75 research outputs found
A cervelleite-like mineral and other Ag-Cu-Te-S minerals [Ag2CuTeS and (Ag,Cu)2TeS] in gold-bearing veins in metamorphic rocks of the Cycladic Blueschist Unit, Kallianou, Evia Island, Greece
No full textA cervelleite-like mineral, two unnamed silver
sulfotellurides in the system Ag-Cu-Te-S [Ag2CuTeS,
(Ag,Cu)2TeS], Te-rich polybasite and cadmian tetrahedrite
occur in gold-bearing quartz veins in metapelites and faults
within brecciated marbles of the Cycladic Blueschist Unit
in the Kallianou area (southern Evia Island, Greece). The
quartz veins and faults are discordant to syn-metamorphic
structures and formed during ductile to brittle deformation in
the final stages of exhumation of the Styra Nappe extrusion
wedge (~21 Ma). Te-rich polybasite (up to 7.4 wt. % Te),
cadmian tetrahedrite (up to 12.4 wt. % Cd), together with
electrum (23–54 wt. % Ag) and the sulfotellurides, are the
main silver carriers in the mineralization. The two unnamed
sulfotellurides, Ag2CuTeS and (Ag,Cu)2TeS are believed to
be new quaternary minerals in the system Ag-Cu-Te-S.
These minerals and the cervelleite-like phase could have
exsolved from galena during cooling (below 200°C). Initial
temperatures for the formation of the sulfotellurides, in the
form of hessite-intermediate solid solution, at Kallianou may
be up to 300°C under logfS2 values between~ −11.5 to −8.3,
and logfTe2 from~ −14.8 to −7.8. The values of logfTe2 and
logfS2 during re-equilibration (at ~200°C) were constrained
to −19.5 to −15.2 and to −15.8 to −11.5 respectively
First occurrence of gasparite-(Ce), chernovite-(Y) and retzian-(Ce) in Greece: mineral chemistry and implications for REE enrichment in the Varnavas Mn-oxide ore
No full textThis work describes the mineral chemistry and mode of occurrence of a complex REE arsenate-phosphate
mineral association, namely gasparite-(Ce) Ce(AsO4), chernovite-(Y) Y(AsO4), retzian-(Ce),
Mn2REE(OH)4(AsO4), as well as the phosphates, monazite-(Ce), Ce(PO4). The association is reported from
the metamorphosed Mn-oxide mineralization within a Triassic volcaniclastic unit from Varnavas, NE Attica
(Greece), part of the Attic-Cycladic Blueschist Unit. The REE minerals were investigated using electron
probe microanalysis (WDS) and scanning electron microscopy.
The Varnavas Mn ores have lenticular or banded textures; the ore bodies, only a few meters wide, are
enclosed in piemontite-schists similar to those described from the Alps in Switzerland, Italy, and Austria
(Stouraiti et al., 2022; Pagliaro et al., 2022). Gasparite-(Ce) is a common mineral hosting REE in the ore;
it occurs as minute isometric grains up to 15 μm in size associated with chernovite-(Y) and retzian-(Ce),
enclosed in todorokite-pyrolusite-hollandite ore. The Mn-rich rocks are also profoundly enriched in arsenic
reaching concentrations of 2000 ppm. The geochemical data of the previously mentioned phases demonstrate
a P-As substitution between monazite-(Ce) and gasparite-(Ce), as well as for the pair xenotime-(Y)
and chernovite-(Y) (Pagliaro et al., 2022). The above suggest that both monazite-(Ce) and xenotime-(Y)
reacted with As-bearing hydrothermal fluids that converted them to arsenates (Ondrejka et al., 2007).
The presence of ardennite-(As) in rocks surrounding the Mn-ore indicates the deposition of As under
strongly oxidizing conditions. Notably, all the studied REE-minerals display a pronounced positive Ceanomaly
in the normalized REE patterns, which is known to result from oxidation of Ce on Mn-oxides.
