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

Abstract

Stable isotope compositions of quartz ( δ 18O quartz) and fluid inclusion waters ( δ 18O FI and δD FI) were analysed from Profitis Ilias, a low-sulphidation epithermal gold mineralisation deposit on Milos island, Greece, to establish if δ 18O FI 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. The data show systematic variations with sample elevation. Samples from the highest elevations (ca. 650 m asl) have the lightest δ 18O FI (−7.3‰) and δD FI (−68.0‰) whilst the deepest (ca. 210 m asl) are isotopically heavier ( δ 18O FI, −0.3‰; δD FI, −19.0‰). Relative changes in δ 18O FI closely parallel those in δD FI. δ 18O quartz shows an opposite trend, from the lightest values (+13.9‰) at the lowest elevations to the heaviest (+15.1‰) at the highest elevations. δ 18O FI 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 δD FI (−33.3‰ to −50.5‰) and δ 18O FI (−4.1‰ to −6.2‰). Gold contents also increase in the same zone (up to 50 ppm Au). Comparable correlations in δ 18O quartz or δ 18O calculated (estimated geothermal fluid from fluid inclusion homogenisation data) are absent. δ 18O calculated are always 5–10‰ heavier than δ 18O FI. Comparison with the present day geothermal field shows that δD FI and δ 18O FI 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 δ 18O calculated and δ 18O fluid rather than diffusive posttrapping reequilibration. The evidence suggests that in low-temperature epithermal systems, δ 18O FI may represent a better record of fluid process and the isotopic composition of the paleogeothermal fluid than temperature-corrected quartz data.