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Papers by DIMITRIOS ALFIERIS

Ore Geology Reviews, Sep 1, 2019

Calc-alkaline volcanic rocks in western Milos (pyroclastics and shallow subvolcanic intrusives/la... more Calc-alkaline volcanic rocks in western Milos (pyroclastics and shallow subvolcanic intrusives/lava domes) were emplaced during the Upper Pliocene to Pleistocene times under transitional submarine to locally subaerial conditions, forming a series of volcanic edifices. During the eruptive phase a combination of vent clearing pyroclastic flows and water supported volcaniclastic products created a poorly consolidated pumice pile (pumice cone). Shortly after, it has been intruded by shallowly emplaced rhyolitic domes (cryptodomes). This dominantly acid explosive and intrusive activity has been followed by effusive volcanism that formed numerous dacitic-rhyodacitic-rhyolitic and andesitic domes and lava domes, emplaced shallowly underneath or on the sea-floor, creating a series of hyaloclastitic products intercalated with shallow marine sediments. Locally some of the domes gave origin to bedded pumiceous tuffs (dome-top tuff cones), while other domes and lavas raised above the sea level becoming thus subaerial. All the subvolcanic high-level intrusive and partially extrusive volcanic products represent the equivalents of a deeper magma chamber that fed the volcanic succession, and were the heat source required to drive the hydrothermal convective system responsible for the mineralization. Geochronological data, of several of these intrusions suggest that they are partially coeval with, and partially younger than the pyroclastic succession into which they were emplaced. Methodological collection of existing information, geological mapping of surface outcrops, geochemistry, X-ray diffraction analysis, drill holes logging, air-photos and satellite imagery structural interpretation, alteration and ore mineralogical studies, were combined in order to better understand and interpret the magmatic-hydrothermal environment of ore deposition at western Milos. The dominant structural features at western Milos are from one side a series of steep NE striking fault / lineament zones, and from the other side a series of dilational NW trending ones (fault jogs), into which, composite volcanic centers, domes and collapse calderas have been developed. They together have promoted and localized fluid upflow along steep, near vertical feeder conduits, producing in such a way zones of intense silicification, brecciation and veining. With respect to those structures, subsidiary E-W and N-S trending faults were also created and are considered to have enhanced the permeability of the volcanic and volcano-sedimentary sequences, through a further dismembering and dislocation. Of particular interest can be characterized the quasi E-W trending lineament along the central part of western Milos (Ammoudaraki-Plakota-Rivari lineament), which subdivided it into two sectors, from a geological, hydrothermal and mineralogical point of view, the northern and southern sectors. The tectonic structures which controlled the principal eruptive centres are the same structures which controlled and localized hydrothermal discharge that formed the various deposits and mineralization. The alteration patterns observed at western Milos correspond at least to two distinct and partially overlapped environments, namely the volcanic-hydrothermal and geothermal environments, the first corresponding to a high sulfidation (HS) mineralizing system, and the second one to an intermediate sulfidation (IS) system. Hydrothermal alteration at the northern part of western Milos is dominated by a typical acid-sulfate submarine pool, characterized by extreme acid leaching and formation of residual quartz, grading outwards to Na-alunite, dickite/kaolinite ±diaspore± pyrophyllite and then to kaolinite-illite-sericite assemblages.The observed elongated shape of the silica bodies in the northern sector of western Milos suggests that a major structural control was exercised by E-NE striking faults. In the southern sector the ore bearing quartz-adularia zone is flanked by an illite± smectite alteration assemblage which grades to peripheral alteration dominated by carbonates-chlorite assemblages. The abundance of quartz and adularia increases toward the upflow zone, whereas clay abundance increases toward the margin of the upflow zone with smectite dominating at 200o C. A long-lived zone of hydrothermal flow with an initial submarine to a later subaerial steam heating throughout the southern sector and partially at the northern sector (Kondaros area) occurred. Any model for the development of the regional alteration in western Milos must account for the extend of the silicification and advanced argillic alterations, the timing of emplacement of the major mineralized veins, and the presence of late advanced argillic alteration (of steam heated nature), that obliterated some of the previous features, by overprinting them. The development of a well framed vein system mainly at the southern sector and sub- ordinately at the northern one, together with a well localized breccias field at…

