Epithermal Base Metal - Silver Mineralization at Gomish-Tappeh Deposit, Southwest of Zanjan

Gomish-Tappeh Zn-Pb-Cu (Ag) deposit is located in northwestern part of Urumieh-Dokhtar volcano-plutonic zone، 90 km southwest of Zanjan. Exposed rocks at the area include Oligo-Miocene volcano-sedimentary and sedimentary sequences as well as Pliocene dacitic subvolcanic dome، rhyodacitic volcanics and andesite porphyry dykes. The main mineralization at Gomish-Tappeh deposit has occurred in a steeply deeping normal fault and fracture system defined by NE-SW trend in three stages including hydrothermal breccias، silicic-sulfidic، silicic-sulfidic-carbonate veins and veinlets and late banded veins (rich in silica and specularite). Host rocks to mineralization include dacitic crystal lithic tuff، dacitic subvolcanic dome، and specifically acidic tuff. Paragenetic minerals at the deposit consist of pyrite، arsenopyrite، chalcopyrite، bornite، galena، low-Fe sphalerite، tetrahedrite، tennantite and specularite. The main alteration types at the area are silicic، silicic-sulfidic، sericitic، carbonate، argillic and propylitic. Based on element distribution and frequency patterns in the ore samples، among base metals، Zn، Pb، Cu and Ag show the highest concentrations. Average grades in the ore veins at Gomish-Tappeh deposit are: 6% Zn، 4% Pb، 2% Cu، 88 ppm Ag and 44 ppb Au. Fluid inclusion microthermometric studies on quartz crystals of the first and second stages of mineralization indicate homogenization temperatures of 260-367 °C، salinities of 9. 1-16. 9 wt% NaCl equiv.، and approximate mineralization depth of 956 m below the paleowater table. Considering high salinity fluids and base metal contents، it is likely that base metals and silver were transported by chloride complexes. Fluid inclusion studies، hydrothermal breccias، banded-colloform-crustiform textures and amorphous silica indicate that boiling is the main factor for instability of the complexes and eventually، ore deposition.