Shallow submarine epithermal Pb-Zn-Cu-Au-Ag-Te mineralization on western Milos Island, Aegean Volcanic Arc, Greece: Mineralogical, geological and geochemical constraints

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. © 2013

Structural role of tellurium in the minerals of the pearceitepolybasite group

The crystal structure of a Te-rich polybasite has been refined by means of X-ray diffraction data collected at room temperature (space group P3m1; R = 0.0505 for 964 observed reflections and 94 parameters; refined formula Ag 14.46Cu1.54Sb1.58As0.42S 9.67Te1.33). The structure comprises stacking of [(Ag, Cu)6(Sb, As)2(S, Te)7]2- A and [Ag9Cu(S, Te)2(S, Te)2]2+ B layer modules in which Sb forms isolated SbS3 pyramids, as occurs typically in sulfosalts, Cu links two S atoms in a linear coordination and Ag occupies sites with coordination ranging from quasi linear to almost tetrahedral. The silver d 10 ions are found in the B layer module along two-dimensional diffusion paths and their electron densities evidenced by means of a combination of a Gram-Charlier development of the atomic displacement factors and a split model. The Te-for-S substitution occurs at the same structural sites that Se substitutes for S in selenopolybasite and the Te occupancy at one of these sites is 0.49, thus suggesting the possibility that 'telluropolybasite' could be found in nature. © 2013 The Mineralogical Society

Understanding gold-(silver)-telluride-(selenide) mineral deposits

Gold-(silver)-telluride (selenide) ores occur as epithermal orogenic and intrusion related deposits. Although Te and Se are chalcophile elements and share geochemical affinity with Au, formation of selenides and other elements Ag-Au require acidic or reducing environments. The thermodynamic stability conditions for Au and Agtellurides and native tellurium indicate an epithermal environment. Analysis of mineral paragenensis, textures and compositional variation in tellurides/selenides suggest petrogenetic processes involving interaction with fluids leading to Au scavenging and entrapment in tellurides, changes in chemistry/rates of fluid infiltration and attaining equilibrium in a given assemblage

Re-Os systematics of löllingite and arsenopyrite in granulite-facies garnet rocks: Insights into the metamorphic history and thermal evolution of the Broken Hill Block during the Early Mesoproterozoic (New South Wales, Australia)

Löllingite and arsenopyrite aggregates occur in spessartine-almandine garnet rocks (garnetite) metamorphosed to granulite facies, which are spatially associated with Pb-Zn-Ag mineralization in the giant Broken Hill deposit, southern Curnamona Province, New South Wales, Australia. Sulfarsenide and sulfide minerals comprise löllingite and coexisting arsenopyrite ± galena ± tetrahedrite that occur interstitial to garnet crystals. Löllingite formed first while gold-bearing löllingite, which occurs as rare relicts in arsenopyrite, was destroyed to produce arsenopyrite ± detectable micro-inclusions of invisible gold. Standard mineral separation procedures produced pure separates of löllingite, arsenopyrite, and mixtures of arsenopyrite ± löllingite and löllingite ± arsenopyrite. In a plot of 187Re/188Os versus187Os/188Os, samples of löllingite and löllingite ± arsenopyrite have 187Re/188Os ratios between 6.87 and 7.40 and 187Os/188Os ratios between 0.8506 and 0.8651, whereas arsenopyrite and arsenopyrite ± löllingite samples have higher 187Re/188Os ratios (7.14 to 11.32) and more radiogenic 187Os/188Os ratios (0.8828 to 0.9654). Thirteen analyses of arsenopyrite and arsenopyrite ± löllingite define a Model 1 isochron with an age of 1574 ± 38 Ma (2σ; MSWD = 1.4, initial 187Os/188Os ratio of 0.666 ± 0.006), whereas the five löllingite and löllingite ± arsenopyrite samples define a Model 1 isochron with an age of 1707 ± 290 Ma (2σ; MSWD = 0.32, initial 187Os/188Os ratio of 0.652 ± 0.036) that is indistinguishable from the arsenopyrite age. Rhenium and Os contents are extremely high for löllingite and arsenopyrite (Re = 120–475 ppb; Os = 65–345 ppb), likely as a result of concentration of Re and Os in these minerals during granulite-facies metamorphism from the inferred exhalite protolith. Petrographic observations combined with the Model 1 Re-Os ages and literature SHRIMP U-Pb ages of monazite in garnetite suggest that arsenopyrite formed on the retrograde path at the expense of löllingite. Cooling from peak Olarian P-T conditions (∼800 °C at 1602 Ma) to at least 550 °C (first temperature of stability of arsenopyrite) at ca. 1574 Ma occurred at a rate of ∼9 °C/Myr, which is similar to the rate of cooling determined for previously published SHRIMP U-Pb ages from successive monazite generations (McFarlane & Frost 2009). These results are consistent with the late phase of retrograde metamorphism that began between ca. 1590 and 1575 Ma

