The Zhireken porphyry Mo ore-magmatic system (eastern Transbaikalia): U–Pb age, sources, and geodynamic setting

Abstract

Two intrusive complexes are recognized in the Zhireken deposit: Amanan and ore-bearing porphyry. According to the ages obtained by U–Pb zircon dating (Amanan complex—162.6 ± 1.4 Ma, granites and monzonite-porphyry of the ore-bearing complex—159.0 ± 1.6 and 157.5 ± 2.9 Ma), the Amanan complex formed at the end of collision, and the ore-bearing porphyry complex, during the change of the geodynamic regime by the postcollisional (rift) one. The rocks of two complexes have high contents of LILE and LREE and low contents of HFSE and HREE. The (87Sr/86Sr)0ratio in the gabbro and granites of the Amanan complex is 0.70501 and 0.70534, respectively, and that in the rocks of the porphyry complex is within 0.70451–0.70633. The Amanan gabbro, gabbro-diorites, and granites are characterized by εNd(T) = –1.4, –1.8, and –10.3, respectively, and the rocks of the ore-bearing complex, by εNd(T) = –3.7 to +1.0. The model TNd(DM) age of the Amanan granites is 1.5 Ga, and that of the granites and porphyry of the ore-bearing complex is 1.0–0.8 Ga. The Pb isotope ratios in the rocks of the Amanan and porphyry complexes are: 206Pb/204Pb = 18.086–18.136 and 18.199–18.442, 207Pb/204Pb = 15.487–15.499 and 15.506–15.545, and 208Pb/204Pb = 38.046–38.256 and 38.230–38.456. The results of geological, geochemical, and isotope studies admit that magmas were generated from juvenile and ancient crusts. Melts probably ascended from a depth of no less than 55 km during the melting of crust thickened as a result of tectonic deformations (in the upper horizons) and during the basic-magma supply (in the lower horizons). Juvenile mafic crust is considered to be the major source of fluid components and metals. Favorable conditions for the ore generation in the magmatic system during the formation of the porphyry complex arose at the previous stage, during the formation of the Amanan complex, which we regard as a preparatory stage in the evolution of the long-living ore-magmatic system.© 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.