Molybdenite Re-Os geochronology and conditions of formation of potassic and sodic-calcic alteration associated with the Plaka porphyry Mo-Cu system, Lavrion, Greece

Abstract

The Plaka porphyry Mo-Cu system occurs in the world-class Lavrion Ag-Pb-Zn district in Attica, southern Greece. It is spatially associated with a granodiorite porphyry that intruded the Attic-Cycladic Crystalline Complex in the late Miocene, along the footwall of the Western Cycladic detachment fault. A Re-Os age of 9.51 ± 0.04 Ma indicates that molybdenite formed during the early stage of the granodiorite porphyry intrusion and that subsequent cooling was very rapid. Brittle deformation and hydrothermal fluid flow created a network of A-, B-, diopside-actinolite and D- veins, associated with potassic-, sodic-calcic- and sericitic alterations. Potassic alteration is characterized by secondary biotite + K-feldspar + quartz + magnetite ± apatite, contains disseminated molybdenite, pyrite, and chalcopyrite, and formed at 420–500 °C, at pressures up to 530 bars (< 5.3 km depth) from hydrothermal fluids that underwent phase separation. Sodic-calcic alteration is devoid of Cu-Mo mineralization and, consists of diopside + actinolite + oligoclase/andesine + titanite + magnetite ± epidote-allanite ± chlorite ± quartz, which corresponded to a temperature range of between 350 and < 500 °C. Primary magnetite, titanite and biotite crystallized between the nickel‑nickel oxide (NNO) and hematite-magnetite (HM) buffers, indicating fairly oxidizing conditions for the granodioritic magma. Hydrothermal biotite plots closer to the HM buffer suggesting increasing oxygen fugacity during exsolution of the hydrothermal fluids associated with potassic alteration. The system evolved toward more reducing conditions during sericitic alteration and associated pyrite-molybdenite mineralization. A combination of evaporated seawater and magmatic fluids likely caused formation of the sodic-calcic alteration through the decarbonation of the host marble.