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Title

Ore Formation During Jurassic Subduction of the Tethys Along the Eurasian Margin: Constraints from the Kapan District, Lesser Caucasus, Southern Armenia

Authors
Selby, David
Published in Economic Geology. 2019
Abstract The Kapan mining district in the southernmost Lesser Caucasus is one of the unique locations along the Central metallogenic Tethyan belt where ore-forming processes were associated to magmatic arc growth during Jurassic Neotethys subduction along the Eurasian margin. Three ore deposits of the Kapan district were investigated in this study, including Centralni West, Centralni East and Shahumyan. The ore deposits are hosted by Middle Jurassic andesitic to dacitic volcanic and volcaniclastic rocks of tholeiitic to transitional affinities below a late Oxfordian unconformity, which is covered by calc-alkaline to transitional Late Jurassic-Early Cretaceous volcanic rocks interlayered with sedimentary rocks.. The mineralization consists of veins, subsidiary stockwork and partial matrix-replacement of breccia host rocks, with chalcopyrite, pyrite, tennantite-tetrahedrite, sphalerite and galena as the main ore minerals. Centralni West is a dominantly Cu deposit, and its host rocks are altered to chlorite, carbonate, epidote and sericite. At Centralni East, Au is associated with Cu, and the Shahumyan deposit is enriched in Pb and Zn as well as precious metals. Both deposits contain high-sulfidation state mineral assemblages with enargite and luzonite. Dickite, sericite and diaspore prevail in altered host rocks in the Centralni East deposit. At the Shahumyan deposit, phyllic to argillic alteration with sericite, quartz, pyrite and dickite is dominant along polymetallic veins, and advanced argillic alteration with residual quartz is locally developed. The lead isotopic composition of sulfides and alunite (206Pb/204Pb = 18.17-18.32, 207Pb/204Pb = 15.57-15.61, 208Pb/204Pb = 38.17 - 38.41) indicates a common metal source for the three deposits, and suggests that metals were derived from magmatic fluids that were exsolved upon crystallization of Middle Jurassic intrusive rocks or leached from Middle Jurassic country rocks. The δ18O values of hydrothermal quartz (8.3 to 16.4‰) and the δ34S values of sulfides (2.0 to 6.5‰) reveal a dominantly magmatic source at all three deposits. Combined oxygen, carbon and strontium isotopic compositions of hydrothermal calcite (δ18O = 7.7 to 15.4‰, δ13C = -3.4 to 0.7‰, 87Sr/86Sr = 0.70537 to 0.70586) support mixing of magmatic-derived fluids with seawater during the last stages of ore formation at Shahumyan and Centralni West. 40Ar/39Ar dating of hydrothermal muscovite at Centralni West and of magmatic-hydrothermal alunite at Shahumyan yield, respectively, a robust plateau age of 161.78±0.79 Ma and a disturbed plateau age of 156.14±0.79 Ma. Re-Os dating of pyrite from the Centralni East deposit yields an isochron age of 144.7±4.2 Ma, and a weighted average age of the model dates of 146.2±3.4 Ma, which are younger than the age of the immediate host rocks. Two different models are offered depending on the reliability attributed to the disturbed 40Ar/39Ar alunite age and the young Re-Os age. The preferred interpretation is that the Centralni West Cu deposit is a volcanogenic massive sulfide deposit, and the Shahumyan and Centralni are parts of porphyry-epithermal systems, which are broadly coeval or which were formed within a short time interval in a nascent magmatic arc setting, before the late Oxfordian. Alternatively, but less likely, the three deposits could represent different mineralization styles successively emplaced during evolution and growth of a magmatic arc during a longer time frame between the Middle and Late Jurassic.
Keywords Lesser CaucasusArmeniaTethys metallogenic beltKapan districtOre deposits
Identifiers
Full text
Structures
Research group Mineral Resources and Geofluids
Projects FNS: 200020-121519, 200020-138130, and 200020-168996
SCOPES Joint Research Project IZ73Z0-128324
Citation
(ISO format)
MEDERER, Johannes et al. Ore Formation During Jurassic Subduction of the Tethys Along the Eurasian Margin: Constraints from the Kapan District, Lesser Caucasus, Southern Armenia. In: Economic Geology, 2019. https://archive-ouverte.unige.ch/unige:116857

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Deposited on : 2019-05-02

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