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 paleo-geothermal processes. Previous studies show that mineralisation at Profitis Ilias resulted from extreme boiling and vaporisation and a zone located at approximately 430 m asl represents the transition between a liquid- and vapour-dominated system [Mineral. Dep. 36 (2001) 43]. The deposit is also closely associated with an active geothermal system, whose waters have a well-characterised stable isotope geochemistry [Pagel and Leroy (1991) Source, transport and deposition of metals. Balkema, Rotterdam, 107–112]. The samples were collected over an elevation interval of 440 m (210 to 650 m asl) to give information on the liquid- and vapour-segments of the paleo-system. The data show systematic variations with sample elevation. Samples from the highest elevations (c. 650 m asl) have the lightest δ18OFI (–7.3 ‰) and δDFI (–68.0 ‰) whilst the deepest (c. 210 m asl) are isotopically heavier (δ18OFI –3.7 ‰; 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. δ18OFI show correlations with other parameters. For example, variable fluid inclusion homogenisation temperatures in the vapour-dominated part of the system, correlate with a rapid shift in δDFI (–33.3 to –50.5 ‰) and δ18OFI (–4.1 to –6.2 ‰) and gold contents also increase in the same zone (up to 50 ppm). Comparable correlations in δ18Oquartz or δ18Ocalculated (estimated geothermal fluid from fluid inclusion homogenisation data) are absent. δ18Ocalculated are always 5 to 10 ‰ heavier than δ18OFI. Comparison with the modern geothermal system shows that δDFI–δ18OFI are similar. Isotope data for the modern system and fluid inclusion waters fall on linear trends sub-paralleling 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 post-trapping equilibration. The evidence suggests, that in low-temperature epithermal systems, δ18OFI may represent a better record of fluid process and the isotopic composition of the geothermal fluid than temperature-corrected quartz data

Molecular, chemical and morphological evidence for hematite biogenicity at the Quaternary Cape Vani Mn-(Ba-Fe) deposit, Milos, Greece

Η γένεση ιζηματογενών πετρωμάτων πλούσιων σε σίδηρο, κυρίως αιματίτη, μέσω βιολογικών ή αβιοτικών διεργασιών, η κάποιου συνδυασμού αυτών, παραμένει άλυτο επιστημονικό πρόβλημα. Στην εργασία αυτή χρησιμοποιήσαμε έναν συνδυασμό αναλυτικών μεθόδων που συμπεριλαμβάνουν οπτική μικροσκοπία, ηλεκτρονική μικροσκοπία σάρωσης και μικροανάλυση (SEM-EDS), φασματοσκοπία Raman και φασματομετρία μάζας χρόνου πτήσης δευτερογενών ηλεκτρονίων (time-of-flight secondary ion mass spectrometry, TOF-SIMS), για να προσδιορίσουμε την χωρική κατανομή και την σύσταση εν δυνάμει γεωβιολογικών δεικτών (π.χ. μικροβιακά μικροαπολιθώματα, ιχνοστοιχεία, οργανικές ιοντικές ενώσεις) σε αιματιτικούς μικροστρωματόλιθους και μικροδομές από αιματίτη που μοιάζουν με oncolites (ονκόλιθους). Οι μικροδομές αυτές προσδιορίστηκαν σε υλικό πλούσιο σε σίδηρο που αποτελεί συνδετικό υλικό παράλιου κροκαλοπαγούς Τεταρτογενούς ηλικίας από την ευρύτερη περιοχή του ακρωτηρίου Βάνι, στη ΒΔ Μήλο. Ο προσδιορισμός μορφολογικών, χημικών και οργανικών μοριακών ιοντικών βιοδομών σε συνδυασμό με πιθανά μικροαπολιθώματα από αιματίτη, υποδηλώνουν μικροβιακή μεσολάβηση στον σχηματισμό των τελευταίων, και υποδεικνύουν ότι μικροστρωματόλιθοι από αιματίτη μπορεί να χρησιμοποιηθούν ως βιοδείκτες για τη ύπαρξη σιδηρούχων βίο- υλικών στη Γη, και για την ανίχνευση τους σε εξωγήινα υλικά. Η μελέτη των αιματιτικών μικροστρωματόλιθων του ακρωτηρίου Βάνι μπορεί να συμβάλει στην κατανόηση των λειτουργιών των μικροοργανισμών στην γένεση τόσο σύγχρονων όσο και παλαιών κοιτασμάτων σιδήρου.Many aspects of the biotic or abiotic origin of iron-rich sedimentary rocks consisting mainly of hematite, an important indicator for exobiology, remain unresolved. Here, we use combined optical microscopy, scanning electron microscopy and energy dispersive X-ray spectrometry (SEM-EDS), Raman spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS), to image the spatial distribution, and determine the composition, of potential biogenic markers (microbial microfossils, trace elements, organic ion species) in hematitemicrostromatolites and oncolite-like microstructures. These structures are identified in iron-rich material cementing a Quaternary fossil-beach conglomerate deposit in the Cape Vani area, NW Milos Island, Greece. The combined detection of morphological, chemical, and molecular organic-ion, biomarkers closely associated with possible hematite microfossils within microstromatolite laminae, strongly supports microbial mediation for their formation, and indicates that hematitic microlamination may be used as a biosignature for Fe-rich biomats on Earth and for their detection in extra-terrestrial materials. The Cape Vani hematitemicrostromatolites may contribute to growing our understanding of the function of microorganisms in the genesis of modern and ancient Fe deposits

Fluid inclusion thermometric and stable isotope (δ18O-δD) evidence for “cryptic boiling” in the Profitis Ilias epithermal gold deposit, Milos : a potential exploration tool for epithermal gold

