Mineralogical and fluid inclusion constraints on the evolution of the Plaka intrusion-related ore system, Lavrion, Greece

Abstract

The intrusion-centered, polymetallic Plaka ore system, Lavrion, Greece, is genetically related to the emplacement of a Miocene biotite ± hornblende granodioritic body and granodiorite porphyry dikes/sills within metamorphic Mesozoic rocks of the Attic-Cycladic Crystalline Belt. The ore system includes porphyry, breccia-type, skarn, skarn-free carbonate replacement and vein-type ores enriched in Mo, W, Pb, Zn, Cu, As, Sb, Bi and Ag. Early pyrite-molybdenite (±chalcopyrite ± pyrrhotite) mineralization was deposited in multidirectional and sheeted quartz veins within the silicically and sericitically altered granodiorite intrusive. The emplacement of granodiorite porphyry dikes/sills was followed by the introduction of base metal sulphides during hydrothermal brecciation of the porphyritic rocks. Skarn- and skarn-free carbonate replacement ores developed contemporaneously with these magmatic-hydrothermal events and are characterized by the presence of pyrrhotite, pyrite, Fe-rich sphalerite and Bi-Ag-rich galena. A late Pb-As-Sb-Cu-Ag-rich banded vein (Filoni 80) is composed of pyrrhotite, chalcopyrite, arsenopyrite, sphalerite (both Fe-rich and Fe-poor), lollingite, argentian tetrahedrite, bournonite, pyrargyrite, miargyrite, native arsenic, semseyite, heteromorphite, veenite, falkmanite and fluorite, quartz and siderite. The ore mineralogy of the Plaka deposit is generally consistent with reducing and low sulphidation fluid conditions during metal deposition. Fluid inclusion data indicate that molybdenite deposition was associated with boiling between 270 and 360 °C and at a pressure of 50–200 bars, corresponding to a depth of approximately 1 km. Dilution of a saline fluid by mixing with less saline water resulted in the formation of a moderately saline (14–18 wt% NaCl equiv.), low-temperature (190–220 °C) fluid and the precipitation of sulphides and sulphosalts in the Filoni 80 vein. The evolution from early porphyry-style mineralization to the later vein ores likely formed either as a result of continuous syn-mineralization uplift of the Plaka granodiorite in relation to post-orogenic regional uplift during the Upper Miocene, or as a simple cooling of the high-temperature hydrothermal system.