Magmatic and hydrothermal evolution of the Skouries Au-Cu porphyry deposit, northern Greece

Abstract

The Skouries Au-Cu porphyry deposit is located in northern Greece and hosted by quartz monzonites and monzogranites of early Miocene age (∼20 Myr). The host rocks show geochemical similarities to other mafic to felsic intrusions in the district that have a similar strike direction, but which are 6 Myr older and lack evidence of economic mineralization. The Skouries magmas probably formed by fractional crystallization of mafic mantle-derived melts, from which the ore-forming fluids were released during second boiling accompanied by massive feldspar crystallization at ∼65 wt% SiO₂. Drill core samples record the dominant potassic alteration with A- and B-type veins, which are locally overprinted by chlorite-sericite alteration and related C-type veins, transitioning into sericitic alteration assemblages and D-type veins. The ore mineralization is characterized by chalcopyrite, bornite, pyrite, magnetite and accessory minerals, such as tellurides and PGE minerals. Incompatible trace element ratios of the host rocks, that are commonly used as tracers of magma fertility (e.g., Sr/Y, La/Sm), vary between the alteration-types and may therefore not generally provide a record of magmatic processes. Fluid inclusion and Ti-in-quartz thermometry yielded a temperature range of 520 to >600 °C for the A-type veins and 420 to 500 °C for the B-type veins. Decreasing fluid inclusion entrapment pressures suggest relatively rapid uplift of the hydrothermal system during the early- to main porphyry stage. Fluid inclusion compositions indicate that early K- and Cl-rich fluids caused the potassic alteration resulting in a strong mobilization of REEs, following a decrease in fluid salinity and temperature with proceeding porphyry evolution. Systematic variations in trace element contents (e.g., As, Ag, Pb) and ratios (e.g., As/Sb, Zn/Pb) of pyrite record fluid temperature changes and suggest early phase separation as a major ore-forming process. The common occurrence of native Au as inclusions in pyrite and chalcopyrite are indicative of early Au oversaturation in the fluid, which we relate to sulfide precipitation and phase separation, destabilizing the AuHS⁰ or Au(HS)₂^- complex, leading to the accumulation of Au particles. The formation of such Au colloids in fluids may thus reflect an important step towards the hydrothermal enrichment of Au in porphyry environments.