The inclusions of the REE-arsenates in the Mn-ore and the existence of the monazite and xenotime in the
adjacent Mn-rich host rock, may indicate a short fluid transport of leached REE from the REE-phosphate
minerals and oxidative deposition in the local Mn-oxide accumulations
Mineralogy, geochemistry and C-O isotope study of the iron oxide deposit at Grammatiko, NE Attica, Greece: ore genesis in an extensional setting
No full textThe Grammatiko Iron oxide deposit (NE Attica region, Greece) is one of the economically most significant iron deposits in the Attic-Cycladic metallogenic province which was extensively mined until 1912. Mineralization is hosted in marbles and developed in the form of replacement bodies. These bodies are structurally controlled and follow large subvertical fracture zones with a NNE-trending strike that are associated with the Late Miocene extensional tectonic regime in the region. The mineral paragenesis consists mainly of Fe-Mn-Ba minerals including hematite, Mn-goethite, pyrolusite, hollandite, barite as well as traces of sulfides (e.g. galena). Gangue minerals are mostly calcite, ankerite and minor quartz and muscovite. Supergene minerals include limonite and Fe-Mn and Zn- oxy-hydroxides. Bulk ore analyses show concentrations of Mn up to 3 wt% and high contents of Pb and Cu. Microscopic observations, textural relationships and detailed SEM/EDS analyses showed minor contents of sulfur (<1 wt %) in some coarse grains with tetrahedral morphologies possibly indicating hematite formation after primary pyrite. Carbon and oxygen isotope analyses of carbonates collected along a traverse extending from the ore-bearing zone to the unaltered/non-mineralized marble show δ13C values between −10.5 and −4.5‰ and δ18O values from 20.9 to 24‰. For the non-mineralized marble host rock, δ13C values range from 2.2 to 2.8‰ with δ18O values between 26 and 30.3‰, respectively. The isotopic composition records the variable degree of hydrothermal alteration of the marble host rock as well as the characteristics of the ore-forming fluid. Unmineralized marbles exhibit a marine signature whereas vein calcite in the mineralized parts reflects a contribution of isotopically light carbon, likely from oxidized organic matter. In summary, δ13C and δ18O values from the Grammatiko mineralized carbonates record a complex interaction between marine carbonate and surface (meteoric) fluid
Oscillatory Zoned Gersdorffite and the Ni-Bi-Au Association at Clemence Mine and Km 3 Locality, Lavrion District, Greece
No full textVein-type Pb-Ni-Bi-Au-Ag mineralization at Clemence mine in Kamariza and the Km 3 locality in the Lavrion area was synchronous with the intrusion of a Miocene granodiorite body and related felsic and mafic dykes and sills within marbles and schists that constitute the footwall of the Western Cycladic Detachment System. The Ni-Bi-Au association at Clemence mine consists of initial deposition of pyrite and arsenopyrite followed by an intergrowth of native gold-bismuthinite and oscillatory zoned gersdorffite. Oscillatory zoning in gersdorffite is related to variable As, Ni and Fe contents, indicating fluctuation of arsenic and sulfur fugacity in the hydrothermal fluid. A late evolution towards higher sulfur fugacity in the mineralization is evident by the deposition of chalcopyrite, tennantite, enargite and galena rimming gersdorffite. At the Km 3 locality, the observed Ni sulfides and sulfarsenides, vaesite, millerite, ullmannite and polydymite, are enclosed in gersdorffite and/or galena. The gersdorffite is homogenous and contain less Fe (up to 2 wt %) than that from the Clemence mine (up to 9 wt %). Bulk analyses for Clemence ore reveal Au and Ag grades exceeding both 100 g/t, Pb and Zn > 1 wt %, Ni up to 9700 ppm, Co up to 118 ppm, Sn > 100 ppm and Bi > 2000 ppm. The Km 3 mineralization is enriched in Mo (up to 36 ppm), Ni (>1 wt %) and Co (up to 1290 ppm). Despite local variations, both districts are characterized by a common fluid evolution similar to the broad Lavrion area
The Controlled-source Audio-frequency Magnetotellurics (CSAMT/AMT) method tested in the search for deep Fe-Skarn mineralization at Kimmeria, Xanthi, N. Greece
No full textThe CSAMT/AMT method was used for the first time in Greece by HSGME and was
field-tested on the ongoing exploration carried out at the Kimmeria Public Mining
Area. The skarn mineralization (Fe+Cu±W±Bi) is related to the Xanthi pluton and
develops along the contact of the granodiorite and the adjacent metamorphic rocks.
In the magnetotelluric methods (MT), electromagnetic fields are utilized to investigate
the electrical properties of the earth and to determine resistivity structures. The
electromagnetic fields can be of natural origin (AMT) or created by artificial sources
(CSAMT). The AMT fields result from lightning discharges (1 Hz-10 kHz) and the CSAMT
far field is created by alternating current run through a grounded cable at frequencies
between 0.25 Hz-8 kHz at a large distance (6.5 Km) from the investigation area.