“Cryptic boiling” has been disclosed throughout the vertical extent of the Profitis Ilias epithermal system (~450 m) by assessing systematic variations in fluid inclusion and stable isotope (δ18O-δD) data in relation to elevation. Highest gold grades (Au: 2-57.9 g/t) are preferentially concentrated in the upper 200-250 m of the system in a “steam-dominated zone” that is separated from a lower “liquid-dominated zone” (Au < 2 g/t). This distribution of gold may be due to colloidal transport in a boiling epithermal system. The identification of the base of the “steam-dominated zone” using fluid inclusion and stable isotope data may potentially guide exploration drilling to depths with high-grade gold ore in productive epithermal systems

Fossilized microorganisms preserved as fluid inclusions in epithermal veins, Vani Mn-Ba deposit, Milos Island, Greece

Fossilized microorganisms preserved as fluid inclusions are found in barite silica-Mn oxide veins in the marine rift basin-related Quaternary Mn-Ba deposit of Vani, Milos . Basin fill consists of 35-50 m thick sequence of glauconitic sediments sandwiched between volcaniclastic sandy tuffs, and, bedding-parallel barite Mn oxide( silica) horizons, pebble horizons, and massive gravel. Exhalative barite-rich deposits characteristic of sea-floor venting, such as white smoker(sulphate) structures in glauconitic sediments, feeder veins, bedding-conformable horizons, and extensive microbial mat- related structures in sandy tuffs, were recognized. The feeder veins host the microfossils and consist chiefly of banded barite and minor colloform quartz, Fe-oxyhydroxides, and hollandite-group minerals and MnO2 phases, and display epithermal textures characteristic of open-space precipitation. Curvilinear, branched filamentous microfossils with distinct segmentation of septa and a turgid appearance of knob-like outgrowths occur associated with spheroidal spore-like microfossils and small twisted microstructures. Both filamentous and spheroidal microstructures are filled with aqueous (liquid + vapour) and/or hydrocarbon phases. Oil and solid hydrocarbons in the fluid inclusions may represent decomposed biological material. Chitin was detected by the pigment Wheat Germ Agglutinin conjugated with Fluorescein Isothiocyanate (WGA-FITC) in some of the microfossils, indicating that they are fossilized fungi; a fungal interpretation is further supported by microfossil morphology. Smaller, often twisted filamentous microfossils with a simpler morphology in which chitin was not detected probably represent fossilized prokaryotes and, if so, prokaryotes and eukaryotes co-existed in the geothermal system of Vani. Fluid inclusion microthermometry shows that microfossils were trapped at temperatures of ~100°C in boiling water, probably evolved seawater. Preservation of microfossils occurred at shallow sub-marine conditions of <10 m depth. Our results show that fluid inclusions may contain valuable palaeobiological information and can be used both for establishing biogenicity but also for the reconstruction of the palaeoenvironment of fossilized microorganisms

The Profitis Ilias deposit, Milos island, Greece : a case study of boiling in an epithermal system recorded by stable isotope and fluid inclusion data

Stable isotope (δD and δ18O) and microthermometric data are presented for the Profitis Ilias adularia-sericite-type epithermal gold deposit on Milos Island, Greece. The deposit was discovered by Midas Enterprise SA, and estimated reserves are 5 million tonnes grading at 4.4 g/t Au and 43 ppm Ag. Microther-mometric data show that the system boiled. Stable isotope ratios were determined on inclusion-fluids (δD and δ18O) and vein quartz (δ18O). The inclusion-fluid data show fractionation trends consistent with single–stage steam separation and are broadly comparable to the currently active Milos geothermal system. The trends and comparisons in the fluid-mineral data are more difficult to interpret. This suggests that data derived by ana-lysing both δD and δ18O on inclusion-fluids can provide a better framework for interpreting fluid processes in epithermal systems than current methodologies based on calculating δ18O from fluid-quartz equilibria

Evidence of Mn-oxide biomineralization, Vani Mn deposit, Milos, Greece

We present evidence that precipitation of primary Mn-oxide minerals in the Vani volcanic hosted hybrid epithermal-VMS-type Mn-oxide and barite deposit was in part biogenically mediated. Manganese-oxides pseudomorphically replace small (1-5 mu m) spherical cell-like structures, and branching filamentous constructions (< 60 mu m long) probably representing manganese oxidizing bacteria. In addition, silicified consortia of spherical (5-10 mu m), filamentous, sheathed, septate and spiral (similar to 50-200 mu m) fossilized bacteria, proposed to represent photosynthetic thermophilic cyanobacteria, were found in quartz paragenetically related to the Mn ore. Fluid inclusions indicate formation temperatures around 100 degrees C. XRD and EMP analyses suggest X-ray-amorphous hollandite-group like Mn-oxide phases, and poorly crystalline todorokite and vernadite. These findings suggest a biological link between bacterial and mineralization processes

Marble-hosted submicroscopic gold mineralization at asimotrypes area, mount pangeon, Southern Rhodope core complex, Greece

Gold mineralization at Asimotrypes, Mount Pangeon, Greece, occurs within amphibolite facies rocks of the Southern Rhodope Core Complex, one of the largest metamorphic core complexes in the world. Exhumation of the complex resulted from middle Eocene to middle Miocene northeast-southwest-oriented extension in the northern Aegean and was controlled by the Kerdylion detachment zone. Host rocks are mylonitic, impure dolomite marbles of marine provenance (δ13C = 1.9 - 2.9%), which are intercalated with paramica schists, and amphibolites, and intruded by early Miocene syntectonic granitoids. In the Asimotrypes area, metamorphic rocks and granitoids exhibit flat mylonite-type ductile fabrics with consistent top-to-the-southwest sense of shear, as does the entire Complex. © 2011 Society of Economic Geologists, Inc