Mineralogy, geochemistry and fluid evolution of a fossil hydrothermal system in the Paleogene Mendejin volcanic sequence, East Azarbaijan, Iran

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The Mendejin area is one prominent hydrothermal 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, jarosite, 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 CO2 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% equivalent NaCl) resulted in the extensive alteration of the volcanic country rocks and precipitation of hydrothermal assemblages at Mendejin.