Fingerprinting fluid evolution and metal distribution in the Skouries porphyry Au-Cu deposit (NE Greece) by mineral micro-analysis

The Skouries deposit in NE Greece is a platinum-group element enriched (⌀=149 ppb Pd, ⌀=30 ppb Pt) Au-Cu porphyry system hosted by monzonite-syenite intrusions. The porphyry stockwork consists of quartz-rich A- and B-type veins associated with potassic alteration, followed by massive sulfide-bearing veins related to chlorite-sericite alteration, and D-type veins associated with sericitic alteration. The mineralization is dominated by chalcopyrite, pyrite, bornite, and magnetite associated with native gold and Bi-Pd-rich telluride inclusions. The mineral chemistry (EPMA and LA-ICP-MS) of chalcopyrite, pyrite, magnetite, and hydrothermal quartz was used to track the physiochemical fluid evolution, and to define mechanisms enhancing metal enrichment at Skouries.

Textural differences of hydrothermal quartz generations, as reflected by distinct CL-SEM intensities, were linked to Ti-in-quartz-thermometry and quartz trace element ratios, such as Ge/Ti, Sb/Ti and Al/Ti. These ratios record temperatures around 600°C for the A-type veins, suggest a reopening of veins by a second, hotter fluid pulse (>600°C), which was followed by a decrease in temperature below 600°C for the later B-type veins. Furthermore, the sulfide mineralization is characterized by systematic variations in trace element concentrations (e.g., As, Ag, Au) and ratios (e.g., Au/Cu, Co/TI, Se/Te, As/Sb) fingerprinting physiochemical changes in the hydrothermal fluid system. Therefore, sulfide and quartz micro-analysis can help decipher small-scale processes within the porphyry system and allow to establish mechanisms controlling the metal endowment of porphyry deposits on a larger scale.