Mineralogical, stable isotope, and fluid inclusion studies of spatially related porphyry Cu and epithermal Au-Te mineralization, Fakos Peninsula, Limnos Island, Greece

The Fakos porphyry Cu and epithermal Au-Te deposit, Limnos Island, Greece, is hosted in a ~20 Ma quartz monzonite and shoshonitic subvolcanic rocks that intruded middle Eocene to lower Miocene sedimentary basement rocks. Metallic mineralization formed in three stages in quartz and quartz-calcite veins. Early porphyry-style (Stage 1) metallic minerals consist of pyrite, chalcopyrite, galena, bornite, sphalerite, molybdenite, and iron oxides, which are surrounded by halos of potassic and propylitic alteration. Stage 2 mineralization is composed mostly of quartz-tourmaline veins associated with sericitic alteration and disseminated pyrite and molybdenite, whereas Stage 3, epithermal-style mineralization is characterized by polymetallic veins containing pyrite, chalcopyrite, sphalerite, galena, enargite, bournonite, tetrahedrite-tennantite, hessite, petzite, altaite, an unknown cervelleite-like Ag-telluride, native Au, and Au-Ag alloy. Stage 3 veins are spatially associated with sericitic and argillic alteration. Fluid inclusions in quartz from Stage 1 (porphyry-style) mineralization contain five types of inclusions. Type I, liquid-vapor inclusions, which homogenize at temperatures ranging from 189.5°C to 403.3°C have salinities of 14.8 to 19.9 wt. % NaCl equiv. Type II, liquid-vapor-NaCl, Type III liquid-vapor-NaCl-XCl 2 (where XCl is an unknown chloride phase, likely CaCl 2), and Type IV, liquid-vapor-hematite ± NaCl homogenize to the liquid phase by liquid-vapor homogenization or by daughter crystal dissolution at temperatures of 209.3 to 740.5 °C, 267.6 to 780.8 °C, and 357.9 to 684.2 °C, respectively, and, Type V, vapor-rich inclusions. Stage 2 veins are devoid of interpretable fluid inclusions. Quartz from Stage 3 (epithermal-style) veins contains two types of fluid inclusions, Type I, liquid-vapor inclusions that homogenize to the liquid phase (191.6 to 310.0 °C) with salinities of 1. 40 to 9. 73 wt. % NaCl equiv., and Type II, vapor-rich inclusions. Mixing of magmatic fluids with meteoric water in the epithermal environment is responsible for the dilution of the ore fluids that formed Stage 3 veins. Eutectic melting temperatures of -35.4 to -24.3 °C for Type I inclusions hosted in both porphyry- and epithermal-style veins suggest the presence of CaCl 2, MgCl 2, and/or FeCl 2 in the magmatic-hydrothermal fluids. Sulfur isotope values of pyrite, galena, sphalerite, and molybdenite range from δ 34S = -6.82 to -0.82 per mil and overlap for porphyry and epithermal sulfides, which suggests a common sulfur source for the two styles of mineralization. The source of sulfur in the system was likely the Fakos quartz monzonite for which the isotopically light sulfur isotope values are the result of changes in oxidation state during sulfide deposition (i.e., boiling) and/or disproportionation of sulfur-rich magmatic volatiles upon cooling. It is less likely that sulfur in the sulfides was derived from the reduction of seawater sulfate or leaching of sulfides from sedimentary rocks given the absence of primary sulfides in sedimentary rocks in the vicinity of the deposit. Late-stage barite (δ 34S = 10.5 per mil) is inferred to have formed during mixing of seawater with magmatic ore fluids. Petrological, mineralogical, fluid inclusion, and sulfur isotope data indicate that the metallic mineralization at Fakos Peninsula represents an early porphyry system that is transitional to a later high- to intermediate-sulfidation epithermal gold system. This style of mineralization is similar to porphyry-epithermal metallic mineralization found elsewhere in northeastern Greece (e.g., Pagoni Rachi, St. Demetrios, St. Barbara, Perama Hill, Mavrokoryfi, and Pefka). © 2012 Springer-Verlag