The CSAMT survey was conducted in the eastern part, targeting a skarn zone with Fe-
Cu mineralization in the vicinity of the old mine. The results show a highly conductive
body with resistivity values of 5-20 Ohm*m, dipping under the granodiorite, at a depth
of 200 m, which is probably interpreted as a mineralized skarn zone. The investigation
depth of the method reached 800 m. The AMT survey conducted over a skarn zone in
the western sector of the area showed a good agreement with IP data with a
succession of conductive layers from 40-200 m depth with resistivities of 10-25
Ohm.m and probable correlation to the mineralized zones. The methods confirmed
their advantages in terms of investigating at depth and resolution
The White Marbles of the Tomb of Christ in Jerusalem: Characterization and Provenance
No full textIn this work, samples of the white marbles enclosing the Tomb of Christ, as well as samples from the interior marble facades of the Holy Aedicule structure surrounding the Tomb of Christ in the Church of Resurrection in Jerusalem, are investigated using petrographic and isotopic analysis. The aim is to characterize the marble samples and investigate their provenance. The results demonstrate that all examined marble samples originate from Proconnesos (Marmara island), and can be attributed to the so-called Proconnesos-1 variety. Published maximum grain size (MGS) and isotopic (δ18O and δ13C) values of Proconessos quarries are compared with the respective values displayed by the marble samples of the Holy Aedicule, aiming to achieve—to a certain degree—intra-site discriminations. A number of ancient quarries are excluded through this double parameter criterion as sources for the examined Holy Aedicule marbles. The discussion of petrographic and isotopic results in relation to historical testimonies and previously published archaeometry results, regarding the mortars of the Holy Aedicule, reveal that Proconnesos marble was the material of choice used at different construction phases of the Holy Aedicule, from the time of Constantine the Great and throughout the centuries, both for the cladding of the Holy Tomb and the interior facings of the Tomb Chamber and the Chapel of the Angel
Moganite occurrence in quartz varieties (Chalcedony and Carnelian) from Evros Region, NE Greece
No full textExtended Abstract
Samples of gem varieties of quartz from the Evros region in north-eastern Greece were
spectroscopically analysed to determine their micro- to cryptocrystalline mineralogical properties
using a Renishaw inVia micro-Raman microscope. The micro-Raman instrument provides point-andshoot
Raman capabilities with a spatial resolution of 1 μm integrated into a standard microscope. The
frequency shifts were calibrated with an internal Si reference sample.
Samples analysed included different types of quartz, such as chalcedony and carnelian. During the
micro-Raman analysis, the samples of chalcedony and carnelian showed different ratios of
quartz/moganite, indicating heterogeneous formation conditions. This observation indicates that
nanocrystalline to microcrystalline moganite is present in the quartz [1].
The bands at 465 (Si-O bending vibrations), 209 and 129 cm-1 (lattice vibrations) are the
characteristic bands of α-quartz [2]. The peak at 503 cm-1 corresponds to the symmetrical stretching
vibrations of the Si-O bonds in moganite. The proportion of moganite in the chalcedony was I503/I465
= ~25%, while it was particularly high in the carnelian samples with I503/I465 = ~35%. Both samples
show a weak band at 395 cm-1, which could indicate the presence of the Fe+3 - O vibrational mode,
possibly due to the colorant substance [3]. The peaks at 2851 and 2930 cm-1, which only occur in the
carnelian sample, correspond to the OH- vibrational modes. Τhe presence of moganite in SiO2 varieties has been interpreted as an indication of their formation
age. According to Lee et al., [4], the presence of moganite suggests that the SiO2 varieties are younger
than -100Ma. This fact is consistent with the available geochronological data for the formations
hosting the studied SiO2 varieties, which indicate an Oligocene age [5,6]. This is evidenced by the
transformation of thermodynamically unstable moganite into quartz, or by the dissolution process of
moganite during silica-water interactions at ambient pressure [6-9]. Moganite also exhibits a
relatively intense band at 463 cm–1 that overlaps with the main 465 cm–1 mode of quartz at 465 cm–1 while the 503 cm–1 peak of moganite shows no interference [10].
