Timing and metal sources for carbonate-hosted Zn-Pb mineralization in the Franklinian Basin (North Greenland): Constraints from Rb-Sr and Pb isotopes |
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| Institution: | 1. Department of Petrology and Economic Geology, GEUS - Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 København K, Denmark;2. Department of Mineralogy, Technische Universität Bergakademie Freiberg, Brennhausgasse 14, 09596 Freiberg, Germany;3. Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland;1. Department of Earth Sciences, National Cheng Kung University, 70101 Tainan, Taiwan;2. Earth Dynamic System Research Center, National Cheng Kung University, 70101 Tainan, Taiwan;3. School of Geography, South China Normal University, 510631 Guangzhou, China;4. Institute of Earth Sciences, Academia Sinica, 11529 Taipei, Taiwan;5. Department of Oceanography, National Sun Yat-sen University, 80424 Kaohsiung, Taiwan;6. Guangzhou Climate & Agro-meteorology Center, Guangzhou 511430, China;1. Grupo Recursos Minerales, Dpto. Ciencias de la Tierra, Universidad de Zaragoza, c/ Pedro Cerbuna 12 (Edificio Geológicas), 50009 Zaragoza, Spain;2. Department of Geological Sciences, Swedish Museum of Natural History, Frescativagen 40, Box 50007, SE-104 05 Stockholm, Sweden;3. CADIC-CONICET, c/ Houssay 200, 9410 Ushuaia, Tierra del Fuego, Argentina |
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| Abstract: | Carbonate-hosted Zn-Pb (± barite, fluorite) occurrences in the Franklinian Basin of North Greenland were studied using the Rb-Sr method, applied to sphalerite, and combined Sr and Pb isotope analysis of ore and gangue minerals, to place constraints on their age and genesis. The occurrences are located in the easternmost part of the basin in Peary Land and Kronprins Christian Land, and are hosted by Upper Ordovician to lowermost Silurian dolostones of the Turesø Formation.Sphalerite samples from the Zn-Pb occurrence at Børglum, in Peary Land, reflect undisturbed Rb-Sr systems and return an isochron age of 388 ± 4 Ma (MSWD = 1.6, 87Sr/86Sri = 0.70930 ± 1). Sphalerites and their fluid inclusion fractions from an adjacent Zn-Pb occurrence, at Tvilum, exhibit various degrees of isotopic disturbance caused by secondary fluid overprint. They yield no age, however, reconstructive modelling of their Rb-Sr and Pb-Pb signatures indicates original isotope systematics consistent with the Børglum data. Likewise, due to a lack of suitable samples, no Rb-Sr age could be determined for a third occurrence in southeastern Peary Land at Løgum, however, the time-integrated Pb isotopic evolution of fluorite and calcite from this mineralization over c. 390 Ma is compatible with modelled initial signatures for Tvilum and Børglum ores. Consequently, a contemporaneous formation at c. 390 Ma can be assumed for all studied locations in Peary Land. For the Zn-Pb occurrence in Kronprins Christian Land farther east, a Rb-Sr sphalerite age of c. 360–365 Ma has been obtained through a combined isochron and paleomixing line approach.The absolute ages obtained at the studied occurrences, corresponding to Middle to Upper Devonian ages, are in no conflict with the age of the host dolostones of the Turesø Formation, which placed a maximum age limit for the mineralization. Actually, the fact that mineralization was emplaced at least 50 Ma after deposition of the host rocks, which would have been completely lithified by then, warrants the classification of the studied occurrences as epigenetic Mississippi Valley-Type, rather than diagenetic Irish-type. Furthermore, the obtained ages are contemporaneous to the Middle or Upper Devonian to the Lower Carboniferous Ellesmerian Orogeny, indicating that the mineralization likely formed from basinal brines expelled by tectonism and/or hydraulic head caused by Ellesmerian orogenic uplift, as previously suggested for the Polaris carbonate-hosted Zn-Pb deposit (Cornwallis Island, Canada).Pb isotope systematics of base metal mineralization in the Franklinian Basin point to principal metal sources located in the crystalline basement and in basement-derived clastic sediments. These two reservoirs define radiogenic and unradiogenic end components from which lead was mobilized and mixed in different proportions during discrete periods of hydrothermal activity. Distinct thorogenic Pb isotope signatures indicate that specific local sources (lower crustal basement, carbonate rocks and possibly organic-rich shales) were also involved in mineralization. |
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