The Yermakovsky beryllium deposit,Western Transbaikal region,Russia: Geochronology of igneous rocks |
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Authors: | D A Lykhin V I Kovalenko V V Yarmolyuk E B Sal’nikova A B Kotov I V Anisimova Yu V Plotkina |
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Institution: | 1.Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry,Russian Academy of Sciences,Moscow,Russia;2.Institute of Precambrian Geology and Geochronology,Russian Academy of Sciences,St. Petersburg,Russia |
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Abstract: | The sequence of rock and ore formation at the Yermakovsky beryllium deposit is established on the basis of geological relationships
and Rb-Sr and U-Pb isotopic dating. The Rb-Sr age of amphibolitefacies regional metamorphism is determined for quartz-biotite-plagioclase
schist (266 ± 18 Ma) and dolomitized limestone (271 ± 12 Ma) of the Zun-Morino Formation. The U-Pb zircon age of premineral
gabbro is 332 ± 1 Ma. The Rb-Sr age of gabbro is somewhat younger (316 ± 8.3 Ma), probably owing to the effect of Hercynian
metamorphism on sedimentary rocks of the Zun-Morino Formation and gabbroic intrusion that cuts through it. The U-Pb zircon
age of gneissose granite of the Tsagan Complex at the Yermakovsky deposit is 316 ± 2 Ma, i.e., close to the age of metamorphism
superimposed on gabbro rocks. The U-Pb zircon age of preore granitic dikes, estimated at 325 ± 3 and 333 ± 10 Ma, is close
to the age of gabbro. The Ar/Ar age of amphibole from a granitic dike (302.5 ± 0.9 Ma) probably displays a later closure of
this isotopic system or the effect of superimposed processes. The Rb-Sr age of alkali syenite intrusion is 227 ± 1.9 Ma. The
U-Pb zircon age of alkali leucogranite stock pertaining to the Lesser Kunalei Complex is 226 ± 1 Ma, while the Rb-Sr age of
beryllium ore is 225.9 ± 1.2 Ma. These data indicate that beryllium ore mineralization is closely related in space and time
to igneous rocks of the Lesser Kunalei Complex dated at 224 ± 5 Ma and varying from gabbro to alkali granite in composition.
Thus, the preore Hercynian magmatism at the Yermakovsky deposit took place ∼330 Ma ago and was completed by metamorphism dated
at 271–266 Ma. The ore-forming magmatism and beryllium ore mineralization are dated at 224 ± 5 Ma. Postore magmatic activity
is scarce and probably correlated with tectonic melange of host rocks. |
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