Moganite is a low-temperature precipitate that coexists with fine-grained quartz [4]. In Schmidt et
al. [9], the peak at 503 cm-1, of moganite is described as a temperature dependent vibration band that
tends to be shifted to lower Raman shifts at higher temperatures. The moganite-specific Raman band
was found exactly at a wavenumber of 503 cm-1 in the samples analysed in this study. Moganite
formation is associated with the presence of silanol in hydrothermal fluids. This is evident from our
Raman spectroscopy results, which show a proportion of quartz/moganite. In addition, Caggiani et
al., [11] suggest that a peak at 504 cm−1 also indicates the presence of silanol. Furthermore, the
occurrence of peaks at wavenumbers 2851 and 2930 cm-1 suggests the presence of OH- within the
quartz (var. carnelian) crystal structure. These observations not only confirm the presence of moganite
by the Si-O peak at 503 cm-1, but also indicate the presence of Si-OH- resulting from the hydrothermal
process, which requires silica-rich fluids known as silanol [12]. Lee et al., [4] found that carnelian is
formed mainly from a mixture of higher-temperature hydrothermal fluids and shallower, lowertemperature,
meteoric water. These were formed in Tertiary magmatic-hydrothermal environments
throughout Greece, more specifically in hydrothermally altered igneous rocks that host several silica
varieties, most of which yield gem- or semi-precious quality material [13]. The results presented here
correspond to the same statement confirmed by micro-Raman analysis. In addition, stability studies
of moganite during thermal treatment show that moganite disappears at high temperatures [11].
Similarly, moganite is observed as an intermediate phase in the diagenesis of permineralised fossil
wood [14], suggesting a hydrothermal influence on the rock at low temperatures. Based on all the
above, moganite could indicate a low-temperature precipitating mineral coexisting in relation to α-
quartz
Conditions de transport des métaux de base (Cu, Pb, Zn), critiques (Mo, Re, Ga, Ge, Te) et précieux (Au, Ag) du systéme porphyre-épithermal de Pagoni Rachi (NE Gréce)
No full textLes gisements de type porphyre-épithermal représentent prés de 75% du Cu, 95% du Mo,
et prés de 20% de l'Au exploités dans le monde. Nombre de ces gisements se concentre au
sein de la Ceinture Méetallogénique Téthysienne et notamment dans la région SE des Balkans-Rhodopes. Le NE de la Gréce préesente une occurrence de ces gisements parmi les plus riches en métaux accessoires dans le monde. Ceci rend cette région propice pour la prospection et la compréhension des processus d'enrichissements dans ce type de systéme métallogénique associé a un contexte géodynamique particulier - post-collision et amincissement de la croute continentale.
Le site de Pagoni Rachi présente une granodiorite altérée encaissant un stockwork minéralisé en métaux de base (Cu, Pb, Zn), précieux (Au, Ag) et critiques (Mo, Re, Ga, Ge, Te),
caractéristique des systémes porphyres et épithermaux. La présence de molybdénite enrichie en Re (> 4.7 %wt) avec la présence de rhéniite (ReS2) en plus du cortége métallique, rend cette minraléisation atypique. Son étude permettra de mieux contraindre les conditions du transport hydrothermal notamment des métaux critiques (Mo, Re, Ge, Ga, Te). L'étude de terrain et pétrographique indique une succession de veines caractéristique des systémes porphyres de type : A/M a quartz-hématite-pyrite/magnétite-hématite, B a quartz- chalcopyrite-molybdénite-rhéniite, D a pyrite, E a quartz-calcite-sphalérite-galéne associées a un systéme épithermal tardif. L'imagerie des veines de quartz par cathodoluminescence révéle des réouvertures de veines, une croissance oscillatoire des cristaux de quartz, ainsi que des textures de recristallisation dynamique témoignant d'une transition progressive d'un domaine de déformation ductile a cassant. L'étude des inclusions
fluides dans le quartz indique une circulation continue de fluides pendant la cristallisation et la recristallisation du quartz, marquépar une ébullition vers 420C associée a la formation de la minéralisation a Cu, Mo, et Re, puis par une dilution par des fluides de surface associé a l'evénement épithermal a métaux critiques. Ce scénario sera testé par des analyses isotopiques par SIMS de l'oxygéne sur quartz et calcite, ainsi que par l'analyse des éléments traces dans le quartz et les inclusions fluides
Manganese metallogenesis in the Hellenic arc: Case studies from a Triassic rift-related volcaniclastic succession of the Cycladic Blueschist Unit, Greece
