Abstract: The Çöpler porphyry-epithermal gold deposit is associated with middle Eocene intrusive rocks relatedto an extensional tectonic phase in the Tethyan Alpine-Himalayan orogenic belt. The intrusive rocks (granodioriteporphyry and diorite porphyry) were emplaced in Late Paleozoic–Mesozoic metapelite and metacarbonate rockscreating the porphyry-epithermal Cu-Au deposit and hydrothermal alteration zones (potassic, phyllic, propylitic,argillic). The mineralogic-petrographic and geochemical properties of the extensive argillic alteration zones (notsupergene alteration described previously) were determined by optical and scanning electron microscopy (SEM andSEM-EDS), X-ray diffraction (XRD), O-H isotope geochemistry and K-Ar dating. Within the primary porphyritictexture, fine-grained clay and quartz crystals are present as homogeneous and partly micro-laminations in the moreintensely argillized granodiorite porphyry. Smectite and mixed-layer illite-smectite (I-S) minerals exhibit flaky/sponge-like and fibrous shapes, respectively, whereas illites and kaolinites show platy/tabular crystal shapes. Quartzand jarosite have euhedral crystals, cristobalites and crandallites (first determined here) are shown as worm-likeand fine-grained granular aggregates, respectively, developed within the pores indicating direct precipitation fromthe hydrothermal fluids. The samples from the argillic zone display quartz + I-S in the inner parts (advanced argilliczone) close to phyllic zone, whereas quartz + smectite + kaolinite associations are towards outer parts (argilliczone). Smectites have a dioctahedral composition (d060<1.500 Å, octahedral Al=1.47-1.66 a.p.f.u). I-S mineralshave a high illite component (I85-S15) with R3 type ordering of interlayering. The tetrahedral Al and interlayer Kcontents are 1.66-1.71 and 0.58-0.75, respectively. Oxygen and hydrogen isotope composition of I-S indicate thelow temperature conditions from the magmatic water dominant fluids. The data demonstrate the argillic alteration was widely distributed and developed under low temperature (<200 ºC),acidic conditions in association with the aluminum phosphate and iron phosphate minerals.