Phase separation,fluid mixing,and origin of the greisens and potassic episyenite associated with the Água Boa pluton,Pitinga tin province,Amazonian Craton,Brazil |
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| Authors: | Régis Munhoz Krás Borges Raimundo Netuno Nobre Villas Kazuo Fuzikawa Roberto Dall’Agnol Marcos Assunção Pimenta |
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| Institution: | 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland, Australia;1. Department of Geology, Saint Mary''s University, Halifax NS B3H 3C3, Canada;2. Department of Earth Sciences, Laurentian University, Sudbury ON P3E 2C6, Canada;3. Department of Geology and Mineralogy, Mongolian University of Science and Technology, P.O. Box 46/672 Ulaanbaatar, Mongolia;4. Department of Earth Science, National Taiwan Normal University, 88 Tingzhou Road Section 4, Taipei 11677, Taiwan;5. Department of Geological Sciences, University of Manitoba, Winnipeg, MBR3T 2N2, Canada;1. PaleoProterozoic Mineralisation Group, University of Johannesburg, Auckland Park 2006, South Africa;2. ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, TAS 7001, Australia;3. Materials Research Department, iThemba LABS, National Research Foundation, Somerset West 7129, South Africa;4. AGH University of Science and Technology, Faculty of Physics & Applied Computer Science, Al. A. Mickiewicza 30, 30-059 Krakow, Poland;5. Vinogradov Institute of Geochemistry and Analytical Chemistry, SB RAS, Irkutsk, Russia;1. Université de Lorraine, CNRS, GeoRessources, Boulevard des Aiguillettes B.P. 70239, F-54506 Vandoeuvre-lès-Nancy, France;2. BRGM-French Geological Survey, 3, Av. Claude Guillemin, BP 36009, 45060 Orléans Cedex 2, France;3. Département des Sciences de la Terre, Université de Genève, Rue des Maraîchers 13, 1205 Genève, Switzerland;4. Institut des Sciences de la Terre, Université de Lausanne, Quartier UNIL-Mouline, Bâtiment Géopolis, CH-1015 Lausanne, Switzerland;5. Ecole Nationale Supérieure des Mines de Nancy, Parc de Saurupt, F-54042 Nancy, France;6. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Xianlin University Town, Nanjing 210046, China;1. Universidade de Brasília, Instituto de Geociências, Brasília, DF 70910-900, Brazil;2. Department of Geological Sciences and Geological Engineering, Queen''s University, Kingston, Ontario K7L 3N6, Canada |
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| Abstract: | Based on petrographical data, three types of greisen have been characterized at the western border of Água Boa pluton: siderophyllite–topaz–quartz greisen (greisen 1), fluorite–phengite–quartz greisen (greisen 2) and quartz–chlorite–phengite greisen (greisen 3). Episyenites were also identified.Two fluids of independent origin interacted with the same protolith – a hornblende-biotite alkali feldspar granite – and produced both the greisens and potassic episyenite: (1) an acid, low-salinity (4–12 wt.% NaCl eq.), F-rich, relatively hot (400–350 °C) reduced aqueous-carbonic fluid (CH4–H2O–NaCl–FeCl2 ± KCl), which by immiscibility gave rise to fluid IA (aqueous) and IC (carbonic); and (2) a lower salinity (2–4 wt.% NaCl eq.) and temperature (200–150 °C) aqueous fluid (H2O–NaCl), which was responsible for all dilution processes. Fluid 1 seems to have had a magmatic-hydrothermal origin, while fluid 2 is probably surface-derived (meteoric water?). An alkaline, F-poorer and diluted equivalent of fluid IA was interpreted to have caused the episyenitization of the granite host rock as well as the formation of phengite-rich greisen 3. The continuos interaction of this fluid with the potassic episyenite produced a moderate- to high-salinity (20–24 wt.% NaCl eq.), low-temperature (200–100 °C) fluid (H2O–NaCl–CaCl2 ± KCl), leading to the formation of chlorite-rich zone of greisen 3 and late silicification of potassic episyenite.In the greisen 1, decreasing F-activity and increasing oxygen fugacity, as the system cooled down, favored the formation of a topaz-rich inner zone, which grades into a siderophyllite-rich zone outwardly. Greisen 2 was formed under more oxidizing conditions by fluids poorer in F than those trapped in the siderophyllite-rich zone.The oxidation of aqueous-carbonic fluid took place at three distinct stages: (i) below the FMQ buffer; (ii) between the FMQ and NNO buffers; and (iii) above the NNO buffer.The dissolution cavities generated during the episyenitization process increased the permeability of the altered rocks, resulting in an increase of the fluid/rock ratios at the potassic episyenite and greisen 3 sites.All these fluids were trapped under pressure conditions of <1.0 kbar, representing shallow crustal levels and are consistent with those that have been estimated for the Pitinga tin–granites.The oxygen fugacity, F-activity gradients and salinity variations that occurred during the cooling of the hydrothermal system, in addition to differences in permeability, were important factors in the formation of distinct greisens. They not only controlled the fluid compositional changes, but also caused the cassiterite and sulfide precipitation at the greisen sites. |
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