Is the Palea Kavala Bi-Te-Pb-Sb±Au district, northeastern Greece, an intrusion-related system?

Intrusion-related gold systems are generally characterized by a Au-Bi-Te ± Sn-W metal assemblage genetically linked to the emplacement of granitoids. The Palea Kavala ore system, Greece, consists of ~. 150 minor Fe-Mn (Pb ± Zn ± Ag), Fe-Mn-Au, Fe-As-Au, Fe-Cu-Au, and Bi-Te-Au occurrences that occur primarily in quartz-calcite-sulfide veins (hypogene mineralization), or as supergene bodies, in overlapping zones centered on the ~. 21-22. Ma granodioritic Kavala pluton, which intrudes metamorphic rocks of the Paleozoic Rhodope metamorphic core complex. The pluton consists mostly of granodiorite with lesser amounts of diorite, tonalite and monzodiorite, which was emplaced along the regional E-W trending Kavala-Komotini fault. The recently discovered, ~4km long, E-W trending so-called Kavala vein is a sheeted quartz vein system of Bi-Te-Pb-Sb±Au mineralization that crosscuts the Kavala pluton and the schists and gneisses of the Rhodope Massif. The Kavala vein system is comprised of quartz with lesser amounts of K-feldspar, plagioclase and muscovite. Quartz-sericite-pyrite alteration is pervasive but minor kaolinite is also present. Pyrite (~5% of vein volume) contains inclusions of tetradymite (some gold-bearing), bismuthinite, and cosalite. Sulfur isotope values (n=27) of pyrite from the Kavala and Chalkero veins, as well as pyrite and galena from Garizo Hill Fe-Mn-Pb vein range from -1.9 to 1.0‰ (with one outlier of -4.6‰) and suggest a magmatic sulfur source. Homogenization temperatures (Th) of type I (two-phase aqueous liquid-vapor) and type II (three-phase, H2O-CO2-rich) fluid inclusions that homogenize into the liquid phase in quartz from the Kavala and Chalkero veins range from 216.0° to 420.0°C (n=216) and 255.7° to 414.0°C (n=112), respectively. The Th of type III (two-phase aqueous liquid-vapor), which homogenize into the vapor phase, ranges from 210.4° to 323.4°C (n=28). The salinities of type I and type II inclusions range from 15.9 to 22.6wt.% NaCl equiv. and 5.5 to 11.2wt.% NaCl equiv., respectively. Eutectic temperatures of -58.5° to -44.3°C for type I inclusions suggest the presence of appreciable CaCl2 in addition to NaCl. Clathrate melting temperatures for type II inclusions of ~-56.7°C indicate that CO2 is the major component of the gaseous phase, however up to ~6% CH4 is present in some inclusions. The presence of a zoned metallogenetic district centered on Bi-Te-Pb-Sb±Au mineralization within the Kavala pluton, the presence of both magnetite and ilmenite in the Kavala pluton, and the two high-temperature, high-salinity, immiscible carbonic and aqueous fluids associated with the Kavala and Chalkero veins are consistent with them being part of an intrusion-related gold system that formed along the ilmenite-magnetite buffer. © 2011 Elsevier B.V

