The Bizmilen skarn-type iron deposit is one of the major operating mines in east-central Turkey. Middle Eocene (46.3-42.0 Ma) plutonic rocks (quartz diorite) were intruded into Triassic-Cretaceous limestones and Upper Cretaceous ophiolites forming skarns (garnet, diopside, epidote, scapolite and tremolite), iron mineralization and clay-bearing alteration zones (argillic alteration). Clay-rich rocks of the argillic zone, contain mainly quartz, calcite, dolomite, feldspar and clay/phyllosilicate minerals comprising dioctahedral smectite, kaolinite, mixedlayered illite-smectite (I-S) and illite, and trioctahedral vermiculite, serpentine/chrysotile, talc, chlorite and mixed-layered chlorite-smectite (C-S). Na-Ca smectite, I-S, illite and kaolinite are developed at both the quartz diorite-limestone contact and within the quartz diorite body. However, phlogopite, vermiculite, serpentine/chrysotile, talc and chlorite occur at the quartz diorite-ophiolite contacts, indicating the distributions of clay/phyllosilicate minerals are influenced primarily by the host-rock composition. Chlorite geothermometry data suggest similar to 300 degrees C for the retrograde stage of skarn formation and similar to 120 degrees C for the late argillic alteration overprint, and the formation of low-temperature clays (smectite, vermiculite, C-S) in areas of skarn formation. Kaolinite 1 M (one-layer monoclinic) and I-S (I-75-S-25, R1 ordering) are indicative of low-temperature ( < 200 degrees C) argillic alteration conditions. Optical and electron microscopy studies of kaolinite, smectite and I-S indicate a completely hydrothermal (neo-formation) origin, with the absence of any detrital input. The major and trace/REE wholerock analyses of clays show they have a composition between the analyses of quartz diorite and ophiolite rocks (i.e., serpentinite and serpentinized peridotite). However, the analyses are closest to those of the quartz diorite. The calculated delta O-18 and delta D values of fluids in equilibrium with the clays (using the average values of the fluid inclusion homogenization temperatures) indicate a pre-dominantly magmatic water source. Assuming the clayforming fluids originated from magmatic water, the delta 18O(V-SMOW) and delta DV-SMOW compositions indicate < 200 degrees C for smectite during the supergene or low-temperature argillic stage, but in excess of 200 degrees C for I-S and kaolinite during the hypogene or high-temperature argillic stage. Ar-40/Ar-39 age data of illite and I-S (37.5-25.5 Ma) indicate alteration started during the late Eocene, similar to 4 Ma after intrusion of the pluton at 42 Ma, and continued to the late Oligocene, with a duration of similar to 12 Ma. The age data imply a long period of cooling at depth, without exhumation, during epithermal alteration after the retrograde skarn stage. The data show that hydrothermal clays formed from magmatic fluids in a hydrothermal system that persisted well after the intrusion of the igneous body and are indicative of how long hydrothermal systems can persist in areas of mineralization.