The Copler porphyry-epithermal gold deposit is associated with middle Eocene intrusive rocks related to an extensional tectonic phase in the Tethyan Alpine-Himalayan orogenic belt. The intrusive rocks (granodiorite porphyry and diorite porphyry) were emplaced into Late Paleozoic-Mesozoic metapelite and metacarbonate rocks creating 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 (not supergene alteration described previously) were determined by optical and scanning electron microscopy (SEM and SEM-EDS), X-ray diffraction (XRD), O-H isotope geochemistry and K-Ar dating. Within the primary porphyritic texture, fine-grained clay and quartz crystals are present as homogeneous and partly micro-laminations in the more intensely 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. Quartz and jarosite have euhedral crystals, cristobalites and crandallites (first determined here) are shown as worm-like and fine-grained granular aggregates, respectively, developed within the pores indicating direct precipitation from the hydrothermal fluids. The samples from the argillic zone display quartz + I-S in the inner parts (advanced argillic zone) close to phyllic zone, whereas quartz + smectite + kaolinite associations are towards outer parts (argillic zone). Smectites have a dioctahedral composition (d 060 < 1.500 A, octahedral Al= 1.47-1.66 a. p. f. u). I-S minerals have a high illite component (I 85 -S 15) with R3 type ordering of interlayering. The tetrahedral Al and interlayer K contents are 1.66-1.71 and 0.58-0.75, respectively. Oxygen and hydrogen isotope composition of I-S indicate the low temperature conditions from the magmatic water dominant fluids. K/Ar age data from jarosite-bearing sample (43.6 +/- 1.0 My) indicates that the argillic alteration started during or shortly after (< 1 Ma) the plutonic intrusion. The data demonstrate the argillic alteration was widely distributed and developed under low temperature (< 200 C-o), acidic conditions in association with the aluminum phosphate and iron phosphate minerals.