| Περίληψη: | Hydrothermal fluid flow and associated mineralization in Syros Island at the Attico-Cycladic was studied comprehensively through microstructures, ore petrography and chemistry, fluid inclusions and both stable and radiogenic isotopes in order to classify the observed ore deposits, determine the pressure and temperature conditions during their formation and the possible source of the ore-bearing fluids. Fluid flow was controlled using N-S and NW trending fault s and channeled at permeability and lithological contrast interfaces between the Cyclades Blueschist Unit schists and the calcic or dolomitic marbles, as well as internally between the latter.
Two different types of mineralization were reported, i.e., metamorphozed and hydrothermal. The hydrothermal hypogene carbonate-replacement- and vein-type ores are composed of massive aggregates or disseminations of pyrite+sphalerite+chalcopyrite (stage I), followed by galena+tetrahedrite (stage II). Pyrite is deposited adjacent to the contacts of the schists and marbles, and galena further away from the contact. The vein-type ores comprise an older NE-trending set which contains fine-grained calcite, milky quartz, pyrite and sphalerite, and a younger NW-trending set of coarse-grained calcites, clear quartz, galena and cleophane (low XFeS%) that crosscut the former. Milky, clear quartz and calcite were deposited from low- to moderate-temperature (T = 129º to 229ºC) and low- to moderate-salinity (1.2 to 14.3 wt percent NaCl equivalent) ore fluids. Cleophane was formed at temperatures of 125º to 163ºC and salinity of 3.5 to 11.5 wt percent NaCl equivalent. Fluid inclusion study suggests that two boiling episodes have occurred, at ~175° and ~220oC, at Syros hydrothermal system.
Deposition of Syros hydrothermal ore deposits was controlled by structural discontinuities as a result of a crack-seal mechanism under nearly hydrostatic pressures. Syros ore-bearing fluids have experienced intense wall-rock interaction, leading their initial δ18ΟΗ2Ο values to equilibrate with the ones obtained from the host metamorphic rocks. The marbles of the CBU in Syros, during the wall-rock interaction have experienced decarbonation and infiltration of the magmatic fluids in the marbles by single-stage phase separation.
Thus, a magmatic signature is proposed for the Syros ore fluids related to carbonate-replacement and vein-type ore deposits. Temperatures of 198° to 222°C and 158° to 186°C, representing stages I and II, were calculated based on the pyrite-sphalerite, pyrite-chalcopyrite and galena-sphalerite isotopic pairs. Rb-Sr dating on sphalerite (stage I) suggests that the Syros veins were deposited at 9.93 ± 0.19 Ma, while Rb-Sr dating on gaunge minerals, i.e., calcite, quartz and muscovite suggests deposition at 9.40 ± 0.35 Ma.
The analytical results of our study were also compared to published results, as well as additional data, from Tinos, Mykonos and Serifos plutons and related vein-type ores in order to recognize geochemical similarities in the formation of the Syros ore deposits. In these ores, the mineralizing fluids of magmatic origin have mixed with more dilute, low temperature Miocene meteoric waters and were highly exchanged with a metasedimentary source. Lead due to hydrothermal circulation was remobilized by the magmatic ore-forming fluids from both the Basal Unit and Cycladic Blueschist Unit, however in Tinos and Syros the BU had greater contribution in respect to Serifos and Mykonos. Syros ores have almost identical 87Rb/86Sr and 87Sr/86Sr(i) values and plot closer to Tinos leucogranite, which is suggested as a possible source of Syros ore-bearing fluids. A common source for Re, Os and most possibly Fe, is also attributed to Tinos leucogranite for its possible satellite Syros ores in Azolimnos, Tourlos and Rozos (stage I-pyrite from Syros).
We have distinguished three major, different phase separation events, due to fluid immiscibility, which most probably occurred at ~175º±10°C, ~250º±10°C and ~325º±10°C. Phase separation episodes are considered to be the result of the resurgent, low-pressure retrograde boiling and CO2-effervescence. The calculated trapping pressures ranged from ~220 to ~380 bars, at ~250ºC and ~325ºC, where phase separation has occurred, which correspond to a depth of deposition ranging from ~1.0 to ~2.8 km. This implies that ore deposition of the Syros hydrothermal system was contemporaneous with tectonic uplift of ~1.7 km, at the central part of the Attico-Cycladic Metallogenic Massif.
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