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ISOTOPIC COMPOSITION OF Pb AND SOURCE OF ORE MATTER OF BASE MATAL

DEPOSITS, SOUTH-EAST TRANSBAIKALIA

Tatarnikov S.A., Dril S.I., Vladimirova T.A., Sasim S.A.

Institute of Geochemistry SB RAS, Irkutsk, Russia, stat@igc.irk.ru

Forming of numerous base metal Pb-Zn deposits in East Transbaikalia is associated with episodes of intraplate magmatic activity which was the most intensive in the Jurassic-Cretaceous. To reveal sources of ore matter for these deposits, we studied Pb isotopic composition of sulfide minerals of some typical and well studied deposits of this type.

Isotopic composition of ore Pb was investigated for Mylnikovsko-Khorkirinskoe and Pochekuevskoe deposits. Both deposits are found in the zone of Kir-Kirinskiy massif of rare-metal morion granites of the Kukulbey complex. The first deposit is located in the zone of dolomites and limestones of the Nortuysk suite among Paleozoic granites. The age of suite is Riphean (Sanin, Zorina, 1978, Geological structure…, 1997). The second deposit is found in the zone tectonic dislocations and confined to limestones and schist of Birkinskaya suite (Arkhangelskaya, 1963, Geologic structure…, 1997).

The Akatuy deposit is found on north part of Aleksandorv-Zavodskaya rift depression close to the Akatuy massif of monzonites and Akatuy intrusive complex. All ore bodes are located in the Riphean limestone rocks of the Nortuysk suite.

Fig. 1. Points of isotopic composition of Pb of ores: 1 – Mylnikovsko-Khorkirinskoe; 2- Pochekuevskoe; 3- Akatuy; 4 – area of trachybasalt compositions of Late Jurassic - Early Cretaceous rift depression, East Transbaikalia; 5 – area of granites compositions of Kir-Kirinskiy massif of the Kukulbei complex East Transbaikalia; 6 – area of monzonites compositions of the Akatuy massif, East Transbaikalia. Incut shows lines of mixing of ore Pb of Pochekuevskoe (I), Mylnikovsko-Khorkirinskoe (II) and Akatuy (III) deposit. Areas of evolution of Pb isotopic composition of rocks of passive continental margins (PCM) and island arcs (IA) calculated from data (6,8), lines of evolution for upper continental crust and N-MORB – from data (9); SK – lines of Pb isotopic evolution from model by Stacey-Kramers.


Results obtained from studies of isotopic composition of Pb in galena, blendes and pyrites are shown on figure 1 for three deposits. All isotopic compositions of ore Pb are anomalous and can be interpreted as lines of mixing of Pb of different geochemical reservoirs: earth's crust and mantle. Lines of mixing of Pb for Milnikovsko-Khorkirinskoe and Pochekuevskoe deposits (trend I and II) determine the evolution of ore Pb for these deposits. The matter of upper crust with μ=10,55 and higher can be regarded as the crust source of lead. Composition of terrigenous sediments of passive continental margins satisfy this condition (Fig.1). The metaterrigenous rocks of the Birkinskaya suite of the Argunsky superterrane composing the major part of host rocks of deposits in the Klichka ore district can be regarded as terrigenous sediments of the passive continental margin (Parfenov, Popeko, Tomurtogoo, 1999). Lines of mixing of ore Pb of Akatuy deposit (trend III on incut) indicate other nature of crustal Pb source. The Neoproterozoic carbonate rocks of the Nortuysk suite can be regarded as such source. They host the mineralization. Nature of mantle source of matter can be confidently determined for ore Pb of the Akatuy deposit, as the trend evolution in its less radiogenic part adjoins the field of composition of gabbro- monzonite- monzonitic rock series of Akatuy massif. Granites of the Kir-Kirinskiy massif can be considered as the endogenous source for Milnikovsko-Khorkirinskoe and Pochekuevskoe deposits.

Investigations are supported by grant RFBR 05-05-64332, 06-05-64754, 08-05-00660.



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