MIOCENE MAGMATISM AND MINERALIZATION OF LAVRION, WESTERN CYCLADES (GREECE)

Miocene igneous activity in the Lavrion area of the western Cyclades, Greece is the result of back-arc extension of the Aegean. The Western Cyclades Detachment System (WCDS) likely extends and terminates here, placing ultramylonitic marbles over schists of the Cycladic Blueschist Unit along a SW-directed detachment. Metagabbros reside atop the marbles and represent relict and dismembered ophiolites. Weakly to undeformed granoitoid sills and plutons pierce the marble and schists indicative of syn- to post-tectonic intrusion. Whole rock geochemistry of the Plaka pluton, related dykes, and several older (Triassic?), sill-like orthogneisses, which penetrate the blueschists, yields similar compositions with varying amounts of quartz, biotite, and plagioclase as the constituent minerals; muscovite, potassium feldspar and hornblende were present in lesser amounts. All felsic rocks possess slightly elevated LREE patterns. High TiO₂ and relatively low K₂O and Na₂O in comparison to silica content suggest an I-type origin for the pluton. Despite similarities in geochemistry and field observations, the dykes and sills plot separately from the pluton and orthogneiss samples when comparing Th/U to SiO₂ values. Weak to strong weathering, silicification and feldspar alteration are the most common secondary products, with a portion of the pluton exhibiting silicification and significant quartz veining. Near the detachment horizon, fluid flow focussed away from the intrusion that lead to carbonate-hosted alteration and supergene ores. Pb-Ag-Zn supergene ore deposits are typically strata bound carbonate-replacement type, with local sulphide veinlet occurrences. Galena and sphalerite are visible, and are known to be the main ore-bearing minerals in addition to smithsonite, goethite and hematite. Calcitic and dolomitic rocks proximal to intrusions are high in Fe. Due to fluid leaching from surrounding rocks and its precipitation with the ores, brecciated carbonates tend to have lower Fe concentrations. Additionally, clinopyroxene-garnet bearing skarn veins in low-strain tension gashes adjacent to the pluton have also resulted from the Miocene magmatism. The Lavrion area shares strikingly similar geology and tectonic features with the southern termination of the WCDS on Serifos island.