geology, mineralization and geochemistry of the aqkand cu occurrence (north of zanjan, tarom-hashtjin zone)

introduction the aqkand cu occurrence, 48 km north of zanjan, is located in the tarom subzone of the western alborz-azerbaijan structural zone. apart from small scale geological maps of the area, i.e., 1:250,000 geological maps of bandar-e-anzali (davies, 1977) and 1:100,000 geological maps of hashtjin (faridi and anvari, 2000) and a number of unpublished perlite exploration reports, prior to this research no work has been done on cu mineralization at aqkand. the present paper provides an overview of the geological framework, the mineralization characteristics, and the results of geochemistry study of the aqkand cu occurrence with an application to the ore genesis. identification of these characteristics can be used as a model for exploration of this type of copper mineralization in the tarom area and elsewhere. materials and methods detailed field work has been carried out at different scales in the aqkand area. about 35 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 6 samples from ore zones at the aqkand occurrence were analyzed by icp-ms for trace elements and ree compositions at kimia pazhuh alborz co., isfahan, iran. results and discussion the oldest units exposed in the aqkand area are eocene volcanic rocks which are overlain unconformably by oligocene acidic rocks. the eocene units consist of lithic and vitric tuff with intercalations of andesitic basalt lavas (equal to karaj formation, hirayama et al., 1966). the andesitic basalt lavas show porphyritic texture consisting of plagioclase and altered ferromagnesian minerals set in a fine-grained groundmass. the oligocene acidic rocks consist of rhyolite-rhyodacite, perlite, pitchstone and ignimbrite. these rocks are exposed as domes and lava flows. the rhyolite-rhyodacite lavas usually show onion-skin weathering and locally display flow bands. rapid cooling of rhyolitic-rhyodacitic lavas has resulted in the formation of volcanic glasses (obsidian). hydration of these volcanic glasses by hydrothermal fluids caused perlite formation which is located in the lower parts of the rhyolitic-rhyodacitic domes. copper mineralization at aqkand occurs as cu-bearing quartz-fluorite veins in eocene andesitic basalt lavas. the main ore vein reaches up to 50 m in length and average of 2 m in width. it has nw-trend and mostly dips ne. six stages of mineralization can be distinguished at the aqkand cu occurrence. stage-1 is characterized by <5 mm fluorite vein-veinlets. clast of this stage have been recognized in the hydrothermal cements of stage-2. stage-2 is the most abundant, widespread, and economically important ore forming stage at aqkand and is represented by quartz and chalcopyrite veins (up to 10 mm wide) and breccias cement. stage-3 is marked by <2 mm wide vein and veinlets of quartz with oligist that usually cut stage-2 mineralization, and, in turn, are cut by stage-4 quartz veins. stage-4 is represented by 2 mm wide individual or sets of quartz veins. no sulfide minerals are recognized with stage-4. stage-5 is dominated by chlorite as vein-veinlets and vug infill. malachite, azurite and fe-hydroxides formed during the supergene stage (stage-6). they are usually show vein-veinlet and vug infill textures. the hydrothermal alteration assemblages at aqkand grade from proximal quartz and chlorite to distal sericite, epidote, calcite and chlorite (propylitic alteration). the quartz and chlorite alteration types are spatially and temporally closely associated with cu mineralization. the propylitic alteration marks the outer limit of the hydrothermal system. the ore minerals at aqkand formed as vein-veinlet and hydrothermal breccia cements, and show vein-veinlet and disseminated textures. chalcopyrite is the main ore which is accompanied by minor oligist. malachite, goethite and lepidocrocite are supergene minerals. quartz, fluorite and chlorite are present in the gangue minerals that represent vein-veinlet, breccia, vug infill, replacement, colloform, cockade, crustiform and plumose textures. comparison of chondrite normalized (nakamura, 1974) ree patterns of eocene fresh and barren andesitic basalt lavas, oligocene rhyolitic-rhyodacitic lavas and the mineralized samples at aqkand indicate that ore-forming fluids are probably related to oligocene rhyolitic-rhyodacitic lavas. however, leaching of some elements from the host andesitic basalt lavas may have also been involved in mineralization. the geological, mineralogical, geochemical, textural and structural characteristics of the aqkand occurrence reveals that copper mineralization at aqkand is an example of epithermal base metal mineralization within the tarom-hashtjin zone. acknowledgements the authors are grateful to the university of zanjan grant commission for research funding. mr. a.a. atashnema is also appreciated for his help during the field work. journal of economic geology reviewers and editor are also thanked for their constructive suggestions on alterations to the manuscript. references davies, r.g., 1977. geological map of bandar-e-anzali, scale 1:250,000. geological survey of iran. faridi, m. and anvari, a., 2000. geological map of hashtjin, scale 1:100,000. geological survey of iran. hirayama, k., samimi, m., zahedi, m. and hushmand-zadeh, a., 1966. geology of the tarom district, western part (zanjan area north-west iran), geological survey of iran, tehran, report 8, 31 pp. nakamura, n., 1974. determination of ree, ba, fe, mg, na and k in carbonaceous and ordinary chondrites. geochimica et cosmochimica acta, 38(5): 755–773.