پديد آورندگان :
فيضي، مريم نويسنده دانشكده علوم,گروه زمين شناسي,دانشگاه زنجان,ايران Feyzi, Maryam , ابراهيمي، محمد نويسنده دانشكده علوم,گروه زمين شناسي,دانشگاه زنجان,ايران Ebrahimi, Mohammad , كوهستاني، حسين نويسنده دانشكده علوم,گروه زمين شناسي,دانشگاه زنجان,ايران Kouhestani, Hossein , مختاري، علي اصغر نويسنده دانشكده علوم,گروه زمين شناسي,دانشگاه زنجان,ايران Mokhtari, Mir Ali Asghar
چكيده لاتين :
Introduction
The Aqkand Cu occurrence, 48 km north of Zanjan, is located in the Tarom subzone of the Western AlborzAzerbaijan structural zone. Apart from small scale geological maps of the area, i.e., 1:250,000 geological maps of BandareAnzali (Davies, 1977) and 1:100,000 geological maps of Hashtjin (Faridi and Anvari, 2000) and a number of unpublished perlite exploration reports, prior to this research no work has been done on Cu mineralization at Aqkand.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 ICPMS 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 finegrained groundmass. The Oligocene acidic rocks consist of rhyoliterhyodacite, perlite, pitchstone and ignimbrite. These rocks are exposed as domes and lava flows. The rhyoliterhyodacite lavas usually show onionskin weathering and locally display flow bands. Rapid cooling of rhyoliticrhyodacitic 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 rhyoliticrhyodacitic domes.
Copper mineralization at Aqkand occurs as Cubearing quartzfluorite 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 NWtrend and mostly dips NE. Six stages of mineralization can be distinguished at the Aqkand Cu occurrence. Stage1 is characterized by <5 mm fluorite veinveinlets. Clast of this stage have been recognized in the hydrothermal cements of stage2. Stage2 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. Stage3 is marked by <2 mm wide vein and veinlets of quartz with oligist that usually cut stage2 mineralization, and, in turn, are cut by stage4 quartz veins. Stage4 is represented by 2 mm wide individual or sets of quartz veins. No sulfide minerals are recognized with stage4. Stage5 is dominated by chlorite as veinveinlets and vug infill. Malachite, azurite and Fehydroxides formed during the supergene stage (Stage6). They are usually show veinveinlet 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 veinveinlet and hydrothermal breccia cements, and show veinveinlet 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 veinveinlet, 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 rhyoliticrhyodacitic lavas and the mineralized samples at Aqkand indicate that oreforming fluids are probably related to Oligocene rhyoliticrhyodacitic 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 TaromHashtjin 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 BandareAnzali, 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 HushmandZadeh, A., 1966. Geology of the Tarom District, Western Part (Zanjan area northwest 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.
