A new occurrence of cervelleite-like phases and Te-polybasite from gold-bearing veins in metamorphic rocks of the cycladic blueschist unit, Greece

Category Mineral processing
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Voudouris, Panagiotis۱; Spry, Paul۲
Holding Date 21 September 2008

Au-Ag-Te-bearing milky quartz veins in the Kallianou area (southern Evia Island, Greece), are hosted in mica schists and marbles of the Cycladic Blueschist Unit in the backarc region of the Hellenic subduction zone. The quartz veins (up to 3m thick and 100m long) generally strike NW-SE and are discordant with respect to syn-metamorphic structures. The veins formed during the Miocene, under ductile to brittle deformation, and in the footwall block of an exhumed metamorphic core complex. Ore minerals in the veins occur in masses (up to 10 vol %) to disseminations, filling fractures or cementing brecciated quartz fragments. The ore mineralogy consists of pyrite, arsenopyrite, löllingite, sphalerite, chalcopyrite, tetrahedrite, galena, gold, pearceite, sylvanite, bornite and argentite. The main gangue minerals include quartz and calcite, whereas wallrock alteration consists of chlorite, muscovite, albite and calcite. Mineralogical studies show the Au-Ag-Te-bearing milky quartz veins in the Kallianou area (southern Evia Island, Greece), are hosted in mica schists and marbles of the Cycladic Blueschist Unit in the backarc region of the Hellenic subduction zone. The quartz veins (up to 3m thick and 100m long) generally strike NW-SE and are discordant with respect to syn-metamorphic structures. The veins formed during the Miocene, under ductile to brittle deformation, and in the footwall block of an exhumed metamorphic core complex. Ore minerals in the veins occur in masses (up to 10 vol %) to disseminations, filling fractures or cementing brecciated quartz fragments. The ore mineralogy consists of pyrite, arsenopyrite, löllingite, sphalerite, chalcopyrite, tetrahedrite, galena, gold, pearceite, sylvanite, bornite and argentite. The main gangue minerals include quartz and calcite, whereas wallrock alteration consists of chlorite, muscovite, albite and calcite. Mineralogical studies show the presence of previously unrecognized silver sulfotellurides, hessite and Te-polybasite in association with pyrite, chalcopyrite, galena, electrum and fahlore-group minerals. Electrum (35.3-50.2 apfu Ag) is closely associated with galena and chalcopyrite in fracture fills, surrounding and/or occurring as inclusions in pyrite. EMPA data indicated elevated Bi content (up to 0.44 wt%). Te-rich polybasite (up to 7.4 wt% Te), as well as zincian tetrahedrite and ferrian tennantite (with up to 1.3 wt% Ag), occur as inclusions in galena and chalcopyrite. The Ag-sulfotellurides (up to 40μm in length) are minerals of the cervelleite-group (up to 0.24 apfu Cu), as well as unnamed minerals with compositions approximating Ag2CuTeS and (Ag,Cu)2TeS (up to 0.23 wt% Au). They are enclosed in galena or occur at the grain boundaries between galena-chalcopyrite and galena-pyrite. The Kallianou occurrence of the cervelleite-like phases resembles that in the nearby Panormos Bay Au-Ag-Te deposit, Tinos Island, where Cu-bearing cervelleite is accompanied by Ag-, Au-, and Cu-bearing tellurides. The Kallianou deposit is one of several metamorphic rock-hosted intrusion-related sulfide occurrences in Greece that formed during extensional conditions and during the exhumation of metamorphic core compexes. In this respect, it resembles reduced intrusion-related mineral deposits elsewhere in the world.