Abstract

Introduction Qoyjeh Yeylaq Pb-Zn (Ag) deposit, 120 km southeast of Zanjan, is located in the Urumieh-Dokhtar magmatic arc. Apart from small scale geological maps of the area, i.e., 1:250,000 geological maps of Kabudar Ahang (Bolourchi and Hajian, 1979) and 1:100,000 geological maps of Marzban (Majidifard and Shafei, 2006) and a number of unpublished Pb-Zn exploration report, prior to this research no work had be done on Pb-Zn (Ag) mineralization at Qoyjeh Yeylaq. The present paper provides an overview of the geological framework, the mineralization characteristics, and the results of geochemistry study of the Qoyjeh Yeylaq deposit with an application to the ore genesis. Identification of these characteristics can be used as a model for exploration of this type of Pb-Zn (Ag) mineralization in this area and elsewhere.   Materials and methods Detailed field work has been carried out at different scales in the Qoyjeh Yeylaq area. About 26 polished- thin and thin sections from host rocks and mineralized and altered zones were studied by conventional petrographic and mineralogic methods at the University of Zanjan. In addition, a total of 11 samples from fresh and altered host rocks and ore zones at the Qoyjeh Yeylaq deposit were analyzed by ICP-MS for trace elements and REE compositions at Zarazma Co., Tehran, Iran.   Results and Discussion The host rocks at Qoyjeh Yeylaq consist of Oligo-Miocene volcano-sedimentary rocks which are overlain conformably by Oligo-Miocene sedimentary rocks. Volcanic rocks are mostly basaltic andesite and andesite lava flows. Basaltic andesites with porphyritic texture consist predominantly of plagioclase (70 vol.%) and clinopyroxene (25 vol.%) phenocrysts with accessory hornblende (<5 vol.%) crystals. The andesites consists of plagioclase (75 vol.%), hornblende (15 vol.%), and clinopyroxene (10 vol.%) phenocrysts set in a fine-grained groundmass. The Oligo-Miocene sedimentary units consist of alternation of sandstone, red marl, and siltstone as well as medium-bedded to massive limestone with interlayers of tuff and shale. The Miocene sedimentary units consist mostly of alternations of red and green marl and red to grey sandstone. Mineralization at Qoyjeh Yeylaq occurs as quartz-sulfide veins in Oligo-Miocene basaltic andesite and andesite lavas. The ore zone reach up to 150 m in length and 10 m in width. It has NNW-trending and mostly dip 70-80^o to SW. Three stages of mineralization can be distinguished at Qoyjeh Yeylaq deposit. Stage-1 is the most abundant, widespread, and economically important ore forming stage at Qoyjeh Yeylaq and is represented by quartz and sulfide (galena, sphalerite, and chalcopyrite) veins (up to 5 mm wide) and breccias cement. Stage-2 is represented by 2 mm wide individual or sets of late calcite veins and veinlets that usually cut stage-1 mineralization. No sulfide minerals are recognized with stage-2. Covellite, cerussite, and Fe-oxides and hydroxides formed during the supergene stage (stage-3). They are usually show replacement and vug infill textures. The hydrothermal alteration assemblages at Qoyjeh Yeylaq grade from proximal quartz and calcite to distal sericite, epidote, calcite and chlorite (propylitic alteration). The quartz and calcite alteration types are spatially and temporally closely associated with Pb-Zn (Ag) mineralization. The propylitic alteration marks the outer limit of the hydrothermal system. The ore minerals at Qoyjeh Yeylaq formed as vein-veinlet and hydrothermal breccia cements, and show vein-veinlet, vug infill, and disseminated textures. Galena, sphalerite, and chalcopyrite are the main ore minerals; covellite, cerussite, and goethite are supergene minerals. Quartz, and calcite are present in the gangue minerals that represent vein-veinlet, breccia, vug infill, and replacement textures.  Comparison of Chondrite normalized (Nakamura, 1974) REE patterns of Oligo-Miocene fresh and altered basaltic andesite and andesite lavas, and the mineralized samples at Qoyjeh Yeylaq indicate that mineralization is probably genetically related with basaltic andesite and andesite lavas. In this case, leaching of some elements from the host basaltic andesite and andesite lavas may have involved in mineralization. The geological, mineralogical, geochemical, textural and structural characteristics of the Qoyjeh Yeylaq deposit reveals that mineralization at Qoyjeh Yeylaq is an example of intermediate-sulfidation type of epithermal base metal (Ag) mineralization.