Ore Geology Reviews, Apr 1, 2019

Ore Geology Reviews, Sep 1, 2013

ABSTRACT Milos Island contains several epithermal deposits (e.g., Profitis Ilias–Chondro Vouno Pb... more ABSTRACT Milos Island contains several epithermal deposits (e.g., Profitis Ilias–Chondro Vouno Pb–Zn–Ag–Au–Te–Cu, Triades–Galana–Agathia–Kondaros Pb–Zn–Ag–Bi–W–Mo ± Cu–Au, and Katsimoutis–Kondaros–Vani Pb–Zn–Ag–Mn) of Late Pliocene to Early Pleistocene age. These deposits are hosted in calc-alkaline volcanic rocks emplaced as a result of three successive magma pulses in an emergent volcanic edifice: submarine rhyolitic to rhyodacitic cryptodomes at ca. 2.7. Ma (Profitis Ilias–Chondro Vouno), submarine to subaerial andesite to dacite domes at ca. 2.2 to 1.5 Ma (Triades–Galana–Kondaros–Katsimouti–Vani). Hydrothermal alteration of the volcanic rocks includes advanced argillic- (both hypogene and steam-heated), argillic, phyllic, adularia-sericite and propylitic types. In the northern sector (Triades–Galana–Agathia–Kondaros), initial magma degassing derived from andesitic–dacitic intrusives along NE–SW to E–W trending faults resulted in the development of pre-ore hypogene advanced argillic alteration (dickite, alunite, ± diaspore, pyrophyllite, halite, and pyrite) in a submarine environment. Mineralogical data indicate common features among the Profitis Ilias–Chondro Vouno, Kondaros–Katsimoutis–Vani and Triades–Galana mineralized centers, all of which are characterized by the presence of galena, Fe-poor sphalerite, and chalcopyrite as well as abundant barite, adularia, sericite and, to a lesser extent, calcite, which are typical of intermediate-sulfidation epithermal type deposits. Locally, at Triades–Galana and Kondaros–Agathia, high-sulfidation conditions prevailed as suggested by the presence of coexisting enargite and covellite. The high silver and gold content of the western Milos deposits is derived from Ag-bearing sulfosalts (polybasite, pearceite, pyrargyrite, freibergite) and tellurides. Gold at Profitis Ilias, both as native gold and silver-gold tellurides, is present in base-metal precipitates within multicomponent blebs, which recrystallized to form hessite, petzite, altaite, coloradoite, and native gold. Mineralogical evidence (e.g. microchimney structures, copper sulfides, widespread occurrence of barite, aragonite) suggests that precious metal mineralization in western Milos mineralization formed in a submarine setting.We present information on the surface distribution of Au, Ag, Cu, Pb, Zn, As, Sb, Hg, Mo, Bi, W and Cd at western Milos. Gold is enriched at Profitis Ilias–Chondro Vouno deposits and to a lesser extent at Triades–Galana. Arsenic is absent from the southern sector but shows elevated concentrations together with molybdenum, bismuth and tungsten at the northern sector (Triades–Galana, Vani deposits). The differences in precious and base metal abundances may be related to the depths at which the deposits are exposed, and/or different sources of magma. The metal signatures of the Triades–Galana and Agathia–Kondaros–Katsimouti–Vani (Mo–Bi–W–As–Hg–Ag–Au) occurrences compared to Profitis Ilias (Te–Au–Ag) reflect different sources of magma (dacite–rhyodacite for Profitis Ilias, andesite–dacite for Triades–Galana, and dacite for Kondaros–Katsimoutis). The enrichment of Te, Mo, W, and Bi in the deposits is a strong indication of a direct magmatic contribution of these metals.At western Milos, precious and base-metal vein mineralization was deposited during episodic injection of magmatic volatiles and dilution of the hydrothermal fluids by seawater. The mineralization represents seafloor/sub-seafloor precipitation of sulfides that formed in stockwork zones. Base and precious metal mineralization formed from intermediate- to high-sulfidation state fluids and mostly under boiling conditions as indicated by the widespread occurrence of adularia associated with metallic mineralization. We speculate that the widespread occurrence of boiling and the shallow depth of the precious- and base-metal emplacement prevented the formation of seafloor massive sulfides.