Re-Os isotope evidence for mixed source components in carbonate-replacement Pb-Zn-Ag deposits in the Lavrion district, Attica, Greece

The Lavrion ore district contains carbonate-replacement and vein-type Pb-Zn-Ag deposits as well as low-grade porphyry Mo, Cu-Fe skarn, and minor breccia-hosted Pb-Zn-Cu sulfide mineralization. These ore types are spatially related to a Late Miocene granodiorite intrusion (7 to 10 Ma), and various sills and dikes of mafic to felsic composition. Samples of sphalerite and pyrite from the Ilarion carbonate replacement deposit, and galena from Vein 80 (vein-type mineralization) in the Adami deposit show heterogeneous Re-Os values. These values were partially disturbed by hydrothermal activity associated with the formation of hydrothermal veins (e.g., Vein 80). A plot of initial 187Os/188Os versus 1/Oscommon ratios for pyrite and sphalerite from the Ilarion deposit form a mixing line (r2 = 0.78) between high concentration crustal-like and low concentration mantle-like end-members, or two crustal end-members one of which was more radiogenic than the other. Based on the Re-Os systematics and previously published geological and geochemical evidence, the most plausible explanation for the Re-Os isotope data is that ore-forming components were derived from mixed sources, one of which was a radiogenic crustal source from schists and carbonates probably near intrusion centers and the other, intrusive rocks in the district that are less radiogenic. Although the Re and Os concentrations of galena from Vein 80 are above background values they cannot be used as a chronometer. However, the results of the current study suggest that although pyrite, sphalerite, and galena are poor geochronometers in this ore deposit, due to partial open-system behavior, they still yield valuable information on the origin of the source rocks in the formation of bedded replacement and vein mineralization in the Lavrion district. © 2013 Springer-Verlag Wien

Mineralogy and geochemical environment of formation of the Perama Hill high-sulfidation epithermal Au-Ag-Te-Se deposit, Petrota Graben, NE Greece

The Perama Hill deposit is a high-sulfidation Au-Ag-Te-Se epithermal system hosted in silicic- and argillic altered andesitic rocks and overlying sandstones, which were emplaced on the eastern margin of the Petrota graben, northeastern Greece. The deposit evolved from an early stage silica-pyrite rock and argillic alteration followed by the deposition of sulfide-, sulfosalt- and telluride-bearing quartz-barite veins and stockworks. Early ore formation is characterized by a high-sulfidation-type enargite-galena-bearing ore assemblage (consisting of enargite, watanabeite, Fe-free sphalerite, covellite, kesterite, bismuthinite, selenian bismuthinite, lillianite homologues, kawazulite-tetradymite, goldfieldite, and native gold), followed by the formation of an intermediate-sulfidation-type tennantite-bearing assemblage characterized by ferrian/zincian tennantite, tellurobismuthite, tetradymite, melonite, native tellurium, Au-Ag-tellurides (calaverite, krennerite, sylvanite, hessite, petzite, stützite), altaite and electrum. Quartz, barite, kaolinite, sericite and minor aluminum-phosphate-sulfate minerals are gangue minerals. Fluid inclusion data demonstrate that the ore system evolved from an initial high temperature (up to 330°C) and low salinity (up to 4.9 wt.% NaCl equiv.) fluid towards a cooler (200°C) and very low salinity (0.7 wt.% NaCl equiv.) hydrothermal fluid suggesting progressive cooling and dilution of the ore fluid. The ore minerals at Perama Hill reflect variable fS2 and fTe2 conditions during base and precious metal deposition. Early ore deposition took place at ~300°C, at logfS2 values between ≈-8.2 and -5.5, and logfTe2 from -11.8 and -7.8. Late ore deposition occurred at logfS2 = -11.8 to -9.8 and logfTe2 of ≈-9.2 and -7.8. These data and paragenetic studies indicate a shift towards higher logfTe2 and lower logfS2 and logfSe2 values for the mineralizing fluids with time. The kawazulite/tetradymitess-gold association at Perama Hill suggests that it formed from a sulfide melt in the Bi-Au-Se-Te system as Au was scavenged from the hydrothermal ore-forming fluid at elevated temperatures. The presence of tellurides, and Bi- and Sn-bearing minerals in the ore system is compatible with direct deposition of metals from the vapor phase of a degassing magmatic (porphyry) body. © 2011 Springer-Verlag

