The origin, age and duration of hydrothermal alteration associated with iron skarn mineralization determined from clay/phyllosilicate minerals, Bizmişen-Erzincan, East-Central Turkey

Abstract

The Bizmişen 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, mixed-layered 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 ~300 °C for the retrograde stage of skarn formation and ~120 °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 (I75-S25, R1 ordering) are indicative of low-temperature (<200 °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 whole-rock 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 δ18O and δD 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 clay-forming fluids originated from magmatic water, the δ18OV-SMOW and δDV-SMOW compositions indicate <200 °C for smectite during the supergene or low-temperature argillic stage, but in excess of 200 °C for I-S and kaolinite during the hypogene or high-temperature argillic stage. 40Ar/39Ar age data of illite and I-S (37.5–25.5 Ma) indicate alteration started during the late Eocene, ~4 Ma after intrusion of the pluton at 42 Ma, and continued to the late Oligocene, with a duration of ~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.