Mineralium Deposita

Drill core samples from the Profitis Ilias Pb-Zn-Cu-Ag-Au vein mineralization on Milos Island, Gr... more Drill core samples from the Profitis Ilias Pb-Zn-Cu-Ag-Au vein mineralization on Milos Island, Greece provide new insights into (i) the metal sources, (ii) the primary vertical metal(loid) distribution, and (iii) the supergene enrichment processes in a transitional shallow-marine to subaerial hydrothermal environment. Metal contents of unaltered and altered host rocks combined with Pb isotope analyses of hydrothermal sulfides suggest that most metal(loid)s were derived by leaching of basement rocks, whereas the distinct enrichment of Te is related to the addition of Te by a magmatic fluid. The trace element contents of base metal sulfides record decreasing Au, Te, Se, and Co, but increasing Ag, Sb, and Tl concentrations with increasing elevation that can be related to progressive cooling and fluid boiling during the hypogene stage. The formation of base metal veins with porous pyrite hosting hessite inclusions at ~ 400 m below the surface was triggered by vigorous fluid boiling. By ...

Goldschmidt2022 abstracts, 2022

Proceedings of The 1st International Electronic Conference on Mineral Science, 2018

Bulletin of the Geological Society of Greece, 2017

The metallic mineralization in Kontaros-Katsimouti area is an epithermal Pb-Zn Ag mineralization... more The metallic mineralization in Kontaros-Katsimouti area is an epithermal Pb-Zn Ag mineralization located along the NW-trending Kondaros-Katsimouti-Vani fault, NW Milos Island, Greece. It is hosted within propylitically and argillically altered dacitic flow dome and volcaniclastic sandstone and shows features typical of intermediate sulfidation deposits like colloform banding, cockade breccias and gangue adularia, Mnrich carbonates and amethystine quartz. The Kondaros-Katsimouti system evolves at higher elevation into the Vani Ag-Pb mineralization, which occurs proximal to the Vani manganese deposit. The metallic mineralogical assemblage at Kondaros-Katsimouti includes mainly galena and sphalerite and minor pyrite. Silver is present in the form of Ag-(Cd) rich tetrahedrite (up to 23.1 wt. % Ag) and polybasite included in galena. Bulk ore analyses indicate enrichment in W (up to 424 mg/kg) and Mo (up to 24 mg/kg), similarly to the other neighboring mineralizations in western Milos (e...

Bulletin of the Geological Society of Greece, 2013

Epithermal-altered volcanic rocks in Greece host gem-quality amethyst veins in association with v... more Epithermal-altered volcanic rocks in Greece host gem-quality amethyst veins in association with various silicates, carbonates, oxides, sulfides and halides. Host rocks are Oligocene to recent calc-alkaline to shoshonitic lavas and pyroclastics of intermediate- to acid composition. The amethyst-bearing veins occur in the periphery of porphyry-type and/or high-sulfidation epithermal mineralized centers in northern Greece (e.g. Sapes, Kirki, Kornofolia/Soufli, Lesvos island) and on Milos island in the active Aegean Volcanic Arc. Hydrothermal alteration around the quartz veins includes sericitic, K-feldspar (adularia), argillic, propylitic and zeolitic types. Precipitation of amethyst in the northern Greece occurrences, took place during the final stages of the magmatic-hydrothermal activity from near-neutral to alkaline fluids, as indicated by the presence of gangue adularia, calcite, smectite, chlorite, sericite, pyrite, zeolites (laumontite, heulandite, clinoptilolite), analcime and ...

