ORIGINAL PAPER Mineralogy, geochemistry and fluid evolution of a fossil hydrothermal system in the Paleogene Mendejin volcanic sequence, East Azarbaijan, Iran A. Karimzadeh Somarin & D. R. Lentz Received: 6 February 2006 / Accepted: 10 March 2008 / Published online: 24 April 2008 # Springer-Verlag 2008 Abstract The Mendejin area is one prominent hydrother- mal alteration zone in association with Paleogene volcano- sedimentary sequences in NW Iran. The volcanic sequence at Mendejin ranges in composition from basalt to andesite, dacite and rhyolite. Sulfide mineralization and related hydrothermal alteration is associated with the late Mendejin pluton that discordantly intrudes into this cogenetic volcanic sequence. The common hypogene hydrothermal alteration types observed at Mendejin include; phyllic, propylitic, carbonatization, sulfidation, silicification (veins) and argillic types with locally abundant calcite, quartz, tourmaline, sericite, chlorite, kaolinite, illite, alunite, jar- osite, and gypsum. Phyllic alteration (without calcite and epidote) is paragenetically early and it is followed by propylitic alteration (with epidote and little calcite) and carbonatization (with high calcite and no epidote) reflecting a gradual increase of CO 2 in the hydrothermal system. Mass balance of the hydrothermal alteration within the various volcanic rocks indicates that most elements released during alteration were locally fixed in the low-temperature hydrothermal minerals. As a result, geochemical changes are not as prominent as mineralogical transformations. There is no net mass change during phyllic, argillic, silicic vein and carbonate alteration of andesite and basalt. However, propylitic alteration and carbonatization of dacite caused net mass additions of 8% and 17%, respectively. The high-field-strength elements (HFSE) like Ti, Zr, Hf, Th, Nd, Y, La, Ce, Sm and Lu were the immobile elements during hydrothermal alteration at Mendejin. However, the rare-earth elements (REE) were mobilized during carbonate alteration. Fluid inclusions in quartz and calcite from various assemblages show that hydrothermal minerals were deposited from a low salinity (0.35 to 4.34 wt% equivalent NaCl) hydrothermal solution at 385°C to 150°C. Fluid-wall rock reactions along with boiling of hydrothermal solution and mixing of relatively saline fluid (4.34 wt% equivalent NaCl) with a low-salinity groundwater (0.35 wt% equiva- lent NaCl) resulted in the extensive alteration of the volcanic country rocks and precipitation of hydrothermal assemblages at Mendejin. Introduction Continental collision between the Afro-Arabian continent and the Iranian microcontinent during closure of the Tethys ocean in the Late Cretaceous resulted in the development of a volcanic arc in NW Iran (Mohajjel and Fergusson 2000; Babaie et al. 2001; Karimzadeh Somarin 2005). Emplacement of subduction-related granitic intrusions into Miner Petrol (2008) 94:123–143 DOI 10.1007/s00710-008-0008-3 Editorial handling: R. Abart A. Karimzadeh Somarin (*) Department of Geology, University of Tabriz, 51664 Tabriz, Iran e-mail: somarina@brandonu.ca D. R. Lentz Department of Geology, University of New Brunswick, Fredericton, NB, Canada Present address: A. Karimzadeh Somarin Department of Geology, Brandon University, Brandon, MB R7A 6A9, Canada