liquid-rich inclusions vary from 145 to 399 °C for the ore stage, and 112 to 263 °Cforthe 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 post-ore stage, respectively. Rare vapour-rich inclusions in ore stage quartz homog- enise between 368 and 399 °C and estimates of eutectic melting )25 to )38 °C) indicate the presence of Ca and Mg in the ore ¯uids. Sample elevation versus ¯uid in- clusion T h ±salinity relationships show 1) a high-salinity trend, where moderate-temperature 300±250 °C) and moderate-salinity brines 3 wt% NaCl equiv.) trend to high-salinity up to 15 wt% NaCl equiv.) ¯uids with lower 25±50 °C) homogenisation temperatures, and 2) a high-T h trend where moderate-salinity and moder- ate-temperature brines 200±250 °C; 3 wt% NaCl equiv.) develop into low-salinity <1 wt% NaCl equiv.), high-temperature >350 °C)¯uids.Thesetrendsarebest explained by extreme boiling and vapourisation phe- nomena between 200 and 250 °C. The 430±450 m asl metres above sea level) level marks the transition be- tween a lower liquid-dominated segment of the system where only the 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 close spatial association between mineable gold grades and the upper segment of the system. Depth-to-boiling curves suggest that the paleo-surface was 200 m above the present summit of Pro®tis Ilias. Comparison of the mineralisation and ¯uid geochemistry at Pro®tis Ilias with that of the nearby modern geothermal system in- dicates that the processes of metal mineralisation have probably been continuous since the Late Pliocene. Introduction The purpose of this work is to present the ®rst published geological, mineralogical and ¯uid inclusion data of a productive gold-bearing epithermal system in the Plio- cene±Quaternary south Aegean active volcanic arc SAAVA). We have attempted to decipher the mecha- nisms of gold deposition and to address the ¯uid sour- ces) by comparing the ancient geothermal system with a nearby modern equivalent. Mineralium Deposita 2001) 36: 32±44 Ó Springer-Verlag 2001 Editorial handling: P. Lattanzi S. P. Kilias &) National University of Athens, Department of Geology, Section of Economic Geology and Geochemistry, Panepistimiopolis, Ano Ilisia, 157 84 Athens, Greece e-mail: kilias@geol.uoa.gr J. Naden á T. J. Shepherd British Geological Survey, Keyworth, Notts NG12 5GG, UK I. Cheliotis á H. Constandinidou Institute of Geology and Mineral Exploration, 70 Mesoghion St., 11527 Athens, Greece J. Crossing RGC Exploration, P.O. Box 322, Victoria Park, WA 6100, Australia I. Simos 1 Midas S. A., Adamas, GR 84801, Milos, Greece Present address: 1 Silver & Baryte Ores Mining Co, Amerikis 21A, 106 72 Athens, Greece