Hypogene Zn carbonate ores in the Angouran deposit

Authors: Maria Boni H Albert Gilg Giuseppina Balassone Jens Schneider Cameron R Allen Farid Moore

Publish Date: 2007/07/24

Volume: 42, Issue: 8, Pages: 799-820

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Abstract

The worldclass Angouran nonsulfide Zn–Pb deposit is one of the major Zn producers in Iran with resources estimated at about 18 Mt at 28 Zn mainly in the form of the Zn carbonate smithsonite This study aims to characterize these carbonate ores by means of their mineralogy and geochemistry which has also been extended to the host rocks of mineralization and other local carbonate rock types including the prominent travertines in the Angouran district as well as to the local spring waters Petrographical chemical and stable isotope O H C Sr data indicate that the genesis of the Zn carbonate ores at Angouran is fairly distinct from that of other “classical” nonsulfide Zn deposits that formed entirely by supergene processes Mineralization occurred during two successive stages with the zinc being derived from a preexisting sulfide ore body A first main stage of Zn carbonates stage I carbonate ore is associated with both preexisting and subordinate newly formed sulfides whereas a second stage is characterized by supergene carbonates Zn and minor Pb coexisting with oxides and hydroxides stage II carbonate ore The coprecipitation of smithsonite with galena pyrite and arsenopyrite as well as the absence of Fe and Mnoxides/hydroxides and of any discernible oxidation or dissolution of the sphaleriterich primary sulfide ore shows that the fluids responsible for the main stage I carbonate ores were relatively reduced and close to neutral to slightly basic pH with high fCO2 Smithsonite δ18OVSMOW values from stage I carbonate ore range from 183 to 236‰ while those of stage II carbonate ore show a much smaller range between 243 and 249‰ The δ13C values are fairly constant in smithsonite of stage I carbonate ore 32–60‰ but show a considerable spread towards lower δ13CVPDB values 46 to −112‰ in stage II carbonate ore This suggests a hypogene formation of stage I carbonate ore at Angouran from lowtemperature hydrothermal fluids probably mobilized during the waning stages of Tertiary–Quaternary volcanic activity in an environment characterized by abundant travertine systems throughout the whole region Conversely stage II carbonate ore is unambiguously related to supergene weathering as evidenced by the absence of sulfides the presence of FeMnoxides and arsenates and by high δ18O values found in smithsonite II The variable but still relatively heavy carbon isotope values of supergene smithsonite II suggests only a very minor contribution by organic soil carbon as is generally the case in supergene nonsulfide depositsThanks are due to M Sadeghi Shiraz University for collecting part of the samples and to AE Annels and M Pittuck SRK for providing carbonate samples from the mining district We thank also R Mohammadi Niaei IZMDC and S Modabberi for help during fieldwork We are indebted to F Daliran Karlsruhe University Germany and G Borg Halle University Germany for fruitful discussions We acknowledge the help of U Struck LMU Munich Germany and W Stichler GSF Neuherberg Germany in measuring isotope compositions of carbonate and water samples respectively J Cleverley JCU Townsville Australia kindly provided a revised version of his Thermo2000 database This study was partly financed by funds of Università di Napoli to GB and MB

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