Do fluid inclusions preserve δ18O values of hydrothermal fluids in epithermal systems over geological time? Evidence from paleo- and modern geothermal systems, Milos island, Aegean Sea

Stable isotope compositions of quartz (δ18Oquartz) and fluid inclusion waters (δ18OFI and δDFI) were analysed from Profitis Ilias, a low-sulphidation epithermal gold mineralisation deposit on Milos island, Greece, to establish if δ18OFI preserve a record of paleogeothermal processes. Previous studies show that mineralisation at Profitis Ilias resulted from extreme boiling and vaporisation with a zone located at approximately 430 m above sea level (asl) representing the transition between liquid- and vapor-dominated systems [Miner. Depos. 36 (2001) 32]. The deposit is also closely associated with an active geothermal system, whose waters have a well-characterised stable isotope geochemistry [Pflumio, C., Boulegue, J., Liakopoulos, A., Briqueu, L., 1991. Source, Transport and Deposition of Metals. Balkema, Rotterdam]. The samples were collected over an elevation interval of 440 m (210–650 m asl) to give information on the liquid and vapor dominated sections of the paleosystem. ta show systematic variations with sample elevation. Samples from the highest elevations (ca. 650 m asl) have the lightest δ18OFI (−7.3‰) and δDFI (−68.0‰) whilst the deepest (ca. 210 m asl) are isotopically heavier (δ18OFI, −0.3‰; δDFI, −19.0‰). Relative changes in δ18OFI closely parallel those in δDFI. δ18Oquartz shows an opposite trend, from the lightest values (+13.9‰) at the lowest elevations to the heaviest (+15.1‰) at the highest elevations. δ18OFI shows correlations with other parameters. For example, variable fluid inclusion homogenisation temperatures in the vapor-dominated part of the system correlate with a rapid shift in δDFI (−33.3‰ to −50.5‰) and δ18OFI (−4.1‰ to −6.2‰). Gold contents also increase in the same zone (up to 50 ppm Au). Comparable correlations in δ18Oquartz or δ18Ocalculated (estimated geothermal fluid from fluid inclusion homogenisation data) are absent. δ18Ocalculated are always 5–10‰ heavier than δ18OFI. Comparison with the present day geothermal field shows that δDFI and δ18OFI are similar. Isotope data for the modern system and fluid inclusion waters fall on linear trends subparalleling the meteoric water line and project towards seawater values. Numerical modelling favours kinetically controlled fractionation to explain differences in δ18Ocalculated and δ18Ofluid rather than diffusive posttrapping reequilibration. The evidence suggests that in low-temperature epithermal systems, δ18OFI may represent a better record of fluid process and the isotopic composition of the paleogeothermal fluid than temperature-corrected quartz data.