Ore Geology Reviews, 2017

The Mn-Ba-Pb deposit at Aspro Gialoudi in NW Milos is shown to be a fossil inhalative-exhalative ... more The Mn-Ba-Pb deposit at Aspro Gialoudi in NW Milos is shown to be a fossil inhalative-exhalative hydrothermal deposit that represents the deepest part of the Vani succession at the western extremity of the main Vani manganese deposit. The geology of the Vani-Aspro Gialoudi area is characterized by Upper Pliocene-Lower Pleistocene dacitic and rhyodacitic lava domes, which are overlain by the Vani volcaniclastic unit considered to be part of the 2.66–1.44 Ma magmatic event at Milos Island. The presence of in-situ and intrusive hyaloclastite breccias surrounding the coherent lava domes at Aspro Gialoudi and Vani areas indicates submarine emplacement for the domes. The dacitic-rhyodacitic domes are variously altered (mainly propylitic and/or argillic alteration, silicified and in some cases locally exhibiting adularia alteration). Both Aspro Gialoudi and main Vani deposit are located proximal to fault systems: the main Vani manganese deposit is adjacent to the NW-trending Kondaros-Katsimouti-Vani Dome fault, whereas the Aspro Gialoudi deposit is adjacent to the relatively minor NE-trending fault on the west coast of Milos. At Aspro Gialoudi, mineralization took place in a subseafloor and/or seafloor environment and is characterized by a stratabound Mn-barite-rich deposit mainly within a package of propylitized intrusive hyaloclastites and within the overlying sandstones. Banded epithermal veins trending NE-SW and composed of chalcedonic silica/quartz + barite + Mn-oxide + sulfides crosscut the dacitic lavas, the hyaloclastites and the overlying volcaniclastic sequence at Aspro Gialoudi and are considered to represent the feeder zones of the manganese-barite mineralization. Within the veins, early sulfide (galena-sphalerite) barite and quartz deposition is followed by manganese oxides and aragonite, thus resembling the epithermal-style Pb-Zn-Ag-Mn mineralization across the NW-trending Katsimoutis-Kondaros-Vani fault. Mineralization in Aspro Gialoudi and Vani deposits seems to be controlled by alternating cycles of deposition of sulfides and hydrothermal manganese oxides within the faults. Manganese deposition in both deposits formed in a similar manner, namely by transport of hydrothermal fluids through the adjacent fault systems into a reservoir of volcanoclastic sandstone and hyaloclastites to produce a deposit initially consisting of principally of pyrolusite and occasionally ramsdellite, which were subsequently replaced by cryptomelane, hollandite, coronadite and hydrohaeterolite. Precipitation of hydrothermal manganese oxides took place very quick and under microbial Mn(II) oxidation. Compositional data show that metallic elements most enriched in the Aspro Gialoudi and Vani manganese deposits relative to the average continental crust, lie in the sequences Pb > Cd > Mn > As > Sb > Zn > W > Tl > Ba > Cu > Mo > Co > Bi and As > Sb > Pb > Mn > Tl > Cd > Zn > W > Cu > Ba > Mo > Co, respectively. Mineralogical and geochemical (e.g. REE) data from both Aspro Gialoudi and main Vani deposit are taken to indicate mainly a seawater source for the hydrothermal fluids. These two deposits are genetically and spatially related to base- and precious metal intermediate-sulfidation epithermal mineralization. They formed successively by similar processes and are considered to be integral parts of the same hydrothermal system.

Chemical Geology, 2021

Boiling is a crucial process triggering ore formation in magmatic-hydrothermal systems and contro... more Boiling is a crucial process triggering ore formation in magmatic-hydrothermal systems and controlling the enrichment of precious and rare metal(loid)s in epithermal-porphyry mineralizations. Steep physicochemical gradients during boiling of hydrothermal fluids at shallow water depths caused metal(loid) precipitation along a 3 km long Pb-Zn-Ag vein system on Milos Island in the South Aegean Volcanic Arc. We present new trace element and Pb, Sr, and S isotope data from sulfides providing insights into the diversity of mineralization processes in shallow-marine hydrothermal systems. Lead and Sr isotope compositions of sulfides and sulfates reflect the mixing of fluids that reacted with metamorphic basement and the volcanic host rocks, whereas some of the S were derived from seawater. Investigation of mineralized samples along the Kondaros-Vani fault zone revealed distinct chemical variations that represent a vertical profile through the boiling zone of a hydrothermal system. Boiling d...