The geochemistry of carbonate-replacement Pb-Zn-Ag mineralization in the Lavrion district, Attica, Greece: Fluid inclusion, stable isotope, and rare earth element studies

Strata-bound carbonate-replacement Pb-Zn-Ag deposits in the Lavrion district, Greece, are spatially related to a late Miocene granodiorite intrusion (7-10 Ma) and various sills and dikes of mafic to felsic composition. The Plaka granodiorite contains porphyry molybdenum mineralization and is locally associated with a Ca-Fe skarn. Carbonate-replacement deposits occur predominantly in marbles (Upper and Lower Marble of the Basal unit), Kaesariani schists, and along a major detachment fault that separates the Basal unit from the Upper unit. Orebodies are mainly strata bound carbonate-replacement, although sulfides also occur in veins. The mineralogy of carbonate-replacement deposits is dominated by base metal sulfides and sulfosalts of Ag, Bi, Sn, Sb, As, and Pb, particularly at Plaka and Kamariza. Carbonates are intergrown with earlier formed sulfides and sulfosalts but are more abundant late in the paragenetic sequence with fluorite and barite. Fluid inclusion studies of sphalerite, fluorite, calcite, and quartz in carbonate-replacement deposits suggest that they were deposited from 132° to 365 deg;C from CO 2-poor, low- to high-salinity fluids (1-20 wt % NaCl equiv). Carbon and oxygen isotope compositions of calcite (δ13C = -15.6 to -1.5% and δ18O = -9.2 to +17.3%) intergrown with sulfides reflect variable exchange of the ore-bearing fluid with the Upper and Lower Marbles and proximity to the Plaka granodiorite. Post-Archean Australian Shale (PAAS)-normalized rare earth and yttrium patterns of the Upper and Lower Marbles, and calcite intergrown with sulfides show positive Eu and negative Ce anomalies as well as Y/Ho ratios between 40 and 80. Normalized rare earth and yttrium patterns of fluorite also have positive Eu and negative Ce anomalies. Such anomalies for both the carbonates and fluorite reflect the high pH or high fO2 conditions of the late-stage hydrothermal fluids and the likely derivation of calcium from marine carbonates (precursors of the Upper and Lower Marbles). The range of sulfur isotope compositions for sulfides (δ34S = -4.9 to +5.3%, with one outlier of 9.4%) in carbonate-replacement and vein deposits is due likely to a magmatic sulfur source with a contribution of reduced seawater sulfate. Sulfur isotope compositions of barite from carbonate-replacement range from δ34S = 17.2 to 23.7 per mil and reflect Miocene seawater sulfate values. If a magmatic source of sulfur is assumed along with an average temperature of 250°C for the ore-forming fluids, as based on fluid inclusion studies, sulfides in carbonate-replacement deposits were deposited at values of log fO2 = -41 to -36 and a pH = 5.8 to 9.1. However, the range of sulfur isotope values does not rule out the possibility that sulfur in sulfides could have been produced by the reduction of seawater sulfate with no contribution from a magmatic source. The carbonate- replacement deposits resemble manto-type sulfide deposits in Mexico, central Colorado, South Korea, Nevada, and northern Greece. © 2011 by Economic Geology