TRACE AND RARE-EARTH ELEMENT REDISTRIBUTION AND O-H ISOTOPE GEOCHEMISTRY IN THE HYDROTHERMAL ALTERATION FACIES FROM THE CENTRAL URUMIEH-DOKHTAR MAGMATIC ARC, IRAN

The Urumieh-Dokhtar Magmatic Arc, part of the Tethyan Eurasian Metallogenic Belt, contains several exemplar copper porphyry deposit systems. We examine the propylitic, potassic (Cu-bearing) and phyllic alteration facies in the Eshtehard area and present new data on element mobility, mass transfer and the non‒CHARAC behavior of REE during alteration accompanying mineralization. Incompatible element abundances in hydrothermally‒altered volcanic sequences are characterized by: (i) depletion of alkalis and alkaline earth elements (Ba, Sr), (ii) variable behavior of first series transition elements (depletion in Cr, Co and Ni; enrichment in Zn and Cu), (iii) depletion of HREE, Hf and Th relative to U and LREE, and (iv) negative Ce and Eu anomalies, reflecting reducing conditions. The simultaneous occurrence of M and W shapeds in normalized trace element patterns and variations in isovalent element ratios (K/Ba, K/Rb, Y/Ho, Sr/Eu, Zr/Hf and Eu/Eu*) are interpreted as imperfect tetrad effect reactions between REE³⁺ and F-and Cl-rich fluids in hydrothermal systems. Propylitic–potassic samples have lower T₄ tetrad‒effect values (<0.32) than phyllic samples (>0.37). The effect of relatively low temperature fluids resulted in low T₄ tetrad‒effect values. Mass (ΔM) and volume (ΔV) were slightly depleted in the propylitic (‒3.80% and ‒5.66%), potassic (‒3.48% and ‒5.66%) and phyllic (‒2.52% and ‒2.91%) zones with isocon slopes of 1.06, 1.06 and 1.03, respectively. Fluid inclusion study yields homogenization temperatures ranging from 275-596 ºC and salinity ranges of 14.7‒62.1 wt.% in the alteration zones. The potassic–phyllic samples have δ¹⁸O and δD_fluid between +5.3 to +7.1‰ and ‒75 to ‒62 ‰, respectively. We conclude that the most reasonable agent for ore formation in the Eshtehard area is a magmatic fluid that underwent boiling and/or exchange with hydrous minerals.