Porphyry-epithermal veins hosting Re-rich molybdenite and rheniite (ReS2) from the Maronia Cu-Mo +/- Re +/- Au porphyry in Thrace, NE Greece, provide new insights into the hydrothermal processes causing extreme Re enrichment. Quartz trace element chemistry (Al/Ti, Ge/Ti), Ti-in-quartz thermometry, and cathodoluminescence imaging reveal multiple quartz generations in consecutive hydrothermal quartz-sulfide veins associated with potassic, sericitic, and argillic alteration. Fluid inclusions in different quartz generations indicate that phase separation and fluid cooling are the main ore-forming processes in the porphyry stage (similar to 500 - 350 degrees C), whereas mixing of a vapor-rich fluid with metalliferous (e.g., Pb, Zn, Au) meteoric water forms the epithermal veins (similar to 280 degrees C). These processes are recorded by trace element ratios in pyrite that are sensitive to changes in fluid temperature (Se/Te), fluid salinity (As/Sb, Co/As), and mixing between fluids of magmatic and meteoric origin (Se/Ge). Highly variable intra-grain delta S-34 values in pyrite record S isotope fractionation during SO2 disproportionation and phase separation, emphasizing the importance of in situ delta S-34 analysis to unravel ore-forming processes. High delta S-34 (similar to 4.5 parts per thousand) values of sulfides are indicative of low SO42-/H2S fluid ratios buffered by the local host rocks and mixing of the magma-derived fluid with meteoric water. The formation of Re-rich molybdenite (similar to 6600 ppm) is favored by cooling and reduction of a magma-derived, high-temperature (similar to 400 degrees C), oxidized, and Re-rich fluid triggering efficient Re precipitation in early veins in the potassic alteration zone. The systematic temporal fluid evolution therefore reveals that coeval cooling and reduction of oxidized Re-rich fluids cause extreme Re enrichment at the Maronia porphyry system.