ABSTRACT The Vani Ag prospect is a high-grade epithermal mineralization located along the NW-tren... more ABSTRACT The Vani Ag prospect is a high-grade epithermal mineralization located along the NW-trending Kondaros-Katsimouti-Vani fault, NW Milos Island, Greece. The prospect is hosted in calc-alkaline dacite domes and volcaniclastic sandstones and represents the NW extension of the Pb-Zn-Ag-Mn Katsimoutis-Kondaros mineralization. It occurs proximal to the Vani exhalative manganese deposit. The Ag content of the prospect is derived from Ag-bearing phases (native silver, argentite/acanthite, silver halides and argentian covellite). Mineralogical evidence like the presence of skeletal habits of sulfides, presence of hydrothermal anglesite, covellite and silver halides that were formed after dissolution of the primary silver and lead-bearing minerals, verifies earlier work that mineralization along the Kondaros-Katsimoutis fault is the product of seawater oxidation and was formed in a submarine setting after reaction of hydrothermal fluids with seawater.

Ore Geology Reviews

Abstract Greece hosts a variety of magmatic-hydrothermal ore deposits/prospects with porphyry- an... more Abstract Greece hosts a variety of magmatic-hydrothermal ore deposits/prospects with porphyry- and epithermal styles playing a major role in its total gold endowment. These deposit types are mainly clustered in two areas, the Rhodope- and Attico-Cycladic massifs, and formed from about 33 Ma to the Pleistocene, as a result of back-arc extension in the Aegean Sea, metamorphic core complex formation, and contemporaneous post-subduction and arc magmatism. In the Serbo-Macedonian massif, porphyry Cu-Au deposits include Skouries, Fisoka, Tsikara, Vathi and Gerakario. Causative intrusives are Oligocene to Miocene granodiorites to monzonites. Gold and PGE mineralization is associated with potassic alteration of the intrusives. In the eastern Rhodope massif and the NE Aegean islands, porphyry prospects occur at Pagoni Rachi, Konos Hill, Myli, Melitena (west Thrace), Fakos, Sardes, and Kaspakas (Limnos island) and Stypsi (Lesvos island). Mineralization is associated with Oligocene to Miocene subvolcanics of calc-alkaline to shoshonitic affinity. Feature of these prospects, which they share in common with several porphyry Au-only systems, is their shallow depth of emplacement, the presence of potassic/sodic-calcic and/or phyllic alteration, a strong epithermal overprint, their low Cu content, an extreme Re-enrichment, the multistage introduction of Au, the presence of banded quartz veinlets, and the local presence of tourmaline. New discoveries of porphyry-style mineralization at King Arthur, St. Philippos and Aisymi, increases the gold potential in west Thrace. High-intermediate sulfidation epithermal Au-Ag polymetallic deposits/prospects overprint and/or occur laterally from porphyry-style mineralization, where they are spatially associated with lithocaps of advanced argillic alteration. High-intermediate sulfidation Au-Ag epithermal mineralization at Perama Hill, Mavrokoryfi and Pefka in west Thrace, and at Pterounda, Mesotopos and Megala Therma on Lesvos island is controlled by steeply-dipping extensional faults within volcanic rocks, without any obvious genetic relationship to spatially-related porphyry-style mineralization. Polymetallic epithermal deposits and prospects contain critical and energy critical metals (e.g., Te, Se, Bi, Sb, In, Ge and Ga), which may be considered as by-products. In the Attico-Cycladic area, porphyry Mo-W mineralization occurs as sheeted quartz veins and stockworks cutting a potassic- to sericitic-altered Miocene granodiorite stock in the Lavrion district. Bonanza grade Au- and/or Ag-rich veins with epithermal affinities crosscut metamorphic rocks at Lavrion, and on Syros, Tinos, Antiparos and Anafi islands. Milos island is characterized by shallow submarine volcanic-hosted IS-HS epithermal Au-Ag-Te and base metal deposits. Antimony-As-Ag-Au deposits/prospects on Chios, Samos and Kos islands in the eastern Aegean Sea, indicate the potential for Carlin-style mineralization in Greece. Several factors played a role to the metal endowment of the Aegean porphyry-epithermal systems: magma fertility in the source regions, depth of emplacement of causative intrusives, relative contribution of mantle versus crustal material, redox state of subduction-related magmas, and physico-chemical fluid conditions at the site of ore deposition.