No full textThe Triassic volcaniclastic sequence in the Cycladic Blueschist Unit (CBU) of Greece is known to contain several occurrences of metamorphosed Mn mineralization. Despite previous literature reports, a comprehensive and holistic review of the origin of these occurrences is hitherto lacking. Here we revisit the Mn metallogenic system of the CBU through a comprehensive study of Mn mineralisation at Varnavas area, Northern Attica, and its correlation with a similar occurrence at Mparades Hill, Andros island. Manganese mineralogy at both localities is manifested in a typical high-P metamorphic silicate assemblage dominated by piemontite, spessartine garnet, and minor pyroxmangite (rhodonite). Whereas at Andros Mn-rich subdomains contain brecciated braunite micronodules, preservation of tetravalent Mn oxides in a similar nodular form is documented from Varnavas, comprising dominant todorokite along with lesser hollandite, pyrolusite, and minor Mn-bearing hematite. The contrasting Mn oxide mineralogy and comparable textural characteristics at the two sites are tentatively interpreted as the result of locally incomplete reduction of precursor Mn(IV) phases during metamorphism. Bulk Mn concentrations of the studied materials generally do not exceed the value of 10 wt% Mn, unless small-scale subsampling of Mn-rich domains is undertaken. Key geochemical characteristics of the Mn-rich rocks include low transition metal concentrations (sum of Co + Ni + Cu + Zn between 0.02 and 0.06 wt%); positive-sloping, PAAS-normalised REE spidergrams at a maximum Ndsn/Ybsn ratio of 0.3; positive Ce anomalies, albeit of variable magnitude across individual samples; and high As (up to 1930 ppm) and Ba (up to 2767 ppm) contents. A combination of structural, geochemical and mineralogical evidence from the two localities along with published results from similar occurrences elsewhere (e.g., Kythnos) suggest that Mn-oxide accumulation in the CBU is genetically linked to hydrothermal venting in a back-arc rift setting during the Triassic. The geochemical variability recorded is attributed primarily to the variable mixing of a hydrothermal-sourced, hydrogenous metalliferous component that precipitated broadly contemporaneously with the deposition of the host tuffs. Low transition metal contents coupled with substantial enrichments in elements such as Ba, As, Pb and REY, attest to the felsic/intermediate character of back-arc magmatism/volcanism and associated hydrothermal activity. Primary Mn precipitates are thought to have been in the form of tetravalent Mn assemblages, which may locally be partially preserved through metamorphism, as appears to be the case in the Varnavas occurrence. The fully oxic and hydrogenous character of the precursor Mn(IV) oxy-hydroxides is supported by the consistently positive Ce anomalies observed in practically all samples from both sites. Although the Mn concentrations and distribution of the studied assemblages are sub-economic at best, we consider possible that their geographically widespread occurrence may still hold untapped potential for future discovery of commercially viable Mn ores sensu stricto, both in the Hellenic arc and in other regions of similar geotectonic and metallogenic activity
Hydrothermal Manganese mineralization in a Triassic back-arc riftrelated volcaniclastic succession of the Cycladic Blueschist Unit, Greece
No full textThe Cycladic Blueschist Unit (CBU) of the Aegean (Greece) contains several occurrences of
metamorphosed Manganese mineralization within a Triassic volcaniclastic sequence. The latter
includes quartz-mica schists intercalated with bimodal metavolcanics and blue-grey marble layers.
SHRIMP U-Pb zircon dating has documented the Triassic age (~242 Ma) of the volcanic rocks.
Herein we revisit the Mn metallogenic system of the CBU through an extensive study of Mn
mineralization at Varnavas area, Northern Attica, and a similar occurrence at central Andros Island
(Mparades hill). Manganese mineralogy at both localities is manifested in a typical high-P
metamorphic silicate assemblage dominated by piemontite, spessartine garnet, and minor
pyroxmangite (rhodonite). At Andros, Mn-rich subdomains contain brecciated braunite
micronodules. The preservation of similar nodular form is documented from Varnavas, comprising
dominant todorokite, lesser hollandite, pyrolusite, and minor Mn-bearing hematite. The
contrasting Mn oxide mineralogy at the two sites is tentatively interpreted as the result of locally
incomplete reduction of precursor Mn(IV) phases during metamorphism. Common geochemical
characteristics of the Mn-rich rocks include low transition metal concentrations; positive-sloping,
PAAS-normalized REE spidergrams; positive Ce anomalies of variable magnitude across individual
samples; and high As, Ba, Pb. The geochemical variability recorded is ascribed to the varying
mixing of a hydrothermal-sourced, hydrogenous metalliferous component that precipitated
penecontemporaneously with the deposition of the host tuffs. The primary Mn precipitates are
thought to have been in the form of tetravalent Mn assemblages, which may locally be partially
preserved through metamorphism, as appears to be the case in the Varnavas occurrence. All
these reveal the interplay between the felsic/intermediate back-arc volcanism and associated
hydrothermal activity and the Mn mineralization within the rift setting of the CBU domain
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